Influence of Doping and Temperature on Solvatochromic Shifts in

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Influence of Doping and Temperature on Solvatochromic Shifts in Optical Spectra of Carbon Dots Claas J. Reckmeier, Yu Wang, Radek Zboril, and Andrey L. Rogach J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12294 • Publication Date (Web): 23 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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The Journal of Physical Chemistry

Influence of Doping and Temperature on Solvatochromic Shifts in Optical Spectra of Carbon Dots Claas J. Reckmeier,† Yu Wang,‡ Radek Zboril‡ and Andrey L. Rogach*,† †

Department of Physics and Materials Science & Centre for Functional Photonics, City

University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong ‡

Regional Centre of Advanced Technologies and Materials, Department of Physical

Chemistry, Faculty of Science, Palacky University, 78371 Olomouc, Czech Republic

Abstract Solvatochromic shifts in nitrogen doped and nitrogen-sulfur co-doped carbon dots are studied by analyzing absorption, photoluminescence excitation and photoluminescence emission spectra and their emission lifetimes in two different solvents – protic water (H2O) and aprotic dimethyl sulfoxide (DMSO). We identify three emission bands belonging to the sp2 hybridized core, the edge and the functional surface groups of carbon dots as well as surface attached fluorophores that emit within the edge band energy range. Edge and surface bands show opposite solvatochromic shifts solely depending on the doping atom used. We are able to reproduce emission shifts observed in DMSO by heating CDs in H2O from 7 to 87 °C while reducing polarity and hydrogen bonding strength of the solvent. Intrinsic edge band transitions are found to be strongly influenced by the solvent polarity, as the charge transfer processes dominate. Surface band transitions are found to be influenced especially by hydrogen bonding between the carbon dots and the solvent. Together, these processes lead to characteristic, solvatochromic blue- and redshifts of the emission bands. Furthermore, we observe strong emission quenching in the edge band in DMSO but emission enhancement in the surface band. This is assigned to quenched organic fluorophores which are formed during the carbon dot synthesis, leaving only intrinsic edge band emission while enhancing the radiative decay in the surface band. As a result, the edge band of nitrogen-sulfur co-doped carbon dots switches from excitation independent, fluorophore-like emission to excitation dependent emission associated with intrinsic edge states.

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1. INTRODUCTION Since the first report on carbon dots (CDs) in 2004,1 these nanoparticles have attracted a great deal of attention due to their distinct advantages such as bright blue luminescence, good chemical stability, excellent biocompatibility, low toxicity, and rather simple synthetic routes.2-3 Commonly, colloidal CDs have been described as para-crystalline carbon, consisting of sp2-hybridized carbon domains or graphene flakes within the crystalline core, surrounded by an sp3 amorphous carbon frame and a large variety of functional surface groups. Nevertheless, their exact structure is still to be revealed, also because it may vary depending on synthetic routes.4 The majority of CDs exhibit an excitation-dependent photoluminescence (PL), which is commonly ascribed to the emission originated from bandgap transitions of conjugated π-domains, and the emission from surface defects.3-5 Within the former process, conjugated π-domains are isolated by creation of C-sp2 hybridized islands, leading to domain size-dependent energy bandgaps which can localize electron-hole pairs.4 Fu et al.6 were able to recreate emission profiles of CDs by using a model system consisting of polycyclic aromatic hydrocarbons, while Song et al.7 assigned their PL to both crystalline cores and organic fluorophores IPCA and TPA (see below). Citric acid – the common precursor of CDs – can form blue emitting fluorophores with various other precursors.7-11 In particular, citric acid and ethylenediamine can form the fluorescent molecule 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-α]pyridine-7-carboxylic acid (IPCA)7 in the typical synthesis of nitrogen-doped-CDs, and 5-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2a]pyridine-3,7-dicarboxylic acid (TPA)10 can be formed as the result of reaction of citric acid and L-cysteine in the typical synthesis of nitrogen-sulfur co-doped-CDs. However, it is impossible to ascribe all the observed emissions properties of CDs solely to the energy bandgap related origin or to such organic fluorophores. CDs possess non-perfect sp2 domains which can give rise to defects and surface states, resulting in broad and low energy PL emissions. The PL characteristics of surface defect related emission can be tuned by surface passivation or functionalization.5, 12-13 Rhee and co-workers13 succeeded in control of the PL of CDs through the surface modification employing a series of para-substituted anilines, providing new energy levels for CDs exhibiting yellow-to-red PL emissions with very narrow spectral widths. Heteroatom doping is yet another widely used strategy to manipulate the PL properties of CDs.14 To date, a variety of elements such as N,15-18 S,19-21 Se,21-22 P,23-24 Cl,25 and Gd26 have been doped into CDs to achieve different emission color and to improve their PL quantum 2 ACS Paragon Plus Environment

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yields (QYs). Among them, N and S are the most frequently used elements for the synthesis of doped CDs. Doping can not only change the PL intensity and emission color of CDs, but also influence the reactivity or sensitivity of CDs to the environment. Yang et al.22 reported on the utilization of Se-doped graphene dots as a reversible fluorescent switch for the detection of oxidative hydroxyl radical (•OH) and reductive glutathione with a high selectivity and stability. Kozak et al.27 prepared N-doped CDs using cationic surfactant cetylpyridinium chloride as precursor, exhibiting tunable blue-green-yellow PL emission relying on the solvent polarity. In this study, we choose two types of commonly used doped CDs (i.e., N-doped CDs and N,S-co-doped CDs) as a model system to systematically explore their PL properties in two solvents of different polarity – in protic H2O and aprotic dimethyl sulfoxide (DMSO) – and at the different temperatures ranging from 7 to 87 °C (or 280 to 360 K). Heated H2O has a polarity comparable to DMSO, and a reduced hydrogen bonding.28-33 We observed characteristic, band specific solvatochromic shifts of CDs that depend on dopant, temperature and polarity. We derive new insights on the recombination processes in CDs and further elucidate the specific influence of doping, fluorophores, solvent, and temperature on their core, edge and surface emission bands.

2. MATERIALS AND METHODS N-CDs were synthesized following the previously reported hydrothermal method by Zhu et al.15 1.051 g of citric acid and 335 µL of ethylenediamine were dissolved in 10 mL of Milli-Q water under vigorous stirring. The resulting solution was transferred to a 25 mL Teflon-lined stainless steel autoclave and heated at 200 oC for 5 h in an electric oven. In the pyrolysis process, citric acid acted as a carbon source while ethylenediamine endowed N-CDs with nitrogen dopant. After the reaction, the brown-black product was purified by dialysis to obtain colloidal solution of CDs as a stock solution with the concentration of 1.0 mg/mL. N,S-CDs were synthesized using a hydrothermal method according to the established protocol reported by Dong et al.20 1.83 g (9.5 mmol) of citric acid and 1.0 g (8.3 mmol) of Lcysteine were mixed in 5 mL Milli-Q water under vigorous stirring, and heated at 70 oC for 12 h until formation of a thick syrup. The syrup was placed in a 25 mL Teflon-lined stainless steel autoclave and annealed at 200 oC for 3 h with a heating rate of 10 oC/min. Citric acid 3 ACS Paragon Plus Environment


