TOPOTACTIC NANOCHEMISTRY APPROACH TO SILVER SELENIDE NANOWIRES

• Silver selenide Ag2Se

• Silver ion superionic conductor

• Photoconductor

• Thermoelectric - large Seebeck coefficient

• Thermochromic 133°C alpha-beta phase transition

• Therefore interesting to synthesize nanowires of silver selenide

• Idea is to synthesize c-Se nanowires and topotactically convert them with Ag+ to c-Ag2Se nanowires with shape retention - similar for ZnSe, Be2Se3

Unique Features of Selenium

• Intrinsic Optical Chirality• Highest Photoconductivity (s = 8 x 104 S/cm for t-Se)• Piezoelectric and Nonlinear Optical (NLO) Properties• Thermoelectric Properties • Useful Catalytic Properties (Halogenation, Oxidation) • Reactivities to Form Other Functional Materials such

as ZnSe, CdSe and Ag2Se

Trigonal Selenium (t-Se)

Se Chain

Growth of c-Se Nanowires from a-Se Seeds

100 oC

R.T.

a-Se

100 oC

a-Set-Set-Se

a-Se

(t-Se)

Various Stages of Se Wire Growth

Nanowires of t-Se with ~30 nm

XRD

Absorption Spectra of t-Se Nanowires

~30 nm wires ~10 nm wires

Photoresponse of t-Se Nanowire

Synthesis of Silver

Nanowires

AgNO3

+HO(CH2)2OH

PVP:Ag=1:1

160-180 oC

CH2 CH( )N O

n

PtCl2

(PVP)

Mechanism: Chemistry versus Art

PtCl2

(CH2OH)2

Pt seeds

AgNO3

PVP

PVP ? Growth

Various Stages of Wire Growth

10 min 20 min

40 min 60 min

Silver Nanowires with ~40 nm

XRD

Bi-Crystalline Structure

3Se(s) + Ag+(aq) + 3H2O 2Ag2Se(s) + Ag2SeO3 (aq) + 6H+(aq)

+ AgNO3

<30 nm)

+ AgNO3>40 nm)

t-Se

(tetragonal Ag2Se)

(orthorhombic Ag2Se)

0.78

0.70

0.49 0.49

0.71

0.44

0.44

TOPOTACTIC TRANSFORMATION OF ORIENTED c-Se NWS TO ORIENTED C-Ag2Se NWS

PXRD MONITORING OF TOPOTACTIC CONVERISON OF c-Se NWs TO c-Ag2Se NWs

• Rapid solution-solid phase reaction

• Complete in less than 2 hours

• Samples washed with hot water to remove Ag2SeO3 by product

• Time evolution of PXRD shows c-Se converts to c-Ag2Se

3Se(s) + Ag+(aq) + 3H2O 2Ag2Se(s) + Ag2SeO3 (aq) + 6H+(aq)

Tetragonal -Ag2Se (~30 nm)

EDX

Orthorhombic -Ag2Se (>40 nm)

FILMS - FORM?

• Supported - substrate type and effect of interface

• Free standing - synthetic strategy

• Epitaxial - lattice matching - tolerance

• Superlattice - artificial

• Patterned - chemical or physical lithography

FILMS - WHEN IS A FILM THICK OR THIN?

• Monolayer - atomic, molecular thickness

• Multilayer - compositional superlattice - scale - periodicity

• Bulk properties - scale - thickness greater than e,h)

• Quantum size effect - 2D confinement - free electron behavior in third dimension - quantum wells

THIN FILMS ARE VITAL IN MODERN TECHNOLOGY

• Protective coatings

• Optical coatings, electrochromic windows

• Filters, mirrors, lenses

• Microelectronic devices

• Optoelectronic devices

• Photonic devices

THIN FILMS ARE VITAL IN MODERN TECHNOLOGY

• Electrode surfaces

• Photoelectric devices, photovoltaics, solar cells

• Xerography, photography

• Electrophoretic and electrochromic ink, displays

• Catalyst surfaces

• Information storage, magnetic, magneto-resistant, magneto-optical, optical memories

