Water Intercalation for Seamless, Electrically Insulating, and Thermally

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Water Intercalation for Seamless, Electrically Insulating and Thermally Transparent Interfaces Yanlei Wang, and Zhiping Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10173 • Publication Date (Web): 31 Dec 2015 Downloaded from http://pubs.acs.org on January 5, 2016

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ACS Applied Materials & Interfaces

Water Intercalation for Seamless, Electrically Insulating and Thermally Transparent Interfaces Yanlei Wang and Zhiping Xu* Applied Mechanics Laboratory, Department of Engineering Mechanics, and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China *

Email: [email protected]

ABSTRACT: The interface between functional nanostructures and host substrates is of pivotal importance in the design of their nanoelectronic applications as it conveys energy and information between the device and environment. We report here an interfaceengineering approach to establish a seamless, electrically insulating, while thermally transparent interface between graphene and metal substrates by introducing water intercalation. Molecular dynamics simulations and first-principles calculations are performed to demonstrate this concept of design, showing that the presence of interfacial water layer helps to unfold wrinkles formed in the graphene membrane, insulate the electronic coupling between graphene and the substrate, and elevate the interfacial thermal conductance. The findings here lay the ground for a new class of nanoelectronics setups through interface engineering, which could lead to significant improvement in the performance of nanodevices such as the field-effect transistors.

KEYWORDS: Nanoelectronics; Interface engineering; Water intercalation: Wrinkles; Electronic coupling; Thermal dissipation

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INTRODUCTION Recent development in the electron-device community has witnessed notable advances in designing and fabricating nanostructures-based devices, which hold great promises in transcending Moore’s law and pushing the limit of downsizing devices beyond the foreseen limits of conventional silicon integrated circuits. By following the bottom-up concept of device design, these tiny functional components can be engineered at the molecular level before being assembled into integrated systems, enabling a broad spectrum of well-tailored material properties and device functions.1-2 As an representative example of the versatile two-dimensional (2D) material family, graphene has been readily implemented into nanoelectronic devices such as the field-effect transistor (FET).3-4

Figure 1. (a) A schematic illustration of graphene-FET device with intercalated water layer (IWL) on the metal substrate.3 (b) The top- and side-view of the molecular structures of IWL, where an ordered hydrogen bond network is formed.

A typical graphene FET setup consists of a gate and a graphene channel connecting source and drain electrodes (Fig. 1a).3 Graphene can be prepared by mechanical exfoliation,5 chemical vapor deposition (CVD) growth on metals and dielectrics,6-7 and thermal decomposition of SiC.8 Among these procedures, CVD growth on metal substrates is relatively cheap and scalable, providing high-quality, wafer-scale samples to afford the high-yield essential for very large-scale integrated (VLSI) systems. However, a

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subsequent transfer process is necessary after the growth on catalytic metal surfaces to deposit graphene onto insulating substrates for device integration. This process may create defects in graphene and thus reduce the performance. Moreover, structural distortions such as the formation of wrinkles during the growth or transfer processes also lead to imperfections in the device setup, which not only result in incomplete contact with the substrate, i.e. formation of buckles or crumples,9 but also perturb the electronic structure and transport characteristics of graphene itself.10 A seamless interface between graphene and the substrate that is electrically insulating is essential for transistor applications. However, by weakening the interfacial electronic coupling by choosing insulators with low surface charge and defects, heat transport through the contact with substrate that is the major pathway for heat dissipation will also be reduced. Limited capability of power dissipation reduces or even breaks down the device performance and thus is a critical issue to be considered in device design.11-12 In this work, we propose a device setup by introducing water intercalation within the gallery between graphene and the metal substrate, yielding a seamless, electrically insulating interface, without sacrificing the capability of thermal dissipation through the contact with substrate (Fig. 1b). We prove this concept of device by performing molecular dynamics (MD) simulations and density functional theory (DFT) based first-principles calculations, and discuss potential issues in the practical implementation. RESULTS AND DISCUSSION Water intercalation helps graphene wrinkles to unfold. Graphene grown or deposited on solid surfaces is often observed to form a wrinkled structure, which modifies the electronic transport within the membrane.10 The structures of wrinkles are determined by the competition between interfacial interaction (adhesion, friction), graphene elasticity (in-plane stretching, out-of-plane bending), and the geometry (the contour length of graphene compared to its contact length with substrate).9 Mechanical stability of the wrinkles is maintained by the interfacial adhesion and friction, while perturbation exceeding the shear strength could unfold them. We expect that water intercalation between graphene and the substrate weakens the interfacial adhesion, and drive sliding motion of graphene relative to the substrate, which could help to destabilize the wrinkles

