grain boundary grooving

why does it occur and why is it important

surface energy

surface tension

they are the same

Surface energy is controlled by bond strength (strong intermolecular forces make it larger).  The reversible work of formation of a surface of unit area.

• Terrace
  
• Monatomic step
  
• Adatom (mobile)
  
• Step-adatom
  
• Step vacancy
  
• Kink
  
• Kink, step and terrace atoms have large equilibrium concentrations on real surfaces (hard to get a perfect surface)

A rough surface may be homo or heterogeneous (formation of air pockets under droplet) both of which have varying effects on the contact angle measured.  It is therefore important to know effects of scale of roughness relative to size of drop

gives flux of molecules striking a surface of unit area at a given ambient pressure

Wenzel: homogeneous

Cassie-Baxter: heterogeneous

Petal effect: liquid wets grooves of leaf and adhesive force between water and solid is very high (until weight is above a critical value)

Lotus effect: surface is heterogeneous with air and solid therefore wetting is not able to occur and adhesive forces are extremely low so water rolls off easily ex. self cleaning glasses, windshield

X-ray photoelectron spectroscopy (XPS)

• Provides information about the elemental surface composition (core electronic levels of adsorbates of the surface)
• Electronic binding energies change as a result of changes in chemical environment.
• Application: determination of the oxidation states of elements at the surface (oxidation tends to raise binding energy)

• Involves atomic ionization (electrons with binding energies less than that of incident may be ejected), electron emission, analysis of emitted Auger electrons (by considering kinetic energy of electrons)
• Suitable for studying the composition of solid and liquid surfaces
• A series of peaks (number of species v. energy) is used to analyze.  The height of the peak corresponds to concentration and the kinetic energy at which the peak exists is unique for various elements.
• AES is not completely quantitative becomes some of the electrons are lost due to impacts with other atoms.

• A tip is brought near the surface of a sample and a voltage is placed across it.  The closer it is to the surface, a current will flow.  The machine attempts to measure a constant current therefore the tip will displace accordingly to follow the contour of the surface
• Advantages: good for looking at semiconductors because surface contours are corrugated
• Disadvantages: demanding, highly susceptible to vibrations, not good for metals, cannot determine interatomic distance, does not give compositional information

• Used for small samples and allows one to accurately measure the distance between an adsorbed later and the surface, surface structure, and adsorbate arrangement
• Relatively insensitive to defects and composition
• Range of electron wavelength is the same as atomic spacing necessary for diffraction
• Intensities recorded as a function of incident electron beam energy

tilt: entirely edge dislocations

D=b/sinx

twist: entirely screw dislocations

D=b/2sinx

• If atoms are ionized and transfer electrons into the solid the value decreases
• adsorption of negative ions increases it

• more open or rough surfaces due to higher heats of adsorption
• lower temperatures typically but sometimes when molecules must dissociate in order to absorb onto surface, S may increase with T indicating an activated process

• weakly adsorbed and low heat of adsorption
• bonding between molecules and surface is weaker than molecule-molecule bonds
• adsorbate retains identity as no bonds are broken (overlayer and substrate have independent lattices) and van der Waals forces link them
• Consider Langmuir and BET models here

• must fill a single layer of surface before building up
• O₂, CO, CO₂ tend to follow this trend

can build up without coverage [Θ=number of occupied adsorption sites/total number of possible sites]. Valid for high adsorbate pressures

heat of adsorption decreases logarithmically with coverage (Chemisorption)

• strongly adsorbed, high heat of adsorption (which depends very much on the surface structure)
• tends to have a long residence time
• adsorbate-adsorbate interactions are lower than that of adsorbate surface
• the overlayer and substrate do not have independent lattices
• bonds are changed/broken
• heat of adsorption decreases with increasing coverage because of the repulsion interaction between adsorbate molecules close to each other
• Substrate surface relaxation and reconstruction can be removed and a bulk-like equilibrium can be obtained

Growth of a few monolayers before island growth becomes energetically favaorable

(island growth)

γI+γF>γS this results in the formation of 3D nuclei and they form by direct impingement of atoms on the surface at one of the possible active sites like steps, kinks or dislocations.  Fast deposition rates and low substrate temperature promote island growth.

γl+γf<γs (film surface energy, interface, substrate surface energy. In homoepitaxy, the interfacial energy is very low and will change with film thickness.  As successive monolayers are deposited, dislocations form to induce strain relief.

• significant in thin films
• oriented growth of a crystalline material on the single crystal surface of another material

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• these are composed of terraces separated by steps which may also have kinks
• w(h_tk_tl_t) x (h_sk_sl_s)
• w-width of atoms in terrace
• "compact step notation"

• reduce the amount of surface area exposed
• alter local surface atomic geometry to reduces surface free energy
• expose surface planes that have low surface free energy

bond lengths may change or surface atoms relocate to optimize the strength of the adsorbate-substrate bond.  Because dangling bonds cannot easily be satisfied except by drastically rearranging atoms.  Involves larger displacements of surface atoms and occurs when there is a less stable metal surface and is much more prevalent in semiconductors

1. ionization of large binding energy molecules
2. formation of chemisorbed intermediates
3. reduction of activation energy

2Pt+H2>>2PtH   2Pt+O2>>2PtO2

H and O are chemisorbed to the surface

σ0-total sites, σ-occupied sites, τ-residence time, F-flux

(1-σ/σ0)F - fraction adsorbed

(1-σ/σ0)Fτ - coverage (σ)

Fτ=kP