Thesis (Ph. D.)--University of Chicago, Dept. of Chemistry, December 2000. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--Michigan State University. Dept. of Chemistry, 2006. / Title from PDF t.p. (viewed on June 19, 2009) Includes bibliographical references (p. 267-278). Also issued in print.

Thesis (PhD)--Stellenbosch University, 2003. / ENGLISH ABSTRACT: A tunable narrow-bandwidth laser source combined with a supersonic gas jet as sample is wellsuited for obtaining high-resolution spectra of cold isolated molecules and complexes. In the present study such a laser source in the vacuum ultraviolet was applied to the spectroscopic investigation of rare carbon monoxide (CO) isotopomers and CO-noble gas van der Waals complexes in supersonic gas pulses seeded with natural CO gas. Tunable coherent vacuum ultraviolet radiation was generated by two-photon resonant fourwave sum-frequency mixing of two pulsed dye laser beams in a magnesium vapour medium. Laser induced fluorescence excitation spectra of the A(v'=3)-X(v"=0) vibronic band of CO molecules in a noble gas (neon or argon) jet were obtained by measuring the total undispersed fluorescence from the irradiated sample volume in dependence of the excitation wavelength. The dynamics of the flow-cooling process in the supersonic jet were investigated and the experimental parameters optimised using the rotational temperature of 12C160as determined from rotational line intensities. Rotational temperatures as low as 2 ± I K were observed. Spectroscopic detection of the rare 12C170and 12C180isotopomers was facilitated by the low rotational temperature and high spectral resolution. Six rotational lines of 12C170and four of 12C180were detected in the A(v'=3)-X(v"=0) vibronic band. This demonstrates the low detection limit (circa 3 parts per million) obtained in the experiment. The line wavelengths were determined to an accuracy of 0.2 pm using the well-documented 12C160and 13C160lines for calibration. The spectral results on 12C170are, to our knowledge, the first rotationally resolved laboratory measurements published on the A-X band of this isotopomer. Accurate wavelength data of the stable isotopomers of CO is of importance in the interpretation of astrophysical observations of CO in the interstellar medium. The newly determined 12C170wavelengths were successfully applied to a recent problem in astrophysics (Astrophys. J. Lett. 2003). The conditions in a supersonic jet facilitate the study of weakly bound van der Waals complexes, of which CO-noble gas complexes are prototypes. However, there is no experimental data available on the electronic excitation spectra of the CO-noble gas complexes, lying in the vacuum ultraviolet region. In the present experiment evidence of extensive complexation of the CO in the noble gas jet has been found, but in the spectral region around the A(v'=3)-X(v"=O) band of CO no distinct spectral features that could be associated with these complexes were observed. Having considered the existing knowledge on CO and CO-noble gas complexes and experimental studies on the excitation and dissociation dynamics of Iz-noble gas complexes, we regard complex induced inter-system crossing or electronic predissociation as the most likely causes for these observations. The results on the rare CO isotopomers demonstrate the potential of our experimental setup for high-resolution, isotope and state selective spectroscopy in the vacuum ultraviolet with a high sensitivity for fluorescent species. The availability of the now well-characterised experimental setup in our laboratory opens the way for further investigation of molecular or complex species with spectroscopic features in the vacuum ultraviolet region. / AFRIKAANSE OPSOMMING: Vakuum ultraviolet laser spektroskopie van CO molekules in 'n supersoniese gasstraal: 'n Afstembare smal bandwydte laserbron en 'n supersoniese gasstraal as monster is 'n geskikte kombinasie vir hoë-resolusie spektroskopie van geïsoleerde afgekoelde molekules en komplekse. In hierdie studie is so 'n laserbron in die vakuum ultraviolet gebruik in die spektroskopiese ondersoek van skaars koolstofmonoksied (CO) isotopomere en CO-edelgas van der Waals komplekse in supersoniese gaspulse wat 'n klein persentasie natuurlike CO gas bevat. Afstembare koherente vakuum ultraviolet lig is verkry deur twee-foton resonante vier-golf som-frekwensie vermenging van twee gepulseerde kleurstoflaserbundels in 'n magnesiumdamp medium. Laser-geïnduseerde fluoressensie opwekkingspektra van die A(v'=3)-X(v"=0) vibroniese band van die CO molekules in die edelgasstraal (neon of argon) is uitgemeet deur die totale fluoressensie van die beligte gasmonster, sonder golflengteskeiding, te meet as funksie van die opwekkingsgolflengte. Die dinamika van die vloeiverkoelingsproses in die supersoniese gasstraal is ondersoek en die eksperimentele parameters geoptimeer deur gebruik te maak van die rotasionele temperatuur van 12Cl60 soos bepaal uit die intensiteitsverhoudings van die rotasielyne. Rotasionele temperature tot so laag as 2 ± 1 K is waargeneem. Spektroskopiese waarneming van die skaars 12C170 and 12Cl80 isotopomere is moontlik gemaak deur die lae rotasionele temperatuur en die hoë spektrale resolusie. Ses rotasielyne van 12C170 en vier van 12C180 is waargeneem in die A(v'=3)-X(v"=0) vibroniese band. Dit demonstreer die lae deteksielimiet (ongeveer 3 dele per miljoen) wat bereik kon word. Die golflengtes van die lyne is bepaal met 'n akkuraatheid van 0.2 pm deur die bekende lyne van 12C160en 13C160vir kalibrasie te gebruik. Die resultate ten opsigte van 12C170 is sover vasgestel kon word die eerste rotasioneel-opgeloste laboratorium metings van die A-X band van hierdie isotopomeer. Akkurate golflengte data vir die stabiele CO isotopomere is van belang vir die interpretasie van die astrofisiese waarnemings van CO in die interstellêre medium. Die nuwe 12C170 golflengtes is suksesvol aangewend in die oplossing van 'n onlangse interpretasieprobleem in astrofisika (Astrophys. J. Lett. 2003). Die toestande in 'n supersoniese gasstraal maak die bestudering van swak-gebonde van der Waals komplekse