Igor I. Vlasov

Large-area diamond deposition by microwave plasma

Abstract Diamond films of up to 1 mm thickness and of 30 cm2 area have been grown in a 8 kW MW plasma reactor ASTeX PDS-19 using CH4/H2/O2 gas mixtures. Growth rate, film thickness radial profiles, diamond morphology and quality were evaluated in dependence substrate temperature (T=690–900° C), and feed gas composition (CH4/H2=2.5%, O/C=0-0.7). Translucent diamond wafers have been produced without any sign of nondiamond carbon phases. Raman diamond peak being as narrow as 2.5 cm−1. The presence of some impurities (N, H, Si and Al) was detected and analyzed with SIMS and IR optical absorption spectroscopy. An interesting type of diamond growth instability at certain deposition conditions is observed that manifests itself in the form of accelerated growth of selected diamond crystallites of very big lateral size (≈ 1 mm), and of more perfect structure compared to the rest of the film.

Nitrogen and Luminescent Nitrogen‐Vacancy Defects in Detonation Nanodiamond

An efficient method to investigate the microstructure and spatial distribution of nitrogen and nitrogen-vacancy (N-V) defects in detonation nanodiamond (DND) with primary particle sizes ranging from approximately 3 to 50 nm is presented. Detailed analysis reveals atomic nitrogen concentrations as high as 3 at% in 50% of diamond primary particles with sizes smaller than 6 nm. A non-uniform distribution of nitrogen within larger primary DND particles is also presented, indicating a preference for location within the defective central part or at twin boundaries. A photoluminescence (PL) spectrum with well-pronounced zero-phonon lines related to the N-V centers is demonstrated for the first time for electron-irradiated and annealed DND particles at continuous laser excitation. Combined Raman and PL analysis of DND crystallites dispersed on a Si substrate leads to the conclusion that the observed N-V luminescence originates from primary particles with sizes exceeding 30 nm. These findings demonstrate that by manipulation of the size/nitrogen content in DND there are prospects for mass production of nanodiamond photoemitters based on bright and stable luminescence from nitrogen-related defects.

Core-shell designs of photoluminescent nanodiamonds with porous silica coatings for bioimaging and drug delivery I: fabrication

A multifunctional core-shell nanocomposite platform consisting of a photoluminescent nanodiamond (ND) core with uniform porous silica coatings is presented. This design intended for drug delivery applications allows simultaneous stable fluorescent imaging with high loading capacity of bioactive molecules. Despite irregularly shaped starting cores, well-dispersed and uniformly shaped nanocomposite particles can be produced. Moreover, after optimization of the silica source-to-diamond ratio, the thickness of the porous layer can be tuned by adjusting the ethanol amount, allowing rational nanoparticle size control. The ND key property, photoluminescence, is not quenched regardless of coating with thick silica layers. The high loading capacity for incorporation of active agents, provided by the introduced porous layer, is demonstrated by adsorption of a hydrophobic model drug to the composite particles. The loading degree, as compared to a pure ND, increased by two orders of magnitude from 1 wt% for the ND to >100 wt% for the composite particles. Combining these two material classes, which both have well-documented excellent performance especially in biomedical applications, for the NDs with emphasis, but not exclusively, on imaging and mesoporous silica (MSN) on drug delivery, the advantages of both are shown here to be synergistically integrated into one multifunctional nanocomposite platform.

Ablation of CVD diamond with nanosecond laser pulses of UV-IR range

Etch rates of CVD diamond upon irradiation by nanosecond (5‐9 ns) pulses at three diVerent wavelengths 1078, 539 and 270 nm at laser fluences in the range 1‐1000 J/cm2 were measured. A Nd:YAP laser system operated at first, second and fourth harmonics was used in the ablation experiments. Both shallow (<15 microns) and through holes were etched in a 95-mm thick free-standing diamond film grown by microwave plasma CVD. The ablation rate was found to be wavelength-independent, this result being ascribed to surface blackening caused by amorphization/graphitization as confirmed by Raman analysis. The maximum etch rate approached 600 nm/pulse. The etch rate depended on the crater depth, which was ascribed to the eVect of laser‐plasma interaction inside the deep channel. The possibility of cutting trenches of high aspect ratio has been demonstrated. In a separate experiment, a batch of thin diamond films diVering in thermal conductivity (k=2‐5 W/cmK ) was ablated with a KrF excimer laser (l=248 nm). No dependence of ablation rate on film quality was observed, which could be explained assuming grain boundaries to be the main source of thermal resistance.