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(Fig. 2). To demonstrate this effect, we carry out full-atom molecular dynamics (MD) simulations for a graphene sheet supported on a 79.4 nm-thick copper substrate with a (111) surface at 300 K. To represent the surface roughness, we construct a substrate with grooved surface pattern. The depth of grooves is 0.65 nm and their lateral distance is 12.0 nm.13 A graphene membrane is then deposited onto this surface. 2D periodic boundary conditions (PBCs) are used for the metal substrate, with lateral dimensions of 523.3 (x) and 3.1 (y) nm. Periodic ripples with amplitude of 0.3 nm are initialized in our simulation model of graphene to represent the irregular nanostructures formed during the growth or deposition process, caused by intrinsic or thermal fluctuations. The contour length of deposited graphene is 490.0 nm, which is smaller than the span of substrate so that its wrinkled structures can fully relax without restriction from the PBC (Fig. 2a).

Figure 2. (a) MD simulation snapshots showing the formation and unfolding of wrinkles on Cu and IWL/Cu substrates. (b) Thermal fluctuation induced lateral force (per atom) on the graphene sheet deposited on substrate (top) and single carbon atoms in the sheet (b). These forces promote diffusion of the graphene sheet and local atomic vibrations, which could activate and assist the unfolding of wrinkles.

We perform equilibrium MD simulations by coupling the graphene/Cu(111) hybrid with a thermal bath at 300 K using the Berendsen thermostat. Our simulation results show that wrinkles remain after graphene makes contact with the substrate, and further develop on 4


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the Cu (111) surface, resulting in coalescence of the small ripples, and the formation of a single wrinkle. This final wrinkle is stabilized on the substrate by the surface interaction and the self-contact formed due to van der Waals interaction within the graphene sheet, although notable thermal fluctuation of the structures is observed in the simulations. To demonstrate the effect of intercalated water layers (IWLs), we perform additional MD simulations by preparing a thin layer of water molecules on the Cu (111) substrate before rippled graphene is deposited according to the experimental evidence of mono- or bilayer water formation at the interface.14-16 The simulation results show that, in contrast to the bare interface, wrinkles unfold and the graphene forms a perfect contact with the IWL-covered substrate (Fig. 2a). More interestingly, water molecules in the IWL arrange into a monolayer 2D quasi-ice lattice as has been recently identified for water under various nanoconfined environments (Fig. 1b).17 The ordered organization of 2D IWL maintains the structural stability of graphene and the interface, yielding a seamless and flat graphene-substrate contact. To obtain more insights into the stability and unfolding dynamics of wrinkled graphene on substrate, we calculate the fluctuation of lateral force induced to a deposited graphene sheet, which could drive collective motion of the graphene sheets and unfold the wrinkles. The results in Fig. 2b show that the amplitude of fluctuating force is elevated by about eight folds with the presence of IWL. The force on each single carbon atoms that promotes local atomic motion also increases after introducing the IWL (Fig. 2b). These facts suggest that the mobile water molecules in the IWL boost the sliding motion of graphene sheet, help to destabilize the metastable structures of graphene, and lead to a flat, seamless interface. Water intercalation weakens electronic coupling between graphene and metal substrates. Electronic coupling between graphene and metal substrates can be classified into weak and strong interactions according to the contrast in interfacial bonding strengths.18 DFT calculations using the local density approximation (LDA) for exchangecorrelation functional show significant hybridization between the π electrons in graphene and d electrons in nickel, with an interfacial energy of 403.62 meV, while the weak interaction between π and d electrons in copper leads to a much lower interfacial energy of 140.00 meV.19 For the latter case, Fermi level of the contact aligns with that in 5



