Showing papers on "Quantum dot published in 2005"

Solar energy conversion by dye-sensitized photovoltaic cells

Abstract: The quality of human life depends to a large degree on the availability of energy. This is threatened unless renewable energy resources can be developed in the near future. Chemistry is expected to make important contributions to identify environmentally friendly solutions of the energy problem. One attractive strategy discussed in this Forum Article is the development of solar cells that are based on the sensitization of mesoscopic oxide films by dyes or quantum dots. These systems have already reached conversion efficiencies exceeding 11%. The underlying fundamental processes of light harvesting by the sensitizer, heterogeneous electron transfer from the electronically excited chromophore into the conduction band of the semiconductor oxide, and percolative migration of the injected electrons through the mesoporous film to the collector electrode will be described below in detail. A number of research topics will also be discussed, and the examples for the first outdoor application of such solar cells wi...

Optical gain and stimulated emission in nanocrystal quantum dots

Abstract: The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.

Solution-processed PbS quantum dot infrared photodetectors and photovoltaics

Abstract: In contrast to traditional semiconductors, conjugated polymers provide ease of processing, low cost, physical flexibility and large area coverage1. These active optoelectronic materials produce and harvest light efficiently in the visible spectrum. The same functions are required in the infrared for telecommunications (1,300–1,600 nm), thermal imaging (1,500 nm and beyond), biological imaging (transparent tissue windows at 800 nm and 1,100 nm), thermal photovoltaics (>1,900 nm), and solar cells (800–2,000 nm). Photoconductive polymer devices have yet to demonstrate sensitivity beyond ∼800 nm (refs 2,3). Sensitizing conjugated polymers with infrared-active nanocrystal quantum dots provides a spectrally tunable means of accessing the infrared while maintaining the advantageous properties of polymers. Here we use such a nanocomposite approach in which PbS nanocrystals tuned by the quantum size effect sensitize the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) into the infrared. We achieve, in a solution-processed device and with sensitivity far beyond 800 nm, harvesting of infrared-photogenerated carriers and the demonstration of an infrared photovoltaic effect. We also make use of the wavelength tunability afforded by the nanocrystals to show photocurrent spectra tailored to three different regions of the infrared spectrum.

PbSe Nanocrystal Solids for n- and p-Channel Thin Film Field-Effect Transistors

Abstract: Initially poorly conducting PbSe nanocrystal solids (quantum dot arrays or superlattices) can be chemically "activated" to fabricate n- and p-channel field effect transistors with electron and hole mobilities of 0.9 and 0.2 square centimeters per volt-second, respectively; with current modulations of about 10(3) to 10(4); and with current density approaching 3 x 10(4) amperes per square centimeter. Chemical treatments engineer the interparticle spacing, electronic coupling, and doping while passivating electronic traps. These nanocrystal field-effect transistors allow reversible switching between n- and p-transport, providing options for complementary metal oxide semiconductor circuits and enabling a range of low-cost, large-area electronic, optoelectronic, thermoelectric, and sensing applications.

Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

Abstract: We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for ...

Quantum Dots as Cellular Probes

Abstract: ▪ Abstract Robust and bright light emitters, semiconductor nanocrystals [quantum dots (QDs)] have been adopted as a new class of fluorescent labels. Six years after the first experiments of their uses in biological applications, there have been dramatic improvements in understanding surface chemistry, biocompatibility, and targeting specificity. Many studies have shown the great potential of using quantum dots as new probes in vitro and in vivo. This review summarizes the recent advances of quantum dot usage at the cellular level, including immunolabeling, cell tracking, in situ hybridization, FRET, in vivo imaging, and other related technologies. Limitations and potential future uses of quantum dot probes are also discussed.

Single-shot read-out of an individual electron spin in a quantum dot

Abstract: Spin is a fundamental property of all elementary particles. Classically it can be viewed as a tiny magnetic moment, but a measurement of an electron spin along the direction of an external magnetic field can have only two outcomes: parallel or anti-parallel to the field. This discreteness reflects the quantum mechanical nature of spin. Ensembles of many spins have found diverse applications ranging from magnetic resonance imaging to magneto-electronic devices, while individual spins are considered as carriers for quantum information. Read-out of single spin states has been achieved using optical techniques, and is within reach of magnetic resonance force microscopy. However, electrical read-out of single spins has so far remained elusive. Here we demonstrate electrical single-shot measurement of the state of an individual electron spin in a semiconductor quantum dot. We use spin-to-charge conversion of a single electron confined in the dot, and detect the single-electron charge using a quantum point contact; the spin measurement visibility is ∼65%. Furthermore, we observe very long single-spin energy relaxation times (up to ∼0.85 ms at a magnetic field of 8 T), which are encouraging for the use of electron spins as carriers of quantum information.

