Tunable Broadband Nanocarbon Transparent Conductor by

Dec 29, 2016 - Optical transparent and electrical conducting materials with broadband transmission are important for many applications in optoelectron...
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Tunable Broadband Nanocarbon Transparent Conductor by Electrochemical Intercalation Jiayu Wan,†,⊥ Yue Xu,†,⊥ Burak Ozdemir,‡,⊥ Lisha Xu,† Andrei B. Sushkov,§ Zhi Yang,∥ Bao Yang,∥ Dennis Drew,§ Veronica Barone,*,‡ and Liangbing Hu*,† †

Department of Materials Science and Engineering, §Department of Physics, and ∥Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, United States ‡ Department of Physics and Science of Advanced Materials Program, Central Michigan University, Mount Pleasant, Michigan 48859, United States S Supporting Information *

ABSTRACT: Optical transparent and electrical conducting materials with broadband transmission are important for many applications in optoelectronic, telecommunications, and military devices. However, studies of broadband transparent conductors and their controlled modulation are scarce. In this study, we report that reversible transmittance modulation has been achieved with sandwiched nanocarbon thin films (containing carbon nanotubes (CNTs) and reduced graphene oxide (rGO)) via electrochemical alkali-ion intercalation/deintercalation. The transmittance modulation covers a broad range from the visible (450 nm) to the infrared (5 μm), which can be achieved only by rGO rather than pristine graphene films. The large broadband transmittance modulation is understood with DFT calculations, which suggest a decrease in interband transitions in the visible range as well as a reduced reflection in the IR range upon intercalation. We find that a larger interlayer distance in few-layer rGO results in a significant increase in transparency in the infrared region of the spectrum, in agreement with experimental results. Furthermore, a reduced plasma frequency in rGO compared to few-layer graphene is also important to understand the experimental results for broadband transparency in rGO. The broadband transmittance modulation of the CNT/rGO/ CNT systems can potentially lead to electrochromic and thermal camouflage applications. KEYWORDS: broadband, electrochemical intercalation, alkali-ion, infrared transmittance, tunable

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tions on how to achieve broadband TCEs are crucial for functional IR-transparent conductors and devices. Carbon materials such as CNTs and graphene have attracted tremendous attention recently as transparent conducting materials for optoelectronic devices due to their simultaneously high conductivity and transmittance.1,9,24−26 Like many other new-generation TCE materials, carbon-based transparent conductors are potentially low cost, resourceful, appropriate for large-scale processing and patterning, stable, and flexible. Moreover, there are several advantages of carbon materials over all other transparent conductor materials. First, thin CNTs and graphene layers are not only transparent in the visible range but also quite transparent in a broad range (i.e., visible to IR range).27,28 Second, the transmittance and/or conductivity of carbon-based transparent conductors can be modulated with electric fields,29−31 electrical double layers,32−34 and intercalation species.35−39 These modification methods not only

imultaneously optical transparent and electrical conducting materials are ubiquitously used in optoelectronic devices, such as solar cells, organic light-emitting devices (OLEDs), touch screen displays, and smart glass.1,2 Highperformance doped metal oxides such as indium tin oxide (ITO) have dominated the transparent conductor electrodes (TCEs) market.3 However, an increasing research effort exists to replace ITO in the past decades due to the scarcity of indium sources, the high growth cost of ITO, and the brittle nature of the ceramics. Metal nanowires,3−5 metal grids,6,7 conducive polymers,8 carbon nanotubes (CNTs),9 graphene,10,11 and their hybrids12−14 have been intensively investigated as highperformance TCEs. Despite the success of these materials, most studies on TCs are focused on the visible range. Simultaneous transparency and conductivity is also of great importance in other regions of the electromagnetic spectrum, such as the infrared region.15,16 IR-transparent conductors17 are needed in multijunction solar cells,18 IR imaging and sensing,19 and IR emission devices20,21 for military, astronomy, and telecommunication applications.22,23 Thus, further investiga© 2016 American Chemical Society

Received: October 25, 2016 Accepted: December 29, 2016 Published: December 29, 2016 788

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Figure 1. (a) Schematic of alkali-ion intercalation in a CNT/rGO thin film. (b) Schematic of the CNT/rGO/CNT sandwich structure. Largescale printed CNT/rGO/CNT film on a PET substrate before Li-ion intercalation (c) and after Li-ion intercalation (d). (e) Typical transmittance change of carbon film before and after Li-ion intercalation for selected wavelengths of 550 nm (visible range) and 2 μm (infrared range).

