Strain-Gradient Effect in Gas Sensors Based on Three-Dimensional

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Strain Gradient Effect in Gas Sensors based on 3D Hollow Molybdenum Disulfide Nanoflakes Min-A Kang, Jin Kyu Han, Sam Yeon Cho, Sang Don Bu, Chong-Yun Park, Sung Myung, Wooseok Song, Sun Sook Lee, Jongsun Lim, and Ki-Seok An ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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ACS Applied Materials & Interfaces

Strain Gradient Effect in Gas Sensors based on 3D Hollow Molybdenum Disulfide Nanoflakes Min-A Kang†,‡,§,||, Jin Kyu Han†,||, Sam Yeon Cho⊥, Sang Don Bu⊥, Chong-Yun Park§, Sung Myung†,*, Wooseok Song†, Sun Sook Lee†, Jongsun Lim†, Ki-Seok An† †

Thin Film Materials Research Center, Korea Research Institute of Chemical Technology,

Daejeon 305-600, Republic of Korea ‡

Department of Energy Science and §Department of Physics, Sungkyunkwan University, Suwon,

Gyeonggi-do 440-746, Republic of Korea, ⊥

Department of Physics, Chonbuk National University, Jeonju 561-756, Republic of Korea.

*Corresponding author: [email protected] (Sung Myung)

KEYWORDS: Transition metal dichalcogenide, chemical vapor deposition, MoS2, nanoflakes, gas sensing

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ABSTRACT A novel three-dimensional (3D) transition metal dichalcogenide (TMD) structure consisting of seamless hollow nanoflakes on two dimensional (2D) basal layers was synthesized by a one-step chemical vapor deposition method. Here, we demonstrate that the as-grown nanoflakes are formed on an organic promoter layer which served as a positive template and swollen at the grain boundaries by the bubbling effect. TMD nanosheets with hollow nanoflakes are successfully applied as chemical sensors, and it was found that their gas adsorption property is strongly related to the internal strain gradient resulting from the variation in the lattice parameter. This result is consistent with the theoretical prediction in previous studies. Our chemical vapor deposition-based approach is an efficient way to generate TMD-based nanostructures over a large surface area for various practical applications, such as chemical sensors.

1. INTRODUCTION Owing to their high surface-to-volume ratio and superior physical properties, such as the occurrence of charge density waves and high electron conductivity, two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant interest as materials that can be used in a variety of advanced electronic devices.1 For instance, 2D TMDs have been utilized in advanced devices such as high-performance gas sensors,2 ultra-capacitors,3 lithium ion batteries,4 and hydrogen evolution reactions (HER).5 Research on gas adsorption using 2D TMD has focused on improving sensitivity to gas exposure through layer number control6 and vertically standing structure fabrication.7 Recently, it was proved theoretically through density functional theory calculation that the gas adsorption characteristics of the 2D TMD material molybdenum disulfide (MoS2) are closely related to the internal strain of TMD nanosheets.8,9 In


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addition, the strain caused by the curvature is known to be generated easily than that arising from rigid thin film or nanoflake.10 These studies suggest that the gas response to MoS2 is adjusted by strain modifications. However, the experimental proof of the gas adsorption characteristics of strain-induced MoS2 is still insufficient, and synthetic methods for optimizing MoS2 nanosheets for chemical sensors with high performance are still required. In the previous work, we reported that the optimized promoter thickness for MoS2 nanoflakes growth was 7 nm.11 In this study, we synthesized strain-engineered three-dimensional (3D) hollow MoS2 nanoflakes using a one-step chemical vapor deposition (CVD) method. Here, basal MoS2 thin film and vertically standing MoS2 nanoflakes on basal film were synthesized simultaneously by using organic promoter layer during single CVD process. Hollow MoS2 nanoflakes were formed by the bubbling effect of an organic promoter used during the CVD process. The strain of vertically standing MoS2 hollow nanoflakes was controlled by adjusting the lateral size and number of layers. Furthermore, thickness of the promoter layers was employed to investigate the surface morphology of vertically standing MoS2 hollow nanoflakes contributed to the enhancement of gas sensing properties and chemical sensors based on the vertically standing MoS2 hollow nanoflakes with a high surface-to-volume ratio were demonstrated.

