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.

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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 perspectives to the future developments of MNCs, in particular, for the application of MNCs in the nanocatalysis science.

2. Photoemission Mechanism of Metal Nanoclusters

2.1. Size-Dependent Photoemission As the saying goes, “gold will glitter forever!” The first experimental observation of photoluminescence from bulk gold metal can date back to as early as 1969 [29]. Mooradian reported that bulk gold and copper film could generate photoemission when excited under a high-energy laser. The emission was arising from the interband transitions between electrons in conduction-band states below the Fermi level and holes in the d bands generated by optical excitation, as shown in Figure2. This is the first proposed energy band structure to explain the photoemission mechanism of transition bulk metal. The energy gap was elucidated to 2.0 eV for copper and 2.2 eV for gold with the help of emission spectra. While photoemission was observed from bulk metal, the emission was generated 10 under an extreme condition, and the quantum yield was very low (10− ), which was not suitable for practical application. Metal nanoparticles with a size range from 2 to 50 nm exhibit intense colors due to the surface plasmon resonance (SPR); in some cases, photoluminescence was also observed in these small metal nanoparticles, which contrasts the conclusion that MNCs with SPR effect generally do not produce fluorescence [2]. In 1998, Wilcoxon et al. observed relatively intense photoluminescence when the size of the metal nanocluster was sufficiently small [47]. Nonluminescent gold particles can also become luminescent by partial etching with potassium cyanide (KCN). The photoemission with large Stokes shift is ascribed to the electron and hole interband recombination process. When the particle size was further reduced to 2 nm or less, the continuous band structure will be broken down into discrete energy levels according to quantum confinement mechanics, exhibiting molecular-like properties and not exhibiting typical plasmonic properties. Indeed, these very small transition metal clusters with precisely defined architectures are first studied in heterogeneous catalysis [48], which did not attract much more attention from photochemists and photophysicists. With the rapid development of synthetic nanochemistry, more and more people focused on the peculiar optoelectronic properties of these tiny metal clusters, even for still low quantum yield (QY) confined in a gas matrix at low temperature [49,50]. By combining solid-state principles as well as a molecular model, Link et al. improved the energy band structure model to explain the electron transition process and subsequent PL emission, which was attributed to the intraband (sp–sp) and interband (sp–d) transitions as illustrated in Figure3[30]. Nanomaterials 2020, 10, 261 4 of 24 Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 24 Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 24

Figure 2. Schematic band structure of a noble metal showing the excitation and recombination Figure 2. SchematicSchematic band band structure structure of of a a noble noble metal metal showing the excitation and recombination transitions. Reprinted with permission from Ref. [29]. Copyright (1969) American Physical Society. transitions.transitions. Reprinted Reprinted with with permi permissionssion from from Ref. Ref. [29]. [29]. Copyright Copyright ( (1969)1969) American American Physical Physical Society. Society.

Figure 3.3. Solid-state model for the originorigin ofof thethe twotwo luminescenceluminescence bands:bands: The high-energy band is Figure 3. Solid-state model for the origin of the two luminescence bands: The high-energy band is proposed to bebe duedue toto thethe radiativeradiative interbandinterband recombinationrecombination between the sp andand d-bands,d-bands, while the proposed to be due to the radiative interband recombination between the sp and d-bands, while the low-energylow-energy bandband is is thought thought to to originate originate from from radiative radiative intraband intraband transitions transiti withinons within the sp-band the sp-band across low-energy band is thought to originate from radiative intraband transitions within the sp-band theacross HOMO-LUMO the HOMO-LUMO gap (highest gap (highest occupied occupied molecular molecular orbital andorbital lowest and unoccupiedlowest unoccupied molecular molecular orbital). across the HOMO-LUMO gap (highest occupied molecular orbital and lowest unoccupied molecular Noteorbital). that Note intraband that intraband recombination recombination has to involve has to theinvolve prior the nonradiative prior nonradiative recombination recombination of the hole of orbital). Note that intraband recombination has to involve the prior nonradiative recombination of inthe the hole d-band in the createdd-band aftercreated excitation after excitation with an with (unexcited) an (unexcited) electron electron in the sp-band.in the sp-band. Reprinted Reprinted with the hole in the d-band created after excitation with an (unexcited) electron in the sp-band. Reprinted permissionwith permission from Ref.from [ 30Ref.]. Copyright[30]. Copyri (2002)ght (2002) American American Chemical Chemical Society. Society. with permission from Ref. [30]. Copyright (2002) American Chemical Society. With thethe progress progress of wetof chemicalwet chemical synthesis synthesi strategy,s strategy, luminescent luminescent water-soluble water-soluble metal nanoclusters metal With the progress of wet chemical synthesis strategy, luminescent water-soluble metal withnanoclusters different sizeswith weredifferent successfully sizes were synthesized successfully with improvedsynthesized PL emissionwith improved efficiency PL and emission tunable nanoclusters with different sizes were successfully synthesized with improved PL emission colorsefficiency [26 ,and51–54 tunable]. The colors photoemission [26,51–54]. wavelength The photoemission could be wavelength varied from could the ultravioletbe varied from (UV) the to efficiency and tunable colors [26,51–54]. The photoemission wavelength could be varied from the near-infraredultraviolet (UV) (NIR) to near-infrared region by controlling (NIR) region the sizesby co ofntrolling metal NCsthe sizes [1]. Inof themetal early NCs 2000s, [1]. In Dickson the early et ultraviolet (UV) to near-infrared (NIR) region by controlling the sizes of metal NCs [1]. In the early al.2000s, first Dickson achieved et the al. preparationfirst achieved of the water-soluble preparation Au of NCswater-soluble with high Au fluorescence NCs with in high solution fluorescence using a 2000s, Dickson et al. first achieved the preparation of water-soluble Au NCs with high fluorescence biocompatiblein solution using dendrimer a biocompatible (OH-terminated dendrimer poly(amidoamine) (OH-terminated (PAMAM)) poly(amidoamine) as a capping (PAMAM)) agent [26,51 as,52 a]. in solution using a biocompatible dendrimer (OH-terminated poly(amidoamine) (PAMAM)) as a Thecapping emission agent wavelength [26,51,52]. The of as-prepared emission wavelength Au NCs could of as-prepared be adjusted Au from NCs UV could to the be NIRadjusted region from by capping agent [26,51,52]. The emission wavelength of as-prepared Au NCs could be adjusted from changingUV to the theNIR metal region/polymer by changing ratio during the metal/polymer the synthesis. ratio Benefiting during from the synthesis. the progress Benefiting of soft-ionization from the UV to the NIR region by changing the metal/polymer ratio during the synthesis. Benefiting from the massprogress spectrometry of soft-ionization (MS) technology, mass spectrometry the cluster (MS) size technology, could be precisely the cluster identified. size could By be ingeniously precisely progress of soft-ionization mass spectrometry (MS) technology, the cluster size could be precisely creatingidentified. the By linear ingeniously correlation creating between the the linear emission correla intensitytion between at a certain the emission wavelength intensity and the at intensitya certain identified. By ingeniously creating the linear correlation between the emission intensity at a certain ofwavelength mass spectra and at the a certain intensity cluster of mass size, thespectra emission at a certain wavelength cluster of disize,fferent the clustersemission was wavelength determined. of wavelength and the intensity of mass spectra at a certain cluster size, the emission wavelength of Thedifferent correlation clusters between was determined. the size of Au The nanodots correlation and emissionbetween energiesthe size wasof Au established nanodots and and fit emission a simple differentscaling relation clusters of wasE determined./N1/3 (where TheE correlationis the Fermi between energy the of bulksize goldof Au and nanodotsN is the numberand emission of Au energies was establishedFermi and fit a simpleFermi scaling relation of EFermi/N1/3 (where EFermi is the Fermi energies was established and fit a simple scaling relation of EFermi/N1/3 (where EFermi is the Fermi energy of bulk gold and N is the number of Au atoms) [26], as shown in Figure 4. The energy of bulk gold and N is the number of Au atoms) [26], as shown in Figure 4. The

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size-dependent scaling of excitation and emission energies with EFermi/N1/3 confirms the good matching with the jellium model where the gap between the discrete 5d valence band and the 6sp conduction band decreases with the increasing cluster size [1]. If this model is rational, the coinage metals at the same family with the similar size or the close atom numbers should show very similar

NanomaterialsPL emission.2020 However,, 10, 261 the size distribution and optical properties of stable “magic” number5 ofAg 24 NCs were found to be obviously different to the gold NCs. Recently, Udaya Bhaskara Rao et al. successfully synthesized Ag NCs with very similar particle size (Ag7 and Ag8) using atoms)mercaptosuccinic [26], as shown acid in (H Figure2MSA)4. Theas capping size-dependent ligands; scaling they ofshow excitationed distinguished and emission PL energies emission with at 1/3 EapproximatelyFermi/N confirms 440 nm the goodand 650 matching nm for with Ag the7 and jellium Ag8 modelNCs, respectively. where the gap Only between one thesilver discrete atom 5ddifference valence in band cluster and structures the 6sp conduction could lead band to such decreases a big change with the of increasingemission color cluster from size blue [1]. to If thisred model[55]. This is rational, could not the coinagebe simply metals exchanged at the same only family by a withsize-dependent the similar sizequan ortum the closeconfinement atom numbers effect should(QCE). show very similar PL emission. However, the size distribution and optical properties of stable “magic”Even number though AgMNCs NCs shows were foundstrong tosize-dependent be obviously PL diff emissionerent to properties, the gold NCs. the quantum Recently, lifetime Udaya Bhaskaraand QY calculated Rao et al. by successfully theoretical synthesizedcalculation are Ag NCsmuch with less verythan similarthe experimental particle size observed (Ag7 and values, Ag8) usingand if mercaptosuccinic we are only considering acid (H2MSA) the contribution as capping ligands; of metal they to showedPL emission, distinguished the wavelength PL emission of PL at approximatelyemissions calculated 440 nm by and the 650 theoretical nm for Ag 7modeland Ag should8 NCs, lie respectively. in a near-infrared Only one range, silver atominstead diff erenceof the inobserved cluster structuresvisible wavelength could lead emission. to such a bigThese change disagreements of emission between color from experimental blue to red [and55]. theoretical This could notdata be indicate simply the exchanged invalidity only of metal-centered by a size-dependent quantum quantum confinement confinement mechanics. effect (QCE).

