Abstract
We report a scanning tunneling microscopy and noncontact atomic force microscopy study of close-packed 2D islands of tetrafluorotetracyanoquinodimethane (F4TCNQ) molecules at the surface of a graphene layer supported by boron nitride. While F4TCNQ molecules are known to form cohesive 3D solids, the intermolecular interactions that are attractive for F4TCNQ in 3D are repulsive in 2D. Our experimental observation of cohesive molecular behavior for F4TCNQ on graphene is thus unexpected. This self-assembly behavior can be explained by a novel solid formation mechanism that occurs when charged molecules are placed in a poorly screened environment. As negatively charged molecules coalesce, the local work function increases, causing electrons to flow into the coalescing molecular island and increase its cohesive binding energy.
Highlights
IntroductionSurface functionalization via molecular self-assembly is a potentially useful technique for tuning the properties of graphene layers.1À3 Previous studies have shown that functionalization via tetrafluorotetracyanoquinodimethane (F4TCNQ) adsorbates is effective at p-doping graphene4À7 (as well as other substrates8À13) through the transfer of electrons from the substrate to the adsorbed molecules
Surface functionalization via molecular self-assembly is a potentially useful technique for tuning the properties of graphene layers.1À3 Previous studies have shown that functionalization via tetrafluorotetracyanoquinodimethane (F4TCNQ) adsorbates is effective at p-doping graphene4À7 through the transfer of electrons from the substrate to the adsorbed molecules
We report a combined scanning tunneling microscopy (STM), noncontact atomic force microscopy, and theoretical investigation into the selfassembly characteristics of F4TCNQ molecules on a graphene substrate supported by insulating hexagonal boron nitride (BN)
Summary
Surface functionalization via molecular self-assembly is a potentially useful technique for tuning the properties of graphene layers.1À3 Previous studies have shown that functionalization via tetrafluorotetracyanoquinodimethane (F4TCNQ) adsorbates is effective at p-doping graphene4À7 (as well as other substrates8À13) through the transfer of electrons from the substrate to the adsorbed molecules. We report a combined STM, noncontact atomic force microscopy (nc-AFM), and theoretical investigation into the selfassembly characteristics of F4TCNQ molecules on a graphene substrate supported by insulating hexagonal boron nitride (BN) This provides a nearly ideal setting to study molecular self-assembly on isolated graphene. 12 ’ 12168–12173 ’ 2015 www.acsnano.org in a poorly screened environment as BN is a wideband-gap insulator that only weakly interacts with graphene.18À21 We observe that, unlike previous measurements of F4TCNQ on graphene supported by metals, F4TCNQ molecules on graphene/BN assemble into tightly packed 2D islands This unexpected result can be explained by a unique self-assembly mechanism that is based on heterogeneous lateral charge transfer driven by local work function differences
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