Silicon nanoparticles synthesized in the gas phase are studied. From time-resolved photoluminescence measurements we determine, quantitatively, the size-dependence of the oscillator strength of the nanoparticles. We investigate experimentally the absorption and photoluminescence emission of nanoparticle ensembles with a broad size distribution. Using a model which accounts for size-effects in both oscillator strength and quantum-confinement, we are able to calculate absorption and emission spectra of ensemble samples. From these results we have determined, whether silicon nanoparticles should be regarded as indirect or direct semiconductors. Moreover, we systematically study the influence of the particle size-distribution on the optical spectra.
The excitonic fine structure of silicon nanoparticles is investigated by time-resolved and magnetic-field-dependent photoluminescence. The results are analyzed using the common model of an excitonic fine structure consisting of a bright and a dark exciton. We find that the radiative recombination rates of both excitons differ only by a factor of eight. Therefore, we observe a thermal crossover in the character of the emission from bright-exciton-like to dark-exciton-like. The validity of our model is further supported by magnetic-field-dependent measurements, in which effects of state mixing are observed.
Microfabricated Lamellar grating interferometers (LGI) require fewer components compared to Michelson interferotemeters and offer compact and broadband Fourier transform spectrometers (FTS) with good spectral resolution, high speed and high efficiency. This study presents the fundamental equations that govern the performance and limitations of LGI based FTS systems. Simulations and experiments were conducted to demonstrate and explain the periodic nature of the interferogram envelope due to Talbot image formation. Simulations reveal that the grating period should be chosen large enough to avoid Talbot phase reversal at the expense of mixing of the diffraction orders at the detector. Optimal LGI grating period selection depends on a number of system parameters and requires compromises in spectral resolution and signal-to-bias ratio (SBR) of the interferogram within the spectral range of interest. New analytical equations are derived for spectral resolution and SBR of LGI based FTS systems.
We have studied SnOx nanoparticles fabricated by gas-phase condensation and in-flight sintering using Raman and photoluminescence (PL) spectroscopy. We are able to identify various vibrational states of the rutile phase of the SnOx crystal. By thorough analysis of the vibrational modes, we are able to determine the bond lengths of the O–O and Sn–O bonds for the substoichiometric SnO1.5, leading, together with x-ray diffraction data, to a full characterization of the SnO1.5 lattice. In absorption and photoluminescence spectra, we observe a finite density of states inside the band gap due to oxygen vacancies, giving rise to a midgap luminescence peak. Our results suggest that the defect related luminescence efficiency is limited by nonradiative recombination processes and by the oxygen vacancy density. We therefore conclude that the PL intensity has a maximum around a stoichiometry of SnO1.7.
Standard FT-IR spectrometers are large, usually static, and expensive and require operation by qualified personnel. The presented development involves achievements in MEMS technologies and electronics design to address size, speed and power requirements and develop a fully integrated miniaturized FT-IR spectrometer. A suitably matched interaction of multiple new components - source, interferometer, detector and control and data processing - develops unique MEMS based spectrometers capable of reliable operation and finally results in compact, robust and economical analyzers. The presented system now aims at a high performance level to measure in the range between 5000-750 cm -1 at a spectral resolution better than 10 cm -1. The Michelson interferometer design and the desired performance put several demands on the MOEMS device. Amongst these, a mirror travel of ± 500 m and a minimal dynamic deformation of < /10 peak-to peak in combination with a large mirror aperture of 5 mm were the most challenging goals. However, a signal-to-noise ratio of 1000 is required to qualify a FT-IR system as a sensor for industrial applications e.g. process control. The purpose of the system, presented in this work, is to proof that this is feasible on the basis of MEMS technology and it is demonstrated that most of these specifications could be already met
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