Nanosurf easyScan E-STM User manual

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Operating Instructions
easyScan E-STM
Version 2.1

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‘NANOSURF’ AND THE NANOSURF LOGO ARE TRADEMARKS OF NANOSURF AG, REGISTERED AND/OR
OTHERWISE PROTECTED IN VARIOUS COUNTRIES.
© JANUARY 2004 BY NANOSURF AG, SWITZERLAND, PROD.:BT00655, V2.1R1

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Table of Contents
Introduction 4
What is STM? .......................................................................................4
Scanning with the E-STM .....................................................................5
(Un)packing and Installation 8
Unpacking the instrument .....................................................................8
Hardware Installation ............................................................................9
Software installation............................................................................11
System requirements 11
Installation Procedure 11
Simulated microscope 14
Preparing for Measurement 15
Preparing and installing the STM tip ..................................................15
Preparing the sample .........................................................................17
Installing the sample ...........................................................................19
First Measurements 20
Starting the microscope......................................................................20
Approaching the tip.............................................................................21
1. Coarse approach by hand 21
2. Fine approach by piezo motor 22
3.Automatic approach 23
Start measurement .............................................................................24
Adjusting the sample’s tilt coordinates ...............................................25
Achieving atomic resolution................................................................27
Snapshots of images ..........................................................................30
STM Measurement modes .................................................................30
Judging the quality of the images .......................................................31
The graphite surface ...........................................................................33
Measuring Gold ..................................................................................34
Finish measuring ................................................................................36
Turning off and storing the instrument 36
Maintenance 38
Scan head 38
Scan electronics 39
Problems and Solutions 40
Technical Data 44

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Introduction
This manual gives instructions on how to set up and use your Nanosurf
easyScan E-STM system.The aim is to help people who don’t have a good
knowledge of physics to get pictures of atomic resolution easily.This intro-
duction chapter gives some general information on the scanning tunneling
microscopy technique, and its implementation in the easyScan STM. The
next chapter, (Un)packing and Installation, should be read when installing
your easyScan system. The chapters Preparing for Measurement and First
Measurementsshouldbe readbyallusers,astheycontainusefulinstructions
for your everyday measurements. The other chapters give more informa-
tion for advanced users.Those who need a more detailed description of the
functions of the easyScan software should refer to the Software Reference
manual.
What is STM?
Microscopy is one of the most exciting scientific techniques. The insight
into small dimensions has led to a new understanding of the structure of
materials and forms of life.
Withthehelp ofthe scanningtunneling microscope(STM) itis possible to
look into the fascinating world of the atoms.This completely new micros-
copy technique works without focusing elements and features atomic reso-
lution (laterally and vertically).
The Scanning Tunneling Microscope was developed by Gerd Binnig and
Heinrich Rohrer in the early 80’s at the IBM research laboratory in
Rüschlikon, Switzerland. For this revolutionary innovation Binnig and
Rohrer were awarded the Nobel prize in Physics in 1986.
In the STM, a small sharp conducting tip is scanned across the sample’s
surface, so close that the so-called ‘tunneling current’ can flow. With the
helpofthatcurrentthetip-surfacedistancecanbecontrolledvery precisely.
Therefore an enormous resolution is achieved so that the atomic arrange-
ment of metallic surfaces can be ‘probed’.
INTRODUCTION WHAT IS STM?

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To be able to get such excellent pictures of atomic resolution is almost in-
credible,consideringthatthesizeoftheatominrelationtothetip,isthatof
a golf ball to a mountain!
Scanning with the E-STM
In the E-STM, a platinum-iridium tip is moved in three dimensions using
piezo-crystal translators that are driven with sub-nanometer precision.
INTRODUCTIONSCANNING WITH THE E-STM

