Nanocrystal Quantum Dots Research Articles

Spectroscopic and Device Aspects of Nanocrystal Quantum Dots.

The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminescent solar concentrators; and control of intraband relaxation for enhanced carrier multiplication in advanced QD photovoltaics. We examine the considerable recent progress on these multiple fronts of nanocrystal research, which has resulted in the first commercialized QD technologies. These successes explain the continuing appeal of this field to a broad community of scientists and engineers, which in turn ensures even more exciting results to come from future exploration of this fascinating class of materials.

Electronic properties of an exciton in CdTe/CdSe/CdTe/CdSe type-II nano-heterostructure

In this study, we have carried out a detailed theoretical investigation on the binding energy of an exciton in type-II CdTe/CdSe core/shell/well/shell (CSWS) nanocrystal quantum dot (NCQD) in the strong confinement region. The calculations are based on the effective mass approximation, and the coulombic interaction between electron and hole is introduced using Hartree approximation. With these theoretical basis, the coupled Poisson–Schrodinger equations are solved in a self consistent iterative manner. In strong confinement regime, the binding energy variation with core radius in type-II NCQD shows a peak. And this peak widens for larger well width and inner shell thickness. Our study suggests that, this anomalous behavior of exciton binding energy is due to an effect called ‘positional flip of exciton’, caused by the faster tunneling of hole to the inner layer in comparison with electron. Our results can be applied in laser and optoelectronic engineering for designing more efficient optoelectronic devices.

Mechanochemical approach for the capping of mixed core CdS/ZnS nanocrystals: Elimination of cadmium toxicity

The wet mechanochemical procedure for the capping of the CdS and CdS/ZnS quantum dot nanocrystals is reported. l-cysteine and polyvinylpyrrolidone (PVP) were used as capping agents. When using l-cysteine, the dissolution of cadmium(II) was almost none for CdS/ZnS nanocrystals. Moreover, prepared CdS- and CdS/ZnS-cysteine nanosuspensions exhibited unimodal particle size distributions with very good stability, which was further supported by the zeta potential measurements. The Fourier-transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy showed the successful embedment of cysteine into the structure of the nanocrystals. Additionally, the optical properties were examined, and the results showed that the cysteine nanosuspension has promising fluorescence properties. On the other hand, PVP was not determined to be a very suitable capping agent for the present system. In this case, the release of cadmium(II) was higher in comparison to the l-cysteine capped samples. The nanosuspensions were successfully used for in vitro studies on selected cancer cell lines. Using fluorescence microscopy, it was evidenced that the nanocrystals enter the cell and that they can serve as imaging agents in biomedical applications.

(Invited) Luminescence of Lanthanide-Doped Nanocrystals

Nanocrystals (NCs) doped with luminescent ions form an emerging class of materials. They serve as probes in physics, chemistry and biology and demonstrate unprecedented flexibility in spectral conversion (including up- and downconversion) for applications ranging from solar cells and white light LEDs to photodynamic therapy. In this presentation the synthesis and optical properties of lanthanide doped nanocrystals (both quantum dots and insulator nanocrystals) will be discussed with a focus on design strategies to tailor the luminescence properties and to enhance spectral conversion efficiencies.

Surface Charge Control of Quantum Dot Blinking

A characteristic property of colloidal semiconductor nanocrystal quantum dots (QDs) is their emission intermittency. Although a unifying theory of QD photoprocesses remains elusive, the importance of charged states is clear. We now report a new approach to directly study the role of surface charge on QD emission by adding metal ions to individual, core-only QDs immobilized in aqueous solution in an agarose gel. The CdTe QDs show very stable emission in the absence of metal ions but a dramatic and reversible increase in blinking due to the presence of trivalent metal ions. Our results support a charge-separation model, in which the major blinking pathway is the surface trapping of electrons; transiently bound metal ions close to the QD surface enhance this process.

Measuring and Predicting the Internal Structure of Semiconductor Nanocrystals through Raman Spectroscopy.

Nanocrystals composed of mixed chemical domains have diverse properties that are driving their integration in next-generation electronics, light sources, and biosensors. However, the precise spatial distribution of elements within these particles is difficult to measure and control, yet profoundly impacts their quality and performance. Here we synthesized a unique series of 42 different quantum dot nanocrystals, composed of two chemical domains (CdS:CdSe), arranged in 7 alloy and (core)shell structural classes. Chemometric analyses of far-field Raman spectra accurately classified their internal structures from their vibrational signatures. These classifications provide direct insight into the elemental arrangement of the alloy as well as an independent prediction of fluorescence quantum yield. This nondestructive, rapid approach can be broadly applied to greatly enhance our capacity to measure, predict and monitor multicomponent nanomaterials for precise tuning of their structures and properties.

