DESC:FIELD OF THE INVENTION
[0001] The present disclosure pertains to technical field of gas detection. In particular, the present disclosure relates to method of detecting gases or vapors using relative variance of resistance fluctuations of graphene field effect transistor.

BACKGROUND OF THE INVENTION
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Graphene is pure carbon in the form of thin layer of carbon atoms arranged in a honeycomb structure. Graphene is a pure two-dimensional material having all its atoms exposed to surface, which makes the conductance of graphene extremely sensitive to the environment and the adsorbed molecules on its surface can significantly modify its electrical characteristics.
[0004] Several unique properties of graphene make it exceptionally suitable for sensor applications. For example, graphene is highly conductive even in very low carrier density regimes and exhibits extremely low levels of Johnson-Noise compared to semiconductor based sensors. Graphene has few crystal defects which ensures a low level of excess (1/f) noise caused by their thermal switching. Graphene allows four-probe measurements on a single-crystal device with electrical contacts that are ohmic and have low resistance. All these factors combine to give a very large signal-to-noise ratio in graphene sensors even at room temperatures enable them to detect changes in local charge concentration by less than one electron charge. Further, the exceptional surface-to-volume ratio and the ability to strongly tune the conductivity by the gate in graphene transistors may make them promising for gas sensing applications.
[0005] Graphene based sensors and their sensitivity to gases such as NH3, NO2, CO, CO2, O2, H2, etc. have been demonstrated and disclosed in several publications. However, specific detection of individual chemical species in gaseous mixtures by these graphene sensors utilizing all the above-mentioned sensing parameters is difficult to attain. Further, these graphene sensors require very long time to reset back to its pristine condition and the resetting procedure often involves heating the sensing system at high temperature and/or exposing the system to UV radiation. Exposure to UV radiation affects the sensitivity of the sensor system and often causes permanent alteration to the device characteristics. Most of these sensor devices exhibit hysteresis which further limits the stability of the sensing material. Further, the response is often not reproducible and the sensors are difficult to regenerate.
[0006] Accordingly, there exists a need in the art for an improved graphene sensor that facilitates high specificity, fast reset time and high reproducibility. There also exists a need for a graphene sensor that is capable of detecting ultra-low concentrations of different gases, and exhibits faster response time than known graphene sensors in response to rapid changes in the concentration of the gas to be measured.
[0007] The present invention satisfies the existing needs, as well as others, and generally overcomes the deficiencies found in the prior art.

OBJECTS OF THE INVENTION
[0008] It is an object of the present disclosure to provide a graphene based sensor for detection of gas molecules.
[0009] It is a further object of the present disclosure to provide a graphene based sensor for detection and differentiation of individual gas molecules in a gas mixture.
[0010] It is another object of the present disclosure to provide a graphene based sensor having faster response time for detection of gas molecules and a shorter recovery time for enabling subsequent detection.
[0011] It is another object of the present disclosure to provide a graphene based sensor that is extremely reproducible and achieves very uniform responses.
[0012] It is another object of the present disclosure to provide a graphene based sensor that can respond to ultra-low concentrations of gases to be detected.
[0013] It is another object of the present disclosure to provide a graphene based sensor that can detect a broad range of gas molecules with high sensitivity and selectivity.
[0014] It is another object of the present disclosure to provide a method of detection of gas molecules.

