Chemicals<\/p>\n
Cadmium oxide (CdO, 99.5%, trace metals basis), zinc acetate 99.99%, trace metals basis), oleylamine (70%, technical grade), oleic acid (90%, technical grade), myristic acid (MA, 98%), 1-octadecene (ODE, 90%, technical grade), 1-octanethiol (OT, \u226598.5%), 1-dodecanethiol (DDT, 98%), selenium (Se, \u226599.99%, trace metals basis) and sulfur (S, \u226599.0%) were purchased from Sigma-Aldrich. Tri-n-octylphosphine (TOP, 97%) was purchased from Strem Chemicals. All chemicals were used as received without further purification.<\/p>\n
Preparation of precursors<\/p>\n
All chemical procedures were performed under an inert atmosphere using the Schlenk line technique. Stock solutions of 0.5\u2009M zinc oleate, 0.5\u2009M cadmium oleate and 2\u2009M solutions of TOPSe and TOPS were prepared before the synthesis of the QDs. To prepare the zinc oleate stock solution, 20\u2009mmol of zinc acetate and 20\u2009ml of oleic acid were combined in a flask and degassed under vacuum at 140\u2009\u00b0C for 2\u2009h. After purging with nitrogen, the concentration of the precursor was adjusted to 0.5\u2009M by adding ODE. This solution was stored at 100\u2009\u00b0C under an inert atmosphere for future use. To prepare the 0.5\u2009M\u2009cadmium oleate solution, 10\u2009mmol of CdO, 10\u2009ml of oleic acid and 10\u2009ml of ODE were combined and degassed at 110\u2009\u00b0C under vacuum. The mixture was then gradually heated to 300\u2009\u00b0C until the solution became transparent. Afterwards, the flask was cooled to 110\u2009\u00b0C and vacuum-degassed once more to remove any residual moisture. Stock solutions of 2\u2009M TOPSe and TOPS were made by dissolving 10\u2009mmol of Se and S in 5\u2009ml of TOP at 80\u2009\u00b0C, and these solutions were stored in a glove box for further use.<\/p>\n
Synthesis of type-(I\u2009+\u2009II) QDs<\/p>\n
CdSe (r\u2009=\u20092.6\u2009nm)\/Cd1\u2212xZnxSe (l\u2009=\u20091.7\u2009nm)\/CdS (h\u2009=\u20092.2\u2009nm)\/ZnS (d\u2009=\u20090.3\u2009nm) type-(I\u2009+\u2009II) QDs were synthesized following the procedure described previously23<\/a>,24<\/a>. In a typical synthesis, 0.1\u2009mmol of cadmium oleate and 6\u2009ml of ODE were loaded into a reaction flask, degassed at 110\u2009\u00b0C and purged with nitrogen. After heating to 310\u2009\u00b0C, 0.2\u2009mmol of TOPSe was injected, followed by a gradual addition of 1\u2009ml of TOP, forming CdSe cores with a radius of 1.5\u2009nm. The core radius was then increased to 2.6\u2009nm by adding 0.25\u2009mmol of cadmium oleate and 0.25\u2009mmol of TOPSe. To form a compositionally graded Cd1\u2212xZnxSe layer, 0.4\u2009mmol of zinc oleate, 0.19\u2009mmol of cadmium oleate and 0.38\u2009mmol of TOPSe were injected at 310\u2009\u00b0C, followed by a second injection of 1.31\u2009mmol of zinc oleate and 1.52\u2009mmol of TOPSe. After a 30\u2009min reaction, the Cd1\u2212xZnxSe barrier layer was complete. For the CdS layer, 4\u2009mmol of OT and cadmium oleate were added stepwise. The ZnS shell was grown by injecting 2\u2009mmol of OT and 4\u2009mmol of zinc oleate, followed by an additional 2\u2009mmol of zinc oleate after 30\u2009min. After cooling, the QDs were purified and redispersed in toluene for further use.<\/p>\n Synthesis of ccg-QDs<\/p>\n The synthesis of compact compositionally graded ccg-QDs with the CdSe\/Cd1-xZnxSe\/ZnSe1-ySy structure (rCdSe\u2009=\u20092.6\u2009nm, lCdZnSe\u2009=\u20092.7\u2009nm, dZnSeS\u2009=\u20091.7\u2009nm, and x and y vary from 0 to 1 in the radial direction towards QD periphery) follows the procedure described previously31<\/a>,37<\/a>, with minor modifications. The CdSe core with a radius of 2.6\u2009nm was prepared using the same method as for the type-(I\u2009+\u2009II) QDs. To grow the compositionally graded Cd1-xZnxSe layer on top of the CdSe core, a mixture of 0.75\u2009mmol of cadmium oleate and 2.5\u2009mmol of TOPSe was slowly injected into the reaction flask over 40\u2009min, accompanied by a stepwise injection of zinc oleate (total 5\u2009mmol). For the growth of the compositionally graded ZnSe1-ySy layer, 5\u2009mmol of zinc oleate was first added, followed by the dropwise injection of 2.5\u2009mmol of TOPSe over 1\u2009h. During this step, 0.25\u2009mmol, 0.5\u2009mmol and 0.75\u2009mmol of TOPS were injected at 15\u2009min, 30\u2009min and 45\u2009min, respectively. Finally, 1\u2009mmol of TOPS and an equivalent amount of zinc oleate were injected, and the reaction was allowed to proceed for an additional 30\u2009min.