Nonradiative decay of excitons is a competing process to Multi-Exciton Generation (MEG) in nanoparticles. Nonradiative decay of single excitons with sufficient energy to generate bi-excitons in Cd₂₀ Se₁₉ and Cd₈₃ Se₈₁ nanoparticles was studied using Tully's Molecular Dynamics with Quantum Transitions (MDQT) method and a CdSe pseudopo- tential. Exciton decay rates increase with increases in nanoparticle temperature and density of lower-lying excitonic states. There did not appear a significant effect of size on energy decay rates. The decay dynamics generally follow a gradual decay with transitions between nearby states. This is punctuated by periodic, short-lived periods of rapid downhill tran- sitions that result in a large proportion of excess exciton energy being transferred to the vibrational motion of the nanoparticle. The time for relaxation to below the 2.0E[subscript g] cutoff was on the order of 1ps. / text

Doctor of Philosophy / Department of Chemistry / Christine Aikens / Thiolate-protected noble metal nanoparticles (~2 nm size) are efficient solar photon harvesters, as they favorably absorb within the visible region. Clear mechanistic insights regarding the photo-physics of the excited state dynamics in thiolate-protected noble metal nanoclusters are important for future photocatalytic, light harvesting and photoluminescence applications. Herein, the core and higher excited states lying in the visible range are investigated using the time-dependent density functional theory method for different thiolate-protected nanoclusters. Nonadiabatic molecular dynamics simulations are performed using the fewest switches surface hopping approach with a time-dependent Kohn-Sham (FSSH-TDKS) description of the electronic states with decoherence corrections to study the electronic relaxation dynamics. Calculations on the [Au₂₅ (SH)₁₈]⁻¹ nanocluster showed that relaxations between core excited states occur on a short time scale (2-18 ps). No semiring or other states were observed at an energy lower than the core-based S₁ state, which suggested that the experimentally observed picosecond time constants could be core-to-core transitions rather than core-to-semiring transitions. Electronic relaxation dynamics on [Au₂₅ (SH)₁₈]⁻¹ with different R ligands (R = CH₃, C₂H₅, C₃H₇, MPA) [MPA = mercaptopropanoic acid] showed that all ligand clusters including the simplest SH model follow a similar trend in decay within the core states. In the presence of higher excited states, R= H, CH₃, C₂H₅, C₃H₇ demonstrated similar relaxations trends, whereas R=MPA showed a different relaxation of core states due to a smaller LUMO+1-LUMO+2 gap. Overall, the S₁ state gave the slowest decay in all ligated clusters. An examination of separate electron and hole relaxations in the [Au₂₅ (SCH₃)₁₈]⁻¹ nanocluster showed how the independent electron and hole relaxations contribute to its overall relaxation dynamics. Relaxation dynamics in the Au₁₈(SH)₁₄ nanocluster revealed that the S₁ state has the slowest decay, which is a semiring to core charge transfer state. Hole relaxations are faster than electron relaxations in the Au₁₈(SH)₁₄ cluster due its closely packed HOMOs. The dynamics in the Au₃₈(SH)₂₄ nanocluster predicted that the slowest decay, the decay of S₁₁ or the combined S₁₁-S₁₂, S₁-S₂-S₄-S₇ and S₄-S₅-S₉-S₁₀ decay, involves intracore relaxations. The phonon spectral densities and vibrational frequencies suggested that the low frequency (25 cm⁻¹) coherent phonon emission reported experimentally could be the bending of the bi-icosahedral Au₂₃ core or the “fan blade twisting” mode of two icosahedral units. Relaxation dynamics of the silver nanoparticle [Ag₂₅ (SR)₁₈]⁻¹ showed that both [Ag₂₅(SH)₁₈]⁻¹ and [Au₂₅ (SH)₁₈]⁻¹ follow a common decay trend within the core states and the higher excited states.

