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This thesis focuses on the synthesis, properties and applications of two types of contorted organic molecules: contorted molecular ribbons and conjugated corrals. We utilized the power of reaction chemistry to writing information into conjugated molecules with contorted structures and studied “structure-property” relationships. The unique properties of the molecules were expressed in electronic and optoelectronic devices such as field-effect transistors, solar cells, photodetectors, etc. In Chapter 2, I describe the design and synthesis of a new graphene ribbon architecture that consists of perylenediimide (PDI) subunits fused together by ethylene bridges. We created a prototype series of oligomers consisting of the dimer, trimer, and tetramer. The steric congestion at the fusion point between the PDI units creates helical junctions, and longer oligomers form helical ribbons. Thin films of these oligomers form the active layer in n-type field effect transistors. UV−vis spectroscopy reveals the emergence of an intense long-wavelength transition in the tetramer. From DFT calculations, we find that the HOMO−2 to LUMO transition is isoenergetic with the HOMO to LUMO transition in the tetramer. We probe these transitions directly using femtosecond transient absorption spectroscopy. The HOMO−2 to LUMO transition electronically connects the PDI subunits with the ethylene bridges, and its energy depends on the length of the oligomer. In Chapter 3, I describe an efficiency of 6.1% for a solution processed non-fullerene solar cell using a helical PDI dimer as the electron acceptor. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor−acceptor interfaces, indicating that charge carriers are created from photogenerated excitons in both the electron donor and acceptor phases. Light-intensity-dependent current−voltage measurements suggested different recombination rates under short-circuit and open-circuit conditions. In Chapter 4, I discuss helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor-acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometers in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells. In Chapter 5, I compare analogous cyclic and acyclic π-conjugated molecules as n-type electronic materials and find that the cyclic molecules have numerous benefits in organic photovoltaics. We designed two conjugated cycles for this study. Each comprises four subunits; one combines four electron-accepting, redox-active, diphenyl-perylenediimide subunits, and the other alternates two electron-donating bithiophene units with two diphenyl-perylenediimide units. We compare the macrocycles to acyclic versions of these molecules and find that, relative to the acyclic analogs, the conjugated macrocycles have bathochromically shifted UV-vis absorbances and are more easily reduced. In blended films, macrocycle-based devices show higher electron mobility and good morphology. All of these factors contribute to the more than doubling of the power conversion efficiency observed in organic photovoltaic devices with these macrocycles as the n-type, electron transporting material. This study highlights the importance of geometric design in creating new molecular semiconductors. In Chapter 6, I describe a new molecular design that enables high performance organic photodetectors. We use a rigid, conjugated macrocycle as the electron acceptor in devices to obtain high photocurrent and low dark current. We directly compare the macrocyclic acceptor devices to an acyclic control device; we find that the superior performance of the macrocycle originates from its rigid, conjugated, and cyclic structure. The macrocycle’s rigid structure reduces the number of charged defects originating from deformed sp2 carbons and covalent defects from photo/thermo-activation. With this molecular design we are able to suppress dark current density while retaining high responsivity in an ultra-sensitive non-fullerene organic photodetector. Importantly, we achieve a detectivity of ~10^14 Jones at near zero bias voltage. This is without the need for extra carrier blocking layers commonly employed in fullerene-based devices. Our devices are comparable to the best fullerene-based photodetectors, and the sensitivity at low working voltages (< 0.1 V) is a record for non-fullerene OPDs.

Thesis (MScEng (Electical and Electronic Engineering))--University of Stellenbosch, 2008. / A Cryogenic CMOS-based Control System for Testing Superconductor Electronics P.C. van Niekerk Department of Electrical and Electronic Engineering University of Stellenbosch Private Bag X1, 7602 Matieland, South Africa Thesis: M.Sc.Eng. (E&E) March 2008 A complete control system, with accompanying software, is designed to interface superconductive digital and sensory circuits for use in cryogenic vacuumed environments. It acts as an inter-mediator between superconductor electronics and room temperature electronics for research purposes. In order to facilitate low bit-error rate communications with superconductive electronics, the system is designed to have ultra low-noise current and voltage sources for transmitting data to superconductor electronics. Very high sensitivity voltage inputs are also implemented for data extraction from superconductor electronics. It implements both digital as well as analog design components, including ADC and DAC devices. The data is transmitted via a USB cable connection at 1Mbaud to a computer where the data is processed by specially designed software and graphically displayed for user interfaced research. Extensive research is done on the electronic components, such as CMOS devices, for functioning in an average temperature of 70 Kelvin inside cryogenic environments. This is done to reduce the thermal noise and heat transfer to superconductor electronics. An integrated temperature control system also ensures a stable environment for the electronics to operate at 70 K.

