STUDENT PROJECTS

Introduction
It may be hard for the public to imagine exactly what a Scanning Tunneling Microscope (STM) is, so let me make this following analogy. STM is like an archeologist. An archeologist gets right down to the base surface of the Earth and creates a detailed record of what was observed at the surface. This is what STM does and there is no better machine in science today that will allow you to create topography of an unknown surface, while finding microscopic properties and variations. This is what makes the STM as well as STM research an exciting and beneficial field to study.

Purpose of the Project and Collaborators
I had the great opportunity to take advantage of this new field while I attended Peking University in Beijing, China over the summer of 2002 for one month. During my stay, I was able to work with an exciting group of people, at Peking University’s School of Physics, led by Prof. Hang Ji. I was able to work specifically with two undergraduate students, Chen Shou and Li Junyi. While I was not able to fully develop a complete understanding of what the research can encompass, I was able to learn the basic capabilities of the teaching version of the STM, build tips to the STM as well as complete simple experiments to learn techniques.

Equipment
To be more specific about what STM actually is, you first have to be able to visualize what the microscope looks like. Unlike any microscope you may have used in a high-school science class, the STM consists of a tip with a one electrode diameter and a sample with a connected current. The tip is capable of moving back and forth over a small surface using a voltage connected to both the tip and the sample. The sample is on a platform that is raised up to the tip for scanning. The tip and sample and enclosed in a small cylinder, which is then enclosed inside of a vacuum sealed bell jar, which is set for 1x10^-5 Pa. This is the STM that I was able to work with for the duration of my stay, which I worked experiments on, replace tips, and other such tasks.

There is such a STM in this research lab, which can weigh up to one ton and contain a variety of other machines, including an ultra-high vacuum (UHV), a scanning electron microscope (SEM), a low energy electron diffractor (LEED), and an Auger electron spectroscope (AES). The entire machine is hung and supported by a spring system, to avoid the slightest vibration which could possibly damage the sample or break the tip. Currently, the larger STM is being used for studies on protein stands and DNA.

All the parts of the STM are created separately and then placed together. The tip I the very heart of the STM and is of a simple construction. The tip is Tungsten (W) in a form similar to pencil lead, with a diameter of 2mm. Tungsten is used to make the tips because it is a conductive, yet corrosive material. To create the tip, we used a U-shaped test tube filled with a solution of sodium hydroxide (NaOH). We began with a 5M NaOH solution to eat away at the Tungsten and hopefully create a concave, skinny tip. A power source is hooked at one end to a different piece of Tungsten (non-corrosive) input that is submerged into the NaOH at one end and the other power output is connected to the piece of Tungsten to be used for our tip. Our piece is submerged about 2mm into the NaOH. We then changed to a 1M NaOH solution to refine our tip and to break the uneven and bulky end off and to create a one-electron diameter tip end. It is safe to wash off the NaOH solution with tap water once the process is complete, as long as care is taken to not touch the tip so that it does not break or get damaged in any way. The tip is then checked under a microscope to ensure that it has taken the correct shape. If it fits the ideal, we are able to mount it into the STM and begin scanning.

To create a scanning sample, you must also choose something that is conductive so that a tunneling current can be detected with the tip. For my tests that I ran, I used a base of HOPG (graphite), then surrounded it with a copper foil and then coated it with a chemical DDME. DDME is an organic material that was created by the Peking University School of Chemistry and was given to us to examine its structure.

To create this sample, the HOPG and copper are held over an open test tube that is wrapped in a coiled nickel wire, with DDME held in the test tube. This is all contained inside a vacuumed bell jar held at 1x10^-3Pa. Sending a voltage of 8-10V through the nickel wire, the temperature inside the vacuum reaches and maintains at 100ºC and the DDME is left to vaporize for 8 minutes. If this process does not create a coating of DDME that is of a suitable thickness and evenness, the voltage could be adjusted to between 8-12V or the vaporization process could have been extended up to 25 minutes. If the coating turns out too thick, the sample will not be able to conduct the given voltage evenly or maybe not at all and the tip would not be able to detect the current. If the sample turns out too thin, when it is scanned, the tip will only read the HOPG surface and will not detect the DDME material.

