Showing papers on "Quantum dot published in 2000"
TL;DR: Control of the growth kinetics of the II–VI semiconductor cadmium selenide can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one, with aspect ratios as large as ten to one.
Abstract: Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties1,2 that depend sensitively on both size and shape3,4, and are of both fundamental and technological interest In contrast to the syntheses of zero-dimensional systems, existing preparations of one-dimensional systems often yield networks of tubes or rods which are difficult to separate5,6,7,8,9,10,11,12 And, in the case of optically active II–VI and III–V semiconductors, the resulting rod diameters are too large to exhibit quantum confinement effects6,8,9,10 Thus, except for some metal nanocrystals13, there are no methods of preparation that yield soluble and monodisperse particles that are quantum-confined in two of their dimensions For semiconductors, a benchmark preparation is the growth of nearly spherical II–VI and III–V nanocrystals by injection of precursor molecules into a hot surfactant14,15 Here we demonstrate that control of the growth kinetics of the II–VI semiconductor cadmium selenide can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one, with aspect ratios as large as ten to one This method should be useful, not only for testing theories of quantum confinement, but also for obtaining particles with spectroscopic properties that could prove advantageous in biological labelling experiments16,17 and as chromophores in light-emitting diodes18,19
4,288 citations
TL;DR: In this article, solution phase syntheses and size-selective separation methods to prepare semiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm and monodisperse to ≤ 5%, are presented.
Abstract: ▪ Abstract Solution phase syntheses and size-selective separation methods to prepare semiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm and monodisperse to ≤5%, are presented. Preparation of monodisperse samples enables systematic characterization of the structural, electronic, and optical properties of materials as they evolve from molecular to bulk in the nanometer size range. Sample uniformity makes it possible to manipulate nanocrystals into close-packed, glassy, and ordered nanocrystal assemblies (superlattices, colloidal crystals, supercrystals). Rigorous structural characterization is critical to understanding the electronic and optical properties of both nanocrystals and their assemblies. At inter-particle separations 5–100 A, dipole-dipole interactions lead to energy transfer between neighboring nanocrystals, and electronic tunneling between proximal nanocrystals gives rise to dark and photoconductivity. At separations <5 A, exchange interactions cause otherwise insulating ass...
4,116 citations
TL;DR: In this article, the authors 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.
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.
2,535 citations
TL;DR: Using pulsed laser excitation of a single quantum dot, a single- photon turnstile device that generates a train of single-photon pulses was demonstrated.
Abstract: Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.
2,224 citations
TL;DR: It is demonstrated that light amplification is possible using silicon itself, in the form of quantum dots dispersed in a silicon dioxide matrix, which opens a route to the fabrication of a silicon laser.
Abstract: Adding optical functionality to a silicon microelectronic chip is one of the most challenging problems of materials research. Silicon is an indirect-bandgap semiconductor and so is an inefficient emitter of light. For this reason, integration of optically functional elements with silicon microelectronic circuitry has largely been achieved through the use of direct-bandgap compound semiconductors. For optoelectronic applications, the key device is the light source--a laser. Compound semiconductor lasers exploit low-dimensional electronic systems, such as quantum wells and quantum dots, as the active optical amplifying medium. Here we demonstrate that light amplification is possible using silicon itself, in the form of quantum dots dispersed in a silicon dioxide matrix. Net optical gain is seen in both waveguide and transmission configurations, with the material gain being of the same order as that of direct-bandgap quantum dots. We explain the observations using a model based on population inversion of radiative states associated with the Si/SiO2 interface. These findings open a route to the fabrication of a silicon laser.
2,204 citations
TL;DR: A novel and direct method is described for conjugating protein molecules to luminescent CdSe−ZnS core−shell nanocrystals (Quantum Dots) for use as bioactive fluorescent probes in sensing, imaging, immunoassay, and other diagnostics applications.
