Effective hole conductivity in nitrogen-doped CVD-graphene by singlet oxygen treatment under photoactivation conditions

Nitrogen substitutional doping in the π-basal plane of graphene has been used to modulate the material properties and in particular the transition from hole to electron conduction, thus enlarging the field of potential applications. Depending on the doping procedure, nitrogen moieties mainly include graphitic-N, combined with pyrrolic-N and pyridinic-N. However, pyridine and pyrrole configurations of nitrogen are predominantly introduced in monolayer graphene:N lattice as prepared by CVD. In this study, we investigate the possibility of employing pyridinic-nitrogen as a reactive site as well as activate a reactive center at the adjacent carbon atoms in the functionalized C–N bonds, for additional post reaction like oxidation. Furthermore, the photocatalytic activity of the graphene:N surface in the production of singlet oxygen (1O2) is fully exploited for the oxidation of the graphene basal plane with the formation of pyridine N-oxide and pyridone structures, both having zwitterion forms with a strong p-doping effect. A sheet resistance value as low as 100 Ω/□ is reported for a 3-layer stacked graphene:N film.

www.nature.com/scientificreports/ catalytic activity of the graphene surface, such as the well investigated activated generation of reactive oxygen species (ROS) [28][29][30][31] . Among these new "catalytic" capabilities, the production of singlet oxygen ( 1 O 2 ) by both (i) activating the dissociation of peroxydisulfate 28 and (ii) the photosensitize excitation of molecular oxygen 30 is of particular interest. In fact, singlet oxygen (also known as singlet delta oxygen 1 O 2 ( 1 Δg)), being a non-radical reactive oxidizing specie, does not intervene on the conjugated double bond as it happens for radical species (oxygen atoms, OH radicals, ozone) and shows higher reactive selectivity also because of its electrophilic nature.
In this paper, we report on N-doped graphene, grown by CVD, and its subsequent chemical modification to improve the p-type conductivity, taking advantage of the presence of nitrogen functionalities as new active sites for selective reactions, without affecting the sp 2 graphene network. As for the CVD growth of N-doped graphene on copper foil, the major challenges are how to control the insertion of the different N-functionalities. Specifically, in order to realize an increase in p-type doping, we need to minimize the graphitic nitrogen in favour of pyrrolic and, even better, pyridinic nitrogen; this is because each nitrogen substituting a graphitic carbon increases the number of conjugated π-electrons, thus improving the n-type conductivity. It is important to underline that N-graphitic atom, like carbon atom in pure graphene, has a low reactivity. Among the different methodologies proposed in literature, we opted for the CVD growth of N-functionalized graphene (graphene:N) via N atoms embedded into Cu substrate and used as a nitrogen solid source 24 . We show that this approach allows obtaining N-graphene layers with small grain size and therefore a high density of structural defects, such as grain boundaries and carbon vacancies, which are the preferred sites for the inclusion of pyridinic-and pyrrolic-nitrogen. From this point of view, in other words, graphene grain-boundary defects do not damage the material properties but rather improve them 32 . The post-growth treatment of N-doped graphene to realize chemical modifications at nitrogen reactive sites is performed by photoirradiation with Xe lamp of N-graphene sample in air. The chemistry, involving the interaction of singlet oxygen photo-catalytically activated on N-graphene, is highly selective towards pyridine active sites leaving the basal plane components intact. We show that the pyridine ring almost disappear in favour of pyridine N-oxide and pyridone structures, both having a strong p-doping effect.

