Two-dimensional Quantum Dot Arrays Research Articles

Soliton nanoantennas in two-dimensional arrays of quantum dots

We consider two-dimensional (2D) arrays of self-organized semiconductor quantum dots (QDs) strongly interacting with electromagnetic field in the regime of Rabi oscillations. The QD array built of two-level states is modelled by two coupled systems of discrete nonlinear Schrödinger equations. Localized modes in the form of single-peaked fundamental and vortical stationary Rabi solitons and self-trapped breathers have been found. The results for the stability, mobility and radiative properties of the Rabi modes suggest a concept of a self-assembled 2D soliton-based nano-antenna, which is stable against imperfections In particular, we discuss the implementation of such a nano-antenna in the form of surface plasmon solitons in graphene, and illustrate possibilities to control their operation by means of optical tools.

Array of tunneling-coupled quantum dots as a terahertz range quantum nanoantenna

A terahertz range nanoantenna of a new type was conceptualized. The physical mechanism underlying the concept is population waveguiding, the Rabi wave, in tunneling-coupled one-dimensional and two-dimensional arrays of quantum dots (QDs) excited by the light wave traveling along the antenna. The terahertz component of the emission is stimulated by the tunneling current induced on the Rabi frequency by the motion of quantum transitions along the QD array. We calculated the antenna characteristics for the Rabi-wave loop nanoantenna and found its emission characteristics to be electrically tunable via varying the intensity and phase velocity of the light wave. The conceptualized antennas are promising for many practical applications due to their ability of electrical scanning of the antenna radiation pattern.

Multiple-Time-Scale Motion in Molecularly Linked Nanoparticle Arrays

We explore the transport of electrons between electrodes that encase a two-dimensional array of metallic quantum dots linked by molecular bridges (such as α,ω alkaline dithiols). Because the molecules can move at finite temperatures, the entire transport structure comprising the quantum dots and the molecules is in dynamical motion while the charge is being transported. There are then several physical processes (physical excursions of molecules and quantum dots, electronic migration, ordinary vibrations), all of which influence electronic transport. Each can occur on a different time scale. It is therefore not appropriate to use standard approaches to this sort of electron transfer problem. Instead, we present a treatment in which three different theoretical approaches-kinetic Monte Carlo, classical molecular dynamics, and quantum transport-are all employed. In certain limits, some of the dynamical effects are unimportant. But in general, the transport seems to follow a sort of dynamic bond percolation picture, an approach originally introduced as formal models and later applied to polymer electrolytes. Different rate-determining steps occur in different limits. This approach offers a powerful scheme for dealing with multiple time scale transport problems, as will exist in many situations with several pathways through molecular arrays or even individual molecules that are dynamically disordered.

Model of tunnelling through periodic array of quantum dots in a magnetic field

A two-dimensional periodic array of quantum dots with two laterally coupled leads in a magnetic field is considered. The model of electron transport through the system based on the theory of self-adjoint extensions of symmetric operators is suggested. We obtain the formula for the transmission coefficient and investigate its dependence on the magnetic field.

SUBSTRATE MODULATED GRAPHENE QUANTUM DOTS

We propose a method to use gapped graphene as barriers to confine electrons in gapless graphene and form a good quantum dot, which can be realized on an oxygen-terminated SiO 2 substrate partly hydrogen-passivated. In particular, we use deposited ferromagnetic insulators as contacts which give rise to spin-dependent energy spectrum and transport properties. Furthermore, we upgrade this method to form two-dimensional quantum dot arrays, whose coupling strength between neighboring dots can be uniquely anisotropic. Compared to complexity of other approaches to form quantum dot in graphene, the setup suggested here is a promising candidate for practical applications.

Dipolar transformations of two-dimensional quantum dots arrays proven by electron energy loss spectroscopy

Dipolar transformations of two-dimensional arrays of quantum dots are investigated theoretically. Homogeneous and non-homogeneous two-dimensional distributions are modeled by considering sections of the quantum dot array with different confinement potential. The dipolar transformations are tested by means of simulation of the dispersion of electrons of a beam traveling parallel to the plane of the quantum dot array. We calculate the electron energy loss function as a function of the temperature for different non-homogeneous distributions, considering several confinement potentials and coupling potentials among the quantum dots. Our results indicate that it would be possible by electron energy loss spectroscopy experiments to detect these dipolar transformations.

Damage-free top-down processes for fabricating two-dimensional arrays of 7 nm GaAsnanodiscs using bio-templates and neutral beam etching

The first damage-free top-down fabrication processes for a two-dimensionalarray of 7 nm GaAs nanodiscs was developed by using ferritin (a protein whichincludes a 7 nm diameter iron core) bio-templates and neutral beam etching. Thephotoluminescence of GaAs etched with a neutral beam clearly revealed that theprocesses could accomplish defect-free etching for GaAs. In the bio-template process,to remove the ferritin protein shell without thermal damage to the GaAs, wefirstly developed an oxygen-radical treatment method with a low temperature of280 °C.Then, the neutral beam etched the defect-free nanodisc structure of the GaAs using the iron coreas an etching mask. As a result, a two-dimensional array of GaAs quantum dots with a diameter of ∼ 7 nm, a height of ∼ 10 nm, a high taper angle of88° and a quantum dotdensity of more than 7 × 1011 cm − 2 was successfully fabricated without causing any damage to the GaAs.

