Covalent Carbene Functionalization of Graphene: Toward Chemical

Subscriber access provided by La Trobe University Library

Article

Covalent Carbene-Functionalization of Graphene: Towards Chemical Band Gap Manipulation. Toby Sainsbury, Melissa Passarelli, Mira Naftaly, Sam Gnaniah, Steve J. Spencer, and Andrew John Pollard ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10525 • Publication Date (Web): 29 Jan 2016 Downloaded from http://pubs.acs.org on February 2, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22

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

ACS Applied Materials & Interfaces

Covalent Carbene-Functionalization of Graphene: Towards Chemical Band Gap Manipulation. Toby Sainsbury*, Melissa Passarelli, Mira Naftaly, Sam Gnaniah, Steve J. Spencer, Andrew J. Pollard. National Physical Laboratory, Teddington, London, TW11 0LW, United Kingdom. E-mail: [email protected] Abstract: In this work we employ dibromocarbene (DBC) radicals to covalently functionalize

solution-exfoliated

graphene

via

the

formation

of

dibromocyclopropyl adducts. This is achieved using a basic-aqueous/organic biphasic reaction mixture to decompose the dibromocarbene precursor, bromoform, in conjunction with a phase transfer catalyst to facilitate ylide formation and carbene migration to graphene substrates. DBC-functionalized graphene (DBC-graphene) was characterized using a range of spectroscopic and analytical techniques to confirm the covalent nature of functionalization. Modified optical and electronic properties of DBC-graphene were investigated using UV-vis spectroscopy, analysis of electrical I-V transport properties and using non-contact terahertz time-domain spectroscopy (TTDS). The implications of the carbene functionalization of graphene are considered in the context of scalable radical functionalization methodologies for bulk-scale graphene processing and controlled band-gap manipulation of graphene. Keywords: graphene, carbene, functionalization, band-gap, dibromocarbene Introduction The covalent chemical functionalization of graphene is an established means by which its surface chemistry may be manipulated as well as the intrinsic electronic properties modified.1,2 Tuneable surface chemistry is known to enable dispersion and integration within solvent systems and at condensed phase substrates through direct chemical compatibilization and designed bonding interactions.3 In recent years multiple strategies have been utilized for the covalent functionalization of graphene.4 These strategies include the use of radical species including nitrene, carbene and aryl intermediates to react with the basal plane of graphene.5 A much sought consequence from such covalent functionalization is the ability to modify the surface chemistry of graphene and also to induce controlled modification to the electronic properties.6 This will allow the compatibilized dispersion of graphene within solvent systems and polymer matrices as well as defined modification

1 ACS Paragon Plus Environment


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

of the electronic energy states through doping and designed functionalization.4 Through controlled functionalization strategies it is clear that a balance may be achieved between the introduction of surface chemical groups, which add beneficial points for dispersion, mechanical attachment or chemical function and detrimental effects as the extent of functionalization negates the originally attractive mechanical and electrical intrinsic properties of pristine graphene. In view of this, exploration of solution-phase radical functionalization strategies which can be controlled through reaction time or reactant concentration are of great interest for the scalable functionalization of graphene and related nanomaterials. Here we report the covalent functionalization of exfoliated graphene nanosheets using solution generated dibromocarbene (DBC) radicals. Addition of DBC groups to the sp2 hybridized graphene lattice via the formation of dibromocyclopropyl adducts was achieved using a two-phase synthetic procedure involving the base hydrolysis of the DBC precursor; bromoform, in conjunction with tertiary amine phase transfer catalyst, as depicted in Scheme 1. The DBC functionalization of the exfoliated graphene nanosheets was characterized using Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), timeof-flight secondary ion mass spectrometry (ToF SIMS), thermogravimetric analysis (TGA), and UV-Vis spectroscopy (UV-Vis). The impact of the functionalization on the electronic properties of graphene was investigated using terahertz time-domain spectroscopy (TTDS) and examined in a device configuration to determine the transport characteristics of the DBC-functionalized graphene relative to pristine unmodified graphene. The combined characterization and analysis of DBC-functionalized graphene in this work allows us to form a complimentary framework for the characterization of alternative radical systems and the impact of reactive chemical species on the intrinsic properties of graphene.


Page 2 of 22

Page 3 of 22

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


Scheme 1. DBC-functionalization of graphene via formation of dibromocyclopropyl adducts tangential to the lattice plane.

