五边形截面的Ag纳米线局域表面等离子体共振模式

五边形截面的单晶Ag纳米线对ZnO量子点荧光具有增强的现象. 为解释这一现象, 利用时域有限差分法对五边形截面的Ag纳米线的局域表面等离子体共振模式进行了理论模拟. 结果表明, 五边形截面的Ag纳米线在紫外区域存在两个消光峰, 分别由Ag纳米线的横向偶极共振(340 nm)和四极共振(375 nm)引起; 这两个消光峰与ZnO量子点荧光增强峰相一致, 而且随着Ag纳米线的半径增大而红移; 消光峰对应的共振模式取决于Ag纳米线的截面形状; 根据Ag纳米线电场增强倍数与激发光波长变化关系曲线可知, 最大增强电场位于五边形截面的顶点处, 而边线处电场增强较小. 理论模拟的结果较好地解释了Ag纳米线/ZnO量子点体系的荧光增强现象, 也为Ag纳米线在提高半导体材料发光效率、生物探测等方面的应用提供有益的参考.     

Coulomb blockade and hopping transport behaviors of donor-induced quantum dots in junctionless transistors

The ionized dopants, working as quantum dots in silicon nanowires, exhibit potential advantages for the development of atomic-scale transistors. We investigate single electron tunneling through a phosphorus dopant induced quantum dots array in heavily n-doped junctionless nanowire transistors. Several subpeaks splittings in current oscillations are clearly observed due to the coupling of the quantum dots at the temperature of 6 K. The transport behaviors change from resonance tunneling to hoping conduction with increased temperature. The charging energy of the phosphorus donors is approximately 12.8 meV. This work helps clear the basic mechanism of electron transport through donor-induced quantum dots and electron transport properties in the heavily doped nanowire through dopant engineering.     

双量子点结构中Majorana费米子的噪声特性

Majorana费米子是其自身的反粒子, 在拓扑量子计算中有着重要的应用. 利用粒子数表象下的量子主方程方法, 研究双量子点与Majorana费米子混合结构的电子输运特性, 特别是散粒噪声. 有无Majorana费米子耦合的电流与散粒噪声存在明显差别: 有Majorana费米子耦合时稳态电流差呈反对称, 噪声谱呈现相干振荡并且低频噪声显著增强. 量子点与Majorana费米子对称弱耦合时, 零频噪声由"峰"变为"谷", 并且"边谷"展宽逐渐减小; 当对称强耦合时, 零频噪声的谷深增加, "边谷"向高频端移动. 改变系统与电极的耦合强度时, 零频噪声由谷变成峰. 因此, 稳态电流结合散粒噪声可以探测双量子点结构中Majorana费米子是否存在.     

Indication of unusual pentagonal structures in atomic-size Cu nanowires

We present a study of the structural and quantum conductance properties of atomic-size copper nanowires generated by mechanical stretching. The atomistic evolution was derived from time-resolved electron microscopy observations and molecular dynamics simulations. We have analyzed the quantum transport behavior by means of conductance measurements and theoretical calculations. The results suggest the formation of an unusual and highly stable pentagonal Cu nanowire with a diameter of approximately 0.45 nm and approximately 4.5 conductance quanta.     

Temperature dependence of photoluminescence of semiconductor quantum dots upon indirect excitation in a SiO2 dielectric matrix

The processes of excitation and relaxation of confined excitons in semiconductor quantum dots upon indirect high-energy excitation have been considered. The temperature behavior of photoluminescence of quantum dots in a SiO₂ dielectric matrix has been described using a model accounting for the process of population of quantum-dot triplet states upon excitation transfer through mobile excitons of the matrix. Analytical expressions that take into account the two-stage and three-stage schemes of relaxation transitions have been obtained. The applicability of these expressions for analyzing fluorescence properties of semiconductor quantum dots has been demonstrated using the example of silicon and carbon nanoparticles in the thin-film SiO₂ matrix. It has been shown that the complex character of the temperature dependences of the photoluminescence upon indirect excitation can be an indication of a multistage relaxation of excited states of the matrix and quantum dots. The model concepts developed in this study allow one to predict the form of temperature dependences of the photoluminescence for different schemes of indirect excitation of quantum dots.

