Nanoscience Midterm

nanometer scale

10^-9 meter

meter → 10³ millimeter → 10^6 micrometer → 10^-9 nanometer

Basic structure of C60

60 carbons forming a soccer ball shape - from 20 Hexagons and 12 pentagons

Basic structure of graphite

sheets of carbon-carbon bonds that are held together by weak Van der Wals forces which gives graphite its brittle property.

sp² hybridized - each carbon has 3 bonds

Basic structure of diamond

crystalline structure of carbon-carbon bonds, which gives diamond its hardness

sp³ hybridized - each carbon has 4 bonds

Who discovered C60 first?

Three scientists at Rice University - Kroto, Curl, Smalley

How C60 was discovered

Rotating graphite disk was struck with laser beam

turned the graphite into carbon atoms (carbon plasma)

pulse helium into chamber

carbon atoms cool and cluster together forming C60, C70, etc..

What is the method to produce sufficient C60 for people to experiment on?

They use the arc discharge method to produce sufficient C60 for people to experiment on:

• two graphite rods in chamber

• Voltage passed between them creating electric arc between the two rods - high temperatures vaporize the graphite creating carbon atoms

• The carbon atoms cool and form C60 in the soot which collects on the sides of the container

Chemical chromatography:

• used to isolate the C60 → the soot is dissolved and then run through column where C60 comes out at different time than other isotopes.

What was the experimental evidence that suggested the fullerene molecular

structure exist?

• Mass spectroscopy: shows a peak at C60 indicating that C60 is a stable form of carbon

• Nuclear Magnetic resonance: shows that all the carbons in C60 have the same chemical environment which shows that it must be a symmetrical → proves that it must be the ball shape they predicted.

the two main properties of a superconductors

• They conduct electricity with no resistance, so no energy is lost during while it is being conducted

• They are diamagnetic so they repel magnetic fields from inside

examples of the how superconductors are used

• Maglev trains: trains that use superconductors to float the train above the rail which reduces friction, these trains can go extremely fast (Japan one goes over 600 miles per hour)

• Superconducting power lines: less energy is lost during transmission

what is the difference between a single crystal and an amorphous solid?

Single crystals have long range, continuous, and periodic lattice structures, they don’t have breaks or abnormalities in the structure.

Amorphous solids doesn’t have long range order like glass

what is the difference between a regular crystal and a nanocrystal?

The only difference is their size, a nanocrystal needs to have one dimension less than 100 nm

• higher surface to volume ratio: nanocrystals usually have lower melting point

• Quantum confinement: light emission changes with size

• higher surface to volume ratio: nanocrystals have more chemical reactivity

What is basic idea of synthesis nanocrystals by the “arrested” nucleation

process?

arrested nucleation uses surfactants with hydrophilic head and hydrophobic tail. These surfactants will make a circle around the inorganic substance when placed in an organic solution. The surfactants will make a boundary of sorts, so the inorganic phase stays one particular size.

Why the color of the light emitted from the nanocrystal changes with the

size of the nanocrystal?

the color of light changes because of quantum confinement, which is a property of nanomaterials.

Nanomaterials have the ability to change their band gap depending on their size, so a different band gap means when the electron falls from high energy to low energy, the energy released will be a different wavelength - resulting in a different color

Smaller nanoparticle - bigger band gap - bluer light

larger nanoparticle - smaller band gap - redder light

What is n-type and p-type doping?

Pure silicone is sp³ hybridized in a crystal lattice structure

N-doped: Si is doped with group 5 elements, they will form four bonds with Si and 1 electron is left to carry charge, so it carries a negative charge

• N, P, As

P-doped: Si is doped with group 3 elements, they will form three bonds and leave a hole that has a positive charge.

• B, Al, Ga

What is a p-n junction?

The P-N junction is when a P type semiconductor is put with an N type semiconductor. The free e- from N side diffuses to the P side. This makes the N side more positive and the P side more negative.

• depletion region: area where e- have filled holes creating the electric gradient

• Electric gradient: pointing from the N side to the P side

what is the basic structure of a photovoltaic solar cell?

The solar cell has a front electrode on top with an antireflective coating. Then there is a small N type semiconductor which is put next to the P type conductor, and then there is a bottom electrode.

The incoming photons will excite the e- only if their energy is greater than the semiconductors band gap

it excites the e- from valence to conduction band, which creates an electron hole pair

that e- is free to conduct electricity

• PN junction creates the elctric potential

  • after sunlight: photons excite e-

why is quantum dot useful for solar cell?

Quantum dots are so useful because solar cells are actually not very efficient since the sun has a large spectrum of light, so not all the waves meet the minimum energy required and the waves that have too much energy will waste the leftover energy after exciting one electron.

Quantum dot’s size can be manipulated to change the band gap’s size. So quantum confinement allows us to capture more of the sun’s electromagnetic spectrum, exciting more electrons.

Multiple layers of QDs are are stacked, decreasing bandgap energy from top to bottom - lower energy photons can pass through and be used on lower layers.

What is a liquid crystal

A liquid crystal flows like a liquid but it’s molecules have orientational order like a solid.

• applying voltage changes physical orientation of molecules

What is the basic structure of a LCD?

