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PART 4. Scanning Tunneling Microscopy and Spectroscopy. Contents PART 4. Scanning Tunneling Microscopy and Spectroscopy Contents 1. INTRODUCTION ................................................................................................................................. 4-2 2. PREPARATION FOR OPERATION ................................................................................................. 4-3 2.1. BASIC PROCEDURE OF THE INSTRUMENT PREPARATION FOR STM OPERATION............................. 4-3 2.2. ELECTROMECHANICAL CONFIGURATION ....................................................................................... 4-4 2.3. LOADING SCANNER CALIBRATION PARAMETERS .......................................................................... 4-4 2.4. MANUFACTURING THE TIP............................................................................................................. 4-6 2.5. INSTALLING THE TIP INTO THE HOLDER......................................................................................... 4-8 2.6. CENTERING THE SCANNER............................................................................................................. 4-9 2.7. PREPARING AND INSTALLING A SAMPLE ...................................................................................... 4-10 2.8. INSTALLING THE MEASURING HEAD............................................................................................ 4-12 2.9. PERFORMING THE PRELIMINARY SAMPLE TO TIP APPROACH ...................................................... 4-13 2.10. INSTALLATION OF A PROTECTIVE HOOD ...................................................................................... 4-13 2.11. INSTRUMENT TURNING ON........................................................................................................... 4-14 3. CONSTANT CURRENT MODE ....................................................................................................... 4-15 3.1. SETTING THE INSTRUMENT FOR WORKING IN STM MODES......................................................... 4-15 3.2. LANDING THE SAMPLE TO THE TIP............................................................................................... 4-16 3.3. SETTING “FEEDBACK GAINFACTOR WORKING LEVEL .............................................................. 4-19 3.4. SWITCHING THE FEEDBACK SIGNAL OVER .................................................................................. 4-20 3.5. SETTING THE SCANNING PARAMETERS ........................................................................................ 4-21 3.6. SCANNING ................................................................................................................................... 4-24 3.7. SAVING THE OBTAINED RESULTS ................................................................................................ 4-27 3.8. TERMINATION OF OPERATION...................................................................................................... 4-27 4. CONSTANT HEIGHT MODE. ATOMIC RESOLUTION ON GRAPHITE ............................... 4-29 5. STM SPECTROSCOPY ..................................................................................................................... 4-33 5.1. I(V) SPECTROSCOPY .................................................................................................................... 4-33 5.2. MODULATION TECHNIQUE OF THE SCANNING TUNNELING SPECTROSCOPY. BARRIER HEIGHT IMAGING ...................................................................................................................................... 4-34 5.2.1. SPM Mode Setup .......................................................................................................... 4-34 5.2.2. Setting the Piezo-oscillator Operating Frequency........................................................ 4-35 5.2.3. Scanning ....................................................................................................................... 4-37 4-1
Transcript
Page 1: PART 4. Scanning Tunneling Microscopy and Spectroscopyusers.auth.gr/eczss/AFM/4.scanning_tunneling_microscopy_pro.pdfScanning Tunneling Microscopy and Spectroscopy 1. Introduction

PART 4. Scanning Tunneling Microscopy and Spectroscopy. Contents

PART 4. Scanning Tunneling Microscopy and Spectroscopy

Contents

1. INTRODUCTION .................................................................................................................................4-2 2. PREPARATION FOR OPERATION .................................................................................................4-3

2.1. BASIC PROCEDURE OF THE INSTRUMENT PREPARATION FOR STM OPERATION.............................4-3 2.2. ELECTROMECHANICAL CONFIGURATION .......................................................................................4-4 2.3. LOADING SCANNER CALIBRATION PARAMETERS ..........................................................................4-4 2.4. MANUFACTURING THE TIP.............................................................................................................4-6 2.5. INSTALLING THE TIP INTO THE HOLDER.........................................................................................4-8 2.6. CENTERING THE SCANNER.............................................................................................................4-9 2.7. PREPARING AND INSTALLING A SAMPLE......................................................................................4-10 2.8. INSTALLING THE MEASURING HEAD............................................................................................4-12 2.9. PERFORMING THE PRELIMINARY SAMPLE TO TIP APPROACH ......................................................4-13 2.10. INSTALLATION OF A PROTECTIVE HOOD......................................................................................4-13 2.11. INSTRUMENT TURNING ON...........................................................................................................4-14

3. CONSTANT CURRENT MODE.......................................................................................................4-15 3.1. SETTING THE INSTRUMENT FOR WORKING IN STM MODES.........................................................4-15 3.2. LANDING THE SAMPLE TO THE TIP...............................................................................................4-16 3.3. SETTING “FEEDBACK GAIN” FACTOR WORKING LEVEL..............................................................4-19 3.4. SWITCHING THE FEEDBACK SIGNAL OVER ..................................................................................4-20 3.5. SETTING THE SCANNING PARAMETERS........................................................................................4-21 3.6. SCANNING ...................................................................................................................................4-24 3.7. SAVING THE OBTAINED RESULTS ................................................................................................4-27 3.8. TERMINATION OF OPERATION......................................................................................................4-27

