CARBON xxx (2013) xxx– xxx Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon Review Impermeability of graphene and its applications Vikas Berry * Department of Chemical Engineering, Kansas State University, 1005 Durland Hall, Manhattan, KS 66506, USA ARTICLE INFO ABSTRACT Article history: This review discusses the genesis of impermeability in graphene and its extraordinary Received 2 April 2013 applications in fluid-encasement for wet electron-microscopy, selective gas-permeation, Accepted 26 May 2013 nanopore-bio-diffusion, and barrier coating against rusting and environmental hazards. Available online xxxx As the thinnest material, graphene is composed of sp2 hybridized carbon atoms linked to one another in a 2D honeycomb lattice with high electron-density in its aromatic rings, which blocks-off all molecules. This phenomena, in combination with its strong structure (C–C bond energy = 4.9 eV and intrinsic strength = 43 N/m) makes graphene the most impermeable membrane (thinnest membrane that is impermeable). Apart from the appli- cations mentioned above, graphene coatings have enabled fundamental studies on chem- ical processes and fluid structures. For example, graphene can allow electron imaging of nanocrystal nucleation process and water-lattice-structure due to its impermeability. Along with being the strongest, most conductive, and optically-absorbing material (2.3% optical absorbance), graphene’s impermeability opens a wide range of exciting opportunities. Ó 2013 Elsevier Ltd. All rights reserved. Contents 1. Introduction . ................................................................................. 00 2. Impermeability of graphene. ................................................................... 00 3. Impermeable graphene microchambers for TEM imaging . .......................................... 00 4. Impermeable graphene membranes as protective coating . .......................................... 00 5. Stabilizing molecular monolayers ................................................................... 00 6. Graphene membranes for separation applications . ................................................. 00 7. Graphene pore ionic diffusion . ................................................................... 00 8. Conclusion ..................................................................................... 00 References . ............................................................................... 00 1. Introduction material (Fig. 1) [9]. Graphene’s overlapping p-orbitals with large scattering length imparts it with a high room-tempera- Graphene – a single atom thick sheet of sp2 hybridized carbon ture mobility [13–16]. Furthermore, it exhibits high optical atoms arranged in honeycomb lattice – is the thinnest 2D absorptivity (2.3%) [17], high thermal conductivity (25· silicon) * Fax: +1 785 532 7372. E-mail address: [email protected]. 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.05.052 Please cite this article in press as: Berry V. Impermeability of graphene and its applications. Carbon (2013), http://dx.doi.org/10.1016/ j.carbon.2013.05.052 2 CARBON xxx (2013) xxx– xxx Fig. 1 – Graphene lattice structure: sp2 hybridized carbon atoms arranged in a 2D honeycomb lattice. (Bottom) the molecular structure with rough electronic density distribution: while graphene is relatively transparent to electrons, it is practically impermeable to all molecules at room temperature. Geometric pore (0.064 nm) is also small enough not to allow molecules to pass through. [18], and high mechanical strength (strongest nanomaterial) than the vdw diameter of small molecules like helium and [19]. It has been used in several extraordinary applications, hydrogen: 0.28 nm and 0.314 nm (bond-length including ultrafast photo-detectors [20], single molecule (0.074 nm) + 2r (0.12 nm)), respectively. detectors [21,22], ultrafast FETs [14,23–25], single-bacterial/ Since the permeance of a membrane (Q = flux/(DP)=C.Pr/ DNA detectors [26], hydrogen visualization-templates for (DP), whereqffiffiffiffiffiffi C, Pr and DP are the particle collision rate q 8kT TEM [27], and tunable spintronic [28–30] devices. Further, it c ¼ 4 pm , the probability of diffusion (Pr) and pressure dif- possesses a unique electronic structure [31,32], which can ference (DP), respectively, increases with reduction in the be controlled chemically [33,34] and geometrically [35]. membrane thickness, and graphene is the thinnest 2D mate- Although it is only one atom thick, an interesting property rial, along with the fact that it does not allow any molecule to of graphene is its impermeability. Graphene’s p-orbitals forms pass through, it can safely be graded as the most imperme- a dense [36], delocalized cloud that blocks