Size-Dependent Electronic Properties of Uniform Ensembles of

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Size-Dependent Electronic Properties of Uniform Ensembles of Strongly Confined Graphene Quantum Dots Zhiqiang Ji, Enkeleda Dervishi, Stephen K. Doorn, and Milan Sykora J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00119 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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The Journal of Physical Chemistry Letters

Size-Dependent Electronic Properties of Uniform Ensembles of Strongly Confined Graphene Quantum Dots Zhiqiang Ji,1 Enkeleda Dervishi,2 Stephen K. Doorn,2 Milan Sykora1,*

1

Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States. 2

Materials Physics and Applications Division, Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

*Address correspondence to [email protected]

ACS Paragon Plus Environment

1


ABSTRACT. Electronic structure of a series of bottom-up synthesized GQDs smaller than 2 nm was investigated by ensemble level spectro-electrochemistry, yielding insights not available previously from ensemble studies. The results show that for the strongly confined GQDs the dependence of the band gap on the GQD size deviates from the prediction of the standard Dirac fermion model, but agrees well with the models explicitly accounting for the electron-electron interactions. The HOMO/LUMO energy levels are found to be distributed nearly symmetrically around the 0V value vs. NHE, becoming more positive/negative, with increasing GQD size. The exciton binding energies are found to follow power dependence on the number of carbon atoms per GQD, with the experimental values falling within the range of ~0.1-0.6 eV. Given the broad accessibility of the described experimental tools and methods, our work opens a path to more systematic examination of quantum confinement effects in GQDs.

Table of content graphic GQD(1) GQD(2) GQD(3) C60H23(COOH) C132H36(COOH)2 C204H48(COOH)2

N

N 200

-1 -1 (-3.5)

250

-2 (-2.5) -2 -1 -1 (-3.5)

LUMO

0 (-4.5) 0 (1) (2) (3)

HOMO

1 (-5.5) 1

N(2)

(1)

2 (-6.5) 2 0

50

100

-1 0 -1 (-2.5) (-3.5) -2 -2

50

100

-2 (-2.5) -2

0 (-4.5) -10 -1 (-3.5)

2 (-6.5) 2

(3)

1 (-5.5) 1 2 (-6.5) 2

150

200

250

150

200

250

50

100 0 -2 (-2.5) -2

N 150 50

200 100

250 150

200

250

-1 -1 (-3.5) LUMO

LUMO

HOMO (1) (2) (3)

HOMO

0 (-4.5) 0

(1) (2) (3) 1 (-5.5) 1 (1)

2 (-6.5) 2

(2) (1)

(3)(2)

(3)

N LUMO

(1) (2) (3)

HOMO LUMO

(-5.5) 01 (-4.5) 01 2 (-6.5) 1 (-5.5) 12

0 (-4.5) 0

0

E(V) vs. NHE

150

E(V) vs. NHE (vac)

100

E(V) vs. NHE

50

E(V) vs. NHE (vac)

0

E(V) vs. NHE

-2 (-2.5) -2

E(V) vs. E(V)NHE vs. NHE

E(V) vs. (vac)(vac) E(V) vs. NHE (vac) E(V)NHE vs. NHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(1) (2) (1) (3)

(2)

HOMO (3)

(1)

(2)

(3)


2

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Thanks to its remarkable properties graphene has been described as the electronic material of the future.1-4 However, the absence of an electronic band gap in bulk graphene presents a significant barrier to its effective utilization in many electronic and optoelectronic applications. One way an electronic bandgap can be introduced into graphene is through exploitation of reduced dimensionality effects. Nanometer-size graphene fragments called Graphene Quantum Dots (GQDs) retain many of the remarkable properties of bulk graphene, but unlike bulk graphene, they possess a size-dependent, non-zero electronic band gap.5 According to theory,5-11 the sizedependence of graphene electronic properties is a consequence of quantum confinement (QC) effects similar to those observed in inorganic nanocrystals, but with quite different scaling laws, sensitivity to its shape, edge structure and functionalization. Systematic experimental studies of these effects are an important step towards utilization of GQDs in practical applications and understanding and potential exploitation of the interplay between the QC and relativistic effects in strongly confined GQDs.12 In spite of many exciting theoretical predictions and important practical implications, quantitative experimental studies of QC effects in GQDs have so far been limited. This is, in part, due to the difficulties with reproducible preparation of structurally uniform GQDs and challenges with their experimental characterization. The methods for preparation of GQDs are classified typically into “top-down” and “bottom-up” approaches. The “top-down” approaches based on physical “cutting” of exfoliated graphene sheets using electron/ion beam can produce structures, one at a time, with limited reproducibility and control over the specific size, shape and edge structure. The “top-down” approaches based on chemical “cutting” of exfoliated graphene by strong acids and/or bases can produce large quantities of GQD fragments rapidly but yield non-uniform ensembles of structures with distributions of shapes, sizes and edge structures, typically randomly functionalized with –OH, =O, -O-, NH2 or


