ECASIA SPECIAL ISSUE PAPER X-ray and UV photoelectron spectroscopy of Ag nanoclusters Eleonora Bolli 1,2 | Alessio Mezzi 1 | Luca Burratti 2 | Paolo Prosposito 2 | Stefano Casciardi 3 | Saulius Kaciulis 1 1 Institute for the Study of Nanostructured Materials, ISMN-CNR, Rome, Italy 2 Department of Industrial Engineering, University of Rome Tor Vergata, Rome, Italy 3 Department of Occupational and Environmental Medicine, Epidemiology and Hygiene, National Institute for Insurance against Accidents at Work (INAIL), Rome, Italy Correspondence Eleonora Bolli, Institute for the Study of Nanostructured Materials, ISMN-CNR, 00015 Rome, Italy. Email: eleonora.bolli@ismn.cnr.it The main purpose of the present work is to analyze a series of Ag nanoparticles (NPs) with different size or ligand functionalization by using X-ray photoelectron spectros- copy (XPS) and to identify the differences in the band-shape and energy peak posi- tion of photoemission spectra due to the particle dimension. A transmission electron microscopy characterization was performed, to verify the consistency of the results. Three types of samples were prepared starting from AgNO 3 water solution and adding different capping agents. In the first two cases, the formation of NPs was pro- moted by the reduction of silver ions Ag +1 to metallic Ag 0 through the addition of sodium borohydride, whereas in the last case, it was triggered by the exposure to UV light. Depending on the size of the NPs, a different physical behavior can be recog- nized. NPs with diameter of about 5 nm are characterized by the phenomenon of localized surface plasmon resonance (LSPR). The other type of samples having a diameter of about 1.5 nm presents discrete energy levels instead of electronic bands, and in this case, a typical fluorescence phenomenon can be observed. In the latter case, we can refer to such systems as nanoclusters. The XPS analyses were focused on the Ag 3D spectra looking for the possible shifts of the Ag doublet as a function of the particles size. The ultraviolet photoelectron spectroscopy with He II source was used for the investigation of possible changes in the valence band. KEYWORDS Ag nanoclusters, Ag nanoparticles, TEM, UPS, XPS 1 | INTRODUCTION In the last decade, nanomaterials have attracted the attention of sci- entific community for their unusual features with respect to the same bulk material. They present interesting and innovative properties related to the small dimensions that allow their use in many fields such as biology, material science, physics, energy, and medicine with a huge number of applications, 1–3 spanning from energy to sensors and from bio-labeling to drug delivery and personalized medicine. Among the most studied nanomaterials, there are metal nanoparticles (NPs) and nanoclusters (NCs) with a mean diameter above and below 2 nm, respectively. The partial or total confinement of electrons in these two types of nanomaterials confer them very interesting physical and chemical properties. Optical features are probably the most studied for the ease of measurement and for their use in numerous fields of applications such as biomedicine, 4,5 biotechnology, 6,7 energy, 8–10 optics, and optoelectronics. 11–18 Silver NPs (Ag NPs) represent an example of noble metal NPs with peculiar properties, according to the principle for which the material properties can change as a function of its size. It is well known that bulk Ag is a good electrical and thermal conductor and has a highly reflective surface. However, the reduced size (d ≤ 100 nm) of Ag NPs allows the phenomenon of localized surface plasmon resonance (LSPR) 19 or quantum effects, like photoluminescence (PL), if the size approaches the dimension of 2 nm. 20 In X-ray photoelectron spectroscopy (XPS), the metallic Ag 3D 5/2 peak lies at a binding energy (BE) of 368.2 eV and has a very small Received: 2 October 2019 Revised: 4 March 2020 Accepted: 16 March 2020 DOI: 10.1002/sia.6783 Surf Interface Anal. 2020;1–6. wileyonlinelibrary.com/journal/sia © 2020 John Wiley & Sons, Ltd. 1