Showing papers on "Quantum dot published in 2008"
TL;DR: In this paper, three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell, (ii) polymer−smiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell.
Abstract: The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future resear...
2,434 citations
TL;DR: This work reports on electron transport in quantum dot devices carved entirely from graphene, demonstrating the possibility of molecular-scale electronics based on graphene.
Abstract: The exceptional electronic properties of graphene, with its charge carriers mimicking relativistic quantum particles and its formidable potential in various applications, have ensured a rapid growth of interest in this new material. We report on electron transport in quantum dot devices carved entirely from graphene. At large sizes (>100 nanometers), they behave as conventional single-electron transistors, exhibiting periodic Coulomb blockade peaks. For quantum dots smaller than 100 nanometers, the peaks become strongly nonperiodic, indicating a major contribution of quantum confinement. Random peak spacing and its statistics are well described by the theory of chaotic neutrino billiards. Short constrictions of only a few nanometers in width remain conductive and reveal a confinement gap of up to 0.5 electron volt, demonstrating the possibility of molecular-scale electronics based on graphene.
2,032 citations
TL;DR: Two major findings are highlighted: ability to tune the photoelectrochemical response and photoconversion efficiency via size control of CdSe quantum dots and improvement in the photoconversions efficiency by facilitating the charge transport through TiO2 nanotube architecture.
Abstract: Different-sized CdSe quantum dots have been assembled on TiO2 films composed of particle and nanotube morphologies using a bifunctional linker molecule. Upon band-gap excitation, CdSe quantum dots inject electrons into TiO2 nanoparticles and nanotubes, thus enabling the generation of photocurrent in a photoelectrochemical solar cell. The results presented in this study highlight two major findings: (i) ability to tune the photoelectrochemical response and photoconversion efficiency via size control of CdSe quantum dots and (ii) improvement in the photoconversion efficiency by facilitating the charge transport through TiO2 nanotube architecture. The maximum IPCE (photon-to-charge carrier generation efficiency) obtained with 3 nm diameter CdSe nanoparticles was 35% for particulate TiO2 and 45% for tubular TiO2 morphology. The maximum IPCE observed at the excitonic band increases with decreasing particle size, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection. The maximum power-conversion efficiency
1,608 citations
TL;DR: The synthesis and development of state-of-the-art QD probes and their use for molecular and cellular imaging are discussed and key issues for in vivo imaging and therapy, such as nanoparticle biodistribution, pharmacokinetics, and toxicology are examined.
1,116 citations
TL;DR: Results clearly demonstrate that the unique nanotubes structure can facilitate the propagation and kinetic separation of photogenerated charges, suggesting potentially important applications of the inorganic QDs sensitized TiO2 nanotube-array films in solar cell applications.
Abstract: Novel CdS quantum dots (QDs) sensitized TiO2 nanotube-array photoelectrodes were investigated for their photoelectrochemical (PEC) performance. The highly ordered TiO2 nanotube arrays were synthesized by anodic oxidation and CdS QDs were deposited into the pores of the nanotube arrays by a sequential chemical bath deposition method. It is found that the CdS QDs deposited in the pores of the TiO2 nanotube arrays may significantly increase the liquid junction PEC short circuit photocurrent (from 0.22 to 7.82 mA/cm2) and cell efficiency (up to 4.15%). These results clearly demonstrate that the unique nanotube structure can facilitate the propagation and kinetic separation of photogenerated charges, suggesting potentially important applications of the inorganic QDs sensitized TiO2 nanotube-array films in solar cell applications.
1,058 citations
TL;DR: In this article, the use of sputtered, amorphous inorganic semiconductors as robust charge transport layers and demonstrate devices capable of operating at current densities exceeding 3.5 cm−2 with peak brightness of 1,950 cm−m−2 and maximum external electroluminescence efficiency of nearly 0.1%, which represents a 100-fold improvement over previously reported structures.
