Quantum Dot Heterostructures Research Articles

Bi2WO6/Ti3C2 MXene Quantum Dots Heterostructure-Based Label-Free Photoelectrochemical Aptamer Sensor for Ultrasensitive Kanamycin Monitoring

As Kanamycin (KAN) is an amino-glycoside antibiotic and has seriously negative effects towards human health, it is urgent to develop an effective way for KAN monitoring. Herein, Bi2WO6/Ti3C2 MXene quantum dots (Bi2WO6/Ti3C2 QDs) heterostructure was synthesized by electrostatically driven assembly and hydrothermal method. On the basis of a Bi2WO6/Ti3C2 QDs heterostructure, a novel aptamer-based anodic photoelectrochemical (PEC) sensing platform was constructed. Through the formation of Au–S bond, the mercapto-group modified aptamers were immobilized as a recognition unit for KAN. Under visible light irradiation, this PEC platform exhibits excellent signal-on photocurrent in the presence of KAN. A log-linear response is observed over a KAN concentration range from 0.01 nM to 50 nM, with a detection limit of 0.005 nM. Moreover, this aptamer-sensor can accurately detect KAN in animal product samples, with recoveries ranging from 94.2% to 101.3%. Overall, this work demonstrates the considerable potential of component reconstitution in PEC biosensors for real sample analysis, providing a new perspective on PEC sensor development.

Low mass loading of graphene quantum dots on titanium nitride nanotube arrays for boosting capacity and operating voltage of symmetric supercapacitor in an aqueous electrolyte

Compared to other supercapacitors, aqueous supercapacitors offer reliability, a long lifecycle, and affordability. However, the significant challenge in advancing aqueous supercapacitors lies in their limited operating voltage window. This limitation has resulted in a notable reduction in energy density. Therefore, contemporary electrode materials exhibiting high electrochemical activity and wide operating potential windows are imperative. This study introduces a technique that not only boosts energy density but also signifies a significant breakthrough for metal nitride-based aqueous supercapacitors. This method broadens the operating voltage range and necessitates optimization of the heterostructure design. A distinctive titanium nitride/graphene quantum dot (TiN/GQD) heterostructure was synthesized through electrophoretic deposition of GQDs onto plasma-nitrided TiO2 nanotubes. This novel heterostructure demonstrates the capability to function across an extended voltage range of 2 V. It is noteworthy that the TiN/GQD nanocomposite has excellent bonding at the interface of TiOC and a large surface area, resulting in an exceptional areal capacitance of 190 mF cm−2 at a current density of 0.2 mA cm−2 in 0.5 M Na2SO4 aqueous solution. It also displays excellent cycle stability (91.5 % after 6000 galvanostatic charge-discharge cycles). Moreover, the optimum GQDs deposition time (15 min) was determined to achieve the most efficient electrochemical performance. Furthermore, a symmetric TiN/GQD-15 supercapacitor device was assembled, reaching a high energy density of 8.2 Wh kg−1 at a power density of 77 W kg−1.

Dangling Bonds as Possible Contributors to Charge Noise in Silicon and Silicon-Germanium Quantum Dot Qubits.

Spin qubits based on Si and Si1-xGex quantum dot architectures exhibit among the best coherence times of competing quantum computing technologies, yet they still suffer from charge noise that limit their qubit gate fidelities. Identifying the origins of these charge fluctuations is therefore a critical step toward improving Si quantum-dot-based qubits. Here, we use hybrid functional calculations to investigate possible atomistic sources of charge noise, focusing on charge trapping at Si and Ge dangling bonds (DBs). We evaluate the role of global and local environment in the defect levels associated with DBs in Si, Ge, and Si1-xGex alloys, and consider their trapping and excitation energies within the framework of configuration coordinate diagrams. We additionally consider the influence of strain and oxidation in charge-trapping energetics by analyzing Si and GeSi DBs in SiO2 and strained Si layers in typical Si1-xGex quantum dot heterostructures. Our results identify that Ge dangling bonds are more problematic charge-trapping centers both in typical Si1-xGex alloys and associated oxidation layers, and they may be exacerbated by compositional inhomogeneities. These results suggest the importance of alloy homogeneity and possible passivation schemes for DBs in Si-based quantum dot qubits and are of general relevance to mitigating possible trap levels in other Si, Ge, and Si1-xGex-based metal-oxide-semiconductor stacks and related devices.

