Raman microscope can pinpoint graphene defects and contaminants with roughly 10-nm resolution

OCTOBER 14, 2011 Materials Analysis: Imaging method reveals nanometer-sized defects and contaminants on graphene Mitch Jacoby ACS Nano PINPOINT RESOLUTION Positioning a metal scanning probe tip at the laser focal point of a confocal Raman microscope leads to tip-enhancement of Raman signals….

OCTOBER 14, 2011

Materials Analysis: Imaging method reveals nanometer-sized defects and contaminants on graphene

Mitch Jacoby

ACS Nano

PINPOINT RESOLUTION Positioning a metal scanning probe tip at the laser focal point of a confocal Raman microscope leads to tip-enhancement of Raman signals. This procedure, depicted schematically on a graphene micrograph, can pinpoint graphene defects and contaminants with roughly 10-nm resolution.

Structural defects and impurities in graphene can be pinpointed with improved resolution—in the low-nanometer range—by a spectroscopy method that can quickly image large areas of the film, researchers in Switzerland report (ACS Nano, DOI: 10.1021/nn2035523). The advance may lead to routine quality control methods to assess the purity and structural integrity of graphene films used in electronics applications.

Research teams in labs around the globe are driving intensely to develop a host of electronic applications that exploit graphene’s superlative electronic, optical, and other properties. A glance through the tables of contents of leading journals shows that each issue brings new reports of graphene-based circuitry, transistors, supercapacitors, photovoltaic cells, and other devices.

Because graphene’s electronic properties are altered by structural defects and contaminants and by the locations of those imperfections relative to the edges of the film, researchers building graphene-based devices must image them to determine whether the ultrathin films of carbon are pristine and free from such imperfections.

The vibrational specificity of confocal Raman spectroscopy has made it the standard for graphene analysis, although electron microscopy and scanning probe methods have been used as well. However, even confocal Raman spectroscopy has inadequate spatial resolution to view some graphene imperfections and impurities.

Now, Johannes Stadler, Thomas Schmid, and Renato Zenobi of the Swiss Federal Institute of Technology (ETH), Zurich, have demonstrated a variation of Raman spectroscopy imaging that reveals the locations of chemical impurities, such as carbonaceous materials and hydrogen-coated graphene, and structural defects, such as rumpled regions and folds, all with roughly 10-nm resolution. Called tip-enhanced Raman spectroscopy (TERS), the combined spectroscopy and scanning probe technique outperforms the spatial resolution of the conventional confocal method by one to two orders of magnitude.

By positioning an etched silver scanning probe tip at the laser focal point of a Raman microscope, the ETH team confines the probe process to a tiny region immediately adjacent to the apex of the tip. In addition, by appropriately aligning the film, probe tip, and direction of irradiation, the group selectively enhances Raman signals from defects, which boosts the contrast of those features relative to the background signal from pristine graphene. In contrast, conventional Raman imaging cannot distinguish small corrugated regions of graphene from surrounding flat ones.

“This paper brings new opportunities to scanning probe-based technology and demonstrates its capacity to map the positions of graphene defects and contaminants with exceptional resolution,” comments Chanmin Su, director of technology at Bruker Nano, a Santa Barbara, Calif.-based instrument maker.

Su notes that carrying out chemical characterization on the nanometer scale was identified as a grand challenge by the U.S. National Nanotechnology Initiative nearly a decade ago. This study makes important and newsworthy progress toward that goal, Su asserts. “As a leader in scanning probe technology, we eagerly look forward to using TERS as a general tool for chemical identification,” he adds.