Qing-Hai Tan

Raman and photoluminescence spectra of two-dimensional nanocrystallites of monolayer WS2 and WSe2

Defects strongly modify optical properties in pristine and nanostructured two-dimensional (2D) materials. The ion implantation technique can be used to gradually introduce defects in semiconductor to obtain nanocrystallites (NCs) with different domain sizes. Here, we present a detailed study on the Raman and photoluminescence spectra of 2D NCs of monolayer WS2 (1L WS2) and 1L WSe2 prepared by ion implantation. With increasing ion dosages, both and modes of 1L WS2 exhibit a downshift in frequency and an asymmetrical broadening toward lower frequency, while the mode in 1L WSe2 NCs exhibits an opposite behavior, showing asymmetrical broadening and peak shift toward higher frequency. This behavior is well understood by phonon quantum confinement of the out-of-plane optical branch whose frequency displays a minimum at Γ in pristine 1L WSe2. After the ion implantation, phonons from the Brillouin zone boundary are revealed in the Raman spectra, and the corresponding assignments are identified by resonant Raman spectra at low temperature. The defects can act as trapping centers of free carriers, which result in a sharp decrease of photoluminescence (PL) emission from A exciton with increasing ion dosage. The PL peak from A-exciton in both 1L WS2 and 1L WSe2 NCs blueshifts with increasing the ion dosage due to the quantum confinement effect of smaller NC size. The ion-implantation results in a new emission peak of defect-bound neutral excitons below the A-exciton peak in both 1L WS2 and 1L WSe2 NCs. Its relative intensity to the A exciton increases with increasing the ion dosage and finally vanishes along with the A exciton. These results offer a route toward tailoring the optical properties of 2D materials by controlling the size of 2D NCs.

Determining layer number of two-dimensional flakes of transition-metal dichalcogenides by the Raman intensity from substrates

Transition-metal dichalcogenide (TMD) semiconductors have been widely studied due to their distinctive electronic and optical properties. The property of TMD flakes is a function of their thickness, or layer number (N). How to determine the N of ultrathin TMD materials is of primary importance for fundamental study and practical applications. Raman mode intensity from substrates has been used to identify the N of intrinsic and defective multilayer graphenes up to N = 100. However, such analysis is not applicable to ultrathin TMD flakes due to the lack of a unified complex refractive index (ñ) from monolayer to bulk TMDs. Here, we discuss the N identification of TMD flakes on the SiO2/Si substrate by the intensity ratio between the Si peak from 100 nm (or 89 nm) SiO2/Si substrates underneath TMD flakes and that from bare SiO2/Si substrates. We assume the real part of ñ of TMD flakes as that of monolayer TMD and treat the imaginary part of ñ as a fitting parameter to fit the experimental intensity ratio. An empirical ñ, namely, ñ(eff), of ultrathin MoS2, WS2 and WSe2 flakes from monolayer to multilayer is obtained for typical laser excitations (2.54 eV, 2.34 eV or 2.09 eV). The fitted ñ(eff) of MoS2 has been used to identify the N of MoS2 flakes deposited on 302 nm SiO2/Si substrate, which agrees well with that determined from their shear and layer-breathing modes. This technique of measuring Raman intensity from the substrate can be extended to identify the N of ultrathin 2D flakes with N-dependent ñ. For application purposes, the intensity ratio excited by specific laser excitations has been provided for MoS2, WS2 and WSe2 flakes and multilayer graphene flakes deposited on Si substrates covered by a 80-110 nm or 280-310 nm SiO2 layer.

Layer-number dependent high-frequency vibration modes in few-layer transition metal dichalcogenides induced by interlayer couplings

Two-dimensional transition metal dichalcogenides (TMDs) have attracted extensive attention due to their many novel properties. The atoms within each layer in two-dimensional TMDs are joined together by covalent bonds, while van der Waals interactions combine the layers together. This makes its lattice dynamics layer-number dependent. The evolutions of ultralow frequency ( -1 ) modes, such as shear and layer-breathing modes have been well-established. Here, we review the layer-number dependent high-frequency (>50 cm -1 ) vibration modes in few-layer TMDs and demonstrate how the interlayer coupling leads to the splitting of high-frequency vibration modes, known as Davydov splitting. Such Davydov splitting can be well described by a van der Waals model, which directly links the splitting with the interlayer coupling. Our review expands the understanding on the effect of interlayer coupling on the high-frequency vibration modes in TMDs and other two-dimensional materials.

Controllable Synthesis of Two-Dimensional Ruddlesden–Popper-Type Perovskite Heterostructures

Two-dimensional Ruddlesden-Popper type perovskites (2D perovskites) have recently attracted increasing attention. It is expected that 2D perovskite-based heterostructures can significantly improve the efficiency of the optoelectronic devices and extend the material functionalities; however, rational synthesis of such heterostructures has not been realized to date. We report on a general low-temperature synthetic strategy for the synthesis of 2D perovskite-based lateral and vertical (n-CH3(CH2)3NH3)2PbI4/(n-CH3(CH2)3NH3)2(CH3NH3)Pb2I7 heterostructures for the first time. A combination of solution synthesis and gas-solid phase intercalation approach allows us to efficiently synthesize both lateral and vertical heterostructures with great flexibility. X-ray diffraction, photoluminescence, and photoluminescence excitation mapping and electrical transport measurement studies reveal the successful synthesis of lateral and vertical heterostructures with precisely spatial-modulation control and distinguishable interfaces. Our studies not only provide an efficient synthetic strategy with great flexibility, enabling us to create 2D perovskite-based heterostructures, but also offer a platform to investigate the physical processes in those heterostructures.

Moiré Phonons in Twisted Bilayer MoS2

The material choice, layer thickness, and twist angle widely enrich the family of van der Waals heterostructures (vdWHs), providing multiple degrees of freedom to engineer their optical and electronic properties. The moiré patterns in vdWHs create a periodic potential for electrons and excitons to yield many interesting phenomena, such as Hofstadter butterfly spectrum and moiré excitons. Here, in the as-grown/transferred twisted bilayer MoS2 (tBLMs), one of the simplest prototypes of vdWHs, we show that the periodic potentials of moiré patterns also modify the properties of phonons of its monolayer MoS2 constituent to generate Raman modes related to moiré phonons. These Raman modes correspond to zone-center phonons in tBLMs, which are folded from the off-center phonons in monolayer MoS2. However, the folded phonons related to crystallographic superlattices are not observed in the Raman spectra. By varying the twist angle, the moiré phonons of tBLM can be exploited to map the phonon dispersions of the monolayer constituent. The lattice dynamics of the moiré phonons are modulated by the patterned interlayer coupling resulting from periodic potential of moiré patterns, as confirmed by density functional theory calculations. The Raman intensity related to moiré phonons in all tBLMs are strongly enhanced when the excitation energy approaches the C exciton energy. This study can be extended to various vdWHs to deeply understand their Raman spectra, moiré phonons, lattice dynamics, excitonic effects, and interlayer coupling.