Raman Metrology and Instrumentation

Summary

We develop measurement capabilities and protocols to validate and advance the science of Raman spectroscopy and work to establish NIST as a unique resource to the academic, industrial, and the National Metrology communities for such measurements. Working collaboratively, the ultimate goal is to develop Raman spectroscopy into a quantitative, SI-traceable technique critical for many applications including sensing, security and forensics.

Description

Raman spectroscopy/microscopy is a powerful optical technique for rapid, non-destructive, label-free characterization of materials. It works under ambient conditions, often without requirement of any sample preparation. Applications span microelectronics, pharmaceutical, security and fundamental investigations of nanomaterials such as the novel 2D and nanocarbon materials. 

Despite a century of development, Raman spectroscopy remains a qualitative analytical technique. A major obstacle in the use of Raman data in regulatory applications is the lack of traceable quantification and the absence of certified reference and documentary standards.  This project aims—by working with international collaborators at National Metrology Institutes (NMIs), academia and instrument manufacturers—to improve measurement reliability, establishing traceability to the SI Units mole and meter, and to develop measurement protocols for consideration within ISO as documentary standards. 

Furthermore, repeatability and measurement error analysis from Raman data are lacking on technologically important materials such as graphene and carbon nanotubes.  These low dimensional materials are prototypical materials from which all others are benchmarked.  This project will help U.S. industry accelerate the development and commercialization of advanced electronics and photonics based on low dimensional carbon materials for future information, communication, and sensor technologies.  

International Round Robins

Using VAMAS, the Versailles Project on Advanced Materials and Standards, which supports world trade in products dependent on advanced materials technologies, through International collaborative projects aimed at providing the technical basis for harmonized measurements, testing, specifications, and standards, we established a Technical Working Area (TWA) on Raman Spectroscopy and Microscopy.  This framework enables multiple round robin (RRs) or interlaboratory comparisons (ILCs) to occur simultaneously. Presently, studies are underway for spectral calibration, spatial resolution and composition differentiation. Once the protocols are tested and issues identified and rectified, the documents are brought into the ISO process for full development as a documentary standard.

Of note, a separate TWA on Graphene and Related 2D Materials has a joint projects with the Raman TWA, especially the Structural characterization of CVD-grown graphene: Coverage on substrate, number of layers, level of disorder, where Raman spectroscopy is the main characterization method.

Other international work is ongoing under the BIPM, CCQM Working Group on Surface Analysis (CCQM-SAWG). A pre-pilot study occurred on the Raman confocal volume measurements and presently a Task Group is just formed to elevate Raman spectroscopy from just the SAWG to CCQM more broadly.

Rapid, Accurate and Precise Characterization

Rapid, quantitative techniques for assessing defect type/density in graphene and other 2D materials could broadly impact the field by providing a needed standard for materials quality. Working collaboratively with a team from Howard and Columbia Universities, crystals with varying defect type/density, well characterized by STM/TEM, will be used to ‘benchmark’ other techniques such as Raman spectroscopy. While Raman provides highly specific vibrational signatures for defects, such as the graphene D-band, its ability to distinguish different defect types remains infantile.  Strategies including exploiting resonance effects to identify phonon anomalies as a function of defect type and density.  Further work on twisted graphene stacks will be used to identify phonon positions as a metric for twist angles prior to device fabrication.