Thermal nanoprobes

Figure 7.12. Typical graphs of thermal nanoprobe cantilever deflection versns heating voltage used to construct a cantilever deflection-tip temperature calibration curve (a) polycaprolactone (b) poly(ethylene terephthalate) (reproduced with permission of Anasys Instruments Inc.).

A field emission source uses a needle-like tungsten or carbon tip as the cathode, shown in Fig. 14.28. The tip is only nanometers wide, resulting in a very high electric field at the tip. Electrons can tunnel out of the tip with no input of thermal energy, resulting in an extremely narrow beam of electrons. Electron beams from heated filaments have a focal (cross-over) diameter of about 50 p,m while a field emission source has a crossover diameter of only about 10 nm. Field emission sources can serve as probes of surfaces at the nanometer scale (an Auger nanoprobe). 

Chemical methods of material processing were known for years, existing in parallel with physical and other methods of film deposition. Recent advances in electron microscopy and scanning nanoprobe microscopy (STM, ATM) have revealed that some of the materials produced by the chemical methods have distinctive nanocrystalline structure. Furthermore, due to the achievements of colloid chemistry in the last 20 years, a large variety of colloid nanoparticles have become available for film deposition. This has stimulated great interest in further development of chemical methods as cost-effective alternatives to such physical methods as thermal evaporation magnetron sputtering chemical and physical vapor deposition (CVD, PVD) and molecular beam epitaxy (MBE). 

The silicon nanoprobes, as used in micro/nano-TA, are a far more recent development, and consequently the spatial resolution that can be achieved with these probes is not yet well established but must be of the same order for topographic imaging as conventional AFM tips because the sharpness of the tip is the same. The capabilities for thermal property imaging remain, at the time of writing, unexplored. 

At the time of writing, there are two major types of probe, both of which are described above the Wollaston and the nanoprobes. The Wollaston probe is very robust, can be used at temperatures of 00 °C, and can provide the calorimetric measurement required for the thermal force-distance curves described above as well as AC and DC thermal imaging. Its disadvantages are that the spatial resolution is of the order of a micro-meter and can be used only in contact mode. 

The nanoprobes are less robust, but as robust as most conventional silicon AFM probes, and can be used in aU of the conventional AFM imaging modes, including contact, tapping, and pulsed-force mode. When used in this way, the spatial resolution is similar to that in conventional AFM microscopy, (i.e., of the order of 1 nm). For L-TA the spatial resolution is of the order of 100 nm or better. Their disadvantages are that they cannot, at the time of writing, be used for calorimetric measurements or thermal imaging. This is because the heater is located at the top of the pyramid that forms the tip rather than immediately adjacent to the surface. However, they perform well for local TMA. The maximum temperature is -250 °C undoubtedly more versions of these probes will become available in the future with a wider range of capabilities. 

The process of temperature calibration has been discussed above and is also mentioned below in reference to the use of nanoprobes (see Section 7.8.3). In essence, a series of standard materials is used to construct a calibration curve this is the procedure routinely adopted in many thermal analysis techniques. It is preferable that the surfaces of the calibrants are smooth. Polymers are often available in films or coupons that readily lend themselves to use as calibrants. However, as discussed previously, polymers have relatively broad melting transitions (sometimes very broad), and so polymer samples must be selected with as narrow a melting transition as possible. Simple organic materials such as benzoic acid can be obtained in high-purity form with very sharp melting transitions but are usually presented in a granular form not very suitable for use in L-TA. Consequently, such materials are more effectively melted onto a very flat surface and then allowed to crystallize before being removed. 

At the time of writing there is only one manufacturer of micro/nano-TA equipment Anasys Instruments, (www.anasysinstruments.com). They supply the hardware and software for local thermal analysis and thermal imaging that can be interfaced with the most popular types of atomic force microscope. More recently they have launched an instrument based on an optical microscope, the Vesta system, which is simpler to use than an atomic force microscope but the spatial resolution is limited to approximately 1.5 micrometers, see Fig. 7.22. The Wollaston probes are supplied by Veeco (www.veeco.com) and are, therefore, compatible only with Veeco AFMs. The nanoprobes are supplied by Anasys Instruments and can be used with most popular makes of AFM. In addition, Anasys Instruments supply calibration kits containing temperature standards. More recently they have launched an instrument based on an optical microscope, the Vesta system, which is simpler to use than an atomic force microscope, but the spatial resolution is limited to approximately 1.5 pm. 

Metrology activities for thermal, mechanical properties, magnetism, micromagnetic modeling, and thermodynamics of nanostructures have been initiated. Nanoprobes to study nanometer material structures and devices with nanometer length scale accuracy and picosecond time resolution have been developed and others are in development. 

A variety of probe types have been developed for SECM-SICM. Eor one type of probe, gold was deposited onto one side of a nanopipette, which was then insulated entirely with aluminum oxide the nanopipette opening and a UME of the deposited gold were subsequently exposed via an EIB mill. A similar but simpler technique was also developed and used in SECM-SICM experiments— a nanopipette with a thin gold layer thermally evaporated on one side. Double-barrel carbon nanoprobes have also been fabricated, in which one barrel of the nanopipette is filled with a pyro-lized carbon electrode for the measurement of the faradaic current of SECM analysis and the other barrel filled with electrolyte solution to simultaneously measure the ion current for feedback and SICM analysis.269... 

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