Effect of Amine-Based Organic Compounds on the Work-Function

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Effect of Amine-Based Organic Compounds on the Work-Function Decrease of Graphene Ki Chang Kwon, Ho Jun Son, Yeun Hee Hwang, Jeong Hyeon Oh, Tae-Won Lee, Ho Won Jang, Kyungwon Kwak, Kwangyong Park, and Soo Young Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10473 • Publication Date (Web): 24 Dec 2015 Downloaded from http://pubs.acs.org on January 3, 2016

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The Journal of Physical Chemistry

Effect of Amine-Based Organic Compounds on the Work-Function Decrease of Graphene Ki Chang Kwon,† Ho Jun Son,‡ Yeun Hee Hwang,∥ Jeong Hyeon Oh,‡ Tae-Won Lee,‡ Ho Won Jang,† Kyungwon Kwak,∥ Kwangyoung Park,*,‡ Soo Young Kim*,‡

† Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea ‡ School of Chemical Engineering and Materials Science, Chung-Ang University, 84 Heukseokro, Dongjak-gu, Seoul 06974, Republic of Korea ∥Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea

CORRESPONDING AUTHOR FOOTNOTE K. P. ([email protected]), Tel: 82-2-820-5330, Fax: 82-2-824-3495. S. Y. K. ([email protected]), Tel: 82-2-820-5875, Fax: 82-2-824-3495.

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ABSTRACT We have demonstrated that amine and alkyl groups, applied by a simple spin-coating method, can provide an n-type doping effect on graphene sheets. The organic compounds used in this work are based on amine, phenyl amine, butylphenyl amine, benzoylphenyl amine, and tolylvinylphenyl amine groups. The increase of sheet resistance, decrease of transmittance and work function, as well as the G peak shift to higher wavenumbers and the 2D peak shift to lower wavenumbers in the Raman spectra indicate that graphene was doped to n-type after the graphene sheet was spin-coated by amine-based compounds. In particular, graphene doped with butylphenyl amine showed the strongest n-type effect among all the samples because butylphenyl amine had the strongest binding energy with graphene sheet and dispersion property in a nonpolar solvent, suggesting that binding energy with graphene sheet and the degree of dispersion in solvents are important factors in the doping process. Molecular calculations based on density function theory confirmed the n-type property of graphene doped with amine-based compounds. These results suggest that amine and alkyl groups play a crucial role for n-type doping of graphene.

KEYWORDS: graphene, graphene doping, amine functionalization, DFT calculation, work function.


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INTRODUCTION Graphene, a one-atom-thick sheet of carbon atoms with a two-dimensional honeycomb lattice, has recently attracted significant attention because of its many excellent properties such as high thermal conductivity, high mobility of charge carriers, and excellent ballistic transport properties.1-4 Ever since uniform and large-scale graphitic materials were first successfully synthesized by the chemical vapor deposition (CVD) method, there have been many reports on the application of graphene layers to the transparent electrodes of light-emitting diodes, fieldeffect transistors, and organic photovoltaic cells. The use of graphene as a transparent electrode requires low sheet resistance, high transmittance, and a modulated work function to reduce the injection barrier; consequently, doping methods have been developed for graphene. Boron and nitrogen atoms, which are located on the left and right side of carbon in the periodic table, are used as p- or n-type dopants during the process of synthesizing graphene.5-7 Chemical doping is an efficient Fermi-level engineering technique without basal plane reactions, resulting in no damage to the carbon network.8-10 The spontaneous surface-charge-transfer doping method uses the negative Gibbs free energy and the electronegativity difference between absorbed materials and carbon atoms.11-13 Metal chlorides, organic compounds, and metal oxide layers are used as dopants in these doping techniques.14-23 Metal chlorides including AuCl3, RhCl3, IrCl3, and the like, are well-known p-type dopants that reduce the sheet resistance while increasing the workfunction.24-26 As n-type dopants, amine-based organic compounds are usually used to decrease the work-function of graphene. Recently, amine-containing materials such as N,N,N ′ ,N ″ ,N ″ pentamethyldiethylenetriamine (PMDTA) and its derivatives have been used to change the surface properties of graphene oxide.27-30 However, the effect of amine-based organic compounds on the decrease of the work function of graphene sheets was not fully investigated.


