Correct Evaluation of the Effect of Transverse Effective Charges on Phonons in AlAs Quantum Dots

An improved valence force field model (VFFM) is suggested to calculate the phonon modes in both bulk specimens and quantum dots (QDs) of AlAs taking account of the effect of transverse effective charges (TCs) correctly.The resultant dispersions of AlAs bulk phonons are in accord better with the results carefully fitted to the experimental data by using 11-parameters rigid-ion model, than those got by ordinary VFFM, especially in the region of near F point. For AlAs QDs, TCs are evaluated bond by bond for each phonon mode of QD and its effect on the change of the force on atoms is taken into account to modify further the phonon spectrum. The frequency spectra and densities of phonon states of different irreducible representations calculated by using improved VFFM are compared with the results of ordinary VFFM. The correct evaluation of the TCs is not only important in calculating the phonon spectrum of both bulk and QD specimens accurately, but is also in the further discussion of the electron-phonon (e-ph) interaction, which can be directly related to TCs of ions in QD.     

Vibration Spectrums of Polar Interface Optical Phonons in GaAs/AlAs Cylindrical Quantum Dots

The dispersions of the top interface optical phonons and the side interface optical phonons in cylindrical quantum dots are solved by using the dielectric continuum model. Our calculation mainly focuses on the frequency dependence of the IO phonon modes on the wave-vector and quantum number in the cylindrical quantum dot system. Results reveal that the frequency of top interface optical phonon sensitively depends on the discrete wave-vector in z direction and the azimuthal quantum number, while that of the side interface optical phonon mode depends on the radial and azimuthal quantum numbers. These features are obviously different from those in quantum well, quantum well wire, and spherical quantum dot systems. The limited frequencies of interface optical modes for the large wave-vector or quantum number approach two certain constant values, and the math and physical reasons for this feature have been explained reasonably.     

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

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.     

Strong Electron-Phonon Interaction Effect in Quantum Dots

Strong-coupling polaron in a parabolic quantum dot is investigated by the Landau-Pekar variational treatment. The polaron binding energy and the average number of virtual phonons around the electron as a function of the effective confinement length of the quantum dot are obtained in Gaussian function approximation. It is shown that both the polaron binding energy and the average number of virtual phonons around the electron decrease with increasing the effective confinement length. The results indicate that the polaronic effects are more pronounced in quantum dots than those in two-dimensional and three-dimensional cases.     

Effect of the Electron—Phonon Coupling on Barrier D^— Quantum Dots in Magnetic Fields

The influence of the electron-phonon coupling of the energy of low-lying states of the barrier D^- center,which consists of a positive ion located on the z-azis at a distance from the two-dimensional quantum dot plane and two electrons in the dot plane bound by the ion,is investigated at arbitrary strength of maguetic field by mading use of the method of few-body physics.Discontinuous ground-state energy transitions induced by the magnetic field are reported.The dependence of the binding energy of the D^- ground state on the quantum dot radius is obtained.A considerable enhancement of the binding is found for the D^- ground state,which results from the confinement of electrons and electron-phonon coupling.     

量子点中磁激子的极化子效应

采用推广的LLP方法研究了自组织量子点中磁激子的极化子效应。考虑带电粒子和声子的相互作用,得到了激子能量随磁场的变化关系。结果表明,激子-声子的相互作用降低了激子的能量,但影响很小;极化子效应在没有外磁场时较明显,随着外磁场的增加,这种效应变得越来越弱。     

Polaron Effect of a Two Electron Parabolic Quantum Dot

Polaron induced double electron in a quantum dot is investigated using the exact diagonalization techniques and the compact density-matrix approach. The dependence of nonlinear optical processes on the incident photon energies and the polaronic effect are brought out. The linear, third order non-linear optical absorption coefficients and the refractive index changes of singlet and triplet states as a function of photon energy are obtained with and without the inclusion of polaronic effect. It is found that the geometrical confinement and the effect of polaron have great influence on the optical properties of dots.     

Four-Electron Systems in a Coupled Double-Layer Quantum Dots

Making use of the method of few-body physics, the energy spectrum of a four-electron system consisting in a vertically coupled double-layer quantum dot as a function of the strength ofa magnetic field is investigated. Discontinuous ground-state transitions induced by an external magnetic field are shown. We find that, in the strong coupling case, the ground-state transitions depend not only on the external magnetic field B but also on the distance d between double-layer quantum dots. However, in the case of weak coupling, the ground-state transitions occur in the new sequence of the values of the magic angular momentum. Hence, the interlayer separation d and electron-electron interaction strongly affect the ground state of the coupled quantum dots.     

Kondo Effect Versus Magnetic Coupling in Indirectly Coupled Double Quantum Dots

We theoretically investigate a device consisting of two quantum dots(QDs) side-coupled to a quantum wire which has many physicalingredients of an artificial heavy fermion system. An extra parameter, the distance L between the two QDs, is introduced and it plays an important role on the competition of the Kondo temperature and magnetic coupling. Three different phases are found: antiferromagnetic phase, Kondo phase with spin S=1/2, and Kondo phase with S=1, depending on the distance L, the magnetic coupling, and the Kondo temperature. Quantum transport properties are qualitatively different for different phases: for the S=1 Kondo and the antiferromagnetic phases, the conductance tends to the unitary value 2e²/h; for the S=1/2 Kondo phase the conductance is strongly dependent on the distance.     

