Surface decorated quantum dots: Synthesis, properties and role in herbal therapy

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.

ABSTRACTThe influence of a nanoengineering procedure on the properties of single- and multi-layer self-assembled InAs quantum dot (QDs) has been studied using photoluminescence, transmission electron microscopy (TEM), and electroluminescence. Optical properties of QDs were optimized by shape engineering via adjustment of a GaAs overlayer thickness prior to a heating (truncation) step. TEM micrographs have confirmed that the employed growth procedure results in truncated pyramidal QDs covered entirely by AlAs with smooth top interfaces. Truncated QDs with two-monolayer-thick AlAs capping have demonstrated a strong blue shift of ground state (GS) energies and up to 10 meV larger separation between GS and first excited state energy levels as compared to non-capped QDs. We believe that AlAs capping, in combination with truncation procedure, results in significant suppression of carrier transport between QDs within the same layer as well as between QD layers. A record high characteristic temperature for GS lasing threshold, T0 = 380 K up to 55 °C, as well as a maximum saturated gain of 16 (5.3 per layer) cm-1, were measured for a 1.22 μm edge-emitting lasers with this truncated triple layer QD gain medium. A maximum saturated gain, 27 (3.9 per layer) cm-1, and T0 = 110 K have been demonstrated in a 1.19 μm lasers with truncated seven layer QD medium.

Self-assembled Quantum Dots (QDs) have recently attracted great attention for enabling a novel type of non-volatile electronic nanomemory, the QD-Flash. Such a memory architecture represents the ultimate memory cell, exhibiting endurance and fast access times (~ns). To ensure non-volatile operation, the storage time for carriers has to be of the order of 10 years (or longer) at room temperature, which mainly depends on the depth of the localization potential provided by QDs, but also on their capture cross-section. By choosing an appropriate III/V material combination for QDs and matrix, surpassing the non-volatility limit of 10 years is feasible: type-II antimony-based QDs showing hole confinement are very suitable for such application, leading to a higher carrier localization energy and an increased carrier storage capacity for single QD, with respect to type-I heterostructures. Retention times of more than 10 years for holes in InGaSb/AlP/GaP QDs have been predicted, making thus such QDs promising candidates for the QD-Flash. This work aims to demonstrate the Metalorganic Chemical Vapor Epitaxy (MOVPE) of InGaSb/AlP/GaP QDs and their use as storage units for the future QD-Flash. The storage time of such QDs is improved by one order of magnitude compared to the previous value for In0.5Ga0.5As/GaP QDs. InGaSb QDs are embedded in GaP np-diodes, where an additional AlP barrier is employed, and are electrically characterized. A localization energy Eloc = (1.15 ± 0.02) eV for holes captured in QDs is measured by using the Deep-Level Transient Spectroscopy (DLTS) technique. This work investigates growth strategies in order to tune the QD structural and optical properties for maximum hole retention times in QDs. A growth interruption (GRI) after deposition of InGaSb and an Sb-flush prior to the QD growth are found to be very effective. By using an Sb-flush, the surface diffusion is controlled leading to more homogeneous QDs and to reduction of defect formation. Taking advantage of a ripening process during GRI, the QD size is increased and the luminescence remarkably improved. The use of antimony in QD growth is found to decrease the associated QD capture cross-section by one order of magnitude with respect to the one of In0.5Ga0.5As/GaP QDs/GaP QDs, i.e. from (8 ± 5) x 10^(-10) to (9 ± 5) x 10^(-11) cm^(2). This is the first time that engineering of the QD capture cross-section has been utilized to increase hole retention times in QDs. It leads to a hole storage time of almost 1 hour at room temperature, which is to date the record for MOVPE-grown QDs. Such material combination for QDs and matrix represents therefore an excellent candidate for the future QD-Flash.

