Graphene and Graphene Analogs toward Optical, Electronic

Spotlight on Applications www.acsami.org

Graphene and Graphene Analogs toward Optical, Electronic, Spintronic, Green-Chemical, Energy-Material, Sensing, and Medical Applications M. Reza Rezapour,† Chang Woo Myung,† Jeonghun Yun, Amirreza Ghassami, Nannan Li, Seong Uk Yu, Amir Hajibabaei, Youngsin Park,* and Kwang S. Kim* Center for Superfunctional Materials, Department of Chemistry, School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea ABSTRACT: This spotlight discusses intriguing properties and diverse applications of graphene (Gr) and Gr analogs. Gr has brought us two-dimensional (2D) chemistry with its exotic 2D features of density of states. Yet, some of the 2D or 2D-like features can be seen on surfaces and at interfaces of bulk materials. The substrate on Gr and functionalization of Gr (including metal decoration, intercalation, doping, and hybridization) modify the unique 2D features of Gr. Despite abundant literature on physical properties and well-known applications of Gr, spotlight works based on the conceptual understanding of the 2D physical and chemical nature of Gr toward vast-ranging applications are hardly found. Here we focus on applications of Gr, based on conceptual understanding of 2D phenomena toward 2D chemistry. Thus, 2D features, defects, edges, and substrate effects of Gr are discussed first. Then, to pattern Gr electronic circuits, insight into differentiating conducting and nonconducting regions is introduced. By utilizing the unique ballistic electron transport properties and edge spin states of Gr, Gr nanoribbons (GNRs) are exploited for the design of ultrasensitive molecular sensing electronic devices (including molecular fingerprinting) and spintronic devices. The highly stable nature of Gr can be utilized for protection of corrosive metals, moisture-sensitive perovskite solar cells, and highly environment-susceptible topological insulators (TIs). Gr analogs have become new types of 2D materials having novel features such as half-metals, TIs, and nonlinear optical properties. The key insights into the functionalized Gr hybrid materials lead to the applications for not only energy storage and electrochemical catalysis, green chemistry, and electronic/spintronic devices but also biosensing and medical applications. All these topics are discussed here with the focus on conceptual understanding. Further possible applications of Gr, GNRs, and Gr analogs are also addressed in a section on outlook and future challenges. KEYWORDS: graphene, graphene nanoribbon, graphene analogs, adsorption, intercalation, circuit patterning, hybrid materials, energy materials

1. INTRODUCTION Since the advent of Gr, two-dimensional (2D) chemistry has emerged with noncovalent adsorption on Gr, intercalation inside Gr bilayer to few-layers, and functionalization of Gr.1,2 The techniques developed for synthesizing Gr can be grouped into several methods including mechanical cleavage, epitaxial growth, chemical vapor deposition (CVD), and organic synthesis methods. As compared with the 3D bulk system, the boundaries of the bulk systems or the interface between two different types of bulk systems are often related to 2D systems. Two-dimensional materials show the characteristic density of states (DOS) which is nearly constant to energy variation due to the confinement effect, in contrast to those of 3D materials which are nearly proportional to the 3/2th power of the energy variation. Since these characteristic DOSs can lead to new types of materials with exotic electronic properties, the physical and chemical behaviors are quite different from the bulk properties. In this regard, the isolation of 2D materials has been an © 2017 American Chemical Society

intriguing topic. However, it has been known that the conventional long-range positional order is absent in 2D systems.3 Nevertheless, it was suggested that long-range orientation order may exist. Thus, 2D solids have been characterized by quasi-long-range positional order and true long-range orientation order, so as to show almost all the properties of crystals. The Kosterlitz−Thouless transition, which might be responsible for order−disorder transitions in 2D systems, must be viewed as an upper bound for 2D quasilong-range positional order, and so the grain boundary (GB) formation and vacancy condensation mechanisms have appeared.4,5 In this regard, GB and dislocation have been critical issues in the preparation of practical 2D materials. Received: February 27, 2017 Accepted: July 5, 2017 Published: July 5, 2017 24393

DOI: 10.1021/acsami.7b02864 ACS Appl. Mater. Interfaces 2017, 9, 24393−24406

Spotlight on Applications

ACS Applied Materials & Interfaces

Figure 1. Various types of Gr defects including grain boundaries (GB) (typical width and height of each defect are described). Each GB can be differentiated by different populations of adatoms because their adsorption energies depend on the GB type. For example, as compared with the absolute binding energy 0.4 eV of a gold adatom on pristine Gr, gold adatoms on stitched GBs are 0.4−0.7 eV higher, those on the upper/lower edges of overlapped GBs are 0.3−0.6 eV/0.1 eV higher, and those on wrinkles are ∼0.1 eV higher because of the enhanced C sp3 character. Reprinted with permission from ref 6. Copyright 2014 American Chemical Society.

Figure 2. Typical HRTEM images of Gr holes (a) at room temperature and (b) at 800 °C. The edges are color-coded to differentiate the types of edge configurations (red, armchair; yellow, zigzag; green, zz57; white, mixed/unidentified edge types). (c) Schematic representation of zigzag, armchair, and zz-57 Gr edges. Reprinted with permission: Panels a and b from ref 11. Copyright 2015 American Chemical Society. Panel c from ref 12. Copyright 2015 American Chemical Society. Panel d from ref 13. Copyright 2014 Nature Publishing Group.

wrinkle) and GBs (stitched and overlapped) are main line defects.6 Figure 1 illustrates the shape and size of each defect. Despite that defects are inevitable in large-sized Gr, wideranging local regions of Gr are almost 2D crystal-like. Thus, Gr is eventually found to be a really practical 2D material with fascinating electronic properties. 2.2. Gr Edges and Gr Nanoribbons. As GBs and dislocations are formed in Gr, edges are also easily formed or constructed. While GBs and dislocations can have better chemical activities which are often useful for functionalization, edges are also quite useful due to their unique electronic/ spintronic properties. The one-dimensional (1D) edge structure in nanosized 2D materials determines their various mesoscopic properties. Therefore, the atomic configuration of Gr edges influences the electrical, magnetic, and chemical properties of Gr nanostructures, in particular, graphene nanoribbons (GNRs).7 This implies that realistic device fabrication requires precise engineering of Gr edges. However, imaging and analysis of the intrinsic nature of Gr edges can be illusive due to contamination problems and measurementinduced structural changes to Gr edges. Gr edges are studied typically via either scanning tunneling microscopy (STM),8 aberration-corrected transmission electron microscopy (ACTEM),9 or micro-Raman spectroscopy.10 As an example, the effect of temperature on Gr edge terminations at the atomic scale has been examined using an in situ heating holder within an AC-TEM.11 Although zigzag terminations survive below 400 °C (Figure 2a), above 600 °C (Figure 2b)12 most of the zigzag terminations are recombined into zz57, which is paramagnetic

Nowadays, thanks to the usefulness of Gr with unusual material features, Gr functionalization such as adsorption, intercalation, and doping toward device applications has attracted great attention. Since Gr properties and applications have already been well-discussed in the literature, we rather focus on how conceptual understanding of 2D chemistry of Gr and Gr analogs is related to various applications. We will discuss the following: (section 2) Graphene: Structural Properties of Defects, Edges, Nanoribbons, Metal Substrates, and Patterned Circuits; (section 3) Graphene-Based Applications: Photoluminescence, Electronics, Spintronics, Magnetics, Superconductors, and Protecting Materials; (section 4) Graphene-Based Green Chemistry and Energy Materials; (section 5) Graphene-Based Sensing and Medical Applications; and (section 6) Outlook and Future Challenges.

2. GRAPHENE: STRUCTURAL PROPERTIES OF DEFECTS, EDGES, NANORIBBONS, METAL SUBSTRATES, AND PATTERNED CIRCUITS 2.1. Gr Defects. As discussed in the Introduction, 2D materials due to their quasi-crystal nature are prone to having defects such as GBs and dislocations even within a small-sized region. Indeed, a Gr sheet has often unavoidable defects which significantly affect the electronic, physical, and chemical properties of Gr. In particular, CVD grown Gr which is most widely used due to large-scale synthesis shows substantial defects. Among all kind of defects which are formed due to multiple seeds and differential thermal expansion coefficients, the line defects often appear as they can form two separated grains. Wrinkles (standing collapsed wrinkle and folded 24394




Figure 3. DFT calculated band structures of Gr on metal (111) substrates (EF, 0 eV; black, C pz character; MAJ/MIN, majority-/minority-spin bands of Gr). Reprinted with permission from ref 19. Copyright 2009 American Physical Society.

