Enhanced photo-luminescence of CdX (X= S, Se, Te) Q-dots embedded in ordered mesoporous silica thin films

DIEGO ARIEL ONNA; M. L. MARTINEZ RICCI; SARA A. BILMES

Kyoto

Conferencia; XVIII edition of the International Sol-Gel Conference; 2015

International Sol-Gel Society

Photo-luminescence (PL) of Q-dots can be tuned by the particle diameter, crystallinity anddensity of surface states (SS). In particular, SS provide a non-radiative recombination and/or aradiative decay at low energies, far from the excitonic emission. In this work we explore theformation and the PL of Q-dots grown in ordered mesoporous oxides (OMPO) where pores act asnanoreactors leading to confined quasi-spherical nanoparticles (NPs) whose size is limited by porediameter fixed by the molecular template.OMPO films were prepared following the conventional protocols [1], using differentamphiphillic templates to tune pore radios (3-9nm). Q-dots were grown by successive2+impregnations in Cd and X precursor solutions (X= S, Se, Te). Later the Q-dots could bephotoetched by irradiating with a monochromatic wavelength source, this second step allowsmodifying their size and eliminating some SS through photocorrosion of surface defects [3]. Sampleswere characterized by UV-Vis, SEM, HRTEM, XRR and Environmental Ellipsometric Porosimetry(EEP). Steady state PL was measured at different excitation wavelengths in order to get themaximum intensity for each system.The UV-Vis spectra of as-prepared Q-dots in SiO2-OMPO showed how the size of Q-dotsdepends on the number of cycles of successive impregnations (Fig. 1 Left). The position of shoulderin the region of 350 ? 450 nm (first exciton state) shift to a longer wavelength as the Q-dots sizeincreases. Also the amount of scattering was very low what means the Q-dots were synthesizedwithin the pores of the SiO2-OMPO and not over the SiO2 layer. The absence of particles over thesurface of the SiO2 -OMPO was confirmed by SEM. For SiO2-OMPO with 2 cycles to synthesizedQ-dots, the HRTEM images showed an homogeneous filled of the pores with diameters below thepore diameter, that proved the pores effectively act as nano-reactors limiting the growing size of theQ-dots within the pores without using any template or capping to limit the growth (Fig. 1 Right).The amount of Q-dots was determined with XRR, the first shoulder shift corresponds to theelectronic density increment caused by the filling of the pore (Fig. 1 Center). Using this shift, it wascalculated a loading of 35% of the total porous volume. The EEP numeric modeling revealed thatthe Q-dots were only inside the pores and the connections among the pores (usually callednecks)were empty. Both results were in agreement with the HRTEM images.The PL spectra of as-prepared Q-dots in SiO2-OMPO exhibits one or two PL bands in the500 - 700nm range assigned to bulk defects (BD) or to SS[2]. Usually the BD band PL was higherthan the SS band PL. The relation between these two bands could vary through the exposure forseveral hours to air and ambient light, producing relative increase of the SS band but a decrease of - Kyoto, Japan - September 6-11, 2015both bands as overall effect. Although keeping the SiO2-OMPO with Q-dots in the dark, producedin the short term an increase of both bands and in the long term a relative increase of the SS band.The exposure to weak polychromatic light produced a soft photocorrotion that cleaned the surfacethis originate more quenching by the oxygen. The presence of oxygen generated surface defectswhich can passivate the surface, so the SS band PL increased and the oxygen quenching decrease.In the case of exposing the SiO2-OMPO with Q-dots to an intense monochromaticwavelength source, the PL intensity and band location depends on the activation wavelength andtime illumination (Fig. 2). Irradiation (370 nm) for less than 10 min leads to a noticeable increase ofBD PL. Further irradiation promotes a BD-excitonic (BD-EXC) interconversion developingnarrow excitonic band at higher energies in agreement with the literature [3]. The recorded spectraafter 2 days in dark does not show changes contrary to the fast decay present in Q-dots insuspensions.In summary, inclusion of CdX Q-dots in SiO2-OMPO films gives persistent enhanced PL ofQ-dots tuned by the pore diameter and the irradiation treatment and provides an environmentalfriendly platform for the design of optical devices based on Q-dots.