More information on our research projects:
- Metal nanowire films
- Nano-chirallity and chiroptical effects
- Magneto-transport in magnetic nanoparticle films
We have developed a wet-chemical technique for the deposition of high aspect ratio gold/silver nanowire mesh films by spreading small metal seed nanoparticles at the desired locations followed by deposition of a nanowire growth solution on a substrate. Ultra-thin gold-silver nanowires spontaneously form on the substrate. These nanowires can be further coated with silver to obtain controllable conductivity and transparency of the film, for various transparent electrode applications (various displays, lighting, smart windows etc...)
The mechanism of the nanowire formation involves formation of a tubular surfactant template which is induced by the metal seeds that initiate the nanowire growth reaction. This reaction proceeds trough catalytic reduction of gold and silver ions by ascorbate on the pre-fromed metal seed particles, until all the seed particles become connected into ultrathin nanowires within the tubular surfactant structures.
Our work in nano-chirality is about connecting chiral molecules and inorganic nanostructures in various ways. We are primarily interested in chiroptical properties of such hybrid structures. The main spectroscopy technique that we use is circular dichroism (CD).
A typical system that we study is a colloidal noble metal (gold, silver) nanoparticle with strong surface plasmon excitations or a semiconductor quantum dot with discrete electron-hole excitations coated with chiral (bio)molecules, such as amino acids. In many cases we observe circular dichroism peaks at wavelengths corresponding to electronic transitions of the inorganic core (plasmons, excitons). Our aim is to study and understand the mechanisms leading to the induction of the chiroptical effects in the nanocrystal cores. We believe that circular dichroism spectroscopy of such systems may reveal new information on the interaction of molecules with metal and semiconductor surfaces and about the molecular conformation at these surfaces.
An interesting case is when the nanocrystals are made of material which crystallizes in a chiral symmetry group, as in the famous case of quartz. In this case interesting chiroptical effects are expected. We have recently demonstrated that intrinsically chiral mercury sulfide (Cinnabar) nanocrystals can be prepared with high enantiomeric excess using chiral thiolated molecules such as cysteine or penicilamine. Later we showed that also intrinsically chiral Tellurium and Selenium nanocrystal can be prepared at high enantiomeric excess using similar methods.
In another set of projects we have studied magnetite (Fe3O4) nanocrystals. Magnetite has very interesting electronic properties – it is ferrimagnetic and was claimed to be half-metallic. Hence, it is supposed to be a strongly spin-polarized conductor. The bulk material shows a metal-insulator phase transition around 120K and we were able to observe this phenomenon around 100K in 5 nm nanocrystals. In one recent work we have prepared magneto-resistive double tunnel junctions from a spin-filtering substrate on which we scattered isoltaed colloidal magnetite nanocrystals and a scanning tunneling micriscope's (STM) tip above the particle. By tuning the temperature in a variable temperature STM we were able to slow down the magnetization switching of small magnetite nanocrystals (~10 nm) and observe current fluctuations due tot he magnetization switching of the nanocrystal.
We have recently prepared films of colloidal nickel nanoparticles which were conducting and exhibited substantial Extraordinary Hall Effect signals (in collaboaration with the group of Alexander Gerber (TAU School of Physics). Such films may be printed using inkjet printing techniques and thus be integrated in 3D printed electronic systems.