Shengli Chang

Adsorption of gas molecules on monolayer MoS2 and effect of applied electric field

AbstractUsing first-principles calculations, we investigate the adsorption of various gas molecules (H2, O2, H2O, NH3, NO, NO2, and CO) on monolayer MoS2. The most stable adsorption configuration, adsorption energy, and charge transfer are obtained. It is shown that all the molecules are weakly adsorbed on the monolayer MoS2 surface and act as charge acceptors for the monolayer, except NH3 which is found to be a charge donor. Furthermore, we show that charge transfer between the adsorbed molecule and MoS2 can be significantly modulated by a perpendicular electric field. Our theoretical results are consistent with the recent experiments and suggest MoS2 as a potential material for gas sensing application.

Growth of Millimeter-Size Single Crystal Graphene on Cu Foils by Circumfluence Chemical Vapor Deposition

A simply and reproducible way is proposed to significantly suppress the nucleation density of graphene on the copper foil during the chemical vapor deposition process. By inserting a copper foil into a tube with one close end, the nucleation density on the copper foils can be reduced by more than five orders of magnitude and an ultra-low nucleation density of ~10 nucleus/cm2 has been achieved. The structural analyses demonstrate that single crystal monolayer graphene with a lateral size of 1.9 mm can be grown on the copper foils under the optimized growth condition. The electrical transport studies show that the mobility of such single crystal graphene is around 2400 cm2/Vs.

Bandgap tuning in armchair MoS2 nanoribbon.

We report on the first-principles calculations of bandgap modulation in armchair MoS(2) nanoribbon (AMoS(2)NR) by transverse and perpendicular electric fields respectively. In the monolayer AMoS(2)NR case, it is shown that the bandgap can be significantly reduced and be closed by transverse field, whereas the bandgap modulation is absent under perpendicular field. The critical strength of transverse field for gap closure decreases as ribbon width increases. In the multilayer AMoS(2)NR case, in contrast, it is shown that the bandgap can be effectively reduced by both transverse and perpendicular fields. Nevertheless, it seems that the two fields exhibit different modulation effects on the gap. The critical strength of perpendicular field for gap closure decreases with increasing number of layers, while the critical strength of transverse field is almost independent of it.

Magnetic and electronic properties of α-graphyne nanoribbons.

Based on the first-principles calculations, we investigate the magnetic and electronic properties of α-graphyne nanoribbons (NRs). We show that all the armchair α-graphyne NRs are nonmagnetic semiconductors with band gaps as a function of ribbon widths. The zigzag α-graphyne NRs are found to have magnetic semiconducting ground state with ferromagnetic ordering at each edge and opposite spin orientation between the two edges. Under the application of transverse electric field, we further predict the existence of half-metallicity in the zigzag NRs which strongly depends on the width of the ribbon.

Non-line-of-sight multiscatter propagation model

A propagation model that describes the characteristics of multiscatter radiation in atmosphere is presented. The model is based on the Monte Carlo method; each scattering process is set as an event of probability. LOWTRAN7 is used to calculate the atmospheric coefficients, and Mie theory is used to calculate the scattering characteristics of the particles. It is shown that the multiscatter model matches the single-scatter model perfectly when the scattering count is 1, and the formula for the single-scatter approximation is modified for the non-line-of-sight (NLOS) problem. It is also shown that the duration of the impulse response is about 8 micros, the proportion of single-scatter irradiance is very small, and the average scattering count is 3.85 instead of 1 when the range is close to 1 km (weather conditions, field of view, and elevation angle are given). All these characteristics are presented for what is, to our knowledge, the first time. This model is wavelength-independent; 0.254 microm is chosen as the wavelength of simulation.

Influence of carrier concentration on piezoelectric potential in a bent ZnO nanorod

The influence of carriers on the piezoelectric potential in a bent ZnO nanorod is investigated using finite difference method. The distributions of carriers and the electrical potential in the nanorod are obtained. The results shows that the positive piezoelectric potential in stretched side of the bent nanorod is significantly screened by the carriers and the negative potential in compressed side is well preserved when considering a moderate carrier concentration of 1×1017 cm−3. The calculation results agree with the experimental results that only negative pulses are observed in the nanogenerator experiment using the as-grown ZnO nanorods. Further investigation shows that when the carrier concentration is 1×1018 cm−3, the piezoelectric potential in the nanorod is almost completely screened by the redistributed carriers.