Figure 4. AEPES intensity maps. (a) Gr/Cu/Ir(111) obtained in the vicinity of the K point (EF = 0). (b1−4) Annealing temperature-dependent π band of the Fe-adsorbed single-layer Gr (SLG): (b1) clean SLG, (b2) incompletely Fe-adsorbed SLG at 300 °C, (b3) completely Fe-adsorbed SLG at 600 °C, and (b4) annealing above 1050 °C. The Dirac point arising from the Fe intercalation is denoted by a red arrow. (c) Epitaxial Gr on Ir (Gr/ Ir). (d) Epitaxial Gr with Cs intercalated into Gr/Ir. Reprinted with permission: Panel a from ref 22. Copyright 2014 Nature Publishing Group. Panels b1−4 from ref 23. Copyright 2014 Royal Society of Chemistry. Panels c and d from ref 24. Copyright 2013 Nature Publishing Group.

spectra of Gr vary widely. Ni15 and Cu16,17 substrates have been the most widely used for the growth of Gr using CVD. Various other metal substrates, e.g., Pt, Ru, Co, Ir, Rh, Ag, Pd, and Au, have also been studied.18 The bindings between Gr and metal surfaces have been investigated using density functional theory (DFT) calculations19 in which nonlocal van der Waals interaction is found to be important. According to ab initio calculation, the intercalation of a Ni (3d8) atom having an unfilled d orbital between stacked benzene rings shows a strong binding energy, while that of a Cu (3d10) atom having filled d orbitals shows a very weak binding energy.20 The binding energies for noble metals of Cu(3d10), Ag(4d10), and Au(5d10) toward Gr are all weak. Ni, Pd, Ru, Rh, and Co surfaces are considered “strongly” interacting due to hybridization of metal d with C pz orbitals, while Cu, Au, Ag, Pt, and Ir as “weakly” interacting. DFT band structures of Gr on metal substrates (Figure 3) show that the bands of Gr on Au, Cu, Ag, and Al are slightly doped. However, for Gr on Ni, the Dirac bands are disrupted with mixed character of the C pz and Ni d bands with the spin dependency (the majority and minority bands are slightly different). Furthermore, because Ni is ferromagnetic, Gr on Ni(111) shows the effective magnetic moment on the Gr layer which can act as the spin filtering system. Also the large Rashba effect ∼0.23 eV induced by a large electric field at Gr/Ni(111) interface was reported in a magnetic linear dichroism study.18 Because the Ni surface strongly interacts with Gr, Gr tends to grow as a monolayer to a few layers on a Ni surface, while the

(Figure 2c). After the annealing process, the zigzag termination is not recovered because of its unstable energy. It has been shown that zigzag edge structures narrower than 7 nm exhibit an electronic band gap of about 0.2−0.3 eV (Figure 2d), which is the signature of interaction-induced spin ordering along their edges.13 Moreover, upon increasing the ribbon width, a semiconductor-to-metal transition is revealed, indicating the switching of the magnetic coupling between opposite ribbon edges from the antiferromagnetic to the ferromagnetic configuration. In this regard, GNRs utilizing unique properties of zigzag edges can be useful spintronic devices. Zigzag GNRs (zGNRs) have localized magnetic (or spin) edge states, either antiparallel or parallel. This makes zGNR a promising candidate as a basic block of spintronic devices that manipulate electron spin rather than charge. In the past, GNRs were made by using e-beam lithography, zipping CNT, and so on. However, recently, nearly defect-free bottomup chemical synthesis has become possible.8,14 2.3. Gr on Metal Substrates: Band Structure Modification. Though a single-layer Gr (SLG) can be exfoliated from graphite, it can be made only in scanty amounts. Furthermore, even though defect-free SLG is made within a small-sized segment, the SLG is highly susceptible to being damaged and deteriorated by environments. Thus, SLG needs a supporter, which is usually metal for practical large-scale synthesis. In general, depending on the metal substrate, the growth pattern (the number of layers, corrugation, and moirè pattern), electronic structure, magnetism, and lattice vibration 24395



ACS Applied Materials & Interfaces weak interaction between a Cu surface and Gr19 allows the formation of a SLG on the Cu surface. The Ni hexagonal (111) surface commensurates the Gr lattice with 1.2% mismatch, so Gr isotopically grows on a single-crystal Ni(111) surface. As discussed above, noble metals having filled d orbitals or sp valence electrons, such as Au, Cu, and Ag, interact very weakly with Gr, so the intercalation of noble metals between Gr and Ni substrate tends to restore the properties of pristine Gr. Angleresolved photoelectron spectroscopy (ARPES) and STM studies showed that the intercalation of Au preserves the Dirac point near the Fermi energy (EF) and its linear dispersion.21 Though noble metals decouple Gr from the Ni(111) surface, they fail to fully restore the Dirac point. With significant n-doping, quadratic dispersions of Cu- and Agintercalated systems result in substantial band gaps. In contrast to Ni, Ir interacts weakly with Gr. Thus, Gr in Gr/Ir shows well-preserved linear dispersion with energy gaps due to avoided crossings among the main π band and the replica bands which appear due to the additional periodicity of the moiré lattice.22 Since Gr in Gr/Ir is slightly p-doped, Gr is not involved in band hybridization with Ir. After intercalation of Cu in Gr/Ir [i.e., Gr/Cu/Ir(111)], Gr is n-doped with the Dirac point at 0.69 eV below EF, and the resulting hybridization between Gr π and Cu 3d bands opens a band gap, while the π band still has a linear dispersion. Figure 4a illustrates the ARPES intensity map of Gr on Cu/Ir (Gr/Cu/Ir) showing the energy gap of 0.36 eV that appears at the Dirac point.22 On the other hand, interaction of Gr with Fe is strong. Intercalation of magnetic Fe through bilayer Gr (BLG) formed on the SiC(0001) surface is found to induce a symmetry breaking in charge, spin, and interaction energies of two Gr layers.23 The physical origin of this symmetry breaking is due to the spin injection from Fe atoms into interfacial Gr. A diffusive and fuzzy nature of ARPES of the Gr/Fe/SiC(0001) system is ascribed to spin-split bands because of Coulomb and spin exchange interactions. Upon Fe intercalation and annealing above 300 °C, the well-defined linear π band from SLG (Figure 4b1) in ARPES shows the diffused Dirac point at ∼0.45 eV below EF due to n-doping by Fe (Figure 4b2). Fe intercalation is completed at 600 °C, and then, unlike two sharp split bands of BLG, much diffused two split bands appear with its Dirac point at 0.25 eV below EF arising from the p-doping effect (Figure 4b3). Above the annealing temperature 1050 °C, the unique core levels for both C 1s and Si 2p are restored before Fe adsorption (Figure 4b4). Owing to strong interaction of alkali metals with Gr, their intercalation in Gr/Ni(111) opens significant band gaps, [Na (1.3 eV), K (0.8 eV), and Cs (0.7 eV)]; see Figure 4d, as compared to Figure 4c which represents the electronic structure of epitaxial Gr on Ir (Gr/Ir).24 The ARPES study of Na intercalation between Gr and Ni(111) substrate shows that the electronic band structures near the K point have a band gap of 0.5 eV.25 The Na atoms break the chemical interaction of the Gr and the Ni substrate, changing the electronic structure of the Gr/Ni(111) system (Figure 5a) toward that of a quasi-free-standing Gr. According to DFT calculations, a simple adsorption of Na atoms on top of the Gr/Ni(111) system (Figure 5b) does not recover the Dirac-cone band structure of Gr with slight n-doping 0.21 eV. When the adsorbed Na atoms fully penetrate beneath the Gr layer (Figure 5c), Na intercalation recovers the linear dispersion with large ndoping ∼1.2 eV, indicating a large charge transfer from the Na/ Ni(111) surface to the Gr layer. While the Na atoms interact

Figure 5. Majority-spin band structures of (a) Gr/Ni(111), (b) Na(0.63 monolayer)/Gr/Ni(111), and (c) Gr/Na(0.63 monolayer)/ Ni(111). The filled-red/open-green circles represent C-/N-derived surface states that contain more than 20%/40% of charge in the Gr/ topmost-Ni layer. The vertical solid line in the surface Brillouin zone represents the calculated line. EF is set to 0, and the bands of ideal Gr are given as the reference by the thin solid purple lines. Reprinted with permission from ref 25. Copyright 2014 American Physical Society.

strongly with the Ni substrate, the Na−Na interaction is so weak that the Na-derived states are empty at approximately 3 eV above EF. 2.4. Patterned Gr Circuits. After Gr is grown using CVD, the metal substrate is etched and the suspended Gr is transferred onto polymer stamps for fabrication. An important issue in fabricating Gr-based electronic and optical devices is the necessity of producing intricate patterned structures for wide-ranging applications. This fabrication requires formation of clean boundaries around Gr, i.e., neat interfaces between Gr and the remaining non-Gr region. Thin epitaxial graphite grown on silicon carbide by vacuum graphitization can be patterned with nanolithography methods.26 However, device fabrication is not easy by just using lithographic techniques and a wild etching process. For example, Gr and boron nitride lateral heterojunctions were exploited for circuitry (Figure 6a).27 However, they are not useful for tailored patterning of Gr. Thus, a new technique for one-pot synthesis of Gr/ amorphous-carbon (a-C) heterostructures has been developed from a solid source of polystyrene via selective photo-crosslinking process (Figure 6b). Gr was grown from neat polystyrene regions, while the patterned cross-linked polystyrene regions turn into a-C because of a large difference in their thermal stability.28 CVD growth of the Gr/a-C heterostructure using cross-linking-driven chemical patterning of a solid carbon source of polystyrene has been achieved. Flexible devices fabricated from the Gr/a-C heterostructure show high mobility and excellent mechanical stability against bending as well as the quantum Hall effect in Gr/a-C lateral heterostructures, which confirmed the well-defined Gr/a-C interface.