Figure 2. Photo images and microscope images of (a) pristine, (b) lithiated, and (c) delithiated CNT/rGO/CNT film, respectively. Reversible transmittance change in the visible range (wavelengths of 450, 550, 750 nm). The length of the scale bars in (a) and (b) are 1 cm and 50 μm, respectively. Transmittance obtained in (c) pristine, lithiated, and delithiated CNT/rGO/CNT network and (d) pristine, sodiated, and desodiated CNT/RGO/CNT network.

visible range (450 nm) to the mid-infrared range (5 μm) can be achieved. We also show the reversibility of the process by optical and in situ Raman spectroscopy. Our calculations in Liintercalated few-layer graphene indicate that interband transitions are significantly reduced along with absorption upon intercalation in the visible spectrum. The calculations, for few-layer graphene, also show that a plasma edge appears within the visible region due to increased conduction electron density, which causes high reflectance at lower energies. This result is in contrast to reduced graphic oxide (rGO) experimental results, where an increase in transmittance occurs in a broad energy region including the infrared. This effect is attributed, at least partly, to a reduction in plasma frequency

improve the overall transmittance and conductivity of the thin carbon films but also provide possible tunability in a broad range. It is exciting to envision a transparent conductor thin film with reversibly tunable transmittance in the broadband used for smart windows in energy-saving buildings.40 Additionally, a tunable transmittance in the broadband can also be potentially applied for thermal camouflage.41 Thus, our goal here is to achieve a carbon-based transparent conductor with reversible modulation of transmittance in a broad range. To this end, we designed a CNT/rGO/CNT sandwich structure and achieved reversible broadband transmittance modulation through electrochemical alkali-ion intercalation. We have been able to demonstrate that upon ion intercalation in these structures, a wide transmittance modulation from the 789

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Figure 3. (a) Schematic of in situ Raman with a planar battery of a CNT/rGO/CNT thin film with an ion intercalation and deintercalation process. Raman spectrum of a (b) pristine, lithiated, delithiated and a (c) pristine, sodiated, and desodiated CNT/rGO/CNT thin film.

respectively. The drastic reversible modulation in the three typical wavelengths demonstrates ion intercalation as an effective tuning method in the visible range. Note that in all cases the transmittance after deintercalation did not fully recover its original value, which should be mainly caused by irreversible reactions after Li-ion intercalation with residual functional groups on rGO.42 The doping effect simultaneously increased the conductivity of the sandwich film, which was manifested as a decrease of sheet resistance after intercalation. A typical change of sheet resistance from 3.0 kΩ/sq to 2.1 kΩ/ sq after Li-ion intercalation was observed. Na-ion intercalation and deintercalation in the CNT/rGO/CNT network exhibits a similar transmittance modulation in the visible range, as shown in Figure 2d. The overall performance of the CNT/rGO/CNT sandwich film after intercalation is comparable with the highest performance carbon-based network transparent conductors (Supplementary Figure S2). Although the reversibility of the ion intercalation process is well-known for rGO or graphite in battery electrodes,43 until now, a reversible intercalation/deintercalation of ions in pure rGO network thin films on a substrate has not been demonstrated. The key difference between rGO in battery electrodes and in transparent conductive thin films is that in battery electrodes rGO is surrounded by functional additives such as carbon black and polymer binder (i.e., PVDF), while in the thin film the rGO network is tightly attached to a transparent substrate (i.e., glass). Just like in any battery electrode materials, when ion intercalation/deintercalation occurs, rGO experiences a significant volume expansion/ shrinking. In normal batteries, the polymer binder holds the position of adjacent rGO flakes during the intercalation/ deintercalation process, while in planar batteries, the rGO network holds itself only through van der Waals forces without a binder. Therefore, during the volume expansion/shrinking process, the rGO network is prone to breakage, which leads to an irreversible ion intercalation process. During the ion deintercalation process, the rGO network breaks and peels off (Supplementary Figure S3, in contrast to cyclable CNT/rGO/ CNT in Supplementary Figure S4). This result demonstrates the necessity of CNTs in our design in order to preserve the structural integrity of the rGO network. To further confirm the reversible modulation of the CNT/ rGO/CNT film via ion intercalation, we conducted in situ Raman spectroscopy on the planar battery film. Figure 3a illustrates the schematic of the in situ Raman setup with the planar battery. The CNT/rGO/CNT film is connected to a Cu

