Friday, 22 November, 2013
Atomic ruptures in a conductive carbon net
Graphene, a monolayer hexagonal net of carbon atoms, is the thinnest substance known and highly praised for its electrical conduction. A new microscopy method reveals ultrafine rupture lines only one atom wide that hinder free charge flow. Great hopes see graphene as the base material for future electronic devices. Therefore enormeous efforts are underway to manufacture graphene in defect-free quality as is standard with silicon, the workhorse material of present-day chips and computers.
“It’s impossible to confine infrared light at nanometer scales because infrared wavelengths are many micrometers,” said Zhe Fei, a graduate student in professor Basov’s lab at the University of California in San Diego, USA, and the first author of the paper. “We used infrared light to excite surface plasmons with a wavelength of 100 nanometers that partially reflect at a defect line.” Each defect line—although it itself remains invisible—appears in the form of twin lines which are very easy to detect. The method is a badly needed tool for assuring the absence of defects in improved graphene material, and this is an important milestone in the development of the "material of future electronics".
Graphene can be obtained by diverse fabrication techniques, among which chemical vapour deposition (CVD) is one of the most promising. However, numerous defect or rupture lines intersect the monolayer and present barriers to electronic transport even though their width is less than 1 nm only. The nature and physical properties of these—just one atom wide—ruptures remains challenging to characterize directly and conveniently. The present study proves that an infrared nanoscope method not only visualizes the line defects in high contrast, but also probes the electronic properties of individual grain boundaries. Quantitative analysis reveals that line defects form 10-20 nm wide electronic barriers that reflect plasmons by as much as 8%. These results uncover a microscopic mechanism that is responsible for the low electron mobility observed in CVD-grown graphene. The nanoscope employs surface plasmons, which are ripples of electron waves oscillating at infrared frequencies, that emanate from a probing tip. These plasmons are reflected from a defect line and cause plasmon interference which is recorded by the tip.
“We are at a cross-roads. Graphene optoelectronics and information processing are very promising. We like to see our work contribute to future technology,” Basov said. “There also is entirely new, fundamental science coming out of this. By monitoring plasmons, we learn what electrons do in this new form of carbon, how fundamental interactions govern their properties. This is a path of inquiry.”
Two CeNS groups participated in this work: Both group leaders, Rainer Hillenbrand and Fritz Keilmann, are cofounders of CeNS spin-off Neaspec GmbH, producer of the infrared nanoscope apparatus used for plasmon interferometry. Incidentally, both teams had already a year ago independently demonstrated plasmons on graphene, and published their findings back-to-back in the same issue of Nature.