Experimental Chemicals and reagents used were purchased from Sigma Aldrich and were used as received. Sonication was carried out using a Branson 2510EMT sonic bath. Transmission Electron Microscopy (HR-TEM) images were acquired using a JEOL-2010 operated at 200 kV. Samples for TEM were prepared by evaporating a drop of a dilute suspension of graphene onto a lacy-carbon copper TEM grid. Scanning Electron Microscopy (SEM) images were acquired using a ZEISS Supra at an accelerating voltage of 5 kV and a nominal working distance of 2.5 mm. Atomic Force Microscopy (AFM) measurements were made using an Asylum research Cypher system in tapping mode using PPP-NCHR silicon cantilevers at a resonant frequency of ∼290 kHz. Fourier Transform Infrared spectroscopy (FT-IR) measurements were recorded using a Nicolet 6700, with a diamond ATR accessory. Raman spectra were recorded using a Renishaw inVia Raman Microscope at λ = 514.5 nm and a total laser power incident on the sample of 5 mW laser excitation. X-ray Photoelectron Spectroscopy (XPS) was performed using a Kratos Axis Ultra DLD system using an Al monochromated X-ray source operated at 15 kV, 5 mA emission. Analysis conditions used were 160 eV pass energy, 1 eV steps, 0.2 sec dwell per step and 2 sweeps. Samples for XPS were prepared by evaporation of graphene from solution onto Si-wafer substrates. X-ray diffraction measurements were conducted using a Siemens D5000 diffractometer in conjunction with a Cu-kα X-ray tube (40 kV, 40 mA) filtered using a Ni filter and anti-scatter and divergences slits of 1mm under standard θ-2θ conditions. ToFSIMS analysis was performed in the negative ion mode using a ToF-SIMS IV mass spectrometer (IONTOF) with a 25 keV Bi3+ primary ion source, a target current of 0.125 pA, and with a final ion dose of 8.2 x 1010 ions per cm2. Optical Absorption measurements were recorded using a Perkin Elmer Lambda 850 using a 2 mm path-length quartz cuvette. Thermogravimetric Analysis (TGA) was performed using a Perkin-Elmer Pyris-1 TGA system in air. The temperature was scanned from 30 to 1100 oC at 10 oC/min. Terahertz Time-Domain Spectroscopy (TTDS) measurements used a standard configuration spectrometer incorporating a femtosecond laser, four off-axis mirrors, a biased GaAs emitter, and electro-optic detection with a ZnTe crystal and balanced photodiodes. The maximum dynamic range of the system was 5000 in amplitude, and the frequency resolution in the experiments was 7.5 GHz. The samples were measured in transmission by placing at the focus of the THz beam. Samples for terahertz analysis were prepared by forming nanocomposites at identical mass fractions (0.1 wt.%) within a terahertz transparent carrier matrix; polytetrafluoroethylene (PTFE). This was achieved by dispersing a known mass of the solution exfoliated carbon material



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

(dispersed in DMF) and PTFE powder within chloroform, sonicating for 1h and then evaporating the materials to dryness. Each material sample (50 mg) was then additionally dried using Schlenk apparatus to remove trace moisture and then compressed to a pellet (7 mm) using a Carver hydraulic press (10T, 10 min.). Measurements were carried out in dry air in order to eliminate water absorption lines from the recorded spectra. The amplitude and phase of the THz signal as a function of frequency are obtained from the measured time-domain data of THz electric field by applying the Fourier Transform using a standard FFT application (OriginPro 8), refractive indices and absorption coefficients of the samples were calculated using standard equations.7 Preparation of Dibromocarbene Functionalized Graphene Graphene was solution-exfoliated according to a derivation of a published procedure.8 This involved bath-sonication of graphite flakes (typically 10 µm lateral size) in benzene (23 mL, 2.56 × 10-1 mol) in a round-bottomed flask (50 mL) at an initial concentration of 3 mg/mL for 48 h. This was accompanied by cooling using an immersion cooler in order to maintain ambient solution temperature (20 oC) and prevent decomposition and/or pressure build-up. This was followed by the addition of bromoform (10 mL, 1.14 × 10-1 mol) and the phase transfer catalyst trihexylamine (2 mL, 5.9 x 10-3 mol) to the dispersion and a further period of sonication to ensure complete mixture (2 h). The suspension was then centrifuged at 1500 rpm for 45 minutes to remove any aggregated material. The suspension of exfoliated graphene was left to equilibrate for 24 h to allow any insoluble material or aggregates to precipitate. The concentration of the suspension was determined to be 0.5 mg.mL-1 by evaporation and weighing of three 2.5 mL aliquots of the suspension. The translucent/grey supernatant fraction (18 mL) was retained and was added drop-wise to a stirring aqueous solution of sodium hydroxide (0.125 M, 10 mL) and allowed stir for 48 h at 70 oC. The organic phase was retained and filtered using membrane filtration apparatus (Whatman, Anodisc 200 nm). The residual solid product was then washed with toluene (500 mL) and tetrahydrofuran (500 mL) and was then re-suspended in toluene (30 mL). The organic phase was washed a further three times using water (distilled, deionized, 18.2 M.Ω.cm) and was dried over calcium carbonate (5 g). The retained organic phase containing the suspended dibromocarbene-functionalized graphene (DBC-graphene) was evaporated to dryness and stored under vacuum for several days for subsequent analysis using Schlenk apparatus. A control experiment was run under identical conditions to the DBC functionalization above with the absence of the dibromocarbene precursor; bromoform. Preparation of Graphene Oxide (GO) Graphite oxide was prepared according to the Hummers method.9 Powdered graphite (0.50 g) and


