Overview of Overview of NanowiresNanowires and and NanotubesNanotubes

Mildred S. DresselhausMITApril 25, 2003

1st International Symposium on Nanomanufacturing

Al

Al2O3

Barrier Layer

Pores

Motivation towards Nanotechnology

• Device miniaturization, reducing physical sizes• Exploiting enhanced surface effects by increased

surface/volume ratio (e.g. catalysts)• Utilization of biological objects in inorganic

nanostructures for various sensors and novel functions

• Novel phenomena in low-dimensional structures not observed in their bulk counterparts

Science Introduction

• Nanostructures (< 30 nm) have become an exciting research field– New quantum phenomena occur at this length scale– New structure – property relations are expected– Promising applications are expected in optics, electronics, thermoelectric,

magnetic storage, NEMS (nano-electro-mechanical systems)

• Low-dimensional systems are realized by creating nanostructures thatare quantum confined in one or more directions and exhibit properties different from their 3D counterparts

3DBulk Semiconductor

2DQuantum Well

1DQuantum Wire

0DQuantum Dot

E

D. O

. S.

D. O

. S.

D. O

. S.

D. O

. S.

E E E

Nanotechnology R&D Funding in the USAFiscal year 2000 2001 2002 2003

(all in million $)__________________________________________________________________________________________________________________________________________________________________

National Science Foundation 97 150 199 221Department of Defense 70 125 180 201Department of Energy 58 88 91.1 139.3National Institutes of Health 32 39.6 40.8 43.2 NASA 5 22 46 51 NIST 8 33.4 37.6 43.8 Environmental Protection Agency - 5.8 5 5Depart. of Transportation/FAA - 2 2Department of Agriculture - 1.5 1.5 2.5Department of Justice - 1.4 1.4 1.4

TOTAL 270.0 466.7 604.4 710.2

Other NNI participants are: DOC, DOS, DOTreas, NOAA, NRC, USG

M.C. Roco, NSF, 4/30/02

0

500

1000

1500

2000

2500

1997 1998 1999 2000 2001 2002

mil

lion

s $

/ ye

ar

W. Europe

Japan

USA

OthersTotal

Senate Briefing, May 24, 2001 (M.C. Roco), updated on February 5, 2002

• U.S. begins FY in October, six month before EU & Japan in March/April

• U.S. does not have a commanding lead as it was for other S&T megatrends(such as BIO, IT, space exploration, nuclear)

Context Context –– Nanotechnology in the WorldNanotechnology in the WorldGovernment investments 1997Government investments 1997--20022002

Note:

A “Nano Tool-box”

To fabricate/probe nanostructures

Nanofabrication

Top-down Method

- create nanostructures out of macrostructures

Bottom-up Method

- self assembly of atoms or molecules into nanostructures

(a) Bi Nanowire (b) Bi Nanotube (c) Bi Atomic Line

Various Nanostructures can occur in 1DEach have different structure/properties

Selectivity is therefore important

K. MikiETL,Japan

Peidong YangUC Berkeley

Dresselhaus Group(MIT)

Wiring on Si with ultra-fine Bi linesStructure different from 3D Bismuth

Bi nanoline

• Features:• over 300 nm long• 1 nm (3 Si dimers) wide• without kink• in terrace (not on top layer)K. Miki et al. Surf. Sci. 421 (1999) 397

Self-Assembled Nanopores in Aluminafor growing nanowires/nanotubes

• Applications– Templates for ordered arrays of

nanowires and nanotubes– 2D photonic crystal– High density magnetic storage media– Filters and gas sensors

Al

Al2O3

Barrier Layer

Pores

Free-standing wires

Nanowire array

SEM image of the surface of an anodic aluminatemplate with self-assembled nanopore structure

Template DissolutionDensities of 1011 nanowires/cm2

are achieved

Single Crystal Nanowires

10 nm

TEM and electron diffraction of a 40-nm nanowire (Bi0.85Sb0.15)

X-ray diffraction of Bi nanowire arrays with different diameters.

Quantum Confinement Produces New Materials Classes

• Bi – Group V element– Semimetal in bulk form– The conduction band (L-electron)

overlaps with the valence band (T-hole) by 38 meV

Decreasing wire diameter

Semimetal Semiconductor

• Bi nanowire– Semimetal-semiconductor

transition at a wire diameter about 50 nm due to quantum confinement effects

SemimetalSemimetal--Semiconductor TransitionSemiconductor TransitionWe utilize novel properties in applications

Why nanostructures are useful for thermoelectrics

ZTS T

?2??

