Cuilan Ren

Effects of tube diameter and chirality on the stability of single-walled carbon nanotubes under ion irradiation

Using molecular dynamics method, we investigated the influence of tube diameter and chirality on the stability of single-walled carbon nanotubes (CNTs) under ion irradiation. We found that in the energy range below 1 keV, the dependence of CNT stability on the tube diameter is no longer monotonic under C ion irradiation, and the thinner (5, 5) CNT may be more stable than the thicker (7, 7) CNT, while under Ar irradiation, the CNT stability increases still monotonically with the CNT diameter. This stability behavior was further verified by the calculations of the threshold ion energies to produce displacement damage in CNTs. The abnormal stability of thin CNTs is related to their resistance to the instantaneous deformation in the wall induced by ion pushing, the high self-heating capacity, as well as the different interaction properties of C and Ar ions with CNT atoms. We also found that under ion irradiation the stability of a zigzag CNT is better than that of an armchair CNT with the same diameter. This is because of the bonding structure difference between the armchair and the zigzag CNTs with respect to the orientations of graphitic networks as well as the self-healing capacity difference

First-principles study of the effect of phosphorus on nickel grain boundary

Based on first-principles quantum-mechanical calculations, the impurity-dopant effects of phosphorus on Σ5(012) symmetrical tilt grain boundary in nickel have been studied. The calculated binding energy suggests that phosphorus has a strong tendency to segregate to the grain boundary. Phosphorus forms strong and covalent-like bonding with nickel, which is beneficial to the grain boundary cohesion. However, a too high phosphorus content can result in a thin and fragile zone in the grain boundary, due to the repulsion between phosphorus atoms. As the concentration of phosphorus increases, the strength of the grain boundary increases first and then decreases. Obviously, there exists an optimum concentration for phosphorus segregation, which is consistent with observed segregation behaviors of phosphorus in the grain boundary of nickel. This work is very helpful to understand the comprehensive effects of phosphorus.

Compositional and structural studies of DC magnetron sputtered SiC films on Si(111)

Abstract Polycrystalline SiC films were deposited on Si(111) by reactive DC magnetron sputtering using a four inch elemental silicon target. Composition of the deposited films was studied by AES (auger electron spectroscopy) and RBS (Rutherford backscattering spectroscopy) which showed that stoichiometric SiC could be obtained. Both AES depth profile and RBS indicated the existence of a transition layer. XRD (X-ray diffraction) analysis revealed the formation of polycrystalline SiC films at a substrate temperature as low as 1123 K, and the absence of other peaks in XRD patterns except the SiC(111) peak implied that the films were (111) oriented. Furthermore, the films were found to consist of columnar nanometer sized crystallites by cross-section TEM (transmission electron microscopy) study.

Investigation and modeling of the infrared optical properties of direct current sputtered SiC films on silicon

Silicon carbide films were reactively dc sputtered onto Si(111) substrates using a silicon target in a mixed CH4/Ar atmosphere. Non-Rutherford backscattering using a high energy incident He+ beam (4.3 MeV for carbon analysis) and Auger electron spectroscopy were employed to analyze the composition of the films. Structural investigations of the stoichiometric SiC films showed that they were composed of microcrystalline and amorphous SiC. The formation mechanism of the microcrystalline and amorphous SiC during our deposition process was discussed. The optical behavior of the SiC film was studied by infrared (IR) reflectance in the range of 400–4000 cm−1. The experimental IR reflectance in this range was fitted by calculating the complex dielectric function of the films based on effective medium theory, in which the SiC films were assumed to consist of homogeneously distributed SiC (amorphous and crystalline). The fitting of the experimental data using our model is quite satisfactory; thus the assumed model ...

First-Principles Study of Vacancies in Ti3SiC2 and Ti3AlC2

MAX phase materials have attracted increased attention due to their unique combination of ceramic and metallic properties. In this study, the properties of vacancies in Ti3AlC2 and Ti3SiC2, which are two of the most widely studied MAX phases, were investigated using first-principles calculations. Our calculations indicate that the stabilities of vacancies in Ti3SiC2 and Ti3AlC2 differ greatly from those previously reported for Cr2AlC. The order of the formation energies of vacancies is VTi(a) > VTi(b) > VC > VA for both Ti3SiC2 and Ti3AlC2. Although the diffusion barriers for Ti3SiC2 and Ti3AlC2 are similar (~0.95 eV), the properties of their vacancies are significantly different. Our results show that the vacancy–vacancy interaction is attractive in Ti3AlC2 but repulsive in Ti3SiC2. The introduction of VTi and VC vacancies results in the lattice constant c along the [0001] direction increasing for both Ti3SiC2 and Ti3AlC2. In contrast, the lattice constant c decreases significantly when VA are introduced. The different effect of VA on the lattice constants is explained by enhanced interactions of nearby Ti layers.

