Aligned Quantum Dots Research Articles

Unambiguous spectral characterization on triplet energy transfer from quantum dots mediated by hole transfer competing with other carrier dynamics.

Triplet generation by quantum dots (QDs)-sensitized molecules emerges great potential in many applications. However, the mechanism of triplet energy transfer (TET) is still fuzzy especially due to the complicated energy level alignment of QDs and molecules or trap states in QDs. Here, CdSe QDs and 5-tetracene carboxylic acid (TCA) molecules are selected as the triplet donor and acceptor, respectively, to form a TET system. By tuning the band gap of CdSe, the CdSe-TCA complex is exactly designed to present a Type-II like alignment of relative energetics. Coupling the transient absorption and time-resolved fluorescence spectra, all carrier dynamics is distinctly elucidated. Quantitative analysis demonstrates that hole transfer persisting for ∼ 2 ps outcompetes all other carrier dynamics such as electron trapping (∼100 ps level), charge recombination (∼ 5 ns) and the so-called "back transfer charge recombination" (∼50 ns), and thus leads to a hole-transfer-mediated TET process. The low TET yield (∼34.0%) ascribed to electron behavior can be further improved if electron trapping and charge recombination are efficiently suppressed. The observation on distinguishable carrier dynamics attributed to legitimate design of energy level alignment facilitates a better understanding of the TET mechanism from QDs to molecules as well as further development of photoelectronic devices based on such TET systems.

Perovskite Plasticity: Exploiting Instability for Self‐Optimized Performance

AbstractHalide perovskites display outstanding photoluminescence quantum yield, tunable emission, and simple deposition, which make them attractive for optoelectronics. At the same time, their facile ion migration and transformation under optical, electrical, and chemical stress are seen as a major limitation. Mixed halide perovskites are particularly problematic since optical excitation can cause changes in the bandgap that are detrimental for solar cell and light‐emitting diode efficiency and stability. In this work, instead of preventing such changes, photo‐induced halide segregation in perovskites is exploited to enable responsive, reconfigurable, and self‐optimizing materials. The mixed halide perovskite film is trained to give directional light emission using a nanophotonic microlens; through a self‐optimized process of halide photosegregation, the system mimics the training stimulus. Longer training leads to more highly directional emission, while different halide migration kinetics in the light (fast training) and dark (slow forgetting) allows for material memory. This self‐optimized material performs significantly better than lithographically aligned quantum dots because it eliminates lens‐emitter misalignment and automatically corrects for lens aberrations. The system shows a combination of mimicking, improving over time, and memory, which comprise the basic requirements for learning, and allow for the intriguing prospect of intelligent optoelectronic materials.

Photoluminescence of compact GeSi quantum dot groups with increased probability of finding an electron in Ge

The photoluminescence (PL) of the combined Ge/Si structures representing a combination of large (200–250 nm) GeSi disk-like quantum dots (nanodisks) and four-layered stacks of compact groups of smaller (30 nm) quantum dots grown in the strain field of nanodisks was studied. The multiple increase in the PL intensity was achieved by the variation of parameters of vertically aligned quantum dot groups. The experimental results were analyzed on the basis of calculations of energy spectra, electron and hole wave functions. It was found that the quantum dot arrangement in compact groups provides the effective electron localization in Δx,y-valleys with an almost equal probability of finding an electron in the Si spacer and Ge barrier. As a result, the main channels of radiative recombination in the structures under study correspond to spatially direct optical transitions.

Effect of energy transfer on the optical properties of surface-passivated perovskite films with CdSe/ZnS quantum dots

Surface passivation is an effective method to protect the surfaces and improve the luminescence properties of perovskite (PS) films. CdSe/ZnS core-shell quantum dots (QDs) have been employed for surface passivation of PS films because of their size-dependent tunable bandgaps. Herein, the energy transfer (ET) behavior of CH3NH3PbI2Br PS films covered with CdSe/ZnS QDs (QD/PS hybrid structures) is characterized by using photoluminescence (PL) and time-resolved PL spectroscopy. The PL decay time and the integrated PL intensity of the QD/PS hybrid structure increase compared with those of the bare PS films, owing to ET from the QDs to the PS and reduced charge traps. The ET efficiency increases from ~7% to 63% for the QD/PS hybrid structure when the core diameter of the QDs decreases from 6.5 to 2.7 nm, respectively. This can be explained by the charge transfer rate enhancement due to the control of energy level alignment of QDs. These results allow us to understand fundamental mechanisms such as ET from QDs to PS films as a function of the size of the QD.

Structure and Charge Carrier Dynamics in Colloidal PbS Quantum Dot Solids.

The ligand exchange process is a key step in fabrications of quantum dot (QD) optoelectronic devices. In this work, on the basis of grazing incidence X-ray scattering techniques, we find that the ligand exchange process with halide ions changes the PbS QD superlattice from face-centered-cubic to body-centered-cubic stacking, while the QD crystal lattice orientation also changes from preferentially "edge-up" to "corner-up". Thus, the QDs' shape is supposed to be the main factor for the alignment of QDs in close packed solids. Moreover, we tailor the alignment of the close packed solids by thermal treatments and further investigate their inner charge carrier dynamics by pump-probe transient absorption experiments. An overall better structure alignment optimizes the charge carrier hopping rate, as confirmed by the time dependence of the photon bleaching peak shift. The QD solid treated at 100 °C shows the best inner structure alignment with the best charge carrier hopping rate.

Capillary-bridge mediated assembly of aligned perovskite quantum dots for high-performance photodetectors

A liquid bridge induced assembly strategy to align perovskite QDs in one direction for high-performance photodetectors.

Correlation between photoluminescence and morphology for single layer self-assembled InGaAs/GaAs quantum dots

Single layer self-assembled InGaAs quantum dots (QDs) are manipulated by using different arsenic species on GaAs (100) surface. The As4 molecules are experimentally observed to be more promising than As2 to promote the formation of one-dimensionally-aligned QD-chain arrays. The lateral alignment of QDs and the corresponding formation of dot chains are explained by the anisotropic surface kinetics in combination with the different reactivities of the two molecules with bonding sites on the GaAs (100) surface. Photoluminescence (PL) measurements demonstrate that the spectra of the QD-chains broaden to higher energy and increases in intensity with increasing excitation laser power. The PL band of the QD-chains also exhibits a 9 meV reduction in linewidth as temperature increases starting from 8 K. These observations confirm an efficient lateral coupling between neighboring QDs and thereafter polarized QD emission, whereas the randomly distributed QDs grown with As2 show no preferential polarization. Such QD-chains exhibiting anisotropic properties have the potential for nanophotonics applications like electro-optic modulators with very low drive voltage and ultra-wide bandwidth operation.

Spin and charge Nernst effect in a four-terminal double quantum dot system

With the increase of integration scale, heat dissipation becomes one of the major problems in high density electronic devices and circuits. Controlling and reusing the heat energy in such miniaturized structures are essential topics for current and future technologies. With the development of microfabrication technology and low-temperature measurement technology in the last two decades, the thermoelectric measurement in low-dimensional sample has been feasible, and the thermal transport has received more and more attention. For the multi-terminal device, there is a novel thermoelectric phenomenon, called the spin Nernst effect, in which spin currents (or spin voltages) are generated perpendicularly to the temperature gradient. The spin Nernst effect has been confirmed experimentally, and has been theoretically studied in a variety of materials. In this paper, the spin and charge Nernst effect in a pair of vertically aligned quantum dots attached to four leads are studied in the Coulomb blockade regime based on the nonequilibrium Green's function technique. We focus on the influences of magnetic configuration and intra-dot (inter-dot) Coulomb interaction on the spin and charge Nernst effect. It is found that the signs and the magnitudes of spin and charge Nernst effect can be modulated by adjusting the magnetization directions of ferromagnetic electrodes. When the magnetic moments in the 1 and 3 electrodes are turned to antiparallel alignment, the pure spin Nernst (without charge Nernst) effect can occur by applying a transverse temperature gradient. Conversely, the spin and charge Nernst effect disappear if the magnetic moments of lead 1 and lead 3 are in the case of parallel configuration. Except for left and right thermal leads, we investigate the effect of the middle lead (lead 4) on the property of the Nernst effect. We find that when the normal metal lead 4 is transferred to ferromagnetic metal, the spin and charge Nernst effect both can be obtained simultaneously. In the end of the paper, we study the influences of intra-dot and inter-dot Coulomb interaction on the spin dependent Nernst coefficient. Through numerical calculations, we demonstrate that the magnitude of the Nernst effect is less dependent on the polarization strength of ferromagnetic electrodes, but can be remarkably enhanced by the Coulomb blockade. The spin Nernst coefficient is predicted to be more than two orders of magnitude larger than that of the case of zero Coulomb interaction. All the results indicate that the proposed four-terminal double quantum dot nano system is a promising candidate for spin caloritronic device.

