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Pressure confinement effect in MoS2 monolayers Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
Fangfei Li1, Yalan Yan1, Bo Han1, Liang Li1, Xiaoli Huang1, Mingguang Yao1, Yuanbo Gong1, Xilian Jin1, Baoli Liu2, Chuanrui Zhu2, Qiang Zhou1* and Tian Cui1
With ever increasing interest on the layered materials, molybdenum disulfide has been wildly investigated due to its unique optoelectronic properties. Pressure is an effective technique to tune the lattice and electronic structure of materials so that high pressure study can disclose new structural and optical phenomena. In this study, taking MoS2 as an example, we investigate the pressure confinement effect on monolayer MoS2 by in situ high pressure Raman and photoluminescence (PL) measurements. Our results reveal a structural deformation of monolayer MoS2 starting from 0.84 GPa, which is evidenced by the splitting of E12g and A1g modes. Further compression leads to a transition from 1H-MoS2 phase to a novel structure evidenced by the appearance of two new peaks located at 200 and 240 cm-1. This is a distinct feature of monolayer MoS2 compared with bulk MoS2. The new structure is supposed to have a distorted unit with the S atoms sided within a single layer like that of metastable 1T’-MoS2. However, unlike the non-photoluminescent 1T’-MoS2 structure, our monolayer shows a remarkable PL peak and a pressure induced blue shift up to 13.1 GPa. This pressure tuned behavior might enable the development of novel devices with multiple phenomena involving the strong coupling of the mechanical, electrical and optical properties of layered nanomaterials.
Introduction Since the discovery of graphene in 2004,1 two-dimensional (2D) materials with similar layered hexagonal structure like graphene have attracted considerable attention, which is stimulated by the successful mechanical exfoliation2,3 and chemical synthesis4−8 in the past ten years. Among these 2D materials, monolayer molybdenum disulfide (MoS2), as one of the important transition metal dichalcogenides, is a kind of semiconductor with strong light absorption and emission ability and has received extensive attention. In natural bulk 2H-MoS2, Mo atom has a 6-fold coordination environment and are hexagonally packed between two trigonal atomic layers of S atoms, forming strongly bonded 2D S-Mo-S sandwiches (1H-MoS2), which are loosely coupled to each other by weak van der Waals bonding. The optical absorption spectra of few layered MoS2 show a blue-shift when the layer number is reduced until only one single layer is left because of the quantum confinement effect2, 9-11. When thinned to monolayer, MoS2 has a 1.8eV direct band gap and a similar 2D hexagonal honeycomb lattice like the graphene if look along its c axis, but distinct from graphene, the unique crystal structure of MoS2 embraces a high-symmetry valley since the lack of an inversion centre. The conduction and valence band
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edges have two energy-degenerate valleys at the corner of the first Brillouin zone. Another aspect is that monolayer MoS2 has a strong spin-orbit coupling from the d orbitals of the heavy metal atoms,12 and this coupling links spin and momentum. So, valley polarization could be achieved by using circularly polarized light because of the above distinctions of monolayer MoS2: the direct band gap, broken inversion symmetry and spin-orbit coupling. Monolayer MoS2 was first mechanically isolated in 2010,2,3 after that the monolayer as well as the multi-layer materials have been wildly explored to learn and improve the optical and transport properties due to their potential applications in many fields, like valleytronics, spintronics and opto-electronics and so on.12–20 Among various approaches, pressure is considered to be an effective way to change the structural, electrical and optical properties of materials, but only few high pressure studies of layered materials has been published in the past ten years. Proctor studied the mechanical characteristics of monolayer, bilayer, and few-layer graphene by high pressure Raman spectroscopy;21 Nicolle studied the pressure-mediated doping process by Raman vibrational studies of several layers of graphene;22 Nayak and Chi investigated the high pressure electrical transport, structural and vibrational
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properties of single crystal and powder MoS2,23, 24 separately. They both found a new high pressure 2Ha phase transformed from the initial 2Hc phase, which is a first-order phase transformation. Furthermore, they also discovered a pressureinduced metallization of semiconducting MoS2. From the high pressure point of view, monolayer materials are naturally ultrasensitive sensors to study the pressure effect on crystal and electrical structures thanks to the unique 2D structure and the absence of the interference of interlayer interactions. The properties of monolayer MoS2, such as the photoluminescence (PL), valley and spin properties, are closely related to the crystal and electrical structure, which should be easily affected by pressure. On the other hand, the quantum confinement effects dramatically affect the properties of MoS2 cluster when the cluster size was reduced to a few nanometers. That is to say, when the distance between atoms is getting shorter under pressure, the response of monolayer MoS2 due to the pressure confinement effect is intriguing. To date, only few studies have been focusing on monolayer MoS2 under high pressure, and the related band gap modulation, as well as the pressure confinement effect on monolayer MoS2 is still limited. In the present study, we investigate the micro-PL and Raman spectra of monolayer MoS2 under high pressure. A blue shift of PL and splitting of Raman vibrations were observed, suggesting a new pressure induced structural transformation of monolayer MoS2 from 1HMoS2 phase to a novel structure. The underlying mechanism has also been discussed.
Fig. 1. Photos of the monolayer 1H-MoS2 sitting on silicon wafer. (a) the monolayer shown in red box is isolated through a mechanical exfoliation method and is enlarged in (b), the size of the sample is about 10×4 micro which is estimated from ruler scale; (c) Photo of the mechanically exfoliated sample placed in the gasketed chamber of a DAC (culet size 400 µm), and the pressure is 6.6 GPa with argon used as PTM; (d) the monolayer 1H-MoS2 from a CVD method indicated by a red arrow, several homogeneous monolayer 1H- MoS2 in triangle shape with large size can be found on the silicon wafer.
