Energy Levels Of Quantum Dots Research Articles

Quantum dot refrigerator driven by photon

A model of quantum dot refrigerator driven by photon, which consists of two two-level quantum dots, a photon reservoir and two leads, is proposed in this paper. Comparing with previous studies, we consider the transitions of electrons between different energy levels in a single quantum dot, which is more practical.Based on the theory of master equation and the assumption of weak coupling, we derive the expression of the cooling rate and the coefficient of performance of the refrigerator and obtain the condition of the tight coupling of the refrigerator operation. Next, we plot numerically the performance characteristic curves between the cooling rate and the coefficient of performance in the case of the tight coupling and in the general case. We find that the curves between the cooling rate and the coefficient of performance are opened loops for tight coupling, but they are closed loops in the general case. And we gain the conclusions that the refrigerator can be reversible under the condition of the tight coupling, while it can be irreversible in the general case. Then the optimally operating range of the refrigerator is determined. Finally, the effect of the temperature of the photon reservoir, transition coefficient, and temperature ratio on the performance of refrigerator under the conditions of the maximum cooling rate are studied, and also the coefficient of performance under the maximum cooling rate, the maximum coefficient of performanceand the cooling rate under the maximum coefficient of performanceare analyzed in detail.

Spin thermoelectric transport in a four-quantum-dot ring: Influence of quantum dots magnetization

Non-equilibrium Green’s function method is applied to study the spin thermoelectric effects in a four-quantum-dot ring with two of the quantum dots magnetized. The influence of magnetic configuration on the spin thermoelectric transport through the system is investigated. In some magnetic configurations, a significant spin Seebeck coefficient [Formula: see text] can be generated with the variation of the quantum dot (QD) energy level. Appropriately tuning the QD energy level can let the present device work as a pure spin-up (spin-down) or a pure-spin-current thermal generator. The combined effect of the magnetization and magnetic flux (or Rashba spin-orbit coupling) is also investigated. Finally, the charge figure of merit [Formula: see text] and spin figure of merit [Formula: see text] are evaluated, it is found that the magnitude of the spin figure of merit can be comparable to that of the charge counterpart.

Study and Development of Photovoltaic Structures Based on Quantum Dot Solids of PbS with Various Ligands

The photoconductivity of condensates of lead-sulfide quantum dots (QDs)—QD solids—with various organic ligands is studied. It is demonstrated that the QD solid photoconductivity increases exponentially with a reduction in length of ligand molecules and does not depend on their chemical structures, since it is governed by hopping transport of charge carriers. In contrast, the photocurrent in photovoltaic ITO/PEDOT: PSS/PbS/ZnO/Al elements depends on the ligand structure, since this structure sets the positions of QD energy levels and thus affects the efficiency of charge carrier transfer to electrodes. The difference between mechanisms of generation of photoconductivity and photovoltaic currents is discussed.

Electron transport through two one-dimensional multi-quantum dot arrays coupled to four leads

Employing the non-equilibrium Green’s function technique, we have obtained the formula for dc current of two one-dimensional multi-quantum dot arrays, which couple to each other via tunneling coupling between two quantum dots connected to four leads, respectively. The retarded Green’s function is a staircase type, terminating at the four leads. Furthermore, the four quantum dots case is demonstrated. The influence of inter-dot coupling strength and quantum dot energy level on the transmission probability for TL, TM and TN branches is investigated. A non-resonant band is observed. By adjusting energy levels of quantum dots, a resonance emerges in the region of the non-resonance band. The system can be used as a quantum switch.

Pressure dependent optical properties of quantum dot with spin orbit interaction and magnetic field

In this paper, a detailed investigation of the effects of the interplay of temperature, hydrostatic pressure, Rashba spin orbit interaction, and external magnetic field on the optical properties for quantum dot is presented. We obtained the exact wavefunctions and energy levels of a harmonic potential quantum dots taking Rashba spin orbit interaction into account. We have obtained the linear and nonlinear absorption coefficients and relative refractive index change for varying incident photon energies using density matrix theory. Counter intuitive red shift in the absorption coefficient peaks with increase in hydrostatic pressure is demonstrated. The role of temperature and hydrostatic pressure on variation of effective charge carrier mass is investigated.

Polarization switch in an elliptical micropillar – quantum dot system induced by a magnetic field in Faraday configuration

In this paper we study the effect of applying a magnetic field on an elliptical microcavity pillar with quantum dots embedded, in the presence of external laser excitation. To obtain the system dynamics we use the matrix density formalism, taking into account realistic parameters and including losses. Our results show that it is possible to use the magnetic field strength to control the polarization of the photons inside the cavity, making our system behave like a photon polarization switch. We also report the best set of parameters where this is possible. Our results also indicate that we can use the polarization of the cavity photons to look into the fine structure of the energy levels of quantum dots.

