Dangling-bond Band Research Articles

On the spectral features of dangling bonds in CH4/H2O amorphous ice mixtures.

Dangling bond (DB) bands in IR spectra, above 3600 cm-1, are a source of information on structural properties of amorphous water ice, and especially on ice mixtures of water and other frozen gases. We deal in this paper with the spectroscopic behavior of DB bands of CH4/H2O mixtures. We use ab initio methodology to predict theoretical results which are compared with experimental results. Our model mixtures are created by inserting a variable number of molecules of either species into a cell of appropriate size to reach an initial density of 1 g cm-3, which can be modified by including an empty space at the top, to simulate pores. The cell is taken as a unit cell for a solid state calculation The structure of the mixture is optimized and the IR spectrum is calculated for the converged geometry. We find two different kinds of dangling bonds, in which the O-H stretching responsible for this mode is directed either to an empty space of a pore or towards a nearby CH4 molecule, with which some interaction takes place. The spectral characteristics of these two DB types are clearly different, and follow satisfactorily the pattern observed in experimental spectra. Estimated band strengths for these DB bands are given for the first time.

Modulation of the electronic and magnetic properties of a GaN nanoribbon from dangling bonds

Though GaN nanoribbons (GaNNRs) with H atoms terminating both edges are nonmagnetic semiconductors, the extra dangling bond bands around the Fermi level lead to a transition from semiconducting to metallic, except for the armchair edge GaNNRs (AGaNNRs) with bare N and Ga edges, which are still nonmagnetic semiconductors due to the strong coupling of the dangling bonds of dimeric N and Ga atoms at the same edge. The larger difference in the charge density (ρup-ρdown) for edge bare N atoms and decaying for N sub-lattices away from the edge, as well as the smaller difference in the charge density for edge bare Ga atoms and without decaying for Ga sub-lattices away from the edge is consistent with the magnetic moment of a GaNNR with bare N edge being larger than that of a GaNNR with bare Ga edge. The magnetic moment of a zigzag edge GaNNR (ZGaNNR) with bare N (Ga) edge has nearly half the value of the magnetic moment of a AGaNNR with bare N (Ga) edge. Such a relationship also exists in the number of extra dangling bond states appearing around the Fermi level in the band structures. For ZGaNNRs, the magnetic moment of bare N and Ga edges is larger than either bare N edge or bare Ga edge, but smaller than their sum, implying that there exists an interaction between the dangling bonds at both edges of bare N and Ga edges.

First-principles of wurtzite ZnO (0001) and (0001̄) surface structures

The surface structures of wurtzite ZnO (0001) and (0001̄) surfaces are investigated by using a first-principlescalculation of plane wave ultra-soft pseudo-potential technology based on density functional theory (DFT). The calculatedresults reveal that the surface energy of ZnO–Zn is bigger than that of ZnO–O, and the ZnO–Zn surface ismore unstable and active. These two surfaces are apt to relax inward, but the contractions of the ZnO–Zn surface aresmaller than the ZnO–O surface. Due to the dispersed Zn4s states and the states of stronger hybridization between theZn and O atoms, the ZnO–Zn surface shows n-type conduction, while the O2p dangling-bond bands in the upper partof the valence cause the ZnO–O surface to have p-type conduction. The above results are broadly consistent with theexperimental results.

Interaction of CH4 and H2O in ice mixtures

Ice mixtures of methane and water are investigated by means of IR spectroscopy in the 14-60 K range. The spectroscopic research is focused on the symmetry-forbidden nu(1) band of CH(4) and the dangling bond bands of water. The nu(1) band is visible in the spectra of the mixtures, revealing a distorted methane structure which co-exists with the normal crystalline methane. The water dangling bond bands are found to increase their intensity and appear at red-shifted frequency when distorted methane is present. Methane adsorbed on water micropores or trapped inside the amorphous solid water structure is assumed to be responsible for these effects. CH(4) mobility in water ice depends on the deposition method used to prepare the samples and on the temperature. After warming the samples to 60 K, above the methane sublimation point, a fraction of CH(4) is retained in the water ice. An adsorption isotherm analysis is performed yielding the estimation of the desorption energy of CH(4) on H(2)O amorphous surfaces.

