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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 8190−8196

Encapsulating Chemically Doped Graphene via Atomic Layer Deposition A. Black,†,‡,⊥ F. J. Urbanos,†,‡ M. R. Osorio,† R. Miranda,†,‡,§ A. L. Vázquez de Parga,†,‡,§ and D. Granados*,† †

IMDEA Nanociencia, 28049 Madrid, Spain Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain § Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain ‡

S Supporting Information *

ABSTRACT: Controlling graphene’s doping will be critically important for its incorporation into future electronic and optoelectronic devices. Noncovalent functionalization through adsorption of organic molecules on graphene’s surface has proved to be a promising route for achieving p- or n-type doping. However, due to the poor adhesion of the molecules, these tend to desorb over time under standard environmental conditions or in the presence of certain solvents. The resulting reversal in the achieved chemical doping is a major obstacle to using organic molecules as noncovalent graphene dopants. In this work, we present a simple method for achieving long-term p- and n-doping of graphene devices through vapor phase evaporation of organic molecules, followed by encapsulation under an inert Al2O3 film. This film, grown via an optimized atomic layer deposition process, ensures long-term doping stability, as confirmed by electrical transport and Raman spectroscopy measurements. The doping is maintained even after storing the devices for six weeks in ambient conditions and immersing them in a dopant removing solvent, demonstrating that the film is as an effective barrier against environmental degradation of the doped devices. KEYWORDS: graphene, chemical doping, atomic layer deposition, encapsulation, thin film growth

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INTRODUCTION Reliably controlling graphene’s doping, which entails customizing its charge carrier type and concentration, is critical if graphene is to ever be incorporated into commercial electronic and optoelectronic devices.1,2 Covalent p- and n-type doping has been achieved through substitution of carbon atoms in graphene’s lattice.3,4 Unfortunately, this approach severely distorts the atomic lattice, drastically reducing the charge carrier mobility and conductivity. In contrast, noncovalent functionalization, for example through molecular adsorption, does not significantly alter graphene’s lattice and therefore hardly perturbs its outstanding mechanical and electrical properties while allowing charge transfer and thus doping to occur.5 This approach has been adopted in many studies over the past few years. For example, 7,7,8,8-tetra-cyanoquinodimethane (TCNQ) has been used as a p-dopant,6,7 with the resulting graphene utilized as a conductive anode in graphene/ organic solar cells.8 Nitric acid has also been widely used as a strong graphene p-dopant, generally by soaking the graphene in the acid.9,10 However, subsequent studies showed that the doping was not robust, returning to lower doping levels over the course of several days through exposure to ambient conditions.11 Similar work has been carried out to n-dope © 2018 American Chemical Society

graphene. Vapor phase evaporation of ethylenediamine (EDA), containing an electron donating amine group, resulted in ndoped graphene devices.12 Nevertheless, after exposure to ambient conditions, doping loss occurred over the course of several days. Doping achieved through noncovalent functionalization is impermanent due to the weak adhesion of the molecules on graphene’s surface. These tend to desorb over time through exposure to ambient conditions or specific solvents. One way of preventing environmental degradation is by encapsulating devices below an inert film such as Al2O3 grown by atomic layer deposition (ALD).13 Growing ALD films on graphene, however, is exceedingly difficult due to the lack of reactive functional groups on its surface. Several approaches have been investigated to circumvent this drawback. Continuous Al2O3 films were grown via ALD by seeding the entire graphene surface with a self-assembled monolayer (SAM) molecule14,15 or by pretreating the graphene surface with a mild plasma.16,17 However, using SAMs requires the entire graphene surface to Received: December 8, 2017 Accepted: February 20, 2018 Published: February 20, 2018 8190