Seebeck Coefficient ConductivityTemperature

ThermalConductivity

ZT ~ 3 for conventional refrigerators

Difficulties in increasing ZT in bulk materials:

S ? ? ? ? ?

? ? ? ? S ? and ??

? A limit to Z is rapidly obtained in conventional materials

? So far, best bulk material (Bi0.5Sb1.5Te3)has ZT ~ 1 at 300 K

Low dimensions give independent control of variables:? Enhanced density of states due to quantum confinement effects

? Increases S without reducing ?? Boundary scattering at interfaces reduces ? more than ?? Possibility of carrier pocket engineering to further improve ZT

Anodic Alumina on Silicon Wafers -Motivation

• Elimination of unfavorable fabrication steps.• Rigid substrate, affords extended crack-free films.• Low surface roughness.• Solution to the barrier layer problem for electrical connects.• Incorporation into electronic, optical devices on the wafer.• Combined nano-scale and micro-scale patterning.

Fabrication of the porous alumina templates on silicon substrates is an appealing improvement over the conventional fabrication process for the following reasons:

Alumina Templates on Silicon Wafers

Si (100)

patterned conducting layer

thermal evaporation

Si (100)

patterned conducting layer

electrochemical

Si (100)

patterned conducting layer

Al

polishing

anodization

Si (100)

Si (100)

deposition

porous alumina

nanowires

Al

selective etch patterning

Si (100)

Advantages of the New ProcessAdvantages of the New Process

? The highhigh--qualityquality porous alumina films prepared on silicon substrates are rigidrigid and easy to handleeasy to handle.

? This method affords very large arealarge area films (70 cm2) with a very flat interfaceflat interface.

? Films were grown on conducting (Ti) and insulating (SiO2) substrates.

? Both adhesion to adhesion to andand separation from separation from the substrate are possible.

?? Pores are accessible from both endsPores are accessible from both ends.

?? Growth ofGrowth of nanowiresnanowires in the templates was demonstrated by two different methods.

? The porous films were patternedpatterned using lithography techniques.

Towards Multi-Component Nanowire Arrays

By means of lithography, a design allowing deposition of both p-type and n-type components on the same support could be achieved.

patterned titaniumback electrodes

andhot-end junction

deposit p-type nanowirescold-end junction

deposited and patterned

Si(100)

Si(100)

Si(100)

POTENTIOSTAT

Si(100)

POTENTIOSTAT

deposit n-type nanowires

Structural Features

Ti

Si

Al

Oblique View Cross Section

The new Al2O3/Si structure conserves the dense uniform porous morphology with long channels, but consists of a thinner barrier layer.

ForComparison

Characterization of Characterization of NanowiresNanowires ArraysArrays

Filled alumina template

Silver paint contacts

Al

Si (100)

Pt/Ti

V

2-Point Resistance Setup Seebeck Coefficient Measurement

V

V

Si (100)

Pt/TiHeater

Filled alumina template

Gold film

Thermocouple

Growth of Semiconductor Nanowiresby Vapor Liquid Solid (VLS) method

a) FESEM image of GaP nanowires. The inset is a TEM image of the end of one of these wires.

b) TEM image of a GaP wire. The [111] lattice planes are resolved, showing that wire growth occurs along this axis.

5 ? m

50 n m

10 nm

Peidong Yang et al.

Laser ablation overcomes thermodynamic equilibrium constraints, and enables liquid nanocluster formation.

*SEM images of ZnO nanowire arrays grown on a sapphire substrate, where (a)shows patterned growth, (b) shows a higher resolution image of the parallel alignment ofthe nanowires, and (c) shows the faceted side-walls and the hexagonal cross-section of thenanowires. For nanowire growth, the sapphire substrates were coated with a 1.0 to 3.5nmthick patterned layer of Au as the catalyst, using a TEM grid as the shadow mask. Thesenanowires have been used for nanowire laser applications (Huang et al., 2001a).*Patterned growth can be arranged.*Proper selection of nanowires and substrate materials can led to facets, useful for nanowire lasers.

ZnO Nanowires on Sapphire by VLS method

Nano-Lasers using ZnO Nanowires

ZnO nanowires grown by VLS method.