2. EXPERIMENTAL SECTION 2.1 CVD Growth of hollow and vertical MoS2 nanoflakes on a basal MoS2 layer. Prior to the conventional CVD synthesis process, 5, 10, 15, 20-tetrakis(4-hydroxyphenyl)-21H, 23H-porphyrin (p-THPP) thin films used as an organic promoter layer were deposited on SiO2/Si substrates by thermal evaporation. First, a 6 nm- p-THPP layer was deposited on a 300 nm


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SiO2/Si substrate in order to make the entire surface uniform. Then, the substrate was placed at the center of the heating zone of a CVD furnace. A 0.1 M Mo solution was prepared by dissolving ammonium heptamolybdate (Sigma Aldrich, 99%) in 10 mL distilled water. The solution was subsequently coated onto UV-treated SiO2 substrates by spin-coating at 2,000 rpm for 30 s, and a Mo film was placed at a position 2 cm from the organic promoter layer. S powder (0.2 g) (Samchun, 98.0%) was used as the sulfur source and located upstream in the reactor. The synthesis of v-MoS2/MoS2-TF was then carried out on the 4 cm × 4 cm promoter layer in the CVD furnace at various temperature conditions between 550°C and 800°C under 1.4 Torr, while introducing Ar (500 sccm) as a carrier gas for 5 min. After the CVD synthesis, the furnace was cooled down to room temperature.

2.2 Transfer method of MoS2 nanosheets with hollow nanoflakes. PMMA was first spin-coated at 2,000 rpm for 30 s on the v-MoS2/ MoS2-TF. Then, the SiO2 layer was completely removed in 0.1 M NaOH solution, and the v-MoS2/MoS2-TF with the PMMA layer was transferred onto the target substrate, such as a TEM grid. Subsequently, the PMMA layer was removed by rinsing with acetone. The detailed structure of the v-MoS2/MoS2TF was observed using a FETEM (JEM-2100F JEOL, Daejeon Center of KBSI).

2.3 Measurement of the resistance change of a basal MoS2 layer with MoS2 nanoflakes. In order to measure the resistance response of the synthesized v-MoS2/MoS2-TF, 100 nm Au electrodes (a 5 nm Cr layer was pre-deposited as an adhesion layer) with a width of 2 mm and a length of 100 µm were deposited by e-beam evaporation using a shadow mask. Gas sensors based on v-MoS2/MoS2-TF were investigated utilizing a Keithley-4200 semiconductor parameter


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analyzer for measurement and data collection. NH3 (NH3 27% with N2 73%) and NO2 (NO2 10% with N2 90%) gases were introduced into the sensing chamber (volume ~ 2 L) to measure the resistance signal of the v-MoS2/MoS2-TF. The mixed gases used for the data collection were N2based, 0.5, 1.3, 2.7, 5.4, 8.1, 10.8, and 13.5 ppm NH3 and 0.15, 0.5, 1, 2, 3, 4, and 5 ppm NO2, respectively. The v-MoS2/MoS2-TF samples were placed at a distance of 2 cm from the gas inlet to quickly come into contact with the analytes. All the gas sensing measurements were carried out at room temperature.

3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization of hollow v-MoS2 nanoflakes on basal MoS2 nanosheets. Figure 1a shows the process of fabricating vertically standing MoS2 (v-MoS2) nanoflakes on the carbon promoter layer and transferring v-MoS2 grown on MoS2 thin films (MoS2-TF) onto a SiO2/Si substrate to fabricate MoS2-based gas sensors.11-13 As shown in Figure 1b, field emission scanning electron microscopy (FE-SEM) was utilized to analyze surface morphologies of vMoS2 flakes grown at various growth temperatures. Initially, at temperatures below 550°C, neither v-MoS2 nanoflakes nor MoS2-TF could be observed. Figure S-1 exhibits no existence of v-MoS2 nanoflakes and the formation of amorphous phase of v-MoS2/MoS2-TF grown at temperature below 550oC. As the growth temperature reached 550°C, v-MoS2 nanoflakes initiated to grow exclusively on the organic promoter layer. Furthermore, the size of the v-MoS2 nanoflakes increased between 600°C and 700°C. However, v-MoS2 nanoflakes could not be observed above 800°C. Figure 1c displays an optical image of a large-scale area (4 cm × 4 cm)