moontlik. Hoewel CO-edelgas van der Waals komplekse as prototipes beskou word, is daar geen eksperimentele data beskikbaar oor die elektroniese opwekkingspektra van hierdie spesies, wat in die vakuum ultraviolet gebied lê, nie. In hierdie studie is daar eksperimentele getuienis gevind vir uitgebreide kompleksering van CO in die edelgasstraal, maar in die spektraalgebied rondom die A(v'=3)-X(v"=O) band van CO is geen duidelike spektrale kenmerke wat met hierdie komplekse geassosieer kan word, waargeneem nie. Na oorweging van die bestaande kennins oor CO en CO-edelgas komplekse en eksperimentele studies oor die opwekking en dissosiasie-dinamika van Iz-edelgas komplekse, beskou ons kompleksgeïnduseerde intersisteemoorgange of elektroniese predissosiasie as die waarskynlikste redes vir hierdie waarnemings. Die resultate oor die skaars CO isotopomere toon die potensiaal van ons eksperimentele opstelling vir hoë-resolusie, isotoop- en toestandselektiewe spektroskopie in die vakuum ultraviolet met uitstekende sensitiwiteit vir fluoresserende spesies. Die beskikbaarheid van hierdie nou deeglik gekarakteriseerde eksperimentele opstelling in ons laboratorium maak verdere ondersoek na molekulêre of kompleks-spesies met spektroskopiese kenmerke in die vakuum ultraviolet moontlik.

Van der Waals materials and inorganic nanoparticles are two categories of nanomaterials that have been widely investigated in the past two decades. Both of them have been considered to be promising as candidates for the next generation electrical, optical, and mechanical applications. However, both of them have a few limitations that greatly affect the performance of devices, e.g. zero bandgap for graphene; poor contact quality, low mobility and quantum efficiency for MoS2; and poor interparticle conductivity for nanoparticles. This thesis tries to explore a new way of combining these two categories of material into hybrids, so that the intrinsic limitations of materials from each category will be overcome by the other materials that are introduced into the hybrid. This thesis consists of five parts. The first part (Chapter 1) introduces the background and motivation of the thesis. The second part (Chapters 2, 3, 4, and 5) describes the detailed processes and methods, starting from preparing each element to the assembly of these element into a hybrid structure device. This part also includes understanding the mechanisms of 2D and 3D self-assembly of nanoparticles. The third part (Chapter 6 and 7) describes two examples of hybrid structures, including the investigation of electron or molecule transfer inside the hybrid. The fourth part (Chapter 8) introduces other findings and technical innovations, including alternative ways of thin film nanoparticle self-assembly/deposition, and fabrication methods for the band structure analysis of transition metal dichalcogenides by angle resolved photo-electron spectroscopy. The fifth part (Chapter 9) describes several possible future work directions that could be investigated to improve the understanding of the nanoparticle assembly and translating the conceptual device into real applications.

One of the fastest growing fields in condensed matter physics is that of two-dimensional materials; compounds that promise to revolutionize nanotechnology due to the ability to easily isolate clean atomically thin sheets of conducting material for use in atomic-scale circuits. Since the initial demonstration of the electric-field effect in nanocircuits fabricated from mechanically exfoliated graphene, the number of available compounds that can be isolated and used in atomically thin circuits has exponentially grown to include diverse electrical properties from metals and insulators to superconductors and magnets. The bulk compounds from which flakes are isolated are known as van der Waals materials named for their intrinsic structural anisotropy resulting in weak van der Waals chemical bonds in one dimension. Since this field is relatively young, there are a multitude of branching opportunities for experimental advancement. In this work, we begin by addressing a significant technical challenge within the two-dimensional community; contacting and measuring air-sensitive two-dimensional materials. We developed a novel technique for embedding metal electrodes in atomically thin insulating flakes used to simultaneously contact and preserve a wide-array of air-sensitive two-dimensional materials. Using this technique, we proceed to explore the properties of a diverse set of van der Waals compounds in both three dimensions and two dimensions. We investigate the nature of superconductivity in the two-dimensional limit by quantifying the fragility of the superconducting state in a single atomic sheet of NbSe2. In combination with theoretical time-dependent Ginzburg-Landau simulations, we show that the dissipation in two-dimensional NbSe2 can be accurately described by vortex dynamics, including the poorly understood low-temperature metallic-like state. We examine how superconductors proximitize with normal metals through measurements on atomic-scale normal metal/insulator/superconductor tunnel junctions fabricated from van der Waals materials, demonstrating agreement with Blonder- Tinkham-Klapwijk theory. In addition, in junctions fabricated from graphene and NbN, a high-critical- field superconductor, we gain an understanding of Andreev processes in graphene under large magnetic fields. Finally, we provide a detailed characterization Re6Se8Cl2 and CrSBr, two new van der Waals compounds. In Re6Se8Cl2, we develop a novel strategy for doping in van der Waals compounds with labile ligands, demonstrating a semiconducting to superconducting transition upon electron doping. In CrSBr, we discover a well-developed semiconducting gap along with strong coupling between magnetic order and transport properties, unique among van der Waals magnets. Further, we find the semiconducting and magnetic properties persist down to 2 layers of CrSBr, with the observation of air-stability, establishing it as a promising material platform for increasing the applicability of van der Waals magnets.

