Doctor of Philosophy, The Ohio State University, 2005, Chemistry

Contact mode atomic force microscopy (AFM) was used to intentionally scratch a monolayer of stilbene, biphenyl, nitrobiphenyl, terphenyl, and nitroazobenzene deposited on a pyrolyzed photoresist film (PPF). The force was set to completely remove the monolayer but not to damage the underlying PPF surface. A line profile determined across the scratch with tapping mode AFM permitted determination of the monolayer thickness from the depth of the scratch. A statistical process was devised to avoid user bias in determining the monolayer thickness, and was used to determine the thickness as a function of derivatization parameters. Carbon/molecule/metal molecular junctions were fabricated by metal deposition of titanium or copper onto monolayers of biphenyl, fluorene, and nitrobiphenyl, and multilayers of NBP covalently bonded to sp2 carbon substrates. The electronic behavior of Ti junctions was extremely dependent on residual gas pressure during e-beam deposition, due to the formation of a disordered Ti oxyhydroxide deposit. Junction resistance decreased with decreasing residual gas pressure, and the hysteresis and rectification observed previously for relatively high deposition pressure was absent for pressures below 5 x 10-7 torr. Ti junctions made at low residual gas pressure had resistances and current/voltage characteristics similar to those of junctions with Cu top contacts, with the latter exhibiting high yield and good reproducibility. The current/voltage characteristics of both the Ti and Cu junctions fabricated with low residual gas pressure were nonlinear, and showed a strong dependence on the thickness of the molecular layer. Carbon/molecule/Cu molecular electronic junctions were fabricated by metal deposition of copper onto films of various thicknesses of fluorene, biphenyl and nitrobiphenyl covalently bonded to flat, graphitic carbon. A “crossed wire” junction configuration provided high device yield and good junction reproducibility. The current/voltage curv (open full item for complete abstract)

Committee: Franklin McCreery (Advisor) Subjects:

Doctor of Philosophy (PhD), Ohio University, 2010, Electrical Engineering (Engineering and Technology)

This dissertation work is focused on the study of two molecular electronic systems: (BETS)2GaCl4 and a Nanoscale Protein Molecular Wire. The detailed structural and electronic study of these systems was carried out using a custom built Ultra-High Vacuum Low Temperature Scanning Tunneling Microscope (UHV-LT-STM). The STM is capable of studying these systems with a spatial and energy resolution that has never been achieved before. λ-(BETS)2GaCl4 is a charge transfer superconductor in bulk crystals. Using a unique deposition procedure, a single sheet of the superconductor was formed on a Ag(111) metal surface. Molecular resolution imaging of the well ordered molecular layer revealed the exact arrangement of the molecules on the surface. The superconductivity of the layer was confirmed by scanning tunneling spectroscopy and spectroscopic mapping. The superconducting signal remained down to a chain of only four molecular pairs, which was only 3.5 nm long. The structure of the nanoscale protein molecular wire is revealed by high resolution STM imaging. Single proteins are imaged and individual amino acid side chains are resolved. Electronic and vibrational properties are explored using the spectroscopic capability of the STM. The STM images, in combination with high resolution vibrational spectroscopy, allow for the first direct sequencing of individual proteins. These two systems have unique properties that could be used as key components in the new class of Nano/Bio electronics. By developing a better understanding of the properties of these systems at the nanoscale, this PhD study answers questions about the electronic and structural properties of these systems and points towards their future electronic and bio-medical applications.

Committee: Savas Kaya PhD (Advisor); Saw-Wai Hla PhD (Advisor); Wojciech Jadwisienczak PhD (Committee Member); Ralph Whaley PhD (Committee Member); Avinash Kodi PhD (Committee Member); Ido Braslavsky PhD (Committee Member) Subjects: Electrical Engineering; Physics

Doctor of Philosophy, The Ohio State University, 2011, Materials Science and Engineering

