2 D Islanding of Dodecane on an Au111

2 D Islanding of Dodecane on an Au(111) Surface: An Investigation Using He beam Reflectivity and Monte Carlo Modeling Timothy C. Arlen 1, Craig J. D. Webster 2, and Peter V. Schwartz 1 1 Physics Department, California Polytechnic State University, San Luis Obispo, CA, 93401, USA 2 Physics Department, Princeton University, Princeton, NJ 08544, USA Abstract Helium Reflectivity Data of Dodecane / Au(111) Dodecane is deposited at sub-monolayer coverages onto a Au(111) surface forming two-dimensional (2 D) islands. The islands sublimate Increased specularity indicates ordering of molecules into islands to a 2 D gas at higher substrate temperatures. We observe island formation and subsequent 2 D sublimation between substrate temperatures of 40 and 350 K, using low energy helium reflectivity. A computer model of the sub-monolayer islanding process using Monte Carlo simulations shows significant agreement with experimental data, and yields an intermolecular potential of 0. 10 0. 03 e. V, (about half that of the bulk substance) and a significantly higher corrugation potential of 0. 3 0. 1 e. V. Helium reflectivity records molecular islanding as an increase in specular reflectivity resulting from the overlapping of the “shadow region”. The shadow region is the effective loss of surface area in the area near the molecule due to long range attraction between the He atom and the molecule. Reflectivity vs. Temperature curves for six different coverages. Black dots indicate decreasing temperature. In two instances, the temperature was increased again (red dots). Reflectivity vs. Time curves immediately after a molecular dose of about 5% coverage onto a clean Au(111) surface. Crystal temperature is indicated by each curve. Monte Carlo Modeling Results The molecule’s “shadow” or “footprint” is about 9 times the molecular area, resulting in a reflectivity of about 50% for this surface. When the molecules are ordered, the overlap of the “shadow region” results in a reflectivity increase to about 75%. Theoretical Max T = 0. 167 7% T = 0. 208 T = 0. 250 We modeled thermal motion of the dodecane molecules and the helium reflectivity signal using a Monte Carlo model (below). 5% 3% T = 0. 125 T = 0. 292 T = 0. 333 Temperature Specularity vs. temperature curves generated by the computer model, at specified surface coverages. Temperatures are in theoretical units of (molecular interaction potential)/k. B Dodecane molecules (blue) in a hexagonal array of possible locations. Yellow sites represent sites that are shadowed (rendered unreflective) due to the long-range molecular attraction between the dodecane molecule and the incident helium atoms. Iterations Specularity vs. time (iterations per molecule) curves generated by the computer model for different temperatures. Temperatures are in theoretical units of (molecular interaction potential)/k. B We compared the experimental data with the model results. The location of the “knee” of the temperature vs. specularity allowed us to calculate an intermolecular potential of 0. 10 0. 03 e. V. Comparing the specularity recovery rates allowed us to infer a corrugation energy of 0. 3 0. 1 e. V of dodecane on the Au(111) surface. This work may be helpful in understanding the hierarchy of energies involved in the self-assembly process of organic molecules on metal surfaces which we found increase in strength for our subject accordingly: adsorbate-adsorbate, corrugation of adsorbate on substrate, adsorbate-substrate.