D-band Narrowing Research Articles

A new insight into the structural modulation of graphene oxide upon chemical reduction probed by Raman spectroscopy and X-ray diffraction

Lately, Raman spectroscopy has become powerful tool for quality assessment of graphene analogues with identification of intensity ratio of Raman active D-band and G-band (ID/IG ratio) as a vital parameter for quantification of defects. However, during chemical reduction of graphitic oxide (GrO) to reduced GrO (RGrO), the increased ID/IG ratio is often wrongly recognized as defect augmentation, with “formation of more numerous yet smaller size sp2 domains” as its explanation. Herein, by giving due attention to normalized peak height, full-width half-maxima and integrated peak area of Raman D- and G-bands, and compliment the findings by XRD data, we have shown that in-plane size of sp2 domains actually increases upon chemical reduction. Particularly, contrary to increased ID/IG ratio, the calculated decrease in integrated peak area ratio (AD/AG ratio) in conjunction with narrowing of D-band and broadening of G-band, evinced the decrease in in-plane defects. Finally, as duly supported by reduction induced broadening of interlayer-spacing characteristic XRD peak and narrowing of ~ 43° centered XRD hump, we have also shown that the sp2 domains actually expands in size and the observed increase in ID/IG ratio is indeed due to increase in across-plane defects, formed via along-the-layer slicing of graphitic domains.

First principles calculations of the electronic structure and magnetic properties of Y(Fe,M)9.2 and Y(Fe,M)9.2C (M= Si, Ga, Zr)

The preferential site substitution of the Fe by Si, Ga and Zr in the Y(Fe,M)9.2 and Y(Fe,M)9.2C compounds, and the doping effects on the magnetic properties have been studied by the first-principles calculations. It is found that the doping of the Si or Zr can improve the thermodynamic stability of the 1:9 phase, while the substitution of the Fe by Ga makes it unstable. Si atom tends to enter the 3g crystal site and Zr prefers to occupy the 2e site when Y(Fe,M)9.2 and their carbides are synthesized. Although the substitution of the Fe by Si and Zr will reduce the total magnetic moments of the YFe9.2 and their carbides, the volumetric and the d-band narrowing effects caused by the doping can still modify the electron density distributions of the Fe near the Fermi level, improving the magnetic ordering temperature of the non-carbonated compound YFe9.2. The calculated magnetic ordering temperatures of Y(Fe,M)9.2C decrease with the increasing content of the doping elements M due to the stronger hybridization of the d bands in the carbides. For the rare-earth(RE) iron based intermetallics REFe9.2 with the TbCu7-type structure, it is suggested that Zr is able to stabilize the phase and enhance the magnetic ordering temperature, indicating the possible further application in the field of permanent magnets, which has not been reported before.

Metal-Insulator Transition of strained SmNiO3 Thin Films: Structural, Electrical and Optical Properties

Samarium nickelate (SmNiO3) thin films were successfully synthesized on LaAlO3 and SrTiO3 substrates using pulsed-laser deposition. The Mott metal-insulator (MI) transition of the thin films is sensitive to epitaxial strain and strain relaxation. Once the strain changes from compressive to tensile, the transition temperature of the SmNiO3 samples shifts to slightly higher values. The optical conductivity reveals the strong dependence of the Drude spectral weight on the strain relaxation. Actually, compressive strain broadens the bandwidth. In contrast, tensile strain causes the effective number of free carriers to reduce which is consistent with the d-band narrowing.

Angle-dependent ultraviolet photoelectron spectroscopy of sputter cleaned polycrystalline noble metals

Polycrystalline noble metal films are commonly used in practical applications across a variety of different fields. However, the surface electronic structure of the noble metals has primarily only been studied on single crystal substrates. In addition, sputter cleaned polycrystalline noble metal films are commonly used substrates in ultraviolet photoelectron spectroscopy (UPS) studies, but have yet to be systematically studied in terms of their photoemission anisotropy. The angle-dependence of the valence band spectra of sputter cleaned polycrystalline Au, Ag and Cu were studied using angle-resolved UPS. It is found that the photoemission is anisotropic with respect to photoelectron take-off angle. The results for Ag and Cu are in good agreement with previous reports of surface d-band narrowing in polycrystalline noble metal films. However, significant anisotropies in the d-band, s-band and Fermi edge of sputter cleaned Au are observed, which cannot be attributed to surface d-band narrowing alone. The unusual results for Au are attributed to drastic changes in the film morphology near the surface as a result of sputter cleaning.

Surface structural characteristics and tunable electronic properties of wet-chemically prepared Pd nanoparticles

The ligand substitution reaction, Pd L(3,2,1)-edge and S K-edge x-ray absorption fine structure (XAFS), XAFS simulations, and valence-band and core-level x-ray photoelectron spectroscopy (XPS) have been used to systematically study the surface chemical and electronic properties of wet-chemically prepared Pd nanoparticles of varied size, molecular capping, and metal composition. It was found that the replacement of weakly interacting capping molecules (amine and tetra-alkylphosphonium bromide) with strongly binding thiols caused a considerable change in the surface bonding of Pd nanoparticles. However, the Pd d-electron counts (number of d electrons) remained almost unchanged before and after ligand substitution, which is unexpected since Pd atoms normally lose electrons to the more electronegative S atoms. XAFS results and simulations provided useful insights into the surface structural characteristics of Pd nanoparticles and satisfactorily accounted for the unexpected d-electron behavior involved in the ligand substitution process. XPS valence and core-level spectra further revealed a size-dependent d-band narrowing and presented complementary information to XAFS about the surface electronic properties of Pd atoms. The small weakly bound Pd nanoparticles seem inevitably to have a net d-electron depletion due to the influence of the surface effect (chemical adsorption by oxygen), which is more significant than the d-electron enriching nanosize effect. However, it was demonstrated that by forming Pd-Ag alloy nanoparticles, a net increase of the Pd d-electron counts can be realized. Therefore, it is illustrated that by manipulating the surface, size, and alloying effects, the electronic properties of Pd nanoparticles can be possibly tuned.

Electronic structures and magnetism of Fe nanowires on Cu(001) and Ag(001): A first-principles study

The electronic structures and magnetism of Fe nanowires along the [110] direction on Cu(001) and Ag(001) [Fe(nw)/Cu(001) and Fe(nw)/Ag(001)] are investigated by using the all-electron full-potential linearized augmented plane wave method in the generalized gradient approximation. It is found that the magnetic moment of Fe atom for the Fe(nw)/Cu(001) is 2.99 μB, which is slightly smaller than that (3.02 μB) for the Fe(nw)/Ag(001) but much larger than that (2.22 μB) for the bcc iron. The great enhancement of magnetic moment in the Fe nanowires can be explained by the Fe d-band narrowing and enhancement of the spin-splitting due to a reduction in coordination number. From the calculated spin-polarized layer-projected density of states, it is found that the Fe 3d-states are strongly hybridized with the adjacent Cu 3d-states in the Fe(nw)/Cu(001), and there exists a strong hybridization between the Fe sp- and the adjacent Ag 4d-states in the Fe(nw)/Ag(001).

Spin polarization of Nb/Ni nanocontacts measured by Andreev reflection in planar thin-film geometry

Andreev reflection in point contacts between a ferromagnet and a superconductor has been developed in recent years as a method to measure the degree of spin polarization P of the ferromagnet. We have developed the method for planar thin-film edge contacts prepared by a seven-step process. Measurements of the conductance of an Nb/Cu edge junction showed the expected doubling of conductance for bias levels in the range ±1.1mV, corresponding to the superconducting energy gap of the Nb film (TSC=6.5K). For Ni, however, the conductance falls to 20% of its normal value within the energy gap. A fit of the data gives P=70%, which is much greater than the currently accepted value of 46%. Possible explanations for the elevated spin polarization in these planar junctions are d-band narrowing and electron transfer to incorporated oxygen.

Tight-binding electronic structure calculations for the TiFe(001) surface

We present a self-consistent tight-binding calculation of the electronic structure for the (001) surface of TiFe in the CsCl (B2) structure. The results were obtained using a three-center s-p-d orthogonal tight-binding Hamiltonian, within the surface Green function matching formalism. We have analyzed the local density of states (LDOS) for cases where Fe or Ti forms the surface layer. When Fe forms the atomic surface layer, our calculations show that the corresponding LDOS consists predominantly of bonding states, like in the bulk, but with a value at the E F enhanced by ∼160% with respect to the bulk. When Ti forms the atomic surface layer, the corresponding LDOS consists mainly of antibonding states, but with a value at E F raised compared with the bulk by ∼350%; a strong d-band narrowing was found in this case. Additionally, the trends to surface segregation and chemical activity for these surfaces are discussed.

