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~100x100nm
~10x10nm
~1000x1000nm ~100000x100000nm
Anders Mikkelsen
e-mail: [email protected]
Synchrotron Radiation Research, Fysicum
www.sljus.lu.se
Experimental methods are based on:
Atomic Force Microscope
• Scanning Tunneling Microscopy (STM)
• Scanning Tunneling Spectroscopy (STS)
• Scanning Tunneling Luminesence (STL)
• Atomic Force Microscopy (AFM)
• Magnetic Force Microscopy (MFM)
• Electrostatic Force Microscopy (EFM)
• Capacitance Microscopy (CM)
• Scanning Nearfield Optical Microscopy(SNOM)
• ...
Some Scanning Probe Techniques
• Scanning Tunneling Microscopy (STM)
Resolve individual atoms, measure electrical properties, induce photo luminesence.
Only conducting samples.
• Atomic Force Microscopy (AFM)
Resolution limit ~STM, but more difficult to achieve.
Any sample type.
Some Scanning Probe Techniques
Imaging with SPM
Red blood cells DNA on lipid-bilayer Butterfly wing
Carbon nanotube III-V nanowire Magnetic domains
Nanomanipulation
and electron density
waves by STM:
Quantum Corrals (Don Eigler IBM)
Condensed matter & Chemistry
applications of SPM
• Range: 10x10nm – 10x10mm
• Resolution: ~0.01nm
• Typical environment: UHV
• Samples: Conducting
• Topography
• Geometric structure
• Electronic structure
• Vibrational structure
• Magnetic structure
• Manipulate atoms / molecules / nanostructures
• Range 10x10nm – 100x100mm
• Resolution: 1 – 0.01nm
• Typical environment: Air/Liquid
• Samples: All types
• Topography
• Geometric structure
• Friction
• Adhesion
• Hardness
• Manipulate molecules / nanostructures
STM AFM
• Scanning Tunneling Microscopy (STM)
• Scanning Tunneling Spectroscopy (STS)
• Scanning Tunneling Luminesence (STL)
• Atomic Force Microscopy (AFM)
• Magnetic Force Microscopy (MFM)
• Capacitance Microscopy (CM)
• Scanning Nearfield Optical Microscopy(SNOM)
• ...
Some Scanning Probe Techniques
The Scanning Tunneling Microscope
Two ways of forming the STM image:
a) is almost always used!
The expermental setup:
a) I constant, feedback loop active,
Z variation measured
b) Z constant, feedback loop idle,
I variation measured
STM systems at Fysicum
• Omicron STM1 (in C163)
High resolution
• Omicron XA STM + PEEM (in C162)
20mm scan range, low noise, precise positioning
• JEOL STM (in C161)
• Exploratory
• Omicron VT STM (in C164)
Variable Temperature 15-1500K
Sources available:
• Ga, In and As MBE sources
• Atomic Hydrogen source
• Oxygen, Ammonia
from 1-10mbar to 1bar
Experimental issues STM tips made by
electrochemical etching STM stage with magnetic damping
Tripod STM with magnetic damping
STM stage –side view
A) Tripod scanner
B) Sample stage
C) Magnetic damping
D) Current amplifier
STM top view
Scanner design
SPM scanners are made from a piezoelectric
material PZT, Lead Zirconium Titanate.
PZT expands and contracts proportionally to an
applied voltage.
In tripod design three piezos
are used to move in XYZ
Piezoelectric scanner:
• based on images of known surfaces calibrate the
piezoelectric response to convert [Volts] to [nm]
• the response is continuous down to atomic lengths
scales
• unfortunately, the response is highly non linear
(hysteresis)
Scanner designs
Precise coarse movement with piezos:
Piezo tube can also be used as scanner
Clean surfaces
• Sputtering the surface with Ar-ion bombardment (to remove contamination on the surface)
• After Sputtering: Annealing to high temperatures to smooth the surface
• Chemical cleaning by O2 treatment at higher temperature (burns away the carbon)
• Or by H2 treatment at high temperatures
Alternatively:
• evaporate well-ordered films on a substrate
(Attard 1.9)
Watching Surface Quality:
Surface Crystallography: LEED (low energy electron diffraction)
Chemical Surface composition: AES, XPS
Cleaning surfaces Keeping them clean
Pressure (mbar)
1000
100
10-3
10
10-11
10-13
10-17
1
10-1
10-2
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-12
10-14
10-15
10-16
8 km 10-3 km
30 km
90 km
278-460 km
The pressure gap
Vacuum Vacuum is measured in units of pressure. The SI unit of pressure is the pascal (symbol
Pa), but vacuum is usually measured in torrs (symbol Torr), named for Torricelli, an
early Italian physicist (1608 - 1647). A torr is equal to the displacement of a
millimeter of mercury (mmHg) in a manometer with 1 torr equaling 133.3223684
pascals above absolute zero pressure.