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acted as the carbon source, while L-cysteine provided the nitrogen and sulfur dopants for N,S-CDs. After reaction, the obtained black product was neutralized with 1.0 mol/L NaOH solution and dissolved in 20 mL Milli-Q water. The solution was centrifuged at 6000 rpm to remove the precipitate. The resulted supernatant was purified by filtering out large sized carbon nanoparticles using a syringe filter with 0.22 µm pores, and the filtrate was reserved as a stock solution with the concentration of 2.0 mg/mL. X-ray diffraction (XRD) data were recorded at room temperature with a PANalytical X'Pert Pro Multipurpose Diffractometer using Cu Ka radiation at 40 kV and 40 mA. Fourier transformed infrared (FTIR) spectra were measured on a Bruker TENSOR 27 FTIR spectrometer. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained on a JEOL 2010F microscope operated at 200 kV (LaB6 cathode, resolution 0.19 nm) using ultrathin carbon film coated copper grids. Solutions of CDs for optical measurements were prepared by diluting the high concentrated stock solutions with appropriate solvent. The concentration was kept constant from there on to ensure comparability between measurements. Absorption spectra were recorded using a Shimadzu UV-3600 UV-VIS-NIR absorption spectrometer with a temperature controlled cell holder. Photoluminescence (PL), absolute PL quantum yield (QY), photoluminescence excitation (PLE) and fluorescence decay (lifetime) measurements were performed on an Edinburgh Instruments FLSP920 spectrometer with a PMT detector (R928P, Hamamatsu). Excitation source for PL and PLE measurements was a 450 W Xe900 continuous xenon arc lamp. All PLE spectra were excitation corrected. The PL QY was measured by an absolute method using an integrating sphere (Edinburgh Instruments). For time resolved measurements, an EPL-405 405 nm, 5 mW and an EPLED-320 320 nm, 5 mW picosecond pulsed diode lasers (Edinburgh Instruments) were used. Fluorescence decay was fitted with a multi-exponential function and lifetime was calculated by taking a weighted average. Temperature dependent spectra were recorded in a cryostat Optistat DN2 (Oxford Instruments) mounted in the FLSP920 spectrometer. Before heating the samples, H2O was degassed via evacuation to avoid creation of bubbles during the measurement.


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3. RESULTS 3.1. Structural characterization of CDs

Figure 1. (a) Representative TEM image of N-CDs. (b) Exemplary HRTEM images of NCDs and N,S-CDs used in this work, with the corresponding size histograms. Figure 1a shows a representative transmission electron microscopy (TEM) image of the NCDs, with particle diameters in the range of 2 – 5 nm. A crystalline structure with a lattice fringe distance of 0.32 nm can be observed from the HRTEM images of an exemplary single N-CD (Figure 1b), corresponding to the spacing between graphene layers (002 facet). TEM images of N,S-CDs (not shown) reveal the particle size range from 2.5 to 6.5 nm. The HRTEM image of an exemplary single N,S-CD indicates a high crystallinity with a lattice fringe distance of 0.21 nm (Figure 1b), corresponding to the in-plane lattice spacing of graphene (100 facet). Size histograms of N-CDs and N,S-CDs obtained as a results of the statistical analysis of the corresponding TEM images are presented in Figure 1b. XRD patterns of the N-CDs and N,S-CDs (Figure S7) display a broad peak centered at 25°, corresponding to a lattice spacing of 0.34 nm, similar to the graphite lattice spacing, which confirms the graphene structure of CDs and agrees well with the TEM results. The absence of other characteristic long range graphite peaks is related to the presence of small crystalline cores with few graphene layers and disordered surface structures.15, 20, 34 FTIR spectra of N-CDs and N,S-CDs (Figure S5, S6) confirm the presence of the oxygencontaining groups (O-H, -COO-, C-O, C=O, epoxy), C-H, C=C as well as the nitrogen 5 ACS Paragon Plus Environment


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containing, heteroatom bound N-H and N-C groups. In addition, C-NH-C bond signal is present for N-CDs and C-S bond signal appears for N,S-CDs, confirming the incorporation of nitrogen and sulfur into the respective dots. This has been previously confirmed in literature by XPS measurements on respective identical CDs, identifying pyrollic and pyridinic nitrogen and C-S-C as bonding configuration.20 3.2. Absorption spectra of CDs

Figure 2. Temperature dependent absorption spectra of N- and N,S-CDs in H2O and DMSO. a) N- and N,S-Doped CDs in H2O; b) N- and N,S-Doped CDs in DMSO; c) N-Doped CDs in H2O and DMSO; d) N,S-Doped CDs in H2O and DMSO. Temperatures of solutions for all spectra are indicated in the frames.

Figure 2 compares temperature dependent absorption spectra of N- and N,S-CDs in H2O and DMSO. In aprotic solvent DMSO, the polarity is reduced and the hydrogen bonding (donation) between solvent and CDs is prohibited. All CDs have three major absorption features, as discussed in detail below. In general, absorption spectra represent the sum of 6 ACS Paragon Plus Environment

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different absorbing species. Especially in the UV region, many organic compounds show strong absorption. We therefore focus on the distinctive spectral characteristics in this region that can be linked to structural features in the CDs. The highest energy absorption peak located at 238 nm is generally considered as π-π* transitions within the sp²-hybridized graphitic core of CDs (Figure 2a).15, 20, 35-38 The π-π* core band originates from conjugated π-orbitals of carbon rings, and is thus associated with the (crystalline) carbon core. Its location at 238 nm suggests a large bandgap that is associated with small sp2 hybridized carbon clusters embedded in an sp3 hybridized carbon matrix.6, 38 The second absorption feature of CDs located around 340 nm originates from n-π* edge transitions (Figure 2a-d). These transitions are associated with the edge of the CD core and are the main emissive centers for the bright luminescence in CDs.5, 15, 20, 37-38 We will denote them as edge states or edge band here as they form the boundary between sp2 and sp3 hybridized carbon. In addition, n-π* transitions require non-binding orbitals from outside of the sp2-hybridized core, which are formed mainly by functional surface groups that connect to the sp2 carbon lattice.5,

38-41

Upon excitation, an electron moves from the non-binding

orbital into the anti-bonding π* orbital of the CD core. This is also valid for the picture of an isolated graphene flake completely embedded in an sp3 carbon matrix. Wang et al.5 reported a change of emission type upon reduction of the CD surface groups, which highlights the strong interaction between the edge band and surface groups in CDs. Recently, Sudolská et. al.35 reported that π-π* transitions of charge transfer character contribute to the n-π* absorption peak. We will count them towards the edge states, as the excitation energies are similar and they transfer charge from inner to outer lying graphene flakes of the carbon core into the edge band. The third absorption feature starting from about 400 nm forms a plateau or a shoulder within the low energy tail of the edge peak (Figure 2a-d). It is related to the surface states or

surface band of CDs that belong to an ensemble of low energy transitions created mostly by functional surface groups attached to the edge of the carbon core. Wang et al.5 made an assumption that these surface groups create emissive states within the edge, and can therefore be defined as sub-bandgaps within the edge band. This, again, highlights the interaction possibilities between the edge and surface band related photophysical processes. Characteristic for surface states in CDs is the weak absorption strength and broad, excitation 7 ACS Paragon Plus Environment


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dependent emission in the region following the edge absorption. The transition from pure edge transitions to pure (intrinsic) surface state transitions is overlapping without clear boarder. Surface groups are in a direct contact with the solvent. Later, when discussing emission, surface state transitions are considered as dominating when strong emission redshifts are observed.

Effect of doping on the core π-π* absorption peak: N-CDs in H2O show a well pronounced shoulder (238 nm) that flattens out upon increase of temperature. N,S-CDs have a flat shoulder at 239 nm which shows no significant change with temperature. There is no major difference in the shoulder positions between N- and N,S-doped CDs, indicating that doping has no influence on high energy transitions within the core. The π-π* peak could not be recorded in DMSO, because the solvent absorption dominates from 300 nm towards smaller wavelength.

