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PtSe2 heterostructures
Journal Pre-proof Lattice vibration properties of MoS2/PtSe2 heterostructures Kuilong Li, Tianyi Wang, Wenjia Wang, Xingguo Gao PII:
S0925-8388(19)34438-X
DOI:
https://doi.org/10.1016/j.jallcom.2019.153192
Reference:
JALCOM 153192
To appear in:
Journal of Alloys and Compounds
Received Date: 17 August 2019 Revised Date:
25 November 2019
Accepted Date: 26 November 2019
Please cite this article as: K. Li, T. Wang, W. Wang, X. Gao, Lattice vibration properties of MoS2/ PtSe2 heterostructures, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.153192. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Kuilong Li: Resources, Formal analysis, Writing - Original Draft. Tianyi Wang: Data Curation, Visualization, Investigation. Wenjia Wang: Conceptualization,
Methodology,
Software,
Supervision,
acquisition. Xingguo Gao: Writing - Review & Editing
Funding
Lattice vibration properties of MoS2/PtSe2 heterostructures Kuilong Li1,2*, Tianyi Wang3, Wenjia Wang1*, and Xingguo Gao1
1
School of Electronic and Information Engineering (Department of Physics), Qilu
University of Technology (Shandong Academy of Sciences), Jinan, 250353, People's Republic of China
2
Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech
and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, People's Republic of China
3
School of Physics, Shandong University, Jinan, 250110, People's Republic of China
*Corresponding author: Wenjia Wang, [email protected]; Kuilong Li, [email protected]
1
ABSTRACT
In this study, large-area contiguous triplelayer PtSe2, monolayer MoS2, and MoS2/PtSe2 heterostructures were fabricated using chemical vapor deposition combined with the transfer technology. The temperature-dependent Raman spectrum demonstrates that the in-plane and out-of-plane PtSe2- and MoS2-related active modes all display red shift with increasing temperature. In addition, the temperature coefficient deduced from the MoS2/PtSe2 heterostructure is much smaller than the corresponding value obtained from the pristine MoS2 and PtSe2 films, demonstrating a better thermal stability of the heterostructure. A strong interlayer coupling in triplelayer PtSe2 and small thermal expansion mismatch together with the layer interaction between MoS2 and PtSe2 are ascribed to this softening of phonon shifts. Such study firstly contributes to the fundamental exploration of the lattice vibration properties of the MoS2/PtSe2 heterostructure and offers significant guidance for the PtSe2-based device design and fabrication. Keywords: MoS2/PtSe2 heterostructure; Raman spectrum; Temperature coefficient; Lattice vibration
2
1. Introduction In the past decade years, the research related to transition metal dichalcogenides (TMDCs) with the common formula of MX2, where M denotes a transition metal ranging from group 4 to group 10 and X represents a chalcogen atom (S, Se or Te), is a hot and eye catching field owing to their distinctive properties such as layer-dependent energy bandgap, weak van der Waals bonding between adjacent layers, high carrier mobility, anisotropic in-plane structure, unique optical properties and so on [1-5]. In turn, these materials bring revolutionary changes in the fields of electronics
and
optoelectronics
including
field
effect
transistors
(FETs),
photodetectors, sensors, catalysis, nano-renewable energy devices, and memory devices [4, 6-10]. Among them, semiconducting prototypical MX2 with group VIB transition metals (M = Mo, W) have been extensively explored. Unfortunately, the experimental carrier mobility obtained from the related devices is always one order of magnitude lower than the theoretical value [11], which seriously hinders the further device improvement. In addition, the energy bandgap of MoS2, WS2 etc. only covers the visible to near-infrared spectrum (<1.0 µm), which limits their applications in the longer wavelength. Recently, the new discovered group-10 TMDCs with strong interlayer interaction, tunable bandgap from 0.0 to 1.60 eV and high carrier mobility at room temperature have aroused an increasing research attention [12-16]. PtSe2, as a typical member of the group-10 TMDCs, has now come under the spotlight due to its tunable bandgap covering from monolayer 1.20 eV to bulk 0.0 eV, implying its potential broad spectral range similar to black phosphorus (BP) [17-20]. The superior nonlinear optical performance and ultrafast dynamics of layered PtSe2 3
indicate its promising potential in nanophotonic devices such as infrared detectors, optical switches, and saturable absorbers [21]. The PtSe2/GaN heterojunction device has demonstrated a high responsivity of 193 mA·W–1, an ultrahigh specific detectivity of 3.8 × 1014 Jones, and linear dynamic range of 155 dB [22]. Similarly, L. H. Zeng et al. have reported a PtSe2/GaAs photodetector with a responsivity as high as 262 mA·W–1 and broadband sensitivity from deep ultraviolet to near-infrared light [23]. Meanwhile, PtSe2 posses a better air stability than BP and high room-temperature carrier mobility (4000 cm2·V−1·s−1), which make it an excellent candidate in the application of ultrathin and flexible electronic devices [24-26]. The back-gate configured few-layer PtSe2 based FETs have exhibited carrier mobility up to about 210 cm2·V−1·s−1 [24], which is comparable to that of BP. The forward-current cutoff frequency of wafer-scale PtSe2 MOSFETs reaches to 42 ± 5 MHz [27]. All these characteristics mentioned above make few-layer PtSe2-based van der Waals heterostructures (vdWHs) promising for fabricating functional electronic and optoelectronic devices. Furthermore, it is important and meaningful to explore the temperature-dependent vibration properties of PtSe2 and related vdWHs, since the performances of the related devices are closely associated with the lattice vibration properties because of the electron−phonon interaction and self-heating phenomenon [28]. In this paper, high quality monolayer-MoS2/triplelayer-PtSe2 heterostructures on SiO2/Si substrate were fabricated using CVD growth method combined with the transfer technology. The lattice vibration properties were investigated using 4
