With the capability of measuring physical, mechanical, electrical and other properties with nanometer resolution, atomic force microscopy (AFM) is a versatile technique. The advent of AFM-Raman spectroscopy in the late 1990s added to this versatility, enabling the addition of chemical information, including molecular orientation in polymers. Since that time, demand for the technique has grown rapidly, with both specialized AFM providers and broader-based surface science instrument suppliers offering AFM-Raman systems. Consequently, Raman companies, in particular, HORIBA and Renishaw, have formed partnerships with AFM providers to supply coupled systems. Applications for AFM-Raman range from materials research, including graphene research and polymer characterization, to life science, such as biophysics and cell analysis. AFM-Raman has matured as a technique, making it more accessible to nonspecialists through automated and simplified systems. Also driving market growth is tipped-enhanced Raman spectroscopy (TERS), or nano-Raman. TERS utilizes a metallic tip to increase spatial resolution, resulting in measurements in the subnanometer range and an enhanced Raman signal.

NT-MDT, a Russia-based supplier of AFM and optical spectroscopy systems, introduced its first AFM-Raman platform in 1998 and today offers fourth generation systems. According to Pavel Dorozhkin, PhD, head of Product Management and Applications at NT-MDT, the company has an installed base of several hundred AFM-Raman systems, with the majority sold in the last seven years. “We usually consider the ‘AFM-Raman’ term more generally,” said Dr. Dorozhkin. “NT-MDT integrates AFM with variety of optical microscopy and spectroscopy techniques, not only with Raman. This includes fluorescence microscopy, SNOM [scanning near-field optical microscopy], FLIM [fluorescence lifetime imaging microscopy], Rayleigh microscopy, IR spectroscopy and others.” Customer requirements can vary widely. “To address the diverse performance criteria, this leads to the development of multiple unique solutions—in hardware, in software and in electronics.”

AFM-Raman is available as part of the company’s NTEGRA SPECTRA and SPECTRUM platforms. Designed for advanced research applications, the NTEGRA SPECTRA enables multiple measurement options. Describing the system, Dr. Dorozhkin told IBO, that it offers “a full set of advanced AFM modes, flexible Raman solutions (wavelength ranges, detectors, spectrometers, etc.), SNOM, FLIM, controlled environment, electrochemistry, large sample measurements, etc.” NT-MDT’s systems can be used with various companies’ Raman systems.

As AFM-Raman has matured as a technique, more standardized solutions have become available, allowing for wider adoption. NT-MDT’s latest AFM-Raman platform is the SPECTRUM AFM, an automated system with capabilities for AFM, Raman and SNOM. Automated features include sample positioning, probe removal and system alignment. The SPECTRUM represents a focused offering for more common applications. “Considering the current market needs, the newest AFM-Raman product line was designed with emphasize on compactness, ease of use and automation for routine material analysis,” explained Dr. Dorozhkin.

One of the main applications of AFM-Raman is the colocalization of topographic and chemical information. “[It is] when AFM and Raman (or other optical) maps are obtained simultaneously from the sample area, without the tip enhancement of optical signal.” He described the major developments for such applications as automation, ease of use, AFM speed and quantitative results. NT-MDT systems can perform AFM measurements and Raman mapping in less than a minute, according to Dr. Dorozhkin. The SPECTRUM is designed to make AFM-Raman mapping routine and allow for multiple measurement options. “AFM measurements, done simultaneously with Raman mapping are, of course, not limited to only topography: electrical, nanomechanical and other physical measurements are also available,” he explained. “With the development of our new Hybrid mode (real-time force-curve measurements with online quantitative data analysis), Raman is now integrated with quantitative AFM nanomechanical imaging.”

Limited by the optical diffraction limit, AFM-Raman is capable of resolution only in the upper nanometer range. In contrast, TERS generally can provide resolution to as low as 10 nm. Lower resolutions have also been achieved. However, TERS remains a specialist technique for the most part largely due to TERS tips and issues related to their reliability and durability. Many researchers make their own TERS probes. “TERS probe technology is critical,” stated Dr. Dorozhkin. “In the last couple of years, many leading research groups have published different approaches in fabricating reliable, long lasting TERS probes,” he explained, adding that NT-MDT has adapted some of these approaches and offers guaranteed TERS tips. “The Tip Enhanced Raman Scattering technique is really awaiting the universal, reliable TERS probe applicable for most samples.” With such a tip, he said, TERS will become a more routine technique.