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of the v-MoS2/MoS2-TF. This observation confirmed that the organic promoter layer played a significant role in the uniform growth of v-MoS2 nanoflakes over a large-scale area. To investigate the structural characteristics of the v-MoS2/MoS2-TF, Raman spectra were measured using a 514-nm laser excitation wavelength under ambient conditions. Figure 1d exhibits representative Raman spectra for v-MoS2/MoS2-TF grown at various temperatures using an organic promoter layer. There are two prominent Raman features, namely the A1g and the E12g phonon modes, for v-MoS2/MoS2-TF grown at temperatures between 550°C and 800°C. The A1g phonon mode is generated by an out-of-plane vibration mode of the S atoms, and the E12g phonon mode is attributed to the in-plane vibration mode of S atoms and Mo atoms.7 As shown in the Raman spectra of v-MoS2/MoS2-TF grown at 550°C and 600°C, asymmetric and broad Raman spectra were observed. This result can be explained that the degree of crystallinity of specific MoS2 is not perfect yet. Upon increasing the temperature up to 700°C, symmetric A1g and E12g phonon modes were observed, which indicates that the degree of crystallinity becomes perfect at 700°C. In the Raman spectrum of v-MoS2/MoS2-TF grown above 800°C, two sharp A1g and E12g phonon modes were observed. Further, X-ray photoelectron spectroscopy (XPS) was employed for the surface compositional analysis of v-MoS2/MoS2-TF. In this analysis, XPS spectra were recorded with a monochromatic Al Kα (Ephoton = 1486.6 eV) source and normal emission geometry. As shown in Figure 1e, Mo 3d3/2 and Mo 3d5/2 peaks were located at a binding energy of 232.5 eV and 229.3 eV, respectively, in the Mo 3d core level spectrum. The S 2p1/2 and S 2p3/2 peaks were observed at a binding energy of 163.3 eV and 162.0 eV, respectively, which confirmed that v-MoS2/MoS2-TF was successfully synthesized using the promoter layer. The Mo 3d core level spectrum of vMoS2/MoS2-TF grown at 550°C and 600°C showed a prominent extra peak at 236 eV. This


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feature can be understood by the presence of MoO3 in v-MoS2/MoS2-TF. Additionally, the absence of the MoO3 peak could be observed in the Mo 3d spectrum of v-MoS2/MoS2-TF grown above 700°C, which correspond with the Raman results mentioned earlier. The surface morphology of v-MoS2/MoS2-TF grown at various temperatures was observed in detail by non-contact atomic force microscopy (AFM).14 Figure 2a presents AFM topographic images of v-MoS2/MoS2-TF grown at 550, 600, 700, and 800°C. v-MoS2 nanoflakes were not observed below a temperature of 550°C. However, v-MoS2 nanoflakes with a high areal density were observed on MoS2-TF grown at 550, 600, and 700°C. The areal density of v-MoS2 nanoflakes gradually decreased for growth temperatures above 700°C, which was consistent with the results from the FE-SEM analysis shown in Figure 1. As shown in Figure 2b, line profile scans were performed to accurately analyze the height of individual v-MoS2, as indicated by the yellow lines in the AFM topographic images. The width of v-MoS2 nanoflakes grown at 550, 600, and 700°C were 215, 150, and 116 nm, respectively. v-MoS2 nanoflakes grown at 550°C were relatively thick with a small diameter. In the case of v-MoS2 grown at 700°C, the areal density and width of v-MoS2 nanoflakes decreased, while the height of v-MoS2 nanoflakes increased than those grown at lower temperatures. It should be noted that the height, width, and areal density of the v-MoS2 nanoflakes are closely related to the temperature of the CVD process. Figure 2c and d show the variation of surface morphology of v-MoS2 nanoflakes as a function of growth temperature. Initially, v-MoS2 nanoflakes were not observed for the sample grown at temperature below 550°C. As the temperature increased, v-MoS2 nanoflakes were successfully synthesized at temperatures between 550°C and 800°C. At temperatures above 800°C the nanoflakes could not be observed. For v-MoS2 grown between 550°C and 800°C, it was found


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that the height tended to increase while the width and areal density tended to decrease with increasing growth temperature.