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graphene and copper at different interlayer distances, reflecting the nature of interfacial electronic states.19 To explore the role of IWL in modulating this interfacial electronic coupling, we construct a thin slab model of Cu (111) surface with four atomic layers. The graphene-Cu (111) interface is set up by considering a unit cell shown in Fig. 3a. The inplane lattice constant aC-Cu is set to 2.55 Å according to the lattice constant of bulk copper aCu = 3.61 Å. The interlayer ordering between graphene and Cu (111) surface follows the most stable top-fcc configuration, where carbon atoms align with the 1st and 3rd atomic layers of copper vertically.19 The differential charge density plots from our DFT calculations show that as water intercalation is introduced, the electronic coupling is reduced and the interlayer distance between graphene and Cu increases from 2.27 to 6.11 Å (Fig. 3b). The charge transfer between graphene and the substrate, calculated following Bader’s atoms-in-molecules approach,20 shows remarkable reduction from 0.0442e to 0.0017e per atom, signifying a much weakened π-d coupling. These results are consistent with recent Raman mapping and scanning Kelvin probe microscopy studies, which show that ultrathin IWL of 0.4 nm can suppress charge transfer between the graphene-mica contact to a high degree, and the Fermi level is modulated by the watergated charge doping.21

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Figure 3. (a) Atomic structure of the top-fcc interface and copper represented by four atomic layers. (b) Differential charge density plots of the graphene/Cu and graphene/IWL/Cu interfaces. (c, d) Band structures of the graphene/Cu and graphene/IWL/Cu hybrids, as well as their decompositions into graphene and Cu, respectively.

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To obtain more insights into the changes in interfacial coupling, we decompose the band structures calculated into atomic orbitals of carbon and Cu atoms (Figs. 3c and 3d), respectively. The results show that electronic coupling between graphene and Cu (111) substrate is mainly attributed to charge transfer at the interface, as indicated by the shifts of Fermi levels for electrons in graphene and copper after the contact forms. This interaction between π and d orbitals is significantly reduced with the presence IWL. This conclusion is further validated by our additional calculations using the van der Waals density functional (vdW-DF) (Fig. S1).22 The graphene-Ni (111) interface is also explored by following the same procedure. The results yield consistent conclusions that the hybridization between electrons in graphene and Ni is removed and the strong interfacial electronic coupling is significantly reduced as the IWL is introduced (Fig. S2). The water intercalation approach would thus be expected to be valid for other metal substrates. Ordered water intercalation enhances thermal dissipation across the interface. Graphene-based nanoelectronics has shown promising features including ultrahigh charge mobility beyond 10000 cm2V-1s-1 at room temperature.5 At high current, electron flow in the device could lead to significant overpopulation of phonons, impeding transport and enhancing scattering with electrons as a result of strong electron-phonon interactions. Moreover, local temperature rise resulting from Joule heating creates local hot spots confined to nanoscale dimensions, especially at defective sites. According to recent reports, the power density in these nanoelectronic transistors and circuits could approach very high values beyond 1 GWm-3.11, 23 This excess amount of thermal energy, if not dissipated efficiently, could result in breakdown of the device in performance, nucleation of defects, reconstruction of surfaces and interfaces, and even materials failure.24-25 As other heat dissipation processes such as radiation and air-cooling can be neglected in nanoelectronic devices, thermal conduction across the interfaces with the substrate and electrodes becomes the major pathway. The weakening of interfacial coupling we discussed above helps to maintain the intrinsic properties of graphene. However, it may also reduce the efficiency of heat dissipation through the interface, and an interface with efficient heat dissipation capability is necessary to maintain the thermal stability in operation. 8


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We show here that the water intercalation does not sacrifice the heat dissipation efficiency, and surprisingly, the interfacial thermal conductance (ITC) can be further elevated by the presence of IWLs. To see this, we carry out non-equilibrium molecular dynamics (NEMD) simulations to quantify the ITC, as the electronic contribution is expected to be negligible due to the weak coupling. The ITC is also known as the Kapitza conductance, which is defined as κI = J/∆TA. Here J is the heat flux transport across the interface with area A, and ∆T is the temperature difference. A supercell of 3.10×5.62 nm is constructed. Our calculation results show that for the bare graphene-Cu (111) interface, κI = 9.76 ± 0.34 MWm-2K-1, which can be enhanced by almost two folds as the IWL is introduced (Fig. 4a). Moreover, we find an almost linear relation between κI and the number density of intercalated water molecules nW, that is, κI(nW) = 1.05nW + 10.52 MWm-2K-1. Simulation snapshots show that water molecules aggregate into an ordered cluster with area increasing with the number density nw.

