Heated-Tip Atomic Force Microscope
The Atomic Force Microscopy or AFM has always been a key technique for the measurement and analysis of samples when nanoscale topography is involved. It offers a number of complementary probing modes that extend an AFMs measurement capability to a wide range of material and transport properties of surfaces, including hardness, friction, conductivity and adhesion. Sample temperature controlled AFM extends the study of surface morphology and properties to include changes in the material phases. Recently, silicon microfabricated AFM cantilevers that have integrated heaters. They have likewise become commercially available. These cantilevers were initially developed for probe based data storage by researchers at IBM Zurich. With the availability of these cantilevers, AFM measurements can be performed where the tip is heated as opposed to the sample. Heated Tip AFM , or the HT-AFM, refers to AFM operation where a heated tip is utilized instead of a normal tip. This is done in order to locally heat the sample surface. A number of AFM modes can accommodate a heated tip to yield new information tied to the thermal properties of the sample. The HT-AFM mode was used to improve the discrimination of the two components in the phase image, which occurs when the tip is heated near the transition temperature of one of the components. The three micrometer scan size images show the high resolution capabilities of the heated tips as well as the ability to locally modify the sample surface.
Compared to substrate heating methods, the HT-AFM method offers some distinct advantages. Primarily, it has the ability to heat to higher temperatures. With substrate heating, a significant temperature rise of the materials in contact with the sample can take place. This has been known to commonly cause problems with the calibration of the scanner as well as the possibility of damaging the scanner at high temperatures. Another advantage with the heated tip is that it exhibits much less thermally induced drift. With substrate heating, when the large area that is heated, spatial displacements of many microns can be observed over small temperature ranges. Other advantages of this technique include the ability operate at very high heating rates due to the low thermal mass. Also, the ability to perform multiple measurements on specimen are highly sensitive to thermal history. A specific technique within the HT-AFM family of techniques is nano Thermal Analysis or the nano-TA. This local thermal analysis technique enables one to obtain quantitative information on melt temperature or glass transition temperatures. The HT-AFM method has been used for wide range of applications primarily focused within the polymer and pharmaceutical industries. The measurements that have been demonstrated in the polymer industry include analysis of blend samples, thin films, and nano-scale inclusions. In the pharmaceutical industry, HT-AFM has been used to map crystallinity and polymorphs as well as for the identification of components in compressed tablets and tablet coats.
The HT- AFM method can be used to not only to differentiate between material components or phases within a material. It also allows for the manipulation of the micro and nanostructure of materials that could be used to study diffusion rates and produce controlled nanoscale features. In the nanocomposite field, the method can be employed to investigate the lateral forces exerted during particle-matrix debonding and by studying particle matrix adhesion, which consequently could lead to the design of improved interfaces. From testing the nanometer scale thermomechano response of an energetic material to analyzing polymeric composites, the high resolution imaging capability, wide range temperatures and heating rates offer unique new capabilities material analyses.