Dimensional Superlattice Research Articles

Simple cubic self-assembly of PbS quantum dots by finely controlled ligand removal through gel permeation chromatography.

The geometry in self-assembled superlattices of colloidal quantum dots (QDs) strongly affects their optoelectronic properties and is thus of critical importance for applications in optoelectronic devices. Here, we achieve the selective control of the geometry of colloidal quasi-spherical PbS QDs in highly-ordered two and three dimensional superlattices: Disordered, simple cubic (sc), and face-centered cubic (fcc). Gel permeation chromatography (GPC), not based on size-exclusion effects, is developed to quantitatively and continuously control the ligand coverage of PbS QDs. The obtained QDs can retain their high stability and photoluminescence on account of the chemically soft removal of the ligands by GPC. With increasing ligand coverage, the geometry of the self-assembled superlattices by solution-casting of the GPC-processed PbS QDs changed from disordered, sc to fcc because of the finely controlled ligand coverage and anisotropy on QD surfaces. Importantly, the highly-ordered sc supercrystal usually displays unique superfluorescence and is expected to show high charge transporting properties, but it has not yet been achieved for colloidal quasi-spherical QDs. It is firstly accessible by fine-tuning the QD ligand density using the GPC method here. This selective formation of different geometric superlattices based on GPC promises applications of such colloidal quasi-spherical QDs in high-performance optoelectronic devices.

Growth Optimization of Multi-layer Thin Film Thermoelectric Materials based on Bi2Te3 / WS2 superlattice Structure

Heat is the most ubiquitous form of energy on planet Earth. Every day, the sun continuously strikes the Earth’s surface with 120,000 Terawatts of energy. This solar energy is more than 10,000 times the amount of energy produced worldwide. With the scarcity of fossil fuels looming on the horizon and its adverse effect on the environment many researchers, from academia to industry, are exploring cleaner, greener and more efficient renewable energy technologies. Thermoelectricity can provide an alternative to hazardous fossil fuels as its electricity is produced directly from heat with no moving parts or working fluid. The efficiency of any thermoelectric material is given by a quantity called the figure of merit ZT. For thermoelectric (TE) devices to be competitive with fluid-based and other energy related devices, ZT greater than 2 is usually sought. Here, we report on the fabrication of thin film thermoelectric materials based on Bi2Te3/WS2 superlattice layer structure using RF magnetron sputtering deposition method. Quantum confinement in these low dimensional and ultrathin superlattices can enhance the density of states near the fermi level resulting in higher ZT value. The thermoelectric figure of merit can be enhanced by controlling the layer thickness close to the phonons mean free path. This way heat carrying phonons with different wavelengths can be scattered efficiently resulting in lower lattice thermal conductivity.

Photonic Band Gap Alteration in LiNbO3-SiO2 Based 1D Periodic Multilayered Structure via Plate Wave

Acoustic control on refractive index of periodic nanostructures facilitate a colocalize far-reaching control of light propagation. Static tuning of photonic band gap in lithium niobate and silicon dioxide based one dimensional superlattice through the GHz plate wave perturbation is reported. Acoustic field confinement in proposed structure produces field pattern of mechanical strain, analysed for fundamental symmetric plate mode within the elastic limit constraints of material. Contraction and rarefaction raises and lowers down the refractive index of material along the periodicity of structure. Using appropriate photo-elastic relations, the modified refractive index for both the materials of unit cell of periodic structure is estimated at different instants of perturbation. It is found that the refractive index contrast of unit cell gets altered with acoustic propagation and leads to photonic band gap modification. The available photonic band gap for transverse electric waves in both unperturbed and acoustically perturbed structure is calculated using Bloch’s theorem and transfer matrix method.

Chemically Triggered Formation of Two-Dimensional Epitaxial Quantum Dot Superlattices

Two dimensional superlattices of epitaxially connected quantum dots enable size-quantization effects to be combined with high charge carrier mobilities, an essential prerequisite for highly performing QD devices based on charge transport. Here, we demonstrate that surface active additives known to restore nanocrystal stoichiometry can trigger the formation of epitaxial superlattices of PbSe and PbS quantum dots. More specifically, we show that both chalcogen-adding (sodium sulfide) and lead oleate displacing (amines) additives induce small area epitaxial superlattices of PbSe quantum dots. In the latter case, the amine basicity is a sensitive handle to tune the superlattice symmetry, with strong and weak bases yielding pseudohexagonal or quasi-square lattices, respectively. Through density functional theory calculations and in situ titrations monitored by nuclear magnetic resonance spectroscopy, we link this observation to the concomitantly different coordination enthalpy and ligand displacement potency of the amine. Next to that, an initial ∼10% reduction of the initial ligand density prior to monolayer formation and addition of a mild, lead oleate displacing chemical trigger such as aniline proved key to induce square superlattices with long-range, square micrometer order; an effect that is the more pronounced the larger the quantum dots. Because the approach applies to PbS quantum dots as well, we conclude that it offers a reproducible and rational method for the formation of highly ordered epitaxial quantum dot superlattices.

