Annealing Effects after Nitrogen Ion Casting on Monolayer and

Article pubs.acs.org/JPCC

Annealing Effects after Nitrogen Ion Casting on Monolayer and Multilayer Graphene Ki-jeong Kim,† Sena Yang,‡ Youngchan Park,‡ Myungjin Lee,‡ BongSoo Kim,*,† and Hangil Lee*,‡ †

Beamline Research Division, Pohang Accelerator Laboratory (PAL), Pohang 790-784, Republic of Korea Department of Chemistry, Sookmyung Women’s University, Seoul 140-742, Republic of Korea

‡

ABSTRACT: The modification of the electronic properties of a coverage-dependent graphene layer by nitrogen ions irradiation was investigated using core-level photoemission spectroscopy (CLPES). Here we describe preparation of monolayer and multilayer epitaxial graphene (EG) functionalized by nitrogen ions irradiation with 100 eV to minimize the damage of graphene layer as we annealed up to 1300 K to track the surface property changes using CLPES. As a result, on the monolayer EG, we found that pyridinic nitrogen mainly existed on the surface. On the multilayer EG, we observed the formation of graphitic nitrogen remaining as a major species confirmed using N 1s core-level spectra. Through a work function change (ΔΦ) measurement, both systems indicated p-type doping properties with 4.71 (monolayer EG) and 4.87 eV (multilayer EG) of work function values after N2 ion irradiation. Interestingly, we observed that monolayer EG maintained its p-type doping character, whereas multilayer EG changed the doping character from p-type to n-type as we annealed both systems up to 1300 K due to the discrepancy of the electron charge transfer. We will systematically demonstrate these variations of electronic properties for both monolayer and multilayer EG.

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INTRODUCTION In 2010, A. K. Geim and K. S. Novoselov were awarded the Nobel Prize in Physics for their landmark work on graphene, which demonstrated the outstanding electronic and mechanical properties of this material.1 Its potential applications are expected to yield new electrical devices, and industries have supported significant research in graphene-related studies seriously.2−8 By controlling the band gap effectively, which can be achieved through a variety of processes, its versatility can be improved.9,10 The zero band gap in graphene is a consequence of the identical environments of the two carbon atoms in the graphene unit cell. Unfortunately, it is not easy to manipulate the atoms in a unit cell or manipulate them using the lithographic modification techniques because the distance between the carbon atoms is only 1.4 Å apart.3 Instead, chemical-modified reactions that form superstructures using chemical functionalized molecule and atom may be used to introduce a potential difference into the band gap.11−13 Recently, many research groups have focused on chemical doping using nitrogen for various applications such as biosensors, fuel cells, and so on.14−16 Recently, L. Zhao et al. grew N-doped graphene films using chemical vapor deposition on copper foil substrates, indicating that the individual nitrogen atoms were incorporated as graphitic dopant, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant.17−22 In this work, we introduced low-energy nitrogen ions (100 eV) as a method for changing the monolayer and multilayer © 2013 American Chemical Society

graphene surfaces properties to minimize the surface damage. In addition, we identified three nitrogen species that induced such changes on few-layer epitaxial graphene (EG) on SiC(0001): N2, pyridinic nitrogen, and graphitic nitrogen, described previously.23 Through the consecutive sample annealing up to 1300 K, we could pursue that the surface property changes and these properties were monitored by corelevel photoemission spectroscopy (CLPES).

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EXPERIMENTAL SECTION

Monolayer and multilayer EG were grown on 6H-SiC(0001) substrates by thermal annealing under ultrahigh vacuum (UHV) conditions. The nitrogen-doped (ND = 9 × 1017 cm−3) Si-terminated 6H-SiC(0001) sample was purchased from Cree Research (USA). Samples were prepared 2 mm × 12 mm in size with a thickness of 0.25 mm. The 6H-SiC(0001) annealing temperature was monitored by infrared pyrometer, assuming an emissivity of 0.90. After overnight sample outgassing at 1200 K, samples were annealed at 1200 K under a Si flux (1 Å/min) to make a Si-rich surface on 6HSiC(0001) for 5 min. After annealing at 1450 K, a carbon-rich (6√3 × 6√3)R30° electron diffraction pattern was observed. A well-reconstructed 1 × 1 surface with a 12 Å thick (about four layers) graphene layer was found after further annealing at 1500 K. The average thickness of the EG was consistent with the monolayer EG, as confirmed by the attenuation of the Si 2p Received: October 9, 2012 Revised: January 13, 2013 Published: January 17, 2013 2129

dx.doi.org/10.1021/jp309964m | J. Phys. Chem. C 2013, 117, 2129−2134

The Journal of Physical Chemistry C

Article

Figure 1. C 1s CLPES spectra measured at photon energy of 500 eV for clean monolayer EG (a) and multilayer EG (e). (b,f) After the nitrogen ion irradiation with the energy of 100 eV on panels a and e. Panels c (and g) and d (and h) are after annealing of panel b (and f) at the temperature of 750 K and at the temperature of 1300 K for 2 min, respectively.

