S. Tóth

Grain boundary fine structure of ultrananocrystalline diamond thin films measured by Raman scattering

Structural units of the grain boundaries in ultrananocrystalline diamond thin films with different grain sizes were investigated using Raman spectroscopy. Characteristic peaks of well-defined molecular structural building blocks were detected in the near-infrared excited Raman spectra of these materials by limiting the excitation volume to the size of the crystallites using an optical microscope and surface enhanced Raman spectroscopy. The analysis of the spectra provides evidence for the presence of aromatic hydrocarbons and different sp3 CHx groups in grain boundaries of these materials.

Multi-band structure of amorphous carbon luminescence

Abstract Structured photoluminescence (PL) spectra of hydrogenated amorphous carbon (a-C:H) layers prepared from benzene are presented. The spectra contain bands with peak positions in the range of 4.34–4.50, 3.93–4.01 and 3.64–3.79 eV in the ultraviolet region, and additional luminescence bands are in the energy regions of 3.19–3.28 and 2.85–2.96 eV, besides the previously mostly measured band, with a peak in the 2.28–2.48 eV photon energy range. Relative efficiency of these bands depends on the deposition conditions. Each of the new bands could be excited above a given photon energy and, therefore, the overall spectral shape as well as the structured feature of a-C:H luminescence varies with excitation energy. It is supposed that the opening of new radiation recombination transitions with increasing excitation energy explains the appearance of new luminescence bands.

Composite character of the photoluminescence in hydrogenated amorphous carbon films

Abstract Composite photoluminescence (PL) spectra, consisting of numerous characteristic bands, are presented for hydrogenated amorphous carbon (a-C:H) layers, prepared from methane or benzene. Some of these appear already in the visible range of excitation, the others can be excited by UV light only. Three peaks with maximum position in the range 4.34–4.50, 3.93–4.01 and 3.64–3.70 eV are in the ultraviolet region. Additional peaks appear in the ranges 3.17–3.22 and 2.85–2.92 eV beside the well-known broad PL band with maximum in the 2.1–2.33 eV range. The relative intensities vary on deposition conditions. Our results measured on numerous samples strongly suggest the existence of some type of intrinsic radiative centers.

Sp2 carbon defects in nanocrystalline diamond detected by Raman spectroscopy

The properties of nanocrystalline diamond (NCD) thin films are significantly affected by the defects found in the interfacial regions between the diamond crystallites (in the so called grain boundaries). Dominant sources of these defects are the sp2 hybridized carbon atoms that terminate the sp3 diamond lattice and interconnect the neighboring crystallites. The detailed evaluation of these structural units is of great importance for practical applications of NCD. Since sp2 hybridized C atoms have high Raman scattering cross-section, Raman spectroscopy could be a valuable method for the determination of bonding configuration of these defects. In this work near-infrared excited Raman spectroscopy and surface-enhanced Raman spectroscopy were used to investigate the sp2 structural units in grain boundaries of different NCD thin films.

Creation of deep blue light emitting nitrogen-vacancy center in nanosized diamond

This paper reports on the formation of complex defect centers related to the N3 center in nanosized diamond by employing plasma immersion and focused ion beam implantation methods. He+ ion implantation into nanosized diamond “layer” was performed with the aim of creating carbon atom vacancies in the diamond structure, followed by the introduction of molecular N2+ ion and heat treatment in vacuum at 750 °C to initiate vacancy diffusion. To decrease the sp2 carbon content of nanosized diamond formed during the implantation processes, a further heat treatment at 450 °C in flowing air atmosphere was used. The modification of the bonding properties after each step of defect creation was monitored by Raman scattering measurements. The fluorescence measurements of implanted and annealed nanosized diamond showed the appearance of an intensive and narrow emission band with fine structures at 2.98 eV, 2.83 eV, and 2.71 eV photon energies.