Valley-dependent tunneling through electrostatically created quantum dots in heterostructures of graphene with hexagonal boron nitride

Valley-dependent tunneling through electrostatically created quantum dots in heterostructures of graphene with hexagonal boron nitride

Atomistic approach to the strain field in finite-sized heterostructures

The numerical code AS_LBFG_v2 developed to simulate the atomic elastic relaxation in finite-sized heterostructures is introduced. To minimize the elastic energy the L-BFGS algorithm is utilized. A user-defined initial atomic configuration, without restrictions on the shape of the matrix embedding quantum dot (QD), can be implemented in the code. By enlarging the pyramidal QD heterostructure towards mesoscopic size, the code generates a strain field distribution similar to that obtained by continuum model for weakly polar semiconductors. The strain field obtained for semi-torus, and spherical shape QDs embedded in finite-sized matrices is also presented. The dependence of the strain field on the cap thickness and the distribution of misfit dislocations in the relaxed configuration are simulated by the code. Program summaryProgram titles:AS_LBFG_v2, AS_Calc_v2CPC Library link to program files:https://doi.org/10.17632/53dshkskct.1Code Ocean capsule:https://codeocean.com/capsule/5342364/tree/v1 and https://codeocean.com/capsule/9050452/tree/v1Licensing provisions: MIT licenseProgramming language: FORTRANNature of problem: An initial constraint atomic configuration of a heterostructure is relaxed by imposing Keating's elastic interaction. The strain field and distribution of misfit dislocations are simulated for various quantum dot shapes and heterostructure materials.Solution method: The elastic energy of the heterostructure is minimized by an L-BFGS algorithm. The strain field is calculated by a discrete mathematical expression at each atomic location.

Improvement in Performance of InAs Surface Quantum Dot Heterostructure-Based H2S Gas Sensor by Introducing Buried Quantum Dot Layer

In this work, we have demonstrated InAs surface quantum dot (SQD) based H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S gas sensors. The epitaxial growth of the strain-coupled and uncoupled InAs/GaAs QD heterostructures is done using the solid-source Molecular Beam Epitaxy (MBE) tool. For both types of heterostructures, the coverage of the InAs monolayer (ML) for the SQD layer varies from 0.9 ML to 2 ML. The ML coverage of the buried quantum dots (BQDs) layer for the coupled heterostructures is kept constant (2.7 ML). The atomic force microscopy results demonstrated that the coupled heterostructures have higher quantum dot density in the SQDs layer in comparison to the uncoupled one due to strain propagation from the BQDs towards the SQD layer. The sensor fabricated using the coupled heterostructure with 2 ML SQDs has demonstrated better performance than the uncoupled one for various concentrations (1 ppm -1000 ppm) of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S gas due to inter-dot carrier tunneling between BQDs and SQDs layer. The coupled InAs gas sensor showed the best sensing properties at room temperature (45.9% sensor response @ 100ppm H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S). We have demonstrated the selectivity of the sensor towards H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S among various target gasses like CO, CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O, and NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and the stability over a longer period of time with only 3% deviation (within acceptable limit). These findings have the potential to promote the fabrication of high-performance gas sensors using SQDs-based coupled heterostructures.

Quantum dot heterostructures on N-doped graphene with accelerated diffusion kinetics for stable lithium-ion storage

The high energy density and low self-discharge rate of lithium-ion batteries make them promising for large-scale energy storage. However, the practical development of such electrochemical energy storage systems relies heavily on the development of anode materials with high multiplier capacity and stable cycle life. Here, a simple and efficient one-step hydrothermal method is used to obtain stannide heterostructures, which are loaded on N-doped graphene (SnS2/SnO2@NG) that promotes Li+ diffusion for fast charge transfer. It is demonstrated that the built-in electric field generated by the electron transfer from electron-rich SnS2 to SnO2 in the stannide heterojunction collectively provides abundant cation adsorption sites, accelerating the migration of Li+ thus improving the electrochemical reaction kinetics. Besides, the SnS2/SnO2 nanoparticles have high structural stability, and the heterojunction compressive stresses obtained from density functional theory (DFT) calculations can significantly limit the structural damage. When applied as anodes in Li+ batteries with 300 cycles at 0.5 A/g, we achieved a high reversible capacity of 892.73 mAh/g. The rational design of low-cost batteries for energy storage and conversion can benefit from the quantitative design of fast and persistent charge transfer in a stannide heterostructure.

