One-dimensional Band Structure Research Articles

Intra- and Intermolecular Band Dispersion in an Organic Crystal

The high crystallinity of many inorganic materials allows their band structures to be determined through angle-resolved photoemission spectroscopy (ARPES). Similar studies of conjugated organic molecules of interest in optoelectronics are often hampered by difficulties in growing well-ordered and well-oriented crystals or films. We have grown crystalline films of uniaxially oriented sexiphenyl molecules and obtained ARPES data. Supported by density-functional calculations, we show that, in the direction parallel to the principal molecular axis, a quasi-one-dimensional band structure of a system of well-defined finite size develops out of individual molecular orbitals. In contrast, perpendicular to the molecules, the band structure reflects the periodicity of the molecular crystal, and continuous bands with a large dispersion were observed.

Influence of the R-substituents on the properties of [Ni(R2pipdt)(dmit)] complexes and crystal structure where R = CH2C6H5

[Ni(R2pipdt)(dmit)], (R2pipdt = 1,4-disubstituted-piperazine-3,2-dithione, R = CH2C6H5; dmit = 1,3-dithiole-2-thione-4,5-dithiolate) (1b) has been prepared and characterised and the properties compared with those of the known complex 2b belonging to the same class where R = Pr i. Cyclic voltammetry of 1b and 2b was carried out and compared with that of the respective R2pipdt ligand precursors (1a and 2a ). The nature of the R-groups of the pipdt ligand exerts an effect on the redox potentials and confirmed the position of the LUMO as mainly on the R2pipdt part of the complex. Accordingly the low frequency absorption, assigned to the HOMO-LUMO transition which has inter-ligand charge-transfer character, is found for 1b at lower frequency when compared to the corresponding transition of 2b. In situ EPR was carried out on electroreduced radical species of the R2pipdt ligand precursors (1a, 2a ) and corresponding complexes (1b, 2b ). This revealed considerable delocalisation of the unpaired electron on the R2pipdt ligand in 1b and 2b with coupling constants to N and H comparable with those of 1a and 2a . Complex 1b crystallised in the space group Pnma and shows an essentially planar complex (with out-of-plane R groups) pi stacked at a distance of 3.65(1) A. Such a one-dimensional structure is not achieved in the case of 2b, where the complex units are almost parallel and head-to-tail with each other forming dimers and this difference in solid-state packing is apparent in the diffuse reflectance spectrum of each. Plane-wave DFT calculations for 1b revealed a highly one-dimensional band structure with considerable band dispersion along the direction of greatest molecular interaction via pi-stacking.

Tunable band structures in uniaxial multilayer stacks

We demonstrate highly tunable one-dimensional photonic band structures under normal incidence. The system consists of a multilayer film that alternates two anisotropic layers. The band-structure characteristics of this multilayer stack, such as location and width of stop bands, can be tailored by altering the relative orientation of the optical axes of the adjacent layers. This structure can be realized by employing liquid crystals and transparent electrodes. In some cases, we observe the ability of this structure to distinguish between different linear polarizations. We propose a depolarizing and filtering device based on this multilayer medium.

Band structure effects on one-dimensional resonant tunneling in STM tips made of carbon nanotubes

In recent experiments, attempts were made to use carbon nanotubes to replace the normal metal tips in the scanning tunneling microscope (STM), and stable atomic images were observed. However, does the one-dimensional characteristic band structure of the carbon nanotube (CNT) affect the tunneling? We present a theoretical analysis of the one-dimensional resonance tunneling model using the nonequilibrium Green's function method. The results clearly imply that the Van Hove singularities of the CNT probe play an important role in the tunneling process. The resonance curve is quite different from the one with a metallic tip; new peaks and peak splittings are induced. So these characteristics must be considered seriously if one uses a nanoprobe as the STM tip. We also notice that a sharp peak will appear near the first Van Hove singularity, which resembles the Kondo peak.