Structure and Electronic Properties of Saturated and Unsaturated Gallium Nitride Nanotubes

The atomic and electronic structures of saturated and unsaturated GaN nanotubes along the [001] direction with (100) lateral facets are studied using first-principles calculations. Atomic relaxation of nanotubes shows that appreciable distortion occurs in the unsaturated nanotubes. All the nanotubes considered, including saturated and unsaturated ones, exhibit semiconducting, with a direct band gap. Surface states arisen from the 3-fold-coordinated N and Ga atoms at the lateral facets exist inside the bulklike band gap. When the nanotubes are saturated with hydrogen, these dangling bond bands are removed from the band gap, but the band gap decreases with increasing the wall thickness of the nanotubes.

Vibrational properties of Na nanoclusters self-assembled on Si(1 1 1) surface

We have investigated structural and vibrational properties of Na nanoclusters self-assembled on the Si(1 1 1)- 7 × 7 surface at room temperature mainly using high-resolution electron-energy-loss spectroscopy. We observe three characteristic loss peaks L 1, L 2, and L 3 ascribed to an interband transition, a local atomic vibration, and another interband transition from an Na nanocluster-induced state, respectively. The spectral change of L 1 with Na coverage θ suggests that the S 1 dangling bond band is gradually filled up to open a band gap with increasing θ up to 1.1 eV when all three loss peaks completely disappeared. The relatively high loss energy E l = 243 meV of L 2 with a narrow linewidth of 32 meV indicates the only Na–Si atomic vibrational mode with Na atoms occupying the tilted on-top sites above Si rest atoms. Furthermore the extremely weak loss peak L 3 visible at a coverage range unique only to the Na nanoclusters proves the presence of a Na nanoclusters-induced electronic state N 2. These observations provide explanation to most unresolved spectral behavior of earlier photoemission study and evidence for the atomic structure of the Na nanocluster.

Formation of compact solid water after ion irradiation at 15 K

We used infrared absorption spectroscopy to study the effects of ion irradiation on the morphology/porosity of amorphous water ice. Thin icy films (about 0.25 μm) of amorphous water were irradiated with 200 keV protons at 15 K. Both the behaviour of the OH dangling bond feature and the ability to trap carbon monoxide (CO) were used to investigate the evolution of icy samples after ion irradiation. We show that the intensity of the OH dangling bond feature decreases after ion irradiation and that the amount of absorbed carbon monoxide decreases as the fluence of impinging ions increases. The results obtained indicate that the porosity of amorphous water ice decreases after ion irradiation. Furthermore, icy mixtures such as H 2 O:CO 2 , H 2 O:CO, and H 2 O:CH 4 were irradiated with 200 keV H + , 30 and 200 keV He + ions. Also in these cases, the intensity of the OH dangling bond band decreases after ion irradiation. However, when a second molecular species is present in the ice sample, this decrease is slower. Here we present the experimental results and discuss their relevance to our understanding of the properties of interstellar water ice. In particular, we suggest that, because of cosmic ion bombardment, water ice in interstellar grain mantles is compact in structure.

Ultrafast carrier dynamics inSi(111)7×7dangling bonds probed by time-resolved second-harmonic generation and two-photon photoemission

By combining time-resolved two-photon photoemission (2PPE) and optical second-harmonic generation (SHG), the dynamics of surface electronic states of $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ has been studied. In the experiment the normally unoccupied part of the adatom dangling-bond bands are populated by $1.55\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ pump pulses. The 2PPE results show very fast relaxation within these bands with lifetimes ranging between 40 and $150\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ depending on energy relative to the Fermi level. The regime of high excitation densities was investigated by means of time-resolved SHG. With increasing pump fluence, we observed filling of the surface bands and a concomitant increase of the optical relaxation times from 0.5--$2\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. The results are interpreted in terms of electron-electron scattering within a two-dimensional metallic system that is isolated from the bulk semiconductor.

