Atomically precise bismuth oxido nanoclusters: cerium doping for optical modification and supramolecular self-assembly on Au(111)

Abstract

The functionalization of atomically precise bismuth oxido nanoclusters (BiO-NCs) by partial substitution of bismuth with cerium, chiral modification by ligand substitution, and their 2D-supramolecular self-assembly on surfaces such as Au(111) and HOPG(0001) was studied. Starting from [Bi₃₈O₄₅(NO₃)₂₀(dmso)₂₈](NO₃)₄·4dmso (C-1) and its cerium doped counterpart [Bi₃₈O₄₅(NO₃)₂₄(dmso)₂₈]:Ce·1.5dmso (C-1:Ce), soluble BiO-NCs with a size of 2 nm are obtained by reaction with methacrylate and chiral carboxylates to give [Bi₃₈O₄₅(L)₂₄] and [Bi₃₈O₄₅(L)₂₄]:Ce with L = McO⁻ (C-2 and C-2:Ce), Boc-L-Phe–O⁻ (C-3 and C-3:Ce), and Boc-L-Ala–O⁻ (C-4 and C-4:Ce). The cerium doping content determined using ICP-OES and the oxidation state of cerium determined via XP- and EPR spectroscopy were studied, showing Ce(III) for the carboxylate functionalized BiO-NCs, whereas mixed valency Ce(III)/Ce(IV) was detected for the nitrate C-1:Ce. However, doping of BiO-NCs with cerium results in a change of absorption from the UV to the visible light region for all compounds, whereby the red-shift is concluded to originate from the MLCT from Ce(III) to oxygen. The self-assembly of chiral BiO-NCs [Bi₃₈O₄₅(Boc-L-Phe–O)₂₄] (C-3) and [Bi₃₈O₄₅(Boc-L-Phe-O)₂₄]:Ce (C-3:Ce) on Au(111) and HOPG(0001) was analyzed. STM images of BiO-NCs revealed a stepwise one-dimensional arrangement of BiO-NCs and STS analysis proved significant changes in the electronic energy gap as a result of ≈1 ω% Ce(III) doping (C-3E_g = 3.5 eV vs.C-3:CeE_g = 2.6 eV).