Work-Function Decrease of Graphene Sheet Using Alkali Metal

Subscriber access provided by Milner Library | Illinois State University

Article

Work-Function Decrease of Graphene Sheet Using Alkali Metal Carbonates Ki Chang Kwon, Kyoung Soon Choi, Buem Joon Kim, Jong-Lam Lee, and Soo Young Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3069927 • Publication Date (Web): 30 Nov 2012 Downloaded from http://pubs.acs.org on December 2, 2012

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.

The Journal of Physical Chemistry C 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 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

The Journal of Physical Chemistry

Work-Function Decrease of Graphene Sheet Using Alkali Metal Carbonates Ki Chang Kwon and Kyoung Soon Choi School of Chemical Engineering and Materials Science, Chung-Ang University 221 Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea

Buem Joon Kim and Jong-Lam Lee*

Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Republic of Korea

Soo Young Kim* School of Chemical Engineering and Materials Science, Chung-Ang University 221 Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea

*

Corresponding authors. J.-L.L. ([email protected]); S.Y.K. ([email protected]);

ACS Paragon Plus Environment

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

Page 2 of 21

ABSTRACT

A chemical approach was applied to decrease the work function of few-layer graphene. Li2CO3, K2CO3, Rb2CO3, and Cs2CO3 were used as n-doping materials. The sheet resistance of graphene doped with carbonate salt slightly increased from 1100 Ω/sq to 1700 - 2500 Ω/sq and the transmittance of doped graphene with 0.1 M alkali metal at 550 nm decreased from 96.7 % to 96.1-94 % due to the formation of metal particles on the surface of graphene. A higher sheet resistance and lower transmittance were obtained at a higher concentration of alkali metal carbonate. The G peak in the Raman spectra was shifted to a lower wavenumber after alkali metal carbonate doping and the intensity ratio of the carbon double bond to the carbon single bond decreased with doping in the X-ray photoemission spectroscopy spectra, suggesting the charge transfer from metal ions to graphene sheets. Ultraviolet photoemission spectroscopy data showed that the work function of the graphene sheets decreased from 4.25 eV to 3.8, 3.7, 3.5, and 3.4 eV for the graphene doped with Li2CO3, K2CO3, Rb2CO3, and Cs2CO3, respectively. This suggested that spontaneous chemical combination occurred between the carbon atoms and alkali metal, thereby decreasing the work function.

Keywords: Graphene, Chemical vapor deposition (CVD), Work-function engineering, X-ray photoemission spectroscopy (XPS), Ultraviolet photoemission spectroscopy (UPS)


2

Page 3 of 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


1. Introduction Graphene, a molecule comprising a single layer of carbon, has attracted extensive interest in the wide scientific community for its extraordinary thermal, mechanical, electrical, and other properties such as high mobility of charge carriers, anomalous quantum Hall effect, superior thermal/electrical conductivity, and ballistic transport properties.1-6 One of the potential applications of large-area graphene film is an electrode of an electronic device such as organic solar cells (OSCs), organic light-emitting diodes (OLEDs), and light-emitting diodes (LEDs).7-10 For synthesizing a graphene film with large-area, high transmittance, low sheet resistance, and low cost, many researchers have focused on the chemical vapor deposition (CVD) method using metal catalysts such as Cu, Ni, and Ru, roll-to-roll production, and low-temperature growth method.11-17 It is reported that the synthesis of high-quality graphene can be achieved by controlling the individual grain and grain boundaries of the metal catalyst surface.14 According to previous reports, pristine graphene (P-G) films are acceptable for next generation anode or cathode materials in display devices, OSCs, and vertical-LEDs because of their transmittance and sheet resistance. Although the graphene was considered a candidate for electron transport layer or cathode in optoelectronic devices, the efficiency of devices based on graphene cathode was very low compared with the devices with commercial cathodes.18-22 The work function of graphene is reported to be 4.2 – 4.6 eV.23-25 The performance of organic optoelectronic devices also relies on efficient carrier injection between active layers and the electrodes in addition to the excellent conductivity and transparency of electrodes. Therefore, the low efficiency of devices with graphene cathode is thought to be induced by energy level differences and Schottky contact between the graphene electrode and the other active layers in electronic devices.


