Ultrafast Nonlinear Spectroscopy Study of Interfaces and Surfaces

Interfaces and surfaces are ubiquitous in people’s daily life and they play a pivotal role in many research fields such as molecular electronics, photovoltaics, and environmental chemistry. However, due to the lack of interface-specific probes, there are plenty of questions remain unsolved for interfaces. During the past few decades, thanks to the development of ultrafast pulsed laser technology, nonlinear spectroscopy has merged to be a powerful technique to characterize the properties of interfaces. In this dissertation, several novel nonlinear spectroscopic methods will be discussed and used to perform static as well as kinetic study of interfaces. First, the major emphasis of this dissertation is to understand the electronic structure and charge transfer dynamics at the interface formed by an organic semiconductor – poly(3-hexylthiophene-2,5-diyl) (P3HT) and a metal – gold or an inorganic semiconductor – silicon. Using the interface specific sum frequency generation spectroscopy (SFG), a method of measuring the band gap of buried interfaces is established. The result demonstrates that the electronic structure at buried interface differs from that of the bulk. By combining the transient absorption spectroscopy and vibrational sum frequency generation (VSFG) spectroscopy, the first dynamical electric-field-induced VSFG signal is observed and for the first time, a spectroscopic evidence of direct electron transfer at complex polymer/metal interfaces is presented. Aided by heterodyne detection, a phase rotation approach has been established to disentangle the pure molecular response from the electronic nonresonance to interfacial charge transfer. The first fourth order three-dimensional SFG spectroscopy (3D SFG) is introduced and used to measure the interstate vibrational coherences from a Re(diCN-bpy)(CO)3Cl monolayer adsorbed on gold surface. It is learned that the surface attachment induces both homogeneous and inhomogeneous dephasing dynamics of the vibrational mode. However, the coherence is preserved upon surface attachment. Other than applying to air/solid and solid/solid interfaces, nonlinear spectroscopy is also powerful to learn air/aqueous interfaces. VSFG is used to explore the protein adsorption kinetics at air/salt water interface under environmentally relevant conditions. In combination of surface pressure measurement, a novel “salting up” phenomenon is proposed, and the role of ions is discussed. Moreover, a critical surface coverage needs to be satisfied to induce the conformational change of proteins at the surface. Overall, nonlinear spectroscopy has been proved to be an ideal candidate for non-destructive and interface specific characterization method. By choosing the appropriate method, both static and kinetic information can be achieved

Modeling spontaneous charge transfer at metal/organic hybrid heterostructures

Hybrid materials are crucial in photovoltaics where the overall efficiency of the heterostructure is closely related to the level of charge transfer at the interface. Here, using various metal / poly(3-hexylthiophene)(P3HT) heterostructure models, we reveal that the level of spontaneous charge transfer and electronic coupling at these interfaces depend on the conformational regularity of the organic polymer deposited on the metal substrate. Using ab-initio quantum chemical calculations based on density functional theory (DFT) and heterodyne vibrational sum frequency generation (HD-VSFG) measurements, we show that inducing regio-randomness into the organic polymer modifies the intensity of interfacial electronic states, level of hybridization, density of interfacial charge transfer and the electronic wave function of the material. We present the HD-VSFG responses of the metal/P3HT heterojunctions containing both regio-regular and regio-random P3HT structures and show that the amount of non-resonant signal is closely related to the level of the spontaneous charge transfer at the interface. Thus, by measuring the non-resonant response of the metal/P3HT heterojunctions, the level of spontaneous charge transfer at the interface can be determined

Direct observation of the intermediate in an ultrafast isomerization.

Using a combination of two-dimensional infrared (2D IR) and variable temperature Fourier transform infrared (FTIR) spectroscopies the rapid structural isomerization of a five-coordinate ruthenium complex is investigated. In methylene chloride, three exchanging isomers were observed: (1) square pyramidal equatorial, (1); (2) trigonal bipyramidal, (0); and (3) square pyramidal apical, (2). Exchange between 1 and 0 was found to be an endergonic process (ΔH = 0.84 (0.08) kcal mol-1, ΔS = 0.6 (0.4) eu) with an isomerization time constant of 4.3 (1.5) picoseconds (ps, 10-12 s). Exchange between 0 and 2 however was found to be exergonic (ΔH = -2.18 (0.06) kcal mol-1, ΔS = -5.3 (0.3) eu) and rate limiting with an isomerization time constant of 6.3 (1.6) ps. The trigonal bipyramidal complex was found to be an intermediate, with an activation barrier of 2.2 (0.2) kcal mol-1 and 2.4 (0.2) kcal mol-1 relative to the equatorial and apical square pyramidal isomers respectively. This study provides direct validation of the mechanism of Berry pseudorotation - the pairwise exchange of ligands in a five-coordinate complex - a process that was first described over fifty years ago. This study also clearly demonstrates that the rate of pseudorotation approaches the frequency of molecular vibrations

Software-defined Design Space Exploration for an Efficient DNN Accelerator Architecture

Deep neural networks (DNNs) have been shown to outperform conventional machine learning algorithms across a wide range of applications, e.g., image recognition, object detection, robotics, and natural language processing. However, the high computational complexity of DNNs often necessitates extremely fast and efficient hardware. The problem gets worse as the size of neural networks grows exponentially. As a result, customized hardware accelerators have been developed to accelerate DNN processing without sacrificing model accuracy. However, previous accelerator design studies have not fully considered the characteristics of the target applications, which may lead to sub-optimal architecture designs. On the other hand, new DNN models have been developed for better accuracy, but their compatibility with the underlying hardware accelerator is often overlooked. In this article, we propose an application-driven framework for architectural design space exploration of DNN accelerators. This framework is based on a hardware analytical model of individual DNN operations. It models the accelerator design task as a multi-dimensional optimization problem. We demonstrate that it can be efficaciously used in application-driven accelerator architecture design. Given a target DNN, the framework can generate efficient accelerator design solutions with optimized performance and area. Furthermore, we explore the opportunity to use the framework for accelerator configuration optimization under simultaneous diverse DNN applications. The framework is also capable of improving neural network models to best fit the underlying hardware resources

Revealing Hidden Vibration Polariton Interactions by 2D IR Spectroscopy

We report the first experimental two-dimensional infrared (2D IR) spectra of novel molecular photonic excitations - vibrational-polaritons. The application of advanced 2D IR spectroscopy onto novel vibrational-polariton challenges and advances our understanding in both fields. From spectroscopy aspect, 2D IR spectra of polaritons differ drastically from free uncoupled molecules; from vibrational-polariton aspects, 2D IR uniquely resolves hybrid light-matter polariton excitations and unexpected dark states in a state-selective manner and revealed hidden interactions between them. Moreover, 2D IR signals highlight the role of vibrational anharmonicities in generating non-linear signals. To further advance our knowledge on 2D IR of vibrational polaritons, we develop a new quantum-mechanical model incorporating the effects of both nuclear and electrical anharmonicities on vibrational-polaritons and their 2D IR signals. This work reveals polariton physics that is difficult or impossible to probe with traditional linear spectroscopy and lays the foundation for investigating new non-linear optics and chemistry of molecular vibrational-polaritons