When an atom is lacking in a crystalline lattice, the thermal, electronic, and mechanical properties of the material change. In nanomaterials, these properties are not always easily measurable and theoretical studies can help researchers understand and predict modifications of the structure and properties of materials at the nanoscale. G. Guisbiers of the Catholic University of Louvain recently studied vacancies formation in nanoparticles with the help of classical thermodynamics.
Thermodynamics is a top-down approach that avoids many-body calculations. It is known to be relevant when the thermal fluctuations, proportional to the inverse square root of the number of particles in the system, are small. Thus, issues concerning nanoparticles with diameters down to 4 nm can be addressed by thermodynamics, as reported in the February 17th issue of the Journal of Physical Chemistry (DOI: 10.1021/jp108041q; p. 2616).
Guisbiers uses a universal equation linking the bulk property of a material and the size and shape of the particle to deduce the corresponding property in nanoparticles for a number of metals and semiconductors. Results show that vacancies form more easily in nanosized materials than in the bulk. In energetic considerations, this can be related intuitively to the augmentation of the surface- to-volume ratio, since vacancies can be thought of as internal surfaces.
“Even if the vacancy concentration of a nanoparticle increases compared to its bulk vacancy concentration, this model can also explain why nanomaterials appear to be perfect. It is due to the limited number of atoms in a particle,” said Guisbiers.
The predicted increase in concentration of vacancies matches several experimental results. It leads to bond length contraction. This induces that nanomaterials are harder and possess higher yield strengths than bulk materials, a phenomenon known as the Hall–Petch effect. The higher concentration of vacancies also lowers phonon frequencies, an effect that is observed experimentally. It results in lower electronic and thermal conductivities due to enhanced electron and phonon scattering.
Guisbiers said that this study could help in the understanding of the formation of nanopores, widely used in nanotechnology applications.
