Stoichiometry dependent electron transport and gas sensing properties of indium oxide nanowires

• Published in: Nanotechnology 2013-06, Vol.24 (22), p.225704-225704
• Main Authors: Gali, Pradeep, Sapkota, Gopal, Syllaios, A J, Littler, Chris, Philipose, U
• Format: Article
• Language: English
• Subjects: Ammonia - analysis; Carrier density; Channels; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Cross-disciplinary physics: materials science; rheology; Electric Conductivity; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Electronic transport in multilayers, nanoscale materials and structures; Equipment Design; Exact sciences and technology; Gas sensors; Indium - chemistry; Indium oxides; Materials science; Methods of nanofabrication; Nanocrystalline materials; Nanoscale materials and structures: fabrication and characterization; Nanowires; Nanowires - chemistry; Oxygen - analysis; Physics; Quantum wires; Stoichiometry; Transport; Vapor phases
• ISSN: 0957-4484; 1361-6528
• DOI: 10.1088/0957-4484/24/22/225704

The effect of stoichiometry of single crystalline In2O3 nanowires on electrical transport and gas sensing was investigated. The nanowires were synthesized by vapor phase transport and had diameters ranging from 80 to 100 nm and lengths between 10 and 20 μm, with a growth direction of [001]. Transport measurements revealed n-type conduction, attributed to the presence of oxygen vacancies in the crystal lattice. As-grown In2O3 nanowires were shown to have a carrier concentration of 5 × 1017 cm−3, while nanowires that were annealed in wet O2 showed a reduced carrier concentration of less than 1016 cm−3. Temperature dependent conductivity measurements on the as-grown nanowires and analysis of the thermally activated Arrhenius conduction for the temperature range of 77-350 K yielded an activation energy of 0.12 eV. This is explained on the basis of carrier exchange that occurs between the surface states and the bulk of the nanowire, resulting in a depleted surface layer of thickness of the order of the Debye length (LD), estimated to be about 3-4 nm for the as-grown nanowires and about 10 times higher for the more stoichiometric nanowires. Significant changes in the electrical conductance of individual In2O3 nanowires were also observed within several seconds of exposure to NH3 and O2 gas molecules at room temperature, thus demonstrating the potential use of In2O3 nanowires as efficient miniaturized chemical sensors. The sensing mechanism is dominated by the nanowire channel conductance, and a simple energy band diagram is used to explain the change in conductivity when gas molecules adsorbed on the nanowire surface influence its electrical properties. Less stoichiometric nanowires were found to be more sensitive to oxidizing gases while more stoichiometric nanowires showed significantly enhanced response to reducing gases.