Imaging the atom world: how do we see scales hundred thousand times smaller?
While we work at the nanometer scale, we need to be able to image objects on this scale in order to ultimately synthesize and manufacture at that scale. There are different types of microscopes available today that produce stunning pictures of atoms on surfaces or molecules moving in biological cells. All the sophisticated imaging techniques available today have each their own application range. Often we forget that this gives us still only a limited picture to what is really happening to atoms and molecules at the nanometer scale at a given temperature and pressure.
How can we see at the nanometer scale? With our eyes we can see a fraction of a millimeter, let’s say one tenth of a millimeter or one hundred micrometers which is a little more than the diameter of a single hair. If we want to see at smaller scales than this, we use a lens. The lens bends the light rays or refracts and the object appears larger. Optical microscopes work on this principle. Lenses work well as long as we do not have to work at scales smaller than the wavelength of light which is about half a micrometer. At this scale, we have to take into account the wave nature of light, so things are different. Optical microscopes help us to image about hundred times smaller than what we can see with our eyes. If we want to track a molecule or a particle that is still smaller, we can improve the resolution by labeling it with a fluorescent molecule. The fluorescence can be detected and we can follow the movement of the molecule or particles in liquids for example. If we need higher resolution, we can use electrons instead.
Using electrons to image: Thinking of electrons one might be puzzled as to how electrons, which are particles, can be used to image things at very small scales. At small scales things are different, drastically different. It’s the realm of quantum physics. Quantum particles propagate like waves and interact like particles. When the sample is thin enough and the electron comes in at high speed it can go through the sample. The path of the electron can be influenced by magnetic and electric fields and this effect is used in producing lenses for electrons. The advantage of electrons is that their wavelength is several orders of magnitude smaller than optical wavelengths and the microscope resolution is not limited by the wavelength but by the performance of the lenses. With electron microscopes we can increase the resolution by more than a factor of thousand when compared to optical microscopes. We can see one tenth of a nanometer with a high resolution electron microscope (HRTEM), the length scale of the atom world. We can see planes of atoms. Electrons are also reflected off and in fact there is a type of electron microscope that works in reflection mode (SEM). SEM’s have about ten fold smaller resolution than TEMs. The drawback of electron microscopes is that electrons are charged and we need vacuum environment in order for the electrons to propagate freely. There are SEMs available today that do not need high vacuum and can function in a partial nitrogen or argon atmosphere with reduced resolution. This makes it particularly interesting to observe growth processes at the nanometer scale. Non conducting samples are more difficult to observe with electrons and often need to be coated with a conducting metal layer. The electron microscope also needs a high voltage generator (10kV to several 100kV) to accelerate the electrons. All together electron microscopes are rather large instruments.
Using a surface sensor to image: during the last two decades, as manufacturing technology improved and powerful computers became available, a new type of microscope was developed. This microscopy is based on the principle that by scanning a sensor close to a surface (Scanning Probe Microscopy), the surface can be imaged point by point in a sequential manner. To achieve significant resolution, the sensor position needs to be controlled with a high accuracy near to the surface. This was first achieved by measuring the electron current between the conducting sensor tip and a conducting surface. Electrons are delocalized in clouds around the core of the atom. When the tip and the sample are close to each other the electron clouds overlap and a current flows between them that can be measured. As the distance between the tip and the surface gets large because of some topography on the surface, the current is reduced; the overlap of the electrons clouds is smaller. The distance dependence of the electron current is exponentially decreasing and can be used to measure the distance between the tip and the sample. The microscope that works on this principle is called the scanning tunneling Microscope (STM). It can image atoms on surfaces so long as electrons can flow between the tip and the sample. This current between two objects close to each other, but which do not touch each other yet, is called electron tunneling current. Alternatively, by using a cantilever and a laser beam to monitor the position of the cantilever when contacting the surface, it becomes possible to image insulating surfaces (scanning force microscopy (SFM) or atomic force microscopy (AFM)). Several different versions and types of sensors have been developed to image different properties of surfaces. Scanning probe microscopes come in small sizes and combine piezoelectric scanners, signal electronics and a computer to control the image scanning. They can be used in normal atmosphere although high resolution imaging needs high vacuum environment or a controlled atmosphere. Scanning force microscopes have been widely used and were instrumental for example in designing new memory technologies. They have been become popular due to their relative ease of use and the fact that cantilevers can be batch fabricated using standard semiconductor technology resulting in high performance probes at a relatively low price.Working at the nanometer we need to image. We can image using sophisticated microscopes. We need to keep in mind however that each microscope has its own limitations. In general, we need vacuum environment or a controlled atmosphere, high electrical conductivity, a thin sample or a small surface roughness. These are far away from real life conditions and much remains to be done before these imaging technologies can be adapted to more complex situations.
Further reading: ‘Soft Machines: Nanotechnology and Life’ by Richard A.L. Jones (Oxford University Press 2004)