Effect of Nb Doping on Chemical Sensing Performance of Two

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Effect of Nb doping on chemical sensing performance of two-dimensional layered MoSe2 Sun Young Choi, Yonghun Kim, Hee-Suk Chung, Ah Ra Kim, Jung-Dae Kwon, Jucheol Park, Young Lae Kim, Se-Hun Kwon, Myung Gwan Hahm, and Byungjin Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14551 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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ACS Applied Materials & Interfaces

Effect of Nb doping on chemical sensing performance of two-dimensional layered MoSe2 Sun Young Choi,1,5,† Yonghun Kim,1,† Hee-Suk Chung,2,† Ah Ra Kim,1 Jung-Dae Kwon,1 Jucheol Park,3 Young Lae Kim,4 Se-Hun Kwon,5,* Myung Gwan Hahm,6,* and Byungjin Cho1,*

1

Department of advanced Functional Thin Films, Surface Technology Division, Korea Institute of Materials Science (KIMS), 797 Changwondaero, Sungsan-Gu, Changwon, Gyeongnam 51508, Republic of Korea

2

Analytical Research Division, Korea Basic Science Institute, Jeonju, Jeollabuk-do, 54907, Republic of Korea

3

Structure Analysis Group, Gyeongbuk Science & Technology Promotion Center, Future Strategy Research Institute, 17 Cheomdangieop 1-ro, Sangdong-myeon, Gumi, Gyeongbuk 39171, Republic of Korea

4

Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115, United States

5

School of Materials Science and Engineering, Pusan National University, 30 Jangjeon-Dong Geumjeong-Gu, Busan 609-735, Republic of Korea

6

Department of Materials Science and Engineering, Inha University, 100 Inharo, Nam-Gu, Incheon 22212, Republic of Korea

KEYWORDS. two-dimensional layered MoSe2, Nb doping, chemical sensing †

These authors contributed equally to this work.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected]; [email protected]

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ABSTRACT Here, we report that Nb doping of two-dimensional (2D) MoSe2 layered nanomaterials is a promising approach to improve their gas sensing performance. In this study, Nb atoms were incorporated into a 2D MoSe2 host matrix, and the Nb doping concentration could be precisely controlled by varying the number of Nb2O5 deposition cycles in the plasma enhanced atomic layer deposition process. At relatively low Nb dopant concentrations, MoSe2 showed enhanced device durability as well as NO2 gas response, attributed to its small grains and stabilized grain boundaries. Meanwhile, an increase in the Nb doping concentration deteriorated the NO2 gas response. This might be attributed to a considerable increase in the number of metallic NbSe2 regions, which do not respond to gas molecules. This novel method of doping 2D transition metal dichalcogenide-based nanomaterials with metal atoms is a promising approach to improve the performance such as stability and gas response of 2D gas sensors.


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Introduction Two-dimensional (2D) atomic-layered materials such as graphene and transition metal dichalcogenides (TMDs) have attracted much attention as promising materials for chemical sensing devices owing to their large surface-to-volume ratios and extremely sensitive surfaces.1–4 Graphene-based chemical sensors have been intensively investigated in the past few years since a field effect transistor-based sensing platform with mechanically exfoliated single-layer graphene exhibited an excellent gas sensing capability. However, in principle, the zero-bandgap of graphene gives rise to its weak switching characteristics.5 This inherent drawback of graphene can be overcome by using TMDs in sensing applications. For example, a MoS2 sensor showed a strong response upon exposure to triethylamine and a highly selective reactivity to a range of analytes compared to carbon-based nanomaterials (carbon nanotubes and graphene).1 Besides the fundamental studies on gas sensing characteristics of mechanically exfoliated TMDs,1–4 recent advancements in large-scale synthesis processes using chemical vapour deposition (CVD) have accelerated the development of gas sensing devices for practical applications.6–10 More recently, a few methods such as the development of vertically aligned 2D structures,11 nanoparticle decoration,12–14 surface chemical functionalization,15,16 and the development of 2D heterostructures17–19 and nanocomposites20,21 have been proposed for enhancing the gas sensitivity of devices. Nevertheless, these techniques rarely address the longterm stability of gas sensing devices. Incorporation of substitutional impurities (stable dopants) into a host lattice can modulate its electrical properties through the donation of electrons or holes to the conduction or valence bands, respectively. In this context, the lattice doping of 2D layered nanomaterials would be an effective method to realize practical gas sensors with reliable and reproducible gas sensing properties. Herein, we investigated the effect of Nb doping on 2D MoSe2 host films to realize their highly sensitive and stable gas sensing characteristics when exposed to NO2 gas. The Nb doping concentration could be precisely controlled by using a plasma enhanced atomic layer deposition 3 ACS Paragon Plus Environment