In general terms, this thesis is about tuning coupled electronic and nuclear (or mechanical) dynamics in the nanoscale. With “tuning” we mean changing parameters to achieve a specific phenomenon or functionality. This is not a trivial task in this context, because the dynamics of the systems we consider depend nontrivially on the parameters. To be more concrete, we consider two systems which are “complimentary” in many aspects. We start by studying nonradiative decay of an electronic excitation in a minimal example from supramolecular chemistry: a molecular dimer. Each monomer in our model has two electronic states and the respective potential energy surfaces (PESs) are harmonic. Electronic de-excitation occurs in the monomeric level through well-localized regions in the nuclear space which we call ``NRD channels\'\'. The monomers interact via transition dipole-dipole interaction. The decay dynamics of the monomer are trivial due to its harmonic PESs and simple NRD channel. However, the dimer shows distorted and nontrivially coupled PESs conferring rather complex decay dynamics on it. Depending on the position of the NRD channel, we find that the NRD lifetime can exhibit a completely different dependence on the intermolecular-interaction strength. The extension to larger aggregates and the implications to the quantum yield of molecular systems will be discussed. Our findings suggest design principles for molecular systems where a specific fluorescence quantum yield is desired. The most part of this thesis is about a nanoscale rotor driven by charge tunneling. The rotor consists of electronic islands linked to a bearing via insulating arms. The islands can exchange electrons via tunneling with flanking electronic leads. An uniform electrostatic field brings about the coupling between electronic and mechanical degrees of freedom. Moreover, coupling to an environment lead to dissipation in the mechanical dynamics. In the literature one can identify two generic models of this type of rotor [1-3], which we refer to as “mean-field” and “stochastic” models in this thesis. In the mean-field model the system is described by a set of deterministic differential equations involving the average charge on the electronic islands, and therefore charge fluctuations are not taken into account. In the stochastic model the rotor is described by Fokker–Planck equations which fully take into account the charge fluctuations. We start by showing and comparing the dynamics of these models. The models show interesting phenomenology and predict useful functionality to the rotor. However, it is often unclear which assumptions are made upon the system when using these models. To clarify this matter we derived the models using the “orthodox” theory of single electron tunneling [4]. Next, we go on and propose experimental devices which can be described by these models. The parameter ranges accessible using these devices are estimated. Turning our attention back to functionality, we show how to introduce a preferred direction of rotation, which is useful in the context of motors. In the outlook we also show how to recast the system as a current rectifier. [1] A. Y. Smirnov, S. Savel’ev, L. G. Mourokh and F. Nori; Phys. Rev. E 78 031921 (2008). [2] A. Croy and A. Eisfeld; EPL (Europhysics Lett. 98 68004 (2012). [3] A. Smirnov, L. Murokh, S. Savel’ev and F. Nori; Bio-mimicking rotary nanomotors; volume 7364 (2009); doi:10.1117/12.821567; URL http://dx.doi.org/10.1117/12. 821567. [4] B. L. Altshuler, P. A. Lee and W. R. Webb; Mesoscopic phenomena in solids; volume 30; Elsevier (2012).

