Structural Dynamics and Electronic Properties of Semiconductor Quantum Dots: Computational Insights

This thesis is devoted to theoretically study the systems of single quantum dots (QDs) embedded in various photonic nanostructures, in close relation with experimental data. The word single is essential: due to their nanometer size, QDs have mainly been studied as ensembles. It is only recently that the experimental progress, in particular in the domain of QDs growth, has allowed to address single QDs. This helped to observe that the confined states in a QD resemble those of an atom, leading in turn to the macro-atom picture for QDs. It opened the path to the study of cavity quantum electrodynamics (CQED) in the solid state, as the analogous of a system of an atom in an electromagnetic cavity. However, already at the level of the QD (i.e. without a solid state cavity), the macro-atom picture has been proven to be oversimplified. Various effects related to the solid state environment of the QD have been experimentally and theoretically evidenced, like the influence of acoustic and optical phonons. These are channels of decoherence for the otherwise well isolated and confined QD levels. When adding the cavity, these effects are possibly enhanced, which can lead to very large deviations from the CQED ideal picture. Therefore the clear understanding of these effects is of essential importance, especially in view of the application of solid state systems to quantum information science. In Chapter 2, we address the radiation-matter coupling between two QDs in a nanocavity. This field of research combines the high quality of light resonators to the nonlinearities of the semiconductor material. It is promising in view of manipulating and transferring single long-lived excitations, an essential goal for quantum information science. Then, we study the deviations of a QD from the idealized macroatom picture. First, in Chapter 3, we include the influence of acoustic phonons on the light emitted by a QD-cavity system. We show that the QD modified susceptibility leads to the qualitative explanation of the cavity feeding effects, which is one of the most striking deviations of the QD-cavity system from the ideal CQED picture. In addition to phonons are extended two-dimensional states populated by the non resonant pumping laser in the QD's wetting layer (WL). In Chapters 4 and 5 we build a Monte-Carlo model of the excitation-decay mechanism in a QD-cavity system. We use it to describe the influence of WL states on the dynamics of the QD, leading in turn to a non-CQED behavior. This approach is particularly successful in the case of large QD-cavity detuning, where it predicts both time-dependent and spectral characteristics of the emitted light. Finally, in Chapter 6, we discuss our conclusions, emphasizing in particular that the knowledge of the actual solid state environment of the QD leads to a better control over the parameters allowing to reach the CQED regime. We also discuss the possible extensions of this work, in particular in relation to the analysis of the stimulated emission regime.

The processes of excitation and relaxation of confined excitons in semiconductor quantum dots upon indirect high-energy excitation have been considered. The temperature behavior of photoluminescence of quantum dots in a SiO2 dielectric matrix has been described using a model accounting for the process of population of quantum-dot triplet states upon excitation transfer through mobile excitons of the matrix. Analytical expressions that take into account the two-stage and three-stage schemes of relaxation transitions have been obtained. The applicability of these expressions for analyzing fluorescence properties of semiconductor quantum dots has been demonstrated using the example of silicon and carbon nanoparticles in the thin-film SiO2 matrix. It has been shown that the complex character of the temperature dependences of the photoluminescence upon indirect excitation can be an indication of a multistage relaxation of excited states of the matrix and quantum dots. The model concepts developed in this study allow one to predict the form of temperature dependences of the photoluminescence for different schemes of indirect excitation of quantum dots.

Chemosensors, able to signal the presence of target analytes with luminescence enhancements, can be designed around electron and energy transfer processes. Specifically, the association of a target analyte with a complementary receptor can be transduced into a significant change in the emission intensity of an organic fluorophore on the basis of these processes. The photophysical properties of semiconductor quantum dots, however, are significantly more attractive than those of organic dyes. As a result, the identification of strategies to adapt similar transduction mechanisms to quantum dots can lead to the emergence of luminescent chemosensors with improved performances. In this context, we developed two strategies based on electron and/or energy transfer to probe pH and signal receptor–substrate complementarities with luminescent quantum dots. In particular, we coated preformed CdSe–ZnS core–shell quantum dots with pH-sensitive [1,3]oxazines, in one instance, and 4,4′-bipyridinium dications, in the other. Both species exchange electrons and/or energy with the nanoparticles upon excitation, and hence quench their emission. However, their absorption wavelengths, redox potentials and distance from the nanoparticles can be designed to vary in response to pH changes or the presence of complementary receptors. These modifications alter the quenching ability of the organic ligands and culminate into significant enhancements in the luminescence of the inorganic nanoparticles. Thus, our transduction mechanisms can successfully be exploited to signal the presence of target analytes with luminescence changes, and can ultimately lead to the development of innovative luminescent chemosensors based on the unique photophysical properties of semiconductor quantum dots.

