Interacting Quantum Dot Research Articles

Size- and surface functionalization-driven molecular interaction of CdSe quantum dots with jack bean urease: multispectroscopic, thermodynamic, and AFM approach.

Quantum dots (QDs) with distinctive optical properties have been extensively researched and developed for usage in solar cells, imaging, drug delivery, cellular targeting, etc. But the inevitable production of QDs can lead to their unavoidable release and increased environmental concentration. Depending on morphological and surface properties, QDs at the nano-bio interface considerably impact the activity and structure of bio-molecules. The present study investigates the interaction of metalloenzyme jack bean urease (JBU) and bi-sized CdSe QDs (2.43nm and 3.63nm), surface-functionalized to mercaptopropionic acid (MPA) (-COOH), L-cysteine (CYS), L-glutathione (GSH), N-acetyl L-cysteine (NAC) (-COOH, -NH2), and cysteamine hydrochloride (CYST) (-NH2) to assess any alterations in JBU's binding, microenvironment, structure, exciton lifetime, and activity. JBU catalyzes the hydrolysis of urea to produce ammonia and carbon dioxide; any changes in its properties could threaten the survival of several microbes and plants. Spectroscopy techniques such as UV-Vis, fluorescence, circular dichroism, synchronous, time-resolved fluorescence, atomic force microscopy, and JBU activity assay were studied. Results suggested highly spontaneous and energy-favored interactions, which involved static quenching and hydrophobic forces of varied magnitude, dependent on QDs properties. The size, surface modifications, and dosage of QDs significantly impacted the secondary structure and activity of JBUs. Even though the larger sizes of the relevant modifications demonstrated stronger binding, the smaller sizes had the greatest impact on α-helicity and activity. CYST-capped QDs with an average number of the binding site (n) = 1, reduced α-helicity by 16% and activity by 22-30% at 7nM concentration. In contrast, MPA-capped QDs with n < 1 had the least effect on α-helical structure and activity. The smaller GSH-capped QDs increased the activity by 9%, via partially restoring JBU's α-helical content. The study thus thoroughly analyzed the impact of varied-size and surface-functionalized QDs on the structure and function of JBU, which can be exploited further for several biomedical applications.

Steady-state dc transport through an Anderson impurity coupled to leads with spin-orbit coupling

We study the steady-state dc transport characteristics of a system comprised of an interacting quantum dot, modelled as an Anderson impurity, coupled to two metallic non-interacting leads with Rashba SOC, using an interpolative perturbative approach (IPA). The single-particle spectra, current and differential conductance are obtained in weak and strong coupling regimes over a wide range of SOC and bias values. Extensive benchmarking of the IPA validates the method in the linear as well as non-linear response regime. The universal, zero bias ($V_{sd}=0$) peak with a width proportional to the Kondo scale ($T_K$) and two non-universal finite bias peaks around $V_{sd}=\pm U$ in the zero temperature differential conductance show a clear separation with increasing $U$ or increasing SOC. In the strong coupling regime, increasing temperature induces melting of the zero bias peak leading to a crossover from a three-peak conductance to a two-peak conductance. Recent experiments find the emergence of a two-peak structure by increasing SOC at a fixed temperature. Our results appear to provide a qualitative explanation of these observations as a SOC tuned crossover from weak/intermediate to strong coupling, and a simultaneous crossover from low $T/T_K$ to high $T/T_K$ ratio. We also reproduce the experimentally observed temperature dependence of the zero bias conductance.

Entanglement between quantum dots electronic spins and circular polarized cavity photons due to the spin–orbit interaction

A protocol for entanglement formation between electronic spins in the system of interacting quantum dots and circular polarized cavity photons was suggested. The entangled states are formed due to the spin–orbit interaction in the system consisting of two different types of quantum dots spatially arranged in a diamond like configuration resonantly interacting with left and right circular polarized cavity field. Time evolution of the degree of entanglement (concurrence) in the presence of interaction of electrons localized in the quantum dots with fermionic reservoir and cavity leakage was obtained.

Josephson Diode Effect in Supercurrent Interferometers.