due to smaller conductivity or larger effective mass of electrons in rGO.

RESULTS AND DISCUSSION In order to achieve reversible optical modulation of the largescale rGO network thin film, we designed a CNT/rGO/CNT sandwich structure that reinforces the bonding of the rGO network (Figure 1a,b). The advantages of the CNT/rGO/CNT structure are as follows: (1) the two CNT layers provide a mechanical protection to the rGO network in between and prevent the cracking of rGO during ion intercalation/ deintercalation; (2) the CNT network mechanical protection layer is significantly porous and thus allows the ions to access the rGO interlayers; (3) the CNT network is also electrically conductive, thus contributing to the fast and uniform ion intercalation throughout the entire rGO network thin film (Figure 1c,d); (4) although a single-walled CNT has some absorption features in the IR range, the optical density of thin CNT film is very low (Supplementary Figure S1); thus it is quite transparent to photons in the IR regime. Thus, the addition of a CNT network would not affect the optical modulation of rGO with an ion intercalation process in the visible and IR regime we are interested in. A typical transmittance change in both the visible range (550 nm) and IR range (2 μm) before and after Li-ion intercalation in the CNT/rGO/CNT film is shown in Figure 1e. We first demonstrate the optical modulation of CNT/rGO/ CNT in the visible range with Li-ion intercalation. As shown in Figure 2a−c, photo images (upper images) of a CNT/rGO/ CNT thin film (gray strip) on a planar battery before intercalation, after intercalation, and after deintercalation are taken. The region inside the yellow dashed line box is the CNT/rGO/CNT thin film confined inside the polydimethylsiloxane (PDMS) reservoir, in which the electrolyte is accessible; thus enabling the ion intercalation/deintercalation process. Clearly, our results successfully demonstrate a reversible modulation of a CNT/rGO/CNT film in the visible range via Li-ion intercalation. Figure 2a−c (lower images) show the microscope images of the sandwich film in the pristine, lithiated, and delithiated states. We also measured the optical transmittance change of the CNT/rGO/CNT film by analyzing the gray-scale image of the films at three different wavelengths in the visible range (450, 550, 750 nm). The transmittance change before intercalation, after intercalation, and after deintercalation is 68.5%, 85.6%, 72.1% for 450 nm, 70.4%, 93.4%, 75.8% for 550 nm, and 79.5%, 99.1%, 81.2% for 750 nm, 790

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Figure 4. (a) Schematic of IR transmission characterization with a CNT/rGO/CNT film by an IR camera. False-color image of the CNT/ rGO/CNT film before and after (b) Li-ion intercalation and (c) Na-ion intercalation. Transmission change of the CNT/rGO/CNT before and after (d) Li-ion intercalation (inset: a simplified schematic) and (e) Na-ion intercalation, as a function of energy in the infrared range.