Figure 2. N 1s CLPES spectra measured at photon energy of 500 eV. (a) Monolayer EG and (e) multilayer EG after irradiation with 100 eV nitrogen ions. N 1s CLPES spectra acquired after annealing at 750 K (b) (and (f)), 1000 K (c) (and (g)), and 1300 K (d) (and (h)) for 2 min, respectively.

emission spectra (−20 V sample bias) and valence band spectra were measured at photon energies of 130 eV. All spectra were recorded in the normal emission mode. The photoemission spectra were carefully analyzed using a standard nonlinear leastsquares fitting procedure with Voigt functions.26,27

signal and low-energy electron diffraction (LEED) measurements.11,24,25 The graphene/SiC surface was irradiated with 100 eV energetic nitrogen ions for 10 min at 300 K in a UHV chamber. Nitrogen ions (N2+) were generated by an ion gun (PHP 3000), and the sample current was measured to be 0.4 μA/cm2 during ion beam irradiation. The estimated number of nitrogen ions was 2.5 × 1012 ions/cm2·sec. The effects of the nitrogen ion irradiation on the graphene/SiC surface properties were investigated using core-level photoelectron spectroscopy (CLPES). The C 1s and N 1s core-level spectra were measured taken before and after the nitrogen ion irradiation at photon energy of 500 eV with a total resolution of 200 meV at the 8A2 HRPES Beamline at the Pohang Accelerator Laboratory (PAL). The binding energy (BE) and the spectral resolutions of the N 1s core-level were calibrated by measuring the Au 4f7/2 corelevel. The surface properties immediately after nitrogen ion irradiation have been previously described. Secondary electron

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RESULTS AND DISCUSSION We focused on describing the changes of C 1s and N 1s corelevel spectra according to the annealing temperature because Si 2p spectra were not shown the remarkable spectral changes depending on nitrogen ion irradiation and annealing effect. Figure 1 shows the C 1s core-level spectra obtained before and after nitrogen ion irradiation of the monolayer or multilayer EG as a function of annealing temperature. As shown in Figure 1a (monolayer EG) and Figure 1e (multilayer EG), four distinct peaks were observed at 285.5, 285.1, 284.7, and 283.6 eV, which were well-assigned, respectively, to sp2 carbon atoms at 2130



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399.0 (N2′), 400.6 (N3′), and 402.6 eV (N4′). As shown in Figure 2e, we can assign N1′, N2′, N3′, and N4′ as a pyridinic structure, intermediate states, a graphitic structure, and molecular nitrogen, respectively. Considering BE, we also confirmed that the N4′ peak corresponds to molecular nitrogen (N2) based on the previous study.23 However, it is difficult to observe this molecular nitrogen (N4′) species on the monolayer EG after irradiation of N2 ion because it is less likely to intercalate into the monolayer EG. Therefore, we could observe it only on the multilayer EG. Besides, the intensity of molecular nitrogen (N4′) is being decreased as the annealing temperature is increased on the multilayer EG, and it indicates that the intercalated N2 escaped from the EG upon annealing at 1300 K. Interestingly, as the annealing temperature is increased to 1300 K, we found that the intensity of N3′ feature increased as the annealing temperature increased, whereas that of N1′ feature remained, indicating that a graphitic structure (N3′) is a major species. These results are completely opposite to the results from monolayer EG. As a result, we can demonstrate that it could be occurred a large charge transfer between new N2 ion irradiated on surface and multilayer graphene.10 Through CLPES results, we could verify that the pyridinic nitrogen species were found to be the major species on the monolayer EG and the graphitic nitrogen species were the major component on the multilayer graphene. Figure 3 displays the plots for the relative intensity variation of each nitrogen features varying with the annealing temperature shown in Figure 2 to show clearly the intensity variation among nitrogen bonding features. As shown Figure 3a, on monolayer EG, we observed the clear change among three nitrogen bonding features. Increasing the annealing temper-

