Self-assembly Of Organic Molecules On HOPG Surface By Scanning Tunneling Microscope

Organic molecules can form different self-assemblies which dependents on molecularstructure, molecule-molecule interaction and molecule-substrate interaction. In thisdissertation, we systematically investigated the morphologies of a series of molecularself-assembled monolayers directed by changing the chemical structure, solutionconcentration and solvent by scanning tunneling microscopy (STM). A simple effectivemethod has been developed for the fabrication of molecular self-assembled monolayers withhigh thermal stability.Our research contents and main conclusions have been summarized as follows:1. The self-assembled structures of four bromobenzene derivatives with different numberof alkyl chains and substituted bromine atoms adsorbed on graphite surface were investigatedby STM. The bromine atoms are in bright contrast and can be identified in STM images. Forthe three molecules with one substituted bromine atom (BPB-15, BHB-16and BOB-18), thepolarity and the anti-parallel pattern of the neighboring phenyl head groups in one lamellagive rise to the electrostatic attractions. Odd-even effect on the self-assemblies of the threebromobenzene derivatives was observed due to the different lengths of alkyl chains. TheDDB-18molecules with two substituted bromine atoms adsorb on graphite surface in aparallel misalignment. A halogen-halogen interaction is formed between the bromine atoms ofthe neighboring DDB-18molecules. The STM images of the four bromobenzene derivativesobtained under positive surface bias resemble the calculated HOMO electron density contourssomewhat.2. The self-assembly structural transformation of4,4′-dihexadecyloxy-benzophenon(DHB) in different concentrations of phenyloctane was studied by STM. Experiments showthat with the concentration increasing until the phenyloctane solution completely volatilized,DHB molecules change from completely adsorption to half adsorption. The interactionbetween DHB and the graphite surface decreases during such transformation.3. The self-assembly structure of5,6-dioctyloxy-2,1,3-benzothiadiazole (BDT) inphenyloctane and n-octylic acid was observed by STM, respectively. The DBT moleculesformed the self-assembled monolayer with single component in phenyloctane. In n-octylicacid solution, DBT molecules coadsorbed with n-octylic acid by the hydrogen bonds.