Metal organic framework photonic and electronic device for gas sensing

Field of the invention

The invention relates to chemically sensitive vertical metal organic framework (MOF) based micro-cavities/capacitors (MCCs) and field-effect transistors (ChemFETs).

Background of the invention

MOFs are an emerging class of porous materials suitable for gas sensing applications, and have attracted strong academic research efforts due to their unique physical properties: high crystallinity on a (sub)-nanometre scale, perfectly uniform porosity yielding record internal surface areas (up to 6000 m2/g), and tuneable opto electronic properties. Furthermore, chemical tailorability is a key-advantage of these crystalline solids regarding gas sensing applications, inter alia, and distinguishes them from the commercially most successful group of miniaturized gas sensors, which are based on semiconducting metal oxide (SMOx) chemi-resistors.

Along with issues in achieving high chemical selectivity towards the targeted analytes, the high working temperature (typ. > 400°C) of SMOx sensors forms a major challenge in gas sensing applications [Dey (2018) Materials for Advanced Technology 229, 206-217; Kreno et a/. (2012) Chemical Reviews 112, 1105-1125; Allendorf et at. (2020) Chemical Reviews 120, 8581-8640]. Furthermore, SMOx sensors can suffer from limited lifetimes, and are prone to produce questionable data because of high cross-sensitivities and base line drifts [Lewis 8i Edwards. (2016) Nature News 535, 29-31]. Therefore, the demand for a new generation of ideally miniaturized, low-cost, selective and sensitive gas sensors is pressing. This holds, in particular, for detection of volatile organic compounds (VOCs) in the ppb-ppm range in a wide range of applications, e.g., monitoring breath biomarkers is promising as a diagnostic method (e.g., lung cancer detection by concentration changes of exhaled VOCs) [Peng et at. (2009) Nature Nanotechnology 4, 669-673], in air quality monitoring, checking food freshness, and monitoring the exposure to toxic chemicals.

Adsorption of gas molecules into a MOF's pores increases its effective refractive index (n _M 0F) and dielectric constant (EMOF) . Various device concepts, transducing these changes into a physical measurable signal, have been proposed as gas sensing devices. Typically, an optical or electrical read-out is used, in particular, if the device design aims at applications requiring miniaturization/micro-integration. Photonic 2 crystals (PhCs), i.e., periodic structures with alternating high-n/low-n materials, with the MOF typ. used as low-n material, have been demonstrated. A change in n _M 0 _F upon gas adsorption causes an optical shift of the PhC's optical gap (formed upon interference), which can be detected via a suitable optical read-out (spectrometer, photo-diode, ...) [Zhang et a/. (2020 ) Angewandte Chemie International Edition 59, 1674-1681]. This has particularly been shown for repeatedly vertically stacked MOF/metal-oxide bi-layers, constituting a distributed Bragg reflector (DBR) [Hinterholzinger et al. (2012) J. Mater. Chem. 22 , 10356-10362]. Furthermore, optical resonators, surrounded by a MOF material, have been realized. This holds for vertical micro-cavities with the MOF coated on top of the resonator [Kumari, (2018) PhD thesis, University of Otago, Dunedin, New Zealand], thus, functioning as a gas filter; and for horizontal (ring-) oscillators, in which the MOF is coated around the wave-guide of the oscillator to alter its resonance condition upon gas adsorption [Tao et a/. (2017) Sci. Rep. 7, 1-8]. Optical resonators with the MOF inside the resonator as chemically sensitive optical medium, are much more difficult to realize , and so far, have only been realized as Fabry-Perot interferometers not necessarily showing clear resonances. For instance, a planar silver mirror coated with a MOF material, referred to as "open cavity", was reported [Kumari cited above]. Furthermore, a horizontal Fabry-Perot interferometer with light coupled in/out of a MOF single crystal was shown [Zhu et at. (2019) ACS Appl. Mater. Interfaces 11, 4393-4398]. In both cases, gas uptake in the MOF alters the interference condition of the interferometric structure. To detect the change in a MOF's dielectric properties upon gas adsorption, typically, devices with the MOF material coated around inter-digitated electrodes (IDEs) are used, and the change in the electrical capacitance between these electrode is read-out [Allendorf et al. (2020) Chemical Reviews 120, 8581-8640; Zhang et ai. (2020) Angewandte Chemie International Edition 59, 1674-1681; Sachdeva et al. (2017) small 13, 1604150]. Furthermore, plate capacitor geometries with the MOF material sandwiched in-between two metal electrodes were realized, e.g., with the MOF deposited by drop-casting from a nanocrystal suspension [Homayoonnia & Zeinali. (2016) Sensors and Actuators B 237, 776-786], or placed in form of pressed pellets [Weiss et al. (2016) Microporous and Mesoporous Materials 220, 39-43], but in either case at least a few micrometre thick to reduce the risk of electrical leakage between the attached electrodes. Montanez et al. (2019) ACS appl. Mater interfaces 11, 4393-4398, describes a metal-organic framework single crystal coupled to an optical fiber for chemical detection. 3

