Photoluminescent Carbon Nanostructures - Chemistry of Materials

Review pubs.acs.org/cm

Photoluminescent Carbon Nanostructures Ondřej Kozák,† Mária Sudolská,† Goutam Pramanik,‡ Petr Cígler,*,‡ Michal Otyepka,† and Radek Zbořil*,† †

Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University in Olomouc, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic ‡ Laboratory of Synthetic Nanochemistry, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v.v.i., Flemingovo nám. 2, 166 10 Prague 6, Czech Republic ABSTRACT: Photoluminescent nanosized allotropes of carbon have attracted considerable interest because of their diverse optical properties depending on their crystal structure, size, and morphology, and chemical functionalization. Here, we present the first critical review covering the photoluminescence (PL) properties, their control, and origin in various carbon allotropes and their composites. Different mechanisms by which carbon nanostructures exhibit PL are discussed, involving excitonic PL in carbon nanotubes, thermally activated delayed fluorescence in spherical fullerenes, the presence of impurity−vacancy color centers in nanodiamonds, aromatic sp2 domains in reduced graphene oxide, and surface chromophores or defect-related PL in carbon dots. We critically analyze the intrinsic and external effects affecting the PL properties (spectral shift, decay, quantum yield) from both experimental data and theoretical calculations. The key parameters addressed include, for example, the type and content of impurity elements in nanodiamonds (NV and SiV centers), chemical composition in reduced graphene oxides, external effects (temperature, solvent) in C60 fullerene, structural type (single-wall versus multi-wall carbon nanotubes), and the roles of doping and surface functional groups in the PL behavior of carbon/ graphene dots.

1. INTRODUCTION Carbon is one of the most important chemical elements. As the main building block of organic compounds, it is essential for all living organisms.1 In addition, carbon and its compounds are vital in the energy and chemical industries, and are therefore essential for the functioning of modern societies.2 Accordingly, the Web of Science database shows that carbon is the most extensively studied of the elements. Since the 1980s, research activities in several fields have focused on nanoscale structures and phenomena, revealing many previously unforeseen possibilities. Studies on the chemistry of carbon led to the discovery of some of the first well-characterized nanostructures, and the discovery of the nanosized carbon allotropes known as the spherical fullerenes,3 together with progress in microscopy techniques,4 helped spur the nanotechnology boom. Nanocarbon materials exhibit several unique physicochemical properties. Here we focus on their optical characteristics and particularly their photoluminescence (PL), which is closely related to their electronic properties. The two best-known crystalline allotropes of carbon are graphite and diamond, which are made up of sp2- and sp3hybridized carbon atoms, respectively.1 Bulk graphite and diamond can both be considered to consist of effectively infinite carbon networks, and they do not exhibit PL. However, if the symmetry and/or size of these networks is reduced, the resulting structural rearrangements usually change the materi© 2016 American Chemical Society

al’s electronic properties and thus its optical behavior. This Review is concerned with the PL of carbon allotropes that are like diamond and graphite in that they consist of networks of sp2- or sp3-hybridized carbon atoms but unlike diamond and graphite in that the networks are finite and only extend over a nanoscale distance in at least one dimension. For convenience, we refer to such allotropes as nanosized carbon allotropes (NCAs). Table 1 lists the NCAs that have attracted the greatest interest from the scientific community and specifies their dimensionality. Photoluminescence is the emission of light from electronically excited states that occurs spontaneously after the system absorbs light. The state-of-the-art techniques allow researchers to non-destructively examine various PL characteristics of a matter. The process of PL can be basically described in terms of its spectral positioning, dynamics, and efficiency. Steady-state, time-resolved, and quantum yield (QY) measurements respectively are used to gain the relevant information. In steady-state arrangement, the sample is continuously excited, and two basic types of spectra are obtainedemission and excitationdepending on which monochromator is scanning. The spectra provide information about the energy difference Received: April 6, 2016 Revised: May 28, 2016 Published: May 29, 2016 4085

DOI: 10.1021/acs.chemmater.6b01372 Chem. Mater. 2016, 28, 4085−4128

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Chemistry of Materials

refluxing in a strongly acidic environment.8 Bottom-up methods include chemical vapor deposition (CVD)9 and various chemical procedures in which carbon-rich precursors undergo heat treatment or severe dehydration.10 Specific NCAs with particular properties can be obtained by carefully applying one or more of these methods, together with appropriate postpreparative treatments and functionalization steps. Photoluminescent NCAs with different architectures, sizes, morphologies, and structures will naturally also have different PL properties in terms of spectral shapes, PL decay, and QY. While some NCAs exhibit narrow emission bands in the red− near-infrared (NIR) range (e.g., carbon nanotubes, or CNTs),11 others emit mostly blue or green light and have a full width at half-maximum intensity (fwhm) in excess of 100 nm (CDs).12 The lifetimes of NCAs are typically in the picosecond−nanosecond range, but their QYs vary widely, from the rather inefficient spherical fullerenes13 to ultrabright CDs with QYs above 80%.14 However, there are some optical properties that are common to several structurally different members of this large family of materials, demonstrating that they are quite similar in certain respects. This is particularly true for the NCAs described in next four sections, namely nanotubes, spherical fullerenes, graphene and its derivatives, and CDs. All of these materials consist primarily of sp2hybridized carbon atoms and thus possess π-electrons that can undergo optical transitions in the UV−vis−NIR region. As noted above, reducing the dimensions and/or symmetry of a bulk material will typically change its optical properties.15 In the case of photoluminescent NCAs, such symmetry reductions can be achieved by reducing the dimensionality of the bulk material. Alternatively, they can be induced by introducing structural disorders, for instance by functionalization to create regions of sp3-hybridized carbon centers surrounding embedded aromatic domains. This produces band gaps, making the material photoluminescent.16−18 In addition to the effects of structural disorder, the functional groups themselves can influence the mechanism of PL directly, as will be shown several times throughout this Review. The mechanism of PL in NDs is fundamentally different because NDs consist almost exclusively of sp3-hybridized carbon atoms. Their PL is due to the presence of luminescent centers, which are typically impurity−vacancy pairs.19 In more general terms, there are five known mechanisms by which nanocarbon structures can exhibit PL. Some of these mechanisms appear to be specific to particular nanostructure classes such as CNTs (excitonic PL),20 spherical fullerenes (thermally activated delayed fluorescence, or TADF),21 and nanodiamonds (PL arising from impurity−vacancy centers).19 In addition, several classes of NCAs owe their PL to the presence of aromatic sp2 clusters or surface defects. Figure 1 shows selected classes of photoluminescent NCAs and the mechanisms that have been invoked in the literature to explain their PL. Large libraries of materials can be prepared by functionalizing NCAs with heteroatoms or functional groups. The most interesting such functionalized derivatives that have been reported to date are also covered in this Review. The ease with which NCAs can be functionalized and linked to other moieties or NCAs is also important in their practical applications. Because of their high brightness and biocompatibility, the most important potential applications in NCAs are likely to be in biological and medical tools such as imaging agents or sensing probes.11,22−27

Table 1. Overview of the Most Common Nanosized Carbon Allotropes dimensionalitya

nanosized carbon allotrope

3D/quasi-2D 3D/quasi-1D 3D/quasi-0D 2D 2D/quasi-1D 2D/quasi-0D

multi-layer graphene nanotube, nanowire, nanohorn fullerene, nanocage, multi-layer graphene dot, nanodiamond graphene, graphyne,b graphdiyne nanoribbon (nanobelt) graphene dot

a

Here, a structure that only extends across the diameter of a single atom in a given dimension is not considered to have any presence in that dimension. bGraphyne has not yet been prepared and has therefore only been studied theoretically.

between the excited and ground states and usually serve as the first indicator of a material’s applicability. Time-resolved measurements are carried out under pulsed excitation, and the decay is recorded after the excitation pulse terminates. The obtained PL lifetime characteristics are generally indispensable for rigorous discussion on the PL mechanism and for the description of important PL-related phenomena such as quenching. The efficiency of the PL process is commonly described by the QYa ratio between the numbers of emitted and absorbed photons. Besides the possibility of comparing emission intensity with standards of known QY, absolute QY determination is possible when the sample is placed inside an integration sphere. In principle, all emitted light is then brought to the detector. QY is an important PL characteristic for assessing a material’s applicability.5 We do not intend to review the optical properties of all the NCAs listed in Table 1, because some of them do not emit light, and the optical properties of some others have not yet been adequately described in the literature. The materials selected for inclusion in this Review have two unifying characteristics: high carbon content and interesting optical propertiesspecifically, PL. This means that the Review does not exclusively focus on pure forms of carbon; heteroatomcontaining derivatives of NCAs that exhibit PL are also covered, including nitrogen vacancy-containing nanodiamonds (NDs), graphene derivatives such as graphene oxide (GO), reduced graphene oxide (rGO), and halogenated graphenes (GF, GCl), and carbon dots (CDs), which often contain oxygen and nitrogen functionalities as well as hydrogen atoms. There is a degree of terminological ambiguity relating to the “quasi-0D” NCAs, which have been referred to as “nano-graphene”, “nanographene oxide”, “graphene (nano)dots”, “carbon dots”, and so on. However, these materials are quite similar to one another in many respects and are therefore covered together in a single section; for convenience, they are collectively referred to as “carbon dots”. Each section of this Review focuses on a particular class of NCAs and their derivatives, detailing (i) their general physicochemical properties, (ii) methods for their preparation, (iii) their PL properties including their typical spectral, decay, and QY characteristics as well as the origin of their PL, (iv) their functionalization and the potential for tuning their PL, and (v) their applications. Various techniques have been developed for preparing NCAs. As with other nanomaterials, these methods can be divided into two basic categories: bottom-up and top-down. Top-down routes involve physical or chemical disruption of a bulk carbon precursor, typically graphite or amorphous carbon. This may be achieved by laser ablation,6 arc discharge,7 or 4086


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(classified in terms of two indices, n and m), which unambiguously describe the orientation of the planar graphene-like lattice relative to the tube’s central axis30 (see Figure 2a).