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Furthermore, the role of amine groups and their derivatives in graphene doping techniques needs more explanation. Therefore, we conclude that experimental work and theoretical analyses are necessary to thoroughly elucidate the role of amine groups in graphene doping. In this study, we investigated the effect of amine-based organic compounds on graphene doping. The well-known PMDTA (N1) was prepared as a reference sample. Four kinds of samples including those terminating with phenyl groups (N2), butylphenyl groups (N3), benzophenyl groups (N4), and 4-(1-phenyl-2-(4-tolyl)vinyl)phenyl groups (N5) were also prepared to identify their degree of doping performance as a function of the different terminating groups. Four-point probe techniques and ultraviolet (UV)-visible spectroscopy were used to measure the changes of sheet resistance and transmittance. Contact angle and field emission scanning electron microscopy (FESEM) measurements were performed to investigate the surface change after organic doping. The Raman spectra were measured to identify the positions of G and 2D peaks, which are indices of the degree of doping in graphene networks. The core-level spectra and secondary-electron cutoff spectra were obtained using synchrotron radiation photoemission spectroscopy (SRPES) and ultraviolet photoemission spectroscopy (UPS) to investigate the change of composition and work function after the doping process. The simulated and calculated data were used to verify that the amine-based organic compounds are efficient graphene doping materials. Based on these experimental and theoretical data, the effects of the amine-based organic compounds on n-type graphene doping are discussed and a possible doping mechanism is suggested.


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EXPERIMENTAL METHODS Graphene film preparation. Graphene samples were grown on a 25-µm-thick copper foil in a quartz tube furnace, using a CVD method involving methane (CH4) and hydrogen (H2) gas. Under a vacuum of 90 mTorr, the furnace was heated without a gas flow for 30 min. Before the growth of graphene, the copper foil was preheated at 950 °C for 30 min. To obtain a large-singlecrystal copper surface, H2 gas was supplied to the furnace under a vacuum of 150 mTorr at a rate of 33 cm3/min (sccm). After the preheating step, a gas mixture of CH4 and H2 was supplied at a ratio of 200 sccm: 33 sccm under ambient conditions for 10 min to synthesize the graphene. After 10 min of growth, the furnace was cooled to room temperature at a rate of 10–15 °C/min, under 33 sccm of H2 flow. Poly(methyl methacrylate) (PMMA) was then spin-coated onto the graphene, and the PMMA-coated foil was heated on a hot plate at 180 °C for 1 min, after which O2 plasma was used to etch the graphene on the other side of the copper foil. The sample was then immersed in a ferric chloride (1M FeCl3) bath at room temperature for 12–18 h to etch away the copper foil. Then the remaining sample was carefully dipped into a de-ionized water bath about 7–9 times to remove any residual etchant. The graphene sheets were then transferred onto an arbitrary substrate. The PMMA was removed by immersion in an acetone bath at 50 °C for 30 min after the graphene layer had completely adhered to the target substrate. Doping procedures. After the graphene sheet was transferred to a glass slide or a Si/SiO2 wafer substrate, the sample was placed on the spin-coater. The chemical structures of the purchased (N1, PMDTA) and synthesized amine-based organic compounds (N2, N3, N4, and N5) are displayed in Supporting information. The synthetic methodes are explanined in Supporting Information. The amine-based organic compounds (denoted as N1, N2, N3, N4, and N5) were separately dissolved in chloroform at three different concentrations: 10, 20, and 40 mM (i.e., a


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total of 15 different solutions were prepared). Then, 1 mL of each of the solutions with the different concentrations of amine-based organic compounds was pipetted on top of the transferred graphene substrates, allowed to stand for 1 min, and then spun at 2500 rpm for 1 min. Finally, the samples were annealed at 150 °C on a hot plate for 2 min to evaporate the solvent. Characterization. The sheet resistance was measured in a standard state using a four-point probe technique (Keithley 2612A multimeter, U.S.A.). UV-visible spectra were recorded on a JASCO V-740 photo-spectrometer. FESEM (JEOL, JSM-5410LV, Japan) images of the pristine and doped graphene films were also obtained. Raman spectra of graphene were obtained with a Lab RAM HR (Horiba JobinYvon, Japan) at an excitation wavelength of 514.54 nm. SRPES experiments were performed in an ultra-high vacuum chamber (base pressure of ~10-10 Torr) in a 4D beam line, equipped with an electron analyzer and a heating element, at the Pohang Acceleration Laboratory. The onset of photoemission, corresponding to the vacuum level at the surface of the graphene, was measured by using incident photon energy of 250 eV with a negative bias on the sample. The results were corrected for charging effects by using Au 4f as an internal reference. Geometry optimization. The calculations were carried out with DMol3 (version 6.1) software package to determine the periodic structure.31 All Periodic Boundary Condition (PBC) calculations employed the DNP (double numerical with d and p polarization) basis set (comparable to a 6-31G(d, p) Gaussian-type basis set) with the generalized gradient approximation including the exchange-correlation of the Perdew–Burke–Ernzerhof (PBE) functional provided in DMol3.32 To describe the dispersive interaction between graphene and the amine-based organic compounds, we employed the Tkatchenko-Scheffler (TS) method for DFT dispersion correction.31 TS is a parameter-free method for an accurate determination of long-


