Recent experiments have shown that photoluminescence decay of silicon nanocrystals can be described by the stretched exponential function. We show here that the associated decay probability rate is the one-sided Levy stable distribution which describes well the experimental data. The relevance of these conclusions to the underlying stochastic processes is discussed in terms of Levy processes.
We show that photoluminescence properties of PbS nanocrystal thin films are directly related to film morphology and nanocrystal density. In densely packed PbS nanocrystal films, low-temperature donor-to-acceptor energy transfer is mainly responsible for the photoluminescence spectra narrowing and shift toward longer wavelengths. At elevated temperatures, back energy transfer is proposed to be responsible for an unusual photoluminescence intensity temperature dependence. In thin films with a low PbS nanocrystal density, the energy transfer is suppressed, and the effect is dramatically reduced.
We used the theory of finite periodic systems to explain the photoluminescence spectra dependence on the average diameter of nanocrystals embedded in oxide matrices. Because of the broad matrix band gap, the photoluminescence response is basically determined by isolated nanocrystals and sequences of a few of them. With this model we were able to reproduce the shape and displacement of the experimentally observed photoluminescence spectra.
We study the dependence of the quantum yield of photoluminescence of a dense, periodic array of semiconductor nanocrystals (NCs) on the level of doping and NC size. Electrons introduced to NCs via doping quench photoluminescence by the Auger process, so that practically only NCs without electrons contribute to the photoluminescence. Computer simulation and analytical theory are used to find a fraction of such empty NCs as a function of the average number of donors per NC and NC size. For an array of small spherical NCs, the quantization gap between 1S and 1P levels leads to transfer of electro…
The photoluminescence spectra of spherical CdTe nanocrystals with zincblende structure are studied by size-selective spectroscopic techniques. We observe a resonant Stokes shift of 15 meV when the excitation laser energy is tuned to the red side of the absorption band at 2.236 eV. The experimental data are analyzed within a symmetry-based tight-binding theory of the exciton spectrum, which is first shown to account for the size dependence of the fundamental gap reported previously in the literature. The theoretical Stokes shift presented as a function of the gap shows a good agreement with the…
Silicon nanocrystals with diameters between 1 and 3 nm and surfaces passivated by chlorine or a mixture of chlorine and hydrogen were modeled using density functional theory, and their properties compared with those of fully hydrogenated nanocrystals. It is found that fully and partially chlorinated nanocrystals are stable, and have higher electron affinity, higher ionization energy and lower optical absorption energy threshold. As the hydrogenated silicon nanocrystals, chlorinated silicon nanocrystals doped with phosphorus or boron require a high activation energy to transfer an electron or h…
Single cesium lead bromide (CsPbBr3) nanocrystals show strong photoluminescence blinking, with on- and off- dwelling times following power-law distributions. We investigate the memory effect in the photoluminescence blinking of single CsPbBr3 nanocrystals and find positive correlations for successive on-times and successive off-times. This memory effect is not sensitive to the nature of the surface capping ligand and the embedding polymer. These observations suggest that photoluminescence intermittency and its memory are mainly controlled by intrinsic traps in the nanocrystals. These findings …
We study the low-temperature magneto-photoluminescence (PL) from individual CdSe nanocrystals. Nanocrystals having a small "bright" exciton fine structure splitting ($<$0.5 meV) exhibit a conventional left- and right-circularly polarized Zeeman PL doublet in applied magnetic fields. In contrast, nanocrystals with large fine structure splitting ($>$1 meV) show an anomalous magneto-PL polarization, wherein the lower-energy peak becomes circularly polarized with increasing field, while the higher-energy peak remains linearly polarized. This unusual behavior arises from strong mixing between the a…
The exciton spin dynamics and polarization properties of the related emission are investigated in colloidal CdSe/CdS dot-in-rod (DiR) and spherical core/shell nanocrystal (NC) ensembles by magneto-optical photoluminescence (PL) spectroscopy in magnetic fields up to 15 T. It is shown that the degree of circular polarization (DCP) of the exciton emission induced by the magnetic field is affected by the NC geometry as well as the exciton fine structure and can provide information on nanorod orientation. A theory to describe the circular and linear polarization properties of the NC emission in mag…
We study structural and chemical transformations induced by focused laser beam in GaAs nanowires with axial zinc-blende/wurtzite (ZB/WZ) heterostucture. The experiments are performed using a combination of transmission electron microscopy, energy-dispersive X-ray spectroscopy, Raman scattering, and photoluminescence spectroscopy. For the both components of heterostructure, laser irradiation under atmospheric air is found to produce a double surface layer which is composed of crystalline arsenic and of amorphous GaO$_{x}$. The latter compound is responsible for appearance of a peak at 1.76 eV i…
We develop a phenomenological theory of inhomogeneous broadening in zero-dimensional systems and apply it to study photoluminescence (PL) spectra of silicon nanocrystals measured at helium and room temperatures. The proposed approach allowed us to explain experimentally observed PL peak asymmetry, linear dependence of the peak width on its maximum and anomalous alteration of spectral characteristics with temperature increase.