Effect of doping on the edge absorption peak: Nitrogen and sulfur are both heteroatoms that can be incorporated into the carbon rings. X-ray photoelectron spectroscopy (XPS) analysis shows that these doping atoms are rarely bound to 3 carbon atoms15, 20, 38-43 and therefore are most likely located at the edge of the carbon lattice. The edge absorption peak in H2O is located at 341 nm for N- and at 345 nm for N,S-CDs (Figure 2a). While the energies are comparable, the shape of the edge absorption peaks differs significantly. In H2O, the edge peak of N-CDs is broader and of a lower absorption strength than that of N,S-CDs. The additional S-doping leads to an increased edge absorption with a much stronger decreasing tail of surface states compared to N-CDs. Absorption strength of N- and N,S-CDs drops with increased temperature in H2O (Figure 2a). This decrease is stronger for N,S-CDs indicating that the enhanced edge absorption peak is more sensitive to higher temperatures. In DMSO, the absorption strength decreases with temperature on a similar scale (Figure S1).

Effect of doping on the surface state absorption region: Surface state absorption spectra for N- and N,S-doped CDs in H2O are shown in Figure 2a (from 400 nm onwards). N-CDs have significantly larger surface state absorption strength and show a slowly decreasing plateau extending to about 550 nm. The broad edge peak creates a region where transitions from edge and surface states overlap. In contrast, N,S-CDs only have a narrow transition zone into surface states at around 400 nm (Figure 2a). Surface state absorption strength of N,S-CDs is


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weaker and extends to just about 500 nm. Sulfur doping therefore enhances absorption of edge states but decreases the surface state absorption. As it will be discussed later on, in N,SCDs we observe higher energy, blue edge band emission and weaker emission from low energy surface states which might indicate a strong contribution of organic fluorophores.

Effect of solvent on the edge absorption peak: The peak maxima of the edge absorption shift upon changing the solvent from H2O to DMSO. For N-CDs (Figure 2c), the peak is redshifted by 6 nm (341 nm in H2O, 347 nm in DMSO) without a change in shape. Increased temperature does not lead to any further shifts. For N,S-CDs (Figure 2d), the edge peak in DMSO is slightly blueshifted by 2 nm (343 nm in DMSO, 345 nm in H2O) and narrower with a small shoulder appearing at 359 nm. Increased temperature in H2O triggers an edge peak blueshift by 1.5 nm (at 67 °C) towards the peak position in DMSO. Heated DMSO causes an additional blueshift of 1 nm. Enlarged plots illustrating these spectral shifts are presented in Figure S2. As heated H2O has a comparable polarity to DMSO28-30, 33 and edge states have no direct contact with the solvent, we can relate an environment of lower polarity to a blueshift of the edge absorption peak for N,S-doped CDs. The narrowing of the edge peak (Figure 2d) indicates the loss of certain transitions and is observed only in DMSO. The redshift in DMSO for N-CDs (Figure 2c) appears to be solvent related as no shifts in heated H2O are observed. The overall shape of the edge peak remains similar in both solvents. This indicates that the lost transitions of N,S-CDs in DMSO can be related to transitions introduced by the sulfur doping. Sulfur doping is known to increase spin density in graphene sheets42 and for introduction of additional energy levels39 enhancing emission intensity. As discussed later (see Figure 5d), strong PL quenching is observed in DMSO indicating a significant loss of emissive pathways. N-doping is known to decrease the bandgaps and the emission energy in CDs.44-47 The observed redshift of the edge absorption peak in N-CDs in DMSO can therefore be due to an influence of DMSO on the energy states created by N-doping. In addition, the opposite solvatochromic shifts with a polarity change of the solvent in N- and N,S-CDs, could indicate the existence of two different contributing fluorophore species associated with CDs . This is in agreement with the formation of IPCA and TPA in N- and N,S-CDs demonstrated in recent studies6,9 and is discussed in more detail later.

Effect of solvent on the surface state absorption region: In DMSO, the surface state absorption strength increases for N- and N,S-CDs as compared to those dissolved in H2O (Figure 2c,d). N-CDs show a significantly broader transition region from edge into surface states with the surface state absorption extending up to 700 nm (550 nm in H2O) (Figure 2c). 9 ACS Paragon Plus Environment


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Furthermore, absorption strength in DMSO increases with temperature in the region ranging from 370 nm to 500 nm (Figure 2c). In H2O, no temperature dependence was found. For N,SCDs in DMSO (Figure 2d), the transition from edge to surface states shows as an abrupt and early broadening at 375 nm (400 nm in H2O) with surface state absorption extending up to 550 nm (500 nm in H2O). The narrow transition zone from 375 nm to 405 nm in DMSO shows increasing absorption strength with temperature (Figure 2d). The overall absorption strength of surface states increases in DMSO for N,S-CDs, but is small compared to the increase observed for N-CDs (Figure 2b). Fluorophore contribution to surface state absorption can be ruled out, as surface states show an excitation dependent emission which is not a characteristic feature of CD related fluorophores.7-8

3.3. Photoluminescence excitation spectra and corresponding band structure of CDs

Figure 3. Temperature dependent PLE spectra of N- and N,S-CDs in H2O and DMSO at different emission detection wavelengths. a) N-Doped CDs at main emission (440nm) and b)


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at the wavelengths corresponding to redshifted emission peaks. c) N,S-Doped CDs at main emission (420nm) and d) at the wavelengths corresponding to redshifted emission peaks.

To explore solvent dependent emission shifts, or solvatochromism, and to connect absorption with emission for the previously discussed core, edge and surface bands, PLE measurements have been conducted. While absorption gives measure for light absorbed at each wavelength, PLE provides estimation of the number of the emitted photons. In the PLE spectra presented in Figure 3, we distinguish between those detected at the high energy, blue part of the emission spectrum (main emission peak of CDs, see Figure 4) and the low energy, red part of the emission spectrum (redshifted emission peaks and surface state emission see Figure 4). This allows us to examine the emission band structure that contributes to the emission of photons in different energy regimes.

Effect of doping on the PLE bands: Figure 3 shows PLE spectra of N and N,S-CDs. The shape of the PLE spectrum in H2O appears similar for both kinds of CDs. The core band is located at 251 nm for N-CDs (Figure 3a) and 255 nm for N,S-CDs (Figure 3c). The emissive bands are only slightly shifted in respect to the absorption peaks (see Figure 1a: 238 nm and 239 nm, respectively). The position of the core band does not change for lower energy detection wavelengths (Figure 3b,d). Considering the strong absorption at 250 nm, emissive contribution is low. This high energy excitation results in emission of photons with energy similar to the edge or surface band emission. It therefore indicates that excitation and emission pathways allow charge carriers to be transported from the core to the edge or surface band. The edge band is the dominating emission center of CDs. In H2O, the edge band has little shape and position variations between N and N,S-CDs. N-CDs show a narrower edge peak expanding at half maximum from 319 nm to 378 nm with the maximum located at 345 nm (Figure 3a). The maximum position remains almost the same for all lower energy detection wavelengths (346 nm, Figure 3b). N,S-CDs show a broader edge band expanding from 312 nm to 378 nm with a maximum at 353 nm (Figure 3c). This corresponds to the observed more intense and higher energy emission of N,S-CDs compared to N-CDs, as more transitions contribute to the emission. A small but continuous shift with increasing detection 11 ACS Paragon Plus Environment