Majorana oscillations modulated by Fano interference and degree of nonlocality in a topological superconducting-nanowire–quantum-dot system

We explore theoretically the influence of Fano interference in the so-called Majorana oscillations in a T-shaped hybrid setup formed by a quantum dot (QD) placed between conducting leads and side-coupled to a topological superconducting nanowire (TSNW) hosting zero-energy Majorana bound states (MBSs) at the ends. Differential conductance as a function of the external magnetic field reveals oscillatory behavior. Both the shape and amplitude of the oscillations depend on the bias-voltage, degree of MBSs non-locality and Fano parameter of the system determining the regime of interference. When the latter is such that direct lead-lead path dominates over lead-QD-lead path and the bias is tuned in resonance with QD zero-energy, pronounced fractional Fano-like resonances are observed around zero-bias for highly non-local geometries. Further, the conductance profiles as a function of both bias-voltage and QD energy level display "bowtie" and "diamond" shapes, in qualitative agreement with both previous theoretical and experimental works. These findings ensure that our proposal can be used to estimate the degree of MBS non-locality, thus allowing to investigate their topological properties.

Energy Levels in a Single-Electron Quantum Dot with Hydrostatic Pressure

In this article we present a study of the effects of hydrostatic pressure on the energy levels of a quantum dot with an electron. A quantum dot is modeled using an infinite potential well and a two-dimensional harmonic oscillator and solved through the formalism of second quantization. A scheme for the implementation of a quantum NOT gate controlled with hydrostatic pressure is proposed.

Hot Charge Carrier Extraction from Semiconductor Quantum Dots

This article focus on the recent work carried out on hot charge carrier extraction from colloidal quantum dots (QDs). It has been contemplated that the quantum dot based solar cell (QDSC) can reach exceptionally high efficiencies if hot charge carriers can be extracted. However, the process requires excellent charge carrier separation from energetically hot states as compared to relaxation within QDs energy levels. Also, most of the studies were being carried out on thermal charge carrier extraction from QDs and very limited information was available on hot charge carrier extraction. In this Feature Article, we have discussed the work carried out on hot charge carrier extraction from QDs and the challenges involved in the process.

Crystal Growth, Exponential Optical Absorption Edge, and Ground State Energy Level of PbS Quantum Dots Adsorbed on the (001), (110), and (111) Surfaces of Rutile-TiO2

It is important to investigate the dependencies of the optical absorption and the ground state energy level on the size of the semiconductor quantum dots (QDs) on fully studied single crystal TiO2 surfaces. The present study focuses on the systems comprising PbS QDs on (001), (110), and (111) surfaces of single crystal rutile-TiO2. By the optical absorption characterization, the average diameter of PbS QDs on a (001) surface is independent of the number of adsorption cycles, although those on (110) and (111) surfaces increase with the number of cycles. The rate of adsorption of PbS QDs on a (001) surface is higher than those grown on (110) and (111) surfaces. The results suggest that the crystal growth is caused by the difference of the surface energy of the substrate. The exponential optical absorption edge suggests that the structural disorder of PbS QDs on (001) and (110) surfaces increases as the number of adsorption cycles increases. On the other hand, that on a (111) surface decreases as the number ...

Enhanced visible-light photocatalysis via back-electron transfer from palladium quantum dots to perylene diimide

The electronic-coupling interaction between noble-metal cocatalysts and host semiconductor nanocrystals has been found to be effective for the utilization of the solar energy. However, electron transfer (ET) mechanism between noble metals and self-assemblies has not been elucidated clearly. Here, we revealed a mechanism of back-electron-transfer-enhanced photocatalysis, which contributed to the visible-light photocatalytic improvement of perylene diimide (PDI) assembly for phenolic degradation and hydrogen generation. By compared with the energy level of PDI and Pd quantum dots (QDs), it seemed to be disadvantageous for ET from PDI assembly to Pd QDs, but surface photovoltage spectra showed that this can come true under visible-light irradiation, indicative of formation of a new Schottky barrier resulting from other light-induced transition species. Absorption spectra showed that an additional electronic state existed above the conduction band of PDI assembly originating from PDI radical anions. Such active species provided the light-driven dissipative structure with stable energy and electron flow through two opposite types of ET pathway: one is ET from light-excited PDI anions to Pd QDs and the other is direct electron injection from plasma Pd QDs accumulated with high-energy electrons to PDI neutral molecules. It has been found that continuous ET from PDI assembly to Pd QDs caused the plasma resonance of the Pd QDs to be higher in energy to overcome interfacial energy barrier. And then back-ET from Pd QDs to neutral PDI molecules occurred and led to formation of more PDI radical anions that were important for the improvement of the photocatalytic activity. These findings provide a new strategy for the development of highly efficient visible-light photocatalysts based on self-assembly for energy production.

