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Carrier Dynamics in Single Luminescent Silicon Quantum Dots
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.ORCID iD: 0000-0003-3833-9969
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: urn:nbn:se:kth:diva-174149ISBN: 978-91-7595-665-7 (print)OAI: oai:DiVA.org:kth-174149DiVA: diva2:858215
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
List of papers
1. Fabricating single silicon quantum rods for repeatable single dot photoluminescence measurements
Open this publication in new window or tab >>Fabricating single silicon quantum rods for repeatable single dot photoluminescence measurements
2011 (English)In: Physica Status Solidi A-applications and materials science, ISSN 1862-6319, Vol. 208, no 3, 631-634 p.Article in journal (Refereed) Published
Abstract [en]

A fabrication method for a matrix pattern of laterally separated silicon quantum rods was developed, consisting of a three-step recipe utilizing electron beam lithography (EBL), reactive ion etching (RIE), and oxidation. Photoluminescence (PL) measurements -images, spectra, and blinking-verified that the presented method results in a high number of luminescing single silicon quantum rods in well defined positions on the sample. These are suitable for single dot spectroscopy and repeatable measurements, even using different measurement methods and instruments. [GRAPHICS] Colorized scanning electron microscope images of undulating silicon nanowalls for controlled single quantum rod fabrication.

Place, publisher, year, edition, pages
Malden: Wiley-VCH Verlagsgesellschaft, 2011
Keyword
nanocrystal, quantum dot, quantum rod, silicon, single dot spectroscopy
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-32127 (URN)10.1002/pssa.201000556 (DOI)000288177600030 ()2-s2.0-79952526662 (Scopus ID)
Note
QC 20110407Available from: 2012-03-21 Created: 2011-04-07 Last updated: 2015-10-01Bibliographically approved
2. Exciton lifetime measurements on single silicon quantum dots
Open this publication in new window or tab >>Exciton lifetime measurements on single silicon quantum dots
2013 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 24, no 22, 225204- p.Article in journal (Refereed) Published
Abstract [en]

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 mu s) 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.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-124446 (URN)10.1088/0957-4484/24/22/225204 (DOI)000319326600005 ()2-s2.0-84877760460 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20130710

Available from: 2013-07-10 Created: 2013-07-05 Last updated: 2017-12-06Bibliographically approved
3. Optical absorption cross section and quantum efficiency of a single silicon quantum dot
Open this publication in new window or tab >>Optical absorption cross section and quantum efficiency of a single silicon quantum dot
2013 (English)In: Nanotechnology VI, SPIE - International Society for Optical Engineering, 2013, 876607- p.Conference paper, Published paper (Refereed)
Abstract [en]

Direct measurements of the optical absorption cross section (sigma) 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 mu s. By recording the photoluminescence rise time at different photon fluxes the absorption cross could be extracted yielding a value of 1.46x10(-14)cm(2) under 405 nm excitation wavelength. The PL quantum efficiency is found to be about 9% for the specified single silicon quantum dot.

Place, publisher, year, edition, pages
SPIE - International Society for Optical Engineering, 2013
Series
Proceedings of SPIE, ISSN 0277-786X ; 8766
Keyword
Single silicon quantum dot, absorption cross section, quantum efficiency (QE), photoluminescence (PL) decay, luminescence rise time, silicon nanocrystals
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-128505 (URN)10.1117/12.2017483 (DOI)000323346100002 ()2-s2.0-84881188084 (Scopus ID)978-0-8194-9563-1 (ISBN)
Conference
Conference on Nanotechnology VI, APR 24-25, 2013, Grenoble, France
Note

QC 20130913

Available from: 2013-09-13 Created: 2013-09-12 Last updated: 2015-10-01Bibliographically approved
4. Non-radiative decay in Si/SiO2 quantum dots in transition from dark to bright exciton states
Open this publication in new window or tab >>Non-radiative decay in Si/SiO2 quantum dots in transition from dark to bright exciton states
(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences Nano Technology
Identifiers
urn:nbn:se:kth:diva-174096 (URN)
Note

QS 2015

Available from: 2015-09-30 Created: 2015-09-30 Last updated: 2015-10-01Bibliographically approved
5. Near-Unity Internal Quantum Efficiency of Luminescent Silicon Nanocrystals with Ligand Passivation.
Open this publication in new window or tab >>Near-Unity Internal Quantum Efficiency of Luminescent Silicon Nanocrystals with Ligand Passivation.
Show others...
2015 (English)In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 9, no 7, 7097-7104 p.Article in journal (Refereed) Published
Abstract [en]