We review spin properties of semiconductor quantum dots and their effect on optical spectra. Photoluminescence and other types of spectroscopy are used to probe neutral and charged excitons in individual quantum dots with high spectral and spatial resolution. Spectral fine structure and polarization reveal how quantum dot spins interact with each other and with their environment. By taking advantage of the selectivity of optical selection rules and spin relaxation, optical spin pumping of the ground state electron and nuclear spins is achieved. Through such mechanisms, light can be used to process spins for use as a carrier of information.

Correlations between excitons, that is, electron-hole pairs, have a great impact on the optoelectronic properties of semiconductor quantum dots and thus are relevant for applications such as lasers and photovoltaics. Upon multiphoton excitation, these correlations lead to the formation of multiexciton states. It is challenging to observe these states spectroscopically, especially higher multiexciton states, because of their short lifetimes and nonradiative decay. Moreover, solvent contributions in experiments with coherent signal detection may complicate the analysis. Here we employ multiple-quantum two-dimensional (2D) fluorescence spectroscopy on colloidal CdSe1-xSx/ZnS alloyed core/shell quantum dots. We selectively map the electronic structure of multiexcitons and their correlations by using two- and three-quantum 2D spectroscopy, conducted in a simultaneous measurement. Our experiments reveal the characteristics of biexcitons and triexcitons such as transition dipole moments, binding energies, and correlated transition energy fluctuations. We determine the binding energies of the first six biexciton states by simulating the two-quantum 2D spectrum. By analyzing the line shape of the three-quantum 2D spectrum, we find strong correlations between biexciton and triexciton states. Our method contributes to a more comprehensive understanding of multiexcitonic species in quantum dots and other semiconductor nanostructures.

Luminescent quantum dots: Synthesis, optical properties, bioimaging and toxicity

The unique photophysical and optoelectronic properties of semiconductor quantum dot (QD) nanocrystals have paved the way for their diverse use in several application fields within the chemical, physical, and biology domains. In this work, we report for the first time on the synthesis of alloyed heavy metal-free MnInPSeS QDs via the hot-injection organometallic pyrolysis of metal precursors, organic surfactants, and ligands. To make the QDs biocompatible, stable in aqueous media, and solution-processable, the QDs were capped with 3-mercaptopropionic acid via a ligand exchange reaction. The QDs emitted in the UV/vis region and had a fluorescence quantum yield of 41 %. The QDs were drop-casted on screen-printed carbon electrodes (SPCEs) and from the cyclic voltammetry (CV) analysis, the QDs exhibited efficient electron transfer and enhanced electroactive surface area whilst from the electrochemical impedance spectroscopy (EIS) analysis, the QDs exhibited lower charge transfer resistance which was indicative of enhanced charge transport process. A novel electrochemical nanosensor was thereafter developed for the synthetic cathinone, 3,4-methylenedioxypyrovalerone (MDPV) by electropolymerizing the drug in the presence of dopamine on the QDs/SPCE surface. Under optimized experimental conditions, the developed molecularly imprinted polymer (MIP)@MnInPSeS QDs/SPCE detected MDPV ultrasensitively within the concentration range of 6 nM–600 μM with buffered K3[Fe(CN)6]/K4[Fe(CN)6] redox probe. The limit of detection and limit of quantification obtained were 0.28 nM and 1.39 nM, respectively. An MDPV fingerprint was also obtained electrocatalytically with the MIP@QDs/SPCE nanosensor using differential pulse voltammetry (DPV). Electrochemical profiling analysis showed that MDPV can be selectively detected in drug mixtures containing either known illicit drugs or known cutting agents.

