Guoan Tai

Synthesis of Atomically Thin Boron Films on Copper Foils

Two-dimensional boron materials have recently attracted extensive theoretical interest because of their exceptional structural complexity and remarkable physical and chemical properties. However, such 2D boron monolayers have still not been synthesized. In this report, the synthesis of atomically thin 2D γ-boron films on copper foils is achieved by chemical vapor deposition using a mixture of pure boron and boron oxide powders as the boron source and hydrogen gas as the carrier gas. Strikingly, the optical band gap of the boron film was measured to be around 2.25 eV, which is close to the value (2.07 eV) determined by first-principles calculations, suggesting that the γ-B28 monolayer is a fascinating direct band gap semiconductor. Furthermore, a strong photoluminescence emission band was observed at approximately 626 nm, which is again due to the direct band gap. This study could pave the way for applications of two-dimensional boron materials in electronic and photonic devices.

Boron Nitride Nanostructures: Fabrication, Functionalization and Applications.

Boron nitride (BN) structures are featured by their excellent thermal and chemical stability and unique electronic and optical properties. However, the lack of controlled synthesis of quality samples and the electrically insulating property largely prevent realizing the full potential of BN nanostructures. A comprehensive overview of the current status of the synthesis of two-dimensional hexagonal BN sheets, three dimensional porous hexagonal BN materials and BN-involved heterostructures is provided, highlighting the advantages of different synthetic methods. In addition, structural characterization, functionalizations and prospective applications of hexagonal BN sheets are intensively discussed. One-dimensional BN nanoribbons and nanotubes are then discussed in terms of structure, fabrication and functionality. In particular, the existing routes in pursuit of tunable electronic and magnetic properties in various BN structures are surveyed, calling upon synergetic experimental and theoretical efforts to address the challenges for pioneering the applications of BN into functional devices. Finally, the progress in BN superstructures and novel B/N nanostructures is also briefly introduced.

Ni induced few-layer graphene growth at low temperature by pulsed laser deposition

We have used pulsed laser deposition to fabricate graphene on catalytic nickel thin film at reduced temperature of 650 °C. Non-destructive micro-Raman spectroscopic study on our samples, measuring 1x1 cm2 each, has revealed few-layer graphene formation. Bi-, tri-, and few-layer graphene growth has been verified by High Resolution Transmission Electron Microscopy. Our experimental results imply that the number of graphene layers formation relies on film thickness ratios of C to Ni, which can be well controlled by varying the laser ablation time. This simple and low temperature synthesizing method is excellent for graphene based nanotechnology research and device fabrication.

Enhanced gas-flow-induced voltage in graphene

We find experimentally that gas-flow-induced voltage in monolayer graphene is more than twenty times of that in bulk graphite. Examination over samples with sheet resistances ranging from 307 to 1600 Ω/sq shows that the induced voltage increases with the electric resistance and can be further improved by controlling the quality and doping level of graphene. The induced voltage is nearly independent of the substrate materials and can be well explained by the interplay of Bernoulli’s principle and the carrier density dependent Seebeck coefficient. The results demonstrate that graphene has great potential for flow sensors and energy conversion devices.

Ultrathin Nanoribbons of in Situ Carbon-Coated V3O7·H2O for High-Energy and Long-Life Li-Ion Batteries: Synthesis, Electrochemical Performance, and Charge–Discharge Behavior

The ever-growing demands of Li-ion batteries (LIBs) for high-energy and long-life applications, such as electrical vehicles, have prompted great research interest. Herein, by applying an interesting one-step high-temperature mixing method under hydrothermal conditions, ultrathin V3O7·H2O@C nanoribbons with good crystallinity and robust configuration are in situ synthesized as promising cathode materials of high-energy, high-power, and long-life LIBs. Their capacity is up to 319 mA h/g at a current density of 100 mA/g. Moreover, the capacity of 262 mA h/g can be delivered at 500 mA/g, and 94% of capacity can be retained after 100 cycles. Even at a large current density of 3000 mA/g, they can still deliver a high capacity of 165 mA h/g, and 119% of the initial capacity can be kept after 600 cycles. Importantly, their energy density is up to 800 Wh/kg, which is 48-60% higher than those of conventional cathode materials (such as LiCoO2, LiMn2O4, and LiFePO4), and they can maintain an energy density of 355 Wh/kg at a high power density of 8000 W/kg. Furthermore, based on ex situ X-ray diffraction and X-ray photoelectron spectroscopy technology, their exact charge-discharge behavior is reasonably described for the first time. Excitingly, it is found for the first time that the as-synthesized V3O7·H2O@C nanoribbons are also great promising cathode materials for Na-ion batteries.

