Revelation of graphene-Au for direct write deposition and characterization
© Bhandari et al; licensee Springer. 2011
Received: 9 December 2010
Accepted: 15 June 2011
Published: 15 June 2011
Graphene nanosheets were prepared using a modified Hummer's method, and Au-graphene nanocomposites were fabricated by in situ reduction of a gold salt. The as-produced graphene was characterized by X-ray photoelectron spectroscopy, ultraviolet-visible spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy (HR-TEM). In particular, the HR-TEM demonstrated the layered crystallites of graphene with fringe spacing of about 0.32 nm in individual sheets and the ultrafine facetted structure of about 20 to 50 nm of Au particles in graphene composite. Scanning helium ion microscopy (HIM) technique was employed to demonstrate direct write deposition on graphene by lettering with gaps down to 7 nm within the chamber of the microscope. Bare graphene and graphene-gold nanocomposites were further characterized in terms of their composition and optical and electrical properties.
Graphene, structurally known as a monatomic layer of allotropic-carbon atoms in a hexagonal honeycomb two-dimensional lattice system, has always been a potential candidate for various applications due to its remarkable structural, physical, and electronic properties [1–9]. The zero density of state at the Fermi level without an energy gap offered by graphene, and a linear, rather than parabolic, energy dispersion around the Fermi level has been well understood in the past. The material has also been investigated in a combination with other carbon structures to yield composites with superior properties [10, 11].
The composites of metal nanoparticles on graphene sheets also provide a new way to develop catalytic, magnetic, and optoelectronic materials. Moreover, adhesion of such metal nanoparticles to the graphene prevents their aggregation in dry state . Recently, Kamat et al.  have used solution-based approach of chemical reduction of AuCl4 - ions in graphene suspensions to fabricate gold (Au) nanoparticles-graphene hybrid assemblies. In yet another report, Goncalves et al.  demonstrated how presence of oxygen functionalities at the graphene surface provides reactive sites for the nucleation and growth of Au nanoparticles (AuNPs). These graphene/Au nanocomposites act as potential substrates for surface-enhanced Raman scattering. Min et al.  have also used a surface-chemistry-based approach for investigating the influence of surface functionalization on the growth of Au nanostructures on graphene thin films by utilizing various pyrene derivatives containing different functional groups.
But in comparison to these reports, the work presented here highlights a simpler route to obtain stable Au nanoparticles-graphene nanocomposites. It also demonstrates the capability of direct labeling on nanocomposite by use of scanning helium ion microscopy (HIM).
The demonstration of imaging by helium (He) ions is relatively a new technique to characterize the surfaces at sub-nanoscale with extraordinary additional advantages of in situ ion lithography, nano-patterning, device prototyping, fabrication of quantum dots, beam-induced chemistry, and milling at nanoscale [15, 16]. Such a diverse usage is possible due to the light mass of the He ion and high speed, which results in smaller interaction volume with the surface layers and therefore in better resolution and potential milling feature size. From the perspective of sputtering and patterning, the result is a reduced proximity effect in the surface layer. The light ion mass results in low energy transfer and hence a relatively lower sputtering yield compared to gallium. Exploiting the method of nano-patterning of graphene with helium ions leads considerable promise for a number of applications in nanoscale electronics, optoelectronics, and mechanics. It has been emphasized [17–22] that in an application like high-speed field-effect transistors, there is a strong need for graphene to be patterned at the nanoscale. Patterned graphene can form complex extended geomenies and can be readily contacted electrically, yielding a well-controlled connection between microscale and nanoscale systems and devices.
Hydrogen tetrachloroaurate (HAuCl4) was purchased from Aldrich (St. Louis, MO, USA). Sodium borohydride (NaBH4) was acquired from Merck (Darmstadt, Germany). Inorganic transparent electrodes of SnO2:F-coated glass (Pilkington, sheet resistance of 14 Ω/sq) were cleaned in a soap solution, 30% HCl solution, double-distilled water, acetone, and trichloroethylene (in that order) prior to use. Deionized water (resistivity ≈ 18.2 MΩ cm) obtained through Milli-Q system, nitric acid (HNO3) (Merck), sulfuric acid (H2SO4) (Merck), and toluene (Spectrochem, Hyderabad, India)were used as solvents.
