Crystalline Quantum Dots Research Articles

Crystalline quantum dots of covalent organic frameworks for fast and sensitive detection of uranium.

A crystalline quantum dot of a COF was prepared for the first time by the original BRB method and a novel pathway for online monitoring of the COF reaction rate was proposed. The quantum dot can respond to uranyl ion quickly and sensitively and is of great potential in uranium detection.

Synthesis, optical and luminescence behavior of Zn1-xPaxTe (0 ≤ x ≤ 0.09) ceramics prepared via poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] assisted microwave approach

Here, we report the influence of protactinium (Pa) on the optical behavior of ZnTe quantum dots. A nanocrystalline Zn1-xPaxTe (0 ≤ x ≤ 0.09) quantum dots prepared via poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] assisted microwave approach. The X-ray diffraction showed that the ZnTe crystal preserves their zinc blende cubic structure at all Pa ions concentrations. The transmission electron microscopy revealed that the Pa dopants lead to an increase of the QDs size from 2.9 ± 0.5 nm to 8.3 ± 0.2 nm. The doping of Pa ions in the ZnTe QDs gives rise to a red shift of the optical absorption and luminescence spectral peaks. A remarkable improvement of the luminescence intensity and a decrease of the luminescence spectral width take place as a result of the doping of ZnTe with Pa ions. The quantum yield improved from 37 % to 95 %. The Stokes shift decreases with the increase of the Pa ions, implying the inhibition of the crystal defects due to the decrease of the number of Zn2+ vacancies. These exceptional comportments may pave the way for developing high quality laser diodes based on Zn1-xPaxTe semiconducting crystalline quantum dots.

Sn:In2O3 and Sn:In2O3/NiS2 Core–Shell Nanowires on Ni, Mo Foils and C Fibers for H2 and O2 Generation

Sn:In2O3 nanowires have been grown by the vapor liquid solid mechanism on Si, Ni, Mo, and C fibers. These were used to obtain Sn:In2O3/NiS2 core–shell nanowires by the deposition of 10 nm Ni over the Sn:In2O3 nanowires followed by post growth processing under H2S between 100 and 200 °C. The Sn:In2O3/NiS2 nanowires have diameters of ≈100 nm and lengths up to ≈100 μm and consist of cubic bixbyite Sn:In2O3 surrounded by 3 nm NiS2 crystalline quantum dots with a cubic crystal structure. Higher temperatures of 300–500 °C result in the formation of NiS2 quantum dots and cubic In3S4 branches around the Sn:In2O3. We find that the p-type NiS2 in contact with n-type Sn:In2O3 NWs gives rectifying current–voltage (IV) characteristics due to the formation of a p–n heterojunction with a straddling type band alignment where electrons are confined to the n-type Sn:In2O3 core and holes in the p-type NiS2, as shown by self-consistent Poisson–Schrodinger calculations in the effective mass approximation. The gas evolution of...

Vapour-liquid-solid growth: Nanowire-quantum dot epitaxy.

The synthesis of crystalline quantum dots epitaxially incorporated into silicon nanowires holds promise for future device applications in various areas of opto- and quantum electronics.

Growth of Nb2O5 quantum dots by physical vapor deposition

The crystalline quantum dots of Nb2O5 in the range 1–20nm have been grown by vacuum thermal evaporation using the bottom-up approach of early nucleation and growth stage. The dot sizes were controlled by growth time for an optimized constant molecular flux rate of growth. The dots were characterized by TEM and UV/Vis absorption spectroscopy. The variation of optical band gap of dots scaling with inverse square of their size demonstrates the quantum confinement effect in the dots. The reduced mass of electron–hole pair (exciton) was determined to be 0.532me from this behavior.

