DIRECT VAPOR PHASE PROCESS FOR SYNTHESIS OF MICROSPHERES IN

NETWORK OF NANOWIRES

BACKGROUND

[0001] In recent years there is tremendous amount of research being performed in developing nano-materials for sensors applications. Up to now, semiconducting metal oxide sensors have been widely investigated due to their small dimensions, low cost, and high compatibility with microelectronic processing. The nanoscale materials exhibit quantum confinement effects with distinct electronic, optical, chemical and thermal properties. One reason is that the surface-to-bulk ratio for the nano-sized materials is much greater than that for bulk counterparts wherein only a small fraction of the species adsorbed near the grain boundaries is active in modifying the electrical transport properties [S. Kan et al., Nature Materials, 2, 155 (2003)].

[0002] Nanostructure based sensors systems (ultraviolet radiation or gas) exhibit a fast response with a substantially higher sensitivity and selectivity than polycrystalline and crystalline bulk film based sensors. The shape of the nanostructures thus guides the application of a specific material under study [A. P. Alivisatos, Science 271, 933 (1996)]. The main advantage of combining such structures of varied dimensions and structures facilitate to fabricate versatile sensors for biological and chemical species in which receptors are added selectively (comparable diameter) for a particular type of bio chemical sensing. The sensors consisting of nanostructures [B. Liu, T. Ren, J. Zhang, H. Chen, J. Zhu, C. Burda, Electrochemistry Communications 9, 551 (2007)] with large surface area to volume ratio (spheres) have better response characteristics and higher sensitivity [X.L. Cheng, H. Zhao, L.H. Huo, S. Gao, J.G. Zhao, Sensors and Actuators B 102, 248 (2004)].

[0003] Conventional techniques for fabrication of nanoscale materials, however, typically fail to provide methods for preparation at reasonable temperatures. Further, each conventional technique for fabrication of a nanoscale material typically provides only one type of nanoscale architechture (e.g., spheres or wires) and, thus, fails to provide methods for substantially simultaneous preparation of a plurality of nanoscale materials (e.g., spheres and wires) in a single operation. Moreover, each conventional technique for fabrication of such

materials typically provides only one scale architechture (e.g., nanoscale or microscale) and, thus, fails to provide methods for substantially simultaneous preparation of a plurality of scales (e.g., nanoscale and microscale) in a single operation.

[0004] Therefore, there remains a need for methods and compositions that overcome these deficiencies and that effectively provide for preparation at reasonable temperatures, substantially simultaneous preparation of a plurality of nanoscale materials, and/or substantially simultaneous preparation of a plurality of scales.

SUMMARY

[0005] In accordance with the purpose(s) of the invention, as embodied and broadly described herein, the invention, in one aspect, relates to interconnected networks of transition metal oxide microspheres and nano wires.

[0006] Thus, disclosed are compositions comprising an interconnected network of transition metal oxide microspheres and nanowires. In one aspect, the interconnected network is fabricated in a single step.

[0007] Also disclosed are interconnected network of transition metal oxide microspheres and nanowires further comprising a pair of conductive input/output electrodes disposed in spaced- apart relationship, the interconnected network positioned in a region between the electrodes in conductive contact with the electrodes, thereby forming at least one programmable conductive pathway between the pair of electrodes.

[0008] Also disclosed are devices for detecting an environmental influence onto a sensor, by means of detecting a change in an electrical conductivity of a sensor layer of the sensor, wherein the sensor has a first electrode, a second electrode, and a sensor layer, which comprises: an excitation unit for generating electrical potentials, and a sensor layer having a conductivity that is dependent on environmental influences, wherein the sensor layer comprises an interconnected network of transition metal oxide microspheres and nanowires.

[0009] Also disclosed are methods for making an interconnected network of transition metal oxide microspheres and nanowires comprising the steps of evaporating a transition metal in

an inert gas/oxygen flowstream at temperature of less than about 900 ⁰ C; and depositing a transition metal oxide film on a substrate positioned downstream in a gas pressure gradient from the location of the evaporation step, wherein the transition metal oxide film comprises an interconnected network of transition metal oxide microspheres and nanowires.

[0010] Also disclosed are direct vapor phase deposition methods comprising the steps of evaporating a metal in an inert gas/oxygen flowstream at temperature of less than the temperature required for thermal deposition of the same metal; and depositing a metal oxide film on a substrate positioned downstream in a gas pressure gradient from the location of the evaporation step.

[0011] Also disclosed are products produced by the disclosed methods.

[0012] While aspects of the present invention can be described and claimed in a particular statutory class, such as the system statutory class, this is for convenience only and one of skill in the art will understand that each aspect of the present invention can be described and claimed in any statutory class. Unless otherwise expressly stated, it is in no way intended that any method or aspect set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not specifically state in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including matters of logic with respect to arrangement of steps or operational flow, plain meaning derived from grammatical organization or punctuation, or the number or type of aspects described in the specification.

BRIEF DESCRIPTION OF THE FIGURES

[0013] The accompanying figures, which are incorporated in and constitute a part of this specification, illustrate several aspects and together with the description serve to explain the principles of the invention.

[0014] Figure 1 is a schematic showing the growth of ZnO nanostructures.

[0015] Figure 2 shows ZnO nanostructures thick film on the walls of the tube furnace.

[0016] Figure 3 is an image of ZnO nanostructured thick film.

[0017] Figure 4 shows SEM of the nanostructure ZnO thin film (a) and (b) micro spheres with a network of nanowires, (c) and (d) enlarged view of ZnO nanowires.

[0018] Figure 5 shows (a) SEM of ZnO micro spheres with a network of nanowires, (b) enlarged view of nanowires.

[0019] Figure 6 (a) and Figure 6 (b) show SEM of ZnO micro spheres evolving into hexagons.

[0020] Figure 7 shows XPS general scan of ZnO nanostructure consisting of microspheres and nanowires corresponding to morphology shown in Figure 4.

[0021] Figure 8 shows PL spectrum of ZnO nanostructure thin film. The film comprises nanostructures corresponding to band gap of 3.2 eV. Note that the defect related emission at 512 nm is absent.

[0022] Figure 9 shows a schematic of a UV sensor formed from nanostructured ZnO thin film.

[0023] Figure 10 /- V characteristics of ZnO UV sensor with and without UV illumination.

[0024] Figure 11 shows photoresponse measurements of UV sensor as a function of electrode spacing. The inset shows the FWHM of photoresponse curves as a function of electrode spacing.

[0025] Figure 12 shows the rise and decay time of UV sensor for different electrode spacing.

[0026] Figure 13 shows SEM of the ZnO sample that contained network of nanowires connecting large microspheres.

[0027] Figure 14 shows SEM of the ZnO sample containing only nanowires.

[0028] Figure 15 shows photo luminescence spectra of ZnO nanowire and nanowire -microsphere morphology samples.

[0029] Figure 16 shows the photoresponse of ZnO with nanowire-microsphere morphology for electrode spacing of 80 μm. Inset: Corresponding 1 -V characteristics.

[0030] Figure 17 shows 1-V characteristics of ZnO UV sensors with nanowire-microsphere (top) and nanowire (bottom) morphology without (Left) and with (right) UV illumination.

[0031] Figure 18 shows the decay time ZnO UV sensors with nanowire-microsphere morphology and nanowire morphology at different O ₂ pressures.

[0032] Figure 19 shows schematic of catalyst free growth mechanism showing the growth of ZnO nanostructure

[0033] Figure 20 shows SEM of the (a) ZnO nanowire with the Zn catalyst on the edge of the nanowire involved in the growth in the form of catalyst, (b) EDX analysis of Zn droplets and ZnO nanowire. Note due to EDX accuracy the Zn imaging is done on the area exposing only Zn droplets.

[0034] Figure 21 shows (a) SEM of ZnO nanowires on AI2O3 substrates, (b) ZnO nanowire showing the hexagon with dimensions of 25 nm in diameter.

[0035] Figure 22 shows (a) SEM of ZnO nanowires on PLD-ZnO nucleation layer (NL) on AI ₂ O ₃ substrates, (b) and (c) well aligned ZnO nanorods.

[0036] Figure 23 shows XRD spectra of ZnO nanowires and ZnO nanowires on PLD-ZnO nucleation layer (NL) on AI ₂ O ₃ substrates (a) θ-2θ and (b) rocking curve plots.

[0037] Figure 24 shows Room temperature PL spectra of ZnO nanowires and ZnO nanowires on PLD-ZnO nucleation layer (NL) on AI ₂ O ₃ substrates showing the laser excitation, the dominant excitonic emission and the green band emission. Inset shows the blue light emission from the ZnO nanowires under laser excitation.

[0038] Figure 25 shows (a) SEM of a ZnO nanowire ~ 45nm in diameter, (b) the illuminated ZnO nanowire under electron beam (c) CL spectrum of ZnO nanowire indicating excitonic and green emission.

[0039] Figure 26 shows XPS spectra of ZnO nanowire film showing (a) Zn 2p and (b) Ols lines.

[0040] Figure 27 shows SEM OfV ₂ O ₅ wires on (a) Mo and (b) FS substrates

[0041] Figure 28 shows SEM OfV ₂ Os wires on (a) Si (111) substrate growing from a nanosized nucleation centers (marked), (b) AFM of squared location in (a). Note the orientation of the nanosized nucleation centers and the orientation of micron sized belts, (c) Height analysis of the nucleation centers OfV ₂ O ₅ .

[0042] Figure 29 shows XRD of V ₂ Os on Si substrate. VO ₂ monoclinic plane is indicated by highlighting.

[0043] Figure 30 shows PL OfV ₂ O ₅ on Si, FS and Mo substrates.

[0044] Figure 31 shows I-V OfV ₂ O ₅ on FS substrate without and under propanol exposure. The inset shows the schematic of the device.

[0045] Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or can be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

DESCRIPTION

[0046] The present invention can be understood more readily by reference to the following detailed description of the invention and the Examples included therein.

[0047] Before the present compounds, compositions, articles, systems, devices, and/or methods are disclosed and described, it is to be understood that they are not limited to specific synthetic methods unless otherwise specified, or to particular reagents unless otherwise specified, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be

limiting. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, example methods and materials are now described.

[0048] All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided herein can be different from the actual publication dates, which can need to be independently confirmed.

A. DEFINITIONS

[0049] As used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a functional group," "an alkyl," or "a residue" includes mixtures of two or more such functional groups, alkyls, or residues, and the like.

[0050] Ranges can be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as "about" that particular value in addition to the value itself. For example, if the value "10" is disclosed, then "about 10" is also disclosed. It is also understood that each unit between two particular units are also disclosed. For example, if 10 and 15 are disclosed, then 11, 12, 13, and 14 are also disclosed.

[0051] As used herein, the terms "optional" or "optionally" means that the subsequently described event or circumstance can or can not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0052] As used herein, the term "microsphere" refers to a generally round particle having a diameter in the range of microparticles (i.e., microscale). In one aspect, a microsphere can have a diameter of from about 0.5 μm to about 100 μm, for example, from about 0.5 μm to about 50 μm, from about 0.5 μm to about 20 μm, from about 0.5 to about 10 μm, from about 0.5 μm to about 3 μm, from about 0.5 μm to about 2 μm, from about 0.5 μm to about 1 μm, or from about 1 μm to about 3 μm. It is understood that a plurality of microspheres can have an average diameter in the aforementioned ranges. It is also understood that a microsphere can be approximately spherical, ovoid, oblong, cubic, or three-dimensional polygonal.

[0053] As used herein, the term "nanowire" refers to a generally linear structure having at least one dimension (e.g., width or cross-sectional diameter) in the range of nanoparticles (i.e., nanoscale). In one aspect, a nanowire can have a width or cross-sectional diameter of from about 5 nm to about 200 nm, for example, from about 10 nm to about 100 nm, from about 20 nm to about 80 nm, from about 40 nm to about 70 nm, from about 30 nm to about 60 nm, or from about 30 nm to about 65 nm. It is understood that a plurality of nano wires can have an average width or cross-sectional diameter in the aforementioned ranges. Typically, a nanowire can have a length in the range of nanoparticles or microparticles. In one aspect, a nanowire can have a length of from about 5 nm to about 5000 nm, from about 100 nm to about 5000 nm, from about 500 nm to about 5000 nm, from about 1000 nm to about 5000 nm, from about 100 nm to about 500 nm. In a further aspect, a nanowire can have a length of from about 0.5 μm to about 10 μm, from about 1 μm to about 10 μm, from about 1 μm to about 5 μm, or from about 2 μm to about 3 μm. It is understood that a plurality of nanowires can have an average length in the aforementioned ranges. It is also understood that the cross-sectional shape of a nanowire can be approximately round, oval, oblong, square, rectangular, substantially triangular, or polygonal.

