Electronic Spectra of Graphene in Far- and Deep Ultraviolet Region

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C: Physical Processes in Nanomaterials and Nanostructures

Electronic Spectra of Graphene in Far- and Deep Ultraviolet Region. Attenuated Total Reflection Spectroscopy and Quantum Chemical Calculation Study Krzysztof B. Bec, Yusuke Morisawa, Kenta Kobashi, Justyna Grabska, Ichiro Tanabe, and Yukihiro Ozaki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08089 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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The Journal of Physical Chemistry

Electronic Spectra of Graphene in Far- and Deep- Ultraviolet Region. Attenuated Total Reflection Spectroscopy and Quantum Chemical Calculation Study

Krzysztof B. Beć,*1 Yusuke Morisawa,*2 Kenta Kobashi1, Justyna Grabska1, Ichiro Tanabe3 and Yukihiro Ozaki*1

Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan 1

Department of Chemistry, School of Science and Engineering, Kindai University, Kowakae, HigashiOsaka, Osaka 577-8502, Japan 2

3 Graduate

School of Engineering Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-8531,

Japan

Corresponding Authors. Email: [email protected] Email: [email protected] Email: [email protected]

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Abstract We measured the electronic spectra of graphene nanostructures (flakes and platelets) extending into far-ultraviolet (FUV) region by attenuated total reflection far- and deepultraviolet (ATR-FUV-DUV) spectroscopy in the region of 2.76-8.55 eV (450-145 nm). The major absorption of graphene appears in the DUV region (4.7 eV), as already reported, however, we observed a new peak in the FUV region; visible clearly in the case of the flakes at 7.5-7.7 eV (165-161 nm) and less pronounced in the spectrum of the platelets at 6.6-6.7 eV (188-185 nm). Graphene flakes (thickness 1-2 nm; sub-micrometers of side dimension) and nanoplatelets (thickness 6-8 nm; several micrometres of side dimension) give notably different ATR absorbance spectra in the spectral region studied. This discrepancy is reduced upon applying mechanical pressure on the samples. These observations can evidence that the morphology as well as electronic structure of graphene can be manifested in the FUV-DUV region. Quantum chemical calculations were applied to several molecular models incorporating the expected principal structural features of graphene nanostructures. Based on the results of timedependent density functional theory (TD-DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) calculations, it was possible to consistently reproduce the experimental spectral variations in terms of both band positions and intensities. The spectral differences result from the differences in the die area, ordering and the number of layers; structural factors which separate nanoflakes and nanoplatelets. These results provide insights into the probable origins of the spectral variability of graphene nanostructures as well as the molecular orbitals involved in a FUV -* transition of graphene nanostructures.

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1. Introduction Electronic states of graphene have been of keen interest over the last two decades.1,2,3,4,5,6,7,8,9,10,11 The unique properties of graphene are strongly correlated with its specific electronic structure featuring conjugated  system.12 Accordingly, an extensive amount of information about graphene has been gathered, with significant contributions from theoretical and computational investigations. For example, analytical formulation of graphene’s band structure and density of states has been reported in the literature.13,14 Yet, many open questions still remain in the area of electronic states of graphene and also in a broader scope of carbon nanostructures. This includes higher energy transitions. Despite significant progress in the theoretical description of its electronic property, graphene proved to be a difficult system for conducting experimental studies with very scarce spectral data available above 6.2 eV (below 200 nm). Obviously, graphite has been investigated for longer, including early reflectance measurements.15 Even recent studies still debate over graphite optical properties in UV region (2.48-6.20 eV; 500-200 nm);16 difficulties can be noticed due to the influences of experimental conditions, i.e. sample origin and form.16 Thus, knowledge about FUV electronic transitions of graphene have remained limited, with an incomplete experimental background. Much of the studies have focused on single-layer graphene and planar properties.17,18,19 On the other hand, graphene nanostructures have attracted much attention due to possible applications. For example, recent attention is being noticed on the morphology and spatial arrangement of the composites involving carbon nanostructures.20,21,22 This kind of substrates can be yielded relatively easily and offer straightforward and cost-effective applications in designing new materials, i.e. nanocomposites with polymer.23 Graphene nanostructures, i.e. nanoflakes or nanoplatelets are comprised of short stacks of platelet-shaped graphene sheets that are identical to those found in the walls of carbon nanotubes, but in a planar form. In the role of composite constituents, they offer enhanced barrier properties and improved mechanical properties, i.e. stiffness, strength, and surface hardness. The graphene nanoflakes are in between oligolayered graphene sheet and higher carbon carbon structures (i.e. graphite),24,25 and their optical26,27,28,29 and transitional30,31,32 properties are of high interest of general physical chemistry for wider understanding of carbon structures. The morphology and physicochemical properties of graphene have been studied by spectroscopy, i.e. UV-Vis,33 infrared,34 Raman,35,36 circular dichroism,37 and X-ray photoelectron spectroscopy,38 as well as theoretical calculations, focusing not only on electronic structure and closely related properties2,5,6,11,12,17,19,39,40,41 but also on morphology18,19,39,40,42 and interactions.41,43,44 Electronic properties of graphene not only remain in the center of attention of physical chemistry, but also are essential for advanced material chemistry.45,46,47 For example, graphene properties are 2 ACS Paragon Plus Environment


