Lateral Quantum Dot Research Articles

Synthesis from aqueous solutions and optical properties of Ag–In–S quantum dots

Size-selected series of Ag–In–S and Ag–In–S:Zn quantum dots (QDs) with intense broadband photoluminescence were synthesized in aqueous solutions in the presence of glutathione and mercaptoacetic acid. The bands observed in their Raman spectra are assigned to the corresponding bond vibrations. A noticeable bandgap broadening with addition of a zinc agent at the second stage of the synthesis procedure implies incorporation of Zn into the QD lattice rather than the QD surface passivation by a ZnS shell. The stability of the synthesized QD solutions during long-term storage at room temperature is discussed.

Supramolecular Assembly of U(IV) Clusters and Superatoms with Unconventional Countercations

Superatoms are nanometer-sized molecules or particles that form ordered lattices, mimicking their atomic counterparts. Hierarchical assembly of superatoms gives rise to emergent properties in lattices of quantum dots, p-block clusters, and fullerenes. Here, we introduce a family of uranium-oxysulfate cluster anions whose hierarchical assembly in water is controlled by two parameters: acidity and the lanthanide or transition-metal countercation. In acid, larger LnIII (Ln = La-Ho) link hexamer (U6) oxoclusters into body-centered cubic frameworks, while smaller LnIII (Ln = Er-Lu and Y) promote linking of 14 U6 clusters into hollow superclusters (U84 superatoms). U84 assembles into superlattices including cubic-closest packed, body-centered cubic, and interpenetrating networks, bridged by interstitial countercations and U6 clusters. Divalent transition metals (TM = MnII and ZnII) charge-balance and promote the fusion of 10 U6 and 10 U monomers into a wheel-shaped cluster (U70). Dissolution of U70 in organic media reveals (by small-angle X-ray scattering) that differing supramolecular assemblies are accessed, controlled by TMII-linking of U70 clusters. Magnetic measurements of these assemblies reveal Curie-Weiss behavior at high temperatures, without pairing of the 5f2-electrons down to 2 K.

Control of a spin qubit in a lateral GaAs quantum dot based on symmetry of gating potential

We study the influence of quantum dot symmetry on the Rabi frequency and phonon induced spin relaxation rate in a single electron GaAs spin qubit. We find that anisotropic dependence on the magnetic field direction is independent of the choice of the gating potential. Also, we discover that relative orientation of the quantum dot, with respect to the crystallographic frame, is relevant in systems with ${\bf C}_{1{\rm v}}$, ${\bf C}_{2{\rm v}}$, or ${\bf C}_{n}$ ($n\neq4r$) symmetry. To demonstrate the important impact of the gating potential shape on the spin qubit lifetime, we compare the effects of an equilateral triangle, square, and rectangular confinement with the known results for the harmonic potential. In the studied cases, enhanced spin qubit lifetime is revealed, reaching almost six orders of magnitude increase for the equilateral triangle gating.

Near-infrared lateral photoresponse in InGaAs/GaAs quantum dots

Infrared photodetectors with In(Ga)As quantum dot (QD) active element functioning on interband and intersubband transitions are currently actively investigated, however, the vertical sensors were mostly reported. In the current study, a multilayer In0.4Ga0.6As/GaAs QD photodetector structure allowing the lateral photocurrent detection at normal incidence has been prepared depositing top contact. In order to have a comparison, a heterostructure with only a stack of In0.4Ga0.6As/GaAs wetting layers (WL) has been grown. In-depth photoelectrical characterization shows an effective broad-band photodetection related to the interband transitions between quantum-confined levels in QDs ranging from 1.03 to 1.38 eV (0.9–1.2 μm) that covers much wider infrared range in comparison to that from WLs (1.27–1.38 eV). Photoluminescence spectroscopy confirms the existence of QD transitions, observed as intense QD emission which peaked at 1.12 eV (∼1 μm) redshifted in comparison to the WL structure. The mechanisms of photoconductivity are modelled and discussed, comparing both the structures. We also show that our QD stack has an order lower contribution from defects compared to similar QD structures investigated before. At the same time, our structures demonstrate appropriate device characteristics at room temperature, such as the wide dynamic range from 10–2 to 103 μW cm−2 and a high photoresponsivity up to 20 A W−1 at low excitation intensities over 10–2–10–1 μW cm−2, while at higher excitation intensities the responsivity is reduced, exhibiting a strong spectral dependence. Thereby, our results show that the grown multilayer In0.4Ga0.6As/GaAs QD heterostructure is of relevant interest for application in lateral QD photodetectors.