State of the Art of Continuous and Atomistic Modeling of Electromechanical Properties of Semiconductor Quantum Dots.

The main intent of this paper is to present an exhaustive description of the most relevant mathematical models for the electromechanical properties of heterostructure quantum dots. Models are applied both to wurtzite and zincblende quantum dot due to the relevance they have shown for optoelectronic applications. In addition to a complete overview of the continuous and atomistic models for the electromechanical fields, analytical results will be presented for some relevant approximations, some of which are unpublished, such as models in cylindrical approximation or a cubic approximation for the transformation of a zincblende parametrization to a wurtzite one and vice versa. All analytical models will be supported by a wide range of numerical results, most of which are also compared with experimental measurements.

Graphene/III–V Quantum Dot Mixed-Dimensional Heterostructure for Enhanced Radiative Recombinations via Hole Carrier Transfer

Fabrication of high quantum efficiency nanoscale device is challenging due to increased carrier loss at surface. Low dimensional materials such 0D quantum dots and 2D materials have been widely studied to mitigate the loss. Here, we demonstrate a strong photoluminescence enhancement from graphene/III-V quantum dot mixed-dimensional heterostructures. The distance between graphene and quantum dots in the 2D/0D hybrid structure determines the degree of radiative carrier recombination enhancement from 80% to 800% compared to the quantum dot only structure. Time-resolved photoluminescence decay also shows increased carrier lifetimes when the distance decreases from 50 to 10 nm. We propose that the optical enhancement is due to energy band bending and hole carrier transfer, which repair the imbalance of electron and hole carrier densities in quantum dots. This 2D graphene/0D quantum dot heterostructure shows promise for high performance nanoscale optoelectronic devices.

One-step solution-induced impregnation synthesis of strongly coupled platinum–molybdenum/silicon carbide quantum dots heterostructures encapsulated on reduced graphene oxide sheet as electrocatalyst for hydrogen evolution reaction

Strongly coupled platinum-based transition-metal oxide/carbon hybrids and the development of quantum-dot structures of hybrid catalysts are cost-effective and maximize accessible active sites. However, a significant obstacle still exists for the rational proposal and simple synthesis of hybrid quantum-dot material catalysts. Herein, novel PtxMo1-xSiC quantum dots encapsulated in reduced graphene oxide (rGO) (PtxMo1-xSiC QDs @rGO) for catalyzing the hydrogen evolution reaction (HER) were fabricated through a simple solution-induced impregnation method. The optimized Pt5Mo95SiC QDs @rGO catalyst only require overpotentials of 18 mV and 25 mV to deliver current densities of 10 mA cm−2 and 250 mA cm−2 in acidic media, respectively. The synergistic effects of the inner PtxMo1-xSiC QDs networks and outer conductive rGO sheets that promote electron transfer are responsible for the outstanding HER performance. This work presents a novel method for producing an extremely effective HER catalyst for applications on large-scale.

Enhanced optical properties and dark I-V characteristics of silicon nanowire-carbon quantum dots heterostructures

Herein, we report a silicon nanowire (SiNW) array-carbon quantum dot (CQD) heterostructure photovoltaic device via direct coating of CQD on chemically-etched SiNW arrays aided by Ag. By using carbon quantum dots layer as a competent element for surface passivation and modification for SiNWs, the solar cells efficiency is improved. The 1.6 times absorption enhancement has been recorded for nitrogen doped CQD decorated pyramidal SiNW heterostructure in comparison to that of CQDs coated silicon nanowires on the planar surfaces. Inclusion of nitrogen doped CQDs into the pyramidal SiNW arrays gives enhanced absorption intensity which can act as a good absorber layer in solar cell. The heterostructure also displays a significant photoluminescence in the blue region as probed by using time-resolved photoluminescence (TRPL) technique which provide the insight into the recombination mechanism in the synthesized heterostructures and is discussed in detail.