<\/p>\n Synthesis of g-QDs<\/p>\n The following procedure was used to prepare quasi-type-II thick-shell g-QDs with the CdSe\/CdS\/ZnS structure (rCdSe\u2009=\u20092.6\u2009nm, hCdS\u2009=\u20093.2\u2009nm, dZnS\u2009=\u20090.3\u2009nm). The CdSe core, with a radius of 2.6\u2009nm, was synthesized using the same method as for the type-(I\u2009+\u2009II) QDs. Before growing the CdS layer, 3\u2009ml of oleylamine was injected into the flask, and the reaction temperature was lowered to 270\u2009\u00b0C. Over the next 3\u2009h, 2.8\u2009mmol of OT was slowly injected along with a stepwise addition of 1.2 equivalents of cadmium oleate. After the CdS layer growth, the temperature was raised to 300\u2009\u00b0C, and the outer ZnS layer was formed by adding 1.5\u2009mmol of OT and 3\u2009mmol of zinc oleate.<\/p>\n Synthesis of thin-shell CdSe\/ZnS QDs<\/p>\n CdSe\/ZnS QDs with rCdSe\u2009=\u20092.5\u2009nm and dZnS\u2009=\u20090.5\u2009nm were synthesized as follows. A mixture of 1\u2009mmol of CdO, 700\u2009mg of MA and 15\u2009ml of ODE was added to a reaction flask and degassed under vacuum. The flask was then back-filled with nitrogen and heated to 270\u2009\u00b0C, maintaining this temperature for 30\u2009min to achieve a clear solution. TOPSe (0.5\u2009mmol) was injected to form CdSe cores with a radius of 2.5\u2009nm. To grow the ZnS shell, 2\u2009mmol of zinc oleate and 1.5\u2009ml of DDT were injected into the flask, and the temperature was raised to 300\u2009\u00b0C. After 30\u2009min, 2\u2009mmol of OT and 2\u2009mmol of zinc oleate were added, and the reaction continued for an additional hour.<\/p>\n ASE measurements<\/p>\n The QD film samples were prepared on a glass substrate using a multi-step coating and crosslinking process with 1,8-diaminooctane as a linking agent32<\/a>. The film thickness was ~300\u2009nm. The substrate was attached to a copper holder acting as a heat sink. The sample assembly was mounted on the cold finger of a cryostat (Janis Research, VPF-100) and cooled with liquid nitrogen during ASE measurements.<\/p>\n The QD sample was excited using the second harmonic of a Q-switched Nd:YLF laser (Spectra-Physics, Empower), with a photon energy of 2.4\u2009eV and a pulse duration of 230\u2009ns (FWHM). The laser repetition rate was reduced from 1,000\u2009Hz to 250\u2009Hz using a chopper.<\/p>\n The laser beam was focused into a thin stripe (100\u2009\u00b5m\u2009\u00d7\u20094\u2009mm) on the QD film using a cylindrical lens. The stripe was aligned approximately perpendicular to the sample\u2019s edge and positioned to be partially truncated by it. Emission from the sample\u2019s edge was collected along the excited stripe\u2019s direction. The collected light was then spectrally dispersed using a Czerny-Turner spectrograph (Acton 500i) and analysed with a nitrogen-cooled charge-coupled device (CCD) camera (Roper Scientific).<\/p>\n Fabrication of a DFB laser<\/p>\n The one-dimensional SiO2-DFB grating was fabricated on a glass substrate using laser interferometric lithography. The grating period (\u039bDFB) was 345\u2013355\u2009nm. The groove height was 50\u2009nm, and the groove width was 0.7\u039bDFB.<\/p>\n The glass substrate was cleaned via sonication in acetone and isopropyl alcohol. A 50-nm-thick SiO2 layer was then deposited using electron beam evaporation. The anti-reflection coating (ARC), i-CON-16, was spin-coated at 3,000\u2009rpm for 30\u2009s, forming a 150-nm-thick film, which was subsequently annealed at 190\u2009\u00b0C for 1\u2009min.