     

Nonlinear thermoelectric response due to energy-dependent transport properties of a quantum dot

Quantum dots are useful model systems for studying quantum thermoelectric behavior because of their highly energy-dependent electron transport properties, which are tunable by electrostatic gating. As a result of this strong energy dependence, the thermoelectric response of quantum dots is expected to be nonlinear with respect to an applied thermal bias. However, until now this effect has been challenging to observe because, first, it is experimentally difficult to apply a sufficiently large thermal bias at the nanoscale and, second, it is difficult to distinguish thermal bias effects from purely temperature-dependent effects due to overall heating of a device. Here we take advantage of a novel thermal biasing technique and demonstrate a nonlinear thermoelectric response in a quantum dot which is defined in a heterostructured semiconductor nanowire. We also show that a theoretical model based on the Master equations fully explains the observed nonlinear thermoelectric response given the energy-dependent transport properties of the quantum dot.     

Zero-phonon linewidth and phonon satellites in the optical absorption of nanowire-based quantum dots

The optical properties of quantum dots embedded in a catalytically grown semiconductor nanowire are studied theoretically. In comparison to dots in a bulk environment, the excitonic absorption is strongly modified by the one-dimensional character of the nanowire phonon spectrum. In addition to pronounced satellite peaks due to phonon-assisted absorption, we find a finite width of the zero-phonon line already in the lowest-order calculation.     

Carrier transport in III–V quantum-dot structures for solar cells or photodetectors

According to the well-established light-to-electricity conversion theory,resonant excited carriers in the quantum dots will relax to the ground states and cannot escape from the quantum dots to form photocurrent,which have been observed in quantum dots without a p–n junction at an external bias.Here,we experimentally observed more than 88% of the resonantly excited photo carriers escaping from In As quantum dots embedded in a short-circuited p–n junction to form photocurrent.The phenomenon cannot be explained by thermionic emission,tunneling process,and intermediate-band theories.A new mechanism is suggested that the photo carriers escape directly from the quantum dots to form photocurrent rather than relax to the ground state of quantum dots induced by a p–n junction.The finding is important for understanding the low-dimensional semiconductor physics and applications in solar cells and photodiode detectors.     

Spin interactions,relaxation and decoherence in quantum dots

We review recent studies on spin decoherence of electrons and holes in quasi-two-dimensional quantum dots, as well as electron-spin relaxation in nanowire quantum dots. The spins of confined electrons and holes are considered major candidates for the realization of quantum information storage and processing devices, provided that sufficiently long coherence and relaxation times can be achieved. The results presented here indicate that this prerequisite might be realized in both electron and hole quantum dots, taking one large step towards quantum computation with spin qubits.     

Negative differential conductance in InAs wire based double quantum dot induced by a charged AFM tip

We investigate the conductance of an InAs nanowire in the nonlinear regime in the case of low electron density where the wire is split into quantum dots connected in series. The negative differential conductance in the wire is initiated by means of a charged atomic force microscope tip adjusting the transparency of the tunneling barrier between two adjoining quantum dots. We confirm that the negative differential conductance arises due to the resonant tunneling between these two adjoining quantum dots. The influence of the transparency of the blocking barriers and the relative position of energy states in the adjoining dots on a decrease of the negative differential conductance is investigated in detail.     

Fabrication and optical properties of type‐II InP/InAs nanowire/quantum‐dot heterostructures

The growth and optical properties of InAs quantum dots on a pure zinc blende InP nanowire are investigated. The quantum dots are formed in Stranski–Krastanov mode and exhibit pure zinc blende crystal structure. A substantial blueshift of the dots peak with a cube‐root dependence on the excitation power is observed, suggesting a type‐II band alignment. The peak position of dots initially red‐shifts and then blue‐shifts with increasing temperature, which is attributed to the carrier redistribution among the quantum dots. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Transmission electron microscopy study of vertical quantum dots molecules grown by droplet epitaxy

The compositional distribution of InAs quantum dots grown by molecular beam epitaxy on GaAs capped InAs quantum dots has been studied in this work. Upper quantum dots are nucleated preferentially on top of the quantum dots underneath, which have been nucleated by droplet epitaxy. The growth process of these nanostructures, which are usually called as quantum dots molecules, has been explained. In order to understand this growth process, the analysis of the strain has been carried out from a 3D model of the nanostructure built from transmission electron microscopy images sensitive to the composition.     

Spin states of holes in Ge/Si nanowire quantum dots

We investigate tunable hole quantum dots defined by surface gating Ge/Si core-shell nanowire heterostructures. In single level Coulomb-blockade transport measurements at low temperatures spin doublets are found, which become sequentially filled by holes. Magnetotransport measurements allow us to extract a g factor g approximately 2 close to the value of a free spin-1/2 particle in the case of the smallest dot. In less confined quantum dots smaller g factor values are observed. This indicates a lifting of the expected strong spin-orbit interaction effects in the valence band for holes confined in small enough quantum dots. By comparing the excitation spectrum with the addition spectrum we tentatively identify a hole exchange interaction strength chi approximately 130 microeV.     

