Vladimir A. Karavanskii

On the mechanism of laser-induced desorption–ionization of organic compounds from etched silicon and carbon surfaces

The laser-induced desorption/ionization of organic compounds from etched carbon and silicon substrate surfaces was investigated. Two different etching procedures were used. Silicon surfaces were etched either by galvanostatic anodization to produce porous silicon or by a hyperthermal (∼5 eV) F-atom beam to produce nonporous silicon. Atomic force microscopy (AFM) images showed that both etching procedures yielded surfaces with sub-micrometer structures. Highly oriented pyrolytic graphite was etched with hyperthermal O atoms. A 337 nm ultraviolet (UV) laser and a 3.28 μm infrared (IR) laser were used for desorption. Analytes were deposited on the substrates either from the liquid or the gas phase. Mass spectra were obtained provided that three conditions were fulfilled. First, sufficient laser light had to be absorbed. When the IR laser was employed, a thin physisorbed solvent layer was required for sufficient laser light absorption to occur. Though the required fluence of IR and UV light differed by a fact...

Structural and optical properties of porous nanocrystalline Ge

Nanocrystalline Ge films were prepared by isotropic chemical etching on single-crystalline Ge substrates with 100 and 111 orientations. The structural and optical properties have been investigated by transmission electron microscopy (TEM), electron diffraction (ED), Raman photoluminescence (PL), and infrared spectroscopy. The average size of nanocrystals (NCs) was estimated by fitting of the Raman spectra using a phonon-confinement model developed for spherical semiconductor NCs. Considered collectively TEM, ED, and Raman results indicate that all films contain high density of 3–4 nm diameter, diamond-structured Ge NCs with disordered surfaces. There are indications that surface of nanoparticles is mainly hydrogen terminated even for air-stabilized samples. Red PL is observed at room temperature upon excitation by 1.96 eV with peak energy of ∼1.55 eV and correlates well with recent theoretical calculations of the enlarged optical gap in Ge NCs of similar size.

On the origin of the 2.2-2.3 eV photoluminescence from chemically etched germanium

The photoluminescence (PL) at B2.2–2.3 eV from Ge-based nanocrystalline materials is described in the literature as nanocrystal size-independent. We have observed visible luminescence from two different types of stain-etched Ge samples, one prepared after Sendova-Vassileva et al. (Thin Solid Films 255 (1995) 282) in a solution of H2O2:HF at 50:1 volume ratio, and the other in a solution of HF:H3PO4:H2O2 at 34:17:1 volume ratio. Energydispersive X-rayanalysis (EDX), Raman and FTIR spectroscopy, and the near edge X-ray absorption structure (XANES), indicate that the chemically etched Ge layers of the former type of samples are composed of non-stoichometric Ge oxides, i.e. GeOx (0oxo2), and free from anyGe nanoconstructions. It is also suggested from XANES that the latter type of chemically etched Ge samples comprise 8–9 nm nanocrystals of Ge, surface-covered with mainly oxygen. Photoluminescence occurred at B2.3 eV for all samples. The PL behavior of the latter type of chemically etched Ge on annealing in different chemical environments (air or H) allowed us to conclude that the PL from these materials, as well as that from those Ge-based nanocrystalline materials reported in the literature, is from GeOxs. r 2002 Elsevier Science B.V. All rights reserved.

Gas Chromatography/Surface-Assisted Laser Desorption Ionization Mass Spectrometry of Amphetamine-like Compounds

A variety of amphetamine-like compounds were analyzed by gas chromatography/surface-assisted laser desorption ionization mass spectrometry, GC/SALDI-MS. In the SALDI method, compounds are adsorbed on a solid SALDI substrate and directly ionized from the substrate by means of a laser pulse. The interfacing of a SALDI ion source with a gas chromatograph is presented here for the first time. The end of the GC column is situated 20 mm from the silicon substrate in the vacuum of the ion source of a time-of-flight mass spectrometer, and the compounds eluted from the GC capillary are adsorbed onto the nanostructured silicon surface. The mass spectra show very low levels of background noise and no reagent ions. GC/SALDI-MS detection limits are several orders of magnitude lower than those previously reported for GC/MS analysis of amphetamine-like compounds. The extent of fragmentation is under experimental control by changing the laser fluence.

Photoluminescence relaxation kinetics in vapor etched porous silicon

New porous silicon preparation technique has been suggested and realized with using vapor etching of silicon in iodine and HF contained vapors. It has been shown that vapor etching allows the preparation of luminescent porous layers on heavy doped (n++ and p++ type) silicon. Comparison of Raman and CW excitation PL measurements of vapor etched porous layer with typical anodized luminescent porous silicon indicated that they have in general similar structural and PL properties. Time resolved photoluminescence measurements reveal however that excitation recombination for iodine contained vapor etched samples is two times faster with higher photoluminescence efficiency, which can be interpreted as increasing of radiative recombination rate for luminescence centers in new nanocrystalline silicon.