Computational Modeling of Geometry Dependent Phonon Transport in Silicon Nanostructures

Recent experiments have demonstrated that thermal properties of semiconductor nanostructures depend on nanostructure boundary geometry. Phonons are quantized mechanical vibrations that are the dominant carrier of heat in semiconductor materials and their aggregate behavior determine a nanostructure\u27s thermal performance. Phonon-geometry scattering processes as well as waveguiding effects which result from coherent phonon interference are responsible for the shape dependence of thermal transport in these systems. Nanoscale phonon-geometry interactions provide a mechanism by which nanostructure geometry may be used to create materials with targeted thermal properties. However, the ability to manipulate material thermal properties via controlling nanostructure geometry is contingent upon first obtaining increased theoretical understanding of fundamental geometry induced phonon scattering processes and having robust analytical and computational models capable of exploring the nanostructure design space, simulating the phonon scattering events, and linking the behavior of individual phonon modes to overall thermal behavior. The overall goal of this research is to predict and analyze the effect of nanostructure geometry on thermal transport. To this end, a harmonic lattice-dynamics based atomistic computational modeling tool was created to calculate phonon spectra and modal phonon transmission coefficients in geometrically irregular nanostructures. The computational tool is used to evaluate the accuracy and regimes of applicability of alternative computational techniques based upon continuum elastic wave theory. The model is also used to investigate phonon transmission and thermal conductance in diameter modulated silicon nanowires. Motivated by the complexity of the transmission results, a simplified model based upon long wavelength beam theory was derived and helps explain geometry induced phonon scattering of low frequency nanowire phonon modes

Cutting Edge Nanotechnology

The main purpose of this book is to describe important issues in various types of devices ranging from conventional transistors (opening chapters of the book) to molecular electronic devices whose fabrication and operation is discussed in the last few chapters of the book. As such, this book can serve as a guide for identifications of important areas of research in micro, nano and molecular electronics. We deeply acknowledge valuable contributions that each of the authors made in writing these excellent chapters

Nonlinear Optical Studies of Bulk and Thin Film Complex Materials

Nonlinear optical studies of bulk and thin film materials provide a vast playground for physical and dynamical characterization. In this thesis, we have implemented experimental methods to probe novel phase transitions in single crystals using rotational anisotropic second harmonic generation (RASHG) and carrier dynamics in thin films with time-resolved pump-probe reflectivity. Furthermore, a novel low temperature ultra-high vacuum system coupled to nonlinear optics has been developed to extend lab capabilities. Doping (Bi1-xSbx)2Se3 with antimony, the surface electronic reconstruction near x=80% was identified with RASHG by deviations in the six-fold and three-fold polarization anisotropic patterns. Development of RASHG techniques to include temperature control and vacuum conditions lead to the exploration of the temperature-dependent electronic phase transition of IrTe2. In IrTe2 the surface electronic transition at Tc~280 K measured by SHG is immediate and completes within the experimental temperature resolution. Comparing the surface temperate response to corresponding bulk measurements, the surface electronic transition occurs four times faster indicating the surface precedes the bulk transition. With time-resolved pump-probe reflectivity, an acoustic phonon mode in La0.67Sr0.33MnO3/SrTiO3 has been identified along with an anomalous low frequency oscillation unreported in similar systems

Confocal and cavity-enhanced spectroscopy of semiconductor van der Waals heterostructures

