Increasing Interlayer Coupling Prevented the Deformation in

Subscriber access provided by Warwick University Library

C: Physical Processes in Nanomaterials and Nanostructures

Increasing Interlayer Coupling Prevented the Deformation in Compressed Multilayer WSe 2

Yuan-Bo Gong, Qiang Zhou, Yan Liu, Xinpeng Fu, Mingguang Yao, Xiaoli Huang, Yanping Huang, Hanxue Gao, Fangfei Li, and Tian Cui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02093 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Increasing Interlayer Coupling Prevented the Deformation in Compressed Multilayer WSe2 Yuanbo Gong, Qiang Zhou*, Yan Liu, Xinpeng Fu, Mingguang Yao, Xiaoli Huang, Yanping Huang, Hanxue Gao, Fangfei Li*, Tian Cui State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China

Corresponding Authors * Qiang Zhou: E-mail: [email protected]. * Fangfei Li: E-mail: [email protected].

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT High pressure investigations on transition-metal dichalcogenides (TMD) have been considered as an efficient way to disclose their unique crystalline and electronic properties. Here we studied the vibrational behaviors of pressurized multilayer WSe2 from two layers (2TL) to six layers (6TL) by Raman spectroscopy. The intralayer and interlayer vibrations of WSe2 all show monotonous blue shift without any discontinuity. Due to the strong interlayer coupling interactions no structural transition occurs, but nondegeneration splitting of shear mode vibrations coming from pressure-induced in-plane deformation is observed. As the interlayer coupling increases in thicker WSe2, the in-plane deformation is suppressed and takes place at higher pressure. The monotonous increase of force constants and elastic constants suggested a stable structure of WSe2 within our studied pressure range.

2


Page 2 of 28

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Transition-metal dichalcogenides (TMD) materials MX2 (M=transition metal, X=S, Se, Te) have attracted extensive attention due to their graphene-like structures and excellent optoelectronic properties.1-7 With the success in preparation of two-dimensional (2D) monolayer or multilayer TMDs, their remarkable properties including electronic, excitonic, structural and mechanical performance have been investigated in recent years.8-11 Unlike graphene or other 2D materials, the layered TMD has a “trilayer (TL)” unit composed of three atomic layers linked by covalent bonds between them, and then the adjacent TLs are coupled via weak van der Waals (vdW) interactions.6, 12 From bulk to monolayer, the TMDs give distinct electronic and optical properties depending on the variation of layer number. For instance, the indirect-to-direct bandgap transition from bulk to monolayer TMDs have been found in MoS2 etc.13-14 Monolayer MoS2 is suggested to be possible valleytronics and field effect transistors.15 With layer number decreases the structural stability is also affected due to the thickness dependent interlayer coupling effect. Thus, systematically exploring the relationship between interlayer coupling and structure of layered TMD materials are indispensable, and also helpful for further comprehending the inherent properties of layered materials. The hydrostatic pressure may induce a structural transition from 2Hc to 2Ha in bulk MoS2, and a pressure induced layer sliding in multilayer MoS2 comes much earlier compared with the bulk MoS2.16-17 In sharp contrast to MoS2, the bulk MoSe2 undergoes an anisotropic 2D layered network to a three-dimensional (3D) structure 3



without any structural transition even up to 60 GPa.18 Their different high pressure behaviors suggested that, pressure engineering is an effective way to distinguish hidden interaction characters of various TMDs and to investigate the relations between layer numbers, layer distances, layer sliding and structural stabilities etc. As a representative TMD material, tungsten diselenide (WSe2) has attracted great attention in the prospective applications including novel optoelectronic devices and new energy materials due to its strong photoluminescence (PL) emission, ambipolar transport, phototonus and photocurrent properties.3 High pressure electronic study of WSe2 was performed as early as fifteen years ago.19 The X-ray diffraction (XRD) and first principles calculation pointed out an isostructural transition in WSe2 around 35~40 GPa.8,

20

While using a different pressure transmitting medium (PTM) the

isostructural transition in bulk WSe2 is found to start from 28.5 GPa and complete until 61.7 GPa, which experiences a very broad pressure range.21 It reveals a dominant role of in-plane strain over the out-of-plane compression in achieving the transition. The pressure induced K-Λ crossing in monolayer WSe2 has been discovered later.22-23 Recently single photon emission in monolayer WSe2 was achieved by applying hydrostatic pressure, coming from the excitons bound to deep-level defects.24 For multilayer WSe2, Zhao’s investigation at ambient conditions indicated that the nanoscale interlayer frictional characteristics is independent of number of layers.25 However the high pressure behaviors and characters have not been reported till now. In this paper, we performed high pressure Raman study on multilayer WSe2 from two layers (2TL) to six layers (6TL). The intralayer vibrations of E12g and A1g modes, 4


Page 4 of 28

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


as well as the interlayer shear and breath modes were all observed. The high pressure evolutions of these vibration modes disclosed a thickness dependent in-plane structural deformation at different pressures. The relationship between layer number and interlayer coupling effect, as well as the structural stability of multilayer WSe2 under pressure has been discussed.

