Ru-Zhi Wang

R-graphyne: a new two-dimensional carbon allotrope with versatile Dirac-like point in nanoribbons

A novel two-dimensional carbon allotrope, rectangular graphyne (R-graphyne) with tetra-rings and acetylenic linkages, is proposed by first-principles calculations. Although the bulk R-graphyne exhibits metallic property, the nanoribbons of R-graphyne show distinct electronic structures from the bulk. The most intriguing feature is that band gaps of R-graphene nanoribbons oscillate between semiconductor and metal as a function of width. Particularly, the zigzag edge nanoribbons with half-integer repeating unit cell exhibits unexpected Dirac-like fermions in the band structures. The Dirac-like fermions of the R-graphyne nanoribbons originate from the central symmetry and two sub-lattices. The extraordinary properties of R-graphene nanoribbons greatly expand our understanding on the origin of Dirac-like point. Such findings uncover a novel fascinating property of nanoribbons, which may have broad potential applications for carbon-based nano-size electronic devices.

A photoelectrochemical investigation on the synergetic effect between CdS and reduced graphene oxide for solar-energy conversion.

CdS modified with reduced graphene oxide (RGO) has been widely demonstrated to be effective in the field of solar-energy conversion. However, the inherent mechanism of this superior property is still not thoroughly understood. Thus the photoelectrochemical method was employed to systemically investigate the synergetic effect between CdS and RGO. The result shows that the photoelectrochemical properties of RGO/CdS samples are sensitive to the relative ratio of RGO to CdS, and the photoelectrode with 1.0 wt% ratio of RGO possesses the best photoelectrochemical performance. Further investigation demonstrates that the synergetic effect between CdS and RGO directly influences the charge-transport property and band-structure of the composite, which is also supported by the X-ray photoelectron spectroscopy data and first-principle simulation, respectively.

Energy gaps in nitrogen delta-doping graphene: A first-principles study

First-principles calculations are performed to study the modulation of energy gaps in nitrogen delta-doping (N δ-doping) graphene and armchair-edge graphene nanoribbons (AGNRs). The energy gap of graphene only opens at a large nitrogen doping content. For AGNRs, the energy gaps tend to decrease with the N δ-doping, and an interesting transition from direct to indirect bandgap is observed. Moreover, the effects of N δ-doping on energy gaps incline to decease with the reduction of the doping content. Our results may help to design novel graphene-based nanoelectronics devices by controlling N δ-doping of graphene.

Thermal expansions in wurtzite AlN, GaN, and InN: First-principle phonon calculations

Using the first-principle phonon calculations under the quasiharmonic approximation, thermal expansions in III-nitrides with wurtzite AlN, GaN, and InN are reported. The results showed that it is different for each thermal expansion of three III-nitrides at low temperatures, which is consistent with their Gruneisen parameters as the function of temperature. Below 50 K, negative thermal expansions occur in InN, while GaN and AlN follow the rule of positive thermal expansion. To seek the origin of positive/negative thermal expansion distinction, the mode Gruneisen parameters and the phonon spectra are investigated. They indicate that different low-frequency phonon vibration modes correspond to the change of thermal expansions. Below 5 THz, the significant weighted negative values of mode Gruneisen parameters, caused by the weakening of mixing-mode constituted with two transverse acoustic (TA) modes and a small overlapped part of optical modes, directly lead to the negative thermal expansion at low temperatures.

Wurtzite-type CuInSe2 for high-performance solar cell absorber: ab initio exploration of the new phase structure

CuInSe2 (CIS) has been widely studied because of its potential applications in photovoltaics, and the phase structure is believed to significantly affect its electronic and optical properties. A new wurtzite-type phase of CIS is predicted by density functional theory calculations combined with evolutionary methodology. In contrast to the common chalcopyrite CIS, Cu atoms of the predicted phase form new bonds with Se atoms derived from the interaction with the second nearest neighbor due to symmetry, and such new bonding results in beneficial band structure for both electron transition and transport because of the delocalized Cu-d electrons. The calculated absorption spectrum of the new phase further reveals an improvement in light absorption index over that of the chalcopyrite phase under near-infrared and visible light. Thus the wurtzite-type CIS has advantageous electronic and optical properties and is a highly efficient active layer material for high-performance solar cells.

