Quantum Dot Superlattices Research Articles

Role of Superlattice Phonons in Charge Localization Across Quantum Dot Arrays.

Understanding charge transport in semiconductor quantum dot (QD) assemblies is important for developing the next generation of solar cells and light-harvesting devices based on QD technology. One of the key factors that governs the transport in such systems is related to the hybridization between the QDs. Recent experiments have successfully synthesized QD molecules, arrays, and assemblies by directly fusing the QDs, with enhanced hybridization leading to high carrier mobilities and coherent band-like electronic transport. In this work, we theoretically investigate the electron transfer dynamics across a finite CdSe-CdS core-shell QD array, considering up to seven interconnected QDs in one dimension. We find that, even in the absence of structural and size disorder, electron transfer can become localized by the emergent low-frequency superlattice vibrational modes when the connecting neck between QDs is narrow. On the other hand, we also identify a regime where the same vibrational modes facilitate coherent electron transport when the connecting necks are wide. Overall, we elucidate the crucial effects of electronic and superlattice symmetries and their couplings when designing high-mobility devices based on QD superlattices.

Machine Learning-Driven Optimization of Quantum Dot Superlattices for Enhanced Photonic Properties

Machine Learning-Driven Optimization of Quantum Dot Superlattices for Enhanced Photonic Properties

Synthesis and Characterization of Epitaxially-Fused Quantum Dot Superlattices at the Single Grain Limit

Epitaxially-fused superlattices of colloidal quantum dots (QD epi-SLs) may exhibit electronic minibands and high-mobility charge transport, but doing so requires epi-SLs with extremely high degrees of spatial and chemical (compositional) uniformity. In this talk, I will discuss both our efforts to understand and improve the epi-SL formation process, as well as recent charge transport studies on single epi-SL grains.Epi-SLs are synthesized from a parent SL structure, typically composed of QDs with long insulating ligands. Over the past several years, we have developed insight into the physical and chemical mechanisms by which this parent SL structure is converted into an epi-SL. I will present detailed, multi-model structural characterization of the SL structures, which unveils the astonishing choreographic process by which millions of QDs rotate and translate, in concert, to form the epi-SL.This structural transformation is initiated by chemical changes to the QD ligand layer, usually through manual introduction of liquids into the SL film. Mechanistic studies into this ligand-exchange process reveal a multi-step chemical sequence that initiates with a simple acid-base reaction. We exploit this insight to fabricate epi-SLs by means of photochemical triggering (using ultraviolet illumination) rather than alternative “hands-on” techniques which suffer from poor experimental control and pronounced chemical inhomogeneity and structural damage in the films. The use of light to trigger the SL conversion process enables much finer control in both the temporal and spatial domains, opening the door to much larger-area SL films and direct photopatterning of epi-SLs.In the latter portion of the talk, I will present recent work charge transport studies in individual, highly-ordered PbSe QD epi-SL grains. One technical challenge in making these devices is the inherent mismatch in using traditional microfabrication techniques to make single-grained devices of air-sensitive materials. Here, we demonstrate the air-free fabrication of microscale field-effect transistors (μ-FETs) with channels consisting of single PbSe QD epi-SL grains (~ 1 -10 µm grain sizes) and analyze charge transport phenomena in these samples. The devices exhibit p-channel or ambipolar transport with a hole mobility as high as 3.5 cm2 V–1 s–1 at 290 K and 6.5 cm2 V–1 s–1 at 170–220 K, one order of magnitude larger than that of previous QD solids.

Molecularly Imprinted Viral Protein Integrated Zn-Cu-In-Se-P Quantum Dots Superlattice for Quantitative Ratiometric Electrochemical Detection of SARS-CoV-2 Spike Protein in Saliva.

