Atomic-scale structure and formation of self-assembled In(Ga)As quantum rings

We present an atomic-scale analysis of the indium distribution of self-assembled (In,Ga)As quantum rings (QRs), which are formed from InAs quantum dots by capping with a thin layer of GaAs and subsequent annealing. We find that the size and shape of QRs as observed by cross-sectional scanning tunneling microscopy (X-STM) deviate substantially from the ring-shaped islands as observed by atomic force microscopy on the surface of uncapped QR structures. We show unambiguously that X-STM images the remaining quantum dot material whereas the AFM images the erupted quantum dot material. The remaining dot material shows an asymmetric indium-rich crater-like shape with a depression rather than an opening at the center and is responsible for the observed electronic properties of QR structures. These quantum craters have an indium concentration of about 55% and a diameter of about 20 nm, which is consistent with the observed electronic radius of QR structures. Based on the structural information from the X-STM measurements, we calculate the magnetization as a function of the applied magnetic field. We conclude that, although the real QR shape differs strongly from an idealized circular-symmetric open ring structure, Aharonov–Bohm-type oscillations in the magnetization can be expected.     

Bound states in continuum: Quantum dots in a quantum well

We report on the existence of a bound state in the continuum (BIC) of quantum rods (QR). QRs are novel elongated InGaAs quantum dot nanostructures embedded in the shallower InGaAs quantum well. BIC appears as an excited confined dot state and energetically above the bottom of a well subband continuum. We prove that high height-to-diameter QR aspect ratio and the presence of a quantum well are indispensable conditions for accommodating the BIC. QRs are unique semiconductor nanostructures, exhibiting this mathematical curiosity predicted 83 years ago by Wigner and von Neumann.     

Size and temperature effects on electric properties of CdTe/ZnTe quantum rings

The electronic properties of CdTe/ZnTe quantum rings (QRs) are investigated as functions of size and temperature using an eight-band strain-dependent k·p Hamiltonian. The size effects of diameter and height on the strain distributions around the QRs are studied. We find that the interband transition energy, defined as the energy difference between the ground electronic and the ground heavy-hole subbands, increases with the increasing QR inner diameter regardless of the temperature, while the interband energy decreases with the increasing QR height. This is attributed to the reduction of subband energies in both the conduction and the valence bands due to the strain effects. Our model, in the framework of the finite element method and the theory of elasticity of solids, shows a good agreement with the temperature-dependent photoluminescence measurement of the interband transition energies.     

Features of fluorescence of CdSe/ZnS semiconductor quantum rods in multicomponent solutions with pentylcyanobiphenyl

The fluorescence of CdSe/ZnS semiconductor quantum rods (QRs) in the CdSe/ZnS-hexane pentylcyanobiphenyl and CdSe/ZnS-methyl methacrylate pentylcyanobiphenyl multicomponent systems has been investigated. It is shown that the presence of pentylcyanobiphenyl affects differently the QR fluorescence in each composite. The QR fluorescence in hexane solutions is efficiently quenched in the presence of pentylcyanobiphenyl, whereas in the composites based on methyl-methacrylate the QR fluorescence is enhanced as a result of the introduction of pentylcyanobiphenyl in the mesomorphic state. This difference is explained by the different forms of pentylcyanobiphenyl in these systems. The composites based on structured porous polyethylene films containing QRs in a mesomorphic medium have been obtained for the first time. The measured degree of polarization of the QR fluorescence characterizes the composite as strongly ordered.     

Polarization surface-charge density of single semiconductor quantum rods

Electrostatic force microscopy was used to determine that single CdSe quantum rods (QRs) have a permanent polarization surface-charge density, an unexpected observation for supposedly well-shaped particles. The surface charge results from a slight angle between the QR sides and the direction of internal electric polarization. By contrast, despite the large dipole moment expected for CdSe QRs, none was observed. The unavoidable presence of permanently charged surfaces on CdSe QRs has the potential to impede the development of novel devices incorporating these materials.     

Quantum rings in tilted magnetic fields

The electronic states of semiconductor quantum rings (QRs) under tilted magnetic fields are studied in the framework of the effective mass and envelope function approximations. For an axial field, the orbital Zeeman contribution prevails leading to the well-known Aharanov–Bohm spectrum, but it slowly decreases as the magnetic field direction declines. For an in-plane field, only the diamagnetic shift survives and it leads to the formation of double quantum well solutions, this result being relevant for experimental techniques which use in-plane magnetic fields to determine the spin of QR ground states. We also investigate the magnetic response of partially overlapped QRs, which are characteristic of high-density samples of self-assembled rings, and find that the spectrum is quite sensitive to ring coupling.     

Spatial correlation of two particles in semiconductor quantum ring

We analyze the effect of the spatial correlation on the ground state energy of two particles (two electrons and exciton–hole pair) confined in quantum ring (QR) with a soft-edge-barrier confinement potential. Starting from the Schrödinger variational principle we derive a one-dimensional differential equation for the spatial pair correlation function (SPCF), which we solve numerically by using the shooting method. The effect of the repulsive core in a self-assembled GaAs/InAs QR on the two-electron and exciton SPCF is analyzed. A comparative analysis of the dependencies of the two-particle ground state energies on the inner and outer radii of GaAs/InAs QRs with different confinement potential shapes is presented.     

