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Scanning Probe Microscopy

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Scanning Probe Microscopy Principle of Operation, Instrumentation, and Probes One chamber UHV system with variable temperature STM based on a flow cryostat design. – PowerPoint PPT presentation

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Title: Scanning Probe Microscopy


1
Scanning Probe Microscopy Principle of
Operation, Instrumentation, and Probes

2
Scanning Probe Microscopy-STM
  • The principle of electron tunneling was proposed
    by Giaever. He envisioned that if a potential
    difference is applied to two metals separated by
    a thin insulating film, a current will flow
    because of the ability of electrons to penetrate
    a potential barrier.
  • R. Young developed field emission topograph
    profiler.
  • Binnig and Rohler introduced vacuum tunneling
    combined with lateral scanning. The vacuum
    provides the ideal barrier for tunneling. The
    lateral scanning allows one to image surfaces
    with exquisite resolution, lateral-less than 1 nm
    and vertical-less than 0.1 nm, sufficient to
    define the position of single atoms.

3
Calibration standards
Crystals as rulers
h d111 0,31 nm
STM Image of HOPG (Highly Oriented Pyrolytic
Graphite) Image size 10nm x10nm
AFM image of silicon (111) single atomic steps
with native oxide
4
  • Since the introduction of the STM in 1981
    and AFM in 1985 by Binnig and Rohler , many
    variations of probe based microscopies, referred
    to as SPMs, have been developed.

5
Scanning Probe Microscopy
Family of SPM
  • STM scanning tunneling microscope
  • AFM Atomic force microscope
  • FFM (or LFM) (Lateral or Friction) force
    microscope
  • SEFM Scanning electrostatic force microscope
  • SFAM scanning force acoustic microscope
  • AFAM atomic force acoustic microscope
  • SMM scanning magnetic microscope
  • MFM magnetic force microscope
  • SNOM scanning near field optical microscope
  • SThM scanning thermal microscope
  • SEcM scanning electrochemical microscope
  • SKpM scanning Kelvin Probe microscope
  • SCPM scanning chemical potential microscope
  • SICM scanning ion conductance microscope
  • SCM scanning capacitance microscope

6
Non-contact scanning probe microscope (SPM)
  • Scanning tunneling microscope
  • Atomic force microscope
  • Scanning near field optical microscope
  • Scanning magnetic force microscope

7
Optical microscope and Transmission electron
microscope
Operating voltage 50100kV Wave length
0.0040.006 nm Resolution lt1nm
8
Laser autofocus system
9
Field-ion microscope
Radius of the tip 10nm Magnification 106 Operat
ing voltage 100kV Resolution lt1nm
10
Field emission profiler -- R.Young
Operating voltage 100V Resolution
vertical3nm Lateral400nm
11
Principle of operation of the STM made by Binnig
and Rohrer
JT ? VT exp(-Af1/2d)
JT ? the tunnel current, a sensitive function of
the gap width d VT ? the bias voltage f? the
average barrier height (work function) A ?
constant 1.025 eV-1/2 Å-1. With a work
function of a few eV, JT changes by an order of
magnitude for every angstrom change of d.
12
The tip is scanned over a surface while the
tunneling current changes with it, so the surface
is measured.
  • A sharp metal tip to the surface 0.31 nm
  • At a convenient operating voltage (10mV1V)
  • The tunneling current varies from 0.2 to 10 nA
    (which is measurable).

13
Nanoscope STM consists of three main parts
  • the head which houses the piezoelectric tube
  • scanner for three dimensional motion of the
    tip and
  • the preamplifier circuit (FET input amplifier)
    mounted on top of the head for the tunneling
    current,
  • the base on which the sample is mounted, and the
    base support, which supports the base and head

14
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15
Scanning Probe Microscopy-STM
Principle of operation of a commercial STM, a
sharp tip attached to a piezoelectric tube
scanner is scanned on a sample
The motion of the tip due to external vibrations
is proportional to the square of the ratio of
vibration frequency to the resonant frequency of
the tube. Therefore, to minimize the tip
vibrations, the resonant frequencies of the tube
are high at about 60 kHz in the vertical
direction and about 40 kHz in the horizontal
direction.
16
Scanning Probe Microscopy-STM
  • STM cantilevers with sharp tips are typically
    fabricated from metal wires of tungsten (W),
    platinum iridium (Pt-Ir), or gold (Au) and
    sharpened by
  • grinding, cutting with a wire cutter or razor
    blade,
  • field emission/ evaporator,
  • ion milling, fracture, or
  • electrochemical polishing/etching.
  • The two most commonly used tips are made from
    either a Pt-Ir (80/20) alloy or tungsten wire.

