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.
Highlights
Since the first observation of strong luminescence in free-standing, laboratory prepared ultrasmall silicon crystals/ nanoparticles,[1,2,3,4] concerted experimental,[5,6,7] theoretical, simulation, and computation efforts[8–16] have been initiated to discern why and how the luminescence commences and to present structural prototypes that may explain the keys to synthesis and the optical, electronic, and mechanical characteristics of the material
The results show that increasing the size of the network causes the transition potential barrier to a monotonic drop to the 0.06 eV barrier and the molecular well to collapse on one side, turning into a half-open well, which leads to an excited state-ground state conical crossing that quenches emission
We use the time dependent density functional theory (TDDFT) to calculate the ground state and excited state energy surfaces using the B3LYP functional and the 6-311G(d,p) basis set in the quantum computational package GAMESS (General Atomic and Molecular Electronic Structure System)
Summary
Since the first observation of strong luminescence in free-standing, laboratory prepared ultrasmall silicon crystals/ nanoparticles,[1,2,3,4] concerted experimental,[5,6,7] theoretical, simulation, and computation efforts[8–16] have been initiated to discern why and how the luminescence commences and to present structural prototypes that may explain the keys to synthesis and the optical, electronic, and mechanical characteristics of the material. It would be interesting to analyze prototypes that have networks of multiple molecular sites to probe if multiplicity of sites would alleviate the problem It is not clear how such multiplicity would affect the height of the potential barrier and the TD symmetry and elasticity or strain in the particle.[27]. We use laboratory and synchrotron UV excitation as well as comprehensive advanced atomistic simulation and computations to calculate the energy surfaces of the ground state and excited states in a 1-nm silicon particle for a variety of networks of coupled surface Si–Si molecular emission sites. The multiplicity of molecular sites increases the structural strain and instability, causing the excited particle to structurally relax to lower states with a variety of symmetries, which causes the absorbed energy to be transferred into the top of the transition barrier region of those lower states, followed by strong fast emission without traversing the barrier. A direct-like nanoform of silicon with strong fluorescence activity would be highly useful for integration of electronics and optics
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