Nonlinear optical rectification of GaAs/Ga_(1-x)Al_(x)As quantum dots with Hulthén plus Hellmann confining potential

We investigate the nonlinear optical rectification(NOR) of spherical quantum dots(QDs) under Hulthén plus Hellmann confining potential with the external tuning elements. Energy and wavefunction are determined by using the Nikiforov–Uvarov method. Expression for the NOR coefficient is derived by the density matrix theory. The results show that the applied external elements and internal parameters of this system have a strong influence on intraband nonlinear optical properties. It is hopeful that this tuning of the nonlinear optical properties of GaAs/Ga_(1-x)Al_(x)As QDs can make a greater contribution to preparation of new functional optical devices.

Effect of In_xGa_(1-x)As Interlayer on Surface Morphology and Optical Properties of GaSb/InGaAs Type-Ⅱ Quantum Dots Grown on InP (100) Substrates

The effects of indium composition in InGaAs interlayer and on optical properties of GaSb/InGaAs QD material on morphology of GaSb/InGaAs quantum dots （QDs） system are studied. AFM images show that the change of the indium composition in InGaAs interlayer can alter the GaSb QD morphology. It is found that low indium composition in InGaAs interlayer can promote the formation of QDs, while high indium composition can inhibit the formation of QDs. The photoluminescence （PL） spectra of GaSb/InGaAs QDs at 8 K under low excitation power indicate that the third root of the excitation power is linear with the peak position, which provides a direct evidence for their luminescence belonging to type-Ⅱ material optical transition. The PL spectra at 8 K under an excitation power of 90row show that the optical properties of GaSb/InGaAs QD material system can be affected by the indium composition in the InGaAs interlayer, and the PL peak position is linear with the indium composition. The optical properties of GaSb/InGaAs QDs can be improved by adjusting the indium composition in the InGaAs interlayer.

Transport Properties of Two Coupled Quantum Dots Under Optical Pumping

Using the Keldysh-Green function,we present a theoretical study on the electron transport properties of two coupled quantum dots under optical pumping. Plateaus in the I-V curve and resonant peaks in the transmission coefficient occur and can be explained by the local electron density of states in the quantum dots. The effects of the optical pumping frequency and intensity on the transport properties of the system are also discussed. The electron dynamical localization phenomenon occurs when the optical pumping frequency is equal to the discrete hole energy level. This result can be used to realize optical control switches.

Polaron effect on the optical rectification in spherical quantum dots with electric field

The polaron effect on the optical rectification in spherical quantum dots with a shallow hydrogenic impurity in the presence of electric field is theoretically investigated by taking into account the interactions of the electrons with both confined and surface optical phonons. Besides, the interaction between impurity and phonons is also considered. Numerical calculations are presented for typical Zn1-xCdxSe/ZnSe material. It is found that the polaronic effect or electric field leads to the redshifted resonant peaks of the optical rectification coefficients. It is also found that the peak values of the optical rectification coefficients with the polaronic effect are larger than without the polaronic effect, especially for smaller Cd concentrations or stronger electric field.

MoS2 Nanosheets Sensitized with Quantum Dots for Room-Temperature Gas Sensors

The Internet of things for environment monitoring requires high performance with low power-consumption gas sensors which could be easily integrated into large-scale sensor network.While semiconductor gas sensors have many advantages such as excellent sensitivity and low cost,their application is limited by their high operating temperature.Two-dimensional(2D)layered materials,typically molybdenum disulfide(MoS2)nanosheets,are emerging as promising gas-sensing materials candidates owing to their abundant edge sites and high in-plane carrier mobility.This work aims to overcome the sluggish and weak response as well as incomplete recovery of MoS2 gas sensors at room temperature by sensitizing MoS2 nanosheets with PbS quantum dots(QDs).The huge amount of surface dangling bonds of QDs enables them to be ideal receptors for gas molecules.The sensitized MoS2 gas sensor exhibited fast and recoverable response when operated at room temperature,and the limit of NO2 detection was estimated to be 94 ppb.The strategy of sensitizing 2D nanosheets with sensitive QD receptors may enhance receptor and transducer functions as well as the utility factor that determine the sensor performance,offering a powerful new degree of freedom to the surface and interface engineering of semiconductor gas sensors.

