Morphology, Mechanical Stability, and Protective Properties of

Article pubs.acs.org/Langmuir

Morphology, Mechanical Stability, and Protective Properties of Ultrathin Gallium Oxide Coatings Frank Lawrenz,† Philipp Lange,‡ Nikolai Severin,‡ Jürgen P. Rabe,*,‡ Christiane A. Helm,*,† and Stephan Block*,†,§ †

Institut für Physik, Ernst-Moritz-Arndt Universität, Felix-Hausdorff-Str. 6, D-17487 Greifswald, Germany Department of Physics & IRIS Adlershof, Humboldt-Universität zu Berlin, Newtonstr. 15, D-12489 Berlin, Germany § Applied Physics, Chalmers University of Technology, Fysikgränd 3, SE-412 96 Gothenburg, Sweden ‡

S Supporting Information *

ABSTRACT: Ultrathin gallium oxide layers with a thickness of 2.8 ± 0.2 nm were transferred from the surface of liquid gallium onto solid substrates, including conjugated polymer poly(3-hexylthiophene) (P3HT). The gallium oxide exhibits high mechanical stability, withstanding normal pressures of up to 1 GPa in contact mode scanning force microscopy imaging. Moreover, it lowers the rate of photodegradation of P3HT by 4 orders of magnitude, as compared to uncovered P3HT. This allows us to estimate the upper limits for oxygen and water vapor transmission rates of 0.08 cm3 m−2 day−1 and 0.06 mg m−2 day−1, respectively. Hence, similar to other highly functional coatings such as graphene, ultrathin gallium oxide layers can be regarded as promising candidates for protective layers in flexible organic (opto-)electronics and photovoltaics because they offer permeation barrier functionalities in conjunction with high optical transparency.

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INTRODUCTION Conjugated polymers are considered to be promising for organic electronic applications, for example, in OLEDs and organic solar cells.1−4 However, such devices suffer frequently from degradation under ambient conditions and therefore must be protected.5,6 Protective coatings are required to comply with diverse demands such as transparency for the used radiation, chemical inertness, and low wear over a sufficiently long time span.7 While the maximum allowable permeation rates for organic optoelectronics are still not well defined, limits for water vapor transmission rates (WVTR) of 10−6 g m−2 day−1 and oxygen transmission rates (OTR) of between 10−5 and 10−3 cm3 m−2 day−1 are often cited as such.8−10 To approach those values, recent strategies involve barrier coatings made of metal oxides and nitrides such as Al2O3,11,12 ZrO2,13,14 SiO2,15 TiO2,16,17 and SiN, deposited using techniques such as sputtering,16,18 atomic layer deposition,13 and chemical vapor deposition.19 However, these deposition methods require extreme conditions such as high temperatures, making it necessary to deposit an additional buffer layer to protect the organic electronics.20 Also, the potential of single and a few layers of graphene has been investigated, yet so far only on small lateral scales.21 Here, we investigate the protective properties of nanometerthick gallium oxide (GaOx) layers transferred onto a substrate from the surface of a liquid Ga drop, which provide the © 2015 American Chemical Society

advantage of being processable near room temperature. Liquid Ga is known to build a 5-Å-thick surface oxide layer under ambient conditions within less than a microsecond, which passivates the surface of liquid Ga.22,23 Hence, in contrast to recently published approaches that deposit GaOx layers that are a few micrometers thick onto substrates,24,25 we aim to directly transfer the nanometer-thick native GaOx. This avoids the requirement of working under vacuum, which is necessary for deposition based on evaporation,26−28 chemical vapor deposition,29 sputtering,30 pulsed laser deposition,31 or atomic layer deposition32 and the requirement of working or annealing at temperatures far above 100 °C, which is a common feature of solution deposition and some other recently published studies on GaOx deposition.26−34 The morphology and mechanical stability of the thin GaOx layers are assessed using scanning force microscopy. Moreover, the permeation of oxygen and water through the GaOx layers is determined by monitoring the photodegradation of conjugated polymer poly(3-hexylthiophene) (P3HT) covered by the GaOx layer under ambient conditions. Received: March 7, 2015 Revised: April 27, 2015 Published: May 6, 2015 5836

