Post on 04-Apr-2018
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Scanning Probe Microscopy
(SPM)
Real-Space
Surface Microscopic Methods
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SPM Principle
Probes that are nanosized(accomplishedmicrolithographically),
scanning and feedback mechanisms
that are accurate to the subnanometerlevel (achieved with piezoelectricmaterial), and
highly sophisticated computer controls(obtained with fast DACs (digital analogconverters, etc.).
Consists of
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Schematic of SPM Principle
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Resolution Comparison
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3 Axis Cylindrical Piezo
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SPM Tree Conventional STMTopography of conductive surfaces, I-V spectroscopy (e.g., local band gaps)
Ballistic Electron Emission Microscopy and Spectroscopy
Subsurface investigations, e.g., of metal/semiconductor interfaces
STM Scanning Tunneling PotentiometryScanning Tunneling Microscopy Surface potential studies (e.g., study of grain boundaries)
Photovoltaic and Photoassisted Tunneling SpectroscopySurface electron-hole pair recombination during photo-excitation
Inelastic Electron Tunneling
STM induced photon emissions (study of heterostructures)
Conventional SFM (atomic force microscopy AFM)
Topography of mainly non-conducting surfaces, force spectroscopy
(S)LFM (Lateral force mapping of surfaces)
SFM Friction studies, local material distinction ("Chem. Force Microscopy")Scanning Force Microscopy (S)EFM (Electrostatic Force Microscopy)
Non-contact electrostatic force mapping, (e.g., study of charge decay)
SPM (S)MFM (Magnetic Force Microscopy)Scanning Probe Microscopy Contact and non-contact technique used to study magnetic domains
Rheological Force Microscopy
Contact sinusoidal modulation (distance or force) methods
(S)UFM (Ultrasonic Force Microscopy)
non-linear surface effects (e.g., true non-contact interactions, or rheology)(S)PFM (Pulsed Force Microscopy); rheology and adhesion force mapping
SCAM (Scanning Capacitance Microscopy); measuring of trapped charges
SECM (Scanning Electrochemical Microscopy); spatial variations of Faradaic currents or potential changes)
SNOM (Scanning Near-field Optical Microscopy); optical properties, luminescence
SMM (Scanning Micropipette Microscopy); local ion concentration (e.g., transport processes in membranes)
SCM (Scanning Calorimetric Microscopy); local heat transfer coefficients and transition temperatures
SNAM (Scanning Near-field Acoustic Microscopy); topography and rheology
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The Three Basic SPM Systems
Scanning Tunneling Microscope (STM) Scanning Force Microscope (SFM)
Scanning Nearfield
Optical Microscope (SNOM)
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Scanning Tunneling Microscopy (STM)
Signal: Tunnel CurrentThe tunnel current depends
on the tip-sample distance,
the barrier height, and the
bias voltage. Studying the
bias dependence provides
important spectroscopic
information on the occupiedand unoccupied electronic
states (-> local LDOS
studies).
FT
FS
FS
FT
Positive sample bias: Net tunnelingcurrent arises from electrons that tunnel from
occupied states of the tip into unoccupied
states of the sample
Negative sample bias: Net tunneling
current arises from electrons that tunnel fromoccupied states of the sample into unoccupied
states of the tip.
The tunnel current is strongly distance, Dz, dependent
zAexpVI2/1
biasF
A = const.
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Conventional STM
Tunneling Current, I
Bias Voltage, V
Conductive Sample
STM Tip
Piezo Scanner
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STM Modes of Operations
Examples:
Constant height imaging or variable current mode (fast scan mode)The scan frequency is fast compared to the feedback response, whichkeeps the tip in an average (constant) distance from the sample surface.Scanning is possible in real-time video rates that allow, for instance, thestudy of surface diffusion processes.
Differential tunneling microscopyTip is vibrated parallel to the surface, and the modulated current signal isrecorded with lock-in technology.
Tracking tunneling microscopyScanning direction is guided by modulated current signal (e.g., steepestslope).
Scanning noise microscopyUse current noise as feedback signal at zero bias. Nonlinear alternating-current tunneling microscopy
Conventionally, STM is restricted to non-conducting surfaces. A highfrequency AC driving force causes a small number of electrons to tunnelonto and off the surface that can be measured during alternative half-cycles(third harmonics).
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Scanning Force Microscopy (SFM)
Sample
SFM Tip
Piezo Scanner
z
Force: FN = kN* z
Ppring constant: kNSpring deflection: z
Interaction or force
dampening field
Contact Method: Non-Contact Method:
Sample
Input Modulation Cantilever Response
Scan
Cantilever
4-Quadrant
PhotodiodeLaser
fully elastic viscoelastic
Friction
Topography
50/50 PS/PMMA blend annealed at 180 oC for 1 week
Spinodal Decomposition of PS/PMMA Blend
PSPMMAPMMA
PS
10 m
complex flow pattern over time
SFM Topography SFM Lateral Force
2D spinodal decomposition
different from bulkNote: The bright spots (PS phase/lateral force image) represent
spinodal frustration points of PMMA.
50/50 PS/PMMA blend annealed at 180 oC for 1 week
Spinodal Decomposition of PS/PMMA Blend
PSPMMAPMMA
PS
10 m
PSPMMAPMMA
PS
10 m10 m
complex flow pattern over time
SFM Topography SFM Lateral ForceSFM Topography SFM Lateral Force
2D spinodal decomposition
different from bulkNote: The bright spots (PS phase/lateral force image) represent
spinodal frustration points of PMMA.
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Rheological SFM
Sample
SFM Tip
Piezo sinusoidallymodulated either in x or z
z
Load:
FN = kN*z
Lateral Force:
FL = kL*x
x
Input Modulation Signal
Response Modulation
Signal
Amplitude
TimeTime Delay
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Topography Modes of SPM
Constant deflection (contact mode)Analog to the constant current STM mode. The deflection of the cantileverprobe is used as the feedback signal and kept constant.
Constant dampening (AM detection, intermittent contact mode in air orliquid)The response amplitude of sinusoidally modulated cantilevers allow
feedback in the pseudo-non-contact regime (intermittent contact) due tofluid dampening.
Constant frequency shift (FM detection, non-contact mode in ultrahighvacuum)Similar to the FM radio, the frequency is measured and frequency shifts areused as feedback system. This approach works only in vacuum where fluid-dampening effects can be neglected.
Variable deflection imaging (contact mode)Analog to the variable current STM (constant height) mode. Uses fast scanrates compared to the force deflection feedback (close to zero). Sensitive tolocal force gradients such as line defects. Improved high resolutioncapability (atomic resolution).
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SFM Force Spectroscopy
Sample
F(D) forces acting on the tip
linearly ramped voltage
applied to piezo
D = Do - vt
F(D)
0
D
jump in contact
jump out of contact
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Cantilevers Probes for SFM
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Scanning Near-field OpticalMicroscopy (SNOM)
SNOM Principle (Pohl et al. 1984): A tinyaperture, illuminated by a laser beam from the
rear side, is scanned across a samle surface,
and the intensity of the light transmitted throughthe sample is recorded. To achieve high lateral
resolution (first experiments provided already
tens of nanometer resolution), the aperture had
to be nanometer sized, and maintained at ascanning distance of less than 10 nm from the
sample surface (i.e., within the evanescent field).
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SNOM Schematic Examples
Small aperture
Evanescent Field
Regime
Illumination
Objective
Detector
Sample
Illumination
Objective
Detector
Sample
Illumination Mode Reflection Mode
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SNOM