METHOD TO CONTROL DEPOSITION OF ORGANIC MOLECULES AND ORGANIC ELECTRONIC DEVICE

The present invention provides a method to control deposition of organic molecules on a substrate. The present invention further provides an organic electronic device, which comprises a substrate with organic molecules deposited by such method.

Background

Organic semiconductors are in the focus of increasing research activities due to their promising potential applications in electronics and optoelectronics, e. g. organic light emitting diodes (OLED), organic field effect transistors (OFET), photovoltaic devices and organic semiconductor lasers. Among the different explored materials, small molecular weight organic molecules have been recognized as promising candidates for applications due to superior device properties such as carrier mobility. Predominantly, small molecular weight organic molecules are deposited by physical vapour deposition (PVD). This method allows tailoring of the device structure with excellent uniformity and sharp interfaces, provides high material purity and a high efficiency of utilizing raw material.

In many device applications, films of organic molecules are deposited on the substrate in dimensions measuring in inches, and are laterally patterned to meso-scale, i.e. in dimensions measuring in tens of nanometers to micrometers, to functionalize the devices. The technologies for patterning have been well developed for inorganic semiconductors by a combination of lithography and etching.

Photolithographic patterning of organic semiconductor devices is difficult due to the degradation or complete failure of the devices after exposure to water vapour, oxygen or solvents and developers used in the removal or patterning of the photo-resists. Several methods have been developed to overcome this problem. Forrest et al.(Shtein M, Peumans P, Benziger JB, Forrest SR (2003) Micropatterning of small molecular weight organic semiconductor thin films using organic vapor phase deposition. J App Phys 93: 4005-4016) demonstrated a resolution of the pattern in the micrometers range by using shadow masks. However, the method as described requires precise shadow mask fabrication and handling in the vacuum, which is particularly difficult with respect to fabricating shadow masks for miniature devices with a size of few micrometers, particularly if the shadow masks have a large area up to inches.

Alternatively, a stamping method has been applied for patterning devices with a resolution of tens of micrometers, but results obtained by this method are not uniform and areas with defects frequently appear.

Excimer laser photoablation has been used for high resolution patterning of organic devices, but this method is not suitable for mass production due to its low speed.

Selective area deposition of atoms or molecules on a substrate is another valuable tool in the fabrication of semiconductor devices. Predefinition of an energy favourable nucleation site, which allows controlled deposition of the atoms or molecules, is performed by lithographically or self-assembly patterning of the substrate. For the nucleation site, a material is used to which the atoms or molecules have a greater binding energy compared to the substrate, such that atoms or molecules brought in contact with the nucleation sites deposit on these sites. As a result, deposition is controlled

by diffusion or adsorption/desorption processes. This method allows the fabrication of complex, self-aligned device structures, greatly simplifies the subsequent processing and is, at present, used in inorganic semiconductor fabrication, inorganic nanotube formation and for controlling crystal growth and cluster formation. An example of selective deposition of organic molecules on a substrate and formation of organic electronic devices is given in GB024376.0.

US2006/0234059 A1 describes a method of fabricating a surface modified electrode in which a layer of functional organic material is disposed on a surface of and in contact with a first conductive layer by physical vapour deposition, and US2004/0229051 describes an electro-optical device which comprises a transparent multilayer coating with an organic layer, the latter being deposited by vapour deposition. The multilayer coating provides a barrier to moisture and oxygen and provides chemical resistance.

Different methods of controlling crystal growth of organic materials and fabrication of organic electronic devices comprising such crystals are also known.

For example, a method of forming an organic semiconductor thin film with large crystal grains is described in JP2006339604. A thin film of an organic semiconductor precursor is converted into the thin film of the organic semiconductor by desorption, while crystal growth is performed by moving the thin film inside a system having a temperature gradient according to defined conditions. US2007/0117298 A1 discloses a method of manufacturing a field effect transistor, wherein anisotropic drying of a solution containing organic molecules in a channel results in growth of large, highly oriented crystals, however, formation of the channels is

labour intense and only a low resolution can be achieved. US2003/0062845 A1 describes crystallisation of an amorphous semiconductor film by low speed laser annealing for providing a semiconductor region for a TFT, which results in high costs and is less suitable for mass production. The same disadvantages apply to the method of fabricating an organic electro luminescent display (OELD) as described in US2007/0187676 A1 , wherein a switching transistor and a driving transistor of an organic light emitting diode (OLED) of the OLED comprise silicon islands, which are crystallised by low speed Excimer laser annealing, and to the method of fabricating a patterned small molecule semiconductor layer by laser annealing, as described in US2004/0110093.