Figure 5. (a) Top and side views of the LiC6 structure: yellow and gray balls represent C and Li atoms, respectively. (b) Electronic structure of six layers of graphene using the optimized interlayer separation of 3.311 Å labeled as “Graphene (6)” (black dotted line) and using an increased interlayer separation of 4.00 Å labeled as “Graphene** (6)” (red dotted line). (c) Electronic structure of six layers of LiC6. Transmission of (d) six and (e) 20 layers of graphene and LiC6 and reflectance of (f) six layers and (g) 20 layers of graphene and LiC6. All the results are obtained from the electronic structure of six layers, except “Graphene* (20)”, which is obtained by using the electronic structure of bulk graphite. “Graphene** (20)” is obtained from the electronic structure of six layers of graphene with 4 Å of increased interlayer spacing instead of the LDA-optimized value of 3.311 Å. Blue arrow in (e) and (f) denote the intraband transitions and interband transitions of the intercalated graphene.

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before and after Li-ion and Na-ion intercalation, respectively. In both cases, it is obvious that after intercalation a drastic increase in transmission has occurred in both the near-infrared region and the mid-infrared region. The chemical nature of rGO is highly dependent on the synthetic routes, although it is generally characterized by the presence of different and randomly spaced defects and chemical groups, which broaden Raman peaks and increase the interlayer distances and resistance in the material. These effects make reliable electronic structure calculations challenging, as the atomic structure of the material is not known. Therefore, in order to understand the excellent transmittance modulation of the sandwich film with ion intercalation, we started by calculating the reflection and transmission of a few layers of pristine graphene (six and 20 layers) and LiC6 in between glass. Figure 5a shows the schematic top view and side view of LiC6. Figure 5b and c show the electronic structure of pristine and Liintercalated six-layer graphene, respectively. After Li-ion intercalation, we observed an upshifted Fermi energy. This Fermi energy upshift leads to Pauli-blocking of photons with an energy less than 2EF (∼3 eV), resulting in the increased transmittance upon intercalation in the visible range (blue dashed lines in Figure 5b and c). We first discuss the broadband transmittance of the original graphitic film, without intercalation. The intercalated six-layered graphene shows an increased broadband transmission (Figure 5d). In order to understand the effect of thickness in few-layer graphene, we considered 20 layers of graphene in addition to six layers. For 20 layers of graphene, transmission is calculated by using the electronic structure of six layers of graphene and bulk graphite (solid and dashed red lines labeled “Graphene (20)” and “Graphene* (20)” in Figure 5e). For energies larger than 1 eV both results are identical; however below 1 eV, results begin to differ, and at 0.3 eV the transmission obtained from the electronic structure of six layers of graphene is about 10% lower than the transmission obtained by using the electronic structure of bulk graphite. This is due to a slightly larger absorption (ε2) of six layers of graphene with respect to bulk graphite below 1 eV, caused by a broken degeneracy of the energy bands around the Fermi level. However, in contrast to the results of our calculations, the experimentally observed transmission of rGO increases toward 0.3 eV. Importantly, the interlayer distance of rGO is larger than graphite, which affects transmission in the IR region through a weaker interlayer coupling. This can be observed through the marked difference in transmission below 1 eV of 20 layers of graphene obtained from the electronic structure of (i) six layers of graphene, (ii) AB-stacked bulk graphite, and (iii) six layers of graphene with an artificially increased interlayer spacing of 0.4 nm. As seen in Figure 5e, in the latter case, transmission increases toward 0.3 eV in agreement with the experimental observation in rGO. The difference in transmission between the results for 20 layers of graphene with different interlayer spacing is large (about 30%). Therefore, an increase in the interlayer separation and decoupling of the layers can result in significant gain in the IR region transparency in few-layer graphene due to a reduction in interband transitions with increased degeneracy of the bands around the Fermi level, as shown in Figure 5b. Since, rGO is known to have a larger interlayer spacing than graphite and the experimental material consists of a network of rGO layers, the layers are expected to be decoupled and the experimental result of increased transmission toward 0.3 eV is comparable to expanded few-

