Since their first synthesis in 1989 [1], many research groups have studied indium phosphide (InP) colloidal quantum dots (CQDs) as alternatives to cadmium (Cd) based CQDs because InPs are less toxic and more eco-friendly than Cd-based CQDs [2–4]. Among the increasing applications of InP CQDs, the most important one has been the development of light emitting diodes for flat panel displays. Recently, it has been reported that by precisely controlling the shell thickness of CQDs and using hydrofluoric acid during their synthesis InP/ ZnSe/ZnS, CQDs can show photoluminescence quantum yield (PLQY) values of nearly 100 % [5]. However, green and blue InP-based quantum dot light emitting diodes (QLEDs) still suffer from low PLQY because of mismatch between electron and hole injection efficiency [6]. To achieve whole InP-based QLED displays, novel strategies to dramatically enhance the PLQY, such as anisotropic growth for nanoplate formation or ligand exchange for carrier injection improvement, should be introduced [7].

Relatively few studies on the application of InP-based CQD lasers have been reported because of the difficulty of controlling the Auger recombination, even though these materials have wide applications in biophotonics such as bioimaging and photocatalysts, etc., due to their non-toxic property [8,9]. Amplified spontaneous emission (ASE) and lasing from InP-based CQDs have already been reported in distributed feedback (DFB) laser structures. Gratings of DFB structures were formed holographically in polymer-dispersed liquid crystals, and an InP/ZnS CQD film was spin-coated on a grating sample [10]. However, whole thickness of sample was 7 µm and pulsed laser was irradiated onto a stripe with 50 µm width on the top-surface of the sample. Compact and monolithic integration of CQD layer with substrate is essential to the electrical operation of the lasing; therefore, CQD film should be decreased to sub-micrometer thickness [11]. Here, we report ASE from an InP/ZnS thin film with 400 nm thickness formed on glass grating, as shown in Fig. 1(a). By spin-coating an InP CQD films on glass gratings at various coating speeds, and irradiating films with nanosecond laser pulses of 430 nm wavelength, we were able to observe ASE in samples with over 400 nm thickness.

Figure 1. (a) Schematics of an InP/ZnS CQD thin film coating on glass gratings. (b) Cross-sectional TEM image of the fabricated sample. (c) Calculated electric field distribution of a guiding mode in a CQD-coated grating structure. Lumerical FDTD was used in the simulation. Dipole sources were used to find out the guiding mode. Period and thickness of grating structure corresponded to 390 and 150 nm, respectively. The thickness of CQD film was 350 nm. (d) Magnified TEM image of synthesized InP/ZnS CQDs. The diameter of synthesized QDs corresponds to about 3.5 nm.

The glass substrate was patterned by e-beam lithography (JBX-6000 FS, JEOL) and etched by an inductively coupled plasma reactive ion etcher. E-beam resist (ZEP520A, ZEON Corporation) with 200 nm thickness was spin-coated on glass substrate, and gratings with 350 nm period were patterned to make Cr hard mask for plasma etching. After fabrication, we saw well-patterned glass gratings with thicknesses of 142 nm and periods of 360 nm in transmission electron microscopy (TEM) images, as shown in Fig. 1(b). Figure 1(c) shows electric field distribution of the guiding mode formed in the glass grating structure. Simulation was performed using a finite-difference time-domain simulator (Lumerical FDTD).

InP/ZnS QD synthesis was performed using slightly modification of the method of Peng et al [12]. First, 0.2 mmol of tris (trimethylsilyl) phosphine of octadecine (ODE) and 2.4 mmol of 1-octylamine were mixed to prepare a phosphorus precursor. Second, an indium precursor was separately prepared in a three-neck flask by mixing 0.4 mmol ODE indium acetate, 1.54 mmol ODE myristic acid, and 4 g of ODE. This mixed solution was heated to 188 °C. The phosphorus precursor solution was injected into the reaction solution at 188 °C and grown at 178 °C. For ZnS shell growth, zinc stearate (0.1 M in ODE) and sulfur (0.1 M in ODE) were mixed. This mixture was injected into the reaction solution at 150 °C for 10 min. The reactor was heated to 220 °C for 30 min. In Fig. 1(d), we can see that InP/ZnS CQDs were clearly well-synthesized. The synthesized CQDs were spin-coated on the fabricated glass grating at various coating speeds to make films with different thicknesses.

3.1. Optical measurement setup

Figure 2 shows the photoluminescence (PL) measurement setup used in the experiment. The PL signals were measured and found to be reflection type. Nanosecond pulse laser with 430 nm wavelength was irradiated via optical fiber coupler from the right side of the setup, incident to the fabricated sample. PL signals from the sample were directed into the monochromator by a dichroic mirror and saved as spectra in the monochromator. The pumping pulse laser was blocked by the laser cut-off filter in front of the monochromator.

Figure 2. (Left) photography and (Right) schematics of PL measurement setup.

3.2. PL measurement

Figures 3(a)-(c) show PL spectra measured from different samples with different InP/ZnS CQD film thicknesses. Three samples with different CQD thickness were prepared to determine the minimum CQD thickness that shows ASE signals in the experiment. Each CQD layer was spin-coated at speeds of 1000, 700, and 600 rpm. The thicknesses of the samples were measured by UV-vis spectrophotometer and found to be 360, 400, and 450 nm, respectively. From the measured PL spectra, we find that ASE can be observed in samples with over 400 nm thickness at 631 nm wavelength. However, the intensity of ASE measured from the sample with 400 nm thickness was very weak; ASE measured from the sample with 450 nm thickness was clearly seen. This means that the optical gain of the CQD layer with 400 nm thickness can overcome the optical loss and amplify the spontaneous emission at wavelength of 631 nm.

Figure 3. PL spectra measured from of InP/ZnS CQD films that spin-coated for 40 s at (a) 1,000 rpm, (b) 700 rpm, and (c) 600 rpm. We can see ASE signals from sample (b) and (c) that have the thickness of 400 and 450 nm, respectively.

In conclusion, we synthesized InP/ZnS CQDs and spin-coated them on glass grating samples to make CQD thin films. From cross-sectional TEM images, we saw that dense InP/ZnS thin films were well-formed, and we were able to observe ASE from an InP/ZnS CQD thin film with over 400 nm thickness by pumping the fabricated sample with a nanosecond pulse laser. We expect that the observation of ASE from the sample with dense CQD thin film can promote the emergence of electrically-pumped InP-based lasers and expand the applications of InP CQDs in the future.

This study was supported by the National R&D Program, through the National Research Foundation of Korea, funded by the Ministry of Science and ICT (NRF-2021R1F1A1062182), the Basic Science Research Program through the National Research Foundation of Korea, funded by the Ministry of Education (NRF-2020R1A6A1A03047771), and a Korea Institute for Advancement of Technology grant funded by the Korean Government (MOTIE) (P0008458, The HRD Program for Industrial Innovation).

The authors declare no conflicts of interest.