US2001 /0029103 A1 describes a method of fabricating an organic semiconductor film by applying a solution containing an organic semiconductor material and a solvent to a substrate and evaporating the solvent in order to crystallise the organic semiconductor material and to form an organic semiconductor film with a large, continuous area greater than 1 cm ² . This method is merely intended to produce continuous organic semiconductor films, as it results in a low resolution, and is work intense, particularly if contamination of the solution is to be prevented, which adds to the production costs. US2002/0025690 A1 describes a method of manufacturing a semiconductor device comprising a patterned organic film, which is patterned with a low resolution by forming an etching stop film over the organic film and etching, which is a labour intense process. EP1617492 A1 describes a method of growing single layers of organic molecules with a low resolution on silicon oxide nanostructures formed by lithography. US2007/0215868 describes formation of discontinuous layers of photosensitive devices comprising organic photoconductive materials disposed in a stack between a first and second electrode. The discontinuous layer is formed by low speed annealing of an amorphous

polymer film; islands of amorphous film are deposited by jet-based or mask-based printing techniques and are then thermally annealed to create crystals.

In summary, at present, a controlled deposition of organic molecules on arbitrary substrates is difficult to achieve, and the methods of depositing organic molecules on substrates are work intense, costly, result in a low resolution of patterns and/or are slow. Each of these disadvantages also has a negative impact on an organic electronic device, which comprises a substrate with organic molecules fabricated by these methods.

Summary of the invention

Accordingly, the present invention provides a method of site selective deposition of organic molecules on a substrate, particularly at least partially controlled by a substrate topography. Furthermore, an organic electronic device comprising a substrate with organic molecules deposited by such method is provided.

As will be appreciated, if an area of selective deposition of organic molecules is at least partially controlled by the topography of the substrate rather than by chemical properties of a material of the topography and/or the substrate, a material of the substrate may be substantially any material. Furthermore, a material providing the topography may be the same or a different material to that of the substrate.

A first aspect of the invention provides a method to control deposition of organic molecules on a substrate, comprising the steps of (i) forming a topography of the substrate,

(ii) depositing organic molecules by vapour deposition to form at least one organic nucleation site controlled by the topography, and

(iii) further depositing organic molecules at the organic nucleation site by vapour deposition.

By depositing the organic molecules to form the at least one organic nucleation site and further depositing the organic molecules at the organic nucleation site, an organic pattern may be formed.

As will be appreciated, the topography may be a pattern provided on the substrate or a pattern provided by the substrate. For example, in particular embodiments the substrate can have three dimensional profiles provided therein to create recesses and/or raised portions of the substrate.

Although other suitable methods of depositing the organic molecules may be used, e.g. depositing the organic molecules from a solution comprising the latter and a suitable solvent, and at least partially evaporating the solvent, vapour deposition is preferred. As will be appreciated, vapour deposition provides an efficient use of material, particularly raw materials such as the organic molecules. In addition, an amount of work necessary to reduce or prevent contamination, as is required when depositing the organic molecules from the solution et cetera, is reduced.

In particular, controlled deposition of the organic molecules to form the at least one organic nucleation site and further depositing organic molecules at the latter advantageously abolishes or at least reduces a requirement to remove organic molecules from areas of deposition other than the desired area(s). As a result, formation of the desired organic pattern is cost efficient, has a reduced processing time and provides an efficient use of material.

Without wishing to be bound by theory, it is believed that the organic molecules of the organic nucleation site provide pi-pi interaction with further organic molecules which will lead to sites selective deposition.

In addition, layer by layer deposition of organic molecules may advantageously be achieved on amorphous substrates following formation of the nucleation sites comprised of the organic molecules. Additionally, the layers may have a highly ordered structure.

In embodiments, the topography comprises at least one first and second level, e.g. a first and second height or the like, and the at least one organic nucleation site is formed substantially at a transition between the first and second level. As will be appreciated, the transition may be a transition surface, an edge, a curve, a point or area of contact between the first and second level or the like.