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other chemical groups. In spite of ongoing efforts to optimize these methods, the control over reproducibility and structural uniformity remain limited. The “bottom up” approaches, based on piece-by-piece chemical synthesis of GQDs from small molecular fragments,13-16 can reproducibly yield uniform ensembles of GQDs in large quantities. However, unless functionalized at the edges by bulky or polar groups17-18 the synthesized structures tend to aggregate due to strong π-π interactions between the individual GQDs. The aggregated GQDs are difficult to characterize and incorporate into applications. Low reproducibility and structural non-uniformity of GQDs prepared by “top-down” methods severely limit the information the ensemble level studies can reveal about the QC effects in GQDs. On a single nanostructure level these problems limited studies mostly to structures larger than 5 nm.19 The need for peripheral functionalization of the “bottom-up” synthesized GQDs, on the other hand, has constrained their studies primarily to optical spectroscopy, providing only partial information about their electronic structure. Recently, we developed a surface-assisted “bottom-up” GQD synthesis that addresses many of the above challenges.20-21 Our approach reproducibly yields uniform ensembles of well-defined structures with sizes less than 5 nm, while mitigating the aggregation problems. The synthesized GQDs can be effectively studied using commonly accessible ensemble level characterization methods. Building on this advance, in this work, we experimentally investigate the effect of QC on the electronic structure of series of GQDs with sizes less than 2 nm (≤ 204 carbon atoms). We show how the electronic band gaps, absolute band offsets, densities of states and exciton binding energies of these GQDs vary with their size. We also show that the experimental results are in a good agreement with the predictions of theory.


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The surface-assisted synthesis approach used in preparation of GQDs is schematically summarized in Fig. 1a (see methods section, SI and refs.

20-21

for more details). Briefly, a GQD

precursor (P), functionalized with one or more carboxylic acid functional groups, is synthesized by established methods in solution.13 The P is then chemisorbed onto a surface of a metal oxide, such as nanocrystalline tin-doped indium oxide (nc-ITO), whereby the carboxylic groups function as surface anchors. Subsequently, P is converted into a GQD in-situ by oxidation with FeCl3 (Scholl process).22-23 Figure 1b shows structures of the GQDs synthesized and studied in this work. The surface assisted synthesis of GQD(1) and GQD(3) is described here for the first time. In the GQDs the number of conjugated carbon atoms (excluding the carbon atoms comprising the carboxylic groups) varies from 60 to 132 and 204. All GQDs are fully benzenoid; i.e., they have 6n π electrons with n = 60, 132 and 204. Consequently, according to theory, no zero-energy edge states are expected to be present in any of the structures.24 The sizes of the structures were determined, as described previously,19 as an average of the lateral dimensions of the GQDs and found to be 0.97, 1.51 and 1.62 nm, respectively. To confirm the structural identities of the synthesized GQDs, these were investigated in-situ, on nc-ITO, by MALDI mass spectroscopy (MS). The details of the method were described previously.1 Figures 2a and 2b show the results for the GQD(1) and GQD(2), respectively. Both spectra are dominated by a single peak consistent with the calculated mass and isotope distribution of the investigated GQDs. Lack of additional peaks indicates the completion of the dehydrogenation reaction and no significant contamination. We were unable to complete the MALDI-MS analysis of the GQD(3) because of its poor desorption from the nc-ITO surface. However, we were able to confirm the completeness of the conjugation of the synthesized GQDs by using infrared (IR) spectroscopy. As shown previously,25 an absence of a combination band at