3.2 Atomic structures of hollow and vertically standing MoS2 nanoflakes. Field emission transmission electron microscopy (FE-TEM) was carried out to understand the growth mechanism of v-MoS2 nanoflakes using an organic promoter layer and to investigate the atomic arrangement in nanoflakes of various sizes. Figures 3a-c show high-resolution FE-TEM images of v-MoS2 nanoflakes with lateral sizes of 28, 40, and 60 nm, respectively. In these TEM images, hollow-shaped v-MoS2 nanoflakes with the layered structure were observed. Interestingly, v-MoS2 hollow nanoflakes with a 28 nm lateral size possess the multi-layer structure with a spherical and polyhedral shape, as shown in Figure 3a. The crystal structure of vMoS2 hollow flakes with a lateral size of more than 40 nm (Figures 3b and c) was formed into the polyhedral shapes only. Notably, these observations suggest that a lateral size of approximately 30 nm is a significant value for determining the shape of the v-MoS2 hollow nanoflakes. Figure 3d shows the number of layers of the v-MoS2 hollow nanoflakes as a function of the lateral size, which was obtained from the FE-TEM images. The number of layers of the vMoS2 hollow nanoflakes decreased linearly with increasing the lateral size of the nanoflakes. The relationship between this lateral size and the number of layers is closely related to the growth mechanism of the v-MoS2 hollow nanoflakes. Based on the above results, we propose that the formation of the hollow structure of v-MoS2 nanoflakes is induced by the transformation of an organic promoter into a positive template during the CVD process. Here, Mo and S sources are supplied at a constant rate to each positive promoter layer. Thereby, nanoflakes with small lateral size possess multilayers than large-size nanoflakes since they are in an unstable energy state.


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Figure 3e shows the lattice distance between Mo atoms with respect to the layer position. The lattice distance from the innermost MoS2 layer was similar to the lattice distance (~ 2.72 Å) of the previously reported MoS2 regardless of the layer position.15 This phenomenon indicates that v-MoS2 hollow nanoflakes are formed after the formation of a positive template at grain boundaries between two MoS2 grains. Simultaneous growth of v-MoS2 hollow nanoflakes and positive organic template is related to the lattice expansion phenomenon which the lattice distance increases. Thus, we suggest that the v-MoS2 hollow nanoflakes are developed on the positive template originating from a conversion of the organic promoter layer. Exceptionally, the v-MoS2 hollow nanoflakes with a diameter of 28 nm showed the lattice distance difference of about 22% due to the curvature of the positive template, suggesting that the large strain gradients were formed with the v-MoS2 nanoflakes.16 This result implies that the lateral size of the nanoflakes have a strong influence on the lattice distance of the nanoflakes (Figure 3f). The structural properties of the hollow MoS2 nanostructures (NS) have been reported through molecular dynamics (MD) simulation to be related to their lateral dimension and chirality.17 The strain energy of armchair and zigzag MoS2 NS calculated using classical MD simulation is as follows: =

− ( ) ( )

where, ENS and Eunit(MoS2) are the total free energy of hollow MoS2 NS and free energy of MoS2 unit crystal, respectively. n(MoS2) is the number of MoS2 unit layer in hollow MoS2 NS. The relationship between strain energy and NS diameter (d) shows roughly 1/d2, which is consistent with the density-functional-based tight-binding (DFTB) calculation.18 Therefore, it is suggested that the large strain gradient characteristic in MoS2 nanoflake with small diameter is closely related to this strain energy. In addition, the size-dependent nature of the Mo-Mo distance in the


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MoS2 nanoflake mentioned in Figure 3e is the result predicted by the zigzag MoS2 NS, which is the first time we have presented an experimental proof. Based on these results, the proposed growth mechanism of v-MoS2 nanoflakes is as follows: First, MoS2 grains begin to grow in-plane on the promoter layer, and the promoter swells between basal MoS2 grains. Then, v-MoS2 nanoflakes are developed on the swollen promoter which acts as a positive template, as shown in Figures 3d-f. Finally, the v-MoS2 nanoflakes are formed by the vaporization of the promoter during the heat treatment at high temperatures.11-13 Here, the decrease of width and area density at high temperature was resulted from the expansion of the basal MoS2 grains. Thus, the width and area density of v-MoS2 are inversely proportional to the growth temperature, while the height of v-MoS2 nanoflakes is proportional to the growth temperature. Moreover, we observed the evolution of the length and width of v-MoS2 nanoflakes as a function of the reaction time (Figure S-2 in the Supplementary Information) to clarify the formation of the v-MoS2 nanoflakes. The error bars display the variation in size of ten v-MoS2 nanoflakes obtained in each condition. Here, the length of v-MoS2 nanoflakes was proportional to the reaction time, but the width of the v-MoS2 nanoflakes was inversely proportional to reaction time. This confirmed that the length of v-MoS2 nanoflakes increased, since the MoS2 grains increase in the in-plane direction with increasing reaction time. This result showed consistent behavior to the case of v-MoS2 nanoflakes obtained in annealing process with various growth temperature conditions.

