Supporting Information Improvement of Gas-Sensing Performance of ...
Supporting Information Improvement of Gas-Sensing Performance of LargeArea Tungsten Disulfide Nanosheets by Surface Functionalization Kyung Yong Ko1, Jeong-Gyu Song1, Youngjun Kim1, Taejin Choi1, Sera Shin1, Chang Wan Lee1, Kyounghoon Lee2, Jahyun Koo3, Hoonkyung Lee3, Jongbaeg Kim2, Taeyoon Lee1, Jusang Park1*, 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 Mechanical Engineering, Yonsei University,
50 Yonsei-Ro, Seodaemun-Gu, Seoul, 03722, Rep. of Korea 3
Department of Physics, Konkuk University,
120 Neungdong-ro, Gwangjin-gu, Seoul 05029, Rep. of Korea
*
Corresponding author. E-mail: [email protected], [email protected]
S2 A. Observation of roughness of PEALD WO3 and thickness of WS2 nanosheets The scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of WO3 films (20, 30, 50 cycles of PEALD) were studied to compare the roughness of each WO3 film. The PE-ALD WO3 films exhibit smooth and continuous surface. The measured root mean square (r.m.s.) values of each film show very small variations with increasing thickness. After sulfurization, the WS2 nanosheets were transferred to new SiO2 substrates. The AFM image of monolayer (1L) WS2 nanosheets is shown in Figure S1(g). The inset of Figure S1(g) shows the corresponding quantitative AFM height profile. The measured height of the 1L WS2 nanosheet is 0.9 nm. As shown in Figures S1(h–i), the thickness values of the bilayer (2L) and tetra-layer (4L) WS2 nanosheets were measured to be 1.5 and 3 nm, respectively (the height profiles are shown in the insets of Figures S1(h) and S1(i), respectively). The height of the 1L WS2 nanosheet on SiO2 is 0.9 nm, and the thickness difference between adjacent layers is approximately 0.7 nm.
S3
Figure S1. (a–c) SEM images and (d–f) AFM images of WO3 films (top-view). WO3 films were synthesized by (a, d) 20, (b, e) 30, and (c, f) 50 cycles of PE-ALD. The r.m.s. values are shown in AFM images. (g–i) AFM images of transferred WS2 nanosheets; the inset display the height profile of (g) 1L, (h) 2L, and (i) 4L WS2 nanosheets.
S4 B. Raman shift and peak difference of 1L, 2L, and 4L WS2
Figure S2. (a) Raman shift of 2LA(M), E12g, and A1g modes as the number of layers increased from 1L to 4L. (b) Peak difference (frequency difference) of A1g-E12g and A1g-2LA(M).
S5 C. Raman mapping results of 1L, 2L, and 4L WS2 To further investigate uniformity, we studied the Raman mapping of 1L, 2L, and 4L WS2. We observed the uniformity of the A1g peak position, A1g peak intensity, E12g+2LA peak intensity, and difference between A1g and E12g+2LA peaks. The Raman mapping results show that the synthesized WS2 nanosheets have good uniformity.
Figure S3. Raman mapping data of (a–d)1L, (e–h)2L, and (i–l)4L WS2. (a, e, i) A1g peak position, (b, f, j) A1g peak intensity at 417.5 cm-1 (1L), 418.5 cm-1 (2L), and 419.4 cm-1 (4L), (c, g, k) E12g+2LA peak intensity at 355 cm-1 (1L), 353.5 cm-1 (2L), and 352 cm-1 (4L), and (d, h, l) peak difference for 1L, 2L, and 4L WS2 between A1g and E12g+2LA peaks.
S6 D. Schottky barrier between WS2 and metal contact (Cr/Au) Figure S4(a) shows the non-linear I-V curve of the pristine WS2 4L gas sensor, which is considered as evidence for the existence of a considerable level of Schottky barrier. As shown in Figures S4(b-c), the UPS analysis was conducted to confirm the Schottky barrier between WS2 and metal contact. The work function of pristine 4L WS2 (4.39 eV) was obtained through the UPS results, and it is smaller than the other reported values of the work function for WS2 multilayers, which range from 4.7 eV to 5.1 eV. A detailed formula for the work-function calculation is given in the manuscript concerning Figure 4. As schematically illustrated in Figure S4(d), the large work-function difference between WS2 and Au metal contact is favorable to form a Schottky barrier to the conduction band of WS2. The metal contact consists of Cr/Au (5/50 nm). Since the thickness of Cr is less than 5 nm, the net work function of the metal contact is dominantly influenced by the Au.1
S7 Figure S4. (a) I-V characteristics of the pristine 4L WS2 gas sensor. UPS results of pristine 4L WS2 surfaces at the (b) low-binding-energy region and (c) high-binding-energy region. (d) Energy-band alignment of pristine 4L WS2 calculated from UPS data.
