Graphene Quantum Dot‑Mediated Atom‑Layer Semiconductor Electrocatalyst for Hydrogen Evolution

The hydrogen evolution reaction performance of semiconducting 2H-phase molybdenum disulfide(2H-MoS_(2))presents a significant hurdle in realizing its full potential applications.Here,we utilize theoretical calculations to predict possible functionalized graphene quantum dots(GQDs),which can enhance HER activity of bulk MoS_(2).Subsequently,we design a functionalized GQD-induced in-situ bottom-up strategy to fabricate near atom-layer 2H-MoS_(2) nanosheets mediated with GQDs(ALQD)by modulating the concentration of electron withdrawing/donating functional groups.Experimental results reveal that the introduction of a series of functionalized GQDs during the synthesis of ALQD plays a crucial role.Notably,the higher the concentration and strength of electron-withdrawing functional groups on GQDs,the thinner and more active the resulting ALQD are.Remarkably,the synthesized near atom-layer ALQD-SO_(3)demonstrate significantly improved HER performance.Our GQD-induced strategy provides a simple and efficient approach for expanding the catalytic application of MoS_(2).Furthermore,it holds substantial potential for developing nanosheets in other transition-metal dichalcogenide materials.

Qubits based on semiconductor quantum dots

Semiconductor quantum dots are promising hosts for qubits to build a quantum processor. In the last twenty years, in- tensive researches have been carried out and diverse kinds of qubits based on different types of semiconductor quantum dots were developed. Recent advances prove high fidelity single and two qubit gates, and even prototype quantum algorithms. These breakthroughs motivate further research on realizing a fault tolerant quantum computer. In this paper we review the main principles of various semiconductor quantum dot based qubits and the latest associated experimental results. Finally the future trends of those qubits will be discussed.

Cross-Polarized Single Photon Emission from Single Semiconductor Quantum Dots with V-Type Level Driven by Pulse Field

The statistic properties of photon emissions from single semiconductor quantum dots with V-type leveldriven by pulses are investigated theoretically.Based on quantum regression theorem and master equations,the dynamicequations of the second-order correlation function of the photon emissions are deduced.The calculated results reveal thatthe efficiency of single photon emissions from two orthogonal polarization eigenstates(|x〉and |y〉)reaches the maximumwhen the input pulses area is about π,and the probability of the cross-polarized single photon emission from |x〉and |y〉decreases with increasing of pulse width.

Selective Synthesis and Advanced Characteristic of CdSe Semiconductor Quantum Dots by Aqueous Phase

This work mainly investigated the influences of some factors, such as, synthesis methods, precursor alteraatives, and vacuum heat-treating process, etc, on the fluorescent characteristics of the semiconductor quantum dots synthesized by aqueous phase. The research results indicate that the fluorescent characteristic of water- solution sample prepared from Na2 SO3 precursor was sensitive to water bath heating time, and specially, its photohuninescence spectrum shows the unique phenomenon of double excitation and emission peaks. Meanwhile, the fluorescent characteristic of water- solution sample prepared from NaBH4 precursor is slightly influenced by water bath heating time, and the sugface of CdSe quantum dots could be passivated by the excessive amount of NaBH4 precursor, which results in the effective decrease of surface traps and great enhancement of quantum yield. Furthermore, the fluorescent emission peaks of samples could be sharpened by vacuum heat-treating process, with its spectral full width at half of maximum （FWHM） around 30- 40 run, so the emission peaks become redshifi, of which the intensity greatly increases.

Spin manipulation in semiconductor quantum dots qubit

Thirty years of effort in semiconductor quantum dots has resulted in significant developments in the control of spin quantum bits（qubits）. The natural two-energy level of spin states provides a path toward quantum information processing. In particular, the experimental implementation of spin control with high fidelity provides the possibility of realizing quantum computing. In this review, we will discuss the basic elements of spin qubits in semiconductor quantum dots and summarize some important experiments that have demonstrated the direct manipulation of spin states with an applied electric field and/or magnetic field. The results of recent experiments on spin qubits reveal a bright future for quantum information processing.

Single Photon Scattering in a Pair of Waveguides Coupled by a Whispering-Gallery Resonator Interacting with a Semiconductor Quantum Dot

The single photon scattering properties in a pair of waveguides coupled by a whispering-gallery resonator in- teracting with a semiconductor quantum dot are investigated theoretically. The two waveguides support four possible ports for an incident single photon. The quantum dot is considered a V-type system. The incident direction-dependent single photon scattering properties are studied and equal-output probability from the four ports for a single photon incident is discussed. The influences of backscattering between the two modes of the whispering-gallery resonator for incident direction-dependent single photon scattering properties are also pre- sented.

