Abstract:
The inventors experimentally demonstrated NO 2  gas sensing performance of multilayer black phosphorous (BP) field effect transistors. The BP sensors were sensitive to NO 2  concentration down to 5 ppb making them comparable in sensitivity to the best 2D material based sensors. Raman spectroscopy comparison revealed no apparent change in the spectra before and after exposure to NO 2 , which shows that thick BP flakes can maintain their relative stability after sensing. Moreover, the BP device sensing performance fitted well with the Langmuir Isotherm for molecules adsorbed on a surface, which confirms charge transfer as the dominant mechanism for sensing. The systematic increase in conductance with increasing NO 2  concentrations suggests NO 2  molecules withdraw electrons and dope BP flakes with holes. These results lay the ground work for BP to be applied to various sensing applications including chemical, gas, and bio-sensors.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Patent Application No. 62/332,358; filed on May 5, 2016; entitled “BLACK PHOSPHORUS GAS SENSORS.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with government support under Grant Number N00014-11-1-0880 awarded by the Office of Naval Research (ONR). The government has certain rights in this invention. 
     
    
     BACKGROUND 
       [0003]    The utilization of black phosphorus and its monolayer (phosphorene) and few-layers in field-effect transistors has attracted a lot of attention to this elemental two-dimensional material. Various studies on optimization of black phosphorus field-effect transistors, PN junctions, photodetectors, and other applications have been demonstrated. There is no experimental verification of chemical sensing based on black phosphorus devices. 
         [0004]    One or more of the following references may be referred to in this disclosure. The references are incorporated herein by reference.
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Chem., Int. Ed ., DOI: 10.1002/anie.201409400.   Kamalakar, M. V.; Madhushankar, B. N.; Dankert, A.; Dash, S. P., Low Schottky Barrier Black Phosphorus Field-Effect Devices with Ferromagnetic Tunnel Contacts. Small, DOI:10.1002/smll.201402900.   Li, L.; Ye, G. J.; Tran, V.; Fei, R.; Chen, G.; Wang, H.; Wang, J.; Watanabe, K.; Taniguchi, T.; Yang, L. et al. Quantum Oscillations in Black Phosphorus Two-dimensional Electron Gas.  arXiv  2014 , arXiv:  1411.6572.   Buscema, M.; Groenendijk, D. J.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A., Photovoltaic effect in few-layer black phosphorus PN junctions defined by local electrostatic gating.  Nat. Commun.  2014, 5.   Du, Y.; Liu, H.; Deng, Y.; Ye, P. D., Device Perspective for Black Phosphorus Field-Effect Transistors: Contact Resistance, Ambipolar Behavior, and Scaling.  ACS Nano  2014, 8 (10), 10035-10042.   Engel, M.; Steiner, M.; Avouris, P., Black Phosphorus Photodetector for Multispectral, High-Resolution Imaging.  Nano Lett.  2014, 14, 6414-6417.   Buscema, M.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A., Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors.  Nano Lett.  2014, 14, 3347-3352.   Youngblood, N.; Chen, C.; Koester, S. J.; Li, M., Waveguide-integrated black phosphorus photodetector with high responsivity and low dark current.  Nat. Photonics  2015, doi: 10.1038/nphoton.2015.23.   Yuan, H.; Liu, X.; Afshinmanesh, F.; Li, W.; Xu, G.; Sun, J.; Lian, B.; Ye, G.; Hikita, Y.; Shen, Z. et al. Broadband Linear-Dichroic Photodetector in a Black Phosphorus Vertical p-n Junction.  arXiv  2014 , arXiv:  1409.4729.   Wood, J. D.; Wells, S. A.; Jariwala, D.; Chen, K.-S.; Cho, E.; Sangwan, V. K.; Liu, X.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C., Effective Passivation of Exfoliated Black Phosphorus Transistors against Ambient Degradation.  Nano Lett.  2014, 14, 6964-6970.   Ziletti, A.; Carvalho, A.; Campbell, D. K.; Coker, D. F.; Castro Neto, A. H., Oxygen Defects in Phosphorene.  Phys. Rev. Lett.  2015, 114, 046801.   Kou, L.; Frauenheim, T.; Chen, C., Phosphorene as a Superior Gas Sensor: Selective Adsorption and Distinct I-V Response.  J. Phys. Chem. Lett.  2014, 5, 2675-2681.   Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H., Nanotube Molecular Wires as Chemical Sensors.  Science  2000, 287, 622-625.   Qi, P.; Vermesh, O.; Grecu, M.; Javey, A.; Wang, Q.; Dai, H.; Peng, S.; Cho, K. J., Toward Large Arrays of Multiplex Functionalized Carbon Nanotube Sensors for Highly Sensitive and Selective Molecular Detection.  Nano Lett.  2003, 3, 347-351.   Zhang, D.; Liu, Z.; Li, C.; Tang, T.; Liu, X.; Han, S.; Lei, B.; Zhou, C., Detection of NO2 down to ppb Levels Using Individual and Multiple In2O3 Nanowire Devices.  Nano Lett.  2004, 4, 1919-1924.   Zou, X.; Wang, J.; Liu, X.; Wang, C.; Jiang, Y.; Wang, Y.; Xiao, X.; Ho, J. C.; Li, J.; Jiang, C. et al. Rational Design of Sub-Parts per Million Specific Gas Sensors Array Based on Metal Nanoparticles Decorated Nanowire Enhancement-Mode Transistors.  Nano Lett.  2013, 13, 3287-3292.   Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S., Detection of individual gas molecules adsorbed on graphene.  Nat. Mater.  2007, 6, 652-655.   Abbas, A. N.; Liu, G.; Liu, B.; Zhang, L.; Liu, H.; Ohlberg, D.; Wu, W.; Zhou, C., Patterning, Characterization, and Chemical Sensing Applications of Graphene Nanoribbon Arrays Down to 5 nm Using Helium Ion Beam Lithography.  ACS Nano  2014, 8, 1538-1546.   Li, H.; Yin, Z.; He, Q.; Li, H.; Huang, X.; Lu, G.; Fam, D. W. H.; Tok, A. I. Y.; Zhang,   Q.; Zhang, H., Fabrication of Single- and Multilayer MoS2 Film-Based Field-Effect Transistors for Sensing NO at Room Temperature.  Small  2012, 8, 63-67.   He, Q.; Zeng, Z.; Yin, Z.; Li, H.; Wu, S.; Huang, X.; Zhang, H., Fabrication of Flexible MoS2 Thin-Film Transistor Arrays for Practical Gas-Sensing Applications.  Small  2012, 8, 2994-2999.   Late, D. J.; Huang, Y.-K.; Liu, B.; Acharya, J.; Shirodkar, S. N.; Luo, J.; Yan, A.; Charles, D.; Waghmare, U. V.; Dravid, V. P. et al. Sensing Behavior of Atomically Thin-Layered MoS2 Transistors.  ACS Nano  2013, 7, 4879-4891.   Perkins, F. K.; Friedman, A. L.; Cobas, E.; Campbell, P. M.; Jernigan, G. G.; Jonker, B. T., Chemical Vapor Sensing with Monolayer MoS2 . Nano Lett.  2013, 13, 668-673.   Liu, B.; Chen, L.; Liu, G.; Abbas, A. N.; Fathi, M.; Zhou, C., High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Monolayer MoS2 Transistors.  ACS Nano  2014, 8, 5304-5314.   U.S. Department of Environmental Protection Agency. http://www.epa.gov/Air/Nitrogenoxides/.   Xi, L.; Rahbarihagh, Y.; Hwang, J. C. M.; Han, L.; Yuchen, D.; Ye, P. D., Temporal and Thermal Stability of Al2O3-Passivated Phosphorene MOSFETs.  IEEE Electron Device Lett.  2014, 35, 1314-1316.   Doganov, R. A.; O&#39;Farrell, E. C. T.; Koenig, S. P.; Yeo, Y.; Ziletti, A.; Carvalho, A.; Campbell, D. K.; Coker, D. F.; Watanabe, K.; Taniguchi, T. et al. Accessing the transport properties of pristine few-layer black phosphorus by van der Waals passivation in inert atmosphere.  arXiv  2014 , arXiv:  1412.1274.   Zhang, Y.; Kolmakov, A.; Chretien, S.; Metiu, H.; Moskovits, M., Control of Catalytic Reactions at the Surface of a Metal Oxide Nanowire by Manipulating Electron Density Inside It.  Nano Lett.  2004, 4, 403-407.   Low, T.; Rodin, A. S.; Carvalho, A.; Jiang, Y.; Wang, H.; Xia, F.; Castro Neto, A. H., Tunable optical properties of multilayer black phosphorus thin films.  Phys. Rev. B  2014, 90, 075434.   Takahashi, T.; Gunasekara, N.; Ohsawa, H.; Ishii, H.; Kinoshita, T.; Suzuki, S.; Sagawa; T; Kato, H.; Miyahara, T.; Shirotani, I., Angle-resolved photoemission study of black phosphorus: Interlayer energy dispersion.  Phys. Rev. B  1986, 33, 4324-4326.   Han, C. Q.; Yao, M. Y.; Bai, X. X.; Miao, L.; Zhu, F.; Guan, D. D.; Wang, S.; Gao, C. L.; Liu, C.; Qian, D. et al. Electronic structure of black phosphorus studied by angle-resolved photoemission spectroscopy.  Phys. Rev. B  2014, 90, 085101.   Yu, W. J.; Li, Z.; Zhou, H.; Chen, Y.; Wang, Y.; Huang, Y.; Duan, X., Vertically stacked multiheterostructures of layered materials for logic transistors and complementary inverters. Nat. Mater. 2012, 12, 246-252.   Kopf, M.; Eckstein, N.; Pfister, D.; Grotz, C.; Kruger, I.; Greiwe, M.; Hansen, T.; Kohlmann, H.; Nilges T., Access and in situ Growth of Phosphorene-Precursor Black Phosphorus,  J. Cryst. Growth  2014, 405, 6-8.   
 