Abstract: A novel and direct method is described for conjugating protein molecules to luminescent CdSe−ZnS core−shell nanocrystals (Quantum Dots) for use as bioactive fluorescent probes in sensing, imaging, immunoassay, and other diagnostics applications. The approach makes use of a chimeric fusion protein designed to electrostatically bind to the oppositely charged surface of capped colloidal quantum dots (QDs). Preparation of protein-modified QD dispersions with high quantum yield, little or no particle aggregation, and retention of biological activity was achieved. Combining the advantages of lipoic acid capped CdSe−ZnS quantum dots (photochemical stability, a wide range of size-dependent emission wavelengths, and aqueous compatibility) with facile electrostatic conjugation of bioactive proteins, this type of hybrid bioinorganic conjugate represents a powerful fluorescent tracking tool for diverse applications. The design and preparation of a model QD/protein conjugate based on E. coli Maltose Binding Protein is...
1,719 citations
TL;DR: Bulk quantities of defect-free silicon nanowires with nearly uniform diameters were grown to a length of several micrometers with a supercritical fluid solution-phase approach, and visible photoluminescence due to quantum confinement effects was observed, as were discrete optical transitions in the ultraviolet-visible absorbance spectra.
Abstract: Bulk quantities of defect-free silicon (Si) nanowires with nearly uniform diameters ranging from 40 to 50 angstroms were grown to a length of several micrometers with a supercritical fluid solution-phase approach. Alkanethiol-coated gold nanocrystals (25 angstroms in diameter) were used as uniform seeds to direct one-dimensional Si crystallization in a solvent heated and pressurized above its critical point. The orientation of the Si nanowires produced with this method could be controlled with reaction pressure. Visible photoluminescence due to quantum confinement effects was observed, as were discrete optical transitions in the ultraviolet-visible absorbance spectra.
1,535 citations
TL;DR: Transport measurements are performed that provide evidence for a coupling between the centre-of-mass motion of the C60 molecules and single-electron hopping—a conduction mechanism that has not been observed previously in quantum dot studies.
Abstract: The motion of electrons through quantum dots is strongly modified by single-electron charging and the quantization of energy levels1,2. Much effort has been directed towards extending studies of electron transport to chemical nanostructures, including molecules3,4,5,6,7,8, nanocrystals9,10,11,12,13 and nanotubes14,15,16,17. Here we report the fabrication of single-molecule transistors based on individual C60 molecules connected to gold electrodes. We perform transport measurements that provide evidence for a coupling between the centre-of-mass motion of the C60 molecules and single-electron hopping18—a conduction mechanism that has not been observed previously in quantum dot studies. The coupling is manifest as quantized nano-mechanical oscillations of the C60 molecule against the gold surface, with a frequency of about 1.2 THz. This value is in good agreement with a simple theoretical estimate based on van der Waals and electrostatic interactions between C60 molecules and gold electrodes.
1,507 citations
TL;DR: It is observed that in the quantum-confined regime, the Auger constant is strongly size-dependent and decreases with decreasing the quantum dot size as the radius cubed.
Abstract: We have resolved single-exponential relaxation dynamics of the 2-, 3-, and 4-electron-hole pair states in nearly monodisperse cadmium selenide quantum dots with radii ranging from 1 to 4 nanometers. Comparison of the discrete relaxation constants measured for different multiple-pair states indicates that the carrier decay rate is cubic in carrier concentration, which is characteristic of an Auger process. We observe that in the quantum-confined regime, the Auger constant is strongly size-dependent and decreases with decreasing the quantum dot size as the radius cubed.
1,312 citations
TL;DR: Recent research on lead salt quantum dots is reviewed, which suggests that lead salt nanostructures may be the only materials in which the electronic energies are determined primarily by quantum confinement.
