Scanning Probe Microscopy - The Application of SmartSPM 1000 Scanning Probe Microscope for Polymer Research by AIST-NT

::AZoNanotechnology Article

Topics Covered

Introduction
Application of AFM for Polymer Research
SmartSPM from AIST-NT
Examples of Application of SmartSPM for Polymer Research
Polymer Membranes
Molecular Resolution
Block Copolymers
Self-Assembled Polymer Meso-Structures
Photovoltaic Polymer Materials
Conclusion

Introduction

Since the introduction of Scanning Probe Microscopy (SPM) it has been applied to the research of polymer materials. Both Scanning Tunneling Microscopy (STM) and contact mode Atomic Force Microscopy (AFM) proved to be a very useful tool for the analysis of the crystalline polymers with molecular resolution. One of the major advantages of the SPM and AFM over SEM, specifically for polymer research, is better resolution provided by the AFM. Another great advantage of SPM over the Charged Particle Microscopy is the possibility to perform high-resolution imaging of mostly non-conductive polymer materials "as is", without coating a polymer sample with conductive layer, which is usually necessary for SEM.

Application of AFM for Polymer Research

The application of the AFM for polymer research flourished after the introduction of dynamic scanning modes (also known as semicontact mode) where a cantilever excited at its resonant frequency is in intermittent contact with the surface of the sample. This mode provides more detailed information not only on a sample's topography, but also on the mechanical and adhesion properties of that sample. It does this by means of analyzing the phase shift of the cantilever's oscillations relative to the excitation vibration. Introduction of the semicontact scanning mode also allowed for the investigation of very soft materials which could not be explored by means of contact mode AFM or STM.

SmartSPM from AIST-NT

In 2007 AIST-NT Inc introduced to the market a new Scanning Probe Microscope possessing several unique features that make it the first choice for polymer research:

• AIST-NT's SmartSPM 1000 features a unique high frequency scanner having the range of 100x100x15 micron and unmatched resonant characteristics (10-20 kHz-in XY and up to 40 kHz in Z - these are by far the best characteristics in industry). The high resonant frequency scanner makes the SPM less sensitive toward mechanical vibrations and allows measurements to be performed faster than any other AFM. It also allows a much more precise control over the tip-sample interaction. The latter being very important for soft polymer sample measurements.
• A low noise registration system based on 1300 nm IR laser allows accurate measurements of features having the height in the Angstrom range which makes possible molecular-resolution measurements. The use of IR laser enables a more accurate measurement of light-sensitive materials, which is of great importance for the organic photovoltaic material research.
• AIST-NT's SmartSPM 1000 makes the alignment of the laser, probe and photodiode easy, fast, reproducible and operator-independent. For standard probes the alignment procedure usually takes less than 45 seconds! With AIST-NT's SmartSPM 1000 replacement of the probe is no longer a limiting factor in SPM research.
• AIST-NT's SmartSPM 1000's extra-safe, automated landing procedure allows fast, safe landing even with extremely fragile ultra-sharp probes which are required for ultimate resolution imaging. With AIST-NT's SmartSPM 1000 even the highest resolution imaging may be started in less than 5 minutes from the installation of the probe.
• AIST-NT's SmartSPM 1000 is designed to be combined with optics for performing simultaneous AFM and Raman mapping of the sample and carrying out TERS (Tip Enhanced Raman Scattering) measurements, thus providing the researchers with Chemical analysis having the resolution far below the diffraction limit.

Examples of Application of SmartSPM for Polymer Research

Polymer Membranes

Crystalline polymers like polypropylene and polyethylene play an extremely important role in modern industry. The processing of these materials often results in final products that have fascinating features and properties. One example of such a product is a membrane Celgard 2400 which is produced from isotactic polypropylene and is widely used in battery manufacturing.

High resolution topography and respective phase images of Celgard 2400 membrane obtained with AIST-NT's SmartSPM 1000 are presented in Fig.1.

Figure 1. 2x2 µm topography (top) and the phase image of Celgard 2400 membrane. Both fibrillar and lamellar structures are clearly seen.

