1 © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1. Introduction Isomerizable molecules produce precise dipole modula- tion and nanoscale mechanics, [1–7] which can be employed to build molecular valves, [2,8] molecular hydroswitches, [9] molecular cargo lifting, [10] and molecular shuttles. [11–13] Covalent Functionalization of Dipole-Modulating Molecules on Trilayer Graphene: An Avenue for Graphene-Interfaced Molecular Machines Phong Nguyen, Junwen Li, T. S. Sreeprasad, Kabeer Jasuja, Nihar Mohanty, Myles Ikenberry, Keith Hohn, Vivek B. Shenoy,* and Vikas Berry* Similarly, several biosystems leverage molecular mechanics and dipolar behavior. For example, flagella strands (2–100 nm) on the outer cell wall of bacteria rotate to propel bac- teria in solutions. Amongst mechanically actuating molecules, photoswitable (photoisomerizable) molecules are interesting since these systems can be switched optically. The mechanics of these molecules are characterized by modulation of the dipole moment, response at high frequencies, and the absence of heat production. An extensively studied photoswitchable molecule is azobenzene, which photoisomerizes between trans and cis states, where the two configurations assume different dipole moments ( trans = 0D; cis = 3D). There are several examples of electron-tunneling modulation through junctions with azobenzene; [14–17] however, studies on its “covalent interfacing” to apply dipole-induced potential to change the carrier properties of the interfaced substrate are limited. In 2011, Kim et al. [18] showed that azobenzene mol- ecules attached vertically on graphene via π-π interfacing DOI: 10.1002/smll.201300857 The molecular dipole moment plays a significant role in governing important phenomena like molecular interactions, molecular configuration, and charge transfer, which are important in several electronic, electrochemical, and optoelectronic systems. Here, the effect of the change in the dipole moment of a tethered molecule on the carrier properties of (functionalized) trilayer graphene—a stack of three layers of sp 2 -hybridized carbon atoms—is demonstrated. It is shown that, due to the high carrier confinement and large quantum capacitance, the trans-to- cis isomerisation of ‘covalently attached’ azobenzene molecules, with a change in dipole moment of 3D, leads to the generation of a high effective gating voltage. Consequently, 6 units of holes are produced per azobenzene molecule (hole density increases by 440 000 holes μm -2 ). Based on Raman and X-ray photoelectron spectroscopy data, a model is outlined for outer-layer, azobenzene-functionalized trilayer graphene with current modulation in the inner sp 2 matrix. Here, 0.097 V are applied by the isomerisation of the functionalized azobenzene. Further, the large measured quantum capacitance of 72.5 μF cm -2 justifies the large Dirac point in the heavily doped system. The mechanism defining the effect of dipole modulation of covalently tethered molecules on graphene will enable future sensors and molecular-machine interfaces with graphene. Molecular Machines P. Nguyen, Dr. T. S. Sreeprasad, K. Jasuja, N. Mohanty, M. Ikenberry, Prof. K. Hohn, Prof. V. Berry Department of Chemical Engineering Kansas State University, 66506, USA E-mail: vberry@k-state.edu J. Li, Prof. V. B. Shenoy Department of Materials Science and Engineering University of Pennsylvania, 19104, USA E-mail: vshenoy@seas.upenn.edu small 2013, DOI: 10.1002/smll.201300857