Linear and Nonlinear Optical Properties in Spherical Quantum Dots

We calculate the energy eigenvalues and the sate functions of one-electron Quantum Dot （QD） by using a combination of Quantum Genetic Algorithm （QGA） and Hartre-Fock-Roothaan （HFR） method. The linear and the third-order nonlinear optical absorption coefficients for the 1s-1p, 1p-1d, and 1d-1f transitions are examined as a function of the incident photon energy for three different values of the stoichiometric ratio. The results show that the stoichiometric ratio, impurity, relaxation time, and dot size have great influence on the optical absorption coefficients of QDs.

Linear and Nonlinear Optical Absorptions of a Donor Impurity in Spherical Quantum Dots

An investigation of the optical properties of a hydrogenic donor in spherical parabolic quantum dots hasbeen performed by using the matrix diagonalization method.The optical absorption coefficient between the ground(L=0) and the first excited state (L=1) have been examined based on the computed energies and wave functions.The results are presented as a function of the incident photon energy for the different values of the confinement strength.These results show the effects of the quantum size and the impurity on the optical absorption coefficient of a donorimpurity quantum dot.

Preparation and Optical Properties Assessment of CdSe Quantum Dots

CdSe quantum dots (QDs) (2 - 3 nm) were synthesized by chemical precipitation method. Optical and structure properties of the products were investigated by scanning tunneling microscope (STM), X-ray diffraction (XRD), and ultra violet-visible (UV-Vis) spectrophotometer. The results show that high-quality cubic CdSe QDs were obtained. It is also obtained that temperature is one of the most important factors the affect on the particle size and optical properties of the prepared QDs samples.

Optical properties of core/shell spherical quantum dots

In this study,the effects of quantum dot size on the binding energy,radiative lifetime,and optical absorption coefficient of exciton state in both GaN/AlxGa1-xN core/shell and AlxGa1-xN/GaN inverted core/shell quantum dot structures are studied.For the GaN/AlxGa1-xN core/shell structure,the variation trend of binding energy is the same as that of radiation lifetime,both of which increase first and then decrease with the increase of core size.For AlxGa1-xN/GaN inverted core/shell structure,the binding energy decreases first and then increases with core size increasing,and the trends of radiation lifetime varying with core size under different shell sizes are different.For both structures,when the photon energy is approximately equal to the binding energy,the peak value of the absorption coefficient appears,and there will be different peak shifts under different conditions.

Synthesis and characterization of ZnS-based quantum dots to trace low concentration of ammonia

In the present work,a solution-based co-precipitation method has been adopted to synthesize pure and cobalt-doped ZnS quantum dots and characterized by XRD,SEM,TEM with EDX,FTIR and gas sensing properties.XRD analysis has shown a single phase of ZnS quantum dots having a zinc blend structure.TEM and XRD line broadening indicated that the average crystallite size in the sample is in the range of 2 to 5 nm.SEM micrographs show spherical-shaped quantum dots.FTIR studies show that cobalt has been successfully doped into the ZnS cubic lattice.EDX spectra have analyzed the elemental presence in the samples and it is evident that the spectra confirmed the presence of cobalt(Co),zinc(Zn),oxygen(O),and sulphur(S)elements only and no other impurities are observed.The ZnS-based quantum dot sensors reveal high sensitivity towards 50 ppm of ammonia vapors at an operating temperature of 70℃.Hence,ZnS-based quantum dots can be a promising and quick traceable sensor towards ammonia sensing applications with good response and recovery time.