temperature-dependent Raman spectrum. We have found that all the Raman active modes displayed a red shift with increasing temperature. However, the temperature coefficients extracted from the heterostructure were much smaller than the corresponding value obtained from the pristine MoS2 and PtSe2 materials. The intrinsic origin was also investigated by the aid of calculation. This study offers fundamental information of the temperature-dependent vibration properties of the less-explored group 10 TMDCs and the related heterostructures, and provides a promising guidance for future device design and fabrication. 2. Experiments The MoS2/PtSe2 heterostructures investigated in this work were prepared by CVD approach combined with the transfer process. Firstly, monolayer MoS2 and triplelayer PtSe2 were grown on SiO2 (200 nm)/Si substrates, respectively. The schematic diagram of the CVD system for PtSe2 growth was shown in Fig. 1(a). During the continuous multilayer PtSe2 films growth process, platinum tetrachloride (PtCl4) and selenium (Se) were employed as the Pt and Se sources, respectively. Se powders were placed at zone 1 and heated to the melting point of Se (200 ℃) while PtCl4 powders at zone 2 were heated up to 310 °C. Then, the evaporated Se and Pt precursors were transported onto the SiO2/Si substrate at zone 3 maintained at 500 °C by the aid of mixed Ar/H2 carrier gas with a flow rate of 210/25 sccm. Meanwhile, the growth pressure in the tube furnace was kept at 300 Pa. After 7 minutes, the desirable large-area continuous PtSe2 film (over 1 cm × 1 cm) was successfully deposited on the substrate. Large-area monolayer MoS2 films were grown using CVD method with 5
MoO3 (0.05 mg, 99%, Alfa Aesar) and sulfur(0.8 g, 99%) powder as precursors and Ar gas as the carrier gas. The growth temperature was 800 ℃ for 2 minutes. Then MoS2/PtSe2 heterostructures were fabricated by transferring the monolayer MoS2 onto the grown PtSe2 thin film with the mature PMMA method [29]. The as grown monolayer MoS2/SiO2/Si sample was spinning coated by PMMA, and then immersed into KOH solution to etch SiO2 oxide, leaving the MoS2 together with PMMA floating on the surface of the solution. Subsequently, a 4-min cleaning step in the de-ionized water was performed to remove the residual K+ on the sample. Then, the MoS2/PMMA was transferred onto the grown PtSe2/SiO2/Si sample and dried at 70 ℃. After this process, the PMMA layer was washed away by acetone. Eventually, the sample was put into vacuum chamber at 10−6 mbar for 48 h to drive off impurities adsorbed on the surface. In this work, Raman spectrums were obtained from a confocal RENISHAW system and the used pump laser is 514.5 nm. A heating stage was employed to adjust the temperature from 300 K to 510 K with a step of 30 K at a vacuum level of 10-3 mbar. Meanwhile, in order to weaken local laser heating influence, laser power was kept as low as 0.50 mW during the experiments. The related room temperature photoluminescence (PL) was also implemented in the Raman system. A Dimension-3100 atomic force microscopy (AFM) system was utilized to characterize the sample thickness. The x-ray photoelectron spectroscopy (XPS) was taken in a VG ESCALAB 220i-XL system and C 1s peak (284.8 eV) was used to calibrate the measured core-level binding energy in order to eliminate the sample surface 6
differential charging effect. 3. Results and discussion Fig. 1(b) shows the AFM image of the PtSe2 sample, and the height profile is displayed in the inset. Obviously, the thickness of PtSe2 film is about 2.00 nm, corresponding to triple layers. The PL results shown in Fig. 1(c) have identified the existence of monolayer MoS2, due to the only direct energy band for monolayer while indirect band for bilayer or thicker MoS2 [30]. Meanwhile, the small full width at half maximum (FWHM) ~ 35 nm of the peak located at 680 nm and the high peak intensity demonstrate high crystalline quality. The low-temperature (10 K) PL result shown in the inset image displays the typical three excition peaks corresponding to B excition, A neutral excition and A- trion excition just as reported in the literature [31, 32]. Fig. 1(d) displays the optical image of the MoS2/PtSe2 sample, indicating the successful construction of large area heterostructure. Then XPS measurements were taken on the prepared MoS2/PtSe2 heterojunction to estimate the related elemental binding energy and the corresponding spectra are depicted in Fig. 1(e) and 1(f). Fig. 1(e) shows the core-level XPS spectrum of Pt 4f and Se 3d. The peak binding energy for Pt 4f7/2, Pt 4f5/2, Se 3d5/2, and Se 3d3/2 are determined to be 73.58 eV, 76.90 eV, 54.89 eV, and 55.64 eV, respectively, in consistence with the reported value [33]. Meanwhile, the Se: Pt atomic ratio is estimated to be 2.05 derived from the XPS results, which is close to the stoichiometric PtSe2. In the Mo 3d spectrum shown in Fig. 1(f), Mo 3d5/2 and Mo 3d3/2 peaks locate at 230.16 eV and 233.36 eV, respectively [34]. Besides, the peak at 227.37 eV corresponds to the binding energy of S 2s, which 7