[0054] As used herein, the term "interconnected network" refers to a composition of one or more materials wherein the various components of the composition are in contact. In one

aspect, a network can comprise generally spherical components (particles) and generally linear components (wires). The components can be on the microscale and/or on the nanoscale. In a further aspect, the contact can be physical contact, chemical contact, electrical contact, and/or photonic contact. In a further aspect, the components can be in communication via such contact (e.g., chemical, electrical, or photonic information can be transferred from one component to one or more other components via the interconnected network).

[0055] Disclosed are the components to be used to prepare the compositions of the invention as well as the compositions themselves to be used within the methods disclosed herein. These and other materials are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc. of these materials are disclosed that while specific reference of each various individual and collective combinations and permutation of these compounds can not be explicitly disclosed, each is specifically contemplated and described herein. For example, if a particular compound is disclosed and discussed and a number of modifications that can be made to a number of molecules including the compounds are discussed, specifically contemplated is each and every combination and permutation of the compound and the modifications that are possible unless specifically indicated to the contrary. Thus, if a class of molecules A, B, and C are disclosed as well as a class of molecules D, E, and F and an example of a combination molecule, A-D is disclosed, then even if each is not individually recited each is individually and collectively contemplated meaning combinations, A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F are considered disclosed. Likewise, any subset or combination of these is also disclosed. Thus, for example, the sub-group of A-E, B-F, and C- E would be considered disclosed. This concept applies to all aspects of this application including, but not limited to, steps in methods of making and using the compositions of the invention. Thus, if there are a variety of additional steps that can be performed it is understood that each of these additional steps can be performed with any specific embodiment or combination of embodiments of the methods of the invention.

[0056] It is understood that the compositions disclosed herein have certain functions. Disclosed herein are certain structural requirements for performing the disclosed functions, and it is understood that there are a variety of structures that can perform the same function

that are related to the disclosed structures, and that these structures will typically achieve the same result.

B. DIRECT VAPOR PHASE TECHNIQUES

[0057] The Direct Vapor Phase technique disclosed herein is a new methodology and process to fabricate complicated shapes and structures in a single operation. In short, this technique is a modification of the vapor transport and thermal evaporation method in which the chemical elements from the precursors are transported to the substrate location through the gas pressure gradient. In this technique, the evaporation can be carried out at much lower temperatures (-600-800 ⁰ C), as opposed to 1800-2000 ⁰ C in thermal evaporation. Localized regions are formed on the substrates upon evaporation of constituent elements. This process is simple, rapid, scalable, and cost effective, as opposed to processes such as MOCVD and MOVPE, wherein precursors are used in a chemical reaction between the activated oxygen to grow structures in presence of catalyst. This method can be applied to batch processing on a large scale and does not require any catalyst.

[0058] One principle behind nanostructure based sensors is the detection of small concentrations by measuring changes in electrical conductance produced by the adsorption of the chemical species onto specific shaped nanostructure [O.K. Tan, W. Cao, Y. Hu, W. Zhu, Ceramics International 30, 1127 (2004)]. Metal -Oxide nanostructured thin films having spheres of diameter approximately 40-65 nm prepared by sol-gel dip coating method for gas sensing applications have been studied and reported [C. Ge, C. Xie, S. Cai, Mater. Sci. Eng. B (2006), doi: 10.1016/j.mseb.2006.10.006]. Zhang et. al. [Y. Zhang, K. Yu, D. Jiang, Z. Zhu, H. Geng, L. Luo, Applied Surface Science, 242, 212 (2005)], have reported the fabrication of humidity sensors based on metal oxide nanorods and nanowires grown by vapor phase transport process on pre patterned platinum electrode substrates. However, most of the fabrication technology and research to date has been focused on demonstration of either nanowire or nanorods, or nanobelts or nanospheres "alone" structure based sensors. The combination of different shapes and dimensions of nanostructures achieved by process varying conditions leads to a completely different domain of applications [G. C. Yi, C. Wang, W. Park, Semicond. Sci. Technol. 20, S22-S34 (2005)].

[0059] The compositions and methods disclosed herein can exhibit at least the following advantages: (1) additional steps are not required to combine different shapes of active elements to add functionality; (2) the techniques can combine both spheres and wires in the micro- and/or nano-region; (3) because the wires are of nano dimensions and typically spaced about 10-30 nm, there is no need to put down contacts for electrical conduction by lithography technique; (4) the process can be easily adapted to most of the metal oxide materials; (5) the process can be used to alloy different material to alter the band gap so that different functionalities can be achieved; and (6) the process does not require high investment in setting up the technique

[0060] The compositions and methods disclosed herein can be employed in at least the following commercial applications: (1) ultra violet radiation sensing; (2) chemical sensors; (3) biohazard detection; (4) missile defense applications for UV exhaust monitoring from missiles; (5) flame sensors; (6) fire detection; (7) UV index monitoring (health); and (8) nanotechnology vendors marketing nanomaterials for laboratory applications.

C. ULTRA VIOLET SENSORS BASED ON NANOSTRUCTURED ZNO

[0061] Zinc oxide (ZnO) is a promising wide bandgap semiconductor for applications in ultra violet (UV) light emitting devices and sensors [U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Do, S. Doan, V. Avrutin, S.-J. Cho, H. Morkoc, J. Appl. Phys. 98, 041301 (2005)]. ZnO is a direct band-gap (Eg = 3.37 eV) semiconductor with a large exciton binding energy (60 me V) [Y.S. Park, CW. Litton, T.C. Collins, D.C. Reynolds, Phys. Rev. 143, 512 (1966)], exhibiting near UV emission, transparent conductivity [D. Ravinder, J.K. Sharma, J. Appl. Phys. 58, 838 (1985)] and piezoelectricity [J. S. Wang, K.M. Lakin, Appl. Phys. Lett. 42, 352 (1983). The high exciton binding energy in ZnO crystal can ensure an efficient excitonic emission at room temperature under low excitation intensity, which has potential applications in high efficiency light emitting diodes and UV lasers. ZnO has been widely reported as a visible blind UV sensor [G. Goncalves, A. Pimentel, E. Fortunato, R. Martins, EX. Queiroz, R.F. Bianchi, R.M. Faria, J. Non-Crystalline Solids 352, 14.44 (2006)] over a wide range of applications in military and non-military arenas [http://www.mda.millmdalink/pdf/materials.pdf] that includes missile plume detection for

hostile missile tracking, flame sensors, UV source monitoring, and calibration [http://gtresearchnews.gatech.edu/newsrelease/nanohelices.ht m]. Research in the sensor area has lead many researchers to explore the possibility of widening the band gap of ZnO by alloying with Cd [T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78, 1237 (2001)] and Mg so as to cover UV-A, UV-B and UV-C region of ultra violet region [S. S. Hullavarad, S. Dhar, B. Varughese, I. Takeuchi, T. Venkatesan, R.D. Vispute, J. Vac. Sci. Technol. A 23, 982 (2005)]. ZnO possess unique figures of merit, such as low thin-film growth temperatures (100-750 ⁰ C) [SJ. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50, 293 (2005)], and radiation hardness [F. D. Auret, S.A. Goodman, M. Hayes, M.J. Legodi, H.A. van Laarhoven, D.C. Look, Appl. Phys. Lett. 79, 3074 (2001)], which can be important for practical optoelectronic devices. Despite the challenges of reliable p-type doping in ZnO, there have been reports on fabrication of photodetectors [S. Liang, H. Sheng, Y. Liu, Z. Huo, Y. Lu, H. Shen, J. Cryst. Growth 225, 110 (2001)], quantum wells [T. Makino, A. Ohtomo, CH. Chia, Y. Segawa, H. Koinuma, M. Kawasaki, Physica E: Low-dimensional Syst. Nanostruct. 21, 671 (2004)], and superlattices [N.B. Chen, CH. Sui, Mater. Sci. Eng. B, 126, 16 (2006)] based on ZnO. As predicted [R.F. Service, Science 276, 895 (1997)], the observation of room-temperature UV lasing from the ordered, nano-sized ZnO crystals provides an important step for the development of practical blue -UV laser. Thus, ZnO nanoscale structures, such as one-dimension nanowires, are attracting more attention because of their enormous potential as fundamental building blocks for nanoscale electronic [M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292, 1897 (2001)] and photonic devices due to enhanced sensitivity offered by quantum confinement effects [N.I. Kovtyukhova, T.E. Mallouk, Chem. - Eur. J. 8, 4354 (2002)]. Sensors consisting of nanostructures [B. Liu, T. Ren, J. Zhang, H. Chen, J. Zhu, C Burda,

Electrochem. Commun. 9, 551 (2007)] with large surface area to volume ratio (spheres) typically have better response characteristics and higher sensitivity [X.L. Cheng, H. Zhao, L.H. Huo, S. Gao, J.G. Zhao, Sensor Actuator B 102, 248 (2004)]. ZnO nanostructured thin films consisting of spheres of diameter approximately 40-65 nm prepared by sol-gel dip coating method for gas sensing applications have been reported [C Ge, C Xie, S. Cai, Mater. Sci. Eng. B, Doi: 10.1016/ j.mseb.2006.10.006 (2006)]. Zhang et al. [Y. Zhang, K. Yu, D.

Jiang, Z. Zhu, H. Geng, L. Luo, Appl. Surf. Sci. 242, 212 (2005)], have reported the fabrication of humidity sensors based on ZnO nanorods and nanowires grown by vapor phase transport process on pre patterned platinum electrode substrates. The formation of different shapes of nano-structures depends largely on temperature, pressure, heating/cooling rates and the saturation of reactive elements in the gaseous phase during the reaction. There have been reports on fabrication of complex ZnO structures with morphology like mushroom [H. Wang, C. Xie, D. Zeng, Z. Yang, J. Colloid Interface Sci. DOI: yjcis 11765 (2006)], spheres [A. Umar, S.H. Kim, Y.H. Im, Y.B. Hahn, Superlattic. Microstruct. 39, 238 (2006)], ellipsoids, flowers and propellers [J. Liu, X. Huang, Y. Li, Q. Zhong, L. Ren, Mater. Lett. 60, 1354 (2006)]. Luo et al. [L. Luo, Y. Zhang, S.S. Mao, L. Lin, Sensor Actuator A, 127, 2006, 201 (2005)] have reported the fabrication of UV photodiode by forming a heterojunction of n- ZnO nanowires of diameter (70-120 nm) with p-type Si. The UV responsivity of 70 mA W ^"1 under UV illumination of 365 nm at unusually higher (20 V) reverse bias has been observed.

[0062] Nanostructured materials continue to be of great interest for fabricating sensors [L.F. Dong, Z. L. Cui and Z.K. Zhang, Nanostructured Materials 8, 815 (1997)] for many practical applications. These materials have unique properties because of the confinement of electrons at the nanoscale and large surface area and offer opportunities for tuning and enhancing sensor performance. Most chemical (i.e., gas) or radiation sensors made of nanoscale materials are two terminal devices that exhibit a change in the conductance due to the adsorption of a chemical species or the exposure to a radiation [A. Vaseashta and D. Dimova- Malinovska, Science and Technology of Advanced Materials, 6, 312 (2005); O.K. Tan, W. Cao, Y. Hu, W. Zhu, Ceramics International 30, 1127 (2004)]. The number of sensors made of metal oxides continues to grow because of the diversity of compositions, structures, and sensor/transducer modalities, the ease of processing, and compatibility with microelectronic processing. Nanostructure based sensors tend to exhibit a fast response with a substantially higher sensitivity and selectivity than bulk film based sensors [S. Kudera, L.Carbone, M.Zanella, R. Cingolani, W. J. Parak, L. Manna, Phys. Stat. Sol. (a) 203, 1329 (2006)]. Nanostructures of various shapes and sizes have been used for fabricating sensors in pursuit of integration at the nanoscale, to take advantage of a specific, size/shape related

performance, or simply to leverage an existing simple nanostructure fabrication process [A. P. Alivisatos, Science, 271, 933 (1996)].

