Session U10: Focus Session: Surfaces and Interfaces in Electronic Materials II

Nanoscale Patterning of Electrochemically Deposited Metallic Features on Si, Ge, InP and GaAs Surfaces

Nanostructured materials continue to be the focus of intense research due to their promise of innumerable practical applications as well as advancing the fundamental understanding of these intriguing materials. In particular, the need for metallic features of increasingly smaller size regimes has imposed stringent demands upon chemists to produce a variety of highly functional materials with reduced dimensions.While much effort has been expended towards the synthesis of nanoscale structures, one of the most challenging aspects for the nanoscale materials community is the question of how to `wire in' these functional elements with the real world. In this talk, we will describe recent work towards the synthesis and nanoscale patterning of metallic structures on semiconductor surfaces such as silicon, germanium, gallium arsenide and indium phosphide. Through simple and efficient galvanic displacement reactions on these interfaces, complex metal nanostructures form spontaneously, and can be patterning via self-assembling soft block copolymer materials. The self-assembled materials direct transport of reagents to the semiconductor so that the reaction takes place in a spatially defined manner, with precise control over the quantity of reagent delivered. Even mixtures of reagents can be 'sorted out' by these interfaces to produce nanoscale ($\sim $10 nm) domains of different chemical functionalities, simultaneously. We will describe these and related approaches towards precise patterning of semiconductor surfaces, entirely via wet-chemical processes that are compatible with existing fabrication strategies.   

Scanning Tunneling Microscopy Study of Molecular Structure: Controlled Monolayer Formation on Graphite at the Liquid-solid Interface

The self-assembly of heptadecanoic acid \textbf{1} and racemic 2-bromoheptadecanoic acid \textbf{2} mixtures on the basal plane of a graphite surface has been studied using scanning tunneling microscopy at the liquid-solid interface. The domain structure varies as a function of the ratio of coadsorbed molecules. At lower concentration of acid \textbf{2}, heptadecanoic acid controls the surface structure by forming a template with fixed lamellar axis-molecular axis angle and domains with alternating R- and S-enantiomer molecular rows. Increasing the concentration of acid \textbf{2} leads to the segregation of chiral domains. The inter-correlation between heptadecanoic acid and 2-bromoheptadecanoic acid determines the 2D chiral configuration in the mixed monolayer. A model based on energetically favorable molecular conformations is proposed and will be discussed.   

Static and Dynamic Aspects of Surfactant Surface Aggregates studied by AFM

Using AFM, we show that surfactants form micellar aggregates of varying morphology, depending on the surface structure. While all previous studies were limited to atomically flat substrates, we achieve imaging the micelles on rough gold. By gradually annealing these surfaces, we show the influence of roughness on the aggregate structures. For crystalline gold (111), aligned, hemi-cylindrical micelles that recognize the symmetry axes of the gold lattice are found. With increasing roughness, the degree of organization of the aggregates decreases. We also show that the micellar pattern on HOPG and gold(111) surfaces changes with time and responds to perturbations in a self-healing way. Our results suggest that this organization happens at the molecular scale. Theoretical analysis for HOPG, however, show that the micelle orientation cannot be explained on the molecular level, but the anisotropic van der Waals interaction between micelles and HOPG has to be considered as well [1]. \newline \newline [1] Saville, D. A.; Chun, J.; Li, J.-L.; Schniepp, H. C.; Car, R.; Aksay, I. A., accepted by Physical Review Letters.   

Accelerated Molecular Dynamics Simulation of Alkane Desorption

Thermal desorption has been the focus of much surface science research. Studies of alkanes on graphite$^{1}$ and gold$^{2}$ have shown prefactors that are constant with alkane chain length but vary by over six orders of magnitude. Other studies on magnesium oxide$^{3}$ and gold$^{4}$ show a prefactor that increases with increasing chain length. We have developed an all-atom model to study alkane desorption from graphite. Transition state theory is used to obtain rate constants from the simulation. Accelerated MD is used to extend the desorption simulation to experimentally relevant temperatures. Our results show a prefactor that increases with increasing chain length. We predict that it will become constant as internal conformational changes occur significantly. We examine the effect of desorption environment through varying the alkane surface coverage. 1. K.R. Paserba and A.J. Gellman, \textit{J. Chem. Phys. }\textbf{115}, 6737 (2001). 2. S.M. Wetterer et al., \textit{J. Phys. Chem.} \textbf{102}, 9266 (1998). 3. S.L. Tait et al., \textit{J. Chem. Phys. }\textbf{122}, 164707 (2005). 4. K.A. Fichthorn and R.A. Miron, \textit{Phys. Rev. Lett. }\textbf{89}, 196103 (2002).   

Atomic force microscopy and fluorescence correlation spectroscopy studies of interfacial fluids.