Semiconductor quantum dots have long been described as artificial atoms because they have discrete energy states analogous to the energy levels of natural atoms. In recent years, it has become possible to create coupled pairs of quantum dots that are analogous to natural diatomic molecules. These artificial molecules have received a great deal of attention because of the potential applications in novel optoelectronic and spintronic devices, including the possibility of scalable implementations of quantum information processing. Just as in natural diatomic molecules, tunneling of electrons or holes between the two dots creates delocalized molecular orbitals. In natural diatomic molecules the molecular ground state has bonding orbital character and the first excited molecular state has antibonding character. Artificial quantum dot molecules were believed to behave in a similar way. However, recently Dr. Doty from the University of Delaware, Dr. Climente from Universitat Jaume I (Castellon, Spain), and their collaborators in Canada and the US, have experimentally verified and explained the existence of an antibonding molecular ground state for holes in artificial quantum dot molecules. The coherent coupling of quantum dots leads to the formation of delocalized molecular orbitals that appear in photoluminescence spectra under electric fields as ant crossings. The orbital character of the molecular states cannot be measured at zero magnetic field. However, a recent discovery by Doty and coworkers at the Naval Research Lab revealed that, when a magnetic field was applied, the resonant changes in the Zeeman splitting that depended on the orbital character of the molecular states appeared. Using these changes to identify the molecular orbital character, Doty and coworkers demonstrated that the orbital character of the molecular ground state reverses as a function of the distance between the dots. Holes confined in the molecular states of dots separated by 2 nm have the expected bonding molecular ground state; when the dots are separated by three or more nanometers, the molecular ground state becomes antibonding. “This was a surprising discovery for us,” said Doty. “Until we looked closely at the orbital character of the molecular states we had simply assumed that the molecular ground state was always a bonding orbital.” To explain this surprising result, Climente and coworkers utilized a four-bandk · papproximation which shows that the parity along the molecular axis is broken by the spin-orbit interaction in the valence band. “This leads to the mixing of bonding and antibonding heavy- and light-hole components of the spinor which destabilizes (stabilizes) the otherwise pure bonding (antibonding) states, leading to the state reversal” Climente told Nanospotlight. He continued explaining that the molecular ground states are found to have up to ~95% antibonding character as a result of this mixing, “This is about 10 times higher than the largest value observed in natural molecules, so we can speak about a novel kind of molecular state whose properties are still to be explored”. Asp3d5s* tight binding multimillion atom calculation was applied to probe theoretically a real case. “We considered self-assembled InGaAs/GaAs double quantum dot structures and included strain, structural asymmetries as well as vertical electric fields. The results are in qualitative agreement with thek · ptheory and predict a bonding-to-antibonding ground state reversal at interdot distances ofd ~ 2 nm” says Climente. This discovery drastically changed the previous conception of the single-particle ground state of holes in coupled quantum dots. “This paves the way for more accurate simulations of device performance for applications in optics, transport, or quantum information” Climente mentioned. He continued saying that the tunneling rate, which is an important parameter for the quantum computation and transport can now be flexibly tuned from large values down to zero using a magnetic field. “This is a consequence of the spinor nature of holes” says Climente, “and it shows that we can greatly manipulate the properties of artificial molecules through the spin-orbit interaction”. As a matter of fact, this discovery provides new tools for engineering the spatial distribution of molecular wavefunctions in regions with varying material parameters. “Wavefunction engineering can be used to control magnetic, spin, and optical properties, so this discovery will enable the design of wide variety of materials for novel optoelectronic applications,” said Doty (Fig. ​(Fig.11). Figure 1 Dissociation spectrum of a hole in an artificial molecule (solid line). Note the state reversal atd ~ 1.5 nm, implying a bonding-to-antibonding ground state transition. This never occurs for electrons in artificial or natural diatomic molecules, where ... This work is also featured in the OAtube nanotechnology journal,http://www.oatube.org/2008/09/jiclimente.html. This journal is a new kind of open access effort that offers video access to science.

Chapter 2 - Luminescent Quantum Dots, Making Invisibles Visible in Bioimaging

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

In search of strategies to photoactivate the luminescence of semiconductor quantum dots, we devised a synthetic approach to attach photocleavable 2-nitrobenzyl groups to CdSe-ZnS core-shell quantum dots coated with hydrophilic polymeric ligands. The emission intensity of the resulting nanostructured constructs increases by more than 60% with the photolysis of the 2-nitrobenzyl appendages. Indeed, the photoinduced separation of the organic chromophores from the inorganic nanoparticles suppresses an electron-transfer pathway from the latter to the former and is mostly responsible for the luminescence enhancement. However, the thiol groups anchoring the polymeric envelope to the ZnS shell also contribute to the photoinduced emission increase. Presumably, their photooxidation eliminates defects on the nanoparticle surface and promotes the radiative deactivation of the excited quantum dots. This effect is fully reversible but its magnitude is only a fraction of the change caused by the photocleavage of the 2-nitrobenzyl groups. In addition, these particular quantum dots can cross the membrane of model cells and their luminescence increases by ~80% after the intracellular photocleavage of the 2-nitrobenzyl quenchers. Thus, photoswitchable luminescent constructs with biocompatible character can be assembled combining the established photochemistry of the 2-nitrobenzyl photocage with the outstanding photophysical properties of semiconductor quantum dots and the hydrophilic character of appropriate polymeric ligands.