Abstract: Nanocrystals or quantum dots of the IV−VI semiconductors PbS, PbSe, and PbTe provide unique properties for investigating the effects of strong confinement on electrons and phonons. The degree of confinement of charge carriers can be many times stronger than in most II−VI and III−V semiconductors, and lead salt nanostructures may be the only materials in which the electronic energies are determined primarily by quantum confinement. This Account briefly reviews recent research on lead salt quantum dots.
1,057 citations
Book•
01 Jan 2000
TL;DR: Theoretical Aspects of Strained-layer Quantum-Well Lasers are discussed in this article, as well as their application in the synthesis and characterization of nanostructured materials.
Abstract: Volume 1: Synthesis and Processing HG Jiang, ML Lau, VL Telkamp, and EJ Lavernia, Synthesis of Nanostructured Coatings by High Velocity Oxygen Fuel Thermal Spraying KE Gonsalves, SP Rangarajan, and J Wang, Chemical Synthesis of Nanostructured Metals, Metal Alloys, and Semiconductors J Costa, Nanoparticles from Low-Pressure and Low-Temperature Plasma CD Johnson, M Noh, H Sellinschegg, R Schneidmiller, and DC Johnson, Kinetic Control of Inorganic Solid State Reactions Resulting from Mechanistic Studies Using Elementally Modulated Reactants EJ Gonzalez and GJ Piermarini, Low Temperature Compaction on Nanosize Powders WH Weinberg, CM Reaves, BZ Nosho, RI Pelzel, and SP denBaars, Strained-layer Heteroepitaxy to Fabricate Self-assembled Semiconductor Islands JJ McClelland, Nanofabrication via Atom Optics KC Kwaitkowski and CM Lukehart, Nanocomposites Prepared by Sol-Gel Methods: Synthesis and Characterization Q Yitai, Chemical Preparation and Characterization of Nanocrystalline Materials DJ Duval and SH Risbud, Semiconductors Quantum Dots-Progress in Processing ITH Chang, Rapid Solidification Processing of Nanocrystalline Metallic Alloys KL Choy, Vapor Processing of Nanostructured Materials Volume 2: Spectroscopy and Theory JM Cowley and JCH Spence, Nanodiffraction M-I Baraton, FT-IR Surface Spectrometry of Nanosized Particles P Milani and CE Bottani, Vibrational Spectroscopy of Mesoscopic Systems RM Taylor II and R Superfine, Advanced Interfaces to Scanning-probe Microscopes R Blick, Microwave Spectroscopy on Quantum Dots E Meyer and R Luthi, Tribological Experiments with Friction Force Microscopy M J Yacaman and JA Ascencia, Electron Microscopy Techniques Applied to Study of Nanostructured Materials and Ancient Materials K Ounadjela and RL Stamps, Mesoscopic Magnetism in Metals DJ Whitehouse, Tools of Nanotechnology: Nanometrology VGasparian, M Ortuno, G Schon, and U Simon, Tunneling Times in Nanostructures SB Sinnott, Theory of Atomic-Scale Friction D Ahn, Theoretical Aspects of Strained-layer Quantum-Well Lasers LR Ram Mohan, I Vurgaftman, and JR Meyer, Wavefunction Engineering: A New Paradigm in Quantum Nanostructure Modeling Volume 3: Electrical Properties J Smolines and G Ploner, Electron Transport and Confining Potentials in Semiconductor Nanostructures MA Reed, JW Sleight, and MR Deshpande, Electron Transport Properties of Quantum Dots U Simon and G Schon, Electrical Properties of Chemically Tailored Nanoparticles and Their Applications in Microelectronics RP Andres, S Datta, DB Janes, CP Kubiak, and R Reifenberger, Design, Fabrication, and Electronic Properties of Self-assembled Molecular Nanostructures TP Sidiki and CM Sotomayor Torres, Silicon-based Nanostructures PV Kamat, K Murakoshi, Y Wada, and S Yanagida, Semiconductor Nanoparticles FM Peeters and J DeBoeck, Hybrid Magnetic-Semiconductor Nanostructures OI Micic and AJ Nozik, Colloidal Quantum Dots of III-V Semiconductors VV Moshchalkov, Y Bruynseraede, L Van Look, MJ Van Bael, Y Bruynseraede, and A Tonomura, Quantization and Confinement Phenomena in Nanostructured Superconductors M Graetzel, Properties and Applications of Nanocrystalline Electronic