S. B. Zhang

Fermi-level tuning of epitaxial Sb2Te3 thin films on graphene by regulating intrinsic defects and substrate transfer doping.

High-quality Sb2Te3 films are obtained by molecular beam epitaxy on a graphene substrate and investigated by in situ scanning tunneling microscopy and spectroscopy. Intrinsic defects responsible for the natural p-type conductivity of Sb2Te3 are identified to be the Sb vacancies and Sb(Te) antisites in agreement with first-principles calculations. By minimizing defect densities, coupled with a transfer doping by the graphene substrate, the Fermi level of Sb2Te3 thin films can be tuned over the entire range of the bulk band gap. This establishes the necessary condition to explore topological insulator behaviors near the Dirac point.

Ab initio design of Ca-decorated organic frameworks for high capacity molecular hydrogen storage with enhanced binding

Ab initio calculations show that Ca can decorate organic linkers of metal-organic framework, MOF-5, with a binding energy of 1.25 eV. The Ca-decorated MOF-5 can store molecular hydrogen (H2) in both high gravimetric (4.6 wt %) and high volumetric (36 g/l) capacities. Even higher capacities (5.7 wt % and 45 g/l) can be obtained in a rationally designed covalent organic framework system, COF-α, with decorated Ca. Both density functional theory and second-order Moller–Plesset perturbation calculations show that the H2 binding in these systems is significantly stronger than the van der Waals interactions, which is required for H2 storage at near ambient conditions.

Unintentional doping and compensation effects of carbon in metal-organic chemical-vapor deposition fabricated ZnO thin films

Carbon is a typical impurity in thin films fabricated by metal-organic chemical-vapor deposition (MOCVD). The role of carbon in undoped and nitrogen-doped ZnO thin films was studied experimentally and theoretically to understand the possible compensation effects. ZnO thin films are fabricated by low-pressure MOCVD using diethylzinc, nitric oxide (for nitrogen-doped films), or oxygen precursors (for undoped films). Compared with sputtering-fabricated ZnO film, the carbon concentration in the MOCVD-fabricated ZnO film is very high. Furthermore, the MOCVD-fabricated ZnO:N film has an even higher carbon concentration than the undoped ZnO. Considering the signal observed previously by Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, it is possible that the incorporated carbon has formed complexes with doped nitrogen. The first-principles calculations predict that the formation energy for carbon interstitial (Ci) is relatively high. However, due to the large binding energy between C...

Organic salts as super-high rate capability materials for lithium-ion batteries

First-principles calculation reveals that organic salts could be super-high rate capability electrode materials for Li-ion batteries. We show that di-lithium terephthalate, an anode material demonstrated recently by experiment, has low Li diffusion barrier (EA). A resonant bonding model for the low EA is developed, which leads to the prediction that di-potassium terephthalate (K2TPA) has even lower EA (150 meV), with diffusion rate orders of magnitude higher than that in Li-intercalated graphite. The calculated anode voltage (0.62 V), specific energy density (209 mA·h/g), and volume change upon lithiation (5%) make K2TPA a promising anode material for power-intensive applications such as electric-vehicles.

Cyclodextrin driven hydrophobic∕hydrophilic transformation of semiconductor nanoparticles

Quantum dots (QDs) have been the subject of considerable study in theoretical physics, and water soluble QDs now appear to have numerous applications in biological tagging, molecular electronic devices, and nanoscale engineering. The work reported here supports the notion that the aliphatic chains of the trioctylphosphine oxide molecules decorating these (CdSe)ZnS core-shell QDs are stabilized by the hydrophobic cyclodextrin (CD) lumen. Photoluminescence studies show a redshift of over 15nm in the emission wavelength of the QDs upon complexation with the CD, and first-principles calculations reveal an exothermic exchange of the S in the ZnS shell with the CD hydroxyl oxygen. Unlike simple water-driven surface transformations, the directed bonding of hydroxyl groups to the ZnS shell results in stable structures, verified by photoluminescence and Fourier transform infrared spectroscopy.

