Browsing by Author "Urquhart, Stephen"
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Item Connecting Molecular Conformation to Aggregation in P3HT Using Near Edge X-ray Absorption Fine Structure Spectroscopy(American Chemical Society, 10/11/2017) Martinson, Mercedes; Urquhart, Stephen; Eger, Shaylin; Murcia, Victor; Ade, Harald; Collins, BrianCarbon 1s Near Edge X-ray Absorption Fine Structure (NEXAFS) and UV-vis spectroscopy are used to examine differences between highly aggregated and poorly aggregated forms of the polymer poly(3-hexylthiophene) (P3HT), based on as-cast and annealed regio-random and regio-regular P3HT samples. UV-vis spectra show characteristic signatures of unaggregated P3HT in regio-random P3HT, and of H-aggregation in regio-regular P3HT samples. Distinct spectroscopic differences, including energy shifts, are observed in the NEXAFS spectra of aggregated P3HT relative to the unaggregated forms. These differences are reproduced with Transition – Potential Density Functional Theory (TP-DFT) calculations which explore aggregation and molecular confirmation. Differences in the NEXAFS spectra of P3HT are assigned to thiophene backbone twisting in the unaggregated forms of P3HT, and to various degrees of chain planarization in aggregated forms of P3HT that also correlate to the exciton bandwidth. These results open up the prospect of charactering conformation and related difficult to assess structural details through NEXAFS spectroscopy and correlative theory and electronic structure analysis.Item Internal molecular conformation of organic glasses: A NEXAFS study(American Institute of Physics, 7/16/2021) Zuhaib, Amara; Urquhart, StephenThe origin of the exceptional stability of molecular glasses grown by physical vapor deposition (PVD) is not well understood. Differences in glass density have been correlated with thermodynamic stability for thin films of N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) grown by PVD at specific substrate temperatures below the glass transition temperature. However, the relationship between the internal conformation of glass molecules and the thermodynamic properties of molecular glasses is not well studied. We use carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectroscopy to examine different TPD sample preparations in which differences in the thermodynamic stability of the glass are known. Density functional theory simulations of the NEXAFS spectra of TPD allow us to attribute spectroscopic differences to changes in the internal conformation of the TPD molecule and relate this conformation to the stability of the TPD glass. This provides a direct experimental measurement of the internal conformation of molecules forming an organic glass.Item Linear Dichroism in the NEXAFS Spectra of n-Alkane Crystalline Polymorphs(Elsevier, 2019-04) Perera, Sahan; Wang, Jian; Urquhart, StephenLinear Dichroism in Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is a useful tool for studying molecular orientation and for clarifying spectroscopic assignments. n- Alkane molecules can form single crystals with different polymorphs; orthorhombic and monoclinic in the case of even chain lengths between n-C28H58 and n-C40H82. These polymorphs exhibit different linear dichroism in their NEXAFS spectra; in particular a backbone-oriented transition contributes to the low-energy “C-H” band in the monoclinic polymorph.Item Silicon 1s Near Edge X-ray Absorption Fine Structure Spectroscopy of Functionalized Silicon Nanocrystals(American Chemical Society, 2016) Ritchie, Andrew; Cao, Wei; Dasog, Mita; Purkait, T.K.; Senger, Curtis; HU, Yongfeng; Xiao, Q.F.; Veinot, Jon; Urquhart, Stephen; Urquhart, StephenSilicon 1s Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of silicon nanocrystals have been examined as a function of nanocrystal size (3 – 100 nm), varying surface functionalization (hydrogen or 1-pentyl termination), or embedded in oxide. The NEXAFS spectra are characterized as a function of nanocrystal size and surface functionalization. Clear spectroscopic evidence for long range order is observed Si-NCs that are 5-8 nm in diameter or larger. Energy shifts in the silicon 1s NEXAFS spectra of covalently functionalized silicon nanocrystals with changing size are attributed to surface chemical shifts and not to quantum confinement effects.Item Systematic Investigation of π–π Interactions in Near-Edge X-ray Fine Structure (NEXAFS) Spectroscopy of Paracyclophanes(2017) Perera, Sahan; Urquhart, StephenNEXAFS spectroscopy has potential for study of packing and order in organic materials but only if intermolecular effects are understood. This work studies how π–π interactions between adjacent unsaturated groups affect their NEXAFS spectra, with a broader goal of building a general understanding of the role of intermolecular effects in NEXAFS spectroscopy. These effects are examined using paracyclophane (PCP) molecules, in which the benzene–benzene separation distance is controlled by varying the length of the alkyl groups separating the benzene rings. NEXAFS spectroscopy and density functional theory (DFT) simulations are used to examine spectroscopic changes related to the strength of these π–π interactions. A characteristic red shift is observed as adjacent benzene rings get closer together. This shift is attributed to Coulombic and orbital interactions between the adjacent benzene rings, mediated through overlapping π/π* orbitals.Item Temperature Dependence in the NEXAFS Spectra of n-Alkane(American Chemical Society, 11/16/2018) Urquhart, Stephen; Perera, Sahan; Sadegh, Shokatian; Wang, JianThe near edge X-ray absorption fine structure (NEXAFS) spectra of orthorhombic single crystals of n-octacosane (n-C28H58), recorded at room temperature (298 K) and at cryogenic temperatures (93 K), show distinct differences. The characteristic carbon 1s → σ*C–H band in the NEXAFS spectrum of n-C28H58 is broader and has a lower-energy onset in its room temperature spectrum than in its NEXAFS spectrum recorded at cryogenic temperatures. Density functional theory simulations show that nuclear motion and molecular disorder contribute to the observed spectral broadness and are the origin of the low-energy onset of the C–H band in the room temperature spectrum