Nature of the Bottom t2g-Block Bands of Layered Perovskites. Implications for the Transport Properties of Phases Where These Bands Are Partially Filled

The electronic structure of a series of double layer (Sr(3)V(2)O(7), KLaNb(2)O(7), Li(2)SrNb(2)O(7), RbLaNb(2)O(7), Rb(2)LaNb(2)O(7)) as well as triple layer (Ca(4)Ti(3)O(10), K(2)La(2)Ti(3)O(10), Sr(4)V(3)O(9.7), CsCa(2)Nb(3)O(10)) Dion-Jacobson and Ruddlesden-Popper phases and quadruple layer A(n)()B(n)(O(3)(n)(+2) phases (Sr(2)Nb(2)O(7) as well as the low-temperature and room-temperature structures of Sr(2)Ta(2)O(7)) has been studied by means of a first-principles density functional theory approach. The results are rationalized on the basis of a simple tight-binding scheme, which provides a simple yet precise scheme allowing the correlation of the crystal structure details and the nature of the bottom t(2g)-block band levels. Both the quantitative and the qualitative approaches are used to analyze the nature of the carriers in intercalated samples of the d(0) semiconducting phases as well as those of the metallic d(x) (0 < x < or = 1) systems. The Ruddlesden-Popper and Dion-Jacobson materials with partially filled t(2g)-block bands must be genuine two-dimensional metals except when the M-O(ap) distances of the outer layer octahedra are similar and the band filling is not low. The conducting electrons in these phases are almost equally distributed among the different layers. It is shown that the A(n)()B(n)O(3)(n)(+2) phases with partially filled bands are potentially interesting materials because they are structurally two-dimensional materials exhibiting one-dimensional band structure features. Finally, the possible application of the simple scheme to related materials such as layered perovskite oxynitrides and the effect of disorder are briefly discussed.

The emission characteristics of liquid-crystal lasers

— Liquid-crystal lasers exhibit narrow linewidth, large coherence area, and low threshold laser emission. Moreover, the wavelength of the laser line can be readily tuned using a variety of different external stimuli, including electric fields. These combined features make them particularly attractive as compact tunable laser light sources. Recent experimental results with regards to the emission characteristics of chiral nematic photonic band-edge lasers are discussed. This type of liquid-crystal laser consists of a self-organizing one-dimensional photonic band structure and a gain medium in the form of a laser dye. Some of the generic features that are observed for these lasers are discussed, including the typical emission linewidth of the laser line, the change in emission energy of the laser for high excitation energies and high pump repetition rates, and the dependence of the excitation threshold and slope efficiency on the cell thickness. In addition, how the performance changes when either the molecular structure of the chiral nematic host or the gain medium is varied is considered. To conclude, results are presented on the laser emission for a wide-temperature-range blue phase I band-edge laser which consists of a self-organizing three-dimensional photonic band structure.

Study of the band structure of the poly (para-phenylene) and its alkoxyl derivatives

In this paper, the one-dimensional band structures of poly (para-phenylene) (PPP) and its alkoxyl derivatives are calculated by an extended Hückel method (BICON-CEDiT code). The results of the band gaps Eg of these materials are in agreement with the available experimental data. In addition, we found the band gaps of PPP's alkoxyl derivatives become wider as the carbon atoms increase in alkoxyl side chain. The calculation indicates that the heteroatom and side groups have an obvious effect on the band structure of polymer, which is also consistent with the experimental results. The results are useful for synthesizing PPP with different emitting wavelength.

Real-space investigation of the metal-insulator transition ofSi(557)−Au

The Si 557 surface with Au adsorbates consists of a well ordered array of atomic chains, which exhibit interesting one-dimensional 1D metallic band structure with two nearly half-filled 1D bands. This system was recently found to undergo a metal-insulator transition below room temperature Phys. Rev. Lett. 91, 196403 2003 . The structural and electronic changes upon the phase transition have been investigated in detail using scanning tunneling microscopy and spectroscopy STM/STS with the guide of first-principles calculations. While the detailed bias-dependent STM images at room temperature are well simulated based on the present structure model, the characteristics of low-temperature images are not successfully reproduced. This casts doubts on the structure model of the low-temperature phase based on the step-edge buckling. The spatially resolved density of states DOS in STS measurements indicates clearly that only the step-edge Si chains are metallic, which exhibit strong reductions of DOS near Fermi level upon the metal-insulator transition together with the periodic lattice distortion. The band-gap opening on this metallic chain is shown to be symmetric in filled and empty states with a consistent gap size with the previous photoemission result. This result denies the recently proposed mechanism of the phase transition based on the dynamic fluctuation of the step-edge buckling. The effect of vacancy defects on the local lattice structures is also discussed for both the roomand low-temperature phases.