Figure 4.4. (a) Excitation (dashed) and emis emissionsion (solid) spectra spectra of of different different sizes of of PAMAM-Au NCs. Excitation and emission maxima shift to longer wave wavelengthslengths with increasing in initialitial Au concentration, suggesting that increasing increasing the the nanocluste nanoclusterr size size leads leads to to lower lower energy energy emission. emission. (b) (Correlationb) Correlation of the of thenumber number of Au of Auatoms atoms per per cluster cluster (N) (N) with with the the ph photoemissionotoemission energy energy of of Au Au NCs. Reprinted withwith permission fromfrom Ref.Ref. [[26].26]. CopyrightCopyright (2004)(2004) AmericanAmerican PhysicalPhysical Society.Society.

2.2. Size-IndependentEven though MNCs Photoemission shows strong size-dependent PL emission properties, the quantum lifetime and QY calculated by theoretical calculation are much less than the experimental observed values, and if weWith are only sophisticated considering synthetic the contribution techniques, of metalatom-p torecise PL emission, MNCs thewere wavelength achieved for of PLa number emissions of calculatedmetal nanoclusters by the theoretical (referred modelto as M shouldnLm, where lie in n a and near-infrared m are the range,numbers instead of metal of the atoms observed and ligands visible wavelengthin the cluster). emission. Very recently, These disagreements Luo and his betweenco-workers experimental synthesized and a series theoretical of glutathione data indicate (GSH) the invalidityprotected ofAu metal-centered NCs with precisely quantum atomic confinement compositions mechanics. (Au29SG27, Au30SG28, Au36SG32, Au39SG35, Au43SG37), and, to one’s surprise, they all exhibited completely the same emission at approximately 2.2.610 Size-Independentnm [56], which is Photoemission completely contradictory to the explanation of the classical QCE mechanism. Furthermore, the emission wavelength could be readily regulated from 600 nm to 810 nm by just With sophisticated synthetic techniques, atom-precise MNCs were achieved for a number of metal fine-tuning the surface ligands’ coverage [57], suggesting the pivotal role of surface-protective nanoclusters (referred to as MnLm, where n and m are the numbers of metal atoms and ligands in the ligands to tune the PL of Au NCs. In addition, the Au NCs with the same number of core atoms but cluster). Very recently, Luo and his co-workers synthesized a series of glutathione (GSH) protected different protecting ligands show different PL properties [33,58,59]. These results clearly indicate Au NCs with precisely atomic compositions (Au29SG27, Au30SG28, Au36SG32, Au39SG35, Au43SG37), that the metal core is not the only determining factor for the photoemission of MNCs. Other and, to one’s surprise, they all exhibited completely the same emission at approximately 610 nm [56], components of MNCs including the nature of coordinate ligands, valence states of surface metal which is completely contradictory to the explanation of the classical QCE mechanism. Furthermore, the emission wavelength could be readily regulated from 600 nm to 810 nm by just fine-tuning the surface ligands’ coverage [57], suggesting the pivotal role of surface-protective ligands to tune the PL of Au NCs. In addition, the Au NCs with the same number of core atoms but different protecting ligands show different PL properties [33,58,59]. These results clearly indicate that the metal core is not the only determining factor for the photoemission of MNCs. Other components of MNCs including Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 24 Nanomaterials 2020, 10, 261 6 of 24 atoms, and assembly architectures of nanoclusters also make a significant difference on the emission theproperties nature of coordinateMNCs. ligands, valence states of surface metal atoms, and assembly architectures of nanoclusters also make a significant difference on the emission properties of MNCs. 2.2.1. Ligand Effect 2.2.1. Ligand Effect It is well accepted that the organic/inorganic scaffolds and/or surface protective ligands were used Itto is stabilize well accepted the metal that nanocl the organicusters,/inorganic since naked scaff oldsMNCs and would/or surface strongly protective interact ligands with each were other used toand stabilize aggregate the metalirreversibly nanoclusters, as to reduce since nakedtheir su MNCsrface wouldenergy. strongly Solid matrices interact including with each noble other andgas aggregatematrices [60], irreversibly glasses [61], as to and reduce zeolites their surface[62–64] energy.are widely Solid selected matrices for including incubating noble silver gas matricesnanoclusters [60], glasseswith only [61 a], few and atoms. zeolites However, [62–64] arethe widelylarge size selected of solid for matrices incubating impedes silver thei nanoclustersr further application with only in a fewbiomedicine atoms. However, science. theThe large organic size oftemplates solid matrices that encapsulated impedes their MNCs further were application first achieved in biomedicine using science.water-soluble The organicdendrimer templates (PAMAM) that as encapsulated the scaffold by MNCs Dickson were et firstal. in achieved 2002 [51]. using Since water-soluble then, various dendrimersynthesis strategies (PAMAM) including as the scatheff oldtemplate-assisted by Dickson et method al. in 2002 and [51ligand-induced]. Since then, etching various have synthesis been strategiesdeveloped including to fabricate the organic template-assisted ligands-capped method lumi andnescent ligand-induced MNCs [10,14]. etching The have formation been developed of MNCs to fabricatewith different organic capping ligands-capped ligands in luminescent solution have MNCs been [10 accomplished,14]. The formation in various of MNCs ways, with as shown different in cappingFigure 5 ligands[34]. The in synthesis solution haveparameters, been accomplished such as the reduction in various method ways, as of shown metal inions, Figure the 5initial[ 34]. ratio The synthesisof reactants, parameters, and the reaction such as thetemperature reduction have method a profound of metal ions,effect the on initial the generation ratio of reactants, of luminescent and the reactionMNCs. temperature have a profound effect on the generation of luminescent MNCs.

Figure 5. Representative luminescent noble–metal nanoclus nanoclustersters scaled as a function of their emission wavelength superimposedsuperimposed overover the the spectrum. spectrum. Protected Protected molecules molecules show show diff erentdifferent capabilities capabilities to tune to thetune emission the emission wavelength wavelength of metallic of metall nanoclustersic nanoclusters from from current current reports. reports.

Prominently, thethe emission emission wavelength wavelength of theof ligand-protectedthe ligand-protected MNCs MNCs exhibits exhibits a strong a ligand-strong dependentligand-dependent effect. Foreffect. example, For exampl the emissione, the emission wavelength wavelength of DNA of oligomer-capped DNA oligomer-capped Ag NCs couldAg NCs be variedcould be from varied the bluefrom to the NIR blue region to NIR by playing region withby playing the nucleotide with the sequence nucleotide [65 ].sequence In addition, [65]. theIn photoluminescenceaddition, the photoluminescence intensity is also dependentintensity is on al theso nucleotidedependentsequence. on the nucleotide The photoluminescence sequence. The of DNA-AgNCsphotoluminescence can be of enhanced DNA-AgNCs 500-fold can when be placedenhanced in proximity 500-fold towhen guanine-rich placed in DNA proximity sequences. to Basedguanine-rich on this discovery,DNA sequences. Yeh et al. Based designed on this a fluorescent discovery, probe Yeh toet detect al. designed specific nucleica fluorescent acid targets probe [66 to]. Thisdetect fluorescence specific probenucleic can acid easily targets reach high [66]. signal-to-background This fluorescence ratios probe (S/ Bcan ratios) easily (>100) reach upon targethigh binding,signal-to-background since it does not ratios rely (S/B on Forster ratios) energy (>100) transferupon target for quenching. binding, since Wu et it al.does systematically not rely on studiedForster theenergy ligand’s transfer role for in thequenching. fluorescence Wu ofet goldal. systematically nanoclusters [studied33]. They the found ligand’s that role the photoluminescencein the fluorescence intensityof gold nanoclusters was scaled with[33]. theThey electron found donationthat the photoluminescence capacity of thiol ligands. intensity The was photoluminescence scaled with the waselectron attributed donation to thecapacity charge of transfer thiol ligands. from the Th ligandse photoluminescence to the metal nanoparticle was attributed core (i.e.,to the LMNCT) charge throughtransfer thefrom Au-S the bonds.ligands These to the ligands metal withnanopartic electron-richle core atoms(i.e., LMNCT) (e.g., N, O) through or groups the (e.g., Au-S –COOH, bonds. NHThese2) canligands largely with promote electron-rich fluorescence atoms via(e.g., surface N, O) interactions, or groups (e.g., as shown –COOH, in Figure NH26) .can Luo largely et al. demonstrated that even only the aggregated Au(I) thiolate complexes induced by solvents or cations promote fluorescence via surface interactions, as− shown in Figure 6. Luo et al. demonstrated that couldeven only generate the aggregated strong photoluminescence. Au(I)−thiolate complexes The formation induced of aurophilic by solvents bonds or provided cations could the impetus generate for aggregation;strong photoluminescence. then, denser and The more formation rigid aggregates of aurophilic were formed. bonds The emissionprovided fromthe theimpetus aggregates for was attributed to ligand-to-metal charge transfer (LMCT) or ligand-to-metal metal charge transfer aggregation; then, denser and more rigid aggregates were formed. The emission− from the aggregates

Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 24 Nanomaterials 2020, 10, 261 7 of 24 was attributed to ligand-to-metal charge transfer (LMCT) or ligand-to-metal−metal charge transfer (LMMCT) from the sulfur atom in the thiolate ligands to the Au atoms, and subsequent radiative (LMMCT) from the sulfur atom in the thiolate ligands to the Au atoms, and subsequent radiative relaxation, which was most likely via a metal-centered triplet state. Pyo et al. reported an efficient relaxation, which was most likely via a metal-centered triplet state. Pyo et al. reported an efficient strategy to enhance the photoluminescence of gold clusters (Au22(SG)18) by rigidifying its gold shell strategy to enhance the photoluminescence of gold clusters (Au (SG) ) by rigidifying its gold shell with bulky tetraoctylammonium (TOA) cations [67]. The luminous22 quantum18 yield could improve up with bulky tetraoctylammonium (TOA) cations [67]. The luminous quantum yield could improve to approximately 60% at room temperature. The luminescence was ascribed to the LMMCT triplet up to approximately 60% at room temperature. The luminescence was ascribed to the LMMCT state from the gold shell, which exhibits a strong rigidifying effect. They conclude that the triplet state from the gold shell, which exhibits a strong rigidifying effect. They conclude that the inter-complex aurophilic Au(I)···Au(I) interactions are unlikely in the well-separated dimeric gold inter-complex aurophilic Au(I) Au(I) interactions are unlikely in the well-separated dimeric gold shell shell of the Au25(SG)18 cluster··· encapsulated in dendrimer (PAMAM), since these as-synthesized of the Au (SG) cluster encapsulated in dendrimer (PAMAM), since these as-synthesized AuNCs AuNCs were25 well-separated18 by the cavity of the dendrimer, and the luminescence can be ascribed to were well-separated by the cavity of the dendrimer, and the luminescence can be ascribed to the the ligand-to-metal charge transfer (LMCT) effect, rather than the LMMCT effect [26]. ligand-to-metal charge transfer (LMCT) effect, rather than the LMMCT effect [26]. Londono-Larrea et Londono-Larrea et al. successfully synthesized water-soluble naked gold nanoclusters (AuNCnaked) al. successfully synthesized water-soluble naked gold nanoclusters (AuNCnaked) by using only NaOH by using only NaOH (the reductant) and HAuCl4 [68]. They found that the nonluminescent (the reductant) and HAuCl4 [68]. They found that the nonluminescent AuNCnaked could be transferred AuNCnaked could be transferred to strong luminescent when passivated with different thiols and to strong luminescent when passivated with different thiols and adenosine monophosphate. The adenosine monophosphate. The photoluminescence of the passivated NCs was clearly attributed to photoluminescence of the passivated NCs was clearly attributed to the ligand–AuNC surface interaction. the ligand–AuNC surface interaction.

Figure 6. (a) Weak fluorescence of [Au25(SR)18]- with different R groups (–C2H4Ph, –C12H25, and –C6H13). Figure 6. (a) Weak fluorescence of [Au25(SR)18]- with different R groups (–C2H4Ph, –C12H25, and (b) Possible Interactions of amine and carboxyl groups of glutathione (–SG) ligands to the gold –C6H13). (b) Possible Interactions of amine and carboxyl groups of glutathione (–SG) ligands to the goldsurface. surface. Reprinted Reprinted with withpermission permission from from Ref. Ref.[33]. [Copyright33]. Copyright (2010) (2010) Americ Americanan Chemical Chemical Society. Society.

TheseThese resultsresults indicatedindicated that thethe surfacesurface ligandsligands of MNCsMNCs werewere alsoalso pivotalpivotal toto theirtheir photoluminescencephotoluminescence properties.properties. Ligand-to-metalLigand-to-metal chargecharge transfertransfer (LMCT)(LMCT) andand ligand-to-metalligand-to-metal−metalmetal − chargecharge transfertransfer (LMMCT(LMMCT oror LMNCT)LMNCT) mechanismsmechanisms werewere proposedproposed toto understandunderstand thethe principleprinciple ofof thethe luminescenceluminescence process.process. However, However, some issues areare stillstill didifficultfficult toto understand.understand. ForFor example,example, thesethese MNCsMNCs withwith identicalidentical sizesize could could be be luminescent luminescent or or nonluminescent, nonluminescent, and and they they are are very very sensitive sensitive to the to delicatethe delicate change change of reaction of reaction parameters. parameters.

2.2.2.2.2.2. Metal Valence StateState CorrelatedCorrelated PhotoemissionPhotoemission InIn additionaddition toto thethe clustercluster sizesize andand surface-cappingsurface-capping ligands,ligands, aa surfacesurface metalmetal valencevalence statestate waswas alsoalso aa considerableconsiderable factorfactor forfor synthesizingsynthesizing highlyhighly luminescentluminescent MNCs.MNCs. ZhouZhou etet al.al. observedobserved thatthat thethe completelycompletely didifferentfferent luminescence luminescence properties properties of glutathioneof glutathione protected protected gold gold nanoparticles nanoparticles (GS-Au (GS-Au NPs) withNPs) identicalwith identical size but size di butfferent different in metal in metal valence valence states states [44]. The[44]. luminescent The luminescent GS-Au GS-Au NPs withNPs with a core a sizecore ofsize approximately of approximately 2 nm 2 become nm become nonluminescent nonluminescent after treatmentafter treatment by NaBH by NaBH4, noting4, noting that thethat core the sizecore wassize sustained. was sustained. If this isIf truethis (sameis true size), (same obviously size), theobviously metal centered the metal quantum centered confinement quantum econfinementffect (QCE) caneffect not (QCE) answer can this not peculiar answer observation.this peculiar Theobservation. authorsconcluded The authors that concluded the high contentthat the ofhigh gold(I) content (40–50%) of gold(I) in the (40–50%) luminescent in the nanoparticles luminescent nanoparticles verified by X-ray verified photoelectron by X-ray photoelectron spectroscopy (XPS)spectroscopy was responsible (XPS) was for responsible the unique opticalfor the properties unique optical of the luminescentproperties of gold the nanoparticles. luminescent gold The luminescencenanoparticles. lifetimeThe luminescence highly depends lifetime on highly the excitation depends wavelength; on the excitation when wavelength; excited at approximately when excited 420at approximately nm, it emits orange 420 nm, colors it emits with aorange long lifetime colors ofwith several a long microseconds lifetime of atseveral approximately microseconds 565 nm, at

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approximately 565 nm, while, if excited at 530 nm, the same color is emitted with a short lifetime while,with nanoseconds if excited at 530 (Figure nm, the 7). same This color interesting is emitted observation with a short of lifetime dual PL with emissions nanoseconds with (Figuredifferent7). Thislifetimes interesting indicates observation the presence of dual of two PL emissionscompletely with different different chromophores. lifetimes indicates The authors the presence pointed oftwo out completelythat the degeneration different chromophores. in energy of Thethe authorstriplet an pointedd singlet out excited that the states degeneration in the luminescent in energy of gold the tripletnanoparticles and singlet led to excited the same states wavelength in the luminescent emissions golddue to nanoparticles the change of led metal to the charge same valence wavelength state. emissions due to the change of metal charge valence state. Wu et al. also observed that the fluorescence Wu et al. also observed that the fluorescence of Au25(SC2H4Ph)18 could be largely enhanced by of Au (SC H Ph) could be largely enhanced by increasing their oxidation charge state (from 1 increasing25 their2 4 oxidation18 charge state (from −1 up to +2) using oxidants such as O2, H2O2, Ce(SO−4)2, upetc. to [69,70].+2) using In these oxidants reports, such all as the O 2evidence,H2O2, Ce(SO confirmed4)2, etc. the [69 influence,70]. In theseof the reports, valence allstate the of evidence a metal confirmedion or core theon the influence PL emissions, of the valence and the state PL emis of asion metal of ionMNCs or core was onoriginat the PLed emissions, from the interband and the PL or emissionintraband of transitions MNCs was of originated the outmost from shell the interbandelectrons. orHowever, intraband it transitionsis important of theto outmostnote that shell the electrons.increasing However, of the electropositivity it is important toof notethe thatmetal the core, increasing i.e., the of high the electropositivity charge state of ofmetals, the metal can core,also i.e.,promote the high the charge interactions state of between metals, canthe also surface promote ligands the interactionsand metal betweencore, which the surfacecannot ligandscompletely and metalexclude core, the which role of cannot ligand completely assembly to exclude tune the the PL role emission. of ligand assembly to tune the PL emission.

FigureFigure 7.7.A possibleA possible optical optical transition transition scheme scheme for orange-emitting for orange-emitting GS-AuNPs whereGS-AuNPs the luminescence where the originatesluminescence from originates transitions from between transitions d and spbetween bands. d When and sp the bands. NPs are When excited the at NPs 420 are nm, excited the electrons at 420 willnm, bethe relaxed electrons from will triplet be relaxed states in from sp bands triplet to somestates groundin sp bands states to in some d bands, ground leading states to microsecond in d bands, emission.leading to Once microsecond the excitation emission. wavelength Once the is shifted excitation to 530 wavelength nm, the electrons is shifted will to be 530 decayed nm, the from electrons singlet excitedwill be decayed states in from the sp singlet band toexcited singlet states states in inthe ground sp band states to singlet and give states out in a ground nanosecond states emission.and give Theout tripleta nanosecond and singlet emission. excited statesThe intriplet the luminescent and singlet gold excited nanoparticles states in are the degenerate luminescent in energy. gold Reprintednanoparticles with are permission degenerate from in energy. Ref. [44 ].Reprinted Copyright with (2010) permission American from Chemical Ref. [44]. Society. Copyright (2010) American Chemical Society. 2.2.3. Self-Assembly Governed Photoemission 2.2.3.The Self-Assembly intrinsic defect Governed of luminous Photoemission MNCs was their limited quantum yield, since these ultra-small clustersThe with intrinsic high defect surface-to-volume of luminous MNCs ratio easilywas their suff limiteder from quantum the solvent yield, molecules since these and ultra-small oxygens, whichclusters could with quenchhigh surface-to-volume their photoluminescence ratio easily [71 ].suffer Many from MNCs the onlysolvent exhibit molecules photoluminescence and oxygens, underwhich lowcould temperature quench their and photoluminescence are nonluminescent [71] at. Many room temperature,MNCs only exhibit which largelyphotoluminescence limits their practicalunder low application temperature [72]. and Aggregation-induced are nonluminescent emission at room (AIE) temperature, is an efficient which strategy largely for limits improving their thepractical photoluminescence application [72]. performance Aggregation-induced of lumigens. emission It has attracted (AIE) is tremendous an efficient attentionstrategy for since improving the first observationthe photoluminescence in organic chromophores performance by of Tang’s lumigens. group It in has 2001 attracted [73]. Generally, tremendous the aggregation attention ofsince organic the dyesfirst observation could lead to in photoluminescence organic chromophores quenching by Tang’s due togroup the formationin 2001 [73]. of detrimentalGenerally, the species aggregation such as excimersof organic [74 dyes,75]. could In contrast, lead to the photoluminescenc luminescence ofe (luminigens) quenching due AIEgens to the is formation conspicuously of detrimental enhanced viaspecies aggregation such as due excimers to the strong[74,75]. restriction In contrast, of the the intramolecular luminescence vibrations of (luminigens) (RIV) and AIEgens rotations is (RIR)conspicuously [76]. Recently, enhanced this fascinating via aggregation AIE effect due has beento the regarded strong as restriction an efficient strategyof the intramolecular for optimizing thevibrations photoemission (RIV) and performance rotations (RIR) of nanoclusters [76]. Recently, by directingthis fascinating their spatial AIE effect construction has been [ 45regarded,56,77,78 as]. Thean efficient alternation strategy of nanoarchitectures for optimizing the greatly photoemissi influenceson performance the charge of transfer nanoclus andters energy by directing conduction their spatial construction [45,56,77,78]. The alternation of nanoarchitectures greatly influences the charge

Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 24 Nanomaterials 2020, 10, 261 9 of 24 transfer and energy conduction process of MNCs, and thus their photoelectric performance. However, the physical origin of AIE effect is not clearly addressed. In fact, if we carefully checked process of MNCs, and thus their photoelectric performance. However, the physical origin of AIE effect the structures and PL properties of AIEgens, they showed very similar PL properties with MNCs, is not clearly addressed. In fact, if we carefully checked the structures and PL properties of AIEgens, such as a very strong solvent effect and large Stock shift. It is possible that their PL origins come they showed very similar PL properties with MNCs, such as a very strong solvent effect and large from the same physical principle. Stock shift. It is possible that their PL origins come from the same physical principle. Jianping Xie’s group and Erkang Wang’s group first reported that the nonluminescent Jianping Xie’s group and Erkang Wang’s group first reported that the nonluminescent oligomeric oligomeric Au(I)−thiolate complexes and weak luminescent CuNCs in aqueous solution could Au(I) thiolate complexes and weak luminescent CuNCs in aqueous solution could generate strong generate− strong luminescence upon aggregation induced by weak polar solvents or divalent cations luminescence upon aggregation induced by weak polar solvents or divalent cations [56,79], and their [56,79], and their luminescence enhancement was assigned to the AIE effect. Very recently, Jia et al. luminescence enhancement was assigned to the AIE effect. Very recently, Jia et al. demonstrated that demonstrated that the photophysical features of the thiolated AgNCs were dependent on their the photophysical features of the thiolated AgNCs were dependent on their morphology, which was morphology, which was controlled by the solvents-induced aggregation [41]. The ordered structure controlled by the solvents-induced aggregation [41]. The ordered structure of AgNCs assemblies was of AgNCs assemblies was well defined by XRD and TEM techniques. Subsequently, Benito et al. well defined by XRD and TEM techniques. Subsequently, Benito et al. observed the mechanochromic observed the mechanochromic luminescence properties in copper iodide clusters [46]. Two kinds of luminescence properties in copper iodide clusters [46]. Two kinds of crystalline polymorphs with green crystalline polymorphs with green and yellow emission were obtained. Upon mechanical grinding, and yellow emission were obtained. Upon mechanical grinding, the green emissive polymorph exhibits the green emissive polymorph exhibits great modification of its emission from green to yellow, as great modification of its emission from green to yellow, as shown in Figure8. XRD analyses indicated shown in Figure 8. XRD analyses indicated that the crystalline packing was damaged, and an almost that the crystalline packing was damaged, and an almost complete amorphous state was formed, complete amorphous state was formed, which implies an assembly architecture-dependent emission which implies an assembly architecture-dependent emission property of CuNCs. Obviously, the property of CuNCs. Obviously, the change of PL properties triggered by simply grounding cannot change of PL properties triggered by simply grounding cannot be simply answered by metal-centered be simply answered by metal-centered QCE, since individual CuNCs remains unchanged. The QCE,authors since suggested individual that CuNCs the Cu-Cu remains interactions unchanged. were The responsible authors suggested for the thatluminescence the Cu-Cu interactionsproperties. wereUpon responsible mechanical forstimulation, the luminescence destruction properties. of the crystalline Upon mechanical structure led stimulation, to shortening destruction of the Cu of−Cu the crystalline structure led to shortening of the Cu Cu bond in the cluster core and resulting red shift bond in the cluster core and resulting red shift of− emission wavelength from green to yellow. These ofresults emission definitely wavelength established from greenthe important to yellow. role These of cuprophilic results definitely interactions established in the themechanochromic important role ofmechanism cuprophilic of interactionsCuNCs. A similar in the me mechanochromicchanism was used mechanism by Zhang of to CuNCs. explain A the similar PL emission mechanism of metal was usednanoclusters by Zhang (Cu, to Au, explain Ag) theself-assemblies PL emission with of metal different nanoclusters assembly (Cu, morphology Au, Ag) self-assemblies[45,78,80–83]. These with dias-synthesizedfferent assembly nonluminescent morphology [45 individual,78,80–83]. TheseCuNCs as-synthesized could generate nonluminescent strong photoemission individual CuNCswhen couldassembled generate to a strong crystalline photoemission packing whenstructure, assembled similar to to a crystallinethe AIE effect, packing noting structure, that the similar emission to the AIEwavelength effect, noting could that be easily the emission adjusted wavelength by controlling could the be assembly easily adjusted structure. by controlling As shown thein Figure assembly 9, structure.CuNCs exhibit As shown yellow-green in Figure9, CuNCsemission exhibit in nano yellow-greensheets but emission blue inemission nanosheets in butnanoribbons. blue emission A inmechanochromic nanoribbons. A property mechanochromic was also property observed was in alsowhich observed the blue in emissive which the nanoribbon blue emissive could nanoribbon transfer couldto yellow transfer emissive to yellow with emissiveless crystallinity. with less crystallinity.

FigureFigure 8.8.( a()a Molecular) Molecular structure structure of clustersof clusters 1G and 1G 1Yand and 1Y the and mean the of mean selected of bondselected lengths. bond Hydrogen lengths. atomsHydrogen have atoms been omitted have been for clarity. omitted (b )for Photos clarity. of the (b) ground Photos (left) of the and ground intact (right) (left) crystallineand intact powder(right) ofcrystalline 1G under powder ambient of 1G light under and ambient under UVlight irradiation and under atUV 312 irradiation nm (UV at lamp) 312 nm at (UV room lamp) temperature. at room Reprintedtemperature. with Reprinted permission with from permission Ref. [46]. Copyright from Ref. (2014) [46]. AmericanCopyright Chemical (2014) American Society. Chemical Society. The relationship between the compactness of assemblies and the emission was summarized as follows.The relationship (1) “High between compactness the compactness reinforces the of cuprophilicassemblies and Cu(I) the Cu(I)emission interaction was summarized of inter- and as ··· intra-NCs,follows. (1) and “High meanwhile, compactness it suppresses reinforces intramolecular the cuprophilic vibration Cu(I)···Cu(I) and rotation interaction of the cappingof inter- ligand and ofintra-NCs, 1-Dodecanethiol and meanwhile, (DT), thus it suppresses enhancing intramolecular the emission intensity vibration of and Cu rotation NCs. (2) of The the emission capping ligand energy dependsof 1-Dodecanethiol on the distance (DT), of thus Cu(I) enhancingCu(I); the the improved emissioncompactness intensity of Cu increases NCs. (2) the The average emission Cu(I) energyCu(I) ··· ··· distancedepends by on inducing the distance additional of inter-NCsCu(I)···Cu(I); cuprophilic the improved interaction, compactness and therewith increases leads to the the blueaverage shift ofCu(I)···Cu(I) NCs emission” distance [45]. by Noting inducing that additional it is counterintuitive inter-NCs cuprophilic for improving interaction, the compactness and therewith could lead leads to theto the increase blue ofshift Cu(I) of NCsCu(I) emission” distance. The[45]. authors Noting explain that itthis is counterintuitive abnormal phenomenon for improving by inducing the ··· additionalcompactness inter-NCs could lead cuprophilic to the increase interaction of Cu(I)··· betweenCu(I) the distance. neighboring The authors NCs. However, explain this it needs abnormal to be

Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 24 Nanomaterials 2020, 10, 261 10 of 24 phenomenon by inducing additional inter-NCs cuprophilic interaction between the neighboring NCs. However, it needs to be reiterated that cuprophilic interactions could only generate when the reiterated that cuprophilic interactions could only generate when the adjacent Cu Cu distances are in adjacent Cu···Cu distances are in the range of the van der Walls interaction distance··· (generally less thethan range 3.6 ofÅ); the however, van der the Walls distance interaction between distance these (generallytwo neighboring less than NCs 3.6 Å);is at however, a nanometer the distance scale, betweenwhich is these far twobeyond neighboring the effective NCs is distance at a nanometer of metallophilic scale, which interactions is far beyond [32,84,85]. the effective Thus, distance the ofrationality metallophilic of the interactions LMCT and/or [32, 84LMMCT,85]. Thus, model the was rationality challenged. of the However, LMCT and the/or fact LMMCT remains model that the was challenged.change of spacing However, distance the fact between remains adjacent that the surface change ligands of spacing in varied distance morphologies between adjacent is definitively surface ligandsconfirmed, in varied which morphologies further evidenced is definitively the paramount confirmed, role which of surface further ligand evidenced packing the to paramount tune the PL role ofproperties. surface ligand packing to tune the PL properties.