Abstract: Colloidal quantum dots, with their tunable luminescence properties, are uniquely suited for use as lumophores in light-emitting devices for display technologies and large-area planar lighting1,2,3,4,5,6,7,8,9,10. In contrast to epitaxially grown quantum dots, colloidal quantum dots can be synthesized as highly monodisperse colloids and solution deposited over large areas into densely packed, solid-state multilayers, which have shown promise as efficient optical gain media11. To be a viable platform for colour-tunable electrically pumped lasers, the present-generation quantum-dot LEDs must be modified to withstand the extended, high-current-density operation needed to achieve population inversion. This requirement necessitates a quantum-dot LED design that incorporates robust charge transport layers. Here we report the use of sputtered, amorphous inorganic semiconductors as robust charge transport layers and demonstrate devices capable of operating at current densities exceeding 3.5 A cm−2 with peak brightness of 1,950 Cd m−2 and maximum external electroluminescence efficiency of nearly 0.1%, which represents a 100-fold improvement over previously reported structures8,10. The authors show that metal oxide and colloidal quantum dots can be combined to fabricate monochrome LEDs with a brightness that matches that of the best organic-based quantum-dot LEDs, but with the advantage of improved shelf-life robustness. The reported maximum external electroluminescence efficiency is nearly 0.1%, which represents a 100-fold improvement over previously reported structures
868 citations
TL;DR: Semiconductor nanoparticles, or quantum dots (QDs), have unique photophysical properties, such as size-controlled fluorescence, have high fluorescence quantum yields, and stability against photobleaching, which enable the use of QDs as optical labels for the multiplexed analysis of immunocomplexes or DNA hybridization processes.
Abstract: Semiconductor nanoparticles, or quantum dots (QDs), have unique photophysical properties, such as size-controlled fluorescence, have high fluorescence quantum yields, and stability against photobleaching. These properties enable the use of QDs as optical labels for the multiplexed analysis of immunocomplexes or DNA hybridization processes. Semiconductor QDs are also used to probe biocatalytic transformations. The time-dependent replication or telomerization of nucleic acids, the oxidation of phenol derivatives by tyrosinase, or the hydrolytic cleavage of peptides by proteases are probed by using fluorescence resonance energy transfer or photoinduced electron transfer. The photoexcitation of QD-biomolecule hybrids associated with electrodes enables the photoelectrochemical transduction of biorecognition events or biocatalytic transformations. Examples are the generation of photocurrents by duplex DNA assemblies bridging CdS NPs to electrodes, and by the formation of photocurrents as a result of biocatalyzed transformations. Semiconductor nanoparticles are also used as labels for the electrochemical detection of DNA or proteins: Semiconductor NPs functionalized with nucleic acids or proteins bind to biorecognition complexes, and the subsequent dissolution of the NPs allows the voltammetric detection of the related ions, and the tracing of the recognition events.
817 citations
TL;DR: Ensemble and single molecule fluorescence studies show that oligonucleotide encapsulated Ag nanoclusters exhibit significantly greater photostability and higher emission rates than commonly used cyanine dyes.
Abstract: Single-stranded oligonucleotides stabilize highly fluorescent Ag nanoclusters, with emission colors tunable via DNA sequence. We utilized DNA microarrays to optimize these scaffold sequences for creating nearly spectrally pure Ag nanocluster fluorophores that are highly photostable and exhibit great buffer stability. Five different nanocluster emitters have been created with tunable emission from the blue to the near-IR and excellent photophysical properties. Ensemble and single molecule fluorescence studies show that oligonucleotide encapsulated Ag nanoclusters exhibit significantly greater photostability and higher emission rates than commonly used cyanine dyes.
797 citations
TL;DR: Novel CdSe-CdS core-shell quantum dots with thick crystalline shells with 68% of which do not blink when observed individually at 33 Hz for 5 min are synthesized, establishing a direct correlation between shell thickness and blinking occurrences.
