Self-consistent Tight-binding Calculations Research Articles

Covalent functionalization of ZnO surfaces: A density functional tight binding study

We have demonstrated the covalent functionalization of 101̱0-ZnO surfaces with carboxylic acids by employing self-consistent charge density functional tight binding (SCC-DFTB) calculations. We have found two thermodynamically stable surface configurations: (i) a monolayer coverage with a bidentate chelating ligand and (ii) a half-monolayer coverage with a bidentate bridging ligand. In both cases, the electronic band structures show the presence of covalent surface/adsorbate interactions. Besides, a nonbonding carboxylate character is verified for the bidentate adsorbate. Our results are consistent with infrared spectroscopy experiments on functionalized ZnO nanostructures, and open possibilities for further investigations on functionalized ZnO-based materials for bio/chemical sensing.

Structure and Properties of Single-Nano Diamond Particles

Revised structure of the primary particle of single-nano detonation diamond (SNBD) are constructed by merging our own experimental observations with the results from self-consistent charges density functional tight binding (SCC-DFTB) calculations by Barnard. Internal geometrical structure is characterized by triply-decked core-shell structure with graphene layers on the surface, electronic structure by strong electrostatic potential fields of both signs over the facets, and surface structure by intermixed graphene/diamond patches. Surprisingly high stability of colloidal solution of SNBD is interpreted in terms of strong hydration over the charged facets. Rapid aggregation of primary particles by the changes in pH and the addition of electrolytes are mentioned by invoking ligand exchange reactions that take place on the charged facets involving hydrated water molecules and protons or metallic ions. In this way a considerably improved picture on the characteristic behaviors of this novel nanocarbon emerged. The most remarkable feature of SNBD is that diamond polarizes in nano.

Tight-binding theory of manganese and iron oxides

The electronic structure is found to be understandable in terms of the free-atom term values and universal interorbital coupling parameters, since self-consistent tight-binding calculations indicate that the Coulomb shifts of the $d$-state energies are small. The special-points averages over the bands are seen to be equivalent to the treatment of local octahedral clusters, so that all properties are given by algebraic formulas in terms of these parameters. The cohesive energy per manganese for MnO, ${\text{Mn}}_{2}{\text{O}}_{3}$, and ${\text{MnO}}_{2}$, in which manganese exists in the valence states ${\text{Mn}}^{2+}$, ${\text{Mn}}^{3+}$, and ${\text{Mn}}^{4+}$, is very nearly the same and dominated by the transfer of manganese $s$ electrons to the oxygen $p$ states. There are small corrections, 1.37 eV per Mn in all cases, from the couplings of the minority-spin states. Transferring one majority-spin electron from an upper cluster state to a nonbonding oxygen state adds 1.67 eV to the cohesion for ${\text{Mn}}_{2}{\text{O}}_{3}$ and the two transfers add twice that for ${\text{MnO}}_{2}$. The electronic and magnetic properties are consistent with this description and appear to be understandable in terms of the same parameters.

Screening and polaronic effects induced by a metallic gate and a surrounding oxide on donor and acceptor impurities in silicon nanowires

We present self-consistent tight binding calculations of the electronic structure of donor and acceptor impurities in silicon nanowires surrounded by a gate oxide (SiO2 or HfO2) and a metallic gate. These environments efficiently screen the potential of the impurities so that their ionization energy strongly decreases with respect to the case of freestanding nanowires. It is also shown that the carriers trapped by the impurities form a polaron due to the response of the ions in the surrounding oxide layer. We predict that the polaron shift represents a large part of the impurity ionization energy, in particular, in HfO2. Our work demonstrates the importance of screening and polaronic effects on the transport properties in nanoscale devices based on Si nanowires.

Local electronic structure of threading screw dislocation in wurtzite GaN

The atomic and electronic structure of the full-core threading screw dislocation in wurzite GaN has been investigated using a self-consistent density-functional tight-binding calculation. It is shown that the atoms are severely strained in a hexagonal atomic core, and an extra charge transfer of 0.12 e occurs at the core atoms from Ga to N, in addition to the typical charge transfer of 0.56 e for bulk GaN. Filled and unfilled gap states are found to be spread over the entire band gap. The p states of the core N-atom mostly contribute to the tail states of valence and conduction bands, whereas the deep levels are heavily localized at the Ga and N core atoms. The coexistence of acceptor and donor gap states in the vicinity of the screw dislocations could be an origin of the leakage currents observed in GaN-based devices.