Modified Photoluminescence by Silicon-Based One-DimensionalPhotonic Crystal Microcavities

Photoluminescence (PL) from one-dimensional photonic band structures isinvestigated. The doped photonic crystal with microcavities arefabricated by using alternating hydrogenated amorphous silicon nitride(a-SiNx:H/a-SiNy:H) layers in a plasma enhanced chemicalvapour deposition (PECVD) chamber. It is observed that microcavitiesstrongly modify the PL spectra from active hydrogenated amorphoussilicon nitride (a-SiNz:H) thin film. By comparison, the wideemission band width 208 nm is strongly narrowed to 11 nm, and theresonant enhancement of the peak PL intensity is about two orders ofmagnitude with respect to the emission of the λ/2-thick layer ofa-SiNz:H. A linewidth of Δλ = 11 nm and a quality factorof Q = 69 are achieved in our one-dimensional a-SiNz photoniccrystal microcavities. Measurements of transmittance spectra of theas-grown samples show that the transmittance resonant peak of a cavitymode at 710 nm is introduced into the band gap of one-dimensionalphotonic crystal distributed Bragg reflector (DBR), which furtherverifies the microcavity effects.

Preparation and Characterization of π-Stacking Quinodimethane Oligothiophenes. Predicting Semiconductor Behavior and Bandwidths from Crystal Structures and Molecular Orbital Calculations

A series of new quinodimethane-substituted terthiophene and quaterthiophene oligomers has been investigated for comparison with a previously studied quinoid oligothiophene that has demonstrated high mobilities and ambipolar transport behavior in thin-film transistor devices. Each new quinoidal thiophene derivative shows a reversible one-electron oxidation between 0.85 and 1.32 V, a quasi-reversible one-electron second oxidation between 1.37 and 1.96 V, and a reversible two-electron reduction between -0.05 and -0.23 V. The solution UV-vis-NIR spectrum of each compound is dominated by an intense (epsilon congruent with 100 000 M(-1) cm(-1)) low energy pi-pi transition that has a lambda(max) ranging between 648 and 790 nm. All X-ray crystal structures exhibit very planar quinoidal backbones and short intermolecular pi-stacking distances (3.335-3.492 A). Structures exhibit a single pi-stacking distance with parallel cofacial stacking (sulfur atoms of equivalent rings pointed in the same direction) or with alternating distances and antiparallel cofacial stacking (sulfur atoms of equivalent rings pointed in the opposite direction). Examples of the layered and herringbone-packing motifs are observed for both the parallel and the antiparallel cofacial stacking. Analysis of the X-ray structures and molecular orbital calculations indicates that all of these compounds have one-dimensional electronic band structures as a result of the pi-stacking. For structures with a unique pi-stacking distance, a simple geometric overlap parameter calculated from the shape of the molecule and the slip from perfect registry in the pi-stack correlates well with the transfer integrals (t) calculated using molecular orbital theory. The calculated valence (633 meV) and conduction (834 meV) bandwidths for a quinoid quaterthiophene structure are similar to those calculated for the benchmark pentacene and indicate that both hole and electron mobilities could be significant.

Magnetic-Field-Induced Suppression of Electronic Conduction in a Superlattice

We use a magnetic field applied along the axis of a semiconductor superlattice (SL) as a controllable means of creating a one-dimensional band structure. We demonstrate that the current flow through the SL is strongly suppressed when the electron motion perpendicular to the SL axis is strongly confined by the quantizing magnetic field. By modeling this behavior using semiclassical and nonequilibrium Green's function methods, we show that the observed quenching arises from a qualitative change in electron dynamics caused by increasing quantum confinement.