Hybrid nanoparticles (NPs) consisting of an n-type CdSe/ZnS quantum dot (QD) as a core and p-type π-conjugate molecules as a shell were fabricated, and their nanoscale photoluminescence (PL) and photoresponsive molecular electronic characteristics were investigated. π-Conjugated macromolecular dioctyloxybenzodithiophene-based polythiophene (P3000) or a single carbazole (CB) molecule with insulating molecular blocks were attached to the QD surface. The nanoscale PL characteristics for the single QD were drastically changed through close contact with P3000 owing to energy and charge transfer effects. However, for the hybrid QD-CB NP, the PL of the QD was dominant because of weak energy transfer resulting from the relatively longer insulating molecular block between the QD and the CB molecule. From time-resolved PL spectra, the exciton lifetimes of the QD in the hybrid QD-P3000 and QD-CB NPs were clearly different because of a variation in the energy transfer rate. The photocurrents of the single hybrid QD-P3000 NP were considerably higher and actively responded to both forward and reverse biases due to the energy and charge transfer effects, while those of the single QD-CB NP exhibited diode characteristics. The QD-based hybrids show distinct nanoscale PL features and photoresponsive molecular electronic characteristics depending on the structures of π-conjugated molecules. For n–p-junctioned QD-P3000 hybrid NPs, the PL intensity from the green QD drastically decreased and that of the π-conjugated P3000 was weakly changed due to energy and charge transfer effects. The photocurrent of the QD-P3000 was much higher and more symmetric than that of the QD-CB NP, which had an insulating molecular block, suggesting more active charge transfer between the p-type P3000 and the n-type QD. The nanoscale PL and molecular optoelectronic properties of the hybrids of QDs and π-conjugated molecules could be tuned by their relative distance and the degree of spectral overlapping. An enhanced control over the optical properties of quantum dots, achieved by attaching organic molecules, has been demonstrated by Kwang-Sup Lee and Jinsoo Joo from Hannam University/Korea University and their collaborators in South Korea and the United States. The addition of nanometer-sized quantum dots dramatically alters the properties of a material, which is of use in applications such as solar cells. At the same time, it is important to exert effective control over these modified properties. This can be achieved by surrounding the quantum dots with other materials that interact with them. In their study of cadmium selenide quantum dots, the researchers use organic molecules for this purpose and demonstrate that the light emission from the quantum dots depends strongly on the type of molecule attached to them. This enables a broader flexibility when designing quantum dots for specific electronic or optical applications.

Highly luminescent CdTe quantum dots (QDs) are the most widely used nanoparticles for making fluorescent biosensors and optoelectronic devices. In this paper, we present the successful synthesis of CdTe QDs in the aqueous phase using mercaptosuccinic acid (MSA) as a capping ligand. The CdTe QDs were formed by the reaction of the Te precursor (in TeO2) with Cd2+/MSA in solution at 80 oC for 30 min, then annealing in an autoclave at different temperatures (110 - 150 oC) and time intervals (60 - 180 min). The synthesis of QDs in water has many advantages such as the simplicity, environmental friendliness, and using cheap chemical agents, and particularly make the CdTe QDs suitabble for bioapplications because of the biocompatibility. The high quality of the synthesized CdTe QDs was confirmed by X-ray diffraction (XRD) and their optical properties. The XRD pattern shows the cubic zinc blende structure of the synthesized CdTe QDs. Additionally, the influences of the synthesis parameters, namely the annealed temperature, time to the optical properties of the obtained QDs were systematically investigated with regard to the growth rate and particle size. As a result, CdTe QDs become bigger causing the red-shifted of peaks in their absorption and photoluminescence spectra with increasing annealed temperature and time. This provides evidence for the quantum confinement effect.

Variation of the steady-state optical properties of quantum dots (QDs) caused by photoinduced charge transfer is a widely known phenomenon which hinders QD applications requiring long-term stability of the optical signal, for example, bioimaging, single-photon sources, molecular tracking, and lasing. Intense light irradiation may provoke QD photodarkening, when their photoluminescence (PL) quantum yield (QY) is reduced, or photobrightening, when the QY increases and QD PL becomes brighter. In order to understand the nature of these processes and to determine the optimal operating conditions for QDs with different core sizes and shell thicknesses, we have systematically studied the photodarkening and photobrightening phenomena for different wavelengths and intensities of irradiation. The results have shown that changes in the core size only slightly alter the effect of light irradiation on the QD optical properties, whereas the irradiation wavelength and shell thickness are the major factors that determine the variation of the QD PL QY. We have found that the observed photoinduced processes are independent of the irradiation intensity, which proves the single-photon origin of these processes. We have also used comparative data on irradiation of QD solution and QDs embedded in polymer matrices to develop a general model describing the mechanism of the QD sensitivity to intense irradiation and elucidating the role of surface ligands in this mechanism. Our findings determine the strategy for increasing QD optical stability and optimization of operational conditions for QD-based devices.

