Bias- and Gate-Tunable Gas Sensor Response Originating from

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Bias and gate-tunable gas sensor response originating from modulation in Schottky barrier height of graphene/MoS van der Waals heterojunction 2

Hiroshi Tabata, Yuta Sato, Kouhei Oi, Osamu Kubo, and Mitsuhiro Katayama ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14667 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Applied Materials & Interfaces

Bias and Gate-Tunable Gas Sensor Response Originating from Modulation in Schottky Barrier Height of Graphene/MoS2 van der Waals Heterojunction Hiroshi Tabata*, Yuta Sato, Kouhei Oi, Osamu Kubo, and Mitsuhiro Katayama Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

ABSTRACT: We report on the gas-sensing characteristics of a van der Waals heterojunction consisting of graphene and an MoS2 flake. To extract the response actually originating from the heterojunction area, the other gas-sensitive parts were passivated by gas barrier layers. The graphene/MoS2 heterojunction device demonstrated a significant change in resistance, by a factor of greater than 103, upon exposure to 1 ppm of NO2 under a reverse-bias condition, which was revealed to be a direct reflection of the modulation of the Schottky barrier height at the graphene/MoS2 interface. The magnitude of the response demonstrated strong dependences on the bias and back-gate voltages. The response further increased with increasing reverse bias. Conversely, it dramatically decreased when measured at a large forward bias or a large positive back-gate voltage. These behaviors were analyzed using a metal-semiconductor-metal diode model consisting of graphene/MoS2 and counter Ti/MoS2 Schottky diodes.

KEYWORDS: heterojunction, graphene, MoS2, Schottky barrier, thermionic emission, gas sensor, metal-semiconductor-metal diode

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1. INTRODUCTION Since the pioneering works of Kong et al.1 and Li et al.2, nanomaterials (e.g., carbon nanotube3, different inorganic nanowires4, graphene5,6, and other 2D materials7) have been investigated for use as chemical sensors. They represent remarkable changes in their electric conductance under gas exposure and have proven to be excellent sensing materials because of their distinguished electric properties and large surface-to-volume ratios. Although they continue to exhibit problems for practical use, such as a poor gas selectivity, an interference from humidity or other gas species, and a current drift due to irreversible gas adsorption, these problems are being overcome by functionalizing or coating the nanomaterials with appropriate molecules/polymers for molecular recognition8,9 and eliminating the undesirable interferences from other gases10. It is also important to explore the novel functions of nanomaterials as a transducing material, because these determine the basic performance of the gas sensor. Recently, van der Waals (vdW) vertical heterojunctions of graphene and semiconductors have attracted considerable attention as emerging transducers for chemical sensors.11–17 Similar to the majority of interfaces between a conventional metal and a semiconductor, the interface between graphene and a semiconductor in a vdW heterojunction leads to the formation of a Schottky barrier (SB) caused by the difference in the work function. However, unlike conventional metals, the Fermi level, and thereby the work function of graphene, are extremely sensitive to its carrier density and thus the external electric field caused by its low density of states. Therefore, the Schottky barrier height (SBH) at such an interface can be tuned by electrostatic18,19 or chemical doping.20 For gas sensor applications, the heterojunction transduces the number of adsorbed molecules on the graphene surface into an exponential change in current through the interface via modulation of the SBH.21 This type of sensor is known as a Schottky diode gas sensor13,14,21,22 and is expected to have a greater response than chemiresistive