Self-Assembled PbSe Nanowire:Perovskite Hybrids.

Inorganic semiconductor nanowires are of interest in nano- and microscale photonic and electronic applications. Here we report the formation of PbSe nanowires based on directional quantum dot alignment and fusion regulated by hybrid organic-inorganic perovskite surface ligands. All material synthesis is carried out at mild temperatures. Passivation of PbSe quantum dots was achieved via a new perovskite ligand exchange. Subsequent in situ ammonium/amine substitution by butylamine enables quantum dots to be capped by butylammonium lead iodide, and this further drives the formation of a PbSe nanowire superlattice in a two-dimensional (2D) perovskite matrix. The average spacing between two adjacent nanowires agrees well with the thickness of single atomic layer of 2D perovskite, consistent with the formation of a new self-assembled semiconductor nanowire:perovskite heterocrystal hybrid.

Atom probe tomography analysis of InAlGaAs capped InAs/GaAs stacked quantum dots with variable barrier layer thickness

In this work, we analyse by Atom Probe Tomography (APT) the composition distribution of InAs/GaAs stacked quantum dots (QDs) with InAlGaAs capping layers. Two different GaAs barrier layer thicknesses have been considered, 20 nm and 30 nm. Our results have shown the vertical alignment of QDs in the structure with reduced barrier layer and the unexpected formation of InGaAs quantum rings (QRings) surrounded by an Al-rich area in this sample. The APT data show that the QRings contain an amount of In atoms that is double the In composition in the QDs. The reduced strain measured in the QRings with regard to the QDs suggests that this change in morphology is promoted by the reduction in the strain of the nanostructure. The behaviour of Al during the process of formation of the QRings is also discussed.

Enhanced photocarrier generation in large-scale photonic nanostructures fabricated from vertically aligned quantum dots

We demonstrate enhanced photocarrier generation using photonic nanostructures fabricated by a wet etching technique with vertically aligned quantum dots (QDs). Using photoluminescence excitation spectroscopy, we found that the photocarrier generation in Ge/Si QDs placed close to the surface is enhanced below the band gap energy of crystalline silicon. The enhancement is explained by light trapping owing to the photonic nanostructures. Electromagnetic wave simulations indicate that the photonic nanostructure with a subwavelength size will be available to light trapping for efficient photocarrier generation by increasing their dip depth.

Excitons in quantum dot molecules: Coulomb coupling, spin-orbit effects, and phonon-induced line broadening

Excitonic states and the line shape of optical transitions in coupled quantum dots (quantum dot molecules) are studied theoretically. For a pair of electrically tunable, vertically aligned quantum dots we investigate the coupling between spatially direct and indirect excitons caused by different mechanisms such as tunnel coupling, Coulomb coupling, coupling due to the spin-orbit interaction, and coupling induced by a structural asymmetry. The peculiarities of the different types of couplings are reflected in the appearance of either crossings or avoided crossings between direct and indirect excitons, the latter ones being directly visible in the absorption spectrum. We analyze the influence of the phonon environment on the spectrum by calculating the line shape of the various optical transitions including contributions due to both pure dephasing and phonon-induced transitions between different exciton states. The linewidth enhancement due to phonon-induced transitions is particularly pronounced in the region of an anticrossing and it strongly depends on the energy splitting between the two exciton branches.