The nonlinear optical properties of coupling and decoupling graphene layers

Third-order optical nonlinearities of graphene from monolayer to multilayers were investigated in the femtosecond regime, and the contribution of interlayer coupling to the nonlinearities was studied. The nonlinear refractive index γ of the order of 10−9 cm2/W and the nonlinear absorption coefficient β of 10−6 cm/W were obtained. By systematically investigating the nonlinear optical properties with the number of layers and comparing the coupling graphene with the decoupling superimposed graphene, we found that the coupling of interlayers has large effect upon the nonlinear refraction. These results provide an effective approach for developing graphene-based nonlinear photonic devices.

Study of effects of obstacle on non-line-of-sight ultraviolet communication links

This paper studies the effects of the obstacle on non-line-of-sight ultraviolet communication links using multiple-scatter model based on a Monte Carlo method. On the condition that transmitter beam and receiver FOV just pass the top of the obstacle, and ranges is fixed, the received energy density is at its maximum. The path loss increases when the transmitter or the receiver is much near to the obstacle, because the nearby common scattering volumes decrease intensively. The optimal received range decreases with the increasing of the distance between transmitter and obstacle. The predictions are validated with experimental measurements. This work can be used for the guidance of UV system design and network technology to apply in complex surroundings, such as mountain, buildings, etc.

Vectorized polarization-sensitive model of non-line-of-sight multiple-scatter propagation.

The existing Monte-Carlo-based non-line-of-sight (NLOS) multiple-scatter propagation model is extended to include polarization and also vectorized to improve the simulation speed by about 500 times. This model is validated by the noncoplanar single-scatter model; the results show a perfect match. Numerical examples for various polarization setups are obtained, and results show that the single-scatter and multiple-scatter signals are all polarization dependent. Therefore, NLOS polarized UV communication with a high data rate is achievable--the polarizing information is coded by a time-dependent polarizer, influenced by the atmospheric channel, and decoded according to the distribution characteristics of the scattered signals after the time-independent analyzers.

Detection of TNT in acetone using Raman spectroscopic signature

The detection of explosive agents is becoming more important and receiving much greater emphasis for homeland defense. Raman spectroscopy is a well established tool for vibration spectroscopic analysis and can be applied to the field of explosives identification and detection. The major bands of the Raman spectroscopy of industrial TNT (Trinitrotoluene, CH3C6H2(NO2)3) are analyzed and seven prominent peaks, that is 1616.9cm-1 (C=C aromatic stretching vibration), 1533.9cm-1 (NO2 asymmetric stretching vibration), 1360.1cm-1 (NO2 symmetric stretching vibration ), 1210.5cm-1 (C6H2-C vibration), 822.9cm-1 (nitro-group scissoring mode), 792.3cm-1 (C-H out-of-plane bend), and 326.7cm-1 (framework distortion mode) are used to identify the TNT. The Raman spectroscopes of TNT solved in acetone at different mass ratios are studied, and the TNT in the solution can be detected correctly according the relative distance, intensity, and peak area of the seven peaks. The TNT prominent peaks appear clearly in high level solution (the mass ration of TNT and acetone is more than 1:10). With the decrease of TNT concentration in solution, the signature of TNT becomes more and more weak. The low detection limit of TNT is limited by the noise of the instrument (NXR FT-Raman accessory module with Nicolet 5700 FT-IR spectrometer is used for our experiments. The low detection limit in our experiments is mass ratio 1:200, which is about 4mg/mL). The prominent peak heights are discussed in consideration of the TNT concentration. Taking one of the acetones peaks (1716.9cm-1) as the internal standard line, the relative height of the prominent TNT peaks is almost proportional to the concentration of the TNT in the solution. A fitting curve for the relations of prominent peak height according to the concentration is proposed with multinomial fitting method, which can be used to analyze the concentration of TNT more accurately.