Scanning Probe Microscopy (SPM)

Real-Space

Surface Microscopic Methods

SPM Principle

• 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.).

Consists of

Schematic of SPM Principle

Resolution Comparison

3 Axis Cylindrical Piezo

SPM Tree Conventional STMTopography of conductive surfaces, I-V spectroscopy (e.g., local band gaps)

Ballistic Electron Emission Microscopy and SpectroscopySubsurface 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 TunnelingSTM 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 MicroscopyContact 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 SNOM (Scanning Nearfield Optical Microscopy)

SNAM (Scanning Near-field Acoustic Microscopy); topography and rheology

The Three Basic SPM Systems

Scanning Tunneling Microscope (STM) Scanning Force Microscope (SFM)

Scanning Nearfield Optical Microscope (SNOM)

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 occupied and unoccupied electronic states (-> local LDOS studies).

T

S

S

T

Positive sample bias: Net tunneling current 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 from occupied states of the sample into unoccupied states of the tip.

The tunnel current is strongly distance, Dz, dependent

zAexpVI 2/1bias

A = const.

Conventional STM

Tunneling Current, I

Bias Voltage, V

Conductive Sample

STM Tip

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 microscopyTip is vibrated parallel to the surface, and the modulated current signal is recorded with lock-in technology.

• Tracking tunneling microscopyScanning direction is guided by modulated current signal (e.g., steepest slope).

• Scanning noise microscopyUse current noise as feedback signal at zero bias.

• Nonlinear alternating-current tunneling microscopyConventionally, 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)

Sample

SFM Tip

Piezo Scanner

z

Force: FN = kN*z

Ppring constant: kN

Spring deflection: z

Interaction or force dampening field

Contact Method: “Non-Contact” Method:

Sample

Input M odu latio n C antilever R esponse

S can

C antilever

4-Q uadrantP hotodiode

L aser

fu lly e la s tic visco e las tic

Topography

50/50 PS/PMMA blend annealed at 180 oC for 1 weekSpinodal Decomposition of PS/PMMA Blend

PSPMMA PMMA PS

10 m

complex flow pattern over time

SFM Topography SFM Lateral Force

2D spinodal decomposition different from bulk

Note: 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 weekSpinodal Decomposition of PS/PMMA Blend

PSPMMA PMMA PS

10 m

PSPMMA PMMA PS

10 m10 m

complex flow pattern over time

SFM Topography SFM Lateral ForceSFM Topography SFM Lateral Force

2D spinodal decomposition different from bulk

Note: The bright spots (PS phase/lateral force image) represent spinodal frustration points of PMMA.

Rheological SFM

Sample

SFM Tip

Piezo sinusoidally modulated 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

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 fluid-dampening 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

Sample

F(D) forces acting on the tip

linearly ramped voltageapplied to piezo

D = Do - vt

F(D)

0

D

jump in contact

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

Small aperture

Evanescent FieldRegime

Illumination

Objective

Detector

Sample

Illumination

Objective

Detector

Sample

Illumination Mode Reflection Mode

SNOM