Polyethylene (CH2)x vs. Polyacetylene (CH)x

σ = 10–16 Ω–1cm–1 Insulator

σ = 10–5 Ω–1cm–1 Similar to pure Si

σ = 10–9 Ω–1cm–1 “Modest”

West p.362-4 Smart & Moore p.405-8 Polyacetylene, A 1-D Conductor

• (CH)x, conjugated π system • Organic solids are usually insulators (e.g. ) • Conjugated systems are exceptions

• (CH)x, semiconductor (Eg = 1.9 eV; Eg = 1.1 eV for Si) • Synthesized by Ziegler-Natta catalysis • above 1%: semiconductor to metal transition, σ up to 103 Ω–1cm–1 6 –1 –1 • Doped (CH)x more conducting than Cu (σ = 10 Ω cm ) on a weight-for-weight basis

δ+ δ– • acceptors: Br2, I2, SbF5, WF6 → (CH)x Br δ– δ+ • Electron donors: Alkali metals → (CH)x M • SC to “synthetic metal” at several mol % doping • Combination of properties: high σ with mechanical properties Batteries

+ – (CHNax)n → (CHNax–1)n + nNa + ne – – (CHIy)n + ne → (CHIy–1)n + nI

nNa+ + nI– → nNaI

(CHNay)n (CHIx)n

+ – Li and ClO4 : West p.364 • Electroluminescence • Beyond GaP • Patterned array similar to ink-jet printing, on any substrate • Active displays, low power (for darker images), fast refresh rates, better contrast • Traffic signs, lights, displays • Salt-like, can be difficult to process, lifespan issues, burn-in • poly(p-phenylene vinylene) (PPV) • polyfluorenes

• Control hν and solubility by tailoring side groups 1-D Inorganic Materials

NbS3 (CH)x

• 1D channels for injection of guests – • Oxidation of chains: Br2, AsF5 (e acceptor) • Reduction of chains: Li (e– donor) • One-dimensional character for electronic conductivity • migration in channels more easily blocked by defect or immobile impurity than 2D or 3D materials Transition Metal Trichalcogenides

Eur. J. Solid State Inorg. Chem. 1992, 29, 111

• TiS3, NbS3, MoS3 • Face-sharing trigonal prisms • Nb4+, d1 4+ 2– 2– • Crystal structure: Nb (S2) S • Distorted trigonal prisms: intrachain Nb-Nb pairs, d(Nb-Nb) = 3.04Å

• Half-filled dz2 splits into two bands, Eg ~ 0.5 eV, semiconductor • Associated chains via eight-coordinate Nb • More of a layered compound with van der Waals gap n • Chemical intercalation with BuLi → LixNbS3, 0 ≤ x ≤ 3 Graphite & its Intercalates

• Classic host compound 2 • sp hybridized; pz’s overlap into π, π* band • Filled π VB, empty π* CB, overlapped ⇒ semimetal • Reversible intercalation (usually) into van der Waals gap – + + – • Alkali cations (C8 K ), halide anions (C8 Br ), – amines, FeCl3, HSO4 , …

– + • Graphite + K(melt or vapor) → C8 K (bronze) • Interlayer spacing increases to ~ 5.4Å

• C8K, vacuum, Δ → C16K → C24K → → • Tuning filling of bands, ∴ thermal, electrical properties

West p.184-5, p.207-8 Smart & Moore p.399-402 Graphene

• Single layer of graphite • One-atom-thick planar sheet of honeycomb of sp2-bonded atoms • “Discovered” in 2004 by scotch tape method • Known earlier, e.g. graphite abrasion • Semi-metal • σ = 106 Ω–1cm–1, but decreased by phonons from substrate • 200× stronger than steel • Potential uses: transistors, single molecule gas detection, transparent conducting electrodes West p.209-211 Smart & Moore p.402 Buckyball

• C60: 12 pentagons, 20 hexagons: truncated Ih • crystallizes in fcc lattice • a = 14.2Å

• Td and Oh intersticial sites empty • sp2 hybridized C, 1.40Å, 1.45Å bond lengths (cf. C=C: 1.33Å; C–C: 1.54Å) • Former for C-C bonds that join two hexagons • More π-bonding, -like, dihapto ligand West p.31-3 Smart & Moore p.403-4 KxC60 Fulleride

• Injection of K/Rb/Cs into C60 intersticial sites → “fulleride”

• x = 0, 6: insulator (degree of filling of t2g C60 band) • x = 3: all sites of ccp lattice occupied

• Metal @ RT, superconductor below Tc ~ 18K • Chemical or electrochemical intercalation; reversible Higher Fullerenes

• Pentagons introduce curvature • Higher fullerenes: add graphitic hexagons • Still 12 pentagons at icosahedron vertices

• 20 triangular faces C70 Carbon Nanotubes

• Form on C electrode surface of dc arc • 500 Torr He • High aspect ratio: several mm length, 2 to 25 nm diameter • Ends are open or capped by semi- spherical or polyhedral graphite domes

• Single wall or concentric cylinders • Spacing between layers close to interlayer distance of graphite (3.4Å) • Cylinders are rotationally disordered wrto each other • A: Armchair; Z: zigzag; H: helical/chiral

Smart & Moore p.404-5 Single-Walled Nanotubes (SWNTs)

• Multi-walled nanotubes (MWNTs) are metals, varying diameter, difficult to fill • SWNTs band gap 0 to 2eV, depends on helicity • Synthesis: (i) packing anode with Ni and graphite; (ii) laser ablation of graphite; (iii) CVD on Ni particle film • Self-assembles into bundles (ropes) of tens of nanotubes

• Molecular wires • Intramolecular FET (n-type usually, becomes p-type when exposed to O2) • Nanotechnology, electronics, optics, thermally conducting, huge specific strength