Abstract: At a single-molecule level, fluorophore emission intensity fluctuates between bright and dark states. These fluctuations, known as blinking, limit the use of fluorophores in single-molecule experiments. The dark-state duration shows a universal heavy-tailed power-law distribution characterized by the occurrence of long non-emissive periods. Here we have synthesized novel CdSe-CdS core-shell quantum dots with thick crystalline shells, 68% of which do not blink when observed individually at 33 Hz for 5 min. We have established a direct correlation between shell thickness and blinking occurrences. Importantly, the statistics of dark periods that appear at high acquisition rates (1 kHz) are not heavy tailed, in striking contrast with previous observations. Blinking statistics are thus not as universal as thought so far. We anticipate that our results will help to better understand the physico-chemistry of single-fluorophore emission and rationalize the design of other fluorophores that do not blink.
779 citations
TL;DR: The demonstrated photoluminescence adds another dimension to the versatility of carbon-based emitters, and is suggested that the tethered modifier stabilizes the surface of the carbon nanoparticles helping to generate energy traps that emit light when stimulated.
Abstract: Quantum dots are semiconductor nanocrystals that inherently fluoresce at specific wavelengths in the visible, enabling a number of potential applications to be realized. However, conventional quantum dots are based on metallic elements, which has raised concerns over toxicity, stability and high cost. As a result, the search for more benign substitutes is a worthwhile yet challenging undertaking. Recently a new type of visible emitters has been reported exclusively based on functionalized carbon nanoparticles. The carbon dots were 5 nm in diameter and were produced via laser ablation of graphite. Surface oxidation with nitric acid and subsequent covalent grafting of organic moieties afforded light-emitting derivatives. Notably, the light emitted by these dots depends on the wavelength of light used for excitation. It was suggested that the tethered modifier stabilizes the surface of the carbon nanoparticles helping to generate energy traps that emit light when stimulated, an effect described as emission from passivated surfaces. Because of its origin the emission is size-dependent, i.e., the smaller the size of the dots the better their photoluminescence efficiency. In another intriguing approach, photoluminescent carbon dots 3 nm in size were directly fabricated by electrochemical shocking of multi-wall carbon nanotubes. The demonstrated photoluminescence adds another dimension to the versatility of carbon-based
765 citations
TL;DR: The superior figures of merit of these new fluorescent probes, together with the demonstration of cellular imaging, indicate their enormous potential for demanding fluorescence-based imaging and sensing applications such as high speed super-resolution single molecule/particle tracking and highly sensitive assays.
Abstract: Highly fluorescent conjugated polymer dots were developed for demanding applications such as fluorescence imaging in live cells. These nanoparticles exhibit small particle diameters, extraordinary fluorescence brightness, and excellent photostability. Single particle fluorescence imaging and kinetic studies indicate much higher emission rates (∼108 s−1) and little or no blinking of the nanoparticles as compared to typical results for single dye molecules and quantum dots. Analysis of single particle photobleaching trajectories reveals excellent photostability—as many as 109 or more photons emitted per nanoparticle prior to irreversible photobleaching. The superior figures of merit of these new fluorescent probes, together with the demonstration of cellular imaging, indicate their enormous potential for demanding fluorescence-based imaging and sensing applications such as high speed super-resolution single molecule/particle tracking and highly sensitive assays.
TL;DR: This paper reports the preparation of highly stable aqueous suspensions of Si QDs using phospholipid micelles, in which the optical properties of Si nanocrystals are retained, which paves the way for silicon quantum dots to be a valuable optical probe in biomedical diagnostics.
Abstract: Luminescent silicon quantum dots (Si QDs) have great potential for use in biological imaging and diagnostic applications. To exploit this potential, they must remain luminescent and stably dispersed in water and biological fluids over a wide range of pH and salt concentration. There have been many challenges in creating such stable water-dispersible Si QDs, including instability of photoluminescence due their fast oxidation in aqueous environments and the difficulty of attaching hydrophilic molecules to Si QD surfaces. In this paper, we report the preparation of highly stable aqueous suspensions of Si QDs using phospholipid micelles, in which the optical properties of Si nanocrystals are retained. These luminescent micelle-encapsulated Si QDs were used as luminescent labels for pancreatic cancer cells. This paves the way for silicon quantum dots to be a valuable optical probe in biomedical diagnostics.