A novel strategy is reported for highly sensitive, rapid, and selective detection of nuclear matrix protein NMP22 using two-color quantum dots based on fluorescence resonance energy transfer (FRET). Quantum dots (QDs) are highly advantageous for biological imaging and analysis, particularly when combined with (FRET) properties of semiconductor quantum dot (QDs) are ideal for biological analysis to improve sensitivity and accuracy. In this FRET system narrowly dispersed green emitting quantum dot CdTe core is used as a donor and labelled by monoclonal (mAb) antibody, while orange emitting quantum dot CdTe/CdS core shell is used as an accepter and labelled by polyclonal (pAb) antibody. The quantum dots are labelled by antibodies using EDC/NHS as crosslinking agent. Bovine serum albumin (BSA) solution was added to block nonspecific binding sites. The fluorescence intensity of QDs accepter decreased linearly with the increasing concentrations of NMP22 from 2–22 pg mL−1 due to FRET system and fluoroimmunoassay reaction. This method has good regression coefficient (R2 = 0.998) and detection limit was 0.05 pg mL−1. The proposed FRET-based immunosensor provides a quick, simple and sensitive immunoassay tool for protein detection, and can be considered as a promising approach for clinical applications. The proposed FRET-based immunosensor provides a quick, simple and sensitive immunoassay tool for protein detection, and can be considered as a promising approach for clinical applications.

The size dependence of optical and electronic properties of semiconductor quantum dots (QDs) have been extensively studied in various applications ranging from solar energy conversion to biological imaging. Core/shell QDs allow further tuning of these properties by controlling the spatial distributions of the conduction-band electron and valence-band hole wave functions through the choice of the core/shell materials and their size/thickness. It is possible to engineer type II core/shell QDs, such as CdTe/CdSe, in which the lowest energy conduction-band electron is largely localized in the shell while the lowest energy valence-band hole is localized in the core. This spatial distribution enables ultrafast electron transfer to the surface-adsorbed electron acceptors due to enhanced electron density on the shell materials, while simultaneously retarding the charge recombination process because the shell acts as a tunneling barrier for the core localized hole. Using ultrafast transient absorption spectroscopy, we show that in CdTe/CdSe-anthraquinone (AQ) complexes, after the initial ultrafast (~770 fs) intra-QD electron transfer from the CdTe core to the CdSe shell, the shell-localized electron is transferred to the adsorbed AQ with a half-life of 2.7 ps. The subsequent charge recombination from the reduced acceptor, AQ(-), to the hole in the CdTe core has a half-life of 92 ns. Compared to CdSe-AQ complexes, the type II band alignment in CdTe/CdSe QDs maintains similar ultrafast charge separation while retarding the charge recombination by 100-fold. This unique ultrafast charge separation and slow recombination property, coupled with longer single and multiple exciton lifetimes in type II QDs, suggests that they are ideal light-harvesting materials for solar energy conversion.

Surface effects and interactions with capping ligands strongly influence the properties of semiconductor quantum dots (QDs), opening up possibilities for new technologies, e.g. molecule sensing, and limiting the efficiencies of other applications, e.g. based on QD luminescence. By computing and analyzing in detail the infrared and Raman spectra of two ligands showing qualitatively different bonding to the QD surface, we demonstrate that vibrational spectroscopy constitutes a powerful tool for studying surface–ligand interactions due to its high sensitivity to changes in molecular structure. By forming a covalent bond with a CdSe QD, a single molecule of trimethylphosphine oxide (TMPO) exhibits a strong red-shift in the frequency of P═O that transforms from a double to a single bond. In addition, interaction with the QD breaks TMPO's C3v symmetry, splitting the signals arising from the three methyl groups. The interaction of a methylamine molecule with the CdSe QD is weaker and occurs via a coordination bond. Nevertheless, a strong blue-shift is seen in the frequency of the NH2 wagging mode, arising due to steric hindrance of this motion induced by the QD proximity. The theoretical predictions agree well with the available experimental data, establish the mechanisms for QD–ligand interactions, and provide specific guidelines for vibrational analysis of QD surfaces.

We have designed organic ligands able to adsorb on the surface of CdSe-ZnS core-shell quantum dots and switch the luminescence of the inorganic nanoparticles in response to hydroxide anions. These compounds incorporate a [1,3]oxazine ring within their molecular skeleton, which reacts with the nucleophilic hydroxide anion to generate a 4-nitrophenylazophenolate chromophore. The chromogenic transformation activates an energy transfer pathway from the quantum dot to the adsorbed chromophores. As a result, the luminescence intensity of the coated nanoparticles decreases significantly in the presence of hydroxide anions. In fact, this mechanism can be exploited to probe the pH of aqueous solutions. Indeed, an increase in pH from 7.1 to 8.5 translates into a 35% decrease in the luminescence intensity of the sensitive quantum dots. Thus, our operating principles for luminescence switching can efficiently transduce a chemical stimulation into a change in the emissive response of semiconductor nanoparticles. In principle, this protocol can be extended from hydroxide anions to other target analytes with appropriate adjustments in the molecular design of the chromogenic ligands. It follows that luminescent chemosensors, based on the unique photophysical properties of semiconductor quantum dots, can eventually evolve from our design logic and choice of materials.