Molecular electronics is the study of charge transport through single molecules or molecular ensembles. Molecular electronic junctions consist of single molecules or an ensemble of molecules positioned between two conducing contacts. To fabricate and measure the electronic properties of molecular junctions, several techniques have been employed such as scanning tunneling microscopy, conducting probe atomic force microscopy, and vapor deposition of top contacts. Charge transport observed through molecular junctions has been shown to exhibit technologically important phenomena such as rectification, conductance switching, and orbital gating. The primary focus of the field of molecular electronics is to understand the effect of molecular properties, such as structure and molecular orbitals, on charge transport mechanisms through molecular junctions. In this dissertation, the various techniques to fabricate and characterize molecular junctions are discussed, along with an introduction to charge transport mechanisms expected to control transport through molecular junctions. More specifically, this dissertation is primary focused on the fabrication and characterization of molecular junctions fabricated through the formation of an electronic contact on a molecular layer through physical vapor deposition. A common problem with this technique is structural damage to the molecular layer or metal penetration through the molecular layer during the contact formation. To overcome these limitations, a novel fabrication technique was developed and employed to fabricate reproducible molecular junctions through a physical vapor deposition technique without molecular damage or metal penetration. Termed surface diffusion mediated deposition (SDMD), the technique remotely deposits a metallic contact adjacent to and about 10 – 100 nm away from the molecular layer. Surface diffusion causes the metallic contact to migrate towards and onto the molecular layer to form an electronic contact. (open full item for complete abstract)

Committee: Gerald S. Frankel ScD (Advisor); Richard L. McCreery PhD (Advisor); Roberto Myers PhD (Committee Member); Nitin P. Padture PhD (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2010, Physics

As with traditional semiconductor electronic and photoelectronic devices, successful implementation of organometallic device architectures requires understanding and engineering of molecule-electrode interfaces. The intramolecular and electronic structure of C60 stabilized in monolayer films on a Cu(100) surface has been studied with low temperature scanning tunneling microscopy (STM) and spectroscopy. In contrast to other single crystal metal surfaces, C60 adopts four unique orientations on the Cu(100) surface due to a reconstruction of the underlying copper atoms. Spectroscopy and spatial imaging of molecular orbital resonances from LUMO to LUMO+3 for the four distinct adsorption geometries are presented. Shifts in the MO resonances indicate different degrees of electronic molecule-surface hybridization for the four geometries. Additionally, nanoscale modulation of the relative work function is observed across the film which correlates with the proposed atomic-scale electrode reconstruction.

Committee: Jay Gupta PhD (Advisor); Louis DiMauro PhD (Committee Member); Ratnasingham Sooryakumar PhD (Committee Member); Stroud David PhD (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2009, Chemistry

Carbon-based molecular electronic junctions using “cross-bar” geometry were fabricated by covalently bonding organic molecules to graphitic carbon bottom electrodes, pyrolyzed photoresist films (PPF), followed by electron-beam evaporation of metal oxides or metals on top of molecules as top electrodes. For the electrical measurements, a bias was applied between the carbon bottom electrode and the metal top electrode and current and voltage signals were recorded. Carbon/nitroazobenzene (NAB)/TiO2/Au molecular electronic junctions have been previously reported to exhibit robust conductance switching and rectification, which was attributed to redox reactions in the NAB and TiO2 layer. When Ag or Cu is substituted for Au as top contact, dramatically different current-voltage curves and conductance switching behavior result. When the carbon electrode is biased negative, an apparent junction breakdown consistently occurs, leading to a high conductance state which is stable at least for several hours; upon scanning to a positive bias, the conductance returns to a low state and the cycle may be repeated for hundreds of times. The results were consistent with a mechanism based on solid-state oxidation of Ag or Cu metal, transport of their ions through the junction, and reduction of metal ions at the carbon electrode to form conductive metal filaments. Interfacial energetics at the carbon/TiO2 interface in carbon/TiO2/Au junctions were studied by inserting molecular layers with different dipoles or thin layers of SiO2 or Al2O3 between carbon and TiO2. When a low κ material, either an alkane molecular layer or a SiO2 layer, was inserted between carbon and TiO2, the current increases significantly for negative bias (carbon relative to Au), resulting in apparent asymmetry in current-voltage curves. The results were explained by a mechanism of electron injection barrier reduction via energy level alignment at the carbon/TiO2 interface. By using a thin low- κ material as an inser (open full item for complete abstract)

