Robustness of quantum dot power-law blinking

Photon emission from quantum dots (QDs) and other quantum emitters is characterized by abrupt jumps between an "on" and an "off" state. In contrast to ions and atoms however, the durations of bright and dark periods in colloidal QDs curiously defy a characteristic time scale and are best described by a power-law probability distribution, i.e., ρ(τ) ∝ τ(-α). We controllably couple a single colloidal QD to a single gold nanoparticle and find that power-law blinking is preserved unaltered even as the gold nanoparticle drastically modifies the excitonic decay rate of the QD. This resilience of the power law to change provides evidence that blinking statistics are not swayed by environment-induced variations in kinetics and provides clues toward the mechanism responsible for universal fluorescence intermittency.     

Peptide-mediated constructs of quantum dot nanocomposites for enzymatic control of nonradiative energy transfer

A bottom-up approach for constructing colloidal semiconductor quantum dot (QDot) nanocomposites that facilitate nonradiative Fo?rster-type resonance energy transfer (FRET) using polyelectrolyte peptides was proposed and realized. The electrostatic interaction of these polypeptides with altering chain lengths was probed for thermodynamic, structural, and morphological aspects. The resulting nanocomposite film was successfully cut with the protease by digesting the biomimetic peptide layer upon which the QDot assembly was constructed. The ability to control photoluminescence decay lifetime was demonstrated by proteolytic enzyme activity, opening up new possibilities for biosensor applications.     

Electrical and optical control of entanglement entropy in a coupled triple quantum dot system

We investigated theoretically the entanglement creation through tunneling rate and fields in a four-level triple quantum dot molecule based on InAs/GaAs/AlGaAs heterostructure in both steady state and transient state. We demonstrate that the entanglement entropy among the QDM and its spontaneous emission fields can be controlled by coherent and incoherent pumping field and tunnel-coupled electronics levels. The results may provide some new possibilities for technological applications in solid-state quantum information science, quantum computing, teleportation, encryption, compression codec, and optoelectronics.     

Decoherence of nuclear spin quantum memory in a quantum dot

Recently, an ensemble of nuclear spins in a quantum dot have been proposed as a long-lived quantum memory. A quantum state of an electron spin in the dot can be faithfully transfered into nuclear spins through controlled hyperfine coupling. Here we study the decoherence of this memory due to nuclear spin dipolar coupling and inhomogeneous hyperfine interaction during the storage period. We calculated the maximum fidelity of writing, storing, and reading operations. Our results show that nuclear spin dynamics can severely limit the performance of the proposed device for quantum information processing and storage based on nuclear spins.     

Tunneling control of cavity linewidth narrowing via quantum interference in triangular quantum dot molecules

A scheme for tunneling control of cavity linewidth narrowing by quantum interference in triangular-type triple quantum dots (TQDs) is proposed. In such system, quantum interference induced by tunneling between the TQDs can result in the appearance of two transparency windows and a steep dispersion. Furthermore, when the sample is embedded in a ring cavity, an ultranarrow transmission peak is obtained within the narrowed transparency windows. And by varying the tunneling, the linewidth and the position of the ultranarrow transmission peak can be engineered. Because no coupling laser is required, the scheme proposed here is more convenient for future experiments and applications in optics, and may be useful in designing novel optoelectronic devices.     

An active lasing region with a quantum well and a quantum dot array

A combined active lasing region of the new type, containing an In_0.2Ga_0.8As quantum well (QW) and a single-layer array of InAs quantum dots (QDs) located outside the QW, was studied. In this system, the QW accumulates the injected charge carriers and the QD array serves as a radiator. The energy levels of electrons and holes in a QD were calculated. It is shown that the QDs can be filled by the resonance tunneling of holes from the QW to an unoccupied QD. The electron energy level in an unoccupied QD is markedly higher than that in the QW, but occupation of the QD by a hole leads to a resonance of the electron levels. Theoretical conclusions agree with the results of observations on a prototype laser with a combined active region.     

ZnO hollow quantum dot: a promising deep-UV light emitter

We establish an analytic model to illustrate the energy bandgap of ZnO hollow quantum dots (HQDs) with negative curvature surface from the perspective of nanothermodynamics. It was found that the bandgap of ZnO HQDs shows a pronounced blue-shift as comparable to those of bulk counterpart and free nanocrystals. Furthermore, the photoelectric properties of ZnO HQDs can be effectively modulated by three independent dimensions, including the outer surface, the inner surface and the shell thickness. Strikingly, the emission wavelength of ZnO HQDs can be extended into the deep-ultraviolet (DUV) region, which suggests this kind of nanostructure could be expected to be applicable for the new-generation, compact, and environmentally friendly alternative DUV light emitter.     

