This is my Ph.D. defense presentation that I gave on Oct. 24, 2006.

1. Novel ZnS NanostructuresSynthesis, Growth Mechanism, and Applications Ph.D. Defense presented by Daniel F. Moore Advisor – Dr. Zhong L. Wang School of Materials Science and Engineering Georgia Institute of Technology October 24, 2006

2. Outline Applications ZnS applications ZnS nanostructure applications Models ZnS crystal structure Synthesis and growth Novel ZnS Nanostructures Nanobelts/Nanowires Aligned ZnS Nanowires Nanohelices UltralongZnS/SiO2 nanowires

5. Understanding and Synthesizing One-Dimensional Nanostructures Why? Facilitate a rational design method of creating new materials Enables controlling their properties New nanoscale materials help understand the variety of novel properties and morphologies that arise with working with on the nanoscale

6. ZnS Crystallographic and Growth Models Applications Models Crystallographic Structure Synthesis and Growth Novel ZnS Nanostructures Nanobelts/Nanowires Aligned ZnS Nanowires Nanohelices UltralongZnS/SiO2 nanowires

7. ZnS Crystal Structures Zinc Blend ABCABC Wurtzite ABABAB

8. ZnS Wurtzite (0111) (0001) Wurtzite crystal structure projected along [2110]

9. Why Wurtzite?Ostwald’s Rule of Stages The formation of the new stable phase takes place by consecutive steps from one phase to another with increasing thermodynamic stability Under certain conditions, the metastable phase has a higher nucleation rate than the stable phase and nucleates first Ostwald’s rule holds when where μs is the supersaturation = kTln(P/P0s), bs is a geometric factor, σ is the surface energy

10. Why Wurtzite?Ostwald’s Rule of Stages If a reaction can result in several products, it is not the most stable state with the least amount of free energy that is initially obtained, but the least stable one, lying nearest to the original state in free energy

11. ZnS Nanosaw – Phase Transformation Before After Illumination of the sample with the electron beam introduces stacking faults into the material. These stacking faults take the form of the zinc blend crystal and twins

12. ZnS Nanostructures - Synthesis Substrate Source Materials Cooling Water Cooling Water Tube Furnace Pump Carrying Gas Thermal evaporation Vapor Deposition Controllable Parameters System Pressure, Temperature, Reaction Time, Source and substrate materials

13. ZnS Growth - Gradients

14. Growth MechanismVapor-Liquid-Solid Au catalyst Source vapor Nanowire Au catalyst Without Au Au Source material sublimates forming a vapor phase Catalyst (for example, gold) melts forming a liquid droplet The droplet saturates when source vapor keeps coming Solute precipitates out from the droplet forming a solid one-dimensional nanostructure 20 μm Au ball ZnS wire 200 nm

15. Growth MechanismVapor-Solid Nanowire Nanowire Source vapor Source vapor Seed Seed Source material sublimates into a vapor phase Source vapor deposits onto the substrates forming a seed The seed serves as a preferential growth site for multiple one-dimensional nanostructures creating “weed”-like growth

16. ZnS VS and VLS growth VLS and VS growth can occur in the same temperature zone and on the same substrate

17. Growth Mechanism - Model Growth Species in vapor (2) Ad(de)sorption onto surface (1) Diffusion from source to vapor (5) Diffusion of by- products (4) Inclusion of growth species in the crystal structure (3) Surface diffusion of growth species Rate limiting steps are the inclusion of the adsorbed species into the crystal and the ad(de)sorption onto and from the growth surface

18. Growth MechanismModel The growth of the nanostructure with both VS and VLS growth depends on the super-saturation of the species in the vapor, the size of the species, the accommodation coefficient, the size of the growth plane, the growth temperature, and the enthalpy of formation VS VLS VLS VS nw = area number density of adatoms on the surface Dw= surface diffusivity of the growth species on the surface Ω = atomic volume of the adatoms ∆Hevap = evaporation enthalpy Rside = effective impingement rate of adatoms on the surface α = accommodation coefficient of the surface σ = super-saturation in the vapor (P-P0)/P0 P0 = equilibrium vapor pressure λ = effective diffusion distance = √(Dsτs)

