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Reversible Photochemical Control of Doping Levels in Supported Graphene Hai I. Wang, Marie-Luise Braatz, Nils Richter, Klaas-Jan Tielrooij, Zoltan Mics, Hao Lu, Nils-Eike Weber, Klaus Müllen, Dmitry Turchinovich, Mathias Kläui, and Mischa Bonn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00347 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

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The Journal of Physical Chemistry

Reversible Photochemical Control of Doping Levels in Supported Graphene

Hai I. Wang1#, Marie-Luise Braatz1,2#, Nils Richter1,2, Klaas-Jan Tielrooij3,4, Zoltan Mics4, Hao Lu4, Nils-Eike Weber5&, Klaus Müllen4, Dmitry Turchinovich4, Mathias Kläui1,2*, Mischa Bonn4*

1

Institute of Physics, University of Mainz, Staudingerweg 7, 55128 Mainz, Germany;

2

Graduate School of Material Science in Mainz, University of Mainz, Staudingerweg 9, 55128 Mainz,

Germany; 3

ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860

Castelldefels (Barcelona), Spain; 4

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany;

5

Carbon Materials Innovation Center (CMIC), BASF SE, 67056 Ludwigshafen, Germany.

* Correspondence to: [email protected] (T: +49 6131 39 23633), [email protected] (T: +49 6131 379161) # These two authors contributed to the work equally &

Current address: Scienta Omicron GmbH, Limburger Str. 75, 65232 Taunusstein, Germany

1 ACS Paragon Plus Environment


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Abstract

Controlling the type and density of charge carriers in graphene is vital for a wide range of applications of this material in electronics and optoelectronics. Up to now, chemical doping and electrostatic gating have served as the two most established means to manipulate the carrier density in graphene. Although highly effective, these two approaches require sophisticated graphene growth or complex device fabrication processes to achieve both the desired nature and doping densities, with generally limited dynamic tunability and spatial control. Here, we report a convenient and tunable optical approach to tune the steady-state carrier density and Fermi energy in graphene by photochemically controlling the concentration of adsorbed molecular O2 - a p-dopant in graphene - using fs pulsed laser irradiation in the UV range. As an all-optical approach, it allows spatial control over doping levels. Combined terahertz (THz) spectroscopy and electrical device measurements reveal that the Fermi level in laserilluminated graphene can be controllably and reversibly tuned between p- and n-type in a large range (over ~ 600 meV from -420 to +180 meV, by readily tuning the peak intensity and the duration of the laser irradiation treatment. Furthermore, we demonstrate that our photochemical approach for doping of graphene allows to optically write doping structure with spatial control. Given the ease, effectiveness and simplicity of the method, this photochemical doping mechanism offers a simple, reversible approach to control the steady-state electronic and optical properties of graphene.


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1. Introduction Graphene, a single layer sp2-hybridized carbon allotrope, has been considered as an attractive material for electrical1-2 and optical applications3-4 since first reports in 20045. With an extremely high dc charge carrier mobility6-7 and a large optical transmittance across a wide range of the electromagnetic spectrum8-9, it is very promising for applications including, for instance, high-frequency communications devices10, transparent electrodes11-12 and ultra-broadband photodetectors13. Both the electronic and optical properties of graphene depend strongly on the charge carrier density and hence the precise position of the Fermi level. Undoped graphene with the Fermi level at the Dirac point behaves as a semiconductor or a semimetal with a low density of states at the Fermi surface. On the other hand, shifting the Fermi level away from the Dirac point leads to a “metallic” behavior with a high carrier density of either holes or electrons and a resulting increase of the conductance.14-16 Additionally, along with a change of the carrier density and corresponding electronic transport properties, the interband and intraband optical absorption can be strongly tuned by doping17-18, enabling new optical technologies, such as ultrafast and highly sensitive photodetectors4,

