Correlating structural, mechanical, and charge transport properties of molecular monolayers and surfaces.

摘要

In this dissertation I present experimental studies of the correlation between the structural, mechanical, and electrical properties of organic monolayers, graphite, and GaAs using conductance-atomic force microscopy (C-AFM).;I studied three kinds of molecular monolayers. The first kind is alkanethiol self-assembled monolayers (SAMs), a saturated molecular film representing the "resistor" type element in electronics; the second kind is trans-stilbene based SAMs, a molecular system which has been proposed for molecular switches; the third kind is oligothiophene Langmuir-Blodgett (LB) monolayers, a prototype molecular system with conjugated groups which has shown potential for low cost optoelectronic devices.;In alkanethiol SAMs on Au(111), a correlation between charge transport and molecular tilt changes caused by pressure has been found. The junction I-V characteristics are sensitive to the load applied by the tip, which causes film thickness and current to change in a step-wise manner. We found that the tunneling decay constant beta through molecules of fixed chain length is significantly smaller than that for the case where the distance between electrodes is changed by using alkanethiol molecules of different lengths. We propose that changes in S-Au binding structure occur due to the steric forces acting between close packed molecules under pressure, which displace the S from hollow sites to bridge sites and possibly others. Such changes can produce much larger changes in the tunneling characteristics.;The second kind of monolayer I studied is o-(trans-4-stilbene)alkylthiol SSAMs on Au(111). Heating in a nitrogen gas atmosphere at 120° C for 1 h caused the morphology of the sample surface to change from a uniform molecular film consisting of small grains 10 ~ 20 nm in size to a heterogeneous surface consisting of well-packed and flat islands 100 ~ 200 nm in size and disordered structures. The flat islands exhibit substantial reduction (50%) in friction supporting the existence of good ordering. NEXAFS measurements reveal an almost upright molecular orientation for samples both before and after heating, with substantial increase of ordering after heating. Conductance-AFM measurements revealed more than 2 orders of magnitude enhancement in conductivity on the islands than the disordered phase on the heated samples. Conductance-AFM measurements on non-heated samples showed a very low level current ( 1 pA). We propose that the conductance enhancement is a result of a better pi-pi stacking between trans-stilbene units of neighboring molecules as a result of improved ordering in islands. This makes possible inter-molecule conduction in addition to the conduction along the molecules.;The third kind of monolayer I studied is (4-(5""-Decyl-[2,2';5',2";5",2"';5"',2""] penta-thiophene-5-yl)-butyric acid (C10-5TBA) LB monolayer. Structural, frictional and electrical properties of C10-5TBA LB monolayers were investigated using C-AFM. Two types of structures were found to coexist in the monolayer: the smooth, compact islands and porous structures. While high resolution AFM friction images reveal a crystalline nature of the compact islands, no lattice was resolved on the porous structures under the same imaging conditions. Although the topographic heights of these structures are almost identical, the friction of the crystalline island is significantly lower than that of the porous structures, and the current of the crystalline islands is one order of magnitude higher than that of the porous structures. A lateral inter-molecule conduction was proposed to explain the substantial enhancement in the conductivity of the crystalline islands.;Using a similar approach, I have also studied highly oriented pyrolytic graphite (HOPG), which can be viewed as a stack of high conjugated molecular layers. We found that: (1) the conductivity of HOPG has a dependency on the pressure exerted by the AFM tip, which causes the current density to change very little at pressures less than 1.5 GPa, and increases linearly at pressures greater than 1.5 GPa. (2) For an interesting suspended monolayer flake on graphite, the floating graphene contains two distinct regions C1 and C2. Region C1 is supported by the underlying layer B, while underneath the region of C2 layer B is missing. When an external load is applied, C2 deforms more than C1, and the height difference reaches a saturation value roughly equal to the spacing of a single graphene layer. The current density of C1 is approximately one order of magnitude greater than that of C2 at low loads. When the C1-C2 height difference decreases and finally reaches a saturation value as a result of increased loads, the current density of C2 becomes equal to that of C1. (3) A freshly cleaved HOPG surface exhibits regions with conductivity varying by orders of magnitude. The load dependence of the current on two neighboring terraces suggests that the interlayer interaction may play an important role. (Abstract shortened by UMI.)