J. Appl. Phys. 127, 231101 (2020); https://doi.org/10.1063/1.5143819 127, 231101

© 2020 Author(s).

Surface states of carbon dots and theirinfluences on luminescenceCite as: J. Appl. Phys. 127, 231101 (2020); https://doi.org/10.1063/1.5143819Submitted: 07 January 2020 . Accepted: 04 June 2020 . Published Online: 17 June 2020

Hui Ding , Xue-Hua Li, Xiao-Bo Chen , Ji-Shi Wei, Xiao-Bing Li, and Huan-Ming Xiong

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Surface states of carbon dots and their influenceson luminescence

Cite as: J. Appl. Phys. 127, 231101 (2020); doi: 10.1063/1.5143819

View Online Export Citation CrossMarkSubmitted: 7 January 2020 · Accepted: 4 June 2020 ·Published Online: 17 June 2020

Hui Ding,1 Xue-Hua Li,1 Xiao-Bo Chen,2 Ji-Shi Wei,3 Xiao-Bing Li,1 and Huan-Ming Xiong3,a)

AFFILIATIONS

1Key Laboratory of Coal Processing and Efficient Utilization of Ministry of Education and College of Chemical Engineering,

China University of Mining and Technology, Xuzhou, 221116 Jiangsu, People’s Republic of China2School of Engineering, RMIT University, Carlton, VIC 3053, Australia3Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University,

Shanghai 200433, People’s Republic of China

a)Author to whom correspondence should be addressed: hmxiong@fudan.edu.cn

ABSTRACT

Luminescent carbon dots (CDs) have received increasing attention from many fields during the past decade. Unfortunately, the luminescentmechanisms of CDs remain unclear due to insufficient experimental and theoretical knowledge, which significantly hinders thedevelopment of CDs with desired optical properties. Currently, surface states of CDs, which are based on synergistic hybridization betweenthe carbon backbones and the connected functional groups, have been considered as the dominant luminescence origins. This tutorialpaper, thus, aims to offer an overview of the key features on the surface of CDs, such as particle size, surface functional groups, defects andheteroatom doping, and their influences on the photoluminescence of CDs. In addition, optical characteristics of surface state-derivedluminescence emissions of CDs are also summarized. Finally, the potential approaches of characterizing surface states of CDs are intro-duced, followed by an outlook of synthesizing high-quality CDs through modulation of the surface states.

Published under license by AIP Publishing. https://doi.org/10.1063/1.5143819

I. INTRODUCTION

As emerging luminescent nanomaterials, carbon dots (CDs)have attracted extensive attention from multiple disciplines sincetheir discovery by Xu et al.1 Over the past decade, a large number ofhigh-quality CDs have been produced through hydrothermal, solvo-thermal, or microwave-assisted thermal treatment of various rawmaterials ranging from laboratory-produced chemicals to naturalproducts.2–6 The obtained CDs are spherical or nearly spherical inshape with a diameter below 10 nm and possess a graphite coredecorated with various functional groups on the surface, such ascarboxyl, amide, hydroxyl, and carbonyl moieties.7–9 In contrast totraditional semiconductor quantum dots (QDs), CDs exhibit lowproduction costs, low toxicity, high stability, and high biocompatibil-ity, making them a promising candidate in biologicalapplications.10–12 Furthermore, a broad range of outstanding opticaland electrical properties are also observed in CDs, such as excitation-dependent photoluminescence (PL) emission, and photo-inducedelectron transfer and redox, thereby widening their applications inchemical sensing, photocatalysis, drug delivery, energy conversion,

etc.13–15 Although great achievements have been made in CDsrelated fields, the physical mechanisms behind their PL emissionremain a matter under debate, which heavily hinders the preparationof CDs with desired optical properties and their potential applica-tions in the related fields.16–18 As such, there is an urgent need toimprove the understanding of PL mechanisms of CDs.

To address such a challenge, the relationships between theunique optical features and the complicated structures/componentsof CDs should be well understood. However, existing literaturesreport the development of thousands of carbon precursors andhundreds of approaches to produce CDs under various reactionconditions, making them exhibit a significant diversity in structure,components, and optical properties.12,19,20 In addition, the obtainedCDs are always a mixture of molecules, oligomers, polymer chains,polymer clusters, and carbon core owing to their incompletecarbonization.21–24 Such facts indicate that CDs suffer much morecomplicated structures and components than expected,25 therebybringing about more complex PL origins. Therefore, it remains agreat challenge to establish the complicated structure–property

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relationship in CDs. Nonetheless, three kinds of models have beenput forward to explain PL origins of CDs, i.e., carbon core states,surface states, and molecular fluorescence.26 Among them, thesurface states-derived PL emission, which correlates with both thecarbon backbones and the connected functional groups, has beenconsidered as the main luminescence origins of CDs. However,surface states are not well defined and remain an ambiguousconcept,27–29 making people confused about the numerous relativesof surface states because of their diversity and uncertainty.Furthermore, the surface states-derived PL possesses uniqueoptical characteristics from carbon core states and molecularfluorescence.30–32 Therefore, systematically summarizing thesurface state of CDs and their related optical characteristics isurgently needed for the development of CDs with promisingproperties.

In this tutorial, we first outline the key structures and compo-nents of CDs related to surface states, including particle sizes,surface functional groups, defects and heteroatom doping, andtheir influences on PL. We also summarize the optical characteris-tics of surface states-related PL of CDs, such as circumstance-dependent and broadened emission band. Subsequently, we intro-duce the potential approaches of characterizing surface states ofCDs. In the conclusion, we discuss the potential development andchallenges in relation to the preparation of high-quality CDsthrough modulating their surface states.

II. CLASSIFICATION OF CDS

“Carbon dots” are first proposed by Sun et al. based on theconcept of quantum-sized carbon nanoparticles, which graduallybecome a universal name for all the 0D carbon-based nanomaterialwith fluorescence.33 According to the specific carbon core structure,surface groups, and properties, CDs can be classified into four differ-ent categories: graphene quantum dots (GQDs), carbon quantumdots (CQDs), carbon nanodots (CNDs), and carbonized polymerdots (CPDs),3,5,34 as shown in Fig. 1. The GQDs are small graphenefragments with a larger horizontal dimension than that in height.They consist of a single or a few layered graphene sheets with well-defined graphene lattices that are connected to functional groups onthe edges, which endow GQDs with quantum confinement effectsand edge effects.35–37 The CQDs are always quasi-spherical in shapewith obvious crystal lattices in the carbon core and plenty of chemi-cal groups on the surface. They show intrinsic state luminescenceand size-dependent PL, making possible the regulation of emissionwavelength of CQDs through tuning the particle size.31 The CNDsare small sized highly carbonized nanoparticles with some chemicalgroups on the surface, but usually without distinct crystal latticesstructure and polymer features. The luminescence of CNDs ismainly originated from the defect/surface state and subdomain statewithin the graphitic carbon core without quantum confinementeffect of the particle size.38 In the last category, the CPDs contain acarbon core with aggregated or cross-linked functional groups/

FIG. 1. Classification of CDs and their possible structures: including graphene quantum dots (GQDs), carbon quantum dots (CQDs), carbon nanodots (CNDs), and car-bonized polymer dots (CPDs). Reprinted with permission from Xia et al. Adv. Sci. 6(23), 1901316 (2019). Copyright 2019 Wiley-VCH.

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polymer chains attached on the surface. The carbon core can be clas-sified into four subcategories, including two kinds of completely car-bonized cores similar to CNDs or CQDs, a paracrystalline carbonstructure consisting of tiny carbon clusters surrounded by polymerframes, and a highly dehydrated crosslinking and close-knit polymerframe structure.39 The optical features and mechanisms of CPDs arealso dominated by the surface state, the subdomain state, the molec-ular state, and the crosslink enhanced emission (CEE) effect, respec-tively. Obviously, at least three kinds of CDs have surfacestates-controlled PL emission, including GQDs, CNDs, and CPDs.

III. KEY FACTORS DETERMINING SURFACESTATE-DERIVED PL OF CDs

Theoretically, when CDs have a smaller size than their excitonradius, they would exhibit particle size-dependent PL behavior, whichessentially originates from the energy gap transition of conjugatedπ-domains in the sp2-carbon-constructed core.34 However, in mostcases, the PL spectra of CDs present the quantum size effect inconspic-uously but show strong dependence on their chemical compositionsand structures,22 indicating that the surface state is a very complicatedsystem. Differing from the carbon core state and molecular fluores-cence, the surface state is synergistic hybridization of carbon backbonesand connected functional groups within a CD, and their energy gap

correlates with the extent of the π-electron system and surface chemis-try.2,40 Therefore, both sp3- and sp2-hybridized carbons and othersurface defects of CDs can serve as a capture center for excitons andthus result in surface states-related fluorescence,13 which includes parti-cle size, surface functional groups, and defects and heteroatom doping.