Experimental Method
Results and Discussion
Monolayer 1H-MoS2 films were prepared both by standard mechanical exfoliation from MoS2 single crystal (SPI supplies, USA) and by a chemical vapor deposit (CVD) method. The monolayer 1H-MoS2 films were deposited on a freshly cleaned 30 µm thick silicon wafer with 300nm of silicon oxide. The photos of monolayer 1H-MoS2 are shown in Figure 1. The layer number of MoS2 was carefully identified by optical contrast, Raman and PL spectroscopy.25, 26 Both the Raman and PL spectra were collected using a micro-Raman spectrometer (Horiba-JY T64000) excited with a solid state green laser (λ = 532 nm) in a back-scattering configuration. The signal was dispersed by a 1800 g/mm grating under triple subtractive mode with a spectra resolution of 1 cm−1. The 100x and 50x objective lens were used to collect the spectra under air condition and high pressures, respectively. To avoid the damage of sample by heating or oxidizing under exposure, the laser power reaching the diamond anvil was set to lower than 0.65 mw. High pressure confinements on the monolayer MoS2 samples were performed in a gasketed symmetric diamond anvil cell (DAC). Pressure was obtained using the ruby fluorescence method.27 The T301 stainless gasket was preindented to a thickness of 50µm and a hole of 160µm in diameter was drilled in the center of the gasket. The silicon wafer supporting the monolayer 1H- MoS2 was cut to a size of 100 µm x100 µm and moved into the gasketed chamber. Liquid argon or 4:1 methanol- ethanol liquid was used as the pressure
Raman spectroscopy is considered to be an efficient method to study lattice vibrations of crystalline materials and has been widely used in the investigation of bulk and monolayer MoS2.23-26, 28, 29 There are four Raman-active vibration modes in the center of Brillouin zone for bulk 2Hc-MoS2 crystallizing in a hexagonal space group P63/mmc (z = 2). The two internal 'molecular' modes (A1g and E12g) locate at 408.5 cm-1 and 383.4 cm-1, respectively, and two external 'lattice' modes with E1g at 280 cm-1 and E22g at around 33 cm-1 are associated with movement of the layer as a rigid body. With the thickness reduction of ultrathin MoS2, the in plane optical vibration of the Mo-S bond E12g mode stiffens, while the out of plane optical vibration of S atoms A1g mode and in-plane optical vibration of the rigid atomic bond E22g mode show opposite evolution, and E22g mode finally disappears when there is only one single layer 1H-MoS2 left.25 Figure 2 shows the Raman and PL spectra of the monolayer samples used in this study with the high signal-noise ratio. The b band is attributed to a two-phonon Raman process of successive emission of a dispersive quasi-acoustic phonon and a dispersionless transverse-optical phonon, both of which propagate along the c axis.29 The Raman spectrum of bulk MoS2 is presented for comparison (Fig. 2a), there is about 5cm1 blue shift of the A1g mode for both mechanically exfoliated
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transmission medium (PTM) for a hydrostatic pressure condition.
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Fig. 2. (a) Raman spectra of bulk and monolayer MoS2 films at room conditions. Insert shows the photoluminescence (PL) spectrum of monolayer MoS2 (by CVD); (b) the schematics of Raman-active modes is presented for a direct view; (c) the enlarged low frequency vibration E22g mode of bulk MoS2, which is absent in the monolayer MoS2; (d) comparison of the internal molecular modes A1g and E12g vibrations, the spacing between A1g and E12g modes of monolayer MoS2 is smaller than that of bulk MoS2. and CVD grown monolayer 1H-MoS2. No E22g vibration signal is detected for the monolayer samples, which is enlarged shown in Fig. 2(c). The peak positions of E12g mode between the exfoliated and CVD grown monolayer are very close to each other, and the frequency difference between E12g and A1g modes of the two samples is 20.8 cm-1, which exactly comes from the 1H-MoS2 monolayer film. The high PL intensity (the inset of Fig. 2a) also proves that our monolayer sample is of good quality. Bulk MoS2 has been widely studied under high pressure in the past ten years.23, 24, 30-34 Recently, the electrical measurements on both powder and single crystal layered MoS2 pointed out that the metallization pressure of bulk MoS2 was higher than 23 GPa, and the transition into a new 2Ha phase predicated by Hromadova et al was confirmed by the splitting of E22g and E12g vibrations and X-ray diffraction measurements.34 Analysis on the vibration properties of monolayer MoS2 together with bulk layered MoS2 will help us to understand the experimental results. In the case of bulk layered MoS2, the out of plane A1g mode and in plane E12g mode show opposite frequency dependence on the number of layers. With the increase of layers, the frequency of A1g mode increases because the binding force caused by the weak interlayer interaction is getting stronger. The reason for the softening of the E12g mode is that the dielectric screening increases, as the increased screening effect reduces the long-range
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Coulomb interaction between the effective charges, so the restoring force on the atoms decreases.35 When hydrostatic pressure is applied, the atoms in bulk MoS2 are getting closer, the frequency of all the related vibrations increases with pressure because the interaction between atoms increases. Our studies show that similar splitting of E22g and E12g vibration modes for the bulk layered MoS2 were detected above 18GPa, but different pressure evolution was observed in monolayer MoS2 compared with bulk MoS2. Figure 3 shows the Raman shift as a function of pressure up to 10.3GPa for monolayer MoS2 and 36GPa for the bulk. At ambient pressure, the peaks of A1g and E12g vibrations of monolayer can be well fitted by Lorentz functions, while after 0.84GPa the asymmetric profiles of these two modes suggest a peak splitting. From Fig. 3(a) it is clearly seen that at 1.74GPa new split peaks emerge for both A1g and E12g modes, and then all these internal modes exhibit blue shift with the increase of pressure but at different rates. We present the Raman spectra of both CVD grown monolayer 1H-MoS2 in Fig. 3(a) and the exfoliated monolayer 1H-MoS2 in supporting information Fig. S1, which gives similar results. In sharp contrast, no splitting of A1g mode can be observed in the bulk MoS2 even through the 2Hc to 2Ha transition in this study, which is consistent with previous studies; the E12g mode gives a higher frequency vibration when the 2Ha phase appears due to the layer sliding and change in the
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interlayer stacking.23, 24
Fig. 3. (a) Raman spectra of CVD grown monolayer MoS2 at different pressures, multiple peak fitting with Lorentz profile curves are shown as distinct color; (b) Raman spectra of bulk MoS2 at different pressures, the shadow region is used just block the noise background at high pressures. On the other hand, the newly emerged peak (indicated as “E12g split” in Fig. 4) appearing at lower energy side shows a 2 cm-1 downshift in frequency compared with the starting E12g mode. As pressure increases, both E12g and E12g split exhibit blue-shift but with different pressure coefficients, leading to a 9 cm-1 difference between the two modes when pressure reaches 20 GPa (Fig. 4c). Notably the