ZnCdSe量子点的激子行为研究

用金属有机化学气相沉积(MOCVD)的方法在晶格失配较小的情况下制备了ZnCdSe量子点。并用原子力显微镜(AFM)和极低温度下的发光光谱确认了量子点的形成。原子力显微镜观测的形貌变化发现。随着生长后时间的增加,量子点的尺寸逐渐增大。而密度减小,这是由于熟化过程作用的结果。随着量子点生长完毕与加盖层之间间隔时间的增加,量子点的发光峰位明显红移,且由变温光谱得到的激子束缚能逐渐变小。这可以解释为随着间隔时间的增加,量子点的熟化过程导致量子点的尺寸增大,量子限域效应减弱所致。     

Charged-Exciton Complexes in Quantum Dots

It is known experimentally that stable charged-exciton complexes can exist in low-dimensional semiconductor nanostructures. Much less is known about the properties of such charged-exciton complexes since three-body problems are very difficult to be solved, even numerically. Here we introduce the correlated hyperspherical harmonics as basis functions to solve the hyperangular equation for negatively and positively charged excitons (trions) in a harmonic quantum dot. By using this method, we have calculated the energy spectra of the low-lying states of a charged exciton as a function of the radius of quantum dot. Based on symmetry analysis, the level crossover as the dot radius increases can be fully explained as the results of symmetry constraint.``     

退火温度对聚对苯乙炔MOPPV-ZnSe量子点复合材料太阳电池性能影响

利用原位缩合法制备了聚(2-甲氧基-5辛氧基)对苯乙炔(MOPPV)-ZnSe量子点复合材料,通过对复合材料的X射线衍射、透射电子显微镜、扫描电子显微镜、紫外可见吸收光谱等研究,发现聚合物MOPPV与ZnSe量子点以包覆形式有效地复合在一起,复合材料中ZnSe量子点结晶性良好,尺寸约为4 nm;且两者之间发生光诱导电荷转移,复合材料随着退火温度的升高,其吸收光谱范围发生红移。通过对MOPPV-ZnSe复合材料光电性能的研究发现,复合材料光电性能随着退火温度的升高逐渐表现出明显的二极管特性,转换效率出现先增大后减小的趋势,且在160℃时转换效率达到最大为0.3726%。     

Charged-Exciton Complexes in Quantum Dots

It is known experimentally that stable charged-exciton complexes can exist in low-dimensional semiconductor nanostructures. Much less is known about the properties of such charged-exciton complexes since three-body problems are very difficult to be solved, even numerically. Here we introduce the correlated hyperspherical harmonics as basis functions to solve the hyperangular equation for negatively and positively charged excitons (trions) in a harmonic quantum dot. By using this method, we have calculated the energy spectra of the low-lying states of a charged exciton as a function of the radius of quantum dot. Based on symmetry analysis, the level crossover as the dot radius increases can be fully explained as the results of symmetry constraint.     

Energy Spectra of Three-Electron Quantum Dots

By making use of the method of few-body physics, the energy spectra of three electrons confined by two-dimensional and three-dimensional quantum dots (QDs) are investigated. Using the present results, the size and shape effects of QDs on the spectra are revealed.     

球型量子点中的电子拉曼散射

研究了球型半导体量子点中的电子拉曼散射.讨论了初态为导带全满,价带全空时的电子跃迁过程,给出了电子拉曼散射的跃迁选择定则。通过计算GaAs和CdS材料球型量子点中电子及空穴参与拉曼散射的微分散射截面,分别比较了电子和空穴的不同影响,发现电子对拉曼散射的贡献要远大于空穴的贡献;当选取不同量子点半径时,拉曼散射微分散射截面变化也非常明显;量子点尺寸不变的条件下,改变入射光子能量,可以发现,微分散射截面随入射光子能量增大而减小。     

非对称量子点中磁极化子的声子平均数

采用线性组合算符方法研究了非对称量子点中强、弱耦合磁极化子的声子平均数的性质。导出了非对称量子点中强、弱耦合磁极化子的声子平均数和振动频率随量子点的横向和纵向受限强度,电子-声子耦合强度的变化关系。对RbCl晶体进行数值计算,结果表明非对称量子点中强、弱耦合磁极化子的声子平均数和振动频率随量子点的横向和纵向受限强度和电子-声子耦合强度的增大而迅速增大。     

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 has been 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 donor impurity quantum dot.     

Energy Spectra of Excitons Bound to a Neutral Acceptor in Quantum Dots

The energy spectra of the ground state for an exciton (X) trapped by a neutral acceptor (A⁰) in a quantum dot with a parabolic confinement have been calculated as a function of the electron-to-hole mass ratio σ by using the hyperspherical coordinates. We find that the (A⁰,X) complex confined in a quantum dot has in general a larger binding energy than those in a two-dimensional quantum well and a three-dimensional bulk semiconductor, and the binding energy decreases with the increase of the electron-to-hole mass ratio.     

红、绿CdSe@ZnS量子点配比对三波段标准白光LED器件的影响

将一步法合成的具有梯度合金结构的红光、绿光CdSe@ZnS量子点与硅胶均匀混合后,作为光转换层涂覆到蓝色InGaN LED芯片上,制备了不含荧光材料的三波段白光LED器件。研究了峰值为650 nm和550 nm的高效率红、绿量子点在硅胶中的含量及配比对白光LED色坐标以及效率的影响。结果表明,当红、绿量子点配比为2：3时,可得到发射纯正白光的QDs-LED器件,色坐标为（0.322 8,0.335 9）、色温为5 725 K,功率效率为26.61 lm/W,显色指数为72.7。光谱中红、蓝、绿三色发光峰的半高宽分别为30,25,38 nm,表明器件具有很好的单色性和高色纯度。     

Quantum Size Effects on Two Electrons and Two Holes in Double-Layer Quantum Dots

We propose a procedure to solve exactly the Schrodinger equation for a system of two electrons and two holes in a double-layer quantum dot by using the method of few-body physics. The features of the low-lying spectra have been deduced based on symmetry. The binding energies of the ground state are obtained as a function of the electron-to-hole mass ratio σ for a few values of the quantum dot size.