Up-conversion photoluminescence (UCPL) refers to the elementary process where low-energy photons are converted into high-energy ones via consecutive interactions inside a medium. When additional energy is provided by internal thermal energy in the form of lattice vibrations (phonons), the process is called phonon-assisted UCPL. Here, we report the exceptionally large phonon-assisted energy gain of up to ~ 8k_BT (k_B is Boltzmann constant, T is temperature) on all-inorganic lead halide perovskite semiconductor colloidal nanocrystals that goes beyond the maximum capability of only harvesting optical phonon modes. By systematic optical study in combination with a statistical probability model, we explained the nontrivial phonon-assisted UCPL process in perovskites nanocrystals, where in addition to the strong electron-phonon (light-matter) coupling, other nonlinear processes such as phonon-phonon (matter-matter) interaction also effectively boost the up-conversion efficiency.

Fundamental understandings on the dynamics of charge carriers and excitonic quasiparticles in semiconductors are of central importance for both many-body physics and promising optoelectronic and photonic applications. Here, we investigated the carrier dynamics and many-body interactions in two-dimensional (2D) transition metal dichalcogenides (TMDs), using monolayer WS₂ as an example, by employing femtosecond broadband pump-probe spectroscopy. Three time regimes for the exciton energy renormalization are unambiguously revealed with a distinct red-blue-red shift upon above-bandgap optical excitations. We attribute the dominant physical process in the three typical regimes to free carrier screening effect, Coulombic exciton-exciton interactions and Auger photocarrier generation, respectively, which show distinct dependence on the optical excitation wavelength, pump fluences and/or lattice temperature. An intrinsic exciton radiative lifetime of about 1.2 picoseconds (ps) in monolayer WS₂ is unraveled at low temperature, and surprisingly the efficient Auger nonradiative decay of both bright and dark excitons puts the system in a nonequilibrium state at the nanosecond timescale. In addition, the dynamics of trions at low temperature is observed to be significantly different from that of excitons, e.g., a long radiative lifetime of ~ 108.7 ps at low excitation densities and the evolution of trion energy as a function of delay times. Our findings elucidate the dynamics of excitonic quasiparticles and the intricate many-body physics in 2D semiconductors, underpinning the future development of photonics, valleytronics and optoelectronics based on 2D semiconductors.

We review lattice vibrational modes in atomically thin two-dimensional (2D) layered materials, focusing on 2D materials beyond graphene, such as group Ⅵ transition metal dichalcogenides, topological insulator bismuth chalcogenides, and black phosphorus. Although the composition and structure of those materials are remarkably different, they share a common and important feature, i.e., their bulk crystals are stacked via van der Waals interactions between "layers" while each layer is comprised of one or more atomic planes. First, we review the background of some 2D materials (MX₂, M = Mo, W; X = S, Se, Te. Bi₂X₃, X = Se, Te. Black phosphorus), including crystalline structures and stacking order. We then review the studies on vibrational modes of layered materials and nanostructures probed by the powerful yet nondestructive Raman spectroscopy technique. Based on studies conducted before 2010, recent investigations using more advanced techniques have pushed the studies of phonon modes in 2D layered materials to the atomically thin regime, down to monolayers. We will classify the recently reported general features into the following categories: phonon confinement effects and electron-phonon coupling, anomalous shifts in high-frequency intralayer vibrational modes and surface effects, reduced dimensionality and lower symmetry, the linear chain model and the substrate effect, stacking orders and interlayer shear modes, polarization dependence, and the resonance effect. Within the seven categories, both intralayer and interlayer vibrational modes will be discussed. The comparison between different materials will be provided as well.

The electron–hole exchange interaction significantly influences the optical properties of excitons and radiative decay. However, exciton dynamics in luminescent carbon dots (Cdots) is still not clear. In this study, we have developed a simple and efficient one-step strategy to synthesize luminescent Cdots using the pyrolysis of oleylamine. The sp² clusters of a few aromatic rings are responsible for the observed blue photoluminescence. The size of these clusters can be tuned by controlling the reaction time, and the energy gap between the π–π^* states of the sp² domains decreases as the sp² cluster size increases. More importantly, the strong electron–hole exchange interaction results in the splitting of the exciton states of the sp² clusters into the singlet-bright and triplet-dark states with an energy difference ΔE, which decreases with increasing sp² cluster size owing to the reduction of the confinement energy and the suppression of the electron–hole exchange interaction.

Strain engineering provides an effective mean of tuning the fundamental properties of semiconductors for electric and optoelectronic applications. Here we report on how the applied strain changes the emission properties of hetero-structures consisting of different crystalline phases in the same CdS nanobelts. The strained portion was found to produce an additional emission peak on the low-energy side that was blueshifted with increasing strain. Furthermore, the additional emission peak obeyed the Varshni equation with temperature and exhibited the band-filling effect at high excitation power. This new emission peak may be attributed to spatially indirect exciton recombination between different crystalline phases of CdS. First-principles calculations were performed based on the spatially indirect exciton recombination, and the calculated and experimental results agreed with one another. Strain proved to be capable of enhancing the anti-Stokes emission, suggesting that the efficiency of laser cooling may be improved by strain engineering.

We report on a Te-seeded epitaxial growth of ultrathin Bi₂Te₃ nanoplates (down to three quintuple layers (QL)) with large planar sizes (up to tens of micrometers) through vapor transport. Optical contrast has been systematically investigated for the as-grown Bi₂Te₃ nanoplates on the SiO₂/Si substrates, experimentally and computationally. The high and distinct optical contrast provides a fast and convenient method for the thickness determination of few-QL Bi₂Te₃ nanoplates. By aberration-corrected scanning transmission electron microscopy, a hexagonal crystalline structure has been identified for the Te seeds, which form naturally during the growth process and initiate an epitaxial growth of the rhombohedralstructured Bi₂Te₃ nanoplates. The epitaxial relationship between Te and Bi₂Te₃ is identified to be perfect along both in-plane and out-of-plane directions of the layered nanoplate. Similar growth mechanism might be expected for other bismuth chalcogenide layered materials.