Summary of the invention

The invention is summarised in the following statements:

1. A gas sensor comprising subsequently arranged from bottom to top:

- a bottom first layer (1) of a reflective or semi-reflective conductive material or a bottom first layer (1) which is a distributed Bragg reflector,

- a second layer (2) with a thickness between 10 and 500 nm, of a material which is transparent or semi-transparent to ultra-violet, visible, or/and near-infrared light, wherein optionally the order of layer (1) and (2) is switched,

- a third layer (3) of a porous sensing material specifically binding a volatile analyte,

- an optional fourth layer (4) of an organic polymer, a metal-oxide film or a hybrid organic-inorganic film with a maximum one-side roughness of 20 nm and thickness between 0 and 100 nm,

- a top fifth layer (5) of a gas permeable, semi-reflective , and conductive material.

2. The gas sensor according to statement 1, wherein the top fifth layer (5) is partially transparent.

3. The gas sensor according to statement 2, wherein the top fifth layer (5) has an optical thickness of less than 0.5.

4. The gas sensor according to any one of statements 1 to 3, wherein the third layer (3) of porous sensing material consists of a metal-organic framework, a covalent-organic framework, or a zeolite.

5. The gas sensor according to any one of statements 1 to 4, wherein the third layer (3) of porous sensing layer material has a thickness between 5, 10 or 20 nm up to 100, 500, 1000 nm.

6 The gas sensor according to statement 4 or 5, wherein the metal-organic framework is a zeolitic imidazolate framework (ZIF).

7. The gas sensor according to any one of statements 1 to 6, wherein the fifth layer as a thicknesses between 10 to 200 nm.

8. The gas sensor according to any one of statements 1 to 7 wherein the bottom first layer (1) is a reflective or semi-reflective conductive material.

9. The gas sensor according to any one of statements 1 to 8, where the first and/or the fifth layer (5) are metals. 10. The sensor according to any one of statements 1 to 9 wherein the first layer

(1) is patterned into source and drain electrodes, and the second layer (2) is a 4 semiconductor and transparent or semi-transparent to ultra-violet, visible, and/or near-infrared light .

11 The sensor according to any one of statement 1 to 10, further comprising an additional non-conductive passivation layer, wherein said passivation layer is positioned between the second (2) and the third (3) layer.

12. A method of detecting an analyte in a gas comprising the steps of :

- contacting a gas comprising the analyte with the sensor according to any one of statement 1 to 11, and

- measuring the optical and/or electrical property changes of the sensing material upon contact with the gas comprising the analyte.

13. The method according to statement 12,

- wherein binding of the analyte results in a change of the refractive index of the porous sensing material and

-wherein the analyte gas is detected by measuring the change of the sensor's optical resonance upon the contacting the gas with the sensor,

-wherein a change of optical resonance is indicative of the presence of the analyte in the gas.

14. The method according to statement 12,

- wherein binding of the analyte results in a change of the dielectric constant of the porous sensing material and

- measuring the change of electric capacity upon the contacting the gas with the sensor,

- wherein a change of electric capacity is indicative of the presence of the analyte in the gas.

15. The method according to statement 12, using a sensor comprising a first layer (1) patterned into source and drain electrodes, wherein binding of the analyte results in a change of the dielectric constant of the porous sensing material and measuring the change of current between source and drain electrodes upon contacting the gas with the sensor, wherein a change of current is indicative of the presence of the analyte in the gas.

