Mechanism and enhancement of photoluminescence from silicon nanocrystals implanted in SiO2 matrix

Photoluminescence (PL) spectra of Si nanocrystals (NCs) prepared by 130 keV Si ions implantation onto SiO2 matrix were investigated as a function of annealing temperature and implanted ion dose. PL spectra consist of two PL peaks, originated from smaller Si NCs due to quantum confinement effect (QCE) and the interface states located at the surface of larger Si NCs. The evolution of number of dangling bonds (DBs) on Si NCs was also investigated. For hydrogen-passivated samples, a monotonic increase in PL peak intensity with the dose of implanted Si ions up to 3×1017 ions /cm2 is observed. The number of DBs on individual Si NC, the interaction between DBs at the surface of neighbouring Si NCs and their effects on the efficiency of PL are discussed.     

Structures,Oxidation, and Charge Transport of Phosphorus‐Doped Germanium Nanocrystals

The doping of semiconductor nanocrystals (NCs) is crucial for the optimization of the performance of devices based on them. In contrast to recent progress on the doping of compound semiconductor NCs and silicon NCs, the doping of germanium (Ge) NCs has lagged behind. Here it is shown that Ge NCs can be doped with phosphorus (P) during synthesis by a nonthermal plasma. It is found that there are more P atoms in the NC near‐surface region than in the NC core. P doping modifies the surface state of Ge NCs. Compressive strain can be incuced in Ge NCs by P which can explain the P‐doping‐enhanced oxidation resistance of Ge NCs. Stable dispersions of P‐doped Ge NCs in acetonitrile can be cast to produce films for field‐effect transistors (FETs). FET analysis shows that the electrical conductivity and electron mobility of a Ge‐NC film increase with the increase of the P doping level, although the electrical activation efficiency of P in the Ge‐NC film is low. Finally, atomic layer deposition of aluminum oxide at the surface of P‐doped Ge NCs is shown to improve the performance of the FETs.     

Optical spectroscopy of lanthanides doped in wide band-gap semiconductor nanocrystals

Currently, tripositive lanthanide (Ln³⁺) ions doped wide band-gap semiconductor nanocrystals (NCs) have been the focus of research interest due to their distinct optical properties and potential applications in optical devices and luminescent biolabels. Because of the low absorptions of parity-forbidden 4f-4f transitions for Ln³⁺, it is highly anticipated that the luminescence of Ln³⁺ ions embedded in wide band-gap NC lattices can be sensitized efficiently via exciton recombination in the host. For this purpose, the successful incorporation of Ln³⁺ into the lattices of semiconductor NCs is of utmost importance, which still remains intractable via conventional wet chemical methods. Here, the most recent progress in the optical spectroscopy of Ln³⁺ ions doped wide band-gap semiconductor NCs is discussed. Much attention was focused on the optical properties including electronic structures, luminescence dynamics, energy transfer as well as the up-conversion emissions of Ln³⁺ ions in ZnO, TiO₂, SnO₂ and In₂O₃ NCs that were synthesized in our laboratory using wet chemical methods.     

Analytical formula for two‐dimensional ring artefact suppression

Ring artefacts are the most disturbing artefacts when reconstructed volumes are segmented. A lot of effort has already been put into better X‐ray optics, scintillators and detectors in order to minimize the appearance of these artefacts. However, additional processing is often required after standard flat‐field correction. Several methods exist to suppress artefacts. One group of methods is based on minimization of the Tikhonov functional. An analytical formula for processing of a single sinogram was developed. In this paper a similar approach is used and a formula for processing two‐dimensional projections is found. Thus suppression of ring artefacts is organized as a two‐dimensional convolution of `averaged' projections with a given filter. Several approaches are discussed in order to find elements of the filter in a faster and accurate way. Examples of experimental datasets processed by the proposed method are considered.     

Automatically Purification of Aqueous CdTe Nanocrystals in Water-ethanol Co-environment

Purification is a separated post-treatment step after the synthesis of nanocrystals (NCs) in order to exclude excess ligands and monomers in NC solution. The common purification process involves many manipulations, such as concentrating, addition of anti-solvents and centrifugation, which are troublesome and time consuming. In this work, we originally integrate NC synthesis and NC purification in one-pot via selecting water-ethanol co-environment for NC synthesis and NC purification. Our research shows that NCs can grow in water-ethanol mixture. When growing into critical size, NCs will automatically precipitate from the solution. Element analysis demonstrates that precipitates fraction fits well with stoichiometric of ligand-capped NCs. Excess monomers are left in supernatant, and thus achieving automatically purification of NCs in the water-ethanol co-environment. By adjusting the volume ratios of water and ethanol in bi-solvent system, different-sized purified NCs can be controlled. Besides, this water-ethanol co-environment can be used in both thermal-promoted and hydrazine-promoted growth.     

