Time-resolved electronic and optical properties of a thiolate-protected Au₃₈ nanocluster

Abstract

Density functional theory and density matrix theory are employed to investigate the time-dependent optical and electronic properties of an Au₁₄ nanocluster protected by six cyclic thiolate ligands, Au₄(SCH₃)₄. The Au₁₄[Au₄(SCH₃)₄]₆ nanocluster, i.e. Au₃₈(SCH₃)₂₄, is equivalent to a truncated-octahedral face-centred cubic Au₃₈ core coated by a monolayer of 24 methylthiol molecules. The electronic and optical properties, such as density of states, linear absorption spectra, nonradiative nonadiabatic dissipative electronic dynamics and radiative emission spectra were calculated and compared for the core Au₁₄ and thiolate-protected Au₃₈(SCH₃)₂₄ nanocluster. The main observation from computed photoluminescence for both models is a mechanism of radiative emission. Specifically, a strong contribution to light emission intensity originates from intraband transitions inside the conduction band (CB) in addition to interband LUMO → HOMO transition (HOMO: highest occupied molecular orbital and LUMO: lowest unoccupied molecular orbital). Such comparison clarifies the contributions from Au core and methylthiol ligands to the electronic and optical properties of the Au₃₈(SCH₃)₂₄ nanocluster.

Keywords:

• ultrafast spectroscopy
• photoluminescence
• protected Au nanoparticles
• thiolate ligands
• density matrix
• photoexcited dynamics
• nonradiative relaxation
• multilevel Redfield theory

Acknowledgements

Authors thank S. Huang, H. Yao, S. Jensen, and Yulun Han for editing and thorough discussion.

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Additional information

Funding

This work was supported financially by the South Dakota Governor's Office of Economic Development, NSF award EPS-0903804, DOE, BES – Chemical Sciences, NERSC Contract No. DE-AC02-05CH11231, allocation Award 86898, ‘Computational Modeling of Photo-catalysis and Photoinduced Charge Transfer Dynamics on Surfaces’. Computational resources of USD High Performance Computing facilities operated by Douglas Jennewein, the Dean's Opportunity Fund of the College of Arts and Sciences, University of South Dakota.