16. A gas sensor comprising subsequently arranged from bottom to top:

- a bottom first layer (1) of a reflective, semi-reflective metal, a semi-transparent material, a distributed Bragg reflector, or a conductive oxide, 5

- a second layer (2) with a thickness between 10 and 500 nm, of a semiconductive material and/or of a material which is transparent to ultra-violet, visible, or/and near- infrared light, wherein optionally the order of layer 1 and 2 is switched,

- a third layer (3) of a porous sensing material specifically binding a volatile analyte,

- an optional fourth layer (4) of an organic polymer, a metal-oxide film or a hybrid organic-inorganic film with a maximum one-side roughness of 20 nm and thickness between 0 and 100 nm,

- a top fifth layer (5) of a gas permeable and, semi-reflective or conductive material.

17. The gas sensor according to statement 16, wherein the third layer of porous sensing material consists of a metal-organic framework, a covalent-organic framework, or a zeolite.

18. The gas sensor according to statement 16 or 17, wherein the third layer of porous sensing layer material has a thickness between 5, 10 or 20 up to 100, 500, 1000 nm.

19. The gas sensor according to statement 16 or 17, wherein the metal-organic framework is a zeolitic imidazolate framework (ZIF).

20. The gas sensor according to any one of statements 16 to 19, wherein

- the first layer is a reflective metal or a distributed Bragg reflector,

- the second layer has a maximum thickness of 500 nm and being transparent to ultra-violet, visible, and/or near-infrared light, and

- the fifth layer is semi-reflective and gas permeable and has a thicknesses between 10 to 200 nm.

21. The gas sensor according to any one of statements 16 to 20, where the first and the fifth layer are conductive.

22. The sensor according to statement 21, wherein the first layer is patterned into source and drain electrodes, and the second layer 2 is a semiconductor.

23. The sensor according to statement 21 or 22, further comprising an additional non-conductive passivation layer, wherein said passivation layer is positioned between the second and the third layer.

24. A method of detecting an analyte in a gas comprising the steps of :

- contacting a gas comprising the analyte with the sensor according to any one of statement 16 to 23, and

- measuring the optical and/or electrical property changes of the sensing material upon contact with the gas comprising the analyte.

25. The method according to statement 24,

- wherein binding of the analyte results in a change of the refractive index of the porous sensing material and 6

-wherein the analyte gas is detected by measuring the change of the sensor's optical resonance upon the contacting the gas with the sensor,

-wherein a change of optical resonance is indicative of the presence of the analyte in the gas.

26. The method according to statement 24,

- wherein binding of the analyte results in a change of the dielectric constant of the porous sensing material and

- measuring the change of electric capacity upon the contacting the gas with the sensor,

- wherein a change of electric capacity is indicative of the presence of the analyte in the gas.

27. The method according to statement 14, using a sensor comprising a first layer patterned into source and drain electrodes, wherein binding of the analyte results in a change of the dielectric constant of the porous sensing material and measuring the change of current between source and drain electrodes upon contacting the gas with the sensor, wherein a change of current is indicative of the presence of the analyte in the gas.

Detailed description

Brief description of the figures

Figure 1: Device structures: Embodiments of the (a,b) MOF-MCC, and (c,d) MOF- ChemTFT. 1: metallic or DBR layer, 2: (semiconducting) spacer layer, 2b: passivation layer, 3: nano-porous layer, 4: smoothening layer, 5: gas-permeable and conductive layer.

Figure 2: MCC optical properties and Ethanol sensing: (a) Optical tuning of MCCs by thickness variation of oxide spacer layer, e.g., ZnO, w/ and w/o a present ZIF-8 thin film for gas adsorption, (b) Example of ethanol sensing: Reflection spectra of a Ag/ZIF-8/AIOx/ZnO/Ag micro-cavity in equilibrium with an ethanol vapor of varying partial pressure p/p°. The right curve indicates the red-shift of the resonance position, *** A _res , with increasing p/p°. (c) Optical Transfer-Matrix Model (TMM) calculations of the micro-cavity for varying ZnO thickness and ZIF-8 refractive index. Figure 3: 2 ^nd order micro-cavities: (a) Rough MOF films in Ag/MOF/Ag micro cavities yield optical resonances of poor quality, (b) By increasing the overall thickness of the micro-cavity d _cav , the sensor can be read out via its sharper higher 7 orders, which is illustrated on an Ag(30)/ZIF-8(50)/Al0x(8)/Zn0(170)/Ag(160) stack with a sharp 2 ^nd order resonance at ca. 520 nm.

