Xiaomu Wang

Highly anisotropic and robust excitons in monolayer black phosphorus

Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of ∼0.9 eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices.

Black Phosphorus Radio-Frequency Transistors

Few-layer and thin film forms of layered black phosphorus (BP) have recently emerged as a promising material for applications in high performance nanoelectronics and infrared optoelectronics. Layered BP thin films offer a moderate bandgap of around 0.3 eV and high carrier mobility, which lead to transistors with decent on-off ratios and high on-state current densities. Here, we demonstrate the gigahertz frequency operation of BP field-effect transistors for the first time. The BP transistors demonstrated here show respectable current saturation with an on-off ratio that exceeds 2 × 10(3). We achieved a current density in excess of 270 mA/mm and DC transconductance above 180 mS/mm for hole conduction. Using standard high frequency characterization techniques, we measured a short-circuit current-gain cutoff frequency fT of 12 GHz and a maximum oscillation frequency fmax of 20 GHz in 300 nm channel length devices. BP devices may offer advantages over graphene transistors for high frequency electronics in terms of voltage and power gain due to the good current saturation properties arising from their finite bandgap, thus can be considered as a promising candidate for the future high performance thin film electronics technology for operation in the multi-GHz frequency range and beyond.

Black Arsenic–Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties

New layered anisotropic infrared semiconductors, black arsenic-phosphorus (b-AsP), with highly tunable chemical compositions and electronic and optical properties are introduced. Transport and infrared absorption studies demonstrate the semiconducting nature of b-AsP with tunable bandgaps, ranging from 0.3 to 0.15 eV. These bandgaps fall into the long-wavelength infrared regime and cannot be readily reached by other layered materials.

Van der Waals heterostructures: Stacked 2D materials shed light

A powerful strategy to leverage and combine the optoelectronic characteristics of different 2D materials is to stack them into vertical van der Waals heterostructures. This approach is now used to realize efficient light-emitting devices.

Band Gap Opening of Bilayer Graphene by F4-TCNQ Molecular Doping and Externally Applied Electric Field

The band gap opening of bilayer graphene with one side surface adsorption of F4-TCNQ is reported. F4-TCNQ doped bilayer graphene shows p-type semiconductor characteristics. With a F4-TCNQ concentration of 1.3 x 10(-10) mol/cm(2), the charge transfer between each F4-TCNQ molecule and graphene is 0.45e, and the built-in electric field, E(bi), between the graphene layers could reach 0.070 V/A. The charge transfer and band gap opening of the F4-TCNQ-doped graphene can be further modulated by an externally applied electric field (E(ext)). At 0.077 V/A, the gap opening at the Dirac point (K), DeltaE(K) = 306 meV, and the band gap, E(g) = 253 meV, are around 71% and 49% larger than those of the pristine bilayer under the same E(ext).

High‐Performance Graphene Devices on SiO2/Si Substrate Modified by Highly Ordered Self‐Assembled Monolayers

Graphene fi eld-effect transistors (GFETs) are considered promising devices as a result of graphene’s notably high mobility, fl exibility, and ultrathin nature. [ 1 , 2 ] Indeed, high mobility up to 200 000 cm 2 V − 1 s − 1 and extraordinarily large mean free length were observed in clean suspended graphene at 5 K. [ 3 ] Unfortunately, the applications of graphene are usually confi ned by a standard SiO 2 /Si substrate through charge transfer, adsorbates, and electron phonon scattering. [ 4–8 ] It is a common desire to search for high quality substrates to accommodate graphene devices where the deleterious effects of the substrate can be minimized and the high mobility achieved in free standing graphene can be largely preserved. This is crucial: i) to obtain high performance devices for electronic applications, ii) to reveal the intrinsic properties of graphene, and iii) to understand the mystery of scattering origins, and the charge transfer and transport mechanisms in graphene devices. The groups of Hong and Dean have pioneered the search for novel substrates by using epitaxial ferroelectric dielectric (Pb(Zr 0.2 Ti 0.8 )O 3 (PZT)) and hexagonal boron nitride (h-BN) substrates. [ 9 , 10 ] The charge carrier mobilities in graphene devices fabricated on these substrates are found to be 70 000 and 64 000 cm 2 V − 1 s − 1 at high carrier density, respectively. However, it is generally believed that utilization of surface/interface engineering to modify the existing commercially available substrates, for example, silicon wafers with thermally grown silicon dioxide, is more useful because of the convenient and mature production process, low leakage current, small surface roughness, best optical contrast (especially for a 300 nm thick SiO 2 layer), and low cost. Only a few research efforts following this strategy have been reported, among which the groups of Lafkioti and Liu found that suppression of unintentional substrate doping effects can be achieved by insertion of hydrophobic self-assembled layers (hexamethyldisilane,

