1. Xiamen Key Laboratory of Ultra-Wide Bandgap Semiconductor Materials and Devices, Department of Physics, School of Science, Jimei University, Xiamen 361021, China
2. Department of Microelectronics and Integrated Circuits, Xiamen University, Xiamen 361005, China
hlzhu@jmu.edu.cn
yangwf@xmu.edu.cn
Show less
History+
Received
Accepted
Published
2022-01-07
2022-03-18
2022-10-15
Issue Date
Revised Date
2022-06-27
PDF
(5886KB)
Abstract
Two-dimensional (2D) WS2 films were deposited on SiO2 wafers, and the related interfacial properties were investigated by high-resolution X-ray photoelectron spectroscopy (XPS) and first-principles calculations. Using the direct (indirect) method, the valence band offset (VBO) at monolayer WS2/SiO2 interface was found to be 3.97 eV (3.86 eV), and the conduction band offset (CBO) was 2.70 eV (2.81 eV). Furthermore, the VBO (CBO) at bulk WS2/SiO2 interface is found to be about 0.48 eV (0.33 eV) larger due to the interlayer orbital coupling and splitting of valence and conduction band edges. Therefore, the WS2/SiO2 heterostructure has a Type I energy-band alignment. The band offsets obtained experimentally and theoretically are consistent except the narrower theoretical bandgap of SiO2. The theoretical calculations further reveal a binding energy of 75 meV per S atom and the totally separated partial density of states, indicating a weak interaction and negligible Fermi level pinning effect between WS2 monolayer and SiO2 surface. Our combined experimental and theoretical results provide proof of the sufficient VBOs and CBOs and weak interaction in 2D WS2/SiO2 heterostructures.
Tungsten disulfide (WS ), one of two-dimensional (2D) layered transition metal dichalcogenides (TMDCs), has been extensively investigated for its extraordinary mechanical, electronic and optoelectronic properties [1–12]. Structurally, WS is a typical hexagonal layered compound with strong in-plane and weak out-of-plane interaction, resulting in the absence of dangling bonds on the surface [13-15]. Unlike gapless graphene, 2D layered WS exhibits a moderate and variable bandgap (1−2 eV). When cleaved down to monolayer (1L), energy bandgap of layered WS converts from indirect to direct due to the quantum confinement effect [16, 17]. In addition, WS monolayer presents strong valley-contrasting spin splitting near the valence and conduction band edges owing to the absence of inversion symmetry and strong spin−orbit coupling [18]. These intriguing properties make 2D WS not only a testing platform for various physical phenomena (valley polarization [19], valley Hall effect [20], valley Zeeman effects [21], etc.), but also a promising material in a wide range of applications, such as field-effect transistors (FETs) [22, 23], flexible electronics and optoelectronics [24-26], and various types of sensors [23, 27–30].
Heterostructures of 2D layered WS with insulating substrates are of fundamental importance in the electronic and optoelectronic applications [31, 32]. Among various insulating substrates, silicon dioxide (SiO ) is the most widely used one. On one hand, high-quality 2D WS films were almost grown or exfoliated on the SiO substrates. For example, WS films with 400 µm single crystal domain have been fabricated on SiO wafers by using atmospheric pressure chemical vapor deposition (CVD) [6]. Moreover, many previous works have demonstrated the high-performance WS -based FETs and photodetectors on SiO substrates [22, 23]. Iqbal et al. [22] have successfully tailored the electronic and optoelectronic properties of a WS -based FET by using the selective chemical doping. An unprecedented high on/off ratio of 108 and a high-mobility of 255 cm2·V−1·s−1 at room temperature were obtained in their work [22]. Huo et al. [23] demonstrated that the photoresponsivity (R ) and external quantum efficiency (EQE) of FET based on the WS /SiO heterostructure could be significantly enhanced to 884 AW−1 and 1.7×105 %, respectively, due to the inclusion of molecular NH . Most recently, the WS /SiO heterostructures were also adopted to fabricate the non-volatile flash memory devices, where the layered WS films were used as charge trapping stack layers [33].