Figure 4: Smoothening (optical spacer) layer: Demonstration of improvement of the optical quality of Ag/ZIF-8/Ag micro-cavities by deposition of thin, transparent polymer films of varying thickness on top of the MOF.

Detailed description of the Invention.

It should be emphasized that the below-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Any variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

It should be further noted that ratios, thicknesses, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a thickness range of "about 10 nm to about 2000 nm" should be interpreted to include not only the explicitly recited thicknesses of about 10 nm to about 2000 nm, but also include individual thicknesses (e.g., 10, 20, 30, ..., 100, 200, 300, ..., and 2000 nm) within the indicated range. The term "about" can include traditional rounding according to significant figures of numerical values. In addition, the phrase "about 'c' to y'" includes "about 'c' to about ’y".

"reflective " in the context of the present invention means all light is reflected by a material.

"semi reflective" in the context of the present invention means that a part (up to 10 %, up to 25, up to 50%, up to 80%, up to 95%, up to 98%) of incoming light is reflected by a material.

"Transparent material" in the context of the present invention means all light passes through the material. 8

"Semi-transparent material" refers to materials wherein part of the incoming light (up to 5 %, up to 25%, up to 50%, up to 80%, up to 95%) passes through a material, whereby the remainder of the incoming light is reflected or absorbed.

An alternative way of describing transparency is "optical depth" which is the product of the absorption coefficient of a material and the thickness of the material. For the top layer of the devices of the present invention the optical depth is typically 0.5 or below.

The prior art discloses proto-type plate capacitor structures i.e., electrode/MOF/electrode structures [Homayoonnia & Zeinali. (2016) Sensors and Actuators B 237, 776-786; Weiss et at. (2016) Microporous and Mesoporous Materials 220, 39-43]. However, either the porous electrodes used there are too thick (macroscopic silver paste droplets deposited on MOF films [Sachdeva et al. (2017) small 13, 1604150], or the surface roughness of the structures is way too high (80 nm gold layers deposited on rough MOF pellets [Weiss et al. (2016) Microporous and Mesoporous Materials 220, 39-43]), to enable these structures functioning as optical micro-cavities. The present invention demonstrates the feasibility of structures capable of functioning as both capacitive and optical gas sensors. This is achieved by:

(1) Applying a well-designed metal top electrode functioning as gas permeable optical mirror in a vertically stacked micro-cavity including a porous MOF film to determine its selective gas sensitivity as well as optical spacer layers to fine-tune its optical resonance. Neither the devices of Homayoonnia &. Zeinali. (2016) Sensors and Actuators B 237, 776-786 or of Weiss et al. (2016) Microporous and Mesoporous Materials 220, 39-43, nor a structure in which the MOF is used as gas filter on top of a vertical micro-cavity [Kumari cited above], offer this functionality.

(2), the devices of the present invention are not plate-capacitors formed by electrode deposition around macroscopic MOF pellets as in Weiss et al. cited above, but they are built sequentially as vertical thin film stacks, offering the advantage of miniaturization and high parallelization/integration, which is a requirement for realization of micro-integrated gas sensor arrays. Furthermore, compared to other purely optical gas sensor concepts based on vertical photonic crystals, the device design disclosed herein does not require comprehensive multi bi-layer stacking, which makes its fabrication more simple, while possessing a higher sensitivity as can be shown by optical TMM calculations; apart from its advantage of a dual read-out (optical and capacitive). 9

The MOF micro-cavity/capacitor is furthermore the basis for chemically sensitive field effect transistors (MOF-ChemFETs), illustrated by FIGs. lc and Id, since basically the same layer sequence is used. Prior art designs are gas sensitive field effect transistors with a suspended gate possibly coated with a MOF [DE102011075396], which however, do have an air gap between the coated gate and semiconductor, hence, requiring advanced/expensive fabrication. Applying a thin gas permeable conductive top electrode directly on top of the insulating, porous MOF layer, which constitutes (a part of) the gate dielectric, makes the fabrication of the MOF-ChemTFT much more simple. Also it clearly differs from another prior art design, in which the porous MOF is part of the conductive gate electrode [CN 110579526], since the MOF-ChemFETs disclosed herein uses the MOF as (part of) the transistors dielectric. In principle, this positioning enables a dual read-out of the gas sensor, which is due to changes in the gate's work function (Wf _G ) and the MOF's dielectric constant (EMOF) when in contact with a gas, a property which neither of the prior art designs possess.

