Nanostructure Formation: 1-D

• Whiskers and fibers

• Nanorods

• Nanowires

• Carbon Nanotubes

1-D nanostructure term: Aspect ratio = length ÷ diameter

4 : 1 6 : 1 12 : 1

36 : 1

1-D Nanostructures

Fun fact…

All these nano words are made up as scientists make new structures and there’s not much consistency yet…

What word would you invent??

Let’s give this set of notes some context…

What are some uses of 1-D nanostructures?

• Carbon NT fibers• Transistors use CNT

instead of Silicon• Solar cells use tall thin

strands of silicon to trap light

• Spontaneous Growth

• Template-based Synthesis

• Electrospinning

• Lithography

Techniques for 1-D Nanostructure Formation

Bottom-up

Top-down

Spontaneous Growth: Evaporation (dissolution)-Condensation

(AKA ROCK CANDY ANALOGY AGAIN)

• Anisotropic Growth (aniso = uneven)– Side effects:

• Growth imperfections• Accumulation of impurities

Evaporation-Condensation Growth Example

“Nanobelts”

Helical Nanostructures & “Nanorings”

Evaporation-Condensation Growth Example

VLS or SLS Growth• VLS: Vapor-Liquid-Solid• SLS: Solution-Liquid-Solid

– 1st Step with both: Impurity or catalyst introduced

1. Growth species evaporated2. Growth species diffused and dissolved into droplet3. Droplet (with growth species) deposits on growth

surface4. Growth species diffused and precipitated onto growth

surface

VLS Growth: Control of Nanowire Size• Dependent on size of liquid catalyst droplets

– Small droplets: • thin layer of catalyst on substrate• anneal at high temperature

• Spontaneous Growth

• Template-based Synthesis

• Electrospinning

Techniques for 1-D Nanostructure Formation

Bottom-up

Template-Based Synthesis• Used for polymers, metals, semiconductors,

oxides• Membranes as templates

Methods:1. Electroplating & Electrophoretic Deposition2. Colloid Dispersion, Melt, or Solution Filling

(may also involve chemical reactions)

Electrochemical Deposition

• Only for electrically conductive materials– Metals, alloys, semiconductors, some polymers

• Electroplating is electrochemical deposition

Electrochemical DepositionMethod:1. Charged growth species moves through solution in one

direction when electric field applied2. Charged growth species reduced at deposition surface (also

an electrode)

http://www.flashscience.com/chemistry/copper_plating.htm

If you put a template at the cathode, a metal will fill the pore, creating a nanowire!

• Product examples:– Metals: Ag, Ni, Co, Cu, Au nanowires: <10 nm- 200 nm dia– Semiconductors: CdSe, CdTe– Polymers: polyporrole

CVD / Centrifugation(it’s sort of like the Gravitron ride at a carnival!)

• Chemical Vapor Deposition (CVD)– Diffuse gas through porous material; heat

• Centrifugation– Combine sol and porous material– Spin at 1500 rpm for 60 minutes– Heat to 650 ˚C for 60 minutes– Forcecentrifugation > Forcerepulsion between particles

http://www.youtube.com/watch?v=4F2gfuKwn6k http://www.youtube.com/watch?v=ewmdPNfyBzI

• Spontaneous Growth

• Template-based Synthesis

• Electrospinning

Techniques for 1-D Nanostructure Formation

Bottom-up

http://mrsec.umd.edu/Research/Seeds.html#Template

Electrospinning• Electrical forces at surface overcome surface tension (aka electricity

destroys the surface tension that was holding the drop of liquid together before)

• Fiber can be directed or accelerated by electrical forces• Product: 30+ types of polymer, 40 -500 nm diameter

– Can be collected in sheets or other forms– Morphology depends on:

• Solution concentration• Applied electric field strength• Feeding rate of precursor solution

Electrospinning

http://nano.mtu.edu/documents/Electrospinning.swf