13, 19-21

,

surface plasmons22-25, terahertz modulators26-28 etc. In this respect, for developing high-performance electrical and opto-electronic devices based on graphene, a primary requirement is to manipulate the nature and degree of doping of the graphene in an efficient, simple, controllable and reversible manner. Current prevailing approaches to achieve tuning of the Fermi level position and corresponding carrier densities in graphene include (i) chemical doping, either by heteroatom substitutions in graphene29-36 or via the adsorption of molecular adsorbates onto graphene37-40 and (ii) external gating5, 7. Regarding (i), the chemical doping process by replacing a single carbon to nitrogen or boron atom, has been realized by adding precursor gasses during the graphene growth process by the chemical vapor deposition (CVD) method30, 35-36, and proven effective to control the carrier density30, 33, 35-36, 41. One important drawback of such substitutional doping is that it does not allow for dynamic control over the Fermi energy, and the introduction of lattice imperfections or defects in the lattice can increase the carrier scattering rate and correspondingly reduce the carrier mobility.35, 41 Additionally, chemical doping by adsorption of molecular adsorbates onto graphene has also been widely employed.37-40, 42-43 The success of such doping relies on the charge transfer process taking place at the adsorbate/graphene



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interfaces. The Brus group demonstrated that physical adsorption of halogen38-39 and oxygen37,

42

molecules onto graphene results in hole doping and the degree of doping can be controlled by the surfaces coverage of the molecules, yet only over a limited range of Fermi energies.37-39,

42-43

Furthermore, a tunable Fermi level is obtained using (ii) external gating electrostatically5 or electrochemically14-15,

44

. These gating methods, although highly successful, require sophisticated

nanofabrication techniques to create a doping structure with high spatial resolution, low-noise electrical inputs. These samples are typically also quite vulnerable. Very recently, a few reports have demonstrated optical gating by controlling the type and density of the charge carriers in graphene via photo-induced charge transfer between the graphene and an adjacent material (adsorbates or substrates).45-51 As an approach that combines the advantages of simple fabrication with doping reversibility, such optical gating can provide major advantages over conventional doping methods. For example, in contrast to doping by other atom species it does not necessarily induce defects that act as scattering centers, thus potentially preserving the high mobility of the charge carriers in graphene.48, 50-51 Moreover, this method can provide a control of the doping profile in space by focusing a laser beam on graphene and moving the focus on a selected path.49, 51 However, the photo-induced graphene-substrate charge transfer effect critically depends on the nature of the substrate, and has been reported only a limited number of materials (BN49-51 and TiO248) so far. Additionally, the operation of the device requires also a backgate. Alternatively, controlling the doping in graphene can be also achieved photochemically via a photo-induced desorption of molecular adsorbates using UV light treatment.52-54 For instance, the photo-induced O2 desorption has been reported to offer an easy route to control the adsorbate concentration and thus the doping densities, as adsorbed O2 is a p-dopant for graphene.37, 54-57 A major advantage of this effect is that it does not require a backgate and does not put any constraints on the substrate. However, in the most cases the reported photo-induced O2 desorption results in a rather moderate range of Fermi level tuning, and is often limited to only one type of carrier (i.e. only p-doping)37, 54. So there is a clear need for a flexible and reversible doping method that allows for accessing doping levels in both, the n- and p-doped regimes.


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Here we demonstrate a backgate-free method that allows us to optically control the Fermi level in graphene in a wide range (up to ~600 meV, across the Dirac point) with spatial control. Our strategy is based on employing nitrogen atoms to induce a fixed electron n-doping level in graphene, while dynamically controlling the amount of molecular oxygen (a p-dopant) adsorbed on the graphene surface by pulsed UV laser irradiation treatment. By balancing these two doping effects, the overall doping level in graphene can be readily tuned over a wide energy range (up to ~600 meV), from a strong p-type doping (-420 meV) in air to the intrinsic, n-type regime (+180 meV) for nitrogen-doped graphene, i.e. the doping level can be controlled across the Dirac point. Additionally, the doping level in graphene can be dynamically and reversibly controlled by tuning the partial pressure of O2 and the fluence of the UV laser irradiation. Finally, we demonstrate that we can further exploit this method to effectively gate graphene devices with spatial control. By focusing UV laser light on selected areas in a graphene device, we can “write” a local n-doping profile, and thus optically construct a lateral p-n junction. Our results not only provide physical insights into the photochemical doping effect in graphene, but also establish a reversible, convenient and controllable method to tune the doping level and potentially spatial profiles in graphene devices, which paves the way for novel optoelectronic applications based on graphene.

