Benjamin Bruhn

Controlled fabrication of individual silicon quantum rods yielding high intensity, polarized light emission

Elongated silicon quantum dots (also referred to as rods) were fabricated using a lithographic process which reliably yields sufficient numbers of emitters. These quantum rods are perfectly aligned and the vast majority are spatially separated well enough to enable single-dot spectroscopy. Not only do they exhibit extraordinarily high linear polarization with respect to both absorption and emission, but the silicon rods also appear to luminesce much more brightly than their spherical counterparts. Significantly increased quantum efficiency and almost unity degree of linear polarization render these quantum rods perfect candidates for numerous applications.

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.

Coexistence of 1D and quasi-0D photoluminescence from single silicon nanowires.

Single silicon nanowires (Si-NWs) prepared by electron-beam lithography and reactive-ion etching are investigated by imaging optical spectroscopy under variable temperatures and laser pumping intensities. Spectral images of individual Si-NWs reveal a large variability of photoluminescence (PL) along a single Si-NW. The weaker broad emission band asymmetrically extended to the high-energy side is interpreted to be due to recombination of quasi-free 1D excitons while the brighter localized emission features (with significantly variable peak position, width, and shape) are due to localization of electron-hole pairs in surface protrusions acting like quasi-0D centers or quantum dots (QDs). Correlated PL and scanning electron microscopy images indicate that the efficiently emitting QDs are located at the Si-NW interface with completely oxidized neck of the initial Si wall. Theoretical fitting of the delocalized PL emission band explains its broad asymmetrical band to be due to the Gaussian size distribution of the Si-NW diameter and reveals also the presence of recombination from the Si-NW excited state which can facilitate a fast capture of excitons into QD centers.

Spectroscopy of carrier multiplication in nanocrystals

Carrier multiplication in nanostructures promises great improvements in a number of widely used technologies, among others photodetectors and solar cells. The decade since its discovery was ridden with fierce discussions about its true existence, magnitude, and mechanism. Here, we introduce a novel, purely spectroscopic approach for investigation of carrier multiplication in nanocrystals. Applying this method to silicon nanocrystals in an oxide matrix, we obtain an unambiguous spectral signature of the carrier multiplication process and reveal details of its size-dependent characteristics-energy threshold and efficiency. The proposed method is generally applicable and suitable for both solid state and colloidal samples, as well as for a great variety of different materials.

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.

Multi-chromatic silicon nanocrystals

Silicon nanocrystals (SiNCs) have great potential to become environmental friendly alternatives to heavy-metal containing nanocrystals for applications including medical imaging, lighting and displays. SiNCs exhibit excellent photostability, non-toxicity and abundant resources, but their often reported inefficient and spectrally limited light emission seriously impair their applications. Here we demonstrate a new method that converts SiNCs into an efficient and robust multi-chromatic phosphor. Using ~15 keV electron-beam irradiation of oxide-capped SiNCs, we introduce several types of color centers into the nanocrystal’s oxide shell with efficient blue, green and red emission bands, together yielding warm-white photoluminescence, even for a single SiNC. Introduced centers are not native to the original system and we relate them to known defects in silica. Unlike in the silica host, however, here the centers are efficiently optically excitable. Provided further optimization and up-scaling of this method, e-beam irradiated SiNCs can be of great interest as white phosphors for applications such as LEDs.