A. Particle size

In general, particle size observed from transmission electronmicroscopy (TEM) presents the overall size of CDs, which includesa conjugated graphite core and an amorphous shell consisting ofsome functional groups.41 The inner effective conjugation length orsp2 domain size in carbon cores, which is highly dependent on theparticle size of CDs, has a significant influence on the optical prop-erties of CDs.42 On one hand, the energy gap transitions of aπ-electron system display intrinsic PL emission and make someCDs presenting the quantum size effect to a certain extent.43 Onthe other hand, the π-electron system in carbon cores can alter theenergy gaps of CDs through strong coupling interaction withsurface groups, such as carboxyl and carbonyl groups, consequentlyresulting in the particle size-dependent PL arisen from surfacestates. For example, Pang et al. prepared a series of CDs withtunable PL from blue to orange wavelength via a controllable wetoxidation method [Fig. 2(a)].44 Detailed characterizations on the

FIG. 2. (a) Optical images of as-prepared multicolor fluorescent CDs under UV light. (b) Model for the tunable PL of CDs with increase in size. (c) A possible growthmechanism for the metal-cation-functionalized CDs (CND1) and non-metal-cation-functionalized CDs (CND2). (d) Schematic representation of a CD core and its smalldomains made of fully sp3- and sp2-hybridized carbon atoms. (e) HOMO and LUMO states of a fully sp2-hybridized carbon domain. ( f ) Three-plate capacitor model usedto estimate the displacement of the domain layers upon the redistribution of electron density. (a) and (b) are reprinted with permission from Bao et al. Adv. Mater. 27(10),1663 (2015). Copyright 2015 Wiley-VCH. (c) is reprinted with permission from Qu et al. Adv. Mater. 28(18), 3516 (2016). Copyright 2016 Wiley-VCH. (d)–(f ) are reprintedwith permission from Tepliakov et al. ACS Nano 13(9), 10737 (2019). Copyright 2019 American Chemical Society.

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microstructure and composition showed that PL of the as-obtainedCDs is of surface-state emission, and that their energy gaps createdby surface oxidation are significantly determined by the π-electronsystem [Fig. 2(b)]. The larger the π-electron system facilitatessmaller energy gaps of the surface states in CDs. That is, theincrease in particle size leads to a red shift in the emission wave-length of CDs. Such a phenomenon was also reported by Qu’sgroup.45 They produced orange emissive CDs with a quantum yield(QY) of 20% through solvothermal treatment of citric acid (CA)and urea in dimethylformamide (DMF), as shown in Fig. 2(c).Compared with blue emissive CDs that were hydrothermally syn-thesized from the same carbon source, the as-obtained CDs possessa larger sized conjugated sp2-domain, which is the basis of energygap orange emission arisen from the surface state. Afterward, Sunand co-workers synthesized multiple color emissive CDs throughcontrolled thermal pyrolysis of CA and urea in DMF.46 By increas-ing the temperature and CA/urea ratio, PL color of the as-producedCDs could be continuously tuned from blue to red, covering theentire light spectrum. The red shift of CDs emission was ascribed tothe increased π-electron system and improved content of carboxylgroups on the CD surface. In addition to the experimental evi-dence, the influence of the conjugated sp2-domain on the surfacestates-related PL of CDs was also demonstrated by the simulatedcalculations. In Figs. 2(d)–2(f ), Tepliakov et al. built a simple,comprehensive, and accurate semianalytical model of opticalcenters for revealing the true nature of emission centers of CDs.47

The results show that the domain hybridization factor could deter-mine the localization of electrons and the electronic energy gapinside the domains, thereby causing the partially sp2-hybridizedcarbon domains-determined optical properties of CDs.

Based on these reports, it can be seen that the particle size,which is highly associated with the size of conjugated π domain inCDs, plays a determining role in PL properties derived fromsurface states, especially tuning the emission peak toward longerwavelength regions. However, it is should be noted that onlythrough strong coupling interaction with surface functional groups,can the conjugated sp2-domain influence the surface states-relatedoptical properties of CDs.

B. Surface defects

Structurally, the sp2 conjugated frameworks in the carbon coreof CDs are usually accompanied by a large number of non-perfectsp2 domains. Those sites will create or induce surface energy trapsthat can serve as capture centers for excitons, thus giving rise tosurface defect state-related fluorescence.48 As such, surface defectsof CDs are responsible for their multicolor emissions that arelocated in the visible light spectrum. According to previous reports,surface defects of CDs are highly complicated, which are closelyassociated with both sp2 and sp3 hybridized carbons and otherheteroatom-containing functional groups, such as dangling bonds,non-radiative states, and oxygen and nitrogen-related functionalgroups.49 The bright surface defect-derived fluorescence of CDs isresulted from the recombination of electron–hole pairs in thestrongly localized π and π* electronic levels of the sp2 sites, whichstay between the energy gap of the σ and σ* states of the sp3

matrix, thereby resulting in weak absorption in the near ultraviolet-visible region but strong emissions in the visible region.

1. Oxygen-containing functional groups

Most reported CDs, owning to complete carbonization,possess abundant oxygen-containing functional groups over theirsurface, including epoxide, hydroxyl, carbonyl, carboxyl, and sulf-oxide.3 Such functional groups are connected with carbon back-bones, which not only impart high solubility of CDs in polarsolvents, but also introduce diverse surface defects with differentenergy levels,50,51 thus resulting in numerous electronic transitionpossibilities and various PL properties of CDs. Hu et al. prepared aseries of CDs with tunable PL emission across the entire visiblespectrum by simple adjustment of reagents and synthesis condi-tions.52 They postulated that surface epoxides or hydroxyls caninduce significant local distortion and then create new energy levelsbetween n–π* gaps, consequently causing a wide range of excitationenergies and excitation wavelength-dependent PL [Fig. 3(a)]. Toexplore the electronic structure of CDs, Yang and co-workers firstused ultrafast spectroscopy to unravel the common origin of threekinds of typical green fluorescent CDs synthesized by top-downor bottom-up methods.53 Based on the obtained results inFigs. 3(d)–3(f ), the special edge states consisting of carbon atomson the edge of the carbon backbone and functional groups withCvO are responsible for the PL emission of those CDs. In thisregard, by purifying a CD mixture obtained via hydrothermallytreating an aqueous solution of urea and p-phenylenediamine, ourgroup obtained eight batches of CDs with a similar particle sizedistribution and graphite structure but exhibiting PL color varyingfrom blue to red [Fig. 3(b)].38 The distinct PL redshift was found tocoincide with the increasing content of carboxyl on the CD surface,implying that surface oxidation could produce defects and thusresult in surface state-related PL [Fig. 3(c)]. Meanwhile, it was con-cluded that the energy gaps of CDs between the lowest unoccupiedmolecular orbital (LUMO) and the highest occupied molecularorbital (HOMO) reduced with the increasing degree of surfaceoxidation, which is in agreement with the density functional theory(DFT) calculations reported by Chien et al.54 In light of theaforementioned surface state emission mechanisms, Li’s group dis-solved orange emissive CDs in dimethyl sulfoxide (DMSO) solventwithout any further treatment55 and realized 10% efficient near-infrared (NIR) emission CDs excited in an NIR-I window(λabs = 715 nm, λem = 760 nm) for the first time. Regarding PLmechanisms, the SvO and CvO functional groups attached to theouter layers and the edges of CDs increase surface oxidation andthe occurrence of discrete energy levels, thereby contributing to theNIR absorption band and enhanced NIR fluorescence [Fig. 3(g)].

Recently, Zhang et al. unraveled the relationships between PLand surface structures through semiquantitatively tuning oxygenfunctional groups on the CD surface.56 The results of experimentsand theoretical calculations reveal that the emission wavelength ofCDs is significantly determined by both the delocalization extentof the π-electron system and carbonyl-group content, while QYs ofCDs are heavily dependent on the carbonyl-group content, as shownin Fig. 3(h). Remarkably, the carboxyl groups in CDs have noobvious effects on the surface state-related PL owing to no conjugate

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effects between carboxyl groups and the π-electron system, whichdiffers from previous reports. These instructive works indicate thatoxygen-containing functional groups play an important role in deter-mining PL properties of CDs derived from surface states.

