9a. Nanotechnology

Definition nanotechnology:

“Th nd r t ndin & ntr l f m tt r t dim n i n f 1 100 nm h r niq“The understanding & control of matter at dimensions of ~1-100 nm, where unique phenomena enable novel applications”

But when does the history of nanotechnology start, or has it even started yet?

1959: Famous lecture by Nobel Laureate Richard Feynman “There’s Plenty Room at the Bottom”

Very respected and clever scientist who, e.g., developed quantum electrodynamics and was recruited to the Manhattan project as a 25-year-old

Still, when he proposed manipulating individual atoms to make new small structures with very different properties, he was met with skepticism…

Today, his prophetic lecture, in which he, e.g., predicted the discovery of electron-beam etching & nano-sized circuits in powerful computers, is legendary!

9a. Nanotechnology

1700s-1800s: (Traditional) film photography has its basis in a nano-sized material!basis in a nano-sized material!

Specifically: photographic film consists of emulsion of Ag-halide particles in gelatin g p gdeposited on transparent & flexible (polyester) substrate

When light hits (“exposes”) Ag-halide particles: they decompose and Ag nano-particles form. Subsequently “developed” in chemical solution & dark (silver-particle-rich) regions formed where light hit (the “negative” effect)

Fi f ll d l h hFirst to successfully produce color photograph: James Clark Maxwell (i.e. the same guy who developed the electromagnetic theory!)

9a. Nanotechnology

1661: Irish-born chemist Robert Boyle (“the father of modern chemistry”) criticizes Greek philosopher Aristotle’s antique belief that matter composed of 4 elements: Earth fire water and airelements: Earth, fire, water and air

Writes “The Sceptical Chymist”, in which he instead suggests that tiny particles of matter combine in

ri t f rm “ rp l ”various ways to form “corpuscles” (≈ nano-particles)

9a. Nanotechnology

~400 AD: Roman glass makers manage to fabricate unique type of fascinating glass One famous example: The Lycurgustype of fascinating glass. One famous example: The Lycurgus cup (resides in British museum in London)

Made from standard soda-lime glass (SiO2 + NaO + CaO), g ( 2 ),

but with small amount (ppm-range) of {Au+Ag} nano-particles(~70 nm in diameter) mixed in

When viewed in reflected light (eg daylight) cup appears green

But when a light is shone into the cup and transmitted through the glass, it appears red, due to the scattering effect of the {Au+Ag} nano-particles (as recently detected with modern STM technique)q )

Only a handful of ancient glasses showing this effect are known, all of them Roman

9a. Nanotechnology

Beginning of life on Earth (~4 billion years ago!):ago!):

E.g., red abalone (seaweed-eating snail) creates its own extremely strong & colorful shell by

i i C CO i d b i k h ldorganizing CaCO3 into nano-structured bricks held together by carbohydrate-protein glue

Remember that bulk CaCO3 is the material of soft and brittle chalk!

∴ Mechanical∴ Mechanical properties of nano-structures can be completely differentcompletely differentthan for macroscopic materials

9a. Nanotechnology

Nano-structured bricks (formed from evenly spaced nucleation sites) on top of which stacks of hexagonalnucleation sites) on top of which stacks of hexagonal “tiles” of CaCO3 grow in Christmas tree-like pattern

Top & bottom surfaces of each layer of tiles (but not the edges) separated and bound together by (carbohydrate-protein) glue

Under stress tiles of CaCO can slide laterally & absorbUnder stress, tiles of CaCO3 can slide laterally & absorb energy. Due to its structure, the abalone shell is capable of absorbing great deal of energy without failing (shattering)

Currently an inspiration for the development of new types of ceramic materials for use as bullet-stopping armor

The abalone shell investigation is one of a growingThe abalone shell investigation is one of a growing number of science-mimicking-nature, so-called biomimetic, projects

9a. Nanotechnology

Ok, nanotechnology not new branch of science, but field in which current scientific activity is enormous due to promise of new & exciting phenomenascientific activity is enormous due to promise of new & exciting phenomena and applications

Remember definition:

“The understanding & control of matter at dimensions of ~1-100 nm, where unique phenomena enable novel applications”

Herein, one representative topic from the two areas defining nanotechnology will be presented:

1. The control and patterning of matter on the nano-scale: The scanning probe microscope

2 U d di i h h l h bl l2. Understanding new unique phenomena on the nano-scale that enable novel applications: The quantization of conductance at the nano-level and the realization of nano-wires and nano-sized circuits

9a. Nanotechnology

The Atomic Force Microscope (AFM)

I d 1985 G d Bi i ( i f iInvented 1985: Gerd Binnig (co-inventor of scanning tunneling microscope, STM) and Calvin Quate from California, Christoph Gerber from Zurich, SwitzerlandSwitzerland

AFM technological pinnacle of microscopy: provides true 3D surface profile on nm-scale, no sample pre-treatment required, works in ambient & liquid environments

→ excellent tool for studying biological processes in excellent tool for studying biological processes in action at molecular level:

E.g. Paul Hansma (and colleagues) succeeded in b i bl d l i i hi bl d llobserving blood-clotting process within blood cells,

and presented findings in 33-min movie, assembled from AFM pictures taken every 10 s!

9a. Nanotechnology

The imaging AFM

AFM consists of a cantilever with a sharp tip at its end (typically composed of Si or SiN) with a tip size (radius of curvature) ~nm-range

Tip is brought into close proximity of sample surfacesample surface…

…Repulsive force between tip and sample (at close distance) leads to deflection of ( )cantilever according to Hooke’s law (F = - kx)

D fl ti d b l b hi hDeflection measured by laser beam, which is reflected from top of cantilever into a photo-detector (e.g. array of photodiodes)

9a. Nanotechnology

The imaging AFM

If tip scanned at constant height: significant risk that tip collides with surface resulting in damage

f f db k h i l d dj→ often feedback mechanism employed to adjust tip-to-sample distance to keep force constant(=constant deflection) between tip & sample

Sample mounted on piezoelectric stage, which can move sample in z direction to maintain constant force and also in x and y directions for scanning offorce, and also in x and y directions for scanning of sample area

The resulting map of height (height (x,yx,y)) represents the g p g (g ( ,y,y)) ptopography of the sample

(topographic scan of “flat” glass surface →)

9a. Nanotechnology

The imaging AFM

Several modes of AFM operation: contact mode, non-contact mode, and dynamic contact mode

Contact modeContact mode: tip surface force kept constantContact modeContact mode: tip-surface force kept constant during scanning by maintaining constant deflection

NonNon--contact modecontact mode: (i) cantilever oscillated close to its resonance frequency

(ii) oscillation gets modified by tip-sample forces → changes provide information about sample→ changes provide information about sample characteristics

Dynamic contact modeDynamic contact mode (e.g. tapping mode): cantilever oscillated such that it yy ( g pp g )comes in contact with, and is detached from, sample each cycle. Prevents tip from sticking to surface (often covered with liquid surface layer at ambient conditions)

9a. Nanotechnology

The manipulating AFMS i b i ( AFM)Scanning probe microscopes (e.g. AFM) initially used only for high-resolution imaging & detailed surface characterization

Soon realized that they could change, interact and control nano-scale matter. Well-known early example: The writing of IBM logo with Xenon atoms using STM (22 h work for entire logo or ~38 min/atom; carried out at 4K and imaged

Research group at Lund university, Sweden: proposed to build nano-objects with

STM (22 h work for entire logo, or 38 min/atom; carried out at 4K, and imaged with AFM)!

larger molecular-sized building blocks and assemble them with AFM in ambient conditions!