Figure 1. Five known mechanisms of photoluminescence in nanosized carbon allotropes, and representative allotropes in which each mechanism occurs. Note that the mechanistic assignments are not exclusive; some mechanisms, particularly those based on aromatic domains and surface groups/defects, may occur in more NCA classes than are shown here.

This Review focuses on NCAs’ PL characteristics and mechanisms, and the relationships of these characteristics and mechanisms to the NCAs’ structure, functionalization, and other physicochemical properties. It presents state-of-the-art knowledge based on the most recent scientific results available in the literature and emphasizes the common aspects of the mechanisms of PL in different types of NCAs. We hope that it will be a valuable introduction to the topic of PL in carbonbased materials for students and researchers new to the field, while also providing a comprehensive overview of the literature on the most important nanosized photoluminescent carbon allotropes.

Figure 2. (a) Relationship between the structure of a single-walled carbon nanotube and the direction in which the parent graphene sheet is rolled up relative to the tube’s central axis (denoted here as the “SWCNT axis”, black line). The roll-up vector that is perpendicular to the SWCNT axis connects the position (0,0) and the position whose index values define the chirality of the SWCNT [(9,4) in this case]. Reprinted with permission from ref 47. Copyright 2010 Springer. (b) A photoluminescence excitation−emission map of semiconducting SWCNTs. SWCNTs with different chiralities emit at different wavelengths in response to different excitations. Reprinted with permission from ref 11. Copyright 2009 Springer.

2. CARBON NANOTUBES Carbon nanotubes (CNTs), also known as cylindrical fullerenes or buckytubes, were first discovered in 1991,28 and they have been the most extensively studied NCAs since then. It is a quasi-1D structure consisting of graphite-like hexagonal network in a form of hollow cylinder or multiple concentric hollow cylinders. Its diameter ranges from a few nanometers to a few tens of nanometers, depending on the number of concentric layers. We distinguish between single- and multiwalled CNTs (SWCNTs and MWCNTs, respectively). In MWCNTs, the coaxial graphene cylinders are separated by approximately the graphite interplanar distance (0.34 nm).29 CNTs have extraordinary mechanical, electrical, and thermal properties and also exhibit remarkable optical characteristics because their excitation can lead to the formation of a strongly bound and luminescent exciton.30 CNTs are structurally related to other forms of carbon such as graphite (3D) and its 2D constituent material graphene. Although all CNTs consist of well-ordered sp2-hybridized carbon atoms, their properties vary widely depending on their chirality, i.e., the orientation of the carbon lattice relative to the tube’s longitudinal axis. Importantly, the chirality of a CNT determine whether it behaves as a semiconductor or a metal.30,31 Separation and coating protocols have been developed that make it possible to study individual CNTs and determine their chiral vectors

There are two widely used approaches for preparing CNTs.32 Arc-discharge33 and laser ablation34 techniques involve the evaporation of carbon atoms from a solid source and their subsequent condensation in the form of SWCNTs or MWCNTs. On the other hand, CVD35 involves passing a flow of gaseous hydrocarbons through a tube furnace containing a heated catalyst over which CNTs are grown. Although both approaches have been developed extensively over the past two decades, several issues relating to their implementation remain to be addressed. In particular, methods for the large-scale production of high-quality CNTs with controllable chirality are needed to unlock the full practical potential of these materials.31 The fundamental impossibility of preparing uniform nanotubes also necessitates the use of postsynthesis separation techniques such as chromatography, dielectrophoresis, or ultracentrifugation.36 CNTs were first reported to exhibit PL by Riggs et al. in 2000.37 The origins and mechanisms of PL in CNTs are 4087


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treatment.69 Remarkably, Akizuki et al.70 have recently shown that individual SWCNTs with accidentally or intentionally embedded localized states exhibit unique excited-state dynamics that can result in efficient one-phonon-assisted upconversion PL. Although interest in MWCNTs waned somewhat following the first preparation of SWCNTs in 1993,71,72 MWCNTs are complementary materials to SWCNTs in many scientific and practical respects.31 MWCNTs consist of two or more concentric SWCNTs, which makes them more mechanically stable (see Figure 3a). This could be beneficial in certain

complex because nanotubes can have a wide range of geometrical and chemical arrangements, which affect their PL properties. In addition, their PL is very sensitive to extrinsic effects arising from their environment and bundling. The fluorescence may be intrinsic to the CNTs themselves or attributable to some other fluorophore that is somehow linked to the CNTs. However, this Review only deals with intrinsic PL. CNTs only exhibit strong PL in solution; in the solid state, their bundling completely suppresses their emissions. The solubilization/individualization of CNTs can be induced by techniques such as ultrasonic agitation in a solution of an amphiphile (noncovalent functionalization),38,39 oxidation,40,41 or some other form of covalent functionalization.42−44 The latter two approaches typically also changes the density of electronic states in the nanotubes, which may cause the absorption and emission spectra of the functionalized derivatives to be shifted relative to those of the nonfunctionalized parent CNTs. As noted above, the properties of CNTs are largely determined by their chirality. Non-functionalized semiconducting SWCNTs (s-SWCNTs) fluoresce, but metallic SWCNTs (m-SWCNTs) do not, and in fact they quench PL from adjacent s-SWCNTs in CNT bundles. Unlike bulk direct-gap materials, in which photon absorption usually leads to interband transitions, excited electrons in a CNT are confined in a quasi-1D system, leading to the formation of strongly bound and highly mobile excitons. The binding energies of these electron−hole (e-h) pairs depend on the nanotube’s diameter and chirality as well as the dielectric constant of the surroundings, but are typically in the range of 0.2−0.5 eV.45 The PL of individualized SWCNTs exhibits monoexponential decay kinetics, confirming its excitonic origin.20 The band gap of an s-SWCNT is typically on the order of ∼1 eV, and the radiative recombination of excitons across a band gap of this magnitude causes the emission of a photon in the NIR range.46 The exact emission wavelength depends on the chirality of the SWCNT, as shown in Figure 2b. Excitonic PL from SWCNTs also depends strongly on their diameter in a rather complex manner. In general, the diameter affects the exciton−phonon interactions, the exciton dephasing time, and the separation energy between bright and dark excitons.48 The exciton dynamics in individual s-SWCNTs appear to be strongly influenced by defect-related trap states, resulting in a wide range of observed decay times (20−200 ps).20 The PL efficiency of SWCNTs measured in ensembles is usually quite low (typically on the order of 0.01−1%)46,49−51 and depends strongly on the environmental conditions. If the SWCNT is well separated from other quenching tubes and oxygen, its QY may approach 20%.52 The QY is limited by the presence of “dark” states existing below the lowest bright state.53−56 Depending on the SWCNT’s quality, non-radiative recombination of excitons at defect sites can cause substantial further reductions in its PL efficiency.57−60 However, it was recently found that intentionally introducing a rather small number of defects can strongly brighten the PL of SWCNTs by creating an emissive deep trap below the dark state.61−63 Such defects can be introduced via covalent functionalization (doping) with oxygen,64,65 diazonium,66,67 or hexanoic acid,68 which changes the hybridization of the functionalized carbon atoms from sp2 to sp3 and thus disrupts the symmetry of the SWCNT lattice, changing its electronic structure.61 In addition, covalently functionalized SWCNTs (notably, dialkylated SWCNTs) may exhibit new PL peaks as a result of thermal

Figure 3. (a) High-resolution transmission electron microscopy image of a small MWCNT (scale bar 1.9 nm). (b) Schematic depiction of intershell energy transfer within a MWCNT resulting in PL quenching. Reprinted with permission from ref 74. Copyright 2013 Springer.

applications such as the construction of tips for scanning tunneling microscopes.73 Their PL properties are usually very different to those of SWCNTs. In principle, it could be difficult to detect any emission from MWCNTs because any metallic CNT within the overall structure would quench its emissions very effectively47,74 as depicted in Figure 3b. It was also shown that the inner shell in double-walled CNTs (DWCNTs) exhibits excitonic PL only when its diameter falls within a very narrow interval around 0.93 nm.75 However, MWCNTs can achieve PL in the UV−vis range via a different mechanism, as shown in Figure 4a: oxidation,40,41 thiolation,44 halogenation,16 or amination/amidation43 of the outer shells of short MWCNTs yield strongly fluorescent MWCNTs with QYs of up to 25%.43 Importantly, visible (VIS) PL may also be observed in similarly functionalized SWCNTs.42,76,77 The mechanism of VIS PL from CNTs is the subject of ongoing debate. Two different structural features that could account for its occurrence have been identified: (i) defects and functional groups that act as luminescent trapping sites for the excitation energy,41,42,44,76,78,79 and (ii) sp2 carbon clusters embedded in an sp3 carbon matrix, which were previously suggested to be responsible for the PL of GO by Eda et al.18 The latter hypothesis was strongly supported by several studies conducted on GO in the 1990s.80−83 Both possibilities were discussed by Riggs et al. in their pioneering report on PL in CNTs37 and recently evaluated by Qian and co-workers both experimentally and computationally.16,40 Density functional 4088