TL;DR: The concept of using quantum dots and nanoparticles to yield electrons and radicals in photodynamic and radiation therapies as well their combination is reviewed in this article.
TL;DR: In this paper, the authors demonstrate that non-classical states of light can be coherently generated using a quantum dot strongly coupled to a photonic crystal resonator, and demonstrate that the capture of a single photon into the cavity affects the probability that a second photon is admitted.
Abstract: Quantum dots in photonic crystals are interesting because of their potential in quantum information processing and as a testbed for cavity quantum electrodynamics. Recent advances in controlling and coherent probing of such systems open the possibility of realizing quantum networks originally proposed for atomic systems. Here, we demonstrate that non-classical states of light can be coherently generated using a quantum dot strongly coupled to a photonic crystal resonator. We show that the capture of a single photon into the cavity affects the probability that a second photon is admitted. This probability drops when the probe is positioned at one of the two energy eigenstates corresponding to the vacuum Rabi splitting, a phenomenon known as photon blockade, the signature of which is photon antibunching. In addition, we show that when the probe is positioned between the two eigenstates, the probability of admitting subsequent photons increases, resulting in photon bunching. We call this process photon-induced tunnelling. This system represents an ultimate limit for solid-state nonlinear optics at the single-photon level. Along with demonstrating the generation of non-classical photon states, we propose an implementation of a single-photon transistor in this system.
TL;DR: Continuous-wave fluorescence spectra and fluorescence transients of PbS quantum dots coupled to titanium dioxide nanoparticles are consistent with electron transfer for small quantum dots, and implications of this for future photovoltaics will be discussed.
Abstract: Injection of photoexcited electrons from colloidal PbS quantum dots into TiO(2) nanoparticles is investigated. The electron affinity and ionization potential of PbS quantum dots, inferred from cyclic voltammetry measurements, show strong size dependence due to quantum confinement. On the basis of the measured energy levels, photoexcited electrons should transfer efficiently from the quantum dots into TiO(2) only for quantum-dot diameter below approximately 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of PbS quantum dots coupled to titanium dioxide nanoparticles are consistent with electron transfer for small quantum dots. The measured electron transfer time is surprisingly slow ( approximately 100 ns), and implications of this for future photovoltaics will be discussed. Initial results obtained from solar cells sensitized with PbS quantum dots are presented.
TL;DR: The description of the latest attempts at incorporating phthalocyanines into photovoltaic devices for solar energy conversion and onto quantum dots for photodynamic therapy or quantum computing is described.
Abstract: This review paper gives a brief overview on how the outstanding chemical and physical properties of phthalocyanines and phthalocyanine derivatives are being studied and employed in order to construct state-of-the-art technological devices. In a first instance, a short account on how the nature of the phthalocyanine structure and its organization in condensed phases play an important role in their conducting and ultraviolet-visible absorption properties is presented. Consequently, these basic electronic and photophysical features of phthalocyanines allow us to explain why phthalocyanine-based multicomponent covalent or noncovalent donor-acceptor systems may give rise to very interesting photophysical properties, in particular in terms of their ability to generate very long-lived photoinduced charge-separated states. A concise survey on the organization of these multifunctional systems shows how a profound understanding of the morphology at the nanometer-scale of these phthalocyanine-based molecular materials is needed in order to control their physical properties in condensed phases. All the previously mentioned chemical and physical features combined together led us to the description of the latest attempts at incorporating phthalocyanines into photovoltaic devices for solar energy conversion and onto quantum dots for photodynamic therapy or quantum computing. © 2008 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 8: 75–97; 2008: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20139
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TL;DR: In this paper, the status of research on the formation of nanowire structures via highly anisotropic growth of nanocrystals of semiconductor and metal oxide materials with an emphasis on the structural characterization of the nucleation, initial growth, defects and interface structures.