Size-dependent intrinsic radiative decay rates of silicon nanocrystals at large confinement energies

We study ultrafast photoluminescence (PL) dynamics of Si nanocrystals (NCs). The early-time PL spectra (<1 ns), which show strong dependence on NC size, are attributed to emission involving NC quantized states. The PL spectra recorded for long delays (>10 ns) are almost independent of NC size and are likely due to surface-related recombination. Based on instantaneous PL intensities measured 2 ps after excitation, we determine intrinsic radiative rate constants for NCs of different sizes. These constants sharply increase for confinement energies greater than approximately 1 eV indicating a fast, exponential growth of the oscillator strength of zero-phonon, pseudodirect transitions.     

Energy dispersion of localized states in light-sensitive nanocrystals

The kinetics of the formation and thermal destruction of color centers in CuCl and AgCl nanocrystals (NCs) distributed in a glass matrix is described on the basis of the band model of an NC with colloidal color centers and with hole traps of one species. The possibility of experimentally determining the relative depth distribution of hole states in light-sensitive NCs in glass is demonstrated. The observed energy dispersion of localized hole states and its variation in NCs are associated, in accordance with Dexter’s idea, with large-scale thermal fluctuations of the crystal field. The presence of an excess charge on a colloidal particle and its influence on localized hole states are presumed.     

Ultrasound-assisted synthesis of nanocomposites based on aromatic polyamide and modified ZnO nanoparticle for removal of toxic Cr(VI) from water

In this study, novel nanocomposites (NCs) of aromatic polyamide (PA) and surface modified ZnO nanoparticle with s-triazine heterocyclic ring was introduced for efficient removal of toxic hexavalent chromium (VI) from aqueous solution. The surface of ZnO nanoparticle was modified by s-triazine core silane coupling agent (ZnO-TSC) and PA/ZnO-TSC NCs with different amount of ZnO-TSC nanoparticles (0, 5, 10 and 15 wt%) were prepared by ultrasonic irradiation. The synthesized PA/ZnO-TSC NCs were characterized by FT-IR, XRD, FE-SEM, TEM and TGA methods. TEM images showed that ZnO nanoparticles were dispersed homogeneously in the polymer matrix. The adsorption experiments were carried out in batch mode to optimize various parameters like contact time, pH and concentration of metal ion that influence the adsorption rate. The maximum uptakes of Cr(VI) at pH 4.0 was 72%, 81%, 89% and 91% for pure PA, NC5%, NC10% and NC15%, respectively. The kinetic of adsorption was investigated and the pseudo second-order model is an appropriate model for interpretation of adsorption mechanism of Cr(VI) ions.     

Electron-microscopy study of the distribution of dopants (Ta) in Si-C nanocomposite films

At present, silicon-carbon nanocomposites (NCs) are used in everyday life as hardwearing anticorrosive coatings, because they have a very high diffusion barrier. In addition, the electrical conductivity of a film can be changed by incorporating metal impurities into a NC structure because of its porosity; in this case, the physicochemical properties are retained. The aim of this paper is to study a model of metal-impurity (Ta) incorporation into a Si-C NC matrix using methods of analytical transmission electron microscopy. An understanding of this problem gives an opportunity not only to obtain information on how the metal is incorporated into the NC matrix, but also serves as the first step toward controlling electrical conduction in these structures.     

Nanocomposite materials based on poly(vinyl chloride) and bovine serum albumin modified ZnO through ultrasonic irradiation as a green technique: Optical,thermal, mechanical and morphological properties

In this project, physicochemical properties of poly(vinyl chloride) (PVC) reinforced by ZnO nanoparticles (NPs) were studied. Firstly, ZnO NPs were modified with bovine serum albumin (BSA) as an organo-modifier and biocompatible substance through ultrasound irradiation as environmental friendly, low cost and rapid means. Nanocomposite (NC) films were prepared by loadings of various ratios of ZnO/BSA NPs (3, 6 and 9 wt%) inside the PVC. Structural morphology and physical properties of the ZnO-BSA NPs and NC films were investigated via Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis (TGA), transmission electron microscopy and field emission scanning electron microscopy. According to the obtained information from the TGA, an increase in the thermal stability can be clearly observed. Also the results of contact angle analysis indicated with increasing percent of ZnO/BSA NPs into PVC the hydrophilic behaviors of NCs were increased.     