3.3 Crystal structure and gas absorption properties of hollow v-MoS2 nanoflakes on basal MoS2 nanosheets.


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The effect of the thickness of the promoter layer for the surface morphology of v-MoS2 nanoflakes was also investigated. Figures 4a (i)-(iii) display FE-SEM images of v-MoS2 nanoflakes synthesized on the organic promoter layers with a thickness of 1.5 nm, 3 nm, and 6 nm, respectively. FE-SEM images showed that the thickness of the promoter layer was inversely proportional to the areal density of the v-MoS2 nanoflakes on the promoter layer. This result indicates that the thickness of the organic promoter layer can be utilized for the control of the areal density of the v-MoS2 nanoflakes. Raman spectroscopy was explored to characterize the chemical bonds of v-MoS2 nanoflakes. The intensity ratio of the A1g phonon mode (out-of-plane vibration) and the E12g phonon mode (in-plane vibration) was calculated through Raman spectra of v-MoS2 nanoflakes in Figure 4b. This ratio is known as the signature of vertical growth, indicating the presence of exposed MoS2 edge sites.13 The Raman peak intensity ratio of the A1g and E12g modes (A1g/E12g) increased with increasing thickness of the promoter layer. For example, v-MoS2 nanoflakes grown on a 6 nm promoter layer had a relatively high A1g/E12g value of 22.4% compared to v-MoS2 nanoflakes grown on a 1.5 nm promoter layer. In addition, we also confirmed that tri-layer MoS2 were synthesized on a 1.5 nm promoter layer through the frequency difference between the A1g and E12g modes in the Raman spectra. Moreover, the number of layers increased gradually as the thickness of the promoter layer increased from 1.5 nm to 6 nm (Figure 4b i-iii). As shown in Figures 4c and d, the detailed microstructure of vMoS2 nanoflakes grown on a 1.5 nm and 6 nm promoter layer was also examined using FE-TEM. Compared with v-MoS2 nanoflakes grown on the 6 nm promoter layer, the area density of the vMoS2 nanoflakes on the 1.5 nm promoter layer was increased and the small size of v-MoS2 nanoflakes was observed. The selective area electron diffraction (SAED) patterns of the v-MoS2 nanoflakes grown on the 1.5 nm promoter showed polycrystalline features with (100), (103),


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(110), (201), and (205) reflections (inset in Figures 4c-i). Whereas, (100) and (110) reflections were observed in the SAED patterns of nanoflakes grown on the 6 nm promoter layer (inset in Figures 4d-i). The basal MoS2 TF and v-MoS2 nanoflakes on the promoter layer with a thickness of 1.5 nm consisted of crystalline and amorphous phase, respectively (inset Figures 4c-ii). Here, the electron diffraction from amorphous phase and polycrystalline phase exhibits fuzzy rings and the ring patterns assigned as (100), (110), and (201) reflections, respectively. However, in case of MoS2 TF and v-MoS2 nanoflakes on the 6 nm promoter, SAED patterns showed only polycrystalline features (inset in Figures 4c-ii). Here, ring patterns assigned as (100), (110), (201), (213) reflections are observed (Table S-1 in Supporting Information). Interestingly, SAED patterns in the FE-TEM analysis showed that both v-MoS2 nanoflakes grown on promoter layers with a thickness of 1.5 nm and 6 nm possess a crystalline structure. This finding suggests that the thickness of the promoter is closely related to the crystallinity of the basal MoS2 TF. The size distributions of v-MoS2 nanoflakes are shown in the insets of Figures 4c and d. The average sizes of v-MoS2 nanoflakes grown on 1.5-nm and 6-nm promoter layer are estimated to be 28 ± 9 nm and 45 ± 10 nm, respectively. In the FE-TEM image of the MoS2 layer grown on the 1.5 nm promoter, a few crystalline MoS2 nanosheets were observed in the basal plane. We determine that this crystal structure corresponds to the typical appearance of crystalline MoS2 nanosheets.15 Additionally, we could observe the crystal structure over the whole area in the basal MoS2 TF grown on the 6 nm promoter. These results imply that the thickness of the promoter layer have a significant influence on the degree of crystallinity of v-MoS2 nanoflakes and MoS2-TF. In previous studies, hollow-shaped materials were utilized to significantly enhance the sensitivity of sensors owing to their large surface area and high number of active sites for