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multiplicity and sharpness of the peaks. These authors’ results also showed that structural modifications which changed the numbers of defects and functional groups in the MWCNTs barely affected their PL properties, suggesting that their emission cannot be attributed to defects or functionalization. Instead, it was suggested that their multiple sharp emission bands might be due to interband transitions between van Hove singularities in the density of states. Much scientific research is conducted with the aim of developing materials and methods with practical applications. CNTs have several interesting properties that give them numerous potential applications in nanomedicine,84 optoelectronics,85 energy storage,86 and other fields.31 The most important potential applications of CNTs that take advantage of their PL are in imaging23,87,88 and sensing.22,89 As mentioned above, non-functionalized and individualized s-SWCNTs with small band gaps of ∼1 eV exhibit PL in the NIR range (800− 2000 nm). They thus emit within the transparency window of biological tissues, making them suitable for biological imaging.11 Numerous research groups have developed in vitro and ex vivo imaging methods that exploit this phenomenon.90−93 However, there are several factors that have limited the use of CNTs for biological imaging in vivo. Extrinsic problems such as inter-bundle quenching mean that as-prepared SWCNTs typically cannot reach a QY of PL above 10−2, which are needed to achieve adequate optical contrast.51,91 Moreover, non-functionalized (hydrophobic) CNTs exhibit appreciable toxicity.94,95 While functionalized CNTs are relatively non-toxic,96 covalent functionalization often shifts their emission deeper into the VIS region (i.e., beyond the biological window) because it changes the mechanism of PL as discussed above. Both the QY and toxicity issues can be addressed via non-covalent functionalization, whereby CNTs are coated with robust amphiphilic, biocompatible, and stable agents such as surfactants and polymers.11,93 In vivo imaging has been successfully performed using non-covalently functionalized SWCNTs coated with agents including PEGylated phospholipids,97 bovine serum albumin,98 1,2-distearoyl-snglycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)5000] (DSPE-mPEG),99 and poly(maleic anhydride-alt1-octadecene)-poly(ethylene glycol) methyl ether (C18-PMHmPEG).100,101 The applications of s-SWCNTs in biosensing generally exploit the high environmental sensitivity of their NIR PL.102,103 Because of the low tissue transparency of ultraviolet (UV) and VIS light, CNTs with PL in the UV−vis range (due to introduced defects or covalent functionalization) are not very useful in biological applications. They do, however, have potential uses in chemical and environmental sensing.43,104−106

Figure 4. (a) Photoluminescence emission spectra of thiol-functionalized carbon nanotubes in DMF. The spectra were acquired by exciting samples at 400, 440, 480, and 520 nm. Inset: optical photograph obtained using 5 mW laser excitation at 514.5 nm with a sapphire filter placed in front of the camera objective. Reprinted from ref 44. (b) The calculated excitation and emission maximum wavelengths of model compounds with different numbers of aromatic rings (N = 1−10, 19, 37). Reprinted with permission from ref 16. Copyright 2012 Royal Society of Chemistry.

theory (DFT) calculations showed that even a low number of sp2 carbon clusters in a model (0,6) SWCNT could shift the band gap toward the VIS range, whereas a comparable number of defects in the same SWCNT model only shifted the band gap slightly such that it remained in the NIR range.40 Additional calculations suggested that the VIS PL in functionalized CNTs could originate from clusters containing 9−37 fused aromatic rings16 (see Figure 4b). However, both defect sites and sp 2 carbon clusters can be formed during functionalization, and it is not possible to confidently assign the observed VIS PL to either of them alone due to the lack of unambiguous experimental data. It has thus been suggested that the coexistence of defect sites and sp2 carbon clusters in SWCNTs could expand the band gap into the UV−vis region. In contrast to excitonic PL in the NIR region, which has subnanosecond decay times, the lifetimes of excited states in the confined domains of functionalized CNTs are typically on the order of a few nanoseconds. Interestingly, Kim et al.74 observed emission from MWCNTs in the 400−600 nm range that resembled the NIR emission from SWCNTs in terms of the

3. SPHERICAL FULLERENES (C60, C70) Spherical fullerenes, also known as buckyballs, were the first NCAs to be predicted, discovered, and studied.3,107−109 These soccer ball-shaped molecules typically consist of 12 pentagonal rings of carbon atoms together with various numbers of hexagonal rings that can be arranged in different ways. The most stable species in this class are C60 (Buckminsterfullerene) and C70.3 Spherical fullerenes are unique among the allotropes of carbon because of their closed formthe molecules are edgeless and chargeless, and have no dangling bonds or unpaired electrons.110 Their optical properties are related to the number of carbon atoms in the molecule: as this number increases, the edge of the molecule’s optical absorption shifts toward lower energies. 111 Consequently, the allotrope 4089


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Chemistry of Materials representing the terminal stage of this increase in size grapheneappears to be black. Spherical fullerenes are typically produced by condensation from carbon plasma. Carbon may be vaporized by laser ablation,3 arc discharge,112 or using various plasma torches in the arc-jet plasma method.113−115 It has also been reported that small quantities of fullerenes can be produced by nonequilibrium plasma methods employing microwaves,116 lowpressure glow discharge,117 or atmospheric-pressure Ar−He mixed-gas plasmas.118 Spherical fullerenes exhibit strongly environment-dependent PL. The lowest energy singlet electronic excited state of C60 lies about 1.7 eV above the ground state, so its relaxation is accompanied by emission in the VIS-NIR boundary region,119 with lifetimes of about 10−10−10−9 s.13,120 In keeping with the general principles of PL, this process is primarily influenced by the solvent121,122 and temperature.123,124 At room temperature, the PL is very weak, and its characterization is hampered by the very low solubility of fullerenes. The fluorescent singlet excited state of C60 is deactivated very effectively (yield >99%) via intersystem crossing to a triplet state, and only a minor fraction of excited singlet states return radiatively to the ground state; the QY of this process is typically on the order of 10−4.13,125 Moreover, fullerenes exhibit extremely low extinction coefficients for low-energy excitations.122 Because fullerenes are good electrophiles,126 they can form both ground- and excitedstate complexes with many aromatic solvents, resulting in strong solvatochromism.122 Because the PL of unmodified fullerenes is subject to the drawbacks described above, fullerene-based nanocomposites are considered to be more promising materials for use in PLdependent applications. Functionalization and hybridization of fullerenes can yield derivatives with improved solubility in polar solvents, resistance to aggregation, and QYs as well as boosted emission ranges.127,128 The most common mono- and bisfunctionalized fullerenes are shown together with their PL spectra in Figure 5a. As can be seen, their functionalization affects both the shapes of their spectra and the intensity of their PL.81 Fullerenes can be hybridized with electron-donating or electron-accepting moieties to build photosensitized, charge separation-inducing assemblies with applications in photovoltaic,129 photocatalytic,130 and photosensing systems.131 Fullerene-based composites such as fullerene-containing films, polymers, and inorganic matrixes form another group of potentially useful materials.127 For example, incorporating C60 into the molecular cage of zeolite VPI-5 yielded a white lightemitting composite.132 It was also shown that hierarchical C70 structures (microrods built from C70 units) exhibit considerably stronger PL than unstacked C70 molecules, and that both the intensity of emission and spectral shapes of these assemblies can be easily tuned by varying the solvent used in their selfassembly by drop-drying (see Figure 5b).133 This was attributed to the presence of solvent molecules in the C70 crystals and the unique architectures of the hierarchical structures. In addition to prompt fluorescence occurring immediately after photon absorption and excited-state relaxation to the S1 energy level, unimolecular fluorescence can also occur via the so-called TADF mechanism in certain systems.134 TADF involves intersystem crossing from a singlet to a triplet state (S1→T1), followed by a second intersystem crossing back to S1, from which the system is de-excited by photon emission. The S1→T1→S1 cycle may be repeated several times before emission finally occurs (see Figure 6). Spherical fullerenes are

Figure 5. (a) Photoluminescence spectra of C60 (black) and some of its more common derivatives. Relative intensities reflect the corresponding quantum yields. Reprinted from ref 125. (b) PL spectra of C70 powder, C70-P (hierarchical C70 structures), C70-P-TMB, C70-P-HA, and C70-P-TMB/HA, where TMB and HA represent the solvents (1,3,5-trimethylbenzene and n-heptanol, respectively) used in the drop-drying process. Reprinted with permission from ref 133. Copyright 2015 American Chemical Society.