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wavelength is observed (Figure 3d, blue arrow). Heating the CDs in H2O from 7°C to 87°C leads to an edge band shift that differs with doping. N-CDs in heated H2O redshift by 5 nm (from 345 nm to 350 nm) towards the maximum position found in DMSO (Figure 3a,b, cyan arrows). This is valid for all detection wavelengths. In contrast, the edge band of N,S-CDs in H2O is almost unaffected by heating (only 1 nm redshift) for the detection wavelength of 420 nm (main emission). However, emission redshifts by up to 5 nm for lower energy detection wavelengths towards 520 nm (Figure 3c,d, cyan arrows). As discussed later, a strong contribution to the edge band by fluorophore emission in N,S-CDs could be an additional reason for the weak temperature dependence of this band. The surface band is observed in the PLE spectra by setting the detection wavelength to longer wavelengths (lower energy) within the surface band emission range. The emissive contribution of the surface band increases in H2O for both, N- and N,S-CDs, with decreasing emission energy (Figure 3b,d, blue upward arrows). A plateau is formed from about 400 nm to 450 nm followed by a decrease to almost zero within the next 50 nm. This plateau originates from the decreasing contribution of the interband transitions from the edge to the surface band and the increasing contribution of intrinsic surface band transitions. On the high energy side of the plateau, interband transitions contribute the most, while contribution of intrinsic emission is low. At longer emission wavelengths, interband contribution is reduced but intrinsic absorption and emission are strong. The sum of both contributions leads to the formation of the plateau until intrinsic transitions decrease as well. For N,S-CDs, the plateau has a noticeable downward slope (Figure 3d). This can be attributed to sulfur doping, which reduces the interband emission contribution by enhancing intrinsic edge band emission. The low energy intrinsic surface band transitions correspond to the strongly redshifted emission spectra of CDs (see Figure 2 for absorption range of surface states and Figure 4 and 5 for the corresponding emission). High energy transitions within the core and the edge bands result in strong blue emission with a weak red tail formed by interband transitions between the edge and the surface band. What further highlights the importance of interband transitions in Nand N,S-CDs, we find that heating of solvent H2O (87 °C) leads to an intensity increase of the surface band plateau (Figure 3b,d). Due to the elevated temperatures, the number of interband transitions from edge to the surface as well as the absorption strength are increased (see Figure 2c,d). Furthermore, the transition region between the edge and the surface bands becomes notably broader (Figure 3b,d), which indicates an increased interaction between the edge and surface bands. 12 ACS Paragon Plus Environment

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Effect of solvent on the PLE bands: While the previous discussion was focused on the samples in H2O, Figure 3a-d also show the PLE spectra of N- and N,S-CDs in DMSO at 27 °C (red) and at 87 °C (orange). In order to compare effects of both temperatures and solvents, here and in the following discussions we will refer to ‘heated’ H2O or DMSO if the temperature is 87 °C or to ‘cold’ H2O or DMSO if the temperature is 27 °C. Detection wavelengths were chosen in a similar way as in H2O. The high energy (blue) and lower energy wavelengths were chosen to correspond to the main features of the emission spectrum. This ensures comparability with the PLE spectra recorded in H2O. The core band in DMSO cannot be recorded completely, as the absorption of DMSO increases rapidly for wavelengths shorter than 300 nm. The small peak that can be seen between 250 nm and 300 nm has to be considered as the tail of the core band. All PLE spectra of N- and N,S-CDs are redshifted in DMSO compared to H2O (Figure 3a-d). Judging by the tail, the core band appears to be redshifted as well. Since it cannot be excited directly in DMSO, its overall contribution to the emission has to be considered as low. The core band tail, however, increases with longer detection wavelengths. The edge band shows a significant redshift in DMSO for N- and N,S-CDs and an additional change of shape for N,S-CDs. For N-CDs the maximum position of the edge band shifts by 10 nm to the red from 345 nm in H2O to 355 nm in DMSO (Figure 3a). At 87 °C, N-CDs in heated H2O redshift only by 5 nm, while N-CDs in heated DMSO redshift by additional 12 nm to 368 nm. For N,S-CDs, the additional sulfur doping leads to a larger edge band redshift of 20 nm from 353 nm in H2O to 373 nm in DMSO (Figure 3c), while heating redshifts the peaks only by an additional 2 nm (turquoise and orange arrows in Figure 3c). In general, spectral shifts in heated H2O appear to move in the direction of the band positions in DMSO. The edge band of N,S-CDs in DMSO is significantly narrower compared to the edge band in water (Figure 3c). The strong redshift combined with the narrowing indicates a high energy transition cut-off in the edge band of N,S-CDs in DMSO. At low energy detection wavelengths, the edge band maximum for N-CDs in H2O (345 nm) and DMSO (355 nm) remains at the same position (Figure 3b) but redshifts for N,S-CDs in DMSO from 373 nm almost continuously to 390 nm with increasing detection wavelength (Figure 3d, red horizontal arrow). In H2O, the peak redshifts only marginally with a notable decrease on the high energy side. The contribution of surface bands to emission increases strongly for N- and N,S-CDs with increasing detection wavelength (Figure 3b,d, red upward arrows). The surface band plateau 13 ACS Paragon Plus Environment


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observed for CDs in H2O is also found in DMSO. Due to the strong surface band contribution in DMSO, the tail of the edge band in N-CDs is broadened towards longer wavelengths (Figure 3b). In N,S-CDs, this effect appears even stronger (Figure 3d). For the detection wavelength of 520 nm, surface and edge bands merge to a single broad peak with a shoulder on the red side. This suggests a strong overlap of the edge and surface bands for N,S-CDs in DMSO and is consistent with the overall redshift of the edge band. In N-CDs, the peak broadening suggests a smaller overlap between both bands. Heating of CDs in DMSO further enhances the redshift and band broadening. For N-CDs in heated (87 °C) DMSO (Figure 3b), the overlap of edge and surface bands increases. For N,S-CDs in heated DMSO, the edge and surface bands appear completely merged with no shoulder at detection wavelength of 520 nm (Figure 3d). To summarize the discussion on PLE spectra, we need to recall that doping with nitrogen and sulfur leads to incorporation of these heteroatoms into the edge of the carbon lattice and into functional groups at the surface of the CDs.20, 41-42, 44, 48 Nitrogen is bound to the edge of the sp2-hybridized carbon core mostly in pyridinic and pyrrolic form15, 20, 40 and is known to increase the charge carrier density42 and redshift emission spectra.44-47 When nitrogen is bound to the surface as a functional group like amine, it lowers the bandgap of graphene quantum dots through charge transfer with the carbon lattice.44 This supports that the edge and surface bands have interconnecting transition pathways over most of the corresponding excitation and emission regime, especially in the picture of surface groups creating subbandgaps in the edge band. Heating increases the emissive contribution of the surface band (Figure 3b) which broadens the edge band for all detection wavelengths. Sulfur doping in N,S-CDs creates additional energy levels39 that enhance emission from the edge band. With the detection wavelength set to longer wavelengths, a larger number of connecting transitions between the edge and the surface band contribute to the emission. This adds to the observed redshift of the edge band with increasing detection wavelength. Similarly, the edge peak redshifts with increased temperature, as energy loss due to interband transition from an edge to a surface state is enhanced and the emissive contribution of the surface band increases accordingly (Figure 3d). With the CD model consisting of polycyclic aromatic hydrocarbons proposed by Fu et al.6, this corresponds to an increased energy transfer to small bandgap emitters. The effect of DMSO on the PLE bands of CDs can be summarized as a redshift of the edge band, reduced emissive contribution of high energy transitions (especially in N,S-CDs), and a 14 ACS Paragon Plus Environment

