Seeing Nanostructures
Nanotechnology is the science of small things. Nanostructures can be as small as 1/1000 th the diameter of a human hair !!! Wow! That’s small.
Three questions generally arise when people first encounter nanotechnology.
- Why would we want to make or study such small things ?
- How would we make such small things ?
- How would we see these nanostructures ?
Questions 1) and 2) are answered elsewhere on this site. Here let's concentrate on how we “see” nanostructures.
First it takes some pretty sophisticated instruments to see nanostructures.
Optical (light) Microscopes focus visible light through “lenses” to make a magnified image. They work essentially like a magnifying glass. Some of you may even own a microscope, or have used one in school. Precision optical microscopes used in nanotechnology can cost up to $50,000. But even with the most precision, most sophisticated optical microscope, one problem remains—light waves are “big”, at least on the scale of nanostructures. As the resolution power of these instruments is limited to about half of the wavelength of light, they can only reveal features down to ~250 nm.
When we talk about seeing small stuctures, it is important to distinguish between “resolution” and “magnification”. We can “blow up” (magnify) an image (e.g. a picture) as much as we want - make it as big as a poster on your wall - but that does not make the image any sharper or increase our ability to resolve small structures – i.e. to have sharp edges and to distinquish separately closely spaced objects. Blowing up a picture too big just gives you a fuzzy big picture; that is called “empty magnification” and it does us little good. What is important is the ability to shaply see structures that are close to each other. This latter is called resolution and is the most important property of any microscope.
Optical microscopes give us a top-down, flat, "airplane" view of the surface. It is difficult to learn much about 3-D objects with a high powered optical microsocope because they have very low "depth of field"- i.e only objects at a certain, very narrow height will be in focus. For a high magnification optical microscope, this "depth of field" can be less than 1 micro meter- anything taller than 1 micrometer is out of focus and blurry.
With a super high quality optical miciroscope, we see and resolve structures down to about 250 nm. That still leaves a lot that we can’t see. For those, we need an electron microscope !
Electron Microscopes use electron beams instead of visible light, enabling resolution of features down to a few nm. Several different types of EMs exist, including Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) . Electron Microscopes use a beam of high energy electrons to probe the sample. Electrons do not suffer the same resolution limits that light does, so we can “see” features as small as 0.1 nm. This is the size of an individual atom. Electronic signal processing is used to create a picture of what the sample would look like if we could see it. While electron microscopy offers finer resolution of features than does optical microscopy, it requires vacuum conditions in order to maintain a focused electron beam. This makes electron microscopy inconvenient for examining many biological samples, which must first be preserved and coated with layers of metal atoms. Another advantage of electron microscopes is that they have both high magnification and high depth of field. We can see objects as in apparent three dimensions. This is again due to the short "wavelenth" of electrons. You may have seen some really "monster" like pictures of bugs that highlight the imaging capabilities of the scanning electron microscpe. High quality electron microscopes can cost from $250,000 to $1,000,000 ! They are one of the most useful instruments in our laboratories.
Scanning Probe Microscopes (SPM) of various types trace surface features by movement of a very fine pointed tip mounted on a flexible arm across a surface. SPM enables resolution of features down to ~1 nm in height, allowing imaging of single atoms under ideal conditions. Scanning Tunneling Microscopes (STM) measure current (i.e., electron flow) between the probe tip and sample, essentially acting like a tiny voltmeter. This method requires that the sample be electrically conductive. Atomic Force Microscopes (AFM - sometimes call Scanning Force Microscopes) measure interaction forces between probe tip and sample, providing information on the mechanical properties of surfaces. They can measure forces of 10-9 Newton. (For comparison, the force exerted by an apple is ~1 N.) AFMs are widely used to measure surface topography of many types of sample and do not require special conditions such as conductive surfaces or vacuum.
Scanned probe microscopes and particularly AFMs basically see things by touching. Imagine you have your right hand in a dark box with a mystery object and you are trying to figure out what the object is, without looking. One systematic way to do this would be to touch every point on a grid , say 30 points wide and 30 points deep, covering the entire floor of the box. Imagine that with your left hand, you record the the “height” ( or any other physical property) at each grid point on a piece of graph paper. You could then make a 3-d graph surface, or a 2-d plot with colors indicating height. After touching and recording 900 points, you would have a “picture” of the object. That is exactly what an atomic force microscope does, except the AFM uses a very fine point instead of a finger, and is built on a mechanism that can reproducibly move the tip less than 0.1 nm between points. Scanning probe microscopes can actually ‘feel” the bumps due to individual atoms and molecules ! !
Amazing Creatures with Nanoscale Features-Part 1 developed by our colleagues at Penn State's Center for Nanotechnology Education and Utilization.
This animation is an introduction to microscopy, scale, and applications of nanoscale properties. It introduces some of the tools that are used by scientists to visualize samples that are smaller than what we can see with our eyes. This includes the optical microscope, scanning electron microscope, and the atomic force microscope. In this animation, you will take a closer look at a butterfly wing at different magnifications and see features at the nanoscale that give the butterfly unique properties. Then, you will learn how scientists and engineers are able to mimic these structures through engineering techniques.
http://www.cneu.psu.edu/edToolsActivities.html
Ethan Allen, U. Washington
Lynn Rathbun, Cornell