Optical Properties Of Quantum Dots Research Articles

Optoelectronic Properties of Semiconductor Quantum Dot Solids for Photovoltaic Applications

Quantum dot (QD) solids represent a new type of condensed matter drawing high fundamental and applied interest. Quantum confinement in individual QDs, combined with macroscopic scale whole materials, leads to novel exciton and charge transfer features that are particularly relevant to optoelectronic applications. This Perspective discusses the structure of semiconductor QD solids, optical and spectral properties, charge carrier transport, and photovoltaic applications. The distance between adjacent nanoparticles and surface ligands influences greatly electrostatic interactions between QDs and, hence, charge and energy transfer. It is almost inevitable that QD solids exhibit energetic disorder that bears many similarities to disordered organic semiconductors, with charge and exciton transport described by the multiple trapping model. QD solids are synthesized at low cost from colloidal solutions by casting, spraying, and printing. A judicious selection of a layer sequence involving QDs with different size, composition, and ligands can be used to harvest sunlight over a wide spectral range, leading to inexpensive and efficient photovoltaic devices.

Structural and physical properties of InAlAs quantum dots grown on GaAs

Quantum dots (QDs), which have particular physical properties due to the three dimensions confinement effect, could be used in many advanced optoelectronic applications. We investigated the properties of InAlAs/AlGaAs QDs grown by molecular beam epitaxy on GaAs/Al0.5Ga0.5As layers. The optical properties of QDs were studied by low-temperature photoluminescence (PL). Two bandgap transitions corresponding to the X-Sh and X-Ph energy structure were observed. The QDs structure was investigated using high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM). HRXRD investigations showed that the layers grew epitaxially on the substrate, with no relaxation. HRTEM investigations confirmed the epitaxial nature of the grown structures. In addition, it was revealed that the In atoms aggregated in some prismatic regions, forming areas of high In concentration, that were still in perfect registry with the substrate.

The optical properties of quantum dots in anisotropic media

The optical properties of cadmium selenide quantum dots were investigated in the nematic liquid crystals, which were used as an anisotropic medium. The effects of anisotropic media on the optical properties of immersed quantum dots were studied through ultra-violet absorption/transmission spectroscopy. The optical band gap and refractive index of immersed quantum dots were calculated for isotropic and anisotropic medium. The influences of anisotropic medium on the optical properties of quantum dots were discussed in terms of anchoring of liquid crystals molecules on the surfaces of immersed quantum dots.

Enantioselective cytotoxicity of ZnS:Mn quantum dots in A549 cells.

Chirality strongly influences many biological properties of materials, such as cell accumulation, enzymatic activity, and toxicity. In the past decade, it has been shown that quantum dots (QDs), fluorescent semiconductor nanoparticles with unique optical properties, can demonstrate optical activity due to chiral ligands bound on their surface. Optically active QDs could find potential applications in biomedical research, therapy, and diagnostics. Consequently, it is very important to investigate the interaction of QDs capped with chiral ligands with living cells. The aim of our study was to investigate the influence of the induced chirality of Mn-doped ZnS QDs on the viability of A549 cells. These QDs were stabilized with D- and L-cysteine using a ligand exchange technique. The optical properties of QDs were studied using UV-Vis, photoluminescence (PL), and circular dichroism (CD) spectroscopy. The cytotoxicity of QDs was investigated by high content screening analysis. It was found that QDs stabilized by opposite ligand enantiomers, had identical PL and UV-Vis spectra and mirror-imaged CD spectra, but displayed different cytotoxicity: QDs capped with D-cysteine had greater cytotoxicity than L-cysteine capped QDs.

AgCl-doped CdSe quantum dots with near-IR photoluminescence.

We report the synthesis of colloidal CdSe quantum dots doped with a novel Ag precursor: AgCl. The addition of AgCl causes dramatic changes in the morphology of synthesized nanocrystals from spherical nanoparticles to tetrapods and finally to large ellipsoidal nanoparticles. Ellipsoidal nanoparticles possess an intensive near-IR photoluminescence ranging up to 0.9 eV (ca. 1400 nm). In this article, we explain the reasons for the formation of the ellipsoidal nanoparticles as well as the peculiarities of the process. The structure, Ag content, and optical properties of quantum dots are also investigated. The optimal conditions for maximizing both the reaction yield and IR photoluminescence quantum yield are found.