being studied in the context of development of novel sensors48,49 and nanoelectronics.50 Recently, Zheng et al. reported potential usefulness of FUV photovoltaic detector with p-type graphene and aluminium nitride AlN.51 Tu et al. has developed FUV promoted oxidative micro photoetching of graphene oxide.52,53 In these investigations, basic knowledge about electronic states of graphene and graphene oxide are significantly important in advancing the technology. Fairly recent development of attenuated total reflection far- and deep-ultraviolet (ATR-FUVDUV) spectroscopy54,55,56,57,58 has enabled feasible spectroscopy studies of condensed matters in the FUV region, unlike conventional spectroscopy59 limited to gas phase measurements54 or requiring specific approaches, i.e. external reflection, ultra-thin film or synchrotron techniques.54,60,61 ATR-FUV-DUV spectroscopy does not require sample pre-treatment, and thus, it is applicable for a variety of liquid and solid samples under normal conditions.54 The technique offers a wide operating spectral range (2.76-8.55 eV; 450-145 nm) and a number of other advantages,55,62 i.e. a sample may be hold in open air environment.54,58 So far, ATR-FUV-DUV spectroscopy has allowed robust investigations of water and aqueous solutions,58 organic molecules,63,64,65 polymers,23,66 semiconductors67, ionic liquids,68 and inorganic materials (i.e. TiO2).67 It has also demonstrated its potential in elucidating surface effects of various types of samples,69 due to the typical penetration depth of evanescent wave in the range of 40-100 nm.55,54 Although the determination of the exact effective sample thickness is difficult due to surface property effects,70 it may be safely assumed that the penetration depth in any case is less than 100 nm for the spectrometer used in the study; with a bit less confidence the penetration may be estimated to be less than 50 nm in the present case. In this work we report the FUV-DUV spectra of graphene acquired with the use of ATR technique. Although the sensitivity level of the experimental technique is not yet adequate for the measurement of a single layer graphene, we succeeded in obtaining spectra of two kinds of nanostructures of graphene, flakes and platelets. The experimental spectra of these graphene nanostructures uncover a peak in the FUV region, of which position and intensity varies between these two kinds of samples. In the case of the flakes, the FUV peak of moderate intensity appears at 7.5-7.7 eV (165-161 nm), while in the spectrum of the platelets it is much weaker and located at around 6.6-6.7 eV (188-185 eV). Moreover, we found that the morphology of graphene structure correlates with its electronic spectrum in the FUV region. Graphene flakes (thickness 1-2 nm; sub-micrometers side dimension) and nanoplatelets (thickness 6-8 nm; several micrometres of side dimension) manifest differences in their ATRFUV-DUV absorbance, and furthermore, respond to the application of mechanical pressure in different ways in the DUV vs. FUV regions. These two kinds of graphene nanoparticles differ in their size significantly; the flakes should be considered oligo-layered graphene (approximately 25 layers) while the platelets, in this case counting ca. 15-20 graphene layers, get closer to the properties of graphite.16,24,25 For the interpretation of the observed spectral patterns we 3 ACS Paragon Plus Environment

