In the first study, superconducting 2D NbSe₂ was examined towards its prototypical demonstration as a transition-edge sensor, where photoexcitation caused a thermodynamic phase transition in NbSe₂ from the superconducting state to the normal state. The efficacy of the optical absorption was found to depend on the wavelength of the incoming radiation used, which ranged from the ultra-violet (405 nm), visible (660 nm), to the infrared (1060 nm). In the second case involving WSe₂, the UV-ozone treatment revealed the presence of localized excitonic emission in 1L WSe₂ that was robust and long-lived. Our third material platform dealt with hybrid 0D-2D ensembles based on graphene and WSe₂, specifically graphene–endohedral, WSe₂–fullerene (C₆₀), and WSe₂–Au nanoparticles, and exhibited exceptional performance gains achieved with both types of hybrid structures. Next, we investigated WSe₂ based mixed dimensional hybrids. Temperature T-dependent and wavelength λ-dependent optoelectronic transport measurements showed a shift in the spectral response of 1L WSe₂ towards the SPR peak locations of Au-Sp and Au-BP, fostered through the plexciton interactions. Models for the plexcitonic interactions are proposed that provide a framework for explaining the photoexcited hot charge carrier injection from AuNPs to WSe₂ and its influence on the carrier dynamics in these hybrid systems. Last, we studied interactions of vdWs hybrid structures composed of WSe₂ with 0D buckminsterfullerene (C₆₀) spheres. Our results indicate that the C₆₀-WSe₂ vdWs hybrid heterostructure appears to be an attractive architecture for enabling charge transfer and high performance photodetection capabilities. T-dependent electrical transport measurements after C₆₀ deposition revealed a dominant p-type conduction behavior and a significant ×10³ increase in WSe₂ field-effect mobility, with a maximum field-effect mobility of 281 cm²V⁻¹s⁻¹ achieved at 350 K and room-T mobility of 119.9 cm²V⁻¹s⁻¹ for the C₆₀-WSe₂ hybrid.

This thesis is concerned with weakly bound neutral and ionic clusters. Spectra of the region near the S1fS0 electronic origin of four neutral van der Waals molecules - aniline-argon, phenol-argon, chlorobenzene-argon and fluorobenzene-argon - were obtained using resonance enhanced multiphoton ionization (REMPI). These spectra indicate that Fermi resonances between van der Waals stretching and bending motions are important in these molecules. Effective Hamiltonians are constructed that describe well the low frequency vibrations. In order to better discuss the low frequency van der Waals motions of aromatics bound to one and two rare gas atoms a simple model for the vibrations is developed. The model enables expression of van der Waals frequencies in terms of fundamental molecular properties and enables facile comparison of effective force constants in a variety of van der Waals molecules. The model is successfully employed to explain van der Waals vibrational structure associated with the origin region of aniline-(argon)2 using van der Waals potential parameters derived from the aniline-(argon)1 spectrum. REMPI and emission spectra of larger clusters of aniline and argon are also reported and discussed. Using atom-atom potentials, equilibrium structures for aniline-(argon)n (n=l, 2, 3) are calculated. The calculations prove useful in the analysis of the spectra.The BfX transitions of the cation complexes fluorobenzene+-argon and chlorobenzene+-argon have been investigated. The cations were prepared by resonance enhanced multiphoton ionization of the neutral van der Waals molecules. A time delayed tunable dye laser was then used to dissociate the cations, loss of an argon atom being the dominant process. When the second laser was tuned to a cation resonance the dissociation cross section increased markedly, allowing characterization of BfX transition. The resulting spectra are presented and discussed.