Electron tunneling in 1D from I to III
Time independent Schrödinger equation: 1D model of tunneling from
one side (I) to the other (III)
Y(x) oscillate in region I and III
Y(x) decay exponentially in region II
outgoing current density
incomming current density T=
T~exp(-2ka), k2~(V0-E)
T falls of exponentially with barrier width
Sensitivity of STM
Tunneling current is proportional to exp(-2Kd) ; K=(2mf)/h
For average work function f of ~4eV, K=1Å-1
=>
Changing d ~0.1 nm will change current by an order of magnitude!
Typical currents: 0.1-1nA Typical voltages: 0.1-4V
0.001
0.002
0.005
0.01
0.21 0.23 0.25 0.27 0.29 0.31 0.33 0.35 0.37 0.39
Distance between tip and sample, d in nm
log(N
om
aliz
ed T
unn
elin
g c
urr
ent)
Mophology of samples
What are we imaging at the atomic
scale?
Local Density of States
• Density of state (DOS) means the "number of states at a
particular energy level", i.e. the distribution of states over
energy.
dE
dNE E)(
• LDOS (x,y,E)
gives the density
of electrons of a
certain energy at
that particular
spatial location.
Electron tunneling in 1D from I to III
Time independent Schrödinger equation: 1D model of tunneling from
one side (I) to the other (III)
Y(x) oscillate in region I and III
Y(x) decay exponentially in region II
outgoing current density
incomming current density T=
T~exp(-2ka), k2~(V0-E)
T falls of exponentially with barrier width
Bardeen’s tunneling current
formalism
Basic assumptions:
Fermi’s golden rule + Coupling between surface & STM tip, is very low
=>
Treat tunneling as a slight perturbation of the electronic structure.
Then the tunneling current can be computed as the overlap of
wave functions of the sample and the tip
Tunneling current:
Fermi function Tunneling matrix elements
yt/s ; t/s are wave functions and density of states of tip/sample
Further simplifications
(Tersoff & Hamann)
Further assume an S-wave tip with
constant density of states:
Assume low voltage:
This means:
I depends on the local density of states of the sample near the fermi level
which can be calculated by standard ab-initio theoretical methods!
S-wave tip
s is evaluated at the center of curvature
of the tip
Energy levels involved in
tunneling
(b)
(c)
(d)
(a)
Density of states
Energy level diagram for sample
and tip:
(a) Independent sample and tip.
(b) Sample and tip at equilibrium,
separated by a small vacuum
gap.
(c) Positive sample bias: electron
tunnel from the tip to the sample.
(d) Negative sample bias: electron
tunnel from the sample into the
tip.
What are we imaging in STM?
Some general concepts can also be derived:
Metals:
High density of states at atoms => atoms appear as bright protrusions
Insulators:
No conduction possible => we crash
Semiconductors and thin oxides:
Complex electronic structure at fermi level => be careful!
e-
d d
Positive Bias Positive Bias Negative Bias
Negative Bias
FS
FT 1eV
Measuring empty or filled states
by bias switching
Potential Barrier Potential Barrier
Tip Sample Sample Tip
Filled Electronic Valence Band States
Filled state imaging Empty state imaging
e- FS
FT
GaAs(110) – an example of different
filled / empty state imaging
High density of states in conduction band
above Ga atoms
High density of states in valence band
above As atoms
We can change bias to image different
species.
LEED pattern
Surface Crystallography in the Virtual Lab
• common practice: trial and error modeling
– set up models and compare with experiment
• STM
• vibrational spectroscopy (EELS)
• core-level binding energies
• LEED
– set up a number of reasonable models and calculate their energy the one with the lowest energy is most stable (“ab initio” thermodynamics)
• Ab-initio theoretical method assumptions
– Quantum mechanics with a few simplifying approximations
– Basic structural model (density, type and symmetry of atoms)
– Thermodynamc equilbrium
– S-wave STM tip
Favorable model from theory
7 Pd5O4 cells on undistorted 3 layer substrate (6x8=48 Pd atoms per layer)
Three fold Oxygen
Four fold Oxygen
4 fold Pd atoms are above Pd-Pd rows
Comparison of STM images
Experimental STM image
Theoretically simulated STM image
Tip influences sample...
A recent example..
Wires are deposited via a dry deposition
method onto a flat substrate outside
vacuum.
Subsequently the sample is inserted into
the STM vacuum chamber and annealed to
~350-400C in the presence of atomic
hydrogen.
Note:
Anneal temperature is sufficiently low that wires and devices should not melt!
High resolution STM imaging
of wurtzite InAs/InP nanowires
Overview STM images of InAs
nanowires
STM image of InAs NW STM image of wire bundles
STM traces
Corrugation in STM will be smaller on bundles!
