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

3.2. Preparation 4090 3.3. Intermediate Layers 4090 3.4. Support 4090 3.5. Modification 4091 3.5.1. Silica Membrane Modification 4091 3.5.2. Membrane Structure Modification 4092 3.6. Operational Stability 4092 4. Zeolite Membranes 4092 4.1. Membrane Growth Methods 4093 4.2. Permeation and Gas Transport 4093 4.3. Defect Site Diffusion/Nonzeolitic Pores 4094 4.4. Thin Films 4094 4.5. Zeolite Membrane Modification 4094 4.6. CO2 Sequestration in H2 Separations 4095 4.7. Manufacturing 4095 5. Carbon-Based Membranes 4096 5.1. Carbon Membrane Preparations 4097 5.2. Carbon Membrane Post-treatment 4097 5.3. Carbon Membrane Module Construction 4097 5.4. Selective Surface Flow Membranes 4097 5.5. Disadvantages of Carbon Membranes 4098 5.6. Molecular Sieving Carbon Membranes 4098 5.7. Carbon Nanotubes 4099 6. Polymer Membranes for H2 Separations 4100 6.1. Dense Polymeric Membranes 4100 6.2. Hydrogen Selective Polymeric Membranes 4101 6.3. H2 versus CO2 Selective Polymeric Membranes 4103

Membranes for hydrogen separation.

We introduce a Gaussian approximation potential (GAP) for atomistic simulations of liquid and amorphous elemental carbon. Based on a machine learning representation of the density-functional theory (DFT) potential-energy surface, such interatomic potentials enable materials simulations with close-to DFT accuracy but at much lower computational cost. We first determine the maximum accuracy that any finite-range potential can achieve in carbon structures; then, using a hierarchical set of two-, three-, and many-body structural descriptors, we construct a GAP model that can indeed reach the target accuracy. The potential yields accurate energetic and structural properties over a wide range of densities; it also correctly captures the structure of the liquid phases, at variance with a state-of-the-art empirical potential. Exemplary applications of the GAP model to surfaces of ``diamondlike'' tetrahedral amorphous carbon ($\mathit{ta}$-C) are presented, including an estimate of the amorphous material's surface energy and simulations of high-temperature surface reconstructions (``graphitization''). The presented interatomic potential appears to be promising for realistic and accurate simulations of nanoscale amorphous carbon structures.

Machine learning based interatomic potential for amorphous carbon

The present status of development of the density-functional-based tightbinding (DFTB) method is reviewed. As a two-centre approach to densityfunctional theory (DFT), it combines computational efficiency with reliability and transferability. Utilizing a minimal-basis representation of Kohn–Sham eigenstates and a superposition of optimized neutral-atom potentials and related charge densities for constructing the effective many-atom potential, all integrals are calculated within DFT. Self-consistency is included at the level of Mulliken charges rather than by self-consistently iterating electronic spin densities and effective potentials. Excited-state properties are accessible within the linear response approach to time-dependent (TD) DFT. The coupling of electronic and ionic degrees of freedom further allows us to follow the non-adiabatic structure evolution via coupled electron–ion molecular dynamics in energetic particle collisions and in the presence of ultrashort intense laser pulses. We either briefly outline or give references describing examples of applications to ground-state and excited-state properties. Addressing the scaling problems in size and time generally and for biomolecular systems in particular, we describe the implementation of the parallel ‘divide-and-conquer’ order-N method with DFTB and the coupling of the DFTB approach as a quantum method with molecular mechanics force fields.

https://iopscience.iop.org/article/10.1088/0953-8984/14/11/313/pdf

Atomistic simulations of complex materials: ground-state and excited-state properties

Using a combined experimental and theoretical approach, we address C 1s core-level shifts in amorphous carbon. Experimental results are obtained by x-ray photoelectron spectroscopy (XPS) and electron-energy-loss spectroscopy (EELS) on thin-film samples of different atomic density, obtained by a pulsed-laser deposition growth process. The XPS spectra are deconvoluted into two contributions, which are attributed to sp(2)- and sp(3)-hybridized atoms, respectively, separated by 0.9 eV, independent or atomic density, The sp(3) hybridization content extracted from XPS is consistent with the atomic density derived from the plasmon energy in the EELS spectrum. In our theoretical study, we generate several periodic model structures of amorphous carbon of different densities applying two schemes of increasing accuracy in sequence. We first use a molecular-dynamics approach, based on an environmental-dependent tight-binding Hamiltonian to quench the systems from the liquid phase. The final model structures are then obtained by further atomic relaxation using a first-principles pseudopotential plane-wave approach within density-functional theory. Within the latter framework. we also calculate carbon 1s core-level shifts for our disordered model structures. We find that the shifts associated to threefold- and fourfold- coordinated carbon atoms give rise to two distinct peaks separated by about 1.0 eV. independent of density, in close agreement with experimental observations. This provides strong support for decomposing the XPS spectra into two peaks resulting from sp(2)- and sp(3)-hybridized atoms. Core-hole relaxations effects account for about 30% of the calculated shifts.

