Studies of self assembled monolayers on highly oriented pyrolytic graphite using scanning tunneling microscopy and computational simulation 3

CHAPTER 3 CALIBRATION OF SCANNING TUNNELING MICROSCOPY USING HIGHLY ORIENTED PYROLYTIC GRAPHITE AS STANDARD 3.1 Purpose of Calibration The scanning tunneling microscope STM is the ide

CHAPTER 3 CALIBRATION OF SCANNING TUNNELING MICROSCOPY USING

HIGHLY ORIENTED PYROLYTIC GRAPHITE AS STANDARD

3.1 Purpose of Calibration

The scanning tunneling microscope (STM) is the ideal instrument for surface

investigations of matter on an atomic scale The most quantitative information

obtained by STM is the geometry and physics of surface topography, including the

roughness of microstructures [1, 2] Incorrect lateral calibration of the STM scanning

elements will cause the images to be stretched, compressed or even skewed Thus it is

crucial to have accurate calibration to achieve reliable STM images

Several factors can cause the distortion of the STM images, including

temperature and humidity which affect the piezoceramic’s sensitivity, hysteresis of the

system, thermal drift, the sharpness of the STM tip, and noise In order to minimize

these effects, a very sharp Pt/Ir tip was used throughout scanning process The

vibration isolation system is in operation during the experiment to isolate the vibration

from the environment The experimental room is equipped with air-conditioners and

dehumidifier to control the temperature and humidity respectively

Some works have been carried out regarding the calibration of STM Jorgensen

and coworkers [3] developed a technique based on detection of the reciprocal unit cell

in Fourier space for estimating the lateral calibration factors and the drift in the fast

scanning direction Desogus and coworkers used a capacitive sensor coupled to a

STM piezoscanner for accurate measurements [4] Lapshin also described a practical

method to find automatically the calibration coefficients and residual

nonorthogonality of STM [5] Wang et al employed the combination of STM and

X-Ray interferometer for calibration [2] There are also some computational methods

developed for data analysis to remove the distortion [6, 7] However, these methods

are not suitable for our experiments, either due to their complexity or incompatibility

with our instruments Here we present a simple calibration procedure by improving

the original calibration method developed by Veeco Instruments

3.2 Piezoelectricity and Scanner

Piezoelectricity is the ability of some materials (deleted) to generate an electric

potential in response to applied mechanical stress This may take the form of a

separation of electric charges across the crystal lattice The piezoelectric effect is

reversible, exhibiting the direct piezoelectric effect (the production of electricity when

stress is applied) as well as the converse piezoelectric effect (the production of stress

and/or strain when an electric field is applied) Lead zirconate titanate (PZT) crystal is

a ceramic perovskite material that shows a marked piezoelectric effect [8]

The effect finds useful applications such as the production and detection of sound,

generation of high voltages, electronic frequency generation, microbalance, and ultra

fine focusing of optical assemblies It is also the basis of operation of the scanning

probe microscope, such as STM and AFM

Scanner serve as the “movers and shakers” of every scanning probe microscope

Typically, they consist of a hollow tube made of piezoelectric material such as PZT

[9] Piezo materials contract or elongate when a negative or positive voltage is applied

depending upon the orientation of the material’s polarized grain structure The

scanners are used to manipulate the sample-tip movement very precisely to scan the

sample surface In some models (e.g., Small Sample MultiMode) the scanner tube

moves the sample, with the tip set stationary In other models (e.g., Dimension SPMs)

the sample is stationary and the tip is moved

The different scanners do not react exactly the same to a voltage Because of

slight variations in the orientation and size of the piezoelectric granular structure

(polarity), material thickness, etc., each scanner has a unique property [9] This

property is conveniently measured in terms of sensitivity, a ratio of piezo

voltage-to-piezo movement Sensitivity is not a linear relationship, because piezo

scanners exhibit a larger sensitivity (i.e., more movement per volt) at higher voltages

than they do at lower voltages This non-linear relationship is determined for each

scanner crystal As the scanner ages, its sensitivity will decrease and periodic

recalibration becomes necessary

The response curve of piezo-materials to applied voltage curve is presented in Fig

3.1 This curve typifies scanner sensitivity across the full range of movement The

vertical axis denotes the voltage applied to the scanner [9] while the horizontal axis

denotes the scanner movement At higher voltages, the scanner’s sensitivity increases

