Dawei He

Exciton-exciton annihilation in MoSe2 monolayers

We investigate the excitonic dynamics in MoSe2 monolayer and bulk samples by femtosecond transient absorption microscopy. Excitons are resonantly injected by a 750-nm and 100-fs laser pulse, and are detected by a probe pulse tuned in the range of 790 - 820 nm. We observe a strong density-dependent initial decay of the exciton population in monolayers, which can be well described by the exciton-exciton annihilation. Such a feature is not observed in the bulk under comparable conditions. We also observe the saturated absorption induced by exciton phase-space filling in both monolayers and the bulk, which indicates their potential applications as saturable absorbers.

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. Here we show that black phosphorus has room-temperature charge mobilities on the order of 10(4) cm(2) V(-1) s(-1), which are about 1 order of magnitude larger than silicon. We also demonstrate strong anisotropic transport in black phosphorus, where the mobilities along the armchair direction are about 1 order of magnitude larger than in the zigzag direction. A photocarrier lifetime as long as 100 ps is also determined. These results illustrate that black phosphorus is a promising candidate for future electronic and optoelectronic applications.

Luminescence Properties of Green-Emitting Phosphor (Ba1-x, Srx)2 SiO4:Eu2+ for White LEDS

Abstract The (Ba 1- x Sr x ) 2 SiO 4 :Eu 2+ green-emitting phosphors were synthesized by conventional solid-state reaction in a CO-reductive atmosphere, and their luminescent properties were investigated. The XRD data show that the Ba/Sr ratio not only affects the lattice parameters, but also influences the emission peak. The excitation spectra indicate that this phosphor can be effectively excited by UV light from 370 to 470 nm. The emission band is due to the 4f 6 5d 1 →4f 7 transition of the Eu 2+ ion. With an increase in x , the emission band shifts to longer wavelength and the reason was discussed. The emission spectra exhibit a satisfactory green performance under different excitation wavelength (380, 398, 412, 420, 460 nm). (Ba 1- x Sr x ) 2 SiO 4 :Eu 2+ is a promising phosphor for green white-lighting-emission diode by ultraviolet chip.

Optical identification of MoS 2 /graphene heterostructure on SiO 2 /Si substrate

Chemical vapor deposition (CVD) method is considered to be an efficient way to prepare Van der Waals heterostructure. However, accurately and hurtlessly identifying the layers number of MoS(2) in the heterostructure is still a challenge. Here, we calculated the expected contrast between MoS(2)/graphene heterostructure and underlying SiO(2)/Si substrate by using a Fresnel law based model. And we indicated that contrast at blue and green incident light is ideal for visibility and layer number detecting. Our measured value showed good agreement with calculated ones. And Raman spectrum helped to confirm our speculation.

Graphene and Nanostructured Mn3O4 Composites for Supercapacitors

Mn3O4 nanoparticles and graphene/Mn3O4 composites were synthesized by a facile method. The Mn3O4 nanoparticles were uniformly distributed on graphene nanosheets, which were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The capacitive properties of the Mn3O4, graphene and graphene/Mn3O4 composites have been investigated by galvanostatic charge/discharge, cyclic voltammetry (CV) and electrochemical impendence spectroscopy (EIS). The specific capacitance of Mn3O4 reached 190 F/g at a scan rate of 10 mV/s in 5 M KOH electrolyte., owing to pseudocapacitance. The G/Mn3O4 nanocomposites achieved a specific capacitance of 140 F/g (27.6% increases than that of graphene), a good rate property and good cycle stability, making them a promising electrode material as supercapacitors.

High performance asymmetric supercapacitors with ultrahigh energy density based on hierarchical carbon nanotubes@NiO core–shell nanosheets and defect-introduced graphene sheets with hole structure

In this work, we report a fast and cost-effective cobalt catalyzed gasification strategy to obtain defect-introduced graphene sheets (DGNs) with hole structure. Compared with the pristine graphene, the porous DGNs display much more outstanding capacitive behaviors. An electrode based on the DGNs shows an ultrahigh specific capacitance of 256 F g−1 at 1 A g−1 and 148 F g−1 even at 20 A g−1. In addition, hierarchical carbon nanotubes@NiO (CNT@NiO) core–shell hybrids were fabricated via a facile chemical bath deposition method, followed by thermal annealing. The resulting CNT@NiO electrode shows a considerable specific capacitance of 1000 F g−1 at 1 A g−1. For the first time, an advanced asymmetric supercapacitor (ASC) device was successfully fabricated consisting of a porous DGN-based negative electrode and a hierarchical CNT@NiO core–shell nanosheet-based positive electrode. The device exhibits a high specific capacitance of 108 F g−1 at 0.5 A g−1 and an excellent cycling stability, with 93.5% capacitance retention after 10 000 cycles at 6 A g−1. Due to its unique microstructures, the CNT@NiO//DGNs ASC device displays a supreme energy density of 38.1 W h kg−1 at a power density of 500 W kg−1 and even retains an energy density of 16.2 W h kg−1 at 16 000 W kg−1 (voltage window of 1.6 V). These results indicate that our ASC device is extremely valuable for energy storage applications and predict future trends toward the realization of graphene-based materials used in supercapacitors.