the interface layer weakly interacting with the carbon atoms of the underlying substrate (marked S2), the (6√3 × 6√3)R30° interface layer (buffer layer marked S1), the EG layer (marked G), and the SiC (marked SiC).28,29 After irradiation with 100 eV nitrogen ions for 10 min, a new peak was observed with a BE of 286.8 eV (marked S3), as shown in Figure 1b,f. The chemical shift of S3 indicated that this species incorporated the N atoms.30,31 We confirmed that this feature remained after annealing at 1300 K. On the monolayer EG, G was decreased and S1 and SiC components were increased after nitrogen ion irradiation because the top graphene layer was modified by the nitrogen ion. Whereas the intensity of S2 was maintained with a similar intensity, a new S3, a nitrogen-ion-induced species, appeared. After nitrogen ion irradiation, the spectra were a little bit broadened, while annealing lead again to a sharp peak. Until the annealing up to 750 K, the similar structure was maintained. After 1300 K annealing, the relative intensity of S1 and SiC was remarkably decreased. Also, the intensity of S3 was decreased a bit, but it still existed after 1300 K annealing, and the sharp peak of G was increased. In the case of multilayer EG, the intensity of G′ (same origin as G on monolayer EG) was decreased after the nitrogen ion irradiation. Similarly, S1′ (same origin as S1 on monolayer EG), S2′ (same origin as S2′ on monolayer EG), and SiC were increased, and S3′ (same origin as S3 on monolayer EG) appeared after nitrogen irradiation. Unlike the monolayer EG, there is no big difference in the multilayer EG upon increasing the annealing temperature to 1300 K. Hence, we could explain more direct information about nitrogen-incorporated species as we precisely analyzed the results of N 1s core-level spectra. Figure 2 shows the N 1s core-level spectra taken after N2 irradiation on monolayer and multilayer EG with 100 eV accelerated nitrogen ions (N2+). First, nitrogen-ion-irradiated monolayer EG taken at 300 K shows three distinct N 1s peaks located at 398.4 (N1), 399.0 (N2), and 400.6 eV (N3), as shown in Figure 2a. From these binding energies, it can be clearly described that the characters of N 1s bonding features indicate the formations of −CN−C−, CxN, and −C−N−C− bonds, respectively. As the same as the previous result,23,32 these peaks can be assigned to the nitrogen atoms of a pyridinic nitrogen structure (N1), intermediate states (N2), and a graphitic nitrogen structure (N3). Among these bonding features, we focused on the intensity variation between a pyridinic nitrogen peak (N1) and a graphitic nitrogen peak (N3), which we want to confirm depending on the condition of graphene layered structure and the annealing temperature. We increased the annealing temperature up to 1300 K to track the variation of electronic structures. The samples were annealed stepwise at 750, 1000, and 1300 K for 2 min, respectively. After each treatment, the CLPES spectra were collected to track the changes in the nitrogen-ion-induced species on EG. As we expected, we could confirm the dramatic intensity change between N2 ion deposited on monolayer EG and that on multilayer EG. On the monolayer EG, we found that the intensity of the N1 feature relatively increased with the increased annealing temperature, whereas that of the N3 feature remained without significant change, as shown in Figure 2a−d. This indicates that N1 bonding feature is a major species on monolayer EG after annealing at 1300 K for 2 min. As mentioned above, we could confirm that pyridinic nitrogen structure is the major species on the monolayer EG. On the contrary, nitrogen-ion-irradiated multilayer EG showed four distinct N 1s peaks occurred at 398.4 (N1′),

Figure 3. Relative intensity changes of each nitrogen components upon the annealing temperature on the monolayer graphene (a) and the multilayer graphene (b). Each component represents a pyridinic nitrogen structure (N1 and N1′), intermediate states (N2 and N2′), a graphitic nitrogen structure (N3 and N3′), and molecular nitrogen (N4′). 2131



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Scheme 1. Ball-and-Stick Schematic Model of Chemical Structures for Nitrogen-Doped Graphene (Monolayer EG and Multilayer EG) As a Function of Annealing Temperaturea

a

Top panel: N2-ion-irradiated monolayer EG taken after annealing of (a) 300 and (b) 1300 K for 2 min, respectively. Bottom panel; N2 ion irradiated multilayer EG taken after annealing of (c) 300 K and (d) 1300 K for 2 min, respectively. The blue-, red-, green-, and purple-colored balls indicate pyridinic nitrogen, graphitic nitrogen, intermediate nitrogen, and molecular nitrogen, respectively.

Figure 4. Variations in the secondary edge upon treatments of the (a) monolayer EG and (b) multilayer EG under a sample bias of −20 V. Black spectrum: as-grown graphene, red spectrum: nitrogen ions on graphene, green spectrum: annealing at 750 K for 2 min, blue spectrum: annealing at 1000 K for 2 min, and purple spectrum: annealing at 1300 K for 2 min. (c) Work function changes of the monolayer EG and (d) the multilayer EG as a function of annealing temperature.

ature up to 1300 K, pyridinic nitrogen feature (N1) is being increased, whereas intermediate bonding feature (N2) is being decreased. Interestingly, graphitic nitrogen bonding features does not depend on the annealing temperature. From this result, on monolayer EG, we could exactly demonstrate that the intermediate nitrogen bonding feature desorbs as the annealing temperature is increased; then, the stable pyridinic and graphitic nitrogen bonding features exist after 1300 K annealing temperature.