Emission spectrum from ZnO nanowires.

Peidong Yang et al

Wavelength (nm)370 380 390 400

Inte

nsity

(a.u

.)

Nanowire UV Nanolaser

UV Laser Output

Excitation

Tunable Bandgap in Nanowires

InP nanowire

diameter ?

energy ?

M. S. Gudiksen et al., J. Phys. Chem B 106, 4036 (2002)

? light emission canoccur at a p-n junction from the same material

Unique Properties of Carbon Nanotubes

• Size: Nanostructures with dimensions of ~1 nm diameter (~20 atoms around the cylinder)

• Electronic Properties: Can be either metallic or semiconducting depending on the diameter or orientation of the hexagons

• Mechanical: Very high strength and modulus. Good properties on compression and extension

• Heat pipe and electromagnetic pipe• Single nanotube spectroscopy yields structure• Many applications are being attempted worldwide

Graphene sheet SWNT

Roll up

Applications for NanotubesScanning tips and Electronics

• STM/AFM tips• Direct Analysis of DNA• Semiconductor devices• Field Emitters

S.J. Tans et al. Nature, 393, 49 (1998)

AFM image of Immunoglobulin G resolved by nanotube tips

Field Emitter

Transistor

Imaging biological molecules

New Materials

Fullerene C60 inside nanotubes

2 2

1 21

( , )3

tan2

t

h

L ad n nm m

C na ma n mm

n m

? ?

? ?

? ? ? ? ???? ? ? ?? ?? ??

? ? ?

(4,2)

Rolling up graphene layer

0? ? ?

0 30?? ? ?

30? ? ?armchair

zigzag

chiral

Nanotubes

(n-m) = 3q metallic(n-m) = 3q ?1 semiconducting

SynthesisS. Iijima, Nature 354 56 (1991)

• Arc Discharge• Laser Ablation

5-20mm diameter carbon rod Nd-Yb-Al-garnet Laser, 1200?

Y. Saito et al, Phys. Rev. 48 1907 (1993)

A. Thess et al. Science273 483 (1996)

Arc Method: Y. Saito

50-120A DC

500torr He

Challenges for Carbon Nanotube Research

• Control synthesis process to produce tubes with same diameter and chirality

• Until control of synthesis process is achieved, develop effective separation methods:? metallic from semiconducting? by diameter? by chirality

• Develop method for large-scale, cheap synthesis• Improve nanotube characterization and manipulation • Develop commercial scale applications

Resonant Raman spectra for isolated single-wall carbon nanotubes grown on Si/SiO2 substrate by the CVD method

RBM

SemiconductingMetallic

(n,m) identification? RBM=248/dtEii ? Elaser

A. Jorio et al., Phys. Rev. Lett. 86, 1118 (2001)

Raman signal from Raman signal from one one SWNT indicates a strong resonance processSWNT indicates a strong resonance processOtherwise it would not be possible to observe spectraOtherwise it would not be possible to observe spectra

'

Single Nanotube SpectroscopySingle Nanotube Spectroscopy

SEPARATION PROCESSSEPARATION PROCESS

AcidAcid--treated SWNT material nontreated SWNT material non--covalentlycovalently functionalizedfunctionalizedwith with octadecylamineoctadecylamine (ODA) and dispersed in (ODA) and dispersed in tetrahydrofurantetrahydrofuran(THF) is immersed in a water bath to accelerate precipitation.(THF) is immersed in a water bath to accelerate precipitation.VacuumVacuum--dried precipitate and supernatant are studied.dried precipitate and supernatant are studied.

The ODA organization on metallic (M) SWNTs is selectivelyThe ODA organization on metallic (M) SWNTs is selectivelydestabilized promoting their preferential precipitation, whiledestabilized promoting their preferential precipitation, whilethe supernatant is enriched in semiconducting (S) SWNTs.the supernatant is enriched in semiconducting (S) SWNTs.

What is the relative amount of precipitate and supernatantWhat is the relative amount of precipitate and supernatantSWNT material compared to asSWNT material compared to as--supplied samplesupplied sample ??????