In terms of testing the fundamental and novel phenomena of 2D layered WS , superconductivity in CVD-grown monolayer WS is achieved on the SiO substrate by strong electrostatic electron doping of an electric double-layer transistor [34]. By adopting the bilayer WS /SiO heterostructure, Nayak et al. [19] have achieved a record-high valley polarization of 80% at room temperature. Strong valley Zeeman effect of dark excitons in monolayer WS /SiO heterostructure has also been demonstrated in a tilted magnetic field [21]. All above reports revealed the importance and suitability of SiO as the insulating substrate for the promising application of 2D WS . Then, Ulstrup et al. [35] used photoemission electron microscopy (PEEM) to investigate the real- and momentum-space electronic structures of electrically contacted monolayer WS stacked on SiO substrates. However, the interfacial properties of the WS /SiO heterostructure, especially the atomic-scale insight into the interfacial interactions and band offsets at the interface, remain poorly understood.
In this study, the interfacial properties of 2D layered WS grown on SiO /Si wafers were studied by X-ray photoelectron spectroscopy (XPS) and first-principles calculations. By using the experimental direct (indirect) method, the VBO value of 3.97 eV (3.86 eV) was obtained at 1L WS /SiO interface, while the CBO was obtained to be 2.70 eV (2.81 eV). When increasing the WS thickness to its bulk limit, the VBO and CBO were enhanced by 0.48 and 0.33 eV, respectively, consistent with the results of the first-principles calculations. Moreover, the binding energy of about 75 meV per S atom was obtained, implying a weak interfacial interaction between the layered WS and SiO surface.
2 Experimental details
Atomically thin WS layers were deposited on 300 nm SiO /Si substrates by using ultrahigh vacuum dc magnetron sputtering. High purity W target and S particles were used as the reaction sources. More growth details can also be found in our previous works [8, 36, 37]. S partial pressure was controlled to be 3.0×10−5 millibars and the substrate temperature is set to be 680 °C. Ar pressure of 1.1×10−3 millibars and a low sputtering power of only 8 W were adopted to achieve the thickness controllable growth. When growing for 30 s under above conditions, the WS monolayer could be obtained. The surface image and thickness of as-grown monolayer WS films were further taken by atomic force microscopy (AFM) using the Bruker Dimension Icon system. Raman and photoluminescence spectra were taken to confirm the thickness and crystal quality of WS films and performed by a Thermo Scientific DXR microscope with a 514.5 nm laser. Atomic structure was observed by using the TEM JEOL 2100 system. XPS system (VG ESCALAB 220i-XL) was mainly adopted to investigate the interfacial properties of the layered WS /SiO heterostructures. The relative sensitivity factors for W and S elements are respectively 9.8 and 1.67 in the XPS measurement. C 1s signal from the sample surface was used to correct the core-level binding energy.
3 Results and discussion
Fig.1(a) shows the AFM image of as-grown monolayer WS film on the SiO substrate. The cross sectional height of about 0.91 nm along with the white dashed line in Fig.1(a) confirms the formation of the 1L WS on the SiO substrate. Raman spectrum of 1L WS on SiO wafer shows the (in-plane) and (out-of-plane) peaks at 357.0 and 418.8 cm−1, respectively, as given in Fig.1(c). The stronger intensity of peak and the frequency difference of about 61.8 cm−1 [38] further indicate the formation of the 1L WS . A strong exciton emission peak at 611 nm was obtained in the photoluminescence spectrum, as shown in Fig.1(d), implying the high quality of the monolayer WS film. The typical transmission electron microscopy (TEM) images, as shown in Fig.1(e) and Fig.1(f), further confirm the hexagonal 2H phase of the as-grown WS layer. The periodic arrangement of W and S atoms were clearly observed, indicating the successful fabrication of the high-quality layered WS film. The lattice constant of 2H-WS crystal is 0.317 nm, similar with the reported values [8].
