Theoretical study of phenol adsorption on the (8, 0) silicon carbide nanotube

Phenol adsorptions on solid surfaces have attracted considerable attention due to their potential applications. Through density functional theory (DFT) methods, we study phenol adsorption on a semiconducting (8, 0) silicon carbide nanotube (SiCNT). We find that the hydroxyl group of phenol prefers to attach to the Si atom of SiCNT. The calculated adsorption energy is −0.494 eV, and 0.208 electrons are transferred from the adsorbate to the nanotube. Interestingly, the O–H bond of the adsorbed phenol can be split on the SiCNT, in which the H atom of the O–H group in the phenol is transferred from the Si atom to its neighboring C atom. Furthermore, we also explore the π–π interaction between the aromatic ring of the phenol and the hexagons of the SiCNT. The calculated adsorption energy is about −0.285 eV with a neglectable charge transfer (0.064 e). On the basis of the calculated band structures, we find that the electronic properties of the adsorbed SiCNT by the phenol are little changed. The present results might be helpful not only to provide an effective way to convert or remove phenol but also to widen the application fields of the SiCNT.     

Adsorption of transition-metal atoms on boron nitride nanotube: a density-functional study

Adsorption of transition atoms on a (8,0) zigzag single-walled boron nitride (BN) nanotube has been investigated using density-functional theory methods. Main focuses have been placed on configurations corresponding to the located minima of the adsorbates, the corresponding binding energies, and the modified electronic properties of the BN nanotubes due to the adsorbates. We have systemically studied a series of metal adsorbates including all 3d transition-metal elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and two group-VIIIA transition-metal elements (Pd and Pt). We found that many transition-metal atoms can be chemically adsorbed on the outer surface of the BN nanotubes and that the adsorption process is typically exothermic. Upon adsorption, the binding energies of the Sc, Ti, Ni, Pd, and Pt atoms are relatively high (>1.0 eV), while those of V, Fe, and Co atoms are modest, ranging from 0.62 to 0.92 eV. Mn atom forms a weak bond with the BN nanotube, while Zn atom cannot be chemically adsorbed on the BN nanotube. In most cases, the adsorption of transition-metal atoms can induce certain impurity states within the band gap of the pristine BN nanotube, thereby reducing the band gap. Most metal-adsorbed BN nanotubes exhibit nonzero magnetic moments, contributed largely by the transition-metal atoms.     

Tuning the electronic structures of semiconducting SiC nanotubes by N and NHx (x=1,2) groups

We investigate the stable configurations and electronic structures of silicon carbide nanotubes (SiCNTs) decorated by N and NHx (x=1,2) groups by using first-principles calculations. We find that these groups can be chemically incorporated into the network of SiCNTs in different ways, accompanied with the formation of N-C and N-Si bonds. The adsorbing energy of N and NHx (x=1,2) groups on (5,5) and (8,0) SiCNTs ranges from -1.82 to -7.19 eV. The electronic structures of SiCNTs can be effectively modified by these groups and display diverse characters ranging from semiconducting to semimetallic, depending on the chirality of SiCNTs as well as the way of the incorporation of these functional groups. The relationship between the electronic structures and the configurations of these functionalized SiCNTs is also addressed by performing projected density of states combined with Milliken population analysis. These results are expected to open a way to tune the electronic structures of SiCNTs which may have promising applications in building nanodevices.     

Adsorption of hydrogen molecules on the platinum-doped boron nitride nanotubes

Adsorption of hydrogen molecules on platinum-doped single-walled zigzag (8,0) boron nitride (BN) nanotube is investigated using the density-functional theory. The Pt atom tends to occupy the axial bridge site of the BN tube with the highest binding energy of -0.91 eV. Upon Pt doping, several occupied and unoccupied impurity states are induced, which reduces the band gap of the pristine BN nanotube. Upon hydrogen adsorption on Pt-doped BN nanotube, the first hydrogen molecule can be chemically adsorbed on the Pt-doped BN nanotube without crossing any energy barrier, whereas the second hydrogen molecule has to overcome a small energy barrier of 0.019 eV. At least up to two hydrogen molecules can be chemically adsorbed on a single Pt atom supported by the BN nanotube, with the average adsorption energy of -0.365 eV. Upon hydrogen adsorption on a Pt-dimer-doped BN nanotube, the formation of the Pt dimer not only weakens the interaction between the Pt cluster and the BN nanotube but also reduces the average adsorption energy of hydrogen molecules. These calculation results can be useful in the assessment of metal-doped BN nanotubes as potential hydrogen storage media.     