Journal Name DOI: 10.1039/C5NR00580A E12g split and A1g split modes of our monolayer exhibit similar pressure evolutions as those of the intrinsic E12g and A1g modes observed in bulk MoS2 upon compression (Fig. 4b). Since the exfoliated monolayer 1H-MoS2 is placed nearby multilayer or bulk MoS2, to preclude possible interferential signal from these nonmonolayer MoS2, CVD grown monolayer 1H-MoS2 film which has a large and homogeneous area and is well isolated from others has been studied. The results confirm that the splitting of E12g and A1g modes is an intrinsic feature of monolayer 1H-MoS2. For the monolayer 1H-MoS2, the intra-layer interaction of SMo-S should be the main reason for the vibration frequency changes due to the absence of interlayer interaction. Meanwhile, the possible interactions between the monolayer and silicon wafer substrate, as well as the monolayer and PTM should also be considered. In the study on graphene, Nicolle et al. observed electron transfer in monolayer and bilayer graphene when immersed in alcohol mixture, while such effect is absent when Ar or N2 is used as PTM.22 In contrast, we have not observed any difference in the Raman spectra and their pressure dependence when the PTM was changed from 4:1 methanol- ethanol to liquid argon. On the other hand, early studies by Zhu et al. showed that when uniaxial tensile strain was applied to monolayer and bilayer MoS2 placed on a flexible substrate (polyethylene terephthalate), the split of E12g mode is observed, indicating a Van der Waals coupling between the sample and substrate [48]. In our case, the splitting of E12g mode may be an indication of strain increase in monolayer 1H-MoS2. Meanwhile, since the A1g mode is related to the movements of two S atoms in opposite
Fig. 4. (a) Pressure dependence of the A1g and E12g Raman vibrations of CVD grown monolayer MoS2; (b) Pressure dependence of the A1g and E12g modes for the bulk (hollow symbols) and the data from CVD grown monolayer MoS2 (hollow filled with × symbols) is also presented for comparison; (c) Pressure evolution of frequency differences between different vibration modes of monolayer (symbol filled with × symbols) and bulk MoS2(symbols without filling). The newly emerged peaks on the shoulder of A1g and E12g modes are denoted by “A1g split” and “E12g split”, respectively. A1g splitting presents the frequency difference between A1g split and A1g, and similar expressions have been used for other modes shown in (c)
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direction and perpendicular to the basal plane of MoS2 unit, splitting of A1g mode suggests that the vibration of the corresponding atoms has been changed. The interaction between silicon substrate and bottom S layer in MoS2 film should be different from the upper S layer which suspends in PTM and may possesses more freedom of motion. Thus, the Mo or S atom sliding within monolayer should be much easier under pressure, leading to deformation of the structure and unequal changes of Mo-S bond length, which may cause the splitting of the related vibration modes. In addition, from the peak fitting of A1g and E12g modes, an inverse of the peak intensity can be observed above 4.39 GPa, as shown in Fig. 3 (a). At 1.74 GPa the peak on the left shoulder of A1g is lower than the right one which grows to a dominant peak at above 4.39 GPa. As to the E12g mode, the peak on the left shoulder turns into the dominant one at above 4.39 GPa, while the right one decreases its intensity obviously. Such a inversion in the peak intensity also suggests that the increase of strain or deformation in monolayer MoS2 is probably becoming more significant. When pressure increases higher than 5.8 GPa, two new peaks emerge at around 200 cm-1 and 240 cm-1, as shown in Fig. 5(a). These two new peaks stiffens under pressure with a different increasing pressure coefficients, the low frequency vibration increases with a slop of 1.27 cm-1/GPa while the high frequency vibration shows a lower value of about 0.89 cm-1/GPa. It should be noted that these two new peaks should come from the monolayer sample, but not from the silicon wafer, PTM or diamond anvils around the sample in the DAC, since we have carefully checked all background Raman signals and no similar peaks from them can be detected. Moreover, these two peaks are also absent in the bulk MoS2 from neither our bulk MoS2 nor early studies. We thus conclude that these two peaks are from monolayer MoS2, which have not been observed before. Generally speaking, emergence of new vibration modes in crystals always indicates a structural transformation. As shown above, the Raman spectra of 2Ha structure is similar to 2Hc-MoS2, the only difference is the splitting of E2g1 and E2g2 modes to a higher energy. Thus, these two new peaks observed in this study should not belong to 2Hc or 2Ha phase. A very recent report on high pressure optical study of a distorted 1T’ monolayer MoS2, in which the Mo atom is coordinated by six S atoms in a distorted octahedral arrangement (1T’-MoS2), shows an analogous vibrations of J1, J2 and J3, but the corresponding positions are different which locate at 150 cm-1, 225 cm-1 and 325 cm-1, respectively.36-39 This 1T’-MoS2 is related with the 1H-MoS2 via intralayer atomic plane gliding. Therefore, we believe that a new phase occurs above 5.8 GPa in our monolayer 1H-MoS2. As monolayer MoS2 is constructed by S-Mo-S unit in a long-ranged periodicity, upon compression the distortion of the unit cell will cause the intralayer sliding of S atoms like that in monolayer 1T’ MoS2.37 It should also be noted that our Raman spectra clearly indicate the occurrence of a phase transition, while to determine the structure of the new phase requires additional crystal structure data. Another important property of monolayer is the electronicrelated behavior under high pressure. As the bulk MoS2 is an
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indirect-gap semiconductor with the band gap of 1.29 eV at room conditions. As thickness decreases, it exhibits a crossover from an indirect - to direct-gap (1.88 eV) semiconductor in the monolayer limit, accompanying with a dramatic enhancement in luminescence quantum efficiency [2,9]. Such quantum confinement effect has been explained by the fact that the electron movement is confined by the size of nanomaterials and thus the electronic structure was changed when the size of nanomaterials decreases [45].
Fig. 5. (a) Raman spectra of CVD grown monolayer MoS2 at different pressures. The open circles are experimental data, and the broad lines are smoothed guide lines for eyes; (b) the pressure dependence of Raman frequencies for the three modes. The E1g mode is measureable at pressure higher than 11.48 GPa, while the two new peaks are detectable above 5.8 GPa. As shown in Fig. 2(a), PL spectra of the monolayer 1H-MoS2 recorded at ambient pressure exhibit a strong peak at 1.83 eV. We find that the PL peak positions show blue shift and the fluorescence intensity decreases with the increase of pressure. The PL is hard to be detectable at above 13.1 GPa. The band gap can be extracted from the PL peaks under pressures by fitting them with Gaussian functions and the results are shown in Fig. 6(a). Obviously, the band gap increases in a way similar to that of the results reported by Dou et al.,40 but our pressure coefficient is different from theirs. For comparison, the value is 1.90 eV at 5 GPa in Ref. 40, while our value is 1.95 eV at similar pressure. Very recent high pressure studies on the optical and vibrational behaviors of 1H-MoS2 and 1T’ MoS2 also reported similar PL evolution under pressure up to 16.1 GPa.36 We compare our data with the band gap of 1H-MoS2 monolayers in Ref. 36 and the data from Ref. 40 in Fig 3(b). For details, below 5 GPa, the band gap measured in our experiment is lower than that reported by Nayak, and such difference becomes negligible as the pressure increases higher than 5 GPa. It is worth mentioning that Nayak et al. predict a metallization of the monolayer
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Journal Name DOI: 10.1039/C5NR00580A Mo d orbitals and S p orbitals may lead to a crossover from direct- to indirect transition,36, 40 thus the decreased intensity of PL under pressure can be explained. In addition, we can see that a turning point occurs in the curve of PL intensity as a function of pressure at 6GPa (Fig. S4 in supporting information). Below 6 GPa the PL intensity decreases rapidly, while above 6 GPa the decrease of the intensity slows down. This is consistent with the Raman results, further suggesting a structure transition at this pressure. Several theoretical and experimental studies on the mechanical strain effect on monolayer MoS2 have shown that the band gap decreases with strain increasing which strongly depends on the type of applied strain [42-45]. In our study, the observed increase of band gap indicates that pressure confinement effect plays a primary role on modifying the electronic structure of monolayer MoS2.
