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Exploring the microscopic world: Scanning Tunneling Microscopy Cover photo. http://researcher.ibm.com/researcher/view_project.php? id=4245 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

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Exploring the microscopic world: Scanning Tunneling Microscopy

Cover photo. http://researcher.ibm.com/researcher/view_project.php?id=4245

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.

http://www.ieap.uni-kiel.de/surface/ag-kipp/stm/stm.htm

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

Figure 4.

http://www.nanoscience.com/education/STM.html

Classically, a particle will bounce of the wall. When a

particle is small enough to exhibit quantum behavior, it

can tunnel through the wall. Its wave function can

extend beyond the barrier if it is small enough. If the

wave function does, the electron can essentially exist

beyond the barrier.

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.

http://www.nobelprize.org/educational/physics/microscopes/scanning/gallery/10.html

http://researcher.ibm.com/researcher/view_project_subpage.php?id=4253

ConclusionScanning 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.

[1] IBM, Scanning Tunneling Microscope [Online].

Available:http://www-03.ibm.com/ibm/history/ibm100/us/en/icons/microscope/

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

http://www.nanoscience.com/education/STM.html

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

http://www.gammavacuum.com/operation.asp

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

http://www.britannica.com/EBchecked/topic/526582/scanning-tunneling-microscope-STM/

285634/Applications

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

http://www.youtube.com/watch?v=0wF4f2YadoA

[6] History of the Microscope, Who invented the microscope? A complete Microscope History

[Online]. Available: http://www.history-of-the-microscope.org/history-of-the-microscope-who-

invented-the-microscope.php

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

http://www.nobelprize.org/educational/physics/microscopes/scanning/