Multiple Quantum Dots Research Articles

Strain Balanced Epitaxial Stacks of Quantum Dots and Quantum Posts

AbstractThe self assembly of quantum dots by heteroepitaxy of lattice‐mismatched semiconductors is based on elastic energy relaxation, which spontaneously occurs at the growth front when the largest atoms in the crystal cluster together. Because a larger covalent radius is related to weaker bonds, and this is in turn fundamentally related to smaller bandgaps, the formation of quantum dots leads to a confinement potential for electrons and/or holes. This effect has applications ranging from ultralow threshold diode lasers to highly efficient solar cells and usually requires the stacking of multiple quantum dots. The number of layers is limited by the stress accumulated during growth due to the larger covalent radius of the atoms that constitute the quantum dots. This accumulated stress can be relieved by introducing in the epitaxial layers compensating atomic species with a smaller covalent radius, enabling a reduction of spacer layer thickness. In the limit of sub‐nanometer spacer thickness, the quantum dots, which have a tendency to line up vertically, fuse into a quantum post. The current efforts to optimize the properties of strain balanced quantum dot stacks and quantum posts are reviewed.

Coulomb gap in graphene nanoribbons

We investigate the density- and temperature-dependent conductance of graphene nanoribbons with varying aspect ratio. Transport is dominated by a chain of quantum dots forming spontaneously due to disorder. Depending on ribbon length, electron density, and temperature, single or multiple quantum dots dominate the conductance. Between conductance resonances, cotunneling transport at the lowest temperatures turns into activated transport at higher temperatures. The density-dependent activation energy resembles the Coulomb gap in a quantitative manner. Individual resonances show signatures of multilevel transport in some regimes, and stochastic Coulomb blockade in others.

SiO<sub>2</sub> Beads with Quantum Dots: Synthesis and Bioapplication

Aqueous CdTe and organic CdSe/ZnS quantum dots (QDs) were encapsulated into SiO2beads using various controlled sol-gel processes. Ligand exchange is a key to adjust the QD number in each bead and get a high photoluminescence (PL). Namely, partial ligand exchange from thioglycolic acid to 3-mercaptopropyotrimethoxysilane (MPS) on the CdTe QDs enables retention of the initial PL efficiency of the QDs in water, while the simultaneous addition of a poor solvent (ethanol) resulted in regulated assembly of the QDs through condensation of hydrolyzed MPS. SiO2beads thus prepared had, for example, a diameter of 17 nm and contained 3 QDs each. The PL efficiency of these beads was 30%, while the initial PL efficiency was 38% in a colloidal solution. In addition, a method including surface silanization, phase transfer and self-assembly, and SiO2shell growth has been developed to incorporate multiple hydrophobic CdSe/ZnS QDs into SiO2beads where they are well suited for bio-application due to their high brightness, less-cytotoxic, and non-blinking nature. To investigate the potential use for labeling in bio-applications, SiO2beads with multiple QDs were conjugated with IgG using streptavidin-maleimide as linkers. This preparation method is an important step towards fabricating intensely emitting biocompatible SiO2beads impregnated with semiconductor QDs.

Non-Fermi-liquid quantum impurity physics from non-Abelian quantum Hall states

We study the physics of electron tunneling between multiple quantum dots and the edge of a quantum Hall state. Our results generalize earlier work [G. A. Fiete, W. Bishara, C. Nayak, Phys. Rev. Lett. 101, 176801 (2008)] in which it was shown that a single quantum dot tunnel coupled to a non-Abelian quantum Hall state can realize a stable multi-channel Kondo fixed point at low-energy. In this work, we investigate the physics of multiple dots and find that a rich set of possible low-energy fixed points arises, including those with non-Fermi liquid properties. Previously unidentified fixed points may also be among the possibilities. We examine both the situation where the dots are spatially separated and where they are in close proximity. We discuss the relation to previous work on two-impurity Kondo models in Fermi liquids and highlight new research directions in multiple quantum impurity problems.