S8 E. Gas-sensing results of ANF WS2 gas sensors (1L, 2L) Figures S5(a-b) show the current change of the ANF 1L WS2 gas sensor. The current decreased when acetone gas was supplied, while the current increased under NO2 exposure. Figures S5(c-d) show the gas-sensing results of the ANF 2L WS2 gas sensor, which shows better response compared to the ANF 1L WS2 gas sensor.
Figure S5. Gas sensing results of ANF (a-b) 1L and (c-d) 2L WS2 gas sensors under (a,c) acetone and (b,d) NO2 exposure.
S9 F. NO2 sensing (1 ppm) result of ANF 4L WS2
Figure S6. Response of the ANF 4L WS2 gas sensor for 1 ppm NO2.
S10 G. Fourier transform infrared spectroscopy (FTIR) Various FTIR peaks corresponding to PVP characterized by pyrrolidinyl (1463 and 1424 cm-1), CH and CH2 vibrations (2923, 1460 and 1374 cm-1), C-N vibrations (1285-1295 cm-1 and 1018 cm-1), and C=O vibrations (1650-1660 cm-1) clearly appeared in the FTIR spectra of pure PVP.2,3 However, these peaks indicating the presence of PVP molecules hardly appear in the spectra of the surface of Ag NWs.
Figure S7. FTIR spectra of pure PVP and Ag nanowires after several round of centrifugation with methanol.
S11 Supporting references (1)
Hwang, W. S.; Remskar, M.; Yan, R.; Protasenko, V.; Tahy, K.; Chae, S. D.; Zhao, P.; Konar, A.; (Grace) Xing, H.; Seabaugh, A.; Jena, D. Transistors with Chemically Synthesized Layered Semiconductor WS2 Exhibiting 105 Room Temperature Modulation and Ambipolar Behavior. Appl. Phys. Lett. 2012, 101, 013107.
(2)
Liu, H.; Zhang, B.; Shi, H.; Tang, Y.; Jiao, K.; Fu, X. Hydrothermal Synthesis of Monodisperse Ag2Se Nanoparticles in the Presence of PVP and KI and Their Application as Oligonucleotide Labels. J. Mater. Chem. 2008, 18, 2573.
(3)
Bryaskova, R.; Pencheva, D.; Nikolov, S.; Kantardjiev, T. Synthesis and Comparative Study on the Antimicrobial Activity of Hybrid Materials Based on Silver Nanoparticles (AgNps) Stabilized by Polyvinylpyrrolidone (PVP). J. Chem. Biol. 2011, 4, 185–191.
School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-Ro, Seodaemun-Gu, Seoul, 03722, Rep. of Korea 2
School of Mechanical Engineering, Yonsei University,
50 Yonsei-Ro, Seodaemun-Gu, Seoul, 03722, Rep. of Korea 3
Department of Physics, Konkuk University,
120 Neungdong-ro, Gwangjin-gu, Seoul 05029, Rep. of Korea
*
Corresponding author. E-mail: [email protected], [email protected]
S2 A. Observation of roughness of PEALD WO3 and thickness of WS2 nanosheets The scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of WO3 films (20, 30, 50 cycles of PEALD) were studied to compare the roughness of each WO3 film. The PE-ALD WO3 films exhibit smooth and continuous surface. The measured root mean square (r.m.s.) values of each film show very small variations with increasing thickness. After sulfurization, the WS2 nanosheets were transferred to new SiO2 substrates. The AFM image of monolayer (1L) WS2 nanosheets is shown in Figure S1(g). The inset of Figure S1(g) shows the corresponding quantitative AFM height profile. The measured height of the 1L WS2 nanosheet is 0.9 nm. As shown in Figures S1(h–i), the thickness values of the bilayer (2L) and tetra-layer (4L) WS2 nanosheets were measured to be 1.5 and 3 nm, respectively (the height profiles are shown in the insets of Figures S1(h) and S1(i), respectively). The height of the 1L WS2 nanosheet on SiO2 is 0.9 nm, and the thickness difference between adjacent layers is approximately 0.7 nm.
S3
Figure S1. (a–c) SEM images and (d–f) AFM images of WO3 films (top-view). WO3 films were synthesized by (a, d) 20, (b, e) 30, and (c, f) 50 cycles of PE-ALD. The r.m.s. values are shown in AFM images. (g–i) AFM images of transferred WS2 nanosheets; the inset display the height profile of (g) 1L, (h) 2L, and (i) 4L WS2 nanosheets.
S4 B. Raman shift and peak difference of 1L, 2L, and 4L WS2
Figure S2. (a) Raman shift of 2LA(M), E12g, and A1g modes as the number of layers increased from 1L to 4L. (b) Peak difference (frequency difference) of A1g-E12g and A1g-2LA(M).
