Drastic enhancement of the Raman intensity in few-layer InSe by uniaxial strain

Abstract

The vibrational and electronic properties of 2-dimensinal (2D) materials can be efficiently tuned by external strain due to their good stretchability. Resonant Raman spectroscopy is a versatile tool to study the physics of phonons, electrons and their interactions simultaneously, which is particularly useful for the investigation of strain effect on 2D materials. Here, for the first time, we report the resonant Raman study of strained few-layer InSe (γ-phase). Under ~ 1% of uniaxial tensile strain, one order of magnitude enhancement of Raman intensity for longitudinal optical (LO) phonon is observed, while other modes exhibit only modest change. Further analysis demonstrates that it arises from the intraband electron-phonon scattering channel for LO phonon in resonance. The large enhancement of Raman intensity provides us a sensitive and novel method to characterize the strain effect and a mapping of the strain distribution in a wrinkled sample is demonstrated. In addition, we observed sizable redshifts of firstorder optical phonon modes. The shift rate exhibits phonon mode dependence, in excellent agreement with density functional perturbation theory (DFPT) calculations.Our study paves the way for sensitive strain quantification in few-layer InSe and its application in flexible electronic and optoelectronic devices. 3 / 41 I. INTRODUCTION Mechanical cleavage of graphene [1] by K. S. Novoselov et al. arouses tremendous research interest in 2D materials. A variety of 2D semimetals and semiconductors have been discovered ever since, such as transition metal dichalcogenides (TMDCs) [2], silicone [3], stanine [4] and black phosphorus [5,6]. Atomically thin indium selenide (γphase) joins the family lately with unique electronic properties [7-9]. Quantum Hall effect was observed in the high quality few-layer InSe electronic devices [9]. Strong quantum confinement in the out-of-plane direction gives rise to layer-dependent bandgap [7], covering a large range of visible and near infrared regions. Few-layer InSe has promised great application potentials in electronics and optoelectronics [10-12]. The mechanical stretchability of 2D materials opens the door for straining, to continuously and reversibly tune their lattice constants and electronic properties [13]. Raman spectroscopy is a crucial diagnostic tool to evaluate the strain effect. Phonon softening and splitting are commonly observed in 2D materials under uniaxial tensile strain, such as graphene [14,15], TMDCs [15,16] and black phosphorus [18,19], indicating the weakening of the bond strength and the symmetry-breaking. The band structure and electronic properties of 2D materials can be engineered efficiently via strain as well. For example, prominent strain-induced shift of the band gap and indirectto-direct bandgap transition were observed in multilayer TMDCs [20]. Owing to the small Young's modulus ( ~ 45 N/m) [21], the bandgap of few-layer InSe can be easily tuned by uniaxial tensile strain with shift rate up to 90-150meV/% [22,23]. Therefore, 5 / 41 II. EXPERI...

High‐Performance Near‐Infrared Photodetectors Based on Surface‐Doped InSe

“…Four characteristic Raman peaks are observed at 115, 177, 198, and 227 cm −1 from the undoped InSe, corresponding to the A 1 ′, E′, A 2 ″, and A 1 ′ vibrational modes, respectively. [28,29,[31][32][33] After the AuCl 3 doping, the A 2 ″ peak intensity decreases, and the peak is redshifted from 198 to 191 cm −1 , while other peaks are unchanged, similar to the previous observation in the Ti 4+ -attached InSe. [28] The ratio of A 2 ″ peak to E′ peak intensity decreases from ≈2.5 to ≈1, which reveals that the electron density is reduced.…”

“…[28] The redshift of the A 2 ″ peak is due to the strain induced in InSe by the formation of Au NPs. [28,32] The spatial maps of the intensity ratio of the A 2 ″ peak to E′ peak (A 2 ″/E′) and the Raman shift of the A 2 ″ peak suggest that the treated surface is relatively uniform ( Figure S2, Supporting Information). The scanning electron microscope measurements further confirm the formation of NPs, which were measured as ≈33 ± 5 nm in diameter ( Figure S3, Supporting Information).…”

Strain Effect on Thermoelectric Performance of InSe Monolayer

“…Previous theoretical calculations demonstrated that InSe monolayer can sustain a tensile strain over 20%, which is much larger than our predicted strains [20]. In the experiment, applying a strain on 2D materials are mostly through their interaction with substrates, which can be induced from heating [56], the lattice mismatch between epitaxial thin films [57], or bending of the 2D material on substrate [58, 59]. Actually, it is experimentally more common to apply uniaxial strain instead of biaxial strain.…”

Anomalous Low Thermal Conductivity of Atomically Thin InSe Probed by Scanning Thermal Microscopy

“…The latter assignment is confirmed by micro‐Raman spectroscopy images (Figure S2, Supporting Information) revealing a Raman redshift of the vibrational modes A 1 ′(Γ 1 2 ) and A 1 ′(Γ 1 3 ) at the microwell edges. Tensile strain in bent InSe can shift these Raman modes [ 14,64 ] and enhance anharmonic phonon scattering, reducing phonon group velocity and heat capacity. [ 31 ] Specifically, a reduction of κ from 25.9 to 13.1 W m −1 K −1 was predicted under a strain of 6%.…”