Figure 6. Simplified Jablonski diagram of prompt fluorescence and TADF processes; krS and kDF are the rate constants of prompt fluorescence and TADF, respectively; kISC and kRISC are the intersystem crossing and reverse intersystem crossing rate constants, respectively; knrS and knrT are the non-radiative decay constants for the singlet and triplet states, respectively; ΦPF, ΦDF, ΦRISC, and ΦISC represent the prompt fluorescence efficiency, TADF efficiency, reverse intersystem crossing efficiency, and intersystem crossing efficiency, respectively. Reprinted with permission from ref 137. Copyright 2014 Wiley VCH.

the only NCAs discussed in this Review that are known to exhibit TADF.21,135 This mechanism is especially important in C70,136 and it has already been employed in the construction of fullerene-based organoelectronic devices,137 temperature sensors,138 and oxygen sensors.139 4090


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4. GRAPHENE AND ITS DERIVATIVES Graphene is a two-dimensional crystalline form of carbon in which sp2-bonded carbon atoms are arranged in a planar honeycomb lattice. It attracted immense scientific interest following its first isolation by A. K. Geim and K. S. Novoselov in 2004.140,141 However, it had been investigated using theoretical methods for many decades beforehand and used to describe the structural properties of various carbon allotropes.142 It can be regarded as the “building block” of many other allotropesconceptually, one can envisage wrapping a graphene sheet up into a ball to form a spherical fullerene, rolling it up to form a nanotube, or stacking graphene sheets to form graphite (see Figure 7). It has attracted

band gap, which is generally done by breaking the lattice’s symmetry.144 The desire to prepare specific functional materials with non-zero band gaps has prompted the synthesis of many different graphene derivatives,145−149 which differ from the parent material in terms of their structure and/or chemical composition. Excluding CNTs and spherical fullerenes, which are discussed in the preceding sections, we can identify three main structural derivatives of graphene: multi-layer graphene,150 graphene nanoribbons (armchair or zigzag),151 and graphene dots (nanoflakes).152 Porous graphene resembles graphene with periodically missing phenyl rings (see Figure 8),

Figure 8. (a) STM image of porous graphene, with a superimposed depiction of its chemical structure. Reprinted from ref 165. (b) Structures of graphyne (left) and graphdiyne (right) showing a unit cell of each material. Reprinted with permission from ref 153. Copyright 2013 Royal Society of Chemistry.

Figure 7. (a) Graphene as a 2D building block for carbon allotropes of all other dimensionalities (quasi-0D spherical fullerenes, quasi-1D nanotubes, 3D graphite). Reprinted from ref 142. (b) A graphene sheet could be rolled up to form a nanotube or cut along one axis to form a nanoribbon, or a small portion of the sheet could be excised to form a nanodot. Reprinted with permission from ref 151. Copyright 2011 American Chemical Society.

and its band gap was determined to be in the UV region.153 Another predicted graphene analogue known as graphyne, which consists of sp- and sp2-bonded carbon atoms, has not yet been synthesized but has been studied theoretically.154 However, a diacetylenic 2D carbon allotrope closely related to graphyne termed graphdiyne has been prepared on a copper surface by Li et al.155 A huge family of materials can be obtained by chemical functionalization of graphene, including graphene oxide (GO) and reduced graphene oxide (rGO),156 graphane (hydrogenated graphene),157 the graphene halidesi.e., graphene fluoride (GF) or graphene chloride (GCl)148or graphene covalently or non-covalently functionalized with small organic/ inorganic molecules or polymers.158−160 In addition, many

considerable attention because of its extraordinary properties such as high transparency, excellent conductivity, strength, stability, and thinness; its unique combination of attractive characteristics suggests that it could potentially supplant widely used materials such as silicon in many applications.140 However, single-layer graphene has no band gap,143 which limits its use in electronic and optoelectronic technologies. Consequently, there is great interest in ways of opening its 4091


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Chemistry of Materials different materials can be prepared by the chemical functionalization of nanoribbons and nanodots, further extending the range of graphene-derived nanomaterials. An alternative way of modifying graphene’s electronic structure is by substitutional chemical doping,160,161 which involves replacing a carbon atom in the graphene network with a heteroatom having a different number of electrons, such as nitrogen, phosphorus, or sulfur (n-type doping), or boron (ptype doping).162−164 While single-layer graphene can be isolated by Scotch tape exfoliation of graphite,140 most attempts at mechanical exfoliation of graphite typically yield stacks of graphene sheets, or they produce single sheets with yields that are too low for practical use.156 The main methods for graphene synthesis are thus CVD,166 epitaxial growth,167 and reduction of graphene derivatives, which are typically obtained by chemical exfoliation of a graphitic precursor.168 Although CVD was originally only applicable in syntheses involving exclusively gaseous starting materials, this limitation has been overcome by the development of methods for growing either pristine or doped graphene from solid carbon sources (e.g., polymer films or small molecules) deposited on a metal catalyst.169 While CVD and epitaxial growth can produce good-quality single-layer graphene in rather small quantities, the reduction of graphene derivatives provides bulk quantities of defect-containing graphene-like sheets.156 These materials, which are often referred to as rGOs, feature graphene domains delimited by residual oxygen functionalities such as epoxy, hydroxyl, and carboxyl groups, or other structural asymmetries. Several methods have been used successfully to produce graphene derivatives of different dimensionalities (multi-layer graphene, nanoribbons, and nanodots). The most studied form of multi-layer graphene, i.e., bilayer graphene, can be prepared by CVD170 or from solid carbon sources (films) on insulating substrates.171 Graphene nanoribbons (GNRs) can be prepared by lithographic process,172 CVD,173 or splitting of CNTs,174 or derived chemically from various sources.175−177 Graphene nanodots (GNDs) can be regarded as a class of CDs and will be fully covered in the following section. Nitrogen-doped graphene materials are usually obtained by annealing178 or irradiation179 of pristine graphene under an NH3 atmosphere or by unzipping N-doped CNTs.180 Because of their reasonable water dispersibility, non-zero band-gap energy, and good processability, graphene derivatives such as GO are among the most promising graphene-based materials for many applications. The first step in the preparation of GO usually involves oxidizing graphite to graphite oxide. Several effective procedures for doing this have been known for decades.181−183 The oxidation of graphite introduces oxygen functionalities into the stacked graphene sheets and thus disrupts the symmetry of their sp2 arrangement. In addition, it causes an increase in the interplanar distance (i.e., the distance between adjacent sheets) that depends on the number of intercalated water molecules.184 Water-soluble sheets of GO containing various oxygen functional groups (see Figure 9a) can then be liberated by sonication.185 Similarly, GF can be obtained by fluorinating graphite and then sonicating the resulting graphite fluoride in a suitable solvent.186 As mentioned above, libraries of functional materials can be prepared by combining structural and chemical approaches for modifying graphene. Important materials obtained in this way include GO nanoribbons187 and GO nanodots.188 Interestingly, Qian et al.189 have shown that GO

Figure 9. (a) Structural model of graphene oxide including five- and six-membered lactol rings (blue), a tertiary alcohol ester (purple), and hydroxyl (black), epoxy (red), and ketone (green) functionalities, as proposed by Gao et al. Reprinted from ref 190. (b) Various kinds of nitrogen dopants in graphene. Reprinted from ref 179. (c) Structure of single sheet graphitic carbon nitride (g-C3N4) constructed from tri-striazine units. Reprinted with permission from ref 191. Copyright 2014 Elsevier B.V.

sheets of different sizes having distinct emission properties can be prepared by controlled oxidative cutting of MWCNTs. Because of its zero band gap and rapid charge carrier relaxations, graphene does not generally exhibit PL; however, PL can occur via the relaxation of non-thermalized hot electrons under ultrashort excitation in pristine graphene192 or from thermalized hot electrons in electrostatically doped (gated) graphene.193 However, for practical applications it is necessary to open the band gap of graphene by derivatizing it in some way as described above. While the band gap of bilayer graphene is reportedly tunable,194 it has not yet been reported to exhibit spontaneous PL. By lowering the dimensionality of graphene (basically to GNRs or GNDs), its band gap can theoretically be tuned from zero to that of benzene due to increasing quantum confinement. However, the band gap is not solely dependent on the physical size of the graphene fragment. For example, it has been shown that the edge structure of GNRs and GNDs (armchair versus zigzag) has a profound impact on their band gap.17,195 Additionally, the most common methods for cutting larger structures inevitably yield chemically functionalized products. For instance, the oxidation of graphitic material not only cleaves the bonds between carbon atoms but 4092


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Chemistry of Materials also disrupts the π network by causing oxygen functional groups to become bonded to some of the carbon centers in the graphene plane.196 Consequently, the long-distance symmetry of the graphene sp2 arrangement is interrupted by sp3hybridized carbon atoms, giving rise to quantum-confined graphene-like states.197 The optoelectronic properties of such materials, which have both sp2 and sp3 bonding, are determined by the π states of the aromatic domains.198 Therefore, the PL of such systems is often attributed to the radiative recombination of e-h pairs confined in the sp2 clusters, which are assumed to be the luminescence centers.18,199,200 Both experimental data and DFT calculations performed by Gong et al.201 confirm that the functionalization-induced structural changes are more important than the functional groups themselves in tuning the PL properties of graphene derivatives. The energy of the emitted light is determined by the spatial confinement of the eh pair, which depends on the size of the embedded aromatic islands. The PL of functionalized graphene derivatives could thus potentially be tuned by controlling the sizes of the residual sp2 structures within the sp3 network. This is essentially the approach that has been used to modulate VIS emissions from CNTs (see previous section) and is outlined in Figure 10.