Scanned gate microscopy of a one-dimensional quantum dot

We analyze electrostatic interaction between a sharp conducting tip and a thin one-dimensional wire, e.g., a carbon nanotube, in a scanned gate microscopy (SGM) experiment. The problem is analytically tractable if the wire resides on a thin dielectric substrate above a metallic backgate. The characteristic spatial scale of the electrostatic coupling to the tip is equal to its height above the substrate. Numerical simulations indicate that imaging of individual electrons by SGM is possible once the mean electron separation exceeds this scale (typically, a few tens of nm). Differences between weakly and strongly invasive SGM regimes are pointed out.     

Functional characterization and atomic force microscopy of a DNA repair protein conjugated to a quantum dot

Quantum dots (QDs) possess highly desirable optical properties that make them ideal fluorescent labels for studying the dynamic behavior of proteins. However, a lack of characterization methods for reliably determining protein-quantum dot conjugate stoichiometry and functionality has impeded their widespread use in single-molecule studies. We used atomic force microscopic (AFM) imaging to demonstrate the 1:1 formation of UvrB-QD conjugates based on an antibody-sandwich method. We show that an agarose gel-based electrophoresis mobility shift assay and AFM can be used to evaluate the DNA binding function of UvrB-QD conjugates. Importantly, we demonstrate that quantum dots can serve as a molecular marker to unambiguously identify the presence of a labeled protein in AFM images.     

Bandlike transport in strongly coupled and doped quantum dot solids: a route to high-performance thin-film electronics

We report bandlike transport in solution-deposited, CdSe QD thin-films with room temperature field-effect mobilities for electrons of 27 cm(2)/(V s). A concomitant shift and broadening in the QD solid optical absorption compared to that of dispersed samples is consistent with electron delocalization and measured electron mobilities. Annealing indium contacts allows for thermal diffusion and doping of the QD thin-films, shifting the Fermi energy, filling traps, and providing access to the bands. Temperature-dependent measurements show bandlike transport to 220 K on a SiO(2) gate insulator that is extended to 140 K by reducing the interface trap density using an Al(2)O(3)/SiO(2) gate insulator. The use of compact ligands and doping provides a pathway to high performance, solution-deposited QD electronics and optoelectronics.     

Fast control of nuclear spin polarization in an optically pumped single quantum dot

Highly polarized nuclear spins within a semiconductor quantum dot induce effective magnetic (Overhauser) fields of up to several Tesla acting on the electron spin, or up to a few hundred mT for the hole spin. Recently this has been recognized as a resource for intrinsic control of quantum-dot-based spin quantum bits. However, only static long-lived Overhauser fields could be used. Here we demonstrate fast redirection on the microsecond timescale of Overhauser fields on the order of 0.5 T experienced by a single electron spin in an optically pumped GaAs quantum dot. This has been achieved using coherent control of an ensemble of 10(5) optically polarized nuclear spins by sequences of short radiofrequency pulses. These results open the way to a new class of experiments using radiofrequency techniques to achieve highly correlated nuclear spins in quantum dots, such as adiabatic demagnetization in the rotating frame leading to sub-μK nuclear spin temperatures, rapid adiabatic passage, and spin squeezing.     

Colloidal quantum dot photovoltaics: the effect of polydispersity

The size-effect tunability of colloidal quantum dots enables facile engineering of the bandgap at the time of nanoparticle synthesis. The dependence of effective bandgap on nanoparticle size also presents a challenge if the size dispersion, hence bandgap variability, is not well-controlled within a given quantum dot solid. The impact of this polydispersity is well-studied in luminescent devices as well as in unipolar electronic transport; however, the requirements on monodispersity have yet to be quantified in photovoltaics. Here we carry out a series of combined experimental and model-based studies aimed at clarifying, and quantifying, the importance of quantum dot monodispersity in photovoltaics. We successfully predict, using a simple model, the dependence of both open-circuit voltage and photoluminescence behavior on the density of small-bandgap (large-diameter) quantum dot inclusions. The model requires inclusion of trap states to explain the experimental data quantitatively. We then explore using this same experimentally tested model the implications of a broadened quantum dot population on device performance. We report that present-day colloidal quantum dot photovoltaic devices with typical inhomogeneous linewidths of 100-150 meV are dominated by surface traps, and it is for this reason that they see marginal benefit from reduction in polydispersity. Upon eliminating surface traps, achieving inhomogeneous broadening of 50 meV or less will lead to device performance that sees very little deleterious impact from polydispersity.     

Spin filling of valley-orbit states in a silicon quantum dot

We report the demonstration of a low-disorder silicon metal-oxide-semiconductor (Si MOS) quantum dot containing a tunable number of electrons from zero to N = 27. The observed evolution of addition energies with parallel magnetic field reveals the spin filling of electrons into valley-orbit states. We find a splitting of 0.10?meV between the ground and first excited states, consistent with theory and placing a lower bound on the valley splitting. Our results provide optimism for the realisation in the near future of spin qubits based on silicon quantum dots.     