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The sample to be examined approaches the tip within a distance of 1 nano-
meter (1nm= 1 / 1 000 000 000 m). Classical physics would prohibit the
appearance of electrons in the small gap between a tip and a sample, but if
a sharp tip and a conducting surface are put under a low voltage (U~0.1V)
a very small tunneling current (I~1nA) may nevertheless flow between tip
and sample. This tunneling current is due to a quantum physics effect.
The strength of the tunneling current depends exponentially on the dis-
tance between the tip and the sample (usually referred to as z-distance).
This extreme dependence on the z-distance makes it possible to measure
thetip-sample movementvery precisely.Oneofthe threepiezocrystals, the
z-piezo, can now be used in a feedback loop that keeps the tunneling cur-
rent constant by appropriately changing the z-distance.
To obtain an image of the sample, the tip is scanned using the x- and y-
piezo crystals.The feedback loop will now let the tip follow the structure of
the sample's surface. A height image can now be made by recording the
position of the z-feedback loop as a function of the x-y piezo position.This
‘landscape’(ortopography)oftheatomicsurfaceisthendrawnonthecom-
puter screen line by line.
x
z
I = const.
The feedback loop maintains a constant tunneling current between the tip and the sample
during motion in the x-direction by changing the z-direction (viewed from top)
INTRODUCTION SCANNING WITH THE E-STM

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Scan-images of graphite as views from the ‘side’ and the ‘top’
The sample can also be scanned in a second mode: When the feedback
loop is slowed down very much (P-Gain=0, I-Gain=2), the tip scans at a
fixed distance from the sample (constant height mode).This time the vari-
ations in the tunneling current are measured and drawn line by line on the
computerscreen.However,thismodeonlyworkswhenthesampleisatomi-
cally flat, because the tip would otherwise ‘crash’ in to the sample
What is so special about the Nanosurf easyScan E-STM?
The E-STM was designed to allow people without training as a physicist to
do experiments in the world of atoms. Its design is compact, simple and
comfortable to operate. With the E-STM, it is possible to do any STM
experiment which can be carried out in air. The special sample approach
system allows simple handling of samples and tips, whilst at the same time
providing maximum stability of the tip-sample distance. All functions can
be carried out using a computer and the E-Line software.
INTRODUCTIONSCANNING WITH THE E-STM

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(Un)packing and Installation
This chapter describes the installation of the Nanosurf easyScan E-STM.
Important!
To enable measurements of atomic resolution to be made the following
precautions must be taken to keep equipment dust and grease free:
•Never toucheither thewirefortips (figure components, 8e), the sample(8
g) nor the open part of the STM scan head (2) with your fingers.
• Only touch the sample holder (8 f) at the black plastic end.
Unpacking the instrument
Before unpacking the instrument suitcase, check for the following items:
Documents and
software(9)
1
2
3
54 8
67
c d
bfg
e
a
Components:The easyScan DFM system
(UN)PACKING AND INSTALLATION UNPACKING THE INSTRUMENT

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1- easyScan E-SPM electronics
2- STM scan head
3- Power supply LPS-1
4- RS232 cable between computer and control electronics
5- Mains cable
6- Vibration isolation platform
7- Magnifying cover with 10x magnifier
8-Toolset,containinga:wirecutterb:half-roundpliers,c:pointedtweez-
ers, d: rounded tweezers, e: 30 cm Pt/Ir 0.25mm wire for tips, f: sample
holder, g: sample set: HOPG (graphite), gold thin film, three empty
sample supports
9- easyScan E-Line CD, containing the easyScan E-Line software
- this manual
- software reference manual
- a test sheet and a sheet with the calibration information.
- a Case for storing and transporting the instrument
Additionally, the following material is needed that is not included with the
contents of delivery:
•AcomputerthatmeetstherequirementsdescribedinthesectionSoftware
Installation.
• Solvent to clean the tools (e.g. ethanol, acetone)
Hardware Installation
Important!
- Please check that the mains voltage corresponds to that of the power sup-
ply (3).
- Make sure that your mains connection is protected against excess voltage.
(UN)PACKING AND INSTALLATIONHARDWARE INSTALLATION

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Chooseasteadytablewhereyoucanworkundisturbed.To ensurethefault-
less operation of the STM it has to be kept away from vibrations, heat
emission and air current.
- Put the STM scan head (2) onto the vibration isolation platform (6). Fix
the scan head cable under the strain relief clip.
- Connect the STM scan head to the easyScan E-SPM electronics (1).
- Make sure your computer is turned off. Then connect the control elec-
tronics to a free serial port (or RS232, or COM Port) on your computer
with the RS232 cable (4). If you are using the ‘USB adapter: ES’, DO
NOT connect it now.
- Connect the power supply (3) to the E-SPM electronics, and plug in the
mains cable (5).
- Finally turn on the power supply.
Now the E-SPM electronics and the microscope are connected, and the
LED on the E-SPM electronics flashes.
(UN)PACKING AND INSTALLATION HARDWARE INSTALLATION