2 Experimental Section 2.1 Sample information The samples used in this study are nitrogen-doped graphene (N-graphene) fabricated by the CVD method and details are described in ref.36, 58 To confirm the nitrogen doping, Raman spectroscopy was employed. As shown in Figure S1 (see support information (SI)), with the introduction of nitrogen atoms into the graphene, an additional resonance, the so called “D-band” at ~1362 cm-1 is observed. For quantifying the doping concentration, we further characterized the 3 samples used in this work by X-ray photoelectron spectroscopy (XPS) measurements, and the nitrogen doping concentration was determined to be ~ 2.5% (data not shown). While the intrinsic N-doping is expected to result in n-type



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graphene41, the adsorption of oxygen gives rise to overall p-type doping with Ef ~420 meV below the Dirac point as inferred from electrical transport measurements (see section 3.2).

2.2 Studying carrier dynamics and doping level in graphene using optical–pump terahertz (THz)-probe (OPTP) spectroscopy In order to simultaneously monitor the doping level in graphene, we first employ contact-free optical– pump terahertz (THz)-probe (OPTP) spectroscopy.14-16, 59-63 For OPTP study, the graphene samples are subsequently transferred onto fused silica substrate (device type I). The measurements are conducted either in vacuum (< 1.4 *10-4 mBar), or in a dry N2 purged environment inside a measurement chamber with control over the O2 partial pressure between 4.5 ± 0.3% and 20.8% (in air) by tuning the flux of N2 flow. The measurement and sample geometry (type I) are sketched in Figure 1 (A). After photoexcitation by a 400 nm laser pulse with ~50 fs duration and 500 Hz repetition rate, the photoinduced THz conductivity is monitored as a function of the delay time between the optical pump and THz probe pulses. The optical pump has a twofold role: on the one hand it provides the photochemical treatment, and on the other hand it serves as the pump to excite photo-carriers in graphene. The THz probe pulse is sensitive to these photo-carriers in such a way that we can extract information on the steady state carrier density. Indeed, several groups have recently shown that the doping level in graphene has a major impact on the photo-induced THz conductivity.14-16, 62, 64 For heavily doped graphene, regardless of doping type, graphene shows a “metal-like” characteristic: after photoexcitation with moderate fluence16, the photo-generated hot carriers heat up the carriers at the Fermi surface, thereby reducing the intraband conductivity, i.e. leading to photo-induced THz transparency.14-16 On the other hand, graphene with the Fermi level situated at the Dirac point behaves “semiconductor-like”, and exhibits positive photoconductivity (i.e. photo-induced THz absorption).1416

Therefore, the magnitude and sign of the OPTP photo-induced THz conductivity in graphene

directly reflect the doping level in graphene.

2.3 Device characterization


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For electrical transport measurements, the graphene samples are transferred onto Si/SiO2 (device type II). We have sputtered contact pads on graphene with a shadow mask (Pt/Au: 5 nm/100 nm), with a distance of approximately 600 µm between them. The contact pads were then contacted with silver paste in two-point configuration.