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~4050 cm-1 in the IR spectrum of a GQD is an indication of the lack of free-rotating (unconjugated) benzene rings. Comparison of the IR spectra of the surface adsorbed P before and after the conversion to a GQD showed in all cases a complete disappearance of the 4050 cm-1 band. The absorption spectra of the nc-ZrO2/GQD films are shown in Fig. 2c.26 The appearance of broad, visible absorption bands is consistent with the formation of large conjugated aromatic systems. The bands in the UV to visible region show distinct peaks, which according to Clar’s classification,27-28 are labeled p and β in the order of increasing energy. This assignment is consistent with the recent analysis of the emission spectra of similar GQDs.29 When comparing the absorption spectra of GQD(1-3) both absorption peaks progressively red shift across this series, consistent with the prediction of decrease in the energy gap with increasing GQD size.9 Figure 2d shows Raman spectra of the GQDs(1-3) produced with 532 nm excitation. The spectra of all GQDs are dominated by two main peaks; one at ~1600 cm-1, attributed to the so-called Gband and one at ~1310-1340 cm-1 attributed to the so-called D-band.30-31 Several additional smaller combination peaks are observed at higher frequencies: ~2620 (2D), ~2900 (D+G), ~3190 (2G). The inset of the figure shows the plot of the ratio of intensities of the main D and G peaks as a function of the GQD size. Based on previous studies of size-dependent properties of GQDs,32-33 the observed increase in I(D)/I(G) is consistent with the increase in the size of the conjugated region of the GQD. The electronic structure of the GQDs was investigated by a spectro-electrochemical approach described in our previous work.20 Briefly, GQDs synthesized on a surface of a conductive, transparent nc-ITO were immersed in a liquid electrolyte and exposed to an electrochemical potential by means of a variable potentiostat. The effect of the applied potential on the absorption properties of the nc-ITO/GQD film was simultaneously monitored by linear absorption


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spectroscopy (Fig. 3a). Any variation in the absorption of the film was recorded as a difference in the absorption in the presence and the absence of the applied potential. The observed (fully reversible) changes in the differential absorption of the nc-ITO/GQD(1-3) films, upon exposure to a potential in the range 0 to +1.5 V, relative to the standard hydrogen electrode (NHE), are shown in Figure 3(c)-(e). The differential absorption spectra of all samples are dominated by a strong negative signal (bleach) at ~3.1 eV for GQD(1), 2.5 eV for GQD(2) and 2.1 eV for GQD(3). In addition to the bleach, all spectra show a pronounced positive signal (induced absorption, IA) at lower energies. The observed IA results from changes in the GQD’s electronic structure following a charge injection, as shown schematically in Fig. 3b. We note that in previous studies it was shown that charging of bulk graphene34 leads to bleaching of the electronic transitions near the Dirac point in a fashion similar to the bleaching of band-edge electronic transitions observed for GQDs here. However, no significant IA was observed in bulk graphene in the visible-to-near infrared spectral range. The observed variations in the differential absorption were globally fit with a modified Nernst expression in the form of Eq. (1): 𝛥𝐴(𝜆) = ∑ ∆𝐴𝑖 (𝜆) = ∑ 𝜀𝑖 (𝜆)[𝛽𝑖 ⁄(𝛽𝑖 + 1)] , where

(1)

𝛽𝑖 = 𝑒𝑥𝑝[𝛼𝑖 (𝐸 − 𝐸𝑖0 )⁄𝑘𝐵 𝑇].