electrode on one side and to Li/Na metal and a Cu electrode on the other side. After the planar batteries are sealed, they are integrated with an electrochemical station and Raman spectrometer. The He−Ne laser (633 nm) shoots directly on the back side of the planar battery and focuses on the surface of the CNT/rGO/CNT thin film through the glass. Figure 3b shows the Raman spectrum of the hybrid film before and after intercalation and after deintercalation of Li, respectively. As shown in Figure 3b, the Raman spectrum of the sandwich film shows two typical peaks, which correspond to the D and G peaks of graphitic materials. After Li-ion intercalation, both peaks diminish and disappear after intercalation. This result is very similar to the Raman spectrum of Li-intercalated graphite at the LiC6 stage.44 After full delithiation, both D and G peaks are recovered, demonstrating the reversible Li intercalation and deintercalation in the sandwich film. The small peaks in both intercalated states in Figure 3b are probably a result of the electrolyte.45 The sandwich film interacting with Na-ions shows similar characteristics to those in the Li-ion case, demonstrating its successful reversible intercalation. We then used an IR camera to qualitatively demonstrate the modulation of the sandwich thin film via intercalation. The temperature distribution of the samples was characterized by a temperature-controlled isothermal hot plate beneath the sample and then measuring the resulting temperature distribution with an FLIR Merlin MID IR camera, as shown in Figure 4a. At a fixed hot plate temperature (55−60 °C), the surface temperature distribution of the thermalized samples was detected by the IR camera. The glass substrate is quite transparent in the IR range (the emitted radiation region), but the CNT/rGO/CNT film coating on the glass surface is not as transparent. As a result, the non-IR-transparent coating will block the radiation to the IR camera, causing a temperature difference on the thermal map (Figure 4b,c). The black region is the Cu electrodes overlaid on the rGO network, which indicates that the metal totally blocks the IR radiation. The pink region marks the lower temperature display caused by the relatively low transparency of the rGO coating, while the orange region is the glass substrate. Before ion intercalation, the color difference corresponds to the temperature difference caused by different IR transparencies on the surface. The average temperature difference between the film-coated area and the glass substrate is 0.5−0.6 °C. However, after Li-ion or Na-ion intercalation, the temperature difference between the film-coated area and the glass is only 0−0.1 °C, indicating that intercalation greatly improves the IR transparency of the sandwich nanocarbon filmcoated area. We additionally measured the transmission spectra of the CNT/rGO/CNT film and the Li-ion intercalated film with a Bomem DA3 FTIR spectrometer. In this case, the film was coated on a BaF2 substrate instead of glass since BaF2 has a much wider transparent window (0.2 to 12 μm in wavelength). The Cu thin film electrode is evaporated on top of the sandwich thin film to assist Li-ion intercalation. Two spectrometer configurations were used. In the mid-infrared, we used a Globar source, KBr beam splitter, and MCT detector. In the near-infrared, we used a quartz lamp, quartz with TiO2 film beam splitter, and InSb detector (simple schematic in Figure 4d inset). Special care was taken to minimize degradation of the intercalated graphene. The two spectra merge very well without scaling, which is an evidence of good reproducibility. As shown in Figure 4d and e, the black and red lines depict the transmission spectrum of sandwich film 792

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response in optical transmission but also achieved reversible optical modulation via ion intercalation/deintercalation. The experimental results in intercalated reduced graphene oxide show an increase in transmission in the infrared as well as in the visible region, in contrast to calculated results for intercalated few-layer graphene. This disagreement can be understood by considering a reduced plasma frequency and also an increased interlayer spacing in rGO in addition to possible empty spaces in the large network of rGO flakes. Assisted by interference of light, the plasma frequency downshift quenches the reflectance in the infrared region in rGO, in contrast to what would occur in few-layer graphene. We additionally show that the sandwich thin film is also highly scalable with a flexible polyethylene terephthalate (PET) substrate via an all-solution-based process. The demonstration of the scalable, printable, broadbandresponsive, carbon-based sandwich film is significant in optoelectronic and electrochromic49 devices for consumer electronics, communication, and military uses.

