Exploring the Microscopic World: Scanning Tunneling Microscopy

Exploring the microscopic world: Scanning Tunneling Microscopy

Cover photo.

Abstract

Scanning tunneling Microscopy has revolutionized the study of atoms. This unique microscopic technique allows images to be taken in such high resolution that atoms themselves can be seen. Utilizing its powerful imaging ability, scientists are pushing the boundaries of their knowledge of the microscopic world. Since its invention, there have been high hopes and endless possibilities of what might be discovered and what benefit would arise from it. New insights into how the building blocks of matter function at the fundamental level are being explored every day. STM has changed the field of nanotechnology forever, and it is continuing to teach scientists more and more in various fields including engineering, physics, and chemistry.

Introduction

The world of the extremely small is very much unknown. Similar to the stars in the sky, a scientist can’t reach out and touch anything, or physically change what he is looking at. Things cannot be affected, only observed. Just as a powerful telescope is vital to an astronomer, a powerful microscope is essential to exploring the very small. Scanning tunneling microscopy (STM) has opened doors to the microscopic world. It has provided new insights to what actually goes on at the nanoscale level. STM is a revolutionary technique that allows for images of just a few nanometers in length to be taken with incredible resolution. Taking advantage of quantum tunneling, ultra high vacuum conditions, and extremely low temperatures, the building blocks of the universe, the atoms themselves are being understood and even manipulated. STM has created a whole new style of research, with applications in engineering, physics, chemistry, and nanotechnology. STM is literally teaching us more about this microscopic world that we know so little about.

History

In 1981, two IBM researchers, Gerd Binnig and Heinrich Rohrer, revolutionized the science of the very, very small with their invention of the scanning tunneling microscope [1]. Never before could scientists peer into this microscopic world with such precision. With the ability to take images all the way down to the size of atoms themselves, STM has opened up entirely new fields for the study of matter. Figure 1 shows an example of an image taken of graphite. STM is widely regarded as the instrument that opened the door to nanotechnology and a wide range of explorations in many diverse fields [1]. This instrument, just five years after its invention, would win Binnig and Rohrer the Nobel prize in physics in 1986 [2]. In addition to changing the field of microscopic study forever, it also led to the invention of other sorts of high resolution microscopic instruments that together continue to teach us more about the world of the very small.

Figure 1. This is an STM image of graphite. The outlines in the picture are the hexagonal structures of the carbon atoms of graphite bonded together. Brighter spots correspond to more electrons reaching the tip, distinguishing the layers of the graphite.

How it works

STM synonymously stands for both the scanning tunneling microscope, as well as the scanning tunneling microscopic technique. This technique allows for images of just a few nanometers to be taken with surprisingly clear resolution. It does so by taking advantage of quantum tunneling, a process to be explained soon. Basically, an atomically sharp tip, that is a tip with one atom at the very end, is brought very close to a conductive surface. Figure 2 shows a picture of an STM tip. The tip slowly scans across the surface at a distance of only an atom's diameter [7]. At this small distance electrons can tunnel into the tip, from the surface, producing a measurable current. Figure 3 illustrates this principle.

Figure 2. This is a picture of an STM tip taken by a different type of microscope.

Figure 3. An atomically sharp tip is brought within nanometers of a surface. Once close enough, the electrons will tunnel the gap between surface and tip.

Tunneling is a quantum mechanical phenomenon that only occurs with particles small enough to exhibit quantum behavior, in this particular case electrons. Real, everyday objects (macro scale) are governed by classical/Newtonian mechanics, not quantum mechanics. In other words tunneling is not a phenomenon one can observe with the eyes. Tunneling is the process of a particle overcoming a barrier that classically it cannot overcome. To put it in simple terms let’s look at an example. Imagine a person throwing a tennis ball at a wall. Classically what will happen? The ball will, no matter what, bounce back unless it is thrown with enough force to overcome the barrier that is the wall. If the tennis ball were replaced with an electron, and the wall replaced with a metal barrier, the electron has two options. Classically it will bounce off the metal just as the tennis ball would bounce off the wall. Now however, it also has the option of tunneling, in which it will pass through the wall and continue on the other side, without disturbing the wall.

The phenomenon of tunneling occurs at the quantum level due to particle-wave duality. Really small particles, like electrons, have noticeable wave-like properties as well as classic particle-like properties. In fact, every object has wave like properties. The effects of these properties in objects we deal with everyday are just so small they become negligible. The wave nature of particles is only significant at quantum levels. What this wave nature yields in the case of STM is a way to explain tunneling. All the electrons have wave-functions, described by graphs like Figure 4. The wave-function governs the probability that an electron can exist at a given place in time. The higher peaks give places where it is more probable for the electron to exist. If the wave-function extends beyond the barrier, then the electron has a good chance of existing beyond the barrier. Because there are so many electrons in a particular sample, some will exist beyond the wall. The few of the many electrons in the sample that do tunnel to the tip produce a current. It is this current that is mapped out to produce the image.

The Machine

The STM apparatus itself is usually constructed into three chambers. While other variations exist, this is the most common. Figure 5 shows an example. There is usually one chamber for sample preparation and loading (called a sample chamber or prep chamber), one for tip preparation and loading (called a load lock chamber), and one for the microscope (called the STM chamber), in which the actual imaging occurs. The three chambers are connected, but isolated from each other with gate valves, to be opened only when tips or samples are moved in and out of the STM chamber. The chambers are isolated from each other due to uneven pressures in the three chambers. In order to get high resolution images, the STM chamber is kept at very low pressures.

Figure 5. This is a picture of an STM instrument. The leftmost chamber is the preparation chamber, where samples to be imaged are prepared. The middle chamber is the STM chamber where the actual imaging occurs. The rightmost chamber (much smaller than the other two) is the load lock, where STM tips are loaded into the STM chamber.

The microscope is very sensitive. Remember, it is taking nanoscale images. Even the slightest vibrations in the surroundings can disturb images. Because of this sensitivity, the STM apparatus is pumped down to ultra-high vacuum (UHV) conditions. It is not uncommon for the STM chamber to be at pressures of 10-11 Torr, an incredibly low pressure. To put this into perspective, atmospheric conditions, what you and I are likely feeling right now, is about 760 Torr. The STM Chamber is engineered to withstand pressures of 13 orders of magnitude less than atmosphere! These low pressures are referred to as ultra-high vacuum conditions. Keep in mind that even at ultra-high vacuum, these low temperatures do not produce a perfect vacuum. In a theoretical sense, vacuum is empty space, the existence of nothing in a given area. There are still molecules in the air of the chamber at these low pressures, but a significantly lower amount than at atmospheric conditions. Ultra-high vacuum is obtained using a combination of various pumps. The ion pump is generally the pump utilized by STM to achieve its lowest pressures. Ion pumps remove gases from their environment by charging the molecules and then attracting them with a high magnetic field [3]. This removes the air from chamber, by sucking it into the pump. The absence of the molecules in the space of the chamber is what generates the low pressure.

In addition to low pressures, the chamber is also built to withstand low temperatures. At the fundamental level all atoms vibrate. The STM is attempting to image these vibrating atoms. It is easy to see that reducing the vibration will be a great help in getting clearer images. To reduce the drift of the molecules the STM can be cooled all the way to 4K (-269 °C or -452 °F) using liquid helium [4]. That’s 4 degrees above absolute zero! These low temperatures reduce significantly the thermal drift of the molecules because the lower the temperature, the slower the molecules will vibrate. With as many molecules out of the way as possible and the instrument as cold as can be, electrons have fewer obstacles to overcome in tunneling the space between surface and tip. With a good flow of electrons into the STM probe, scientists can get images of high resolution.

Applications

Binnig and Rohrer’s breakthrough invention was the starting point for research in nanotechnology [1]. It has found applications in engineering, physics, chemistry, and materials science. It is a tool scientists use to learn about the nature of the nanoscale world. In learning more and more about the building blocks of matter, scientists have high hopes in developing new, smarter technologies, whose foundations stand in the newfound knowledge of the atomic world. More than 20 years ago, an IBM researcher named Dr. Eigler, using STM succeeded in moving atoms themselves. He rearranged them to make the smallest logo ever designed as seen in Figure 6. Understanding the properties, movement, and interaction of various materials at the nanoscale is essential for one day building smaller, faster, and more energy efficient processes and memory devices [5]. With the ability to understand and manipulate atoms, scientists hope to develop personalized health care and targeted treatments and therapies. Many industrial processes make use of chemical reactions. These reactions however are very inefficient at the fundamental level. In some cases, one out of a trillion molecules will react the way they are wanted to. STM is teaching scientists why reactions like this are inefficient. If they can discover ways to make the yield increase to even one out of a million, a company’s production will have increased by a factor of one million. It is discoveries like these that make STM such an important tool.

Figure 6 (includes all 3 images). These are images taken with STM after the repositioning of atoms. The IBM logo was the first time atoms have ever been moved in this fashion before.

Conclusion

Scanning tunneling microscopy has created a new way in which scientists can observe the world. The building blocks of our universe can be observed and even manipulated with STM. Making use of this technique, scientists and engineers can research and design new materials and products from a nanoscale foundation. For a long time, the microscopic world was just like the cosmic world, the atoms like the stars in the sky. You couldn’t touch them, or change them, only observe them. STM has invalidated this analogy. The stars of the microscopic world are being studied and even moved like the stars in the sky may never be. STM and the instruments and techniques it has paved the way for will continue to make discoveries in this microscopic world, and these discoveries will likely continue to amaze us.

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[2] Nanoscience Instruments, Scanning Tunneling Microscopy [Online]. Available:

[3] Gamma Vacuum, Ion Pump Technology [Online]. Available:

[4] Encyclopædia Britannica, scanning tunneling microscope (STM) [Online]. Available:

[5] YouTube, IBM Celebrates 20th Anniversary of Moving Atoms [Online]. Available:

[6] History of the Microscope, Who invented the microscope? A complete Microscope History [Online]. Available:

[7] Nobel Prize, The Scanning Tunneling Microscope [Online]. Available: