1.2. Chemical means to manufacture nanoparticles

1.2. Chemical means to manufacture nanoparticles

[Poole-Owens 4.5.2, Atkins]

1.2.1. Liquid-phase chemistry

In addition to the “physical” means to manufacture nanoparticles described above (based on

evaporation or sputtering and involving a vacuum) there are also numerous purely chemical ways to

manufacture nanoparticles.

- One important difference: physical ways can attain very good purity (at least if using ultra high

vacuum), chemical methods: risk of having impurities high

- On the other hand chemical methods can almost always be easily scaled upwards to attain high

yields, in physical this is often difficult.

There is a variety of ways to make nanoparticles in solution by chemical means (“traditional

chemistry”).

Introduction to Nanoscience, 2005 JJ J � I II × 1

The basic approach is to have some compound, typically a halide, containing a metal atom, as well

as a reducing agent which removes the other parts of the compound.

halide is group VII element i.e. F, Cl, Br, I. Remember alkali halides. Reducing agent is something which donates electrons to the

species. Since Cl likes to have electrons, the electrons free the Cl from the metal compound into a Cl− species. See Atkins p. 263

An example is making Mo in toluene (C7H8) solution

MoCl3 + 3 NaBEt3H =⇒ Mo + 3 NaCl + 3 BEt3 +3

2H2 (1)

where Et is the ethyl radical ·C2H5. This reaction makes first free Mo atoms which then coalesce

to form nanoparticles of Mo in the size range 1 - 5 nm, and works at room temperature.

Al nanoparticles can be made by decomposing Me2EtNAlH3 in toluene solution and heating the

solution to 105oC for 2 h (Me is the methyl radical ·CH3). Titanium isopropoxide (C12H28O4Ti) is

added to the solution and acts as a catalyst, which leads to the production of 80 nm particles.

Many similar reactions exist for other metals.


A problem with these methods is that the metal particles can keep growing in size if they meet each

other; they then tend to aggregate by sintering, i.e. joining together with each other spontaneously

[Zhu and Averback, Phil. Mag. Lett. 73 (1996) 27].

In the vacuum methods the sintering is controlled simply by the time by which the clusters are

allowed to grow. In the chemical methods this can also be done by adding a surfactant to the

solution. This surfactant will coat the particles and prevent further aggregation of them.

After the nanoparticles have formed what one then has is a colloid, i.e. a disperse phase of small

particles of one material (the metal) in another (the liquid containing the other chemicals). In

particular, a colloid of a solid in a liquid is called a sol. A colloid of solids or liquid particles in gases

is called an aerosol.

See Atkins p. 752. surfactant = surface active species which stabilize the particles

Usually the particles produced by chemical means are polydisperse, i.e. contain a range of

sizes. But with careful control of the reactions and species it is also sometimes possible to make

monodisperse nanoparticles, i.e. ones which all have the same size.

In the vacuum methods one can always make monodisperse particles by using a mass

spectromometer, but this means most of the produced particles are lost, i.e. is expensive.


Basic science: only few needed, who cares, vs. large-scale production

1.2.2. Thermolysis

Nanoparticles can also be made by decomposing solids or liquids at high temperatures having metal

cations, or molecular anions or metal organic compounds. This is called thermolysis.

For example Li particles can be made by decomposing lithium azide, LiN3. The material is placed

in a quartz tube in vacuum and heated to 400oC in an apparatus like the following:

- FIG 4.26 from Poole-Owens

At about 370oC the LiN3 decomposes and releases N2 gas. The remaining Li atoms coalesce and

form a colloid of metal nanoparticles with sizes < 5 nm.

A more complex example is forming gold nanoparticles from a thiolate complex,

[C14H29(CH3)3N][Au(SC12H25)2] at 180 degrees C for 5 h under an N2 atmosphere. This produces

gold nanoparticles of average size 26 nm which are directly passivated by alkyl groups derived from

the precursor complex. [Nakamoto, Chem Commun (Camb). 2002 Aug 7;(15):1622-3.]


These methods typically produce nanoparticles in a mixture with the surrounding organic material.

The nanoparticles can then be isolated by dissolving the organic material with usual chemical

methods.

Here is an example of Au nanoparticles produced by the Nakamoto group:

which are almost monodisperse.

[Nakamoto, Proceedings of the Gold2003 conference, http://www.gold.org/discover/sci indu/gold2003/pdf/s36a1275p901.pdf]


1.3. Background: polycrystallinity

- All elemental metals and most metal alloys are polycrystalline in their natural state after

manufacturing, i.e. they consist of individual crystalline grains randomly oriented with each other.

- Typical grain size: 10 – 100 µm.

- Looks something like this, except the scale is much much bigger:


[http://mstd.nrl.navy.mil/6320/6324/magneticthinfilms1.html]


1.4. Ball milling

A completely mechanical way of making nanocrystalline metals is that of ball milling. This is a

method where starting point materials are placed in a mill where they collide with macroscopic

ceramic balls.

Citation: “Ball mills are basically hollow horizontal cylinders that are partially filled with ceramic balls and the precious metal ink

compositional mixture. These mills are rotated at a speed that allows the balls to cascade through the mixture, separating the

precious metal particle clumps into individual particles.”

Funny analogy to gas condensation: replace noble gas with balls...

The balls have high velocities, and collide with the material of interest, This raises the internal

potential energy of the system leading to defect production.

First the strain is increased, then dislocations are produced, and finally the dislocations produce

new grain boundaries which are smaller than the initial ones. Thus an increasingly small grain size

is achieved.

It is even possible to make alloys out of initially pure elements. For instance starting from pure Ni


and Mo production of a nanocrystalline fcc Ni60Mo40 alloy was achieved, with average grain size

of the order of 3 nm. The minimum grain size of these materials clearly correlates with the melting

temperature:

The powder particle size after ball milling are typically in the micron range.

A nice advantage of the method is that it can impart quite high energy to the material, meaning


that it is possible to form nanocrystals out of immisicble elements (materials which can not be

mixed in thermodynamic equilibrium).

The ball milling method produces a powder of the milled material.


1.5. Manufacturing of bulk nanocrystalline materials

[Own knowledge, http://nanotechweb.org/dl/wp/nanocrystalline materials WP.pdf, ...]

1.5.1. Pressing bulk materials from nanopowders

During the previous lecture it was described how one can deposit nanoclusters to make thin

nanocrystalline films. An obvious question is whether one can make bulk nanocrystalline materials,

either from single clusters or a powder produced in ball milling.

- In principle this can be done using deposition, but very slow

There are several techniques to make bulk nanocrystalline materials directly, provided one has

enough nanoparticles as starting material. This is called consolidation of the powders.

Most approaches involve using standard powder metallurgy techniques on nanoparticles.

- Powder metallurgy (e.g. hot isostatic pressing): take a powder and put it under high

temperature/pressure. This enables forming a metal of desired shape.


- For nanoparticles problem is their thermal stability: grain size tends to grow at high temperatures,

i.e. nanocrystallinity may be lost.

- Hence very low temperatures often need to be used, room temperature and even 77 K (liquid

nitrogen temperature) processing is often used. [Youssef, APL 85 (2004) 929].

- On the other hand at low temperatures it is problematic to obtain a fully dense material, one with

no voids between the grains.


1.6. Mechanical properties of nanocrystalline films

[http://nanotechweb.org/dl/wp/nanocrystalline materials WP.pdf]

- Metals are usually considered to be ductile (malleable), but not so elastically hard

Explain ductile

- Ceramics are usually considered to be elastically hard but brittle.

An often used overall describition of what happens when grain sizes move into the nanoscale is that

metals get stronger and harder and more brittle, while ceramics become more ductile.