DOI: 10.1021/acs.langmuir.5b00871 Langmuir 2015, 31, 5836−5842

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Fluorescence Measurements. For reflectivity and fluorescence measurements, an inverted microscope (Axiovert 100 TV with a LD plan-neofluar 40×/0.6 object lens, Zeiss, Oberkochen, Germany) was used. The measurements were performed either under continuous nitrogen flow (to protect the samples from photodegradation) or under ambient conditions. The samples were illuminated through the mica substrate with a HBO 50 microscope lamp (Zeiss). The light intensity at the sample was about 5.6 W/cm2. Fluorescence was excited with a band-pass width of 445−565 nm (Laser Components, Olching, Germany). The light collected from the sample passed through a 585 nm long-pass filter (Laser Components) and was detected by an SC4022 CCD camera (EHD, Damme, Germany).

MATERIALS AND METHODS

Materials. Gallium (99.99%; Haines & Maassen Metall Handelsgesellschaft mbH, Bonn, Germany) was used as received. Soda-lime glass microscope slides (Roth, Karlsruhe, Germany; silica content >75%) were cleaned by the RCA procedure.35 Mica used for measurements of mechanical stability was purchased from SPI Supplies (West Chester, PA, USA). Mica (V1 optical quality) used for fluorescence and reflectivity measurements was purchased from Ratan Mica Exports (Jharkhand, India). The polished Si(100) wafers were a generous gift of Wacker Siltronic AG (Burghausen, Germany). The wafers were covered with a roughly 2-nm-thick native silicon oxide layer. Regioregular poly(3-hexylthiophene) (P3HT) (SigmaAldrich, Munich, Germany) was dissolved in chloroform (SigmaAldrich) at 1 g/L. Sample Preparation. Gallium layers were deposited on mica, silicon wafers, and soda-lime glass slides. However, as the lowest gallium layer surface roughness was observed on mica surfaces (Results section), most of the experiments focused on mica as the supporting substrate. Directly before gallium deposition, the silicon wafers and soda-lime glass slides were cleaned according to the RCA standard, and mica was cleaved. Gallium layers were deposited using one of three different deposition methods: dragging, squeezing, or spin coating. In all cases, gallium was first incubated at 50 °C for at least 30 min using a C-MAG HS 7 thermostat and an ETS D5 thermometer (IKA, Staufen, Germany). Afterward, a Pasteur pipet (Roth, Karlsruhe, Germany) was used to place a 50 μL drop of liquid gallium on the surface. In the pipetting method, the Pasteur pipet was used to drag the gallium drop over the surface. In the squeezing method, the drop of liquid gallium was sandwiched between two freshly cleaved mica surfaces. After applying normal pressure by squeezing with the fingers, both mica surfaces were separated again and one of them was used for the scanning force microscopy (SFM) experiments. For spin coating deposition, the drop was applied to a freshly cleaved mica surface mounted on a homemade spin coater. The sample was spin coated for 1 min at 50 rpm, leading to an effective acceleration of 1000g acting parallel to the mica surface. After sample preparation, the samples were transferred into the scanning force microscope, equipped with a heating stage to keep the sample temperature constant during the experiments. Experiments were conducted either at 20 or 50 °C, below and above the melting temperature of bulk gallium (29.76 °C),22 respectively. For the quantification of permeation through Ga layers, thin P3HT films were spin-coated (spin-coater SCI 20, Novocontrol, Montabaur, Germany) from chloroform solution onto freshly cleaved muscovite mica under ambient conditions under illumination with red light. Subsequently, thin gallium layers were pipetted (see above) onto the polymer films, and P3HT photodegradation was assessed by fluorescence microscopy (see below). Scanning Force Microscopy (SFM). SFM imaging was performed using a Multimode microscope (Veeco/Digital Instruments, Santa Barbara, CA, USA) equipped with a Nanoscope IIIa controller and a heating unit as well as using a JPK microscope (NanoWizard III; JPK Instruments, Berlin, Germany). The images were recorded using conventional tapping mode (TM) and contact mode (CM) imaging in air using standard tapping mode cantilevers (OMCL-AC160TS, k ≈ 40 N/m, f ≈ 320 kHz, tip curvature radius 2 μN). Layer deterioration sets in during the application of a normal force of 2 μN (b6 + b7). The application of even larger forces abrades the layer completely (>2 μN, b7). Averaged height profiles (d) of the CM measurement reveal that the layer height remains unchanged even for normal forces as high as 2 μN, implying negligible compressibility of the layer. The line sections (e) taken in (b) and (c) strongly resemble the first height profile (a) not only in height but also in fine structure. Because this is also observed for all Fn < 2 μN (Supporting Information), it indicates that imaging does not affect the topography of the layer until the normal force exceeds the threshold value. We attribute the dip in height profile 6 to an artifact due to a tilt of the cantilever tip stuck at the layer edge while scanning from the right to the left in contact.

