Compound Semiconductor Quantum Dots Research Articles

Fabrication of Quantum Dot LED Display with Multinary Compound Semiconductor Quantum Dots

Fabrication of Quantum Dot LED Display with Multinary Compound Semiconductor Quantum Dots

Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

The Properties of the Polaron in III-V Compound Semiconductor Quantum Dots Induced by the Influence of Rashba Spin-Orbit Interaction

The Properties of the Polaron in III-V Compound Semiconductor Quantum Dots Induced by the Influence of Rashba Spin-Orbit Interaction

A Two-Pathway Model for the Evolution of Colloidal Compound Semiconductor Quantum Dots and Magic-Size Clusters.

A fundamental understanding of formation pathways is critical to the controlled synthesis of colloidal semiconductor nanocrystals. As ultrasmall-size quantum dots (QDs) sometimes emerge in reactions along with magic-size clusters (MSCs), distinguishing their individual pathway of evolution is important, but has proven difficult. To decouple the evolution of QDs and MSCs, an unconventional, selective approach has been developed, along with a two-pathway model that provides a fundamental understanding of production selectivity. For on-demand production of either ultrasmall QDs or MSCs, the key enabler is in how to allow a reaction to proceed in the time prior to nucleation and growth of QDs. In this prenucleation stage, an intermediate compound forms, which is the precursor compound (PC) to the MSC. Here, the two-pathway model and the manipulation of such PCs to synthesize either ultrasmall QDs or binary and ternary MSCs are highlighted. The two-pathway model will assist the development of nucleation theory as well as provide a basis for a mechanism-enabled design and predictive synthesis of functional nanomaterials.

Photoluminescence of InGaAs/GaAs Quantum Nanodisk in Pillar Fabricated by Biotemplate, Dry Etching, and MOVPE Regrowth

The III–V compound semiconductor quantum dot (QD) photonic devices have attracted considerable attention due to their stable operation at high temperature, low threshold current, and high-speed modulation. The photoluminescence (PL) energy of QDs depends on the dot sizes and the energy height of barriers. It is difficult to encourage the growth of low indium content QDs because of small lattice distortions between the InGaAs well regions and GaAs barrier regions. In conventional self-organized QD devices produced by Stranski–Krastanow growth methods, the injected carriers are first captured by the wetting layers and then dropped to the QDs with relaxation to the ground state for QD devices. Our proposed unique quantum disk structures have no wetting layers because the quantum well (QW) region is fully etched by using a low-damage dry etching process. We produced 30 nm diameter and 9 nm thick InGaAs nanodisks using a biotemplate via neutral beam etching and metal–organic vapor phase epitaxy (MOVPE). We observed the InGaAs nanodisks via scanning electron microscopy and measured their photoluminescence (PL). To confirm the quantum confinement effects of InGaAs nanodisks, their energies and transient PL behaviors were measured as a function of temperature. The combined low-damage dry etching and MOVPE regrowth processes form an important technique, as they promise a low-damage interface and regrowth ability. This unique technology is attractive for carrier transportation dynamics and spintronics of hybrid QDs and QW nanostructures, and it is easy to adapt for industrial production systems.

Transfer-printed single-photon sources coupled to wire waveguides

Photonic integrated circuits (PICs) are attractive platforms to perform large-scale quantum information processing. While highly-functional PICs (e.g. silicon based photonic-circuits) and high-performance single photon sources (SPSs, e.g. compound-semiconductor quantum dots (QDs)) have been independently demonstrated, their combination for single-photon-based applications has still been limited. This is largely due to the complexities of introducing SPSs into existing PIC platforms, which are generally realized with different materials and using distinct fabrication protocols. Here, we report a novel approach to combine SPSs and PICs prepared independently. We employ transfer printing, by which multiple desired SPSs can be integrated in a simple pick-and-place manner with a theoretical waveguide coupling efficiency >99%, fulfilling the demanding requirements of large-scale quantum applications. Experimentally, we demonstrated QD-based SPSs with high waveguide coupling efficiencies, together with the integration of two SPSs into a waveguide. Our approach will accelerate scalable fusion between modern PICs and cutting-edge quantum technologies.