Fast Energy Transfer in CdSe Quantum Dot Layered Structures: Controlling Coupling with Covalent-Bond Organic Linkers

Quantum dot (QD) solids and arrays hold a great potential for novel applications which are aimed at exploiting quantum properties in room-temperature devices. Careful tailoring of the QD energy levels and coupling between dots could lead to efficient energy-harvesting devices. Here, we used a self-assembly method to create a disordered layered structure of QDs, coupled by covalently bonded organic molecules. Energy transfer rates from small (donor) to large (acceptor) QDs are measured. Best tailoring of the QDs energy levels and the length of the linking molecules results in an energy transfer rate as high as 30 ps–1. Such rates approach energy transfer rates of the highly efficient photosynthesis complexes and are compatible with a coherent mechanism of energy transfer. These results may pave way for new controllable building blocks for future technologies.

Разработка и исследование фотовольтаических структур на основе слоев квантовых точек PbS с различными лигандами

AbstractThe photoconductivity of condensates of lead-sulfide quantum dots (QDs)—QD solids—with various organic ligands is studied. It is demonstrated that the QD solid photoconductivity increases exponentially with a reduction in length of ligand molecules and does not depend on their chemical structures, since it is governed by hopping transport of charge carriers. In contrast, the photocurrent in photovoltaic ITO/PEDOT: PSS/PbS/ZnO/Al elements depends on the ligand structure, since this structure sets the positions of QD energy levels and thus affects the efficiency of charge carrier transfer to electrodes. The difference between mechanisms of generation of photoconductivity and photovoltaic currents is discussed.

Spin-Seebeck effect and spin polarization in a multiple quantum dot molecule

In this work, we study the conductance and the thermoelectric properties of a quantum dot embedded between two metallic leads with a side-coupled triple quantum dot molecule under a magnetic field. We focus on the spin polarization and thermoelectric quantities. Our results show the possibility of design an efficient spin filter device besides a noticeable enhancement of the Seebeck coefficient driven by the asymmetry in the quantum dots energy levels and a tunable pure spin Seebeck effect is obtained. This behavior also holds in the interacting case, where a pure spin Seebeck effect can be obtained for fixed values of the embedded quantum dot energy level. Our findings could lead to the implementation of a new pure spin energy conversion and capable spin filter devices working with weak magnetic fields.

Fano resonances in Majorana bound states–quantum dot hybrid systems

We consider a quantum wire, containing two Majorana bound states (MBS) at its ends that are coupled to a current lead on one side and to a quantum dot (QD) on the other side. Using the method of full counting statistics we calculate the conductance and the zero-frequency noise. Using an effective low-energy model, we analyze in detail the Andreev reflection probability as a function of the various system parameters and show that it exhibits a Fano resonance (FR) line shape in the case of a weakly coupled QD as a function of the QD energy level when the two MBS overlap. The asymmetry parameter changes sign as the bias voltage is tuned through the MBS overlap energy. The FR is mirrored as a function of the QD level energy as long as tunneling to the more distant MBS is negligible. However, if both MBS are coupled to the lead and the QD, the height as well as the asymmetry of the line shapes cease to respect this symmetry. These two exclusive cases uniquely distinguish the coupling to a MBS from the coupling to a fermionic bound state that is shared between the two MBS. We complement the analysis by employing a discretized one-dimensional p-wave superconductor (Kitaev chain) for the quantum wire and show that the features of the effective low-energy model are robust towards a more complete Hamiltonian and also persist at finite temperature.

Size-dependent photoluminescence of PbS QDs embedded in silicate glasses

PbS quantum dots (QDs) with a mean radii of 1.27 nm to 7.32 nm were precipitated in alkaline-earth silicate glasses enriched in sulfur. A transmission electron microscope image and absorption spectra evidenced the narrow size dispersion of PbS QDs and discrete energy levels of PbS QDs. Upon above-band-gap excitation, symmetry of the photoluminescence from PbS QDs was strongly dependent on their size. Photoluminescence from small PbS QDs showed a long-wavelength tail and vice versa for large PbS QDs. The size dependence of full width at half maximum, PL energy and Stokes shift on the size of PbS QDs suggested that the electron/hole trap states of PbS QDs and the defects states on the interface between PbS QDs and glass matrix have great effect on the photoluminescence properties of PbS QDs.

