High-Performance Gas Sensor Using a Large-Area WS2xSe2â2x

Subscriber access provided by - Access paid by the | UCSB Libraries

Energy, Environmental, and Catalysis Applications

High-Performance Gas Sensor using Large-Area WS2xSe2-2x Alloy for Low-Power Operation Wearable Applications Kyung Yong Ko, Sangyoon Lee, Kyunam Park, Youngjun Kim, Whang Je Woo, Donghyun Kim, Jeong-Gyu Song, Jusang Park, Jung Hwa Kim, Zonghoon Lee, and Hyungjun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10455 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

High-Performance Gas Sensor using Large-Area WS2xSe2-2x Alloy for Low-Power Operation Wearable Applications Kyung Yong Ko1, Sangyoon Lee1, Kyunam Park1, Youngjun Kim1, Whang Je Woo1, Donghyun Kim1, Jeong-Gyu Song1, Jusang Park1, Jung Hwa Kim2, Zonghoon Lee2, and Hyungjun Kim1*

1

School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-Ro, Seodaemun-Gu, Seoul, 03722, Rep. of Korea

2

School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Uljugun, Ulsan, 44919, Rep. of Korea.

*

Corresponding author. E-mail: [email protected]

KEYWORDS: transition metal dichalcogenide alloy, gas sensors, electronic sensitization, low-power operation, wearable application

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

1

ABSTRACT: Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted considerable attention as promising building blocks for a new generation of gassensing devices because of their excellent electrical properties, superior response, flexibility, and low power consumption. Owing to their large surface-to-volume ratio, various 2D TMDCs, such as MoS2, MoSe2, WS2, and WSe2, have exhibited excellent gas-sensing characteristics. However, exploration toward the enhancement of TMDC gas-sensing performance has not yet been intensively addressed. Here, we synthesized large-area uniform WS2xSe2-2x alloys for room-temperature gas sensors. As-synthesized WS2xSe2-2x alloys exhibit an elaborative composition control owing to their thermodynamically stable sulfurization process. Further, utilizing uniform WS2xSe2-2x alloys over a large area, we demonstrated improved NO2-sensing performance compared to WSe2 on the basis of an electronic sensitization mechanism. The WS0.96Se1.04 alloy gas sensor exhibits 2.4 times enhanced response for NO2 exposure. Further, we demonstrated a low-power wearable NO2-detecting wristband that operates at room-temperature. Our results show that the proposed method is a promising strategy to improve 2D TMDC gas sensors and has a potential for application in advanced gas-sensing devices.


Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

As modern technology has enabled more convenient life, human beings have been more exposed to environmental threats such as gas explosion accidents and air pollution by emission gases. Among the various air pollutants, NO2, which is the most common toxic gas, is emitted from power plants and vehicles, and contributes to the formation of acid rain, ground-level ozone, and fine dust (particle) pollutants. Furthermore, continuous NO2 exposure may cause respiratory diseases such as respiratory irritation, chronic bronchitis, and emphysema. To prevent exposure to air pollutants, indoor/outdoor environmental monitoring systems have been considered as a special precaution. With the advent of the Internet of Things (IoT), the use of various sensors has soared and power consumption for the operation of sensors has become a great concern.1 Thus, low-power gas sensors are in high demand for indoor/outdoor environmental monitoring systems such as wearable sensing devices and mobile healthcare systems. Various metal oxide-based semiconductor gas sensors have attracted considerable attention because of their simple structure and high sensitivity. Despite their mature technical status, they have inadequate specifications for use in low-power gas-sensing systems, because the conventional oxide-based semiconductor gas sensor consumes tens or hundreds of milliwatts as thermal energy, which is required to desorb gas molecules on the surface of the sensing material. Atomically thin two-dimensional (2D) layered materials such as graphene,2 graphitic carbon nitride,3,4 black phosphorous,5 and transition metal dichalcogenides (TMDCs; MX2: M = Mo, W; X = S, Se)6–9 have attracted much attention as promising gas-sensing materials because of their large surface-to-volume ratio, excellent electrical conductivity, and susceptible surfaces. Some fundamental studies on TMDC gas sensors have been reported utilizing MoS2,6 WS2,10 WSe2,11 and MoSe2.12 In particular, these TMDCs exhibit excellent adsorption/desorption properties even at room-temperature, because TMDCs have an inert



Page 4 of 37

3

surface along the out-of-plane direction. On the basis of this unique property, TMDCs can be exploited as low-power gas sensors without heating. Recently, some researchers have attempted to enhance the gas-sensing performance of TMDCs to realize practical TMDC gas sensors. Generally, the gas-sensing performance of semiconducting TMDCs can be enhanced by sensitization. Some of the chemical sensitization of TMDCs includes Ag-nanowire functionalized WS29 and Pd-nanoparticle functionalized MoS2.13 For these examples, the adsorption of gas molecules is increased on the TMDC surface because of the catalytic effect. Another gas sensor enhancement method, which is widely known, is electronic sensitization mechanism. Since the response of semiconducting

chemiresistive

gas

sensors

depends

on

the

change

ratio

of

resistance/conductivity of sensing material, initial status of resistance/conductivity (baseline) is important factor to calculate response. Thus, the electronic sensitization has widely used as a strategy for the semiconducting metal oxide gas sensor enhancement.14–16 Recently, electronic sensitization of Nb-doped MoSe2 was reported, in which a small amount of Nb dopant effectively controls the carrier concentration of MoSe2, resulting in a significant enhancement in gas-sensing response.17 Although this method shows enhanced gas-sensing properties, the precise control of doping concentration is difficult to achieve, and highly Nbdoped MoSe2 shows metallic properties that can attenuate the gas-sensing response. Meanwhile, synthesis of ternary semiconducting alloys of TMDCs such as MoxW1-xS2,18 MoxW1-xSe2, and WS2xSe2-2x19 have been intensively studied because the bandgap of TMDCs can be precisely controlled by modulating the composition ratio. This bandgap engineering technology of TMDCs alloys is valuable for the realization of the various applications such as photodiodes,20 phototransistors,21,22 solar cells,23 PN diodes,24 field-effect transistors,19 and hydrogen evolution reactions.25 Nevertheless, the electronic property modulation of TMDC alloys has been rarely studied for the enhancement of gas-sensing performance. A recent


Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

study of WS2xSe2-2x alloy synthesis by chemical vapor deposition (CVD) using WS2 and WSe2 powder reported significant bandgap modulation and control of carrier concentration depending on the composition ratio of n-type WS2 and p-type WSe2.19 However, the previous synthesis method cannot be applied to the fabrication of practical gas sensors because of the lack of large area composition uniformity and layer-number uniformity which can affect gas sensor performance. Here, we report a composition-controllable two-step synthesis process for large-area WS2xSe2-2x alloys, which consists of WSe2 synthesis and consequent sulfurization. The systematic composition modulation of WS2xSe2-2x (0 < x < 1) alloys was demonstrated, and the precisely controlled bandgap was confirmed as a function of the composition. Because the bandgap modulation results in a change in carrier concentration in the TMDC alloy, the gassensing property of WS2xSe2-2x alloy was significantly enhanced; the WS0.96Se1.04 alloy gas sensor exhibited an approximately twofold better response for NO2 at 500 ppm, compared to the WSe2 gas sensor. Further, we designed a room-temperature flexible NO2 gas detector, showing its potential for low-power gas-sensing systems.



Page 6 of 37

5

Results and Discussion

Figure 1. (a) Schematic images of the synthesis process of WS2xSe2-2x alloy depending on the sulfurization temperature. XPS spectra for (b) W 4f, (c) Se 3d, and (d) S 2p core levels of 1L WS2xSe2-2x alloys with different sulfurization temperatures. All XPS spectra are normalized by W 4f7/2 peak intensity. (e) Sulfur ratio vs. sulfurization temperature of 1L WS2xSe2-2x alloys. Figure 1a shows a summarized schematic illustration of the sulfurization process. To control the composition of the WS2xSe2-2x alloy, the sulfurization temperature was controlled from 500 °C to 1100 °C. During the sulfurization process, the Se atoms were spontaneously substituted with S atoms in the S-rich environment. The atomic composition of 1L WS2xSe2-2x alloys depending on the sulfurization temperature was confirmed by XPS measurement on the W 4f, Se 3d, and S 2p binding energies. Figure 1b exhibits similar peak intensities, which correspond to the W 4f7/2 and W 4f5/2 binding energy of W4+ at 32.9 eV and 34.9 eV. There is no obvious difference in intensity regarding W atoms among the WSe2 and WS2xSe2-2x alloys,


Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

which can be understood as the W atoms being thermodynamically stable and remaining intact during the sulfurization process. The evolutionary variation in XPS peaks is observed from the binding energy related to Se and S, as shown in Figure 1c and 1d. The peaks corresponding to the Se 3d5/2 and Se 3d7/2 are observed at 54.9 eV and 55.8 eV in Figure 1c. These Se 3d peaks diminished with increasing sulfurization temperature. Figure 1d shows the Se 3p3/2, S 2p3/2, and S 2p1/2 peaks (161.2 eV, 162.6 eV, and 163.8 eV). With increasing sulfurization temperature, S 2p peaks become prominent, whereas the Se 3p peak decreases, indicating the Se atoms were substituted by S atoms continuously. In particular, the WSe2 was completely converted to the WS2 at 1100 °C of sulfurization. The XPS spectra were resolved by multi-peak Lorentzian fitting to calculate the quantitative composition ratio, as shown in Figure S2 (see Supporting Information). Figure 1e represents the calculated S composition, x. The calculated x value depends on the sulfurization temperature, and yields x = 0.18 for 500 °C, x = 0.25 for 600 °C, x = 0.33 for 700 °C, x = 0.48 for 800 °C, x = 0.60 for 900 °C, x = 0.77 for 1000 °C, and x = 1 for 1100 °C. In addition, the calculated stoichiometry is 2 ((S + Se)/W). Because the conversion is governed by the sulfurization temperature, the WS2xSe2-2x alloy exhibits thermodynamically controlled composition. Therefore, the sulfurization reaction mechanism can be described by the following equations:

WSe() + H S( ) + H( ) → WS Se () + H Se( ) + H( ) , where 0 < x < 1 (1) WSe() + 2H S( ) + H( ) → WS() + 2H Se( ) + H( ) , where x = 1 (2)



Page 8 of 37

7

Figure 2. Evolution of Raman spectra in the 1L WS2xSe2-2x alloys as a function of the sulfur ratio: (a) Full-range Raman spectra; the enclosed area of (a) is magnified in (b). (c) Raman 1

spectra peak position shifts with increasing sulfur ratio in WS2xSe2-2x alloys for the A g and E

1

1

2g

modes of WS2 and the A g mode of WSe2. (d) PL spectra of 1L WS2xSe2-2x alloys. All

Raman and PL spectra were excited by a 532-nm laser. (e) Bandgap of WS2xSe2-2x alloys depending on the S composition. To further evaluate the systemic composition control of the WS2xSe2-2x alloys, we studied the Raman and PL using a 532-nm laser, as shown in Figure 2. The normalized Raman spectra of composition-controlled 1L WS2xSe2-2x alloys are shown in Figure 2a. All the Raman peaks are normalized by the intensity of the Si Raman peak. WSe2 (x = 0) exhibits the overlapped peaks of in-plane (E12g) and out-of-plane (A1g) at 250 cm−1. With increasing ratio


Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 of S composition (x) from 0 to 0.60, the strong E12g peak intensity of WSe2 decreases and becomes inconspicuous at 0.77 of S composition (x). The intensity of the A1g mode of WSe2 is decreased by coupling with the A1g mode of WS2. The Raman peaks related to E12g and A1g modes of WSe2 appear at 0.18 of S composition (x), and the intensity of the peaks becomes prominent with increasing S composition (x), as shown in Figure 2b. In addition, the secondorder resonance modes of WSe2 at 375 cm−1 and 395 cm−1 are exhibited in the Se-rich alloy region and disappear as the sulfur ratio increases.26 Further, the Raman peak position shift is shown in Figure 2c with increasing S composition (x). The substitution of S and Se atoms occurred in the lattice, which could lead to lattice distortion and variation in the Raman peak position. The E12g mode of WSe2 shows a strong resonance at 250 cm−1 for the Se-rich phase, and a gradual blue shift (up to 256.2 cm−1) is observed as the sulfur ratio increases. Similarly, The A1g mode of WS2 follows the same trend and changes from a weak resonance at 415.6 cm−1 in Se-rich WS2xSe2-2x alloy (x = 0.18) to 420 cm−1 in WS2. The E12g mode of WS2 shows indistinguishable variation in position near 355.3 cm−1 as the sulfur ratio increases.19 Figure 2d presents the normalized PL spectra of the composition-controlled 1L WS2xSe2-2x alloys. The 1L WSe2 represents a strong PL peak at 1.61 eV. As the value of x increases from 0 to 1, the PL peak position gradually increases to 1.94 eV, as shown in Figure 2d and 2e. The linear transition of PL peaks, corresponding to gradual bandgap modulation of the WS2xSe2-2x alloy, is attributed to the small lattice mismatch between WSe2 and WS2.27,28 These continuous Raman and PL peak shifts confirm that the sulfurization temperature is the key factor in the composition-controlled synthesis of WS2xSe2-2x alloy. Further, to examine the large-area uniformity, we observed Raman spectra at five different points along the length of the samples (sample size: 1 × 5 cm2), as shown in Figure S3a and S3b (see Supporting Information). The variations in the relative peak intensities depending on position are small: between 0.3% and 2% for all samples. In addition, the optical band gap uniformity was



Page 10 of 37

9

confirmed by the spatial PL mapping (Figure S3c and S3d).

Figure 3. Sulfurization time dependency of 3L WS2xSe2-2x alloy: (a) Sulfurization process to various sulfurization times (5 min, 15 min, 30 min, and 60 min). Raman spectra of 3L WS2xSe2-2x alloy at different sulfurization times, which were sulfurized at (b) 700 °C, (c) 800 °C, (d) 900°C, and (e) 1000 °C.

To further study the sulfurization process, we carried out different sulfurization processes as a function of sulfurization time (5 min, 15 min, 30 min, and 60 min), as illustrated in Figure 3a. Each sulfurization process was carried out at different sulfurization temperatures (700 °C, 800 °C, 900 °C, and 1000 °C). In Figure 3b–3e, the Raman spectra of 3L WSe2xSe22x

alloy under different sulfurization processes can be observed. Figure 3b and Figure 3c,

which display the Raman spectra of sulfurized 3L WSe2 at 700 °C and 800 °C, show dominant Raman peaks related to WSe2 at 250 cm−1, whereas weak E12g and A1g Raman


Page 11 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 peaks of WS2 are exhibited at ~350 cm−1 and ~418 cm−1. Figure 3d shows increased E12g and A1g Raman peaks of WS2 at 900 °C of sulfurization temperature. The WS2 Raman peaks become dominant at 1000 °C of sulfurization temperature as shown in Figure 3e. Interestingly, although the sulfurization time is different, all the Raman peak intensities of WS2xSe2-2x alloys, which were prepared at the same sulfurization temperature, are equivalent. Thus, we infer that the sulfurization process reaction finishes before 5 min and the reaction is saturated at the prolonged time. Sulfurization time control at a fixed sulfurization temperature is a predictable method to modulate the composition of WS2xSe2-2x alloy. However, our results indicate that the sulfurization reaction is completed in a short time (5 min). Thus, we propose that the temperature-controlled sulfurization process is a relatively reliable way to prepare composition-controlled WS2xSe2-2x alloy.

Figure 4. HRTEM images of (a) WSe2 and (b) WS0.96Se1.04 alloy, and (inset) SAED pattern. (c) HAADF STEM image of WS0.96Se1.04 alloy. EDS mapping of HAADF TEM images: (d) HAADF TEM image of WS0.96Se1.04 alloy, and EDS elemental maps of (e) W, (f) S, and (g) Se in HAADF images.



Page 12 of 37

11

Figure 4a and Figure 4b are the HRTEM images of 3L WSe2 and 3L WS0.96Se1.04 alloy, which is sulfurized 3L WSe2 at 800 °C. The WSe2 and WS0.96Se1.04 alloy exhibit honeycomblike structures, with lattice spacing of 0.284 nm and 0.172 nm for the (100) and (110) planes. The ring pattern of selected area electron diffraction (SAED) represents polycrystalline structure of WS0.96Se1.04 alloy, as shown in the inset of Figure 4b. Figure 4c shows the highresolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image of the WS0.96Se1.04 alloy. The W atoms are clearly resolved in the HAADF image as brighter spots, whereas the S and Se atoms are difficult to classify, becausethe intensity is proportional to the square of the atomic weight (Z). Therefore, the order of the predicted relative intensities of the atoms in the HAADF image from low to high could be S (Z = 16), Se (Z = 34), W (Z = 74). Energy-dispersive X-ray spectroscopy (EDS) mapping analysis represents WS0.96Se1.04 alloy has uniform distribution of W, S, and Se atoms, as shown in Figure 4d-4g.


Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12

Figure 5. Schematic image of the (a) WS2xSe2-2x alloy on the SiO2/Si substrate. Schematic image of the (b) WS2xSe2-2x alloy gas sensors. (c) Photographic image of reference sample (Pd on SiO2 substrate) and WS0.96Se1.04 alloy gas sensor. (d) I-V characteristics of WSe2, WS0.66Se1.34 alloy, WS0.96Se1.04 alloy, and WS2 gas sensors. (e) Energy band alignment of WSe2, WS2xSe2-2x alloy, WS2, and Pd. (f) Energy band diagram of WSe2 and WS2xSe2-2x alloy gas sensor junctions with Pd electrodes. (g) Gas-sensing property of WS0.96Se1.04 alloy gas sensor with NO2 exposure (10, 20, 50, 100, 200, and 500 ppm). Inset: Energy-band diagram of WS2xSe2-2x alloy gas sensor under bias and eff ect of NO2 gas exposure. (h) Response of WSe2, WS0.66Se1.34 alloy, WS0.96Se1.04 alloy, and WS2 gas sensor for NO2 exposure as a function of gas concentration.



