Two-dimensional Quantum Dot Research Articles

Two-dimensional ruddlesden-popper hybrid perovskite quantum dots for optical limiting application

The perovskite-based materials with optical tuning properties are very important for many practical applications such as the solid-state lighting technology, solar cell, and catalysis. In this work, we used the top-down method to prepare the two-dimensional (2D) (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 (CPI, n = 1, 2) hybrid perovskite quantum dots (QDs), and reported that the 2D CPI QDs featured an emission band covering the ultraviolet (UV) and blue region, with almost all the emission intensity in the UV region. This UV emission spectrum, along with the UV absorption spectrum, means a strong quantum confinement, as revealed by the X-ray photoelectron spectroscopy (XPS). In addition, the nonlinear optical scattering and absorption properties based respectively on the optical and photoacoustic z-scan characterizations were observed in the 2D CPI QDs, the results of which revealed a two-photon absorption coefficient of up to 6.5 × 102 cm/GW. This work reveal the potential of 2D CPI QDs in the high optical limiting applications.

Remote Capacitive Sensing in Two-Dimensional Quantum-Dot Arrays.

We investigate gate-induced quantum dots in silicon nanowire field-effect transistors fabricated using a foundry-compatible fully depleted silicon-on-insulator (FD-SOI) process. A series of split gates wrapped over the silicon nanowire naturally produces a 2 × n bilinear array of quantum dots along a single nanowire. We begin by studying the capacitive coupling of quantum dots within such a 2 × 2 array and then show how such couplings can be extended across two parallel silicon nanowires coupled together by shared, electrically isolated, "floating" electrodes. With one quantum dot operating as a single-electron-box sensor, the floating gate serves to enhance the charge sensitivity range, enabling it to detect charge state transitions in a separate silicon nanowire. By comparing measurements from multiple devices, we illustrate the impact of the floating gate by quantifying both the charge sensitivity decay as a function of dot-sensor separation and configuration within the dual-nanowire structure.

Thermoelectric properties of finite two-dimensional quantum dot arrays with band-like electronic states

The thermal power ( PF = S 2 G e ) depends on the Seebeck coefficient ( S ) and electron conductance ( G e ). The enhancement of G e will unavoidably suppress S because they are closely related. As a consequence, the optimization of PF is extremely difficult. Here, we theoretically investigated the thermoelectric properties of two-dimensional quantum dot (QD) arrays with carriers injected from electrodes. The Lorenz number of 2D QD arrays in the resonant tunneling procedure satisfies the Wiedemann-Franz law, which confirms the formation of minibands. When the miniband center is far away from the Fermi level of the electrodes, the electron transport is in the thermionic-assisted tunneling procedure (TATP). In this regime, G e in band-like situation and S in atom-like situation can happen simultaneously. We have demonstrated that the enhancement of G e with an increasing number of electronic states will not suppress S in the TATP. • The formation of minibands of 2D QD arrays is confirmed by the Wiedemann-Franz law in the resonant tunneling procedure. • The electron transport between the electrodes is in the thermionic-assisted tunneling procedure (TATP), as the miniband center is far away from the Fermi level of the electrodes. • In TATP, G e in band-like situation and S in atom-like situation can happen simultaneously. • The enhancement of G e with an increasing number of electronic states will not suppress S in the TATP.

Optical properties of a two-dimensional GaAs quantum dot under strain and magnetic field

It is well known that the use of the strain plays an important role in the material properties. Strain effect is a potential tool for altering the atomic positions and defect formations. It can adjust the electronic structures and lattice vibrations. It can also affect the phase transition of the structure, the physical and chemical properties. Consequently, in this paper, we have studied the effects of strain and magnetic field on optical properties of a two-dimensional quantum dot. The Hamiltonian of our system consists of the Bychkov–Rashba, Dresselhaus and strain-dependent terms. Using the diagonalization method, we have obtained the energy levels and wave functions of the system and thereby the refractive index changes and absorption coefficient in the presence of different strains. According to the results, it is found that an anti-crossing magnetic field is 7.37 T and it does not depend on the strain. Also, the energy transition increases with enhancement in the strain and decreases with increase in the magnetic field. The refractive index changes and absorption coefficient shifts toward lower (higher) energies for negative (positive) strain. The energy transition has lower values for negative strain at fixed magnetic field and confinement length.