Committee: Richard McCreery Dr. (Advisor); Prabir Dutta Dr. (Committee Member); Yiying Wu Dr. (Committee Member); James Coe Dr. (Committee Member); Ann Christy Dr. (Committee Member) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2008, Chemistry

Carbon based heterojunctions were constructed by covalent bonding of an organic molecular layer onto a graphitic carbon electrode and vapor deposition of metal oxide and metal on top of the organic layer. Organic molecules were bonded through the electrochemical reduction of aryl diazonium tetrafluoroborate salts on the patterned carbon substrate to form ‘bottom' contact first. Then, metal oxide was electron beam vaporized onto the molecular layer to form the ‘top' contact. Finally, a conducting Au layer was deposited on the oxide by the same method to complete this molecular heterojunction. The prepared junctions were characterized by Raman spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM) layer by layer. The initial carbon surface was atomic flat and this flatness did not alter much after derivatization of ~2nm fluorene layer. Deposition of 10nm TiO2 onto the molecules increased the RMS surface roughness to 1.46nm, which was small compared to a 10nm oxide layer. Raman spectra of chemisorbed nitroazobenzene (NAB) layer on carbon substrate before and after the top contact deposition showed no destructive effect from oxides or Au vaporization. SEM study showed the ebeam vaporized Au layer was porous, which allowed gas transfer into the oxide layer. In situ Raman spectroscopy verified NAB reduction in a carbon/NAB/TiOx/Au junction, which revealed the possibility of solid state redox reaction with a barrier presenting in a dry molecular junction. However, electronic properties of a carbon/NAB/TiO2/Au junction did not show distinguish differences from a junction with nonredox active molecules: carbon/Fl/TiO2/Au. The replacement of semiconducting TiO2 with insulting Al2O3 altered the junction properties greatly, which implied that it was the oxide layer dominated junction electric properties. Compared to Al2O3, TiO2 is not only semiconducting but also reducible and the reduced TiIII is much more conductive then TiIV. A carbon/Fl/TiO2/Au (open full item for complete abstract)

Committee: Richard McCreery (Advisor); Prabir Dutta (Committee Member); Heather Allen (Committee Member) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2004, Chemistry

Carbon based molecular electronic junctions were prepared by covalently bonding a molecular layer to a conducting graphitic carbon substrate through the electrochemical reduction of aryl diazonium etrafluoroborate salts. The “top-contact” is made by electron beam evaporation of metals, such that the molecular layer is sandwiched between the two electrodes. The carbon substrate is fabricated by pyrolyzing commercially available photoresist in a reducing atmosphere. It is possible to pattern the photoresist before pyrolysis by simple photolithographic techniques, thus introducing the potential to prepare patterned carbon films to support various analytical applications. An investigation was conducted using Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) to evaluate the interaction between vapor deposited metals and the molecular layer. Evidence for the formation of a covalent bond between the molecular layer and the metal top-contact was found in molecular junctions incorporating a layer of 4-nitroazobenzene (NAB) and vapor deposited titanium. A strong interaction between the NO2 group of the NAB and the deposited titanium was observed using Raman Spectroscopy, in that the peaks associated with the NO2 group decrease in intensity immediately following deposition. The nature of this interaction was realized from the XPS spectra where a peak in the N1s region characteristic of a Ti-N bond was observed. Bias-induced structural changes of the 4.5 nm thick NAB layer within a PPF/NAB/Ti/Au molecular electronic junction were observed using Raman spectroscopy through the semi-transparent Ti/Au top-contact. To our knowledge, this experiment is unprecedented in the literature. The observed spectral changes are consistent with a reversible redox process over a +3 to –1 volt range (PPF relative to Ti/Au). At potentials negative of –1V the NO2 group is permanently reduced to a para-substituted amine. This structure undergoes a reversible redox process over a +3 to –3 (open full item for complete abstract)

Committee: Richard McCreery (Advisor) Subjects: Chemistry, Analytical

Master of Science, Miami University, 2007, Chemistry and Biochemistry

The first chapter of this thesis reports the e-beam lithography process, including procedure, instrumentation, sample preparation, appropriate dosage and final parameters of electrode writing done at the University of Cincinnati. The second chapter shows the synthetic strategies of di/trimetal molecules for potential use in molecular electronics on the fabricated electrodes conducted at Miami University. The final two chapters deal with the synthesis, characterization, and potential applications of metal-organic frameworks that incorporate two novel ligands: a trigonal planar carboxylate ligand and a tetrahedral tetrazolate-based ligand.