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(PEALD)-based Nb2O5 deposition process. The high-resolution transmission electron microscopy (HR-TEM) analysis revealed that Nb doping resulted in an increase in the number of grains in MoSe2 and stabilized the grain boundaries, thereby increasing the surface-tovolume ratio. On the other hand, an increase in the Nb doping concentration decreased the gas response. This is attributed to the considerable increase in the number of NbSe metallic regions on the sensing channel. This novel approach is expected to lead to high-performance 2D gas sensors.

Experimental section Synthesis of 2D TMDs materials C-plane sapphire was cleaned using a typical cleaning process (sonication in acetone, isopropyl alcohol, and deionized water for 10 min each). First, 3 nm-thick layers of molybdenum trioxide (MoO3) were deposited on the sapphire substrate using a thermal evaporator. Subsequently, niobium pentoxide (Nb2O5) layers were deposited on the pre-deposited MoO3 film for Nb doping. The thickness of these Nb2O5 layers was precisely controlled by varying the number of deposition cycles in the plasma-enhanced atomic layer deposition (PEALD) process. The Nb2O5 layer thickness was 0.8 and 1.6 nm when one and five PEALD cycles were used, respectively. The samples were loaded at the centre of a furnace and selenium powder was placed on a source flange heater. The temperature of the furnace and source flange heater was increased to ~1000 and 500 ℃, respectively. Thermal selenization was carried out for an hour under a flow of Ar/H2 mixing gases and at a pressure of 800 Torr. Mixed layers consisting of MoO3 and Nb2O5 finally resulted in the formation of the Nb-doped MoSe2 layered structure (MoxNb1-xSe2). The PEALD process used in this study enabled a delicate doping of Nb atoms on the MoSe2 matrix. We also prepared MoSe2 as a reference material along with the Nb-doped MoSe2 obtained after one and five PEALD cycles (denoted as MoSe2:Nb 1C and MoSe2:Nb 5C). 4 ACS Paragon Plus Environment

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ADF-STEM characterization The atomic-resolution annular dark-field scanning transmission electron microscopy (ADFSTEM) images were captured using a scanning transmission electron microscope (JEOL-ARM 200F) operating at 200 kV with a spherical aberration corrector (CEOS GmbH). The optimal size of the electron probe was less than 1.0 Å . The collection semi-angles of the ADF detector were varied from 90 to 200 mrad. The scanning rate of each frame was typically 6 µs per pixel and each frame consisted of 512×512 pixels. The STEM-energy dispersive X-ray spectroscopy (EDS) analysis was performed using an EDS detector (SDD type 80T) and an analysis software (AZtecTEM, Oxford).

Fabrication of gas-sensing devices Using a rectangular active shadow mask, MoO3 films (3 nm) were thermally deposited on the pre-cleaned sapphire substrate. For Nb doping, Nb2O5 films were deposited using one and five PEALD cycles that correspond to 0.8 and 1.6 nm. The oxide-based precursor films were selenized by a thermal CVD process. Then, using a shadow mask with an interdigitated electrode (IDE) array structure (having an electrode width of 400 μm and a gap of 100 μm), a Au/Cr film (50/3 nm) (for electrode) was deposited on the MoSe2-based 2D materials. Cr acts as an adhesion layer for Au.

Gas sensing measurement The gas sensing responses of the 2D MoSe2-based devices were tested in the presence of NO2 gas diluted with ultrahigh purity (99.999%) N2 gas in a closed chamber system. Their resistance changes were monitored at intervals of 1 s at a voltage of 5 V (supplied by a Keithley 2401 source meter). The devices were exposed to NO2 gas for only 1 min. After that, recovery process 5 ACS Paragon Plus Environment


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of the sensing measurement was carried out by supplying N2 gas for 30 min and maintaining a temperature of 150 °C during the entire process to accelerate the recovery speed. The devices were stored under a vacuum condition during the measurement of their long-term stabilities.

