Novel ZnS NanostructuresSynthesis, Growth Mechanism, and Applications<br />Ph.D. Defense presented by<br />Daniel F. Moore...
Outline<br />Applications<br />ZnS applications<br />ZnS nanostructure applications<br />Models<br />ZnS crystal structure...
ZnS Applications<br />- Electrodes<br />- Insulator<br />- Phosphor (ZnS)<br />- Insulator<br />SUBSTRATE<br />- Electrode...
Monochrome TFEL structure</li></li></ul><li>One dimensional applications<br />INSULATOR<br />DRAIN<br />SOURCE<br />METAL ...
Understanding and Synthesizing One-Dimensional Nanostructures<br />Why?<br />Facilitate a rational design method of creati...
ZnS Crystallographic and Growth Models<br />Applications<br />Models<br />Crystallographic Structure<br />Synthesis and Gr...
ZnS Crystal Structures<br />Zinc Blend<br />ABCABC<br />Wurtzite<br />ABABAB<br />
ZnS Wurtzite<br />(0111)<br />(0001)<br />Wurtzite crystal structure projected along [2110]<br />
Why Wurtzite?Ostwald’s Rule of Stages<br />The formation of the new stable phase takes place by consecutive steps from one...
Why Wurtzite?Ostwald’s Rule of Stages<br />If a reaction can result in several products, it is not the most stable state w...
ZnS Nanosaw – Phase Transformation<br />Before<br />After<br />Illumination of the sample with the electron beam introduce...
ZnS Nanostructures - Synthesis<br />Substrate<br />Source Materials<br />Cooling Water<br />Cooling Water<br />Tube Furnac...
ZnS Growth - Gradients<br />
Growth MechanismVapor-Liquid-Solid<br />Au catalyst<br />Source vapor<br />Nanowire<br />Au catalyst<br />Without Au<br />...
Growth MechanismVapor-Solid<br />Nanowire<br />Nanowire<br />Source vapor<br />Source vapor<br />Seed<br />Seed<br />Sourc...
ZnS VS and VLS growth<br />VLS and VS growth can occur in the same temperature zone and on the same substrate<br />
Growth Mechanism - Model<br />Growth Species in vapor<br />(2) Ad(de)sorption onto surface<br />(1) Diffusion from source ...
Growth MechanismModel<br />The growth of the nanostructure with both VS and VLS growth depends on the super-saturation of ...
Nanobelts, Nanowires, and Nanosaws<br />Nanobelt Synthesis<br />Source Temperature: 1050 C<br />Reaction Time: 60 minutes<...
Models
Novel ZnS Nanostructures
Nanobelts/Nanowires
Aligned ZnS Nanowires
Nanohelices
Ultralong ZnS/SiO2 nanowires</li></li></ul><li>ZnS Nanobelts<br />20 μm<br />Nanobelts have a rectangular cross-section<br...
ZnSNanosaws and Nanocombs<br />3 μm<br />[0110]<br />[0001]<br />300 nm<br />Wurtzite ZnS nanosaws produced by polar surfa...
ZnS Nanocombs<br />The secondary growth occurs off of the Zn-terminated plane of the material<br />
ZnS Nanobelt/Nanosaws - Growth<br />Nanosaws form at high temperature zones, closer to the source material<br />Closer to ...
Vertically Aligned ZnS Nanowires<br />ZnS Aligned Nanowire Synthesis<br />Two Step Process<br />1st Source Temperature: 75...
Models
Novel ZnS Nanostructures
Nanobelts/Nanowires
Aligned ZnS Nanowires
Nanohelices
Ultralong ZnS/SiO2 nanowires</li></li></ul><li>Aligned ZnS Nanowires<br />2 μm<br />1 μm<br />Crystal Orientation-Ordered ...
Aligned ZnS Nanowires<br />
Aligned ZnS Nanowires - Formation<br />Multiple nucleation of CdSe forms a polycrystalline film<br />ZnS wires form off of...
Aligned ZnS Nanowires - Growth<br />The initial deposit of CdSe forms a polycrystalline film<br />This film on the silicon...
Hierarchical Structured ZnS Nanohelices<br />ZnS Hierarchical Nanohelices Synthesis<br />Source Temperature: 1000 C<br />S...
Models
Novel ZnS Nanostructures
Nanobelts/Nanowires
Aligned ZnS Nanowires
Nanohelices
Ultralong ZnS/SiO2 nanowires</li></li></ul><li>(0111)<br />Emphasizing the polar planes<br />(0001)<br />Spontaneous Polar...
ZnSNanohelix<br />2 μm<br />2 μm<br />Hierarchical structured nanohelices of ZnS, D. F. Moore, Y. Ding, and Zhong L. Wang,...
ZnSNanohelix<br />2 um<br />2 um<br />20 um<br />
ZnSNanohelix - XRD<br />
10 um<br />ZnSNanohelix – Hierarchical Structure<br />
ZnSNanohelix – Branch Structure<br />
ZnSNanohelix - Initial branch<br />beam direction<br />
ZnSNanohelix – Branch Model<br />The initial branch growth is not very energetically favorable<br />Secondary growth occur...
