Systematic First-principles Study Research Articles

Electronic structures, stabilities, and spectroscopies of the fullerene derivatives C 68X 4 (X = H, F, Cl)

A systematic first-principle study based on the density functional theory has been performed on the electronic structures and stabilities of fullerene C 68 and its derivatives C 68X 4 (X = H, F, Cl). By searching the 6332 classical and 43 nonclassical isomers of C 68, the ground state is found to be the classical isomer 6290 bearing C 2 symmetry. However, after attaching X atoms to the active sites of C 68, the heptagon-containing nonclassical derivatives C 68(c)-2 C 68H 4, C 68(c)-2 C 68F 4, and C 68(c)-3 C 68Cl 4 are predicted to be the most stable among all derivatives. The chemical deriving could affect the electronic structures distinctly, and enhance the stability of fullerene C 68. The Mulliken charge populations of the most stable C 68X 4 are calculated, showing that different X atoms added to C 68 will cause remarkably different charge populations. The IR, Raman, and NMR spectra of the most stable C 68X 4 are also calculated and presented to facilitate future experimental identification.

Carbon and proton shielding tensors in methyl halides

The series of methyl halides, CH(3)X (X = F, Cl, Br, and I), is prototypic for demonstrating the s.c. normal halogen dependence of light-atom nuclear magnetic resonance shielding constants in the presence of halogen atoms of varying electronegativity. We report a systematic experimental and first-principles theoretical study of the (13)C and (1)H shielding tensors in this series. The experimental shielding constants were obtained from gas-phase NMR experiments and the anisotropies were determined using liquid crystal NMR spectroscopy. After taking into account rovibrational effects and solute-solvent interactions, this provided the currently best experimental estimates for the full shielding tensors. Quantum chemical calculations were carried out at ab initio and density functional theory levels, involving relativistic corrections taken into account at the leading-order Breit-Pauli perturbation level. Anharmonic and harmonic vibrational corrections were performed. The main trends of the shielding constants and anisotropies of the nearby light (13)C and (1)H nuclei as functions of the halogen mass, were confirmed to be mainly due to relativistic spin-orbit effects. For carbon, also the scalar relativistic effects are important for quantitative results. Thermal averaging at 300 K decreases the magnitude of all the parameters but exhibits partial cancellation between the nonrelativistic and smaller relativistic rovibrational averages. For the shielding anisotropy, the relativistic terms add to the negative rovibrational effect. Overall, the current experimental and theoretical results are in excellent agreement for all the shielding parameters, setting a standard for further investigations of normal halogen dependence.

Effect of Aromatic Coupling on Electronic Transport in Bimolecular Junctions

We have performed a systematic first-principles study on conductance−voltage characteristics of bioligo(phenylene ethynylene)−monothiol molecular junctions as recently reported by Wu et al.[Nature Nanotech. 2008, 3, 569]. It is found that the molecular conductance is very sensitive to the vertical distance between two molecules as well as the titled angle between two molecular planes. By comparing with experimental results, key structure parameters for bimolecular junction are determined, indicating that in the experimental devices, the vertical distance between two molecules is around 0.30 nm, the two planar molecules have a cofacial arrangement, and the length of the molecular bridge is about 2.88 nm. The underlying mechanism for electron transport in these aromatically coupled bimolecular junctions has also been discussed.

Chemisorption of small fullerenesCn(n=28,32,36,40,44,48,60)on theSi(001)−c(2×1)surface

We present a systematic first-principles study using density-functional theory on chemisorption of small fullerenes ${\text{C}}_{n}$ $(n=28,32,36,40,44,48,60)$ on the $\text{Si}(001)\text{\ensuremath{-}}c(2\ifmmode\times\else\texttimes\fi{}1)$ reconstructed surface. The most stable adsorption structures were identified and the size-dependent adsorption strength was calculated. Detailed analysis on the geometric and electronic structures indicates that fullerenes can be strongly anchored on the substrate. Unlike adsorption of small unsaturated organic molecules on Si surfaces, where adsorption on top of Si dimers was reported to be favored, the trench channel and the two adjacent Si dimer sites were found to be the energetically preferred binding sites for fullerenes. Our results indicate that the adsorption strength is largely determined by the local curvature of fullerene molecules near the binding sites, the geometric fitness between fullerenes and adsorption sites and the stability of fullerene electronic structures. In addition, charge transfer from the substrate to fullerenes was found to be relatively small due to the highly covalent nature of the C-Si bonds formed upon fullerene chemisorption.