Structure and properties of high-temperature annealed CVD diamond

Abstract Effects of high temperature, up to 1700 °C, annealing in vacuum of CVD diamond on its structure, optical and mechanical properties are investigated. Translucent polycrystalline diamond films of thickness 0.06–1.0 mm were grown by microwave plasma CVD method, and examined with transmission electron microscopy, optical absorption spectroscopy, and three-point bending technique to measure the fracture strength. A progressive darkening of the samples, with appearance of absorption features specific for graphite-like material, was observed upon annealing at temperatures above 1300 °C. The formation along grain boundaries of amorphous carbon and/or well crystallized graphite layers, 5–20 nm thick, as well as intra-granular graphite islands, was directly observed with TEM. This internal diamond-graphite transformation process can be described by two activation energies, both values being much less than those known for the surface graphitization of diamond. The fracture strength of the diamond films increases up to 50% with annealing temperature (1460–1640 °C), this being ascribed to a build up of compressive stress as a result of local diamond-graphite conversion.

Effect of high temperature annealing on optical and thermal properties of CVD diamond

Abstract Structural changes in diamond films of different qualities caused by annealing in vacuum up to 1600°C have been studied by IR and UV-visible optical absorption, Raman and photoluminescence spectroscopy. An internal degradation of the diamond films and a strong optical absorption enhancement in the whole UV-vis-IR range take place at T>1300°C, and correlate with the loss of bonded hydrogen. At least 25% of the total amount of hydrogen is found to be in the unbound state in some of the as-grown (untreated) films. The diamond darkening is ascribed to appearance of graphite-like phases presumably at grain boundaries. Activation energy of GB transformation process is much lower (250–530 kJ/mol) compared to surface graphitization of single crystal diamond. No evidence of charge transfer altering the concentration of substitutional nitrogen N0S and NA−ND (in B-doped films) upon annealing was found. Thermal conductivity measured by laser flash technique remains almost constant (20 W/cmK) even after annealing to 1575°C, then catastrophically drops because of crack development in the film.

Stress mapping of chemical-vapor-deposited diamond film surface by micro-Raman spectroscopy

A confocal Raman spectroscopy was used to measure intrinsic stress distribution on the growth surface within individual grains of chemical-vapor-deposited diamond film. Polarization analysis of the Raman line shape revealed that even in high quality (2.8 cm−1 linewidth), free-standing film of 0.6 mm thickness, small regions exist where high local stresses (both compressive and tensile) develop. The stressed regions tend to appear near crystal edges and grain boundaries. A strong gradient in defect or impurity concentrations is supposed to cause the stress fluctuations observed.

Predicting the distribution and stability of photoactive defect centers in nanodiamond biomarkers

Recent observations of photoactive silicon-vacancy (Si-V) color centers in diamond nanoparticles less than 10 nm in size has prompted interest in this material for optical labeling in biomedical applications. In order to be useful in such situations, these Si-V defects need to be in sufficient concentrations, and must be stable with respect to diffusion at room and body temperature. In this paper, density functional tight binding simulations are used to systematically examine the configuration, distribution and thermodynamic stability of the neutral and charged Si and Si-V centers in representative diamond nanocrystals. The results indicate that the stability of neutral Si-V is superior to other Si-related defects when the particle surfaces are suitability passivated, in agreement with PL measurements. Based on these results we show the size dependence, and thermal stability of this defect make fluorescent nanodiamond an ideal candidate for biomarkers.

Fabrication of CVD Diamond Optics with Antireflective Surface Structures

Polycrystalline CVD diamond is an excellent material for advanced optical applications, especially in the IR spectral range. However, one drawback of diamond is the significant reflection loss of 29%, caused by its high refraction index n = 2.4. We fabricated subwavelength, “moth-eye” antireflective structured (ARS) surfaces (two-dimensional array of pyramids) by filling with CVD diamond the inverted pyramids etched in a Si substrate, followed by the substrate removal to obtain the patterned diamond replica. An increase in IR transmission up to T = 80% was observed at wavelengths λ > 10 μm for the ARS surfaces compared to T = 71% for flat surfaces even at the non-optimized geometry of the surface relief. A further increase in transmission could be achieved by combining ARS and a single layer AR coating of amorphous carbon.

Electrodeposition of nanostructured diamond-like films by oxidation of lithium acetylide

Diamond-like carbon (DLC) films have been deposited by anodic oxidation of 4 M solution of lithium acetylide in dimethylsulfoxide on the surface of stainless steel or nickel electrode at room temperature and moderate anodic current densities (0.2–2.0 mA/cm2) in the range of electrode potentials 0.3–2.5 V (vs. sat. Ag|AgCl reference electrode). Electrodeposited DLC coatings represented complete and optically transparent films of a thickness 50–100 nm having dark island inclusions with a diameter 0.8–5.0 μm. The concentration and average size of these particles increased with the prolongation of deposition time. Micro-Raman spectra obtained by the focusing of laser beam onto these dark inclusions are characterized by a broad peak centered at 1500 cm−1 and weak peak at 1200 cm−1. With a defocused laser beam, there appear two well-distinguished peaks on the integrated Raman spectra – at 1530 and 1130 cm−1. Analysis of Raman spectra with the use of a Breit–Wigner–Fano lineshape and spectrum deconvolution indicates that the electrodeposited films consist of diamond-like nanostructured carbon with a high content (70–80%) of sp3 phase.