Department of Electrical Engineering, University of North Texas, Denton, TX 76203, USA.
Nanotechnology. 2013 Jun 7;24(22):225704. doi: 10.1088/0957-4484/24/22/225704. Epub 2013 May 3.
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 × 10(17) cm(-3), while nanowires that were annealed in wet O2 showed a reduced carrier concentration of less than 10(16) 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.
研究了单晶 In2O3 纳米线的化学计量比对其输运性能和气体传感性能的影响。纳米线通过气相传输法合成，直径为 80-100nm，长度为 10-20μm，生长方向为[001]。输运测量表明，纳米线呈 n 型导电，这归因于晶体点阵中存在氧空位。未经退火的 In2O3 纳米线的载流子浓度约为 5×10(17)cm(-3)，而在湿氧中退火后的纳米线的载流子浓度则降至小于 10(16)cm(-3)。对未经退火的纳米线进行了温度依赖的电导率测量，并对 77-350K 温度范围内的热激活 Arrhenius 导电机理进行了分析，得出的激活能为 0.12eV。这可以解释为表面态和纳米线体相之间的载流子交换导致了厚度约为德拜长度(LD)的耗尽表面层，对于未经退火的纳米线，LD 约为 3-4nm，而对于更化学计量的纳米线则高 10 倍左右。在室温下，单个 In2O3 纳米线在接触 NH3 和 O2 气体分子后的几秒钟内，其电导也发生了显著变化，这表明 In2O3 纳米线有望作为高效的小型化化学传感器使用。传感机制主要由纳米线沟道电导决定，采用简单的能带图来解释当气体分子吸附在纳米线表面并影响其电性能时，电导率的变化。发现化学计量比较低的纳米线对氧化性气体更敏感，而化学计量比较高的纳米线对还原性气体的响应明显增强。