is the characteristic of the stoichiometric MoS2 [35]. The absence of Pt-O and Mo-O bonds in the XPS spectrum illustrates the high crystal quality of the materials. Raman spectroscopy as a powerful and non-destructive tool has been widely employed to investigate the properties of 2D materials and related vdWHs such as atomic bonding, material quality, thermal conductivity and so on [36-42]. Fig. 2 exhibits the room-temperature Raman spectra of the triplelayer PtSe2, monolayer MoS2, and the MoS2/PtSe2 heterostructure. For triplelayer PtSe2, mainly two Raman active modes located at around 177.01 cm−1 and 206.05 cm-1 are observed, which can be ascribed to the Eg and Ag vibration modes [43], respectively. The former one represents the atom vibration of Se within the plane while the latter one stands for the vibration of Se atoms outside the plane. The less prominent peak around 233.01 cm−1 is assigned to a couple between the A2u and Eu infrared active modes, which are longitudinal optical (LO) modes involving the out-of-plane and in-plane motions of Pt and Se atoms [43]. In the monolayer MoS2 Raman spectra, there are also two characteristic Raman modes A1g (~403.50 cm−1) and E2g1 (~385.00 cm−1) existed, which correspond to the out-of-plane movement of S atoms and in-plane vibrations of Mo and S atoms, respectively. Meanwhile, the Raman frequency difference (∆ω) between both modes is often utilized to evaluate the layer number of MoS2 film (∆ω ~19 cm-1 for monolayer MoS2), which is attributed to the different interlayer Van der Waals restoring force and the influence of stacking-induced structure changes [44]. Herein, ∆ω in this study is about 18.50 cm-1, indicating the grown MoS2 film is monolayer just as verified from the PL results. Furthermore, all the PtSe2- and 8
MoS2-based Raman active modes mentioned above are observed from the MoS2/PtSe2 heterojunction region, Eg-177.01 cm-1, Ag-206.09 cm-1, LO-233.16 cm-1, A1g-403.45 cm-1, and E2g1-382.21 cm-1 in detail. In comparison with those of the independent PtSe2 and MoS2 films, in the heterostructure, all the PtSe2-related Raman modes and MoS2-related A1g peak almost keep the same, while the MoS2-related E2g1 peak exhibits an obvious red shift by about 2.79 cm-1, which is closely associated with the stress introduced during the transfer process. Before and after transfer, the little change of FWHM of the Raman peaks implies the transfer process exerted small effect on the material quality. Subsequently, the temperature dependent Raman characterization on the triplelayer PtSe2, monolayer MoS2, and MoS2/PtSe2 samples were explored from room temperature 300 K to 510 K as displayed in Fig. 3. Fig. 3(a) shows that the three active Raman modes of PtSe2 films, Eg, Ag, and LO follow a systematic red shift with increasing the temperature. While for monolayer MoS2 shown in Figure 3(b), both A1g and E2g1 modes have presented a similar trend with PtSe2. In Fig. 3(c), all the observed Raman modes soften with the temperature rising. In order to further investigate the vibration properties of the materials, the change of Raman peak positions versus the temperature are plotted in Fig. 4(a) and 4(b). The related results extracted from triplelayer PtSe2 materials in both pristine PtSe2 and MoS2/PtSe2 heterostructure are displayed in Fig. 4(a), and those associated with MoS2 material in both independent monolayer MoS2 and MoS2/PtSe2 heterostructure are exhibited in Fig. 4(b). Herein, the data of the peak positions ω(T) versus temperature T are just 9
fitted linearly because the second and higher order temperature coefficient terms are only significant at high temperatures [45]. The formula is as follows ω T = ω + χΔT
(1)
where ω0 is the peak position of the related active modes at 300 K, χ is the first-order temperature coefficient, and ∆T is the temperature difference relative to 300 K. All the obtained χ value are summarized in Table 1. Specifically, the measured χ value (absolute value) of Eg and Ag in PtSe2 film are 0.01567 cm-1/K and 0.00991 cm-1/K, respectively. While in the MoS2/PtSe2 sample, the corresponding value are reduced to 0.01424 cm-1/K and 0.00834 cm-1/K, respectively. Recently, Dattatray J. Late et al. have reported the temperature coefficient of Eg and Ag modes in few layer PtSe2 nanosheets synthesized by wet chemistry about -0.014 cm-1/K and -0.008 cm-1/K [46], respectively, which are close to our experimental results. In comparison with monolayer MoS2, the absolute χ value of A1g and E2g1 modes in the MoS2/PtSe2 heterostructure are also reduced from 0.02141 cm-1/K to 0.01455 cm-1/K and 0.01680 cm-1/K to 0.01388 cm-1/K, respectively. Specially, in both standalone MoS2 and PtSe2 films, the observed softening of the Raman peaks with increasing temperature is primarily contributed to the variation of the harmonic pulsation by varying the lattice parameters (as a consequence of the thermal expansion), anharmonic potential constant as well as phonon occupation [37, 47]. An important and interesting phenomenon in this study is that either the PtSe2-based or the MoS2-based active Raman modes in the MoS2/PtSe2 heterostructure show smaller temperature coefficients than those in the corresponding 10