Quantized magneto-thermopower in tunnel-coupled ballistic channels: sign reversal andoscillations

The quantized electron-diffusion thermoelectric powerS is studied for a general one-dimensional band structure in ballistic channels withapplications to a single-quantum-well channel and tunnel-coupled double-quantum-wellchannels in a perpendicular magnetic field. We find a field-induced sign reversal ofS and oscillations in double-well channels.

Development of one-dimensional band structure in artificial gold chains.

The ability of a scanning tunneling microscope to manipulate single atoms is used to build well-defined gold chains on NiAl(110). The electronic properties of the one-dimensional chains are dominated by an unoccupied electron band, gradually developing from a single atomic orbital present in a gold atom. Spatially resolved conductance measurements along a 20-atom chain provide the dispersion relation, effective mass, and density of states of the free electron-like band. These experiments demonstrate a strategy for probing the interrelation between geometric structure, elemental composition, and electronic properties in metallic nanostructures.

Anisotropicab-plane optical response of the charge-density-wave superconductorP4W14O50

We report the polarized optical reflectance of ${\mathrm{P}}_{4}{\mathrm{W}}_{14}{\mathrm{O}}_{50},$ the only charge-density-wave (CDW) superconductor in the monophosphate tungsten bronze family, over a wide frequency range. The 300-K infrared spectra are dominated by two-dimensional behavior: high reflectance with weak phonons in the conducting $\mathrm{ab}$ plane and low reflectance with strong vibrational modes along the interlayer direction. An anisotropic intra-${t}_{2g}\stackrel{\ensuremath{\rightarrow}}{d}d$ excitation $(\ensuremath{\sim}10000 {\mathrm{cm}}^{\ensuremath{-}1})$ reveals the ``hidden'' one-dimensional band structures in ${\mathrm{P}}_{4}{\mathrm{W}}_{14}{\mathrm{O}}_{50}.$ The variable temperature $\mathrm{ab}$-plane spectra clearly show a suppression of the optical conductivity along the b axis below 140 K, giving rise to charge localization and anisotropic CDW gap formation at $\ensuremath{\sim}1400 {\mathrm{cm}}^{\ensuremath{-}1}.$ Although the oscillator strength is redistributed among the free-carrier response, the CDW gap absorption, and the $\stackrel{\ensuremath{\rightarrow}}{d}d$ transition in the density wave states, the spectral weight is largely conserved below the plasma frequency. The $\mathrm{ab}$-plane phonons become well resolved in the CDW states due to reduced screening. Based upon these observations, ${\mathrm{P}}_{4}{\mathrm{W}}_{14}{\mathrm{O}}_{50}$ is another example of a superconductor with an unusual normal state.

Ethylene adsorption on Ge(100)-(2×1): A combined angle-resolved photoemission and thermal desorption spectroscopy study

Ethylene adsorption on vicinal, single-domain Ge(100)-(2×1) has been investigated by thermal desorption spectroscopy (TPD) and angle-resolved photoemission (ARUPS) using linearly polarized synchrotron radiation. Thermal desorption experiments show that chemisorbed C2H4 desorbs from Ge(100) nondissociatively around 393 K with a high temperature shoulder which is tentatively assigned to step site desorption. The ethylene saturation coverage is strongly temperature dependent. Adsorption at 90 K saturates at 0.38 monolayer (ML), whereas adsorption at 170 K leads to a saturation coverage of approximately 1 ML. This behavior is explained by an adsorption barrier for coverages exceeding 0.38 ML. ARUP spectra for a dilute and the saturated ethylene monolayer reveal clear differences. Using photoemission selection rules a highly (C2v) symmetric adsorption geometry with a C–C bond axis parallel to the Ge–Ge dimer axis is found for the dilute layer; whereas a reduced C2 adsorption symmetry is found for the saturated ethylene layer. The comparison of photoemission spectra for C2H4 on Ge(100) and Si(100) shows that C2H4 is di-σ bound to the dangling bonds of a single Ge–Ge dimer. For two molecular orbitals, 1b3u and 1b2g, one-dimensional band structures with dispersion widths of 0.5 and 0.39 eV, respectively, along the Ge–Ge dimer rows are found which present a straightforward explanation for the observed symmetry reduction and adsorption behavior.