Figure 4. (a) The ITC measured as a function of the areal density of water molecules in the IWL. (b) VDOS of atoms in graphene, IWL and Cu, calculated from equilibrium MD simulations.

To obtain more insights into this enhancement, we calculate the vibrational density of states (VDOS) for atoms in graphene, IWL and Cu from the Fourier transform of their

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atomic velocity autocorrelation functions (Fig. 4b). Clearly we see from the results that the VDOS of IWL bridges those in graphene and Cu, which suggests that the IWL effectively improves the thermal diffusion at the interface and enhances the interfacial thermal conduction.26 To quantify this effect, we define an overlap between the VDOS in graphene and Cu as SG-Cu = ∫fG(ω)fCu(ω)dω/[∫fG(ω)dω ∫fCu(ω)dω], where f(ω) is the VDOS as a function of mode frequency ω. Our results from equilibrium MD simulations show that with the IWL, the overlap between is significantly enhanced, with SG/IWL/Cu defined as SG/IWL+SIWL/Cu is ~45% higher than SG/Cu at the bare interface, which explains the significant enhancement in ITC. To implement this concept of design in practice, we are aware that the presence of ultrathin water film has been reported for various substrates, with thickness ranging from that of a single water molecule to several nanometers.27 The interlayer gallery is also known to be able to accommodate nanometer-thick IWLs, with water molecules diffusing in at a humid condition.16, 21 Although the microscale dynamics of interfacial wetting is not clear and whether water in the environment could spontaneously enter the interface is an open question,28 one can deposit graphene on to a wetted metal surface. Our additional simulation with a finite-size graphene sheet deposited onto copper (111) surfaces with a water monolayer shows that the graphene/IWL/metal hybrid is thermodynamically stable at 300 K. In practical applications following the device setup proposed in this work, a precise control of the water thin film, by changing the humidity for example, is necessary to finely tune the device performance.15, 29 Recent studies shows that the thickness of IWL tIWL has no effect on the Kapitza resistance between few-layer graphene and water,30 and thus our calculations provide a reference for the performance of interfacial thermal dissipation. However, it should also be noticed that a critical value of tIWL may be essential to effectively weaken the dielectric coupling across the interface.31-32 Although we only consider metallic substrates in this work, the conclusions hold in general for other hosts such as oxides and other dielectrics that are widely used.15, 33 It should also be noticed that there have been efforts to intercalate gas molecules such as N2, O2, NO, Ar, H2, etc. into the interlayer space between graphene and the substrate.34-36 Compared to these gas-phase species, the approach based on IWL is more feasible due to the natural presence of thin water film on substrate or in the interface, while the 10


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intercalation of gas usually requires a complicate procedure. The IWLs between graphene oxide layers has been studied recently.37-38 Additional benefits of using water as the intercalating species include its high dielectric constant.39 Moreover, the hydrogen bonds lead to a condensed phase of water in the IWL, which is the key to enhance the interfacial thermal conductance. Intercalation using gas molecules, for example Ar, lead to decrease in the conductance, although the amplitude of reduction is not significant.33 CONCLUSION In brief, we propose an electronic device setup based on the intercalating water layer between graphene and metallic substrates, which can be further extended to other devices with nanoscale interfaces. The intercalated water with ordered structures promotes thermal dissipation from the device, insulate electronic coupling between the device and substrate, and unfold wrinkled structures in the graphene membrane. This theoretically proved concept lays the ground for a wide spectrum of electronic applications where nanostructures, especially 2D materials that recently draw much attention in the community, are used as the functional components.

