AVS2016 Session NS-MoA: Nanophotonics, Plasmonics, and Energy

Bandgap tuning beyond 1.3 µm in GaAsSb based nanowires by incorporation of dilute amount of N is reported, for realizing nanoscale optoelectronic devices in the telecommunication wavelength region. Vertical GaAs/GaAsSbN/GaAs core-shell configured nanowires are grown on Si (111) substrates using plasma assisted molecular beam epitaxy. Effects of N incorporation and thickness of the shell layers on the micro-photoluminescence spectral peak shifts have been studied. Annealing in N₂ ambient led to enhanced spectral intensity, which is attributed to the annihilation of defects. Shifts and changes in the spectral shapes of the Raman spectra prior to and after annealing have been used to ascertain the nature of the defects being annihilated during the growth. I-V measurements also provided further support to the annihilation of predominantly point defects on annealing. Results from the transmission electron microscopy study on the planar defects will also be presented.

Nanostructures made of semiconducting metal oxides such as TiO₂, ZnO, SnO₂, WO₃, etc. have a remarkably versatile application spectrum, serving applications in sensing, catalysis, photocatalysis and solar cells. Metal oxide nanostructures abound in electronic defects originating in their high surface area and the method of fabrication. Such defects include dangling bonds, grain boundaries and color centers which in turn may act as shallow or deep trapping sites for electrons and/or holes. These defects have been more or less uniformly been viewed negatively in the literature for their deleterious effects on light harvesting and charge transport. However, a recently emerging view is that the defects also provide an opportunity to engineer sensitivity and much-needed selectivity in sensor designs, particularly with regards to the detection and quantification of small molecules.

We used highly ordered TiO₂ nanotube arrays (TNA) grown by low-cost electrochemical anodization as platforms to perform the selective sensing of alcohols without the use of external binding receptors. TNA membranes were placed in the active coupling gap of a microwave ring-type resonator [1]. By monitoring the resonator's Quality factor (Q) and resonance frequency (f₀) as a function of time following light illumination of the nanotube membrane, we were able to distinguish between the methanol, ethanol and isopropanol [2].

Our work also brings in focus a hitherto underexplored topic in nanomaterials - namely the leveraging of the interactions of microwaves and semiconductor nanostructures to build better sensors and diagnostic platforms [2]. Extension of this concept enabled us to detect a molecular monolayer by monitoring the interactions of microwaves with semiconductors, and also enabled us to use the molecular monolayer to tune the electronic interactions of the surface of wide bandgap TiO₂ with external analytes in the service of VOC sensing as well as extremely low-level photodetection.

REFERENCES

1. Zarifi MH, Mohammadpour A, Farsinezhad S, Wiltshire BD, Nosrati M, Askar AM, Daneshmand M and Shankar K, TRMC Using Planar Microwave Resonators: Application to the Study of Long-lived Charge Pairs in Photoexcited Titania Nanotube Arrays, Journal of Physical Chemistry C, 119 (25), 14358-14365, 2015.

2. Zarifi MH, Farsinezhad S, Abdolrazzaghi M, Daneshmand M and Shankar K, Selective microwave sensors exploiting the interaction of analytes with trap states in TiO2 nanotube arrays, Nanoscale, DOI: 10.1039/c5nr06567d, 2016.

Artificial metamaterials – metallo-dielectric composites tailored on deep-subwavelength scale – enable implementation of electromagnetic responses not found in nature, leading to potentially useful applications as well as yielding new insights into the fundamental nature of light. Here we show how we have leveraged ultrasmooth planar nanoplasmonic waveguides deposited by ion-beam-assisted sputter deposition to implement easy-to-fabricate bulk metamaterials operating at visible and near-ultraviolet wavelengths and having refractive indices ranging from highly anisotropic and positive [1] to quasi-isotropic and negative [2]. Exploiting these structures to tailor the flow of light in exotic ways, we realize devices ranging from high-contrast, near-field nanoparticle optical sensors working in the visible, to the first implementation of a Veselago flat lens [3] functioning in the near ultraviolet. Substituting Al for Ag as the constituent plasmonic metal of choice, we investigate the extension of bulk metamaterial operation into the far-ultraviolet, for lithographic applications beyond the diffraction limit.

[1] T. Xu and H.J. Lezec, Nat. Comm. 5, 4141 (2014). [2] T. Xu, A. Agrawal, M. Abashin, K.J. Chau, and H.J. Lezec, Nature 497, 470 (2013). [3] V.G. Veselago, Sov. Phys. Usp. 10, 509 (1968).

Plasmonic nanostructures are of great interests recently due to their ability to concentrate light to small volume. They have many potential applications in optical communication, disease diagnosis, and chemical sensing. Therefore it is extremely important to investigate the plasmonic hot spots both theoretically and experimentally. While it is theoretically predicted that the optimal hot spot is a sub-5 nm gap between two metallic particles , due to the difficulties in fabrication of sub-5 nm structures, most of the studies on hot spot behaviors at that scale are theoretical only. Therefore, it is essential to find a way to fabricate hot spots with sub-5 nm gap sizes deterministically and reliably as the experimental platform to probe and utilize those hot spots.

Recently, we have successfully fabricated gap plasmonic structure with precisely controlled nano-gap by using collapsible nano-fingers. First, a nano-finger array in flexible polymer (i.e. nanoimprint resist) is fabricated using nanoimprint lithography (NIL), and metallic caps, such as gold disks, are deposited on the top of each finger using electron-beam evaporation. Second, atomic-layer deposition (ALD) is used to coat a thin conformal dielectric layer. Finally, the nano-finger sample is dipped into Ethanol (water works too) and air-dried. When the Ethanol dries up, the capillary force makes the nano-fingers close together. The ALD-coated dielectric layer serves as the spacer to define the gaps between the metallic particles. If we use TiO2 as an example, each atomic layer of TiO2 is only about 1Å thick, which means the gap between the metallic particles can be precisely controlled with an accuracy of 2 Å and as small as 2 Å. For the first time, we can reliably achieve such small gaps deterministically and precisely. It is the ideal experimental platform to probe the rich sciences at the gap plasmonic hot spots.

As the polarized light shone on the dimer-like structure, it will trigger dipole-like charge distribution inside gold nanoparticle. Based on classical electromagnetic theory, field at gap center increases as the gap gets smaller. However, as gap size reduces, for sub-5 nm gap structure, electron tunneling between two gold nanoparticles becomes significant, which would cancel part of the charge in opposite sides and hence reduce the field. The competing factors result in an optimal gap size for the strongest optical field enhancement. But such a small gap structure has not been fabricated reliably until recently we demonstrated how to define and scale sub-5nm gaps by using collapsible nano-fingers