Showing papers on "Quantum dot published in 1997"

(CdSe)ZnS Core-Shell Quantum Dots - Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites

Abstract: We report a synthesis of highly luminescent (CdSe)ZnS composite quantum dots with CdSe cores ranging in diameter from 23 to 55 A. The narrow photoluminescence (fwhm ≤ 40 nm) from these composite dots spans most of the visible spectrum from blue through red with quantum yields of 30−50% at room temperature. We characterize these materials using a range of optical and structural techniques. Optical absorption and photoluminescence spectroscopies probe the effect of ZnS passivation on the electronic structure of the dots. We use a combination of wavelength dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, small and wide angle X-ray scattering, and transmission electron microscopy to analyze the composite dots and determine their chemical composition, average size, size distribution, shape, and internal structure. Using a simple effective mass theory, we model the energy shift for the first excited state for (CdSe)ZnS and (CdSe)CdS dots with varying shell thickness. Finally, we characterize the...

Block copolymer lithography: Periodic arrays of ~1011 holes in 1 square centimeter

Abstract: Dense periodic arrays of holes and dots have been fabricated in a silicon nitride–coated silicon wafer. The holes are 20 nanometers across, 40 nanometers apart, and hexagonally ordered with a polygrain structure that has an average grain size of 10 by 10. Spin-coated diblock copolymer thin films with well-ordered spherical or cylindrical microdomains were used as the templates. The microdomain patterns were transferred directly to the underlying silicon nitride layer by two complementary techniques that resulted in opposite tones of the patterns. This process opens a route for nanometer-scale surface patterning by means of spontaneous self-assembly in synthetic materials on length scales that are difficult to obtain by standard semiconductor lithography techniques.

A single-electron transistor made from a cadmium selenide nanocrystal

Abstract: The techniques of colloidal chemistry permit the routine creation of semiconductor nanocrystals, whose dimensions are much smaller than those that can be realized using lithographic techniques. The sizes of such nanocrystals can be varied systematically to study quantum size effects or to make novel electronic or optical materials with tailored properties. Preliminary studies of both the electrical and optical properties of individual nanocrystals have been performed recently. These studies show clearly that a single excess charge on a nanocrystal can markedly influence its properties. Here we present measurements of electrical transport in a single-electron transistor made from a colloidal nanocrystal of cadmium selenide. This device structure enables the number of charge carriers on the nanocrystal to be tuned directly, and so permits the measurement of the energy required for adding successive charge carriers. Such measurements are invaluable in understanding the energy-level spectra of small electronic systems, as has been shown by similar studies of lithographically patterned quantum dots and small metallic grains.

Transport in nanostructures

Abstract: 1 Introduction 2 Quantum confined systems 3 Transmission in nanostructures 4 Quantum dots and single electron phenomena 5 Interference in diffusive transport 6 Temperature decay of fluctuations 7 Non-equilibrium transport and nanodevices

Mesoscopic electron transport

Abstract: Preface L.P. Kouwenhoven, et al. Introduction to Mesoscopic Electron Transport L.P. Kouwenhoven, et al. Geometric Phases in Mesoscopic Systems - From the Aharonov-Bohm Effect to Berry Phases A. Stern. Delocalization, Inelastic Scattering and Transport Due to Interactions Y. Imry. Electron Transport in Quantum Dots L.P. Kouwenhoven, et al. Magnetotunneling Spectroscopy: Studying Self-Organized Quantum Dots and Quantum Chaology L. Eaves. Shot Noise in Mesoscopic Systems M.J.M. de Jong, C.W.J. Beenakker. Admittance and Nonlinear Transport in Quantum Wires, Point Contacts, and Resonant Tunneling Barriers M. Buttiker, T. Chirsten. Transport Theory of Interacting Quantum Dots H. Schoeller. Transport in a One-Dimensional Luttinger Liquid M.P.A. Fisher, L.I. Glazman. The Proximity Effect in Mesoscopic Diffusive Conductors D. Esteve, et al. Mesoscopic Effects in Superconductivity R. Fazio, G. Schoen. Ultrasmall Superconductors D.C. Ralph, et al. The Superconducting Proximity Effect in Semiconductor-Superconductor Systems: Ballistic Transport, Low Dimensionality and Sample Specific Properties B.J. van Wees, H. Takayanagi. Scanning Probe Microscopes and Their Applications L.L. Soh, et al. Quantum Point Contacts Between Metals J.M. van Ruitenbeek. Conductance Quantization in Metallic Nanowires N. Garcia, et al. Quantum Optics Y. Yamamoto. Topics in Quantum Computers D.P. DiVincenzo.