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or chemical-field-effect-transistor (FET)-type sensors. The Schottky diode gas sensor based on the graphene/semiconductor heterojunction has several advantages compared to conventional Schottky diode gas sensors. For example, the adsorption sites on the graphene surface are proximate to its interface owing to the atomic thickness of graphene, promising efficient transducing. Moreover, a large-scale heterostructure can be easily obtained using a large area of graphene. Several groups have reported Schottky diode gas sensors using graphene/semiconductor heterojunction.11,13–15,21 For example, Singh et al.21 demonstrated a reverse-biased graphene/Si heterojunction diode-based sensor with ultrahigh sensitivity to NO2 and NH3 gas. They determined the magnitude of the change in the SBH at the graphene/Si heterojunction due to molecular adsorption by capacitance-voltage (C-V) measurement and explained that the observed response arose from the gas-induced modulation in the SBH. However, the interpretation remains ambiguous because the change in current measured under a reverse-bias condition was not as large as expected from the change in the SBH in the thermionic emission model. Further, not only the graphene/p-Si device but also the graphene/n-Si device demonstrated a p-type behavior, i.e., an increase (decrease) in current upon the exposure of oxidizing (reducing) gas, despite the opposite change in their SBHs. To date, there has been no report of gas sensors using a graphene/semiconductor heterojunction with the response directly reflecting the SBH modulation at the interface; this remains a challenge. More recently, a new type of vdW heterojunctions consisting of graphene and two-dimensional layered semiconductors (2DLSs), such as transition-metal dichalcogenides (TMDCs), have emerged and been studied for different device applications.23–30 The vdW interface of graphene/2DLS heterojunctions are expected to have a weaker Fermi level pinning (FLP) effect, compared to graphene/bulk semiconductor interfaces, without special treatment to obtain an inert



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surface; this is because 2DLSs do not have inherent dangling bonds on their surface. This enables more effective modulation of the SBH. Further, if the 2DLSs are atomically thin, the work function of graphene and thereby the SBH at the interface can be tuned and even can be eliminated via the back-gate voltage24 while holding the graphene exposed. This could be advantageous for chemical sensing applications because the setting of a suitable initial SBH for the type of analyte gas (oxidizing or reducing) provides improved gas-sensing responses.21 However, there have been limited studies on the graphene/2DLS heterojunctions for gas sensing applications16,17 and none of these have focused on the gas-induced modulation of the SBH at the heterojunctions. This is primarily because of the unignorable contributions from the gas-sensitive parts other than the heterojunction. To reveal their potential for gas sensing applications, it is necessary to understand the gas-sensing characteristics actually originating from the graphene/2DLS heterojunction. We present the gas-sensing characteristics arising from gas-induced modulation in the SBH at the graphene/2DLS heterojunction. We utilize a device based on a vdW heterojunction consisting of graphene and molybdenum disulfide (MoS2), which has been one of the most studied 2DLSs. Hereinafter, we refer to the graphene/MoS2 heterojunction as “GMH” and the device based on the GMH as a “GMH device”. The response originating from the GMH area is extracted by passivating the gas-sensitive parts, except for the GMH area, with gas barrier layers. The GMH device demonstrated a significant increase in resistance, by a factor greater than 103, upon exposure to 1 ppm of NO2 under reverse-biases, which was revealed to be a direct reflection of the NO2-induced increase in the SBH at the GMH. The response also demonstrated strong dependences on the bias and back-gate voltages. These behaviors were analyzed using a metal-semiconductor-metal (MSM) diode model31 considering graphene/MoS2 and counter Ti/MoS2 Schottky diodes.


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2. EXPERIMENTAL SECTION To fabricate the GMH device, few-layer MoS2 flakes, mechanically exfoliated from a bulk crystal, were transferred onto p+-Si substrates covered with a 285 nm thermally grown SiO2 layer. Chemical vapor deposition (CVD)-grown graphene on copper foil was then transferred onto an MoS2 flake of interest by a dry transfer method32 with a manual manipulator under microscope observation such that the graphene edge partially overlapped the MoS2 flake. The extra areas of the MoS2 flake and graphene were removed using conventional photolithography and O2 plasma etching (50 W, 30 s). Metal contact pads consisting of Ti/Au (20 nm/50 nm) were attached on the MoS2 flake and graphene by electron beam deposition. The thickness of the MoS2 flake and graphene were determined by atomic force microscopy (AFM) and Raman spectroscopy using a confocal Raman microscope. The gas barrier layers for the passivation of specific areas of the GMH device were formed by photolithographic patterning of a double layer of resist consisting of poly(methyl methacrylate) (PMMA) (200 nm in thickness) and SU-8 (an epoxy-based photoresist) (3.3 m in thickness).33 Here, the PMMA layer acted as a virtual gas barrier layer and the SU-8 layer formed an insoluble pattern to protect the PMMA layer from developing agents. All electric measurements, including those of the device characteristics and sensor response, were conducted in a sensing chamber equipped with manual probes and a source meter (Keithley 2636) controlled by a LabVIEW program developed at our lab, at room temperature, at ambient pressure, and under a continuous gas flow of N2 or NO2 gas (1ppm) diluted with N2. In the measurements, the graphene electrode of the device was always set as the source electrode. Prior to the measurements, the device was heated at 200 °C for 1 h in a vacuum (approximately 10-3 Pa) to desorb adsorbate molecules such as H2O and O2 from the device surface.



