Nanowire-based photovoltaic devices have the advantages over planar devices in light absorption and charge transport and collection.Recently,a new strategy relying on type-Ⅱ band alignment has been proposed to facilitate efficient charge separation in core/shell nanowire solar cells.This paper reviews the type-Ⅱ heterojunction solar cells based on core/shell nanowire arrays,and specifically focuses on the progress of theoretical design and fabrication of type-Ⅱ Zn O/Zn Se core/shell nanowire-based solar cells.A strong photoresponse associated with the type-Ⅱ interfacial transition exhibits a threshold of 1.6 e V,which demonstrates the feasibility and great potential for exploring all-inorganic versions of type-Ⅱ heterojunction solar cells using wide bandgap semiconductors.Future prospects in this area are also outlooked.
Yiyan CaoZhiming WuJianchao NiWaseem.A.BhuttoJing LiShuping LiKai HuangJunyong Kang
Undoped ZnO and doped ZnO films were deposited on the MgO(111) substrates using oxygen plasma-assisted molecular beam expitaxy. The orientations of the grown ZnO thin film were investigated by in situ reflection high-energy electron diffraction and ex situ x-ray diffraction(XRD). The film roughness was measured by atomic force microscopy, which was correlated with the grain sizes determined by XRD. Synchrotron-based x-ray absorption spectroscopy was performed to study the doping effect on the electronic properties of the ZnO films, compared with density functional theory calculations.It is found that, nitrogen doping would hinder the growth of thin film, and generate the NOdefect, while magnesium doping promotes the quality of nitrogen-doped ZnO films, inhibiting(N_2)Oproduction and increasing nitrogen content.
A zinc oxide thin film in cubic crystalline phase, which is usually prepared under high pressure, has been grown on the Mg O(001) substrate by a three-step growth using plasma-assisted molecular beam epitaxy. The cubic structure is confirmed by in-situ reflection high energy electron diffraction measurements and simulations. The x-ray photoelectron spectroscopy reveals that the outer-layer surface of the film(less than 5 nm thick) is of ZnO phase while the buffer layer above the substrate is of ZnMgO phase, which is further confirmed by the band edge transmissions at the wavelengths of about 390 nm and 280 nm, respectively. The x-ray diffraction exhibits no peaks related to wurtzite ZnO phase in the film. The cubic ZnO film is presumably considered to be of the rock-salt phase. This work suggests that the metastable cubic ZnO films, which are of applicational interest for p-type doping, can be epitaxially grown on the rock-salt substrates without the usually needed high pressure conditions.
By using the first-principles calculations, structural and electronic properties of Au and Ti adsorbed WS2 monolayers are studied systematically. For Au-adsorbed WS2, metallic interface states are induced in the middle of the band gap across the Fermi level. These interface states origin mainly from the Au-6s states. As to the Ti adsorbed WS2, some delocalized interface states appear and follow the bottom of conduction band. The Fermi level arises into the conduction band and leads to the n-type conducting behavior. The n-type interface states are found mainly come from the Ti-3d and W-5d states due to the strong Ti–S hybridization. The related partial charge densities between Ti and S atoms are much higher and increased by an order of magnitude as compared with that of Au-adsorbed WS2. Therefore, the electron transport across the Ti-adsorbed WS2 system is mainly by the resonant transport, which would further enhances the electronic transparency when monolayer WS2 contacts with metal Ti. These investigations are of significant importance in understanding the electronic properties of metal atom adsorption on monolayer WS2 and offer valuable references for the design and fabrication of 2D nanodevices.
Electronic structure and spin-related state coupling at ferromagnetic material(FM)/MgO(FM = Fe, CoFe, CoFeB)interfaces under biaxial strain are evaluated using the first-principles calculations. The CoFeB/MgO interface, which is superior to the Fe/MgO and CoFe/MgO interfaces, can markedly maintain stable and effective coupling channels for majorityspin ?_1 state under large biaxial strain. Bonding interactions between Fe, Co, and B atoms and the electron transfer between Bloch states are responsible for the redistribution of the majority-spin ?_1 state, directly influencing the coupling effect for the strained interfaces. Layer-projected wave function of the majority-spin ?_1 state suggests slower decay rate and more stable transport property in the CoFeB/MgO interface, which is expected to maintain a higher tunneling magnetoresistance(TMR) value under large biaxial strain. This work reveals the internal mechanism for the state coupling at strained FM/MgO interfaces. This study may provide some references to the design and manufacturing of magnetic tunnel junctions with high tunneling magnetoresistance effect.
The effect of Ag nanoislands on the Raman of graphene was investigated in this work.Compared with that on the bare silicon wafer,Raman enhancement was observed in the graphene film that covered on Ag/Si surface with nanoscale Ag islands,which would be induced by the localized plasmon resonance in Ag nanostructures.The interaction between the graphene sheet and Ag/Si substrate was further studied.The peak shift and line shape of Raman spectroscopy indicated a nonuniform strain distribution in the Ag/Si supported graphene film.
HU WeiHUANG ZhiYiZHOU YingHuiCAI WeiWeiKANG JunYong
A tiny number of Zn atoms were deposited on Si(111)-(797) surface to study the evolution process of Zninduced nanoclusters. After the deposition, three types(type I, II, and III) of Zn-induced nanoclusters were observed to occupy preferably in the faulted half-unit cells. These Zn-induced nanoclusters are found to be related to one, two, and three displaced Si edge adatoms, and simultaneously cause the depression of one, two, and three closest Si edge adatoms in the neighboring unfaulted half-unit cells at negative voltages, respectively. First-principles adsorption energy calculations show that the observed type I, II, and III nanoclusters can reasonably be assigned as the Zn3Si1, Zn5Si2, and Zn7Si3 clusters,respectively. And Zn3Si1, Zn5Si2, and Zn7Si3 clusters are, respectively, the most stable structures in cases of one, two, and three displaced Si edge adatoms. Based on the above energy-preferred models, the simulated bias-dependent STM images are all well consistent with the experimental observations. Therefore, the most stable Zn7Si3 nanoclusters adsorbed on the Si(111)-(797) surface should grow up on the base of Zn3Si1 and Zn5Si2clusters. A novel evolution process from Zn3Si1 to Zn5Si2, and finally to Zn7Si3 nanocluster is unveiled.