[0063] Zinc oxide (ZnO) is a transparent, piezoelectric, direct band-gap (Eg = 3.37 eV) semiconductor with a large exciton binding energy (60 me V) [Y. S. Park, C. W. Litton, T. C. Collins, D. C. Reynolds, Phys. Rev., 143, 512 (1966)]. ZnO absorbs and emits in the UV band. Most of the ZnO based UV sensor research to date has dealt with vapor deposited thin (or thick) films or clusters (or forests) of nanostructures such as nanowires, nanorods, or nanobelts. Thin/thick film sensors have exhibited better performance than that of nanosensors but require very small (a few microns) spacing between electrical contacts which can only be produced photo lithographically. Effective lateral growth and assembly of nanostructures over a large area are difficult to accomplish. Smaller physical area sensors made by bridging a very narrow gap between two contacts by ZnO nanostructures exhibit very limited performance. Making sensors with a wider gap between the contacts involves dispensing or spin coating a networked layer of shorter nanowires between the contacts. This process typically does not produce a structure with structural integrity or good electrical continuity. For example, ZnO nanostructured sensors for ultra violet, gas (O ₂ [Q. Fl. Li, Q. Wan, Y. X. Liang, T. H. Wang, Appl. Phys. Lett. 84, 4556 (2004)], ethanol [Q. Wan, Q. H. Li, Y. J. Chen, and T. H. Wang, X. L. He, J. P. Li, C. L. Lin, Appl. Phys. Lett, 84, 3654 (2004)] and hydrogen storage [Q. Wan, C. L. Lin and X. B. Yu, T. H. Wang, Appl. Phys. Lett, 84, 124 (2004)]) and bio sensing [R. Gabl, H. D. Feucht, H. Zeininger, G. Eckstein, M. Schreiter, R. Primig, D. Pitzer, W. Wersing, Biosensors and Bioelectronics, 19, 615-620 (2004)] have been fabricated by dispersing nanowires in a liquid and then spin coating them onto a substrate. Time constants larger than 150 seconds have been reported for UV photoconductive response and decay for UV sensors [K. Keem, H. Kim, G.T. Kim, J. S. Lee, B. Min,K. Cho, M.Y. Sung, Skim, Photocurrent in ZnO nanowires grown from Au electrodes, Appl. Phys. Lett., 84, 4376 (2004)]. It has been suggested that a combination of different shapes and dimensions of nanostructures achieved by using novel fabrication processes could be used to explore different domains of applications [Yi, C. Wang, W. Park, Semicond. Sci. Technol. 20, S22 — S34 (2005)].

D. CATALYST-FREE DIRECT VAPOR PHASE GROWTH OF HEXAGONAL ZNO NANOWIRES ON C-AL ₂ O ₃

[0064] There is a great deal of interest in the development of optical and semiconductor nanowires, because the quantized geometrical shapes lead to strong confinement of electrons, holes in reduced dimensions that have potential applications in nanoscale electronics and optoelectronic devices [J. Johnson, H.J. Choi, K. P. Knutsen, R.D. Schaller, P. Yang, R.J. Saykally, Nature Materials 1, 106 (2002)] such as highly quantum efficient lasers [M. H. Huang, et al, Science, 292, 1897 (2001); X. Duan, Y. Huang, R. Agarwal, CM. Lieber, Nature, 421, 241 (2003); Y. Zhang, R.E. Russo, S.S. Mao, Appl. Phys. Lett., 87, 043106 (2005)] and non-linear optical converters [J. C. Johnson, et al., Nano Lett., 2, 279 (2002)]. It is generally accepted that the low dimensional structures (where the size in one direction is equivalent to or smaller than the de Brogue wavelength) are useful materials for investigating the dependence of electrical and thermal transport or mechanical properties on the dimensionality and quantum confinement [Y.N. Xia, P.D. Yang, Y. G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15, 353 (2003)]. They also play an important role as functional units in fabricating the electromechanical devices [Z. Fan and J. G. Lu., Appl. Phys. Lett., 86, 032111 (2005)]. It is becoming increasingly clear that, in addition to size, the shape of nanomaterials plays an important role in determining their electronic, optical, and catalytic properties.

[0065] There are several electronic and optoelectronic materials that have exciting applications if developed in the form of nanostructures. Particularly, zinc oxide (ZnO) can be useful for UV emitting devices and sensors upon exploiting its direct band-gap (Eg = 3.37 eV) and exciton binding energy (60 me V) [Y. S. Park, C. W. Litton, T. C. Collins, D. C. Reynolds, Phys. Rev., 143, 512 (1966)], transparent conductivity (by Aluminum doping) [D. Ravinder and J. K. Sharma, J. Appl. Phys., 58, 838 (1985)] and piezoelectricity [J. S. Wang and K. M. Lakin, Appl. Phys. Lett., 42, 352 (1983)]. The high exciton binding energy in ZnO crystal can ensure an efficient excitonic emission at room temperature under low excitation intensity. As was predicted [R. F. Service, Science 276, 895 (1997)], the observation of room- temperature UV lasing from the ordered, nano-sized ZnO crystals provides an important step for the development of practical blue -UV laser using ZnO.

[0066] To date, deposition techniques (with or without catalyst [W. Lee, M. C. Jeong, J.M. Myoung, Nanotechnology, 15, 1441 (2004); M. Wei, D. Zhi, J. L. MacManus-Driscoll, Nanotechnology, 16, 1364 (2005)]) including Metal-organic chemical vapor deposition (MOCVD) [P. Souletie and B. W. Wessels, J. Mater. Res., 3, 740 (1988)], Metal-organic vapor phase epitaxy (MOVPE) [W. Park, G. Yi, M. Kim, S. Pennycook, Adv. Mater., 14, 1841 (2002); W. I. Park, S. J. An, G. C. Yi and H.M. Jang, J. Mater. Res., 16, 1358 (2001)], and direct thermal evaporation are known for fabrication of various kinds of ZnO based nanostructures [Z. W. Pan, Z.R. Dai, ZX. Wang, Science 291, 1947 (2001)]. The alloying of ZnO with Mg to enhance the band gap in nanostructures has also been reported [W. I. Park, G. C. Yi, H. M. Jang, Appl. Phys. Lett., 79, 2022 (2001)]. Different kinds of ZnO nanostructures have been realized [Z.L. Wang, J. of Physics, 26, 1 (2006) and references therein], such as nanorods, nanowires, nanobelts, and nanonails, nanowalls, nanohelixes, nanorings, mesoporous single-crystal nanowires, and polyhedral cages [Z. L. Wang, Mater. Today 7, 26 (2004)]. Although growth and fabrication of ZnO nanostructures have advanced significantly, further efforts are needed for establishment of controlled and reproducible nanostructures for technology development and application demonstrations.

[0067] In the disclosed catalyst- free processes, the source material Zn itself initiates the catalytic process and assists the formation of ZnO nanostructures. This process is simple, rapid, scalable, and cost effective, as opposed processes such as MOCVD and MOVPE, wherein precursors of zinc element (Diethyl Zinc) are used to provide the Zn in a chemical reaction between the activated oxygen to grow ZnO nanostructures in presence of catalyst. The other well method to grow nanostructures is a Vapor-Liquid-Solid (VLS) technique, which requires growth temperatures close to that of eutectic melting point of the alloy that forms during initial stages of growth of nanostructure on the substrate. Technologically, catalyst free growth of ZnO nanostructures has a great potential as opposed to techniques that use catalyst. The catalysts containing foreign elements like Ni, Sn, Au are not totally consumed in the process and affect the opto -electronic properties in the form of luminescence yield quenching or generation of metal induced defects [R. S. Wagner and W. C. Ellis, Appl. Phys. Lett., 4, 89 (1964)]. (A comprehensive review detailing nanostructure growth methods, materials and applications is available [Kuchibhatla SVNT et al., One dimensional

nanostructured materials, Prog. Mater. Sci. (2007), doi:10.1016/j.pmatsci.2006.08.001].) Using a catalyst- free DVP approach, ZnO nanostructures have been grown on variety of substrates. The ability to grow high purity and luminescent ZnO nano wires can greatly increase the versatility of fabrication process for building nanoscale photonic and electronic devices. Thus, the effect of growth temperature, mass of Zn, and carrier gas composition (ratio of oxygen to argon) on the formation of nano wires are disclosed. Specific shapes and size of ZnO nanowires observed in the disclosed growth process and can be exploited for various applications such as optically pumped UV lasers, detectors, FETS, MEMS, and bio- and chemical-sensing applications.

E. V ₂ O ₅ MICRO AND NANOSTRUCTURES BY DIRECT VAPOR PHASE SYNTHESIS

[0068] V2O5 is an n-type semiconductor at room temperature and is thermodynamically stable among all oxides of vanadium at normal temperature and pressure [N. Kimizuka, M. Saeki, M. Nakahira, Materials Research Bulletin, 5, 403 (1970)]. The attractive electronic transport properties OfV ₂ Os fibers are well suited for the construction of multi functional and novel devices, which can be operated under ambient conditions [A. D. Rata, A. R. Chezan, C.

Presura, T. Hibma, Surface Science, 532-535, 341]. Thick films OfV ₂ Os fiber networks are employed as anti-static protection layers in photographic industry, and as charge storage material in lithium-ion and magnesium batteries [J. Livage, Chem. Mater. 3, 578 (1991)], electrochromic devices [S. J. Yoo, J. W. Lim, Y. E. Sung, Solar Energy Materials & Solar Cells 90, 477 (2006)], for quality control in food industry [V. Venugopal, Biosens.

Bioelectron. 17, 147 (2002)], and medical diagnosis [G. Preti, J.N. Labows, J.G. Kostelc, S. Aldinger, R. Daniele, J. Chromatogr. 432,1 (1988)]. Freestanding sheets of V ₂ Os are demonstrated as sheet actuators [G. Preti, J.N. Labows, J.G. Kostelc, S. Aldinger, R. Daniele, J. Chromatogr. 432,1 (1988)] in the form of nanofϊbers used in the applications of artificial muscles where the length of V ₂ Os electrode varied with applied potential. V ₂ Os is sensitive to ethanol, 1-propanol, ammonia and amines like ethylamine and trimethylamine. V ₂ Os nanofϊber based gas sensors are good for detecting organic amines with high sensitivity and selectivity at room temperature [I. Raible, M. Burghard, U. Schlecht, A. Yasuda, T. Vossmeyera, Sensors and Actuators B 106, 730 (2005)]. Understanding the fabrication of nanostructures of V ₂ Os and their properties can be important for enhancing the known

applications of the material. Moreover, such a study, when combined with the studies of other materials, can add to the understanding of processes involved in nanostructure formation [S. S. Hullavarad, N. V. Hullavarad, P. C. Karulkar, A. Luykx, P. Valdivia, Nano Scale Research Letters (2007) DOI 10.1007/sl 1671-007-9048-6].

[0069] V2O5 structures are produced by various methods such as pellet formation by powder compaction, thin films by thermal evaporation [M. A. Sobhan, M. R. Islam, K. A. Khan, Applied Energy 64, 345 (1999)], and pulsed laser deposition [J. M. McGraw, J. D. Perkins, J.-G. Zhang, P. Liu, P.A. Parilla, J. Turner, D. L. Schulz, C. J. Curtis, D. S. Ginley, Solid State Ionics 113-115, 407 (1998)], and nanoparticles by ball milling [J. L. Guimaraes, M. Abbate, S .B. Betim, M. C. M. Alves, Journal of Alloys and Compounds 352 16 (2003)] and hydrothermal synthesis [J. Liu, X. Wang, Q. Peng, Y. Li, Sensors and Actuators B 115, 481 (2006)].

[0070] Disclosed herein are micro- and nano-sized V2O5 structures produced by by Direct Vapor Phase (DVP) methods. Such techniques have also been used for fabricating nanostructures of materials such as ZnO [S. S. Hullavarad, et al., Under Internal Review, 2007]. In short, this technique is a modification of the vapour transport and thermal evaporation method in which the chemical elements from the precursors are transported to the substrate location through the gas pressure gradient. In this technique, the evaporation is carried out at much lower temperatures (-600-800 ⁰ C) as opposed to 1800-2000 ⁰ C in thermal evaporation. Localized regions are formed on the substrates upon evaporation of constituent elements.

F. COMPOSITIONS

[0071] In one aspect, the invention relates to a composition comprising an interconnected network of transition metal oxide microspheres and nano wires. In a further aspect, the interconnected network is fabricated in a single step. That is, by using direct vapor phase despositon processes, an interconnected network of microspheres and nano wires can be provided in a single operation. Thus, the microspheres can be embedded in an interconnected network that is produced in a single deposition step.

[0072] In one aspect, the microspheres and the nanowires can be prepared from the same transition metal, for example, from vanadium, titanium, tin, hafnium, or zinc. In one aspect, the microspheres and the nanowires can be prepared from zinc. In one aspect, the microspheres and the nanowires comprise the same transition metal oxide, for example, an oxide of vanadium, titanium, tin, hafnium, or zinc. That is, networks of microspheres and nanowires OfV ₂ O ₅ , TiO ₂ , SnO, SnO ₂ , HfO ₂ , and ZnO can be prepared. In one aspect, the transition metal oxide is zinc oxide.

[0073] In a further aspect, the microspheres and the nanowires can be prepared from different transition metals, for example, from two or more of vanadium, titanium, tin, hafnium, or zinc. In a further aspect, the microspheres and the nanowires comprise different transition metal oxides, for example, two or more oxides of vanadium, titanium, tin, hafnium, or zinc (e.g., V ₂ O ₅ , TiO ₂ , SnO, SnO ₂ , HfO ₂ , and ZnO). It is also contemplated that the tranisiton metal oxide(s) can be doped with a further tranision metal or transition metal oxide (e.g., Indium).