Silica-coated nanocomposites of magnetic nanoparticles and quantum dots.

Abstract: Quantum dots (QDs) and magnetic nanoparticles (MPs) are of interest for biological imaging, drug targeting, and bioconjugation because of their unique optoelectronic and magnetic properties, respectively. To provide for water solubility and biocompatibility, QDs and MPs were encapsulated within a silica shell using a reverse microemulsion synthesis. The resulting SiO2/MP-QD nanocomposite particles present a unique combination of magnetic and optical properties. Their nonporous silica shell allows them to be surface modified for bioconjugation in various biomedical applications.

Controlling the spontaneous emission rate of single quantum dots in a two-dimensional photonic crystal.

Abstract: We observe large spontaneous emission rate modification of individual InAs quantum dots (QDs) in a 2D photonic crystal with a modified, high-$Q$ single-defect cavity. Compared to QDs in a bulk semiconductor, QDs that are resonant with the cavity show an emission rate increase of up to a factor of 8. In contrast, off-resonant QDs indicate up to fivefold rate quenching as the local density of optical states is diminished in the photonic crystal. In both cases, we demonstrate photon antibunching, showing that the structure represents an on-demand single photon source with a pulse duration from 210 ps to 8 ns. We explain the suppression of QD emission rate using finite difference time domain simulations and find good agreement with experiment.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

Abstract: We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Phonon scattering and energy relaxation in two-, one-, and zero-dimensional electron gases

Abstract: We report on calculations of inlrasubband and intersubband phonon scattering in quantum-confined electron gases based on lattice-matched In x Ga 1-x As/InP quantum wells. Dimensionality effects on the emission of acoustic phonons are studied comparing the scattering times of two-, one-, and zero-dimensional electron gases as a function of the lateral confinement. Optical phonon scattering in quantum wells and wires is discussed using a phenomenological broadening of the one-dimensional density of states. The energy relaxation rates of heated electron gases due to phonon emission and absorption have been calculated for lattice temperatures T 1 between 0.3 and 20 K. For a given healing power per electron, the electron temperature T e in a quantum wire can be greater or smaller than that in the corresponding quantum well, depending on the electron density n s , while the energy relaxation in quantum dots with significant quantization energies is always slower than in the corresponding wells and wires.

Exciton-photon strong-coupling regime for a single quantum dot embedded in a microcavity.

Abstract: We report on the observation of the strong-coupling regime between the excitonic transition of a single GaAs quantum dot and a discrete optical mode of a microdisk microcavity. Photoluminescence is performed at various temperatures to tune the quantum dot exciton with respect to the optical mode. At resonance, we observe a clear anticrossing behavior, signature of the strong-coupling regime. The vacuum Rabi splitting amounts to 400 microeV and is twice as large as the individual linewidths.

Deterministic Coupling of Single Quantum Dots to Single Nanocavity Modes

Abstract: We demonstrate a deterministic approach to the implementation of solid-state cavity quantum electrodynamics (QED) systems based on a precise spatial and spectral overlap between a single self-assembled quantum dot and a photonic crystal membrane nanocavity. By fine-tuning nanocavity modes with a high quality factor into resonance with any given quantum dot exciton, we observed clear signatures of cavity QED (such as the Purcell effect) in all fabricated structures. This approach removes the major hindrances that had limited the application of solid-state cavity QED and enables the realization of experiments previously proposed in the context of quantum information processing.

Single quantum dot coupled to a scanning optical antenna: a tunable superemitter.

Abstract: The interaction of a single quantum dot with a bowtie antenna is demonstrated for visible light. The antenna is generated at the apex of a ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ atomic force microscopy tip by focused ion beam milling. When scanned over the quantum dot, its photoluminescence is enhanced while its excited-state lifetime is decreased. Our observations demonstrate that the relaxation channels of a single quantum emitter can be controlled by coupling to an efficiently radiating metallic nanoantenna.

Trajectory Surface Hopping in the Time-Dependent Kohn-Sham Approach for Electron-Nuclear Dynamics

Abstract: The mean-field treatment of electron-nuclear interaction results in many qualitative breakdowns in the time-dependent Kohn-Sham (TDKS) density functional theory. Examples include current-induced heating in nanoelectronics, charge dynamics in quantum dots and carbon nanotubes, and relaxation of biological chromophores. The problem is resolved by the trajectory surface-hopping TDKS approach, which is illustrated by the photoinduced electron injection from a molecular chromophore into ${\mathrm{TiO}}_{2}$, and the excited-state relaxation of the green fluorescent protein chromophore.