3. Results and Discussion 3.1 Monitoring the photochemical doping effect in graphene using THz spectroscopy We start by monitoring the evolution of the graphene THz photoconductivity as a function of pumpprobe delay time after excitation with a short (~50 fs) pump pulse of 400 nm light. We then keep repeating these pump-probe scans continuously over 8 hours in a N2 purged atmosphere with a stable O2 partial pressure of 4.5 ± 0.3% (compared to 20.8% in air). In Figure 1(B), we show the ultrafast carrier dynamics at several times (laboratory time) after initiating irradiation. Initially, the photoconductivity is negative, turning positive after approximately 10 minutes and negative once more after several hours. Figure 1(C) summarizes the peak values of these dynamics as a function of photochemical treatment duration. The dynamics can be divided into three regimes: starting with an initial negative sign, we observe a reduction of the signal magnitude and then a sign change from negative to positive. Remarkably, the positive photoconductivity signal peaks after around one hour to subsequently decrease again with further laser exposure time and it finally switches back to the negative side after 3 hours of pumping. In this state the dynamics are saturated and are stable for over ~5 hours of measurement time. We further investigate the stability of the photoconductivity by exposing the sample to air (O2 concentration 20.8%) while terminating optical excitation. The photoconductivity measurement is used to track the dynamics in a given time. In order to minimize the effect of optical excitation during the measurements, we only monitor the maximum change of the photoconductivity peak value within a short laser excitation period of less than 10 seconds, at any given time of interest. As demonstrated in the Figure 1(D), we find that the photoconductivity in graphene is completely reversible by exposing graphene to air after terminating the laser irradiation treatment. In case of terminating the optical excitation while keeping the O2 concentration at 4.5 ±0.2 %, the photoconductivity is partially reversible (see Figure S2(A) in SI). Additionally, we have



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conducted the same measurements in vacuum as a control measurement. In vacuum, the sign change of the photoconductivity is also observed twice, however, with a slightly larger photoconductivity after saturation (discussed in more detail in section 3.3). By terminating the laser irradiation treatment and keeping the sample in vacuum, we find that the doping level remains constant; the photoconductivity does not show any reversal for up to 12 hours (see Figure S2(B) for sample G, and Figure S3(A) for sample C). In contrast, when the sample is exposed to air while blocking the excitation beam, we observe a complete recovery of the photoconductivity in graphene to the initial doping level as shown Figure S3(B): a trend that is comparable to what is observed in Figure 1(D). At the first glance, a trivial and plausible explanation for the altering of THz conductivity sign might be attributed to photothermal effect, by which the Fermi energy could be gradually shifted up, either by heating up the sample or thermally kicking out O2 via laser illumination. However, the controlled measurements as discussed previously reveal a stable doping in vacuum over at least 12 hours and doping switching back to initial state under ambient conditions in tens of minutes. These observations directly rule out the photothermal model and indicate a distinct chemical origin of the phenomenon. Here we can rationalize the change of the sign of the photoconductivity, its stability and reversibility in various controlled environments in the following manner: the chemical nitrogen-dopants giving rise to n-doping are invariant to laser irradiation. The adsorption of molecular O2 introduces p-doping in graphene37,

54

, and this effect dominates under ambient conditions, so that, at the start of our

experiment, the graphene used in this study is p-doped. The initial p-doped sample (“metallic phase” marked as gradient red in Figure 1(C)) displays a negative photoconductivity (or “photo-induced THz transparency”) due to the pump-induced conductivity reduction in the graphene originating from carrier heating.14-16, 62-63 Upon 400 nm pulsed laser illumination, molecularly adsorbed O2 is removed from the surface54, and accordingly the Fermi level of the graphene sample gradually shifts towards the charge neutrality point. As the Fermi level reaches the vicinity of the Dirac point14-16, the sample shows “semiconductor-like” characteristics, with positive photoconductivity. A further photo-induced decrease of the levels of adsorbed oxygen, results in the doping of graphene reaching its intrinsic doping status: an n-type nature for the N-graphene used in this study. This leads again to a “metalliclike” phase and thus negative photoconductivity (the gradient green area in Figure 1(C)).14-16 It is