It is possible to confine vibrational modes to silicon nanocrystals by encapsulating them within hydrogenated amorphous silicon. This is not because of the small impedance mismatch between materials but, rather, is due to higher order moments in the distribution of density and stiffness in the amorphous phase--i.e. it is a result of material substructure. The concept is elucidated using an idealized one-dimensional setting and then demonstrated for a realistic nanocrystalline geometry. Beyond the immediate focus on amorphous encapsulation, this offers the prospect of specifically engineering h…
Tin-containing nanocrystals, embedded in silicon, have been fabricated by growing an epitaxial layer of Si_{1-x-y}Sn_{x}C_{y}, where x = 1.6 % and y = 0.04 %, followed by annealing at various temperatures ranging from 650 to 900 degrees C. The nanocrystal density and average diameters are determined by scanning transmission-electron microscopy to ~ 10^{17} cm^{-3} and ~ 5 nm, respectively. Photoluminescence spectroscopy demonstrates that the light emission is very pronounced for samples annealed at 725 degrees C, and Rutherford back-scattering spectrometry shows that the nanocrystals are predo…
Silicon nanocrystals are studied by time-resolved fluorescence spectroscopy. After laser excitation the bright and dark exciton ground state levels are populated at random, but subsequently the decay curves reveal a thermalization between these levels. The characteristic thermalization time is found to be approximately 100 ns for temperatures below 100 K and surprisingly increases for higher temperatures. The decay curves are analyzed using a simple two-state model for the bright and dark exciton ground states.
Engineering semiconductors to enhance carrier multiplication (CM) could lead to increased photovoltaic cell performance and a significant widening of the materials range suitable for future solar technologies. Semiconductor nanocrystals (NCs) have been proposed as a favourable structure for CM enhancement, and recent measurements by transient absorption have shown evidence for highly efficient CM in lead chalcogenide and CdSe NCs. We report here an assessment of CM yields in CdSe and CdTe NCs by a quantitative analysis of biexciton and exciton signatures in transient photoluminescence decays. …
It is widely accepted that the properties of most semiconductor nanocrystals can be tuned by their core size, shape and material. In covalent semiconductor nanocrystal materials, such as silicon, germanium or carbon, certain degree of tunability of the properties can be also achieved by the surface ligands. In particular, covalently bonded ligand species on the surface of such a nanocrystal (i) contribute to the density of states of the core via orbital delocalization; (ii) might introduce strain via ligand-to-ligand steric hindrance and (iii) will cause charge transfer from/to the core. In th…
We report on the possibility of describing the absorption and emission characteristics of an ensemble of silicon nanocrystals (NCs) with realistic distributions in the NC size, by the sum of the reponses of the single NCs. The individual NC responses are evaluated by means of ab-initio theoretical calculations and the summation is performed by taking into account the trend of the optical properties as a function of NC size and oxidation degree. The comparison with experimental results shows a nice matching of the spectra, also without any tuning of the parameters. Finally, the possibility of a…
In semiconductors, quantum confinement can greatly enhance the interaction between band carriers (electrons and holes) and dopant atoms. One manifestation of this enhancement is the increased stability of exciton magnetic polarons in magnetically-doped nanostructures. In the limit of very strong 0D confinement that is realized in colloidal semiconductor nanocrystals, a single exciton can exert an effective exchange field $B_{\rm{ex}}$ on the embedded magnetic dopants that exceeds several tesla. Here we use the very sensitive method of resonant photoluminescence (PL) to directly measure the pre…
In most of the realistic ab initio and model calculations which have appeared on the emission of light from Si nanocrystals, the role of surface oxygen has been usually ignored, underestimated or completely ruled out. We investigate theoretically, by density functional theory (DFT/B3LYP) possible modes of oxygen bonding in hydrogen terminated silicon quantum dots using as a representative case of the Si29 nanocrystal. We have considered Bridge-bonded oxygen (BBO), Doubly-bonded oxygen (DBO), hydroxyl (OH) and Mix of these oxidizing agents. Due to stoichiometry, all comparisons performed are un…
Although the structural phase transitions in single-crystal hybrid methyl-ammonium (MA) lead halide perovskites (MAPbX3, X = Cl, Br, I) are common phenomena, they have never been observed in the corresponding nanocrystals. Here we demonstrate that two-photon-excited photoluminescence (PL) spectroscopy is capable of monitoring the structural phase transitions in MAPbX3 nanocrystals because nonlinear susceptibilities govern the light absorption rates. We provide experimental evidence that the orthorhombic-to-tetragonal structural phase transition in a single layer of 20-nm-sized 3D MAPbBr3 nanoc…