Page 14 of 37

13

Next, we fabricated gas sensors using the synthesized large-area 3L WSe2 and WS2xSe2-2x alloy. As schematically shown in Figure 5a and Figure 5b, the WS2xSe2-2x alloy gas sensor was fabricated with thermally evaporated metal electrodes (Pd 50 nm).In this study, we selected the Pd as a metal electrode material which have been widely used for the p-type WSe2. Figure 5c shows a photograph of a reference sample (Pd on SiO2 substrate) and the gas sensors fabricated using the large-area 3L WS0.96Se1.04 alloy. The linear I-V characteristics of the gas sensors are shown in Figure 5d. Interestingly, the WS2xSe2-2x alloy and WS2 gas sensors exhibit lower current level than that of the WSe2 gas sensor. These reduced I-V characteristics of the WS2xSe2-2x alloy and WS2 gas sensors mean the increased resistance of the sensing materials, which results in the conversion from WSe2 to WS2xSe2-2x alloy. Figure 5e shows the predicted change in energy band alignment of WS2xSe2-2x depending on the S composition (x), which refers to the previous calculation studies.27,28 The literature values for the work-function of Pd range from 5.2 eV to 5.6 eV,29–31 whereas the reported values of the electron affinity and energy difference between vacuum level and valence band maximum for WSe2, WS2xSe2-2x alloy, and WS2 range from 3.5 eV to 3.9 eV and from 5.1 eV to 5.7 eV, respectively.27,28 On this basis, the work function of Pd is closest to the valence band of WSe2 and WS2xSe2-2x alloy, which would result in a negligible Schottky barrier for hole carriers, thereby causing the linear I-V characteristics shown in Figure 5d. As the S composition (x) is increased, the conduction band minimum and valence band maximum can be reduced.27,28 From this energy band alignment, the contact resistance between the valence band of WS2xSe2-2x alloy and Pd electrode can be increased compared to that of WSe2.19,32 By forming the junction with Pd electrodes, the energy band of WSe2 and WS2xSe2-2x alloy can be illustrated as shown in Figure 4f. The high Schottky barrier between Pd and the conduction band is favorable for blocking the electron transport, whereas the hole transport can flow through the valence band.


Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14

The gas-sensing property was evaluated with cyclic NO2 exposure (10–20–50–100–200– 500 ppm) at room-temperature, as shown in Figure 5g. After NO2 exposure for 20 min, N2 exposure was followed to recover the gas sensor for 50 min. All of the gas sensors (WSe2, WS0.66Se1.34 alloy, WS0.96Se1.04 alloy, and WS2) show similar current-change behavior upon NO2 exposure, as shown in Figure 5g, Figure S4b, Figure S4c, and Figure S4d. The current increases upon NO2 exposure, because NO2 gas acts as an electron acceptor (hole donor), which is known as the charge transfer mechanism for both WSe2 and WS2.33,34 The withdrawn electrons from WSe2 and WS2xSe2-2x alloy by adsorption of NO2 molecules increase the current of the gas sensor, because the hole concentration, which dominantly contributes to charge transport, can be increased. The gas-sensing mechanism of the WS2xSe22x

alloy gas sensor can be understood by the schematic energy-band diagram shown in the

inset of Figure 5g. The hole carrier transport of the WS2xSe2-2x alloy gas sensor is dominant under a constant voltage bias (+1 V). After NO2 exposure, the adsorbed NO2 molecules withdraw electrons from the WS2xSe2-2x alloy, resulting in an upward-bent valence band, leading to the current increase (increase in hole transport). Further, we can calculate the power consumption of WS0.96Se1.04 alloy gas sensor using result of Figure 5g. Our WS0.96Se1.04 alloy gas sensor consumes only 0.75 µW ~21 µW to operate (P = V x I; V = + 1 V, I = 0.75 µA ~ 21 µA), which is proper to apply the low-power operation of gas sensor applications. The response of the WSe2, WS0.66Se1.34 alloy, WS0.96Se1.04 alloy, and WS2 gas sensors was calculated for a quantitative estimate of sensor response. The sensor response is defined as

(%) =

− "

"

× 100 % =

∆

× 100 % (1)

"



Page 16 of 37

15

Here, I0 and Ig are the currents of the gas sensor before and after exposure to the target gas, respectively. The responses of the WSe2, WS0.66Se1.34 alloy, WS0.96Se1.04 alloy, and WS2 gas sensors are shown in Figure 5h for 10, 20, 50, 100, 200, and 500 ppm NO2. The WSe2 gas sensor indicates a positive response to NO2 sensing, increasing from 280% to 1113%. Meanwhile, the WS0.66Se1.34 and WS0.96Se1.04 alloy gas sensors show a considerably superior NO2-sensing ability. The WS0.66Se1.34 and WS0.96Se1.04 alloy gas sensors show 1793% and 2621% of response to 500 ppm NO2 exposure, respectively, which are ~1.6 and ~2.4 times higher than that of the WSe2 gas sensor. We performed further gas-sensing measurement under exposure of NH3 and CO gases to find selectivity which is important factor for the realtime application, as shown in Figure S5. The WS0.96Se1.04 alloy gas sensor shows negative response to NH3 and CO, because the NH3 and CO molecules are known as the electron donor.34 As summarized in Figure S5c, The WS0.96Se1.04 alloy gas sensor exhibits the highest response value to NO2, which means a good selectivity to various gases (NO2, NH3, and CO). Meanwhile, the NO2-response of WS2 gas sensor is much lowered and it is considered that result of significantly increased contact resistance as shown in Figure 5d. The gas sensing responses rely on the current (resistance) change of WSe2, WS0.66Se1.34 alloy, WS0.96Se1.04 alloy, and WS2 gas sensors upon the gas exposure. Total resistance (RTotal) of gas sensor is sum of two times the contact resistance (RC) and sensing material resistance (RS).7,35 Thus, we considered that the choice of proper electrode metal, which has low contact resistance, is also important for high performance gas sensor. The response value of our gas sensor to NO2 is superior to that observed in the recent works on 2D TMDC gas sensors.9,36–38 The comparison of room-temperature NO2-sensing properties of 2D TMDCs was summarized in Table 1 of Supporting Information. Further, the WS0.96Se1.04 alloy gas sensor exhibits good repeatability under 2 ppm NO2 as shown in Figure S6.


Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


16

We assumed that there are two possible reasons for the enhancement in the NO2-sensing response of the WS0.96Se1.04 alloy gas sensor. First, the increased response of WS2xSe2-2x alloy gas sensors can be caused by the electronic sensitization mechanism, which is based on the controlled carrier concentration. As previously reported, the hole concentration of WS2xSe2-2x alloy can be decreased through the sulfurization process of p-type WSe2, because WS2 is generally known as an n-type semiconductor.19 In addition, with the help of a previous calculated study based on the Bader charge analysis, we can estimate the charge accumulation at every S atom (0.031e), along with the charge depletion at every Se atom (0.058e) for the anion-mixed case of WS2xSe2-2x alloy.28 Because of the decreased hole carrier concentration of WS2xSe2-2x alloy, the variation in hole carrier concentration of WS2xSe2-2x alloy can be higher than that of WSe2 upon NO2 exposure, which acts as a hole donor (electron acceptor). Second, the degree of charge transfer to NO2 molecules, which depends on the kind of material, can be changed during the sulfurization process. From the previous calculated study, the NO2 molecule behaves as charge acceptor, which obtains 0.116 e and 0.178 e from WSe2 and WS2, respectively.33,34 However, the second opinion can be proved by a calculation study regarding the adsorption energy and degree of charge transfer for WS2xSe2-2x alloy. This needs much effort with other calculation study group, and we hope that this topic could be valuable as a follow-up study.



Page 18 of 37

17

Figure 6. (a) Photograph of flexible WS0.96Se1.04 alloy gas sensor. (b) NO2-sensing response of flexible WS0.96Se1.04 alloy gas sensor before and after the bending test. (c) Summarized NO2-sensing response of the flexible WS0.96Se1.04 alloy gas sensor. (d), (e) Wearable NO2detecting wrist band system composed of the flexible WS0.96Se1.04 alloy gas sensor, microcontroller board (Arduino Uno), breadboard, LEDs, resistors, and wires. Inset: Circuit design of the NO2 detector. Different states are exhibited using the LEDs upon (f) 0 ppm, (g) 10 ppm, (h) 20 ppm, (i) 50 ppm, and (j) 100 ppm of NO2 exposure. Next, we designed a flexible NO2 gas sensor to demonstrate the practical potential of advanced gas-sensing devices using 2D TMDCs. An as-synthesized 3L WS0.96Se1.04 alloy gas sensor was transferred to a PET substrate by a standard PMMA transfer process. Figure 6a shows the fabricated flexible gas sensor. To evaluate the bending stability of our flexible gas sensor, a cyclic bending test was carried out as shown in the inset of Figure 6b (bending radius: 2.5 mm). As shown in Figure 6b, a slightly increased NO2-sensing response was exhibited after 5000 cycles of the bending test. The value of the sensing response further ACS Paragon Plus Environment

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18

increased after 10000 cycles of the bending test. Figure 6c shows the summarized NO2sensing response for the bending test. The detailed sensing results of flexible WSe2 gas sensor are shown in Figure S7. This increased NO2-sensing response after the bending test can be attributed to the result of increased reactive edge sites such as wrinkles or microcracks by strain force.36,39,40 Generally, the edge sites of 2D TMDCs have stronger adsorption energy than the basal plane. Recent work demonstrated that more edge sites of TMDCs obtained higher response.41 Similarly, the flexible MoS2 gas sensor exhibits slightly increased response after bending test.37 In addition, we analyzed the stability of the flexible gas sensor over time to estimate the operational shelf life. We stored the flexible gas sensors at the vacuum environment by sealing in vacuum package at room-temperature. After three months, the response of the flexible WS0.96Se1.04 and WSe2 gas sensors exhibited 1076% and 512% of response to 100 ppm NO2, which are only 5% and 7% variation to the initial response value as shown in Figure 6c. Thus, we can expect that our flexible gas sensor has excellent stability for real-life applications. Furthermore, we prepared a wearable NO2-detecting wristband to demonstrate the applicability of wearable sensing devices using 2D WS0.96Se1.04 alloy. Figure 6d shows the wearable NO2-detecting wristband system, which is composed of the flexible WS0.96Se1.04 alloy gas sensor, microcontroller board, breadboard, LEDs, resistors, and wires. In this work, we used the Arduino Uno, which is a well-known open-source microcontroller board based on the ATmega328P. The inset of Figure 6d shows the circuit design of the NO2 detector module. The resistance of the WS0.96Se1.04 alloy gas sensor inputs into the microcontroller as an analog signal, and the digital signals output to five LEDs depending on the states. The WS0.96Se1.04 alloy gas sensor and the LED module were placed on the wearable wristband, which was prepared using PET, as shown in Figure 6e. In order to demonstrate the roomtemperature wearable NO2 detector, the NO2 gas was delivered by a gas line using a mass



Page 20 of 37

19

flow controller system, as shown in Figure 6f and Movie S1 (see Supporting Information). Depending on the NO2 concentration, the resistance of WS0.96Se1.04 is changed; the different states are shown by the LEDs. In Figure 6f–6j, the different states using the LEDs with NO2 exposure (0 ppm, 10 ppm, 20 ppm, 50 ppm, and 100 ppm) are displayed. During the operation of the NO2 detector, the first white LED turns on to show that the system is working normally (Figure 6f). The second green LED turns on when the NO2 gas is detected over 10 ppm (Figure 6g). The third and fourth green LEDs turn on when the NO2 gas is detected over 20 ppm and 50 ppm (Figure 6h and 6i). If the NO2 gas is detected over 100 ppm, the red LED turns on. Interestingly, our NO2 detector consumes only 2–3 µW to operate the sensing part of the system (Ps = Vc2xRs/(Rs + RL)2), whereas the SnO2-based commercial semiconductor gas sensor, which is integrated with a metal heater, consumes over 900 mW. Thus, we believe that the 2D TMDC-based high-performance gas sensor will surpass the previous semiconductor gas sensor in the field of low-power sensing systems.