Page 4 of 22

Page 5 of 22

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


NaNO3 (0.25 g, 2.94 x 10-3 mol) were combined in H2SO4 (95.0-98.0%, 16.5 mL). After 10 min of stirring, KMnO4 (1.5 g, (.5 x 10-3 mol) was slowly added over 30 min to the stirring solution which was maintained at 0 oC in an ice bath. The mixture was then allowed to stir for 30 min at 35 oC in an oil bath. Water (distilled, deionized, 18.2 M.Ω.cm)(23 mL) was added slowly to the solution which increased in temperature to 98 oC, and was then followed by the addition of a further aliquot of water (distilled, deionized, 18.2 M.Ω.cm)(70 mL). H2O2 (30 %, 1.75 mL) was then added to the stirring solution. The resulting dark solution was then divided into equal aliquots in plastic centrifuge tubes and was centrifuged (10 min, 3000 rpm) in order to precipitate the oxidised graphite material. Graphite oxide (GO) deposited on the bottom of the centrifuge tubes was then washed 3 times with HCl solution (10 %) following by ethanol. Each washing step was followed by centrifugation and removal of the supernatant fraction using a Pasteur pipette. Residual GO powder was allowed to dry overnight (30 oC, oven) and was recovered as a dark black solid. XPS analysis of GO prepared by this technique was used to determine a nominal C/O ratio of 1.22, and were found to have an average flake size of 1 µm determined by SEM and AFM. Results and Discussion A critical factor for the efficient functionalization of graphene is the efficiency of the reactantsubstrate interaction. Optimization of the exfoliation process is therefore essential in order to present a maximum concentration of few-layer graphene substrates for chemical functionalization. Solution-phase exfoliation of graphene may be routinely achieved by ultrasonication within solvent systems appropriately matched in terms of surface energy and solubility parameters.8,10 In this work, exfoliated graphene was produced by ultrasonication of graphite in the carrier solvent for the carbene functionalization reaction, benzene. Ultrasonication followed by centrifugation and retention of the supernatant yielded material which showed single-layer to few-layer nanosheets, Figure 1.



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

Figure 1. (a) TEM image of exfoliated graphene on a lacey carbon TEM grid. Inset showing several few-layer graphene nanosheets. (b) AFM image of exfoliated graphene, (c) linescan profile of exfoliated graphene indicating average height profile of ∼1.2 nm.

Characterization of exfoliated graphene using TEM indicated that the material consisted of singlelayer to few-layer graphene. Lateral dimensions of the nanosheets were of the order of 500 nm-2 μm in diameter, with smaller fragments also present. The inset in Figure 1a shows a typical isolated graphene sheet and identifies that the 2-D structure of the material is intact and devoid of any major 6 ACS Paragon Plus Environment

Page 6 of 22

Page 7 of 22

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


deformations or dislocations which may be observed as large holes or damage caused by the energy of the ultrasonic preparative treatment. AFM analysis of the exfoliated graphene sheets indicates the average height profile corresponding to an approximate height of 1.2 nm, Figure 1b,c. These factors are of importance for any technological application of graphene as an extended crystalline lattice structure for mechanical or electron/phonon transport dictate the viability of technological applications.11-13 Additional images of exfoliated graphene sheets using AFM and SEM are included in the supporting information, Figure S1,2, supporting the conclusions from the TEM analysis. Following exfoliation, graphene nanosheets were functionalized using a bi-phasic benzene and aqueous sodium hydroxide reaction system.14-16 The carbene precursor, bromoform, and the phasetransfer catalyst, trihexylamine, were combined with an organic suspension of exfoliated graphene in benzene. The addition of a phase transfer catalyst, trihexylamine, facilitates the migration of carbenes generated at the phase boundary by the formation of ylide intermediates. Br2C−-N+(C6H13)3 ylides in-turn react with graphene substrates yielding the dibromocyclopropyl adducts tangential to the lattice plane.15 Following functionalization with dibromocarbene (DBC) groups, DBCfunctionalized graphene was characterized using XPS, XRD, FTIR, Raman spectroscopy, ToF SIMS, TGA, UV-Vis, and TTDS. Figure 2a shows the XPS survey spectra of pristine graphene and DBC-graphene. In both spectra, the dominant feature of the C1s peak of sp2 hybridized graphene is evident at 284.6 eV. In the spectrum of DBC-graphene, a series of substantial peaks corresponding to Br 3s, 3p1/2/3p3/2 and 3d are present at 257.8 eV, 190.8/184.3 eV, and ∼ 69.6 eV respectively. Analysis of the C1s core level spectrum for DBC-graphene, Figure 2b, indicates significant asymmetric broadening relative to the C1s spectrum for pristine graphene (Supporting Information, Figure S3). Deconvolution of the DBC-graphene C1s peak identifies peaks at 285.4 eV and 286.4 eV which are assigned to the C-Br and C-OH bonding.16 Carbon to oxygen bonding is likely to be present since CBr2 groups are susceptible to hydrolysis to form ketone and hydroxyl groups. In view of this, a third peak present at 287.8 eV cannot accurately be assigned, as both C=O and CBr2 groups are known to exist in this energy range.17 Overall, C-Br bonding in the C1s spectrum along with additional Br peaks in the survey spectrum of DBC-graphene identifies strong evidence for the covalent attachment of CBr2 species to the basal plane of graphene and is in direct agreement with recent reports of halogenated graphene in the literature.18-20 Analysis of the Br 3d peak shows that the peak consists of a doublet at 70.1 eV and 69.2 eV indicating covalent C-Br bonding and corresponds with reports in the literature concerning covalent bromine functionalization.16,19



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

Figure 2. (a) Survey spectra of pristine graphene and DBC-functionalized graphene nanosheets. (b) Core level C1s spectrum for DBC-graphene. (c) Core level Br 3d spectrum for DBC-graphene. (d) XRD spectra of unmodified

graphene

and

DBC-functionalized

graphene.