Crystal engineered nanowires with many
different structures STM imaging with atomic resolution possible along micron long wires!
Twinned ZB{110} Twinned ZB{110}
WZ{11-20} WZ{10-10}
Twinned ZB{110} Twinned ZB{110} Twin super lattice
WZ{11-20} WZ{11-20} WZ{10-10}
200 nm
Scanning Electron Microscopy
Scanning Tunneling Microscopy(STM)
ZB{110} WZ{11-20} WZ {10-10}
Atomic scale structure imaged on all different
facets of the same wire
Twinned ZB{110} Twinned ZB{110} Twin super lattice
WZ{11-20} WZ{11-20} WZ{10-10}
Imaging down into trenches of a twin plane
superlattice surfaces with atomic resolution
Twinned ZB{110} Twinned ZB{110} Twin super lattice
WZ{11-20} WZ{11-20} WZ{10-10}
100x100nm2
10nm
{111}B
{111}A
2nm
Wurtzite {11-20}
Zincblende {110}
Spectroscopy across the Wurtzite / Zincblende
interface
Twinned ZB{110} Twinned ZB{110} Twin super lattice
WZ{11-20} WZ{11-20} WZ{10-10}
• Scanning Tunneling Microscopy (STM)
• Scanning Tunneling Spectroscopy (STS)
• Scanning Tunneling Luminesence (STL)
• Atomic Force Microscopy (AFM)
• Magnetic Force Microscopy (MFM)
• Capacitance Microscopy (CM)
• Scanning Nearfield Optical Microscopy(SNOM)
• ...
Some Scanning Probe Techniques
STM on an AFM
Original AFM was based on a STM as
transducer.
Atomic Force Microscopy (AFM)
or
Scanning Force Microscopy (SFM)
AFM probe surface with a sharp tip
Forces between tip and cantilever deflects
tip.
A detector measures the deflection
Two basic operational modes:
Contact mode
Non-contact mode
AFM vs STM
Kubo and Nozoye. Physical
Review Letters, 86(9), 1801–1804, 2001
STM NC-AFM
Low temperature STM and NC-AFM
images on graphite
PNAS, 100(22), 12539–12542, 2003
Forces: simple view
E. Meyer, H. J. Hug, R. Bennewitz; “Scanning Probe Microscopy: The Lab on a Tip”
(some discussion on forces also in Attard 1.13)
Forces between tip and surface
during AFM
Detecting Cantilever Deflection
Photodiodes
Laser Diode
Mirror
Cantilever
and Tip Feedback
X-Y-P PiezoScanner
Image
Common detection schemes Quartz resonator
AFM detection system Quite simmilar to our STM experimental setup!
Some of our AFMs
EduScope AFM
Specifications
X-Y scanning range 50x50 μm2
Z scanning range 6 μm
Resolution AFM mode
1-10nm
JHK NANOWIZARD II Bio
AFM
Atomic lattice resolution on inverted
microscope (< 0.055 nm RMS z noise level)
100 x 100 x 15 µm3 scan range
All standard AFM modes – contact mode,
Lateral Force Microscopy (LFM), AC modes, force modulation,
force spectroscopy, force mapping, nanomanipulation,
nanolithography, etc.
Omicron Q-plus sensor AFM/STM
AFM image of biomolecules
Human chromosone 190 nm-long DNA strands
AFM image of blood clotting
• Polymerization of fibrin catalyzed by the presence of
thrombin
• Mechanism for blood clotting
B. Drake et al., Science 243, 1586 (1989)
High resolution AFM of supramolecular
assembly of the photosynthetic
complexes in native membranes
Scheuring, S., Sturgis, J.N., 2005. Chromatic adaptation of photosynthetic
membranes. Science 309, 484–487.
AFM tips
Three common types of AFM tips
Commercial, from silicon or silicon nitride:
-Standard chip size: 1.6 x 3.6 x 0.4 mm (more than
1000 probes from aSiwafer)
-High reflective Au coating (reflective property is 3 times
better in comparison with uncoated cantilevers)
-Typical curvature radius of a tip: 10 nm
-Cantilever length: 100 -200 μm
-Cantilever width: 10 -40 μm
-Cantilever thickness: 0,3 -2 μm
-Available for NONCONTACT, SEMICONTACT
and CONTACT modes
-Triangular (V shaped) and rectangular
cantilevers
-Available with conductive TiN, W2C, Pt, Au and
magnetic Co coatings
Different cantilever shapes
Tip Artifacts
Double tip
Overestimate object size
Underestimate object size
Tip shape imaged
AFM image with Double tip
Tip artifacts
AFM Modes
E. Meyer, H. J. Hug, R. Bennewitz; “Scanning Probe Microscopy: The Lab on a Tip”
By: B. Resel
Non contact AFM
Detection scheme:
Static deflection
Dynamic Mode: Amplitude change
Frequency shift
High resolution AFM in non contact mode
(frequency modulated)
Si(111)-(7x7) AFM vs. STM
AFM STM
(A)The tip is approached to the surface until contact occurs.
