Diamond-like carbon (DLC) is a metastable form of amorphous carbon with significant sp3 bonding. DLC is a semiconductor with a high mechanical hardness, chemical inertness, and optical transparency. This review will describe the deposition methods, deposition mechanisms, characterisation methods, electronic structure, gap states, defects, doping, luminescence, field emission, mechanical properties and some applications of DLCs. The films have widespread applications as protective coatings in areas, such as magnetic storage disks, optical windows and micro-electromechanical devices (MEMs).

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Diamond-like amorphous carbon

Electrical current can be completely spin polarized in a class of materials known as half-metals, as a result of the coexistence of metallic nature for electrons with one spin orientation and insulating nature for electrons with the other. Such asymmetric electronic states for the different spins have been predicted for some ferromagnetic metals--for example, the Heusler compounds--and were first observed in a manganese perovskite. In view of the potential for use of this property in realizing spin-based electronics, substantial efforts have been made to search for half-metallic materials. However, organic materials have hardly been investigated in this context even though carbon-based nanostructures hold significant promise for future electronic devices. Here we predict half-metallicity in nanometre-scale graphene ribbons by using first-principles calculations. We show that this phenomenon is realizable if in-plane homogeneous electric fields are applied across the zigzag-shaped edges of the graphene nanoribbons, and that their magnetic properties can be controlled by the external electric fields. The results are not only of scientific interest in the interplay between electric fields and electronic spin degree of freedom in solids but may also open a new path to explore spintronics at the nanometre scale, based on graphene.

https://arxiv.org/pdf/cond-mat/0611600.pdf

Half-metallic graphene nanoribbons

The last decade witnessed significant progress in angle-resolved photoemission spectroscopy (ARPES) and its applications. Today, ARPES experiments with 2-meV energy resolution and $0.2\ifmmode^\circ\else\textdegree\fi{}$ angular resolution are a reality even for photoemission on solids. These technological advances and the improved sample quality have enabled ARPES to emerge as a leading tool in the investigation of the high-${T}_{c}$ superconductors. This paper reviews the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds, with the purpose of providing an updated summary of the extensive literature. The low-energy excitations are discussed with emphasis on some of the most relevant issues, such as the Fermi surface and remnant Fermi surface, the superconducting gap, the pseudogap and $d$-wave-like dispersion, evidence of electronic inhomogeneity and nanoscale phase separation, the emergence of coherent quasiparticles through the superconducting transition, and many-body effects in the one-particle spectral function due to the interaction of the charge with magnetic and/or lattice degrees of freedom. Given the dynamic nature of the field, we chose to focus mainly on reviewing the experimental data, as on the experimental side a general consensus has been reached, whereas interpretations and related theoretical models can vary significantly. The first part of the paper introduces photoemission spectroscopy in the context of strongly interacting systems, along with an update on the state-of-the-art instrumentation. The second part provides an overview of the scientific issues relevant to the investigation of the low-energy electronic structure by ARPES. The rest of the paper is devoted to the experimental results from the cuprates, and the discussion is organized along conceptual lines: normal-state electronic structure, interlayer interaction, superconducting gap, coherent superconducting peak, pseudogap, electron self-energy, and collective modes. Within each topic, ARPES data from the various copper oxides are presented.

https://arxiv.org/pdf/cond-mat/0208504.pdf

Angle-resolved photoemission studies of the cuprate superconductors

Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the ‘memristor’ (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron–ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance. Nanoscale metal/oxide/metal devices that are capable of fast non-volatile switching have been built from platinum and titanium dioxide. The devices could have applications in ultrahigh density memory cells and novel forms of computing.

Memristive switching mechanism for metal/oxide/metal nanodevices.

A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

Photoelectron spectroscopy measurements and density functional theory calculations are combined to determine structures of ${\mathrm{Nb}}_{n}^{\phantom{\rule{0ex}{0ex}}\ensuremath{-}}$ ( $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3--8$) clusters. A detailed comparison between observed and calculated electronic binding energies shows that the clusters have low-symmetry compact 3D structures and the lowest possible total spin, except for the three- and five-atom clusters which are in triplet states. We find evidence for the coexistence of two isomers of ${\mathrm{Nb}}_{8}^{\phantom{\rule{0ex}{0ex}}\ensuremath{-}}$ under some experimental conditions. This approach shows great promise for structural characterization of small clusters.

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Photoelectron Spectra and Geometric Structures of Small Niobium Cluster Anions.

Photoelectron spectra of mass-separated Nbn− clusters reveal an even/odd alternation for n=6–17, indicating a closed electronic shell of the neutral even-numbered clusters. The HOMO–LUMO gap of Nb8, Nb10, and Nb16 is found to be larger than that of the other even-numbered clusters, which correlates with the low H2 reactivities of these species. The spectrum of Nb15− is different from all other clusters in this size range, which might be an indication for a geometric bcc shell closing. The influence of the electronic structure of the clusters on the reactivity is discussed.

Photoelectron spectra of Nbn− clusters: Correlation between electronic structure and hydrogen chemisorption

New look at the mechanism for alkali-metal promotion.

Investigating ${\mathrm{LaNi}}_{1\ensuremath{-}x}{M}_{x}{\mathrm{O}}_{3}$ ( $M\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{Mn}$ and Fe), we identify a characteristic evolution of the spectral function with increasing disorder in the presence of strong interaction effects across the metal-insulator transition. We discuss these results vis-\`a-vis existing theories of electronic structure in the simultaneous presence of disorder and interaction.

Disorder Effects in Electronic Structure of Substituted Transition Metal Compounds

Half a monolayer (ML) of Mn deposited above 270 K on the (100) surfaces of Cu and Ni form ordered surface alloys of c(2\ifmmode\times\else\texttimes\fi{}2) structure. Their electronic structure is studied in a combined experimental and theoretical work. The experimental approach, comprising angle-resolved photoemission and inverse photoemission, characterizes both systems as ideal cases of well-ordered magnetic surface alloys. A large atomiclike splitting between majority- and minority-spin Mn-3d states is measured: 5.5 eV for the Cu-based system and 5.25 eV for the Ni-based system. The large splittings are direct evidence that Mn develops a high local magnetic moment in these systems. Our first-principles band-structure calculations of c(2\ifmmode\times\else\texttimes\fi{}2) CuMn/Cu(100) and c(2\ifmmode\times\else\texttimes\fi{}2) NiMn/Ni(100) corroborate this finding and give values of 3.75${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ and 3.5${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ , respectively, for the Mn moments. We find that the measured splittings are even larger than the ones calculated from first principles. The difference amounts to 2.7 eV and 1.7 eV for the Cu-based and Ni-based system, respectively. We suppose that the splitting measured in photoemission and inverse photoemission is increased by a Coulomb energy U due to the enhanced localization of the Mn-3d states in the surface alloy. This high localization can be quantified by the small band dispersion of 110\ifmmode\pm\else\textpm\fi{}60 meV measured for the Mn minority d band in the Cu-based system. We also investigated the work-function change upon surface-alloy formation. By comparing our results with our ab initio calculations we identified the magnetism as the source of the work-function change.

Wolfgang Eberhardt

Papers

Photoelectron Spectra and Geometric Structures of Small Niobium Cluster Anions.

Photoelectron spectra of Nbn− clusters: Correlation between electronic structure and hydrogen chemisorption

New look at the mechanism for alkali-metal promotion.

Disorder Effects in Electronic Structure of Substituted Transition Metal Compounds

Electronic structure of two-dimensional magnetic alloys: c(2×2) Mn on Cu(100) and Ni(100)