The core-level XPS spectra of W 4f and S 2p were taken from 1L and bulk WS /SiO interfaces and shown in Fig.2. During the XPS fitting, we followed the rule that the full width at half maximum (FWHM) of both components are comparable and the intensity ratios follow the expect quantum mechanically predicted ratio when XPS doublet have narrow separations. For W 4f spectrum of 1L WS , the peaks at 33.3 and 35.5 eV originate respectively from the 2H-WS W4+ 4f and W4+ 4f orbitals [39]. For S 2p spectrum of 1L WS , the well resolved peaks at 163.0 and 164.2 eV originate respectively from the S2− 2p and S2− 2p orbitals. Based on the quantitative analysis of XPS peaks, the W/S ratio is further calculated to be about 1:2, revealing the correct stoichiometry and high quality of the as-grown layered WS [40]. The peaks of W 4f spectrum at 36.5 and 38.3 eV may be W6+ 4f and W6+ 4f orbitals originated from the minor WO contributions (~2%) [39]. Furthermore, C 1s signal from the sample surface was used to correct the core-level binding energy in this study, then the existence of the small amounts of WO will not impact the determination of the band alignment of the WS /SiO interfaces. Additionally, the W 5p peak overlapping with the W6+ 4f is also observed at 38.9 eV. For bulk WS , the W4+ 4f , W4+ 4f , S2− 2p and S2− 2p peaks are located at 33.4, 35.6, 163.0, and 164.1 eV, respectively, similar with those of 1L WS . The W6+ 4f , W6+ 4f and W 5p peaks are located at 36.4, 38.3 and 39.0 eV, respectively. On the whole, W 4f and S 2p spectra show similar distributions for 1L and bulk WS grown on the SiO substrates. It is noted that W 4f and S 2p spectra from bulk WS show much stronger intensities than those from 1L WS , which further implies from another point of view the formation of the 1L and bulk WS in our experiments. Above obtained W 4f and S 2p spectra agree well with the current literature values [8, 39–41]. More characterization on our 2D WS can also be found in our previous works [8, 36, 37].
XPS has been proved to be an efficient and noninvasive technique for acquiring the interfacial properties of the heterojunction systems. We first adopted the indirect method proposed by Kraut et al. [42] to investigate the band alignment at 1L WS /SiO interface. The relative energy positions between the core and valence levels are assumed to be unaltered both in the bulk and at its interface. Then the valence band offset (VBO) can be calculated by
where is the binding energy difference between the core-level and valence band maximum (VBM) of bulk WS ; is the binding energy difference between the core-level and VBM of bulk SiO ; is the core-level difference between upper 1L WS and lower SiO substrate. Figures 3(a) and (b) show the core-level and valence band spectra, obtained respectively from bulk WS and bulk SiO . Fig.3(c) is the core-level spectra of S 2p and Si 2p obtained from the 1L WS /SiO heterostructure. The VBM was derived by the intersection of the linear regressions of the leading edge of valence band and the baseline of its spectra. for WS and SiO was taken to be the binding energy of S2− 2p and Si 2p peaks, respectively. The binding energy differences , , and have been illustrated in Fig.3(a)−(c). Using Eq. (1), the valence band offset (VBO) of 1L WS /SiO heterostructure was calculated to be 3.86 eV. Then, the conduction band offset (CBO) at 1L WS /SiO interface can be given by
where and are respectively the bandgap of SiO and 1L WS . Based on the O 1s loss energy spectrum, as shown in Fig.3(d), the bandgap of SiO is obtained to be 9.05 eV, consistent with the previous results [43]. Since the electronic bandgap of 2.38 eV was obtained for 1L WS in the scanning tunneling spectroscopy (STS) measurement [16], the CBO at 1L WS /SiO interface is then obtained to be 2.81 eV.
As a comparison, the direct method used by Santoni et al. [44] was further adopted to derive the band offsets of the WS /SiO heterostructures. This method requires the alignment of the XPS spectra of bare SiO , 1L and bulk WS /SiO to a common reference before comparing their relative VBM positions. Here, we used the Si 2p spectrum (104.50 eV) from the bare SiO wafer as a calibration to align the valence bands of bare SiO , 1L WS /SiO and bulk WS /SiO samples [45, 46], as shown in Fig.4. Since that, will be equal to in Eq. (1). Further substituting and by and , respectively, Eq. (1) will change to
Therefore, the direct method could deduce both the band offsets at the 1L and bulk WS /SiO interfaces as compared to the indirect method. The leading edge of valence band of clean SiO wafer is obtained to be 5.88 eV. The Si 2p and valence band spectra taken from 1L WS /SiO heterostructure were shown in Fig.4(b). As compared to Fig.4(a), some new electronic states at ~ 3.03 eV are clearly observed, which is owing to the formation of hybridized W 5d−S 3p states [47]. These hybridized W 5d−S 3p states indicates the formation of long-range ordered 2H−WS film. The valence band edge of 1L WS /SiO heterostructure is obtained to be 1.91 eV, as illustrated in Fig.4(b). Further considering the edge of O 2p band (5.88 eV) from the SiO wafer, the VBO of 3.97 eV at 1L WS /SiO interface was obtained. Taking into account the same bandgap values of the 1L WS (2.38 eV) and SiO (9.05 eV) as the previous calculation yields the CBO value of 2.70 eV at 1L WS /SiO interface. Therefore, the band offsets of the 1L WS /SiO heterostructure obtained from the direct method are in agreement with those from the indirect Kraut ’s method, indicating the validity of the experimental XPS results. Furthermore, the VBO derived from the direct method agree well with the PEEM results of 4.0 eV [35]. However, there are still small differences between two methods, which might be owing to ignoring the quantum size effects in the 1L WS sample by the indirect method [48].