Coulomb blockade and hopping conduction in graphene quantum dots array

We show that the low-temperature electron transport properties of chemically functionalized graphene can be explained as sequential tunneling of charges through a two-dimensional array of graphene quantum dots (GQDs). Below 15 K, a total suppression of current due to Coulomb blockade through a GQD array was observed. Temperature-dependent current-gate voltage characteristics show Coulomb oscillations with energy scales of 6.2--10 meV corresponding to GQD sizes of 5--8 nm, while resistance data exhibit an Efros-Shklovskii variable range hopping arising from structural- and size-induced disorder.

Photonic implementation for the topological cluster-state quantum computer

An implementation of the topological cluster-state quantum computer is suggested, in which the basic elements are linear optics, measurements, and a two-dimensional array of quantum dots. This overcomes the need for nonlinear devices to create a lattice of entangled photons. Whereas the thresholds found for computational errors are quite satisfactory (above ${10}^{\ensuremath{-}3}$), the estimates of the minimum efficiencies needed for the detectors and quantum dots are beyond current technology's reach. This is because we rely heavily on probabilistic entangling gates, which introduces loss into the scheme irrespective of detector and quantum-dot efficiencies.

Spin dynamics in two-dimensional arrays of quantum dots with different shapes

Electron paramagnetic resonance and spin echo methods are used to probe the spin dynamics in two-dimensional quantum dot (QD) arrays with different shape of nanoclusters. Two types of QD structures were investigated: 1) with single shaped QDs (hut-clusters, having the ratio of height h to lateral size l, h/l = 0.1), and 2) with two groups of QDs, hut- and dome-clusters (h/l = 0.2). Both types of structures demonstrate the EPR signals from electrons localized in QD layers.The orientation dependence of EPR line width for first type structures is well described by model of spin relaxation through precession in the effective magnetic field, arising during tunneling between QDs due to structure-inversion-asymmetry. In the experiments on structures with dome-clusters the additional peculiarity, which can not be explained in the framework of precession model, is observed. The different orientation dependencies can be explained by different localization degree of electrons in investigated structures. Spin echo measurements give the longest spin decoherence time for structures with single shaped QDs.

Random Telegraph Signals in Two-Dimensional Array of Si Quantum Dots

Silicon-quantum-dots (Si-QDs) with an areal density as high as ∼1012cm-2 were self-assembled on thermally-grown SiO2 by low pressure CVD using Si2H6, in which OH-terminated SiO2 surface prior to the Si CVD was exposed to GeH4 to create nucleation sites uniformly. After thermal oxidation of Si-QDs surface, two-dimensional electronic transport through the Si-QDs array was measured with co-planar Al electrodes evaporated on the array surface. Random telegraph signals were clearly observed at constant applied bias conditions in dark condition and under light irradiation at room temperature. The result indicates the charging and discharging of a dot adjacent to the percolation current path in the dots array.

Formation of two-dimensional InAs quantum dot arrays by self-organized anisotropic strain engineering on InP (3 1 1)B substrates

The authors report the formation of two-dimensional InAs quantum dot (QD) arrays by self-organized anisotropic strain engineering of InAs/InGaAsP superlattice (SL) templates on InP (3 1 1)B substrates by chemical-beam epitaxy (CBE). The SL template and InAs QD growth conditions are studied in detail for optimized QD ordering. Excellent photoluminescence emission up to room temperature is achieved from buried QD arrays. The emission wavelength is tuned from above 1.9 μm to the 1.55 μm telecom wavelength region through the insertion of ultrathin GaAs interlayers beneath the QD arrays.

Strong to weak localization transition and two-parameter scaling in a two-dimensional quantum dot array

The transition from strong to weak localization behavior was observed in two-dimensional Ge/Si quantum dot structure under the variation in the quantum dot occupancy, their areal density and annealing of the structures at 480–625 °C. To clarify the carrier transport mechanism and separate the hopping and diffusive regimes, the temperature dependence of conductance and conductance nonlinearity were analyzed in the structures with different correlations between the disorder and interaction. It was shown that the change in the relative disorder without significantly changing the interaction keeps the system inside the one-parameter scaling. The role of the interaction in two-parameter scaling was revealed by observing the shift of Gell-MannLow scaling function for the samples with large variation in the Coulomb interaction.

Bandgap calculation in Si quantum dot arrays using a genetic algorithm

In this work we present a fast and accurate genetic algorithm to determine the envelope functions and eigenenergies of the ground states of electrons and holes in low-dimensional complex semiconductor structures. We have developed the theoretical formalism of the algorithm in a general way in order to make it easy to include arbitrary nonparabolic and anisotropic band profiles in the calculations. From these results, calculation of the bandgaps of nanostructures can be carried out efficiently. Besides presenting and testing the algorithm, we calculate the ground state of electron and holes in two-dimensional quantum dot arrays, taking nonparabolicity and anisotropy into account.

