Nanocrystal Radius Research Articles

Size-Dependent Trap-Assisted Auger Recombination in Semiconductor Nanocrystals

The acceleration of Auger-type multicarrier recombination in semiconductor nanocrystals impedes the development of many quantum-dot photonics, solar-cell, lighting, and lasing technologies. To date, only multiexciton and charged-exciton Auger recombination channels are known to show strong size dependence in nanocrystals. Here, we report the first observation of strongly accelerated "trap-assisted" Auger recombination rates in semiconductor nanocrystals. Trap-assisted Auger recombination in ZnO nanocrystals, involving the recombination of conduction-band electrons with deeply trapped holes via nonradiative energy transfer to extra conduction-band electrons, has been probed using time-resolved photoluminescence and transient absorption spectroscopies. We demonstrate that this trap-assisted Auger recombination accelerates dramatically with decreasing nanocrystal size, having recombination times of >1 ns in the largest nanocrystals but only ~80 ps in the smallest. These trap-assisted Auger recombination rates are shown to scale with inverse nanocrystal radius squared (1/τ(Auger) ~ R(-2)). Because surface carrier traps are ubiquitous in colloidal semiconductor nanocrystals, such fast trap-assisted Auger recombination is likely more prevalent in semiconductor nanocrystal photophysics than previously recognized.

Study of the growth and switching kinetics on ferroelectric nanocrystals of copolymer vinylidene fluoride and trifluoroethylene on an atomic force microscope

The growth of nanocrystals obtained from Langmuir-Blodgett films of ferroelectric copolymer consisting of 70% vinylidene fluoride and 30% trifluoroethylene has been investigated by atomic force microscopy (AFM). The radius and concentration of nanocrystals are found to depend on the annealing time of the film. A model for nanocrystal growth is proposed which yields adequate time dependences for nanocrystal size parameters. The switching kinetics of individual ferroelectric nanocrystals with an average diameter of 100–200 nm and a height of 15–20 nm has been investigated in the piezoelectric response mode. It is shown that the switching of nanocrystals has an activation character.

Coulomb matrix elements for the impact ionization process in nanocrystals: An envelope function approach

We propose a method for calculating Coulomb matrix elements between exciton and biexciton states in semiconductor nanocrystals based on the envelope function formalism. We show that such a calculation requires proper treatment of the Bloch parts of the carrier wave functions which, in the leading order, leads to spin selection rules identical to those holding for optical interband transitions. Compared to the usual (intraband) Coulomb couplings, the resulting matrix elements are additionally scaled by the ratio of the lattice constant to the nanocrystal radius. As a result, the Coulomb coupling between exciton and biexciton states scale as 1/R^2. We present also some statistical estimates of the distribution of the coupling magnitudes and energies of the coupled states The number of biexciton states coupled to exciton states form a certain energy range shows a power-law scaling with the ratio of the coupling magnitude to the energy separation. We estimate also the degree of mixing between exciton and biexciton states. The amount of biexciton admixture to exciton states at least 1 eV above the multiple exciton generation threshold can reach 80% but varies strongly with the nanocrystal size.

Optical properties of GaAs nanocrystals: influence of an electric field

A study of the electronic and optical properties of the hydrogen-terminated GaAs nanocrystals Ga₆₈As₆₈H₉₆ and Ga₉₂As₈₀H₁₀₈ is presented. In this study, their dielectric functions, refractive indices, and absorption coefficients were calculated using density functional theory (DFT). The influence of a uniform external electric field on the optical properties of the nanocrystals was also explored. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for each nanocrystal were studied in the absence and the presence of the uniform external electric field. Our results indicate that the HOMO-LUMO gap decreases with increasing electric field strength. The calculated density of states revealed that the main reason for this shrinking gap is an increase in the delocalization of the gallium π-orbitals under the influence of an increasing external electric field. The permanent dipole moment and the polarizability of the nanocrystals under the induced electric field increased with increasing nanocrystal radius. The induced electric field caused a noticeable redshift in the absorption peaks. The electric field also increased the absorption intensity, particularly when the field strength was >0.25 V/Å.