Synthesis of ZnS QDs with Different Values of pH and White Light Generation from Fabrication of TPD: PMMA/ZnS Hybrid Devices

Quantum dots (QDs) of zinc sulfide (ZnS) was prepared by chemical reaction with different potential of hydrogen (pH) and used to fabricate organic quantum dot hybrid junction device. The optical properties of QDs were characterized by ultraviolet-visible (UV-Vis.) and photoluminescence (PL) spectrometer. The results show that the prepared QDs were nanocrystalline with defects formation. The energy gap (Eg)calculated from PL were 3.64, 3.53 and 3.35 eV for pH=8, 10 and 12 respectively. This decreasing of energy gaps is results of the effect the pH solution increases, which in turn leads to the shifted of the PL spectrum toward red shifted, which makes the energy bands at surface states are shallow bands. Fabrication of EL-device from ZnS QDs with different pH value was effective in efficient white light generation and can be the color of emerged light with different intensities.

Temperature-dependent optical properties of lead selenide quantum dot polymer nanocomposites.

The optical properties of PbSe quantum dots (QDs) in AB9093 epoxy nanocomposite are examined with respect to temperature over a range of 0°C-80°C, a useful working range for many QD-based sensors and devices, and results are compared to QDs in toluene solution. A complete characterization of QD optical properties is provided as a function of temperature, including the absorption spectrum, first excitonic (1-s) absorption peak intensity and wavelength, fluorescence intensity, and peak wavelength. QD optical properties in toluene were found to be more sensitive to temperature as compared to those in AB9093. Interestingly, 1-s and fluorescence peak wavelength variation with temperature are reversed in AB9093 as compared to those in toluene solution. Results for the fluorescence properties of Lumogen F Red 305 dye in toluene are presented for comparison. The dye was found to have similar sensitivity to temperature to that of the QDs in terms of fluorescence peak wavelength shift, but the fluorescence peak intensity was far less variant. These results can be used to build a temperature sensor or as a guide to building other types of QD-based devices to be more robust against changes in ambient temperature.

Ag colloids and arrays for plasmonic non-radiative energy transfer from quantum dots to a quantum well

Non-radiative energy transfer (NRET) can be an efficient process of benefit to many applications including photovoltaics, sensors, light emitting diodes and photodetectors. Combining the remarkable optical properties of quantum dots (QDs) with the electrical properties of quantum wells (QWs) allows for the formation of hybrid devices which can utilize NRET as a means of transferring absorbed optical energy from the QDs to the QW. Here we report on plasmon-enhanced NRET from semiconductor nanocrystal QDs to a QW. Ag nanoparticles in the form of colloids and ordered arrays are used to demonstrate plasmon-mediated NRET from QDs to QWs with varying top barrier thicknesses. Plasmon-mediated energy transfer (ET) efficiencies of up to ∼25% are observed with the Ag colloids. The distance dependence of the plasmon-mediated ET is found to follow the same d−4 dependence as the direct QD to QW ET. There is also evidence for an increase in the characteristic distance of the interaction, thus indicating that it follows a Förster-like model with the Ag nanoparticle-QD acting as an enhanced donor dipole. Ordered Ag nanoparticle arrays display plasmon-mediated ET efficiencies up to ∼21%. To explore the tunability of the array system, two arrays with different geometries are presented. It is demonstrated that changing the geometry of the array allows a transition from overall quenching of the acceptor QW emission to enhancement, as well as control of the competition between the QD donor quenching and ET rates.

Encapsulation of highly confined CdSe quantum dots for defect free luminescence and improved stability

This is the first report on the generation of trap states and their effective elimination in highly confined CdSe quantum dots in order to obtain enhanced and stable optical properties prepared by aqueous route. Surface plays an important role in optical properties of quantum dots (QDs) and surface modification of quantum dots can improve optical properties. In present work luminescent CdSe QDs were prepared using 2-Mercaptoethanol (2-ME) as stabilizing agent and encapsulated by polymer. Different concentrations of 2-ME were used to tune the emission spectra with respect to their reduced size. Addition of 2-ME to CdSe QDs enhances the trap emission and quenching band edge emission due to (i) increased surface to volume ratio and; (ii) presence of high concentration of sulfide ions as confirmed from EDX analysis as sulfide ions possesses the hole scavenging characteristics. Polymer encapsulation of QDs was carried out to make them stable and to improve their optical properties. Even though there are previous reports addressing the improved optical properties by polymer encapsulation and silica encapsulation but experimentally it has not been reported yet experimentally. In this work we have synthesized and characterized water soluble polymer encapsulated QDs and proved the facts experimentally. Photoluminescence spectroscopy clearly reveals the role of polymer encapsulation in boosting the optical properties of CdSe QDs. FTIR spectra validate the presence of biocompatible functional groups on CdSe4/PEG (Polymer encapsulated QDs).