Strong Electronic Coupling in Two-Dimensional Assemblies of Colloidal PbSe Quantum Dots

Thin films of colloidal PbSe quantum dots can exhibit very high carrier mobilities when the surface ligands are removed or replaced by small molecules, such as hydrazine. Charge transport in such films is governed by the electronic exchange coupling energy (beta) between quantum dots. Here we show that two-dimensional quantum dot arrays assembled on a surface provide a powerful system for studying this electronic coupling. We combine optical spectroscopy with atomic force microscopy to examine the chemical, structural, and electronic changes that occur when a submonolayer of PbSe QDs is exposed to hydrazine. We find that this treatment leads to strong and tunable electronic coupling, with the beta value as large as 13 meV, which is 1 order of magnitude greater than that previously achieved in 3D QD solids with the same chemical treatment. We attribute this much enhanced electronic coupling to reduced geometric frustration in 2D films. The strongly coupled quantum dot assemblies serve as both charge and energy sinks. The existence of such coupling has serious implications for electronic devices, such as photovoltaic cells, that utilize quantum dots.

Electronic spectrum of a two-dimensional quantum dot array in the presence of electric and magnetic fields in the Hall configuration

We report calculations of the electronic spectrum of a two-dimensional lattice of coupled quantum dots, subject to external electric and magnetic fields in the Hall configuration. The quantum dots array was modeled by a periodic superposition of truncated, parabolic potential wells. By adopting the Landau gauge, a single-particle Hamiltonian was formulated, and its eigenfunctions were obtained as appropriately symmetrized, magnetic field-dependent Bloch functions. The magnetic field was consistently included in the corresponding Wannier functions, which were approximated by the eigenvectors of an isolated quantum dot in the presence of the external magnetic field, and multiplied by the Peierls's phase.

Scaling of entanglement at a quantum phase transition for a two-dimensional array of quantum dots

Using the Hubbard model, the entanglement scaling behavior in a two-dimensional itinerant system is investigated. It has been found that, on the two sides of the critical point denoting an inherent quantum phase transition (QPT), the entanglement follows different scalings with the size, just as an order parameter does. This fact reveals the subtle role played by the entanglement in QPT as a fungible physical resource.

Time evolution of initially localized states in two-dimensional quantum dot arrays in a magnetic field

The evolution of non-stationary localized states |Ψ(t=0)〉 is investigated in two-dimensional tight binding systems of N potential wells with and without a homogeneous field perpendicular to the plane. Most results are presented in analytical form, what is almost imperative if the patterns are as complex as for rings in a magnetic field, where the qualitatively different features arise depending on rational or irrational numbers. The systems considered comprise finite linear chains (N=2,3), finite rings (N=3–6), infinite chains, finite rings (N=3–6) in a magnetic field, and rings with leads attached to each ring site. The position of the particle at time t is described by the projection of the wave function Pm(t)=|〈m|Ψ(t)〉|2 onto the localized basis function at site m. For finite chains and rings with N=3,4,6 the time evolution is periodic, whereas it is non-periodic for N=5 and N greater then 6. Rings in a magnetic field show a rich spectrum of different features depending on N and the number of flux quanta through the ring, including periodic oscillation and rotation of the charge as well as non-periodic charge fluctuations.

Hopping photoconduction and its long-time kinetics in a heterosystem with Ge quantum dots in Si

The effect of interband-transition-inducing illumination on the hole hopping conduction along a two-dimensional array of Ge quantum dots in Si was studied. It is found that the photoconductance has either positive or negative sign depending on the initial filling of quantum dots with holes. In the course of illumination and after switching off the light, long-time photoconduction kinetics was observed (102−104s at T=4.2 K). The results are discussed in terms of a model based on the spatial separation of nonequilibrium electrons and holes in a potential relief formed by positively charged dots. The effect of equalization of potential barrier heights as a result of photohole capture by the charged quantum dots during the process of illumination and relaxation is suggested as an additional factor for explaining the phenomenon of persistent conduction.

Imaging of quantum array structures with coherent and partially coherent diffraction

Recent achievements in experimental and computational methods open the possibility of measuring and inverting the diffraction pattern from a finite object of submicrometer size. In this paper the possibilities of such experiments for two-dimensional arrays of quantum dots are discussed. The diffraction pattern corresponding to coherent and partial coherent illumination of a sample was generated. Test calculations based on the iterative algorithms were applied to reconstruct the shape of the individual islands in such a quantum structure directly from its diffraction pattern. It is demonstrated that, in the case of coherent illumination, the correct shape and orientation of an individual island can be obtained. In the case of partially coherent illumination, the correct shape of the particle can be obtained only when the coherence of the incoming beam is reduced to match the size of the island.