4. CONSTANT HEIGHT MODE. ATOMIC RESOLUTION ON GRAPHITE ...............................4-29 5. STM SPECTROSCOPY .....................................................................................................................4-33

5.1. I(V) SPECTROSCOPY ....................................................................................................................4-33 5.2. MODULATION TECHNIQUE OF THE SCANNING TUNNELING SPECTROSCOPY. BARRIER HEIGHT

IMAGING......................................................................................................................................4-34 5.2.1. SPM Mode Setup ..........................................................................................................4-34 5.2.2. Setting the Piezo-oscillator Operating Frequency........................................................4-35 5.2.3. Scanning .......................................................................................................................4-37

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PART 4. Scanning Tunneling Microscopy and Spectroscopy

1. Introduction

The Scanning Tunneling Microscopy is intended for the investigation of the surfaces properties of conductive materials with resolution down to atomic scale. The tunnel current recorded during scanning is small enough (0.5 pA -50 nA) to allow investigating samples with low conductance, biological objects in particular.

Application of special modes allows to investigate the surface distribution of various electrical characteristics, such as work function, local density of electron states, etc.

One of the STM limitations is the complexity of interpretation of the obtained results, since the STM image is determined not only by the topography, but also by the local electrical characteristics.

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Chapter 2. Preparation for Operation

2. Preparation for Operation

This section describes general preparation procedures to perform STM measurements.

2.1. Basic Procedure of the Instrument Preparation for STM Operation

Preparation of the instrument for operation using STM modes can be divided into the following basic operations:

Step 1. Electromechanical Configuration (see page 4-4)

Step 2. Loading Scanner Calibration Parameters (see page 4-4)

Step 3. When the Equivalent Scanner is used it shall be prepared for operation as described in the Attachment. (see i. «Preparing Scanner for Operation with Equivalent»).

Step 4. Manufacturing the Tip (see page 4-6)

Step 5. Installing the Tip into the Holder (see page 4-8)

Step 6. Centering the Scanner (see page 4-9)

Step 7. Preparing and Installing a Sample (see page 4-10)

Step 8. Installing the Measuring Head (see page 4-12)

Step 9. Performing the Preliminary Sample to Tip Approach (see page 4-12)

Step 10 Installation of a Protective (see page 4-13)

Step 11. Instrument Turning on (sees page 4-14)

ATTENTION! Switch the controller off before connecting/disconnecting cables. Disconnecting or connecting while the device is operating may damage its electrical circuit.

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PART 4. Scanning Tunneling Microscopy and Spectroscopy

2.2. Electromechanical Configuration

1. Set up the scanner using the following procedure: undo the spring clips (Fig. 2-1) and insert the scanner into the positioning device of the approach unit (Fig. 2-2). Make sure the truncated part of the carriage is facing the user.

Fig. 2-1 Fig. 2-2

2. Connect the scanner to the corresponding terminal of the approach unit, labeled SCANNER.

3. Connect the STM head to the corresponding terminal of the switching unit, HEAD, located on the base of the instrument.

4. Connect the switching unit cables HEAD and SCANNER to the corresponding terminals of the SPM controller.

2.3. Loading Scanner Calibration Parameters

Turn on the computer and than launch the control program.

Upon starting the program the Default.par file which contains calibration parameter for a specific scanner is loaded by default. If only one scanner is supplied with the instrument then the Default.par file contains the parameters for this particular scanner.

If several scanners are included in the package then Default.par stores parameters corresponding to one of the scanners.

After changing the scanner the related parameter file (par-file) shall be loaded.

To load a par-file for the scanner complete the following operations:

1. In the Main menu select successively the following items Settings → Calibrations → Load Calibrations (Fig. 2-3).

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Chapter 2. Preparation for Operation

Fig. 2-3

This will open a dialog box with a list of par-files contained in the PARFiles folder as in the example shown in Fig. 2-4.

The names of the par-files have the following pattern: XXXXXX-YY-ZZZ.par

where XXXXXX – identifying code;

YY – model year;

ZZZ – serial number.

Fig. 2-4

2. Choose the par-file corresponding to the installed scanner.

3. Click the Open button to load the par-file.

If you prefer the current scanner parameters to load by default at the program start-up save the file as Default.par. Proceed as follows:

1. In the Main menu select the items: Settings → Calibrations → Save Calibrations (Fig. 2-5).

Fig. 2-5

2. The Save As dialog box opens (Fig. 2-6). Save the file as Default.par.

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PART 4. Scanning Tunneling Microscopy and Spectroscopy

Fig. 2-6

2.4. Manufacturing the Tip

The tip is the sharpened end of a platinum-iridium (PtIr), or platinum-rhodium (PtRo) (with platinum content of about 80 %) or tungsten (W) wire, 8-10 mm long with a diameter of 0.25 - 0.5 mm.