Furthermore, the valence band spectrum of bulk WS2/SiO2 interface was taken and shown in Fig.4(c), where the Si 2p spectrum from the SiO substrate was also given as the reference. The electronic states from the hybridized W 5d−S 3p orbitals are developed at ~ 2.68 eV. The valence band edge is obtained to be 1.43 eV. Therefore, the formation of bulk WS shifts the valence band edge by 0.48 eV towards the Fermi level in comparison with that of the 1L WS /SiO interface. Then, the VBO value of 4.45 eV at the bulk WS /SiO interface was obtained. When the bandgap values of bulk WS (1.57 eV) and SiO (9.05 eV) were employed [17], the CBO value of 3.03 eV at bulk WS /SiO interface is then determined. Therefore, the VBO (CBO) value at bulk WS /SiO interface is about 0.48 eV (0.33 eV) larger than that of 1L WS /SiO interface, which will be further discussed based on the following first-principles calculations. In addition, we further collected the XPS spectrum at different positions of different samples, and the average VBOs of 1L and bulk WS /SiO interfaces are about 4.10 and 4.49 eV, respectively. The consistent VBOs prove the accuracy of the experiments and the uniformity of the as-grown WS . It is known that SiO is a good gate insulator for FETs due to its high surface or interfacial quality. Meanwhile, sufficient VBO and CBO at the interface of SiO /semiconductor are still necessary to act as barriers for both electron and hole injections to avoid gate leakage. Clearly, the VBO and CBO values of the WS /SiO interfaces are sufficient ( 1.0 eV) for the suppression of the leakage current, indicating the applicability of the WS /SiO heterojunction for high performance FETs.
To further investigate the interfacial interaction in the WS /SiO heterostructures, first-principles calculations were performed by using Vienna ab initio simulation package (VASP) [49]. The exchange-correlation function adopted in this calculation was the Perdew−Burke−Ernzerhof (PBE) function [50]. The van der Waals effect was included in the calculation by the DFT-D3 approach [51]. To obtain appropriate band alignment, Heyd−Scuseria−Ernzerhof (HSE06) calculations were performed to simulate the density of states (DOSs) of the WS /SiO heterostructure [52]. Possible artificial dipole interactions were eliminated by the dipole correction [53]. For 1L WS , the optimized lattice constant and HSE06 bandgap are 3.19 Å and 2.37 eV, respectively. For bulk alpha-SiO (α-SiO2), the optimized lattice constant and HSE06 bandgap are 5.02 Å and 7.53 eV, respectively. All the basic parameters obtained in this calculations agree well with the reported values [47, 54].
To simulate experimentally relevant situations, the amorphous SiO (a-SiO ) bulk structure was further generated through the molecular dynamics (MD) simulations based on the VASP code following the same procedure as shown in detail in literatures [55, 56]. The WS /SiO heterostructure was further simulated by constructing a repeated-slab model, including a-SiO , (3×3)-WS , and a 20 Å vacuum region. The optimized configuration of 1L WS /a-SiO interface was shown in Fig.5(a), in which the (3×3) WS monolayer was located on the 4.53% compressed a-SiO surface. The binding energy between 1L WS and the a-SiO surface is obtained to be only 75 meV per S atom, confirming the weak interaction at the 1L WS /SiO interface. Moreover, 1L WS almost keeps its intact hexagonal network. Our result is in agreement well with the case of the graphene weakly adsorbed on the SiO surface without forming the covalent bond between them [57]. Furthermore, most recent research about the novel fluorescence aging behavior induced by strain release in the WS /SiO heterostructure [58] also proved from another side the weak interaction at the WS /SiO interface as demonstrated in this study.
