QD Formation Research Articles

Modification of Nitrogen Doped Carbon Quantum Dots Emissive States Using Plasma-Induced Non-Equilibrium Electrochemistry

Highly luminescent nanomaterials with ambient stability, environmental friendliness, low-cost and abundance are in demand for various applications in biological sensing, imaging and optoelectronics [1-6]. Many of the photoluminescent nanomaterials are semiconductors and usually contain toxic elements, heavy and expensive metals which have limited their applications. Carbon nanoparticles are a promising alternative to semiconductor nanocrystals as next generation green nanomaterials due to excellent biocompatibility, low cytotoxicity and solution processability which results in ease of production and incorporation in devices [7]. In the current work, nitrogen doped carbon quantum dots (N-CQDs) are synthesized by plasma-induced non-equilibrium electrochemistry utilizing a direct-current atmospheric pressure microplasma. The direct current microplasmas used in this work serve as a reliable and highly reproducible synthesis method for tuning the optical properties of carbon-based quantum dots. The outcome of precursors and discharge current affecting the surface chemistry and optical properties are studied using various characterization techniques, which have contributed to determine the mechanisms leading to QD formation from the plasma-induced reactions at the interface. The synthesized N-CQDs are crystalline with graphitic core doped with nitrogen and functionalized surface. The luminescence which originates from the combined effect of core and surface states can be finely controlled by changing synthesis conditions. These N-CQDs have potential to be used as an active material in next-generation solar cells or even as down-converters for high energy photons to be absorbed by a lower bandgap transporter [8]. The particularly high photoluminescence quantum yield (33% to 68%) can be exploited for optoelectronic applications.

Metastable quantum dot for photoelectric devices via flash-induced one-step sequential self-formation

The first metastable phase Ag, ZnS: α-In 2 S 3 QDs were synthesized by ultrafast light-material interaction. The multiple irradiation of millisecond flash pulses facilitated thermodynamic non-equilibrium superheating and quenching for metastable QD formation, as well as sequential self-formation of α-In 2 S 3 QD nucleation, Ag-doping and ZnS-passivation by photo-responsive ionic kinetics. Upon multiple illumination of flash pulses, the synthesis mechanism of Ag, ZnS: α-In 2 S 3 QD was experimentally proved by atomic-resolution transmission electron microscopy (AR-TEM), scanning TEM (STEM) and diverse spectral analysis. To verify the metastable phase QD formation by superheating/quenching in reaction solution, the localized surface plasmon (LSP) properties and instantaneous temperature increment were theoretically calculated using finite-difference time-domain (FDTD) method. Finally, optoelectronic performance and long-term stability of as-synthesized QDs were evaluated by demonstrating the broad wavelength metal-semiconductor-metal (MSM) photoelectric device. • The first ultrafast light-induced metastable quantum dots were synthesized by one-step sequential self-formation. • Upon multiple illumination of flash pulses, synthesis mechanism of Ag, ZnS: α-In2S3 QD was experimentally proved. • The localized surface plasmon properties were theoretically calculated using finite-difference time-domain method. • Optoelectronic performance and long-term stability of QDs were evaluated by demonstrating MSM photoelectric device.

The effect of rare earth on color conversion properties of Cd–S–Se quantum dot embedded silicate glasses for white LED

Abstract Various rare earth (RE) oxides such as Pr2O3, Dy2O3, Ho2O3, and Er2O3 were introduced into Cd–S–Se quantum dot embedded glasses (QDEGs) to investigate their effect on the color conversion properties of the Cd–S–Se QDEGs. The absorption and emission peak wavelengths due to the excitonic transition of the Cd–S–Se QDs were modified by the introduction of RE oxides, implying their influence on QD formation. The photoluminescence of the various QDEGs RE oxides under 450 nm excitation were monitored and compared with the un-doped QDEGs. White LEDs (wLEDs) were also fabricated with the RE-doped QDEGs and their color conversion properties, such as quantum yield (QY), luminous efficacy, correlated color temperature (CCT) and color rendering index (CRI) were examined. Transmission electron microscope (TEM) revealed that the size and spatial distribution of the QDs were improved by the RE-oxides. Their effect on QD formation within the silicate glass was discussed using Raman spectroscopy.

