Communication pubs.acs.org/cm
Solution−Liquid−Solid Approach to Colloidal Indium Nitride Nanoparticles from Simple Alkylamide Precursors Niladri S. Karan,‡ Yang Chen,‡ Zhihui Liu, and Rémi Beaulac* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322, United States S Supporting Information *
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ditions which limit the control of the characteristics (size, shape, monodispersity, etc.) of the nanomaterials. Here, we report a solution-based approach yielding colloidal InN nanomaterials of high quality and which does not rely on the use of insoluble inorganic precursors. The approach is based on the in situ reduction of In(III)-amine molecular complexes by the injection of alkylamide precursors at high temperature and proceeds through the formation of metallic indium nanoparticles that further catalyze the growth of InN nanorods (NRs) through the well-established solution−liquid− solid (SLS) growth mechanism.32−34 In a typical synthesis (described fully in the Supporting Information), InBr3 is solubilized in a mixture of oleylamine (OLA) and a highboiling-point aliphatic solvent (octadecene or hexadecane) under air-free conditions in a three-neck round-bottom flask by heating to 220 °C; this In-OLA solution is kept under constant vigorous stirring and water-cooled-condenser reflux conditions throughout the course of the reaction. An alkylamide precursor solution is separately prepared under air-free conditions by deprotonating OLA with a mixture of n-butyllithium (n-BuLi) and tetramethylethylenediamine (TMEDA). This alkylamide solution is injected promptly into the In−OLA solution to initiate the reaction. After injection, the solution temperature drops to 178 °C and quickly rises back to 210 °C, where it is maintained until the reaction is stopped. The solution changes from colorless to brownish-black over the course of the first few minutes, indicative of the formation of InN nanocrystals. The reaction is stopped after 10 min by removing the heat source and letting the solution cool down to room temperature. The resulting solution is then sonicated in ethanol for 2 min, followed by a 5 min centrifugation to precipitate the solids; this sonication/centrifugation cycle is then repeated a second time, after which the sample is suspended in a nonpolar solvent, toluene or tetrachloroethylene (TCE). The sample is either analyzed directly (“as-prepared”), further functionalized with oleylamine as described below (“as-prepared, f unctionalized”), or further purified to eliminate In(0) from the sample (vide inf ra). For the latter, the as-prepared sample is sonicated in dilute nitric acid and then centrifuged. The resulting sample (“acidtreated”) is then washed following the same protocol described above. The sample can be directly converted into a stable colloidal suspension by heating the solid in OLA at 80 °C for 3 h. The resulting mixture is then washed by sequential polar/ nonpolar solvent treatments to yield stable suspensions of InN
olloidal semiconductor nanostructures are fascinating free-standing chemical objects that combine the general characteristics and stability of inorganic semiconducting materials with the processability of molecular compounds.1,2 In the limit of the smallest crystal sizes, colloidal nanostructures are further characterized by electronic quantum confinement, which allows significant tuning of their optical, chemical, and charge transport properties by simply changing the size and shape of the crystallites.2−4 To this day, II−VI semiconductors are by far the most commonly studied form of quantumconfined colloidal nanomaterials, owing to highly efficient and optimized chemical approaches yielding highly monodisperse colloidal suspensions.1,2,5 Although not as much studied in the colloidal form, III−V semiconductors possess many desirable qualities such as wide bandgap variations across the series (0.2 eV for InSb to 3.4 eV for GaN), high carrier mobilities, large dielectric constants, and large thermal conductivities that are well-suited for high-performance light-emitting devices, photodetectors, and integrated electronic circuitry.6 Group-III nitrides (AlxGayInzN, x + y + z = 1) have also recently played a major role in the development of efficient blue/violet solidstate light-emitting devices.7−9 Unfortunately, the synthesis of high-quality colloidal III−V nanoparticles using solution-based approaches remains a challenge to this day, a fact generally attributed to the higher covalency of the III−V lattices compared to that of II−VIs which complicates the separation of nucleation and growth processes.4,10−14 An interesting case in point, indium nitride has proven surprisingly difficult to prepare from solution-based approaches.15−22 Arguably the least understood of the III−V materials,23 InN nevertheless offers tremendous potential for future optoelectronic or electronic applications24−27 due, among other things, to its low-energy direct bandgap (0.7 eV),28 large electron affinity (6 eV),25,29 and unusually small electron effective mass (
