PingAn Hu

Synthesis of Few-Layer GaSe Nanosheets for High Performance Photodetectors

Two-dimensional (2D) semiconductor nanomaterials hold great promises for future electronics and optics. In this paper, a 2D nanosheets of ultrathin GaSe has been prepared by using mechanical cleavage and solvent exfoliation method. Single- and few-layer GaSe nanosheets are exfoliated on an SiO(2)/Si substrate and characterized by atomic force microscopy and Raman spectroscopy. Ultrathin GaSe-based photodetector shows a fast response of 0.02 s, high responsivity of 2.8 AW(-1) and high external quantum efficiency of 1367% at 254 nm, indicating that the two-dimensional nanostructure of GaSe is a new promising material for high performance photodetectors.

Highly Responsive Ultrathin GaS Nanosheet Photodetectors on Rigid and Flexible Substrates

The first GaS nanosheet-based photodetectors are demonstrated on both mechanically rigid and flexible substrates. Highly crystalline, exfoliated GaS nanosheets are promising for optoelectronics due to strong absorption in the UV-visible wavelength region. Photocurrent measurements of GaS nanosheet photodetectors made on SiO2/Si substrates and flexible polyethylene terephthalate (PET) substrates exhibit a photoresponsivity at 254 nm up to 4.2 AW(-1) and 19.2 AW(-1), respectively, which exceeds that of graphene, MoS2, or other 2D material-based devices. Additionally, the linear dynamic range of the devices on SiO2/Si and PET substrates are 97.7 dB and 78.73 dB, respectively. Both surpass that of currently exploited InGaAs photodetectors (66 dB). Theoretical modeling of the electronic structures indicates that the reduction of the effective mass at the valence band maximum (VBM) with decreasing sheet thickness enhances the carrier mobility of the GaS nanosheets, contributing to the high photocurrents. Double-peak VBMs are theoretically predicted for ultrathin GaS nanosheets (thickness less than five monolayers), which is found to promote photon absorption. These theoretical and experimental results show that GaS nanosheets are promising materials for high-performance photodetectors on both conventional silicon and flexible substrates.

Back gated multilayer InSe transistors with enhanced carrier mobilities via the suppression of carrier scattering from a dielectric interface.

The back gate multilayer InSe FETs exhibit ultrahigh carrier mobilities, surpassing all the reported layer semiconductor based electronics with the same device configuration, which is achieved by the suppression of the carrier scattering from interfacial coulomb impurities or surface polar phonons at the interface of an oxidized dielectric substrate. The room-temperature mobilities of multilayer InSe transistors increase from 64 cm(2)V(-1)s(-1) to 1055 cm(2)V(-1)s(-1) using a bilayer dielectric of poly-(methyl methacrylate) (PMMA)/Al2O3. The transistors also have high current on/off ratios of 1 × 10(8), low standby power dissipation, and robust current saturation in a broad voltage range.

Monolayer hexagonal boron nitride films with large domain size and clean interface for enhancing the mobility of graphene-based field-effect transistors.

Viable and general techniques that allow effective size control of triangular-shaped, single-crystal, monolayer h-BN domains grown by the CVD method, direct optical visualization of h-BN domains, and the cleaning of the h-BN surface to achieve reliable graphene device quality are reported for the first time. This study points to a critical role of the interfacial properties between the graphene and the monolayer h-BN in determining reliable, enhanced graphene-device performance.

Carbon Nanostructure-Based Field-Effect Transistors for Label-Free Chemical/Biological Sensors

Over the past decade, electrical detection of chemical and biological species using novel nanostructure-based devices has attracted significant attention for chemical, genomics, biomedical diagnostics, and drug discovery applications. The use of nanostructured devices in chemical/biological sensors in place of conventional sensing technologies has advantages of high sensitivity, low decreased energy consumption and potentially highly miniaturized integration. Owing to their particular structure, excellent electrical properties and high chemical stability, carbon nanotube and graphene based electrical devices have been widely developed for high performance label-free chemical/biological sensors. Here, we review the latest developments of carbon nanostructure-based transistor sensors in ultrasensitive detection of chemical/biological entities, such as poisonous gases, nucleic acids, proteins and cells.

Carrier control of MoS2 nanoflakes by functional self-assembled monolayers.

