Amide-Assisted Synthesis of Iron Germanium Sulfide (Fe2GeS4) Nanostars: The Effect of LiN(SiMe3)2 on Precursor Reactivity for Favoring Nanoparticle Nucleation or Growth.

Olivine Fe2GeS4 has been identified as a promising photovoltaic absorber material introduced as an alternate candidate to iron pyrite, FeS2. The compounds share similar benefits in terms of elemental abundance and relative nontoxicity, but Fe2GeS4 was predicted to have higher stability with respect to decomposition to alternate phases and, therefore, more optimal device performance. Our initial report of the nanoparticle (NP) synthesis for Fe2GeS4 was not well understood and required an inefficient 24 h growth to dissolve an iron sulfide impurity. Here, we report an amide-assisted Fe2GeS4 NP synthesis that directly forms the phase-pure product in minutes. This significant advance was achieved by the replacement of the poorly understood hexamethyldisilazane (HMDS) additive and TMS2S by the conjugate base, lithium bis(trimethylsilyl)amide (LiN(SiMe3)2), and elemental S, respectively. We hypothesized that fragments of both TMS2S and HMDS had carried out the roles that Brønsted bases play in amide-assisted NP syntheses and were necessary for Ge incorporation. Convolution of this role with the supply of S in TMS2S caused the iron sulfide impurities. Separating these effects in the use of LiN(SiMe3)2 and elemental S resulted in synthetic control over the ternary phase. Herein we explore the Fe-Ge-S reaction landscape and the role of the base. Its concentration was found to increase the reactivities of the Fe, Ge, and S precursors, and we discuss possible metal-amide intermediates. This affords tunability in two areas: favorability of NP nucleation versus growth and phase formation. The phase-purity of Fe2GeS4 depends on the molar ratios of the cations, base, and amine as well as the Fe:Ge:S molar ratios. The resultant Fe2GeS4 NPs exhibit an interesting star anise-like morphology with stacks of nanoplates that intersect along a 6-fold rotation axis. The optical properties of the Fe2GeS4 NPs are consistent with previously published measurements showing a measured band gap of 1.48 eV.