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  • 1.
    Bruhn, Benjamin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    Fabrication and characterization of single luminescing quantum dots from 1D silicon nanostructures2012Doktoravhandling, med artikler (Annet vitenskapelig)
    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).

  • 2.
    Bruhn, Benjamin
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik. University of Amsterdam, Netherlands .
    Qejvanaj, Fatjon
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Gregorkiewicz, Tom
    University of Amsterdam, Netherlands .
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Temporal correlation of blinking events in CdSe/ZnS and Si/SiO2 nanocrystals2014Inngår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 453, s. 63-67Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Well passivated single Si/SiO2 nanoparticles obey mono exponential blinking statistics, whereas CdSe/ZnS quantum dots follow an apparent (truncated) power-law. Log-normal distributions are found to describe the interval length histograms at least as well as power-laws, while at the same time being more physically feasible and significantly easing the determination of the exponential cutoff in the ON-time distribution. The correlation of an ON- (OFF-)interval with its temporally displaced ON (OFF) neighbors, as well as that of intermixed intervals (ON with OFF and OFF with ON neighbors) has been studied. As expected from purely random processes, the correlation coefficients for events in silicon nanocrystals equal zero, whereas positive correlations between the pure and negative correlations between the mixed states in CdSe quantum dots hint at a switching process between two distinct blinking regimes that are slower than the blinking itself.

  • 3.
    Bruhn, Benjamin
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Qejvanaj, Fatjon
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Sychugov, Ilya
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Blinking Statistics and Excitation-Dependent Luminescence Yield in Si and CdSe Nanocrystals2014Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, nr 4, s. 2202-2208Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    ON-OFF intermittency or blinking is a phenomenon observed in single quantum emitters, which reduces their overall light emission. Even though it seems to be a fundamental property of quantum dots (QDs), substantial differences can be found in the blinking statistics of different nanocrystals. This work compares the blinking of numerous single, oxide-capped Si nanocrystals with that of CdSe/ZnS core-shell nanocrystals, measured under the same conditions in the same experimental system and over a broad range of excitation power densities. We find that ON- and OFF-times can be described by exponential statistics in Si QDs, as opposed to power-law statistics for the CdSe nanocrystals. The type of blinking (power-law or monoexponential) does not depend on excitation but seems to be an intrinsic property of the material system. Upon increasing excitation power, the duty cycle of Si quantum dots remains constant, whereas it decreases for CdSe nanocrystals, which is readily explained by blinking statistics. Both ON-OFF and OFF-ON transitions can be regarded as light-induced in Si/SiO2 QDs, while the OFF-ON transition in CdSe/ZnS nanocrystals is not stimulated by photons. The differences in blinking behavior in these systems will be discussed.

  • 4.
    Bruhn, Benjamin
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Sangghaleh, Fatemeh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Fabricating single silicon quantum rods for repeatable single dot photoluminescence measurements2011Inngår i: Physica Status Solidi A-applications and materials science, ISSN 1862-6319, Vol. 208, nr 3, s. 631-634Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 5.
    Bruhn, Benjamin
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik.
    Valenta, Jan
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik.
    Controlled fabrication of individual silicon quantum rods yielding high intensity, polarized light emission2009Inngår i: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 20, nr 50, s. 1-5Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Elongated silicon quantum dots (also referred to as rods) were fabricated using a lithographic process which reliably yields sufficient numbers of emitters. These quantum rods are perfectly aligned and the vast majority are spatially separated well enough to enable single-dot spectroscopy. Not only do they exhibit extraordinarily high linear polarization with respect to both absorption and emission, but the silicon rods also appear to luminesce much more brightly than their spherical counterparts. Significantly increased quantum efficiency and almost unity degree of linear polarization render these quantum rods perfect candidates for numerous applications.

  • 6.
    Bruhn, Benjamin
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik.
    Valenta, Jan
    Sangghaleh, Fatemeh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik.
    Blinking Statistics of Silicon Quantum Dots2011Inngår i: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 11, nr 12, s. 5574-5580Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 7.
    Bruhn, Benjamin
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Valenta, Jan
    Sychugov, Ilya
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Mitsuishi, Kazutaka
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Transition from silicon nanowires to isolated quantum dots: Optical and structural evolution2013Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, nr 4, s. 045404-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The evolution of the structural and optical properties of a silicon core in oxidized nanowalls is investigated as a function of oxidation time. The same individual nanostructures are characterized after every oxidation step in a scanning electron microscope and by low-temperature photoluminescence, while a representative sample is also imaged in a transmission electron microscope. Analysis of a large number of recorded single-dot spectra and micrographs allows to identify delocalized and localized exciton emission from a nanowire as well as confined exciton emission of a nanocrystal. It is shown how structural transitions from one-to zero-dimensional confinement affect single-nanostructure optical fingerprints.

  • 8.
    Bruhn, Benjamin
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Valenta, Jan
    Sychugov, Ilya
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Mitsushi, Kazutaka
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Transition fromsilicon nanowires to isolated quantum dots: Optical and structural evolution2009Manuskript (preprint) (Annet (populærvitenskap, debatt, mm))
  • 9.
    Göthelid, Mats
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Tymczenko, Michael
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Chow, Winnie
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Ahmadi, Sareh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Yu, Shun
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Bruhn, Benjamin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Stoltz, Dunja
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    von Schenck, Henrik
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Weissenrieder, Jonas
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Sun, Chenghua
    Surface concentration dependent structures of iodine on Pd(110)2012Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, nr 20, s. 204703-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We use photoelectron spectroscopy, low energy electron diffraction, scanning tunneling microscopy, and density functional theory to investigate coverage dependent iodine structures on Pd(110). At 0.5 ML (monolayer), a c(2 x 2) structure is formed with iodine occupying the four-fold hollow site. At increasing coverage, the iodine layer compresses into a quasi-hexagonal structure at 2/3 ML, with iodine occupying both hollow and long bridge positions. There is a substantial difference in electronic structure between these two iodine sites, with a higher electron density on the bridge bonded iodine. In addition, numerous positively charged iodine near vacancies are found along the domain walls. These different electronic structures will have an impact on the chemical properties of these iodine atoms within the layer.

