Valence and Magnetic Investigations of Alkali Metal-Doped Europium Sulfide William L. Boncher, † Edward A. Gö rlich, ‡ Krzysztof Tomala, ‡ Julie L. Bitter, § and Sarah L. Stoll* ,† † Department of Chemistry, Georgetown University, Washington, D.C. 20057, United States ‡ M. Smoluchowski Institute of Physics, Jagellonian University, Krakow, Poland § Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States * S Supporting Information ABSTRACT: Using europium sesquioxide nanowires as a starting material for the synthesis of europium sulfides, we have discovered alkali metal doping Eu 1-x Na x S for the first time. Alkali metal doping stabilizes the NaCl structure type of EuS at surprisingly low temperatures. We investigated the europium valence through XPS and Mö ssbauer spectroscopy as a function of dopant in Eu 1-x Na x S for x =0-0.5. In addition, based on magnetic studies, the ferromagnetic ordering is diminished by the presence of the nonmagnetic dopant, causing a decrease in the ordering temperature. KEYWORDS: europium sulfide, sodium-doping, XPS, 151 Eu Mö ssbauer ■ INTRODUCTION There has been a strong interest in the synthesis of nanoscale europium sulfide (EuS) due to its properties as an intrinsic magnetic semiconductor. 1-5 This stems from the search for novel luminescent, 4,5 magnetic, 6-8 and photomagnetic proper- ties observed in this class of materials. EuS has a strong magneto-optical Kerr effect, 9 colossal magnetoresistive effects, 9 and demonstrated spin-filtering effects with potential applica- tions in spintronics. 10,11 The most common method for synthesizing EuS nanoparticles is by thermal decomposition of dithiocarbamate complexes in coordinating solvents. 2 However, this results in ligand capped nanoparticles, which can hinder some applications by creating a barrier against charge or ion transport when incorporated into devices. 12 By utilizing preformed Eu 2 O 3 nanowires, we have investigated the oxide as a templating precursor to form uncapped nano- structured EuS at reduced temperatures through sulfurization reactions, a combination of anion replacement and europium reduction. Chemical transformation of nanostructures can be an effective synthetic tool and is likely to broaden the range of materials synthesized with morphological control. 13,14 Many synthetic transformations have been demonstrated, including galvanic replacement, oxidation, and diffusion, but perhaps the most successful technique is cation exchange. 15 One of the earliest examples is the remarkable exchange of cadmium in CdSe with silver to form Ag 2 Se, which is reversible at room temperature. 15 Nanoparticles of CdS have been undergone cation-exchange with copper and lead, 16 as well as platinum and palladium. 17 The effect has also been demonstrated in nanowires, for example ZnS nanowires have been transformed to sulfides of copper, silver, antimony, and bismuth, 18 maintaining the morphology of the parent structure. Conversion chemistry for ‘anion exchange’ has also been demonstrated for the reaction of oxides with nitrogen and sulfur sources to form metal nitrides 19-23 and sulfides. 24,25 Frequently anion exchange is associated with the formation of hollow nanomaterials, depending on the diffusivity of the cation versus anion. 13 It is also possible to form core-oxide/shell- sulfide structures from nanowires of metal oxides. 26 The conversion of oxide nanowires to sulfides was initially explored as an approach to prepare sulfur analogs of carbon nanotubes, where layers of a given material such as MoS 2 , WS 2 , or NbS 2 (for example) compose the concentric graphene sheets. Another advantage of anion conversion is that nanowire arrays of metal oxides, which are easily prepared such as for ZnO, can be converted to sulfide nanowires of materials such as ZnS, which are quite difficult to prepare. Here, we have studied the dependence of temperature on the anion exchange process for Eu 2 O 3 to EuS and discovered that the reduced reaction temperatures for chemical conversion of nanostructures has allowed for the formation of alkali metal doped europium sulfide, Eu 1-x Na x S for x =0-0.5. Alkali metal doping of europium sulfide appears to stabilize the rock salt structure of EuS at surprisingly low temperatures (350 °C) and causes significant changes in cell constants and the magnetic properties. We have explored the synthesis and characterization Received: August 10, 2012 Revised: October 16, 2012 Published: October 18, 2012 Article pubs.acs.org/cm © 2012 American Chemical Society 4390 dx.doi.org/10.1021/cm3025507 | Chem. Mater. 2012, 24, 4390-4396