15:30-16:00

Controlling the crystallographic polarity of wurtzite structure thin-films

The wurtzite structure is crystallographically polar (asymmetrical), which gives rise to anisotropy in a wide range of materials properties: electronic, piezoelectric, chemical, mechanical, and interfacial. Moreover, there are many technologically important materials that have the wurtzite structure, such as GaN, ZnO, AlN, α-SiC, and CdS. Therefore, it would very convenient to be able to measure the polarity AND control it. We have developed several ways to do both.

The ability to measuring the crystallographic polarity is extremely important since it is the first step towards controlling the polarity. We have developed two different polarity measurement techniques, both of which are non-destructive and based on the photoelectric effect. One method uses the diffraction of photoelectrons to interpret the crystal structure.¹ The other method uses photoelectrons from the valence band as a compass that point to the crystallographic north.² These two measurement techniques enable us to develop methods to control the polarity.

Currently we are focusing on physical vapor deposition (PVD) thin-films of ZnO for our polarity controlling studies. Firstly, we have found that dopants can be use to invert the polarity.³ Here, the type of dopant and concentration are the important parameters. We believe that this is related to partitioning of the dopant.

Lastly, we have found that the polarity can be controlled during PVD by applying a bias to the substrate. We believe that the applied bias interacts with the internal electric field in the wurtzite structure and thus directs its configuration. This particular method is promising because it utilizes the intrinsic nature of the wurtzite structure, and so it is not specific to a particular material. Rather, this method is applicable to the family of wurtzite materials.

¹ J.R. Williams et al., Surf. Sci. 605, 1336 (2011).

² N. Ohashi et al., Appl. Phys. Lett. 94, 122102 (2009).

³ Y. Adachi et al., Thin Solid Films 519, 5875 (2011).

Speaker

Dr. Jesse Williams, ICYS-MANA Researcher, NIMS

Chair

Dr. Naoki Ohashi, Division Director, Environment and Energy Materials Division, NIMS

16:00-16:30

Nanocomposites for sustainable photocatalysis

Enough energy from sunlight strikes our planet in one hour to provide all the energy needed for human activity in one year. Harvesting this energy relies on efficient conversion and storage, a considerable challenge given that the technology also needs to be cheap, robust and simple to manufacture. One approach, modelled on photosynthesis, uses photocatalysts to split water into hydrogen and oxygen, enabling storage of energy as a fuel. For this, semiconductor nanoparticles are usually combined with one or more cocatalysts that stabilize charge separation and act as catalytic sites for gas evolution. Platinum is the material of choice for hydrogen evolution. However, in addition to being scarce and expensive, mining and refining of platinum causes considerable environmental problems. For sustainable hydrogen production from photocatalytic water splitting, an alternative cocatalyst is essential.

This proposal aims to create a platform capability for assembling hybrid multi-anion photocatalysts. Here, early transition metal carbides and nitrides (e.g. WC) will be exploited for their platinum-like catalytic properties. By combining these materials with semiconductors, a new range of nanocomposite photocatalysts will be created. This will be achieved through a simple one-pot procedure based on differences in metal oxophilicity. Photocatalytic semiconductors are typically oxides, oxynitrides or oxysulfides of oxophilic metals such as Ti or Zr. Soluble precursors to these semiconductors will be combined in a biopolymer gel with a soluble tungsten salt. On heating under nitrogen, oxide nanoparticles of both metals will nucleate, coupled with decomposition of the biopolymer to a carbon-rich matrix. The difference in oxophilicity will then induce selective carbothermal reduction of the tungsten oxide nanoparticles. N- and S-containing biopolymers can be used to generate oxynitride or oxysulfide semiconductors.

The multi-anion materials generated in this project will represent a new paradigm in photocatalyst synthesis. Furthermore, the simple procedures and readily available starting materials will enable hydrogen production in a much more economically viable and sustainable manner.

Speaker

Dr. Zoe Schnepp, ICYS-Sengen Researcher, NIMS

Chair

Dr. Naoto Shirahata, MANA Independent Scientist, NIMS