Particulate, Structural, and Optical Properties of D-Glucose-Derived Carbon Dots Synthesized by Microwave-Assisted Hydrothermal Treatment

Environmentally friendly and visible fluorescent carbon dots (CDs) have received attention as alternatives to compound semiconductor quantum dots containing toxic elements. Hydrothermal reaction of water-soluble carbohydrates can provide an eco-friendly and facile synthesis approach to CDs. In this study, we have attempted to efficiently produce uniform CDs using a microwave heating system that can maintain the reaction solution at a constant temperature. We synthesized CDs from D-glucose through hydrothermal treatment at 200°C for 5–60 min. From transmission electron microscopy, the particle size of the CDs increased in proportion to the hydrothermal duration. Furthermore, the absorbance and maximum photoluminescence intensity of the CDs aqueous dispersions (10 mg L−1) increased with an increase in hydrothermal duration. The maximum photoluminescence intensity of the dispersions decreased at pH < 4 and pH > 10; the absorbance at the excitation wavelength remained unchanged. These results indicate that the number of emission sites and absorption sites increased with an increase in the particle size and that emission sites were located near the surface of the obtained CDs.

ZnO Quantum Dot Decorated Zn2SnO4 Nanowire Heterojunction Photodetectors with Drastic Performance Enhancement and Flexible Ultraviolet Image Sensors.

Here we report the fabrication of high-performance ultraviolet photodetectors based on a heterojunction device structure in which ZnO quantum dots were used to decorate Zn2SnO4 nanowires. Systematic investigations have shown their ultrahigh light-to-dark current ratio (up to 6.8 × 104), specific detectivity (up to 9.0 × 1017 Jones), photoconductive gain (up to 1.1 × 107), fast response, and excellent stability. Compared with a pristine Zn2SnO4 nanowire, a quantum dot decorated nanowire demonstrated about 10 times higher photocurrent and responsivity. Device physics modeling showed that their high performance originates from the rational energy band engineering, which allows efficient separation of electron-hole pairs at the interfaces between ZnO quantum dots and a Zn2SnO4 nanowire. As a result of band engineering, holes migrate to ZnO quantum dots, which increases electron concentration and lifetime in the nanowire conduction channel, leading to significantly improved photoresponse. The enhancement mechanism found in this work can also be used to guide the design of high-performance photodetectors based on other nanomaterials. Furthermore, flexible ultraviolet photodetectors were fabricated and integrated into a 10 × 10 device array, which constitutes a high-performance flexible ultraviolet image sensor. These intriguing results suggest that the band alignment engineering on nanowires can be rationally achieved using compound semiconductor quantum dots. This can lead to largely improved device performance. Particularly for ZnO quantum dot decorated Zn2SnO4 nanowires, these decorated nanowires may find broad applications in future flexible and wearable electronics.

Highly Luminescent ZnO Quantum Dots Made in a Nonthermal Plasma

Nonthermal plasmas allow the preparation of ligand‐free quantum dots combining high production rates with superior crystalline quality and luminescence properties. Here, ZnO quantum dots are produced in a radiofrequency capacitively‐coupled plasma, exhibiting size dependent photoluminescent quantum yields up to 60% after air exposure—the highest reported to date for any compound semiconductor quantum dots prepared in the gas phase. Systematic studies indicate the importance of the surface for the observed luminescence behavior. The high luminescent quantum yields in the visible range of the spectrum and the ligand‐free, scalable synthesis make these quantum dots good candidates for light emitting applications.

Highly Luminescent Covalently Linked Silicon Nanocrystal/Polystyrene Hybrid Functional Materials: Synthesis, Properties, and Processability

Silicon nanocrystals (SiNCs) have received much attention because of their exquisitely tunable photoluminescent response, biocompatibility, and the promise that they may supplant their CdSe quantum dot counterparts in many practical applications. One attractive strategy that promises to extend and even enhance the utility of SiNCs is their incorporation into NC/polymer hybrids. Unfortunately, methods employed to prepare hybrid materials of this type from traditional compound semiconductor (e.g., CdSe) quantum dots are not directly transferable to SiNCs because of stark differences in surface chemistry. Herein, the preparation of chemically resistant SiNC/polystyrene hybrids exhibiting exquisitely tunable photoluminescence is reported and material processability is demonstrated by preparing micro and nanoscale architectures.