XRD was also used to identify modifications to the graphene lattice, as shown in Figure 2d, and to support the XPS data. Material is condensed for analysis to present indicative analysis of the bulk functionalized sample rather than individual dispersed nanosheets. The (002) reflection for graphene (strictly speaking this is graphite) is present at 26.70 o, Figure 2d. This is accompanied by the additional (100), (101), (004) and (103) reflections at 42.60o, 44.77o, 54.85o, and 60.11o respectively.


Page 8 of 22

Page 9 of 22

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


Following DBC functionalization, substantial changes are now evident in the spectrum of DBCgraphene indicative of covalent bonding of C-Br2 groups relative to the original pristine graphene material, Figure 2d. The (002) peak has shifted 0.07o to 26.63o, with a substantial increase in the FWHM value from 0.50o to 0.63o. This broadening of the (002) peak is assigned to the lattice becoming less ordered as CBr2 groups are added and which we assume identifies the fact that the covalent functionalization results in rehybridization of C-C atom pairs across the lattice plane. This is accompanied by a significant decrease in the intensity of the (100) and (101) peaks, a marked increase in the intensity of the (004) peak and the appearance of a number of peaks assigned to C-Br refection’s at 18.22o, 28.10o, and 35.70o. Modification to the spectra of DBC-graphene relative to pristine graphene is indicative of the functionalization of the graphene lattice with CBr2 groups and is in agreement with the XPS characterization as well as reports on bromine modified graphene in the literature.19,20 FTIR spectroscopy was used to characterise the DBC-graphene relative to pristine graphene in order to elucidate the precise bonding structure of the functionalized material, Figure 3a. The spectrum of the graphene starter material is featureless and indicates no pre-existing functionality to the basal plane, Figure 3a(i). The spectrum of bromoform exhibits bands due to the C-H stretching and bending vibrations at 3020 cm-1 and 1142 cm-1 respectively, while the C-Br stretch is evident at 656 cm-1 with a shoulder at 692 cm-1, Figure 3a(ii). Following DBC functionalization, the spectrum of DBCgraphene exhibits a substantial peak at 1080 cm-1 assigned to the C-C stretching of cyclopropyl groups tangential to the graphene plane, Figure 3a(iii). This is consistent with a report in the literature concerning the covalent binding of C-Br2 groups to carbon nanotubes by the formation of dibromocyclopropyl groups and supports the assertion of covalent bonding of C-Br2 groups in this work.22 Evidence of the cyclopropyl group is also accompanied by the appearance of a strong vibration at 475 cm-1 assigned to C-Br2 bonding.23 Notably, the large shift in the C-Br peak from 656 cm-1 in bromoform to 475 cm-1 in DBC-functionalized graphene is consistent with covalent binding of dibromo groups to carbon nanotubes and h-BN and precludes the suggestion of physisorbed species.22,24 As the covalent attachment of DBC groups results in rehybridization of the C atoms via the formation of cyclopropyl groups, Raman spectroscopy was used to probe the degree of disorder of the functionalized material, Figure 3b. The spectrum of the graphene starting material, condensed in the graphitic phase for bulk analysis, includes the G-peak at 1580 cm-1 due to the in-plane E2g mode and the 2D peak at 2725 cm-1 due to second-order zone boundary phonons, Figure 3b(i). Following DBC functionalization, the G- and 2D-peaks have red shifted to 1575 cm-1 and 2705 cm-1 respectively,



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

which is indicative of both mechanical strain in the lattice and a doping effect of the bound DBC groups due to the formation of cyclopropyl groups and rehybridization of C atoms out of the lattice plane, Figure 3b(ii).25 This is accompanied by the appearance of a significant D-peak, located at ∼1360 cm-1 due to first-order zone boundary phonons and a decrease in intensity and broadening of the 2D peak at 2705 cm-1.26,27 To evaluate that the Raman scattering signals correspond to the addition of the C-Br2 groups following DBC functionalization and not simply damage induced disorder, Time-of-Flight-Secondary-Ion-Mass-Spectrometry (ToF SIMS) was performed, Figure 3c.

Figure 3. (a) FTIR spectra of (i) pristine graphene (ii) bromoform, (iii) DBC-functionalized graphene. (b) Raman spectra for (i) pristine graphene (ii) DBC-functionalized graphene. (c) ToF-SIMS spectra of (i) pristine graphene, indicating a featureless spectrum in the region of Br isotopes (inset), and (ii) DBC-graphene, showing peaks 79

80

due to the Br and Br isotopes. (d)(i) TGA traces for graphene and DBC-graphene indicating decomposition o

onsets above 400 C. (ii) Derivative TGA traces for graphene and DBC-graphene indicating modified decomposition profiles following DBC-functionalization.

ToF-SIMS analysis reveals the presence of bromine in DBC-graphene, Figure 3c. Bromine has two stable isotopes that are two mass units apart and relatively equal in abundance (79Br, 50 % and 81Br, 49 %). The ToF-SIMS spectrum of DBC-graphene shows the bromine anion isotopes at m/z 78.92 and m/z 80.92, Figure 3c(ii). The integrated, Poisson-corrected, and background subtracted intensities 10 ACS Paragon Plus Environment