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strong overlap of the edge and the surface bands. To understand the effect of DMSO, it is important to recall that DMSO and H2O are both polar solvents but DMSO has only half the polarity of H2O.28-30, 33 When H2O is heated to 87°C, its polarity decreases towards the level of DMSO. The influence of hydrogen bonding (H-bonding) on the edge band can be neglected as the CD edge is not in direct contact with the solvent. The influence of Hbonding on the surface band is discussed later. Accordingly, the change of polarity therefore has the largest influence on the edge in N- and N,S-CDs. While in H2O sulfur doping of N,SCDs enhances edge band transitions, in DMSO high energy transitions within the edge band are cut-off and the overlap with surface states is increased. Sudolská et al.35 showed that π-π* transitions of charge transfer character contribute largely to the high energy side of the edge absorption band. These polarity sensitive transitions appear to be quenched in DMSO for Nand N,S-CDs, especially in the presence of sulfur in N,S-CDs. Fluorophores formed during the synthesis of CDs7-8, 11 can contribute as well to a quenching of higher energy emission. Fluorophore IPCA7 (N-CDs) and TPA10 (N,S-CDs) have a similar excitation band and emission within the high energy part of the edge band. Quenching of this emission, as shown later (Figure 5c,d), would therefore appear as a redshift or even cut-off of the edge band with only intrinsic CD emission remaining. In DMSO, the redshift of the edge band results in an increased overlap with the surface band. Possible explanations for this interaction is the charge transfer between the carbon edge and functional amine groups at the surface of CDs44, interband transitions between the edge and sub-bandgaps, and energy transfer from large to small band gap graphene flakes6. In addition, it adds a polarity sensitive interband transition by charge transfer from the edge to the surface band that is enhanced in DMSO or heated H2O.

3.4. Photoluminescence spectra of CDs



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Figure 4. Solvent and excitation dependent emission spectra of a) N-CDs and b) N,S-CDs at selected excitation wavelengths indicated on frames. a) PL spectra of N-CDs are blueshifted in DMSO for the edge emission but redshifted for the surface emission. b) PL spectra of N,SCDs are redshifted in DMSO for the edge emission but blueshifted for the surface emission.

The effect of different solvents on PL spectra can provide useful information on the nature of radiative transitions. Many processes related to the emission couple to the solvent’s polarity and hydrogen bonding ability,32, 49 and solvatochromism can be related to the fluorescence mechanisms down to the molecular level.50 In the following, we will discuss the influence of the solvents H2O and DMSO on the emissive properties of N- and N,S-CDs, whose typical emission spectra are shown in Figure 4. DMSO has a lower polarity than H2O and cannot donate hydrogen bonds. For a better comparison of the excitation and solvent dependence of the PL spectra, we will only track the emission peak positions, as shown in Figure 5.


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Figure 5. Solvent mediated excitation dependent emission shifts (a,b) and emission quenching (c,d) for N-CDs (a,c) and N,S-CDs (b,d) in H2O and DMSO. The characteristic shifts for N- and N,S-CDs at the surface and edge, respectively, indicate the existence of different transitions and demonstrates their solvent dependence. The emission is quenched by half in N-CDs (c) and much stronger in N,S-CDs (d) for the edge transitions, while the surface emission is enhanced in DMSO (c,d). Connecting lines are a guide to the eye.

N-CDs in DMSO show a solvent dependent emission blueshift of several nanometers in comparison to H2O (Figure 4a and Figure 5a) for the excitation within the edge band. The largest emission blueshifts in DMSO compared to H2O are 6 nm and 5 nm at 360 nm and 375 nm excitation, respectively. At other excitation wavelengths an emission blueshift of 4 nm is found. However, upon excitation of intrinsic surface band transitions, the emission of N-CDs redshifts in DMSO compared to H2O (Figure 5a). This opposite solvatochromic shift of the edge and the surface band emission is a strong indication of two different radiative processes taking place. H2O and DMSO influence these processes differently. In addition, the 17 ACS Paragon Plus Environment


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solvatochromic redshift of the surface band emission in DMSO is larger than the opposite blueshift of the edge band emission (Figure 5a). The general excitation dependence of N-CDs in H2O and DMSO remains on the same level. The excitation dependent emission redshift increases slowly until 390 nm, which marks the transition zone, and redshifts strongly from 405 nm onwards when surface states are excited. The Stokes shift is the largest when the sample is excited on the high energy side of the edge peak (assigned to π-π* charge transfer), and decreases with increasing excitation wavelength (Figure 5a). A small increase of the Stokes shift is observed at the beginning of the surface band (transition zone) when emission strongly redshifts and more excited states decay via the low energy surface band. For lower energy excitation within the surface band the Stokes shift decreases again. Emission of N,S-CDs also shows solvatochromic shifts (Figure 4b and Figure 5b). However, these shifts are exactly opposite to those observed for N-CDs. In DMSO, the N,S-CDs show a redshift of the edge band and a blueshift of the surface band. Interestingly, the large redshift of the edge band in DMSO appears to be enhanced by a switch from excitation independent emission (in H2O) to excitation dependent emission in DMSO (Figure 5b). In H2O, emission shifts only a few nanometers in case of the edge band excitation (until 375 nm), which has been previously assigned to the sulfur doping20 and fluorophore emission.10-11 In DMSO however, the emission of N,S-CDs already shows a continuous excitation dependent redshift within the edge band starting from 330 nm (Figure 5b). A strong redshift of the edge band has already been observed in our PLE data (Figure 3c). The biggest redshift of 14 nm is found at 375 nm excitation in DMSO (435 nm emission).The switch to the excitation dependent emission in DMSO (Figure 5b) can be due to the quenching of organic fluorophores that are otherwise emissive in H2O. Thus, only intrinsic, excitation dependent emission from the edge band of CDs remains. Upon excitation in the surface band (405 nm), a strong solvatochromic blueshift is observed in DMSO (Figure 5b). The largest emission blueshift is 20 nm at 405 nm excitation; it becomes subsequently smaller with increasing excitation wavelength. The sudden and strong emission blueshift in the surface band can be explained by the solvent dependence of edge and surface band in N,S-CDs (PLE spectra, Figure 3c,d). In H2O at 405 nm excitation (Figure 5b), low energy transitions in the surface band are emitting (64 nm Stokes shift), while in DMSO higher energy edge band transitions are emitting (44 nm Stokes shift). In addition, a continuous (excitation dependent) redshift is observed in DMSO until the large emission redshifts into the surface states occur. At the same time, N,S-CDs in H2O are already redshifted into the surface states (Figure 5b). Therefore, the same excitation 18 ACS Paragon Plus Environment

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wavelength excites different bands in H2O and DMSO. At longer excitation wavelengths, surface states are excited in both solvents, whereas the emission in DMSO remains blueshifted (Figure 5b). The large Stokes shift observed at high energy excitation within the edge band shows that a part of the excitation energy is lost non-radiatively before the emission occurs. The energy levels can shift depending on the solvent. The smallest Stokes shifts are achieved for N,SCDs, where sulfur contributes to recombination resulting in higher PL QY. In H2O, we observe the smallest Stokes shift of 60 nm at 390 nm excitation for N-CDs and just 41 nm at 390 nm for N,S-CDs. In DMSO, we observe a Stokes shift of 56 nm at 390 nm excitation (NCDs) and 44 nm at 405 nm excitation (N,S-CDs). Both excitations correspond to the end of the edge band. We can therefore conclude that the smallest Stokes shifts are observed at the lowest energy edge band transition, while they increase for the higher energy surface band transitions. For low energy surface band transitions, a decreasing Stokes shift is observed suggesting a similar behaviour as in the edge band. Edge and surface band transitions of N- and N,S-CDs in DMSO are subject to PL quenching and enhancement, respectively (Figure 5c,d). The different doping appears to influence the magnitude of quenching and the transition to enhanced surface states. Edge transitions of NCDs have an absolute PL QY of 28% in H2O at 345 nm excitation; this value drops to 19% in DMSO (Figure 5c). N,S-CDs are highly luminescent in H2O with an absolute PL QY of 78% at 370 nm excitation, while the value drops to 7% in DMSO (Figure 5d). In the following discussion, the relative numbers for quenching and enhancement refer to the drop or increase in PL intensity of CDs in DMSO compared to their counterparts in H2O, as shown in Figure 5c,d. For N-CDs, excitation within the high energy site of the edge band (compare with Figure 3a,b) leads to the peak emission intensity of only 38% in DMSO as compared to H2O (Figure 5c). Within the transition zone between the edge and the surface band, the PL intensity then increases by 71% at 390 nm excitation. Upon excitation of the surface states, the PL intensity in DMSO further increases exceeding the intensity in H2O from 420 nm excitation onwards. The intensity enhancement in the surface band settles at about 115% at 435 nm and 450 nm excitation without increasing as fast as before. For N,S-CDs, the intensity ratio increase between the edge and the surface states is much steeper, and the transition from quenching to enhancement happens over a short range of excitation wavelengths (Figure 5d). This is in agreement with the redshifted edge band in DMSO and its vicinity to the surface band (see Figure 3d). Emission intensity in DMSO is only 2% at 19 ACS Paragon Plus Environment