Correlation effects at idealSiC{0001}−(1×1)surfaces

We report on studies of the surface band structure of clean and unreconstructed $6H\text{\ensuremath{-}}\mathrm{SiC}{0001}\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ surfaces by angle-resolved photoelectron spectroscopy (ARPES). Using light induced desorption of hydrogen from the H-terminated, unreconstructed $6H\text{\ensuremath{-}}\mathrm{SiC}{0001}$ surfaces we were able to prepare these highly metastable surfaces. On both Si face and C face we observe a surface band with $(1\ifmmode\times\else\texttimes\fi{}1)$ periodicity derived from the unsaturated surface dangling bonds. In both cases the surface band is located below the Fermi level indicating a semiconducting surface. On the (0001) surface the Si dangling bond band is located $\ensuremath{\approx}0.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the valence-band maximum (VBM), whereas the C dangling bond band is located $\ensuremath{\approx}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the VBM. The dispersion of the surface bands amount to 0.2 and 0.7 eV, respectively. The experimental observations are discussed in the light of earlier theoretical studies on the surface electronic structure of hexagonal SiC surfaces. It is suggested that the electronic properties of these surfaces are governed by strong correlation effects.

Angle-resolved photoemission from stoichiometric GaN(0 0 0 1)-1 × 1

We present results from an angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) investigation using synchrotron radiation of a stoichiometric Ga-polar GaN(0001)-1×1 sample grown by metal-organic chemical vapour deposition (MOCVD). The electronic structure of the surface was investigated with ARUPS along the Γ¯–K¯–M¯, Γ¯–M¯, and Γ–A directions of the main symmetry lines of the surface and bulk Brillouin zones. We find one surface state close to the valence band maximum (VBM) at Γ, dispersing down and emerging into the projected band gap in the region around K¯. From a comparison with a density functional theory (DFT) calculation [F.-H. Wang, P. Krüger, J. Pollmann, Phys. Rev. B 64 (2001) 035305-1], three additional surface related features are proposed, but no strongly dispersing Ga dangling bond band is observed. We find the VBM at 3.45eV below the Fermi energy.

Angle-resolved photoemission study of the valence band structure of ZnO()

The valence band structure of the () surface of wurtzite ZnO has been investigated by angle-resolved photoelectronspectroscopy utilizing polarized synchrotron radiation. The energetic position and thedispersion width of the O 2p dangling-bond (DB) state are determined along the highsymmetry axes of the surface Brillouin zone (SBZ). The O 2p DB is 3.7 eV below the point and disperses to higher binding energies with a total dispersion width of eV. Comparison with the projected bulk bands from the earlier theoretical studies revealsthat the DB state is a surface resonance in the whole SBZ. Polarization-dependentmeasurements have also been carried out along the axis suggest that the DB band with odd symmetry with respect to the mirror plane ofthe crystal surface has a larger dispersion than the band with even symmetry.

Electronic structure and electron dynamics at Si(100)

The electronic structure and electron dynamics at a Si(100) surface is studied by two-photon photoemission (2PPE). At 90 K the occupied Dup dangling-bond state is located 150±50 meV below the valence-band maximum (VBM) at the center of the surface Brillouin zone Γ and exhibits an effective hole mass of (0.5±0.15)me. The unoccupied Ddown band has a local minimum at Γ at 650±50 meV above the VBM and shows strong dispersion along the dimer rows of the c(4×2) reconstructed surface. At 300 K the Ddown position shifts comparable to the Si conduction-band minimum by 40 meV to lower energies but the dispersion of the dangling-bond states is independent of temperature. The surface band bending for p-doped silicon is less than 30 meV, while acceptor-type defects cause significant and preparation-dependent band bending on n-doped samples. 2PPE spectra of Si(100) are dominated by interband transitions between the occupied and unoccupied surface states and emission out of transiently and permanently charged surface defects. Including electron–hole interaction in many-body calculations of the quasi-particle band structure leads us to assign a dangling-bond split-off state to a quasi-one-dimensional surface exciton with a binding energy of 130 meV. Electrons resonantly excited to the unoccupied Ddown dangling-bond band with an excess energy of about 350 meV need 1.5±0.2 ps to scatter via phonon emission to the band bottom at Γ and relax within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds.