       SUMMARY 
       [0043]    Disclosed herein is chemical sensing of nitrogen dioxide (NO 2 ) using field-effect transistors based on multilayer black phosphorus. Black phosphorus sensors exhibited increased conduction upon NO 2  exposure and excellent sensitivity for detection of NO 2  down to 5 parts per billion (ppb). Moreover, when the multilayer black phosphorus field-effect transistor was exposed to NO 2  concentrations of 5, 10, 20, and 40 ppb, the transistor&#39;s relative conduction change followed the Langmuir isotherm for molecules adsorbed on a surface. Additionally, based on an exponential conductance change, the rate constants for adsorption and desorption of NO 2  on black phosphorus were extracted for different NO 2  concentrations, and they were in the range of 130-840 sec. These results shed light on electronic and sensing characteristics of black phosphorus. 
         [0044]    In one embodiment, the invention provides a black phosphorus gas sensor having repeatable sensing performance. The sensor includes a substrate, a black phosphorus flake supported by the substrate, and a plurality of electrical contacts supported by the substrate and coupled to the black phosphorus flake. The black phosphorus flake can have a thickness greater than 15 nanometers. 
         [0045]    In another embodiment, the invention provides a method of sensing a with the black phosphorus gas sensor. The method includes providing the black phosphorus gas sensor at a location, exposing the black phosphorus gas sensor to a gas, and conducting a current from a first electrode to a second electrode of the plurality of electrodes and through the black phosphorus flake. The conduction is based on the concentration of the gas. 
         [0046]    Also disclosed is a method of manufacturing the black phosphorus gas sensor. 
         [0047]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0048]    This patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
           [0049]      FIG. 1  depicts a scheme of a multilayer BP FET. 
           [0050]      FIG. 2  depicts an optical image of the multilayer BP flake between two Ti/Au electrodes. 
           [0051]      FIG. 3  depicts the I d -V d  curves of the device at different back gate voltages and an inset of an I d -V g  curve. 
           [0052]      FIG. 4  shows an atomic force microscope (AFM) height profile of the BP flake revealing a height of ˜55 nm. 
           [0053]      FIG. 5  depicts a bias stress test of a BP multilayer flake in air. 
           [0054]      FIG. 6  depicts a Raman spectroscopy taken on a multilayer BP flake before (red) and after (black) exposure to 800 ppb NO 2  for ˜30 minutes. 
           [0055]      FIG. 7  depicts the relative conductance change (ΔG/G 0 ) vs time in seconds for a multilayer BP sensor showing a sensitivity to NO 2  concentrations (5-40 ppb). The inset of  FIG. 7  depicts a zoomed in image of a 5 ppb NO 2  exposure response with identification of points in time where the NO 2  gas is switched on and off. 
           [0056]      FIG. 8  depicts the measured ΔG/G 0  plotted vs. NO 2  gas concentration. 
           [0057]      FIG. 9  depicts the measured time-dependent conductance change of a multilayer BP FET (black) flushed with argon after being exposed to 20 ppb NO 2 . 
           [0058]      FIG. 10  depicts the rate constants (τ) vs. different concentrations of NO 2 . 
           [0059]      FIG. 11  depicts the I d -V g  curves of multilayer BP FET under different concentrations of NO 2 . 
           [0060]      FIG. 12  depicts the measured I ON  vs. NO 2  concentration (red) and the corresponding fitted Langmuir isotherm (black). 
           [0061]      FIG. 13  depicts the I d -V d  curves of multilayer BP FET at different concentrations of NO 2 . 
           [0062]      FIG. 14  depicts the measured G vs. NO 2  concentration (red) and the corresponding fitted Langmuir isotherm (black). 
           [0063]      FIG. 15  depicts the I d -V g  curve of a ˜10 nm flake BP FET under a 50 mV V d . 
           [0064]      FIG. 16  depicts the optical image of the device in  FIG. 15  after repeated measurements in air. 
           [0065]      FIG. 17  depicts an optical microscope image of a vertical FET comprised of a bottom monolayer CVD graphene electrode/BP/top (Ti/Au) electrode with a P ++  Si/300 nm SiO 2  back gated structure. 
           [0066]      FIG. 18  depicts the I d -V G  curve for the device in  FIG. 17 . 
           [0067]      FIG. 19  depicts an optical microscope image of a vertical FET comprised of a bottom monolayer CVD graphene electrode/BP/top (Ti/Au) electrode with a P ++  Si/300 nm SiO 2  back gated structure. 
           [0068]      FIG. 20  depicts the I d -V G  curve for the device in  FIG. 19 . 
           [0069]      FIG. 21  depicts the relative conductance change (ΔG/G 0 ) vs. time in seconds for a multilayer BP sensor for a first time sensing. 
           [0070]      FIG. 22  depicts the relative conductance change (ΔG/G 0 ) vs. time in seconds for a multilayer BP sensor in response to various concentrations of NO 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0071]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
