Shape Control: Selective Adhesion

Selective adhesion involves the use of appropriate capping agent(s) to change the free energy of various crystallographic surfaces, & kinetically control the growth rates of various facets of seed. The introduction of an agent that selectively adheres to a particular crystal facet can be used to effectively lower the energy & slow the growth rate of that facet relative to others.

This strategy allows the appearance of highly anisotropic shapes, characterised by larger surface areas which render them metastable, high energy forms). Selective adhesion effects have not been directly observed during nanocrystal growth, but theoretical studies lend credence to the concept.

Nanocrystal Growth (II): Thermodynamic & Kinetic Considerations

Thermodynamic Considerations
Shape of crystal is dependent on relative specific surface energies associated with facets of crystal. At equilibrium, a crystal has to bound by facets giving a minimum total surface energy (Wulff facets theorem).
Hence, the equilibrium shape often reflect the intrinsic symmetry of corresponding lattice (i.e. cubic materials → cubic morphologies). In addition, the equilibrium shape, although faceted, has low aspect ratio to minimize surface A.

Kinetic Considerations
Growth rate of a crystal facet depends exponentially on surface energy, so at high growth rates (kinetically controlled growth regime), high energy facets grow more quickly, leaving behind low energy planes as facets of the product. Hence, nanocrystal shapes may be controlled via the use of appropriate capping agent(s) to change the free energy of various crystallographic surfaces, to kinetically control growth rates of various facets of seed .

Nanocrystal Growth (I): Step-by-Step

(I) Transformation to monomers → (II) Supersaturation → (III) Nucleation/ Precipitation → (IV) Growth → (V) Ostwald Ripening → (VI) Aggregation

(I) Transformation of chemical precursors into active atomic or molecular species (monomers) via selected method (e.g. chemical, photochemical etc)
E.g. AgNO3 + Reducing Agent → Ag(0) [Chemical Reduction]
AgNO3 + hv → Ag(0) [Laser Ablation]

(II) Supersaturation - For particular solvent, there is a certain solubility of solute; In the presence of excess solute, precipitation occurs (supersaturation of monomers).

(III) Nucleation - When the concentration of monomers (atoms, ions, molecules) is sufficiently high, they aggregate into small clusters (or nuclei). Nucleation is thermodynamically driven i.e. supersaturated solution is not stable in energy.

(IV) Growth - Nuclei grow by molecular addition (soluble species deposit on the solid surface) relives the supersaturated step. When concentration drop below critical level, nucleation stop and particle continue to grow until equilibrium concentration of species reached.

(V) Ostwald Ripening - When reactants are depleted, particles grow by Ostwald ripening (large particles continue to grow at the expense of smaller ones). Smaller particles have higher surface energy which promotes dissolution.

(VI) Secondary Growth (Aggregation) - Rate of particle growth by aggregation is larger than by molecular addition. For nanocrystal growth, this step is stemmed by the addition of surface adsorbing species (e.g. surfactants) to afford stabalization (steric or electronic).

Introduction to Nanoworld

For the uninitiated, Nanoscience deals with materials that falls within the size regime of 1 - 100nm and typically consisting of 100 to 10000 atoms (Recall that 1 mol = 6x10²³ atoms). The laws of classical chemistry (i.e for bulk materials) and quantum chemistry (i.e. for small number of atoms) were once thought to be all it needs to describe the world... until nano made its entrance.

Although nanomaterials mark a transition range between between a single atom and a macroscopic bulk sample, they exist very much in a realm of their own, exhibiting unique properties. (e.g. surface plasmon resonance, Coloumb blockade, increased catalytic performance etc.) This is primarily due to:

1. high dispersity (surface area/ volume ratio) &
2. size quantization of its charge carriers. Spatial confinement of the charge carriers (e^- for metals, exitons for semiconductors) causes the valance and conduction bands to split into discrete, quantized levels similar to those in atoms/molecules.

These properties, as well as the versatility with which they can be manipulated, have rendered nanomaterials immense potential for application in many fields including nanoelectronics, optoelectronics, photovoltaics & catalysis.

At the Beginning

I got acquainted with ‘nano’ in the fourth year of my undergraduate studies, where I’d to do a final year project. However, ‘nano’ had long manifested in my life i.e. my ‘Arial font 8’ handwriting, ‘B5 = A3’ syndrome, ‘9 slides in 1 page’ tendencies… So it didn’t come as a surprise when I got fascinated with the idea of doing a ‘nano’ project, even when I haven’t had the slightest idea what ‘nano’ is about.

Despite sound knowledge in most fields of Chemistry (Organic, Inorganic, Organometallics, Analytical, Physical...), I took the leap of faith & ventured down the road less travelled... now here it is... my nano world.