Tunable strength saddle-point contacts impact on quantum rings transmission

A particular subject of investigation is the role of several sadle-point contact (QPC) parameters on the scattering properties of an Aharonov–Bohm–Aharonov–Casher quantum ring (QR) under Rashba-type spin orbit interaction. We discuss the interplay of the conductance with the confinement strengths and height of the QPC, which yields new and tunable harmonic and non-harmonics patterns, while one manipulates these constriction parameters. This phenomenology may be of utility to implement a novel way to modulate spin interference effects in semiconducting QRs, providing an appealing test-platform for spintronics applications.     

Comparative study between different quantum infrared photodetectors

This paper presents a theoretical analysis for the dark current characteristics of different quantum infrared photodetectors. These quantum photodetectors are quantum dot infrared photodetectors (QDIP), quantum wire infrared photodetectors (QRIP), and quantum well infrared photodetectors (QWIP). Mathematical models describing these devices are introduced. The developed models accounts for the self-consistent potential distribution. These models are taking the effect of donor charges on the spatial distribution of the electric potential in the active region. The developed model is used to investigate the behavior of dark current with different values of performance parameters such as applied voltage, number of quantum wire (QR) layers, QD layers, lateral characteristic size, doping quantum wire density and temperature. It explains strong sensitivity of dark current to the density of QDs/QRs and the doping level of the active region. In order to confirm our models and their validity on the practical applications, a comparison between the results obtained by proposed models and that experimentally published are conducted and full agreement is observed. Several performance parameters are tuned to enhance the performance of these quantum photodetectors through the presented modeling. The resultant performance characteristics and comparison among them are presented in this work. From the obtained results we notice that the total dark current in the QRIPs can be significantly lower than that in the QWIPs. Moreover, main features of the QRIPs such as the large gap between the induced photocurrent and dark current open the way for overcoming the problems of quantum dot infrared photodetectors.     

Comparison studies of infrared photodetectors with a quantum-dot and a quantum-wire base

This paper mainly presents a theoretical analysis for the characteristics of quantum dot infrared photodetectors (QDIPs) and quantum wire infrared photodetectors (QRIPs). The paper introduces a unique mathematical model of solving Poisson’s equations with the usage of Lambert W functions for infrared detectors’ structures based on quantum effects. Even though QRIPs and QDIPs have been the subject of extensive researches and development during the past decade, it is still essential to implement theoretical models allowing to estimate the ultimate performance of those detectors such as photocurrent and its figure-of-merit detectivity vs. various parameter conditions such as applied voltage, number of quantum wire layers, quantum dot layers, lateral characteristic size, doping density, operation temperature, and structural parameters of the quantum dots (QDs), and quantum wires (QRs). A comparison is made between the computed results of the implemented models and fine agreements are observed. It is concluded from the obtained results that the total detectivity of QDIPs can be significantly lower than that in the QRIPs and main features of the QRIPs such as large gap between the induced photocurrent and dark current of QRIP which allows for overcoming the problems in the QDIPs. This confirms what is evaluated before in the literature. It is evident that by increasing the QD/QR absorption volume in QDIPs/QRIPs as well as by separating the dark current and photocurrents, the specific detectivity can be improved and consequently the devices can operate at higher temperatures. It is an interesting result and it may be benefit to the development of QDIP and QRIP for infrared sensing applications.     

A highly effective method for synthesizing hybrid Pt-CdSe nanocomposite

Hybrid Pt-CdSe nanocomposite was fabricated by a two-step chemical route. Cadmium selenide (CdSe) quantum rods (QRs) were prepared by a one-pot approach with tunable size. After ligand exchange, CdSe QRs were loaded with monodisperse 1.9 nm Pt nanopaticles in aqueous solution. Transmission electron microscopy (TEM) revealed the morphology of the Pt-CdSe nanostructure, and the decreased photoluminescence (PL) intensity demonstrated that electron and hole separation can be enhanced after loading Pt on CdSe QRs. X-ray photoelectron energy spectrum (XPS) was applied to confirm the existence of Pt and detect the Pt mass concentration of 3%.     

The effects of geometrical parameters on carrier states and optical transitions in a quantum ring

Using a numerical method via the electron effective mass theory, a model of a quantum ring (QR) with a shape very close to the real one and taken from an experimental work, we investigate the electron states in a semi-conductor QR, studying the influence of the ring’s geometrical parameters on the electron spectrum and on the optical transitions. Our hetero structure evolves from a single quantum dot (QD) to a QR. We find that the one-electron ground state presents an absolute minimum when studied as a function of the ring radius. The reliability of the calculations is checked with experimental data.     

Interband photoconductivity of Ge/Si structures with self-organized quantum rings

At low temperatures a lateral photoconductivity (PC) of Ge/Si (1 0 0) self-organized quantum rings (QRs) structures as a function of interband light intensity has been investigated for different values of lateral voltage and temperature. In contrast to self-organized Ge/Si quantum dots (QDs) structures (grown at the same conditions) where the stepped PC was registered, for QRs structures essential smoothing of PC steps was observed. Such behavior is determined by decreasing of strain potential around QRs in conductive Si matrix due to a transfer of Ge atoms from the center of QDs to its periphery accompanied by Ge/Si intermixing.     

