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TOPOTACTIC NANOCHEMISTRY APPROACH TO
SILVER SELENIDE NANOWIRES
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Silver selenide Ag2Se
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Silver ion superionic conductor
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Photoconductor
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Thermoelectric - large Seebeck coefficient
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Thermochromic 133°C alpha-beta phase transition
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Therefore interesting to synthesize nanowires of silver selenide
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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
Se Chain
Trigonal Selenium (t-Se)
Functional Materials such
as ZnSe, CdSe and Ag2Se
Growth of c-Se Nanowires from a-Se Seeds
100 oC
100 oC
a-Se
R.T.
a-Se
t-Se
(t-Se)
t-Se
a-Se
Various Stages of Se Wire Growth
Nanowires of t-Se with f~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
PtCl2
( CH2 CH )n
O
N
(PVP)
PVP:Ag=1:1
160-180 oC
Mechanism: Chemistry versus Art
PtCl2
AgNO3
(CH2OH)2
PVP
Pt seeds
PVP ?
Growth
Various Stages of Wire Growth
10 min
20 min
40 min
60 min
Silver Nanowires with f~40
nm
XRD
Bi-Crystalline Structure
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
t-Se
0.44
0.49
(f<30 nm)
+ AgNO3 (f>40 nm)
0.44
(tetragonal Ag2Se)
0.78
0.70
(orthorhombic Ag2Se)
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
3Se(s) + Ag+(aq) + 3H2O
• Time evolution of PXRD
shows c-Se converts to cAg2Se
+
2Ag2Se(s) + Ag2SeO3 (aq) + 6H
(aq)
Tetragonal a-Ag2Se (f~30 nm)
EDX
Orthorhombic a-Ag2Se (f>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
l(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-
• PO43- +2e-  PO33- + O2-
anode
cathode
• Overall electrochemistry: potential control of
oxide thickness
• Oxide anions diffuse through growing layer of
aluminum oxide
• 2Al3+ + 3O2-  g-Al2O3 (annealing)  a-Al2O3
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 + 3PO332Al + 3C2O42-  Al2O3 + 6CO + 3O2-
ANODIC OXIDATION OF LITHOGRAPHIC
PATTERNED Al TO PERIODIC NANOPOROUS Al2O3
ANODIC OXIDATION OF LITHOGRAPHIC
PATTERNED Al TO PERIODIC NANOPOROUS Al2O3
40V
60V
80V
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 AMPHIPHILES
CURRENT 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
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•
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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%
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 < 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 pretexturing directs
periodic pore
formation