3. GRAPHENE-BASED PHYSICAL APPLICATIONS: PHOTOLUMINESCENCE, ELECTRONICS, SPINTRONICS, MAGETICS, SUPERCONDUCTORS, AND PROTECTING MATERIALS 3.1. Photoluminescence of Gr on Cu Substrate. Gr has diverse unique electrical and thermal properties mainly because of twodimensionality, light C mass, and metallicity. Recent studies realized that those interesting properties are altered depending on the substrate with which Gr interacts. Nevertheless, Gr has not been considered as a useful optical material because of its zero band gap. Since excited 24396




Figure 6. (a) Raman Gr 2D band showing a stark contrast between two regions of Gr (lighter areas)/h-BN (darker areas) on a Si/SiO2 substrate. (b) Illustration of growth mechanism for Gr/amorphous-carbon (a-C) heterostructures. (c) Raman maps of Gr/a-C line pattern based on the Raman frequency of ωG. Reprinted with permission: Panel a from ref 27. Copyright 2012 Nature Publishing Group. Panels b and c from ref 28. Copyright 2015 American Chemical Society. electron and hole pairs are easily screened by free electrons in metals, the luminescence efficiency of metals including Gr is very low in noble metals such that only band-to-band transitions are significant. Indeed, no direct experimental evidence was reported except some signatures or for a very short time scale ∼30 fs transient state.29 Though the photoluminescence (PL) of Gr quantum dots and Gr oxide (GO) exhibits a broad luminescence, the excitonic luminescence was not observed. However, recently, the excitonic PL of Gr on a Cu(111) was observed, where Cu interacts with Gr, creating large van Hove singularities (vHs).30 Micro-PL spectrum of a Gr sheet synthesized on a Cu(111) surface (Figure 7) shows a strong and sharp PL peak near

tin-oxide (ITO), pristine Gr has higher resistance and lower work function which highly limit the performance of the devices. Therefore, a p-type doping of Gr is required to reduce the operation voltage and increase the hole injection from the anode to the organic layers in the device. As a useful method for the p-doping of Gr, chloroform can be used because it is spontaneously intercalated at the interface between the Gr/SiO2 interface.31 The intercalation of chloroform occurs spontaneously. The structure of chloroform-intercalated Gr is energetically more stable than that of the unintercalated Gr. The desorption energies depend on the position of the intercalation site, specifically whether the intercalation site is beneath the basal planes or inside the wrinkles of the Gr. The advantages of this doping method lie not only in the sustainable process but also in the fact that this simple, strong, large-scale, and defect-free process does not sacrifice transmittance. Here, the number of processing steps is reduced, because the p-doping occurs simultaneously with the removal of the supporting layer on the Gr. The band gap opening of pristine Gr is vital for its application in fabricating logical devices of semiconductors in the digital device industry. An intriguing technique to make a reasonably wide band gap in BLG is dual-doping with FeCl3 acceptor and K donor.32 This method properly controls the Dirac point shift for BLG FET application. A transport measurement was reported for BLG with a magnetic intercalant of FeCl3.33 Each FeCl3 layer behaves as an electron acceptor in the intercalated compound; thus the Gr layers will be hole-doped, where the doping amount would vary depending on the intercalation stage. By putting the elements of lower electron affinity than carbon, for instance potassium atoms, at the opposite side of the BLG, the resulting electron doping will suppress the shift of the Dirac point originally appearing in the FeCl3-adsorbed BLG (aBLG) system (Figure 8a). The effect of dual-doping on the electrical properties of BLG FETs is investigated.34 The doping process consists of both n-doping by aminopropyltriethoxysilane-modified SiO2/Si substrate and p-doping by evaporating 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). Figure 8b shows the effects of top/bottom doping on the electrical properties of SLG and BLG FETs, respectively. The subsequent deposition of F4-TCNQ onto BLG leads to a huge increase in the off-resistance (12.2 kΩ), which implies that p-doping from the top effectively breaks the inversion symmetry of BLG, thus leading to band gap opening. The aforementioned findings show that SLG and BLG sheets as well as other well-studied Gr/semiconductor multilayers or intercalation techniques could be utilized to construct the fundamental blocks of logic devices that are diodes31,35 and transistors.36−38 For example, in order to utilize Gr to construct OLEDs, the lower work function of Gr with respect to the traditional transparent electrode material such as ITO can be increased by using a chemical p-type dopant trifluoromethanesulfonic acid (CF3SO3H, TFMS) on Gr.39 Dissolving spin-coated TFMS in a nitromethane can p-dope the Gr surface, which highly reduces the sheet resistance (∼70% decrease), increases the work function (0.83 eV increase), and shows a great air stability. Within this TFMS-doped four-layered Gr anode, a green

Figure 7. Micro-PL spectrum of Gr grown on a Cu (111) surface measured at 4.2 K. The inset depicts temperature-dependent micro-PL spectra of Gr. Reprinted with permission from ref 30. Copyright 2017 American Chemical Society. 3.161 eV (the lowest energy emission peak, denoted as P) and many peaks around 3.18 eV (denoted as MP). The DFT calculations showed that the Dirac band of Gr is slightly n-doped with a finite band gap of 13.5 meV. Although the interfacial interaction of Gr and Cu slightly disturbs the Dirac band near EF, the interaction becomes strong at 1.5 eV below (Gr π and Cu d orbitals) and above (Gr π* and Cu sp orbitals) EF near the M point of the Brillouin zone. This strong orbital hybridization below and above EF produces a new vHs with the flattened band near the midpoint of M−K, giving rise to an enhanced DOS. Additionally, according to the temperature-dependent band gap and emission line width broadening of Cu(111)−Gr, the exciton− phonon interaction is dominant for out-of-plane acoustic mode over optical modes.30 3.2. Gr Electronics: Semicondctors, Field Effect Transistors, and Organic Light Emitting Diodes. Since Gr shows high transmittance and high flexibility, Gr-based flexible electronic devices, such as Gr-based organic light emitting diodes (OLEDs), field effect transistors (FETs), and organic optoelectronics, have been developed. Compared to the traditional transparent electrode material indium− 24397




Figure 8. (a) Schematic illustrations of bilayer Gr (BLG) doped with FeCl3 and K and dual-doped by FeCl3 and K. The onsite potential difference (Δ) and Dirac point shifts (δεD) obtained by DFT calculation are represented. (b) Schematic dual-doping process for BLG consisting of both ndoping by (aminopropyl)triethoxysilane-modified SiO2/Si substrate (bottom) and p-doping by evaporating 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4-TCNQ) (top). (c) Chemical structures of NH2-functionalized self-assembled monolayers (SAM) on a SiO2/Si substrate (bottom) and F4-TCNQ (top). (d) Electronic band structures of BLG on an untreated, NH2−SAM-modified SiO2/Si substrate and F4TCNQ deposited BLG on a NH2−SAM-modified SiO2/Si substrate (from left to right). Reprinted with permission: Panel a from ref 32. Copyright 2011 American Chemical Society. Panels b−d from ref 34. Copyright 2012 John Wiley & Sons, Inc.

Figure 9. (a) Schematic illustration and calculated spin-magnetization density isosurfaces for the zGNR under antiparallel magnetic fields. (b) Spin structure in a noncollinear magnetic domain wall obtained from a first-principles calculation for the 8-zGNR. (c) Spin-polarized I−V curves in the antiparallel configuration for the 32-zGNR. (d) 1D porphyrin array M−PAn (metal porphyrin array nanoribbon); Cr_PA3; nanodevice Au-{Cr-PAn}Au in which Cr-PAn is linked to two Au(111) electrodes via Au−S bond; and band structures of H-PAnS and Cr-PAn. (e) Geometric and band structures of 2D 2,4,6-tri-(1,3,5-triazinyl)methyl radical polymer (C4N3; C, gray; N, blue). Reprinted with permission: Panels a−c from ref 40. Copyright 2008. Nature Publishing Group. Panel d from ref 43. Copyright 2011 American Chemical Society. Panel e from ref 44. Copyright 2010 Wiley-VCH. phosphorescent OLEDs has been fabricated, with a lower operating voltage and higher efficiency (104.1 cd/A; 80.7 lm/W) than ITO anodes. Therefore, molecular intercalation or surface doping of Gr also provides a superior method for flexible electronic devices fabrication to the conventional materials. 3.3. Gr Nanoribbon Spintronics and Half-Metallic Gr Analogs. Spintronic devices are very important for futuristic information technology. Suitable materials for such devices should have a half-metallic property so that only one type of spin passes

through the device. In half-metals which have the metallic nature for one electron’s spin and the non-metallic nature for the other, the current can be completely spin-polarized. There have been some reports on 1D and 2D half-metals. One of them is GNR under strong electric field or magnetic field. The advantage of using carbon materials as spintronic devices is that the spin relaxation length and spin lifetime are considerably long because of weak spin−orbit and hyperfine interactions. In addition, organic materials are usually inexpensive light materials and would be candidates for flexible devices. 24398