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employed quantum mechanical calculations. The study required considering a number of models to reflect the major structural features which were expected to play a role in the case of observed spectral variability. The investigation of the influence of carbon nanonscale morphology on FUV transitions needed consideration of numerous models, and thus we based the majority of theoretical investigation on semi-empirical calculations at Zerner's Intermediate Neglect of Differential Overlap (ZINDO) level of theory.71 To validate the consistency and reliability of chosen semi-empirical approach, selected models were also subjected to ab initio treatment by means of time-dependent density functional theory (TD-DFT). The calculated spectra consistently reproduced the trends observed experimentally. The symmetry of the molecular orbitals involved in DUV and FUV transitions of the studied models was analysed and the observed trends indicate that similar correlations may occur in real graphene nanostructures. Thus, we measured an absorption peak in FUV region of carbon nanostructures and observed spectral variability in the form of band shifts and intensity changes. These observations were reproduced by quantum mechanical calculations, yielding new insights into the possible origin of the observed FUV features of graphene nanostructures. Both absorption peaks, the DUV and FUV ones, originate from -* transitions. A closer analysis of the calculation results of the model molecules suggest a higher level of delocalization of the electron density specific to the excited states relevant for FUV transitions as opposed to DUV transitions. Further, the conclusion was drawn that the size of graphene planes and their spatial arrangement appear to be strongly manifested in the electronic spectrum. However, the number of graphene layers seems to be less meaningful.

2. Materials and Methods 2.1 Experimental Graphene nanostructures were purchased from Strem Chemicals, Inc. The following samples were used; nanoflakes (grapheme aggregates, sub-micron particles, surface area 300 m2/g) and nanoplatelets (6-8 nm thick, 5 m wide, carbon content of >99.5 wt%). The samples were used as received. The details on the design and properties of the ATR-FUV-DUV spectrometer used in this study have already been reported,55,62 and only the basics will be explained here. The ATR spectrometer for this study used an internal reflective element (IRE) made of sapphire (~8 mm of path length, UV grade, Opto-line, Tokyo), a radiation source in a form of 30 W deuterium lamp. The angle of beam incidence was 70°. During the measurements, the interior of the spectrometer was continuously purged by dry nitrogen flow (3 dm3∙min-1). The spectra were measured in the spectral region of 2.76-8.55 eV (450-145 nm). 4 ACS Paragon Plus Environment


2.2 Quantum chemical calculations A selection of molecular models featuring the structural principles which we expected to play a role in the observed spectral variability was constructed. The major structural parameters of graphene used for the design of the models were taken from literature.3 The validity of the models, including those with and without hydrogen termination was evaluated and discussed in detail (Results and Discussion Section). Quantum mechanical calculations of vertical transition energies and oscillator strengths were mostly based on Zerner's Intermediate Neglect of Differential Overlap (ZINDO)71 theory in ZINDO/s variant. To verify the selected models were also subjected to additional timedependent density functional theory (TD-DFT)72 calculations. CAM-B3LYP (Coulomb-attenuating method, Becke three-parameter exchange, Lee–Yang–Parr correlation)73 density functional approach with Pople 6-31+G(d,p)74 basis set were used, for maintaining reasonable levels of the computational cost. The calculations were performed with the use of Gaussian 16 Rev.A.03 package75. The calculated vertical transition energies and oscillator strengths were subsequently convoluted by a Lorentzian spectral profile with full-width at half-maximum (FWHM) parameter of 1.2 eV. The calculated molecular orbitals were projected with the contour isovalue of 0.02 e/Å3. The assumed colour scheme for the presented orbital contours is as follows: red for positive and blue for negative values. 3. Results and Discussion 3.1 ATR-FUV-DUV spectra of graphene nanostructures. Experimental ATR-FUV-DUV spectra of graphene flakes and nanoplatelets are presented in Figure 1. These spectra can be roughly divided into three regions of interest; (A) the main peak at 4.5-5 eV, (B) the region below the main peak and (C) the region above the main peak. The main peak (A) is likely analogous to the Fresnel peak commonly measured in various studies of nanocarbon structures.15,16,24 Its position varies significantly among these reports, influenced by the sample type, measurement technique and experimental conditions;24 the present work adds new data in this regard with the use of ATR technique. In the literature the peak position of graphene varies; for example, Bhandari et al.76 observed the peak at 240 nm (5.17 eV), Sahu et al.77 reported the peak of pure graphene at 266 nm (4.65 eV). On the other hand, Zhang et al.78 and Pan et al.79 in their studies of graphene quantum dots observed two strong peaks at 320 nm (3.88 eV) and 227 nm (5.46 eV). Due to apparent variability, we will first discuss and explain the spectral differences observed between the samples measured in the present study. Despite general broadness of the spectral outlines, the experimental results yielded for graphene flakes allow for the identification of a DUV band at 4.65 eV (266 nm). This band likely corresponds to the peaks reported by the earlier studies of UV spectra of graphene by Sahu et al.,77 and also other reports.24,80,81 In the case of nanoplatelets, the corresponding peak observed in the present 5 ACS Paragon Plus Environment