3


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 4 of 21

Lowering the work function of the graphene cathode to form an ohmic contact with an active layer is needed to enhance the charge injection, thereby improving the device efficiency. Controlling the electronic structure in graphene has been investigated to lower the work function by using ammonia gas during the growth of graphene, self-assembled monolayer, or metal oxide layer.26-31 Chemical doping was reported to be an effective method in graphene doping due to the absence of any damage on the graphene sheet. Some metals have a low work function such as Li (2.93 eV), K (2.29 eV), Rb (2.261 eV), and Cs (2.14 eV).32 For effective and easy use of these reactive metals as dopants for graphene, alkali metal carbonate salts are suitable because metal carbonates have good solubility for water and metal ions with poor electron properties are spontaneously combined with carbon atoms on graphene sheet due to the negative Gibbs free energy. Therefore, the use of Li2CO3, K2CO3, Rb2CO3, and Cs2CO3 as n-type dopants could be considered effective for lowering the work function of graphene sheet. Furthermore, such surface charge transfer induced by chemical doping is expected to control the Fermi level of graphene without companying substitutional impurities or basal plane reactions. In the present study, metals with relatively low work functions were used as n-type dopants to lower the work function of graphene sheet. Li2CO3, K2CO3, Rb2CO3, and Cs2CO3 were dissolved in deionized (DI) water and the graphene sheet was soaked in these solutions for n-type doping. Four-point probe method was used to measure the sheet resistance of the doped graphene sheet. Atomic force microscopy (AFM) was used to measure the thickness and surface roughness of the graphene layer. The optical properties of the metal-doped graphene layer were characterized with UV-visible spectroscopy and optical microscopy (OM). Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM) were used to confirm the alkali metal doping. Raman spectroscopy and X-ray photoemission spectroscopy (XPS) were used to characterize the doping mechanism and properties of the graphene sheets. ACS Paragon Plus Environment

4

Page 5 of 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


The changes in work function after alkali metal carbonates doping were measured by using ultraviolet photoemission spectroscopy (UPS). On the basis of these measurements, the influence of alkali metal carbonates on the change in work function in graphene sheets is discussed.

2. Experimental Section 2.1 Preparation of graphene Few-layer graphene samples were grown on 25-µm-thick copper foil in a quartz tube furnace system using a CVD method involving methane (CH4) and hydrogen (H2) gas. Under vacuum conditions of 90 mTorr, the furnace was heated without gas flow for 30 min. Before the growth of graphene, a copper foil was preheated at 950 °C for 30 min. In order to obtain a large single crystal copper surface, H2 gas was supplied to the furnace at 33 cm3/min (sccm) under 150 mTorr. After the preheating step, a gas mixture of CH4:H2 = 200 sccm : 33 sccm was supplied at ambient condition for 10 min to synthesize the graphene. After 10 min growth, the furnace was cooled down to room temperature in a rate of 10-15 oC/min under 33 sccm H2 flow. 2.2 Transfer of graphene After growth of the graphene, poly[methyl methacrylate] (PMMA, 46 mg/mL in chlorobenzene) was spin-coated on the graphene-coated copper foil. The PMMA-coated foil was placed on 180 o

C-heated hot plate for 1 min, after which O2 plasma was used to etch graphene on the other side

of the copper foil. The sample was then immersed in a ferric chloride (1M FeCl3) bath at room temperature to etch away the copper foil for 12-18 hours. After etching the copper foil, the remaining PMMA-coated graphene was carefully dipped into a DI water bath about 7-9 times to remove any residual etchant. The PMMA-coated graphene sheets were then transferred onto an