Figure 10. (a) Structure, fabrication, and transport properties of a ferric chloride (FeCl3)n monolayer inside BLG. (b) Schematic view of crystal structure of C6CaC6 on SiC. Reprinted with permission: Panel a from ref 33. Copyright 2011 American Chemical Society. Panel b from ref 48. Copyright 2016 American Chemical Society. First-principles simulations showed that spin-valve devices based on GNRs exhibit super-magnetoresistance (>106% at room temperature) and generate highly spin-polarized currents (Figure 9a−c),40 far more effective than the full-sized Gr-based spin-valve device using the epitaxial Gr grown on the Ni (111) surface which showed a magnetoresistance of ∼110%.41 Meanwhile, electric field controlled spin filtering is also interesting because of much easier precise control with much less energy consumption than the magnetically controlled one. The magnetic/nonmagnetic behaviors of zGNR under an external transverse electric field were thus studied, while the edges of zGNR were functionalized by appropriate functional groups.42 The threshold electric field to attain either a half-metallic or nonmagnetic feature is drastically reduced by introducing proper functional groups to the edges of zGNR. From the current−voltage characteristics of the edgemodified zGNR under an in-plane transverse electric field, a remarkable perfect spin filtering feature was predicted. Alteration of magnetic properties by tuning the transverse electric field would be a promising way to construct magnetic/nonmagnetic switches. Carbon-based organic half-metals are also useful for spintronics. One-dimensional chromium porphyrin array (PA) is shown to be halfmetallic in the presence of magnetic field (Figure 9d). A spintronic device based on Cr−PAn is a synthesizable framework.43 As yet, there is no experimental report of 2D pure organic half-metals in normal conditions. 2,4,6-Tri(1,3,5-triazinyl)methyl (C4N3) radical polymer (Figure 9e) is a 2D organic half-metal as a good candidate for a perfect spin filter with a large magnetoresistance.44 3.4. Magnetic and Superconducting Metal-Intercalated Gr Layers. Intercalation of inorganic and organic molecules into graphite has been an intensely focused research topic in the past several decades. This unique intercalation between layered crystalline materials through van der Waals interactions alters the host material significantly, providing new chemical and physical functionality. Various exotic phenomena such as superconductivity and quasi-low dimensional magnetism, have been studied in bulk form of graphite intercalates. In addition, understanding the detailed mechanism of the alkali metal ions intercalation process in the graphite host becomes a vital issue in ion-battery applications, where graphitic carbon is used as an electrode. A periodic arrangement of an alternating sequence of intercalant and Gr layers can exhibit a variety of exotic electronic properties ranging from magnetism to superconductivity, which can lead to a new avenue to create few-layer Gr intercalates with novel functionality. Indeed, such high-quality Gr layers intercalated with magnetic molecules were synthesized and characterized. As an example, a ferric chloride (FeCl3)n island monolayer inside BLG was synthesized, and its transport properties were reported (Figure 10a).33 Shubnikov de Haas (SdH) quantum oscillations in the magnetoresistances originate from microscopic domains of intercalated and unintercalated regions.

A slight upturn in resistance related to magnetic transition was observed. Alkali and alkali-earth metal graphite intercalation compounds are known to be superconducting with Tc ∼ 1 K, showing an anisotropic type II superconducting state.45 CaC6 shows the highest transition temperature among them, with Tc = 11.5 K.46 One can expect that the small units of the graphite intercalation compound including intercalated BLG are also superconducting. Theoretical study emphasized the importance of noninteger electron donation per metal atom to Gr layer in strong electron−phonon coupling. Li-doped Gr was predicted to have relatively high Tc which was proven by experiment, with Tc ∼ 5.9 K.47 Other doped Gr systems, such as Cadoped or K-doped BLG, were also proven to be superconducting.48 For example, the electrical transport measurements with the in situ four-point probe method in ultrahigh vacuum under zero or nonzero magnetic field for C6CaC6 fabricated on SiC substrate (Figure 10b) reveals that the zero-resistance state occurs in C6CaC6 with the onset temperature (Tconset) of 4 K, while the Tconset gradually decreases upon applying the magnetic field. This directly proves the superconductivity origin of the zero resistance in C6CaC6. 3.5. Gr To Protect Surface Electronic States of Topological Insulators. A newly devised technical method is to preserve the intrinsic surface states by growing thin topological insulator (TI) films on a prepatterned Gr device. Preserving the unique surface states of TI was one of the unresolved issues to utilize TI materials in device applications where their intrinsic surface states play a key role. Transport studies are demanded for such applications to ensure the presence of surface states not hampered by the dominant bulk conduction, and well-protected from any contamination in ambient atmosphere. The surface states of TI are highly vulnerable to external perturbations by chemical environment such as interfacial contact with other materials or molecules. Thus, a TI film is fabricated in ultrahigh vacuum (UHV) on a prepatterned Gr device. Since the inert nature of Gr gives no significant lattice mismatching issue with Bi2Se3, a thin film of Bi2Se3 was epitaxially grown on prepatterned Gr. The TI−Gr interface, not deformed by any interfacial strain effect even at the early stage of growth, has the well-preserved surface states with the nearly negligible interfacial interaction between TI and Gr substrate. The transport data from the device formed on the heterointerface reveal the existence of topological surface states in a highly n-doped TI system. The surface states stemming from the buried interface between TI and Gr remain intact, easily detectable despite the bulk conduction, and are well-protected from environmental contamination. It provides an efficient and practical means to utilize surface states of TI in device applications.49 The enhanced surface states were detected despite the bulk states in measurements of the Hall effect and SdH oscillations. The study provides a step forward to amplify the topological surface states by tuning the interface between TI and Gr into a measurable current for device applications. Since this has been made possible with 24399




Figure 11. (a) Molecular interactions and synthetic mechanism for Ptn/DNA/rGO and PtD/DNA/rGO (Ptn, Pt nanoparticles; PtD, Pt nanodendrites). (b) Illustrations depicting the structure of the rGO-Mg nanolaminates where rGO layers prevent O2 and H2O from penetrating, while allowing the diffusion of H2. Reprinted with permission: Panel a from ref 55. Copyright 2013 Nature Publishing Group. Panel b from ref 58. Copyright 2016 Nature Publishing Group. thin TI film formed on Gr prepatterned on any arbitrary insulating substrates, one of the pending technological huddles in designing TIbased microelectronic devices has been securely removed.

attractive forces (electrostatic, dispersive, and inductive interactions) and repulsive forces (exchange repulsion).54 Since DNAs have π moiety of nucleobases, these can be utilized for the interactions with Gr, which can be utilized for the stabilization of metals on Gr. Thus, such π systems are relevant to nanomaterial design and nanodevice fabrication, because subtle changes in the electronic characteristics of the π systems can lead to dramatic effects in structure and properties of nanosystems. Gr can form hybrid materials with Pt and DNA through π−π interactions (between nucleobase and Gr), metal cation−π intercations (between Pt cation and nucleobase or Gr),54 and metal cation−oxygen (anion) interactions (between Pt cation and DNA phosphate group or O group in rGO). The interactions between DNA and GO may provide an efficient approach for positioning Pt ions to form the Pt nanoparticles on the surface of rGO. Pt nanoparticles formed are well-surrounded by Gr and DNA. The hybrid materials are not chemically modified compounds but simply hybrid materials. Pt clusters are actually noncovalently surrounded by Gr and DNA. In this way, Pt clusters are stable without being washed out and play a much better catalytic role due to large surface area arising from ultrafine ∼1 nm-sized nanoparticles (Ptn) or nanodendrimers (PtDs) (Figure 11a). Thus, these hybrid materials are useful for electrochemical cells such as fuel cells.55,56 Gr with metal sulfides clusters is also exploited as electrocatalysts such as hydrogen evolution reaction (HER).57 The Ptn/DNA/rGO composite exhibits larger increases in oxygen reduction reaction (ORR) onset potential, ORR half-wave potential, specific activity, and mass activity for the ORR than Ptn/rGO and commercial 20 wt % Pt/C catalysts.55 The composite displays an excellent accelerated durability test and a long-term CV stability in acidic media and also exhibits environmental stability and durability over a wide pH range. The good corrosion resistance and high conductivity further account for the observed enhanced performance. In contrast, when Pt4+ ions are used in the synthetic procedure, a nanodendritic structure is formed. This nanodendritic structure, despite that the particle sizes are not so small as those in the Pt2+ case, drastically increases the ORR catalytic activity (much higher than the U.S. Department of Energy (DOE) target value) due to the multiple active facets. As compared to the Pt particles/rGO and 20 wt % Pt/C catalysts (fuel cell grade), the PtDs/DNA/rGO hybrid at room temperature maintains a high ORR onset potential (1.01 V), ORR half-wave potential (0.9 V), specific activity (0.60 mA/cm2), mass activity (0.40 mA/μgPt), high durability, and high CV stability.56 Furthermore, the mass activity of the PtDs/DNA/rGO at 80 °C (1.01 mA/μgPt at 0.9 V vs RHE) is found to be more than twice the target value for ORR electrocatalysts (0.44 mA/μgPt at 0.9 V vs RHE at 80 °C) set by DOE. The PtDs/DNA/rGO hybrid exhibits highly stable mass activity in various pH solutions (pH = 1−13), which can be applied for fuel cells, supercapacitors, secondary metal−air batteries, and water electrolysis. Interest in hydrogen fuel, as an instance, is growing for automotive applications; however, safe, dense, solid state hydrogen storage remains a scientific challenge. An environmentally stable and