DOI: 10.1021/acsami.7b18709 ACS Appl. Mater. Interfaces 2018, 10, 8190−8196

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of Method 1 for direct ALD growth. (b) Optical, (c) SEM, and (d) AFM images of EDA functionalized device encapsulated in Al2O3 film grown using Method 1. Scale bars in panels b and c, 10 μm; in panel d, 5 μm. (e) Schematic of Method 2 for direct ALD growth. (f) Optical, (g) SEM, and (h) AFM images of EDA functionalized device encapsulated in Al2O3 film grown using Method 2. Scale bars in panels f and g, 10 μm; in panel h, 5 μm. In all images, white dotted lines mark the area covered by the graphene device. annealed in Ar at 1000 °C for 3 h prior to growth in a gas mixture of 100/50/2 Ar/H2/CH4 for 10 min, followed by rapid cooling in Ar. The monolayer graphene was removed from the copper foil and transferred to a substrate using a PMMA mediated wet etching method.25 The substrate consisted of a degenerately p-doped Si wafer covered in a 285 nm SiO2 layer. Prior to graphene transfer the substrate was patterned with 5/50 nm Cr/Au electrodes. Devices were fabricated via laser writing optical lithography, using AZ 5214 (MicroChemicals) image reversal resist and oxygen plasma to etch the graphene to a size of 8 × 50 μm, followed by washing in n-methylpyrrolidone at 80 °C. Vapor Phase Doping. Vapor phase functionalization began by wetting clean room tissue paper with the desired doping liquid. The wet paper and sample were placed on a Petri dish next to each other, and the Petri dish was placed on a hot plate. The Petri dish was covered by a glass beaker and the hot plate heated, resulting in the evaporation of the liquid dopant and condensation on the inside of the glass beaker.12 Placing the sample on the hot Petri dish ensured that no liquid condensed on it. Functionalization was completed after the liquid dopant had fully evaporated from the tissue paper. The doping (change in charge carrier concentration) was proportional to the amount of liquid used, as shown in Figure S3, with larger quantities of doping requiring longer evaporation times. EDA was evaporated at 70 °C, with typical volumes and times in the range of 2−8 mL, 15−30 min. Nitric acid (60% by volume in water) was evaporated at 50 °C with typical volumes and times in the range 2−4 mL, 10−20 min. Electrical Characterization. Electrical measurements were carried out in ambient using a Keithley 4200-SCS parameter analyzer connected to a probe station. All measurements were carried out using a two-point configuration with a third electrode back gate. Charge carrier mobilities were calculated using the maximum (minimum) transconductance g = dId/dVg in the electron (hole) branches, where Id is the source-drain current and Vg is the gate voltage. The mobility is thus calculated as μ = (eL/CoxVdW)g, where W and L are the device width and length, Cox = 11.9 nF/cm2 is the SiO2 capacitance, Vd is the source-drain voltage, and e the elemental charge. For the undoped, EDA, and nitric acid doped samples, all electrical measurements were performed on 8−12 devices before and after doping and multiple times after film growth to monitor changes over time. Samples were measured immediately after chemical doping.

be covered by molecules, preventing customization of the charge carrier intensity, and plasma treatments damage both the graphene and any organic molecules adhered to its surface. Therefore, neither of these methods is appropriate for tailoring the chemical doping of the graphene prior to ALD growth. Another approach for growing Al2O3 films on chemically doped graphene consists of evaporating a thin metallic layer of Al onto the sample prior to ALD growth.18,19 Although this method can yield homogeneous and compact films, the resulting electronic properties of the graphene can vary widely because they are very sensitive to the evaporation conditions of the Al film.20 Recently, methods for directly growing Al2O3 ALD films on graphene have garnered attention, without the use of any extrinsic seeding layer or plasma treatment. These methods involve an in situ preseeding of the sample with a layer of water in the ALD growth chamber, followed by conventional ALD growth.16,21−23 Covering the sample in a thin layer of water prior to growth ensures that Al2O3 nucleation is homogeneous over the entire surface. In this work, we explored noncovalent doping of graphene through the evaporation of liquid organic molecules: nitric acid and EDA for p- and n-doping, respectively. Long-term doping was achieved by encapsulating the chemically functionalized devices beneath an Al2O3 film, grown directly on the graphene via an optimized ALD method using in situ preseeding with water. The resulting films were homogeneous and compact over the entire sample surface and protected the doping in graphene devices even after six weeks of exposure to ambient conditions and immersion in a dopant removing solvent. Similar growth methods to the one developed in this work should also be applicable to ALD growth of other high-k materials such as HfO.24

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EXPERIMENTS AND METHODS

Graphene Growth and Fabrication. Graphene was grown on a copper foil within a CVD furnace at 10 mbar. The copper foil was first 8191


