Lecture 14 - Stanford Universitydionne.stanford.edu/MatSci202_2011/Lecture14_ppt.pdf · Tunable electronic properties and CNT-FETs tuned from 10 meV (~124 microns) to 1eV (1241 nm).

Lecture 14Lecture 14

Carbon-based & Semiconducting Nanomaterials

Suggested reading: FahlmanSuggested reading: Fahlman

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

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

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

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

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

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

Trivia continues!

Conversion of diamond to graphite at room temperature is:g p p

a) Impossible

b) Spontaneous

c) Possible by doping with holes

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)

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

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

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

Fullerene Nanomachines

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

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

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

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

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

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

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

VLS CNT Growth

VLS CNT Growth

NanObama

Vertically aligned SWNTs

MWNTs

aligned SWNTs

grown from square

regions of Fe regions of Fe catalysts

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

Wrapping (10,0) SWNT (zigzag)Wrapping (10,0) SWNT (zigzag)

(0,0)Ch = (10,0)

a1a2

y

a2 x

Wrapping (10,0) SWNT Wrapping (10,0) SWNT (Animation)(Animation)(Animation)(Animation)

(0,0)Ch = (10,0)

a1a2

y

a2 x

Wrapping (10,10) SWNT Wrapping (10,10) SWNT (armchair)(armchair)(armchair)(armchair)

(0,0)

Ch = (10,10)

a1a2

y

a2 x

Wrapping (10,10) SWNT Wrapping (10,10) SWNT (Animation)(Animation)(Animation)(Animation)

(0,0)

Ch = (10,10)

a1a2

y

a2 x

Wrapping (10,5) SWNT (chiral)Wrapping (10,5) SWNT (chiral)

(0,0)

Ch = (10,5)

a1a2

y

a2 x

Wrapping (10,5) SWNT Wrapping (10,5) SWNT (Animation)(Animation)(Animation)(Animation)

(0,0)

Ch = (10,5)

a1a2

y

a2 x

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

Density of States

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)

Optical Properties

Dai group, Stanford

Mechanical Properties

Size control

Semiconducting NanoparticlesSize control

Shape control

~ 5 nm

O ti l t lOptical control

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

CdS l

Quantum Confinement

CdSe nanocrystals

Exciton Bohr Radii

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

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