Hui-Juan Cui

Hinge-like structure induced unusual properties of black phosphorus and new strategies to improve the thermoelectric performance

We systematically investigated the geometric, electronic and thermoelectric (TE) properties of bulk black phosphorus (BP) under strain. The hinge-like structure of BP brings unusual mechanical responses such as anisotropic Youngs modulus and negative Poissons ratio. A sensitive electronic structure of BP makes it transform among metal, direct and indirect semiconductors under strain. The maximal figure of merit ZT of BP is found to be 0.72 at 800 K that could be enhanced to 0.87 by exerting an appropriate strain, revealing BP could be a potential medium-high temperature TE material. Such strain-induced enhancements of TE performance are often observed to occur at the boundary of the direct-indirect band gap transition, which can be attributed to the increase of degeneracy of energy valleys at the transition point. By comparing the structure of BP with SnSe, a family of potential TE materials with hinge-like structure are suggested. This study not only exposes various novel properties of BP under strain, but also proposes effective strategies to seek for better TE materials.

Octagraphene as a versatile carbon atomic sheet for novel nanotubes, unconventional fullerenes, and hydrogen storage

We study a versatile structurally favorable periodic sp2-bonded carbon atomic planar sheet with C4v symmetry by means of the first-principles calculations. This carbon allotrope is composed of carbon octagons and squares with two bond lengths and is thus dubbed as octagraphene. It is a semimetal with the Fermi surface consisting of one hole and one electron pocket, whose low-energy physics can be well described by a tight-binding model of π-electrons. Its Youngs modulus, breaking strength, and Poissons ratio are obtained to be 306 N/m, 34.4 N/m, and 0.13, respectively, which are close to those of graphene. The novel sawtooth and armchair carbon nanotubes as well as unconventional fullerenes can also be constructed from octagraphene. It is found that the Ti-absorbed octagraphene can be allowed for hydrogen storage with capacity around 7.76 wt. %.

Tinselenidene: a Two-dimensional Auxetic Material with Ultralow Lattice Thermal Conductivity and Ultrahigh Hole Mobility

By means of extensive ab initio calculations, a new two-dimensional (2D) atomic material tin selenide monolayer (coined as tinselenidene) is predicted to be a semiconductor with an indirect gap (~1.45 eV) and a high hole mobility (of order 10000 cm2V−1S−1), and will bear an indirect-direct gap transition under a rather low strain (<0.5 GPa). Tinselenidene has a very small Young’s modulus (20–40 GPa) and an ultralow lattice thermal conductivity (<3 Wm−1K−1 at 300 K), making it probably the most flexible and most heat-insulating material in known 2D atomic materials. In addition, tinseleniden has a large negative Poisson’s ratio of −0.17, thus could act as a 2D auxetic material. With these intriguing properties, tinselenidene could have wide potential applications in thermoelectrics, nanomechanics and optoelectronics.

Highly efficient light management for perovskite solar cells

Organic-inorganic halide perovskite solar cells have enormous potential to impact the existing photovoltaic industry. As realizing a higher conversion efficiency of the solar cell is still the most crucial task, a great number of schemes were proposed to minimize the carrier loss by optimizing the electrical properties of the perovskite solar cells. Here, we focus on another significant aspect that is to minimize the light loss by optimizing the light management to gain a high efficiency for perovskite solar cells. In our scheme, the slotted and inverted prism structured SiO2 layers are adopted to trap more light into the solar cells, and a better transparent conducting oxide layer is employed to reduce the parasitic absorption. For such an implementation, the efficiency and the serviceable angle of the perovskite solar cell can be promoted impressively. This proposal would shed new light on developing the high-performance perovskite solar cells.