The sharpness of the tip can be evaluated by imaging a reference sample with known surface characteristics, for example Highly Oriented Pyrolytic Graphite (HOPG).

There are two techniques of manufacturing an STM tip:

− By cutting the wire apex with scissor (PtIr, PtRo) (see below);

− By electrochemical etching (W, Pt, PtIr, PtRo).

The simplest STM tip manufacturing technique consists in cutting the wire apex with the scissors. In that case the apex radius of curvature is less than 10 nm.

Sharp-edged scissors and tweezers with kinks on the interior surface, to be found in the toolkit supplied with the microscope, are used to cut the wire.

ATTENTION! Do not use the wire cutting scissors for other purposes.

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Chapter 2. Preparation for Operation

Fig. 2-7

Apex forming procedure:

1. Clamp the wire with the tweezers so that it projects beyond its edge for 2-3 mm (Fig. 2-7).

2. Cut the wire at an angle of 10-15 degrees as close to its apex as possible and simultaneously pull the scissors along the wire axis to separate the part being cut off. This is done to avoid the contact between the cutting edges of the scissors and the tip apex. This procedure implies rather a tearing off the wire in the last moment than truncating it. This results in a sharp apex, formed at the wire’s end (Fug. 2-8).

Fug. 2-8. Typical shape of a wire cutoff (apex of the tip)

3. Check the resulting cutoff shape using the optical microscope with 200-x magnification

(if possible). Repeat the cutting process, if necessary.

ATTENTION! Avoid any contact with the tip apex in order not to damage it.

The overall length of the tip should not exceed 10 mm.

After cutting the wire you may anneal its apex in the flame of an alcohol lamp for 1-2 seconds to remove organic substances. Check the apex of the tip using the optical microscope1: the cutoff section should be bright, with no traces of black or dust.

1 This procedure is optional.

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PART 4. Scanning Tunneling Microscopy and Spectroscopy

2.5. Installing the Tip into the Holder

While installing the tip into the holder, it is convenient to use tweezers of two types: narrow tweezers with smooth tips and wide tweezers with kinks.

To insert the blunt end of the tip into the holder do the following:

1. Turn the STM head face down and place it on a plane surface.

2. Take the wide tweezers in one hand, holding the narrow one with the tip in it in the other.

ATTENTION! Avoid contacting a sharpened apex with any surfaces during installation.

3. Squeeze the pressure spring using the wide tweezers (Fig. 2-9 a).

4. Insert the tip's blunt end into the holder so that the sharpened end projects beyond the edge of the holder no more than 3-4 mm (Fig. 2-9 b).

a) b)

Fig. 2-9. Installing the tip into the holder

5. Release the spring. The tip should be fixed firmly in the holder.

NOTE. The quality of the tip sharpness and firmness of the tip fixation in the clamp are the primary factors in determining the quality of results obtained with STM.

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Chapter 2. Preparation for Operation

2.6. Centering the Scanner

Before installing a sample it is recommended to adjust the scanner with respect to the measuring head so that the tip coincides approximately with the axis of the scanner. This will reduce the surface slope during scanning.

NOTE. This procedure is optional.

Scanner centering procedure:

1. Wind the manual approach knob counter-clockwise (Fig. 2-7) to move the scanner to its lowest position.

Fig. 2-10. Manual approach knob

2. Install the measuring head with its legs into the seats of the replaceable base (Fig. 2-11).

Fig. 2-11

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PART 4. Scanning Tunneling Microscopy and Spectroscopy

3. Looking from a side, set the tip-sample distance to 2-3 mm, by rotating the approaching knob (Fig. 2-10) counter-clockwise.

4. Looking from top, make the apex of the tip coincide with the axis of the scanner, using micrometric screws of the positioning device (Fig. 2-12).

Fig. 2-12. 1 – central orifice in the sample stage (sitting on the scanner axis)

2 – micrometric screws of the positioning device

5. Retract the scanner by rotating the approaching knob clockwise.

6. Take the measuring head off the replaceable base.

2.7. Preparing and Installing a Sample

To fix samples during the STM investigations special substrates with electric contact for sample biasing are used (Fig. 2-13).

Fig. 2-13. Substrate with a spring contact

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Chapter 2. Preparation for Operation

Sample preparation procedure:

1. Take a clean substrate. Cut off a strip of a double-sided adhesive tape, slightly wider than the sample.

2. Stick the adhesive tape to the substrate, smooth its surface out with the back of the tweezers to remove air bubbles between the substrate and the adhesive tape.

3. Put the sample, for example, a graphite plate (HOPG), on the adhesive tape and carefully press it with tweezers in several places (not touching the area intended for the investigation) (Fig. 2-14).

NOTE. After the sample is fixed on the substrate, a noticeable vertical drift of the sample can occur within one hour (due to the slow relaxation of sticky tape). This drift should be taken into account. If a minimal drift is required due to the nature of the investigation, prepare the sample beforehand (at least in an hour before the investigation).

