Fluorinated Graphene Enables the Growth of Inorganic Thin Films by

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Fluorinated Graphene Enables the Growth of Inorganic Thin Films by Chemical Bath Deposition on Otherwise Inert Substrates. Woo-Kyung Lee, Sandra C Hernandez, Jeremy T Robinson, Scott G. Walton, and Paul E. Sheehan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12440 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016

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ACS Applied Materials & Interfaces

Fluorinated Graphene Enables the Growth of Inorganic Thin Films by Chemical Bath Deposition on Otherwise Inert Substrates. Woo-Kyung Lee,1,* Sandra C. Hernández,2 Jeremy T. Robinson,3 Scott G. Walton,2 and Paul E. Sheehan1

1. Chemistry Division, U.S. Naval Research Laboratory, Washington DC, 20375 2. Plasma Physics Division, U.S. Naval Research Laboratory, Washington DC, 20375 3. Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington DC, 20375

ABSTRACT: Chemically modified graphenes (CMGs) offer a means to tune a wide variety of graphene’s exceptional properties. Critically, CMGs can be transferred onto a variety of substrates, thereby imparting functionalities to those substrates that would not be obtainable through conventional functionalization. One such application of CMGs is enabling and controlling the subsequent growth of inorganic thin films. In the current study, we demonstrated that CMGs enhance the growth of inorganic films on inert surfaces with poor growth properties. Fluorinated graphene transferred onto polyethylene enabled the dense and homogeneous deposition of cadmium sulfide (CdS) film grown via chemical bath deposition (CBD). We showed the coverage of CdS film can be controlled by the degree of fluorination from less than 20% to complete coverage of the film. The approach can also be applied to other technologically important materials such as ZnO. Finally, we demonstrated that an electron beam SF6 plasma could pattern fluorine onto a graphene/PE sample to selectively grow CdS films on the fluorinated region. Therefore, CMG coatings can tailor the surface properties of polymers and control the growth of inorganic thin films on polymers for the development of flexible electronics.

KEYWORDS: graphene, chemically modified graphene, surface engineering, chemical bath deposition, Cadmium Sulfide

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INTRODUCTION The widespread interest in chemically modifying graphene comes from the ability of chemistry to extend and tune graphene’s many beneficial electronic, mechanical, and tribological properties.1,

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While the physical properties of chemically modified graphenes (CMGs) are

inherently interesting,1 CMGs can also serve as a chemical tool, enabling the rapid functionalization of materials that are otherwise inert. For example, one can prepare a desired chemistry on graphene and then transfer that chemistry in toto onto a target material, circumventing the limitations of the target’s surface chemistry and associated development costs.3, 4 Alternately, one could first apply graphene to the target material and then leverage the chemical flexibility of carbon to impart new functionalities onto the surface.3 Graphene is ideal for chemically modifying surfaces since it adds only a single layer of carbon atoms (less than the thickness of adventitious carbon),5 presents new surface properties,6,

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can be essentially

impermeable to chemical reactants,8 and can be patterned with macroscale gradients9 or with atomic resolution.10, 11 Perhaps most critically, graphene can adhere to many different substrates through van der Waals interactions, its large surface-to-volume ratio ensuring a strong bond.12 Consequently, using graphene one can transfer chemical functionalities to a surface that would otherwise be difficult or impossible to generate with the original material. Chemically modified graphenes offer several critical surface properties. For instance, both oxidized graphene (OG) and fluorinated graphene (FG) can serve as chemical linkers to other molecules such as poly(methyl methacrylate), amines, or biomolecules.13,

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As a second

example, graphene’s hydrophobicity will vary with the coverage of the chemical group, with increased fluorine content increasing its hydrophobicity or increased oxygen content reducing its hydrophobicity.15 Consequently, gradients in chemical coverage can push or pull water or dimethyl methylphosphonate (DMMP) droplets across the graphene surface.9 As a final example, ferromagnetic coatings are even possible using partially-hydrogenated or nitrophenylfunctionalized graphene.16, 17 Overall, CMGs provide a versatile method to modify a substrate’s properties or add new functionalities. One critical application of surface chemistry is enabling the subsequent growth of inorganic thin films.18 Many groups have examined how the chemical functionalization of graphene modifies such growth. Hernández et al. showed nitrogen moieties can be employed to selectively electrodeposit gold on the surface of graphene.19 Jung et al. reported that UV ozone-treated 2 ACS Paragon Plus Environment