  • 10.
    Pevere, Federico
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Bruhn, Benjamin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Sangghaleh, Fatemeh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Hormozan, Yashar
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Sychugov, Ilya
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Effect of X-ray irradiation on the blinking of single silicon nanocrystals2015Inngår i: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 212, nr 12Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 11.
    Pevere, Federico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Sangghaleh, Fatemeh
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Bruhn, Benjamin
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Sychugov, Ilya
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Linnros, Jan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Rapid Trapping as the Origin of Nonradiative Recombination in Semiconductor Nanocrystals2018Inngår i: ACS Photonics, E-ISSN 2330-4022, Vol. 5, nr 8, s. 2990-2996Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 12.
    Pevere, Federico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    von Treskow, Carl
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Marino, Emanuele
    Van der Waals – Zeeman Institute, University of Amsterdam, The Netherlands.
    Anwar, Monib
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Bruhn, Benjamin
    Sychugov, Ilya
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Linnros, Jan
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Fotonik.
    X-ray radiation hardness and influence on blinking in Si and CdSe quantum dots2018Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 113, nr 25, artikkel-id 253103Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 13.
    Sangghaleh, Fatemeh
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Bruhn, Benjamin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Schmidt, Torsten
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Exciton lifetime measurements on single silicon quantum dots2013Inngår i: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 24, nr 22, s. 225204-Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 14.
    Sangghaleh, Fatemeh
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Bruhn, Benjamin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Schmidt, Torsten
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik.
    Exciton lifetime measurementson single silicon quantum dots: explanation of stretched exponentialdecay2011Manuskript (preprint) (Annet vitenskapelig)
  • 15.
    Sangghaleh, Fatemeh
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Bruhn, Benjamin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Sychugov, Ilya
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik, Materialfysik, MF.
    Optical absorption cross section and quantum efficiency of a single silicon quantum dot2013Inngår i: Nanotechnology VI, SPIE - International Society for Optical Engineering, 2013, s. 876607-Konferansepaper (Fagfellevurdert)
    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.

  • 16.
    Sychugov, Ilya
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Sangghaleh, Fatemeh
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Bruhn, Benjamin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Pevere, Federico
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Luo, Jun-Wei
    Zunger, Alex
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Material- och nanofysik.
    Strong Absorption Enhancement in Si Nanorods2016Inngår i: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 16, nr 12, s. 7937-7941Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 17. Valenta, Jan
    et al.
    Bruhn, Benjamin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Materialfysik, MF (Stängd 20120101).
    Coexistence of 1D and Quasi-0D Photoluminescence from Single Silicon Nanowires2011Inngår i: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 11, nr 7, s. 3003-3009Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Single silicon nanowires (Si-NWs) prepared by electron-beam lithography and reactive-ion etching are investigated by imaging optical spectroscopy under variable temperatures and laser pumping intensities. Spectral images of individual Si-NWs reveal a large variability of photoluminescence (PL) along a single Si-NW. The weaker broad emission band asymmetrically extended to the high-energy side is interpreted to be due to recombination of quasi-free 1D excitons while the brighter localized emission features (with significantly variable peak position, width, and shape) are due to localization of electron hole pairs in surface protrusions acting like quasi-0D centers or quantum dots (QDs). Correlated PL and scanning electron microscopy images indicate that the efficiently emitting QDs are located at the Si-NW interface with completely oxidized neck of the initial Si wall. Theoretical fitting of the delocalized PL emission band explains its broad asymmetrical band to be due to the Gaussian size distribution of the Si-NW diameter and reveals also the presence of recombination from the Si-NW excited state which can facilitate a fast capture of excitons into QD centers.

  • 18. Valenta, Jan
    et al.
    Bruhn, Benjamin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Materialfysik, MF.
    Polarization of photoluminescence excitation and emission spectra of silicon nanorods within single Si/SiO2 nanowires2011Inngår i: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 8, nr 3, s. 1017-1020Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Polarization properties of individual silicon nanowires are studied using an optical micro-spectroscopy setup equipped with a Fresnel rhomb to rotate the polarization of the exciting laser and the analyzer to characterize the polarization of emitted photoluminescence. The Si nanowire samples are prepared by electron-beam lithography, plasma etching and oxidation. The fabricated wires are embedded in SiO2 and oriented parallel to the Si substrate. Due to the fluctuating wire diameter (around 5 nm) the very long wires (several tens of μm) are effectively divided into an array of quantum rods (prolate ellipsoids). These structures have strong photoluminescence under UV-blue excitation at room temperature. The degree of photoluminescence linear polarization of both excitation and emission is very high, between 0.9-1, and reveals relatively low fluctuations at different spots of the wires. Experimental results are compared with available theoretical models leading to the conclusion that the high polarization degree is mostly due to surface charges (dielectric confinement) with smaller contribution of quantum confinement effects.

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