         [0072]    Recently, the rediscovery of black phosphorus (BP) as a single-element, two-dimensional (2D) layered material has sparked the interest of scientists in various fields. Electronic and optical properties showed great promise for using BP in numerous applications. The field-effect transistor (FET) of few-layer BP exhibited high charge mobility, anisotropic transport behavior, high operating frequencies, and relatively high current on/off ratios, making BP a potential candidate for future electronics. The recently reported device optimization techniques of BP FETs have yielded transistors with even better performance (e.g. higher mobility and lower contact resistance). In Addition, optical applications including photovoltaics (PV), photodetectors, and imaging devices were created using BP FETs with different device structures. Moreover, passivation and stability of black phosphorus has also been studied. On the other hand, other applications such as chemical sensing of BP remain only theoretically explored. 
         [0073]    Chemical sensing using various nanomaterials is a promising application, due to the inherent large surface-to-volume ratios. A variety of nanomaterials including carbon nanotubes, nanowires, and graphene were extensively studied for chemical and gas sensing applications. In the 2D family, both exfoliated and chemical-vapor-deposited (CVD) MoS 2  with various thicknesses were used for chemical sensing. The sensitivity of these sensors varied significantly depending on flake thickness, metal contacts, method of synthesis, and other factors. For example, chemical sensitivity of MoS 2  FETs to nitrogen dioxide (NO 2 ) varied from a few hundred parts per million (ppm) in exfoliated samples to a few parts per billion (ppb) in monolayer CVD samples. 
         [0074]    NO 2  is a common gas produced as a byproduct in industrial plants and vehicles. According to the U.S. Department of Environmental Protection Agency (EPA), exposure to NO 2  concentrations larger than 53 ppb can cause possible health problems. Consequently, detection of this gas with sensitivities better than the aforementioned limit has value. BP, being a 2D material, is predicted to be sensitive to various chemicals with comparable or better sensitivities than MoS 2  and graphene because the adsorption energies, of molecules such as NO 2  and NO are larger with BP than with graphene and MoS 2 . It is believed that there is yet no systematic experimental verification of gas and/or chemical sensing of BP FETs. 
         [0075]    The chemical sensing performance of multilayer BP FET to NO 2  gas is disclosed herein. Specifically, the stability of BP sensors by Raman spectroscopy of flakes before and after sensing is studied, which revealed no difference in the spectra, indicating the multilayer BP was stable for the timeframe, and repeated sensing. In one implementation, the BP FET was exposed to varying concentrations of NO 2  and the relative conductance change in the device was monitored. The BP FET showed a systematic increase in conductance with varying concentrations, indicative of hole doping charge transfer caused by NO 2  molecules. The multilayer BP sensor exhibited a clear conductance change to NO 2  concentrations as low as 5 ppb comparing favorably with the performance of almost all other 2D sensors including monolayer MoS 2 . Moreover, the device showed a good recovery to the original conductance after flushing the device with argon, suggesting a reversible adsorption and desorption of NO 2 . The relative conductance change fitted fairly well with Langmuir Isotherm for molecular adsorption on a surface. This implies that NO 2  molecular adsorption via site binding and charge transfer are the sensing mechanisms for the some BP devices. Additionally, the adsorption and desorption rates of NO 2  molecules on the BP surface was studied and the rate constants for various NO 2  concentrations were derived. The drain current vs. drain voltage (I d -V d ) and drain current vs. gate voltage (I d -V g ) of the BP FET under varying concentrations of NO 2  was examined, which showed a systematic increase in conductance and good consistency with Langmuir Isotherm. 
         [0076]      FIG. 1  shows a schematic of a multilayer BP FET  10  used for chemical sensing (may also be referred to herein as BP sensor  10 ). First, chemically synthesized BP flakes  15  were exfoliated, using a scotch tape, on a P 2+  Si/300 nm SiO 2  substrate  20  and subsequently patterned with contact metals (0.5 nm Ti/50 nm Au) as source  25  and drain 30 electrodes. In the back gate configuration, the P 2+  Si acts as a back gate and the 300 nm SiO 2  is the dielectric.  FIG. 2  reveals an optical image showing the multilayer BP FET  10  used for NO 2  sensing. The I d -V d  curves of the device at different back gate voltages and an inset of an I d -V g  curve are shown in  FIG. 3 . The linearity of I d -V d  curves suggests Ohmic contacts between Au and multilayer BP. The nondepletable performance of the multilayer BP flake is due to the electric field screening effect in thick BP flakes.  