Letter pubs.acs.org/NanoLett
Doping Monolayer Graphene with Single Atom Substitutions Hongtao Wang,‡,†,∥ Qingxiao Wang,†,∥ Yingchun Cheng,§ Kun Li,† Yingbang Yao,† Qiang Zhang,† Cezhou Dong,‡ Peng Wang,‡ Udo Schwingenschlögl,§ Wei Yang,‡ and X. X. Zhang*,† †
Advanced Nanofabrication, Imaging and Characterization Core Lab, King Abdullah University of Science and Technology (KAUST), Thuwal 239955, Kingdom of Saudi Arabia ‡ Institute of Applied Mechanics, Zhejiang University, Hangzhou 310027, China § Materials Science and Engineering, King Abdullah University of Science and Technology, Thuwal 239955, Kingdom of Saudi Arabia S Supporting Information *
ABSTRACT: Functionalized graphene has been extensively studied with the aim of tailoring properties for gas sensors, superconductors, supercapacitors, nanoelectronics, and spintronics. A bottleneck is the capability to control the carrier type and density by doping. We demonstrate that a two-step process is an efficient way to dope graphene: create vacancies by highenergy atom/ion bombardment and fill these vacancies with desired dopants. Different elements (Pt, Co, and In) have been successfully doped in the single-atom form. The high binding energy of the metal-vacancy complex ensures its stability and is consistent with in situ observation by an aberration-corrected and monochromated transmission electron microscope. KEYWORDS: Graphene, vacancy, metal-vacancy complex, doping, transmission electron microscopy
F
The two-step process includes creating desired vacancies in graphene followed by dopant deposition. The first step is realized in a pulsed laser deposition (PLD) chamber using single-layer graphene as substrate (Figure 1a) and can be benefited from a more controllable atom/ion source. The high energetic particles are ejected from the target surface during laser ablation with a strong forward-directed velocity and bombard the free-standing graphene, which generates defects by knocking off carbon atoms, as schematically shown in Figure 1b. Recent atomistic simulations show that the defect structure has a strong correlation with the energy of the bombarding species.19,20 It is noted that monovacancies and bivacancies are most probably created for ions with energy in the order of 100 eV,19,20 which is similar to the kinetic energy of the species generated in the PLD process. Figure 1c is a typical highresolution transmission electron microscope (HRTEM) image of graphene bombarded with energetic gold particles in PLD. The created defects vary from single vacancies to holes of a few nanometers because the emitted particles have a distribution both in size and energy.21 It is noted that the sample is free of Au contamination after bombardment. A lighter element, boron, was also tested as a target material and similar results were obtained. Further refinement of the process may filter out large particles to selectively generate vacancies for dopant embedding. Nevertheless, simultaneously creating holes of
unctionalized graphene has been extensively studied both experimentally and theoretically with the aim of tailoring properties for sensors, superconductors, supercapacitors, nanoelectronics, and spintronics.1−6 A bottleneck is the capability to control the carrier type and density by doping, which has already played a critical role in tailoring electronic properties of semiconductors and laid the foundation for microelectronics.7 Much effort has been devoted to N or B doping either during graphene synthesis or by post annealing in environment with N- or B-containing species.8−13 A recent attempt was to dope graphene by ion implantation, analogous to the process of large-scale silicon technology.14 However, direct adoption of the traditional technology by ion implantation is of limited success. Also, the strong covalent C−C bonds render a substitutional doping by diffusion less efficient and less compatible with tight thermal budget.15 Here, we show that a two-step process is an efficient way to dope graphene with single atoms: create vacancies by high-energy atom/ion bombardment and fill these vacancies with desired dopants. Different elements (Pt, Co, and In) have been successfully doped in the single-atom form. The high binding energy of the metal-vacancy complex ensures its stability and is consistent with in situ observation by an aberration-corrected and monochromated transmission electron microscope. The nondependence on the type of dopants endows wider applications such as spintronics and optoelectronics3,16−18 and ensures extendibility from single atoms to complex molecules. The presented doping method can easily fit in the current device technology. © 2011 American Chemical Society
Received: September 11, 2011 Revised: November 6, 2011 Published: December 2, 2011 141
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Figure 1. (a) HRTEM image of a monolayer graphene taken at 60 kV with an exposure time of 2 s. The single-layer thickness is confirmed by the intentionally captured free edge. The inset shows the filtered image. Details about the microscope configuration, the imaging condition and the sample preparation method can be found in the Supporting Information. (b) Schematic illustration of the high-energy particle bombardment process. (c) HRTEM image of graphene after bombardment. The arrows indicate (1) a vacancy; (2) a piece of graphene flipped over on the edge of a big hole; and (3) a hole of 1 nm size.