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3. RESULTS AND DISCUSSION Figures 1(a) and 1(b) display the schematic and optical microscope images of the GMH device used for this investigation. This includes a gas barrier layer covering the graphene-uncovered MoS2 area and Ti/MoS2 contact. All experimental data presented below were obtained from this device. (The results of the devices before forming a gas barrier layer and after forming another gas barrier layer are provided in the supporting information.) From the AFM image, the thickness of the MoS2 flake was estimated to be 5 nm, corresponding to an eight-layer MoS2 sheet.34 The Raman spectrum in Fig. 1(c) indicates that the transferred graphene sheet was a bi-layer graphene.35 As indicated in the band diagram in Fig. 1(d), the device structure can be regarded as an MoS2 channel FET with asymmetric contacts of graphene and Ti/Au electrodes. These contacts form back-to-back Schottky diodes with different barrier heights, as displayed in the equivalent

Figure 1. (a) Schematic and (b) optical microscope images of the GMH device with a gas barrier layer. (c) Raman spectrum of transferred graphene. (d) MSM diode model for n-type MoS2 with graphene and Ti asymmetric contacts and its band diagram.


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circuit in Fig. 1(d). Figure 2(a) displays the time-dependent sensor responses upon the exposure of 1 ppm NO2 gas under different biases and at VBG = 0 V. The sensor response ( ) was defined as the change in resistance (∆ ) normalized by initial resistance before gas exposure ( ∆ ⁄

) ( ≡

). The device operated as a typical n-type device, which indicated an increase in resistance

to oxidizing NO2 gas. A comparison among the sensor responses of identical devices before and after forming the gas barrier layer indicated that the gas barrier layer successfully suppressed the responses of the covered areas including the MoS2 channel and Ti/MoS2 contact. (See Figs. S1 and S2 and Table S1 in the supporting information.) It also suggested that the contribution of the graphene area was negligible, even though the graphene remained exposed. We thus concluded that the observed response originated mainly from the GMH area. It has not yet been determined whether the entire GMH area was responsible for the sensor response. However, based on the experimental results presented in the supporting information (See Figs. S1 and S2 and Table S1 in the supporting information.), we can speculate that the graphene contact edge on the MoS2 was the most plausible response area. As displayed in Fig. 2(a), the magnitude of the sensor response varied significantly with the bias polarity and magnitude of the bias voltage. For positive biases, that is, when reverse bias was applied at the GMH diode, the sensor response was greater than 103 at bias voltage VDS = 1 V and further increased at VDS = 3 V. Conversely, the responses under a negative-bias (VDS = -1 V) were



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Figure 2. (a) Time-dependent sensor responses of GMH under different bias conditions (VDS = -1, 1, and 3 V) in linear scale (top) and semilogarithmic scale (bottom). (b) IDSVDS curves for GMH device measured before and after NO2 exposure at VBG = 0 V. Solid curves are best-fit curves using MSM diode model. (c) Band diagrams before and after NO2 exposure. Depending on bias condition, the barrier, which is the main factor limiting the drain current, switches between the GMH and Ti/MoS2 contacts. Red circles indicate bottleneck barriers limiting drain current. The top of the valence band of MoS2 is omitted. (d) ∆ -VDS curve calculated from IDS-VDS curves before and after NO2 exposure displayed in (b). Solid curve was also calculated from fitted curves displayed in (b). two orders of magnitude less than those under the positive biases. Figure 2(b) displays the drain current (IDS)-bias voltage (VDS) curves for the device measured before and after a 10 min exposure to NO2. In the curve obtained before the exposure, current rectification, such that a considerably larger current flowed at a positive bias than at a negative bias, was observed with the rectification ratio at VDS = +/- 3 V equal to approximately 103. Conversely, from the curve obtained after exposure, it can be observed that the drain current under positive bias decreased significantly, whereas under negative bias, this changed only minimally, particularly below -1.5 V.