In this work, we demonstrate a silicon nanocrystal Field Effect Transistor (ncFET). Its operation is similar to that of a Tunnelling Field Effect Transistor (TFET) with two barriers in series. The tunnelling barriers are fabricated in very thin silicon dioxyde and the channel in intrinsic polycrystalline silicon. The absence of doping eliminates the problem of achieving sharp doping profiles at the junctions, which has proven a challenge for large-scale integration and in principle allows scaling down the atomic level. The demonstrated ncFET features a 10$^4$ on/off current ratio at room tempe…
The decay of the optical phonon into the two phonons of smaller energy is calculated for Si nanocrystals. The rate of the process is in the range of 1 to 10 ps. Such anharmonic phonon decay may control the energy relaxation rate of excited carriers in Si/SiO$_2$ nanocrystals. Relevance of the phonon decay to the experimentally observed hot carrier photoluminescence is discussed.
Strain has a crucial effect on the optical and electronic properties of nanostructures. We calculate the atomistic strain distribution in silicon nanocrystals up to a diameter of 3.2 nm embedded in an amorphous silicon dioxide matrix. A seemingly conflicting picture arises when the strain field is expressed in terms of bond lengths versus volumetric strain. The strain profile in either case shows uniform behavior in the core, however it becomes nonuniform within 2-3 Ådistance to the nanocrystal surface: tensile for bond lengths whereas compressive for volumetric strain. We reconsile their coex…
The dynamics of the luminescence decay from germanium nanocrystals embedded in crystalline silicon has been studied for temperatures varied between 16 K and room temperature. At room temperature the characteristic decay time is of the order of 50 nanoseconds while it extends into the microsecond range at low temperatures. The decay is dominated by non-radiative processes, which show a typical thermal activation energy of a few meV.
Carrier spins in semiconductor nanocrystals are promising candidates for quantum information processing. Using a combination of time-resolved Faraday rotation and photoluminescence spectroscopies, we demonstrate optical spin polarization and coherent spin precession in colloidal CsPbBr3 nanocrystals that persists up to room temperature. By suppressing the influence of inhomogeneous hyperfine fields with a small applied magnetic field, we demonstrate inhomogeneous hole transverse spin-dephasing times (T2*) that approach the nanocrystal photoluminescence lifetime, such that nearly all emitted ph…
A modified phonon confinement model considering the size distribution, an improved phonon dispersion curve and a confinement function is developed for the calculation of size dependent Raman spectra of the silicon (Si) nanocrystals. The model is capable in simultaneous calculation of the Raman shift, intensity and linewidth. The calculated size dependent redshift and linewidth of Raman spectra are in good agreement with the available experimental data in literature and better than previously reported theoretical results. The rapid rise in the redshift and linewidth for relatively smaller Si na…
Silicon (Si) nanoparticles (NPs) were synthesized by transforming Si wafer surface to ammonium silicon hexafluoride (ASH) or (NH4)2SiF6 under acid vapor treatment. Si-NPs are embedded within the polycrystalline (ASH) layer formed on the Si surface exhibit a strong green-orange photoluminescence (PL). Difference measurements revealed a major double component spectra consisting of a broad band associated with the ASH-Si wafer interfacial porous oxide layer and a high energy band attributable to Si-NPs embedded in the ASH. The origin of the latter emission can be explained in terms of quantum/spa…
We present a temperature dependent photoluminescence study of silicon optical nanocavities formed by introducing point defects into two-dimensional photonic crystals. In addition to the prominent TO phonon assisted transition from crystalline silicon at ~1.10 eV we observe a broad defect band luminescence from ~1.05-1.09 eV. Spatially resolved spectroscopy demonstrates that this defect band is present only in the region where air-holes have been etched during the fabrication process. Detectable emission from the cavity mode persists up to room-temperature, in strong contrast the background emi…
The epitaxial growth of germanium on silicon leads to the self-assembly of SiGe nanocrystals via a process that allows the size, composition and position of the nanocrystals to be controlled. This level of control, combined with an inherent compatibility with silicon technology, could prove useful in nanoelectronic applications. Here we report the confinement of holes in quantum-dot devices made by directly contacting individual SiGe nanocrystals with aluminium electrodes, and the production of hybrid superconductorsemiconductor devices, such as resonant supercurrent transistors, when the dot …