Fig. 6. High pressure PL spectra of CVD grown monolayer MoS2 (a) and the pressure dependence of band gap extracted from the Gaussian fittings for the PL peaks (b). A polynomial fit to the experimental data with Eg=1.83+0.04×P−0.001×P2. The Eg indicates optical band gap and P is pressure.
As the distorted 1T’-MoS2 monolayer is metallic which has a 0 eV band gap at ambient conditions and should exhibit no PL signal [36]. However, our monolayer MoS2 gives a clear blue shifted PL spectrum under high pressure although the intensity decreases obviously upon compression. It thus excludes the possibility of transformation into 1T’- MoS2. From the theory point of view, there are investigations on the probable distortion of metastable 1T-MoS2 monolayer, in which the Mo atom is also coordinated by six S atoms like that in 1H-MoS2, but in an octahedral arrangement other than the trigonal prismatic arrangement in 1H-MoS2.38, 39, 46, 47 Due to the clusterization of Mo atoms in different ways, the 1T-MoS2 monolayer can be distorted and gives other types of single layer structure with distinct electronic and optoelectronic properties.38 In our study, clear splitting in both A1g and E2g1 modes together with
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new emerged vibrations at 200 and 240 cm-1 in the Raman spectra indicate a pressure induced distortion of initial 1H-MoS2.
Conclusions We have investigated the micro-PL and Raman spectra of monolayer MoS2 under high pressure. A structural deformation of monolayer MoS2 was revealed above 0.84 GPa, which is evidenced by the splitting of A1g and E2g1 modes at this pressure. At higher pressure of 5.8 GPa, the appearance of two new peaks located at 200 and 240 cm-1 suggest a structural transformation might take place. This new structure is supposed to have a distorted unit with S atoms sliding within a single layer, which is similar to the metastable 1T’-MoS2
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ARTICLE MoS2 at about 68 GPa based on their calculations. In fact, such pressure-induced blue shift in the PL position of monolayer MoS2 suggests a pressure confinement effect on nanomaterials. Previously, Wu et al. has reported an exciton emission investigation in InGaAs quantum dots, indicating that the pressure induces a blue shift of the conduction band minimum and causes a increase of band gap.41 For the monolayer 1H-MoS2, the PL peak originates from direct K-K interband transition between the conduction band minimum and valence band maximum at the K point in Brillouin zone, which are contributed primarily from the d orbitals of Mo atoms.42 When pressure increases, these orbitals may move away from Fermi level and lead to an increase of band gap. On the other hand, the p orbital of S atoms is known to contribute mainly to the valence band maximum at Г point and extend out of the MoS2 plane which exhibits interlayer coupling, upon compression. The pressure induced intralayer hybridization between
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Journal Name structure. The monolayer MoS2 shows a remarkable PL peak and the emission energy exhibits a blue shift upon compression, indicating the band gap of monolayer MoS2 broadens as pressure increases. Such distinct features of monolayer MoS2 compared with bulk MoS2 should be related to the unique pressure-induced confinement effect in the monolayer. Such pressure tuned behavior might enable the development of novel devices with multiple phenomena involving the strong coupling of the mechanical, electrical and optical properties of layered nanomaterials.
Acknowledgements This work was supported by the National Basic Research Program of China (no. 2011CB808200), the NSFC (nos.11274137, 91014004, 11474127, 11004074), National Found for Fostering Talents of Basic Science (No. J1103202).
Notes and references 1
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ARTICLE DOI: 10.1039/C5NR00580A S. K. Pati and C. N. R. Rao, Angew. Chem., 2010, 49 (24), 4059– 4062. 15 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nat. Nanotech., 2011, 6 (3) ,147–150. 16. H. S. Lee, S. W. Min, Y. G. Chang, M. K. Park, T. Nam, H. Kim, J. H. Kim, S. Ryu and S. Im, Nano Lett., 2012, 12 (7), 3695– 3700. 17. M. Ghorbani-Asl, N. Zibouche, M. Wahiduzzaman, A. F. Oliveira, A. Kuc and T. Heine, Sci. Rep., 2013, 3, 2961. 18. H. L. Zeng, J. F. Dai, D. Xiao and X. D. Cui, .Nat. Nanotech., 2012, 7 (8), 490–493. 19. K. Behnia, Nat. Nanotech., 2012, 7 (8), 488–489. 20 T. Cao, G. Wang, W. P. Han, H. Q. Ye, C. R. Zhu, J. R. Shi, Q. Niu, P. H. Tan, E. G. Wang, B. L. Liu and J. Feng, Nat. Commun., 2012, 3, 887. 21 J. E. Proctor, E. Gregoryanz, K. S. Novoselov, M. Lotya, J. N. Coleman and M. P. Halsall, Phys. Rev. B, 2009, 80 (7), 073408. 22 J. Nicolle, D. Machon, P. Poncharal, O. Pierre-Louis and A. SanMiguel , Nano Lett., 2011,11 (9), 3564–3568. 23 A. P. Nayak, S. Bhattacharyya, J. Zhu, J. Liu, X. Wu, T. Pandey, C. Q. Jin, A. K. Singh, D. Akinwande and J. F. Lin, Nat. Commun., 2014, 5, 3731. 24 Z. H. Chi, X. M. Zhao, H. D. Zhang, A. F. Goncharov, S. S. Lobanov, T. Kagayama, M. Sakata and X. J. Chen, Phys. Rev. Lett., 2014, 113 (3), 036802. 25 H. L. Zeng, B. R. Zhu, K. Liu, J. H. Fan, X. D. Cui and Q. M. Zhang, Phys. Rev. B, 2012, 86 (24), 241301. 26 Y. Y. Zhao, X. Luo, H. Li, J. Zhang, P. T. Araujo, C. K. Gan, J. Wu, H. Zhang, S. Y. Quek and M. S. Dresselhaus, Nano Lett., 2013, 13 (3), 1007−1015. 27 H. K. Mao, J. Xu and P. M. Bell, J. Geophys. Res., 1986, 91, 4673. 28 T. J. Wieting and J. L. Verble, Phys. Rev. B, 1971, 3, 4286-4292. 29 T. Sekine, K. Uchinokura, T. Nakashizu, E. Matsuura and R. Yoshizaki, J. Phys. Soc. Jpn., 1984, 53, 811. 30 S. Sugai and T. Ueda, Phys. Rev. B, 1982, 26 (12), 6554-6558. 31 R. Aksoy, Y. Z. Ma, E. Selvi, M. C. Chyu, A. Ertas and A. White, J. Phys. Chem. Solids, 2006, 67 (9-10), 1914–1917. 32 H. H. Guo, T. Yang, P. Tao, Y. Wang and Z. D. Zhang, J. Appl. Phys., 2013, 113 (1), 013709. 33 N. Bandaru, R. S. Kumar, D. Sneed, O. Tschauner, J. Baker, D. Antonio, S. N. Luo, T. Hartmann, Y. S. Zhao and R. Venkat, J. Phys. Chem. C., 2014,118 (6), 3230−3235. 34 L. Hromadova, R. Martonak and E. Tosatti, Phys. Rev. B.,2013, 87 (14), 144105. 35 A. Molina-Sanchez and L. Wirtz, Phys. Rev. B., 2011, 84 (15), 155413. 36 A. P. Nayak, T. Pandey, D. Voiry, J. Liu, S. T. Moran, A. Sharma, C. Tan, C. H. Chen, L. J. Li, M. Chhowalla, J. F. Lin, A. K. Singh and D. Akinwande, Nano Lett., 2015, 15 (1), 346–353. 37 G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. W. Chen and M. Chhowalla, Nano Lett., 2011, 11 (12),5111-5116. 38 S. J. Sandoval, D. Yang, R. F. Frindt and J. C. Irwin, Phys. Rev. B, 1991, 44 (8) , 3955−3962. 39 T. Hu, R. Li and J. Dong, J. Chem. Phys. 2013, 139 (17), 174702174707.
