Disorder and optical properties of amorphous carbon

Tight-binding calculations shed light onto the link between disorder and optical properties in pure amorphous carbon films. It is shown that the Urbach energy E_U is an excellent probe of disorder in such films. Our striking finding is that E_U varies non-monotonically with sp³ fraction and optical gap, contrary to what is seen experimentally in hydrogenated samples. Firm evidence is provided, supported by experimental measurements, that in dense films the higher the sp³ fraction the lower the disorder. The Urbach energy has a value of ∼ 0.09 eV for the 100% sp³-bonded network, it reaches a maximum of ∼ 0.3 eV for 65% sp³ content, and it then declines to low values for low-sp³ content films. Analysis of cluster distributions and bond-angle and -length distortions reveals that the Urbach edge is associated to both topological and structural disorder. Another notable finding is that the edge is rather insensitive to the spin density due to unpaired sp² sites.     

Impact of structural short-range order and tetrahedral germanium on electronic and dielectric properties in phase-change materials

The optoelectronic properties of prototypal Telluride amorphous phase-change materials (GeTe and Ge₂Sb₂Te₅) are investigated from ab initio molecular dynamics simulations. Local tetrahedral germanium geometries are identified from topological angular constraint counting and this permits to relate exactly their contribution to targeted properties. The analysis of our computation reveals that the dominant population of tetrahedral Ge contributes to the tail of the valence and conduction band but with an increased electronic localization for the latter, whereas residual (essentially octahedral) geometries induce an overall constant localization at $$ except close to the Fermi gap, where p electrons are largely delocalized. The detailed calculation of the atomic dipoles in the amorphous state indicates that tetrahedral Ge leads to lower momenta, especially in a-GeTe, and corresponding Ge-based correlations with Wannier centers also indicate the dual nature of the local geometries. These features which drive optical and dielectric contrast exemplify the unique properties of phase-change materials, and represent an obvious breakdown of the well-known Zachariasen rule stating that the short-range order is similar in crystals and glasses.     

Atomic and electronic structure of crystalline–amorphous carbon interfaces

Interfaces of diamond with amorphous carbon (a-C) are investigated using tight-binding molecular dynamics simulations. Such interfaces are found to be stable, with a-C atoms covalently bonded to the diamond surfaces. The atomic and electronic structure of the a-C region is consistent with previous results on pure a-C and does not depend critically on the diamond face exposed. However, surface properties influence the relative stability of interfaces with high density a-C. In this case, the interfacial region is small and very dense homogeneous a-C grows on diamond. At lower densities, carbon atoms nucleate on diamond surfaces and create an intermediate region between diamond and a-C. The shape of diamond crystals embedded in a-C is predicted using the Wulff construction with appropriately defined energies for surfaces with overlayer material. These predictions are verified by empirical potential simulations of nanodiamond inclusions in a-C matrix. The electronic density of states and the dielectric function, calculated for our samples containing a-C/diamond interfaces, show that optoelectronic properties of these composite materials are dominated by a-C.     

Structural and electronic properties of highly photoconductive amorphous carbon nitride

We have studied highly photoconductive amorphous carbon nitride (a-CN_x) films prepared by RF-reactive magnetron sputtering. a-CN_x is attractive as a potentially high performance photoconductive semiconductor suited to apply for such as an electron photography, ultraviolet (UV) detector and electron emitter, etc. In this paper, it is reported about the effects of high magnetic field on the sputtering process of a-CN_x films. The experimental results show that films prepared in the higher magnetic field made by samarium cobalt magnets have less CN triple bonding states, less electron spin density N_S=10¹⁸ cm⁻³ and smoother surface morphology than that made by the usual magnetron sputtering using ferrite magnets. From these results, high magnetic field is effective to prepare hard and stable a-CN_x films. We will also report a model of electronic density of states of a-CN_x especially stressed on the Urbach energy region. A preliminary experiment on electroluminescence is also reported.     

Tuning the electronic and optical properties of monatomic carbon chains

The electronic, mechanical and optical properties of the monatomic carbon chains recently fabricated are investigated by hybrid density functional calculations. It is shown that the chain owns a direct band gap of 2.21 eV and the ultimate strength of 12.2 nN, which are in good agreements with experiments. The light absorption shows only one sharp peak in a wide photonic energy range up to 10 eV and exhibits a strong anisotropic feature as the absorption of the visible light polarized along the chain axis is five orders of magnitude stronger than that of the light polarized perpendicularly to the chain. By applying elastic strains on the chain, the band gap can be continuously changed from 1.58 to 3.86 eV with the absorption peak shifting correspondingly. Then a realistic graphene-chain device is proposed and is verified to be capable of applying strains up to 9% on the chain, suggesting potential applications in nano-size electro-optical and polarization devices with tunable working wavelengths from 345 to 561 nm.     