Page 10 of 22

Page 11 of 22

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


for these peaks were 1.81 x 105 and 1.85 x 105, which calculates to a relative abundance of 49.5 % and 50.5 %, respectively. The mass deviation for expected mass of the bromine isotopes was 3.6 ppm for the 79Br isotope and 12.8 ppm for the 81Br isotope. These bromine peaks were not found in the graphene sample, Figure 3c(i) and are in excellent agreement with a report concerning the covalent functionalization of graphene nanoplatelets (GNPs).28 The bromine ion (Br2+) and a number of potassiated- and sodiated-bromine clusters were also detected in the DBC-graphene sample. The unique isotope pattern of bromine was used to validate peak assignments and further confirm the assertion of DBC functionalization of the graphene. Thermogravimetric analysis (TGA) was employed to quantify the extent of DBC functionalization by monitoring the loss of DBC groups as a function of temperature; Figure 3d. Figure 3d(i) shows the TGA trace of pristine graphene, which exhibits an onset of mass-loss indicative of the decomposition of the lattice structure at ∼ 530 oC, reaching a maximum rate of loss at ∼ 570 oC. In the case of DBCgraphene, the onset of an initial mass-loss of approximately 30% occurs at ∼ 420 oC reaching a maximum rate at 520 oC assigned to the loss of the DBC adducts bound to the graphene lattice, Figure 3d(i). This percentage loss due to functional groups equates to approximately 1 DBC groups per every ∼37 C atoms, yielding an approximate density of 1 DBC unit per nm2, or approximately 3 atomic percent functionalization. Even for bi-layer graphene, a functionalization density of 1.5 atomic percent is quite substantial when considering manipulation of electronic states in graphene or even for mechanically interfacing within polymer matrices or at substrates. At approximately 540 o

C, decomposition of the sample occurs at a substantially lower rate which is believed to be

indicative of the lattice decomposition. Notably, this temperature is markedly lower than the decomposition of the pristine graphene sample. This is rationalised by the fact that DBC functionalization creates sp3 carbon atoms with enhanced susceptibility to decomposition and thus gives rise to a step feature in the trace, this is taken to be indicative of the DBC groups and then the lattice respectively. This is emphasised by analysis of the derivative mass loss traces in Figure 3d(ii). In the case of pristine graphene (Figure 3d(ii)), a single peak is evident at ∼570 oC is indicative of the carbon lattice structure, while in the case of DBC-graphene, the derivative trace displays two distinct modes of decomposition corresponding to the DBC groups and the graphene lattice at 520 oC and 780 oC respectively (Figure 3d(ii)). Notably, TGA analysis also supports the assertion of covalent functionalization of the graphene lattice due to the degradation of the sample above 400 oC thus precluding any suggestion of physisorbed residues which evolve at substantially lower temperatures. Characterization of DBC functionalization using a range of spectroscopic and analytical techniques has confirmed the presence of covalently bound DBC adducts. It is therefore of interest to



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

investigate the effect of DBC functionalization on the intrinsic electronic properties. It is well known that a range of reactive chemistries can be used to functionalize graphene and manipulate their electronic properties,1-6 however the majority of experimental work concerning this topic involves device based investigations and this precludes extensive spectroscopic characterization of modified materials.6,26 A number of excellent examples exist which document the covalent functionalization of graphene using halogen species as well as carbenes and thus validate the predictions of energetically favourable chemical reactions as well as the impact on electronic states.5,26,27 Therefore, a great deal of motivation exists to investigate the covalent functionalization of exfoliated graphene following spectroscopic characterization in terms of electronic behaviour. Here we probe the DBC functionalization of graphene by examining the electronic states of the material in solution, device configuration and using a non-contact optical analysis of the material in nanocomposite configuration using THz irradiation. Figure 4a shows UV-Vis spectra of pristine graphene and DBC-graphene. The spectrum of graphene displays a band at 265 nm indicative of the π−π* transition of the delocalized π electrons.28 Following functionalization, the π−π* band has blueshifted to 260 nm along with the appearance of a significant shoulder at 222nm. This dual feature is taken to be indicative of the modification of the graphene lattice in a manner whereby delocalized electron zones exist amongst well defined functional groups. This position of the π−π* band is indicative of functionalization induced disorder as the wavelength of the delocalized system decreases and thus this feature not only compounds the covalent modification of graphene with DBC groups but also verifies a perturbation to the electronic states.28 The extent of the small shift in π−π*band by 5 nm corresponds to a discrete energy shift of ∼ 90 meV. This feature is significant, as unlike spectroscopies which probe local bonding structure largely based on σ bonding, the UV-Vis analysis displays evidence that the discrete levels of functionalization have an effect, albeit discrete, on the delocalized π electron system. To probe the basic transport properties of DBC-graphene versus pristine graphene, nanosheets were deposited across interdigitated electrodes as described in the supporting information. A substantial decrease in conductivity between pristine graphene and DBC-graphene is evident from the I-V curves, Figure 4b. Following DBC functionalization, we found that the I-V characteristics had become markedly non-linear and that the conductivity for DBC-graphene had decreased approximately two orders of magnitude from 7.14 S/cm in pristine graphene to 5.13 x 10-2 S/cm. The decrease in conductivity and more particularly the profile of the I-V curve points towards significant structural perturbation of the graphene and supports the combined spectroscopic evidence for covalent functionalization. To extend the investigation of the modified electronic properties, we used TTDS.