SUMMARY OF THE INVENTION
[0015] The present disclosure provides a graphene field effect transistor that exhibits faster response time for detection of gas molecules and a shorter recovery time for enabling subsequent detection. In an aspect, the present disclosure provides a field effect transistor capable of detecting one or more gases or vapors, wherein the field effect transistor can include (a) a substrate; (b) a graphene layer configured on the substrate; (c) a source and drain electrode, wherein the source and drain electrodes are configured to be in contact with the graphene layer; and (d) a gate electrode disposed at a distance from the graphene layer, and further configured to be in contact with the graphene layer.
[0016] According to embodiments, the field effect transistor of the present disclosure can effect a change in low frequency noise upon exposure to a gas and/or vapor which can be used as a sensing parameter for selective detection of one or more gases or vapors.
[0017] In an embodiment, the present disclosure provides a method for detection of one or more gases or vapors using relative variance (?R2/R2) of 1/f noise of a graphene field effect transistor as a sensing parameter.
[0018] In another embodiment, the present disclosure provides a sensor module for sensing vapors and gases, wherein the sensor module can include (a) a graphene field effect transistor that exhibits a change in low frequency noise in presence of a gas or vapor; and (b) a signal processing module for measuring relative variance of 1/f noise of the graphene field effect transistor.
[0019] In another aspect, the present disclosure provides a method for detecting one or more gases or vapors, wherein the method can include the steps of: (i) exposing a field effect transistor to at least one vapor or gas, wherein the field effect transistor comprising: (a) a substrate; (b) a graphene layer configured on the substrate; (c) a source and drain electrode, wherein the source and drain electrodes are configured to be in contact with the graphene layer; and (d) a gate electrode disposed at a distance from the graphene layer, and further configured to be in contact with the graphene layer; (ii) measuring a change in 1/f noise of the graphene field effect transistor; and (iii) determining the presence of the at least one vapor or gas.
[0020] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0022] FIG.1 illustrates optical microscope image of a graphene field effect transistor with multiple contact pads in accordance with embodiments of the present disclosure.
[0023] FIG.2 depicts noise spectrum of a “graphene field effect transistor” (device under test “DUT”) kept in vacuum, in a bandwidth of 1Hz to 1 KHz in accordance with embodiments of the present disclosure. Inset is small time segment of voltage fluctuation of the DUT after digital signal processing (DSP).
[0024] FIG.3 shows resistance fluctuation time-series of a pristine graphene device when exposed to methanol vapor in accordance with embodiments of the present disclosure.
[0025] FIG. 4 shows the % change in resistance (R) of the DUT plotted as a function of time in accordance with embodiments of the present disclosure.
[0026] FIG. 5 shows the % change in relative variance (?R2/R2) of resistance fluctuations of the DUT plotted as a function of time in accordance with embodiments of the present disclosure.
[0027] FIG. 6 illustrates comparison of behavior of resistance (R) and relative variance (?R2/R2) of the DUT in accordance with embodiments of the present disclosure.
[0028] FIG. 7 shows real time detection of different amount of methanol vapor using relative variance of resistance fluctuations of DUT in accordance with embodiments of the present disclosure.
[0029] FIG. 8 shows the average saturated value of relative variance (?R2/R2) of resistance fluctuations of the DUT at pristine state versus volume of methanol gas present in the measurement chamber in accordance with embodiments of the present disclosure.
[0030] FIG. 9 shows real time gas sensing using normalized resistance fluctuations with same amount of methanol (100 ppm), which shows reproducibility of the sensor in accordance with embodiments of the present disclosure.
[0031] FIG. 10 shows the noise spectrum of the DUT multiplied with frequency f, gamma (?) in four different cases, i.e., the DUT at pristine condition, DUT exposed to methanol, chloroform, and mixture of methanol and chloroform in accordance with embodiments of the present disclosure. For pristine device, the gamma is frequency independent, whereas, for all three other cases, the shows some characteristic signature.
[0032] FIG. 11 shows the parameter gamma (?) of the DUT when exposed to chloroform and methanol simultaneously in accordance with embodiments of the present disclosure. The spectrum shows information regarding the presence of both chloroform and methanol.
[0033] FIG.12 shows comparison of noise of the DUT while it was exposed to different gases in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION
[0034] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0035] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0036] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[0037] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[0038] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
[0039] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.
[0040] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0041] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
[0042] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.
[0043] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0044] Embodiments of the present disclosure pertain to sensors that use graphene as a sensor material. However, it is to be appreciated that considerations taken herein below may equally apply to other sensors or sensor materials, and may for example generally apply to sensor materials that can produce low frequency noise (1/f noise) upon adsorption of gas molecule(s) at the surface of the sensor material.
[0045] In an aspect, the present disclosure provides a field effect transistor capable of detecting one or more gases or vapors, wherein the field effect transistor can include (a) a substrate; (b) a graphene layer configured on the substrate; (c) a source and drain electrode, wherein the source and drain electrodes are configured to be in contact with the graphene layer; and (d) a gate electrode disposed at a distance from the graphene layer, and further configured to be in contact with the graphene layer.
[0046] As used herein, the term “Graphene” refers to material that is more than 95% carbon by weight and includes at least one, one-atom-thick planar layer including SP2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The graphene material may contain one layer of carbon atoms or a plurality of layers of carbon atoms. In an exemplary embodiment, the graphene material can include a “single layer” graphene that comprises one-carbon atom thick layer of graphene.
[0047] As used herein, the term “Source” is the contact region where majority of carriers flow into a field effect transistor (FET).
[0048] As used herein, the term “Drain” is the contact region where majority of carriers flow out of a field effect transistor (FET).
[0049] As used herein, the term “Gate” refers to contact, capacitively coupled to the field effect transistor (FET) that is biased to control the conductivity of the channel between the Source and Drain.
[0050] According to embodiments, the field effect transistor of the present disclosure can effect a change in low frequency noise upon exposure to a gas and/or vapor which can be used as a sensing parameter for selective detection of one or more gases or vapors.
[0051] According to embodiments, the substrate of the field effect transistor can be electrically insulating and it can preferably be silica or hexagonal boron nitride. The substrate can enable the graphene layer to be provided on a well defined surface, and also it can allow the graphene layer to stay 2-dimensional, continuous and to remain in an intended position. The graphene layer can be disposed on the substrate by chemical vapor deposition technique.
[0052] In an embodiment, the substrate can contain a dielectric substrate on its surface. In an exemplary embodiment, the dielectric substrate can be a 300-nm thermally grown silicon dioxide.
[0053] In an embodiment, the graphene layer can be a continuous mono crystalline graphene layer which can have uniform thickness. Further, the graphene layer may be epitaxial in nature and it can be grown epitaxially on the substrate. The graphene layer may preferably be surface treated in order to enhance the chemical selectivity so that only selected types of gas molecules are adsorbed and detected by the graphene layer. The surface treatment can also prevent certain types of gas molecules from adsorbing onto the surface of the graphene layer. The surface treatment may comprise deposition of metal particles or polymers on the surface of the graphene layer.
[0054] The graphene layer can be electrically connected to the source electrode and the drain electrode. The gate electrode can be disposed on an electrically insulating surface of a gate substrate and it can be positioned at a distance from the graphene layer so that the gate electrode and the graphene layer are separated by a gap through which a gas molecule to be detected can reach the graphene layer.
[0055] In an embodiment, the circuitry of the field effect transistor can include a current source arranged to flow a constant current between the source electrode and the drain electrode and a voltage source arranged to supply a constant voltage to the gate electrode. In another embodiment, the circuitry of the field effect transistor may include a voltage source arranged to apply a constant voltage between the source electrode and the drain electrode.
[0056] According to embodiments, the field effect transistor of the present disclosure can effect a change in low frequency noise upon exposure to a gas and/or vapor which can be used as a sensing parameter for selective detection of one or more gases or vapors.