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40 X. M. Dou, K. Ding, D. S. Jiang and B. Q. Sun, ACS Nano, 2014, 8 (7), 7458–7464. 41 X. F. Wu, H. Wei, X. M. Dou, K. Ding, Y. Yu, H. Q. Ni, Z. C. Niu, Y. Ji, S. S. Li, D. S. Jiang, G. C. Guo, L. X. He, and B. Q. Sun, Europhys. Lett., 2014, 107, 27008. 42. H. Conley, B. Wang, J. Ziegler, R. F. Haglund, S. T. Pantelides, K. I. Bolotin, Nano Lett. 2013, 13, 3626–3630. 43. Q. Yue, J. Kang, Z. Z. Shao, X. A. Zhang, S. L. Chang, G. Wang, S. Q. Qin, and J. B. Li, Phys. Lett. A, 2012, 376, 1166-1170. 44. P. Johari, and V. B. Shenoy, ACS Nano, 2012, 6 (6), 5449–5456. 45. C. R. Zhu, G. Wang, B. L. Liu, X. Marie, X. F. Qiao, X. Zhang, X. X. Wu, H. Fan, P. H. Tan, T. Amand, and B. Urbaszek, Phys. Rev. B, 2013, 88 (12), 121301/1-5. 46. G. Eda, T. Fujita, H. Yamaguchi, D. Voiry, M. Chen, and M. Chhowalla, ACS Nano, 2012, 6, 7311–7317. 47. E. Benavente, M. A. Santa Ana, F. Mendizábal, and G. González, Coord. Chem. Rev., 2002, 224, 87–109.
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Pressure confinement effect in MoS2 monolayers Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
Fangfei Li1, Yalan Yan1, Bo Han1, Liang Li1, Xiaoli Huang1, Mingguang Yao1, Yuanbo Gong1, Xilian Jin1, Baoli Liu2, Chuanrui Zhu2, Qiang Zhou1* and Tian Cui1
With ever increasing interest on the layered materials, molybdenum disulfide has been wildly investigated due to its unique optoelectronic properties. Pressure is an effective technique to tune the lattice and electronic structure of materials so that high pressure study can disclose new structural and optical phenomena. In this study, taking MoS2 as an example, we investigate the pressure confinement effect on monolayer MoS2 by in situ high pressure Raman and photoluminescence (PL) measurements. Our results reveal a structural deformation of monolayer MoS2 starting from 0.84 GPa, which is evidenced by the splitting of E12g and A1g modes. Further compression leads to a transition from 1H-MoS2 phase to a novel structure evidenced by the appearance of two new peaks located at 200 and 240 cm-1. This is a distinct feature of monolayer MoS2 compared with bulk MoS2. The new structure is supposed to have a distorted unit with the S atoms sided within a single layer like that of metastable 1T’-MoS2. However, unlike the non-photoluminescent 1T’-MoS2 structure, our monolayer shows a remarkable PL peak and a pressure induced blue shift up to 13.1 GPa. This pressure tuned behavior might enable the development of novel devices with multiple phenomena involving the strong coupling of the mechanical, electrical and optical properties of layered nanomaterials.
Introduction Since the discovery of graphene in 2004,1 two-dimensional (2D) materials with similar layered hexagonal structure like graphene have attracted considerable attention, which is stimulated by the successful mechanical exfoliation2,3 and chemical synthesis4−8 in the past ten years. Among these 2D materials, monolayer molybdenum disulfide (MoS2), as one of the important transition metal dichalcogenides, is a kind of semiconductor with strong light absorption and emission ability and has received extensive attention. In natural bulk 2H-MoS2, Mo atom has a 6-fold coordination environment and are hexagonally packed between two trigonal atomic layers of S atoms, forming strongly bonded 2D S-Mo-S sandwiches (1H-MoS2), which are loosely coupled to each other by weak van der Waals bonding. The optical absorption spectra of few layered MoS2 show a blue-shift when the layer number is reduced until only one single layer is left because of the quantum confinement effect2, 9-11. When thinned to monolayer, MoS2 has a 1.8eV direct band gap and a similar 2D hexagonal honeycomb lattice like the graphene if look along its c axis, but distinct from graphene, the unique crystal structure of MoS2 embraces a high-symmetry valley since the lack of an inversion centre. The conduction and valence band
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edges have two energy-degenerate valleys at the corner of the first Brillouin zone. Another aspect is that monolayer MoS2 has a strong spin-orbit coupling from the d orbitals of the heavy metal atoms,12 and this coupling links spin and momentum. So, valley polarization could be achieved by using circularly polarized light because of the above distinctions of monolayer MoS2: the direct band gap, broken inversion symmetry and spin-orbit coupling. Monolayer MoS2 was first mechanically isolated in 2010,2,3 after that the monolayer as well as the multi-layer materials have been wildly explored to learn and improve the optical and transport properties due to their potential applications in many fields, like valleytronics, spintronics and opto-electronics and so on.12–20 Among various approaches, pressure is considered to be an effective way to change the structural, electrical and optical properties of materials, but only few high pressure studies of layered materials has been published in the past ten years. Proctor studied the mechanical characteristics of monolayer, bilayer, and few-layer graphene by high pressure Raman spectroscopy;21 Nicolle studied the pressure-mediated doping process by Raman vibrational studies of several layers of graphene;22 Nayak and Chi investigated the high pressure electrical transport, structural and vibrational
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properties of single crystal and powder MoS2,23, 24 separately. They both found a new high pressure 2Ha phase transformed from the initial 2Hc phase, which is a first-order phase transformation. Furthermore, they also discovered a pressureinduced metallization of semiconducting MoS2. From the high pressure point of view, monolayer materials are naturally ultrasensitive sensors to study the pressure effect on crystal and electrical structures thanks to the unique 2D structure and the absence of the interference of interlayer interactions. The properties of monolayer MoS2, such as the photoluminescence (PL), valley and spin properties, are closely related to the crystal and electrical structure, which should be easily affected by pressure. On the other hand, the quantum confinement effects dramatically affect the properties of MoS2 cluster when the cluster size was reduced to a few nanometers. That is to say, when the distance between atoms is getting shorter under pressure, the response of monolayer MoS2 due to the pressure confinement effect is intriguing. To date, only few studies have been focusing on monolayer MoS2 under high pressure, and the related band gap modulation, as well as the pressure confinement effect on monolayer MoS2 is still limited. In the present study, we investigate the micro-PL and Raman spectra of monolayer MoS2 under high pressure. A blue shift of PL and splitting of Raman vibrations were observed, suggesting a new pressure induced structural transformation of monolayer MoS2 from 1HMoS2 phase to a novel structure. The underlying mechanism has also been discussed.