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worth commenting that as demonstrated in ref.54, although the detailed mechanism is unclear, hot carriers are proposed to be responsible for the photo-induced desorption of oxygen. In agreement with this proposition, we observe that excitation using 800nm light, for which the hot carrier excess energy is reduced, photo desorption still occurs, but with lower probability (See Figure S4 in SI). In order to shed light on the transport properties (carrier density, carrier scattering time etc.) in graphene, we have conducted THz domain spectroscopy in order to measure the THz conductivity spectra of graphene at the initial (p-doping) and the final n-doping regime. From the measured spectra the scattering time and the Fermi-level can be inferred. The details of the principles are discussed in the SI and data summary are shown in Figure S5. In short, we find that in the final n-doped regime the graphene conductivity can be well modeled by the Drude model with a scattering time of ~20 fs and Fermi energy ~150 ±75 meV. These numbers are consistent with the results of the electrical transport measurement (discussed in the next section 3.2), in which the final Fermi level is independently quantified to be ~180 meV. For the initial doping, the strong p-doping results in a substantially enhanced scattering, the rate of which is too fast to be reliably determined from the conductivity obtained within the THz window of our experiment. To summarize, the THz domain spectroscopy data are fully consistent with OPTP dynamics and electrical transport results (Ef ~420 meV below Dirac point for the initial p-doped graphene) that we will discuss in the next section.

3.2 Confirming the photochemical doping effect by electrical transport measurements To confirm the photochemical control of the doping level and type in graphene, we have characterized electrical transport in graphene devices (device “type II”, sketched in Figure 2(A)) during photochemical treatment. Firstly, we track the resistance change induced by pulsed laser treatment with N2 purging (O2 partial pressure 4.5 ± 0.3%), as schematically shown in Figure 2(A). Before the laser irradiation treatment, the resistance of graphene in air is constant over time. Upon N2 purging, we observe a small increase (about 1% relative change) and saturation of resistance within ~ 25 mins (see SI Figure S6). This observation is consistent with the Fermi level shifting towards the Dirac point, and which we attribute to small amounts of O2 desorption during N2 purging, but is negligible compared to the resistance change induced by photochemical treatment (> 360% relative change). In line with the



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observation of little effect of N2 purging treatments on doping level in graphene, we observed a nearly constant THz conductivity in N2 purging or vacuum atmosphere over 12 hours as shown in Figure S7. Laser pulse irradiation treatment is therefore essential to induce a change in the doping level in graphene. Indeed, as we turn on the laser (first with a fluence of 2 µJ/cm2 and then gradually up to 20 µJ/cm2) and continue N2 purging during the measurement, as shown in Figure 2(B) we first observe an increase of the two-point resistance until the Fermi level is at the Dirac point and then a drop after ~ 60 mins, in line with the photochemical doping from p-doping to Dirac point (at which the resistance is highest) and then further to n-doping. Remarkably, as shown in Figure 2(C) after blocking the pump laser, the change of the resistance is completely reversed by exposing the sample to air, consistent with OPTP results in Figure 1. The change of doping type by photochemical treatment is further supported by the resistance measurement itself by blocking the laser (labeled in gray in Figure 2(B)) while keeping the purging on (O2 partial pressure 4.5 ± 0.3%): we observe a transient resistance decay (the dip indicated by the black arrow) before the highest resistance point, and a transient resistance increase (the peak indicated by the red arrow) after the highest resistance point. In line with our previous explanations, this observation can be rationalized by the fact that O2 re-adsorption takes place after blocking pulsed laser irradiation, resulting in the downwards shift of Fermi level in graphene. For the p-doped situation (before the highest resistance point), such a shift leads to a resistance decrease because of an enhanced density of states for holes; and conversely results in a resistance increase in the n-doped case. The observation that the resistance change for p- or n-doped graphene by blocking the laser goes in different directions, implies that photocurrent has little contribution to the resistance change in our measurements; otherwise the resistance should be always higher in the dark, independent of the nature of the dopant. To fundamentally and independently confirm the change (from p- to n-type) of carrier type in graphene at the initial and saturated doping, we perform back-gate (BG) dependent measurements. As shown in Figure 2 (D), before the photochemical doping, the sample is heavily p-doped with the Dirac point at a back-gate voltage beyond 150 V. Based on the simple capacitance model5,