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reaction with specific gases.19 Up to date, gas sensors based on MoS2 thin film showed relatively low gas response due to their low specific surface area.20-22 In this study, v-MoS2 nanoflakes with a large surface-to-volume ratio were used to improve the gas response during exposure to NH3 or NO2 molecules. The lower inset in Figure 5a shows a MoS2 junction between the source and drain electrodes with a 2-mm-wide and 100-µm-long MoS2 channel. When applying this design, we can obtain uniform MoS2-based devices based on v-MoS2/MoS2-TF. Figure 5a shows the I-V curves for devices based on v-MoS2/MoS2-TF using promoter layers with a thickness of 1.5 nm and 6 nm. I-V curves indicate that the electrical conductance of the device based on the vMoS2/MoS2-TF grown by using a 6 nm promoter was approximately five times higher than that of the device based on the v-MoS2/MoS2-TF synthesized using the 1.5 nm promoter layer. This result can be understood by the fact that v-MoS2 nanoflakes grown on the 6 nm promoter were composed of crystalline phase over the whole area as mentioned in Figure 4d. The bias current in MoS2-based devices increased during exposure to NO2 gas and decreased during exposure to NH3. Here, NO2 molecules absorbed on the MoS2 surface and acted as electron acceptors, while absorbed NH3 molecules acted as electron donors. Figures 5b and c show the gas sensing response for NH3 and NO2 molecules of v-MoS2/MoS2-TF at room temperature. The gas response (%) is defined as ∆R/RN2 = (Rgas – RN2) /RN2, where Rgas and RN2 presents the resistances of the device to the analyte gas and under N2 gas atmosphere. The response of v-MoS2/MoS2-TF seemed highly reliable, as a result of confirming the gas sensing response of the only annealed promoter layer (Figure S-3 in the Supplementary Information). In the case of NH3 molecules, a sensor response with positive resistance changes was observed during exposure to NH3 gas with a concentration of 0.5 ppm, and the sensor response increased as the gas concentration increased to 13.5 ppm. The gas sensors based on v-MoS2/MoS2-TF using the 1.5 nm promoter had a


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response to NH3 gas similar to that of the sensor based on v-MoS2/MoS2-TF grown with the 6 nm promoter. In the case of NO2 gas, the minimum gas concentration for the gas detection was 0.15 ppm; here, a resistance change was decreased. As the gas concentration was increased gradually up to 5 ppm, the gas response upon exposure to NO2 decreased sequentially. For example, the response of a v-MoS2/MoS2-TF grown on the 1.5 nm promoter improved by approximately 44% for the injection of 5 ppm NO2 gas, compared to the response of the sensors based on the v-MoS2/MoS2-TF grown on the 6 nm promoter. These results show that the differences in the resistance resulting from the response to NO2 gas related to the areal density of v-MoS2 nanoflakes were higher than those resulting from the response to NH3 gas. This is presumably caused by the different surface adsorption energies of NO2 and NH3 gas molecules on the surface of v-MoS2/MoS2-TF. As shown in the FE-TEM results in Figure 3, the average size of v-MoS2 nanoflakes grown on a 1.5 nm promoter was 28 ± 9 nm, and the lattice strain gradient calculated from lattice distance difference in this v-MoS2 was about 22%. The strain gradient affected the adsorption of NO2 gas molecules on the MoS2 surface. Owing to the lattice strain, the adsorption distance was reduced by a modification of adsorption characteristics, such as bonding configuration, which is known to originate from the localization of electron densities. In recent theoretical results,8,9 in the case of MoS2 with a large strain gradient, many NO2 gas molecules were easily adsorbed on the MoS2 surface. However, the strain gradient has a relatively small effect on the adsorption of NH3 gas. These results are consistent with our gas response during exposure to NH3 or NO2 molecules. Thus, it can be suggested that the difference in gas reactivity of the v-MoS2/MoS2-TF-based sensors is due to the difference in surface adsorption energy between NO2 and NH3 molecules. It


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is believed that the TMDs synthesized in this study will be a promising sensor material for applications requiring precise and accurate NO2 gas sensing.

