(Invited) Manipulation of the Energy Band Levels in Plasma-Produced Energy Materials
Authors: Mariotti Davide 
Published: February 2020

Progress in energy conversion and energy storage strongly rely on our ability to produce “new” and “old” materials with improved properties. Materials properties are always closely linked to the synthesis process and materials integration in application devices is also critical to the overall performance. Therefore, materials synthesis, properties and device integration are intimately interrelated [1]. In addition, the electronic energy band structure of materials largely determines their behaviour and impact the materials performance in application devices [2-4]. For instance, the bandgap of semiconductor is crucial for a large variety of applications in solar energy conversion. However, the full energy band diagram (EBD) requires accurate knowledge of a range of other critical parameters in addition to bandgap. These include the conduction and valence band edge location and Fermi level; together these are decisive for a very wide range of applications and the ability to manipulate the corresponding energy levels represent an attractive opportunity.
Quantum confinement in nanoscale materials offers one possible avenue for the manipulation of the EBD whereby bandgap engineering, the ability to tailor the bandgap of nanoscale semiconductors, has inspired a considerable excitement at the visionary technological opportunities it presents. However, the impact of quantum confinement on the other energy band parameters has received little attention, in part due to measurement challenges as well as due to the higher sensitivity and precision required. Another potential avenue for EBD manipulation is also offered through doping, alloying as well as introducing “defects” in the crystal structure of materials. These, and in particular “defects”, are also as effective as quantum confinement strategies to modify energy levels to suit specific application.
The synthesis of such materials, i.e. with strong quantum confinement and/or with engineered defects, has been explored and widely reported. However, many approaches have also revealed limitations closely bound to thermodynamic limit of solubility or crystal structure stability. Plasma-based synthesis however can break through the boundaries of these limitations and achieve composition that are beyond those expected by phase diagrams for instance. More specifically, atmospheric pressure microplasmas (APMs) are highly versatile and amenable to explore materials with untested composition or “exotic” structures and morphologies [5-7]. APMs can therefore contribute to progress materials science by pushing the boundaries beyond current synthetic capabilities. APM processes can be easily integrated in the application devices fabrication steps so that materials properties can be verified within the relevant application context [8-10].
Here we will report on various synthesis achievements with APMs demonstrating first the synthesis of both strongly quantum confined structures as well as materials with unexpected doping levels and chemical compositions [11-14]. We will then provide preliminary results on the ability of controlling defects in metal oxides. Corresponding changes in the EBD parameters will be evaluated, compared and discussed with a range of analytical techniques. These results will also offer the opportunity to discuss challenges in the measurement methods.

Acknowledgments

This work is supported by EPSRC (awards n. EP/R023638/1, EP/M024938/1, EP/M015211/1)

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