20. Models

21. Novel ZnS Nanostructures

22. Nanobelts/Nanowires

23. Aligned ZnS Nanowires

24. Nanohelices

26. ZnSNanosaws and Nanocombs 3 μm [0110] [0001] 300 nm Wurtzite ZnS nanosaws produced by polar surfaces, D. F. Moore, C. Ronning, C. Ma, and Z. L. Wang, Chem. Phys. Letts., 385 (2004) 8-11.

27. ZnS Nanocombs The secondary growth occurs off of the Zn-terminated plane of the material

28. ZnS Nanobelt/Nanosaws - Growth Nanosaws form at high temperature zones, closer to the source material Closer to the source material also has higher concentration of the growth species in the atmosphere The higher concentration of the growth species contributes to the secondary growth The secondary growth forms on the more chemically active plane (here, the Zn-terminated plane)

30. Models

31. Novel ZnS Nanostructures

32. Nanobelts/Nanowires

33. Aligned ZnS Nanowires

34. Nanohelices

36. Aligned ZnS Nanowires

37. Aligned ZnS Nanowires - Formation Multiple nucleation of CdSe forms a polycrystalline film ZnS wires form off of the c-plane of the CdSe

38. Aligned ZnS Nanowires - Growth The initial deposit of CdSe forms a polycrystalline film This film on the silicon serves as the deposition surface for the ZnS nanowires Growth occurs because of the lattice match between (0001)-CdSe and (0001)-ZnS and continues because of low growth energy of the growth direction

40. Models

41. Novel ZnS Nanostructures

42. Nanobelts/Nanowires

43. Aligned ZnS Nanowires

44. Nanohelices

46. ZnSNanohelix 2 μm 2 μm Hierarchical structured nanohelices of ZnS, D. F. Moore, Y. Ding, and Zhong L. Wang, Angew. Chemie International Ed.,Vol. 118, 5274-5278

47. ZnSNanohelix 2 um 2 um 20 um

48. ZnSNanohelix - XRD

49. 10 um ZnSNanohelix – Hierarchical Structure

50. ZnSNanohelix – Branch Structure

51. ZnSNanohelix - Initial branch beam direction

52. ZnSNanohelix – Branch Model The initial branch growth is not very energetically favorable Secondary growth occurs off of the Zn-terminated plane [0002] [0002] 115º Twin (0113) (0111) [2110]

53. ZnS Nanohelices - Growth Nanohelices form with longer growth time, lower pressure, and the presence of a gold catalyst ZnS nanohelix formation is not explained solely by the polarity of the surfaces; surface tension may be used to explain the bending Secondary growth is first along a non-favorable direction because it is growing off of the (0111) plane of the helical wire. It soon switches to more favorable growth

55. Models

56. Novel ZnS Nanostructures

57. Nanobelts/Nanowires

58. Aligned ZnS Nanowires

59. Nanohelices

61. UltralongZnS/SiO2 Nanowires Flow gas 200 μm Start Finish 100 μm 200 μm 1 cm Nanowires align in the direction of the flow gas in the furnace Nanowires appear to span the entire growth substrate

62. UltralongZnS/SiO2 Nanowires 0002 10 nm 0002 0110

63. Ultralong ZnS/SiO2 Nanowires 15 min 30 min 45 min Direction of flow gas 10 μm 20 μm 10 μm 90 min 120 min 60 min 20 μm 100 μm 100 μm

64. Ultralong ZnS/SiO2 Nanowires -TEM Measurements The SiO2 shell size increases with increasing growth time; the ZnS core decreases, but not significantly

65. Ultralong ZnS/SiO2 Nanowires- X-ray Diffraction

66. Ultralong ZnS/SiO2 Nanowires- Photoluminesence The intensity of the 532 nm peak increases with increasing time until it disappears with the longest run time The 3.30 eV peak seen in the 30 min spectrum is explained by pure silica 90 min 75 min 60 min 45 min 30 min 110 min