2. Nitrogen-related functional groups

Apart from the abovementioned one, nitrogen-related func-tional groups, which are commonly present in the form of amino,amidic, pyridinic, pyrrolic, graphitic, nitro, amidine, etc.,57–59 can

FIG. 3. (a) Illustration of tunable PL emission from CDs containing different O-related surface groups. (b) A photograph of as-separated multicolor fluorescent CDs underUV light. (c) Model for the tunable PL of CDs with different degrees of oxidation. (d)–(f ) TA spectra for microwave-synthesized CDs, electrochemically synthesized CDs,and solvothermally synthesized GQDs at 400 nm excitation, respectively. (g) Schematic of the structure and energy level alignments of non-treated CDs (left column) andCDs modified with SvO/CvO-rich molecules (right column). (h) Representation of the proposed PL mechanism and structure–property relationship of CDs. (a) isreprinted with permission from Hu et al. Angew. Chem. Int. Ed. 54(10), 2970 (2015). Copyright 2015 Wiley-VCH. (b) and (c) are reprinted with permission from Ding et al.ACS Nano 10(1), 484 (2016). Copyright 2016 American Chemical Society. (e) and (f ) are reprinted with permission from Wang et al. ACS Nano 8(3), 2541 (2014).Copyright 2014 American Chemical Society. (g) is reprinted with permission from Li et al. Adv. Mater. 30(13), 1705913 (2018). Copyright 2018 Wiley-VCH. (h) is reprintedwith permission from Liu et al. J. Phys. Chem. Lett. 10(13), 3621 (2019). Copyright 2019 American Chemical Society.

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be embedded in both carbon core or amorphous shell due tothe similar electronic structure and atom radius of N toC. These groups are more likely to undergo transition from theground state to the lowest excited singlet state of electrons, thusgreatly improving the surface-states-related luminescence propertiesof CDs.

The absorption/emission wavelength of CDs can be redshiftedto long-wavelength ranges through doping N-containing groups inthe form of amino, pyridinic, hydrazine, or graphitic N into thesurface/core of CDs.60 For instance, Permatasari et al. developedpyrrolic-N-rich CDs that give an absorption peak centered at650 nm using CA and urea as raw materials.61 The increasedamount of pyrrolic functional groups led to a high electron densityand resulted in delocalized electron waves spreading through theCD surface, thus enabling redshift of the absorption wavelength ofthe CDs [Fig. 4(a)]. Likely, by functionalizing the surface of CDswith amino, Tetsuka and co-workers designed highly luminescentCDs with tunable PL color from blue to yellow under single-wavelength UV excitation [Fig. 4(b)].62 These edge-terminatedamino moieties systematically modified the electronic structureof CDs through their effective orbital resonance with thegraphene core, thus largely improving the optical features of CDs

[Figs. 4(c) and 4(d)], which was also in agreement with the time-dependent density functional theory (TDDFT) calculationsreported by Kundelev’s group.63 They conducted a comprehensivequantum chemical DFT analysis of absorption and PL of amino-functionalized CDs, as shown in Fig. 4(e). The results reveal thatthe surface perylene- and pyrene-based subunits with differentnumbers of amino groups not only shift their absorption and thePL spectrum to longer wavelengths but also preserve the high PLintensity inherent to CDs. Specifically, the increase in red shift withthe functionalization degree of the CDs was attributed to thecharge transfer from amino groups to CD cores, while the excita-tion wavelength-dependent emission color is ascribed to the pres-ence of different kinds of amino subunits on the CD surface. Inaddition, the distinct PL redshift was also observed in CDs with anincreasing content of graphitic N. In recent work, Otyepka andco-workers prepared full-color-emission CDs through thermaltreatment of a formamide solution of urea and CA, followed bycolumn chromatography separation based on the surface chargediscrepancy of CDs.64 The obtained red-emitting CDs possess alarger content of graphitic N than the other CDs, which could gen-erate midgap states between HOMO and LUMO, thus resulting inredshifted absorption and emission [Fig. 4(f )].

FIG. 4. (a) Schematic diagram of the pyrrolic-N-rich CDs with strong NIR absorption. (b) A photograph of as-prepared PL tunable CDs under UV light. (c) Normalized PLemission and selected UV/VIS absorption spectra of CDs. (d) Schematic illustrations of structures used for theoretical calculations with different functional groups. (e)Perylene- and pyrene-based functional subunits of amino-functionalized CDs with one (Pe1 and Py1), two (Pe2 and Py2), and three (Pe3 and Py3) amino groups. (f )Schematic illustrations of the PL tunable CDs with increasing content of graphitic N. (a) is reprinted with permission from Permatasari et al. ACS Appl. Nano Mater. 1(5),2368 (2018). Copyright 2018 American Chemical Society. (b)–(d) are reprinted with permission from Tetsuka et al. Adv. Mater. 24(39), 5333 (2012). Copyright 2012Wiley-VCH. (e) is reprinted with permission from Kundelev et al. J. Phys. Chem. Lett. 10(17), 5111 (2019). Copyright 2019 American Chemical Society. (f ) is reprinted withpermission from Hola et al. ACS Nano 11(12), 12402 (2017). Copyright 2017 American Chemical Society.

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In addition, some nitrogen-containing functional groups canalso contribute to PL improvement of CDs, which includes amine,pyrrolic, and graphitic groups.63 Zhai et al. used citric acid and avariety of amine molecules as carbon source [Fig. 5(a)] to produceCDs with different N contents.65 It was found that the primaryamine acts as surface passivation agents, which traps surface elec-trons for more efficient radiative recombination, thus considerablyenhancing the fluorescence of the CDs. Thereafter, Qian et al. syn-thesized highly efficient blue-emitting CDs through a solvothermalreaction,66 and the obtained results suggest that both amino andpyrrolic-N result in the improvement in the luminescence proper-ties because they could promote radiative recombination by gener-ating more protonation processes or aromatic structures [Fig. 5(b)].Recently, through adjusting reaction solvents, our group reported aseries of highly luminescent CDs with tunable emission over theentire visible region and gradually narrowed full width at halfmaximum (FWHM) [Figs. 5(c) and 5(d)].67 After detailed

characterization on structure and chemistry of the CDs, the tunablePL was proven to be derived from their surface state in which theenergy gaps could be controlled by both the π-electron system andthe graphitic nitrogen content. However, apart from the typicalsurface-state controlled emission, PL QYs of these CDs was slightlydependent on the particle size of CDs, indicating that the improvedcontents of graphitic nitrogen atoms within carbon cores make asignificant contribution to PL QYs. It is attributed to that fact thatthe graphitic nitrogen content can enhance the rigidity of thedomains and thus reduce non-radiative relaxation through molecu-lar vibrations on CD surfaces. In addition to these experimentalresults, some groups also performed theoretical studies on theeffects of N doping types and positions on optical features of CDs.As depicted in Fig. 5(e), Niu et al. employed a time-dependentdensity functional theory to unveil that center doping creates mid-states that render non-fluorescence.68 However, edge N-containingfunctional groups would regulate the energy levels and increase the

FIG. 5. (a) Schematic representation of CD synthesis in the presence of various amines. (b) Emission spectra of CDs synthesized from different amines. (c) A photographof multicolor CDs synthesized with a solvent-engineered strategy and their corresponding normalized PL spectra (d). (e) Schematic illustrations of how different types of Ndoping effect PL emission of CDs. (a) is reprinted with permission from Zhai et al. Chem. Commun. 48(64), 7955 (2012). Copyright 2012 Royal Society of Chemistry. (b) isreprinted with permission from Qian et al. Chem. Eur. J. 20(8), 2254 (2014). Copyright 2014 Wiley-VCH. (c) and (d) are reprinted with permission from Ding et al. Small14(22), 1800612 (2018). Copyright 2018 Wiley-VCH. (e) is reprinted with permission from Niu et al. Nanoscale 8(46), 19376 (2016). Copyright 2016 Royal Society ofChemistry.

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radiation transition probability, thus enhancing PL intensity. Theresults could address some inconsistency observed in differentexperiments.

3. Other organic molecules/polymers

Some special organic molecules, such as polyethylene glycol(PEG), polyethylenimine (PEI), polyvinylpyrrolidone (PVP),glucose, arginine, Vitamin C (VC), acetaldehyde, or histidine, werealso confirmed to induce additional surface defects,19 which caninteract with the intrinsic energy gap core via electron and/orenergy transfer, consequently changing the optical behaviors ofCDs. Zhang’s group first synthesized blue-emitting CDs throughlow temperature pyrolysis of PEG400.69 Then, these blue lumines-cent CDs were thermally treated in the presence of PEG1000 and2,20-(ethylenedioxy)-bis(ethylamine) (EDA), resulting in the pro-duction of green and orange emissive CDs with enhanced QY,respectively. In this reaction, the surface passivation agents, i.e.,PEG1000 and EDA, alter the electronic structure, introduce new

energy gaps of low energy, and dramatically change the surfacestates of CDs. As a result, long-wavelength emissive CDs withstrong surface oxidation and high N doping as well as increasedQY could be achieved, as shown in Fig. 6(a). Similarly, using PEG,primary amine, glucose, arginine, histidine, biotin, and folic acid assurface passivation reagents, Ali et al. realized the emission wave-length spanning of CDs from 550 to 750 nm because of the addi-tional surface defects induced by new polar functional groups[Fig. 6(b)].70 Recently, Zhen et al. developed a simple approach toengineer the surface states of CDs with blue emission by conjugat-ing them with VC and acetaldehyde and yielded controllable PLred shifts of CDs by 175 nm and 285 nm [Fig. 6(c)].71

Experimental characterization results validated that carbonyl andCvN groups as sub-fluorophores are the origins of green and redemissions of CDs, respectively.