Surface-enhanced emission from single semiconductor nanocrystals

The fluorescence behavior of single CdSe(ZnS) core-shell nanocrystal (NC) quantum dots is dramatically affected by electromagnetic interactions with a rough metal film. Observed changes include a fivefold increase in the observed fluorescence intensity of single NCs, a striking reduction in their fluorescence blinking behavior, complete conversion of the emission polarization to linear, and single NC exciton lifetimes that are >10(3) times faster. The enhanced excited state decay process for NCs coupled to rough metal substrates effectively competes with the Auger relaxation process, allowing us to observe both charged and neutral exciton emission from these NC quantum dots.     

Charge retention in quantized energy levels of nanocrystals

Understanding charging mechanisms and charge retention dynamics of nanocrystal (NC) memory devices is important in optimization of device design. Capacitance spectroscopy on PECVD grown germanium NCs embedded in a silicon oxide matrix was performed. Dynamic measurements of discharge dynamics are carried out. Charge decay is modelled by assuming storage of carriers in the ground states of NCs and that the decay is dominated by direct tunnelling. Discharge rates are calculated using the theoretical model for different NC sizes and densities and are compared with experimental data. Experimental results agree well with the proposed model and suggest that charge is indeed stored in the quantized energy levels of the NCs.     

Exchange-coupled donor dimers in nanocrystal quantum dots

Doping of semiconductor nanocrystals (NCs) is expected to enable the control of key NC properties, yet its practical exploitation requires an understanding of exchange interactions when multiple dopants are incorporated in a single NC. Here, we experimentally probe the exchange of donor dimers in NCs via a deviation of their triplet-state magnetic resonance from Curie paramagnetism. We show that the exchange coupling of the closely spaced donors can be well described by effective mass theory, which allows the consideration of statistical effects crucial in NC ensembles. While a dimer induces discrete states in a NC, their energy splitting differs by up to 3 orders of magnitude for randomly placed dimers in a NC ensemble, due to an enormous dependence of the exchange energy on the dimer configuration.     

High efficiency carrier multiplication in PbSe nanocrystals: implications for solar energy conversion

We demonstrate for the first time that impact ionization (II) (the inverse of Auger recombination) occurs with very high efficiency in semiconductor nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump intensities, for which less than one exciton is initially generated per NC on average, results in the formation of two or more excitons (carrier multiplication) when pump photon energies are more than 3 times the NC band gap energy. The generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (picosecond) time scale and occurs with up to 100% efficiency depending upon the excess energy of the absorbed photon. Efficient II in NCs can be used to considerably increase the power conversion efficiency of NC-based solar cells.     

Photophysical manifestations of interactions of quantum dots with ortho-phenanthroline molecules

The interaction of CdSe/ZnS nanocrystals (NCs) (solubilized by trioctylphosphine oxide (TOPO) molecules) with ortho-phenanthroline (OP) molecules has been investigated. It is shown that OP molecules can replace TOPO; in this case, bonding of OP molecules with the nanocrystal surface does not lead to the formation of NC aggregates. It is established that the difference in the evolution of the spectral-luminescence NC characteristics in a mixture with OP is independent of the NC size. A model of the interaction of NCs with OP molecules is proposed.     

Origins of visible-light emissions in hydrogen-coated silicon nanocrystals: Role of passivating coating

We present a theoretical investigation of the electronic and optical properties of hydrogen-coated silicon nanocrystals (Si:H NCs). On one hand, the density-functional theory (DFT) is used to both calculate the total energy and relax the NCs. On a second hand, the tight-binding method, which includes the minimal sp³-basis set within the second-nearest-neighbor interaction scheme, is applied to calculate the electronic structures, oscillator strength (OS) and recombination rate (RR) versus the NC size, coating and atomic relaxation. Three main findings are reported: (i) The quantum confinement in these NCs do follow similar rule to the case of a single-particle in a box, where the confinement energy decays in power-law with the increasing NC's size. (ii) The coating is shown to play the essential role in creation of large band-gap energy lying within the visible-light energy spectrum. (iii) The surface atomic relaxation is found to reduce the band-gap energy by about 150 meV and enhance both OS and RR. Our claims are corroborated by the available experimental data.     