pristine material just as labeled in Table 1, indicating a better thermal stability of the heterostructure, which is beneficial for the related device applications. In particular, for triplelayer PtSe2 films, the absolute χ value of Ag mode obtained from the heterostructure is reduced by about 16% compared to that of the pristine film while the corresponding value of Eg mode is reduced by about 9.1%. For the underneath multilayer PtSe2 in the heterostructure, the up-covered MoS2 layer can induce compression of the interlayer spacing in PtSe2, leading to a strong interlayer coupling and in turn reducing the absolute χ value of Ag mode, just as reported in 2D PtS2 films that the absolute temperature coefficient decreases with increasing the layer number [48]. In addition, a type-℃ band alignment was formed at the MoS2/PtSe2 heterojunction just as published in previous report with a conduction and valence band offset about 0.86 eV and 0.66 eV [49], respectively. Then electrons would transfer from the unintentionally n-doped MoS2 to PtSe2 to recombine with holes, and in turn, this doping effect contributes to this effect. For the monolayer MoS2, the change of the substrate exerts a profound effect on the lattice vibration. Due to the large thermal expansion coefficient (TEC) mismatch between MoS2 (1.2×10-5 K-1) and SiO2 (5.5×10-7 K-1), additional compressive strain is introduced in monolayer MoS2 during the temperature-dependent Raman measurements [50]. As compared with MoS2-on-SiO2 film, the TEC difference between MoS2 (1.2×10-5 K-1) and PtSe2 (5.7×10-6 K-1) is smaller [51], that is to say the introduced in-plane compressive strain with increasing temperature in the MoS2/PtSe2 heterostructure is smaller than that in MoS2/SiO2, which consequently reduces the χ value of the in-plane strain sensitive 11
E2g1 mode. Subsequently, Vienna ab initio simulation package (VASP) code was used to take a calculation and van der Waals interaction between layers was treated with the vdW-DFT module [52]. The cutoff energy for the plane-wave basis expansion was set to be 520 eV, and the Monkhorst–Pack k-mesh was selected as 17×17×1 [53]. The convergence of the total energy was set to be better than 10-5 eV/atom. The electron exchange between S and Se atoms shown in Fig. 5 implies the formation of interlayer coupling between MoS2 and PtSe2 layers. Apparently, the electron density around S atoms is enlarged illustrated by the yellow cloud while that around Se atoms is reduced as labeled by the light blue region, indicating electrons are transferred from Se atoms to S atoms which agree with the property that the electronnegativity of S is 2.50, larger than that of Se atoms 2.40. Then the electron exchange results in the formation of layer-coupling between MoS2 and PtSe2. Consequently, this interaction also contributes to the reduction of the temperature coefficient, especially the out-of-plane related A1g mode. 4. Conclusions In this study, large-area contiguous triplelayer PtSe2 and monolayer MoS2 were successfully grown on SiO2/Si substrates, and MoS2/PtSe2/SiO2/Si heterostructures were fabricated combined with the transfer technology. The temperature dependent Raman spectrums illustrate that all the in-plane and out-of-plane active modes display red shift with increasing temperature. Nevertheless, the temperature coefficients deduced from the MoS2/PtSe2 heterostructure are much smaller than the 12
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18
Figure Captions Fig. 1. (a) The schematic structure of the chemical vapor deposition system with three zones. (b) The atomic force microscopy of the pristine PtSe2 film with the height profile in the inset, which demonstrates the thickness is about 2.0 nm. (c) The room-temperature photoluminescence of the monolayer MoS2, and the related result measured at 10K in the inset. (d) The optical image of the MoS2/PtSe2 sample. (e) and (f) are the XPS core-level binding energy spectrum of Pt 4f, Se 3d, and Mo 3d, respectively, obtained from MoS2/PtSe2 heterojunction.
Fig. 2. The room-temperature Raman spectrum taken from the pristine triplelayer PtSe2, monolayer MoS2, and MoS2/PtSe2 heterostructure.
Fig. 3. The temperature-dependent Raman spectrum of the pristine PtSe2 sample (a), monolayer MoS2 sample (b), and MoS2/PtSe2 sample (c). The corresponding active Raman modes are also labeled.
Fig. 4. The Raman peak shifts versus temperature T extracted from pristine PtSe2, monolayer MoS2, and MoS2/PtSe2 heterostructures and the related linear fitting curves for PtSe2-related modes (a) and MoS2-related modes (b).
Fig. 5. Isosurfaces of charge accumulation (yellow) and depletion (light blue) of the MoS2/PtSe2 heterostructure. 19
Table 1. The obtained experimental temperature coefficient χ value of the samples. Sample
PtSe2-χ of Eg (cm-1/K)
PtSe2-χ of Ag (cm-1/K)
MoS2-χ of E2g1 (cm-1/K)
MoS2-χ of A1g (cm-1/K)
PtSe2/SiO2/Si
-0.01567
-0.00991
-
-
MoS2/SiO2/Si
-
-
-0.01680
-0.02141
MoS2/PtSe2
-0.01424
-0.00834
-0.01388
-0.01455
20
Fig. 1. K. L. Li et al.
21
Fig. 2. K. L. Li et al.
22
Fig. 3. K. L. Li et al.
23
Fig. 4. K. L. Li et al.
24
Fig. 5. K. L. Li et al.
25
Highlights
Large area continuous triplelayer-PtSe2 and monolayer-MoS2 are grown by chemical vapor deposition. The MoS2/PtSe2 heterostructure is fabricated using transfer technology. The temperature-dependent Raman spectrum demonstrates the PtSe2- and MoS2-related active modes display red shift with increasing temperature. The temperature coefficient deduced from the MoS2/PtSe2 heterostructure is much smaller than the corresponding value obtained from the pristine MoS2 and PtSe2 films. A strong interlayer coupling in triplelayer PtSe2 and charge transfer between MoS2 and PtSe2 are ascribed to this softening of phonon shifts.