Photoluminescence of Polymethylmethacrylate/(Zn,Cu,Ag)S:Eu3+ Compositions

Luminescent (Zn,Cu,Ag)S:Eu3+ nanoparticles (quantum dots, QDs) are synthesized upon decomposition of thioacetamide metal complexes in methylmethacrylate during block polymerization upon heating. The QD formation is confirmed by a corresponding band in the optical absorption spectrum of polymer compositions. Photoluminescence of polymethylmethacrylate/(Zn,Cu,Ag)S:Eu3+ compositions manifests itself as a complex band, related to crystal structure defects formed during doping of ZnS with copper and silver ions, and a set of narrow luminescence bands of Eu3+ ions. The luminescence is excited due to the interband transition and electronic transitions from the valence band to ZnS defect levels and intrinsic energy absorption by Eu3+ ions. The bond between Eu3+ ions and ZnS particles is confirmed by the dependence of the ZnS broadband luminescence intensity on the Eu3+ ion concentration and the increase in the intensity of the narrowband luminescence of Eu3+ ions as a result of energy transfer from the doped ZnS crystal levels to the energy levels of Eu3+ ions.

Plasma Induced Non-Equilibrium Electrochemistry for Synthesis of Nitrogen Doped Carbon Quantum Dots Applied to Third-Generation Solar Cells

Highly luminescent nanomaterials with ambient stability, environmental friendliness, low-cost and abundance are in demand for various applications in biological sensing, imaging and optoelectronics [1-6]. Many of the photoluminescent nanomaterials are semiconductors and usually contain toxic elements, heavy and expensive metals which have limited their applications. Carbon nanoparticles are a promising alternative to semiconductor nanocrystals as next generation green nanomaterials due to excellent biocompatibility, low cytotoxicity and solution processability which results in ease of production and incorporation in devices [7]. In the current work, nitrogen doped carbon quantum dots (N-CQDs) are synthesized by one-step atmospheric pressure microplasma process. The direct current microplasmas used in this work serve as a reliable and highly reproducible synthesis method for tuning the optical properties of carbon-based quantum dots in colloid form which result as being highly stable and environmentally-friendly. The outcome of precursors and discharge current affecting the particle morphology and optical properties are studied using various characterization techniques, which have contributed to determine the mechanisms leading to QD formation from the plasma-induced reactions at the interface. The synthesized N-CQDs are crystalline with graphitic core doped with nitrogen and functionalized surface. The particle size and luminescence can be finely controlled to give either excitation dependent or fixed wavelength emission in the visible region. The synthesis conditions can be easily controlled by changing the precursors or the discharge current of the plasma to give rise to required particle size and absorbance in the ultraviolet-visible region. These N-CQDs have potential to be used as an active material in next-generation solar cells or even as down-converters for high energy photons to be absorbed by a lower bandgap transporter [8]. The possible synthesis mechanisms have been analysed and potential chemical pathways leading to the formation of the QDs are described. The particularly high photoluminescence quantum yield (33% to 68%) can be exploited for applications.

Role of Sb on the vertical-alignment of type-II strain-coupled InAs/GaAsSb multi quantum dots structures

The implementation of GaAs0.8Sb0.2 as CL to obtain type-II strain-coupled InAs MQD structures has been examined and compared to similar structures without Sb or without strain coupling. First, it has been demonstrated that capping with GaAsSb prevents the formation of In-rich agglomerations that hampered the QD formation as it has been observed in the sample without Sb. Instead, it promotes the vertical alignment (VA) of almost all QDs with a high density of QD columns. Second, there is a preferential Sb accumulation over the dots together with an undulation of the growth front, contrary to the observed in the uncoupled structure. In case of a deficient covering of GaAsSb, as occurs for giant QDs, In-rich agglomerations may develop. Each VAQD column consists of a sequence of alternating quantum blocks of pyramid-shaped In(Ga)As separated by GaAsSb blocks that rest over them. These Sb-rich blocks are not homogeneous accumulating around the pyramidal apex like a collar. Between the columns, there is an impoverishment of In and Sb compared to the uncoupled sample. These columns can behave as self-aligned nanowires with type II band alignment between self-assembled InAs and GaAsSb quantum blocks that opens new opportunities for novel devices.

Probing the bacterial detoxification of cadmium to form cadmium sulfide quantum dots and the underlying mechanism

A comprehensive study of the mechanism involved in the extracellular detoxification of cadmium via investigating the impact of microbial growth and environmental parameters on CdS QD formation.