Quantum rings formed in InAs QDs annealing process

InAs quantum dots (QDs) were grown by molecular beam epitaxy in the Stranski-Krastanow growth mode. The samples were placed between two undoped GaAs slices and annealed in nitrogen ambient at different temperature. Effect of annealing temperature on the evolution of QDs morphology is investigated by the AFM. This behavior can be attributed to the mechanisms of QDs ripening, intermixing and segregation in the annealing process. A number of QDs have evoluted into the uniform distribution quantum rings (QRs) when the sample was annealed at the temperature of 800 °C. The results indicated that high density and uniform QRs can be obtained by the post-growth technique.     

Self-assembled MnN superstructure

Self-assembled MnN nanoislands have been prepared on Cu(001) substrate. The nanoislands show a square shape and a well-defined size. They are regularly arrayed with a periodicity of (3.5+/-0.1) nanometer and form a two-dimensional square superstructure. The MnN island superstructure is stabilized by a short-range mechanism. A structural model has been proposed to explain the self-assembly and the high quality of the superstructure.     

A theoretical model for quantum nanostructures electronic wave functions, magnetic field effects

Analytical solutions of electronic wave functions in symmetric quantum ring (QR), quantum wire (QWR) and quantum dots (QD) structures are given using a parabolic coordinates system. The solutions for low-energy states are combinations of Bessel functions. The density of states of perfect 1D QR and QWR are shown to be equivalent. The continuous evolution from a 0D QD to a perfect 1D QR can be precisely described. The sharp variation of electronic properties, related to the build up of a potential energy barrier at the early stage of the QR formation, is studied analytically. Paramagnetic and diamagnetic couplings to a magnetic field are computed for QR and QD. It is shown theoretically that magnetic field induces an oscillation of the magnetization in QR.     

Thermodynamic model of metal-induced self-assembly of Ge quantum dots on Si substrates

We have investigated the nucleation thermodynamics and kinetics of the Ge quantum dot (QD) self-assembly on the Au-patterned Si substrates based on the surface chemical potential theory. It is find that the minimum chemical potential on the substrate surface is located at the center site of the square lattice constructed by Au islands, which indicates that the nucleation of QD is thermodynamically favorable at the center site. The nucleation probability of QD at the center site is kinetically calculated by the mechanochemical potential-based approach. The influence of the surface orientation of Si substrates on the QD shape is addressed by the surface chemical potential theory.     

Self-assembly of block copolymers grafted onto a flat substrate:Recent progress in theory and simulations

Block copolymers are a class of soft matter that self-assemble to form ordered morphologies on the scale of nanometers, making them ideal materials for various applications. These applications directly depend on the shape and size of the self-assembled morphologies, and hence, a high degree of control over the self-assembly is desired. Grafting block copolymer chains onto a substrate to form copolymer brushes is a versatile method to fabricate functional surfaces. Such surfaces demonstrate a response to their environment, i.e., they change their surface topography in response to different external conditions. Furthermore, such surfaces may possess nanoscale patterns, which are important for some applications; however, such patterns may not form with spun-cast films under the same condition. In this review, we summarize the recent progress of the self-assembly of block copolymers grafted onto a flat substrate. We mainly concentrate on the self-assembled morphologies of end-grafted AB diblock copolymers, junction point-grafted AB diblock copolymers(i.e.,Y-shaped brushes), and end-grafted ABA triblock copolymers. Special emphasis is placed on theoretical and simulation progress.     

New self-limiting assembly model for Si quantum rings on Si(100)

We propose a new self-limiting assembly model for Si quantum rings on Si(100) where the ring's formation and evolution are driven by a growth-etching competition mechanism. The as-grown ring structure in a plasma enhanced chemical vapor deposition system has excellent rotational symmetry and superior morphology with a typical diameter, edge width, and height of 150-300, 10, and 5 nm, respectively. Based on this model, the size and morphology can be controlled well by simply tuning the timing procedure. We suggest that this growth model is not limited to certain material system, but provides a general scheme to control and tailor the self-assembly nanostructures into the desired size, shape, and complexity.     

基于机器视觉的量子点STM形貌图像识别研究

为减轻量子点表面形貌分析过程中的人工工作,使量子点的STM图像分析更加自动化,基于机器视觉对衬底的斜切角及量子点的形貌特性展开研究.利用腐蚀和边缘检测提取台阶形状,并通过反三角变换计算斜切角.利用二值化和阈值下降对量子点的数量与空间坐标进行提取,在此基础上,通过邻域密度计算分析其均匀性,并在解决图像中的粘连问题后找出量子点的尺寸.实验结果显示,与人工统计相比,斜切角、量子点计数及尺寸的平均误差分别为5.02%, 0.7788%及1.12%;并实现量子点均匀性的自动化统计与分析.基于机器视觉算法的自动识别过程,对协助研究者分析量子点表面形貌有实际意义.