Gustavo M. Dalpian

Electron-induced stabilization of ferromagnetism in Ga1 xGdxN

SUMMARY In summary, we have investigated in detail the elec-tronic and magnetic properties of Ga 1−x Gd x N. We findthat because of the coupling between the Gd f and hosts states, this system shows some unique behavior that isdrastically different from TM-doped GaN. The exchangesplitting at the CBM is negative. The coupling betweenGd atoms is found to be antiferromagnetic, if no donorsare present, but it become ferromagnetic when enoughdonors are present in the system. We also proposed amodel that may explain the colossal magnetic momentsobserved in this system in the very dilute limit, showingthat it should be directly related to the polarization ofdonor electrons. ACKNOWLEDGMENTS The work at NREL is funded by the U.S. Departmentof Energy, Office ofScience, Basic EnergySciences, underContract No. DE-AC36-99GO10337 to NREL. [1] I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76,323 (2004).[2] D. D. Awschalom and R. K. Kawakami, Nature (London)408, 923 (2000).[3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Fer-rand, Science 287, 1019 (2000).[4] S. Dhar, O. Brandt, A. Trampert, L. Daweritz, K. J.Friedland, K. H. Ploog, J. Keller, B. Beschoten, and G.Guntherodt, Appl. Phys. Lett. 82, 2077 (2003).[5] J. I. Pankove and T. D. Moustakas, Gallium Nitride(GaN) I (Academic, San Diego, 1998).[6] D, C, Look, J. W. Hemsky, and J. R. Sizelove, Phys. Rev.Lett. 82, 2552 (1999).[7] N. Teraguchi, A. Suzuki, Y. Nanishi, Y.-K. Zhou, M.Hashimoto and H. Asahi, Sol. State Comm. 122, 651(2002).[8] S. Dhar, O. Brandt, M. Ramsteiner, V. F. Sapega, andK. H. Ploog, Phys. Rev. Lett. 94, 037205 (2005).[9] H. Asahi, Y. K. Zhou, M. Hashimoto, M. S. Kim, X.J. Li, S. Emura, and S. Hasegawa, J. Phys.: Condens.Matter 16, S5555 (2004).[10] J. M. D. Coey, M. Venkatesan, and C. B. Fitzgerald,Nature Materials 4, 173 (2005).[11] S.-H. Wei and H. Krakauer, Phys. Rev. Lett. 55, 1200(1985); D. J. Singh, Planewaves, Pseudopotentials, andthe LAPW Method, (Kluwer, Boston, 1994).[12] H. J. Monkhorst and J. P. Pack, Phys. Rev. B 13, 5188(1976).[13] D. X. Li, Y. Haga, H. Shida, T. Suzuki, Y. S. Kwon andG. Kido, J. Phys.: Condens. Matter 9, 10777 (1997).[14] A. Janotti, S.-H. Wei, and L. Bellaiche, Appl. Phys. Lett.82, 766 (2003).[15] S.-H. Wei and Alex Zunger, Phys. Rev. B 48, 6111(1993).[16] J. P. Perdew and Y. Wang, Phys. Rev. B 45, 13244(1992).[17] H. Hashimoto, S. Emura, R. Asano, H. Tanaka, N. Ter-aguchi, A. Suzuki, Y. Nanishi, T. Honma, N. Umesaki,H. Asahi , Phys. Stat. Sol. (c) 0, 2650 (2003).[18] W. R. L. Lambrecht, Phys. Rev. B 62, 13538 (2000).[19] G. M. Dalpian, S.-H. Wei, X. G. Gong, A. J. R. da Silva,and A. Fazzio, cond-mat/0504084.[20] Y. K. Zhou, M. S. Kim, X. J. Li, S. Kimura, A. Kaneta,Y. Kawakami, Sg. Fujita, S. Emura, S. Hasegawa and H.Asahi , J. Phys.: Condens. Matter 16, S5743 (2004).

Origin of FM Ordering in Pristine Micro- and Nanostructured ZnO

An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reported previously. Though this property was attributed to the presence of defects, systematic experimental and theoretical studies to pinpoint its origin and mechanism are lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisingly we find that annealing as-prepared samples at low temperature (high temperature) in flowing oxygen actually enhances (diminishes) the FM ordering. For these reasons, we have prepared, annealed in different environments, and measured the ensuing magnetization in micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab initio calculations that a range of magnetic properties in ZnO can result, depending on the sample preparation and annealing conditions. For example, within the same ZnO sample we have observed ferro- to para- and diamagnetic responses depending on the annealing conditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calculations.

Surface magnetization in non-doped ZnO nanostructures

We have investigated the magnetic properties of non-doped ZnO nanostructures by using ab initio total energy calculations. Contrary to many proposals that ferromagnetism in non-doped semiconductors should be induced by intrinsic point defects, we show that ferromagnetism in nanostructured materials should be mediated by extended defects such as surfaces and grain boundaries. This kind of defects creates delocalized, spin-polarized states that should be able to warrant long-range magnetic interactions.