layer graphene. Furthermore, sensitivity of the optical transitions to interlayer spacing and also possibly to stacking of the layers in the IR region of the spectrum is an interesting aspect, considering the recent efforts to find a reliable experimental method for the determination of the complex refractive index of few-layer graphene where wavelengths in the visible region have been used.46 We then discuss the transmittance modulation of rGO film with ion intercalation. Upon intercalation, transmission increases by about 20−30% for 20 layers from the infrared to the visible regions of the spectrum. In the infrared region of 0.3−1.6 eV, the description of intraband transitions becomes important in order to study reflection and transmission. In our calculations, the system with 20 layers of LiC6 shows decreasing transmission (solid blue line in Figure 5e) for energies below 2 eV due to intraband transitions, in good agreement with recent experimental results within the visible range.35 This behavior shifts to lower energies (below 1 eV) for six layers. Calculations show that the increased transparency for few-layer graphene is assisted by interference due to internal reflection (Supplementary Figure S6). This effect, which is highly dependent on plasma frequency, quickly disappears as the number of layers increases. However, in Figure 4, in Li-intercalated rGO we do not observe this decrease in transmission within the visible or infrared regions, although the thickness of the experimental rGO film is larger than few-layer graphene. Therefore, there is clearly a significant difference between intercalated rGO and few-layer graphene in terms of plasma frequency, which affects reflectance and transmittance enormously. For example, a decrease in the plasma frequency of about 1.4 eV results in an increased transmission for both six and 20 layers, and similar to experimental results, a flat transmission in the IR region is obtained at 1 eV of plasma frequency for 20 layers (dashed blue lines in Figure 5d and e). These results seem to indicate that plasma frequency is lowered in intercalated rGO flakes with respect to few-layer graphene, thus reducing the reflectance significantly (Figure 5f and g) and therefore exhibiting improved transparency in the infrared region. A larger effective mass and/or smaller conduction electron density in intercalated rGO compared to graphite can result in lower plasma frequency. The intercalated large-scale rGO network presents a significantly higher sheet resistance (2.1 kΩ/sq) than intercalated few-layer graphene (about 3 Ω/sq). This difference can be caused by increased resistivity due to the intrinsic lower conductivity of rGO to graphene and/or the connections between rGO flakes in the large network of flakes. The observed almost constant transmission of Li-intercalated rGO in the IR region can be understood with a relatively small amount of reduction in plasma frequency as well as a decoupling of the graphene layers due to increased interlayer separation. For example, the transmission of Na-intercalated rGO increases toward 0.3 eV instead of being constant; this can be understood with a further reduction in plasma frequency due to lower Na concentration compared to Li in rGO. We note that the effect of CNT layers on reflectance and transmittance is ignored in our discussion. However, our CNT thin layers are quite transparent in both the infrared and visible region with low optical density.47,48