S5 C. Raman mapping results of 1L, 2L, and 4L WS2 To further investigate uniformity, we studied the Raman mapping of 1L, 2L, and 4L WS2. We observed the uniformity of the A1g peak position, A1g peak intensity, E12g+2LA peak intensity, and difference between A1g and E12g+2LA peaks. The Raman mapping results show that the synthesized WS2 nanosheets have good uniformity.
Figure S3. Raman mapping data of (a–d)1L, (e–h)2L, and (i–l)4L WS2. (a, e, i) A1g peak position, (b, f, j) A1g peak intensity at 417.5 cm-1 (1L), 418.5 cm-1 (2L), and 419.4 cm-1 (4L), (c, g, k) E12g+2LA peak intensity at 355 cm-1 (1L), 353.5 cm-1 (2L), and 352 cm-1 (4L), and (d, h, l) peak difference for 1L, 2L, and 4L WS2 between A1g and E12g+2LA peaks.
S6 D. Schottky barrier between WS2 and metal contact (Cr/Au) Figure S4(a) shows the non-linear I-V curve of the pristine WS2 4L gas sensor, which is considered as evidence for the existence of a considerable level of Schottky barrier. As shown in Figures S4(b-c), the UPS analysis was conducted to confirm the Schottky barrier between WS2 and metal contact. The work function of pristine 4L WS2 (4.39 eV) was obtained through the UPS results, and it is smaller than the other reported values of the work function for WS2 multilayers, which range from 4.7 eV to 5.1 eV. A detailed formula for the work-function calculation is given in the manuscript concerning Figure 4. As schematically illustrated in Figure S4(d), the large work-function difference between WS2 and Au metal contact is favorable to form a Schottky barrier to the conduction band of WS2. The metal contact consists of Cr/Au (5/50 nm). Since the thickness of Cr is less than 5 nm, the net work function of the metal contact is dominantly influenced by the Au.1
S7 Figure S4. (a) I-V characteristics of the pristine 4L WS2 gas sensor. UPS results of pristine 4L WS2 surfaces at the (b) low-binding-energy region and (c) high-binding-energy region. (d) Energy-band alignment of pristine 4L WS2 calculated from UPS data.
S8 E. Gas-sensing results of ANF WS2 gas sensors (1L, 2L) Figures S5(a-b) show the current change of the ANF 1L WS2 gas sensor. The current decreased when acetone gas was supplied, while the current increased under NO2 exposure. Figures S5(c-d) show the gas-sensing results of the ANF 2L WS2 gas sensor, which shows better response compared to the ANF 1L WS2 gas sensor.
Figure S5. Gas sensing results of ANF (a-b) 1L and (c-d) 2L WS2 gas sensors under (a,c) acetone and (b,d) NO2 exposure.
S9 F. NO2 sensing (1 ppm) result of ANF 4L WS2
Figure S6. Response of the ANF 4L WS2 gas sensor for 1 ppm NO2.
S10 G. Fourier transform infrared spectroscopy (FTIR) Various FTIR peaks corresponding to PVP characterized by pyrrolidinyl (1463 and 1424 cm-1), CH and CH2 vibrations (2923, 1460 and 1374 cm-1), C-N vibrations (1285-1295 cm-1 and 1018 cm-1), and C=O vibrations (1650-1660 cm-1) clearly appeared in the FTIR spectra of pure PVP.2,3 However, these peaks indicating the presence of PVP molecules hardly appear in the spectra of the surface of Ag NWs.
Figure S7. FTIR spectra of pure PVP and Ag nanowires after several round of centrifugation with methanol.
S11 Supporting references (1)
Hwang, W. S.; Remskar, M.; Yan, R.; Protasenko, V.; Tahy, K.; Chae, S. D.; Zhao, P.; Konar, A.; (Grace) Xing, H.; Seabaugh, A.; Jena, D. Transistors with Chemically Synthesized Layered Semiconductor WS2 Exhibiting 105 Room Temperature Modulation and Ambipolar Behavior. Appl. Phys. Lett. 2012, 101, 013107.
(2)
Liu, H.; Zhang, B.; Shi, H.; Tang, Y.; Jiao, K.; Fu, X. Hydrothermal Synthesis of Monodisperse Ag2Se Nanoparticles in the Presence of PVP and KI and Their Application as Oligonucleotide Labels. J. Mater. Chem. 2008, 18, 2573.
(3)
Bryaskova, R.; Pencheva, D.; Nikolov, S.; Kantardjiev, T. Synthesis and Comparative Study on the Antimicrobial Activity of Hybrid Materials Based on Silver Nanoparticles (AgNps) Stabilized by Polyvinylpyrrolidone (PVP). J. Chem. Biol. 2011, 4, 185–191.
