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Blinking Statistics of Silicon Quantum Dots
KTH, School of Information and Communication Technology (ICT), Material Physics.
KTH, School of Information and Communication Technology (ICT), Material Physics.ORCID iD: 0000-0003-3833-9969
KTH, School of Information and Communication Technology (ICT), Material Physics.ORCID iD: 0000-0002-5260-5322
2011 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 11, no 12, 5574-5580 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2011. Vol. 11, no 12, 5574-5580 p.
Keyword [en]
Blinking, intermittency, silicon, quantum dot, nanocrystal, photoluminescence, statistical aging, bleaching
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:kth:diva-58819DOI: 10.1021/nl203618hISI: 000297950200081Scopus ID: 2-s2.0-83655164324OAI: oai:DiVA.org:kth-58819DiVA: diva2:474197
Note
QC 20120109Available from: 2012-03-21 Created: 2012-01-09 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Fabrication and characterization of single luminescing quantum dots from 1D silicon nanostructures
Open this publication in new window or tab >>Fabrication and characterization of single luminescing quantum dots from 1D silicon nanostructures
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Silicon as a mono-crystalline bulk semiconductor is today the predominant material in many integrated electronic and photovoltaic applications. This has not been the case in lighting technology, since due to its indirect bandgap nature bulk silicon is an inherently poor light emitter.With the discovery of efficient light emission from silicon nanostructures, great new interest arose and research in this area increased dramatically.However, despite more than two decades of research on silicon nanocrystals and nanowires, not all aspects of their light emission mechanisms and optical properties are well understood, yet.There is great potential for a range of applications, such as light conversion (phosphor substitute), emission (LEDs) and harvesting (solar cells), but for efficient implementation the underlying mechanisms have to be unveiled and understood.Investigation of single quantum emitters enable proper understanding and modeling of the nature and correlation of different optical, electrical and geometric properties.In large numbers, such sets of experiments ensure statistical significance. These two objectives can best be met when a large number of luminescing nanostructures are placed in a pattern that can easily be navigated with different measurement methods.This thesis presents a method for the (optional) simultaneous fabrication of luminescent zero- and one-dimensional silicon nanostructuresand deals with their structural and optical characterization.Nanometer-sized silicon walls are defined by electron beam lithography and plasma etching. Subsequent oxidation in the self-limiting regime reduces the size of the silicon core unevenly and passivates it with a thermal oxide layer.Depending on the oxidation time, nanowires, quantum dots or a mixture of both types of structures can be created.While electron microscopy yields structural information, different photoluminescence measurements, such as time-integrated and time-resolved imaging, spectral imaging, lifetime measurements and absorption and emission polarization measurements, are used to gain knowledge about optical properties and light emission mechanisms in single silicon nanocrystals.The fabrication method used in this thesis yields a large number of spatially separated luminescing quantum dots randomly distributed along a line, or a slightly smaller number that can be placed at well-defined coordinates. Single dot measurements can be performed even with an optical microscope and the pattern, in which the nanostructures are arranged, enables the experimenter to easily find the same individual dot in different measurements.Spectral measurements on the single dot level reveal information about processes that are involved in the photoluminescence of silicon nanoparticles and yield proof for the atomic-like quantized nature of energy levels in the conduction and valence band, as evidenced by narrow luminescence lines (~500 µeV) at low temperature. Analysis of the blinking sheds light on the charging mechanisms of oxide-capped Si-QDs and, by exposing exponential on- and off-time distributions instead of the frequently observed power law distributions, argues in favor of the absence of statistical aging. Experiments probing the emission intensity as a function of excitation power suggest that saturation is not achieved. Both absorption and emission of silicon nanocrystals contained in a one-dimensional silicon dioxide matrix are polarized to a high degree. Many of the results obtained in this work seem to strengthen the arguments that oxide-capped silicon quantum dots have universal properties, independently of the fabrication method, and that the greatest differences between individual nanocrystals are indeed caused by individual factors like local environment, shape and size (among others).

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. xv, 60 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2012:15
Keyword
silicon, quantum dot, nanocrystal, nanowire, nanostructure, photoluminescence
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-102524 (URN)978-91-7501-486-9 (ISBN)
Public defence
2012-10-12, Sal E, Forum KTH-ICT, Isafjordsgatan 39, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Note

QC 20120920

Available from: 2012-09-20 Created: 2012-09-19 Last updated: 2012-09-20Bibliographically approved
2. Carrier Dynamics in Single Luminescent Silicon Quantum Dots
Open this publication in new window or tab >>Carrier Dynamics in Single Luminescent Silicon Quantum Dots
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Bulk silicon as an indirect bandgap semiconductor is a poor light emitter. In contrast, silicon nanocrystals (Si NCs) exhibit strong emission even at room temperature, discovered initially at 1990 for porous silicon by Leigh Canham. This can be explained by the indirect to quasi-direct bandgap modification of nano-sized silicon according to the already well-established model of quantum confinement.

In the absence of deep understanding of numerous fundamental optical properties of Si NCs, it is essential to study their photoluminescence (PL) characteristics at the single-dot level. This thesis presents new experimental results on various photoluminescence mechanisms in single silicon quantum dots (Si QDs).

The visible and near infrared emission of Si NCs are believed to originate from the band-to-band recombination of quantum confined excitons. However, the mechanism of such process is not well understood yet. Through time-resolved PL decay spectroscopy of well-separated single Si QDs, we first quantitatively established that the PL decay character varies from dot-to-dot and the individual lifetime dispersion results in the stretched exponential decays of ensembles. We then explained the possible origin of such variations by studying radiative and non-radiative decay channels in single Si QDs. For this aim the temperature dependence of the PL decay were studied. We further demonstrated a model based on resonance tunneling of the excited carriers to adjacent trap sites in single Si QDs which explains the well-known thermal quenching effect.

Despite the long PL lifetime of Si NCs, which limits them for optoelectronics applications, they are ideal candidates for biomedical imaging, diagnostic purposes, and phosphorescence applications, due to the non-toxicity, biocompability and material abundance of silicon. Therefore, measuring quantum efficiency of Si NCs is of great importance, while a consistency in the reported values is still missing. By direct measurements of the optical absorption cross-section for single Si QDs, we estimated a more precise value of internal quantum efficiency (IQE) for single dots in the current study. Moreover, we verified IQE of ligand-passivated Si NCs to be close to 100%, due to the results obtained from spectrally-resolved PL decay studies. Thus, ligand-passivated silicon nanocrystals appear to differ substantially from oxide-encapsulated particles, where any value from 0 % to 100 % could be measured. Therefore, further investigation on passivation parameters is strongly suggested to optimize the efficiency of silicon nanocrystals systems.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. xviii, 73 p.
Series
Trita-ICT, 2015:09
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-174149 (URN)978-91-7595-665-7 (ISBN)
Public defence
2015-10-23, SAL C, Electrum 229, KTH-ICT, Electrum 229, KTH-ICT, Kistagången 16, Kista, 10:00 (English)
Opponent
Supervisors
Note

QC 201501001

Available from: 2015-10-01 Created: 2015-10-01 Last updated: 2015-10-01Bibliographically approved

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Sangghaleh, FatemehLinnros, Jan

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