Valence band structure of the ZnO[formula omitted] surface studied by angle-resolved photoemission spectroscopy

The electronic band structure of the ZnO(1 0 1¯ 0) surface has been studied by angle-resolved photoemission spectroscopy utilizing synchrotron radiation. Photon-energy-dependent measurements and K- and O2-adsorption studies revealed that the O 2p dangling-bond state exists at 3.7eV below the Fermi level at the Γ¯ point. Polarization-dependent measurements show that the state has dominant contribution of the O 2px orbital (x is parallel to the (1 2¯ 1 0) direction) at the Γ¯ point. It is found that the O 2p dangling-bond band lies within the projected bulk bands along the ΓX¯ and ΓX'¯ axes in the surface Brillouin zone. This result settles a controversial issue on the energetic position of the O 2p dangling-bond band, which has been in dispute among theoretical studies. The dispersion widths are found to be 0.8 and 0.5eV along the ΓX¯ and ΓX'¯ directions, respectively.

Electron correlation effects on SiC(111) and SiC(0001) surfaces

The possibility is discussed that, independent of the polytype, large electronic correlationeffects give rise to semiconducting SiC surfaces with [111] or [0001] orientations. The mostimportant surface translational symmetries and 3 × 3 are considered. The discussion is based on the detailed knowledge of the surfaceatomic geometries and the electronic structures derived for these geometries usinga local density approximation for exchange and correlation. It is argued thatthe resulting half-filled, weakly dispersive dangling-bond bands within thefundamental gaps are split according to a Hubbard interaction parameterU, and that the surface systems undergo a Mott–Hubbard transition. Such a physical pictureprovides a qualitatively correct account of the single-particle excitation spectra eitherinferred from angle-resolved photoemission and inverse photoemission experiments orobtained from scanning tunnelling spectroscopy. The gap opening and hence the HubbardU parameter are discussed in terms of their dependence on the surface reconstruction as wellas the SiC polytype.

Angle-resolved photoelectron spectroscopy study of the anion-derived dangling-bond band onZnO(101¯0)

The band structure of surface electronic states on the $\mathrm{ZnO}(101\ifmmode\bar\else\textasciimacron\fi{}0)$ surface has been investigated by angle-resolved photoelectron spectroscopy utilizing synchrotron radiation. Photon-energy dependent measurement and the K and ${\mathrm{O}}_{2}$ adsorption studies have been carried out to identify the surface-localized O $2p$ dangling-bond state. It is found that the state exits at 3.7 eV below the Fermi level at the $\overline{\ensuremath{\Gamma}}$ point in the surface Brillouin zone and shifts to the higher binding-energy side by 0.8 and 0.5 eV along the $\overline{\ensuremath{\Gamma}X}$ and $\overline{\ensuremath{\Gamma}{X}^{\ensuremath{'}}}$ axes, respectively. The O 2p dangling-bond band is found to locate below the upper edge of the projected bulk bands along these two high-symmetry axes. The present study settles a controversial issue on the energetic position of the O 2p dangling-bond band, which has been in dispute among theoretical studies.

Doping into one-dimensional dangling-bond bands of natural quantum-wire-like Ga 2Se 3 semiconductors

The doping properties of Ga 2Se 3 are investigated by calculating the formation energies and electronic structures, based on the first-principles pseudopotential total energy method in the local density approximation. It is shown that doped V(IV)-family atoms have the tendency to locate at Se(Ga) sites and P(Si) atom becomes the suitable p(n)-type dopant among various V(IV)-family atoms. One can expect that P doping imports holes into the valence-band top and induces the one-dimensional conductive property for Ga 2Se 3 semiconductor.

Atomic and electronic structure of silicate adlayers on polar hexagonal SiC surfaces

Structural and electronic properties of silicate adlayers on $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$-reconstructed C-terminated $(0001\ifmmode\bar\else\textasciimacron\fi{})$ and Si-terminated (0001) surfaces of hexagonal $6H\ensuremath{-}\mathrm{SiC}$ have been studied using the ab initio pseudopotential supercell method. Two significantly different structural models, previously suggested on the basis of a quantitative low-energy electron diffraction (LEED) analysis for the two adsorbate systems, have been investigated. Both of these models have been considered for both surfaces and the four respective structures have been optimized by total energy minimization calculations. The two structures with the lowest formation energy confirm the interpretation of the LEED data. In addition, they allow us to address the physical origin of the distinctly different reconstruction models for the two surfaces. The electronic structure of these surfaces according to local density approximation calculations is presented and discussed. Both models yield a number of oxygen-induced bound states and resonances within the projected valence band region and a band of localized dangling-bond states within the projected gap. Within the local density approximation, this dangling-bond band turns out to be half-filled in both cases, giving rise to metallic surfaces in contradiction to experiment. Therefore, the systems have also been studied within the framework of the Hubbard model and by employing the local-spin-density approximation. In both cases semiconducting surfaces are obtained in agreement with experiment. The dangling-bond bands resulting within the Hubbard-model calculation, in particular, are in quantitative agreement with most recent angle-resolved photoemission spectroscopy data for a ${\mathrm{Si}}_{2}{\mathrm{O}}_{3}$ silicate adlayer on $\mathrm{SiC}(0001\ifmmode\bar\else\textasciimacron\fi{}).$

In situoptical spectroscopy of Ga dimers on GaP, GaAs, and GaSb by surface chemical modulation

The optical characterization of the surface anisotropy of three Ga-based semiconductor compounds using an in situ technique based on the chemical modulation of the surface is described. In this technique, the anisotropic optical reflectivity is modulated by a periodic variation of the surface stoichiometry using valved pulsed cells for group-V elements (As, P, Sb). The substrate is maintained at sufficiently high temperature in order to obtain rapid desorption of group-V molecules from the surface during flux interruptions. Linearly polarized light, reflected at near normal incidence by the sample, is collected separately along one of the two principal axes of the crystal, [110] and [11\ifmmode\bar\else\textasciimacron\fi{}0]. The change in the surface coverage induces a change in the intensity of the reflected light, and the normalized variation $\ensuremath{\Delta}R/R$ is recorded as a function of wavelength. Spectra for (001) surfaces in the 1--3 eV range have been obtained with this method for a set of Ga-based binary III-V compounds GaP, GaAs, and GaSb, showing well-defined features for light polarized along the [110] direction, parallel to Ga dimers. These observed maxima are attributed to transitions between the occupied Ga dimer and the unoccupied dangling bond bands, at characteristic energies in each material.

In situobservation of surface optical anisotropy on InP, InAs, and InSb by chemical modulation spectroscopy

In situ observation of surface optical anisotropy due to In dimers in a set of three In-based semiconductor binary compounds\char22{}InP, InAs, and InSb\char22{}grown by molecular-beam epitaxy is reported. We used an optical reflection technique based on the chemical modulation of the surface, that permits the measurement with light linearly polarized in one selected polarization, usually [110] or [11\ifmmode\bar\else\textasciimacron\fi{}0], typically parallel to the group-III and -V dimers. Spectra for (001) surfaces in the 1\char21{}3-eV range were obtained through this technique, and the results are compared to those previously obtained for a set of Ga-based binary compounds, GaP, GaAs and GaSb. For both sets, well-defined features for light polarized along [110] have been observed, that are attributed to transitions between the occupied group-III dimer and the unoccupied dangling-bond bands.

Temperature dependent photoemission studies of Si(100)2 × 1

We have studied the clean Si(100)2 × 1 surface with angle-resolved direct and inverse photoemission spectroscopy (ARUPS and KRIPES) as a function of temperature in the range 300 < T < 1215 K. We note four effects: (i) The dangling bond surface-state emission observed in ARUPS around −0.8 eV below the Fermi level ( E F) in normal emission gradually loses intensity as a function of temperature due to broadening, however, it is clearly discernible at 1215 K and appears at the same energy position (within 0.1 eV). We take this observation as evidence for the existence of asymmetric dimers over the whole temperature range studied. The reason for the observed broadening with temperature could be due either to an increasing buckle-flip frequency or to an increasing influence of electron-phonon interaction. (ii) The emission at the border of the surface Brillouin zone (SBZ) from a feature around −1.0 eV and the small emission feature at the Fermi level in ARUPS in normal emission, which had been observed to be significant on highly-doped two-domain Si(100)2 × 1 surfaces but less pronounced on single-domain surfaces, vanish completely for T > 450 K. (iii) The dangling bond surface-state emission seen in KRIPES around +0.35 eV above E F in normal emission has completely disappeared above 450 K. (iv) The dangling bond band in KRIPES, well described by models based on asymmetric dimers, is not affected by the temperature increase.