Pressure Induced Semiconductor-Semimetal Transition in WSe2

Jul 29, 2010 - A pressure induced semiconductor-semimetal phase transition on tungsten diselenide has been studied using in situ electrical resistivit...
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J. Phys. Chem. C 2010, 114, 14251–14254

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Pressure Induced Semiconductor-Semimetal Transition in WSe2 Bao Liu, Yonghao Han, and Chunxiao Gao* State Key Lab of Superhard Materials, Jilin UniVersity, Changchun 130012

Yanzhang Ma Department of Mechanical Engineering, Texas Tech UniVersity, Lubbock, Texas 79409

Gang Peng, Baojia Wu, Cailong Liu, Yue Wang, Tingjing Hu, Xiaoyan Cui, Wanbin Ren, Yan Li, Ningning Su, Hongwu Liu, and Guangtian Zou Institute of Atomic and Molecular Physics, Jilin UniVersity, Changchun 130012 ReceiVed: May 6, 2010; ReVised Manuscript ReceiVed: July 14, 2010

A pressure induced semiconductor-semimetal phase transition on tungsten diselenide has been studied using in situ electrical resistivity measurement and first-principles calculation under high pressure. The experimental results indicate that the phase transition takes place at 38.1 GPa. The first-principles calculations performed by CASTEP code based on the density functional theory illustrate that the indirect band gap of WSe2 vanishes at 35 GPa, which results in an isostructural phase transition from semiconductor to semimetal in WSe2. According to the pressure dependence of partial density of states, the semimetallic character of WSe2 is mainly caused by W-Se covalent bonding rather than van der Waals bonding. 1. Introduction Transition metal dichalcogenides (TMDCs), MX2 (M ) Cr, Mo, W, Ti, Nb, Ta, etc., X ) S, Se, Te, etc.), have been a research area for many theoretical and experimental studies during the last 40 years. WSe2, which belongs to the large family of layered TMDCs, is composed of two-dimensional sheets stacked on top of one another, as shown in Figure 1. Each sheet is trilayered with a transition-metal atom W in the middle that is covalently bonded to chalcogen atoms Se located in the top and bottom layers. Weak van der Waals forces exist between the sandwich sheets along the c-axis.1 Due to these weak forces, shearing takes place more easily even under high pressure. So WSe2 is well-suited to extreme pressure lubrication.2 As a potential material for optoelectronic and photovoltaic devices and applications,3-6 electrical and optical anisotropic properties of WSe2 have been investigated in detail7 and some optical studies8-10 and theoretical calculations11,12 have suggested a semiconductor with an indirect band gap in a broad range from 0.35 to 1.3 eV. Isostructural phase transition brings a new way for understanding the mechanics and the driving forces of high-pressure phase transitions.13 For many pressure induced structure phase transitions, X-ray diffraction can be used to find the structure modification. But for pressure induced isostructural phase transition, the evidence of transition is not sensitive to X-ray diffraction, and in situ physical property measurement has been used to determine this type of transition.14 Due to the limitation of high pressure experimental technique, most of isostructural phase transitions were studied by theoretical calculations.15-17 TMDCs prefer isostructural phase transition under high pressure such as TiS2.18 Because of difficulty for in situ physical property measurement, there are quite a few reports on the phase transition of WSe2 under high pressure. Vaidya et al. measured * Corresponding Author, Electronic mail: [email protected].