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Figure 2. Resistance versus gate voltage transfer curves before and after chemical functionalization and after Al2O3 film growth for (a) undoped, (b) nitric acid doped, and (c) EDA doped devices. Histograms showing measured (d) intrinsic doping level and (e) hole and (f) electron charge carrier mobility for numerous devices at different stages in the doping and encapsulation process. ALD Film Growth. Following electrical measurements, doped samples were immediately placed in the ALD growth chamber (Ultratech Fiji). All process steps for both growth methods were carried out at 80 °C using water and trimethyl aluminum (TMA) as precursor gases with chamber purge times of 60 s after each water or TMA pulse. At this temperature, water on the graphene surface will be in the liquid state, increasing the likelihood of subsequent Al2O3 nucleation.24 Growth Method 1 (Figure 1a) consisted of 150 pulses of water prior to growing the 60 nm of Al2O3 by successive water/TMA pulses. Growth Method 2 (Figure 1f) consisted of 150 pulses of water followed by 50 pulses of TMA, then 150 pulses of water, before finally growing 60 nm of Al2O3 by successive water/TMA pulses. After ALD growth, devices were kept in ambient conditions with a temperature of 22−25 °C and relative humidity of 50−60%. Raman Characterization. Scanning Raman spectroscopy was carried out using a 488 nm continuous wave laser excitation source. Light was focused onto the sample and collected via a 40× magnification, 0.65 numerical aperture objective and directed toward an electron multiplied silicon CCD detector (Andor Newton EM) passing through a 0.5 m diffraction spectrometer.

ments extremely difficult. Similar results were obtained for undoped and nitric acid doped graphene, indicating that the chemical functionalization does not significantly affect the film growth. Taken together, the optical, SEM, and AFM images confirm that the Al2O3 film grown via Method 1 is rough and porous. It should be noted that despite the drawbacks of this growth method, the film exhibits the same morphology throughout the sample surface, indicating that equivalent nucleation has been achieved in regions with and without graphene, in contrast to what has been observed using conventional ALD growth methods, as shown in Figure S1. To improve film quality, several changes were implemented in growth Method 2, as shown in the schematic of Figure 1e. We speculated that the debris on top of the film may have been caused by excessive water remaining on the sample surface after the initial water seeding. Therefore, 50 TMA pulses were added after the 150 pulses of water seeding to ensure full reaction of the seeding water film into Al2O3. To further homogenize nucleation over the sample area and prevent the sort of small pinholes observed with Method 1, we introduced a second water seeding process (150 water pulses) following the 50 TMA pulses and prior to regular film growth. The water molecules in this second seeding process may be helping to fill in the small holes remaining in the initially nucleated Al2O3 film, thus covering the entire sample surface and allowing for the growth of a conformal, homogeneous, compact, and smooth film. Devices encapsulated under an Al2O3 film using Method 2 are shown in the optical, SEM, and AFM images in Figures 1f− h, respectively. The optical and SEM images clearly reveal that the resulting film is much smoother and homogeneous over the entire surface of the sample, including Au electrodes, graphene device, and SiO2 substrate, free of the debris and pinholes present in films grown using Method 1.

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RESULTS AND DISCUSSION Figure 1 shows two EDA functionalized graphene devices beneath Al2O3 films grown using growth Method 1 (Figures 1b−d) and Method 2 (Figures 1f−h). Growth Method 1, which utilized 150 water seeding pulses (see schematic in Figure 1a), resulted in a rough film as observed in the optical image of Figure 1b. The dark blue spots and bright green spots correspond to large height and/or composition differences within the film. These differences are seen even more clearly in the SEM image of Figure 1c, showing white patches of debris which appear to be on top of the film. Small dark spots appear to be pinholes in the film and are more clearly observed in the AFM image of Figure 1d, along with the huge height difference caused by the debris on top of the film. The debris is not well adhered and easily moved by the AFM tip, making measure8192