Control of electron localization in a coupled quantum dot structure

Abstract

We systematically investigate the dynamical behaviour of an electron in a double quantum dot system under the influence of an external AC field. It is assumed that the quantum dot confined structure exhibits a non-negligible Coulomb charging energy, inversely proportional to its small capacitance. The dynamic evolution of the system is obtained by numerically solving the coupled, nonlinear, equations derived from the time-dependent Schrödinger equation. We find cases where the electron is localized in the initially placed dot when both effects of the Coulomb charging energy and the external field are present, even though if either effect is absent the electron will tunnel between dots. We also show that we can pre-select the shape and rise time of a semi-infinite, pulsed, AC field in order to transfer an electron from the initially placed dot to the other dot and localize it there.     

Superlocalization spectral imaging microscopy of a multicolor quantum dot complex

The key factor of realizing super-resolution optical microscopy at the single-molecule level is to separately position two adjacent molecules. An opportunity to independently localize target molecules is provided by the intermittency (blinking) in fluorescence of a quantum dot (QD) under the condition that the blinking of each emitter can be recorded and identified. Herein we develop a spectral imaging based color nanoscopy which is capable of determining which QD is blinking in the multicolor QD complex through tracking the first-order spectrum, and thus, the distance at tens of nanometers between two QDs is measured. Three complementary oligonucleotides with lengths of 15, 30, and 45 bp are constructed as calibration rulers. QD585 and QD655 are each linked at one end. The measured average distances are in good agreement with the calculated lengths with a precision of 6 nm, and the intracellular dual-color QDs within a diffraction-limited spot are distinguished.     

Independent control of InAs quantum dot density and size on AlxGa1–xAs surfaces

Decoupling of InAs quantum dot (QD) size and density on Al_xGa_1?xAs surfaces (x = 0, 0.15, 0.30, and 0.45) is achieved by using a low growth rate and careful control of the temperature. The deposition rate of 0.01 μm/h, instead of 0.05 μm/h, allows the QDs to ripen with additional InAs deposition while the substrate temperature (490–520 °C) determines the QD density. On the GaAs surface, an increase of 10 °C results in an order of magnitude lower QD density. The increase of Al in the Al_xGa_1?xAs surfaces results in a higher dot density, lower dot size, and an increased size distribution. All surfaces show reduced QD density with increasing temperature and an identical zero dot density temperature at 523 °C. The GaAs surface shows increasing QD height with temperature while the Al_xGa_1?xAs surfaces show the opposite trend, but the InAs volume fraction in QDs for all surfaces decreases with increasing temperature, implying a more stable wetting layer. Increasing Al content also increases the InAs volume fraction in QDs, implying the wetting layer for all but the 520 °C samples is less than one monolayer. Photoluminescence samples demonstrate ground state QD energies above the GaAs bandedge.     

Multimode emission and optical power in a semiconductor quantum dot laser

Effect of spatial hole burning (SHB) and multi-longitudinal-mode generation on high power operation of a quantum dot (QD) laser is studied. We use a set of rate equations for confined carriers in QDs, free carriers in the optical confinement layer and photons. The threshold currents and output powers of modes are computed numerically. The power of the main mode is reduced due to lasing of higher-order modes and spatially nonuniform carrier distribution. As a new mode turns on, kinks appear in the light-current curves (LCCs) of existing modes. SHB reduces the total optical power of a laser and contributes to nonlinearity of the overall LCC. The effect is more significant when any of the parameters of the structure is close to its critical tolerable value. The LCC becomes more linear with improving QD-size uniformity or increasing surface density of QDs or cavity length.     

Strong extinction of a far-field laser beam by a single quantum dot

Through the utilization of index-matched GaAs immersion lens techniques, we demonstrate a record extinction (12%) of a far-field focused laser beam by a single InAs/GaAs quantum dot. This contrast level enables us to report for the first time resonant laser transmission spectroscopy on a single InAs/GaAs quantum dot without the need for phase-sensitive lock-in detection.     

Thermal behavior of a quantum dot nanocomposite as a color converting material and its application to white LED

We present a novel nanocomposite, a mixture of a CdSe/CdS/ZnS red quantum dot (QD), an Sr(2)SiO(4):Eu green phosphor and silicone resin for a color converting material. The temperature rise and the optical characteristics of the nanocomposite due to the addition of the QD have been investigated in terms of QD content ratio and the mixing components. The experimental results suggested that a small addition of QDs generated a large amount of heat during light conversion at the wavelength of QD emission. Considering the temperature rise in a nanocomposite, we applied 0.2 wt% QDs on an InGaN blue LED chip. As a result, we could achieve a white LED device with a high color rendering index of 83.2, a high luminance of 65.86 lm W(-1) and a moderate temperature increase of 94?°C. The white LED converted by the newly developed QD-phosphor nanocomposite has great potential in future illumination.