Controlling Energy Spectra and Whispering Gallery Modes of Electrons in a Few Electrons Lateral QD

In this work, we study energy spectra and Whispering Gallery Modes (WGMs) of electrons in Quantum Dot (QD). It is shown that the WGMs can exist between two lowest energy levels of a QD, s- and p-states by taking into account the Coulomb interaction between the electrons in the QD, since in a few electron QD, Coulomb Interaction plays a crucial role in its energy spectrum. In literature, lateral QDs are usually modeled by a 2D harmonic oscillator. However, our calculations reveal that some degenerate energy levels would be created due to the height of the QD. The effect of the dimension of the QD, and magnetic field are also investigated on the energy spectra.

Investigation on Surface Properties of Mn-Doped CdSe Quantum Dots Studied by X-ray Photoelectron Spectroscopy

In this work, we report on the effects of incorporating manganese (Mn) dopant into different sizes of cadmium selenide (CdSe) quantum dots (QDs), which improves the electronic and optical properties of the QDs for multiple applications such as light-emitting diodes, lasers, and biological labels. Furthermore, the greener inverse Micelle method was implemented using organic ligand, which is oleic acid. This binding of the surface enhanced the QDs’ surface trap passivation of Mn-doped CdSe, which then increased the quantity of the output. In addition, the inverse Micelle technique was used successfully to dope Mn into CdSe QDs without the risk of Mn dopants being self-purified as experienced by wurtzite CdSe QDs. Also, we report the X-ray photoelectron spectroscopy (XPS) results and analysis of zinc blended manganese-doped cadmium selenide quantum dots (Mn-doped CdSe QDs), which were synthesized with physical sizes that varied from 3 to 14 nm using the inverse Micelle method. The XPS scans traced the existence of the Se 3d and Cd 3d band of CdSe crystals with a 54.1 and 404.5 eV binding energy. The traced 640.7 eV XPS peak is proof that Mn was integrated into the lattice of CdSe QDs. The binding energy of the QDs was related to the increase in the size of the QDs.

Preparation of non-oxidized Ge quantum dot lattices in amorphous Al2O3, Si3N4 and SiC matrices

The preparation of non-oxidized Ge quantum dot (QD) lattices embedded in Al2O3, Si3N4, SiC matrices by self-assembled growth was studied. The materials were produced by magnetron sputtering deposition, using different substrate temperatures. The deposition regimes leading to the self-assembled growth type and the formation of three-dimensionally ordered Ge QD lattices in different matrices were investigated and determined. The oxidation of the Ge QDs in different matrices was monitored and the best conditions for the production of non-oxidized Ge QDs were found. The optical properties of the Ge QD lattices in different matrices show a strong dependence on the Ge oxidation and the matrix type.

Spectroscopy of Quantum Dot Orbitals with In-Plane Magnetic Fields

We show that in-plane magnetic-field-assisted spectroscopy allows extraction of the in-plane orientation and full 3D size parameters of the quantum mechanical orbitals of a single electron GaAs lateral quantum dot with subnanometer precision. The method is based on measuring the orbital energies in a magnetic field with various strengths and orientations in the plane of the 2D electron gas. From such data, we deduce the microscopic confinement potential landscape and quantify the degree by which it differs from a harmonic oscillator potential. The spectroscopy is used to validate shape manipulation with gate voltages, agreeing with expectations from the gate layout. Our measurements demonstrate a versatile tool for quantum dots with one dominant axis of strong confinement.