Stoichiometry Control in Quantum Dots: A Viable Analog to Impurity Doping of Bulk Materials

A growing body of research indicates that the stoichiometry of compound semiconductor quantum dots (QDs) may offer control over the materials' optoelectronic properties in ways that could be invaluable in electronic devices. Quantum dots have been characterized as having a stoichiometric bulk-like core with a highly reconstructed surface of a more flexible composition, consisting essentially of ligated, weakly bound ions. As such, many efforts toward stoichiometry-based control over material properties have focused on ligand manipulation. In this issue of ACS Nano, Murray and Kagan's groups instead demonstrate control of the conductive properties of QD arrays by altering the stoichiometry via atomic infusion using a thermal evaporation technique. In this work, PbSe and PbS QD films are made to show controlled n- or p-type behavior, which is key to developing optimized QD-based electronics. In this Perspective, we discuss recent developments and the future outlook in using stoichiometry as a tool to further manipulate QD material properties in this context.

Miniband Formation in Superlattices of Colloidal Quantum Dots and Conductive Polymers

We study the bound states and quasi-bound states in a 3- dimensional superlattice formed by an alternating structure of colloidal quantum dots and conductive polymers. Calculations of transmission coefficient of injected holes and electrons vs. their energy, by means of a transfer-matrix implementation1, are performed. In this study, TiO2, CdSe, CdS and GaN quantum dots are investigated as in a 1-dimensional array of ten quantum dots, interspersed in conductive polymers PDCTh and MEH- PPV. These compound semiconductor quantum dots are selected in view of their potential applications in single-photon detectors, solar cells, etc.2,3 Discrete energy levels in such periodic nanoscale structures form minibands;4 moreover, for injection energies close to the barrier height, these minibands have substantial width and large transmission coefficients implying that they may be used to improve the charge transport efficiency in previously-identified c lasses of electronic and optoelectronic devices incorporating ensembles of colloidal quantum dots embedded in conductive polymers.

Size-selected compound semiconductor quantum dots by nanoparticle conversion

We have developed a novel technology, called nanoparticle conversion, for producingcompound semiconductor quantum dots (QDs) in which the dot size, surface density,position, and the materials system are all independently controlled. Nanoparticleconversion also lends itself to spatially controlled positioning of QDs. To demonstrate thistechnology we report the formation of InP QDs using nanoparticle conversion. We haveproduced QDs on substrates of different types by converting randomly and lithographicallypositioned nanoparticles into compound semiconductors in a chemical vapour depositionsystem. Electron microscopy and atomic force microscopy measurements reveal that themorphology of these QDs is similar to that of QDs produced by other techniques.Photo- and cathodoluminescence measurements show that the converted nanoparticlesexhibit properties and behaviours typical of semiconductor QDs. These includequantum confinement, free-to-bound recombination and blinking. Production ofmulti-component QDs like InP, GaN, and InAsP on various substrates like Si,SiO2, and sapphire show that this technology can produce a wide variety of differenttypes of QD on different substrates with minimal need for process optimization.

Highly efficient radiative recombination of electron–hole pairs localized at compound semiconductor quantum dots embedded in Si

We have studied the optical properties of compound semiconductor quantum dots (CSQDs) embedded in Si. Both photoluminescence and electroluminescence spectra were found to be associated with an inhomogeneously broadened band in the near-infrared. A long decay lifetime of luminescence was observed, which is in support of an indirect transition in both k- and real-space. Strong localization of electron–hole pairs was found to occur due to a deep potential well created by the built-in electric dipole at the III–V/Si interface. A Si-based light-emitting diode with GaSb-CSQDs in the active layer showed a high value of quantum efficiency. Light amplification was also observed under pulsed laser excitation.