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range interactions from self-consistent field electronic structure. The convergence criterion was set to ∆E < 10–6 hartree for the geometry optimizations, and the program option for a ‘fine’ grid size (corresponding to a k-point separation of 0.04 Å–1) was used. Graphene layers are separated by 20 Å vacuum to form supercell. We used an 8×8 hexagonal graphene supercell. Electrostatic potential calculation. The electrostatic potential calculations provided in DMol3 were performed on periodic structures to obtain the work functions of pristine graphene and doped graphene. The average potentials were calculated along the c-direction perpendicular to the graphene layer. The work function (W) could be determined with the equation: W = Vvacuum−EF, where Vvacuum is the potential in vacuum (defined as the average potential in the middle of two graphene layers) and EF is the Fermi energy obtained directly from the electronic state output file.


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RESULTS AND DISCUSSION Figure 1a shows the change of transmittance of pristine graphene (P-G) and doped graphene. The transmittance of P-G at 550 nm is about 96.7%, which is similar to the previously reported values.33 After spin-coating the amine-based organic compounds on graphene sheets, the transmittance values at 550 nm decreased from 96.7 to 96.1, 91.8, 92.8, 91.5, and 95.3% for N1, N2, N3, N4, and N5, respectively. It seems that organic compounds on the surface of graphene degrade the transmittance value regardless of the dopants’ termination groups. The sheet resistance (Rsh) values are given in Figure 1b, which shows that Rsh increased from 700 to 2950, 3020, 3850, 2500, and 2710 Ω/sq after doping with N1, N2, N3, N4, and N5, respectively. The Rsh of all the doped samples was higher than that in the P-G case regardless of the doping material. Graphene is reported to have p-type properties. Therefore, the increase of Rsh indicates hole depletion or n-type doping of graphene, which means that electrons were spontaneously transferred from the amine-based organic compounds to the graphene sheets. The n-type doping means hole depletion in this article. Figure 1c shows the change of water contact angle that resulted from doping with amine-based organic compound solutions. The contact angle increased from 27.2° of bare glass to 81.5° after the graphene layer was transferred to the glass substrate. After amine-based organic compounds were spin-coated onto the graphene, the water contact angle changed slightly because of alkyl chains and benzene rings in the dopants. These results indicate that the amine-based organic compounds were well-dispersed and that the amine group could modulate the electron state of graphene.


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Figure 1. (a) The transmittance and (b) the variation in sheet resistance after doping with aminebased compounds at a concentration of 20 mM. The transmittance values decreased and the sheet resistance values increased after doping. (c) The water contact angle difference after doping with amine-based compounds at a concentration of 20 mM. The values of water contact angle for the doped samples are similar to the values for the pristine graphene. In order to verify the existence of organic compounds, the graphene was exposed to 5 kV for 10 s. The FESEM images of P-G and doped graphene after electron beam irradiation are displayed in Figure 2a to 2f. Before electron beam irradiation, the specific characteristics of graphene—namely, wrinkles, PMMA residues, and ripples—are shown with remarkable clarity in P-G. It is shown that there is no difference between the P-G and the doped graphene. After electron beam treatment, burned spots were found on the organic-doped graphene—but not on


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the P-G. This results prove that organic compounds are, in fact, coated onto the surface of the graphene.

Figure 2. The FESEM images of (a) P-G, (b) N1, (c) N2, (d) N3, (e) N4, (f) N5 doped graphene exposed to 5 kV electrons for 10 s. The scale bar is 1 µm. Figure 3a shows the Raman spectra of P-G and doped graphene samples. The D peak (1350 cm-1), G peak (1580 cm-1), and 2D peak (2700 cm-1) of graphene are related to the various disorders, the doping state, and the number of layers of graphene, respectively.34-35 The vertical dotted lines in Figure 3a indicate the positions of the G and 2D peaks in the P-G sample, and are similar to previously reported values.34 In the case of the doped graphene samples, the G peak positions were shifted to higher wavenumbers and the 2D peak positions were shifted to lower wavenumbers compared to those of the P-G case regardless of the type of dopant. To compare the degree of peak shift according to the type of dopant, the position of the G and 2D peaks are plotted against each other in Figure 3b.