Fig. 2-14. Prepared substrate with a HOPG sample installed

4. To clean and smooth the graphite sample, do the following:

a. Stick a piece of the adhesive tape (slightly wider than the sample) on the surface of the graphite sample;

b. Smooth it out to remove air bubbles;

c. With an abrupt movement remove the adhesive tape together with the top layer of graphite. The molecular layers of graphite have a spalling angle in relation to the surface layer, at which the layers are easily removed. Therefore the smoothness of the surface will depend on the direction, in which you remove a layer. Find the best direction to obtain the smoothest surface and remember (or mark) it, to use it later for cleaning the surface of pyrolitic graphite samples.

5. Use tweezers to turn the contact spring, used to apply the bias voltage and fix it to the

edge of the sample so that the center of the sample remains free (Fig. 2-14).

6. Install the substrate with the sample on the sample stage, sliding it in sideways under the fixing clips. The substrate is inserted in such a manner as to make the spring contact face the operator (Fig. 2-15). Make sure that the spring contact does not touch any metal parts of the tip holder.

7. Insert the voltage feeding contact terminal into BV connector on the approach unit.

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PART 4. Scanning Tunneling Microscopy and Spectroscopy

ATTENTION! Switch off the controller before connecting/disconnecting any cable. Connecting or disconnecting cables while the device is operating may damage the electronic circuit.

Fig. 2-15 1 – spring contact, 2 – BV contact terminal

2.8. Installing the Measuring Head

1. Insert the measuring head support legs into the corresponding seats of the multipurpose mount (Fig. 2-16).

Fig. 2-16. Installing the STM head

2. Connect the STM head to the terminal labeled HEAD of the switching unit (if step has not been done).

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Chapter 2. Preparation for Operation

2.9. Performing the Preliminary Sample to Tip Approach

1. Looking from a side, approach the sample to the tip apex to a distance of 2-3 mm, by rotating the approaching knob.

2. Looking from top, position the tip above the area intended for investigation, by rotating the positioner screws.

NOTE. It is recommended to position the sample beforehand so that the investigated area is as close to the scanner axis as possible. In case the investigated area is far from the axis, a surface sloping can occur during scanning, restricting the application of certain techniques and complicating their implementation.

3. Then, while looking from a side, reduce the distance between the sample and the tip apex down to 0.5-1 mm (Fig. 2-17).

0.5–

1 mm

Fig. 2-17. The STM tip – sample clearance after a preliminary approach

2.10. Installation of a Protective Hood

It is recommended to work with a hood in the following cases:

− if it is necessary to obtain a high resolution image in XY plane or along Z axis;

− when working at controlled temperature;

− for protection against temperature drifts;

− for reduction of acoustic noise level.

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PART 4. Scanning Tunneling Microscopy and Spectroscopy

To install the protective hood proceed as follows:

1. Install the fixing clip of the measuring head cable into its holder built into the approach unit (Fig. 2-18).

Fig. 2-18. Installation of the STM head cable into its holder

2. Mount the protective hood onto the platen of the approach unit.

3. Ground the protective hood by plugging the special cable of the approach unit into the grounding jack on the hood (Fig. 2-19).

Fig. 2-19 Protective hood mounted on the approach unit 1 – grounding jack

2.11. Instrument Turning on

1. Turn on the computer and launch the control program.

2. Turn on the SPM controller with the power switch located on the front panel. If turning on is successful, a green "tick" appears in the monitor screen bottom left corner.

3. Turn on the vibration isolation system.

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Chapter 3. Constant Current Mode

3. Constant Current Mode

The Constant Current Mode implies maintaining a constant value of tunnel current during scanning, using the feedback loop system. The feedback signal, fed to the Z-channel of the scanner, traces the surface topography.

To maintain the feedback while operating the instrument in STM modes, two signals are used:

Iprlow a signal proportional to the value of the tunnel current flowing through the tip. This signal is used to maintain the feedback when approaching the sample to the tip;

Iprlog a signal proportional to the logarithm of value of the tunnel current flowing through the tip. This signal is used to maintain the feedback during scanning.

3.1. Setting the Instrument for Working in STM Modes

To switch the device onto the STM modes, click the switch button, selecting the device electronic configuration and select Tunnel Current in the basic operation panel (Fig. 3-1).

Fig. 3-1

NOTE:

− The switch button, selecting the device electronic configuration can be set in one of the four states: Custom, Contact, SemiContact and Tunnel Current;

− If Tunnel Current state is set, the device will be automatically configured for measurements in STM modes: IprLog signal is switched to the feedback input, the oscillator is set to off position, and Bias V constant voltage is fed to the sample;

− If Custom state is set, the user can customize the device freely, setting up various configurations and implementing different states of the device.

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PART 4. Scanning Tunneling Microscopy and Spectroscopy

3.2. Landing the Sample to the Tip

1. Switch the Approach tab (the button on the main operations panel) (Fig. 3-2).

Fig. 3-2

2. Check the state of the automatic Set Point parameter setup button. The Auto SetPoint button should be pressed in, as shown in Fig. 3-3.

Fig. 3-3

3. Set the Bias V parameter value to 0.1 V as shown in Fig. 3-4.

Fig. 3-4

4. Launch the approach procedure by clicking the Landing button.

As a result:

- Set Point parameter value will be automatically set to 0.1 nA;

- IprLow will be set as the feedback input signal;

- The feedback will be switched-on and Z-piezo-scanner will be fully protracted. Z-scanner protraction will be monitored in the piezotube protraction analogue indicator, located in the lower left corner of the program main window. The length of a color strip represents the level of the scanner protraction;

- The step motor will start, moving the scanner with the sample towards the tip.

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Chapter 3. Constant Current Mode

5. While the approaching is in progress, observe the changes of IprLow signal in the oscilloscope window and the state of the scanner protraction indicator, waiting for the termination of the process.

After a while, if the parameters of approaching are correctly set, the approaching will stop and the following will occur:

- IprLow signal will increase up to Set Point parameter value. The feedback will drive the Z-scanner in a position at which IprLow signal is equal to Set Point signal, this position corresponding approximately to one half of the scanner protraction range;

- The length of the color strip of the indicator will shorten, taking some intermediate position (Fig. 3-5);

- The step motor will stop;

- The increase of IprLow signal up to the value of Set Point parameter will be represented on IprLow (t) trace in the oscilloscope window;

- the message "…Approach Done." will appear in the journal (pos. 2 in Fig. 3-5);

Fig. 3-5. Completing of approach procedure 1 – indicator of scanner protraction; 2 – journal

- The feedback input signal will switch onto IprLog.

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PART 4. Scanning Tunneling Microscopy and Spectroscopy

Special cases

Self-oscillations

It may happen that performing the approaching and increasing the IprLow signal up to the Set point parameter value leads to a substantial growth of the IprLow signal noise (for example, as it is shown in Fig. 3-6). This means that there is an oscillation in the feedback system due to excessively high gain factor (FB Gain parameter). In this case it is necessary to reduce the FB Gain down to 50%-70% of the threshold value. Adjustment of the FB Gain parameter is described below, in item 3.3 “Setting “Feedback Gain” Factor Working Level” on page 4-19.

Fig. 3-6

Selecting and setting up the Set Point parameter manually

If the Auto SetPoint button is not active, it is necessary to set the value of the Set Point parameter manually. The value is set in the input field located on the panel of main parameters.

The recommended initial value of the Set Point is 0.1 nA.

While selecting an optimal Set Point value, it is necessary to account for the following:

The value of the Set Point parameter determines the value of the tunnel current, flowing between the tip and the sample surface.

The greater is the Set Point value, the greater is the value of the tunnel current and the more intense is the tip-sample surface interaction. Therefore the level of the tip-sample surface interaction can be set by modifying the Set Point value. This provides the possibility to obtain surface topography images correlated to different tip-sample interaction intensities.

Choosing a too large value for the Set Point parameter, corresponding to a high tunnel current, may result in damaging the tip as well as the surface during scanning.

The operation of the feedback system may prove unstable if the Set Point parameter value is too little.

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Chapter 3. Constant Current Mode

3.3. Setting “Feedback Gain” Factor Working Level

The larger is the gain factor (FB Gain parameter), the higher is the rate of feedback processing. However, if the gain factor is too large (let us call this value the threshold value), the operation of the feedback system becomes unstable, and the IprLow signal starts oscillating.

To provide a stable system operation, the gain factor level should be set to no more than 60%-70% of the threshold value, at which oscillation occurs.

To set the operating level of the feedback gain factor, do the following:

1. Double click the left mouse button in the input field of FB Gain parameter on the basic operations panel (Fig. 3-7). Increase the FB Gain value using the popup slider, while observing the level of IprLow signal by means of the program oscilloscope.

NOTE. The same relates to IprLog signal.

Fig. 3-7

2. Determine the value of FB Gain factor, at which the oscillation starts. The oscillation

onset is registered by the abrupt increase of the variable component of IprLow signal (Fig. 3-8).

Fig. 3-8

3. Decreasing the FB Gain parameter set the operating FB Gain value to 60%-70% of the

value at which the oscillation of IprLow signal starts.

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PART 4. Scanning Tunneling Microscopy and Spectroscopy

3.4. Switching the Feedback Signal Over

The feedback may be provided by one of the two signals: IprLow or IprLog. At the sample to tip approaching the IprLow signal is used by default, whereas the IprLog signal is used during scanning. Switching over is performed automatically.

