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Epoxide -assisted sol-gel process

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  1. Epoxide-assisted sol-gel process Consideration of application on low-temperature processing of oxide TFT

  2. Introduction Alkoxide precursors are… High reactivity with water, gel formation Expensive, moisture-sensitive, material limitation Conventional sol-gel Metal alkoxide + Water Epoxide-assisted sol-gel Metal salt + Water + Epoxide (Nitrates, chlorides) Hydrolysis Condensation Metal hydroxide Metal oxide Gelation initiator Metal salt precursors are… Relatively low reactivity with water, cannot form gel Inexpensive, facile

  3. Introduction Gelation initiator Epoxides – cyclic ether with three ring atoms strained ring makes epoxides more reactive than other ethers Propylene oxide General gelation initiator +MCl or M(NO3) → Oxide gel

  4. Epoxide Role of PO in sol-gel system Basically, PO is an acid scavenger (ex: reaction with HCl) Cl- Ring-opening + Cl- H H+ Proton attaches on epoxide oxygen 1-chloro-2-propanol

  5. Epoxide Role of PO in sol-gel system PO consumes protons from hydrated metal ions (ex: reaction with Fe(H2O)63+) Aqua ion is a strong acid + H + Fe(H2O)63+ + Fe(OH)(H2O)52+ Hydrolysis

  6. Epoxide-assisted sol-gel Role of PO in sol-gel system In case of nitrate precursor, H2O is stronger nucleophile than NO3- H + + H + H2O + H+ 1,2-propanediol

  7. Epoxide-assisted sol-gel Role of PO in sol-gel system In case of nitrate precursor, Cl- is stronger nucleophile than H2O + H + Cl- 1-chloro-2-propanol

  8. Epoxide-assisted sol-gel Reaction products analyzed by NMR (Fe2O3 gel system) Nitrate precursor ~80% 1,2-propanediol & small amount of 1-nitroxy-2-propanol 2-nitroxy-2-propanol Chloride precursor Unreacted propylene oxide 38% 1-chloro-2-propanol 30% 2-chloro-2-propanol 8% 1,2-propanediol 25%

  9. Epoxide-assisted sol-gel TG-DTG analysis (CuZnAl oxide system) 1) Nitrate precursor 180-350°C : 35% weight loss → Combustion of residual organic matters Water evaporation peak is not distinct → Water is consumed during ring-open reaction of PO

  10. Epoxide-assisted sol-gel TG-DTG analysis (CuZnAl oxide system) 2) Chloride precursor ~100°C : Water evaporation peak 176-220°C : 20% weight loss → Oxidation of residual organic compounds 300-800°C : 20% weight loss → Interaction between CuO, ZnO, and Al2O3 (Likely)

  11. Epoxide-assisted sol-gel DTA analysis Exothermic peak at ~200°C → Confirms organic oxidation Endothermic peak < 200°C → Confirms water evaporation

  12. Epoxide-assisted sol-gel Precursor effects With nitrate precursor, too much water inhibits gelation (PO is consumed by water) With anhydrous chloride prcursor, water should be added into solution for gel formation

  13. Epoxide-assisted sol-gel Solvent effects 1) Polar protic solvents Most suitable Gel formation 2) Polar aprotic solvents Monolith formation Gelation can be faild by fast precipitation 3) Nonpolar solvents Metal salts are insoluble

  14. Epoxide-assisted sol-gel Epoxide effects Gelation speed PO>TMO>DMO TMO,DMO: Rectanglular ring structure Less strain – harder to open ring More bulky DMO Steric effect – disturb nucleophilic attack

  15. Epoxide-assisted sol-gel Water content: (ex:Ruthenium oxide system) High water content (>200 mol/mol) → Gel is not formed because the precursor concentration was too low Low water content (<30mol/mol) → Gelatinous precipitate was obtained because the hydrolysis reaction could not proceed well

  16. Epoxide-assisted sol-gel Epoxide type: The gel time – 1,2-epoxybutane > propylene oxide (bulkier molecular structure has a lower protonationrate) Epoxide content: <5 mol/mol: epoxide was not sufficient >20 mol/mol: soft gel is formed (increased alcohol content)

  17. Future works Epoxide-assisted sol-gel : Bulk-based sol-gel technique Solution is converted into gel in this process How can it be used for solution-processed thin film transistor?

  18. Future works Sol-gel on chip concept (Alkoxide + Water) Using epoxide-assisted sol-gel (Hydrated metal salt + PO) H2O H2O H2O H2O Spin-cast alkoxides Zn(OH) 2 Zn(NO3)2·6H2O Zn(OH) 2

  19. Future works • Merits of “PO-steam annealing” • By using metal salts instead metal alkoxides, • Precursors are inexpensive • Precursor is easy to be made, material is not limited • Process is facile, drybox is not required • Thin-film transistor can be fabricated at low-temperature like sol-gel on chip process

  20. Summery • Inexpensive metal salt precursors can be used for sol-gel reaction if epoxide assists reaction • Epoxideshydrolize metal salt precursors by proton scavenge by ring-open reaction of epoxides • Epoxide-assisted sol-gel process can be combined with steam-annealing process – this can be applied to spin-casted thin-film process and used for fabrication of high-performance thin-film transistor at low temperature

  21. Experiments Combustion Al2O3 – Al(NO3) 3 350°C Precursors are dissolved separately Reference Combustion

  22. Experiments Compared with previous data Al(NO3) 3350°C Precursors are dissolved at once

  23. Experiments Combustion Al2O3 – AlCl3 350°C Reference Combustion

  24. Experiments IZO TFT at 350°C annealing Increasing Zn

  25. Experiments IZO TFT at 350°C annealing In:Zn = 6:4

  26. Experiments Contact resistance measurement Line TLM pattern Circular TLM pattern Less accurate Accurate Can be patterned by shadow mask Photolithography required

  27. Experiments Contact resistance measurement