Conclusion In conclusion, this work provided a solid foundation to prove that the sulfurization process can serve as a reliable approach to form large-area uniform WS2xSe2-2x alloys with systemically controlled composition and an optical bandgap. Further, the significantly improved gas-sensing performance of large-area WS2xSe2-2x alloy was demonstrated; it overcomes the limitations of 2D TMDC gas sensors. The WS0.96Se1.04 alloy gas sensor exhibited over 2.4 times enhanced response (2621%) to 500 ppm NO2, compared to a WSe2 gas sensor. These are the highest values of response to NO2 for 2D TMDCs gas sensors reported to date. In addition, we showed the potential of advanced gas-sensing devices by demonstrating the low-power-operating wearable NO2 detector. We anticipate that this work could advance technical approaches for tailoring the composition of atomically thin-layered


Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

2D TMDC alloys and provide critical and imperative scientific insights for gas sensor applications.

Methods and Materials Synthesis of WS2xSe2-2x Alloy: The synthesis of WSe2 was carried out in a tube-type furnace system. The WSe2 was synthesized on clean SiO2 (285 nm)/Si substrates by CVD using tungsten hexachloride (WCl6) and diethyl selenide (DESe). Large-area, uniform WSe2 was produced at 700 °C of growth temperature and 1.0 Torr of base pressure, and this growth condition was fixed for reproducibility. The layer number was controlled by deposition time. The sulfurization process of WSe2 for the preparation of WS2xSe2-2x alloy was conducted in a tube-type furnace system using hydrogen sulfide (H2S) and hydrogen (H2) gas (Supporting Information, Figure S1a). The sulfurization process was carried out through three consecutive steps, as shown in Figure S1b. Step I: The chamber was heated to the target sulfurization temperature under Ar (50 s.c.c.m.) flow. When the temperature reached 200 °C, the flow of H2S (20 s.c.c.m.) and H2 (20 s.c.c.m.) was started, and Ar flow was stopped. Step II: After reaching the target temperature, the H2S and H2 gas flow was maintained for 30 min of sulfurization time. The sulfurization process was systemically conducted by controlling the sulfurization temperature (500 ºC, 600 ºC, 700 ºC, 800 ºC, 900 ºC, 1000 ºC, and 1100 ºC). After the sulfurization process, the chamber was cooled without a change in gas flow to 200 ºC. Step III: The H2S and H2 lines were closed, and Ar (50 s.c.c.m.) flowed into the chamber. Through the sulfurization process, the WSe2 converted to WS2xSe2-2x (0 < x < 1) alloy and WS2 as illustrated in Figure 1a.

Gas Sensor Fabrication: Pd electrodes (50 nm) were deposited on the WSe2 and WS2xSe22x

alloy using a thermal evaporator. A stainless steel shadow mask was used to form the

electrode patterns. The shadow mask was designed as an interdigitated electrode. The



Page 22 of 37

21

dimensions of the substrate were 15 mm × 25 mm, and the channel width-to-length ratio of the electrode was approximately 300 with a 100-µm distance of channel length. To fabricate the flexible gas sensor, synthesized WS2xSe2-2x alloy was transferred onto polyethylene terephthalate (PET) substrate before depositing the Pd electrodes. We used a wet transfer process using a poly(methyl methacrylate) (PMMA) supporting layer. After transfer of the PMMA/WS2xSe2-2x alloy film, the PMMA was removed by dipping in acetone solution for ~1 h; the sample was then rinsed in isopropyl alcohol.

Characterization of WS2xSe2-2x Alloy: Raman spectroscopy and photoluminescence (PL) were measured using a LabRam ARAMIS (HORIBA, 532-nm wavelength laser). X-ray photoelectron spectroscopy (XPS) was measured using a K-Alpha model (Thermo Scientific Co.). Atomic force microscopy (AFM) was measured using a multimode scanning microscope (Veeco), and a Tecnai G2 F20 S-TWIN (accelerating voltage, 80 kV) were employed to characterize the transmission electron microscopy (TEM) images of the WS2xSe2-2x alloy. I-V curves were measured using a source meter (Keithley 2400).

Gas-Sensing Experiment: The sensing performance of the fabricated sensors was monitored in a sealed gas-sensing chamber. An electrical feed-through and gas inlet and outlet were installed in the chamber. We used N2 as a purging gas. The target gas was diluted with N2, and the operating temperature was room-temperature (27 °C). The operating temperature was controlled using a furnace system. Also, the gas sensors measured under dark environment to avoid light effect by sealing the gas-sensing chamber using aluminum foil. The concentration of each gas was adjusted by controlling the flow rates of the target gas and balance gas (N2), which was achieved using mass-flow controllers. The target gas and purging gas were exposed for 20 min and 50 min for each cycle of the gas-sensing test, respectively. Before target-gas injection, the gas sensors were stabilized under N2 exposure for ~1 h. Input voltage (+1 V) was biased to the source/drain electrodes of the sensors, and


Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


22

the real-time output signal (current) was acquired by the source meter (Keithley 2400). Both the WSe2 and WS2xSe2-2x alloy gas sensors were tested under the same sensing setup and gas flow conditions.

ASSOCIATED CONTENT

Supporting Information. Schematic illustration of the synthetic equipment for WS2xSe2-2x alloy, Resolved XPS spectra of WS2xSe2-2x alloy, Uniformity of large-area synthesized WS2xWSe2-2x alloy, NO2-sensing results of WSe2, WS0.66Se1.34 alloy, and WS2 gas sensor, Gas-sensing response of WS0.96Se1.04 alloy gas sensor upon NH3 and CO2, Comparison of NO2-sensing results of 2D TMDCs, Cyclic NO2-sensing result, Stability of flexible gas sensors.

AUTHOR INFORMATION

Corresponding Author Hyungjun Kim* School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-Ro, Seodaemun-Gu, Seoul, 03722, Rep. of Korea *E-mail: [email protected], Tel: 82-2-2123-5773, Fax: 82-2-313-2879

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT



Page 24 of 37

23

This work was supported by the following: the Materials and Components Technology Development Program of MOTIE/KEIT. [10080527, Development of commercialization technology of high sensitive gas sensor based on chalcogenide 2D nano material]; and a grant from the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (No. NRF-2014R1A2A1A11052588); Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology(No.2015R1D1A1A01060064).


Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


24

REFERENCES (1)

Swan, M. Sensor Mania! The Internet of Things, Wearable Computing, Objective Metrics, and the Quantified Self 2.0. J. Sens. Actuator Networks 2012, 1, 217–253.

