Surface is defined as a few topmost atomic layers of the material, which is in contact with its surrounding (vacuum, air, other material...).
An interface is the boundary between two phases.
Large objects: - small surface-to-volume ratio A:V
- physical and chemical properties primarily defined by the bulk
Small objects: - large surface-to-volume ratio A:V
- properties strongly influenced by the surface
Example: sphere (nanoparticles)
V=4/3 π r3 , A=4 π r2
A/V = 3/r
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Surfaces and interfaces
Comparison of Ag crystal surfaces with different orientations:
Ag, fcc (100)
Ag, fcc (110)
Ag, fcc (111)
Density of atoms Number of broken Surface energy:on surface: bonds:
1.19 ∙ 1019 at/m2 4 bonds γ(100) = 6.78 J/m2
8.41 ∙ 1019 at/m2 5 bonds γ(110) = 5.99 J/m2
6.87 ∙ 1019 at/m2 3 bonds γ(111) = 2.94 J/m2
Surfaces and interfaces
Crystal structure description:
The Bravais lattice are the distinct lattice types which when repeated can fill the whole space.
4 types of unit cell:
P – primitive
I – body centred
F – face centred
C – side centred
7 crystal classes
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Surfaces and interfaces
Crystal lattices at surfaces
14 bravais lattices in 3D are replaced by 5 bravais lattices in 2D!
1-oblique, 2-rectangular, 3-centered rectangular (rhombic), 4-hexagonal, and 5-square
The structure and properties of surfaces differ significantly from that of bulk material!
Why?
... in other words, „Interfaces and surfaces are where the action happens.. “
Surfaces and interfaces
The atoms in surface layers will occupy new equilibrium positions and thusdifferent structure than that of the bulk due to the broken periodicity of the bulkmaterial: the surfaces are eather relaxed or reconstructed.
e.g. Pt (100) surface: cubic to hexagonal
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surface interlayer relaxation in metals:
metal surface relaxation (%)
12 23 34
Al(110) -9 +5 -2
Ag(110) -9.5 +6 -3.5
Cu(110) -8.5 +2.3 -0.9
Fe(211) -10.5 +5.1 -1.7
Surfaces and interfaces
12
23
34
Dimension Structural element
0 point defects: adatoms, vacancies, amorphous coverages, edge atoms
3 deformed regions in the substrate or a thick film (mosaic structures,
agglomerates, strained regions)
dimensionality of defects:
real surfaces are characterized by terrases, steps and knees:
Surfaces and interfaces
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possible surfaces:
Surfaces and interfaces
different binding places at surfaces:
adsorption is binding of atoms or molecules from the gas phase to the substrate
adsorption energy: the energy released after an atom is immobilized at the surface physisorption characterized by a weak interaction between adsorbate and the surface
Surfaces and interfaces
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List of surface science techniques
Techniques
1 AEAPS Auger Electron Appearance Potential Spectroscopy
Usually it is easier to obtain information on bulk than on surface properties:
- because of the smaller quantity of atoms,
1 cm2 surface ≈ 1015 atoms
1 cm3 volume ≈ 1023 atoms
- and difficulty of cleaning and keeping surfaces clean.
At ambient pressure each surface atom is hit about 100 000 000 times per second by a gas particle.
(at 10-6 mbar once per second, at 10-9 mbar once per day)
How to clean the sample surface and how to keep the surface clean for the duration of the experiments (minutes, hours or
even days) ? ? ? ?
Ultra-high vacuum
The development of ultra-high vacuum (UHV) techniques opened the possibility to study well defined surfaces of different materials.
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Reaching UHV conditions can be quite challenging and requires:
• use of UHV pumps, i.e. ion pumps, titanium sublimation pumps, cryopumps,
• use of clean UHV compatible materials (low outgassing and low vapor pressure
materials), i.e. metals, special ceramics, glass, teflon,
• use of special seals and gaskets,
• minimization of the surface area,
• avoiding pits of trapped gas - virtual leaks,
• usage of short and high cross-section tubing,
• baking of the UHV system to remove water and other surface contaminants.
Ultra-high vacuum
Final sample preparation has to be done in UHV!
Methods:
- Heating (for samples covered with e.g. oxide, not stable at higher temperatures – this contamination layers are decomposed or evaporate.) Pt, Si
- Reaction with O2 or H2 (if the contamination layer forms a volatile compound with oxygen or hydrogen, it can be removed by extended heating in this atmosphere) hydrocarbons on metals or sulfur on sulfides
- Ion sputtering (top layer of a sample is removed) most used method, followed by annealing
- Cleaving in UHV (fresh sample surface is exposed)
- MBE, PLD or similar growing methods
Ultra-high vacuum
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Scanning probe microscopy
Optical microscopy
- uses visible light (λ=400 – 700 nm)
- resolution around 200 nm (difraction limited)
(Scanning and Transmision) electron microscopy
- uses electron beam (λ=3.7 pm @ 100 keV)
- SEM resolution < 1 nm (size of interaction volume)
- TEM atomic resolution (aberration limited)
Scanning probe microscopy
- uses sharp physical probe that scans the specimen
- atomic resolution possible (STM, AFM, ...)
- real space and real time imaging
Scanning Probe Microscopy (SPM) is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen.
AFM, atomic force microscopyBEEM, ballistic electron emission microscopyCFM, chemical force microscopyC-AFM, conductive atomic force microscopyEFM, electrostatic force microscopyESTM electrochemical scanning tunneling microscopeFMM, force modulation microscopyKPFM, kelvin probe force microscopyMFM, magnetic force microscopyMRFM, magnetic resonance force microscopySNOM, scanning near-field optical microscopyPFM, Piezoresponse Force MicroscopyPSTM, photon scanning tunneling microscopy PTMS, photothermal microspectroscopy/microscopy SECM, scanning electrochemical microscopySCM, scanning capacitance microscopySGM, scanning gate microscopySICM, scanning ion-conductance microscopySPSM spin polarized scanning tunneling microscopySSRM, scanning spreading resistance microscopySThM, scanning thermal microscopySTM, scanning tunneling microscopySVM, scanning voltage microscopySTP, scanning tunneling potentiometrySHPM, scanning Hall probe microscopySXSTM synchrotron x-ray scanning tunneling microscopy
Scanning probe microscopy
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Scanning near field optical microscopy
conventional (far-field) optical microscopy uses lenses diffraction limits on theresolution due to the lenses used (Abbé’s diffraction barrier): a point is transformedinto an Airy function two points resolved (Rayleigh criterion) theoreticalresolution limit λ/2 cannot be overcome without switching to radically newtechniques, i.e. near field optics
near field optics resolution beyondAbbé’s criterion: illumination through asub-wavelength sized aperture & spacimenwithin the near-field regime (i.e. within ½of the diameter of the hole) to the source
The effect: Light cannot diffract before itinteracts with the sample and theresolution is determined by the diameter ofthe apperture.
Scanning near field optical microscopy
Depending upon the sample being imaged, there are multiple modes of operation:
Transmission: Light source travels through the probe aperture, and transmits through sample.Requires a transparent sample.
Reflection: Light source travels through the probe aperture, and reflects from the surface. Lowerlight intensity, and tip-dependent, but allows for opaque samples.
Collection: Sample is illuminated from large outside light source, and the probe collects thereflected light.
Illumination/Collection: The probe both illuminates the sample and collects the reflected light.
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Scanning near field optical microscopy
Lateral resolution: down to 20 nmVertical resolution: 2-5 nm
To study, e.g., refractive index, chemical structure, local stress...
20 nm x 20 nm intensity distribution of aVertical-Cavity Surface-Emitting Laser(VCSEL)
1 um x 1 um Tobacco Mosaic Virus
Atomic force microscopy
The AFM raster-scans the probe over a small area of the sample, simultaneouslymeasuring the local property (height, friction, magnetism...).
The probe is a tip at the end of a cantilever which bends in response to the forcebetween the tip and the sample. A laser beam is reflected into a split photo-diode andthe image is formed by recording the differential signal.
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Atomic force microscopy
Most common types of interaction:
• contact mode “contact” means the repulsive regime (above the x-axis); maindrawback of remaining in contact with the sample are strong lateral forces due to thecontact
• tapping mode cantilever oscilated at its resonant frequency (hundreds of kHz andamplitudes of the order of 20 nm only fractions of oscilation periods „in contact“ lateral forces largely reduced (convenient for soft samples): “phase imaging” is possible,based on measuring the phase difference between the cant. oscilations and the detectedoscilations.
• non-contact mode cantilever oscilated at distances 50-150 Å above suface, i.e. in theattractive regime main drawback: possibility of jumping into contact mode
• lift mode scanning at distances where the topographic image is not possible
imaging of long-range (magnetic and electrostatic) forces
• lateral force microscopy (LFM) 4-segment photo-diode detection of thecantilever torsion variation in friction between the tip and the sample will twist thecantilever (possible causes: adhesion, surface roughness, elastic properties, chemicalcomposition, local elecrostatic interaction, chemical interaction). If the interaction isknown, it can be directly measured.
•...
Atomic force microscopy
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Atomic force microscopy
Atomic resolution, works with conductive or non-conductive samples, in air orvacuum...
Growth hillock at the calcite (104) surface observed by contact AFM in situ during crystal growth.
Coexisting metastable reconstructions of the Si(111) surface
100 um x 100 um steel surface
topography magnetic force
HDD100 um
Atomic force microscopy
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Atomic force microscopy
Hook’s law: F = k • D (valid for small displacements)
k spring constant, D deflection
D > 0 (F >0) repulsive; D < 0 (F < 0) attractive
for rectangular cantilever: k = ET3W/4L3
E elasticity modulus, T, W & L dimensions
resonant frequency fr = 0.162 (E/)½T/L2
density of cantilever
Force spectroscopy: measure nanoscale contacts, atomic bonding, Van der Waals forces,and Casimir forces, dissolution forces in liquids and single molecule stretching andrupture forces.
