Tuning the electronic structure and stability of Au₃₈(SR)₂₄ nanoclusters via site-selective palladium doping

This study presents a systematic computational investigation into the structural, energetic, electronic, and optical properties of palladium-doped isomers of Au₃₈(SR)₂₄ nanoclusters, with a specific focus on the previously unexplored isomer [Au₃₈(SR)₂₄]_T. Using density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, we demonstrate that single-atom Pd doping at the central site of the Au₁₃ icosahedron in the T-isomer, forming [Au₃₇Pd₁(SR)₂₄]_T, is energetically feasible and yields a stable cluster. However, a critical, isomer-dependent thermodynamic limit is discovered: the incorporation of a second Pd atom to form [Au₃₆Pd₂(SR)₂₄]_T is significantly less favorable compared to its stable analogue in the well-studied Q-isomer ([Au₃₆Pd₂(SR)₂₄]_Q). This doping limit is rationalized by the distinct kernel architectures: the flexible T-isomer kernel (one icosahedron plus three Au₄ units) provides poor electronic stabilization for a second Pd dopant (frontier orbital contribution <20%) and exhibits lower dynamic rigidity, whereas the rigid bi-icosahedral kernel of the Q-isomer offers strong electronic stabilization (∼40-50% orbital contribution) and resists distortion. Structurally, Pd doping induces a pronounced contraction of the Au-Au bonds within the T-isomer's icosahedral shell, a response enabled by its structural flexibility, while the rigid Q-isomer shows a minimal geometric change. Electronically, Pd doping effectively narrows the HOMO-LUMO gap in both systems by modulating the Kohn-Sham frontier orbital energy levels, primarily through the hybridization of Pd-4d states with the original Au-sp and Au-d orbitals. This electronic restructuring induces a distinct redshift in the optical absorption spectra. Analysis of the density of states and the spatial distributions of the frontier orbitals confirms significant Pd contributions, revealing a notable difference in orbital composition between the T and Q isomer series. These findings not only validate the viability of Pd doping in the [Au₃₈(SR)₂₄]_T isomer, but also, more importantly, establish the kernel architecture as a decisive factor in determining the dopant capacity and structural response of nanoclusters, providing a fundamental principle for the isomer-specific design of doped nanoclusters with tailored properties.