Strain-induced Dirac cone-like electronic structures and semiconductor–semimetal transition in graphdiyne

By means of first-principles calculations combined with the tight-binding approximation, the strain-induced semiconductor-semimetal transition in graphdiyne is discovered. It is shown that the band gap of graphdiyne increases from 0.47 eV to 1.39 eV with increasing the biaxial tensile strain, while the band gap decreases from 0.47 eV to nearly zero with increasing the uniaxial tensile strain, and Dirac cone-like electronic structures are observed. The uniaxial strain-induced changes of the electronic structures of graphdiyne come from the breaking of geometrical symmetry that lifts the degeneracy of energy bands. The properties of graphdiyne under strains are found to differ remarkably from that of graphene.

Modulation of the two-photon absorption by electric fields in HgCdTe photodiode

We demonstrate the tunability of the two-photon absorption (TPA) coefficient by adjusting the electric field in a HgCdTe (MCT) photodiode with cutoff wavelength of 5.2μm. The TPA coefficient was measured by using a picosecond pulsed laser with wavelength of 7.92μm. An enhancement of the TPA coefficient occurs in the space charge region of the MCT pn junction, which can be attributed to the Franz–Keldysh effect induced by the built-in electric field. By applying a reverse bias to intensify the built-in field, the TPA coefficient is found to be further enhanced by a factor of 18.9. This electric field dependence of the TPA coefficient has been fairly interpreted by the pn junction model with the Franz–Keldysh effect included.

First-principles study on electronic and magnetic properties of twisted graphene nanoribbon and Möbius strips

The geometrical, electronic, and magnetic properties of tw isted zigzag-edged graphene nanoribbons (ZGNRs) and novel graphene Möbius strips (GMS ) are systematically investigated using first-principles density functional calcula tions. The structures of ZGNRs and GMS are optimized, and their stabilities are examined. The m olecular energy levels and the spin polarized density of states are calculated. It is fo und that for twisted ZGNRs, the atomic bonding energy decreases quadratically with the inc rease of the twisted angle, and the HOMO-LUMO gap are varying in a sine-like behavior with th e twisted angle. The calculated spin densities reveal that the ZGNRs and GMS have antiferromagnetic ground Preprint submitted to Elsevier 11 May 2014 states, which persist during the twisting. The spin flips on t he zigzag edges of GMS are observed at some positions. PACS:31.15.A-, 31.15.ap, 31.15.ej, 75.25.-j, 75.10.Pq

Effects of rapid thermal annealing on the properties of GaNxAs1−x

We report the effect of rapid thermal annealing on the valence-band splitting behavior of GaNxAs1−x films grown by molecular beam epitaxy. The light- and heavy-hole valence-band splitting induced by the elastic strain is observed in both photomodulated reflectance and photoluminescence spectra for as-grown GaNxAs1−x epilayers. The valence-band splitting energy increases with the nitrogen composition. This splitting is decreased with the increase of annealing temperature by the rapid thermal annealing temperature process. These properties have been well explained by strain relaxation model including both in-plane strain at GaAs/GaNAs interface and internal strain in GaNAs epilayer. These strain effects have been confirmed by x-ray diffraction and Raman measurements.

Measurement of Minority Carrier Lifetime in Hg1-xCdxTe Photodetector

The transient photo-generated excess minority carrier lifetime in Hg1-xCdxTe n+p junction photodetector are measured using improved photo-induced open-circuit voltage decay (OCVD) technique. The lifetimes we extracted from the decay curve of the photovoltage are from tens to hundreds ns. A comparison of the relationship between composition of Cd and minority carrier lifetime show the lifetimes become longer with the increasing of Cd composition.

Rapid Thermal Annealing Effect On Valence-band Splitting Behavior in GaNxAs1-x/GaAs

Our investigation is about rapid thermal annealing (RTA) effect on valence-band splitting (VBS) of GaN_xAs_1-x. The observed VBS between light-and heavy-hole induced by lattice strain in GaN_xAs_1-x epilayers is gradually weakened by the RTA process. This could be attributed to strain relax caused by nitrogen reorganization after RTA.