Nanostructured Materials - Synthesis, Methods, Properties & Applications
MATERIALS SYNTHESISlawm/263 5.pdf · 2019-02-22 · 374 MATERIALS SYNTHESIS Bulk Materials •Solid...
Transcript of MATERIALS SYNTHESISlawm/263 5.pdf · 2019-02-22 · 374 MATERIALS SYNTHESIS Bulk Materials •Solid...
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MATERIALS SYNTHESISBulk Materials• Solid state synthesis – “shake & bake”• Chemical vapor transport• Sol Gel Synthesis• Melt growth
Thin Films• Chemical Vapor Deposition• Laser Ablation• Sputtering• Molecular Beam Epitaxy
Nanomaterials• Nanocrystals• Carbon Nanotubes• Nanowires
Just a few of the huge number of different methods!
Reading: West 4
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SOLID STATE SYNTHESISMix starting materials, heat, compress …. presto!• Historically the most common method to make polycrystalline samples.
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SOLID STATE SYNTHESISrelies on enhanced solid-state diffusion at high temps.
4MgO + 4Al2O3 → 4MgAl2O4
1. area of contact between the reacting solids
2. the rate of nucleation of the product phase
3. the rates of ion diffusion through the various phases
2Al3+ - 3Mg2+ + 4MgO → MgAl2O4
3Mg2+ - 2Al3+ + 4Al2O3 → 3MgAl2O4
at MgO/MgAl2O4 interface:
at Al2O3/MgAl2O4 interface:
factors:
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CHEMICAL VAPOR TRANSPORT (CVT)Crystal growth by the thermal transport of volatile compounds generated from nonvolatile elements or compounds.
• Popularized by Schafer (1971).
1. synthesis of new compounds2. growth of large single crystals3. purification of compounds
+ transport agent (often I2)
tube furnace
sealed quartz tube
Ti(s) + 2I2(g) TiI4(g) Ti + S2 TiS2
TiI4(g) + S2(g) TiS2(s) + 2I2(g)
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Ti (using I2)
Au (using I2) Typical transporting agents include:I2, Br2, Cl2, HCl, NH4Cl, H2, H2O, TeCl4, AlCl3, CO, S2
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SOL GEL (solution-gelation) PROCESSLow-temp, solution phase method that uses hydrolysis and polycondensation reactions to form an inorganic network solid.
• makes ceramic and glassy materials, usually from M(OR)x solutions.
M-OR + H2O M-OH + ROH (hydrolysis)M-OR + M-OH [M-O-M]n + ROH (condensation)
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SiO2 from tetraethyl orthosilicate (TEOS)
sol gel
acid-catalyzedreaction
mechanism:
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aerogels
xerogel containing CdSe/CdS quantum
dots
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MELT GROWTHCrystallization from the liquid, without solvent.
→ Production of semiconductor grade silicon from silica (sand).
The Siemens process
SiO2 + 2C Si + 2CO2250°C
Si + 3HCl SiHCl3 + H2300°C
98-99%(metallurgical grade)
2SiHCl3 Si + 2HCl + SiCl41150°C
99.999999999%(semiconductor grade)
Siemens process makes polycrystalline SG-Si
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Czochralski (cho-HRAL-ski) process
CZ-Si
makes single crystal ingots from polycrystalline SG-Si
• good for making doped Si
• suffers from O and C impurities from the crucible
>1420°C
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Bridgman-Stockbarger and float zone techniques
• melt touches nothing→ fewer O and C impurities
• FZ ingots limited to ~150 mm
makes use of the greater solubility of impurities in the melt than the crystal
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SAND TO SILICON
H. Föll
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CHEMICAL VAPOR DEPOSITION (CVD)Reactive thermolysis of volatile molecular precursors to grow a thin film layer.
a hot wall low-pressure CVD (LP-CVD) reactor:
• metals• alloys• oxides, nitrides, carbides• semiconductors (III-V, II-VI)• carbon (even diamond!)• many others
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TYPES OF CVD
• atmospheric pressure CVD (AP-CVD)• LP-CVD >10-6 Torr• ultrahigh vacuum (UHV-CVD) <10-8 Torr
by pressure:
by reactor type:• hot wall• cold wall
• plasma enhanced CVD (PE-CVD)• laser-assisted CVD (LA-CVD)• hot-wire CVD (HW-CVD)
by precursor degradation method: hot wire system
plasma system
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PE-CVD for SiGe growth
Hot wire CVD for a-Si
Danny Chrastina
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EXAMPLESpolysilicon
SiH4 → Si + 2H2 LP-CVD, 600-1000°C, 10-100s of nm min-1
silicon nitride (Si3N4)
3SiH4 + 4NH3 → Si3N4 + 12H2 LP-CVD, 600-800°C, 10s of nm min-1
gallium arsenide (GaAs)
Ga(Me)3 + AsH3 → GaAs + 3CH4 LP-CVD, 400-800°C in H2 carrier gas
• example of metal organic CVD (MO-CVD)
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IMPORTANT STEPS IN CVD
physisorption, chemisorption → reaction → nucleation → growth
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GaAs: Overall Reaction Pathway (350-500°C)
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PULSED LASER DEPOSITION (PLD)Stoichiometric mass transfer by ablation of a solid target using a pulsed laser in HV or UHV.
good for materials of complex stoichiometry
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Vapor-Liquid-Solid Nanowire Growth
metal catalysts
alloyliquid
vapornanowire
Alloying
Nucleation
Growth
I II III
I
IIIII
Unidirectional growth is the consequence of an anisotropy in solid-liquid interfacial energy.Y. Wu et al. J. Am. Chem. Soc. 2001, 123, 3165
800 deg. In-situ TEM
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Nanowires grown by PLD
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Korgel Group, UT, Austin, Department of Chemical Engineering
Solution-liquid-solid (SLS) nanowire growth
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SPUTTERINGBombardment of high-energy ions (usually argon) ejects atoms from a negatively-charged source to create a thin film coating.
