Scanning thermal conduction microscopy with dual cantilever resistive probe

非常小的物体的热性质与我们所熟悉的较大物体的热性质不同。例如，像碳纳米管这样的小物体的大小与传递热量的声波（“声子”）的波长相当。因此，碳纳米管的热传导是由量子效应定义的。同时，这些纳米材料越来越多地用于制造有用的物体，如复合材料、晶体管和激光器。这些有用的器件和材料的热性能是极其重要的：热激光将是低效的，并将在短时间内失效。复合材料通常需要满足重要的热性能要求，同时还需要具有强度、重量轻和韧性。目前用于在纳米尺度上测量材料热性能的方法是不充分的。这个项目是关于在纳米尺度上测量热性能的新技术的发展。它是继格拉斯哥成功开发用于“扫描热显微镜”（SThM）的定量温度计之后发展起来的。这些纳米温度计在“原子力显微镜”（AFM）悬臂探针的末端包含一个小电阻温度计。虽然它们已经被证明是非常有能力在纳米尺度上测量温度的一种手段，但它们在测量导热性方面却很糟糕。这是因为热导率的测量既需要温度计又需要加热器，两者的要求是非常不同的：温度计不能变热（否则它会改变样品的温度），它必须尽可能弱地耦合样品的热量（否则它会使样品冷却）。对加热器的要求正好相反。它必须能够根据需要变得尽可能热，并且与样品强耦合，以便向样品注入显着的功率。提出的解决方案是使用与前面描述的类似类型的温度计，并将其与新设计的加热器集成在距离温度计已知的距离处。在对样品进行扫描以测量热性能变化时，温度计和加热器都需要与样品保持接触。这是通过使用带有两个尖端的微机械AFM悬臂来实现的，该悬臂在加热器和温度计元件之间保持恒定的横向分离，同时允许它们独立地“骑”过颠簸。用于制造加热器和温度计的材料，以及用于将它们与表面耦合的尖端形状将被独立优化。传感器本身并不是一个测量系统。测量必须在受控的环境中进行。在这种情况下，真空用于最小化降低空间分辨率的表面水膜，并防止通过空气传导从加热器到温度计的热量直接耦合。需要一个校准系统来确保测量是定量的，而不仅仅是漂亮的图片。需要构造电子器件来控制样品的输入功率，并对由此产生的温度变化进行灵敏的测量。最后，传感器必须与样品进行可控接触并四处移动，以便绘制样品热性能随位置的变化图。这个项目试图完成所有这些事情。测量系统本身的发展将由一组重要的三种材料的测量驱动，这些材料预计具有极端的热性能。它们是纳米晶体金刚石、碳纳米管和石墨烯薄膜。所有这些材料都是由碳制成的，但尺寸不同。它们都被用于实际应用中，它们的热性能很重要。

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.