Les complexes de van der Waals sont des entités constituées de différentes molécules liées par des interactions faibles (~kJ/mol). Ces agrégats présentent une réactivité particulière et jouent un rôle essentiel dans les phénomènes de solvatation et nucléation. Des moyens expérimentaux et théoriques pour étudier les complexes de van der Waals et en particulier ceux contenant de l’acétylène ont donc été développés dans ce travail. Nous avons utilisé et amélioré un montage expérimental appelé FANTASIO+ (Fourier trANsform, Tunable diode and quadrupole mAss spectrometers interfaced to a Supersonic expansIOn). Ce montage, composé d’un jet supersonique et d’un spectromètre à temps de déclin permet la production et la détection de ce genre de complexes. Le jet supersonique consiste en une détente adiabatique d’un gaz et assure par refroidissement à quelques Kelvins la production de complexes. La spectrométrie à temps de déclin mesure l’absorption d’un laser infra-rouge par ces molécules cibles assurant ainsi leur détection. <p>Une diode laser nous a permis d’exciter deux fois l’étirement CH de l’acétylène. Nous avons pu détecter et analyser le spectre de vibration-rotation des complexes suivants :C2H2-Ne, C2H2-Ar, C2H2-Kr, C2H2-CO2, C2H2-N2O, et C2H2-C2H2. La molécule C2H2-CO2 et des isotopologues de C2H2-C2H2 ont également été étudiés à plus basse énergie durant un séjour à Calgary au Canada. Nos études ont démontré que ces complexes restaient liés à une énergie pouvant aller jusqu’à 130 fois l’énergie d’interaction entre les deux monomères. L’obtention de données à haute résolution spectrale permet également d’obtenir des données de références pour la validation de modèles théoriques et la planétologie. En particulier, la première détection de C2H2-Kr permettra peut-être une future observation de cet agrégat dans des atmosphères planétaires comme par exemple Titan. <p>Pour avoir une approche globale de ces systèmes nous nous sommes tournés vers les outils de la chimie quantique pour caractériser l’interaction entre les entités du complexe. Des tests méthodologiques approfondis nous ont permis d’évaluer avec exactitude les surfaces d’énergie potentielle intermoléculaire des complexes contenant une molécule d’acétylène et un atome de krypton ou de xénon. <p><p>van der Waals complexes are molecular systems in which the units or molecules are held together by weak interactions (~kJ/mol). These complexes present a peculiar reactivity and play a critical role in solvation and nucleation. Theoretical and experimental means were developed in this work to study such systems and in particular, complexes containing acetylene. In the context of this work the FANTASIO+ (Fourier trANsform, Tunable diode and quadrupole mAss spectrometers interfaced to a Supersonic expansIOn) experimental set-up was used and improved. This set-up, composed of a supersonic expansion and a cavity ring-down spectrometer, provides a way to produce and detect these complexes. The supersonic expansion is an adiabatic expansion which produces the complexes by cooling of the gas to few Kelvin. The CRDS set-up detect those complexes by infra-red laser absorption.<p>Using laser diode to doubly excite the CH stretch of acetylene, one then succeeded to observe and analyze the ro-vibrational spectra of the following complexes: C2H2-Ne, C2H2-Ar, C2H2-Kr, C2H2-CO2, C2H2-N2O, et C2H2-C2H2. The C2H2-CO2 and isotopologues of C2H2-C2H2 were also studied at lower energy during a three months stay in Calgary, Canada. Our studies demonstrated that complexes stayed bound even at an energy 130 times higher than the energy holding the entities together. The high resolution data obtained during this work is also useful to validate theoretical models and planetology. The first detection of the C2H2-Kr complex, in particular, could allow its future detection in other atmospheres, i.e. on Titan.<p>To have a global approach to these systems, the quantum chemistry tools were used to characterize the interaction between the partners of the complexes. Numerous methodological tests allowed us to accurately evaluate the intermolecular potential energy surfaces of the complexes containing an acetylene molecule and a krypton or a xenon atom. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished