Introduction
It is reported that the first magnifying spectacles were built in 13th century. It took roughly 400 years till the first microscope was invented. This instrument had only one lens and a long tube. So, the invention of a compound lens system for magnification took another 10 years when in 1609 Galileo invented a double lens telescope. This invention was remarkable in that it used a new concept, i.e. use of compound lenses, and thus lead to the making of better microscopes.
Anton van Leeuwenhoek of Holland (1632-1723) is known as the father of the microscope. He developed methods of polishing to create small lenses with high curvatures and large magnification. He used these lenses in microscopes and was the first who reported observing bacteria and other forms of life previously unseen. Today better understanding of optics, the science of light, and development of new methods and techniques in making better optical instruments has brought us to a point where our ability to see small objects under a microscope is only limited by the physics of optics. This limit, called the diffraction limit, is due to the bending of light (diffraction) around objects whose dimension is comparable to the wavelength of the illuminating light. As a result of this limitation, the optical microscope's resolution is roughly 0.1 mm - too large to let us see even viruses, let alone atoms. To read more about this limitation, visit the pages on Limiting Vision.
This limit is roughly the size of the wavelength of the probe (i.e. light for visible microscopes). deBroglie's wave description of matter objects allows a way out of this limitation. The electron's mass is 9.11x10-31 kg. So, even a fast moving electron's momentum is small enough to produce a rather small deBroglie wavelength (see pages on Quantum Mechanics for further explanation on deBroglie waves). In this way we could probe nm scale objects using electrons. This was utilized back in the 1930s when the first electron microscope was invented. This invention was most likely responsible for Richard Feynman's vision of the bottom-up approach. But it took another 50 years and the invention of Scanning Tunneling Microscope (probe microscopy) to open up the new investigations in Nanotechnology. Today a number of imaging devices allow researchers and technologists to create pictorial representations of objects as small as a single atom.
Below we will examine three of the most commonly used imaging devices in nanotechnology: the electron microscope, tunneling microscope, and atomic force microscope. Each of these has its own advantages and drawbacks. So, in a way, these tend to compliment each other even in the same application. It is worth mentioning that today near-field optical microscopes and other forms of visible microscopes have been invented that make their own claims in competing with electron or probe microscopes. All of these instruments are creating a richer world for us to "view".
Electron Microscope
This device operates similarly to a conventional optical microscope. Instead of imaging photons, as in the visible microscope, electron microscopes use electrons. In these instruments electron beams are accelerated to tens or hundreds of kilo-electron volts (keV). Using electric and magnetic fields the moving charged electrons are manipulated same as photons are in conventional optics. These instruments operate in two forms: transmission and scanning. In the Transmission Electron Microscope (TEM) the electron beam passes through the object being examined and then is imaged on a sensitive detector. In the Scanning Electron Microscope (SEM) the beam is scanned over the object and the reflected (scattered) beam is then imaged by the detector. In the case of TEM the sample needs to be very thin so as to allow for the passage of the electrons. The SEM requires highly conductive samples so as not to create blurring due to the electric field of deposited charges. (In both of these instruments high resolution imaging requires a conductive sample.) In most applications, the sample is coated with a thin layer of a metal, such as gold, before it is imaged in the microscope. Because of this, these devices have a limited application, especially for examining dynamic samples, such as in life forms.
In both SEM and TEM electrons are generated in the portion of the microscope that is referred to as the electron gun. There are two processes used to generate an electron beam. The most common of these is thermionic emission. In this method a metal with a high melting point (tungsten 3643 or molybdenum 2890 K) wire is heated by passing an electric current through it. At temperatures of around 2200 K, in addition to light, electrons are ejected from the metal surface similar to when water molecules leave the surface of a pan of water as it gets hotter and hotter. To create a beam of electrons the filament (heated wire) is held at a voltage below (negative biased) another metal plate that has a hole to allow the electrons to exit. In electron microscopes the gun is in fact a triode made of the filament (cathode) a negatively biased grid (often called the Wehnelt grid) and a positively biased anode plate (see diagram below). Thus, the effect of the grid is to concentrate the ejected electrons through a small opening that serves as the effective illuminating spot size of the electron beam.
The second method for creating an electron beam is called field emission. The field emitting gun is similar to the one described above, but the filament is made to have a very sharp tip. Also, the grid is held at a much higher potential so that the electrons at the tip experience a very large electric field that draws them away from the tip. The cathode is then used to accelerate the ejected electrons. The effect of the high field at the sharp tip is to lower the barrier height and to allow for tunneling of electrons across the vacuum barrier. This process results in the creation of far larger electron currents than is possible by thermionic emission. Although an aside, it is worth noting that thermionic emission is a self limiting process. This is not the case for field emission.
