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Nanostructured Materials: New Generation Field Emitters

Nanostructured Materials: New Generation Field Emitters. Dr. Mahendra A. More. Center for Advanced Studies in Material Science and Condensed Matter Physics , Department of Physics, University of Pune, Pune – 411007. India. Outline of the talk. Introduction: Field Emission

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Nanostructured Materials: New Generation Field Emitters

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  1. Nanostructured Materials: New Generation Field Emitters Dr. Mahendra A. More Center for Advanced Studies in Material Science and Condensed Matter Physics, Department of Physics, University of Pune, Pune – 411007. India

  2. Outline of the talk . . . • Introduction: Field Emission Why Nanomaterials as Field emitters • Field Emission from Sn doped ZnO Nanowires • Photo-enhanced Field Emission from CdSnanocombs • Photo-enhanced Field Emission from TiO2 nanotubes

  3. Introduction . . . University of Pune, Pune INDIA (Established 1948)

  4. Introduction . . . Department of Physics, University of Pune, Pune INDIA Center for Advanced Studies in Materials Science and Condensed Matter Physics

  5. Introduction . . . Scanning Electron Microscope Scanning Porbe Microscope (AFM) Vibrating Sample Magnetometer X-ray Diffractometer (AX8, Bruker)

  6. Introduction . . . TEM: Techni G2 Ultratwin – FEI

  7. Potential barrier at metal-vacuum interface Introduction . . . Field emission: Quantum mechanical tunneling of electrons from condensed phase into vacuum under influence of strong electrostatic field. Requirement: Electric field ~ 106 -107 V/cm Pressure < 1 x 10-9 mbar Fowler-Nordheim (F-N) equation: Relation between the emission current density (J) and the applied field (E) J = A (β2E2/Φ) exp (−BΦ3/2/βE) Where A and B are constants, Φis work function of the material and β is field enhancement factor.

  8. Introduction . . . Conventional field emitters: Emitter is a ‘micro tip’ (a sharp needle) with apex radius ~ 500 Å For a microtip (single emitter): Elocal = β E, whereβ = 1 / k r r = radius at tip apex, and β = geometrical factor (cm-1) 1-D Nanostructures (wires, rods, needles, tubes, etc) High aspectratio High value of β For an assembly of Nanostructures Elocal = β Eav, and Eav = V/d Where, V = applied voltage, d = cathode-anode separation, and β = field enhancement factor (dimensionless) Lowering operating voltage and relaxation of UHV condition

  9. Introduction . . . Field emitter as Electron Source(s) • Practical considerations: • Low turn on voltage • High current density • Stable electron emission • Small optical size • Reproducibility and Ease of Fabrication • Material properties: • High aspect ratio (l/r) • Good electrical conductivity • High chemical, mechanical stability • Applications: • Electron source in SEM and TEM (Monoenergetic e-beam) • Flat panel Displays (An array of 1-D Nanostructures) • Electron source for Portable X-ray tubes and • High Power GHz Microwave Tubes

  10. UHV all metal Field Emission Microscope equipped with Retarding Field Energy Analyzer UHV all metal Field Emission Microscope (FEM) Introduction . . . Field Emission and Ion Microscopy Laboratory Indigenous Instrumentation:

  11. Introduction . . . UHV all metal Field Emission Microscope equipped with a load lock chamber facility Front view of the working chamber showing view port, micro-manipulator, electrical feedthroughs (BNC)

  12. a Standard deviation = ± 20.2 nm Average: 158 nm b Standard deviation = ± 6.8 nm Average: 88.5 nm 5µm 5µm FE characteristics of ZnO Nanostructures . . . Field emission from electrochemically Sn-doped ZnO nanowires Nanowires diameter ~ 80 nm to 150 nm length of several micrometers, 2 to 20 μm. Thin Solid Films 519 (2010) 184

  13. ZnO wurtzite unit cell FE characteristics of ZnO Nanostructures . . . Why ZnO? • Wide direct band gap 3.37 eV • Large exciton binding energy (60 meV) • Chemically and thermally stable • High mechanical strength • Low electron affinity • High aspect ratio in nanostructured form • Excellent electron emission competence ZnO wurtzite unit cell Unique material for which number nanovariants such as Nanowires, Nanoneedles, Nanorods, Nanocombs, Nanoflowers, Nano/microbelts, terapods, etc. have been prepared.