Heterostrukturen zweidimensionaler Materialien zeichnen sich durch vielverprechende elektronische und optische Eigenschaften aus. Eine sehr bekannte Klasse dieser Heterostrukturen ist aus Monolagen von Übergangsmetalldichalkogeniden aufgebaut. Innerhalb der einzelnen Lagen finden sich unter Photoanregung stark gebundene Exzitonen mit valley-spezifischen optischen Auswahlregeln. Diesevalleytronischen Charakteristiken werden an langlebige Interlagenexzitonen vererbt, die sich in künstlich arrangierten Heterostrukturen erzeugen lassen und ein statisches Dipolmoment senkrecht zur Ebene besitzen. In dieser Arbeit wurden Heterostrukturen aus Molybdändiselenid und Wolframdiselenid mit Methoden der optischen Spektroskopie untersucht. Basierend auf Messungen der Frequenzverdoppelung, differentieller Reflektion, sowie energie- und zeitaufgelöster Photolumineszenz konnte das atomare Register gewachsener Heterobilagen bestimmt werden. In diesem Zusammenhang wurden die optischen Übergänge als Faltung von Interlagenexzitonen in verschiedenen Spin- und Valleykonfigurationen modelliert. Daraufhin wurde die Licht-Materie-Wechselwirkung dieser Kanäle mit einem kryogenen Rasterscanresonator untersucht. Nach der Charakterisierung des Systems wurde seine Längendurchstimmbarkeit genutzt, um die Rekombinationsrate zu verstärken. Diese Messungen erlaubten es, die Raten der Licht-Materie-Wechselwirkung für alle drei Zerfallskanäle zu berechnen. Zusätzlich wurde die Absorption der Interlagenexzitonen in einem hochreflektiven Resonatorsystem gemessen. Der Wert der Absorption stimmt mit den vorangegangenen konfokalen Messungen überein und weist auf eine Dehnungsempfindlichkeit der Polarisation von Interlagenexzitonen hin. Zuletzt wurden Heterobilagen und Heterotrilagen im Hinblick auf den Einfluss einer zusätzlichen Lage auf die optischen Übergänge miteinander verglichen. Konfokale Spektroskopie wurde durch numerische Berechnungen der Dichtefunktionaltheorie ergänzt. Magnetolumineszenz-Experimente ermöglichten es, zusammen mit den theoretischen Vorhersagen, impulsdirekte Übergänge in der Heterobilage von impulsindirekten Übergängen in der Heterotrilage zu unterscheiden. Diese Erkenntnisse tragen zum Verständnis der optischen Eigenschaften von Heterostrukturen aus Übergangsmetalldichalkogeniden bei und ermöglichen die Konstruktion komplexerer vertikaler Anordnungen.Heterostructures of layered two-dimensional materials have attracted much attention in modern solid state physics due to their promising transport and optical properties. A very prominent class of heterostructures are build of monolayer transition metal dichalcogenides. After photoactivation, the single-layer components host tightly bound excitons with valley-contrasting optical selection rules. Inheriting these valleytronic characteristics, long-lived interlayer excitons with a permanent out-of-plane dipole moment can arise in artificially assembled heterostructures. Within this thesis, heterostructures of molybdenum diselenide and tungsten diselenide were studied with optical spectroscopy. Based on measurements of second harmonic generation, differential reflectance as well as energy and time-resolved photoluminescence, the atomic registry of grown heterobilayers was determined. In this context, the optical transitions were modeled as a convolution of interlayer excitons in various spin and valley configurations. Subsequently, the light-matter coupling of these recombination channels was probed by a cryogenic scanning cavity setup. After an initial characterization of the system, its length-tunability was used to enhance the interlayer exciton recombination in the limit of weak coupling. These measurements enabled to infer the light-matter coupling rates for all decay channels. In addition, interlayer exciton absorption was tested in a high finesse scanning cavity system. The measured absorption strength is consistent with the initial confocal studies and indicates a strain-susceptibility of interlayer exciton polarization. Finally, heterobilayer and heterotrilayer structures were investigated in direct comparison, probing the influence of an additional layer of molybdenum diselenide upon the optical transitions. Confocal spectroscopy methods were complemented by numerical calculations using density functional theory. Magneto-luminescence experiments were performed and enabled, along with theorical predictions, to differentiate the momentum direct transition in the heterobilayer from momentum-indirect transition in the heterotrilayer. These results shed light on the intrinsic optical properties of transition metal dichalcogenide heterostructures and facilitate the design of more complex vertical arrangements