[0074] In one aspect, the microspheres embedded in the disclosed interconnected networks can have an average diameter of from about 0.5 μm to about 100 μm, for example, from about 0.5 μm to about 10 μm, from about 0.5 μm to about 5 μm, from about 0.5 μm to about 3 μm, or from about 0.6 μm to about 2 μm.

[0075] In one aspect, the nanowires embedded in the disclosed interconnected networks can have an average cross-sectional diameter of from about 5 nm to about 200 nm, for example, from about 10 nm to about 100 nm, from about 20 nm to about 80 nm, or from about 30 nm to about 65 nm. In a further aspect, the nanowires embedded in the disclosed interconnected networks can have an average length of from about 5 nm to about 10 μm, for example, from about 50 nm to about 10 μm, from about 100 nm to about 10 μm, from about 1 μm to about 10 μm, from about 1 μm to about 5 μm, or from about 2 μm to about 3 μm.

[0076] It is understood that the disclosed compositions can be used in connection with the disclosed devices, methods, and products.

G. DEVICES

[0077] Devices that take advantage of the interconnected network micro- and nano-structure of the disclosed compositions are also disclosed. In one aspect, devices for detecting an environmental influence having a sensor layer comprising an interconnected network of transition metal oxide microspheres and nanowires are disclosed. Such devices can leverage the interconnected network structure to observe, e.g., electromagnetic radiation and/or chemical species via an excitation unit coupled to the network micro- and nano-structure. In a further aspect, micro- and/or nano -computing devices can be provided having at least one or a multiplicity of programmable conductive pathways between electrodes, wherein the programmable conductive pathways comprise an interconnected network of transition metal oxide microspheres and nanowires. Such devices can leverage the interconnected network structure to provide massively parallel programmable conductive pathways that cannot typically be achived with bulk materials.

[0078] It is understood that the disclosed devices can be used in connection with the disclosed compositions, methods, and products.

1. DETECTION DEVICES

[0079] In one aspect, the invention relates to a device for detecting an environmental influence onto a sensor, by means of detecting a change in an electrical conductivity of a sensor layer of the sensor, wherein the sensor has a first electrode, a second electrode, and a sensor layer, which comprises an excitation unit for generating electrical potentials, and a sensor layer having a conductivity that is dependent on environmental influences, wherein the sensor layer comprises an interconnected network of transition metal oxide microspheres and nanowires. Typically, the interconnected network is coupled (e.g., chemical, electrical, and/or photonic communication) with the first and second electrodes.

a. SENSOR LAYER

[0080] In one aspect, the sensor layer comprises one or more disclosed interconnected networks. The disclosed devices can be provided from the same transition metal as disclosed in the interconnected networks. For example, in one aspect, vanadium, titanium, tin,

hafnium, or zinc can be used. In one aspect, zinc is used. The disclosed devices can be provided as the same transition metal oxides as disclosed in the interconnected networks. For example, a sensor layer can be provided as an oxide of vanadium, titanium, tin, hafnium, or zinc (e.g., V ₂ O ₅ , TiO ₂ , SnO, SnO ₂ , HfO ₂ , and ZnO). In one aspect, the sensor layer comprises zinc oxide.

[0081] In a further aspect, the devices can be provided from different transition metals, for example, from two or more of vanadium, titanium, tin, hafnium, or zinc. In a further aspect, the sensor layers of the disclosed devices comprise different transition metal oxides, for example, two or more oxides of vanadium, titanium, tin, hafnium, or zinc (e.g., V ₂ O ₅ , TiO ₂ , SnO, SnO ₂ , HfO ₂ , and ZnO). It is also contemplated that the tranisiton metal oxide(s) can be doped with a further tranision metal or transition metal oxide (e.g., Indium).

b. EXCITATION UNIT

[0082] In one aspect, the excitation unit is a means for generating, observing, and/or measuring one or more of chemical, electrical, and/or photonic stimuli in the sensor layer. Examples include detectors for photons, chemical reactions, ion radiation, and/or electron radiation. Typically, the excitation unit is coupled (e.g., chemical, electrical, and/or photonic communication) with the sensor layer by means of the first and second electrodes.

c. ELECTRODES

[0083] It is understood that electrodes known to those of skill in the art can be employed in connection with the devices of the invention. Examples include commercially available microelectrodes and commercially available conducting pastes (e.g., silver, gold, and graphite; selection depends on the growth process compatibility). Typically, the electrodes couple (e.g., chemical, electrical, and/or photonic communication) the excitation unit with the sensor layer.

[0084] In one aspect, the electrodes comprise a conducting metal electrode with a defined gap of less than about 100 μm, although it is understood that the gap can be provided at other sizes, as appropriate for the application. In a further aspect, the electrodes comprise two conducting metal electrodes with a defined gap of less than about 100 μm. In a further

aspect, the electrodes comprise a conducting metal paste electrode with a defined gap. In a further aspect, the electrodes comprise two conducting metal paste electrodes with a defined gap of less than about 100 μm.

d. SENSITIVITY

[0085] Typically, the disclosed interconnected networks of transition metal oxide microspheres and nanowires are more sensitive than conventional compositions used in comparable devices. In one aspect, the device exhibits a dark current of less than about 1 x 10 ^"9 A at 1 V, for example, less than about 7.5 x 10 ^"10 A, less than about 5 x 10 ^"10 A, less than about 2.5 x 10 ^"10 A, or about 1 x 10 ^"10 A at 1 V.

[0086] In a further aspect, the device exhibits a greater sensitivity than a comparable device comprising the same transition metal oxide nanowires in the absence of microspheres (e.g., nanowires only or nanowires and nanospheres). That is, a comparable device comprising the same transition metal oxide nanowires in the absence of microspheres have fewer more absorption sites than found in microspheres. In a further aspect, the device exhibits a decreased detection time than a comparable device comprising the same transition metal oxide nanowires in the absence of microspheres (e.g., nanowires only or nanowires and nanospheres).

[0087] Without wishing to be bound by theory, it is believed that a greater number of absorption sites (for e.g., chemical, electrical, and/or photonic stimuli) results in an enhanced sensitivity and/or a decreased detection time for device with a sensor layer comprising an interconnected networks of microspheres and nanowires, as compared to devices comprising nanowires in the absence of microspheres.

e. ANALYTES

[0088] In one aspect, the device can detect electromagnetic radiation, for example, visible, ultraviolet, or infrared light. The selection of light (e.g., visible, ultraviolet, or infrared) depends on the band gap of the material under consideration. In a further aspect, the electromagnetic radiation is ultraviolet light. That is, when contacted by electromagnetic

radiation, the sensor layer produces information (e.g., chemical, electrical, or photonic signal) that can be observed by the excitation unit.

[0089] In one aspect, the device can detect the presence of a chemical species. That is, when contacted by a chemical species, the sensor layer produces information that can be observed by the excitation unit. In a further aspect, the chemical species is a gas. In various further aspects, the chemical species is one or more of oxygen, carbon monoxide, carbon dioxide, propanol, ethanol, or ammonia. In a further aspect, the environmental influence is the presence of a bio-hazardous (i.e., toxic) chemical species.

2. COMPUTING DEVICES

[0090] In one aspect, the invention relates to a microelectronic device. In a further aspect, as the device can comprise a micro- and nano-scale interconnected network, this technology can scale down to much smaller sizes than that typically achieveable by conventional techniques, thereby reducing electronic device size and fabrication costs by several orders of magnitude. The disclosed computing devices can be used in connection with, e.g., memory and as logic gates.

[0091] The embedded microspheres of the interconnected networks can, in one aspect, be thought of as a set of discontinuous islands of conducting material deposited onto a substrate (e.g. silicon dioxide). The presence of nano wires in the interconnected networks can provide conductive links between these islands. One or more leads (i.e., conductive input/output electrodes) spaced apart around the perimeter of the device can carry signals to and from the device. Due at least in part to the microsphere components, the micro- and nano-scale interconnected networks can be addressed using current lithographic tools.

[0092] As the precise placement of components is disordered, the disclosed computing devices are not typically programmed like conventional computers. Instead, the disclosed computing devices can be trained (with, e.g., software methods) to carry out specific logical functions. Even if computing processes using the disclosed computing devices are only a few percent efficient, such computing methods can result in very high logic densities, thereby providing much more powerful electronic devices.

[0093] The disclosed interconnected networks can also be employed as components in a nanocomputing system. In one aspect, an interconnected network of transition metal oxide microspheres and nanowires can further comprise a pair of conductive input/output electrodes disposed in spaced-apart relationship; the interconnected network positioned in a region between the electrodes in conductive contact with the electrodes, thereby forming at least one programmable conductive pathway between the pair of electrodes. In a further aspect, an interconnected network can further comprise a multiplicity of pairs of spaced-apart conductive input/output electrodes in conductive contact with the interconnected network.

[0094] In a further aspect, the microstructure of the microspheres embedded in the interconnected network can provide convenient sites for coupling electrodes, thereby addressing the at least one programmable conductive pathway. In a further aspect, the at least one programmable conductive pathway is addressed using lithographic tools.

H. METHODS

[0095] Methods for providing micro- and/or nano-structured interconnected networks are disclosed. In one aspect, direct vapor phase deposition methods are disclosed. In certain aspects, physical vapor deposition can be accomplished by any thin film evaporation technique, including such techniques as thermal evaporation, electron beam evaporation and/or direct current/magnetron sputtering. In a further aspect, methods for making an interconnected network of transition metal oxide microspheres and nanowires are disclosed.

[0096] It is understood that the disclosed methods can be used in connection with the disclosed compositions, methods, and devices.

1. DIRECT VAPOR PHASE DEPOSITION

[0097] In one aspect, the invention relates to direct vapor phase deposition methods comprising the steps of evaporating a metal in an carrier gas/oxygen flowstream at temperature of less than the temperature required for thermal deposition of the same metal; and depositing a metal oxide film on a substrate positioned downstream in a gas pressure gradient from the location of the evaporation step. In a further aspect, the metal is a transition metal, for example, vanadium, titanium, tin, hafnium, or zinc. In one aspect, the metal oxide

film comprises an interconnected network of microspheres and nanowires. In one aspect, the method can be performed in the absence, or substantial absence, of exogenous catalyst.

[0098] In one aspect, the evaporating temperature is less than about about 1200 ⁰ C, for example, less than about about 1100 ⁰ C, less than about about 1000 ⁰ C, less than about about 900 ⁰ C, less than about about 800 ⁰ C, less than about about 700 ⁰ C, less than about about 600 ⁰ C, or from about 600 ⁰ C to about 800 ⁰ C.

[0099] In one aspect, the temperature required for thermal deposition of the same metal is greater than about 1800 ⁰ C, for example, greater than about 1900 ⁰ C, greater than about 2000 ⁰ C, or from about 1800 ⁰ C to about 2000 ⁰ C.

[00100] In one aspect, the carrier gas is an inert gas, for example, nitrogen, helium, neon, argon, krypton, or xenon. In various aspects, the carrier gas is mixed with an oxidant, for example, air, oxygen gas, ozone, nitrous oxide, or a peroxide. In one aspect, the carrier gas is nitrogen. In a further aspect, the acrrier gas is argon. In one aspect, the oxidant is oxygen gas.

2. METHODS FOR MAKING INTERCONNECTED NETWORKS

[00101] In one aspect, the invention relates to methods for making an interconnected network of transition metal oxide microspheres and nanowires comprising the steps of evaporating a transition metal in an carrier gas/oxygen flowstream at temperature of less than about 900 ⁰ C; and depositing a transition metal oxide film on a substrate positioned downstream in a gas pressure gradient from the location of the evaporation step, wherein the transition metal oxide film comprises an interconnected network of transition metal oxide microspheres and nanowires. In a further aspect, the the evaporating and depositing steps comprise direct vapor phase deposition. Typically, the preparation is performed as a single operation. Thus, in one aspect, the microspheres and nanowires are produced substantially simultaneously. In one aspect, the method can be performed in the absence, or substantial absence, of exogenous catalyst.

[00102] While conventional techniques typically employ a temperature in excess of

1300 ⁰ C (e.g., 1800-2000 ⁰ C), the disclosed methods can provide the film by using a

temperature of less than about 900 ⁰ C, for example, less than about 850 ⁰ C, less than about 800 ⁰ C, less than about 750 ⁰ C, less than about 700 ⁰ C, less than about 6500 ⁰ C, less than about 600 ⁰ C, from about 600 ⁰ C to about 900 ⁰ C, from about 600 ⁰ C to about 800 ⁰ C, from about 600 ⁰ C to about 700 ⁰ C, from about 700 ⁰ C to about 900 ⁰ C, from about 700 ⁰ C to about 800 ⁰ C, about 850 ⁰ C, about 800 ⁰ C, about 750 ⁰ C, about 700 ⁰ C, about 650 ⁰ C, or about 600 ⁰ C.