  14. Results: 10 A/cm2 : 1.62 V/m 360 A/cm2: 2.36 V/m FE characteristics of ZnO Nanostructures . . . Vertically aligned ZnO nanorods: Synthesis route: Thermal evaporation Applied Surface Science, 256 (2010) 6157-6163.

  15. b a FE characteristics of ZnO Nanostructures . . . Sn doped ZnO nanwires: Synthesis route: Electrochemical route Thin Solid Films 19 (2010) 184

  16. FE characteristics of ZnO Nanostructures . . . a Standard deviation = ± 20.2 nm Average: 158 nm b Standard deviation = ± 6.8 nm Average: 88.5 nm 5µm 5µm Sn doped ZnO nanwires:

  17. a b FE characteristics of ZnO Nanostructures . . . Field emission current density versus applied field (J-E) plot Folwer-Nordheim (F-N) plot Results: 10 µA/cm2: 0.68 V/µm and 100 µA/cm2: 1.1 V/µm (0.5 % Sn) 10 µA/cm2: 1.72 V/µm and 100 µA/cm2: 2.25 V/µm (2.0 % Sn).

  18. c 1cm FE characteristics of ZnO Nanostructures . . . Field emission from Sn-doped ZnO nanowires Current stability movie Field emission current versus time (I-t) stability plot Inset: FE image recorded at the outset of the measurement.

  19. FE characteristics of ZnO Nanostructures . . . Photo Enhanced Field emission from Sn-doped ZnO nanowires Photoluminescence Spectrum showing enhanced emission in visible region for 2% Sn doped ZnO nanowires

  20. b a FE characteristics of ZnO Nanostructures . . . Photo Enhanced Field emission from Sn-doped ZnO nanowires Illuminating the specimen with visible light (Halogen lamp, 1300 watt). The cathode was illuminated from ‘front side’. The intensity of incident light at the cathode site was found to be ~ 80 watts Journal of Physical Chemistry, 114( 2010) 3843

  21. FE characteristics of ZnO Nanostructures . . . • ZnO nanovariants: • Tetrapods (a single isolated pod) (APL 2006, 2007) • Nanowires, nanoparticles, nanoneedles(Nanotechnology 2007) • Marigold, belts and multipods(Journal of Physical Chemistry B, 2008) • Randomly oriented nanowires (Thin Solid Films 2008, Ultramicroscopy 2009) • Vertically aligned nanorods (Applied Surface Science 2010) • Al-doped nanostructures (Journal of Nanoparticle Research 2009) • Sn doped ZnO nanowires (TSF 2010, JPCC 2010) • Cu-ZnO nanocomposites(Materials Chemistry and Physics 2010)

  22. FE characteristics of CdS Nanostructures . . . Synthesis of CdS nanoforms (nanocombs, nanobelts and nanowires) by Thermal Evaporation nanobelts nanocombs nanowires CdS Ar IN Ar OUT 29 cm 30 cm 31 cm CdS = 50 mg Substrate = Si Catalyst used (Au) = 30 Å Temp = 1000o C Deposition time = 40 mins 3. Position of Si w. r. t. source = 29, 30 and 31 cm for nanocombs, nanobelts and nanowires, respectively

  23. FE characteristics of CdS Nanostructures . . . Nanoscale Letters 3 (2010) 1078

  24. FE characteristics of CdS Nanostructures . . . • TEM image of the CdS nanocomb and (b) HRTEM image of an individual nanocomb with ED pattern as inset. Nanoscale Letters 3 (2010) 1078

  25. b a FE characteristics of CdS Nanostructures . . . Field emission current density versus applied field (J-E) plot Folwer-Nordheim (F-N) plot Results: Turn on field (0.1µA/cm2) = 0.26 V/µm J max= 14.6 μA/cm2at a field of 0.65 V/µm Nanoscale Letters 3 (2010) 1078

