lecture 14 - stanford universitydionne.stanford.edu/matsci202_2011/lecture14_ppt.pdf · tunable...
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Lecture 14Lecture 14
Carbon-based & Semiconducting Nanomaterials
Suggested reading: FahlmanSuggested reading: Fahlman
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TriviaIn Group 14, all of the elements have at least one solid
h i h bi di d phase with a cubic diamond structure except:
PbSnGe
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TriviaIn Group 14, all of the elements have at least one solid
h i h di d bi phase with a diamond cubic structure except:
PbSnGe
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TriviaIn Group 14, all of the elements have at least one solid
h i h di d bi phase with a diamond cubic structure except:
PbSnGe
Legend has it that Napoleon’s armies were defeated in Russia because:Russia because:
Maps were misleading Horse hooves frozeCoat buttons crumbled
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TriviaIn Group 14, all of the elements have at least one solid
h i h di d bi phase with a diamond cubic structure except:
PbSnGe
Legend has it that Napoleon’s armies were defeated in Russia because:Russia because:
Maps were misleading Horse hooves frozeCoat buttons crumbled
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Tin Phase Changes
“Grey tin” or α-Sndi d bi l
“White tin” or β-Snl l • diamond cubic crystal structure
•dull grey powdery material • not stable at room temperature
• tetragonal crystal structure•Malleable, ductile •Metallic, density = 7.36 g/cm3 p
• not metallic, density = 5.77g/cm3
•Stable below 13.2oC
, y g•stable at and above room temperature
Transformation first recognized on organ pipes in medieval European cathedrals
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Allotropy and Carbon
Diamond: •Each C atom forms single
Graphite: •stacks of planar sp2 hybridized sheets•Each C atom forms single
bonds of length 1.54 Angstroms with 4
dj C
stacks of planar sp hybridized sheets•Each C atom has 3 nearest neighbors at 1.42 Angstroms
2 h b id bi l l i i adjacent C atoms•Measurement of thermal conductivity can identify
•sp2 hybrid orbital overlap gives rise to σ bonds between neighbors•Remaining perpendicular p orbitalsy y
fake specimensg p p p
overlap to form π bonds delocalized over the plane
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Trivia continues!
Conversion of diamond to graphite at room temperature is:g p p
a) Impossible
b) Spontaneous
c) Possible by doping with holes
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Trivia continues!
Conversion of diamond to graphite at room temperature is:g p p
a) Impossible
b) Spontaneous (ΔGo=-2.9 kJ/mol, but doesn’t occur at an observable rate)
c) Possible by doping with holes (nh=0.25/atom)H. Nakayama & H. Katayama-Yoshida, J. Phys.: Condens. Matter 15 (2003)
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Carbon-based Nanomaterials
Carbon allotropes require extreme synthetic techniques:• Laser vaporization (fullerenes & nanotubes)• Laser vaporization (fullerenes & nanotubes)
• Arc discharge methods (fullerenes & nanotubes)• Pyrolysis (fullerenes & nanotubes)
• Chemical vapor deposition (nanotubes)
The precursor (graphite) require significant dissociation p (g p ) q genergies prior to self-assembly (contains strong covalent
bonds)
How were they ever discovered?y
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Fullerenes: the celestial spheres that fell to earth
1980s: Experiments aimed at simulating the f b h d
Smalley Kroto
environment of a carbon-rich red giant star
S Soot + C60,70,76,84
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Applications
Superconductors: M C60MRI Contrast agents (Gd3+@C60-(OH)x)
Superconductors: M3C60
K3C60: Tc=18K, Rb3C60: Tc=29KCsRb2C60:Tc=33K
Drug delivery agents: Slow release of lung cancer drug release of lung cancer drug
Paclitaxel for liposome aerosol drug delivery
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Fullerene Nanomachines
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Large Scale Synthesis of Fullerenes (1989)
Huffman and Kratschmen: arc-evaporation of graphite
electrodes via resistive heating in an atmosphere of heating in an atmosphere of
~100 atm He
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Proposed Mechanism for Fullerene Growth
Ring: enhanced
Linear if 10
garomaticity/stab
ility of planar rings when 4n+2 n<10,
unless n=6
rings when 4n+2 π electrons
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Isolated Pentagon Rule
Buckyballs represent the smallest fullerene that obey the “isolated pentagon rule”:
• An energetic requirement that no pentagons share an edge • All pentagons surrounded by hexagons
•Calculations show that π bonds shared between 6-membered rings have large bong orders and positive resonance energies rings have large bong orders and positive resonance energies
high aromaticity
b d h d b t dj t 5 b d i h • π bonds shared between adjacent 5-membered rings have negative bond energies and small bond orders, leading to lower
thermodynamic stability
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Fi di d i 1991 b Iiji
Carbon Nanotubes
First discovered in 1991 by Iijima•Electric arcs from cylindrical electrodes produced multi-walled nanotubes (MWNTs) instead of fullerenes( )• In an attempt to fill the tubes with metal, he doped the electrode. Instead, got SWNTs.