Page 12 of 22

Page 13 of 22

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


TTDS is an optical based spectroscopic technique which can probe the intrinsic carrier dynamics in the time domain in a non-contact configuration. The terahertz spectral region of the electromagnetic spectrum lies between microwave and infra-red frequencies and allows analysis of the complex dielectric permittivity at frequencies between 100 GHz and 3 THz. The analysis of materials using TTDS is based on the effect of a material on the amplitude and phase of THz pulses generated by a femtosecond pulsed laser within a standard spectrometer configuration. This technique is known to be exceptionally sensitive to the free carrier concentration within insulating matrices and has been used in studies to examine the response of carbon nanotubes within rubber, analysis of single and multi-walled carbon nanotubes as well as the analysis of graphene-polymer multilayer heterostructures.29-31 A full synopsis of this technique and related embodiments may be found in reference 7. To set a frame of reference for this analysis, we characterized samples across a range of electrical behaviour from conductive to insulating, and thus from high absorptivity to low absorptivity across the terahertz range studied (0-3.5 THz). The range of samples includes: pristine exfoliated graphene, DBC-graphene (derived from the same graphite starting material as the pristine graphene), and GO (derived from the same graphite starter material). Samples are prepared by forming nanocomposites at identical mass fractions (0.1 wt.%) within a terahertz transparent carrier matrix: polytetrafluoroethylene (PTFE). Described in the supporting information are additional samples of GO and carbon black (CB) which were used in order to cross-reference the analysis, Figure S4. We speculate that this analysis may be used to investigate the free carrier concentration in graphene and derived forms of graphene, and therefore may form the basis for contactless conductivity evaluation in bulk samples.



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

Figure 4. (a) UV-Vis spectra of graphene, and DBC-functionalized graphene. (b) I-V transport characteristics for graphene and DBC-functionalized graphene showing a voltage sweep to ± 0.2V. (c) Absorption coefficient spectra for graphene, DBC-graphene and graphene oxide (GO). (d) Refractive index, n, for graphene, DBC-graphene and graphene oxide (GO) materials.

Figure 4c indicates high absorption for pristine graphene, across the range 0-3.5 THz. In contrast to this, a markedly lower absorption across the THz range was exhibited for the GO sample, which is expected on account of the insulating nature of the oxidized nanosheets and the relative absence of free carriers. It is noted that the absorption of low-energy photons by carbon nanomaterials increases as a function of frequency as well as mass fraction.29 Consequently, we observe a greater degree of noise at higher frequencies and derive the optimum trends in material behaviour between 0.0-2.0 THz, Figure 4c. Notably, the absorption for DBC-graphene matches that of GO initially and then between 0.5-1.0 THz the absorption increases until it is midway between graphene and GO i.e. midway between a conductor and insulator. Based on electrical transport characterization as well as the combined spectroscopic evidence, we assign this behaviour to a modification to the conductive character of the graphene sheets induced by DBC functionalization. Analysis of the refractive index for the samples indicated clear differences in dielectric behaviour between the samples, Figure 4d. Optical absorption in the graphene/PTFE nanocomposite samples is 14 ACS Paragon Plus Environment

Page 14 of 22

Page 15 of 22

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


due to the interaction of incident photons with free carriers and thus is indicative of conductivity. Refractive index values are highest for graphene and lowest for GO. This trend is taken to be indicative of the lowering of the free carrier concentration as a function of chemical functionalization. The refractive index of GO at ∼1.48 is frequency independent across the 0.0-3.5 THz range. Graphene, G, exhibits an initial refractive index value of ∼1.70 which decreases across the frequency range. DBC-graphene, at an identical mass fraction loading and derived from the pristine G starter material, exhibits a refractive index originating at ∼1.54. Most significantly, an initial frequency independent region from 0-1.0 THz then decreases as a function of frequency above ∼1.1 THz in a manner similar to graphene. This transition in behaviour above a certain frequency, i.e. behaviour transitioning from that of GO to G following DBC functionalization is tentatively speculated to be indicative of modification of the band gap. Importantly, as part of the TTDS analysis of DBC-graphene relative to pristine graphene and GO, we have validated the materials behaviour by cross-referencing large flake size graphene, designated G10, and small flake size graphene, designated G1, as well as the derived forms of GO respectively designated GO10 and GO1 respectively, as described in the Supporting Information. Similar THz absorption and refractive index trends were observed for conductive graphene samples and insulating GO samples respectively. However, we do observe a fall-off in the absorption of the smaller flake size graphene sample G1 relative to the larger sample, G10 (Figure S4), as well as lower refractive index values for G1 graphene across the THz range. We speculate that using TTDS may be utilized to gain further insight into the mechanisms of THz absorption by free carriers and consequently this may be utilized to characterise extended crystallinity or defectivity in graphene samples and warrants further study. Taking into account the evidence presented from the UV-Vis analysis and I-V characteristics of the DBC-graphene we suggest that the functionalization procedure has indeed modified the intrinsic electronic states in the material. The exploratory research described here warrants further investigation and will be used by us to form the basis for more controlled functionalization experiments that investigate sequential functionalization in order to tune electronic properties. Conclusion In this work we have demonstrated the use of carbene radicals to covalently functionalize graphene in an additive fashion. The covalent DBC addition reaction has been characterized using FTIR, Raman spectroscopy, ToF SIMS, TGA, while UV-Vis, I-V transport analysis, and TTDS have been used to evaluate the electronic transport properties. We have found that the electronic properties of graphene have been substantially modified as a result of the DBC-functionalization. The addition of DBC groups to graphene provides the means for subsequent chemical derivatization of the bromide



