Graphene and 2D Nanostructures

Tuesday, April 29, 2014 9:20 AM in Room Tiki

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9:20 AM TS4-5 Synthesis, Properties, and Application of Two-dimensional Nano-Yttrium Oxide
Xingliang He, Hong Liang (Texas A&M University, US)
Two-dimensional (2D) nanomaterials have attractive great attention due to their unique chemical, electrical, mechanical, and physical properties. In this research, 2D yttrium oxide (Y2O3) nanosheets (NS) was synthesized and characterized. Through in situ doping of copper, an interesting phase transformation from multiphase Y2O3 NS to single-phase cubic Y2O3 NS was observed. The multiphase Y2O3 NS-based metal-semiconductor (M-S) junction was found to be a Schottky barrier-based diode. The single-phase cubic Y2O3 NS was found to have negative resistance as an M-S junction. This was due to localized redox reaction/junction electronics. A case study was conducted to use the 2D Y2O3 NS as additives into a polishing slurry. Results showed that the global planarization was improved by 30%. This presentation discusses the structure-property of the new 2D Y2O3 NS and their potential applications for surface modification.
9:40 AM TS4-6 Toward Growth of Few Layer Hexagonal Boron Nitride via Pulsed Laser Deposition
Nicholas Glavin (Air Force Research Laboratory and Birck Nanotechnology Center, Purdue University, US); Michael Check, Michael Jespersen (University of Dayton Research Institute, US); Jamie Gengler (Spectral Energies, LLC, US); Timothy Fisher (School of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, US); Andrey Voevodin (Materials and Manufacturing Directorate, Air Force Research Laboratory, US)

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

10:00 AM TS4-7 Reduction and Healing of Graphene Oxyde in Carbon Monoxide Atmosphere
Cristian Ciobanu (Colorado School of Mines)

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.

10:40 AM TS4-9 Mobility and Preferential Edge-Site Binding of Metal Adatoms on Graphene
Trevor Hardcastle, Che Seabourne (University of Leeds, UK); Recep Zan (Manchester, UK); Rik Brydson (University of Leeds, UK); Uschi Bangert (Manchester, UK); Quentin Ramasse (SuperSTEM Laboratory, Daresbury, UK); Konstantin Novoselov (Manchester, UK); Andrew Scott (University of Leeds, UK)

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)

11:00 AM TS4-10 High Energy Density Asymmetric Supercapacitor Based on Nitrogen Doped Graphene
Fitri Sari, Jyh-Ming Ting (National Cheng Kung University, Taiwan)

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.

11:20 AM TS4-11 Graphene-based Supercapacitors
Richard Kaner, Lisa Wang, Jeeyoun Hwang, Sergey Dubin, Mengping Li, Haosen Wang (University of California, Los Angeles, US); Maher El-Kady (Cairo University, Egypt); Mir Mousavi (Tarbiat Modares University, Iran)
Graphene is the ultimate two-dimensional material consisting of a single layer of sp2 hybridized carbon. Chemical synthetic methods are needed in order to scale its synthesis for applications. Here we explore converting graphite into graphene oxide sheets, which readily disperse in water.1 Using a 780 nm laser in an inexpensive LightScribe dvd drive, we can convert graphene oxide into a form of graphene with both high surface area and high conductivity.2 This laser-scribed graphene can be patterned and used to make electronic devices such as sensors. When an electrolyte is combined with two pieces of laser-scribed graphene, a high performance supercapacitor is formed. These supercapacitors exhibit high power, good energy density and long cycle life.3 They can be combined in series to increase voltage or in parallel to increase capacitance. By patterning the graphite oxide deposited on a plastic substrate, flexible microsupercapacitors can be made. These microsupercapacitors exhibit very high power along with enhanced energy density.4

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).

12:00 PM TS4-13 Characterization of 2D Nanomaterials with Spectroscopic Imaging Ellipsometry
Peter Thiesen (Accurion GmbH, Germany); Gregory Hearn (Accurion Inc., US); Ursula Wurstbauer, Alexander Holleitner, Bastian Miller, Eric Parzinger (Technische Universität München, Germany); Ulrich Wurstbauer (Columbia University, US); Christian Roling (Technische Universität München, Germany)

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)

12:20 PM TS4-14 Effect of Laser Irradiation on Structural and Electrical Properties of CVD Grown Graphene
Kartik Ghosh, M Langhoff, Anagh Bhaumik (Missouri State University, US); W Mitchel (Air Force Research Laboratory, AFRL/RXA, WPAFB, US); G Tompa, N Sbrockey, E Gallo, T Salagaj (Structured Materials Industries Inc., US)

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.