Single hole spin relaxation probed by fast single-shot latched charge sensing

Hole spins have recently emerged as attractive candidates for solid-state qubits for quantum computing. Their state can be manipulated electrically by taking advantage of the strong spin-orbit interaction (SOI). Crucially, these systems promise longer spin coherence lifetimes owing to their weak interactions with nuclear spins as compared to electron spin qubits. Here we measure the spin relaxation time T1 of a single hole in a GaAs gated lateral double quantum dot device. We propose a protocol converting the spin state into long-lived charge configurations by the SOI-assisted spin-flip tunneling between dots. By interrogating the system with a charge detector we extract the magnetic-field dependence of T1 ∝ B−5 for fields larger than B = 0.5 T, suggesting the phonon-assisted Dresselhaus SOI as the relaxation channel. This coupling limits the measured values of T1 from ~400 ns at B = 1.5 T up to ~60 μs at B = 0.5 T.

Maximal thermal entanglement using three-spin interactions

Three-spin interactions in three-qubit systems at thermal equilibrium can be used for simple and efficient creation of maximally entangled states. We do not require set of gates to achieve this goal; rather, maximal thermal entanglement naturally arises by appropriately tuning the interactions present in the system. Within the broad range of parameter regimes found, we identify the ones accessible in triple quantum dot and triangular optical lattice, thus opening a way toward simple implementation of maximally entangled states with different types of three-spin interactions. Our results suggest tight connection between the presence of W type of entanglement and magnetization, enabling experimental detection of the W state.

Dopant incorporation in ultrasmall quantum dots: a case study on the effect of dopant concentration on lattice and properties of SnO2 QDs

Although impurity doping of nanocrystals is essential in controlling their properties for various applications, but the doping mechanism of ultrasmall semiconductor QDs is not yet understood. In this study, the effect of concentration of Ni2+ ions on the lattice and properties of 1.4 nm SnO2 QDs, prepared via chemical-precipitation route has been studied in detail. The quantum dot lattice contracted at maximum Ni concentration of 10% due to the incorporation of Ni ions at the substitutional sites on the surface of SnO2 lattice, while it began to expand gradually as the Ni concentration was further decreased from 10 to 1%, due in part to the Ni ions located in the core of the host lattice, occupying interstitial sites between Sn and O atoms leading to the expansion of the lattice with the decrease in the amount of strain and dislocation density. High resolution transmission electron microscopy confirms the presence of such dislocations in pure and doped SnO2 QDs. Optical investigation shows that the Ni doping in SnO2 lattice leads to a blue shift in the absorption wavelength. The concentration quenching effect of the PL emission with Ni doping is explained in detail. An enhancement in the photocatalytic activity has been achieved with optimum Ni incorporation in SnO2. The work also successfully correlates photoluminescence quenching and enhanced photocatalytic activity with the defect production happened in SnO2 system on Ni doping.

Enhancing the exciton emission of CsPbCl3 perovskite quantum dots by incorporation of Rb+ ions

All-inorganic perovskite quantum dots have wide application prospects in the fields of lighting and display. However, the photoluminescence quantum yields of violet-blue emission for the CsPbCl3 quantum dots is still low. In this work, the Rb+ ions were successfully incorporated into the lattice of CsPbCl3 quantum dots with partially substituting the sites of Cs+ ions by the modified hot injection method. It was interesting to observe that the photoluminescence quantum yields of CsPbCl3 quantum dots was improved from 5.7% to 13% through the optimization of Rb+ doping concentration. The absorption and emission peaks of CsPbCl3 quantum dots shifted to blue, originating from lattice substitution of Cs+ with Rb+. Further investigation on the photoluminescence dynamics of the quantum dots revealed that the enhanced photoluminescence quantum yields resulted from the reduced defects in CsPbCl3:Rb quantum dots. This work offers a new approach to improve the optical performance of perovskite quantum dots.

Lattice Dynamics and Thermal Stability of Cubic-Phase CsPbI3 Quantum Dots.