(2)

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. (3)

Malik, R.; Tomer, V. K.; Dankwort, T.; Mishra, Y. K.; Kienle, L. Cubic Mesoporous Pd-WO3loaded Graphitic Carbon Nitride (g-CN) Nanohybrids: Highly Sensitive and Temperature Dependent VOC Sensors. J. Mater. Chem. A 2018, 6, 10718–10730.

(4)

Malik, R.; Tomer, V. K.; Chaudhary, V.; Dahiya, M. S.; Sharma, A.; Nehra, S. P.; Duhan, S.; Kailasam, K. An Excellent Humidity Sensor Based on In–SnO 2 Loaded Mesoporous Graphitic Carbon Nitride. J. Mater. Chem. A 2017, 5, 14134–14143.

(5)

Abbas, A. N.; Liu, B.; Chen, L.; Ma, Y.; Cong, S.; Aroonyadet, N.; Köpf, M.; Nilges, T.; Zhou, C. Black Phosphorus Gas Sensors. ACS Nano 2015, 9, 5618–5624.

(6)

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.

(7)

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.

(8)

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.; Rao, C. N. R. Sensing Behavior of Atomically Thin-Layered MoS2 Transistors. ACS Nano 2013, 7, 4879–4891.

(9)

Ko, K. Y.; Song, J. G.; Kim, Y.; Choi, T.; Shin, S.; Lee, C. W.; Lee, K.; Koo, J.; Lee, H.; Kim, J.; Lee, T.; Park, J; Kim, H. Improvement of Gas-Sensing Performance of Large-Area Tungsten Disulfide Nanosheets by Surface Functionalization. ACS Nano

2016, 10, 9287–9296. (10)

Huo, N.; Yang, S.; Wei, Z.; Li, S.-S.; Xia, J.-B.; Li, J. Photoresponsive and Gas Sensing Field-Effect Transistors Based on Multilayer WS2 Nanoflakes. Sci. Rep. 2015,

4, 5209. (11)

Hong, Y.; Kang, W.-M.; Cho, I.-T.; Shin, J.; Wu, M.; Lee, J.-H. Gas-Sensing



Page 26 of 37

25

Characteristics of Exfoliated WSe 2 Field-Effect Transistors. J. Nanosci. Nanotechnol.

2017, 17, 3151–3154. (12)

Late, D. J.; Doneux, T.; Bougouma, M. Single-Layer MoSe2 Based NH3 Gas Sensor.

Appl. Phys. Lett. 2014, 105, 233103. (13)

Sarkar, D.; Xie, X.; Kang, J.; Zhang, H.; Liu, W.; Navarrete, J.; Moskovits, M.; Banerjee, K. Functionalization of Transition Metal Dichalcogenides with Metallic Nanoparticles: Implications for Doping and Gas-Sensing. Nano Lett. 2015, 15, 2852– 2862.

(14)

Kugishima, M.; Shimanoe, K.; Yamazoe, N. C2H4O Sensing Properties for Thick Film Sensor Using La2O3-Modified SnO2. Sensors Actuators, B Chem. 2006, 118, 171–176.

(15)

Liu, L.; Zhang, Y.; Wang, G.; Li, S.; Wang, L.; Han, Y.; Jiang, X.; Wei, A. High Toluene Sensing Properties of NiO-SnO2 Composite Nanofiber Sensors Operating at 330 ??C. Sensors Actuators, B Chem. 2011, 160, 448–454.

(16)

Liu, L.; Zhong, Z.; Wang, Z.; Wang, L.; Li, S.; Liu, Z.; Han, Y.; Tian, Y.; Wu, P.; Meng, X. Synthesis, Characterization, and m-Xylene Sensing Properties of Co-ZnO Composite Nanofibers. J. Am. Ceram. Soc. 2011, 94, 3437–3441.

(17)

Choi, S. Y.; Kim, Y.; Chung, H. S.; Kim, A. R.; Kwon, J. D.; Park, J.; Kim, Y. L.; Kwon, S. H.; Hahm, M. G.; Cho, B. Effect of Nb Doping on Chemical Sensing Performance of Two-Dimensional Layered MoSe2. ACS Appl. Mater. Interfaces 2017,

9, 3817–3823. (18)

Song, J.-G.; Ryu, G. H.; Lee, S. J.; Sim, S.; Lee, C. W.; Choi, T.; Jung, H.; Kim, Y.; Lee, Z.; Myoung, J.-M.; Dussarrat, C.; Lansalot-Matras, C.; Park, J.; Choi, H.; Kim, H. Controllable Synthesis of Molybdenum Tungsten Disulfide Alloy for Vertically Composition-Controlled Multilayer. Nat. Commun. 2015, 6, 7817.

(19)

Duan, X. X.; Wang, C.; Fan, Z.; Hao, G.; Kou, L.; Halim, U.; Li, H.; Wu, X.; Wang, Y.; Jiang, J.; Pan, A.; Huang, Y.; Yu, R.; Duan, X. Synthesis of WS2xSe2-2x Alloy Nanosheets with Composition-Tunable Electronic Properties. Nano Lett. 2016, 16, 264–269.

(20)

Li, Y.; Xu, C.-Y.; Wang, J.-Y.; Zhen, L. Photodiode-Like Behavior and Excellent Photoresponse of Vertical Si/Monolayer MoS2 Heterostructures. Sci. Rep. 2015, 4,


Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


26

7186. (21)

Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. Single-Layer MoS2 Phototransistors. ACS Nano 2012, 6, 74–80.

(22)

Lee, H. S.; Min, S. W.; Chang, Y. G.; Park, M. K.; Nam, T.; Kim, H.; Kim, J. H.; Ryu, S.; Im, S. MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap. Nano Lett. 2012, 12, 3695–3700.

(23)

Tsai, M. L.; Su, S. H.; Chang, J. K.; Tsai, D. S.; Chen, C. H.; Wu, C. I.; Li, L. J.; Chen, L. J.; He, J. H. Monolayer MoS2 Heterojunction Solar Cells. ACS Nano 2014, 8, 8317–8322.

(24)

Li, X.; Lin, M. W.; Basile, L.; Hus, S. M.; Puretzky, A. A.; Lee, J.; Kuo, Y. C.; Chang, L. Y.; Wang, K.; Idrobo, J. C.; Li, A.-P.; Chen, C.-H.; Rouleau, C. M.; Geohegan, D.B.; Xiao, K. Isoelectronic Tungsten Doping in Monolayer MoSe2 for Carrier Type Modulation. Adv. Mater. 2016, 8240–8247.