Computational studies of doped nanostructures

One of the most challenging issues in materials physics is to predict the properties of defects in matter. Such defects play an important role in functionalizing materials for use in electronic and optical devices. As the length scale for such devices approaches the nano-regime, the interplay of dimensionality, quantum confinement and defects can be complex. In particular, the usual rules for describing defects in bulk may be inoperative, i.e. a shallow defect level in bulk may become a deep level at the nanoscale. The development of computational methods to describe the properties of nanoscale defects is a formidable challenge. Nanoscale systems may contain numerous electronic and nuclear degrees of freedom, and often possess little symmetry. In this review, we focus on new computational methods, which allow one to predict the role of quantum confinement on the electronic, magnetic and structural properties of functionalized nanostructures. We illustrate how these methods can be applied to nanoscale systems, and present calculations for the electronic, magnetic and structural properties of dopants in semiconductor nanocrystals and nanowires.

Barrier-free substitutional doping of graphene sheets with boron atoms: Ab initio calculations

Using ab initio methods, we propose a simple and effective way to substitutionally dope graphene sheets with Boron. The method consists of selectively exposing each side of the graphene sheet to different elements. We first expose one side of the membrane to Boron, while the other side is exposed to Nitrogen. Proceeding this way, the B atoms will be spontaneously incorporated into the graphene membrane, without any activation barrier. In a second step, the system should be exposed to a H-rich environment, that will remove the CN radical from the layer and form HCN, leading to a perfect substitutional doping.

Charge storage in oxygen deficient phases of TiO2: defect Physics without defects

Defects in semiconductors can exhibit multiple charge states, which can be used for charge storage applications. Here we consider such charge storage in a series of oxygen deficient phases of TiO2, known as Magnéli phases. These Magnéli phases (TinO2n−1) present well-defined crystalline structures, i.e., their deviation from stoichiometry is accommodated by changes in space group as opposed to point defects. We show that these phases exhibit intermediate bands with an electronic quadruple donor transitions akin to interstitial Ti defect levels in rutile TiO2. Thus, the Magnéli phases behave as if they contained a very large pseudo-defect density: ½ per formula unit TinO2n−1. Depending on the Fermi Energy the whole material will become charged. These crystals are natural charge storage materials with a storage capacity that rivals the best known supercapacitors.

Liquid separation by a graphene membrane

The behavior of liquids separated by a single graphene membrane has been studied with extensive molecular dynamics (MD) simulations at ambient conditions. With the help of appropriate empirical potentials, we have exploited two liquid phases forming distinct systems; say XGY, where G stands for graphene and X (Y) represents water (W), benzene (B), or acetonitrile (A). Our MD simulations revealed important changes in the wettability patterns of these liquids near the graphene surface. For instance, WGW exhibits strong density oscillations in a thin interfacial region with thickness of ∼2.4 nm. In the case of BGB and AGA the oscillating-density interfacial region extends beyond ∼3 nm and ∼5 nm, respectively, under ambient conditions. More interestingly, our findings indicate that a liquid at one side of the graphene sheet can affect the degree of wetting on the other side, by means of dispersion interactions through the graphene membrane. These systems can offer a useful framework to understand the structur...

First principles investigations on the electronic structure of anchor groups on ZnO nanowires and surfaces

We report on density functional theory investigations of the electronic properties of monofunctional ligands adsorbed on ZnO-(1010) surfaces and ZnO nanowires using semi-local and hybrid exchange-correlation functionals. We consider three anchor groups, namely thiol, amino, and carboxyl groups. Our results indicate that neither the carboxyl nor the amino group modify the transport and conductivity properties of ZnO. In contrast, the modification of the ZnO surface and nanostructure with thiol leads to insertion of molecular states in the band gap, thus suggesting that functionalization with this moiety may customize the optical properties of ZnO nanomaterials.

Effects of side-chain and electron exchange correlation on the band structure of perylene diimide liquid crystals: a density functional study.

The structural and electronic properties of perylene diimide liquid crystal PPEEB are studied using ab initio methods based on the density functional theory (DFT). Using available experimental crystallographic data as a guide, we propose a detailed structural model for the packing of solid PPEEB. We find that due to the localized nature of the band edge wave function, theoretical approaches beyond the standard method, such as hybrid functional (PBE0), are required to correctly characterize the band structure of this material. Moreover, unlike previous assumptions, we observe the formation of hydrogen bonds between the side chains of different molecules, which leads to a dispersion of the energy levels. This result indicates that the side chains of the molecular crystal not only are responsible for its structural conformation but also can be used for tuning the electronic and optical properties of these materials.

Impurity-induced phase stabilization of semiconductors

We propose an approach to stabilize the cubic zinc-blende (ZB) phase of semiconductor compounds that are usually more stable in the hexagonal wurtzite (WZ) phase. This approach is based on impurity doping and we take advantage of the band offset between the ZB and WZ phases. We show that introduction of donors should stabilize the one with lower conduction band (ZB), whereas holes should stabilize the one with higher valence band (WZ). A mechanism to invert the valence band offset is proposed in order to stabilize the ZB phase through holes. We used GaN, ZnO, and AlN as examples.