FigureFigure 9. 9.(a ()a TEM) TEM image image of theof the ribbons ribbons from from Cu NCs’Cu NCs’ self-assembly. self-assembly. (b) HRTEM (b) HRTEM image image of the ribbons.of the Inset:ribbons. the FourierInset: the transform Fourier transform image. (c) image. Small-angle (c) Small-angle region of XRDregion pattern. of XRD ( dpattern.) Steady-state (d) Steady-state absorption (black)absorption and emission(black) and (red) emission spectra (r ofed) the spectra ribbons of inthe chloroform. ribbons in ch Inset:loroform. thefluorescent Inset: the fluorescent image with 365image nm excitation.with 365 nm (a’ excitation.) TEM image (a’) ofTEM the image sheets of from the sheets Cu NCs’ from self-assembly. Cu NCs’ self-assembly. (b’) HRTEM (b’ image) HRTEM of the sheets.image (ofc’) the Small-angle sheets. (c’ region) Small-angle of XRD region pattern. of XRD (d’) Steady-state pattern. (d’) absorptionSteady-state (black) absorption and emission (black) and (red) spectraemission of the(red) sheets spectra in chloroform. of the sheets Inset: in chloro the fluorescentform. Inset: image the withfluorescent 365 nm image excitation. with Reprinted365 nm withexcitation. permission Reprinted from Ref.with [permission45]. Copyright from (2015) Ref. [45]. American Copyright Chemical (2015) Society.American Chemical Society.

3.3. Our Our P P Band Band Intermediate Intermediate State State (PBIS) (PBIS) Model Model Dominates Dominates the the Photoluminescence Photoluminescence Emission Emission of ofMNCs MNCs EvenEven though though the the metal-centeredmetal-centered free-electronfree-electron model based based on on the the QCE QCE and, and, subsequent subsequent LMCT LMCT andand/or/or LMMCT LMMCT mechanism,mechanism, in in some some sense, sense, explai explainedned some some important important PL emission PL emission phenomena, phenomena, the theelucidation elucidation and and origin origin of ofoptoelectronic optoelectronic properti propertieses is isdiverse diverse and and contradictory, contradictory, at atevery every point point invitinginviting inquiry inquiry and and debatedebate overover aa decade.decade. Since 2 2014,014, this this continuous continuous and and long-term long-term study study in in my my groupgroup and and collaborators collaborators hashas beenbeen carriedcarried out to understand the the nature nature of of the the photoluminescence photoluminescence emissionemission ofof metalmetal nanoclustersnanoclusters andand relatedrelated quantumquantum nanostructures. We We first first provided provided the the key key evidencesevidences that that the the distributiondistribution ofof surface-protectingsurface-protecting ligands ligands on on the the metal metal core core played played a aparamount paramount rolerole to to tune tune the the optoelectronic optoelectronic propertiesproperties ofof noblenoble metal NCs. However, However, the the basic basic chemical chemical principle principle hiddenhidden behind behind abnormalabnormal opticaloptical phenomenaphenomena has been troubling troubling us, us, such such as as its its room room temperature temperature phosphorescencephosphorescence enhancement, enhancement, surfacesurface ligand selectiv selectivity,ity, unprecedented unprecedented large large Stokes Stokes Shift, Shift, tunable tunable opticaloptical absorption, absorption, newbornnewborn electronicelectronic band struct structure,ure, etc. etc. Very Very recently, recently, using using metal metal NCs NCs as as a a modelmodel system, system, by by judiciously judiciously manipulatingmanipulating the delicate surface surface ligand ligand interactions interactions at at the the nanoscale nanoscale interfaceinterface of of a singlea single metal metal nanocluster, nanocluster, superlattice superlattice and and mesoporous mesoporous materials, materials, together together with awith careful a controlcareful ofcontrol the solvophobicity of the solvophobicity and solvophilicity and solvophi oflicity the ligands,of the ligands, the resulting the resulting interplay interplay of various of noncovalentvarious noncovalent interactions interactions can lead tocan the lead precise to the modulation precise modulation of optoelectronic of optoelectronic properties properties of metal NCs. of Ametal completely NCs. A new completely p band intermediate new p band stateintermediate (PBIS) model state was(PBIS) proposed model was to understand proposed to the understand origin of PL emissionthe origin of allof relatedPL emission quantum of nanodots,all related which quantum completely nanodots, challenges which thecompletely metal-centered challenges quantum the mechanicsmetal-centered for the quantum elucidation mechanics of PL for emission the elucidation of metal of NCs. PL emission We definitively of metal NCs. identify We thatdefinitively the PBIS identify that the PBIS stems from the overlapping of p orbitals of the paired or more adjacent stems from the overlapping of p orbitals of the paired or more adjacent heteroatoms (O and S) from the heteroatoms (O and S) from the surface-protected ligands on the metal NCs, which can be surface-protected ligands on the metal NCs, which can be considered as a dark state at the metal–NCs interface to activate the triplet site of the surface chromophores. Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 24 Nanomaterials 2020, 10, 261 11 of 24 considered as a dark state at the metal–NCs interface to activate the triplet site of the surface chromophores. InIn our our earlierearlier investigations,investigations, inin orderorder to fully understand the the structure structure of of luminescent luminescent MNCs, MNCs, wewe used used water-soluble water-soluble polymerspolymers poly(methacrylicpoly(methacrylic acid) (PMAA) as templates templates to to encapsulate encapsulate AgNCs AgNCs with small size (2 5 nm) [86]. By precisely designing the experiments’ parameters, we found that with small size (2−5 nm) [86]. By precisely designing the experiments’ parameters, we found that the thephotoluminescence photoluminescence of AgNCs of AgNCs showed showed high highselectiv selectivityity on the on surface-anchoring the surface-anchoring ligands ligandsand strong and strongdependence dependence on the on valence the valence state stateof surface of surface meta metal.l. That That was, was, the the strong strong fluorescence fluorescence of of carboxyl-protectedcarboxyl-protected AgNCs AgNCs disappeared disappeared when when the the surface surface functional functional ligands ligands were changedwere changed to sulfonic to acidsulfonic groups, acid even groups, though even the though metal corethe metal was sustained,core was sustained, indicating indicating the key role the of key ligand role type of ligand on the regulationtype on the of regulation PL properties. of PL Basedproperties. on these Based experimental on these experimental results, a core-shell results, a structural core-shell modelstructural was proposedmodel was to proposed understand to understand the nature the of photoluminescencenature of photoluminescence of Ag NCs. of Ag As NCs. shown As shown in Figure in Figure10, the fluorescence10, the fluorescence from the AgNCsfrom the was AgNCs attributed was to attributed ligand-to-metal to ligand-to-metalmetal charge− transfermetal charge (LMMCT) transfer from − Ag(I)-carboxylate(LMMCT) from Ag(I)-carboxylate complexes (the oxygen complexes atom (the in theoxygen carboxylate atom in ligandsthe carboxylate to the Ag(I) ligands ions) to to the the AgAg(I) atoms ions) and to the subsequent Ag atoms radiative and subsequent relaxation. radiative In this relaxation. report, we In also this followed report, we the also classical followed QCE the to elucidateclassical QCE the PL toemission elucidate of the MNCs. PL emission However, of MNCs. we highlighted However, the we pivotal highlighted role of the surface pivotal ligands role of to regulatesurface ligands the PL propertiesto regulate of the MNCs. PL properties of MNCs.

FigureFigure 10. 10.(a )(a UV) UVvis−vis absorption absorption of theof freshlythe freshly prepared prepar Ag-carboxylateed Ag-carboxylate NCs using NCs PMAA using asPMAA scaffold as at − variousscaffold radiation at various times. radiation (b) PL times. spectra (b of) PL the spectra Ag-carboxylate of the Ag-carboxylat NCs. Inset imagese NCs. showInset theimages corresponding show the photographscorresponding of Ag-carboxylatephotographs of NCsAg-carboxylate under room NCs and under UV light room exposure and UV at lightλ = 365 exposure nm. (c )at Schematic λ = 365 illustrationnm. (c) Schematic of the structures illustration of our of luminescent the structures Ag-carboxylate of our luminescent with Ag+ -carboxylateAg-carboxylate complexes with shell.Ag+-carboxylate (d) TEM image complexes of freshly shell. prepared(d) TEM image Ag-carboxylate of freshly prepared NCs (histogram Ag-carboxylate describes NCs the (histogram statistical distributiondescribes the of statistical the particle distributi size, whichon of the is approximately particle size, which 3.5 nm). is approximately Reprinted with 3.5 permissionnm). Reprinted from Ref.with [86 permission]. Copyright from (2014) Ref. American[86]. Copyri Chemicalght (2014) Society. American Chemical Society.