Figure 1. Crystal structure of WSe2. Blue balls represent tungsten atoms, and yellow balls represent selenium atoms.

the variation of resistance with pressure and did not find the clear indication of phase transition up to 27 GPa.19 In another study, Selvi et al. carried out a high pressure X-ray diffraction study that illuminates that no structural phase transformation was clearly observed up to 35.8 GPa.20 Additionally, Patel et al. performed optical absorption, temperature dependence of resistance, and thermoelectric power measurements at pressures of 0, 2, 4, and 6 GPa.21 On the basis of the pressure dependence of band gap, they predicted that WSe2 would experience a semiconductor-metal phase transformation above 6 GPa. However, because of the limitation of the high pressure device, no direct evidence of the predicted phase transition was observed. To verify whether the phase transition exists in WSe2, in the present study, we carried out the in situ resistivity measurement and the first-principles calculation under high pressure. Both the theoretical and experimental results suggested that the WSe2 sample undergoes an isostructural semiconduc-

10.1021/jp104143e  2010 American Chemical Society Published on Web 07/29/2010

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tor-semimetal phase transition under high pressure. The mechanism of the phase transition in WSe2 was also understood by calculated partial density of states (PDOS). 2. Experimental and Calculation Method High-pressure experiments were served by a diamond anvil cell (DAC) with the anvil culet of 400 µm in diameter. A sheet of T-301 stainless steel used as a gasket was indented to 40 µm in thickness. A hole with 200 µm in diameter was drilled in the center of the indentation by an electric discharge machine and served as a sample chamber. A piece of ruby crystal the size of 5 µm was used as the pressure calibrator. The sample studied here was polycrystalline WSe2 powder brought from Alfa Aesar Co. with a purity of 99.8%. Van der Pauw electrodes were integrated on one diamond anvil for in situ resistivity measurement under high pressure. The electrode manufacturing process has been reported in our previous work.22,23 The sample thickness under pressure was determined by a micrometer with the precision of 0.5 µm, and the deformation of diamond anvils was taken into account.24 The temperature dependence of resistivity measurements was conducted by placing the DAC into a tropical drying cabinet, lasting for more than 10 min to make the thermal balance. The electrical current was provided by KEITHLET 2400 Source Meter, and the voltage drop was measured by a KEITHLEY 2700 Multimeter. The first-principles calculation was performed on the basis of the density functional theory and pseudopotential methods,25 which are implemented in the CASTEP code. The electron-ion interaction was described by Vanderbilt-type ultrasoft pseudopotentials.26 The exchange and correlation terms were described with generalized gradient approximations (GGA) in the scheme of Perdew-Burke-Ernzerhof.27 The geometric optimization of the unit cell was carried out with the BFGS minimization algorithm in the CASTEP code.28 The configurations of W and Se were 4f145f56s2 and 3d104s24p4, respectively. Integration in the Brillouin zone was performed using special k points generated with 9 × 9 × 2 mesh parameter grids. The one-electron valence state was expanded on a basis of plane wave with a cutoff energy of 272 eV. The space group was P63/mmc, two W atoms were located at 2c sites ((1/3, (2/3, (1/4), and four Se atoms at 4f sites ((1/3, (2/3, (u) and [(2/ 3, (1/3, (A(u+1/2)], with the lattice constant a ) b ) 3.282 Å, c ) 12.960 Å, and the internal parameter u ) 0.62. 3. Results and Discussion A. In Situ Resistivity Measurement under High Pressure. The pressure dependence of resistivity at room temperature is shown in Figure 2. Under high pressure, the resistivity of WSe2 decreases linearly by 5 orders of magnitude. At pressure above 35.7 GPa, the slope of resistivity vs pressure becomes relatively even. Around 38.1 GPa, an abnormal change appears, which means that an underlying phase transition exists herein.29 Previous X-ray diffraction study has proved that no structural transformation occurred in this pressure range,20 so an isostructural phase transition could be expected. During decompression, following a hysteresis cycle, the abnormal change is recovered at 36 GPa and the resistivity does not return to its original magnitude. In the pressure range from 0 to 27 GPa, our result agrees well with the result of pressure dependence of resistance of single crystal WSe2,19 which indicates that there is no clear indication of any structural transition up to 27 GPa. B. Temperature Dependence of Resistivity. For confirming the phase transition of WSe2 at 38.1 GPa, the temperature

Figure 2. Pressure dependence of the electrical resistivity of WSe2 at room temperature. The inset figure shows enlargements of the pressure dependence near 38 GPa.