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315 nm excitation and barely reaches 8% at 375 nm excitation (Figure 5d). In the transition zone, the overlap of the edge and the surface bands (compare with Figure 3d) leads to an abrupt and strong increase of the emission intensity, with 40% at 390 nm and 95% at 405nm excitation. With the excitation of intrinsic surface states, significant emission enhancement is observed (Figure 5d) reaching a relative intensity of 108% at 420 nm and 426% at 435 nm with a further increase at 450 nm (not shown). Apparently, sulfur doping in N,S-CDs contributes to a stronger quenching and enhancement compared to N-CDs. In DMSO, emissive edge band transitions of the CDs are quenched. Furthermore, surface attached fluorophores which are in a direct contact with the solvent are another likely source for the strong quenching observed for the edge band emission, which can be caused both by polarity change and changes in the hydrogen bonding configuration. In contrast, intrinsic surface state transitions appear to benefit from these changes, resulting in the emission enhancement in DMSO. A detailed discussion of these observations is provided in the next section.

4. Discussion 4.1. The origin of solvatochromism and the PL band specific quenching in CDs To explain the observed solvatochromic and PL quenching effects in CDs, it is important to understand the influence of doping on CD’s structure and emission. While undoped CDs containing only carbon and oxygen atoms show a very weak emission, introduction of Ndoping commonly results in bright luminescent CDs.15, 20, 25, 48 Nitrogen bound in pyridinic and pyrrolic form at the core edge15, 20, 40 increases charge carrier mobility40 and induces charge transfer of electrons towards the edge.25 Nitrogen bound as part of the functional surface groups lowers the amount of low energy trap states51 and can interact with the core edge by charge transfer as well, as it was shown for related graphene quantum dots.44 Graphitic bound nitrogen is energetically not favorable and is therefore rarely found in CDs.43 Furthermore, functional surface groups such as carbonyl and carboxyl can create subbandgaps enhancing lower energy emission; these are possibly major contributors to the surface states of CDs.5 The presence of negatively charged surface groups is in line with the negative zeta potentials of CDs studied here: -30.4 mV for N-CDs and -50.9 mV for N,SCDs.


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Similar to nitrogen, sulfur is found in the edge structures of the core (thiophene sulfur) and surface groups (oxide sulfur) where it introduces additional energy levels and increases the density of states.39, 52 Sulfur is not as electronegative as nitrogen and on a similar level to carbon. While nitrogen doping causes an increased electron density that enables charge transfer, sulfur can increase the spin density of carbon atoms in the edge structure enhancing the electron transfer.42 The ability of CDs and closely related graphene quantum dots to transfer charge carriers to nearby acceptors has been extensively documented.15, 25, 40-41, 48 In a recent study, Gosh et al.53 reported that the emission of CDs originates from dipole emission centers and found a strong electron-phonon coupling, while Yu et al.54 reported weak electron-phonon coupling from their temperature dependent measurements. All these studies show that charges can be moved within and between the energy bands of CDs. As charge transfer processes are highly sensitive to the solvent and its polarity, a part of the observed spectral shifts of N- and N,S-CDs and the corresponding change in PL intensity can be attributed to charge transfer processes. For transitions created by functional surface groups, hydrogen bonding between CDs and solvent has to be taken into account as well. Furthermore, especially for citric acid derived CDs, contribution of organic fluorophores to the observed spectral properties has to be taken into account.6,7,10 Emission of fluorophores often depends on the solvent. Different fluorophores are created depending on the specific choice of precursors in the synthesis of CDs;7-8,

10

if and where these fluorophores are

incorporated into the CDs remains under investigation, while our results on the solvent depending quenching suggest their possible location at the edge or the surface of CDs, as will be discussed below. The emissive contribution of fluorophores is overlapping with the intrinsic edge band processes of CDs.

Edge band processes: The edge absorption peak involves two kinds of transitions.35 The ππ*charge transfer originates from excitations inside sp2 hybridized stacked carbon core and transports charges from inner to outer graphene sheets. The n-π* transition involves charges from edge atoms or surface groups being excited into hybridized π*-orbitals of the core. Both transitions require movement of charges and therefore create excited state dipoles53 which in turn are sensitive to polarity. N-doping in the edge of the carbon core or as a part of surface groups improves localisation and transfer of charges. The strong electronegativity of nitrogen can cause local charge density variations at the edge of the carbon lattice, making it favourable for charges to recombine radiatively in H2O. Additional sulfur doping enhances localisation of charges around the edge and makes radiative recombination to the dominant 21 ACS Paragon Plus Environment


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decay channel in H2O. In DMSO however, excited states around the edge decay strongly via non-radiative channels. Strong quenching of N,S-CD emission (Figure 5d) and to a lesser extent of N-CD emission (Figure 5c) is observed. Especially transitions with high excitation energies are quenched. Notably, as seen in Figure 5, the remaining radiative edge emission is blueshifted for N-CDs but redshifted for N,S-CDs. Sulfur appears to provide additional low emissive, low lying energy states in N,S-CDs which are observable in DMSO and lead to the excitation dependent redshift starting from shorter wavelengths (Figure 5b). On the other hand, N-CDs show excitation dependent emission over the entire edge band in both H2O and DMSO (Figure 5a). The switch from excitation independent to dependent emission is a strong indication that fluorophores associated with CDs are quenched in DMSO. The low remaining intensity in the edge band might originate from intrinsic edge states, while strong excitation independent fluorophore emission is prohibited. This is further supported by the observed high energy cut-off in the edge band (Figure 3c). For N-CDs, the PL quenching in DMSO is not as strong and excitation dependent emission in the edge band remains detectable, which suggest that fluorophore amount might be low in this case. This is further supported by the overall low absolute PL QY (28%) of N-CDs compared to the very high absolute PL QY of N,S-CDs (72%), with probably much higher fluorophore concentration. The specific quenching of emission in the edge band energy region indicates that fluorophores are not incorporated into the core (and thus not protected) but are rather attached to the edge or the surface and are in direct contact with the solvent. This should be considered in applications such as ion detection through quenching of CD emission.