Epitaxially Self‐Assemblied Quantum Dot Pairs

The present paper reviews the recent efforts and achievements regarding the fabrication techniques of self‐assembled quantum dot pairs. Quantum dot pairs, the simplest but most‐investigated quantum dot molecules, have attracted significant attention due to their potential applications in quantum information technologies. Coupled quantum dot pairs have been used to implement quantum qubits, quantum gates, and exciton‐spin memory. In the last several decades, the development of epitaxial growth has advanced the design and control of how novel quantum dot pairs form at nanoscale. Both vertically and laterally aligned quantum dot pairs can be grown using the molecular beam epitaxy technique. In this review, we provide assess various growth methods of aligned quantum dot pairs fabricated by molecular beam epitaxy. We highlight the recent development of novel growth techniques and discuss the morphological, electrical, and optical properties of quantum dot pairs. Using advanced epitaxial growth techniques to fabricate well‐controlled quantum dot pairs opens the door to developing of advanced quantum information technologies and understanding the unveiled physics and properties of artificial quantum molecules.

Photoelectrochemical cell/dye-sensitized solar cell tandem water splitting systems with transparent and vertically aligned quantum dot sensitized TiO2 nanorod arrays

The present work reports fabrication of vertically aligned CdS sensitized TiO2 nanorod arrays grown on transparent conducting oxide substrate with high transparency as a photoanode in photoelectrochemical cell for water splitting. To realize an unassisted water splitting system, the photoanode and dye-sensitized solar cell tandem structures are tried and their electrochemical behaviors are also investigated. The hydrothermally grown TiO2 nanorod arrays followed by CdS nanoparticle decoration can improve the light absorption of long wavelength light resulting in increased photocurrent density. Two different techniques (electrodeposition and spray pyrolysis deposition) of CdS nanoparticle sensitization are carried out and their water splitting behaviors in the tandem cell are compared.

Improving the characteristics of intermediate-band solar cell devices using a vertically aligned InAs/GaAsSb quantum dot structure

This study demonstrates the feasibility of improving the optical properties of a vertically aligned quantum dot (QD) structure and the performance of a quantum dot intermediate band solar cell (QD-IBSC) by capping a GaAsSb layer on the InAs QDs. Experimental results indicate that dot-size uniformity is significantly improved due to the strain modification in the evolution of the successive vertically aligned dot layer growth. A solar cell device with an InAs/GaAsSb columnar dot structure increases the short-circuit current density (Jsc) by 8.8%, compared to a GaAs reference cell. This dot structure also increases quantum efficiency by up to 1200nm through the absorption of lower-energy photons. The InAs/GaAsSb QD-IBSC also improves the open-circuit voltage (Voc), indicating a reduction in misfit defect density and recombination current density. The results of this study confirm the ability of a columnar InAs/GaAsSb QD structure to enhance the device performance.

Antibonding ground state of holes in double vertically coupled Ge/Si quantum dots

The existence of the antibonding ground state of holes in artificial molecules, which are formed by the vertically coupled Ge/Si quantum dots, has been proved experimentally. This phenomenon is absent in natural molecules and double quantum dots containing electrons. It is a consequence of spin-orbit interaction and deformation effects in the valence band of vertically aligned quantum dots.