Results and discussion
To enhance the performances of graphene employed as a transparent conductor, a uniquely accessible route is to reduce the sheet resistance by operating a chemical doping. Therefore, it is essential to fingerprint the structure of the graphene layer and to understand the possible chemical modification for achieving high efficiency and stability in the charge transport process. For the nitrogen doped graphene (G:N), since the presence of C-N hetero-bonds into the aromatic platform, the main question is: are there possibilities to functionalize the aryl carbon in such a way as to generate new electron-withdrawing groups, thus improving p-doping? The outcomes arising from the present study confirm positively the hypothesis we set out. Figure 1 is a schematic representation of a crystalline grain, i.e. the graphene flakes, of the nitrogen-graphene (G:N) sheet as prepared and after aerobic photo-oxidation with singlet oxygen generated by Xe-lamp irradiation. In the schemes, in addition to the typical structural defects (carbon vacancy and Stone-Wales defects) and oxygen chemical functionalities (hydroxyl, carbonyl, carboxyl), the highlights are on the nitrogen functionalities. In as prepared G:N the N-functionalities are mainly graphitic (quaternary), pyridinic and pyrrolic; whereas light irradiation treatment in ambient conditions yields a more p + -doped graphene with some pyridinic groups converted to pyridine N-oxy and pyridone groups.
Looking ahead, from the side of the growth and the doping chemistries, both Raman and XPS data confirm (provided evidence) that graphene (G:N) incorporates N atoms during the growth and contains the "new" N-functionalities without introducing significantly C-sp 3 defects in the C-sp 2 basal-plane. Here, it is important to underline that Pyridinic-N and Pyrrolic-N occur mainly at the boundary of C-vacancy sites or, even more, at the edge of the graphene grains 19 . Therefore, since the Pyridinic-N is responsible for the activation of reactive sites leading to p + -doping, a higher defects density (C-vacancies and small grains) in G:N introduced during the growth allows for higher p-doping 33 . Figure 2a shows a photograph of the CVD reactor while performing the nitrogen plasma for the copper foil nitridation. Figure 2b shows an optical emission spectrum of the nitrogen plasma glow. The emission intensity of the first positive system is used for the evaluation of the nitrogen atoms amount in the plasma downstream where the copper foils are positioned 34,35 .
Optical microscopy and Scanning Raman spectroscopy were used for quality mapping the graphene layer (Fig. 2c). The microscope image shows the surface of the deposited graphene on copper foil after oxidizing by long time exposure to wet-air. The oxidation of the copper foil in correspondence of the graphene grain boundary decorates and draws the polycrystalline morphology of the graphene, whereas the isolated dark dots that spread throughout the graphene surface are local defective graphene spots, eg. carbon vacancies 13 . The size of graphene grains is around 10 µm. Two typical spectra with G (∼1582 cm -1 ) and 2D (∼2718 cm -1 ) peaks are observed without apparent D (∼1350 cm -1 ) peaks indicating the high quality of the graphene layer. The ratio I(2D)/I(G) = 1.8 and the G and 2D peak bandwidths of, respectively, 14 cm -1 and 27 cm -1 are fingerprint of a single layer graphene (blue line). The Raman spectrum with I(2D)/I(G) ∼1 (red line), recorded at the grain centre, is representative of bilayer. Most of the area is covered with single layer graphene, whereas, the presence of the small double layer island in centre of each grain confirms the under-layer nucleation mechanism described by Nie et al. 36 The distribution density of double layer islands on graphene can be evidenced by enhanced optical contrast of the microscope image of graphene transferred on 300 nm SiO 2 /Si reported in Figure S1 together with the relative Raman analysis. Figure 3 reports the XPS C1s and N1s acquired on single layer graphene as grown on copper and as transferred on Corning glass before and after Xe-lamp light irradiation. The quantitative estimation of the nitrogen content is about 1.5 at % the amount of carbon. Therefore, the labelling of the components used in the C1s-peak deconvolution resembles that of the pure pristine graphene (i.e. without nitrogen) 13 . Besides the small contribute . The presence of carbon-to-nitrogen bonds adds a new peak at 286.0 eV ascribed to N = C bonds that are located in the π-conjugated graphene structure [37][38][39] . On the other side, significant changes are observed on N1s peak deconvolution. Specifically, in the pristine graphene:N (G:N) grown on Cu substrate, besides the contribute at high binding energy (404.6 eV) due to adsorbed nitrogen 40,41 , it is recognized the presence of the three typical components of pyridinic (N p , 398.8 eV), pyrrolic (N pr , 399.8 eV) and graphitic/quaternary (N gr , 401.5 eV) nitrogen bonds 19,40 . The peak of graphitic nitrogen (N gr ) is much lower than pyridinic (N p ) and pyrrolic (N pr ) nitrogen, thus the n-type doping with two p-electrons at the graphene π-cloud is low. Almost equivalent peak intensities are observed for N p and N pr signals for as grown graphene on copper foil. Following the graphene:N single layer transfer on Si/SiO 2 substrate and the subsequent Xe-lamp irradiation, the N1s peak deconvolution needs two additional contributions at 400.7 eV and 402.3 eV assigned to Pyridone (N pd ) and Pyridine N-oxide (N ox ), respectively 19,38 . The appearance of these new N-functionalities takes place at the expense of pyridine nitrogen as evidenced by the strong reduction of the N p energy peak, while N pr peak, and, thus, the amount of pyrrolic nitrogen structures remains unchanged.
The observed evolution of the N1s peaks can be read in the chemical processes involving the interaction of G:N with singlet oxygen. In fact, the formation of singlet oxygen occurs on pristine G:N during both (a) the solubilisation of cupper foil by ammonium peroxydisulfate and (b) the light irradiation by Xe-lamp. As for the generation of single oxygen by interaction of G:N layer (still on TRT) with ammonium peroxydisulfate during the phase of copper solubilisation, the chemistry has been described in detail in previous studies 28,31,42 . The proposed chemistry is outlined in the following overall reactions (Eqs. (1) and (2)): www.nature.com/scientificreports/ and is based in the recombination of superoxide radicals ( O − 2 ) for the formation of singlet oxygen ( 1 O 2 ). Here, graphene:N surface is the catalyst that activate the hydrolysis of ammonium persulfate for the formation of superoxide radicals. Specifically, mainly pyrrole nitrogen acts on the neighbouring carbon atoms by increasing the charge density, thus generating an adsorption and activation center to form singlet oxygen 42 .
The graphene:N shows its catalytic action also in the photogeneration of reactive oxygen species (ROS) and, in particular, of singlet oxygen by molecular oxygen. In a recent paper, Yao and coworkers 29 have evaluated the role of graphene functionalization in determining the ROS generation. In particular, they underline that the generation of 1 O 2 is driven by graphene photoexcitation with energy larger than the excitation energy of O 2 -ground state ( 3 Σ g -) to 1 O 2 ( 1 Δ g ), which is of approximately 0.97 eV. This photosensitization mechanism has been reported as the most common means of singlet oxygen generation 43 . It is detailed that the energy transfer to O 2 ( 3 Σ g -) from an excited state of a sensitizer, which is formed by the light absorption of light in a specific wavelength region, results in the formation of both excited states O 2 ( 1 Σ g -) and 1 O 2 ( 1 Δ g ). And, followed by the very fast spin-allowed 1 Σ g -→ 1 Δ g deactivation that results in the complete formation of 1 O 2 ( 1 Δ g ). Moreover, it has been demonstrated that the graphene doping by heteroatoms, e.g., nitrogen, increase the photocatalytic efficiency 30,44 . The photochemical production mechanism of singlet oxygen on graphene:N can be depicted as follows: www.nature.com/scientificreports/ Thus, the completion of the 1 O 2 -treatment process is carried out also on substrate-transferred graphene:N (glass or SiO 2 /Si substrates) by irradiation with Xe lamp. The effectiveness of the photocatalytic aerobic oxidation via singlet oxygen has been exploited by many for the realization of selective oxidative processes towards organic and biological molecules 30 .
Furthermore, nitrogen-doped graphene is an active catalyst also in the oxygen reduction reaction (ORR), in which again the singlet oxygen can play an important role 45,46 . In particular, since the high lifetime of 1 O 2 in the gas phase, reducing singlet oxygen is expected to facilitate the first electron transfer in ORR. The general scheme for the complete reduction of O 2 / 1 O 2 molecules to OHis as follows: Nevertheless, the same oxidative processes can occur on the reactive sites present on the graphene surface; among these, the pyridine site is important as it can be oxidized simultaneously by aerobic photoirradiation to N-oxy pyridine and pyridine 27 , as schematized in Fig. 4.
