#nanoelectronics

Graphene enables clock rates in the terahertz range

Researchers pave the way for graphene-based nanoelectronics of the future

Graphene -- an ultrathin material consisting of a single layer of interlinked carbon atoms -- is considered a promising candidate for the nanoelectronics of the future. In theory, it should allow clock rates up to a thousand times faster than today's silicon-based electronics. Scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) and the University of Duisburg-Essen (UDE), in cooperation with the Max Planck Institute for Polymer Research (MPI-P), have now shown for the first time that graphene can actually convert electronic signals with frequencies in the gigahertz range -- which correspond to today's clock rates -- extremely efficiently into signals with several times higher frequency. The researchers present their results in the scientific journal Nature.

Today's silicon-based electronic components operate at clock rates of several hundred gigahertz (GHz), that is, they are switching several billion times per second. The electronics industry is currently trying to access the terahertz (THz) range, i.e., up to thousand times faster clock rates. A promising material and potential successor to silicon could be graphene, which has a high electrical conductivity and is compatible with all existing electronic technologies. In particular, theory has long predicted that graphene could be a very efficient "nonlinear" electronic material, i.e., a material that can very efficiently convert an applied oscillating electromagnetic field into fields with a much higher frequency. However, all experimental efforts to prove this effect in graphene over the past ten years have not been successful.

How to bend and stretch a diamond

A transistor of graphene nanoribbons

Breakthrough in Nanoelectronics

Transistors based on carbon nanostructures: what sounds like a futuristic dream could be reality in just a few years' time. Scientists have now produced nanotransistors from graphene ribbons that are only a few atoms wide.

Graphene ribbons that are only a few atoms wide, so-called graphene nanoribbons, have special electrical properties that make them promising candidates for the nanoelectronics of the future: While graphene -- a one atom thin, honeycomb-shaped carbon layer -- is a conductive material, it can become a semiconductor in the form of nanoribbons. This means that it has a sufficiently large energy or band gap in which no electron states can exist: it can be turned on and off -- and thus may become a key component of nanotransistors.

The smallest details in the atomic structure of these graphene bands, however, have massive effects on the size of the energy gap and thus on how well-suited nanoribbons are as components of transistors. On the one hand, the gap depends on the width of the graphene ribbons, while on the other hand it depends on the structure of the edges. Since graphene consists of equilateral carbon hexagons, the border may have a zigzag or a so-called armchair shape, depending on the orientation of the ribbons. While bands with a zigzag edge behave like metals, i.e. they are conductive, they become semiconductors with the armchair edge.

Side-by-side deposition of atomically flat semiconductor sheets enhances solar cell conversion efficiency

Super thin photovoltaic devices underpin solar technology and gains in the efficiency of their production are therefore keenly sought. KAUST researchers have combined and rearranged different semiconductors to create so-called lateral p-n heterojunctions—a simpler process they hope will transform the fabrication of solar cells, self-powered nanoelectronics as well as ultrathin, transparent, flexible devices.

Two-dimensional semiconductor monolayers, such as graphene and transition-metal dichalcogenides like WSe2 and MoS2, have unique electrical and optical properties that make them potential alternatives to conventional silicon-based materials. Recent advances in material growth and transfer techniques have allowed scientists to manipulate these monolayers. Specifically, vertical stacking has led to ultrathin photovoltaic devices but requires multiple complex transfer steps. These steps are hampered by various issues, such as the formation of contaminants and defects at the monolayer interface, which limit device quality.

"Devices obtained using these transfer techniques are usually unstable and vary from sample to sample," says lead researcher and former visiting student of Associate Professor, Jr-Hau He, Meng-Lin Tsai, who adds that transfer-related contaminants significantly affect device reliability. Electronic properties have also proven difficult to control by vertical stacking.