Junctions S Mitsui, Nanostructured Fabrication Using Electron Beam and Its Applications to Nanometer Devices Volume 4: Optical Properties DD Notle, MR Melloch, Y Ding, M Dinu, KM Kwolek, and I Lahiri, Photorefractive Semiconductor Nanostructures F Gonella and P Mazzoldi, Metal Nanocluster Composite Glasses D Thomas, Porous Silicon W Chen, Fluorescence, Thermoluminescence, and Photostimulated Luminescence of Nanoparticles VM Shalaev, Surface-enhanced Optical Phenomena in Nanostructured Fractal Materials VI Klimov, Linear and Nonlinear Optical Spectroscopy of Semiconductor Nanocrystals S Vijayalakshmi and H Grebel, Nonlinear Optical Properties of Nanostructures SS Li and MZ Tidrow, Quantum Well Infrared Photodetectors W Tan and R Kopelman, Nanoscopic Optical Sensors and Probes Volume 5: Organics, Polymers, and Biological Materials PJ Stang and B Olenyuk, Transition-Metal-Mediated Self-Assembly of Discrete Nanoscopic Species with Well-Defined Shapes and Geometries M Gomez-Lopez and JF Stoddart, Molecular and Supramolecular Nanomachines AC Benniston and PR Mackie, Functional Nanostructures Incorporating Responsive Modules A Archut and F Voegtle, Dendritic Molecules: Historical Developments and Future Applications PM Ajayan, Carbon Nanotubes J Sloan and MLH Green, Encapsulation and Crystallization Behavior of Materials Inside Carbon Nanotubes H Kasai, HS Nalwa, S Okada, H Oikawa, and H Nakanishi, Fabrication and Spectroscopic Characterization of Organic Nanocrystals G Liu, Polymeric Nanostructures B Wessling, Conducting Polymers as Organic Nanometals E Nakache, N Poulain, F Candau, AM Orecchioni, and JM Irache, Biopolymers and Polymers Nanoparticles and Their Biomedical Applications T Bayburt, J Carlson, B Godfrey, M Shank-Retzlaff, and SG Sligar, Structure, Behavior, and Manipulation of Nanostructure Biological Assemblies TM Cooper, Biomimetic Thin Films
TL;DR: It is proposed that this semiconductor device uses a single quantum dot as active medium embedded in a p- i- n junction and surrounded by a microcavity has the unique potential to generate pairs of entangled photons at a well-defined repetition rate.
Abstract: We propose a new method of generating nonclassical optical field states. The method uses a semiconductor device, which consists of a single quantum dot as active medium embedded in a $p$- $i$- $n$ junction and surrounded by a microcavity. Resonant tunneling of electrons and holes into the quantum dot ground states, together with the Pauli exclusion principle, produce regulated single photons or regulated pairs of photons. We propose that this device also has the unique potential to generate pairs of entangled photons at a well-defined repetition rate.
TL;DR: This result proves that a single quantum dot acts like an artificial atom, with a discrete anharmonic spectrum, and finds the photon-emission events from a cluster of several dots to be uncorrelated.
Abstract: Maxwell's equations successfully describe the statistical properties of fluorescence from an ensemble of atoms or semiconductors in one or more dimensions But quantization of the radiation field is required to explain the correlations of light generated by a single two-level quantum emitter, such as an atom, ion or single molecule The observation of photon antibunching in resonance fluorescence from a single atom unequivocally demonstrated the non-classical nature of radiation Here we report the experimental observation of photon antibunching from an artificial system--a single cadmium selenide quantum dot at room temperature Apart from providing direct evidence for a solid-state non-classical light source, this result proves that a single quantum dot acts like an artificial atom, with a discrete anharmonic spectrum In contrast, we find the photon-emission events from a cluster of several dots to be uncorrelated
TL;DR: It is found that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.