Loosening Quantum Confinement: Observation of Real Conductivity Caused by Hole Polarons in Semiconductor Nanocrystals Smaller than the Bohr Radius

We report on the gradual evolution of the conductivity of spherical CdTe nanocrystals of increasing size from the regime of strong quantum confinement with truly discrete energy levels to the regime of weak confinement with closely spaced hole states. We use the high-frequency (terahertz) real and imaginary conductivities of optically injected carriers in the nanocrystals to report on the degree of quantum confinement. For the smaller CdTe nanocrystals (3 nm < radius < 5 nm), the complex terahertz conductivity is purely imaginary. For nanocrystals with radii exceeding 5 nm, we observe the onset of real conductivity, which is attributed to the increasingly smaller separation between the hole states. Remarkably, this onset occurs for a nanocrystal radius significantly smaller than the bulk exciton Bohr radius a(B) ∼ 7 nm and cannot be explained by purely electronic transitions between hole states, as evidenced by tight-binding calculations. The real-valued conductivity observed in the larger nanocrystals can be explained by the emergence of mixed carrier-phonon, that is, polaron, states due to hole transitions that become resonant with, and couple strongly to, optical phonon modes for larger QDs. These polaron states possess larger oscillator strengths and broader absorption, and thereby give rise to enhanced real conductivity within the nanocrystals despite the confinement.

Kinetic transition in the growth of Al nanocrystals in Al-Sm alloys

The formation of Al nanocrystals from an amorphous Al92Sm8 alloy involves kinetic phenomena with very different characteristic length and timescales, including initial nucleation and later growth and coarsening. Insight into these processes can be derived from the evolution of the sizes of nanocrystals as a function of time. Synchrotron small angle x-ray scattering (SAXS) experiments provide information about the evolution of the nanocrystal size distribution, particularly at times after nucleation has reached saturation. Accurately interpreting the distribution of intensity measured using SAXS requires a nanoparticle model consisting of nanocrystalline core of pure Al surrounded by a shell enriched in Sm. With this approach, statistical parameters derived from SAXS are independent of detailed assumptions regarding the distribution of Sm around the nanocrystals and allow the maximum radius of nanocrystals within the distribution to be determined unambiguously. Sizes determined independently using transmission electron microcopy are in excellent agreement with the SAXS results. The maximum radius obtained from SAXS is proportional to the cube root of time at large sizes and long times, consistent with a coarsening model. The diffusivity of Al within the Al-Sm alloy is obtained from a quantitative analysis of the coarsening process. Further analysis with this diffusivity and a particle growth model provides a satisfactory account for the particle size evolution at early times before the kinetic transition to coarsening.

ZnO coated nanospring-based chemiresistors

Chemiresistors were constructed using 3-D silica nanospring mats coated with a contiguous film of ZnO nanocrystals. Chemiresistors with an average ZnO nanocrystal radius &amp;lt;3 nm, or &amp;gt;20 nm, were found to exhibit a relative change in conductance of a factor of 50 upon exposure to a gas flow of 20% O2 and 80% N2 with ∼500 ppm of toluene and an operational temperature of 400 °C. Samples with an average ZnO nanocrystal radius of 15 nm were found to be the most responsive with a relative conductance change of a factor of 1000. The addition of metal nanoparticles (average radius equal to 2.4 nm) onto the surface of the ZnO nanocrystals (average radius equal to 15 nm) produced a relative change in conductance of a factor of 1500. For the optimum conditions (T = 400 °C, grain size ∼15 nm) well-defined spikes in conductance to explosive vapors (TNT, TATP) were obtained for 0.1 ms exposure time at ppb levels.

Elaboration and Characterization of AuCl Semiconductor Nanocrystals Embedded in KCL Single Crystals

In this work, we investigated the structural and the optical properties of AuCl semiconductor nanocrystals embedded in KCl single crystals. The AuCl nanocrystals were obtained by doping the KCl crystalline matrix with gold using the Czochralski method. The X-ray diffraction showed the formation of the AuCl nanocrystals in KCl with a tetragonal structure. The average radius of the AuCl nanocrystals is estimated using the Scherrer formula. The photoluminescence spectra presented a band emission situated at 580 nm leading to the formation of the AuCl nanocrystals within KCl.