Creating the Atmosphere
All of the STM systems are run through a program on DOS, including the teaching version of the STM that I was able to use. The DS289 program version 3.0 for Scanning Tunneling Microscopy Systems is a data acquisition program that was created by the Peking University School of Physics in 1996. However, it is possible to create individual programs to run through the original program to be specified to the substance which is being scanned. One of the undergraduates, Chen Shou, is currently in the process of writing such a program for DDME.

To use the teaching version of the STM, I found it just to be a series of tasks to operate the machinery, but careful observations to be able to have useable results and a clear picture. Every time the STM is turned on, the sample must be raised up to the tip, which is about 3.3 mm from the base or whenever the tunneling current is detected. This is done to protect the tip and the sample from damage from any vibrations there may be when the STM is not in use. The process of raising the sample is known as the course approach and is done at a very cautious rate of 250nm per second with the tip waiting to detect the tunneling current, while the retracting process is done at a faster rate of about 800nm per second. This is because the tip needs time to detect the tunneling current and if the sample moves too quickly, it will most likely crash into the tip. While the sample is being raised, the voltage needs to be set to its maximum (about 980 mV) and the current at around 0.2 nAmps. This is to ensure that the tip can detect the current the moment it appears and that the sample does not come too close. The current indicates the distance between the tip and the sample material, so the smaller the current, the larger the distance, but with the voltage, the bigger the voltage, the bigger the distance.

Take a Picture
This STM program scans the surface of a given compound at an adjustable rate and can cover an adjustable area. To begin with, the area is set to cover a square distance of 200nm by 200nm and to scan an area of 1nm by 200nm in 2000ms or 2 seconds. The current and voltage are also adjustable, but unlike the area and scanning times which are set for the entire duration of the scan, we are able to change the voltage and current throughout the experiment so that clearer image may be achieved. To begin with, the voltage is set between 500 and 500 mV and the current is set between .5 and .6 nAmps. Once the tunneling current stage is reached, the sample needs to be adjusted either a little closer or a little further away from the tip so that the z-piezo is at 100V. The z-piezo is the measured distance between the tip and the sample in volts and is able to adjust itself while scanning to the peaks and valleys of the surface. With all coordinates set, the experiment is able to be run and the tip can take its course over the sample.

This image will give you the broadest range available with this particular area of the sample, and the goal is to narrow it down to a very small and smooth part of this sample so that the structure of the sample is clearly seen. With the first attempt, the amplification of the tip is set for 400 times and as the area gets narrowed down the amplification becomes larger as to get closer to the surface. With the first resulting image of the 200 by 200 unit, smooth spots are able to be spotted and the rough terrain is avoided. The next move is to cut the scanning area to 100 by 100 nm and to begin another scan. This is done by adjusting the x-piezo and y-piezo to create the desired origin and then cutting the available range in half (from the 200 capacity to 100) and begin to scan again. The goal area for the ending result is a two by two nm area of the surface with an amplification of 20,000 times.

Results
When learning how to use the STM, I began with simply scanning the HOPG base and I was able to create topographical images of that surface. After doing this, I was able to measure the distance from atom to atom and compare it to previous data to determine how ideal my HOPG sample was compared to others. In general, when working with a new material, this is the first task to complete so that you have some background and ground data to work from.

Acknowledgements: I would like to thank the National Science Foundation for providing this INT-REU opportunity so I was able to study abroad and so research I would not be able to work on at my home university. The Peking University School of Physics for hosting our research and providing us with wonderful opportunities. Finally, I would like to acknowledge the hard work that Chen Shou and Li Junyi had done in preparing for my arrival and for taking time to teach me how to use this technology.