Spin polarization dynamics in perovskite nanocrystals

Seit dem kürzlichen Aufkommen von kolloidalen Blei-Halogenid Perowskit Nanokristallen (LHP NCs) begeistern sie die Branche durch ihre faszinierenden optischen Eigenschaften. Sie besitzen daher großes Potenzial optische Anwendungen wie Strahler, Solarenergiekollektoren und Spintronik zu reformieren. CsPbI3, eine anorganische Verbindung unter LHPs, zeigt besonders ausgeprägte Spin-Bahn-Kopplung, was zu signifikanter Feinstrukturspaltung und folglich zu lediglich zweifach entarteten Valenz- (VB) und Leitungsbändern (CB) führt. Hierdurch bestehen perfekte Bedingungen für maximal effiziente optische Spinausrichtung (oS), wobei Ladungsträger durch zirkular polarisierte Strahlung in bekannte VB- und CB-Zustände angeregt werden. Im Gegensatz dazu stehen konventionelle II-VI und III-V Halbleiter, worin sich dieselben auf mehrere VB- und CB-Zustände aufteilen, was die erreichbare oS um 50% gegenüber LHPs verringert. Das Potenzial der enormen Induktion von zirkularem Dichroismus in CsPbI3 und dessen Vielzahl an faszinierenden optischen Eigenschaften macht diese Verbindung zu einem Modellsystem für fundamentale Spinforschung. Das Wissen über Spindynamiken von Ladungsträgern ist ausschlaggebend für ein grundlegendes Verständnis elektronischer Prozesse in dieser Materialklasse. In dieser Arbeit wird die Dynamik von Ladungsträger-Spinrelaxation (LSR) und dessen zugrundeliegenden theoretischen Mechanismen mittels zeitaufgelöster differenzieller Transmissionsspektroskopie (DTS) beleuchtet. Dabei stellt sich heraus, dass die intrinsisch achiralen NCs beträchtlichen zirkularen Dichroismus kurz nach Anregung durch zirkular polarisierte Laserpulse aufweisen. Darauffolgende LSR gleicht das präparierte Spin-Ungleichgewicht aus. Energetischere optische Anregung bewirkt ein Abkühlen der Ladungsträger zur Bandlücke durch Phononenemission. Dabei entsteht eine große Nicht-Gleichgewichts-Phononenpopulation, welche den Wirkungsquerschnitt der Ladungsträger-Phononenstreuung vergrößert. Der Elliott-Yafet- (EY) Mechanismus, den ich der maßgeblichen LSR in CsPbI3 zuordne, besagt, dass LSR durch Ladungsträger-Phononenstreuung erfolgt. Die gemessene Ensemble-Spinpolarisation veringert sich dementsprechend erheblich mit erhöhter LSR während des Abkühlvorgangs der Ladungsträger. Temperaturabhängige DTS offenbart, dass die LSR-Geschwindigkeit bei Raumtemperatur derer bei kryogenen Temperaturen um eine Größenordnung übersteigt. Entsprechende Raten enthüllen einen klaren und adequaten Zusammenhang jeweils zur Phononenbesetzung und EY-Funktionalität. Der Entzug von Elektronen aus den CsPbI3 NCs durch Beimischung eines Elektron-Absorbermoleküls erlaubt die fast ausschließliche Beobachtung der Loch-Spinrelaxation, welche sich als langsamer, als die der Elektronen erweist.Recently, colloidal lead halide perovskite nanocrystals (LHP NCs) have emerged and impress the community with their intriguing optical properties ever since. They demonstrate great potential to reform optical applications such as light emitting devices, solar energy harvesting and spintronics. Among LHPs, the all-inorganic compound CsPbI3 exhibits particularly strong spin-orbit coupling, leading to significant fine structure splitting, which makes both, valence (VB) and conduction band (CB) only two-fold degenerate. This renders perfect conditions for maximally efficient optical orientation, whereupon charge carriers are excited into precisely known VB and CB states by circularly polarized radiation. This is in contrast to conventional II-VI and III-V semiconductors, where circularly photoexcited charge carriers are distributed among multiple VB and CB states, dropping their maximally achievable optical orientation by as much as 50% compared to LHPs. The potential of photoinducing tremendous circular dichroism into CsPbI3 NCs through optical orientation, in combination with their multiplicity of intriguing optical properties, make them a model system for fundamental spin studies. The knowledge about the spin dynamics of charge carriers is crucial for a profound comprehension of electronic processes in this material class. In this thesis, charge carrier spin polarization dynamics and underlying theoretical mechanisms are elucidated in colloidal CsPbI3 NCs by employing time-resolved differential transmission spectroscopy (DTS). Thereby, the intrinsically achiral NCs are found to exhibit considerable circular dichroism shortly after excitation with a circularly polarized laser pulse. Subsequent charge carrier spin relaxation equilibrates the prepared spin imbalance. Elevated photoexcitation energy causes charge carriers to cool down to the band gap via phonon emission. Thereby, a large non-equilibrium phonon population develops, increasing the carrier-phonon scattering cross-section. The Elliott-Yafet (EY) mechanism, which I assign to govern spin relaxation in CsPbI3 NCs, predicts that spin relaxation is a consequence of carrier-phonon scattering. Accordingly, the investigated ensemble spin polarization is measured to diminish significantly in the process of carrier cooling at an increased spin relaxation rate. Temperature-dependent DTS reveals that room temperature spin relaxation dynamics are one order of magnitude faster than at cryogenic temperatures. The corresponding rates reveal a clear and adequate correlation to phonon occupation and EY functionality, respectively. The removal of electrons from the CsPbI3 NCs through admixture with an electron scavenger molecule permits the almost exclusive investigation of hole spin relaxation, which is revealed to occur slower compared to that of electrons

Nanoscale thermal transport. II. 2003–2012

A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of ~1 nm , the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interfaces between materials. Major advances in the physics of phonons include first principles calculation of the phonon lifetimes of simple crystals and application of the predicted scattering rates in parameter-free calculations of the thermal conductivity. Progress in the control of thermal transport at the nanoscale is critical to continued advances in the density of information that can be stored in phase change memory devices and new generations of magnetic storage that will use highly localized heat sources to reduce the coercivity of magnetic media. Ultralow thermal conductivity—thermal conductivity below the conventionally predicted minimum thermal conductivity—has been observed in nanolaminates and disordered crystals with strong anisotropy. Advances in metrology by time-domain thermoreflectance have made measurements of the thermal conductivity of a thin layer with micron-scale spatial resolution relatively routine. Scanning thermal microscopy and thermal analysis using proximal probes has achieved spatial resolution of 10 nm, temperature precision of 50 mK, sensitivity to heat flows of 10 pW, and the capability for thermal analysis of sub-femtogram samples.United States. Air Force Office of Scientific Research. Multidisciplinary University Research Initiative (FA9550-08-1-0407