UNIMO Technology UDR-204 Manual de instrucciones descargar pdf (Pagina 174)

Scanning Tunneling Microscopy (STM)

Spectroscopy with the STM

154 MultiMode SPM Instruction Manual Rev. B

9.3.2 Operation of STS

In the following sections, the operation of the spectroscopic functions of the NanoScope III STM

will be discussed. Additional information can be obtained from the Command Reference Manual.

STS Plot

There are several items that you should be aware of when using the NanoScope to acquire

spectroscopic plots with the STS Plot commands. The spectroscopic capabilities can provide

information that can help to distinguish different species, although exact species identiﬁcation is

difﬁcult, especially in air. The spectroscopic plots should aid in comparative studies between

samples or between different regions on a sample, but they will not reveal the precise make-up of

that sample.

A comparison to STM imaging reveals two somewhat conﬂicting requirements. The sample and tip

should produce consistent STM images. The images should repeat well from frame-to-frame and

be fairly free of noise or areas on the surface that appear unstable. For current-versus-voltage type

plots, the spectroscopic plots may be smooth and repeat well, but switching back to the STM

imaging mode reveals noisy images because tip quality and uniformity are probably more critical

for imaging than for making the simpler spectroscopic plots. This is generally not true for the I vs.

S plots. Some general recommendations for acquiring spectroscopic plots are:

1. Low settings for the Integral gain are preferred. Since the tip is not tracking any topography,

the lower gains are acceptable and tend to make the plots more stable.

2. The maximum input range of ±100nA on the NanoScope with the standard preamps will

restrict the current-versus-voltage plots.

3. The spectroscopic functions do not adjust for any loss of bias voltage due to IR losses caused

by the input impedance of the preamplifier. This has different effects for the STS i(v) and

STS i(s) functions which are discussed individually below:

STS i(v)

The spectroscopic plots do not correct for reductions in bias voltage caused by the preampliﬁer,

which for current-versus-voltage is typically small, anyway. For example, a 1V scan on the bias

voltage, producing a 50nA response in the tunneling current would have a 5 percent error at the

extremes of the scan (50 nA X 1 MΩ = 50mV or 5 percent of 1V).

Input impedance of the preampliﬁer leads to reduced bias voltages at increased tunneling currents.

This effect can cause the I vs. S plot to be inaccurate. For example, a bias voltage of 20mV will

restrict the upper limit of the tunneling current to 20nA since that level of current would effectively

reduce the bias voltage to zero. Even before the tunneling current gets to 20nA, the reduced bias

voltage will make the measured current appear lower than it should be for a given tip height.

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UNIMO Technology UDR-204 Manual de instrucciones Pagina 174