Figure 11. (a) Schematic depiction of graphene nanodots and graphene oxide from citric acid and the variation of the corresponding PL spectra with the excitation wavelength. Reprinted with permission from ref 203. Copyright 2012 Elsevier B.V. (b) Maps of the fluorescence intensity for (1) GO and (2) oxidation debris. Reprinted with permission from ref 204. Copyright 2013 Royal Society of Chemistry.

Figure 10. (a−c) Structural models of GO at different stages of reduction showing the increasing extent of sp2 domains (indicated by zigzag lines and dark clusters). (d) Representative band structure of GO at different positions of the GO sheet. The local band structure is determined by the size of the sp2 domain. Reprinted with permission from ref 18. Copyright 2010 Wiley VCH.

graphite exhibited broad λex-independent emission whereas the associated oxidation debris exhibited shifting (λex-dependent) PL similar to that frequently observed for CDs (see section 5). Of course, the bottom-up synthetic approach used by Dong et al.202 (CA→GNDs→GO) is very different from the top-down method used by Thomas et al.203 to prepare graphene dots (graphite→GO→GNDs). However, the very different PL behaviors observed in these two cases strongly suggest that the mechanism of emission from these nanostructures may involve more than just radiative recombination of electrons and holes confined in differently sized aromatic clusters. Gokus et al.205 attributed the PL of oxygen plasma-treated graphene to carbonyl group-related localized electronic states at the oxidation sites. Galande et al.206 claim that spectral features observed in GO arise from carboxylic acid groups that are

Xin et al.202 reported that the PL of GO could be tuned by performing controlled reductions with hydrazine. As the hydrazine concentration is raised, the size of the small sp2 domains increases, reducing the band gap and red-shifting the PL. Because there is no straightforward way of preparing uniformly sized sp2 clusters, this approach should produce a wide range of cluster sizes, and the resulting material should have broad emission peaks with excitation wavelength-dependent energies. In keeping with this hypothesis, Dong et al.203 reported that GNDs prepared from citric acid (CA) by incomplete carbonization exhibit λex-independent emission whereas the emission from GO prepared from CA by complete carbonization is λex-dependent (see Figure 11a). However, Thomas et al.204 reported contradictory observations (see Figure 11b): they found that GO prepared from 4093


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Figure 12. (a) Suggested PL mechanisms in GO and rGO. The broad yellow-red emission is assigned to numerous disorder-induced defect states on GO, while the narrower blue emission originates from confined cluster states in rGO. Adapted from ref 209 with modification. (b) Implied relaxation of excited carriers within the context of a disordered band model of GO. Reprinted with permission from ref 210. Copyright 2013 American Chemical Society.

Despite the important studies discussed above, it is still difficult to unambiguously assign the PL of individual graphene derivatives to any simple electronic process, and the mechanisms of PL remain somewhat unclear. As noted by Zhang et al.,214 concentration-related effects such as aggregation and the self-rolling-up of graphene-family 2D crystals should also be taken into account when assessing the optical properties of these materials because they can influence both the intensity of emission and the spectral shape. The newly discovered family of halogenated graphenes has attracted considerable interest because its members emit across a wide range of frequencies, from the UV region in the case of GF215 to the VIS and NIR regions in the cases of graphene fluoroxide216 and fluorinated graphite oxide,217 respectively. Another graphene derivative, GF, which consists of sp3 carbon atoms with covalently attached fluorine atoms186,218 and has the formula (CF)n, was identified as a fluorophore exhibiting broad luminescence in the UV and VIS regions in response to excitation at near-UV wavelengths.215,219 Experimental measurements suggested that the optical band gap of GF is around 3.8 eV. However, high-level theoretical calculations including many body electron−electron and electron−hole correlation effects indicated that the optical band gap of perfect GF should be greater than 5 eV.220,221 This apparent discrepancy could be due to the presence of defects in the experimental material that would introduce midgap states,222 modulating the apparent optical band gap.223 It should be noted that the material’s exciton binding energy was rather high, amounting to ∼1.8 eV.221,224 The luminescence properties of GF can be controlled by altering its degree of fluorination, which can be achieved by exposing it to alkaline media201 and nucleophilic dipolar solvents,225 by thermal annealing,1,11 and by fluorinating graphene.218,226 The changes in luminescence induced by these treatments were attributed to the formation of sp2 islands that emit as a consequence of the recombination of confined e-h pairs.201 Quantum dots prepared from GF exhibited bright blue PL.227 The fluorinated graphenes are hydrophobic materials, which would limit their applicability in polar or aqueous media and hence in biological and medical applications. However, their hydrophilicity can be enhanced by introducing polar functional groups or by preparing them by fluorinating GO. It is worth noting that fluorination of GO enhances its luminescence.217 Overall, fluorinated graphenes

electronically coupled with nearby atoms of the graphene sheet to form quasi-molecular fluorophores similar to polycyclic aromatic compounds. Systematic research in this area during past few years has revealed the indisputably significant role of functional groups such as C−O, CO, and OC−OH in the PL of GO.207 Electronic states arising from structural defects and inherent imperfections in the graphene lattice208 also appear to contribute to the observed radiative recombination in some cases. Chien et al.209 linked the blue-shifting of PL caused by GO reduction to increased blue emission from small sp2 domains at the expense of yellow−red emission arising from the numerous disorder-induced defect states present on asprepared GO (see Figure 12a). Conversely, on the basis of subpicosecond time-resolved PL measurements, Exarhos et al.210 argued that these observations can be explained by an increase in non-radiative e-h recombination (quenching) at the expense of radiative processes, which would curtail the relaxation-related blue→red spectral migration (see Figure 12b). The higher nonradiative transition rate is attributed to the introduction of larger and/or more numerous sp2 clusters during the reduction process. In principle, this should be accompanied by a pronounced decline in the QY; unfortunately, the authors did not perform experiments to determine whether such a decline actually occurred. DFT calculations performed recently by Kumar et al.211 showed that the π and π* tail band states are mainly localized at epoxy and carbonyl groups, respectively. Given this result, and assuming that the PL energy is determined by the difference in energy between the densityof-states peaks of the π and π* bands, the broad disorderrelated emission from GO/rGO could potentially be tuned by manipulating the epoxy to carbonyl ratio. Biroju et al.212 have suggested that the photoluminescent emission of chemically derivatized graphene can be tuned by controlled treatment with hydrogen and oxygen because their spectroscopic investigations have shown that green emissions are related to COOH and CO edge functionalities whereas blue emissions are due to localized states in sp2/sp3 domains and epoxy-related in-plane functionalities. On the other hand, Du et al.213 assigned the VIS PL from GO to the formation of excimers between the GO plane and the oxidative debris. This assignment was justified on the basis of investigations into the pH- and concentrationdependence of the PL together with absorption data and timeresolved PL measurements. 4094


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5. CARBON DOTS AND GRAPHENE DOTS Even though carbon dots (CDs) are the most recently discovered family of photoluminescent carbon-based nanomaterials, they are now also the most intensely studied. They were first isolated by Xu et al.249 in 2004 as a byproduct of crude arcdischarge soot purification. Since then, a huge library of methods for preparing CDs has been developed.10,250 Of the known functional NCAs, CDs appear to be the most amenable to simple, large-scale, cost-effective, and eco-friendly production. The ubiquity of suitable precursors has been demonstrated by preparing CDs from green tea,251 orange peel waste,252 and overcooked barbecue food,253 among other things. Moreover, large quantities of highly fluorescent dots can be obtained in just 2 min using a domestic microwave oven.254 Unfortunately, the ready availability of large quantities of CDs is often counterbalanced by their poor homogeneity and definability. In addition to being readily prepared, CDs exhibit low toxicity, making them attractive for biological applications.255−257 Unlike the other materials discussed in this Review, CDs are not narrowly defined in terms of their physicochemical properties. In particular, the structure, chemical composition, and surface chemistry of different CDs can vary considerably. These properties are often not even precisely determined because CDs are very small ( 0.1) with and without charge-transfer character, respectively. (b) Structures of the 3L and 3Le models (vacuum). (c) Electron density differences between the ground and excited states for the electronic transitions at 263 and 295 nm in 3L. Charge depletion and charge accumulation areas are indicated in blue and red, respectively. Reprinted with permission from ref 292. Copyright 2015 American Chemical Society. 4098


