Scanning thermal conduction microscopy with dual cantilever resistive probe

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The thermal properties of very small things differ from those of the larger objects with which we are familiar. For example, the size of a small object such as a carbon nanotube is comparable to the wavelength of the sound waves which transport heat ("acoustic phonons"). The thermal conduction of a carbon nanotube is therefore defined by quantum effects. At the same time these nanomaterials are increasingly used to make useful objects, such as composites, transistors and lasers. The thermal behaviour of these useful devices and materials is extremely important: A hot laser will be inefficient and will fail in a short time. A composite material will very often have an important thermal specification to meet, as well as being required to be strong, light and tough. The methods currently used to measure the thermal properties of materials at the nanoscale are inadequate.This project is concerned with the development of a new technology for the measurement of thermal properties at the nanoscale. It follows on from the successful development of quantitative thermometers at Glasgow which are used in "Scanning Thermal Microscopes" (SThM). These nano-thermometers incorporate a small resistance thermometer on the end of an "Atomic Force Microscope" (AFM) cantilever probe. Although they have proved to be very capable as a means to measure temperature at the nano-scale, they are bad at measuring thermal conductivity. This is because measurement of thermal conductivity requires both a thermometer and a heater, and the requirements of the two are very different: A thermometer must not get hot (or it would change the temperature of the sample) and it must couple heat out of the sample as weakly as possible (or it would cool the sample down). The requirements for a heater are the exact opposite. It must be capable of becoming as hot as necessary and also be strongly coupled to the sample, so as to inject a significant power into the sample. The solution proposed is to use a similar type of thermometer to that previously described, and to integrate it with a newly - designed heater, situated at a known distance from the thermometer. Both the thermometer and heater need to be held in contact with the sample as they are scanned over the it to measure changes in thermal properties. This is accomplished by use of a micromachined AFM cantilever with two tips, which maintains a constant lateral separation between the heater and thermometer elements whilst permitting them to "ride" over bumps independently. The materials used to fabricate heater and thermometer, along with the shape of the tip used to couple them to the surface will be independently optimised.A sensor is not, by itself, a measurement system. The measurement must be made in a controlled environment. In this case a vacuum is used to minimise the surface water film which degrades spatial resolution and to prevent direct coupling of heat from heater to thermometer by conduction through the air. A system for calibration is required to ensure that measurements are quantitative, not just pretty pictures. Electronics needs to be constructed to control the power input to the sample and to make a sensitive measurement of the resulting temperature change. Lastly the sensor must be brought into controlled contact with the sample and moved around, so as to map the changes in thermal properties of the sample as a function of position. This project seeks to accomplish all of these things. Development of the measurement system itself will be driven by the measurement of an important set of three materials which are expected to have extreme thermal properties. These are nanocrystalline diamond, carbon nanotubes and graphene films. All of these materials are made from carbon, but have different dimensionality. All of them are used in practical applications in which their thermal properties are important.