King Fahad University of Petroleum and Minerals Electrical Engineering Department

EE 340-03 Electromagnetics Student Research Term Project 101/2010-2011

Scanning Tunneling Microscope (STM); its Structure and Applications in Nanotechnology

Mohammed Sawlan- 279295, Junior Student EE340-03 Electromagnetics

Abstract- the ability of producing a high resolution, three dimensional image of a material surface in a nano scale and manipulating its discrete atoms has been one of the most powerful breakthroughs in the history of science. The scanning tunneling microscope (STM) is an electron microscope that uses a single atom tip to scan/manipulate materials’ surfaces in nanometer scale. Many scientists believe that the invention of STM was the birth of nanoscience. This report will emphasize on the main components of STM, its theory of operation, its vital applications in nanotechnology, and the future improvement of STM operation and precision.

I. DISCOVERY: In 1986, the Nobel Prize for physics was granted for

the two inventors of STM named Gerd Binnig, a German physicist, and Heinrich Rohrer, a Swiss physicist, during their work at International Business Machines (IBM), an American multinational computer and IT corporation.[3]

II. INTRODUCTION:

STM has opened a new era for materials engineering that meets today’s and future’s demands. It can scan a material surface with a 0.1 nm width and 0.01 nm depth resolutions. Having this precision, STM is a powerful instrument that can image and dislocate individual atoms of different materials and therefore reengineer the materials surfaces to meet the desired demands of science and industry. The device can operate at harsh conditions such as from near zero Kelvin up to hundred degrees Celsius and can be placed in an ultra-high vacuum (a desirable condition to prevent contaminations), air, water, and various other liquid or gases. [7]

III. STM STRUCTURE:

STM consists of scanning tip, piezoelectric tube, tunneling current amplifier, and a computer to process and display the data, as shown in Figure 1.

A. Scanning Tip:

The tip of STM is considered as the most important part of the device. The smaller the tip radius is the more

precise and better scanning the STM performs. Its manufacturing requires lots of high precision and it is accomplished in different stages. The first one is the mechanical sharpening that uses a wire cutter and tweezers to hold the wire and produce sharp mini tips of 1000 nm radius. Then, a field emission is utilized to cleanly sharp the tip by applying a high DC voltage (200-600V), producing 10-100 nm radius tips. Finally, electrochemical etching is used usually in NaOH to produce 10-30 nm radius tips. STM tips are usually made of Tungsten because of the possibility of using Electro-chemical etching techniques to create single-atom tips. Other materials like Platinum-iridium and gold can be utilized. [2]

Fig.1. Scanning Tunneling Microscope Structure

B. Pizoelectrical tube:

A pizoelectrical tube or transducer is a ceramic positioning device that expands or contracts in response to a change in applied voltage. It’s used in STM to accurately position the scanning tip within 2 nanometers above the surface and also to pass the tunneling current to be processed by the computer. Examples are piezocrystals (quartz, BaTiO3) and Piezoceramics (Pb(Ti,Zr)O3). [5][4]

C. Tunneling Current Amplifier:

After applying a high basing voltage between the tip and the material surface, a tunneling current based on quantum mechanics theory will follow through a current

King Fahad University of Petroleum and Minerals Electrical Engineering Department

EE 340-03 Electromagnetics Student Research Term Project 101/2010-2011

amplifier in order to make the results out of this current readable by the computer. [7]

D. Computer Processing:

The computer is used to maintain the tip position over the sample, control scanning the sample, and also acquire data out of the scanning. In addition, further image processing to enhance the scanned image and several quantitative measurements on the data can be achieved by the help of special enhancing applications. [7]

IV. ITS THEORY OF OPERATION: In classical physics, an object bounces back at a high

potential when it doesn’t have enough energy. However, in quantum mechanics, its wave function dies off exponentially through that potential. So, by making the distance short enough, there will be a noticeable probability of finding that particle across that potential. This particle can be considered as the electron coming out of the scanning tip of the STM and tunnel through the air (high potential) to the surface.

Quantum mechanics predicts that the wave function

dies exponentially based on Schrödinger’s equation to obtain: I α ����� , where I is the current (A), d is the distance(m) from

the tip to the surface, and k is the material constant. For example, when d changes by 1 Å, the current

changes by a factor of about 10. Therefore, the tunneling current is very sensitive to the gap distance, making it more precise to detect the minor changes in distance by current inspection. [7][3]

V. ITS NANOTECHNOLOGY APPLICATIONS:

A. Surface Science Studies: By processing the amount of tunneling current with

respect of the tip-surface gad distance, STM has a great impact on surface science. STM has been used to study metals and semiconductors such as Si(100) which is the core substrate material for microelectronics device fabrication. Other examples are the inventions of Si(111) 7x7, Au(111), and SiC(0001) 3x3. [8]

B. Metrological Applications:

STM images have improved the nanotopography of surfaces by providing high precision analysis. For instance, a diamond-turned Al substrate was analyzed by STM to be used in a high capacity hard disc drive.[8]

C. Manipulation of Atoms:

In 1989, Don Eigler in IBM was able to show an image of 35 Xenon with its atoms arranged to the word IBM. This dislocating of atoms was performed with STM by increasing the tunneling current(increasing the bias voltage) and decreasing the tip-surface gap, therefore creating an attractive bond between the tip atom and the a single atom in the material. This invention has created ongoing breakthroughs in materials engineering.[1]

VI. WINDOW TO FUTURE:

The properties of the scanning tips reflect the performance of STM scanning. Scientists believe that that Carbon Nanotubes (CNT’s) have a great potential to be used as scanning tips for STM because of their great electromagnetic and mechanical properties such as highly conductive to electricity, ulra-thin with a range of 1.5 to 15 nm radius, crash-proof material because of the high strength of C-C bond, and the stable electronic structure of the tip. [6]

VII. CONCLUSION

STM technology of scanning surfaces at nanometer scale motivated many other microscopy techniques to be developed such as Photon Scanning Microscopy (PSTM), utilizing an optical tip to tunnel photons; Scanning Tunneling Potentiometry(STP), measuring electrical potential across a surface; and Spin Polarized Scanning Tunneling Microscopy (SPSTM), utilizing ferromagnetic tip to tunnel spin-polarized electrons into a magnetic sample. [7]

ACKNOWLEDGMENTS:

I sincerely thank my EE340-Electomagnetic instructor, Dr. Husain Masoudi for his ongoing support, feedback, and efforts that made writing this report possible.

REFRENCES:

[1] “Individual Atom Manipulation” University of Wisconsin 2008. [2] Jorgen Muller “Scanning Tunneling Microscopy Applications”

<http://www.e-basteln.de/index_q.htm>. [3] Lucas Carlson “Scanning Tunneling Microscope” Reed College, USA

march 2004. [4] Martin Pickel “Scanning Tunneling Microscope” Ulrich Starke 2000. [5] Peter Bedrossian “Scanning Tunneling Microscope: Opening a New Era

of Materials Engineering” Science& Technology, Harvard University, USA 1995.

[6] Sarah A. Burke “What Properties of Carbon Nanotubes Might Make the Appropriate Use as STM tips?” Oxford University 1999.

[7] “Scanning Tunneling Microscope” Wikipedia Foundation Inc. 2010. [8] Tit-wah Hui “Scanning Tunneling Microscope: Some Applications”

<http://www.chembio.uoguelph.ca/educmat/chm729/stmpage/stmappl.htm>.