Surface band processes: It has been reported previously that functional surface groups interact with the edge of graphene quantum dots via charge transfer.44 Especially, oxygen and nitrogen containing surface groups may act as electron donors and cause an overall negative zeta potential of the CDs. N,S-CDs studied here have a more negative zeta potential (50.9 mV) than N-CDs (-30.4 mV). The surface groups generally introduce a large amount of low energy transitions with low absorption strength, possibly by creating specific subbandgaps within the edge5. Surface states in DMSO show an increased absorption strength (Figure 2c,d) indicating a favorable change of dipole moment enhancing absorption. Upon the surface band excitation, the charge is redistributed or transferred between or within functional groups and their corresponding energy bandgaps. Surface state emission of N-CDs is redshifted in DMSO (Figure 5a), but blueshifted in N,S-CDs (Figure 5b). Similar to the edge band, sulfur doping results in an opposite emission shift. The PL intensity ratio of N22 ACS Paragon Plus Environment

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and N,S-CDs is increasing in DMSO with longer excitation wavelengths, as more intrinsic surface states are excited (Figure 5c,d). In contrast to the edge band, fluorophores do not contribute to any emission within the surface band. In H2O however, where fluorophores are not quenched, they possibly enhance the non-radiative energy transfer from surface groups, thus lowering their overall emission. When quenched in DMSO, surface band transitions are less disturbed and stronger emission is detected. Furthermore, functional surface groups of CDs are in direct contact with the solvent while screening the core and the edge from direct interactions. Therefore, the absence of solvent–CD hydrogen bonding and lower polarity in DMSO can further contribute to the enhancement of surface band emission. Solute-solvent interactions by hydrogen (H-) bonding between the CD surface and the solvent can change the spectral properties of CDs in addition to the solvent polarity. In general, change of the solvent from a non-polar to a polar protic or aprotic one causes a bathochromic (red) shift.32 H-bonds form more readily in the protic solvent H2O which is a potent H-bond donator (1.17 H-bond acidity parameter) but weak acceptor (0.18 H-bond basicity), while the aprotic solvent DMSO can only accept H-bonds (0.76 H-bond basicity).31 In most cases, specific interactions between H2O and the solute redshift emission of fluorophores stronger than between DMSO and the solute.49 This is obviously not the case for N-CDs as their surface band emission is blueshifted in H2O. For N,S-CDs, the emission is redshifted in H2O, while the amount of available surface groups is expected to be similar to N-CDs. Furthermore, H-bonds tend to quench emission in most cases,32 which appears to be the case for fluorophores attached to the CD surface. However, interactions of H-bonds with the functional surface groups of N- and N,S-CDs have a different impact on the related energy bandgaps and cause the opposite solvatochromic shifts of the surface band (Figure 5c,d). In order to further investigate the solvatochromic shift in CDs it is important to evaluate contributions of solvent polarity and H-bonding.

4.2 Temperature and lifetime dependence of CD emission By heating H2O and DMSO, the polarity can be modulated (reduced) and the H-bonding weakened. This allows us to evaluate their specific influence on the emission of CDs. As reported previously, CDs do not show any strong temperature dependence of their emission,54 leaving the influence of solvent related effects to consider. With increasing temperature, Hbonding between H2O molecules is weakened, which decreases order and in turn decreases 23 ACS Paragon Plus Environment


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the solvent polarity. The dielectric constant of H2O drops from about 86 to 58 when the temperature is increased from 5°C to 90°C.28 The dielectric constant of DMSO decreases only slightly from about 48 to 43 when the temperature changes from 20°C to 60°C.29-30, 33 We note that the polarity of H2O at 87°C is close to the polarity of DMSO at 27°C. Furthermore, H-bonding with CDs is reduced in heated H2O and DMSO. The largest difference of (cold) and heated H2O with DMSO lies within the high H-bond accepting ability of DMSO. However, when DMSO is heated, the difference to H2O can be assumed as negligible because the H-bonding is overall weakened.

Figure 6. Temperature dependent peak emission shifts of (a) N-CDs and (b) N,S-CDs in H2O and DMSO. All CD samples in heated H2O shift towards the position in room temperature DMSO, while samples in heated DMSO only show significant shifts for N-CDs. Connecting lines are a guide to the eye.

Effect of temperature on the emission shifts: Figure 6 shows the temperature dependent peak emission shifts of N- and N,S-CDs in H2O and DMSO. For N-CDs, the emission in heated H2O (87°C) blueshifts within the edge to the beginning of the surface band and moves towards the emission position observed in cold DMSO (27°C) (Figure 6a). The largest emission blueshift of 5 nm occurs at 365 nm and 375 nm excitation. Specifically at 375 nm excitation, N-CDs in heated H2O emit at the same wavelength as they do in cold DMSO (Figure 6a). Notably, the largest temperature triggered blueshift occurs exactly at the position where the largest solvent dependent emission shift between N-CDs in cold H2O (7 °C) and DMSO was observed (Figure 5). In the surface band, the blueshift decreases which is due to the increased emission redshift of N-CDs in DMSO at low energy excitation (compare with 24 ACS Paragon Plus Environment

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Figure 5a). In heated DMSO (87°C), N-CDs are significantly redshifted over the entire edge band but slightly blueshifted in the surface band. For N,S-CDs in heated H2O, the emission similarly shifts towards the peak position observed in cold DMSO (Figure 6b). In the edge band, a small emission redshift of 3 nm is observed. In the surface band however, the emission shift in heated H2O changes direction and is strongly blueshifted by 13 nm as compared to cold H2O. Noteworthy here as well, the strong temperature triggered blueshift occurs at exactly the excitation wavelength where the solvent dependent blueshift in DMSO occurred (see Figure 5b). At lower excitation wavelengths, the emission blueshift in heated H2O decreases, as mainly intrinsic surface states are excited (Figure 6b) which do not shift as strong in DMSO (see Figure 5b). In addition, we checked the reproducibility of the emission spectra compared before and after heating (Figure S3, S4). In H2O, heating has no significant impact on N- and N,S-CDs. In DMSO, heating has no impact on edge band emission but surface band emission shows a slightly broader tail. The position of the maximum remains unchanged, however. Effect of temperature on the PL lifetime of CDs: PL lifetimes of N-CDs are 12 ns / 8 ns in cold/heated H2O at the edge excitation (320 nm) and 12 ns / 9 ns at the surface excitation (405 nm), respectively. The PL lifetimes of N-CDs in cold DMSO are similar to those in heated H2O: 7 ns and 9 ns in the edge and surface band, respectively. The decrease of the NCD lifetime in heated H2O and cold DMSO is likely due to quenching (compare Figure 5c) and increased non-radiative decay, as properties of heated H2O are more similar to DMSO. PL lifetime of N-CDs in heated DMSO drops slightly to 6 ns under the edge band excitation (320 nm) and remains unchanged (9 ns) for the surface excitation/emission. PL lifetimes of N,S-CDs in cold H2O are 10 ns for the edge and the surface band excitation/emission and are therefore slightly shorter than those observed for N-CDs (12 ns). The PL lifetime of N,S-CDs drops significantly in cold DMSO. In the edge band a lifetime of 3 ns is found (320 nm excitation) and for the surface band (405 nm excitation), lifetime is 5 ns at 450 nm detection wavelength and increases to 6 ns at 500nm detection wavelength. This is in agreement with the observed strong fluorescence quenching in the edge band and the PL recovery and enhancement in the surface band (Figure 5d). In heated H2O, N,S-CDs experience a similar PL lifetime drop to 4 ns in the edge band. At the surface band excitation, PL lifetime remains at about 8 ns. This is in agreement with the observed lifetimes in DMSO.