Crystal structure and conducting properties of a SbF 6 salt of BEDT-BDTBF

A single-crystalline electric conducting salt [BEDT-BDTBF] 2[SbF 6][PhCl] 0.5 was grown by electrochemical oxidation of BEDT-BDTBF in chlorobenzene. This salt showed a room temperature conductivity of 0.42 S cm −1 along the stacking c-axis and semiconducting behavior with an activation energy of E a=0.183 eV. From X-ray crystal structure analysis and calculated band structure, it is revealed that the inter-column interaction p3 connecting the weakly dimerized donor columns plays an important role in the quasi one-dimensional band structure of this salt.

Ultraviolet photoelectron spectroscopy of thin films of new materials for multilayer organic light emitting diodes

The electronic structure of thin films of new heterocyclic compounds was studied by means of ultraviolet photoelectron spectroscopy (UPS) and near edge X-ray absorption fine structure (NEXAFS). The complete one-dimensional valence electronic band structure has been determined from the UPS spectra. The spectral features that correspond to the top of the valence band and that dominate the electronic properties of this material were assigned to the different molecular eigenstates by comparison with simulated spectra from quantum chemical calculations. NEXAFS measurements were carried out on the same films.

Electronic band structures of low-dimensional adsorbate systems: Rare-gas adsorption on transition metals

Angle-resolved photoemission data are dis-cussed for five different Xe adlayers which exhibit electronic structures of different dimensionalities. Xe adsorption on Ni (110)-(1 × 2)-3Hand the (\(\)×\(\)) R30° Xe layer on Ru (001) reveal two-dimensional (2D) Xe-derived band structures that are characteristic for hexagonal rare-gas layers. Different Xe 5p dispersion widths on Ni and on Ru are found due to the difference in the Xe-Xe nearest-neighbor distance. For three rare-gas systems (two different Xe coverages on hydrogen-modified Pt (110)-(1 × 2)-H and Kr step decoration on a Pt (997) surface) true one-dimensional (1D) band structures are found. For Xe step adsorption on Pt (997), electronic localized (0D) behavior is observed due to an enlarged Xe-Xe separation. The qualitative differences of the band structures in the case of 2D, 1D and 0D rare-gas systems are demonstrated and are explained by the different dimensionalities of the various structures.

Angle resolved ultraviolet photoelectron spectroscopy of oriented sexiphenyl layers

Ultra-thin films of oriented p-sexiphenyl (6P) on a GaAs(001) wafer were studied by angle resolved ultraviolet photoelectron spectroscopy (ARUPS) using UV photons in the energy range between 20 and 60 eV. The samples were prepared under ultra-high vacuum by controlled evaporation onto cleaned GaAs substrates, which were thermostated at enhanced temperature during the deposition process. The thickness of the deposited films played a very important role for the orientation of 6P molecules. For layers having a thickness of about 35 Å, the molecules were oriented perpendicular to the substrate surface and for films of about 300-Å thickness the molecules lost their upright orientation. The orientation of 6P molecules was monitored both by the anisotropy as visible in the measured ARUPS spectra and independently by atomic force microscopy (AFM) investigations carried out after UPS measurements. From the experimental photoelectron spectra, the full one-dimensional valence band structure of oriented p-sexiphenyl layers has been determined. While valence states between 2 and 5 eV show only weak dispersion, the bands between 5 and 11 eV show a clear momentum dispersion, which indicates the high degree of order in the investigated 6P layers.

STM study of single-walled carbon nanotubes

The electronic density of states of atomically resolved single-walled carbon nanotubes (SWNTs) have been investigated using scanning tunneling microscopy (STM). Peaks in the density of states due to the one-dimensional nanotube band structure have been characterized and compared with the results of tight-binding calculations.