Fig. 1. Photos of the monolayer 1H-MoS2 sitting on silicon wafer. (a) the monolayer shown in red box is isolated through a mechanical exfoliation method and is enlarged in (b), the size of the sample is about 10×4 micro which is estimated from ruler scale; (c) Photo of the mechanically exfoliated sample placed in the gasketed chamber of a DAC (culet size 400 µm), and the pressure is 6.6 GPa with argon used as PTM; (d) the monolayer 1H-MoS2 from a CVD method indicated by a red arrow, several homogeneous monolayer 1H- MoS2 in triangle shape with large size can be found on the silicon wafer.
Experimental Method
Results and Discussion
Monolayer 1H-MoS2 films were prepared both by standard mechanical exfoliation from MoS2 single crystal (SPI supplies, USA) and by a chemical vapor deposit (CVD) method. The monolayer 1H-MoS2 films were deposited on a freshly cleaned 30 µm thick silicon wafer with 300nm of silicon oxide. The photos of monolayer 1H-MoS2 are shown in Figure 1. The layer number of MoS2 was carefully identified by optical contrast, Raman and PL spectroscopy.25, 26 Both the Raman and PL spectra were collected using a micro-Raman spectrometer (Horiba-JY T64000) excited with a solid state green laser (λ = 532 nm) in a back-scattering configuration. The signal was dispersed by a 1800 g/mm grating under triple subtractive mode with a spectra resolution of 1 cm−1. The 100x and 50x objective lens were used to collect the spectra under air condition and high pressures, respectively. To avoid the damage of sample by heating or oxidizing under exposure, the laser power reaching the diamond anvil was set to lower than 0.65 mw. High pressure confinements on the monolayer MoS2 samples were performed in a gasketed symmetric diamond anvil cell (DAC). Pressure was obtained using the ruby fluorescence method.27 The T301 stainless gasket was preindented to a thickness of 50µm and a hole of 160µm in diameter was drilled in the center of the gasket. The silicon wafer supporting the monolayer 1H- MoS2 was cut to a size of 100 µm x100 µm and moved into the gasketed chamber. Liquid argon or 4:1 methanol- ethanol liquid was used as the pressure
Raman spectroscopy is considered to be an efficient method to study lattice vibrations of crystalline materials and has been widely used in the investigation of bulk and monolayer MoS2.23-26, 28, 29 There are four Raman-active vibration modes in the center of Brillouin zone for bulk 2Hc-MoS2 crystallizing in a hexagonal space group P63/mmc (z = 2). The two internal 'molecular' modes (A1g and E12g) locate at 408.5 cm-1 and 383.4 cm-1, respectively, and two external 'lattice' modes with E1g at 280 cm-1 and E22g at around 33 cm-1 are associated with movement of the layer as a rigid body. With the thickness reduction of ultrathin MoS2, the in plane optical vibration of the Mo-S bond E12g mode stiffens, while the out of plane optical vibration of S atoms A1g mode and in-plane optical vibration of the rigid atomic bond E22g mode show opposite evolution, and E22g mode finally disappears when there is only one single layer 1H-MoS2 left.25 Figure 2 shows the Raman and PL spectra of the monolayer samples used in this study with the high signal-noise ratio. The b band is attributed to a two-phonon Raman process of successive emission of a dispersive quasi-acoustic phonon and a dispersionless transverse-optical phonon, both of which propagate along the c axis.29 The Raman spectrum of bulk MoS2 is presented for comparison (Fig. 2a), there is about 5cm1 blue shift of the A1g mode for both mechanically exfoliated
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transmission medium (PTM) for a hydrostatic pressure condition.