Cubic-phase CsPbI3 quantum dots (QDs) have been recently synthesized with merits of excellent optoelectronic performance. However, vital properties of cubic CsPbI3 including lattice dynamics and stability at high temperature remain poorly explored. We fabricate cubic CsPbI3 QDs and study their lattice dynamic and thermal stability to 700 K. We obtain Raman modes of cubic CsPbI3 QDs from 300 to 500 K at ultra-low-frequency range down to 15 cm-1, consistent with first-principles calculations. Above 550 K, the modification of Raman features suggests sample degradation. Consistently, temperature-dependent photoluminescence measurements indicate the absence of other luminescence phases up to 700 K. With increasing temperature, the CsPbI3 QD photoluminescence peak has a blue shift with exponentially decreasing intensity, showing faster electronic degradation than structural degradation. Our work provides detailed investigation of CsPbI3 QD lattice dynamics, band gap, and their high-temperature behavior, potentially useful for their emerging optoelectronic applications.

Phonon-assisted relaxation and decoherence of singlet-triplet qubits in Si/SiGe quantum dots

We study theoretically the phonon-induced relaxation and decoherence of spin states of two electrons in a lateral double quantum dot in a SiGe/Si/SiGe heterostructure. We consider two types of singlet-triplet spin qubits and calculate their relaxation and decoherence times, in particular as a function of level hybridization, temperature, magnetic field, spin orbit interaction, and detuning between the quantum dots, using Bloch-Redfield theory. We show that the magnetic field gradient, which is usually applied to operate the spin qubit, may reduce the relaxation time by more than an order of magnitude. Using this insight, we identify an optimal regime where the magnetic field gradient does not affect the relaxation time significantly, and we propose regimes of longest decay times. We take into account the effects of one-phonon and two-phonon processes and suggest how our theory can be tested experimentally. The spin lifetimes we find here for Si-based quantum dots are significantly longer than the ones reported for their GaAs counterparts.

Fano effect in the transport of an artificial molecule

The Fano effect is a ubiquitous phenomenon arising from interference between a discrete energy state and an energy continuum. We explore this effect in an artificial molecule, namely, two lateral quantum dots (QDs) fabricated from a two-dimensional electron gas system and coupled in series. When the coupling between the leads and QDs is small, the charge stability diagram of the system shows a honeycomb lattice structure that is characteristic of a double QD system. As the coupling increases, a honeycomb structure consisting of the Fano resonances emerges. A numerical simulation based on the T-matrix method can satisfactorily reproduce our experimental observation. This report constitutes a clear example of the ubiquitous nature of the Fano effect in mesoscopic transport.

Comparison of Phonon Damping Behavior in Quantum Dots Capped with Organic and Inorganic Ligands.

Surface ligand modification of colloidal semiconductor nanocrystals has been widely used as a means of controlling photoexcited-state generation, relaxation, and coupling to the environment. While progress has been made in understanding how surface ligand modification affects the behavior of electronic states, less is known about the influence of surface ligand modification on phonon behavior, which impacts relaxation dynamics and transport phenomena. In this work, we compare the dynamics of optical and acoustic phonons in CdTe quantum dots (QDs), CdTe/CdSe core/shell QDs capped with octadecylphosphonic acid ligands, and CdTe QDs capped with Se2- to ascertain how ligand exchange from native aliphatic ligands to single-atom Se2- ligands affects phonon behavior. We use transient absorption spectroscopy and observe modulations in the kinetics of excited-state decay due to QD lattice vibrations from both optical and acoustic phonons, which we describe using the damped oscillator model. The longitudinal optical phonons have similar frequencies and damping behavior in all three samples. In contrast, the longitudinal acoustic phonon mode in the Se2--capped CdTe QDs is severely damped, much more so than in CdTe and CdTe/CdSe QDs capped with the native aliphatic ligands. We attribute these differences in the acoustic phonon behavior to the differences in how the QD dissipates vibrational energy to its surroundings as a function of ligand identity. Our results indicate that these inorganic surface-capping ligands enhance not only the electronic but also the mechanical coupling of nanocrystals with their environment.