• vacuum (10-7 T), line-of-sight method
• argon ions created by high voltage dc or rf plasma
• produces hard, dense coatings
negative
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MOLECULAR BEAM EPITAXY (MBE)UHV technique for producing very high quality epitaxial layers with monolayer thickness control.
• ultrahigh vacuum (10-10 Torr), medium temp, line-of-sight method
• UHV gives long mean free path → “molecular beams” and ultrapure crystals
• slow growth rate (1 ML s-1) promotes extremely high quality crystals
• very expensive
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Brookhaven Nat’l Lab
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Reflection high-energy electron diffraction (RHEED)
3-100 keV electrons used to monitor monolayer growth
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• Ewald sphere crosses many lattice rods, resulting in many spots separated by small diffraction angle differences. However, few of these are close to the grazing exit angles, so only a few spots are seen on the RHEED screen.
• The crossing of Ewald sphere with lattice rod can be very poorly defined RHEED spots are broadened into vertical streaks
wave vector of 50 keV electron ~120 Å-1 large Ewald sphere
no diffraction condition in 3rd dimension reciprocal lattice rods
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SAFIRE
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GaAsAlAs
SC multilayer stack – “multiple quantum well” – grown by MBE
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QUANTUM WELLS
• can design transition energies• high radiative efficiency• low laser thresholds• low surface recombination
A finite potential well with only discrete energy levels.
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MULTIJUNCTION SOLAR CELLS
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410William R. Wiley Environmental Molecular Sciences Laboratory (PNNL)
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SEMICONDUCTOR QUANTUM DOTSNanometer size crystals with size-dependent properties.
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Synthesis of colloidal QDs
http://nanocluster.mit.edu/research.php
Synthesis Goals:• monodispersity• crystallinity• size control• shape control• doping• surface passivation• colloidal stability
crystalline inorganic core
oily organicligand shell
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COLLOIDAL SC QUANTUM DOTS
oleate-capped QDs
PbO + 2OA → Pb(OA)2 + H2O
Pb(OA)2 + TOP-Se → PbSe+ by-products
for PbSe quantum dots100-350°C
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Hot injection method
Temp
Time
inject TOP-Se
nucleation
growthquench
Pb(OA)2 + (C8H17)3PSe PbSe dotsODE, 180°C
5% polydispersity(unit cell roughness)
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Native surface ligands
Oleic acid
TOPO DDT
Functions:• control size and shape• prevent aggregation• passivate surface states strong PL• doping?• isolate QD from its environment (chemical stability)
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QUANTUM CONFINEMENT
energy gap depends on size - “particle in a box”
discrete e- transitions -“artificial atoms”
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smaller dots → larger bandgap → bluer absorption & emission
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MULTIPLE EXCITON GENERATION
larger currents for solar cells
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NUCLEATIONNucleation = localized creation of a distinct thermodynamic phase.
simple examples: liquid in gas, solid in liquid, solid in gas, ZB in WZ
heterogeneous nucleation: nucleation assisted by a nucleation site, usually a surface (e.g., substrate, suspended particle, dust, etc.)
homogeneous nucleation: nucleation without a nucleation site. Harder to achieve than heterogeneous nucleation.
The driving force for nucleation is the lowering of free energy.
e.g., freezing
G = H – TS
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Freezing of supercooled liquids is thermodynamically favoredbut kinetically hindered
SUPERCOOLINGsupersaturationenergeticskinetics
Nucleationand
Growth
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NUCLEATION and GROWTHconsider a spherical amorphous solid nucleating from a homogeneous supersaturated liquid:
3 24( ) 43 vG r G r r
energy liberatedby forming volume (-)
energy required to form interface (+)
critical nucleus r*: * 2
v
rG
r<r* - nucleus dissolvesr>r* - nucleus grows
volume free energyinterfacial free energy
vG
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OSTWALD RIPENING
Initially the solid particles draw their material from the solution/melt. Later, as the solution/melt becomes depleted, the particles compete with
each other for growth. In Ostwald ripening, small particles shrink and supply atoms to larger particles, which grow.
time
thermodynamically driven ; atoms on surface are less stable than atoms in the bulk b/c lower coordination number (poorer bonding), so system can lower energy by reducing surface area.
1
time
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OSTWALD RIPENINGconsider two spherical nuclei w/ N1 and N – N1 atoms (N constant):
with atomic volume, 2/3
2/3 2/31 1 1 1
3( , ) 4 ( )4vG N N N N G N N N
minimum G occurs when the two equal spheres combine into one big sphere:
nucleus 1
dissolves grows
2/31/3 2/334 (2 1)
4G N
423
N
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CARBON NANOTUBES: ARC DISCHARGE
424
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LASER ABLATION of CNTs
425
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CVD of CNTs
426
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427
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Science 19 November 2004:Vol. 306. no. 5700, pp. 1362 - 1364
Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes
428
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CNT DEVICES: FIELD EFFECT TRANSISTOR
429
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CNT-BASED LOGIC
430
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CNT FIELD EMITTERS
431
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432