1st International Symposium on Nanomanufacturing

Overview of and Nanotubes Mildred S. Dresselhaus MIT April 25, 2003

Al2O3

Pores Al Barrier Layer Motivation towards

• 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 that are quantum confined in one or more directions and exhibit properties different from their 3D counterparts D. O. S. D. O. S. D. O. S. D. O. S.

E E E E 3D 2D 1D 0D Bulk Semiconductor Quantum Well Quantum Wire Quantum Dot Nanotechnology R&D Funding in the USA

Fiscal year 2000 2001 2002 2003 (all in million $) ______National Science Foundation 97 150 199 221 Department of Defense 70 125 180 201 Department of Energy 58 88 91.1 139.3 National 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 5 Depart. of Transportation/FAA - 2 2 Department of Agriculture - 1.5 1.5 2.5 Department 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 Context – Nanotechnology in the World Government investments 1997-2002

2500 W. Europe Japan 2000 USA Others 1500 Total 1000

millions $ / year 500

0 1997 1998 1999 2000 2001 2002 Note: • 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)

Senate Briefing, May 24, 2001 (M.C. Roco), updated on February 5, 2002 A “Nano Tool-box”

To fabricate/probe nanostructures

Nanofabrication

Top-down Method Bottom-up Method - create nanostructures - self assembly of out of macrostructures atoms or molecules into nanostructures Various Nanostructures can occur in 1D Each have different structure/properties Selectivity is therefore important

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

Dresselhaus Group Peidong Yang K. Miki (MIT) UC Berkeley ETL,Japan Wiring on Si with ultra-fine Bi lines Structure different from 3D Bismuth

Bi nanoline

• Features: • over 300 nm long • 1 nm (3 Si dimers) wide • without kink K. Miki et al. Surf. Sci. 421 (1999) 397 • in terrace (not on top layer) Self-Assembled Nanopores in Alumina for growing nanowires/nanotubes

Al2O3

Pores SEM image of the surface of an anodic alumina Al template with self-assembled nanopore structure Barrier Layer

• Applications – Templates for ordered arrays of nanowires and nanotubes – 2D photonic crystal Nanowire array – High density magnetic storage media – Filters and gas sensors Densities of 1011 nanowires/cm2 Template Dissolution are achieved Free-standing wires Single Crystal Nanowires

X-ray diffraction of Bi nanowire arrays with different diameters. 10 nm TEM and electron diffraction of a 40-nm nanowire (Bi0.85Sb0.15) Quantum Confinement Produces New Materials Classes

• Bi • Bi nanowire – Group V element – Semimetal-semiconductor – Semimetal in bulk form transition at a wire diameter – The conduction band (L-electron) about 50 nm due to quantum overlaps with the valence band confinement effects (T-hole) by 38 meV

Semimetal Semiconductor

Decreasing wire diameter

Semimetal-Semiconductor Transition We utilize novel properties in applications Why nanostructures are useful for thermoelectrics

Difficulties in increasing ZT in bulk materials: Seebeck Coefficient Conductivity Temperature S ? ?? ? ? S 2 ? T ZT ? ? ? ?? S ? and ?? ? Thermal A limit to Z is rapidly obtained in Conductivity ? conventional materials ZT ~ 3 for conventional refrigerators ? 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

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

• 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. Alumina Templates on Silicon Wafers

thermal evaporation Al Si (100) Si (100) patterned conducting layer patterned conducting layer

electrochemical Al anodization

polishing Si (100)

patterned conducting layer Si (100) porous alumina

selective etch patterning deposition Si (100) Si (100) nanowires Advantages of the New Process

? The high-quality porous alumina films prepared on silicon substrates are rigid and easy to handle.

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

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

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

? Pores are accessible from both ends.

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

? The porous films were patterned 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.

deposit p-type nanowires cold-end junction deposited and patterned Si(100)

Si(100) POTENTIOSTAT

deposit n-type nanowires patterned titanium Si(100) back electrodes and Si(100) hot-end junction POTENTIOSTAT Structural Features

Oblique View Cross Section

Al Ti

Si For Comparison

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

2-Point Resistance Setup Seebeck Coefficient Measurement

V V V

Al

Si (100) Si (100)

Pt/Ti Heater Pt/Ti

Filled alumina template Filled alumina template

Silver paint contacts Gold film Thermocouple Growth of Semiconductor Nanowires by Vapor Liquid Solid (VLS) method

50 n m

5 ? m

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

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 10 nm are resolved, showing that wire growth occurs along this axis. Peidong Yang et al. ZnO Nanowires on Sapphire by VLS method

*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 of the nanowires, and (c) shows the faceted side-walls and the hexagonal cross-section of the nanowires. For nanowire growth, the sapphire substrates were coated with a 1.0 to 3.5nm thick patterned layer of Au as the catalyst, using a TEM grid as the shadow mask. These nanowires have been used for nanowire applications (Huang et al., 2001a). *Patterned growth can be arranged. *Proper selection of nanowires and substrate materials can led to facets, useful for nanowire . Nano-Lasers using ZnO Nanowires

Nanowire UV UV Laser Output

ZnO nanowires grown by VLS method. Excitation Intensity (a.u.)