[00103] The disclosed methods can be used in connection with the same transition metals as disclosed in the interconnected networks. For example, in one aspect, vanadium, titanium, tin, hafnium, or zinc can be used. In one aspect, zinc is used. The disclosed methods can be used in connection with the same transition metal oxides as disclosed in the interconnected networks. For example, an oxide of vanadium, titanium, tin, hafnium, or zinc (e.g., V ₂ O ₅ , TiO ₂ , SnO, SnO ₂ , HfO ₂ , and ZnO) can be produced. In one aspect, zinc oxide can be produced.

[00104] In a further aspect, the methods can be used in connection with different transition metals, for example, from two or more of vanadium, titanium, tin, hafnium, or zinc. In a further aspect, the methods can be used to produce different transition metal oxides, for example, two or more oxides of vanadium, titanium, tin, hafnium, or zinc (e.g., V2O5, TiO ₂ , SnO, SnO ₂ , HfO ₂ , and ZnO). It is also contemplated that the tranisiton metal oxide(s) can be doped with a further tranision metal or transition metal oxide (e.g., Indium).

[00105] In one aspect, the carrier gas is an inert gas, for example, nitrogen, helium, neon, argon, krypton, or xenon. In various aspects, the carrier gas is mixed with an oxidant, for example, air, oxygen gas, ozone, nitrous oxide, or a peroxide. In one aspect, the carrier gas is nitrogen. In a further aspect, the acrrier gas is argon. In one aspect, the oxidant is oxygen gas.

I. USES

[00106] Also provided are uses of the disclosed compositions and products. In one aspect, the invention relates to a use of a disclosed composition or product to in ultraviolet radiation sensing. In a further aspect, the invention relates to a use of a disclosed composition

or product to in chemical sensors. In a further aspect, the invention relates to a use of a disclosed composition or product to in biohazard detection. In a further aspect, the invention relates to a use of a disclosed composition or product to in missile defense applications for UV exhaust monitoring from missiles. In a further aspect, the invention relates to a use of a disclosed composition or product to in flame sensors. In a further aspect, the invention relates to a use of a disclosed composition or product to in fire detection. In a further aspect, the invention relates to a use of a disclosed composition or product to in UV index monitoring. In a further aspect, the invention relates to a use of a disclosed composition or product to in nanotechnology vendors marketing nanomaterials for laboratory applications. It is also contemplated that the disclosed compositions and products will prove useful in further applications.

J. KITS

[00107] Also provided are kits related to the disclosed compositions. In one aspect, the invention relates to a kit comprising at least one composition comprising an interconnected network of transition metal oxide microspheres and nano wires. The disclosed kits can further comprise one or more electrodes or other sensing devices. It is understood that the disclosed kits can be used in connection with the disclosed methods of using.

K. EXPERIMENTAL

[00108] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices and/or methods claimed herein are made and evaluated, and are intended to be purely exemplary of the invention and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ⁰ C or is at ambient temperature, and pressure is at or near atmospheric.

1. PREPARATION OF INTERCONNECTED NETWORK OF TRANSITION METAL OXIDE MICROSPHERES AND NANOWIRES

[00109] ZnO sheets consisting of spheres in nano wires were synthesized in a horizontal tube furnace (Lindberg) by a self catalyst Direct Vapor Phase (DVP) technique. Figure 1 shows the schematic of the growth set up. The source material Zn (99.9%) in granular form was placed at the center of the furnace (800 ⁰ C, heating rate 10 ⁰ C min ^"1 ). Double side polished AI2O3 (0001) samples were used as substrates for optical characterization. In the initial stage, the furnace was flushed by Ar gas and was later stabilized with a flow rate of 40- 50 seem. When the furnace reaches 420 ⁰ C, the Zn metal evaporates and O ₂ gas was introduced with the combined gas mixture of 60 seem. The evaporated Zn metal forms ZnO and was deposited on the AI2O3 substrates and also on the walls of the tube furnace. The process was carried out for 90 minutes and samples were pulled out after furnace was cooled down to room temperature. ZnO nanostructures were characterized by Environmental and Hitachi Scanning Electron Microscopy (Electro Scan and S-4800), and Photoluminescence (PL) measurements to monitor the morphology (Laser Science, Inc, Model VSL-337 ND-S, 337 nm, 6 mW and Ocean Optics SD5000 spectrometer) and the band gap. Sensors were fabricated on a glass plate with silver conducting paste contacts with varying gaps in the range of 80-250 μm. The photo response measurements were carried out using Xe arc lamp, Thermo Oriel monochromator set up and a com- mercial UV sensor read out by Solartech, Inc. The experimental set up was calibrated with standard SiC and AlGaN UV detectors and the output power of the Xe arc lamp was measured by Newport standard power meter. The X-ray Photoelectron Spectroscopic (XPS) measurements were performed using Kratos Axis 165 spectrometer at a vacuum of 4 x 10 ^"10 Torr with non-monochromatic Mg Kx radiation. All binding-energies were calibrated with respect to C is at 284.6 eV.

a. RESULTS AND DISCUSSIONS

[00110] The thick films could be easily peeled off from the walls of the furnace as shown in Figure 2. Figure 3 shows the photograph of flexible sheets of ZnO which were used for UV sensor fabrication. SEM images in Figure 4(a) and Figure 4(b) show the

morphology of the ZnO films. The films contain micro spheres in the range of 600nm - 2μm embedded in the network of intricate nanowires. The enlarged images in Figure 4(c) and Figure 4(d) reveal the network of nanowires that are 30-65 nm in diameter and a few microns in length. Additional images are shown in Figure 5.

[00111] To investigate the process mechanism of formation of spheres alone in detail, the overall growth temperature was lowered from 800 ⁰ C to 600 ⁰ C (process temperature for the formation of ZnO microspheres in network of nanowires is 800 ⁰ C). Figure 6(a) and Figure 6(b) show the SEM of low temperature processed ZnO in the identical set up. From the micrograph, it is evident that the size of sphere is around 2 μm (similar size as in Figure 4(a)) before the spheres turn shaping into a perfect hexagon of size about 5 pm. The formation of radially spherical ZnO spheres at a lower growth temperature of 650 ⁰ C has been reported and the authors attempted to explain the evolution of spheres into nanorods when the samples were annealed to 1000 ⁰ C [S. Ding, J. Guo, X. Yan, T. Lin, K. Xuan, J. Crystal Growth 284, 142 (2005)]. The radius of the spheres was calculated to be 400 nm from the formula proposed by Ding et al. [S. Ding, J. Guo, X. Yan, T. Lin, K. Xuan, J. Crystal Growth 284, 142 (2005)]:

where σ^v is surface free energy of liquid-vapor, V _L , is molar volume and σ is vapor phase supersaturation. The inflation of spheres continues with the process as more vapor gets condensed on to the small spheres. The mechanism for the formation of nanowires and spherical structure in a one step process (as observed in the present investigation) in the absence of any catalyst or a reducing agent is not very clear. However, without wishing to be bound by theory, it is believed that that complex nature of DVP process in which the rate of supersaturation of reactive elements varies, plays a role in defining the shape and structure of the resulting material.

[00112] Mo et al. [M. Mo., J.C. Yu, L. Zhang, S.A. Li, Adv Mater. 17, 756 (2005)] have observed formation of ZnO nanorods embedded in micro hemispheres and spheres by hydrothermalthermo lysis of Zn(en ²⁺ ) in the presence of long chain polymer of — poly(sodium-

4 styrenesulfonate) — (PSS) and the authors noted that such formation of multiple shapes is due to the presence of an appropriate amount of water soluble long chain polymer. In self catalyst DVP technique, the supersaturation of reactive elements like Zn vapor and the oxygen favors the formation of nanowire- and sphere-like morphology. There have been reports [A. Umar, S.H. Kim, Y.-S. Lee, K.S. Nahm, Y.B. Hahn, J. Crystal Growth 282, 131 (2005)] on the fabrication of ZnO nano wires of diameter 200 nm at a lower growth temperature of 650 ⁰ C, and the reaction was carried out in N ₂ atmosphere. The formation of nanowire or spheres or both of shapes in vapor phase can depend on the temperature, carrier gas, and the type of substrates. It is clear from the two experiments that the nature of carrier gas used in the vapor deposition process can affect the shape of structures due to difference in masses of processing gases.

[00113] Figure 7 shows the XPS general scan spectrum for the ZnO nanostructure film consisting of nanowire and microspheres. The spectrum depicts the core levels at 90.3 eV and 531.5 eV corresponding to Zn 3p and Ols and Zn Auger lines Zn LMM b, c, d [CD. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Mullenberg, Handbook of X-ray

Photoelectron Spectroscopy, (Perkin-Elmer Corp., Eden Prairie, MN, USA, 1979)]. The ZnO thin films were investigated by the room temperature PL spectroscopy. The excitation energy of the laser was 3.6 eV corresponding to a wavelength of 340 nm. As shown in Figure 8, the dominant peak was observed at λ = 383 nm which is attributed to the recombination of free excitons through an exciton — exciton collision process (D ⁰ X) corresponding to 3.2 eV [U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Do, S. Doan, V. Avrutin, S. -J. Cho, H. Morkoc, J. Appl. Phys. 98, 041301 (2005)]. The inset of Figure 8 shows the bright blue/violet emission from the ZnO microsphere-nanowires network film under laser illumination. The exciton peak has the full width at half maximum (FWHM) of 15 nm. Interestingly, the green emission at λ = 512 nm (2.41 eV) is absent in the PL spectrum indicating the absence of any nonstoichiometry between Zn and O.

b. PREPARATION OF A UV SENSOR

[00114] The nanostructure ZnO film was pressed on to the silver conducting paste electrode with a defined gap (Figure 9). The current-voltage characteristics were carried out

both with and without illumination, as shown in Figure 10. The dark (background) current of the nano -structure ZnO film device was 1 x 10 ^"10 A at 1 V. This dark current is comparatively better than the thin film based UV sensor fabricated from ZnO and MgZnO UV sensitive materials [W. Yang, S. S. Hullavarad, B. Nagaraj, I. Takeuchi, R.P. Sharma, T. Venkatesan, R.D. Vispute, H. Shen, Appl. Phys. Lett. 82, 3424 (2003)]. A lower dark current is desirable for a better sensor. When a ZnO nanowire is exposed to air, the negative space charge layer is created when the adsorbed oxygen molecule captures an electron from the conduction band and therefore the device exhibits higher resistivity. When the photon energy is larger than the band gap energy E _g , the incident radiation is absorbed in the ZnO nanostructured UV sensor, creating electron-hole pairs. The photogenerated, positively charged hole neutralizes the chemisorbed oxygen responsible for the higher resistance, increasing the conductivity of the device. As a consequence, the conductivity in the material increases, giving rise to photocurrent. The sensor exhibited a photo current of 1 x 10 ^"8 A, at 1 V under UV illumination at 383 nm. The UV to Visible rejection ratio was found to be two orders of magnitude. The responsivity R of the UV sensor at 1 V corresponding to a dark current of 1 x 10 ^"10 A was calculated to be 50 mA W ^"1 from the relation:

R = ^Iph

^P o>

where, the Iph is the photon induced current at A = 383 nm with the power output Po _p is the incident optical power which was measured independently at the ZnO UV sensor location. Figure 11 shows the photo response measurements of nanostructure ZnO film device for varying electrode spacing of 80, 130, 200 and 250 μm. The response for the largest spacing of 250 microns is enhanced 20 times for clarity. The photo response signal output (mW/cm ² ) was found to increase with the reducing electrode spacing. The full width at half maximum (FWHM) of the photo response curves are measured to be 76, 71, 54 and 55 nm for the electrode spacing of 80, 130, 200 and 250 μm, respectively. The effect of shapes and sizes of nanostructures on the photoresponse properties needs to be investigated and is seldom reported in the literature.

