Effects of pulse duration in laser processing of diamond-like carbon films

Experimental studies of the interaction between amorphous hydrogenated carbon (a-C:H) film and short and ultrashort laser pulses in the near-infrared and visible spectral ranges (150 ns and 1064 nm, 10 ns and 1078 nm, 300 ps and 1078 nm, 220 ps and 539 nm, 100 fs and 800 nm.) are reported. The influence of the irradiation conditions (pulse duration, wavelength, laser intensity and the number of laser shots) on the structure and thickness of the laser-induced graphitized layer has been investigated. The effects of heat dissipation on the annealing duration and on the graphitized layer thickness are discussed for the case of laser processing with short pulses. It was found that the resulting morphology of the irradiated a-C:H film surface was determined by the concurrence of three processes: change of the mass density induced by structural transformations, multiple spallations of thin layers, and material evaporation. The laser-induced spallation is asserted to be the main factor limiting the laser microprocessing reproducibility for the examined a-C:H film; its effects were found to increase dramatically for longer (150 ns) laser pulses. The ablation (evaporation) rates of the a-C:H films and glassy carbon were revealed to be similar for femtosecond and picosecond pulses, but they essentially differed for nanosecond pulses. The ablation process demonstrated the same main features for both materials: (i) increase of the ablation rate with the pulse duration, and (ii) saturation of the ablation rate with fluence for picosecond and nanosecond pulses.     

Laser ablation of diamond particles suspended in ethanol: Effective formation of long polyynes

Polyynes were synthesized by laser ablation of diamond particles with various sizes suspended in ethanol. Chain length distributions and total yields of polyynes produced were compared with those produced from graphitized diamond particles and graphite particles. The relative amounts of long polyynes such as C₁₄H₂ and C₁₆H₂ produced from diamond particles were found to be larger than those from graphitized diamond particles and graphite particles. From the change of the chain length distribution with the laser irradiation time, it is concluded that the long polyynes are produced directly from diamond particles at the initial stage of ablation. Furthermore, the total yield of polyynes was found to increase with the size of diamond particles and decrease as their graphitization proceeds. Possible mechanisms of these results are discussed.     

Diamond photoconductors: operational lifetime and radiation hardness under deep-UV excimer laser irradiation

The first study of long term pulse exposure and fluence level on the performance of CVD diamond photodetectors subjected to 193 nm excimer laser radiation has been performed. Whilst diamond is considered ‘radiation hard’ it is shown that damage to detector performance can be provoked at laser fluence levels considerably below that required for graphitisation or ablation. However, the application of defect passivation treatments prior to device use acts to considerably reduce the damaging effect of the radiation, such that devices suitable for stable laser monioring applications can be realised.     

Laser-induced phase transitions in ion-implanted diamond

Results are reported on the study of phase transformations in D⁺ (deuterium) ion-implanted diamond single crystals induced by nanosecond pulses of a KrF excimer laser (λ=248 nm). Multipulse laser irradiation at fluences lower than the graphitization thresholds resulted in progressive annealing, pronounced in an increase of the optical transmission and surface contraction. A non-linear defect distribution in a near-surface layer was found to strongly affect the annealing and graphitization processes; higher annealing efficiency and higher graphitization thresholds were observed under irradiation conditions when a laser beam was incident onto a buried defective layer through diamond, i.e., onto a ‘back’ side of the diamond sample opposite to the ion-implanted side. The influence of non-uniform laser-induced heating of defective diamond material on characteristic features of the phase transitions is discussed.     