Abstract: The tremendous interest in nanoscale structures such as quantum dots (zero-dimension) and wires (quasi-one-dimension) stems from their size-dependent properties. One-dimensional (1D) semiconductor nanostructures are of particular interest because of their potential applications in nanoscale electronic and optoelectronic devices. For 1D semiconductor nanomaterials to have wide practical application, however, several areas require further development. In particular, the fabrication of desired 1D nanomaterials with tailored atomic structures and their assembly into functional devices are still major challenges for nanotechnologists. In this review, we focus on the status of research on the formation of nanowire structures via highly anisotropic growth of nanocrystals of semiconductor and metal oxide materials with an emphasis on the structural characterization of the nucleation, initial growth, defects and interface structures, as well as on theoretical analyses of nanocrystal formation, reactivity and stability. We review various methods used and mechanisms involved to generate 1D nanostructures from different material systems through self-organized growth techniques including vapor–liquid–solid growth, oxide-assisted chemical vapor deposition (without a metal catalyst), laser ablation, thermal evaporation, metal-catalyzed molecular beam epitaxy, chemical beam epitaxy and hydrothermal reaction. 1D nanostructures grown by these technologies have been observed to exhibit unusual growth phenomena and unexpected properties, e.g., diameter-dependent and temperature-dependent growth directions, structural transformation by enhanced photothermal effects and phase transformation induced by the point contact reaction in ultra-thin semiconductor nanowires. Recent progress in controlling growth directions, defects, interface structures, structural transformation, contacts and hetero-junctions in 1D nanostructures is addressed. Also reviewed are the quantitative explorations and predictions of some challenging 1D nanostructures and descriptions of the growth mechanisms of 1D nanostructures, based on the energetic, dynamic and kinetic behaviors of the building block nanostructures and their surfaces and/or interfaces.
TL;DR: In this article, the integration of a micrometre-sized magnet with a semiconductor device has enabled the individual manipulation of two single electron spins, which may provide a scalable route for quantum computing with electron spins confined in quantum dots.
Abstract: The integration of a micrometre-sized magnet with a semiconductor device has enabled the individual manipulation of two single electron spins. This approach may provide a scalable route for quantum computing with electron spins confined in quantum dots.
TL;DR: In this article, a power-law distribution of on-and off-times observed in colloidal semiconductor quantum dots, nanorods, nanowires and some organic dyes is discussed.
Abstract: Virtually all known fluorophores exhibit mysterious episodes of emission intermittency. A remarkable feature of the phenomenon is a power-law distribution of on- and off-times observed in colloidal semiconductor quantum dots, nanorods, nanowires and some organic dyes. For nanoparticles, the resulting power law extends over an extraordinarily wide dynamic range: nine orders of magnitude in probability density and five to six orders of magnitude in time. Exponents hover about the ubiquitous value of -3/2. Dark states routinely last for tens of seconds—practically forever on quantum mechanical timescales. Despite such infinite states of darkness, the dots miraculously recover and start emitting again. Although the underlying mechanism responsible for this phenomenon remains a mystery and many questions persist, we argue that substantial theoretical progress has been made.
TL;DR: Semiconductor quantum dots and metal nanoparticles have extensive applications, e.g., in vitro and in vivo bioimaging, and toxic effects of NPs and their clearance from the body are discussed.
Abstract: We review the syntheses, optical properties, and biological applications of cadmium selenide (CdSe) and cadmium selenide-zinc sulfide (CdSe-ZnS) quantum dots (QDs) and gold (Au) and silver (Ag) nanoparticles (NPs). Specifically, we selected the syntheses of QDs and Au and Ag NPs in aqueous and organic phases, size- and shape-dependent photoluminescence (PL) of QDs and plasmon of metal NPs, and their bioimaging applications. The PL properties of QDs are discussed with reference to their band gap structure and various electronic transitions, relations of PL and photoactivated PL with surface defects, and blinking of single QDs. Optical properties of Ag and Au NPs are discussed with reference to their size- and shape-dependent surface plasmon bands, electron dynamics and relaxation, and surface-enhanced Raman scattering (SERS). The bioimaging applications are discussed with reference to in vitro and in vivo imaging of live cells, and in vivo imaging of cancers, tumor vasculature, and lymph nodes. Other aspects of the review are in vivo deep tissue imaging, multiphoton excitation, NIR fluorescence and SERS imaging, and toxic effects of NPs and their clearance from the body.
