Changes In Electronic Band Structure Research Articles

Potassium Doped Single Wall Carbon Nanotubes: Resistance under Pressure

We report in situ measurements of four-probe de resistance (R) of K-doped purified single wall carbon nanotube (SWNT) “buckypaper” as a function of quasi-hydrostatic pressure. Doped samples show completely different behavior compared to that of pristine nanotubes in the pressure range up to 90 kbar. The characteristic increase in the resistance of pristine buckypaper above 10 kbar, associated with the formation of kinks or/and twists of tubes, is not observed in K-doped samples. This may originate from 1) a substantial change in electronic band structure of donor intercalated nanotubes, 2) completely different transport properties of defect structures, or 3) higher stiffness of doped SWNT's which prevents formation of kinks and twists in this pressure range. On deintercalation, the pristine behavior of R(P) is restored, establishing the reversibility of potassium vapor-transport doping.

Raman and RAS Measurements on Uniaxially Strained Thin Semiconductor Layers

A sensitive and reliable method was developed for the determination of electron and phonon deformation potentials of thin layers and layered structures. A device was constructed and used to apply a line force at the center of a narrow stripe cut from a semiconductor wafer. The induced strain along the stripe was calculated from the maximum deflection at the center of the stripe. Thin layers of Si, SiGe and InGaAs were studied and the information on the phonon deformation potentials was deduced from the phonon frequency shift measured by Raman spectroscopy. Strain dependent changes in electronic band structure were monitored by means of Reflectance Anisotropy Spectroscopy (RAS). The RAS spectra consisted of sharp peaks at or near bandgap energies which increased rapidly with even a small increase of strain. The uniaxial strain device used with RAS is shown to provide a very sensitive technique for the detection of small amounts of strain. This method could be also applied for the study of superlattices.

Lattice and electronic band structure changes across the surface ferroelectric transition

The origin of the surface ferroelectric phase transition in crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%) is explored. We report a uniaxial doubling of the surface Brillouin zone in the conduction band dispersion across the surface ferroelectric phase transition. The temperature dependent changes in the electronic structure occur primarily in the conduction band and are accompanied by a surface dipole reorientation.

Magnetism in the NdLa alloy system

We have investigated the phase diagram of the dhcp neodymium-lanthanum alloy system through heat capacity and neutron scattering measurements. A change from the incommensurate antiferromagnetic ordering of neodymium to ferromagnetic ordering is reported. This is attributed to changes in electronic bandstructure as a result of alloying.

Optical Spectroscopies of Lithium-Intercalated Compounds

AbstractLayered compounds are known to form lithium intercalation complexes as electron donor systems. A charge transfer which can strongly affect the electronic properties of the host lattice, and a change of preferential crystallographic parameters without destruction of the original structure are the main effects occuring during intercalation. Optical spectroscopies such as Raman scattering, far-infrared reflectivity, absorption measurements and photoluminescence have been carried out for the study of electronic and structural modifications. Upon lithium intercalation, lattice dynamics and electronic band structure change in numerous layered compounds. The optical properties of transition-metal dichalcogenides, non-transition metal chalcogenides and transition-metal oxides are presented and discussed with the aim of a better understanding of the intercalation process and establish some guide lines for improving the performances of these materials in their most important applications.

Atomic microstructure and electronic properties of a-SiNx:H deposited by radio frequency glow discharge

a-SiNx:H films of various composition x were deposited by rf glow discharge (GD). The deposition rate was analyzed for three ranges of gas flow ratio R = [NH3]/[SiH4] depending on the deposition mechanism. Properties of these films were measured by means of x-ray photoelectron spectroscopy (XPS), infrared (IR) absorption, optical absorption, and the temperature dependence of electrical conductivity. The composition x was determined by XPS. For large values of R, x was found to be saturated at 1.7. The variation of H content was detected by IR absorption. The variation of coordinating atoms of Si with increasing x was deduced from the variation of XPS spectra of the Si 2p core-level and the shift of Si-H stretching vibration frequency in IR absorption. Based on the random bonding model and assuming bonding units to the central Si atom to be Si, N, and NH, probabilities of Si tetrahedra with various coordinating units were obtained. The results indicate that there are many Si—Si bonds for the stoichiometric x=1.33 and that the concentration of Si—Si bonds diminishes at around the saturation value x=1.7. These results seem to imply that the presence of Si—Si bonds to some extent is a prerequisite condition for film deposition by GD. Experimental results of optical absorption analyzed by Tauc relation revealed the presence of two kinds of x region whose properties are quite different. For x<1.5, its optical absorption is similar to a-Si:H modified by the presence of N. With increasing x, the optical band gap EO increases and B coefficient in the Tauc plot decreases. At about x=1.5, Si—Si bonding effectively disappears and the optical absorption abruptly changes to that similar to β-Si3N4. Considering these results, the change of electronic band structure with x was deduced on the basis of the atomic structure obtained above and by the tight binding approximation. For x<1.5, the optical band gap is due to Si—Si bonding, the energy gap of which increases, and the linear band tail becomes broad with increasing x. From the observed temperature dependence of conductivity, variations of the activation energy and pre-exponential factor are obtained with x up to x=1.0. For the decrease of conductivity with x, the contribution from the pre-exponential factor is much larger than that from the activation energy. This result can be understood by a transport mechanism in the electronic band structure obtained above. Finally, it is concluded that the electronic properties of a-SiNx:H deposited by GD ran.