Electron and hole spectrum taking into account deformation and polarization in the quantum dot heterostructure InAs/GaAs

In the paper InAs spherical quantum dots in a GaAs matrix were investigated. The energies of electrons and holes in single- and multi-band models (with strong, weak, and intermediate spin-orbit interaction) were calculated taking into account both the deformation of the quantum-dot matrix and the polarization charges on the quantum dot surface. The dependence of the energy levels of electrons and holes on the radius of the quantum dot is considered. It is shown that the deformation effects are stronger than polarization for the electron. For holes those effects are opposites. The energies of electrons and holes have been compared in all approximation models.

Molecular Collapse States in Graphene/WSe_{2} Heterostructure Quantum Dots.

In relativistic physics, both atomic collapse in a heavy nucleus and Hawking radiation in a black hole are predicted to occur through the Klein tunneling process that couples particles and antiparticles. Recently, atomic collapse states (ACSs) were explicitly realized in graphene because of its relativistic Dirac excitation with a large "fine structure constant." However, the essential role of the Klein tunneling in the ACSs remains elusive in experiment. Here we systematically study the quasibound states in elliptical graphene quantum dots (GQDs) and two coupled circular GQDs. Bonding and antibonding molecular collapse states formed by two coupled ACSs are observed in both systems. Our experiments supported by theoretical calculations indicate that the antibonding state of the ACSs will change into a Klein-tunneling-induced quasibound state revealing deep connection between the ACSs and the Klein tunneling.

All-Optically Controlled Artificial Synapses Based on Light-Induced Adsorption and Desorption for Neuromorphic Vision.

Artificial synapses with the capability of optical sensing and synaptic functions are fundamental components to construct neuromorphic visual systems. However, most reported artificial optical synapses require a combination of optical and electrical stimuli to achieve bidirectional synaptic conductance modulation, leading to an increase in the processing time and system complexity. Here, an all-optically controlled artificial synapse based on the graphene/titanium dioxide (TiO2) quantum dot heterostructure is reported, whose conductance could be reversibly tuned by the effects of light-induced oxygen adsorption and desorption. Synaptic behaviors, such as excitatory and inhibitory, short-term and long-term plasticity, and learning-forgetting processes, are implemented using the device. An artificial neural network simulator based on the artificial synapse was used to train and recognize handwritten digits with a recognition rate of 92.2%. Furthermore, a 5 × 5 optical synaptic array that could simultaneously sense and memorize light stimuli was fabricated, mimicking the sensing and memory functionality of the retina. Such an all-optically controlled artificial synapse shows a promising prospect in the application of perception, learning, and memory tasks for future neuromorphic visual systems.

Surface Stoichiometry Control of Colloidal Heterostructured Quantum Dots for High-Performance Photoelectrochemical Hydrogen Generation.

Manipulating the separation and transfer behaviors of charges has long been pursued for promoting the photoelectrochemical (PEC) hydrogen generation based on II-VI quantum dot (QDs), but remains challenging due to the lack of effective strategies. Herein, a facile strategy is reported to regulate the recombination and transfer of interfacial charges through tuning the surface stoichiometry of heterostructured QDs. Using this method, it is demonstrated that the PEC cells based on CdSe-(Sex S1- x )4 -(CdS)2 core/shell QDs with a proper Ssurface /Cdsurface ratio exhibits a remarkably improved photocurrent density (≈18.4mA cm-2 under one sun illumination), superior to the PEC cells based on QDs with Cd-rich or excessive S-rich surface. In-depth electrochemical and spectroscopic characterizations reveal the critical role (hole traps) of surface S atoms in suppressing the recombination of photogenerated charges, and further attribute the inferior performance of excessive S-rich QDs to the impeded charge transfer from QDs to TiO2 and electrolyte. This work puts forward a simple surface engineering strategy for improving the performance of QDs PEC cells, providing an efficient method to guide the surface design of QDs for their applications in other optoelectronic devices.

InAs quantum dot-in-a-well heterostructures with InGaAs, GaAsN and GaAsSb well using digital alloy capping layer

In this work, the authors introduced a novel approach called digital alloy capping layer (DACL) and investigated its effect on the optical and structural properties of InAs quantum dot-in-a-well (DWELL) heterostructures. In DACL, a conventional thick well layer is digitized equally with different compositions analogous to short-period-superlattice (SPS). The DACL approach's effect has been studied experimentally and theoretically on DWELL heterostructures with InxGa1-xAs as the well material. The photoluminescence (PL) study reveals that DACL observes a red-shift of ∼55 nm as compared to AACL approached heterostructures. High-resolution X-ray diffraction (HRXRD) results reveal higher In-content, controlled In-out diffusion from InAs QD, and improved in-plane strain in DACL samples compared to the analog sample. The study has been extended to QD heterostructures with GaAs1-xNx and GaAs1-ySby as well materials, and comprehensive analysis has been carried out. Hence, the DWELL heterostructures with the DACL approach can be utilized to fabricate infrared photodetector devices.

