ICMCTF2015 Session TS4: Graphene and 2D Nanostructures

The use of graphene in electrochemical and gas sensors is reported. Graphene was first obtained using a chemical vapor deposition technique. In a typical process, TiO₂, Pt, or Ni was deposited on copper foil substrates to produce different porous graphene deposits. The resulting graphene also has a controllable number of layers, which is critical for use in electrochemical sensor. The obtained graphene was characterized using atomic force microscopy, scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. Selected graphene was then fabricated into sensor to detect Na₂S₂O₄. Cyclic voltammetry and differential pulse voltammetry test were carried out to examine the sensor performance. The effects of the morphology, the exposed graphene edges on the selectivity, sensitivity, and recovery properties of the sensor are addressed and discussed. The sensor performance was also correlated to these characteristics and improvement of the sensitivity was then achieved. We show that the graphene exhibits excellent response to NO₂ gas and a food additive molecule of Na₂S₂O₄ at room temperature.

Physical vapor deposition (PVD) is an expensive and complicated way to process materials, however, the exquisite control of structure and composition and freedom from many thermodynamic constraints make it worthwhile. This is evidenced by the common use of sputter deposition in a significant fraction of the total fabrication steps required to produce ubiquitous commercial electronic devices. By employing well-known theories of film growth and making the necessary conditions a reality through the use of surface engineering technology, thin film microstructures become very tailorable via PVD. This is of particular importance in the field of two dimensional (2D) materials, where processing of continuous and uniform films with thicknesses on the order of 1 nm is currently a major challenge. Transition metal dichalcogenides (TMDs), such as MoS₂ are currently under extensive study as high-performance 2D semiconductors. Using PVD techniques, which are easy to integrate into existing semiconductor device fabrication processes, 2D TMDs can be grown on diverse substrates including SiO₂, graphene, metals, and polymers. This is surprising as it is more common for such thin films to form isolated islands on substrates. Using a variety of in situ and conventional ex situ materials characterization tools such as Raman spectroscopy and X-ray photoelectron spectroscopy, the mechanisms governing continuous growth of TMDs have been examined for all of the substrates listed above. These substrates possess a broad range of surface energies. It appears that under some conditions a continuous metal monolayer is formed initially on higher surface energy substrates, and normal TMD growth continues on that interfacial metal layer. This is typically observed at high temperature where preferential desorption of the chalcogen atoms during growth is enhanced. Initial studies also suggest that growth mechanisms for Mo-based (MoS₂ and MoSe₂) TMDs are different than that which occurs for analogous W-based compounds (WS₂ and WSe₂). A comprehensive model allowing prediction of other TMD microstructures is put forward based on the experimental studies. This fundamental understanding will be used to show how the crystalline domain size within 1-5 nm thick TMD films can be manipulated during PVD growth over at least one order of magnitude.

Unique electronic properties of graphene originate from its unusual linear Dirac-cone dispersion. Phonons – quanta of lattice vibrations – in two-dimensional crystals also reveal features different from those in bulk materials. We discovered that the phonon thermal conductivity of suspended graphene can be exceptionally high – above ~2000 W/mK at room temperature – exceeding that of the basal graphite planes [1]. We explained this fact by quenching of the phonon scattering processes in two-dimensional systems and resulting anomalously long mean free path of the low-frequency acoustic phonons in graphene [2-3]. In this talk, I will review results of our studies of thermal properties of graphene and describe practical applications of graphene-based materials as thermal coatings for heat removal from advanced electronics. Specific examples of thermal coating applications will include graphene and few-layer graphene heat spreaders for high-power-density GaN electronics [4], graphene-enhanced thermal interface materials [5], graphene – copper heterostructures and prototype interconnects [6] and graphene laminate coatings on flexible substrates [7]. Our results suggest that thermal coatings and thermal management materials can become the first major commercial applications of graphene.

[1] A.A. Balandin, et al., Nano Lett., 8, 902 (2008); [2] S. Ghosh, et al., Nature Mat., 9, 555 (2010); [3] A.A. Balandin, Nature Mat., 10, 569 (2011); [4] Z. Yan, et al., Nature Comm., 3, 827 (2012); [5] K.F. Shahil and A.A. Balandin, Nano Lett., 12, 861 (2012); [6] P. Goli, H. Ning, K.S. Novoselov and A.A. Balandin, Nano Lett., 14, 1497 (2014); H. Malekpour, K.-H. Chang, D. L. Nika, K. S. Novoselov and A. A. Balandin, Nano Lett., 14, 5155 (2014).