sp(2)/sp(3) hybridization ratio in amorphous carbon from C 1s core-level shifts: X-ray photoelectron spectroscopy and first-principles calculation

The structure of amorphous carbon at densities of 20, 26, 29, and 32 ${\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$ has been investigated using Car-Parrinello ab initio molecular dynamics The networks were generated by rapidly cooling samples of 125 atoms to form a solid amorphous structure The important structural attributes of each network were found to be in good agreement with experimental observations In particular, the fraction of ${\mathrm{sp}}^{2}$ atoms was found to have a linear dependence on the density of the network The relationship between stress, an important parameter in experimental synthesis, and the structural properties of each network, was also investigated A nonlinear dependence of stress on ${\mathrm{sp}}^{3}$ fraction was observed The electronic properties of each network is also explored with a widening of the band gap and a corresponding increase in the optical gap observed with increasing density (or ${\mathrm{sp}}^{3}$ fraction) of the network The simulations also provided qualitative support for the widely used experimental method for determining the ${\mathrm{sp}}^{3}$ fraction from the carbon core loss edge in electron-energy-loss spectra

Ab initio simulations of the structure of amorphous carbon

First-principles quantum molecular-dynamics simulations of the structure of hydrogenated amorphous carbon, a-C:H, at two densities (2.0 and 2.9 ${\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$) have been carried out using the Car-Parrinello method. The results for the low-density structure show good agreement with experiment in the manner in which the hydrogen is incorporated, as judged by agreement with published vibrational density of states and neutron-diffraction data at various levels of deuteration. The simulation reproduces the position and magnitude of the hydrogen features in the pair-correlation function $G(r)$ obtained from neutron diffraction. The nonhydrogenated carbon atoms are predominantly ${\mathrm{sp}}^{2}$ hybridized and the hydrogen atoms are largely ``sandwiched'' between layers of ${\mathrm{sp}}^{2}$ atoms. The simulated high-density structure has a majority of nonhydrogenated carbon atoms with ${\mathrm{sp}}^{3}$ hybridization. The results of this study show that a useful test for confirming the high-density a-C:H structure is the presence of a small C-C-C bond angle and the occurrence of C-H bond stretching frequencies above 3100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.

Ab initio simulation of structure in amorphous hydrogenated carbon

Car–Parrinello molecular dynamics is applied to the liquid quench simulation of the amorphous networks of BN, AlN and AlBN2. Structural and elastic properties were determined. It is found that AlN has a stronger tendency to chemical order than BN, driven by the greater energy penalty for “wrong bonds.” AlN, however, has a stronger tendency to form an amorphous structure as judged by the energy difference between the crystalline and amorphous states. There is some experimental evidence for an amorphous form of AlN. BN was simulated at two densities, 2.0 and 3.0 g/cm3. Even at the higher density, the fraction of tetrahedral coordination remained low, in contrast to AlN, enabling us to predict that the tetrahedral amorphous form of BN does not form under liquid quench conditions. The amorphous network with the formula AlBN2 has an intermediate tendency to form a tetrahedral structure and has a relatively high elastic modulus. This material is predicted to be of value for application as a wear resistant coating.

Ab initio study of structure in boron nitride, aluminum nitride and mixed aluminum boron nitride amorphous alloys

A tight-binding simulation of the atom-by-atom deposition of amorphous carbon (a-C) at 100 eV incident energy is presented. More than 500 atoms were deposited. Chains are observed to form on the surface, some of which are sputtered. The good agreement with the experimental sputter frequency data and observation that all such clusters are linear provides strong support for the existence of these chains and the direct emission model of sputtering. The bulk of the grown film is a-C with a tetrahedral bonding fraction of 20%. Experiments have shown that at this incident energy of 100 eV, tetrahedral a-C is the preferred structural form rather than the a-C produced by this simulation. This discrepancy is attributed to the short range of the interatomic potential.

https://iopscience.iop.org/article/10.1088/0953-8984/14/4/307/pdf

Surface structure and sputtering in amorphous carbon thin films: a tight-binding study of film deposition

The electron microscope provides an ideal environment for the structural analysis of small volumes of amorphous and polycrystalline materials by enabling the collection of scattering information as a function of energy loss and momentum transfer. The scattered intensity at zero energy loss can be readily processed to a reduced density function, providing information on nearest-neighbour distances and bond angles. A method for collecting and processing the scattered intensity, which allows for the collection of an energy-loss spectrum for a range of momentum transfers, is discussed. A detailed structural determination from a reduced density function alone is difficult and it is shown that a more detailed structural model can be obtained by combining the experimental reduced density function with model structures obtained from molecular dynamics based on first-principles quantum mechanics. This method is applied to tetrahedral amorphous carbon, as an example of a monatomic network, and to aluminium nitride, as a prototype for a binary amorphous alloy.

Experimental and theoretical characterization of structure in thin disordered films.

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