Solution-processable colloidal quantum dots (QDs) are promising materials for the development of rapid and low-cost, next-generation quantum-sensing diagnostic systems. In this study, we report on the synthesis of multinary Zn-Cu-In-Se-P (ZCISeP) QDs and the application of the QDs-modified electrode (QDs/SPCE) as a solid superlattice transducer interface for the ratiometric electrochemical detection of the SARS-CoV-2-S1 protein in saliva. The ZCISeP QDs were synthesized through the formation of In(Zn)PSe QDs from InP QDs, followed by the incorporation of Cu cations into the crystal lattice via cation exchange processes. A viral-protein-imprinted polymer film was deposited onto the QDs/SPCE for the specific binding of SARS-CoV-2. Molecular imprinting of the virus protein was achieved using a surface imprinting electropolymerization strategy to create the MIP@QDs/SPCE nanosensor. Characterization through spectroscopic, microscopic, and electrochemical techniques confirmed the structural properties and electronic-band state of the ZCISeP QDs. Cyclic voltammetry studies of the QDs/SPCE superlattice confirmed efficient electron transport properties and revealed an intraband gap energy state with redox peaks attributed to the Cu1+/2+ defects. Binding of SARS-CoV-2-S1 to the MIP@QDs/SPCE cavities induced a gating effect that modulated the Fe(CN)6 3-/4- and Cu1+/2+ redox processes at the nanosensor interface, producing dual off/on ratiometric electrical current signals. Under optimal assay conditions, the nanosensor exhibited a wide linear detection range (0.001-100 pg/mL) and a low detection limit (0.34 pg/mL, 4.6 fM) for quantitative detection of SARS-CoV-2-S1 in saliva. The MIP@QDs/SPCE nanosensor demonstrated excellent selectivity against nonspecific protein targets, and the integration with a smartphone-based potentiostat confirmed the potential for point-of-care applications.

SI/GE Quantum Dot Channel FETs for Multi-Bit Computing

This paper presents quantum dot channel (QDC) FETs in quantum wire and coupled quantum dot configurations for cryogenic operation with multi-state operation. It also describes gate-all-around (GAA) quantum dot channel (QDC) FETs that exhibit potential multi-state characteristics at room temperature. FETs with cladded Si and Ge quantum dot layers as a transport channel have been fabricated. The formation of a quantum dot superlattice (QDSL) when SiOx-cladded Si and/or GeOx-cladded Ge quantum dots (QD) are assembled results in mini-energy sub-bands in the conduction and valence band. The intra-mini-energy band transitions results in significant changes in the drain current when gate and/or drain voltages are varied. This novel feature provides a pathway for 16-/32-state logic in CMOS-X configuration. The gate-defined Si quantum dot FETs, comprising of tunnel barrier coupled, have been reported for quantum computing at cryogenic temperatures.

Cavity‐Enhanced Superfluorescence Stimulates Coherent Energy Transfer in a Perovskite Quantum Dot Superlattice

AbstractThe exploration of cooperative states in many‐body systems is a key research area. It focuses on superfluorescence (SF), a phenomenon linked with radiative dipole coupling, and its intersection with classical lasing effects. This produces a unique lasing‐field‐hybrid cooperative dipole (LCD) state through optical cavity‐enhanced superfluorescence (CESF). A coherent energy transfer is demonstrated between two such states within a perovskite quantum dot (QD) superlattice. This results in competitive luminescence timing dynamics. The findings reveal that stimulated energy transfer occurs when two cooperative cavity‐exciton states coexist, controllable via a dual‐pulse pump technique. This understanding is vital for advancing quantum phenomena knowledge and enhancing optoelectronic devices.

PbI2 Passivation of Three Dimensional PbS Quantum Dot Superlattices Toward Optoelectronic Metamaterials.

Lead chalcogenide colloidal quantum dots are one of the most promising materials to revolutionize the field of short-wavelength infrared optoelectronics due to their bandgap tunability and strong absorption. By self-assembling these quantum dots into ordered superlattices, mobilities approaching those of the bulk counterparts can be achieved while still retaining their original optical properties. The recent literature focused mostly on PbSe-based superlattices, but PbS quantum dots have several advantages, including higher stability. In this work, we demonstrate highly ordered 3D superlattices of PbS quantum dots with tunable thickness up to 200 nm and high coherent ordering, both in-plane and along the thickness. We show that we can successfully exchange the ligands throughout the film without compromising the ordering. The superlattices as the active material of an ion gel-gated field-effect transistor achieve electron mobilities up to 220 cm2 V-1 s-1. To further improve the device performance, we performed a postdeposition passivation with PbI2, which noticeably reduced the subthreshold swing making it reach the Boltzmann limit. We believe this is an important proof of concept showing that it is possible to overcome the problem of high trap densities in quantum dot superlattices enabling their application in optoelectronic devices.