Compositional and morphological study of reactive ion beam deposited AlN thin films

Abstract AlN thin films were prepared on Si(1 0 0) substrates by reactive ion beam coating (RIBC). The composition and morphology of these films have been analyzed through non-Rutherford backscattering (NBS), Auger electron spectroscopy (AES) and atomic force microscopy (AFM). It was found that the A1/N ratio and the surface morphology depend on the deposition conditions. We also concluded that those films which are nearly stoichiometric all have smooth surfaces. When beyond stoichiometry, the surplus A1 atoms will condense to form protruding tips.

Optical study of low energy ion-beam-assisted deposited diamond-like carbon films

Abstract Diamond-like carbon (DLC) films were prepared on single crystal silicon wafers by low energy ion-beam-assisted deposition (IBAD). The microstructure characteristics, thickness, optical band gap, sp3 fraction and so on, of these DLC films have been analyzed by infrared reflection, Raman and ultraviolet-visible optical spectroscopy experiments. It was found that the infrared reflectivity of the as-deposited DLC films depends on both the wave number and deposition conditions. The measured infrared reflection spectra were fitted by a Bruggeman effective medium approximation (EMA) method. The calculated spectra coincided very well with the experimental data, and it was demonstrated that the as-deposited DLC films were mainly composed of sp3 bonded carbon atoms. The Raman spectra showed a broad asymmetric band in the range of 1500–1580 cm−1, which is a typical characteristic of an amorphous diamond-like structure. The optical band gap of the DLC films was determined to be 0.54–1.0 eV from UV transmittance spectra.

Strain-controlled interface engineering of binding and charge doping at metal-graphene contacts

Strain effects on tuning the interface binding as well as the charge doping at metal-graphene contacts have been investigated by using density functional theory calculations. A realizable tensile strain is found to be very effective in enhancing the interface binding as well as shifting the Fermi level. Particularly, an enhancement of the binding energy up to 315% can be achieved because of the dipole-dipole interaction. Our results presented here show that strain is an efficient way to overcome the weak binding problem at metal-graphene interface, and will motivate active experimental efforts in improving the performance of graphene-based devices.

IR studies of reactive DC magnetron sputtered SiC films on silicon using effective medium theory

Abstract Silicon carbide films were reactively DC sputtered on Si(111) substrates using a silicon target in a mixed CH4/Ar atmosphere. Auger electron spectroscopy (AES) and non-Rutherford backscattering in which an incident He+ beam with high energy (4.3 MeV for carbon analysis) was employed to analyze the composition of the films. The dependence of the optical behavior of the SiC films on their compositions were studied by IR reflectance in the range of 400 to 4000 cm−1. The experimental IR reflectance in this range was fitted by calculating the complex dielectric function of the films based on effective medium theory (EMT), in which the SiC films were consisted of homogeneously distributed SiC (amorphous and crystalline). The fitting of the experimental data using our model is quite satisfactory, which means the model in our simulation is reliable in explaining the IR optical properties of DC sputtered SiC films.

Theoretical study of the substitutional solute effect on the interstitial carbon in nickel-based alloy

The carbon binding in nickel-based alloy with 3d, 4d and 5d transition metal solutes is investigated by using first-principles methods. The first nearest neighbor carbon exhibits repulsive behaviors with most metals except Cr element, which are analyzed from both mechanical and chemical aspects. It shows that the size factor from metal solute is one of the main reasons affecting 1NN carbon-metal binding. Further electronic structures analysis shows that the hybridization of C 2p(z)-M 3d(z)(2) states plays an important role in C-M bonding. The introduced vacancy enhances carbon bonding to most metal solutes through local strain change and charge redistribution. Among all the metal solutes, Cr shows its affinity to carbon which coincides with the previous experimental observation that chromium carbides are commonly precipitated in nickel-based alloys. The present study helps to understand the carbon-metal solute interaction in nickel-based alloys.