Raman Signatures of Polytypism in Molybdenum Disulfide

Abstract

Since the stacking order sensitively affects various physical properties of layered materials, accurate determination of the stacking order is important for studying the basic properties of these materials as well as for device applications. Because 2H-molybdenum disulfide (MoS2) is most common in nature, most studies so far have focused on 2H-MoS2. However, we found that the 2H, 3R, and mixed stacking sequences exist in few-layer MoS2 exfoliated from natural molybdenite crystals. The crystal structures are confirmed by HR-TEM measurements. The Raman signatures of different polytypes are investigated by using 3 different excitation energies 2 which are non-resonant and resonant with A and C excitons, respectively. The low-frequency breathing and shear modes show distinct differences for each polytype whereas the highfrequency intra-layer modes show little difference. For resonant excitations at 1.96 and 2.81 eV, distinct features are observed which enable determination of the stacking order.3 Polytypism, a special type of polymorphism in layered materials, refers to different stacking sequences of monolayers with the same structure. 1-3 Since stacking sequence is one of the key attributes of layered materials, the effects of different stacking sequences on the electronic and other properties of 2-dimensional (2D) materials are of great interest. Although the crystal structure of each layer is identical, the properties of few-layer crystals are sensitively dependent on the stacking sequence due to different inter-layer interactions. For example, in the case of graphene, two stable stacking orders, ABA (Bernal) and ABC (rhombohedral) stacking orders, predominantly exist in nature. 4,5 Several studies have revealed the influence of the stacking order on transport 6,7 and optical properties. [8][9][10][11][12] Hence, identifying the stacking sequences has become an important issue, and Raman spectroscopy has proven to be a reliable and easy characterization tool to identify stacking orders in few-layer graphene. [9][10][11]13 Since the Raman spectrum reflects the phonon dispersion and the electronic band structure, it is an ideal tool for fingerprinting the polytypes of graphene without complicated sample preparations.In layered transition metal dichalcogenides (TMDs), polymorphism and polytypism are more important than in graphene because of more complex crystal structures. Among TMD materials, molybdenum disulfide (MoS2) is the most extensively studied. Thanks to a finite bandgap, electronic applications such as field effect transistors 14,15 or photodetectors 16,17 are explored.Single-layer MoS2 comprises a monolayer of Mo atoms sandwiched between two sulfur layers, forming a 'trilayer' (TL). 18,19 Each TL is connected via weak van der Waals interactions. Like other 2D materials, MoS2 has several polymorphs. For single-layer MoS2, there are two types of polymorphs: trigonal prism (1H-MoS2) and octahedral coordination (1T-MoS2). Since 1T-MoS2 is metastable, only the trigonal phase is found in natural bulk ...

Determination of band offsets at GaN/single-layer MoS2 heterojunction

“…1(a) and 1(b) represents 2LA mode. 21 The separation between E 1 2g and A 1 g phonon modes is observed to be 19.5 6 0.5 cm À1 , which confirms that the MoS 2 on c-sapphire deposited by CVD is a single-layer. [22][23][24] The intensity ratio (I A /I E ) of A 1 g and E 1 2g modes decreases from sample A to B which could be due to the unintentional doping of MoS 2 monolayer in sample B.…”

Interlayer bond polarizability model for stacking-dependent low-frequency Raman scattering in layered materials

“…They have been reported in recent works. 24,[49][50][51][52][53] The adopted stacking nomenclature here can be extended to group-6 MX 2 of any thickness, and it works equally well for h-BN, 68 GaSe 69,70 and NbSe 2 . 46 All four stacking types can be found in bulk GaSe crystals, where AA ′ , AB, ABC and AA ′ B ′ B are historically named as β -2H, ε-2H, γ-3R, δ -4H polytypes, respectively.…”

“…5b), only the highest-frequency mode S 1 is observed in AA ′ (or AB) stacking type, only the lowest-frequency S 2 mode is observed in ABC stacking type, while both S 1 and S 2 modes can be observed with nearly equal intensities in AA ′ B ′ B stacking type; in 4L MoSe 2 (Fig. 5c) 53 Thus careful parameter fitting is needed to quantitatively account for experimental Raman measurements.…”

“…7,9,33 They can be categorized into two types: in-plane shear and out-of-plane breathing vibration modes. Due to their greater sensitivity to interlayer coupling, LF Raman modes can directly probe the interfacial coupling, and they have been found as more effective indicators of the layer thickness [34][35][36][37][38][39][40][41][42][43][44][45][46] and stacking 24,[47][48][49][50][51][52][53][54][55] for diverse 2D materials. LF Raman spectroscopy is a rapidly developing field of research and has accelerated due to recent development of volume Bragg gratings for ultra-narrow optical filters with bandwidth about 1 cm -1 , which allows efficient cut-off of the excitation laser light without employing expensive triple monochromators for LF Raman measurements.…”

Stacking-controllable interlayer coupling and symmetric configuration of multilayered MoS2

“…[3][4][5]11,16,17 To the best of our knowledge, few studies of stacking-orientation-controlled samples of more than three layers and subsequent characterization with PL and Raman spectroscopy have been conducted. [10][11][12][13]18 Furthermore, studying the nonlinear optical properties, such as SHG or four-wave mixing (FWM), as a function of the layer number and stacking orientation of the multi-stacked crystals grown by CVD have scarcely been sought. [4][5][6] The minute variations of the band structures according to the geometry and order of stacking are sufficient to modulate the nonlinear optical susceptibility, and hence, versatile MoS 2 crystals synthesized by CVD play a crucial role in understanding fundamental optoelectronic properties.…”