In order to manually switch over the feedback signal (for example, in scanning low-conductive samples the IprLow signal is preferable), perform the following:

1. Break the feedback loop by releasing the button (Fig. 3-9).

Fig. 3-9

2. Change the feedback input signal (Fig. 3-10).

Fig. 3-10

3. Set the value of the Set Point parameter to:

- 1 nA for IprLog signal (Fig. 3-11);

- 0.1 nA for IprLow signal.

Fig. 3-11

4. Close feedback loop by clicking the button (Fig. 3-12).

Fig. 3-12

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Chapter 3. Constant Current Mode

3.5. Setting the Scanning Parameters

Open the Scan tab by clicking the button on the main operations panel.

Fig. 3-13

The upper part of the Scan tab contains the scanning parameters control panel (Fig. 3-13). The software oscilloscope displaying the trace of the signal measured during scanning is located below. Still below is the scanned images viewer.

SPM Mode Setup

To set up the operating SPM mode click on Mode box and select Constant Current in the drop-down list (Fig. 3-14). The device will be configured automatically.

Fig. 3-14

Selecting the area of scanning:

Recommendations for the selection of the initial size of the scanned area:

− If some preliminary information on the sample surface properties is available, suggesting that the surface roughness does not exceed the limits of z-range of the scanner, the maximal scan size can be set;

− When data on the surface properties are not available, it is recommended to start the scanning from a smaller area, 0.5÷1 μm, for example. Based on the results of the scanning of this small area, the optimal values for Scan Rate, Set Point, FB Gain can be chosen. The scan size can then be increased.

By default, the Scan Size parameter is automatically set to maximum value.

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To change the scan size and to select another area within the limits of the maximum possible area, perform the following:

1. Click the button in the Data Viewer toolbar (Fig. 3-15) to change the size and position of the scan area.

2. Change the area size and position using the mouse (pos. 1 in Fig. 3-15).

NOTE. Changing the scan area size will be automatically reflected in input fields of Scan Size parameter (Fig. 3-13).

3. Click the button. Make sure that the tip can touch the surface in any point within the selected area of scanning without "hitting" it anywhere. To do this click the left mouse button and, move the cursor within the limits of a selected area, keeping pressed the mouse button (pos. 2 in Fig. 3-15). The movement of the cursor reflects the actual movement of the tip relative the sample surface. The degree of piezo-scanner protraction should be controlled using the indicator at the bottom of the window.

Fig. 3-15. Data Viewer window 1 – the limits of the selected area of scanning,

2 – cursor indicating the position of the tip relative the sample surface

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Chapter 3. Constant Current Mode

Setting the scan area, number of data-points, scan resolution

The values of the Scan Size (the size of the area of scanning), the Point Number (number of points on X and Y axes) and the Step Size (the step of scanning) parameters can be changed using the switch button displaying the selected option (Fig. 3-16) and the input fields, displaying the current value of the set parameter, which are located next to the switch button.

Fig. 3-16

When setting parameters Point Number, Scan Size and Step Size, consider the following:

− While altering Point Number: Scan Size alters;

Step Size does not alter.

− While altering Scan Size: Step Size alters;

Point Number does not alter.

− While altering Step Size: Scan Size alters;

Point Number does not alter.

Use one of the following methods to change a parameter value:

− Double click the relevant input field and set the required parameter value using the popup slider;

− Enter the required value into the input field using the keyboard.

Setting of scanning speed

Selection of the optimal value for scanning speed depends on surface properties of the sample under study, scanning area dimensions and external conditions. Surface with smooth topography can be scanned at higher speed than that with uneven topography and high overfalls.

At the start, it is recommended to set the line scanning frequency (parameter Frequency) within 0.5÷2.0 Hz (see Fig. 3-16).

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3.6. Scanning

As an example we shall consider the process of scanning of a Highly-Oriented Pyrolitic Graphite (HOPG) sample.

Scanning Start

The sample surface scanning can be started once all preparatory operations are performed, the tip is landed to the sample, the set point is selected, and the scanning parameters are set.

To start scanning click the Run button located in the left part of the SCAN tab control panel.

After the Run button is pressed:

− The process of line-by-line scanning of the sample surface starts, followed by a line-by-line representation of the resulting image in the Data Viewer window (Fig. 3-17);

Fig. 3-17

− The lines of the scanned surface topography profile will be displayed one by one in the oscilloscope window (Fig. 3-17);

− Some buttons will disappear from the SCAN tab control panel, replaced by a number of new ones (Fig. 3-18): Pause, Restart, Stop.

Fig. 3-18

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Chapter 3. Constant Current Mode

Modifying the Parameters during Scanning

Slope subtraction:

− It is evident that the sample in the above example (Fig. 3-17) has some inclination along X axis, and correspondly the line scan exhibits a finite slope;

− The slope can be subtracted during scanning (i.e. in on-line mode), using the Subtract switch button. By default this button is in None state (Fig. 3-18).

Clicking this button and selecting Plane in the drop-down menu will subtract the plane. The image, which looked like shown on Fig. 3-17 before this procedure, will look like shown on Fig. 3-19. A current line of the scanning profile displayed will be modified accordingly.

