Capturing complex reaction pathways step by step : organic molecules on the Si(001) surface

• Main Author: Rahnejat, K. A.
• Other Authors: Schofield, S. R.
• Published: University College London (University of London) 2015
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.746050

Experts expect that inherent limitations in semiconductor electronics will become apparent within the next couple of decades. Consequently, the construction of novel electronic-devices that surpass existing technologies in both miniaturisation and intrinsic functionality is becoming increasingly important. The assembly of nanoscale circuitries comprised of individual, synthetically-tailored molecules designed to substitute conventional electronic-components is a popular approach to tackling this challenge. The incorporation of functional molecules with existing silicon-based electronics is the most credible route towards realising this goal in the near-future. To this end, we present a scanning tunnelling microscopy (STM) study of acetophenone—an archetypal, small, aromatic molecule—adsorbed onto the Si(001) surface. An elevated imaging bias is used to induce sequential structural-transitions in hundreds of adsorbates simultaneously. Specifically developed analytical-software was used to catalogue and analyse the transition sequences that were recorded. These transitions described long, complex surface-bound reaction-pathways in a step-by-step manner. Density functional theory (DFT) analysis reveals that acetophenone adsorbates adopt a remarkable 24 distinct adsorbate structures. Moreover, the captured reaction-pathways are up to six discreet steps in length and belong to one of three separate branches determined upon adsorption. These transitions are governed by predictable pivoting motions about a strong O–Si bond formed with the substrate. Our fundamental studies have informed the direct manipulation of individual adsorbates using precisely targeted voltage-pulses affording the control of specific transitions. In addition, we explore changes in the adsorption behaviour of these molecules after minor chemical modification through substitution of function groups. Finally, we construct and study 2D systems of overlapping wavefunctions by manufacturing dangling bonds (DB) on Si(001):H.