Article pubs.acs.org/Langmuir
Tailoring the Mechanics of Ultrathin Carbon Nanomembranes by Molecular Design Xianghui Zhang,* Christof Neumann, Polina Angelova, André Beyer, and Armin Gölzhaü ser Physics of Supramolecular Systems and Surfaces, University of Bielefeld, Bielefeld 33615, Germany S Supporting Information *
ABSTRACT: Freestanding carbon nanomembranes (CNMs) with a thickness between 0.6 and 1.7 nm were prepared from selfassembled monolayers (SAMs) of diverse polyaromatic precursors via low-energy electron-induced cross-linking. The mechanical properties of CNMs were investigated using AFM bulge test, where a pressure difference was applied to the membrane and the resulting deflection was measured by atomic force microscopy. We found a correlation between the rigidity of the precursor molecules and the macroscopic mechanical stiffness of CNMs. While CNMs from rigid and condensed precursors like naphthalene and pyrene thiols prove to exhibit higher Young’s moduli of 15−19 GPa, CNMs from nonfused oligophenyls possess lower Young’s moduli of ∼10 GPa. For CNMs from less densely packed SAMs, the presence of defects and nanopores plays an important role in determining their mechanical properties. The finite element method (FEM) was applied to examine the deformation profiles and simulate the pressure−deflection relationships.
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of 4−6 nm thick freestanding glassy carbon films.18 Point deflection using an atomic force microscope (AFM) probe was employed to determine the elastic modulus of nanoparticle monolayer, 11 graphene monolayer,19 and 10 nm thick amorphous carbon membranes.20 AFM was further employed to measure the biaxial creep compliance of ultrathin polymer films by nanobubble inflation.21−23 Bulge test using an AFM to measure the membrane’s deflection was also utilized to study mechanical properties of 1 nm thick CNMs,24 nanocrystalline graphene,25 graphene,26 ultrathin polymer carpet,27 and crosslinked gold nanoparticles.28 As the membrane thickness decreases to nanometers, surface effects should be considered and a thickness-dependent pressure−deflection in bulge tests was explored in theory.29 Self-assembly of a molecular monolayer at a solid surface is a complex process, and the understanding of underlying mechanisms determining the mechanical characteristics of molecular monolayers is very limited.30−32 It is known that formation of alkanethiol SAMs on gold induces a compressive surface stress whose magnitude depends on the surface structure of underlying substrate.33 Similarly, for aromatic SAMs a compressive surface stress is also expected, and the
INTRODUCTION Freestanding nanomembranes with nanoscale thickness are promising two-dimensional (2D) materials for nanotechnology.1,2 They can contribute to a variety of applications, such as separation of gases or macromolecules,3,4 sensors,5,6 fuel cells, and electrochemical capacitors.7 Several methods have been developed to fabricate different types of freestanding nanomembranes, such as organic synthesis,8 covalent self-assembly in solution,9 spin-assisted layer-by-layer (LBL) assembly,10 selfassembly of nanoparticle monolayers at the liquid/air interface,11 polymerization of organometallic monolayers at the liquid/air interface,12 selective etching of the sacrificial layer on which the functional layer is deposited,13,14 cross-linking of alkyne-containing self-assembled monolayers (SAMs) on solid surfaces,15 and cross-linking of aromatic SAMs.16 For the latter, it has been recently demonstrated that a variety of polyaromatic molecules could be used to fabricate freestanding carbon nanomembranes (CNMs) with a thicknesses between 0.5 and 3 nm as well as with and without nanopores.17 To investigate the mechanics of such ultrathin membranes, mechanical characterization methods, such as microtensile test, point deflection method, bubble inflation technique, and bulge test, must be adapted to the nanoscale by combining them with nanoanalytical tools. For example, microelectromechanical systems (MEMS) inside a scanning electron microscope (SEM) have been used to measure the uniaxial tensile stress © 2014 American Chemical Society
Received: May 20, 2014 Revised: June 17, 2014 Published: June 19, 2014 8221
dx.doi.org/10.1021/la501961d | Langmuir 2014, 30, 8221−8227
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Supporting Information). Pt-coated AFM probes with a spring constant of 0.1 N/m were used. Finite Element Analysis. The finite element method (FEM) is employed for the simulation of the pressure−deflection behavior of freestanding carbon nanomembranes. The FEM in this study is done with the structural mechanics of the MEMS module from the commercial software COMSOL Multiphysics (version 4.0). The nanomembrane was modeled as a homogeneous structure. The edges of the freestanding membrane were considered to be clamped edges, and the boundary conditions were fixed at membrane edges for zero degree of freedom. The material properties, such as the Young’s modulus, the initial stress, and the thickness, were taken from the experimental results. As boundary condition the four edges were fixed to zero. Because of the high aspect ratio, a swept mesh was chosen. The predefined mesh size was selected to extremely fine. A predefined pressure was applied to the bottom face, and the displacement of the membrane was simulated.
macroscopic mechanical properties of the resulting CNMs Young’s modulus, viscoelastic behavior, and the residual stressare closely related to the molecular motif and structural order of pristine SAMs and the cross-linking process. Despite a number of surface analytical investigations,34−36 details of the elementary steps of the cross-linking and molecular structures of CNMs are not yet fully known. Here we report the variation of the precursor molecules for the preparation of CNMs in order to tailor their mechanical properties. Imaging of freestanding CNMs was done using helium ion microscopy (HIM) and their intactness and dimensions was precisely evaluated. We use AFM bulge tests to determine Young’s moduli and residual stress of freestanding CNMs from different precursor molecules, and the deformation shape of a pressurized membrane is compared with the finite element method (FEM) analysis. In order to explore factors that determine the macroscopic mechanical properties of freestanding 2D materials, we consider the properties of their molecular building units, the structure and packing density of pristine SAMs and the steric hindrance encountered by molecules in the process of cross-linking.
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RESULTS AND DISCUSSION Figure 1A shows the aromatic molecules used as building units for the purpose of this study. They all have one or two surface
EXPERIMENTAL SECTION
Preparation of SAMs and Fabrication of Freestanding CNMs. The synthesis of precursor molecules used in this study was described elsewhere.17 For SAM preparation, 300 nm thermally evaporated gold on mica substrates (Georg Albert PVD-Coatings) were employed. The substrates were cleaned in a UV/ozone cleaner (UVOH 150 LAB FHR) for 3 min, immersed in absolute ethanol for 20 min and blown dry in a stream of nitrogen. The details of SAM preparation of each precursor molecule are presented in the Supporting Information. Each cross-linking of SAMs was achieved in high vacuum (