In Eq. (1) ΔA is a differential absorbance, ΔAi is a differential absorbance associated with an injection of an ith charge, E is the applied potential, kB is the Boltzman constant, T is temperature. The adjustable parameters εi, Ei0, and αi are respectively: scaling factor, standard redox potential and a non-ideality factor associated with the ith charge injection processes. A more detailed discussion of the parameters can be found in SI and in our previous work.20


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The panels (f)-(h) of Fig. 3 show the results of the global fitting of the Eq. (1) to the data shown in panels (c)-(e), respectively. Very good agreement between the experiment and the best global fit was achieved when the number of terms of Eq. (1) used in the fitting was one, two and three for GQD(1), GQD(2) and GQD(3), respectively. The increase in the number of required fitting terms is an indication that in the case of GQD(2-3) multiple charge injections took place in the studied potential range. The potentials extracted from the fits are attributed to the standard potentials for the first, second and third oxidation (hole injection). The extracted parameters and the processes they are associated with are summarized in Table 1. To determine the potential necessary for injection of an electron into the GQDs, we performed a similar experimental study and analysis on the same films in the negative potential range, 0 to 1.8 V vs. NHE (see SI).35 In this potential range clear bleaching of the absorption was observed for GQD(2) and GQD(3), but not for GQD(1). The results of the fitting of the Eq. (1) comprising a single charge injection term led to high quality fits. The lack of significant change in the absorption observed in the case of GQD(1) indicated that the injection of an electron into the GQD(1) requires a more negative potential than -1.8V, and was outside of the usable range of the nc-ITO electrode. We were, however, able to estimate this potential from the optical measurements and the calculated exciton binding energy for GQD(1), as described below. The extracted reduction (electron injection) potentials for GQD(1-3) are included in Table 1. The results of the above analysis are summarized graphically in Fig. 4a, which shows the hole and electron injection potentials for GQD(1-3), plotted as a function of the number of GQD carbon atoms, N. The plot shows that the E1,ox0 (HOMO) and E1,red0 (LUMO) energy levels are distributed nearly symmetrically around the 0V value vs. NHE, with the LUMO becoming more positive and HOMO more negative with increasing N (GQD size). The HOMO potentials determined here are


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in a good agreement with the ionization potentials obtained previously for similar structures by photoelectron yield spectroscopy (grey symbols in the main plot).36 The plot also reveals the increase in the density of states in the valence band of the GQD(3) compared to GQD(2). With the knowledge of the oxidation and reduction potentials in GQDs we can calculate the magnitude of the electronic (quasi-particle) bandgap for each of the GQDs using an expression Eg(el)GQD = E1,red0 - E1,ox0 and, for the first time, experimentally investigate the relationship between the Eg(el)GQD and the size of the GQDs with sizes 2 nm, prepared by physical “top down” approach from exfoliated bulk graphene (blue symbols). In that work the authors showed that, for the GQDs with armchair edges, the best fit to the experimental data follows a dependence Eg(el)GQD = 1.57±0.21/L1.19±0.15 (solid blue line in Fig. 4b). According to the Dirac fermion model, the electron confinement energy in GQDs can be expressed as ΔEc = Eg(el)GQD - Eg(el)Bulk = πħνf/L = ~2 eV•nm/L,37 where νf (~106 m/s) is the Fermi velocity of electrons in graphene and L is the lateral dimension of a GQD. Since for bulk graphene Eg(el)Bulk = 0,38 the model predicts that the Eg(el)GQD should scale with 1/L (ΔEc = Eg(el)GQD = ~2/L), which is in a reasonable agreement with the experimental results of Ritter et.al.19 We find that when the experimental data obtained in the present work for GQD(1-3) are included, the best fit to both sets of data yields a dependence Eg(el)GQD = 3.30±0.25/L1.42±0.19 (red solid line). While the dependence is still in qualitative agreement with the theory the adjustable parameters in the equation clearly deviate from the predicted 2/L dependence. The observed discrepancy between the theory and the experiment is attributed to previously identified limitations


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of the standard Dirac fermion model for GQDs with less than several hundred atoms.11 For these strongly confined structures use of atomistic models, which explicitly account for the electronelectron and excitonic (excite state electron-hole) interactions, is needed to achieve more quantitative agreement with the experiment.39-40 In Figure 4c we plot the electronic Eg(el)GQD (red solid symbols) as well as an optical bandgap, Eg(op)GQD (obtained by analysis of ground state absorption spectra as described in SI) (hollow red symbols) as a function of N-1/2. Also incuded are Eg(op)GQD values for GQDs studied previously in solutions (gray triangles).17, 41 In addition to the experimental data the plot also shows EgGQD calculated for GQDs with N1eV), of similar sizes.43 They are only moderately higher than Ebs of single-wall carbon nanotubes with diameters comparable to the lateral size of GQDs (~0.02-0.1 eV).43 The relatively low Ebs of GQDs together with high brightness and no PL blinking as reported recently,29 suggest a potential of GQDs for exploitation in optoelectronics.