Light has two waves, electric wave and magnetic wave - polarized light only has one component of light

• LCD takes unpolarized light from backlight and passes through polarizing filter so it only has one wave form

• Then this one wave component passes through liquid crystal, if no voltage applied crystal will naturally flip wave, if voltage applied then crystal will align and wave won’t change.

next polarizing filter is 90 degrees rotated so only rotated wave can pass - turning pixel on

Color is added as LCDs use red, green, blue filters over these pixels to produce images

What is the function of the liquid crystal in a LCD

The liquid crystal changes orientation based on if voltage is applied. So when no voltage is applied, the polarized wave passes through and the liquid crystal will naturally flip the wave. Since the wave is flipped, it will pass through the polarized filter, and the pixel will turn on. But if voltage is applied, the crystal will align and the wave will not flip, so it cannot pass through the polarizing filter, and the pixel will not turn on.

How quantum dots are used in some LCD’s?

Because of quantum confinement different band gaps produce different colors, so it can be used to create better filters and produce brighter and better colors.

What is the structure of a hydrogen fuel cell?

Hydrogen fuel cell has one hydrogen electrode where H2 is being pumped in and one oxygen electrode (where o2 is being pumped in) and a membrane separates the two with electrolyte mixture in between.

• anode: negative electrode coated with platinum where Hydrogen splits into H+ and free e-

• cathode: positive electrode, coated with catalyst and O2 inputted. combines with H+ and e- to form H2O

• Electrolyte: only allows H+ to pass so e- have to flow through the circuit

  • external circuit: conductive path from anode to cathode where e- flows to generate energy

What are the chemical reactions at the two electrodes of a hydrogen fuel

cell?

Anode: H2 → 2H+ + 2e-

cathode: 1/2O2 + 2H+ + 2e- → H2O

What is a lithium-metal battery?

A lithium metal battery uses pure lithium metal as the anode

What are the advantages and disadvantages of Li-metal battery?

advantages: Has a high energy yield per unit weight

disadvantages: with repeated charging Li deposits unevenly leading to dendrites forming - they can pierce the separator and since Li is so reactive → fires/explosions.

What is the basic structure of a Li-ion battery?

Li-ion battery has an anode which contains graphite to store the Li+ atoms and the cathode has a metal oxide (LiCoO2) which gets the Li+ atoms. The electrolyte only allows Li+ to pass and doesn’t allow e- to pass so they pass through the circuit.

What is charging/discharging process?

Charging:

anode: 6C + XLi+ + Xe- → LixC6

cathode: LiCoO2 → XLi + Xe- + Li1-xCoO2

Discharging:

anode: LixC6 → 6C + XLi+ +Xe-

cathode: XLi+ + Xe- + Li1-xCoO2 → LiCoO2

Why Si is attractive as an electrode for Li-ion battery? Why not bulk Si?

Si is the anode because it has a higher theoretical yield (10X more than graphite)

Bulk Si isn’t feasible because it expands when absorbing Li. It requires a lot of energy to break stable Si-Si bonds and this causes builk Si to swell, crack, and loose electrical contact

What is special about nanoparticles of Si for Li storage

Nanoparticles of Si have a lower surface to volume ratio, so they are able to lower the energy barrier for rxn so they can break Si-Si bonds more easily and they can absorb mechanical stress better.

How the VLS mechanism is used to explain the growth of Si nanowires?

The VLS mechanism involves three phases:

• there is a Si substrate, and on it a nanoparticle of gold is placed.

• Then Si gas and crack it so that Si atoms enter the gold alloy

• Once the gold alloy becomes saturated, the Si will precipitate on the bottum

  • This precipitate grows upwards, creating a nanowire.

Who first observed carbon nanotubes?

Sumino Iijima

What are the 3 main techniques to make CNTs?

Arc Discharge

Laser Ablation

Chemical Vapor Deposition (CVD)

Arc Discharge

Similar to making C60, two graphite are placed in chamber and electrical current is passed through creating an electric arc, this vaporizes the carbon and when it cools it collects on the graphite rods as CNTs.

Laser Ablation

A high powered laser blasts the target graphite inside the high temp furnace, vaporized carbon condenses into CNT in a cooled collector

CVD

carbon containing gas passes over metal catalyst nanoparticles - gas molecules break down and carbon grows from catalyst as nanotubes.

What is the basic structure of a carbon fibers?

They are a graphene sheet rolled into a tube shape

What makes carbon fibers useful? What are the main applications?

Carbon fiber is made up of a lot of graphite flakes put together. The carbon fiber is a composite, combines 2+ materials to get the best of both worlds - uses carbon fiber & resin to keep smooth.

main applications: high intensity sports gear, sports car, airplanes

Why CNT may be better than carbon fiber?

CNT may be better than carbon fiber because it is a continuous graphene sheet so the whole thing is held together by C-C bonds which are very strong. While carbon fiber is weaker since it is made up of multiple flakes which aren’t all held together by C-C bonds.

What is the basic structure of an Atomic Force Microscope (AFM)?

AFTs have have a flexible cantilever moves the sharp tip across a surface, shifts in cantilever change laser’s reflection. This creates a picture of surface.

How can CNT improve the performance of an AFM?

A CNT can put a nanowire on the tip of the cantilever, this nano diameter can reach deep and narrow areas, offering atomic level resolution

CNTs are durable and flexible, they can probe with out breaking or damaging samples.