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To further clarify the emission origin of MNCs, we separately discussed the functions of surface ligandsTo further and clarifymetal thecore emission to the origin photoluminescence. of MNCs, we separately Metal-centered discussed emission the functions (MCE) of surface and ligandsligand-centered and metal emissions core to the photoluminescence.(LCE) were simultan Metal-centeredeously observed emission in AgNCs (MCE) and[87]. ligand-centered Luminescent emissionswater-soluble (LCE) AgNCs were simultaneouslyprotected by sulfydryl observed ligand in AgNCss with [87 different]. Luminescent functional water-soluble groups (carboxyl, AgNCs protectedamino, alkyl) by sulfydryl were ligandssuccessfully with differentfabricated functional using groupsa modi (carboxyl,fied cyclic amino, reduction alkyl)− weredecomposition successfully fabricatedapproach. using Two distinct a modified photoemissions cyclic reduction with decompositionemission peaks approach.at approximately Two distinct 580 and photoemissions 665 nm were − withobserved, emission as shown peaks in at Figure approximately 11. The emission 580 and at 665 approximately nm were observed, 580 nm with as shown a small in Stokes Figure shift 11. (30 The emissionnm), narrow at approximately peak width (full 580 widt nm withh at half a small maxima Stokes (FWHM) shift (30 approximately nm), narrow peak30 nm), width short (full lifetime width at(1.6 half ns), maxima and low (FWHM) quantum approximately yield (<1%) was 30 nm), ligand-ind short lifetimeependent, (1.6 since ns), andit could low be quantum observed yield in all (< 1%)of wasthese ligand-independent, as-synthesized Ag sinceNCs it couldwith bedifferent observed capping in all of theseligands. as-synthesized In contrast, AgNCs the withemission different at cappingapproximately ligands. 665 In contrast,nm with thea large emission Stokes at shift approximately (>200 nm), broad 665 nm peak with width a large (FWHM Stokes approximately shift (>200 nm), broad100 nm), peak relative width (FWHM longer approximatelylifetime (180 ns), 100 nm),and relativehigher quantum longer lifetime yield (180 (approximately ns), and higher 10%) quantum was yieldligand-dependent, (approximately and 10%) it was could ligand-dependent, only be observed and in it couldAgNCs only protected be observed by incarboxyl. AgNCs protectedThey were by carboxyl.attributed They to MCE were and attributed LCE, respectively. to MCE and The LCE, MCE respectively. was highly The dependent MCE was on highly the metal dependent core, and on the the metalLCE was core, highly and the related LCE was to the highly surface related ligands to the and surface solvent ligands environment. and solvent Accordantly, environment. the emission Accordantly, at theapproximately emission at approximately 580 nm was pH 580 independent, nm was pH independent,and the emission and theat approximately emission at approximately 665 nm exhibits 665 nm a exhibitsstrong apH-dependent strong pH-dependent effect (Figure effect (Figure 12a–c). 12 Baseda–c). Based on these on these observations, observations, a new a new ligand ligand synergistic synergistic emission effect was proposed to understand the PL emission of MNCs. That is, the amino-correlated emission effect was proposed to understand the PL emission of MNCs. That is, the amino-correlated nπ* nπ* state provides a pivot to bridge the carboxyl correlated ππ* and nπ* states to enhance the charge state provides a pivot to bridge the carboxyl correlated ππ* and nπ* states to enhance the charge transfer transfer efficiency between different surface electronic states. Consequently, the photoluminescence efficiency between different surface electronic states. Consequently, the photoluminescence quantum quantum yields were significantly improved (approximately 1% to 10%), as shown in Figure 12. yields were significantly improved (approximately 1% to 10%), as shown in Figure 12. Very recently, our Very recently, our unpublished results showed that the assignment on the emission at unpublished results showed that the assignment on the emission at approximately 580 nm is probably approximately 580 nm is probably not right. Thus, the ligand synergistic emission mechanism needs not right. Thus, the ligand synergistic emission mechanism needs further optimization. further optimization.

FigureFigure 11.11. ExcitationExcitation andand emissionemission spectra of th thee narrow approximately 580 580 nm nm emission emission (a (a) )and and broadbroad approximatelyapproximately 665665 nmnm emissionemission ((bb).). Inset images show show the the co correspondingrresponding photographs photographs of of D-penicillamine-cappedD-penicillamine-capped silversilver nanoclustersnanoclusters (DPA-AgNCs) (DPA-AgNCs) under under room room and and UV UV light light exposure exposure at λat= 365λ=365 nm. nm. (c) ( Schematicc) Schematic illustration illustration of theof the metal-centered metal-centered and and ligand-centered ligand-centered emission emission mechanisms mechanisms of DPA-cappedof DPA-capped AgNCs. AgNCs. (d) Time-resolved(d) Time-resolved luminescence luminescence decay decay profiles profiles of DPA-AgNCs of DPA-AgNCs in water in solution water measuredsolution measured at 665 and at 580 665 nm and (inset), 580 nm excited (inset), at 400exci andted at 550 400 nm, and respectively. 550 nm, respectively. Reprinted withReprinted permission with frompermission Ref. [87 from]. Copyright Ref. [87]. (2019) Copyrigh Americant (2019) Chemical American Society. Chemical Society.

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Figure 12. ((aa)) Photoluminescence Photoluminescence spectra spectra of of as-synthesiz as-synthesizeded DPA-AgNCs DPA-AgNCs in different different pH conditions. conditions. (Figureb) PL intensity 505. nm. of (b DPA-AgNCs) PL intensity upon of DPA-AgNCs cyclic switching upon of thecyclic pH switching between 4.6 of and the 9.7.pH (betweenc) PL spectra 4.6 and and 9.7. PL intensity(c) PL spectra (inset) and of DPA-AgNCs PL intensity with (inset) changing of DPA-Ag pH values.NCs with PL spectra changing were pH excited values. at 400PL nm;spectra pH valuewere overexcited the at range 400 nm; 1.5 pH12. (valued) Schematic over the illustration range 1.5− of12. the (d) ligand Schematic synergistic illustration effect of and the pH-induced ligand synergistic conical − intersections.effect and pH-induced Reprinted withconical permission intersections. from Ref.Reprinted [87]. Copyright with permission (2019) American from Ref. Chemical [87]. Copyright Society. (2019) American Chemical Society. If the metal-centered QCE dominates the PL emission of MNCs, the optical properties should not be significantlyIf the metal-centered affected by theQCE solvent dominates effect. the However, PL emis it ission not of the MNCs, case.Very the opti recently,cal properties we unexpectedly should observednot be significantly a strong solvent-induced affected by the enhancement solvent effect eff. ectHowever, of carboxyl-protected it is not the case. water-soluble Very recently, AgNCs, we indicatingunexpectedly the invalidityobserved ofa thestrong well-accepted solvent-indu metalced centered enhancement QCE model effect [88 ].of Thecarboxyl-protected emission QY of AgNCswater-soluble could AgNCs, be largely indicating improved the from invalidity approximately of the well-accepted 1% to 40% metal by a simple centered solvent-stimulated QCE model [88]. strategyThe emission (Figure QY 13 of). TheAgNCs fluorescence-phosphorescence could be largely improved dual from solvoluminescence approximately 1% (SL) to 40% of water-soluble by a simple metalsolvent-stimulated nanoclusters (NCs)strategy at room(Figure temperature 13). The fluo wasrescence-phosphorescence observed. The photoluminescence dual solvoluminescence was originated from(SL) theof self-assemblywater-soluble ofmetal surface nanoclusters ligands, which (NCs) was at induced room bytemperature solvent stimulation, was observed. that is, The the clusteringphotoluminescence of surface was ligands, originated as illustrated from the in Figureself-assembly 14. The of clustering surface ligands, of surface which carbonyl was groupsinduced was by promotedsolvent stimulation, in two different that ways:is, the (1) clustering the strong of interaction surface ligands, between carboxylateas illustrated ligands in Figure and the 14. metal The core,clustering and (2)of surface strong ncarbonylπ* interactions groups was between promoted these in two two different adjacent ways: carbonyl (1) the groups strong [89 interaction–91]. The → clusteringbetween carboxylate of carbonyl ligands groups couldand the lead metal to the core, extension and (2) of strong conjugation n → π and* interactions efficient delocalization between these of electronstwo adjacent in overlapped carbonyl groups C=O double[89–91]. bonds The clustering between neighboringof carbonyl groups carbonyl could groups. lead to In the addition, extension the molecularof conjugation conformation and efficient becomes delocalization more rigid, of which electrons could in largely overlapped restrict theC=O intramolecular double bonds vibrations between (RIV)neighboring and rotations carbonyl (RIR). groups. Finally, In addition, the photoluminescence the molecular conformation was largely enhanced. becomes more We first rigid, proposed which thatcould the largely exact chromophorerestrict the ofintramolecular metal NCs for vibrations aggregation-induced (RIV) and emissionrotations (AIE) (RIR). mechanics Finally, wasthe originatedphotoluminescence from the clusteringwas largely carbonyl enhanced. groups. We first This proposed interpretation that the gives exact a completely chromophore new of insight metal intoNCs thefor luminescentaggregation-induced principle emission of MNCs, (AIE) but we mechanics cannot completely was originated preclude from the the role clustering of the metal carbonyl core forgroups. the PL This emission. interpretation gives a completely new insight into the luminescent principle of MNCs, but we cannot completely preclude the role of the metal core for the PL emission.

Nanomaterials 2020, 10, 261 14 of 24 Nanomaterials 2020, 10, x FOR PEER REVIEW 14 of 24

Figure 13.13.( a)( UVa) UVvis− absorptionvis absorption and ( b)and photoluminescence (b) photoluminescence spectra of polymethylspectra of vinyl polymethyl ether-alt-maleic vinyl − ether-alt-maleic acid-capped silver nanoclusters (PMVEM-Ag NCs) in the varied volume fraction fd acid-cappedether-alt-maleic silver acid-capped nanoclusters silv (PMVEM-Ager nanoclusters NCs) (PMVEM-Ag in the varied NCs) volume in the fraction varied fvolumed of DMSO fraction in the f of DMSO in the mixed solvent (fd = VDMSO/VDMSO+water). Photoluminescence was excited at 365 nm. (c) mixedof DMSO solvent in the (f d mixed= VDMSO solvent/VDMSO (fd +=water VDMSO). Photoluminescence/VDMSO+water). Photoluminescence was excited at was 365 nm.excited (c) Correlations at 365 nm. ( ofc) theCorrelations emission intensitiesof the emission of the two intensities peaks centered of th ate approximatelytwo peaks centered 460 and approximatelyat approximately 530 nm 460 versus and approximately 530 nm versus fd. (Inset) Photographs of PMVEM-Ag NCs at different fd under visible fapproximatelyd. (Inset) Photographs 530 nm ofversus PMVEM-Ag fd. (Inset) NCs Photographs at different off d PMVEM-Agunder visible NCs (top at row) different and UV fd under (bottom visible row) light.(top row) (d) Time-resolvedand UV (bottom luminescence row) light. ( decayd) Time-resolved profiles of PMVEM-Ag luminescence NCs decay inDMSO profiles measured of PMVEM-Ag at 460 andNCs 530 in DMSO nm (inset), measured respectively. at 460 Reprinted and 530 nm with (inset), permission respectively. from Ref. Reprinted [88]. Copyright with permission (2017) American from ChemicalRef. [88]. Copyright Society. (2017) American Chemical Society.