Coexistence of strongly and weakly confined energy levels in (Cd,Zn)Se quantum dots: Tailoring the near-band-edge and defect-levels for white light emission

We demonstrate white light emission (WLE) from (Cd,Zn)Se system, which is a composite of Zn alloyed CdSe quantum dot and ZnSe-amorphous (ZnSe-a) phase. Detailed structural and photoluminescence emission studies on pure CdSe and (Cd,Zn)Se show cubic zinc blende structure in the size range of 2.5 to 5 nm. (Cd,Zn)Se quantum dots (QDs) also have a significant fraction of ZnSe-a phase. The near-band-edge green-emission in crystalline CdSe and (Cd,Zn)Se is tunable between 500 to 600 nm. The (Cd,Zn)Se system also exhibits a broad, deep defect level (DL) red-emission in the range 600 to 750 nm and a sharp ZnSe near-band-edge blue-emission (ZS-NBE) between 445 to 465 nm. While DL and CdSe near-band-edge (CS-NBE) emissions significantly shift with the size of QD due to strong confinement effect, the ZS-NBE show minimal change in peak position indicating a weak confinement effect. The intensities of ZS-NBE and DL emissions also exhibit a strong dependence on the QD size. A gamut of emission colors is obtained by combining the CS-NBE with the ZS-NBE emission and broad DL emission in (Cd,Zn)Se system. Interestingly, we find the convergence of Commission Internationale de l'Eclairage (CIE) coordinates towards the white light with increasing Zn concentration in CdSe. We demonstrate by combining these three emissions in a proper weight ratio WLE can be achieved. Cd1-yZnySe (y = 0. 5; QD size ∼4.9 nm) alloy with a maximum quantum yield of 57% exhibits CIE coordinates of (0.39, 0.4), color rendering index (CRI) of 82, correlated color temperature (CCT) of 3922 K, and Duv of 0.0078 which is very promising for white light applications.

Electrostatically confined trilayer graphene quantum dots

Electrically gating of trilayer graphene (TLG) opens a band gap offering the possibility to electrically engineer TLG quantum dots. We study the energy levels of such quantum dots and investigate their dependence on a perpendicular magnetic field $B$ and different types of stacking of the graphene layers. The dots are modeled as circular and confined by a truncated parabolic potential which can be realized by nanostructured gates or position-dependent doping. The energy spectra exhibit the intervalley symmetry ${E}_{K}^{e}(m)=\ensuremath{-}{E}_{{K}^{\ensuremath{'}}}^{h}(m)$ for the electron $(e)$ and hole $(h)$ states, where $m$ is the angular momentum quantum number and $K$ and ${K}^{\ensuremath{'}}$ label the two valleys. The electron and hole spectra for $B=0$ are twofold degenerate due to the intervalley symmetry ${E}_{K}(m)={E}_{{K}^{\ensuremath{'}}}[\ensuremath{-}(m+1)]$. For both ABC $[\ensuremath{\alpha}=1.5$ (1.2) for large (small) $R]$ and ABA $(\ensuremath{\alpha}=1)$ stackings, the lowest-energy levels show approximately a ${R}^{\ensuremath{-}\ensuremath{\alpha}}$ dependence on the dot radius $R$ in contrast with the $1/{R}^{3}$ one for ABC-stacked dots with infinite-mass boundary. As functions of the field $B$, the oscillator strengths for dipole-allowed transitions differ drastically for the two types of stackings.

The effects of pressure and temperature on the exchange energy of a parabolic quantum dot under a magnetic field

The combined effects of pressure and temperature on the energy levels of a parabolic GaAs quantum dot under a magnetic field have been studied. The exact diagonalization method was used to solve the two-electron quantum dot Hamiltonian and to obtain the eigenenergies. In addition, we investigated the effects of pressure and temperature on the singlet-triplet exchange energy (J=ET−Es) of the quantum dot as a function of a magnetic field. The magnetic field-parabolic confinement (ωc−ω0) phase diagram of the quantum dot was calculated. The comparisons show that our results are in very good agreement with the previously published works.

Reversible Electrochemistry of Mercury Chalcogenide Colloidal Quantum Dot Films.

The absolute positions of the energy levels of colloidal quantum dots of Hg(S, Se, Te), which are of interest as mid-infrared materials, are determined by electrochemistry. The bulk valence bands are at -5.85, -5.50, and -4.77 eV (±0.05 eV) for zinc-blend HgS, HgSe, HgTe, respectively, in the same order as the anions p-orbital energies. The conduction bands are conversely at -5.20, -5.50, and -4.77 eV. The stable ambient n-doping of Hg(S, Se) quantum dots compared to HgTe arises because the conduction band is sufficiently lower than the measured environment Fermi level of ∼ -4.7 eV to allow for n-doping for HgS and HgSe quantum dots even with significant electron confinement. The position of the Fermi level and the quantum dots states are reported for a specific surface treatment with ethanedithiol and electrolyte environment. The positions are however sensitive to different surface treatments, providing an avenue to control doping. Electrochemical gating is further used to determine the carrier mobility in the films of the three different systems as a function of CQD size. HgSe and HgS show increasing mobility with increasing particle sizes while HgTe shows a nonmonotonous behavior, which is attributed to some degree of aggregation of HgTe QDs.