Borophene and BC2N quantum dot heterostructures: ultrasensitive humidity sensing and multifunctional applications

Hydrogenated borophene and BC2N quantum dot heterostructures have been synthesized via facile ultrasonication. The sensitivity of the fabricated borophene-BC2N sensor is the highest among all the reported typical 2D-material chemiresistive sensors.

Theoretical Study on Vertically Coupled Stranski-Krastanov (SK) on Sub Monolayer (SML) InAs Quantum Dot Heterostructures Employing Ternary and Quaternary Barrier Materials

This work presents the impact of barrier spacer on the structural and optical properties of strain-coupled Stranski-Krastanov (SK) on Sub Monolayer (SML) quantum dot (QD) heterostructures. Various ternary and quaternary materials have been employed as the barrier layer of SK-SML QD heterostructures. In the coupled SK-SML QDs, the residual strain propagates from the SML seed layer towards the top SK dots, introducing defects and dislocations. After employing the ternary (GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> ) and quaternary (In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.21</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-.0.21-x</sub> As and In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.18</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.82</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> ) materials, the residual strain reduces, reducing the defects which thereby helps in increasing the crystalline quality of the heterostructure. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Nextnano</i> software has been used to compute the structures' strain, energy band profile, probability density functions, and emission wavelength. Two strain components, viz. hydrostatic as well as biaxial strain, have been computed and compared for all the heterostructures to understand the distribution of the strain profile. The emission wavelength is red-shifted for the SK-SML QD heterostructures with ternary and quaternary materials as a barrier layer as compared to that of the GaAs barrier. Moreover, type-I and II energy band profiles are observed for Sb-based barrier material, appropriate for various optoelectronic applications. This is a comparative study of SK-SML QDs with various barrier materials helps minimize the strain and defects and improve the device performance.

ZnO–CoS heterostructure quantum dots for enhanced electrocatalytic nitrogen reduction to ammonia

In 0.1 M Na2SO4, ZnO–CoS quantum dots attained a high NH3 yield of 33.03 μg h−1 mgcat.−1 and a high FE of 11.7% at −0.20 V vs. a reversible hydrogen electrode, with high electrochemical and structural stability.

CoS2 enhanced SnO2@rGO heterostructure quantum dots for advanced lithium-ion battery anode

The dual transition metals CoS2–SnO2@ reduced graphene oxide heterostructure quantum dots (shorten as CoS2–SnO2@rGO QDs) is intentionally synthesized by hydrothermal method. The compositions, grain sizes and contents of the synthesized CoS2–SnO2@rGO QDs are identified and calculated from XRD pattern. The layered structures and crystal sizes (<10 nm) of CoS2–SnO2 heterostructure quantum dots are confirmed by TEM high-resolution image. Moreover, the CoS2–SnO2@rGO QDs deliver higher specific capacity of 940.4, 781.9, 673.8, 506.6 and 303.1 mAh g−1 at 0.2, 0.5, 1.0, 2.0 and 4.0 A g−1, respectively, and remained 521.4 mAh g−1 (300 cycles) with 98.9% Coulomb efficiency under 1.0 A g−1 cycling. The DFT (density functional theory) calculation results suggest that the Li ions should diffuse through two possible paths with relatively lower Ebar values of 0.519 and 0.566 eV in CoS2–SnO2@rGO QDs. Combining with XRD, XPS, TEM and DFT calculation, we therefore assign the excellent electrochemical performances of CoS2–SnO2@rGO QDs to its highly active CoS2–SnO2 interfaces for electrode reactivity enhancement, to the characteristic heterointerfaces for “extra” Li+ ions storage/release, and to super conductive/layered rGO network for better ionic/electronic transport. The successful synthesis of CoS2–SnO2@rGO QDs may provide a strategy for dual/multi-transition metal heterojunction to enhance lithium-ion battery electrode materials.