<p>This work presents first-principles calculations of long-range intermolecular dispersion energies between two atoms or molecules as expressed in terms of the C₆ dipole-dipole dispersion coefficients. In a series of publications, it has been shown by us that the complex linear polarization propagator method provides accurate <em>ab initio</em> and first-principles density functional theory values of the C₆ dispersion coefficients in comparison with those reported in the literature. The selected samples for the investigation of dispersion interactions in the electronic ground state are the noble gases, <em>n</em>-alkanes, polyacenes, azabenzenes, and C₆₀. It has been shown that the proposed method can also be used to determine dispersion energies for species in their respective excited electronic states. The C₆dispersion coefficients for the first <em>π</em> → <em>π*</em> excited state of the azabenzene molecules have been obtained with the adopted method in the multiconfiguration self-consistent field approximation. The dispersion energy of the <em>π</em> → <em>π*</em> excited state is smaller r than that of the ground state. It is found that the characteristic frequencies ω₁ defined in the London approximation of <em>n</em>-alkanes vary in a narrow range and that makes it possible to construct a simple structure-to-property relation based on the number of -bonds for the dispersion interaction in these saturated compounds. However, this simple approach is not applicable for the interactions of the <em>π</em>-conjugated systems since their characteristic frequencies <em>ω</em>₁ vary strongly depending on the systems.</p> / Report code: LIU-TEK-LIC-2006:2

This work presents first-principles calculations of long-range intermolecular dispersion energies between two atoms or molecules and of electronic circular dichroism spectra of chiral molecules. The former is expressed in terms of the C6 dipole-dipole dispersion coefficients Δε, and the latter is given in terms of the extinction coefficient. In a series of publications, the complex linear polarization propagator method has been shown to be a powerful tool to provide accurate ab initio and first-principles density functional theory results. This was the case not only for the C6 dispersion coefficients but also for the electronic circular dichroism at an arbitrary wavelength ranging from the optical to the X-ray regions of the spectrum. The selected samples for the investigation of dispersion interactions in the electronic ground state are the noble gases, n-alkanes, polyacenes, azabenzenes, alkali-metal clusters, and C60. It is found that the values of C6 for the sodium-cluster-to-fullerene interactions are well within the error bars of the experiment. The proposed method can also be used to determine dispersion energies for species in their respective excited electronic states. The C6 dispersion coefficients for the first π → π* excited state of the azabenzene molecules have been obtained with the adopted method in the multiconfiguration self-consistent field approximation. The dispersion energy of the π → π* excited state is smaller than that of the ground state. It is found that the characteristic frequencies ω1 defined in the London approximation of n-alkanes vary in a narrow range which makes it possible to construct a simple structure-to-property relationship based on the number of π-bonds for the dispersion interaction in these saturated compounds. However, this simple approach is not applicable to the interactions of the π-conjugated systems since, depending on the systems, their characteristic frequencies ω1 can vary greatly. In addition, an accomplishment of calculations of the electronic circular dichroism spectra in the near-edge X-ray absorption has been demonstrated.

In the absorption spectra, photoabsorption cross sections and oscillator strengths were measured. Quantum defects were calculated to identify the assignments of absorption features. The measured quantities were also compared with previous results. In fluorescence excitation spectra, ultraviolet and visible emissions were observed, and fluorescence cross sections were measured in the excitation region between 140 and 170 nm. The ultraviolet emission is due to the transition B¡¦2£G¡÷X2£S and the visible emission is associated with B¡¦2£G¡÷B2£S and E2£U+¡÷A2£U+ transitions. In the presence of foreign gas, the collisional quenching effect plays an important role on the fluorescence intensity of C(0) and D(0).We measure the quenching half pressure P1/2 of D(0) respectively. The potential well depth of van der Walls molecule NO-M can be derived from the measured quenching half pressure. In contrast to quenching, the fluorescence intensity of C(1), B(9), B(10) and B(11) are enhanced as the pressure of foreign gas is increased. The enchancement of the fluorescence intensity is attributed to the collisional removal of predissociation.

