Bright Silicon Nanocrystals from a Liquid Precursor: Quasi-Direct Recombination with High Quantum Yield

Abstract

Silicon nanocrystals (SiNCs) with bright bandgap photoluminescence (PL) are of current interest for a range of potential applications, from solar windows to biomedical contrast agents. Here, we use the liquid precursor cyclohexasilane (Si6H12) for the plasma synthesis of colloidal SiNCs with exemplary core emission. Through size separation executed in an oxygen-shielded environment, we achieve PL quantum yields (QYs) approaching 70% while exposing intrinsic constraints on efficient core emission from smaller SiNCs. Time-resolved PL spectra of these fractions in response to femtosecond pulsed excitation reveal a zero-phonon radiative channel that anticorrelates with QY, which we model using advanced computational methods applied to a 2 nm SiNC. Our results offer additional insight into the photophysical interplay of the nanocrystal surface, quasi-direct recombination, and efficient SiNC core PL.

Luminescence mechanism in hydrogenated silicon quantum dots with a single oxygen ligand

“…Though the momentum kconservation is relaxed, the transition still shows an indirect nature and the observed photoluminescence (PL) lifetime is found to be at the scale of tens or hundreds of microseconds. [22][23][24][25] In contrast, the PL lifetime of direct QDs only reaches a few nanoseconds, according to previous experimental results. 22 Despite much effort, the mechanism behind luminescence is still unclear, especially the role played by the passivation ligand, and this is of great interest and practical importance.…”

“…According to Einstein coefficients, the lifetime of spontaneous emission is related to the energy difference between two states involved in the emission process as well as oscillator strength. The spontaneous emission factor can be written as , According to Equation (2)-(3), the emission lifetime of Si=O double bonded and hydrogenpassivated Si-QDs of different diameters can be calculated and the results are given in Figure 5, where it can be seen that hydrogen passivated Si-QDs have an emission lifetime of around 4 to 200 μs, exhibiting an indirect band gap feature [22][23][24][25] , which varies with respect to size. It seems abnormal that 1.3-H 64 has even lower oscillator strength as well as longer emission lifetime compared to larger hydrogen passivated Si-QDs.…”

Structure and Properties of Heavily B and P Codoped Amorphous Silicon Quantum Dots

“…The simulated QDs were prepared by a three-step procedure: (1) The QD of a particular size and doping with no surface saturation was slowly heated until diffusion of all of the constituent atoms was observed, which was typically ∼1500 K, and then again slowly cooled until diffusion completely seized, typically ∼700 K. (2) The temperature of the QD was set to 0 K, and its structure was locally optimized. (3) The metallic character of the QD was rectified by removal of the gap states, which was the critical step of the procedure.…”

“…Although the optical quality of Si QDs had been very low for many years, it has been improving quickly in recent years. For example, the luminescence quantum yield approaches 70% around 800 nm. , The high luminescence quantum yield in combination with some specific features arising from the indirect nature of the energy band structure such as the long luminescence lifetime and the large Stokes shift bring out new applications in Si QDs, for example, as phosphors for a luminescent solar concentrator , and for time-gated imaging of biosubstances …”

“…For example, the luminescence quantum yield approaches 70% around 800 nm. 2,3 The high luminescence quantum yield in combination with some specific features arising from the indirect nature of the energy band structure such as the long luminescence lifetime and the large Stokes shift bring out new applications in Si QDs, for example, as phosphors for a luminescent solar concentrator 4,5 and for time-gated imaging of biosubstances. 6 The property of Si QDs has been controlled predominantly by the size and the surface termination.…”