Also, on multilayer EG, we found that graphitic nitrogen feature (N3′) is being increased, whereas intermediate bonding feature (N2) and molecular nitrogen feature are being decreased, as shown in Figure 3b. We observed that the pyridinic nitrogen bonding feature does not depend on the annealing temperature, and intermediate nitrogen and molecular nitrogen bonding features also desorb as the annealing temperature is increased; then, the stable pyridinic and graphitic nitrogen bonding features also exist after 1300 K annealing temperature. 2132



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irradiated by the N2 ion energy of 100 eV, but only the multilayer EG showed the n-type doping character after annealing up to 1300 K. This discrepancy is attributed to the different doping geometries in monolayer and multilayer EG.

Scheme 1 displays the schematic model of nitrogen-doped graphene (monolayer EG and multilayer EG) including two stable nitrogen bonding features (pyridinic and graphitic nitrogen bonding features), as obtained in Figure 2. The work-function measurements shown in Figure 4 provide the evidence of the formation of charge-transfer complexes upon nitrogen ion irradiation. The measured kinetic energies were converted after correcting for the applied bias and the analyzer work function so that the sample work function was obtained from the intersection between the baseline of the spectrum and the linear fit to the tail of the sample secondary electron cutoff. The work function as a function of annealing temperature was measured after nitrogen ion irradiation by monitoring of the low kinetic energy cutoff region, as shown in Figure 4. In general, the global doping effects on the monolayer EG can be confirmed by measuring the difference between the secondary electron edges, which is the same as the work function difference.30 The lower spectra shown in Figure 4a,b correspond to the reference spectrum of pristine monolayer EG and multilayer EG with a work function of 4.30 and 4.32 eV.32 After irradiation with 100 eV nitrogen ions, we clearly observed that each secondary electron edge shifted by 410 (pristine monolayer EG, see Figure 4a) and 550 meV (pristine multilayer EG, see Figure 4b) toward higher kinetic energies with respect to the secondary electron edge energies. It is noted that the work functions abruptly increased after nitrogen ion irradiation to 4.71 (monolayer EG) and 4.87 eV (multilayer EG), indicating p-type doping. To track the variation of electronic properties for both systems, we increased the annealing temperature of the two different graphene systems up to 1300 K. As shown in the Figures, we could observe the remarkable discrepancy between the monolayer and multilayer EG systems. Overall, the secondary energy cutoffs shifted to lower kinetic energies, indicating that the work function decreased on both monolayer and multilayer EG (see Figure 4a,b). However, on the monolayer EG, the p-type doping character still remained after annealing at 1300 K, whereas the doping character changed from p-type to n-type doping after annealing at 900 K on the multilayer EG. Figure 4c,d displays the work function changes of the monolayer (up to 4.34 eV) and multilayer EG (up to 3.93 eV) after annealing at higher temperature, 1300 K. As stated above, we confirmed that pyridinic nitrogen is dominant on the monolayer EG, whereas the graphitic nitrogen is major species on the multilayer EG through CLPES spectra, as shown in Figure 2. In addition, it is difficult to transfer the charge between nitrogen in the pyridinic structure and carbon in graphene. On the contrary, it is easy to transfer the charge between nitrogen in the graphitic structure and carbon in graphene because nitrogen and carbon are organized in the same layer. Furthermore, it is likely to precede the electron charge-transfer because graphitic nitrogen has more negativecharge character.33 Therefore, we can explain the larger change of work function on the multilayer EG than that on the monolayer EG, which is well-matched with N 1s core-level spectra.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-54-279-1535; Fax: +82-54-279-1599; E-mail: [email protected] (B.K.). Tel: +82-2-710-9409; Fax: +822-2077-7321; E-mail: [email protected] (H.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0087138, 2010-0008608, and 2012000875). The experiments at the PLS were partially supported by MEST, POSTECH, and FEL.

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REFERENCES

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CONCLUSIONS Conclusively, we confirmed that pyridinic nitrogen is dominant on the monolayer EG and graphitic nitrogen is the major species on the multilayer EG as we annealed the nitrogen ionirradiated monolayer and multilayer EG to 1300 K. Monolayer EG and multilayer EG show the p-type doping characters as 2133



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Annealing Effects after Nitrogen Ion Casting on Monolayer and

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