Probing the electronic structure of multi-walled carbon nanotubes by transient optical transmittivity

High-resolution time resolved transmittivity measurements on horizontally aligned free-standing multi-walled carbon nanotubes reveal a different electronic transient behavior from that of graphite. This difference is ascribed to the presence of discrete energy states in the multishell carbon nanotube electronic structure. Probe polarization dependence suggests that the optical transitions involve definite selection rules. The origin of these states is discussed and a rate equation model is proposed to rationalize our findings.     

The relationship between structure and mechanical properties of hydrogenated amorphous carbon films

There is increasing interest in the relations between Raman fit parameters and the mechanical properties of diamond-like carbon films. The present work describes these relations in hydrogenated diamond-like carbon films (a-C:H) deposited by an ion beam source operated at varied discharge voltages, i.e. kinetic carbon species energies. A number of highly distinct relations between Raman fit parameters and mechanical properties are identified for the a-C:H films investigated. For example the nanohardness (H) and reduced elastic modulus (E) increase almost linearly with an increase in full width at half maximum of the G-band (FWHM (G)). The film elasticity, expressed as H³/E² increases with increasing FWHM (G). In addition, H and E increase linearly with decreasing intensity ratio of the D-band and the G-band (I_D/I_G). H and E also increase with the G-band dispersion (Disp. (G)), i.e. the rate of change of the G-band position vs. excitation energy. Hydrogen contents in all films are approximately equal and range from 21.2 to 23.5 at.% over the entire set of investigated samples.     

Effects of acetylene addition on mechanical and dielectric properties of amorphous carbon films

The para-xylene added with acetylene from 15% to 50% was plasma polymerized at 50 to 150 W to deposit the a-C:H films. After the films were annealed from 200 to 400 °C, the network structure, hardness and dielectric constant of films were analyzed by FT-IR, Raman, nanoindentor and capacitance–voltage plot, respectively. Those measured results suggest that hydrocarbon bonds and oxygen related bonds of the a-C:H film effectively reduce and the number of ordered aromatic rings increases with decreasing the deposition power after annealing at 400 °C. In addition, both the dielectric constant and the hardness, respectively, increase up to 2.82 and 2.37 GPa, but the adhesion strength decreases with increasing the C₂H₂ concentration and deposition power. Therefore, the a-C:H films not only have a lower dielectric constant, but also have enough mechanical strength for the IC processing.     

Relationship between bonding structure and mechanical properties of amorphous carbon containing silicon

Unhydrogenated amorphous carbon films with different silicon concentrations were synthesized by magnetron sputtering, and the corresponding evolution of inter-atomic bonding configurations, surface roughness and mechanical properties like hardness, modulus and stress was analyzed. Introducing silicon into amorphous carbon not only reduced the sp²-hybridized carbon bonding, it also helped to reduce residual stress. Both the hardness and elastic modulus suffered degradation when the silicon concentration was low. But these properties recovered when silicon dosage increased. Surface roughness increased when silicon concentration was low, but decreased when the silicon dosage increased. Such changes in the mechanical properties were closely related to the carbon and silicon inter-atomic interaction. The amorphous carbon network was modified by silicon, and affected by deposition kinetics. The mismatch in the atomic size and bond length, and the alteration of the carbon hybridization were determined to be the basis for the changes in the mechanical properties.     

A theoretical evaluation of the effect of water on the electronic properties of low density amorphous carbon nanoparticles

Monte Carlo simulations combined with first-principles calculations were carried out to investigate the structure and electronic properties of amorphous carbon nanoparticles containing between 287 and 467 atoms, isolated and in an aqueous environment. Carbon interactions were described in terms of the semiemperical Tersoff potential and the interactions of the nanoparticles with water were described by an all-atom Lennard–Jones potential. The nanostructures were generated from low mass density amorphous carbon phases simulated under ambient conditions. Our study has indicated that the formation energy of these nanomaterials is slightly affected by the initial configuration of the amorphous phase. The large number of dangling bonds present in the nanostructures leads to huge magnetic moments, in a 31–42 μ_B range, calculated for the amorphous nanostructures containing distinct sp²/sp³ ratios. Also, these nanoparticles exhibit strong interaction energies with the aqueous environment, which are in agreement with the high reactivity expected for these amorphous systems. Most importantly, the calculated magnetism appears to be reduced between 1% and 14% in the presence of water, depending on both the nanoparticle density and surface.     

Tuning electronic structure and optical properties of ZnO monolayer by Cd doping

Structural, electronic and optical properties of Cd-doped ZnO monolayer have been investigated using first-principles density-functional theory based on the local density approximation plus Hubbard U approach, which precisely predicts the band gap. The formation energy of doped system is negative, implying a stable incorporation of Cd. The band gaps of Cd-doped ZnO monolayer decrease with increasing Cd concentration. Furthermore, Cd doping is found to result in a red-shift of the absorption peaks, enhancing the visible light absorption. These findings demonstrate that Cd-doped ZnO monolayer could display potential application in optoelectronic and photocatalytic fields.     

Effects of matrix structure on mechanical properties of carbon/carbon composites

Mechanical properties of carbon/carbon composites prepared by thermal-gradient CVD technique were investigated by three-point flexural tests. The mechanical properties are strongly influenced by the matrix structure, which in turn depends on the deposition conditions employed. The lower fracture stress and elastic modulus of isotropic structure and columnar structure are due to the low matrix density and to the presence of matrix cracks, respectively. The transition structure shows relatively high modulus and stress. The fracture strain remains almost constant over the whole ranges investigated. The role of the matrix crack during the fracture is discussed.     

Effects of needle-punched felt structure on the mechanical properties of carbon/carbon composites

The effects of needle-punched felt structure, including mass ratio of non-woven cloth to short-cut fiber web, PAN-based carbon fiber types of non-woven cloth and thickness of unit (one layer of non-woven cloth and short-cut web was named as a unit), on the flexural properties of C/C composites from pressure gradient CVI are discussed. Results show that flexural strength and modulus increase when mass ratio of non-woven cloth to short-cut fiber web changes from 7:3 to 6:4 and that PAN-based carbon fiber types of non-woven cloth strongly influence the flexural properties. The strength of C/C composites is not linear with the strength of non-woven cloth carbon fiber because of the important interface between carbon fiber and matrix carbon. It is suitable to choose T300 or T700 as reinforcing carbon fiber for C/C composites in the present study. An optimum unit number per cm of the needle-punched felts for higher flexural properties exists.     

单斜态Na0.2TiO2电子结构和光学性质的研究

采用基于密度泛函理论的平面波赝势方法,对Na0.2TiO2的能带结构、电子态密度和电荷密度进行了分析计算。结果表明,Na0.2TiO2属于一种间接带隙半导体,禁带宽度为2.81eV;其电子态密度主要由Ti的3d层电子和O的2p层电子的能态密度决定;该材料对紫外线有较强的宽频吸收,拓展了光谱响应范围,提高太阳能利用转化率和光催化效率;H-O键比Na-O键结合的更紧密,Na原子与O原子形成的是离子性较强而共价键较弱的混合键,Ti-O键的共价作用要强于Na-O键。因此,可以通过阳离子交换来调变钛酸钠的键特性及稳定性。     

Effect of ion bombardment and hydrogen pressure during deposition on the optical properties of hydrogenated amorphous carbon thin films

Carbon-based thin films are ideal materials for several state-of-the-art applications, such as protective materials and as active films for organic electronics, medical, optoelectronic devices. In this work, we study in detail the effect of the ion-bombardment and the hydrogen partial pressure during deposition on the optical properties of hydrogenated amorphous carbon (a-C:H) thin films grown onto c-Si substrates by rf magnetron sputtering. The optical properties of the a-C:H films were investigated by phase modulated Spectroscopic Ellipsometry in a wide spectral region from the NIR to the Vis-far UV (0.7-6.5 eV). A dispersion model based on two Tauc-Lorentz oscillators, has been applied for the analysis of the measured < ε(ω)> of the a-C:H films to describe the π-π* and σ-σ* interband electronic transitions, that can describe accurately the optical properties of all amorphous carbons. The applied V_b influences the bombardment of the growing thin films with Ar ions affecting the content of sp² and sp³ hybridized carbon bonds in the films. As it was found, the increase of the applied negative voltage reduces the optical transparency of the a-C:H films. Also, the H incorporation has been found to change only the energy position of the σ-σ* transitions. Finally, from the study of the refractive index n(ω = 0 eV) it has been found that the increase of the ion bombardment during the films deposition is correlated to an increase in the films density.     

