Dilute magnetic semiconductors (DMSs) that are formed by the partial replacement of cations in semiconductors by magnetic transition metal ions have drawn considerable attention because of their potential use in spintronic devices. [1,2] Many reports on the observation of ferromagnetism (FM) at room temperature in transition-metal-doped ZnO, TiO 2 , and other semiconducting oxides have been published.[3±14] However, the origin of FM in these materials is still not well understood. Mechanisms based on carrier-mediated FM [15,16] and percolation of bound magnetic polarons [3,17,18] have been proposed. The presence of a certain level of free carriers is required for carrier-mediated FM, whereas the bound magnetic polaron model is also applicable to the insulating state. In order to explore the mechanism of FM, epitaxial thin films of Co-doped ZnO have been grown using ultrasonic-assisted solution chemical vapor deposition (UASCVD). The technique is a simple, soft, combinatorial process that allows for growth at low temperatures using either inorganic [19] or organic precursors. [20] Deposition can be carried out in the absence of a vacuum and this is particularly useful for preventing evaporation of cations of high volatility. First, the influence of growth temperature on the presence or absence of a well-known broad feature in undoped ZnO films at ca. 500 nm (green band), which arises because of structural defects, namely Zn interstitials (Zn_ i ) [21] or oxygen vacancies (V_ o ), [22] is investigated. While simple convention would suggest doubly charged Zn_ i _ and V_ o _, here the convention of singly charged defects is used because the conductivity of ZnO due to native defects is found to be correlated to singly charged defects of Zn_ i [23] Figure 2 shows magnetization (M) versus field (H) curves at room temperature for Zn 0.98 Co 0.02 O films prepared at 400 and 500 C. The film prepared at 400 C shows a saturation M value (M s ) of over 0.4 l B /Co (l B : Bohr magneton), whereas the film grown at 500 C has barely a trace signal above the background. Both the films are highly resistive (resistivity q » 10 4 X cm), which is indicative of near-oxygen stoichiometry and suggests that FM does not depend upon the presence of a significant carrier concentration. The net moment per Co atom, although an order of magnitude lower than the absolute Co moment, is comparable to that found in many previous reports and can be interpreted either in terms of a predominately paramagnetic response (most Co ions are decoupled) or whether Co ions can couple antiferromagnetically or ferromagnetically depending on the local environment. Venkatesan et al. [24] present clear evidence for the latter, which implies that the magnetism is likely to be macroscopically homogenous.

Single walled carbon nanotube (SWCNT) films are candidates for use as transparent electrodes, especially where low-cost, flexible materials are desired. Chemical doping is a critical step in fabricating conductive films as doping substantially decreases the sheet resistance within SWCNTs and at tube−tube junctions. Despite the importance of chemical doping, surprisingly little effort is devoted to developing doping chemistry. Concentrated acid solutions are typically used to dope SWCNT films. Although they are effective at reducing the sheet resistance of SWCNT films, this method is plagued by two critical drawbacks. The first is that concentrated acid baths, such as HNO3, are extremely harsh and will damage virtually any device technology. Second, the film resistance is unstable and rises dramatically over time. These drawbacks make implementation of SWCNT transparent, conducting films in technological applications extremely difficult. Here, we report an alternative doping scheme that utilizes a single-electron oxidant (triethyloxonium hexachloroantimonate) to effectively dope the SWCNT films. As evidenced by optical and electrical measurements, the compound effectively p-dopes SWCNT films. In addition to the effective doping, the resultant film resistance is stable over time. The films doped with triethyloxonium hexachloroantimonate outperform nitric acid doped films by a factor of 2.5 over time. This study introduces a new category of chemical dopants that yield stable, transparent, and conductive SWCNT films suitable for technological applications.

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