Multi-layer nanoarrays sandwiched by anodized aluminium oxide membranes: an approach to an inexpensive, reproducible, highly sensitive SERS substrate.

A large-scale sub-5 nm nanofabrication technique is developed based on double layer anodized aluminium oxide (AAO) porous membrane masking. This technique also provides a facile route to form multilayer nano-arrays (metal nanoarrays sandwiched by AAO membranes), which is very challenging for other techniques. Normally the AAO mask has to be sacrificed, yet in this work it is preserved as a part of the nanostructure. The preserved AAO layers as the support for the second/third layer of the metal arrays provide a high-refractive index background for the multilayer metal arrays. This background concentrates the local E-field more significantly and results in a much higher Surface-Enhanced Raman Spectroscopy (SERS) signal than single layer metal arrays. This technique may lead to the advent of an inexpensive, reproducible, highly sensitive SERS substrate. Moreover, the physical essence of the plasmonic enhancement is unveiled by finite element method based numerical simulations. Enhancements from the gaps and the multilayer nanostructure agree very well with the experiments. The calculated layer-by-layer electric field distribution determines the contribution from different layers and provides more insights into the 3D textured structure.

Wide range bandgap modulation based on ZnO-based alloys and fabrication of solar blind UV detectors with high rejection ratio.

Theoretical calculations on formation energies of MgZnO, BeZnO and BeMgZnO alloys are presented. The ternary alloy MgZnO (BeZnO) is found to be unstable with high Mg (Be) contents. However, the quaternary system BeMgZnO is predicted to be stable with small Be/Mg atom ratio. Subsequently, a wurtzite Be0.17Mg0.54Zn0.29O alloy with a bandgap of 5.15 eV has been acquired experimentally. Its bandgap is in the middle of solar blind region and thus it is an ideal material for realizing a high rejection ratio solar blind ultraviolet (UV) detector, which has long been a problem. A metal-semiconductor-metal (MSM) structured solar blind UV detector based on this material is then fabricated, realizing a much higher rejection ratio than reported MgZnO-based detectors. One more interesting thing is, as a complicated quaternary system, BeMgZnO can maintain its crystal quality in a wide compositional range, which is not happening in MgZnO and BeZnO. To get some microscopic insight into the Be-Mg mutual stabilizing mechanism, more calculations on the lattice constants of BeZnO and MgZnO alloys, and the coordination preference of Be ions in alloy were conducted. The a-axis lattice compensation and 4-fold coordination preference of Be atom are confirmed the major origins for Be-Mg mutual stabilizing in ZnO lattice.