Growth of Acetone Molecular Lines on the Si(001)(2×1)-H Surface: First-Principle Calculations Marco Gallo,* ,† Edgar Martínez-Guerra,* ,‡ and Jairo A. Rodríguez § † Facultad de Ciencias Químicas, UASLP, Av. Manuel Nava No. 6, Zona Universitaria San Luis Potosí, SLP 78210, Me ́ xico ‡ Facultad de Ciencias Físico-Matema ́ ticas, UANL, San Nicola ́ s de los Garza, N.L. 66451, Mé xico § Departamento de Física, Universidad Nacional de Colombia, Bogota ́ D.C. 5997, Colombia ABSTRACT: Recent experimental work has shown that addition of acetone molecules to hydrogen-terminated Si(001) surfaces leads to the formation of one-dimensional molecular structures through a chain reaction mechanism. These structures are observed experimentally to be parallel to dimer rows on the Si(001)(2×1)-H surface. Using periodic density functional theory calculations, we have studied the initial steps of the radical chain mechanism of these reactions, and we have determined whether (or not) perpendicular growth could be possible. Our results show that, while the calculated difference of 0.03 eV between parallel and perpendicular attachment may not be enough to exclude growth between rows on Si(001)(2×1)-H at room temperature, the growth between dimers rows is kinetically less favorable because of the additional energy barrier associated with the hydrogen diffusion to the adjacent dangling bond on the same Si dimer (intradimer hopping motion), a necessary step to avoid steric repulsion between close proximity adsorbed acetone molecules. 1. INTRODUCTION Currently, there is a great interest in the chemical modification of silicon surfaces by chemisorbing organic molecules to create a new class of electronic devices that keep the well-known properties of silicon and add the useful properties of organic molecules as light emission and absorption. When exploring this concept, the main goal is to develop a new kind of hybrid molecular devices for applications in molecular electronics and sensors. However, it is clear that a previous understanding of the silicon organic chemistry is essential for the assembling of these hybrid molecular devices in different functional patterns into complex circuits. 1,2 One of the requirements for these hybrid molecular devices is the stability of the organic molecules against a range of voltages and currents, which have been found to induce fragmentation or desorption. In this regard, the strength and stability of the Si-O bond opens the possibility for molecules containing the carbonyl CO group (acetone) to be good candidates for attachment to the Si surface, in this case for use in device applications where the transport of charge through the molecule is important. 3 Nowadays, it is well-known that the Si-Si dimers on the surface present nucleophilic and electrophilic reactivities with their up and down dimer ends, respectively. Thus, different reaction patterns are possible when the surface is exposed to organic molecules susceptible to these two types of reactivities. Molecules such as alcohols, 4-7 amines, 8,9 and phosphines 10 react preferably with down-buckled silicon dimers, while molecules like borane 11 are bonded to up-buckled dimer ends through a lone pair orbital of the surface. From an organic domain, acetone is attractive due to its high dielectric constant and its ability to separate ionic charges fairly well. This organic molecule is itself an ambiphilic reactant. However, because the Si(001) surface presents ambipolar character, many structures can be formed on it. Energetically it is not possible to induce growth of acetone molecular lines on this surface. However, chain reactions of some organic molecules initiated by a dangling bond site on the H-terminated Si(001) surfaces have emerged as one of the most promising approaches to developing a molecular line connecting two points. 12 Until now, many studies have shown molecular line growth with different organic molecules (alkenes, alkynes, and aldehydes) on the Si(001)(2×1) surface through chain reactions based on a hydrosilylation process. 12-17 The chemistry for this class of reactions is quite clear now. A parallel-row chain reaction starts on a dangling bond site where the organic molecule through a C-C or C-O bond binds to the surface, creating a Si-C or a Si-O bond and an organic group with a carbon center radical (metastable state). Next, the intermediate C-centered reactive radical abstracts a hydrogen atom from the nearest Si dimer in the same row to generate a more stable structure where the molecule is chemically Received: March 17, 2012 Revised: August 15, 2012 Published: August 15, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 20292 dx.doi.org/10.1021/jp3025914 | J. Phys. Chem. C 2012, 116, 20292-20299