different sizes paves a way for studying the rich physics of graphene, such as edge configuration and evolution.22−28 The vacancy structures were characterized using an aberration-corrected and monochromated transmission electron microscope. The microscope was operated at 60 kV in order to minimize electron beam damage. The monochromator was activated to reduce the electron energy spread to less than 0.2 eV. Figure 2a−c shows the HRTEM images of a monovacancy (V1), a bivacancy (V2), and a trivacancy (V3), respectively. Corresponding atomic models are presented in Figure 2d−f. The simulated HRTEM images (Figure 2g−i) based on these models agree with the experiments, endorsing their validity. The monovacancy is crucial to the magnetic properties of graphene.1,29 It is also an ideal trapping site for B and N atoms, which have similar atomic radii as carbon. Interestingly, noble and transition metal atoms are usually trapped by larger vacancies. The defect creation process is simulated by molecular dynamics (MD) with the setup given in Figure 2j. It has been found that the type of defect created depends on both the energy transferred from the projectile and the location upon collision, that is, the head-to-head, the headto-bond, or the head-to-hexagon positions (Figure 2k). A linear relationship is approximately maintained between the transferred energy and the kinetic energy (Ek) of the incoming atoms in the high energy range (Figure 2l). For the head-tobond collision, only bivacancies can be created for Ek > 150 eV (Figure 2m). For the head-to-head collision, monovacancies are created when 150 eV < Ek < 250 eV (Figure 2n). At higher energy (Ek > 250 eV), multivacancies are created with four carbon atoms being knocked off (Figure 2o). If hitting the center of a hexagonal ring, the Au atom will penetrate the graphene due to similar sizes. Multivacancies with five or six
Figure 2. HRTEM images of (a) a monovacancy, (b) a bivacancy, and (c) a trivacancy. Scale bar: 1 nm. (d−f) Atomic models and (g−i) simulated HRTEM images for the three different vacancy types in (A− C). (j) Atomic setup for MD simulations. (k) Locations of the headto-head (red triangle), head-to-bond (blue circle), and head-tohexagon (purple hexagon) collisions. (l) Transferred energy as a function of the kinetic energy of the Au projectile. The transferred energy is defined as the difference in kinetic energy of the Au atom before and after collision. Various vacancies are created from singleatom bombardments on (m) the bond, (n,o) the carbon atom and (p,q) the center of the hexagonal ring. The smaller atoms indicate the original locations of missing atoms. The letters in (l) indicate the threshold transferred energy above which the corresponding type of vacancy begins to be generated. Full MD simulation results are summarized in the Supporting Information.
atoms missing can be formed only at very high energy (Ek > 350 eV) (Figure 2p,q). This simulation shows that monovacancies or bivacancies can be selectively created if the kinetic energy of the incoming atoms is confined within a certain energy range. It is also found that the physisorption is restricted to the low energy end (Ek < 2 eV). For higher kinetic energies, Au atoms either bounce back or directly penetrate the graphene, which explains why the sample is free of Au contamination after bombardment. The threshold knock-on energy for ejecting an in-lattice carbon atom is 15−20 eV,22,30,31 which is well below the transferred energy (Figure 142
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The second step can be realized in various ways. In our experiment, Pt atoms were deposited by electron beam in a focused ion beam (Helios 400s). Cobalt and indium were deposited using a conventional sputtering tool. More details can be found in the Supporting Information. Figure 3a−f shows trapped Pt atoms in a bivacancy and a trivacancy with atomic configurations derived from the HRTEM images, while the pristine graphene is free of Pt adatoms. The migration barrier for metal adatoms has been calculated to be in the range of 0.2−0.8 eV, which is low enough to ensure a high mobility even at room temperature.3,35 In contrast, a density functional theory (DFT) study reveals higher binding energies of the Pt-vacancy complexes (Figure 2g), which makes the complex stable enough to survive the prolonged electron beam irradiation (Figure 3h−j). It is also interesting to observe that the edgetrapped Pt atoms migrate easily along the edge. These edgetrapped Pt atoms are a strong evidence of edge-doping, which is critical to graphene nanoribbon devices.4,25,38 Figure 4a−c shows a typical dopant trapping process in Codoped graphene, which uncovers