FIG. 4  shows an atomic force microscope (AFM) image of the multilayer BP sensor  10  and a height profile revealing a thickness of ˜55 nm. The use of thick BP flakes may stabilize the BP sensor  10  and it was recently applied to other applications of BP such as imaging. Generally, a stable performance over the sensing experiment is required for reliable sensing. Specifically, BP FETs  10 , using relatively thin BP, have displayed degradation in performance under ambient conditions due to the oxidation of phosphorus. Recently, several solutions were developed to encapsulate and passivate BP FETs  10  to maintain good performance under ambient conditions. See Wood et al., Xi et al., and Doganov et al. These methods are not applicable for sensing applications since direct exposure of the device active material to the chemical is required. Thick BP flakes were used in the sensing experiments described herein to enhance the stability of the device and reduce degradation under exposure to NO 2 . In one implementation, the thick BP flakes have a thickness greater than 50 nm. Although thinner BP flakes (e.g., BP flakes smaller than 15 nm) may theoretically offer better sensing performance because of the larger surface-to-volume ratio and larger bandgaps (i.e., reduced charge density), thinner flakes maybe more affected by oxidation than thicker flakes. On the basis of the inventors&#39; experiments, the inventors observed that continuous electrical measurements of thin BP flakes in air made the BP FET fail (see  FIGS. 15 and 16 ). On the other hand, a bias stress test of a thick BP flake in air showed relatively stable performance with a conductance variation less than 4% (see  FIG. 5 ). To isolate the effect of NO 2  on BP from other species, such as oxygen and water vapor, the sensing experiments were carried out in an argon environment (i.e. NO 2  diluted in argon). Nonetheless, the relatively stable conductance value in air is promising for more practical sensing applications. To further investigate the effect of NO 2  exposure on BP flakes, the inventors compared the Raman spectrum of BP flakes before and after exposure to NO 2 . Due to the anisotropic nature of BP, care was taken to keep the laser polarization, for a specific flake, in the same direction in all Raman measurements.  FIG. 6  shows a Raman spectroscopy taken on a multilayer BP flake before and after exposure to 800 ppb NO 2  for ˜30 min. It can be seen that all peaks associated with BP (A 1   g  at ˜362 cm −1 , A 2   g  at ˜466 cm −1 , and B 2g  at ˜440 cm−1) remain in the same positions and show similar relative peak intensity ratios before and after exposure. This suggests minimal chemical degradation of multilayer BP flakes used in the experiments during NO 2  exposure. 
         [0077]    The NO 2  sensing experiment starts by loading the device in a gas chamber while flushing the system with argon for a time period (e.g., 10 mins). Afterwards, the NO 2  gas is diluted with argon to produce various concentrations and the BP sensor  10  is subsequently exposed to the desired concentration. After the exposure to a certain NO 2  concentration, the system is flushed with argon for 300 seconds to partially recover BP sensor  10  and to observe a conductance change opposite to the NO 2  exposure period. Finally, when the BP sensor  10  is exposed to all desired concentrations, the gas chamber is flushed with argon until the device recovers to the original conductance value. 
         [0078]      FIGS. 7 and 8  show the results from the above-described sensing experiment. In  FIG. 7 , the relative conductance change ΔG/G 0  is plotted vs. time (where ΔG=G−G 0 , G is the instantaneous conductance of the device, and G 0  is the conductance of the device before exposure to NO 2 ). The inset of  FIG. 7  illustrates the point in time where the BP sensor  10  is exposed to 5 ppb NO 2  concentration (i.e., ON) and when the BP sensor  10  is flushed with argon (i.e., OFF). It can be clearly observed that the BP sensor  10  responds to NO 2  concentrations down to 5 ppb evident by a conductance change of 2.9%. The relatively high sensitivity to NO 2  for multilayer BP is very interesting and may be further improved by reducing the layer number and increasing the surface-to-volume ratio. Additionally, it can be seen that the conductance change is monotonic and systematically increases as the concentration increases from 5 ppb to 40 ppb. After exposure to all NO 2  concentrations, the BP sensor  10  recovers while being flushed with argon in a period of ˜35 mins and can be used again for another round of sensing ( FIG. 7 ). In  FIG. 8 , the measured ΔG/G 0  is plotted vs. NO 2  gas concentration. ΔG/G 0  is extracted by taking the difference of the ON and OFF values ( FIG. 7  inset) for each concentration. The data points in  FIG. 8  fit with Langmuir Isotherm for molecules adsorbed on a surface with equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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                           21.8 
                           
