Quantum Beats between Spin-Singlet and Spin-Triplet Interlayer Exciton Transitions in WSe-MoSe Heterobilayers.

The long-lived interlayer excitons (IXs) of semiconducting transition metal dichalcogenide heterobilayers are prime candidates for developing various optoelectronic and valleytronic devices. Their photophysical properties, including fine structure, have been the focus of recent studies, and the presence of two spin states, namely, spin-singlet and spin-triplet, has been experimentally confirmed. However, the existence of the interaction between these states and their nature remains unknown to date.

Gradient Type-II CdSe/CdSeTe/CdTe Core/Crown/Crown Heteronanoplatelets with Asymmetric Shape and Disproportional Excitonic Properties.

Characterized by their strong 1D confinement and long-lifetime red-shifted emission spectra, colloidal nanoplatelets (NPLs) with type-II electronic structure provide an exciting ground to design complex heterostructures with remarkable properties. This work demonstrates the synthesis and optical characterization of CdSe/CdSeTe/CdTe core/crown/crown NPLs having a step-wise gradient electronic structure and disproportional wavefunction distribution, in which the excitonic properties of the electron and hole can be finely tuned through adjusting the geometry of the intermediate crown. The first crown with staggered configuration gives rise to a series of direct and indirect transition channels that activation/deactivation of each channel is possible through wavefunction engineering.

Structural and Optical Properties of Phase-Pure UO, α-UO, and α-UO Epitaxial Thin Films Grown by Pulsed Laser Deposition.

Fundamental understanding of the electronic, chemical, and structural properties of uranium oxides requires the synthesis of high-crystalline-quality epitaxial films of different polymorphs of one material or different phases with various oxygen valence states. We report the growth of single-phase epitaxial UO, α-UO, and α-UO thin films using pulsed laser deposition. Both oxygen partial pressure and substrate temperature play critical roles in determining the crystal structure of the uranium oxide films.

Photoluminescence Intensity Fluctuations and Temperature-Dependent Decay Dynamics of Individual Carbon Nanotube sp Defects.

Recent demonstration of room temperature, telecommunication wavelength single photon generation by sp defects of single wall carbon nanotubes established these defects as a new class of quantum materials. However, their practical utilization in development of quantum light sources calls for a significant improvement in their imperfect quantum yield (QY∼10-30%). PL intensity fluctuations observed with some defects also need to be eliminated.

Stable Graphene-Two-Dimensional Multiphase Perovskite Heterostructure Phototransistors with High Gain.

Recently, two-dimensional (2D) organic-inorganic perovskites emerged as an alternative material for their three-dimensional (3D) counterparts in photovoltaic applications with improved moisture resistance. Here, we report a stable, high-gain phototransistor consisting of a monolayer graphene on hexagonal boron nitride (hBN) covered by a 2D multiphase perovskite heterostructure, which was realized using a newly developed two-step ligand exchange method. In this phototransistor, the multiple phases with varying bandgap in 2D perovskite thin films are aligned for the efficient electron-hole pair separation, leading to a high responsivity of ∼10 A W at 532 nm.

Electrothermal Control of Graphene Plasmon-Phonon Polaritons.

Graphene plasmons are known to offer an unprecedented level of confinement and enhancement of electromagnetic field. They are hence amenable to interacting strongly with various other excitations (for example, phonons) in their surroundings and are an ideal platform to study the properties of hybrid optical modes. Conversely, the thermally induced motion of particles and quasiparticles can in turn interact with electronic degrees of freedom in graphene, including the collective plasmon modes via the Coulomb interaction, which opens up new pathways to manipulate and control the behavior of these modes.

Strong Acoustic Phonon Localization in Copolymer-Wrapped Carbon Nanotubes.

Understanding and controlling exciton-phonon interactions in carbon nanotubes has important implications for producing efficient nanophotonic devices. Here we show that laser vaporization-grown carbon nanotubes display ultranarrow luminescence line widths (120 μeV) and well-resolved acoustic phonon sidebands at low temperatures when dispersed with a polyfluorene copolymer. Remarkably, we do not observe a correlation of the zero-phonon line width with (13)C atomic concentration, as would be expected for pure dephasing of excitons with acoustic phonons.

Prolonged spontaneous emission and dephasing of localized excitons in air-bridged carbon nanotubes.

The bright exciton emission of carbon nanotubes is appealing for optoelectronic devices and fundamental studies of light-matter interaction in one-dimensional nanostructures. However, to date, the photophysics of excitons in carbon nanotubes is largely affected by extrinsic effects. Here we perform time-resolved photoluminescence measurements over 14 orders of magnitude for ultra-clean carbon nanotubes bridging an air gap over pillar posts.

Quantum light signatures and nanosecond spectral diffusion from cavity-embedded carbon nanotubes.

Single-walled carbon nanotubes (SWCNTs) are considered for novel optoelectronic and quantum photonic devices, such as single photon sources, but methods must be developed to enhance the light extraction and spectral purity, while simultaneously preventing multiphoton emission as well as spectral diffusion and blinking in dielectric environments of a cavity. Here we demonstrate that utilization of nonpolar polystyrene as a cavity dielectric completely removes spectral diffusion and blinking in individual SWCNTs on the millisecond to multisecond time scale, despite the presence of surfactants. With these cavity-embedded SWCNT samples, providing a 50-fold enhanced exciton emission into the far field, we have been able to carry out photophysical studies for the first time with nanosecond timing resolution.