EXPERIMENTAL SECTION Preparation of CNT/rGO/CNT Network. GO solution was prepared using an improved Hummer’s method.50 Printable GO ink is obtained by adding ethanol to the as-prepared GO solution (2.5 mg/mL), with a ratio (v/v) of 3:5). Printable CNT ink is prepared by adding sodium dodecylbenzenesulfonate (SDBS) in a CNT water solution, followed by probe sonication for 5 min.51 CNT/GO/CNT thin films are coated on precleaned glass or a PET substrate with spray coating or Meyer rod coating methods. GO in the CNT/GO/CNT sandwich on a glass substrate is thermally reduced in a tube furnace at 300 °C (1 °C/min ramp). The reduction process was carried out under H2/Ar (5%/95%) in a tube furnace. GO in the CNT/GO/CNT sandwich on a PET substrate is chemically reduced with HI solution at 90 °C for 1 min. Fabrication of a Planar Battery with a CNT/rGO/CNT Network. A planar battery for alkali-ion intercalation was fabricated in an argon-filled glovebox. A 50 nm Cu thin film was thermally evaporated with the sandwich film as current collector. One molar LiPF6 and NaPF6 in (1:1 v/v) ethylene carbonate/diethyl carbonate as the electrolyte, pure lithium metal, and sodium metal as the counter electrode are used for Li/Na-ion intercalation, respectively. The planar battery was covered with glass and sealed with PDMS before being taken out from the glovebox. In Situ Optical Characterization with a Planar Battery. A portable electrochemical workstation (Biologic SP-150) was connected with as-fabricated planar batteries for reversible intercalation/ deintercalation of alkali ions in a CNT/rGO/CNT network for both transmittance and Raman measurements. For in situ transmittance measurement, the planar battery was connected with an optical microscope in transmission mode (Nikon Eclipse Ti-U). Narrow-band filters (Thorlabs Inc.) were added in the optical path to obtain transmittance measurements at 450, 550, and 750 nm. The transmittance of the network was obtained from analysis of the gray-scale images taken by CCD camera. For in situ Raman measurement, the planar battery is combined with a Raman spectrometer (Horiba Jobin Yvon, a 633 nm He−Ne laser source). Note that the laser was focused at the back side of the planar battery, to minimize the signal from the electrolyte. Infrared Transmittance Measurement. The CNT/rGO/CNT networks are coated on glass or BaF2 substrates for IR transmittance measurements. For both measurements, samples are intercalated inside an Ar-filled glovebox by directly contacting Li/Na metal with Cu, then adding electrolyte. The intercalation process takes about 10 min to achieve uniform and thorough intercalated states. For thermal mapping with an IR camera, the IR radiation at wavelengths ranging from 1 to 5.4 μm emitted from the sample surface was detected by the IR camera, which has a sensor resolution of 320 × 256 pixels and is connected to a computer via ThermaCAM Research software.

CONCLUSION In conclusion, we designed and fabricated a CNT/rGO/CNT sandwich thin film, which not only demonstrated its broadband 793

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reflection of bare and potassium-intercalated few-layer graphene.57,59,60 The equations for reflection coefficient (r) and transmission coefficient (t) are defined as

For the infrared transmission spectrum measurement, samples were glued by rubber cement to a metallic circular aperture. The sample with the aperture was then placed in an argon-filled chamber in the optical path for transmission measurement. Raw transmission spectra of samples were divided by raw transmission of an identical empty hole. DFT Calculation Methods. Density-functional theory calculations within the local density approximation (LDA) are carried out using the Quantum-Espresso software employing a plane-wave basis set with periodic boundary conditions.52 Norm-conserving pseudopotentials are used to replace core electrons of C and Li atoms. The kinetic energy cutoff of the wave functions is 140 Ry. The Monkhorst−Pack grid of k-points is used.53 Structure optimization is carried out with a k-point grid of 14 × 14 × 1 for six layers of graphene and 8 × 8 × 1 for LiC6. Marzari−Vanderbilt smearing with a smearing width of 0.02 Ry is used for Li-intercalated metallic systems.54 The convergence threshold of stress and interatomic forces for structure optimizations are 0.5 kbar and 10−3 Ry/bohr. Separations along the vacuum direction for six layers of graphene and LiC6 are 12.4 and 13.1 Å, respectively. For the density-of-states calculations, 64 × 64 × 1 and 40 × 40 × 1 k-point grids are used for six layers of graphene and Liintercalated structures, respectively. The LiC6 gallery structure in six layers of graphene has the stoichiometry Li0.83C6, due to the absence of Li atoms on the surfaces with an AA stacking of layers of both C and Li atoms (Figure 5a). The Yambo code55 is used to obtain optical interband transitions within the random-phase approximation (RPA), and intraband transitions in the metallic system (intercalated) are taken into account through the Drude model with a damping energy of 0.3 eV.56,57 Exchange−correlation contributions to the full polarizability are neglected in the RPA, and the problem is reduced to finding independent-particle polarizabilities. The dielectric function obtained with the Drude model for intraband transitions in a metal can be expressed as wD2 ϵ(w) = 1 − , w(w − iΓ)