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Software installation
System requirements
The System requirements for the easyScan E-Line software are:
• PC with a Pentium 133 MHz processor or higher
• a free COM-Port or USB port.*
• Windows 95 or higher
• 8 MB RAM or more (16 MB recommended)
• graphics adapter with 800 x 600 resolution and 16-bit colours (‘high
color’) or better (resolution of 1024 x 768 recommended)
* The Nanosurf ‘USB Adapter: ES’ option must be acquired to use the
USB port.
Installation Procedure
- Turn on your computer and start Windows.
Do not run any other program while installing the scan software.
- If you are using the USB Adapter, install it first according to the instruc-
tions included with the adapter.
- Insert (the backup copy of) your E-Line CD.
Windows NT/2000/XP
- Make sure you have administrator privileges before installing the soft-
ware.
All operating systems
- Start the ‘setup.exe’ program on the CD.
The following screen will now appear:
(UN)PACKING AND INSTALLATIONSOFTWARE INSTALLATION

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- Select the button ‘Install’, to install the data acquisition program ‘easy-
Scan’ on your computer.
Setup will ask for the directory in which the program files are to be copied:
- Put them in the proposed directory, unless the ‘Program Files’ directory
has a different name in your language of the Windows operating system.
Afterwards setup will ask for the start menu entry or program group in
which ‘easyScan’ is to be placed:
(UN)PACKING AND INSTALLATION SOFTWARE INSTALLATION

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- Accept the proposed name by clicking ‘OK’ or type another name.
The installation setup will now ask for the ’COM-Port’:
- Select the serial port to which you have connected the E-SPM electronics
(See section hardware installation, figure components, 1)
The setup program will start copying files onto your hard-disk.
After successful installation you will get this confirmation:
Important!
The E-Line software CD delivered with the instrument contains calibra-
tion information specific to your instrument, therefore you should there-
fore always keep (a backup copy of) the CD delivered with the instrument.
(UN)PACKING AND INSTALLATIONSOFTWARE INSTALLATION

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Simulated microscope
You can start the easyScan software without having the microscope con-
nected to your computer. We recommend using this simulation to explore
the easyScan-system (measurements and software) ‘off-line’.
When the simulate microscope mode is started, the following dialog box
appears:
By clicking ‘Start Simulation’ a simulation of the microscope is started.
This imitates most of the functions of the real microscope. The sample is
replacedbyamathematicaldescriptionofasurface.Youcannowfollowthe
instructions in the chapter First measurements.
-To explore the system in the ‘Simulate Microscope’ mode with the micro-
scope connected, activate it in the menu ‘Options’.
(UN)PACKING AND INSTALLATION SOFTWARE INSTALLATION

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Preparing for Measurement
The easyScan hard- and software have been set up according to the instruc-
tions in the section (Un)packing and Installation.
Preparing and installing the STM tip
The STM tip can be prepared and installed by yourself. This is the most
difficult part of your preparations. It usually needs patience and some prac-
tise to get the first good tip. Only an accurately cut tip enables optimal
measurements.Therefore, cutting and installing should be carried out with
great care.
- First ensure that the cutting part of the wire cutters (figure components, 8
a), the half-round pliers (8 b) and the pointed tweezers (8 c) have been
cleaned with ethanol.Touch the Pt/Ir wire (8 e) with these tools only.
- hold the end of the wire firmly with the pliers and cut a piece off approxi-
mately 5mm long.
- Still holding this piece of wire with the pliers, place the cutters at the free
end, as obliquely as possible. (See picture below)
- Close the cutters until you can feel the wire, then pull in the direction
shownbelow.Thetipneedstobetornoffratherthancleanlycutthrough,
in order to get the required sharp tip.
half-round
pliers
wire cutter
pulling and cutting
direction
wire
PREPARING FOR MEASUREMENTPREPARING AND INSTALLING THE STM TIP