制备条件对Ni-P非晶态结构及其对催化硝基苯加氢的影响

采用化学还原法制备了Ni-P非晶态催化剂,研究了制备过程中pH值及P/Ni摩尔比对催化剂结构及硝基苯催化加氢合成苯胺的影响。结果表明,在Ni-P非晶态催化剂的制备过程中,制备液初始pH值和P/Ni摩尔比是影响催化剂物化性质的主要因素。高pH值下制备的Ni-P催化剂为非晶态结构,适宜的制备液初始pH值为11.0,此时催化剂的热稳定性好,比表面积为13.7m2/g。当P/Ni摩尔比低于4时,Ni-P催化剂中Ni-P非晶态相的含量较少,适宜的P/Ni摩尔比为4。添加P/Ni摩尔比为4,在制备液pH值为11.0条件下制备0.2gNi-P非晶态催化剂,反应3h后硝基苯的转化率及苯胺的收率分别为55.2%和53.8%。     

Dispersion relations and optical properties of amorphous carbons

A dispersion model based on two Tauc–Lorentz oscillators (2-TL), to describe the π–π^⁎ and σ–σ^⁎ transitions, is applied to a wide variety of amorphous carbons grown by various vapor deposition techniques. The application of identical analysis to the various samples enables the quantitative comparison between various forms of amorphous carbon. The model is applied to spectroscopic ellipsometry data and can describe accurately the optical properties of all amorphous carbons. A validation is performed based on wide-range electron energy-loss spectroscopy spectra. This approach, universal for all carbons, extends the single TL model and can determine accurately the energy position of the π–π^⁎ transition and estimate fairly the σ–σ^⁎ transitions in addition to the E_g; these parameters as well as the refractive index are correlated to the sp³ content and density of all amorphous carbons.     

Microstructure and electronic properties of pulsed-discharge-deposited amorphous carbon-nitride films

Amorphous carbon-nitride films were grown on the nitridated-diamond substrates by pulsed-discharge of pure nitrogen gas using graphite rods as the discharge electrodes. The deposition conditions were optimized by monitoring the discharge plasma with optical-emission spectroscopy. It is demonstrated that the films were mainly a mixture of nanosized carbon and carbon-nitride with sp² and sp³ phases. Preliminary results show that the deposited carbon-nitride films exhibit semiconductor behavior and have a cold-cathode-emission property, which make them possible to be superior electronic materials. Improvement in the conductivity and field-emission properties was observed after chemical etching with hydrofluoric acid. The activation energy for electrical conduction of the HF-treated film decreased from 3.42 to 1.19 eV. The similar threshold voltages were obtained for the carbon-nitride films before and after chemical etching, which were 4.0 and 3.5 V/μm, respectively. However, the emission current density after etching increased by one order of magnitude.     

A correlation between the microstructure and optical properties of hydrogenated amorphous carbon films prepared by RF magnetron sputtering

Comparative and systematic studies of the effect of the radiofrequency (RF) bias on the microstructure and the optical properties of hydrogenated amorphous carbon (a-C:H) have been carried out on films deposited by RF magnetron sputtering under different RF power varying from 10 to 250 W applied to the graphite target, leading to a negative bias voltage at the target in the range of −60 to −600 V.A combination of infrared (IR) absorption experiments, which give information about the local microstructure (i.e. C–C and C–H bonding), and optical transmission measurements in the UV-visible and near IR, from which we determined the optical gap E₀₄ and the refractive index n, are applied to fully characterize the samples in their as-deposited state. The results show first that the films deposited at low RF power (i.e. low negative bias) exhibit a more open microstructure (polymeric character) with a lower density than those deposited at high RF power (i.e. high negative bias). They also indicate that the total bonded H content as well as the sp³/sp² ratio of carbon atoms bonded to H decrease with increasing RF power leading to the formation of higher proportions of C-sp² sites. The same tendency is observed for the optical gap E₀₄. On the contrary, the refractive index increases with increasing RF power, suggesting the densification of the films in going to a higher RF power.     

Optical properties and new carbon forms of sputtered amorphous carbon films

Amorphous carbon films were deposited by r.f. magnetron sputtering at various bias voltages V_b applied on Si substrate. We studied the optical properties of the films using in situ spectroscopic ellipsometry (SE) measurements in the energy region 1.5–5.5 eV. From the SE data analysis the dielectric function ε(ω) of the a-C films was obtained, providing information about the electronic structure and the bonding configuration of a-C films. Based on the SE data the films are classified in three categories. In Category I and II belong the films developed with V_b≥0 V (rich in sp² bonds) and −100≤V_b<0 V (rich in sp³ bonds), respectively. The dielectric function of the films belonging in these two categories can be described with two Lorentz oscillators located in the energy range 2.5–5 eV (π–π^*) and 9–12 eV (σ–σ^*). A correlation was found between the oscillator strength and the sp² and sp³ contents. The latter were calculated by analyzing the ε(ω) with the Bruggeman effective medium theory. In films deposited with V_b<−100 V (Category III), the formation of a new and dense carbon phase was detected which exhibits a semi-metallic optical behavior and the ε(ω) can be described with two oscillators located at ∼1.2 and ∼5.5 eV.