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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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