TEM image of MWNTTEM image of MWNT
STM image of SWNT
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VLS CNT Growthi) C dissolves on molten catalytic
i lnanoparticleii) C precipitates on highly supersaturated
catalyst, forming C strings/polygonsy g g p ygiii) C nuclei form graphitic islands on the
catalyst surface, which aggregate
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VLS CNT Growth
iv) At low temperatures the graphitic iv) At low temperatures, the graphitic islands cannot lift off the catalyst surface, resulting in graphite-
l d l lencapsulated metal nanoclusters
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VLS CNT Growth
v) At high temperatures (500-1200oC) v) At high temperatures (500-1200 C), when the diameter of the island becomes ~1/2 that of the catalyst, the
hi i l lif ff h l graphitic nucleus lifts off the catalyst to form the SWNT endcap
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VLS CNT Growth
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VLS CNT Growth
NanObama
Vertically aligned SWNTs
MWNTs
aligned SWNTs
grown from square
regions of Fe regions of Fe catalysts
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Th l i l d i i f SWNT’ i f lli
Electrical Conductivity
The electrical conductivity of SWNT’s varies from metallic to semiconducting, depending on the way a graphene sheet is rolled:
Chirality Vector: C=na1+ma2 (n,m integers)D=nanotube diameter
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Wrapping (10,0) SWNT (zigzag)Wrapping (10,0) SWNT (zigzag)
(0,0)Ch = (10,0)
a1a2
y
a2 x
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Wrapping (10,0) SWNT Wrapping (10,0) SWNT (Animation)(Animation)(Animation)(Animation)
(0,0)Ch = (10,0)
a1a2
y
a2 x
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Wrapping (10,10) SWNT Wrapping (10,10) SWNT (armchair)(armchair)(armchair)(armchair)
(0,0)
Ch = (10,10)
a1a2
y
a2 x
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Wrapping (10,10) SWNT Wrapping (10,10) SWNT (Animation)(Animation)(Animation)(Animation)
(0,0)
Ch = (10,10)
a1a2
y
a2 x
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Wrapping (10,5) SWNT (chiral)Wrapping (10,5) SWNT (chiral)
(0,0)
Ch = (10,5)
a1a2
y
a2 x
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Wrapping (10,5) SWNT Wrapping (10,5) SWNT (Animation)(Animation)(Animation)(Animation)
(0,0)
Ch = (10,5)
a1a2
y
a2 x
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Electrical Conductivity
Zig-zag SWNTs: metallic when n/3 integer; semiconducting for all other ng g g gArmchair SWNTs: Always metallicChiral SWNTs: metallic when (2n+m)/3 integer; otherwise semiconducting
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Density of States
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Without dopants, the bandgap of semiconducting SWNTs can be Tunable electronic properties and CNT-FETs
tuned from 10 meV (~124 microns) to 1eV (1241 nm).
For CNTs with an average diameter of 1 4nm the ON current for a FET is 2100 For CNTs with an average diameter of 1.4nm, the ON current for a FET is 2100 microAmps per micron at VDS=VGS-VT=1.3V. The highest drive current in a p-CMOS
is 650 microAmps per micron. The transconductance (ratio of the output current variation to the input voltage ( p p gvariation) for a CNTFET is 2300uS/um, versis 650uS/um for a Si pCMOS FET. CNTFETS can outperform Si CMOS! (Challenge is controlling the synthesis)
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Optical Properties
Dai group, Stanford
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Mechanical Properties
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Size control
Semiconducting NanoparticlesSize control
Shape control
~ 5 nm
O ti l t lOptical control
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Bandgap increases from bulk value based on a 1/R2 confinement
Quantum Confinementg p
energy term. Also, must include Coulombic interaction of excitonsand correlation energy:
n2
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CdS l
Quantum Confinement
CdSe nanocrystals
Exciton Bohr Radii
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CdS, CdSe, ZnS, ZnSe, CdTe, ZnO, TiO2, etc.
Nanocrystal Synthesis 2
Example: CdSeDimethylcadmium is dissolved in a mixture of trioctylphosphine (TOP) and
trioctylphosphine oxide (TOPO).y p p ( )Solution is heated and vigorously stirred
Selenium source – usually Se dissolved in TOP or TOPO – is injected quickly and at room temperature
• widespread nucleation of TOPO-stabilized CdSe quantum dots•The room-temperature Se-TOP solution prevents further nucleation or growth
•Reaction can be heated for further growthg
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Nanocrystal-based PV
Some cool applications
40
60
7 x 56 nm
Nanocrystal based PV
Nanocrystal-based Photosynthesis
20
407 x 34 nm
EQE (%
)
0400 500 600 700
7 x 7 nm
Wavelength (nm)W. Huynh, J. Dittmer, A. P. Alivisatos, Science, 2002, 295, 2425.
Single Electron TransistorsTransistors