Detailed Observation of Multiphoton Emission Enhancement from a Single Colloidal Quantum Dot Using a Silver-Coated AFM Tip.

The enhancement of multiphoton emission from a single colloidal nanocrystal quantum dot (NQD) interacting with a plasmonic nanostructure was investigated using a silver-coated atomic force microscopy tip (AgTip) as the plasmonic nanostructure. Using the AgTip, which exhibited a well-defined localized surface plasmon (LSP) resonance band, we controlled the spectral overlap and the distance between the single NQD and the AgTip. The emission behavior of the single NQD when approaching the AgTip at the nanometer scale was measured using off-resonance (405 nm) and resonance (465 nm) excitation of the LSP. We directly observed the conversion of the single-photon emission from a single NQD to multiphoton emission with reduction of the emission lifetime at both excitation wavelengths as the NQD-AgTip distance decreased, whereas a decrease and increase in the emission intensity were observed at 405 and 465 nm excitation, respectively. By combining theoretical analysis and the numerical simulation of the AgTip, we deduced that the enhancement of the multiphoton emission was caused by the quenching of the single-exciton state due to the energy transfer from the NQD to the AgTip and that the emission intensity was increased by enhancement of the excitation rate due to the electric field of the LSP on the AgTip. These results provide evidence that the photon statistics and the photon flux from the single NQD can be manipulated by the plasmonic nanostructure through control of the spectral overlap and the distance.

Temperature-dependent excitonic photoluminescence excited by two-photon absorption in perovskite CsPbBr3 quantum dots.

Recently, lead halide perovskite quantum dots have been reported with potential for photovoltaic and optoelectronic applications due to their excellent luminescent properties. Herein excitonic photoluminescence (PL) excited by two-photon absorption in perovskite CsPbBr3 quantum dots (QDs) has been studied at a broad temperature range, from 80 to 380 K. Two-photon absorption has been investigated and the absorption coefficient is up to 0.085 cm/GW at room temperature. Moreover, the PL spectrum excited by two-photon absorption shows a linear blue-shift (0.32 meV/K) below the temperature of 220 K. However, for higher temperatures, the PL peak approaches a roughly constant value and shows temperature-independent chromaticity up to 380 K. This behavior is distinct from the general red-shift for semiconductors and can be attributed to the result of thermal expansion, electron-phonon interaction and structural phase transition around 360 K. The strong nonlinear absorption and temperature-independent chromaticity of CsPbBr3 QDs observed in temperature range from 220 to 380 K will offer new opportunities in nonlinear photonics, light-harvesting, and light-emitting devices.

Segmented flow reactor for synthesis of quantum dot nanocrystals and plasmonic nanoparticles

This research presents an automated method of synthesizing semiconductor and metal nanoparticles using flow rector synthesis as a new alternative to the batch method of synthesizing nanoparticles. Experiments were carried out to determine the optimal flow rates of reagents droplets. The reactor was successfully applied to the synthesis of colloidal solutions of semiconductor (CdSe) and metal (Ag) nanoparticles. This instrument is applicable both in material science laboratories and in industry.

Nonlinear optical response of planar and spherical CdSe nanocrystals

The nonlinear optical response of a colloidal solution of planar CdSe semiconductor nanocrystals (nanoplatelets) is studied for the first time. The nonlinear optical response of these nanoparticles is compared to that of spherical CdSe nanocrystals (quantum dots). The photoinduced nonlinearity is attributed to the optical generation of long-lived charge carriers in the nanoobjects under study. It is shown that, upon the exposure of a cell with the solution of nanoparticles to focused continuous-wave (cw) laser radiation with a wavelength of 473 nm, the nonlinear optical responses of CdSe nanoplatelets and quantum dots are somewhat different at identical optical densities at the above-indicated wavelength. The differences are supposedly associated with a higher diffusion rate of spherical nanoparticles in the solution because of their smaller size compared to that of nanoplatelets.

Fabrication and optical studies of epoxy-quantum-dot nanocomposites

Light emitting quantum dots were dispersed, particle-particle distance~50±8 nm, in an epoxy resin through the incorporation of ethylene glycol surfactant into the organic passivating layer of the quantum-dot nanocrystals. Evaporation of the wetting solvent enabled the ethylene glycol surfactant to incorporate into the hydrophobic layer of the semiconductor nanocrystals by interfacial interaction driven by van der Waals forces. The chemical structure effects on the degree of dispersion for the quantum-dot nanocrystals in an epoxy resin were studied. The passive optical properties of the resulting quantum-dot-epoxy nanocomposites were investigated using a light emitting diode as an excitation source. Well-dispersed nanocrystals in an epoxy showed both enhanced photo-stability and superior light emission intensity compared to the less-dispersed nanocrystals in an epoxy for 20 day evaluation.