Declaration of interests ☑The authors declare that they have no known competing financial interestsor personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34438-X
DOI:
https://doi.org/10.1016/j.jallcom.2019.153192
Reference:
JALCOM 153192
To appear in:
Journal of Alloys and Compounds
Received Date: 17 August 2019 Revised Date:
25 November 2019
Accepted Date: 26 November 2019
Please cite this article as: K. Li, T. Wang, W. Wang, X. Gao, Lattice vibration properties of MoS2/ PtSe2 heterostructures, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.153192. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Kuilong Li: Resources, Formal analysis, Writing - Original Draft. Tianyi Wang: Data Curation, Visualization, Investigation. Wenjia Wang: Conceptualization,
Methodology,
Software,
Supervision,
acquisition. Xingguo Gao: Writing - Review & Editing
Funding
Lattice vibration properties of MoS2/PtSe2 heterostructures Kuilong Li1,2*, Tianyi Wang3, Wenjia Wang1*, and Xingguo Gao1
1
School of Electronic and Information Engineering (Department of Physics), Qilu
University of Technology (Shandong Academy of Sciences), Jinan, 250353, People's Republic of China
2
Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech
and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, People's Republic of China
3
School of Physics, Shandong University, Jinan, 250110, People's Republic of China
*Corresponding author: Wenjia Wang, [email protected]; Kuilong Li, [email protected]
1
ABSTRACT
In this study, large-area contiguous triplelayer PtSe2, monolayer MoS2, and MoS2/PtSe2 heterostructures were fabricated using chemical vapor deposition combined with the transfer technology. The temperature-dependent Raman spectrum demonstrates that the in-plane and out-of-plane PtSe2- and MoS2-related active modes all display red shift with increasing temperature. In addition, the temperature coefficient deduced from the MoS2/PtSe2 heterostructure is much smaller than the corresponding value obtained from the pristine MoS2 and PtSe2 films, demonstrating a better thermal stability of the heterostructure. A strong interlayer coupling in triplelayer PtSe2 and small thermal expansion mismatch together with the layer interaction between MoS2 and PtSe2 are ascribed to this softening of phonon shifts. Such study firstly contributes to the fundamental exploration of the lattice vibration properties of the MoS2/PtSe2 heterostructure and offers significant guidance for the PtSe2-based device design and fabrication. Keywords: MoS2/PtSe2 heterostructure; Raman spectrum; Temperature coefficient; Lattice vibration
2
1. Introduction In the past decade years, the research related to transition metal dichalcogenides (TMDCs) with the common formula of MX2, where M denotes a transition metal ranging from group 4 to group 10 and X represents a chalcogen atom (S, Se or Te), is a hot and eye catching field owing to their distinctive properties such as layer-dependent energy bandgap, weak van der Waals bonding between adjacent layers, high carrier mobility, anisotropic in-plane structure, unique optical properties and so on [1-5]. In turn, these materials bring revolutionary changes in the fields of electronics
and
optoelectronics
including
field
effect
transistors
(FETs),
photodetectors, sensors, catalysis, nano-renewable energy devices, and memory devices [4, 6-10]. Among them, semiconducting prototypical MX2 with group VIB transition metals (M = Mo, W) have been extensively explored. Unfortunately, the experimental carrier mobility obtained from the related devices is always one order of magnitude lower than the theoretical value [11], which seriously hinders the further device improvement. In addition, the energy bandgap of MoS2, WS2 etc. only covers the visible to near-infrared spectrum (<1.0 µm), which limits their applications in the longer wavelength. Recently, the new discovered group-10 TMDCs with strong interlayer interaction, tunable bandgap from 0.0 to 1.60 eV and high carrier mobility at room temperature have aroused an increasing research attention [12-16]. PtSe2, as a typical member of the group-10 TMDCs, has now come under the spotlight due to its tunable bandgap covering from monolayer 1.20 eV to bulk 0.0 eV, implying its potential broad spectral range similar to black phosphorus (BP) [17-20]. The superior nonlinear optical performance and ultrafast dynamics of layered PtSe2 3
indicate its promising potential in nanophotonic devices such as infrared detectors, optical switches, and saturable absorbers [21]. The PtSe2/GaN heterojunction device has demonstrated a high responsivity of 193 mA·W–1, an ultrahigh specific detectivity of 3.8 × 1014 Jones, and linear dynamic range of 155 dB [22]. Similarly, L. H. Zeng et al. have reported a PtSe2/GaAs photodetector with a responsivity as high as 262 mA·W–1 and broadband sensitivity from deep ultraviolet to near-infrared light [23]. Meanwhile, PtSe2 posses a better air stability than BP and high room-temperature carrier mobility (4000 cm2·V−1·s−1), which make it an excellent candidate in the application of ultrathin and flexible electronic devices [24-26]. The back-gate configured few-layer PtSe2 based FETs have exhibited carrier mobility up to about 210 cm2·V−1·s−1 [24], which is comparable to that of BP. The forward-current cutoff frequency of wafer-scale PtSe2 MOSFETs reaches to 42 ± 5 MHz [27]. All these characteristics mentioned above make few-layer PtSe2-based van der Waals heterostructures (vdWHs) promising for fabricating functional electronic and optoelectronic devices. Furthermore, it is important and meaningful to explore the temperature-dependent vibration properties of PtSe2 and related vdWHs, since the performances of the related devices are closely associated with the lattice vibration properties because of the electron−phonon interaction and self-heating phenomenon [28]. In this paper, high quality monolayer-MoS2/triplelayer-PtSe2 heterostructures on SiO2/Si substrate were fabricated using CVD growth method combined with the transfer technology. The lattice vibration properties were investigated using 4