layers with areas of up to 10 μm only, which were difficult to locate optically on the surface because of their small lateral extension. Layers with much larger areas were created using the squeezing and spin-coating methods (Figure 2b,c), which allowed layer sizes of up to 104 μm2 (Figure S1 in the Supporting Information). For such strongly extended GaOx layers we still observed a constant layer thickness of approximately 3 nm (section S1 in the Supporting Information). These layers can barely be recognized using optical microscopy in a transmission setup, which indicates high transparency of the 3-nm-thick GaOx layers. Mechanical Stability of the Gallium Oxide Layers. We addressed the mechanical stability of the GaOx layers using CM-SFM imaging. Strikingly, the stability of the layers was so high that it was possible to employ cantilevers with high spring constants (k ≈ 40 N/m as provided by the manufacturer), which are typically used for TM-SFM imaging. This made it possible to use the same cantilever to locate a GaOx layer in TM imaging, then to apply a defined normal force in CM imaging, and finally to switch back to TM imaging to assess the induced morphological changes of the GaOx layer. In a typical experiment (Figure 3), normal forces of up to 1.5 μN did not have any influence on the morphology of the GaOx layers to which they were applied (traces 1−5 in Figure 3b). That is, no differences in topography were observed between TM images taken before and after the application of mechanical stress. A further increase in normal force led to a deformation of the GaOx layer edge (trace 6 in Figure 3b) and then finally to a complete layer destruction at even higher normal forces (trace 7 in Figure 3b). Both CM and TM images of GaOx layers show apparently the same surface patterns as long as the layers do not become destroyed by the application of normal forces larger than the threshold values (Fn ≈ 2 μN). This is also reflected by the high similarity of line sections taken along the same lines in TM and CM images. (See Figure 3e for one representative example and the Supporting Information for a similarity analysis.) Replication of Buried Structures. Next we investigated how far the GaOx layers, in addition to their high mechanical stability, allow the replication of surface features buried below the layer. These experiments were motivated by the observation that the RMS roughness of the GaOx layers reproduces that of 2

the underlying substrate (Figure 1). We addressed this question by depositing GaOx layers on soda-lime glass, which occasionally exhibits defects (due to their fabrication and surface cleaning processes) that are micrometer-sized in their lateral directions but only a few nanometers in height. These defects are not always visible but are observed frequently in TM-SFM imaging (Figure S3). Figure 4a shows a representative example of such a defect that is partially covered by a GaOx layer. Cross sections of the uncoated part of the defect show that its corrugation has a depth of 1.1 nm. A comparison of the uncoated and coated parts reveals that the layer replicates the geometrical features of the corrugation: the SFM image shows no lateral distortion of the coated defect (e.g., ripples in the layer or discontinuities in the “corrugation valley”), and cross sections of the coated part show the same corrugation depth as observed on the uncoated area. Moreover, the sections themselves are shifted by roughly 3 nm to higher values in coated areas, which matches the expected layer height. Protection of Thin P3HT Films. Finally, we assessed the ability of GaOx layers to protect conjugated polymers from degradation. Because of the limited lateral size of the GaOx layers, commercially available permeation tests such as MOCON sensors and the calcium test were not applicable.7,36 5838


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Figure 4. TM-SFM topography image (8 μm × 8 μm) of a fewmicrometers-long, nanometer-deep defect on soda-lime glass, partially covered by a GaOx layer (a). The layer replicates the topography of the defect. The areas uncovered and covered with a GaOx layer are labeled with (i) and (ii), respectively. Averaged cross sections (b) taken at the bare (i) or covered defect (ii) show profiles similar in relief yet offset vertically by 2.5 nm, which is close to the expected layer height of 2.8 nm (c). These averaged profiles were made of 100 height sections taken perpendicular to the bare defect (highlighted with the dashed black line in a) and gallium oxide-covered part (highlighted with the dashed red line in a), respectively. For better comparison, the x-axis orientation of profile ii (in b) is reversed.