The fabrication of nanochain structure of gold nanoparticles and its application in ractopamine sensing

The illegal food additives including ractopamine and melamine throw a serious threat to human health. In this paper, the ractopamine and melamine were first used to form the nanochain structure of citrate-stabilized gold nanoparticles (AuNPs) with a convenient and inexpensive method. The fabricated nanochain structure consisting of several AuNPs was characterized by Scanning Electron Microscopy. A new longitudinal surface plasma resonance, which could be adjusted from visible to near infrared range, was observed in absorption spectra due to the aggregation of AuNPs. This could be well explained by Finite Different Time Domain algorithm theoretically. As confirmed by Fourier Transform Infrared Spectroscopy, the complex formed by hydrogen-bonding interaction between melamine and ractopamine could effectively promote the aggregation of AuNPs that was useful to develop the sensitivity and selectivity for the detection of ractopamine. Hence, the plasmonic coupling phenomenon of gold nanochain could be applied in bio-assay for ractopamine through the change of solutions color and optical absorption band with naked eye or absorption spectra. The linear range was broadened to (1.23 × 10(-7)M, 1.11 × 10(-6)M) and the limit of detection was extended to 4.10 × 10(-8)M (S/N=3). More importantly, this time-saving method will be promising in rapid and selective detection of β-agonist for clinical applications.

Influence of Polymer/Fullerene-Graphene Structure on Organic Polymer Solar Devices

The dependence of device performance has been measured for solar cells using blends containing the conjugated polymer, poly (3-hexylthiophene) (P3HT) with two different functionalized electron acceptor commercially available [6,6]-phenyl C-60 butyric acid methyl ester (PC60BM) and graphene derivative to increase the carrier mobility which was produced in this laboratory. Performance was characterized by short-circuit current–open circuit voltage output of the devices. The interrelationships between UV-Vis absorption, fill factor and energy conversion efficiency (ECE) have been simulated. It is observed that the ECE of the device with low concentration of solution-processable functionalized graphene (SPFGraphene) has been increased significantly compared with the device without SPFGraphene. The improved performance is attributed to the presence of semiconducting graphene which provides charge-transport pathways for carriers to be extracted and hence boosts carrier mobility in organic photovoltaic (OPVs) devices. However, when the concentration of SPFGraphenes increased up to certain degree, the performance of the device decreased. By analysis on the intensity dependent photocurrent, the decreased performance is attributed to the PC60BM-graphene which causes bimolecular recombination of free carriers. In addition, the active layer films were also investigated by XRD and SEM in order to understand the crystallite and morphology of the hybrid films.

Synthesis and Microwave Absorption Properties of Polyaniline-Graphene Nanocomposites

To develop novel microwave absorbing materials, polyaniline (PANI) composites filled with graphene was prepared, and the morphology, crystallographic structures and microwave parameters of the composites were investigated by SEM, XRD, FTIR and HP8722ES network analyzer. The uniform dispersion of graphene significantly enhanced the formation of conductive pathways in the PANI arrays and the interfacial affinity between PANI and graphene increases the number of conductive pathways of electron transfer. The composites show great microwave absorbing properties of −17dB. This study reveals that the PANI-Graphene composite is promising for applications as microwave absorbing material.

Synthesis of ZnO inverse opals with high crystalline quality by a three-dimensional colloidal crystal template-assisted hydrothermal method over a seed layer

In this paper, zinc oxide (ZnO) inverse opal structures were fabricated on a seed layer using three-dimensional (3D) colloidal crystal templates. The 3D colloidal crystal templates hindered the diffusion of zinc ions during hydrothermal growth thereby restricting the formation of porous ZnO structures at typical Zn2+ concentrations leading to rods. Instead of porous ZnO microstructures, ZnO nanobowl films were exclusively grown using the 3D colloidal templates on the photoresist-patterned seed layer at the initial Zn2+ concentrations. 3D porous ZnO with different filling fractions were successfully created by a hydrothermal method at a Zn2+ concentration of 0.15 M with different adjuvants. The addition of sodium citrate yielded ZnO having inverse opal structures and showing variable reflection peaks according to the photonic stop band, which were controlled by the diameter of the template colloidal spheres. ZnO inverse opals prepared with sodium citrate as an adjuvant also showed superior ultraviolet photoluminescence intensity values compared to their adjuvant-free and polyethylenimine counterparts. The hydrothermal method resulted in ZnO inverse opal structures with significantly enhanced crystalline quality compared to those prepared by electrodeposition methods. ZnO inverse opals prepared by large-diameter colloidal crystal templates showed slightly better properties than those prepared using small-diameter templates. The ZnO inverse opals conformally coated by a thin layer of TiO2 were used as photoanodes for water splitting. The ZnO inverse opals prepared by the hydrothermal method showed significantly higher photocurrent values than those synthesized by electrodeposition. These ZnO inverse opals showed maximum photocurrent values of ca. 0.9 mA cm−2 comparable to the best results of other ZnO-based microstructures.