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Consequently, the rectification direction was inverted and the rectification ratio at VDS = +/-3 V was reduced to 0.1. This asymmetric current reduction at a positive bias strongly indicates that the SBH at the GMH increased owing to the NO2 exposure. To explain the change in the rectification behavior in more detail, we analyzed the IDS-VDS curves before and after gas exposure using the MSM diode model31 displayed in Fig. 1(d), where the back-to-back Schottky diodes of the GMH and Ti/MoS2 contacts were considered. Note that the curve before exposure appears to represent the characteristics of a single Schottky diode consisting of a Ti/MoS2 Schottky contact and GMH Ohmic contact; however, we assume the GMH has an SB with a low, yet nonzero barrier height, because the graphene/MoS2 contact in a similar contact geometry has been previously reported to have an obvious contact barrier.23,24 In this model, the current across the barriers (

) follows the thermionic emission model and can

be written as, 2 exp

sinh 2

2 exp

,

(1)

2

where q, η, kB, and T are the elemental charge, ideality factor, Boltzmann constant, and absolute temperature, respectively.

is the effective voltage, which is the sum of the voltage drops on

and Ti/MoS2 diode (

the GMH diode (

), and also equals

, where

is the total

series resistance including the resistances of the graphene electrode and MoS2 channel. At low bias voltages, because (

).

and

is reasonably small, the voltage drop on the series resistance is negligible are the saturation currents for the GMH and Ti/MoS2 diodes and are

defined as36



∗

where

∗

,

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exp

,

(2)

are the effective Richardson constant for 2D semiconductors37, and

, and

the effective contact area and SBH for the GMH (Ti/MoS2) diode, respectively. The solid lines in Fig. 2(b) indicate the best-fit curves using Eq. 1 for the “before” and “after” curves without considering the series resistance

. These fitted curves were in satisfactory agreement with the

experimental curves at negative and small positive biases, whereas they deviated as the positive bias increased, particularly above approximately 1 V in the “before” curve. This discrepancy is attributed to the tunneling current through the SB and the bias-induced barrier lowering21,38 at the GMH interface, which leads to decreasing the effective SBH as the positive bias increases. We obtained the following parameters from the best-fit curves: η = 7.02, 7.23 × 10-9 A for the “before” curve, and η = 17.3,

= 1.94 × 10-6 A, and

= 4.0 × 10-10 A, and

=

= 6.84 × 10-9 A for

the “after” curve. From these parameters and Eq. 2, the NO2-induced increment in the SBH of the GMH (Ti/MoS2) diode is obtained as follows,

≡

where respectively.

_

and

_

_

ln

_

are

and

_ _

,

(3)

after(before) NO2 exposure,

was estimated to be approximately 0.26 eV at the GMH diode, whereas the

SBH of the Ti/MoS2 diode changed minimally owing to the gas barrier layer. The variation of the SBH at the GMH diode was in satisfactory agreement with the Fermi level shift in graphene of 0.22 eV, which was approximated from the shift in the transfer curve of another graphene FET device measured in the same condition.39,40 (For details, see Fig. S4 in the supporting information.) This indicated that the increase in the SBH at the GMH interface directly reflected the NO2-


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induced modulation of the work function of graphene owing to weak FLP at the GMH interface. For both curves, η >> 1 was commonly observed, which has been attributed to the spatial inhomogeneity of the SBH at the heterojunction interface41–44 and the presence of a tunneling barrier36 of a vdW gap between the graphene and MoS2. The increase in η after NO2 exposure could suggest the expansion of the vdW gap, which was induced by an intercalation of NO2 molecules at the interface. The right (left) vertical line in Fig. 2(b) indicates the voltage where the first-order derivative of the best-fit curve for the before (after) curve has a local maximum. At this voltage, the barrier that acts as a bottleneck, limiting the current through the GMH device, switches between the barriers at the two diodes31. Using these voltages as boundaries, we divided the IDS–VDS curves into three regimes (I, II, III) with respect to the bottleneck barriers. Figure 2(c) depicts the band diagrams before and after gas exposure corresponding to each regime. In Regime I, the SBH at the Ti/MoS2 diode

always set the magnitude of the drain current, even though the SBH at the GMH diode increased. Because the SBH at the Ti/MoS2 contact was passivated,