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graphene, primarily OG, enhances the growth of cadmium telluride (CdTe) films during closespaced sublimation.20 Wang et al. found that graphene functionalized with physisorbed 3,4,9,10perylene tetracarboxylic acid (PTCA) enhanced the deposition of aluminum oxide by atomic layer deposition, while pristine graphene did not.21 Similarly, Jagtiani et al. showed the presence of nitrogen, oxygen, and fluorine moieties on graphene allow for the deposition of HfO2 via atomic layer deposition.22 The goal of these and much related work was to couple different materials to the graphene to improve its performance in electronic devices. Recently Seo et al. sought a different goal—using graphene as a chemical coating.23 They reported that ozonetreated graphene improved the growth of cadmium sulfide (CdS) film on glass deposited via chemical bath deposition (CBD); however, they did not examine its use on inert substrates that did not otherwise support growth.23 CdS, a II-VI compound semiconductor with a direct band gap of ~2.42 eV,23 has been widely used for thin film photovoltaic devices, especially as a window layer in CuInSe or CdTe heterojunction based solar cells.24 CBD is a common deposition technique for CdS since it is scalable, cost-effective, non-destructive, and generally produces high quality films. For instance, CBD-grown CdS has been successfully employed in highly efficient photovoltaic cells.25 CdS is one of the easier materials to deposit by CBD, generally producing homogeneous films with minimal constraints on the choice of substrate. 23, 24, 26 That said, its deposition onto hydrophobic polymer substrates remains a challenge due to their poor adhesion property.26, 27 Here, we examine more broadly the engineering of an interface to support crystal growth. The primary interest is to understand how graphene can enable growth on otherwise inert substrates, which chemical functionalities enhance CBD growth, how the degree of functionalization and duration of growth impacts film coverage, and whether this approach can be applied to various materials. Additionally, graphene’s chemistry can change with the underlying substrate,28, 29 and so it must be taken into account. Earlier examinations of crystal growth on graphene were typically concerned with modifying the graphene and not the substrate, so the substrate’s impact on growth was not explicitly considered. Finally, we selectively and locally grew CdS films in regions fluorinated using a low-energy plasma. The results show that chemically-modified graphene is a flexible tool for transferring chemical functionality onto otherwise inert substrates. 3 ACS Paragon Plus Environment


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RESULTS Our examination of CBD growth on CMG films required at least two substrates, one that would and one that would not support growth. Many materials were tested by immersing them for 90 min in the CdS growth solution followed by visual inspection, where cadmium sulfide’s vibrant yellow color immediately would reveal any film growth. As noted earlier, CdS grows on many substrates, so we chose SiO2 for its cleanliness and technological importance. Figure 1a-SiO2 shows the typical homogeneous growth on a bare SiO2 substrate. Finding an inert substrate was more challenging, but it was found that hot-pressed polyethylene (PE), noted for its low surface energy, did not support CdS film growth (Figure 1a-PE). With these substrates in hand, we could explore how the substrate impacts CBD growth. Single-layer graphene (SLG), OG, and FG were prepared on both substrates. On the polyethylene substrates (Figure 1), uniform and dense CdS films formed on those substrates covered with OG and FG, but no growth was observed on those covered with pristine graphene. On the SiO2 substrates, growth was observed on all samples whether or not the sample was covered and whether or not the graphene was functionalized. Note that growth on pristine graphene depended on the underlying substrate, with growth when SiO2 was the substrate but not when polyethylene was, an effect not typically seen in thin films. Since SiO2 supported robust film growth regardless of coating, the focus was placed on the more interesting, and useful, capability of enabling growth on the inert PE substrates. The qualities of these films were studied in greater detail with AFM and XPS. Figure 1a-PE shows again that growth on bare PE was minimal. For the SLG/PE samples, growth was not continuous showing either random precipitation from the solution or otherwise scattered growth on defects in the graphene (Figure 1b-PE) which are typically terminated with functional groups such as carboxylic acid. Because of this scattered growth, AFM analysis showed that the sample’s RMS roughness tripled from 15.51 nm to 44.45 nm. For the OG/PE samples, continuous CdS films were formed on OG (Figure 1c-PE). The RMS roughness of the CdS film on OG/PE was 16.28 nm. Similarly, heavily-fluorinated graphene (≈ 40 atm. % F) grew a smooth, thicker (> 100 nm) yellow-orange CdS film (Figure 1d-PE) of fairly high quality. Indeed, AFM revealed a homogeneous film with an average grain size of 36.2±5.8 nm (Figure 1d-PE) and smoothing of a