Abstract: Quantum dots or rings are artificial nanometre-sized clusters that confine electrons in all three directions. They can be fabricated in a semiconductor system by embedding an island of low-bandgap material in a sea of material with a higher bandgap. Quantum dots are often referred to as artificial atoms because, when filled sequentially with electrons, the charging energies are pronounced for particular electron numbers; this is analogous to Hund's rules in atomic physics. But semiconductors also have a valence band with strong optical transitions to the conduction band. These transitions are the basis for the application of quantum dots as laser emitters, storage devices and fluorescence markers. Here we report how the optical emission (photoluminescence) of a single quantum ring changes as electrons are added one-by-one. We find that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.
TL;DR: In this paper, the authors used single molecule confocal microscopy to study fluorescence intermittency of individual ZnS overcoated CdSe quantum dots (QDs) excited at 488 nm.
Abstract: Single molecule confocal microscopy is used to study fluorescence intermittency of individual ZnS overcoated CdSe quantum dots (QDs) excited at 488 nm The confocal apparatus permits the distribution of “on” and “off” times (ie, periods of sustained fluorescence emission and darkness) to be measured over an unprecedentedly large dynamic range (109) of probability densities, with nonexponential behavior in τoff over a 105 range in time scales In dramatic contrast, these same τoff distributions in all QDs are described with remarkable simplicity over this 109-fold dynamic range by a simple inverse power law, ie, P(τoff)∝1/τoff1+α Such inverse power law behavior is a clear signature of distributed kinetics, such as predicted for (i) an exponential distribution of trap depths or (ii) a distribution of tunneling distances between QD core/interface states This has important statistical implications for all previous studies of fluorescence intermittency in semiconductor QDs and may have broader implications for other systems such as single polymer molecules
TL;DR: Modulation doping of a semiconducting single-walled carbon nanotube along its length leads to an intramolecular wire electronic device that can be tuned into n-type, exhibiting single-electron charging and negative differential conductance at low temperatures.
Abstract: Modulation doping of a semiconducting single-walled carbon nanotube along its length leads to an intramolecular wire electronic device. The nanotube is doped n-type for half of its length and p-type for the other half. Electrostatic gating can tune the system into p-n junctions, causing it to exhibit rectifying characteristics or negative differential conductance. The system can also be tuned into n-type, exhibiting single-electron charging and negative differential conductance at low temperatures. The low-temperature behavior is manifested by a quantum dot formed by chemical inhomogeneity along the tube.
TL;DR: In this paper, the Coulomb blockade for electron tunneling is overcome completely by the Kondo effect, and the conductance reaches the unitary limit value in a semiconductor quantum dot when a small magnetic field is applied.
Abstract: We observe a strong Kondo effect in a semiconductor quantum dot when a small magnetic field is applied. The Coulomb blockade for electron tunneling is overcome completely by the Kondo effect, and the conductance reaches the unitary limit value. We compare the experimental Kondo temperature with the theoretical predictions for the spin- 12 Anderson impurity model. Excellent agreement is found throughout the Kondo regime. Phase coherence is preserved when a Kondo quantum dot is included in one of the arms of an Aharonov-Bohm ring structure, and the phase behavior differs from previous results on a non-Kondo dot.
TL;DR: It is reported that electrically contacted single-walled carbon nanotubes can serve as powerful probes of Kondo physics, demonstrating the universality of the Kondo effect.