The invention relates to as sensor comprising subsequently arranged from bottom to top:

- a bottom first layer (1) of a reflective, semi-reflective metal, a semi-transparent material, a distributed Bragg reflector, or a conductive oxide,

- a second layer (2) with a thickness between 10 and 500 nm, of a semiconductive material and/or of a material which is transparent to ultra-violet, visible, or/and near- infrared light, wherein optionally the order of layer 1 and 2 is switched,

- a third layer (3) of a porous sensing material;

- an optional fourth layer (4) of an organic polymer, a metal-oxide film or a hybrid organic-inorganic film with a maximum one-side roughness of 20 nm and thickness between 0 and 100 nm,

- a top fifth layer (5) of a gas permeable and, semi-reflective or conductive material. Examples of a reflective material suitable in the context of the present invention are silver, gold, aluminium, or chromium.

Examples of semi -reflective metals suitable in the context of the present invention are silver, gold, aluminium, or chromium,

Examples of a semi-transparent materials suitable in the context of the present invention are conductive polymers like (doped) PEDOT,

Examples of a distributed Bragg reflector suitable in the context of the present invention are stacks of alternating low-n/high-n oxides, e.g., SiOx/TiOx or AlOx/ZnO, 10

Examples of conductive oxides suitable in the context of the present invention are indium tin oxide (ITO) or n-doped ZnO like aluminium-doped ZnO (AZO).

In specific embodiments the thickness of the second layer is between 5 and 100 nm. In these sensors porous material typically have a pore size of < 5 nm (microporous) or < 50 nm (mesoporous).

In specific embodiments the maximum one-side roughness of the fourth layer is < 10 nm.

In embodiments of the gas sensor, the third layer of porous sensing material consists of a metal-organic framework, a covalent-organic framework, or a zeolite.

Examples of a metal-organic framework suitable in the context of the present invention are zeolitic imidazolate frameworks (ZIFs) such as ZIF-8, ZIF-11, ZIF-67, ZIF-90, ZIF-74, or MOFs built from Cu2+ dimers such as HKUST-1, CuBDC, or CuCDC. or MOFs built from Mg metal nodes such alpha-magnesium formate (alpha- MgFm), or M-MOF-74 with the metal M being zinc (Zn), magnesium (Mg), cobalt (Co), or nickel (Ni).

In embodiments of the gas sensor, the third layer of porous sensing layer material has a thickness between 5, 10 or 20 up to 100, 500, 1000 nm.

In preferred embodiments the thickness is between 50 and 200 nm.

In embodiments of the gas sensor, the metal-organic framework is a zeolitic imidazolate framework (ZIF). Examples of zeolitic imidazolate framework (ZIF) are ZIF-8, ZIF-11, ZIF-67, ZIF-90.

In embodiments of the gas sensor,

- The first layer is a reflective metal or a distributed Bragg reflector,

- The second layer has a maximum thickness of 500 nm and being transparent to ultra-violet, visible, and/or near-infrared light, and

- the fifth layer is semi-reflective and gas permeable and has a thicknesses between 10 to 200 nm.

In specific embodiments the second layer has a maximum thickness of 50 nm.

In specific embodiments the fifth layer has a thicknesses between 20 to 50 nm.

In embodiments of the gas sensor the first and the fifth layer are conductive.

In embodiments of the gas sensor the first layer is patterned into source and drain electrodes, and the second layer 2 is a semiconductor.

In embodiments of the gas sensor, the sensor, further comprises an additional (non- conductive) passivation layer, wherein said passivation layer is positioned between the second and the third layer . 11

The invention further relates to method of detecting an analyte gas comprising the steps of : contacting a gas comprising the analyte with the sensor as described above, and measuring the optical and/or electrical property changes upon contact with the analyte gas.

