ICMCTF2014 Session TS4: Graphene and 2D Nanostructures

Two dimensional (2D) hexagonal boron nitride (h-BN) is a unique dielectric material that can be utilized in configurations with other two-dimensional materials for next generation electronic systems. 2D boron nitride possesses key advantages over other dielectric materials in graphene-based devices by providing an atomically smooth surface free of dangling bonds and minimal sites for absorbed surface impurities. Testing of h-BN in graphene devices has been limited to exfoliation techniques and the search for synthetic synthesis of 2D h-BN is continuing with CVD technologies being recently reported, where ammonia borane decompositions at 1000°C was used [1]. Alternatively, pulsed laser deposition (PLD) techniques have a potential to create few layer h-BN at lower temperatures than CVD methods by utilizing a high energy of the plasma plumes. In this work a pulsed laser deposition from BN targets in Ar/N₂ background gas was used to explore possibilities for the h-BN growth at 700°C substrate temperatures. Substrates of Si (111), Al₂O₃ (0001) and basal planes of pyrolitic graphite were used to impose a hexagonal template for h-BN film growth. Laser power, repetition rate, background pressure, and sample-target geometry arrangements were varied to optimize film stoichiometry and structure. X-ray photoelectron spectroscopy analysis had shown BN formation but also indicated nitrogen deficiency and oxygen presence in the films. This could be corrected by Ar/N₂ mixture adjustments and inducing higher plasma ionization at the substrate with biasing. Raman spectroscopy indicated a formation of h-BN. The study had pointed one major challenge in the 2D h-BN growth by PLD, where energetic laser plasma plumes create a competition between high adatom surface mobility needed for crystalline film growth and collision induced displacements and disordering of the grown films. Possible approaches to overcome this challenge are discussed.

[1] L. Song et al., Nano Letters 10 (2010) 3209

Graphene oxide holds promise as a carbon-based nanomaterial that can be produced inexpensively in large quantities. However, its structural and electrical properties remain far from those of the graphene sheets obtained by mechanical exfoliation or by chemical vapor deposition—unless efficient reduction methods that preserve the integrity of the parent carbon-network structure are found. Here, the authors use molecular dynamics and density functional theory calculations to show that the oxygen from the main functional groups present on graphene oxide sheets is removed by the reducing action of carbon monoxide; the energy barriers for reduction by CO are very small and easily overcome at low temperatures. Infrared and Raman spectroscopy experiments confirm the reduction in CO atmosphere and also reveal a strong tendency for CO to heal vacancies in the carbon network. Our results show that reduced graphene oxide with superior properties can be obtained through reduction in CO atmosphere.

Recent scanning transmission electron microscopy (STEM) observations [1 - 4] of metal-doped graphene have shown that the metal atoms bind exclusively to edge sites and contaminated regions, but not to the pristine regions of graphene. It was hypothesised from this that metal adatoms are very mobile on graphene at room temperature and therefore quickly migrate randomly across the lattice until they bind to more energetically-favourable edge sites by the time the samples reach the microscope. To test this hypothesis, we used density functional theory to optimise the structures of Al, Au and Cr atoms on the adsorption and edge sites of monolayer, bilayer and trilayer graphene and compared their energies and bonding characters. Then we calculated the migration energy barriers between the adsorption sites. It was found that Al, Au and Cr atoms form very weak bonds at the adsorption sites but form strong chemical bonds at the edge sites, and the migration activation barriers were all found to be very small: within an order of magnitude of kT at T = 300 K. These theoretical predictions are in striking agreement with the STEM observations. The implications of this are very broad. Much of nanotechnology relies on the passive manipulation physical matter on the microscopic level by means of harnessing naturally occurring processes under controlled conditions. Preferential edge-site binding is one such process which could be exploited in contexts such as patterned nanoscale devices, systematic edge-decoration of 2D nanoribbons and other nanoscale constructions where site-dependent bonding tendencies are an important ingredient in the fabrication process.