Silicon photoluminescence and lasing have been critical issues to breakthrough bottlenecks in the understanding of luminescence mechanisms. Unfortunately, long-standing disputes about the exciton recombination mechanism and fluorescence lifetime remain unresolved, especially about whether silicon nanocrystals (Si NCs) can realize fast direct-bandgap-like optical transitions. Here, using ground-state and excited-state density functional theory (DFT), we obtained intrinsic phonon-free optical transitions at sizes from Si22 to Si705, showing that very small Si NCs can realize a strong direct opti…
Yb3+-doped inorganic metal-halide perovskites (Yb3+:CsPbX3, X = Cl, Br) have recently been discovered to display highly efficient quantum cutting, in which the energy from individual blue or UV photons absorbed by the material is re-emitted in the form of pairs of near-infrared photons by Yb3+ dopants. Experimental photoluminescence quantum yields approaching 200{%} have been reported. As the first quantum-cutting materials that combine such high photoluminescence quantum yields with strong, broadband absorption in the visible, these materials offer unique opportunities for enhancing the effic…
Titanium dioxide (TiO2) nanocrystals were prepared by wet chemical method and characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Raman scattering (RS) and photoluminescence (PL). The X-ray diffraction shows the formation of nanocrytalline TiO2 of average sizes 7 nm and 15 nm for two samples. The x-ray diffraction, transmission electron microscopy (TEM) and Raman scattering shows that the TiO2 nanocrystals has anatase crystal structure for both samples. The PL intensity of the smaller particle is more, which has been attributed to defects and particle size variati…
In recent years many experiments have demonstrated the possibility to achieve efficient photoluminescence from Si/SiO2 nanocrystals. While it is widely known that only a minor portions of the nanocrystals in the samples contribute to the observed photoluminescence, the high complexity of the Si/SiO2 interface and the dramatic sensitivity to the fabrication conditions make the identification of the most active structures at the experimental level not a trivial task. Focusing on this aspect we have addressed the problem theoretically, by calculating the radiative recombination rates for differen…
The nucleation and structure of silicon nanocrystals formed by different preparation conditions and silicon concentration (28 - 70 area %) have been studied using Transmission Electron Microscopy (TEM), Energy Filtered TEM (EFTEM) and Secondary Ion Mass Spectroscopy (SIMS). The nanocrystals were formed after heat treatment at high temperature of a sputtered 10 nm thick silicon rich oxide on 3 nm SiO$_2$ layer made by Rapid Thermal Oxidation (RTO) of silicon. Nanocrystals precipitate when the excess silicon concentration exceeds 50 area %. Below this percentage amorphous silicon nanoclusters we…
We present the room temperature (RT) near-infrared (NIR) photoluminescence (PL) properties of Si/Ge nanowire (NW) grown silicon wafers which were treated by vapor of HF:HNO3 chemical mixture. This treatment activates or enhances the PL intensity at NIR region ranging from 1000 nm to 1800 nm. The PL consists of a silicon band-edge emission and a broad composite band which was centered at around 1400-1600 nm. The treatment modifies the wafer surface particularly at defect sites particularly pits around NWs and NW surfaces by etching and oxidation of Si and Ge. This process can induce spatial con…
Silicon nanocrystals are produced using a two-stage gold ion implantation technique. First stage implantation using low energy ions leads to the formation of an amorphous Si (a-Si) layer. A subsequent high energy Au irradiation in the second stage is found to produce strained Si NCs. An annealing at a temperature as low as 500$^o$C is seen to result in strain free NCs showing quantum confinement effects. Higher temperature annealing of the samples is found to result in growth in size from recrystallization of the a-Si matrix. Raman Scattering, X-ray diffraction (XRD) and Rutherford Backscatter…
Silicon nanocrystals (Si NCs) have shown great promise for electroluminescent and photoluminescent applications. In order to optimize the properties of Si NC devices, however, electronic transport in Si NCs films needs to be thoroughly understood. Here we present a systematic study of the temperature and electric field dependence of conductivity in films of alkyl-ligand-terminated Si NCs, which to date have shown the highest potential for device applications. Our measurements suggest that the conductivity is limited by the ionization of rare NCs containing donor impurities. At low bias, this i…