TL;DR: In this article, a single-step synthetic method for highly luminescent and stable quantum dots (QDs) was demonstrated by using the reactivity difference between Cd and Zn precursors.
Abstract: We demonstrate a single-step synthetic method for highly luminescent (i.e., quantum yield up to 80%) and stable quantum dots (QDs) by using the reactivity difference between Cd and Zn precursors and that between Se and S precursors. A wide range of emission wavelengths (500−610 nm) with a narrow fwhm (<35 nm) is obtained by changing the ratios of the precursors. Under the reaction conditions selected, Cd- and Se (with a bit of S)-based cores are formed first and Zn- and S-based shells are formed successively; therefore, the QDs have a core/shell structure with composition gradients, which relieve the lattice mismatch between core and shells. The QDs are characterized using the combined techniques of HR-TEM, UV–vis, PL spectroscopy, and ICP-AES. The QDs also have energy gradients depending on their compositions in a radial direction, which energetically confine carriers (electrons and holes) to the cores. This leads to the stability of QDs during their surface passivation from oleic acid to mercaptopropion...
TL;DR: A detailed study of the energetics of periodic graphene antidot lattices is presented, the level structure of a single defect is analyzed, the exchange coupling between a pair of spin qubits is calculated, and possible avenues for further developments are identified.
Abstract: Antidot lattices, defined on a two-dimensional electron gas at a semiconductor heterostructure, are a well-studied class of man-made structures with intriguing physical properties. We point out that a closely related system, graphene sheets with regularly spaced holes ("antidots"), should display similar phenomenology, but within a much more favorable energy scale, a consequence of the Dirac fermion nature of the states around the Fermi level. Further, by leaving out some of the holes one can create defect states, or pairs of coupled defect states, which can function as hosts for electron spin qubits. We present a detailed study of the energetics of periodic graphene antidot lattices, analyze the level structure of a single defect, calculate the exchange coupling between a pair of spin qubits, and identify possible avenues for further developments.
TL;DR: It is shown, based on measurements in high magnetic fields, that defects are the dominant source of light from Si nanocrystals, and it is shown that it is possible to control the origin of the photoluminescence in a single sample.
Abstract: Silicon dominates the electronics industry, but its poor optical properties mean that III–V compound semiconductors are preferred for photonics applications. Photoluminescence at visible wavelengths was observed from porous Si at room temperature in 1990, but the origin of these photons (do they arise from highly localized defect states or quantum confinement effects?) has been the subject of intense debate ever since. Attention has subsequently shifted from porous Si to Si nanocrystals, but the same fundamental question about the origin of the photoluminescence has remained. Here we show, based on measurements in high magnetic fields, that defects are the dominant source of light from Si nanocrystals. Moreover, we show that it is possible to control the origin of the photoluminescence in a single sample: passivation with hydrogen removes the defects, resulting in photoluminescence from quantum-confined states, but subsequent ultraviolet illumination reintroduces the defects, making them the origin of the light again.
TL;DR: It is found that relaxation is markedly slowed with increasing ZnSe shell thickness, which could prove useful for more efficient photovoltaic or infrared devices.
Abstract: Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of approximately 0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness.
TL;DR: A partial cation-exchange method is described in which this lability to controllably synthesize PbSe/CdSe/ZnS core/shell/shell NQDs are used to produce bright, biocompatible near-infrared optical labels.
Abstract: Infrared-emitting nanocrystal quantum dots (NQDs) have enormous potential as an enabling technology for applications ranging from tunable infrared lasers to biological labels. Notably, lead chalcogenide NQDs, especially PbSe NQDs, provide efficient emission over a large spectral range in the infrared, but their application has been limited by instability in emission quantum yield and peak position on exposure to ambient conditions. Conventional methods for improving NQD stability by applying a shell of a more stable, wider band gap semiconductor material are frustrated by the tendency of lead chalcogenide NQDs toward Ostwald ripening at even moderate reaction temperatures. Here, we describe a partial cation-exchange method in which we take advantage of this lability to controllably synthesize PbSe/CdSe core/shell NQDs. Critically, these NQDs are stable against fading and spectral shifting. Further, these NQDs can undergo additional shell growth to produce PbSe/CdSe/ZnS core/shell/shell NQDs that represent...