the gap within its able material. Via a similar argument, graphene with con- aromatic rings (Fig. 1). This creates a repelling field, which trolled pores can form the ultimate thin membrane for gas does not allow even the smallest molecules, like hydrogen separation or fluid purification. This property is gaining atten- and helium, to pass through even when 1–5 atm pressure tion since it has already shown a wide range of applications, difference is imposed across its atomic thickness at room as discussed in this review. Further, other properties of graph- temperature [4]. The ability to withstand such pressure differ- ene can be combined with its impermeability to develop cor- ences (6 atm) in graphene is a result of its high strength responding applications. For example, the flexible [37] and (breaking strength = 42 N/m) and Young’s modulus (1 TPa) stretchable [19] carbon–carbon bonds in graphene can allow [19], which retains the structural integrity of graphene. While conformal coating on corrugated surfaces to make them the theoretical studies have shown that there is no gap in impermeable [12]. The high electron-transparency and con- electron-density around the aromatic rings to allow mole- formability can be employed to construct atomically-thin cules to pass, it can be seen the geometric gap calculated impermeable liquid cells for transmission electron micro- from van der Waals (vdw) radius of carbon will be smaller scope (TEM) imaging. The strength of graphene to withstand than the size of He (Fig. 1). The C–C bond length [13] of high pressure difference and its high electrical conductivity 0.142 nm in graphene implies that considering the nuclei enables liquid retention and reduction in electrical charging alone, the pore size would be 0.246 nm. Now if we add the during electron-microscopy imaging. Moreover, chemical vdw radius of carbon of 0.11 nm, this geometric pore size interfaces of graphene with membranes can be used to con- would decrease to 0.064 nm. This geometric gap is smaller trol its permeance, and it can be used as a protective coating Please cite this article in press as: Berry V. Impermeability of graphene and its applications. Carbon (2013), http://dx.doi.org/10.1016/ j.carbon.2013.05.052 CARBON xxx (2013) xxx– xxx 3 to prevent metal-corrosion and to quarantine hazardous the upper bound of the probability of transmittance of a He dN 2d nanomaterials. The next section will discuss the imperme- atom through graphene ( dt Nv, where d is the depth of the ability of graphene, followed by its applications. microchamber and v is the velocity of He atom) to be 10À11. With estimated tunneling barrier [38] of 8 eV [39] (or 3.5 eV 2. Impermeability of graphene for the temporary bond-breaking ‘window’ mechanism [40]) for He transmittance through graphene, the probability of Bunch et al. tested the permeance of several gases through a transmittance of He is orders of magnitude lower at 10À335 micro-chamber capped with a graphene sheet [4] (Fig. 2). (or 10À212): pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! Here, the graphene sheet blister depresses and stretches un- À2x 2mðV À EÞ der a negative pressure difference of À93 kPa across the p ¼ exp – ð2Þ monolayer sheet (on a 2 · 2 lm2 hole) by applying vacuum h (0.1 Pa) for 4 days and releasing the system to atmospheric where x = 0.3 nm, V is 8 eV (or 3.5 eV), and E is 25 meV (room pressure of different gases. The leak rate values are calculated temperature). Therefore, for all practical purposes, graphene by measuring the pressure change: is impermeable [41,42]. Further, the permeance of graphene dN V dP is related with the route of its synthesis, since it influences ¼ ð1Þ the defects on graphene. For example, CVD graphene is ex- dt kBT dt pected to have Stone–Wales defects, which reduce the barrier For He, a leak rate of 105–106 atoms/s is measured for the to He diffusion. Although, this reduction in barrier height (cal- chamber; this rate does not change with number of graphene culated to be between 6 and 9.2 eV) is not sufficient to allow layers. Importantly, estimated diffusion through the silica ready passage of He [38]. However, other defects can signifi- microchamber yields similar leak rate. This implies that the cantly reduce the barrier to He penetration, as per calcula- permeation is not through the graphene sheet, but through tions; for example vacancy defects 555777 divacancy, 858 the microchamber walls. Further, Bunch et al. also estimated divacancy, tetravacancy, hexavacancy, and decavancy induce Fig. 2 – (a) Schematic of a graphene sealed microchamber. (Inset) optical image of a single atomic layer graphene drumhead on 440 nm of SiO2. The dimensions of the microchamber are 4.75 · 4.75 lm · 380 nm. (b) Side view