Chemically or micro-mechanically exfoliated 2D graphene has been a subject of considerable research during the last decade due to its unique electrical, thermal and mechanical properties. Porous graphene-based nanostructures combine multiple advantages such as lightweight, large surface areas and pore volumes, as well as thermal and chemical stability. Therefore, these nanostructures have the potential to be used in a series of applications including gas storage/separation, water purification and as catalyst substrates. In the present work, a nanoporous (average pore width ~0.7 nm) graphene sponge-like structure with large-specific surface area (~350 m²/g) and high-oxygen content (~15 at.%) was synthesized by wet chemical reduction of graphene oxide in combination with freeze-drying. The surface morphology and elemental composition of the spongy graphene were studied by Scanning Electron Microscopy combined with Energy Dispersive X-ray Spectroscopy, while the respective structure was investigated by X-ray Diffraction analysis. Textural properties, including specific surface area (determined by the Brunauer-Emmet-Teller method), micropore volume and surface area (determined by the Carbon Black statistical thickness method), as well as pore size distribution (determined by the Quenched Solid Density Functional Theory method) were deduced from nitrogen gas adsorption/desorption data obtained at 77 K and pressures up to 1 bar. The obtained characterization results were used as a basis for the comparison of the spongy graphene with commercially available graphene nanoplatelets of high-crystallinity. In addition their potential for gas sorption applications was preliminary assessed by low-pressure carbon dioxide, methane and hydrogen sorption measurements.

Ni matrix reinforced with ceramic particles and fibers have been widely used in automobile and aerospace industries because of their excellent mechanical properties, wear resistance and corrosion resistance [1]. Graphene incorporated into the Ni matrix are expected to exhibit even higher hardness and strength, since the wear mechanism of graphene is proposed to be due to breakage of in-plane bonds between carbon atoms and shearing at the interface of graphene layers [2]

Nickel/graphene metal matrix composite coatings were deposited by pulse electro co-deposition method from a Watt's type electrolyte. The influence of the graphene concentration in the electrolyte on the particle co-deposition and distribution, the surface morphology, microstructure, microhardness, tribological features of nanocomposite coatings were studied. Microhardness of the composite coating was measured using a Vicker's microhardness indenter. SEM, EDS and XRD analysis were used to determine chemical composition and structure of composite coatings. The tribological behaviors of the electrodeposited nano composite coatings sliding against ceramic balls were examined on a CSM Instrument. The tribological behavior of the resultant composite coating was tested by a reciprocating ball-on disk method at constant load but varying sliding speeds for determination the wear loss and friction coefficient features against a counterface. All the friction and wear tests were performed without lubrication at room temperature and in the ambient air (with a relative humidity of 55–65 %). The change in wear mechanisms by changing graphene nanosheets content was also comprehensively studied. Special attention was given to the effect of sliding speed, graphene nanosheet content on the tribological performance of the nanocomposites.

References

[1] J. Xu, J. Tao, S. Jiang, Z. Xu, Applied Surface Science, 254 (2008), pp. 4036–4043

[2] W. Zhai, X. Shi, M. Wang, Z. Xu, J. Yao, S. Song, Y. Wang, Wear, 310 (2014) 33–40

Providing reliable coatings for rotating/sliding electrical contacts, suffering from mechanical wear is extremely important in various applications. Apart from graphene’s unique ability to drastically reduce friction and wear in mechanical systems, we have investigated its potential to serve as an excellent electrical conductor in sliding contacts. We have looked at gold surface rubbing against titanium nitride coated steel ball in an pin-on-disc macroscale tribometer while monitoring the electrical resistance of the gold/titanium nitride contact during the test. We have observed that in presence of few layer graphene, even under high contact pressures (up to 0.5 GPa contact pressure), both friction and wear significantly reduces by as much as 2-3 times and 2 orders of magnitude respectively both in humid and in dry environments. More importantly, the contact resistance stays low and stable (within 5% deviation) for longer period (4000 cycles). These results open a new niche for graphene films to be used as flexible, transparent, and wear resistant conducting electrodes where the atomically thin nature of graphene coupled with superior mechanical and electrical performance makes graphene very unique and fitting.

ACKNOWLEDGMENTS

Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Graphene oxide (GO) was reported to be bactericidal in saline whereas its activity in nutrient broth was controversial. By performing antibacterial assays under comparable conditions, we show that bare GO intrinsically kills both bacteria and mammalian cells but the non-covalent adsorption by biomolecules commonly found in biologically relevant environments on GO basal planes readily deactivates its activity. Similarly, graphene and reduced graphene oxide (rGO) have demonstrated bactericidal activity to varying extent and also readily adsorb a variety of substances onto their basal planes via non-covalent adsorption. Such a dilemma makes the use of graphene and graphene-derivitized forms for practical pathogen control a challenge. To address this, we exploit the photochemical/photothermal properties of rGO. Using a layer-by-layer thin film based on rGO, >90% airborne bacteria, including antibiotic-tolerant persisters, on contact were killed within 10 min upon solar irradiation. Further experiments suggest that solar light in the near-infrared region may play dominant roles in the observed activity. This work suggests that rGO’s photothermal/photodynamic property confers rGO-based surface coating with effective antibacterial activity upon minutes of solar irradiation and sun may be alternative to a laser for potent photothermal/photodynamic cytotoxicity at ultralow irradiance.