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Figure 3. The Raman spectroscopy analysis of pristine graphene sheets and sheets doped with different amine-compounds. (a) The wide peak investigation including the D, G, and 2D bands. (b) Scatter plots of the position of the G band peak and the 2D band peak in the Raman spectra using data extracted from (a); the red straight dashed line is included to guide the eye and to aid in making the comparison. (c) I2D/IG intensity ratio of P-G and each of the graphene sheets doped with amine-compounds. The same experiments were performed five times, and the peak shift of each sample is displayed with the same color. It can be seen that the G and 2D peaks of graphene doped with N3 experienced the greatest shift of all the samples. In general, the shift of the G and 2D peaks was affected by doping and by the strain of the lattice structures. Both electron and hole doping in graphene make the G peak shift to a higher wavenumber.36 A calculation based on density functional theory (DFT) revealed that the position of the 2D peak shifted to lower wavenumber for an increasing electron concentration.37 Therefore, the shifts of the G and 2D peaks to higher


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and lower wavenumbers, respectively, indicate n-type doping. The red dashed line was inserted to highlight the degree of n-type doping of the graphene. The ratio of the 2D peak intensity to the G peak intensity (I2D/IG) decreased after dopant solution treatment as shown in Figure 3c. It has been reported that electron doping and lattice defects can decrease the I2D/IG ratio of doped graphene sheets.38 Therefore, the results shown in Figure 3c indicate that amine-based organic compounds cause graphene to have n-type properties, and that the graphene doped with N3 should have more pronounced n-type properties than the other four samples.

Figure 4. The synchrotron radiation photoemission spectroscopy (SRPES) analysis of pristine graphene sheets and sheets doped with different amine compounds at a 20 mM concentration. (a) C 1s core level spectra. The position of the C=C bond peak shifted about 0.2 eV to higher binding energy in the case of doped graphene compared with the pristine case. (b) N 1s core level spectra. No nitrogen peak occurred in the case of pristine graphene. Figure 4a shows the C 1s core level spectra of P-G and of graphene doped with various aminebased organic compounds that were spin-coated from solutions with a concentration of 20 mM.


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The C 1s peak of P-G was separated into four components: carbon (C=C) at 284.6 eV, sp3 carbon (C–C) at 285.8 eV, C−O bond at 287.8 eV, and a carbonyl group (C=O) near 289.0 eV.39 Some oxide functional groups were present on the surface of graphene despite the fact that the graphene had been synthesized by a CVD method; this seems to have occurred during the wet transfer operation that included hot acetone bath cleaning processes. After the doping process, the C=C bond peaks shifted about 0.2 eV to higher binding energies, suggesting that graphene captures electrons from amine-based organic compounds. The new N 1s peak is found as well, which was not found in P-G, as shown in Supporting Information. Furthermore, the carbonoxygen bond peaks have disappeared except in the case of N1-doped graphene, suggesting that dopants exist on the surface of the graphene. The smaller molecular structure of N1 compared to the other dopants is thought to make it possible to find the C−O and C=O bonds in the N1 sample. The peaks around 285.9 eV in the N2 to N5 cases correspond to the C–N bond in synthesized amine-based compounds.40 Figure 4b shows the N 1s spectra of graphene doped with the same amine-based organic compounds at a concentration of 20 mM. The appearance of the N 1s peak is attributed to the formation of thin films of the dopant solutions. The peak at 399.5 eV corresponds to the N–C bond, and the peak at 400.7 eV corresponds to the N–C–H bond (aminelinkage). The N 1s peak of N3 located in higher binding energy among samples, indicating that the N3 compound has the highest binding energy between amine based compounds and the graphene sheets. Similar phenomena were found in graphene doped from solutions with a concentration of 10 and 40 mM as shown in Supporting Information, respectively. On the basis of SRPES results, the atomic ratio of C 1s versus N 1s in P-G and doped graphene are displayed in Supporting Information. The N 1s peak was not observed in P-G so that the ratio of carbon is 100%. However, the amount of nitrogen increased after the dopant solution was spin-coated on


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graphene. The amount of nitrogen in each sample increases as a function of dopant solution concentration, which is consistent with other N-doped carbon materials fabricated by using amide or amine group compounds.41-42 It is interesting to note that the N 1s level in N3 doped graphene with a concentration of 40 mM is the highest among the samples. These results indicate that graphene is well-doped by amine-based compounds.