Fabrication of molecular spin devices with ferromagnetic electrodes coupled with a high spin molecule is an important challenge. This doctoral study concentrated on realizing a novel molecular spin device by the bridging of magnetic molecules between two ferromagnetic metal layers of a ferromagnetic-insulator-ferromagnetic tunnel junction on its exposed pattern edges. At the exposed sides, distance between the two metal electrodes is equal to the insulator film thickness; insulator film thickness can be precisely controlled to match the length of a target molecule. Photolithography and thin-film deposition were utilized to produce a series of tunnel junctions based on molecular electrodes of multilayer edge molecular electrodes (MEME) for the first time. In order to make a microscopic tunnel junction with low leakage current to observe the effect of ~10,000 molecules bridged on the exposed edge of a MEME tunnel barrier, growth conditions were optimized; stability of a ~2nm alumina insulator depended on its ability to withstand process-induced mechanical stresses. The conduction mechanism was primarily 1) tunneling from metal electrode to oranometalic core by tunneling through alkane tether that acts as a tunnel barrier 2) rapid electron transfer within the oranometalic Ni-CN-Fe cube and 3) tunneling through alkane tether to the other electrode. Well defined spin-states in the oranometalic Ni-CN-Fe cube would determine electron spin-conduction and possibly provide a mechanism for coupling. MEME with Co/NiFe/AlOx/NiFe configurations exhibited dramatic changes in the transport and magnetic properties after the bridging of oranometalic molecular clusters with S=6 spin state. The molecular cluster produced a strong antiferromagnetic coupling between two ferromagnetic electrodes to the extent, with a lower bound of 20 erg/cm,2 that properties of individual magnetic layers changed significantly at RT. Magnetization, ferromagnetic resonance and magnetic force microscopy studies were performed. Transport studies of this configuration of MEME exhibited molecule-induced current suppression by ~6 orders by blocking both molecular channels and tunneling between metal leads in the planar 25μm2 tunnel junction area. A variety of control experiments were performed to validate the current suppression observation, especially critical due to observed corrosion in electrochemical functionalization step. The spin devices were found to be sensitive to light radiation, temperature and magnetic fields. Along with the study of molecular spin devices, several interesting ideas such as ~9% energy efficient ultrathin TaOx based photocell, simplified version of MEME fabrication, and chemical switching were realized. This doctoral study heralds a novel molecular spin device fabrication scheme; these molecular electrodes allow the reliable study of molecular components in molecular transport.

In this thesis, a combination of density functional theory (DFT) based calculations and nonequilibrium Green’s functions are employed to investigate electron transport in molecular switches, molecular cords and nanoscale devices.   Molecular electronic devices have been proposed as an approach to complement today’s silicon based electronic devices. However, engineering of such miniature devices and design of functional molecular components still present significant challenges.   First, the way to connect a molecule to conductive electrodes has to be controlled. We study, in a nanoelectrode-nanoparticle platform, how structural changes affect the measured conductance and how current fluctuations due to these structural changes can be decreased. We find that, for reproducible measurements, it is important to have the molecules chemically bonded to the surfaces of adjacent nanoparticles. Furthermore, we show by a combination of DFT and theoretical modeling that we can identify signals from single-molecules in inelastic electron spectroscopy measurements on these devices.   Second, active elements based on molecules, some examples being switches, rectifiers or memory devices, have to be designed. We study molecular conductance switches that can be operated by light and/or temperature. By tuning the substituents on the molecules, we can optimize the shift of the most conducting molecular orbital and increase the effective coupling between the molecule and the electrodes when going from the OFF to the ON-state of the switches, giving high switching ratio (up to three orders of magnitude). We also study so called mechanoswitches that are activated by a mechanical force elongating the molecules, which means that these switches could operate as sensors.   Furthermore, we have studied two different classes of compounds that may function either as rigid molecular spacers with a well-defined conductance or as molecular cords. In both cases, we find that it is of great importance to match the conjugation of the anchoring groups with the molecular backbone for high conductance.   The last part of the thesis is devoted to another interesting semiconductor material, diamond. We have accurately calculated the band structure and effective masses for this material. Furthermore, these results have been used to calculate the Hall coefficient, the resistivity and the Seebeck coefficient.

Molecular-based quantum cellular automata (m-QCA), as an extension of quantum-dot QCAs, offer a novel alternative in which binary information can be encoded in the molecular charge configuration of a cell and propagated via nearest-neighbor Coulombic cell-cell interactions. Appropriate functionality of m-QCAs involves a complex relationship between quantum mechanical effects, such as electron transfer processes within the molecular building blocks, and electrostatic interactions between cells. In the first part of this document, the influence of structural distortions in single m-QCA is addressed within a minimal model using an diabatic-to-adiabatic transformation. Thus, it is shown that even small changes of the classical square geometry between driver and target cells, such as those induced by distance variations or shape distortions, can make cells respond to interactions in a far less symmetric fashion, modifying and potentially impairing the expected computational behavior of the m-QCA. The model has been further extended to consider time-dependent external electric fields in which a special emphasis is given to the profiles in which this external parameter can interact with the associated molecular complex. The results of the model have been validated by a direct comparison with first-principle calculations allowing to conclude the plausibility to induce the intra-molecular charge transfer process in a controllable manner via the interaction with the external electric field. The influence played by the electric field profile in the response of the molecular complex is also investigated. The results suggests a major role played by this variable in terms of the time length in which the intra-molecular charge transfer can be observed. In the second part, first-principle theoretical calculations of the self-assembly properties and electronic structure of Ferrocene-functionalized complexes have been carried out. Hence, five different molecular complexes which offer a potential playground to realistic implement the m-QCA paradigm have been investigated. The main emphasis is given to study the interaction between localized charge-carrier molecular states and the delocalized surface states. The results of these calculations demonstrate the possibility to obtain real systems in which intra-molecular charge localization can be combined with self-assembly scaffolding and absorbed on either Highly oriented pyrolytic graphite (HOPG) or metallic-surfaces. Finally, the validation of these findings is carried out via comparison with accesible experimental results and opening the gate to plausible strategies where the paradigm can be implemented.