Fig.5(b) presents the calculated total DOSs of 1L WS /SiO interface and the partial DOSs of Si, O, W and S atoms. The valence bands of SiO substrate are located below −3.17 eV, and the conduction bands are above 4.82 eV. The contributions to conduction band minimum (CBM) and VBM of 1L WS /SiO heterostructure mainly come from W and S atoms of 1L WS . No surface or interfacial electronic states were found in the energy region from −3.17 eV to 4.82 eV, indicating the totally separated electronic states of the upper 1L WS and lower SiO substrate and negligible Fermi level pinning effect. The separated DOS distributions and negligible Fermi level pinning effect further imply the weak interfacial interaction at the 1L WS /SiO interface. Furthermore, the VBO (3.17 eV) and CBO (2.58 eV) at 1L WS /SiO interface could be identified and illustrated in Fig.5(b). Additionally, it should be noted that the weak interaction also shows a tiny modification of the bandgaps of the 1L WS (2.24 eV) and the SiO substrate (7.99 eV) obtained in the WS /SiO heterostructure, similar with the observation in the graphene/SiO heterostructure [57].
Above XPS results show that the VBO (CBO) value at bulk WS /SiO interface is about 0.48 eV (0.33 eV) larger than that of 1L WS /SiO interface. To illuminate the experimental findings, partial DOSs of W 5d and S 3p calculated from 1L and bulk WS were presented in Fig.5(c). The low-energy electronic states were taken as references to align the partial DOSs. Compared with the W 5d and S 3p states of 1L WS , some new electronic states emerge around the VBM and CBM and further reduce the bandgap of bulk WS . According to the previous theoretical calculation [47], these electronic states originate from the splitting of valence and conduction band edges due to the interlayer orbital coupling in bulk WS . Then, the band offsets for the bulk WS /SiO heterostructure will be enlarged correspondingly. The shift-up energy around the VBM is observed to be about 0.47 eV, which agrees well with the above XPS result of 0.48 eV. By using the photoemission electron microscopy, Keyshar et al. [59] also obtained the shift-up energy around the VBM of about 0.48 eV between 1L and bulk WS . Thus, the VBO at bulk WS /SiO interface can be calculated to be 3.64 eV. Since the bandgap of 1.52 eV for bulk WS is obtained in this calculation, the CBO value of 2.83 eV could be obtained at bulk WS /SiO interface.
We also constructed another configuration of 1L WS /O-terminated (2×2) -SiO (0001) heterostructure, as shown in Fig.6(a). The binding energy between 1L WS and the compressed O-terminated (2×2) -SiO (0001) surface is obtained to be 165 meV per S atom. The interlayer space between 1L WS and SiO substrate is 3.09 Å. Small binding energy and wide interlayer space confirm that the interaction at the 1L WS /SiO interface is week. The calculated partial DOSs of W, S, Si, and O atoms [as shown in Fig.6(b)] also exhibit the totally separated electronic states of the upper 1L WS and lower SiO substrate and negligible Fermi level pinning effect. And the VBO (3.52 eV) and CBO (2.22 eV) of this configuration could be identified, which agrees well with the theoretical results of the 1L WS /a-SiO heterostructure.
Fig.7(a) and (b) show the experimental band alignments of the 1L and bulk WS /SiO heterostructures derived using the indirect and direct method. For comparison, theoretical band alignments calculated from the 1L WS /a-SiO heterostructure were also shown in Fig.7(c). It can be seen that WS /SiO heterostructures have a Type I alignment, where the conduction and valence band edge of 1L and bulk WS films are both located within the bandgap of the SiO substrate. For 1L WS /SiO heterostructure, the results from both experimental indirect and direct methods exhibit the similar distributions. On the whole, the theoretical VBOs and CBOs are consistent with the experimental XPS band offsets, except that the theoretical VBOs and CBOs are about 0.8 and 0.2 eV smaller than the experimental results, respectively. Since the bandgap difference between the experimental (9.05 eV) and theoretical (7.99 eV) is 1.06 eV, the numerical discrepancies between the theoretical and experimental VBOs and CBOs might be just due to the narrower theoretical bandgap of SiO . Thus, the experimental band offsets from XPS are agreement well with the first-principles calculations, which also confirms the weak interaction at the WS /SiO interface. The sufficient VBOs and CBOs and weak interaction reveal that the integration strategy of 2D WS /SiO heterostructure still play an important role in fabrication of the next-generation electronic and optoelectronic devices.