Figure 5. (a) The ultraviolet photoemission spectroscopy (UPS) spectra of graphene doped with different amine compounds at a concentration of 20 mM. The secondary threshold electron cutoff energy is lowest in the N3 doped sample. (b) The summary of the variation of the work function of the doped graphene sheets as functions of the kind of dopant and the concentrations.


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The UPS spectra of P-G and graphene doped with 20 mM solutions are shown in Figure 5a. The onset of the secondary-electron cutoff energy was determined by extrapolating two straight lines from the background and threshold in the spectra.43 The work function value decreased from 4.3 to 3.9, 3.9, 3.7, 4.05, and 4.0 eV for N1, N2, N3, N4, and N5 doped graphene, respectively. A similar work function decrease after doping was also found in the 10 mM and 40 mM cases as shown in Supporting Information, respectively. The thickness of amine-based compounds is lower than 3 nm. Furthermore, it is thought that the organic compounds cannot perfectly cover the whole surface of graphene due to the steric hindrance. Therefore, the work function values are thought to come from the doped graphene. The calculated work function values are displayed in Figure 5b for each dopant concentration. Figure 5b shows that lower work function values were obtained when the more concentrated dopant solutions were used. Among the amine-based organic compounds, the work function decrease was largest in the case of graphene doped with N3 from a 40 mM solution. The amine-based organic compounds were dissolved in chloroform which is frequently used as a nonpolar solvent in organic synthesis reactions. The N3 compound—unlike the other compounds—has its phenyl groups terminated by alkyl chains; this suggests that the N3 compound could be easily dissolved in chloroform solvent because of its nonpolar chain termination, which is supported by the fact that it displayed the highest UV-visible absorption intensity, as shown in Supporting Information. Therefore, it is thought that the N3 compound was uniformly spin-coated on the graphene sheets, inducing the strongest doping effect among the all compounds. Furthermore, it suggests that the aliphatic group is also important for graphene doping due to its excellent solubility in nonpolar solvents.


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Figure 6. Calculated electrostatic potential diagram along the c-axis for (a) pristine, (b) N1, (c) N2, (d) N3, (e) N4, (f) N5 doped graphene. Energy minimum extended structure geometries are shown in the inset of each figure. DFT calculations for P-G and doped graphene sheets were used to probe changes in the work function when the different amine-based organic compounds were introduced as dopants. The geometry optimization and the electrostatic potential calculations for P-G and doped graphene are shown in Figure 6. Since the average potential in the middle of the layers is close to zero, there would be negligible interaction between the graphene layers. The transformation of the graphs at a fractional coordinate value of about 0.2 is caused by the organic compounds; different electrostatic potential diagrams were obtained for different organic compounds. Also,


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when organic molecules are adsorbed, graphene sheets are bent due to the dispersion interaction between the graphene sheet and the organic molecule. The extended structure geometries with minimum energy are shown in the inset of each figure. The figure shows that the calculated work functions for each of the doped graphene samples are smaller than that of P-G. Thus, it is confirmed that the amine-based organic compounds induced n-type doping in graphene. The computational results are well-matched with the experimental results in the 10 mM case except for N3-doped graphene; there is a large work function difference between the experimental and computational results in the N3 case. This difference is thought to arise from the fact that the two types of results were arrived at under different conditions: the experimental results were obtained from the solution phase, whereas the computational results were performed in the gas phase. It is widely known that organic molecules dissolve readily in organic solvents when many alkyl chains are attached.44 The N3 compound has more aliphatic chains than other organic molecules so that it has better solubility as shown in Supporting Information. Therefore, N3 has a higher probability of being adsorbed on the graphene sheet. Furthermore, N3 has higher binding energy between amine based compound and graphene sheet as shown in Figure 4b. As a result, the smallest work function was attained in the case of N3-doped graphene. The detailed vacuum potential and Fermi energy are described in Supporting Information.