temperature-dependent Raman spectrum. We have found that all the Raman active modes displayed a red shift with increasing temperature. However, the temperature coefficients extracted from the heterostructure were much smaller than the corresponding value obtained from the pristine MoS2 and PtSe2 materials. The intrinsic origin was also investigated by the aid of calculation. This study offers fundamental information of the temperature-dependent vibration properties of the less-explored group 10 TMDCs and the related heterostructures, and provides a promising guidance for future device design and fabrication. 2. Experiments The MoS2/PtSe2 heterostructures investigated in this work were prepared by CVD approach combined with the transfer process. Firstly, monolayer MoS2 and triplelayer PtSe2 were grown on SiO2 (200 nm)/Si substrates, respectively. The schematic diagram of the CVD system for PtSe2 growth was shown in Fig. 1(a). During the continuous multilayer PtSe2 films growth process, platinum tetrachloride (PtCl4) and selenium (Se) were employed as the Pt and Se sources, respectively. Se powders were placed at zone 1 and heated to the melting point of Se (200 ℃) while PtCl4 powders at zone 2 were heated up to 310 °C. Then, the evaporated Se and Pt precursors were transported onto the SiO2/Si substrate at zone 3 maintained at 500 °C by the aid of mixed Ar/H2 carrier gas with a flow rate of 210/25 sccm. Meanwhile, the growth pressure in the tube furnace was kept at 300 Pa. After 7 minutes, the desirable large-area continuous PtSe2 film (over 1 cm × 1 cm) was successfully deposited on the substrate. Large-area monolayer MoS2 films were grown using CVD method with 5
MoO3 (0.05 mg, 99%, Alfa Aesar) and sulfur(0.8 g, 99%) powder as precursors and Ar gas as the carrier gas. The growth temperature was 800 ℃ for 2 minutes. Then MoS2/PtSe2 heterostructures were fabricated by transferring the monolayer MoS2 onto the grown PtSe2 thin film with the mature PMMA method [29]. The as grown monolayer MoS2/SiO2/Si sample was spinning coated by PMMA, and then immersed into KOH solution to etch SiO2 oxide, leaving the MoS2 together with PMMA floating on the surface of the solution. Subsequently, a 4-min cleaning step in the de-ionized water was performed to remove the residual K+ on the sample. Then, the MoS2/PMMA was transferred onto the grown PtSe2/SiO2/Si sample and dried at 70 ℃. After this process, the PMMA layer was washed away by acetone. Eventually, the sample was put into vacuum chamber at 10−6 mbar for 48 h to drive off impurities adsorbed on the surface. In this work, Raman spectrums were obtained from a confocal RENISHAW system and the used pump laser is 514.5 nm. A heating stage was employed to adjust the temperature from 300 K to 510 K with a step of 30 K at a vacuum level of 10-3 mbar. Meanwhile, in order to weaken local laser heating influence, laser power was kept as low as 0.50 mW during the experiments. The related room temperature photoluminescence (PL) was also implemented in the Raman system. A Dimension-3100 atomic force microscopy (AFM) system was utilized to characterize the sample thickness. The x-ray photoelectron spectroscopy (XPS) was taken in a VG ESCALAB 220i-XL system and C 1s peak (284.8 eV) was used to calibrate the measured core-level binding energy in order to eliminate the sample surface 6
differential charging effect. 3. Results and discussion Fig. 1(b) shows the AFM image of the PtSe2 sample, and the height profile is displayed in the inset. Obviously, the thickness of PtSe2 film is about 2.00 nm, corresponding to triple layers. The PL results shown in Fig. 1(c) have identified the existence of monolayer MoS2, due to the only direct energy band for monolayer while indirect band for bilayer or thicker MoS2 [30]. Meanwhile, the small full width at half maximum (FWHM) ~ 35 nm of the peak located at 680 nm and the high peak intensity demonstrate high crystalline quality. The low-temperature (10 K) PL result shown in the inset image displays the typical three excition peaks corresponding to B excition, A neutral excition and A- trion excition just as reported in the literature [31, 32]. Fig. 1(d) displays the optical image of the MoS2/PtSe2 sample, indicating the successful construction of large area heterostructure. Then XPS measurements were taken on the prepared MoS2/PtSe2 heterojunction to estimate the related elemental binding energy and the corresponding spectra are depicted in Fig. 1(e) and 1(f). Fig. 1(e) shows the core-level XPS spectrum of Pt 4f and Se 3d. The peak binding energy for Pt 4f7/2, Pt 4f5/2, Se 3d5/2, and Se 3d3/2 are determined to be 73.58 eV, 76.90 eV, 54.89 eV, and 55.64 eV, respectively, in consistence with the reported value [33]. Meanwhile, the Se: Pt atomic ratio is estimated to be 2.05 derived from the XPS results, which is close to the stoichiometric PtSe2. In the Mo 3d spectrum shown in Fig. 1(f), Mo 3d5/2 and Mo 3d3/2 peaks locate at 230.16 eV and 233.36 eV, respectively [34]. Besides, the peak at 227.37 eV corresponds to the binding energy of S 2s, which 7
