Development and testing of an optoelectronic sensor based on Ti nanotube.

Project leaders

Mauro Zarrelli, Teodor Sisianu

Agreement

MOLDOVA - ASM-not in force - Academy of Sciences of Moldova

Call

CNR/ASM 2011-2012

Department

Molecular Design

Thematic area

Chemical sciences and materials technology

Status of the project

New

Research proposal

Nanotechnology and nanoscale materials are a new and exciting field of research. The inherently small size and unusual optical, magnetic, catalytic, and mechanical properties of nanoparticles not found in bulk materials permit the development of novel devices and applications previously unavailable. One of the earliest applications of nanotechnology that has been realized is the development of improved chemical and biological sensors. Remarkable progress has been made in the last years in the development of optical nanosensors and their utilization in life science applications. To fulfil the promise of ubiquitous sensor systems providing situational awareness at low cost, there must be a demonstrated benefit that is only gained through further miniaturization.
NanoSensors can improve the world through diagnostics in medical applications; improved health and safety and security for people; and improved environmental monitoring. On this line of argument, in last years, IMCB-CNR group has been involved in research activities focused on the development of novel opto-chemical nano-sensors employing near field effects to enhance the overall performance of the final device. Aim of the present proposal is to develop and tests a new class of opto-chemical sensors whose excellent sensing performance are related to an enhancement effect of the optical near field induced by semiconductive structures of zinc oxide.
Although the general principle of the detection mechanism is appreciated, the size of the change of electric conductivity is largely determined by the structural type of the semiconductor, the nature and concentration of surface reactive centers, and the real structure of the material: the size, structure, and degree of agglomeration of crystallites, specific surface area, and pore geometry (Rumyantsevaa et al. 2008 Russian Journal of General Chemistry, 78-5).
In principle, any semiconducting oxide can be exploited as a sensor by monitoring changes of its resistance during interaction with the detected gas molecules at an operating temperature typically above 200 °C. Because tin oxide  offers high sensitivity at conveniently low operating temperatures, attention has been concentrated on this material although lately many studies extended also to other oxides. In fact, several commercial devices based on SnO2 for detecting low concentration of both flammable, i.e. CH4 and H2S and NOx, gases, are available. SnO2 sensors can be referred to as the best-understood prototype of oxide based gas sensors. Nevertheless, highly specific and sensitive SnO2 sensors are not yet available. It is well known that sensor selectivity can be fine-tuned over a wide range by varying the SnO2 crystal structure and morphology, dopants, contact geometries, operation temperature or mode of operation, etc. The electric conductivity of oxide semiconductors is extremely sensitive to the composition of the surface, which reversibly varies as a consequence of surface reactions involving chemisorbed oxygen (O2–, O2–, O–) and the gas mixture components, proceeding at 100–500°C. (Barsan, et al., 1999  Fresenius' Journal of Analytical Chemistry, 365, 287–304. ).
For the proposed near-field opto-chemical sensors the reflectometric configuration has always been used (Pisco et al., 2006  Journal of Lightwave Technology, 24-12). The basic principle is to take advantage from the changes in the optical properties of the sensitive overlays induced by chemical interaction with target analytes to produce modulated light signals. The reflectometric configuration is essentially based on a low finesse and extrinsic Fabry Perot interferometer and it uses a tin oxide thin  layer  deposited at the distal end of a properly cut and prepared fiber to produce a FP cavity. The thin film acts thus as an optical cavity where the first mirror is given by the fiber/sensitive layer interface whereas the second one is given by the sensitive layer/external medium interface. The principle of operation of the refractometric sensor is based on the fact that the changes in the optical and geometric properties of the layer modify the power level reflected by the fiber/layer interface.
As a matter of the fact, studies carried out in the past about the optical properties of tin dioxide showed that absorption region of SnO2 films lies in the wavelength range as low as 250– 300 nm, while in the VIS and NIR wavelength range it results to be transparent (Ansary et al., 1997 Journal of Materials Science: Materials in Electronics. 8-1). When the target analyte is injected in the test ambient, the interaction between its molecules and the sensitive material leads to changes in the reflectance at the fiber-film interface and thus in the sensor output signal.  Titanium dioxide nanotubes are widely used in devices to purify air, to make self-cleaning surfaces, in photovoltaics and sensors, and in biomedicine. Now, researchers in Moldova report on a new way to control the inner diameters of the tubes by simply changing the electrolyte temperature during processing. The new result could widen the potential applications for these nanotubular structures even further. Ion Tiginyanu's team at TUM and the Academy of Sciences, Moldova, began by anodizing titanium sheets below 0 °C in an electrolyte containing ethylene glycol and hydrofluoric acid. This technique produces self-organized surface nucleation layers with ordered arrays of nanochannels distributed in a 2D hexagonal lattice. Tiginyanu and colleagues discovered that each pore at the surface represents a starting point for the subsequent growth of a double-walled titania nanotube. These individual nanotubes can then be easily detached from the network and studied separately. Indeed, the Moldova team found that individual titania tubes luminescence thanks to a micro-cavity effect, where light follows closed trajectories inside the tubular structures.

Research goals

The main objective of the proposed collaboration between the Laboratory of Nanotechnologies (ASM) and the IMCB is to develop and later test optoelectronic sensor based on titanium oxide by mean of  refractometry configuration.
 
The scientific experience of ASM group (Laboratory of Nanotechnologies and Department of Microelectronics TUM) regarding the titania nanotubes fabrication technologies and the know-how acquired by the Advanced Material Group of IMCB-CNR within area of optoelectronic sensor based on SnO2 will be synergistically employed to achieve the target result.
 
The main stages of the project will be:
a)                the elaboration of methods of synthesis for titanium dioxide nanotubes
b)               morphological characterization of obtained titanium oxide nanotube;
c)                set-up of optoelectronic based sensor with titanium oxide nanotube layer;
d)               titanium oxide nanotube based sensor testing (morphology, electrical properties);

Last update: 07/06/2025