METHODS Molecular Dynamics Simulations. All MD simulations are performed using the largescale atomic/molecular massively parallel simulator (LAMMPS).40 The time step for integrating Newtonian equations of motion is 0.5 fs, which is validated to assure energy conservation and give a converged prediction for thermal conductivities. The interatomic interactions in copper and graphene are described using the embeddedatom method (EAM) potential41 and adaptive intermolecular reactive empirical bond order (AIREBO) potential functions, respectively.42 Non-covalent van der Waals interactions between carbon atoms in graphene and Cu atoms in the substrate are calculated using a Lennard-Jones type of function with parameters σC-Cu = 0.30825 nm, and εC-Cu = 0.02578 eV.43 The EAM and AIREBO potential functions and parameters are known to predict well structural, mechanical and thermal properties of both Cu and carbon nanostructures.33 The extended simple point charge model (SPC/E) is used for water.44 The interactions between water, carbon and Cu are modeled using the Lennard-

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Jones 12-6 potential function with parameters εC-O, σC-O and εCu-O, σCu-O. The LorentzBerthelot mixing rules are used to model the interactions between water, carbon and Cu atoms, that is, εC-O = (εC-CεO-O)1/2, σC-O = (σC-C + σO-O)/2 and εCu-O = (εCu-CuεO-O)1/2, σCu-O = (σCu-Cu + σO-O)/2, where εC-C = 0.004555 eV, σC-C = 0.3851 nm, εCu-Cu = 0.167 eV, σCu-Cu = 0.2314 nm are the parameters for interactions between carbon and Cu atoms.43 In preparing the graphene/metal hybrids, we use the f.c.c. lattice constants of metals to determine the size of supercells, which introduce pre-strain of 4.6% and 2% for the graphene/copper and graphene/nickel interfaces, respectively. The strain energies in graphene are 63.35 meV and 11.38 meV per atom, which are close to the thermal energy at 300 K and neglected in our discussions. Interfacial Thermal Conductance Calculations. We calculate the values of ITCs following our previous work.33 The system is firstly equilibrated at 300 K by coupling to a Berendsen thermostat, where the damping time constant τ is set to 100 ps. We then heat up graphene while keeping a 2 nm-thick copper slab at the far end at 300 K. The temperature profile across the interface is measured after the steady state is reached, and finally, we use the Fourier law to calculate the interfacial thermal conductance as κI = J/A∆T, where J and ∆T are the heat flux and temperature drop across the interface, respectively. A is the area of interface. Thermal conductivities calculated from MD simulations usually depend on the size of the model as it can be much shorter than the phonon mean free path lp in crystals.45 However, as we are focusing on the heat conduction across the interface between graphene and copper where the interfacial scattering is strong enough to limit lp, this size effect is not expected to have significant impact. This is verified by performing additional MD simulations with larger supercells. First-Principles Calculations. The interface between graphene and copper is modeled by using a unit cell in Fig. 3a, which contains a graphene sheet and a face-centered-cubic (FCC) Cu or Ni substrate with (111) surfaces. A vacuum layer of 20 Å is used in the direction normal to the interface, representing the isolated slab boundary condition. The structure and properties of this hybrid system are subsequently investigated using planewave basis-set-based density functional theory (DFT) methods. The local (spin) density approximation (LDA)46 and van der Waals density functional (vdW-DF)22 are used in our calculations for they are reported to predict relatively reasonably binding behaviors of 12


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graphene-metal interfaces, while the generalized gradient approximation (GGA) cannot predict the correct interfacial binding between graphene and metal.19,

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We use the

Quantum ESPRESSO package for all calculations reported here.48 For all results presented, energy cutoffs of 38 and 380 Ryd are used for plane-wave basis sets and charge density grids, respectively. 32 Monkhorst–Pack sampling k-points are used in each in-plane direction for Brillouin zone integration. These settings have been verified to achieve a total energy convergence less than 1 meV/atom. For geometry relaxation, the force on atoms is converged below a threshold of 0.01 eVÅ-1.

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ASSOCIATED CONTENT Supporting Information Available: Details of Computational Methods, band structures of the graphene/Cu (111) and graphene/IWL/Cu (111) hybrids calculated using the van der Waals density functional (vdW-DF), spin-polarized band structures of graphene/Ni (111) and graphene/IWL/Ni (111) hybrids calculated using LDA, and their decomposition onto graphene and Ni, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China through Grants 11222217 and 11472150, and the State Key Laboratory of Mechanics and Control of Mechanical Structures (Nanjing University of Aeronautics and Astronautics) through Grant No. MCMS-0414G01. The computation was performed on the Explorer 100 cluster system at Tsinghua National Laboratory for Information Science and Technology.

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Water Intercalation for Seamless, Electrically Insulating, and Thermally

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