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film during growth. The RMS roughness of FG-covered PE before deposition, 13.34 ±1.44 nm, was halved to 7.34±2.26 nm after the deposition of CdS film. It appears that the CdS particles flatten out the corrugations of the FG samples, presumably due to the high degree of nucleation producing multiple growth sites that then coalesce. Finally, chemical analysis of this CdS film obtained by XPS showed typical peaks of CdS (≈ 1:1 atomic %, Supporting Information, S1) indicating a well-formed stoichiometric film. The continuity of the surface and the decreased roughness demonstrate that the FG coating significantly improves the growth of CdS film. The thickness and coverage of the CdS films on the FG/PE samples depended both on the duration of the deposition and on the extent of fluorination. The thicknesses of films grown for different durations were measured by scraping off the films with a tweezer and then scanning across the boundaries with an AFM (Figure 2a). For heavily fluorinated graphene, film growth plateaus at a thickness of approximately 100 nm (Figure 2b) after 60 min when the reagents are depleted in the precursor mixture.30 So, using a 60 min growth as a baseline, we examined the correlation between extent of fluorination and CdS film coverage. As previously reported, the extent of fluorination may be raised by increasing the XeF2 exposure from 5 to 15 min, shifting the approximate stoichiometry from C8F to C1.5F when grown on SiO2.31 Increased fluorination clearly improves the morphology of the CdS film as shown by scanning electron microscopy (SEM) (Figures 2d-f). A flooding analysis of these images performed with the imaging software WSxM32 extracted the coverage of the CdS film (Figure 2c), showing a linear increase with fluorine exposure up to 15 min where complete coverage was achieved. Overall, CdS coverage ranged from approximately 20% with pristine graphene, gradually increasing from 30% to 100% with the addition of fluorine. In addition, RMS roughness was decreased from 49.09±7.9 nm for 5 min FG and 30.36±2.9 nm for 10 min FG to 7.34±2.3 nm for 15 min FG, supporting that CdS film becomes smoother as the coverage is increased by fluorination. The optical transmittance of FG/PE samples was characterized by Craic spectrophotometer and the CdS coverage directly affects the transmission spectra at the absorption edge ranged from 500 to 550 nm (Figure 2g). Coating a material with fluorinated graphene could be a general strategy for enhancing deposition in CBD. In addition to CdS, this approach improves the growth of zinc oxide (ZnO), a wide band gap semiconductor with a high transmittance in visible light and luminescence in visible range of spectrum.33-35 ZnO tends to grow more slowly than CdS, requiring a much



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longer required period of 12 hours. On uncoated PE, some ZnO background growth was observed leading to a coverage of ≈20% (S2), presumably due to the adsorption of ZnO particles. Pristine graphene provided little benefit, yielding only scattered growth (31% coverage, Figure 3a) when immersed in the ZnO growth solution. In contrast, heavily fluorinated FG on PE yielded a continuous ZnO film that was 140 nm thick as measured by AFM (Figure 3b). Visually, the gray ZnO films appear almost identical to PE and so are hard to identify; however, the optical transmission spectra clearly indicate the formation of ZnO film on FG/PE sample with a sharp absorption edge around 375 nm (red line in Figure 3c) due to the ≈3.3 eV band gap