Evolution of In‐based compound semiconductor quantum dots on Si (001)

The morphological evolution of InAs and InSb quantum dots grown on Si (001) by molecular beam epitaxy was studied by reflection high-energy electron diffraction and scanning probe microscopy. For the InSb/Si system, the maximum density of 1�×�1010�cm–2 was achieved at a growth temperature of 200�°C with the average base length and dot height of 80 nm and 1 nm, respectively. A broad luminescence band extending over 1.1–1.4�μm was observed at low temperature from the In-based quantum dots embedded in Si. The decay lifetimes of luminescence were of the order of submicroseconds, which indicates an indirect transition both in real- and in k-space. Excessive thermal budget after the growth only resulted in the development of luminescence features characteristic of In-doped Si.

Ion beam synthesis of buried Zn-VI quantum dots in SiO2 – grazing incidence small-angle X-ray scattering studies

Grazing incidence small-angle X-ray scattering was used to study ion-beam synthesized Zn-VI compound-semiconductor quantum dots (QDs), buried in a SiO2 matrix. The ZnTe and ZnS QDs were formed by successive ion implantation of constituent atoms, at high ion doses and subsequent annealing at different temperatures in the 1070–1370 K range. In Zn and Te implanted SiO2, small nano-crystals were formed at higher annealing-temperatures, a bimodal size distribution of nano-particles was observed for both materials, which could be explained by an interplay of Ostwald ripening and enhanced diffusion in the irradiation-damaged region.

Atomic Ordering in Self-assembled Epitaxial and Endotaxial Compound and Element Semiconductor Quantum Dot Structures: The First Review

AbstractAlthough the main international research thrust on self-assembled epitaxial semiconductor quantum dots is currently being directed towards random alloy quantum dots, the suggestion is made that atomically ordered quantum dots which are grown by either epitaxy or endotaxy may in addition to their larger quantum confinement potentials possess superior long term structural stability. Such atomically ordered quantum dots should, therefore, be superior to random alloy quantum dots as far as prospective device applications are concerned. The basis for this suggestion is simple thermodynamic considerations. These considerations seem to explain our transmission electron microscopical observations of epitaxially grown atomically ordered In(Sb,As), (In,Ga)Sb, (Cd,Zn)Se, (Cd,Mn,Zn)Se quantum dots and Pb(Se,Te) quantum dot predecessor islands. Atomic ordering in (In,Ga)P quantum dot structures, as recently observed by other authors, does not seem to contradict our thermodynamic considerations. Endotaxially grown atomically ordered (In,Si,As) and (Sn,Si) quantum dots in Si matrices are briefly discussed as an even more unconventional approach to nanostructures with applications in electronics, photonics, information storage, and sensing.

Atomic Ordering in Self-assembled Epitaxial II-VI and IV-VI Compound Semiconductor Quantum Dot Systems

ABSTRACTTransmission electron microcopy (TEM) in both the parallel illumination and scanning probe mode revealed atomically ordered entities within a 5 to 250 nm range in a IV-VI and a II-VI compound semiconductor quantum dot (QD) system. While the II-VI system was a nominal [001] CdSe/(Mn0.1Zn0.9)Se multilayer structure with the QDs embedded, the IV-VI system nominally consisted of [111] PbSe islands. The comparison of photoluminescence (PL) spectra from the CdSe/(Mn0.1Zn0.9)Se structure with those of a reference structure, that was grown to the same nominal specification under otherwise identical conditions except that no Mn was incorporated into the cladding layers, revealed for the former sample two peaks at approximately 2 and 2.1 eV. We tentatively attribute these two PL peaks to two groups of atomically ordered entities.

Electronic structure of CdTe nanocrystals: a tight-binding study

We present a symmetry-based calculation of the electronic structure of a compound semiconductor quantum dot (QD) in the sp3s* tight-binding model including the spin–orbit interaction. The Hamiltonian matrix is diagonalized exactly for CdTe QD sizes up to 60Å. The surface dangling bonds are passivated by hydrogen through a careful analysis of the density of states and wave functions. The calculated size dependence of the energy gap shows a reasonable agreement with the available experimental data. Our symmetry analysis indicates that, in contrast with a reported prediction of the three-band effective-mass model, the fundamental interband transition remains dipole-allowed in CdTe nanocrystals.