4. GRAPHENE-BASED GREEN CHEMISTRY AND ENERGY MATERIALS Noncovalent functionalization of Gr with metals can exploit the characteristics of metal clusters. Metal nanoparticles tend to aggregate. In order to have large active surface areas of metals which are critical for catalytic reactions, the aggregation between metal nanoparticles should be prohibited. This is possible by protecting metal nanoparticles from their self-aggregation by using a Gr supporter. Gr can surround nanoparticles with the contacting interfaces involved in charge-transfer-driven Coulomb interactions and dispersion-driven van der Waals interactions so that the aggregation between neighboring nanoparticles is hindered. Here, we show two examples: (1) metal oxide nanoparticles and (2) metal nanoparticles with Gr. 4.1. Gr−Metal Oxide Hybrids. As metal oxides are dispersed on reduced GO (rGO), the size of metal oxide nanoparticles can be nanometer scale without aggregation, showing much stability under reduction/oxidation electrochemical environments. Then, their catalytic activity and durability can drastically increase as compared with their aggregated materials. Thus, highly electrochemically active Gr hybrid materials with SnO2, MnO2, Mn3O4, Co3O4, and Fe3O4, etc., have been exploited for electrochemical energy storage such as lithiumion batteries and supercapacitors.50 Magnetite-rGO (M-rGO) composites are also applied for green chemistry and medical imaging. As an example, arsenic is one of the most toxic chemical elements which is widely present in industrial wastes as well as in wide areas in the world. To get rid of arsenic from water, the M-rGO composites were synthesized via chemical reduction with magnetite particle size of ∼10 nm.51 Owing to this small size, each magnetic nanoparticle is in a single magnetic domain, and the magnetization can randomly flip direction due to the temperature effect. In the absence of an external magnetic field, the magnetization appears to be zero on average, because the typical time between magnetic flips (Néel relaxation time) is shorter than the magnetism measurement time. Thus, the composites are super-paramagnetic at room temperature. Since M-rGO composites are highly waterdispersible and show high binding capacity for As(III) and As(V) because of increased surface area of magnetities, the composites to which As(III)/As(V) is bound can be easily separated by an external magnetic field. Thus, the composites demonstrate near complete arsenic removal within 1 ppb, which would be practical for arsenic separation from flowing water.51,52 This material also shows clear NMR imaging for medical applications because of the superparamagnetism phenomena of ∼10 nm-sized magnetite nanoparticles.53 4.2. Gr−Metal Hybrids and Gr−DNA−Pt Hybrids for Energy Storage and Energy Conversion. The π−π interactions are of importance in device and sensing applications of Gr sheets. The strength of the π interactions is determined by the combined effect of 24400




Figure 12. (a) Schematic representation of hydrazine sensing mechanism at Gr nanobelt/glassy carbon electrode. (b) Schematic illustration of the novel biosensor platform based on changes in fluorescence resonance energy transfer (FRET) between Gr quantum dots and pyrene-functionalized molecular beacon probes for detection of miRNAs. Reprinted with permission: Panel a from ref 63. Copyright 2016 The Royal Society of Chemistry. Panel b from ref 66. Copyright 2015 American Chemical Society.

Figure 13. (a) Schematic of a nanochannel device with an armchair GNR through which a single-stranded DNA passes. (b) Δg in terms of E − EF and Vb for DNA bases and methylated DNA bases physisorbed on GNR. (c) Schematic of a typical Gr nanopore device layout. (d) Double-stranded DNA current blockades are larger for Gr nanopores (blue) than for SiNx pores (red) due to their thin membranes. The largest blockade signals were measured with the smallest pores of ∼3 nm. I0 is the open pore current. (e) Schematic view of a metallic zigzag Gr nanoribbon with a nanopore. (f) Four different bases yielding very different current modulations. Variations in base rotation result in a spread of the conductance modulations. Shaded areas mark the regions of overlap. Reprinted with permission: Panel a from ref 72. Copyright 2011 Nature Publishing Group. Panel b from ref 73. Copyright 2014 American Chemical Society. Panel c from ref 74. Copyright 2010 Nature Publishing Group. Panel d from ref 75. Copyright 2013 National Academy of Sciences USA. Panels e and f from ref 76. Copyright 2012 American Chemical Society. unsurpassed hydrogen storage material comprised of Mg nanocrystals encapsulated by atomically thin and gas-selective rGO sheets was reported by Urban and co-workers (Figure 11b).58 They showed that rGO sheets function as a protective layer against Mg nanocrystal oxidation by preventing the permeation of O2 and H2O, while still allowing hydrogen to easily penetrate, diffuse along the layers, and be released.

its chemical environment. Gr has inherently low electrical noise due to the quality of its crystal lattice and its very high electrical conductivity. These properties make Gr an ideal candidate for the Gr-based electrochemical sensors, electronic sensors, optical sensors, and nanopore sensors for biological or chemical detection.59 Novel nanostructures of Gr including nanobelts,60 nanoflakes, nanoribbons, and nanodots are emerging as interesting alternatives to conventional Gr.61 Gr nanobelts were synthesized from natural graphite. It has been shown that an electrochemical sensor based on Gr nanobelts is applicable for the detection of dopamine,62 hydrazine,63 and bisphenol.64 The schematic of Gr-nanobelts-based electrochemical hydrazine sensor used in a sensing mechanism is shown in Figure 12a. Previously, Gr and GO were used to sense RNA as a molecular beacon.65 In a recent study66 based on Gr quantum

5. GRAPHENE-BASED SENSING AND MEDICAL APPLICATIONS 5.1. Gr-Based Electrochemical Sensing. Owing to its extraordinary electrical, chemical, optical, mechanical, and structural properties, such as high electron mobility at room temperature, ballistic electron transport, and large surface area per unit volume, Gr can be used for sensing devices which are highly sensitive to adsorbed molecular species and changes in 24401




Figure 14. (a) Fabrication procedure for transparent Gr electrode array. (b) Assembly after release of the device and etching of a SiO2 protection layer. (c) Comparison between transparent neural electrodes and opaque neural electrodes. The Gr-based transparent electrodes can utilize seethrough imaging. (d, e) Optical coherence tomography (OCT) image captured through the implanted transparent Gr electrode array and opaque platinum electrode array, respectively. Reprinted with permission from ref 77. Copyright 2016 Nature Publishing Group.

the feasibility of DNA sequencing using a fluidic nanochannel functionalized with a GNR was demonstrated72 in which GNR forms a bridge across the nanochannel between opposing tip electrodes (Figure 13a). It is shown that transmission vs electron channel energies (E − EF) at varying bias voltage (Vb) distinguishes different molecules by their molecular fingerprints.73 Thus, once the relative differential conductance (Δg with respect to the pristine-GNR, where the differential conductance is g = dI/dVb) is measured at varying Vb and E − EF, the distinction between different molecules becomes clear. Figure 13b illustrates the application of the aforementioned 2D molecular electronic spectroscopy (2D MES) method to distinguish four different types of nucleobases, adenine (A), cytosine (C), guanine (G), and thymine (T), as well as methylated nucleobases, 5-methylcytosine (5mC), and 5-hydroxymethylcytosine (5hmC). It shows clearly that the variation of Δg is different for each nucleobase as well as the methylated forms. Another interesting Gr-based sequencing device is Gr nanopore. As illustrated in Figure 13c−f, there are two main techniques to utilize such a device to sequence DNA strand via measuring either the ionic current or the electronic conductance. In the first method, when a voltage is applied across the membrane, an ionic current is induced through the pore.74 Since DNA is strongly negatively charged, it can be driven in a head-to-tail fashion through the nanopore by an electric field. While a molecule translocates, it excludes ions from the pore volume, resulting in a temporal decrease in ionic current (Figure 13d). The magnitude and duration of the current blockade provide information on the diameter and length of the molecule, respectively.75 The second method employs monitoring of the current through a narrow Gr nanostructure that contains a nanopore through which a DNA molecule translocates.76 The nanoribbon current was found to be modulated due to electrostatic interactions between nucleotides and Gr pore, causing a change in local density of

dots and pyrene-functionalized molecular beacon probes (pyMBs), a novel biosensor platform was developed for detection of micro-RNAs (Figure 12b). Many different preparation methods for Gr are known so far, but only a few of them are applicable in terms of sensor preparation. rGO obtained by the reduction of GO is inexpensive and easily scalable. Due to a more defective structure, its electrical properties are not so outstanding as those of pristine Gr, but are still suitable for sensitive gas detection.67 rGO with a more defective structure often shows an improved adsorption of gas molecules.68 Furthermore, rGO can be dispersed in solutions, simplifying the transfer to a sensor setup by spraying, printing, and casting methods.69 5.2. Gr-Based DNA Sequencing. Gr’s unique 2D characteristics and flexible nature as well as its ultrasensitive surface to molecular absorption, make it a promising candidate for many possible applications in the fields of single-molecule detection, biotechnology, and medicine, such as disease/tumor detection, drug delivery, DNA sequencing, and more. In recent years, different approaches have been proposed to employ Gr nanodevices for DNA sequencing and molecular sensing using nanochannels, nanopores, and nanogaps and also to employ the physisorption of DNA on Gr nanostructures.70 In the case in which a nucleobase is stacked onto Gr, the π interactions between the nucleobase and Gr play a crucial role in selfassembling and nanorecognition. The binding energy (∼20 kcal/mol per each base)71 is strong enough to hold nucleobases on Gr and reduce noises in measurement and weak enough to translocate a single-stranded DNA (ssDNA) over GNR in a nanochannel. It is well-known that Gr offers ballistic conductance for charge carriers and a narrow GNR provides a few channels for electron transport. Meanwhile, defect-free narrow GNRs have been synthesized via chemical synthesis techniques.8,14 Therefore, GNR would be a promising tool to design and fabricate ultrasensitive sensing devices. Employing these characteristics, 24402




Figure 15. (a) C3N4 sheets, two-photon fluorescence spectra for the C3N4 sheets dispersed in water for red-light excitation. (b) Comparison of phosphate-buffered saline (PBS) and C3N4 sheets for the enhancement in repairing cranial bone defect under red light in vivo: Analysis of bone regeneration in critical size cranial defects under red light and 3D μ-CT images after 3 days or 4 weeks of PBS and C3N4 sheet-assisted treatments. Regenerated bone is indicated with a yellow color. Each figure is a representative example among three different experiments (n = 3). Reprinted with permission from ref 80. Copyright 2017 American Chemical Society.

devices have been widely used. However, photonics based on multiphotons exploiting nonlinear optics, which has been used for medical imaging, has hardly been utilized for medical treatment. There is a common knowledge that infrared light penetrates deep into skin, enhances the blood flow due to nitric oxide release, and promotes tissue healing. On the other hand, ultraviolet light is known to be harmful by causing skin cancer. Therefore, two-photon absorption has not been considered as a possible medical treatment. Nevertheless, a remarkable effectiveness on two-photon-steered bone regeneration has been reported.80 Carbon nitride (C3N4) sheets through sonication of bulk C3N4 exhibit unprecedented cell proliferation rates and osteogenic differentiation through two-photon excitation due to ∼12-fold increase in alkaline phosphatase (ALP) activity, 5fold increase in phosphorylated runt-related transcription factor 2 (p-Runx2), and 13−21-fold increase in mRNA gene expressions of bone sialoprotein (BSP), ALP, and osteocalcin (OCN). As a result, highly effective human bone marrowderived mesenchymal stem cells (hBMSCs) driven mice bone regeneration is achieved (91% recovery over 36% recovery in normal condition after 4 weeks). This fast bone regeneration, shown in Figure 15, is attributed to both red-light absorption (driving deep penetration into tissues) and two-photoninduced blue light stimulation driven photocurrent enhancement (driving cell stimulation). Two-photon C3N4 sheets enhance hBMSCs differentiation to form new bone. This demonstrates the therapeutic potential of irradiated C3N4 sheets in bone regeneration and fracture healing. This work utilizing two-photon systems is clearly distinctive as it shows remarkable advantages over the normal-light systems. This affirms the potential of C3N4 sheets in cells for developing bone formation and directing hBMSCs toward bone regeneration. This new and efficient two-photon-assisted material approach could open a wide range of opportunities toward the development of hBMSC-driven fracture repair treatments.

states in the Gr near the pore. Base specificity (that is, different nanoribbon current when a different base is inserted in the pore) is attributed to different coupling strengths of nucleobases with GNR (Figure 13f). 5.3. Gr-Based Transparent Neural Electrode Array. Transparent neural electrode arrays have the potential to provide an optimal platform for various applications, including optogenetics and neural imaging. Transparent neural electrodes allow simultaneous observation of cells immediately beneath electrode sites during optical or electrical stimulation. Gr has a UV-to-IR transparency of over 90%, in addition to its high electrical and thermal conductivity, flexibility, and biocompatibility that make it a promising candidate for transparent neural electrodes. Recently It has been described how to fabricate and implant a Gr-based microelectrocorticography (μECoG) electrode array and subsequently use this alongside electrophysiology, fluorescence microscopy, optical coherence tomography (OCT), and optogenetics.77 The Gr-based broadwavelength transparent neural electrode array provides fullfield optogenetic stimulation on any optically accessible tissue that maximizes the intensity of light stimulus delivered to the brain. Panels a and b of Figure 14 illustrate the procedure of fabricating a transparent Gr electrode array. In addition to optogenetics, transparent neural electrode arrays are capable of monitoring biological changes such as immune responses (e.g., microglial activation) and the development of collagen or other scar tissue directly underlying the electrode surface via in vivo imaging, and possibly even using these observations to correlate with electrical impedance measurements. Furthermore, because of the increased light delivery efficiency, the transparent electrode array will require less electrical power for the light source to achieve an optical stimulus effect similar to that achieved by the metal electrode array. Reduced power would be beneficial to minimize the heat dissipation from the light source, which could damage underlying tissue. The capabilities of the transparent neural electrode are contrasted with those of the opaque metal-based electrode in Figure 14c−e. 5.4. Gr Analogs for Bone Regeneration. Gr analogs have been exploited for biosensing78 and biomedical applications such as drug delivery and tissue engineering.79 Here we particularly present an example of the use of Gr analogs for bone regeneration. For medical treatment, optical and photonic

6. OUTLOOK AND FUTURE CHALLENGES It is of importance to understand how Gr functionalization (including metal decoration, metal substrates, intercalation, doping, and hybridization) modify the unique 2D features of 24403



ACS Applied Materials & Interfaces Author Contributions

Gr. In this way, the electronic and physical properties of Gr can be controlled toward the given purpose such as highly effective novel electronic device applications. This understanding would also help to control the electronic properties of other 2D materials such as BN, MoS2, MoSe2, and Bi2Se3, etc. It is also important to study the chemical, electrochemical, and phtochemical properties of Gr by functionalization, in particular, as hybrid materials, for the development of energy storage and conversion materials such as batteries, supercapacitors, fuel cells, solar cells, display panels, molecular/ biomolecular sensing, and medical treatments. In this way, the functionalized hybrid materials can be widely utilized for green chemistry, electrochemistry, and optoelectronic devices. For practical Gr device fabrication, exquisite circuit boards patterning is required. In situ TEM and STM setups are able to monitor atomic structures and measure the properties simultaneously. Patterning Gr by chemical methods is also highly useful due to its quick and easy technology. The patterndriven Gr circuitries could be further developed in ultrafine approach for more compact device fabrication. The bottom-up chemical synthesis approach for defect-free circuitries would be a very sophisticated approach in the future. While Gr is highly useful for its 2D properties, the Gr edge structures show unique 1D properties. Thus, the precise atomic edge manipulation of Gr would provide edge structure− property relationships, which would lead to the tailored Gr nanodevices showing specific electrical and thermal transport properties. This tailored circuits patterning could be possible with both in situ AC-TEM/STM measurement setups and bottom-up chemical synthesis circuitries. Specifically designed Gr circuits could lead to the development of practical molecular sensing, fingerprinting, and DNA sequencing based on 2D molecular electronics spectroscopy. Owing to the highly stable nature, Gr can be utilized for protection of the surface of environment-susceptible materials including TIs and perovskite solar cells. Gr analogs can be further developed to expand the use of novel 2D features. Such materials include efficient half-metals, nonlinear optical materials, and TIs, etc. The stacked Gr and Gr analog materials are another direction for the extended use of Gr. For example, C3N4-rGO hybrid can be used as metal-free electrochemical catalysts for ORR.81 Gr− TI heterostructures can be exploited as highly sensitive photodetectors.82 Gr−boron nitride stack heterojunction shows tunable diodes.83 Fabrication of Gr−boron nitride stack on Co/MgO electrodes shows extremely long spin lifetime useful for spin-valve devices.84 Gr-superconductor hybrid devices exhibit ballistic Josephson junctions in edgecontacted Gr.85 All the aforementioned exotic characteristics of Gr, patterned Gr, and Gr analogs will be further utilized in diverse future high-tech research fields including IT−BT fused science and technology as well as industrial applications and medical applications including brain information communications.

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†

M.R.R. and C.W.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (National Honor Scientist Program, Grant 2010-0020414).

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.S.K). *E-mail: [email protected] (Y.P). ORCID