Results and discussions

Figure 1a shows the schematic of the atomic structures for the MoSe2-based layered materials (MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C). It illustrates that Nb atoms, which thermally diffused to the Mo sites, formed a ternary TMD alloy with an atomic configuration of

Figure 1. (a) Schematic atomic structures of MoSe2 (top), MoSe2:Nb 1C (middle), and MoSe2:Nb 5C (bottom). (b) Raman spectra recorded of the assynthesized MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C films. Cross-sectional HR-TEM images of the (c) MoSe2, (d) MoSe2:Nb 1C, and (e) MoSe2:Nb 5C films. Scale bar is 5 nm. 6 ACS Paragon Plus Environment

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W−Nb−Se. This is because, Mo atoms (closed black circles), vertically sandwiched by selenium atoms (closed yellow circles), could be substituted by the Nb atoms (closed red circles). The formation of 2D layers by the CVD process was confirmed by Raman spectroscopy (532 nm). Figure 1b shows the Raman spectra of the MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C samples. The Raman spectrum of MoSe2 showed two typical vibrational modes; A1g and E12g corresponding to the out-of-plane vibration of Se atoms and the in-plane vibration of Mo and Se atoms, respectively.22,23 The addition of substitutional Nb atoms to the MoSe2 matrix (MoSe2:Nb 1C) caused lattice vibrations (the E12g peak of NbSe2). The MoSe2:Nb 5C film with a relatively large number of Nb atoms showed relatively weak intensity and band broadening because of the overlapping between the Mo-Se and Nb-Se vibrational modes. However, the A1g mode of NbSe2 was not observed even in the Raman spectrum of the MoSe2:Nb 5C, indicating that the vibration of the spatially distributed Nb-Se atomic bonds might be too weak. Figures 1c–1e show the cross-sectional high resolution TEM images of the as-synthesized 2D layered crystal structures. These images were captured to determine the number and quality of layers deposited on the sapphire substrate. The number of layers in the MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C films were measured to be approximately 1–2, 2, and 2–3, respectively. It clearly indicates that an increase in the Nb doping concentration resulted in a slight increase in the number of layers in the 2D films. Notably, the highly-doped MoSe2:Nb 5C sample showed more disconnected layers. It might be because of the lattice distortion due to the variation in the lattice distance between Mo-Se and Nb-Se. Such distortion damages the perfect honey comb structure of ternary TMDs.24,25



a

Se

MoSe2

b

Mo

Se

Se Se

c

MoSe2:Nb 1C Mo

Se

Se

MoSe2:Nb 5C Nb

Nb

Mo

Nb

Nb

5

10

Intensity (a.u.)

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4

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3

10

Se

Se

d

Mo

e

Se

Mo

Nb

f

Mo Nb

Nb

2

10

MoSe2

MoSe2:Nb 1C

MoSe2:Nb 5C

1

10

0

3

6

Depth (nm)

9

0

3

6

Depth (nm)

9

0

3

6

Depth (nm)

9

Figure 2. TEM-EDS spectra of the (a) MoSe2, (b) MoSe2:Nb 1C, and (c) MoSe2:Nb 5C films. Nb dopant atoms (orange arrow in Figure 1b and 1c) were detected on only the MoSe2:Nb 1C and MoSe2:Nb 5C samples. SIMS depth profiles of Mo, Nb, and Se atoms on (d) MoSe2, (e) MoSe2:Nb 1C, and (f) MoSe2:Nb 5C To determine the composition of the 2D layered materials, EDS and secondary ion mass spectrometry (SIMS) depth profile analyses were carried out. As shown in Figures 2a–2c, Mo and Se were found in the entire sample while Nb was detected in only two samples i.e. in MoSe2:Nb 1C and MoSe2:Nb 5C. From the EDS data, atomic ratio of each film were also measured as follows: Mo1.07Se2, Mo0.71Nb0.14Se2, and Mo0.66Nb0.16Se2. It was also obvious that vacant sites of transition metal atoms (Mo or Nb) gradually increased with increasing Nb doping concentration. Thus, the related defect chemistry of the synthesized 2D materials also should be carefully considered. As expected, MoSe2:Nb 5C had a higher Nb concentration than MoSe2:Nb 1C. The elemental depth profiling of Mo, Nb, and Se atoms for the three samples were obtained from the SIMS analysis (Figures 2d–2f). As compared to MoSe2, in the Nbdoped MoSe2 samples (MoSe2:Nb 1C and MoSe2:Nb 5C), Mo atoms were observed in deeper regions, indicating an increase in the total number of layers in the Nb-doped MoSe2 samples. These results are consistent with the TEM analysis results (Figure 1c–1e). The important observation made from the SIMS elemental analysis is that the concentration of Nb in the three samples followed the order: MoSe2 < MoSe2:Nb 1C < MoSe2:Nb 5C (for clarity, the dotted 8 ACS Paragon Plus Environment

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grey line was used as the reference level). The presence of C and H in the samples is attributed to the substrate impurities and exposure to hydrogen gas during the synthesis process (Figure S1 in Supplementary Information (SI)). The X-ray photoelectron spectroscopy (XPS) analysis could not detect the presence of Nb in MoSe2:Nb 1C because of the low concentration of Nb in this sample (Figure S2 in SI).

