Dynamic transition of nanosilicon from indirect to direct-like nature by strain-induced structural relaxation

Abstract

Silicon nanoclusters exhibit light emission with direct-like ns–µs time dynamics; however, they show variable synthesis and structure, optical, and electronic characteristics. The widely adopted model is a core–shell in which the core is an indirect tetrahedral absorbing Si phase, while the shell is a network of re-structured direct-like H–Si–Si–H molecular emitting phases, with the two connected via back Si–Si tetrahedral bonds, exhibiting a potential barrier, which significantly hinders emission. We carried out first-principles atomistic computations of a 1-nm Si nanoparticle to discern the variabilities. Enlarging the network reduces the potential barrier monotonically to a finite limit not sufficient for strong emission to proceed while inducing a path to quenching of emission via a conical crossing between the excited and ground states. However, enlarging the network is found to induce strain and structural instability, which causes structural relaxation that creates a direct path for emission without crossing the barrier. Following emission, the particle relaxes back to the indirect ground structure, which completes the cycle. The results also confirm the pivotal role of HF/H2O2 etching in synthesizing the core–shells and affording control over the molecular network. Measurements using synchrotron and laboratory UV excitation of thin films of 1-nm Si particles show good agreement with the simulation results. It is plausible that the relaxation is behind the stimulated emission, gain, or microscopic laser action, reported earlier in macroscopic distributions of 1- and 3-nm Si nanoparticles.

Time–thermo-dynamics of anti-Stokes and Stokes scattering and luminescence in 1-nm silicon nanoparticles: Toward an optical nanorefrigerator

“…However, being trapped in the excited state, the particle experiences strong strain and structural instability, which triggers non-radiative, structural relaxation to lower energy manifolds, allowing the trapped energy to be strongly released radiatively, i.e., creating a direct path for emission without crossing the barrier and effectively mimicking the lowering of the barrier. 33 The time delay observed is in agreement with the calculated delays using the model. The emission is fast but the population transfer is sensitive to temperature.…”

“…The relaxed configuration (Td symmetry) shows five atoms constituting a single tetrahedral core and 24 atoms constituting a H-terminated reconstructed surface or shell (Si 29 H 24 ), with six reconstructed dimers, and a center atom. 33 The Si 29 H 24 particle is a mix of solid and molecule states, exhibiting solid-like behavior as well as molecule-like behavior.…”

“…Nanosystems may also show some surprises when testing the second law of thermodynamics under non-equilibrium conditions. [32][33][34] Scientists found that in a silica nanoparticle trapped and cooled, heat flows from cold to hot. 32 After fast deposition of energy in the 1-nm nanoparticle using femtosecond light, the particle suffers structural relaxation into an equilibrium geometry with reduced bandgap, releasing light with less heat and entropy of the system.…”

“…32 After fast deposition of energy in the 1-nm nanoparticle using femtosecond light, the particle suffers structural relaxation into an equilibrium geometry with reduced bandgap, releasing light with less heat and entropy of the system. 33 A 50-nm-thick membrane of silicon nitride creates more entropy when measuring time more accurately. 34 Among all semiconductors, silicon is the most challenging since it is an indirect bandgap material.…”

“…Yet, silicon is highly rewarding at the nanoscale. 33,[35][36][37][38] For instance, sub-3-nm, in general, and 1-nm Si nanoparticles, in particular, are unique by being not a pure solid bulk, but rather a mix of solid and molecule natures. Being at the transition of solid and molecule phases, much of the characteristics that are most applicable to bulk may not be fully applicable, affording novel characteristics.…”