Self-consistent analysis of electron transport in GaN/AlGaN super lattice nanostructure for light emission

Schrödinger and Poisson’s equation have been solved self-consistently to investigate the electron transport in GaN/AlGaN super lattice nanostructure. Effect of applied bias along with spatial variation in effective mass of GaN and AlGaN has been taken in to account for realizing the electron confinement and recombination in one dimensional super lattice nanostructure. We have investigated in detail the influence of applied bias and Aluminum mole composition on Eigen energy, probability density and recombination rate. Here, we have determined the electron concentration through self-consistent solutions of Schrodinger and Poisson’s equations. The electron concentration deduced was 1.75×1020cm−3 under applied bias voltage of 5V with Aluminum mole fraction of 20%. The probability density along X direction clearly reveals that electrons are confined very well within the central region of superlattice nanostructure.

Ultra-low thermal conductivity in graphene nanomesh

Graphene nanomesh (GNM), a new nanostructure of graphene, has attracted extensive interest recently due to the promising chemical, electronic and photonic applications. In this paper, another important property – thermal conductivity is systematically investigated by using molecular dynamics simulations. The thermal conductivity (κ) is found to be extremely low, up to more than 3 orders lower than the pristine single layer graphene. Roughly, κ decreases exponentially with increasing porosity and linearly with decreasing neck width, and is not temperature sensitive in the range of 300 K–700 K. κ of GNMs is found to be even up to 200-fold lower than the graphene nanoribbons (GNR), a potential thermoelectric material, of the same neck width and boundary-to-area ratio. The extremely low κ in the GNM makes it a potential candidate for thermoelectrics. The phonon participation spectra show that the low κ in GNM is due to the localization and phonon back scattering around the nanopores. We also find that the phonon coherence in two dimensional superlattice GNM indeed exists, but is not as important as in the one dimensional superlattices. The isotope effect is negligible. The thermal conductivity reduction by edge passivation increases with increasing neck width and porosity.

Plasmon modes of a massive Dirac plasma, and their superlattices

We explore the collective density oscillations of a collection of charged massive Dirac particles, in one, two and three dimensions and their one dimensional superlattice. We calculate the long wavelength limit of the dynamical polarization function analytically, and use the random phase approximation to obtain the plasmon dispersion. The density dependence of the long wavelength plasmon frequency in massive Dirac systems is found to be different compared to systems with parabolic, and gapless Dirac dispersion. We also calculate the long wavelength plasmon dispersion of a 1d metamaterial made from 1d and 2d massive Dirac plasma. Our analytical results will be useful for exploring the use of massive Dirac materials as electrostatically tunable plasmonic metamaterials and can be experimentally verified by infrared spectroscopy as in the case of graphene [L. Ju. et. al., Nat. Nanotechnol. 6, 630 (2011)].

Effect of interdiffusion on electronic states of strain-free Gaussian-shaped double quantum ring superlattice

The effect of interdiffusion on band structure and Bloch amplitudes of three dimensional superlattice composed of Gaussian shaped double quantum rings is investigated using the Fourier transformation to the momentum space. It is shown that the sequence of the principal quasimomentum vectors which correspond to the energy increase depends on superlattice symmetry. Due to interdiffusion both the first and the second minibands of conduction band shift to the region of higher energies and broaden, meanwhile the difference between the behaviors of the dispersion curves in different directions of momentum space gradually disappears. The obtained results indicate to the opportunity of purposeful manipulation of structure characteristics by means of interdiffusion both quantitively and qualitatively.

Effect of interdiffusion on electronic states in one-layer quantum ring superlattice

The effect of interdiffusion on band structure and Bloch amplitudes of two dimensional superlattice composed of initially cylindrical quantum rings is investigated in the framework of adiabatic approach and using transformation to in-plane momentum space. It is shown that the adiabatic approximation is applicable even if the ring’s height is equal to its radius, because of weak localization of electron in the superlattice plane comparing with the localization in perpendicular direction. The consideration of the dependence of effective mass on spatial coordinate and time leads to the energy correction up to 10meV. It is shown that energy minibands rise and become wider due to interdiffusion and the dependence of Bloch amplitudes on quasimomentum direction is more pronounced in the case of diffused potential profile.