Atomic structure and energy of threading screw dislocations in wurtzite GaN

Atomic structure and energy of the screw dislocation b = <000c> in the full core configuration has been investigated with a self-consistent density functional tight binding calculation. A 288-atom cluster was used and the dangling bonds saturated with pseudo-hydrogen. The line energy is comparable to the values obtained using a supercell. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Different wavelength oscillations in the conductance of5dmetal atomic chains

Combining ab-initio and self-consistent parametrized tight-binding calculations we analyze the conductance properties of atomic chains of 5d elements like Au, Pt and Ir. It is shown that, in addition to the even-odd parity oscillations characteristic of Au, conduction channels associated with the almost full d bands in Pt and Ir give rise to longer periods which could be observed in sufficiently long chains. The results for short chains are in good agreement with recent experimental measurements.

A tight-binding study of surface magnetic anisotropy of the Co (0001) and its perturbation by Cu and CO

Recent experiments show that on rather thick Co films deposition of Cu or adsorption of CO or C can switch the magnetization direction along the surface normal. We present semi-empirical self-consistent tight-binding calculations for a semi-infinite hcp Co(0001) crystal. It appears that the contribution of the magnetic anisotropy-energy from the surface layer favors the in-plane alignment of magnetic moments. Various surface perturbations (Cu deposition or CO adsorption, artificial suppression of surface magnetization), however, reduce this contribution considerably or even change the sign of the electronic part of the magnetic anisotropy energy, thus making conditions for perpendicular magnetization more favourable.

Electronic transport properties of free-base tape-porphyrin molecular wires studied by self-consistent tight-binding calculations

We investigated by theoretical calculations the electronic states and the transport properties of one of the oligoporphyrin molecular wires, the free-base tape-porphyrins. Inside these molecules the adjacent building blocks, referred to as porphyrin macrocycles, are linked by three conjugating C-C bonds aligned in parallel. We found that the origin of the extremely small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps of these molecules is the strong coupling between the unoccupied $\ensuremath{\pi}$ orbitals of each macrocycle. Then we considered the molecular bridges where these porphyrin molecules are bridged between the aluminum electrodes. We found that the conductances have large values and that the current increases nearly in proportion to the applied bias. These features are explained by the strong hybridization of the HOMO with the electrode wave functions and by the wide bandwidth of the tape porphyrins. Finally we studied the current distribution inside the tape-porphyrin wire. The current is found to flow in an anisotropic way, i.e., it seems to flow along the spatial distribution of the HOMO of the tape porphyrins, reflecting the strong hybridization of the HOMO with the electrode wave functions.

Theoretical Predictions of Electronic Transport Properties of Differently Conjugated Porphyrin Molecular Wires

Based on the self-consistent tight-binding calculations and the Keldysh Green's functions technique, we predicted the transport properties of three types of differently conjugated porphyrin molecular wires, i.e., the tape-porphyrin, the butadiyne-linked porphyrin, and the edge-fused porphyrin wires. We found that among the three types of molecular wires the tape-porphyrin wires are the most conductive because of their extremely small HOMO–LUMO energy gaps. The other two types of wires are found to form semiconductive devices. For all the types of porphyrin wires the current is found to bypass the zinc atom sites because the zinc-related energy levels are away from the Fermi level. Then we examined the transport properties of their junctions where the butadiyne-bridged porphyrin molecule is inserted in the infinite tape-porphyrin wire. Their transport properties are found to be easily tuned from metallic to semiconductive by increasing the number of butadiyne-linked-porphyrin building blocks.

