Synthesis of Widely Tunable and Highly Luminescent Zinc Nitride Nanocrystals

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry C. This journal is © The Royal Society of Chemistry 2014 Synthesis of Widely Tunable and Highly Luminescent Zinc Nitride Nanocrystals Peter N. Taylor,* Michael A. Schreuder, Tim Smeeton, Alastair J. D. Grundy, James A. R. Dimmock, Stewart E. Hooper, Jonathan Heffernan and Matthias Kauer Electronic Supplementary Information Methods Synthesis All nanocrystals were prepared in a nitrogen atmosphere glove box and handled using standard air–free methods. The general method for the formation of oleylamine capped Zn3N2 nanocrystals is as follows - A 50 ml round bottomed flask containing 1-octadecene (30 ml) and oleylamine (1 ml) was heated to 225oC. Ammonia gas was bubbled through the reaction at a rate of 5ml/min. After 5 minutes diethylzinc (102 µl, 1.0 mmol) was rapidly injected, after a further 5 minutes additional diethylzinc (102 µl, 1.0 mmol) was added. Injections were continued in this fashion until the desired size of nanocrystal was obtained. The data in figures 2, 5 and S3 comes from measurement of small samples of the reaction mixture which were removed immediately before each diethylzinc addition and diluted with toluene. The above procedure was altered by either changing the ammonia flow rate or the size of the individual diethylzinc injections to observe the impact of these reaction parameters on the synthesis. In a typical purification, 15 ml of the reaction mixture was centrifuged to remove any insoluble material. The resulting solution was then treated with toluene (6 ml), isobutyronitrile (15 ml) and acetonitrile (8 ml) . The mixture was centrifuged and the top layer was discarded. To the remainder, toluene (15 ml), isobutyronitrile (15 ml) and acetonitrile (9 ml) was added, the mixture was centrifuged and again the top layer was discarded. Finally, toluene (10ml), isobutyronitrile (15 ml) and acetonitrile (15ml) was added to give a solid which was subsequently dried under reduced pressure (Found: C, 29.1; H, 5.0; N, 8.3; Zn, 55.8). A typical synthesis of nanocrystalline Zn3N2 in the absence of oleylamine is as follows – A 50 ml round bottomed flask is charged with 1-octadecene (20 ml) and heated to 225oC. Diethylzinc (92 µl, 0.9 mmol) was injected rapidly followed by ammonia gas (15 ml, 0.6 mmol) which was added via a gas syringe into the solution, after 4 minutes the addition of diethylzinc and ammonia was repeated. After a total of 15 additions each of diethylzinc and ammonia the temperature was increased to 295 oC while ammonia was bubbled through the mixture at rate of 5ml/min, heating was maintained at this temperature for 60 minutes. The resulting dark black precipitate was isolated by centrifugation and washed with toluene (3 x 20 ml) before being dried under reduced pressure (Found: C, 3.5; H, 1.1; N, 12.8; Zn, 82.8). The solid was analysed by X-ray diffraction (XRD) without any further purification. The procedure outlined above results in samples of zinc nitride without any obvious impurities in the XRD (see figure S1). Methods using a constant flow of ammonia and alternative temperatures tended to give samples contaminated with either zinc metal or zinc amide, both of which were clearly visible in the XRD patterns of the final products. Infrared photoluminescence of a thin film of the solid was measured, giving a broad and weak peak centred at 1120 nm with a FWHM of 364 nm. TEM-energy dispersive x-ray analysis (EDX) of an aggregate of nanocrystals from this sample identified the presence of zinc and nitrogen with an average atomic Zn:N ratio of 3.1:2.0. Optical spectroscopy All absorption spectra were measured using a Perkin Elmer Lambda 950 UV-VIS spectrometer. Visible PL emission spectra on colloidal samples were obtained using a Horiba Scientific Fluoromax-4 spectrometer. The PLQY measurements were performed using standard methods employing Nile Red in 1,4-dioxane (PLQY 70%) as a reference. Infrared PL Infrared PL was performed at room temperature. The excitation source was a 4 mW He/Ne laser (~633nm) and the emitted photoluminescence was measured with a Princeton Instruments/Acton SpectraPro 500i spectrometer and a Princeton Instruments OMA-V 1024/LN InGaAs linear photodiode array. X-ray diffraction Thin films of the powder samples were deposited on amorphous silicon substrates and covered with a thin (25 µm) layer of Kapton® film. The measurements were performed using a Bruker D2 Phaser diffractometer at room temperature with a wavelength of 1.541 Å. Transmission Electron Microscopy In order to perform the TEM analysis, dilute solutions of purified nanocrystals were deposited on lacy carbon or ultra-smooth carbon grids. The grids were mounted in the TEM sample holder under a nitrogen atmosphere and red light conditions to reduce the oxidation process. The TEM analysis was carried out on either a LaB6 JEOL 2100 microscope operating at 200 kV or a JEOL ARM200F microscope operating at 80kV. Figure S1 – XRD pattern of zinc nitride prepared from the reaction of ammonia with diethylzinc in the absence of oleylamine. Sticks show the peaks reported for Zn3N2 with an anti-bixbyite crystal structure [PDF 35-0762]. Figure S2- TEM images taken before (left) and after 1-2 minutes of exposure to the electron beam (right). Figure S3 – Evolution of the maximum emission wavelength during the course of the reaction, diethylzinc was added in either 0.5, 1.0 or 2.0 mmol portions at five minute intervals. .

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