Transcript 2_Intro_SPM Methods

Scanning Probe Microscopy
(SPM)
Real-Space
Surface Microscopic Methods
SPM Principle
Consists of
• Probes that are nanosized (accomplished
microlithographically),
• scanning and feedback mechanisms
that are accurate to the subnanometer
level (achieved with piezoelectric
material), and
• highly sophisticated computer controls
(obtained with fast DACs (digital analog
converters, etc.).
Schematic of SPM Principle
Resolution Comparison
3 Axis Cylindrical Piezo
SPM Tree
Conventional STM
Topography 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 Microscopy
Scanning Tunneling Potentiometry
Surface potential studies (e.g., study of grain boundaries)
Photovoltaic and Photoassisted Tunneling Spectroscopy
Surface 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
Scanning Force Microscopy
Friction studies, local material distinction ("Chem. Force Microscopy" )
(S)EFM (Electrostatic Force Microscopy)
Non-contact electrostatic force mapping, (e.g., study of charge decay)
SPM
Scanning Probe Microscopy
(S)MFM (Magnetic Force 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
SNOM (Scanning Near-field Optical Microscopy); optical properties, luminescence
SCM
(Scanning
Calorimetric
Microscopy);
local heat transfer coefficients and transition temperatures
SNOM
(Scanning
Nearfield Optical
Microscopy)
SNAM (Scanning Near-field Acoustic Microscopy); topography and rheology
SCAM (Scanning Capacitance Microscopy); measuring of trapped charges
SECM (Scanning Electrochemical Microscopy); spatial variations of Faradaic currents or potential changes)
SMM (Scanning Micropipette Microscopy); local ion concentration (e.g., transport processes in membranes)
The Three Basic SPM Systems
Scanning Tunneling Microscope (STM)
Scanning Force Microscope (SFM)
Scanning Nearfield
Optical Microscope (SNOM)
Scanning Tunneling Microscopy (STM)
Signal: Tunnel Current
Positive sample bias: Net tunneling
current arises from electrons that tunnel from
occupied states of the tip into unoccupied
states of the sample
FT
FS
Negative sample bias: Net tunneling
current arises from electrons that tunnel from
occupied states of the sample into unoccupied
states of the tip.
FS
The 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 occupied
and unoccupied electronic
states (-> local LDOS
studies).

I  Vbias exp  AF
1/ 2
z

A = const.
FT
The tunnel current is strongly distance, Dz, dependent
Conventional STM
STM Tip
Bias Voltage, V
Tunneling Current, I
Conductive Sample
Piezo Scanner
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, which
keeps the tip in an average (constant) distance from the sample surface.
Scanning is possible in real-time video rates that allow, for instance, the
study of surface diffusion processes.
• Differential tunneling microscopy
Tip is vibrated parallel to the surface, and the modulated current signal is
recorded with lock-in technology.
• Tracking tunneling microscopy
Scanning direction is guided by modulated current signal (e.g., steepest
slope).
• Scanning noise microscopy
Use current noise as feedback signal at zero bias.
• Nonlinear alternating-current tunneling microscopy
Conventionally, STM is restricted to non-conducting surfaces. A high
frequency AC driving force causes a small number of electrons to tunnel
onto and off the surface that can be measured during alternative half-cycles
(third harmonics).
Scanning Force Microscopy (SFM)
Force: FN = kN*z
Ppring constant:
kN
Spring deflection: z
Contact Method:
“Non-Contact” Method:
SFM Tip
z
Interaction or force
dampening field
Sample
Piezo Scanner
Spinodal Decomposition of PS/PMMA Blend
50/50 PS/PMMA blend annealed at 180 oC for 1 week
4-Quadrant
Photodiode
Laser
Cantilever
Topography
PMMA
PS
PMMA
PS
F r ic t
ion
Scan
10 m
SFM Topography
complex flow pattern over time
SFM Lateral Force
2D spinodal decomposition
different from bulk
Note: The bright spots (PS phase/lateral force image) represent
spinodal frustration points of PMMA.
 
Sample
  
Input Modulation
 
Cantilever Response
 
fully elastic
  
viscoelastic
Rheological SFM
Amplitude
Response Modulation
Signal
Lateral Force:
FL = kL*x
Load:
FN = kN*z
SFM Tip
z
Input Modulation Signal
Sample
x
Piezo sinusoidally
modulated either in x or z
Time
Time Delay
Topography Modes of SPM
•
•
•
•
Constant deflection (contact mode)
Analog to the constant current STM mode. The deflection of the cantilever
probe is used as the feedback signal and kept constant.
Constant dampening (AM detection, intermittent contact mode in air or
liquid)
The response amplitude of sinusoidally modulated cantilevers allow
feedback in the pseudo-non-contact regime (intermittent contact) due to
fluid dampening.
Constant frequency shift (FM detection, non-contact mode in ultrahigh
vacuum)
Similar to the FM radio, the frequency is measured and frequency shifts are
used as feedback system. This approach works only in vacuum where fluiddampening effects can be neglected.
Variable deflection imaging (contact mode)
Analog to the variable current STM (constant height) mode. Uses fast scan
rates compared to the force deflection feedback (close to zero). Sensitive to
local force gradients such as line defects. Improved high resolution
capability (atomic resolution).
SFM Force Spectroscopy
linearly ramped voltage
applied to piezo
F(D)
jump in contact
D = Do - vt
0
D
F(D) forces acting on the tip
Sample
jump out of contact
Cantilevers Probes for SFM
Scanning Near-field Optical
Microscopy (SNOM)
SNOM Principle (Pohl et al. 1984): A tiny
aperture, illuminated by a laser beam from the
rear side, is scanned across a samle surface,
and the intensity of the light transmitted through
the 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 a
scanning distance of less than 10 nm from the
sample surface (i.e., within the evanescent field).
SNOM Schematic Examples
Illumination
Detector
Illumination
Small aperture
Objective
Evanescent Field
Regime
Sample
Objective
Sample
Detector
Illumination Mode
Reflection Mode
SNOM