Synthesis of N-Doped Graphene by Chemical Vapor Deposition and

Mar 27, 2009 - To realize graphene-based electronics, various types of graphene are required; thus, modulation of its electrical properties is of grea...
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NANO LETTERS

Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties

2009 Vol. 9, No. 5 1752-1758

Dacheng Wei,†,‡ Yunqi Liu,*,† Yu Wang,† Hongliang Zhang,†,‡ Liping Huang,†,‡ and Gui Yu† Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China, and Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China Received October 30, 2008; Revised Manuscript Received March 5, 2009

ABSTRACT To realize graphene-based electronics, various types of graphene are required; thus, modulation of its electrical properties is of great importance. Theoretic studies show that intentional doping is a promising route for this goal, and the doped graphene might promise fascinating properties and widespread applications. However, there is no experimental example and electrical testing of the substitutionally doped graphene up to date. Here, we synthesize the N-doped graphene by a chemical vapor deposition (CVD) method. We find that most of them are few-layer graphene, although single-layer graphene can be occasionally detected. As doping accompanies with the recombination of carbon atoms into graphene in the CVD process, N atoms can be substitutionally doped into the graphene lattice, which is hard to realize by other synthetic methods. Electrical measurements show that the N-doped graphene exhibits an n-type behavior, indicating substitutional doping can effectively modulate the electrical properties of graphene. Our finding provides a new experimental instance of graphene and would promote the research and applications of graphene.

Graphene, a two-dimensional form of carbon with atoms arranged in a honeycomb lattice, is the basic building block for graphitic materials of all other dimensionalities. Although it was only discovered very recently,1 it has received great interest since it could provide an excellent object for condensed physics and material science.2 It combines high electron mobility with atomic thickness and promises widespread applications;3-7 thus, it has been considered as one of the most promising materials candidate for future nanoelectronics.7 To realize the graphene-based circuits, various types of graphene are needed; thus, modulation of its electrical properties is of great technological importance. Doping it with other elements is a promising way to achieve this goal. Doping is a common approach to tailor the electronic properties of the semiconductor materials. For instance, after doping with N or B atoms, carbon nanotubes (CNTs) become n-type or p-type, respectively.8,9 Doping can also dramatically alter the electrical properties of graphene. Theoretic studies show that the substitutional doping can modulate the band structure of graphene,10-14 leading to a metal-semiconductor * To whom correspondence should be addressed. E-mail: liuyq@ iccas.ac.cn. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences. 10.1021/nl803279t CCC: $40.75 Published on Web 03/27/2009

 2009 American Chemical Society

transition;10-12 thus, the applications of graphene can be largely improved and expanded. The doped graphene promises many fascinating properties and widespread potential applications such as superconduction,15 ferromagnetism,16 etc. Therefore, intensive theoretic researches are focused there, and many theoretic models of the substitutionally doped graphene have been established.10-12 However, these researches only rest on theory, and we are still lack of an experimental example of the substitutionally doped graphene. This would partly attribute to the limitations of the synthetic method. To date, mechanical exfoliation,1 thermal decomposition of SiC,17 oxidation of graphite,18 liquid-phase exfoliation of graphite6,19 have been used to produce graphene. Chemical vapor deposition (CVD) is a general method to grow the graphitic20 or the doped graphitic21 thin films for decades. In the early 1990s, Johansson et al. successfully produced few-layer graphite by CVD,20 and recently, CVD has been widely used to synthesis the graphene.22,23 Here, we demonstrate the synthesis of the N-doped graphene by the CVD method, which is hard to produce by other methods, thus we realize the synthesize of the substitutionally doped graphene. Moreover, we provide the experimental measurement of the electrical properties of the N-doped graphene, which shows n-type behavior, indicating the modulation of

Figure 1. SEM and TEM characterization of the N-doped graphene. (a) SEM image of the N-doped graphene produced by CVD. (b) TEM image and (c) low resolution TEM image of the N-doped graphene. The background is the lacey carbon TEM grid. We can find the graphene sheet is crumpled with many ripples. (d-f) HRTEM images of the N-doped graphene on the Cu catalyst with 2 graphitic layers (d), 3 graphitic layers (e), and 4-5 graphitic layers (f), respectively.