ZnS Nanohelices - Growth<br />Nanohelices form with longer growth time, lower pressure, and the presence of a gold catalys...
Ultra-long Core-Shell ZnS-SiO2 Nanowires<br />Core-Shell ZnS-SiO2 Nanowires Synthesis<br />Source Temperature: 1000 C<br /...
Models
Novel ZnS Nanostructures
Nanobelts/Nanowires
Aligned ZnS Nanowires
Nanohelices
Ultralong ZnS/SiO2 nanowires</li></li></ul><li>UltralongZnS/SiO2 Nanowires<br />10 μm<br />10 μm<br />Si<br />O<br />Zn<br...
UltralongZnS/SiO2 Nanowires<br />Flow gas<br />200 μm<br />Start<br />Finish<br />100 μm<br />200 μm<br />1 cm<br />Nanowi...
UltralongZnS/SiO2 Nanowires<br />0002<br />10 nm<br />0002<br />0110<br />
Ultralong ZnS/SiO2 Nanowires<br />15 min<br />30 min<br />45 min<br />Direction of flow gas<br />10 μm<br />20 μm<br />10 ...
Ultralong ZnS/SiO2 Nanowires<br />-TEM Measurements<br />The SiO2 shell size increases with increasing growth time; the Zn...
Ultralong ZnS/SiO2 Nanowires- X-ray Diffraction<br />
Ultralong ZnS/SiO2 Nanowires- Photoluminesence<br />The intensity of the 532 nm peak increases with increasing time until ...
UltralongZnS/SiO2 Nanowires- Formation Mechanism<br />
Ultralong ZnS/SiO2 Nanowires - Growth<br />Increasing the amount of catalyst has a variety of effects; including a higher ...
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ZnS Nanostructures: Synthesis, Characterization, and Theory - Defense Presentation

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

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  • Good morning, my name is Daniel Moore and I am presenting on Novel ZnS Nanostructures. I have been working on this topic under the advisement of Dr. Z. L. Wang. I’d also like to welcome the members of my committee – Dr. Snyder, Dr. Summers, Dr. Wong, and Dr. Nie.
  • Copper and aluminum doped ZnS typically can be used to created green emitting LEDs.ZnS is the most common phosphor material used for Electroluminescent Display devices. ELDs work by electroluminescence – the result of radiative recombination of electrons and holes. Essentially, ELDs are “lossy capacitors” that become electrically charged and then lose their energy in the form of light. The insulator layer is necessary to prevent arcing between the two conductive layers.ELD are commonly used as backlights in LCDs, providing a gentle even illuminationEL devices have low power consumption when compared with neon signs, and have a wide range of applications such as their use on advertising boards and safety signs. Because an EL layer can be very thin (around 1 mm thick), it can be used as decoration added to everyday items, including clothing and accessories such as bags and earphone cords
  • Nanowires have been shown to be used in an array of applications, including field electronic transistors and related devices.
  • Planes are polarized due to the semi-ionic nature of the bonding in the crystalThe different terminated species also lead to the planes having different chemical activities
  • Ever since Ostwald stated his Rule of stages in 1897, attempts have been made to derive a theoretical basis for the rule. Now, even after 100 years, an undisputed theoretical basis for the rule has not been formulated successfully, even though field and laboratory observations supporting the rule are now too numerous to mention. A general consensus is that the rule can be explained through an understanding of nucleation theory and reaction kinetics. How-ever, there is no rigorous basis for the rule in fact, i.e., in principle, there is no reason for the rule not to be violated, and occasional exceptions to the rule have been cited. In fact, one school of thought has attempted to formulate the rule on the basis of irreversible thermodynamics. In natural systems, manifestations of the Ostwald rule, may in fact contain components both of kinetics, irreversible thermodynamics, and equilibrium thermodynamics. (See, for example, Duffy, 1993). Because quantitative evaluations of alteration rates in complex natural systems have hardly ever been made, it is not possible at this time to comment further on the applicability of each approach, and whether or not they can be reconciled, particularly when the complexity of the problem is fully appreciated. Perhaps, the strongest argument for preferring a kinetic treatment for the Ostwald rule, is evidence for the simultaneous growth of two or more phases of the same composition. Experimental results on a zeolitic system, cited by Barrer (1982), provide substantiation, although further examples are needed to lend credibility to this position.
  • deltaG(wurtzite)=376700-191.9T=324.31 kJ/mol at room temp=180.36 at 750CdeltaG(sphalerite)=374200-190.4T=322.22 kJ/mol at room temp=179.39 at 750CThe transition from sphalerite to wurtzite is nearly athermal, from room temperature up to the transition temperature at 1020 C.Sphalerite very rarely exists without stacking faults.