Electronic structure of pulsed laser deposited carbon thin films monitored by photoluminescence

Abstract The influence of pressure of methane atmosphere on the pulsed laser deposited (PLD) amorphous carbon layers (a-C:H) were studied, on the structure on atomic scale as well as on the electronic levels near the band edges. Raman scattering, measured by two different probing wavelengths, show the absence of graphitic ordering of sp 2 hybridized carbons. The samples prepared at low methane pressure are characterized with spectra, similar to the spectra of diamond-like carbon. Infrared absorption of PLD films is dominated in the CH bond stretching region by the bands due to sp 3 CH 3 and sp 3 CH 2 vibrations, however, a weak, widely distributed band also appears over 3000 cm −1 . Photoluminescence (PL) can be excited in these samples with decreasing wavelength below 400 nm (over 3.1 eV) and a steep increase of PL intensity occurs below 250 nm (over 4.9 eV). Some distinct PL bands can be distinguished in wavelength region of 270–450 nm (4.6–2.7 eV), the appearance and intensity of which shows correlation with the applied methane pressure.

Nickel-Silicon Related Color Center Formed in Nanodiamond Grains under CVD Growth

Formation of optical centers in nanodiamond grains with narrow, near-infrared emission at room temperature is one of the most important challenges nowadays. Our aim was to form a metal-related color center through the CVD growth process of nanodiamond. Previously undocumented photoluminescence (PL) system with 865 nm zero-phonon line (ZPL) and 2 nm full width at half maximum (FWHM) was successfully created in nanodiamond grains. According to the detailed analysis of the spectral features of the ZPL and quasilocal modes of the vibronic sideband, a complex center containing Ni and Si atoms could be accounted for these PL features. The inclusion of Ni and Si impurity atoms in the complex optical center was strengthened by micro-Raman spectroscopy performed in the frequency range due to quasilocal vibrations of the vibronic sideband.

Influence of microwave plasma parameters on light emission from SiV color centers in nanocrystalline diamond films

Abstract Zero phonon line (ZPL) shape, position and integral intensity of SiV defect center in diamond is presented for nanocrystalline diamond (NCD) films grown at different conditions, NCD films of average grain sizes from ~50 nm up to ~180 nm have been deposited onto c-Si wafer at substrate temperature of 700 and 850oC from mixture with different CH4 and H2 ratios using MWCVD process. Light emission of SiV defect center and Raman scattering properties of NCD samples were measured on a Renishaw micro-Raman spectrometer with 488 nm excitation. Scanning electron microscopy images were used for monitoring surface morphology and for the analysis of the average grain sizes. Sample thickness was determined by in situ laser reflection interferometry. Characteristics of SiV ZPL are discussed in light of the morphology, bonding structure and average grain size of NCD films.

Raman Analysis of Diamond-Like Carbon Films Deposited onto Corrosion Resistant Alloys Used for Coronary Stent Fabrication

The structure of diamond-like carbon thin films prepared by radio frequency chemical vapor deposition onto the surface of different corrosion resistant alloys (304, 316L and Iconel 617) used for coronary stent fabrication was investigated by Raman spectroscopy excited by visible (488 nm) and near-infrared (785 nm) light. The effect of electrochemical polishing of the alloy surface and that of Ar+ ion pre-treatment on the structure of the forming film was also studied in case of substrate made of 316L material. It was found that in spite of simultaneous preparation the character of the formed amorphous carbon films depends on the type of the alloy. The layer developed on the surface of Fe-rich stainless steels (316L and 304) showed the most diamond-like character, while the Ni-rich Inconel alloy promoted the formation of layer with high graphitic sp2 cluster content. The Ar+ pre-treatment enhanced the adhesion and the diamond-like properties of the film while the electrochemical polishing was found to be unfavorable concerning the diamond-like character of the layer.

Experimental Study of Spectral Parameters of Silicon-Vacancy Centers in MWCVD Nanodiamond Films Important for Sensing Applications

Microwave enhanced chemical vapor deposition (MWCVD) was used to prepare nanodiamond (ND) films with different process parameters containing silicon-vacancy (SiV) centers. The effect of the local environment on the spectral parameters of SiV center’s zero phonon line emission, like peak position and full width at half maximum (FWHM), being important for many applications, have been studied in films having different morphological and structural properties. Relationships between the residual internal stress of the nanodiamond films and the emission parameters of the SiV centers were found and explained by shifting of the electronic levels of ground and excited states of individual SiV centers within the probed SiV ensembles due to the internal stress.