Physical and electrical characteristics of Si/SiC quantum dot superlattice solar cells with passivation layer of aluminum oxide

In this work, we numerically simulate the silicon (Si)/silicon carbide (SiC) quantum dot superlattice solar cell (SiC-QDSL) with aluminum oxide (Al2O3-QDSL) passivation. By exploiting the passivation layer of Al2O3, the high photocurrent and the conversion efficiency can be achieved without losing the effective bandgap. Based on the two-photon transition mechanism in an AM1.5 and a one sun illumination, the simulated short-circuit current (Jsc) of 4.77 mA cm−2 is very close to the experimentally measured 4.75 mA cm−2, which is higher than those of conventional SiC-QDSLs. Moreover, the efficiency fluctuation caused by the structural variation is less sensitive by using the passivation layer. A high conversion efficiency of 17.4% is thus estimated by adopting the QD’s geometry used in the experiment; and, it can be further boosted by applying a hexagonal QD formation with an inter-dot spacing of 0.3 nm.

GaAs/GaP quantum dots: Ensemble of direct and indirect heterostructures with room temperature optical emission

We describe the optical emission and the carrier dynamics of an ensemble of self-assembled GaAs quantum dots embedded in GaP(001). The QD formation is driven by the 3.6% lattice mismatch between GaAs and GaP in the Stranski-Krastanow mode after deposition of more than 1.2 monolayers of GaAs. The quantum dots have an areal density between 6 and 7.6 × 1010 per cm−2 and multimodal size distribution. The luminescence spectra show two peaks in the range of 1.7 and 2.1 eV. The samples with larger quantum dots have red emission and show less thermal quenching compared with the samples with smaller QDs. The large QDs luminescence up to room temperature. We attribute the high energy emission to indirect carrier recombination in the thin quantum wells or small strained quantum dots, whereas the low energy red emission is due to the direct electron-hole recombination in the relaxed quantum dots.

Two-Step Nucleation and Growth of InP Quantum Dots via Magic-Sized Cluster Intermediates

We report on the role of magic-sized clusters (MSCs) as key intermediates in the synthesis of indium phosphide quantum dots (InP QDs) from molecular precursors. Heterogeneous growth from the MSCs directly to InP QDs was observed without intermediate sized particles. These observations suggest that previous efforts to control nucleation and growth by tuning precursor reactivity have been undermined by formation of these kinetically persistent MSCs prior to QD formation. The thermal stability of InP MSCs is influenced by the presence of exogenous bases as well as choice of the anionic ligand set. Addition of a primary amine, a common additive in previous InP QD syntheses, to carboxylate-terminated MSCs was found to bypass the formation of MSCs, allowing for homogeneous growth of InP QDs through a continuum of isolable sizes. Substitution of the carboxylate ligand set for a phosphonate ligand set increased the thermal stability of one particular InP MSC to 400 °C. The structure and optical properties of the MSCs with both carboxylate and phosphonate ligand sets were studied by UV–vis absorption spectroscopy, powder XRD analysis, and solution 31P{1H} and 1H NMR spectroscopy. Finally, the carboxylate-terminated MSCs were identified as effective single-source precursors (SSPs) for the synthesis of high-quality InP QDs. Employing InP MSCs as SSPs for QDs effectively decouples the formation of MSCs from the subsequent second nucleation event and growth of InP QDs. The concentration dependence of this SSP reaction, as well as the shape uniformity of particles observed by TEM suggests that the stepwise growth from MSCs directly to QDs proceeds via a second nucleation event rather than an aggregative growth mechanism.

Self-assembled Ge QDs Formed by High-Temperature Annealing on Al(Ga)As (001)

This work studies the spontaneous self-assembly of Ge QDs on AlAs, GaAs, and AlGaAs by high temperature in-situ annealing in molecular beam epitaxy (MBE). The morphology of Ge dots formed on AlAs are observed by atom probe tomography, which revealed nearly spherical QDs with diameters approaching 10 nm and confirmed the complete absence of a wetting layer. Reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) of Ge annealed under similar conditions on GaAs and Al0.3Ga0.7As surfaces reveal the gradual suppression of QD formation with decreasing Al-content of the buffer. To investigate the prospects of using encapsulated Ge dots for upconverting photovoltaics, in which photocurrent can still be generated from photons with energy less than the host bandgap, Ge QDs are embedded into the active region of III-V PIN diodes by MBE. It is observed that orders of magnitude higher short-circuit current is obtained at photon energies below the GaAs bandgap compared with a reference PIN diode without Ge QDs. These results demonstrate the promise of Ge QDs for upconverting solar cells and the realization of device-quality integration of group IV and III-V semiconductors.

