PART 4. Scanning Tunneling Microscopy and Scanning Tunneling Microscopy and Spectroscopy 1....

Click here to load reader

download PART 4. Scanning Tunneling Microscopy and Scanning Tunneling Microscopy and Spectroscopy 1. Introduction

of 38

  • date post

    10-Aug-2020
  • Category

    Documents

  • view

    4
  • download

    0

Embed Size (px)

Transcript of PART 4. Scanning Tunneling Microscopy and Scanning Tunneling Microscopy and Spectroscopy 1....

  • 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

    4-1

  • 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.

    4-2

  • 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.

    4-3

  • 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).

    4-4

  • 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.

    4-5

  • 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