[00115] The device response in terms of rise and decay time at room atmosphere when the UV illumination turned ON and OFF was studied using a modified circuitry that involves a sensitive oscilloscope. Figure 12 shows the rise and decay time of ZnO micro sphere -nanowires network film device under UV illumination. The rise time was observed to be 6.1, 5, 2.3, 3.9 seconds while the decay time was observed to be 55, 41, 0.5 and 1.7 seconds for electrodes spacing of 80, 130, 200 and 250 μm, respectively. It is interesting to note that the rise and decay times are faster for the electrode spacing of 200 gm whereas the corresponding photo response was poor. Keem et al. [K. Keem, H. Kim, G. T. Kim, J. S. Lee, B. Min, K. Cho, M.-Y. Sung, S. Kim, Appl. Phys. Lett. 84, 4376 (2004)] have studied the photo response of ZnO nanowires grown on pre -patterned Ti/ Au electrodes with a spacing around 50 microns and measured photocurrents with the trapping time of electrons ranging from 10 ms to several hours through the nanowires excited by both the above and below gap light and concluded that the photo current is surface-related rather than bulk related. Without wishing to be bound by theory, it is believed that that such faster rise and decay times observed in the photoresponse measurements result from the characteristic structure [S.E. Ahn, J.S. Lee, H. Kim, S. Kim, B.H. Kang, K.H. Kim, G.T. Kim, Appl. Phys. Lett. 84, 5022 (2004)]. A comparative study of the effect of oxygen on UV sensors in the nano and the bulk regime is necessary to resolve the issues encountered in this study. The photoconductivity of ZnO and hence its response is known to depend on the presence of oxygen in the atmosphere [Q. H. Li, Q. Wan, Y.X. Liang, T. H. Wang, Appl. Phys. Lett. 84, 4556 (2004)]. It is interesting to study the effect of background oxygen pressure on the rise and decay time of the UV ZnO nanostructure sensor.

c. CONCLUSIONS

[00116] Fabrication of ZnO UV sensors are demonstrated by a simple route without employing the tedious clean room procedure of inter-digitized electrode formation. The active ZnO in the form of a flexible sheet consisted of micro spheres in a matrix of nanowires. The morphology of the deposited structures indicated that the sizes of ZnO nano-wires in the range of 30-65 nm and the micro spheres in the range of 600 nm-2 Rm. The PL measurements indicated the exciton bandgap at 383 nm corresponding to band gap of 3.2 eV. The photoresponse measurements indicated that the multiple shapes involving

the spheres in network of nano wires and the electrode spacing affect the sensor responsivity. This technique has a potential for scale up to fabricate UV sensors for mass production.

2. ZNO FILMS COMPRISING MICROSPHERES IN NANOWIRES

[00117] Thick ZnO films were fabricated in a horizontal tube furnace by catalyst free direct vapor deposition method. Granular Zn (99.9%) was placed at the center of the furnace and was evaporated in Ar and O ₂ mixture at 800 ⁰ C. The process was carried out for 90 minutes and resulted in the formation of thick films of ZnO on the tube walls downstream. Deposition conditions could be varied to obtain different types of depositions. The details of synthesis and structural, optical properties of ZnO nanostructures are discussed elsewhere [S. S. Hullavarad, P. C. Karulkar, R.D. Vispute, R. Heng, T. Venkatesan, Zinc Oxide and Related Materials, edited by J. Christen, C. Jagadish, D. C. Look, T. Yao, F. Bertram, Mater. Res. Soc. Symp. Proc. 957, Warrendale, PA, 2007, paper # 0957-K06-07]. Thick films of ZnO were peeled off after the furnace was cooled down to room temperature. ZnO UV sensors were fabricated by forming electrodes of silver conducting paste on glass plates with pad spacing of 80 μm and placing the ZnO nanostructured films on to the electrodes. The photoresponse measurements were carried out by exposing the ZnO UV sensors to the light emerging from a monochromator (Thermo Oriel) equipped with a Xe arc lamp source. The light output from the sensor was measured on a (Solar-PMA2200) radiometer [http://www.solar.com/pma2200.htm]. The I-V characteristics were measured with and without UV illumination using Keithley 2700 IV Source meter after allowing the device under test to reach a steady state in about 15 minutes before recording the I-V characteristics. The I-V characteristics were also measured in a vacuum chamber in oxygen environment at different pressures. An oscilloscope was used for measuring photocurrent rise or decay characteristics.

a. RESULTS AND DISCUSSION

[00118] Scanning electron micrographs taken from the ZnO films are shown in Figure

13. Under a particularly selected set of deposition conditions, the morphology of the films comprised 2-5 μm diameter spheres with 30-100 nm diameter nanowires of hexagonal cross-

section growing out of them. Thus, the dense network of nanowires interconnected the microspheres. Under different deposition conditions, thin films containing only a large network of nanowires were formed. (Figure 14).

[00119] The photo luminescence spectra of the two samples (Figure 15) were very similar and exhibited the peak at -384 nm. This is the well known photo luminescence peak for ZnO and is attributed to the recombination of free excitons at the room temperature through an exciton-exciton collision process (D ⁰ X) corresponding to 3.2 eV (384 nm) direct bandgap. A noticeable feature of the spectra in Figure 15 is the absence of the green band for the ZnO sample with the nanowire -microsphere morphology. This indicates the absence of Zn-to-0 non-stoichiometry. The green band (512 nm) is observed in the photo luminescence spectra of ZnO containing oxygen vacancy (i.e., excess Zn) and originates from the transitions between the singly ionized oxygen vacancies and photo-excited holes [D. Banerjee, J. Y. Lao, D. Z. Wang, J. Y. Huang, Z. F. Ren, D. Steeves, B. Kimball, M. Sennett, Appl. Phys. Lett., 83, 2061 (2003)]. Thus the nanowire-microsphere samples considered here were stoichiometric. The fact that the devices were responding to the UV radiation was verified by studying the wavelength dependence of the photocurrent response of ZnO samples with the nanowire-microsphere morphology for 80 μm electrode spacing (Figure 16). The inset shows the I- V characteristics. (The devices with large spacing between contacts exhibited poorer photoresponse, which did not scale with electrode spacing. While not wishing to be bound by theory, it is believed that this is due to the series resistance of the device film.)

[00120] A dark current of IxIO ^"10 A was observed at 1 V. This dark current is comparatively better than that observed in thin film ZnO and MgZnO UV sensors [S. S. Hullavarad, S. Dhar, B. Varughese, Takeuchi, T. Venkatesan, R.D.Vispute, J. Vac. Sci. Technol. A 23, 982 (2005)]. The device exhibited a photocurrent of IxIO ^"8 A at IV under UV illumination at 384 nm which corresponds to a light-to-dark photoconductivity ratio of about 100. This is significantly better than the values reported in the literature [L. Luo, Y. Zhang, S. S. Mao, L. Lin, Sensors and Actuators A 12, 201 (2006)].

[00121] Figure 17 shows the I-V characteristics of ZnO UV sensors with nanowire- microsphere morphology (top) and nanowire morphology (bottom) without (left) and with

(right) UV illumination. When a ZnO surface is exposed to oxygen or air, oxygen is adsorbed on its surface. Each adsorbed oxygen ties up an electron from the conduction band. This reduces the number of electrons available for conduction near the surface and the effect would be pronounced especially in nanostructures.

[00122] Thus the exposure to oxygen reduces ZnO conductivity and therefore the device exhibits higher resistivity. An electron-hole pair is generated when a photon of energy equal to or higher than the band gap is absorbed in the material. Adsorbed oxygen and the photo-generated hole contribute to local charge neutrality and thus freeing up the photo- generated electron to contribute to enhancing the conductivity of the device. Thus ZnO surfaces exposed to oxygen is also photoconductive. The behavior of the samples made in this study is thus consistent with the well known dependence of conductivity and photoconductivity of ZnO on exposure to oxygen [Q. Fl. Li, Q. Wan, Y. X. Liang, T. H. Wang, Appl. Phys. Lett. 84, 5456 (2004); R. J. Collins and D. G. Thomas, Phys. Rev. 112, 388 (195*)].

[00123] Figure 18 illustrates the decay times for ZnO UV sensors with nanowire- microsphere morphology and nanowire morphology under different oxygen pressures. The decay times varied from 39 s to 5.2 s corresponding to a vacuum of 0.1 Torr to oxygen pressure filled to the atmospheric pressure for UV sensors made of ZnO with the nanowire— microsphere morphology. Whereas the decay times for UV sensors made of ZnO with nanowire morphology were much longer for the same pressure range: 141s to 15 s.

[00124] The diversity of photoconductivity results found in the literature and the results obtained in this work are intriguing. The characterization done here is obviously not enough to thoroughly understand the mechanisms responsible for the different behaviors. Conductivity, photoconductivity, and the sensitivity to oxygen all depend on the microstructure. Surface effects and hence the surface area probably plays a very important role since the UV absorption depth is only 40 nm for ZnO. Nanometer scale structures are affected by surface effects more than larger dimension structures and hence it is conceivable that they are less sensitive to other effects. Other obvious factors that affect the behavior are quality and type of the contact, series resistances, and parasitics such as the edge effects.

Slow current transients and instabilities are well known in solid state devices, particularly in solar cell materials and silicon on insulators (SOI) [P. C. Karulkar, SOI Technology, S. Christoloveann (Ed.), (Electrochem. Society, 1994), p. 209 and references therein.]. They are respectively caused by grain boundaries and parasitic edge effects.

[00125] Without wishing to be bound by theory, it is believed that that the conductivity and photoconductivity in ZnO structures, that have unprotected, uncontrolled surfaces with unknown near surface band structures, are influenced by the microstructural features because they determine the surface area. Sputtered or evaporated thin films typically exhibit poorer performance, because they are structurally and stoichiometrically more imperfect in addition to having uncontrolled surface electronic structure.

b. CONCLUSIONS

[00126] UV Sensing properties of ZnO films exhibiting different morphology were studied. Samples exhibiting a micro-nano morphology (microspheres networked by nano wires) and nano morphology (network of nano wires) were fabricated by vapor phase growth. Simple, two terminal conductors were constructed using silver paste contact, and photoconductivity, as well as its dependence on the exposure to oxygen, was characterized. The conductivity was found to vary with increasing the oxygen pressure, demonstrating that the surface oxygen species controlled the electric transport through the individual ZnO nano wires. The existence of micro spheres in a network of nano wires does improve the performance by imparting better photoconductivity ratios and faster decays. Such a mixed micro-nano morphology material is not only useful for rudimentary sensors but may help in understanding the electronic material behavior.

3. CATALYST FREE PREPARATION OF HEXAGONAL ZNO NANOWIRES

[00127] ZnO nanowires were synthesized in a horizontal tube furnace (Lindberg) by a self catalyst Direct Vapor Phase (DVP) technique. The schematic (Figure 19a) shows the process of DVP growth of ZnO wires and nanorods. In short, this technique is a modification of the vapor transport and thermal evaporation methods the chemical elements from the precursors react and then transported to the substrate location through the gas pressure

gradient. However, in DVP technique the concentration of the precursors, amount of gas flow and most importantly the location of the substrate is chosen such that the final products are directly deposited on to substrates. In this technique, the evaporation is carried out at much lower temperatures (-600-800 ⁰ C) as opposed to 1800-2000 ⁰ C in thermal evaporation. Localized regions are formed on the substrates upon evaporation of constituent elements. Figure 19(b) shows the schematic of the growth set up. The source material Zn (99.9%) in granular form was placed at the center of the furnace in an alumina boat (800 ⁰ C, heating rate 10 ⁰ C min ^"1 , assuming the temperature of substrate as that of boat). Double side polished AI2O3 (0001) and pulsed laser deposited (PLD) ZnO - nucleation layer (NL) of thickness 100 nm on AI ₂ O ₃ were used as substrates. In the initial stage, the furnace was flushed by Ar gas and was later stabilized with a flow rate of 40-50 seem. When the furnace temperature reaches 420 ⁰ C (close to the melting point of Zn), O ₂ gas was introduced with the combined gas mixture of 60 seem. The evaporated Zn metal reacts with O ₂ /Ar gas admixture yielding growth of ZnO nanostructures. The process was carried out for 90 minutes, and samples were pulled out after furnace was cooled down to room temperature.

[00128] The ZnO nanostructures were characterized with X-ray diffraction

(SIEMENS -D5000 Diffractometer), Environmental and Hitachi Scanning Electron Microscopy (Electro Scan and S-4800), and Photoluminescence (Laser Science, Inc, Model VSL-337 ND-S, 337 nm, 6mW and Ocean Optics SD5000 spectrometer) and Energy Dispersive X-Ray Spectroscopy (EDXS) measurements were carried out at Natural Science Facility, University of Alaska Fairbanks (UAF) on new set of samples prepared at Office of Electronic Miniaturization (OEM), UAF. Cathodo luminescence (CL) measurements were carried out in SEM at a magnification of 5000 with an electron beam of energy 10 keV and excitation power of 10 μW. The X-ray Photoelectron Spectroscopic (XPS) measurements were performed using Kratos Axis 165 spectrometer at a vacuum of 4X10 ¹⁰ Torr with non- monochromatic Mg Ka radiation. All binding energies were calibrated with respect to C Is at 284.6 eV.

[00129] The reaction furnace after it cooled down from 800 ⁰ C, has consisted of two distinct and well separated ZnO products. The area around the Zn metal source and the substrates were completely filled with snow white ZnO nanomaterial.