Laser induced sponge-like Si in Si-rich oxides for photovoltaics

We show that a sponge-like structure of interconnected Si nanowires embedded in a dielectric matrix can be obtained by laser annealing of silicon rich oxides (SRO). Due to quantum confinement, the large bandgap displayed by these percolated nanostructures can be utilized as a tandem stage in 3rd generation thin-film solar cells. Well passivated by the SiO₂ dielectric matrix, they are expected to overcome the difficulty of carrier separation encountered in the case of isolated crystalline quantum dots. In this study PECVD grown SRO were irradiated by a cw Ar⁺ laser. Raman spectroscopy has been used to assess the crystallinity of the Si nanostructures and thus to optimize the annealing conditions as dwell times and power densities. In addition, Si plasmon imaging in the transmission electron microscope was applied to identify the sponge-like structure of phase-separated silicon.

Raman spectroscopy of very small Cd1−xCoxS quantum dots grown by a novel protocol: direct observation of acoustic‐optical phonon coupling

Glass‐embedded Cd1−xCoxS quantum dots (QDs) with mean radius of R ≈ 1.70 nm were successfully synthesized by a novel protocol on the basis of the melting‐nucleation synthesis route and herein investigated by several experimental techniques. Incorporation of Co2+ ions into the QD lattice was evidenced by X‐ray diffraction and magnetic force microscopy results. Optical absorption features with irregular spacing in the ligand field region confirmed that the majority of the incorporated Co2+ ions are under influence of a low‐symmetry crystal field located near to the Cd1−xCoxS QD surface. Electron paramagnetic resonance data confirmed the presence of Co2+ ions in a highly inhomogeneous crystal field environment identified at the interface between the hosting glass matrix (amorphous) and the crystalline QD. The acoustic‐optical phonon coupling in the Cd1−xCoxS QDs (x ≠ 0.000) was directly observed by Raman measurements, which have shown a high‐frequency shoulder of the longitudinal optical phonon peak. This effect is tuned by the size‐dependent sp‐d exchange interaction due to the magnetic doping, causing variations in the coupling between electrons and longitudinal optical phonon. Copyright © 2013 John Wiley & Sons, Ltd.

Size‐Tunable Polymeric Nanoreactors for One‐Pot Synthesis and Encapsulation of Quantum Dots

Hydrophilic polymeric nanoparticles are synthesized through a Bergman cyclization- mediated intramolecular chain collapse of structurally well-defined linear polymers, and then used as size-tunable nanoreactors to fabricate and encapsulate quantum dots in a one-pot reaction. Crystalline quantum dots are formed in all of these nanoreactors and visualized by transmission electron microscopy. Smaller nanoreactors produce one quantum dot each while larger nanoreactors form a number, resulting in fluorescence quenching. By controlling the molecular weight of the linear polymer precursor, a variable number of nanocrystals are fabricated and assembled in a single nanoreactor.

Distributed luminescence from alkyl-capped silicon quantum dots

Orange luminescence attributable to a core of silicon atoms in alkyl-capped crystalline quantum dots excited at λa=355 and 405 nm is investigated as a function of applied intensity and time. The intensity of luminescence displays a linear power dependence on the intensity of the applied field, from which an exponent n=0.94±0.02 commensurate with single-photon absorption is derived. The dependence of luminescence on time is observed to be strongly nonexponential and is optimally accounted for by a probability density function which describes a continuous distribution of two decay times: the behavior is characteristic of a pair of elementary steps connected with light emission within a distribution of local environments, or a single rate process supported by two environments. Nonlinear least-squares fits to the time dependent luminescence formulated on this basis with a Gaussian, Lorentzian, or log-normal distribution of rates return most probable lifetimes T¯1=21±1 μs and T¯2=3.7±0.8 μs. The widths of the distributions vary between σ1=0.01–0.03 μs−1 and σ2=0.14–1.1 μs−1 associated with 1/T¯1 and 1/T¯2, respectively.

Nonthermal laser-induced formation of crystalline Ge quantum dots on Si(100)

The effects of laser-induced electronic excitations on the self-assembly of Ge quantum dots on Si(100)-(2×1) grown by pulsed laser deposition are studied. Electronic excitations due to laser irradiation of the Si substrate and the Ge film during growth are shown to decrease the roughness of films grown at a substrate temperature of ∼120 °C. At this temperature, the grown films are nonepitaxial. Electronic excitation results in the formation of an epitaxial wetting layer and crystalline Ge quantum dots at ∼260 °C, a temperature at which no crystalline quantum dots form without excitation under the same deposition conditions.

