NCSU [110] [001] [110] Si GaAs 2 nm

NCSU The World of Atoms Instructor: Dr. Gerd Duscher www4.ncsu.edu/~gjdusche www4.ncsu.edu/~gjdusche Office: 2156 Burlington Nuclear Lab. Office Hours: Tuesday: 10-12pm NCSU

The World of Atoms Objective Johann Wolfgang von Goethe ( ): Faust Faust is searching for the first principles, for "that inner force which holds the world together," “was die Welt im Innersten zusammenhält”. So do we, in this lecture

NCSU Atomic Structure Bohr Model that is too simple

NCSU Atomic Structure probability 1 Bohr Model Nucleus distance Wavemechanic Model Nucleus electrons probability 1 radial distance Energy

NCSU Some Links Bonding : ds.html (pictures of different kinds of bonding) ds.html Orbitals : (computer simulation of electron positiond for certain orbitals) Crystal Structure : (Pictures of several crystal structures)

NCSU have discrete energy states tend to occupy lowest available energy state. 3 Electrons... Adapted from Fig. 2.5, Callister 6e. Electron Energy States increasing energy n=1 n=2 n=3 n=4 1s 2s 3s 2p 3p 4s 4p 3d

NCSU have complete s and p subshells tend to be unreactive. Adapted from Table 2.2, Callister 6e. Stable Energy Configurations Z Element Configuration 2 He 1s 2 10 Ne 1s 2 2s 2 2p 6 18 Ar 1s 2 2s 2 2p 6 3s 2 3p 6 36 Kr 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6

NCSU Why? Valence (outer) shell usually not filled completely. Most elements: Electron configuration not stable. Adapted from Table 2.2, Callister 6e. Survey of Elements Element Hydrogen Helium Lithium Beryllium Boron Carbon... Neon Sodium Magnesium Aluminum... Argon... Krypton Electron configuration 1s 1 2 (stable) 1s 2 2s 1 1s 2 2s 2 1s 2 2s 2 2p 1 1s 2 2s 2 2p s 2 2s 2 2p 6 (stable) 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2 1s 2 2s 2 2p 6 3s 2 3p s 2 2s 2 2p 6 3s 2 3p 6 (stable)... 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 (stable) Atomic #

NCSU Columns: Similar Valence Structure Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions. Adapted from Fig. 2.6, Callister 6e. The Periodic Table He Ne Ar Kr Xe Rn inert gases accept 1e - accept 2e - give up 1e - give up 2e - give up 3e - F Li Be Metal Nonmetal Intermediate H Na Cl Br I At O S Mg Ca Sr Ba Ra K Rb Cs Fr Sc Y Se Te Po

NCSU Bonding attractive force F A repulsive force F R Net force F N repulsion attraction 0 force F repulsive energy E R attractive energy E A net energy E N repulsion attraction 0 potential energy E

NCSU bond length, r bond energy, E o melting temperature, T m T m is larger if E o is larger. Properties from Bonding: T m F F r r larger T m smaller T m Energy (r) r o E o = “bond energy” Energy (r) r o r unstretched length

NCSU 16 Elastic modulus, E E ~ curvature at r o Properties from Bonding: E cross sectional area A o D L length,L o F undeformed deformed r larger Elastic Modulus smaller Elastic Modulus energy r o unstretched length E is larger if E 0 is more negative. coefficient of thermal expansion,   ~ symmetry at r o smaller  larger   r o D L length,L o unheated, T 1 heated, T 2 r energy  is smaller if E o is more negative

NCSU Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaCl Ionic Bonding Na (metal) unstable Cl (nonmetal) unstable electron + - Coulomb attraction Na (cation) stable Cl (anion) stable

NCSU Ionic Bonding + - Electron density difference

NCSU 9 predominant bonding in ceramics give up electronsacquire electrons Examples: Ionic Bondings He - Ne - Ar - Kr - Xe - Rn - F 4.0 Cl 3.0 Br 2.8 I 2.5 At 2.2 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 H 2.1 Be 1.5 Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 Ti 1.5 Cr 1.6 Fe 1.8 Ni 1.8 Zn 1.8 As 2.0 CsCl MgO CaF 2 NaCl O 3.5

NCSU requires shared electrons example: CH 4 C: has 4 valence e, needs 4 more H: has 1 valence e, needs 1 more Electronegativities are comparable. Covalent Bonding shared electrons from carbon atom shared electrons from hydrogen atoms H H H H C CH 4 H H H H C enhanced electron density