Spectrally resolved photoluminescence (PL) decays were measured for samples of colloidal, ligand-passivated silicon nanocrystals. These samples have PL emission energies with peak positions in the range 1.4-1.8 eV and quantum yields of 30-70%. Their ensemble PL decays are characterized by a stretched-exponential decay with a dispersion factor of 0.8, which changes to an almost monoexponential character at fixed detection energies. The dispersion factors and decay rates for various detection energies were extracted from spectrally resolved curves using a mathematical approach that excluded the effect of homogeneous line width broadening. Since nonradiative recombination would introduce a random lifetime variation, leading to a stretched-exponential decay for an ensemble, we conclude that the observed monoexponential decay in size-selected ensembles signifies negligible nonradiative transitions of a similar strength to the radiative one. This conjecture is further supported as extracted decay rates agree with radiative rates reported in the literature, suggesting 100% internal quantum efficiency over a broad range of emission wavelengths. The apparent differences in the quantum yields can then be explained by a varying fraction of "dark" or blinking nanocrystals.

Keyword
dispersion factor, lifetime, nonradiative channel, photoluminescence decay, radiative rate
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-171884 (URN)10.1021/acsnano.5b01717 (DOI)000358823200046 ()26083194 (PubMedID)2-s2.0-84938153097 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20150814

Available from: 2015-08-14 Created: 2015-08-10 Last updated: 2017-12-04Bibliographically approved
6. Blinking Statistics of Silicon Quantum Dots
Open this publication in new window or tab >>Blinking Statistics of Silicon Quantum Dots
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
Keyword
Blinking, intermittency, silicon, quantum dot, nanocrystal, photoluminescence, statistical aging, bleaching
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-58819 (URN)10.1021/nl203618h (DOI)000297950200081 ()2-s2.0-83655164324 (Scopus ID)
Note
QC 20120109Available from: 2012-03-21 Created: 2012-01-09 Last updated: 2017-12-08Bibliographically approved
7. Effect of X-ray irradiation on the blinking of single silicon nanocrystals
Open this publication in new window or tab >>Effect of X-ray irradiation on the blinking of single silicon nanocrystals
Show others...
2015 (English)In: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 212, no 12Article in journal (Refereed) Published
Abstract [en]

Photoluminescence (PL) intermittency (blinking) observed for single silicon nanocrystals (Si-NCs) embedded in oxide is usually attributed to trapping/de-trapping of carriers in the vicinity of the NC. Following this model, we propose that blinking could be modified by introducing new trap sites, for example, via X-rays. In this work, we present a study of the effect of X-ray irradiation (up to 65 kGy in SiO) on the blinking of single Si-NCs embedded in oxide nanowalls. We show that the luminescence characteristics, such as spectrum and life-time, are unaffected by X-rays. However, substantial changes in ON-state PL intensity, switching frequency, and duty cycle emerge from the blinking traces, while the ON- and OFF- time distributions remain of mono-exponential character. Although we do not observe a clear monotonic dependence of the blinking parameters on the absorbed dose, our study suggests that, in the future, Si-NCs could be blinking-engineered via X-ray irradiation.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2015
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-174101 (URN)10.1002/pssa.201532652 (DOI)000366589900006 ()2-s2.0-84949591107 (Scopus ID)
Note

QC 20151001

Available from: 2015-09-30 Created: 2015-09-30 Last updated: 2017-12-01Bibliographically approved
8. Biexciton Emission as a Probe of Auger Recombination in Individual Silicon Nanocrystals
Open this publication in new window or tab >>Biexciton Emission as a Probe of Auger Recombination in Individual Silicon Nanocrystals
Show others...
2015 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 13, 7499-7505 p.Article in journal (Refereed) Published
Abstract [en]

Biexciton emission from individual silicon nanocrystals was detected at room temperature by time-resolved, single-particle luminescence measurements. The efficiency of this process, however, was found to be very low, about 10-20 times less than the single exciton emission efficiency. It decreases even further at low temperature, explaining the lack of biexciton emission line observations in silicon nanocrystal single-dot spectroscopy under high excitation. The poor efficiency of the biexciton emission is attributed to the dominant nonradiative Auger process. Corresponding measured biexciton decay times then represent Auger lifetimes, and the values obtained here, from tens to hundreds of nanoseconds, reveal strong dot-to-dot variations, while the range compares well with recent calculations taking into account the resonant nature of the Auger process in semiconductor nanocrystals.

Keyword
Augers, Efficiency, Optical waveguides, Silicon, Temperature, Auger recombination, Biexciton emission, Luminescence measurements, Poor efficiencies, Room temperature, Semiconductor nanocrystals, Silicon nanocrystals, Single dot spectroscopy
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-166326 (URN)10.1021/acs.jpcc.5b01114 (DOI)000352329500060 ()2-s2.0-84926436244 (Scopus ID)
Funder
Swedish Research CouncilCarl Tryggers foundation
Note

QC 20150508

Available from: 2015-05-08 Created: 2015-05-07 Last updated: 2017-12-04Bibliographically approved

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