In embodiments of this method, the gas comprising the analyte is contacted with a sensor comprising a bottom light reflective layer, a transparent layer, a transparent porous layer, an optional transparent smoothening layer and a top semi -reflective layer and wherein the analyte gas is detected by measurement of optical resonance changes.

In embodiments of this method, the gas comprising the analyte is contacted with a sensor comprising a capacity bottom conductive layer, a semiconductive layer, a porous layer and a top conductive layer, wherein the analyte gas is detected by measurement in changes in electric capacity upon contact with a gas analyte.

The present invention relates to a vertical MOF micro-cavity/capacitor (MOF-MCC) which allow an optical and electrical read-out in the same device, i.e., enabling a dual read-out which can be optimized regarding high chemical sensitivity and selectivity of the device by the presence and properties of the various layers as illustrated in FIG. la and lb.

Reference will now be made in detail to the description of the embodiments as illustrated in the drawings FIG. la and lb, illustrating the MOC-MCC. The vertical stacking includes, next to the substrate (e.g. glass), a combination of minimum four elements. Disposed on the substrate is a bottom mirror (layer 1) composed of a metal layer (FIG. la) or a low-n/high-n DBR (FIG. lb), subsequently one (or more) transparent material layer(s) (FIG. la,b, layer 2) and a porous material layer (FIG. la,b, layer 3). A metal layer forms the top mirror (FIG. la,b, layer 5) of the MOF- MCC. Furthermore, a thin transparent smoothening layer (FIG. la,b, layer 4) can be included in-between the porous layer (layer 4) and the top metal mirror (layer 5). Several complementary combinations of the transparent layer(s) and porous material layer are envisaged . The porous material layer typically consists of a metal-organic framework, a covalent-organic framework, or a zeolite.

Gas molecules permeate through the top mirror (FIG. 1, layer 5) and, if present, the smoothening layer (FIG. 1, layer 4), get adsorbed by the porous layer, and thus, increase its refractive index (n _M 0F) and dielectric constant (EMOF) . Therefore, the 12 optical resonance, which forms between the bottom and top mirror at wavelength A _res , red-shifts by AA _res as a function of the gas concentration; which can be detected in both reflection and transmission, with light entering the device either from top-to- bottom or from bottom -to -top. The spectral position of the resonance, A _res , can be tuned by the thicknesses and materials refractive indexes of layers 2, 3, and 4. Furthermore, the electrical capacitance between the metal top mirror (FIG. 1, layer 5), also referred to as top-electrode, and any other conductive layer underneath the porous layer (FIG. 1, layer 5), referred to as bottom-electrode, can be used to detect changes in E _MOF upon gas adsorption (AC).

Apart from its dual-readout capability, the MOF-MCC offers the following advantages over the above described state-of-the art that makes it interesting for use in commercial applications.

Compared to vertically stacked MOF-based DBRs, the MOF-MCC possesses much less layers (well working DBRs need at least 3...5 bi-layer to create a photonic gap), i.e., it is more simple to fabricate. Nonetheless, higher sensitivities AA _res /Ac regarding resolving changes in the gas concentration are in principle possible with the MOF- MCC. Furthermore, spectrally sharp resonances (for micro-cavities with high Q- factors) enable strong light Intensity changes at wavelengths within their FWHMs already at small resonance shifts, i.e., for a given MOF and gas analyte, the lower limit of detection is lower than in reported DBRs.

Compared to reported MOF-based 2- or 3-dimensional PhCs or grating structures, fabrication of the MOF-MCC does not require any advanced micro-structuring techniques or complicated synthesis for self-assembling of MOF crystals/particles. Apart from the MOF layer 3, the spectral properties of the micro-cavity can be tuned by the layers 2 and 4, here referred to as optical spacer layer(s), which enables at least 1 more degree of freedom to optimize the device's optical read-out, and thus, its sensitivity. In particular, at given MOF thickness d _MOF , the MCC's resonance A _res can be freely tuned by varying the thickness of the optical spacer layers without altering the capacitive read-out capabilities of the device.