                             C 
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         [0000]    (where C is the concentration in ppb). The fitting further confirms that charge transfer is the sensing mechanism for NO 2  sensing in the multilayer BP sensor  10 . Moreover, repeated sensing experiments revealed very similar sensing performances of the same BP sensor  10  suggesting a stable performance over the timeframe of the experiment (see  FIGS. 21-22 ). It should be noted that the BP sensor  10  is not specifically selective to NO 2 . To improve the selectivity of the BP sensor to NO 2 , further efforts could be applied to modify the BP surface or sensor structure to be selective to a specific chemical species. 
         [0079]    One can define the BP sensor  10  response time as the time required to change the conductance after introducing either NO 2  (conductance increase) or argon (conductance decrease) in a specific range by 90%. Accordingly, the inventors calculated the response time of the BP sensor  10 , in one implementation, to be in the range of ˜280-350 seconds for different concentrations. This value is an indication of the rate the molecules are adsorbed on BP surface and it is comparable to other reports using other 2D materials, such as MoS 2  as gas sensors. To further analyze the adsorption and desorption of NO 2  molecules on BP, one can extract the absorption/desorption rate constants (τ) of the multilayer BP device. The first order rate equation is of the form: 
         [0000]        G=G   f +( G   0   −G   f ) e   −t/τ ,  (Eq. 2)
 