Quantum-Confined and Enhanced Optical Absorption of Colloidal PbS Quantum Dots at Wavelengths with Expected Bulk Behavior.

Nowadays it is well-accepted to attribute bulk-like optical absorption properties to colloidal PbS quantum dots (QDs) at wavelengths above 400 nm. This assumption permits to describe PbS QD light absorption by using bulk optical constants and to determine QD concentration in colloidal solutions from simple spectrophotometric measurements. Here we demonstrate that PbS QDs experience the quantum confinement regime across the entire near UV-vis-NIR spectral range, therefore also between 350 and 400 nm already proposed to be sufficiently far above the band gap to suppress quantum confinement. This effect is particularly relevant for small PbS QDs (with diameter of ≤4 nm) leading to absorption coefficients that largely differ from bulk values (up to ∼40% less). As a result of the broadband quantum confinement and of the high surface-to-volume ratio peculiar of nanocrystals, suitable surface chemical modification of PbS QDs is exploited to achieve a marked, size-dependent enhancement of the absorption coefficients compared to bulk values (up to ∼250%). We provide empirical relations to determine the absorption coefficients at 400 nm of as-synthesized and ligand-exchanged PbS QDs, accounting for the broadband quantum confinement and suggesting a heuristic approach to qualitatively predict the ligand effects on the optical absorption properties of PbS QDs. Our findings go beyond formalisms derived from Maxwell Garnett effective medium theory to describe QD optical properties and permit to spectrophotometrically calculate the concentration of PbS QD solutions avoiding underestimation due to deviations from the bulk. In perspective, we envisage the use of extended π-conjugated ligands bearing electronically active substituents to enhance light-harvesting in QD solids and suggest the inadequacy of the representation of ligands at the QD surface as mere electric dipoles.

Enzyme-free fluorescence sensing of catechins in green tea using bifunctional graphene quantum dots

The fluorescent graphene quantum dots (GQDs) provide an amusing nanoplatform, which integrates unique optical properties of quantum dots and prominent electronic properties of carbon materials.

Quantum dots: from fluorescence to chemiluminescence, bioluminescence, electrochemiluminescence, and electrochemistry.

During the past decade, nanotechnology has become one of the major forces driving basic and applied research. As a novel class of inorganic fluorochromes, research into quantum dots (QDs) has become one of the fastest growing fields of nanotechnology today. QDs are made of a semiconductor material with tunable physical dimensions as well as unique optoelectronic properties, and have attracted multidisciplinary research efforts to further their potential bioanalytical applications. Recently, numerous optical properties of QDs, such as narrow emission band peaks, broad absorption spectra, intense signals, and remarkable resistance to photobleaching, have made them biocompatible and sensitive for biological assays. In this review, we give an overview of these exciting materials and describe their potential, especially in biomolecules analysis, including fluorescence detection, chemiluminescence detection, bioluminescence detection, electrochemiluminescence detection, and electrochemical detection. Finally, conclusions are made, including highlighting some critical challenges remaining and a perspective of how this field can be expected to develop in the future.

Ultrafast, Polarized, Single-Photon Emission from m-Plane InGaN Quantum Dots on GaN Nanowires.

We demonstrate single-photon emission from self-assembled m-plane InGaN quantum dots (QDs) embedded on the side-walls of GaN nanowires. A combination of electron microscopy, cathodoluminescence, time-resolved microphotoluminescence (μPL), and photon autocorrelation experiments give a thorough evaluation of the QD structural and optical properties. The QD exhibits antibunched emission up to 100 K, with a measured autocorrelation function of g(2)(0) = 0.28(0.03) at 5 K. Studies on a statistically significant number of QDs show that these m-plane QDs exhibit very fast radiative lifetimes (260 ± 55 ps) suggesting smaller internal fields than any of the previously reported c-plane and a-plane QDs. Moreover, the observed single photons are almost completely linearly polarized aligned perpendicular to the crystallographic c-axis with a degree of linear polarization of 0.84 ± 0.12. Such InGaN QDs incorporated in a nanowire system meet many of the requirements for implementation into quantum information systems and could potentially open the door to wholly new device concepts.