SiO 2 beads with quantum dots: Preparation and stability investigation for bioapplications

SiO 2 beads with quantum dots (QDs) including hydrophilic CdTe or hydrophobic CdSe/ZnS were prepared through sol–gel procedures. These beads contained single or multiple QDs by controlling the ligand exchange and the QD assembly during incorporation. Namely, SiO 2 beads with CdTe QDs retained high photoluminescence (PL) efficiencies up to 40% when thioglycolic acid was partially replaced by 3-mercaptopropyltrimethoxysilane. Multiple CdSe/ZnS QDs were assembled into a SiO 2 bead (∼7 QDs in each bead) through surface silanization using tetraethyl orthosilicate instead of organic ammine. The stability of CdTe QDs and luminescent SiO 2 beads was investigated by measuring the degradation of PL intensity against the concentration of phosphate-buffered saline (PBS) buffer solutions. The Stern–Volmer quenching constant K sv of QDs depended strongly on the properties of the QDs and the thickness and porous type of SiO 2 shell. Red- and green-emitting CdTe QDs exhibited K sv of 2.02 × 10 −2 and 8.93 × 10 −2 mM −1, respectively. The II-type pores (cylindrical pores) in the SiO 2 shell resulted in quick degradation while the IV-type pores (ink-bottle-shaped pores) led the QDs a high stability. Because both two kinds of QDs in SiO 2 beads revealed high PL efficiency and stability in buffer solution, these luminescent beads should be utilizable for bioapplications.

Controlling polarization anisotropy of site-controlled InAs/InP (100) quantum dots

We report on the shape and polarization control of site-controlled multiple and single InAs quantum dots (QDs) on InP pyramids grown by selective-area metal-organic vapor phase epitaxy. With increasing growth temperature the QDs elongate causing strong linear polarization of the photoluminescence. With reduced pyramid base/pyramid top area/QD number, the degree of polarization decreases, attributed to the symmetric pyramid top, reaching zero for single QDs grown at lower temperature. This control of linear polarization is important for entangled photon sources operating in the 1.55 μm wavelength region.

Realization of theSU(N)Kondo Effect in a Strong Magnetic Field

In this Letter we suggest a realization of the SU(N) Kondo effect, using quantum dots at strong magnetic field. We propose using edge states of the quantum Hall effect as pseudospin that interact with multiple quantum dots structures. In the suggested realization one can access each pseudospin separately and hence may perform a set of experiments that were impossible until now. We focus on the realization of SU(2) and SU(3) Kondo effects and find in the unitary limit a conductivity of 3/4 quantum conductance in the SU(3) case.

Hyperspectral Line Scanning Microscopy for High-Speed Multicolor Quantum Dot Tracking

One of the fundamental goals in observing protein-protein interactions on the cell membrane is achieving nanometer scale spatial resolution along with temporal resolution sufficient to study live cell behavior. Traditional fluorescence microscopy methods have been unsuccessful in studying these interactions due to the diffraction limit with visible light. Single particle tracking techniques using quantum dots have provided single particle localizations to well below the diffraction limit, however, clustering of multiple particles limits the unique identification and thus tracking of individual particles throughout the (possibly dynamic) clustering process. This problem can be solved by tracking multiple quantum dot colors using a high-speed hyperspectral microscope.We have developed a high-speed hyperspectral microscope based on a line scanning design that is capable of up to 30 frames/sec with 128 spectral channels per pixel (ranging from 500-800 nm) with a sample area of ∼30 υm2. We describe the details of the microscope optical design details and layout, and show the results of excitation and emission path characterization. We image RBL-2H3 cells with IgE that has been conjugated with various colors of QDs (to give a near 1:1 QD:IgE ratio) and image at 30 frames per second. By imaging in both the presence and absence of crosslinking DNP-BSA, we demonstrate the ability to identify dynamic interactions and/or oligomers at spatial scales below the diffraction limit.

Plasmonic enhanced electro-optic stub modulator on a SOI platform

Abstract The performance of a plasmonic enhanced stub modulator coupled to an underlying Si-waveguide was investigated using finite element simulations. The transmission of the system is controlled by changing the absorption coefficient of the material filling the stub, which modifies both the power transmitted by the stub itself and the field profile, and hence the coupling of this field into the single-mode output waveguide. An extinction ratio of 8.5 dB with an insertion loss of 8.5 dB can be achieved via electro-absorption derived from the quantum confined Stark effect (QCSE), assuming that the stub is filled with Ge/SiGe multiple quantum wells (MQWs) or Ge quantum dots (QDs) in a silicon matrix. The effect is of potential interest for application in electro-absorption modulators for integrated photonics, since the sub-wavelength dimensions of the device offer low power operation and high switching speeds.

Triple quantum dot device designed for three spin qubits

Electron spin confined in quantum dots is a promising candidate for experimental qubits. Aiming at realizing a three spin-qubit system, we designed split micromagnets suitable for the lateral triple quantum dots. From numerical simulations of the stray magnetic field distribution, field gradients ∼0.8 T/μm and differences of in-plane components ∼10 mT can be attained, which enable the electrical and addressable manipulation of three qubits. Furthermore, this technique can be applied for up to 25 qubits in realistic multiple quantum dots. For the first step of implementing such three-qubit systems, a relevant triple quantum dot device has been fabricated and characteristic charge states were observed.

Highly luminescent SiO 2 beads with multiple QDs: Preparation conditions and size distributions

Multiple CdTe quantum dots (QDs) were incorporated in SiO 2 beads using a reverse micelle route (i.e., a water-in-oil emulsion) and a precursor solution of SiO 2-coated CdTe QDs as the water phase. The QDs in the beads retained their initial photoluminescence efficiency (56%) because they had a thin SiO 2 shell that prevented removal of the ligands from their surface during incorporation. Because of the SiO 2 sol in the precursor solution, the size of the water pools in water-in-oil emulsion increased to 700 ± 320 nm compared to 25 ± 5 nm when an aqueous CdTe QD solution was used. The size of the beads prepared in the water pools depended strongly on the preparation parameters (water phase injection speed, tetraethyl orthosilicate concentration, reaction time). The beads were easily separated into three size ranges (60 ± 23, 160 ± 40, and 650 ± 200 nm) by filtering for further application.

Suppression of Kondo screening by the Dicke effect in multiple quantum dots

The interplay between the coupling of an interacting quantum dot to a conduction band and its connection to localized levels has been studied in a triple quantum dot arrangement. The electronic Dicke effect, resulting from quasi-resonant states of two side-coupled non-interacting quantum dots, is found to produce important effects on the Kondo resonance of the interacting dot. We study in detail the Kondo regime of the system by applying a numerical renormalization group analysis to a finite-U multi-impurity Anderson Hamiltonian model. We find an extreme narrowing of the Kondo resonance, as the single-particle levels of the side dots are tuned towards the Fermi level and "squeeze" the Kondo resonance, accompanied by a strong drop in the Kondo temperature, due to the presence of a supertunneling state. Further, we show that the Kondo temperature vanishes in the limit of the Dicke effect of the structure. By analyzing the magnetic moment and entropy of the three-dot cluster versus temperature, we identify a different local singlet that competes with the Kondo state, resulting in the eventual suppression of the Kondo temperature and strongly affecting the spin correlations of the structure. We further show that system asymmetries in couplings, level structure or due to Coulomb interactions, result in interesting changes in the spectral function near the Fermi level. These strongly affect the Kondo temperature and the linear conductance of the system.

Thermal Rectification Effects of Multiple Semiconductor Quantum Dot Junctions

On the basis of the multiple-energy-level Anderson model, we theoretically study the thermoelectric effects of semiconductor quantum dots (QDs) in the nonlinear response regime. The charge and heat currents in the sequential tunneling process are calculated by Keldysh Green's function technique. The thermal rectification effect can be observed for such a multiple QD junction system, whereas the rectification efficiency is significantly affected by the tunneling rate, size fluctuation, and location distribution of QDs. We also find that the charge current rectification with respect to temperature bias can be observed.

Effects of AlGaAs energy barriers on InAs/GaAs quantum dot solar cells

We have studied the effects of AlGaAs energy barriers surrounding self-assembled InAs quantum dots in a GaAs matrix on the properties of solar cells made with multiple quantum dot layers in the active region of a photodiode. We have compared the fenced dot samples with conventional InAs/GaAs quantum dot solar cells and with GaAs reference cells. We have found that, contrary to theoretical predictions, the AlGaAs fence layers do not enhance the transport properties of photogenerated carriers but instead suppress the extraction of the carriers excited in the dots by light with wavelengths longer than the cutoff wavelength of the GaAs matrix material. Both the standard quantum dots and the fenced dots were found to give solar cell performance comparable to the GaAs reference cells for certain active region thicknesses but neither showed enhancement due to the longer wavelength absorption or improved carrier transport.

Hydrogenic impurity states in zinc-blende symmetric InGaN/GaN multiple quantum dots

Based on the effective-mass approximation, the ground-state donor binding energy of hydrogenic impurity in a cylindrical zinc-blende (ZB) symmetric InGaN/GaN multiple quantum dots (QDs) is investigated variationally. It is found that the donor binding energy has a maximum value when the impurity is located at the center of the QDs. Numerical results also show that, for the impurity located at the center of the middle QD, the donor binding energy has a minimum value with increase in the middle barrier width. The donor binding energy is insensitive to the increment of the middle barrier width when the middle barrier width is large. Moreover, we find that the donor binding energy is basically invariable with increase in the Indium (In) composition when the impurity is located at the center of the middle barrier.