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

groups and opens the scope of the modified graphene to reactions such as reductive coupling, nucleophilic substitution as well as oxidation of bromide groups to generate sites for esterification.32 It is clear that substantial opportunities exist for graphene which can be controllably functionalized.1,3,5 These include the attachment of molecules, oligomers, nanoparticles and to promote adhesion and surface interaction with substrates. It is also clear that the manner of functionalization has important implications for the manipulation of electronic properties.26,27 By using a non-oxidative, structurally non-destructive techniques for functionalization, the use of reactive radical chemical functionalization has a great deal of potential. Reactive radical functionalization is anticipated for bulk exfoliated graphene as well as wide-area graphene by thermolyis/photolysis of radical precursors as well as plasma processing, and therefore would allow scale-up technologies to potentially be explored. By firstly confirming the viability of the reaction methodology described in this work we envisage subsequent studies to investigate sequential functionalization using radical chemistry and the corresponding controllable manipulation of electronic states in graphene. Supporting Information AFM analysis of exfoliated graphene (Figure S1). SEM analysis of exfoliated graphene (Figure S2). XPS C1s core level spectrum for graphene (Figure S3). Terahertz Time Domain Spectroscopy (TTDS) spectra showing additional graphene, graphene oxide and carbon black samples (Figure S4), analysis of absorption rate for graphene, DBC-graphene and GO (Figure S5), Refractive index spectra showing additional samples graphene, GO and carbon black (Figure S6), and additional repeated spectra of absorption coefficient and refractive index for graphene, DBC-graphene and GO (Figure S7). Corresponding Author * Telephone: +44 20 8943 6434, E-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgements The authors acknowledge funding through the UK National Measurement System (NMS) strategic capability programme.


Page 16 of 22

Page 17 of 22

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


References 1. Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.; Kim, N.; Kemp, K. C.; Hobza, P.; Zboril, R.; Kim, K. S. Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications Chemical Reviews 2012, 112, 6156-6214. 2. Englert, J. M.; Dotzer, C.; Yang, G.; Schmid, M.; Papp, C.; Gottfried, J. M.; Steinrück, H.-P.; Spiecker, E.; Hauke, F.; Hirsch, A. Covalent Bulk Functionalization of Graphene Nature Chemistry. 2011, 3, 279-286. 3. Hirsch, A.; Englert, J. M.; Hauke, F. Wet Chemical Functionalization of Graphene Acc. Chem. Res. 2013, 46, 87-96. 4. Criado, A., Melchionna, M., Marchesan, S. and Prato, M. The Covalent Functionalization of Graphene on Substrates Angew. Chem. Int. Ed. 2015, 54, 10734-10750. 5. Park, J.; Yan, M. Covalent Functionalization of Graphene with Reactive Intermediates Acc. Chem. Res. 2013, 46, 181-189. 6. Zhang, H.; Bekyarova, E.; Huang, J.-W.; Zhao, Z.; Bao, W.; Wang, F.; Haddon, R. C.; Lau, C. N. Aryl Functionalization as a Route to Band Gap Engineering in Single Layer Graphene Devices Nano Lett., 2011, 11, 4047-4051. 7. Naftaly, M. Terahertz Metrology, Norwood: Artech House, 2015. 8. Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; DE, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H.-Y.; Lee, K.; Kim, G. T.; Duesberg, G. S.; Hallam, T.; Boland, J. J.; Wang, J. J.; Donegan, J. F.; Grunlan, J. C.; Moriarty, G.; Schmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen, K.; McComb, D. W.; Nellist, P. D.; Nicolosi, V. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials Science 2011, 331, 568-571. 9. Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide J. Am. Chem. Soc. 1958, 80, 1339-1339. 17 ACS Paragon Plus Environment


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 22

10. Hernandez, Y.; Lotya, M.; Nicolosi, V.; Blighe, F. M.; De, S.; Duesberg, G.; Coleman, J. N. HighYield Production of Graphene by Liquid-Phase Exfoliation of Graphite Nat. Nanotechnol. 2008, 3, 563-568. 11. Anagnostopoulos, G.; Androulidakis, C.;

Koukaras, E. N.;

Tsoukleri, G.; Polyzos, I.;

Parthenios, J.; Papagelis, K.; Galiotis, C. Stress Transfer Mechanisms at the Submicron Level for Graphene/Polymer Systems ACS Appl. Mater. Interfaces. 2015, 7, 4216-4223. 12. Zhang, B.; Fan, L.; Zhong, H.; Liu, Y.; Chen, S. Graphene Nanoelectrodes: Fabrication and Size-Dependent Electrochemistry J. Am. Chem. Soc., 2013, 135, 10073-10080. 13. Malekpour,H.; Chang, K.-H.; Chen, J.-C.; Lu, C.-Y.; Nika, D. L.; Novoselov, K. S.; Balandin A. A. Thermal Conductivity of Graphene Laminate Nano Lett., 2014, 14, 5155-5161. 14. Doering, W. E.; Hoffman, A. K. The Addition of Dichlorocarbene to Olefins J. Am. Chem. Soc. 1954, 76, 6162-6165. 15. Kimura, Y.; Isagawa, K.; Otsuji, Y. An Effective Method for the Generation of Dibromocarbene and Its Application to Organic Synthesis Bull. Univ. Osaka Prefecture, Ser. A, Engineering and Natural Sciences. 1979, 28, 49-56. 16. Chanunpanich, N.; Ulman, A.; Strzhemechny, Y. M.; Schwarz, S. A.; Janke, A.; Braun, H. G.; Kraztmuller, T. Surface Modification of Polyethylene through Bromination Langmuir 1999, 15, 2089-2094. 17. Baker, M. V.; Watling, J. D. Functionalization of Alkylsiloxane Monolayers via Free-Radical Bromination Langmuir 1997, 13, 2027-2032. 18. Chua, C. K.; Ambrosi, A.; Pumera, M. Introducing Dichlorocarbene in Graphene Chem. Commun. 2012, 48, 5376-5378. 19. Zheng, J.; Liu, H.-T.; Wu, B.; Di, C.-A.; Guo, Y.-L.; Wu, T.; Yu, G.; Liu, Y.-Q.; Zhu, D.-B. Production of Graphite Chloride and Bromide Using Microwave Sparks Sci. Rep. 2, 2012, 662. 20. Li, Y.; Chen, H.; Voo, L.-Y.; Ji, J.; Zhang, G.; Zhang, G.; Zhang, F.; Fan, X. Synthesis of Partially Hydrogenated Graphene and Brominated Graphene J. Mater. Chem. 2012, 22, 15021-15024. 18 ACS Paragon Plus Environment