TL;DR: Stable solution-processed photovoltaic devices having 3.6% power conversion efficiency in the infrared are reported, and diffusion of electrons and holes over hundreds of nanometers through the PbSe colloidal quantum dot solid is chiefly responsible for the high external quantum efficiencies obtained in this new class of devices.
Abstract: Half of the sun’s power lies in the infrared. As a result, the optimal bandgaps for solar cells in both the single-junction and even the tandem architectures lie beyond 850 nm. However, progress in low-cost, large-area, physically flexible solar cells has instead been made in organic and polymer materials possessing absorption onsets in the visible. Recent advances have been achieved in solution-cast infrared photovoltaics through the use of colloidal quantum dots. Here we report stable solution-processed photovoltaic devices having 3.6% power conversion efficiency in the infrared. The use of a strongly bound bidentate linker, benzenedithiol, ensures device stability over weeks. The devices reach external quantum efficiencies of 46% in the infrared and 70% across the visible. We investigate in detail the physical mechanisms underlying the operation of this class of device. In contrast with drift-dominated behavior in recent reports of PbS quantum dot photovoltaics, we find that diffusion of electrons and ...
TL;DR: It is shown that a single quantum dot coupled to a photonic crystal nanocavity can facilitate controlled phase and amplitude modulation between two modes of light at the single-photon level.
Abstract: Optical nonlinearities enable photon-photon interaction and lie at the heart of several proposals for quantum information processing, quantum nondemolition measurements of photons, and optical signal processing. To date, the largest nonlinearities have been realized with single atoms and atomic ensembles. We show that a single quantum dot coupled to a photonic crystal nanocavity can facilitate controlled phase and amplitude modulation between two modes of light at the single-photon level. At larger control powers, we observed phase shifts up to π/4 and amplitude modulation up to 50%. This was accomplished by varying the photon number in the control beam at a wavelength that was the same as that of the signal, or at a wavelength that was detuned by several quantum dot linewidths from the signal. Our results present a step toward quantum logic devices and quantum nondemolition measurements on a chip.
TL;DR: The synthesis of a size series of copper indium selenide quantum dots of various stoichiometries exhibiting photoluminescence from the red to near-infrared (NIR) is reported.
Abstract: We report the synthesis of a size series of copper indium selenide quantum dots (QDs) of various stoichiometries exhibiting photoluminescence (PL) from the red to near-infrared (NIR). The synthetic method is modular, and we have extended it to the synthesis of luminescent silver indium diselenide QDs. Previous reports on QDs luminescent in the NIR region have been primarily restricted to binary semiconductor systems, such as InAs, PbS, and CdTe. This work seeks to expand the availability of luminescent QD materials to ternary I−III−VI semiconductor systems.
TL;DR: In this article, a spherical quantum dot with parabolic confinement subjected to an external electric field with the presence of an impurity, the linear and third-order nonlinear optical absorption coefficients as well as refractive index changes have been calculated.
Abstract: In the present work, the case of a spherical quantum dot with parabolic confinement subjected to an external electric field with the presence of an impurity, the linear and third-order nonlinear optical absorption coefficients as well as refractive index changes have been calculated. The numerical method we are using for the calculation of the energy levels and the corresponding wave functions is the potential morphing method in the effective mass approximation. As our results indicate an increase of the electric field and/or the position of the impurity and/or the quantum dot radius redshifts the peak positions of the total absorption coefficient and total refractive index changes. Additionally, an increase of the position of the impurity and/or the quantum dot radius decreases the total absorption coefficient and increases the total refractive index changes. An increase also of the electric field decreases the total absorption coefficient but does not significantly affect the peak values of the total refractive index changes. Finally, an increase of the optical intensity considerably changes the total absorption coefficient as well as the total refractive index changes.