Внезонное поглощение электромагнитного излучения электронами с оптическим фононным участием в сверхрешетках квантовых точек

Physics Institute of Azerbaijan National Academy of Sciences In this paper, we investigate the absorption of electromagnetic radiation by a free electron gas interacting with lattice vibrations in a quantum dot superlattice. It is assumed that the electron gas in the quantum dot superlattice is limited by the anisotropic parabolic potential. The absorption of light by free carriers with the participation of phonons is calculated in the second order of the perturbation theory. When calculating the absorption coefficient, the matrix elements of the electron-photon and electron-phonon interactions (with polar and nonpolar optical phonons) are used. We can expect three possible transitions in the absorption coefficient for electron-polar phonon scattering: (1) a transition due to the size subband levels for only the x direction, (2) a transition due to the size subband levels for only the y direction and (3) a transition due to the size subband levels for both the x direction and the y direction. For electron-nonpolar phonon scattering we can expect оnly one possible transitions in the, the absorption coefficient: a transition due to the size subband levels for both the x direction and the y direction.

Effect of Extended Defects on AlGaN Quantum Dots for Electron-Pumped Ultraviolet Emitters.

We study the origin of bimodal emission in AlGaN/AlN QD superlattices displaying a high internal quantum efficiency (around 50%) in the 230-300 nm spectral range. The secondary emission at longer wavelengths is linked to the presence of cone-like domains with deformed QD layers, which originate at the first AlN buffer/superlattice interface and propagate vertically. The cones originate at a 30°-faceted shallow pit in the AlN, which appears to be associated with a threading dislocation that produces strong shear strain. The cone-like structures present Ga enrichment at the boundaring facets and larger QDs within the conic domain. The bimodality of the luminescence is attributed to the differing dot size and composition within the cones and at the faceted boundaries, which is confirmed by the correlation of microscopy results and Schrödinger-Poisson calculations.

Non-monotonic thermal conductivity modulation in colloidal quantum dot superlattices via ligand engineering

Colloidal quantum dots (QDs), which consist of inorganic cores surrounded by soft organic ligands, can self-assemble into superlattices exhibiting long-range order. Their tunability, in terms of size, shape, and ligand properties, makes them promising for applications in solar cells, photodetectors, and light-emitting diodes. However, the complex interplay between ligand stiffness, QDs cores-ligands coupling strength, and its impact on QDs' morphology and thermal properties is not well understood. This work employs molecular dynamics simulations to investigate the possibility of modulating the thermal conductivity of QDs superlattices via ligand engineering. It is found that the structural stability of QDs superlattices depends significantly on the interaction strength between the QDs cores and ligands. At lower interaction strengths, the instability manifests itself in a random fusion of the QDs cores, while at intermediate strengths a stable simple cubic lattice structure is maintained. Higher interaction strengths lead to amorphization in the surface regions of QDs core. We observed a nonlinear trend in the thermal conductivity with varying QD-ligand interaction strengths due to competing factors: fusion of QDs cores at lower interaction strengths and increased crosslinking interaction among ligands at higher interaction strengths. The influence of ligand stiffness on thermal conductivity was found to be minimal. This study provides a deep insight into the role of ligand stiffness and interaction strength on structural dynamics and thermal transport in QDs superlattice and demonstrates the feasibility of engineering thermal transport in QDs superlattice via ligand engineering.

The future of quantum technologies: superfluorescence from solution-processed, tunable materials.

One of the most significant and surprising recent developments in nanocrystal studies was the observation of superfluorescence from a system of self-assembled, colloidal perovskite nanocrystals [G. Rainò, M. A. Becker, M. I. Bodnarchuk, R. F. Mahrt, M. V. Kovalenko, and T. Stöferle, "Superfluorescence from lead halide perovskite quantum dot superlattices," Nature, vol. 563, no. 7733, pp. 671-675, 2018]. Superfluorescence is a quantum-light property in which many dipoles spontaneously synchronize in phase to create a collective, synergistic photon emission with a much faster lifetime. Thus, it is surprising to observe this in more inhomogenous systems as solution-processed and colloidal structures typically suffer from high optical decoherence and non-homogeneous size distributions. Here we outline recent developments in the demonstration of superfluorescence in colloidal and solution-processed systems and explore the chemical and materials science opportunities allowed by such systems. The ability to create bright and tunable superfluorescent sources could enable transformative developments in quantum information applications and advance our understanding of quantum phenomena.