the proposed two-step mechanism. We observe that a vacancy traps a Co atom after some time, which confirms the theoretical predictions.3 In agreement with previous studies,2,3,5 our DFT results show that the Pt-vacancy complexes are nonmagnetic for different vacancies Vn (n = 1−4), while all Co−Vn complexes are magnetic. Figure 4d,e shows the band structure and density of states obtained for a Co−V2 complex. The spin polarization in the Co−V2 complex is reflected by a splitting into spin-up and spin-down bands. The local density of states at the Fermi level is mainly attributed to the localized states around the Co atom, as shown in Figure 4f. The charge density distribution in between Co and C atoms is much less than that between neighbor C atoms (Figure 4g). The charge difference in Figure 4h indicates that the C charge far away from the Co atom is the same as in the bivacancy, that is, a slight p-doping. According to a Löwdin charge analysis, graphene can be either p-doped with
Figure 3. HRTEM images of a Pt atom trapped in (a) a bivacancy and (b) a trivacancy. (c,d) Atomic models and (e,f) simulated HRTEM images for the Pt-vacancy complexes in (a,b). (g) Binding energies for different configurations. For the monovacancy, the Pt atom resides out of the lattice plane in order to minimize the energy of lattice distortion due to atom size mismatch. (h−j) Video clips show the evolution of the Pt-vacancy complexes (indicated by arrows) under 60 keV electron irradiation. The electron beam current density is estimated to be 7 × 106 e/s·nm2, i.e., 100 A/cm2. The graphene sheet finally collapsed after an observation time span of ∼16 min. Scale bar: 1 nm.
2l) from the energetic particles (typically ∼100−200 eV in PLD).21 In comparison, the maximum transferred energy from 200 keV electrons is about 43 eV. Therefore, prolonged electron irradiation is required for creating vacancies or holes in TEM,22,26−28,31−35 which is usually accompanied by rapid defect reconstruction.33−37
Figure 4. In situ observation of the Co atom trapping process. (a) HRTEM image (decomposed from a video) of a vacancy (a white spot indicated by the arrow) before trapping a Co atom. (b) A Co atom was trapped to form a Co-vacancy complex. (c) The complex was stable under electron irradiation. Scale bar: 1 nm. (d) Spin up and (e) spin down band structures and density of states of the Co-bivacancy complex. (f) Local density of state isosurface (0.1), (g) charge density isosurface (0.1), (h) charge difference isosurface (±0.01 for red/blue color) and (i) spin density isosurface (0.001 e/Bohr3) of the Co-bivacancy complex. 143
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Pt and Co dopants or n-doped with In dopants. The total spin moment of a Co−V1 complex is 1.0 μB with only 0.44 μB being contributed by the Co atom.2 A different scenario is found for the Co−V2 complex: the total spin moment is 1.56 μB where the Co atom is contributing 1.50 μB (mainly from the 3d orbitals, according to a Löwdin charge analysis). The rest spin moment originates from the C 2p orbitals, as revealed by the localized spin density depicted in Figure 4i. The above observation suggests that the Co atom in the Co−V2 complex behaves as a free atom. The localized moments of the Co atom in graphene has important consequence in electrical conductivity by coupling to the conduction electrons through Kondo effect.1 The electronic structures of Pt−V2 and In−V2 can be found in the Supporting Information. On the basis of our initial results, we believe that one of the most efficient atomic doping techniques for graphene will rely on controllable vacancy generation by an atom/ion beam with narrow energy distribution and manipulable dosage, and dopant deposition by a less energetic atom/ion source. Besides, the physical bombardment provides a clean and efficient way to fabricate free edges in graphene and thus opens a door to study the interaction between the introduced atoms/molecules and graphene, which may help understand the functionalization mechanisms of edges.4,22−24,39−41
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ASSOCIATED CONTENT
S Supporting Information *
The methods of sample preparation, microscopy, and numeric simulation are included in the supporting material. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. Author Contributions ∥ The authors contributed equally to this work.
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ACKNOWLEDGMENTS H.T.W. and W.Y. acknowledge the financial support from the National Science Foundation of China (Grant 10832009; Grant 11090333) and Science Foundation of Chinese University (Grant 2011QNA4038).
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