67. UltralongZnS/SiO2 Nanowires- Formation Mechanism

68. Ultralong ZnS/SiO2 Nanowires - Growth Increasing the amount of catalyst has a variety of effects; including a higher number of catalyst sites and greater competition for the incoming growth species The extra gold catalyst forms a eutectic with the silicon substrate and helps form the SiO2 shell

69. Conclusions A model for the basic growth of ZnS nanostructures was presented Novel ZnS nanostructures were presented and characterized including nanobelts, nanosaws, orientationally aligned nanowires, nanohelices, and core-shell nanowires Growth process can be controlled and affected by changing the local pressure, temperature, vapor concentration, catalyst, and substrate This research allows for a more fundamental understanding of the processes that affect nanostructure growth

70. Acknowledgements My advisor – Dr. Zhong L. Wang My committee – Dr. Nie, Dr. Snyder, Dr. Summers, and Dr. Wong Group members – Dr. Xudong Wang, Dr. Puxian Gao, Dr. William Hughes

71. Publications Hierarchical Structured Nanohelices of ZnS, D.F. Moore, Y. Ding, and Z.L. Wang, AngewandteChemie International Edition, Vol 118, 5274-5278, 2006 Growth of anisotropic one-dimensional ZnS nanostructures, D.F. Moore and Z.L. Wang, J. Materials Chemistry, 2006 (16) 3898-3905 Nanobelt and nanosaw structures of II-VI semiconductors, C. Ma, D. F. Moore, Y. Ding, J. Li and Z. L. Wang, Int. J. Nanotechnology 1 (2004) 431-451 Crystal Orientation-Ordered ZnS Nanowire Bundles, D. F. Moore, Y. Ding, and Zhong L. Wang, J. Am. Chem. Soc., 126 (2004) 14372-14373 Wurtzite ZnS nanosaws produced by polar surfaces,D. F. Moore, C. Ronning, C. Ma, and Z. L. Wang, Chem. Phys. Letts., 385 (2004) 8-11 Single-Crystal CdSe Nanosaws, C. Ma, Y. Ding, D. F. Moore, X. D. Wang and Z. L. Wang, J. Am. Chem. Soc., 126 (2004) 708-709 (featured in Nature 427 (2004) 497) Nanobelts, Nanocombs, and Nano-windmills of Wurtzite ZnS, C. Ma, D. F. Moore, J. Li and Z. L. Wang, Adv. Mater., 15, (2003) 228-231

72. Questions? Novel ZnS Nanostructures Daniel F. Moore

73. Appendix Novel ZnS Nanostructures Daniel F. Moore

74. Gas Gas Alloy drop Alloy drop ZnS Substrate Substrate Why Do Small Dots Initiate Nanowire Growth?- A Thermodynamic Model (1) Stage 1: pre-initiation Stage 2: post-initiation VAGAV: Free energy of the alloy phase VGGGV: Free energy of the gas phase AASγAS: Free energy of the substrate/alloy interface AAGγAG: Free energy of the alloy/gas interface AGSγGS: Free energy of the gas/substrate interface VAGAV: Free energy of the alloy phase VGGGV: Free energy of the gas phase VZGZV: Free energy of the ZnS phase AAZγAZ: Free energy of the alloy/ZnS interface AZSγZS: Free energy of the ZnS/substrate interface AZGγZG: Free energy of the ZnS/gas interface AAGγAG: Free energy of the alloy/gas interface AGSγGS: Free energy of the gas/substrate interface * V: volume; GV: free energy per volume; A: surface area; γ: free energy of the interface per area

75. Free energy of the two stages: Gas Gas Alloy drop Stage 1: Stage 2: Alloy drop ZnS Substrate Substrate Free energy change upon the precipitation of ZnS: Assuming: The alloy composition remains the same: ZnS has the same contact area between the alloy and the substrate:AAS= AAZ = AZS Why Do Small Dots Initiate Nanowire Growth?- A Thermodynamic Model (2)

76. In order to initiate precipitating of ZnS, ΔG has to be negative, so: where volume and surface energies can be treated as constants: Why Do Small Dots Initiate Nanowire Growth?- A Thermodynamic Model (3) Small AAS is easier to satisfy above equation Small gold dots are thermodynamic favorable sites for the initiation of nanowire growth.