Compared to capacitive MOF-based gas sensors with IDEs, the MOF-MCC possesses significantly higher AC/Co under gas adsorption (with Co being the base line capacitance), i.e., also a higher sensitivity AC/Ac regarding resolving changes in the gas concentration c. Furthermore, and in contrast to demonstrated MOF plate capacitor sensors, the present material between the bottom-electrode and the porous layer (layer 2 in FIG. la) can be used to reduce electrical leakage issues, which might happen if the porous layer 3 is thin. If the porous layer is sufficiently insulating, the 13 optical spacer layers can be composed of a transparent conductive oxide, which removes their contribution to the sensor's capacitance but yet enables optical tuning of the micro-cavity.

In contrast to the MOF-MCC disclosed here, state-of-the-art MOF plate capacitor sensors ^12,13 do not enable an optical read-out because the employed metal electrodes are not suitable to function as high-quality mirrors, which however, is required to obtain micro-cavities. This holds in particular for the top mirror, which also has to be gas permeable. In this invention, metal thin films with thicknesses in the range of 10 to 200 nm and directly deposited on the layer stack are used, matching both requirements.

The sensitivity AA _res /Ac of the MOF-MCC in FIG. la,b can be enhanced by increasing the thickness of the porous layer 3, d _MOF , which however, might reduce the Q-factor (increase the FWHM) of the optical resonance due to roughness of the porous layer (see FIG. 3a). To overcome this issue, a thin gas-permeable smoothening layer, e.g., consisting of a transparent and porous polymer, can be applied on top of the porous material layer to level off its roughness (see FIG. 4). Alternatively, the micro-cavity can be tuned such that the optical read-out is performed on higher order resonances with relatively lower FWHMs (see FIG. 3b).

By choosing a suitable material, the present spacer layer(s) protect(s) the bottom mirror underneath from chemical degradation, i.e., also function as a diffusion barrier.

Compared to reported MOF-based 2- or 3-dimensional PhCs or grating structures, fabrication of the MOF-MCC does not require any advanced micro-structuring techniques or complicated synthesis for self-assembling of MOF crystals/particles.

EXAMPLES

Example 1. Functioning of the MOF-MCC.

Referring to FIGS. 2a and 2b, shown are examples of the MOF-MCC, which are Ag/AIOx/ZnO/Ag and Ag/ZIF-8/AIOx/ZnO/Ag stacks (left to right reads top to bottom of the stack). The tunability of the optical properties and VOC sensing capability are demonstrated in the following. FIG. 2a (top panel) shows reflection spectra of reference stacks without a porous MOF layer. 40 nm and 160 nm silver layers are used as top and bottom mirror, respectively. By increasing the thickness of the ZnO layer from 30 to 90 nm, the resonance peak at A _res is red-shifted from 390 nm to ca. 550 nm. In FIG. 2a (bottom panel), spectra of similar devices with, however, a 30 14 nm thin MOF (ZIF-8) film deposited by MOF-CVD underneath the top mirror are shown. Compared to their reference devices of FIG. 2a (top panel), the resonance peaks are each red-shifted by ca. 30 to 50 nm due to presence of the thin porous MOF layer. On the example of a Ag(40)/ZIF-8(30)/Al0x(10)/Zn0(50)/Ag(80) stack, FIG. 2b illustrates measured reflection spectra (left) and the determined adsorption properties (right) of such a micro-cavity in equilibrium with an ethanol (EtOH) vapor of varying partial pressure p/p°. The optical shift of the resonance position A _res under varying EtOH vapor partial pressure resembles the known EtOH adsorption isotherm for ZIF-8 [Tu etal. (2015) Adv. Fund. Mater. 25, 4470-4479], evidencing the proper functioning of the MOF-MCC sensor design. As illustrated in FIG. 2c, the observed strength of the sensor response (max. Dl~6.8 nm for p/p°~0.7) can further be simulated using optical Transfer-Matrix Model (TMM) calculations, assuming a change in the refractive index of the 30 nm thin ZIF-8 layer from 1.35 to 1.45 upon EtOH adsorption.