Structure and magnetism of cobalt clusters

The lowest-energy geometric structures and isomers of freestanding ${\mathrm{Co}}_{N}$ clusters $(4&lt;~N&lt;~60)$ and their corresponding magnetic moments are calculated using an evolutive algorithm based on a many-body Gupta potential and a self-consistent $\mathrm{spd}$ tight-binding method, respectively. We found an icosahedral growth pattern for the global minimum with some hcp and fcc structures for some particular sizes, whereas for the second isomer, distorted icosahedral structures are obtained in general. With the aim to study the possible coexistence of isomers within the experimental cluster beam we assumed an equilibrium distribution and calculated for each cluster size the different coexistent structures and the relative populations at room temperature. Our results show that the coexistence is present only at some particular sizes, in agreement with chemical-adsorption and photoionization experiments. Our self-consistent tight-binding calculations considering $3d,$ $4s,$ and $4p$ valence electrons for the magnetic properties show that the magnetic moments for the global minima and the second isomers are in general very similar except in a small region of $20&lt;N&lt;40$ atoms where the magnetic moment of the global minimum is smaller than that of the second isomer. We compare our results for the magnetic behavior of the global minimum with theoretical calculations available in the literature as well as with experimental results.

Interpretation and theory of tunneling experiments on single nanostructures

We discuss the interpretation of tunneling experiments on single molecules or semiconductor quantum dots weakly coupled to metallic electrodes. We identify the main features in the current-voltage curves and in the conductance using an extension of the theory of single charge tunneling. We analyze important quantities, such as the charging energy and the quasiparticle gap, providing simple rules to interpret the experiments. We discuss the limitations of the capacitance model to describe the system. We show that at a bias larger than the band-gap energy of the nanostructure the tunneling of both electrons and holes must be taken into account. We use self-consistent tight-binding calculations to illustrate these points and provide a comparison with recent experimental results on InAs nanocrystals.

Single-particle tunneling in semiconductor quantum dots

We present a calculation of single-charge tunneling in a semiconductor quantum dot based on a full self-consistent tight-binding calculation of the charging energies, applicable to quantum dots of realistic size (up to 8 nm diameter). Comparison with recent tunneling spectroscopy experiments on InAs nanocrystals shows excellent agreement and allows an unambiguous assignation of the conductance peaks. For bias voltages V larger that the band gap of the quantum dot we show that both electrons and holes can tunnel into the quantum dot, leading to specific features in the $I(V)$ curves.

Tight-binding electronic structure calculations for the TiFe(001) surface

We present a self-consistent tight-binding calculation of the electronic structure for the (001) surface of TiFe in the CsCl (B2) structure. The results were obtained using a three-center s-p-d orthogonal tight-binding Hamiltonian, within the surface Green function matching formalism. We have analyzed the local density of states (LDOS) for cases where Fe or Ti forms the surface layer. When Fe forms the atomic surface layer, our calculations show that the corresponding LDOS consists predominantly of bonding states, like in the bulk, but with a value at the E F enhanced by ∼160% with respect to the bulk. When Ti forms the atomic surface layer, the corresponding LDOS consists mainly of antibonding states, but with a value at E F raised compared with the bulk by ∼350%; a strong d-band narrowing was found in this case. Additionally, the trends to surface segregation and chemical activity for these surfaces are discussed.

Doping screening of polarization fields in nitride heterostructures

Using self-consistent tight-binding calculations, we show that modulation doping can be used to screen macroscopic polarization fields in nitride quantum wells. The blue-shift of photoluminescence peak as well as the reduction of radiative recombination lifetime at increasing doping density is explained and correlated to polarization-field screening. The field-induced ionization of the dopants and its relation with alloy composition in the heterostructure barriers is also analyzed.

ELECTRONIC LOCAL DENSITY OF STATES FOR THE TiNi(001) SURFACE

We present a self-consistent tight-binding calculation of the electronic structure for the (001) surface of TiNi in the CsCl (B2) structure. The results were obtained using a three-center s–p–d orthogonal tight-binding Hamiltonian fitted to first-principles calculations, within the surface Green function matching formalism. We have analyzed the local density of states (LDOS) for cases with Ni or Ti at the surface layer. For the case where Ni is at the atomic surface layer we find that the corresponding LDOS consists predominantly of bonding states, like in the bulk but with a value at EF reduced by 38% with respect to the bulk; for this surface a strong peak in the LDOS was found at -1.4 eV below EF. For the case where Ti is at the atomic surface layer the corresponding LDOS consists mainly of antibonding states, but with a value at EF higher than in the bulk by 30%. Comparatively, the case where Ni is at the surface layer presents lower values of LDOS at EF and d holes with respect to the case where Ti is at the surface layer, and therefore more chemical activity can be expected for the Ti surface.