is the characteristic of the stoichiometric MoS2 [35]. The absence of Pt-O and Mo-O bonds in the XPS spectrum illustrates the high crystal quality of the materials. Raman spectroscopy as a powerful and non-destructive tool has been widely employed to investigate the properties of 2D materials and related vdWHs such as atomic bonding, material quality, thermal conductivity and so on [36-42]. Fig. 2 exhibits the room-temperature Raman spectra of the triplelayer PtSe2, monolayer MoS2, and the MoS2/PtSe2 heterostructure. For triplelayer PtSe2, mainly two Raman active modes located at around 177.01 cm−1 and 206.05 cm-1 are observed, which can be ascribed to the Eg and Ag vibration modes [43], respectively. The former one represents the atom vibration of Se within the plane while the latter one stands for the vibration of Se atoms outside the plane. The less prominent peak around 233.01 cm−1 is assigned to a couple between the A2u and Eu infrared active modes, which are longitudinal optical (LO) modes involving the out-of-plane and in-plane motions of Pt and Se atoms [43]. In the monolayer MoS2 Raman spectra, there are also two characteristic Raman modes A1g (~403.50 cm−1) and E2g1 (~385.00 cm−1) existed, which correspond to the out-of-plane movement of S atoms and in-plane vibrations of Mo and S atoms, respectively. Meanwhile, the Raman frequency difference (∆ω) between both modes is often utilized to evaluate the layer number of MoS2 film (∆ω ~19 cm-1 for monolayer MoS2), which is attributed to the different interlayer Van der Waals restoring force and the influence of stacking-induced structure changes [44]. Herein, ∆ω in this study is about 18.50 cm-1, indicating the grown MoS2 film is monolayer just as verified from the PL results. Furthermore, all the PtSe2- and 8
MoS2-based Raman active modes mentioned above are observed from the MoS2/PtSe2 heterojunction region, Eg-177.01 cm-1, Ag-206.09 cm-1, LO-233.16 cm-1, A1g-403.45 cm-1, and E2g1-382.21 cm-1 in detail. In comparison with those of the independent PtSe2 and MoS2 films, in the heterostructure, all the PtSe2-related Raman modes and MoS2-related A1g peak almost keep the same, while the MoS2-related E2g1 peak exhibits an obvious red shift by about 2.79 cm-1, which is closely associated with the stress introduced during the transfer process. Before and after transfer, the little change of FWHM of the Raman peaks implies the transfer process exerted small effect on the material quality. Subsequently, the temperature dependent Raman characterization on the triplelayer PtSe2, monolayer MoS2, and MoS2/PtSe2 samples were explored from room temperature 300 K to 510 K as displayed in Fig. 3. Fig. 3(a) shows that the three active Raman modes of PtSe2 films, Eg, Ag, and LO follow a systematic red shift with increasing the temperature. While for monolayer MoS2 shown in Figure 3(b), both A1g and E2g1 modes have presented a similar trend with PtSe2. In Fig. 3(c), all the observed Raman modes soften with the temperature rising. In order to further investigate the vibration properties of the materials, the change of Raman peak positions versus the temperature are plotted in Fig. 4(a) and 4(b). The related results extracted from triplelayer PtSe2 materials in both pristine PtSe2 and MoS2/PtSe2 heterostructure are displayed in Fig. 4(a), and those associated with MoS2 material in both independent monolayer MoS2 and MoS2/PtSe2 heterostructure are exhibited in Fig. 4(b). Herein, the data of the peak positions ω(T) versus temperature T are just 9
fitted linearly because the second and higher order temperature coefficient terms are only significant at high temperatures [45]. The formula is as follows ω T = ω + χΔT
(1)
where ω0 is the peak position of the related active modes at 300 K, χ is the first-order temperature coefficient, and ∆T is the temperature difference relative to 300 K. All the obtained χ value are summarized in Table 1. Specifically, the measured χ value (absolute value) of Eg and Ag in PtSe2 film are 0.01567 cm-1/K and 0.00991 cm-1/K, respectively. While in the MoS2/PtSe2 sample, the corresponding value are reduced to 0.01424 cm-1/K and 0.00834 cm-1/K, respectively. Recently, Dattatray J. Late et al. have reported the temperature coefficient of Eg and Ag modes in few layer PtSe2 nanosheets synthesized by wet chemistry about -0.014 cm-1/K and -0.008 cm-1/K [46], respectively, which are close to our experimental results. In comparison with monolayer MoS2, the absolute χ value of A1g and E2g1 modes in the MoS2/PtSe2 heterostructure are also reduced from 0.02141 cm-1/K to 0.01455 cm-1/K and 0.01680 cm-1/K to 0.01388 cm-1/K, respectively. Specially, in both standalone MoS2 and PtSe2 films, the observed softening of the Raman peaks with increasing temperature is primarily contributed to the variation of the harmonic pulsation by varying the lattice parameters (as a consequence of the thermal expansion), anharmonic potential constant as well as phonon occupation [37, 47]. An important and interesting phenomenon in this study is that either the PtSe2-based or the MoS2-based active Raman modes in the MoS2/PtSe2 heterostructure show smaller temperature coefficients than those in the corresponding 10
pristine material just as labeled in Table 1, indicating a better thermal stability of the heterostructure, which is beneficial for the related device applications. In particular, for triplelayer PtSe2 films, the absolute χ value of Ag mode obtained from the heterostructure is reduced by about 16% compared to that of the pristine film while the corresponding value of Eg mode is reduced by about 9.1%. For the underneath multilayer PtSe2 in the heterostructure, the up-covered MoS2 layer can induce compression of the interlayer spacing in PtSe2, leading to a strong interlayer coupling and in turn reducing the absolute χ value of Ag mode, just as reported in 2D PtS2 films that the absolute temperature coefficient decreases with increasing the layer number [48]. In addition, a type-℃ band alignment was formed at the MoS2/PtSe2 heterojunction just as published in previous report with a conduction and valence band offset about 0.86 eV and 0.66 eV [49], respectively. Then electrons would transfer from the unintentionally n-doped MoS2 to PtSe2 to recombine with holes, and in turn, this doping effect contributes to this effect. For the monolayer MoS2, the change of the substrate exerts a profound effect on the lattice vibration. Due to the large thermal expansion coefficient (TEC) mismatch between MoS2 (1.2×10-5 K-1) and SiO2 (5.5×10-7 K-1), additional compressive strain is introduced in monolayer MoS2 during the temperature-dependent Raman measurements [50]. As compared with MoS2-on-SiO2 film, the TEC difference between MoS2 (1.2×10-5 K-1) and PtSe2 (5.7×10-6 K-1) is smaller [51], that is to say the introduced in-plane compressive strain with increasing temperature in the MoS2/PtSe2 heterostructure is smaller than that in MoS2/SiO2, which consequently reduces the χ value of the in-plane strain sensitive 11
