M. Koós

Origin of the D peak in the Raman spectrum of microcrystalline graphite

Abstract We have studied the D peak in the Raman spectrum of polycrystalline graphite as a function of excitation energy in the range from 1.16 to 4.3 eV. The D peak disperses with excitation energy (≈50 cm−1/eV) and its intensity decreases with increasing laser energy. The dispersion of the D peak is explained in terms of a novel k → -selective resonant Raman scattering process. The scattering cross-section is resonantly enhanced for phonons on one branch in the phonon dispersion curves which have the same k → -vector as the electronic transition responsible for the resonance.

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.

The role of hydrogen in luminescence of electrochemically oxidized porous Si layer

Abstract Remarkable improvements of photoluminescence (PL) efficiency together with large blueshift of PL peak wavelength have been achieved by electrochemical oxidation. It is known from Fourier transform infrared (FT-IR) measurements that the SiO 2 covering on pore walls has discontinuities where the surface states are terminated by hydrogen. Dramatic degradation of luminescence transitions in anodically oxidized PSL, due to thermal hydrogen desorption, indicates the importance of hydrogen in electrochemical surface passivation.

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.

Polarization memory of photoluminescence in a-C:H film

Abstract Large polarization memory effects of photoluminescence were observed in a-C:H films, its degree decreased from 40 % on the high energy side to zero on the low energy side of the luminescence band. Radiative recombination of strongly localized electron-hole pairs is suggested as being the mechanism responsible for the polarization memory of luminescence.

Reactive pulsed laser deposition of hydrogenated carbon thin films: The effect of hydrogen pressure

Diamondlike properties of amorphous carbon films can be enhanced by applying reactive hydrogen atmosphere during pulsed laser deposition (PLD) as shown by recent studies. The complex phenomena occurring during carbon plume expansion has not been examined yet thoroughly. Therefore we deposited amorphous hydrogenated carbon thin films in hydrogen ambient atmosphere (4×10−4–25Pa) by PLD at room temperature. The deposited films were characterized by Rutherford backscattering spectrometry, elastic recoil detection analysis, Raman spectroscopy, infrared spectroscopy, variable angle spectroscopic ellipsometry, and nanoindentation measurements. At low hydrogen pressures (below ∼0.1Pa), when the mean free path of plasma constituents in the background gas is larger than the target substrate distance, the deposited films show diamondlike properties. At higher pressures (above ∼0.1Pa) the deposited films change their diamondlike carbon character towards to a structure which contains increasing amount of sp2 bonded ca...

Luminescence efficiency enhancement in laser soaked hydrogenated amorphous carbon films

An unexpected effect of laser soaking was observed on the properties of annealed a‐C:H films. The photoluminescence (PL) intensity was increased up to six times and, the spectrum was blue shifted in samples exposed to an unfocused Ar+ laser beam. The time dependence of the PL intensity increase was found to be exponential up to saturation with characteristic times around 103 s. The observed changes were influenced by illumination intensity and sample properties. These results are corroborated by a PL mechanism when excitation and recombination take place on π‐bonded clusters.

Experimental proof for nanoparticle origin of photoluminescence in porous silicon layers

A series of nonlinear phenomena in the excitation intensity dependence of photoluminescence (PL) was observed in porous silicon (PS) at room temperature. From a low level of excitation, the blue shift of the PL spectra was detected followed by complete saturation of the integrated PL intensity, without detectable change in the spectral position. This behavior may well be experimental proof of the nonparticle origin of PS light emission.

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.