4. CONCLUSION In summary, we reported the synthesis of hollow and vertically standing TMD flakes on a basal MoS2 layer and their strain-induced modification of gas adsorption. In this study, basal TMD layers were synthesized, and then TMD hollow nanoflakes were formed on the swollen promoter layer, which acted as a positive template, owing to the bubbling effect at the grain boundary. In this process, the thickness of the promoter layer affected the degree of crystallization of the basal TMD layer and the areal density and size of the hollow TMD flakes. In addition, when we measured the gas adsorption property according to the size of the hollow TMD flakes, it was confirmed that the adsorption response of NO2 gas was controlled, while that of NH3 gas was relatively constant. We speculate that this response difference is due to the different adsorption properties of the gas depending on the strain gradient formed in the MoS2 nanoflake. The growth mechanism of hollow and vertically standing TMD flakes on the organic promoter layer provides a fundamental understanding of the emerging 2D TMD material synthesis, and hollow and vertically standing TMD flakes are expected to be regarded as a promising material for accurate and sensitive gas sensing.

ASSOCIATED CONTENT Supporting Information. FE-SEM image of v-MoS2/MoS2-TF and the gas adsorption property of promoter-based device. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions ||

M.K. and J.K.H. contributed equally. M.K., W.S. and J.L. contributed to synthesis and

characterization of v-MoS2 on the basal MoS2-TF. K.A. contributed to SEM and Raman analysis of v-MoS2. J.K.H. conducted the TEM characterization. S.Y.C. and S.D.B. contributed the AFM characterization. S.S.L. and C.P. performed XPS and electrical experiments and contributed to interpretation of the results. S.M. conceived and designed the experiments, and M.K., J.K.H. and S.M. wrote the manuscript, and all authors discussed the results and commented on the manuscript.

ACKNOWLEDGMENT This research was supported by a grant (2011-0031636) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT and Future Planning, Korea.

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Figure 1. Hollow MoS2 nanoflakes and MoS2 basal layer. (a) A schematic showing the fabrication of v-MoS2 based devices. (b) FE-SEM images of v-MoS2/MoS2-TF grown at various temperatures, i.e., (i) 550°C, (ii) 600°C, (iii) 700°C, and (iv) 800°C. (c) An optical image of a large-scale area of v-MoS2/MoS2-TF. (d) Raman spectra of v-MoS2/MoS2-TF. (e) X-ray photoelectron spectroscopy (XPS) spectra of v-MoS2/MoS2-TF. Mo 3d (left) and S 2p (right) core level spectra of v-MoS2/MoS2-TF grown at various temperatures.


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Figure 2. Surface morphology of hollow v-MoS2 nanoflakes grown on a MoS2-TF. (a) AFM topographic images of v-MoS2/MoS2-TF grown at various temperatures using an organic promoter layer. (b) Height profiles of an individual v-MoS2 flake. (c) Areal density and (d) height and width change of v-MoS2 flakes in respect to different growth temperatures.


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Figure 3. Atomic structure of hollow MoS2 nanoflakes. (a)–(c) FE-TEM images of v-MoS2 with 28, 40, and 60 nm lateral size. (d) Number of v-MoS2 layers as a function of the lateral size observed in the FE-TEM analysis. (e) Lattice distances between Mo atoms according to the layer position. (f) Variations of the lattice distance with lateral size.


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Figure 4. Crystal structure of hollow MoS2 nanoflakes in relation to the thickness of the promoter layer. (a) FE-SEM images of v-MoS2/MoS2-TF grown on organic promoter layers with a thickness of (i) 1.5 nm, (ii) 3 nm, and (iii) 6 nm. All scale bars are 500 nm. (b) Raman spectra of v-MoS2/MoS2-TF grown on the promoter layers with various thicknesses. FE-TEM analysis including SAED, size distribution, and crystallinity of v-MoS2/MoS2-TF grown on the organic promoter layers with a thickness of (c) 1.5 nm and (d) 6 nm.


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Figure 5. Electrical properties and gas adsorption properties of hollow MoS2 nanoflakes and MoS2 thin films. (a) I-V curve of devices based on v-MoS2/MoS2-TF grown on a promoter layer with a thickness of 1.5 nm and 6 nm, respectively. The inset displays an optical image of the device showing the distance between both electrodes. Gas sensing response for NH3 and NO2 molecules at room temperature as a function of (b) time and (c) gas concentration.


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Table of Contents Graphic:


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Strain-Gradient Effect in Gas Sensors Based on Three-Dimensional

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