Study of Growth Parameters for Single InAs QD Formation on GaAs(001) Patterned Substrates by Local Oxidation Lithography

This work studies the selective nucleation of InAs within nanoholes on GaAs(001) substrates patterned by atomic force microscopy local oxidation. The effects of substrate temperature and As4 overpressure during InAs deposition directly on the patterned substrate (without a GaAs buffer layer) are considered. It is found that when InAs is deposited at substrate temperature of 510 °C under low As4 overpressure, a single InAs quantum dot per nanohole is obtained for a broad range of sizes of pattern motifs. The use of these InAs quantum dots as seed nuclei for vertical stacking of optically active single InAs site-controlled quantum dots is investigated.

Electro-optical characterization of Ti/Au–ZnTe Schottky diodes with CdTe quantum dots

In this study the electric and optical spectroscopy techniques have been applied to investigate ZnTe (p-type)–Ti/Au Schottky diodes containing a layer of CdTe self-assembled quantum dots (SAQDs). The reference sample was the ZnTe–Ti/Au diode without dots. Both samples were grown by molecular beam epitaxy technique. Raman measurements confirmed the presence of the CdTe layer while the photoluminescence proved that CdTe quantum dots were formed in the investigated structure. The PL spectra reveal the CdTe QD electron-hole recombination energy equal to 2.1eV at 10K. Based on the temperature PL measurements the activation energy of PL quenching has been determined to be equal to 22meV. Further confirmation for the QD formation has been obtained from the C–V characteristics which exhibited a step related to the charge accumulation at the QD states. DLTS spectra for the sample with QDs yield three hole-related signals with apparent activation energies equal to EH1=0.16eV, EH2=0.2eV and EH3=0.4eV. For the reference ZnTe–Ti/Au diode solely single signal was observed of signature close to the level H3 in the QD sample. Detailed characterization of the traps as well as the PL studies lead to the conclusion that the level H2 is related to the defects located close to the QDs created during the growth while the other traps are associated with defects present in the ZnTe bulk material.

Defect passivation by hydrogen reincorporation for silicon quantum dots in SiC/SiOx hetero-superlattice

The SiC/SiOx hetero-superlattice (HSL) consisting of alternating near-stoichiometric SiC barrier layers for the electrical transport and silicon rich SiOx matrix layers for the quantum dot formation is a promising approach to the realization of silicon quantum dot (Si–QD) absorbers for 3rd generation solar cells. However, additional defect states are generated during post deposition annealing needed for the Si–QD formation causing an increase in sub-band gap absorption and a decrease in PL intensity. Proper passivation of excess defects is of major importance for both the optical and electrical properties of the SiC/SiOx HSL Si–QD absorber. In this work, we investigate the effectiveness of the hydrogen reincorporation achieved with hydrogen plasma in a plasma-enhanced chemical vapor deposition (PECVD) reactor, hydrogen dissociation catalysis in hot-wire chemical vapor deposition (HWCVD) reactor and annealing in forming gas atmosphere (FGA). Both the HSL samples and single layer reference samples are tested. The passivation quality of the hydrogen reincorporation was examined by comparing electrical and optical properties measured after deposition, after annealing and after passivation. In addition, the formation of Si–QDs in SiC/SiOx HSL was evaluated using high resolution transmission electron microscopy. We demonstrated that hydrogen can be successfully reincorporated into the annealed HSL sample and its single layer reference samples. FGA passivation is most effective for SiO1.2 single layers and HSL samples. Passivation with PECVD appeared to be only effective for SiC single layers.

