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Scanning Tunneling Microscopy (STM)

STM-Speciﬁc Information and Operations

Rev. D Dimension 3100 Manual 185

Tunneling Tips

Although the microscope will accept any 0.010" diameter tip, tips made of platinum iridium (PtIr)

and tungsten are used most often. PtIr tips are supplied by Veeco while tungsten tips can be

electrochemically etched from tungsten wire by following instructions at the end of this chapter. 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 which are tip speciﬁc.

PtIr tips seem to give better atomic resolution than tungsten in air and liquids, probably due to the

lower reactivity of platinum. The PtIr tips are not as uniformly shaped as the tungsten tips, so

freshly etched tungsten tips may provide cleaner data when scanning steeply sloped surfaces such

as compact or optical disks. For tunneling on surfaces immersed in conductive liquids, coated tips

have been used. Glass coatings are removed from the very end of the tip by brieﬂy applying a high-

bias voltage.

The quality of the mechanically formed PtIr tips will vary. Some give beautiful images as soon as

tunneling starts; others start poorly but improve over time; while others start noisy and stay noisy.

Some tips have very ﬂimsy points which give an image that is stretched out on one side; this

indicates that the tip is unable to turn around quickly at the ends of the X scan. These tips should be

replaced unless the artifacts can be eliminated by increasing the X-rounding using the Calibrate

command in the Realtime > Microscope menu, or by capturing large scans and using the Ofﬂine >

Modify Zoom to focus on the desired information.

Sample Surface

Samples to be imaged with a scanning tunneling microscope must conduct electricity to some

degree. 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. NanoScopes

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, liquid crystals, and other

materials. 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 (if it can). If oxide

presents a problem, keep the sample covered with oil or operate the microscope in a glove bag ﬁlled

with nitrogen or argon. The standard NanoScope microscope heads were not designed to operate in

UHV.

On samples which are noisy or 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 work well with

some samples. The only 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, falsely engage the tip (use Ctrl-E quickly

after the Motor > Engage command), then lower the tip the rest of the way with the stepper motor.

It may take the motor about a minute or so, but it is better than smashing the tip on the surface of

the sample.

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