Tailoring TMR Ratios by Ultrathin Magnetic Interlayers: A First-principles Investigation of Fe/MgO/Fe

AbstractFor spintronic device applications, large and in particular tunable tunnel magnetoresistance (TMR) ratios are inevitable. Fully crystalline and epitaxially grown Fe/MgO/Fe magnetic tunnel junctions (MTJs) are well suited for this purpose and, thus, are being intensively studied [1]. However, due to imperfect interfaces it is difficult to obtain sufficiently large TMR ratios that fulfill industrial demands (e.g. [2]).A new means to increase TMR ratios is the insertion of ultra-thin metallic buffer layers at one or at both of the Fe/MgO interfaces. With regard to their magnetic and electronic properties as well as their small lattice mismatch to Fe(001), Co and Cr spacer are being preferably investigated.We report on a systematic first-principles study of the effect of Co and Cr buffers (with thicknesses up to 6 ML) in Fe/MgO/Fe magnetic tunnel junctions (MTJs) on the spin-dependent conductance. The results of the transport calculations reveal options to specifically tune the TMR ratio. Symmetric junctions, i.e. with Co buffers at both interfaces, exhibit for some thicknesses much larger TMR ratios in comparison to those obtained for Fe-only electrodes. Further, antiferromagnetic Cr films at a single interface introduce TMR oscillations with a period of 2 ML, a feature which provides another degree of freedom in device applications. The comparison of our results with experimental findings shows agreement and highlights the importance of interfaces for the TMR effect.

Electronic and optical properties of polyicosahedral Si nanostructures: A first-principles study

In a previous molecular dynamics study, we predicted a polyicosahedral Si nanostructure which has a ${\mathrm{Si}}_{20}$ fullerene cage per icosahedral ${\mathrm{Si}}_{100}$ nanodot. The unique cage structure is distinct from the crystalline diamond Si nanostructure. Encapsulating a guest atom into the ${\mathrm{Si}}_{20}$ cage allows us to tune the electronic and optical properties. Here, we report on a systematic first-principles study of the effect of the sodium and iodine doping on the physical properties of the hydrogen-terminated polyicosahedral Si nanostructures. Our calculations reveal the strongly guest-dependent and size-dependent physical properties of the polyicosahedral Si nanostructures: (1) the semiconducting guest-free polyicosahedral nanowire becomes metallic by the sodium and iodine doping, (2) the quantum confinement effect is observed in the icosahedral and polyicosahedral nanodots, and (3) the radiative recombination rate comparable to the luminescent amorphous Si nanostructures is expected from some of the Na- and I-doped polyicosahedral nanostructures. From these results, we assert that the polyicosahedral Si nanostructures are promising candidates for the building blocks of the future nanoscale optoelectronic devices.

First-principles study of half-metallic ferromagnetism and structural stability of CrxZn1−xTe

We report a systematic first-principles study of structural properties, electronic structures and magnetic properties of Cr doped ZnTe with the Cr concentration up to 0.5. We fully optimize all the structures concerned, both ferromagnetic and antiferromagnetic, and their internal atomic positions. Our calculations show that they are half-metallic ferromagnets with half-metallic gaps over 0.6 eV. Our calculated results are consistent with the fact that only those bulk samples with very small doping concentrations can be stable thanks to the entropy effect. On the other hand, up to 20% Cr doping concentrations have been achieved experimentally in thin films fabricated with nonequilibrium methods. Our calculated results indicate that half-metallic ferromagnetism can be realized in high-quality thin film samples of CrxZn1−xTe with nonequilibrium methods. We explain the half-metallic ferromagnetism in terms of a pd hybridization and resulting large exchange splitting of d-dominated bands. Compatible with appropriate semiconductors, these materials, at least some of them, could be useful in spintronics.