Spin Relaxation Benchmarks and Individual Qubit Addressability for Holes in Quantum Dots.

We investigate hole spin relaxation in the single- and multihole regime in a 2 × 2 germanium quantum dot array. We find spin relaxation times T1 as high as 32 and 1.2 ms for quantum dots with single- and five-hole occupations, respectively, setting benchmarks for spin relaxation times for hole quantum dots. Furthermore, we investigate qubit addressability and electric field sensitivity by measuring resonance frequency dependence of each qubit on gate voltages. We can tune the resonance frequency over a large range for both single and multihole qubits, while simultaneously finding that the resonance frequencies are only weakly dependent on neighboring gates. In particular, the five-hole qubit resonance frequency is more than 20 times as sensitive to its corresponding plunger gate. Excellent individual qubit tunability and long spin relaxation times make holes in germanium promising for addressable and high-fidelity spin qubits in dense two-dimensional quantum dot arrays for large-scale quantum information.

Bisphenol S-doped g-C3N4 nanosheets modified by boron nitride quantum dots as efficient visible-light-driven photocatalysts for degradation of sulfamethazine

Antibiotics may pose a great risk to ecosystem and human health. Photocatalysis, as a low-cost and environmentally friendly technology is widely used for the removal of antibiotics from wastewater. Graphitic carbon nitride (g-C3N4) has shown promising prospects in visible light photocatalysis, while its photocatalytic performance is greatly limited because of the sluggish charge separation and transfer. In this work, metal-free boron nitride quantum dots (BNQDs) modified bisphenol S (BPS)-doped g-C3N4 nanosheets (BNQDs/BPS-CN) heterojunction was synthesized to overcome these defects and applied in photodegradation of sulfamethazine (SMZ, a typical antibiotic) under the visible light. Multifarious characterization methods were used to explore the structure, porosity, elemental composition, optical performances, photo-electrochemical properties and photocatalytic performances of as-prepared BNQDs/BPS-CN composites. The degradation efficiency of SMZ with BNQDs/BPS-CN-4 composite reaches 100% within 60 min, and the rate constant is 13.7 times higher than that of pure g-C3N4. This phenomenon is because of the shrinking band gap width and the introduction of electronegative BNQDs, which is conducive to the absorption of visible light and high-efficiency separation of photoexcited charge carriers. Meanwhile, the results of free radical trapping experiments and electron spin resonance characterization prove that the photogenerated holes and superoxide radicals play predominant roles in the photodegradation of SMZ. This study proposes an effective mechanism for the construction of novel visible-light-driven photocatalysts using metal-free two-dimensional materials and quantum dots, which can be applied in the treatment of organic contaminants.

Covalently Linked, Two-Dimensional Quantum Dot Assemblies.

Using nanoscale building blocks to construct hierarchical materials is a radical new branch point in materials discovery that promises new structures and emergent functionality. Understanding the design principles that govern nanoparticle assembly is critical to moving this field forward. By exploiting mixed ligand environments to target patchy nanoparticle surfaces, we have demonstrated a novel method of colloidal quantum dot (QD) assembly that gives rise to 2D structures. The equilibration of solutions of spherical and quasispherical QDs, including CdS, CdSe, and InP, with 2,2'-bipyridine-5,5'-diacrylic acid resulted in the preferential formation of 2D assemblies over the course of days as determined by transmission electron microscopy analysis. Small-angle X-ray scattering confirms the existence of the QD assemblies in solution. The dependence of the assembly on linker properties (length and rigidity), linker concentration, and total concentration was investigated, together with the data point to a mechanism involving ligand redistribution to create a patchy surface that maximizes the steric repulsion of neighboring QDs. By operating in an underexchanged regime, the arising patchiness results in enthalpically preferred directions of cross-linking that can be accessed by thermal equilibration.