Chemisorbed halogen adlayers on Fe(110): electronic band structure and surface geometry

Abstract A series of overlayer structures, starting with a c(3 × 1) geometry are formed during the dissociative chemisorption of chlorine, bromine, and iodine on Fe(110). These adlayers provide an opportunity to systematically examine the corresponding changes in electronic band structure as a function of coverage and adsorbate. At coverage levels greater than θ = 1 3 low-energy electron diffraction patterns obtained from the halogen-covered Fe(110) surface evolve continuously with coverage. The electronic band structure was examined as a function of coverage and adsorbate species via angle-resolved ultraviolet photoelectron spectroscopy. It is concluded that the electronic band structure measurements support an incommensurate structural model for halogen overlayer geometry on Fe(110) in which an overlayer lattice, that is in general incommensurate with the substrate, rotates and compresses with increases in coverage. An alternative interpretation of the diffraction results based on antiphase domain boundaries within the overlayer is not consistent with the photoemission data. The incommensurate model emphasizes adatom-adatom interactions over adatom-substrate interactions while the reverse is true for the antiphase boundary model.

Chemisorbed halogen adlayers on Fe(110): Electronic band structure and surface geometry

A series of overlayer structures, starting with a c(3 × 1) geometry are formed during the dissociative chemisorption of chlorine, bromine, and iodine on Fe(110). These adlayers provide an opportunity to systematically examine the corresponding changes in electronic band structure as a function of coverage and adsorbate. At coverage levels greater than θ = 1 3 low-energy electron diffraction patterns obtained from the halogen-covered Fe(110) surface evolve continuously with coverage. The electronic band structure was examined as a function of coverage and adsorbate species via angle-resolved ultraviolet photoelectron spectroscopy. It is concluded that the electronic band structure measurements support an incommensurate structural model for halogen overlayer geometry on Fe(110) in which an overlayer lattice, that is in general incommensurate with the substrate, rotates and compresses with increases in coverage. An alternative interpretation of the diffraction results based on antiphase domain boundaries within the overlayer is not consistent with the photoemission data. The incommensurate model emphasizes adatom-adatom interactions over adatom-substrate interactions while the reverse is true for the antiphase boundary model.

Sb overlayers on GaAs(110)

The electronic band bending at the interface GaAsSb, as determined by inelastic light scattering, shows strong variations, not only in the submonolayer region, but also at coverages above 10 ML. The latter band bending variations depend strongly on substrate doping and deposition temperature. They are correlated with the crystallization of the Sb overlayer and the corresponding electronic band structure changes.

Zur charakteristischen Streuung langsamer Elektronen an Ni(111) und ihrer Temperaturabhängigkeit im Bereich des ferromagnetischen Curie-Punktes / Characteristic Scattering of Low Energy Electrons from Ni(111) and its Temperature Dependence in a Range Containing the Ferromagnetic Curie-point

Characteristic energy losses (CEL) of slow electrons scattered from a Ni ( 111 ) surface were studied using primary energies Ep in the range 150 ≦ Ep 600 ≦ eV. Special attention was given to the cleanliness of the crystal surface. In particular, CEL were studied as function of temperature T in the range containing the Curie-temperature Tc . The volume loss energy ħωp shows an anomalous variation in the region of Tc superimposed on a linear decrease from T = 100 °C to T = 700 °C, while the surface loss energy ħωs remains constant in this range within the error of measurement ( ±0.1eV); ħωs and ħωs could be identified in connection with optical measurements of VEHSE and ARAKAWA14. A possible explanation of the variation of loss energy ħωp is given in terms of electronic band structure changes.