This makes nanocrystalline materials very interesting from a mechanics point of view, since thus

one can improve on the properties of both metals and ceramics towards the other class of materials.

- On the other hand, at the same time one loses something (ductility in metals, elastic hardness in

ceramics).

This is, however, a simplification, reality is more complex.


On this course we can not go into the whole large field of mechanical properties of nanostructured

materials, but will mention one important example in detail.

1.6.1. Background: yield strength

[Callister]

- The yield strength σy is a standard engineering measure of the strength of a material.

- It is a measure of how much you can draw out a material before it fails for practical purposes.

- Measure of this: stress vs. strain curve (strain is relative elongation) point where an external force

has lead to a permanent elongation of 0.002 after the external pressure on the sample is relieved.


- A few typical values:


1.6.2. Hall-Petch relationship

[http://www.matsci.ucdavis.edu/MatSciLT/EMS-174L/Files/HallPetch.pdf,http://www.fysik.dtu.dk/ schiotz/research.html, http://www.wtec.org/loyola/nano/06 02.htm]


- In traditional metal processing the grain size could be varied by changing the processing conditions.

Then a natural question is, how does the grain size affect the strength of the material

Hall and Petch, working independently around 1950, found that the yield strength of a polycrystalline

material follows the following dependence:

σy = σ0 +K√

d(2)

Here σy is the yield strength described above, d is the mean grain size, and σ0 and K are constants.

- Hall both developed a theory for this and showed it empirically. Petch did independent experiments

on steels on this. The explanation is rather complex but related to dislocation activity in the grains.

Related to dislocation motion being impeded by grain boundaries, dislocation pile up and slip bands. Hall: “There is a critical

stress above which a crack will grow and this stress is proportional to the inverse of the square root of the length of the crack. The

length of the slip band is analogous to the crack length and defines a critical stress required to punch through a grain boundary.

The maximum length of a slip band is related to the mean grain size.”

1.6.3. Validity in nanoscale materials


Equation 2 is interesting with respect to nanoscale materials in that it obviously predicts that as

d −→ 0 ⇒ σy −→ ∞ (3)

I. e. this would indicate that simply by decreasing the grain size one can make a material arbitrarily

strong!

- Natural lower limit: atom size d ∼ 0.2nm

- But this is not sensible: a polycrystalline material with grain size of a single atom would just be a

single crystal and all this would not hold at all!

- There must be a limit where Hall-Petch breaks down!

- In fact for very small grain sizes one can now perform atomistic simulations of the deformation:


- ANIMATION fig/gr16KB T300.gif shown

- These simulations have shown that in fact for very small grain sizes there is a reverse Hall-Petcheffect, i.e. the strength decreases with increasing grain size. This has been found for d . 20 nm:


[Schiotz, PRB 60 (1999) 11971]

-

“The lack of dislocations in small, confined spaces such as single-crystal whiskers has been known for many years (Darken 1961).

Creation of new dislocations is also made difficult as the grain size reaches the lower end of the nanoscale (¡ 10 nm)”

- Crucial question: where does the breakdown from normal to reverse Hall-Petch occur?


- Some literature sources talk about ∼ 20 nm, but issue is still somewhat unclear.

- Some very recent work reports that: [Youssef, APL 85 (2004) 929]

- Ordinary polycrystalline Cu with ∼ 50 µm grain size: σy = 56 MPa

- Nanocrystalline Cu with ∼ 23 nm grain size: σy = 770 MPa

- A truly dramatic improvement of strength!

Comparison of Schiotz simulation and Youssef experiments: Fig. 11 in Shiotz shows about 1600 MPa at 300 K for 15 nm grains,

Youssef has 770 MPa for 23 nm =¿ not so good, not so bad (consider MD time scale problem)

- Reference “Youssef” also claims good ductility (i.e. the material is not becoming brittle), which

is good.

- And proves that the general rule given in the beginning of the section (metals becoming brittle) is

not always true!

