3-D Atomic-Scale Mapping of Manganese Dopants in Lead Sulfide Nanowires

Dopants in nanowires, whether intentional or unintentional, can ultimately control the material’s properties and, therefore, need to be understood on the atomic scale. We study vapor−liquid−solid grown manganese-doped lead sulfide nanowires by atom-probe tomography for the first time for lead salt materials. The three-dimensional chemical concentration maps at the atomic scale demonstrate a radial distribution profile of Mn ions, with a concentration of only 0.18 and 0.01 at. % for MnCl2 and Mn-acetate precursors, respectively. The ability to characterize these small concentrations of dopant atoms in Pb1−xMnxS nanowires (x = 0.0036 and 0.0002), important for spintronic and thermoelectric devices, sets a platform for similar analyses for all nanostructures. First-principles calculations confirm that Mn atoms substitute for Pb in the PbS structure. ■ INTRODUCTION Dilute magnetic semiconductors (DMSs) not only are an ideal test bed for exploring spin-related transport and magnetization but also have the potential to revolutionize the next electronics generation at faster speeds and reduced energy consumption by exploiting spins instead of charges as the main information carriers. To date, the main focus of the DMS research is on wideband-gap semiconductors, where a high Curie temperature has been predicted theoretically and observed experimentally. Alternatively, narrow-band-gap DMSs are less studied and may provide distinct perspectives on magnetotransport mechanisms, which are complex and not well-understood. Low-dimensional DMSs, such as quantum dots (QDs) and nanowires (NWs), facilitate device miniaturization and may also have enhanced magnetoresistance (MR) and higher Curie temperatures, as the quantum confinement effect and restricted transport direction can strengthen the exchange interaction between impurity spins and conduction charges. The QDs, even in the absence of transition-metal ions, exhibit MR effects because of squeezing of electron wave functions and spin blockade. Addition of local spins into low-dimensional semiconductors can create an effective local magnetic field of 100−1000 T and a giant Zeeman splitting on the order of 10−100 meV. NWs, at the smallest dimension with electrical connections, provide an ideal platform for studying magnetotransport. In this article, we report on the successful vapor−liquid− solid (VLS) growth of Pb1−xMnxS NWs and their atomic-scale characterization in three dimensions using atom-probe tomography (APT). PbS has been employed in photodetectors and is recognized as a promising material for photovoltaics. PbS has several unique properties, such as a narrow band gap (0.41 eV at room temperature), high dielectric constant (190), and a large exciton Bohr radius (20 nm). Mn ions are chosen as the dopants because (1) their half-filled d orbital yields a maximum sp−d exchange interaction with the conduction electrons and (2) Mn ions act neither as donors nor as acceptors in PbS if Mn substitutes Pb and thus will not affect the carrier type and concentration in PbS. Several routes to synthesize Mn-doped PbS and PbSe nanostructures have been attempted, including solution synthesis, molecular-beam epitaxy (MBE), and the fusion method in a glass matrix. One major difficulty of doping nanostructures is that the impurities tend to migrate to the surface by a selfpurification process. Additionally, there has been a dearth of effective characterization techniques to determine the spatial distribution of the impurity ions, which has a great impact on the electronic and magnetic properties of the nanostructures. Specifically, magnetic ions that segregate at the surface have much weaker interactions with conduction electrons, and magnetic ions forming clusters result in secondary phases, which can Received: January 5, 2012 Revised: February 13, 2012 Published: February 16, 2012 Article