Nonlinear and dot-dependent Zeeman splitting in GaAs/AlGaAs quantum dot arrays

We study the Zeeman splitting in lateral quantum dots that are defined in GaAs-AlGaAs het- erostructures by means of split gates. We demonstrate a non-linear dependence of the splitting on magnetic field and its substantial variations from dot to dot and from heterostructure to het- erostructure. These phenomena are important in the context of information processing since the tunability and dot-dependence of the Zeeman splitting allow for a selective manipulation of spins. We show that spin-orbit effects related to the GaAs band structure quantitatively explain the ob- served magnitude of the non-linear dependence of the Zeeman splitting. Furthermore, spin-orbit effects result in a dependence of the Zeeman splitting on predominantly the out-of-plane quantum dot confinement energy. We also show that the variations of the confinement energy due to charge disorder in the heterostructure may explain the dependence of Zeeman splitting on the dot position. This position may be varied by changing the gate voltages which leads to an electrically tunable Zeeman splitting.

Robust operating point for capacitively coupled singlet-triplet qubits

Singlet-triplet qubits in lateral quantum dots in semiconductor heterostructures exhibit high-fidelity single-qubit gates via exchange interactions and magnetic field gradients. High-fidelity two-qubit entangling gates are challenging to generate since weak interqubit interactions result in slow gates that accumulate error in the presence of noise. However, the interqubit electrostatic interaction also produces a shift in the local double well detunings, effectively changing the dependence of exchange on the gate voltages. We consider an operating point where the effective exchange is first order insensitive to charge fluctuations while maintaining nonzero interactions. This "sweet spot" exists only in the presence of interactions. We show that working at the interacting sweet spot can directly produce maximally entangling gates and we simulate the gate evolution under realistic 1/f noise. We report theoretical two-qubit gate fidelities above 99% in GaAs and Si systems.

Coherent long-distance displacement of individual electron spins

Controlling nanocircuits at the single electron spin level is a possible route for large-scale quantum information processing. In this context, individual electron spins have been identified as versatile quantum information carriers to interconnect different nodes of a spin-based semiconductor quantum circuit. Despite extensive experimental efforts to control the electron displacement over long distances, maintaining electron spin coherence after transfer remained elusive up to now. Here we demonstrate that individual electron spins can be displaced coherently over a distance of 5 µm. This displacement is realized on a closed path made of three tunnel-coupled lateral quantum dots at a speed approaching 100 ms−1. We find that the spin coherence length is eight times longer than expected from the electron spin coherence without displacement, pointing at a process similar to motional narrowing observed in nuclear magnetic resonance experiments. The demonstrated coherent displacement will open the route towards long-range interaction between distant spin qubits.

Stabilizing Cesium Lead Halide Perovskite Lattice through Mn(II) Substitution for Air-Stable Light-Emitting Diodes.

All-inorganic cesium lead halide perovskite (CsPbX3, X = Cl, Br, and I) quantum dots (QDs), possessing high photoluminescence quantum yields and tunable color output, have recently been endowed great promise for high-performance solar cells and light-emitting diodes (LEDs). Although moisture stability has been greatly improved through separating QDs with a SiO2 shell, the practical applications of CsPbX3 QDs are severely restricted by their poor thermal stability, which is associated with the intrinsically low formation energies of perovskite lattices. In this regard, enhancing the formation energies of perovskite lattices of CsPbX3 QDs holds great promise in getting to the root of their poor thermal stability, which hitherto remains untouched. Herein, we demonstrate an effective strategy through Mn2+ substitution to fundamentally stabilize perovskite lattices of CsPbX3 QDs even at high temperatures up to 200 °C under ambient air conditions. We employ first-principle calculations to confirm that the significantly improved thermal stability and optical performance of CsPbX3:Mn2+ QDs arise primarily from the enhanced formation energy due to the successful doping of Mn2+ in CsPbX3 QDs. Benefiting from such an effective substitution strategy, these Mn2+-doped CsPbX3 QDs can function well as efficient light emitters toward the fabrication of high-performance perovskite LEDs.