Atomic force microscopy (AFM)/Raman is the latest trend in hybrid imaging/chemical analysis instrumentation. 1–4 Enjoying a rapid rise to prominence over the past two years, manufacturers now offer fully integrated systems as well as interfaced
Atomic force microscopy (AFM)/Raman is the latest trend in hybrid imaging/chemical analysis instrumentation. 1–4 Enjoying a rapid rise to prominence over the past two years, manufacturers now offer fully integrated systems as well as interfaced systems, marrying independent AFMs and Raman spectrometers, making this technique available to the routine laboratory either as a full system or as an add-on.
Initially the purview of physicists, AFM/Raman is now seen as a valuable tool for pharmaceutical companies; cell and molecular biologists seeking to link structure with drug or metabolic interactions; chemists working with films, monolayers, and multiphase systems; and designers of new materials, especially in nanotechnology, all of whom need to coordinate chemical information with phase distribution, electrical, mechanical, or magnetic information.
Atomic force microscopy is a member of the larger class called scanning probe microscopy (SPM). In the simplest sense, an SPM operates very much like a phonograph, scanning an atomically sharp tip over the surface of interest in a variety of motions ranging from contact to semicontact and noncontact. Depending on configuration, the tip:surface interaction can sense, and therefore image, diverse information, including topography; location of phases; differences in local elasticity and work function; magnetic fields; and electrical fields dependent on voltage, current, or resistance.5 Magnification can be changed by varying the size of the field scanned.
Raman spectroscopy detects changes in the vibrational state of the atoms within a susceptible molecule. A photon, typically generated by a laser, excites an electron, causing a change in vibrational state. Upon relaxation, part of the excitation energy is released in the form of a signature spectrum in the 0–4000 cm–1 range (approximately 2–10,000 μm). When used in connection with either optical or atomic force microscopy, the Raman spectrometer is fitted with a confocal slit or pinhole, to define the exact plane from which the spectrum is collected. Combining AFM with Raman opens new opportunities for correlating chemical information with other physical, electrical, and magnetic properties.
Interface or integrate?
The NTegra Spectra (NT-MDT, Zelenograd, Russia) leads the field for fully integrated AFM/Raman systems. Built around a high-resolution Solare spectrometer and the highly modular NTegra AFM, it offers motorized controls for the confocal apertures; choices between as many as three different lasers; and multiple dichroics, all easily managed through the same Nova software that runs the AFM. For transmitted light applications, it fits neatly on an Olympus IX-71 microscope (Melville, NY), also providing convenient expansion to nearfield scanning optical microscopy (NSOM). In total, the system offers light and confocal microscopy; more than 40 different AFM/SPM modes; NSOM; and both confocal Raman and fluorescence spectroscopy, all controlled through one piece of software—Nova.
For facilities that already own a Raman spectrometer or practitioners who prefer a specific brand, many of the AFM companies now offer interface options. NTegra has already been mated with both the Renishaw InVia (Wotton-under-Edge, U.K.) and with the Horiba-JY LabRam (Edison, NJ). However, in most instances, the interface is primarily physical. The spectrometer is still run on its own software and the AFM run on its software, with the ability to import spectra.
“Seeing around corners”—the Holy Grail for reflected light samples
Figure 1 - The high aperture head “sees around corners,” ensuring that the Raman and AFM information come from exactly the same location. It also provides simple interfacing to other spectrometers.
Reflected samples have been more of a challenge for AFM/Raman systems. To answer that challenge, NT-MDT designed the high aperture (HA) head. As shown in Figure 1, the optics essentially render the normally opaque tip transparent and ensure that the Raman signal comes from exactly the same location as the AFM image. In addition to extending AFM/Raman to opaque samples, HA also provides a facile link for interfacing.
All Raman experiments face several key challenges, notably, Raman signal strength and locating Raman peaks that are not masked by often-present fluorescence. Today’s spectrometers optimize signal collection through thoughtful use of adjustable entrance slits, holographic gratings, and notch filters.
Combining AFM with Raman presents a different set of challenges. While not significant, they are best met with realistic expectations. The first of these is the difference in acquisition time: AFM images typically take tens of seconds, while the Raman spectra can require hours. To conduct combined AFM/Raman experiments, the systems must be extremely stable, with feedback electronics and device