AFM Confocal Raman & Tip Enhanced TERS Solutions

By AZoM

Table of Contents

Introduction

Nanoscale imaging is a rapidly evolving field. Nowadays there are several techniques available for sample characterisation, but each of them is targeted to extract specific information. It would therefore be ideal to have several of these analysis tools all integrated into the same measuring system to achieve full characterisation of the specimen.   

Simultaneous Confocal-AFM Imaging and TERS

Nanonics Imaging pioneered the field of AFM-Raman/Tip Enhance Raman Spectroscopy (TERS) with the Multiview^TM series. Its hallmark free optical axis allows for seamless integration with any Raman system, whether upright or inverted.  Figure 1 shows the Multiview 4000^TM two probe head mounted on the HORIBA Jobin Yvon Raman Spectrometer (Left) and the Multiview 1000^TM head mounted on the Renishaw Raman Spectrometer (Right).

Figure 1. Multiview 4000^TM two probe head, mounted on the HORIBA Jobin Yvon Raman Spectrometer (left) and Multiview 1000^TM head, mounted on the Renishaw Raman Spectrometer  (right).

Integration Packages are available for a variety of Raman manufactures including Renishaw PLC and HORIBA Jobin Yvon. These state-of-the-art integrations mark the beginning of a new era in high-resolution Raman spectroscopy.   Such Integration Package allow for complete isolation of the AFM from Raman and laser sources and provides the ultimate in AFM performance and optical/confocal Raman/TERS throughput for various samples such as carbon nanotube, graphene flakes, monolayers, bio-surfaces, etc.

Integration Package Solutions

The Integration Package is an optical connection package which can connect either to the Raman microscope or directly to the Raman spectrometer. The advantage is that the AFM sits on its own microscope which is completely isolated from vibrations from the Raman's laser, spectrometer or CCD. This allows both the AFM and the Raman to work under the best operating conditions.

Three configurations for the Integration Package are available:

• Upright microscope
• Inverted microscope
• Dual microscope (a combination of an Upright and an Inverted microscope).

The Nanonics Multiview^TM series is designed to be transparently optically integrated (Fig. 2). The same head can work with either an upright or an inverted microscope and with a dual microscope system where either backscattered or transmission operation is presented.

Figure 2. Integration Package scheme of a Dual Microscope system with an efficient optical connection to external devices such as Raman spectrometers and Raman microscopes (Left). Multiview 4000^TM SPM system mounted on a Dual microscope and a vibration isolation platform of the Integration Package (Right). The Integration Package contains an enclosure for acoustic shielding.

Parallel Imaging

With the combined system, it is possible to record in parallel with Raman, a wide variety of scanned probe imaging modalities. For instance, while the Si Raman peak of a microcircuit is being monitored to detect stress in the silicon, the micro-topography of the circuit can simultaneously be measured by AFM, as well as its NSOM reflectivity or its electrical properties, such as the dopant concentration (Fig. 3).

In addition, Nanonics provides a software displaying all these images at once, for direct and simultaneous comparison and analysis.

Figure 3. Parallel imaging of a Silicon Semiconductor. 9x7 μm² AFM image (left) and Raman Intensity of the same region at 520nm/cm (right).

Conventional Confocal Raman Mapping

There is a serious drawback to Raman Spectroscopy when studying non-smooth surfaces. As with all lens-based microscopy techniques, Raman suffers from the problem of out-of-focus light.

When a sample is scanned conventionally under the illuminating beam of a Raman microscope, the uneven sample surface will scan in and out of the focal plane. As a result the resolution of the Raman mapping is limited by the large area of the unfocussed beam on the sample.

In addition, the point spread function is significantly broader where there are contributions from the out-of-focus light. As a result the Raman spectra of non-flat surfaces can be very misleading, and tends to misrepresent the true information that can be gained by using Raman.

Raman Mapping with Z-Control

The problem of out-of focus light can be solved by using a Z-feedback mechanism. With this feedback in place, the surface of the sample can be kept in the focal plane throughout the scan. All the Nanonics Multiview^TM AFM platforms have a completely free optical axis. This makes them the ideal add-on to any Raman system to provide the Z-control necessary for true high resolution Raman mapping.

Examples of the Power of Integrated Raman/AFM

The difference between Raman mapping with and without Z-control can be seen clearly in the examples below. Here the vibrational mode of diamond at 1334 cm^-1 is represented (Fig. 4).

Figure 4. The pair of images on the top shows the same area mapped with and without Z-control (Top). The advantage of Z-control is made apparent by the differences between the two. The images on the bottom are collages of AFM topography and Raman intensity of the same sample at two different wavelengths (Bottom). Note the differences in the intensity of the two images: the bright spots at the top of the image at 1334 cm^-1 are absent from the image at 1525 cm^-1.

Specialized AFM/Raman/TERS Functional NanoImaging and NanoManipulation of Carbon NanoMaterials protocols are available for a variety of materials such as carbon nanotubes, Graphene, diamonds, etc. No other Raman system has sufficient control of Z position to pick out these differences.

Nanoindentation Correlated with Material Properties

To illustrate the combination of the worlds of AFM and Raman spectroscopy, actual data has been obtained on the Si stress problem mentioned above (Fig. 5).

Figure 5. (a) 14 x 14 μm² AFM height image of a nanoindentation in Si, (b) a line scan through a region of the AFM image is highlighted.

The points on the AFM cross section are points at which Raman microscope spectra were collected. As a result of the nanoindentation, the silicon has been displaced. The question is whether or not these regions correspond to different phases of the silicon that can be correlated with the AFM measurements.

Only Raman microprobe spectroscopy can give this information. The Raman spectra were obtained at the same time as the topography was measured.

Local Stress of MEMs Devices

Raman spectroscopy is a very important technique for measuring silicon strain. On-line AFM can impose finely controlled and well-defined strain on silicon with pressures exceeding megapascals since the area of a probe tip is nanometric. NanoRaman technology is ideal for super-resolution silicon stress measurements in floating structures such as combs and forks.

The on-line AFM allows for defined forces to be imposed on a MEMs cantilever while the on-line Raman measures the shift in the silicon vibrational frequency and silicon strain at the cross (Figs. 6 and 7). No other AFM is capable of such a combination.

Figure 6. AFM imposing forces on a MEMs cantilever with the Raman measuring simultaneously the silicon vibrational frequency and silicon strain at the cross.

Figure 7. Raman shift as a function of local stress location.

Intermittent Contact Mode in Liquids

In addition, the Nanonics SPM/Raman systems can operate in intermittent contact mode even in liquids. Thus, the whole world of NSOM/SPM imaging of biological materials in physiological media can now be directly correlated with Raman spectra.

About Nanonics Imaging

Nanonics Imaging is the premier innovator of AFM and NSOM systems in the SPM market. Since its inception in 1997 and throughout the last ten years Nanonics have introduced to the SPM market new concepts in system functionality which in turn have supported the pursuit of new areas of scientific application.

Nanonics contributions span from the revolutionary approach to NSOM imaging with cantilevered NSOM probes, to the introduction of dual tip/sample scanning AFM systems and from the introduction of the first ever NSOM/AFM cryogenic systems to the first ever, Raman/AFM, Multiprobe AFM and SEM/AFM systems.

This information has been sourced, reviewed and adapted from materials provided by Nanonics Imaging.

For more information on this source, please visit Nanonics Imaging.