A computational study of oxygen-termination of a (6,0) boron nitride nanotube

Abstract  

We performed density functional theory (DFT) calculations to investigate the properties of electronic structures of representative armchair and zigzag silicon carbide nanotubes (SiCNTs). The model structures were optimized and the NMR parameters were calculated at the sites of silicon-29 and carbon-13 atoms in these structures. Our results indicated that different electronic environments could be detected by using the atoms of nanotubes in which the atoms of tips, especially for zigzag SiCNT, exhibit distinctive properties among other atoms.     

In situ x-ray photoelectron spectroscopic and density-functional studies of Si atoms adsorbed on a C60 film

The interaction between C(60) and Si atoms has been investigated for Si atoms adsorbed on a C(60) film using in situ x-ray photoelectron spectroscopy (XPS) and density-functional (DFT) calculations. Analysis of the Si 2p core peak identified three kinds of Si atoms adsorbed on the film: silicon suboxides (SiO(x)), bulk Si crystal, and silicon atoms bound to C(60). Based on the atomic percent ratio of silicon to carbon, we estimated that there was approximately one Si atom bound to each C(60) molecule. The Si 2p peak due to the Si-C(60) interaction demonstrated that a charge transfer from the Si atom to the C(60) molecule takes place at room temperature, which is much lower than the temperature of 670 K at which the charge transfer was observed for C(60) adsorbed on Si(001) and (111) clean surfaces [Sakamoto et al., Phys. Rev. B 60, 2579 (1999)]. The number of electrons transferred between the C(60) molecule and Si atom was estimated to be 0.59 based on XPS results, which is in good agreement with the DFT result of 0.63 for a C(60)Si with C(2v) symmetry used as a model cluster. Furthermore, the shift in binding energy of both the Si 2p and C 1s core peaks before and after Si-atom deposition was experimentally obtained to be +2.0 and -0.4 eV, respectively. The C(60)Si model cluster provides the shift of +2.13 eV for the Si 2p core peak and of -0.28 eV for the C 1s core peak, which are well corresponding to those experimental results. The covalency of the Si-C(60) interaction was also discussed in terms of Mulliken overlap population between them.     

Interactions of hydrogen with Pd and Pd/Ni alloy chain-functionalized single walled carbon nanotubes from density functional theory

Density functional theory is employed to study Pd and Pd/Ni alloy monatomic chain-functionalized metallic single walled carbon nanotubes (SWNT(6,6)) and semiconducting SWNT(10,0), and their interactions with hydrogen molecules. The stable geometries and binding energies have been determined for both isolated chains and chains on SWNT surfaces. We found that continuous Pd and Pd/Ni chains form on SWNTs with geometries close to stable geometries in the isolated chains. Ni alloying improves stability of the chains owing to a higher binding energy to both Pd and C atoms. The physical properties of SWNTs are significantly modified by chain functionalization. SWNT(10,0) is transformed to metal by either Pd or alloy chains, or to a smaller band gap semiconductor, depending on the Pd binding site. From calculations for H(2) interactions with the optimized chain-SWNT systems, the adsorption energy per H atom is found to be about 2.6 times larger for Pd/Ni chain-functionalized SWNTs than for pure Pd chain-functionalized SWNTs. Band structure calculations show that the SWNT(10,0) reverts back to semiconductor and SWNT(6,6) has reduced density of states at the Fermi level upon H(2) adsorption. This result is consistent with the experimentally observed increase of electrical resistance when Pd-coated SWNTs are used as H(2) sensing materials. Finally, our results suggest that Pd/Ni-SWNT materials are potentially good H(2)-sensing materials.     