Interplay of exchange and superexchange in triple quantum dots

Recent experiments on semiconductor quantum dots have demonstrated the ability to utilize a large quantum dot to mediate superexchange interactions and generate entanglement between distant spins. This opens up a possible mechanism for selectively coupling pairs of remote spins in a larger network of quantum dots. Taking advantage of this opportunity requires a deeper understanding of how to control superexchange interactions in these systems. Here, we consider a triple-dot system arranged in linear and triangular geometries. We use configuration interaction calculations to investigate the interplay of superexchange and nearest-neighbor exchange interactions as the location, detuning, and electron number of the mediating dot are varied. We show that superexchange processes strongly enhance and increase the range of the net spin-spin exchange as the dots approach a linear configuration. Furthermore, we show that the strength of the exchange interaction depends sensitively on the number of electrons in the mediator. Our results can be used as a guide to assist further experimental efforts towards scaling up to larger, two-dimensional quantum dot arrays.

Nanostructures in structured light: Photoinduced spin and orbital electron dynamics

We study the coupled spin and orbital dynamics of electrons in one- and two-dimensional quantum dots driven by few cycle pulses of polarization-structured terahertz vector fields. Emphasis is put on the use of radially and azimuthally polarized, cylindrical vector beams for spin-flip processes caused by transitions between electronic states characterized by spin-position entanglement and corresponding injection of spin currents. We demonstrated how different topologies of the beams result in different selection rules and corresponding spin and position dynamics. These results point to new possibilities for spatiotemporal control of the coupled spin and orbital electron dynamics via polarization shaping of the driving electromagnetic pulse.

Path integral molecular dynamics for fermions: Alleviating the sign problem with the Bogoliubov inequality.

We present a method for performing path integral molecular dynamics (PIMD) simulations for fermions and address its sign problem. PIMD simulations are widely used for studying many-body quantum systems at thermal equilibrium. However, they assume that the particles are distinguishable and neglect bosonic and fermionic exchange effects. Interacting fermions play a key role in many chemical and physical systems, such as electrons in quantum dots and ultracold trapped atoms. A direct sampling of the fermionic partition function is impossible using PIMD since its integrand is not positive definite. We show that PIMD simulations for fermions are feasible by employing our recently developed method for bosonic PIMD and reweighting the results to obtain fermionic expectation values. The approach is tested against path integral Monte Carlo (PIMC) simulations for up to seven electrons in a two-dimensional quantum dot for a range of interaction strengths. However, like PIMC, the method suffers from the sign problem at low temperatures. We propose a simple approach for alleviating it by simulating an auxiliary system with a larger average sign and obtaining an upper bound to the energy of the original system using the Bogoliubov inequality. This allows fermions to be studied at temperatures lower than would otherwise have been feasible using PIMD, as demonstrated in the case of a three-electron quantum dot. Our results extend the boundaries of PIMD simulations of fermions and will hopefully stimulate the development of new approaches for tackling the sign problem.

Enhancing electrostatic coupling in silicon quantum dot array by dual gate oxide thickness for large-scale integration

We propose a structure with word/bit line control for a two-dimensional quantum dot array, which allows random access for arbitrary quantum dots with a small number of control signals. To control multiple quantum dots with a single signal, every quantum dot should have a wide operating voltage allowance to overcome the property variations. We fabricate two-dimensional quantum dot arrays using silicon-complementary-metal-oxide-semiconductor technology with an alternating dual-standard gate oxide thickness. The quantum dots are designed to have an allowable operating voltage window of 0.2 V to control the number of electrons, which is a window one order of magnitude wider than that of previous works. The proposed structure enables both easy fabrication and operation for multiple quantum dots and will pave the way for practical use of large-scale quantum computers.

Solving fermion problems without solving the sign problem: Symmetry-breaking wave functions from similarity-transformed propagators for solving two-dimensional quantum dots.