A Josephson diode is a nonreciprocal circuit element that supports a larger dissipationless supercurrent in one direction than in the other. In this Letter, we propose a class of Josephson diodes based on supercurrent interferometers composed of Andreev bound state Josephson junctions or interacting quantum dot Josephson junctions, which are not diodes themselves but possess nonsinusoidal current-phase relations. We show that such Josephson diodes have several important advantages, like being electrically tunable and requiring only time-reversal breaking by a magnetic flux. We also show that our diodes have a characteristic ac response, revealed by the Shapiro steps. Even the simplest realization of our Josephson diode paradigm that relies on only two junctions can achieve efficiencies of up to ∼40% and, interestingly, far greater efficiencies are achievable by concatenating interferometer loops. We hope that our Letter will stimulate the search for highly tunable Josephson diode effects in Josephson devices based semiconductor-superconductor hybrids, 2d materials, and topological insulators, where nonsinusoidal current-phase relations were recently observed.

Creating and detecting poor man's Majorana bound states in interacting quantum dots

We propose and theoretically investigate an alternative way to create the poor man's Majorana bound states (MBSs) introduced in Phys. Rev. B 86, 134528 (2012). Our proposal is based on two quantum dots (QDs) with strong electron-electron interactions that couple via a central QD with proximity-induced superconductivity. In the presence of spin-orbit coupling and a magnetic field, gate control of all three QDs allows tuning the system into sweet spots with one MBS localized on each outer dot. We quantify the quality of these MBSs and show how it depends on the Zeeman energy and interaction strength. We also show how nonlocal transport spectroscopy can be used to identify sweet spots with high MBS quality. Our results provide a path for investigating MBS physics in a setting that is free of many of the doubts and uncertainties that plague other platforms.

Dynamical Behavior of Two Interacting Double Quantum Dots in 2D Materials for Feasibility of Controlled-NOT Operation.

Two interacting double quantum dots (DQDs) can be suitable candidates for operation in the applications of quantum information processing and computation. In this work, DQDs are modeled by the heterostructure of two-dimensional (2D) MoS2 having 1T-phase embedded in 2H-phase with the aim to investigate the feasibility of controlled-NOT (CNOT) gate operation with the Coulomb interaction. The Hamiltonian of the system is constructed by two models, namely the 2D electronic potential model and the matrix model whose matrix elements are computed from the approximated two-level systems interaction. The dynamics of states are carried out by the Crank–Nicolson method in the potential model and by the fourth order Runge–Kutta method in the matrix model. Model parameters are analyzed to optimize the CNOT operation feasibility and fidelity, and investigate the behaviors of DQDs in different regimes. Results from both models are in excellent agreement, indicating that the constructed matrix model can be used to simulate dynamical behaviors of two interacting DQDs with lower computational resources. For CNOT operation, the two DQD systems with the Coulomb interaction are feasible, though optimization of engineering parameters is needed to achieve optimal fidelity.

Pit rim decomposition into multiple quantum dots on surfaces of epitaxial thin films grown on pit-patterned substrates

We report results of dynamical simulations according to an experimentally validated surface morphological evolution model on the formation of regular arrays of quantum dot molecules (QDMs) consisting of 1D arrays of smaller interacting quantum dots (QDs). These QD arrays form along the sides of each pit rim on the surface of a coherently strained thin film epitaxially deposited on a semiconductor substrate, the surface of which consists of a periodic pattern of inverted prismatic pits with rectangular pit openings. We find that this complex QDM pattern results from the decomposition of the pit rim from a “quantum fortress” with four elongated QDs into four 1D arrays of multiple smaller QDs arranged along each side of the pit rim. Systematic parametric analysis indicates that varying the pit opening dimensions and the pit wall inclination directly impacts the number of QDs in the resulting QDM pattern, while varying the pit depth only affects the dimensions of the QDs in the nanostructure pattern. Therefore, the number, arrangement, and sizes of QDs in the resulting pattern of QDMs on the film surface can be engineered precisely by proper tuning of the pit design parameters. Our simulation results are supported by predictions of morphological stability analysis, which explains the pit rim decomposition into multiple QDs as the outcome of a tip-splitting instability and provides a fundamental characterization of the post-instability nanostructure pattern. Our theoretical findings can play a vital role in designing optimal semiconductor surface patterns toward enabling future nanofabrication technologies.