Chemical surface passivation of 3C‐SiC nanocrystals: A first‐principle study

The effect of the chemical surface passivation, with hydrogen atoms, on the energy band gap of porous cubic silicon carbide (PSiC) was investigated. The pores are modeled by means of the supercell technique, in which columns of Si and/or C atoms are removed along the [001] direction. Within this supercell model, morphology effects can be analyzed in detail. The electronic band structure is performed using the density functional theory based on the generalized gradient approximation. Two types of pores are studied: C‐rich and Si‐rich pores surface. The enlargement of energy band gap is greater in the C‐rich than Si‐rich pores surface. This supercell model emphasizes the interconnection between 3C‐SiC nanocrystals, delocalizing the electronic states. However, the results show a clear quantum confinement signature, which is contrasted with that of nanowire systems. The calculation shows a significant response to changes in surface passivation with hydrogen. The chemical tuning of the band gap opens the possibility plenty applications in nanotechnology. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2455–2461, 2010     

Impact of Surface Dipole on Resonant Electron Injection in Scanning Tunneling Spectroscopy

Resonant electron injection and first-principles calculations are utilized to study single-adsorbed selenium (Se) atom on a Si(111)-7×7 surface. Theoretical calculations indicate that a negative dipole of 0.61 eV forms toward the adsorbed Se atom due to electron trans-fer from the associated Si atoms. The formation of surface dipole modifies the effective tunneling barrier height and causes a shift in the energy of the resonant state formed in the vacuum gap between the tip and the sample surface. The experimental data imply that an outward negative surface dipole of 0.61 eV causes a resonant electron injection bias shift to high voltage of about 0.45 V.     

Electronic properties of single-walled carbon nanotubes inside cyclic supermolecules

Possible ways for manipulating carbon nanotubes (CNTs) with cyclic supermolecules are studied using density functional theory. Electronic structure calculations with structure optimizations have been performed for the (4,4) and (8,0) single-walled carbon nanotubes (SWNTs) complexed with crown ethers as well as for the (4,0) SWNT with beta-cyclodextrin. A slight polarization of charge in both the nanotube and the supermolecule is observed upon rotaxane complexation, but the interaction is mainly repulsive, and the systems stay 2.8-3.5 A apart. The supermolecule does not affect the electronic band structure of the nanotube significantly within such a configuration. The situation differs noticeably for chemically cross-linked SWNTs and crown ethers, where a peak arises at the Fermi energy in the density of states. As a result, the band gap of semiconducting CNT(8,0) (0.5 eV) vanishes, and a new conduction channel opens for the metallic CNT(4,4).     

Tuning of optical band gaps: syntheses, structures, magnetic properties, and optical properties of CsLnZnSe(3) (Ln = Sm, Tb, Dy, Ho, Er, Tm, Yb, and Y)

Eight new quaternary selenides CsSmZnSe(3), CsTbZnSe(3), CsDyZnSe(3), CsHoZnSe(3,) CsErZnSe(3), CsTmZnSe(3), CsYbZnSe(3), and CsYZnSe(3) have been synthesized with the use of high-temperature solid-state experimental methods. These compounds are isostructural with KZrCuS(3), crystallizing with four formula units in the orthorhombic space group Cmcm. The structure of these CsLnZnSe(3) compounds is composed of [LnZnSe(3)(-)] layers separated by Cs atoms. The Ln atom is octahedrally coordinated by six Se atoms, the Zn atom is tetrahedrally coordinated by four Se atoms, and the Cs atom is coordinated by a bicapped trigonal prism of eight Se atoms. Because there are no Se-Se bonds in the structure, the oxidation state of Cs is 1+, that of Ln is 3+, and that of Zn is 2+. CsYbZnSe(3) exhibits an antiferromagnetic transition at 11 K, whereas CsSmZnSe(3) does not follow a Curie-Weiss law. The remaining rare-earth compounds are paramagnetic, and the calculated effective magnetic moments of the rare-earth ions agree well with their theoretical values. Optical absorption data on face-indexed single crystals of CsSmZnSe(3), CsErZnSe(3), CsYbZnSe(3), and CsYZnSe(3) demonstrate that the optical band gap changes by more than 0.75 eV with the composition and by as much as 0.20 eV with the crystal orientation. The optical band gaps range from 2.63 eV (CsSmZnSe(3), CsErZnSe(3)) to 1.93 eV (CsYbZnSe(3)) for the (010) crystal face and 2.56 eV (CsErZnSe(3)) to 1.88 eV (CsYbZnSe(3)) for the (001) crystal face. The difference in the optical band gap of the (010) face vs the (001) face varies from +0.05 eV (CsYbZnSe(3)) to +0.20 eV (CsSmZnSe(3)).     