the electrical properties of graphene by the substitutional doping. The N-doped graphene was prepared via a CVD process by using a 25 nm thick of Cu film on a Si substrate as the catalyst. The substrate was placed in a quartz tube with a flow of hydrogen (20 sccm) and argon (100 sccm). When the center of the furnace reached 800 °C, 60 sccm CH4 and 60 sccm NH3 were introduced into the flow as the C source and N source respectively, and then the substrate was rapidly moved to the high temperature region. After 10 min of growth, the sample was cooled to room temperature under H2 ambient. The pristine graphene was synthesized through a similar route without introducing NH3 in the feedstock. The products were characterized by scanning electron microscopy (SEM, Hitachi S-4800, 15 kV), transmission electron microscopy (TEM, Hitachi-2010, 200 kV), X-ray photoelectron spectroscopy (XPS, ESCA Lab220I-XL), XNano Lett., Vol. 9, No. 5, 2009

ray energy dispersive spectrometer (EDS, equipped on SEM and TEM) and Raman spectrometer (Laboratory Ram HR800, with laser excitation at 633 nm). After the CVD process of the N-doped graphene growth, SEM images (Figures 1a and S1, Supporting Information) show that the substrate covered with a layer of large area, continuous, crumpled membrane. TEM images (Figure 1b,c) show the membrane has morphology like a large crumpled paper on a lacey carbon TEM grid, which indicates the membrane is flexible. The rumpling comes from the growth process and the post-treatment process and can also be observed in the graphene produced by CVD in the absence of NH3.20 The high resolution TEM images (HRTEM, Figure 1d-f) reveal the thickness of the graphene sheets and the detailed crystalline structure of the N-doped graphene. According to the layer number, graphene can be distinguished as single-layer or few-layer graphene, and as observed from 1753

most of the products are the few-layer graphene, while singlelayer and graphitic graphene can be occasionally detected.

Figure 2. Raman spectra of the N-doped graphene. The black, red, and green lines correspond to the single-layer, few-layer, and graphite-like graphene, respectively. (Inset) Enlarged spectra of the 2D band.

the HRTEM images, the N-doped graphene, produced here, is predominantly the few-layer graphene (with graphitic layers less than 10) grown on the surface of Cu, and most of them are the graphene with 2-6 graphitic layers. The interlayer separation is about 0.34 nm, and the graphitic layers are curved, interrupted and have many defects. This crystalline morphology was also found in case of N-doped multiwalled CNTs (MWNTs), and would be attributed to the substitution of N atoms.24 Raman spectroscopy is a powerful tool for identifying carbon materials. We measured various parts of the membrane. All of the Raman spectra (Figure 2) have a high intensity of D band (ca. 1328 cm-1), which indicates the doping of the graphitic sheets,25 as the D band only occurs in the sp2 C with defects,26 and N doping introduces large amount of topological defects.24 The G band is located at 1576-1582 cm-1, while the pristine graphene, produced here, is located at 1583-1588 cm-1. There are many factors, which can affect the position of the G band, such as doping,27 layer numbers,28,29 defects,30 strains,31 substrate,32 etc. And our observation is similar to the N-doped CNTs, where N doping causes a downshift of the G band.25 The 2D band is the most prominent feature in the Raman spectrum of graphene, and its shapes is sensitive to the number of layers of graphene.28,33-35 We observed three types of 2D band of the N-doped graphene. In most cases, the shape of 2D band is a broad peak at 2650 cm-1, corresponding to the fewlayer graphene, as bilayer and few-layer graphene has a much broader and up-shifted 2D band compared with single-layer graphene.28,33-35 The other two types can only be occasionally detected. One is a sharp peak at 2620 cm-1. This part of graphene should be single-layer, as the 2D band of singlelayer graphene is located at lower frequency with a shape of a single sharp peak,33,35 and this shape is highly sensitive to identify the single-layer graphene.28 The other type is a broad peak at 2661 cm-1, corresponding to the graphitic graphene. The higher frequency component of the 2D band increases, compared with few-layer graphene, as the further increase in graphitic layers will lead to a significant decrease of the relative intensity of the lower frequency component.28,33 Therefore, based on the Raman characterization, 1754