铯铅溴量子点的合成与发光性质

通过改进的热注射法制备了铯铅溴量子点材料。制得的量子点属于立方相结构,形貌是纳米立方体形状,尺寸均匀,边长约为10 nm,分散良好,在空气中可稳定2个月以上。铯铅溴量子点有较宽的激发光谱和强烈的绿光发射峰,荧光呈双指数过程衰减,平均寿命为纳秒量级。所得的量子点胶体可以通过滴制或旋涂的方法制成均匀的薄膜,在太阳能电池、光电探测器、LED和激光等半导体光电领域都有潜在的应用。     

双栅调控的硅量子线中的库仑振荡效应

基于单电子隧穿和库仑阻塞效应,研究了硅量子线中的单电子输运特性.利用绝缘体上硅薄膜材料作为衬底构建侧栅结构的硅量子线单电子晶体管,通过背栅和侧栅对量子线的电子输运特性进行调制.实验发现,在硅量子线中分别观察到背栅和侧栅调制的单电子效应和库仑振荡现象.从微分电导的二维灰度轮廓图,清楚地观察到了库仑阻塞区,说明由于栅压导致在硅量子线中形成了库仑岛. 关键词： 库仑振荡 单电子效应 硅量子线     

Spectroscopy of spin-orbit quantum bits in indium antimonide nanowires

A double quantum dot in the few-electron regime is achieved using local gating in an InSb nanowire. The spectrum of two-electron eigenstates is investigated using electric dipole spin resonance. Singlet-triplet level repulsion caused by spin-orbit interaction is observed. The size and the anisotropy of singlet-triplet repulsion are used to determine the magnitude and the orientation of the spin-orbit effective field in an InSb nanowire double dot. The obtained results are confirmed using spin blockade leakage current anisotropy and transport spectroscopy of individual quantum dots.     

Photoemission from InAs/GaAs quantum dots decorated with cesium adatoms

An array of non-overgrown InAs/GaAs quantum dots has been decorated with adsorbed metal atoms in situ in ultrahigh vacuum. Their electron and photoemission properties have been studied. The radical modification of the spectra of the threshold emission from the quantum dots with increasing cesium coating has been found. Two photoemission channels have been established; they are characterized by considerably different intensities, spectral locations, and widths of the selective bands. It has been shown that the decoration of the quantum dots makes it possible to control the electronic structure and quantum yield of photoemission, the nature of which is related to the excitation of the electronic states of the GaAs substrate and InAs/GaAs quantum dots.     

Absorption of terahertz radiation in Ge/Si(001) heterostructures with quantum dots

The terahertz spectra of the dynamic conductivity and radiation absorption coefficient in germanium-silicon heterostructures with arrays of Ge hut clusters (quantum dots) have been measured for the first time in the frequency range of 0.3–1.2 THz at room temperature. It has been found that the effective dynamic conductivity and effective radiation absorption coefficient in the heterostructure due to the presence of germanium quantum dots in it are much larger than the respective quantities of both the bulk Ge single crystal and Ge/Si(001) without arrays of quantum dots. The possible microscopic mechanisms of the detected increase in the absorption in arrays of quantum dots have been discussed.     

Inter-dot coupling effects on transport through correlated parallel coupled quantum dots

Transport through symmetric parallel coupled quantum dot system has been studied, using non-equilibrium Green function formalism. The inter-dot tunnelling with on-dot and inter-dot Coulomb repulsion is included. The transmission coefficient and Landaur-Buttiker like current formula are shown in terms of internal states of quantum dots. The effect of inter-dot tunnelling on transport properties has been explored. Results, in intermediate inter-dot coupling regime show signatures of merger of two dots to form a single composite dot and in strong coupling regime the behaviour of the system resembles the two decoupled dots.      

Decoherence-free spin entanglement generation and purification in nanowire double quantum dots

We propose a deterministic generation and purification of decoherence-free spin entangled states with singlet-triplet spins in nanowire double quantum dots via resonator-assisted charge manipulation and measurement techniques. Each spin qubit corresponds to two electrons in a double quantum dot in the nanowire, with the singlet and one of the triplets as the decoherence-free qubit states. The logical qubits are immunized against the dominant source of decoherence-dephasing—while the influences of additional errors are shown by numerical simulations. We analyse the performance and stability of all required operations and emphasize that all techniques are feasible in current experimental conditions.