To increase the strength of the response, i.e., the sensitivity AA _res c, aiming to resolve small changes in a VOC vapor concentration, thicker MOF layers are required. However, the surface quality of thick MOF films might be not sufficient (high roughness) to yield micro-cavities with sharp resonances, as illustrated in FIG. 3a on the example of a Ag(20)/ZIF-8(170)/Ag(160) stack.

Two solutions are suggested: (1) Application of transparent spacers layers underneath the MOF, such that higher order resonances are excited as illustrated in FIG. 3b on an example stack yielding a clear sharp 2 ^nd order at 520 nm. (2) Application of a smooth, thin, transparent, and gas permeable layer on top of the MOF layer to compensate for its roughness, as illustrated in FIG. 4a and 4b. Shown are micro-cavities without and with a 45 nm thin spin-coated polymer layer (d-AZ) on top of a 90 nm thick ZIF-8 film, sandwiched between two Ag mirrors. Due to the polymer-coating, the AFM surface roughness is reduced from ca. 22 to 10 nm, yielding a significantly sharper resonance at A _res ~550 nm (FWHM~65 nm). Variation of the thickness of the smoothening layer, here obtained via varying the spin-coating speed during the polymer deposition, further enables an optical tuning of the micro cavity, which is illustrated in FIG. 4c. With increasing polymer thickness, the resonance A _res red-shifts and its FHWM decreases. 15

Example 2 MOF-ChemFET.

A chemically sensitive field effect transistor (ChemFET, FIG. lc), is obtained by (1) structuring the bottom metallic mirror of the MOF-MCC of FIG. la into two separate parts, functioning as the source and drain contacts of the ChemFET (FIG. lc, layer 1), and (2) using a semi-conductor material for its layer 2. The porous insulating material layer 4 functions as gas-sensitive gate dielectric. To reduce the risk of electrical leakage, another insulting layer 2b can be further included in-between layers 2 and 4. The previously metallic top mirror functions now as gas permeable gate electrode of the ChemFET (Fig. lc, layer 5). Similarly to the MOF-MCC, gas adsorption in the porous material layer increases its dielectric constant (EMOF), thus, capacitance C of the formed metal-insulator- semiconductor structure, which can be detected either directly or by a change in the source-drain current IDS of the ChemFET, if the transistor is switched on via application of voltage to the gate (VGS) and drain (V _D s) electrodes w.r.t. to the source electrode. Furthermore, gas adsorption can trigger a change in the work function of the metallic gate layer Wf _G due to presence of gas molecules at the interface of layers 3 and 5 as well as in the bulk of layer 3. This effect has been reported in literature by Kelvin Probe measurements on metal surfaces coated with MOFs [Pohle et at. (2011) Procedia Engineering 25, 2011; Davydovskaya et al. (2013) Sensors and Actuators B 187, 142; Stassen et al. (2016) Chem. Sci. 7, 5827], but only proposed to be used in field effect transistors with a suspended gate [DE102011075396], i.e., in structures possessing an air gap between the (suspended) gate electrode and semiconductor layer (GasFETs) [DE4239319 ; Hietner et al. (1994) Sensors and Actuators B 22,109]. Apart from those, only (1) a structure similar to a standard CMOS-FET but coated with a MOF layer instead of an actual gate electrode to slightly modulate the potential difference over the Si/Si02/MOF stack via a change in the Wf at the MOF surface [David et al. (2019) Chemistry - A European Journal 25, 13176- 13183], and (2) a design in which a MOF is part of a conductive compound gate electrode [CN110579526], have recently been published. The MOF-ChemFET illustrated in FIGs. lc,d, offers the following advantages over these technologies:

Since the porous (MOF) layer 3 is part of the insulating gate dielectric of the ChemFET, i.e., no air gap is present, its manufacturing is much more simple than of GasFETs with a suspended gate because the latter involves precisely positioning and mounting the top gate electrode which is fabricated separately [DE4239319]. 16

Compared to GasFETs which are only sensitive to changes in Wf _G , altering the transistors threshold voltage V _th , the MOF-ChemFET is sensitive to both changes in WfG (i.e. V _th ) and the dielectric constant of the porous layer (EMOF), i.e., it possesses the capability of a dual read-out. Neither of the designs of David et al. (2019) Chemistry - A European Journal 25, 13176-13183 or of CN110579526 offer this functionality/capability.