Article pubs.acs.org/JPCC
Graphene-Based Vibronic Devices Edson P. Bellido†,‡ and Jorge M. Seminario*,†,‡,§ †
Department of Chemical Engineering, ‡Materials Science and Engineering Graduate Program, and §Department of Electrical and Computer Engineering, Texas A&M University College Station, Texas 77843, United States ABSTRACT: Molecular dynamic simulations are used to model the vibrational bending modes of graphene ribbons of several sizes to calculate frequencies of the ribbons and determine the relationship between the size of the ribbon and their corresponding resonance frequencies. These ribbons can be utilized to fabricate several types of vibronic devices such as NEMS, sensors, terahertz generators, and devices for encoding, transferring, and processing information. The interaction of a graphene vibronic device with water and isopropyl alcohol molecules demonstrates that this device can be used as a very sensitive vibronic molecular sensor that is able to distinguish the chemical nature of the sensed molecule. The electrical properties of the graphene vibronic devices are also calculated for two cases, armchair and zigzag border. The zigzag border demonstrated in this work has the potential to generate THz electrical signals.
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to encode, transfer, and process information,18,19,21 extending and complementing several areas of molecular electronics.24−30
INTRODUCTION Graphene is a promising material for future technological applications in fields ranging from medicine1 to electronics2,3 because of its unique mechanical and electrical properties.4−10 Devices that are investigated and are ubiquitous in many fields today are nanoelectromechanical systems (NEMS).11−15 Device performance improves as the resonance frequency increases and the NEMS mass decreases. Researchers are working to develop smaller NEMS; however, the ultimate performance of these devices is still yet to be achieved. This ultimate performance can be achieved with the development of 2D and 1D NEMS, which are fabricated from carbon nanotubes and graphene, respectively. A characterization of large graphene resonators was done by Bunch et al.16 In their work, they measured amplitude versus frequency in a suspended graphene sheet and observed multiple resonances with the fundamental vibrational mode varying from 1 to 170 MHz. Oscillations were also observed that were not externally driven but were natural thermal oscillations intrinsic to the graphene sheets. It is also important to understand the role vibrations play in electronic and spin transport; vibrational states can be considered to be scattering points that in some cases can limit the operational temperature of spin devices.17 The use of intrinsic vibrational modes to transport information through molecules was demonstrated by Seminario et al.18−23 They develop the vibronics scenario using vibrational modes as carriers of terahertz (THz) signals. Therefore, signals modulate a higher vibrational frequency carrier corresponding to an intrinsic resonance frequency of a large linear molecule; the injected signal was recovered at another point using digital signal processing techniques. Therefore, the natural oscillations of small graphene ribbons can be utilized in several types of devices such as NEMS, which on very small scales can be considered to be vibronic devices. The demonstrated vibronic devices in this study can be utilized as sensors and THz generators and have the potential to become nanodevices able © 2012 American Chemical Society
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METHODOLOGY The graphene vibronic devices and their interaction with other molecules are modeled with classical molecular dynamics (MD) simulations using the LAMMPS31 program. The graphene structures are modeled using the Tersoff potential32−35 VijTersoff = fC (rij)[fR (rij) + bijfA (rij)]
(1)
where f C is smooth cutoff function to limit the range of the potential ⎧1; r < R − D ⎪ ⎪ ⎪ 1 − 1 sin⎛⎜ π r − R ⎞⎟ ; fC (r ) = ⎨ 2 2 ⎝2 D ⎠ ⎪ ⎪ R−D