G U i i f S h C lif i ’ L b f M l l R b iGroup at University of Southern California’s Laboratory for Molecular Robotics (LMR) inspired by proposal: attempts to program AFM as sensory robot capable of nanomanipulation and assembly of nanosystems…

9a. Nanotechnology The manipulating AFM

LMR method: position Au nano-particles (d ~ 5-30 nm)LMR method: position Au nano-particles (d 5-30 nm) accurately & reliably on flat (mica and Si) substrates, in ambient air or liquid environments, using stiff cantilevers (13 N/m) suitable for mechanical pushing in AFM contact mode) p g

First: record topography image in tapping mode to not alter particles

Then: decide desired manipulation trajectory & application of contact mode

When tip is close to particle of manipulation: feedback loop p p p pturned off (= imaging off)

When particle moved to desired position: feedback loop on again (= imaging on)again ( imaging on)

Note that 30-nm particle was pushed up a 10 nm step: first step towards mechanical construction of 3D nano-structures

9a. Nanotechnology

The manipulating AFM

At nano-scale, important forces on macro-scale (notably gravitation) are negligible, while surface effects are prominent → interesting to investigate tip-particle interactions and also understand mechanism of manipulationp p

Relatively distant: amplitude of tip vibration decreases when tip approaches particlewhen tip approaches particle

Close: amplitude approaches zero & cantilever deflection non-zero → particle moves (as long as deflection over gcertain threshold value)

(Nano-particles move by sliding not rolling!)

Understanding characteristic behavior of tip: controllable manipulation in real time without need for post-imaging

9a. Nanotechnology

The manipulating AFMSo what can this nanoparticle manipulation be used for?

Fabrication of 2D structures: Random pattern of p15nm Au ”balls” on a substrate converted into USC logo by sequence of pushing commands

Fun but other examples of more useful stuff...

9a. Nanotechnology

Using 2D nano-manipulation for data storage

H i l f i l l iHorizontal rows of nano-particles translate into ASCII code: presence of particle at node corresponds to “1” and its absence to “0”

Horizontal rows read from top to bottom...

As presented: data storage density ~60 GB/in2, can be further improved by tighter p y gspacing and smaller particle size…

Candidate for rewritable Nano-CD!

9a. Nanotechnology

The LMR group also demonstrated that it is possible to fabricate 3D structures

Pyramid formed by pushing 30 nm Au nano-particle up between 2 others

Manipulation of asymmetrically shaped objects, such as Au nanorods (L = 100 nm, d = 10 nm), more difficult:

Longitudinal pushing yields desired simple translational motion

while transverse pushing often results in combination of translation and rotationof translation and rotation, making structural assembly complicated

9a. Nanotechnology

The manipulating AFM

AFM also used for cutting & bending of soft materials, such as polymers

Direct cutting of DNA plasmids not completely successful, since result rather coarsecoarse

Better approach: use enzyme-coated AFM probe; first select exact site for modification and then apply enzyme to perform exact cutting

AFM l f ll l dAFM also successfully employed to bend soft organic materials, such as carbon nanotubes

9a. Nanotechnology

The manipulating AFM

AFM i l i f i l dAFM manipulation of nanoparticles used to build prototypes of electronic & optoelectronic devices…

Single-electron transistor: placing nanoparticle between two electrodes

Plasmonic nano-waveguide: placing chain of 30 nm Au nanoparticles at equal distances (100 nm), with a fluorescentdistances (100 nm), with a fluorescent (latex) particle at the end. Light is introduced at Au particle at one end; it propagates through chain via nearend; it propagates through chain via near-field effects; and finally it is detected as (latex) fluorescence at other end!

Nano-waveguide able to e.g. provide light to specific molecular machines

9a. Nanotechnology

The manipulating AFM

G i f f li k dGoing from patterns of non-linked nanoparticles to solid (i.e. linked) nanostructures often desirable and can be done

ith 3 diff t hwith 3 different approaches

i) Dithiol connection: organic molecules with S at both ends chemically react with Au, and function

“ l l l ” b i hb i Aas a “molecular glue” between neighboring Au particles. Possible to push group of thiol-linked Au particles as one unit

ii) Self-assembly: pattern of Au nanoparticles function as a template for additional Au deposition when sample dipped into appropriate A t i i l tiAu-containing solution

iii) Sintering: latex nanoparticles in desired geometry is fused together by heating (160oC for 10 min →)

9a. Nanotechnology

The manipulating AFM

Finally, for certain applications important to fix nanostructure to substrate:

i) deposit and manipulate nanoparticles as usual onto ) p p pSi/SiO2 substrate

ii) deposit monolayer of OTS: organic molecule containing silyl group (that chemically attaches tocontaining silyl group (that chemically attaches to Si/SiO2 substrate) and hydrocarbon chain

iii) oxidize (“burn off ”) material: hydrocarbon chain removed, while nanoparticles embedded in new SiO2

iv) 3D Au nanostructures can be fabricated by repeating steps ii) and iii) until a ) y p g p ) )planarized structure is attained, then deposit a new pattern of Au nanoparticles, and than finally remove the supporting SiO2 structure

9b. Nanotechnology

Quantization of conductance at the nano-level

Electrical transport in a bulk metallic wire is given by the following equations:

R = *L/AR = ρ*L/A

S = 1/R = σ*A/L

Th i i i (d l li d) lThe resistance exists since (delocalized) electrons are scattered by impurities, defects, and phonons

A “mean free path” is defined as the averageA mean free path is defined as the average length an electron can travel freely, without hitting against something and losing some kinetic energykinetic energy

But what when happens when ´length of wire´ < ´mean free path´ of electrons?

9b. Nanotechnology

Quantization of conductance at the nano-level

´Length of wire´ < ´mean free path´ of electrons?

T t f l t hift f b iTransport of electrons shifts from being ”diffusive” to become ”ballistic”

If width of wire also shrinks to nanometerIf width of wire also shrinks to nanometer range (i.e., the Fermi wavelength scale), the conductance of an electrode-nanowire-electrode structure becomes quantized:electrode structure becomes quantized:

S = N*(2e2/h)

N b f d i h lN: number of conduction channels

Note that the conductance now is independent on the length of the nanowire!

9b. Nanotechnology

Fabrication of nanowire structures

The fabrication of electrode/nano-wire/electrode structures can be monitored with transmission electron microscope (TEM) p ( )operating at 200 kV

TEM allows for very high resolution on

A miniaturized scanning tunneling microscope

~0.1-nm-scale in ultra-high-vacuum (UHV) chamber...

A miniaturized scanning tunneling microscope (STM) is also added to UHV chamber

A mechanically sharpened STM Au tip is y p pbrought into contact with Au sample deposited onto thin Cu wire substrate

9b. Nanotechnology

Fabrication of nanowire structures

By slowly withdrawing STM tip using piezoelectric transducer: Au electrode/nano-wire/electrodeelectrode/nano-wire/electrode structure formed

The structural changes of nanowire gduring tip withdrawal is monitored in real-time with TEM

Simultaneously, conductance of electrode/nano-wire/electrode structure measured as:

S = It/Vb

9b. Nanotechnology Nanowires and the quantization of conductance at the nanolevel

l l i b l… clear correlation between structural changes & conductance change in steps of (N*2e2/h)!

9b. Nanotechnology

Nanowires and the quantization of conductance at the nanolevel

Hard work (> 300 experiments) proved that one single linear strand of Au atoms indeed exhibits “unit conductance”:

S = 1*(2e2/h)

and that a double strand as expected exhibits aand that a double strand, as expected, exhibits a conductance equal to twice the “unit conductance”

TEM images also showed that length of a single-strand nanowire is 0.89 nm, and consequently consists of 3 Au atoms (Au-Au co seque y co s s s o 3 u o s ( u unearest-neighbor distance in bulk crystal: 0.29 nm)

9b. Nanotechnology

F b i ti n f G ld n n t bFabrication of Gold nanotubes

Alternative fabrication method: Produce very thin Au film in UHVProduce very thin Au film in UHV chamber and bombard it with strong electron beam (~100 A/cm2)

Long Au nanowires with a range of diameters formed (0.6 - 1.3 nm)

Elaborate analysis (using a computer simulation) demonstrated that such nanowires consist of helical multi-shell structures…

9b. Nanotechnology

Characterization of gold nanotubes

Helical multi-shell structure:

Each shell tubular, made of atomic strands h il d h i d h i dthat coil around the axis, and characterized

by the number of atomic strands

E g 14-7-1 structure: 3 coaxial tubes with (↑) Also possible toE.g. 14-7-1 structure: 3 coaxial tubes, with outermost containing 14 atomic strands (↓)

(↑) Also possible to produce single-strand structure with this

th d b t ith lmethod but with larger nearest-neighbor distances: 0.4 nm (compared to 0.29 nm in the bulk)

9b. Nanotechnology

Evaporating nanowires

Possible to fabricate nanowires on flat (mica) substrate with parallel “staircase” steps (one or several atomic layers high) using relatively i l i h i

A vapour of atoms/molecules formed by heating a source of material in vacuum

If substrate placed in the path of the vaporized atoms/molecules: some of these will

simple evaporation techniques:

If substrate placed in the path of the vaporized atoms/molecules: some of these will deposit onto substrate

It substrate at (appropriately) high T: evaporated atoms/molecules diffuse around rapidly l l f ll b l k duntil close to one of “staircase” steps, where they will be locked up with much higher

probability than on flat surface

Why?y

2D attractive surface interaction stronger than 1D

Thin nanowires form parallel to, and nestling in, steps in original substrate surface

9b. Nanotechnology

Ok, nanowires/nanotubes interesting from a fundamental physics standpoint (remember:

quantization of conductivity)

But is there any imminent practical application for nanowires?nanowires?

9b. Nanotechnology

Moore’s law – one example of useful NT

Famous Moore’s law is attributed to Gordon Moore, a co-founder of Intel

Many different forms over the years, but as it was originally stated in 1965:

“The complexity for minimum component costs has increased at a rate of roughly a factor of two per year ... Certainly over the short term this rate can be expected to continue, if not to increase. Over the longer term, the rate of increase is a bit more uncertain, although there is no reason to believe it will not remain nearly constant for at least 10 years. That means by 1975, the y y y ,number of components per integrated circuit for minimum cost will be 65,000. I believe that such a large circuit can be built on a single wafer.”

9b. Nanotechnology

Moore’s lawAssuming that chip “complexity” is proportional to number of transistors, regardless of what they do, Moore’sl b d jlaw can be restated to project doubling of number of transistors per chip every couple of years

Interestingly, it has held the test of time to date (self-prophetic effect!?)

Nevertheless, the current size ofNevertheless, the current size of transistors is so small that nanotechnology production and phenomena currently are becoming p y gcritical in order to keep up with Moore's Law…

9b. Nanotechnology

Nanotubes and Moore’s law!

A few years ago: IBM reports first electronic circuit using one single-walled carbon nanotube (length = 18 μm) as thecarbon nanotube (length = 18 μm) as the base material for all transistors!

The electronic circuit (a ring oscillator: odd number of NOT gates) relatively slow: 52 MHz

B IBM h h hBut IBM researchers expect that such single-walled carbon nanotube based transistors in a near future will allow for

12switching in the THz (1012 Hz) range!

9b. Nanotechnology

Risks with nanotechnology

Exposure to (man-made) nano-sized particles has increased drastically lately, and is expected to continue to increase with development of pnanotechnology

Carbon nanotubes have been found to make h i i l li ( i i k i l ) ftheir way into alveoli (tiny air sacks in lung) of

mice, and cause inflammation

Rats exposed to air containing 20-nm-sized p gnano-particles of teflon for 15 min died within 4 h

hil d l i d i lwhile rats exposed to larger sized particles (130-nm) suffered no ill effects

More studies on the way…

9b. Nanotechnology

Spectacular risks with nanotechnology“Grey goo” term was initially coined by nano-technology pioneer and author Eric Drexler

It refers to hypothetical end-of-world event involving out-of-control self-replicating nano-robots consuming all living matter on Earth while building more of themselves!

Origin to disaster: accidental mutation of self-replicating nano-robots or from deliberate doomsday device

Realistic?

Well, in Britain, the Prince of Wales called upon Royal Society to investigate the “enormous environmentalSociety to investigate the “enormous environmental and social risks” of nanotechnology, leading to much delighted media commentary on grey goo…