(1) Retracting the cantilever stretches the connection of the single biomolecule to the surfaces.
When the force reaches the unbinding force of the complex, the biological interaction is
ruptured and
(2) The cantilever is available for a new force distance curve.
(B) Loading rate dependence of the unbinding forces of the avidin-biotin system under
physiological conditions
( Single Mol. 1 (2000) 285.)
Force spectroscopy
AFM - STM overview
STM AFM
• Range: 10x10nm – 10x10mm
• Resolution: ~0.01nm
• Typical environment: UHV
• Samples: Conducting
• Topography
• Geometric structure
• Electronic structure
• Vibrational structure
• Magnetic structure
• Manipulate atoms / molecules / nanostructures
• Range 10x10nm – 100x100mm
• Resolution: 1 – 0.01nm
• Typical environment: Air/Liquid
• Samples: All types
• Topography
• Geometric structure
• Friction
• Adhesion
• Hardness
• Manipulate molecules / nanostructures
Science applications of SPM
• Atomic scale structure of surfaces and
nanostructures
• Electrical properties on the nanometer scale
• Molecular bonding and structure of individual
molecules
• Morphology during chemical reactions
• Magnetic structure of very small objects
• Electronic, vibrational and geometrical structure
correlation down to the atomic scale
• Diffusion and growth studied directly on the
atomic scale
Technology applications
• Texture / roughness / topography of
materials and lithographic structures
• Electrical vs structural properties of chip
technology
• Magnetic vs structural properties of hard
disk disks
Some examples of SPM applications in CM technology
A cross-sectional scanning tunneling microscopy study of a quantum dot infrared photo-detector structure (Lund + Acreo AB)
Topography
MFM
Images of Over written Track on a Hard Disk TappingMode AFM phase and amplitude images of Celgard 2400
tape (membrane for Lithium batteries)
• Scanning Tunneling Microscopy (STM)
• Scanning Tunneling Spectroscopy (STS)
• Scanning Tunneling Luminesence (STL)
• Atomic Force Microscopy (AFM)
• Electrostatic Force Microscopy (EFM)
• Magnetic Force Microscopy (MFM)
• Capacitance Microscopy (CM)
• Scanning Nearfield Optical Microscopy(SNOM)
• ...
Some (more) Scanning Probe
Techniques
In situ AFM/STM/STS on NW devices combined AFM / STM / STS and conductivity measurements on
individually contacted and biased nanowire heterostructures
• interplay between atomic structure & electrical properties
• local changes of charge distribution upon external biasing
• influence of surface on conductivity
• AFM/STM QPlus sensor
• sample holder with four
separate electrical contacts
Vtip =
2 V
Vtip =
- 2 V
ISD ISD AFM
0.2 µm
Scanning gate microscopy
InAs GaSb contact
STM images on
top of NW
showing height AFM images, 2.6 x 0.5 µm
VSD = - 0.1 V ISD
biased STM tip
as local gate:
• strong gating
dependence of
the InAs part
• hardly any effect
at the GaSb part
AFM image, 3.5 µm x 3 µm
MFM: Magnetic Force Microscope AFM with magnetic probe
magnetic tip
laser photodiode
piezo-element
Magnetic Force Microscopy (MFM)
•Special probes are used for MFM. These are magnetically sensitized by
coating with a ferromagnetic material.
•The tip is oscillated 10’s to 100’s of nm above the surface
•Gradients in the magnetic forces on the tip shift the resonant frequency of the
cantilever .
•Monitoring this shift, or related changes in oscillation amplitude or phase,
produces a magnetic force image.
•Many applications for data storage technology
MFM of cobalt islands
m = -1 m = 0 m = 1
MFM image 100x140x20 nm Co particles
(square lattice; s=300 nm)
0 2.5 mm
0
1.0
degrees
Array of 80x140x18 nm Co nanostructures
200 nm
Electrostatic Force
Microscopy
EFM maps locally charged domains on the sample surface.
EFM can map the electrostatic fields of a electronic circuit as the device is turned on and off.
Electrostatic force microscopy (EFM) applies a
voltage between the tip and the sample while the
cantilever hovers above the surface, not touching
it. The cantilever deflects when it scans over
static charges.
Some material from:
• Dawn Bonnell Scanning Probe Microscopy
and Spectroscopy
Surface vs Nanowire
Surface Science, Invited Prospective 607, (2012) 97
Nanostructure surface studies at the NmC
Nanostructures
Growth substrates