Committee: Hong-Cai Zhou (Advisor) Subjects: Chemistry, Inorganic

Doctor of Philosophy, Case Western Reserve University, 1993, Chemistry

The synthesis and characterization of GePc (OSi(n- C6H13)3]2, ( n- C6H13)3SiOGePcOC(O)CCl3, (n- C6H13)3SiOGePcOH, ( n- C6H13)3SiO[ GePcO]2Si( n- C6H13)3, ( n- C6H13)3SiO[ GePcO]2C(O)CHCl2, and (n- C6H13)3SiO[ GePcO]2H are reported. On an HCl-acidified subphase of pH 4.2, (n- C6H13)3SiO[ GePcO]2C(O)CHCl2 gives a Langmuir monolayer of (n- C6H13)3SiO[ GePcO]2H having a molecular co-area of 1.55 nm2/molecule. The co-area of this monolayer varies with pH on unbuffered subphases. On hydrophilic substrates, (n- C6H13)3SiO[ GePcO]2H gives a Langmuir-Blodgett multilayer. This multilayer has been characterized by transfer ratio, X-ray photoelectron spectroscopy, Auger spectroscopy, polarized and unpolarized attenuated total reflectance Fourier transform infrared spectroscopy, polarized variable angle visible spectroscopy, ellipsometry, X-ray diffraction, and contact angle, resistance and current-voltage measurements. The results of this characterization work show that the multilayer is comprised of HOGePcOGePcOH molecules oriented with their rings parallel to the substrate surfaces. The monolayers of the HOGePcOGePcOH multilayer are ∼6.9 A thick. They probably are slightly wavy and not in full registry with their neighboring monolayers. The multilayer is free of a significant number of defects. It absorbs both strongly and anisotropically in the infrared and visible regions. The multilayer is a semiconductor and is probably a photoconductor. It is apparently lightly doped with O2. It is hydrophilic, stable in air and mechanically robust. The film may be useful for the fabrication of optical, microelectronic and molecular electronic devices

Committee: Malcolm Kenney (Advisor) Subjects: Chemistry, Inorganic

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2007, Photochemical Sciences

Organic light-emitting diodes (OLEDs) could become the leading lighting technology for fabrication of full color flat panel displays and general illumination purposes in the near future. To meet this goal, operational hurdles still need to be addressed in electroluminescent devices. In particular, long-term operation and efficient performance are the main challenges to be tackled. Energy transfer processes play a significant role in the improvement of electroluminescence efficiency by utilizing all of the excited states generated during the electron-hole recombination, in particular non-emissive triplet excitons. In this regard, the advantageous energy transfer features displayed by molecular photonic wires could be of practical importance for the fabrication of efficient OLEDs. In the present work, donor-bridge-acceptor triads with appropriate triplet energy alignment of the components were studied. The systems consisted of materials already successfully used in OLEDs. Specifically, aluminum (III) tris(8-quinolinolate) was used as a triplet energy donor (3Alq3 = 2.17 eV), fluorene oligomers as the connecting bridge (3OF1-4 = 2.86-2.18 eV), and platinum (II) tetraphenylporphyrin as an energy acceptor (3PtTPP = 1.91 eV). The molecular photonic wire behavior of these triads was investigated and OLEDs were fabricated. A rationale for tuning the excited-state energies of Alq3was introduced. The synthetic preparation of tunable derivatives bearing conjugated aryl spacers was realized. Red, green, and blue electroluminescence was obtained from OLEDs fabricated using the Alq3-based materials. Dyad systems Alq3-oligofluorene (n=1-9) were synthesized. Strong electronic coupling was observed for materials comprising short oligofluorene fragments (n=1, 3). These systems were used as components for the construction of triads to study molecular photonic wire behavior. Donor-bridge-acceptor triads with a small number of fluorene units (n=1-4) were prepared. In these systems effic (open full item for complete abstract)

Committee: Pavel Anzenbacher (Advisor) Subjects: