Dot Lattice Research Articles

Tracking electron pathways with magnetic field: Aperiodic Aharonov-Bohm oscillations in coherent transport through a periodic array of quantum dots

We study resonant tunneling through a periodic square array of quantum dots sandwiched between modulation-doped quantum wells. If a magnetic field is applied parallel to the quantum dot plane, the tunneling current exhibits a highly complex Aharonov-Bohm oscillation pattern due to the interference of multiple pathways traversed by a tunneling electron. Individual pathways associated with conductance beats can be enumerated by sweeping the magnetic field at various tilt angles. Remarkably, Aharonov-Bohm oscillations are aperiodic unless the magnetic field slope relative to the quantum dot lattice axes is a rational number.

Electric-field-driven insulating-to-conducting transition in a mesoscopic quantum dot lattice

We investigate electron transport through a finite two dimensional mesoscopic periodic potential, consisting of an array of lateral quantum dots with electron density controlled by a global top gate. We observe a transition from an insulating state at low-bias voltages to a conducting state at high-bias voltages. The insulating state shows simply activated temperature dependence, with strongly gate voltage dependent activation energy. At low temperatures the transition between the insulating and conducting states becomes very abrupt and shows strong hysteresis. The high-bias behavior suggests underdamped transport through a periodic washboard potential resulting from collective motion.

Tunable spin wave dynamics in two-dimensional Ni80Fe20 nanodot lattices by varying dot shape

We demonstrate tunable spin wave spectrum in two-dimensional Ni80Fe20 nanodot lattices by varying dot shape. A single collective mode in elliptical dot lattices transforms into three distinct modes for the half-elliptical, rectangular, and diamond dot lattices, albeit with different peak frequencies and intensities. A drastic change is observed for the triangular dots, where eight modes covering a broad band are observed. Using micromagnetic simulations, we characterized the modes as different localized, extended, and quantized modes, whose frequencies and spatial profiles are determined by a combination of internal field profiles within the nanodots and the stray magnetic field within the lattice.

One-pot synthesis of N-doped graphene quantum dots as a fluorescent sensing platform for Fe3+ ions detection

Herein, a facile bottom–up approach for producing nitrogen doped graphene quantum dots (N-GQDs) was carried out by the carbonization of citric acid through hydrothermal treatment in the presence of ammonia and a fluorescent sensor for determination of Fe3+ ions has been demonstrated. These synthesized N-GQDs consist mostly of 1–3 layers having a uniform size of about 3.5nm, and the nitrogen atoms were successfully introduced into the lattice of graphene quantum dots (GQDs). Due to the introduction of nitrogen atoms, N-GQDs exhibit a bright green luminescence with a higher quantum yield of 30.7%, whereas GQDs show a blue luminescence with a quantum yield of 4.5%. Furthermore, a novel sensor application of N-GQDs was performed for a fluorescent sensing platform for the detection of Fe3+ ions with high sensitivity and selectivity. These N-GQDs are promising for applications for biosensors, biological imaging and optoelectronic devices.

An Explicitly Solvable Model for Tunneling through a Quantum Dots Array in a Magnetic Field

An explicitly solvable model for electron tunneling through a periodic quantum dots array in a magnetic field is suggested. It is based on the theory of self-adjoint extensions of symmetric operators. Two types of arrays are considered: a honeycomb lattice of point-like potentials and a square lattice of quantum dots with parabolic confinement. Ranges of the electron energies corresponding to zero transmission coefficients are observed. The dependence of the positions of these ＂zero transmission bands＂ on the magnetic field is investigated.

Non-stochastic switching and emergence of magnetic vortices in artificial quasicrystal spin ice

Previous studies of artificial spin ice have been largely restricted to periodic dot lattices. Ferromagnetic switching of segments in an applied magnetic field is stochastic in periodic spin ice systems, which makes emergent phenomena, such as the formation of vortex loops, hard to control or predict. We fabricated finite, aperiodic Penrose P2 tilings as antidot lattices with fivefold rotational symmetry in permalloy thin films. Measurements of the field dependence of the static magnetization reveal reproducible knee anomalies whose number and form are temperature dependent, which suggests they mark cooperative rearrangements of the tiling magnetic texture. Our micromagnetic simulations of the P2 tiling are in good agreement with experimental magnetization data and exhibit non-stochastic magnetic switching of segments in applied field, and vortex loops that are stable over an extended field interval during magnetic reversal.

Dot-bound and dispersive states in graphene quantum dot superlattices

We consider a square lattice configuration of circular gate-defined quantum dots in an unbiased graphene sheet and calculate the electronic, particularly spectral properties of finite albeit actual sample sized systems by means of a numerically exact kernel polynomial expansion technique. Analyzing the local density of states and the momentum resolved photoemission spectrum we find clear evidence for a series of quasi-bound states at the dots, which can be probed by optical measurements. We further analyze the interplay of the superlattice structure with dot localized modes on the electron energy dispersion. Effects of disordered dot lattices are discussed too.

Configuration-induced vortex motion in type-II superconducting films with periodic magnetic dot arrays

Using the molecular dynamic method we investigate numerically the current driven vortex motion in a superconducting film with periodic arrays of both ferromagnetic (FM) and anti-parallel ferromagnetic (AFM) dots. The simulation results show that in the absence of thermal fluctuation the vortex motion is dominated by the configurations of the magnetic dot array. This guided vortex motion is only observed at the onset of the depinning of the interstitial vortices. Two guided vortex motion mechanisms are discussed in this work. For the AFM configuration the vortex motion is mainly originated by the existence of magnetic dots with opposite magnetized moments, while for the FM case it is mainly dependent on the degree of the condensed packing of the magnetic dot lattice constant.

Femtosecond laser-induced optical anisotropy in a two-dimensional lattice of magnetic dots

Using pump-probe optical polarimetry we demonstrate that femtosecond laser excitation of a 2D regular lattice of magnetic nanodots effectively changes the optical anisotropy of the array. Study of the dynamics of the femtosecond laser-induced anisotropy reveals four main mechanisms occurring in the electronic, spin, and lattice subsystems. Below 1 ps, a strong Kerr-like nonlinearity causes linear birefringence, with its axis directed along the electric field of the linearly polarized femtosecond laser pump pulse. In addition, a long-living linear birefringence is also induced due to slowly relaxing excitations. Also below 1 ps, ultrafast laser-induced demagnetization of Co leads to a partial breakdown of the circular birefringence of the magnetic nanodots. On the timescale up to 300 ps, optically triggered acoustic modes of the dots drive oscillations of the linear optical birefringence. During this process, the oscillations damp while transferring their energy into acoustic modes of the substrate. On the nanosecond timescale, the signal is dominated by acoustic oscillations at the surface of the substrate.

Lattice and optical property evolution of ultra-small ZnS quantum dots grown from a single-source precursor

Cubic structured ZnS quantum dots (QDs) with sizes from 1.6 to 4.0nm were grown at temperatures of 90–210°C from zinc ethylxanthate single-source precursor. The quantum dots were characterized by X-ray diffraction, high resolution transmission electron microscope, UV–vis absorption spectroscope, photoluminescence spectroscope and micro-Raman spectroscope. Effects of the size on their lattice and optical properties were studied in detail. The lattices contract with a decrease in the quantum dot sizes, especially for quantum dots with sizes smaller than 2.9nm. The contraction rate varies almost linearly with the quantum dot sizes and the maximum size for the contraction is derived to be 4.5nm. The exciton ground state energy increases with a decrease in the quantum dot size. Weak UV emission and strong blue-green emission were observed and transition paths for luminescence were provided. Relative intensity of 1LO and 2LO modes of the quantum dots is sensitive to their sizes, while their frequencies are insensitive to the quantum dot sizes.

Tuning the growth properties of Ge quantum dot lattices in amorphous oxides by matrix type

Self-assembled growth of Ge quantum dot lattices in oxide matrices prepared by the quite simple magnetron sputtering deposition method allows the preparation of a variety of structures tunable by their shape, size and arrangement. The driving mechanism for the self-assembly was attributed to the surface morphology features originating from the quantum dots' growth. Here it is shown specifically that the matrix type is another critical factor that enables the control of the self-assembly process and the tuning of the ordering type and degree of regularity of quantum dot systems. The effectiveness of the matrix factor is demonstrated through the analysis of quantum dot arrangements in amorphous silica, alumina and mullite matrices. Using the same deposition conditions, different ordering types and degrees of disorder were found in the quantum dot systems based on different matrices. The matrix factor is shown to be driven by different matrix tendencies to smooth the surface during the growth of the films. The obtained results are relevant for understanding and tailoring of the self-assembled growth of quantum dot lattices in amorphous systems.

