How to Fabricate a Surface-Grafted Polythiophene on H‑Si(100)2×1 Surface via Self-Assembling and in Situ Surface Polymerization: A Theoretical Guide Xiaojing Yao, † Jinlan Wang,* ,†,‡ Gang Wu, § Jianwei Xu, ∥ and Shuo-Wang Yang* ,§ † Department of Physics, Southeast University, Nanjing 211189, China ‡ Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal University, Changsha 410081, China § Institute of High Performance Computing, Agency for Science, Technology and Research, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632, Republic of Singapore ∥ Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634, Republic of Singapore *S Supporting Information ABSTRACT: Based on density functional theory calculations, we have studied the self-assembled growth of thiophene substituted alkenes, [H 2 CCH-(CH 2 ) n -thiophene] on hydrogen-terminated H- Si(100)2×1 and H-Ge(100)2×1 surfaces into aligned one-dimensional (1D) molecular arrays which are chemically bonded to the surfaces via the alkane chain. The thiophene rings at the top end of the molecular arrays are situated side by side and can undergo an in situ polymerization reaction into polythiophene once radicals are introduced to the thiophene rings, thereby forming polyalkylthio- phene-Si/Ge(100)2×1 surface-grafted polymers. Like most of con- ductive polymers, these surface single polymer chains exhibit semiconducting character and can be made conductive either by p- doping or by applying an external electric field. More importantly, both surface-grafted polymers and substrates retain their electrical properties, and the polythiophene chains are the sole conductive channels in the structures. Our findings put forth a new way to fabricate conductive polymeric molecular wires on traditional semiconducting substrates, and could find potential application in nanoelectronic devices. 1. INTRODUCTION In the past decades, scientists and engineers have been struggling to fabricate single conductive molecular wires or conductive polymer chains on traditional semiconductor substrates in order to satisfy the ongoing miniaturization of electronic devices. Recently, using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM), 1,2 experiments have demonstrated that many olefins are able to self-assemble on a hydrogen-passivated H-Si(100)2×1 surface to form one- dimensional (1D) aligned molecular arrays. For example, Lopinski et al. have proven in their experiments that styrene can grow into 1D molecular array on a H-Si(100)2×1 surface. 3 Via ethenyl group, a styrene molecule bonds easily with a H- empty silicon site, created by atomic force microscopy (AFM) or scanning tunneling microscopy (STM) nanopatterning technologies. The adsorption breaks the ethylene C−C π- bond and generates a C-centered radical that can extract a H atom on the adjacent surface Si atom to form ethane, resulting in a new H-empty silicon site for subsequent styrene adsorption. As such radical chain reaction process, 3−7 a 1D surface absorbed phenyl ethane molecular array is formed eventually. Other examples like allyl mercaptan, 8 long chain alkene (C 8 −C 14 ), 9 benzaldehyde, 10 and phenylacetylene (PA) 11 have been demonstrated to self-assemble and form 1D molecular arrays on H-Si(100)2×1 surfaces. Following the same mechanism, a thiophene-substituted alkene, H 2 CCH-(CH 2 ) n -thiophene [Figure 1a], should be able to adsorb to a H-empty silicon site on either H- Si(100)2×1 or H-Ge(100)2×1 surfaces and further grow into a 1D thiophene-substituted alkane molecular array along the dimer row [011] direction (Figure 1). Based on density function theory (DFT) calculations, in this study we investigated the adsorption reaction paths for H 2 CCH- (CH 2 ) n -thiophene, including ethylene (n = 0), propylene (n = 1), butylene (n = 2), and amylene (n = 3) on H-Si/(100)2×1 and H-Ge/(100)2×1 surfaces. We demonstrate that it is very feasible for these H 2 CCH-(CH 2 ) n -thiophene molecules to adsorb and grow into 1D aligned molecular arrays along [011] direction because the molecular adsorption energies are much higher than activation energies for molecular array growth. Received: August 19, 2016 Revised: October 12, 2016 Published: October 13, 2016 Article pubs.acs.org/JPCC © 2016 American Chemical Society 25612 DOI: 10.1021/acs.jpcc.6b08389 J. Phys. Chem. C 2016, 120, 25612−25619