Femtosecond laser microstructuring in the bulk of diamond

In this paper we report the fabrication of graphitic microstructures in the bulk of diamond using 120-fs-laser pulses at 800 nm wavelength. Polished plates of single crystal diamond and optical quality polycrystalline CVD diamond were used as samples for 3D microstructuring. Under low fluence conditions and focusing a laser beam into the bulk of diamond plates, multipulse irradiation was found to result in the appearance and continuous growth of a laser-modified (graphitized) region from the focal plane towards the laser beam. Controlling the laser fluence and sample translation velocity (scanning beam velocity) allowed high-aspect-ratio ‘graphitic wires’ – microstructures of a few microns in diameter and several hundred micrometers in length – to be fabricated in the bulk of diamond. Physical processes responsible for the continuous growth of microscopic graphitic regions towards a laser beam are discussed. Results of comparative investigations of graphitic microstructures produced by laser pulses of different durations (120 fs and 300 ps) are presented to show the advantages of ultrashort laser pulses in 3D microstructuring of diamond.     

Diamond film growth in an oxygen atmosphere

We propose a new method for the synthesis of diamond films via a hydrogen-free vapor-phase route of pulsed laser ablation of a graphite target in a low-pressure pure oxygen atmosphere. The evidence from microscopic, diffraction and spectroscopic techniques indicates that high-quality diamond crystals can be nucleated and grown epitaxially on sapphire (single-crystal aluminum oxide) substrates without diamond-powder treatment at temperatures lower than 600°C under the optimized growth conditions of oxygen pressure and pulsed KrF-excimer laser ablation. The spectroscopic property of a laser plasma plume produced during pulsed laser ablation of graphite in an oxygen atmosphere was examined by using time-resolved optical emission measurements in order to discuss the vapor-phase reaction and film growth mechanism.     

激光石墨化过程中激光功率和牵伸力对聚丙烯腈基碳纤维化学结构和微观结构的影响

采用自制的高温激光石墨化平台对聚丙烯腈（PAN）基碳纤维进行石墨化处理,分别在不同激光功率和不同牵伸力条件下制备了多种碳纤维实验样品,利用拉曼光谱和XRD射线衍射研究了不同条件下碳纤维样品的化学结构和微观结构。结果表明：在牵伸力不变时,随着激光功率的增大,碳纤维石墨化程度提高,当激光功率增大到一定值时,单方面继续提高激光功率对于提高碳纤维石墨化程度的影响将变小;在激光功率一定时,随着牵伸力的增大,石墨微晶尺寸L_c、L_a均逐渐增大,而d₀₀₂和取向角逐渐减小,在激光石墨化过程中,施加一定的牵伸力可以促使碳纤维中的石墨微晶沿纤维轴方向择优取向,改善微晶尺寸、减少石墨微晶层间距、提高微晶堆砌层数。     

CVD金刚石膜激光打孔温度场有限元仿真

化学气相沉积金刚石膜是聚优异力学、电学、热学性能于一体的材料.金刚石激光打孔过程中存在特有的石墨化现象使得其温度场的分布有别于其他材料,因而,开展CVD金刚石膜激光打孔温度场研究对于理解热加工机理和进行参数优化具有重要的指导意义.本文首先建立了考虑金刚石石墨化过程的激光打孔热力学模型,根据有限元模型开展对CVD金刚石膜激光打孔有限元仿真研究,得到激光打孔温度场和金刚石石墨化的分布规律,并讨论激光能量、脉冲宽度、重复频率对单个脉冲去除量和石墨化程度的影响.仿真结果表明:单个脉冲平均去除量和石墨化程度均随着激光能量、脉冲宽度、重复频率的增加而增加,但激光能量的影响最为显著;当激光能量取0.5～1.6 J,脉冲宽度取300～800 ns,重复频率取30～60 Hz时既可以得到很高的加工效率同时又可以获得热影响区域较小的加工效果.     