Role of self-formed InGaN quantum dots for exciton localization in the purple laser diode emitting at 420 nm

Abstract: Structural analysis was performed on a purple laser diode composed of In0.20Ga0.80N (3 nm)/ In0.05Ga0.95N (6 nm) multiple quantum wells, by employing transmission electron microscopy and energy-dispersive x-ray microanalysis, both of which are assessed from the cross-sectional direction. It was found that the contrast of light and shade in the well layers corresponds to the difference in In composition. The main radiative recombination was attributed to excitons localized at deep traps which probably originate from the In-rich region in the wells acting as quantum dots. Photopumped lasing was observed at the high energy side of the main spontaneous emission bands.

Reversible Tuning of Silver Quantum Dot Monolayers Through the Metal-Insulator Transition

Abstract: The linear and nonlinear (χ(2)) optical responses of Langmuir monolayers of organically functionalized silver quantum dots were measured as a continuous function of interparticle separation under near-ambient conditions. As the distance between metal surfaces was decreased from 12 to ∼5 angstroms, both quantum and classical effects were observed in the optical signals. When the separation was less than 5 angstroms, the optical second-harmonic generation (SHG) response exhibited a sharp discontinuity, and the linear reflectance and absorbance began to resemble those of a thin metallic film, indicating that an insulator-to-metal transition occurred. This transition was reversible.

Quantum-confined Stark effect in single CdSe nanocrystallite quantum dots

Abstract: The quantum-confined Stark effect in single cadmium selenide (CdSe) nanocrystallite quantum dots was studied. The electric field dependence of the single-dot spectrum is characterized by a highly polarizable excited state (∼10 5 cubic angstroms, compared to typical molecular values of order 10 to 100 cubic angstroms), in the presence of randomly oriented local electric fields that change over time. These local fields result in spontaneous spectral diffusion and contribute to ensemble inhomogeneous broadening. Stark shifts of the lowest excited state more than two orders of magnitude larger than the linewidth were observed, suggesting the potential use of these dots in electro-optic modulation devices.

Random Telegraph Signal in the Photoluminescence Intensity of a Single Quantum Dot

Abstract: We propose a model in which the time dependence of the photoluminescence intensity of a single nanosize quantum dot under cw excitation conditions shows a sequence of ``on'' and ``off'' periods similar to a random telegraph signal. In our model the off periods are the times when the dot is ionized and the luminescence is quenched by nonradiative Auger recombination. The duration of the on periods depends on the ionization rate of the dot via thermal or Auger autoionization, and depends strongly on excitation intensity. Numerical simulations reproduce the random intermittency recently observed in the photoluminescence intensity of a single CdSe quantum dot.

Electronic structure and optical properties of PbS and PbSe quantum dots

Abstract: The electronic structure of spherical PbS and PbSe quantum dots is calculated with a four-band envelope-function formalism. This calculation accounts for both exciton energies and wave functions with the correct symmetry of the materials. The selection rules and the strength of the dipole transitions of lead-salt quantum dots are derived accounting for the symmetry of the band-edge Bloch functions of the lead salts. The calculated energies of the optically allowed exciton states are found to be in good agreement with experimental data. The effects of many-body perturbations, such as Coulomb interactions and intervalley scattering, are also discussed.

