| 1. |
- Cavallaro, Sara, et al.
(författare)
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Multiparametric Profiling of Single Nanoscale Extracellular Vesicles by Combined Atomic Force and Fluorescence Microscopy : Correlation and Heterogeneity in Their Molecular and Biophysical Features
- 2021
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Ingår i: Small. - : Wiley. - 1613-6810 .- 1613-6829. ; 17:14
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Tidskriftsartikel (refereegranskat)abstract
- Being a key player in intercellular communications, nanoscale extracellular vesicles (EVs) offer unique opportunities for both diagnostics and therapeutics. However, their cellular origin and functional identity remain elusive due to the high heterogeneity in their molecular and physical features. Here, for the first time, multiple EV parameters involving membrane protein composition, size and mechanical properties on single small EVs (sEVs) are simultaneously studied by combined fluorescence and atomic force microscopy. Furthermore, their correlation and heterogeneity in different cellular sources are investigated. The study, performed on sEVs derived from human embryonic kidney 293, cord blood mesenchymal stromal and human acute monocytic leukemia cell lines, identifies both common and cell line-specific sEV subpopulations bearing distinct distributions of the common tetraspanins (CD9, CD63, and CD81) and biophysical properties. Although the tetraspanin abundances of individual sEVs are independent of their sizes, the expression levels of CD9 and CD63 are strongly correlated. A sEV population co-expressing all the three tetraspanins in relatively high abundance, however, having average diameters of <100 nm and relatively low Young moduli, is also found in all cell lines. Such a multiparametric approach is expected to provide new insights regarding EV biology and functions, potentially deciphering unsolved questions in this field.
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| 2. |
- Liu, Lige, et al.
(författare)
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Cation effect on excitons in perovskite nanocrystals from single-dot photoluminescence of CH3NH3PbI3
- 2019
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:19
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Tidskriftsartikel (refereegranskat)abstract
- The single-dot photoluminescence properties of similar to 7 nm perovskite MAPbI(3) (MA = CH3NH3+) nanocrystals (NCs) were investigated in the 5-295 K temperature range both in spectral and time domains. Repeatable single-dot measurements were facilitated by the use of a protective polymer, which stabilized the NCs. Temperature-induced phase transition and exciton-phonon interactions were revealed as well as the exciton fine structure. A pronounced spectral jump of the emission peak at 140-160 K, indicating a tetragonal-orthorhombic phase transition, was observed. In addition, the emission linewidth of similar to 0.6 meV was measured, which is the narrowest ever recorded for this perovskite material system. A similar to 4.0 meV phonon mode was identified for the NCs at 5 K, defining the linewidth thermal broadening. In general, the presence of MA(+) leads to broader spectra than for Cs+ or FA(+) containing perovskite NCs. It is attributed to higher polarity of this cation, thus it is more susceptible to spectral diffusion, which is clearly observed here. Photoluminescence decay measurements indicated that the recombination from the lowest energy state of the emission level manifold is partially forbidden. This is opposite to Cs+ cation NCs, highlighting the central role of the positive ion in the exchange interaction in perovskites. Finally, delayed luminescence was found to govern the recombination dynamics below room temperature, suggesting an involvement of trap sites for the orthorhombic phase. The reported photophysics of a quantum-confined exciton in this material, which is of interest for various light-converting applications, clarifies the role of the cation in perovskite nanocrystal optical properties.
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| 3. |
- Liu, Lige, et al.
(författare)
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Size-Dependent Phase Transition in Perovskite Nanocrystals
- 2019
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 10:18, s. 5451-5457
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Tidskriftsartikel (refereegranskat)abstract
- Organic-inorganic hybrid perovskites simultaneously possess strong spin- orbit coupling (SOC) and structure inversion asymmetry, establishing a Rashba effect to influence light emission and photovoltaics. Here, we use mechanical bending as a convenient approach to investigate the Rashba effect through SOC in perovskite (MAPbI(3-x)Cl(x)) films by elastically deforming grains. It is observed that applying a concave bending can broaden the line shape of the magnetophotocurrent, increasing the internal magnetic parameter B-0 from 121 to 205 mT, which indicates an enhancement on SOC. Interestingly, the PL lifetime is found to be enlarged from 9.9 to 14.8 ns under this bending, which suggests that introducing compressive strain can essentially increase the Rashba effect through SOC, leading to an increase upon indirect band transition. Furthermore, the PL peak associated with the Rashba effect is shifted from 776 to 780 nm under this mechanical bending. Therefore, mechanical bending provides a convenient experimental method to approach the Rashba effect in hybrid perovskites.
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| 4. |
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| 5. |
- Nestoklon, Mikhail O., et al.
(författare)
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Tight-binding calculations of the optical properties of Si nanocrystals in a SiO(2)matrix
- 2020
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Ingår i: Faraday discussions. - : Royal Society of Chemistry (RSC). - 1359-6640 .- 1364-5498. ; 222:0, s. 258-273
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Tidskriftsartikel (refereegranskat)abstract
- We develop an empirical tight binding approach for the modeling of the electronic states and optical properties of Si nanocrystals embedded in a SiO(2)matrix. To simulate the wide band gap SiO(2)matrix we use the virtual crystal approximation. The tight-binding parameters of the material with the diamond crystal lattice are fitted to the band structure of beta-cristobalite. This model of the SiO(2)matrix allows us to reproduce the band structure of real Si nanocrystals embedded in a SiO(2)matrix. In this model, we compute the absorption spectra of the system. The calculations are in an excellent agreement with experimental data. We find that an important part of the high-energy absorption is defined by the spatially indirect, but direct ink-space transitions between holes inside the nanocrystal and electrons in the matrix.