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amorphous matrix had only minor effects on the observed optical properties. The results discussed above suggest that the PL from CDs should not be attributed to the particle as a whole but to localized units within it. The presence of multiple PL centers within an individual dot was revealed by a study carried out by Xu et al.,282 who observed the emission of multiple photons after single laser pulse excitation. Interestingly and contrarily, Ghosh et al.278 recently assigned the PL from isolated CDs to single quantum emitters by investigating their transition dipole moments. Moreover, these emitters were identified as surface defects based on the fact that materials with different observed crystal structures did not exhibit appreciable differences in their PL. The idea of PL centers embedded within a carbon matrix was developed in publications dealing with amorphous carbon (aC) long before CDs attracted scientific attention (Figure 21).81−83,199,296,297 Such embedded centers may also be responsible for the λex-dependent PL observed by Demichelis et al.80 in hydrogenated a-C films, 10 years before the first CDrelated publications appeared. Although the vast majority of synthesized CDs exhibit λex-dependent (shifting) PL, there have been a few reports describing CDs whose PL exhibits very low λ ex -dependency or is even wholly λ ex -independent.257,298−300 For example, Long et al.300 prepared CDs by applying voltages to carbon paste electrodes of different compositions. The resulting CDs differed considerably in their surface complexity and the λex-dependence of their PL. One of the samples exhibited non-shifting PL, which was attributed to its low degree of oxidation. Conversely, in a study examining two types of CDs with different levels of surface complexity, Zheng et al.300 found that the CDs with the more complex surfaces exhibited lower levels of λex-dependence. In addition, some CDs exhibit both shifting and non-shifting PL depending on the excitation wavelength.299,301,302 It should be noted that non-shifting PL can also originate from molecular fluorophores (see below). Another factor that can influence the UV−vis absorption and PL of CDs is their surface chemistry. As outlined earlier in this section, the surfaces of CDs can be functionalized and passivated in many ways, and the characteristics of their cores can be similarly varied, making their classification rather complex. It is not clear how generally important surface passivation is for the occurrence of PL in CDs. Many bottomup and top-down methods for CD preparation yield products that exhibit PL without any need for separate surface passivation.196,259,261,276,303 On the other hand, CDs prepared by laser ablation of a carbon target showed no PL even after intense refluxing in nitric acid, which generally causes the introduction of oxygen-containing functionalities and thus makes CDs water-soluble. However, these CDs exhibited bright multi-color luminescence after being modified with organic species such as diamine-terminated oligomeric poly(ethylene glycol) (PEG1500N) or poly(propionylethylene-imineco-ethylenimine) (PPEI-EI).12 Their PL was thus attributed to the presence of surface energy traps that become emissive upon stabilization by surface passivation. Clearly, the observation that surface passivation with non-fluorescent species can induce PL in previously non-fluorescent CDs demonstrates that the luminescence of CDs is not solely a function of their core structure. In keeping with these findings, Marzari et al.304 proposed that the optical properties of CDs might be governed by their outer amorphous shells, which can be visualized by

studies that have used bilayer structures as models for CDs,289,293 both of which similarly indicated that excited-state coupling was rather weak and that stacking did not induce appreciable red-shifting (vide inf ra). While there have been multiple computational studies on well-defined model systems as described above, we are only aware of one study that has examined wholly amorphous CDs. Margraf et al.294 used extensive Monte Carlo sampling to generate amorphous CDs with sizes of around 1−2 nm from periodic carbon structures containing only carbon and hydrogen, and from structures containing systematically varied quantities of N and O. This made it possible to study the influence of these heteroatoms on the dots’ overall stability, band gaps, light absorption, and PL. The calculations indicated that both heteroatoms and the hybridization of the carbon atoms (i.e., the sp2/sp3 carbon ratio) contributed relatively little to the observed band-gap variation, and that geometric factors (i.e., the topology of the sp2 conjugated carbon network) had a much stronger effect. Interestingly, the excitation energies corresponding to electronic transitions from the ground state to the first excited state (S0→S1) of the modeled finite-sized amorphous CDs decreased as the nanodots’ diameters increased (Figure 19a), which is consistent with the results

Figure 19. (a) S0→S1 excitation energies of amorphous CDs (1−2 nm) calculated at different levels of theory (semiempirical and DFT methods). (b) Illustrative molecular orbitals corresponding to bandlike (top) and surface (bottom) states of a 2 nm CD. Reprinted with permission from ref 294. Copyright 2015 American Chemical Society.

obtained for series of increasingly large ordered sp2 systems such as polycyclic aromatic hydrocarbons. Additionally, characteristic “band-gap” and “surface” states were identified (Figure 19b). The latter, which are higher in energy than the bulk band-gap states, were responsible for the lowest energy transitions and are thus potentially relevant in the context of PL. Recently, Fu et al.295 used model systems based on three specific PAHs (anthracene, pyrene, and perylene) in an amorphous matrix of poly(methyl methacrylate) (PMMA) to imitate CDs exhibiting shifting fluorescence. Films containing single PAH molecules exhibited CD-like absorption and fluorescence wavelengths, along with the large Stokes shifts typical of CDs. However, excitation wavelength-dependent PL was only observed in films containing all three PAHs (Figure 20a,b). The authors therefore proposed that CDs mostly consist of small sp2 domains within an amorphous matrix of sp3 carbon centers. In addition, phenomena such as exciton selftrapping or energy transfer between individual sp2 clusters may occur (Figure 20c). They also suggested that functionalization of the polyaromatic domains and near-surface defects in the 4099


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Figure 20. (a) Normalized absorption spectra of a PAH-film consisting of anthracene, pyrene, perylene, and PMMA (blue) in a molar ratio of 10:10:1:20, compared to the spectra of CDs dispersed in water (black dashed line). (b) PL spectra of the same samples excited at different wavelengths (indicated). (c) Exciton self-trapping in a pair of pyrene molecules: a free exciton (blue spot) can be self-trapped on a molecule pair as a self-trapped exciton (red spot), reducing the energy of the system. Reprinted with permission from ref 295. Copyright 2015 American Chemical Society.

Figure 21. (a) Models of hexagonal clusters: (1) C6H6, (2) C10H8, (3) C16H10, (4) C24H12, (5) C32H14, (6) C42H16, (7) C54H18, and (8) C66H18. (b) Models of tetrahedral clusters: (1) C5H12, (2) C17H36, (3) C29H36, and (4) C35H36. (c) Energy gaps were computed for tetrahedral and hexagonal carbon clusters using the BLYP density functional. Reprinted with permission from ref 83. Copyright 1998 Elsevier B.V.

high-resolution transmission electron microscopy and fast Fourier transform imaging. Furthermore, based on an extensive study of the intensity, lifetime, time-resolved anisotropy, and quenching of the fluorescence of three different types of CDs, Dekaliuk et al.305 concluded that their PL activity originated entirely from multiple individual surface-exposed emitters. To explain the PL enhancement observed in GNDs functionalized with different diamine molecules, single-layer graphene-like dots consisting of polyaromatic structures with 7−37 aromatic rings and oxygen/nitrogen-containing groups (hydroxyl, carboxyl, amine, and amide) at the edges were modeled using the DFT/TD-DFT approach.289 It was suggested that the observed PL enhancement of the protonated 1,2-ethylenediamine (EDA)-functionalized CDs might be due to indirect proton transfer from a pendant ammonium ion to the conjugated system via carboxyl groups, which could be facilitated by the formation of a cyclic conformation that becomes more heavily populated in the excited state.

Interestingly, no such cyclic structures were detected after protonation in the case of CDs functionalized with 1,3propanediamine or 1,4-butanediamine. The frequently reported pH-susceptibility of PL211,222,223,276,306−311 indicates that CD−solvent interactions have important effects on PL and supports the conclusion that the optical properties of CDs are sensitive to the properties of their surfaces. A transition from acidic to basic conditions can cause a gradual decrease in PL intensity in some cases;211,222,223 in other cases, the highest intensity occurs at neutral pH.276,309 In addition, changes in the pH cause appreciable changes in the emission wavelength in some cases276,307,309 but not in others.306,308 These effects probably vary on a case-by-case basis because different CDs bear different PL-affecting functionalities on their surfaces that respond in different ways to changes in the pH. Yang et al.310 studied the PL of selected CDs at a range of pH values and found that their emissions became red-shifted as the pH increased, which was attributed to 4100