Micro-/nanostructural investigation of laser-cut surfaces of single- and polycrystalline diamonds

Micrometer- to nanometer-scale structures of the cut surfaces of single- and polycrystalline diamonds by a pulsed ultraviolet laser have been thoroughly investigated by scanning and transmission electron microscopy. Within the laser-cut grooves, the processed diamond surfaces are extensively covered with laser-modified debris which consists of complex layered units of graphite with various crystallinities. The units consist of 1) highly oriented graphite, 2) corrugated graphite, and 3) nanocrystalline graphite, which are sequentially located from the surface of the underlying diamond substrate to the center of the grooves. Detailed textural examinations revealed that the highly oriented graphite unit is a product of the initial graphitization of diamond by a solid-state diffusion process, whereas the latter two units are deposition products from the liquid and/or vapor phases of carbon in the later stage. The present study demonstrates that the laser-cutting of diamonds proceeds in a two-step process: 1) extensive graphitization of laser-scanning path and 2) subsequent sublimation of the pre-formed graphite. These processes are basically identical among the three different types of diamonds (single crystal type Ib, single crystal type IIa and nano-polycrystalline aggregate) tested in this study.     

Fabrication of graphitic layers in diamond using FIB implantation and high pressure high temperature annealing

We have used high pressure high temperature annealing (HPHT) for graphitisation of implanted layers in diamond created by 30 keV Ga⁺ focused ion beam. Electron microscopy has been used to investigate the implanted layers. It has been revealed that, unlike annealing at vacuum pressure, the graphitization during HPHT annealing occurred through epitaxial growth of graphite (002) planes parallel to (111) diamond planes. High quality of graphite was confirmed by high resolution electron microscopy and electron energy loss spectroscopy.     

Formation of diamond and graphite at high pressure using glassy carbon source

Experimental study on diamond and graphite formation with presence of metal catalysts under 6 GPa and 1300–1600 °C was carried out using pyrolyzed furfuryl alcohol resin (glassy carbon) or graphite and Mg(OH)₂ mixture. Diamond was formed from glassy carbon pyrolyzed higher than 1500 °C in vacuum. Graphite crystals were dominantly formed when glassy carbons pyrolyzed below 900 °C or graphite containing Mg(OH)₂ higher than 3 wt.% were used as starting carbons even by long reaction time (28 h) or diamond seeding experiments. Degree of graphitization of glassy carbon with catalyst metal was increased markedly under diamond-forming pressure and temperature condition. It was apparent that the lower degree of graphitization of starting carbon is not an essential factor to prevent diamond formation. The results revealed that graphite crystals were grown when starting carbons contained approximately higher than 1000 ppm of hydrogen. It was suggested that if the metal carbon system contains a higher amount of C–O–H fluid than that of threshold, diamond nucleation was prevented and graphite was dominantly precipitated.     

Rapid and low-cost carbon/carbon composites by using graphite slurry impregnated prepregs

A rapid and low-cost carbon/carbon (C/C) composites preparation method is proposed: graphite prepreg-coated carbon fiber fabric (CFF) is formed by hot pressing, followed by hot isostatic pressing and high temperature graphitization, to prepare C/C composite with low porosity and high crystallinity. In this method, the carbon fiber (CF) mass fraction can be precisely regulated in the range of 40–95% by the impregnation process conditions of CFF in graphite prepreg. The graphite particles in the preform were graphitized and bonded with CFF by high temperature graphitization. Finally, a ZrO₂ anti-ablative layer was applied using sol-gel method. The results show that when the CF mass fraction is 50%, the C/C composite with a crystallinity of 92.21 and a porosity of 3.47% can be obtained, with mass ablation rate of 0.23 mg/s and density of 1.62 g/m³. The method can prepare C/C composites with uniform density and high ablation resistance.     