The effect of solvent dependent local field factor in the optical properties of CdTe quantum dots

CdTe quantum dots (QDs) are synthesized at room temperature in aqueous solvents of different dielectric constants and characterized using optical spectroscopy. Absorption spectra of the QDs obtained is used to calculate the size dependent dielectric function of the QDs using Kramers–Kronig relation and iterative matrix inversion method. Effect of solvent dielectric constant on optical properties of QDs is studied theoretically using Maxwell–Garnett effective medium theory. Direct correlation between absorption intensity and solvent dielectric constant is explained on the basis of decreasing local field factor of the solvents. Emission rates of QDs is also found to have dependence on the dielectric constant of the solvent. Spontaneous emission rates of QDs in Ionic liquid environment is studied theoretically using Maxwell–Garnett effective medium theory. Our results show that variation in dielectric constant of Ionic liquids have a significant impact on spontaneous emission properties of the QDs.

Optimization of the recipe for the synthesis of CuInS2/ZnS nanocrystals supported by mechanistic considerations

Abstract CuInS2/ZnS (CIS/ZnS) quantum dots (QDs) with high photoluminescence (PL) were synthesized via a facile solvothermal approach. Gaussian deconvolution of PL spectra, transmission electron microscopy, and time-resolved PL spectroscopies were used to characterize the emission properties of the prepared CIS and CIS/ZnS QDs. It was found that the growth of ZnS can reduce the surface defect acting as traps to minimize donor-acceptor emissions, and the contribution of band to donor/acceptor transition becomes a dominating emission with the increase of shell growth time. The blue shift of PL emission wavelength of CIS/ZnS QDs underwent two steps: the dramatic blue shift originated from the decreased fraction donor-acceptor transition due to the reduction of surface defects at the beginning and the subsequently mild blue-shift with the time from the interdiffusion of CIS and ZnS. The effect of trioctylphosphine (TOP) and dodecanethiol (DDT) as ligands during shell growth on the optical properties of QDs were investigated and compared. The PL quantum yield (QY) of CIS core affects the final value of CIS/ZnS QDs, and the higher PL QY is achieved while using CIS core with higher PL QY. Based on the selected ligand DDT, the reaction parameters, such as CIS core reaction time, shell growth time, and Zn/Cu feed molar ratio, were further optimized. CIS/ZnS QDs with high PL QY can be obtained with a Zn/Cu feed molar ratio larger than 4, shell growth time of 30 to 90 min, and shell growth temperature 220°C–240°C, and the maximum value was up to about 80% by adjusting the above-mentioned parameters.

Effects of thickness and V/III ratio of low temperature capping layer growth to the optical properties of InAs quantum dots

To achieve increased device performance, the authors systematically explored the dependence of the optical characteristics of InAs quantum dot (QD) ensembles grown via molecular beam epitaxy with different V/III ratios and thicknesses of the low temperature GaAs capping layers. In addition, the paper discusses the mechanism behind the dependence. Experiments showed the QD optical properties were significantly dependent on the GaAs capping layer V/III ratio and the initial GaAs capping layer thickness. An optimized V/III ratio and GaAs capping thickness is reported.

Droplet etching of deep nanoholes for filling with self-aligned complex quantum structures.