5


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 6 of 21

arbitrary substrate. PMMA was removed by acetone bath at 50 °C for 30 min after the PMMA/graphene layer had completely adhered onto the target substrate. 2.3 Doping of transferred graphene Four alkali metal carbonate powders were purchased from Sigma-Aldrich: Li2CO3, K2CO3, Rb2CO3, and Cs2CO3. It is reported that solubility of K2CO3, Rb2CO3, or Cs2CO3 in DI water is hundred times higher than that of Li2CO3 in DI water.32 Therefore, each metal carbonate powder was dissolved in DI water at a concentration of 0.1 M, 0.5 M, and 1 M (in the case of Li2CO3, 0.01 M, 0.05 M, and 0.1 M). After dissolving the metal carbonate powder in each solvent, the doping solution at different concentrations was poured into plastic petri dishes in which the transferred graphene was soaked for n-type doping for 30 min. 2.4 Characterization of graphene and doped graphene films Optical micrographs were acquired by a digital camera (Lumix DMC-LX5, Panasonic). UVvisible spectra were recorded on a JASCO V-740 photospectrometer with a wavelength range from 400 nm to 900 nm. Field emission SEM (JEOL, JSM-5410LV, Japan) and TEM (JEOL2100F) images of the P-G and doped graphene films were also obtained. Raman spectroscopy spectra of graphene were obtained with LabRAM HR (Horiba Jobin Yvon) at an excitation wavelength of 514.54 nm. The XPS was carried out on a Sigma Probe model (ThermoVG, U.K.) operating at a base pressure of 5 × 10-10 mbar at 300 K with a non-monochromatized Al Kα line at 1486.6 eV, a spherical sector analyzer of 180 degree, a mean diameter of 275 mm, an analysis area of 15 µm to 400 µm, and multichannel detectors. Corrections due to charging effects were performed by using C 1s as an internal reference and the Fermi edge of a gold sample. UPS was performed on a PHI 5000 versa probe with a He I (21.2 eV) source.


6

Page 7 of 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


3. Results and Discussion Figure 1(a) shows the transmittance spectra of the graphene sheets doped with alkali metal carbonate at a concentration of 0.1 M (Li2CO3 is 0.01 M) in the wavelength of 400 ~ 900 nm. The transmittance of P-G at 550 nm was about 96.7 %, but decreased to between 96.1 % for Li2CO3 and 94.0 % for Rb2CO3 for the four alkali metal-doped graphene sheets. The transmittance spectra of 1 M-doped doped graphene sheets (Li2CO3 is 0.1 M) are shown in Fig. 1(b). The 1 M solution-doped graphene sheets showed a lower degree of transparency than the graphene sheets doped with 0.1 M solution, indicating that the transmittance decreased with increasing doping solution concentration. The change of sheet resistance after doping on graphene is shown in Fig. 1(c). The sheet resistance increased from 1100 Ω/sq to 2050, 1750, 2520, and 1500 Ω/sq after doping with Li2CO3, K2CO3, Rb2CO3, and Cs2CO3, respectively. The Cs2CO3-doped sample showed the lowest sheet resistance, but the sheet resistance of all samples was higher than that of P-G. This increased sheet resistance indicated the charge transfer between the graphene sheet and the dopant solution. Graphene is reported to be a p-type semiconducting material.30 Therefore, the n-type dopant solution treatment was considered to have increased the sheet resistance of P-G. The degree of change in the sheet resistance as a function of n-dopants is graphically depicted in the inset of Fig. 1(c). The dramatic increase in the sheet resistance was considered to indicate the combination of carbon atoms and dopant metals. OM images of the doped graphene are shown in Fig. 2(a). The image of P-G is also shown as a reference. The concentration of Li2CO3 was 0.1 M, but that of the other dopants was 1 M. The metal particles on the P-G sheet were uniformly dispersed, indicating that they were formed by the combination of graphene with metal ions. Consequently, the number of metal particles