[0057] Gases or gas molecule(s) of different chemicals can produce distinguishably different effects on the low frequency noise spectra of graphene. Some gases can change the electrical resistance of graphene field effect transistor without changing their low frequency noise spectra while other gases modify the noise spectra with different frequencies. According to embodiments of the present disclosure, gas molecules can be sensed using 1/f resistance fluctuations of a graphene field effect transistor. The relative variance (?R2/R2) of low frequency noise of a graphene field effect transistor can be used as a sensing parameter for selective sensing of various gas molecules. The relative variance (?R2/R2) of low frequency noise of a graphene field effect transistor device can produce reproducible change upon exposure to gas molecules.
[0058] FIG.1 shows optical microscope image of a graphene field effect transistor (FET) with multiple contact pads in accordance with embodiments of the present disclosure. As shown in FIG.1, the graphene field effect transistor can include a substrate, a dielectric substrate disposed on top surface of the substrate, a layer of graphene disposed on top surface of the dielectric substrate, and a source and drain contact on top surface of the layer of graphene.
[0059] In an exemplary embodiment, the graphene field effect transistor device can be constructed on a silicon substrate containing thermally grown silicon dioxide layer on its top surface. Single layer graphene can be mechanically exfoliated on the top surface of the silicon dioxide layer that is previously deposited on the top surface of the silicon substrate. The single layer graphene can be identified and located on the wafer with respect to pre-patterned alignment marks. The thickness of silicon dioxide layer can be chosen for visibility of single layer graphene under optical microscope. Metallic contacts to samples can be provided by electron beam lithography or optical lithography technique.
[0060] In another embodiment, the present disclosure provides a sensor module for sensing vapors and gases, wherein the sensor module can include (a) a graphene field effect transistor that exhibits a change in low frequency noise in presence of a gas or vapor; and (b) a signal processing module for measuring relative variance of 1/f noise of the graphene field effect transistor.
[0061] In accordance with embodiments of the present disclosure, the 1/f noise of the field effect transistor can be measured by employing a signal processing unit which can incorporate a lock in amplifier (LIA), a data acquisition card (DAQ) and a digital anti-aliasing filter. The graphene field effect transistor (FET) can be current biased using the lock in amplifier (LIA) and the voltage fluctuation about the average value can read from the output of the same LIA as a digital data using the data acquisition card (DAQ) at a high sampling rate. The digital data can be passed through the digital anti-aliasing filters and can be decimated to get rid of components of power line frequency and its harmonics and aliasing resulting from discrete fourier transform. The power spectrum of this fluctuation can be calculated, and from the excess noise over the thermal fluctuation background, variance of resistance fluctuation of the graphene FET can be calculated. The variance can be normalized to square of average resistance to afford relative variance (?R2/R2) of low frequency noise which can be a quantity independent of average resistance.
[0062] FIG. 2 depicts power spectral density of resistance fluctuations of a graphene field effect transistor (FET). The Johnson noise arising due to thermal fluctuation is independent of frequency and the excess noise of the FET is 1/f in nature over the measurement bandwidth.
[0063] In another aspect, the present disclosure provides a method for detecting one or more gases or vapors, wherein the method can include the steps of: (i) exposing a field effect transistor to at least one vapor or gas, wherein the field effect transistor comprising: (a) a substrate; (b) a graphene layer configured on the substrate; (c) a source and drain electrode, wherein the source and drain electrodes are configured to be in contact with the graphene layer; and (d) a gate electrode disposed at a distance from the graphene layer, and further configured to be in contact with the graphene layer; (ii) measuring a change in 1/f noise of the graphene field effect transistor; and (iii) determining the presence of the at least one vapor or gas.
[0064] The sensor of the present disclosure provide good repeatability and has a very short recovery time for reaching its pristine condition. The relative variance (?R2/R2) of resistance fluctuation of the graphene sensor can reset to initial value within few minutes upon evacuation of the test gas. The gas sensor of the present disclosure solves the problems of conventional graphene based gas sensors that require desorption of the gas molecules adsorbed on the graphene layer by heat treatment and UV radiation treatment. Further, the present sensor can overcome the hysteresis phenomenon, a slow response speed and unstable characteristics under high temperature and high humidity conditions.