Electron Transport in Quantum Dots

Abstract: The ongoing miniaturization of solid state devices often leads to the question: “How small can we make resistors, transistors, etc., without changing the way they work?” The question can be asked a different way, however: “How small do we have to make devices in order to get fundamentally new properties?” By “new properties” we particularly mean those that arise from quantum mechanics or the quantization of charge in units of eeffects that are only important in small systems such as atoms. “What kind of small electronic devices do we have in mind?” Any sort of clustering of atoms that can be connected to source and drain contacts and whose properties can be regulated with a gate electrode. Practically, the clustering of atoms may be a molecule, a small grain of metallic atoms, or an electronic device that is made with modern chip fabrication techniques. It turns out that such seemingly different structures have quite similar transport properties and that one can explain their physics within one relatively simple framework. In this paper we investigate the physics of electron transport through such small systems.

Intermixing and shape changes during the formation of InAs self-assembled quantum dots

Abstract: The initial stages of GaAs overgrowth over self-assembled coherently strained InAs quantum dots (QDs) are studied. For small GaAs coverages (below 5 nm), atomic force microscopy (AFM) images show partially covered island structures with a regular size distribution which are elongated in the [011] direction. Analysis of the AFM profiles show that a large anisotropic redistribution of the island material is taking place during the initial GaAs overgrowth. Short time annealing experiments together with photoluminescence spectroscopy on annealed QDs are consistent with a Ga and In intermixing during the overgrowth. Surface QDs capped with 5 nm or more GaAs show a strong luminescence intensity indicating that surface QDs are remarkably insensitive to surface recombination effects.

Phase measurement in a quantum dot via a double-slit interference experiment

Abstract: The transport properties of electronic devices are usually characterized on the basis of conductance measurements. Such measurements are adequate for devices in which transport occurs incoherently, but for very small devices—such as quantum dots1,2—the wave nature of the electrons plays an important role3. Because the phase of an electron's wavefunction changes as it passes through such a device, phase measurements are required to characterize the transport properties fully. Here we report the results of a double-slit interference experiment which permits the measurement of the phase-shift of an electron traversing a quantum dot. This is accomplished by inserting the quantum dot into one arm of an interferometer, thereby introducing a measurable phase shift between the arms. We find that the phase evolution within a resonance of the quantum dot can be accounted for qualitatively by a model that ignores the interactions between the electrons within the dot. Although these electrons must interact strongly, such interactions apparently have no observable effect on the phase. On the other hand, we also find that the phase behaviour is identical for all resonances, and that there is a sharp jump of the phase between successive resonance peaks. Adequate explanation of these features may require a model that includes interactions between electrons.

Energy relaxation by multiphonon processes in InAs/GaAs quantum dots

Abstract: Carrier relaxation and recombination in self-organized InAs/GaAs quantum dots (QD's) is investigated by photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy We demonstrate inelastic phonon scattering to be the dominant intradot carrier-relaxation mechanism Multiphonon processes involving up to four LO phonons from either the InAs QD's, the InAs wetting layer, or the GaAs barrier are resolved The observation of multiphonon resonances in the PLE spectra of the QD's is discussed in analogy to hot exciton relaxation in higher-dimensional semiconductor systems and proposed to be intricately bound to the inhomogeneity of the QD ensemble in conjunction with a competing nonradiative recombination channel observed for the excited hole states Carrier capture is found to be a cascade process with the initial capture into excited states taking less than a few picoseconds and the multiphonon (involving three LO phonons) relaxation time of the first excited hole state being 40 ps The |001〉 hole state presents a relaxation bottleneck that determines the ground-state population time after nonresonant excitation For the small self-organized InAs/GaAs QD's the intradot carrier relaxation is shown to be faster than radiative (g1 ns) and nonradiative (\ensuremath{\approx}100 ps) recombination explaining the absence of a ``phonon bottleneck'' effect in the PL spectra

Excitation Spectra of Circular, Few-Electron Quantum Dots

Abstract: Studies of the ground and excited states in semiconductor quantum dots containing 1 to 12 electrons showed that the quantum numbers of the states in the excitation spectra can be identified and compared with exact calculations. A magnetic field induces transitions between the ground and excited states. These transitions were analyzed in terms of crossings between single-particle states, singlet-triplet transitions, spin polarization, and Hund’s rule. These impurity-free quantum dots allow “atomic physics” experiments to be performed in magnetic field regimes not accessible for atoms.