Si(3)AlP: A New Promising Material for Solar Cell Absorber

First-principles calculations were performed to study the structural and optoelectronic properties of the newly synthesized nonisovalent and lattice-matched (Si(2))(0.6)(AlP)(0.4) alloy (Watkins, T.; et al. J. Am. Chem. Soc.2011, 133, 16212). We found that the most stable structure of Si(3)AlP is a superlattice along the ?111? direction with separated AlP and Si layers, which has a similar optical absorption spectrum to silicon. The ordered C1c1-Si(3)AlP is found to be the most stable one among all structures with a basic unit of one P atom surrounded by three Si atoms and one Al atom, in agreement with experimental suggestions. (1) We predict that C1c1-Si(3)AlP has good optical properties, i.e., it has a larger fundamental band gap and a smaller direct band gap than Si; thus, it has much higher absorption in the visible light region. The calculated properties of Si(3)AlP suggest that it is a promising candidate for improving the performance of the existing Si-based solar cells. The understanding on the stability and band structure engineering obtained in this study is general and can be applied for future study of other nonisovalent and lattice-matched semiconductor alloys.     

The interface effect on the lithiation of silicon/graphene composites: The first principles study

To reveal the interaction mechanism between lithium (Li) and silicon/graphene (Si/Gra) interface at the atomic scale, it was calculated that the energy band structure, density of states, charge transfer, radial distribution function and Li diffusion coefficient based on the first principles. The results indicated that the volume expansion of Si was effectively limited by the Si/Gra interface during Li insertion. There appeared the interface effect of Si/Gra on the combination of Li and Si atoms, according to the longer Li-C (2.9 Å) and the larger electron cloud near the Li atom at the Si/Gra interface. The better diffusion channel for Li atoms was constructed at the Si/Gra interface, due to the lower diffusion energy barrier (0.42–0.44 eV) and higher diffusion coefficient (D_Li = 0.784 × 10⁻⁴ cm²/s) for Li⁺ diffusion.     

First-principle studies of Ca-X (X=Si,Ge,Sn,Pb) intermetallic compounds

The structural properties, elastic properties, heats of formation, electronic structures, and densities of states of 20 intermetallic compounds in the Ca-X (X=Si, Ge, Sn, Pb) systems have been systematically investigated by using first-principle calculations. Our computational results indicated that with increasing atomic weight of X, the bulk modulus of Ca-X intermetallic compounds decreases gradually. It was also found that Ca₃₆Sn₂₃ and CaPb are mechanically unstable phases. Results on the electronic energy band and densities of states also indicated that Ca₃Si₄ is an indirect band gap semiconductor with a band gap of 0.598 eV, and Ca₂Si, Ca₂Ge, Ca₂Sn, and Ca₂Pb are direct band gap semiconductors with band gaps of 0.324, 0.265, 0.06, and 0.07 eV, respectively. In addition, it is found that the absolute values of heats of formation for all Ca-X intermetallics are larger than 30 kJ/mol atom.     

Li2B12Si2: the first ternary compound in the system Li/B/Si: synthesis, crystal structure, hardness, spectroscopic investigations, and electronic structure

We present the synthesis, crystal structure, hardness, IR/Raman and UV/Vis spectra, and FP-LAPW calculations of the electronic structure of Li(2)B(12)Si(2), the first ternary compound in the system Li/B/Si. Yellow, transparent single crystals were synthesized from the elements in tin as solvent at 1500 degrees C in h-BN crucibles in arc-welded Ta ampoules. Li(2)B(12)Si(2) crystallizes orthorhombic in the space group Cmce (no. 64) with a=6.1060(6), b=10.9794(14), c=8.4050(8) A, and Z=4. The crystal structure is characterized by a covalent network of B(12) icosahedra connected by Si atoms and Li atoms located in interstitial spaces. The structure is closely related to that of MgB(12)Si(2) and fulfils the electron-counting rules of Wade and Longuet-Higgins. Measurements of Vickers (H(V)=20.3 GPa) and Knoop microhardness (H(K)=20.4 GPa) revealed that Li(2)B(12)Si(2) is a hard material. The band gap was determined experimentally and calculated by theoretical means. UV/Vis spectra revealed a band gap of 2.27 eV, with which the calculated value of 2.1 eV agrees well. The IR and Raman spectra show the expected oscillations of icosahedral networks. Theoretical investigations of bonding in this structure were carried out with the FP-LAPW method. The results confirm the applicability of simple electron-counting rules and enable some structural specialties to be explained in more detail.     

