Exciton Emissions in Bilayer WSe2 Tuned by the Ferroelectric Polymer.

In this work, a hybrid structure of multilayer transition-metal dichalcogenides (TMDs) and a ferroelectric polymer is designed to achieve passive control of optical properties in situ. The electrical polarization in the ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) polymer can regulate the photoluminescence (PL) in bilayer WSe2. The total PL emission intensity is substantially suppressed or enhanced under large gate voltage in bilayer WSe2. This is because electrons transfer between the conduction band K valley and the conduction band Λ valley by the electrostatic field in the P(VDF-TrFE) polymer. This electron transfer further adjusts the proportion of direct and indirect excitons and, in turn, changes the overall optical radiation efficiency. We also illustrate that the engineered PL originates from the external electric-field-dependent transferred electron effect. The theoretical result matches the experimental data well. This work demonstrates a device platform in which passive regulation is achieved using 2D TMDs modulated by polarized ferroelectric materials.

Hexagonal layered group IV-VI semiconductors and derivatives: fresh blood of the 2D family.

New phases of group IV-VI semiconductors in 2D hexagonal structures are predicted and their unusual physical properties are revealed. The structures of monolayer group IV-VI semiconductors are similar to those of blue phosphorene and each unit has the same ten valence electrons. The band gap of 2D hexagonal group IV-VI semiconductors depends on both the thickness and stacking order. Atomic functionalization can induce ferromagnetism, and the Curie temperature can be tuned. Gapped Dirac fermions with zero mass are developed and this makes it exceed that of graphene. The Fermi velocity can be compared to or even above that of graphene.

Linearly Polarized Luminescence of Atomically Thin MoS2 Semiconductor Nanocrystals.

Atomically thin layers of transition-metal dichalcogenides semiconductors, such as MoS2, exhibit strong and circularly polarized light emission due to inherent crystal symmetries, pronounced spin-orbit coupling, and out-of-plane dielectric and spatial confinement. While the layer-by-layer confinement is well-understood, the understanding of the impact of in-plane quantization in their optical spectrum is far behind. Here, we report the optical properties of atomically thin MoS2 colloidal semiconductor nanocrystals. In addition to the spatial-confinement effect leading to their blue wavelength emission, the high quality of our MoS2 nanocrystals is revealed by narrow photoluminescence, which allows us to resolve multiple optically active transitions, originating from quantum-confined excitons (coupled electron-hole pairs). Surprisingly, in stark contrast to monolayer MoS2, the luminescence of the lowest-energy levels is linearly polarized and persists up to room temperature, meaning that it could be exploited in a variety of light-emitting applications.

Strong multiphoton absorption in chiral CdSe/CdS dot/rod nanocrystal-doped poly(vinyl alcohol) films.

Cysteine-capped cadmium selenide/cadmium sulfide (CdSe/CdS) dot/rod nanocrystals (NCs) were synthesized and then doped in poly(vinyl alcohol) (PVA) films. Compared with an L-/D-cysteine-capped NC solution (10-4), the anisotropic factors of the circular dichroism and circular polarized luminescence in the doped PVA films increased by one order of magnitude, probably because of the enhanced anisotropy degree, crystal orientations, and ordered morphologies. The two- and three-photon absorption coefficients of the doped PVA films were determined as 0.58 cm/GW at 800 nm and 2.3×10-4 cm3/GW2 at 1300 nm, respectively. The chiral NC-doped PVA films are promising for applications in chirality-related nonlinear photonic devices.

Prediction of an extremely long exciton lifetime in a Janus-MoSTe monolayer.

The electron-hole separation efficiency is a key factor that determines the performance of two-dimensional (2D) transition metal dichalcogenides (TMDs) and devices. Therefore, searching for novel 2D TMD materials with a long timescale of carrier lifetime has become one of the most important topics. Here, based on time-domain density functional theory (TD-DFT), we propose a brand new TMD material, namely Janus-MoSTe, which exhibits a strong built-in electric field. Our results show that in the Janus-MoSTe monolayer, the exciton consisting of an electron and hole has a relatively wide spatial extension and low binding energy. In addition, a slow electron-hole recombination process is observed, with a timescale on the order of 1.31 ns, which is even comparable to those of van der Waals (vdW) heterostructures. Further analysis reveals that the extremely long timescale for electron-hole recombination could be ascribed to the strong Coulomb screening effect as well as the small overlap of wavefunctions between electrons and holes. Our findings establish the built-in electric field as an effective factor to control the electron-hole recombination dynamics in TMD monolayers and facilitate their future applications in light detection and harvesting.

Observation of intrinsic dark exciton in Janus-MoSSe heterosturcture induced by intrinsic electric field.

The nature of exciton in heterostructure exhibits distinct properties, which makes heterostructure a promising candidate for valleytronic and optoelectronic applications. Therefore, understanding of exciton behaviour in heterostructure is the key approach to design novel devices. Here, we investigate the electronic properties including quasiparticle-energy calculations (on the level of the GW approximation) and optical properties (on the level of the Bethe-Salpeter equation) of Janus-MoSSe based heterostructure. Our results show that the build-in electric field caused by spontaneous polarization of Janus-MoSSe monolayer can significantly affect the interlayer interactions within the heterostructure, giving rise to a bright-to-dark exciton transition. To shed light on this phenomenon, a theoretical model is developed, which illustrates that the dark exciton can be ascribed to a coherence cancellation at the band edge positions induced by the strong interlayer coupling. Our findings may provide a new way for modulating and developing of van der Waals heterostructure that have applications in valleytronic and optoelectronic devices.

Highly Tunable Electronic Structures of Phosphorene/Carbon Nanotube Heterostructures through External Electric Field and Atomic Intercalation.

Black phosphorene (BP)/carbon nanotube (CNT) heterostructures can be classified as either type I or II, depending on the size of the CNTs. An external electric field (Eext) can modulate the interfacial electronic structures and separate the electron and hole carriers of the BP/CNT heterostructures. The giant Stark effect is observed, and the band gap of the semiconducting heterostructures can vary several-fold. The intercalation of 3d transition metals can strongly bond BP and CNTs together. Furthermore, strong ferromagnetism with Curie temperature (TC) above room temperature is predicted. It is expected that these BP/CNT heterostructures will provide new opportunities and applications in the fields of optoelectronics and electronics as well as spintronics.

Anomalously robust valley polarization and valley coherence in bilayer WS2.

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.

Magnetoelectric effects and valley-controlled spin quantum gates in transition metal dichalcogenide bilayers.

In monolayer group-VI transition metal dichalcogenides, charge carriers have spin and valley degrees of freedom, both associated with magnetic moments. On the other hand, the layer degree of freedom in multilayers is associated with electrical polarization. Here we show that transition metal dichalcogenide bilayers offer an unprecedented platform to realize a strong coupling between the spin, valley and layer pseudospin of holes. Such coupling gives rise to the spin Hall effect and spin-dependent selection rule for optical transitions in inversion symmetric bilayer and leads to a variety of magnetoelectric effects permitting quantum manipulation of these electronic degrees of freedom. Oscillating electric and magnetic fields can both drive the hole spin resonance where the two fields have valley-dependent interference, making an interplay between the spin and valley as information carriers possible for potential valley-spintronic applications. We show how to realize quantum gates on the spin qubit controlled by the valley bit.