Preparation of acid-functionalized graphene
For acid functionalization of graphene, a solution with H2SO4:HNO3 in a 3:1 volume ratio (12 ml H2SO4 and 4 ml HNO3) and 2 g graphite powder was made in a flask and refluxed at 40°C for 16 h. The resulting solution was washed with deionized water till the pH was reduced to 5 or 6. As a result, a black colored solution of acid-functionalized graphene was obtained.
Preparation of Au-graphene nanostructures
To fabricate Au-graphene nanostructures, Au nanoparticles were synthesized in situ in graphene suspension by the reduction of gold(III) complex by NaBH4. A concentrated aqueous solution of 0.4 M NaBH4 was first mixed with acid-functionalized graphene suspension in toluene. With continuous stirring, 30 mM of HAuCl4 was then introduced into this suspension. After continuously stirred for 1 h, the resulting Au-graphene composites were collected by centrifugation and washed with water for three times.
Fourier transform infrared spectra for the films were recorded in reflection mode with a Perkin Elmer GX2000 OPTICA spectrophotometer at 28°C, RH ≈ 50% to 53%. I-V measurements of films were carried out on Keithley 238 high-current electrometer characterization system. Absorbance (A) spectra were recorded in the 200- to 800-nm wavelength range in a Perkin Elmer Lambda 25 spectrophotometer (Perkin Elmer, Ferdinand-Porsche-ring, Rodgau, Germany). X-ray photoelectron spectroscopy (XPS) spectra were recorded for the as-synthesized graphene samples using a Perkin Elmer 1257 model PHI, Maple Grove, Minnesota, 55311 U.S.A operating at a base pressure of 3.8 × 10-8 Torr at 300 K with a non-monochromatized AlKα line at 1,486.6 eV, an analyzer pass energy of 60 eV kept for core level spectra and a hemispherical sector analyzer capable of 25-meV resolution. The overall instrumental resolution was about 0.3 eV. The core level spectra were deconvoluted using a non-linear iterative least squares Gaussian fitting procedure. For all fitting doublets, the FWHMs were fixed accordingly.
Surface morphology of the graphene sheets was studied employing a variable pressure scanning electron microscopy (SEM), model: Zeiss EVO MA10 Carl Zeiss SMT AG, Germany. Nanostructural imaging at high magnifications was carried out using HR-TEM model: FEI-Tecnai G2 F 30 STWIN FEI, Achtseweg Noord 5 5651 GG Eindhoven, Netherlands (operated at the electron accelerating voltage of 300 kV). HR-TEM specimens were prepared by dispersing the graphene films on copper grid of 3.05 mm in diameter having a 200-mesh pore size. Further, the surface topography of graphene and graphene-Au composite films was analyzed by HIM (model: Zeiss ORION Carl Zeiss, NTS Corporation Way, Peabody MA 01960, U.S.A.). The He ion capability of the microscope was used to perform the experiments of nanoscale patterning on the surfaces of graphene.
Results and discussion
UV-Vis spectral response
X-ray photoelectron spectroscopy
Deconvoluted contributions of various core level spectra present in Au-graphene nanocomposite
Binding energy (eV)
Peak area (%)
C1s (FWHM = 1.51eV)
C-C in graphene
C-OH in graphene
C-O-C in graphene
C=O in graphene
C(O)O in graphene
O1s (FWHM = 1.22eV)
C(O)O in graphene
C=O in graphene
C-O-Au of composite
C=O in graphene
C-OH in graphene
Au4f (FWHM= 2.03eV)
Au4f (of Au0)
Au4f (of Au+)
Microstructural features induced during synthesis
Nano-patterning on the helium ion microscopy
A simple modified Hummer's method was used to fabricate graphene and graphene-Au nanocomposites. A significant change in I-V characteristics between bare graphene and its Au incorporated nanocomposites has been noticed. An important in situ direct write deposition on nanosheets of graphene has been demonstrated by employing He ions inside the chamber of the microscope.
The authors thank the Director, NPL, New Delhi for his guidance and encouragement. One of the authors (AKS) acknowledges the CSIR travel grant to visit USA in February 2010. M/S Carl Zeiss NTS (USA) is gratefully acknowledged for extending the facility of helium ion microscopy to carry out the experiments of nano-patterning. Mr. K. N. Sood is acknowledged for SEM measurements.
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