Fig. 3-19

Other Subtract functions are described in “SPM Software” manual, part 1.

Fig. 3-20 presents an example of a scanned image after plane subtraction.

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Fig. 3-20. Scanned image example

NOTE. The changes made to the scanned image using Subtract function are not saved in the resulting frames.

Scanning Parameters Setup

The quality of the resulting image of a surface depends essentially on various parameters as the Scan Rate, the Set Point, the feedback gain factor FB Gain and bias voltage BV. Any of these parameters can be modified directly during scanning.

When selecting the scanning parameters it is necessary to remember that the sharpness of the tip and the stability of the tip fixation in the holder are the primary factors, determining the quality of the STM measurements.

Some Recommendations on the Optimization of the Scanning Parameters

The selection of the optimal scan rate value depends on the properties of the investigated object surface, on the scan size and on the environment conditions.

A surface with a smooth topography can be scanned with a higher velocity than a surface with a more pronounced roughness.

It is convenient to begin testing the sample working at very low scan rate and then progressively increase the scan rate until the line profile becomes deformed: at this point the scan rate should be slightly decreased and a good image can be recorded.

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Chapter 3. Constant Current Mode

3.7. Saving the Obtained Results

Once the sample surface is scanned, the resulting surface image is stored in RAM.

To save the obtained data to a hard disk perform the following:

1. Select File → Save option from the main menu.

2. A dialog window will appear. Choose a folder to store the data (by default, it is the folder C:\Program Files\NT-MDT\Nova).

3. Type in a filename and save it with the extension *.mdt.

NOTE. By default, the images obtained are stored in files “NoNameXX.mdt”, where XX is the file index in the folder Nova.

3.8. Termination of Operation

To switch off the microscope perform the following:

1. Retract the sample from the tip to a distance of approximately 2-3 mm. To do this:

a. Switch to the Approach tab (the button on the main operations panel).

Fig. 3-21

b. Double click in the input field of the Moving parameter for the Backward option.

Fig. 3-22

c. Set the value of 2-3 mm using the slider.

Fig. 3-23

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d. Click the Fast button for the Backward option.

Fig. 3-24

2. Switch off the feedback by releasing the FB button.

Fig. 3-25

3. Switch off the SPM Controller.

4. Exit the control program.

5. Turn off the vibration isolation system.

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Chapter 4. Constant Height Mode. Atomic Resolution on Graphite

4. Constant Height Mode. Atomic Resolution on Graphite

When using Constant Height Mode, the tip moves only in a plane so the changes of the current between the apex of the tip and the sample surface reproduces the surface topography.

ATTENTION! When using the Constant Height Mode ("the flying apex") it is possible to scan only very smooth surfaces without any relief, since the feedback during scanning is practically cut-out. Therefore the tip can "hit" the sample, when the roughness is not small enough.

Preparation for operation and measurements of the surface topography using the Constant Current Mode should be performed preliminary (see Chapter 3 “Constant Current Mode” on page 4-15):

− Tunnel Current configuration is selected in the drop down menu;

− the sample is approached to the tip;

− the operating level of the feedback gain factor is set;

− Constant Current mode is selected;

− scanning parameters (scan size, number of points, velocity) are selected and set;

− the scanning is performed, recording the sample topography.

An example of use of Constant Height Mode to acquire the topography of a HOPG sample will be described hereafter.

Setting up the Scanning Parameters

1. Switch over to the Scan windows (the Scan tab on the main operations panel).

2. Select the Constant Height mode in a drop-down Mode menu (Fig. 4-1).

Fig. 4-1

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- The device will be automatically configured to implement the selected mode:

- The feedback gain factor will be decreased to 0.01;

- The IprLow signal will be selected as the signal displayed during scanning. This can be checked by clicking button of the scanning parameters setup, which opens Scan Setup dialog window (Fig. 4-2).

Fig. 4-2

NOTE. When switching over back to measurements using the Constant Current mode, do not forget to increase the FB Gain feedback gain factor, since the former value will not be automatically assigned to this parameter.

3. Set the following values for the scanning parameters (Fig. 4-3, Fig. 4-4):

- Point Number = 128;

- Step Size = 0.2-0.5 A;

- maximal Frequency;

- Plane subtraction.

NOTE. The values of the minimal step and the maximal scan rate can differ for different devices.

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Chapter 4. Constant Height Mode. Atomic Resolution on Graphite

Fig. 4-3

Fig. 4-4

NOTE. The linear dimensions of the scan area should not exceed several nanometers.

4. Set the analog-digital converter gain factor to 10. To do this:

a. Switch over to the settings menu;

b. Select x10 factor in the drop-down list of the Gain button in the dialog window (Fig. 4-5).