The observed changes in nitrogen configurations, as assigned in Fig. 3, can be read in the simplified reaction scheme in Fig. 4, wherein the disappearance of the pyridine peak (N p at 398.8 eV) is in favour of N-oxy www.nature.com/scientificreports/ pyridine (N ox at 402.3 eV) and hydroxyl-pyridone (N pd at 400.7 eV). While the other two pyridone-tautomers (carbonyl and zwitterion) with a protonated nitrogen, which resemble a quaternary nitrogen with a very similar XPS peak position to the graphitic nitrogen, contribute to the observed increased of the peak N gr at 401.5 eV. The scheme emphasizes the role of the delocalization of the positive charge on the aromatic ring in both the oxidation to N-oxide pyridine and the formation of pyridone by the attachment of the OH group to the carbon atom in α-position to the pyridine nitrogen. The C1s XPS spectra shows that the C-OH, peak 3 at 285.5 eV, slightly increases after transferring on substrates. This confirms that carbon atoms close to pyridine nitrogen are the main active sites, among the different doping configurations of nitrogen, capable of further functionalizing graphene in the direction of p-doping 27 . Thus, the possibility of maximizing the introduction of N heteroatoms with pyridinic structure in the sp 2 carbon lattice would maximize the p-doping effect.
The changes in G:N surface structure in the direction of a p-doping have been monitored by Raman spectroscopy and electrical characterization, both sheet resistance (Van der Pauw) and FET measurements. Figure 5 shows the sheet resistance variation, as measured in a 4-probe Van-der-Pauw configuration, during Xe-lamp irradiation exposure of a G:N single layer on Si/SiO 2 . The observed slow kinetics is consistent with complex  Volts) for the light irradiated graphene:N when compared with pristine graphene (V D = 20 Volts). The evaluation of hole carrier mobility has been done using the formula µ h = (L/W)·C -1 ·V D -1 ·(dI D /dV G ), where C = 120 µF·m -2 is the gate capacitance of the silicon dioxide dielectric layer (300 nm thick) [47][48][49] . The measured hole mobility of annealed pristine-G and light irradiated graphene:N are nearly equal and reach an average values of about 1350 cm 2 /(V·sec). Thus, the reduction of sheet resistance in irradiated graphene:N is mainly ascribed to the increase of hole carrier density.
The increase in hole carrier density (p-doping) is confirmed by the Raman analysis of single layer graphene on FET device. Figure 6 shows the Raman spectra of the graphene:N single layer, before and after Xe-lamp irradiation. In these spectra, the following features can be labelled: (i) the strong decrease of the 2D/G intensity ratio (from 1.4 to 0.8); (ii) the shift of the G peak that moves of 24 cm -1 , from 1580 cm -1 to 1604 cm -1 ; (iii) the G peak asymmetry and Lorentzian-deconvolution in two narrow peaks (G1 at 1596 cm -1 and G2 at 1604 cm -1 ) after Xelamp irradiation; and (iv) the high-energy displacement of the 2D peak that moves of 9 cm -1 , from 2675 cm -1 to 2684 cm -1 . Similar results are observed from the Raman analysis of graphene:N transferred on glass before and after Xe lamp irradiation ( Figure S2).
The simultaneous occurrence of these four events can be read in the more general picture of a strong increase in p-type doping (increase in the carrier density of holes) as described in the Raman "master paper" on the doping effect by Das et al. 48 . The observed asymmetry of the G-peak indicates the existence of charge inhomogeneity within the laser probe, i.e., on a scale of 1 µm 50 .
According to ref. 49 , from these important Raman parameters we can estimate an hole doping density of about 1.3 × 10 13 cm -2 for the irradiated graphene:N sample. This value is consistent with the average value of sheet resistance measured for a single layer of G:N after irradiation. In fact, by applying the simple Drude model of electrical conductivity, Rs (Ω/□) = 1/(n·µ·e), where n = 1.3 × 10 13 (cm -2 ), µ = 1350 (cm 2 V -1 s -1 ) and e = 1.6 × 10 -19 (A·sec), we extract the sheet resistance of 350 (Ω/□) that matches the measured values shown in Fig. 7.