77. Model Equations Assumptions: (1) The metal particle in VLS is assumed to be hemispherical (2) steady-state adatom diffusion on the substrate and nanowire sides toward the metal particle (3) The process within the metal particle as well as at the metal-semiconductor interface need not be considered (4) the interwire separation is fairly large so the growing structures are not competing for the vapor species – there is ample material for growth 2 is a quasi-static approach. It is assuming that the nanowire growth rate is slow compared to the velocity of diffusing adatoms 3 is problematic for VLS, since there are clear indications that diffusion through a solid particle or nucleation effects at the interface can be rate-limiting

78. VLS is faster than VS growth By comparing the two we get the following:

79. AuSi Eutectic AuSi alloy with 19.5 atomic % Si and 80.5 % Au melts at T=363 C, while pure Au and pure Si are solid up to 1063 C and 1412 C respectively

80. ZnS Vapor Pressure For T between 900-1250 K At T=1000 C, p=3.34*10-1 Torr For T between 704 and 1006 K At T=750 C, p=2.06*10-5 Torr

81. XRD Crystallite Size Analysis Determined through the peak broadening t is the volume weighted crystallite size, θ the peak position in radians, λ is the x-ray wavelength, ∆d/d is the nonuniform strain in the crystal (uniform strain causes the unit cell to expand/contract in an isotropic manner. This leads to a change in the unit cell parameter and a shift of the peaks, however there is no broadening). Bcosθ vs. 2sinθ is plotted giving a y=mx+b straight line where the intercept b=Kλ/t and the slope is the strain K is the Scherrer constant. It is in the range of 0.87-1.0. 0.9 was used in the calculation The x-rays are formed via copper radiation, giving a wavelength of ~1.315 A The profile that was assumed for the peaks was a Pseudo-Voigt peak, as it best fit the peaks analysed

82. XRD Reference Intensity Ratio

83. TEM – Convergent Beam Electron Diffraction Beam is converged so that the source is no longer a point but is conical The beam is incident on the crystal from multiple angles This allows for the imaging of strain, charge density distribution, and asymmetry

84. II-VI Semiconductors – Covalent and Ionic nature Calculation of effective charge Comparison of calculated radii with experimental

85. ZnS Properties a,b-plane spacing: 3.8227 A c-plane spacing: 6.2607 A Band gap (wurtzite) 3.91 eV (~317 nm) Band gap (Zinc blend) 3.54 eV (~350 nm) Effective mass of electron, hole ~.25m(e), .60m(e) Dielectric constant 8.9 Resistivity 3.5*102 ohm-m Can be doped in both p and n type

86. ZnS Thermodynamic Properties Transition – 1020 C (1293.15 K) Melting point – 1718 C Enthalpy of Formation: -204.6 kJ/mol for zinc blend DeltaG(wurtzite)=376700-191.9T kJ/mol DeltaG(zinc blend)=374200-190.4T kJ/mol

87. ZnS Nanobelt Mechanical Properties (from Li et al “Mechanical Properties of ZnS Nanobelts) Bulk hardness – 1.9 Gpa Bulk elastic modulus – 75 Gpa Nanobelt hardness – 3.4 +-.2 Gpa Nanobelt elastic modulus – 35.9 +-3.5 GPa

88. Four basic steps Electrons tunnel from electronic states at the insulator/phosphor interface Electrons are accelerated to ballistic energies Electrons impact and ionize the luminescent center or create electron-hole pairs that lead to the activation of the luminescent center Luminescent center relaxes toward the ground state and emits a photon - Electrodes - Insulator - Phosphor - Insulator SUBSTRATE - Electrodes Electroluminescent Display Devices (ELD)