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- Hold the tip wire with the tweezers behind the freshly cut tip.
Important!
• Do not touch the end of the tip with anything.
•Take care that the tip wire is straight.
•Donottwistthetip clamp intheopen partof the scanheadnorlift it too
high.
The procedure for mounting the tip under the tip clamp depends on the
type of clamp on your scan head:
Single clamp
(see figure Mounting the tip under the single clamp):
a) b)
2.
1.
Mounting the tip under the single clamp
1. Put the tip wire on the tip holder parallel to the groove in the tip holder,
so that it crosses below the tip clamp (figure a).
2. Move the tip wire sideways until it is in the groove in the tip holder.
(Figure b)
The freshly cut tip should be securely held under the clamp and extend
about 2-3mm beyond the tip holder. The tip is now installed.
PREPARING FOR MEASUREMENT PREPARING AND INSTALLING THE STM TIP

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Double clamp
Always use the following procedure for mounting the tip under the double
clamp (see figure mountingthetip underthe doubleclamp). Using adifferent
procedure may cause a gradual deterioration of the measurement quality
due to loosening of the clamp.
1.
a)
2.
3.
4.
b)
5.
6.
7.
c)
Mounting the tip under the double clamp
1. Hold the tip wire at a slight angle, and move it along the groove against
the first clamp, so that the end of the wire is under the clamp (figure a).
2. Lower the wire so that it lifts the first clamp (figure b).
3. Push the wire in, until it touches the second clamp.
4. Lift the wire.
5. Push the end of the wire below the second clamp.
6. Lower the wire so that it lifts the second clamp (figure c).
7. Push the wire under the second clamp.
The freshly cut tip should be securely held under the clamp and extend
about 2-3mm beyond the tip holder. The tip is now installed.
Preparing the sample
The STM can only examine electrically conductive materials. Nevertheless
thechoice of material is rather small because thesurface of the samplemust
notoxidise,betotallycleanandmirror-liketoobtainusefulresults.Because
of this some of the samples need special preparation.
PREPARING FOR MEASUREMENTPREPARING THE SAMPLE

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Gold thin film
Cleaningthesample isneitherpossible nornecessary.Nevertouch thesam-
plewith your fingersor put it upside down anywhere, this will only make it
unusable faster.
Graphite
The surface of the graphite sample can be cleaned when it is very dirty or
uneven. Due to the layered structure of graphite this can easily be done
using a piece of adhesive tape: (see pictures below)
- Put the sample on the table using the pair of tweezers.
- Stick a piece of adhesive tape gently to the graphite and then pull it off
again:The topmost layer of the sample should stick to the tape.
- Remove any loose flakes with the pair of tweezers.
The graphite sample is now ready for use and may not be touched with the
fingers.
PREPARING FOR MEASUREMENT PREPARING THE SAMPLE

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Installing the sample
Important!
Always store the sample holder in its package, in order to prevent corrosion
(see chapter Maintenance).
- Unpack the sample holder (figure components, 8 f) touching only its black
plastic handle.
- Put the prepared sample onto the magnetic end of the sample holder
using a pair of tweezers. (See below)
- Place the sample holder carefully in the scan head so that it doesn’t touch
the tip (see below).
- Put the sample holder down on to the guide bars first and release it gently
on to the approach motor’s support.
PREPARING FOR MEASUREMENTINSTALLING THE SAMPLE

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First Measurements
All the preparations are now done:
The STM is assembled, the software and a tip are installed and the graphite
sample is ready.
In this chapter step by step instructions are given on how to operate the
microscope and get your first pictures. More detailed explanations of the
software and the system are given in the software reference.
Important!
- Never touch the sample's surface or the tip! Good results depend strongly
on the accuracy of the preparation of the tip and the sample.
-The tip must never come in direct contact with the sample.This damages
the end of the tip, and it has to be cut again.
Starting the microscope
The red LED of the E-SPM electronics (1) should now be flashing when
the easyScan E-Line software has not been started yet.
- Start the easyScan E-Line software on your computer:
The main program window and a message box appear:
Now, your computer is communicating with the control electronics to ini-
tialisethe system.Thisprocess is repeatedevery timethecontrolelectronics
is turned off and on again. When download is completed, the electronic’s
red LEDs change from flashing to constantly shining, the LED on the scan
head shines orange, and some control panels appear (see figure Main pro-
gram window). Now the system is ready to use.
FIRST MEASUREMENTS STARTING THE MICROSCOPE