, and thus the drain

current changed minimally in this regime. Conversely, in Regime III, the SBH at the GMH diode always set the magnitude of the drain current, which resulted in the significant sensor response. Regime II was the transition region where the bottleneck barrier transited from the MoS2/Ti diode to the GMH diode during the gas exposure. As a metric for representing the gas-induced variation in the drain current, we define

≡

ln

_ _

,

(4)



where

is the drain current before (after) exposure, and plot this as a function of bias

_

voltage in Fig. 2(d). Using this equation, the sensor response as

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/

exp

/

defined above can be expressed

. The dotted and solid lines correspond to the values

obtained from the experimental and best-fit curves displayed in Fig. 2(b), respectively. In Regime III, q

obtained from the experimental data increased with the bias voltage, which is

consistent with the result in Fig. 2(a). This indicates that the above-mentioned effects, lowering the effective SBH at the GMH with the positive bias voltage, weakened after NO2 exposure, which resulted in a greater change in the SBH

as the positive bias voltage increased.

Next, we discuss the back-gate dependence of the sensor response of the GMH device. Figure 3(a) displays the time-dependent sensor response measured at VBG = 40 V compared with that at VBG = 0 V. The sensor response S at VBG = 40 V was 6.6 after 10 min exposure to NO2, which

Figure 3. (a) Time-dependent sensor responses of GMH under different gate voltages (VBG = 0 and 40 V) in linear scale (top) and semilogarithmic scale (bottom). (b) IDS-VDS curves for the GMH device measured at VDS = 40 V before and after NO2 exposure.


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was significantly weak compared with that at VBG = 0 V (S = 1600). The corresponding IDS-VDS curves before and after NO2 exposure are displayed in Fig. 3(b). Unlike the IDS-VDS curves measured at VBG = 0 V, NO2 exposure did not cause the inversion of the current rectification direction or a marked change in the rectification ratio at VDS = +/-3 V, and only reduced the current for both polarities by a similar factor of approximately 10. Neither of the curves could be fitted by either Eq.1 for the MSM diode model or an equation for a simple diode model considering a Ohmic contact of GMH and a Schottky diode of Ti/MoS2 contact: exp

⁄

1 , which was likely because of the major participation of

charge injection mechanisms of non-thermionic emission, such as thermionic field emission and field emission. However, assuming the MSM diode model to be valid, the preservation of the rectification ratio after NO2 exposure indicates an ignorable increase in the SBH at the GMH due to the NO2 exposure. This can be interpreted in two fashions. First, a NO2-induced Fermi level shift of graphene at VBG = 40 V, which was estimated from the shift in the transfer curve of the graphene FET device, was 0.07 eV and considerably less than that at VBG = 0 V (0.22 eV), because of the difference in the density of the states where the Fermi level is located. (For details, see Fig. S4 in the supporting information.) This resulted in a small increase in the SBH at the GMH. Secondly, at VBG = 40V, the SBH at the GMH approaches zero and the GMH interface is expected to form an Ohmic contact24. The GMH with Ohmic contact can be regarded as a combined new material instead of a conventional heterojunction. In such a case, the NO2-induced modulation in carrier density of the graphene would be compensated by electron transfer from the MoS2 in contact with the graphene, which suppresses the increase in the SBH at the GMH. Under the conditions of VBG = 40 V, and VDS = 1 V displayed in Fig. 3(a), the injection barrier at the GMH side is considerably small, if any, compared to the ejection barrier at the Ti/MoS2 side, even after



