Shengbai Zhang

MoS2 Nanoribbons: High Stability and Unusual Electronic and Magnetic Properties

First-principles computations were carried out to predict the stability and magnetic and electronic properties of MoS2 nanoribbons with either zigzag- or armchair-terminated edges. Zigzag nanoribbons show the ferromagnetic and metallic behavior, irrespective of the ribbon width and thickness. Armchair nanoribbons are nonmagnetic and semiconducting, and the band gaps converge to a constant value of approximately 0.56 eV as the ribbon width increases. The higher stability of MoS2 nanoribbons, compared with the experimentally available triangular MoS2 nanoclusters, invites the experimental realization of such novel ribbons in true nanoscale.

Effects of Ga addition to CuInSe2 on its electronic, structural, and defect properties

Using a first-principles band structure method we have theoretically studied the effects of Ga additions on the electronic and structural properties of CuInSe2. We find that (i) with increasing xGa, the valence band maximum of CuIn1−xGaxSe2 (CIGS) decreases slightly, while the conduction band minimum (and the band gap) of CIGS increases significantly, (ii) the acceptor formation energies are similar in both CuInSe2 (CIS) and CuGaSe2 (CGS), but the donor formation energy is larger in CGS than in CIS, (iii) the acceptor transition levels are shallower in CGS than in CIS, but the GaCu donor level in CGS is much deeper than the InCu donor level in CIS, and (iv) the stability domain of the chalcopyrite phase increases with respect to ordered defect compounds. Our results are compared with available experimental observations.

A phenomenological model for systematization and prediction of doping limits in II–VI and I–III–VI2 compounds

Semiconductors differ widely in their ability to be doped. As their band gap increases, it is usually possible to dope them either n or p type, but not both. This asymmetry is documented here, and explained phenomenologically in terms of the “doping pinning rule.”

Graphene Oxide as an Ideal Substrate for Hydrogen Storage

Organometallic nanomaterials hold the promise for molecular hydrogen (H(2)) storage by providing nearly ideal binding strength to H(2) for room-temperature applications. Synthesizing such materials, however, faces severe setbacks due to the problem of metal clustering. Inspired by a recent experimental breakthrough ( J. Am. Chem. Soc. 2008 , 130 , 6992 ), which demonstrates enhanced H(2) binding in Ti-grafted mesoporous silica, we propose combining the graphene oxide (GO) technique with Ti anchoring to overcome the current synthesis bottleneck for practical storage materials. Similar to silica, GO contains ample hydroxyl groups, which are the active sites for anchoring Ti atoms. GO can be routinely synthesized and is much lighter than silica. Hence, higher gravimetric storage capacity can be readily achieved. Our first-principles computations suggest that GO is primarily made of low-energy oxygen-containing structural motifs on the graphene sheet. The Ti atoms bind strongly to the oxygen sites with binding energies as high as 450 kJ/mol. This is comparable to that of silica and is indeed enough to prevent the Ti atoms from clustering. Each Ti can bind multiple H(2) with the desired binding energies (14-41 kJ/mol-H(2)). The estimated theoretical gravimetric and volumetric densities are 4.9 wt % and 64 g/L, respectively.

First-principles calculation of band offsets, optical bowings, and defects in CdS, CdSe, CdTe, and their alloys

Using first principles band structure theory we have calculated (i) the alloy bowing coefficients, (ii) the alloy mixing enthalpies, and (iii) the interfacial valence band offsets for three Cd-based (CdS, CdSe, CdTe) compounds. We have also calculated defect formation energies and defect transition energy levels of Cd vacancy VCd and CuCd substitutional defect in CdS and CdTe, as well as the isovalent defect TeS in CdS. The calculated results are compared with available experimental data.

Effects of Na on the electrical and structural properties of CuInSe2

We found theoretically that Na has three effects on CuInSe2: (1) If available in stoichiometric quantities, Na will replace Cu, forming a more stable NaInSe2 compound having a larger band gap (higher open-circuit voltage) and a (112)tetra morphology. The ensuing alloy NaxCu1−xInSe2 has, however, a positive mixing enthalpy, so NaInSe2 will phase separate, forming precipitates. (2) When available in small quantities, Na will form defect on Cu site and In site. Na on Cu site does not create electric levels in the band gap, while Na on In site creates acceptor levels that are shallower than CuIn. The formation energy of Na(InCu) is very exothermic, therefore, the major effect of Na is the elimination of the InCu defects and the resulting increase of the effective hole densities. The quenching of InCu as well as VCu by Na reduces the stability of the (2VCu−+InCu2+), thus suppressing the formation of the “Ordered Defect Compounds.” (3) Na on the surface of CuInSe2 is known to catalyze the dissociation of O2 int...

