The organic materials are envisioned to be used as building blocks for designing of functional devices to broaden and partially replace the current-used silicon-based devices in future. Due to easily processing and environmental friendly properties of organic materials, molecular electronics show a great promise to overcome some difficulties encountered in current-used silicon-based technology, for example, further miniaturization, mechanically folding, self-regeneration, self-repairing, to name a few. At current stage, this field is still in its infancy, and many challenges still remain. In particular, the experimental ability to address these materials at atomic level is strongly required in order to thoroughly understand their intrinsic properties. This thesis is dedicated to...[ Read more ]

The organic materials are envisioned to be used as building blocks for designing of functional devices to broaden and partially replace the current-used silicon-based devices in future. Due to easily processing and environmental friendly properties of organic materials, molecular electronics show a great promise to overcome some difficulties encountered in current-used silicon-based technology, for example, further miniaturization, mechanically folding, self-regeneration, self-repairing, to name a few. At current stage, this field is still in its infancy, and many challenges still remain. In particular, the experimental ability to address these materials at atomic level is strongly required in order to thoroughly understand their intrinsic properties. This thesis is dedicated to the electronic characterization of single molecules, conjugated polymers, and molecular nanostructures at sub-nanometer resolution by utilizing low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS). Within this thesis, I focused on a few representative molecular systems and measured their structural and electronic properties. The results are divided into three parts, which are (1) characterization of the single-molecular super-exchange coupling, capacitance, conductance and metal contact at sub-nanometer resolution, (2) fabrication of poly-p-phenylene oligomers utilizing on-surface Ullmann coupling reaction, and systematically characterization of their electronic properties including band structure, localized excitations and dopant states at sub-nanometer resolution, and (3) fabrication of different molecular nanostructures by using supramolecular self-assembly method and STM manipulation strategy, and investigation of their electronic structure in real space as well as in reciprocal space.

Part one contains three sections. (1) I probed the superexchange coupling at isolated molecules using a novel way. The results confirm, at the single-molecule level, the theoretically predicated exponential decay behavior of the superexchange coupling at sub-nanometer resolution. (2) I characterized the resonant tunneling properties of molecules that are strongly anchored at an electrode. Differential conductance spectroscopy indicates that electron transport occurs through a series of vibronic states of the ionized molecules owing to double-barrier junction tunneling. Using a simple multiple capacitor model, a capacitance of 0.9 aF (3.4 aF) for single (double) molecular group(s) has been derived. (3) I directly measured the single molecular conductance using STM manipulation. Two kinds of molecules on a dual-functional substrate are investigated. For both molecules, the highest conductance is given by molecular end(s) strongly coupled to the electrode(s). The large standard deviation among different measurements on a same molecule indicates that the molecule-tip contact also plays an important role in determining the molecular conductance. (4) I inspected the modulation of electronic states of individual extended conjugated molecules by multiple metal atom contacts. I found that a delocalized empty molecular state is modulated by these multiple contacts in a cumulative manner.

Part two contains four sections. (1) I ultilized the on-surface Ullmann coupling reaction to fabricate conjugated poly-p-phenylene oligomers. For the first time, I observed an intermediate state, demonstrating that the Cu adatoms not only act as catalysts but also form stable intermediate polymers. (2) I developed a method to spatially map the molecular orbitals of polymers; This method allowed one to understand the correlation of polymer structure and electronic states. The dependency on size (N=3n, where N is the number of phenyl rings) allowed us to indirectly obtain data on the band structure of very long chains, essentially the intrinsic data for a polymer. (3) I charaterized the polymers in a kinked, branched and closed topology. I observed the theoretical predicated localized states in brached polymers, and quantified their dependences on branched numbers. These results afford insight into the narrowing of band gap and the enhancement of conductivity of conjugated polymers with branched topology. (4) I investigated the dopant states of conjugated polymers. The data, for the first time, directly revealed (i) the characteristic spatial extension of the dopant states, (ii) a structural deformation of the molecular backbone, and (iii) a localized shallow level within the energy gap of the undoped parent oligomer. These results unambiguously confirmed several key predictions.

Part three contains three sections. (1) I improved the Fourier-transformed STS (FT-STS) technique, a technique for detecting the band structure of 2D materials, which can provide similar information as angle-resolved photoemission spectroscopy. (2) I fabricated three isostructural supramolcular self-assemblies and characterized their modulation of the shockley surface state. I found that three isostructural supramolecular architectures result in the 2D bands with tunable band characteristics of band bottom, bandwidth and bandgap. These results demonstrate that supramolecular structures provide an effective mean to create artificial electronic structures on surfaces. (3) I used STM manipulation to fabricate the molecular grapheme nanostructures by arranging molecular species into a triangular potential array on Cu(111). Using FT-STS technique, I resolved the band structure of the molecular graphene, which displays a massless quasi-particle spectrum. In the next step, I fabricated and characterized artificial aperiodical nanostructures: grapheme nanoribbons with perfect zigzag or armchair edges of different width, single vacancy, Stone-Waals point defects and pentagon-pair octagon defect lines. The site-specific local density of states (LDOS) and LDOS spatial maps associated with these aperiodical phases were probed by experimentally and simulated theoretically. The results unambiguously confirm the predicted spatial localization of the in-gap states associated with these aperiodical structures.

In summary, owing to the high resolution of LT-STM and STS to probe both geometric and electronic properties at the atomic level, several pertinent problems regarding the electronic structure of organic materials have been solved. This fundamental study might be important for putting organic materials into practical use as future electronic components.