Chang Woo Myung: 0000-0002-9480-6982 Kwang S. Kim: 0000-0002-6929-5359 24404



ACS Applied Materials & Interfaces (17) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ö zyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Roll-to-Roll Production of 30-in. Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574−578. (18) Dahal, A.; Batzill, M. Graphene-Nickel Interfaces: A Review. Nanoscale 2014, 6, 2548−2562. (19) Khomyakov, P. A.; Giovannetti, G.; Rusu, P. C.; Brocks, G.; van den Brink, J.; Kelly, P. J. First-Principles Study of the Interaction and Charge Transfer between Graphene and Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 195425. (20) Youn, I. S.; Kim, D. Y.; Singh, N. J.; Park, S. W.; Youn, J.; Kim, K. S. Intercalation of Transition Metals into Stacked Benzene Rings: A Model Study of the Intercalation of Transition Metals into Bilayered Graphene. J. Chem. Theory Comput. 2012, 8, 99−105. (21) Varykhalov, A.; Scholz, M. R.; Kim, T. K.; Rader, O. Effect of Noble-Metal Contacts on Doping and Band Gap of Graphene. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 121101. (22) Vita, H.; Böttcher, S.; Horn, K.; Voloshina, E. N.; Ovcharenko, R. E.; Kampen, Th.; Thissen, A.; Dedkov, Yu. S. Understanding the origin of band gap formation in graphene on metals: graphene on Cu/ Ir(111). Sci. Rep. 2015, 4, 5704. (23) Sung, S. J.; Yang, J. W.; Lee, P. R.; Kim, J. G.; Ryu, M. T.; Park, H. M.; Lee, G.; Hwang, C. C.; Kim, K. S.; Kim, J. S.; Chung, J. W. Spin-Induced Band Modifications of Graphene through Intercalation of Magnetic Iron Atoms. Nanoscale 2014, 6, 3824−3829. (24) Petrović, M.; Šrut Rakić, I.; Runte, S.; Busse, C.; Sadowski, J. T.; Lazić, P.; Pletikosić, I.; Pan, Z.-H.; Milun, M.; Pervan, P.; Atodiresei, N.; Brako, R.; Šokčević, D.; Valla, T.; Michely, T.; Kralj, M. The mechanism of caesium intercalation of graphene. Nat. Commun. 2013, 4, 2772. (25) Park, Y. S.; Park, J. H.; Hwang, H. N.; Laishram, T. S.; Kim, K. S.; Kang, M. H.; Hwang, C. C. Quasi-Free-Standing Graphene Monolayer on a Ni Crystal through Spontaneous Na Intercalation. Phys. Rev. X 2014, 4, 031016. (26) Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud, C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Electronic Confinement and Coherence in Patterned Epitaxial Graphene. Science 2006, 312, 1191−1196. (27) Levendorf, M. P.; Kim, C. J.; Brown, L.; Huang, P. Y.; Havener, R. W.; Muller, D. A.; Park, J. Graphene and Boron Nitride Lateral Heterostructures for Atomically Thin Circuitry. Nature 2012, 488, 627−332. (28) Park, B.; Park, J.; Son, J. G.; Kim, Y. J.; Yu, S. U.; Park, H. J.; Chae, D. H.; Byun, J.; Jeon, G.; Huh, S.; Lee, S. K.; Mishchenko, A.; Hyun, S.; Lee, T. G.; Han, S. W.; Ahn, J. H.; Lee, Z.; Hwang, C.; Novoselov, K. S.; Kim, K. S.; Hong, B. H.; Kim, J. K. A Facile Route for Patterned Growth of Metal-Insulator Carbon Lateral Junction through One-pot Synthesis. ACS Nano 2015, 9, 8352−8360. (29) Mak, K. F.; Shan, J.; Heinz, T. F. Seeing Many-Body Effects in Single- and Few-Layer Graphene: Observation of Two-Dimensional Saddle-Point Excitons. Phys. Rev. Lett. 2011, 106, 046401. (30) Park, Y.; Kim, Y. S.; Myung, C. W.; Taylor, R. A.; Chan, C. C. S.; Reid, B. P. L.; Puchtler, T. J.; Nicholas, R. J.; Singh, L. T.; Lee, G.; Hwang, C.-C.; Park, C.-Y.; Kim, K. S. Two-Dimensional Excitonic Photoluminescence in Graphene on a Cu Surface. ACS Nano 2017, 11, 3207−3212. (31) Kim, H. H.; Yang, J. W.; Jo, S. B.; Kang, B.; Lee, S. K.; Bong, H.; Lee, G.; Kim, K. S.; Cho, K. Substrate-Induced Solvent Intercalation for Stable Graphene Doping. ACS Nano 2013, 7, 1155−1162. (32) Yang, J. W.; Lee, G.; Kim, J. S.; Kim, K. S. Gap Opening of Graphene by Dual FeCl3 Acceptor and K Donor Doping. J. Phys. Chem. Lett. 2011, 2, 2577−2581. (33) Kim, N.; Kim, K. S.; Jung, N.; Brus, L.; Kim, P. Synthesis and Electrical Characterization of Magnetic Bilayer Graphene Intercalate. Nano Lett. 2011, 11, 860−865. (34) Park, J.; Jo, S. B.; Yu, Y. J.; Kim, Y.; Yang, J. W.; Lee, W. H.; Kim, H. H.; Hong, B. H.; Kim, P.; Cho, K.; Kim, K. S. Single-Gate

Bandgap Opening of Bilayer Graphene by Dual Molecular Doping. Adv. Mater. 2012, 24, 407−411. (35) Li, Q.; Tian, Z.; Zhang, X.; Singh, R.; Du, L.; Gu, J.; Han, J.; Zhang, W. Active Graphene−Silicon Hybrid Diode for Terahertz Waves. Nat. Commun. 2015, 6, 7082. (36) Meric, I.; Han, M. Y.; Young, A. F.; Ozyilmaz, B.; Kim, P.; Shepard, K. L. Current Saturation in Zero-Bandgap, Top-Gated Graphene Field-Effect Transistors. Nat. Nanotechnol. 2008, 3, 654− 659. (37) Yun, J.; Lee, G.; Kim, K. S. Electron Transport in Graphene Nanoribbon Field-Effect Transistor under Bias and Gate Voltages: IsoChemical Potential Approach. J. Phys. Chem. Lett. 2016, 7, 2478−2482. (38) Lee, W. H.; Park, J.; Kim, Y.; Kim, K. S.; Hong, B. H.; Cho, K. Control of Graphene Field-Effect Transistors by Interfacial Hydrophobic Self-Assembled Monolayers. Adv. Mater. 2011, 23, 3460−3464. (39) Han, T. H.; Kwon, S. J.; Li, N.; Seo, H. K.; Xu, W.; Kim, K. S.; Lee, T. W. Versatile p-Type Chemical Doping to Achieve Ideal Flexible Graphene Electrodes. Angew. Chem., Int. Ed. 2016, 55, 6197− 6201. (40) Kim, W. Y.; Kim, K. S. Prediction of Very Large Values of Magnetoresistance in a Graphene Nanoribbon device. Nat. Nanotechnol. 2008, 3, 408−412. (41) Cho, Y.; Choi, Y. C.; Kim, K. S. Graphene Spin-Valve Device Grown Epitaxially on the Ni(111) Substrate: A First Principles Study. J. Phys. Chem. C 2011, 115, 6019−6023. (42) Rezapour, M. R.; Yun, J.; Lee, G.; Kim, K. S. Lower Electric Field-Driven Magnetic Phase Transition and Perfect Spin Filtering in Graphene Nanoribbons by Edge Functionalization. J. Phys. Chem. Lett. 2016, 7, 5049−5055. (43) Cho, W. J.; Cho, Y.; Min, S. K.; Kim, W. Y.; Kim, K. S. Chromium Porphyrin Arrays as Spintronic Devices. J. Am. Chem. Soc. 2011, 133, 9364−9369. (44) Lee, E. C.; Choi, Y. C.; Kim, W. Y.; Singh, N. J.; Lee, S.; Shim, J. H.; Kim, K. S. A Radical Polymer as a Two-Dimensional Organic Half Metal. Chem. - Eur. J. 2010, 16, 12141−12146. (45) Dresselhaus, M. S.; Dresselhaus, G. Intercalation Compounds of Graphite. Adv. Phys. 2002, 51, 1−186. (46) Weller, T. E.; Ellerby, M.; Saxena, S. S.; Smith, R. P.; Skipper, N. T. Superconductivity in the Intercalated Graphite Compounds C6Yb and C6Ca. Nat. Phys. 2005, 1, 39−41. (47) Ludbrook, B. M.; Levy, G.; Nigge, P.; Zonno, M.; Schneider, M.; Dvorak, D. J.; Veenstra, C. N.; Zhdanovich, S.; Wong, D.; Dosanjh, P.; Straßer, C.; Stöhr, A.; Forti, S.; Ast, C. R.; Starke, U.; Damascelli, A. Evidence for Superconductivity in Li-Decorated Monolayer Graphene. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 11795−11799. (48) Ichinokura, S.; Sugawara, K.; Takayama, A.; Takahashi, T.; Hasegawa, S. Superconducting Calcium-Intercalated Bilayer Graphene. ACS Nano 2016, 10, 2761−2765. (49) Kim, N.; Lee, P.; Kim, Y.; Kim, J. S.; Kim, Y.; Noh, D. Y.; Yu, S. U.; Chung, J.; Kim, K. S. Persistent Topological Surface State at the Interface of Bi2Se3 Film Grown on Patterned Graphene. ACS Nano 2014, 8, 1154−1160. (50) Wang, D.; Kou, R.; Choi, D.; Yang, Z.; Nie, Z.; Li, J.; Saraf, L. V.; Hu, D.; Zhang, J.; Graff, G. L.; Liu, J.; Pope, M. A.; Aksay, I. A. Ternary Self-Assembly of Ordered Metal Oxide-Graphene Nanocomposites for Electrochemical Energy Storage. ACS Nano 2010, 4, 1587−1595. (51) Chandra, V.; Park, J.; Chun, Y.; Lee, J. W.; Hwang, I. C.; Kim, K. S. Water Dispersible Magnetite-Reduced Graphene Oxide Composites for Arsenic Removal. ACS Nano 2010, 4, 3979−3986. (52) Kemp, K. C.; Seema, H.; Saleh, M.; Le, N. H.; Mahesh, K.; Chandra, V.; Kim, K. S. Environmental Applications using Graphene Composites: Water Remediation and Gas Adsorption. Nanoscale 2013, 5, 3149−3171. (53) Chen, W.; Yi, P.; Zhang, Y.; Zhang, L.; Deng, Z.; Zhang, Z. Composites of Aminodextran-Coated Fe3O4 Nanoparticles and Graphene Oxide for Cellular Magnetic Resonance Imaging. ACS Appl. Mater. Interfaces 2011, 3, 4085−4091. 24405