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Chemistry of Materials the deprotonation of carboxyl and hydroxyl surface groups. In addition, the PL intensity of the studied CDs decreased dramatically under acidic conditions; this was explained as a consequence of the formation of strong intramolecular hydrogen bonds between oxygen-containing groups on the CDs’ surfaces in strongly acidic environments. The presence of such bonds was demonstrated by FTIR spectroscopy experiments conducted at various pH values. Jin et al.312 studied oxygen-containing GNDs and their oxygen/nitrogen-functionalized counterparts, and observed that the PL of oxygenfunctionalized CDs was red-shifted on moving from neutral to basic pH. This was suggested to be due to the deprotonation of the oxygen groups (−COOH, −OH). Accordingly, when N,OCDs were moved from neutral to acidic solutions, their PL wavelength was blue-shifted because the amine groups became protonated. Similar spectral changes were observed and shown to be reversible by Xu et al.311 To help explain the observed pH-dependence of PL, some authors have combined experimental studies with computational modeling. For example, Kozawa et al.200 modeled the effects of pH by performing calculations on protonated and deprotonated variants of a 1.7 nm structure functionalized with several OH and COOH groups located at the edges. The bandgap energies computed for the protonated, neutral, and deprotonated variants of the system decreased in that order, in keeping with the experimentally observed red-shifting of the corresponding CDs’ absorption and PL on being moved from acidic to basic environments. DFT/TD-DFT calculations292 on stacked pyrene/coronene systems bearing oxygen functional groups at their edges revealed that the low-energy absorption bands were slightly blue-shifted (Figure 18a) by the deprotonation of a carboxyl group located at the edge of either the pyrene or the coronene layer. Both the identity and the number of the surface functional groups present on CDs can also strongly affect their PL. For example, Liu et al.313 studied carbon nanodots synthesized hydrothermally using ascorbic acid dissolved in alcohol−water binary systems and found that the as-prepared CDs exhibited blue and yellow dual emission, with the blue and yellow emitters being derived from the ascorbic acid and alcohol, respectively. After separating the two emitter-containing fractions and subjecting them to FTIR analysis, the authors were able to show that the yellow emission was due to the presence of large quantities of surface hydroxyl groups derived from alcohol molecules. The blue and yellow emission intensities also varied with the alcohol−water ratio, further demonstrating the influence of surface functionalization. The broad emission of CDs is frequently considered to stem from a mixture of surface-located emitters and core-embedded aromatic clusters. For example, the PL spectra of CDs prepared by dehydration of acetic acid with P2O5 were deconvoluted using two bands that were assigned to emissions from sp2 clusters (core) and COOH-induced surface states, respectively (see Figure 22a).314 Similarly, Wei and Qiu315 observed convoluted blue and green emission peaks and assigned the green one to the presence of surface defects caused by the introduction of oxygen functional groups. Nguyen et al.316 explained the PL of CDs prepared by laser ablation of graphite powder by suggesting two different pathways of e-h radiative recombination: a slow pathway (>14 ns) from the carbogenic core onto surface states, and a direct, fast recombination pathway (∼1.3 ns) on the surface states. Wen et al.317 identified two types of PL from CDs on the basis of ultrafast timeresolved measurements. The spectrally narrower and bluer

Figure 22. (a) Schematic depiction of core- and surface-related energy states (left panel) and the assigned spectral bands (right panel). Reprinted with permission from ref 314. Copyright 2012 American Chemical Society. (b) Schematic representation of the mechanisms of PL in carbon dots before (LG27, left panel) and after the deesterification of surface carboxyl groups (LG27O, right panel). Reprinted with permission from ref 319. Copyright 2014 Elsevier B.V. (c) HOMO (black lines) and LUMO (blue lines) energy levels of GNDs-(NH2)n. The dotted lines denote the HOMO and LUMO energies of NH2. The insets show the HOMO and LUMO isosurfaces of each system at 0.01 e/Å. Reprinted with permission from ref 312. Copyright 2013 American Chemical Society. (d) Continuously excited PL intensity of surface-oxidized and surface-reduced CDs as a function of time. Reprinted with permission from ref 323. Copyright 2014 American Chemical Society.

intrinsic PL, which decayed rapidly, was assigned to sp2 nanodomains in the core, while the shifting extrinsic PL was linked to emissive surface states. Similarly, Wang et al.318 4101


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Figure 23. Antagonistic (a) and cooperative (b) interactions between frontier orbital hybridization and charge transfer in determining the HOMO− LUMO energy gap and the energy of the first bright exciton, respectively. The x-axis values in (b) are measures of the system’s energy; the label ΔI indicates the increase in oscillator strength due to frontier orbital hybridization. Reprinted with permission from ref 322. Copyright 2015 American Chemical Society.

surfaces of the non-reduced CDs induced high upward band bending, facilitating the separation of electrons and holes and their recombination at surface states. Conversely, the prevailing OH groups in the reduced material induced low upward band bending and thus prevented the e-h pair from undergoing surface recombination. As a result, the dots’ PL was blue-shifted and became more intense, originating mainly from core-related states. Similar blue-shifting and increases in PL intensity as a consequence of reducing surface groups are commonly observed in CDs including GNDs,299,325−328 and were observed by Chien et al.209 in a comparative study on GO and rGO as discussed in the preceding section (see Figure 12a). On the basis of time-resolved single-particle observations, Das et al.323 assigned the PL from reduced CDs to multiple chromophoric units that emitted at multiple intensity levels, whereas the oxidized particles predominantly emitted at a single level (see Figure 22d). Interestingly, all of the studied CDs were susceptible to photobleaching, but the survival time of the reduced dots was approximately twice that of the oxidized ones. It is clear that the surface complexity and degree of oxidation strongly affect the destiny of an e-h pair after photoexcitation, while the surface states probably act as both trapping and recombination centers. Many studies have explored the optical properties of CDs doped with various elements, combinations of elements, or compounds of those elements, including N,329−335 B,336 S,337,338 Mg,14 P,339,340 Gd,341 ZnS,342,343 ZnO,342 and TiO2.343 Doped CDs are generally prepared by synthesis from a precursor that contains the desired dopant,233−239 by exploiting an already doped carbon material,330 or by doping previously prepared CDs, for example by precipitating an inorganic salt over them.342,343 It should be noted that the meaning of doping in the context of CDs is somewhat nebulous because, by definition, they can contain elements other than carbon. Indeed, most elemental analyses of CDs have shown them to contain O, H, and N, and the presence of other minor elements cannot be excluded because it is typically not investigated. This means that almost all CDs could be considered to be doped even though they are not generally regarded as such. While the emission spectra of doped semiconductor nanoparticles usually differ very substantially from those of the parent materials, it is hard to identify any specific effect of particular dopants on the PL properties of CDs because there are few comprehensive experimental studies and

identified three emission states in green-fluorescent GNDs: an in-plane localized intrinsic state with a short lifetime, and two long-lived, functional group-related states that the authors described as being “molecular-like”. In keeping with these observations, Hola et al.319 observed that the PL of gallatederived CDs bearing ester groups on their surfaces was redshifted when the esters were converted into carboxylic acids by hydrolysis, relieving the passivation of the surface carbonyl groups (see Figure 22b). GNDs were also reported to exhibit red-shifting of their PL upon increasing surface functionalization because of charge transfer between the GNDs and their functional groups.312 DFT calculations showed that the bandgap energy fell as the degree of functionalization increased, as shown in Figure 22c. However, DFT studies on the HOMO− LUMO gaps of a series of models representing primary amine edge-functionalized graphene-like dots (i.e., one-layer structures based on a conjugated C36H18 system) showed that these systems only exhibit significant red-shifting if a primary amine group is bound directly to the conjugated system.320 It was therefore concluded that the red-shift of the PL originated from the strong orbital interactions between the aromatic system and adjacent amine groups, which are suppressed by −CH2NH2 functionalization. Similar red-shifts of absorption have been attributed to the presence of hydroxyl and carboxyl groups at the edges of CDs.200,286,321 The red-shifting effects of amine and oxygen functional groups were further confirmed by extensive DFT and GW+BSE calculations on a series of onelayer model GNDs containing 54 carbon atoms that were singly edge-functionalized with selected groups (−CH3, −NH2, −OH, −F, −CHO, −COCH3, and −COOH).322,323 The variation in the HOMO−LUMO gaps of these species was attributed to frontier orbital hybridization (which would reduce the magnitude of the gap) and charge transfer (which would increase the gap) between the functional group and the aromatic system. These two effects could be cooperative or antagonistic in different cases (Figure 23). The authors also concluded that functional groups containing a carbon−oxygen double bond exhibited lower levels of charge transfer and greater levels of frontier orbital hybridization than other groups, making them particularly useful for tuning the dots’ optical properties. Hu et al.324 studied CDs with two degrees of surface oxidation by using NaBH4 as a selective reductant of CO groups. The numerous CO and COOH groups on the 4102


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up to ∼0.50−0.70.337,347−350 Dong et al.337 attempted to explain the excitation-independent fluorescence enhancement (QY = 0.73) observed in CDs co-doped with N and S (synthesized hydrothermally from citric acid and L-cystein) as a consequence of the cooperative effects of nitrogen and sulfur surface functionalities. They demonstrated that N-CDs and N,S-CDs exhibited common UV−vis absorption (well-resolved absorption peaks at ∼240 and 345 nm) and PL-related characteristics (nearly non-shifting PL at 415 nm after excitation at wavelengths ranging from 285 to 400 nm), whereas O-CDs exhibit broad absorption without clearly resolved peaks and shifting PL. Moreover, the N-CDs exhibited significantly higher QY values (0.17) than the O-CDs (0.05), and their PL decay lifetimes were similar to those for N,S-CDs. Based on these observations, the authors concluded that the sulfur atoms enhanced the fluorescent N-states responsible for bright non-shifting PL and eliminated the effects of O-states associated with broad absorption and weak shifting PL (Figure 25).337 A similar QY enhancement was observed by Ding et

robust conclusions to draw on. However, some studies on this topic have been reported. Li et al. reported that pyrrolic Ndoping of GNDs caused their PL to be blue-shifted,265,329 and calculations supporting these findings were presented by Sk et al.17 A similar outcome was reported for quaternary (graphitic) nitrogen-doping.344 On the other hand, Jiang et al.345 reported that the emissions from CDs were red-shifted as the nitrogen content increased, although these authors did not investigate the bonding arrangement in this case. The differences between these results could potentially be due to the use of synthetic precursors with nitrogen centers located at different positions (o-, m-, or p-). Single-particle studies conducted by Das et al.334 showed that N-doping can cause red-shifting of the emissions from graphene quantum dots, and Tang et al.333 characterized the heteroatom-related energy levels of N-doped GNDs prepared from glucose (see Figure 24).