Separation method developed by Fotios Papadimitrakopolousof the University of Connecticut

Determination of nanotube diameter distribution from RBMDetermination of nanotube diameter distribution from RBM

HiPcoHiPco RAMAN RAMAN ArAr ion 514 nm (2.41 eV)ion 514 nm (2.41 eV) 1.36 nm1.04 nm0.78 nm

MM BWFBWF

asas--suppliedsupplied asas--suppliedsupplied? RBM ? dt ? ? G? ? 1517,1532,1564cm?1 (exp. 1524,1548,1571cm?1 )

Continuum approximationContinuum approximation:: H. Kuzmany et al EPJ B 22 307 (2001)The Eii bands are treated as a continuum of states rather than individual VHSs. This model results in

SS SS

MM?? SS

SS SSMMMM

MM

(A)

DISCRETEDISCRETE MODEL FOR RBM PROFILEMODEL FOR RBM PROFILEasas--suppliedsupplied

02

( , ) ( , , )( )

( ) ( )RBM ph t

n m laser ii el laser RBM ii el

L d dI

E E i E E i

? ? ??

?

? ? ??

? ? ? ? ? ? ??

?

Gaussian distribution for tube diameters:? ?dt?d0?? ??exp[??dt?d0?????? ????and ? ??FWHM????ln??

Lorentzian lineshape for phonons: L?? ,? ??????[4? ???? ??Integrated over energy, assuming constant matrix elements

? RBM??239cm??nm/dt????cm?? (fitted for HiPco)A. Kukovecz et al EPJ B 28 223 (2002)Elaser?????eV??? el??meV, ? ph???cm?1

?0=2.90eV, tight-binding & zone-folding ? Eii?0 ? by 1% (? -? coupling at 1nm diameter SWNTs)

A. G. Souza Filho et al (unpublished)

Diameter distribution dDiameter distribution d???? ? ?? ??? ???????? ????????nm nm ?? Ratio (M:S) Ratio (M:S) ????? ????????? ????Ratio (M:S) Ratio (M:S) varies slightly with dvaries slightly with d?? and and ?? and is around and is around (M:S) (M:S) ?? ??????

Elaser ?????? and ?????eV are needed for more precise d??? determination

RBM mode at 514 nm (2.41 eV)RBM mode at 514 nm (2.41 eV)

precipitateprecipitate supernatantsupernatantasas--suppliedsupplied

11%11% 4.4%4.4%

78%78%

89%89% 96%96% 22%22%

(M:S)(M:S)tubestubes=36%:64% (M:S)=36%:64% (M:S)tubestubes=60%:40% (M:S)=60%:40% (M:S)tubestubes=1.9%:98%=1.9%:98%Nanotube ratio after Nanotube ratio after compensatingcompensating for resonance conditionsfor resonance conditions

Comparison of RBM peak areas for EComparison of RBM peak areas for E1111 vsvs EE3333 & E& E22 22 ::(M:S)(M:S)RBMRBM=89%:11% (M:S)=89%:11% (M:S)RBMRBM=96%:4.4% (M:S)=96%:4.4% (M:S)RBMRBM=22%:78%=22%:78%

MM S SS S

SEPARATION EFFICIENCYSEPARATION EFFICIENCY

asas--suppliedsupplied precipitateprecipitate supernatantsupernatantRBM RBM (M:S)=1:2 (M:S)=2.70:2 (M:S)=1:28.8(M:S)=1:2 (M:S)=2.70:2 (M:S)=1:28.8

RBMRBM peaks of S tubes in aspeaks of S tubes in as--supplied sample are very weaksupplied sample are very weakDetermination of precipitate efficiency Determination of precipitate efficiency 2.70:22.70:2 is not accurateis not accurate

Measurements at higher excitation laser energies are neededMeasurements at higher excitation laser energies are neededwhere both M and S tubes equally contribute to the spectrawhere both M and S tubes equally contribute to the spectra

•Enhancement in semiconducting content is by a factor of ~14 in supernatant•Enhancement in metallic content is by a factor of ~2.7 in precipitate

CONCLUDING REMARKSCONCLUDING REMARKS

• Research on nanotubes and nanowires show much interestingscience with potential for applications

• Research at present is mostly focused on demonstration stage

• Little effort has been given so far to integration into systemswith large scale production

• Synthesis control of components, integration of components and scale-up all present major challenges

Raman Spectroscopy of Carbon Nanotubes

• Not destructive, contactless, measurement– Room Temperature– In Air at Ambient Pressure– Quick (1min), Accurate in Energy

• Diameter Selective (Resonant Raman Effect)• Diameter and Chirality dependent phonons

– Characterization of (n,m)– Related Low Dimensional Physics