In general terms, this thesis is about tuning coupled electronic and nuclear (or mechanical) dynamics in the nanoscale. With “tuning” we mean changing parameters to achieve a specific phenomenon or functionality. This is not a trivial task in this context, because the dynamics of the systems we consider depend nontrivially on the parameters. To be more concrete, we consider two systems which are “complimentary” in many aspects. We start by studying nonradiative decay of an electronic excitation in a minimal example from supramolecular chemistry: a molecular dimer. Each monomer in our model has two electronic states and the respective potential energy surfaces (PESs) are harmonic. Electronic de-excitation occurs in the monomeric level through well-localized regions in the nuclear space which we call ``NRD channels\'\'. The monomers interact via transition dipole-dipole interaction. The decay dynamics of the monomer are trivial due to its harmonic PESs and simple NRD channel. However, the dimer shows distorted and nontrivially coupled PESs conferring rather complex decay dynamics on it. Depending on the position of the NRD channel, we find that the NRD lifetime can exhibit a completely different dependence on the intermolecular-interaction strength. The extension to larger aggregates and the implications to the quantum yield of molecular systems will be discussed. Our findings suggest design principles for molecular systems where a specific fluorescence quantum yield is desired. The most part of this thesis is about a nanoscale rotor driven by charge tunneling. The rotor consists of electronic islands linked to a bearing via insulating arms. The islands can exchange electrons via tunneling with flanking electronic leads. An uniform electrostatic field brings about the coupling between electronic and mechanical degrees of freedom. Moreover, coupling to an environment lead to dissipation in the mechanical dynamics. In the literature one can identify two generic models of this type of rotor [1-3], which we refer to as “mean-field” and “stochastic” models in this thesis. In the mean-field model the system is described by a set of deterministic differential equations involving the average charge on the electronic islands, and therefore charge fluctuations are not taken into account. In the stochastic model the rotor is described by Fokker–Planck equations which fully take into account the charge fluctuations. We start by showing and comparing the dynamics of these models. The models show interesting phenomenology and predict useful functionality to the rotor. However, it is often unclear which assumptions are made upon the system when using these models. To clarify this matter we derived the models using the “orthodox” theory of single electron tunneling [4]. Next, we go on and propose experimental devices which can be described by these models. The parameter ranges accessible using these devices are estimated. Turning our attention back to functionality, we show how to introduce a preferred direction of rotation, which is useful in the context of motors. In the outlook we also show how to recast the system as a current rectifier. [1] A. Y. Smirnov, S. Savel’ev, L. G. Mourokh and F. Nori; Phys. Rev. E 78 031921 (2008). [2] A. Croy and A. Eisfeld; EPL (Europhysics Lett. 98 68004 (2012). [3] A. Smirnov, L. Murokh, S. Savel’ev and F. Nori; Bio-mimicking rotary nanomotors; volume 7364 (2009); doi:10.1117/12.821567; URL http://dx.doi.org/10.1117/12. 821567. [4] B. L. Altshuler, P. A. Lee and W. R. Webb; Mesoscopic phenomena in solids; volume 30; Elsevier (2012).

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18288号 / 工博第3880号 / 新制||工||1595(附属図書館) / 31146 / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 田中 一義, 教授 田中 庸裕, 教授 佐藤 啓文 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19741号 / 工博第4196号 / 新制||工||1647(附属図書館) / 32777 / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 田中 庸裕, 教授 佐藤 啓文, 教授 梶 弘典 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

京都大学 / 新制・課程博士 / 博士(工学) / 甲第25014号 / 工博第5191号 / 新制||工||1991(附属図書館) / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 佐藤 徹, 教授 田中 庸裕, 教授 佐藤 啓文 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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GaN-Nanodrähte können mit einer hohen strukturellen Perfektion auf verschiedenen kristallinen und amorphen Substraten gewachsen werden. Sie bieten somit faszinierende Möglichkeiten, sowohl zur Untersuchung von fundamentalen Eigenschaften des Materialsystems, als auch in der Anwendung in optoelektronischen Bauteilen. Obwohl bereits verschiedene Prototypen solcher Bauteile vorgestellt wurden, sind viele grundlegende Eigenschaften von GaN-Nanodrähten noch