In particular embodiments, the second level is a raised level, and the organic nucleation site is formed substantially below said raised level. If, the first level is a lower level, for example a surface of the substrate or a chemically or physically modified substrate, the organic nucleation site may be formed on the lower level, particularly at the transition.

In embodiments, the at least one organic nucleation site is formed by absorbing organic molecules by said topography.

In an embodiment of the invention, the vapour deposition method is physical vapour deposition.

In embodiments, the step of depositing organic molecules by vapour deposition to form at least one organic nucleation site and/or the step of further depositing organic molecules at the organic nucleation site is/are controlled by adjusting at least one factor having an impact on the deposition, e.g. a temperature, such as a temperature of the substrate, of the topography, the organic molecules, or the like, a deposition time, a concentration of organic molecules, a deposition rate of the organic molecules etc., or a combination thereof. Particularly, a temperature of the substrate, a deposition rate of the organic molecules or both is adjusted.

The temperature of the substrate may be in the range of 25°C to 400 ⁰ C, particularly 50 ⁰ C to 250°C. More particularly, the temperature of the substrate may be in the range of 140 ⁰ C to 200°C, such as 160°C to 180 ⁰ C. For example, the temperature of the substrate may be substantially 170°C. It will be appreciated that the temperature may vary with the substrate, organic molecules to be deposited, a size of the substrate and/or the organic pattern to be formed.

In an embodiment of the invention, the organic molecule comprises at least one aromatic portion.

In an embodiment of the invention, the organic molecule is N, N*-dioctyl- 3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8).

The substrate may comprise a material selected from silicon, silicon oxide, plastics, indium tin oxide (ITO), glass, aluminium oxide or derivatives thereof.

In embodiments, the substrate is a solid substrate. Alternatively, the substrate may be flexible. This is particularly advantageous, as the

substrate with the deposited organic molecules may be deformed, e.g. rolled, bent or the like.

In embodiments of the invention, the topography of the substrate is at least partially formed by e-beam lithography, photolithography, soft lithography, scanning probe microscopy writing or the like.

Preferably, the topography is formed by e-beam lithography. As will be appreciated, a high resolution, for example a resolution of approximately 20 nm, can be achieved. Furthermore, a shadow mask is advantageously not required for e-beam lithography.

Photolithography is a commonly used, fast technique for forming patterns with a sub-micrometer resolution.

Soft lithography is a cost efficient method, particularly with regard to mass production, and can be used with various substrate sizes and shapes (such as nonplanar substrates). Depending on the mask used, a resolution of approximately 6 nm (30 nm for industrial applications) can be achieved; furthermore various pattern transfer methods are possible, which allows a good adaptation to different substrates, pattern materials et cetera.

Scanning probe microscopy writing provides a high resolution of under 100 nm (22 nm in laboratory set ups).

In embodiments, the topography is at least partially formed by silicon oxide or gold.

In embodiments of the invention, at least one crystal comprising the organic molecule is grown by the method as hereinbefore and hereinafter

described. Additionally, the crystal can have a defined orientation.

Advantageously, the oriented crystal may be grown on any suitable substrate and the orientation of the crystal is controlled by the substrate topography.

Preferably the controlled deposition of organic molecules on a substrate as hereinbefore and hereinafter described may be followed or preceded by a method of selective growth of organic molecules on a substrate, particularly organic molecules different from the organic molecules used in the method according to the first aspect of the invention, e.g. aromatic molecules, for example N,N'-bis-(1 -naphyl)-N,N'-diphenyl-1 ,1 '-biphenyl'-

4,4'-diamine (NPB), comprising the steps of: creating a pattern of nucleation sites for the organic molecules on the substrate; depositing of organic molecules at the nucleation sites by vapour deposition.

This is particularly advantageous, as different materials, which may have different physical and/or chemical properties, can be controlled to deposit at different areas of deposition. The deposited materials may serve different functions, e.g. reflect or refract electromagnetic waves such as light or the like, reflect or refract photons, conduct electrons, electrically insulate, provide a p- or n-layer or the like. This is particularly advantageous if the substrate is, for example, comprised by an organic electronic device, such as an organic light emitting diode (OLED), organic field effect transistor (OFET), organic semiconductor laser, photovoltaic element or the like.