Influence of the GaN barrier thickness on the optical properties of InGaN/GaN multilayer quantum dot heterostructures

A study of InGaN/GaN multiple layer quantum dot (QD) structures with varying barrier thicknesses is reported. With increasing barrier thickness both a redshift in the photoluminescence (PL) peak energy and increase in the PL decay lifetime is observed. This is attributed to an increase in the size of the internal electric field and the influence on the electronic structure via the quantum confined Stark effect. Theoretical surface integral potential calculations support this interpretation. A minimum barrier thickness of 4 nm appears to be required for the formation of separate homogeneous QD layers.

ZnMgSSe/ZnSe/CdSe quantum dot heterostructures for green laser applications

Cd(Zn)Se/ZnSSe/ZnMgSSe laser heterostructures with an asymmetrical ZnSSe/ZnSe supperlattice waveguide, and multiple (up to 9) electronically-coupled CdSe quantum dot sheets in the active region were designed and grown by molecular beam epitaxy. Internal quantum efficiency η i, internal loss α i , transparency threshold J T and characteristic gain ΓG 0 were found to be: 58.5%, 7 cm − 1 , 1.78 kW/cm 2, and 194 cm − 1 , respectively, in a laser heterostructure with 9 electronically-coupled CdSe quantum dot active layers. A violet-green integrated laser converter with the output pulse power up to 50 mW was fabricated on the basis of the Cd(Zn)Se/ZnSSe/ZnMgSSe quantum dot laser heterostructure. The heterostructures were optically pumped by emission of a violet ( λ = 416 nm) commercial InGaN/GaN pulsed laser diode. The ways for further laser threshold reduction and increasing the efficiency are discussed.

Thermoelectric Effects of Multiple Quantum Dot Junctions in the Nonlinear Response Regime

The thermoelectric effects of semiconductor quantum dots (QDs) embedded into an insulator matrix connected to metallic electrodes are theoretically investigated in the nonlinear response regime. A multilevel Anderson model is used to simulate the multiple QDs junction system. The charge and heat currents in the sequential tunneling process are calculated by the Keldysh Green's function technique. We have demonstrated that thermal rectification and negative differential thermal conductance behaviors can be observed for the multiple QD junction system in the absence of phonon heat current.

Violet–green laser converter based on MBE grown II–VI green lasers with multiple CdSe quantum dot sheets, pumped by InGaN laser diode

AbstractThe violet–green laser converter based on a molecular‐beam‐epitaxy (MBE) grown CdSe quantum dot (QD) laser heterostructure pumped by a commercial InGaN laser diode (LD) emission has been fabricated and studied in detail. The optimized II–VI laser heterostructure consists of asymmetrical ZnSe/ZnSSe superlattice (SL) waveguide and active region comprising five CdSe QD sheets (QDS) placed in the centre of 2‐nm‐thick ZnSe quantum wells. The new laser structure design provides both a high homogeneity of optical pumping of the CdSe QDS due to tunnelling of charge carriers between the QDS separated by 5‐nm‐thick ZnSe/ZnSSe/ZnSe barriers and high optical confinement factor. Optimization of both cavity length of the II–VI laser and parameters of optical focusing system to obtain a narrow stripe with the length slightly exceeding the cavity length has been performed. As a result, the maximum achieved quantum efficiency and pulse output power in green have been as high as 8% and 65 mW, respectively.

Thermoelectric and thermal rectification properties of quantum dot junctions

The electrical conductance, thermal conductance, thermal power and figure of merit (ZT) of semiconductor quantum dots (QDs) embedded into an insulator matrix connected with metallic electrodes are theoretically investigated in the Coulomb blockade regime. The multilevel Anderson model is used to simulate the multiple QDs junction system. The charge and heat currents in the sequential tunneling process are calculated by the Keldysh Green function technique. In the linear response regime the ZT values are still very impressive in the small tunneling rates case, although the effect of electron Coulomb interaction on ZT is significant. In the nonlinear response regime, we have demonstrated that the thermal rectification behavior can be observed for the coupled QDs system, where the very strong asymmetrical coupling between the dots and electrodes, large energy level separation between dots and strong interdot Coulomb interactions are required.