4 Conclusions
In summary, the interfacial properties of 2D layered WS grown on SiO /Si wafers were investigated using XPS and first-principles calculations. By using the experimental direct (indirect) method, the VBO value of 3.97 eV (3.86 eV) was obtained at 1L WS /SiO interface, while the CBO was obtained to be 2.70 eV (2.81 eV). Furthermore, the VBO (CBO) value at bulk WS /SiO interface is about 0.48 eV (0.33 eV) larger than that of 1L WS /SiO interface. Therefore, the WS /SiO heterostructures have a Type I energy-band alignment. The band offsets obtained experimentally and theoretically are consistent except the narrower theoretical bandgap of SiO . Moreover, the binding energy of 75 meV per S atom and the totally separated partial DOSs demonstrate a weak interaction and negligible Fermi level pinning effect between WS monolayer and SiO surface. Our experimental and theoretical results ensure the practical applications of 2D WS /SiO heterostructures in next-generation electronic and optoelectronic devices.
Y.J. Zhan, Z.Liu, S.Najmaei, P.M. Ajayan, J.Lou. Large-area vapor-phase growth and characterization of MoS2 atomic layers on a SiO2 substrate. Small, 2012, 8( 7): 966
[5]
Y.H. Lee, X.Q. Zhang, W.J. Zhang, M.T. Chang, C.T. Lin, K.D. Chang, Y.C. Yu, J.T. W. Wang, C.S. Chang, L.J. Li, T.W. Lin. Synthesis of large-area MoS2 atomic layers with chemical vapor deposition. Adv. Mater., 2012, 24( 17): 2320
[6]
P.Y. Liu, T.Luo, J.Xing, H.Xu, H.Y. Hao, H.Liu, J.J. Dong. Large-area WS2 film with big single domains grown by chemical vapor deposition. Nanoscale Res. Lett., 2017, 12( 1): 558
[7]
A.L. Elías, N.Perea-Lopez, A.Castro-Beltran, A.Berkdemir, R.T. Lv, S.M. Feng, A.D. Long, T.Hayashi, Y.A. Kim, M.Endo, H.R. Gutierrez, N.R. Pradhan, L.Balicas, T.E. Mallouk, F.Lopez-Urias, H.Terrones, M.Terrones. Controlled synthesis and transfer of large-area WS2 sheets: From single layer to few layers. ACS Nano, 2013, 7( 6): 5235
M.X. Ye, D.Y. Zhang, Y.K. Yap. Recent advances in electronic and optoelectronic devices based on two-dimensional transition metal dichalcogenides. Electronics (Basel), 2017, 6( 2): 43
H.M. Hill, A.F. Rigosi, K.T. Rim, G.W. Flynn, T.F. Heinz. Band alignment in MoS2/WS2 transition metal dichalcogenide heterostructures probed by scanning tunneling microscopy and spectroscopy. Nano Lett., 2016, 16( 8): 4831
[17]
Y.Z. Guo, J.Robertson. Band engineering in transition metal dichalcogenides: Stacked versus lateral heterostructures. Appl. Phys. Lett., 2016, 108( 23): 233104
[18]
J.Jadczak, J.Kutrowska-Girzycka, P.Kapuscinski, Y.S. Huang, A.Wojs, L.Bryja. Probing of free and localized excitons and trions in atomically thin WSe2, WS2, MoSe2 and MoS2 in photoluminescence and reflectivity experiments. Nanotechnology, 2017, 28( 39): 395702
[19]
P.K. Nayak, F.C. Lin, C.H. Yeh, J.S. Huang, P.W. Chiu. Robust room temperature valley polarization in monolayer and bilayer WS2. Nanoscale, 2016, 8( 11): 6035
[20]
N.Ubrig, S.Jo, M.Philippi, D.Costanzo, H.Berger, A.B. Kuzmenko, A.F. Morpurgo. Microscopic origin of the valley Hall effect in transition metal dichalcogenides revealed by wavelength-dependent mapping. Nano Lett., 2017, 17( 9): 5719
[21]