Temperature and pH effect on reduction of graphene oxides in aqueous solution

Reduced graphene oxides (RGOs) have usually been obtained by hydrazine reduction, but hydrazine-related compounds are corrosive, highly flammable and very hazardous, and the obtained RGOs heavily aggregated. Here we investigated extensively the effect of temperature and pH value on the structure of RGOs in hydrothermal environments without any reducing agents. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra showed that reduction rate of GOs remarkably increased with the temperature from 100 to 180 °C and with pH value from 3 to 10. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) exhibited the structural transition of the RGOs. Energy-dispersive x-ray analysis (EDX) showed the reduction degree of the RGO samples quantitatively. The results demonstrate that the GOs can be reduced controllably by a hydrothermal reduction process at pH value of 10 at 140 °C, and the large-scale RGOs are cut into small nanosheets with size from several to a few tens of nanometers with increasing temperature and duration. This study provides a feasible approach to controllably reduce GO with different nanostructures such as porous structures and quantum dots for applications in optoelectronics and biomedicals.

Large-area synthesis and photoelectric properties of few-layer MoSe2 on molybdenum foils

Compared with MoS2 and WS2, the selenide analogues have narrower band gaps and higher electron mobilities, which make them more applicable to real electrical devices. Besides, few-layered metal selenides have higher electrical conductivity, carrier mobility and light absorption than the corresponding monolayers. However, the large-scale and high-quality growth of few-layered metal selenides remains a significant challenge. Here, we develop a facile method to grow large-area and highly-crystalline few-layered MoSe2 by directly selenizing the Mo foil surfaces at 550 oC within 60 min under ambient pressure. The atomic layers were controllably grown with the thickness between 3.4 and 6 nm which just met the thickness range required for high-performance electrical devices. Furthermore, we fabricated a vertical p-n junction photodetector composed of few-layered MoSe2 and p-type silicon, achieving photoresponsivity higher than two orders of magnitude than that of the reported monolayer counterpart. This technique provides a feasible approach towards preparing other 2D TMDs for device applications.Compared with MoS2 and WS2, selenide analogs have narrower band gaps and higher electron mobilities, which make them more applicable to real electrical devices. In addition, few-layer metal selenides have higher electrical conductivity, carrier mobility and light absorption than the corresponding monolayers. However, the large-scale and high-quality growth of few-layer metal selenides remains a significant challenge. Here, we develop a facile method to grow large-area and highly crystalline few-layer MoSe2 by directly selenizing the Mo foil surface at 550 °C within 60 min under ambient pressure. The atomic layers were controllably grown with thicknesses between 3.4 and 6 nm, which just met the thickness range required for high-performance electrical devices. Furthermore, we fabricated a vertical p-n junction photodetector composed of few-layer MoSe2 and p-type silicon, achieving photoresponsivity higher by two orders of magnitude than that of the reported monolayer counterpart. This technique provides a feasible approach towards preparing other 2D transition metal dichalcogendes for device applications.

Ultrathin molybdenum boride films for highly efficient catalysis of the hydrogen evolution reaction

All molybdenum borides with various phases such as Mo2B, α-MoB, β-MoB, and MoB2 have been found to possess excellent electrocatalytic hydrogen evolution reaction (HER) activity. Ultrathin two-dimensional (2D) borides are expected to have both maximized surface active sites and fast electron transport, ensuring higher HER activity. Here we report the large-area preparation of ultrathin hexagonal Mo3B films of 6.48 nm thickness on Mo foils by chemical vapour deposition using a mixture of boron and boron oxide powders as the boron source, and hydrogen gas as both the carrier and reducing gas. The ultrathin film exhibits fantastic stability in acidic solution and has a small Tafel slope of 52 mV dec−1 which is the lowest value so far reported for molybdenum boride catalysts. Furthermore, our first-principles calculations show that the ultrathin Mo3B film is metallic, which facilitates fast electron transport along the active edges of the thin film for enhancing the HER activity.

Phonon Trapping in Pearl-Necklace-Shaped Silicon Nanowires

A pearl-necklace-shaped silicon nanowire, in contrast to a smooth nanowire, presents a much lower thermal conductivity due to the phonon trapping effect. By precisely controlling the pearl size and density, this reduction can be more than 70% for the structures designed in the study, which provides a unique approach for designing high-performance nanoscale thermoelectric devices.

Electricity generated from ambient heat across a silicon surface

We report generation of electricity from the limitless thermal motion of ions across a two-dimensional (2D) silicon (Si) surface at room temperature. A typical Si device with Au-Ag electrodes could generate an open-circuit voltage of up to 0.40 V in a 5M CuCl2 solution and an output current of more than 11 μA when a 25 kΩ resistor was loaded into the circuit. A possible momentum transfer process was proposed to explain the electronic excitation, and modified thermionic emission theory was used to explain the experimental results. This finding provides a self-charging technology for energy harvesting from ambient heat.