In addition to introducing new emissive surface defects,organic molecules or polymers could also be used as passivationagents to improve the optical behavior of CDs derived fromthe surface state.43,50 Surface defects, which behave as organic

FIG. 6. (a) Schematic illustration of the synthetic routes for CDs with different emission colors. (b) PL emission spectra of red emissive CDs (left), photograph of CD colloi-dal solution under UV irradiation (middle), and fluorescence image of CDs labeled KB cells (right). (c) Schematic syntheses and applications of blue, green, and red emis-sive CDs. (d) Schematic illustrations of CDs with PEG1500N species attached to the surface. (e) Preparation of surface-passivated CDs by the hydrothermal treatment ofcitric acid in the presence of hyperbranched PEI. (a) is reprinted with permission from Zhang et al. Nanoscale 8(1), 500 (2016). Copyright 2016 Royal Society ofChemistry. (b) is reprinted with permission from Ali et al. ACS Appl. Mater. Interfaces 8(14), 9305 (2016). Copyright 2016 American Chemical Society. (c) is reprinted withpermission from Zheng et al. J. Mater. Chem. B 7(24), 3840 (2019). Copyright 2019 Royal Society of Chemistry. (d) is reprinted with permission from Sun et al. J. Am.Chem. Soc. 128(24), 7756 (2006). Copyright 2006 American Chemical Society. (e) is reprinted with permission from Wang et al. ChemNanoMat. 1(2), 122 (2015).Copyright 2015 Wiley-VCH.

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functional molecules that are individually embedded into solidhosts, render CDs defenseless to external contaminants, and therelated CDs usually suffer from poor photostability and low QYs.72

The surface passivation treatment motivates the localization of elec-tron–hole pairs on the surface states of CDs and eliminates the dissi-pation of photo-induced carriers from surface sites, thus makingpossible the more highly efficient radiative recombination and theenhancement of PL properties of CDs. Accordingly, compared to barecounterparts, surface-passivated CDs become highly optically activeand emit significantly improved PL from visible to near-infrared spec-tral regions.29 Until now, the predominant passivation agents areorganic molecules and polymers. Sun et al. used organic moietiesdiamine-terminated oligomeric poly-(ethylene glycol) (PEG1500N) topassivate as-obtained CDs [Fig. 6(d)].33 The authors stated that theyexhibited obviously increased fluorescence in a solution-like suspen-sion and in the solid state, which is ascribed to that surface passivationcan stabilize emission of CDs from surface energy traps. It should benoted that emissive energy traps must remain bound to the surface ofCDs via quantum confinement when surface passivation is able toproduce strong fluorescence. Additionally, by using citric acid as acarbon source and hyper-branched polyethylenimine (PEI) as asurface passivation agent, Wang and co-workers also produced blue-emitting CDs via a one-step route [Fig. 6(e)].73 The resultant CDspossessed a high QY up to 24.3%, and high photo-, metal-, andsolvent-stability, which is ascribed to the protective surface passivationlayer formed by PEI. Though functional group-terminated CDs wereextensively studied, a large number of issues need to be furtheraddressed. For example, it remains unclear whether light is emitted bysurface groups per se or through their influence on PL by changingthe internal electrical structure of the CDs, and how different func-tional groups have an impact on the PL (i.e., emission wavelength andQY) of CDs at different levels.

C. Heteroatom doping

As many reports declared, heteroatom doping is an effectiveway to improve optical properties of CDs, including the enhance-ment in fluorescence intensity and redshift of emissionwavelength.74–76 Up to now, many types of heteroatoms have beenadopted to adjust the PL features of CDs by tailoring their struc-tural and electronic properties. This section mainly focuses on theadvancements of CDs with non-metal heteroatom doping exceptthat nitrogen already discussed, including preparation methods,optical properties, and their PL mechanisms.

1. Sulfur doping

Sulfur atoms possess similar electronegativity but largeratomic radius than carbon. Therefore, electron transition of S iseasier than that of carbon,76 enabling the effective improvement inoptical properties of CDs by sulfur doping. In addition, each sulfuratom has six valence electrons, which can modify the electronicstructure of CDs. So far, a large number of studies reported thesynthesis of sulfur-doped CDs (S-CDs). Chandra et al. reported thefirst sulfur-doped CDs using thiomalic acid sulfur acid as thecarbon source and the sulfur source in 2013.77 The as-preparedS-CDs exhibited a PL QY of 11.8% without any additional surfacepassivation. The presence of S and O functionality made S-CD

negatively charged and bright blue luminescent. Thereafter, ourgroup synthesized a new type of S-CDs with a low QY of 5.4%from α-lipoic acid via a hydrothermal route.78 The doped S atomsserve as a synergistic role in increasing the content of CvN incarbon cores. Consequently, the optimal N- and S-coped CDs pos-sessed the highest QY of 54% than S-CDs and N-CDs in the bluelight region [Fig. 7(a)], which is resulted from the functionalgroups on the surface and aromatic structures in the carbon core.Recently, Xu and co-worker synthesized 67% efficient blue emissiveS-CDs from sodium citrate and sodium thiosulfate with a simpleand straightforward hydrothermal method [Fig. 7(b)].79 In compar-ison to other S-CDs, QY of these S-CDs was significantly improvedbecause the sulfur atom in CDs plays a catalytic role in oxidationreduction reaction, which introduces more passivated surface detectof CDs and further enhances photoluminescence. In addition toblue color luminescence, red-emitting S-CDs were also reported.Ge et al. used conjugated polymer polythiophene phenylpropionicacid as the carbon source and produced S-CDs with red emission.80

These S-CDs were water-soluble, and carboxylate-terminated, dis-playing a broad absorption band ranging from 400 to 800 nm andan emission peak at 640 nm, which makes them appealing in PLimaging, photoacoustic imaging, and photothermal therapy, asshown in Fig. 7(c). Zhang et al. synthesized N and S co-doped CDsfrom o-phenylenediamine and L-cystine, which possess relatively ahigh PL QY (35.7%) toward a near-infrared fluorescent peak up to648 nm [Fig. 7(d)].81 With the advancement of characterizationtechniques, N, S-dopants were found to not only enrich electronicstates, but also lead to the creation of in-gap trap states that couldact as nonradiative and recombination centers. This conclusionagrees with those proposed in other heteroatom doped CDs.

2. Phosphorus doping

As one important member of the fifth group in the periodictable of elements, phosphorus (P) atoms have a larger radius thanthat of carbon counterparts and have high reactivity owing to theirrich valence electrons.82–84 As a result, P atoms can form substitu-tional defects in a carbon cluster and behave as n-type donors, thusmodifying the electronic and optical properties of CDs. P dopingcan also significantly enhance the luminescence emission efficiencyof CDs. Zhou’s group dramatically increased the QY of CDs from3% to 25% by doping P into carbon cores,85 which is synthesizedfrom phosphorous tribromide and hydroquinone with a solvent-thermal method [Fig. 8(a)]. The high QY may be ascribed to thecoexistence of defect sites and isolated sp2 carbon clusters, which canmore efficiently increase the energy gap into the UV-vis region andproduces stronger visible PL than individual sp2 carbon clusters. ThePL enhancement may also be resulted from the increasing numberof isolated sp2 carbon clusters because the introduction of P intoCDs produces more defects and thus enhances PL intensity. Xuet al. applied a single-step and high-efficiency hydrothermal methodto produce N and P-doped carbon dots (N,P-CDs) from sodiumcitrate and diammonium phosphate with a superior PLQY of 53.8%[Fig. 8(b)],86 excitation-independent emission behavior, and robuststability over a large pH range. Spectroscopic investigations suggestedthat the addition of P and the passivation effect of the oxidizedsurface were primarily responsible for the high PLQY of the N,

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P-CDs. On the other hand, P doping was reported to change theemission wavelength of CDs. Hu’s group first demonstrated that thered color luminescent centers can be created through P incorpora-tion into CDs.87 They believed that P-containing groups can intro-duce a new energy level state above HOMO but below N-state ofCDs (named as P-state) as these groups act as strong electrondonors, as shown in Fig. 8(c). Therefore, the transition between Nand P states produces red (R2) and yellow (R1) emission simultane-ously, presenting rare orange/red dual-emission in CDs. Beyondthese, there are also some different and contradictory viewpoints onthe effect of P doping on the fluorescent properties. Zhi et al. arguedthat P doping has no distinct effects on the optical properties ofP-CDs, including PLQY and fluorescence lifetime, but can signifi-cantly improve their photostability [Fig. 8(d)].88