Structure and friction properties of laser-patterned amorphous carbon films

In the paper we report on laser surface modification of super hard micrometer-thick tetrahedral amorphous carbon (ta-C) films in the regime of single-shot irradiation with KrF laser pulses (wavelength 248 nm, pulse duration 20 ns), aimed at investigations of the laser-induced changes of the structure and surface properties of the ta-C films during graphitization and developing ablation processes. Based on the analysis of surface relief changes in the laser-irradiated spots, characteristics of the single-shot graphitization and ablation of the 2-μm-thick ta-C film are determined. Using Raman spectroscopy, it is found that during the graphitization regime the structure transformation and growth of graphitic clusters occur according to the relationship I(D)/I(G) ∝ L_a², but after reaching the ablation threshold the Tuinstra-Koenig relationship I(D)/I(G) ∝ 1/L_a describes further growth of the graphitic cluster size (L_a) during developing ablation of the ta-C film with nanosecond pulses. The maximal size of graphitized clusters is estimated as L_a = 4–5 nm. The studies of nanomechanical properties of laser-patterned ta-C films using the lateral force microscopy and force modulation microscopy have evidenced lower friction forces (between diamond-coated tips and film surface) and lower stiffness in the laser-graphitized areas. The laser-produced graphitic layer acts as a solid lubricant during sliding of the diamond-coated tips on the ta-C film surface in ambient air (~ 50% RH); the lubricating role of adsorbed water layers is suggested to be significant at low loads on the tips. The results of this work demonstrate that the UV laser surface texturing in the regime of graphitization is a promising technique to control the friction and surface elasticity of super hard amorphous carbon films on the micro and nanoscale.     

Growth of diamond nanocrystals by pulsed laser ablation of graphite in liquid

Nanocrystalline diamond has been successfully synthesized at room temperature and pressure using the novel technique of pulsed laser ablation (Nd:YAG, 532 nm) of a graphite target in water. High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and laser Raman spectroscopy have been used to characterise the nanocrystals and to confirm that they are diamond. Time-averaged optical emission spectroscopy showed the presence of H, C and O atoms in the ablation plume. Imaging of the light emitted from the plume showed that H atoms were formed in two regions, in the liquid above the graphite surface, and also in the air just above the water surface. The results are consistent with the idea that atomic H is a necessary requirement for the growth of diamond via this method.     

A kinetic model of diamond nucleation and silicon carbide interlayer formation during chemical vapor deposition

The presence of thin silicon carbide intermediate layers on silicon substrates during nucleation and the early stages of diamond deposition have been frequently reported. It is generally accepted that the intermediate layer is formed by the bulk diffusion of carbon atoms into the silicon carbide layer and the morphology and orientation of the diamond film subsequently grown on the intermediate layer are strongly affected by that layer. While there have been considerable attempts to explain the mechanism for intermediate layer formation, limited quantitative data are available for the layer formation under the operating conditions conducive to diamond nucleation.This study employs a kinetic model to predict the time evolution of a β-SiC intermediate layer under the operating conditions typical of diamond nucleation in hot filament chemical vapor deposition reactors. The evolution of the layer is calculated by accounting for gas-phase and surface reactions, surface and bulk diffusions, the mechanism for intermediate layer formation, and heterogeneous diamond nucleation kinetics and of its dependence on the operating conditions such as substrate temperature and inlet gas composition. A comparison between the time scales for intermediate layer growth and diamond nuclei growth is also performed. Discrepancies in published adsorption energies of gaseous hydrocarbon precursors on the intermediate layer—ranging from 1.43 to 4.61 eV—are examined to determine the most reasonable value of the adsorption energy consistent with observed saturated thicknesses, 1–10 nm, of the intermediate layer reported in the literature. The operating conditions that lead to intermediate layer growth followed by diamond deposition vs. those that yield heteroepitaxial diamond nucleation without intermediate layer formation are discerned quantitatively. The calculations show that higher adsorption energies, 3.45 and 4.61 eV, lead to larger surface number densities of carbon atoms, lower saturated nucleation densities, and larger intermediate layer thicknesses. The observed saturated thicknesses of the intermediate layer may be reproduced if the true adsorption energy is in the range of 3.7–4.5 eV. The intermediate layer thickness increases by increasing substrate temperature and inlet hydrocarbon concentration and the dependence of the thickness on substrate temperature is especially significant. Heteroepitaxial diamond nucleation without intermediate layer formation reported in experimental results can be readily explained by the significant decrease of the intermediate layer thickness at lower substrate temperatures and at higher diamond nucleation densities. Further, the present model results indicate that the intermediate layer thickness becomes saturated when growing diamond nuclei cover a very small surface area of that layer.     