It is well known that the use of the primitive second-order propagator in path-integral Monte Carlo calculations of many-fermion systems leads to the sign problem. This work will show that by using the similarity-transformed Fokker-Planck propagator, it is possible to solve for the ground state of a large quantum dot, with up to 100 polarized electrons, without solving the sign problem. These similarity-transformed propagators naturally produce rotational symmetry-breaking ground-state wave functions previously used in the study of quantum dots and quantum Hall effects. However, instead of localizing the electrons at positions that minimize the potential energy, this derivation shows that they should be located at positions that maximize the bosonic ground-state wave function. Further improvements in the energy can be obtained by using these as initial wave functions in a ground-state path-integral Monte Carlo calculation with second- and fourth-order propagators.

Monolayer single crystal two-dimensional quantum dots via ultrathin cutting and exfoliating

Two-dimensional (2D) atomically thin quantum dots (QDs) possess extraordinary electrical and optical properties. However, fabricating high quality 2D QDs via a universal and reliable technique remains a challenge. Here, we report a simple strategy to prepare high quality, monolayer single crystal 2D QDs via ultrathin cutting 2D bulk single crystals by ultramicrotome, followed by an exfoliation process. The as-prepared 2D QDs have pristine surface, high quality, high monolayer yield and high photoluminescence quantum yield (the highest photoluminescence quantum yield of WS2 is 18%), which can be used as promising, low toxic, biocompatible, and good cell-permeability fluorescent labeling agents for in vitro imaging.

Intrinsic-strain-induced curling of free-standing two-dimensional Janus MoSSe quantum dots

Motivated by the fascinating properties of both two-dimensional transition metal dichalcogenide quantum dots (TMD QDs) and Janus TMD monolayers, we theoretically explore the equilibrium structures of free-standing Janus MoSSe QDs in which atomic asymmetry of chalcogen is introduced. Two distinct types of spontaneous curling are observed by molecular dynamics simulations, and the curling behavior depends on the size of QD. The bowl-like (tube-like) curling occurs in relatively small (large) MoSSe QDs with different shapes (hexagon and triangle) and edge types (zigzag and armchair). The transition between these two curling types occurs at the sizes of around 10 nm and 13 nm for hexagonal and triangular shapes, respectively. By applying equivalent misfit strains into two adjacent sublayers, finite element analysis reproduces similar curling behavior. This confirms the relaxation of intrinsic strain in Janus structure acting as the predominant driving force of spontaneous curling. In addition, the curvatures of Janus TMD QDs increase from MoSSe to MoSeTe to MoSTe, indicating the positive correlation between the curling and misfit.

Bismuthene quantum dots based optical modulator for MIR lasers at 2 μm

The uniformly sized two-dimensional (2D) bismuthene quantum dots (BiQDs) were fabricated through the liquid-phase exfoliation (LPE) route and characterized comprehensively. By transferring the BiQDs onto a quartz substrate, BiQDs saturable absorber (SA) was prepared and applied in a Tm:YLF bulk laser. To get the best output performance of passive Q-switching (PQS) laser with BiQDs-SA, three output couplers with different transmittance were applied in laser implementation. The stable solid-state 2 μm laser was obtained, generating a maximum peak power of 8.1 W and shortest pulse width of 440 ns with the T = 5% output coupler. To the best of our knowledge, it is the first demonstration of BiQDs-SA for Q-switching operation around 2 μm, implying that the BiQDs-SA can be considered as a promising optical modulator for the mid-infrared (MIR) pulses producing.