It will be appreciated that deposition of the organic molecules at the pattern of nucleation sites may be further controlled by adapting a

temperature of the substrate, adjusting a deposition or growth rate of the organic molecules, adjusting a deposition temperature or the like or may be controlled by dimensions of the substrate, topography and/or the pattern of nucleation sites.

The organic molecules may be deposited at the pattern of nucleation sites by physical vapour deposition or chemical vapour deposition.

The pattern of nucleation sites may comprise a nucleation material having a different surface energy than the substrate, for example, the nucleation material may be gold or the like. If the nucleation material is gold, the substrate may be silicon, silicon dioxide, a derivative thereof or any other suitable material.

The pattern of nucleation sites may be created by e-beam lithography, photolithography, soft lithography, scanning probe microscopy writing and/or the like.

According to a second aspect of the invention, there is provided an organic electronic device comprising a substrate with organic molecules obtainable by the method according to the first aspect of the invention.

In embodiments, the organic electronic device is an organic light emitting diode (OLED), organic field effect transistor (OFET), organic semiconductor laser, photovoltaic element or the like. For example, the organic molecules can be deposited according to the first aspect of the invention to form an active organic layer between an anode and a cathode of an organic light emitting device (OLED), particularly in the form of a crystalline deposit or the like. Additionally or alternatively, the organic

molecules may be deposited to form at least one additional conducting layer, e.g. between the anode and a luminescent layer.

In another example, the organic molecules are deposited according to the first aspect of the invention to form an electron donor and/or an electron acceptor layer, which are arranged substantially between an anode and a cathode of a photovoltaic element.

In yet another example, the organic molecules are deposited according to the first aspect of the invention to form an organic charge-transporting channel, which is located between a source and a drain electrode and on a gate insulator of an organic thin film transistor (OTFT).

An embodiment of the present invention will now be discussed by way of example only with reference to the following figures:

Fig. 1 shows a schematic representation of a fabrication process according to the present invention,

Fig. 2 shows a) the chemical structure of N, N ^* -dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8), b) an atomic force microscopy (AFM) image of a topography of PTCDE-C8 deposited on unpatterned SiO ₂ , and c) a high resolution image from an area indicated in Fig. 2b,

Fig. 3 shows AFM images of PTCDI-C8 deposited on an Au patterned SiO ₂ substrate after a) 10 min and b) 60 min of deposition,

Fig. 4 shows AFM images of a) an Au patterned Au substrate, b) the Au patterned Au substrate following deposition of PTCDI-C8 and c) a height

profile of the Au patterned Au substrate following deposition of PTCDI-C8 along the representative line shown in Fig. 4a and b,

Fig. 5 shows fluorescent microscopy images of PTCDI-C8 deposited to form organic patterns, i.e. a) lines, b) concentric rings and c) antidot arrays,

Fig. 6 shows an AFM image of PTCDI-C8 deposited on an SiO2 substrate patterned with Au lines; PTCDI-C8 crystals are oriented at an angle of 60° relative to the Au lines, and

Fig. 7 illustrates a substrate surface topography evolution by subsequent deposition of different organic molecules to a SiO2 substrate having an Au topography (Fig. 7a to 7c), wherein a) shows an AFM image of the substrate with the topography, b) an AFM image of PTCDI-C8 deposited on the substrate as shown in Fig. 7a, c) an AFM image of and N,N'-bis-(1 - naphyl)-N,N'-diphenyl-1 ,1 '-biphenyr-4,4'-diamine (NPB) subsequently deposited on the substrate as shown in Figure 7b, and Fig. 7d and 7e) show fluorescent microscopy images of the patterned substrate with PTCDI-C8 and NPB as shown in Fig. 7c, excited by d) green light (520 nm) and e) UV (360 nm).