Improvement of solar cell performance using PbS quantum dot superlattices with iodine ligands

We investigated the method to improve the performance of solar cells that use colloidal quantum dots (QDs) as light absorption layers. By replacing ligands of PbS QDs with iodine, adjusting QD concentration in a solvent suitable for the ligands, and preparing QD films by the sedimentation method, photoelectric conversion efficiency of QD solar cells was improved. It is known that by depositing QDs freely onto a substrate in a solvent, QDs are closest packed to form a superlattice. A suitable solvent that aided the free sedimentation of QDs facilitated the QD superlattice formation. Iodine ligands had narrowed QD spacing, promoting intermediate bands formation in the QD superlattices. QD concentration had affected the film quality. To achieve high performance expected for QD superlattice solar cells in future, it is necessary to develop carrier transport layers that extract carriers from high energy bands originating from excited states in QDs.

Giant Superbunching Emission from Mesoscopic Perovskite Emitter Clusters

AbstractPhoton superbunching, as a crucial resource for photonic quantum technologies, is investigated in several systems. This phenomenon can be induced by both coherent collective spontaneous processes such as superfluorescence and superradiance, and cascaded emission. Although superbunching emission induced by superfluorescence and cascaded emission is demonstrated recently in perovskite quantum dot (QD) superlattice and single perovskite QD, the effect itself is not that strong, especially in the former case with a second‐order photon correlation g(2)(0) only slightly larger than 2. Here, the superbunching emission from mesoscopic CsPbBr3 emitter clusters is reported. The higher temperature and excitation power are found to suppress the superbunching effect significantly. Compared with the former results in the literature, under the same experimental conditions, that is, temperature and excitation power, much larger g(2)(0) with the maximum value of 21 being achieved at 1.6 K with an excitation power of 0.1 µW is observed. The discoveries here improve the understanding of superbunching emission in perovskite‐based materials and will enable their application in developing multi‐photon light sources for photonic quantum technologies.

A composite electrodynamic mechanism to reconcile spatiotemporally resolved exciton transport in quantum dot superlattices.

Quantum dot (QD) solids are promising optoelectronic materials; further advancing their device functionality requires understanding their energy transport mechanisms. The commonly invoked near-field Förster resonance energy transfer (FRET) theory often underestimates the exciton hopping rate in QD solids, yet no consensus exists on the underlying cause. In response, we use time-resolved ultrafast stimulated emission depletion (STED) microscopy, an ultrafast transformation of STED to spatiotemporally resolve exciton diffusion in tellurium-doped cadmium selenide-core/cadmium sulfide-shell QD superlattices. We measure the concomitant time-resolved exciton energy decay due to excitons sampling a heterogeneous energetic landscape within the superlattice. The heterogeneity is quantified by single-particle emission spectroscopy. This powerful multimodal set of observables provides sufficient constraints on a kinetic Monte Carlo simulation of exciton transport to elucidate a composite transport mechanism that includes both near-field FRET and previously neglected far-field emission/reabsorption contributions. Uncovering this mechanism offers a much-needed unified framework in which to characterize transport in QD solids and additional principles for device design.

Structural and Temperature Dependence of Emergent Electronic States in PbSe Quantum Dot Superlattices.

Structural and Temperature Dependence of Emergent Electronic States in PbSe Quantum Dot Superlattices.

Design and Simulation of Multi-State D-Latch Circuit Using QDC-SWS FETs

This paper presents a novel D-latch circuit using multi-state quantum dot channel (QDC) spatial wavefunction-switched (SWS) field-effect transistors (FET). The SWS-FET has two or more vertically stacked quantum-well or quantum dot (QD) layers where the magnitude of the gate voltage determines the location of carriers in each channel. Spatial location is used to encode multiple logic states along with the carrier transport in mini-energy bands formed in GeOx-Ge/ SiOx-Si quantum dot superlattice (QDSL), and to obtain 8-states operation. The design is based on the 8-state inverter using QDC SWS-FETs in CMOS-X configuration. This could be a new paradigm for designing flip-flops and registering more complex sequential circuits. The proposed design leads to reduced propagation delay and a smaller Si footprint.

