TOPOTACTIC NANOCHEMISTRY APPROACH TO SILVER SELENIDE NANOWIRES

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TOPOTACTIC NANOCHEMISTRY APPROACH TO SILVER
SELENIDE NANOWIRES

• Silver selenide Ag2Se
• Silver ion superionic conductor
• Photoconductor
• Thermoelectric - large Seebeck coefficient
• Thermochromic 133C 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

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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

Se Chain
Trigonal Selenium (t-Se)
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Growth of c-Se Nanowires from a-Se Seeds
100 oC
100 oC
a-Se
R.T.
a-Se
t-Se
(t-Se)
a-Se
t-Se
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Various Stages of Se Wire Growth
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Nanowires of t-Se with f30 nm
XRD
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Absorption Spectra of t-Se Nanowires
30 nm wires
10 nm wires
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Photoresponse of t-Se Nanowire
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Synthesis of Silver Nanowires
AgNO3 HO(CH2)2OH
PtCl2
(PVP)
PVPAg11
160-180 oC
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Mechanism Chemistry versus Art
PtCl2
AgNO3
(CH2OH)2
PVP
Pt seeds
PVP ?
Growth
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Various Stages of Wire Growth
20 min
10 min
60 min
40 min
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Silver Nanowires with f40 nm
XRD
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Bi-Crystalline Structure
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TOPOTACTIC TRANSFORMATION OF ORIENTED c-Se NWS TO
ORIENTED C-Ag2Se NWS
3Se(s) Ag(aq) 3H2O 2Ag2Se(s)
Ag2SeO3 (aq) 6H(aq)
0.71
AgNO3
0.49
0.49
(flt30 nm)
t-Se
0.44
AgNO3
(fgt40 nm)
0.44
(tetragonal Ag2Se)
0.78
0.70
(orthorhombic Ag2Se)
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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)
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Tetragonal a-Ag2Se (f30 nm)
EDX
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Orthorhombic a-Ag2Se (fgt40 nm)
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FILMS - FORM?

• Supported - substrate type and effect of
  interface
• Free standing - synthetic strategy
• Epitaxial - lattice matching - tolerance
• Superlattice - artificial
• Patterned - chemical or physical lithography

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FILMS - WHEN IS A FILM THICK OR THIN?

• Monolayer - atomic, molecular thickness
• Multilayer - compositional superlattice - scale -
  periodicity
• Bulk properties - scale - thickness greater than
  l(e,h)
• Quantum size effect - 2D confinement - free
  electron behavior in third dimension - quantum
  wells

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THIN FILMS ARE VITAL IN MODERN TECHNOLOGY

• Protective coatings
• Optical coatings, electrochromic windows
• Filters, mirrors, lenses
• Microelectronic devices
• Optoelectronic devices
• Photonic devices

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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

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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

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METHODS OF SYNTHESIZING THIN FILMS

• ELECTROCHEMICAL, PHYSICAL, CHEMICAL
• Cathodic deposition, anodic deposition,
  electroless deposition
• Laser ablation
• Cathode sputtering, vacuum evaporation
• Thermal oxidation, nitridation

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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

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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

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

• PtH3PO4, H2OAl
• Al ? Al3 3e- anode
• PO43- 2e- ? PO33- O2- cathode
• Overall electrochemistry potential control of
  oxide thickness
• Oxide anions diffuse through growing layer of
  aluminum oxide
• 2Al3 3O2- ? g-Al2O3 (annealing) ? a-Al2O3

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ANODIC OXIDATION OF PATTERNED Al DISC TO MAKE
PERIODIC NANOPOROUS Al2O3 MEMBRANE
Aqueous HgCl2 dissolves Al to give Hg and
Al(H2O)63 and H3PO4 dissolves Al2O3 barrier
layer to give Al(H2O)63 - yields open channel
membrane
2Al 3PO43- ? Al2O3 3PO33- 2Al 3C2O42- ?
Al2O3 6CO 3O2-
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ANODIC OXIDATION OF LITHOGRAPHIC PATTERNED Al TO
PERIODIC NANOPOROUS Al2O3
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ANODIC OXIDATION OF LITHOGRAPHIC PATTERNED Al TO
PERIODIC NANOPOROUS Al2O3
40V
60V
80V
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PROPOSED MECHANISM OF ALUMINA PORE FORMATION IN
ANODICALLY OXIDIZED ALUMINUM
SELF ORGANIZED SELF LIMITING GROWTH OF PORES
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Templated synthesis of metal barcoded nanorods
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MESOSCOPIC AMPHIPHILES
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MESOSCOPIC AMPHIPHILESCURRENT CONTROL OF LENGTH
OF POLYMER AND METAL SEGMENTS
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MESOSCOPIC AMPHIPHILES - POLYMERIZATION INDUCED
SHRINKAGE OF Ppy SEGMENT
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MESOSCOPIC AMPHIPHILES - GEOMETRIC PACKING
PARAMETERS
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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

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LIGHT WORK BY THE SILICON SAMURAIWHERE IT ALL
BEGAN AND WHERE IT IS ALL GOING
FROM CANHAMS 1990 DISCOVERY OF PL AND EL
ANODICALLY OXIDIZED p-DOPED Si WAFERS, TO NEW
LIGHT EMITTING SILICON NANOSTRUCTURES, TO SILICON
OPTOELECTRONICS, TO PHOTONIC COMPUTING
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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 G 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

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SEMICONDUCTOR BAND STRUCTURE CHALLENGE,
EVOKING LIGHT EMISSION FROM Si

• EMA Rexciton 0.529e/mo where e 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 lt 5 nm to make Si work

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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