p-Type alpha-Fe2O3 nanowires and their n-type transition in a reductive ambient.

One-dimensional metal oxide nanomaterials have attracted much attention due to their semiconducting behavior and applications in nanodevices such as gas sensors, photoACHTUNGTRENNUNGdiodes, field-effect transistors (FETs), and light-emitting diodes. The control of electrons and holes, that is, nor ptype nature, inside the functional metal oxide is of vital importance in the application of nanodevices. In general, ntype metal oxide nanomaterials are easily formed due to oxygen deficiencies (vacancies), which result in a donor level below but close to the conduction band. By contrast, p-type behavior is hard to achieve except in cases where the material is naturally p type. The common way to form the p type is through diffusion during the growth process or implantation into an as-grown sample, followed by post-annealing at high temperatures in order to eliminate defects. By contrast, by using a reductive ambient, the direct transition between the n and p types of a metal oxide can be ACHTUNGTRENNUNGachieved through a change of dominant carriers on the surface, namely, those of electrons or holes. This n–p switch was first found in Cr2O3 under a treatment of ethanol vapor. In addition, the n–p switch was found in other functional metal oxides, such as SnO2, MoO3, and In2O3, under certain kinds of reductive ambient. However, the details of the n–p transition are still under investigation. a-Fe2O3 is the most stable iron oxide compound material and is widely used in photoelectrodes, gas sensing, catalysts, magnetic recording, and medical fields. Recently, the n–p switch was demonstrated in an a-Fe2O3 thin-film sample after annealing with oxidation and reduction processes, for which the main principle was based on the surface adsorption of oxygen to increase band bending near the surface. It is well known that the surface of a nanowire (NW) is very unstable, due to the large surface-to-volume ratio, and it easily adsorbs foreign molecules for stabilization. By using NWs with unstable surface states, the n–p transition through surface adsorption is likely to be much more easily ACHTUNGTRENNUNGcontrolled and achieved than that with thick films. The divergence of measurement methods between our previous report and Ref. [6] is consistent with our argument. Following our success in synthesizing a-Fe2O3 NWs with ordered oxygen vacancies, in the present study we focus mainly on the electronic properties of those a-Fe2O3 NWs with oxygen vacancy orders in which the p-type nature can be found in as-grown NWs without any additional annealing. After a process of annealing in a reductive ambient, the p-type to n-type transition was observed. The detailed electronic structure of the a-Fe2O3 NWs before and after annealing was measured by electron energy loss spectrometry (EELS). The finding of a p–n transition suggests the potential application of the NWs in future nanodevices. Figure 1a shows an SEM image of a-Fe2O3 NWs synthesized on an Fe64Ni36 substrate at 450 8C for 10 h in an Ar ambient of 100 sccm. It is obvious that a-Fe2O3 NWs can be uniformly formed on a large scale. A typical TEM image of an a-Fe2O3 NW, with a diameter of 18 nm, is shown in Figure 1b. The sequential periodic structure with a regular spacing of 1.45 nm can be observed in the image shown in Figure 1c. The corresponding diffraction pattern, shown in Figure 1d, confirms the phase of a-Fe2O3 with a [001] zone axis. The extra spots can be found in the diffraction pattern after detailed examination and have ten times the distance of the ð330Þ plane, as marked by arrows. Figure 1e shows the corresponding high-resolution TEM image highlighting the ordering structure. The two almost-identical d spacings of 0.25 nm are consistent with the d values of the ð210Þ and ð110Þ planes, thereby indicating the single-crystal nature of the wires with growth along the [110] direction. The longrange ordering phenomenon with a periodicity of 1.45 nm is located and marked by white arrows. The inset in the bottom of Figure 1e shows the corresponding fast Fourier transfer (FFT) of the image, which is accurately consistent with the results of the diffraction pattern. In order to clarify the ordering phenomenon found in both the diffraction pattern and the FFT pattern directly transformed from the high-resolution TEM image, the ideal high-resolution TEM image was processed by using an inverse FFT method through selection of only the matrix reflections from the stoichiometric a-Fe2O3 structure, as shown in Figure 1g. In a comparison of the three high-resolution images in Figure 1e–g, the ambiguous lattice image (dark image) between the two nearest bright dots can be seen (along the white arrows in Figure 1e); this is suggested to be caused by ordered oxygen vacancies. The atomic resolution image ac[*] Y.-C. Lee, Y.-L. Chueh, C.-H. Hsieh, M.-T. Chang, Prof. L.-J. Chou Department of Materials Science and Engineering National Tsing Hua University, Hsinchu, Taiwan 300 (ROC) Fax: (+886)35722366 E-mail: [email protected]