Efficient hole transfer from monolayer WS to ultrathin amorphous black phosphorus.

The newly developed van der Waals materials allow fabrication of multilayer heterostructures. Early efforts have mostly focused on heterostructures formed by similar materials. More recently, however, attempts have been made to expand the types of materials, such as topological insulators and organic semiconductors.

Photocarrier Transfer across Monolayer MoS-MoSe Lateral Heterojunctions.

In-plane heterojuctions formed from two monolayer semiconductors represent the finest control of electrons in condensed matter and have attracted significant interest. Various device studies have shown the effectiveness of such structures to control electronic processes, illustrating their potentials for electronic and optoelectronic applications. However, information about the physical mechanisms of charge carrier transfer across the junctions is still rare, mainly due to the lack of adequate experimental techniques.

Photocarrier dynamics in monolayer phosphorene and bulk black phosphorus.

We report a combined theoretical and experimental study on photocarrier dynamics in monolayer phosphorene and bulk black phosphorus. Samples of monolayer phosphorene and bulk black phosphorus were fabricated by mechanical exfoliation, identified according to their reflective contrasts, and protected by covering them with hexagonal boron nitride layers. Photocarrier dynamics in these samples was studied by an ultrafast pump-probe technique.

A type-I van der Waals heterobilayer of WSe/MoTe.

We present a joint theoretical/experimental study of a van der Waals heterobilayer with type-I band alignment formed by monolayers of WSe and MoTe. Our first-principles computation suggests that both the valence band maximum and the conduction band minimum of the WSe/MoTe heterobilayer reside in the MoTe layer. The type-I band alignment allows efficient transfer of excitons from WSe to MoTe.

Type-I van der Waals heterostructure formed by MoS and ReS monolayers.

We report a van der Waals heterostructure formed by monolayers of MoS and ReS with a type-I band alignment. First-principle calculations show that in this heterostructure, both the conduction band minimum and the valence band maximum are located in the ReS layer. This configuration is different from previously accomplished type-II van der Waals heterostructures where electrons and holes reside in different layers.

Time-Resolved Measurements of Photocarrier Dynamics in TiS3 Nanoribbons.

We report synthesis and time-resolved transient absorption measurements of TiS3 nanoribbons. TiS3 nanoribbons were fabricated by direct reaction of titanium and sulfur. Dynamics of the photocarriers in these samples were studied by transient absorption measurements.

Exciton formation in monolayer transition metal dichalcogenides.

Two-dimensional transition metal dichalcogenides provide a unique platform to study excitons in confined structures. Recently, several important aspects of excitons in these materials have been investigated in detail. However, the formation process of excitons from free carriers has yet to be understood.

Probing charge transfer excitons in a MoSe2-WS2 van der Waals heterostructure.

We show that the van der Waals heterostructure formed by MoSe2 and WS2 provides a unique system with near degenerate interlayer and intralayer excitonic states. Photoluminescence measurements indicate that the charge transfer exciton states are approximately 50 meV below the MoSe2 exciton states, with a significant spectral overlap. The transient absorption of a femtosecond pulse was used to study the dynamics of the charge transfer excitons at room temperature.

Transient Absorption Measurements on Anisotropic Monolayer ReS2.

Anisotropic optical and transport properties of monolayer ReS2 fabricated by mechanical exfoliation are reported. Transient absorption measurements with different polarization configurations and sample orientations reveal that the absorption coefficient and transient absorption are both anisotropic, with maximal and minimal values occurring when the light polarization is parallel and perpendicular to the Re atomic chains, respectively. The maximal values are about a factor of 2.

Tightly Bound Trions in Transition Metal Dichalcogenide Heterostructures.

We report the observation of trions at room temperature in a van der Waals heterostructure composed of MoSe2 and WS2 monolayers. These trions are formed by excitons excited in the WS2 layer and electrons transferred from the MoSe2 layer. Recombination of trions results in a peak in the photoluminescence spectra, which is absent in monolayer WS2 that is not in contact with MoSe2.

Exceptional and Anisotropic Transport Properties of Photocarriers in Black Phosphorus.

One key challenge in developing postsilicon electronic technology is to find ultrathin channel materials with high charge mobilities and sizable energy band gaps. Graphene can offer extremely high charge mobilities; however, the lack of a band gap presents a significant barrier. Transition metal dichalcogenides possess sizable and thickness-tunable band gaps; however, their charge mobilities are relatively low.

Electron transfer and coupling in graphene-tungsten disulfide van der Waals heterostructures.

The newly discovered two-dimensional materials can be used to form atomically thin and sharp van der Waals heterostructures with nearly perfect interface qualities, which can transform the science and technology of semiconductor heterostructures. Owing to the weak van der Waals interlayer coupling, the electronic states of participating materials remain largely unchanged. Hence, emergent properties of these structures rely on two key elements: electron transfer across the interface and interlayer coupling.

Ultrafast charge separation and indirect exciton formation in a MoS2-MoSe2 van der Waals heterostructure.

We observe subpicosecond charge separation and formation of indirect excitons a van der Waals heterostructure formed by molybdenum disulfide and molybdenum diselenide monolayers. The sample is fabricated by manually stacking monolayer MoS2 and MoSe2 flakes prepared by mechanical exfoliation. Photoluminescence measurements confirm the formation of an effective heterojunction.