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study shifts towards lower photon energy, by ca. 0.2 eV (to ca. 4.45 eV; ca. 278 nm); as explained earlier, the dimensions of nanoplatelets should put them in between multi-layered graphene and graphite. There is a noticeable redshift manifested in the maximum of ATR absorbance (Figure 1). Note that the limitation of the ATR technique involves a distortion of measured spectra, due to the convolution of the real and imaginary parts of the complex refractive index of a sample. The corresponding dispersion and absorption curves of graphene have been resolved recently based on elipsometry,82 and they vary significantly in the region of 3-5 eV. Accordingly, the true absorbance curve just at the high photon energy side of the 4.454.65 eV peak should be much less pronounced and the band shoulder should be reaching the baseline level more rapidly. The region (B), below ~4.5 eV features significant difference between the ATR-FUV-DUV spectra of graphene flakes and nanoplatelets (Figure 1). The reason of this discrepancy may originate from the difference in how ATR technique reacts to macro-morphology of these samples, with flakes and platelets forming an optically different medium at the sample-IRE surface The relative difference in the scattering of the incident electromagnetic wave should be primarily accounted for the spectral variability observed in the region B; this may remarkably affect the measured ATR spectrum as reported recently.83 This phenomenon is now under our detailed investigation in the context of nanocarbon samples. Nevertheless, a possibility of a coincident contribution of electronic properties will be discussed in the present study. The region (C), covering the FUV region above the main peak up to ca. 7 eV is highly interesting as a clear difference between the two kinds of graphene samples can be observed. The absorption of the nanoplatelets is higher throughout this region than that of the nanoflakes, with an exception of an FUV peak, which will be discussed below. An interesting fact was noted upon applying mechanical pressure (clamp, 0.5-1.5 kgF) to the sample placed on the IRE (Figure 2). The spectra of these two kinds of graphene samples become largely similar in the 2.75-4.8 eV region (B). However, throughout the entire higher photon energy part (region C; 4.8-8.5 eV) the spectrum of graphene flakes manifests lower intensity than that of nanoplatelets. This difference does not diminish upon the application of mechanical pressure. Finally, there exists a weak band (D) in the region of 6.6-7.7 eV (flakes 7.5-7.7 eV; nanoplatelets 6.6-6.7 eV) (Figure 1). It is clearly visible in the case of nanoflakes, much less so in the case of nanoplatelets (Figure 2). The spectral differences in the DUV and also FUV regions seen between the two kinds of graphene nanostructures may be caused by a few possible reasons. In the present work we considered the nano-morphology of graphene particles, that is the number of dies and their area, stacking scheme, and their orientation (level of disorder) which may influence the electronic states and result in the observed spectral differences. For the purpose of gaining a 6 ACS Paragon Plus Environment


better understanding of the observed spectral features, we employed a quantum chemical calculation study.

Figure 1. Experimental ATR-FUV-DUV spectra of graphene flakes and nanoplatelets. The spectra have been normalized at 4.65 eV (266 nm). Raw experimental spectra are presented in SI (Fig. S1). Arbitrary regions of interest (A-D) discussed in the text are highlighted.


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Figure 2. Experimental ATR-FUV-DUV spectra of graphene flakes and nanoplatelets upon applying mechanical pressure (1.5 kgF). The spectra have been normalized at 4.65 eV (266 nm). Raw experimental spectra are presented in SI (Fig. S2).

3.2 Simulation of FUV-DUV spectra. To evaluate if the observed spectral variability may be connected with the principal structural features characteristic of the two kinds of graphene nanostructures (the size of a layer in a model, the number of layers and their orientation, AA and AB stacking, boundary condition), and to investigate how these features may be manifested in the electronic spectra, we carried out quantum mechanical calculations. Graphene is a complex system from the point of its electronic properties.1,2,5,11,12,14,18,19,24 However, the practical applicability of the molecular models and computational methods form a significant factor in the general feasibility of the study. In order to yield meaningful data on the sample morphology, a selection of molecular models was designed featuring the principal structural features expected to be relevant in this study. Due to practical limitations, the models were attempted to be as simple as possible, without compromising the agreement with the experimental spectra (Figure 3). Despite their simplicity, the selected molecular models allow to capture the most relevant spectral features characteristic of graphene nanostructures, as it will 8 ACS Paragon Plus Environment