17
Schematic of a typical tungsten cantilever with a
sharp tip produced by electrochemical etching.
The resonant frequencies of the tube are high
at about 60 kHz in the vertical direction and
about 40 kHz in the horizontal direction.
A lateral resolution of about 2 nm requires
tip radii on the order of 10 nm.
18
Scanning Probe Microscopy-STM
Schematics of (a) CG(controlled geometry) Pt-Ir
probe, and (b) CG Pt-Ir FIB (focused ion beam)
milled probe.
19
Constant-current mode a feedback network
changes the height of the tip z to keep the
current constant. ? topographic map yielded by
the displacement of the tip.
Constant height mode a metal tip scannes across
a surface at nearly constant height and constant
voltage while the current is monitored ?
topographic map yielded by the change of the
current.
STM can be operated in either the
constant-current or the constant height mode. The
images are of graphite in air.
20
Scanning Probe Microscopy-STM
STM images of evaporated C60 film on a
gold-coated freshly-cleaved mica using a
mechanically sheared Pt-Ir (80-20) tip in
constant height mode.
21
Atomic Force Microscope
Contact
Non-contact mode/Tapping
Tip radius 2 ... 50 nm Force 0.01 nN ... 1
nN sample nearly any sample
22
Atomic force microscope
Principle of operation of the AFM. Sample mounted
on a piezoelectric tube scanner is scanned
against a short tip and the cantilever deflection
is measured, mostly, using a laser deflection
technique. Force (contact mode) or force gradient
(noncontact mode) is measured during scanning.
23
  • Atomic force microscope
  • The AFM combines the principles of the STM and
    the stylus profiler .
  • During initial contact, the atoms at the end of
    the tip experience a very weak repulsive force
    due to electronic orbital overlap with the atoms
    in the sample surface. The force acting on the
    tip causes a cantilever deflection which is
    measured by tunneling, capacitive, or optical
    detectors.
  • In an AFM, the force between the sample and tip
    is detected, rather than the tunneling current.
  • The deflection can be measured to within 0.02 nm,
    so for typical cantilever spring constant of
    10N/m a force as low as 0.2 nN can be detected.
  • The AFM can be used either in a static or
    dynamic mode.

24
Schematics of the four more commonly used
detection systems for measurement of cantilever
deflection. In each set up, the sample mounted on
piezoelectric body is shown on the right, the
cantilever in the middle, and the corresponding
deflection sensor on the left
25
An illustration of the optical beam deflection
system that detects cantilever motion in the AFM.
The voltage signal VA-VB is proportional to the
deflection
26
long-range (up to 100 nm) Van der Waals
,Electrostatic ,Magnetic forces short-range
(fractions of a nm) Chemical forces (bonding
energy, equilibrium distance)
27
AFM cantilever
28
  • The cantilever is characterized by 3 important
    coefficients
  • Spring constant k
  • Eigenfrequency f0
  • Quality factor Q - is typically a few hundred but
    can reach hundreds of thousands in vacuum.

29
  • A variety of silicon and silicon nitride
    cantilevers are commercially available with
  • -micron-scale dimensions,
  • -spring constants ranging from 0.01 to 100N/m,
    and
  • -resonant frequencies ranging from 5 kHz to over
    300 kHz.

30
Spring constant of cantilever
To obtain atomic resolution with the AFM, the
spring constant of the cantilever should be
weaker than the equivalent spring between atoms.
  • The vibration frequencies ? of atoms bound in a
    molecule or in a crystalline solid are typically
    1013 Hz or higher
  • The mass of the atoms m on the order of 10-25
  • Interatomic spring constants k, given by ?2m, on
    the order of 10N/m.
  • Therefore, a cantilever beam with a spring
    constant of about 1N/m or lower is desirable.

31
AFM,a powerful surface tool on atomic/molecular
scales
1N/m, 1ng 0.01nN 0.1nm
100 kHz
0.015N/m
Interatomic forces with one or several atoms in
contact are 2040 or 50100 pN, respectively.
Thus, atomic resolution with an AFM is only
possible with a sharp tip on a flexible
cantilever at a net repulsive force of 100 pN or
lower.
32
Contact (static) Mode
In the contact (static) mode, the interaction
force between tip and sample is measured by
measuring the cantilever deflection.
Tip approach sample A-B-C A-B ? lt10-10N
(attractive force) B-C ? gt10-10N (repulsive
force)
Tip leaves sample C-B-D-A C-B ? gt 10-10N
(repulsive force) B-D ? lt10-10N (attractive
force) D-A ? lt10-10N (repulsive force)
33
NonContact (dynamic) Mode
q(t) ? the deflection of the tip of the
cantilever, It oscillates with an amplitude A at
a distance q(t) to a sample. kts varies in
orders of magnitude during one oscillation cycle,
34
NonContact (static) Mode
35
Tapping mode
Schematic of tapping mode used for surface
roughness measurements
36
  • Tapping mode,a powerful surface tool on
    atomic/molecular scales, because of
  • (1) it has true atomic resolution,
  • (2) it can measure atomic force (so-called atomic
    force spectroscopy),
  • (3) it can observe even insulators, and
  • (4) it can measure mechanical responses such as
    elastic deformation.