Fig. 1 shows a schematic representation of a fabrication process according to the present invention. Silicon (Si) with 300 nm thermal grown oxide is used as a substrate 10 (Fig. 1A). In order to form a topography of the substrate 10, a resist of poly methyl methacrylate (PMMA) 12 is deposited on the substrate 10 (Fig. 1 B). In a second step (Fig.1C), the resist 12 is subjected to e-beam lithography to remove portions of the resist 12 and at least partially expose the substrate 10, i.e. form a pattern

14a, 14b, 14c. A thin layer of chrome (Cr) may be deposited at least on the exposed substrate 10 areas, i.e. the pattern 14a, 14b, 14c, for providing improved adherence properties. Thirdly (Fig. 1 D), gold (Au) 16 is deposited on the pattern 14a, 14b, 14c to form a negative gold (Au) pattern 16a, 16b, 16c on the substrate 10 (or, if applicable, on the Cr layer). The Au pattern 16a, 16b, 16c can have a thickness in the range of 0.5 nm to 500 nm. In a fourth step, the resist 10 is removed by incubation in acetone and sonication, and the Au patterned 16a, 16b, 16c substrate 10 is cleaned in organic solvent. The resulting substrate 10 with the topography, here the Au patterned 16a, 16b, 16c substrate 10, is transferred into vacuum, and molecular layers of organic molecules are deposited by physical vapour deposition (Fig. 1 F). The organic molecules move over the substrate 10 surface and are controlled to nucleate at an edge of the Au pattern 16a, 16b, 16c to form organic nucleation sites 18a, 18b, 18c, 18d, 18e, 18f. Due to strong pi-pi interactions, further organic molecules are deposited at the nucleation sites 18a to 18f, which finally results in formation of an organic pattern 20a, 20b, 20c, 2Od on the substrate 10 (Fig. 1 G).

The chemical structure of N, N ^* -dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) as an example of a typical aromatic molecules with an extended pi- electron system facilitating pi-pi interactions is shown in Fig. 2a. Fig. 2b shows an atomic force microscopy (AFM) image of a surface topography of a PTCDI-C8 film deposited by the method according to the first aspect of the invention on a SiO2 substrate at 170 ⁰ C. The continuous PTCDE-C8 film comprises large planar terraces, which have a size of 0.1 μm to 100 μm and are separated from each other by mono-molecular steps of 2 nm. Molecular resolution with in plane cell parameters of a=0.94 nm and b=0.48 nm, obtained by high resolution AFM as shown in Fig. 2c indicates the high crystalline film.

An AFM image of PTCDI-C8, which is deposited on a SiO2 substrate with a topography, here a pattern of an array of Au dots, after 10 min of PTCDI- C8 deposition is shown in Fig. 3a and after 60 min of PTCDI-C8 deposition is shown in Fig. 3b. The Au dots are 600 nm in diameter, have a height of 10 nm and have a first neighbour distance of 3 μm. However, it will be appreciated that other dimensions and neighbour distances are possible. In an initial stage, two dimensional molecular islands representing organic nucleation sites are deposited substantially exclusively on the substrate at the Au dots (Fig. 3a). Following formation of the organic nucleation sites, further deposition of the organic molecules occurs, as molecules move along the substrate surface and deposit at the nucleation sites due to strong molecular interaction (PTCDI-C8 organic patterns formed within 60 min of deposition are shown in Fig. 3b).

To clarify the deposition mechanism, an Au topography on an Au substrate was used. Au lines with different line widths were patterned on the Au substrate, such that these Au lines define channels extending from an upper height of the Au lines to the substrate surface; an AFM image of the Au patterned Au substrate is shown in Fig. 4a (Au lines = light grey; substrate surface = dark grey). As the substrate and topography have the same material, in this case Au, a difference in binding energy to the deposited organic molecules, as could be the case for different substrate and topography materials, can be excluded. In Fig. 4b, the Au patterned Au substrate following deposition of PTCDI-C8 is shown (AFM image). The organic molecules are deposited in the channels of the Au pattern. A height profile of a cross sectional cut (position indicated by white line in Fig. 4a and 4b) of the Au patterned Au substrate with PTCDI-C8 deposit is shown in Fig. 4c. The height profile illustrates a reversed topography, which indicates that the organic molecules are controlled to deposit in the

channels to form the organic pattern, i.e. below a raised level defined by the upper height of the Au lines.

Fig. 5a to 5c show fluorescent microscopy images of PTCDI-C8 deposited to form different organic patterns, i.e. lines, concentric rings and antidot arrays (Fig. 5a to 5c, respectively). The respective substrate was a silicon oxide substrate with Au pattern (light grey), and the PTCDI-C8 molecules were deposited substantially below a raised level formed by an upper height of the Au pattern; in this case the organic patterns are in the form of an array of lines, of concentric rings with different diameters and an array of spaced dots (Fig. 5, left to right). As will be appreciated, modifications and variations of the organic pattern are possible by designing the topography of the substrate accordingly.