FILM PROPERTIES - ELECTRICAL, OPTICAL, MAGNETIC, MECHANICAL, ADSORPTION, PERMEABILTY, CHEMICAL

• Thickness and surface : volume ratio

• Structure - surface vs bulk, surface reconstruction, roughness

• Hydrophobicity, hydrophilicy

• Composition

• Texture, single crystal, microcrystalline, orientation

• Form, supported or unsupported, nature of substrate

METHODS OF SYNTHESIZING THIN FILMS

• ELECTROCHEMICAL, PHYSICAL, CHEMICAL

• Cathodic deposition, anodic deposition, electroless deposition

• Laser ablation

• Cathode sputtering, vacuum evaporation

• Thermal oxidation, nitridation

METHODS OF SYNTHESIZING THIN FILMS

• ELECTROCHEMICAL, PHYSICAL, CHEMICAL

• Liquid phase epitaxy

• Self-assembly, surface anchoring

• Discharge techniques, RF, microwave

• Chemical vapor deposition CVD, metal organic chemical vapour deposition MOCVD

• Molecular beam epitaxy, supersonic cluster beams, aerosol deposition

ANODIC OXIDATIVE DEPOSITION OF FILMS

• Deposition of oxide films, such as alumina, titania

• Deposition of conducting polymer films by oxidative polymerization of monomer, such as thiophene, pyrrole, aniline

• Oxide films formed from metallic electrode in aqueous salts or acids

ANODIC OXIDATION OF Al IN OXALIC OR PHOSPHORIC ACID TO FORM ALUMINUM OXIDE

• Pt|H3PO4, H2O|Al

• Al Al3+ + 3e- anode

• PO43- +2e- PO3

3- + O2- cathode

• Overall electrochemistry: potential control of oxide thickness

• Oxide anions diffuse through growing layer of aluminum oxide

• 2Al3+ + 3O2- -Al2O3 (annealing) -Al2O3

ANODIC OXIDATION OF PATTERNED Al DISC TO MAKE PERIODIC NANOPOROUS Al2O3 MEMBRANE

2Al + 3PO43- Al2O3 + 3PO3

3-

2Al + 3C2O42- Al2O3 + 6CO + 3O2-

Aqueous HgCl2 dissolves Al to give Hg and Al(H2O)6

3+ and H3PO4 dissolves Al2O3 barrier layer

to give Al(H2O)63+ - yields open channel membrane

ANODIC OXIDATION OF LITHOGRAPHIC PATTERNED Al TO PERIODIC NANOPOROUS Al2O3

40V

60V

80V

ANODIC OXIDATION OF LITHOGRAPHIC PATTERNED Al TO PERIODIC NANOPOROUS Al2O3

PROPOSED MECHANISM OF ALUMINA PORE FORMATION IN ANODICALLY OXIDIZED ALUMINUM

SELF ORGANIZED SELF LIMITING GROWTH OF PORES

Templated synthesis of metal barcoded nanorods

MESOSCOPIC AMPHIPHILES

MESOSCOPIC AMPHIPHILESCURRENT CONTROL OF LENGTH OF POLYMER AND

METAL SEGMENTS

MESOSCOPIC AMPHIPHILES - POLYMERIZATION INDUCED SHRINKAGE OF Ppy SEGMENT

MESOSCOPIC AMPHIPHILES - GEOMETRIC PACKING PARAMETERS

ANODIC OXIDATION OF Si TO FORM POROUS Si: THROWING SOME LIGHT ON SILICON

• Typical electrochemical cell to prepare PS by anodic oxidation of heavily doped p+-type Si

• PS comprised of interconnected nc-Si with H/O/F surface passivation

• nc-Si right size for QSEs and red light emission observed during anodic oxidation

LIGHT WORK BY THE SILICON SAMURAI:WHERE IT ALL BEGAN AND WHERE IT IS ALL GOING

FROM CANHAM’S 1990 DISCOVERY OF PL AND EL ANODICALLY OXIDIZED p-DOPED Si WAFERS, TO NEW LIGHT EMITTING SILICON NANOSTRUCTURES, TO SILICON OPTOELECTRONICS, TO PHOTONIC COMPUTING

ELECTRONIC BAND STRUCTURE OF DIAMOND SILICON LATTICE

• band structure of Si computed using density functional theory with local density and pseudo-potential approximation

• diamond lattice, sp3 bonded Si sites• VB maximum at k = 0, the point in

the Brillouin zone, CB minimum at distinct k value

• indirect band gap character, very weakly emissive behavior

• absorption-emission phonon assisted• photon-electron-phonon three

particle collision very low probability, thus band gap emission efficiency low, 10-5%

SEMICONDUCTOR BAND STRUCTURE: CHALLENGE, EVOKING LIGHT EMISSION FROM Si

• EMA Rexciton ~ 0.529/mo where = dielectric constant, reduced mass of exciton mo = memh/(me + mh)

• Note exciton size within the bulk material defines the size regime below which significant QSEs on band structure are expected to occur, clearly < 5 nm to make Si work

REGULAR OR RANDOM NANNSCALE CHANNELS IN ANODICALLY OXIDIZED SILICON WAFERS

• Anodized forms of p+-type Si wafer

• Showing formation of random (left) and regular (right) patterns of pores

• Lithographic pre-texturing directs periodic pore formation