topotactic nanochemistry approach to silver selenide nanowires silver selenide ag 2 se silver ion...
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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-Se
t-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 On
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