Figure 25. Mechanisms of fluorescence in O-, N-, and N,S-CDs: (1) electrons are excited and trapped by the surface states, (2) excited electrons return to the ground state via a non-radiative route, and (3) excited electrons return to the ground state via a radiative route. Reprinted with permission from ref 337. Copyright 2013 Wiley VCH.

al.,347 who found that N,S-CDs synthesized from α-lipoic acid and EDA exhibited substantially higher QY (0.54) values than singly doped S-CDs (0.06) and N-CDs (0.20), demonstrating the synergistic effect of sulfur atoms. However, in this case all of the different types of CDs exhibited shifting PL. Although the publications cited in the preceding paragraph reported exceptionally high PL QY values, some researchers have prepared dual-doped N,S-CDs with QYs below 0.20. For example, N- and S-co-doped graphene-like dots prepared hydrothermally from oxidized graphene with subsequent incorporation of heteroatoms via the addition of ammonia and powdered sulfur exhibited QY values of around 0.19, compared to 0.10 for singly doped N-CDs. However, the similar PL characteristics of the N-CDs and N,S-CDs (both have emission maxima at 430 nm and exhibit almost nonshifting PL under excitation wavelengths of 310−350 nm) confirmed the hypothesis that sulfur could enhance PL via a cooperative/synergistic mechanism.351 In addition to N,S-CDs, some studies have examined other potentially beneficial codopant combinations such as B/N and P/N.352−354 Sun et al.353 observed bright dual fluorescence from nitrogen- and phosphorus-co-doped CDs synthesized by microwave heating

Figure 24. (a) Schematic depiction of the synthesis of nitrogen-doped GNDs from glucose. (b) Energy levels of nitrogen-doped GNDs. Reprinted with permission from ref 333. Copyright 2013 Royal Society of Chemistry.

It is worth noting that CDs are often intentionally enriched with a doping element or compound to increase the QY of their PL rather than to tune its wavelength.14,330,335−338,340,346 Some dopant combinations have proven superior to others for this purpose. Notably, nitrogen- and sulfur-co-doping has been repeatedly identified as a very efficient way to achieve QYs of 4103


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Figure 26. (a) Structures of selected model N-CDs with amide edge functional groups. From left to right: base model with no further functionalization, epoxidized model system, and pyridinic N-doped model system (top and side views). (b) UNO-CIS calculated UV−vis absorption spectrum of the leftmost model shown in (a). (c) Spectral distribution of the S1 states of 200 conformations (blue) of the leftmost model system and its simulated emission spectrum (red). Reprinted with permission from ref 289. Copyright 2014 American Chemical Society. (d) One- and two-layer models of N,O-CDs. From left to right: 2L, N-free model; g1−2L and g2−2L, graphitic N-doped model; pd-2L, pyridinic N-doped model; pl-2L, pyrrolic N-doped model; am-2L, amino N-doped model. (e) Relative energy levels of the occupied (red lines) and unoccupied (blue lines) molecular orbitals of the two-layer models. HOMO−LUMO energy differences (ΔE) are reported in eV. Reprinted with permission from ref 293. Copyright 2016 American Chemical Society.

pyrrolic, and amine configurations had no appreciable effects on the CDs’ absorption properties. However, the latter structural features did induce appreciable blue-shifts in the dots’ absorption. In addition, the spectral features of the two-layer structures were similar to those for single layers, indicating that the inter-layer interactions (i.e., the coupling of excited states after stacking) are rather weak. A more systematic study on graphitic N-doping was performed by Cuesta et al.,355 who examined systems in which dopant atoms were incorporated into coronene molecules that served as models of sp2 domains in CDs. The calculated HOMO−LUMO gaps for these structures indicated that N-doping caused a red-shift of the absorption in all of the molecules relative to undoped coronene. Graphitic boron-doping was also predicted to cause redshifting. Wang et al.356 calculated the HOMO−LUMO gap energies of a few simple one-layer CD models (42 carbon atoms) containing oxygen, nitrogen, and sulfur heteroatoms to assist in the effective design of hybrid N,S-CD/TiO2 photosensitizers. Their DFT calculations revealed a cooperative effect resulting from the incorporation of nitrogen and sulfur heteroatoms into the carbon lattice, which caused a pronounced reduction of the band gap in the N,S-CD model. While this result and those discussed above have provided important insights, more extensive calculations will be needed to fully elucidate the effects of N- and S-co-doping. The emission QY of most CDs is generally found to be rather moderate and strongly dependent upon the synthetic methods and surface passivation techniques used in their preparation. The overall QY of CDs ensembles can be significantly enhanced by post-preparative fractionation, which

of aniline, EDA, and phosphoric acid. These CDs exhibited QYs of 0.51 and 0.38 for blue emission centered at 450 nm and green emission at 510 nm, respectively, both of which were appreciably higher than the corresponding values for singly doped CDs prepared using the same approach. To our knowledge, only a few studies have focused directly on modeling doping effects in CDs in order to support experimental results and clarify the mechanisms responsible for the observed PL properties of doped CDs. Semiempirical UNO−CI (unrestricted natural orbital-configuration interaction) calculations on a few amide-bearing edge-functionalized GND models (Figure 26a) showed that pyridinic nitrogendoping caused blue-shifted emission. In addition to this “static approach”, classical molecular dynamics (MD) simulations were performed to investigate the conformational freedom of the studied structure and determine how this might influence its PL (Figure 26b,c). The authors concluded that the broad absorption and emission bands originating from the synthesized N-CDs could be attributed to sp2-conjugated domains and that the corresponding transitions had significant π−π* character.289 Sarkar et al.293 used the DFT/TD-DFT approach to model the effects of N-doping on the electronic properties and absorption spectra of N,O-CDs represented by single- and double-layered models consisting of pristine pyrene and coronene subunits as well as their nitrogen- and/or oxygen-functionalized analogues (Figure 26d). Graphitic N-doping caused a significant red-shift that was associated with the formation of energy levels within the original HOMO−LUMO gap (Figure 26e), which reduced the energies of excitations with strong frontier/near-frontier orbital components. In contrast, nitrogen atoms in pyridinic, 4104


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nm.362 Because CDs mostly emit in the blue-green-yellow range, they must generally be excited using light with a wavelength of less than 500 nm, which is outside the biological optical window and would therefore be strongly absorbed by biological tissues. An important exception to this rule was recently reported by Ge et al.,363 who prepared GNDs with red emission peaking at 680 nm and broad absorption over the whole VIS spectrum. One way of overcoming the challenge presented by the biological optical window is to exploit one of the most important advantages of CDs: their capacity for upconversion PL (UCPL), i.e., their ability to emit light of a shorter wavelength than that used for excitation.10,364 This makes it possible to excite CDs with light of a wavelength within the biological optical window. Generally, the two-photon absorption cross section is much smaller than that of onephoton absorption. However, CDs exhibit similar two-photon absorption cross sections to their semi-conducting counterparts (CdSe, CdSe/ZnS), which is promising for biological imaging.25 A typical emission map for UCPL is shown in Figure 28. As is generally the case for down-conversion PL (DCPL), most up-converting CDs exhibit λex-dependent UCPL,284,309,328,330,365 but some do not.307,366 Interestingly, Jia et al.307 discovered a class of CDs exhibiting λex-independent UCPL but λex-dependent DCPL. Two mechanisms have been proposed to explain the occurrence of UCPL in CDs. Cao et al.25 described UCPL as

yields subsets of CDs with different ratios of non-radiative processes.357 It was suggested that doping could increase the QY by facilitating radiative recombination due to an increased density of dopant-related states337 and/or by improving surface passivation via a combination of inorganic surface-doping and organic functionalization.343 This approach has yielded QY improvements that could potentially make CDs viable alternatives to organic fluorescent dyes, whose QY values approach unity in some cases. Thus, doping together with suitable surface coating procedures has afforded CDs with QYs as high as 0.83.14 Post-preparative treatment with submerged liquid plasma in tetrahydrofuran was also found to considerably increase QY values.358 Song et al.359 recently reported a detailed investigation into the synthesis of CDs from citric acid, which has been widely used to prepare CDs with high QYs (close to 0.8). They used EDA as a co-reactant and found that the very strong blue emission of the products (which is commonly observed in the brightest CDs) actually arises from a molecular fluorophore (imidazo[1,2-a]pyridine-7-carboxylic acid, IPCA) that is formed by the condensation of the precursors. The PL of IPCA dominated the overall emission of the CD sample, while the carbon cores formed hydrothermally at higher temperatures (see Figure 27) gave rise to the λex-

Figure 27. Schematic depiction of the products obtained by hydrothermal synthesis using citric acid and EDA at different temperatures, and the dominant sources of fluorescence in each case. Reprinted with permission from ref 359. Copyright 2015 Royal Society of Chemistry.

dependent emission component. Similar findings were obtained by Wang et al.,360 who observed that CDs prepared by the thermal decomposition of citric acid under mild conditions (

Photoluminescent Carbon Nanostructures - Chemistry of Materials

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