It is important to note that the large accelerating voltages used in electron microscopes create electrons moving at velocities for which classical mechanics no longer applies. So, to calculate electron's dynamical variables we then need to rely on Einstein's Relativity Theory. As is illustrated in the table below, even at an acceleration potential of 60 kV relativity makes more than a 2% correction.
|
Accelerating
potential (kV)
|
l (nm
) classical
|
l (nm
) relativistic
|
|
20
|
0.0086
|
0.0086
|
|
40
|
0.0061
|
0.0060
|
|
60
|
0.0050
|
0.0049
|
|
80
|
0.0043
|
0.0042
|
|
100
|
0.0039
|
0.0037
|
|
200
|
0.0027
|
0.0025
|
|
300
|
0.0022
|
0.0020
|
|
400
|
0.0019
|
0.0016
|
|
500
|
0.0017
|
0.0014
|
|
1000
|
0.0012
|
0.0009
|
To calculate the electron wavelength with the relativistic correction we can use the relativistic expressions for the mass and energy to obtain the relativistic relation:
l = [ 1.5 / ( V + 10-6 V2)]1/2 nm
Once electrons are emitted from the gun they are bent by electrodes and electromagnets and brought to focus in the same way that light is refracted in optical lenses. In the case of an electron beam, it is the Lorentz force that bends the electron path:
F = q E + q v x B
In electron microscopes a variety of electromagnets (condenser lens, zoom condenser lens, objective lens, etc.) are used for their optics. In almost all of these, the role of these magnets is the same as the optical elements in a standard optical microscope. (See the STM annimation, created by Ted Simons, to the right.)
For the transmission electron microscopes (TEM) the principle of image formation is very similar to that of an optical microscope. In the TEM the electron beam passes through a thin ("transparent") sample and then forms an image of the sample on a detecting screen. This screen could be a phosphorescent target that glows (i.e. emits light) once electrons strike it, very similar to conventional CRT (cathode ray tube) television screens. In the TEM the magnification of the image is a function of the electron "optics". However, the image created on the fluorescent screen could receive further magnification using a standard optical design. It is worth noting that in TEM it is the diffraction effect that plays the role of refraction in the optical imaging. Also worthy of notice is that the energetic electron beam could lead to other processes and thus produce secondary effects, such as X-ray emission, that could be used for elemental analysis of the sample.
There are a wide variety of TEMs, each suited for a particular application. In general, the accelerating potential ranges from 60 to 200 keV for most common machines. Medium range machines accelerate electrons in the 300 to 400 keV, and the high end machines go as high as 600 to few mega-electron volts. (The Toulouse electron microscope goes as high as 3.5 MV.) The primary advantage of using higher energy electron beams is to penetrate thicker samples. The 100 to 125 kV machines require sample thicknesses no more than a few nm for providing high resolution images. With the MV machines samples as thick as 0.1 mm could be imaged with relatively good resolution. Still, a primary drawback is sample damage at these high energies. Another obvious drawback of these ultra high energy machines is their construction cost and their large operating expenses.
All of these microscopes require good vacuum environments for protecting their electron beam from interact with background matter (air). (See the table to the right for more technical information about vacuum requirements.) Because of this all of the beam generation, steering, and detection apparatus must be enclosed in vacuum maintaining enclosures. Typically, these enclosures are thick stainless steel vessels that are continuously pumped using a collection of vacuum pumps. As the accelerating voltages increases high and ultrahigh vacuum becomes necessary and cause the size of the microscope to increase substantially. The MV microscopes can get as tall as 10 meters or taller.
Image formation in the scanning electron microscope (SEM) is very different. In the SEM a well focused electron beam is scanned in parallel lines, rasters, over the sample. Reflected (and secondary) electrons emitted from the sample are then collected by a fixed detector. Although different SEMs may use different types of electron detectors (electron counters), it is ultimately the number of electrons collected from the target that represent a measure of the image. The collected electron signal acts as a signature of the target at the point where the incident electron beam strikes. Typically, the collected electrons strike a scintillator (phosphorous) that emits light. This light is collected with the aid of a highly sensitive detector, such as a photomultiplier. The photomultiplier signal is further amplified, using conventional electronics, and is used to recreate an image. The ultimate resolution of the SEM is thus very much a function of how tightly the impinging electron beam is focused on the sample and how it is rastered over it. Because of this and that reflected electrons form a less compact region of emission, the secondary electrons are the more preferred signal for imaging.
