Research Article www.acsami.org
Nanopore Creation in Graphene by Ion Beam Irradiation: Geometry, Quality, and Efficiency Zhitong Bai,† Lin Zhang,† Hengyang Li,‡ and Ling Liu*,† †
Department of Mechanical and Aerospace Engineering, Utah State University, Logan, Utah 84322, United States Department of Engineering Mechanics, Dalian University of Technology, Dalian 116023, China
‡
S Supporting Information *
ABSTRACT: Ion beam irradiation is a promising approach to fabricate nanoporous graphene for various applications, including DNA sequencing, water desalination, and phase separation. Further advancement of this approach and rational design of experiments all require improved mechanistic understanding of the physical drilling process. Here, we demonstrate that, by using oblique ion beam irradiation, the nanopore family is significantly expanded to include more types of nanopores of tunable geometries. With the hopping, sweeping, and shoving mechanisms, ions sputter carbon atoms even outside the ion impact zone, leading to extended damage particularly at smaller incident angles. Moreover, with lower energies, ions may be absorbed to form complex ion-carbon structures, making the graphene warped or curly at pore edges. Considering both efficiency and quality, the optimal ion energy is identified to be 1000 eV at an incident angle of 30° with respect to the graphene sheet and 400−500 eV at higher incident angles. All of these results suggest the use of oblique ion beam and moderate energy levels to efficiently fabricate high-quality nanopores of tunable geometries in graphene for a wide range of applications. KEYWORDS: graphene, ion irradiation, nanopore, defect, molecular dynamics
1. INTRODUCTION Nanoporous graphene, a monatomic sheet with nanometersized pores, has been shown to have unparalleled potentials for many applications, including DNA sequencing, water desalination, and phase separation. Due to the thickness of about 0.34 nm, which is comparable to the distance between neighboring DNA nucleotides, nanoporous graphene is able to detect double-stranded DNA at the single-base resolution, which is unreachable by many other methods.1−3 With the unique combination of subnanometer thickness and ultrafine nanopores with tunable sizes, nanoporous graphene can effectively reject salts while maintaining water permeabilities orders of magnitude higher than what may be achieved by common reverse osmosis technologies.4−7 All of these applications require efficient fabrication of nanoporous graphene in a highly controllable manner. Irradiation of graphene by argon gas clusters8 and gold clusters9 has been theoretically proven to be promising, but such experiments are challenging to perform, mainly due to the difficulty of producing gas/metal clusters. Another viable approach is nanosphere lithography, which has been used to produce graphene nanomesh;10 however, the application has been limited because of the performance-inhibiting contaminants introduced during the synthesis. Alternately, ion beam irradiation has emerged with many promises for engineering nanopores in graphene. Fischbein et al. used the focused electron beam from a transmission electron microscope to © 2016 American Chemical Society
produce desired defective features including pores, slits, and gaps in suspended graphene.11 Similar approaches have been developed to drill into graphene using different ion sources, under varied conditions (i.e., suspended or on substrates),1,12−14 and/or for different applications.15−17 Due to the importance of ion beam irradiation in creating nanoporous graphene, atomistic modeling and simulation have been widely employed to provide mechanistic understanding of the physical processes.18,19 Using classical molecular dynamics (MD), Li et al. demonstrated the formation of round nanopores with relatively smooth edges, where heavy ions such as C, Si, and Au were made to collide with the suspended graphene at a range of energy levels.20 Using similar techniques, Wu et al. simulated the irradiation of graphene on a SiO2 substrate and discovered that the substrate deteriorated the quality of the created nanopores due to the secondary sputtering of graphene by high-energy particles, including electrons and argon ions.21 In a later MD study, Zhao et al. investigated the swift heavy ion irradiation of graphene on a SiO2 substrate and concluded that defect creation directly related to the energy deposition in graphene as well as the pressure wave generated from the hot center of the ion track.22 Received: May 24, 2016 Accepted: August 30, 2016 Published: August 30, 2016 24803
DOI: 10.1021/acsami.6b06220 ACS Appl. Mater. Interfaces 2016, 8, 24803−24809
Research Article
ACS Applied Materials & Interfaces