become clear from the agreement between the experimental and simulated spectra. Some of the models were redundant on purpose (featured analogous set of structural principles) to evaluate the consistency of calculated spectra. Therefore, to balance the model size and their number with the computational cost of the method, we based our reasoning on semi-empirical ZINDO calculations. ZINDO calculations have recently been used for the approximation of electronic transitions of graphene.84,85 Budyka et al. have recently used ZINDO calculations to effectively study a variety of complex graphene-based molecules.84 They have concluded that ZINDO based calculation of the absorption spectra yields reasonably reliable results for closedshell molecules. On the other hand, ZINDO method has been used by Suzuki et al. to obtain theoretical circular dichroism spectra of chiral graphene quantum dots.85 Therefore, relatively inexpensive ZINDO calculations are being used in the current research of graphene. Although care needs to be taken, as this approach can deviate the energy of electronic transitions.86 This may strongly vary depending on the studied molecule. Concerning -conjugated systems which are the most relevant for the present study, Suendo and Viridi have recently compared the reliability of a selection of methods in reproducing electronic spectra of chlorophyll molecule.87 They have concluded that the methods based on INDO principle give results of at least comparable quality to TD-DFT, while both surpass the accuracy of calculations by means of configuration interaction singles (CIS).87 Nevertheless, to validate the reliability of the selected computational approach in the treatment of the present molecular systems, we have evaluated the viability of the used methods by comparing semi-empirical ZINDO results with ab initio TDDFT calculations for selected models.


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Figure 3. Selected examples of the molecular models (Mode l, Model II, Model III, Model IV, Model VII, Model IX) designed to reflect the principle structural features of graphene nanostructures. The remaining models are presented in Supporting Information (Fig. S7). 10 ACS Paragon Plus Environment


A comparison of ZINDO and TD-DFT spectra of Model I (Figure 4a) indicates that both methods are capable of reproducing the spectral features of graphene qualitatively. An overestimation of calculated vertical electronic transition energies is a known tendency, and semi-empirical methods consistently work better in this case also for -conjugated systems87 due to implicit parametrization of the wavefunction.88,89 Therefore, the accuracy of the calculated transition energies is very good for ZINDO and the only possible disadvantage may be lack of description of fine features i.e. due to coupling of the electronic transitions to vibrational modes; a factor which is not expected to be of importance in the present case.87 The semiempirical approach consecutively yielded the results of better quality in the presented study. The TD-DFT approach overestimated the position of the major peak by a larger margin than ZINDO. Therefore, our further reasoning will be based on the ZINDO calculations, as these give better possibilities to study more complex models and to focus on explaining the morphology factor.

Figure 4. Comparison of simulated electronic spectra. A: ZINDO and TD-CAM-B3LYP/631+G(d,p) spectra of Model I (Fig. 3A). B: ZINDO spectra of Model I (AA stack) and Model II (AB stack). C: ZINDO spectra of Model III (AA stack) and Model IV (AB stack).

Multi-layered graphenes feature two principle kinds of stacking schemes; AA and AB (Bernal-stacked).90 In the former the layers are aligned, while in the latter the layers are interleaved with the atoms from a given layer being positioned over the centers of hexagons of the adjacent layers.91 Bernal-stacked graphene is in general more stable. In the present study we investigated if these two kinds of graphene stacking forms are differentiated in ZINDO spectra as well. As demonstrated in Figure 4B-C, the stacking scheme does not induce any noticeable dissimilarity. This conclusion is based on the comparison performed for small Models 11 ACS Paragon Plus Environment