The XPS (Figure 3a-c) and EDS (Figure S2-S5, Supporting Information) spectra confirm the doping of the graphene. In the XPS spectra, the peaks at 284.8, 401.6, and 531.9 eV correspond to C 1s of sp2 C, N 1s of the doped N, and O 1s of the absorbed oxygen, respectively, and the atomic percentage of N in the sample is about 8.9 at%. There are three components in the C 1s spectrum of the N-doped graphene. The main peak at 284.8 eV corresponds to the graphite-like sp2 C, indicating most of the C atoms in the N-doped graphene are arranged in a conjugated honeycomb lattice. The small peaks at 285.8 and 287.5 eV reflect different bonding structure of the C-N bonds, corresponding to the N-sp2 C and N-sp3 C bonds, respectively, and would originate from substitution of the N atoms, defects or the edge of the graphene sheets.36 In the pristine graphene, the N 1s peak is absent, while in the N-doped graphene, the N 1s peak has three components, indicating that N atoms are in the three different bonding characters inserted into the graphene network (Figure 3d). The small peaks at 398.2, 400.1 eV correspond to “pyridinic”, “pyrrolic” N, respectively. They refer to the N atoms which are located in a π conjugated system and contribute to the π system with one or two p-electrons, respectively.37,38 The peak at 401.7 eV corresponds to “graphitic” N, which refers to the N atoms replacing the C atoms inside of the graphene layers.38 The peak for “graphitic” N is much higher, thus the N atoms are substitutionally doped into the graphene lattice and mainly in the form of “graphitic” N. The O 1s peak arises from the oxygen or water absorbed on the surface of the N-doped graphene,1,37,39 as this peak obviously decreases after heating in vacuum (Figure S6, Supporting Information). The N doping can also be confirmed by the EDS spectra. An obvious N peak can be clearly detected in each area of the N-doped graphene (Figures S2, S4, Supporting Information), while be absent in case of the pristine graphene (Figures S3, S5, Supporting Information). The N content in the graphene can be controlled by the ratio of NH3 and CH4 in the growth. With lower NH3/CH4 ratios of 1:2 and 1:4, the N content, estimated by XPS, reduced to 3.2 and 1.2 at%, respectively. CVD is a normal technique to produce carbon materials. In the process, the liquid metal act as the catalytic sites for absorption and dissociation of the gas reactants, and then solid graphitic carbon grow from the saturated catalyst by means of precipitation.22 If N-contained reagent is introduced in the growth, N atoms will also absorb, dissociate and then precipitate into the graphitic lattice, thus, i.e. N-doped CNTs are grown in the present of NH3.37 As the doping process accompanies with the recombination of carbon atoms into graphene at high temperature, the N atoms can be substitutionally doped into the graphene lattice, which is hard to realize by other methods. Furthermore, the rapid heating of the supported catalyst is an important factor for producing the doped graphene. The catalyst film tends to aggregate at high temperature. The rapid heating can avoid the aggregation of the catalyst before the growth of the N-doped Nano Lett., Vol. 9, No. 5, 2009

Figure 3. (a) XPS spectra of the pristine graphene and the N-doped graphene. (b) XPS C 1s spectrum and (c) XPS N 1s spectrum of the N-doped graphene. The C 1s peak can be split to three Lorentzian peaks at 284.8, 285.8, and 287.5 eV, which are labeled by red, green, and blue dashed lines. The N 1s peak can be split to three Lorentzian peaks at 401.7, 400.1, and 398.2 eV, which are labeled by red, green, and blue dashed lines. (d) Schematic representation of the N-doped graphene. The blue, red, green, and yellow spheres represent the C, “graphitic” N, “pyridinic” N, and “pyrrolic” N atoms in the N-doped graphene, respectively.

graphene. In case of slow heating, the catalyst film would aggregate before the growth of graphene, thus we can only obtain CNTs or nanocages (Figure S7, Supporting Information). For the electrical measurements, we prepared the bottomgated field-effect transistors (FETs) by using the N-doped graphene and the pristine graphene (Figure 4a,b). The graphene, bridging the source and drain electrodes, behaved as the conducting channel. The channel length (L) and width (W) were about 2 µm and about 8-16 µm, respectively. To obtain a better contact, thermal annealing was performed. We measured more than fifty devices at ambient conditions, and found that the N-doped graphene showed distinguishing features, compared with the pristine graphene. Figure 4c and d show the typical source-drain current (Ids) vs the sourcedrain voltage (Vds) curves at different gate voltage (Vg), and Figure 4e and S8 show the transfer curves of these devices. The pristine graphene shows good conductivity and a linear Ids-Vds behavior, indicating a good ohmic contact between the Au/Ti pads and the graphene. Ids increases slowly with decreasing Vg and the neutrality point is at about 15-20 V, consistent with previous observations, indicating a p-type behavior.1,40,41 Distinguishingly, the N-doped graphene has relatively lower conductivity and larger on/off ratio. Ids was suppressed in low Vds region, thus at low Vds the on/off ratio would further increase (at a fixed Vds of 0.5 V, Ids increased from the 1.5 × 10-8 to 1.2 × 10-5 A as Vg changes from Nano Lett., Vol. 9, No. 5, 2009

-20 to 20 V). This behavior, which should arise from a barrier with the electrodes,42 can also be observed in case of CNT FETs43 and organic FETs.44 More importantly, as shown in Figure 4d and e, Ids decreases with decreasing Vg, thus the conductive behavior changes to n-type after N doping. The carrier mobilities (µ) can be deduced by (1), µ ) (L/WCgVds)(∆Ids /∆Vg)

