EMSE-515 F. Ernst

Scanning Probe Microscopy

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Literature

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Scanning Probe Microscopy: The Lab on a Tip by Ernst Meyer ,Ans Josef Hug ,Roland Bennewitz

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Scanning Probe Microscopy and Spectroscopy : Theory, Techniques, and Applications by Dawn Bonnell (Editor)

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High-Resolution Imaging and Spectrometry of Materials by Frank Ernst, M. Rühle (Editors)

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Introduction

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Scanning Probe Microscopy

• acronym: SPM

• group of surface characterization techniques

⇥ scanning tunneling microscopy (STM)· 1982, Binnig and Rohrer (IBM)· Nobel Prize 1986

⇥ atomic force microscopy (AFM)· 1986, Binnig, Quate, and Gerber

⇥ advanced techniques based on STM and AFM

• new SPM techniques are still being developed

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Principle of SPM

• sharp tip scanning over specimen surface

• motion controlled very precisely by piezoelectric actuators

• local tip–surface interaction:

� measure local surface structure or properties

� or: manipulate

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Tip–Surface Interaction

• force

� electric fields

� magnetic fields

• energy transport

� electron current(tunneling or contact)

� heat current

� photon current

� elastic vibration

� …

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A Typical SPM Experiment

• move tip in three directions x, y, z by electrostatic actuator

• electronic controller:

� maintain tip–specimen distance that yields preset tun-neling current

� record required scanner voltage as a function of x, y

� display as two-dimensional image

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Families of SPM

• scanning tunneling microscopy (STM)and spectroscopy (STS)

• atomic force microscopy (AFM)

• scanning near-field microscopy (SNOM)

• related techniques

� nano-indentation, scratching, hardness,friction, wear

� conductive AFM

� electrochemical STM

� …

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Information Provided by SPM

• lateral range of imaging: 100µm to 10 pm

• surface topography

� structure of perfect crystal surfaces

� but also: defects(point defects, adsorbates, steps,…)

• local electronic structure

• magnetic and electrostatic domain structure

• local mechanical properties

• local electrochemical properties

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Applications in Materials Science

• complimentary toRBS, XPS, SAM, XRD, LEED, SEM, TEM, …

• does not require vacuum(although vacuum often beneficial)

• no irradiation damaging(but other kinds of damaging are possible)

• SPM provides complimentary information(e. g. topography and electronic structure)

• SPM can measure local properties(e. g. hardness, electrical conductivity)

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Basic Concepts

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Interaction

• interaction between scanning probe and sample

• “near-field”

⇤ overcome resolution limits of far-field techniques

� resolution much better than e. g. photon or electronwavelength

• but: resolution limited by shape of probe (tip)

⇤ lateral resolution depends on vertical amplitude of surfacestructure

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Topography

• “typical” shape of probing tip: cone

⇤ topography will not be imaged correctly

� sharp steps will be “smeared out”

� holes with diameter < tip radius will not be imaged

• mathematical description: “convolution” of surface struc-ture with tip shape

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Convolution

P[x] = (S ⇤ T)[x] =Z+⇧

�⇧S[x⌅] · T[x � x⌅]dx⌅

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Near-Field Interaction of STM

• most powerful near-field interaction:tunneling current

• reasons:

� very well localized – decay length as small as the diam-eter of an atom

� no other electrical currents exist that could obscure tun-neling current

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Near-Field Interaction of AFM and SNOM

• near-field interaction:forces between atoms of tip and specimen

• some forces well localized

� comparable to tunneling current

� provide high spatial resolution

• but: also long-range forces

� e. g. AFM: van der Waals forces

� e. g. SNOM: optical far-field

⇤ need to measure short-range forces on the background oflong-range forces

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Far-Field Interactions

• electrostatic forces

⇥ isolated charge:force decays as r�2 with distance r

⇥ not exactly “near-field”

⇥ but: fields of multipoles decay more rapidly

• magnetic forces

⇥ generally far-field

⇥ but: decay length of magnetic forces from e. g. alternat-ing magnetic domains ⌅ domain size

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