Figure 5. Graph showing the P3HT fluorescence intensity dependence on time recorded over areas covered (solid blue line) and uncovered (dotted red line) with a GaOx layer. The insets show fluorescence images of the P3HT film uncovered (wo GaOx) or covered (w GaOx) with a GaOx layer. The images were recorded under nitrogen flow at ta = 5 min before exposure to ambient air (a), at tb = 1.1 d (b), and at tc = 2.9 d (c) after exposure to ambient air. The unprotected P3HT fluorescence bleached within a few minutes after exposure to ambient air. In contrast, P3HT covered with GaOx exhibited fluorescence even after a substantially longer exposure of the sample to ambient air, and its fluorescence decay for t > 0.5 day can be well fitted with the Stern− Volmer equation (solid black line), providing a permeation rate estimation of 0.04 M/day. The initial increase in fluorescence intensity of GaOx-covered P3HT (see also Figure S4) is usually attributed to a process whereupon the P3HT molecules improve their order and exhibit a higher quantum yield (Discussion section), contributing to the contrast inversion observed between (a) and (b).

We therefore applied a recently developed method21 in which layers to be tested are deposited on nanometer-thick layers of fluorescent polymer P3HT. The fluorescence of unprotected polymer was quickly quenched by ambient water and oxygen.5,37 A comparison of fluorescence quenching rates of protected and unprotected polymer layers allows us to prove barrier properties of the layers being tested. Areas of P3HT covered by GaOx layers were identified by bright-field imaging in reflection mode (Figure S5 in the Supporting Information). This approach is based on the different effective reflection coefficients of the mica−P3HT− air, mica−P3HT−GaOx−air, and mica−P3HT−Ga drop−air interfaces leading to different intensities for areas being covered with P3HT, GaOx, or P3HT-GaOx-composites. Finally, the identified areas of P3HT covered with GaOx layers were confirmed by SFM imaging after the fluorescence measurements. Because of the deposition process, areas of P3HT covered by GaOx layers were always located in close proximity to uncovered areas. This partial GaOx layer coverage allowed us to image uncovered and covered P3HT areas simultaneously within the same field of view. This yielded a high measurement comparability between these two conditions, which is a requirement to evaluate protective effects by the GaOx layers. Figure 5 provides a representative example of the time dependence of the fluorescence intensity of a P3HT layer partially covered by GaOx. The uncoated and therefore unprotected polymer became quenched within a few minutes after sample exposure to ambient air (dashed curve in Figure 5). At the same time, the fluorescence intensity of GaOx-coated P3HT increased. The intensity growth saturated after a few hours, followed by a slow decrease in intensity (solid blue curve in Figure 5). The fluorescence intensity decay can be attributed to photoinduced P3HT degradation through interaction with water and oxygen.5,6,21

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DISCUSSION Because liquid Ga is known to build a surface oxide layer even in the presence of trace amounts of oxygen,22 we assume the layers (deposited in this study) to be gallium oxide. In contrast to recently published studies on GaOx layer deposition using evaporation,26−28 chemical vapor deposition,29 sputtering,30 pulsed laser deposition,31 or atomic layer deposition,32 it was not necessary to work under vacuum. Moreover, it was not necessary to anneal the deposited GaOx layers at high temperature, which is often reported for alternative approaches that do not require vacuum conditions, such as sol−gel and chemical solution deposition.33,34 Finally, while most of the mentioned studies focused on a structural characterization of the formed layer (e.g., using scanning tunnelling microscopy,26 electron microscopy,27,29,33 X-ray diffraction,27−29,31,33 X-ray photoelectron spectroscopy,32 the measurement of photo- and electroluminescence,27−30,33 etc.), there are only rare examples of (visco)elastic measurements aimed at understanding the mechanical properties of such thin films.38 No study was found that investigated the protective effects of GaOx layers with respect to mechanical stress or photodegradation. The assumption that the layers consist of oxidized gallium is further supported by several experimental findings discussed in the following text. First, pure Ga should remain in a liquid state under our experimental conditions. Thus, the assumption that the layers consist of pure Ga is in contradiction to their high mechanical stability. In addition, the replication of substrate topography by the layers both below and above the melting temperature of Ga does not imply that the layers are liquid. Therefore, we infer that the liquid Ga drop oxidizes at its interface with ambient air and that moving a drop of liquid Ga along the surface leads to a transfer of Ga oxide sheets onto the surface. We observe them as ultrathin GaOx layers in the SFM 5839