(1)

where Cg is the gate capacitance per unit area (ca. 7 nF•cm-2). The mobilities of the devices we made are in about 300-1200 cm2 V-1 s-1 and in about 200-450 cm2 V-1 s-1 for the pristine and the N-doped graphene, respectively. Our results are close to the CVD grown graphene (100-2000 cm2 V-1 s-1) reported by Reina et al.,23 the chemically exfoliated graphene nanoribbons (100-200 cm2 V-1 s-1) reported by Li et al.,6 and are higher than the reduced graphene oxide (2-200 cm2 V-1 s-1) reported by GomezNavarro et al.45 The high mobilities suggest that the products are of high quality and free of excessive covalent functionalization. However, compared with the mechanically exfoliated graphene, the mobilities are about 1-2 orders of magnitude lower than its best result (1.5 × 104 cm2 V-1 s-1).1 This is possibly attributed to scattering at the doping defects, the growth defects, and the grain boundaries formed in the CVD process,23 consistent with the HRTEM characterization. 1755

Figure 4. Electrical properties of the N-doped graphene. (a) SEM image of an example of the N-doped graphene device. (b) Bird’s-eye view of a schematic device configuration. (c) and (d) Ids/Vds characteristics at various Vg for the pristine graphene and the N-doped graphene FET device, respectively. The insets are the presumed band structures. (e) Transfer characteristics of the pristine graphene (Vds at -0.5 V) and the N-doped graphene (Vds at 0.5 and 1.0 V).

The presumed band structures of the pristine and the N-doped graphene are shown in the insets of Figure 4c and d. As we know, graphene is a zero-gap semiconductor. The band structure of graphene exhibits two bands (the valence band and the conduction band) intersecting at two inequivalent points, K and K′, in the reciprocal space,2 thus it exhibits a good conductivity and a distinct electric field effect with charge concentrations as high as 1013 cm-3 and mobilities as high as 1.5 × 104 cm2 V-1 s-1.1,6,46 Due to its zero band gap, the pristine graphene exhibits a low on/off ratio at room temperature,6,40 and due to the absorption of oxygen or water in air, the pristine graphene usually shows a p-type behavior,1,41 similar like the pristine CNTs in air.47 In case of the N-doped graphene, foreign atoms and topological defects (as indicated by HRTEM images and Raman spectra), which 1756

act as scattering centers, are introduced into the graphene lattice,24 causing the decrease of the conductivity.11,45 More importantly, as clarified by the previous theoretic work,10,11 the doping atoms enter into the lattice of graphene, form the covalent bonding with C atoms and change the lattice structure of graphene. This would largely modify the electrical structure of graphene and suppress the density of states of graphene near the Fermi energy (Fm) level, thus a gap is opened between valence and conduction bands.10,11 In fact, there are many ways to open a band gap in the graphene, such as the charge transport with adsorbed atoms or molecules,13,48 the interaction with the substrate,49 the perpendicular electric field,50 etc. In our case, the gap originates from the substitutional doping, which is similar to the substitutionally doped graphene 8,51 and consistent with Nano Lett., Vol. 9, No. 5, 2009

the theoretic researches.10,11 Moreover, the substituted N atoms can introduce strong electron donor states near Fm.52 Therefore, silimar with the N-doped CNTs,8 the N-doped graphene exhibits an n-type semiconductor behavior, which leads to a decreased conductivity, an improved on/off ratio and a Schottky barrier with the electrodes. In this letter, we provide a CVD technique to produce the N-doped graphene, the first experimental example of the substitutionally doped graphene, which is hard to be produced by other methods. The CVD method is a nondestructive route to produce the graphene and realizes the substitutional doping, as the doping accompanies with the recombination of the carbon atoms into the graphene in the CVD process. By using SEM, TEM, Raman, XPS and EDS, we demonstrate the existence of the N-doped graphene. The CVD method can not only produce the N-doped graphene, but also has the potential to produce the graphene doped with other elements. Moreover, we measure the electrical properties of the N-doped graphene. It behaves like an n-type semiconductor, indicating the doping can modulate the electrical properties of graphene. This research provides a new type of graphene experimentally, which is required for the applications of graphene. We believe it would promote the research and the applications of graphene. Acknowledgment. This work is supported by the National Natural Science Foundation of China (60736004, 60671047, 50673093, 60736004, 20825208), the Major State Basic Research Development Program (2006CB806200, 2006CB932100), the National High-Tech Research Development Program (2008AA03Z101), and the Chinese Academy of Sciences. Supporting Information Available: Complete ref 9, ref 17, ref 19b, ref 22c, ref 27a, ref 33, the experiment section, and Figures S1-S8. This material is available free of charge via the Internet at http://pubs.acs.org.

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