7


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 8 of 21

increased with increasing doping solution concentration, thereby decreasing the transmittance of the graphene film. Figure 2(b) shows the SEM images and EDS spectra of graphene doped with 1 M solution. Dopant particles were observed on the doped graphene surface, but not on the P-G surface. The EDS spectra suggested that these particles originated from the doping solution. Furthermore, it is shown that the content of oxygen increased with dopant. Metal ions in the doping solution and carbon atoms of P-G combined to form a metal-carbon complex on the surface of the graphene sheets, indicating that graphene was doped well with different metal carbonate solutions. Figure 2(c) shows the TEM images of pristine and doped graphene. Selected area electron diffraction (SAED) pattern of pristine graphene is shown in inset of Fig. 2(c). Graphene exhibits typical wrinkled structure with corrugation and scrolling which is intrinsic to graphene. The well-defined diffraction spots in the SAED patterns confirm the crystalline structure of the graphene. It is considered that graphene used in this work might contain one- to few-layer sheets. Black spots were found in doped graphene, indicating that metal particles were dispersed on the surface of graphene. Figure 3(a) shows the XPS C 1s spectra of the n-doped graphene sheets. In order to separate each chemical bonding state, including those in the spectra, the spectral line curve was simulated using an appropriate combination of Gaussian and Lorentzian functions.33 For all fitting, the fullwidth-at-half-maximums were fixed accordingly. The C 1s peak of P-G was separated into four components of sp2 hybridized carbon bond (C=C) at 284.5 eV, sp3 hybridized carbon bond (C-C) at 285.4 eV, C-O bond at 286.6 eV, and a carbonyl group (C=O) near 289.0 eV.27 Even though PG was synthesized by CVD method, some oxide functional groups were present on its surface. These were induced by a wet transfer process using a hot acetone bath, isopropyl alcohol, and methyl alcohol. The C=C bond peak was shifted to a higher binding energy by about 0.15, 0.1, 0.15, and 0.2 eV for Li2CO3, K2CO3, Rb2CO3, and Cs2CO3 doping processes, respectively. The ACS Paragon Plus Environment

8

Page 9 of 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


peak intensity of C-C (IC-C) and C=O bonds increased, but that of C=C bond (IC=C) significantly decreased with the doping process. Figure 3(b) summarizes the peak intensity of each component in the C 1s spectra. The percentage of the carbon single bond significantly increased, but that of the carbon double bond decreased. The largest change of IC=C was induced by cesium. Moreover, the IC=C/IC-C intensity ratio decreased after n-type doping treatment in all samples, as shown in Fig. 3(c). The decrease of the IC=C/IC-C intensity ratio was considered further evidence for n-type doping by combination between carbon and alkali metals. In order to investigate the change of the electronic structure in doped graphene induced by alkali metal carbonate, the UPS spectra were measured. The UPS spectra around the secondary electrons threshold region of the graphene sheets treated by alkali metal carbonate solution are shown in Figs. SI3, SI4, and SI5. The concentration of Li2CO3 was one tenth of that of the other dopants. Gold plate was used for correction values for accuracy. The secondary electron threshold was determined by extrapolation between the background and straight solid lines in the secondary electron threshold region of the UPS spectra.34 The work function (Φ) was determined from the secondary electron threshold using the function of Φ = hν –Eth, where hν and Eth are the photon energy of excitation light (He I discharge lamp, 21.2 eV) and the secondary electron threshold energy, respectively. Figure 4 shows the work function of graphene doped with 1 M (in case of lithium is 0.1 M) solution. The work function of P-G was about 4.25 eV, which is close to that of graphite and carbon nanotubes in a previous report.35 The work function decreased from 4.25 eV to 3.8, 3.7, 3.5, and 3.4 eV for Li2CO3, K2CO3, Rb2CO3, and Cs2CO3, respectively. The work function of graphene doped by Cs2CO3 was similar to that previously reported.14 Among the n-dopants used in this experiment, the work function of graphene doped by 1 M Cs2CO3 was the strongest. The alkali metal with a larger atomic number tends to provide electrons to other