65

reported

previously, we can obtain the Fermi energy Ef ~420 meV below the Dirac point, indicating a strong pdoped nature of the graphene in ambient conditions. As we start the photochemical doping treatment


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(i.e. the fluence gradually increasing from 1 to 20 µJ/cm2), as shown in the back-gate measurements in figure 2(D), we clearly observe the change of doping polarity from the initial p- (with the Dirac point at positive gate voltage), gradually to the final n-doping (with the Dirac point at negative gate voltage 25V, corresponding to Ef ~180 meV above Dirac point), resulting in a large shift of Fermi energy of 600 meV. In line with the doping reversibility based on THz data, the doping switches back to initial p-doping after exposure to air as shown in Figure 2(E). Additionally, based on transport measurements, carrier mobility can be extracted (The method is also discussed in SI and Figure S8).66 Intriguingly, the electron mobility in graphene increases from the initial value of between 6 and 14 (±1), to ~28 ±3 cm2/V*s with photochemical treatment. This observation is consistent with the reported reduced electron mobility in graphene upon O2 adsorption55, 67 which can serve as a scattering channel along with other possible scattering events such as defects, grain boundaries etc., and also consistent with the increase in the scattering rate observed in the terahertz measurements (see section 3.1).

3.3 Controlling the doping level in graphene by the photochemical doping effect The photochemical doping effect shown in this study can, in principle, be used in graphene devices to control the doping density during sample and device fabrications. One particularly relevant question for employing this doping approach in graphene devices is the following: can we effectively tune the doping level in graphene in a controllable and convenient manner? Given the scenario presented above, there are essentially two ways to control the steady-state doping level: by changing the partial pressure of oxygen and/or by changing the UV photon flux. Indeed, as shown experimentally for a given laser fluence, the final achievable doping level depends critically on the gas composition. The oxygen partial pressure can be controlled by the degree of vacuum or the flux of N2 purging during measurements. As visible in Figure 3(A), for the same N-doped graphene sample both the doping saturation rate and photo-induced THz conductivity in the saturation regime follow a trend: in vacuum > in N2 purged chamber (4.5 ±0.2% O2) > in air (20.8 % O2). In Figure 3(B), we show the peak value of the photo-induced THz conductivity after the photochemical treatments at selected values of the O2 partial pressure: the lower the O2 partial pressure, the higher the THz photoconductivity of the sample in the saturated regime, and therefore a more n-doped status of the sample can be reached. This



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observation demonstrates that the subtle equilibrium between O2 adsorption (controlled by the available O2 in the environment) and desorption (controlled by laser pulse fluence) determines the adsorbed O2 concentration and thus the overall doping level in graphene, as schematically shown in Figure 3(C). It is worth commenting that with the reduction of the O2 partial pressure by tuning the N2 flux or vacuum in our study, the humidity of the environment correspondingly decreases. In a previous study from the Brus group, the role of chemical adsorption of water, O2 molecules and the combination of both on graphene doping level has been systematically studied.37 While water molecule adsorption alone shows no impact on the Fermi energy in graphene, it does enhance the hole doping effect of O2. To further verify the scenario that the O2 adsorption-desorption equilibrium controls the doping density in graphene, we investigate the impact of laser fluence on the final, steady-state doping levels for a given oxygen partial pressure 4.5 ±0.2%. To allow for a quantitative comparison between the different excitation levels, the THz photoconductivity dynamics are measured and compared at the same fluence of 2 µJ/cm2, following long-time exposure to the indicated excitation levels (see Figure 3(A)). As expected, graphene exhibits a higher negative THz photoconductivity resulting from a more n-doped character for higher laser fluence, since less O2 is adsorbed. As shown in the inset of Figure 3(D), the photochemical effect eventually saturates at 40 µJ/cm2. At this stage, the adsorbed O2 concentration on graphene is presumably minimized and the intrinsic doping condition is reached in our sample. Finally, we monitor the doping relaxation process from the adsorbed O2 equilibrium conditions from 40 µJ/cm2 to 2 µJ/cm2 as shown in Figure 3(E). After the doping saturation is reached at 40 µJ/cm2, we change the pump fluence to 2 µJ/cm2, and monitor the OPTP dynamics and the corresponding peak value evolution in real time. With decrease of the laser fluence, the O2 concentration equilibrium gradually shifts towards the (re)adsorption direction, until the new equilibrium at 2 µJ/cm2 is established.

