Presentation on theme: "Still Another Semiconductor Definition!. Clathrate Semiconductors Not in the Texts! A research interest for me for about the last 12 years! “New” crystalline."— Presentation transcript:
Still Another Semiconductor Definition!
Clathrate Semiconductors Not in the Texts! A research interest for me for about the last 12 years! “New” crystalline phases of the Group IV Elements: Si, Ge, Sn (not C yet). Few pure elemental phases yet. Mostly compounds, usually with Groups I & II elements (Na, K, Cs, Ba). Interesting properties (possible applications are for use as a thermoelectric material). Clathrate Crystal Structures will be discussed briefly now & contrasted to the diamond structure. More properties as class proceeds.
Group IV Elements The valence electron configurations of the free atoms are: ns 2 np 2 [ n = 2, C; n = 3, Si; n = 4, Ge; n = 5, Sn]
Group IV Crystals Si, Ge, Sn: Their ground state crystal structure is the Diamond Structure Each atom tetrahedrally (4-fold) coordinated (4 nearest- neighbors) with sp 3 covalent bonding Bond angles: Perfect, tetrahedral = 109.5º Si, Ge: Semiconductors Sn: (α-tin or gray tin) - Semimetal
Carbon Crystals C: Graphite & Diamond Structures Diamond An insulator or a wide bandgap semiconductor Graphite A planar structure sp 2 bonding a 2d metal (in plane) The Ground State (lowest energy configuration) is graphite at zero temperature & atmospheric pressure. The graphite-diamond energy difference is VERY small!
Other Group IV Crystal Structures (Higher Energy) C: “Buckyballs” (C 60 ) “Buckytubes” (nanotubes), other fullerenes Graphene
Sn: (β-tin or white tin) - body centered tetragonal lattice, 2 atoms per unit cell. Metallic. Si, Ge, Sn: The Clathrates.
Clathrates Crystalline Phases of Group IV elements: Si, Ge, Sn (not C yet!) “New” materials, but known (for Si) since 1965! –J. Kasper, P. Hagenmuller, M. Pouchard, C. Cros, Science 150, 1713 (1965) As in the diamond structure, all Group IV atoms are 4- fold coordinated in sp 3 bonding configurations. Bond angles: Distorted tetrahedra Distribution of angles instead of the perfect tetrahedral 109.5º Lattice contains hexagonal & pentagonal rings, fused together with sp 3 bonds to form large “cages”.
Pure materials: Metastable, expanded volume phases of Si, Ge, Sn Few pure elemental phases yet. Compounds with Group I & II atoms (Na, K, Cs, Ba). Potential applications: Thermoelectrics Open, cage-like structures, with large “cages” of Si, Ge, or Sn atoms. “Buckyball-like” cages of 20, 24, & 28 atoms. Many varieties. The two most common varieties are: Type I (X 46 ) & Type II (X 136 ) X = Si, Ge, or Sn
Meaning of “Clathrate” ? From Wikipedia, the free encyclopedia: “A clathrate or clathrate compound or cage compound is a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule. The word comes from the Latin clathratus meaning furnished with a lattice.” “For example, a clathrate-hydrate involves a special type of gas hydrate consisting of water molecules enclosing a trapped gas. A clathrate thus is a material which is a weak composite, with molecules of suitable size captured in spaces which are left by the other compounds. They are also called host-guest complexes, inclusion compounds, and adducts.”
Group IV clathrates have the same crystal structure as clathrate-hydrates (ice). Type I clathrate-hydrate crystal structure X 8 (H 2 O) 46 :
Si 46, Ge 46, Sn 46 : ( Type I Clathrates) 20 atom (dodecahedron) cages & 24 atom (tetrakaidecahedron) cages, fused together through 5 atom rings. Crystal structure = Simple Cubic, 46 atoms per cubic unit cell. Si 136, Ge 136, Sn 136 : ( Type II Clathrates) 20 atom (dodecahedron) cages & 28 atom (hexakaidecahedron) cages, fused together through 5 atom rings. Crystal structure = Face Centered Cubic, 136 atoms per cubic unit cell.
Clathrate Building Blocks 24 atom cage: Type I Clathrate Si 46, Ge 46, Sn 46 (C 46 ?) Simple Cubic Type II Clathrate Si 136, Ge 136, Sn 136 (C 136 ?) Face Centered Cubic 20 atom cage: 28 atom cage:
Clathrate Lattices Type I Clathrate Si 46, Ge 46, Sn 46 simple cubic Type II Clathrate Si 136, Ge 136, Sn 136 face centered cubic  direction  direction
Group IV Clathrates Not found in nature. Synthesized in the lab. Not normally in pure form, but with impurities (“guests”) encapsulated inside the cages. Guests “Rattlers” Guests: Group I (alkali) atoms (Li, Na, K, Cs, Rb) or Group II (alkaline earth) atoms (Be, Mg, Ca, Sr, Ba) Synthesis: Na x Si 46 (A theorists view!) –Start with a Zintl phase NaSi compound. –An ionic compound containing Na + and (Si 4 ) -4 ions –Heat to thermally decompose. Some Na vacuum. Si atoms reform into a clathrate framework around Na. –Cages contain Na guests
Type I Clathrate (with guest “rattlers”) 20 atom cage with a guest atom + 24 atom cage with a guest atom  direction  direction
Pure Materials: Semiconductors. Guest-containing materials: –Some are superconducting materials (Ba 8 Si 46 ) from sp 3 bonded, Group IV atoms! –Guests are weakly bonded in cages: A minimal effect on electronic transport –Host valence electrons taken up in sp 3 bonds –Guest valence electrons go to conduction band of host ( heavy doping density). –Guests vibrate with low frequency (“rattler”) modes A strong effect on vibrational properties = Guest Modes Rattler Modes
Possible use as thermoelectric materials. Good thermoelectrics should have low thermal conductivity! Guest Modes Rattler Modes: A focus of recent experiments. Heat transport theory says: The low frequency rattler modes can scatter efficiently with the acoustic modes of the host. The guest vibrations lower the thermal conductivity A good thermoelectric! Clathrates of Interest: Sn (mainly Type I). Si & Ge, (mainly Type II). Recently, “Alloys” of Ge & Si (Type I ).