Quantum-dot Semiconductor Optical Amplifiers in State Space Model

A state space model(SSM) is derived for quantum-dot semiconductor optical amplifiers(QD-SOAs).Rate equations of QD-SOA are formulated in the form of state update equations,where average occupation probabilities along QD-SOA cavity are considered as state variables of the system.Simulations show that SSM calculates QD-SOA′s static and dynamic characteristics with high accuracy.

High-Performance Perovskite Quantum Dot Solar Cells Enabled by Incorporation with Dimensionally Engineered Organic Semiconductor

Perovskite quantum dots(PQDs)have been considered promising and effective photovoltaic absorber due to their superior optoelectronic properties and inherent material merits combining perovskites and QDs.However,they exhibit low moisture stability at room humidity(20-30%)owing to many surface defect sites generated by inefficient ligand exchange process.These surface traps must be re-passivated to improve both charge transport ability and moisture stability.To address this issue,PQD-organic semiconductor hybrid solar cells with suitable electrical properties and functional groups might dramatically improve the charge extraction and defect passivation.Conventional organic semiconductors are typically low-dimensional(1D and 2D)and prone to excessive self-aggregation,which limits chemical interaction with PQDs.In this work,we designed a new 3D star-shaped semiconducting material(Star-TrCN)to enhance the compatibility with PQDs.The robust bonding with Star-TrCN and PQDs is demonstrated by theoretical modeling and experimental validation.The Star-TrCN-PQD hybrid films show improved cubic-phase stability of CsPbI_(3)-PQDs via reduced surface trap states and suppressed moisture penetration.As a result,the resultant devices not only achieve remarkable device stability over 1000 h at 20-30%relative humidity,but also boost power conversion efficiency up to 16.0%via forming a cascade energy band structure.

Energy Structure of Two-Dimensional Graphene-Semiconductor Quantum Dot

Graphene is a newly discovered material that possesses unique electronic properties. It is a two-dimensional singlelayered sheet in which the electrons are free and quasi-relativistic. These properties may open a door for many new electronic applications. In this paper we proposed a flat 2-dimensional circular graphene-semiconductor quantum dot. We have carried out theoretical studies including deriving the Dirac equation for the electrons inside the graphene-semiconductor quantum dot and solving the equation. We have established the energy structure as a function of the rotational quantum number and the size (radius) of the dot. The energy gap between the energy levels can be tuned with the radius of the quantum dot. It could be useful for quantum computation and single electron device application.

Size-dependent optical properties and carriers dynamics in CdSe/ZnS quantum dots

Size-dependence of optical properties and energy relaxation in CdSe/ZnS quantum dots （QDs） were investigated by two-colour femtosecond （fs） pump-probe （400/800 nm） and picosecond time-resolved photoluminescence （ps TRPL） experiments. Pump-probe measurement results show that there are two components for the excited carriers relaxation, the fast one with a time constant of several ps arises from the Auger-type recombination, which shows almost particle sizeindependence. The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence. The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm. The room-temperature decay lifetime is due to the thermal mixing of bright and dark excitons, and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.

Circuit quantum electrodynamics with a quadruple quantum dot

In this theoretical work,we describe a mechanism for the coupling between a plane structure consisting of four quantum dots and a resonator.We systematically study the dependence of the quadruple coupling strength and the qubit decoherence rate and point out the optimized operating position of the hybrid system.According to the transmission given by the input-output theory,the signatures in the resonator spectrum are predicted.Furthermore,based on the parameters already achieved in previous works,we prove that the device described in this paper can achieve the strong coupling limit,i.e.,this approach can be used for system extension under the existing technical conditions.Our results show an effective and promotable approach to couple quantum dot structures in plane with the resonator and propose a meaningful extension method.

Modification of the spontaneous emission of quantum dots near the surface of a three-dimensional colloidal photonic crystal

This paper demonstrates experimentally and numerically that a significant modification of spontaneous emission rate can be achieved near the surface of a three-dimensional photonic crystal. In experiments, semiconductor coreshell quantum dots are intentionally confined in a thin polymer film on which a three-dimensional colloidal photonic crystal is fabricated. The spontaneous emission rate of quantum dots is characterised by conventional and time-resolved photoluminescence （PL） measurements. The modification of the spontaneous emission rate, which is reflected in the change of spectral shape and PL lifetime, is clearly observed. While an obvious increase in the PL lifetime is found at most wavelengths in the band gap, a significant reduction in the PL lifetime by one order of magnitude is observed at the short-wavelength band edge. Numerical simulation reveals a periodic modulation of spontaneous emission rate with decreasing modulation strength when an emitter is moved away from the surface of the photonic crystal. It is supported by the fact that the modification of spontaneous emission rate is not pronounced for quantum dots distributed in a thick polymer film where both enhancement and suppression are present simultaneously. This finding provides a simple and effective way for improving the performance of light emitting devices.