Anisotropy Effects on Polar Optical Vibrations in a Freestanding Wurtzite Cylindrical Quantum Dot

The properties of polar optical phonon vibrations in a quasi-zero- dimensional （QOD） anisotropic wurtzite cylindrical quantum dot （QD） are analyzed based on the dielectric continuum model and Loudon＇s uniaxial crystal model. The analytical electrostatic potentials of the phonon vibrations in the systems are deduced and solved exactly. The result shows that there exist four types of polar mixing optical phonon modes in the QOD wurtzite cylindrical QD systems. The dispersive equations and electron-phonon coupling function for the quasi-confined-half-space （QC-HS） mixing modes are derived and discussed. It is found that once the radius or the height of the QD approach infinity, the dispersive equations of the QC-HS mixing modes in the QOD cylindrical QD can naturally reduce to those of the QC and HS modes in Q2D QWs or Q1D QWWs systems. This has been analyzed reasonably from both of physicM and mathematical viewpoints.

Thiourea functionalized CdSe/CdS quantum dots as a fluorescent sensor for mercury ion detection

CdSe/CdS quantum dots （QDs） functionalized by thiourea （TU） were synthesized and used as a fluorescent sensor for mercury ion detection. The TU-functionalized QDs were prepared by bonding TU via electrostatic interaction to the core/shell CdSe/CdS QDs after capping with thioglycolic acid （TGA）. It was observed that the fluorescence of the functionalized QDs was quenched upon the addition of Hg^2＋. The quantitative detection of Hg^2＋ with this fluorescent sensor could be conducted based on the linear relationship between the extent of quenching and the concentration of Hg^2＋ added in the range of 1-300 μg.L^-1, A detection limit of 0.56 μg.L^-1 was achieved. The sensor showed superior selectivity for Hg^2＋ and was successfully applied to the determination of mercury in environmental samples with satisfactory results

Second-harmonic generation in asymmetric quantum dots in the presence of a static magnetic field

The second-harmonic generation （SHG） coefficient in an asymmetric quantum dot （QD） with a static magnetic field is theoretically investigated. Within the framework of the effective-mass approximation, we obtain the confined wave functions and energies of electrons in the QD. We also obtain the SHC coefficient by the compact-density-matrix approach and the iterative method. The numerical results for the typical GaAs/AlGaAs QD show that the SHC coefficient depends strongly on the magnitude of magnetic field, parameters of the asymmetric potential and the radius of the QD. The resonant peak shifts with the magnetic field or the radius of the QD changing.

Boric Acid-Based Dual Modulation Photoluminescent Glucose Sensor Using Thioglycolic Acid-Capped CdTe Quantum Dots

Most luminescent glucose sensors based on the interaction of glucose with organic boric acids or borates.Herein,a new luminescent glucose sensor is designed using thioglycolic acid-capped CdTe quantum dots in the presence of cheap inorganic boric acid.Both peak position and intensities change upon the addition of glucose because of the interaction of boric acid with glucose and thioglycolic acid-capped CdTe quantum dots,which enables glucose detection by either color change or intensity change.The luminescent intensities change linearly with glucose concentrations in the ranges from 0.03 to 1 mM and 1–25 mM with a detection limit of 10μM(S/N=3).Moreover,glucose concentrations can be conveniently detected by color change in the range from 1 mM–25 mM.It displays a highly selective response to glucose over other interfering but biologically important saccharides,amino acids,and common ions.Graphical Abstract A thioglycolic acid-capped CdTe QD-based sensor can detect glucose with wide linear range by change in intensity or color in the presence of cheap inorganic boric acid.

Exact investigation of the electronic structure and the linear and nonlinear optical properties of conical quantum dots

Intersubband linear and third-order nonlinear optical properties of conical quantum dots with infinite barrier potential are studied. The electronic structure of conical quantum dots through effective mass approximation is determined analytically. Linear, nonlinear, and total absorption coefficients, as well as the refractive indices of GaAs conical dots, are calculated. The effects of the size of the dots and of the incident electromagnetic field are investigated. Results show that the total absorption coefficient and the refractive index of the dots largely depend on the size of the dots and on the intensity and polarization of the incident electromaenetic field.

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.