(i.e more movement per voltage applied), whereas the scanner is “motionless” at zero

volts Plotting each point along the curve describes a second-order, exponential

relationship which provides a rough approximation of the scanner sensitivity

Fig 3.1 Diagram showing the response curve of piezo-materials to applied voltage curve [9] The

vertical axis denotes the voltage applied to the scanner while the horizontal axis denotes the

scanner movement The scanner’s sensitivity is not linear, with larger sensitivity at higher voltage

3.3 Calibration of STM

3.3.1 Calibration Methods

For purposes of calibration, the manufacturer employs various derating and

coupling parameters to model the scanners’ nonlinear characteristics By precisely

determining the points along the scanner’s sensitivity curve, and through the

application of a rigorous mathematical model, the full-range measuring accuracy can

be made (deleted) better than 1 percent

Because the scanner sensitivities vary according to the voltage applied to them,

the reference must be thoroughly scanned at a variety of sizes and angles The user

then specifies the distance between known features on the reference’s surface and a

parameter is recorded to compensate the scanner’s movements The X, Y and Z axes

may be calibrated in any sequential order

The calibration procedure consists of three steps Firstly, a calibration reference

having surface features of precisely known dimensions is scanned These dimensions

are compared with those estimated by the SPM software; and the calibration

parameters are adjusted until the SPM’s dimensions meet the true dimensions of the

reference

The “A” scanner is the smallest scanner, with a total travel distance of

approximately 0.4m along each axis Its compact design provides excellent stability

for atomic scans Based on the previous method developed by the manufacturer, we

developed a relatively simple methodology to perform calibration of the “A” scanner

for scanning tunneling microscope function

3.3.2 Calibration Procedures

The calibration of STM can be conducted with a standard sample: highly ordered

pyrolytic graphite (HOPG) The HOPG needs to be cleaved prior to the scanning

Cleaving was accomplished by the adhering of a tape to the surface and then having it

peeled off, which produced a fresh surface of atoms having a regular lattice The

sample to be used was placed on a puck and then attached to the scanner cap

The surface was engaged by the tip The Intergral Gain and Setpoint are adjusted

to obtain good images The scanner was allowed to stabilize by scanning 2-3 frames

If the scanner has been running at extreme voltages, the stabilization time might take

longer The Setpoint is kept low if possible In our calibration, its value is set between

30-50pA The Z Center Position is set to 0V The scan rate is set quite high (~12Hz)

for atomic scale images to reduce some of the noise due to thermal drift

The condition of the tip is critical for obtaining a good STM image with atomic

resolution Fig 3.2 is a typical STM image of HOPG scanned with a poorly prepared

Pt-Ir tip

Fig 3.2 STM image of a cleaved HOPG scanned with a poorly prepared Pt-Ir tip prior to

calibration (6×6 nm, V bias =80mV, I offset=30pA)

If difficulty is experienced obtaining an image, a different Pt-Ir tip will be

installed until the image of carbon atoms can be roughly observed Fig 3.3 shows a

STM image of HOPG scanned with a good Pt-Ir tip

Fig 3.3 Typical STM image a clean HOPG scanned with a good Pt-Ir tip prior to calibration (6×6

nm, V bias =80mV, I offset =30pA) Image was treated with Lowpass function of the Nanoscope IIIa

software to remove noise

The alignment of the graphite crystal on the working platform was not always

ideal; it resulted in the angle between the measured direction and horizontal direction

Therefore the line containing the highest number of bright carbon atoms was selected

for distance measurement The section analysis (Fig 3.4) of the distance profile along

the white arrow in Fig 3.3 showed that the average distance between neighbouring

bright carbon atoms was 0.267nm

Fig 3.4 Section Analysis of the STM image in Fig 3.3 along the orientation of the white arrow