3. Boron doping

Boron (B) locates next to C in the periodic table of elementsand has similar chemical properties to those of N. Therefore, it is asound strategy to dope B into CDs for changing their electronic

structures and optical properties, which has been widely studied inthe literature.76 Meanwhile, the incorporation of electron-deficientB atoms can accommodate the charge density of C and N bondingmatrix, consequently decreasing the number of surface state bycausing a change in the bonding patterns. As with other nonmetal-lic atoms (N, S, and P), B doping can be used to enhance the lumi-nescence efficiency of CDs. Shan and co-workers synthesized blueluminescent B-doped CDs (B-CDs) from one-pot thermal treat-ment of an acetone solution of BBr3 and hydroquinone.89 ThePLQY of B-CDs is 14.8%, much higher than that of B-free CDs(3.4%), which is attributed to the role of B as an engine in chargetransfer in the excited state. Interestingly, a large PL blue shift wasfound in B-CDs with respect to B-free CDs [Fig. 9(a)], which isascribed to the strong electron-withdrawing character of Choi et al.developed a one-step microwave-assisted synthesis of B and Nco-doped CDs (BN-CDs) using citric acid, ethylenediamine, andboric acid as raw materials.90 The obtained BN-CDs displayed aPLQY of up to 81% [Figs. 9(b) and 9(c)], which is almost twice asbright as that of B-free N-CDs, indicating that the incorporation of

FIG. 7. (a) Scheme of the synthesis of S-doped CDs with strong blue luminescence. (b) Absorption and PL spectra of S-CDs and N,S-CDs and their photograph underUV light. (c) Schematic syntheses and applications of red emissive CDs. (d) Schematic syntheses and structures of red emitting CDs. (a) is reprinted with permission fromXu et al. J. Mater. Chem. A 3(2), 542 (2015). Copyright 2015 Royal Society of Chemistry. (b) is reprinted with permission from Ding et al. Nanoscale 6(22), 13817 (2014).Copyright 2014 Royal Society of Chemistry. (c) is reprinted with permission from Ge et al. Adv. Mater. 27(28), 4169 (2015). Copyright 2015 Wiley-VCH. (d) is reprintedwith permission from Zhang et al. Nano Res. 12(4), 815 (2019). Copyright 2019 Springer Nature.

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B atoms into the CD network has a significant influence on theiroptical properties. The detailed mechanistic exploration confirmedthat B doping increases the fraction of graphitic C and N, therebyproducing more uniform and fewer well-distributed surface defectstates, which account for the observed PL enhancement ofBN-CDs. Meanwhile, the emission behavior of BN-CDs changedfrom excitation-dependent to near excitation-independent due totheir fewer multi-chromophoric units than N-CDs [Fig. 9(d)].Thereafter, Jana et al. reported the synthesis of four kinds ofB-CDs with varying amounts of B content by modified hydrother-mal treatment of a mixture of ascorbic acid with borax, boric acid,sodium borate, and sodium borohydride.91 The dominant absorp-tion band of B-CDs would redshift when more B atoms were grad-ually doped into carbon cores [Fig. 9(e)]. The following theoreticalcalculations suggested that electron-deficient B can cause hugecharge polarization within the carbon surface, which thus results inthe formation of surface defects and, subsequently, anelectronic-transition-related redshift in the absorption spectrum.

4. Fluorine doping

Fluorine (F) has the maximum electronegativity and can stronglyabsorb adjacent electrons, thus increasing the separation of positiveand negative charges.76 In contrast to other heteroatoms, F dopedCDs (F-CDs) have been reported rarely. Some work suggested that

doping F is an efficient route to improve and tune the fluorescenceproperties of CDs. Zuo et al. produced a kind of F-CDs by a solvo-thermal process using 1,2-diamino-4,5-difluorobenzene as the fluorinesource, as shown in Fig. 10(a).92 With respect to F-free CDs, theas-obtained F-CDs exhibited not only an obvious red shift by50 nm, but also a slightly improved QY, indicating that F dopingcan be used to improve PL behavior of CDs. Recently, Yang’s groupdisclosed that red emission F-CDs could be produced by dopingthe electron-withdrawing fluorine atoms into CDs via amicrowave-assisted carbonation route.93 In light of UV-Vis absor-bance and x-ray photoelectron spectroscopy (XPS) analysis, theredshifted PL emission could be attributed to the increased extentof the π-electron system induced by F doping, which will narrowthe energy gap of the π–π* transitions and further result in asmaller energy gap for surface states. As a consequence, redshiftedfluorescence with a lower energy level could be achieved bydoping F into CDs [Fig. 10(b)]. Sun et al. demonstrated the pro-duction of F-doped CDs (F-CDs) from graphene wastes by amicrowave-assisted hydrothermal method.94 The obtained F-CDsexhibited remarkable photostability and distinct PL redshift com-pared to F-free CDs, which were closely related to F doping intoCDs. The redshifted PL could be attributed to the higher degree offluorination because the relatively high surface defect concentrationin F-CDs caused by fluorination could trap more excitons, whichresults in irradiation decay of activated electron and thus a longer

FIG. 8. (a) Schematic syntheses and structures of P-doped CDs with blue luminescence. (b) Schematic showing the synthesis process of strongly photoluminescent (N,P)-CDs. (c) Schematic illustration of the PL mechanism for P-CDs. (d) Schematic syntheses and TEM images of P-CDs from different carbon sources. (a) is reprinted withpermission from Zhou et al., RSC Adv. 4(11), 5465 (2014). Copyright 2014 Royal Society of Chemistry. (b) is reprinted with permission from Xu et al., Nano Res. 11(7),3691 (2018). Copyright 2018 Springer Nature. (c) is reprinted with permission from Hu et al., J. Mater. Chem. C 5(38), 9849 (2017). Copyright 2017 Royal Society ofChemistry. (d) is reprinted with permission from Zhi et al., Carbon 29, 438 (2018). Copyright 2018 Elsevier.

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FIG. 9. (a) Fluorescence spectra of BCQDs and CQDs, and the effect of H2O2 on fluorescence of BCQDs and CQDs. Inset: photographs of BCQDs and CQDs under UVlight. (b) Synthetic scheme for N-CD and BN-CDs. (c) Photographs of N-CDs and BN-CDs solutions as well as their solid powder under UV light. (d) Fluorescence excita-tion–emission map of N-CDs and BN-CDs. (e) Fluorescence emission spectra of B-CDs with different contents of B and their applications for ascorbic acid and Fe3+ detec-tion. (a) is reprinted with permission from Shan et al., Analyst 139(10), 2322 (2014). Copyright 2014 Royal Society of Chemistry. (b)–(d) are reprinted with permission fromChoi et al. Chem. Mater. 28(19), 6840 (2016). Copyright 2018 American Chemical Society. (e) is reprinted with permission from Jana et al., Langmuir 33(2), 573 (2017).Copyright 2017 American Chemical Society.

FIG. 10. (a) Synthesis of F-CDs and undoped CDs and their photographs under UV light. (b) Synthetic scheme for F-CDs and undoped CDs. (a) is reprinted with permis-sion from Zuo et al., J. Phys. Chem. C 121(47), 26558 (2017). Copyright 2017 American Chemical Society. (b) is reprinted with permission from Yang et al., Carbon 128,78 (2018). Copyright 2018 Elsevier.

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wavelength emission in F-CDs. Meanwhile, the remarkable photo-stability was highly likely due to the strong electron-withdrawingability of F, which reduces the electron density of the aromaticstructure in F-CDs. Nevertheless, the underlying exact mechanismsof redshifted PL and improved photostablity upon fluorine dopingneed further investigations.

5. Other non-metals doping

Other non-metallic elements such as silicon, selenium, andhalogen have been used to dope CDs,19 which tailor the structuraland electronic properties of CDs, thus endowing CDs with uniquesurface state-related optical properties.

Qian’s group synthesized silicon-doped CDs (Si-CDs) through asimple and efficient solvent-thermal route using SiCl4 as the siliconsource and hydroquinone as the carbon precursor [Fig. 11(a)].95 QYof the Si-CDs is up to 19%, much higher than that of Si-free CDs(3.4%), which demonstrates that the incorporation of Si can

substantially enhance the PL intensity of CDs. Through systematicallyinvestigating the fluorescence of CDs with experimental and theoreti-cal approaches and comparing the different proposal on the nature offluorescence of CDs, they ascribed the PL enhancement to the forma-tion of more defect sites in the large conjugated C structure resultingfrom the incorporation of a large amount of silicon atoms, conse-quently giving rise to the enhancement of PL intensity. Thereafter,Zhang et al. produced blue light-emission organosilane-functionalizedCDs with a QY of up to 57% through hydrothermal treatment of asolution of citric acid and N-(3-(trimethoxysilyl) propyl) ethylenedi-amine [Fig. 11(b)].96 The attached organosilane functional groups onthe CD surface were crucial for the high QY of Si-CDs because theorganosilane functional groups could not only serve as surface passiv-ation agents to trap surface electrons, but also as electron donors toincrease the electron cloud density, which increases together the prob-ability of electron–hole radiative recombination.