-1

10

Current (A)

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MoSe2:Nb 5C

-3

10

MoSe2:Nb 1C

-5

10

-7

10

MoSe2

-9

10

-4

-2

0

2

Voltage (V)

4

Figure 3. I−V properties of the MoSe2, MoSe2:Nb 1c, and MoSe2:Nb 5c devices over a voltage sweep range of −5 to 5 V.

Next, we investigated the electrical properties of the MoSe2-, MoSe2:Nb 1C-, and MoSe2:Nb 5C-based devices. Figure 3 shows a large difference in the electrical properties of each of these devices in a voltage sweep range of − 5 to 5 V. All the devices showed a linear current-voltage relationship and showed a Ohmic behaviour, indicating the overall resistance of the devices was primarily governed by the 2D channel itself rather than the contact characteristics. An increase in the Nb doping concentration resulted in an abrupt current increase, because of the metallic characteristics of NbSe2.19,24 With an increase in the Nb doping



concentration, the MoSe2 layered structure experienced a doping-induced semiconductor-metal transition.

b 12

0

-10 3 ppm 5 ppm 10 ppm

MoSe2

-20

MoSe2:Nb 1C

50 ppm

MoSe2:Nb 5C

0

50

Gas response (%)

a Gas response (%)

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MoSe2:Nb 1C

9 6

MoSe2

3

MoSe2:Nb 5C

0 0

100

Time (min)

10

20

30

40

50

Gas concentration (ppm)

Figure 4. (a) Transient gas response of the MoSe2, MoSe2:Nb 1c, and MoSe2:Nb 5c devices at NO2 concentrations ranging from 3 to 50 ppm. (b) Comparison of gas responses of the three devices as a function of the NO2 gas concentration. The fitted lines follow a linear relationship between the response and concentration, which agrees well with Langmuir’s isotherm model. All gas-sensing tests were performed at an operating temperature of 150 °C.

In order to investigate the effect of Nb doping on the gas sensing properties of the MoSe2based 2D samples, we fabricated chemoresistive type-gas sensors based on these samples and then investigated their gas sensing properties by monitoring the changes in their resistance upon exposure to NO2 gas. Figure 4a shows the transient response of each device (the MoSe2-, MoSe2:Nb 1C-, and MoSe2:Nb 5C-based devices) under different NO2 concentrations (3, 5, 10, and 50 ppm). As shown in Figure S4 of SI, resistance change of three devices under different NO2 concentration were also plotted as function of time. The gas response was calculated according to the following relation: ΔR/RN2 = (RNO2-R N2)/RN2, where RN2 and RNO2 are the resistances of the sensing devices to the N2 and NO2 analyte gases, respectively. We also tested the 2D sensing devices under air stomsphere condition for recovery process (Figure S5 in SI). We observed the relatively higher gas response under air atmosphere rather than that under N2. Oxygen in air has been well known as electron accepting molecule which increase major hole carrier of the p-type MoSe2. The decrease of base resistance and large gas response during gas 10 ACS Paragon Plus Environment

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reaction might be due to the adsorbed oxygen molecules to 2D TMD materials. But the device showed poor recovery property under air condition. All the gas-sensing measurements were performed at an optimized operating temperature of 150 °C which was determined by considering critical parameters such as response, recovery, and long-term stability. Since NO2 is an electron acceptor, a decrease in the resistance of a device upon being exposed to NO2 gas (negative response) indicates the p-type behaviour of the device. The sensing mechanism of 2D materials depends strongly on the charge-transfer between the adsorbed gas molecules and the 2D materials.8 The MoSe2:Nb 1C device showed the highest gas response among three devices under a wide range of gas concentrations. The NO2 gas responses of the MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C devices were measured to be ~1, 8, and 2 %, respectively at a NO2 concentration of 3 ppm. Response time of three devices was measured to be less than 30 secs which is short enough to detect the presence of toxic chemicals. Nevertheless, we still need to make the response time faster in the case of highly toxic chemical seriously damaging our health. The possible solution is to scale down the device size. Since it is easier to efficiently collect the charge carriers transferred by gas adsorption to 2D TMDs, faster response time can be achieved. Note that the MoSe2 device showed a relatively large baseline drift during the gas-sensing test. This is because the as-synthesized MoSe2 film was more vulnerable to oxidation. Figure 4b summarizes the results of the gas response measurement for each device at different NO2 concentrations. The MoSe2:Nb 1C device showed the highest gas response among all the devices, suggesting that an optimum Nb concentration is necessary for achieving a high gas response. We will discuss the Nb doping effect on 2D sensing materials in more details. Our developed gas sensing device at a specific NO2 gas environment also achieved relatively high response compared to previously reported functionalized or doping methods on 2D-based gas sensor device (Table 1).