We have studied the dynamic structure of thin ($\sim $ few nm) liquid films of a nearly spherical, nonpolar molecule tetrakis(2-ethylhexoxy)silane by using a combination of atomic force microscopy (AFM) and fluorescence correlation spectroscopy (FCS). Ultra-sensitive interferometer-based AFM was used to determine the stiffness (force gradient) and the damping coefficient of the liquid film. The experiments show oscillations in the damping coefficient with a period of $\sim $ 1 nm, which is consistent with the molecular dimension as well as previous x-ray reflectivity measurements. However, it fails to detect any stiffness oscillation, indicating that molecules are layered weakly near the solid-liquid interface. Additionally, we performed FCS experiments for direct determination of the molecular dynamics within the liquid film. From the fluctuation autocorrelation curve, we measure the translational diffusion of the probe molecule. The autocorrelation function cannot be fitted with a single diffusion coefficient indicating that the dynamics may vary in different layers.   

A Single Molecule View of Bi-stilbene Photoisomerization Using Scanning Tunneling Microscopy

The advent of scanning tunnelling microscopy (STM) has permitted a detailed atomic view of organic molecules adsorbed on solid surfaces. With the use of the STM, we present an unprecedented direct single-molecule perspective on the cis-trans photoisomerization of stilbene molecules within ordered-monolayers physisorbed on the Ag/Ge(111) surface. The STM view of the molecular structure transformation upon irradiation provides a direct evidence for the generally accepted one-bond-flip mechanism proposed for the photoisomerization process. We also find that the surface environment produces a profound effect on the reaction mechanism. The reaction is observed to proceed mainly through pairs of co-isomerizing molecules situated at domain boundaries. To explain these observations, we propose a mechanism whereby excitation migrates to the domain boundary and the reaction occurs through a biexciton reaction pathway.   

A combined theoretical and experimental study on the structure of Methylthiolates on the Au(111) surface

Self-assembled monolayers (SAMs) of sulphur containing organic molecules on gold have received enormous attention due to the central role these interfaces play in molecular electronic devices, biosensors, surface coatings and nanolithography. Despite their interest the atomic structure of Methylthiolates on Au(111) surfaces, the simplest SAM in this class, is not fully understood. Here we address this problem with a combined theoretical and experimental study. We show that an asymmetric bridge (quasi on-top) site fits both the X-Ray and Photoelectron Diffraction data better than either the symmetric bridge site or on-top site. To understand this phenomenon we have performed molecular dynamics simulations employing density functional theory within the generalized gradient approximation. We show that at high temperatures the presence of vacancies and gold adatoms tends to favour the quasi on-top site, in spite of the fact that the symmetric bridge site is the lowest energy site at $T=0$.   

Spectroscopic STM study of the binding configuration of benzene molecules on the Si(111)7x7 surface

We have used a home-built UHV STM to study the bonding configuration of benzene molecules chemisorbed onto clean silicon. Our compact STM head is based on the symmetrical, Besocke design. In our design, thermal drift is eliminated to first order, by using the correct combination of materials with known thermal expansion coefficients.$^{ 1}$ This STM head is also capable of being positioned inside of a liquid helium cryostat currently under construction in our lab. Our long-term goal is to use spectroscopic dI/dV imaging to focus on spatial variations of the LDOS and gain valuable information not generally available in topographic imaging.$^{2}$ In particular we will present room-temperature results on the role of restatoms in the binding of benzene molecules to adjacent adatoms. Results on the spatial position and direction of C-C double bonds will also be presented. 1)Stipe \textit{etal}, \textit{Rev. Sci. Instr}. \textbf{70}, 137 (1999). 2)Hamers \textit{etal}, \textit{Phys.Rev.Lett.} \textbf{56}, 1972 (1986).   

STM study of adsorption and dissociation of trichloroethylene molecules on the Si(111)7x7 surface.

We have performed, for the first time, STM studies of the adsorption of trichloroethylene (TCE) on clean silicon. The results were taken with a home-built UHV STM operating at room temperature. Our STM is capable of obtaining both topographic as well spectroscopic images. One of the products of the dissociation has been determined to be single chlorine atoms bound to Si adatoms as evidenced by topographic imaging and tip induced diffusion.$^{1}$ The other product of the dissociation is believed to be a vinyl group attached to an adatom as proposed in a study using EELS and TDS.$^{2}$ Results on the binding site preference (ex. corner vs. middle adatoms) for the Cl and vinyl group will also be presented as well as conclusions about the diffusion of Cl on Si. 1)Nakamura \textit{etal}, \textit{Surf. Sci}. \textbf{487}, 127 (2001). 2)He \textit{etal, Surf. Sci}. \textbf{583}, 179 (2005).   

Decompositional, incommensurate growth of Ferrocene molecules on a Au(111) surface

We have investigated in depth the first layer growth of ferrocene molecules on a Au(111) surface with a low temperature scanning tunneling microscope. Ferrocene molecules adsorb dissociatively and form a two layer structure after being decomposed into fragments. The toplayer unit cell is composed of two tilted cyclopentadienyl rings, while the first layer consists of the remaining fragments. Surprisingly a fourfold symmetry is observed for the top layer while the first layer displays threefold symmetry elements. It is this symmetry mismatch which induces an incommensurability between these layers in all except one surface direction. The toplayer is weakly bonded allowing for an antiferromagnetic ordering as shown by density functional theory calculations.