Critical role of CdSe nanoplatelets in color-converting CdSe/ZnS nanocrystals for InGaN/GaN light-emitting diodes

Here we report CdSe nanoplatelets that are incorporated into color-converting CdSe/ZnS nanocrystals for InGaN/GaN light-emitting diodes. The critical role of CdSe nanoplatelets as an exciton donor for the color conversion was experimentally investigated. The power conversion efficiency of the hybrid light-emitting diode was found to increase by 23% with the incorporation of the CdSe nanoplatelets. The performance enhancement is ascribed to efficient exciton transfer from the donor CdSe nanoplatelet quantum wells to the acceptor CdSe/ZnS nanocrystal quantum dots through Förster-type nonradiative resonance energy transfer.

Size-confined fixed-composition and composition-dependent engineered band gap alloying induces different internal structures in L-cysteine-capped alloyed quaternary CdZnTeS quantum dots.

The development of alloyed quantum dot (QD) nanocrystals with attractive optical properties for a wide array of chemical and biological applications is a growing research field. In this work, size-tunable engineered band gap composition-dependent alloying and fixed-composition alloying were employed to fabricate new L-cysteine-capped alloyed quaternary CdZnTeS QDs exhibiting different internal structures. Lattice parameters simulated based on powder X-ray diffraction (PXRD) revealed the internal structure of the composition-dependent alloyed CdxZnyTeS QDs to have a gradient nature, whereas the fixed-composition alloyed QDs exhibited a homogenous internal structure. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis confirmed the size-confined nature and monodispersity of the alloyed nanocrystals. The zeta potential values were within the accepted range of colloidal stability. Circular dichroism (CD) analysis showed that the surface-capped L-cysteine ligand induced electronic and conformational chiroptical changes in the alloyed nanocrystals. The photoluminescence (PL) quantum yield (QY) values of the gradient alloyed QDs were 27–61%, whereas for the homogenous alloyed QDs, the PL QY values were spectacularly high (72–93%). Our work demonstrates that engineered fixed alloying produces homogenous QD nanocrystals with higher PL QY than composition-dependent alloying.

Enhancement of Optical Efficiency in White OLED Display Using the Patterned Photoresist Film Dispersed With Quantum Dot Nanocrystals

Quantum dot (QD) nanocrystals dispersed in photoresist (PR) film was developed and applied to white organic light-emitting diode (OLED) to improve optical power of red color through down-converting of blue and green light. To integrate to white OLED display panel, the QD dispersed photoresist film was prepared in a thickness of 2 $\mu\hbox{m}$ with high concentration of QDs up to 30 wt%. QDs were dispersed successfully in PR with a matching of nonpolar characteristic for the ligands of QDs and PR as well as a careful mixing process of PR and QD dispersed solutions. We also realized the patterning of QD dispersed PR film with a stripe pattern of 60- $\mu\hbox{m}$ width without a residual layer. The experimental measurement after passing through a 30 wt% QD dispersed PR film and a red color filter in white OLED shows the enhancement of 40.2% in the optical power of red color compared to that from a conventional white OLED without QD dispersed PR film.

Colloidal nanocrystals for quality lighting and displays: milestones and recent developments

AbstractRecent advances in colloidal synthesis of nanocrystals have enabled high-quality high-efficiency light-emitting diodes, displays with significantly broader color gamut, and optically-pumped lasers spanning the whole visible regime. Here we review these colloidal platforms covering the milestone studies together with recent developments. In the review, we focus on the devices made of colloidal quantum dots (nanocrystals), colloidal quantum rods (nanorods), and colloidal quantum wells (nanoplatelets) as well as those of solution processed perovskites and phosphor nanocrystals. The review starts with an introduction to colloidal nanocrystal photonics emphasizing the importance of colloidal materials for light-emitting devices. Subsequently,we continue with the summary of important reports on light-emitting diodes, in which colloids are used as the color converters and then as the emissive layers in electroluminescent devices. Also,we review the developments in color enrichment and electroluminescent displays. Next, we present a summary of important reports on the lasing of colloidal semiconductors. Finally, we summarize and conclude the review presenting a future outlook.

The electrochemical behavior of core-shell CdSe/CdS magic-sized quantum dots linked to cyclodextrin for studies of the encapsulation of bioactive compounds