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the forward and backward sweeps in Figure 2a, was reduced by a factor of 2, from 2.43 × 1012 to 1.17 × 1012 (calculated as ntrap= CoxΔV/e where ΔV is the difference in Dirac Point voltages for the forward and backward sweeps). By drastically reducing the effect of both p-doping contaminants and substrate charge traps, electron scattering centers are significantly reduced, resulting in a significant increase in the electron mobility, shown in Figure 2f, and a much more symmetric transfer curve.29 Similar results have been demonstrated recently, through the growth of an epitaxially self-assembled layer of alkanes which decoupled the graphene from the SiO2 substrate and removed adsorbed contaminants.33 This resulted in a significantly improved charge carrier mobility, reduced transfer curve asymmetry, and reduced intrinsic p-doping. Initial attempts at p-doping graphene devices were carried out by evaporating solid TCNQ powder, resulting in the growth of a thin film.6 Although the TCNQ acted as an efficient p-dopant, it also produced a significant decrease in charge carrier mobility, in particular the electron mobility (note the strong transfer curve asymmetry in Figure S2). This is most likely due to the growth of TCNQ crystals and filaments that strain the graphene surface (see Figure S3). We therefore decided not to use TCNQ as a p-dopant in subsequent experiments. Evaporating nitric acid onto the graphene devices resulted in strong p-doping and reduced resistance, as observed in the red curve of Figure 2b, in agreement with previous reports.9,10 The doping is so strong that the charge carrier density and charge carrier mobility of the device cannot be calculated (because VDP is beyond the measuring range), hence the missing “Doped” point in the red curves of the histogram Figures 2d−f. Al2O3 film growth reduced the p-doping, as expected considering the experiments on the undoped devices, shifting the device’s Dirac point back into the measuring range. The positive doping value of about 6.0 × 1012 cm−2 holds steady for 2 weeks, as shown in Figure 2d. The hole mobility, seen in Figure 2e, is largely unaffected by the ALD growth and exposure to ambient conditions for 2 weeks, remaining between 1200 and 1300 cm2 V−1 s−1 throughout. The electron mobility is significantly lower, below 500 cm2 V−1 s−1. This is not surprising because the nitric acid is a p-dopant, which is most likely acting as an electron charge scattering center.29 EDA evaporation resulted in a strong negative doping of the graphene device, as seen in Figures 2c and d. In addition, a significant increase in both the electron and hole mobilities was measured (Figures 1e and f). This improvement was not observed by Kim et al. in their experiments with EDA doping.12 In our case, the observed improvement in mobility may be a consequence of the EDA eliminating some of the photoresist remnants present on the graphene surface. Indeed, exposing a photoresist covered sample to EDA produced significant photoresist dissolution, as shown in Figure S4. If the EDA is helping to eliminate photoresist remnants and thereby improving charge carrier mobility, it would make sense that Kim et al.12 did not observe such an improvement because they utilized electron beam lithography, a process which is known to be much less contaminating,28 to fabricate their graphene devices. Figure S5 shows that (i) increasing the amount of EDA evaporated increased the device’s n-doping and (ii) doping reversal occurred over the course of several days by exposing EDA doped devices to ambient conditions. Both of these

The optical image in Figure 1f shows orange patches on the substrate, in particular in the vicinity of the graphene device. These could be organoaluminum amide crystals, resulting from a chemical reaction between the adsorbed EDA and the gaseous TMA precursor utilized during film growth. Reactions between these two compounds form polymeric glassy solids with a chelated ring structure with the exact composition depending on the concentrations of the original compounds and reaction conditions.26,27 The amide groups in the resulting crystals are expected to act as efficient electron donors, thus n-doping the graphene. The AFM image in Figure 1h confirms the smoothness of the film in comparison to that seen with Method 1 (Figure 1h). The small bits of debris covering the graphene device are most likely remnants of PMMA or photoresist from the transfer and fabrication process. Although the causes behind the poor film morphology in Method 1 are not fully understood, the fixes adopted in Method 2 proved to be highly effective at growing Al2O3 films with a much better morphology on undoped and EDA/nitric acid doped graphene devices. All subsequent experiments were therefore carried out using Growth Method 2. Growing films on undoped devices allowed us to discern the effect of the Al2O3 on the electrical properties of graphene. Figure 2a shows forward and backward gate sweeps on undoped devices before and after film growth. Prior to film growth, the resistance maximum (Dirac peak) of the device occurs for positive gate voltages, indicating that it is significantly p-doped. Significant hysteresis is also observed, with the Dirac peak shifting toward higher p-doping levels on the backward gate sweep. In addition, the device shows asymmetrical conduction, with an elevated resistance in the electron charge carrier branch (for Vg greater than the Dirac Point voltage VDP). Both the inherent p-doping of the device and conduction asymmetry can be attributed to ambient contaminants such as oxygen and water, and fabrication process contaminants such as photoresist, which is known to strongly p-dope graphene.22,28 In the model proposed by Farmer et al.,29 certain chemical dopants can induce a long-range Coulomb scattering potential in the graphene channel, which manifests itself as an energy barrier at the electrode/graphene interface, suppressing the injection of one type of charge carrier into the device channel. Dopants of p (n)-type will tend to suppress the injection of electrons (holes), resulting in the resistance curve asymmetry observed in Figure 2a for the pristine device. After growth of an encapsulating Al2O3, intrinsic p-doping, hysteresis, and charge conduction asymmetry were significantly reduced. The reduction in intrinsic doping seen in the transfer curve was replicated over a dozen devices, as evidenced in the histogram showing the intrinsic charge carrier density n0 in Figure 2d (calculated as n0 = CoxVDP/e). ALD growth in vacuum at 80 °C could result in the desorption of ambient contaminants such as oxygen and water and possibly even the passivation of contaminating photoresist residues through encapsulation within the Al2O3 film and reduction of charge transfer between them and the graphene. In addition, the reduction in p-doping could be caused by a redox reaction of water molecules occurring in an oxygen deficient environment, with the resulting electrons driven toward unoccupied levels in the graphene.30 The reduction in hysteresis observed in Figure 2a agrees with previous reports and is likely due to the passivation of charge traps at the graphene/SiO2 interface.22,31,32 The estimated charge trap density, calculated from 8193