Influence of the strong metal-support interaction on the CO chemisorption at a Pt/SiO2 catalyst

Pt/SiO2 catalysts prepared by sol-gel processing were studied by transmission electron microscopy, electron diffraction, high-resolution electron microscopy and Fourier-transform infrared spectroscopy. The samples exhibit uniform dispersion of the metal phase with narrow particle size distribution. Treatment with hydrogen in the temperature range 773-923 K yields Pt/Si alloys at lower temperatures and platinum silicides (Pt3Si and Pt12Si5) at higher temperatures. The CO chemisorption shows that with increasing degree of alloying (1) the ν (CO)l and the ν (CO)b band shifts to lower wavenumbers, (2) the intensity of the ν (CO)l band decreases and (3) the ν (CO)b/ν(CO)l intensity ratio increases. The phenomena are interpreted by simultaneous effects of Pt surface dilution, enhanced electron back-donation from platinum to CO and Pt d-band narrowing.

Inverse photoemission and Auger electron spectroscopy of Rh thin films on Cu(100)

The growth of Rh films on Cu(100) has been investigated using Auger electron spectroscopy and momentum- (k-) resolved inverse photoemission (KRIPES) with a view to probing the electronic structure of near-monolayer films and any possible magnetic ordering within these films. A pronounced change in the line-shape of the Rh M4,5VV Auger electron peak as a function of average layer thickness indicates significant d-band narrowing in the films of monolayer or submonolayer thickness relative to thicker films. KRIPES data indicate that in films grown at room temperature the spectra are dominated by density-of-states effects implying poor order, and showing the expected unoccupied d-band feature at the Fermi level. Annealing at 750-900 K leads to significant loss of Rh into the substrate, and KRIPES spectra dominated by surface-localized states showing significant dispersion in parallel momentum more characteristic of s-p-band features.

Surface core level shift of Hf 4f in HfN and HfC

Photoelectron spectra of the Hf 4f core levels from a HfN(100) single crystal and a polycrystalline HfC sample have been measured and analysed. Surface-shifted 4f levels could be identified for both compounds, at a smaller binding energy than the bulk 4f levels in HfN and at a larger binding energy in HfC. The surface core level shifts were extracted by a curve fitting procedure and found to be −0.65 eV and +0.41 eV in HfN(100) and HfC. For HfC the shift is very similar to the previously published surface core level shift of +0.42 eV in polycrystalline Hf metal which agreed with theoretical calculations. For HfN however the results cannot be explained in terms of d-band narrowing effects, or by a simple electrostatic picture.

Soft-x-ray appearance potential spectroscopy study of Pr2Fe14−xCoxB

The praseodymium–iron–boron alloy, Pr2Fe14B, is found to possess excellent permanent-magnet properties. In this paper we report the investigation of the electronic structure of the system Pr2Fe14−x Cox B (x=0, 3, 5, 7, and 8.5) by soft x-ray appearance potential spectroscopy (SXAPS). SXAPS exploits the abrupt changes in the total x-ray emission associated with the thresholds for the excitations of core levels of the atoms in the surface region of the materials. This technique is particularly suitable for investigating the changes in the unoccupied states of rare-earth and transition-metal intermetallics which have high density of empty states at the Fermi level.The Pr M5 -level SXAPS peak in the system is found to exhibit negative chemical shift which progressively decreases as a function of Co concentration. However, the L2,3 levels of Fe and Co show increasing positive chemical shift. The chemical shifts data display a maximum at x=7 in these three cases which correlates well with reported behavior in the lattice constants of alloy systems. These chemical shifts have been discussed on the basis of charge transfer among the constituents of the system. The full width at half-maximum of the L3 peaks of Fe and Co indicates a decreasing trend as a function of Co concentration. These spectral characteristics have been interpreted in terms of the d-band narrowing as a result of interaction between 3d bands of Fe and Co.

Interpretation of Cr (001) photoemission spectra: influence of correlations

Experimental angle-resolved photoemission spectra from Cr (001) are compared with the relevant intensities calculated in a 'one-step model' theory. The calculations reproduce most features observed in the spectra and relate them to the bulk band structure. This confirms the interpretation of the experimental band structure deduced previously from the photoemission data by means of the usual 'three-step model' assuming a free-electron-like final band dispersion. Yet the measured d bandwidth is about 20% smaller than predicted by standard one-electron band-structure calculations and a satellite is seen at about 7 eV below the Fermi level, suggesting that correlation effects are essential to the understanding of photoemission from Cr. Using perturbation theory (limited to second order in U/W where U is the Coulomb integral and W the d bandwidth) within the Hubbard model, the authors calculate the corrections induced by these electronic correlations: good agreement with the 'experimental' band structure, the observed d-band narrowing, the hole lifetimes and the satellite position is obtained for U=1.5 eV.