Both topography and phase images reveal the structure of the film: set of uniaxially oriented fibrils of approximately 20 nm diameter separated by narrow gaps of several nanometers is a dominating topography and functional feature. The 100-300 nm wide lamellar stripes, formed as a result of the annealing stages during the membrane manufacturing, overlaying the fibrillar system are also clearly seen. Due to the fast and accurate feedback loop in AIST-NT's SmartSPM 1000 and high resonance frequency scanner, it is possible to obtain high quality images free from artifacts related to the overreaction of the feedback system.

Molecular Resolution

One of the major advantages of the SPM over an SEM is the possibility to obtain extremely high resolution images down to molecular level. In the same time such measurements require very low noise of the measuring system combined with precise control over the tip-to-sample interaction as excessive force can significantly disturb or completely destroy the subtle molecular order of polymer samples.

Figure 2a, 2b. 165x165 nm topography (left) and phase (right) images of C₃₆H₇₄ lamellae on HOPG. Islands with different orientation of the lamellar stripes are clearly seen.

The images of lamellar stripes of linear alkane C₃₆H₇₄ deposited on the surface of Highly Oriented Pyrolithic Graphite (HOPG), presented in Fig.2 obtained in semicontact mode, demonstrate both excellent noise characteristics of the registration system of AIST-NT's SmartSPM 1000 and precise work of the scanner in the full dynamic range - the 20 nm scans were obtained with the scanner having 100x100 micron full range.

Lamellar stripes of C₃₆H₇₄ molecules having the width of 4.2 nm are clearly seen even in relatively big 165x165 nm scan. Adjacent areas with different orientation of lamellar stripes are well resolved both in topography and phase images and there is no necessity to perform filtering in Fourier space. It is also necessary to note that though the topography images show excellent contrast of the features, the overall height color range is only about 3Å for the 80 and 20 nm scans (Fig. 2c, 2d).

Figure 2c. 82x82 nm topography (left) and phase images of C₃₆H₇₄ lamellae on HOPG. Islands with different orientation of the lamellar stripes are clearly seen. The full false color scale in topography image is 2.5Å

Figure 2d. 21x21 nm topography (left) and phase image of C₃₆H₇₄ lamellae on HOPG. The width of lamellar stripes is 4.1 nm, which is in good agreement with the length of extended C₃₆H₇₄ molecule.

Critical resolution imaging requires both very low thermal / temporal drifts of the AFM and the low noise registration system. Imaging lamellar stripes of linear alkanes is a perfect way to see how big the drifts are. The inclination angles in the 82nm and 21 nm scans taken at the same scan rate of 1 Hz are 64.6° and 63.6° respectively which corresponds to the drift in X direction of about 1Å per minute. The low noise of the registration system of AIST-NT's SmartSPM 1000 is demonstrated in the section analysis of the 21 nm scan (Fig.2e) which shows that the depth of the profile is less than 1Å.

Figure 2e. The low noise of the registration system of AIST-NT's AFM is demonstrated in the section analysis of the 21 nm scan, which shows that the depth of the profile is less than 1Å.

The borders between adjacent lamellar stripes consisting of CH₃ groups on the ends of alkane molecules may be seen both depressed (like in Fig.2) or elevated (Fig.3) in topography images depending on the force exerted by the probe's tip on the sample. This force is controlled through careful selection of the free amplitude of the cantilever vibration and the value of the setpoint and may be maintained with high precision due to the high stability of AIST-NT's SmartSPM 1000.

Figure 3. 52x52 nm topography scan of C₃₆H₇₄ lamellae on HOPG. Due to the very gentle attractive interaction between the probe's tip and the sample, the borders between adjacent lamellar stripes consisting of CH3 groups are seen elevated.

Block Copolymers

An increasing demand in inexpensive and well-controlled nanopattering technologies resulted in significant attention of the research community and the industry to the block copolymers. A variety of self-organized structures resulting from the segregation of respective blocks in these polymers provides a promising route to flexible patterning the surfaces at nanolevel through well established deposition and photolithography techniques.