Influence of annealing conditions on the structural and photoluminescence properties of Ge quantum dot lattices in a continuous Ge + Al2 O3 film

We investigate the structural and photoluminescence (PL) properties of regularly ordered lattices of Ge quantum dots (QDs) formed by self-assembly in a thick film of Ge + Al2O3 mixture. The effects of annealing at different temperatures and different annealing environments (vacuum and air) are studied. We show that the regular ordering of QDs remains unchanged for annealing up to 800 °C in vacuum, while it is preserved up to 700 °C when annealing is performed in air. The inner/crystalline structure of Ge QDs and their shape depend on the annealing conditions. The crystalline grains of γ-Al2O3 form in the initially amorphous Al2O3 matrix after annealing at 800 °C. The PL spectra are found to consist of two main contributions, one from the matrix and the other from Ge QDs. Both contributions depend strongly on the annealing conditions. The annealing treatment of Ge quantum dot lattices formed by self-assembly in alumina matrix influence significantly the internal structure and photoluminescence properties of the films.

Growth of a three-dimensional anisotropic lattice of Ge quantum dots in an amorphous alumina matrix

Simple processes for the preparation of semiconductor quantum dot lattices embedded in dielectric amorphous matrices play an important role in various nanotechnology applications. Of particular interest are quantum dot lattices with properties that differ significantly in different directions parallel to the material surface. Here, a simple method is demonstrated for the fabrication of an anisotropic lattice of Ge quantum dots in an amorphous Al2O3matrix by a self-assembly process. A specific deposition geometry with an oblique incidence of the Ge and Al2O3adparticles was used during magnetron sputtering deposition to achieve the desired anisotropy. The observed Ge quantum dot ordering is explained by a combination of directional diffusion of adparticles from the Ge and Al2O3targets and a shadowing process which occurs during deposition as a result of the specific surface morphology. The prepared material shows a strong anisotropy of the electrical conductivity in different directions parallel to the sample surface.

Effects of tank wall pattern on survival, bone injury rate, and stress response of juvenile Pacific bluefin tuna, Thunnus orientalis

Juvenile Pacific bluefin tuna (PBT), Thunnus orientalis, aged 30 or more days post hatching (≥5cm total length) are at high risk of mortality through contact or collision with tank walls. In this study, survival rates of PBT reared under three tank wall treatments (polka dot pattern, lattice pattern, and dark-green single color (control) were investigated. White tape was attached to the tank's surface to form either a polka dot or a lattice pattern. Fish were then reared in the tanks for 9 days. Fish survival rates in the lattice patterned and polka-dot patterned tanks were higher than that in the control tank. Bone and parasphenoid injury rates in the control group were significantly higher than those in the polka dot and lattice patterned treatment groups. The plasma cortisol content of juvenile PBT reared in the control group was higher than for fish reared in patterned tanks. These results suggest that juvenile PBT mortality from contact or collision with tank walls was reduced when wall visibility was improved by creating patterns on the tank walls.

Ge quantum dot lattices in Al2O3 multilayers

In this article, we show how to produce materials consisting of regularly ordered Ge quantum dot lattices in an amorphous alumina matrix with a controllable Ge quantum dot size, shape, spacing, crystalline structure, and degree of regularity in their ordering. The production of such materials is achievable already at room temperature by magnetron sputtering deposition of a (Ge + Al2O3)/Al2O3 multilayer. The materials show photoluminescence in the visible and ultraviolet light range, a size-dependent blue shift of the photoluminescence peak and an enhancement of its intensity by size reduction, indicating the quantum dot origin of the photoluminescence. The materials also exhibit excellent mechanical properties due to the alumina matrix. Their internal structure is shown to be highly resistive to irradiation with energetic particles for a large range of the irradiation parameters.