Supercurrent through a multilevel quantum dot close to singlet-triplet degeneracy

We investigate two serially aligned quantum dots in the molecular regime of large tunnel couplings $t$. A Zeeman field $B$ is used to tune the energy difference of singlet and triplet spin configurations. Attaching this geometry to BCS source and drain leads with gap $\ensuremath{\Delta}$ and phase difference $\ensuremath{\varphi}$ gives rise to an equilibrium supercurrent $J$. To compute $J$ in the presence of Coulomb interactions $U$ between the dot electrons, we employ the functional renormalization group (FRG). For $B\ensuremath{\approx}t$, where the singlet and (one out of a) triplet spin states are equal in energy, the current exhibits characteristics of a 0-$\ensuremath{\pi}$ transition similar to a single impurity. Its magnitude in the $\ensuremath{\pi}$ phase, however, jumps discontinuously at $B=t$, being smaller on the triplet side. By exploiting the flexibility of the FRG, we demonstrate that this effect is generic and calculate $J$ for realistic experimental parameters $\ensuremath{\Delta}$, $U$, and gate voltages $\ensuremath{\epsilon}$. To obtain a more thorough understanding of the discontinuity, we analytically treat the limit $\ensuremath{\Delta}=\ensuremath{\infty}$, where one can access the exact many-particle states. Finally, carrying out perturbation theory in the dot-lead couplings substantiates the intuitive picture that Cooper-pair tunneling is favored by a singlet spin configuration while inhibited by a triplet one.

Self-assembled InAs quantum dots on anti-phase domains of GaAs on Ge substrates

The authors report the formation of self-assembled InAs quantum dots (QDs) grown on GaAs/Ge substrates having anti-phase domains (APDs) by molecular beam epitaxy. The AFM images of InAs QDs grown on different GaAs thicknesses are shown and compared. The samples with InAs coverage of 1.80MLs with GaAs thickness of 300 and 700nm show non-uniformed size distribution of the dots. Due to anisotropic property of quantum dots, ellipsoidal quantum dots appear. Unexpectedly, most of InAs quantum dots align perpendicularly to anti-phase boundary (APB) and to quantum dot alignment formed in adjacent domains. Photoluminescence spectrum excited by 20mW 476-nm Ar+ laser at 20K does not show emission peak of InAs QDs. This is due to defects in the GaAs buffer layer.

Polarization-dependent strong coupling in elliptical high-Qmicropillar cavities

We present a combined experimental and theoretical study of the polarization-dependent strong-coupling regime between two quantum dots and an asymmetric micropillar cavity. The photoluminescence emission demonstrates that the fundamental cavity mode is split into two linearly polarized cavity modes, both of which are coupled to 45%-aligned quantum dot excitons. We map out various single-exciton and double-exciton coupling regimes, including the full energy dispersion of dual-exciton and dual-cavity emission under $x$ and $y$ detection angle. To explain the complex light-matter coupling we apply an analytical photon Green function approach that successfully reproduces the qualitative features of our experimental data.

The effect of spacer layer thickness on vertical alignment of InGaAs/GaNAs quantum dots grown on GaAs(3 1 1)B substrate

We fabricated multiple stacked self-organized InGaAs quantum dots (QDs) on GaAs (3 1 1)B substrate by atomic hydrogen-assisted molecular beam epitaxy (H-MBE) to realize an ordered three-dimensional QD array. High quality stacked QDs with good size uniformity were achieved by using strain-compensation growth technique, in which each In 0.35Ga 0.65As QD layer was embedded by GaNAs strain-compensation layer (SCL). In order to investigate the effect of spacer layer thickness on vertical alignment of InGaAs/GaNAs QDs, the thickness of GaNAs SCL was varied from 40 to 20 nm. We observed that QDs were vertically aligned in [3 1 1] direction when viewed along [0 1 −1], while the alignment was inclined when viewed along [−2 3 3] for all samples with different SCL thickness. This is due to their asymmetric shape along [−2 3 3] with two different dominant facets thereby the local strain field around QD extends further outward from the lower-angle facet. Furthermore, the inclination angle of vertical alignment QDs became monotonously smaller from 22° to 2° with decreasing SCL thickness from 40 to 20 nm. These results suggest that the strain field extends asymmetrically resulting in vertically tilted alignment of QDs for samples with thick SCLs, while the propagated local strain field is strong enough to generate the nucleation site of QD formation just above lower QD in the sample with thinner SCLs.