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| 6. |
- 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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| 7. |
- 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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| 8. |
- Pevere, Federico
(författare)
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Optical Properties of Single Silicon Quantum Dots
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 9. |
- 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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| 10. |
- Pevere, Federico, et al.
(författare)
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X-ray radiation hardness and influence on blinking in Si and CdSe quantum dots
- 2018
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 113:25
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Tidskriftsartikel (refereegranskat)abstract
- We study the effect of X-ray irradiation on the photoluminescence (PL) efficiency and intermittency (blinking) of single Si/SiO2 and CdSe/CdZnS quantum dots (QDs). Our results show that the PL efficiency of Si nanocrystals is not significantly altered up to a cumulative fluence of 10(20) photons/m(2) (corresponding to similar to 300 kGy of absorbed dose in SiO2), while CdSe particles become completely dark already after a 17 times lower fluence. In both types of QDs, the statistical nature of blinking ON- and OFF-times remains unaltered: mono-exponential for Si and power-law for CdSe QDs. However, the evolution of the blinking parameters with absorbed dose depends on the choice of material. On average, both ON- and OFF-time constants do not vary in Si nanocrystals, highlighting their radiation hardness. Instead, the ON-time exponent increases while the OFF-time exponent decreases with the increasing dose for CdSe dots, confirming their efficiency quenching. Ensemble measurements did not show PL spectral changes neither indicated removal of surface ligands in irradiated CdSe dots. Thus, ionization-generated non-radiative centers in the core-shell system modify blinking of CdSe dots and eventually rapidly quench their emission, in contrast to robust Si/SiO2 nanocrystals. Our study is important for the future use of luminescent QDs in harsh environments, such as space, and the engineering of their blinking properties via ionizing radiation.
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| 11. |
- Sychugov, Ilya, et al.
(författare)
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Luminescent silicon nanocrystals as downconverters for photovoltaic and lighting applications
- 2016
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Ingår i: Asia Communications and Photonics Conference, ACP. - : OSA - The Optical Society.
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Konferensbidrag (refereegranskat)abstract
- Nanocrystals offer new functionalities for optoelectronics. Ensemble and single-dot measurements of chemically-synthesized silicon nanocrystals revealed high quantum yield, narrow homogeneous linewidth, and large Stokes shift, confirming application feasibility for these non-toxic and abundant material nanoparticles. Asia Communications and Photonics Conferene (ACP)
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| 12. |
- Sychugov, Ilya, et al.
(författare)
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Single-dot absorption spectroscopy and theory of silicon nanocrystals
- 2016
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950. ; 93:16
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 13. |
- Sychugov, Ilya, et al.
(författare)
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Strong Absorption Enhancement in Si Nanorods
- 2016
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 16:12, s. 7937-7941
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 14. |
- Sychugov, Ilya, et al.
(författare)
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Ultranarrow Luminescence Linewidth of Silicon Nanocrystals and Influence of Matrix
- 2014
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Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 1:10, s. 998-1005
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 15. |
- Zhou, Jingjian, et al.
(författare)
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Photoluminescence Intensity Enhancement of Single Silicon Quantum Dots on a Metal Membrane with a Spacer
- 2020
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Ingår i: Physica Status Solidi (a) applications and materials science. - : WILEY-V C H VERLAG GMBH. - 1862-6300 .- 1862-6319. ; 217:4
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Tidskriftsartikel (refereegranskat)abstract
- Silicon quantum dots (Si QDs) featuring high photoluminescence (PL) intensity are necessary for the realization of different photonic and photovoltaic devices, such as light-emitting diodes (LEDs) and luminescent solar concentrators (LSCs). Herein, Si QDs on a approximate to 100-200 nm thin silicon dioxide membrane with a metal back-coating are prepared. The dots are formed from the device layer of a silicon-on-insulator (SOI) wafer by etching and thermal oxidation. Aluminum is sputtered on the backside of the membrane, acting as a back-surface mirror, changing the local density of optical modes, as well as the local excitation field. The PL properties of such Si QDs are then characterized at the single-particle level. It is found that the PL yield of single Si QDs on the membrane is enhanced by approximately one order of magnitude, compared with that of Si QDs outside the membrane under the same excitation power. These results indicate that advances in nanofabrication can substantially improve the optical properties of Si QDs, thus paving the way for their application.
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| 16. |
- Zhou, Jingjian, et al.
(författare)
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Wafer-scale fabrication of isolated luminescent silicon quantum dots using standard CMOS technology
- 2020
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 31:50
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Tidskriftsartikel (refereegranskat)abstract
- A wafer-scale fabrication method for isolated silicon quantum dots (Si QDs) using standard CMOS technology is presented. Reactive ion etching was performed on the device layer of a silicon-on-insulator wafer, creating nano-sized silicon islands. Subsequently, the wafer was annealed at 1100 degrees C for 1 h in an atmosphere of 5% H(2)in Ar, forming a thin oxide passivating layer due to trace amounts of oxygen. Isolated Si QDs covering large areas (similar to mm(2)) were revealed by photoluminescence (PL) measurements. The emission energies of such Si QDs can span over a broad range, from 1.3 to 2.0 eV and each dot is typically characterized by a single emission line at low temperatures. Most of the Si QDs exhibited a high degree of linear polarization along Si crystallographic directions [110] abd [(1) over bar 10]. In addition, system resolution-limited (250 mu eV) PL linewidths (full width at half maximum) were measured for several Si QDs at 10 K, with no clear correlation between emission energy and polarization. The initial part of PL decays was measured at room temperature for such oxide-embedded Si QDs, approximately several microseconds long. By providing direct access to a broad size range of isolated Si QDs on a wafer, this technique paves the way for the future fabrication of photonic structures with Si QDs, which can potentially be used as single-photon sources with a long coherence length.
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