  • Thermodynamically, the zinc blend to wurtzite phase transformation occurs at 1020 C (1293 K). Here, the illumination of a region of a nanosaw with a 200 keV electron beam for 10 minutes
  • On the left are measurements of the temperature gradient in the furnace. These were measured at atmospheric pressure, but this is believed to have a minimal impact.On the right is a schematic showing the vapor concentration of the species with and without a carrier gas present.The main point of this slide is to show that the vapor concentration is coupled with the deposition substrate temperature.
  • Combines the incoming of the vapor species (condensation rate J=σP0/(sqrt(2πmkT))) and the inclusion into the crystal structure (α) as rate-limiting stepsMass-transport-limited model. It is a diffusion-deposition model with kinetically hindered growth on the substrate surface and on the sides of the nanowiresAssumptions: (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 growthIn VS The length growth rate of the nanowire can be expressed as the adatom flux from the nanowire sides into the metal particle multiplied by the circumference, divided by the cross-sectional area of the nanowire.The 2 in the VLS comes from that the mantle area of the metal particle divided by the cross-sectional area of the nanowire equals 2.VLS equation is for growth only along the direction encouraged by the metal particle2.667 comes from the formulation for the effective diffusion distance. The real exponential is (Edes-Es). Es=0.1Edes and Edes=(5/6)∆Hevap
  • Nanobelts have a rectangular cross section. There is one fast growth direction, typically the [0001] direction. In the wurtzite structure these planes are the lowest energy to form and this is typically the fast growth direction.This research was presented in the journal Advanced Materials and was featured by Thomson’s Essential Science Indicators as a “Fast Moving Front” in the field of Materials Science in July 2005
  • Features one main growth direction, the fast growth direction, and secondary growth off of at least one of the edges. Typically, the main growth direction of nanosaws and nanocombs is not the [0001] direction.This research was presented in the Journal Chemical Physics Letters
  • The difference in the polarity associated with the Zn surface and the S surface results in different diffraction contrast in the +-(0001) discs. This is compared with a computed pattern to determine which disc (and direction) represents which surface. The simulated pattern is formed by a dynamic simulation conducted by a Bloch wave program. (See Spence, ZuoElectron Microdiffraction)
  • When the growth is adsorption limited, the growth rate is determined in large part by the impingement rate of the growth species onto the growth surface of the nanocrystal. This is directly proportional to the concentration of the growth species in the vapor. In reality, a combination of these two steps provide for the limiting and permitting of growth of the nanostructure.Typically, the VS growth of ZnS one-dimensional nanostructures results in very few imperfections. This probably occurs because the adsorption rate of incoming molecules and the rate of incorporation of that molecule into the crystal structure are comparable. However, if the vapor concentration is high enough and gas-phase supersaturation is high, the incoming molecules adsorb onto the growth surface at a higher rate than they can be incorporated into the forming crystal structure and defects and stacking faults can be formed within the crystal. These sites can serve as preferable nucleation sites for secondary growth causing the growth of nanosaws, nanocombs, and other such nanostructures.This high vapor concentration can be the result of one of two things. First, it could be the result of a higher pressure used in the system. Higher pressure is obtained because of the presence of more molecules. This means that the vapor concentration is higher than if we maintained a lower pressure. Ma et al have shown that in a similar process with CdSe, and a similar result has been shown with ZnS. Second, an increased vapor concentration could be the result of the carrier gas. Within a single experiment, there is a varying gas-phase supersaturation throughout the tube furnace (illustrated in the figure) and this may explain for the different types of growth that is achieved at different distances from the source material.
  • The CdSe is a solid film, but the ZnS is a bundle of aligned nanowiresThis research was presented in the Journal of the American Chemical Society
  • Low-magnification TEM images of bundles of aligned ZnS nanowires and the electron diffraction patterns showing growth along the [0001] direction. The high resolution image shows a uniform lattice structure, without dislocations
  • The [0001]-oriented CdSe has a 6-fold-symmetric a-plane, {10-10}, with an interplanar distance of 0.3724 nm, which matches well to the 6-foldsymmetric Si(111) substrate, with an interplanar distance 0.3839 nm, resulting in a c-axis-oriented growth of CdSe on Si(111). Due to a larger lattice mismatch, single-crystal thin films are not grown, but rather, multiple nucleation of CdSe results in the growth of a polycrystalline CdSe film.ZnS has an a-plane lattice of 0.38227 nmCdSe bandgap of 1.8 eV (~688 nm)In the second step, the CdSe film serves as a substrate for epitaxial growth of the ZnS nanowires. Because both CdSe and ZnS have wurtzite structure, an epitaxial growth along the c-axis would be preferred to minimize the interface lattice mismatch.