  26. J= 1 µA/cm2 FE characteristics of CdS Nanostructures . . . J= 20 µA/cm2 Field emission current versus time (I-t) plot FEM images Nanoscale Letters 3 (2010) 1078

  27. FE characteristics of CdS Nanostructures . . . Photo-sensitive field emission of the CdS nanocombs Nanoscale Letters 3 (2010) 1078

  28. FE characteristics of CdS Nanostructures . . . Photo switching behavior of CdS nanocombs at current (a) ~ 1 µA , (b) ~ 10 µA. Nanoscale Letters 3 (2010) 1078

  29. FE characteristics of CdS Nanostructures . . . Photo-induced FE current stability measurement for long duration (~ 82 minutes) Nanoscale Letters 3 (2010) 1078

  30. FE characteristics of CdS Nanostructures . . . Comparison of photo-sensitivy of various CdS nanoforms

  31. FE characteristics of CdS Nanostructures . . . Results

  32. FE characteristics of TiO2 Nanostructures . . . Synthesis of vertically aligned TiO2 nanotubes: Electrochemical iodization [at constant voltage 30 V] Electrolyte: HF + DMSO Working electrode: Ti foil Counter electrode: Pt foil Temperature: Room temp. Duration: 18 h. Annealing in air at 530 C for 3 h. Ultramicroscopy 111 (2010) 415

  33. FE characteristics of TiO2 Nanostructures . . . XRD pattern of TiO2 nanotubes array SEM images of TiO2 nanotubes EDS pattern of TiO2 nanotubes array Ultramicroscopy 111 (2010) 415

  34. FE characteristics of TiO2 Nanotubes array . . . Turn on field (0.1µA/cm2) = 0.26 V/µm (260V) J max = 14.6 μA/cm2 at a field of 0.65 V/µm Current density versus applied electric field (J-E) plot Inset shows the corresponding Folwer-Nordheim (F-N) plot. Ultramicroscopy 111 (2010) 415

  35. Post Field Emission SEM FE characteristics of TiO2 Nanotubes array . . . Emission current versus time(I-t) plots recorded at different preset values . The inset shows the corresponding FE image Ultramicroscopy 111 (2010) 415

  36. FE characteristics of TiO2 Nanotubes array . . . Photoluminescence Spectrum Emission current versus time: with and without visible exposure. Ultramicroscopy 111 (2010) 415

  37. FE characteristics of Nanostructures . . . • Results and Conclusions: • CdSnanocombs exhibit excellent photo sensitive field emission behaviour as compared to the Sn doped ZnO nanowires and TiO2 nanotubes array • TiO2 nanotubes array delivers more stable emission current than the other nanostructures. Also the post field emission SEM images reveals excellent sturdiness of the array against ion bombardment. • Nanostructures of Optically active Semiconductors are promising materials for New Generation nano/micro Optoelectronic devices

  38. Remarks: • Field Emitters: practical aspects to be resolved • Reproducibility of the “identical” emitter array • Deterioration of the emitters • Relaxation of UHV condition – effect of adsorption • Emission current stability • Life of the emitter • Pulsed field emission • Photo field emission • Dielectric materials ? ? ?

  39. University of Pune Prof. D. S. Joag Research students Farid Jamali Padmakar Chavan NCL, Pune Dr. Vijayamohanan Dr. I. S. Mulla Dr. K. R. Patil ACKNOWLEDGEMENTS

  40. The group Field Ion and Field Emission Laboratory Group From right: Jamali Sheini Farid, Patil Sandip, Ms. Asmita Thorat, Ms. Prachi Nagare, Dr. More M. A. Prof. Joag D. S. Ms. Padmishree Dekhane, Chavan Padmakar, Kashid Ranjit, Shinde Deodatta and Warule Sambhaji Field Ion and Field Emission Laboratory Group From right: Farid Jamali, Sandip Patil, Ms. Asmita Thorat, Ms. Prachi Nagare, Dr. More M. A. Prof. Joag D. S. Ms. Padmishree Dekhane, Chavan Padmakar, Kashid Ranjit, Shinde Deodatta and Warule Sambhaji

  41. Thank you for the attention!

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