Figure 14.14.Schematic Schematic illustration illustration of theof solvent-inducedthe solvent-induced clustering clustering process process of Ag NCs of (top)Ag dependingNCs (top) ondepending the unique on the molecular unique structuremolecular of structure the polymer of the used polymer as a used template as a fortemplate the synthesis for the synthesis of Ag NCs of (bottom-right)Ag NCs (bottom-right) and the energy-leveland the energy-level structure stru of Agcture NCs of Ag in waterNCs in solution water andsolution DMSO and solution DMSO (bottom-left).solution (bottom-left). Reprinted withReprinted permission with frompermissi Ref.on [88 ].from Copyright Ref. [88]. (2017) Copyright American Chemical(2017) American Society. Chemical Society. Very most recently, based on the complimentary characterizations of steady-state absorption, excitation,Very most and time-resolvedrecently, based PL on spectroscopy,the complimentary we definitively characterizations identify of that steady-state the induced absorption, p band intermediateexcitation, and state time-resolved (PBIS) stems PL from spectroscopy, the overlapping we definitively of p orbitals identify of the that paired the orinduced more adjacentp band intermediate state (PBIS) stems from the overlapping of p orbitals of the paired or more adjacent heteroatoms (O and S) from the surface-protected ligands on the metal NCs, which can be

Nanomaterials 2020, 10, 261 15 of 24 Nanomaterials 2020, 10, x FOR PEER REVIEW 15 of 24 heteroatomsconsidered as (O a and dark S) fromstate theat surface-protectedthe metal–NCs interf ligandsace onto theactivate metal the NCs, triplet which site can of be the considered surface aschromophores a dark state atby the enhancing metal–NCs intersystem interface crossing to activate [43]. the Figure triplet 15 site showed of the that surface the water-soluble chromophores and by enhancingoil-soluble intersystemMNCs with crossing identical [43 size]. Figure synthesiz 15 showeded by a that ligand-exchange the water-soluble strategy and oil-soluble (denoted MNCsas Au withNCs@GSH identical and size Au synthesized NCs@DT, respectively) by a ligand-exchange exhibited strategy the strong (denoted solvent-enhanced as Au NCs@GSH PL emission and Au NCs@DT,behavior, and respectively) that it is very exhibited interesting the strong that the solvent-enhanced number and intensity PL emission of PL behavior,emission andwas thatstrongly it is verydependent interesting on the that type the of number used surface-protective and intensity of PLliga emissionnds, even was though strongly the metal dependent core remains on the type intact. of usedThe more surface-protective convincing and ligands, powerful even though evidence the metalfor the core PBIS remains model intact. comes The from more the convincing luminescent and powerfulmesoporous evidence silica nanoparticles for the PBIS model functionalized comes from by thenonluminescent luminescent mesoporous organosilanes silica with nanoparticles amino and functionalizedcarbonyl groups by free nonluminescent of any metals, organosilanes which bears with the aminosame spectroscopic and carbonyl groupsproperties free as of metal any metals, NCs. whichThese bearssurface-modified the same spectroscopic MSNs with propertiestarget organi as metalc functions NCs. showed These surface-modified a very strong and MSNs tunable with targetphotoluminescence organic functions emission showed due ato very the strongclustering and of tunable nonluminescent photoluminescence chromophores emission in the due confined to the clusteringnanospace of [92–99]. nonluminescent It is importantly chromophores noted inthat the the confined selectively nanospace used organosilane [92–99]. It is importantly in the functional noted thatgroups the contains selectively heteroatoms used organosilane with unpaired in the functionallone electrons, groups such contains as oxygen heteroatoms (O), nitrogen with unpaired(N), and lonesulfur electrons, (S), as used such protecting as oxygen (O),ligand nitrogen molecule (N),s andfor sulfurthe synthesis (S), as used of metal protecting NCs ligand[100–102]. molecules These fornonluminescent the synthesis functional of metal NCs groups [100– 102assembled]. These on nonluminescent a non-metallic functional surface groupsgenerated assembled the same on aphotoluminescence non-metallic surface emission generated with theMNCs, same as photoluminescence shown in Figure 16. emission We definitively with MNCs, confirmed as shown that the in Figurepairing 16of. surface We definitively ligands could confirmed serve as that an theexact pairing chromophore of surface for ligandsPL emission, could which serve has as annothing exact chromophoreto do with the formetal PL emission,core. which has nothing to do with the metal core.

Figure 15.15. Solvent-induced ligand-dependent optical absorptionabsorption and emissions.emissions. Photoemission and excitation spectraspectra of of water-soluble water-soluble Au Au NCs@GSH NCs@GSH (a) and(a) oil-solubleand oil-soluble Au NCs@DT Au NCs@DT (c) in mixed (c) in solvents mixed withsolvents diff erentwith different volume fractionsvolume fractions of R (Inset, of R relationship (Inset, relationship between between the luminescence the luminescence intensity intensity and R

(RandEtOH R (R=EtOHVol =EtOH Vol/EtOHVol/VolEtOHEtOH+ H2O + H2O,R, RH2OH2O == VolVolH2O/Vol/VolEtOHEtOH + H2O+ H2O), the), thespectra spectra were were recorded recorded at 0.5 at h 0.5after h afterthe sample the sample preparation). preparation). (b) (Ultraviolet-visibleb) Ultraviolet-visible (UV (UV−vis)vis) absorption absorption spectra spectra of of Au Au NCs@GSH NCs@GSH in − mixed solvents withwith didifferentfferent RREtOHEtOH.. Inset Inset shows shows the the digital digital photos of water-solublewater-soluble AuAu NCs@GSHNCs@GSH and oil-soluble Au NCs@DT in mixed solvents of ethanol and water with varied volume fractions of REtOH andand R RH2OH2O underunder UV UV light. light. (d (d) )Time-resolved Time-resolved luminescence luminescence decay decay profiles profiles of solvent-inducedsolvent-induced luminescent Au NCs@GSH and Au NCs@DT. ReprintedReprinted with permission from Ref.Ref. [[43].43]. Copyright (2019) SpringerSpringer Nature.Nature. GSH: glutathione.

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Figure 16. LigandLigand assembly assembly in inthe the mesoporous mesoporous silica silica nanoparticles nanoparticles (MSNs) (MSNs) free freeof metals of metals and their and theirtunable tunable luminescent luminescent properties. properties. Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) (a) and (a) and TEM TEM (b) (imagesb) images of ofas-synthesized as-synthesized fluorescent fluorescent mesoporous mesoporous silica silicananoparticles. nanoparticles. The inset The shows inset the shows assembly the assemblyof amino ofand amino carbonyl and carbonylgroups in groups the inconfined the confined nanopores. nanopores. (c) Excitation (c) Excitation and andemission emission spectra spectra of ofaminopropyl-functionalized aminopropyl-functionalized MSNs. MSNs. (d()d) Absorption Absorption and emissionand spectraemission of propylsuccinic-spectra of functionalizedpropylsuccinic-functionalized MSNs. Reprinted MSNs with. permissionReprinted with from Ref.permission [43]. Copyright from Ref. (2019) [43].Springer Copyright Nature. (2019) Springer Nature. A descriptive ligand-assembly-mediated interfacial p band intermediate state (PBIS) model was proposedA descriptive to understand ligand-assembly-mediated the origin of the PL emission interfacia of MNCs,l p band as intermediate shown in Figure state 17 (PBIS). At the model nanoscale was interfaceproposed of to the understand metal nanocluster the origin core of or the in thePL confinedemission nanomesopores, of MNCs, as shown the adjacent in Figure ligands 17. At locally the interactnanoscale with interface each other of the to metal form Rydbergnanocluster matter-like core or clustersin the confined by the overlapping nanomesopores, of p-orbitals the adjacent from O,ligands N, S, locally and P interact on proximal with each carbonyl other/thiol to form groups Rydberg with high-energymatter-like clusters lone-pair by electronsthe overlapping [103,104 of]. Thep-orbitals delocalization from O, ofN, the S, high-energyand P on proximal electrons carbonyl/thiol in coupled p orbitalsgroups inwith ligand-directed high-energy molecularlone-pair architectureselectrons [103,104]. produces The a new delocali overallzation lower energyof the state,high-energy the so-called electrons PBIS, which in coupled acts as an p intermediate orbitals in (orligand-directed dark) state to molecular tune PL emissions architectures by intersystem produces crossinga new overall where thelower energy energy gap betweenstate, the the so-called singlet excitedPBIS, which state andacts theas an intermediate intermediate p band(or dark) state state governs to tune the directionPL emissions and theby extentintersystem of the crossing electron transfer.where the Time-dependent energy gap between density the functional singlet excited theory state (TD-DFT) and the calculation intermediate indicated p band that state the governs energy levelthe direction of the p-band and centerthe extent is very of sensitive the electron to the tr localansfer. proximity Time-dependent ligand chromophores density functional at heterogeneous theory interfaces,(TD-DFT) calculation which further indicated confirmed that thethe validityenergy level of the of PBIS the p-band model. center is very sensitive to the local proximityBased ligand on this chromophores model, which at is heterogeneous shown here as interfaces, an atypical which example, further the confirmed abnormal the polymorph- validity of dependentthe PBIS model. emission wavelength phenomenon in anisotropic superlattices could be readily understoodBased on [45 ].this As illustratedmodel, which in Figure is shown 18, in thehere highly as an compact atypical structure example, (nanoribbon), the abnormal due topolymorph-dependent the strong intercalation emission between DTwavelength molecules phenom in the layers,enon thein danisotropic spacing between superlattices two DT moleculescould be isreadily larger understood than that in nanosheets;[45]. As illustrated then, the in overlapping Figure 18, or in the the interactions highly compac betweent structure paired or(nanoribbon), neighboring DTdue molecules to the strong are weakened,intercalation which between answers DT molecules that the blue-shift in the layers, emission the ofd thespacing nanoribbons between istwo due DT to themolecules declined is electronlarger than delocalization that in nanosheets; between then, adjacent the overlapping DT molecules. or the interactions between paired or neighboring DT molecules are weakened, which answers that the blue-shift emission of the nanoribbons is due to the declined electron delocalization between adjacent DT molecules.

