Scanning Tunneling Microscope


STM Setup

The STM must be securely isolated from vibrations in its environment to prevent the sample and/or tip from jumping and even crashing into the surface. Previously, this meant isolating the tip and sample in a vacuum where sound could not travel. The first STM experiments were also performed on levitating superconductors to absorb vibrations from the environment. It has since been shown that the STM will work outside of a vacuum and with much simpler vibration absorbtion techniques. For our laboratory, the STM head was place on a round piece of lead resting on a small inflated tube on an optics table. We found this to be adequate for the imaging we performed in this lab.

Tip Preperation

We experimented with two types of tips-- Platinum Iridium and electrochemically etched Tungsten. While Platinum Iridium tips are much simpler to create, the are also much more expensive. PtIr wire costs around $100 per foot. Tungsten wire is much cheaper at only $2 per foot.

The PtIr tip can be made by simply cutting the wire with wire cutters. We found that cutting the wire on an angle and pulling slightly as we cut gave us the best tip.

The Tungsten tip must be electrochemically etched as shown in the following diagram.

As a potential difference is applied to the platinum electrode, the 5M KOH begins to bubble. Slowly, small pieces of the tungsten wire can be seen falling away from the tip of the tungsten until only a sharp tip remains.

While one might think that the extra work required to make a tungsten tip would result in higher resolution images, we found that tip preparation is one area where you definitely get what you pay for. We never successfully imaged any surface with a tungsten tip. All of the images shown in the results/analysis section were taken with a PtIr tip.

Imaging Surfaces

The procedures for imaging surfaces can be found in better detail in the Burleigh Instructional STM Workbook. Here, I shall only provide a brief and general description of the steps taken in our laboratory.

Pre-imaging Steps:

  1. Mount the sample on the STM sample carriage using the Allen wrench and the provided sample mounts
  2. Prepare a tip as specified above if you do not already have a working tip
  3. Insert the tip and the sample carriage into the STM head
  4. Turn the "Sample Position" dial until the sample is within 0.5 mm of the tip
  5. Set the bias voltage and reference current as specified in the Burleigh Workbook
  6. Press the "Coarse Retract" button to initialize the STM
  7. Press the "Auto Approach" button to have the tip automatically approach to the specified reference current

Steps for imaging surfaces:

  1. Start the Burleigh Instructional STM software
  2. Load a previously created image file to obtain the proper configuration for the software controls
  3. Under the "Collect" menu, choose "scan controls"
  4. Choose "Scan" and watch as the image is displayed on the screen
  5. Press "C" to capture a scan when you have obtained a scan that is to your liking

After Scanning...

There are many filtering options available with the STM software, as well as an "Analysis" menu that allows you to measure separation distances and other features of the lattice structure. Use these features to obtain the best image you can from the raw data.

Back to Table of Contents. Forward to Results and Analysis.