Scm pit

Nanoparticles were drop-cast on indium tin oxide (ITO)-coated glass substrates and characterized with a Solver Next sample-positioning microscope (NT-MDT) using an atomic force microscopy measuring head. To confirm the core–shell morphology, samples were placed in a plasma asher (HPT-100, Henniker plasma) for two 10-s periods, and a few nanometres of material was removed from the surface of the nanoparticles. Topography and surface potential were measured in semi-contact mode using a 20-nm PtIr-coated conductive 0.01–0.025 ohm cm antimony (n)-doped Si cantilever (SCM-PIT, Veeco). The cantilever had a resonant frequency and stiffness of 60–100 kHz and 1–5 N m⁻¹, respectively. The work function of the tip was estimated by measuring the contact potential difference (V_CPD) of a gold standard (5.1 eV) from Digital Instruments Veeco Metrology Group and using the equation:
$$V_{\text{CPD}}=\frac{{\varphi }_{\text{tip}}-{\varphi }_{\text{sample}}}{-e}$$
where ϕ is the work function and e the charge of an electron. The work function of the tip was found to be 5.05 eV (mean V_CPD for gold is 50 mV, from Supplementary Fig. 12e). To confirm, a commercial ITO sample was measured (Supplementary Fig. 12f) and the work function estimated as 4.84 eV (mean V_CPD for ITO is −213 mV).