Nanomaterials 2020,, 1010,, 261x FOR PEER REVIEW 1717 of of 24 Nanomaterials 2020, 10, x FOR PEER REVIEW 17 of 24

Figure 17. Ligand-assembly-mediated p band intermediate state (PBIS) dominates Figurephotoluminescence 17.17.Ligand-assembly-mediated Ligand-assembly-mediated emission. Schematic p band illustp intermediate bandration intermediateof state the (PBIS) ligand dominatesstate exchange (PBIS) photoluminescence processdominates and photoluminescenceemission.solvent-induced Schematic emission illustrationemission. (SIE) propSchematic of theerties ligand of illustAu exchange NCsration (inset: processof thethe ener and ligandgy-level solvent-induced exchange structure emission processof Au NCs (SIE)and in propertiessolvent-inducedwater and of ethanol Au NCsemission mixed (inset: (SIE) thesolution). energy-levelproperties The of structurep Au band NCs offormed(inset: Au NCs the by inener the watergy-level overlapping and ethanolstructure of mixed ofp Auorbitals solution). NCs inof Thewaterelectron-rich p bandand formedethanol sulfur by mixedand the overlappingoxygen solution). heteroatoms ofThe p orbitalsp band of of weformed electron-richll-organized by thesulfur surface overlapping and ligands oxygen of heteroatomsisp usedorbitals as anof well-organizedelectron-richintermediate statesulfur surface or anddark ligands oxygenstate is to used tuneheteroatoms as the an optoelectronic intermediate of well-organized state properties. or dark surface Please state to ligandssee tune more the is optoelectronicdetails used onas thean properties.intermediatepioneering Pleaseconceptual statesee or moredark PBIS state details model. to on tune the Reprinted the pioneering optoelectronic with conceptual permission properties. PBIS from model. Please Ref. Reprinted see [43]. more Copyright with details permission on(2019) the pioneeringfromSpringer Ref. Nature. [43 conceptual]. Copyright PBIS (2019) model. Springer Reprinted Nature. with permission from Ref. [43]. Copyright (2019) Springer Nature.

Figure 18.18. ((aa)) Schematic Schematic structures structures of of a Cu superlatticesuperlattice with different nanoribbon and nanosheetnanosheet Figuremorphology. 18. (a ()b (Schematicb) Corresponding) Corresponding structures emission emission of a spectra Cu spectrasuperlatti of Cu NCsofce Cu withwith NCs nanoribbondifferent with nanoribbonnanoribbon and nanosheet and morphology. nanosheet Themorphology. real distance The(b) betweenrealCorresponding distance two between thiol emission groups two inspectrathiol the grou nanoribbon ofps Cu in theNCs Cunanoribbon with superlattice nanoribbon Cu issuperlattice larger and than nanosheet is thatlarger in morphology.thethan nanosheet that in the The Cu nanosheet real superlattice, distance Cu supe indicatingbetweenrlattice, two the indicating thiol loose grou packing theps loosein the of packing DT nanoribbon molecules of DT Cu onmolecules superlattice the metal on core.the is largermetal The thanemissioncore. thatThe peakinemission the exhibits nanosheet peak a blueexhibits Cushift supe a from rlattice,blue 550shift indicating nm from to 490550 the nm nm loose when to 490 packing the nm morphology when of DT the molecules morphology of the Cu on superlattice theof the metal Cu core.changedsuperlattice The from emission changed a nanosheet peak from exhibits to a nanoribbon,nanosheet a blue shiftto which nanoribbo from implies 550n, nm awhich change to 490 implies ofnm the when packinga change the modelmorphology of the of packing DT molecules.of themodel Cu superlatticeReprintedof DT molecules. with changed permission Reprinted from a from withnanosheet Ref. permission [43 to]. Copyrightnanoribbo from Ref.n, (2019) which[43]. SpringerCopyright implies Nature. a (2019)change Springer of the packing Nature. model of DT molecules. Reprinted with permission from Ref. [43]. Copyright (2019) Springer Nature. The PBIS model waswas alsoalso successfulsuccessful toto explainexplain thethe long-termlong-term debateddebated andand self-contradictoryself-contradictory size-dependentThe PBIS model andand size-independent size-indepen was also successfuldent PL PL phenomena phenomenato explain of the MNCs:of MNCs:long-term the the smaller debatedsmaller the metaltheand metal self-contradictory nanoparticle nanoparticle size, size-dependentthesize, more the more coordinated coordinated and size-indepen unsaturated unsaturateddent metal PL atomsmetal phenomena atoms are exposed are of exposedMNCs: with increasedthe with smaller increased surface-to-volume the metal surface-to-volume nanoparticle rations, size,rations, the resultingmore coordinated in the strong unsaturated binding metal and atoms high aresurface exposed coverage with increasedof surface surface-to-volume coating ligands, rations, resulting in the strong binding and high surface coverage of surface coating ligands,

Nanomaterials 2020, 10, 261 18 of 24 resulting in the strong binding and high surface coverage of surface coating ligands, consequently emitting enhanced PL intensity with low energy due to the maximum overlapping of p orbitals from surface ligands, where the MNCs provided an ideal nanoscale interface for the strong binding interactions of adsorbed molecules, for example, the surface-protective ligands. When the nanoparticle size is fixed, the intensity and wavelength of PL are dependent on ligand coverage or densities: a high Au-S coordination number (CN) and high surface coverage results in stronger PL emission at long wavelengths because of the close packing of surface ligands, whereas a low Au-S CN and a low surface coverage make weak PL emissions with high energy. Obviously, the PL emission of metal nanoclusters is strongly dependent on the close packing or self-assembly of targeted surface ligands; thus, the used template for MNCs must contain electron-rich heteroatoms, including oxygen (O), nitrogen (N), sulfur (S), and phosphorus (P).

4. Summary and Outlooks Numerous and complicated superficial factors, such as the size and type of MNCs, the surface ligands, the valence state of surface metal, the self-assembly, and even the solvent effect affect the luminescent properties of metal nanoclusters, which interferes with the understanding of the luminescent nature of metal nanoclusters. Using metal NCs as a model system, by judiciously manipulating the delicate surface ligand interactions at the nanoscale interface together with a careful calculation of time-dependent density functional theory (TD-DFT), we proposed a completely new p band intermediate state (PBIS) model to elucidate the origin of PL emission of MNCs, which is completely different from the metal-centered quantum confinement mechanics (QCE) for the elucidation of PL emission of metal NCs. This mechanism could be universal to explain the PL emission nature of other quantum dot systems, including carbon and graphene nanodots, transition metal dichalcogenide (TMDC) nanomaterials, luminescent metal organic framework (MOFs), luminescent perovskites, and even organometallic complexes [105–110], because the overlapping of p orbitals at the confined nanointerface and nanospace could not be precluded [111]. Even though the PBIS model could qualitatively describe the PL properties, the exact dynamics and kinetics of electron transition of the excited state were not clear. In the near future, ultrafast time-resolved technology such as transient absorption and fluorescence up-conversion at the femtosecond scale will provide some reliable measurement tools to describe the relaxation and decay pathways of excited electrons. Most importantly, with respect to the paramount importance and universality of ligand and interface interaction at the nanoscale interface, the proof of concept of PBIS not only elucidates the fundamental physical principles driving nanoscale PL emission phenomena, but it also provides completely new insights to understand the interface-confined nanocatalysis on the molecule level. The presence of a new interface state with a low energy level could act as an extra channel to promote election transfer, which reduces the energy barrier of redox reaction, similar to the role of the transition state called in the heterogeneous catalysis [112,113]. At the same time, if the atomic orbitals of reactants interact with the PBIS, the binding strength of the reactant with the metal active site could be optimized (the classical Sabatier principle), which will improve the selectivity and lifetime of catalysts [114]. Of course, PBIS could be self-spontaneously formed between two adsorbed neighboring reactants at the heterogeneous nanoscale interface, which will also accelerate the chemical reaction depending the surface coverage. Hence, this significant conceptual advance will be of immediate interest to a broad readership of researchers in the nanocatalysis science.

Author Contributions: Conceptualization, T.-Q.Y. and K.Z.; writing—original draft preparation, T.-Q.Y., B.P., B.-Q.S., Y.-X.Z., J.-G.J., P.W. and K.Z.; writing—review and editing, T.-Q.Y., P.W. and K.Z.; supervision, P.W. and K.Z.; project administration, K.Z.; funding acquisition, P.W. and K.Z.; All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the NSFC (21872053, 21573074 and 21872052), the Science and Technology Commission of Shanghai Municipality (19520711400), the CAS key laboratory of Low-Coal Conversion Science & Engineering (KLLCCSE-201702), and the JORISS program, the Postdoctoral Science Foundation of China (2018M640360). Nanomaterials 2020, 10, 261 19 of 24

Acknowledgments: K.Z. thanks ENS de Lyon for a temporary position as an invited professor in France. This paper is dedicated to Ming-Yuan He in the occasion of his 80th birthday. Conflicts of Interest: The authors declare no conflict of interest.

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