of ZnO. The spectra from the SLG/PE sample (black and blue line in Figure 3c) do not show a clear absorption edge. Beyond ZnO, both ZnS and PbS were amenable to this approach but were not investigated in detail (S3). As a final example of the utility of CMG to chemically modify a substrate, large-area patterns were prepared by selectively functionalizing graphene with the intent of directing CdS growth. Graphene-covered PE was exposed to a low-energy SF6 plasma through a physical mask to fluorinate only specific regions with 50 µm wide lines. Subsequent exposure to the CdS growth solution led to selective growth on the fluorinated regions. The optical microscope image in Figure 4 shows that lines of CdS film (yellow lines) were deposited on the FG-patterned region, while the CdS film was not formed on the regions with SLG.

DISCUSSION As described in Introduction, graphene’s surface properties are strongly affected by the substrate underneath.28, 29 Previously, several groups have noted that the substrate can affect graphene’s chemistry; however, their observations typically concern covalent reactions.28,

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In contrast,

chemical bath deposition is a physisorption process where changes in surface free energy dictate whether nucleation and growth occur in the solution or at the heterogeneous interface.18 Here, the effect shown in Figure 1 more resembles the wetting transparency of graphene29 which a recent report ascribes to the level of electrostatic doping in graphene and its impact on the water contact angle.37 Specifically, the trapped charges in silicon oxide are known to dope graphene while polyethylene has no mechanism for doping graphene.


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CdS deposition on graphene strongly depends on two aspects—base substrate and chemical modification (Figure 1). When a base substrate has good adhesion for CdS deposition, the formation of CdS film was not strongly dependent upon the chemical nature of graphene on SiO2 (Figure 1-SiO2). Unlike on SiO2 substrate, CdS film was not formed on pure PE or SLG/PE (Figure 1-PE) In general, PE exhibits a poor adhesive property for CBD process probably due to its hydrophobic nature,26,

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and either O2 plasma treatment or wet oxidation is required to

enhance adhesion for CBD.26,

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Our results show that SLG by itself does not promote the

deposition of CdS on PE; however, both FG and OG enabled uniform and homogeneous growth on the otherwise inert PE (Figure 1-PE). To explain how CMGs impact the growth of CdS film we examined (1) roughness change (see Result section) and (2) coverage change with the degree of fluorination after CBD. We chose FG as a model system to discuss these aspects. Our results in Figure 1-FG/PE shows uniform and smooth CdS film compared with pure PE or SLG/PE sample. We believe that the high concentration of nucleation sites on FG promotes the formation of a uniform film. If the nucleation sites are sparse, like pure PE or SLG/PE, scattered CdS particles were deposited, as seen in the higher RMS roughness. However, dense nucleation sites blocks lateral growth of CdS, thereby vertical growth and subsequent coalescence.24 As a result, smoother and thicker CdS film was formed on FG/PE. Figure 2 strongly supports this explanation with the dependence of coverage upon the degree of fluorination. That is, longer fluorination times generate higher concentrations of nucleation sites and so increases the coverage of the CdS film. Simultaneously, RMS roughness was decreased as the coverage was increased. The coverage directly impacts the optical property of CdS film on PE. In Figure 2g, the optical transmission spectra shows that the transmittance of the film decreases as fluorination time increases, indicating that the coverage of film was increased. Also we observed the sharp increase of the spectrum at the absorption edge ranged from 500 to 550 nm with 15 min FG/PE sample, therefore, indicating the formation of CdS film. While fluorinating graphene enhances CdS film growth, providing more nucleation sites, how it does so is not immediately apparent. One possible reason is that ionic complexes such as [Cd(NH3)]2+4, [Cd(OH)2]n, (OH)-, Cd2+, and S2- may be favorable to interact with those sites generated by fluorination. According to CdS growth mechanism,24 deposition is dominated by two processes; (1) ion by ion growth under the CBD temperature below 60°C or (2) cluster by cluster growth above 60°C. Our CBD condition is more likely governed by cluster by cluster 7 ACS Paragon Plus Environment