Hydrogenation: a simple approach to realize semiconductor-half-metal-metal transition in boron nitride nanoribbons.

The intriguing electronic and magnetic properties of fully and partially hydrogenated boron nitride nanoribbons (BNNRs) were investigated by means of first-principles computations. Independent of ribbon width, fully hydrogenated armchair BNNRs are nonmagnetic semiconductors, while the zigzag counterparts are magnetic and metallic. The partially hydrogenated zigzag BNNRs (using hydrogenated BNNRs and pristine BNNRs as building units) exhibit diverse electronic and magnetic properties: they are nonmagnetic semiconductors when the percentage of hydrogenated BNNR blocks is minor, while a semiconductor-->half-metal-->metal transition occurs, accompanied by a nonmagnetic-->magnetic transfer, when the hydrogenated part is dominant. Although the half-metallic property is not robust when the hydrogenation ratio is large, this behavior is sustained for partially hydrogenated zigzag BNNRs with a smaller degree of hydrogenation. Thus, controlling the hydrogenation ratio can precisely modulate the electronic and magnetic properties of zigzag BNNRs, which endows BN nanomaterials many potential applications in the novel integrated functional nanodevices.

Strong Covalency-Induced Recombination Centers in Perovskite Solar Cell Material CH3NH3PbI3

Inorganic-organic hybrid perovskites are a new family of solar cell materials, which have recently been used to make solar cells with efficiency approaching 20%. Here, we report the unique defect chemistry of the prototype material, CH3NH3PbI3, based on first-principles calculation. We found that both the Pb cations and I anions in this material exhibit strong covalency as characterized by the formation of Pb dimers and I trimers with strong covalent bonds at some of the intrinsic defects. The Pb dimers and I trimers are only stabilized in a particular charge state with significantly lowered energy, which leads to deep charge-state transition levels within the band gap, in contradiction to a recent proposal that this system has only shallow intrinsic defects. Our results show that, in order to prevent the deep-level defects from being effective recombination centers, the equilibrium carrier concentrations should be controlled so that the Fermi energy is about 0.3 eV away from the band edges. Beyond this range, according to a Shockley-Read-Hall analysis, the non-equilibrium carrier lifetime will be strongly affected by the concentration of I vacancies and the anti-site defects with I occupying a CH3NH3 site.

Topological Insulator Thin Films of Bi2Te3 with Controlled Electronic Structure

Topological insulator thin films of Bi2Te3 with controlled electronic structure can be grown by regulating the molecular beam epitaxy (MBE) growth kinetics without any extrinsic doping. N- to p-type conversion results from the change in the concentrations of Te-Bi donors and Bi-Te acceptors. This represents a step toward controlling topological surface states, with potential applications in devices.

B80 and B101–103 clusters: Remarkable stability of the core-shell structures established by validated density functionalsa)

Prompted by the very recent claim that the volleyball-shaped B(80) fullerene [X. Wang, Phys. Rev. B 82, 153409 (2010)] is lower in energy than the B(80) buckyball [N. G. Szwacki, A. Sadrzadeh, and B. I. Yakobson, Phys. Rev. Lett. 98, 166804 (2007)] and core-shell structure [J. Zhao, L. Wang, F. Li, and Z. Chen, J. Phys. Chem. A 114, 9969 (2010)], and inspired by the most recent finding of another core-shell isomer as the lowest energy B(80) isomer [S. De, A. Willand, M. Amsler, P. Pochet, L. Genovese, and S. Goedecher, Phys. Rev. Lett. 106, 225502 (2011)], we carefully evaluated the performance of the density functional methods in the energetics of boron clusters and confirmed that the core-shell construction (stuffed fullerene) is thermodynamically the most favorable structural pattern for B(80). Our global minimum search showed that both B(101) and B(103) also prefer a core-shell structure and that B(103) can reach the complete core-shell configuration. We called for great attention to the theoretical community when using density functionals to investigate boron-related nanomaterials.