Bairen Zhu

Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides

We report systematic optical studies of WS2 and WSe2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at multilayers to a direct-gap one at monolayers. A hot luminescence peak (B) is observed at ~0.4 eV above the prominent band edge peak (A) in all samples. The magnitude of A-B splitting is independent of the number of layers and coincides with the spin-valley coupling strength in monolayers. Ab initio calculations show that this thickness independent splitting pattern is a direct consequence of the giant spin-valley coupling which fully suppresses interlayer hopping at valence band edge near K points because of the sign change of the spin-valley coupling from layer to layer in the 2H stacking order.

Exciton Binding Energy of Monolayer WS2

The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 ± 0.01 eV around K valley in the Brillouin zone.

Low-frequency Raman modes and electronic excitations in atomically thin MoS2 films

2g . The latter is distinguished as the compression vibrational mode, similar to the surface vibration of other epitaxial thin films. The opposite evolution of the two modes with thickness demonstrates vibrational modes in an atomically thin crystal as well as a more precise way to characterize the thickness of atomically thin MoS2 films. In addition, we observe a broad feature around 38 cm −1 (5 meV) which is visible only under near-resonance excitation and pinned at a fixed energy, independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in a conduction band at K points in their Brillouin zone.

Anomalously robust valley polarization and valley coherence in bilayer WS2.

Significance Coherence of electronic states is crucial for quantum manipulation through light–matter interactions. To achieve coherence in conventional solid-state systems, extreme conditions such as cryogenic temperatures are required, which is a long-term challenge for practical applications. The emerging atomically thin transition metal dichalcogenides provide an unprecedented platform to explore the interplay of quantum states of spin and valley. In this paper, we demonstrate room-temperature valley coherence and valley polarization in bilayer WS2 with polarization-resolved photoluminescence measurements. The robustness of the valley coherence and valley polarization is understood as the consequence of the coupling of spin, layer, and valley degrees of freedom in bilayer WS2. It inspires new perspectives on quantum manipulations in 2D solid-state systems. We report the observation of anomalously robust valley polarization and valley coherence in bilayer WS2. The polarization of the photoluminescence from bilayer WS2 follows that of the excitation source with both circular and linear polarization, and remains even at room temperature. The near-unity circular polarization of the luminescence reveals the coupling of spin, layer, and valley degree of freedom in bilayer system, and the linearly polarized photoluminescence manifests quantum coherence between the two inequivalent band extrema in momentum space, namely, the valley quantum coherence in atomically thin bilayer WS2. This observation provides insight into quantum manipulation in atomically thin semiconductors.

Resonance Raman scattering in bulk 2H-MX2 (M = Mo, W; X = S, Se) and monolayer MoS2

We have performed a comparative study of resonance Raman scattering in transition-metal dichalcogenides 2H-MX2 semiconductors (M = Mo, W; X = S, Se) and single-layer MoS2. Raman spectra were collected using excitation wavelengths 633 nm (1.96 eV), 594 nm (2.09 eV), 532 nm (2.33 eV), 514 nm (2.41 eV), and 488 nm (2.54 eV). In bulk-MoS2 and WS2, the resonant energies appear to coincide with their exciton excitations. The resonance can be fine tuned by varying sample temperatures, which confirms its excitonic origin in both MoS2 and WS2. Temperature dependence of Raman intensities is analyzed in the context of resonance Raman theory, which agrees well with the existing absorption data. While in WSe2, the resonance has been observed in a wider range of excitations from 633 to 514 nm, which cannot be explained with its excitonic energies of 1.6 and 2.0 eV. It is considered that additional excitonic bands induced by band splitting are involved in the inter-band transitions and substantially extend the resonance energy range. The Raman resonance energy range remains unchanged in single-layer MoS2 compared with that in the bulk sample. However, most phonon modes in single-layer MoS2 are significantly broadened or strongly suppressed under resonance conditions. This change could be related to the modification of acoustic modes by the substrate.

Electronic Raman scattering on individual semiconducting single walled carbon nanotubes.

We report experimental measurements of electronic Raman scattering by electrons (holes) in individual single-walled carbon nanotubes (SWNTs) under resonant conditions. The Raman scattering at low frequency range reveals a single particle excitation feature. And the dispersion of electronic structure around the center of Brillouin zone of a semiconducting SWNT (14, 13) is extracted.