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We have prepared a series of structuraly well defined, fully benzenoid GQDs with sizes less than 2 nm. Using spectroelectrochemistry and optical spectroscopy characterization we have experimentally quantified the effect of QC on the electronic structure of the strongly confined GQDs and compared the results with the predictions of theory. The presented approach and results open a path to a more systematic experimental investigation of the QC effects in GQDs. Conflict of interest: The authors declare no competing financial interest. Supporting Information Available: Details of synthesis, materials fabrication and characterization and details of analysis of experimental results. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgments: Z.J., E. D. S.K.D, and M.S. acknowledge the financial support by the Los Alamos National Laboratory Directed Research and Development (LDRD) program. This work was performed in part at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. We thank Dr. C. Kreller (LANL) for the assistance with nc-ITO films preparation.


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References 1. Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nature Mat. 2007, 6 (3), 183-191. 2. Geim, A. K. Graphene: Status and Prospects. Science 2009, 324 (5934), 1530-1534. 3. Novoselov, K. S.; Fal'ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490 (7419), 192-200. 4. Ferrari, A. C., et al. Science and Technology Roadmap for Graphene, Related TwoDimensional Crystals, and Hybrid Systems. Nanoscale 2015, 7 (11), 4598-4810. 5. Guclu, A. D.; Potasz, P.; Korkusinski, M.; Hawrylak, P. Graphene Quantum Dots. Springer-Verlag: Berlin, 2014. 6. Nakada, K.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Edge State in Graphene Ribbons: Nanometer Size Effect and Edge Shape Dependence. Phys. Rev. B 1996, 54 (24), 1795417961. 7. Son, Y. W.; Cohen, M. L.; Louie, S. G. Energy Gaps in Graphene Nanoribbons. Phys. Rev. Lett. 2006, 97 (21). 8. Zhang, Z. Z.; Chang, K. Tuning of Energy Levels and Optical Properties of Graphene Quantum Dots. Phys. Rev. B 2008, 77 (23). 9. Mandal, B.; Sarkar, S.; Sarkar, P. Exploring the Electronic Structure of Graphene Quantum Dots. J. Nanopart. Res. 2012, 14 (12), 1-8. 10. Mansilla Wettstein, C.; Bonafe, F. P.; Belen Oviedo, M.; Sanchez, C. G. Optical Properties of Graphene Nanoflakes: Shape Matters. J. Chem. Phys. 2016, 144 (22). 11. Ozfidan, I.; Korkusinski, M.; Hawrylak, P. Electronic Properties and Electron-Electron Interactions in Graphene Quantum Dots. Phys. Status Solidi RRL 2016, 10 (1), 13-23. 12. Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Chaotic Dirac Billiard in Graphene Quantum Dots. Science 2008, 320 (5874), 356358. 13. Watson, M. D.; Fechtenkotter, A.; Müllen, K. Big Is Beautiful - "Aromaticity" Revisited from the Viewpoint of Macromolecular and Supramolecular Benzene Chemistry. Chem. Rev. 2001, 101 (5), 1267-1300. 14. Feng, X.; Pisula, W.; Müllen, K. Large Polycyclic Aromatic Hydrocarbons: Synthesis and Discotic Organization. Pure Appl. Chem. 2009, 81 (12), 2203-2224. 15. Yan, X.; Li, L.-s. Solution-Chemistry Approach to Graphene Nanostructures. J. Mater. Chem. 2011, 21 (10), 3295-3300. 16. Yan, X.; Li, B.; Li, L.-S. Colloidal Graphene Quantum Dots with Well-Defined Structures. Acc. Chem. Res. 2013, 46 (10), 2254-2262. 17. Yan, X.; Cui, X.; Li, L.-s. Synthesis of Large, Stable Colloidal Graphene Quantum Dots with Tunable Size. J. Am. Chem. Soc. 2010, 132 (17), 5944-5945. 18. Tan, Y.-Z.; Yang, B.; Parvez, K.; Narita, A.; Osella, S.; Beljonne, D.; Feng, X.; Muellen, K. Atomically Precise Edge Chlorination of Nanographenes and Its Application in Graphene Nanoribbons. Nature Commun. 2013, 4. 19. Ritter, K. A.; Lyding, J. W. The Influence of Edge Structure on the Electronic Properties of Graphene Quantum Dots and Nanoribbons. Nature Mat. 2009, 8 (3), 235-242. 20. Ji, Z.; Doorn, S. K.; Sykora, M. Electrochromic Graphene Molecules. ACS Nano 2015, 9 (4), 4043-4049.


