Crystal Structures Materials can be divided into Amorphous, Single Crystalline, Poly Crystalline according to their crystal structure. Bulk Material composed a repeat pattern of unit cell and thus their properties are determined by their crystal structures Ball-stick model: Ball – Atom, Stick -- Bonding FCC Unit cell BCC Unit cell HCP Unit cell

Crystalline direction and plane

What is Surface and Surface Energy Surface is the place where atoms terminating bulk, i.e. the termination of bulk crystalline structure (Repeating pattern) Surface Energy is coming from unbonded electrons

What is the surface / interface Atoms at surface / Interface has unbonded electrons which will cause extra energy compared with atoms in bulk This extra energy per area is defined as surface / interface energy f The total energy associated with surface / interface E = f * S This energy becomes significant in nano material system

Why surface properties are important for Nano materials d d H Thin Films Total Volume V=H*A Total Surface Area S=A*H/d=V/d Surface-volume ratio S/V = 1/d

Why surface properties are important for Nano materials Nano Particles Total Volume V=6*4/3pR3=8pR3 Total Surface Area S=6*4pR2=24pR2=3V/R Surface-volume ratio S/V = 3/R = 6/d

What happen in real material system? Surface reconfiguration will impact about 2-3 atomic layers in depth. (or 6-8A) In general, atoms in those regions are different than their cousins in bulk and therefore included as surface. For Si, assume surface region are 2-3 atomic layers or roughly one unit cell in depth => Another reason that surface properties are more important for nano materials as most of atoms are at surface

What happen in real material system? % atoms in the surface = 100 * #surface atoms / #bulk atoms Thin layer: #surface atoms ~ #atoms in the unit cell * s/a2 ~ 8*V/d/25A2 #bulk atoms ~ #atoms in the unit cell*V/a3~8*V/125A3 % atoms in the surface = 500/d(A) For particles: #surface atoms ~ #atoms in the unit cell * s/a2 ~ 8*4pR2/25A2 #bulk atoms ~ #atoms in the unit cell*V/a3~8*(4/3)pR3 /125A3 % atoms in the surface = 1500/R(A) Layer thickness 1mm 10nm 2nm % Atoms in the surface 0.05% 5% 25% Si Particle size 1mm 10nm 2nm % Atoms in surface 0.3% 30% 88%

Surface Energy in BCC and FCC Structure Surface energy is determined by how many bonds we have to break in order to create a fresh surface (Cleavage) and thus depending upon crystalline planes? Which crystalline plane shall have lowest surface energy? Why? Closed packed plane has most atoms in plane and thus the out plane bonding is less and weak.

Surface Energy in Diamond Structure such as Silicon Diamond Structures in Cubic (Si, C) Which one is close packed plane?

Surface Reconstruction Surface atoms are re-arranging their position and bonding to minimize the surface energy

Adsorption and Desorption The adsorption of molecules on to a surface is a necessary prerequisite to any surface mediated chemical process. In general, surface reaction can be broken down into the following steps : Transferring of reactants to the active surface Adsorption of one or more reactants onto the surface Surface reaction Desorption of products from the surface Transferring of products away from the surface Absorption and Desorption are equally important.

Physical and Chemical Absorption The basis of distinction is the nature of the bonding between the molecule and the surface. With ... Physical Adsorption : the only bonding is by weak Van der Waals - type forces. There is no significant redistribution of electron density in either the molecule or at the substrate surface. Chemisorption : a chemical bond, involving substantial rearrangement of electron density, is formed between the adsorbate and substrate. The nature of this bond may lie anywhere between the extremes of virtually complete ionic or complete covalent character.

Typical Characteristics of Adsorption Processes Chemisorption Physisorption Temperature Range (over which adsorption occurs) Virtually unlimited (but a given molecule may effectively adsorb only over a small range) Near or below the condensation point of the gas (e.g. Xe < 100 K, CO2 < 200 K) Adsorption Enthalpy Wide range (related to the chemical bond strength) - typically 40 - 800 kJ mol-1 Related to factors like molecular mass and polarity but typically 5-40 kJ mol-1 (i.e. ~ heat of liquefaction) Crystallographic Specificity Marked variation between crystal planes Virtually independent of surface atomic geometry Nature of Adsorption Often irreversible Reversible Saturation Uptake Limited to one monolayer Multilayer uptake possible Kinetics of Adsorption Very variable - often an activated process Fast - since it is a non-activated process

Wetting and Contact Angle The “Young Equation” determines the “contact angle” Balance of forces at the periphery of a drop on a rigid surface The wetting angle,  ranges from 0 (wetting) to  (de-wetting) The engineering importance of surfaces Wetting -- Frying pans and car waxes, Detergents, Lubricants Bonding – Glues, Solders Catalysis -- Adsorption Capillarity -- Tree sap and blood vessels

Film Formation (Spreading) V Spreading: LV+SL interfaces have lower energy than SV Want for painting, coating, soldering, etc. To promote spreading Raise SV: e.g., clean the interface Flux in soldering removes oxides from surface Clean the substrate interface prior to deposit thin film Lower SL: e.g., include reactive species in L Sn in solder forms intermetallic compounds with Cu, Ni or Au Flow catalyst or reactant to form an intermediate layer and prepare for deposition Lower LV: e.g., add surfactant (species that adsorbs at LV interface) Flux in solder coats surface, lowers LV

De-Wetting De-wetting: film of vapor preferred between S and L LV+SV interfaces have lower energy than SL Want for “non-stick” coatings (frying pans, car wax). Passivation Layers to stop moisture. To promote de-wetting Lower SV: e.g., add surfactants or low- coatings to solid Teflon on frying pans Lower LV: e.g., add surfactant (species that adsorbs at LV interface) Raise SL: e.g., remove any possible surfactants or reactive species

Hydrophilic vs. Hydrophobic Hydrophilic (Water Loving): Materials exhibit an affinity for water The surface chemistry allows these materials to absorb water and be wetted forming a water film. Hydrophilic materials also possess a high surface tension and have the ability to form "hydrogen-bonds" with water. Hydrophobic (Water hating): have little tendency to adsorb water and water tends to "bead" on their surfaces. Hydrophobic materials possess low surface tension values and lack active groups in their surface chemistry for formation of "hydrogen-bonds" with water.