Silicon lasers have been the most challenging element for the monolithic integrated Si photonics. Here we report the first successful all-Si laser at room temperature based on silicon nanocrystals (Si NCs) with high optical gains. The active Si NC layer was made from hydrogen silsesquioxane (HSQ) that had undergone a phase separation annealing, followed by high-pressure hydrogen passivation. Using the nanoimprint technique, a second-order Bragg grating was made on the active layer as the distributed feedback (DFB) laser cavity. This Si NC DFB laser device was optically pumped with femtosecond …
We describe the process of Foerster transfer between semiconductor nanoparticles in the presence of a metal subsystem (metal nanocrystals). In the presence of metal nanocrystals, the Foerster process can become faster and more long-range. The enhancement of Foerster transfer occurs due to the effect of plasmon-assisted amplification of electric fields inside the nanoscale assembly. Simultaneously, metal nanocrystals lead to an increase of energy losses during the Foerster transfer process. We derive convenient equations for the energy transfer rates, photoluminescence intensities, and energy d…
We present a theory of the local field corrections to the spontaneous emission rate for the array of silicon nanocrystals in silicon dioxide. An analytical result for the Purcell factor is obtained. We demonstrate that the local-field corrections are sensitive to the volume fill factor of the nanocrystals in the sample and are suppressed for large values of the fill factor. The local-field corrections and the photonic density of states are shown to be described by two different effective permittivities: the harmonic mean between the nanocrystal and the matrix permittivities and the Maxwell-Gar…
Demonstrating the quantum-confined Stark effect (QCSE) in silicon nanocrystals (NCs) embedded in oxide has been rather elusive, unlike the other materials. Here, the recent experimental data from ion-implanted Si NCs is unambiguously explained within the context of QCSE using an atomistic pseudopotential theory. This further reveals that the majority of the Stark shift comes from the valence states which undergo a level crossing that leads to a nonmonotonic radiative recombination behavior with respect to the applied field. The polarizability of embedded Si NCs including the excitonic effects …
The optical properties of metallic tin nanoparticles embedded in silicon-based host materials were studied. Thin films containing the nanoparticles were produced using RF magnetron sputtering followed by ex situ heat treatment. Transmission electron microscopy was used to determine the nanoparticle shape and size distribution; spherical, metallic tin nanoparticles were always found. The presence of a localized surface plasmon resonance in the nanoparticles was observed when SiO2 and amorphous silicon were the host materials. Optical spectroscopy revealed that the localized surface plasmon reso…
The nature of the conducting filaments in many resistive switching systems has been elusive. Through in situ transmission electron microscopy, we image the real-time formation and evolution of the filament in a silicon oxide resistive switch. The electroforming process is revealed to involve the local enrichment of silicon from the silicon oxide matrix. Semi-metallic silicon nanocrystals with structural variations from the conventional diamond cubic form of silicon are observed, which likely accounts for the conduction in the filament. The growth and shrinkage of the silicon nanocrystals in re…
Photoluminescence spectra show that silicon impurity is present in lattice of some nanodiamond grains (ND) of various chondrites as a silicon-vacancy (SiV) defect. The relative intensity of the SiV band in the diamond-rich separates depends on chemical composition of meteorites and on size of ND grains. The strongest signal is found for the size separates enriched in small grains; thus confirming our earlier conclusion that the SiV defects preferentially reside in the smallest (less than 2 nm) grains. The difference in relative intensities of the SiV luminescence in the diamond-rich separates …
We present the design, fabrication and characterization of cubic (3C) silicon carbide microdisk resonators with high quality factor modes at visible and near infrared wavelengths (600 - 950 nm). Whispering gallery modes with quality factors as high as 2,300 and corresponding mode volumes V ~ 2 (λ/n)^3 are measured using laser scanning confocal microscopy at room temperature. We obtain excellent correspondence between transverse-magnetic (TM) and transverse-electric (TE) polarized resonances simulated using Finite Difference Time Domain (FDTD) method and those observed in experiment. These stru…
Selective coherence control and electrically mediated exchange coupling of single electron spin between triplet and singlet states using numerically derived optimal control of proton pulses is demonstrated. We obtained spatial confinement below size of the Bohr radius for proton spin chain FWHM. Precise manipulation of individual spins and polarization of electron spin states are analyzed via proton induced emission and controlled population of energy shells in pure 29Si nanocrystal. Entangled quantum states of channeled proton trajectories are mapped in transverse and angular phase space of 2…
