High-quality manganese-doped zinc sulfide quantum rods with tunable dual-color and multiphoton emissions.

Chemicals Zinc nitrate tetrahydrate (Zn(NO 3 ) 2 ·4H 2 O, 99.8%), Manganese nitrate hydrate (Mn(NO 3 ) 2 ·xH 2 O, 99.99%), Sulfur (S, 99.998% powder), Oleylamine (OAm, technical grade, 70%), 1-Dodecanethiol (DDT, ≥98%), methanol (≥99.5%), ethanol (>99%), isopropyl alcohol (IPA, 99%), and hexane (≥95%), and Coumarin 480 were purchased from Sigma-Aldrich and used without further purification.

Synthesis

Our strategy to synthesize Mn-doped ZnS QRs is based on phosphine-free colloidal chemistry, with the advantages of being fast, simple and environmental friendly. 32 , 38 – 40 The synthesis of Mn-doped ZnS QRs for a typical sample with a diameter of 2.0 nm (sample QR2): First, S precursor solution was prepared by dissolving 4 mmol of S powder in 20 mL OAm and holding the solution at 100 °C; then the solution was continuously stirred for 2 hours before use. Second, Zn/Mn-OAm complex precursor solution was prepared by adding 0.4 mmol of Zn(NO 3 ) 2 , 0.02 mmol of Mn(NO 3 ) 2 , and 0.5 mL of DDT to 15 mL OAm in a flask, which was kept at 160 °C and stirred until a uniform mixture was formed. Third, 2 mL of S-precursor solution was quickly injected into the Zn/Mn-OAm precursor solution through a syringe with continuous stirring. After injection, the temperature was raised to 230 °C with a temperature ramp of 15 °C per minute. After 10 minutes at 230 °C (time started once the temperature was reached), the solution mixture was removed from the heating mantle, and mixed with cold methanol (3 equivalents), ethanol (3 equivalents), and isopropyl alcohol (3 equivalents). Un-reacted starting materials were removed by centrifugation (3,000 rpm for 30 minutes at 4 °C) and the samples were re-dispersed in hexane (1 equivalent) three times. The synthesis of Mn doped ZnS QRs with various Mn levels and diameters was achieved by changing the starting Mn 2+ concentration and reaction times as summarized in Table 1 (in main text), while keeping the other experimental parameters the same. The final products were re-dispersed in hexane for characterizations.

Characterization

Samples for transmission electron microscope

(TEM) analysis were prepared by dropping a diluted hexane solution of doped QRs onto the ultrathin carbon-coated copper grids and air-dried. TEM, high-resolution TEM (HRTEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive X-ray spectroscopy (EDS) were performed on a JEOL JEM 2010F electron microscope operating at 200 kV. For the powder X-ray diffraction (XRD) measurements, the QR samples were dried on a quartz substrate, and it was performed on a PANalytical X’Pert Pro Materials Research X-ray Diffractometer with Cu Kα radiation (λ=1.5418 Å) and scanned at a rate of 0.025 degree per second. ICP-MS was performed on a Thermo X-series Q-ICP-MS with CCT (Collision Cell Technology) instrument. Mn was measured at 55Mn, while Zn was measured at 64Zn, 66Zn and 68Zn. The Zn isotope with the lowest detection limit (66Zn) was selected for reporting, but all three isotopes gave values within 5% of each other. Two standard checks and a blank were measured for every four sample analyses, and each sample was measured for at least three different dilutions. The variation of the Zn/Mn ratio measured in two separate dilutions for each sample was always less than 3%, and typically less than 1.3%. The doping level of Mn in the ZnS QRs was calculated from the atomic ratio of the Mn/(Mn+Zn) measured. Ultraviolet-Visible (UV-Vis) absorption spectra were recorded at room temperature with a JASCO V670 spectrophotometer. Photoluminescence spectra were measured at room temperature using a NanoLog fluorescence-spectrometer manufactured by HORIBA Jobin Yvon equipped with a thermoelectric cooled PMT (R928 in the range 200 nm to 850 nm), a 450 W xenon short-arc lamp, and a flash lamp (for phosphorescence decay lifetime). The emission spectra obtained were corrected using a response file that records the sensitivity of the PMT to different wavelengths of light generated using a standard lamp. The excitation spectra were corrected using the signal from the reference photodiode that records the variation of the intensity of the lamp with excitation wavelength.

Electron paramagnetic resonance

(EPR) study was performed at the EPR Facility at Arizona State University. EPR spectra were recorded at room temperature using a Bruker ELEXSYS 580 X-band spectrometer (Bruker, Silberstreifen, Germany) equipped with a cylindrical mode resonator (Bruker, ER 4103TM) designed for aqueous and high-dielectric samples. Samples were placed in a flat, quartz cell, which was mounted in the resonator. The parameters used were: magnetic field modulation frequency 100 kHz, amplitude 1 mT, microwave power 10 mW, microwave frequency 9.7 GHz and the sweep time 84 seconds. The spectra were obtained by averaging 8 to 12 scans. The time-resolved PL spectroscopic study was performed on a system consisting of an ultrafast laser and streak camera detection system. The intensity of PL as a function of emission wavelength and time for the Mn-doped ZnS QRs samples was recorded simultaneously. The 130-fs light pulses at 650 nm, 700 nm, or 730 nm were generated from a visible OPA pumped by a femtosecond regenerative amplifier system operating between 10 and 250 KHz (Coherent Laser Inc., Verdi 18, Mira900, RegA9000, OPA9400). A femtosecond laser (Maitai, Spectra-Physics, Fremont, CA) with 130-fs pulse and 80 MHz repetition rate, tunable within 690–1020 nm, was used as the excitation source for multiphoton imaging. The laser beam was sent into a FV1000 laser-scanning microscope (Olympus America Inc., PA) and focused onto thr sample with a 60X/IR water objective lens (NA = 1.2). The backward multi-photon signals were collected using the same objective, separated from the excitation laser by a dichroic mirror and detected by two internal spectral detectors (channel 1 and channel 2). The detection range was set as 400–500 nm for channel 1 and 560–660 nm for channel 2, separated by a diachronic mirror (SD560). Microspectroscopy was performed by λ-scan using the internal spectral detector in the confocal scanning box. Details of the optical setup can be found elsewhere. 41 , 42

Supplementary Material 1_si_001 2_si_002