EXAMPLES
[0065] The present invention is further explained in the form of following examples. However it is to be understood that the foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.

Example 1: Detection of methanol
[0066] The graphene FET device (hereinafter “Device Under Test” or DUT) was loaded inside a sensing chamber and the sensing chamber was pumped. The noise of the DUT was measured continuously and it was plotted as a function of time. The sampling rate, and the lock-in time constant was chosen properly to have the noise spectrum over a bandwidth of 1Hz to 1 KHz. After few minutes, precise amount of methanol was applied to the DUT in vapor form inside the chamber. ?R2/R2 was observed to increase sharply and saturated at some value. After a while, methanol vapor was pumped out of the chamber and the ?R2/R2 was observed to come down to the initial base value. After few more minutes the measurement was stopped and the same was repeated with different amount of methanol. Throughout the entire experiment, the resistance of the DUT was also observed to compare the results. While the measurement, the sampling rate was kept 16384, and the raw data was decimated by a factor of 8. Thus, following Nyquist theorem, the upper bound of the measurement bandwidth comes 1024. The lock-in time constant was chosen suitably to have cutoff frequency above this value. The spectral length was 1024, thus the measurement bandwidth spans 1Hz to 1 KHz. For each data point, the sampling time of 3~5 second was found to be appropriate.
[0067] FIG.2 depicts noise spectrum of a “graphene field effect transistor” (device under test “DUT”) kept in vacuum, in a bandwidth of 1Hz to 1 KHz. The Jonson noise arising due to thermal fluctuation (Brown filled circles) is independent of frequency. The excess noise of the device (shown in olive filled circles) is 1/f in nature over this measurement bandwidth. Inset is small time segment of voltage fluctuation of DUT after digital signal processing (DSP). The voltage fluctuation was recorded at rate of 16384 per second.
[0068] FIG.3 shows resistance fluctuation time-series of a graphene field effect transistor device, wherein the blue band depicts resistance fluctuation about the mean value of a pristine graphene device and the red band depicts the resistance fluctuation time series of the same device exposed with methanol vapor. As shown in FIG. 3, the data is vertically shifted for clarity, and a large increase of resistance fluctuation was clearly observed after the device was exposed to methanol vapor. While the vapor was pumped out, the resistance fluctuation was reset back to the initial level.
[0069] The sensing response of the DUT was determined by keeping the DUT for 10 min in vacuum. The change was calculated with respect to the value averaged over this region. Then some specific amount of methanol loaded inside the measurement chamber and R was observed to increase, and gets saturated. Then methanol was pumped out of the measurement chamber. As shown in Fig. 4, the resistance reset to initial range in more than an hour time.
[0070] The sensing response of the DUT was further determined by determining the % change in relative variance (?R2/R2) of resistance fluctuations of the DUT. For first 10 min, the DUT was kept in vacuum. The change was calculated with respect to the value averaged in this region. Then some specific amount of methanol was loaded inside the measurement chamber and ?R2/R2 was observed to increase, and get saturated. Then methanol was pumped out of the measurement chamber. As shown in Fig. 5, the relative variance (?R2/R2) reset to initial value within a minute.
[0071] FIG. 6 illustrates comparison of behavior of resistance (R) and relative variance (?R2/R2) of the DUT. A maximum of 30 % change in R was observed when the DUT was exposed to methanol, whereas the change in ?R2/R2 was ~ 700 %. As shown in FIG. 6, the resistance (R) takes very long time to reset, while ?R2/R2 resets to initial value within a minute. Observed change in resistance is ~ 30%, whereas, the change in ?R2/R2 is ~ 700%, which in turn makes possible faster response time for detection of gas molecules and a shorter recovery time for enabling subsequent detection.
[0072] FIG. 7 depicts the change in ?R2/R2 of the DUT with time, while the sensing experiment was repeated multiple number of times with different amount of methanol. The maximum value of ?R2/R2 obtained for each run was found to scale with the amount of methanol loaded in the measurement chamber. Till some concentration, reset of the DUT was achieved only with pumping, whereas, after some volume (500 ppm), small heating was needed to reset the DUT. The values of ?R2/R2 are scaled with the average value obtained while the DUT was in its pristine state.
[0073] Further, the reproducibility was checked with the DUT since this is an important parameter for any sensor. ?R2/R2 was found to reach the same value upon exposure to same amount of methanol. FIG. 8 shows the plot of ?R2/R2 as a function of time as the DUT was exposed to same amount of methanol vapor consecutively for 3 times, which depicts the reproducibility of the sensing using ?R2/R2 of the DUT.
[0074] FIG. 9 shows real time gas sensing using normalized resistance fluctuations with same amount of methanol (100 ppm), which asserts reproducibility of the sensor.
[0075] The 1/f noise power spectrum Sv(f)multiplied by frequency f defines ?. The ? of the DUT was found to have hump at some frequency, after exposure to methanol, indicating presence of some particular frequency component in the power spectrum of the DUT in presence of methanol in its environment. Furthermore, the experiment was repeated with chloroform and similar hump in ? was observed, but at some different characteristic frequency.

