Article pubs.acs.org/JPCC
Atmospheric Pressure, Temperature-Induced Conversion of Organic Monolayers into Nanocrystalline Graphene Daniel Rhinow,† Nils-Eike Weber,‡ and Andrey Turchanin*,‡ †
Department of Structural Biology, Max-Planck-Institute of Biophysics, Max-von-Laue-Straße 3, D-60439 Frankfurt, Germany Physics of Supramolecular Structures and Surfaces, University of Bielefeld, Universitätsstraße 25, D-33615 Bielefeld, Germany
‡
ABSTRACT: Atomically thin free-standing nanomembranes belong to the emerging class of materials with a great promise for basic research of two-dimensional (2D) systems and applications in nanotechnology. However, their synthesis and characterization remain a frontier challenge in chemical and physical research. Here, we demonstrate that atmospheric pressure (Ar/H2) annealing in the range from 500 to 1000 °C of ∼1 nm thick carbon nanomembranes (CNMs) made of cross-linked aromatic self-assembled monolayers results in their conversion into free-standing sheets of covalently bonded, in-plane oriented graphene nanocrystals. Upon this transformation the electrical characteristics of CNMs evolve from insulating to conducting, which is accompanied by a change of the room temperature sheet resistivity by more than 5 orders of magnitude. We analyze this atmospheric pressure, temperature-induced transformation of CNMs employing various complementary spectroscopic and microscopic techniques, such as X-ray photoelectron spectroscopy, Raman spectroscopy, electron energy loss spectroscopy, optical microscopy, helium ion microscopy, atomic force microscopy, and transmission electron microscopy. In particular we studied the chemical, structural, and electronic properties of CNMs. We provide a comparative analysis of these data with the properties of pristine graphene, graphene oxide, and reduced graphene oxide sheets, which reveals both similarities and differences between these 2D materials. (Figure 1a). These ultrathin 2D carbon films can be separated from their original substrates and transferred onto new substrates8 as well as stacked into multilayer nanocomposites with precisely adjustable thickness, chemical, electrical, and optical properties.29 When transferred to transmission electron microscope (TEM) grids, CNMs with free-standing areas of more than 200 × 200 μm2 can be prepared.30 Vacuum annealing results in conversion of these molecular nanosheets into single/double layer graphene, composed of in-plane oriented covalently bonded graphene nanocrystals8,31 (Figure 1b). In this work we demonstrate that conversion of CNMs into graphene, previously achieved only in ultrahigh vacuum8 (UHV), is also possible by annealing at atmospheric pressure, which significantly simplifies the process. We analyze this temperature-induced transformation using various modes of transmission electron microscopy (TEM). In particular, we employ selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS). We discuss structural and electronic properties of the resulting free-standing nanocrystalline graphene sheets and compare them with the properties of free-standing sheets of single-crystalline graphene obtained by mechanical exfoliation, graphene oxide (GO), and reduced
1. INTRODUCTION Since the first reports on insulated sheets of graphene,1,2 twodimensional (2D) carbon materials like graphene,3,4 graphene oxide,5,6 carbon nanosheets,7,8 and ultrathin polymeric films9,10 have been the subject of intense research. The interest stems from their physical properties, which are related to the low dimensionality, making them promising materials for electronics,11,12 chemical or biological sensors and filters,13−16 nanocomposite materials,3,17 or as ultrathin supports for transmission electron microscopy of single atoms and biomolecules.18−20 However, reliable and scalable fabrication of 2D carbon materials remains challenging. Recent approaches, including chemical exfoliation of graphite,3,21 growth on metal surfaces,22,23 or SiC wafers24 can only partly meet the demand, because of difficulties arising from the reproducible production of free-standing homogeneous sheets with tunable physical and chemical properties, essential for many functional applications. In this respect, approaches based on molecular self-assembly provide a promising alternative route toward the generation of 2D carbon materials with tunable physical and chemical properties.8,25 It has been recently demonstrated that large homogeneous carbon nanomembranes8 (CNMs) can be generated by electron-irradiation induced cross-linking of aromatic self-assembled monolayers26,27 (SAMs). Using biphenyl SAMs as precursors, CNMs (organic nanomembranes28) with a thickness of only ∼1 nm are generated7,8 © 2012 American Chemical Society
Received: February 25, 2012 Revised: April 26, 2012 Published: May 14, 2012 12295
dx.doi.org/10.1021/jp301877p | J. Phys. Chem. C 2012, 116, 12295−12303
The Journal of Physical Chemistry C
Article
Figure 1. Schematic representation of the temperature-induced transformations of CNMs upon annealing. (a) A CNM consists of cross-linked biphenylthiol (BPT) molecules; yellow spheres indicate sulfur atoms. (b) Upon annealing at T > 500 °C the structure is converted into a network of covalently bonded, in-plane oriented graphene nanocrystals; sulfur atoms desorb.