AFM-Raman provider Bruker introduced its first proprietary TERS tips, the IRIS TERS Probes, in 2012. Bruker entered the AFM market through its 2010 purchase of Veeco Instruments (see IBO 8/31/10). Bruker (Veeco) started offering integration of Raman and AFM systems in 2000, according to Thomas Mueller, PhD, director of AFM Development Applications at Bruker. In 2010, it introduced the Innova IRIS (Integrated AFM-Raman Imaging System), which offers TERS.

As Dr. Mueller told IBO, the use of TERS has expanded. He noted that demand for the technique is driven by research applications, but that “increasing maturity has allowed TERS to spread beyond pioneers.” Bruker’s introduction of proprietary TERS probes allows it to offer a complete commercial solution. “It is widely recognized in the community that lack of availability of consistent, high performance TERS probes has been an Achilles heel for further adoption of the technique by a wider community,” he explained. “We have taken that challenge head on and have commercialized TERS probes that provide consistently high TERS signal enhancement.” He added that developments in this area continue to be made. “While we have taken first steps commercializing such tips, pioneers are exploring extensions of the same approach to improve signal to noise in TERS, e.g., through grating coupling.”

For Raman-AFM, Bruker also offers the Dimension Icon AFM. “In addition to our involvement in cutting edge nano-spectroscopy, we also offer micro-Raman integration for our market leading, high resolution Dimension Icon AFM, affording the correlation of chemical information with the nanomechanical maps generated by Bruker’s exclusive PeakForce Tapping mode,” said Dr. Mueller. PeakForce tapping modes include QNM (Quantitative Nanomechanical Property Mapping) for high resolution quantitative nanomechanical information, TUNA (tunneling AFM) for conductivity measurements on soft and fragile samples and KPFM (Kelvin Probe Force Microscopy) for quantitative workfunction mapping. It is for this system’s AFM-Raman capabilities that industrial demand has been especially strong, according to Dr. Mueller.

In addition, AFM-Raman developments promise further refinement of the technique and its capabilities. Describing these developments, Dr. Mueller said they include “research activity, experimental and theoretical, to gain more complete understanding of the actual signals obtained. When comparing this nano-scale spectroscopy with its more established micro-scale parent, differences both in the ensemble size probed and in how the probing is done lead to new phenomena and altered spectra that constitute themselves a rich area for further research.” He also stated that tighter integration of spectroscopy with quantitative nanomechanical probing can be expected.

German firm WITec is a dedicated provider of modular microscope-spectroscopy systems. “The WITec success story began with an innovative cantilever sensor approach for SNOM. Experienced in dealing with low light levels and scanning probe microscopy, it was the next logical step for WITec to move into the Raman and AFM markets,” explained Dr. Sonja Breuninger of WITec Technical Marketing and Public Relations. The company’s alpha 300 and alpha 500 systems are each available with AFM-Raman capabilities, as well as confocal and fluorescence microscopy and SNOM options through modular additions to the platforms. The company introduced its first integrated Raman-AFM microscope in 2003. “One of WITec’s strengths is clearly that, from product development to manufacturing to sales and support everything comes from one source,“ said Dr. Breuninger. “This cohesion is also reflected in our systems, which have all imaging techniques truly integrated in a single instrument with one controller and software.” The company supplies its UHTS 300 Raman system for its AFM-Raman.

Such a platform complements researchers’s needs for different measurement techniques, according to Dr. Breuninger. “[I]n most fields of application, such as nano-carbon research, materials research and also in life science and the pharmaceutical industry, comprehensive sample analysis is required,” she explained. “Therefore, another trend is the combination of different measurement techniques. Modular and flexible instruments allow combinations of various imaging techniques within one microscope system.”

Dr. Breuninger highlighted the need for improved tips for TERS, stating that such a development will enable the routine usage of TERS. “TERS is an interesting and promising technique, but so far it is challenging to obtain commercially available TERS AFM tips that can be conveniently employed while providing reliable results,” she said. Improving ease of the use of TERS is another goal that can be expected to grow demand for the technique. “Another expected development is instrument automation to improve ease-of-use during operation and to increase the measurement speed and throughput.”