Semiconductor Quantum Dots as Target Analytes: Properties, Surface Chemistry and Detection.

Since the discovery of Quantum Dots (QDs) by Alexey I. Ekimov in 1981, the interest of researchers in that particular type of nanomaterials (NMs) with unique optical and electrical properties has been increasing year by year. Thus, since 2009, the number of scientific articles published on this topic has not been less than a thousand a year. The increasing use of QDs due to their biomedical, pharmaceutical, biological, photovoltaics or computing applications, as well as many other high-tech uses such as for displays and solid-state lighting (SSL), has given rise to a considerable number of studies about its potential toxicity. However, there are a really low number of reported studies on the detection and quantification of QDs, and these include ICP–MS and electrochemical analysis, which are the most common quantification techniques employed for this purpose. The knowledge of chemical phenomena occurring on the surface of QDs is crucial for understanding the interactions of QDs with species dissolved in the dispersion medium, while it paves the way for a widespread use of chemosensors to facilitate its detection. Keeping in mind both human health and environmental risks of QDs as well as the scarcity of analytical techniques and methodological approaches for their detection, the adaptation of existing techniques and methods used with other NMs appears necessary. In order to provide a multidisciplinary perspective on QD detection, this review focused on three interrelated key aspects of QDs: properties, surface chemistry and detection.

Interaction properties of biosynthesized cadmium sulphide quantum dots with human serum albumin: further investigation of antibacterial activities and sensing applications.

The synthesis of small-sized quantum dots (QDs) (1-10 nm) via the green route has garnered great interest regarding their prospective use in many biological applications (diagnosis, drug delivery and in vivo sensing); this is difficult to achieve using chemical synthesis methods, which produce larger size QD particles and also require hazardous reagents. Here, we synthesized biogenic cadmium sulphide (CdS) QDs using green tea extract as the reducing agent to produce particles that were homogeneous and a smaller size of 2-4 nm. We also elucidated the (a) protein binding, (b) antibacterial use and (c) sensing applications of biogenic CdS QDs in this present work. The biosynthesized CdS QDs were found to have extensive antibacterial activity against both Gram-negative Escherichia coli and Gram-positive Enterococcus faecalis bacterial strains. The introduction of QDs in biological medium can lead to the formation of protein-QD complexes; therefore we investigated the binding interaction of CdS QDs with the carrier protein human serum albumin (HSA) in vitro. The synthesized CdS QDs quenched the intrinsic fluorescence of HSA through a static quenching mechanism and the binding constant (Kb ) was in the order of 104 M-1 . It was also observed that the presence of biogenic CdS QDs affected the HSA-ligand interactions in vitro. The synthesized CdS made highly effective sensors for tetracycline, rifampicin, and bilirubin with limit of detection (LOD) values of 99, 141 and 29 ng/ml, respectively.

Failure of the mean-field description of magnetic fluctuations in the superconducting quantum dot

The zero-temperature physics of interacting quantum dots attached to superconducting leads is now well understood. The overall qualitative picture is obtained from the static mean-field approximation. The situation drastically changes at non-zero temperatures. No reliable solutions apart from numerical simulations exist there. We show that any static mean-field approximation fails at non-zero temperatures since magnetic fluctuations induce dynamical corrections that lead to broadening of the in-gap state energies to energy bands. Spin-symmetric equilibrium state at non-zero temperatures is unstable with respect to magnetic fluctuations and the zero magnetic field can be reached only as a weak limit of the spin-polarized solution like in a magnetically ordered phase.

Qubit based on spin-singlet Yu-Shiba-Rusinov states

The local magnetic moment of an interacting quantum dot occupied by a single electron can be screened by binding a Bogoliubov quasiparticle from a nearby superconductor. This gives rise to a long-lived discrete spin-singlet state inside the superconducting gap, known as the Yu-Shiba-Rusinov (YSR) state. We study the nature of the subgap states induced by a quantum dot embedded between two small superconducting islands. We show that this system has two spin-singlet subgap states with different spatial charge distributions. These states can be put in a linear superposition and coherently manipulated using electric-field pulses applied on the gate electrode. Such a YSR qubit could be implemented using present-day technology.