370 380 390 400 Wavelength (nm)

Emission spectrum from ZnO nanowires. Peidong Yang et al Tunable Bandgap in Nanowires

InP nanowire

diameter ?

energy ?

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

M. S. Gudiksen et al., J. Phys. Chem B 106, 4036 (2002) Unique Properties of Carbon Nanotubes

Roll up

Graphene sheet SWNT

• 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 Field Emitter Applications for Nanotubes Scanning tips and Electronics

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

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

Imaging biological molecules

AFM image of Immunoglobulin G Fullerene C60 inside nanotubes resolved by nanotube tips

Rolling up graphene layer Nanotubes

armchair ? ??30

zigzag ? ??0

chiral 0?? ??30

(4,2) ? La d??n22??nmm ? ?? ? t ?? Ch ?na12??ma(nm,) ? 3m (n-m) = 3q metallic ?? ? tan? 1 ? (n-m) = 3q ?1 semiconducting ? 2nm? Synthesis S. Iijima, Nature 354 56 (1991) Arc Method: Y. Saito

• Laser Ablation • Arc Discharge

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

500torr He

50-120A DC A. Thess et al. Science Y. Saito et al, Phys. 273 483 (1996) Rev. 48 1907 (1993)

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 Single Nanotube Spectroscopy Resonant Raman spectra for isolated single-wall carbon nanotubes grown

on Si/SiO2 substrate by the CVD method A. Jorio et al., Phys. Rev. Lett. 86, 1118 (2001) RBM

' Metallic Semiconducting (n,m) identification

? RBM=248/dt Eii ? Elaser

Raman signal from one SWNT indicates a strong resonance process Otherwise it would not be possible to observe spectra SEPARATION PROCESS

Acid-treated SWNT material non-covalently functionalized with octadecylamine (ODA) and dispersed in tetrahydrofuran (THF) is immersed in a water bath to accelerate precipitation. Vacuum-dried precipitate and supernatant are studied. The ODA organization on metallic (M) SWNTs is selectively destabilized promoting their preferential precipitation, while the supernatant is enriched in semiconducting (S) SWNTs. What is the relative amount of precipitate and supernatant SWNT material compared to as-supplied sample ???

Separation method developed by Fotios Papadimitrakopolous of the University of Connecticut HiPco RAMAN Ar ion 514 nm (2.41 eV) 1.36 nm 1.04 nm 0.78 nm

M?S M M M S S S M S BWF S

as-supplied as-supplied (A) ?1 ?1 ? RBM ? dt ? ? G? ? 1517,1532,1564cm (exp. 1524,1548,1571cm ) Determination of nanotube diameter distribution from RBM Continuum 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 DISCRETE MODEL FOR RBM PROFILE as-supplied

L(????RBM,??pht)(,,)dd0 I()? ? ? 2 nm (Elaser?Eii?i?el)()Elaser??? RBM?Eiii??el

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) ?1 Elaser?????eV??? el??meV, ? ph???cm ?0=2.90eV, tight-binding & zone-folding ? Eii ?0 ? by 1% (? -? coupling at 1nm diameter SWNTs) A. G. Souza Filho et al (unpublished)

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

Diameter distribution d????? ??? ?????nm ? Ratio (M:S) ????????? Ratio (M:S) varies slightly with d? and ? and is around (M:S) ? ??? RBM mode at 514 nm (2.41 eV) as-supplied precipitate supernatant

78%

11% 89% 4.4% 96% 22%

M S S Comparison of RBM peak areas for E11 vs E33 & E22 : (M:S)RBM=89%:11% (M:S)RBM=96%:4.4% (M:S)RBM=22%:78% (M:S)tubes=36%:64% (M:S)tubes=60%:40% (M:S)tubes=1.9%:98% Nanotube ratio after compensating for resonance conditions SEPARATION EFFICIENCY

as-supplied precipitate supernatant RBM (M:S)=1:2 (M:S)=2.70:2 (M:S)=1:28.8

•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 RBM peaks of S tubes in as-supplied sample are very weak Determination of precipitate efficiency 2.70:2 is not accurate

Measurements at higher excitation laser energies are needed where both M and S tubes equally contribute to the spectra CONCLUDING REMARKS

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

• Research at present is mostly focused on demonstration stage

• Little effort has been given so far to integration into systems with 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