Energy Shift Of Band Research Articles

Anomalous strain dependent effective masses in (111) Si nanowires

First-principles methods are employed to investigate the effect of strain on the carrier effective masses in (111) Si nanowires (SiNWs). It is found that the electron effective masses of SiNWs depend strongly on the applied axial strain. Within a certain range of strain, the electron effective masses are significantly enhanced. Most remarkably, the electron effective masses are anomalous and become infinity at a critical point of strain. This effect is associated with how strain changes the band structure with a single minimum to that with double minima or vice versa. The hole effective masses, on the other hand, decrease with the increase of the tensile strain, as a result of the strain-induced band energy shift in the vicinity of the valence band maximum. It is also shown that the band gaps of SiNWs with diameters ⩾1.16nm increase with the increase of the tensile strain. The tunability of the effective masses and band gaps with strain would be useful in tailoring the transport properties of SiNWs.

Effect of quantum confinement on higher transitions in HgTe nanocrystals

Spectroscopic ellipsometry measurements on 1–10 bilayers of HgTe nanocrystals and polymer were performed in the spectral range from 0.5to5eV. The experiments show that the band model, respectively, band energy shifts due to quantization, can be applied onto nanocrystals with as few as five unit cells in diameter. These nanocrystals exhibit strong transitions at higher critical points. The critical point energies shift up to 0.4eV. It turns out that transitions between bands which are closer to the Fermi energy and have a smaller carrier mass are more strongly affected by quantum confinement.

Piezoelectric Photothermal and Photoreflectance Spectra of InxGa1-xN Grown by Radio-Frequency Molecular Beam Epitaxy

Piezoelectric photothermal spectroscopy (PPTS) measurements were carried out on InxGa1-xN (x=0.01–0.32) thin films grown by radio-frequency molecular beam epitaxy. We found that the band energy shifts to the lower energy side of the spectrum (red shift) with an increase in the indium composition from 0.01 to 0.32. For samples with a lower indium composition, we were able to observe the exciton contribution, and the binding energy was estimated to be 27 meV (x=0.01). Since conventional photoreflectance (PR) spectroscopy was unable to observe signals for the samples with a higher indium content (x=0.13, 0.2, and 0.32), the usefulness of this PPTS method for samples with phase fluctuation is demonstrated.

Influence of annealing temperature on the band structure of sol-gel Ba0.65Sr0.35TiO3 thin films on n-type Si(100)

The annealing temperature effect on the band structure of Ba0.65Sr0.35TiO3 (BST) thin films coated on n-type Si(100) substrate was investigated by ellipsometry and x-ray photoelectron spectroscopy. The band energy shifts of sol-gel BST films annealed at different temperatures are dependent on their developed microstructure. Related to the amorphous BST films annealed at 600°C, the polycrystalline BST film annealed at 700°C exhibits narrow band gap, upwards-moved Fermi level, and downwards-moved conduction band minimum, which are believed to contribute the enhanced field emission of BST-coated Si field emitter arrays.

Initial interface study of Au deposition on GaN(0 0 0 1)

Synchrotron radiation photoelectron spectroscopy (SRPES) has been used to study the electronic structure of the Au/GaN(0 0 0 1) system at the initial growth stage. The peak fitting of Au4f 7/2 core-level and the energy shift of valence band indicate that Au–Ga alloy were formed in the interface reaction. According to the Ga3d signal intensity attenuation vs. the gold film thickness, the early growth mode is considered to be 3D mode above the reaction layer. By using the Linear Augmented Plane Wave method the density of states (DOS) for GaN and Au bulk are calculated within the framework of local functional theory. The theoretical results agree with the valence band structure quite well. The mechanism of interface reaction is discussed based on the experimental and theoretical results.

Control of injected carriers in tetracyano-p-quinodimethane encapsulated carbon nanotube transistors

We examined transistor characteristics of tetracyano-p-quinodimethane encapsulated single-walled carbon nanotubes (TCNQ@SWNTs). In device operations, a clear conversion to a p-type character was observed and the stability of carriers, previously doped into SWNTs, were simultaneously clarified. Because of an energy band shift, between the electrodes and the doped SWNTs induced by the doping, electron injection was achieved only by application of a high source-drain voltage, while holes were easily injected because of decrease in hole barrier height.

Ion stopping cross-section at ferro-paramagnetic phase transition

The influence of temperature on the electronic stopping cross-section for ions in metal foils suffering ferro-paramagnetic phase transition was analysed theoretically within the Bethe theory of stopping combined with the Stoner model for band ferromagnetism. The increase in the cross-section at the Curie temperature was found to depend on energy band shift and on the number of electrons carrying through magnetic interaction. The calculations were compared with experimental data for the slowing down of protons in Ni foil and it was shown that the measured ca.2% increase in cross-section can be approximately explained within the presented model.

