Nanotechnology(Dr Paul May)

What is it? 1 nm = 10-9 m

Or 1 millionth of a mmOr ~5 atoms.

Nanotechnology is making or manipulating things on the nanoscale, i.e. the scale of a few atoms.

(But actually it’s used to mean any microscopic physical process, e.g. making Si chips, manipulating DNA, colloid science, etc.)

Scanning Electron Microscope (SEM) – down to ~100 nm.

Transmission Electron Microscope (TEM) – down to ~5 nm.

How is it done?

• Usually with a Scanning Probe Microscope (STM, AFM,…)

Scanning Tunnelling Microscope (STM) – down to ~0.01 nm.

Direct write ion/electron beams can draw 3D microscopic shapes

Chemical Vapour Deposition of Thin Diamond Films

(or....Grow Your Own Diamonds)

School of ChemistryUniversity of Bristol

Dr Paul May

Diamond - The Ultimate Engineering Material

Extreme Properties:1. Hardest known material2. Highest atomic number density3. Highest Elastic Modulus (stiffest)4. Least compressible5. Highest thermal conductivity (at 300 K)6. Thermal expansion at 300 K < invar7. Transparent from deep UV to far IR 8. Very low coefficient of Friction 9. Chemically inert10. Biologically compatible11. Very good electrical insulator12. When doped, becomes semiconducting13. Radiation hard14. Exhibits 'Negative Electron Affinity'

Uses for Diamond Thin Films1. Wear resistance, hard, little lubrication

• Machine parts • Cogs, gears, bearings • Cutting tools, saws, razors, etc.

2. Transparency IR-UV • Optics, especially IR or high power laser lenses • Windows (Pioneer spacecraft)

3. High Thermal Conductivity • Heat sinks and heat spreaders for PCBs or ICs

4. Stiffness • Reinforcements in composite structures

5. Electrical Properties • Inert Electrochemical Sensors .

6. Semiconducting Properties • High power devices, high V and T (500°C, 400 V)• Computers, denser and therefore faster circuitry

7. Negative Electron Affinity • Ultra-fast switches (transistors, computers,...) • Flat panel displays (TVs, monitors, watches,....)

How to Make Diamond1) Geological• Take carbon (coal), bury it underground

at very high T and high p, wait a few million years... Natural Diamond (gemstones).

2) High Pressure – High Temperature• Copy of Nature's method.• Take graphite, place in a hydraulic press

at 120 kbar, and 3000°C, with a suitable catalyst (Fe, Cr, Pt)... Industrial Diamond

3) Chemical Vapour Deposition, CVD• Since ~1985

Thin Film Diamond (nm, m, mm, cm?)

Recipe for CVD Diamond

• Vacuum Chamber (10s of Torr) • Source of Carbon (methane, ethanol,...) • Activate Gas (hot filament, plasma...) • Substrate (Si, Mo, quartz, etc)

If you do this you get....

... only Graphite or amorphous Carbon.

Need a few more 'tricks of the trade'.

Tricks of the Trade• Excess H2 essential (1% CH4 / 99% H2)

• etches graphite, not diamond• stabilises diamond surface • removes polymers from gas phase

• Hot Substrate (> 700°C)• gives species mobility on surface

• Pre-abrade surface• provides nucleation sites • and/or seed crystals

Now we get Diamond!

Hot Filament Reactor

Microwave Plasma Reactor

SEM ExamplesInitial stages of diamond nucleation on a Ni substrate

A typical polycrystalline CVD diamond film

A cross-section through the film, showing columnar growth

A nanocrystalline film grown with 2% methane

SEM ExamplesA thick (125 µm) W wire coated in ~40 µm of diamond

A thin (25 µm) W wire coated in ~40 µm of diamond

Diamond tubes

Diamond fibre reinforced composites

Ti metal matrixcomposite

PMMA plasticmatrix composite

Diamond biosensors

Replace H termination with a C-C bond, linking the diamond surface to a molecule of your choice, e.g. DNA, proteins, etc.Þ Reusable biosensorsÞ genetic screeningÞ disease diagnosisÞ in situ monitoring (glucose, insulin?)

Electron Emission DevicesDiamond has: a negative electron affinity, or work function ~0.Þ Extraction of electrons from surface easy – no energy loss.Þ Harness those electrons to do something useful….

Field emission devices• Extract electrons using a high

voltage• Electrons accelerated to hit a

phosphor screen • Cheap, bright flat panel displays• But competing with large existing

market, LCDs, Plasmas, OLEDs, etc.

Electron Emission DevicesDiamond has: a negative electron affinity, or work function ~0.Þ Extraction of electrons from surface easy – no energy loss.Þ Harness those electrons to do something useful….

Thermionic emission devices• Extract electrons using heat• Electrons collected on cold

electrode, then returned to other electrode by an electric circuit.

• Converts heat into electricity• Diamond can be CVD coatings or

sprayed nanoparticles.• Very efficient, large area Solar cells!

Diamond-neuron connectionComputer-brain interfaces?

Diamond is biocompatible – cells don’t produce immune response when grown on diamond.Can make patterned diamond grids, mice neurons can be grown to follow the pattern and join up….artificial neural network…’grow your own brain’

Ordered growth of mice neurons on a laminin-coated hydrophilic CVD

diamond surface.C.G. Specht, O.A. Williams, R.B. Jackman, R.

Schoepfer, Biomaterials, 25, (2004) 4073.

Diamond Quantum Computers

Þ Ultrafast computersÞ Unbreakable codesÞ secure communications

The nitrogen-vacancy (NV) defect in diamond lattice.

Þ Single photon sourceÞ very long spin lifetime at RTÞ emits in visible at 637 nm

SummaryDiamond Technology has Arrived...

We should soon see CVD diamond in:• Medical Applications • Car/Aircraft Components • Optics • (Quantum?) Computers • TVs• Engagement rings?

BUT, we still have problems with:• low growth rates • high substrate temperature • n-doping • single crystal growth

Web sitehttp://www.chm.bris.ac.uk/pt/diamond/