  • The [0001]-oriented CdSe has a 6-fold-symmetric a-plane, {10-10}, with an interplanar distance of 0.3724 nm, which matches well to the 6-foldsymmetric Si(111) substrate, with an interplanar distance 0.3839 nm, resulting in a c-axis-oriented growth of CdSe on Si(111). Due to a larger lattice mismatch, single-crystal thin films are not grown, but rather, multiple nucleation of CdSe results in the growth of a polycrystalline CdSe film.ZnS has an a-plane lattice of 0.38227 nmCdSe bandgap of 1.8 eV (~688 nm)
  • ZnO helical structure form because of the dipole across the surface created by the polar planesThey are equally likely to form with the Zn or the O on the inside of the spiral
  • This research was presented in the journal AngewandteChemie International Edition
  • The secondary growth is all approximately the same size, regardless of where on the spine it grows
  • The (01-13) twin is the most common wurtzite twin structure
  • Diffraction pattern comes from the spineThough the secondary growth appears rough, it is stacking fault and dislocation free. It is single crystal.
  • The Surface Tension model proposed by Cahn and Hanneman may help explain the helix formation. This research calculated the relationship of surface tension to “spontaneous bending of thin crystals.”Cahn, J. W.; Hanneman, R. E. Surf. Sci.1964, 1, 387
  • The crystallite size and ratio measurements measure the relative weight fraction. This has to be converted to volume, though the trend will be the same. This is done my multiplying by the ratio of the densities. ZnS density= 4090 kg/m^3 in bulkSiO2 density = 2197.7 kg/m^3 in bulkRatio (ZnS/SiO2) = 1.861Assumptions for RIR – that the SiO2 is crystalline. Though there is some crystalline SiO2 present, this is generally not true as seen in the TEM data. However, the qualitative trend is usefulAssumptions for the crystallite size – The ZnS cores are single crystalline without twins
  • Room-temperature photoluminescence (PL) properties using a 266 nm Nd:YAG Q-switched laser with an average power of 1.9 mW as the excitation light source. A Si photodetector is used to measure the PL.Nd:YAG is an acronym for neodymium-doped yttrium aluminium garnet (Nd:Y3Al5O12).Q-switching, sometimes known as giant pulse formation, is a technique by which a laser can be made to produce a pulsed output beam. An optical switch is inserted in the laser cavity waiting for a maximum population inversion in the neodymium ions before it opens. Then the light wave can run through the cavity, depopulating the excited laser medium at maximum population inversion.
  • The ZnS nanowires forms via the vapor-liquid-solid process. At the same time, the gold forms a eutectic with the silicon and vaporizes, oxidizing in air. This SiO2 the deposits on the outside of the ZnS wire, limiting its radial growth and using the ZnS wire as a template. In this way, the ZnS wire can continue to grow axially through the gold-ZnS alloy and the shell continues to grow larger
  • The reasons for undertaking this research were to understand the formation and fundamental processes influencing nanoscale materials. This was in order to facilitate a rational design method of creating new materials; enables controlling their properties; and develop new nanoscale materials to help understand the variety of novel properties and morphologies that arise with working with on the nanoscale
  • Research has been cited over 200 times
  • Mass-transport-limited model. It is a diffusion-deposition model with kinetically hindered growth on the substrate surface and on the sides of the nanowiresAssumptions: (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 growthEquation for area number density of adatoms on the substrate surface comes from Einstein and brownian motion – see Johansson et al.
  • Equation is from Addamiano, Dell, “The Melting Point of Zinc Sulfide,” Journal of Phys. Chemistry, 1957, Vol. 61, 1020
  • Convergent beam electron diffraction (CBED). This technique differs from conventional electron diffraction in the shape of the incident beam. In normal TEM, the e-beam is a point source that strikes a sample for one direction and is then diffracted, giving diffractions dots. With the CBED technique, the e-beam is changed from a point source to having a conical shape. Now as the cone-shaped e-beam strikes the sample, instead of “seeing” the crystal from only one direction, the e-beam “sees” the crystal from many different directions simultaneously. This results in a change from what were once diffraction dots in conventional TEM to diffraction discs in the convergent beam technique. CBED is a powerful technique because it can yield information such as crystal structure factor, strain, charge density distribution, and most importantly for this investigation, symmetry.The difference in the polarity associated with the Zn surface and the S surface results in different diffraction contrast in the +-(0001) discs. This is compared with a computed pattern to determine which disc (and direction) represents which surface.