Fig. 4-5

5. Select a flat area of the surface on the previously scanned image. To do this:

a. Click the button of the scan area selection on the Data Viewer window toolbar (Fig. 4-6). Select the Active Frame option in the drop-down list. The scanned image will occupy the maximum area of scanning available;

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Fig. 4-6

b. Click the button to change the size and the position of the area of scanning;

c. Moving the area selection frame and modifying its size select a smooth and flat area on the previously scanned image. The selected area should be located near the image center, if possible (Fig. 4-7).

Fig. 4-7

6. Click Run button to start scanning.

Try to change the values of StepSize, Scan Rate, Point Number, Set Point, FB Gain parameters to obtain better results, Fig. 4-8 shows an example of a HOPG surface image with atomic resolution, obtained using the STM.

Fig. 4-8. HOPG surface image

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Chapter 5. STM Spectroscopy

5. STM Spectroscopy

5.1. I(V) Spectroscopy

The procedure of I(V) spectroscopy consists in acquisition of volt-ampere characteristics (VAC) of a tip – sample tunneling junction.

1. Before performing the I(V) spectroscopy it is necessary to perform preliminary STM measurements of the investigated area topography (see Chapter 3 on page 4-15).

2. After the scanning is complete, switch over to the Curves tab ( button on the main operations panel).

Fig. 5-1

3. Set the Bias Voltage (a parameter in Fig. 5-2) on the control panel of the Curves tab as the variable parameter and the Ipr Low (parameter f1(a)) as the signal being measured.

4. Set the voltage range (Min, Max parameters) from -1 V to +1 V.

Fig. 5-2

5. Use the cursor to select a point on the surface, where the measurements will be performed.

6. Click the Run button.

Once the measurements are performed, an additional window will appear, displaying the measured characteristic. A typical plot of the I(V) dependence is shown on Fig. 5-3.

Fig. 5-3

A detailed description of the Curve tab buttons can be found in “SPM Software”, manual, part 1.

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5.2. Modulation Technique of the Scanning Tunneling Spectroscopy. Barrier Height Imaging

Barrier Height or dI/dZ spectroscopy is the representation of the local height of the potential barrier for the electrons (work function).

Before performing the dI/dZ measurements it is necessary to measure the surface topography (see Chapter 3 “Constant Current Mode” on page 4-15). Select the scanning parameters to obtain an image of the best possible quality.

The measurement procedure using dI/dZ spectroscopy mode consists of the following basic operations:

1. Measuring of the surface topography (see Chapter 3 “Constant Current Mode” on page 4-15).

2. SPM mode setup.

3. Setting the piezo-oscillator operating frequency.

4. Scanning.

5.2.1. SPM Mode Setup

1. Switch to the SCAN tab (Scan button on the main operations panel).

2. Select Barrier Height mode in Mode drop-down menu.

Fig. 5-4

3. Connect the oscillator output to the Z-channel of the scanner (Fig. 5-5).

Fig. 5-5

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Chapter 5. STM Spectroscopy

4. In the Scan Setup dialog window (click to open it) choose Mag and Height as forward signals (signal Mag is proportional to local height of potential barrier).

Fig. 5-6

5. Switch the synchronous detector input terminal to measure current. To do this:

a. Open the Additional operation area by clicking on the button in the right upper corner of the control program window.

b. Open the instrument circuitry (the Scheme tab).

c. Set the synchronous detector input terminal switch to Ipr position (Fig. 5-7).

Fig. 5-7

5.2.2. Setting the Piezo-oscillator Operating Frequency

1. Switch to the Resonance tab (Fig. 5-8).

2. Clear the Auto peak find check box.

3. Set the oscillator frequency range (From, To input fields) from 19 to 23 kHz.

4. Set the Low Pass filter switch to 100 Hz.

Fig. 5-8

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5. Click the Run button.

The resonance curve of the shape, shown on Fig. 5-9 will be displayed in the software oscilloscope.

Fig. 5-9

6. Set the oscillator frequency to any of the peaks.

7. Changing the oscillator voltage (the Amplitude parameter) and observing the Mag signal change on the oscilloscope in Additional operation area, set the measured Mag signal value to 2-5 nA (Fig. 5-10).

Fig. 5-10

If at the maximum value of the Amplitude parameter the Mag signal is less than 2-5 nA, then it is possible to amplify the signal being measured by modifying the Gain parameter of the synchronous detector.

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Chapter 5. STM Spectroscopy

5.2.3. Scanning

1. Switch to the Scan tab (Scan button on the main operations panel).

2. Click the Run button to start scanning.

It is worth noting that the value of the measured dI/dZ signal is determined not only by the local height of the potential barrier, but also by the local hardness of the sample. For relatively “soft” samples the change of dZeff tunnel clearance value is less than dZ value of the drift of the apex of the tip, which has a pronounced effect at closer distances.

The simultaneously obtained images of the pyrolitic graphite surface (the Height window) and the change of Mag signal, proportional to the value of the Barrier Height are shown on Fig. 5-11.

Fig. 5-11. Pyrolitic graphite surface image (left) and Mag signal change (right)

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