M. Veres

IR study of the formation process of polymeric hydrogenated amorphous carbon film

Abstract IR spectroscopic studies have been carried out on samples, which were prepared from benzene at relatively high pressure and low self-bias voltage region. The samples show well-resolved spectral features both in the sp3- and the sp2-hybridized spectral regions. The spectral resolution and separation of bands in the sp2 region, and the good signal-to-noise ratio allows Gaussian decomposition of the distinct bands, even in the more crowded sp3 region. The C–H vibrations show two different neighborhoods, a nearly stress-free, and a highly stressed one. By increasing the deposition voltage, the amplitude of the stress-free region decreases.

Characterization of luminescent silicon carbide nanocrystals prepared by reactive bonding and subsequent wet chemical etching

Fabrication of nanosized silicon carbide crystals is a crucial aspect for many biomedical applications. Here, we report an effective fabrication method of silicon carbide nanocrystals based on the reactive bonding method followed by electroless wet chemical etching. Our samples show strong violet-blue emission in the 410–450 nm region depending on the used solvents. Raman and infrared measurements unraveled the surface bonding structure of the fabricated nanoparticles being different from silicon carbide microcrystals. This might give an opportunity to use standard chemistry methods for biological functionalization of such nanoparticles.

Ultraviolet photoluminescence and its relation to atomic bonding properties of hydrogenated amorphous carbon

Abstract We have observed photoluminescence in hydrogenated amorphous carbon (a-C:H) samples at substantially higher energy than the visible band. The emitted light is in the ultraviolet (UV) region, the spectrum consists of three bands with peak positions of ∼4.46 eV, ∼4.01 eV and ∼3.63 eV, depending slightly on sample properties. The UV photoluminescence (PL) has been observed in samples of different band gaps which show either high or low PL efficiency in the visible region. The excitation spectrum of UV luminescence exhibits high efficiency in the photon energy range of 5.6–6.2 eV and a strong decrease at excitations below this energy range. The experimental fact, that, the peak energies of UV bands exceed the optical gap energy of the studied samples, supports the light emission via radiative recombination of localised geminate electron–hole pairs. Strong localisation is expected for the excitation of π–π* transitions in conjugated double bonded fragments of small sizes. An infrared study of the UV light emitting a-C:H films confirms the presence of conjugated double bonds with aromatic and olefinic local configurations as well, however, the unambiguous relation between UV luminescence and small aromatic structures cannot be established. It is more probable that the olefinic fragments with chain lengths of 2–4 give π electronic levels through which UV light emission takes place. The role of twofold co-ordinated carbon sites cannot be excluded yet.

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.

Carbon nano-particles prepared by ion-clustering in plasma

The plasma-assisted chemical vapour deposition technique was used to produce carbon nano-particles in a dusty plasma condition, at increased gas pressure and decreased electric field. The negatively biased upper electrode levitates the forming carbon clusters, which grow in a spherical symmetrical way in the relatively large density of the ions of the plasma. The spheres also attach to each other, but their concentration and mobility being low, the aggregation results in a different structure: a necklace-type chain will be formed. The dust particles, leaving the plasma cover the chamber wall, but a substantial amount of them reach the electrically driven substrate; forming an amorphous carbon film. The cluster systems were investigated by transmission electron microscopy, atomic force microscopy, Raman, infrared and photoluminescence spectroscopic methods.

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.

Optical strength in UV region of amorphous carbon

Abstract Optical strength (damage properties) of amorphous carbon films was investigated under the influence of UV laser pulses in the nanosecond (ns) and femtosecond (fs) region on different laser wavelengths. The damaged area was found to depend linearly on the laser fluence in a narrow region around the damage threshold tested by raising the number of pulses and fluence. The optical damage strength was found to be 2–3 times higher for fs pulses on both wavelengths used. This can be explained by the different weights of the ablation and thermal processes in cases where pulse lengths differ strongly. The fs pulse heats the focal spot less effectively, and cools down before the arrival of the next pulse. The ns pulses are better heaters (proven by the lower threshold), they have smaller crater diameters and slower incubation periods.

In situ investigations of laser and thermally modified As2S3 nanolayers: Synchrotron radiation photoelectron spectroscopy and density functional theory calculations

As-deposited, annealed, and in situ As2S3 nanolayers, illuminated by blue (405 nm) and red (650 nm) laser light, were studied using synchrotron radiation photoelectron spectroscopy and DFT electronic structure calculations. Changes in composition and local atomic coordination occurring in the irradiated region of As2S3 films were monitored by analysis of As 3d and S 2p core levels. These studies show that both the thermal treatment and the red laser illumination of As2S3 nanolayers decrease the concentration of homopolar (As-As and S-S) bonds. From the other hand, an increasing concentration of As-rich structural units (s.u.) with a homopolar As-As bond was observed under in situ blue laser illumination of As2S3 nanolayers. Molecular orbital energies were calculated for different As- and S-centered s.u. and used for the interpretation of the core levels and valence band spectra. The surface local structure of the As2S3 nanolayers and its photoinduced transformation are discussed in detail.