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Page 14 of 21

21. Ji, Z.; Wu, R.; Adamska, L.; Velizhanin, K. A.; Doorn, S. K.; Sykora, M. In Situ Synthesis of Graphene Molecules on TiO2: Application in Sensitized Solar Cells. ACS Appl. Mat. Interf. 2014, 6 (22), 20473-20478. 22. Scholl, R.; Seer, C. The Secession of Aromatic Bounded Hydrogenn and the Attachment of Aromatic Nuclei through Aluminium Chloride. J. Lieb. Ann. Chem. 1912, 394 (1/3), 111-177. 23. Theoretical modeling of the GQDs21 shows that the anchoring groups introduced at the periphery of the GQDs are electronically isolated; i.e., they do not significantly affect the electronic structure of the GQD. 24. McGuire, J. A. Growth and Optical Properties of Colloidal Graphene Quantum Dots. Phys. Status Solidi RRL 2016, 10 (1), 91-101. 25. Wu, J.; Gherghel, L.; Watson, M. D.; Li, J.; Wang, Z.; Simpson, C. D.; Kolb, U.; Müllen, K. From Branched Polyphenylenes to Graphite Ribbons. Macromolecules 2003, 36 (19), 70827089. 26. ZrO2 is a semiconductor with a band gap known to be larger than 5 eV and small density of trap states; therefore no electronic coupling between ZrO2 and GQDs is expected. 27. Clar, E. Polycyclic Hydrocarbons. 1964; Vol. 1. 28. Wassmann, T.; Seitsonen, A. P.; Saitta, A. M.; Lazzeri, M.; Mauri, F. Clar's Theory, PiElectron Distribution, and Geometry of Graphene Nanoribbons. J. Am. Chem. Soc. 2010, 132 (10), 3440-3451. 29. Zhao, S., et al. Single Photon Emission from Graphene Quabtum Dots at Room Temperature. Nature Comm. 2018, 9:3470, 1-5. 30. Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B 2000, 61 (20), 14095-14107. 31. Ferrari, A. C.; Robertson, J. Resonant Raman Spectroscopy of Disordered, Amorphous, and Diamondlike Carbon. Phys. Rev. B 2001, 64 (7). 32. Cancado, L. G., et al. Quantifying Defects in Graphene Via Raman Spectroscopy at Different Excitation Energies. Nano Lett. 2011, 11 (8), 3190-3196. 33. Lucchese, M. M.; Stavale, F.; Ferreira, E. H. M.; Vilani, C.; Moutinho, M. V. O.; Capaz, R. B.; Achete, C. A.; Jorio, A. Quantifying Ion-Induced Defects and Raman Relaxation Length in Graphene. Carbon 2010, 48 (5), 1592-1597. 34. Polat, E. O.; Kocabas, C. Broadband Optical Modulators Based on Graphene Supercapacitors. Nano Lett. 2013, 13 (12), 5851-5857. 35. Application of more negative potential led to a significant loss of transparency of the ncITO electrode. Thus, the experimental window on the reductive side this type of electrode is limited to -1.8 V. 36. Yan, X.; Li, B.; Cui, X.; Wei, Q.; Tajima, K.; Li, L.-s. Independent Tuning of the Band Gap and Redox Potential of Graphene Quantum Dots. J. Phys. Chem. Lett. 2011, 2 (10), 11191124. 37. Berger, C., et al. Electronic Confinement and Coherence in Patterned Epitaxial Graphene. Science 2006, 312 (5777), 1191-1196. 38. Fujii, S.; Enoki, T. Nanographene and Graphene Edges: Electronic Structure and Nanofabrication. Acc. Chem. Res. 2012, 46 (10), 2202-2210. 39. Ozfidan, I.; Guclu, A. D.; Korkusinski, M.; Hawrylak, P. Theory of Optical Properties of Graphene Quantum Dots. Phys. Status Solidi RRL 2016, 10 (1), 102-110. 40. Li, Y.; Shu, H.; Wang, S.; Wang, J. Electronic and Optical Properties of Graphene Quantum Dots: The Role of Many-Body Effects. J. Phys. Chem. C 2015, 119 (9), 4983-4989.