Figure 3. Temperature dependent resistivity of WSe2 plotted in Arrhenius format.

Figure 4. Carrier activation energy as a function of pressure. The inset shows resistivity vs temperature at 35.69 GPa and 42.43 GPa.

dependences of WSe2 resistivity were measured at different pressures. The results are plotted in Arrhenius format and shown in Figure 3. It can be seen that below 38.1 GPa the electrical resistivity of WSe2 decreases with temperature increase and therefore WSe2 shows semiconductor behavior.29 Above 38.1 GPa the electrical resistivity shows a positive relationship with the temperature, which indicates WSe2 has become metallic or semimetallic. In the inset of Figure 4, this conversion in the slope of resistivity vs temperature has been further clearly shown by two R(T) curves at 35.96 and 42.43 GPa. Therefore, the phase transition reflected by abnormal resistivity change should be a pressure induced semiconductor-semimetal transition. From the temperature dependence of resistivity, transport activation energy of WSe2 at a given pressure could be obtained according to the equation

F ) F0 exp(Et /2kT)

(1)

where F0 is the high temperature resistivity, Et is the transport activation energy, k is Boltzmann’s constant, and T is the temperature. According to eq 1, the value of Et was obtained

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Figure 5. (a) Calculated band structure of WSe2 along high-symmetry directions at ambient pressure. (b) Calculated electronic total and partial densities of states at ambient pressure. The sub-bands of the W 5d state were labeled in picture.

Figure 6. (a) Calculated band structure of WSe2 along high-symmetry directions at 35 GPa. (b) Calculated electronic total and partial densities of states at 35 GPa. The sub-bands of the W 5d state were labeled in picture.

by linearly fitting the plot of ln F versus 1000/T. As shown in Figure 4, Et decreases with pressure increasing at an initial rate of ∼-9.29 meV/GPa, decreases more slowly above 26 GPa, and then extrapolates to zero around 36 GPa. As is well-known, Et is determined by defect energy levels in the energy gap.30 Once Et f 0, a further increase in carrier concentration would be weakened and the resistivity would be independent of pressure. Therefore, above 35.7 GPa the pressure dependence of resistivity of WSe2 becomes relatively independent (as shown in Figure 2). C. Band Structure Calculation. To gain further insight into this semiconductor to semimetal phase transition of WSe2, we have performed first-principles ultrasoft pseudopotential band structure calculations. The calculated band structure at ambient pressure along selected high-symmetry lines is shown in Figure 5a. The zero-pressure structure is clearly a semiconductor with an indirect band gap of 1.2 eV. The valence-band maximum (VBM) is found to be located at the Γ point, and the conductionband minimum (CBM) is found to be located about halfway between Γ and K. This result agrees well with the angle-resolved photoemission study.8 The PDOS at ambient pressure is plotted in Figure 5b, which indicates that the band at -15.4 to ∼-12.2 eV is composed of Se 4s. Due to W belonging to the transition-metal group VIB, the intralayer bonding shows mainly covalent character. In this case, the covalent band of WSe2 from -6.9 to -1.4 eV is formed by hybridization of W 5d and Se 4p. For energy reasons, the W 5d band is split into the fully occupied 5dz2 band and unoccupied 5dxy, 5dx2-y2, 5dyz, and 5dzx bands31 (as shown by blue line in Figure 5b), which causes a 1.2 eV band gap. From about -1.4 to 0 eV, the top of valence band is predominantly composed of the W 5dz2 band with a small mixture of the Se 4p band. The conduction band between 0.9 and 5.0 eV also shows considerable W 5dxy, 5dx2-y2, 5dyz, and 5dzx Se 4p covalency. We notice that, at CBM, the density of state is composed of W 5dxy, 5dx2-y2 states and the Se 4p state. With pressure increasing, all energy bands have been expanded, and the conduction bands of WSe2 have been expanded downward into the valence bands. As shown in Figure