Carrier doping of MoS2 nanoflakes was achieved by functional self-assembled monolayers (SAMs) with different dipole moments. The effect of SAMs on the charge transfer between the substrates and MoS2 nanoflakes was studied by Raman spectroscopy, field-effect transistor (FET) measurements, and Kelvin probe microscope (KFM). Raman data and FET results verified that fluoroalkyltrichlorosilane-SAM with a large positive dipole moment, acting as hole donors, significantly reduced the intrinsic n-doping characteristic of MoS2 nanoflakes, while 3-(trimethoxysilyl)-1-propanamine-SAMs, acting as electron donors, enhanced the n-doping characteristic. The additional built-in electric field at the interface between SiO2 substrates and MoS2 nanoflakes induced by SAMs with molecular dipole moments determined the charge transfer process. KFM results clearly demonstrated the charge transfer between MoS2 and SAMs and the obvious interlayer screening effect of the pristine and SAM-modified MoS2 nanoflakes. However, the KFM results were not fully consistent with the Raman and FET results since the externally absorbed water molecules were shown to partially shield the actual surface potential measurement. By eliminating the contribution of the water molecules, the Fermi level of monolayer MoS2 could be estimated to modulate in a range of more than 0.45-0.47 eV. This work manifests that the work function of MoS2 nanoflakes can be significantly tuned by SAMs by virtue of affecting the electrostatic potential between the substrates and MoS2 nanoflakes.

Highly sensitive phototransistors based on two-dimensional GaTe nanosheets with direct bandgap

Highly sensitive phototransistors based on two-dimensional (2D) GaTe nanosheet have been demonstrated. The performance (photoresponsivity, detectivity) of the GaTe nanosheet phototransistor can be efficiently adjusted by using the applied gate voltage. The devices exhibit an ultrahigh photoresponsivity of 274.3 AW−1. The detectivity of 2D GaTe devices is ∼1012 Jones, which surpasses that of currently-exploited InGaAs photodetectors (1011−1012 Jones). To reveal the origin of the enhanced photocurrent in GaTe nanosheets, theoretical modeling of the electronic structures was performed to show that GaTe nanosheets also have a direct bandgap structure, which contributes to the promotion of photon absorption and generation of excitons. This work shows that GaTe nanosheets are promising materials for high performance photodetectors.

Synthesis and characterization of a new hierarchical reinforcement by chemically grafting graphene oxide onto carbon fibers

We proposed a new hierarchical reinforcement consisting of graphene oxide (GO) and carbon fibers (CF). It was confirmed that GO was chemically grafted onto CF via poly(amido amine) dendrimers. The GO grafting significantly changes the surface configuration of CF. The new hierarchical reinforcement has the potential to be applied in high performance polymer matrix composites.

Synthesis of two-dimensional β-Ga2O3 nanosheets for high-performance solar blind photodetectors

Two-dimensional (2D) semiconductors are limited to graphene analogues of layered materials, so it is extremely challenging to fabricate 2D non-layered materials with thicknesses of only a few atomic layers. Here, we report the successful fabrication of 2D Ga2O3 from the corresponding GaSe nanosheets and a solar blind photodetector based on 2D Ga2O3. The as-prepared 2D β-Ga2O3 is polycrystalline and has a thickness of less than 10 nm. Furthermore, we demonstrate a photodetector based on 2D β-Ga2O3, which show a sensitive, fast and stable photoresponse to ultraviolet radiation (254 nm). The responsivity, detectivity and external quantum efficiency of the photodetector are 3.3 A W−1, 4.0 × 1012 Jones and 1600%, respectively, indicating that the 2D Ga2O3 has great potential for application for solar-blind photodetectors.

Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors

A critical challenge for the integration of optoelectronics is that photodetectors have relatively poor sensitivities at the nanometer scale. Generally, a large electrodes spacing in photodetectors is required to absorb sufficient light to maintain high photoresponsivity and reduce the dark current. However, this will limit the optoelectronic integration density. Through spatially resolved photocurrent investigation, we find that the photocurrent in metal-semiconductor-metal (MSM) photodetectors based on layered GaSe is mainly generated from the region close to the metal-GaSe interface with higher electrical potential. The photoresponsivity monotonically increases with shrinking the spacing distance before the direct tunneling happens, which was significantly enhanced up to 5,000 AW−1 for the bottom Ti/Au contacted device. It is more than 1,700-fold improvement over the previously reported results. The response time of the Ti/Au contacted devices is about 10–20 ms and reduced down to 270 μs for the devices with single layer graphene as metallic electrodes. A theoretical model has been developed to well explain the photoresponsivity for these two types of device configurations. Our findings realize reducing the size and improving the performance of 2D semiconductor based MSM photodetectors simultaneously, which could pave the way for future high density integration of optoelectronics with high performances.