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Table 1. Comparison of chemical sensing performances with different 2D sensing materials Sensing materials

Method

Target gas

Temp.

Response

Ref.

Nb doped-MoSe2

thermally activation

NO2

150

8.03 at 3 ppm

this work

NO2

RT

11 at 100 ppm

Cui et al.[26]

NO2

150

5 at 1.2 ppm

Cho et al.[27]

NO2

100

40 at 25 ppm

Ko et al.[28]

NH3

RT

50 at 200 ppm

Dattatray et al.[29]

In-doped SnO2 nanoparticles/rGO MoS2 WS2 MoSe2

one-pot method with reduction of rGO Al metal particle decoration Ag nanowire surface functionalization pristine

MoSe2

a

MoSe2:Nb 1C

b

MoSe2:Nb 5C

c

In conventional bulk oxide-based gas sensing devices, the primary approach to achieve high sensitivity is to effectively reduce the grain size.30,31 Chemical sensitivity can also be altered or improved by adding catalytically active dopants.30–33 These two approaches have been known to be strongly correlated to each other. For instance, the addition of Nb to TiO2 hinders the grain growth, thereby improving the gas sensitivity.32,33 However, none of the previous studies in this context have focused on unveiling the underlying mechanism of the enhanced gas sensing ability of substitutionally doped 2D materials. In order to investigate the effect of Nb doping 12 ACS Paragon Plus Environment

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on the grains of the 2D MoSe2 samples, we analysed the atomic-resolution ADF-STEM images and fast Fourier transform (FFT) patterns of the three samples (MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C). Figures 5a–5c show the low magnification planar ADF-STEM images and FFT patterns (inset) of MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C. The MoSe2 sample showed a single-crystal dot pattern, while the MoSe2:Nb 1C and MoSe2:Nb 5C samples showed polycrystal ring patterns, indicating that only a few grains were present in these samples. The bottom images of Figures 5a-5c clearly show that an increase in the Nb doping concentration resulted in a decrease in the grain size. The high magnification ADF-STEM planar images of MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C show the grain boundaries (Figure S3a–3c in SI) and hexagonal crystal structures consisting of Mo, Nb, and Se (Figure S3d–3f in SI). If selenium percentage is considered nearly the same in the all three samples, the Nb:Mo ratio can be determined from the contrast of the images. In the ADF-STEM images, the darker regions represent Nb atoms. This is because Nb atoms are lighter than Mo atoms and the intensity is proportional to the square of the atomic weight. With an increase in the Nb doping concentration, the darker region on the ADF-STEM images expanded. On the basis of the TEM analysis results, we can conclude that Nb atoms inhibit the grain growth in MoSe2. Thus, we can say that a large number of grains and the stabilized grain boundaries in MoSe2:Nb 1C were responsible for its improved gas sensing response. On the other hand, a further increase in the Nb dopant concentration decreased the gas response (for MoSe2:Nb 5C). Hence, an optimum Nb dopant concentration was required for obtaining the best gas response. However, Nb atoms present in excess amounts in the MoSe2 lattice can form metallic percolation paths consisting of a NbSe2 lattice, which abruptly decreases the total gas response since metallic NbSe2 is rarely responsive to gas molecules.19 This is the reason why the gas response of the heavily Nb-doped MoSe2 device decreased compared to that of the non-doped MoSe2 device.



60

b

MoSe2

Gas response (%)

a Resistance change (R/R0)

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MoSe2:Nb 1C MoSe2:Nb 5C

40 20 0 0

30

60

90

Duration (days)

120

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MoSe2:Nb 1C

20 MoSe2

10 MoSe2:Nb 5C

0

@ NO2 3 ppm

0

30

60

90

Duration (days)

Figure 6. (a) Resistance change data of the MoSe2, MoSe2:Nb 1c, and MoSe2:Nb 5c devices over 120 days. The Nb-doped MoSe2 devices showed negligible resistance change, while the MoSe2 device showed considerably increasing resistance change over the test period (b) Comparison of the long-term stability of the three devices with respect to the gas sensing response.