9


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 10 of 21

materials. Therefore, it is considered that the combination of the carbon atom of graphene with a metal with a larger atomic number shows a lower work function. Based on the experimental results, the coupling between the oxide functional group of carbon in CVD graphene and the alkali metal can be explained as follows. Figure 5 presents the schematic doping mechanism of graphene by alkali metal carbonate. It is reported that Li2CO3, K2CO3, Rb2CO3, and Cs2CO3 have a great tendency to combine oxide group in carbon atom with an alkali metal ion.36-38 The interfacial monolayer of C-O-Cs complexes is reported to be formed during the doping process.18,38 The work function of this interfacial complex is lower than that of decomposed Cs2CO3. Therefore, the interfacial complexes for Li2CO3, K2CO3, and Rb2CO3 were also considered to have formed during the doping processes. These complexes have a low work function and can act as additional dipoles to reduce the value of the work function in graphene sheet. As shown in Fig. SI7, Cs2CO3 decreased the work function of indium tin oxide as well as P-G by forming interfacial complexes on the surface, suggesting that doping mechanism by metal carbonates to P-G is very similar to indium tin oxide. According to theoretical analysis, the carbon atoms combined with the alkali metal via covalent bonding, which upshifted the electrons near the Dirac point of P-G, thereby decreasing the work function of graphene.39 The peak shift of the C 1s peak to a higher binding energy and the decreased IC=C/IC-C intensity ratio, as shown in Figure 3, support the n-type doping by covalent bonding between carbon atoms and alkali metals. Even though the work function of graphene could be lowered by metal carbonates, it is considered that much effort is needed for the practical use of graphene because many devices are very sensitive to the surface states.

4. Conclusions


10

Page 11 of 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


The decreased work function in graphene achieved via chemical doping using four metal carbonates, Li2CO3, K2CO3, Rb2CO3, and Cs2CO3, was investigated. Graphene sheets were synthesized by CVD method in optimized condition. The sheet resistance of doped graphene increased from 1100 Ω/sq to 1700 - 2500 Ω/sq. Because graphene has the properties of a p-type semiconductor, electron donation occurred in the graphene sheets by metal carbonates, thus increasing the sheet resistance. The transmittance of doped graphene at 550 nm also decreased from 96.7 % to 96.1-94 %, suggesting the formation of covalent bonding between alcohol-like functionalized carbon atoms and alkali metal. The sheet resistance was increased and the transmittance was decreased with increasing concentration of metal carbonate. EDS and XPS data revealed covalent bonding between the oxidized carbon atoms of the graphene layer and the alkali metal ions, leading to the formation of metal particles on the surface. The G peak in the Raman spectra was shifted to a relatively lower wavenumber by metal carbonate doping, thus revealing the combinations of oxidized carbon atoms in the graphene surface with alkali metal. According to UPS data, the work function of graphene decreased from 4.2 eV to 3.4 eV via metal carbonate doping. Spontaneous electron donation was considered to have occurred from the metal ions to the specific energy level of graphene, thereby decreasing the work function. Therefore, the metal carbonate doping increased the electron concentration of P-G by covalent bonding, thus decreasing the work function.

Acknowledgements This research was supported by Basic Science Research Program (2011-0008994) and Mid-career Research Program (2011-0028752) through the National Research Foundation of Korea (NRF)


11


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 12 of 21

funded by the Ministry of Education Science and Technology, and in part by the Center for Green Airport Pavement Technology (CGAPT) of Chung-Ang University.

Supporting Information Description Supporting Information Available : Full description of the material. This material is available free of charge via the Internet at http://pubs.acs.org.