6a, the band gap of WSe2 decreases from 1.2 eV to zero when the external pressure reaches 35 GPa. The lowest point of the conduction band is equal with the highest point (Γ point) of the valence band in energy. This band gap closure suggests that the semiconducting WSe2 transforms into a semimetal phase at a pressure of 35 GPa. By analyzing the PDOS (as shown in Figure 6b), we find that, in the semimetal phase of WSe2, the CBM band is dominated by the W 5dx2-y2 band and the W 5dxy band, which intrude downward into the Fermi energy level. Then the W 5dz2 band and the Se 4p band strongly mix at the VBM. The electronic structure of the semimetallic WSe2 is very different from that of the semiconducting one, where the energy band at CBM comes from not only W 5dx2-y2 and W5dxy orbits but also the Se 4p orbit. Pressure increases the mixing between W 5dx2-y2, W 5dxy, and Se 4p at the CBM, the contribution of W 5dx2-y2 and W 5dxy states to the CBM increases gradually with increasing pressure, and then the CBM is dominated by W 5dx2-y2 and W 5dxy states at 35 GPa. Near the Fermi level, the increased mixing of the sub-band of the W 5d states and the Se 4p state indicates a decrease in the W-Se distance with increasing pressure, which contributes to the closure of the band gap as the WSe2 approaches the metallic phase. Combined with the sandwich layer structure and chemical bonds of WSe2 (as shown in Figure 1), we can conclude that the semimetallic phase of WSe2 under high pressure must be attributed to the W-Se covalent bonding in the interlayer rather than van der Waals bonding between layers. The pressure dependence of resistivity (Figure 2) places the pressure of metallization at 38.1 GPa. However, the energy band gap from the first-principle calculation closes at 35 GPa. The difference in transformation pressure is because, we believe, the resistivity measurements are influenced by defect levels in the band gap,30 but in the first principle calculation, there is not any defect state in the band gap. Due to no pressure medium used in the measurement of electric resistivity, the quasihydrostatic pressure environment in the sample chamber can affect the accuracy of pressure measurement. Previous X-ray powder diffraction study has proven no change in structure of

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WSe2 up to ∼36 GPa,20 so we can conclude that this semiconductor-semimetal transition in WSe2 at 38.1 GPa is an isostructural phase transition. Both the experimental and theoretical results suggest that WSe2 undergoes an isostructural semiconductor-semimetal phase transition under high pressure, which results from closure of the band gap. The change in band gap can be reflected by modifications in optical absorption spectra.21 Therefore, our results illustrate the validity of Petal’s prediction. 4. Conclusion In summary, we have performed in situ resistivity measurement and first-principles calculation on WSe2 under high pressure. A pressure induced isostructural semiconductorsemimetal phase transition has been observed by both the resistivity measurement and first-principles calculation under high pressure. The theoretical calculation reveals that the W 5d band plays an important role in this pressure induced semiconductor-semimetal transition of WSe2. The semimetal phase arises from the mixing of the sub-bands of the W 5d state and Se 4p state near the Fermi level. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 10874053, 50802033, 40473034, and 10104008), the National Basic Research Program of China (Grant No. 2005CB724404), the University Innovative Research Team Grant (Grant No. IRT0625), and National Science Foundation (NSF, Grant No. DMR0619215). References and Notes (1) Kertesz, M.; Hoffmann, R. J. Am. Chem. Soc. 1984, 106, 3453. (2) Drummond, C.; Alcantar, N.; Israelachvili, J.; Tenne, R.; Golan, Y. AdV. Funct. Mater. 2001, 11 (5), 34. (3) Prasad, G.; Srivastava, O. J. Phys. D 1988, 21, 1028. (4) Tenne, R.; Wold, A. Appl. Phys. Lett. 1985, 47 (7), 707. (5) Sienicki, W.; Hryniewicz, T. Sol. Energy Mater. Sol. Cells 1996, 43, 67. (6) Lewerenz, H. J.; Heller, A.; DiSalvo, F. J. J. Am. Chem. Soc. 1980, 102 (6), 1877.

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