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(I and II, Figure 4B) as well as large Models (III and IV, Figure 4C). The latter case yielded even less significant discrepancy, indicating that the stacking scheme should not play a major role in the observed spectral variability. Figure 4B-C also demonstrates an anticipated effect of the area of graphene layer. ZINDO spectra of the larger models feature a blueshift of both peaks and an interesting increase in the high-energy shoulder of the major peak. Both of these trends bring the ZINDO spectra closer to the experimental ones. We also conclude that the major features in the ZINDO spectra of  conjugated systems prevail even upon significant change in the model size. The differences in the ZINDO spectra are gradual, as indicated for analogous comparison with the models of intermediate size (Supporting Information). This relatively small dependency on the sheet dimension also indirectly strengthens the viability of the use of smaller models in our case. Another consideration includes the number of stacked graphene layers. As shown in Figure S6A-B (Supporting Information) the change in the number of layers introduces minor spectral shift and bandshape change in the ZINDO spectra. However, these are too insignificant to be directly confronted with the experimental spectral features. Again, a general nondiscernibility between the AA and AB stacked models is confirmed here. Comparison between a model with ordered, parallel layers and a model featuring nonparallel layers (Figure 5A) indicates that only insignificant spectral effects result in the ZINDO simulation. Note that a blueshift of both peaks is observed in the simulated spectra. The pressure effect applied to graphene nanoflakes may induce an aggregation of the flakes with an increased disorder in the arrangement of the planes. The lower ordered model (Figure 5A) featuring the motif of non-parallel planes gives lower intensity at the higher energy shoulder of the major peak. While the trends predicted by ZINDO calculations seem to reflect the experimental observations (Figure 2), the simulated spectral variability is of much lesser magnitude. This may be due to the use of simplistic models, as larger ones tend to give broader and blue-shifted peak (Figure 4B-C). However, further studies are needed for obtaining better understanding of this phenomenon.



Figure 5. Comparison of ZINDO spectra. A: spectra of Model I (ordered) and Model IX (disordered). B: spectra of Model IV (with H-termination) and Model XI (without Htermination).

Figure 6. An experimental ATR-FUV-DUV spectrum of graphene nanoflakes compared with a ZINDO spectrum of Model I. The energy axis in the calculated spectrum was linearly scaled so that the major peak position matches the position of the experimental one (4.65 eV). 13 ACS Paragon Plus Environment

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The role of the plane boundary assumed in the molecular models was assessed as well. In the literature this concern has been investigated before.18 The general conclusion has been drawn that hydrogen-terminated models lead to better results,18 as carbon-terminated models introduce a strong boundary property.18 In our case it can be confirmed (Figure 5B) that although the discrepancy was not fundamental, the ZINDO spectra of H-terminated models gave lower overestimation of the position of simulated peaks. Figs. S4-S5 (Supporting Information) allow comparing the calculated oscillator strengths. It can be noticed that ZINDO yields higher oscillator strengths for both DUV and FUV peaks than TD-DFT (Fig. S4A). All the values calculated by ZINDO for the analogous models remain consistent; the ability of qualitative comparison of the spectral intensities is, therefore, preserved. I.e. the stacking scheme does not alter the oscillator strengths noticeably (Fig. S4B-C). The disordered model yields relatively higher values though (Fig. S5A), as well as the non-hydrogen terminated model (Fig. S5B); however, in the latter case erroneous results should be expected, as it has been explained earlier above. The simulated spectra (Figs. 4-5 and Fig S6 in Supporting Information) reproduced qualitatively the absorption of graphene in the FUV-DUV region, although the vertical energies of corresponding transitions are overestimated, judging from the comparison with the experimental data (Figure 1). Such tendency among various ab initio and semi-empirical methods is known and expected.87 It should be noted again, that the morphology of models used in the present study resemble more that of nanoflakes rather than that of nanoplatelets, but the predicted trends should be helpful in drawing conclusions also about the latter ones. The results of the calculations give consistent and insightful explanation of the spectral features and variability observed experimentally (Figure 1 and Figure 2). The major absorption band (region A) is reproduced throughput all the theoretical spectra (Figs. 4-5 and Fig S6 in Supporting Information) although its shape and width correlates with the structural complexity of the models. The increased absorption of nanoplatelets in the region below 4.5 eV (region B) finds a match in the calculated spectra of the models with large die area (Figure 4B-C). The modelled plane area is still much smaller than the real one, but the trend remains clear. As explained earlier, this experimental difference may be an effect of the surface properties,83 however the present results may suggest that actually two independent effects coincide. An ongoing study is aimed at exploring this feature in detail. The difference in the experimental region C of the nanoflakes and nanoplatelets (Figure 1 and Figure 2) is reflected in the ordering of the layers in the studied models (Figure 5A). Parallel orientation introduces an elevation of the band shoulder from the higher photon energy side. It may be argued that larger dimensions of the nanoplatelets (larger die area, more layers) favours the tendency for parallelization of the planes. Similar effect but of lesser magnitude can be concluded for an increase in the number of layers in the model (Figure S6 in Supporting Information). These two factors correspond reasonably with the dimensional properties of nanoplatelets as opposed to nanoflakes. 14 ACS Paragon Plus Environment