Yang’s group obtained selenium-doped CDs (Se-CDs) byreducing the readily-prepared CDs with NaHSe via a one-step

FIG. 11. (a) Schematic illustration of application of Si-CQDs in bioimaging and biosensor. (b) A schematic of synthesizing organosilane-functionalized CQDs with theirapplication in white LED. (c) A schematic diagram of the preparation process of Se-CDs. The right image shows the typical TEM image of Se-CDs and the digital photo ofSe-CDs solution under UV light. (d) Synthetic scheme of Se-CQDs with green fluorescence and their application in protecting the cells from ROS-induced damage. (e)Fluorescence spectra of halogenated-CQDs in ethanol and following substitution by 1,2-ethylenendiamine in water. (a) is reprinted with permission from Qian et al., ACSAppl. Mater. Interfaces 6(9), 6797 (2014). Copyright 2014 American Chemical Society. (b) is reprinted with permission from Zhang et al., Nanoscale 8(16), 8618 (2016).Copyright 2016 Royal Society of Chemistry. (c) is reprinted with permission from Yang et al., J. Mater. Chem. A 2(23), 8660 (2014). Copyright 2014 Royal Society ofChemistry. (d) is reprinted with permission from Li et al., Angew. Chem. Int. Ed. 56(33), 9910 (2017). Copyright 2017 Wiley-VCH. (e) is reprinted with permission fromZhou et al., RSC Adv. 3(25), 9625 (2013). Copyright 2013 Royal Society of Chemistry.

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hydrothermal reaction and in situ doping treatment [Fig. 11(c)].97

The reduced Se-CDs showed improved PL with redshifted fluores-cence spectrum, higher QY, and longer lifetime than those of oxi-dized CDs, which could be ascribed to the Se-containing surfaceactivity groups that serve as an electron donor. Recently, Xu et al.described a simple method to produce green luminescent selenium-doped CDs (Se-CDs) with a QY of 7.6% through hydrothermaltreatment of a selenocystine aqueous solution [Fig. 11(d)].98

Interestingly, the obtained Se-CDs displayed reversible redox-dependent luminescence, indicating that the oxidation state of Seatoms had a significant effect on the optical behavior of Se-CDsthrough affecting the electron–hole recombination in cores.

Zhou et al. proposed an efficient route to prepare halogenatedCDs (Cl/Br/I-doped CDs) through a solvent-thermal reaction usingcarbon tetrachloride and quinol as precursors.99 All the haloge-nated samples exhibited blue fluorescence in ethanol but with dif-ferent QYs and emission peaks [Fig. 11(e)]. After the substitutionof halogens with diamine, the emission efficiencies of all function-alized CDs were elevated up several times, indicating that thehalogen atoms are incorporated on the CD surfaces and had a sig-nificant influence on the optical properties of CDs. Overall,samples of heteroatom-doped CDs such as P, S, Si, Se, and halogenare scarce, which hinder the exploration of their effects on theoptical properties of CDs in a large part. Accordingly, the effects ofthese heteroatom doping are still ambiguous.

IV. OPTICAL CHARACTRISTICS OF LUMINESCENCEDERIVED FROM SURFACE STATE

As mentioned above, not only surface functional groups butalso carbon cores have a remarkable influence on the PL behaviorof CDs derived from the surface state because both of them can sig-nificantly alter their electronic structures.100–102 The attached func-tional groups on the CD surface are with abundant structuralconfigurations and can introduce more electronic structures withdifferent energy levels, consequently resulting in more recombina-tion possibilities of electrons and holes captured by surfacestates.103 The π-electron system in carbon cores can couple withthese functional groups, synergistically changing the electronicstructures of CDs and their recombination. In addition, the defectcenters of CDs can easily interact with the surrounding environ-ments, altering the electronic structures of CDs, which bringsabout obvious optical changes.30 As a result, surface state-controlled luminescence of CDs typically displayed unique opticalproperties that are distinguished from the carbon core state andmolecular fluorescence.

First, fluorescence quantum yields of CDs may decrease withthe increase in surface defects, which is also companied by PL red-shift to some extent. Xu and co-workers first prepared CDs (namedr-CDs) from 3-(3,4-dihydroxyphenyl)-l-alanine based on acarbonization-extraction strategy.41 The obtained r-CDs mainlyconsisted of carbon cores and exhibited strong blue luminescencewith a PLQY of 6.3% [Fig. 12(a)]. Then, these r-CDs were furtheroxidized to a different kind of CDs (denoted as o-CDs) by HNO3

reflexing, which are with much higher oxidization level relative tor-CDs. Remarkably, o-CDs had a green emission color with a lowQY of 1% [Fig. 12(b)], indicating the surface traps created by the

oxygen-containing loose shell not only lead to a longer wavelengthemission, but also act as a nonradiative recombination center toreduce their PL efficiency. These similar PL changes were alsoreported by Nie’s group.104 They selectively prepared two types ofCDs with different optical properties from chloroform (CHCl3)and diethylamine (DEA) by only varying the reaction conditions.One had respective excitation-independent blue emission with aQY of 17% [Fig. 12(c)], whereas the other possessed distinctiveexcitation-dependent full-color emissions with a QY of 12%[Fig. 12(d)]. After detailed characterization and careful comparisonof the two types of CDs, the newly formed surface functionalgroups, such as CvO and CvN, were confirmed to efficientlyintroduce new energy levels for electron transitions, thus leading tothe continuously adjustable excitation-dependent full-color emis-sions as well as decreased PLQYs. These conclusions were alsoproven by some groups in an opposite way. Zheng et al. found thatafter the surface state was reduced by NaBH4, CDs showeddecreased absorbance intensity at the excitation wavelength, blue-shifted emission, and increased QY [Fig. 12(i)],105 which furtherdemonstrates that when surface defects increase, fluorescence prop-erties of CDs show distinct variations, including the decrease ofQYs and redshift of PL.

Second, the PL spectra of CDs are usually excitation-dependent with a broad full width at half maximum (FWHM) of atleast several dozen up to hundreds of nanometers.106–108 Zhu’sgroup performed a thorough investigation of the surface state ofCDs and found that the certain molecular/chemical groups on CDsurfaces possessed variable channels of radiative energy dissipation,consequently resulting in excitation-dependent and broadenedemission band [Fig. 12(e)].30 Yan’s group synthesized highly lumi-nescent excitation-dependent CDs with three different peaks at375, 450, and 585 nm as altering the excitation wavelength from300 to 500 nm, which are accompanied by broad FWHM valuesover 100 nm nearly covering the entire visible spectra.109 Theyspeculated that multi-component of different surface states consist-ing of increasing oxidation degree and CvN contents could intro-duce more new energy levels to the electronic structures,consequently leading to more electronic transitions and redshiftedemissions. As a proof to the contrary, when these functionalgroups on the surface were eliminated by high-temperature carbon-ization or prolonged UV irradiation, the PL of CDs could resultfrom the intrinsic carbon core state, which exhibited an excitationwavelength-independent emission with a narrowed FWHM of39 nm and improved photostability [Figs. 12(g), 12(h), and 12( j)],further indicating that the unique optical properties of CDs relatedto surface states.110 However, it should be noted that there are alsosome CDs that display excitation-independent emission, which isderived from their surface states. Such a distinct luminescent phe-nomenon can be ascribed to few kinds of surface states of CDsafter careful purification with complicated separation techniquessuch as column chromatography.

At last, the fluorescence intensity and colors of CDs are typi-cally susceptible to the surroundings, such as the pH values, sol-vents, and solid state matrices.111–113 Wu’s group found that the PLintensity of CDs decreases as the pH value decreases in an acidicenvironment, while the PL intensity keeps constant under neutraland alkaline conditions.114 They ascribed the pH-dependent