  • ZnS Nanostructures: Synthesis, Characterization, and Theory - Defense Presentation

    1. 1. Novel ZnS NanostructuresSynthesis, Growth Mechanism, and Applications<br />Ph.D. Defense presented by<br />Daniel F. Moore<br />Advisor – Dr. Zhong L. Wang<br />School of Materials Science and Engineering<br />Georgia Institute of Technology<br />October 24, 2006<br />
    2. 2. Outline<br />Applications<br />ZnS applications<br />ZnS nanostructure applications<br />Models<br />ZnS crystal structure<br />Synthesis and growth<br />Novel ZnS Nanostructures<br />Nanobelts/Nanowires<br />Aligned ZnS Nanowires<br />Nanohelices<br />UltralongZnS/SiO2 nanowires<br />
    3. 3. ZnS Applications<br />- Electrodes<br />- Insulator<br />- Phosphor (ZnS)<br />- Insulator<br />SUBSTRATE<br />- Electrodes<br /><ul><li>Light-Emitting Diode
    4. 4. Monochrome TFEL structure</li></li></ul><li>One dimensional applications<br />INSULATOR<br />DRAIN<br />SOURCE<br />METAL GATE<br />NANOWIRE<br />
    5. 5. Understanding and Synthesizing One-Dimensional Nanostructures<br />Why?<br />Facilitate a rational design method of creating new materials<br />Enables controlling their properties<br />New nanoscale materials help understand the variety of novel properties and morphologies that arise with working with on the nanoscale<br />
    6. 6. ZnS Crystallographic and Growth Models<br />Applications<br />Models<br />Crystallographic Structure<br />Synthesis and Growth<br />Novel ZnS Nanostructures<br />Nanobelts/Nanowires<br />Aligned ZnS Nanowires<br />Nanohelices<br />UltralongZnS/SiO2 nanowires<br />
    7. 7. ZnS Crystal Structures<br />Zinc Blend<br />ABCABC<br />Wurtzite<br />ABABAB<br />
    8. 8. ZnS Wurtzite<br />(0111)<br />(0001)<br />Wurtzite crystal structure projected along [2110]<br />
    9. 9. Why Wurtzite?Ostwald’s Rule of Stages<br />The formation of the new stable phase takes place by consecutive steps from one phase to another with increasing thermodynamic stability<br />Under certain conditions, the metastable phase has a higher nucleation rate than the stable phase and nucleates first<br />Ostwald’s rule holds when<br />where μs is the supersaturation = kTln(P/P0s), bs is a geometric factor, σ is the surface energy<br />
    10. 10. Why Wurtzite?Ostwald’s Rule of Stages<br />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<br />
    11. 11. ZnS Nanosaw – Phase Transformation<br />Before<br />After<br />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<br />
    12. 12. ZnS Nanostructures - Synthesis<br />Substrate<br />Source Materials<br />Cooling Water<br />Cooling Water<br />Tube Furnace<br />Pump<br />Carrying Gas<br />Thermal evaporation<br />Vapor Deposition<br />Controllable Parameters<br />System Pressure, Temperature, Reaction Time, Source and substrate materials<br />
    13. 13. ZnS Growth - Gradients<br />
    14. 14. Growth MechanismVapor-Liquid-Solid<br />Au catalyst<br />Source vapor<br />Nanowire<br />Au catalyst<br />Without Au<br />Au<br />Source material sublimates forming a vapor phase<br />Catalyst (for example, gold) melts forming a liquid droplet<br />The droplet saturates when source vapor keeps coming<br />Solute precipitates out from the droplet forming a solid one-dimensional nanostructure<br />20 μm<br />Au ball<br />ZnS wire<br />200 nm<br />
    15. 15. Growth MechanismVapor-Solid<br />Nanowire<br />Nanowire<br />Source vapor<br />Source vapor<br />Seed<br />Seed<br />Source material sublimates into a vapor phase<br />Source vapor deposits onto the substrates forming a seed<br />The seed serves as a preferential growth site for multiple one-dimensional nanostructures creating “weed”-like growth<br />
    16. 16. ZnS VS and VLS growth<br />VLS and VS growth can occur in the same temperature zone and on the same substrate<br />
    17. 17. Growth Mechanism - Model<br />Growth Species in vapor<br />(2) Ad(de)sorption onto surface<br />(1) Diffusion from source to vapor<br />(5) Diffusion of by- products<br />(4) Inclusion of growth species in the crystal structure<br />(3) Surface diffusion of growth species<br />Rate limiting steps are the inclusion of the adsorbed species into the crystal and the ad(de)sorption onto and from the growth surface<br />
    18. 18. Growth MechanismModel<br />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<br />VS<br />VLS<br />VLS<br />VS<br />nw = area number density of adatoms on the surface<br />Dw= surface diffusivity of the growth species on the surface<br />Ω = atomic volume of the adatoms<br />∆Hevap = evaporation enthalpy<br />Rside = effective impingement rate of adatoms on the surface<br />α = accommodation coefficient of the surface<br />σ = super-saturation in the vapor (P-P0)/P0<br />P0 = equilibrium vapor pressure<br />λ = effective diffusion distance = √(Dsτs)<br />
