| 1. |
- Bruhn, Benjamin, et al.
(författare)
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Blinking Statistics of Silicon Quantum Dots
- 2011
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 11:12, s. 5574-5580
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Bruhn, Benjamin, et al.
(författare)
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Fabricating single silicon quantum rods for repeatable single dot photoluminescence measurements
- 2011
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Ingår i: Physica Status Solidi A-applications and materials science. - Malden : Wiley-VCH Verlagsgesellschaft. - 1862-6319. ; 208:3, s. 631-634
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Pevere, Federico, et al.
(författare)
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Biexciton Emission as a Probe of Auger Recombination in Individual Silicon Nanocrystals
- 2015
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:13, s. 7499-7505
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Pevere, Federico, et al.
(författare)
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Effect of X-ray irradiation on the blinking of single silicon nanocrystals
- 2015
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley-VCH Verlagsgesellschaft. - 1862-6300 .- 1862-6319. ; 212:12
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Pevere, Federico, et al.
(författare)
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Rapid Trapping as the Origin of Nonradiative Recombination in Semiconductor Nanocrystals
- 2018
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Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 5:8, s. 2990-2996
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Sangghaleh, Fatemeh
(författare)
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Carrier Dynamics in Single Luminescent Silicon Quantum Dots
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 7. |
- Sangghaleh, Fatemeh, et al.
(författare)
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Exciton lifetime measurements on single silicon quantum dots
- 2013
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 24:22, s. 225204-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
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| 9. |
- Sangghaleh, Fatemeh, et al.
(författare)
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Near-Unity Internal Quantum Efficiency of Luminescent Silicon Nanocrystals with Ligand Passivation.
- 2015
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 9:7, s. 7097-7104
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
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