silicon oxide8 or a TEM grid,20 and the PMMA was dissolved in acetone to yield a clean CNM. The cleanness of the CNMs after transfer was confirmed by X-ray photoelectron spectroscopy (XPS).8,13 X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded with an Omicron Multiprobe spectrometer using monochromatic Al Kα radiation. Binding energies were calibrated with respect to the substrate Au 4f7/2 signal at 84.0 eV; resolution of the spectra corresponds to 0.9 eV. The thickness was calculated assuming an exponential attenuation of the substrate Au 4f7/2 signal using a photoelectron attenuation length of 36 Å. Raman Spectroscopy. Raman spectra were acquired at an excitation wavelength of 532 nm employing the Thermo Fisher Scientific DXR and LabRam ARAMIS Raman microscopes. Optical, Atomic Force, and Helium Ion Microscopy. Optical microscopy was performed at an Olympus BX51 equipped with a C5060 digital camera; atomic force microscopy (AFM) was conducted with a NT-MDT Ntegra microscope in contact mode employing cantilevers by NT-MDT (Pt-coated, spring constant 0.1 N/m). Helium ion microscopy (HIM) was conducted with a Carl Zeiss Orion Plus. The helium ion beam was operated at 25 kV acceleration voltage. Secondary electrons were collected by an Everhart-Thornley detector. Working distances of 9−10 mm were used. Electrical Measurements. Electrical measurements were done by a standard four-probe setup using Suess probes and a Keithley source measure unit.8,29 Transmission Electron Microscopy. Selected area electron diffraction (SAED) of annealed CNMs, supported by TEM grids with lacey carbon films, was conducted using a Phillips CM120 microscope. Energy-filtered TEM analysis was performed using a phase contrast aberration-corrected electron microscope (PACEM), which is based on the Zeiss Libra 200 platform.32 For EELS analysis the monochromator was tuned to yield a fwhm of the zero-loss beam of 0.5 eV. The collection semiangle was 10 mrad. To record low-loss spectra of CNMs 10 single spectra (0.05 s acquisition time) of the area of interest were recorded and added. Up to 10 spectra taken from different areas were averaged to give the final low-loss spectrum. Coreloss EEL spectra of CNMs were recorded at the carbon K-edge using exposure times up to 80 s.
graphene oxide (RGO). TEM analysis is complemented by Xray photoelectron spectroscopy (XPS) and Raman spectroscopy, optical, helium ion, and atomic force microscopy, and electrical measurements.
2. EXPERIMENTAL METHODS Sample Preparation. CNMs were prepared from 1,1′biphenyl-4-thiol (BPT) SAMs on gold. To this end, BPT monolayers were self-assembled on 300 nm thermally evaporated gold on mica substrates (Georg Albert PVDCoatings). The substrates were cleaned in a UV/ozone-cleaner (FHR), rinsed with ethanol, and blown dry in a stream of nitrogen. BPT SAMs were grown for 72 h in a ∼1 mmol solution of BPT in dry, degassed dimethylformamide (DMF) in a sealed flask under nitrogen.26 The cross-linking of BPT SAMs (conversion into CNMs) was achieved in high vacuum (