ungeklärt, darunter die interne Quanteneffizienz (IQE), welche ein wichtiges Merkmal für optoelektronische Anwendungen darstellt. Die vorliegende Arbeit präsentiert eine detaillierte Untersuchung der Rekombinationsdynamik von Exzitonen, in selbst-induzierten und selektiv gewachsenen GaN Nanodraht-Proben, welche mit Molekularstrahlepitaxie hergestellt wurden. Die zeitaufgelösten Photolumineszenz (PL)-Experimente werden durch Simulationen ergänzt, welche auf Ratengleichungs-Modellen basieren. Es stellt sich heraus, dass die Populationen von freien und gebundenen Exzitonen gekoppelt sind und zwischen 10 und 300 K von einem nichtstrahlenden Kanal beeinflusst werden. Die Untersuchung von Proben mit unterschiedlichem Nanodraht-Durchmesser und Koaleszenzgrad zeigt, dass weder die Nanodraht-Oberfläche, noch Defekte als Folge von Koaleszenz diesen nichtstrahlenden Kanal induzieren. Daraus lässt sich folgern, dass die kurze Zerfallszeit von Exzitonen in GaN-Nanodrähten durch Punktdefekte verursacht wird, welche die IQE bei 10 K auf 20% limitieren. Der häufig beobachtete biexponentiellen PL-Zerfall des Donator-gebundenen Exzitons wird analysiert und es zeigt sich, dass die langsame Komponente durch eine Kopplung mit Akzeptoren verursacht wird. Motiviert durch Experimente, welche eine starke Abhängigkeit der PL-Intensität vom Nanodraht-Durchmesser zeigen, wird die externen Quanteneffizienz von geordneten Nanodraht-Feldern mit Hilfe numerischer Simulationen der Absorption und Extraktion von Licht in diesen Strukturen untersucht. / GaN nanowires (NWs) can be fabricated with a high structural perfection on various crystalline and amorphous substrates. They offer intriguing possibilities for both fundamental investigations of the GaN material system as well as applications in optoelectronic devices. Although prototype devices based on GaN NWs have been presented already, several fundamental questions remain unresolved to date. In particular, the internal quantum efficiency (IQE), an important basic figure of merit for optoelectronic applications, is essentially unknown for GaN NWs. This thesis presents a detailed investigation of the exciton dynamics in GaN NWs using continuous-wave and time-resolved photoluminescence (PL) spectroscopy. Spontaneously formed ensembles and ordered arrays of GaN NWs grown by molecular-beam epitaxy are examined. The experiments are combined with simulations based on the solution of rate equation systems to obtain new insights into the recombination dynamics in GaN NWs at low temperatures. In particular, the free and bound exciton states in GaN NWs are found to be coupled and affected by a nonradiative channel between 10 and 300 K. The investigation of samples with different NW diameters and coalescence degrees conclusively shows that the dominating nonradiative channel is neither related to the NW surface nor to coalescence-induced defects. Hence, we conclude that nonradiative point defects are the origin of the fast recombination dynamics in GaN NWs, and limit the IQE of the investigated samples to about 20% at cryogenic temperatures. We also demonstrate that the frequently observed biexponential decay for the donor-bound exciton originates from a coupling with the acceptor-bound exciton state in the GaN NWs. Motivated by an experimentally observed, strong dependence of the PL intensity of ordered GaN NW arrays on the NW diameter, we perform numerical simulations of the light absorption and extraction to explore the external quantum efficiency of these samples.

Während über hexagonales (alpha) GaN zum ersten Mal 1932 berichtet wurde, gelang erst 1989 die Synthese einer mit Molekularstrahlepitaxie (MBE) auf 3C-SiC epitaktisch gewachsenen, metastabilen kubischen (eta) GaN Schicht. Die vorliegende Arbeit befaßt sich mit der Herstellung der Verbindungen eta-(In,Ga,Al)N mittels RF-Plasma unterstützter MBE auf GaAs(001) und den mikrostrukturellen sowie optischen Eigenschaften dieses neuartigen Materialsystems. Im Vergleich zur hexagonalen bietet die kubische Kristallstruktur auf Grund ihrer höheren Symmetrie potentielle Vorteile für die Anwendung in optischen und elektronischen Bauelementen. Viele wichtige Materialgrößen der kubischen Nitride sind jedoch noch gänzlich unbekannt, da sich die Synthese einkristalliner Schichten als sehr schwierig erweist. Das Ziel dieser Arbeit ist es daher erstens, die technologischen Grenzen der Herstellung von bauelementrelevanten kubischen (In,Ga,Al)N Heterostrukturen auszuweiten und zweitens, einen Beitrag zur Aufklärung der bis dato wenig bekannten optischen und elektronischen Eigenschaften des GaN und der Mischkristalle In GaN zu leisten. Zunächst wird ein optimierter MBE Prozess unter Einsatz einer Plasmaquelle