Although 2D TMD materials show high gas sensitivity, stability should also be considered. Thus, we need to check the device stability when the devices are exposed to ambient conditions for a long period. First, we measured the resistance change of the MoSe 2-based devices, as shown in Figure 6a. The resistance of the bare MoSe2 device gradually increased over a period of ~120 days, indicating that the oxidation of this device was inevitable. On the contrary, the Nb-doped MoSe2 samples experienced negligible oxidation. We also investigated the long-term stability of the gas sensing devices (over a period of 100 days). The Nb-doped MoSe2 devices showed better stability rather than the bare MoSe2 device. More specifically, response change of the MoSe, MoSe2:Nb 1C, and MoSe2:Nb 5C devices were calculated to be ~99.6, 30.5, and 11.3%, respectively. We believe that the strong oxidation of the non-doped MoSe2 device caused the considerable change in its gas sensing response. Addition of Nb along the grain boundaries can meditate defects such as a large number of dangling bonds and vacancies which are more vulnerable to the oxidation.34 Thus, by doping 2D layered MoSe2 with Nb, a stable gas response as well as a long-term stability can be achieved. This approach paves a new path for the realizing high-performance gas sensing devices.


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Conclusions We investigated the effect of Nb doping of the gas sensing properties of 2D-MoSe2 films prepared by a PEALD atomic layer doping process. The NO2 gas sensing response increased dramatically from ~1 to 8% by employing light Nb doping. In addition, an excellent device stability was achieved over a period of ~120 days. These synergistic effects on the gas response and stability can be attributed to the Nb dopants, which inhibited the grain growth in MoSe2, thus increasing the number of grains and the surface-to-volume ratio. On the other hand, a further addition of Nb dopants deteriorated the gas response because of a considerable increase in the metallic portions, which do not function as a sensing channel. An optimum Nb doping of 2D layer nanomaterials can thus be an effective approach for obtaining high-performance gas sensing devices.

ASSOCIATED CONTENT Supporting Information. SIMS depth profile of MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C films; Comparison of detection resolution between XPS and SIMS analysis; Planar ADF-STEM images of MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C films; Transient resistance change of MoSe2, MoSe2:Nb 1C, and MoSe2:Nb 5C devices; Resistance changes and gas responses of MoSe2:Nb 1C device under different recovery gas condition (N2 and air) This material is available free of charge via the Internet at http://pubs.acs.org

Notes The authors declare no competing financial interests.



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Author contributions B.C., S.-H.K., and M.G.H. designed and supervised the experiments. S.Y.C., Y.K., H.-S.C., A.R.K., J.D.K., J.P., Y.L.K. did the experiments. S.Y.C. and B.C. analyzed the data. S.Y.C., Y.K., H.-S.C., S.-H.K., M.G.H., and B.C. co-wrote the paper.

Acknowledgements This study was supported by the Fundamental Research Program (PNK4580 and 4890) of the Korean Institute of Materials Science (KIMS). M. G. H. and B. C. are grateful for the support of the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF2014R1A1A1006214 and NRF-2014R1A1A1036139). H.-S.C. was supported by National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (No. 2015R1C1A1A01052727).

References (1)

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 (2), 668–673.

(2)

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 (6), 4879–4891.

(3)

Late, D. J.; Doneux, T.; Bougouma, M. Single-Layer MoSe2 Based NH3 Gas Sensor. Appl. Phys. Lett. 2014, 105 (23), 233103.

(4)

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. 2014, 4, 5209.


Page 17 of 21

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

(5)


Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6 (3), 183– 191.

(6)

Brien, M. O.; Lee, K.; Morrish, R.; Berner, N. C.; Mcevoy, N.; Wolden, C. a; Duesberg, G. S. Plasma Assisted Synthesis of WS2 for Gas Sensing Applications. Chem. Phys. Lett. 2014, 615, 6–10.

(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 (5), 5304–5314.

(8)

Cho, B.; Hahm, M. G.; Choi, M.; Yoon, J.; Kim, A. R.; Lee, Y.-J.; Park, S.-G.; Kwon, J.-D.; Kim, C. S.; Song, M.; Jeong, Y.; Nam, K.-S.; Lee, S.; Yoo, T. J.; Kang, C. G.; Lee, B. H.; Ko, H. C.; Ajayan, P. M.; Kim, D.-H. Charge-Transfer-Based Gas Sensing Using Atomic-Layer MoS2. Sci. Rep. 2015, 5 (2), 8052.