Example 2: Detection of different gas molecules
[0076] Example 1 was repeated with different test chemical vapors such as methanol, toluene, chloroform and ammonia. The DUT was exposed to vapors of these chemicals and the maximum value of ?R2/R2 was recorded and shown in FIG. 12. It was observed that these vapors exhibited different ?R2/R2 value. For comparison, the noise was first normalized to number of molecules in each case, and it was further normalized by the value obtained for the case of methanol. Detection of ammonia using these schemes was found to be much more sensitive than others.

Example 3: Detection of individual gas molecules in a mixture
[0077] The DUT was exposed to methanol and chloroform separately and the noise spectrum was recorded. Then, same amount of methanol and chloroform was added into the measurement chamber and the spectrum was recorded. Spectrum of all these three cases along with spectrum obtained initially while the DUT was in its pristine state were plotted and shown in FIG. 10. FIG. 10 illustrates the noise spectrum of the DUT multiplied with frequency f, gamma (?) in four different cases, i.e., the DUT at pristine condition, DUT exposed to methanol, chloroform, and mixture of methanol and chloroform. For pristine device, the gamma is frequency independent, whereas, for all three other cases, the ? shows some characteristic signature.
[0078] As shown in FIG. 11, two distinct humps were observed in ? when the DUT was exposed to the mixture, indicating presence of two distinct characteristic frequencies. This allowed to detect different types of molecules from a mixture, given the characteristic frequency is within the measurement bandwidth. The hump at frequencies corresponding to chloroform and methanol are shown by arrows. The plot shows the presence of methanol and chloroform is easily resolvable.

ADVANTAGES OF THE PRESENT INVENTION
[0079] The present disclosure provides a graphene based sensor that is capable of detecting gas molecules within a short period of time.
[0080] The present disclosure provides a graphene based sensor that can be reset for renewed sensing within a short time period.
[0081] The present disclosure provides a graphene based sensor that is capable of detecting various gases in ultra low concentrations.
[0082] The present disclosure provides a graphene based sensor that exhibits excellent specificity to the gas to be detected.
[0083] The present disclosure provides a graphene based sensor that enables accurate differentiation of individual gases in a gas mixture.
[0084] The present disclosure provides a graphene based sensor that overcomes hysteresis phenomenon.
[0085] The present disclosure provides a highly accurate and inexpensive method for detection of gas molecules.
[0086] The present disclosure provides a method of sensing gas molecules that overcomes the drawbacks of the prior art.
,CLAIMS:A field effect transistor for sensing vapors and/or gases comprising:
a substrate;
a graphene layer configured on the substrate;
a source and drain electrode, wherein the source and drain electrodes are configured to be in contact with the graphene layer; and
a gate electrode disposed at a distance from the graphene layer, and further configured to be capacitively coupled to the graphene layer;
wherein, the field effect transistor effects a change in low frequency noise upon exposure to a gas and/or vapor.

2. The field effect transistor of claim 1, wherein the graphene layer is a continuous mono crystalline graphene layer.

3. The field effect transistor of claim 1, wherein the graphene layer has uniform thickness.

4. The field effect transistor of claim 1, wherein the graphene layer is an epitaxial layer.

5. The field effect transistor of claim 1, wherein the substrate is selected from silica, hexagonal boron nitride or a similar suitable dielectric material.

6. The field effect transistor of claim 1, wherein the surface of the substrate is configured with a dielectric substrate.

7. The field effect transistor of claim 6, wherein the dielectric substrate is thermally grown silicon dioxide.

8. The field effect transistor of claim 1, wherein the graphene layer is surface treated graphene layer.

9. The field effect transistor of claim 8, wherein the graphene layer is surface treated by deposition of metal or polymer particles thereon.

10. The field effect transistor of claim 1, wherein the gate electrode is disposed on an electrically insulating surface of a gate substrate.

11. A sensor module for sensing vapors and gases comprising:
(a) a graphene field effect transistor that exhibits a change in low frequency noise in presence of a gas or vapor, wherein the field effect transistor comprises a substrate; a graphene layer configured on the substrate; a source and drain electrode, wherein the source and drain electrodes are configured to be in contact with the graphene layer; and a gate electrode disposed at a distance from the graphene layer, and further configured to be capacitively coupled to the graphene layer; and

(b) a signal processing module for measuring relative variance of 1/f noise of the graphene field effect transistor.

12. A method for detecting one or more gases or vapors, comprising the steps of:
exposing a field effect transistor to at least one vapor or gas, wherein the field effect transistor comprising: (a) a substrate; (b) a graphene layer configured on the substrate; (c) a source and drain electrode, wherein the source and drain electrodes are configured to be in contact with the graphene layer; and (d) a gate electrode disposed at a distance from the graphene layer, and further configured to be in contact with the graphene layer;
measuring a change in 1/f noise of the field effect transistor; and
determining the presence of the at least one vapor or gas.
13. The method of claim 12, wherein the 1/f noise is measured from voltage noise while biasing the field effect transistor with constant current.

14. The method of claim 13, wherein the voltage noise is measured by a four probe method.

15. The method of claim 12, wherein the 1/f noise is measured from current noise of a two terminal field effect transistor biased with a constant voltage.