Phonon-induced superconductivity at high temperatures in electrical graphene superlattices

Due to the chiral nature of the Dirac equation, overlying of an electrical superlattice (SL) can open new Dirac points on the Fermi-surface of the energy spectrum. These lead to novel low-excitation physical phenomena. A typical example for such a system is neutral graphene with a symmetrical unidirectional SL. We show here that in smooth SLs, a semiclassical approximation provides a good mathematical description for particles. Due to the one-dimensional nature of the unidirectional potential, a wavefunction description leads to a generalized Bohr-Sommerfeld quantization condition for the energy eigenvalues. In order to pave the way for the application of semiclassical methods to two dimensional SLs in general, we compare these energy eigenvalues with those obtained from numerical calculations, and with the results from a semiclassical Gutzwiller trace formula via the beam-splitting technique. Finally, we calculate ballistic conductivities in general point-symmetric unidirectional SLs with one electron and one hole region in the fundamental cell showing only Klein scattering of the semiclassical wavefunctions.

Nonlinear optical properties of novel tunable, one dimensional molecular superlattice polymers (heteroarylene methines) containing alternating aromatic and quinoid segments

We have investigated the spectral dispersion of second molecular hyperpolarizability γ (−ω′; ω, −ω, ω) of three derivatives of heteroarylene methine by antiresonant ring interferometric nonlinear spectroscopic technique using femtosecond modelocked Ti:sapphire laser in the spectral range of 725–820 nm. The observed dispersion of γ has been explained in the framework of three-essential states model involving the ground state, a one-photon excited state and a two-photon excited state. The spectral response of the hyperpolarizability has been correlated with the electronic and chemical structures of the three derivatives of heteroarylene methine. The estimated γ values have been compared to the fundamental quantum mechanical limit. We have found for the first time that the heteroarylene methines approach this limit within a factor of 2 while even the best known molecule so far falls short of this limit by a factor of 30.

GRAPHENE: KINKS, SUPERLATTICES, LANDAU LEVELS AND MAGNETOTRANSPORT

We review recent work on superlattices in monolayer and bilayer graphene. We highlight the role of the quasiparticle chirality in generating new Dirac fermion modes with tunable anisotropic velocities in one dimensional (1D) superlattices in both monolayer and bilayer graphene. We discuss the structure of the Landau levels and magnetotransport in such superlattices over a wide range of perpendicular (orbital) magnetic fields. In monolayer graphene, we show that an orbital magnetic field can reverse the anisotropy of the transport imposed by the superlattice potential, suggesting possible switching-type device applications. We also consider topological modes localized at a kink in an electric field applied perpendicular to bilayer graphene, and show how interactions convert these modes into a two-band Luttinger liquid with tunable Luttinger parameters. The band structures of electric field superlattices in bilayer graphene (with or without a magnetic field) are shown to arise naturally from a coupled array of such topological modes. We briefly review some bandstructure results for 2D superlattices. We conclude with a discussion of recent tunneling and transport experiments and point out open issues.

Enhanced tunnel transport in disordered carbon superlattice structures incorporated with nitrogen

The possibility for enhanced tunnel transport through the incorporation of nitrogen in a quasi-one dimensional superlattice structure of amorphous carbon (a−C) made of sp2−C and sp3−C rich phases is shown by using a tight-binding model. The proposed superstructure can be described by a set of disordered graphite-like carbon clusters (acting as quantum wells) separated by a thin layer of diamond-like carbon (barriers) where the variation of the width and depth of the carbon clusters significantly control the electron transmission peaks. A large structural disorder in the pure carbon system, introduced through the variation of the bond length and associated deformation potential for respective carbon phases, was found to suppress the sharp features of the transmission coefficients. A small percentage of nitrogen addition to the carbon clusters can produce a distinct transmission peak at the low energy; however, it can be practically destroyed due to increase of the level of disorder of carbon sites. Whereas pronounced resonance peaks, both for C and N sites can be achieved through controlling the arrangement of the nitrogen sites of increased concentration within the disordered sp2−C clusters. The interplay of disorder associated with N and C sites illustrated the tunable nature of resistance of the structures as well as their characteristic times.