12

Page 13 of 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


References (1) Geim, A. K. Science 2009, 324, 1530-1534. (2) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183-191. (3) Du, X.; Skachko, I.; Barker, A.; Anderi, E. Y. Nat. Nanotech. 2008, 3, 491-495. (4) Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.; Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K. Science 2007, 315, 1379-1379. (5) Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201-204. (6) Gunlycke, D.; Lawler, H. M.; White, C. T. Phys. Rev. B 2007, 75, 085418-1 - 085418-5. (7) Park, H; Rowehl, J. A.; Kim, K. K; Bulovic, V.; Kong, J. Nanotechnology 2010, 21, 505204-1 505204-5. (8) Han, T.-H; Lee, Y.; Choi, M.-R.; Woo, S.-H.; Bae, S.-H.; Hong, B. H.; Ahn, J.-H.; Lee, T.-W. Nat. Photon. 2012, 6, 105-110. (9) Shim, J.-P.; Kim, D.; Choe, M.; Lee, T.; Park, S.-J.; Lee, D.-S. Nanotechnology 2012, 23, 255201-1 - 255201-6. (10) Ko, Y. U.; Cho, S.-R.; Choi, K. S.; Park, Y.; Kim, S. T.; Kim, N. H.; Kim, S. Y.; Chang, S. T. J. Mater. Chem. 2012, 22, 3606-3613. (11) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.Y.; Hong, B. H. Nature 2009, 457, 706-710. (12) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Nano Lett. 2009, 9, 30-35. (13) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E. et al. Science 2009, 324, 1312-1314. (14) Yu, Q.; Jauregui, L. A.; Wu, W.; Colby, R.; Tian, J.; Su, Z.; Cao, H.; Liu, Z.; Pandey, D.; Wei, D. et al. Nat. Mater. 2011, 10, 444-449. (15) Sun, J.; Cole, M. T.; Lindvall, N.; Teo, K. B. K.; Yurgens, A. Appl. Phys. Lett. 2012, 100, 022102-1 - 022102-3. (16) Weatherup, R. S.; Bayer, B. C.; Blume, R.; Ducati, C.; Baehtz, C.; Schlögl, R.; Hofmann, S. Nano Lett. 2011, 11, 4154-4160. (17) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I. et al. Nat. nanotech. 2010, 5, 574-578.

(18) Huang, J.-H.; Fang, J.-H.; Liu, C.-C.; Chu, C.-W. ACS Nano, 2011, 5, 6262-6271. (19) Ji, L.; Rao, M.; Zheng, H.; Zhang, L.; Li, Y.; Duan, W.; Guo, J.; Cairns, E. J.; Zhang, Y. J. Am. Chem. Soc. 2011, 133, 18522-18525.


13


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 14 of 21

(20) Cox, M.; Gorodetsky, A.; Kim, B.; Kim, K. S.; Jia, Z.; Kim, P.; Nuckolls, C.; Kymissis, I. Appl. Phys. Lett. 2011, 98, 123303-1 - 123303-3. (21) 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. ACS Nano 2012, 6, 159-167. (22) Li, Y.; Wang, J.; Li, X.; Geng, D.; Banis, M. N.; Li, R.; Sun, X. Electro. Comm. 2012, 18, 12-15. (23) Park, Y; Choi, K. S.; Kim, S. Y. Phys. Status Solidi A – Appl. Mat. 2012, 209, 1363-1368. (24) Zhou, Y; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J. et al. Science 2012, 336, 327-332. (25) Jeong, H. K.; Kim, K.-J.; Kim, S. M.; Lee, Y. H. Chem. Phys. Lett. 2010, 498, 168-171. (26) Shin, H.-J.; Choi, W. M.; Choi, D.; Han, G. H.; Yoon, S.-M.; Park, H.-K.; Kim, S.-W.; Jin, Y. W.; Lee, S. Y.; Kim, J. M. et al. J. Am. Chem. Soc. 2010, 132, 15603-15609. (27) Benayad, A.; Shin, H.-J.; Park, H. K.; Yoon, S.-M.; Kim, K. K.; Jin, M. H.; Jeong, H.-K.; Lee, J. C.; Choi, J.-Y.; Lee, Y. H. Chem. Phys. Lett. 2009, 475, 91-95. (28) Wei, D.; Liu, Y.; Wang, Y.; Zhang, H.; Huang, L.; Yu, G. Nano. Lett. 2009, 9, 1752-1758. (29) Shi, Y.; Kim, K. K.; Reina, A.; Hofmann, M.; Li, L.-J.; Kong, J. ACS Nano 2010, 4, 2689-2694. (30) Park, J.; Lee, W. H.; Huh, S.; Sim, S. H.; Kim, S. B.; Cho, K.; Hong, B. H.; Kim, K. S. J. Phys. Chem. Lett. 2011, 2, 841-845. (31) Wang, Y.; Tong, S. W.; Xu, X. F.; Özyilmaz, B.; Loh, K. P. Adv. Mater. 2011, 23, 1514-1518.

(32) Lide, D. R. CRC handbook on Chemistry and Physics, CRC press LLC, Cleveland 2008, p.12114. (33) Kim, S. Y.; Hong, K.; Lee, J.-L. Jpn. J. Appl. Phys. 2011, 50, 101602-1 - 101602-4. (34) Kim, S. Y.; Hong, K.; Kim, K.; Yu, H. K.; Kim, W.-K.; Lee, J.-L. J. Appl. Phys. 2008, 103, 076101-1 - 076101-3. (35) Shiraishi, M.; Ata, M. Carbon 2001, 39, 1913-1917. (36) Pickett, W. E. Phys. Rev. Lett. 1994, 73, 1664-1667. (37) Martinelli, R. U. J. Appl. Phys. 1974, 45, 1183-1190. (38) Huang, J.; Xu, Z.; Yang, Y. Adv. Funct. Mater. 2007, 17, 1966-1973.

(39) Davydov, S. Y. Tech. Phys. Lett. 2009, 35, 998-1000.


14

Page 15 of 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


Figure captions. Fig. 1. (a) Transmittance spectra of pristine graphene (P-G) and graphene sheets doped with four kinds of alkali metal carbonate at a concentration of 0.1 M (Li2CO3 is 0.01 M). The transmittance of P-G sheet at 550 nm was about 96.7 %. The graphene doped with Rb2CO3 at 550 nm showed the lowest transmittance of 94.3 %. (b) Transmittance data of P-G and graphene sheets doped with four alkali metal carbonates at a concentration of 1 M (Li2CO3 is 0.1 M). The graphene doped with Rb2CO3 at 550 nm showed the lowest transmittance of 93.4 %. (c) The change of sheet resistance after doping on graphene sheets. The degree of changes in the sheet resistance as a function of n-dopants is depicted in the inset of Fig. 1(c). The largest and smallest increases in the sheet resistance were about 135 % for Rb2CO3 and about 45 % for Cs2CO3, respectively. Fig. 2. (a) OM images of the graphene doped with 1 M solution. Metal particles are uniformly dispersed on the graphene surface. The OM image of P-G is also displayed as a reference. (b) SEM image and EDS spectra of graphene doped with 1 M (Li2CO3 is 0.1 M) solution. The metal particles, which were not observed in P-G, originated from the doping solution. The SEM image and EDS spectrum of P-G is also displayed as a reference. (c) TEM images of graphene. Inset figure shows the SAED pattern of P-G. Fig. 3. (a) XPS C 1s spectra of n-doped graphene film: sp2 carbon (C=C) at 284.45 eV, sp3 carbon (C-C) at 285.4 eV, C–O bond at 286.6 eV, and a carbonyl group (C=O) near 288.8 eV were considered. Small peak shifts are evident in the doped graphene sheets. (b) Peak intensity ratios of each component in the C 1s spectra. (c) Ratio of IC=C to IC-C. The IC=C/IC-C intensity ratio decreased after doping treatment in all samples. Fig. 4. Work function of graphene doped with 1 M solution (Li2CO3 is 0.1 M). UPS data were taken from different sample positions and the results were averaged. Fig. 5. Schematic mechanism for graphene doping by alkali metal carbonate. Metal ions have positive reduction potentials and the covalent bonding between the alcohol-functionalized carbon atoms and the metal ions is a spontaneous reaction. The transfer of electrons from metal ions to the graphene sheet fills in the electrons near the Dirac point of P-G, thereby decreasing the work function of graphene.


15


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 16 of 21

[Figure 1]


16

Page 17 of 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


[Figure 2]

[Figure 3]


17


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

Page 18 of 21

[Figure 4]


18

Page 19 of 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


[Figure 5]


19


Page 20 of 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 ACS Paragon Plus Environment

20

Page 21 of 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


Table of contents Image


21

Work-Function Decrease of Graphene Sheet Using Alkali Metal

Recommend Documents