[00130] To understand the effect of Zn catalyst on the growth of ZnO nanowires, one of the samples was pulled out at earlier stage of the nanowire growth. Figure 20 (a) shows the scanning electron microscopy (SEM) of ZnO nanowire with clearly distinguishable droplets at the tip on some of the nanowires and others being still active in the growth process, where the nanowires are past the droplets. From the Figure the Zn catalyst is found to be ~ 75 nm in diameter whereas ZnO nanowire of much smaller dimension of 50 nm. In order to understand the elemental composition make up of droplets and the wires, the samples with EDXS were examined, and results are shown in Figure 20 (b) (please note that imaging by EDX and separating features at these length scales can be difficult, and the present case, an area where only droplets could be seen without any ZnO background was selected so that it did not interfere during Zn imaging). The EDXS results revealed that the droplets are made up of Zn with two distinct peaks at 1.012 keV and 8.63 keV corresponding to Zn Ka and Zn La, respectively. On the other hand the EDXS showed the additional peak corresponding O K a at 0.523 keV [see http://new.ametek.com/content- manager/fϊles/EDX/Periodic%20Desk%20Mat.pdf] when focused on the area below the droplets confirming the presence of ZnO. In the present case, without wishing to be bound by theory, it is believed that that supersaturation of Zn in the ultra fine droplets condensed on the substrate surface is responsible for nanowire growth activation. For example, growth under higher flux of Zn resulted in multiple nucleations of nanowires with network structure formation as shown in Figure 21 (a). The nature of formation of such a network can thus be explained on the basis of excess Zn on the top of ultra thin nanowire during the vapor-liquid- solid growth. (It appears from Figure 20 (a) that there is some Zn catalyst remained unreacted at a given point of the process that is being utilized at the termination point of a particular nanowire as a seed for fresh growth of ZnO nanowires and then branch out again forming a network of well arranged nanowires). Such nanowire branching termed as "welding" forming three-dimensional interconnecting network of ZnO nanowires has been reported by Gao et. al. [P .X. Gao, CS. Lao, WX. Hughes, Z.L. Wang, Chem. Phys. Lett., 408,174 (2005)]. The networked nanowires form a porous material that is independent of underlying substrate or buffer layer, and it could be useful for high sensitivity gas sensors. By fine tuning the nanowire growth parameters such as flux of ZnO, growth temperature, and Ar/O ₂ flow, it is possible to fabricated well faceted ZnO nanowires as shown in Figure 21(b). It can be clearly

seen that ZnO nanowire has clearly distinguishable facets with [0001 ], [101 1] and [1010]. The dimensions of the nano wires are in the range of 25-60 nm in diameter and about 5 μm long.

[00131] Nanowires with a good control on orientation have been fabricated using template approach. The effect of nucleation layer (template) on the growth morphology of ZnO nanowire has been studied for laser deposited epi-ZnO thin film on AI2O3. The morphology looks completely different for the ZnO nanowires grown on PLD ZnO nucleation layer. As shown in Figure 22 (a), a hexagonal pattern of nano rods of dimensions (500 nm) terminated individually by a needle shaped "syringe-like" nanowires of dimensions (45 nm) can be seen. The enlarged view of the individual nano-rod and a nanowire at different locations has been shown in Figure 22 (b).

[00132] The formation of different dimensions of nanostructures has been reported by various groups [Y. H. Yang, B. Wang, N. S. Xu, G. W. Yang, Appl. Phys. Lett., 89, 043108 (2006)]. For example, a system of vertically aligned nano rods with foam like network of nanowalls below the nanorods grown by vapor phase transport via vapor- liquid-solid mechanism is reported [J. Grabaowska et al. Phys.Rev.B, 71, 115439 (2005)]. Very interestingly, the similar "syringe-like" ZnO needles of same dimensions riding on much larger nanorods have been reported [S. Ye, G.M. Fuge, N.A. Fox, D.J. Riley, M.N.R. Ashfold, Adv.Mater., 17, 2477 (2006)]. In their case, the ZnO nanowires are grown by aqueous method on a PLD grown ZnO thin film. The growth rates and stability of the different crystal planes that affect the morphology of ZnO nanorods have been shown to depend on the solution conditions like pH and oxidation states of Zn. In the present case, the NL- ZnO crystallizes in the thin film form with defined grain boundaries make up the entire

film which is directional along [0001 ]. The grains are of uniform size approximately 0.5-1 μm on a given film that largely depends on the technique and quality of film [R. Yang and Z.L. Wang, Solid State Comm., 134, 741 (2005)]. When the first Zn vapor arises during the nanostructure growth, it finds the grain boundaries and nucleates due to high mobility available as a consequence of a field across the boundaries. However, the electric fields

associated with the grains dictate the nano ZnO growth in horizontal direction along [101 1]

with the higher possibility and along [101 0] direction with the lower degree of freedom of

growth. This preferential disparity of growth with [000 l ] > [lθ l l] > [lθ l θ] directions leads to a rather larger sized nanorods (-500 nm in diameter). The growth of nanowire in the form of needles [R. C. Wang, C. P. Liu, J. L. Huang, S.J.Chen, Appl.Phys.Lett., 86, 251104

(2005)] and castles [X. Wang, J. Song, ZX. Wang, Chemical Physics Letters 424, 86 (2006)] on the larger area ZnO pedestal has been observed. Such a growth of nano wires on nanorods clearly shows that the two stage processes governed by the growth directional disparity along different facets indeed lead to the preferential growth of (0001) planes [R. C. Wang, C P. Liu, J. L. Huang, S.J. Chen, Y.K. Tseng, S.C. Kung, Appl. Phys. Lett., 87, 013110 (2005)]. After critical thickness, the nanorods become nanowires probably due to loss of substrate influence. This quasi-one dimensional structures as shown in the form of needles in Figure 22 (b) can be useful for field emission devices [S. S. Fan, M. Chap line, N. Franklin, T. Tombler, A. Cassell, H. J. Dai, Science 283, 512 (1999)].

[00133] The crystallographic properties of the nanowires were investigated by XRD (θ-

2θ and rocking curve measurements. As shown in Figure 23 (a) the nano ZnO films are

highly oriented along [0001 ] direction showing (0002) planes in both cases. The peaks at 34.3°, 36.2°, 56.56°, and 72.54° correspond to reflections of ZnO hexagonal phase for (002), (110), (101) and (004) planes. The peak at 41.6° corresponds to (0006) plane OfAl ₂ O ₃ substrate. Figure 23 (b) shows the rocking curve scans for ZnO nanowires and ZnO nanowires on PLD-ZnO (NL) on Al ₂ O ₃ substrates. The rocking curve width for ZnO (002) peak is 0.22° for ZnO nanowires on PLD-ZnO (NL) on Al ₂ O ₃ substrates. It can therefore be concluded that the as synthesized ZnO nanowires that grow on the ZnO thin films are well crystallized in hexagonal structure and grow along the c-axis direction.

[00134] The optical properties of ZnO nanowires were investigated by the room temperature photoluminescence spectroscopy. As shown in Figure 24, the dominant peak was observed at λ=386 nm. The laser excitation (3.6 eV) is also shown for comparison. The inset of Figure 24 shows the bright blue/violet emission from the ZnO nanowires under laser

excitation. The dominant peak is attributed to the recombination of donor-bound excitons (D ⁰ X) corresponding to 3.2 eV (386 nm), wide direct bandgap transition of ZnO nanowires at room temperature [U. Ozgur, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Do S. Doan, VAvrutin, S.-J. Cho, H. Morkoc, J. Appl. Phys., 98, 041301 (2005)]. The exciton peak has the sharp full width at half maximum (FWHM) of 10 nm. The narrow width of the dominant emission is rather expected in the nanowires as a consequence of better quantum efficiency. The green emission observed at λ=510 nm (2.42 eV) correspond to deep levels because of transition between the photo-excited holes and singly ionized oxygen vacancies [B.S. Zou, R.B. Liu, F.F. Wang, AX. Pan, L. Cao and ZX. Wang, J. Phys. Chem. B, 110, 12865 (2006)].

[00135] Note that ZnO nanowires in the present work show very weak green emissions, while the ID ZnO nanostructures reported in the literature [D. Banerjee, J. Y. Lao, D. Z. Wang, J. Y. Huang, Z. F. Ren, D. Steeves, B. Kimball, M. Sennett, Appl. Phys. Lett., 83, 2061 (2003); X. Wang, Q. Li, Z. Liu, J. Zhang, Z. Liu, R. Wang, Appl. Phys. Lett., 84, 4941 (2004)] exhibited considerably higher green emission. In comparison to the UV emission, the green emission, associated with intrinsic defects in the ZnO, are much weaker [F. Leiter, H. Alves, D. Pfϊsterer, N. G. Romanov, D.M. Hofmann, B. K. Meyer, Physica B 340-342, 201 (2003)]. Without wishing to be bound by theory, it is believed that that the difference is due to the complete stoichiometry of Zn to O achieved in the DVP process and the absence of any catalyst that alters the oxygen balance due to varied degree of affinity in the growth process. At higher temperatures, the formation of intrinsic defects, such as oxygen vacancy or trap states, is enhanced, resulting in the considerable green emissions [T. W. Kim, T. Kawazoe, S. Yamazaki, M. Ohtsu, T. Sekiguchi, Appl. Phys. Lett., 84, 3358 (2004)]. Therefore, the weaker green bands point out that there was a low concentration of oxygen vacancies in the ZnO nanowires and revealed the high quality of the ZnO nanowires.

[00136] The optical characterization of ZnO nanowires by PL and CL lead to us explore the origin of green emission, from ZnO nanowires. Figure 25 (a) shows the SEM of one of the ZnO nanowires of dimension ~45 nm similar to the one shown in Figure 22 (b). When electron beam is incident on the nanowire there is a luminescence from the nanowire as shown in Figure 25 (b). Figure 25 (c) shows the CL spectrum of ZnO nanowires indicating

the excitonic and green emission. However, CL spectroscopy results indicated enhanced defect related emission (green band) as compared to excitonic emission in contrast to PL studies as shown in Figure 24, which shows week defect related peak. Without wishing to be bound by theory, it is believed that this is due to much higher excitation energy of electrons which ionize electron-hole pairs much easily and hence reducing the excitonic peak there relatively enhancing defect related peak. Earlier studies while profiling of defect related emission as a function of incident electron beam energy in ZnO films on sapphire grown by plasma assisted MBE reported that enhanced green emission is prominent due to higher beam energy of electron beam [Y. S. Jung, W.K. Choi, O.V. Kononenko, G. N. Panin, J. Appl. Phys., 99, 013502 (2006)].

[00137] The XPS results for Zn2p and Ols for ZnO nanowire films on AI2O3 substrates are shown in Figure 26. Chemical states and the presence of any possible compositions were analyzed after deconvoluting the spectra. The films show well-resolved peaks (Figure 26 (a)) at 1022.45 eV and 1045.47 eV corresponding to the doublet of Zn2p (3/2 and 1/2 respectively) as reported for ZnO [L. Jing, Z. Xu, J. Shang, X. Sun, W. Cai, H. Guo, Mater. Sci. and Engin., A 332, 356 (2002); Handbook of X-ray Photoelectron Spectroscopy, CD. Wagner, W.M. Riggs, L.E. Davis, J. F. Moulder, G. E. Mullenberg, Perkin-Elmer Corp., 1979, Eden Prairie, MN, USA]. Figure 26 (b) shows the Ols spectrum in ZnO nanowire film. This asymmetric peak is resolved into three components at 531.1 eV, 532.5 eV and 533.6 eV. The intense peak at 531.1 eV can be attributed to ZnO oxygen whereas the shoulder peaks at 532.5 eV and 533.6 eV have been assigned to the chemisorbed oxygen caused by the surface hydroxyl groups. The relative concentration of Zn/O calculated to be close to 1 from the photoelectron cross-sections and kinematic factors indicating the near perfect stoichiometry achieved in the present process.

4. PREPARATION OF V ₂ O ₅ MICRO AND NANOSTRUCTURES

[00138] V ₂ O ₅ powder (Sigma Aldrich - 99.999% purity) was used as the source. The ultrasonically degreased and acid-etched Molybdenum (Mo), fused silica (FS) and Si (111) were used as the substrates. A fused quartz tube furnace that could be heated to 650 ⁰ C was used. V2O5 source material was placed at the center of the furnace and the substrates were

placed down stream. The furnace was ramped up at 10 °C/min to 650 ⁰ C while maintaining a measured flow of an argon-oxygen mixture. The furnace temperature was ramped down after deposition for a specific period of time and substrates were removed from the tube. X-ray powder diffraction (XRD) analysis of the samples was carried out with Rigaku Geigerflex X- ray Diffractometer system with graphite monochromatized CuKa irradiation (λ = 1.5418A). Scanning Electron microscopy (SEM) images were taken by an Environmental and Hitachi (Electro Scan and S-4800 ). Atomic Force Microscopy (AFM) measurements were carried out using a Pacific Nanotechnology AFM. Photoluminescence (PL) spectra were recorded using a Perkin Elmer LS50B spectrometer. Electrical measurements were carried out using a HP- 4145B semiconductor parameter analyzer.

a. RESULTS

[00139] Figure 27(a-c) show the SEM OfV ₂ Os micro and nanostructures on Mo, FS and Si substrates grown by DVP method indicating the formation of both sized features. The SEM images show 250-300 μm length and 5-6 μm diameter structures for V ₂ Os on Mo and FS substrates. However, for V ₂ Os on Si substrate -500 nm - 2 μm width belts having thickness of a few nanometers and lengths of ~ 50 μm oriented along an angle of 45° are observed.