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CONCLUSION The doping effect of synthesized amine-based compounds on graphene sheets were investigated by means of DFT calculations and experiments. Spots that can be burned by an electron beam, as well as the N 1s peak in the SRPES spectra are found in amine-doped graphene, but not in pristine graphene, indicating that the surface of the graphene became wellcoated with amine-based compounds. After a graphene sheet was spin-coated by amine-based compounds, its Rsh increased from 700 to 3850 Ω/sq, the transmittance values at 550 nm decreased from 96.7 to 91.5%, and the work function decreased from 4.3 to 3.7 eV. Furthermore, the G peak in the Raman spectra shifted to higher wavenumbers and the 2D peak shifted to lower wavenumbers. These results indicate that the amine-based compounds act as n-type dopants in graphene sheets, which was verified by DFT molecular calculations. The increase of Rsh, the decrease of work function, and the Raman peak shift were most pronounced in the N3 sample. UV-visible absorption spectra showed that the N3 compound was easily dispersed in the chloroform solvent due to its nonpolar chain termination. Furthermore, SRPES spectra showed that N3 compound has higher binding energy with graphene sheet. Therefore, the N3 compound was uniformly spin-coated on graphene sheets, inducing the strongest doping effect among all the compounds. Thus, we have demonstrated that amine and alkyl groups play a crucial role in the n-type doping or hole depletion of graphene.


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ASSOCIATED CONTENT Supporting Information Available The skeletal molecular structure, the synthetic method for amine-compound dopant materials, the data of synchrotron radiation photoemission spectroscopy and ultraviolet photoemission spectroscopy, UV-visible absorption spectra, computed Fermi energy, and vacuum potential are available in Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author K. P. ([email protected]), Tel: 82-2-820-5330, Fax: 82-2-824-3495. S. Y. K. ([email protected]), Tel: 82-2-820-5875, Fax: 82-2-824-3495. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2014R1A2A1A11051098). Ki Chang Kwon acknowledges Global PH.D Fellowship Program through the National Research Foundation of Korea funded by the Ministry of Education.


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REFERENCES (1)

Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183-191.

(2)

Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Two-dimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197-200.

(3)

First, P. N.; de Heer, W. A.; Seyller. T.; Berger, C.; Stroscio, J. A.; Moon, J.-S. Epitaxial Graphene on Silicon Carbide. MRS bulletin 2010, 35, 296-305.

(4)

Obraztsov, A. N.; Obraztsova, E. A.; Tyurnina, A. V.; Zolotukhin, A. A. Chemical Vapor Deposition of Thin Graphite Films of Nanometer Thickness. Carbon 2007, 45, 20172021.

(5)

Wang, X.; Li, X.; Zhang, L.; Yoon, Y.; Weber, P. K.; Wang, H.; Guo, J.; Dai, H. NDoping of Graphene Through Electrochemical Reactions with Ammonia. Science 2009, 324, 768-771.

(6)

Panchakaria, L. S.; Subrahmanyam, K. S.; Saha, S. K.; Govindaraj, A.; Krishnamurthy, H. R.; Waghmare, U. V.; Rao, C. N. R. Synthesis, Structure, and Properties of Boron- and Nitrogen-Doped Graphene. Adv. Mater. 2009, 21, 4626-4730.

(7)

Hwang, J. O.; Park, J. S.; Choi, D. S.; Kim, J. Y.; Lee, S. H.; Lee, K. E.; Kim, Y.-H.; Song, M. H.; Yoo, S.; Kim, S. O. Workfunction-Tunable, N-Doped Reduced Graphene Transparent Electrodes for High-Performance Polymer Light-Emitting Diodes. ACS Nano 2012, 6, 159-167.

(8)

Kwon, K. C.; Choi, K. S.; Kim, B. J.; Lee, J.-L.; Kim, S. Y. Work-Function Decrease of Graphene Sheet Using Alkali Metal Carbonates. J. Phys. Chem. C 2012, 116, 2658626591


20

Page 21 of 32

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


(9)

Kwon, K. C.; Choi, K. S.; Kim, C.; Kim, S. Y. Effect of Transition-Metal Chlorides on Graphene Properties. Phys. Status Solidi A 2014, 211, 1794-1800.