Effect of interdiffusion on electronic states and absorption coefficient of semiconductor superlattice of cubic symmetry

Abstract The effect of interdiffusion of Al and Ga atoms on confining potential, band structure and electromagnetic radiation's absorption coefficient of three dimensional superlattice, composed of initially spherical GaAs/Ga 1− x Al x As quantum dots is investigated in the framework of the modified Wood–Saxson potential model. It is shown that the interdiffusion leads to the disappearance of quantum dots’ spherical symmetry, to the widening of the energy minibands and to the significant decrease in the interband absorption coefficient. The obtained results indicate the opportunity to take into account the interdiffusion in the calculations of different characteristics of superlattices.

Thermoelectric power generation materials: Technology and application opportunities

Thermoelectric power sources have consistently demonstrated their extraordinary reliability and longevity for deep space missions (67 missions to date, more than 30 years of life) as well as terrestrial applications where unattended operation in remote locations is required. The development of new, more efficient materials and devices is the key to improving existing space power technology and expanding the range of terrestrial applications. The NASA Jet Propulsion Laboratory is leading collaborative research and development on novel advanced bulk materials capable of long-term operation at temperatures up to 1,300 K at more than 20% conversion efficiency. The research areas include refractory rare earth compounds and bulk three-dimensional nanostructures that emulate results obtained on low dimensional superlattices through “force engineering” and “self-assembling” techniques. Recent experimental results will be highlighted, and progress in transitioning thermoelectric technology to a more flexible, lower-cost modular array configuration suitable for various application opportunities will be discussed.

Preparation of decoherence-free cluster states with optical superlattices

We present a protocol to prepare decoherence-free cluster states using ultracold atoms loaded in a two dimensional superlattice. The superlattice geometry leads to an array of $2\ifmmode\times\else\texttimes\fi{}2$ plaquettes, each of them holding four spin-$1∕2$ particles that can be used for encoding a single logical qubit in the twofold singlet subspace, insensitive to uniform magnetic field fluctuations in any direction. Dynamical manipulation of the supperlattice yields distinct inter- and intraplaquette interactions and permits us to realize one qubit and two qubit gates with high fidelity, leading to the generation of universal cluster states for measurement based quantum computation. Our proposal based on inter- and intraplaquette interactions also opens the path to study polymerized Hamiltonians which support ground states describing arbitrary quantum circuits.

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.

Coherence properties of a Bose-Einstein condensate in an optical superlattice

We study the effect of a one dimensional optical superlattice on the superfluid properties (superfluid fraction, number squeezing, dynamic structure factor) and the quasi-momentum distribution of the Mott-insulator. We show that due to the secondary lattice, there is a decrease in the superfluid fraction and the number fluctuation. The dynamic structure factor which can be measured by Bragg spectroscopy is also suppressed due to the addition of the secondary lattice. The visibility of the interference pattern (the quasi-momentum distribution) of the Mott-insulator is found to decrease due to the presence of the secondary lattice. Our results have important implications in atom interferometry and quantum computation in optical lattices.

Self‐Assembling of Gold and Silver Nanoparticles at a Hydrophilic/Hydrophobic Interface: A Synthetic Aspect and Superstructure Formation

In this review article, we describe a precise synthetic recipe to prepare water‐soluble gold and silver nanoparticles, and present how to prepare two dimensional as well as three‐dimensional superlattices utilizing a hydrophilic and hydrophobic interface. We emphasize the importance of the role of side chain groups of thiol for controlling the properties of nanoparticles at the interface. Through chemical modification of the surface of nanoparticles via ion–pair interaction or covalent‐bonding, it is shown how the phase transfer extraction efficiency is tuned. The interparticle distance in the superlattice can be varied by an appropriate selection of mediator molecules through hydrogen bonding, which is automatically inserted among particles giving ordered structure. Some examples of three dimensional superlattices produced are presented, and the formation mechanism of the superlattices at the hydrophilic/hydrophobic interface is proposed through the van der Waals force calculation. A comparison is made between the calculation and experiments.

Zirconia nanoparticles and nanostructured systems

Stability of rhombic (tetragonal-like) and cubic-like zirconia nanoparticles is investigated using the density functional pseudopotential method in a general gradient approximation. The binding energy (Eb) increases with increasing of rhombic particles, but the cubic-like particle is more favorable than rhombic ones. All studied nanoparticles have nonzero energy distances (ΔE) between occupied and non-occupied states. A simple model of nanostructured zirconia has been constructed using a Zr16O32 cluster as a unit element for a three dimensional superlattice with cubic symmetry. This work was supported by the Russian Foundation of Basic Researches, the grant # 04-02-9700.