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exposure to NO2. Further, the height of the ejection barrier on the Ti/MoS2 side, which electrons in the source electrode see, determines the drain current during the NO2 exposure. Consequently, the marginal change in the SBH at the GMH interface was hidden by the higher and passivated barrier on the Ti/MoS2 side, making the GMH device insensitive upon NO2 exposure. Figure 4(a) displays the comparison between transfer curves of the GMH device before and after NO2 exposure measured at VDS = 1 V. The curve shifted in a positive direction without changing in transconductance after the gas exposure. As mentioned above, the shift did not originate from the carrier modulation in the MoS2 channel, rather, it was a consequence of the modulation in the

Figure 4. (a) Transfer curves of GMH device measured at VDS = 1 V with linear (top) and in semilogarithmic scales (bottom). (b) ∆ – curves calculated from transfer curves before and after NO2 exposure in (a). Dotted parts of lines are less precise because values were obtained from current under the measurement limit. (c) NO2-induced increase in the SBH at the GMH interface , which was calculated from shift in charge neutrality point of graphene ∆ FET.


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SBH at the GMH. Figure 4(b) displays ∆

as a function of the back-gate voltage, which was

calculated from the curves in Fig. 4(a) using Eq. 4. The significant difference in ∆

between

the forward and backward curves originated from the hysteresis in the transfer curve after gas exposure. Thus, the actual value of ∆

without the hysteresis effect should be between the two

curves. The back-gate voltage dependence of ∆

represents a strong tunability of the sensor

response in the range of VBG between -10 V and 10 V. Remembering the modulation in the SBH at the GMH determines the magnitude of the sensor response at VBG = 0 V, the remarkable change in ∆

in this range can be also ascribed to ∆

. The ∆

-VBG curve displayed in Fig. 4(c),

which was obtained from the calculated value of the NO2-induced Fermi level shift of graphene in contact with MoS2 (See the supporting information for details.), supports this interpretation. The large values in ∆

in the range of VBG between -30 V to 2 V in Fig. 4(c) correspond to

the Fermi level shift in the vicinity of the Dirac point in graphene, where the density of states is particularly low. This indicates that tuning the Fermi level position of the graphene with respect to the Dirac point appropriately via the back-gate voltage can maximize the magnitude of the sensor response of the GMH device.

4. CONCLUSIONS In this work, we investigated the gas-sensing characteristics originating from the GMH area of a GMH device utilizing a passivation technique with gas barrier layers. A large sensor response of greater than 103 upon exposure to 1 ppm of NO2 was observed when a reverse bias was applied to the GMH diode at VBG = 0 V. This large response directly reflected the modulation in the SBH at the GMH, which was caused by the NO2-induced increase in the work function of the graphene



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on the MoS2. The magnitude of the response demonstrated the strong dependences on the bias and back-gate voltages. The increase in the response with the magnitude of the reverse bias can be explained by considering the effects (the tunneling current through the SB and the biasinduced barrier lowering) that reduce the effective SBH at the GMH with increasing magnitude of the reverse bias on the NO2-induced modulation in the SBH. Conversely, when a forward bias was applied or when a large positive back-gate voltage of VBG approximately 40 V was applied, even at reverse bias, the sensor response significantly decreased. This is because that in both cases, the drain current was determined by the barrier height at the counter Schottky diode of the MoS2/Ti contact, and the NO2-induced modulation in the SBH at the GMH was not reflected in the sensor response. The large gas sensor response based on the modulation in the SBH and its bias- and gate-tunability confirm a high potential for the GMH device as a transducer for a gas sensor. We believe that these findings will increase the understanding of the sensor characteristics of not only the GMH but also, more generally, heterojunctions between graphene and other 2DLSs.

ASSOCIATED CONTENT Supporting Information. The following Supporting Information is available at no charge on the ACS Publications website at DOI: Structures of as-prepared and gas-barrier-covered devices and their sensor responses, Analysis of the GMH device using the MSM model, and Estimation of Fermi level shift due to NO2 exposure in graphene forming a contact with a MoS2 flake (PDF).


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

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. 

ACKNOWLEDGMENTS

This work was financially supported by a JSPS Grant-in-Aid for Young Scientists (B) (Grant Number 15K21145) and partly by the Iketani Science and Technology Foundation. This work was also partly supported by the MEXT Photonics Advanced Research Center Program.



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Bias- and Gate-Tunable Gas Sensor Response Originating from

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