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Fig. 2. (a) Raman spectra of bulk and monolayer MoS2 films at room conditions. Insert shows the photoluminescence (PL) spectrum of monolayer MoS2 (by CVD); (b) the schematics of Raman-active modes is presented for a direct view; (c) the enlarged low frequency vibration E22g mode of bulk MoS2, which is absent in the monolayer MoS2; (d) comparison of the internal molecular modes A1g and E12g vibrations, the spacing between A1g and E12g modes of monolayer MoS2 is smaller than that of bulk MoS2. and CVD grown monolayer 1H-MoS2. No E22g vibration signal is detected for the monolayer samples, which is enlarged shown in Fig. 2(c). The peak positions of E12g mode between the exfoliated and CVD grown monolayer are very close to each other, and the frequency difference between E12g and A1g modes of the two samples is 20.8 cm-1, which exactly comes from the 1H-MoS2 monolayer film. The high PL intensity (the inset of Fig. 2a) also proves that our monolayer sample is of good quality. Bulk MoS2 has been widely studied under high pressure in the past ten years.23, 24, 30-34 Recently, the electrical measurements on both powder and single crystal layered MoS2 pointed out that the metallization pressure of bulk MoS2 was higher than 23 GPa, and the transition into a new 2Ha phase predicated by Hromadova et al was confirmed by the splitting of E22g and E12g vibrations and X-ray diffraction measurements.34 Analysis on the vibration properties of monolayer MoS2 together with bulk layered MoS2 will help us to understand the experimental results. In the case of bulk layered MoS2, the out of plane A1g mode and in plane E12g mode show opposite frequency dependence on the number of layers. With the increase of layers, the frequency of A1g mode increases because the binding force caused by the weak interlayer interaction is getting stronger. The reason for the softening of the E12g mode is that the dielectric screening increases, as the increased screening effect reduces the long-range
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Coulomb interaction between the effective charges, so the restoring force on the atoms decreases.35 When hydrostatic pressure is applied, the atoms in bulk MoS2 are getting closer, the frequency of all the related vibrations increases with pressure because the interaction between atoms increases. Our studies show that similar splitting of E22g and E12g vibration modes for the bulk layered MoS2 were detected above 18GPa, but different pressure evolution was observed in monolayer MoS2 compared with bulk MoS2. Figure 3 shows the Raman shift as a function of pressure up to 10.3GPa for monolayer MoS2 and 36GPa for the bulk. At ambient pressure, the peaks of A1g and E12g vibrations of monolayer can be well fitted by Lorentz functions, while after 0.84GPa the asymmetric profiles of these two modes suggest a peak splitting. From Fig. 3(a) it is clearly seen that at 1.74GPa new split peaks emerge for both A1g and E12g modes, and then all these internal modes exhibit blue shift with the increase of pressure but at different rates. We present the Raman spectra of both CVD grown monolayer 1H-MoS2 in Fig. 3(a) and the exfoliated monolayer 1H-MoS2 in supporting information Fig. S1, which gives similar results. In sharp contrast, no splitting of A1g mode can be observed in the bulk MoS2 even through the 2Hc to 2Ha transition in this study, which is consistent with previous studies; the E12g mode gives a higher frequency vibration when the 2Ha phase appears due to the layer sliding and change in the
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interlayer stacking.23, 24
Fig. 3. (a) Raman spectra of CVD grown monolayer MoS2 at different pressures, multiple peak fitting with Lorentz profile curves are shown as distinct color; (b) Raman spectra of bulk MoS2 at different pressures, the shadow region is used just block the noise background at high pressures. On the other hand, the newly emerged peak (indicated as “E12g split” in Fig. 4) appearing at lower energy side shows a 2 cm-1 downshift in frequency compared with the starting E12g mode. As pressure increases, both E12g and E12g split exhibit blue-shift but with different pressure coefficients, leading to a 9 cm-1 difference between the two modes when pressure reaches 20 GPa (Fig. 4c). Notably the
Journal Name DOI: 10.1039/C5NR00580A E12g split and A1g split modes of our monolayer exhibit similar pressure evolutions as those of the intrinsic E12g and A1g modes observed in bulk MoS2 upon compression (Fig. 4b). Since the exfoliated monolayer 1H-MoS2 is placed nearby multilayer or bulk MoS2, to preclude possible interferential signal from these nonmonolayer MoS2, CVD grown monolayer 1H-MoS2 film which has a large and homogeneous area and is well isolated from others has been studied. The results confirm that the splitting of E12g and A1g modes is an intrinsic feature of monolayer 1H-MoS2. For the monolayer 1H-MoS2, the intra-layer interaction of SMo-S should be the main reason for the vibration frequency changes due to the absence of interlayer interaction. Meanwhile, the possible interactions between the monolayer and silicon wafer substrate, as well as the monolayer and PTM should also be considered. In the study on graphene, Nicolle et al. observed electron transfer in monolayer and bilayer graphene when immersed in alcohol mixture, while such effect is absent when Ar or N2 is used as PTM.22 In contrast, we have not observed any difference in the Raman spectra and their pressure dependence when the PTM was changed from 4:1 methanol- ethanol to liquid argon. On the other hand, early studies by Zhu et al. showed that when uniaxial tensile strain was applied to monolayer and bilayer MoS2 placed on a flexible substrate (polyethylene terephthalate), the split of E12g mode is observed, indicating a Van der Waals coupling between the sample and substrate [48]. In our case, the splitting of E12g mode may be an indication of strain increase in monolayer 1H-MoS2. Meanwhile, since the A1g mode is related to the movements of two S atoms in opposite
Fig. 4. (a) Pressure dependence of the A1g and E12g Raman vibrations of CVD grown monolayer MoS2; (b) Pressure dependence of the A1g and E12g modes for the bulk (hollow symbols) and the data from CVD grown monolayer MoS2 (hollow filled with × symbols) is also presented for comparison; (c) Pressure evolution of frequency differences between different vibration modes of monolayer (symbol filled with × symbols) and bulk MoS2(symbols without filling). The newly emerged peaks on the shoulder of A1g and E12g modes are denoted by “A1g split” and “E12g split”, respectively. A1g splitting presents the frequency difference between A1g split and A1g, and similar expressions have been used for other modes shown in (c)
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direction and perpendicular to the basal plane of MoS2 unit, splitting of A1g mode suggests that the vibration of the corresponding atoms has been changed. The interaction between silicon substrate and bottom S layer in MoS2 film should be different from the upper S layer which suspends in PTM and may possesses more freedom of motion. Thus, the Mo or S atom sliding within monolayer should be much easier under pressure, leading to deformation of the structure and unequal changes of Mo-S bond length, which may cause the splitting of the related vibration modes. In addition, from the peak fitting of A1g and E12g modes, an inverse of the peak intensity can be observed above 4.39 GPa, as shown in Fig. 3 (a). At 1.74 GPa the peak on the left shoulder of A1g is lower than the right one which grows to a dominant peak at above 4.39 GPa. As to the E12g mode, the peak on the left shoulder turns into the dominant one at above 4.39 GPa, while the right one decreases its intensity obviously. Such a inversion in the peak intensity also suggests that the increase of strain or deformation in monolayer MoS2 is probably becoming more significant. When pressure increases higher than 5.8 GPa, two new peaks emerge at around 200 cm-1 and 240 cm-1, as shown in Fig. 5(a). These two new peaks stiffens under pressure with a different increasing pressure coefficients, the low frequency vibration increases with a slop of 1.27 cm-1/GPa while the high frequency vibration shows a lower value of about 0.89 cm-1/GPa. It should be noted that these two new peaks should come from the monolayer sample, but not from the silicon wafer, PTM or diamond anvils around the sample in the DAC, since we have carefully checked all background Raman signals and no similar peaks from them can be detected. Moreover, these two peaks are also absent in the bulk MoS2 from neither our bulk MoS2 nor early studies. We thus conclude that these two peaks are from monolayer MoS2, which have not been observed before. Generally speaking, emergence of new vibration modes in crystals always indicates a structural transformation. As shown above, the Raman spectra of 2Ha structure is similar to 2Hc-MoS2, the only difference is the splitting of E2g1 and E2g2 modes to a higher energy. Thus, these two new peaks observed in this study should not belong to 2Hc or 2Ha phase. A very recent report on high pressure optical study of a distorted 1T’ monolayer MoS2, in which the Mo atom is coordinated by six S atoms in a distorted octahedral arrangement (1T’-MoS2), shows an analogous vibrations of J1, J2 and J3, but the corresponding positions are different which locate at 150 cm-1, 225 cm-1 and 325 cm-1, respectively.36-39 This 1T’-MoS2 is related with the 1H-MoS2 via intralayer atomic plane gliding. Therefore, we believe that a new phase occurs above 5.8 GPa in our monolayer 1H-MoS2. As monolayer MoS2 is constructed by S-Mo-S unit in a long-ranged periodicity, upon compression the distortion of the unit cell will cause the intralayer sliding of S atoms like that in monolayer 1T’ MoS2.37 It should also be noted that our Raman spectra clearly indicate the occurrence of a phase transition, while to determine the structure of the new phase requires additional crystal structure data. Another important property of monolayer is the electronicrelated behavior under high pressure. As the bulk MoS2 is an
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indirect-gap semiconductor with the band gap of 1.29 eV at room conditions. As thickness decreases, it exhibits a crossover from an indirect - to direct-gap (1.88 eV) semiconductor in the monolayer limit, accompanying with a dramatic enhancement in luminescence quantum efficiency [2,9]. Such quantum confinement effect has been explained by the fact that the electron movement is confined by the size of nanomaterials and thus the electronic structure was changed when the size of nanomaterials decreases [45].