    19. 19. Nanobelts, Nanowires, and Nanosaws<br />Nanobelt Synthesis<br />Source Temperature: 1050 C<br />Reaction Time: 60 minutes<br />Deposition Substrate: Alumina<br />System Pressure: 225 torr<br />Source material: ZnS<br />Nanosaw Synthesis<br />Source Temperature: 1000 C<br />Reaction Time: 10 minutes<br />Deposition Substrate: &lt;111&gt; Silicon<br />System Pressure: 225 torr<br />Source Material: ZnS<br />Form at a higher temperature than nanobelts<br /><ul><li>Applications
    20. 20. Models
    21. 21. Novel ZnS Nanostructures
    22. 22. Nanobelts/Nanowires
    23. 23. Aligned ZnS Nanowires
    24. 24. Nanohelices
    25. 25. Ultralong ZnS/SiO2 nanowires</li></li></ul><li>ZnS Nanobelts<br />20 μm<br />Nanobelts have a rectangular cross-section<br />VS mechanism exhibits “weed”-like growth<br />10 μm<br />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.<br />
    26. 26. ZnSNanosaws and Nanocombs<br />3 μm<br />[0110]<br />[0001]<br />300 nm<br />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. <br />
    27. 27. ZnS Nanocombs<br />The secondary growth occurs off of the Zn-terminated plane of the material<br />
    28. 28. ZnS Nanobelt/Nanosaws - Growth<br />Nanosaws form at high temperature zones, closer to the source material<br />Closer to the source material also has higher concentration of the growth species in the atmosphere<br />The higher concentration of the growth species contributes to the secondary growth<br />The secondary growth forms on the more chemically active plane (here, the Zn-terminated plane)<br />
    29. 29. Vertically Aligned ZnS Nanowires<br />ZnS Aligned Nanowire Synthesis<br />Two Step Process<br />1st Source Temperature: 750 C<br />1st Substrate Temperature: 575 C<br />Reaction Time: 60 minutes<br />Deposition Substrate: &lt;111&gt; Silicon<br />System Pressure: 225 torr<br />Source material: CdSe<br />2nd Source Temperature: 1050 C<br />Reaction Time: 60 minutes<br />System Pressure: 225 torr<br />Source Material: ZnS<br /><ul><li>Applications
    30. 30. Models
    31. 31. Novel ZnS Nanostructures
    32. 32. Nanobelts/Nanowires
    33. 33. Aligned ZnS Nanowires
    34. 34. Nanohelices
    35. 35. Ultralong ZnS/SiO2 nanowires</li></li></ul><li>Aligned ZnS Nanowires<br />2 μm<br />1 μm<br />Crystal Orientation-Ordered ZnSNanowire Bundles, D. F. Moore, Y. Ding, and Zhong L. Wang, J. Am. Chem. Soc., 126 (2004) 14372-14373. <br />
    36. 36. Aligned ZnS Nanowires<br />
    37. 37. Aligned ZnS Nanowires - Formation<br />Multiple nucleation of CdSe forms a polycrystalline film<br />ZnS wires form off of the c-plane of the CdSe<br />
    38. 38. Aligned ZnS Nanowires - Growth<br />The initial deposit of CdSe forms a polycrystalline film<br />This film on the silicon serves as the deposition surface for the ZnS nanowires<br />Growth occurs because of the lattice match between (0001)-CdSe and (0001)-ZnS and continues because of low growth energy of the growth direction<br />
    39. 39. Hierarchical Structured ZnS Nanohelices<br />ZnS Hierarchical Nanohelices Synthesis<br />Source Temperature: 1000 C<br />Substrate Temperature: 750 C<br />Reaction Time: 120 minutes<br />Deposition Substrate: &lt;111&gt; Silicon with 1.5 nm Au<br />System Pressure: 20 torr<br />Source material: ZnS<br />Longer growth time, lower growth pressure, and the presence of gold catalyst<br /><ul><li>Applications
    40. 40. Models
    41. 41. Novel ZnS Nanostructures
    42. 42. Nanobelts/Nanowires
    43. 43. Aligned ZnS Nanowires
    44. 44. Nanohelices
    45. 45. Ultralong ZnS/SiO2 nanowires</li></li></ul><li>(0111)<br />Emphasizing the polar planes<br />(0001)<br />Spontaneous Polarization-Induced Nanohelixes, Nanosprings, and Nanorings of Piezoelectric Nanobelts, X. Y. Kong and Z. L. Wang, NanoLett. 3 (2003) 1625-1631.<br />
    46. 46. ZnSNanohelix<br />2 μm<br />2 μm<br />Hierarchical structured nanohelices of ZnS, D. F. Moore, Y. Ding, and Zhong L. Wang, Angew. Chemie International Ed.,Vol. 118, 5274-5278<br />
    47. 47. ZnSNanohelix<br />2 um<br />2 um<br />20 um<br />
    48. 48. ZnSNanohelix - XRD<br />
    49. 49. 10 um<br />ZnSNanohelix – Hierarchical Structure<br />
    50. 50. ZnSNanohelix – Branch Structure<br />
    51. 51. ZnSNanohelix - Initial branch<br />beam direction<br />
    52. 52. ZnSNanohelix – Branch Model<br />The initial branch growth is not very energetically favorable<br />Secondary growth occurs off of the Zn-terminated plane<br />[0002]<br />[0002]<br />115º<br />Twin<br />(0113)<br />(0111)<br />[2110]<br />