Si quantum dots for solar cell fabrication

Thin film stacks, made of Si-rich SiO alternated with SiO 2 layers, have been deposited by reactive RF sputtering starting from Si and SiO 2 targets, respectively. Crystalline quantum dots (QDs) have been nucleated by Si precipitation from the Si-rich SiO phase using high temperature annealing. PL measurements evidenced a blueshift of the emission peak which has been attributed to a reduction of the Si QD size. Electrical resistivity measurements showed a semiconducting-like behaviour. QD size affect the resistivity values and the activation energies. We have tentatively interpreted the electrical behaviour of this quantum structure by using a Meyer-Neldel Rule conventionally used to explain the electrical properties of nanoporous silicon.

Surface Effects and Quantum Confinement in Nanosized GaN Clusters: Theoretical Predictions

The structure and the electronic properties of stoichiometric (GaN)n clusters (with 6 ≤ n ≤ 48) were investigated by means of quantum-chemical hybrid density functional theory (DFT) using the B3LYP functional. Particular emphasis was put on the investigation of the evolution of the physical properties of the clusters as a function of their size. Two types of model clusters were studied. Cage-type structures were found to be the most stable for smaller cluster sizes, whereas for larger sizes conformations cut out from the GaN wurtzite crystal were favorable. The study of the electronic structure shows that the energy gap of the clusters tends to become larger as the dimensions of the clusters increase. The vertical electronic absorption energies were calculated by means of time-dependent (TD) DFT. For such small clusters, probably due to the predominant amount of surface atoms, well-defined quantum confinement effects, as commonly observed in crystalline quantum dots, are not apparent.

Growth mechanisms of highly mismatched AlSb on a Si substrate

We describe the growth mechanisms of highly mismatched (Δao∕ao=13%) defect-free AlSb on Si(001) substrates. Nucleation occurs during the first few monolayers of AlSb deposition by crystalline quantum dot formation. With continued growth, the islands coalesce into a bulk material with no vertically propagating defects. Strain energy from the AlSb∕Si interface is dissipated by crystallographic undulations in the zinc-blende lattice, as confirmed by high-resolution transmission electron microscopy (TEM) images. Reciprocal space analysis of the TEM images corroborates a crystallographic rotation associated with the undulations. The resulting AlSb material is >98% relaxed according to x-ray diffraction analysis.

Electrodeposited Quantum Dots. 3. Interfacial Factors Controlling the Morphology, Size, and Epitaxy

Electrodeposition of CdSe from nonaqueous solutions is shown to produce epitaxial, 5 nm diameter quantum dots (QDs) on evaporated {111}Au substrate and diffraction-amorphous QDs on evaporated Pd substrate. The QDs were characterized using bright-field and dark-field transmission electron microscopy (TEM), high-resolution TEM, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. It is shown that similar short-range-order elements are present in both the crystalline QDs on Au and the amorphous QDs on Pd. An epitaxial size-limiting mechanism is proposed for the CdSe QDs on Au, on the basis of growing mismatch-induced strain energy upon increase in the QD size, giving rise to arrays of “self-assembled” QDs. We show that this concept, previously demonstrated using MBE and MOCVD techniques, can be successfully applied to electrochemical deposition, resulting in homogeneous QDs of considerably smaller size.

THEORETICAL STUDIES ABOUT ELECTRONIC STRUCTURES OF Si3N4 AND Si QUANTUM DOTS

The studies about the electronic structures of Si 3 N 4 and Si crystalline quantum dots by the tight-binding approximation and the recursion method are presented, in which spherical clusters with high symmetry and optimized surface are used as models of the quantum dots. The variations of the positions of the top of valence band and the bottom of conduction band following the size change of guantum dot are given. The local and average densities of state (DOS) of the central atom in 328-atom Si 3 N 4 quantum dot and 323-atom Si quantum dot have been calculated, and the relations between DOS and spectral structures have been discussed. From the discussions, it is concluded that local DOS of central atom can give good descriptions of optical spectra of quantum dots. which has been proved by experimental results.