In a hybrid structure consisting of a nanorod and a core/shell quantum dot, the interparticle interactions of elements and the effect of the optical response of nanorod on the third-order nonlinear optical response of quantum dot are investigated. The effective parameters in these interactions such as the enhancement factors, type of structural configuration, dimensional ratio, type of background medium, quantum dot size and interparticle distance are studied in the structure for engineering the system to an optimum response. The enhancement factors of field due to the presence of nanorod are obtained in the site of quantum dot, followed by the third-order optical nonlinear response quantum dot is calculated by using the local field theory. It is seen for the situation where quantum dot is perpendicular to the axis of nanorod, the interparticle interactions and enhancement factors have maximum amount at a given aspect ratio of nanorod (about 2.5), depending on the type of surrounding environment. Therefore, the nanorod can produce great nonlinearities atthe quantum dot compared to the spherical nanoparticle into the hybrid structure . As well, it is found that the larger sizes of quantum dot and less interparticle distances cause to the stronger coupling and thereby larger optical nonlinearity in the quantum dot.

Cu-In-S/ZnS:Gd3+ quantum dots with isolated fluorescent and paramagnetic modules for dual-modality imaging in vivo

The utilisation of quantum dots (QDs) as promising materials for next-generation photovoltaics is a recent development. The optical properties of QDs can undergo tuning, and they enhance energy conversion efficiencies. In this study, CuLaSe2 and Zn-doped CuLaSe2 (ZnCuLaSe2) QDs, specifically tailored for quantum dot-sensitized solar cells (QDSSCs). These QDs, which are environmentally friendly as they are free of toxic Cd and Pb elements, exhibit adjustable energy band gaps and improved photoluminescence quantum yields. The incorporation of Zn into CuLaSe2 QDs led to a significant blue shift in optical properties and enhanced photovoltaic performance. The highest power conversion efficiency (PCE) achieved was 2.52% for ZnCuLaSe2 QDs, compared to 1.94% for CuLaSe2 QDs. This improvement is attributed to Zn doping, which enhances charge separation, suppresses surface trap states, and facilitates better electron transfer by modifying the energy band alignment. The synthesis methods have been developed in such a way that they are scalable, and are also compatible with low-cost, eco-friendly production processes; this underscores their feasibility for industrial applications. It can be concluded that the present study fulfills a vital function within the global energy research landscape by identifying two QDs that have the potential to be key components in advancing photovoltaic technology.

An alternative to Western blot analysis using RNA aptamer-functionalized quantum dots

Quantum dots (QDs) and polymeric nanoparticles (NPs) are considered good binomials for the development of multifunctional nanomedicines for multimodal imaging. Fluorescent imaging of QDs can monitor the behavior of QD-labeled NPs in both cells and animals with high temporal and spatial resolutions. The comprehension of polymer interaction with the metallic QD surface must be considered to achieve a complete chemicophysical characterization of these systems and to describe the QD optical properties to be used for their unequivocal identification in the tissue. In this study, by comparing two different synthetic procedures to obtain polymeric nanoparticles labeled with QDs, we investigated whether their optical properties may change according to the formulation methods, as a consequence of the different polymeric environments. Atomic force microscopy, transmission electron microscopy, confocal and fluorescence lifetime imaging microscopy characterization demonstrated that NPs modified with QDs after the formulation process (post-NPs-QDs) conserved the photophysical features of the QD probe. In contrast, by using a polymer modified with QDs to formulate NPs (pre-NPs-QDs), a significant quenching of QD fluorescence and a blueshift in its emission spectra were observed. Our results suggest that the packaging of QDs into the polymeric matrix causes a modification of the QD optical properties: these effects must be characterized in depth and carefully considered when developing nanosystems for imaging and biological applications.