         [0000]    where G is the instantaneous conductance, G f  is the final conductance after the end of an adsorption/desorption period, G 0  is the initial conductance before an adsorption/desorption period, and t is the time.  FIG. 9 , shows the measured conductance decrease associated with NO 2  desorption and the fitted curve when the BP FET  10  is flushed with argon after being in a 20 ppb NO 2  environment. The measured and fitted curves show a nearly perfect agreement with a fitting error of ˜1.5%. This agreement suggests that there is only one time constant, and therefore, only one mechanism associated with NO 2  molecular adsorption/desorption in the experiment.  FIG. 10  plots the adsorption and desorption rate constants (τ) vs. different concentrations of NO 2 . The extracted τ values varied from ˜130 sec for a concentration of 5 ppb to ˜840 sec for a concentration of 40 ppb. It can be observed that τ, which is a metric for how fast the NO 2  adsorption/desorption process is, decreases as the gas concentration increases and saturates at high concentrations. 
         [0080]    The surprisingly high sensitivity of a thick (e.g., approximately 55 nm) multilayer BP flake compared to other multilayer 2D materials such as MoS 2  is worth noting. For instance, He et al. observed a ˜1% change in ΔG/G 0  when exposing an 18 nm MoS 2  flake to 1200 ppb NO 2 , while the inventors observed a 2.9% change in ΔG/G 0  when exposing a 55 nm BP flake to 5 ppb NO 2  (240 times lower in concentration). This difference in response of multilayer samples is potentially material dependent. As was mentioned earlier, the high adsorption energies of NO 2  to BP is one reason causing the high sensitivity of inventors&#39; BP gas sensor  10 . Additionally, it was theoretically predicted and experimentally observed that BP layers have less out-of-plain conductance than other 2D materials, such as graphene and MoS 2 . This low conductance in the out-of-plane direction in BP compared to the in-plane conductance may explain the observed high sensitivity. Since only the top most BP layer and edges are exposed to NO 2  during sensing, and since the metal contact to the multilayer BP flake is mostly to the top layer, the transport and doping of the top layers may dominantly control the conductance of the device because of the low conductance in the out-of-plane direction. 
         [0081]    To further explore the conductance of the out-of-plane direction in BP, the inventors fabricated a vertical structure comprised of a monolayer graphene bottom contact to multilayer BP and a 0.5 nm Ti/50 nm Au top contact (see  FIGS. 19 and 20 ). It can be observed that the ON current value in the vertical BP transistor structure (see  FIGS. 19 and 20 ) is approximately two orders of magnitude lower than the traditional lateral transport FET structure (i.e. 10 μA for V d =0.05V in lateral FET compared to 20-100 nA for V d =0.1 V in vertical FET). Moreover, comparing to a vertical MoS 2  with flake thicknesses in the same range and similar device structure, BP vertical FET structure revealed a vertical current density of 0.5-2.4 A/cm 2  at V d =0.2 V (see  FIGS. 19 and 20 ), while MoS 2  at the same V d  exhibited a current density of −800 A/cm 2 . This notably large difference further supports the explanation and hypothesis about the observed high sensitivity in multilayer BP sensors compared to its other 2D counterparts. 
         [0082]    The inventors further characterize the electronic properties of the multilayer BP FET  10  via observing the change in I d -V d  and I d -V g  curves after exposing the device to different concentrations. First, the device was flushed with argon to clear the system from any contaminants or residual gas species. Then the device was exposed to a specific NO 2  concentration for 500 sec. Subsequently, measurements of I d -V d  and I d -V g  curves were recorded while the device is still exposed to NO 2 . Afterwards, the device was flushed with argon for 300 sec before exposing the device to a new concentration of NO 2 .  FIG. 11  shows the I d -V g  curves of the BP device under different concentrations. An upshift in the curves with increasing NO 2  concentrations associated with extra hole doping was observed.  FIG. 12  plots the on current (I ON ) (defined as current at V g =−60 V) extracted from  FIG. 11  vs. the concentration of NO 2  the BP device was exposed to. The measured data points reveal a saturation behavior at higher concentrations and follow the Langmuir Isotherm with equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       I 
                       ON 
                     