Abstract: The connection of electrical leads to wire-like molecules is a logical step in the development of molecular electronics, but also allows studies of fundamental physics. For example, metallic carbon nanotubes1 are quantum wires that have been found to act as one-dimensional quantum dots2,3, Luttinger liquids4,5, proximity-induced superconductors6,7 and ballistic8 and diffusive9 one-dimensional metals. Here we report that electrically contacted single-walled carbon nanotubes can serve as powerful probes of Kondo physics, demonstrating the universality of the Kondo effect. Arising in the prototypical case from the interaction between a localized impurity magnetic moment and delocalized electrons in a metallic host, the Kondo effect has been used to explain10 enhanced low-temperature scattering from magnetic impurities in metals, and also occurs in transport through semiconductor quantum dots11,12,13,14,15,16,17,18. The far greater tunability of dots (in our case, nanotubes) compared with atomic impurities renders new classes of Kondo-like effects19,20 accessible. Our nanotube devices differ from previous systems in which Kondo effects have been observed, in that they are one-dimensional quantum dots with three-dimensional metal (gold) reservoirs. This allows us to observe Kondo resonances for very large electron numbers (N) in the dot, and approaching the unitary limit (where the transmission reaches its maximum possible value). Moreover, we detect a previously unobserved Kondo effect, occurring for even values of N in a magnetic field.
TL;DR: In this paper, a review of quantum dot properties is presented, focusing on quantum dots in which the electron's dynamics are chaotic or diffusive, giving rise to statistical properties that reflect the interplay between one-body chaos, quantum interference, and electron-electron interactions.
Abstract: A quantum dot is a sub-micron-scale conducting device containing up to several thousand electrons. Transport through a quantum dot at low temperatures is a quantum-coherent process. This review focuses on dots in which the electron's dynamics are chaotic or diffusive, giving rise to statistical properties that reflect the interplay between one-body chaos, quantum interference, and electron-electron interactions. The conductance through such dots displays mesoscopic fluctuations as a function of gate voltage, magnetic field, and shape deformation. The techniques used to describe these fluctuations include semiclassical methods, random-matrix theory, and the supersymmetric nonlinear \ensuremath{\sigma} model. In open dots, the approximation of noninteracting quasiparticles is justified, and electron-electron interactions contribute indirectly through their effect on the dephasing time at finite temperature. In almost-closed dots, where conductance occurs by tunneling, the charge on the dot is quantized, and electron-electron interactions play an important role. Transport is dominated by Coulomb blockade, leading to peaks in the conductance that at low temperatures provide information on the dot's ground-state properties. Several statistical signatures of electron-electron interactions have been identified, most notably in the dot's addition spectrum. The dot's spin, determined partly by exchange interactions, can also influence the fluctuation properties of the conductance. Other mesoscopic phenomena in quantum dots that are affected by the charging energy include the fluctuations of the cotunneling conductance and mesoscopic Coulomb blockade.
TL;DR: It is found that semiconductor nanocrystals prepared as colloids can be made n-type, with electrons in quantum confined states, using an electron transfer approach commonly employed in the field of conducting organic polymers.
Abstract: Colloidal semiconductor nanocrystals combine the physical and chemical properties of molecules with the optoelectronic properties of semiconductors Their colour is highly controllable, a direct consequence of quantum confinement on the electronic states Such nanocrystals are a form of 'artificial atoms' (ref 4) that may find applications in optoelectronic systems such as light-emitting diodes and photovoltaic cells, or as components of future nanoelectronic devices The ability to control the electron occupation (especially in n-type or p-type nanocrystals) is important for tailoring the electrical and optical properties, and should lead to a wider range of practical devices But conventional doping by introducing impurity atoms has been unsuccessful so far: impurities tend to be expelled from the small crystalline cores (as observed for magnetic impurities), and thermal ionization of the impurities (which provides free carriers) is hindered by strong confinement Here we report the fabrication of n-type nanocrystals using an electron transfer approach commonly employed in the field of conducting organic polymers We find that semiconductor nanocrystals prepared as colloids can be made n-type, with electrons in quantum confined states
TL;DR: In this article, the authors reported an efficient electrical injection of spin-polarized carriers from a non-lattice-matched magnetic contact into a semiconductor heterostructure.