Theoretical and experimental analysis of drug release from an ensemble of polymeric particles containing amorphous and nano-crystalline drug

The analysis of the release kinetics from a delivery system containing amorphous and/or nano-crystalline drug requires the evaluation of many parameters among which the water solubility of the amorphous/nano-crystalline drug is one of the most important. This, in turn, needs the determination of the drug melting enthalpy/temperature dependence on the curvature radius of the crystalline phase (thought, for the sake of simplicity, of spherical shape). Accordingly, this paper is aimed to develop a general theoretical approach devoted to model the melting enthalpy/temperature variation with nano-crystals radius, the solubility dependence on nano-crystals radius and the release process from an ensemble of polymeric particles (crosslinked polyvinylpyrrolidone) containing a poorly water soluble drug (nimesulide) in its amorphous and nano-crystalline status. Drug loading was achieved by means of drug and polymer co-grinding. This study allowed the determination of the nanocrystals fraction and size distribution in the co-ground systems, the drug solubility increase of nano-crystals and the estimation of amorphous drug solubility. This approach yielded to the conclusion that, in the case of nimesulide, a significant increase of solubility occurs for the amorphous form and for nanocrystals characterised by radii lower than 4nm.

Simulations of silicon nanocrystals embedded in oxide for nanoelectronic applications

AbstractThe purpose of this paper is to report the results of a study on single electron charging of a nanocrystal floating gate in a metal‐oxide‐semiconductor transistor. Our model is based on the geometrical and physical parameters of hypothetically spherical nanocrystals embedded in oxide (nanocrystal radius, distance between nanocrystals, oxide thickness, bulk band structure). This model can be numerically solved through a two‐dimensional finite element approach.The channel and gate electrodes are described through energy subbands, and the silicon nanocrystal through discrete energy levels. Two types of tunnel transport are considered here: indirect tunneling from the channel to the nanocrystal and then from the nanocrystal to the gate, and direct tunneling from the channel to the gate. We show that the latter type can be neglected for the current range of nanocrystal densities used for memory applications (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Size dependent carrier recombination in ZnO nanocrystals

Experimental and theoretical studies of fluorescence decay were performed for colloidal ZnO nanocrystals. The fluorescence lifetime reduces from 22 ps to ∼6 ps with decreasing nanocrystal radius. We postulate that non-radiative surface states dominate the carrier dynamics in small ZnO nanocrystals and perform Monte Carlo simulations incorporating carrier diffusion and carrier recombination to model the experimental fluorescence decay dynamics. The percentage of excitons undergoing nonradiative decay due to surface trapping is as high as 84% for nanocrystals with 8 nm radius, which explains the ultrafast decay dynamics observed in small ZnO nanostructures even at low temperatures.

Influence of heat treatment conditions on the precipitation and dissolution of lead sulfide nanocrystals in sodium zinc silicate glasses

The influence of the temperature of the primary heat treatment in the case of two-stage heat treatment of glasses on the kinetics of precipitation of PbS nanocrystals and the structure of the glass-ceramic materials prepared has been investigated. The main structural parameters of the glass-ceramic material, such as the radius of lead sulfide nanocrystals, relative volume of the crystalline phase, concentration of nanocrystals, and dispersion of nanocrystal sizes, have been determined under different heat treatment conditions. It has been demonstrated that a variation in the temperature of the primary heat treatment in the range below the glass transition temperature of the glass matrix makes it possible to control the nanocrystal sizes and to obtain close-to-monodisperse size distributions of nanocrystals.