Infrared Quantum Dots

Abstract: Colloidal nanocrystals are quantum-size-effect tunable; offer an abundance of available surface area for electronic and chemical interactions; and are processible from organic or aqueous solution onto substrates rigid or flexible, smooth or rough, flat or curved, inorganic or organic (including biological), crystalline or amorphous, conducting, semiconducting, or insulating. With the benefit of over a decade's progress in visible-light-emitting colloidal-quantum-dot synthesis, physical chemistry, and devices, significant progress has recently been made in infrared-active colloidal quantum dots and devices. This progress report summarizes the state-of-the-art in infrared colloidal quantum dots, with an emphasis on applications and devices. The applications of interest surveyed include monolithic integration of fiber-optic and free-space-communications photonic components with electronic substrates such as silicon and glass; in-vivo biological tagging in infrared spectral bands in which living tissue is optically penetrable to a depth of 5–10 cm; solar and thermal photovoltaics for energy conversion; and infrared sensing and imaging based on non-visible, including thermal, signatures. The synthesis and properties of quantum dots are first reviewed: photoluminescence quantum efficiencies greater than 50 % are achievable in solution, and stable luminescent dots are available in organic and aqueous solvents. Electroluminescent devices based on solution processing have been reported with external quantum efficiencies approaching 1 %. Photoconductive devices have been realized with 3 % internal quantum efficiencies, and a photovoltaic effect was recently observed. Electro-optic modulation achieved by either field- or charge-induced modification of the rate of optical absorption has been demonstrated based both on interband and intersubband (intraband) transitions. Optical gain from these processible materials with a threshold of 1 mJ cm–2 and an optical net modal gain coefficient of 260 ± 20 cm–1 have been reported.

Water‐Soluble Photoluminescent Silicon Quantum Dots

Abstract: For silicon quantum dots to be used in biomedical applications it is essential that they have a substantial photoluminescence quantum yield in the visible region, have a fast radiative recombination rate, and are water soluble and hydrophilic to prevent aggregation and precipitation in a biological environment. The chemical process used to terminate the surfaces of the silicon quantum dots changes the internal electronic structure and thus plays an important role in the resultant emission wavelength and radiative lifetime, and ultimately determines the solubility. [18] Silicon quantum dots with an oxide surface passivation typically display a dipole-forbidden yellow-red emission with radiative lifetimes of 10 3 –10 6 s. [18, 26] This slow rate of recombination limits the use of oxide-passivated silicon quantum dots in biological imaging. However, silicon quantum dots with a hydrogen or carbon surface passivation have electric-dipole-allowed direct band gap transitions that lead to blue photoluminescence with fast recombination rates of 10 8 –10 9 s. [18, 20]

Labelling of cells with quantum dots

Abstract: Colloidal quantum dots are semiconductor nanocrystals well dispersed in a solvent The optical properties of quantum dots, in particular the wavelength of their fluorescence, depend strongly on their size Because of their reduced tendency to photobleach, colloidal quantum dots are interesting fluorescence probes for all types of labelling studies In this review we will give an overview on how quantum dots have been used so far in cell biology In particular we will discuss the biologically relevant properties of quantum dots and focus on four topics: labelling of cellular structures and receptors with quantum dots, incorporation of quantum dots by living cells, tracking the path and the fate of individual cells using quantum dot labels, and quantum dots as contrast agents

Folate-receptor-mediated delivery of InP quantum dots for bioimaging using confocal and two-photon microscopy.

Abstract: A novel method for the synthesis of highly monodispersed hydrophillic InP−ZnS nanocrystals and their use as luminescence probes for live cell imaging is reported. Hydrophobic InP−ZnS nanocrystals are prepared by a new method that yields high-quality, luminescent core−shell nanocrystals within 6−8 h of total reaction time. Then by carefully manipulating the surface of these passivated nanocrystals, aqueous dispersions of folate-conjugated nanocrystals (folate-QDs) with high photostability are prepared. By use of confocal microscopy, we demonstrate the receptor-mediated delivery of folic acid conjugated quantum dots into folate-receptor-positive cell lines such as KB cells. These folate-QDs tend to accumulate in multivescicular bodies of KB cells after 6 h of incubation. Receptor-mediated delivery was confirmed by comparison with the uptake of these particles in folate-receptor-negative cell lines such as A549. Efficient two-photon excitation of these particles and two-photon imaging using these particles a...