Size-Dependent Spectroscopy of InP Quantum Dots

Abstract: The spectroscopic behavior of colloidal InP quantum dots (QDs) has been investigated as a function of the mean QD diameter (which ranged from 26 to 60 A). Absorption spectra show up to three peaks or shoulders which reflect excited state transitions in the QDs. Global photoluminescence (PL) spectra (excitation well to the blue of the absorption onset and which consequently excites most of the QDs in the size distribution) show broad PL emission. The emission and absorption features shift to higher energy with decreasing QD size. Resonant PL spectra (size-selective excitation into the tail of the absorption onset) show increasing fluorescence line narrowing with increasing excitation wavelength; PL and photoluminescence excitation spectroscopy were used to derive the PL red shift as a function of QD size. The resonant red shifts for QDs of a single size were extracted from PL data that reflect the emission from an ensemble of QD diameters. An analysis of the single-dot resonant red shift (difference betwee...

Quantum Confinement and Optical Gaps in Si Nanocrystals

Abstract: Quasiparticle gaps, self-energy corrections, exciton Coulomb energies, and optical gaps in Si quantum dots are calculated from first principles using a real-space pseudopotential method. The calculations are performed on hydrogen-passivated spherical Si clusters with diameters up to 27.2 \AA{} ( $\ensuremath{\sim}800\mathrm{Si}$ and H atoms). It is shown that (i) the self-energy correction in quantum dots is enhanced substantially compared to bulk, and is not size independent as implicitly assumed in all semiempirical calculations, and (ii) quantum confinement and reduced electronic screening result in appreciable excitonic Coulomb energies. Calculated optical gaps are in very good agreement with absorption data.

Stranski-Krastanov growth mode during the molecular beam epitaxy of highly strained GaN

Abstract: It is demonstrated by in situ reflection-high-energy-electron-diffraction studies that the growth of hexagonal GaN on AlN occurs either purely in a layer-by-layer mode or in a Stranski-Krastanov mode, depending on the substrate temperature. Nanometric GaN islands embedded in AlN were fabricated by controlling the growth mode. Electron microscopy and atomic-force microscopy revealed that the dimensions of GaN dots could be varied down to values where zero-dimensional quantum effects are expected: the smallest dots were typically 10 nm wide and 2 nm high. These results open the way to the fabrication of quantum dots in materials with optical properties in the uv wavelength range.

Room-temperature continuous-wave lasing from stacked InAs/GaAs quantum dots grown by metalorganic chemical vapor deposition

Abstract: We report on quantum dot (QD) lasers made of stacked InAs dots grown by metalorganic chemical vapor deposition. Successful growth of defect-free binary InAs/GaAs QDs with high lateral density (dl⩾4×1010 cm−2) was achieved in a narrow growth parameter window. The room-temperature photoluminescence (PL) intensity is enhanced up to a factor of 3 and the PL peak width is reduced by more than 30% when a thin layer of In0.3Ga0.7As is deposited onto the InAs QDs. A QD laser with a single sheet of such InAs/InGaAs/GaAs QDs exhibits threshold current densities as low as 12.7 and 181 A/cm2 at 100 and 300 K, respectively. Lasers with threefold stacked QDs show ground-state lasing and allow for cw operation at room temperature.

Strain distribution and electronic spectra of InAs/GaAs self-assembled dots: An eight-band study

Abstract: Strained epitaxy has been shown to produce pyramidal-shaped quantum dot structures by single-step epitaxy. In this paper we examine the strain tensor in these quantum dots using a valence force field model. We use an eight-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ formalism to find the electronic spectra in the highly strained dots. Results obtained for the conduction-band spectra using the effective-mass approach are shown to have serious errors. This is particularly true for excited states in the conduction band. The dependence of the electronic spectra on the quantum dot size and shape is also reported along with comparisons with published experimental results.

