Research

Synthesis of complex quasi 1D oxide nanostructures

Our activity in synthesis of  quasy-1D nanostructures is focused on  metal oxides  due to their demonstrated feasibility for real world applications in chemical and bio-sensing, optoelectronics, energy conversion and medicine. Namely we are currently concentrate on three classes of the nanostructures :

 a) conventional single crystal well faceted pristine nanorods and nanobelts are particular important for the fundamental studies on the interplay between electron transport  and surface reactivity

b) segmental nanostructures and superlattices, where the composition and morphology of the segments can be tailored.   As an example, the narrow parts of such segmented quasi 1D nanowires can be  made few nanometer thick thus approaching the quantum confinement regime.  From the applications prospective the important advantage of such nanostructures is a smooth transition from the micro-to-nano regimes and as a result-elimination of the parasitic influence of the contact effects on the performance of conductometric device since large size of the microscopic end sections will reduce contact resistance drastically. This work is continuing collaboration with Dr. Y. Lilach (now at PNNL) and with Prof. M. Moskovits group (UCSB) 

d) The surface and /or bulk doping of the nanostructures adds inherent functionality to the device which is called to improve selectivity and sensitivity/reactivity  towards specific target molecules or particular surface reaction.

    The development of new experimental tools, approaches and techniques

   Fundamental surface science of 1D nanostructures implies rigorous requirements on the purity of the nanostructure's surfaces as well as on their crystalline, structural and morphological quality. Due to these requirements we have to use high (HV) and ultra high vacuum (UHV) equipment in our research and ultra pure materials and gases for “dry” vapor-solid, vapor-liquid-solid growth methods to avoid bulk and what is more important surface contamination. For the same reason we avoid the conventional resist based lithography in device fabrication protocols. Therefore the micro fabrication, surface treatment, functionalization and patterning have to be based on HV and UHV depositions methods. In addition, at such a small scale the complexity of the experiment is dictated by strong size dependence of the surface phenomena. Due to this fact it is feasible to perform all measurements and treatments on the same individual nanostructure.

Toward these goals:

    (i) we are developing special hardware and methodologies for making complex contacts to the individual nanostructures using precisely positioned shadow masks.

   (ii) we conduct our  measurements in UHV compatible probe stations and chambers and adopt traditional surface science protocols for the treatment of the nanostructure's surfaces

   (iii) we perform measurements and treatments (as an example surface functionalization) in situ on the same nanostructure, thus results "before" and "after" can be compared directly

   (iiii) we are testing nonplanar sample architectures to eliminate the parasitic influence of the support

Exploring the surface phenomena on quasi 1D metal oxides:

From fundamental transport properties to conductometric nanowire gas sensors

The application of semiconductor oxide nanowires as solid-state chemi- and bio sensors is an area of  apparent technological promise. With the great progress achieved recently in the development of the effective growth techniques and proven sensing performance the focus in research is shifted towards the better understanding of the fundamentals of surface reactivity of   low dimensional nanostructures. One of the most sensitive channels toward electronic changes induced by adsorbates is  electron (hole) transport through the nanowire.  The latter is going to be more significant when the diameter of the nanowires approaches quantum confinement regime. 

We are studying the electron /hole transport in these ultra small chemiresistors and chemi- field effect transistors in a wide temperature range and under the exposure to variety of target molecules. The important information on the chemical reactions which take place at the surface of the nanostructure can be obtained via the kinetic studies under the pulsed exposures.

 We are applying the obtained fundamental knowledge to fabricate "real world" nanosensor prototypes. The latter can be made out of individual nanostructures or out of their network. The problems to address are sensor stability, selectivity and robustness issues.  We are  currently working on the conductometric micro device based on array of different (and /or differently functionalized) nanowires, thus creating the simplistic prototype of the electronic olfactory system.