Special Considerations for STM

Scanning Tunneling Microscopy (STM) requires specialized hardware components, and other considerations, described below:

STM Converter head and tip holder

To perform STM, you need to add a MultiMode 8 STM converter head. The STM converter head (so-called because it “converts” the MultiMode 8 to a scanning tunneling microscope) consists of a rigid ring bisected by a solid support for the tipholder. Because of the converter head’s compact construction, it holds the STM tip rigid, minimizing vibrational noise. Mounted within the converter head is a circuit that contains the preamplifier for the tunneling current and provides interconnections to the MultiMode 8 SPM electronics.

There are three types of STM converter heads available:

• MMSTMC/MMSTMEC: The standard version, which measures current in the nanoampere range:

• MMSTMLC/MMSTMLCE: A special, low-current version, which measures current in the picoampere range. The low-current converter head includes an in-line electronics package.
• MMSECPM: An electrochemistry version:

STM Probes

Probes for the NanoScope STM must be 0.01" in diameter to fit into the tipholder. The two most commonly used tips are made from either a platinum iridium (PtIr, see below) alloy or tungsten. In general, most of the discussion in this manual involving tips and noise reduction applies to both types of tips, but there are some applications that are tip-specific.

PtIr Probes—PtIr probes are mechanically formed and can be purchased directly from Bruker. PtIr probes seem to give better atomic resolution than tungsten, most likely due to the lower reactivity of platinum. The PtIr probes are not as uniformly shaped as the tungsten tips. The quality of the mechanically-formed PtIr probes will vary.

Tungsten Probes—Tungsten probes are electrochemically etched from tungsten wire (see Etching Tungsten Tips). Tungsten probes are more uniformly shaped. They may perform better on samples with steeply sloped features.

Sample Surface

Samples to be imaged with a scanning tunneling microscope must conduct electricity. In many cases nonconductive samples can be coated with a thin layer of a conductive material to facilitate imaging. The sample surface must be conductive enough to allow a few nanoamps of current to flow from the bias voltage source to the area to be scanned. STMs have been used to scan gold, silver, platinum, nickel, copper under oil, chrome plating, doped silicon under oil, conducting polymers, amorphous carbon, blue diamond, diamond-like carbon films, carbon fibers, graphite, iron-oxide compounds, semi-metals, doped semiconductors (molybdenum disulfide), cobalt-chromium compounds, stainless steel and liquid crystals. Oxide layers more than a few atoms thick on the sample tend to affect the scanning and wear down the tip as it is dragged through the oxide. The feedback loop will extend the tip until a tunneling current flows, even if it must push the tip through an oxide layer.

For samples that tend to oxidize, tunneling under oil or scanning in a glove box filled with inert gas can improve the imaging. Silicon oil or paraffin oil (mineral oil) also works well with some samples. One minor problem involved with the use of oil is the increased difficulty in the coarse positioning of the tip. The reflection of the tip comes off the liquid instead of the surface of the sample. It is difficult to tell when the tip is close to the sample surface. The best approach is to lower the tip until it just touches the surface of the oil, then select Engage.

Vibration isolation

The microscope should be isolated from sources of vibration in the acoustic and subacoustic frequencies. This requirement can be relaxed somewhat for large-scale images, but atomic-scale work is very sensitive to ordinary room vibrations.

As a final note, the best way to reduce coupling from vibrations is to eliminate as many sources of vibration as possible. Remember that vibrations can be transmitted to the microscope over the cable. Avoid tension in the cable and keep it away from fans and other noise sources. Also, keep the microscope away from sources of acoustic noise. Loud conversation can disrupt atomic scale images. Air currents can also disturb atomic images, so it may be necessary to run the microscope with a cover on (noting that a cover will trap heat and may cause substantial thermal drift during imaging).

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