ACS Applied Materials & Interfaces (54) Kim, K. S.; Tarakeshwar, P.; Lee, J. Y. Molecular Clusters of πSystems: Theoretical Studies of Structures, Spectra, and Origin of Interaction Energies. Chem. Rev. 2000, 100, 4145−4186. (55) Tiwari, J. N.; Nath, K.; Kumar, S.; Tiwari, R. N.; Kemp, K. C.; Le, N. H.; Youn, D. H.; Lee, J. S.; Kim, K. S. Stable Pt Nanoclusters on Genomic DNA-Graphene Oxide with a High Oxygen Reduction Reaction Activity. Nat. Commun. 2013, 4, 2221. (56) Tiwari, J. N.; Kemp, C. K.; Nath, K.; Tiwari, R.; Nam, H. G.; Kim, K. S. Interconnected Pt-Nanodendrite/DNA/Reduced-Graphene-Oxide Hybrid Showing Remarkable Oxygen Reduction Activity and Stability. ACS Nano 2013, 7, 9223−9231. (57) Chen, S.; Duan, J.; Tang, Y.; Jin, B.; Zhang Qiao, S. Molybdenum Sulfide Clusters-Nitrogen-Doped Graphene Hybrid Hydrogel Film as an Efficient Three-Dimensional Hydrogen Evolution Electrocatalyst. Nano Energy 2015, 11, 11−18. (58) Cho, E. S.; Ruminski, A. M.; Aloni, S.; Liu, Y.-S.; Guo, J.; Urban, J. J. Graphene Oxide/Metal Nanocrystal Multilaminates as the Atomic Limit for Safe and Selective Hydrogen Storage. Nat. Commun. 2016, 7, 10804. (59) Liu, Y.; Dong, X.; Chen, P. Biological and Chemical Sensors Based on Graphene Materials. Chem. Soc. Rev. 2012, 41, 2283−2307. (60) Reznik, I. A.; Gromova, Yu. A.; Zlatov, A. S.; Baranov, M. A.; Orlova, A. O.; Moshkalev, S. A.; Maslov, V. G.; Baranov, A. V.; Fedorov, A. V. Hybrid Structures Based on Quantum Dots and Graphene Nanobelts. Opt. Spectrosc. 2017, 122, 114−119. (61) Sun, J.; Li, Y.; Peng, Q.; Hou, S.; Zou, D.; Shang, Y.; Li, Y.; Li, P.; Du, Q.; Wang, Z.; Xia, Y.; Xia, L.; Li, X.; Cao, A. Macroscopic, Flexible, High-Performance Graphene Ribbons. ACS Nano 2013, 7, 10225−10232. (62) Kannan, P. K.; Moshkalev, S. A.; Rout, C. S. Highly Sensitive and Selective Electrochemical Dopamine Sensing Properties of Multilayer Graphene Nanobelts. Nanotechnology 2016, 27, 075504. (63) Kannan, P. K.; Moshkalev, S. A.; Rout, C. S. Electrochemical Sensing of Hydrazine Using Multilayer Graphene Nanobelts. RSC Adv. 2016, 6, 11329. (64) Kannan, P. K.; Hu, C.; Morgan, H.; Moshkalev, S. A.; Rout, C. S. Electrochemical Sensing of Bisphenol Using a Multilayer Graphene Nanobelt Modified Photolithography Patterned Platinum Electrode. Nanotechnology 2016, 27, 375504. (65) Yi, J. W.; Park, J.; Singh, N. J.; Lee, I. J.; Kim, K. S.; Kim, B. H. Quencher-Free Molecular Beacon: Enhancement of the Signal-toBackground Ratio with Graphene Oxide. Bioorg. Med. Chem. Lett. 2011, 21, 704−706. (66) Zhang, H.; Wang, Y.; Zhao, D.; Zeng, D.; Xia, J.; Aldalbahi, A.; Wang, C.; San, L.; fan, C.; Zuo, X.; Mi, X. Universal Fluorescence Biosensor Platform Based on Graphene Quantum Dots and PyreneFunctionalized Molecular Beacons for Detection of MicroRNAs. ACS Appl. Mater. Interfaces 2015, 7, 16152−16156. (67) Liu, J.; Huang, H.; Zhao, H.; Yan, X.; Wu, S.; Li, Y.; Wu, M.; Chen, L.; Yang, X.; Su, B.-L. Enhanced Gas Sensitivity and Selectivity on Aperture-Controllable 3D Interconnected Macro−Mesoporous ZnO Nanostructures. ACS Appl. Mater. Interfaces 2016, 8, 8583−8590. (68) Zhang, Y.-H.; Chen, Y.-B.; Zhou, K.-G.; Liu, C.-H.; Zeng, J.; Zhang, H.-L.; Peng, Y. Improving Gas Sensing Properties of Graphene by Introducing Dopants and Defects: a First-Principles Study. Nanotechnology 2009, 20, 185504. (69) Eda, G.; Fanchini, G.; Chhowalla, M. Large-Area Ultrathin Films of Reduced Graphene Oxide as a Transparent and Flexible Electronic Material. Nat. Nanotechnol. 2008, 3, 270−274. (70) Heerema, S. J.; Dekker, C. Graphene Nanodevices for DNA Sequencing. Nat. Nanotechnol. 2016, 11, 127−136. (71) Cho, Y.; Cho, W. J.; Youn, I. S.; Lee, G.; Singh, N. J.; Kim, K. S. Density Functional Theory Based Study of Molecular Interactions, Recognition, Engineering, and Quantum Transport in π Molecular Systems. Acc. Chem. Res. 2014, 47, 3321−3330. (72) Min, S. K.; Kim, W. Y.; Cho, Y.; Kim, K. S. Fast DNA Sequencing with a Graphene-Based Nanochannel Device. Nat. Nanotechnol. 2011, 6, 162−165.

(73) Rajan, A. C.; Rezapour, M. R.; Yun, J.; Cho, Y.; Cho, W. J.; Min, S. K.; Lee, G.; Kim, K. S. Two Dimensional Molecular Electronics Spectroscopy for Molecular Fingerprinting, DNA Sequencing and Cancerous DNA Recognition. ACS Nano 2014, 8, 1827−1833. (74) Garaj, S.; Hubbard, W.; Reina, A.; Kong, J.; Branton, D.; Golovchenko, J. A. Graphene as a Subnanometre Trans-Electrode Membrane. Nature 2010, 467, 190−193. (75) Garaj, S.; Liu, S.; Golovchenko, J. A.; Branton, D. MoleculeHugging Gr Nanopores. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 12192−12196. (76) Saha, K. K.; Drndić, M.; Nikolić, B. K. DNA Base-Specific Modulation of Microampere Transverse Edge Currents through a Metallic Graphene Nanoribbon with a Nanopore. Nano Lett. 2012, 12, 50−55. (77) Park, D. W.; Brodnick, S. K.; Ness, J. P.; Atry, F.; KrugnerHigby, L.; Sandberg, A.; Mikael, S.; Richner, T. J.; Novello, J.; Kim, H.; Baek, D. H.; Bong, J.; Frye, S. T.; Thongpang, S.; Swanson, K. I.; Lake, W.; Pashaie, R.; Williams, J. C.; Ma, Z. Fabrication and Utility of a Transparent Graphene Neural Electrode Array for Electrophysiology, in vivo Imaging, and Optogenetics. Nat. Protoc. 2016, 11, 2201−2222. (78) Tiwari, J. N.; Vij, V.; Kemp, K. C.; Kim, K. S. Engineered Carbon-Nanomaterial Based Electrochemical Sensors for Biomolecules. ACS Nano 2016, 10, 46−80. (79) Goenka, S.; Sant, V.; Sant, S. Graphene-based Nanomaterials for Drug Delivery and Tissue Engineering. J. Controlled Release 2014, 173, 75−88. (80) Tiwari, J. N.; Seo, Y.-K.; Yoon, T.; Lee, W. G.; Cho, W. J.; Yousuf, M.; Harzandi, A. M.; Kang, D.-S.; Kim, K.-Y.; Suh, P.-G.; Kim, K. S. Accelerated Bone-Regeneration by Two Photon Photoactivated Carbon Nitride Nanosheets. ACS Nano 2017, 11, 742−751. (81) Tian, J.; Ning, R.; Liu, Q.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Three-Dimensional Porous Supramolecular Architecture from Ultrathin g-C3N4 Nanosheets and Reduced Graphene Oxide: Solution SelfAssembly Construction and Application as a Highly Efficient MetalFree Electrocatalyst for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2014, 6, 1011−1017. (82) Qiao, H.; Yuan, J.; Xu, Z.; Chen, C.; Lin, S.; Wang, Y.; Song, J.; Liu, Y.; Khan, Q.; Hoh, H. Y.; Pan, C. X.; Li, S.; Bao, Q. Broadband Photodetectors Based on Graphene-Bi2Te3 Heterostructure. ACS Nano 2015, 9, 1886−1894. (83) Di Bartolomeo, A. Graphene Schottky diodes: An Experimental Review of the Rectifying Graphene/Semiconductor Heterojunction. Phys. Rep. 2016, 606, 1−58. (84) Drogeler, M.; Franzen, C.; Volmer, F.; Pohlmann, T.; Banszerus, L.; Wolter, M.; Watanabe, K.; Taniguchi, T.; Stampfer, C.; Beschoten, B. Spin Lifetimes Exceeding 12 ns in Graphene Nonlocal Spin Valve Devices. Nano Lett. 2016, 16, 3533−3539. (85) Calado, V. E.; Goswami, S.; Nanda, G.; Diez, M.; Akhmerov, A. R.; Watanabe, K.; Taniguchi, T.; Klapwijk, T. M.; Vandersypen, L. M. K. Ballistic Josephson Junctions in Edge-Contacted Graphene. Nat. Nanotechnol. 2015, 10, 761−764.

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Graphene and Graphene Analogs toward Optical, Electronic

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