Multilayer graphene samples on Corning glass substrates have been prepared by using the layer-by-layer procedure described in ref. 13 . Figure 7 reports the variation of the sheet resistance going from single-layer to three-layer graphene:N (blue dots) without and (red dots) with layer-by-layer light irradiation (each layer is irradiated after transfer). Multiple graphene layers act as parallel resistors, thus, providing a reduction of measured Rs values with the increase of the layer number. It is important to observe that the sheet resistance of the   Fig. 4. Interestingly, the measured sheet resistance reduction is comparable to that obtained by other chemical doping procedures on multilayer graphene 13 .

Conclusions
In summary, using a Nitrogen-doped graphene deposited from a direct CVD method, an efficient p-doping effect has been demonstrated through the selective photochemical activation of pyridinic nitrogen to N-oxy pyridine and pyridone. Notably, the sheet resistance of light-irradiated Nitrogen-doped graphene significantly reduces and can even reach a value as low as 100 Ω/□ for a three-layer graphene. Unlike conventional chemical doping, where the graphene layer is submitted to chemical reaction with strong oxidants (HNO 3 , SOCl 2 ,…) for the covalent doping or it is required to host molecules or particles for the non-covalent doping, the photochemical doping of graphene:N has the advantage to be easily applicable to any combined graphene-semiconductor device architecture. In addition to the capability of doping graphene on any surface device, this light-driven doping strategy has great potential to realize selective-area doping by photolithography mask or laser patterning.

Material and Methods
Graphene:N CVD growth, transfer and photoirradiation process.. The growth of nitrogen-doped graphene occurs in two phases: the plasma nitridation of copper foil and the catalytic CVD-growth. Both processes were carried out in the same quartz tube (i.d.10 cm) equipped with external capacitive-coupled electrodes for the plasma nitridation process and furnace for the CVD growth. The copper foil (25 µm thick, 10x10 cm 2 size) was inserted in a quartz tube of the thermal-furnace CVD reactor. The quartz-tube was evacuated to a vacuum better than 10 -3 torr and heated to 100 °C under an N 2 gas flow of 200 sccm (0.2 torr) that was maintained for 20 min after the temperature was stabilized. The copper nitridation process was carried out in the nitrogen plasma downstream under the following conditions: N 2 flow rate 200 sccm, pressure 0.2 torr, rf (13.56 MHz) power 250 watts, exposure time 30 min. Subsequently, the temperature was raised to 990 °C under an H 2 gas flow of 10 sccm (0.05 torr) that was maintained for 20 min after the temperature was stabilized. The annealing process was followed by the graphene growth: CH 4 (5 sccm) was added to the gas feed for a growth time of 20 min. After the growth phase, the furnace was moved from the growth-zone to realize the rapid cooling of the graphene/copper foil.
The graphene layer was then transferred to the substrate (Corning-glass, SiO 2 /Si) using the thermal release tape (TR-tape) procedure. The tape was placed on top of the graphene/copper foil and pressed by the laminator. The copper was etched away in an ammonium peroxydisulfate, ((NH 4 ) 2 S 2 O 8 ) solution (20 gr/liter) and the floating sheet of graphene/TR-tape was rinsed in DI water and air-dried. Hot-pressing (at ≈100°C) performed the graphene transfer onto substrates, thus the tape was then peeled off. Multilayer graphene samples on Corning glass for sheet resistance measurements were fabricated by the layer-by-layer transfer procedure described in ref. 13 .
The photoirradiation of the samples was performed under ambient laboratory conditions, in air using a 75 watt Xe-lamp (Oriel) with maximum wavelength emission at 470 nm. During light irradiation, the sheet resistance was monitored in real time to ensure the completion of the variation process. www.nature.com/scientificreports/