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growth because our solution temperature was fixed at 75°C. In cluster by cluster growth, CdS film is formed by the chemical reactions among above ionic complexes rather than spontaneous reaction only between Cd2+ and S2-. In fact, our control experiment with 15 min FG and the same CBD conditions at the solution temperature of 45°C clearly shows incomplete CdS deposition with the coverage ranged from 40 to 50% (S4). That the same reaction at 75°C formed complete films (Figure 2f) suggests that defects or functionalized sites on FG induces physisorption of one or more chemical species in the chemical bath, initiating growth. However, what drives the adhesion between ionic complexes and FG is still not clear. Previous report suggests that greater nucleation could occur through an increase in the surface free energy of the graphene.18 Indeed, for polymers that have intrinsically low surface energy, increasing the surface energy through oxidation has been shown to aid uniform growth of inorganic films.26, 27 Here, fluorinating graphene on PE for 15 minutes using XeF2 increased its surface free energy as shown by a decrease in the static contact angle of water from 96.1 ±1.5° to 68.9 ±1.2°. However, when fluorinated via plasma, the water contact angle increased to an average of 116°, not only indicating a lower surface energy but potentially a different organization of the material (to be published). Since CdS films grew equally well on graphene fluorinated using both methods (see Figure 4), surface energy may not be the dominant interaction. Alternately, the fluorine could be labile in the CBD solution which is basic at pH 10 and heated to 75°C. The removal of fluorine would leave a high energy defect that enhanced binding and thus nucleation. To test this, a solution was prepared without sources of cadmium or sulfur ions to avoid CdS growth. Immersing fluorinated graphene in this test solution led to a significant loss of fluorine atoms within the first 10 minutes, shown by an 80% drop in the F/C ratio, the ratio of the C1s and F1s XPS peaks (Figure 5). Specifically, the F/C ratio of FGcovered PE was ≈0.259 after 15 min fluorination but decreased to ≈0.06 after 10 min. These results agree with our previous report on fluorine loss from FG on SiO2 under high pH buffer and at elevated temperatures.38 Loss of fluorine does not restore the graphene but rather leaves extensive defects as seen by Raman spectroscopy (Figure 6). Pristine graphene on PE clearly shows G (≈1590 cm-1) and 2D (≈2670 cm-1) bands overlaid with the PE spectrum (black spectrum) (Figure 6a (red)). After 15 minutes of fluorination (blue), the 2D band becomes broadened. Unlike the typical signature of CMGs on other substrates, a D band from defects 8 ACS Paragon Plus Environment

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(≈1340 cm-1) is not noticeable due to the Raman spectra of CH2 bonds from PE underneath. The submersion in pH 10 solution at 75°C shows the increase of 2D band peak intensity, suggesting that fluorine atoms are removed from FG (6c (green) and 6d (pink)), while the sample from submersion did not show complete recovery of 2D band from the pristine graphene. Therefore this indicates that the sample is not restored to pristine graphene, retaining defects after fluorine removal. The same experiment with fluorinated graphene on SiO2, eliminating the strong Raman background of the PE, shows a clear D band after the submersion in pH 10 solution for 30 min at 75°C (S5), again indicating the presence of defects after fluorine removal. The chemical nature of the remaining, presumably oxygen-rich, defect could not be resolved by XPS. Because of the substrate, XPS survey scans obviously show pronounced O 1s peaks for the graphene on SiO2; however, even the melt-pressed PE contained 8-10 atom.% oxygen, a level slightly less but still comparable to the 8-14 atom.% of oxygen for all the samples—pristine, fluorinated, and defluorinated graphene. Overall, the chemical nature of defects induced by fluorination and how ionic complexes, cadmium, and sulfide ions adhere to them is the subject of ongoing research.

CONCLUSIONS In summary, we demonstrated that chemically modified graphene can engineer surfaces to enhance crystal growth even on inert materials with poor growth properties. Fluorinated graphene on polyethylene enabled the deposition of smooth, high-quality CdS thin films via CBD. Fluorine enhanced the nucleation and growth of CdS, allowing control over film coverage from

Fluorinated Graphene Enables the Growth of Inorganic Thin Films by

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