Theory of random population for quantum dots

Abstract: Carrier capture and recombination in quantum dots are random processes. Conventional rate equation models do not take into account this property. Based on our theory of random population we predict recombination spectra, transients, and gain of quantum-dot ensembles. Even with infinitely fast interlevel energy relaxation excited levels become considerably populated. The impact of a slowdown of energy relaxation is modeled and criteria for a conclusive experimental observation of a finite interlevel-scattering time are given.

Charged Excitons in Self-Assembled Semiconductor Quantum Dots

Abstract: Interband excitations of an ensemble of InAs self-assembled quantum dots have been directly observed in transmission experiments. The dots are embedded in a field-effect structure allowing us to load the dots electrically. We establish an exact correspondence between Coulomb blockade in the device's vertical transport properties and Pauli blocking in the transmission spectra. We observe substantial shifts, up to 20 meV, in the energies of the higher excitations on occupation of the electron ground state. We argue that this is a consequence of an exciton-electron interaction.

Coupled Quantum Dots Fabricated by Cleaved Edge Overgrowth: From Artificial Atoms to Molecules

Abstract: Atomically precise quantum dots of mesoscopic size have been fabricated in the gallium arsenide-aluminum gallium arsenide material system by cleaved edge overgrowth, with a high degree of control over shape, composition, and position. The formation of bonding and antibonding states between two such "artificial atoms" was studied as a function of quantum dot separation by microscopic photoluminescence (PL) spectroscopy. The coupling strength within these "artificial molecules" is characterized by a systematic dependence of the separation of the bonding and antibonding levels, and of the PL linewidth, on the "interatomic" distance. This model system opens new insights into the physics of coupled quantum objects.

Synthesis and optical properties of MoS2 and isomorphous nanoclusters in the quantum confinement regime

Abstract: Highly crystalline nanoclusters of hexagonal (2H polytype) MoS2 and several of its isomorphous Mo and W chalcogenides have been synthesized with excellent control over cluster size down to ∼2 nm. These clusters exhibit highly structured, bandlike optical absorption and photoluminescence spectra which can be understood in terms of the band-structures for the bulk crystals. Key results of this work include: (1) strong quantum confinement effects with blue shifts in some of the absorption features relative to bulk crystals as large as 4 eV for clusters ∼2.5 nm in size, thereby allowing great tailorability of the optical properties; (2) the quasiparticle (or excitonic) nature of the optical response is preserved down to clusters ≲2.5 nm in size which are only two unit cells thick; (3) the demonstration of the strong influence of dimensionality on the magnitude of the quantum confinement. Specifically, three-dimensional confinement of the carriers produces energy shifts which are over an order of magnitude lar...

Tuning self-assembled InAs quantum dots by rapid thermal annealing

Abstract: Blueshifts in the photoluminescence emission energies from an ensemble of self-assembled InAs quantum dots are observed as a result of postgrowth thermal annealing. Enhancement of the integrated photoluminescence emission and narrowing of the full width half-maxima (from 55 to 12 meV) occur together with blueshifts up to 300 meV at annealing temperatures up to 950 °C. Evidence that the structures remain as dots comes form the observation of level filling and photoluminescence excitation studies which reveal LO phonon peaks occurring at multiples of ∼30 meV from the detection energies.