<\/p>\n A 500-nm-thick negative photoresist (NR7-500P, Futurex) was spin-coated on the ARC under the same conditions (3,000\u2009rpm for 30\u2009s) and annealed at 150\u2009\u00b0C for 1\u2009min. The photoresist was then exposed to a 355\u2009nm laser at a 90\u2009mJ dose for 11.4\u2009s using an interferometric lithography system. After exposure, the sample was annealed at 100\u2009\u00b0C for 1\u2009min and developed in RD6 (Futurex) for 40\u2009s.<\/p>\n Inductively coupled plasma (ICP) etching was performed to remove the ARC, SiO2 and photoresist layers, using the following gas flows: O2 (30\u2009sccm) for photoresist etching; CHF3 (45\u2009sccm)\u2009+\u2009O2 (5\u2009sccm) for SiO2 etching; and O2 (30\u2009sccm) for ARC etching.<\/p>\n Characterization of a QD-DFB laser under cw excitation<\/p>\n The fabricated QD-DFB laser was placed inside a cryostat (Janis Research, VPF-100). It was excited at 2.8\u2009eV using a laser diode (Opt Lasers, PLH3D). The pump laser output was electrically modulated to generate pulses with durations ranging from 1\u2009\u03bcs to 10\u2009\u03bcs (FWHM) and repetition rates between 0.1\u2009kHz and 10\u2009kHz. The pump beam was focused onto the sample, forming a spot measuring 150\u2009\u00b5m by 400\u2009\u00b5m. The light emitted from the surface of the QD-DFB laser was spectrally dispersed using a Czerny-Turner spectrograph (Acton 500i) and analysed with a nitrogen-cooled CCD camera (Roper Scientific).<\/p>\n Fabrication of EL-active lasing devices<\/p>\n The fabrication of EL-active lasing devices begins with the preparation of a SiO2-DFB grating on a glass substrate coated with ITO. The fabrication process follows a procedure similar to that used for SiO2-DFB\/glass devices described earlier (see above).<\/p>\n Fabrication of optically excited devices<\/p>\n For devices intended for optical excitation measurements, the DFB\/ITO\/glass substrate was cleaned by sonication in isopropyl alcohol. A 50-nm-thick ZnO electron transport layer was deposited using the sol\u2013gel method and annealed at 200\u2009\u00b0C for 1\u2009h. A densely packed QD film was formed using a multi-step coating and crosslinking process32<\/a>. Each layer was spun at 2,000\u2009rpm from a QD solution in n-octane (50\u2009mg\u2009ml\u22121), resulting in a total QD layer thickness of 150\u2009nm. The QD layer was annealed at 100\u2009\u00b0C for 10\u2009min.<\/p>\n Next, a 50-nm-thick HTL of poly(9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenylamine) (TFB) was deposited by spin-casting at 4,000\u2009rpm for 20\u2009min. A 50-nm-thick HIL of dipyrazino[2,3-f:2\u2032,3\u2032-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) was then deposited using thermal evaporation. Finally, the device was completed with the deposition (also via thermal evaporation) of a 100-nm-thick Ag layer, serving as the anode.<\/p>\n Fabrication of electrically excited devices<\/p>\n Devices intended for EL measurements included an additional dielectric interlayer of LiF, with a current-focusing aperture prepared as a 50-\u00b5m-wide slit. This interlayer was inserted between the HTL and HIL and deposited using thermal evaporation. The slit was defined using a shadow mask. To further enhance the current-focusing effect, the top Ag contact was patterned as a narrow 300-\u00b5m-wide strip, orthogonal to the slit in the LiF layer.<\/p>\n Electrical characterization of EL devices<\/p>\n The current density\u2014voltage (j\u2013V) characteristics of the devices were measured using square voltage pulses generated by a function generator (Tektronix AFG320) and amplified by a high-speed bipolar amplifier (HSA4101, NF Corporation). The voltage across the device was monitored using a Tektronix TDS2024B oscilloscope, connected to the amplifier\u2019s monitoring port. The transient current was determined by measuring the voltage drop across a 10\u2009\u03a9 load resistor placed in the return path.<\/p>\n Fabrication of QD microdisk lasers<\/p>\n