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FIG. 12. (a) PL spectra of reduced CDs, inset: photograph of CDs solution at 365 nm excitation. (b) PL spectra from the oxidized CDs by HNO3 refluxing, inset: photo-graph of oxidized CDs solution at 365 nm excitation. (c) Fluorescence emission spectra of excitation-independent CDs in ethanol excited by different wavelengths. (d)Fluorescence emission spectra of excitation-dependent CDs in ethanol excited by different wavelengths. (e) PL emission spectra of CDs with a surface state controlledemission. ( f ) Variation of PL intensity of GQDs with BR buffer of different pH values. (g) Normalized fluorescence emission spectra of CDs in selected solvents. (h) PLemission spectra of CDs before and after UV laser irradiation by 450 nm. (i) Graphical representation of the synthesis of reduced CDs with blue luminescence fromoriginal CDs. Inset: photographs of CDs solution (left) and the reduced-CDs solution (right) under UV light. ( j) Schematic presentation of a possible structural changeof CDs before and after UV irradiation (upper panel) and proposed photophysical processes taking place in CDs before (left) and after (right) UV irradiation (lowerpanel). (k) Schematic illustration of the preparation and application of CDs. (Optical photographs: The LEDs are trichromatic WLED and red LED. The solidCDs-matrices hybrids are the CDs in PS, PMMA, and APTES Gel. The CDs solutions are the CDs in dichloromethane, acetone, and water.) (m) Schematic diagram ofthe synthesis process of the CDs. (a) and (b) are reprinted with permission from Xu et al., Chem. Eur. J. 19(20), 6282 (2013). Copyright 2013 Wiley-VCH. (c) and (d)are reprinted with permission from Nie et al., Chem. Mater. 26(10), 3104 (2014). Copyright 2014 American Chemical Society. (e) is reprinted with permission from Zhuet al., Nanoscale 7(17), 7927 (2015). Copyright 2015 Royal Society of Chemistry. ( f ) is reprinted with permission from Wu et al., Nanoscale 6(7), 3868 (2014).Copyright 2014 Royal Society of Chemistry. (g) is reprinted with permission from Sciortino et al., J. Phys. Chem. Lett. 7(17), 3419 (2016). Copyright 2016 AmericanChemical Society. (i) is reprinted with permission from Zheng et al., Chem. Commun. 47(38), 10650 (2011). Copyright 2013 Royal Society of Chemistry. (h) and ( j) arereprinted with permission from Sun et al., J. Phys. Chem. Lett. 10(11), 3094 (2019). Copyright 2019 American Chemical Society. (k) is reprinted with permission fromRen et al., Adv. Opt. Mater. 6(14), 1800115 (2018). Copyright 2018 Wiley-VCH. (m) is reprinted with permission from Wei et al., Adv. Opt. Mater. 8(7), 1901938 (2020).Copyright 2020 Wiley-VCH.

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behavior to changes in the surface state induced by protonation–deprotonation [Fig. 12(f )]. Sciortino et al. explored the interactionsof CDs synthesized from citric acid monohydrate and urea with thesurrounding solvent to elucidate the nature of their solvent-dependent fluorescence.115 It was found that the absorption andemission transitions of CDs couple their cores and surface statesthrough transfer of electronic charge from the crystalline core tosurface moieties and back, directly evidencing the central role ofsurface groups in the emissive transitions like amide and carbox-ylic. Based on the sensibility of the CDs to ambient conditions inthe liquid state, the controlled PL could be realized in the solidstate as well, which conquered the well-known aggregation-inducedPL quenching effect. For instance, Ren and co-workers achieved PLtunable CDs-matrix hybrids from green to red through dispersing atype of CDs in different solid matrices, as shown in Fig. 11(k).116

The increased polarity of polymer matrices was believed to have astronger effect on the surface electronic structures of the CDs,resulting in the decline of energy gaps and the bathochromic shiftof PL and absorption peaks. Likewise, by in situ dispersing the CDinto the crystal matrix of NaOH, Wei’s group obtained green fluo-rescent CD powders with an unprecedented efficiency of 76%[Fig. 12(m)],117 which is about 472 times higher than that of CDssynthesized without adding NaOH. The added NaOH could notonly break down the CD clusters into small individual unitsthrough the amide hydrolysis of ester bonds, but also made CDscreated with improved crystallinity. Therefore, the CDs with largerspatial distances and lower defect content could avoid the reso-nance energy transfer of each other and interaction with defectcenters, forming high radiative recombination pathways in the solidstate. These obtained highly luminescent solid state CDs hold greatpotential for optoelectronic applications.

V. POTENTIAL APPROACHES OF CHARACTERIZINGSURFACE STATE OF CDs

As many reports declared, surface state has been considered asone of the main PL origins of CDs. However, due to the complexityin structure and chemistry of CDs, it remains a great challenge toaccurately characterize surface states of CDs. Herein, we attempt toput up with some points of view on how to understand the surfacestates of CDs based on the currently available research findings andcharacterization techniques.

First of all, as-prepared CDs, especially those synthesized via abottom-up route, should be well purified before various spectral andstructural characterizations using various separation techniques likehigh performance liquid chromatography (HPLC) or chromato-graphic column.118–120 This is because the resulting samples arealways a mixture of CDs, and many luminescent centers associatedwith the molecular fluorophores, surface states, and intrinsic corestates contribute together into the overall emission signal of CDs.Zhu’s group reported the first one-pot hydrothermal synthesis ofmulti-emission fluorescent CDs utilizing 2,4-diaminotoluene, EDA,and phosphoric acid,121 which exhibited three fluorescent emissionsat 370, 425, and 505 nm under a single-wavelength UV light of320 nm. After separation with silica chromatographic column, theycollected three CD samples with ultraviolet, blue, and green fluores-cence, respectively. By means of time-resolved emission spectra, they

identified that ultraviolet emission is originated from energy gapstate of CDs, blue emission from the surface defect state, and greenemission from the molecular state [Fig. 13(a)]. Thus, to understandthe mechanisms of their fluorescence emissions, the prerequisite is tochoose CDs with uniform structural and optical properties (i.e.,narrow and excitation-independent luminescence characteristics) asthe subject for study.

Second, some advanced characterization techniques can beapplied to obtain more and deep information about the surfacestate of CDs. According to the literature, the chemical compositionof the surface layer and valence states of atoms on the surface alongwith the charge transfer on surface adsorbents play an importantrole in determining the luminescence properties of CDs related tothe surface state.7 Thus, it is quite useful and imperative to revealconcrete structures and compositions of the surface state of CDsusing a series of advanced spectroscopic techniques such as theauger electron spectrum, x-ray photoelectron spectroscopy(XPS), and the ultraviolet photoelectron spectrometer [Figs. 13(b)and 13(c)].122–124 If necessary, prior to XPS measurements, theresulting CDs can be bombarded by argon ions for a few secondsto investigate their inner structure [Fig. 13(d)].125 Meanwhile, forluminescence dynamics study, time correlated single-photon count-ing, fluorescence up-conversion, and transient pump probe absorp-tion spectroscopy need be used to elucidate the excited-statedynamics of CDs.126 The former is a powerful tool to study com-peting deactivation pathways within the CD particles, while thelatter two ones are conducted to probe the information about thestructure of the excited state energy level and the change of photo-generated carrier populations in corresponding energy levels. As aresult, making smart use of these characterization techniques ispivotal for building the structure–property relationships of CDsand making clear the PL origins of CDs.

Last but not least, in addition to typical assemble measure-ments on CD, a comprehensive study of single CDs can providefundamentally new insight into the PL mechanisms of CDs. Ghoshet al. used a home-built equipment to explore the fundamentaloptical and structural properties of single CDs [Fig. 13(e)],127

which combines various spectroscopic methods with high resolu-tion transmission electron microscopy and with atomic forcemicroscopy. Importantly, they found that photoluminescent CDsbehave as electric dipoles both in absorption and in emission, andthat their emission is originated from the recombination of photo-generated charges on defect centers that involve a strong couplingbetween the electronic transition and collective vibrations of thelattice structure. Such a conclusion is very meaningful for elucidat-ing the PL mechanism of CDs. In our opinions, if this techniquewas combined with other structural and componential spectro-scopic methods, such as in suite Fourier transform infrared spec-troscopy, some new insight beyond thought could be obtained todeepen the understanding of the surface state and their influenceson optical properties.

VI. THEORETICAL STUDY ON SURFACESTATE-DERIVED LUMINESCENCE

The PL mechanisms of CDs remain controversial for manyyears, because both the compositions and structures of CDs are

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complicated and there exist different types of CDs. In fact, theproducts are mostly mixtures of various CDs, with different sizesand surface states. Nevertheless, theoretical modeling and calcula-tions help us deeply understand the mechanisms behind the elec-tronic structures and optical properties of CDs.128 In detail, theinfluences of structural components of CDs on surface state-derived luminescence as revealed by the quantum chemicalapproaches can be ascribed to three aspects, which are used tointerpret PL emission redshift, enhancement, and peak widthbroadening.129

In order to elucidate the mechanisms about tunable PL emis-sions, many factors, including particle sizes, surface functionalgroups, and heteroatom doping, have been modeled by DFT andtime-dependent DFT calculations. For instance, to investigate size-

dependent PL of GQDs, Sk’s group calculated the emission wave-lengths of pristine GQDs with different diameters.128 As shown inFig. 14(a), upon increasing size from 0.46 to 2.31 nm, the PL emis-sion of GQDs can be sensitively tuned from deep blue to NIR, cov-ering the entire visible light region. The distinct PL emissionredshift with increasing size is ascribed to the decrease in energygap resulting from π electron delocalization within the sp2 domain.Meanwhile, the authors predicted the emission wavelength of oxi-dized GQDs as a function of the coverage of –OH and/or –COOHgroups. The results exhibited that both –OH and –COOH groupsattached to edge carbon atoms continuously change the PL emis-sion of GQDs from green to red, which is based on energy gapreduction in a coverage dependent manner. In addition, Liu’sgroup performed DFT calculations to deeply probe the influence of

FIG. 13. (a) Schematic illustration of the preparation and PL emission mechanism of CDs with multicolor emission. (b) Schematic diagram of XPS characterization of theCDs without bombardment by argon. (c) Schematic illustration of different XAS detection schemes: (upper) TEY detection on solid materials in vacuum; (middle) puretransmission and (lower) TIY detection in aqueous CD dispersion. (d) Schematic diagram of XPS characterization of the CDs bombarded by argon ions for 6 s. (e)Schematic illustration of PL measurement and structure characterization of single CDs. (a) is reprinted with permission from Zhu et al. Chem. Mater. 31(13), 4732 (2019).Copyright 2019 American Chemical Society. (b) and (d) are reprinted with permission from Song et al., Nano Lett. 19(8), 5553 (2019). Copyright 2019 American ChemicalSociety. (c) is reprinted with permission from Ren et al., J. Phys. Chem. Lett. 10(14), 3843 (2019). Copyright 2019 American Chemical Society. (e) is reprinted with permis-sion from Ghosh et al., Nano Lett. 14(10), 5656 (2014). Copyright 2014 American Chemical Society.

