XiaoYi Liu

Opening the band gap of graphene through silicon doping for the improved performance of graphene/GaAs heterojunction solar cells

Graphene has attracted increasing interest due to its remarkable properties. However, the zero band gap of monolayered graphene limits its further electronic and optoelectronic applications. Herein, we have synthesized monolayered silicon-doped graphene (SiG) with large surface area using a chemical vapor deposition method. Raman and X-ray photoelectron spectroscopy measurements demonstrate that the silicon atoms are doped into graphene lattice at a doping level of 2.7-4.5 at%. Electrical measurements based on a field effect transistor indicate that the band gap of graphene has been opened via silicon doping without a clear degradation in carrier mobility, and the work function of SiG, deduced from ultraviolet photoelectron spectroscopy, was 0.13-0.25 eV larger than that of graphene. Moreover, when compared with the graphene/GaAs heterostructure, SiG/GaAs exhibits an enhanced performance. The performance of 3.4% silicon doped SiG/GaAs solar cell has been improved by 33.7% on average, which was attributed to the increased barrier height and improved interface quality. Our results suggest that silicon doping can effectively engineer the band gap of monolayered graphene and SiG has great potential in optoelectronic device applications.

Strengthening metal nanolaminates under shock compression through dual effect of strong and weak graphene interface

We use molecular dynamics method to study the strengthening effect of graphene-metal nanolayered composites under shock loading. The graphene interfaces have the advantages of both strong and weak interfacial features simultaneously, which solves a strengthening paradox of interfacial structures. On one hand, the weak bending stiffness of graphene leads to interlayer reflections and weakening the shock wave. On the other hand, the strong in-plane sp2-bonded structures constrain the dislocations and heal the material. The elastic recovery due to graphene interfacial constraints plays an important role in the strengthening effect, and the shock strength can be enhanced by decreasing the interlayer distance. This interface with strong/weak duality should lead to an improved fundamental understanding on the dynamic mechanism of composites with interfacial structures.

Anisotropic growth of buckling-driven wrinkles in graphene monolayer

We theoretically and numerically investigate the growth of buckling-driven wrinkles in graphene monolayers. It is found that the growth of buckling-driven wrinkles in a graphene monolayer is remarkably chirality- and size-dependent. In small sizes, the flexural response of a graphene sheet cannot be accurately described by the classical Euler regime, and the non-continuum effect leads to zigzag-along-preferred buckling. With the increase of size, the width/length ratio α of the compressed region plays an important role in the growth of buckling-driven wrinkles. When α < 0.5, the oblique buckling happens in armchair-along compression; when 0.5 < α < 1.0, the effect of edge warp leads to zigzag-along-preferred buckling. When 1.0 < α < 3.0, the potential energy density difference due to chiral bending stiffness leads to armchair-along-preferred buckling. When α > 3.0, the non-continuum effect and chiral bending stiffness can both be neglected, and the buckling in a graphene monolayer is isotropic. The chirality-along-preferred transition of compressed buckling in a graphene monolayer leads to an improved fundamental understanding of the dynamics mechanism of graphene-based nanodevices, especially for the nanodevices with high frequency response.

Anisotropic propagation and upper frequency limitation of terahertz waves in graphene

Transverse wave propagation in single-layer graphene sheet (SLGS) is studied via molecular dynamics simulation, continuum, and non-continuum analysis. We found that the propagation of transverse waves with frequency over 3 THz is remarkably chirality-dependent. Furthermore, the wave propagation in zigzag direction remains undistorted only when the frequency is below 16 THz, while this threshold is 10 THz in the armchair direction. The minimum permissible wavelength is proposed to explain the frequency limitation due to non-continuity. Our findings lead to an improved fundamental understanding on the vibration of graphene-based nanodevices and have potential applications in design and fabrication of nanoelectromechanical systems.

Anomalous twisting strength of tilt grain boundaries in armchair graphene nanoribbons

The twisting response of armchair graphene nanoribbons with tilt grain boundaries is theoretically and numerically investigated. It is found that the critical instability twist rate of graphene nanoribbons with grain boundaries is generally about 10% higher than that of common armchair graphene nanoribbons when the width of nanoribbons is less than 4.0 nm. Our analytical analysis indicates that the strengthening effect is resulted from the rotation of the compressed direction, deflection of grain boundaries, and the reflexing of the creased angle in nanoribbons: the rotation of the compressed direction induced by grain boundaries improves the buckling strength of nanoribbons due to the chirality-dependent buckling in graphene; the deflection of grain boundaries leads to a nonzero strain in the axle wire of nanoribbons, which eventually decreases the compressed stress; grain boundaries induce a spontaneous creased angle in nanoribbons, which is reflexed under twist loading and impedes the propagation of instability in nanoribbons. Furthermore, we found and demonstrated that grain boundaries changed the transport properties of twisted graphene nanoribbons. It is expected that our findings would improve the fundamental understanding of the strain-engineering of graphene nanoribbons used in nanodevices.

Transformation between divacancy defects induced by an energy pulse in graphene.

The mutual transformations among the four typical divacancy defects induced by a high-energy pulse were studied via molecular dynamics simulation. Our study revealed all six possible mutual transformations and found that defects transformed by absorbing energy to overcome the energy barrier with bonding, debonding, and bond rotations. The reversibility of defect transformations was also investigated by potential energy analysis. The energy difference was found to greatly influence the transformation reversibility. The direct transformation path was irreversible if the energy difference was too large. We also studied the correlation between the transformation probability and the input energy. It was found that the transformation probability had a local maxima at an optimal input energy. The introduction of defects and their structural evolutions are important for tailoring the exceptional properties and thereby performances of graphene-based devices, such as nanoporous membranes for the filtration and desalination of water.

Photovoltaic effect in YBa2Cu3O7−δ/Nb-doped SrTiO3 heterojunctions

The photovoltaic properties of YBa2Cu3O7−δ/Nb-doped SrTiO3 (SNTO) heterostructures were investigated systematically under laser irradiation of different wavelengths from 365 nm to 640 nm. A clear photovoltaic effect was observed, and the photovoltage Voc ranged from 0.1 V to 0.9 V depending on the wavelength. The Voc appeared under laser illumination with a photon energy of 2.4 eV, far below the band gap (3.2 eV) of Nb-doped SrTiO3. The temperature dependencies of the Voc and short-current density showed kinks near the structural phase transition of the Nb-doped SrTiO3. Our findings are helpful for understanding the photovoltaic effect in transition-metal oxide based heterojunctions and designing such photovoltaic devices.

Anisotropic rectifying characteristics induced by the superconducting gap of YBa2Cu3O7−δ/Nb-doped SrTiO3 heterojunctions

In this paper, we investigated the anisotropic rectifying characteristics of a YBa2Cu3O7−δ (YBCO)/Nb-doped SrTiO3 heterojunction in magnetic fields of up to 9 T by rotating the junction from H//c to H//ab of the YBCO film. From the temperature and field dependencies of the diffusion potential Vd, we found that the angle-resolved reductions of Vd from its original value, δVd, were induced by the anisotropic superconducting gap Δ of the YBCO. The anisotropic parameter obtained from Δ was close to that obtained from the angular-dependent upper critical fields of the YBCO. This heterojunction is helpful both in investigating the superconducting gap and in designing sensitive superconducting devices.