(9)

Cho, B.; Kim, A. R.; Park, Y.; Yoon, J.; Lee, Y.-J.; Lee, S.; Yoo, T. J.; Kang, C. G.; Lee, B. H.; Ko, H. C.; Kim, D.-H.; Hahm, M. G. Bifunctional Sensing Characteristics of Chemical Vapor Deposition Synthesized Atomic-Layered MoS2. ACS Appl. Mater. Interfaces 2015, 7 (4), 2952–2959.

(10)

Ou, J. Z.; Ge, W.; Carey, B.; Daeneke, T.; Rotbart, A.; Shan, W.; Wang, Y.; Fu, Z.; Chrimes, A. F.; Wlodarski, W.; Russo, S. P.; Li, Y. X.; Kalantar-zadeh, K. Physisorption-Based Charge Transfer in Two-Dimensional SnS2 for Selective and Reversible NO2 Gas Sensing. ACS Nano 2015, 9 (10), 10313–10323.

(11)

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

(12)

Cho, B.; Yoon, J.; Lim, S.; Kim, A.; Choi, S.-Y.; Kim, D.-H.; Lee, K.; Lee, B.; Ko, H.; Hahm, M. Metal Decoration Effects on the Gas-Sensing Properties of 2D HybridStructures on Flexible Substrates. Sensors 2015, 15 (10), 24903–24913. 17 ACS Paragon Plus Environment


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

(13)

Page 18 of 21

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 (1), 63–67.

(14)

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 (19), 2305–2313.

(15)

Jhon, Y. I.; Kim, Y.; Park, J.; Kim, J. H.; Lee, T.; Seo, M.; Jhon, Y. M. Significant Exciton Brightening in Monolayer Tungsten Disulfides via Fluorination: N-Type Gas Sensing Semiconductors. Adv. Funct. Mater. 2016. 26 (42), 7551-7559.

(16)

Kim, J.-S.; Yoo, H.-W.; Choi, H. O.; Jung, H.-T. Tunable Volatile Organic Compounds Sensor by Using Thiolated Ligand Conjugation on MoS2. Nano Lett. 2014, 14 (10), 5941–5947.

(17)

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 (19), 2994–2999.

(18)

Cho, B.; Yoon, J.; Lim, S. K.; Kim, A. R.; Kim, D.-H.; Park, S.-G.; Kwon, J.-D.; Lee, Y.-J.; Lee, K.-H.; Lee, B. H.; Ko, H. C.; Hahm, M. G. Chemical Sensing of 2D Graphene/MoS2 Heterostructure Device. ACS Appl. Mater. Interfaces 2015, 7 (30), 16775–16780.

(19)

Cho, B.; Kim, A. R.; Kim, D. J.; Chung, H.-S.; Choi, S. Y.; Kwon, J.-D.; Park, S. W.; Kim, Y.; Lee, B. H.; Lee, K. H.; Kim, D.-H.; Nam, J.; Hahm, M. G. Two-Dimensional Atomic-Layered Alloy Junctions for High-Performance Wearable Chemical Sensor. ACS Appl. Mater. Interfaces 2016, 8 (30), 19635–19642.

(20)

Yue, N.; Weicheng, J.; Rongguo, W.; Guomin, D.; Yifan, H. Hybrid Nanostructures Combining graphene–MoS2 Quantum Dots for Gas Sensing. J. Mater. Chem. A 2016, 4 (21), 8198–8203. 18 ACS Paragon Plus Environment

Page 19 of 21

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

(21)


Long, H.; Harley-Trochimczyk, A.; Pham, T.; Tang, Z.; Shi, T.; Zettl, A.; Carraro, C.; Worsley, M. A.; Maboudian, R. High Surface Area MoS2 /Graphene Hybrid Aerogel for Ultrasensitive NO2 Detection. Adv. Funct. Mater. 2016, 26 (28), 5158–5165.

(22)

Tongay, S.; Zhou, J.; Ataca, C.; Lo, K.; Matthews, T. S.; Li, J.; Grossman, J. C.; Wu, J. Thermally Driven Crossover from Indirect toward Direct Bandgap in 2D Semiconductors: MoSe2 versus MoS2. Nano Lett. 2012, 12 (11), 5576–5580.