Interlayer Interactions in Anisotropic Atomically-thin Rhenium Diselenide

In this work, we study the interlayer phonon vibration modes, the layer-numberdependent optical bandgap, and the anisotropic photoluminescence (PL) spectra of atomically thin rhenium diselenide (ReSe2) for the first time. The ultralow frequency interlayer Raman spectra and the polarization-resolved high frequency Raman spectra in ReSe2 allow the identification of its layer number and crystal orientation. Furthermore, PL measurements show the anisotropic optical emission intensity of the material with its bandgap increasing from 1.26 eV in the bulk to 1.32 eV in the monolayer. The study of the layer-number dependence of the Raman modes and the PL spectra reveals relatively weak van der Waal’s interaction and two-dimensional (2D) quantum confinement in the atomically thin ReSe2. The experimental observation of the intriguing anisotropic interlayer interaction and tunable optical transition in monolayer and multilayer ReSe2 establishes the foundation for further exploration of this material in the development of anisotropic optoelectronic devices functioning in the near-infrared spectrum, which is important for many applications in optical communication and infrared sensing.

Novel Field-Effect Schottky Barrier Transistors Based on Graphene-MoS2 Heterojunctions

Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio. However, the carrier mobility in backgate MoS2 FET is rather low (typically 0.5–20 cm2/V·s). Here, we report a novel field-effect Schottky barrier transistors (FESBT) based on graphene-MoS2 heterojunction (GMH), where the characteristics of high mobility from graphene and high on-off ratio from MoS2 are properly balanced in the novel transistors. Large modulation on the device current (on/off ratio of 105) is achieved by adjusting the backgate (through 300 nm SiO2) voltage to modulate the graphene-MoS2 Schottky barrier. Moreover, the field effective mobility of the FESBT is up to 58.7 cm2/V·s. Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature. This provides an opportunity to overcome the limitation of 60 mV/decade for conventional CMOS devices. The FESBT implemented with a high on-off ratio, a relatively high mobility and a low subthreshold promises low-voltage and low-power applications for future electronics.

Synthesis of thin-film black phosphorus on a flexible substrate

We report a scalable approach to synthesize a large-area (up to 4 mm) thin black phosphorus (BP) film on a flexible substrate. We first deposited a red phosphorus (RP) thin-film on a flexible polyester substrate, followed by its conversion to BP in a high-pressure multi-anvil cell at room temperature. Raman spectroscopy and transmission electron microscopy measurements confirmed the formation of a nano-crystalline BP thin-film with a thickness of around 40 nm. Optical characterization indicates a bandgap of around 0.28 eV in the converted BP, similar to the bandgap measured in exfoliated thin-films. Thin-film BP transistors exhibit a field-effect mobility of around 0.5 cm2/Vs, which can probably be further enhanced by the optimization of the conversion process at elevated temperatures. Our work opens the avenue for the future demonstration of large-scale, high quality thin-film black phosphorus.

A spectrally tunable all-graphene-based flexible field-effect light-emitting device

The continuous tuning of the emission spectrum of a single light-emitting diode (LED) by an external electrical bias is of great technological significance as a crucial property in high-quality displays, yet this capability has not been demonstrated in existing LEDs. Graphene, a tunable optical platform, is a promising medium to achieve this goal. Here we demonstrate a bright spectrally tunable electroluminescence from blue (∼450 nm) to red (∼750 nm) at the graphene oxide/reduced-graphene oxide interface. We explain the electroluminescence results from the recombination of Poole–Frenkel emission ionized electrons at the localized energy levels arising from semi-reduced graphene oxide, and holes from the top of the π band. Tuning of the emission wavelength is achieved by gate modulation of the participating localized energy levels. Our demonstration of current-driven tunable LEDs not only represents a method for emission wavelength tuning but also may find applications in high-quality displays.