铁基触媒合成金刚石形成的金属包膜成分的研究

利用电子探针(EPMA)和X射线光电子能谱(XPS)研究了包围金刚石单晶的铁基金属包膜和触媒的成分分布。结果表明,在金刚石生长过程中,接近金刚石单晶的包膜内层中的碳含量是变化的,但均高于接近金刚石的触媒层。然而,与包围金刚石单晶的触媒表面相比,包膜表面碳含量低、铁含量高。分析认为,高温高压下,金刚石生长的碳源主要来自于包膜,但碳并非均匀地在包膜熔体内层向金刚石扩散。结合前期研究发现的“包膜内层无石墨和无定形碳结构”的事实分析,金刚石生长所需的碳极有可能来源于包膜内层铁碳化物的瞬间分解,结果导致包膜表面瞬间碳含量低、铁含量高。     

Submicron laser writing on diamond

A synthesized HPHT IIb diamond plate is irradiated with a KrF excimer laser (wavelength λ=248 nm, pulse duration τ =20 ns). The beam is focused onto the surface with an ultrafluar objective [numerical aperture (NA)=1]. The quality of the optical imaging system is first investigated by performing atomic force microscopy (AFM) measurements on holes ablated into Perspex (PMMA). On diamond, holes are drilled at various pulse energies down to values slightly above the ablation threshold. The hole depth and width are measured by AFM after annealing in air at 600°C during 3 h. Holes with submicrometre diameters are obtained, and suggestions on how to produce even smaller holes are given.     

Development of formation and growth models of CO2 hydrate film

This study aims to develop models to estimate the CO₂ hydrate film formation and growth for different temperature and flow velocity conditions. First, the CO₂ hydrate film thickness at the initial stage of its formation is experimentally measured under different temperature and flow velocity conditions using laser interferometry. Based on the results, the CO₂ hydrate film thickness was found to decrease with increasing temperature and flow velocity. Next, the CO₂ hydrate film formation model and growth model are developed, and the models are verified using the present experimental data. Finally, the long term growth of CO₂ hydrate film thickness is estimated by the proposed growth model of CO₂ hydrate film thickness. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4078–4089, 2016     

Initial stages in the growth of polycrystalline diamond on silicon

Polycrystalline diamond films prepared in a hot filament chemical vapour deposition reactor were investigated with Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) in order to identify chemically and structurally distinguishable phases during nucleation and early stages of diamond growth. This was achieved by investigating a series of films grown under identical conditions for 5 min to 4 h. In addition, the interface between a solid diamond film and its silicon substrate was studied after the film had been removed from the substrate. Carbon deposition commences initially with the simultaneous growth of diamond crystallites along scratches and a layer of microcrystalline graphite covering the remainder of the substrate. Small amounts of SiC could also be identified during the first 100 min of deposition. Once the individual diamond crystallites have grown together to form a continuous layer, the graphitic phase in the spectra is replaced by an amorphous carbon phase which we attribute to the grain boundaries between the crystals. Inspection of the film backside revealed that the amorphous carbon had merely overgrown the microcrystalline graphite which was still present as the major component. Only after prolonged growth times (24 h) did the Raman and XPS spectra exhibit the characteristic diamond features free from any other contributions. On the basis of these observations a model for the initial stages of diamond growth on Si is developed.     

Three-dimensional laser writing in diamond bulk

Laser fabrication of micrometer-scale graphitic structures in the bulk of monocrystalline CVD diamond was investigated. Ultra-short (1 ps) pulses of Ti:sapphire laser were tightly focused inside the sample. Initiation of graphitization in the focal volume and propagation of graphitization front towards the laser beam are considered. Electrical resistivity of the produced graphitic material was measured (3.6-3.9 Ω cm). Problems and strategies of 3D laser writing are discussed paying special attention to graphitization homogeneity, minimization of the wire diameter, control of the structure shape and reduction of the surrounding diamond damage. Laser fabrication of complex microstructures including pillars, plates, hexagonal chain and periodic arrays of straight wires has been demonstrated.