Three-Electron Quantum Dots in Zero Magnetic Field

By using the exact diagonalization method,a system of three electrons confined in a parabolic quantumdot in zero magnetic field is studied.The ground-state electronic structures and orbital and spin angular momentatransitions as a function of the confined strength are investigated.We find that the confinement may cause accidentaldegeneracies between levels with different low-lying states and the inversion of the energy values.The present,resultsare useful to understanding the optical properties and internal electron-electron correlations of quantum dot materials.

Ground State Transitions of Four-Electron Quantum Dots in Zero Magnetic Field

<Abstract>In this paper,we study four electrons confined in a parabolic quantum dot in the absence of magnetic field,by the exact diagonalization method.The ground-state electronic structures and orbital and spin angular momenta transitions as a function of the confined strength are investigated.We find that the confinement may cause accidental degeneracies between levels with different low-lying states and the inversion of the energy values.The present results are useful to understand the optical properties and internal electron-electron correlations of quantum dot materials.

Simple Non-aqueous Fabrication Route for Oleic Acid Capped Luminescent Cadmium Sulphide Quantum Dots at Relatively Low Temperature

In this communication, we report a simplistic non-aqueous fabrication route for oleic-acid capped luminescent cadmium sulphide quantum dots at low temperature. The resulting quantum dots, obtained without using any complicated reagents, are cubic in phase, monodispersed and have excellent optical properties. The simplicity of this fabrication route is also promising to develop other oleic acid-capped metal sulphide quantum dots at relatively low temperature.

Perspective:optically-pumped Ⅲ–Ⅴ quantum dot microcavity lasers via CMOS compatible patterned Si (001) substrates

Direct epitaxial growthⅢ–Ⅴquantum dot(QD)structures on CMOS-compatible silicon substrates is considered as one of the most promising approaches to achieve low-cost and high-yield Si-based lasers for silicon photonic integration.However,epitaxial growth ofⅢ–Ⅴmaterials on Si encounters the following three major challenges:high density of threading dislocations,antiphase boundaries and thermal cracks,which significantly degrade the crystal quality and potential device performance.In this review,we will focus on some recent results related to InAs/GaAs quantum dot lasers on Si(001)substrates byⅢ–Ⅴ/Ⅳhybrid epitaxial growth via(111)-faceted Si hollow structures.Moreover,by using the step-graded epitaxial growth process the emission wavelength of InAs QDs can be extended from O-band to C/L-band.High-performance InAs/GaAs QD microdisk lasers with sub-milliwatts threshold on Si(001)substrates are fabricated and characterized.The above results pave a promising path towards the on-chip lasers for optical interconnect applications.

Steady-state linear optical properties and Kerr nonlinear optical response of a four-level quantum dot with phonon-assisted transition

The linear optical properties and Kerr nonlinear optical response in a four-level loop configuration GaAs/A1GaAs semiconductor quantum dot are analytically studied with the phonon-assisted transition （PAT）. It is shown that the changes among a single electromagnetically induced transparency （EIT） window, a double EIT window and the amplification of the probe field in the absorption curves can be controlled by varying the strength of PAT to. Meanwhile, double switching from the anomalous dispersion regime to the normal dispersion regime can likely be achieved by increasing the Rabi energy of the external optical control field. Furthermore, we demonstrate that the group velocity of the probe field can be practically regulated by varying the PAT and the intensity of the optical control field. In the nonlinear case, it is shown that the large SPM and XPM can be achieved as linear absorption vanishes simultaneously, and the PAT can suppress both third-order self-Kerr and the cross-Kerr nonlinear effect of the QD. Our study is much more practical than its atomic counterpart due to its flexible design and the controllable interference strength, and may provide some new possibilities for technological applications.

Recent progress in epitaxial growth of Ⅲ–Ⅴ quantum-dot lasers on silicon substrate

In the past few decades,numerous high-performance silicon(Si)photonic devices have been demonstrated.Si,as a photonic platform,has received renewed interest in recent years.Efficient Si-basedⅢ–Ⅴquantum-dot(QDs)lasers have long been a goal for semiconductor scientists because of the incomparable optical properties of Ⅲ–Ⅴcompounds.Although the material dissimilarity betweenⅢ–Ⅴmaterial and Si hindered the development of monolithic integrations for over 30 years,considerable breakthroughs happened in the 2000s.In this paper,we review recent progress in the epitaxial growth of various Ⅲ–ⅤQD lasers on both offcut Si substrate and on-axis Si(001)substrate.In addition,the fundamental challenges in monolithic growth will be explained together with the superior characteristics of QDs.