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41. Debije, M. G.; Piris, J.; de Haas, M. P.; Warman, J. M.; Tomovic, Z.; Simpson, C. D.; Watson, M. D.; Mullen, K. The Optical and Charge Transport Properties of Discotic Materials with Large Aromatic Hydrocarbon Cores. J. Am. Chem. Soc. 2004, 126 (14), 4641-4645. 42. Sheng, W.; Sun, M.; Zhou, A.; Xu, S. J. Substrate Effects on Quasiparticles and Excitons in Graphene Nanoflakes. Appl. Phys. Lett. 2013, 103 (14). 43. Scholes, G. D.; Rumbles, G. Excitons in Nanoscale Systems. Nature Mat. 2006, 5 (9), 683696.


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Page 16 of 21

Table 1. Summary of the extracted parameters from the global fit to the spectro-electrochemical data for GQD(1), GQD(2) and GQD(3). Charging Process

Parameter

GQD(1)

GQD(2)

GQD(3)

Hole injection GQD – e- → GQD+•

E1,ox0

1.45 ± 0.05

0.80 ± 0.05

0.85 ± 0.05

GQD+• – e- → GQD2+

E2,ox0

-

1.3 ± 0.1

1.20 ± 0.05

GQD2+• – e- → GQD3+•

E3,ox0

-

-

1.70 ± 0.05

α1,ox

0.11 ± 0.01

0.22 ± 0.01

0.23 ± 0.01

α2,ox

-

0.18 ± 0.02

0.19 ± 0.02

α3,ox

-

-

0.28 ± 0.06

E1,red0

-1.81 ± 0.05*

-1.57 ± 0.07

-1.04 ± 0.05

α1,red

-

0.28 ± 0.03

0.20 ± 0.03

Electron injection GQD + e- → GQD-• *

The value was obtained indirectly from the analysis of the size dependence of the exciton binding energy as discussed in the text. All potential values are shown versus standard NHE as a reference. See SI for more detailed discussion of the extracted α parameters


16

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FIGURES.

Figure 1. (a) Scheme of the surface-assisted, bottom-up synthetic approach developed for preparation of uniform ensembles of strongly confined graphene quantum dots (GQDs). (b) Structures of the GQDs investigated in the present study. The structures are drawn using Clar representations.27-28 All structures are fully benzenoid with 6n π electrons, where n = 60, 132, and 204.


17


m/z 750 600

1000

Energy (eV) 1250

1500 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Absorbance

400

(1) (2) (3) β 200

β

786 788 790 792 794

m/z

p

(a) 0 80

p 1.1

(b)

1622.77

β

p

1.0 0.9

60

0.8

(c) (d)

0.7

40

0.5 1.0 1.5 2.0 2.5

L (nm) 1622

20

1624

1626

Intensity (a.u.)

M+Cl

I(D)/I(G)

Intensity (a.u.)

788.05

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

m/z

D

0 1500

2000

2500

m/z

3000

3500

4000 1000

G

1500

2D D+G 2000

2500

3000

Wavenumber (cm-1)

Figure 2. Structural characterization of GQDs. (a) and (b), MALDI-TOF mass spectra of the surface synthesized NGs 1 and 2 respectively. The molecular weight of the highest intensity mass peak is indicated. The laser energy used in the experiment was 13 μJ. Insets: the experimental (red curves) and calculated (black) isotope distribution for mass peak of C60H24+(a) and C132H37+(b). (c) Absorption spectra of the nc-ITO surface synthesized GQDs 1, 2 and 3, investigated in the present study. The inset shows the photographic image of the GQD/nc-ITO films. (d) Raman spectra of the surface adsorbed GQDs 1, 2 and 3. The color scheme is the same as in (c). The shaded areas indicate regions where the D and G bands and the corresponding combination bands are typically observed in carbon structures. The inset shows the plot of ratio of the intensities of D and G bands for all three GQDs. Raman spectra were collected using 532 nm laser excitation with power of 2 mW.