The studies of the dynamics, phase behavior, interparticle interactions, and hydrodynamics of stimuli-responsive pNIPAm-co-AAc microgels were described in this thesis. Due to their responsiveness to external stimuli, these colloidal particles serve as excellent model systems to probe the relationship between colloidal interactions and phase behavior. As a first step, we established our core experimental methodology, by demonstrating that particle tracking video microscopy is an effective technique to quantify various parameters in colloidal systems. Then we used the technique in combination with a microfluidic device that provides in situ control over sample pH to probe the phase behavior of pNIPAm-co-AAc microgel suspensions. In essence, the experimental set-up enables changes in effective particle volume fractions by changing pH, which can be used to construct the phase diagram. In order to explain the unique features of the microgel phase diagram, we measured the underlying pairwise interparticle potential of pNIPAm-co-AAc microgels directly in quasi-2D suspension and proved that the interactions are pH dependent and can range from weakly attractive to soft repulsive. Finally, the hindered Brownian diffusion due of colloidal particles confined by hard walls was investigated systematically and striking differences between hard sphere and soft sphere were found, with soft pNIPAm-co-AAc microgels showing surprising mobility even under strong confinement.

Thèse doctorat : Chimie Informatique et Théorique : Nancy 1 : 2005. / Titre provenant de l'écran-titre.

This thesis details research undertaken in the investigation of the open-shell open-shell Van der Waals complexes between nitric oxide (NO) and oxygen (O₂) and between nitrogen dioxide (NO₂ ) and oxygen (O</sub>2</sub> ). The Fourier transform microwave spectroscopy technique was used during the experimental work. Spectra were recorded for a mixture of NO and O₂ over the frequency range 7.0 to 18.2 GHz. Detailed ab initio calculations have been performed on the potential energy surface of both NO-O₂ and NO₂-O₂ using Multi-reference Rayleigh Schrodinger Second Order Perturbation Theory. These calculations were performed for the four distinct states of NO-O₂, ²A", ⁴A", ²A' and ⁴A', and for the two distinct states of NO,sub>2</sub>-O₂, ²A and ⁴A. Predicted rotational spectra have been generated by modelling the systems with a rigid-rotor Hamiltonians. The effective Hamiltonians included the rotational Hamiltonian, ℋ_rot, fine structure terms and hyperfine structure terms. It was not possible, for rationalizable reasons, to fit the predicted spectra for NO-O₂ to the experimental observations. Experimental details, calculation methods and rotational theory are discussed in the main body of the thesis. Predicted spectra, relevant calculations and other data is presented in the appendices.

Atom-surface interactions are becoming an integral part of the field of atom optics. Here the role of van der Waals atom-surface interactions in atom wave diffraction and interferometry are investigated. In particular, it is shown that van der Waals interactions influence the intensity and phase of atomic diffraction patterns obtained from material gratings. As a result the atomic diffraction patterns are utilized to make an accurate determination of the interaction strength and verify the spatial variation of the atom-surface potential. A theory for describing the modified atom wave diffraction patterns is developed through an analogy with optical waves. An atom interferometer is used to directly measure the de Broglie wave phase shift induced by atom-surface interactions. More specifically, the phases of the zeroth, first, and second diffraction orders are measured. A proposal for using electromagnetic fields to modify the van der Waals interaction is put forth. Several of the important experimental components for observing such an effect are also demonstrated.

The potential of filling carbon nanotubes in polymers has been widely acknowledged for composites with exceptional new properties owing to the high strength of carbon nanotubes. In the thesis, by employing Materials Studio 4.0 software, the buckling behaviour and vibration of polyethylene and carbon nanotube matrix composites are first discussed using molecular mechanics simulations. The research is aimed to acquire a high strength design of PE-CNT matrix with proper PE/CNT ratio as well as discovering the dynamic characteristics of the PE-CNT composites. Investigation results show that as the number of PE chains increases, the buckling strain and the resonance frequency will decrease. Van der Waals forces are collected to explain the relation of the PE chains to the buckling strain and the resonance frequency of the composites.