2.3. Simulation Details. Initially, the graphene was relaxed at 300 K using the NVT ensemble for 8 ps. The graphene was then irradiated by Si ions with different ion doses (i.e., the number of all incident ions), energies, and incident angles using the NVE ensemble. Regions adjacent to the fixed boundaries were made to have a constant temperature of 300 K using the Berendsen thermostat to allow heat dissipation and to decrease the magnitude of the irradiation-induced pressure waves. Following ion beam irradiation, a 20 ps relaxation was performed to re-equilibrate the graphene to obtain final patterns of the nanoscale defective structures. At each combination of the incident angle and ion energy, nine simulations were performed with different seeds for sampling ion impact sites and the initial velocities of carbon atoms. Most of the results presented in this paper are averaged from these runs. 2.4. Characterization of Pore Geometry. Each nanopore is characterized by two parameters, average radius (r0) and standard derivation (S), both calculated using the coordinates of all atoms forming the edge of the nanopore. The average radius r0 is defined by
Despite significant progress, most of these simulation studies have been focused on the influences of ion dose and ion energy, assuming the ion beam is normal to the plane of graphene. Nevertheless, a recent experiment performed by Wang et al. demonstrated that, at an incident angle of 52°, ion beam irradiation produced nanopores with elliptical shapes.23 This work opened the door to control the shape of the irradiationinduced nanopores, for which mechanistic understanding is still lacking. Here, using molecular dynamics simulations, we demonstrate that, in ion beam irradiation, the geometry of the produced nanopores as well as the quality and efficiency are all controllable by varying the ion energy and incident angle of the ion beam. Analysis of the atomic trajectories further reveals how graphene atoms are sputtered in different areas around the ion impact zone. The results are expected to stimulate and guide experiments and improve control over the creation of nanopores in graphene.
N
r0 =
2. MODELS AND METHODS
∑i = 1 (xi − x0)2 + (yi − y0 )2 + (zi − z 0)2 N
(1)
where (xi, yi, zi) are coordinates of the ith atom residing on the pore edge and (x0, y0, z0) are coordinates of the center which are averaged from the coordinates of all atoms on the edge (N in total). r0 is a quantitative measure of the nanopore size. The standard derivation S is defined by
2.1. Models. The computational model employed in this study is shown in Figure 1a. The graphene was built to have the size of 17.4 ×
N
S=
∑i = 1 (ri − r0)2 N
(2)
where ri stands for the distance from the ith atom to the center. A larger S value indicates a larger deviation from a perfectly circular shape. Both quantities were evaluated for all nanopores; this is discussed in Section 3.2. To account for uncertainties in the simulations, most of the results presented in this paper are averaged from nine simulations. 2.5. Critical Dose. The critical dose is the minimum dose required to induce a stable nanopore in the graphene. To evaluate the critical dose, the number of sputtered atoms was plotted with respect to the ion dose based on MD simulation results. Then, the data points were fitted by the equation:
Figure 1. Ion beam irradiation of graphene: (a) computational model and (b) typical experimental setup.
21.6 nm. All edges were fixed to mimic the clamped boundary conditions.24 Initially positioned 4 nm above the plane of graphene, all incident Si ions were made to bombard the graphene continuously at random sites within a circular region to mimic the mask-protected ion irradiation experiment, as shown in Figure 1b. The system involves five parameters, namely ion energy, ion incident angle, ion dose, dose rate, and the size of the exposure area. This study focused on the first three parameters: ion energy was varied from 100 to 1000 eV (9 levels), incident angle was varied from 90° to 30° with respect to the plane of graphene (4 levels), and the maximum ion dose was 700 for 50−90° and 1200 for 30°. Dose rate was fixed to be one Si ion every 75 fs, and the circular hole in the mask (ion impact zone) was made to have a diameter of 1.5 nm. 2.2. Molecular Dynamics. All MD simulations in this study were performed using LAMMPS.25 Interatomic interactions between carbon atoms in the graphene were determined by the adaptive intermolecular reactive bond-order (AIREBO) potential26 with a cutoff distance of 0.3 nm. On the basis of the second-generation Brenner potential (REBO), the AIREBO potential takes into account the breakage and formation of covalent bonds and has been extensively applied in simulating the formation of nanoscale defective structures in graphene.27 The interactions between the carbon atoms and Si ions were modeled by the Tersoff-ZBL potential,28,29 which combines the three-body Tersoff potential and the Ziegler−Biersack−Littmark (ZBL) universal repulsive potential30 to correctly describe the ion-carbon interaction including bond formation and breakage.20,21 This approach has been successfully applied to simulate several systems of Si−C nanostructures.31,32
