Introduction

Functional materials are materials that encompass some inherent functionality. In other words they have a predictable, and perhaps, manageable change to a given response. For example a material that is used as a photodetector is functional in the sense that it responds in a predictable way to a given optical stimulus. A piezoelectric material is also functional. A sample of given dimensions will produce a known surface charge upon application of a given mechanical stress. This is known as the direct piezoelectric effect. Conversely if an electric field is applied to the piezoelectric material it will change dimensions in a process known as the converse piezoelectric effect. For the purpose of converting vibration (or kinetic energy) into electricity the direct piezoelectric effect is used. Devices have been using this curiosity of the material since early reports in the 1990s and early 2000s [1, 2] and as such it is very much an emerging field.