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Optical Properties of Single Silicon Quantum Dots
KTH, School of Engineering Sciences (SCI), Applied Physics, Photonics. (Nano silicon)ORCID iD: 0000-0001-5304-913X
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

For over 60 years silicon (Si) has dominated the semiconductor microelectronics industry mainly due to its abundance and good electrical and material properties. The advanced processing technology of Si has made it the workhorse for photovoltaics industry as well. However, this material has also a big drawback: it is an indirect-bandgap semiconductor in its bulk form, hence an inefficient light emitter. This has hindered the silicon photonics revolution envisioned in 1980s, where photons were thought to replace electrons inside computer chips.

In parallel with the exponential scaling of Si transistor's size over the years, the discovery of quantum phenomena at the nanoscale raised new hopes for this semiconductor. In the 1990s bright luminescence from nanostructured porous Si was demonstrated claiming the quantum confinement effect as origin of the emission. Since then, an intense research activity has been focused on Si quantum dots (Si-QDs) due to their potential use as abundant and non-toxic light emitters. More precisely, they could be used as fluorescent biolabels in biomedicine, as light-emitting phosphors in e.g. TV screens or as down-converters in luminescent solar concentrators. Nevertheless, in order to realize such applications, it is necessary not only to improve the fabrication of Si-QDs but also to gain a better understanding of their photo-physics. Among different types of optical measurements, those performed at the single-dot level are free of sample inhomogeneities, hence more accurate for a correct physical description.

This doctoral thesis presents a study of the optical properties of single Si-QDs of different type: encapsulated in an oxide matrix, capped with ligands or covered by a thin passivation layer. The homogeneous photoluminescence (PL) linewidth is found to strongly depend on the type of embedding matrix, being narrower for less rigid ones. A record resolution-limited linewidth of ~200 μeV is measured at low temperatures whereas room-temperature values can even compete with direct-bandgap QDs like CdSe. Such narrow PL lines exhibit intensity saturation at high excitation fluxes without any indication of emission from multiexciton states, suggesting the presence of fast non-radiative Auger recombination. Characteristic Auger-related lifetimes extracted from power-dependent decays show a variation from dot-to-dot and confirm the low biexciton quantum efficiency.

For the first time, the absorption curve of single Si-QDs is probed by means of photoluminescence excitation in the range 2.0-3.5 eV. A step-like structure is found which depends on the nanocrystal shape considered and agrees well with simulations of the exciton level structure. Rod-like Si-QDs can exhibit ~50 times higher absorption than spherical-like ones due to local field effects and enhanced optical transitions. In contrast with previous studies, evidence of a direct-bandgap red-shift for small Si-QDs is missing at the single dot level, in agreement with atomistic calculations.

Low-temperature PL decay measurements reveal no triplet-like emission lines, but two ~μs decay constants appearing at low temperatures. They suggest presence of a temperature-dependent fast blinking process based on trapping/detrapping of carriers in the oxide matrix, leading to delayed emission. The proposed model allows to extract characteristic trapping/de-trapping rates for Si-QDs featuring mono-exponential blinking statistics. From PL saturation curves, ligand-passivated Si-QDs do not exhibit such detrimental phenomenon, in agreement with the proposed model.

Last, Si-QDs demonstrate to be very hard against ~10 keV X-ray radiation, in contrast with CdSe-QDs whose PL quenching is correlated with a change in the blinking parameters. This property could be exploited for example in space applications, where radiation-hard materials are required.

To conclude, the results achieved in this thesis will help to understand and engineer the properties of Si-QDs whose application potential has increased after several years of research both at the ensemble and at the single-dot level.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2018. , p. 77
Series
TRITA-SCI-FOU ; 2018:47
Keywords [en]
silicon, nanocrystals, quantum dots, photoluminescence, optics, emission, absorption
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-238859ISBN: 978-91-7873-027-8 (print)OAI: oai:DiVA.org:kth-238859DiVA, id: diva2:1262865
Public defence
2018-12-07, Sal A, Electrum building, 2nd floor, Kistagången 16, Kista (Stockholm), 10:00 (English)
Opponent
Supervisors
Note

QC 20181114

Available from: 2018-11-14 Created: 2018-11-13 Last updated: 2018-11-14Bibliographically approved
List of papers
1. Ultranarrow Luminescence Linewidth of Silicon Nanocrystals and Influence of Matrix
Open this publication in new window or tab >>Ultranarrow Luminescence Linewidth of Silicon Nanocrystals and Influence of Matrix
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2014 (English)In: ACS Photonics, E-ISSN 2330-4022, Vol. 1, no 10, p. 998-1005Article in journal (Refereed) Published
Abstract [en]

The luminescence linewidth of individual silicon nanocrystals was characterized by single-dot spectroscopy, and an ultranarrow linewidth of similar to 200 mu eV at 10 K was found. This value is, in fact, limited by system resolution and represents only the upper limit of the homogeneous linewidth. In addition, the effect of the matrix was investigated for nanocrystals coated with organic ligands, embedded in silicon dioxide, as well as for nanocrystals with only a thin passivating layer. It was found that, depending on the matrix, the room-temperature bandwidth may vary by an order of magnitude, where values as small as similar to 12 meV (similar to 5 nm) at 300 K were detected for nanocrystals with a thin passivation. The observed values for silicon nanocrystals are similar and even surpass some of those for direct-band-gap quantum dots. The narrow linewidth at room temperature enables the use of silicon nanocrystals for nontoxic narrow-band labeling of biomolecules and for application as phosphors in white-light-emitting devices.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2014
Keywords
Silicon nanocrystals, photoluminescence, surface passivation, linewidth
National Category
Nano Technology Materials Engineering
Identifiers
urn:nbn:se:kth:diva-155787 (URN)10.1021/ph500221z (DOI)000343276800013 ()
Funder
Swedish Research CouncilCarl Tryggers foundation
Note

QC 20141114

Available from: 2014-11-14 Created: 2014-11-13 Last updated: 2018-11-13Bibliographically approved
2. 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
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2015 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 13, p. 7499-7505Article 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.