Strain-free epitaxial quantum dots (QDs) are fabricated by a combination of Al local droplet etching (LDE) of nanoholes in AlGaAs surfaces and subsequent hole filling with GaAs. The whole process is performed in a conventional molecular beam epitaxy (MBE) chamber. Autocorrelation measurements establish single-photon emission from LDE QDs with a very small correlation function g (2)(0)≃ 0.01 of the exciton emission. Here, we focus on the influence of the initial hole depth on the QD optical properties with the goal to create deep holes suited for filling with more complex nanostructures like quantum dot molecules (QDM). The depth of droplet etched nanoholes is controlled by the droplet material coverage and the process temperature, where a higher coverage or temperature yields deeper holes. The requirements of high quantum dot uniformity and narrow luminescence linewidth, which are often found in applications, set limits to the process temperature. At high temperatures, the hole depths become inhomogeneous and the linewidth rapidly increases beyond 640 °C. With the present process technique, we identify an upper limit of 40-nm hole depth if the linewidth has to remain below 100 μeV. Furthermore, we study the exciton fine-structure splitting which is increased from 4.6 μeV in 15-nm-deep to 7.9 μeV in 35-nm-deep holes. As an example for the functionalization of deep nanoholes, self-aligned vertically stacked GaAs QD pairs are fabricated by filling of holes with 35 nm depth. Exciton peaks from stacked dots show linewidths below 100 μeV which is close to that from single QDs.

Influence of size and surface state emission on photoluminescence of CdSe quantum dots under UV irradiation

Under UV irradiation, photo-oxidation occurs on the surface of CdSe quantum dots (QDs), thus modifying their QD optical properties. The influence of size and surface state (SS) emission on the photoluminescence (PL) evolution of CdSe QDs was investigated. Colorimetric study of the CdSe QD emission under UV irradiation indicates that the QD size and SS emission will influence the movement of chromaticity parameters. A photo-corrosion effect reduced the QD size and caused blue shift of the band edge (BE) peak. The irradiation-induced blue shift of QD emission always moves the corresponding chromaticity coordinate counterclockwise in the chromaticity diagram. For CdSe QDs with a dominant SS peak, the different evolution between BE and SS peak under UV irradiation will substantively influence the transformation of emission color: the chromaticity coordinate moved faster when PL increased than that when it decreased. Generally, the emission color of CdSe QDs with relatively large size and weak SS peaks is more stable in irradiation process.

Influence of Ionizing Radiation and the Role of Thiol Ligands on the Reversible Photodarkening of CdTe/CdS Quantum Dots.

We investigate the influence of high energy photons and thiol ligands on the photophysical properties of sub-monolayer CdTe/CdS quantum dots (QDs) immobilized in porous silica (PSiO2) scaffolds. The highly disperse, uniform distributions of QDs in a three-dimensional PSiO2 framework ensure uniform interaction of not only radiation but also subsequent surface repassivation solutions to all immobilized QDs. The high optical densities of QDs achieved using PSiO2 enable straightforward monitoring of the QD photoluminescence intensities and carrier lifetimes. Irradiation of QDs in PSiO2 by high energy photons, X-rays, and γ-rays leads to dose-dependent QD photodarkening, which is accompanied by accelerated photooxidative effects in ambient environments that give rise to blue-shifts in the peak QD emission wavelength. Irradiation in an oxygen-free environment also leads to QD photodarkening but with no accompanying blue-shift of the QD emission. Significant reversal of QD photodarkening is demonstrated following QD surface repassivation with a solution containing free-thiols, suggesting reformation of a CdS shell, etching of surface oxidized species, and possible reduction of photoionized dark QDs to a neutral, bright state. Permanent lattice displacement damage effects may contribute toward some irreversible γ radiation damage. This work contributes to an improved understanding of the influence of surface ligands on the optical properties of QDs and opens up the possibilities of engineering large area, low-cost, reuseable, and flexible QD-based optical radiation sensors.

Synthesis and characterization of silicon nanocrystals in SiC matrix using sputtering and PECVD techniques

Synthesis of silicon nanocrystals embedded in a dielectric matrix has attracted considerable interest in third-generation photovoltaics. Till date, significant work has been done on synthesizing and characterization of Si nanocrystals embedded in oxide and nitride dielectric matrices. Recently, silicon carbide based matrices have gathered much attention because of its comparatively lower band gap (2.5 eV) as compared to Si3N4 (5.3 eV) and SiO2 (9 eV) thereby promising a better electrical conductivity. In this work, Si-rich SiC films are synthesized and investigated for possible formation of Si Quantum Dots (QD) within the matrix. A comparative analysis of film growth using variant approaches in sputtering techniques, and PECVD technique are carried out. The effect of annealing at different temperatures and ambience in favouring the Si QD formation, structural and optical properties of the film are studied.This work paves way to analyse the possibility of these nanometric films for future photovoltaic applications.