Optical Spectroscopy of Semiconductor Nanostructures

Abstract: Preface / Kingdom of Nanostructures / Quantum Confinement in Low-Dimensional Systems / Resonant Light Reflection, Transmission & Absorption / Intraband Optical Spectroscopy of Nanostructures / Photoluminescence Spectroscopy / Light Scattering / Nonlinear Optics / Photogalvanic Effects / Conclusion / Appendix A: Tables of Characters of Irreducible Representations / Appendix B: Tables of Nonzero Components of Material Tensors / Appendix C: Electron Hole Exchange Interaction

High-efficiency carrier multiplication through direct photogeneration of multi-excitons via virtual single-exciton states

Abstract: We have previously demonstrated that absorption of a single photon by a nanocrystal quantum dot can generate multiple excitons with an efficiency of up to 100%. This effect, known as carrier multiplication, should lead to substantial improvements in the performance of a variety of optoelectronic and photocatalytic devices, including solar cells, low-threshold lasers and entangled photon sources. Here we present detailed analysis of the dynamics that govern the ultrafast growth of multi-exciton populations in CdSe and PbSe nanocrystals and propose a model of how such populations arise. Our analysis indicates that the generation of multi-excitons in these systems takes less than 200 fs, which suggests that it is an instantaneous event. We explain this in terms of their direct photogeneration via multiple virtual single-exciton states. This process relies on both the confinement-enhanced Coulomb coupling between single excitons and multi-excitons and the large spectral density of high-energy single- and multi-exciton resonances that occur in semiconductor nanocrystals.

Charge-qubit operation of an isolated double quantum dot.

Abstract: We have investigated coherent time evolution of pseudomolecular states of an isolated (leadless) silicon double quantum dot, where operations are carried out via capacitively coupled elements. Manipulation is performed by short pulses applied to a nearby gate, and measurement is performed by a single-electron transistor. The electrical isolation of this qubit results in a significantly longer coherence time than previous reports for semiconductor charge qubits realized in artificial molecules.

Single Quantum Dots in Silica Spheres by Microemulsion Synthesis

Abstract: A new method for the preparation of single quantum dots (QDs) in silica spheres has been developed. The work involves an oil-in-water microemulsion system with cyclohexane as the “oil” phase and Synperonic NP-5 as the surfactant. Reaction parameters such as reactant concentrations, time, and temperature were studied. The method resulted in smooth silica nanoparticles of good monodispersity and high luminescence with single QDs in the center. Elucidating the coating mechanism was attempted. Possible mechanisms include a phase transfer by ligand exchange and a surfactant-supported mechanism. In contrast to sol−gel based methods, this is a straightforward, simple “one-pot” synthesis for such particles. The resulting nanoparticles could be used for further silica growth and assembly of photonic structures, for biolabeling, or for other applications.

Self-assembly of concentric quantum double rings

Abstract: We demonstrate the self-assembled formation of concentric quantum double rings with high uniformity and excellent rotational symmetry using the droplet epitaxy technique. Varying the growth process conditions can control each ring's size. Photoluminescence spectra emitted from an individual quantum ring complex show peculiar quantized levels that are specified by the carriers' orbital trajectories.

Origin of the optical phonon frequency shifts in ZnO quantum dots

Abstract: We carried out nonresonant and resonant Raman spectroscopy of ZnO quantum dots with diameter of 20nm. On the basis of our measurements and comparison with a recently developed theory, we were able to clarify the origin of the observed phonon peak shifts in quantum dots as compared to bulk ZnO. It has been found that the spatial confinement of optical phonons in 20-nm-diameter dots leads to only few cm−1 peak shifts. At the same time, we have demonstrated, that even a low-power ultraviolet laser excitation, required for the resonant Raman spectroscopy of ZnO, leads to strong local heating of the ZnO quantum dots, which results in very large (∼14cm−1) redshifts of the optical phonon peaks. We have estimated from the observed redshift that the local temperature of the quantum dot ensemble is about 700°C. The obtained results are important for identification of phonon peaks in the Raman spectra of ZnO nanostructures.

Encoding electronic properties by synthesis of axial modulation-doped silicon nanowires.

Abstract: We describe the successful synthesis of modulation-doped silicon nanowires by achieving pure axial elongation without radial overcoating during the growth process. Scanning gate microscopy shows that the key properties of the modulated structures-including the number, size, and period of the differentially doped regions-are defined in a controllable manner during synthesis, and moreover, that feature sizes to less than 50 nanometers are possible. Electronic devices fabricated with designed modulation-doped nanowire structures demonstrate their potential for lithography-independent address decoders and tunable, coupled quantum dots in which changes in electronic properties are encoded by synthesis rather than created by conventional lithography-based techniques.