(25)

Gong, Q.; Cheng, L.; Liu, C.; Zhang, M.; Feng, Q.; Ye, H.; Zeng, M.; Xie, L.; Liu, Z.; Li, Y. Ultrathin MoS2(1-x)Se2x Alloy Nanoflakes for Electrocatalytic Hydrogen Evolution Reaction. ACS Catal. 2015, 5, 2213–2219.

(26)

Campbell, P. M.; Tarasov, A.; Joiner, C. A.; Tsai, M.-Y.; Pavlidis, G.; Graham, S.; Ready, W. J.; Vogel, E. M. Field-Effect Transistors Based on Wafer-Scale, Highly Uniform Few-Layer p-Type WSe 2. Nanoscale 2016, 8, 2268–2276.

(27)

Kang, J.; Tongay, S.; Li, J.; Wu, J. Monolayer Semiconducting Transition Metal Dichalcogenide Alloys: Stability and Band Bowing. J. Appl. Phys. 2013, 113.

(28)

Yang, J.-H.; Yakobson, B. I. Unusual Negative Formation Enthalpies and Atomic Ordering in Isovalent Alloys of Transition Metal Dichalcogenide Monolayers. Chem.

Mater. 2018, 30, 1547–1555. (29) Li, S.; Wang, S.; Tang, D. M.; Zhao, W.; Xu, H.; Chu, L.; Bando, Y.; Golberg, D.; Eda, G. Halide-Assisted Atmospheric Pressure Growth of Large WSe2 and WS2 Monolayer Crystals. Appl. Mater. Today 2015, 1, 60–66. (30)

Murata, Y.; Starodub, E.; Kappes, B. B.; Ciobanu, C. V.; Bartelt, N. C.; McCarty, K. F.; Kodambaka, S. Orientation-Dependent Work Function of Graphene on Pd(111).

Appl. Phys. Lett. 2010, 97, 1–4.



Page 28 of 37

27

(31)

Kang, J.; Liu, W.; Sarkar, D.; Jena, D.; Banerjee, K. Computational Study of Metal Contacts to Monolayer Transition-Metal Dichalcogenide Semiconductors. Phys. Rev.

X 2014, 4, 1–14. (32)

Susarla, S.; Kutana, A.; Hachtel, J. A.; Kochat, V.; Apte, A.; Vajtai, R.; Idrobo, J. C.; Yakobson, B. I.; Tiwary, C. S.; Ajayan, P. M. Quaternary 2D Transition Metal Dichalcogenides (TMDs) with Tunable Bandgap. Adv. Mater. 2017, 1702457, 1–8.

(33)

Zhou, C.; Yang, W.; Zhu, H. Mechanism of Charge Transfer and Its Impacts on FermiLevel Pinning for Gas Molecules Adsorbed on Monolayer WS2. J. Chem. Phys. 2015,

142, 214704. (34)

Wang, T.; Zhao, R.; Zhao, X.; An, Y.; Dai, X.; Xia, C. Tunable Donor and Acceptor Impurity States in a WSe 2 Monolayer by Adsorption of Common Gas Molecules. RSC

Adv. 2016, 6, 82793–82800. (35)

Hoefer, U.; Steiner, K.; Wagner, E. Contact and Sheet Resistance of SnO2thin Films from Transmission-Line Model Measurements. “Sensors and Actuators, B: Chemical,” 1995, 26, 59–63.

(36)

Cho, S.-Y.; Kim, S. J.; Lee, Y.; Kim, J.-S. J. J.-S. J.; Jung, W.-B.; Yoo, H.-W.; Kim, J.-S. J. J.-S. J.; Jung, H.-T. Highly Enhanced Gas Adsorption Properties in Vertically Aligned MoS2 Layers. ACS Nano 2015, 9, 9314–9321.

(37)

Zhao, Y.; Song, J. G.; Ryu, G. H.; Ko, K. Y.; Woo, W. J.; Kim, Y.; Kim, D.; Lim, J. H.; Lee, S.; Lee, Z.; Park, J.; Kim, H. Low-Temperature Synthesis of 2D MoS2 on a Plastic Substrate for a Flexible Gas Sensor. Nanoscale 2018, 10, 9338–9345.

(38)

Cui, S.; Wen, Z.; Huang, X.; Chang, J.; Chen, J. Stabilizing MoS2 Nanosheets through SnO2 Nanocrystal Decoration for High-Performance Gas Sensing in Air. Small 2015,

11, 2305-2313. (39)

Park, M.; Park, Y. J.; Chen, X.; Park, Y. K.; Kim, M. S.; Ahn, J. H. MoS2-Based Tactile Sensor for Electronic Skin Applications. Adv. Mater. 2016, 28, 2556–2562.

(40)

Wang, S.; Qin, Z.; Jung, G. S.; Martin-Martinez, F. J.; Zhang, K.; Buehler, M. J.; Warner, J. H. Atomically Sharp Crack Tips in Monolayer MoS2and Their Enhanced Toughness by Vacancy Defects. ACS Nano 2016, 10, 9831–9839.

(41)

Cho, S.-Y.; Kim, S. J.; Lee, Y.; Kim, J.-S.; Jung, W.-B.; Yoo, H.-W.; Kim, J.; Jung, H.-T. Highly Enhanced Gas Adsorption Properties in Vertically Aligned MoS2 Layers. ACS Paragon Plus Environment

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28

ACS Nano 2015, 9, 9314–9321.



Page 30 of 37

29

Graphical TOC Entry


Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1 137x97mm (300 x 300 DPI)



Figure 2 141x119mm (300 x 300 DPI)


Page 32 of 37

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3 141x100mm (300 x 300 DPI)



Figure 4 133x80mm (300 x 300 DPI)


Page 34 of 37

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5 146x128mm (300 x 300 DPI)



Figure6 141x95mm (300 x 300 DPI)


Page 36 of 37

Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ToC 54x49mm (300 x 300 DPI)


High-Performance Gas Sensor Using a Large-Area WS2xSe2â2x

Recommend Documents

High-Performance Gas Sensor Using a Large-Area WS2xSe2â2x