The SPM is an ideal research tool for characterization of the block copolymer films, as has been proven by numerous publications during the last decade. One important fact that has to be taken into account in SPM analysis of block copolymers, especially the ones having the blocks with significantly different mechanical properties, is a fundamental dependence of the measured topography and phase contrast, the latter representing mechanical properties of the sample, on the value of the force exerted on the polymer film by the tip of the SPM probe.

A typical example of such dependence is a well-known SBS- polystyrene-block-butadiene-block-polystyrene triblock copolymer. It is well known that thin films of this copolymer may form a variety of morphological shapes depending on the film thickness, nature of the substrate and annealing. Due to the difference in the glass transition temperature of the constituent blocks, the mechanical properties of the styrene and butadiene parts at room temperature are very different. The polystyrene block is significantly more rigid compared to butadiene one.

In the same time due to the difference in the surface energy of the respective blocks, it is mostly polybutadiene which is present at the polymer-air interface. Therefore, SPM imaging of SBS film reveals (see Fig. 4) either a topography presenting the outer polybutadiene layer with dips corresponding to the polystyrene phase in case of low, mostly attractive interaction, or, in case of hard repulsive one, a reversed topography when the soft polybutadiene is pushed down by the probe's tip and the elevated areas now correspond to significantly more rigid polystyrene phase. One can clearly see in Fig. 4 that features looking like depressions in the image obtained in low force (attractive) regime, become clearly elevated when the interaction force is significantly increased.

Figure 4. 1.5 µm topography scan of thin film of SBS block copolymer on HOPG. In case of the low interaction force between the probe and the film, the soft polybutadiene phase is revealed on the surface while PS phase looks depressed. In case of the higher interaction force between the probe and the film, the soft polybutadiene phase is pushed down and the PS phase looks elevated.

Self-Assembled Polymer Meso-Structures

The self-assembling of different materials at micro- and nano- scale is an extensively researched area of the material science. Polymers are ideal materials for self assembly due to the large size of the molecules and wide variety of physical properties associated with different chemical groups comprising the polymer molecule.

One example of a polymer-assisted self assembly system is micelles formed from 2nm Au nanoparticles functionalized with amfiphyllic block-copolymer molecules. Due to the presence of both hydrophilic (PEO) and hydrophobic [PS] blocks in molecules attached to Au nanoparticles, under certain conditions functionalized nanoparticles self assemble into micelles with PS core and PEO on the surface. High resolution AFM images of such micelles deposited on freshly cleaved HOPG reveal a fine bead-like structure of such micelles (Fig.5). Comparison of the AFM and TEM images of micelles may provide additional information on the deformation of the micelles as a result of deposition on different surfaces.

Figure 5. 1x1 µm topography image of micelles deposited on HOPG. Fine bead-like structure of the micelle is clearly resolved in this image.

Photovoltaic Polymer Materials

Organic photovoltaics is an extensively researched field of material science because these materials may provide cheaper and more efficient direct conversion of the solar light into electricity compared to the conventional silicon-based devices. AIST-NT's SmartSPM 1000's registration system features unique 1300 nm laser which may be very important for the SPM analysis of light sensitive organic materials making possible more accurate measurements of their properties both in the darkness and illumination conditions.

Versatility of the SPM techniques available in AIST-NT's SmartSPM 1000 allows researchers to get better idea about the materials they develop. In Fig.6 one can see topography and friction force images of the photovoltaic composite polymer material. Two different phases are clearlyresolved in friction force image, while bare topography does not provide conclusive information on the sample composition and the distribution of the constituents across the film.

Figure 6. 3.2x3.2 µm topography and lateral force images of a photovoltaic polymer blend.

Conclusion

AIST-NT's SmartSPM 1000 is a powerful and in the same time userfriendly automated scanning probe microscope perfectly suitable for polymer research. High level of automation, the unique scanner's and registration system's parameters allows a researcher to concentrate on experiment rather on instrument set up and to obtain high quality results on numerous polymer systems including light-sensitive materials and supra-molecular structures.

Source AIST-NT

For more information on this source please visit AIST-NT