                     = 
                     
                       
                         
                           1.19 
                           × 
                           
                             10 
                             
                               - 
                               5 
                             
                           
                         
                         
                           1 
                           + 
                           
                             0.836 
                             
                               C 
                                
                               
                                 ( 
                                 ppb 
                                 ) 
                               
                             
                           
                         
                       
                        
                       
                         ( 
                         A 
                         ) 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    which further supports that charge transfer is the main mechanism for the BP FET sensor  10 . The inset of  FIG. 12  shows that 1/I ON  vs. 1/C(ppb) have a linear relationship, which is another representation of the Langmuir Isotherm in  FIG. 12 . I d -V d  curves at different concentrations of NO 2  are plotted in  FIG. 13 . As can be seen, the conductance increases monotonically with increasing NO 2  concentrations. Moreover, the I d -V d  curves maintain their linearity with various NO 2  concentrations indicating minimal effect of Schottky barrier modulation induced by NO 2  exposure under the conditions used in the experiment.  FIG. 14  shows the conductance (G) of the inventors&#39; BP sensor extracted from  FIG. 13  vs. the concentration of NO 2 . The fitted Langmuir Isotherm equation is: 
         [0000]    
       
         
           
             
               
                 
                   
                     G 
                     = 
                     
                       
                         
                           1.6 
                           × 
                           
                             10 
                             
                               - 
                               4 
                             
                           
                         
                         
                           1 
                           + 
                           
                             1.59 
                             
                               C 
                                
                               
                                 ( 
                                 ppb 
                                 ) 
                               
                             
                           
                         
                       
                        
                       
                         ( 
                         S 
                         ) 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    which agrees with the measured data points. Similar to  FIG. 12 , the inset of  FIG. 14  shows a linear dependence of 1/G vs. 1/C(ppb) which agrees with the Langmuir Isotherm fitting. 
         [0083]    BP Synthesis: 
         [0084]    The inventors synthesized BP samples from red phosphorus (Chempur, 99.999+%) and tin/tin (IV) iodide (Sn/SnI 4 =10/5 mg per 250 mg batch) in evacuated (p&lt;10 −3  mbar) silica ampoules according to literature procedures. Subsequently, the temperature of the starting materials was raised to 650° C. in a period of 8 hours and that temperature was held for 5 hours. Then, the oven chamber was cooled down to 550° C. in a period of 7.5 hours and was kept at that temperature for 6 hours. Eventually, the mixture was cooled to room temperature. 
         [0085]    BP Device Fabrication: 
         [0086]    BP flakes were exfoliated using a commercial tape on a P++Si/300 nm SiO 2  substrate with alignment marks patterned. After the flakes were located using optical microscopy, electron beam lithography (EBL) defined electrodes were patterned on the target BP flake. Subsequently, electron beam evaporation of 0.5 nm Ti as an adhesion layer and then 50 nm thick Au layer for contacts were carried out. Then, the sample was soaked in acetone for ˜30 mins to do metal lift-off Afterwards, the devices were manually bonded using indium wire bonds after mounting the substrate on a chip carrier. Finally, devices were loaded inside the gas sensing chamber and measurements were recorded. 
         [0087]    NO 2  Gas Sensing: 
         [0088]    Gas sensing was carried out by exposing the BP FET device to NO 2  gas diluted in argon in a closed chamber. Concentrations of NO 2  were adjusted by changing the flow rates of both gases while keeping the total flow rate constant. For each curve, the device was exposed to the desired concentration for 500 sec and then flushed with argon for 300 sec. Similar procedures were followed to measure I d -V d  and I d -V g  for BP devices. 
         [0089]    Accordingly, the invention provides a new and useful black phosphorus gas sensor. Various features and advantages of the invention are set forth in the following claims.