Band gap narrowing and doping level of heavily doped Germanium nanocrystals deduced from photoconductivity studies

We investigate the photoconductivity of a n⁺-ZnO/n-Ge NCs/p⁺-GaAs junction where the active layer consists of heavily n-doped Ge NCs synthesized in the gas phase. Measurement of a significant current at energies smaller than the band gap of GaAs demonstrates the photogeneration of charge carriers by the Ge NCs. From the correlation of the NC size with the absorption threshold, a narrowing of the direct band gap in the Ge NC thin film is obtained and attributed to the heavy doping of the Ge NCs. A remarkably high electrical activation of ~15% is found for the incorporated P impurities in the NCs.     

Emission of Cu-related complexes in ZnO:Cu nanocrystals

Photoluminescence (PL), its temperature dependence, scanning electronic microscopy (SEM) and X ray diffraction (XRD) have been applied for the comparative study of varying the emission, morphology and crystal structure of ZnO and ZnO:Cu nanocrystals (NCs) versus technological routines, as well as the dependence of ZnO:Cu NC parameters on the Cu concentration. A set of ZnO and ZnO Cu NCs was prepared by the electrochemical (anodization) method at a permanent voltage and different etching durations with follows thermal annealing at 400 °C for 2 h in ambient air. The size of ZnO NCs decreases from 300 nm×540 nm down to 200 nm×320 nm with etching duration increasing. XRD study has confirmed that thermal annealing stimulates the ZnO oxidation and crystallization with the formation of wurtzite ZnO crystal lattice. XRD method has been used for monitoring the lattice parameters and for confirming the Cu doping of ZnO Cu NCs. In ZnO Cu NCs four defect related PL bands are detected with the PL peaks at 1.95–2.00 eV (A), 2.15-2.23  eV (B), 2.43–2.50 eV (C) and 2.61–2.69 eV (D). Highest PL intensities of orange, yellow and green emissions have been obtained in ZnO Cu NCs with the Cu concentration of 2.28 at%. At Cu concentration increasing (≥2.28 at%) the PL intensities of the bands A, B, C decrease and the new PL band peaked at 2.61–2.69 eV at 10 K appears in the PL spectrum. The variation of PL intensities for all PL bands versus temperature has been studied and the corresponding activation energies of PL thermal decay have been estimated. The type of Cu-related complexes is discussed using the correlation between the PL spectrum transformation and the variation of XRD parameters in ZnO Cu NCs.     

Computational modeling of quantum-confined impact ionization in Si nanocrystals embedded in SiO2

Injected carriers from the contacts to delocalized bulk states of the oxide matrix via Fowler–Nordheim tunneling can give rise to quantum-confined impact ionization (QCII) of the nanocrystal (NC) valence electrons. This process is responsible for the creation of confined excitons in NCs, which is a key luminescence mechanism. For a realistic modeling of QCII in Si NCs, a number of tools are combined: ensemble Monte Carlo (EMC) charge transport, ab initio modeling for oxide matrix, pseudopotential NC electronic states together with the closed-form analytical expression for the Coulomb matrix element of the QCII. To characterize the transport properties of the embedding amorphous SiO₂, ab initio band structure and density of states of the α-quartz phase of SiO₂ are employed. The confined states of the Si NC are obtained by solving the atomistic pseudopotential Hamiltonian. With these ingredients, realistic modeling of the QCII process involving a SiO₂ bulk state hot carrier and the NC valence electrons is provided.     

Superior dielectric properties and bandgap modulation in hydrothermally grown Gr/MgO nanocomposite

MgO nanoparticles (NPs) and Gr/MgO nanocomposite (NC) have been synthesized by hydrothermal route. X-ray diffraction (XRD) analysis confirmed the crystalline cubic phase of MgO and Gr/MgO NC. Raman spectroscopy was used to study the defects in the NCs. Electron microscopy study display spherical NPs of MgO on graphene sheets. UV-visible spectroscopy shows a red shift in the absorption band and a significant reduction in the bandgap for Gr/MgO NC. The improvements in dielectric properties of NC can be ascribed to interfacial polarization between rGO and MgO. The rGO in the NCs supports the electron transfer and improves the electrical conductivity.