Enhanced second-order sideband via surface plasmon polaritons in a low-Q microcavity

We investigate the creation of the optical second-order sideband (OSS) from a hybrid system consisting of a single quantum dot (QD), a spherical metallic nanoparticle, and a low-Q microcavity driven with a weak probe and a strong control field. Beyond the conventional linearized description, we solve the nonlinear Heisenberg–Langevin equations for achieving the nonlinear coefficient of the OSS by employing the perturbation technique. It is shown that the hybrid of plasmon and cavity modes induces the formation of the surface plasmon polaritons, resulting in a controllable interaction that can be enhanced into a strong coupling regime even if the cavity–QD interaction is initially in the weak-coupling regime. As a result, enhanced OSS generation and the splitting of the transmission spectra can be observed. In addition, it is also shown that the size of the metal nanoparticle (MNP) has a profound effect on the output OSS spectrum, which might form a precision self-referenced detection scheme for extracting size information of the MNP.

Non-Hermitian many-body topological excitations in interacting quantum dots

Quantum dots are one of the paradigmatic solid-state systems for quantum engineering, providing an outstanding tunability to explore fundamental quantum phenomena. Here we show that non-Hermitian many-body topological modes can be realized in a quantum dot chain by utilizing a gate-tunable modulation of dissipation, and they emerge purely because of the non-Hermiticity. By exactly solving the non-Hermitian interacting description both with exact diagonalization and tensor-networks, we demonstrate that these topological modes are robust even in the presence strong interactions, leading to a strongly correlated topological many-particle state. Our results put forward quantum dot arrays as a platform for engineering non-Hermitian many-body topological modes, and highlight the resilience of non-Hermitian topology to electronic interactions.

General and Efficient C–P Bond Formation by Quantum Dots and Visible Light

General and Efficient C–P Bond Formation by Quantum Dots and Visible Light

Cross-correlation spectra in interacting quantum dot systems

Two-color spin-noise spectroscopy of interacting electron spins in singly charged semiconductor quantum dots provides information on the interquantum dot interactions. We investigate the spin cross-correlation function in a quantum dot ensemble employing a modified semiclassical approach. Spin-correlation functions are calculated using a Hamilton quaternion approach that maintains local quantum mechanical properties of the spins. This method takes into account the effects of the nuclear-electric quadrupolar interactions, the randomness of the coupling constants, and the variation of the electron $g$ factor on the spin-noise power spectra. We demonstrate that the quantum dot ensemble can be mapped on an effective two-quantum dot problem and discuss how the characteristic length scale of the interdot interaction modifies the low-frequency cross-correlation spectrum. We argue that details on the interaction strength distribution can be extracted from the cross-correlation spectrum when applying a longitudinal or a transversal external magnetic field.

Subgap states in superconducting islands

We study an interacting quantum dot in contact with a superconducting island described by the Richardson model with a Coulomb repulsion term controlling the number of electrons on the island. This Hamiltonian admits a compact matrix-product-operator representation and can be efficiently and accurately solved using the density-matrix renormalization group. We systematically explore the effects of the charging energy $E_c$. For $E_c$ comparable to the superconducting gap $\Delta$, the subgap states are stabilized by the combination of Kondo exchange coupling and charge redistribution driven by the Coulomb interaction. The subgap states exist for both even and odd superconductor ground-state occupancy, but with very distinctive excitation spectra in each case. The spectral peaks are not symmetric with respect to the chemical potential and may undergo discontinuous changes as a function of gate voltages.