r=

∑∫ n≠n′

BZ

1 − r01r12e

where r01 =

−2iβ

n0 − n1 ,t n1 + n0 01

,

t=

t01t12e−iβ 1 − r10r12e−2iβ

= 1 + r01, and β is the phase difference due to

the optical path length and also involves the attenuation coefficient. Reflectance results for graphite and 35 layers of bare graphene in a vacuum (using the dielectric function of bulk graphite) can be compared in the infrared as well as the visible region of the spectrum to the experimental results reported in the literature (Supplementary Figure S6).59,61 Here, we considered the systems are contained in between glass with a refractive index of 1.5 instead of a vacuum. With these approximations we find that the reflectance is reduced, and therefore, the transmission is increased with respect to the system being in vacuum and in better agreement with a recent experimental study on transparency of Li-intercalated few-layer graphene.3

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07191. More experiments details (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

⎛ 4πe 2N ⎞1/2 wD = ⎜ ⎟ ⎝ mV ⎠

Liangbing Hu: 0000-0002-9456-9315 Author Contributions ⊥

where w is the frequency, Γ is the scattering rate (damping), N is the number of conduction electrons, m is the optical mass simply taken as free electron mass (see for a discussion the work of Holzwarth et al.58), V is the volume, and wD is the plasma frequency. The independentparticle polarization is used to obtain the RPA dielectric function for the interband transitions, 0 χGG ⃗ ⃗ ′ (q ⃗ , w) = 2

r01 + r10e−2iβ

Jiayu Wan, Yue Xu,and Burak Ozdermir contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS L.H. and V.B. acknowledge the support of NSF through grants CBET-1335979 and CBET-1335944. L.H. also acknowledges the support of NSF through grant CMMI-1300361. We acknowledge the support of the Maryland Nanocenter and its Surface Analysis Center and Fablab. We also acknowledge the help from Y. Xu and Dr. J. Munday for the visible transmittance measurement.

dk ⃗ * ρ ⃗ (q ⃗ , G⃗)ρnn ′ k ⃗ (q ⃗ , G⃗′)fnk ⃗− q ⃗ (1 − fn ′ k ⃗ ) (2π )3 nn ′ k

⎡ ⎤ 1 1 ⎥ ×⎢ − w + εn ′ k ⃗ − εnk ⃗− q ⃗ − iη ⎥⎦ ⎢⎣ w + εnk ⃗− q ⃗ − εn ′ k ⃗ + iη

⃗ ,G⃗ ) = ⟨nk|e ⃗ i(q⃗+G⃗ )·r⃗|n′k⃗ − q⃗⟩, f nk⃗ is the occupation number, where ρnn′(k,q⃗ and εnk⃗ is the Kohn−Sham eigenvalue. In the vanishing momentum limit the macroscopic dielectric function is defined as 1 ϵM(w) = lim q ⃗ → 0 .55 The macroscopic dielectric func−1

REFERENCES

[ϵ (q ⃗ , w) ]G⃗ = 0, G⃗ ′ = 0

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tion of in-plane polarization obtained for the supercell containing six layers of graphene (bare and intercalated) including a large vacuum space is scaled to a volume with a perpendicular distance corresponding to the distance between the six layers plus 3.311/2 Å for each external surface. This distance is half the LDA interlayer distance between sheets, and it is expected that the charge density as well as the dielectric response should be low beyond this distance toward the vacuum. The calculated plasma frequency is 4.8 eV for six layers of LiC6 suspended in a vacuum, which is lower than the bulk value since there is a contribution to conduction electron density due to out-of-plane periodicity in the bulk system.39,58 For example, the reflectance of 16 layers of KC8 is found to be in better agreement with experimental results when a plasma frequency reduced by about 15% from the bulk value is used for the Drude model calculation.57 Reflectance and transmission are calculated by including internal reflections and phase changes, similar to the recent works on optical 794

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