Theoretical Characterization of a Nanocrystal Layer for Nonvolatile Memory Applications

AbstractOn the road to miniaturization, nanocrystal layers are promising as floating gate in nonvolatile flash memories. Although much experimental work has been devoted to the study of these new memory devices, only few theoretical models exist to help the experimentalists to understand the physical phenomena encountered and explain the behavior of the device.We have developed a model based on the geometrical and physical properties of the elementary structure of a nanocrystal flash memory,i.e.one nanocrystal embedded in an oxide between the channel and the gate electrodes. To obtain a fine analysis of the observed phenomena, several specific hypotheses have been taken into account. Concerning the channel, the contribution of the subbands is explicitly included. In the case of an electrode with a quasi-continuum of energy levels, we replace the continuum by equivalent sets of 2D subbands in order to be able to isolate the energy range that really contributes to the charging/discharging of the nanocrystal. The properties of the materials (bulk band structure, dielectric permittivity, …) can be easily set as well as the geometrical specifications of the elementary structure (nanocrystal radius, tunnel and control oxyde thicknesses, …).The behavior of a layer of nanocrystals is described according to a statistical approach starting from single nanocrystal results. This method allows us to take into account the fluctuations of geometrical parameters. Thus we are able to simulate various types of materials for the nanocrystals (Si, Ge, …), the oxide layer (SiO2, HfO2, …) and the electrodes, for both a single nanocrystal and layers of nanocrystals.

Non-square-well potential profile and suppression of blinking in compositionally graded Cd1−xZnxSe/CdxZn1−xSe nanocrystals

Randomly blinking nanocrystals have given rise to numerous and intense theoretical and experimental investigations recently. An experimental breakthrough was finally made by fabricating the non-blinking Cd$_{1-x}$Zn$_x$Se/ZnSe graded nanocrystal [Nature 459 (2009) 686]. Here, we report (1) an unequivocal and detailed theoretical investigation to understand the properties of the potential-well and the distribution of Zn content with respect to the nanocrystal radius and (2) develop a strategy to find the relationship between the photoluminescence (PL) energy peaks and the potential-well due to Zn distribution in nanocrystals. We found that the non-square-well potential can be varied in such a way that one can indeed control the PL intensity and the energy-level difference (PL energy peaks) in any non-free-electron nanocrystals accurately. This implies that we can either suppress the blinking altogether, or alternatively, manipulate the PL energy peaks and intensities systematically to give non-random blinking. Our theoretical strategy developed here is generic and can be applied to any non-free-electron nanocrystals.

Phosphorus doping of ultra-small silicon nanocrystals

P-doped Si nanocrystals (radius≤2 nm) were synthesized by depositing an ultrathin (0.3 nm) P-SiO2 film close to eachSiO layer of SiO/SiO2 multilayers. During annealing P atoms migrate into the Si-rich region. Due to the low diffusivity of Pin SiO2, P atoms segregate in the Si nanocrystal region and are incorporated in the silicon nanostructures.The P level in the Si nanoclusters can be controlled by changing the P content in the P-SiO2 layer.

Luminescence properties of II/VI semiconductor colloidal nanocrystals at collective and single scales

Colloidal nanocrystals are crystalline spheres of semiconductors of a few nanometers, obtained by chemical synthesis. At this size scale, lower than Bohr radius of the exciton, emission properties are dominated by quantum confinement effects and depend crucially on the nanocrystal radius, which can be controlled by adjusting the synthesis parameters. Nanocrystals present high photostability and good quantum efficiency, even at room temperature. Their emission wavelength can be tuned over the whole visible range, making them very attractive solid state light sources which are already used in optoelectronic devices or for biological labeling. The luminescence properties of CdSe colloidal nanocrystals synthesized at the Institute of Materials Science in Hanoi are presented. At collective scale, the emission properties reveal the synthesis quality. Temperature effects from ambient to 4 K on spectra and decay rates will be presented and analyzed in terms of emitting level fine structure. The study of CdSe colloidal quantum dots at the single emitter scale is of great interest as it reveals properties which are hidden by collective studies, such as luminescence "blinking", a random switching from a fluorescent to a non fluorescent state, which is closely related to the crystalline defects of a nanocrystal and its interaction with its environment. We will present the blinking properties of the prepared nanocrystals, and relate them to the nanocrystals synthesis quality and shell quality.

