Fatemeh Sangghaleh

Exciton lifetime measurements on single silicon quantum dots

We measured the exciton lifetime of single silicon quantum dots, fabricated by electron beam lithography, reactive ion etching and oxidation. The observed photoluminescence decays are of mono-exponential character with a large variation (5-45 μs) from dot to dot, even for the same emission energy. We show that this lifetime variation may be the origin of the heavily debated non-exponential (stretched) decays typically observed for ensemble measurements.

Blinking Statistics of Silicon Quantum Dots

The blinking statistics of numerous single silicon quantum dots fabricated by electron-beam lithography, plasma etching, and oxidation have been analyzed. Purely exponential on- and off-time distributions were found consistent with the absence of statistical aging. This is in contrast to blinking reports in the literature where power-law distributions prevail as well as observations of statistical aging in nanocrystal ensembles. A linear increase of the switching frequency with excitation power density indicates a domination of single-photon absorption processes, possibly through a direct transfer of charges to trap states without the need for a bimolecular Auger mechanism. Photoluminescence saturation with increasing excitation is not observed; however, there is a threshold in excitation (coinciding with a mean occupation of one exciton per nanocrystal) where a change from linear to square-root increase occurs. Finally, the statistics of blinking of single quantum dots in terms of average on-time, blinking frequency and blinking amplitude reveal large variations (several orders) without any significant correlation demonstrating the individual microscopic character of each quantum dot.

Optical absorption cross section and quantum efficiency of a single silicon quantum dot

Direct measurements of the optical absorption cross section (σ) and exciton lifetime are performed on a single silicon quantum dot fabricated by electron beam lithography (EBL), reactive ion etching (RIE) and oxidation. For this aim, single photon counting using, an avalanche photodiode detector (APD) is applied to record photoluminescence (PL) intensity traces under pulsed excitation. The PL decay is found to be of a mono-exponential character with a lifetime of 6.5 μs. By recording the photoluminescence rise time at different photon fluxes the absorption cross could be extracted yielding a value of 1.46×10-14cm2 under 405 nm excitation wavelength. The PL quantum efficiency is found to be about 9% for the specified single silicon quantum dot.

Oxidation of nanopores in a silicon membrane: self-limiting formation of sub-10 nm circular openings.

We describe a simple but reliable approach to shrink silicon nanopores with nanometer precision for potential high throughput biomolecular sensing and parallel DNA sequencing. Here, nanopore arrays on silicon membranes were fabricated by a self-limiting shrinkage of inverted pyramidal pores using dry thermal oxidation at 850 °C. The shrinkage rate of the pores with various initial sizes saturated after 4 h of oxidation. In the saturation regime, the shrinkage rate is within ± 2 nm h(-1). Oxidized pores with an average diameter of 32 nm were obtained with perfect circular shape. By careful design of the initial pore size, nanopores with diameters as small as 8 nm have been observed. Statistics of the pore width show that the shrinkage process did not broaden the pore size distribution; in most cases the distribution even decreased slightly. The progression of the oxidation and the deformation of the oxide around the pores were characterized by focused ion beam and electron microscopy. Cross-sectional imaging of the pores suggests that the initial inverted pyramidal geometry is most likely the determining factor for the self-limiting shrinkage.

Luminescent Silicon Nanocrystals as Downconverters for Photovoltaic and Lighting Applications

Nanocrystals offer new functionalities for optoelectronics. Ensemble and single-dot measurements of chemically-synthesized silicon nanocrystals revealed high quantum yield, narrow homogeneous linewidth, and large Stokes shift, confirming application feasibility for these non-toxic and abundant material nanoparticles.