Collapse and revival of entanglement of two-qubit in superconducting quantum dot lattice with magnetic flux and inhomogeneous gate voltage

We study the entanglement of a two-qubit system in a superconducting quantum dot (SQD) lattice in the presence of magnetic flux and gate voltage. The ground state is always in a maximally entangled Bell state for homogeneous gate voltage. In the presence of inhomogeneous gate voltage, the half-integer magnetic flux quantum, completely washes out the entanglement of the system at zero temperature. The entanglement is much higher for the Mott insulating phase. At finite temperature, collapse of entanglement occurs for wider region of magnetic flux.

The first single-sized (∼1 nm) and periodically ordered array of In2Te3semiconductor quantum dots self-assembled in solution

We report the first example of single-sized quantum dots (QD) of a III–VI semiconductor, In2Te3, self-assembled in solution. The nanometer-sized dots (∼1 nm in diameter) with the formula of In8Te12 are arranged into perfectly ordered arrays via an organic passivating and structure-directing molecule (triethylenetetramine or trien) resulting in a single crystal structure of [In8Te12(trien)4] (monoclinic crystal system, space group C2/c). The In8Te12 dots in the [In8Te12(trien)4] structure show a remarkably strong structure-induced quantum confinement effect which results in a very large blue shift of 2.1 eV in the optical absorption spectrum. The observed increase in the band gap is substantially higher than that of the smallest colloidal quantum dots reported to date. The In8Te12 dots are readily dispersible in suitable solvents to form nanoparticles of an average size of ∼8 to 10 nm. The capability of forming periodic crystal lattices of semiconductor quantum dots offers an attractive way to tune the electronic and optical properties in the same (or larger) extent as those of the smallest nanoparticles while having the advantages of precisely controlling the size and stoichiometry. Such a crystal assembly of QDs may find utility as a new type of molecular-based precursors for the fabrication of energy- and cost-effective solution-processed thin film devices.

Reading the signs

Spin coherence of valence holes in semiconductor quantum-dots is governed by interactions with the nuclear spins of the dot lattice. Experiments and theory have revealed an important new ingredient that determines the strength and sign of this coupling.

Tunable Magnonic Spectra in Two‐Dimensional Magnonic Crystals with Variable Lattice Symmetry

AbstractTunable magnonic properties are demonstrated in two‐dimensional magnonic crystals in the form of artificial ferromagnetic nanodot lattices with variable lattice symmetry. An all‐optical time‐domain excitation and detection of the collective precessional dynamics is performed in the strongly magnetostatically coupled Ni80Fe20 (Py) circular dot lattices arranged in different lattice symmetry such as square, rectangular, hexagonal, honeycomb, and octagonal symmetry. As the symmetry changes from square to octagonal through rectangular, hexagonal and honeycomb, a significant variation in the spin wave spectra is observed. The single uniform collective mode in the square lattice splits in two distinct modes in the rectangular lattice and in three distinct modes in the hexagonal and octagonal lattices. However, in the honeycomb lattice a broad band of modes are observed. Micromagnetic simulations qualitatively reproduce the experimentally observed modes, and the simulated mode profiles reveal collective modes with different spatial distributions with the variation in the lattice symmetry determined by the magnetostatic field profiles. For the hexagonal lattice, the most intense peak shows a six‐fold anisotropy with the variation in the azimuthal angle of the external bias magnetic field. Analysis shows that this is due to the angular variation of the dynamical component of magnetization for this mode, which is directly influenced by the variation of the magnetostatic field on the elements in the hexagonal lattice. The observations are important for tunable and anisotropic propagation of spin waves in magnonic crystal based devices.

Influence of annealing conditions on the formation of regular lattices of voids and Ge quantum dots in an amorphous alumina matrix

In this work, the influence of air pressure during the annealing of Ge quantum dot (QD) lattices embedded in an amorphous Al2O3 matrix on the structural, morphological and compositional properties of the film is studied. The formation of a regularly ordered void lattice after performing a thermal annealing process is explored. Our results show that both the Ge desorption from the film and the regular ordering of the QDs are very sensitive to the annealing parameters. The conditions for the formation of a void lattice, a crystalline Ge QD lattice and a disordered QD lattice are presented. The observed effects are explained in terms of oxygen interaction with the Ge present in the film.