Page 19 of 22

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


21. Gao, J.; Bao, F.; Zhu, Q.; Tan, Z.; Chen, T.; Cai, H.; Zhao, C.; Cheng, Q.; Yang, Y.; Ma, R. Attaching Hexylbenzene and Poly(9,9-dihexylfluorene) to Brominated Graphene via Suzuki Coupling Reaction Polym. Chem. 2013, 4, 1672-1679. 22. Yu, J.-G.; Huang, K.-L.; Tang, J.-C. Chemical Attachment of Dibromocarbene to Carbon Nanotubes Physica E 2008, 41, 181-184. 23. Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts: John Wiley and Sons Inc.: New York, 2004. 24. Sainsbury, T.; O’Neill, A.; Passarelli, M. K.; Seraffon, M.; Gohil, D.; Gnaniah, S.; Spencer, S. J.; Rae, A.; Coleman, J. N. Dibromocarbene Functionalization of Boron Nitride Nanosheets: Toward Band Gap Manipulation and Nanocomposite Applications Chem. Mater. 2014, 26, 7039-7050. 25. Niyogi, S.; Bekyarova, E.; Itkis, M. E.; Zhang, H.; Shepperd, K.; Hicks, J.; Sprinkle, M.; Berger, C.; Lau, C. N.; Deheer, W. A.; Conrad, E. H.; Haddon, R. C. Spectroscopy of Covalently Functionalized Graphene Nano Lett. 2010, 10, 4061-4066. 26. Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectra of Graphene and Graphene Layers Phys. Rev. Lett. 2006, 97, 187401-1-4. 27. Farmer, D. B.; Golizadeh-Mojarad, R.; Perebeinos, V.; Lin, Y.-M.; Tulevski, G. S.; Tsang, J. C.; Avouris, P. Chemical Doping and Electron-Hole Conduction Asymmetry in Graphene Devices Nano Lett. 2009, 9, 388-392. 28. Li, J.; Vaisman, L.; Marom, G.; Kim, J.-K. Br Treated Graphite Nanoplatelets for Improved Electrical Conductivity of Polymer Composites Carbon 2007, 45, 744-750. 29. Karlický, F.; Datta, K. K. R.; Otyepka, M.; Zbořil, R. Halogenated Graphenes: Rapidly Growing Family of Graphene Derivatives ACS Nano 2013, 7, 6434-6464. 30. Zan, W. Chemical Functionalization of Graphene by Carbene Cycloaddition: A Density Functional Theory Study Appl. Surf. Sci. 2014, 311, 377-383. 19 ACS Paragon Plus Environment


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

31. Li, D.; Müller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable Aqueous Dispersions of Graphene Nanosheets Nat. Nanotechnol. 2008, 3, 101-105. 32. Rungsawang, R.; Geethamma, V. G.; Parrott, E. P. J.; Ritchie, D. A.; Terentjev, E. M. Terahertz Spectroscopy of Carbon Nanotubes Embedded in a Deformable Rubber J. Appl. Phys. 2008, 103, 123503-123503-4. 33. Dadrasnia, E.; Puthukodan, S.; Lamela, H. Terahertz Electrical Conductivity and Optical Characterization of Composite Non-aligned Single- and Multi-walled Carbon Nanotubes J. Nanophoton. 2014, 8, 083099-1. 34. Xu, Z.; Chen, C.; Wu, S. Q. Y.; Wang, B.; Teng, J.; Zheng, C.; Bao, Q. Graphene-Polymer Multilayer Heterostructure for Terahertz Metamaterials Proc. SPIE 8923, Micro/Nano Materials, Devices, and Systems, 89230C (2013). 35. Friedrich, J. F.; Wettmarshausen, S.; Hanelt, S.; Mach, R.; Mix, R.; Zeynalov, E. B.; MeyerPlath, A. Plasma-Chemical Bromination of Graphitic Materials and its use for Subsequent Functionalization and Grafting of Organic Molecules Carbon 2010, 48, 3884-3894.


Page 20 of 22

Page 21 of 22

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


Table of Contents Graphic



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 22 of 22

Covalent Carbene Functionalization of Graphene: Toward Chemical

Recommend Documents