Chemical Nanosensors

We believe that the employment of new nanomaterials, dual electro-optical transduction and cost effective fabrication methods allow the development of novel chemosensors/ sensor arrays for precise and reliable detection of small molecule gases. Dual transduction, heterogeneous integration of a transducer and analyzing elements with sensory nanomaterial can significantly improve the device's performance: dramatically increase sensitivity and selectivity, reduce response and recovery time, make it robust and cost effective. In particular, we are now working on the development of nanocomposite optical chemosensors for explosives detection, where one-dimensional nanoporous photonic crystal is infiltrated with fluorescent sensory polymers^1,2,8.

Images of the prototypes for explosive detection Mark-I (left) and Mark-II (right).

The capabilities and potential of our first portable prototype (Mark-I) for explosive detection were successfully demonstrated in 2008 year at Army facilities. Later, in 2009, the second improved prototype (Mark-II) was demonstrated to the Army representatives at Emitech' laboratory. Mark-II prototype is capable to detect RDX/PETNT/TNT vapors and particulates in real time mode using our proprietary sampling system based on air-jet technique. Another feature of Mark-II prototype is the sensor mounting on the robotic vehicle and wireless communication with remote terminal. Both prototypes demonstrate much better selectivity to nitroaromatic interferants than commercially available portable detector for explosives.

The video shows detection of RDX with surface concentration of 2 µg per cm² by our Mark-II prototype equipped with Emitech's proprietary sampling system, which allows to detect RDX in real time mode (real time detection is not possible for commercially available portable detector).

Another direction is related to the prototyping of the resistive gas sensors based on point heterocontact between carbon nanotubes and metal wires⁷. In collaboration with a Ukrainian team (Inst.Low Temp.Phys., Kharkov), we proposed to develop sensors for noninvasive human breath analysis and a decease diagnostic. This is a good example, as the combination of a fundamental scientific concept (point contact spectroscopy) with nanoscaled structures provides a novel sensing platform and advanced sensory devices with extremely high sensitivity, good selectivity and fast response/recovery time. These gas sensors are principally distinctive from any existing resistive chemosensors employing sensory films or nanomaterials due to their nano/angstrom conductive channel leading to superior sensing characteristics.

IR Photodetectors and Solar Cells Based on Organic-Inorganic Nanocomposites

SEM image of single walled carbon nanotube (SWNT)- Si heterojunction.

A new generation of IR photodetectors and solar cells are in the focus of Emitech's activity. We were the first to report about photoconductive response of carbon nanotube film illuminated by white/NIR light (not laser pulse) of low intensity¹³. The use of carbon nanotubes as an advanced nanomaterial for photodetectors is very attractive because of their suppressed thermal noise and the opportunity to tune the operational range by varying the nanotube diameter. Recently, we found that the junction between nanotubes and some semiconductors exhibits photoconductivity at far IR light. This project was awarded by a Phase II SBIR grant in 2008. It is remarkable that the similar device can function as a novel type of the hybrid solar cell comprising carbon nanotube- Si heterojuction^4,5, where carbon nanotubes works as an active photosensing component.

SEM image of porous Si surface and cross-section after electrochemical etching.5

We fabricated and tested a novel nanocomposite hybrid solar cell from porous Si infiltrated with small organic molecules ^3,11. The fabrication of the cell is very simple and cost effective procedure. Organic/inorganic components are inexpensive materials and electrochemical etching is a very simple method which does not require any special facilities like high vacuum deposition, lithography or clean rooms. Moreover, this method can be applied to fabricate standalone porous Si thin membrane by reusing the same Si substrate and should considerably reduce the operational cost. In addition, such membrane based solar cells can be prepared as a flexible, lightweight thin film if the appropriate polymer filling is applied. This project was sponsored by NSF SBIR program and were supported by the Boeing Company and several photovoltaic local companies.

Carbon Nanotubes – Polymer Actuators and Field Effect Transistors

Novel and unique phenomena possessing tremendous commercial potential have been discovered in the field of electrical and photo mechanical microactuators by Emitech's team with the support of the University of RI.

Micrograph of a 20×3.5 mm SWNT-Nafion-SWNT cantilever with 0 V bias (left) and with 3.5 V, 28 Hz ac bias (right).

A system composing of an ionomeric polymer (nafion) coated with single-walled carbon nanotube (SWNT) electrodes acts as an electromechanical actuator¹². The actuator gives a sizable mechanical response to low voltages under open-air conditions. The actuator is active under both DC and AC bias and has a strong resonance at low frequencies that is dependent upon the size of the actuator. Remarkably, such a device demonstrates a very long actuation lifetime, much longer than any existing polymer or only carbon nanotube based actuators.

The left micrograph shows a nafion (180 µm)/ SWNT(15µm) bilayer prior to light exposure and the right micrograph shows the same cantilever under illumination of ~ 100 mW/cm2 white light after 20s. 3

Photomechanical actuators¹⁰ is the next scientific and technological breakthrough discovered by us. It was found a strong mechanical response of nafion-SWNT composite on visible-NIR light of low intensity. The figure shows the cantilever displacement on the sub-mm scale under light illumination. Also, you can watch the movie [below, right; use slider to fast forward], demonstrating the cantilever movement induced by light. Such a remarkable photomechanical actuation effect is new and its nature is principally distinctive from existing photostrictive mechanisms in semiconductors, polymers/gels and carbon nanotubes. A cantilever tip displacement can be considered as a gigantic photo elastic response (up to one mm) with respect to semiconductors (several tens nm) at the same conditions.

Flexible field effect transistors (FETs) were fabricated from similar composite material demonstrating an excellent gating effect⁹. To our knowledge, this is the first report about a SWNT based FET fabricated in a micro format with extremely thick SWNT film (10 µm ) and an insulating layer ( ~ 180 µm nafion membrane ). The image on the home page shows the schematic of our flexible field effect transistor fabricated from SWNT- nafion composite.