Fig. 5. (a) Raman spectra of CVD grown monolayer MoS2 at different pressures. The open circles are experimental data, and the broad lines are smoothed guide lines for eyes; (b) the pressure dependence of Raman frequencies for the three modes. The E1g mode is measureable at pressure higher than 11.48 GPa, while the two new peaks are detectable above 5.8 GPa. As shown in Fig. 2(a), PL spectra of the monolayer 1H-MoS2 recorded at ambient pressure exhibit a strong peak at 1.83 eV. We find that the PL peak positions show blue shift and the fluorescence intensity decreases with the increase of pressure. The PL is hard to be detectable at above 13.1 GPa. The band gap can be extracted from the PL peaks under pressures by fitting them with Gaussian functions and the results are shown in Fig. 6(a). Obviously, the band gap increases in a way similar to that of the results reported by Dou et al.,40 but our pressure coefficient is different from theirs. For comparison, the value is 1.90 eV at 5 GPa in Ref. 40, while our value is 1.95 eV at similar pressure. Very recent high pressure studies on the optical and vibrational behaviors of 1H-MoS2 and 1T’ MoS2 also reported similar PL evolution under pressure up to 16.1 GPa.36 We compare our data with the band gap of 1H-MoS2 monolayers in Ref. 36 and the data from Ref. 40 in Fig 3(b). For details, below 5 GPa, the band gap measured in our experiment is lower than that reported by Nayak, and such difference becomes negligible as the pressure increases higher than 5 GPa. It is worth mentioning that Nayak et al. predict a metallization of the monolayer
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Journal Name DOI: 10.1039/C5NR00580A Mo d orbitals and S p orbitals may lead to a crossover from direct- to indirect transition,36, 40 thus the decreased intensity of PL under pressure can be explained. In addition, we can see that a turning point occurs in the curve of PL intensity as a function of pressure at 6GPa (Fig. S4 in supporting information). Below 6 GPa the PL intensity decreases rapidly, while above 6 GPa the decrease of the intensity slows down. This is consistent with the Raman results, further suggesting a structure transition at this pressure. Several theoretical and experimental studies on the mechanical strain effect on monolayer MoS2 have shown that the band gap decreases with strain increasing which strongly depends on the type of applied strain [42-45]. In our study, the observed increase of band gap indicates that pressure confinement effect plays a primary role on modifying the electronic structure of monolayer MoS2.
Fig. 6. High pressure PL spectra of CVD grown monolayer MoS2 (a) and the pressure dependence of band gap extracted from the Gaussian fittings for the PL peaks (b). A polynomial fit to the experimental data with Eg=1.83+0.04×P−0.001×P2. The Eg indicates optical band gap and P is pressure.
As the distorted 1T’-MoS2 monolayer is metallic which has a 0 eV band gap at ambient conditions and should exhibit no PL signal [36]. However, our monolayer MoS2 gives a clear blue shifted PL spectrum under high pressure although the intensity decreases obviously upon compression. It thus excludes the possibility of transformation into 1T’- MoS2. From the theory point of view, there are investigations on the probable distortion of metastable 1T-MoS2 monolayer, in which the Mo atom is also coordinated by six S atoms like that in 1H-MoS2, but in an octahedral arrangement other than the trigonal prismatic arrangement in 1H-MoS2.38, 39, 46, 47 Due to the clusterization of Mo atoms in different ways, the 1T-MoS2 monolayer can be distorted and gives other types of single layer structure with distinct electronic and optoelectronic properties.38 In our study, clear splitting in both A1g and E2g1 modes together with
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new emerged vibrations at 200 and 240 cm-1 in the Raman spectra indicate a pressure induced distortion of initial 1H-MoS2.
Conclusions We have investigated the micro-PL and Raman spectra of monolayer MoS2 under high pressure. A structural deformation of monolayer MoS2 was revealed above 0.84 GPa, which is evidenced by the splitting of A1g and E2g1 modes at this pressure. At higher pressure of 5.8 GPa, the appearance of two new peaks located at 200 and 240 cm-1 suggest a structural transformation might take place. This new structure is supposed to have a distorted unit with S atoms sliding within a single layer, which is similar to the metastable 1T’-MoS2
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ARTICLE MoS2 at about 68 GPa based on their calculations. In fact, such pressure-induced blue shift in the PL position of monolayer MoS2 suggests a pressure confinement effect on nanomaterials. Previously, Wu et al. has reported an exciton emission investigation in InGaAs quantum dots, indicating that the pressure induces a blue shift of the conduction band minimum and causes a increase of band gap.41 For the monolayer 1H-MoS2, the PL peak originates from direct K-K interband transition between the conduction band minimum and valence band maximum at the K point in Brillouin zone, which are contributed primarily from the d orbitals of Mo atoms.42 When pressure increases, these orbitals may move away from Fermi level and lead to an increase of band gap. On the other hand, the p orbital of S atoms is known to contribute mainly to the valence band maximum at Г point and extend out of the MoS2 plane which exhibits interlayer coupling, upon compression. The pressure induced intralayer hybridization between
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Journal Name structure. The monolayer MoS2 shows a remarkable PL peak and the emission energy exhibits a blue shift upon compression, indicating the band gap of monolayer MoS2 broadens as pressure increases. Such distinct features of monolayer MoS2 compared with bulk MoS2 should be related to the unique pressure-induced confinement effect in the monolayer. Such pressure tuned behavior might enable the development of novel devices with multiple phenomena involving the strong coupling of the mechanical, electrical and optical properties of layered nanomaterials.
Acknowledgements This work was supported by the National Basic Research Program of China (no. 2011CB808200), the NSFC (nos.11274137, 91014004, 11474127, 11004074), National Found for Fostering Talents of Basic Science (No. J1103202).
Notes and references 1
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ARTICLE
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