We study interfacial water trapped between a sheet of graphene and a muscovite (mica) surface using Raman spectroscopy and ultra-high vacuum scanning tunneling microscopy (UHV-STM) at room temperature. We are able to image the graphene-water interface with atomic resolution, revealing a layered network of water trapped underneath the graphene. We identify water layer numbers with a carbon nanotube height reference. Under normal scanning conditions, the water structures remain stable. However, at greater electron energies, we are able to locally manipulate the water using the STM tip. KEYWORDSGraphene, water, mica, scanning probe microscopy, atomic resolution, STM, RamanThe interface between water and various surfaces 1,2 at room temperature has been of great interest to scientists due to its relevance in geology, 3 biology, 4 and most recently, electronics. 5,6 It has been demonstrated that water behaves very differently at an interface than it does in the bulk state, forming semi-ordered "hydration layers" close to the solid surface. [7][8][9][10] However, the exact nature of these hydration layers are still not well understood and remains the source of much controversy. 11 Recent studies utilizing AFM and other methods have made 2 progress towards putting some of these controversies to rest, 6,11-14 but atomic-resolution imaging of the interface had not yet been achieved.Graphene 6,[15][16][17][18][19][20] has already been extensively characterized by surface imaging techniques on a variety of substrates, [21][22][23][24][25][26] but only recently has it started to see use as a template for studying other molecules, 13,27,28 Graphene is ideal for coating and trapping volatile molecules for both scanning probe microscopy 13,27,29 and electron microscopy 28 studies in that it is conductive, chemically inert, impermeable, 30 and atomically conforms to most substrates. 31 In this letter, we build upon the work performed by Xu et al. 13 and use the atomic resolution and cleanliness of the ultrahigh vacuum scanning tunneling microscope (UHV-STM) to characterize water confined between monolayer graphene and the mica surface at room temperature. Unlike previous studies of graphene on mica, 6,13,14,27,29,31,32 we use graphene grown on copper via chemical vapor deposition (CVD) 33,34 rather than graphene mechanically exfoliated from graphite. 19 While CVD graphene is inferior to exfoliated graphene in terms of carrier mobility, this drawback is offset by the ability to manufacture large, monolayer sheets and transfer them onto arbitrary substrates. 34Our CVD process uses a methane-to-hydrogen partial pressure ratio of 2:1, as lower ratios give higher monolayer coverage. 35,36 Previous work 33 and the supporting information give more details on our growth procedure. We transfer graphene to mica with polymethyl methylacrylate (PMMA) and use successive deionized (DI) water baths to clean the graphene films from etchant contamination. The final transfer occurs on a freshly cleaved mica surface within a DI bath in co...
We examine the transfer of graphene grown by chemical vapor deposition (CVD) with polymer scaffolds of poly(methyl methacrylate) (PMMA), poly(lactic acid) (PLA), poly(phthalaldehyde) (PPA), and poly(bisphenol A carbonate) (PC). We find that optimally reactive PC scaffolds provide the cleanest graphene transfers without any annealing, after extensive comparison with optical microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Comparatively, films transferred with PLA, PPA, and PMMA have a two-fold higher roughness and a five-fold higher chemical doping. Using PC scaffolds, we demonstrate the clean transfer of CVD multilayer graphene, fluorinated graphene, and hexagonal boron nitride. Our annealing free, PC transfers enable the use of atomically-clean nanomaterials in biomolecule encapsulation and flexible electronic applications. * Correspondence should be addressed to lyding@illinois.edu and epop@stanford.edu. Cu has proven the most fruitful platform for large-area graphene growth, as the low carbon solubility promotes monolayer growth. 8 Nevertheless, most applications using CVD-grown graphene require that the films be transferred to insulating substrates. The predominant graphene transfer approach is by using a poly(methyl methacrylate) (PMMA) scaffold. [12][13][14][15][16][17] In this method, the PMMA polymer coats the graphene, supporting it during Cu removal, underside contaminant cleaning, and placement on its destination substrate. 18, 19 However, PMMA removal from graphene after film transfer has proven challenging. 15 Approaches to remove it by high-temperature Ar/H2 forming gas annealing, 14, 20, 21 O2 based annealing, 15, 22, 23 and in situ annealing 16, 24, 25 have been marginally successful in removing PMMA without affecting the graphene. Furthermore, these processes are all at high-temperature, excluding graphene applications with low thermal budgets, including uses in flexible electronics and biomolecule encapsulation. Another process separated the graphene from the PMMA support by an Au interfacial layer, 26 but that process is subject to effective interfacial Au-graphene wetting. Recent transfer results using thermal release tape (TRT), [27][28][29] poly(bisphenol A carbonate) (PC), 30, 31 and sacrificial polymer release layers 26 required elevated temperature (over 100°C) during transfer and differed considerably in terms of surface contamination and graphene area coverage. To exploit the intrinsic properties of large-area graphene, a room temperature transfer process that comes off more cleanly than the established methods is needed. In print atIn this study, we compare the transfer of graphene with the conventional PMMA polymer scaffold with alternative poly(lactic acid) (PLA), poly(phthalaldehyde) (PPA), PC, and bilayer PMMA/PC scaffolds. We choose both PLA and PPA as scaffolds as they can supposedly be removed by modest heating or acid exposure. Further, we choose PC from its heightened reactivity as a condensation polymer and it...
Scattering scanning near-field optical microscopy (s-SNOM) has emerged as a powerful nanoscale spectroscopic tool capable of characterizing individual biomacromolecules and molecular materials. However, applications of scattering-based near-field techniques in the infrared (IR) to native biosystems still await a solution of how to implement the required aqueous environment. In this work, we demonstrate an IR-compatible liquid cell architecture that enables near-field imaging and nanospectroscopy by taking advantage of the unique properties of graphene. Large-area graphene acts as an impermeable monolayer barrier that allows for nano-IR inspection of underlying molecular materials in liquid. Here, we use s-SNOM to investigate the tobacco mosaic virus (TMV) in water underneath graphene. We resolve individual virus particles and register the amide I and II bands of TMV at ca. 1520 and 1660 cm −1 , respectively, using nanoscale Fourier transform infrared spectroscopy (nano-FTIR). We verify the presence of water in the graphene liquid cell by identifying a spectral feature associated with water absorption at 1610 cm −1 .In biological and life sciences, Fourier transform infrared (FTIR) spectroscopy serves as a ubiquitous noninvasive probe of vibrational fingerprints used to identify chemical compounds and molecular species. 1 This information is the basis for nonperturbative and label-free analysis of cell functionality. 2 For example, small changes in frequencies and line shapes of IR absorption bands due to specific proteins or protein conformations can be used to characterize cells and tissues linked to diseases such as Alzheimer's 3 and cancer. 1,4 Detailed databases document molecular absorp- * To whom correspondence should be addressed
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