Semiconductor nanocrystal quantum dots have been the subject of extensive investigations in different areas of science and technology in the past years. In particular, there are few studies of magic-sized quantum dots (MSQDs), even though they exhibit features such as extremely small size, fluorescence quantum efficiency, molar absorptivity greater than traditional QDs, and highly stable luminescence in HeLa cell cultures, thereby enabling monitoring of biological or chemical processes. The present study investigated the electrochemical behavior of free CdSe/CdS MSQDs using glassy carbon electrode and CdSe/CdS MSQDs immobilized on a gold electrode modified with a self-assembled cyclodextrin monolayer. The MSQDs showed two peaks in aprotic medium. The functionalized film modifier was prepared and characterized by means of cyclic voltammetry and electrochemical impedance spectroscopy using ferricyanide ions as a redox probe. The prepared modified electrode exhibited a stable behavior. The proposed method was successfully applied to encapsulation studies of mangiferin, a natural antioxidant compound, and cyclodextrin associated with the quantum dot, and the response was compared with that of the modified electrode without QD. The fluorescence study revealed that CdSe/CdS quantum dots emit blue light when excited by an optical source of wavelength of 350 nm and a significant increase in fluorescence and absorbance intensity is observed from the core-shell CdSe/CdS MSQDs when quantities of mangiferin are added to the solution containing thiolated cyclodextrin. CdSe/CdS MSQDs are optically and electrochemically sensitive and can be used for the detection and interaction of compounds encapsulated in cyclodextrin.

Exciton and Trion Energy Transfer from Giant Semiconductor Nanocrystals to MoS2 Monolayers

We investigate nonradiative energy transfer (NRET) between CdSe/CdS core/shell “giant” nanocrystal quantum dots (gNQDs) and monolayer domains of molybdenum disulfide (MoS2) grown by chemical vapor deposition. We employ three sets of gNQDs with varied core/shell parameters that exhibit radiative emission from neutral and charged excitons (trions) at different spectral positions from 590 to 660 nm as confirmed by photon statistics of individual nanocrystals. Strong photoluminescence (PL) emission quenching is observed for the donor gNQDs placed on MoS2 domains, indicative of the efficient NRET. Analysis of the double-component PL decays reveals NRET from both neutral excitons and charged trions with the same efficiency. Applying a macroscopic electrodynamics model for the decay of electric-dipole emitters in the vicinity of an ultrathin semiconducting layer with a strong in-plane excitonic polarizability, we confirm high NRET efficiencies from >95% to 85% for dots with diameters from 10 to 20 nm. This demon...

PbS quantum dot based hybrid-organic photodetectors for X-ray sensing

Nanocomposites consisting of conductive polymers and functional nanoparticles have recently been employed in photodetectors and imagers. Here, we present a novel hybrid-organic photodetector (HPD) that was optimized for the detection of X-rays meeting the specific needs of medical imaging. Devices were fabricated using inorganic lead sulfide (PbS) nanocrystal (NC) quantum dots (QDs) and a blend of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Quantum dots convert X-rays directly into charge-carriers that then migrate through the organic blend to the contacting electrodes. The performance of such devices depends on the thickness and probably on the morphology of the active layer. We discuss the synthesis and characterization of the PbS quantum dots, their incorporation into a HPD, and the performance of the HPD in X-ray sensing. Scanning electron microscopy and transmission electron microscopy show the PbS-QD distribution in the organic matrix. We find a strong tendency of the QDs to phase-separate from the organic blend.

Highly Directional Room-Temperature Single Photon Device.

One of the most important challenges in modern quantum optical applications is the demonstration of efficient, scalable, on-chip single photon sources, which can operate at room temperature. In this paper we demonstrate a room-temperature single photon source based on a single colloidal nanocrystal quantum dot positioned inside a circular bulls-eye shaped hybrid metal-dielectric nanoantenna. Experimental results show that 20% of the photons are emitted into a very low numerical aperture (NA < 0.25), a 20-fold improvement over a free-standing quantum dot, and with a probability of more than 70% for a single photon emission. With an NA = 0.65 more than 35% of the single photon emission is collected. The single photon purity is limited only by emission from the metal, an obstacle that can be bypassed with careful design and fabrication. The concept presented here can be extended to many other types of quantum emitters. Such a device paves a promising route for a high purity, high efficiency, on-chip single photon source operating at room temperature.

Improved synthesis dimethylhomoecoerdianthrone (HOCD) and its functionalization through facile amination reactions

An improved synthesis of dimethylhomoecoerdianthrone (HOCD), a dye used for singlet oxygen detection, was reported by isolation of a key intermediate followed by cyclization reaction. Moreover, dehydrogenative amination reaction between HOCD and alkyl amines and diamines resulted in amine functionalized HOCD derivatives with good yield. The observed reactivity depends on the nucleophilicity of amine and electrophilicity of HOCD. DFT calculations suggest that the reaction involves 1,4-Michael addition followed by migration of hydrogen atom due to aromatization. Under the aerated conditions, dehydrogenation occurs to restore the ketone structure. This C-N coupling reaction can be utilized to construct molecular materials with near infrared photonic properties. Finally, we illustrated that the HOCD derives, HOCD-SO3−, can be grafted to semiconductor nanocrystals (quantum dots) to give turn-on fluorescent nanoprobe for highly selective detection of singlet oxygen.