Experimental Methods The few-layer WSe2 films were exfoliated from bulk crystals, then transferred onto Si substrates covered with 300 nm SiO2. The layer number of WSe2 films were initially estimated by optical contrast and then further determined by Raman spectra measurements.25 A series of high-pressure experiments were performed using a symmetric diamond anvil cells (DAC) with culet size of 300 µm or 400 µm. A T301 stainless steel was used as gasket and pre-indented to a thickness of ~50 µm, and then a hole was drilled in the center of pre-indented area. Inert argon served as pressure transmission medium (PTM) was compressively loaded in the sample chamber to offer a hydrostatic pressure condition. The pressure was calibrated by the ruby fluorescence method.26 The sample images of 2TL to 6TL are shown in Figure S-1, and the samples loaded in DAC chamber are also presented. Raman spectra measurements were carried out at room temperature by using a Horiba-JY T64000 micro Raman spectrometer in backscattering configuration. A coherent 532 nm solid state laser was used as excitation light source. The signals at ambient pressure and high pressure were collected by 100× and 50× objective lens, respectively. Then, they were dispersed by an 1800 groove/mm grating under a triple 5



subtractive mode. The spectra resolution was better than 0.8 cm-1. To avoid any laser-induced effect on the sample, the power of incident laser was controlled less than 3 mW.

Results and Discussion The mechanically exfoliated multilayer WSe2 located on Si substrates are shown in Figure 1, and more details on sample images are shown in the Supporting Information (SI). Similar optical contrasts under microscope prevent the possibility to distinguish the layer numbers from each other. The Raman spectroscopy is a convenient and quick way to tell the exact layer number then.25, 27-28 Raman spectra of multilayer WSe2 from 2TL to 6TL as well as the bulk WSe2 at ambient conditions are presented in Figure 2. The high frequency region gives information on intralayer vibrations while ultralow frequency region is from interlayer vibrations.

Figure 1. Images of 4TL WSe2 flake placed on silicon wafer. The sample shape is outlined by yellow dotted line. 4TL-DAC indicates the cut silicon wafer placed in the DAC chamber.

6


Page 6 of 28

Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The bulk WSe2 has a 2Hc type structure with the point group D46h (P63/mmc),25 which possesses four Raman active vibrations including A1g, E1g, E2g1 and E2g2 modes. The E1g mode is forbidden in backscattering experimental geometry when incident laser shoots on a surface perpendicular to its c-axis. For bulk WSe2 the frequencies of the intralayer E2g1 and A1g modes are close to each other, resulting in an envelope peak (Figure 2). There are also other weak second order Raman modes caused by multiphonon bands involving the longitudinal acoustic LA mode,27, 29 which are not shown and discussed here, but the nearby 2LA(M) mode has a considerable intensity comparably. The interlayer E2g2 mode located at 23.5 cm-1 represents vibrations between TL layers moving as a whole unit.25

Figure 2. (a), (b) The Raman spectra of multilayer and bulk WSe2 at ambient pressure. Full spectra are normalized for comparison. The 2LA(M) mode is a second order Phonon mode. (c) Atomic displacements of intralayer and interlayer modes. The dashed lines represent the interlayer vdW interactions. Detailed atomic displacements of S1, S2 and B1, B2 modes are shown in Figure S-2.

7



Page 8 of 28

When thickness decreases down to few layers, the few TL WSe2’s symmetry group is different due to the lack of translation in c-axis direction. To be short (details can be found in SI), we label the E2g2 as S1 mode corresponding to an interlayer shear vibration from the adjacent TLs moving out of phase by 180o. This S1 mode is detected in all multilayer WSe2 and the corresponding frequency shows a descending trend as the thickness decreases, as shown in Figure 2a. When the layer number is more than three, there is another interlayer shear vibration labeled as S2. The interlayer breathing vibration with the lowest frequency is labeled as B1 which exists in all few layer WSe2 except the monolayer, and another interlayer breathing vibration labeled as B2 exists in samples thicker than three layers. The interlayer shear and breathing modes for multilayer WSe2 from 2TL to 6TL are listed in Table 1, except some ultralow vibrations (

Increasing Interlayer Coupling Prevented the Deformation in

Recommend Documents