(10) Kwon, K. C.; Choi, K. S.; Kim, S. Y. Increased Work Function in Few-Layer Graphene Sheets via Metal Chloride Doping. Adv. Funct. Mater. 2012, 22, 4724-4731. (11) Jung, N.; Kim, N.; Jockusch, S.; Turro, N. J.; Kim, P.; Brus, L. Charge Transfer Chemical Doping of Few Layer Graphenes: Charge Distribution and Band Gap Formation. Nano Lett. 2009, 9, 4133-4137. (12) Shin, H.-J.; Choi, W. M.; Choi, D.; Han. G. H.; Moon, S.-M.; Park, H.-K.; Kim, S.-W.; Jin, Y. W.; Lee, S. Y.; Kim, J. M. et al. Control of Electronic Structure of Graphene by Various Dopants and Their Effects on a Nanogenerator. J. Am. Chem. Soc. 2010, 132, 15603-15609. (13) Kwon, K. C.; Kim, S. Y. Extended Thermal Stability in Metal-Chloride Doped Graphene Using Graphene Overlayers. Chem. Eng. J. 2014, 244, 355-363. (14) Kwon, K. C.; Kim, B. J.; Lee, J.-L.; Kim, S. Y. Role of Ionic Chlorine in the Thermal Degradation of Metal Chloride-Doped Graphene Sheets. J. Mater. Chem. C 2013, 1, 253259. (15) Yen, W.-C.; Medina, H.; Huang, J.-S.; Lai, C.-C.; Shih, Y.-C.; Lin, S.-M.; Li, J.-G.; Wang, Z. M.; Chueh, Y.-L. Direct Synthesis of Graphene with Tunable Work Function on Insulators via In Situ Boron Doping by Nickel-Assisted Growth. J. Phys. Chem. C 2014, 118, 25089-25096. (16) Gholizadeh, R.; Yu, Y.-X. Work Functions of Pristine and Heteroatom-Doped Graphenes under Different External Electric Fields: An ab Initio DFT Study. J. Phys. Chem. C 2014, 118, 28274-28282.


21


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 32

(17) Christodoulou, C.; Giannakopoulos, A.; Nardi, M. V.; Ligorio, G.; Oehzelt, M.; Chen, L.; Pasquali, L.; Timel, M.; Giglia, A.; Nannarone, S. et al. Tuning the Work Function of Graphene-on-Quartz with a High Weight Molecular Acceptor. J. Phys. Chem. C 2014, 118, 4784-4790. (18) Yu, Y.-X. A Dispersion-Corrected DFT Study on Adsorption of Battery Active Materials Anthraquinone and Its Derivatives on Monolayer Graphene and h-BN. J. Mater. Chem. A 2014, 2, 8910-8917. (19) Yu, Y.-X. Binding Energy and Work Function of Organic Electrode Materials Phenanthraquinone, Pyromellitic Dianhydride and Their Derivatives Adsorbed on Graphene. ACS Appl. Mater. Interfaces 2014, 6, 16267-16275. (20) Pi, K.; McCreary, K. M.; Bao, W.; Han, W.; Chiang, Y. F.; Li, Y.; Tsai, S.-W.; Lau, C. N. Kawakami, R. K. Electronic Doping and Scattering by Transition Metals on Graphene. Phys. Rev. B 2009, 80, 075406. (21) Dong, X.; Fu, D.; Fang, W.; Shi, Y.; Chen, P.; Li, L.-J. Doping Sing-Layer Graphene with Aromatic Molecules. Small 2009, 5, 1422-1426. (22) Chen, Z; Santoso, I.; Wang, R.; Xie, L. F.; Mao, H. Y.; Huang, H.; Wang, Y. Z.; Gao, X. Y.; Chen, Z. K.; Ma, D. et al. Surface Transfer Hole Doping of Epitaxial Graphene using MoO3 Thin Film. Appl. Phys. Lett. 2010, 96, 213104. (23) Han, C.; Lin, J. D.; Xiang, D.; Wang, C. C.; Wang, L.; Chen, W. Improving Chemical Vapor Deposition Graphene Conductivity using MoO3: an In-Situ Field-Effect-Transistor Study. Appl. Phys. Lett. 2013, 103, 263117. (24) Chen, W.; Chen, S.; Qi, D. C.; Gao, X. Y.; Wee, A. T. S. Surface Transfer P-type Doping of Epitaxial Graphene. J. Am. Chem. Soc. 2007, 129, 10418-10422.