Toward the Intrinsic Limit of the Topological Insulator Bi 2 Se 3

Combining high resolution scanning tunneling microscopy and first principles calculations, we identified the major native defects, in particular the Se vacancies and Se interstitial defects, that are responsible for the bulk conduction and nanoscale potential fluctuations in single crystals of archetypal topological insulator Bi_{2}Se_{3}. Here it is established that the defect concentrations in Bi_{2}Se_{3} are far above the thermodynamic limit, and that the growth kinetics dominate the observed defect concentrations. Furthermore, through careful control of the synthesis, our tunneling spectroscopy suggests that our best samples are approaching the intrinsic limit with the Fermi level inside the band gap without introducing extrinsic dopants.

Scanning secondary-electron microscopy on ferroelectric domains and domain walls in YMnO3

Ferroelectric domain structures in YMnO3 single crystals on the hexagonal polar surface have been investigated by scanning electron microscopy in secondary electron emission mode. The experimental results demonstrate that the domain, as well as domain walls, can be clearly revealed under the operation voltages ranging from 0.6 to 3 kV. Evolution of domain contrasts arising from electron-beam irradiation can be mainly explained by the pyroelectric effect and related charging process. A rich variety of microstructure features of ferroelectric domains can be clearly revealed in YMnO3 by this high-resolution technique.

Enhanced dihydrogen adsorption in symmetry-lowered metal–porphyrin-containing frameworks

Porphyrin is a very important component of natural and artificial catalysis and oxygen delivery in blood. Here, we report that, based on first-principles density-functional calculations, a hydrogen molecule can be adsorbed non-dissociatively onto Ti-, V-, and Fe-porphyrins, similar to oxygen adsorption in heme-containing proteins, with a significant energy gain, greater than 0.3 eV per H(2). The dihydrogen-heme complex will be non-magnetic, as is oxyhemoglobin. In contrast to the backward electron donation of Fe(III)-O(2)(-) in oxyhemoglobin, the dihydrogen binding originates from electron donation from H(2) to the Fe(II). We have identified that the local symmetry of the transition metal center of porphyrins uniquely determines the binding strength, and, thus, one can even manipulate the strength by intentionally and systematically breaking symmetry.

Importance of the correct Fermi energy on the calculation of defect formation energies in semiconductors

Density functional theory (DFT) is a major theoretical tool for the study of defects in semiconductors. However, the results suffer from the often too-small DFT band gap. The calculation of defect formation energy (ΔH) using more sophisticated DFT+GW and hybrid functional methods, however, suggest qualitatively different conclusions about the nature of the DFT error. Here we show that these discrepancies originate from large differences in the representation of the Fermi energy of bulk for the different methods and that they can be largely brought into agreement by correcting the DFT ionization energy.

Theory of the Dirac half metal and quantum anomalous Hall effect in Mn-intercalated epitaxial graphene

The prospect of a Dirac half metal, a material which is characterized by a bandstructure with a gap in one spin channel but a Dirac cone in the other, is of both fundamental interest and a natural candidate for use in spin-polarized current applications. However, while the possibility of such a material has been reported based on model calculations[H. Ishizuka and Y. Motome, Phys. Rev. Lett. 109, 237207 (2012)], it remains unclear what material system might realize such an exotic state. Using first-principles calculations, we show that the experimentally accessible Mn intercalated epitaxial graphene on SiC(0001) transits to a Dirac half metal when the coverage is > 1/3 monolayer. This transition results from an orbital-selective breaking of quasi-2D inversion symmetry, leading to symmetry breaking in a single spin channel which is robust against randomness in the distribution of Mn intercalates. Furthermore, the inclusion of spin-orbit interaction naturally drives the system into the quantum anomalous Hall (QAH) state. Our results thus not only demonstrate the practicality of realizing the Dirac half metal beyond a toy model but also open up a new avenue to the realization of the QAH effect.