Stable Structures of Neutral and Charged Iron Clusters by Self-Consistent Tight-Binding Molecular-Dynamics

By using the simulated-annealing technique with a tight-binding molecular-dynamics, fully optimized structures of iron clusters are determined for the number of atoms, n=2-17. It is found that the clusters with sizes, n=6, 7, and 13, are relatively stable. Iron clusters show an icosahedral growth similar to rare gas clusters. HOMO-LUMO energy gaps are calculated, and electronic shell effects are absent. Additionally, by performing a self-consistent tight-binding molecular-dynamics calculations, we obtain fully optimized structures of positively- and negatively-charged iron clusters. Nearest-neighbor distance of positively- and negatively-charged iron clusters reveals an decrement and increment compared to that of the neutral clusters, respectively. These changes in the interatomic distances are mainly due to the changes in the number of HOMO electrons in anti-bonding molecular-orbitals.

Charge transfer in multicomponent oxides

The transfer of charge between different ions in an oxide plays an essential role in the stability of these compounds. Since small variations in charge can introduce large changes in the total energy, a correct description of this phenomenon is critical. In this work, we show that the ionic charge in oxides can strongly depend on its atomic environment. A model to assign point charges to atoms as a function of their atomic environment has recently been proposed for binary alloys [C. Wolverton, A. Zunger, S. Froyen, and S.-H. Wei, Phys. Rev. B 54, 7843 (1996)] and proven to be very successful in screened solids such as semiconductors and metals. Here, we extend this formalism to multicomponent oxides and we assess its applicability. The simple point-charge model predicts a linear relation between the charge on an atom and the number of unlike neighbors, and between the net value of the charge and the Coulomb field at a given site. The applicability of this approach is tested in a large-supercell self-consistent tight-binding calculation for a random Zr-Ca-O alloy. The observed fluctuations of the ionic charge about the average linear behavior (as a function of the number of unlike neighbors) was larger than 0.25 electrons even when many shells of atomic neighbors were considered in the fit. This variation is significant since it can introduce large errors in the electrostatic energy. On the other hand, for small absolute values of the charge, the ionic charge varied linearly with the Coulomb field, in agreement with previous findings. However, for large Coulomb fields, this function saturates at the formal chemical charge.

Optical and Electronic Properties of Semiconductor 2D Nanosystems: Self-consistent Tight-binding Calculations

A tight-binding models which account for band mixing, strain and external applied potentials in a self-consistent fashion has been developed. This allows us to describe electronic and optical properties of nanostructured devices beyond the usual envelope function approximation. This model can be applied to direct and indirect gap semiconductors thus allowing for instance the self-consistent calculation of band profile and carrier control in pseudomorphic InGaAs/GaAs HEMTs and SiGe/Si MODFETs.

A tight-binding study of the electronic structure of and chemisorbed CO

Experiments on CO chemisorption at the transition-metal alloy find the molecule bound more weakly than at Pt(111). There was, however, disagreement in experimental interpretation supporting either the bulk termination or pure platinum surface. A recent LEED study strongly supports the latter possibility. We have performed extended self-consistent tight-binding calculations for bulk and surface electronic structure of the alloy and for CO chemisorption at selected adsorption sites. For the bulk we get in the initial-state approximation a good agreement with the measured core-level shifts. For CO we find for both the surface models sites at which the adsorption energy is reduced in agreement with measurements, but also sites at which the adsorption energy practically coincides with that at Pt(111). The calculated core-level shifts at Pt sites in both models are close to experimental values. We predict, however, a distinct CLS for Pt atoms at Ti sites in the Pt overlayer which is not reported experimentally. The possible enrichment of the subsurface layer by titanium might reconcile the experimental and theoretical findings. Correlations between surface core-level shifts and reactivity are also discussed.