Atomic structure and electronic properties of folded graphene nanoribbons: A first-principles study

Folded graphene nanoribbons (FGNRs) have attracted great attentions because of extraordinary properties and potential applications. The atomic structure, stacking sequences, and electronic structure of FGNRs are investigated by first-principle calculations. It reveals that the common configurations of all FGNRs are racket-like structures including a nanotube-like edge and two flat nanoribbons. Interestingly, the two flat nanoribbons form new stacking styles instead of the most stable AB-stacking sequences for flat zone. The final configurations of FGNRs are greatly affected by the initial interlayer distance, stacking sequences, and edge styles. The stability of folded graphene nanoribbon depends on the length, and it can only be thermodynamically stable when it reaches the critical length. The band gap of the folded zigzag graphene nanoribbons becomes about 0.17 eV, which provides a new way to open the band gap.

Bandgap engineering of GaN nanowires

Bandgap engineering has been a powerful technique for manipulating the electronic and optical properties of semiconductors. In this work, a systematic investigation of the electronic properties of [0001] GaN nanowires was carried out using the density functional based tight-binding method (DFTB). We studied the effects of geometric structure and uniaxial strain on the electronic properties of GaN nanowires with diameters ranging from 0.8 to 10 nm. Our results show that the band gap of GaN nanowires depends linearly on both the surface to volume ratio (S/V) and tensile strain. The band gap of GaN nanowires increases linearly with S/V, while it decreases linearly with increasing tensile strain. These linear relationships provide an effect way in designing GaN nanowires for their applications in novel nano-devices.

Broadband sensitization of downconversion phosphor YPO4 by optimizing TiO2 substitution in host lattice co-doped with Pr3+-Yb3+ ion-couple

This study demonstrates a feasible and efficient route to alleviate the absorption problem of the terrestrial solar spectrum and enhance broadband luminescence from a promising down conversion powder phosphor YPO4 co-doped with Pr3+-Yb3+ lanthanide ion-couple: incorporating a third sensitizing transition metal ion, e.g., Ti4+. The x-ray powder diffraction results confirm the lattice substitution by the solid-state reaction doping rather than the formation of any secondary phase. The emission spectral results and the luminescence decay curve analysis show that the downconversion luminescence can be enhanced by 200%–300% and the quantum efficiency enhanced by more than 20% at the wavelength of around 980 nm, the best response spectrum for Si-based solar cells, by optimizing TiO2 doping concentration at 7 mol. %.

Designing electronic anisotropy of three-dimensional carbon allotropes for the all-carbon device

Extending two-dimensional (2D) graphene nanosheets to a three-dimensional (3D) network can enhance the design of all-carbon electronic devices. Based on the great diversity of carbon atomic bonding, we have constructed four superlattice-type carbon allotrope candidates, containing sp2-bonding transport channels and sp3-bonding insulating layers, using density functional theory. It was demonstrated through systematic simulations that the ultra-thin insulating layer with only three-atom thickness can switch off the tunneling transport and isolate the electronic connection between the adjacent graphene strips, and these alternating perpendicular strips also extend the electron road from 2D to 3D. Designing electronic anisotropy originates from the mutually perpendicular π bonds and the rare partial charge density of the corresponding carriers in insulating layers. Our results indicate the possibility of producing custom-designed 3D all-carbon devices with building blocks of graphene and diamond.

Si doping at GaN inversion domain boundaries: an interfacial polar field for electron and hole separation

Using first-principles calculations, we investigated the phenomenon of Si doping at the GaN inversion domain boundaries (IDB) perpendicular to the wurtzite [0001] axis. The results reveal that the half monolayer Si doped GaN IDB is more stable than the abrupt monolayer Si doped IDB. This finding is vital to understanding the unique growth mechanism of Si-induced IDBs in N-polar GaN nanowires that are embedded in a Ga-polar layer [Nano Lett., 2012, 12, 6119]. The lower-energy boundary exhibits the characteristics of intrinsic semiconductor and fulfils the electron counting rule. Charge neutrality is achieved by transferring electrons from the Si–N to the Ga–Ga bonds. Moreover, a potential step is induced by the asymmetric substitution of Si for Ga atoms at the interface, which facilitates the spatial separation of excited carriers at this neutral boundary. Our results suggest an alternative strategy for designing novel and highly efficient photovoltaic devices.