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Langmuir images. The transfer of GaOx layers exposes liquid gallium to ambient air, which oxidizes again fast enough to maintain the transfer process. Moreover, for all surfaces investigated (mica, soda-lime glass slides, and silicon wafers, carrying a nanometerthick native silicon oxide layer) we observed a very similar layer thickness of 2.8 ± 0.2 nm, suggesting that the final layer thickness is mainly a property of gallium and is not strongly affected by the supporting material. Relatively large normal forces (Fn ≥ 2 μN), applied by the SFM tip to GaOx, are required to scratch away the layer during contact mode imaging. This implies the GaOx layers to be mechanically very stable. A precise calculation of the contact mechanics on these small length scales is demanding. Still, by approximating the contact radius by the SFM tip radius (on the order of 10 nm), we estimate that a normal pressure on the order of a few GPa has to be applied to the GaOx layers before they are destroyed by friction. Note that this is a very conservative estimation because this implies a very strong deformation of the spherical tip into a half-sphere and because the true contact area of a sphere with a surface should be smaller than the spherical cross-sectional area. A more accurate treatment should therefore lead to even larger normal pressures at the tip apex. Nevertheless, this is a remarkable result, taking into account that the layer with high stability against wear was obtained by simply bringing a droplet of Ga into contact with the surface. For comparison, Sundararajan et al. report that critical normal loads of 20 μN (for an SFM tip radius of 100 nm) are necessary to wear off 3.5-nm-thick diamondlike carbon coatings (DLCs) on silicon wafers.39 Taking into account the larger tip radius, this value is comparable to the 2 μN reported in this study. A larger wear resistance was observed for 10 nm titanium layers on silicon, requiring critical normal loads of up to 600 μN applied using a diamond Berkovich tip (of unspecified radius).40 It is not surprising that the GaOx layers have lower wear resistance than titanium, which is known for its high mechanical strength and its good adhesion to silicon surfaces. Finally, it was demonstrated that GaOx layers are permeation barriers because they protected P3HT layers from degradation. The fluorescence intensity of unprotected P3HT decays fast, which is attributed to chemical quenching by oxygen and/or water molecules.21 In contrast, P3HT covered with GaOx layers showed an initial increase in fluorescence, which is usually attributed to a process whereupon the P3HT molecules pack closer and exhibit a higher quantum yield.37 The initial increase in fluorescence (Figure 5) saturated at t = 0.3 day. We observed a similar fluorescence intensity increase for a P3HT film sandwiched between two mica sheets, which we regarded as impermeable barriers (section S4 in the Supporting Information). Therefore, we assume that, in the case of the GaOx layer, we deal with the superposition of fluorescence intensity increase and decay, with the latter dominating on longer time scales. Collisional quenching is the least efficient among all processes that can quench fluorescence. Therefore, assuming collisional quenching as the only source of fluorescence decay, we obtain an upper limit for the oxygen and water permeation rates through GaOx layers. In this case fluorescence is expected to decay according to the Stern−Volmer equation:41 F (t ) 1 = with F0 kt + 1

k = KSV

ΔQ (t ) Δt

where F0 is the maximum in the fluorescence intensity, F(t) is the intensity at time t, KSV is the Stern−Volmer constant (assumed to be 6.7 mol−1) for oxygen in P3HT),5 and (ΔQ(t)/ Δt) is the rate of the quencher concentration variation over time.21 Covering P3HT with GaOx layers slowed down the decay of fluorescence intensity strongly, which is reflected by an increase in the rate k by a factor of 8 × 103 ± 5.5 × 103 (compared to uncovered areas of P3HT), indicating the protective behavior of the GaOx layers. To calculate the permeation rate through the GaOx layers from the change in the quencher concentration according to eq 1, the surface area A and volume V of P3HT accessible for quencher permeation have to be estimated: P=

ΔQ (t ) V Δt A

(2)

