TOPOTACTIC NANOCHEMISTRY APPROACH TO SILVER SELENIDE NANOWIRES • Silver selenide Ag 2 Se • 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-Ag 2 Se nanowires with shape retention - similar for ZnSe, Be 2 Se 3
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TOPOTACTIC NANOCHEMISTRY APPROACH TO SILVER SELENIDE NANOWIRES Silver selenide Ag 2 Se Silver ion superionic conductor Photoconductor Thermoelectric -
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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
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