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graphitic N on the fluorescence redshift of CDs.130 In this study,four models with the same particle size but different graphite Ncontent were established, which were named GN0, GN1, GN2, andGN3, respectively. Importantly, as the amount of graphite Nincreases from GN0 to GN3, these CD models exhibited redshiftedabsorption wavelength and narrowed gaps between the HOMO andLUMO [Figs. 14(b) and 14(c)], suggesting that the graphitic nitro-gen doping could create middle energy levels within the originalgap of the undoped CDs and thus lead to the pronounced fluores-cence redshift. Besides these examples above, the calculated PL red-shift was also demonstrated in CDs modified with functional

groups (amino, epoxy, and carbonyl) or ones doped with heteroat-oms (B).56,63,90,91

A theoretical study was also carried out to investigate themechanism of PL emission enhancement induced by heteroatomdoping or surface passivation. For instance, to investigate the influ-ences of N and S on optical properties of CDs, Zhang et al. com-puted the electronic structures of bare CDs, S-doped CDs, and N,S-co-doped CDs using the DFT/Perdew–Burke–Ernzerh (PBE) oflevel of theory.81 It can been seen that S or N, S-dopants lead tomore electronic states, and the creation of in-gap trap states thatcould act as nonradiative recombination centers, which accounts

FIG. 14. (a) Calculated emission wavelength (nm) of GQDs as a function of size. (b) Calculated UV-vis absorption spectra and (c) HOMO–LUMO gaps for the N-dopedmodels (GN1, GN2, and GN3) and N-free system GN0. (d) The model structures of CQDs-0, CQDs-NH2, -NMe2, -NEt2, and -NPr2, respectively (from left to right); bottomrows of (d) are corresponding calculated oscillator strength (f ). (e) Calculated absorption spectra and (f ) HOMO and LUMO energy levels of these GQD oxides; each isfunctionalized with two –OH, –OCH3, –COOH, or –COCH3 groups. (g) Structural optimized model for defective GQDs (dGQDs) and oxidized-GQDs (oGQDs). (a) isreprinted with permission from Sk et al., J. Mater. Chem. C 2(34), 6954 (2014). Copyright 2014 Royal Society of Chemistry. (b) and (c) are reprinted with permission fromLiu et al., Chem. Commun. 56(29), 4074 (2020). Copyright 2020 Royal Society of Chemistry. (d) is reprinted with permission from Jia et al., Adv. Sci. 6, 1900397 (2019).Copyright 2019 Wiley-VCH. (e) and (f ) are reprinted with permission from Chen et al., J. Mater. Chem. C 6(25), 6875 (2018). Copyright 2018 Royal Society of Chemistry.(g) is reprinted with permission from Wang et al., Nanoscale 8(14), 7449 (2016). Copyright 2016 Royal Society of Chemistry.

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for remarkable PL enhancement trend observed in heteroatomdoped CDs. For surface passivation-enhanced PL emission mecha-nism, Jia’s group calculated oscillator strength (f ) of another fourkinds of CQDs passivated with amino (NH2), N,N-dimethyl(NMe2), N,N-diethyl (NEt2), and N,N-dipropyl (NPr2), respectively,based on a simple CQDs-0 model consisting of the identical skele-ton structure without any edges.131 The results in Fig. 14(d) showthat oscillator strength significantly increases from 0.446 forCQDs-NH2 to 1.271 for CQDs-NPr2, which is in good agreementwith the QY evolution from 26% to 86%. Moreover, they analyzedthe charge transfer (CT) excited state characters of these CQDs byexamining the degree of spatial separation of the electron clouddensity distributions in the frontier orbitals. It is found that, fromCQDs-0 to -NPr2, the HOMOs become more and more distributedover the passivating edges owing to the increased electron donatingability from passivated functional groups, while the LUMOsremain distributed over the conjugated π-surface, suggesting thepresence of CT state that is beneficial to improve the QY. Hence,they concluded that -NR2 passivation plays a key role in leading tothe increased oscillator strength and present CT excited state,thereby affording high QY of these CQDs.

In addition, the mechanism of broad absorption and emissionlinewidth commonly existing in CDs is studied theoretically. Byselecting a circumcoronene as a GQD model, Chen’s group theoret-ically investigated the effects of the kinds, quantities, and bindingpositions of surface oxygenic functional groups on the opticalproperties of GQDs, as shown in Figs. 14(e) and 14(f ).132 Theyfound that, upon chemisorption, more electronic transitions inGQDs are activated as a result of changing the optical selectionrules, which are responsible for the appearance of new absorptionand emission peaks in the low energy region. Furthermore, theoptical gaps of these GQD models vary significantly from eachother even when they are functionalized with the same number ofoxygen-containing groups, which indicates that binding positionsof these functional groups have a crucial and distinct influence ontheir optical properties. Judging from these DFT results, it is rea-sonable to ascribe both the chemical compositions and bindingpositions to the broad absorption and emission character of CDs.Such a conclusion was also confirmed by Wang’s group.133 UsingDFT and Maximal Localized Wannier Function (MLWF) calcula-tions, they systematically probed the effects of the functionalgroups and disordered structures (i.e., defects and sp3 carbon) onthe electronic structure of GQDs. The results in Fig. 14(g) indicatedthat not only surface functional groups but also disorder structuresare able to induce structural deformation to the aromatic core,leading to irregular hybridization of the π system providingmid-gap states, allowing π*→mid-gap states→ π transitions,which contribute to bathochromic broadening of the integrated PLspectra. Overall, the theoretical study not only provides an explana-tion to observed experimental data but also affords a valuable refer-ence for controllable synthesis and engineering of CDs.

VII. CONCLUSIONS AND REMARK

In this review, we introduced in depth and in detail thecurrent reports on CDs with surface state controlled emission,including their influencing factors, optical characteristics, and

potential characterizing approaches. Although the surface state isreported as main explanations of the PL mechanism, the relatedexploration remains in infancy, which heavily hinders the develop-ment of CDs in mechanistic and practical studies. On one hand,the structure determines performance. But for now, the knowledgeof the surface state is inadequate, especially what the surface stateof CDs is, and what the relationship is between the surface stateand their optical properties. Thus, it is urgent for researchers tomake clear these mysteries, which are useful for guiding the direc-tional synthesis of CDs with desired optical properties. Besides,simulating the possible formation process and inferring reasonablereaction mechanism are beneficial to understand the surface stateof CDs and their influences on PL emission. Therefore, moreadvanced technique to observe the complicated formation processof CDs at an atomic level and characterize their structural compo-nents in situ is highly required. On the other hand, CDs are highlyconsidered as competitive candidates to QDs and organic dyes inthe field of bioimaging, theranostics, and light emitting devices, butthe absorption and emission wavelengths of most reported CDs aremainly located in the blue and green regions, and the FWHM ofCDs is typically more than 100 nm, which greatly restricts theirbroad applications due to the low background-probe fluorescencecontrast and the low color saturation. Thus, it is highly desirable tosynthesize highly efficient emitting CD with absorption/emissionin the NIR region as well as a FWHM as narrow as possible.Engineering surface state may be an efficient way to overcome thisproblem because many reports considered that both the emissionwavelength and FWHM are highly associated with the surfacestates of CDs. With such effort, we expect that the study of PLmechanisms of CDs, especially the surface state, will open new per-spectives toward the designing and synthesis of CDs for practicalapplications.

ACKNOWLEDGMENTS

This work was financially supported by the FundamentalResearch Funds for the Central Universities (No. 2017QNA08).

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J. Appl. Phys. 127, 231101 (2020); doi: 10.1063/1.5143819 127, 231101-21

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