O objetivo desta dissertação é estudar os efeitos estruturais e eletrônicos em fulerenos menores e C60 causados pela dopagem substitucional com boro e nitrogênio para aplicações em eletrônica molecular. Estudamos as propriedades eletrônicas e estruturais de possíveis retificadores moleculares formados por pares de fulerenos menores dopados com boro e nitrogênio. A molécula C@C59N foi estudada e suas propriedades estruturais e eletrônicas comparadas com as do endofulereno N@C60. No estudo da dopagem dos fulerenos utilizamos o método semiempírico Parametric Method 3 (PM3). Foram calculadas as geometrias de equilíbrio e os calores de formação, que serviram para investigar a estabilidade relativa dessas moléculas. Para cada dopante identificamos os sítios de substituição que mais favorecem à estabilidade termodinâmica das moléculas. Dentre todos os fulerenos menores estudados os isômeros do C5o atingiram a maior estabilidade quando comparados com o C60. Com os pares de moléculas mais estáveis obtidas no trabalho anterior, montamos os retificadores em uma estrutura do tipo D-ponte-A, onde D e A representam doador e aceitador de elétrons. Para as moléculas isoladas, calculamos as estruturas eletrônicas através da Teoria do Funcional da Densidade (DFT) com o funcional BLYP e a base 6-31G*. No caso dos pares usamos o método DFT com o funcional BSLYP e a base 3-21G* para obter as geometrias de equilíbrio e as estruturas eletrônicas. Aplicando um campo elétrico sobre as moléculas, investigamos a facilidade de transferência de cargas entre fulerenos. Concluímos que fulerenos menores possuem um grande potencial para construção de um diodo molecular. As propriedades da molécula hipotética C@C59N foram comparadas com as bem Conhecidas C60, C59N e N@C60. A energia de ligação por átomo da molécula é comparável às energias de ligação dos outros fulerenos, em particular do seu isômero N@C60. Devido à tendência dos azafulerenos em formar dímeros, verificamos a estabilidade da molécula N@C60 quando comparada com o dímero N@C60 )2. . Tanto as geometrias quanto as estruturas eletrônicas foram calculados via DFT, BSLYP/6-31G*. Concluímos deste estudo que a molécula C@Ge¡/ é estável energeticamente, como também a interessante possibilidade do uso do dímero (C@C59N)2 como um bit quântico. / The present dissertation is devoted to the study of the effects on small fullerenes and 060 caused by the substitutional doping of boron and nitrogen for applications in molecular electronics. Electronic and structural properties of molecular rectifiers formed by small fullerenes doped with boron and nitrogen have been studied. The molecule C@C59 N has been investigated and its structural and electronic properties compared with those of the endofullerene N@C60 To study the doping of the fullerenes we used the semiempirical method Para­ metric Method 3 (PM3). Ground state conformations and heats of formation were obtained and used to investigate the relative molecular stability. We indentified the most favorable molecular substitution sites for the thermodynamic stability of each dopant. Among all small fullerenes investigated, the isomers of C50reached the largest stability when compared with 060 Molecular rectifiers with a structure of the type D-bridge-A, where D and A indicate electron donor and acceptor, respectively, were built with the most stable pairs found in the previous part of. The Density Functional Theory (DFT) with the functional BLYP and the base 6-31G* was used to calculate the electronic struc­ tures of the isolated molecules. Geometry optimizations and electronic structures of the pairs, were carried by DFT, B3LYP j3 21G*, method. The asymmetry of the charge transfer was assessed through the application of an externai electric field. We concluded that small fullerenes are promising candidates for the construction of molecular rectifiers. The properties of the hypothetical molecule C@C59 N were compared with those well known C60 , C59 N e N@C60 molecules. The binding energy of this molecule is comparable with that of the other fullerenes, in particular with that of its isomer N@C60 Due to the tendency of the azafullerene in forming dimers, the stability of the dimer (C@C59 N)2 was investigated. The molecular conformations and the electronic structures were obtained by the DFT, B3LYP/6-31G*, method. We con­ cluded that (C@C59 N) 2 molecule should be as stable as the azafullerene dimer. Our results point to the interesting possibility of using this system as a quantum bit.

Xue Kun. / "July 2005." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract in English and Chinese.