Synchrotron radiation study on Au/GaN(0001) interface with low coverage

Synchrotron radiation photoemission spectroscopy (SRPES) is used to study the in itial growth mode of the gold deposition on the surface of GaN, the Schottky bar rier height (SBH) and the electronic structure at the interface of the Au/GaN(00 01) system. The results show that at the initial stage chemical reaction exists between the Au and GaN substrate. Over the reaction layer, the growth mode of Au deposition is 3D island mode. The SBH is examined by the SRPES and the result i s 14eV, which is consistent with other experiment reports. Analyzing the ene rgy shift of valence band and the Au core level, the interface chemical reaction is confirmed. The theoretical calculation by linear augmented plane wave method gives the density of states. According to the calculation results, the principl e of the interface reaction is discussed.

Interwell excitons in a lateral potential well in an inhomogeneous electric field

The luminescence of interwell excitons in double quantum wells based on GaAs/AlGaAs semiconductor heterostructures (n-i-n structures) in a lateral trap prepared with the use of an inhomogeneous electric field was studied at helium temperatures. A rather strong and inhomogeneous electric field occurred in the depth of the heterostructure when a current passed through the contact between the conducting tip of a tunneling microscope and the heterostructure surface to the bulk region containing a built-in gate. Because of the Stark shift of energy bands in the electric field, the photoexcited electrons and holes are spatially separated in neighboring quantum wells by a tunnel-transparent barrier and are bound into interwell quasi-two-dimensional excitons. These excitons have a dipole moment even in the ground state. Therefore, electrostatic forces in the inhomogeneous electric field cause the excitons to move in the plane of quantum wells toward the maximum field region and eventually accumulate in the lateral trap artificially prepared in such a way. The maximum trap depth achieved through the inhomogeneous electric field was 13.5 meV, and its lateral size was about 10 μm. It is shown that, in the traps prepared in this way, photoexcited interwell excitons behave with increasing concentration at sufficiently low temperatures (T=2K) in the same fashion as in the lateral traps caused by large-scale fluctuations of the random potential. At concentrations exceeding the percolation threshold, the interwell excitons condense into the lowest energy state in the trap.

Density functional study of the `titanium effect' at metal-ceramic interfaces

A thin interlayer of an early transition metal, e.g. titanium, can enhance the bonding at a metal-ceramic interface considerably. The role of the electronic structure in this phenomenon (the `titanium effect') is studied quantitatively by means of density functional calculations, employing norm-conserving ab initio pseudopotentials and a mixed basis of localized functions and plane waves. The weakly bonding model interface Ag(100)/MgAl2O4(100) is chosen, in order to minimize the lattice mismatch and to concentrate on electronic interactions. The stepwise addition of Ti atoms at the interface from zero to one monolayer of Ti leads to a pronounced enhancement of the bonding strength (Ag/Ti/MgAl2O4). From a comparison to the more strongly bonding systems Ag/Al/MgAl2O4 with a monatomic Al interlayer and Al/MgAl2O4 it is concluded that this enhancement correlates with the reduction of the electron density in the interface layer. It is proposed that the interlayer accommodates Pauli repulsion between O anions and electron-rich Ag atoms by suitable electron redistribution, which is accompanied by pronounced energy band shifts.

Ruthenium-(II)/-(III) terpyridine complexes incorporating imine functionalities. Synthesis, structure, spectroscopic and electrochemical properties †

A new class of ruthenium terpyridine complexes of the type [RuII(trpy)(L1–6)Cl] 1–6 (trpy = 2, 2′∶6′, 2′′-terpyridine; L1–3 = o-−OC6H3(R)C(R′)NCH2C6H5 and L4–6 = o-−OC6H3(R)CH(R′)NNC6H5; where R = H or p-NO2 and R′ = H or CH3) have been synthesized. The “free” ligands incorporating a NH spacer o-−OC6H3(R)C(R′)NNHC6H5 (HL4–6) have undergone imine to azo tautomerism on co-ordination to the ruthenium terpyridine moiety in the complexes 4–6, whereas those having a CH2 spacer (HL1–3) remain unaltered on co-ordination. The diamagnetic, neutral complexes 1–6 exhibit strong MLCT transitions in the visible region and intraligand transitions in the UV region. A significant shift in MLCT band energy has been observed depending on the ligand field strength of the co-ordinated L. The complexes display a reversible ruthenium(III)–ruthenium(II) couple in the potential range of 0.12–0.63 V and a quasi-reversible ruthenium(IV)–ruthenium(III) couple in the range of 1.21–1.85 V versus SCE. The higher ligand field strength of the co-ordinated L4–6 compared to the co-ordinated L1–3 is reflected in the observed metal redox processes. The reduction of the co-ordinated terpyridine has been observed near −1.3 V. The complexes exhibit moderately strong emissions from the lowest energy MLCT bands in the range 661–690 nm in EtOH–MeOH (4∶1 v/v) at 77 K. The quantum yields of the complexes (Φ = 0.006–0.09) are found to be reasonably sensitive to the nature of the co-ordinated L. The oxidised complexes 3+ and 6+ have been isolated in the solid state as their perchlorate salts. The crystal structure of 3+ exhibits pseudo-octahedral trans geometry with regard to the relative disposition of the imine nitrogen (N4) of L3 and the central pyridyl group of the trpy ligand. The one-electron paramagnetic complexes show 1∶1 conductivity and display ligand-to-metal charge transfer bands near 600 and 400 nm and intraligand transitions in the UV region. The observed rhombic EPR spectra at 77 K corresponding to the distorted octahedral geometry have been analysed to furnish values of axial (Δ) and rhombic (V) distortion parameters as well as the energies of the two expected ligand field transitions (ν1 and ν2) within the t2 shell.

Electronic structure of Au-Ag bimetallics: Surface alloying on Ru(001)

We report a study of the electronic structure of Au-Ag overlayers (total coverage varies from 2 to 9 ML) supported on Ru(001). Photoemission spectroscopy with synchrotron radiation and a Mg $K\ensuremath{\alpha}$ x-ray source has been used to study the valence band and the Au $4f$ and Ag $3d$ core levels of these systems. It is found that, in most cases, two-dimensional alloy formation occurs upon deposition at room temperature. The extent of surface alloying depends on the order of deposition, composition, thickness, and annealing temperature. By comparing the valence band and binding energy shifts for bulk and surface alloys, we found that both systems exhibit the same trend: that is, that relative to the d band of pure Au, the alloy d band narrows and the Au $4f$ shifts to higher binding energy upon decreasing Au concentration. However, the surface alloy d band is consistently narrower than that of the bulk with the same stoichiometry. Similar surface bulk behavior is also seen in the Au $4f$ binding energy shift and the Au d-band doublets. This behavior is attributed to the two-dimensional nature (reduction in coordination number of like nearest neighbors on average) of the surface alloy.

Spectroscopic characterization of sol–gel silica obtained by electron irradiation

In the present work, the gelation of tetraethoxysilane (TEOS) was carried out using electron irradiation. The electron source used was a 2 MeV Van de Graaff electron accelerator. Two samples were gelled; for the first one (SiO 2-1), a solution of TEOS was irradiated with a total dose of 324 kGy. Afterwards, NH 4OH was added, and the gelation occurred instantaneously. The second sample (SiO 2-2), was prepared using a solution of TEOS, but in this case the ammonium hydroxide was added to the solution before irradiation. FTIR spectroscopy results show a significant energy band shift for the pre-hydroxylized TEOS irradiated sample. The presence of room temperature stable OH − species was also established by EPR techniques in the SiO 2-2 irradiated sample. An unusually high acidity was observed in both samples.

X-ray absorption spectroscopy on light emitting porous silicon by XEOL and TEY

This paper presents an experimental investigation of the local structure of porous silicon based on X-ray absorption fine structure by two different techniques: Total electron yield (TEY) and X-ray excited optical luminescence (XEOL). The influence of the main parameters of sample preparation on both the photoluminescence and the local structure around Si atoms has been investigated. The dependence of the optical emission energy on the current density shows a saturation limit. The analysis of both the optical emission and the short range order (up to the third coordination shell) confirms the presence of crystalline cores on a scale of a few nanometers in freshly prepared porous silicon samples. Although the local structural parameters obtained by TEY are similar to those reported in recent X-ray absorption studies, a direct connection between optical emission energy and short range order has not always been found. In particular, while the role of HF concentration in determining optical and microstructural properties is clear, a change of current density affects the energy shift of the X-ray excited optical band but not the short range order properties monitored by TEY.

Structure of Densified Lithium Silicate Glasses.

Structure analysis has been performed on permanently densified lithium silicate glasses by means of Raman scattering and ultraviolet (uv) reflection spectra. Higher energy shifts of Raman bands at around 450 and 580 cm-1 and lower energy shift of first uv reflection band were found after densification. It is deduced that these shifts are due to the decrease of Si-O-Si bond angle and the increase of Si-O bond length. The decrease of the magnitude in densification and these band shifts with lithium concentration might be due to obstruction of the structural change by lithium ions.

Coincidence in angular-resolved VLEED spectra: Brillouin zones, atomic shifts and energy bands

It is realised that sharp peaks on the VLEED spectrum come from reflections from band-gap at boundaries of Brillouin zones. Constructing Brillouin zones from the O-Cu(001) angular-resolved VLEED spectra yields information about energy band and lateral atomic shift. The derived information coincides with known quantities detected by other means such as UPS and LEED.

Coupling effect of surface vibration and quantum confinement carriers in porous silicon

This paper deals with the structural characterization and the optical properties of deuterium terminated porous silicon (D-PS) fabricated by electrochemical anodization with DF-ethanol-D6 isotope electrolyte. Both the deuterium terminated and the usual hydrogen terminated porous silicons (H-PS) were characterized with transmission electron microscope analysis and Fourier transform infrared spectroscopy. These characterizations show that the nanometer crystalline structure of D-PS is almost the same as that of H-PS except for the surface termination. Although both PS show the same valence band energy shift, which was confirmed by X-ray photoemission spectroscopy, the photoluminescence spectrum of D-PS was different from that of H-PS. These results indicate that the surface vibration of terminated atoms couples to the excited energy states of quantum-confined carriers, which induces the surface-trapped states of Si nanocrystals.

Optical properties of ultrathin PbI 2 microcrystallite in polymer

We have prepared PbI 2 microcrystallites embedded in polymer, which have a layer structure and are ultrathin crystals consisting of two to nine monolayers. A very large energy shift of the exciton absorption band has been observed in these microcrystallites and interpreted in terms of size confinement of the translational motion of excitons in the c-direction perpendicular to the crystal surface. The simple effective mass approximation is broken down in 2, 3, 4 layer crystallites, because the crystal thickness is smaller compared to the exciton Bohr radius. Secondly, in the Raman spectrum where the excitation energy is resonant to the exciton energy, there appears a new line in the energy region below 20cm −1 which is characteristic of a ultrathin crystal. The Raman shift increases with decreasing the crystal thickness. This line is assigned as due to a longitudinal mode of a rigid-layer phonon. Thirdly, the confinement of the internal motion is studied by measuring a diamagnetic shift of the exciton energy in the magnetic field up to 150 T and a bound exciton luminescence in the ultrathin microcrystallites. Much larger binding energy of the exciton compared to the bulk value is estimated. This fact suggests that the envelope function of the exciton shrinks not only by a strong spacial restriction in the c-direction but also by the dielectric screening from the surrounding polymer. Fourthly, the exciton-phonon interaction is studied by the hole-burning measurement. From the temperature dependence of the hole width, it is suggested that the exciton is scattered by impurities or defects below 40 K and by a LO phonon above 40 K. Finally, the hydrostatic pressure dependence of the exciton energy and the resonant Raman scattering in the energy region of the optical phonons have been measured and the size effect on the atomic bonding between I and Pb is discussed. It is concluded that covalent bonding between Pb and I atoms changes to ionic bonding in the ultrathin crystallites.

Structure and photodissociation spectra of mixed ethene–acetone clusters

Infrared photodissociation spectra of the mixed complexes C2H4–CH3COCH3 and C2H4–(CH3COCH3)2 have been observed after size selecting them by scattering from a helium beam combined with mass spectrometric detection. The excitation of the ν7-out of plane symmetric wagging mode of ethene near its gas phase frequency at 949.3 cm−1 with a cw-CO2 laser leads to a characteristic depletion of the cluster beam. The dissociation spectrum of the 1:1 complex can be explained by two peaks at 950.8 and 961.6 cm−1. Calculations of minimum energy configurations and band shifts based on an empirical site–site potential show that these frequencies can be attributed to the absorption of two different isomers. They correspond to the two different binding patterns of the H atoms of ethene to the O atom of acetone and those of acetone to the C–C group of ethene, respectively. For the 1:2 complex, a large peak at 958.5 cm−1 and a smaller one around 940.5 cm−1 are found which can be explained in a similar manner by several isomers found in the structure calculations.

Multistable states MIS transistor

Mathematical simulation and experimental studies of an MINpn (MISSS) metalconducting insulator-semiconductor bipolar transistor are described in the paper. Such devices generally have switching characteristics. The high-impedance state is associated with a reversebiased p-n junction which sustains high voltages. The breakover occurs when the sum of the small-signal current gains reaches unity. The low-impedance state is associated with an accumulation of the semiconductor surface which increases the electric field across the insulator. The energy bandshift may cause the valence-band edge of the semiconductor to be at a higher energy level than the metal-work function. The metal-insulator-semiconductor (MIS) acts as a minority diode and has a large flow of valence electrons. The second regenerative feedback may create the intermediate stable state, if the positive resistance appears prior to the second breakover. The simulation results show that this intermediate stable state may happen when the oxide thickness is within a certain (tunnelling) range. The larger barrier height device (majority diode) has a greater possibility of exhibiting the intermediate stable state. The narrower basewidth device is more significant in indicating the voltage collapse. The experimental result confirmed the existence of the intermediate stable state.