Enabling metallic behaviour in two-dimensional superlattice of semiconductor colloidal quantum dots

Semiconducting colloidal quantum dots and their assemblies exhibit superior optical properties owing to the quantum confinement effect. Thus, they are attracting tremendous interest from fundamental research to commercial applications. However, the electrical conducting properties remain detrimental predominantly due to the orientational disorder of quantum dots in the assembly. Here we report high conductivity and the consequent metallic behaviour of semiconducting colloidal quantum dots of lead sulphide. Precise facet orientation control to forming highly-ordered quasi-2-dimensional epitaxially-connected quantum dot superlattices is vital for high conductivity. The intrinsically high mobility over 10 cm2 V−1 s−1 and temperature-independent behaviour proved the high potential of semiconductor quantum dots for electrical conducting properties. Furthermore, the continuously tunable subband filling will enable quantum dot superlattices to be a future platform for emerging physical properties investigations, such as strongly correlated and topological states, as demonstrated in the moiré superlattices of twisted bilayer graphene.

Stable and Ultrafast Blue Cavity-Enhanced Superfluorescence in Mixed Halide Perovskites.

Cavity-enhanced superfluorescence (CESF) in quantum dot (QD) system is an ultrafast and intense lasing generated by combination of quantum coupling effect and optically stimulated amplification effect, which can provide a new idea for realizing high quality blue light sources and address the limitation of conventional inefficient blue light sources. Modifying halide composition is a straightforward method to achieve blue emission in perovskite QD system. However, the spectral instability introduced by photoinduced halide phase segregation and low coupling efficiency between QDs and optical cavities make it challenging to achieve stable blue CESF in such halide-doped QD system. Herein, long-range-ordered, densely packed CsPbBr2 Cl QD-assembled superlattice microcavities in which the two core issues can be appropriately addressed are developed. The QD superlattice structure facilitates excitonic delocalization to decrease exciton-phonon coupling, thus alleviating photoinduced phase segregation. By combination of theoretical analysis and temperature-dependent photoluminescence (PL) measurements, the underlying photoinduced phase segregation mitigation mechanism in mixed halide superlattices is clarified. Based on the CsPbBr2 Cl QD superlattices with regularly geometrical structures, in which the gain medium can be strongly coupled to the naturally formed microcavity, stable and ultrafast (3ps) blue CESF with excellent optical performance (threshold ≈33 µJ cm-2 , quality factor ≈1900) is realized.

Adjusting Microscale to Atomic-Scale Structural Order in PbS Nanocrystal Superlattice for Enhanced Photodetector Performance.

An investigation is presented into the effect of the long-range order on the optoelectronic properties of PbS quantum dot (QD) superlattices, which form mesocrystals, for potential use in photodetector applications. By self-assembly of QD nanocrystals on an Si/SiOx substrate, a highly ordered and densely packed PbS QD superlattice with a microscale size is obtained. The results demonstrate that annealing treatment induces mesocrystalline superlattices with preferred growth orientation, achieved by dislodging ligands. The improved orientation and electronic coupling of the mesocrystalline superlattices exhibit superior photodetector performance compared to disordered QD structures and closely packed superlattices. This improved performance is attributed to atomic alignment between QDs, leading to enhanced electronic coupling. The findings suggest that these mesocrystalline superlattices have promising potential for the next generation of QD optoelectronic devices.

Atomic Lattice Resolved Electron Tomography of a 3D Self‐Assembled Mesocrystal

AbstractComplex 3D architectures of nanoscale building blocks can be created by self‐assembly, but characterization of the atomic to mesoscale structure of such materials is limited by the difficulty of visualizing atoms across multiple length scales. Here, scanning transmission electron microscopy (STEM) and full‐tilt tomographic reconstruction are used to image a single‐crystalline region of a 3D epitaxially‐fused PbSe quantum dot (QD) superlattice containing 633 QDs at a spatial resolution of 2.16 Å. The combined real‐space and reciprocal‐space analysis enables 3D mesoscale correlations of atomic lattice and superlattice order across hundreds of nanocrystals in 3D for the first time. Inhomogeneity in QD positional and orientational order reveals that the QD surface layers template the superlattice and that orientational entropy is higher in the interior layers than the surface layers. The measurement and analysis techniques presented here are applicable to a broad range of 3D nanostructured materials.