Quantized Electronic Doping towards Atomically Controlled “Charge-Engineered” Semiconductor Nanocrystals

Abstract

† These authors contributed equally to this work. 'Charge engineering' of semiconductor nanocrystals (NCs) through so-called electronic impurity doping is a long-lasting challenge in colloidal chemistry and holds promise for groundbreaking advancements in many optoelectronic, photonic and spin-based nanotechnologies. To date, our knowledge is limited to a few paradigmatic studies on a small number of model compounds and doping conditions, with important electronic dopants still unexplored in nanoscale systems. Equally importantly, fine tuning of charge engineered NCs is hampered by the statistical limitations of traditional approaches. The resulting intrinsic doping inhomogeneity restricts fundamental studies to statistically averaged behaviours and complicates the realization of advanced device concepts based on their advantageous functionalities. Here we aim to address these issues by realizing the first example of II-VI NCs electronically doped with an exact number of heterovalent gold atoms, a known p-type acceptor impurity in bulk chalcogenides. Single-dopant accuracy across entire NC ensembles is obtained through a novel non-injection synthesis employing ligand-exchanged gold clusters as 'quantized' dopant sources to seed the nucleation of CdSe NCs in organic media. Structural, spectroscopic and magneto-optical investigations trace a comprehensive picture of the physical processes resulting from the exact doping level of the NCs. Gold atoms, doped here for the first time into II-VI NCs, are found to incorporate as nonmagnetic Au + species activating intense size-tuneable intragap photoluminescence and artificially offsetting the hole occupancy of valence band states. Fundamentally, the transient conversion of Au + to paramagnetic Au 2+ (5d 9 configuration) under optical excitation results in strong photoinduced magnetism and diluted magnetic semiconductor behaviour revealing the contribution of individual paramagnetic impurities to the macroscopic magnetism of the NCs. Altogether, our results demonstrate a new chemical approach towards NCs with physical functionalities tailored to the single impurity level and offer a versatile platform for future investigations and device exploitation of individual and collective impurity processes in quantum confined structures.

High Photon Upconversion Efficiency with Hybrid Triplet Sensitizers by Ultrafast Hole‐Routing in Electronic‐Doped Nanocrystals

“…Doping with Au + drastically modifies the PL spectrum and dynamics of the NCs, with the suppression of the band‐edge PL in favor of a broad, long‐lived emission centered at ≈670 nm (1.85 eV). In agreement with the mechanistic scheme drawn in Figure 1D and in the inset of Figure 2B, as well as with previous reports on NCs doped with group 11 metals, [ 36,37 ] this intragap emission, hereafter referred to as Au‐PL, is ascribed to the radiative decay of the CB electron in the d‐states of gold following the transient oxidation of Au + to Au 2+ upon ultrafast localization of the VB hole according to the reaction Au + + h VB → Au 2+ .…”

“…In this work, we realize this regime for the first time by exploiting the ultrafast hole dynamics in CdSe NCs electronically doped with Au + cations. In these systems, similar to Cu + ‐ or Ag + ‐doped NCs, [ 36,37 ] the d‐electrons of Au + impurities (in 5d 10 electronic configuration) introduce intragap hole acceptor states at Δ E VB‐Au ≈ 0.6 eV above the VB maximum that localize the photohole in ≈1–2 ps. [ 36 ] This gives rise to a bound exciton with the delocalized CB electron, whose radiative decay yields the characteristic long‐lived, Stokes‐shifted photoluminescence (PL).…”

“…In these systems, similar to Cu + ‐ or Ag + ‐doped NCs, [ 36,37 ] the d‐electrons of Au + impurities (in 5d 10 electronic configuration) introduce intragap hole acceptor states at Δ E VB‐Au ≈ 0.6 eV above the VB maximum that localize the photohole in ≈1–2 ps. [ 36 ] This gives rise to a bound exciton with the delocalized CB electron, whose radiative decay yields the characteristic long‐lived, Stokes‐shifted photoluminescence (PL). Importantly, since the Au + states are pinned to the host VB, the energy of the bound excitons can be tuned by control of the particle size so as to maximize the energy resonance with the triplet acceptor moiety of choice, 9‐ACA.…”

“…CdSe NCs doped with Au + impurities were synthesized following the method reported in ref. [ 36 ] using oleic acid (OA) as surface ligands. The particle size is 2.3 nm (Figure S1, Supporting Information) so as to maximize the spectral resonance between the emission energy of the doped NCs and the triplet state of the ligand.…”

“…Both doped and undoped NCs show a well‐defined 1S absorption peak confirming the good size dispersion of the NCs and that the incorporation of Au + impurities does not perturb the electronic structure of the NC host, in agreement with previous studies. [ 36 ] The as‐synthesized undoped NCs feature the typically narrow band‐edge PL of CdSe NCs (Stokes shifted of ≈80 meV from the respective 1S absorption peak) and a weak broadband emission at lower energies commonly ascribed to surface traps. [ 39 ] The time decay of the band‐edge PL (Figure 2C) is multiexponential consistent with the core‐only nature of the NCs and features an effective lifetime ≈ 40 ns (Table 1, Supporting Information).…”