[1] R. Zan, Q. M. Ramasse, U. Bangert, K. S. Novoselov Nano Letters 12, 3936 (2012)

[2] R. Zan, U. Bangert, Q. M. Ramasse and K. S. Novoselov Nano Letters 11, 1087 (2011)

[3] R. Zan, U. Bangert, Q. M. Ramasse and K. S. Novoselov Small 7, 2868 (2011)

[4] Q. M. Ramasse, R. Zan, U. Bangert, D.W. Boukhvalov, Y-W. Son and K. S. Novoselov ACS Nano 6, 4063 (2012)

Asymmetric supercapacitor has attracted a lot of attention due to its high specific energy density and power density. Carbon nanomaterials such as activated carbon, carbon nanotubes, and graphene have been demonstrated high performance of supercapacitor. However, there is still a need to improve the energy density of the supercapacitor. Transition metal oxide is one of the way to get high energy density by wide potential windows and high specific capacitance. This study demonstrates an asymmetric supercapacitors using nitrogen-doped graphene (NDG) as negative electrode and SnO₂-rGO nanocomposite as positive electrode. In this experiment, we used facile and effective method to synthesize NDG and SnO₂-rGO nanocomposite, i.e. microwave-assisted hydrothermal, to obtain NDG with ethylene glycol as the reducing agent and ammonia as nitrogen source and to synthesize SnO₂-rGO nanocomposite. X-ray diffraction was used to investigate the crystal structure, X-ray photoelectron spectroscopy was used to observe the content of N-atom. Morphology and structural information were obtained by using scanning electron microscope and transmission electron microscopy analysis. Electrochemical measurement, such as cyclic voltammograms and galvanostatic were used to know performance of supercapacitor.

References:

1. D. Li, M.B. Muller, S. Gilje, R.B. Kaner and G.G. Wallace, “Processable aqueous dispersions of graphene nanosheets”, Nature Nanotech 3, 101 (2008).

2. V. Strong, S. Dubin, M. El-Kady and R.B. Kaner, “Patterning and electronic tuning of laser scribed graphene for flexible all-carbon devices”, ACS Nano 6, 1395 (2012).

3. M.F. El-Kady, V. Strong, S. Dubin and R.B. Kaner, “Laser printing of flexible graphene-based supercapacitors with ultrahigh power and energy densities”, Science 335, 1326 (2012).

4. M.F. El-Kady and R.B. Kaner, “Scalable fabrication of high-power graphene micro-supercapacitors for flexible and on-chip energy storage”, Nature Commun. 4, 1475 (2013).

In the initial period of graphene research, the issue was to identify and characterize crystallites of microscopic scale. High spatial resolution imaging ellipsometry is a nondestructive optical method in thin film metrology with a lateral resolution as small as 1 µm. In a number of papers, Imaging ellipsometry has been applied to characterize graphene flakes with a size of a few micrometers. Ellipsometric contrast micrographs, delta and Psi maps as well as wavelength spectra [1],[2] and single layer steps in multilayer graphene/graphite stacks [3] have been reported.

Molybdenum disulfide is a layered transition metal dichalcogenide. From the point of current research, 2D nanomaterials based on MoS₂ are very promising because of the special semiconducting properties. The bulk material has an indirect 1.2 eV electronic bandgap, but single layer MoS₂ has a direct 1.8 eV bandgap. The monolayer can be used in prospective electronic devices like transistors (MOSFETs) or photo detectors. Delta and Psi Spectra of MoS₂ monolayers as well as maps of the ellipsometric angles will be presented. The practical aspect of single layer identification will be addressed and the capability of ellipsometric contrast micrographs as a fast tool for single layer identification will be demonstrated.

An additional focus will be on the modeling of the optical properties of 2D nanomaterials.

[1] Wurstbauer et al., Appl. Phys. Lett. 97, 231901 (2010)

[2] Matkovic et al. J. Appl. Phys. 112, 123523 (2012)

[3] Albrektsen O. J. OF Appl. Phys. 111, 064305 (2012)

There is a robust research effect on graphene due to its unique properties. For example, graphene exhibits remarkable electrical properties including its ability to travel ballistically over submicron distances. This motivates the scientific community to incorporate graphene into electronic devices. However, this absence of a bandgap diminishes graphene’s utility in many devices. Theoretically, graphene’s charge carriers can be tuned continuously to values as high as 10¹³ cm^-2 and its mobility can be as high as on the order of 120,000 cm²/(V.s). While this entices researchers, there are difficulties due to graphene’s linear IV curve and the lack of control over its doping levels. This research seeks to use laser irradiation to modify CVD grown graphene’s properties. In the first instance, laser irradiation can be used to remove excess residue, thereby allowing graphene to better achieve its mobility potential. Nonetheless, one must remain cognizant that this process may cause differences in graphene’s structural properties that may alter its electrical characteristics. Using field effect transistor measurements and Hall Effect measurements, this research addresses these alterations. It was found that laser irradiation in vacuum causes increased carrier concentration and decreased mobility. Alternatively, annealing the sample in forming gas causes decreased carrier concentration and increased mobility. Hence, it is seen that the doping level of the graphene can be modified using laser irradiation and annealing in different environments.