Fig. 6 shows an AFM image of PTCDI-C8 deposited on an SiO2 substrate with a topography, i.e. a pattern of parallel Au lines (dark grey). PTCDI-C8 crystals are oriented at an angle of substantially 60° relative to a longitudinal axis of the Au lines, and the crystals are substantially parallel to each other. As will be appreciated, modification of the topography of the substrate can be used to control crystal orientation, which advantageously provides a convenient way to grow crystals with arbitrary orientation. As will be appreciated, the performance of organic devices varies with the degree of molecular ordering and crystalline orientation. In addition to growth conditions, orientation controlled organic thin films have superior potential for enhanced performance and provide more unusual properties such as optical and electrical anisotropic characteristics. For example, the anisotropic mobility could be useful in isolating neighbouring components so as to reduce the cross-talk effect in logic circuits or pixel switching elements in displays.

The technique also can be used to separate molecules in pre-defined sites by deposition of another molecule. Fig. 7a to 7c illustrate a substrate surface topography evolution over time by subsequent deposition of different organic molecules to the substrate having a topography, i.e. a patterned substrate. Here, a SiO ₂ substrate is patterned with Au dots, which are 10 nm in height and 0.6 μm in diameter, and which have a first neighbour distance of 1.2 μm, as shown in Fig. 7a. Subsequently, PTCDI- C8 and N,N'-bis-(1 -naphyl)-N,N'-diphenyl-1 ,1 '-biphenyl'-4,4'-diamine (NPB) are deposited on the Au patterned Si oxide substrate. The controlled deposition of PTCDI-C8 molecules on the patterned substrate turns the substrate topography inversed (as shown in Fig. 7b). Fig. 7c shows the AFM topographic image following subsequent deposition of NPB on the patterned substrate with the PTCDI-C8 deposit as shown in Fig. 7b. Whereas the PTCDI-C8 molecules are controlled by the topography of the substrate to deposit on the lower part, here the substrate surface, the NPB molecules selectively deposit on the Au dots, due to the different binding energy between AU and NPB compared to the binding energy between silicon oxide or PTCDI-C8 and NPB. Fig. 7d and 7e) show fluorescent microscopy images of PTCDI-C8 and NPB on the patterned substrate as shown in Fig. 7c, excited by green light (520 nm) and UV (360 nm), respectively. The PTCDI-C8 anti-dots organic pattern (medium grey, Fig. 7d) and the NPB dots pattern (light grey, Fig. 7e) show that the molecules can be controlled to pre-defined sites in meso-scale, i.e. having dimensions measuring in tens of nanometers to micrometers.

In the present work, site-selective deposition of N, N ^* -dioctyl-3,4,9,10- perylene tetracarboxylic diimide (PTCDI-C8) on a substrate with a topography is described. The substrate with the topography may be Au patterned SiO ₂ Or Au patterned Au. By optimizing deposition conditions such as a temperature, e.g. of the substrate, the topography and/or the

like, a deposition time etc., the PTCDI-C8 molecules can be controlled to deposit for example at an edge of the topography of the substrate; the deposited organic molecules provide nucleation sites for other PTCDI-C8 molecules due to strong pi-pi interactions, which will lead to the site selective deposition.

Materials

PTCDI-C8 was purchased from Sigma-Aldrich. Poly (methyl methacrylate) (PMMA), 950K, was purchased from All Reist GmbH. The silicon wafers with a 300 nm thermally oxidized SiO2 surface were purchased from Si- mat Company. All chemicals were used without further purification.

Instruments and characterization

E-beam lithography was performed by LEO VP 1530 field emission scanning electron microscope (SEM) with a Raith Elphy Plus lithography attachment system. Atomic Force Microscopy (AFM) measurements were carried out on a Multimode Nanoscope Ilia instrument (Digital Instrument) operating in tapping mode with silicon cantilevers (resonance frequency in the range of 280-34OkHz). Metal deposition was carried out in a homemade vacuum chamber by heating wolfram (W) wire in a vacuum of 10E-6 mbar; the thickness of the deposited metal was monitored by microbalance. Molecule deposition was performed in a home design ultrahigh Vacuum (UHV) system equipped with a Knudsen cell; the deposition rate can be adjusted by controlling the cell temperature.