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The summary of PL lifetimes for all studied samples can be found in the Supporting Information (Table S1). To explain the observed solvatochromic shifts, we have to consider related inter-molecular photophysical processes occurring in CDs. Charge transfer within the edge band (discussed in section 4.1) is sensitive to polarity and therefore a likely contributor to the solvatochromic shifts. Charge transfer can also occur between the edge and surface states influencing the transition zone between both. Reduced polarity increases energy of transitions in N-CDs but decreases them in N,S-CDs. The characteristic reverse shift of the CD emission under the edge and surface band excitation (Figure 5a,b) indicates a different solvatochromic mechanism in the surface band. The CD surface groups are in direct contact with the solvent and therefore a prime candidate for H-bonding interactions. DMSO is a pure H-bond acceptor and cannot donate H-bonds, while in heated H2O, all H-bonds are strongly weakened.28-30 Negative zeta potential values and FTIR spectra (Figure S5, S6) evidence on the existence of negatively charged surface groups for N- and N,S-CDs. These allow for strong hydrogen bonding between H2O (donation) and the CD surface groups. Any change of the interaction or binding strength, be it by heating or solvent exchange to DMSO can shift the energy levels of the intrinsic surface band states and sub-bandgaps. For N-CDs, surface band emission in cold DMSO is redshifted (Figure 5a) while heat decreases the blueshift of H2O at the end of the edge band (Figure 6a). This indicates a reduction of H-bond donations from the solvent to the CD surface (hot H2O) or its removal (in DMSO) and ultimately leads to a decrease of the surface band energy levels in N-CDs. An exception is observed for N-CDs in hot DMSO where emission is strongly redshifted in the edge band, but is situated very close to cold DMSO (slightly blueshifted) in the surface band (Figure 6a). For N,S-CDs, we may assume the same interactions in edge and surface bands. However, we observe exactly inverse solvatochromic shifts compared to N-CDs (Figure 5b, 6b), possibly caused by the sulfur doping in core, edge and surface structures. Beside the different shift direction, emission shifts in DMSO are reproduced in heated H2O upholding the proposed mechanisms of reduced polarity and hydrogen bonding. The emission of N,S-CDs in the heated DMSO is similar to the emission in cold DMSO within the edge and the surface band. This is in contrast to N-CDs and highlights the possible strong fluorophore quenching in DMSO. Fluorophores attached to the N-CDs (such as IPCA6) could furthermore react 26 ACS Paragon Plus Environment

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differently to heated DMSO, while the fluorophores attached to N,S-CDs (such as TPA9) do not, or are entirely quenched in the edge band. Accordingly, a second solvatochromic mechanism for fluorophores attached to the CDs surface can be assigned. Emission of these fluorophores is limited to the edge band energy range7-8, 10-11 so that they do not contribute to emission shifts of the surface band. Thus, a change of fluorophore emission is only observable within the edge band range. However, fluorophores could serve as an additional acceptor or donor for charge transfer within edge and surface states. In the transition zone between edge and the surface band, the solvent polarity shows a declining but still significant influence. The most striking interaction between fluorophores and DMSO appears to be the strong emission quenching (Figure 5c,d). It can explain the strong redshift of N,S-CDs in the edge band leaving only lower energy intrinsic edge emission. In heated H2O however, only a small redshift is observed (Figure 6b), indicating that reduced polarity and H-bonding contribute to a specific fluorophore – DMSO quenching effect in N,S-CDs only. Comparing cold and heated DMSO, emission does not shift within the surface band suggesting that a hypothetical H-bond donation from surface groups has no effect.



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4.3 Summary of emission processes in CDs and the related solvent interactions

Figure 7: Sketch of the processes in CDs that contribute to the observed emission and solvatochromism. The sp2 hybridized core is shown as graphene or graphite flakes embedded in an amorphous sp3 hybridized carbon matrix. Dopant heteroatoms are shown as colored dots at the edge and within functional groups. Fluorophores are attached to the surface (stars). The drawing is not to scale. The shown processes are (1): π-π* charge transfer from the inner to outer part of the sp2 hybridized core; (2): n-π* transitions from the edge (white arrow) or from surface groups into the core; (3): charge transfer from the edge into the surface states and corresponding sub-bandgaps (orange arrow) or vice versa (blue arrow); Both processes mentioned in (2) and (3) can also involve fluorophores if a transition pathway is possible; (4): intrinsic surface state transitions. These include sub-bandgaps created by functional surface groups; (5): hydrogen bonding interactions between the solvent and the surface. Fluorophores are found to be solvent sensitive and therefore assumed to accumulate at the surface. Figure 7 summarizes all previously discussed interactions in doped N- and N,S-CDs that may contribute to the observed solvatochromism and illustrates the interactions between these different transition pathways. Intrinsic core transitions (1) involve π-π* charge transfer from inner to outer layers of the sp2 hybridized graphene flakes35 within the core. This transports charges to the outer edge of the CDs. At the edge of the CD (2), n-π* transitions move charges from a non-bonding orbital into the π*-orbital of the (outer) sp2 hybridized core. The edge of the CD also functions as the connection to surface attached fluorophores and surface 28 ACS Paragon Plus Environment

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groups. These surface groups are supposed to contribute to a wide range sub-bandgaps responsible for the broad surface state emission. As indicated by (3), charge transfer from the edge to fluorophores or surface group bandgaps (yellow arrow) or vice versa (blue arrow, if energetically possible) create polarity dependent transition pathway. Transitions that are confined in the surface band without a connecting pathway to another processes are indicated as (4). These transitions are most likely not sensitive to polarity but to H-bonding interactions. Consequently, (5) indicates H-bonding interactions of the CD surface with the solvent acting as acceptor or donator. These interactions are blocked in DMSO or in heated H2O. This also includes fluorophores attached to the CD surface. At the same time, the CD edge and core are screened from H-bonding interactions by the surface groups.

5. CONCLUSIONS In conclusion, based on the systematic evaluation of absorption, PLE and emission spectra we identified three emission bands located at the core, edge and surface of the N-doped and N,S-co-doped CDs. A significant contribution of surface attached fluorophores to edge band emission could be elucidated, with no contribution to the surface band emission. We show that emission of edge and surface band undergoes opposite solvatochromic shifts when solvent is changed from H2O to DMSO. Direction of this shift is determined solely by the doping atoms. N-CDs blueshift in the edge band and redshift in the surface band, while N,SCDs redshift in the edge band and blueshift in the surface band. We exploit heating of H2O to reduce its polarity and H-bonding strength to a level similar to DMSO. This allows us to reproduce the emission shifts observed in DMSO and in heated H2O. In combination, these data allow us to relate the characteristic solvatochromic shifts to the originating photophysical processes. Charge transfer is assigned to the intrinsic processes within the edge band, while H-bonding interactions between CDs and solvent have the most influence on the surface band emission. Furthermore, we observe strong fluorescence quenching of the edge band in DMSO but fluorescence enhancement in the surface band. The origin is assigned to quenched, surface attached fluorophores that emit within the edge band energy range. In N,SCDs, where a large fraction of emission is quenched, we find a switch from excitation independent (fluorophore) to excitation dependent (edge state) emission in the edge band. In return, the radiative decay in the surface band is enhanced likely by deactivating fluorophore related traps. 29 ACS Paragon Plus Environment


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ASSOCIATED CONTENT Supporting Information Temperature dependent absorption spectra of N- and N,S-CDs (Figure S1 and S2); impact of temperature on CD stability (Figure S3, S4); FTIR (Figure S5, S6) and XRD (Figure S7) spectra; detailed temperature dependent PL lifetimes with the corresponding trends showing the difference between cold and heated samples (Table S1). This information is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel: +852-3442-9532

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was allowed by NPRP grant No 8-878-1-172 from the Qatar National Research Fund (A Member of the Qatar Foundation), by the Ministry of Education, Youth and Sports of the Czech Republic (LO1305), and by the funding from Palacky University institutional support.


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