Role of Rashba spin-orbit interaction on polaron Zeeman effect in a two-dimensional quantum dot with parabolic confinement

We calculate the energies of the ground and the first excited states of a free polaron and that of a polaron bound to a Coulomb impurity in a quantum dot with harmonic confinement in the presence of Rashba spin-orbit interaction by employing the variation theory of Lee, Low and Pines as modified by Huybrechts for an all-coupling range of the electron-phonon interaction and arbitrary confinement length. We show that in both cases, the Rashba interaction removes the two-fold spin-degeneracy of the first-excited states even in the absence of any applied magnetic field, though the ground state does not show any such spin splitting. The self-energy corrections due to the polaronic effect are however not affected by the Rashba interaction. We also investigate the combined effect of Rashba and polaronic interactions in the presence of an external magnetic field using the Rayleigh-Schrödinger perturbation theory. Application of our results to GaAs and CdS quantum dots shows that the suppression of the phonon-induced size-dependent Zeeman splitting in a quantum dot is reduced by the Rashba coupling.

Digital Designing and Parameter Optimization for Plasmonic Circuits

The use of numerical procedures and algorithms in designing plasmonic circuits based on two-dimensional materials and semiconductor quantum dots is considered. Such mathematical approaches as the basis for a system of automated calculations of semiconductor spherical quantum dot parameters are described, along with the finite-difference–time-domain approach for the full-wave electromagnetic simulation near a surface of nanostructured graphene.

Low-threshold laser medium utilizing semiconductor nanoshell quantum dots.

Colloidal semiconductor nanocrystals (NCs) represent a promising class of nanomaterials for lasing applications. Currently, one of the key challenges facing the development of high-performance NC optical gain media lies in enhancing the lifetime of biexciton populations. This usually requires the employment of charge-delocalizing particle architectures, such as core/shell NCs, nanorods, and nanoplatelets. Here, we report on a two-dimensional nanoshell quantum dot (QD) morphology that enables a strong delocalization of photoinduced charges, leading to enhanced biexciton lifetimes and low lasing thresholds. A unique combination of a large exciton volume and a smoothed potential gradient across interfaces of the reported CdSbulk/CdSe/CdSshell (core/shell/shell) nanoshell QDs results in strong suppression of Auger processes, which was manifested in this work though the observation of stable amplified stimulated emission (ASE) at low pump fluences. An extensive charge delocalization in nanoshell QDs was confirmed by transient absorption measurements, showing that the presence of a bulk-size core in CdSbulk/CdSe/CdSshell QDs reduces exciton-exciton interactions. Overall, present findings demonstrate unique advantages of the nanoshell QD architecture as a promising optical gain medium in solid-state lighting and lasing applications.

The Interphase Layer in Polymer Nanocomposites

Characteristic features of the formation and structure of the interphase layer in polymer nanocomposites are considered. The mutual influence of fillers and matrix on the structure and properties of the interphase layer are shown. The dependence of the characteristics of the interphase layer on the dimension of nanoparticles, namely, one-dimensional (nanotubes), two-dimensional (graphene and layered silicate clays), three-dimensional (metal-containing nanoparticles), and zero-dimensional quantum dots, is taken into account.

Super-statistical description of thermo-magnetic properties of a system of 2D GaAs quantum dots with gaussian confinement and Rashba spin–orbit interaction

We examine the effect of non-equilibrium processes modeled by the introduction of a generalized Boltzmann factor on the thermal and magnetic properties of an array of two-dimensional GaAs quantum dots in the presence of an external uniform and constant magnetic field. The model consists of a single-electron subject to a confining Gaussian potential with a spin–orbit interaction in the Rashba approach. We compute the specific heat and the magnetic susceptibility within the formalism of χ2-superstatistics from the exact solution of the Schrödinger equation. Furthermore, an analytic solution for the partition function allows a study of the impact of the number of subsystems on the superstatistical corrections and confirms that the ordinary thermo-magnetic properties are recovered whenever the thermal distribution can be approximated by a Dirac delta. Also, we found a progressive disappearance of the Schottky anomaly with decreasing number of subsystems, while the specific heat ceases to be a monotonically increasing function with respect to the average temperature when the χ2-distribution is spread over a large range of temperatures. Remarkably, the introduction of fluctuations in the temperature is found to suppress the paramagnetic phase transition that would otherwise appear at low temperatures. Finally, we emphasize that an appropriate construction of the definition of physical observables is crucial for obtaining a correct description of the physics derived from a non-extensive construction of the entropy.