hohen Stickstofflusses vorgestellt, welcher nicht nur die reproduzierbare Epitaxie glatter, einphasiger GaN Nukleationsschichten auf GaAs ermöglicht. Vielmehr können damit auch dicke GaN. Schichten mit glatter Oberflächenmorphologie hergestellt werden, welche die Grundlage komplizierterer eta-(In,Ga,Al)N Strukturen bilden. An einer solchen GaN Schicht mit einer mittleren Rauhigkeit von nur 1.5 nm werden dann temperaturabhängige Reflexions- und Transmissionsmessungen durchgeführt. Zur Auswertung der Daten wird ein numerisches Verfahren entwickelt, welches die Berechnung des kompletten Satzes von optischen Konstanten im Spektralgebiet 2.0 = 0.4 wären grün-gelbe Laserdioden. Zusammenfassung in PostScript / While the earliest report on wurtzite (alpha) GaN dates back to 1932, it was not until 1989 that the first epitaxial layer of metastable zincblende (eta) GaN has been synthesized by molecular beam epitaxy (MBE) on a 3C-SiC substrate. The present work focuses on radio frequency (RF) plasma-assisted MBE growth, microstructure, and optical properties of the eta-(In,Ga,Al)N material system on GaAs(001). Due to their higher crystal symmetry, these cubic nitrides are expected to be intrinsically superior for (opto-) electronic applications than the widely employed wurtzite counterparts. Owing to the difficulties of obtaining single-phase crystals, many important material constants are essentially unknown for the cubic nitrides. The aim of this work is therefore, first, to push the technological limits of synthesizing device-relevant zincblende (In,Ga,Al)N heterostructures and, second, to determine the basic optical and electronic properties of GaN as well as to investigate the hardly explored alloy InGaN. An optimized MBE growth process is presented which allows not only the reproducible nucleation of smooth, monocrystalline GaN layers on GaAs using a high-nitrogen-flow RF plasma source. In particular, thick single-phase GaN layers with smooth surface morphology are obtained being a prerequisite for the synthesis of ternary eta-(Ga,In,Al)N structures. Temperature dependent reflectance and transmittance measurements are carried out on such a GaN film having a RMS surface roughness as little as 1.5 nm. A numerical method is developed which allows to extract from these data the complete set of optical constants for photon energies covering the transparent as well as the strongly absorbing spectral range (2.0 -- 3.8 eV). Inhomogeneities in the refractive index leading to finite coherence effects are quantitatively analyzed by means of Monte Carlo simulations. The fundamental band gap EG(T) of GaN is determined for 5 < T < 300 K and the room temperature density of states is investigated. Systematic studies of the band edge photoluminescence (PL) in terms of transition energies, lineshapes, linewidths, and intensities are carried out for both alpha- and GaN as a function of temperature. Average phonon energies and coupling constants, activation energies for thermal broadening and quenching are determined. Excitation density dependent PL measurements are carried out for both phases in order to study the impact of nonradiative recombination processes at 300 K. A recombination model is applied to estimate the internal quantum efficiency, the (non)radiative lifetimes, as well as the ratio of the electron to hole capture coefficients for both polytypes. It is seen that the dominant nonradiative centers in the n-type material investigated act as hole traps which, however, can be saturated at already modest carrier injection rates. In summary, despite large defect densities in GaN due to highly mismatched heteroepitaxy on GaAs, band edge luminescence is observed up to 500 K with intensities comparable to those of state-of-the-art alpha-GaN. For the first time, thick InGaN films are fabricated on which blue and green luminescence can be observed up to 400 K for x=0.17 and x=0.4, respectively. Apart from bulk-like InGaN films, the first coherently strained InGaN/GaN (multi) quantum wells with In contents as high as 50 % and abrupt interfaces are grown. This achievement shows that a ternary alloy can be synthesized in a metastable crystal structure far beyond the miscibility limit of its binary constituents despite the handicap of highly lattice mismatched heteroepitaxy. The well widths of these structures range between 4 and 7 nm and are thus beyond the theoretically expected critical thickness for the strain values observed. It is to be expected that even higher In contents can be reached for film thicknesses below 5 nm. The potential application of such InGaN/GaN multi quantum wells with x >= 0.4 would thus be diode lasers operating in the green-yellow range. abstract in PostScript