nanomaterials Review Origin of the Photoluminescence of Metal Nanoclusters: From Metal-Centered Emission to Ligand-Centered Emission Tai-Qun Yang, Bo Peng, Bing-Qian Shan, Yu-Xin Zong, Jin-Gang Jiang, Peng Wu * and Kun Zhang * Shanghai Key Laboratory of Green Chemistry and Chemical Processes, College of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China; [email protected] (T.-Q.Y.); [email protected] (B.P.); [email protected] (B.-Q.S.); [email protected] (Y.-X.Z.); [email protected] (J.-G.J.) * Correspondence: [email protected] (P.W.); [email protected] (K.Z.) Received: 23 December 2019; Accepted: 29 January 2020; Published: 4 February 2020 Abstract: Recently, metal nanoclusters (MNCs) emerged as a new class of luminescent materials and have attracted tremendous interest in the area of luminescence-related applications due to their excellent luminous properties (good photostability, large Stokes shift) and inherent good biocompatibility. However, the origin of photoluminescence (PL) of MNCs is still not fully understood, which has limited their practical application. In this mini-review, focusing on the origin of the photoemission emission of MNCs, we simply review the evolution of luminescent mechanism models of MNCs, from the pure metal-centered quantum confinement mechanics to ligand-centered p band intermediate state (PBIS) model via a transitional ligand-to-metal charge transfer (LMCT or LMMCT) mechanism as a compromise model. Keywords: photoluminescence mechanism; metal nanoclusters; quantum confinement effect; ligand effect; p band intermediate state (PBIS); interface state; nanocatalysis 1. Introduction Metal nanoclusters (Au, Ag, Pt, Cu) are a new class of attractive materials owing to their enhanced quantum confinement effect, which endows them with unusual optical and electronic properties [1–6]. Au and Ag NCs in particular have attracted tremendous interest because of their wide applications in single-molecule studies, sensing, biolabeling, catalysis, and biological fluorescence imaging [7–16]. The physical and chemical properties of metal nanoclusters (MNCs) are highly dependent on their size, shape, composition, and even assembly architecture [17–20]. These clusters are composed of a few to hundreds of atoms with a size approaching the Fermi wavelength of an electron (~0.5 nm for Au and Ag), and they possess discrete molecular-like electronic energy band structures, resulting in intense light absorption and emission. Luminescent MNCs exhibit outstanding optical properties including a large Stokes shift (generally large than 100 nm), large two-photon absorption cross-section, good photostability, good biocompatibility, and the emission wavelength could be easily adjusted by controlling their size and surface ligands, which are not present in conventional organic dyes [1,9,11,21]. Their excellent luminous performance is attractive for the applications in biomedicine, since it provides avenues for designing optical sensors, biolabeling, bioimaging, photosensitizers, and light-emitting devices. However, the luminous quantum yield of MNCs is still not competitive when compared to those organic dyes and semiconductor quantum dots (QDs), which always hinder their practical application. One primary reason is the lack of full understanding of the luminescence origin of MNCs at the molecular level, since its photoluminescence properties depend on many factors Nanomaterials 2020, 10, 261; doi:10.3390/nano10020261 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, x FOR PEER REVIEW 2 of 24 Nanomaterials 2020, 10, 261 2 of 24 reason is the lack of full understanding of the luminescence origin of MNCs at the molecular level, since its photoluminescence properties depend on many factors including cluster size, surface includingligands architecture, cluster size, cluster surface assembly ligands structure, architecture, etc. clusterThe preparation assembly structure,and application etc. The of preparationMNCs have andbeen application previously of MNCsreviewed have been[2,8,10,14,15,22–25 previously reviewed]. Herein, [2,8 ,10we,14 ,15are,22 –mainly25]. Herein, focused we are on mainly the focusedphotoluminescence on the photoluminescence emission origin emissionof MNCs. origin It is imperative of MNCs. Itto is clarify imperative the origin to clarify of photoemission the origin of photoemissionof MNCs and of thus MNCs improve and thus their improve luminous their luminousefficacy. effiTwocacy. major Two majorexplanations explanations about about the thephotoluminescence photoluminescence mechanism mechanism have have become become gradually gradually accepted accepted by by researchers. researchers. One One is is the the pure metal quantum confinementconfinement eeffectffect [[1,26–28].1,26–28]. That That is, is, as as the size of MNCs approaches the Fermi wavelength ofof metals (usually <<11 nm), the continuous band of energy level becomes discrete, leading to thethe emergingemerging ofof molecule-likemolecule-like properties.properties. The emission of MNCs was originatedoriginated from intrabandintraband (sp–sp) and interband interband (sp–d) (sp–d) transi transitionstions [29,30]. [29,30]. The The other other explanation explanation is the is thecharge charge transfer transfer on the on theshell shell of metal of metal clusters clusters due dueto the to interaction the interaction betw betweeneen the functional the functional ligands ligands and andthe metal the metal core, core, i.e., i.e., ligand-to-metal charge transfer (LMCT) [31] or ligand-to-metal metal charge transfer (LMMCT: ligand-to-metal charge transfer (LMCT) [31] or ligand-to-metal−metal− charge transfer (LMMCT: emphasize the gold–gold interactions resulted from the relativistic effect) effect) [[32]32] mechanism borrowing from thethe conceptconcept of of PL PL emission emission of of the the transition transition organometallic organometallic complexes. complexes. Both Both of these of these suggest suggest that thethat intraband the intraband (sp–sp) (sp–sp) and interband and interband (sp–d) transitions (sp–d) transitions from metal from NCs metal probably NCs act probably an intermediate act an stateintermediate (or dark state state) (or to dark determine state) to the determine electron the transfer electron [33 ].transfer These [33]. two These explanations two explanations are proposed are toproposed explain to the explain size-dependent the size-dependent and ligand-dependent and ligand-dependent emission emission phenomenon phenomenon respectively, respectively, but they arebut self-contradictedthey are self-contradicted to illustrate to theillustrate size-dependent the size-dependent and size-independent and size-independent emission. emission. In addition, In moreaddition, and more more and studies more show studies that show nonluminescent that nonlum groupsinescent could groups serve could aschromophores serve as chromophores in a certain in solventa certain condition solvent condition or confined or nanospaces. confined nanospaces. In addition, In more addition, and moremore strong and more evidences strong from evidences recent studiesfrom recent tend studies to prove tend the to limitation prove the of limitation metal-centered of metal-centered LMCT and LMMCTLMCT and models LMMCT to explain models the to photoluminescenceexplain the photoluminescence (PL) properties (PL) of properties metal NCs. of metal NCs. So far, twotwo mainmain approaches—theapproaches—the bottom–upbottom–up and top–down strategies—have been developed for the synthesis of emissionemission wavelengthwavelength adjustableadjustable luminescent MNCs with high quantum yield (QY) inin largelarge quantities,quantities, asas illustratedillustrated inin FigureFigure1 [1 [8–10,14,15,34].8–10,14,15,34]. The size effect (including metal corearchitecture) [ 35[35–40],–40], ligand ligand effect effect (charge-donating (charge-donating capability capability [33], [33], chirality chirality [41,42 [41,42],], hydrophily hydrophily [43]), valance[43]), valance stateof state surface of surface metals metals [33,44 ],[33,44], and superlattice and superlattice structures structures of assembly of assembly of metal of clusters metal clusters [45,46] have[45,46] been have intensively been intensively studied. studied. Figure 1. Schematic illustration of synthetic strategies of luminescent AuNCs and effect effect of ligands on their photoluminescence. Reprinted with permissionpermission from Ref. [[14].14]. Co Copyrightpyright (2017) Elsevier. Nanomaterials 2020, 10, 261 3 of 24 It is important to note that especially for the bottom–up synthetic strategy, a common feature of luminescence metal NCs is that the templates used for the synthesis as protective ligands, such as nonconjugated polymers, proteins, amino acid molecules, and even small chemical molecules, always contain electron-rich heteroatoms, including oxygen (O), nitrogen (N), sulfur (S), phosphorus (P), and others, emphasizing the dominant role of the surface ligand to tune the PL properties. However, the exact role of the surface ligand and the dynamic and/or kinetic of electron relax at the nanoscale interface between core–shell structured MNCs was not clarified. In this report, we simply review the present fundamental understanding of the photoluminescence origin of MNCs, especially our recently developed the ligand-centered p band intermediate state (PBIS) model to elucidate the abnormal PL emission phenomena. Finally, we proposed some forward-looking