E2g1 mode. Subsequently, Vienna ab initio simulation package (VASP) code was used to take a calculation and van der Waals interaction between layers was treated with the vdW-DFT module [52]. The cutoff energy for the plane-wave basis expansion was set to be 520 eV, and the Monkhorst–Pack k-mesh was selected as 17×17×1 [53]. The convergence of the total energy was set to be better than 10-5 eV/atom. The electron exchange between S and Se atoms shown in Fig. 5 implies the formation of interlayer coupling between MoS2 and PtSe2 layers. Apparently, the electron density around S atoms is enlarged illustrated by the yellow cloud while that around Se atoms is reduced as labeled by the light blue region, indicating electrons are transferred from Se atoms to S atoms which agree with the property that the electronnegativity of S is 2.50, larger than that of Se atoms 2.40. Then the electron exchange results in the formation of layer-coupling between MoS2 and PtSe2. Consequently, this interaction also contributes to the reduction of the temperature coefficient, especially the out-of-plane related A1g mode. 4. Conclusions In this study, large-area contiguous triplelayer PtSe2 and monolayer MoS2 were successfully grown on SiO2/Si substrates, and MoS2/PtSe2/SiO2/Si heterostructures were fabricated combined with the transfer technology. The temperature dependent Raman spectrums illustrate that all the in-plane and out-of-plane active modes display red shift with increasing temperature. Nevertheless, the temperature coefficients deduced from the MoS2/PtSe2 heterostructure are much smaller than the 12
corresponding value obtained from the MoS2/SiO2/Si and PtSe2/SiO2/Si samples, demonstrating a better thermal stability of the heterostructure. A stronger interlayer coupling in triplelayer PtSe2 and smaller thermal expansion mismatch together with the layer interaction between MoS2 and PtSe2 are ascribed to this enhancement of thermal stability. Such study will contribute to the advanced exploration of the electrical, phonon, and thermal properties of the MoS2/PtSe2 heterostructure and may offer significant guidance for realizing PtSe2-based van del Waals heterostructure electronic, optoelectronic and thermoelectric devices. Acknowledgement This work is supported by the National Natural Science Foundation of China (61804086), and Natural Science Foundation of Shandong Province (ZR2017LF022). References [1]
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18
Figure Captions Fig. 1. (a) The schematic structure of the chemical vapor deposition system with three zones. (b) The atomic force microscopy of the pristine PtSe2 film with the height profile in the inset, which demonstrates the thickness is about 2.0 nm. (c) The room-temperature photoluminescence of the monolayer MoS2, and the related result measured at 10K in the inset. (d) The optical image of the MoS2/PtSe2 sample. (e) and (f) are the XPS core-level binding energy spectrum of Pt 4f, Se 3d, and Mo 3d, respectively, obtained from MoS2/PtSe2 heterojunction.
Fig. 2. The room-temperature Raman spectrum taken from the pristine triplelayer PtSe2, monolayer MoS2, and MoS2/PtSe2 heterostructure.
Fig. 3. The temperature-dependent Raman spectrum of the pristine PtSe2 sample (a), monolayer MoS2 sample (b), and MoS2/PtSe2 sample (c). The corresponding active Raman modes are also labeled.
Fig. 4. The Raman peak shifts versus temperature T extracted from pristine PtSe2, monolayer MoS2, and MoS2/PtSe2 heterostructures and the related linear fitting curves for PtSe2-related modes (a) and MoS2-related modes (b).
Fig. 5. Isosurfaces of charge accumulation (yellow) and depletion (light blue) of the MoS2/PtSe2 heterostructure. 19
Table 1. The obtained experimental temperature coefficient χ value of the samples. Sample
PtSe2-χ of Eg (cm-1/K)
PtSe2-χ of Ag (cm-1/K)
MoS2-χ of E2g1 (cm-1/K)
MoS2-χ of A1g (cm-1/K)
PtSe2/SiO2/Si
-0.01567
-0.00991
-
-
MoS2/SiO2/Si
-
-
-0.01680
-0.02141
MoS2/PtSe2
-0.01424
-0.00834
-0.01388
-0.01455
20
Fig. 1. K. L. Li et al.
21
Fig. 2. K. L. Li et al.
22
Fig. 3. K. L. Li et al.
23
Fig. 4. K. L. Li et al.
24
Fig. 5. K. L. Li et al.
25
Highlights
Large area continuous triplelayer-PtSe2 and monolayer-MoS2 are grown by chemical vapor deposition. The MoS2/PtSe2 heterostructure is fabricated using transfer technology. The temperature-dependent Raman spectrum demonstrates the PtSe2- and MoS2-related active modes display red shift with increasing temperature. The temperature coefficient deduced from the MoS2/PtSe2 heterostructure is much smaller than the corresponding value obtained from the pristine MoS2 and PtSe2 films. A strong interlayer coupling in triplelayer PtSe2 and charge transfer between MoS2 and PtSe2 are ascribed to this softening of phonon shifts.
Declaration of interests ☑The authors declare that they have no known competing financial interestsor personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: