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MoS2 heterostructure grown by pulsed laser deposition
Materials Letters 253 (2019) 187–190
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Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Large-area ZnO/MoS2 heterostructure grown by pulsed laser deposition Binbin Su, Haiping He ⇑, Zhizhen Ye School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 28 April 2019 Received in revised form 10 June 2019 Accepted 13 June 2019 Available online 14 June 2019 Keywords: Semiconductors Optical materials and properties Thin films
a b s t r a c t A simple and efficient method to grow continuous large-area ZnO/MoS2 heterostructures is demonstrated by pulsed laser deposition. The layer number of MoS2 films is controlled by the number of laser pulses. The MoS2 film acts as a closely lattice-matched buffer layer for the subsequent ZnO growth, which improves the structural and optical quality of the ZnO film. ZnO film and multilayer MoS2 forms a type-II heterostructure, and the band offsets between them are quantitatively determined. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Recently, heterostructures formed by two-dimensional (2D) MoS2 and other semiconductors have received considerable attention due to their various potential applications [1,2]. Among these heterostructures, ZnO/MoS2 [3–5] is a promising candidate because ZnO is an outstanding multi-functional semiconductor, which is expected to either significantly enhance the photoelectric properties of MoS2 or to introduce new functions in such heterostructures. Especially, the lattice constant of ZnO (0.325 nm) [6] matches well (only 2% misfit) with that of MoS2 (0.319 nm) [7], which has great advantages in the growth of ZnO/ MoS2 system. However, the fabrication of ZnO/MoS2 heterostructures reported so far usually consisted of several steps, including a manual poly(methyl methacrylate) (PMMA) transfer process [8]. Moreover, large scale fabrication is limited by such a transfer process. A simpler and more efficient method to obtain largearea ZnO/MoS2 heterostructures is thus desired. Pulsed laser deposition (PLD) is widely used for large-area growth of continuous crystalline films and offers a broad dynamic range of operation to control the layer number by using pulsed laser. Some studies have been reported on the growth of 2D MoS2 by PLD [9–11], revealing it an alternative to address the limitations of other methods. In addition, PLD is regarded as one of the most suitable methods for high quality epitaxy of various oxides such as ZnO [12]. In this work, we report direct growth of waferscale ZnO/MoS2 heterostructures by PLD. The underlying MoS2 film acts as a closely lattice-matched buffer layer for the ZnO growth,
⇑ Corresponding author. E-mail address: [email protected] (H. He). https://doi.org/10.1016/j.matlet.2019.06.048 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
leading to improved crystal quality of the ZnO film. The obtained ZnO/MoS2 heterostructure has a type-II band alignment. 2. Experiments 2.1. Growth of ML-MoS2 film The multilayer (ML)-MoS2 films were prepared on a single-side polished Al2O3 (0 0 0 1) substrate by PLD. Two-inch diameter MoS2 target (99.99% purity) was placed 5 cm away from the substrate. A pulsed KrF excimer laser (k = 248 nm, repetition rate = 5 Hz, pulse energy = 100 mJ) was used and the substrate temperature was held at 650 °C. Before the deposition, the chamber was evacuated to a pressure of <103 Pa. After growth, the sample was cooled down to room temperature naturally. 2.2. Growth of ZnO/MoS2 heterostructure The ZnO/MoS2 heterostructure was prepared from a ZnO target (99.99% purity) on the previously grown ML-MoS2 film. A pulsed KrF excimer laser (k = 248 nm, repetition rate = 5 Hz, pulse energy = 300 mJ) was used and the substrate temperature was held at 200 °C. The process was carried out at 1 Pa in an oxygen atmosphere. 2.3. Characterization Scanning electron microscopy (SEM) images were obtained by using an S-4800 SEM (Hitachi) and atomic force microscopy (AFM) images were obtained by using a multimode AFM (Veeco). X-ray diffraction (XRD) patterns were recorded by using an X’pert
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PRO diffractometer (PANalytical) with Cu Ka (k = 0.15406 nm) radiation. Raman spectroscopy was conducted by using a LabRAM HR Evolution system (Horiba Jobin Yvon) with 532.5 nm as the excitation radiation. Photoluminescence (PL) measurements were performed on an FLS920 fluorescence spectrometer (Edinburgh Instruments) with a 325 nm He-Cd laser as the excitation light. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB 250Xi spectrometer (Thermo) using Al Ka (1486.6 eV) radiation.
3. Results and discussion The MoS2 films deposited by different numbers of laser pulses were characterized by Raman spectroscopy. The E12g mode results from the in-plane vibration of Mo and S atoms and the A1g mode
is associated with the out-of-plane vibration of S atoms. The frequency difference (Dk) between these two characteristic Raman modes is closely related to the layer number and can be used to determine the thickness of MoS2 [13]. As shown in Fig. 1a, Dk of 50-pulse sample is about 21.8 cm1, which corresponds to bilayer. For 100-pulse sample, Dk is about 23.5 cm1, and thereby trilayer is expected. When the number of laser pulses exceeds 300, the spectra show a spacing of 25.1 cm1, revealing that bulk-like MoS2 is formed. Fig. 1b shows the surface topography of 50pulse sample probed by AFM. The film is continuous and smooth with a root mean square (RMS) roughness of 0.48 nm. Then, ZnO film was grown on the 300-pulse MoS2 buffer layer on Al2O3 (0 0 0 1) substrate. Considering that the buffer layer should have an enough thickness to facilitate ZnO growth, we choose the 300-pulse sample rather than 50- or 100-pulse ones. Moreover, to avoid oxidation of MoS2 during the ZnO growth, the
Fig. 1. (a) Raman spectra of the MoS2 films deposited on Al2O3 (0 0 0 1) by different numbers of laser pulses. (b) AFM image of 50-pulse MoS2 film.
Fig. 2. SEM images of the ZnO films (a) with and (b) without the MoS2 buffer layer. Insets are the corresponding SEM cross-section images. (c) XRD patterns, (d) XRD rocking curves and (e) room-temperature PL spectra of the ZnO and ZnO/MoS2 films.
B. Su et al. / Materials Letters 253 (2019) 187–190
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Fig. 3. Zn 2p3/2CL spectra for (a) bulk-like ZnO and (e) ZnO/MoS2 samples and Mo 3d5/2CL spectra for (c) bulk-like MoS2 and (f) ZnO/MoS2 samples. Valence band spectra for (b) bulk-like ZnO and (d) bulk-like MoS2 samples. (g) Energy band diagram of the ZnO/MoS2 interface.
temperature of ZnO growth was kept at a relatively low value (200 °C). For the control sample, the deposition of MoS2 buffer layer was skipped. SEM images in Fig. 2a–b show the surface morphologies of the samples with and without the MoS2 buffer layer. They are found to be continuous and compact films with a thickness of 400 nm. As revealed by the XRD patterns in Fig. 2c, both samples exhibit sharp ZnO (0 0 2) diffraction peaks, suggesting good crystallinity of the ZnO films as well as their preferred caxis orientation of growth. The crystal quality of the ZnO films is evaluated by the XRD rocking curves around the ZnO (0 0 2) peak. As shown in Fig. 2d, with MoS2 buffer layer applied, the rocking curve becomes narrower obviously, indicating the improved crystal quality. This is further supported by PL measurements. It is well accepted that the PL linewidth of near band edge serves as an indicator of crystal quality of semiconductors. PL spectra in Fig. 2e show that the near band edge emission of ZnO (3.25 eV) in ZnO/MoS2 sample is narrower than that in ZnO sample, which suggests that MoS2 buffer layer is beneficial to the crystal quality of the ZnO film. The band alignment of the ZnO/MoS2 interface was studied by XPS according to Kraut’s method [14] which is based on the precise determination of the energy difference between the core-level (CL) peak and the valence band edge. Fig. 3a-f show the CL spectra as well as the valence band spectra of the ZnO/MoS2 system. The CL spectra are fitted to Voigt (mixed Lorentzian-Gaussian) line shapes by employing a Shirley background. The valence band maximum (VBM) is obtained from the intercept of the tangent at the leading edge of the valence band spectrum with the base line. The valence band offset (VBO or DEV ) of the interface can be calculated by
MoS2=ZnO MoS2 MoS2 DEV ¼ EMoS2=ZnO Zn 2p3=2 EMo 3d5=2 þ EMo 3d5=2 EVBM ZnO EZnO Zn 2p3=2 EVBM
ð1Þ
where Esi denotes the energy of feature i in sample s. The value of VBO between ZnO and MoS2 from Zn 2p3/2 and Mo 3d5/2CL is estimated to be 2.52 eV, by using Eq. (1). Based on the VBO, the conduction band offset (CBO or DEC ) can be calculated by
DEC ¼ EZnO EMoS2 DE V G G
ð2Þ
where EsG is the band gap of sample s. By using the band gap of ZnO (3.37 eV) [6] and MoS2 (1.20 eV) [15] in Eq. (2), a CBO of 0.35 eV is obtained. Based on the calculated values, the energy band diagram of the ZnO/MoS2 interface can be deduced and is shown in Fig. 3g. It can be seen that the ZnO/MoS2 heterostructure has a type-II staggered band alignment, which is in favor of the charge separation by localizing electrons in ZnO or holes in MoS2. Such a heterostructure is expected to find applications in various high-performance optoelectronic devices, such as solar cells and photodetectors. 4. Conclusions To summarize, we have demonstrated a simple and efficient method to grow ZnO/MoS2 heterostructures by PLD. The number of grown MoS2 layers can be precisely controlled by varying the number of laser pulses, showing the potential of using PLD for layer control of 2D materials. With MoS2 buffer layer, the crystal quality of the ZnO film is improved. In addition, the band alignment of the ZnO/MoS2 interface has been studied by XPS. The ZnO/MoS2 heterostructure reveals a type-II band alignment with a VBO of 2.52 eV and a CBO of 0.35 eV. Such a heterostructure will facilitate the understanding of charge transport and the design of corresponding optoelectronic devices. Declaration of Competing Interest None. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (nos. 91833301 and 51372223), and the Fundamental Research Funds for the Central Universities (no. 2017FZA4007). References [1] X. Duan, C. Wang, A. Pan, R. Yu, X. Duan, Chem. Soc. Rev. 44 (2015) 8859. [2] E. Singh, K.S. Kim, G.Y. Yeom, H.S. Nalwa, ACS Appl Mater. Interfaces 9 (2017) 3223.
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[3] M.S. Kim, S. Roy, J. Lee, B.G. Kim, H. Kim, J. Park, S.J. Yun, G.H. Han, J. Leem, J. Kim, ACS Appl Mater. Interfaces 8 (2016) 28809. [4] X. Liu, Y. Zhang, Q. Liu, J. He, L. Chen, K. Li, F. Jia, Y. Zeng, Y. Lu, W. Yu, D. Zhu, W. Liu, J. Wu, Z. He, K. Ang, Appl. Phys. Lett. 109 (2016) 071602. [5] L. Chen, F. Xue, X. Li, X. Huang, L. Wang, J. Kou, Z.L. Wang, ACS Nano 10 (2016) 1546. [6] Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, H. Morkoçd, J. Appl. Phys. 98 (2005) 041301. [7] Y. Ding, Y. Wang, J. Ni, L. Shi, S. Shi, W. Tang, Phys. B 406 (2011) 2254. [8] S. Park, M.S. Kim, H. Kim, J. Lee, G.H. Han, J. Jung, J. Kim, ACS Nano 9 (2015) 11042. [9] T.A.J. Loh, D.H.C. Chua, ACS Appl Mater. Interfaces 6 (2014) 15966.
[10] M.I. Serna, S.H. Yoo, S. Moreno, Y. Xi, J.P. Oviedo, H. Choi, H.N. Alshareef, M.J. Kim, M. Minary-Jolandan, M.A. Quevedo-Lopez, ACS Nano 10 (2016) 6054. [11] A. Barvat, N. Prakash, G. Kumar, D.K. Singh, A. Dogra, S.P. Khanna, P. Pal, Curr. Appl Phys. 18 (2018) 170. [12] Z. Xu, H.P. He, L.W. Sun, Y.Z. Jin, B.H. Zhao, Z.Z. Ye, J. Appl. Phys. 107 (2010) 053524. [13] C. Lee, H. Yan, L.E. Brus, T.F. Heinz, J. Hone, S. Ryu, ACS Nano 4 (2010) 2695. [14] E.A. Kraut, R.W. Grant, J.R. Waldrop, S.P. Kowalczyk, Phys. Rev. Lett. 44 (1980) 1620. [15] Q.H. Wang, K. Kalantar-Zadeh, A. Kis, J.N. Coleman, M.S. Strano, Nature Nanotechnol. 7 (2012) 699.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Large-area ZnO/MoS2 heterostructure grown by pulsed laser deposition Binbin Su, Haiping He ⇑, Zhizhen Ye School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 28 April 2019 Received in revised form 10 June 2019 Accepted 13 June 2019 Available online 14 June 2019 Keywords: Semiconductors Optical materials and properties Thin films
a b s t r a c t A simple and efficient method to grow continuous large-area ZnO/MoS2 heterostructures is demonstrated by pulsed laser deposition. The layer number of MoS2 films is controlled by the number of laser pulses. The MoS2 film acts as a closely lattice-matched buffer layer for the subsequent ZnO growth, which improves the structural and optical quality of the ZnO film. ZnO film and multilayer MoS2 forms a type-II heterostructure, and the band offsets between them are quantitatively determined. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Recently, heterostructures formed by two-dimensional (2D) MoS2 and other semiconductors have received considerable attention due to their various potential applications [1,2]. Among these heterostructures, ZnO/MoS2 [3–5] is a promising candidate because ZnO is an outstanding multi-functional semiconductor, which is expected to either significantly enhance the photoelectric properties of MoS2 or to introduce new functions in such heterostructures. Especially, the lattice constant of ZnO (0.325 nm) [6] matches well (only 2% misfit) with that of MoS2 (0.319 nm) [7], which has great advantages in the growth of ZnO/ MoS2 system. However, the fabrication of ZnO/MoS2 heterostructures reported so far usually consisted of several steps, including a manual poly(methyl methacrylate) (PMMA) transfer process [8]. Moreover, large scale fabrication is limited by such a transfer process. A simpler and more efficient method to obtain largearea ZnO/MoS2 heterostructures is thus desired. Pulsed laser deposition (PLD) is widely used for large-area growth of continuous crystalline films and offers a broad dynamic range of operation to control the layer number by using pulsed laser. Some studies have been reported on the growth of 2D MoS2 by PLD [9–11], revealing it an alternative to address the limitations of other methods. In addition, PLD is regarded as one of the most suitable methods for high quality epitaxy of various oxides such as ZnO [12]. In this work, we report direct growth of waferscale ZnO/MoS2 heterostructures by PLD. The underlying MoS2 film acts as a closely lattice-matched buffer layer for the ZnO growth,
⇑ Corresponding author. E-mail address: [email protected] (H. He). https://doi.org/10.1016/j.matlet.2019.06.048 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
leading to improved crystal quality of the ZnO film. The obtained ZnO/MoS2 heterostructure has a type-II band alignment. 2. Experiments 2.1. Growth of ML-MoS2 film The multilayer (ML)-MoS2 films were prepared on a single-side polished Al2O3 (0 0 0 1) substrate by PLD. Two-inch diameter MoS2 target (99.99% purity) was placed 5 cm away from the substrate. A pulsed KrF excimer laser (k = 248 nm, repetition rate = 5 Hz, pulse energy = 100 mJ) was used and the substrate temperature was held at 650 °C. Before the deposition, the chamber was evacuated to a pressure of <103 Pa. After growth, the sample was cooled down to room temperature naturally. 2.2. Growth of ZnO/MoS2 heterostructure The ZnO/MoS2 heterostructure was prepared from a ZnO target (99.99% purity) on the previously grown ML-MoS2 film. A pulsed KrF excimer laser (k = 248 nm, repetition rate = 5 Hz, pulse energy = 300 mJ) was used and the substrate temperature was held at 200 °C. The process was carried out at 1 Pa in an oxygen atmosphere. 2.3. Characterization Scanning electron microscopy (SEM) images were obtained by using an S-4800 SEM (Hitachi) and atomic force microscopy (AFM) images were obtained by using a multimode AFM (Veeco). X-ray diffraction (XRD) patterns were recorded by using an X’pert
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PRO diffractometer (PANalytical) with Cu Ka (k = 0.15406 nm) radiation. Raman spectroscopy was conducted by using a LabRAM HR Evolution system (Horiba Jobin Yvon) with 532.5 nm as the excitation radiation. Photoluminescence (PL) measurements were performed on an FLS920 fluorescence spectrometer (Edinburgh Instruments) with a 325 nm He-Cd laser as the excitation light. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB 250Xi spectrometer (Thermo) using Al Ka (1486.6 eV) radiation.
3. Results and discussion The MoS2 films deposited by different numbers of laser pulses were characterized by Raman spectroscopy. The E12g mode results from the in-plane vibration of Mo and S atoms and the A1g mode
is associated with the out-of-plane vibration of S atoms. The frequency difference (Dk) between these two characteristic Raman modes is closely related to the layer number and can be used to determine the thickness of MoS2 [13]. As shown in Fig. 1a, Dk of 50-pulse sample is about 21.8 cm1, which corresponds to bilayer. For 100-pulse sample, Dk is about 23.5 cm1, and thereby trilayer is expected. When the number of laser pulses exceeds 300, the spectra show a spacing of 25.1 cm1, revealing that bulk-like MoS2 is formed. Fig. 1b shows the surface topography of 50pulse sample probed by AFM. The film is continuous and smooth with a root mean square (RMS) roughness of 0.48 nm. Then, ZnO film was grown on the 300-pulse MoS2 buffer layer on Al2O3 (0 0 0 1) substrate. Considering that the buffer layer should have an enough thickness to facilitate ZnO growth, we choose the 300-pulse sample rather than 50- or 100-pulse ones. Moreover, to avoid oxidation of MoS2 during the ZnO growth, the
Fig. 1. (a) Raman spectra of the MoS2 films deposited on Al2O3 (0 0 0 1) by different numbers of laser pulses. (b) AFM image of 50-pulse MoS2 film.
Fig. 2. SEM images of the ZnO films (a) with and (b) without the MoS2 buffer layer. Insets are the corresponding SEM cross-section images. (c) XRD patterns, (d) XRD rocking curves and (e) room-temperature PL spectra of the ZnO and ZnO/MoS2 films.
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Fig. 3. Zn 2p3/2CL spectra for (a) bulk-like ZnO and (e) ZnO/MoS2 samples and Mo 3d5/2CL spectra for (c) bulk-like MoS2 and (f) ZnO/MoS2 samples. Valence band spectra for (b) bulk-like ZnO and (d) bulk-like MoS2 samples. (g) Energy band diagram of the ZnO/MoS2 interface.
temperature of ZnO growth was kept at a relatively low value (200 °C). For the control sample, the deposition of MoS2 buffer layer was skipped. SEM images in Fig. 2a–b show the surface morphologies of the samples with and without the MoS2 buffer layer. They are found to be continuous and compact films with a thickness of 400 nm. As revealed by the XRD patterns in Fig. 2c, both samples exhibit sharp ZnO (0 0 2) diffraction peaks, suggesting good crystallinity of the ZnO films as well as their preferred caxis orientation of growth. The crystal quality of the ZnO films is evaluated by the XRD rocking curves around the ZnO (0 0 2) peak. As shown in Fig. 2d, with MoS2 buffer layer applied, the rocking curve becomes narrower obviously, indicating the improved crystal quality. This is further supported by PL measurements. It is well accepted that the PL linewidth of near band edge serves as an indicator of crystal quality of semiconductors. PL spectra in Fig. 2e show that the near band edge emission of ZnO (3.25 eV) in ZnO/MoS2 sample is narrower than that in ZnO sample, which suggests that MoS2 buffer layer is beneficial to the crystal quality of the ZnO film. The band alignment of the ZnO/MoS2 interface was studied by XPS according to Kraut’s method [14] which is based on the precise determination of the energy difference between the core-level (CL) peak and the valence band edge. Fig. 3a-f show the CL spectra as well as the valence band spectra of the ZnO/MoS2 system. The CL spectra are fitted to Voigt (mixed Lorentzian-Gaussian) line shapes by employing a Shirley background. The valence band maximum (VBM) is obtained from the intercept of the tangent at the leading edge of the valence band spectrum with the base line. The valence band offset (VBO or DEV ) of the interface can be calculated by
MoS2=ZnO MoS2 MoS2 DEV ¼ EMoS2=ZnO Zn 2p3=2 EMo 3d5=2 þ EMo 3d5=2 EVBM ZnO EZnO Zn 2p3=2 EVBM
ð1Þ
where Esi denotes the energy of feature i in sample s. The value of VBO between ZnO and MoS2 from Zn 2p3/2 and Mo 3d5/2CL is estimated to be 2.52 eV, by using Eq. (1). Based on the VBO, the conduction band offset (CBO or DEC ) can be calculated by
DEC ¼ EZnO EMoS2 DE V G G
ð2Þ
where EsG is the band gap of sample s. By using the band gap of ZnO (3.37 eV) [6] and MoS2 (1.20 eV) [15] in Eq. (2), a CBO of 0.35 eV is obtained. Based on the calculated values, the energy band diagram of the ZnO/MoS2 interface can be deduced and is shown in Fig. 3g. It can be seen that the ZnO/MoS2 heterostructure has a type-II staggered band alignment, which is in favor of the charge separation by localizing electrons in ZnO or holes in MoS2. Such a heterostructure is expected to find applications in various high-performance optoelectronic devices, such as solar cells and photodetectors. 4. Conclusions To summarize, we have demonstrated a simple and efficient method to grow ZnO/MoS2 heterostructures by PLD. The number of grown MoS2 layers can be precisely controlled by varying the number of laser pulses, showing the potential of using PLD for layer control of 2D materials. With MoS2 buffer layer, the crystal quality of the ZnO film is improved. In addition, the band alignment of the ZnO/MoS2 interface has been studied by XPS. The ZnO/MoS2 heterostructure reveals a type-II band alignment with a VBO of 2.52 eV and a CBO of 0.35 eV. Such a heterostructure will facilitate the understanding of charge transport and the design of corresponding optoelectronic devices. Declaration of Competing Interest None. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (nos. 91833301 and 51372223), and the Fundamental Research Funds for the Central Universities (no. 2017FZA4007). References [1] X. Duan, C. Wang, A. Pan, R. Yu, X. Duan, Chem. Soc. Rev. 44 (2015) 8859. [2] E. Singh, K.S. Kim, G.Y. Yeom, H.S. Nalwa, ACS Appl Mater. Interfaces 9 (2017) 3223.
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[3] M.S. Kim, S. Roy, J. Lee, B.G. Kim, H. Kim, J. Park, S.J. Yun, G.H. Han, J. Leem, J. Kim, ACS Appl Mater. Interfaces 8 (2016) 28809. [4] X. Liu, Y. Zhang, Q. Liu, J. He, L. Chen, K. Li, F. Jia, Y. Zeng, Y. Lu, W. Yu, D. Zhu, W. Liu, J. Wu, Z. He, K. Ang, Appl. Phys. Lett. 109 (2016) 071602. [5] L. Chen, F. Xue, X. Li, X. Huang, L. Wang, J. Kou, Z.L. Wang, ACS Nano 10 (2016) 1546. [6] Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, H. Morkoçd, J. Appl. Phys. 98 (2005) 041301. [7] Y. Ding, Y. Wang, J. Ni, L. Shi, S. Shi, W. Tang, Phys. B 406 (2011) 2254. [8] S. Park, M.S. Kim, H. Kim, J. Lee, G.H. Han, J. Jung, J. Kim, ACS Nano 9 (2015) 11042. [9] T.A.J. Loh, D.H.C. Chua, ACS Appl Mater. Interfaces 6 (2014) 15966.
[10] M.I. Serna, S.H. Yoo, S. Moreno, Y. Xi, J.P. Oviedo, H. Choi, H.N. Alshareef, M.J. Kim, M. Minary-Jolandan, M.A. Quevedo-Lopez, ACS Nano 10 (2016) 6054. [11] A. Barvat, N. Prakash, G. Kumar, D.K. Singh, A. Dogra, S.P. Khanna, P. Pal, Curr. Appl Phys. 18 (2018) 170. [12] Z. Xu, H.P. He, L.W. Sun, Y.Z. Jin, B.H. Zhao, Z.Z. Ye, J. Appl. Phys. 107 (2010) 053524. [13] C. Lee, H. Yan, L.E. Brus, T.F. Heinz, J. Hone, S. Ryu, ACS Nano 4 (2010) 2695. [14] E.A. Kraut, R.W. Grant, J.R. Waldrop, S.P. Kowalczyk, Phys. Rev. Lett. 44 (1980) 1620. [15] Q.H. Wang, K. Kalantar-Zadeh, A. Kis, J.N. Coleman, M.S. Strano, Nature Nanotechnol. 7 (2012) 699.