Structures, Electronic Properties, Spectroscopies, and Hexagonal Monolayer Phase of a Family of Unconventional Fullerenes C64X4 (X = H, F, Cl, Br)

A systematic first-principles study within density functional theory on the geometrical structures and electronic properties of unconventional fullerene C64 and its derivatives C64X4 (X = H, F, Cl, Br) has been performed. By searching through all 3465 isomers of C64, the ground state of C64 is found to be spherical shape with D2 symmetry, which differs from the parent cage of the recently synthesized C64H4 that is pear-shaped with C3v symmetry. We found that the addition of the halogen atoms like F, Cl, and Br to the pentagon−pentagon fusion vertex of C64 cage could enhance the stability, forming the unconventional fullerenes C64X4. The Mulliken charge populations, LUMO−HOMO gap energies, and density of states are calculated, showing that different halogen atoms added to C64 will cause remarkably different charge populations of the C64X4 molecule; the chemical deriving could enlarge the energy gaps and affect the electronic structures distinctly. It is unveiled that C64F4 is even more stable than C64H4, a...

First-principles study of muonium in A- and B-form DNA

We have carried out a systematic first-principles study of muonium ( μ + e - ) adducts in A- and B-form DNA. Potential trapping sites in the bases cytosine and thymine were considered. Our results indicate that the difference in structural geometry between A- and B-form DNA can lead to substantial deviations in the hyperfine fields at the μ + sites. This could have important implications for the interpretation of muon spin relaxation measurements that have shown evidence for an enhanced electron mobility in A-form DNA, but implicitly assume a negligible difference in the magnitude of the hyperfine fields at the trapped μ + site between A- and B-form DNA.

Atomic strings of group IV, III–V, and II–VI elements

A systematic first-principles study of atomic strings made by group IV, III–V, and II–VI elements has revealed interesting mechanical, electronic, and transport properties. The double bond structure underlies their unusual properties. We found that linear chain of C, Si, Ge, SiGe, GaAs, InSb, and CdTe are stable and good conductor, although their parent diamond (zincblende) crystals are covalent (polar) semiconductors but, compounds SiC, BN, AlP, and ZnSe are semiconductors. First row elements do not form zigzag structures.

First-principles study of crystal structure and stability of Al–Mg–Si–(Cu) precipitates

Al–Mg–Si–(Cu) alloys form the basis of a wide variety of commercial precipitation-hardened alloys, and the observed precipitation sequence in these alloys is complex and involves a wide variety of metastable phases (e.g. GP zones, β″, U1, U2, B ′, β ′). Calculations of metastable phase equilibria in these alloys are virtually nonexistent due to the lack of quantitative information on the thermodynamics of the precipitate phases. We have undertaken an extensive, systematic first-principles study of energetics of all the reported precipitate phases of Al–Mg–Si–(Cu) alloys, using density functional-based calculations in both the local density and generalized gradient approximations. Our calculations help clarify the energetics of the metastable precipitate phases, and in certain cases, provide insight into the compositional changes of precipitates during aging. In addition to energetics, we also examine the relative volumes of the various phases, and discuss cases of significant deviation from that of the solid solution.

Systematic first-principles study of impurity hybridization in NiAl

We have performed a systematic first-principles computational study of the effects of impurity atoms (boron, carbon, nitrogen, oxygen, silicon, phosporus, and sulfur) on the orbital hybridization and bonding properties in the intermetallic alloy NiAl using a full-potential linear muffin-tin orbital method. The matrix elements in momentum space were used to calculate real-space properties: onsite parameters, partial densities of states, and local charges. In impurity atoms that are empirically known to be embrittler (N and O) we found that the 2s orbital is bound to the impurity and therefore does not participate in the covalent bonding. In contrast, the corresponding 2s orbital is found to be delocalized in the cohesion enhancers (B and C). Each of these impurity atoms is found to acquire a net negative local charge in NiAl irrespective of whether they sit in the Ni or Al site. The embrittler therefore reduces the total number of electrons available for covalent bonding by removing some of the electrons from the neighboring Ni or Al atoms and localizing them at the impurity site. We show that these correlations also hold for silicon, phosporus, and sulfur.