According to the manufacturer of the samples used in this study, the presence of oxygen baring terminal groups (-OH, -COOH, =O) may not be fully eliminated. Accordingly, in the present study the influence of the oxygen and oxygen baring groups has been analyzed in a systematic way. Not only the general presence of such addition has been investigated but also the number of attached groups and their spatial arrangement have been taken into account (Fig. S8 in Supporting Information). Fig. 7 presents the results of ZINDO simulation of the effect of such additions. The amount of functional groups in relation to carbon atoms is rather high in these models; real sample should not be expected to be contaminated that much. Thus, these models represent artificially very strong boundary effect; it may be considered the worst scenario. Yet, as it can be noticed, the impact of additions is rather insignificant, particularly in the FUV region (Fig. 7 and Fig. S8). Furthermore, the impact of these additions on the MOs relevant for the DUV transitions in the vicinity of 7.5-8 eV (the only case which could affect FUV region; Fig. 7) is rather low. Fig. S9 (Supporting Information) depicts the initial and final states involved in the selected group of more relevant transitions (on the example of carboxyl group attached), showing that a part of these states remains almost unaltered versus the non-substituted Model I. The rest never tends to be susceptible for localization of MOs in the vicinity of the carboxyl group, although an impact on the MOs symmetry can be noticed. Nevertheless, the final influence of the addition on the absorption spectrum remains low. This further strengthens the confidence about the non-essentiality of the possible presence of oxygen-containing functional groups from the point of view of spectral analysis in DUV region and even less so in FUV region. The conclusion drawn is that the influence of these additions is overall minor and only DUV peak could perhaps be affected to a degree, although not likely significant as well. The FUV peak is even less affected, neither in the relative intensity nor its absolute position. Hence, the final conclusion is that the oxygen contamination, if present due to manufacturing process, may safely be neglected as a factor affecting the observations and conclusions


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Figure 7. Results of ZINDO study of the effect of an addition of oxygen baring functional group (based on the Model I). Panel I: spectra normalized at the DUV peak. Panel II: spectra normalized at the FUV peak. A: non-altered Model I. B: carboxyl (-COOH) group; C: hydroxyl (OH); D: oxygen (=O) atom; E: two oxygen atoms present in both layers but spatially separated; F: four oxygen atoms in both layers and in close proximity. Refer to Fig. S8 in Supporting Information for further details.

Finally, an insight into the assignment of the FUV peak (ca. 7.5 eV; 165 nm; region D) observed in the ATR spectrum of graphene flakes and the corresponding molecular orbitals is available from the calculations. When considering the spectrum of graphene flakes, which can be reasonably well approximated by less complex molecular models, a general conclusion is that even Model I gives a reasonably good result. To see if better agreement between the calculated and experimental spectra is achievable, we performed a uniform scaling of the energy axis in the ZINDO spectrum. We believe that a relative similarity of the states involved in the electronic transitions in the case of -conjugated systems, and well reflected by the models, makes such approach feasible. The energy space between the corresponding states also finds good agreement with the experimental data. In Figure 6 we demonstrate the scaled ZINDO spectrum of Model I (Figure 3) compared with the experimental spectrum of the nanoflakes. The photon energy axis in the calculated spectrum was linearly scaled so that the positions of the major absorption peak in both spectra (experimental and calculated) are the same. This resulted with a scale factor of 0.819 in the present case. This value corresponds very well with the literature reports using INDO principle for simulation of UV spectra of -conjugated systems; i.e. Suendo and Viridi in their study of chlorophyll molecule have obtained the scale factors equal to 0.826, 0.840 and 0.898 for CNDO/s, ZINDO-CI and ZINDO-RPA, respectively.87 In the present work, the calculated energy space between these two peaks matches well with the experimental one; 16 ACS Paragon Plus Environment


