Nanoengineered Quantum Dot Active Medium for Thermally-Stable Laser Diodes

Capture, relaxation and recombination in quantum dots

Polaron dephasing processes are investigated in InAs/GaAs dots using far-infrared transient four wave mixing (FWM) spectroscopy. We observe an oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant excitation of the lowest energy conduction band transition due to coherent acoustic phonon generation. The subsequent single exponential decay yields long intraband dephasing times of 90 ps. We find excellent agreement between our measured and calculated FWM dynamics, and show that both real and virtual acoustic phonon processes are necessary to explain the temperature dependence of the polarization decay. PACS numbers: 78.67.Hc, 78.47.+p, 42.50.Md, 71.38.-k The strong spatial confinement of carriers in semiconductor quantum dots (QDs) leads to striking differences in the carrier-phonon interaction compared with systems of higher dimensionality. In particular, the discrete energy level s tructure in QDs results in long exciton and electron dephasing times [1, 2, 3, 4], making these semiconductor nanostructures highly attractive for implementation in quantum information processing applications. The study of dephasing mechanisms in QDs is commonly carried out using transient four wave mixing (FWM) spectroscopy. Using resonant interband excitation, FWM measurements have revealed the absorption lineshape of single QDs to consist of a narrow zero phonon line (ZPL) and an acoustic phonon-related broadband centred at the same energy. The only intraband FWM study [5] involved resonant excitation of high energy transitions in th e valence band of p-doped QDs yielding dephasing times ∼ 15 ps. However it was not possible to determine the dephasing mechanisms in this case. Intraband studies of the well-resolved lowest energy conduction band electron transitions in InAs/GaAs QDs have provided deep insight into the electron-phonon interaction and carrier relaxation processes in n-doped samples. Clear evidence of strong coupling between electrons and phonons, resulting in polaron formation, has been demonstrated using magneto-transmission measurements [6]. Ultrafast studies [7, 8] of polaron decay have shown that the previously assumed ’phonon bottleneck’ picture is not valid. Compared with semiconductor quantum wells, the intraband population relaxation time in QDs is long (∼ 50 ps) suggesting relatively long dephasing times. However there have been no reports of direct dephasing measurements to date. In the present letter we present the first investigations of i ntraband dephasing in n-doped QDs using degenerate FWM. Our calculations of the absorption lineshape in this case sh ow marked differences in comparison with the interband absorption [9]. The intraband lineshape consists of peaked acoustic phonon sidebands separated by ∼ 1.5 meV from the ZPL, which corresponds to phonons with wavelength close to the dot size, and is reminiscent of the lineshape associated wit h impurity-bound electron transitions [10]. Using pulse durations short enough to excite both the ZPL and acoustic phonon sidebands we find damped oscillations in the FWM signal, indicative of coherent acoustic phonon generation, followed by a single exponential decay. In contrast with the interband case, where the origin of the strong temperature dependence of the excitonic linewidth is still subject to debate, the simple 3 -level structure of the lowest energy conduction band states in InAs QDs permits an accurate simulation of the temperature dependence of the FWM signal. The excellent agreement found between experiment and theory, shows that virtual transitions between the p-states is the dominant dephasing mechanism at high temperature. At low temperature, we have measured an intersublevel dephasing time of T2 ∼ 90 ps. It is also interesting to compare our results with previous intraband dephasing measurements in higher dimensional (quantum well) systems [11]. Here phonon-mediated processes are not significant with the intraband dephasing instead determined by electronelectron interactions, yielding typical dephasing times ∼ 0.3 ps which are approximately 2 orders of magnitude faster than for the QD samples studied here. The relatively long intraband dephasing time in QDs is key to the efficient operation of new types of mid-infrared QD-based devices, such as intersublevel polaron lasers [12] and may be relevant for potential device applications such as qubits for quantum information processors [13]. The investigated samples were grown on (100) GaAs substrates by molecular beam epitaxy in the Stranski-Krastranow mode. They comprise 80 layers of InAs self-assembled QDs separated by 50 nm wide GaAs barriers, thus preventing both structural and electronic coupling between QD layers. The polaron transitions were studied between s-like ground (s) and p-like first excited ( p) states within the conduction band. To populate the s state, the samples were delta-doped with Si 2 nm below each QD layer. The doping density was controlled in such a way that the average doping did not exceed 1 electron per dot (see Ref. [14] for more details). Absorp

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.

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.

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.

We investigated two stacked layers of InP∕GaAs type-II quantum dots by transmission electron microscopy and optical spectroscopy. The results reveal that InP quantum dots formed in two quantum dot layers are more uniform than those from a single layer structure. The thermal activation energies as well as the photoluminescence decays are rather independent of the separation between quantum dot layers and the presence of the second layer. The quantum dot optical emission persists for thermal activation energy larger than the calculated exciton binding energy. The photoluminescence decay is relatively fast for type-II alignment.