Effect of size and indium-composition on linear and nonlinear optical absorption of InGaN/GaN lens-shaped quantum dot

Based on the Schr ¨odinger equation for envelope function in the effective mass approximation, linear and nonlinear optical absorption coefficients in a multi-subband lens quantum dot are investigated. The effects of quantum dot size on the interband and intraband transitions energy are also analyzed. The finite element method is used to calculate the eigenvalues and eigenfunctions. Strain and In-mole-fraction effects are also studied, and the results reveal that with the decrease of the In-mole fraction, the amplitudes of linear and nonlinear absorption coefficients increase. The present computed results show that the absorption coefficients of transitions between the first excited states are stronger than those of the ground states. In addition, it has been found that the quantum dot size affects the amplitudes and peak positions of linear and nonlinear absorption coefficients while the incident optical intensity strongly affects the nonlinear absorption coefficients.

Novel Hybrid SiO2/ZnS Coated CdTe/CdS Quantum Dots and Their Bioapplications

Novel CdTe/CdS quantum dots(QDs)coated with a hybrid of SiO_2 and ZnS were fabricated through a simple two-step approach.The hybrid SiO_2/ZnS coated CdTe/CdS quantum dots was characterized by transmission electron microscopy(TEM),UV and fluorescence spectrometer.Results indicated that the core-shell structure gave the QDs outstanding photoluminescence properties,includinganarrowphotoluminescencespectrum,high photoluminescence(PL)quantum yield and long emission lifetime(average PL lifetime of increased from 26.4 ns to 49.1 ns).Cellular studies showed the QDs had good cytocompatibility with Hela cells as determined by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide(MTT)assay after coating SiO_2/ZnS,and also proved the feasibility of using the hybrid SiO_2/ZnS coated QDs as optical probes for in vitro cell imaging.The synthesis method of QDs is highly promising for the production of robust and functional optical probes for bio-imaging and sensing applications.

Adjusting amplitude of the stored optical solitons by inter-dot tunneling coupling in triple quantum dot molecules

We study the propagation properties of a probe field in an aligned asymmetric triple quantum dot molecule with both sides inter-dot tunneling coupling effect. It is shown that the probe field can form optical soliton due to the destructive quantum interference induced by the quantum inter-dot tunneling coupling effect. Interestingly, these optical solitons can be stored and retrieved by adjusting single or double inter-dot tunneling coupling effect, different from that light memory in the ultra-cold atom system. Furthermore, we also find that the amplitude of the stored optical soliton can be adjusted by the strength of the single or double inter-dot tunneling coupling. It is possible to improve the stability and the fidelity of the optical information in the process of the storage and retrieval in semiconductor quantum dots devices.

Dual-mode emission of single-layered graphene quantum dots in confined nanospace： Anti-counterfeiting and sensor applications

Engineering of the luminescent properties for graphene quantum dots （GQDs） presents two enormous challenges： 1） The bandgap of GQDs is mainly determined by structural defects （size, shape, and the fraction of sp2 and sp3 domains）, which results in non-stoichiometric nature; 2） the preparation methods limit the achievement of an accurate chemical structure of GQDs, leading to many controversial explanations over the relationship between the structural defects and bandgaps. Here, single-layered GQDs with an exact structure are obtained by in-situ reaction of intercalated precursors in the confined nanospace of layered double hydroxides （LDHs）. Subsequently, the structure-property relationship is uncovered, demonstrating the enhanced fluorescence and activated room temperature phosphorescence of the as-pre- pared GQDs-LDHs, which originate from synergistic effects： 1） strong confine- ment provided by the nanospace of LDHs; 2） rich O-containing functional groups on the surface of GQDs resulting from LDH catalysis. Moreover, the colorless nature and dual-emission characteristics of GQDs-LDHs satisfy the preconditions as anti-counterfeiting markers for protecting valuable documents （bank notes, commercial invoices, etc.）. Particularly, owing to the low toxicity of GQDs and the edible property of LDHs, the GQDs-LDHs/gelatin capsules could be the new generation of potential green anti-counterfeiting material in the field of food and drugs.