Abstract: We report efficient electrical injection of spin-polarized carriers from a non-lattice-matched magnetic contact into a semiconductor heterostructure. The semimagnetic semiconductor ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}$ is used as a spin-injecting contact on a GaAs-based light-emitting diode. Spin-polarized electrons are electrically injected across the II-VI/III-V interface, where they radiatively recombine in a GaAs quantum well and emit circularly polarized light. An analysis of the optical polarization which includes quantum confinement effects yields a lower bound of 50% for the spin injection efficiency.
TL;DR: In this paper, a simple etching experiment and electron paramagnetic resonance (EPR) measurements reveal that most of the dopant atoms reside in the surface layers of the inorganic lattice.
Abstract: The synthesis of II−VI semiconductor nanocrystals doped with transition metals has proved to be particularly difficult. In the case of CdSe quantum dots (QDs) produced via high-temperature pyrolysis in trioctylphosphine oxide (TOPO), specially designed precursors used in this study appear to be necessary to successfully incorporate low levels of Mn. A simple etching experiment and electron paramagnetic resonance (EPR) measurements reveal that most of the dopant atoms reside in the surface layers of the inorganic lattice. The dopant dramatically affects 113Cd magic angle spinning (MAS) nuclear magnetic resonance NMR spectra; the observed paramagnetic shift and decreased longitudinal relaxation time are consistent with Mn incorporated in the QDs. Paramagnetic atoms in QDs generate large effective magnetic fields, which implies that magnetooptical experiments can be performed simply by doping. Results from fluorescence line narrowing (FLN) studies on Mn-doped CdSe QDs mirror previous findings on undoped QDs ...
24 Jan 2000
TL;DR: It is demonstrated that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field.
Abstract: New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data. We also demonstrate that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field.
TL;DR: In this paper, the photoluminescence properties of silicon nanocrystals as a function of their size were investigated and a simple theoretical model was developed to describe the PL properties of the silicon nano-crystals.
Abstract: We present results on the photoluminescence (PL) properties of silicon nanocrystals as a function of their size. The nanocrystals are synthesized by laser pyrolysis of silane in a gas flow reactor and deposited at low energy on a substrate after a mechanical velocity and size selection. Both the photoluminescence spectroscopy and yield have been studied as well as the effect of aging of the samples in air. The measurements show that the PL of the silicon nanocrystallites follows the quantum confinement model very closely. The apparent PL yields are rather high (up to 18%). From evaluation of the size distribution obtained by atomic force microscopy it is concluded that the intrinsic PL yield of the nanocrystals can reach almost 100%. These results enabled us to develop a simple theoretical model to describe the PL of silicon nanocrystals. This model can also explain the changes of PL with aging of the sample, just by invoking a decrease of the size of the crystalline core as a result of oxidation.
TL;DR: In this article, the authors show that the luminescence from CdSe quantum dot monolayers can be strongly influenced by the interaction of water molecules adsorbed on the surface.
Abstract: We show that the luminescence from CdSe quantum dot monolayers can be strongly influenced by the interaction of water molecules adsorbed on the surface Light-induced alterations in the surface states following adsorption of water, results in quasi-reversible luminescence changes in the quantum dot The excitonic QY increases by a factor of 20 during the first 200 s of illumination in air (post vacuum) and then steadily decreases to a level 6 times that of the vacuum reference after 5000 s The exciton emission exhibits an exponential blue shift of nearly 16 nm (60 meV) over 1 h of illumination During this time, the line width decreases by 10% during the first 100 s and then slowly increases to 96% of the vacuum reference line width after 5000 s Our model suggests that water molecules adsorbed on the surface of the quantum dot act to passivate surface traps, which results in increased luminescence, similar to an effect well-known for bulk CdSe surfaces In addition, adsorbed water molecules act to oxidi
TL;DR: In this article, the use of nanometre thick silica shells as a means to stabilize metal clusters and semiconductor particles is discussed, and its potential advantages over conventional organic capping agents are presented.