(P is the permeation rate.) SFM imaging revealed that while most of the P3HT films we produced were flat, some of the films exhibited spherical protrusions. We attribute them to P3HT aggregates. Taking into account protrusions for the calculation of surface area and P3HT volume did not have any substantial impact on the results or, correspondingly, the permeation rate estimations (section S6 in the Supporting Information). Permeation rates were found to be less than 3 × 10−6 mol m−2 day−1, which corresponds to an oxygen transmission rate (OTR) of less than 0.08 cm3 m−2 day−1. Note that under our experimental conditions (sufficient time and illumination intensity for reaction) the Stern−Volmer constant for oxygen in P3HT also gives an upper limit for the Stern−Volmer constant for water in P3HT because the quenching efficiency is dominated by exciton diffusion to the quenching center and not by diffusion of the quenching center itself.42 Hence, the estimated molar oxygen permeation rate accounts for an upper limit of the molar water permeation rate, which allows us to estimate a water vapor transmission rate (WVTR) of less than 0.06 mg m−2 day−1. These transmission rates do not fulfill the requirements for the encapsulation of OLEDs but are in good agreement with industrial demands on high barriers (OTR < 1 cm3 m−2 day−1) and ultrahigh barriers (WVTR < 5 mg m−2 day−1).8−10,43−45 Hence, nanometer-thick GaOx layers can serve as protective coatings in organic solar industries or for related applications.

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CONCLUSIONS We showed that moving a liquid Ga droplet along a substrate is a simple approach to transferring native gallium oxide layers from the surface of the Ga droplet onto the substrate under ambient conditions. Optically transparent Ga oxide layers with a homogeneous 2.8 ± 0.2 nm thickness and areas of up to 104 μm2 have been demonstrated. The layers are mechanically stable, enduring normal pressures on the order of 1 GPa. They also provide protection against oxygen and water with oxygen and water permeation rates of less than 0.1 cm3 m−2 day−1. Furthermore, the layers were flexible enough to replicate a surface topography with features that were a few nanometers high. Thus, nanometer-thick gallium oxide layers can be considered to be a promising coating for organic electronics.

