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Article

A Universal Scheme to Convert Aromatic Molecular Monolayers into Functional Carbon Nanomembranes

Polina Angelova, Henning Vieker, Nils-Eike Weber, Dan Matei, Oliver Reimer, Isabella Meier, Simon Kurasch, Johannes Biskupek, Dominik Lorbach, Katrin Wunderlich, Long Chen, Andreas

Terfort, Markus Klapper, Klaus Mullen, Ute Kaiser, Armin Gölzhäuser, and Andrey Turchanin ACS Nano, Just Accepted Manuscript • DOI: 10.1021/nn402652f • Publication Date (Web): 26 Jun 2013

Downloaded from http://pubs.acs.org on July 8, 2013

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A Universal Scheme to Convert Aromatic Molecular Monolayers into

Functional Carbon Nanomembranes

Polina Angelova1, Henning Vieker1, Nils-Eike Weber1, Dan Matei1, Oliver Reimer1,

Isabella Meier1, Simon Kurasch2, Johannes Biskupek2, Dominik Lorbach3,

Katrin Wunderlich3, Long Chen3, Andreas Terfort4, Markus Klapper3, Klaus Müllen3,

Ute Kaiser2, Armin Gölzhäuser1*, and Andrey Turchanin1

1 Faculty of Physics, University of Bielefeld, 33615 Bielefeld, Germany

2Central Facility of Electron Microscopy, University of Ulm, 89081 Ulm, Germany

3Max Planck Institute for Polymer Research, 55128 Mainz, Germany

4Institute of Inorganic and Analytical Chemistry, University of Frankfurt, 60438 Frankfurt, Germany

Keywords: two-dimensional materials, carbon nanomembranes, graphene,

molecular self-assembly, helium ion microscopy

e-mail: ag@uni-bielefeld.de

Tel.: +49-521-1065362

Fax: +49-521-1066002

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Abstract

Free-standing nanomembranes with molecular or atomic thickness are currently

explored for separation technologies, electronics and sensing. Their engineering with

well-defined structural and functional properties is a challenge for materials research.

Here we present a broadly applicable scheme to create mechanically stable carbon

nanomembranes (CNMs) with a thickness of ~0.5 to ~3 nm. Monolayers of

polyaromatic molecules (oligophenyls, hexaphenylbenzene and polycyclic aromatic

hydrocarbons) were assembled and exposed to electrons that crosslink them into

CNMs; subsequent pyrolysis converts the CNMs into graphene sheets. In this

transformation the thickness, porosity and surface functionality of the

nanomembranes are determined by the monolayers, and structural and functional

features are passed on from the molecules through their monolayers to the CNMs

and finally on to the graphene. Our procedure is scalable to large areas and allows

the engineering of ultrathin nanomembranes by controlling the composition and

structure of precursor molecules and their monolayers.

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The creation of functional two-dimensional (2D) materials is a fast growing field within

nanoscience. Nanomembranes,1 i.e. free-standing films with a thickness of a few

nanometers, have been predicted to possess a superior performance in the

separation of materials where they should allow a faster passage of the selected −

gas or liquid − molecules than any conventional filter.2 Nanomembranes were

integrated in stretchable electronics3 and have been applied as mechanical

resonators in nano-electro-mechanical systems;4 further, hybrids of nanomembranes

with fluid lipid bilayers were built as biomimetic interfaces for molecular recognition

and sensing.5

Diverse strategies have been developed to build nanomembranes. Since the 1990's

the so-called Layer-by-Layer (LbL) technique is employed to fabricate membranes for

corrosion protection, sensing and drug delivery.6-7 In the LbL process, an electrically

charged surface is sequentially dipped into positively and negatively charged

polyelectrolytes, leading to the formation of polymeric membranes of well-defined

molecular composition with thicknesses from ~15 nm to several 100 nm. Much

thinner membranes can be made by exfoliating single sheets out of a layered

material.8 Graphene, consisting of a few (~1- 5) layers of carbon atoms, was initially

made by exfoliation9 and is nowadays produced by a variety of techniques,10 allowing

researchers to gain new insights into physics and chemistry in two dimensions.

However, the surface of graphene is homogeneous and chemically inert, which

makes an efficient functionalization with other molecules difficult. On the contrary,

covalent organic frameworks11-12 (COFs) and related two-dimensional systems13-16

are nanomembranes with heterogeneous, chemically reactive surfaces. The first step

in their creation is the synthesis of organic molecules of well-defined size and shape

with functionalities at defined positions. When brought into proximity, adjacent

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molecules form multiple covalent bonds and assemble − like Wang tiles − in a 2D

lattice. COF nanomembranes look very aesthetic; however, their fabrication is time-

consuming and requires high skill in organic synthesis.

In this article, we present a modular and broadly applicable construction scheme to

efficiently fabricate ultrathin carbon nanomembranes (CNMs) that unite the thinness

of graphene with the chemical functionality of a COF and the ease of fabrication of

LbL films. It was found earlier17 that self-assembled monolayers (SAMs) of 1,1’-

biphenyl-4-thiol (BPT) are laterally cross-linked by low-energy electrons and can be

removed from the surface forming 1 nm thick CNMs. When these CNMs are

pyrolized at ~1000K, they transform into graphene.18-19 Motivated by these findings,

we applied similar protocols to a variety of other polyaromatic molecules. Our

scheme (Fig. 1a-c) utilizes a sequence of (i) molecular monolayer assembly on a

solid surface, (ii) radiation induced two-dimensional crosslinking, and (iii) a lift-off of

the network of crosslinked molecules. The product is a free-standing CNM, whose

thickness, homogeneity, presence of pores and surface chemistry are determined by

the nature of the initial molecular monolayer. We investigated three types of thiol-

based precursors: non-fused oligophenyl derivatives (Fig. 1d: 1a-c) which possess

linear molecular backbones providing an improved structural ordering of the formed

SAMs; condensed polycyclic precursors like naphthalene (NPTH), anthracene

(ANTH) and pyrene (MP) mercapto derivatives (Fig. 1d: 2a-f) which are more rigid

and should result in a higher stability and an increased carbon density of the

monolayers; “bulky” molecules, like the noncondensed hexaphenylbenzene

derivative with a propeller-like structure (Fig. 1d: 3a) and extended disc-type

polycyclic aromatic hydrocarbons such as hexa-peri-benzocoronene (HBC)

derivatives (Fig. 1d: 3b-c).20-21 The former are specially functionalized with lo