    53. 53. ZnS Nanohelices - Growth<br />Nanohelices form with longer growth time, lower pressure, and the presence of a gold catalyst<br />ZnS nanohelix formation is not explained solely by the polarity of the surfaces; surface tension may be used to explain the bending<br />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<br />
    54. 54. Ultra-long Core-Shell ZnS-SiO2 Nanowires<br />Core-Shell ZnS-SiO2 Nanowires Synthesis<br />Source Temperature: 1000 C<br />Substrate Temperature: 750 C<br />Reaction Time: 120 minutes<br />Deposition Substrate: &lt;111&gt; Silicon with 20 nm Au<br />System Pressure: 20 torr<br />Source material: ZnS<br />Longer growth time, lower growth pressure, and the significant presence of gold catalyst<br /><ul><li>Applications
    55. 55. Models
    56. 56. Novel ZnS Nanostructures
    57. 57. Nanobelts/Nanowires
    58. 58. Aligned ZnS Nanowires
    59. 59. Nanohelices
    60. 60. Ultralong ZnS/SiO2 nanowires</li></li></ul><li>UltralongZnS/SiO2 Nanowires<br />10 μm<br />10 μm<br />Si<br />O<br />Zn<br />S<br />
    61. 61. UltralongZnS/SiO2 Nanowires<br />Flow gas<br />200 μm<br />Start<br />Finish<br />100 μm<br />200 μm<br />1 cm<br />Nanowires align in the direction of the flow gas in the furnace<br />Nanowires appear to span the entire growth substrate<br />
    62. 62. UltralongZnS/SiO2 Nanowires<br />0002<br />10 nm<br />0002<br />0110<br />
    63. 63. Ultralong ZnS/SiO2 Nanowires<br />15 min<br />30 min<br />45 min<br />Direction of flow gas<br />10 μm<br />20 μm<br />10 μm<br />90 min<br />120 min<br />60 min<br />20 μm<br />100 μm<br />100 μm<br />
    64. 64. Ultralong ZnS/SiO2 Nanowires<br />-TEM Measurements<br />The SiO2 shell size increases with increasing growth time; the ZnS core decreases, but not significantly<br />
    65. 65. Ultralong ZnS/SiO2 Nanowires- X-ray Diffraction<br />
    66. 66. Ultralong ZnS/SiO2 Nanowires- Photoluminesence<br />The intensity of the 532 nm peak increases with increasing time until it disappears with the longest run time<br />The 3.30 eV peak seen in the 30 min spectrum is explained by pure silica<br />90 min<br />75 min<br />60 min<br />45 min<br />30 min<br />110 min<br />
    67. 67. UltralongZnS/SiO2 Nanowires- Formation Mechanism<br />
    68. 68. Ultralong ZnS/SiO2 Nanowires - Growth<br />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<br />The extra gold catalyst forms a eutectic with the silicon substrate and helps form the SiO2 shell<br />
    69. 69. Conclusions<br />A model for the basic growth of ZnS nanostructures was presented<br />Novel ZnS nanostructures were presented and characterized including nanobelts, nanosaws, orientationally aligned nanowires, nanohelices, and core-shell nanowires<br />Growth process can be controlled and affected by changing the local pressure, temperature, vapor concentration, catalyst, and substrate<br />This research allows for a more fundamental understanding of the processes that affect nanostructure growth<br />
    70. 70. Acknowledgements<br />My advisor – Dr. Zhong L. Wang<br />My committee – Dr. Nie, Dr. Snyder, Dr. Summers, and Dr. Wong<br />Group members – Dr. Xudong Wang, Dr. Puxian Gao, Dr. William Hughes<br />
    71. 71. Publications<br />Hierarchical Structured Nanohelices of ZnS, D.F. Moore, Y. Ding, and Z.L. Wang, AngewandteChemie International Edition, Vol 118, 5274-5278, 2006<br />Growth of anisotropic one-dimensional ZnS nanostructures, D.F. Moore and Z.L. Wang, J. Materials Chemistry, 2006 (16) 3898-3905<br />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<br />Crystal Orientation-Ordered ZnS Nanowire Bundles, D. F. Moore, Y. Ding, and Zhong L. Wang, J. Am. Chem. Soc., 126 (2004) 14372-14373<br />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<br />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)<br />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<br />
    72. 72. Questions?<br />Novel ZnS Nanostructures<br />Daniel F. Moore<br />
    73. 73. Appendix<br />Novel ZnS Nanostructures<br />Daniel F. Moore<br />
    74. 74. Gas<br />Gas<br />Alloy drop<br />Alloy drop<br />ZnS<br />Substrate<br />Substrate<br />Why Do Small Dots Initiate Nanowire Growth?- A Thermodynamic Model (1)<br />Stage 1: pre-initiation<br />Stage 2: post-initiation<br />VAGAV: Free energy of the alloy phase<br />VGGGV: Free energy of the gas phase<br />AASγAS: Free energy of the substrate/alloy interface<br />AAGγAG: Free energy of the alloy/gas interface<br />AGSγGS: Free energy of the gas/substrate interface<br />VAGAV: Free energy of the alloy phase<br />VGGGV: Free energy of the gas phase<br />VZGZV: Free energy of the ZnS phase<br />AAZγAZ: Free energy of the alloy/ZnS interface<br />AZSγZS: Free energy of the ZnS/substrate interface<br />AZGγZG: Free energy of the ZnS/gas interface<br />AAGγAG: Free energy of the alloy/gas interface<br />AGSγGS: Free energy of the gas/substrate interface<br />* V: volume; GV: free energy per volume; A: surface area; γ: free energy of the interface per area<br />