The primary advantage of SEM over TEM is that it can image totally non-transparent (opaque) samples. It is therefore the electron microscope of choice for materials characterization, while TEM is most often used for imaging biological samples. However, SEM samples also need to be conductive otherwise electrons can collect on the sample and interact with the electron beam itself resulting in blurring. As stated earlier, nonconductive samples have to first receive a thin metal coating. Gold or aluminum are the most commonly used coatings to cover nonconductive samples. The process of metal coating, of course, does not allow imaging of living/evolving samples. But similar to TEM the scanning electron microscope can also produce X-rays indicative of sample composition, as well as sample characterization using X-ray diffraction.
Scanning Tunneling Microscope (STM)
These devices are referred to as scanning probe microscopes because instead of electrons or photons they use a sensitive scanning probe to examine the sample. The STM was the first scanning probe microscope to demonstrate atomic resolutions. In this device a conducive tip is scanned over the sample's surface at separation distances of a few nanometers. Electrons bound to the conductive tip tunnel though the air gap and jump to the sample surface with a probability that is exponentially related to the air gap thickness (size). As a result, the tunneling electron current represents the size of the air gap. If the tip is scanned over the surface at a constant height, then the current represents the change in the topography of the surface. It is clear, then, that, similar to electron microscopes STM imaging is limited to conductive sample. Since electrons can equally tunnel out of the sample to the tip, a few volts of bias potential is used to favor tunneling in one direction only. By changing the sign of the bias voltage the tunneling current direction can thus be reversed.
In terms of the technology of its construction STM relies on a mechanism that can move the tip at a constant height with great precision. This is accomplished using piezoelectric ceramics for the X, Y, and Z-translation of the tip. These ceramics respond to applied electric fields by mechanical expansion. Clever engineering of the piezo-ceramics can create rather linear and repeatable mechanical response to voltage changes. Equally important for generating high resolution images is a sharp tip. Typically, STM probes are manufactured to have few or one atom at their very tip. The simplest method of creating such sharp tips is through electrolytic or electrochemical etching. In one technique a thin tungsten wire is placed in a solution of potassium hydroxide and, in the presence of platinum, an electric current is passed through it. This causes the atoms to leave the wire into the solution and can create a very sharp point as the wire is removed. Other methods, such as sputtering, could also yield atomic-sharp tips. Libioulle et al. have reported tips with radius of curvature of 5 nm, while Klein et al. report tips with apex of 25 nm or less.
As noted above, same as in the case of electron microscopy STMs can only image conductive surfaces. Another of their drawback is that they require very flat surfaces. Rapidly changing surface topography can easily result in physical contact between the sample and the tip. This, of course, can ruin the tip as well as damage the sample.
Atomic Force Microscope (AFM)
The atomic force microscope is another scanning probe microscope similar to STM, but it functions under very different principles. In the first place, the probe in the AFM is made of a semiconductor, such as silicone. As in the case of the STM this probe ends at a tip with sharp edges with nm scale dimensions. The interaction of the probe with the sample is also electromagnetic, but there is no current flow between the tip and the sample. Instead, the atoms at the tip of the probe interact with the surface atoms of the sample via the Van der Waals force. This is an attractive force that is counteracted by the piezo elements driving the probe in such a way to keep the attractive force constant. In this way the probe is continually scanned over the surface of the sample at a constant height. Because the probe itself has to bend up and down to keep the distance between the tip and the sample a constant, a reflected laser beam from the probe's back serves to monitor the sample's surface topography. This laser beam's reflection is magnified by a cantilever action. So, very small motions of the probe's tip can be turned into larger deflections of the laser beam and thus get recorded.
The primary advantage of the AFM is that
the sample need not be conductive. In fact, AFM can image any variety of samples
from purely conductive to those belonging to insulating surfaces. It can have
nm resolution, depending on the type of the tip used and could respond to
other surface forces. AFMs have been invented in a variety of functions including
modes of operation in which the probe makes "contact" with the sample.
Another form of these probes is the tapping mode in which the frequency of
the contact of the tip with the sample is used as a means of obtaining more
detail information about the sample.
In the tapping mode the amplitude of the oscillating tip is kept a constant and the phase of the oscillation is monitored. From the phase shift information about sample-tip interactions, other useful information such as composition, adhesion, and viscosity can be inferred. Other sample-tip interactions could be magnetic interactions. For these investigations the tip is sputter coated with magnetic material. As the tip is scanning 10 to 100 nm above the sample, its oscillation frequency is shifted through magnetic interaction with the surface. Other forms of this probe microscopy include capacitance force microscopy and thermal microscopy.
All of these scanning probe microscopy techniques have been made possible thanks largely to the integrated chip manufacturing techniques that allow developers to create probe tips in the nm scale. More recent developments have involved the use of a carbon nanotube as the tip.
(Please direct any comments and/or suggestions to Seyffie Maleki at malekis@union.edu. Thanks!)
Last Modified August 13, 2005 malekis@union.edu