M.Van der Donck, M.Zarenia, F.M. Peeters. Strong valley Zeeman effect of dark excitons in monolayer transition metal dichalcogenides in a tilted magnetic field. Phys. Rev. B, 2018, 97( 8): 081109
[22]
M.W. Iqbal, M.Z. Iqbal, M.F. Khan, M.A. Kamran, A.Majid, T.Alharbi, J.Eom. Tailoring the electrical and photo-electrical properties of a WS2 field effect transistor by selective n-type chemical doping. RSC Advances, 2016, 6( 29): 24675
[23]
N.J. Huo, S.X. Yang, Z.M. Wei, S.S. Li, J.B. Xia, J.B. Li. Photoresponsive and gas sensing field-effect transistors based on multilayer WS2 nanoflakes. Sci. Rep., 2014, 4( 1): 5209
R.Bhandavat, L.David, G.Singh. Synthesis of surface-functionalized WS2 nanosheets and performance as Li-ion battery anodes. J. Phys. Chem. Lett., 2012, 3( 11): 1523
[41]
S.Cadot, O.Renault, D.Rouchon, D.Mariolle, E.Nolot, C.Thieuleux, L.Veyre, H.Okuno, F.Martin, E.A. Quadrelli. Low-temperature and scalable CVD route to WS2 monolayers on SiO2/Si substrates. J. Vac. Sci. Technol. A, 2017, 35( 6): 061502
[42]
E.A. Kraut, R.W. Grant, J.R. Waldrop, S.P. Kowalczyk. Precise determination of the valence-band edge in X-ray photoemission spectra-application to measurement of semi-conductor interface potentials. Phys. Rev. Lett., 1980, 44( 24): 1620
[43]
J.G. Tao, J.W. Chai, Z.Zhang, J.S. Pan, S.J. Wang. The energy-band alignment at molybdenum disulphide and high-k dielectrics interfaces. Appl. Phys. Lett., 2014, 104( 23): 232110
[44]
A.Santoni, F.Biccari, C.Malerba, M.Valentini, R.Chierchia, A.Mittiga. Valence band offset at the CdS/Cu2ZnSnS4 interface probed by X-ray photoelectron spectroscopy. J. Phys. D Appl. Phys., 2013, 46( 17): 175101
[45]
F.J. Grunthaner, B.F. Lewis, N.Zamini, J.Maserjian, A.Madhukar. XPS studies of structure-induced radiation effects at the Si/SiO2 interface. IEEE Trans. Nucl. Sci., 1980, 27( 6): 1640
S.Grimme, J.Antony, S.Ehrlich, H.Krieg. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys., 2010, 132( 15): 154104
[52]
J.Heyd, G.E. Scuseria, M.Ernzerhof. Hybrid functionals based on a screened Coulomb potential. J. Chem. Phys., 2003, 118( 18): 8207
O.I. Malyi, V.V. Kulish, C.Persson. In search of new reconstructions of (001) alpha-quartz surface: A first principles study. RSC Advances, 2014, 4( 98): 55599
[55]
W.L. Scopel, A.J. R. da Silva, A.Fazzio. Amorphous HfO2 and Hf1−xSixO via a melt-and-quench scheme using ab initio molecular dynamics. Phys. Rev. B, 2008, 77( 17): 172101
[56]
W.L. Scopel, R.H. Miwa, T.M. Schmidt, P.Venezuela. MoS2 on an amorphous HfO2 surface: An ab initio investigation. J. Appl. Phys., 2015, 117( 19): 194303
[57]
T.C. Nguyen, M.Otani, S.Okada. Semiconducting electronic property of graphene adsorbed on (0001) surfaces of SiO2. Phys. Rev. Lett., 2011, 106( 10): 106801
K.Keyshar, M.Berg, X.Zhang, R.Vajtai, G.Gupta, C.K. Chan, T.E. Beechem, P.M. Ajayan, A.D. Mohite, T.Ohta. Experimental determination of the ionization energies of MoSe2, WS2, and MoS2 on SiO2 using photoemission electron microscopy. ACS Nano, 2017, 11( 8): 8223
RIGHTS & PERMISSIONS
Higher Education Press
AI Summary 中Eng×
Note: Please be aware that the following content is generated by artificial intelligence. This website is not responsible for any consequences arising from the use of this content.
PDF
(5886KB)