    75. 75. Free energy of the two stages:<br />Gas<br />Gas<br />Alloy drop<br />Stage 1:<br />Stage 2:<br />Alloy drop<br />ZnS<br />Substrate<br />Substrate<br />Free energy change upon the precipitation of ZnS:<br />Assuming: <br />The alloy composition remains the same:<br />ZnS has the same contact area between the alloy and the substrate:AAS= AAZ = AZS<br />Why Do Small Dots Initiate Nanowire Growth?- A Thermodynamic Model (2)<br />
    76. 76. In order to initiate precipitating of ZnS, ΔG has to be negative, so:<br />where volume and surface energies can be treated as constants:<br />Why Do Small Dots Initiate Nanowire Growth?- A Thermodynamic Model (3)<br />Small AAS is easier to satisfy above equation<br />Small gold dots are thermodynamic favorable sites for the initiation of nanowire growth.<br />
    77. 77. Model Equations<br />Assumptions: <br />(1) The metal particle in VLS is assumed to be hemispherical <br />(2) steady-state adatom diffusion on the substrate and nanowire sides toward the metal particle <br />(3) The process within the metal particle as well as at the metal-semiconductor interface need not be considered <br />(4) the interwire separation is fairly large so the growing structures are not competing for the vapor species – there is ample material for growth<br />2 is a quasi-static approach. It is assuming that the nanowire growth rate is slow compared to the velocity of diffusing adatoms<br />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<br />
    78. 78. VLS is faster than VS growth<br />By comparing the two we get the following:<br />
    79. 79. AuSi Eutectic<br />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<br />
    80. 80. ZnS Vapor Pressure<br />For T between 900-1250 K<br />At T=1000 C, p=3.34*10-1 Torr<br />For T between 704 and 1006 K<br />At T=750 C, p=2.06*10-5 Torr<br />
    81. 81. XRD Crystallite Size Analysis<br />Determined through the peak broadening<br />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).<br />Bcosθ vs. 2sinθ is plotted giving a y=mx+b straight line where the intercept b=Kλ/t and the slope is the strain<br />K is the Scherrer constant. It is in the range of 0.87-1.0. 0.9 was used in the calculation<br />The x-rays are formed via copper radiation, giving a wavelength of ~1.315 A<br />The profile that was assumed for the peaks was a Pseudo-Voigt peak, as it best fit the peaks analysed<br />
    82. 82. XRD Reference Intensity Ratio<br />
    83. 83. TEM – Convergent Beam Electron Diffraction<br />Beam is converged so that the source is no longer a point but is conical<br />The beam is incident on the crystal from multiple angles<br />This allows for the imaging of strain, charge density distribution, and asymmetry<br />
    84. 84. II-VI Semiconductors – Covalent and Ionic nature<br />Calculation of effective charge<br />Comparison of calculated radii with experimental<br />
    85. 85. ZnS Properties<br />a,b-plane spacing: 3.8227 A<br />c-plane spacing: 6.2607 A<br />Band gap (wurtzite) 3.91 eV (~317 nm)<br />Band gap (Zinc blend) 3.54 eV (~350 nm)<br />Effective mass of electron, hole ~.25m(e), .60m(e)<br />Dielectric constant 8.9<br />Resistivity 3.5*102 ohm-m<br />Can be doped in both p and n type<br />
    86. 86. ZnS Thermodynamic Properties<br />Transition – 1020 C (1293.15 K)<br />Melting point – 1718 C<br />Enthalpy of Formation: -204.6 kJ/mol for zinc blend<br />DeltaG(wurtzite)=376700-191.9T kJ/mol<br />DeltaG(zinc blend)=374200-190.4T kJ/mol<br />
    87. 87. ZnS Nanobelt Mechanical Properties<br />(from Li et al “Mechanical Properties of ZnS Nanobelts)<br />Bulk hardness – 1.9 Gpa<br />Bulk elastic modulus – 75 Gpa<br />Nanobelt hardness – 3.4 +-.2 Gpa<br />Nanobelt elastic modulus – 35.9 +-3.5 GPa<br />
    88. 88. Four basic steps<br />Electrons tunnel from electronic states at the insulator/phosphor interface<br />Electrons are accelerated to ballistic energies<br />Electrons impact and ionize the luminescent center or create electron-hole pairs that lead to the activation of the luminescent center<br />Luminescent center relaxes toward the ground state and emits a photon<br />- Electrodes<br />- Insulator<br />- Phosphor<br />- Insulator<br />SUBSTRATE<br />- Electrodes<br />Electroluminescent Display Devices (ELD)<br />

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