thus relative positions of both DUV and FUV peaks match very well the ones observed experimentally. An apparent overestimation of the intensity of the FUV peak can be noticed (Figure 6). A lower accuracy of prediction of the oscillator strengths as opposed to transition energies is common for semi-empirical methods as well as many ab initio methods;87,92 this is also known in the context of -conjugated systems and may be emphasized in the case of highenergy transitions characterized by a larger energy gap between the ground and excited states. A better consistency and reliability level of the calculated oscillator strengths is achievable with the use of calculations on a higher level of theory, i.e. equation-of-motion coupled cluster singles and doubles (EOM-CCSD);92 due to an extensive computational cost, however, these are unfeasible for investigation of morphology features of complex systems as present in the current study. The rapidly decreasing low energy shoulder of the DUV band is exactly as expected; the experimental spectral outline is raised due to the influence of dispersion curve as explained earlier.82 Moreover, the high-energy part of the experimental ATR spectrum is artificially dampened by the effect of the decreasing penetration depth of the evanescent wave towards higher photon energy. Therefore, the apparent inaccurate reproduction of the intensity ratio between the DUV and FUV peaks by ZINDO can be explained by the distortion of the experimental spectrum due to physical effects.83 Accordingly, we provide the most relevant initial and final states of the Model I (Figure 8), which indicates the anticipated symmetries of the corresponding molecular orbitals of graphene. Both peaks result from -* transitions, and in general the shape of the relevant molecular orbitals (MOs) are similar. An attempt to ascribe the differences between the states responsible for the appearance of the DUV and FUV peaks may be done based on the above results. The ground states remain similar in both cases, while a slightly more consecutive separation between the corresponding excited states can be noticed. An increase in the contribution of the excited states in which the electron density is distributed uniformly among the atomic centres can be concluded in the case of the FUV band (Figure 8). Nevertheless, it should be noted that the artificial character of the molecular models used here limits the potential to generalize these observations onto all kinds of graphene systems. 4. Summary In summary, we measured electronic spectra of graphene nanostructures (flakes, platelets) in the 2.76-8.55 eV region employing the ATR-FUV-DUV technique. The measured spectra uncover the existence of a FUV band of graphene nanostructures. The spectra of these two kinds of samples differ significantly each other; the spectrum of flakes features an FUV peak at ca. 7.5 eV. The FUV peak of nanoplatelets is much less pronounced. On the other hand, the absorption of nanoplatelets rises towards lower photon energy, while that of the flakes does 17 ACS Paragon Plus Environment

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not, and the major DUV peak at ca. 4.8 eV remains well-resolved (Figure 1). The later feature may result from physical (sample-IRE surface properties) and optical factors; we are currently involved in a separate investigation focused on explaining this feature and an article is being prepared. Upon applying mechanical pressure, the spectral differences between the flakes and platelets largely diminish at the lower photon energy band shoulder of the DUV peak. On the other hand, the discrepancy at the high photon energy side of the DUV peak remains visible between these two types of graphene nanostructures. The observed spectral variability may result from principal differences in the molecular structures; the observed FUV spectra also undergo spectral changes. We carried out a preliminary computational study to explore this possibility. Computationally affordable semi-empirical ZINDO calculations allowed taking into consideration a number of molecular models incorporating the set of principal structural features, which we anticipated to be included in the origins of the observed spectral patterns. The simulated spectra provided insights into the correlations between graphene morphology and FUV-DUV spectra. The spectral difference between the nanoflakes and nanoplatelets is likely to be related to the area of graphene planes, their order (parallelization) as well as the number of layers. The stacking scheme (AA or AB) and the number of stacked layers seem to be of lesser importance. Application of mechanical pressure on the studied samples leads to a spectral change, which we may expect to be related to the level of disorder between the graphene layers. Although qualitatively reproduced by ZINDO spectra, the order of magnitude of this effect is lower than the observed one. These simulations provided consistent explanations of the observed spectral variability. An investigation is now being carried out to fully explain the observed phenomena.



Figure 8. The most relevant initial and final states of Model I as yielded in ZINDO calculations. In the brackets the transition energies after linear scaling. Refer to Fig. S7 (Supporting Information) for additional projection of the presented MOs). 19 ACS Paragon Plus Environment

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Acknowledgements This work was supported by the Grant-in-Aid Scientific Research (A) (KAKENHI) from the Japan Society for the Promotion of Science (JSPS) grant number 15H02188. Calculations have been carried out in Wrocław Centre for Networking and Supercomputing (http:/www.wcss.pl), under grant no. 375.

Associated content Supporting Information. Additional experimental spectra (pressure dependence); molecular models used in the QC study and the resulting simulated electronic spectra; detailed drawings of the initial and final states of the selected most relevant electronic transitions; detailed calculated data (models, spectra and molecular orbitals) on the possible influences of oxygen baring groups.



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Electronic Spectra of Graphene in Far- and Deep Ultraviolet Region

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