[00140] Figure 28 shows XRD pattern OfV ₂ O ₅ sample on Si (111) substrates. The

XRD on other substrates resulted in weak and broad peaks around same positions. The orthorhombic phase OfV ₂ O ₅ is detected along with a small peak representing monoclinic phase in experiments.

[00141] The PL studies of the samples indicated the peak emission at 417 nm corresponding to energy gap of 2.9 eV (Figure 29). Irrespective of substrates, the peak emission occurs at same wavelength but with different intensities and sharpness.

[00142] Figure 31 shows the electrical properties characterized by I-V measurements of V ₂ O ₅ ZFS structures for without exposure to propanol and under propanol exposure. The current increases in the latter.

b. DISCUSSION

[00143] From SEM studies it is clear that V ₂ O ₅ tends to grow in the form of 10 μm size width belts having thickness of a few nanometers. The micro belts have an orientation of 45° with respect to the normal to substrate surface (indicated by arrows in Figure 27(c)). In order to confirm the initial arrangement OfV ₂ Os nucleation layer in Si (111) surface, an area was located on the sample that did not have any significant growth as shown in the boxed area of Figure 27 (c). The AFM measurements shown in Figure 27 (d) provide the morphology of this area. The height, angle and grain analysis using NanoRule+ 2.5.05 software (Pacific NanoTechnology) revealed that the nucleation centers are of diameter 60-90 nm (indicated on a chart at three locations in Figure 27 (e)). From the micrograph it may be concluded that the nucleation centers are indeed arranged with an angle of 45° with respect to the substrate surface.

[00144] In case of polycrystalline Mo and amorphous FS substrates the growth morphology results in formation of micron sized features as opposed to a mixture of nano and microcrystalline features on the crystalline silicon. The reactant species in the supersaturation of the gas phase adsorbed on the nucleation layers lead to formation of mixed sizes [S. Pavasupree, Y. Suzukia, A. Kitiyanan, S. Pivsa-Art, S Yoshikawa, J. Solid State Chem. 178, 2152 (2005)]. In the case of thin films, the crystalline substrates offer a defined platform for the growth of thin films either in the form of continuous or in the form of islands. Without wishing to be bound by theory, it is believed that the growth discrepancies leading to different morphologies in the samples could be explained from thermodynamic approach. The supersaturated gas phase conditions lead to the evolution of nucleation centers and the size and shape of these centers are governed by the activation energy of the surface. The nucleation centers tend to grow until the critical density for a given shape is reached onsetting a defined morphology [V.A. Markov, O.P. Pchelyakov, L.V. Sokolov, S.I. Stenin, S.

Stoyanov, Surf. ScL, 250, 229 (1991)]. In the present case, the growth on Si substrate follows the Volmer- Weber growth mode in which the nucleation occurs forming small islands [M. M. R. Evans, J. C. Glueckstein, J. Nogami, Phys. Rev. B 53, 4000 (1996)]. Whereas, for the V2O5 micro-nanostructures on Mo and FS substrates, the surface kinetics favour the growth of long wires that follows Stranski-Krastanov mode according to which nanocrystalline grains

are observed to agglomerate so as to form grains of larger sizes. Cao et. Al [An-Min Cao, Jin- Song Hu, Han-Pu Liang, and Li- Jun Wan, Angew. Chem. Int. Ed., 44, 4391 (2005)], have demonstrated V2O5 nanorods having diameters of about 200 nm and lengths up to 2 μm grown onto microspheres OfV ₂ O ₅ by mediated polyol process. _.

[00145] The appearance of (101), (110), (100), (411), (021) and (412) planes indicated that the V ₂ O ₅ micro-nano structures have orthorhombic phase. There is one small peak at 27.66° corresponding to (002) plane of the VO ₂ monoclinic phase. Whereas, Pavasupree et. al [S. Nishio and M. Kakihana, Chem. Mater, 14, 3730 (2002)] obtained monoclinic B phase VO ₂ nanorods in the as-synthesized samples using hydrothermal method. When these VO ₂ powders were calcined for 4 hours at 400 °C, they observed the phase change to orthorhombic V2O5 nanorods. The parallel but tilted at 45° orientation OfV ₂ O ₅ nanowires on Si substrates is assigned to those of V ⁵⁺ that have the distorted square pyramid form [J. GaIy, A. Ratuszna, J. E. Iglesias, A. Castro, Solid State Sc. 8, 1438 (2006)] with five coordinates [S. Surnev, M. G. Ramsey, F. P. Netzer, Progress in Surface Science, 73, 117 (2003)]. This is similar to the double zig-zag ribbon kind of structure that is formed where the edges of square pyramids are shared by oxygen atoms along [010], and the oxygen corners are shared along [100] giving rise to sheets, as reported by Haber et. Al [J. Haber, M. Witko, R. Tokarz, Appl. Catalysis A; General 157, 3 (1997)]. Therefore, V ₂ O ₅ in the form of a layered structure with (001) cleavage plane and parallel stacked layers of (V ₂ Os) _n that are held together by Van der Waals forces along (001) plane and stacking along [001] direction were formed in these experiments. Fan et. al [H. J. Fan, L. D. Marks, Ultrascopy, 31, 357 (1989)] studied the phase transition in V ₂ O ₅ films using high resolution electron microscope. The V ₂ O ₅ films were irradiated by e-beam for various amounts of time and found the phase changes from V ₂ O ₅ to V ₄ O ₉ to V _δ θi ₃ to VO. However, when a catalyst, such as Ti, is used in the process, V ₂ O ₅ nanorods do not undergo any substantial phase change upon e-beam irradiation [G. N Karakova, G. A. Zenkovets, N. P. Fander, D. -S, Su, R. Schogel, Mat. Sc. Engg. A343, 8 (2003)].

[00146] Many metal oxide catalysts such as vanadium and molybdenum oxides deposited on SiO ₂ exhibit PL in the visible region due to radiative recombination from the charge-transfer excited states of the oxides [H. H. Patterson, J. Cheng, S. Despres, M.

Sunamoto, and M. Anpo J. Phys. Chem. 95, 8813 (1991) and the references therein]. PL exhibits high sensitivity and nondestruction of the surface active sites and is a convenient technique for the investigation of the structure and properties of the surface active sites in metal oxide systems. The PL OfV ₂ Os/ Si shows sharp, smooth-structure with highest intensity peak amongst samples investigated here. The dominant emission in V2O5 occurs due to the exciton recombination. Zhang et. al. [S. G. Zhang, S. Higiashimoto, Y. Yamashita and M. Anpo, J. Phys. Chem B 102, 5590 (1998)], studied the solid-state reaction of the zeolite with V ₂ O ₅ using in-situ PL spectroscopy at 77K and 295K. They found that when the V ₂ O ₅ film sample is excited with 300nm wavelength, the luminescence occurs at -500 nm with fine structures around the peak. These peaks around 500nm and the fine structures were attributed to a vibrational fine structure similar to V ⁵⁺ oxide species highly dispersed in SiO ₂ that has tetrahedral coordination with a VO ₄ unit. The fine structure at the peak arises due to easy destabilization of oxygen in V=O bond at temperatures of 327 ⁰ C to 427 ⁰ C. In another interesting observation, the Franck-Condon principle [M. Iwamoto, H. Furukawa, K. Matsukami, T. Takenaka. and S. Kagawa J. Am. Chem. Soc. 105, 3719 (1983)] was used to study V ₂ OsZSiO ₂ structure, where it was concluded from the spectra that the nuclear distance of the V ⁵⁺ =O ^2" complex becomes longer in the excited states causing the fine structure in the PL measurements. Since no fine structures were observed around the peak, it was concluded that the oxygen in V=O bond in V2O5 micro-nano structures are stable and the nuclear distance of the V ⁵⁺ =O ^2" complex remain unchanged. The PL of V ₂ Os in the form of thin films give emission peak around 500 nm, that of V2O5 pellets give emission ~ 670 nm whereas the disclosed micro-nanostructures give emission - 417 nm.

[00147] The second sample of V ₂ Os micro-nanostructures on Mo substrates shows sharp but medium intensity peak probably due to sparsely placed nanowires and the third sample; V ₂ Os micro-nanostructures on FS substrates shows broad peak amongst the samples investigated. The reason for V ₂ O ₅ /FS emission peak broadening is due to the gap states introduced in the band gap due to defects that have origins in the interface related strain between the V ₂ Os and the amorphous FS surface. The formation of 250-300 μm long V ₂ Os wires on FS with good optical quality leads to applications in optical waveguides in which the high contrast in the refractive index between V ₂ Os and FS results in better signal propagation.

The inherent presence of defects in V2O5/FS structures as shown in broad PL spectra helps in manipulating light in the waveguides as the defects act as scattering centers to bend the light [C. J. Barrelet, A. B. Greytak, C. M. Lieber, Nano Letters 4, 1981 (2004)]. Ansari et. al [Z. A. Ansari, R. N Karekar, R. C. Aiyyer, Thin Solid Films, 301, 82 (1997)] vacuum evaporated V2O5 thin films to fabricate optical waveguides. However, the present work relates to comparative luminescence studies OfV ₂ Os micro-nanostructures on three different substrates. Whereas, from the reported literature on vanadium oxide, in general reveals that the PL spectra of V ⁵⁺ oxides in the form of highly dispersed films on substrates like porous vycor glass (PVG) [M. Anpo, I. Tanahashi, and Y Kubokawa, J. Phys. Chem. 84, 3440 (1980)], SiO ₂ , Al ₂ O ₃ [Y. Kato, H. Yoshida, T. Hattori, Phys. Chem. Chem. Phys., 2, 4231(2000)] and MgO [M. Anpo and M. Che, Adv. CataL, 44, 199 (2000) and references therein], the shape and the vibrational fine structure of emission spectrum were found to be strongly substrate dependent in which the origin of PL spectrum on PVG or SiO ₂ substrates was assigned to the V=O vanadyl group. V ₂ Os on Al ₂ O ₃ and MgO showed no fine structure similar to the present results. And TiO ₂ and ZnO when used as substrates to deposit V ₂ Os, did not exhibit any spectra at all due to the energy transfer from the excited state of the vanadium oxide species (V ⁴⁺ - 0 ^" ) to the substrates semiconductors. The uniform films of V ₂ Os catalyst [J. L. Guimaraes, M. Abbate, S .B. Betim, M. C. M. Alves, Journal of Alloys and Compounds 352 16 (2003)] on SiO ₂ , show that the V ₂ Os species have a well-separated and distorted four- fold coordinated state having (Si-O) ₃ V-O unit structure.

[00148] The electrical conduction in V ₂ Os generally occurs due to hopping of charge between V ⁵⁺ and V ⁴⁺ impurity centers. Muster et. al [J. Muster, G. T Kim, V. Krstic, J. G. Park, Y. W. Park, S. Roth and M Burghard, Adv. Mater, 12, 420 (2000)] carried out electrical measurements through two nano wires of V ₂ Os grown between the pre-patterned electrodes and measured current in the range of few pico-amperes (pA) for 1 volts at room temperature, whereas the current in the nanoregime (nA) was observed to 10 volts for the sample containing scattered wires. Further, propanol gas sensing measurements were carried out in order to prototype demonstrate the feasibility of micro-nano structured V ₂ Os for gas sensor applications. Adsorption of gas species onto the surface of a metal oxide can produce a substantial change in its electrical resistance resulting from the loss or gain of surface

electrons [M. Y. Afridi, J.S. Suchle, M.E. Zaghoul, D.W. Berning, A.R. Hefner, R.E. Cavicchi, S. Semancik, CB. Montgomery, CJ. Taylor, IEEE Sensors Journal, 2, 6 (2002)]. Figure 31 shows the I-V characteristics OfV ₂ OsZFS structures for the control and under propanol exposure. The increase in current is in the range of fraction of nanoamperes, which could be improved by carrying out the measurements in vacuum and controlled atmosphere.

c. CONCLUSIONS

[00149] The formation OfV ₂ Os micro and nanowires having orthorhombic phase has been demonstrated by DVP techniques. V ₂ Os belts oriented at -45° angle with respect to the normal were formed on (111) Si substrate. Whereas, scattered wires of V ₂ Os were seen to grow on Mo and FS substrates Photo luminescence (PL) studies of V ₂ Os wires on these three substrates were carried out that showed strong substrate dependence. The simple route to form these structures will pave the way for fabrication for variety of applications like gas sensors, pressure sensors, dilute magnetic materials, and optoelectronic devices.

[00150] It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.