So bulk nanocrystalline materials are highly promising for use as engineering materials; the main

problem is cost.

- Industry is seriously interested


OMG Kokkola chemicals, Joni Hautojarvi: price target 100/kg, then5 billion annual market potential in 2020.


1.7. Making embedded nanoclusters

- All of the methods described above allow for making free-standing nanoclusters, or clusters in a

solution

- Natural question: can we make clusters directly inside a bulk material?

- If we could, they might give the same functionality as free clusters, but would be embedded in a

rigid, stable environment

- Even better from an application point of view if the material would be Si or something

which can be integrated on a Si chip: then one could utilize the enormous knowledge of

how to integrate chips into making a nanocluster device.

- Ion implantation can do exactly this

1.7.1. Background: Ion implantation


Si wafer

1. Ion implantation

• Ion implantationmeans using an ionaccelerator to shootions into a material• The ions move ran-domly in the materialand then stop downsomewhere inside thesamples• The process isstochastic

Si wafer

2. Implantation profiles

• After a high dose imp-lantation, a depth distri-bution of implanted ionsis formed, with somemean depth R

• Typical energies: 0.1 -1000 keV• Typical mean depth R:10 nm - 10 µm

Numberof ions

Depth


1.7.2. Formation of nanoclusters after implantation

Si wafer

3. Nanocluster formation

• By heating the sampleafter (or during) theimplantaion, one canmake the ions mobile inthe material. They canthen (provided thematerial choice is sui-table) join together toform nanoclusters

Numberof clusters

Depth

Si wafer

4. Ostwald ripening• Moreover, if the heatingtemperature is highenough, the clusters startto emit atoms with someprobability• Emission is more pro-bable the smaller thecluster is (higher curva-ture => less surface bin-ding energy)• This so called Ostwaldripening causes the lar-ger clusters to grow atexpense of the smaller

Numberof clusters

Depth


- Thus in this way one can make embedded nanoclusters inside a sample

- Wide range of substrate materials and clusters have been demonstrated

- Typical example: Ge in SiO2.

- SiO2 may be on a Si wafer, i.e. can be

- Major advantage: accelerators are a standard part of most high-end Si wafer processing facilities,

as is high-temperature annealing ⇒ this method can be used in a normal semiconductor fab with

no need for additional equipment


- For instance blue light emission from Si(-based material) has been demonstrated using this

technique.

- If this would enable Si-based blue lasers that would be a seriously BIG thing.

- But clusters produced in Ostwald ripening are not usually monodisperse

- Possible solution: additional implantation


Si wafer

5. Inverse Ostwald ripening• By doing an additionalion bombardment duringthe Ostwald ripening pro-cess it is possible to hin-der the growth of thelargest clusters, andobtain an almost mono-disperse cluster distri-bution.

Numberof clusters

Depth


1.8. Nanoparticles in nature: aerosols

[Hanna Vehkamaki, private communication]

Finally one should not forget that nature is actually ripe with nanoparticles.

When water drops form in clouds or mists, they nucleate from free water molecules. This means

they start their growth in the nanoscale size regime.

But there are plenty of other examples, and in fact many of the particles stay in the nanometer size

regime.

Examples of important cases:

- water

- water-ammonia-sulphuric acids

- hydrocarbon molecules (e.g. from car exhaust gases)

- viruses and bacteria (aerosols can be alive)


Especially the gas exhaust variety can obviously be a big problem in cities.

But also forests can have significant amounts of aerosol particles. Here is a sample distribution

from Hyytiala, a forestry station in the middle of nowhere some 300 km north of Helsinki.

The yellow and red colors are high concentrations, and the x axis is time of day.

And here is an atom-level computer simulation image of a 200-molecule nonane (C9H20) nanocluster

at 215 K [T. K. Berg and K. Nordlund, Stuart potential simulation].



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1.2. Chemical means to manufacture nanoparticles