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Figure 3. Summary of the GQD spectro-electrochemical studies. (a) Scheme of the spectroelectrochemical experiment. (b) Schematic energy diagram of the change of the GQD electronic structure upon a hole injection. (c)-(e) Difference absorption spectra of nc-ITO/GQD(1), ncITO/GQD(2) and nc-ITO/GQD(3) respectively, recorded as a function potential applied onto the nc-ITO electrode. The difference spectra were generated by subtraction of the spectrum obtained at 0 V from spectra at the indicated potential (all potential step increments are 0.1 V. (f)-(h) Experimental difference absorption spectra, respectively, of the nc-ITO/GQD(1), nc-ITO/GQD(2) and nc-ITO/GQD(3) films for E = 1.5 V (gray trace) and the corresponding best global fit of Eq. (1) to the experimental data (black dashed trace). Also shown are the spectra of components ΔA1 (red trace), ΔA2 (blue trace), and ΔA3 (green trace), corresponding to the difference absorption spectra of GQD+, GQD2+ and GQD3+.


19


N 50

100

150

200

250 0

2

4

6

(a)

(b) LUMO HOMO

1 (-5.5) 1

Eg (eV)

3

2

Ritter et. al. this work

1

(d)

(c)

triang.(t) hex.(t) hex.2(t) exp.

2 el opt opt-sol

1 0 0.00

0.05

0.10

N

TB HF CI ~N-1/2

0.15

0.20 0

3

Eg = a/Lb

(3)

(2)

(1)

2 (-6.5) 2

10

Eg = 2/L

0 (-4.5) 0

4

8

Eg (eV)

-1 (-3.5) -1

0

L (nm)

0 1.2

1.0 0.8 0.6 0.4

Eb (eV)

-2 (-2.5) -2

E(V) vs. NHE

E(V) vs. NHE (vac)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 21

0.2 0.0 100 200 300 400 500 600

-1/2

N

Figure 4. Size-dependence of the GQD electronic properties. (a) Variation in absolute energies of the frontier energy levels in GQDs(1-3) with the number of aromatic carbons, N. The grey symbols are data from ref. 36. The dashed lines are a guide for an eye showing the variation in the HOMOLUMO gap. (b) Variation of the energy gap with the GQD size, L. The red symbols are the HOMOLUMO electronic bandgaps (from panel (a)) for GQDs(1-3). The blue symbols are data from ref. 19

, obtained by STS. The solid red curve is the best fit of equation Eg = a/Lb to both sets of data

and the solid blue line to the data from ref. 19 only. The dashed red line is a best fit to the Eg(opt), for the GQDs(1-3). The solid black curve shows theoretically predicted 2/L dependence. (c) Variation in the GQD band gap with N-1/2. The solid and empty red circles are the Eg(el) and Eg(opt) respectively, for GQDs(1-3). The triangles are the Eg(opt) of GQDs investigated in solutions in previous studies.17, 41 The dashed lines are the average energy gaps of the hexagonal and triangular GQDs calculated by tight-binding (TB), Hartree-Fock (HF) and Configuration Interaction (CI)


20

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approaches.11, 39 The solid line is the Eg~1/N1/2 dependence. (d) Variation in the exciton binding energy, Eb, of a GQD with N. The empty symbols are theoretically calculated Eb’s, for triangular and hexagonal GQDs.11, 39, 42 The solid and dashed curves are the best power law fits to the data. The filled red circles are the experimental Eb’s for GQDs(2-3), obtained from the data in panel (c). The red solid line is the best power law fit to the experimental data. The empty solid circle is an Eb obtained by extending the best fit power law fit to the N = 60.


21

Size-Dependent Electronic Properties of Uniform Ensembles of

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