Effect of activation current variation in the synthesis of quantum dots (QD) of cadmium telluride in colloidal solutions on the change of emission spectra of photoluminescence (PL), the average size of QD and the change in the angular position of refractometric characteristics of surface plasmon resonance (SPR) was studied. The activation current was changed from 0.1 to 0.6 A in steps of 100 mA. For comparison, also performed the synthesis of cadmium telluride QD in colloidal solutions with a change in the duration of the synthesis in the range from 1 to 5 minutes in increments of 1 minute. The average QD size of cadmium telluride was determined by the peak values of the measured PL spectra of the corresponding colloidal solutions according to the modified Bruce formula. The response of the SPR sensor constructed according to the Kretschman scheme was defined as the angular shift of the minimum of the measured refractometric characteristics of the SPR when replacing deionized water with the studied colloidal solutions. The results of the study showed that the increase in the activation current in contrast to the increase in the duration of the synthesis led not only to a decrease in PL intensity, but also to its increase at low currents, which may be due to "electropolishing" of the quantum dot surface. Both technologies for the synthesis of quantum dots were characterized by the presence of slow growth processes, which was associated with the thermodynamics of nucleation processes and manifested in the form of saturation regions on the graphs of PL peak and minimum refractometric characteristics of SPR from synthesis duration and activation current. At different durations of quantum dot synthesis, a dependence (correlation coefficient -0.9992) was established between the response of the SPR sensor and the spectral position of the PL maximum, which was approximated by a linear function with a matching coefficient R 2 = 0.998. The results of the study showed that the use of variation of activation current in stabilizing the duration of synthesis is the dominant condition for creating high-emission light-emitting and light-converting semiconductor systems based on cadmium telluride QD, because with variation of activation current PL intensity was 431.9 abs., 4 times higher than for the case of variation in the duration of synthesis, namely 181.2 abs.one. The results of the study can be used to optimize the technology of manufacturing QD of cadmium telluride with predetermined characteristics.

Biosynthesis of CdSe quantum dots is the process of converting metal ions into semiconductor nanomaterials. Studies have shown that CdSe quantum dots synthesized by Saccharomyces cerevisiae are rich in ribosomal proteins, but the role of ribosomal proteins in the synthesis of CdSe quantum dots remains unclear. In this paper, ribosomal proteins enriched during the synthesis of CdSe quantum dots by S. cerevisiae were screened, and their effects on the synthesis of quantum dots were detected by gene knockout and over-expression. The results reveal that ribosomal protein RPL14B is involved in the synthesis of quantum dots. RPL14B binds cadmium ions during the nucleation of CdSe quantum dots and acts as a template, ultimately regulating the particle size of CdSe quantum dots by change the incubation time of CdCl2. In summary, this study elucidates the mechanism of ribosomal protein RPL14B regulation of CdSe quantum dot biosynthesis, laying a foundation for the precise regulation of CdSe quantum dot synthesis.

In this chapter we present the main concepts and recent advances in the application of semiconductor quantum dots (QD) as agents in bioimaging. We analyze the biologically relevant properties of QDs focusing on the following topics: QD surface treatment and stability, labeling of cellular structures and receptors with QDs, incorporation of QDs in living cells, cytotoxicity of QDs and the influence of the biological environment on the biological and optical properties of QDs. Initially, we consider utilization of QDs as agents in high-resolution bioimaging techniques that can provide information at the molecular level. The diverse range of modern live-cell QD-based imaging techniques with resolution far beyond the diffraction limit of light has been examined. For each technique, we discuss the pros and cons of QD use and deliberate how QDs can be further engineered to facilitate their applications in the respective imaging techniques and to produce significant improvements in resolution. Finally, this chapter provides an analysis of the future prospects for the application of QDs in bioimaging and related areas of biology.

The optical properties of quantum dots (QDs) formed in GaAs or Al0.3Ga0.7As matrices by overgrowth of initial InAs islands formed in the Stranski-Krastanov mode with thin AlAs/InAlAs layers have been studied. It is shown that no transport of carriers between the QDs occurs in the temperature range 10–300 K, so the carrier distribution is of a nonequilibrium nature. The thermal excitation of carriers from the QDs is suppressed by an increase in the energy spacing between the ground and excited states, absence of the level related to the wetting layer, and higher carrier localization energy in the QDs with respect to the continuum states when the Al0.3Ga0.7As matrix is used.