37
Scanning Probe Microscopy-AFM
A commercial small sample AFM/FFM,
38
A large sample AFM/FFM
39
Schematics of a commercial AFM/FFM made by
Digital Instruments Inc. (a) front view, (b)
optical head, (c) base, and (d) cantilever
substrate mounted on cantilever mount (not to
scale)
40
(b) optical head
41
(c) base
42
(d) cantilever substrate mounted on cantilever
mount (not to scale)
43
Block schematic of the feedback control loop of
an AFM
44
The NPL Metrological Atomic Force Microscope
(MAFM)
45
The NPL Metrological Atomic Force Microscope
(MAFM)
46
AFM
(a) A schematic depiction of an atomic force
microscope cantilever and tip interacting with
materials on a surface. Tips typically have
points of 50 nm or less in diameter. (b)
Schematic of multiplexed AFM tips performing
multiple operations in parallel.
47
Probes in Scanning Microscopies
  • ? SPM images are generated through
    measurements of a tip-sample interaction.
  • ? A well-characterized tip is the key element
    to data interpretation and is typically the
    limiting factor.

48
Probes in Scanning Microscopies
49
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50
Scanning Probe Microscope - cantilevers and tips
51
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52
Oxide sharpening of silicon tips. (Si-SiO2 stress
formation reduces the oxidation rate at regions
of high curvature. ) The left image shows a
sharpened core of silicon in an outer layer of
SiO2. The right image is a higher
magnification view of such a tip after the SiO2
is removed.
53
Electron beam deposition (EBD).
54
Carbon Nanotube Tips
SEM micrograph of a multi-walled carbon nanotube
(MWNT) tip physically attached on the
single-crystal silicon, square-pyramidal tip
55
Carbon Nanotube Tips
A MWCNT attached to a tungsten spike
56
Assembled Cantilever Probe (ACP)
ACP Structure comprising two cantilevers glued
together
57
A mechanically cut STM tip (left) and an
electrochemically etched STM tip (right).
58
Probe Tip Performance
A tip model to explain the high resolution
obtained on ordered samples in contact mode in
AFM.
59
Electron beam deposition (EBD) Tips
Measurement of a step height standard using a
normal silicon tip (left) and a silicon tip with
an EBD deposited tip on the end
60
  • The tunneling current is a monotonic function of
    the tip-sample distance and has a very sharp
    distance dependence.
  • In contrast, in AFM, the tip-sample force has
    long and short-range components and is not
    monotonic.
  • ?Jump-to-Contact and Other Instabilities
  • ?Contribution of Long-Range Forces
  • ?Noise in the Imaging Signal (1/f )
  • ?Non-monotonic Imaging Signal (FM modulation)

61
?Jump-to-Contact and Other Instabilities
Plot of tunneling current It and force Fts
(typical values) as a function of distance z
between front atom and surface atom layer
62
Schematic view of 1/ f noise apparent in force
detectors. Static AFMs - from 0.01 Hz to a
few hundred Hz. Dynamic AFMs - around 10 kHz
to a few hundred kHz.
63
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64
Noncontact mode AFM images of Si(111) 77
reconstructed surface obtained using the Si tips
(a) Without and (b) with dangling bond. The scan
area is 99Å99 Å.
65
Low Temperature Scanning Probe Microscopy
  • Probably the most important advantage of
  • the low-temperature operation of scanning probe
  • techniques is that they lead to a significantly
  • better signal-to-noise ratio than measuring at
  • room temperature.

66
One chamber UHV system with variable temperature
STM based on a flow cryostat design.
67
Three-chamber UHV and bath cryostat system for
scanning force microscopy, front view. One
for cantilever and sample preparation, which also
serves as a transfer chamber, One for
analysis purposes, and A main chamber that
houses the microscope.
68
The first demonstration of manipulating atoms was
performed by Eigler and Schweizer (1990), who
used Xe atoms on a Ni(110) surface to write the
three letters IBM (their employer) on the
atomic scale
69
Final artwork greeting the new millennium on the
atomic scale
70
Synthesis of biphenyl?? from two iodobenzene???
molecules on Cu(111) First, iodine is
abstracted from both molecules (i),(j), then the
iodine between the two phenyl groups is removed
from the step (k), and finally one of the phenyls
is slid along the Cu-step (l) until it reacts
with the other phenyl (m) the line drawings
symbolize the actual status of the molecules
71
AFM
(c) AFM image of a quantum corral, a structure
built using AFM manipulation of individual atoms
(from the IBM Image Gallery)
72
AFM
(a) A schematic depiction of an atomic force
microscope cantilever and tip interacting with
materials on a surface. Tips typically have
points of 50 nm or less in diameter. (b)
Schematic of multiplexed AFM tips performing
multiple operations in parallel. (c) AFM image
of a quantum corral, a structure built using AFM
manipulation of individual atoms (from the IBM
Image Gallery)
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