Nonmonotonic size dependence of the dark/bright exciton splitting in GaAs nanocrystals

The dark/bright exciton splitting {Delta}{sub x} in semiconductor nanocrystals is usually caused by electron-hole exchange interactions. Since the electron-hole wave-function overlap is enhanced by quantum confinement, it is generally assumed that {Delta}{sub x} increases monotonically as the quantum-dot size decreases. Using atomistic pseudopotential calculations, we show that in GaAs nanocrystals {Delta}{sub x} scales nonmonotonically with the nanocrystal size. By analyzing the nanocrystal wave functions in terms of contributions from different k points in the bulk Brillouin zone, we identify the origin of such nonmonotonic behavior in a transition of the lowest conduction-band wave function from {lambda} like to X like as the nanocrystal radius decreases below 19{angstrom}. The nonmonotonicity arises because the long-range component of the electron-hole exchange interaction all but vanishes when the electron wave function becomes X like. We also show that the direct/indirect transition induced in GaAs nanocrystals by external pressure results in a sudden reduction in {Delta}{sub x}.

Growth and melting of metallic nanoclusters in glass: A review of recent investigations

The mechanisms of nucleation and growth and the solid-to-liquid transition of metallic nanoclusters embedded in sodium borate glass were recently studied in situ via small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). SAXS results indicate that, under isothermal annealing conditions, the formation and growth of Bi or Ag nanoclusters embedded in sodium borate glass occurs through two successive stages after a short incubation period. The first stage is characterized by the nucleation and growth of spherical metal clusters promoted by the diffusion of Bi or Ag atoms through the initially supersaturated glass phase. The second stage is named the coarsening stage and occurs when the (Bi-or Ag-) doping level of the vitreous matrix is close to the equilibrium value. The experimental results demonstrated that, at advanced stages of the growth process, the time dependence of the average radius and density number of the clusters is in agreement with the classical Lifshitz-Slyozov-Wagner (LSW) theory. However, the radius distribution function is better described by a lognormal function than by the function derived from the theoretical LSW model. From the results of SAXS measurements at different temperatures, the activation energies for the diffusion of Ag and Bi through sodium borate glass were determined. In addition, via combination of the results of simultaneous WAXS and SAXS measurements at different temperatures, the crystallographic structure and the dependence of melting temperature T m on crystal radius R of Bi nanocrystals were established. The experimental results indicate that T m is a linear and decreasing function of nanocrystal reciprocal radius 1/R, in agreement with the Couchman and Jesser theoretical model. Finally, a weak contraction in the lattice parameters of Bi nanocrystals with respect to bulk crystals was established.

Luminescent properties of Eu 3+–Tb 3+-doped SiO 2–SnO 2-based nano-glass–ceramics prepared by sol–gel method

Transparent nano-glass–ceramics of composition 95SiO 2–5SnO 2 mol%, single doped with 0.4 mol% of Tb 3+ and co-doped with 0.4 mol% of Eu 3+ and Tb 3+ have been obtained by thermal treatment of precursor sol–gel glasses. The segregated SnO 2 nanocrystals, precipitated in the insulator SiO 2 glass matrix during heat treatment, constitute a wide band gap quantum-dot system with mean radius of nanocrystals around 2.2–2.8 nm, comparable to the bulk exciton Bohr radius. A fraction of the rare earth ions is incorporated to the SnO 2 nanocrystals and can be excited by energy transfer from the semiconductor host. The efficiency of this energy transfer process is strongly dependent on the nanocrystal radius in the case of the Tb 3+ ions, whereas the dependence is not appreciable in the case of Eu 3+ ions. These results can be explained taking into account the variations of the overlap of the SnO 2 broad visible emission due to oxygen vacancies with the rare earth excitation spectra.

Confinement effect in P doped spherical Si nanocrystals

By means of the effective mass approximation we have computed without any adjustable parameters the energy levels and hyperfine splitting of a P impurity in silicon spherical nanocrystals as a function of the nanocrystal radius. Confinement effects produce an enhancement of the energy levels and of the hyperfine splitting as the crystal radius is decreased. Our results are in good agreement with available experimental data.