Fig 3.5 shows the standard parameters of the graphite surface lattice, where the

real atomic spacing between neighbouring bright carbon atoms is 0.246nm The

difference between the measured distance and the literature value was 8.5%, which

was more than the allowed error of 2.0% Thus, the calibration procedure had to be

carried out to ensure the measuring error of the STM measurements fall within the

allowed value

Fig 3.5 Schematic diagram of atomic spacing on the HOPG surface under STM Each circle

represents one bright carbon atoms

The calibration parameters were listed in Fig 3.6 Among them the ‘X fast sens’,

‘Y fast sens’, ‘X slow sens’ and ‘Y slow sens’ should be modified accordingly We

started with “X fast sens” when the scan angle was set at 0°

The new “X fast sens” value

distance

measured

distance theoretica





= 3.100 2.814

271

0

246

0





The new values of “Y slow sens” (at 0°, vertical direction), “Y fast sens” (at 90°,

horizontal direction) and “X slow sens” (at 90°, vertical direction) were also

calculated After four new parameters were employed, the STM images of HOPG

were captured again after scanning for a certain period of time when whole work

station became stable

Fig 3.6 Calibration parameters of scanner A

The new value of spacing between the adjacent bright carbons was recalculated

If the difference between the experimental result and the literature value was still

larger than 2%, a re-calibration was necessary Fig 3.7 shows the final graphite image

obtained by the STM after repeating several times of calibration procedures The

measured distance between adjacent bright carbon atoms was 0.250nm, as shown by

the section analysis, which was within the allowed range of 0.241 to 0.251nm The

calibration was considered successful and subsequent experiment would be carried

out using the same set of parameters obtained based on the latest calibration

Fig 3.7 STM image of HOPG after calibration with a good Pt-Ir tip (6×6nm, Vbias =80mV,

noise

Fig 3.8 Section Analysis of the STM image in Fig 3.7

3.4 Discussion

The calibration of the STM is particularly important in our experiment and a

chapter is therefore dedicated to this subject The calibration method provided by the

manufacturer has been modified For example, for the parameter of “X fast sens”, we

are supposed to measure the distance horizontally However, in most calibration

procedures it is difficult to position the graphite crystal at the correct angle to carry

out the measurements Instead, the distance along the line of highest atom density

white arrow shown in Fig 3.3 was measured The calibration can be easily done by

repeating that method several times, without spending time on positioning the

graphite crystal on the scanner platform

During the scanning process, the tip moved in two directions: from the top of the

image to the bottom of the image, and in the opposite direction It was observed that

the images obtained when the tip moves in the opposite directions were different This

could be due to the asymmetric form of the STM tip The Pt/Ir tip was usually cut

mechanically along the 45° angle, and therefore the tip did not have a symmetric

shape In addition the STM tip was not perpendicular to the scanning surface in most

cases, as shown in Fig 3.9

Fig 3.9 Illustration of contact between the STM tip and the HOPG surface

As the STM images were scanning-direction dependent, only images at one

scanning direction were collected to ensure the consistency of the experimental results

The STM images can also be affected by the height of the sample When the height of

the sample was changed, the STM head had to be adjusted, which in turn changed the

orientation of Pt/Ir tip and caused distortion of the images It is important to keep the

sample height constant during the whole experiments Hence, the same HOPG crystal

was used throughout the experiment, including calibration, SAMs preparation and

STM measurements

Furthermore, the piezoceramic component is very sensitive to the environment,

such as temperature and humidity The working environment has been equipped with

air conditioner and dehumidifier to provide the constant temperature and humidity for

STM instruments It was also strongly recommend that all the data should be collected

on the same day for one sample

3.5 Conclusion

In short, by modifying the original calibration methods provided by the

manufacturer, we developed the calibration procedures which are more convenient for

users Due to the complexity and sensitivity of the STM components, calibration must

be carried out every time before studying of the surfaces to ensure the reliability of

the experimental results

References

[1] Pope, K.J.; Smith, J.L.; Shapter, J.G Smart Mater Struct 2002, 11, 679

[2] Lin, W.; Kuetgens, U.; Becker, P.; Koenders, L.; Li, D.C.; Cao, M

Nanotechnology 1999, 10, 412

[3] Jorgensen, J.F.; Madsen, L.L.; Garnaes, J.; Carneiro, K.; Schaumburg, K J Vac

Sci Technol B 1994, 12, 1698

[4] Desogus, S.; Lanyi, S.; Nerino, R.; Picotto, G.B J Vac Sci Technol B 1994, 12,

1665

[5] Lapshin, R.V Rev Sci Instrum., 1998, 69, 3268

[6] Andersen, J E T.; Møller, P Surf Coat Technol 1994, 67, 213

[7] Lapshin, R.V Meas Sci Technol 2007, 18, 907

[8] Gandhi, M.V.; Thompson, B.S Smart Materials and Structures, London:

Chapman & Hall, 1992, 1st edition

[9] Digital Instruments, Support Note No 217, Rev D