y = A1e−x / t1 + A 2 e−x / t2 + y0
(3)
where y is the number of sputtered atoms, x is the dose of ions, and A1, A2, t1, t2, and y0 are fitting parameters. Using the fitted equation, we calculated the critical dose using the equation:
y(xcr + 200) − y(xcr) = 5
(4)
where xcr is the critical dose. Physically, beyond the critical dose, no more than 5 carbon atoms may be sputtered with an additional dose of 200 ions. If this is satisfied, the defective structure formed in the graphene is considered stable. 2.6. Sputtering Mechanism. At the end of each ion irradiation process, a carbon atom may be sputtered by an incident Si ion or a neighboring carbon atom or stay unsputtered. Ti,j,k is used to denote the state of the ith carbon atom at the end of the jth simulation, where k stands for the type of the carbon state (1: unsputtered; 2: sputtered by a Si ion; 3: sputtered by a carbon atom). Ti,j,k may take two values (1: the carbon atom is in the state; 0: the carbon atom is not in the state). Because the three types of states are mutually exclusive for any carbon atom in a simulation,
Σ3k = 1Ti , j , k = 1
(5)
Probabilities were evaluated using the atomic trajectories from N simulations with the same incident angle and ion energy. N = 400 in this study. The probability for the ith carbon atom to take state k is 24804
DOI: 10.1021/acsami.6b06220 ACS Appl. Mater. Interfaces 2016, 8, 24803−24809
Research Article
ACS Applied Materials & Interfaces Pi , k =
ΣNj = 1Ti , j , k N
displacements, mainly due to the dangling bonds. Moreover, damage in the center of the ion impact zone is followed by the sputtering of edge atoms, which is consistent with experimental results.33 3.2. Influence of Incident Angle and Ion Energy. Figure 3 shows various defective nanostructures formed by ion
(6)
3. RESULTS AND DISCUSSION 3.1. Formation of a Nanopore in Graphene by Ion Irradiation. As illustrated in Figure 1a, the computational system mimics the mask-protected ion irradiation experiments on suspended graphene (Figure 1b). Continuous irradiation of the graphene with energetic Si ions may sputter carbon atoms and absorb Si ions onto the graphene, leading to sometimes complex defective structures. Because the generation of nanopores is of particular interest, Figure 2 demonstrates a
Figure 3. Examples of the final nanopore structures formed at different ion energies and incident angles (purple sticks: C−C bonds; blue spheres: Si ions; black dashed circle: ion impact zone; i.e., the area directly exposed to ions).
irradiation at different ion energies and incident angles. Incident angle is shown to strongly influence the geometry of the formed nanopore. At 90°, ion irradiation generates circular nanopores slightly larger than the ion impact zone (the dashed circle). This is consistent with an experimental study done by Nagoshi et al. where the produced nanopore was also found to be larger than the ion beam.33 As the incident angle decreases, the generated nanopores become less circular but increasingly elliptical, and the total area of the damaged zone keeps increasing. In other words, smaller incident angles cause more significant defects in elliptical patterns. These results agree well with previous experimental findings. For example, Morin et al. showed that tilting the graphene to 60° with respect to the incident beam allows a significant increase in sputtering yield;34 Gadgil et al. found that sputtering yield roughly increases with 1/sinΘ, where Θ is the angle between the surface and ion beam.35 Another observation is that ion energy mainly controls the number of ions absorbed at the pore edges; at lower energy levels, more ions are absorbed. Apparently, at lower ion energies, the Si ions are less capable of overcoming the attraction by carbon atoms, especially after losing a part of the kinetic energy during the bombardment process, which increases the probability of ion absorption. Ion absorption is also influenced by the incident angle. Comparing the snapshot series at the four angles, we find that the energy threshold for ion absorption increases with decreasing incident angle (∼150 eV at 90°, ∼200 eV at 70°, ∼300 eV at 50°, and ∼500 eV at 30°), demonstrating an interplay between the ion energy and incident angle in the ion beam irradiation of graphene. Note that the energy thresholds shown here are higher bounds of the energy intervals favoring ion absorption, beyond which ions cannot be absorbed. Lower bounds also exist (