Keywords
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: 2018-11-13Bibliographically approved
3. 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
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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: 2018-11-13Bibliographically approved
4. Single-dot absorption spectroscopy and theory of silicon nanocrystals
Open this publication in new window or tab >>Single-dot absorption spectroscopy and theory of silicon nanocrystals
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2016 (English)In: Physical Review B, ISSN 2469-9950, Vol. 93, no 16, article id 161413Article in journal (Refereed) Published
Abstract [en]

Photoluminescence excitation measurements have been performed on single, unstrained oxide-embedded Si nanocrystals. Having overcome the challenge of detecting weak emission, we observe four broad peaks in the absorption curve above the optically emitting state. Atomistic calculations of the Si nanocrystal energy levels agree well with the experimental results and allow identification of some of the observed transitions. An analysis of their physical nature reveals that they largely retain the indirect band-gap structure of the bulk material with some intermixing of direct band- gap character at higher energies.

Place, publisher, year, edition, pages
American Physical Society, 2016
Keywords
Quantum Dots, Photoluminescence Spectroscopy, White-Light, P Method, States, Phosphors, Emission, Energy
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-185829 (URN)10.1103/PhysRevB.93.161413 (DOI)000374951000001 ()2-s2.0-84964689318 (Scopus ID)
Funder
Swedish Research Council, VR 2015-04064
Note

QC 20160512

Available from: 2016-04-28 Created: 2016-04-28 Last updated: 2018-11-13Bibliographically approved
5. Strong Absorption Enhancement in Si Nanorods
Open this publication in new window or tab >>Strong Absorption Enhancement in Si Nanorods
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2016 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 16, no 12, p. 7937-7941Article in journal (Refereed) Published
Abstract [en]

We report two orders of magnitude stronger absorption in silicon nanorods relative to bulk in a wide energy range. The local field enhancement and dipole matrix element contributions were disentangled experimentally by single-dot absorption measurements on differently shaped particles as a function of excitation polarization and photon energy. Both factors substantially contribute to the observed effect as supported by simulations of the light-matter interaction and atomistic calculations of the transition matrix elements. The results indicate strong shape dependence of the quasidirect transitions in silicon nanocrystals, suggesting nanostructure shape engineering as an efficient tool for overcoming limitations of indirect band gap materials in optoelectronic applications, such as solar cells.

Keywords
Absorption, nanocrystals, nanorods, atomistic theory, silicon
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-199754 (URN)10.1021/acs.nanolett.6b04243 (DOI)000389963200092 ()2-s2.0-85006295240 (Scopus ID)
Note

QC 20170120

Available from: 2017-01-20 Created: 2017-01-16 Last updated: 2018-11-13Bibliographically approved
6. Absence of redshift in the direct bandgap of silicon nanocrystals with reduced size
Open this publication in new window or tab >>Absence of redshift in the direct bandgap of silicon nanocrystals with reduced size
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2017 (English)In: Nature Nanotechnology, ISSN 1748-3387, E-ISSN 1748-3395, Vol. 12, no 10, p. 930-932Article in journal (Refereed) Published
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-216620 (URN)000412432500002 ()28945238 (PubMedID)2-s2.0-85030770887 (Scopus ID)
Note

QC 20171102

Available from: 2017-11-02 Created: 2017-11-02 Last updated: 2018-11-13Bibliographically approved
7. Rapid Trapping as the Origin of Nonradiative Recombination in Semiconductor Nanocrystals
Open this publication in new window or tab >>Rapid Trapping as the Origin of Nonradiative Recombination in Semiconductor Nanocrystals
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2018 (English)In: ACS Photonics, E-ISSN 2330-4022, Vol. 5, no 8, p. 2990-2996Article in journal (Refereed) Published
Abstract [en]

We demonstrate that nonradiative recombination in semiconductor nanocrystals can be described by a rapid luminescence intermittency, based on carrier tunneling to resonant traps. Such process, we call it "rapid trapping (blinking)", leads to delayed luminescence and promotes Auger recombination, thus lowering the quantum efficiency. To prove our model, we probed oxide- (containing static traps) and ligand- (trap-free) passivated silicon nanocrystals emitting at similar energies and featuring monoexponential blinking statistics. This allowed us to find analytical formulas and to extract characteristic trapping/detrapping rates, and quantum efficiency as a function of temperature and excitation power. Experimental single-dot temperature-dependent decays, supporting the presence of one or few resonant static traps, and ensemble saturation curves were found to be very well described by this effect. The model can be generalized to other semiconductor nanocrystals, although the exact interplay of trapping/detrapping, radiative, and Auger processes may be different, considering the typical times of the processes involved.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2018
Keywords
quantum dots, photoluminescence, blinking, efficiency, Auger recombination
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-234625 (URN)10.1021/acsphotonics.8b00581 (DOI)000442185900004 ()2-s2.0-85050020891 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20180913

Available from: 2018-09-13 Created: 2018-09-13 Last updated: 2018-11-13Bibliographically approved
8. X-ray radiation hardness and influence on blinking in Si and CdSe quantum dots
Open this publication in new window or tab >>X-ray radiation hardness and influence on blinking in Si and CdSe quantum dots
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-238792 (URN)
Note

QC 20181114

Available from: 2018-11-11 Created: 2018-11-11 Last updated: 2018-11-14Bibliographically approved

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