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ACS Applied Materials & Interfaces observations replicate the results of Kim et al.12 To prevent the molecular desorption responsible for the doping reversal, we grew an encapsulating Al2O3 film using the optimized ALD process. Strong n-doping is evident after film growth, with the residual doping shifting from 0.8 × 1012 to around −5 × 1012, as seen in the histogram of Figure 2d. Once again, the doping proved to be robust and hardly varied after two weeks in ambient conditions. Film growth significantly reduced the hole mobility from an initial value of 2280−1332 cm2 V−1 s−1. This seems to indicate that the polymeric organoaluminum amide crystals, expected to result from chemical reaction between TMA and EDA, act as n-dopants and are also stronger hole scattering centers than the adsorbed EDA molecules. In contrast, film growth resulted in an improvement in electron mobility (from 1299 to 1999 cm2 V−1 s−1). This trend, which was also observed in the undoped devices, is expected due to the passivation of charge traps and elimination of p-doping contaminants, as explained above. Both the electron and hole mobilities decreased after two weeks in ambient conditions. Although the exact mechanism behind this is unknown, it may possibly be due to the reaction between residual EDA and TMA in the Al2O3 film producing more organoaluminum amide. Unreacted byproducts are a common problem within ALD films grown at low temperatures, particularly below 100 °C.13 The protective nature of the encapsulating Al2O3 film is underscored by Figure 3a, which shows that even after six

Scanning Raman spectroscopy maps were carried out to further explore the effects of the chemical doping and encapsulation process. Spectra of EDA and nitric acid doped devices, taken in the same locations before and after doping and film growth, are shown in Figure 4a and b. Both of these

Figure 4. Raman spectra at taken in the same location during different points in the doping and encapsulation process for (a) nitric acid and (b) EDA doped device. Histograms showing evolution of (c) I2D/Ig ratio and (d) G peak position throughout the doping and encapsulation process.

devices show that the graphene D peak is nearly nonexistent throughout all steps of the process, indicating that the doping and encapsulation do not introduce defects or disorder.34 Histograms showing Lorentz fitted I2D/IG ratio and G peak position (Figure 4c and d, respectively) were calculated from Raman scanning maps of the same devices at different stages of the doping and encapsulation process. The point clouds used to calculate these histograms are shown in Figure S6, along with the scanning Raman maps. Charged doping impurities on graphene’s surface cause the I2D/IG ratio to decrease, as confirmed experimentally in multiple studies.35,36 Charged impurities increase electron−electron scattering, thereby decreasing the recombination of photoexcited electron−hole pairs responsible for the 2D peak intensity.37 Given that the G peak intensity is practically constant at these doping levels, a decreased 2D peak intensity will reduce the I2D/IG ratio.37 A strong decrease in I2D/IG ratio is observed in Figure 4c after chemical doping of the devices, in particular in the case of nitric acid. This trend is clearly observable in the I2D/IG maps shown in Figures S6a and b, with the EDA doped device showing a value close to 1.5 and the nitric acid doped device a value close to 1. The low I2D/IG value in the nitric acid sample, apart from having an initially lower pristine value, is due to its very strong p-doping, which is well outside the measuring range of electrical transport measurements, as seen in Figure 2b. Much like the I2D/IG ratio, the position of the G peak, shown in Figure 4d, is also sensitive to charged surface dopants.38,39 The behavior of the G peak is thus compatible with the I2D/IG ratio, with an increase in peak position after chemical doping, also observed in the scanning maps of Figure S6c and d.