NCSU Ni 3 Al–Superalloy Bonds Covalently Ni Al

NCSU 11 Molecules with nonmetals Molecules with metals and nonmetals Elemental solids (RHS of Periodic Table) Compound solids (about column IVA) Examples: Covalent Bonding He - Ne - Ar - Kr - Xe - Rn - F 4.0 Cl 3.0 Br 2.8 I 2.5 At 2.2 Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 H 2.1 Be 1.5 Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 Ti 1.5 Cr 1.6 Fe 1.8 Ni 1.8 Zn 1.8 As 2.0 SiC C(diamond) H 2 O C 2.5 H 2 Cl 2 F 2 Si 1.8 Ga 1.6 GaAs Ge 1.8 O 2.0 column IVA Sn 1.8 Pb 1.8

NCSU Arises from a sea of donated valence electrons (1, 2, or 3 from each atom). Primary bond for metals and their alloys Metallic Bonding

NCSU arises from interaction between dipoles permanent dipoles-molecule induced fluctuating dipoles -general case: -ex: liquid HCl -ex: polymer Van Der Waals Bonding secondary bonding HHHH H 2 H 2 van der Waals bonding ex: liquid H 2 asymmetric electron clouds van der Waals bonding +- van der Waals bonding +- H Cl H van der Waals bonding

NCSU Van Der Waals Bonding permanent dipols fluctuating (Induced) dipols

NCSU 18 Ceramics (Ionic & covalent bonding): Metals (Metallic bonding): Polymers (Covalent & Secondary): large bond energy large T m large E small  variable bond energy moderate T m moderate E moderate  directional Properties van der Waals bonding dominates small T small E large  Summary: Primary Bonds secondary bonding

NCSU Non dense, random packing Dense, regular packing Dense, regular-packed structures tend to have lower energy. Energy And Packing r typical neighbor bond length typical neighbor bond energy energy r typical neighbor bond length typical neighbor bond energy energy

NCSU atoms pack in periodic, 3D arrays typical of: Crystalline materials... -metals -many ceramics -some polymers atoms have no periodic packing occurs for: Noncrystalline materials... -complex structures -rapid cooling crystalline SiO 2 noncrystalline SiO 2 "Amorphous" = Noncrystalline Materials And Packing

NCSU tend to be densely packed. have several reasons for dense packing: -Typically, only one element is present, so all atomic radii are the same. -Metallic bonding is not directional. -Nearest neighbor distances tend to be small in order to lower bond energy. have the simplest crystal structures. We will look at three such structures... Metallic Crystals

NCSU rare due to poor packing (only Po has this structure) close-packed directions are cube edges. Coordination # = 6 (# nearest neighbors) Simple Cubic Structure (sc)

NCSU Coordination # = 8 Close packed directions are cube diagonals. --Note: All atoms are identical; the center atom is shaded differently only for ease of viewing. Body Centered Cubic Structure (bcc)

NCSU Coordination # = 12 Close packed directions are face diagonals. --Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing. Face Centered Cubic Structure (fcc)

NCSU Perovskite Strucutre SrTiO 3 Applications: non-linear resistors (PTC), SMD capacitors, piezoelectric sensors and actuators, ferroelectric memory.

NCSU Some engineering applications require single crystals: Crystal properties reveal features of atomic structure. --Ex: Certain crystal planes in quartz fracture more easily than others. --diamond single crystals for abrasives --turbine blades Crystals as Building Blocks

NCSU Most engineering materials are polycrystals. Nb-Hf-W plate with an electron beam weld. Each "grain" is a single crystal. If crystals are randomly oriented, overall component properties are not directional. Crystal sizes typ. range from 1 nm to 2 cm (i.e., from a few to millions of atomic layers). 1 mm POLYCRYSTALS

NCSU Single Crystals -properties vary with direction: anisotropic. -example: the modulus of elasticity (E) in bcc iron: Polycrystals -properties may/may not vary with direction. -if grains are randomly oriented: isotropic. (E poly iron = 210 GPa) -if grains are textured, anisotropic. 200 mm Single vs Polycrystals E (diagonal) = 273 GPa E (edge) = 125 GPa

NCSU TEMs at NCSU The NEW JEOL 2010F This is a TEM/STEM, which can do everything

NCSU TEMs at NCSU TEM Lab Course at the OLD TEM: Topcon

NCSU STEM at ORNL This STEM provides the smallest beam in the world. It uses the brightest source in the universe, 1000 times brighter than a supernova.