Abstract: The use of nanometre thick silica shells as a means to stabilize metal clusters and semiconductor particles is discussed, and its potential advantages over conventional organic capping agents are presented. Shell deposition depends on control of the double layer potential, and requires priming of the core particle surface. Chemical reactions are possible within the core, via diffusion of reactants through the shell layer. Quantum dots can be stabilized against photochemical degradation through silica deposition, whilst retaining strong fluorescence quantum yields and their size dependent optical properties. Ordered 3D and 2D arrays of a macroscopic size with uniform particle spacing can be created. Thin colloid films can also be created with well-defined interparticle spacing, allowing controlled coupling of exciton and surface plasmon modes to be investigated. A number of future core–shell nanocomposite structures are postulated, including quantum bubbles and single electron capacitors based on Au@SiO2.
TL;DR: In this article, titania and zirconia-supported gold particles of 1−5 nm size were employed in the partial hydrogenation of acrolein, and their structural and electronic properties were characterized by electron microscopy, electron paramagnetic resonance, and optical absorption spectroscopy aimed at disclosing the nature of the active sites controlling the CO vs CC bonds.
Abstract: Titania- and zirconia-supported gold particles of 1−5 nm size, prepared by various routes of synthesis, were employed in the partial hydrogenation of acrolein. In-depth characterization of their structural and electronic properties by electron microscopy, electron paramagnetic resonance, and optical absorption spectroscopy aimed at disclosing the nature of the active sites controlling the hydrogenation of CO vs CC bonds. The structural characteristics of the catalysts, as mean particle size, size distribution, and dispersion, distinctly depend on the synthesis applied and the oxide support used whereby the highest gold dispersion (DAu = 0.78, Au/TiO2) results from a modified sol−gel technique. For extremely small gold particles on titania and zirconia (1.1 and 1.4 nm mean size), conduction electron spin resonance of the metal and paramagnetic F-centers (trapped electrons in oxygen vacancies) of the support were observed. Besides the influence of the surface geometry on the adsorption mode of the α,β-unsat...
TL;DR: An unexpected Kondo effect in a few-electron quantum dot containing singlet and triplet spin states, whose energy difference can be tuned with a magnetic field is reported.
Abstract: The Kondo effect—a many-body phenomenon in condensed-matter physics involving the interaction between a localized spin and free electrons—was discovered in metals containing small amounts of magnetic impurities, although it is now recognized to be of fundamental importance in a wide class of correlated electron systems1,2 In fabricated structures, the control of single, localized spins is of technological relevance for nanoscale electronics3,4 Experiments have already demonstrated artificial realizations of isolated magnetic impurities at metallic surfaces5,6, nanoscale magnets7, controlled transitions between two-electron singlet and triplet states8, and a tunable Kondo effect in semiconductor quantum dots9,10,11,12 Here we report an unexpected Kondo effect in a few-electron quantum dot containing singlet and triplet spin states, whose energy difference can be tuned with a magnetic field We observe the effect for an even number of electrons, when the singlet and triplet states are degenerate The characteristic energy scale is much larger than in the ordinary spin-1/2 case
TL;DR: The correlation between blinking and spectral shifting is consistent with a model of QD ionization as the mechanism for the blinking event, followed by a redistribution of local electric fields that results in spectral shifting.
Abstract: We find a correlation between the dynamics of fluorescence intermittency and spectral diffusion in the spectroscopy of single CdSe nanocrystal quantum dots (QD). A statistical analysis of the data suggests two populations of blinking events: blinking followed by large spectral diffusion shifts and blinking with small or no spectral shifts. Although unexpected from earlier studies, the correlation between blinking and spectral shifting is consistent with a model of QD ionization as the mechanism for the blinking event, followed by a redistribution of local electric fields that results in spectral shifting.