(23)

Horzum, S.; Sahin, H.; Cahangirov, S.; Cudazzo, P.; Rubio, A.; Serin, T.; Peeters, F. M. Phonon Softening and Direct to Indirect Band Gap Crossover in Strained SingleLayer MoSe2. Phys. Rev. B 2013, 87 (12), 125415.

(24)

Kim, A. R.; Kim, Y.; Nam, J.; Chung, H.-S.; Kim, D.-H. J.; Kwon, J.-D.; Park, S. W.; Park, J. J. H.; Choi, S. Y. M.; Lee, B. H.; Park, J. J. H.; Lee, K. H.; Kim, D.-H. J.; Choi, S. Y. M.; Ajayan, P. M.; Hahm, M. G.; Cho, B. Alloyed 2D Metal– Semiconductor Atomic Layer Junctions. Nano Lett. 2016, 16 (3), 1890–1895.

(25)

Yu, Z.; Pan, Y.; Shen, Y.; Wang, Z.; Ong, Z.-Y.; Xu, T.; Xin, R.; Pan, L.; Wang, B.; Sun, L.; Wang, J.; Zhang, G.; Zhang, Y. W.; Shi, Y.; Wang, X. Towards Intrinsic Charge Transport in Monolayer Molybdenum Disulfide by Defect and Interface Engineering. Nat. Commun. 2014, 5, 5290.

(26)

Cui, S.; Wen, Z.; Mattson, E. C.; Mao, S.; Chang, J.; Weinert, M.; Hirschmugl, C. J.; Gajdardziska-Josifovska, M.; Chen, J. Indium-Doped SnO2 Nanoparticle-Graphene Nanohybrides: Simple One-Pot Synthesis and Their Selective Detection of NO2. J. Mater. Chem. A 2013, 1 (14), 4462-4467.

(27)

Cho, B.; Yoon, J.; Hahm, M. G.; Kim, D.-H.; Kim, A. R.; Kahng, Y. H.; Park, S.-W.; Lee, Y.-J.; Park, S.-G.; Kwon, J.-D.; Kim, C. S.; Song, M.; Jeong, Y.; Nam, K.-S.; Ko, H. C. Graphene-Based Gas Sensor: Metal Decoration Effect and Application to a Flexible Device. J. Mater. Chem. C 2014, 2 (27), 5280-5285.



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)

Page 20 of 21

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-sening Performance of Large-Area Tungsten Disulfide Nanosheets by Surface Functionalization. ACS Nano 2016, 10 (10), 9287-9296.

(29)

Late, D, J.; Doneux, T.; Bougouma, M. Single-layer MoSe2 based NH3 gas sensor. Appl. Phys. Lett. 2014, 105 (23), 233103.

(30)

Guidi, V.; Carotta, M. C.; Ferroni, M.; Martinelli, G.; Paglialonga, L.; Comini, E.; Sberveglieri, G. Preparation of Nanosized Titania Thick and Thin Films as GasSensors. Sensors Actuators, B Chem. 1999, 57 (1–3), 197–200.

(31)

Ferroni, M.; Carotta, M. C.; Guidi, V.; Martinelli, G.; Ronconi, F.; Richard, O.; Van Dyck, D.; Van Landuyt, J. Structural Characterization of Nb-TiO2 Nanosized ThickFilms for Gas Sensing Application. Sensors Actuators, B Chem. 2000, 68 (1), 140–145.

(32)

Phanichphant, S.; Liewhiran, C.; Wetchakun, K.; Wisitsoraat, A.; Tuantranont, A. Flame-Made Nb-Doped TiO2 Ethanol and Acetone Sensors. Sensors 2011, 11 (1), 472– 484.

(33)

Zeng, W.; Liu, T.; Wang, Z. Impact of Nb Doping on Gas-Sensing Performance of TiO2 Thick-Film Sensors. Sensors Actuators B Chem. 2012, 166–167, 141–149.

(34)

Ruiz, A.; Calleja, A.; Espiell, F.; Cornet, A.; Morante, J. R. Nanosized Nb-TiO2 Gas Sensors Derived from Alkoxides Hydrolization. IEEE Sens. J. 2003, 3 (2), 189–194.


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Table of contents MoSe2

Gas response (%)

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


MoSe2:Nb 1C

MoSe2:Nb 5C

0

-10 3 ppm 5 ppm 10 ppm

MoSe2

-20

MoSe2:Nb 1C

50 ppm

MoSe2:Nb 5C

0

50

100

Time (min)


Effect of Nb Doping on Chemical Sensing Performance of Two

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