Figure 3. Resistance versus gate voltage transfer curves. (a) Samples immediately after Al2O3 film growth (dotted line) and six weeks later (solid line). (b) Transfer curves of EDA doped samples before (solid line) and after (dashed line) immersion in IPA for 1 h. This causes the EDA doped sample without an encapsulating Al2O3 film, in bright green, to lose its n-doping, as indicated by the black arrow.

weeks of storage in ambient conditions, the graphene devices retain their electronic properties. The undoped device shows a very low intrinsic doping and hole mobility of 1121 cm2 V−1 s−1, whereas the nitric acid and EDA doped devices retain their p- and n-doping with hole mobilities of 1476 and 926 cm2 V−1 s−1, respectively. Figure 3b shows the effect of immersing EDA doped devices in isopropyl alcohol (IPA), an efficient EDA solvent, for 1 h. The n-doping is clearly reversed in the sample without a protective encapsulating film, indicating that the IPA is washing away the EDA molecules. In contrast, the transfer curve of the EDA doped sample covered in an Al2O3 film hardly changes after IPA immersion. This result, taken in conjunction with the constancy of the electrical properties of the devices after 6 weeks in ambient conditions, demonstrates that the film is an effective barrier, protecting the devices from environmental degradation. 8194


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ACS Applied Materials & Interfaces After growing an encapsulating Al2O3 film, the nitric acid doped sample shows an increase (decrease) in the I2D/IG ratio (G peak position), indicating a reduction in the charged impurities on the graphene surface. These measurements are in agreement with the reduced doping observed in electrical transport measurements due to the desorption of contaminating doping impurities and passivation of substrate charge traps. The EDA doped sample, in contrast, exhibits the opposite behavior. The decreased (increased) I2D/IG ratio (G peak position) indicate that the charged impurity concentration increased after film growth on EDA doped samples. Thus, the Raman spectroscopy and electrical transport measurements indicate that the organoaluminum amide crystals, resulting from the reaction of EDA and TMA during film growth, are strongly charged n-doping impurities that preferentially scatter hole charge carriers.

Notes

CONCLUSION Graphene devices were successfully p- or n-doped by evaporating nitric acid or EDA from the liquid. The achieved chemical doping was preserved by growing an encapsulating Al2O3 film on the samples via ALD. Successful film growth hinged on using repeated dosing steps of water and TMA to cover the entire sample surface, ensuring adequate nucleation of the Al2O3. This direct growth method reduced the concentration of environmental p-doping contaminants and passivated substrate charge traps, reducing the intrinsic pdoping, hysteresis, and preferential electron charge scattering present in the devices. Nitric acid doped devices maintained their p-doping after Al2O3 film growth. EDA doped devices were strongly n-doped after film growth, most likely due to the presence of organoaluminum amide crystals on the graphene surface, which were shown to act as hole scattering charged impurity centers. The electrical properties of the chemically doped devices were maintained even after storage in ambient conditions for 6 weeks (and possibly longer) and immersion in a dopant removing solvent for up to 1 h. Thus, encapsulating Al2O3 films, which act as protective barriers preventing environmental degradation, could help to further the integration of chemically doped graphene into future electronic and optoelectronic devices.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Spanish Ministry of Economy, Industry and Competitiveness through Grant FIS2015-67367C2-1-P, SUPERMAN ESP2015-65597-C4-3-R, and the Comunidad de Madrid through Grant S2013/MIT-3007-MAD2DCM and S2013/MIT-2740. D.G. acknowledges RYC-201209864. A.B. acknowledges Graphene Core H2020-FETFLAG2014 and the Universidad Autónoma de Madrid FPI scholarship grant. We would also like to thank Dr. Santiago Casado for help with AFM measurements and Dr. Emilio Pérez for help with interpreting the results. IMDEA Nanoscience acknowledges support from “Severo Ochoa” Programme for Centres of Excellence in R&D (MINECO, Grant SEV-2016-0686).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18709. SEM image of conventional ALD growth method, characterization of TCNQ functionalized samples, image showing effect of EDA functionalization on photoresist and effect of cumulative EDA evaporation, Raman characterization maps and point clouds used to construct Figure 4 (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

A. Black: 0000-0002-0726-5357 Present Address ⊥

A.B.: Institute of Physical Chemistry, University of Hamburg, Grindelalle 117, 20146 Hamburg, Germany. 8195


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