The poor efficiency and stability of blue quantum dot light-emitting diodes (QLED) hinder its practical applications in full-color displays. Insufficient hole injection and excessive surface defects in quantum dots (QD) layer remain the primary challenges limiting the performance of blue devices. Herein, a dual interface modification strategy is proposed to enhance the performance of blue QLED by synergistically regulating both the electronic transport layer (ETL)/QD and hole transport layer (HTL) HTL/QD interfaces. At the HTL/QD interface, the introduction of guanidine sulfamate (GAS) ligands passivates QD surface defects while reducing the hole injection barrier, thereby improving hole injection efficiency in the low-bias region. Meanwhile, at the QD/ETL interface, Guanidine chloride (GACl) ligands are incorporated to passivate interfacial defects, suppress leakage current, and suppress excessive electron injection, thus enhancing hole transport efficiency within the QDs layer. The synergistic effect of bilateral GA-based ligands can simultaneously enhance the hole injection efficiency based upon improving the hole transport efficiency, significantly increasing the radiative recombination ratio during device operation. As a result, the dual-ligand modified blue QLEDs achieve a remarkable improvement in external quantum efficiency (EQE) from 16.6% to 24.3%, and a sevenfold enhancement in operational lifetime.

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.

Semiconductor quantum dots have been extensively used for biomedical applications as efficient biomarkers. At the same time, liposome encapsulation may open other applications of QDs: the possibility of liposome labeling to stimulate the pathway to specific organs as efficient and specific biomarkers. Due to this, the optical properties of QDs after liposomal encapsulation are very important to many applications, for example, in biosensors or theranostic liposomes. The commercial-acquired CdSe/ZnS core/shell quantum dots of two different emission wavelengths (530 and 640 nm) were encapsulated in lecithin liposomes, and characterized by Atomic Force Microscopy, Transmission Electron Microscopy, Dynamic Light Scattering, Photoluminescence, and Raman Spectroscopy. The important changes in optical properties at liposome encapsulation of the studied QDs are demonstrated and discussed.

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.

Quantum dots are the serendipitous outcome of materials research. It is the tiny carbonaceous nanoparticles with diameters ranging from 1 to 10 nm. This review is a brief discussion of the synthesis, properties, and biomedical applicability of quantum dots, especially in herbal therapy. As quantum dots are highly polar, they can be surface decorated with several kinds of polar functionalities, such as polymeric molecules, small functional molecules, and so on. The review also consists of the basic physical and optical properties of quantum dots and their excitation―dependent properties in the application section. We focus on therapeutics, where quantum dots are used as drugs or imaging probes. Nanoprobes for several diagnostics are quite new in the biomedical research domain. Quantum dot―based nanoprobes are in high demand due to their excellent fluorescence, non-bleaching nature, biocompatibility, anchoring feasibility for several analytes, and fast point―of―care sensibility. Lastly, we also included a discussion on quantum dot―based drug delivery as phytomedicine.

We have grown a stack of In‐flushed InAs quantum dot (QD) layers to realize a broadband emission centered at ∼1 µm with a Gaussian‐like spectrum as a light source for optical coherence tomography (OCT). The In‐flush process, which enables control of the height of the QD, was optimized to control the emission wavelength and intensity of self‐assembled InAs/GaAs QDs. A partial capping layer of GaAs was deposited on as‐grown InAs/GaAs QDs with various thicknesses (dcap), and a rapid annealing of the sample was executed with the proper annealing temperature. By optimizing dcap and the annealing temperature for each QD layer, a broadband Gaussian‐like spectrum with a 102‐nm‐bandwidth was obtained. The coherence function of the emission spectrum exhibits a 4.5‐µm axial resolution for OCT images without distinct side lobes. These results demonstrate the effectiveness of the In‐flushed QDs as a light source to obtain high‐quality OCT images. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A new tool for measuring island dimensions and spatial correlations in quantum dot multilayers: Raman scattering interferences

The effect of a high N incorporation in self-assembled InAs quantum dots (QDs) is investigated by analyzing the electronic and structural properties around QD region. Capacitance-voltage profiling and admittance spectroscopy shows that N incorporation into the InAs QD layer leads to drastic carrier depletion in the QD layer and neighboring GaAs layers due to the formation of a deep defect state at 0.34–0.41 eV. The signature of this defect state is similar to those defects observed in strain relaxed QDs or InGaAs/GaAs quantum wells when the InAs deposition thickness exceeds a critical thickness. Accordingly, the N incorporation might result in strain relaxation either by increasing localized strain or by inducing composition inhomogeneities, which provide nucleation sources for strain relaxation. The argument of strain relaxation is supported by transmission electron microscopy that reveals lattice misfits at the QD layer and neighboring GaAs layers.