Graphene Growth on Silver

Electronic characterization of graphene: STS images superimposed on a
three-dimensional rendering of the underlying silver topgraphy
at (a) -200 meV and (b) -60 meV. Both images are 150 x 150 nm.
The difference in the density of states at the dendritic graphene
edges at the two different energies is visible. This behavior is
consistent with an electronic structure that is not perturbed by
the underlying silver substrate.

Users from Northwestern University, working with the Center for Nanoscale Materials EMMD Group, have demonstrated the first growth of graphene on a silver substrate. Unique wave-like electron scattering at the edges of the dendritic graphene also was observed for the first time. 

This behavior is consistent with an electronic structure that is not perturbed by the underlying silver, providing a new system in which graphene is decoupled from its substrate. Because the graphene is electronically decoupled from the silver substrate, the intrinsic properties of graphene can be studied directly. 

This new growth method may enable improved interfacing of graphene with other two-dimensional materials — a vital step for the development of graphene-based circuits and other technologies.

Graphene, a one-atom-thick carbon layer with extraordinary conductivity and strength, holds promise for a range of applications. However, current methods for growing graphene on metals have been unsuccessful with silver. While graphene is conventionally grown on a metal surface by catalytically decomposing hydrocarbons at elevated temperatures, this method is ineffective for silver substrates because the substrates are chemically inert and have a relatively low melting point. Using a graphite carbon source, the team was able to grow graphene by depositing atomic carbon, rather than a carbon-based molecular precursor, onto the substrate. The growth circumvented the need for a chemically active surface and allowed the graphene growth at lower temperatures.

The researchers also found the graphene they grew was electronically decoupled from the underlying silver substrate, allowing the intrinsic properties of graphene to be studied and exploited directly on the growth substrate; this characteristic has not been previously observed with graphene grown on other metals. The researchers observed unique wave-like electron scattering at the edges of the graphene that had previously been observed only on insulating substrates.

Scanning tunneling microscopy (STM) was performed at CNM using an Omicron VT system with electrochemically etched tungsten tips at 55K. Scanning tunneling spectroscopy (STS) was simultaneously collected via periodic modulation to the applied voltage. Raman spectroscopy was taken with a Renishaw InVia Raman Microscope using a 514-nm laser line. Growing graphene on silver under ultrahigh-vacuum conditions could result in exceptionally pure samples that may present opportunities for ultrafast electronics and advanced optics.

B. Kiraly et al., "Solid-source growth and atomic-scale characterization of graphene on Ag(111)," Nat. Comm., 4, 2804 (2013).

Source: http://nano.anl.gov/news/highlights/2014_graphene_on_silver.html

Advancing Graphene for Post-Silicon Computer Logic

Team of UC Riverside researchers pioneer new approach for graphene logic circuits

A team of researchers from the University of California, Riverside’s Bourns College of Engineering have solved a problem that previously presented a serious hurdle for the use of graphene in electronic devices.

Scanning electron microscopy image of graphene device used in the study. The scale bar is one micrometer. The UCR logo next to it is implemented with etched graphene.

Graphene is a single-atom thick carbon crystal with unique properties beneficial for electronics including extremely high electron mobility and phonon thermal conductivity. However, graphene does not have an energy band gap, which is a specific property of semiconductor materials that separate electrons from holes and allows a transistor implemented with a given material to be completely switched off.

A transistor implemented with graphene will be very fast but will suffer from leakage currents and power dissipation while in the off state because of the absence of the energy band gap. Efforts to induce a band-gap in graphene via quantum confinement or surface functionalization have not resulted in a breakthrough. That left scientists wondering whether graphene applications in electronic circuits for information processing were feasible.

The UC Riverside team – Alexander Balandin andRoger Lake, both electrical engineering professors, Alexander Khitun, an adjunct professor of electrical engineering, and Guanxiong Liu and Sonia Ahsan, both of whom earned their Ph.Ds from UC Riverside while working on this research – has eliminated that doubt.

“Most researchers have tried to change graphene to make it more like conventional semiconductors for applications in logic circuits,” Balandin said. “This usually results in degradation of graphene properties. For example, attempts to induce an energy band gap commonly result in decreasing electron mobility while still not leading to sufficiently large band gap.”

Alexander Balandin, a professor of Electrical Engineering

“We decided to take alternative approach,” Balandin said. “Instead of trying to change graphene, we changed the way the information is processed in the circuits.”

The UCR team demonstrated that the negative differential resistance experimentally observed in graphene field-effect transistors allows for construction of viable non-Boolean computational architectures with the gap-less graphene. The negative differential resistance – observed under certain biasing schemes – is an intrinsic property of graphene resulting from its symmetric band structure. The advanced version of the paper with UCR findings can be accessed at http://arxiv.org/abs/1308.2931.

Modern digital logic, which is used in computers and cell phones, is based on Boolean algebra implemented in semiconductor switch-based circuits. It uses zeroes and ones for encoding and processing the information. However, the Boolean logic is not the only way to process information. The UC Riverside team proposed to use specific current-voltage characteristics of graphene for constructing the non-Boolean logic architecture, which utilizes the principles of the non-linear networks.

Roger Lake, a professor of electrical engineering

The graphene transistors for this study were built and tested by Liu at Balandin’s Nano-Device Laboratory at UC Riverside. The physical processes leading to unusual electrical characteristics were simulated using atomistic models by Ahsan, who was working under Lake. Khitun provided expertise on non-Boolean logic architectures.

The atomistic modeling conducted in Lake’s group shows that the negative differential resistance appears not only in microscopic-size graphene devices but also at the nanometer-scale, which would allow for fabrication of extremely small and low power circuits.

The proposed approach for graphene circuits presents a conceptual change in graphene research and indicates an alternative route for graphene’s applications in information processing according to the UC Riverside team.

http://ucrtoday.ucr.edu/17286