Optimising Spectral Quality of Ternary Copper Dichalcogenide Core-Shell Nanoparticle Luminescent Down-Shifting Films in Controlled Environments

The development of luminescent down-shifting films (LDS) for spectral adaptation in controlled environment agriculture systems (CEAs) towards improved crop yields has been of tremendous focus in recent years. Due to their tunable nature, they can better utilise the sun’s energy for effective light transmittance, down-shift emission into photosynthetically active radiation (PAR), while also reducing unwanted wavelengths which contribute to the generation of excessive heat within the CEAs; detrimental to crops grown in hot climates.<br/><br/>Copper-based dichalcogenide quantum dots (QDs) are regarded as environmentally benign materials that display desirable optical properties resulting in their use in LDS films in CEAs. In this project, a facile hot-injection colloidal synthetic protocol is employed. Highly luminescent copper indium sulphide (CIS) nanoparticles synthesised between 180 – 240 °C and excited at 525 nm emit between 619 – 639 nm (orange – red light) of varying luminescent intensities, FWHM between 79 – 90 nm, and peaking at 230 °C after 25 minutes of growth. Particle size is largest at 10 nm under the 240 °C protocol, with triangular morphology and high crystallinity due to their sharp edges and clearly defined uniform inter-particle distances indicative of effective binding of 1-dodecanethiol and oleic acid surfactants.<br/><br/>Energy dispersive spectroscopic (EDS) analysis reveals that at lower synthetic temperatures between 180 – 200 °C an off-stoichiometric copper-deficient particle with respect to indium (Cu_0.59In_0.94S_2.46 at 180 °C), produces negligible to low orange – red luminescence with direct bandgaps between 2.31 – 2.13 eV respectively after 30 minutes of growth. At higher synthetic temperatures between 220 – 240 °C, however, the EDS data indicate a copper-rich particle with respect to indium (Cu_1.26In_0.65S_2.09) with the highest red luminescent intensity at 230 °C. Direct bandgaps decreased from 2.06 to 2.03 eV after 30 minutes of growth. Further characterization using X-ray photoelectron spectroscopy, Raman spectroscopy and time-resolved photoluminescence spectroscopy implicate several defects such as copper vacancies and interstitials influencing the remarkable luminescent properties of CIS.<br/><br/>Additionally, surface shelling the CIS core particles with zinc sulphide (ZnS) further tunes their optical properties for effective spectral adaptation by not only increasing the luminescent intensity through surface passivation and decreasing surface defects, but by blue-shifting its emission by cation-exchange mechanisms at the core’s surface with CIS emission from 634 nm (FWHM 79 nm) at 220 °C, to 620 nm (FWHM 72 nm) CIS/ZnS shell growth at 210 °C for 30 minutes.<br/><br/>The highly luminescent nanoparticles will be embedded into transparent polymers of either poly(lauryl methacrylate-co-ethylene glycol dimethacrylate) (P(LMA-co-EDGM)) or poly(ethylene-vinyl acetate) (P(EVA)) to form LDS films of effective thickness to effect the desired optical properties which can enhance crop yield. The nanocomposite LDS films will be used as a prototype solar window element to evaluate the effectiveness of the QDs to spectrally adapt the solar spectrum to improve certain crop yields under controlled conditions in CEAs.