Scientists from Lawrence Livermore and the Karlsruhe Institute of Technology in Germany have shown that the underlying principle of the field-effect transistor (FET), which revolutionized electronic equipment, can be extended from semiconducting thin films to metallic three-dimensional (3D) materials. By applying electrochemically induced variations in surface charge, they were able to dynamically control the electrical conductivity of centimeter-sized, bulk nanographene samples. Their results were featured on the cover of the June 18, 2014, issue of Advanced Functional Materials.
For this study, researchers filled the pores of a nanoporous bulk material with an electrolyte and imposed a gate potential of less than 1 volt. In the process, they achieved fully reversible changes in conductance of several hundred percent throughout the entire volume of the bulk electrode. The observed conductivity change resulted from the electrochemically induced accumulation or depletion of charge carriers in combination with the large variation in the carrier mobility.
Materials that alter their properties in response to changes in their surface charge are of interest for various electronic applications. A FET, for example, uses an external electric field to control the charge-carrier density and thus conductivity of a semiconducting material. Bulk nanographene has an ultrahigh surface area and is chemically inert, important characteristics for using surface charge to modulate physical properties. The results will allow researchers to explore using bulk graphene materials in such applications as low-voltage, high-power tunable resistors.
Contact: Juergen Biener (925) 422-9081 (biener2@llnl.gov).
An international collaboration called HotQCD, which was led by Livermore physicists, has calculated the properties of the quantum chromodynamics (QCD) phase transition, simulating conditions that occurred during the first microseconds of the big bang. The computationally intensive calculation was supported by the Laboratory’s Computing Grand Challenge Program, which allocates time on high-performance computing systems for compelling large-scale projects such as the one performed by the HotQCD collaboration. Results from the team’s research appeared in the August 18, 2014, edition of Physical Review Letters.
When the universe was less than 1 microsecond old and at a temperature of more than 1 trillion degrees, it transformed from a plasma of quark and gluon particles into bound states of particles known as protons and neutrons. QCD theory, which describes the interactions of quarks and gluons and the strong nuclear forces between protons and neutrons, predicts such a transition during the extreme conditions at the birth of the universe. To calculate the phase transition, the HotQCD team used Livermore’s Vulcan supercomputer, an IBM BlueGene/Q machine that can process up to 5 quadrillion floating-point operations per second (petaflops). With the team’s new algorithm, the researchers could, for the first time, run the calculation in a way that preserves a fundamental symmetry of QCD in which left- and right-handed (or chiral) quarks can be interchanged without altering the equations.
In a 2007 essay in Computing in Science & Engineering, Livermore scientists Ron Soltz and Pavlos Vranas predicted that the QCD phase transition could be calculated given powerful enough computers. “With Vulcan, we could calculate properties that were proposed years before petaflop-scale computers were around,” says Soltz. “The calculation took us several months to complete, but the 2007 estimate turned out to be pretty close.”
The HotQCD collaboration includes researchers from Lawrence Livermore, Los Alamos, and Brookhaven national laboratories; the Institute for Nuclear Theory; Columbia University; Central China Normal University; and Universität Bielefeld in Germany. The team’s research has implications for understanding how the universe evolved during the first microsecond after the big bang and will help scientists interpret data collected at Brookhaven’s Relativistic Heavy Ion Collider and CERN’s Large Hadron Collider.
Contact: Ron Soltz (925) 423-2647 (soltz1@llnl.gov).
A team of Livermore researchers has developed an ultralow-density bulk material with an extremely high surface area, a uniform distribution of pore sizes, and an interconnected nanotubular architecture. “The new material is thermally stable and 10 times stronger and stiffer than traditional aerogels of the same density,” says team lead Monika Biener, a materials scientist in the Laboratory’s Physical and Life Sciences Directorate.
Ultralow-density, porous bulk materials offer many promising applications. To unlock the materials’ full potential, however, scientists must develop mechanically robust architectures to control a material’s form, cell size, density, and composition. “Those characteristics are difficult to achieve with traditional chemical synthesis methods,” says Biener.
To achieve the control required in the bulk material, the researchers used nanoporous gold as a tunable template for atomic layer deposition. The nanotubular network architecture enabled mass transport through two independent pore systems separated by a nanometer-thick 3D membrane. Biener notes that ultralow-density materials have intriguing applications in catalysis, energy storage and conversion, thermal insulation, shock energy absorption, and high-energy-density physics. The team’s work was featured on the cover of the July 23, 2014, issue of Advanced Materials.
Contact: Monika Biener (925) 424-6157 (biener3@llnl.gov).