C59Si on the monohydride Si(100):H-(2 x 1) surface

We propose the use of the Si atom in the experimentally observed C59Si molecule as a possible way to controllably anchor fullerene molecules on a Si surface, due to the formation of a strong bond to one of the Si surface atoms. All our results are based on ab initio total energy density functional theory, and we obtain that the binding energy is on the order of 2.1 eV, approximately 1.4 eV more stable than a C60 bonded in a similar situation. A possible route to obtain such adsorption via a (C59Si)2 dimer is examined, and we find the whole process to be exothermic by approximately 0.2 eV.     

DFT+U study of defects in bulk rutile TiO(2)

We present a systematic study of electronic gap states in defected titania using our implementation of the Hubbard-U approximation in the grid-based projector-augmented wave density functional theory code, GPAW. The defects considered are Ti interstitials, O vacancies, and H dopants in the rutile phase of bulk titanium dioxide. We find that by applying a sufficiently large value for the Hubbard-U parameter of the Ti 3d states, the excess electrons localize spatially at the Ti sites and appear as states in the band gap. At U=2.5?eV, the position in energy of these gap states are in fair agreement with the experimental observations. In calculations with several excess electrons and U=2.5?eV, all of these end up in gap states that are spatially localized around specific Ti atoms, thus effectively creating one Ti(3+) ion per excess electron. An important result of this investigation is that regardless of which structural defect is the origin of the gap states, at U=2.5?eV, these states are found to have their mean energies within a few hundredths of an eV from 0.94 eV below the conduction band minimum.     

Band alignment and defect states in amorphous GaInZnO thin films grown on SiO2/Si substrates

The band alignment and defect states of GaInZnO thin films grown on SiO₂/Si via radio frequency (RF) magnetron sputtering were investigated by using X‐ray photoelectron spectroscopy, reflection electron energy loss spectroscopy, thermally stimulated exo‐electron emission and photo‐induced current transient spectroscopy.The band gap via reflection electron energy loss spectroscopy was 3.2 eV. The defect states via photo‐induced current transient spectroscopy and thermally stimulated exo‐electron emission were at 0.24, 0.53, 1.69 and 2.01 eV below the conduction band minimum of GIZO thin films, respectively. The defect states at 0.24 and 0.53 eV are related to the field‐effect mobility, and the defect stated at 1.69 and 2.01 eV is related to the oxygen vacancy defect. Copyright © 2016 John Wiley & Sons, Ltd.     

Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer-fullerene solar cells

Recent research advances on conjugated polymers for photovoltaic devices have focused on creating low band gap materials, but a suitable band gap is only one of many performance criteria required for a successful conjugated polymer. This work focuses on the design of two medium band gap (~2.0 eV) copolymers for use in photovoltaic cells which are designed to possess a high hole mobility and low highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. The resulting fluorinated polymer PBnDT-FTAZ exhibits efficiencies above 7% when blended with [6,6]-phenyl C(61)-butyric acid methyl ester in a typical bulk heterojunction, and efficiencies above 6% are still maintained at an active layer thicknesses of 1 μm. PBnDT-FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells. PBnDT-FTAZ also highlights other performance criteria which contribute to high photovoltaic efficiency, besides a low band gap.     

Microscopic Kinetics in Poly(Methyl Methacrylate) Exposed to a Single Ultra-Short XUV/X-ray Laser Pulse

We study the behavior of poly(methyl methacrylate) (PMMA) exposed to femtosecond pulses of extreme ultraviolet and X-ray laser radiation in the single-shot damage regime. The employed microscopic simulation traces induced electron cascades, the thermal energy exchange of electrons with atoms, nonthermal modification of the interatomic potential, and a triggered atomic response. We identify that the nonthermal hydrogen decoupling triggers ultrafast fragmentation of PMMA strains at the absorbed threshold dose of ~0.07 eV/atom. At higher doses, more hydrogen atoms detach from their parental molecules, which, at the dose of ~0.5 eV/atom, leads to a complete separation of hydrogens from carbon and oxygen atoms and fragmentation of MMA molecules. At the dose of ~0.7 eV/atom, the band gap completely collapses indicating that a metallic liquid is formed with complete atomic disorder. An estimated single-shot ablation threshold and a crater depth as functions of fluence agree well with the experimental data collected.