Controlling the formation of quantum dot pairs using nanohole templates

Modifying the shape of nanoholes formed by arsenic debt epitaxy by the overgrowth of a thin GaAs buffer is shown to provide a simple and robust method to grow low density lateral In(Ga)As quantum dot pairs (QDPs). We present here a systematic study of the effect of GaAs buffer thickness, InAs deposition amount, substrate temperature and arsenic overpressure on dot nucleation and QDP formation. A (10–30) nm GaAs buffer over nanoholes initially ∼ 10.5 nm deep, (60–80) nm wide results in up to 80% of the nanoholes containing QDPs. The QD pairs are aligned along the [110] direction and have centre-to-centre separation of ∼ 38 nm . These QDPs form following InAs deposition between 1.3 ML and 1.6 ML at 490 °C under an arsenic arrival flux of 0.6 ML/s. From the infilling of the hole prior to QD formation, we estimate a net indium surface flux towards the hole of ∼ 7 times the incident flux. The substrate temperature does not significantly alter the dot distribution over the range (470–510) °C. However, the QDP formation is very sensitive to the arsenic overpressure over the range (0.6–1.2) ML/s because of a partial collapse of the nanohole, due to mass transport as the substrate passes through the (2×4) to c(4×4) surface reconstruction around 500 °C.

Evaluation of multi-stacked InAs/GaNAs self-assembled quantum dots on GaAs (0 0 1) grown using different As species

We have investigated the effect of using different arsenic species on the degree of vertical self-alignment of InAs/GaNAs strain-compensated QDs in molecular beam epitaxy (MBE). We used the pairing ratio as a measure to evaluate the degree of vertical self-alignment. For the spacer layer thickness of >20nm, the QD pairing ratio for As2 sample of 53% was smaller than that for As4 of 62%. For As2 sample, we think that the QD formation is more strongly affected by surface structure with a high step density than pairing effect resulting from strain field propagation.

FE simulation of InGaN QD formation at the edge of threading dislocation in GaN

The stress induced diffusion process of In-Ga segregation in In x Ga 1-x N layer deposited on GaN is simulated step by step by using a 3D nonlinear FE method. From the thermodynamical point of view this process is governed by the driving force induced by the gradient of residual stresses operating in an anisotropic nonlinear elastic structure. The source of stresses we consider is the set of threading dislocations examined in the plane view HRTEM investigation of GaN layer deposited on sapphire.

Comparison between models of insulator and semiconductor thin films islanding

AbstractThe synthesis of self‐organized quantum dots (QD's) can be achieved through bottom up layer by layer deposition processes as chemical vapor deposition (CVD) or physical vapor deposition (PVD). However, QD's may also be synthesized via sol‐gel route, which involves a spontaneous evolution from thin films to discrete QD's without further deposition. The aim of the paper is to discuss and compare the physical phenomena involved in QD's formation which initiate from thin film surface roughening between PVDCVD and sol‐gel synthesis approaches. We propose two simple physical models which are relevant to explain the fundamental differences between those methods. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Laplace DLTS studies on deep levels coexisted with InAs quantum dots

AbstractSelf‐assembled InAs/GaAs quantum dot structures have been investigated in both conventional and Laplace‐transform deep‐level transient spectroscopy (DLTS) experiments. Laplace DLTS technique provides orders of magnitude better energy resolution than conventional DLTS and hence enables us to study the electronic fine structure of the deep level states that are revealed using the conventional DLTS method. Two well‐separated peaks corresponding to excitation energies of 468 meV and 485 meV are determined in the quantum dot sample, which have energy broadenings of 13.7 meV and 22.9 meV, respectively. A fine structure is also observed in the reference sample, but the ratio between the two peaks differs and the energy broadening is too narrow to be resolved even by the Laplace DLTS. The strain relaxation due to the QD formation is proposed to explain our observations. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of InAlAs self-assembled quantum dots on InAlGaAs∕InP for 1.55μm laser applications by metalorganic chemical vapor deposition

We report on the growth of InAlAs self-assembled quantum dots or simply (QDs) on In0.53(Al0.5Ga0.5)0.47As matrices lattice matched on InP (001) substrates using metal organic chemical vapor deposition. We found that (1) a small amount of aluminum strongly affects the morphology of the InAlAs QDs, (2) using a long growth interruption (∼140–190s) before the growth of the QDs is an important element in QD formation, and (3) the AsH3 flow rate during the growth of QDs are very important to get high-quality and high-density QDs on the InAlGaAs surface. A bimodal size distribution of InAlAs QDs is found at low growth temperatures. However, when raising the QD growth temperature, the density of “big” InAlAs QDs becomes lower while the density of “small” QDs becomes much higher. Eventually, a high-density (∼5.1×1010cm−2) unimodal size distribution of In0.92Al0.08As QDs with emission around λ∼1.55μm is realized at the optimized growth temperature condition. The QDs and the In0.53(Al0.5Ga0.5)0.47As barrier are grow...