InP quantum dots: Electronic structure, surface effects, and the redshifted emission

Abstract: We present pseudopotential plane-wave electronic-structure calculations on InP quantum dots in an effort to understand quantum confinement and surface effects and to identify the origin of the long-lived and redshifted luminescence. We find that (i) unlike the case in small GaAs dots, the lowest unoccupied state of InP dots is the ${\ensuremath{\Gamma}}_{1c}$-derived direct state rather than the ${X}_{1c}$-derived indirect state and (ii) unlike the prediction of $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ models, the highest occupied state in InP dots has a $1sd$-type envelope function rather than a (dipole-forbidden) $1pf$ envelope function. Thus explanations (i) and (ii) to the long-lived redshifted emission in terms of an orbitally forbidden character can be excluded. Furthermore, (iii) fully passivated InP dots have no surface states in the gap. However, (iv) removal of the anion-site passivation leads to a P dangling bond (DB) state just above the valence band, which will act as a trap for photogenerated holes. Similarly, (v) removal of the cation-site passivation leads to an In dangling-bond state below the conduction band. While the energy of the In DB state depends only weakly on quantum size, its radiative lifetime increases with quantum size. The calculated $\ensuremath{\sim}300\ensuremath{-}\mathrm{meV}$ redshift and the $\ensuremath{\sim}18$ times longer radiative lifetime relative to the dot-interior transition for the 26-\AA{} dot with an In DB are in good agreement with the observations of full-luminescence experiments for unetched InP dots. Yet, (vi) this type of redshift due to surface defect is inconsistent with that measured in selective excitation for HF-etched InP dots. (vii) The latter type of (``resonant'') redshift is compatible with the calculated screened singlet-triplet splitting in InP dots, suggesting that the slow emitting state seen in selective excitation could be a triplet state.

Nano-structure memory device

Abstract: A memory device and memory incorporating a plurality of the memory devices is described wherein each memory device has spaced apart source and drain regions, a channel, a barrier insulating layer, a nanocrystal or a plurality of nanocrystals, a control barrier layer, and a gate electrode. The nanocrystal which may be a quantum dot, stores one electron or hole or a discrete number of electrons or holes at room temperature to provide threshold voltage shifts in excess of the thermal voltage for each change in electron or hole stored. The invention utilizes Coulomb blockade in electrostatically coupling one or more stored electrons or holes to a channel while avoiding in-path Coulomb-blockade controlled conduction for sensing the stored charge.

Coherent Acoustic Phonons in a Semiconductor Quantum Dot

Abstract: Coherent acoustic phonons in PbS quantum dots are observed using femtosecond optical techniques. This is the first observation of coherent acoustic phonons in a semiconductor quantum dot; the phonons are generated through the deformation-potential coupling to the quantum-dot exciton. The acoustic modes are weakly damped, and we also find extremely weak coupling ( $S\ensuremath{\sim}0.01$) to the optical modes. These conclusions have important consequences for the vibronic nature of the exciton transition in the quantum dot and its dephasing.

Synthesis and characterization of PbSe quantum dots in phosphate glass

Abstract: The controlled synthesis of PbSe nanocrystal quantum dots with narrow size distributions was achieved through phase decomposition of the PbSe solid solution in a phosphate glass host. Structural characterization by electron microscopy and x-ray diffraction shows that the dots have mean diameters between 2 and 15 nm. The exciton Bohr radius aB=46 nm in PbSe, so these quantum dots provide unusual and perhaps unique access to the regime of strong quantum confinement. The optical absorption spectra are compared to the predictions of a theoretical treatment of the electronic structure. The theory agrees well with experiment for dots larger than ∼7 nm, but for smaller dots there is some deviation from the theoretical predictions.

Direct pseudopotential calculation of exciton coulomb and exchange energies in semiconductor quantum dots

Abstract: The effects of electron-hole interaction on the exciton energy of semiconductor quantum dots are calculated using pseudopotential wave functions. A comparison with the widely used, but never tested, effective-mass approximation (EMA) shows that the electron-hole Coulomb energy is significantly ( $\ensuremath{\sim}40%$) overestimated by the EMA, and that the scaling with the dot size $R$ is sublinear in $1/R$. The exchange splitting is much smaller than the Coulomb energy, and in the case of CdSe quantum dots shows significant deviations from the ${1/R}^{3}$ scaling predicted by the EMA.