Interactions of hydrophilic quantum dots with defect-free and defect containing supported lipid membranes

Quantum dots (QDs) are semiconductor nanoparticles with unique optical and electronic properties, whose interest as potential nano-theranostic platforms for imaging and sensing is increasing. The design and use of QDs requires the understanding of cell-nanoparticle interactions at a microscopic and nanoscale level. Model systems such as supported lipid bilayers (SLBs) are useful, less complex platforms mimicking physico-chemical properties of cell membranes. In this work, we investigated the effect of topographical homogeneity of SLBs bearing different surface charge in the adsorption of hydrophilic QDs. Using quartz-crystal microbalance, a label-free surface sensitive technique, we show significant differences in the interactions of QDs onto homogeneous and inhomogeneous SLBs formed following different strategies. Within short time scales, QDs adsorb onto topographically homogeneous, defect-free SLBs is driven by electrostatic interactions, leading to no layer disruption. After prolonged QD exposure, the nanomechanical stability of the SLB decreases suggesting nanoparticle insertion. In the case of inhomogeneous, defect containing layers, QDs target preferentially membrane defects, driven by a subtle interplay of electrostatic and entropic effects, inducing local vesicle rupture and QD insertion at membrane edges.

Enhancement of fluorescence of single quantum dots by encasing in semiconductor and metal nanoparticles

Quantum dots (QDs) are widely investigated in the field of optoelectronics due to their various unique spectral and excellent fluorescence properties. However, QDs suffer from intermittent fluorescence, also known as blinking, that limits their use in optoelectronic devices. The blinking mechanism can be suppressed by numerous processes, and one such process includes the interaction of the QDs with semiconductor nanoparticles (NPs) such as indium tin oxide (ITO) and titanium dioxide (TiO2). By encapsulating the QDs in these NPs, the blinking rate is significantly reduced due to the electron transfer pathway between them. The interaction of QDs with metal NPs such as silver (Ag) and gold (Au) also greatly enhances the fluorescence behavior due to energy transfer and plasmonic effects. This work deals with the electron transfer model that analyzes the effect of radiative recombination, non-radiative recombination, and electron transfer between QDs and the NPs. An analysis of the on and off states for QDs under the influence of considered NPs has also been done. The on and off time for QDs have also been studied, which provide a comprehensive framework of the performance of the QDs interfaced with these NPs. A comparison between the QDs interacting with glass and other semiconductor and metal NPs is also drawn to compare the efficacy of QDs under the influence of different NPs. This analysis postulates the physical mechanism for blinking and ways to curb these mechanisms using the semiconductor and metal NPs. The theoretical study demonstrates the quantitative insights and prerequisites for designing QD-based optoelectronic devices.

Influence of zinc doping on the molecular biocompatibility of cadmium-based quantum dots: Insights from the interaction with trypsin

Doping quantum dots (QDs) with extra element presents a promising future for their applications in the fields of environmental monitoring, commercial products and biomedical sciences. However, it remains unknown for the influence of doping on the molecular biocompatibility of QDs and the underlying mechanisms of the interaction between doped-QDs and protein molecules. Using the “one-pot” method, we synthesized N-acetyl-l-cysteine capped CdTe: Zn2+ QDs with higher fluorescence quantum yield, improved stability and better molecular biocompatibility compared with undoped CdTe QDs. Using digestive enzyme trypsin (TRY) as the protein model, the interactions of undoped QDs and Zn-doped QDs with TRY as well as the underlying mechanisms were investigated using multi-spectroscopy, isothermal titration calorimetry and dialysis techniques. Van der Waals forces and hydrogen bonds are the major driving forces in the interaction of both QDs with TRY, which leading to the loosening of protein skeleton and tertiary structural changes. Compared with undoped QDs, Zn-doped QDs bind less amount of TRY with a higher affinity and then release higher amount of Cd. Zn-doped QDs have a less stimulating impact on TRY activity by decreasing TRY binding and reducing Cd binding to TRY. Taken all together, Zn-doped QDs offer a safer alternative for the applications of QDs by reducing unwanted interactions with proteins and improving biocompatibility at the molecular level.

Photon blockade in a coupled double quantum dot—nonlinear optomechanical system

We propose a scheme to generate strongly sub-Poissonian light via photon blockade in a photonic crystal nanocavity embedded with a χ (2) nonlinear material and two interacting quantum dots (QD). The second- order correlation function at zero time delay g (2)(0) in our scheme can be reduced to g (2)(0) ∼ 10−3 by a large χ (2) nonlinearity and by optimizing the QD and optical cavity detuning. Our proposal provides a highly tunable and efficient method for single photon source on demand which has potential applications in solid state based quantum information processing.