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(11) Groner, M. D.; George, S. M.; McLean, R. S.; Carcia, P. F. Gas diffusion barriers on polymers using Al2O3 atomic layer deposition. Appl. Phys. Lett. 2006, 88, 051907. (12) Carcia, P. F.; McLean, R. S.; Hegedus, S. Encapsulation of Cu(InGa)Se2 solar cell with Al2O3 thin-film moisture barrier grown by atomic layer deposition. Sol. Energy Mater. Sol. Cells 2010, 94, 2375− 2378. (13) Meyer, J.; Görrn, P.; Bertram, F.; Hamwi, S.; Winkler, T.; Johannes, H.-H.; Weimann, T.; Hinze, P.; Riedl, T.; Kowalsky, W. Al2O3/ZrO2 Nanolaminates as Ultrahigh Gas-Diffusion BarriersA Strategy for Reliable Encapsulation of Organic Electronics. Adv. Mater. 2009, 21, 1845−1849. (14) Fakhri, M.; Theisen, M.; Behrendt, A.; Görrn, P.; Riedl, T. Topgate zinc tin oxide thin-film transistors with high bias and environmental stress stability. Appl. Phys. Lett. 2014, 104, 251603-1− 251603-5. (15) Dameron, A. A.; Davidson, S. D.; Burton, B. B.; Carcia, P. F.; McLean, R. S.; George, S. M. Gas Diffusion Barriers on Polymers Using Multilayers Fabricated by Al2O3 and Rapid SiO2 Atomic Layer Deposition. J. Phys. Chem. C 2008, 112, 4573−4580. (16) Fahlteich, J.; Fahland, M.; Schönberger, W.; Schiller, N. Permeation barrier properties of thin oxide films on flexible polymer substrates. Thin Solid Films 2009, 517, 3075−3080. (17) Kim, W.-S.; Ko, M.-G.; Kim, T.-S.; Park, S.-K.; Moon, Y.-K.; Lee, S.-H.; Park, J.-G.; Park, J.-W. Titanium Dioxide Thin Films Deposited by Plasma Enhanced Atomic Layer Deposition for OLED Passivation. J. Nanosci. Nanotechnol. 2008, 8, 4726−4729. (18) Henry, B. M.; Erlat, A. G.; McGuigan, A.; Grovenor, C. R. M.; Briggs, G. A. D.; Tsukahara, Y.; Miyamoto, T.; Noguchi, N.; Niijima, T. Characterization of transparent aluminium oxide and indium tin oxide layers on polymer substrates. Thin Solid Films 2001, 382, 194− 201. (19) da Silva Sobrinho, A. S.; Latrèche, M.; Czeremuszkin, G.; Klemberg-Sapieha, J. E.; Wertheimer, M. R. Transparent barrier coatings on polyethylene terephthalate by single- and dual-frequency plasma-enhanced chemical vapor deposition. J. Vac. Sci. Technol., A 1998, 16, 3190−3198. (20) Ko, Y.; Bang, S.; Lee, S.; Park, S.; Park, J.; Jeon, H. The effects of a HfO2 buffer layer on Al2O3-passivated indium-gallium-zinc-oxide thin film transistors. Phys. Status Solidi RRL 2011, 5, 403−405. (21) Lange, P.; Dorn, M.; Severin, N.; Vanden Bout, D. A.; Rabe, J. P. Single- and Double-Layer Graphenes as Ultrabarriers for Fluorescent Polymer Films. J. Phys. Chem. C 2011, 115, 23057−23061. (22) Regan, M. J.; Tostmann, H.; Pershan, P. S.; Magnussen, O. M.; DiMasi, E.; Ocko, B. M.; Deutsch, M. X-ray study of the oxidation of liquid-gallium surfaces. Phys. Rev. B 1997, 55, 10786−10790. (23) Xu, Q.; Oudalov, N.; Guo, Q.; Jaeger, H. M.; Brown, E. Effect of oxidation on the mechanical properties of liquid gallium and eutectic gallium-indium. Phys. Fluids 2012, 24, 1−18. (24) Nieminen, M.; Niinisto, L.; Rauhala, E. Growth of gallium oxide thin films from gallium acetylacetonate by atomic layer epitaxy. J. Mater. Chem. 1996, 6, 27−31. (25) Fortunato, E.; Gonçalves, A.; Marques, A.; Viana, A.; Á guas, H.; Pereira, L.; Ferreira, I.; Vilarinho, P.; Martins, R. New developments in gallium doped zinc oxide deposited on polymeric substrates by RF magnetron sputtering. Surf. Coat. Technol. 2004, 180−181, 20−25. (26) Hale, M. J.; Yi, S. I.; Sexton, J. Z.; Kummel, A. C.; Passlack, M. Scanning tunneling microscopy and spectroscopy of gallium oxide deposition and oxidation on GaAs(001)-c(2 × 8)/(2 × 4). J. Chem. Phys. 2003, 119, 6719−6728. (27) Zhang, J.; Jiang, F. H. Catalytic growth of Ga2O3 nanowires by physical evaporation and their photoluminescence properties. Chem. Phys. 2003, 289, 243−249. (28) Kim, H. W.; Kim, N. H. Growth of beta-Ga2O3 nanobelts on Ircoated substrates. Appl. Phys. A 2005, 80, 537−540. (29) Kuo, C. L.; Huang, M. H. The growth of ultralong and highly blue luminescent gallium oxide nanowires and nanobelts, and direct horizontal nanowire growth on substrates. Nanotechnology 2008, 19, 155604.

ASSOCIATED CONTENT

S Supporting Information *

Scratch tests showing the 3 nm thickness for extended GaOx layers, further measurements on the mechanical stability of GaOx layers, one example for corrugations that are frequently observed on bare soda-lime glass, a comparison of the initial fluorescence intensity of P3HT layers coated by either GaOx or mica, a comparison of P3HT-GaOx sample morphology with reflectivity and fluorescence imaging, and the results of different models used to estimate water and oxygen permeation rates. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00871.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +49-30-2093-7621. *E-mail: [email protected]. Tel: +49-3834-86-4710. *E-mail: [email protected]. Tel: +46-31-772-33-67. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Fardin Gholami, M. Sc. (Humboldt-Universität) for supporting measurements. Financial support of the European Social Fund (grant number UG 10 022), the state of Mecklenburg-Vorpommern, the Deutsche Forschungsgemeinschaft (He 1616/14-1 and GRK 1947), and the HelmholtzGemeinschaft (Helmholtz-Energie-Allianz) is appreciated.

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REFERENCES

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