22

Page 23 of 32

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


(25) Benayad, A.; Shin, H.-J.; Park, H. K.; Yoon, S.-M.; Kim, K. K.; Jin, M. H.; Jeong, H.-K.; Lee, J. C.; Lee, Y. H. Controlling Work Function of Reduced Graphite Oxide with Auion Concentration. Chem. Phys. Lett. 2009, 475, 91-95. (26) Kwon, K. C.; Choi, K. S.; Kim, C.; Kim, S. Y. Role of Metal Cations in Alkali Metal Chloride Doped Graphene. J. Phys. Chem. C 2014, 118, 8187-8193. (27) Deng, Y.; Li, Y.; Dai, J.; Lang, M.; Huang, X. An Efficient Way to Functionalize Graphene Sheets with Presynthesized Polymer via ATNRC chemistry. J. Polym. Sci. A Polym. Chem. 2011, 49, 1582-1590. (28) Ren, L. L.; Huang, S.; Zhang, C.; Wang. R. Y.; Tjiu, W. W.; Liu, T. Functionalization of Graphene Grafting of Temperature-Responsive Surfaces from Graphene by ATRP “on water”. J. Nanopart. Res. 2012, 14, 940. (29) Yan, J.-L.; Chen, G.-J.; Cao, J.; Yang, W.; Xie, B.-H.; Yang, M.-B. Functionalized Graphene Oxide with Ethylenediamine and 1,-6-Hexanediamine. New Carbon Materials 2012, 27, 370-376. (30) Shanmugharaj, A. M.; Yoon, J. H.; Yang, W. J.; Ryu, S. H. Synthesis, Characterization, and Surface Wettability Properties of Amine Functionalized Graphene Oxide films with Varying Amine Chain Lengths. J. Colloid. Interface Sci. 2013, 401, 148-154. (31) Delley, B. From Molecules to Solids with the DMol3 Approach. J. Chem. Phys. 2000, 113, 7756-7764. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (33) Shi, Y.; Kim, K. K.; Reina, A.; Hofmann, M.; Li, L.-J.; Kong, J. Work Function Engineering of Graphene Electrode via Chemical Doping. ACS nano 2010, 4, 2689-2694.


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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 24 of 32

(34) Podila, R.; Rao, R.; Tsuchikawa, R.; Ishigami, M.; Rao, A. M. ACS nano 2012, 6, 57845790. (35) Sun, T.; Wang, Z.; Shi, Z.; Ran, G. Z.; Xu, W. J.; Wang, Z. Y. Multilayered Graphene Used as Anode of Organic Light Emitting Diodes. Appl. Phys. Lett. 2010, 96, 133301. (36) Wang, Q. H.; Jin, Z.; Kim, K. K.; Hilmer, A. J.; Paulus, G. L. C.; Shih, C.-J.; Ham, M-.H.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T. et al. Understanding and Controlling the Substrate Effect on Graphene Electon-Transfer Chemistry via Reactiviy Imprint Lithography. Nat. Chem. 2012, 4, 724-732. (37) Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S. K.; Waghmare, U. V.; Novoselov, K. S.; Krishnamurthy H. R.; Geim, A. K.; Ferrari, A. C. et al. Monitoring Dopants by Raman Scattering in an Electrochemically Top-Gated Graphene Transistor. Nat. Nanotech. 2008, 3, 210-215. (38) Beams, R.; Cançado, L. G.; Novotny, L. Raman Characterization of Defects and Dopants in Graphene. J. Phys.: Condens. Matter 2015, 27, 083002-083002-6. (39) Kwon, K. C.; Son, P. K.; Kim, S. Y. Ion Beam Irradiation of Few-Layer Graphene and Its Application to Liquid Crystal Cells. Carbon 2014, 67, 352-359. (40) Zafar, Z.; Ni, Z. H.; Wu, X.; Shi, Z. X.; Nan, H. Y.; Bai, J.; Sun, L. T. Evolution of Raman Spectra in Nitrogen Doped Graphene. Carbon 2013, 61, 57-62. (41) Ramanathan, T.; Fisher, F. T.; Ruoff, R. S.; Brinson, L. C. Amino-Functionalized Carbon Nanotubes for Binding to Polymers and Biological Systems. Chem. Mater. 2005, 16, 1290-1295. (42) Jansen, R. J. J.; van Bekkum, H. XPS of Nitrogen-Containing Functional Groups on Activated Carbon. Carbon 1995, 33, 1021-1188.


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(43) Kwon, K. C.; Kim, C.; Le, Q. V.; Gim, S.; Jeon, J.-M.; Ham, J. Y.; Lee, J.-L.; Jang, H. W.; Kim, S. Y. Synthesis of Atomically Thin Transition Metal Disulfides for Charge Transport Layers in Optoelectronic Devices. ACS nano 2015, 9, 4146-4155. (44) Coughlin, J. E.; Henson, Z. B.; Weich, G. C.; Bazan, G. C. Design and Synthesis of Molecular Donors for Solution-Processed High-Efficiency Organic Solar Cells. Acc. Chem. Res. 2013, 47, 257-270.


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Table of Contents


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Effect of Amine-Based Organic Compounds on the Work-Function

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