3.4 Controlling the spatial doping profile in graphene by the photochemical doping effect Besides the ability to tune the graphene doping level in a wide range, the optical means for controlling the Fermi level provides a specific control of the doping profile in space by focusing the laser beam on


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graphene. For that, we demonstrate that we can optically “write” and control the spatial doping profile in graphene samples. The measurement protocol is sketched in Figure 4(A) based on graphene devices with the separation of source and drain electrode ~2 mm. By illuminating only one electrode of the device with a beam diameter of ~1mm we can tune the doping density and polarity in the illuminated area while the rest of the graphene remains p-doped. Based on back-gate measurements, indeed we observe that a second Dirac point is developed with the focused beam illumination on one electrode (2 µJ/cm2 for 1 hour) as shown in Figure 4(B). With a higher fluence 20 µJ/cm2 treatment for 1 hour, the doping in the illuminated area is shifted further to n-doped regime while the rest remains p-doped. This results in a lateral p-n junction optically written in our graphene, with 2 characteristic Dirac points with one at negative and the other in positive back-gate voltage. Clearly, the spatial resolution of the doping structures based on our photochemical doping method can be further improved by nearfield optics or confocal microscopy, down to micrometer or even nanometer. Along these lines, indeed it has been shown very recently that spatial control of charge doping can be achieved with the application of either a focused beam of light or STM tip voltage excitations in graphene/boron nitride hetero-structures.49, 51 As discussed in the introduction, such a photo-induced doping effect relies on the charge transfer process from the photo-excited defect states in boron nitride to graphene in conjunction with a gate electric field. Our photochemical doping method, being complementary to the work in ref.50-51, on the other hand, provides a purely chemical way to tune the Fermi energy in graphene without a backgate, independent of the choice of substrate.

4. Conclusion In summary, we have demonstrated a photochemical method to reversibly tune the doping level in graphene in a wide range up to 600 meV. The underlying mechanism relies on photochemically manipulating the concentration of adsorbed O2 on graphene thus tuning the position of the Fermi level. By controlling the O2 adsorption-desorption equilibrium at the graphene surface via tuning the oxygen partial pressure or laser irradiation fluence and duration, we show that the doping level and type (i-, nor p-) in graphene can be manipulated between its ambient doping and intrinsic doping condition in a convenient and reproducible way. Even more importantly, we demonstrate that photochemical doping



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provides an effective approach to control both the doping density and spatial doping profile in graphene-based devices, and allows for an easy optical writing of a lateral graphene p-n junction. Compared to other doping approaches, our method allows for flexible choice of substrates and there is no requirement for a backgate or nanostructuring, paving the way for novel applications.

Supporting Information Raman characterization; Doping stability and reversibility test at different atmosphere; Doping reversibility test; Photochemical doping effects: 800nm vs 400nm; THz time domain spectroscopy characterizations; Pure N2 purging effect on the graphene resistance; Pure N2 purging or vacuum effect on THz photoconductivity; Graphene mobility estimation before/after photochemical treatment

The authors declare no competing financial interest.

Acknowledgements The work was financially supported by the DFG (Priority Program Graphene SPP 1459, KL1811), the ERC (Starting Grant MASPIC ERC-2007-StG 208162 and Advanced Grant NANOGRAPH) the European Science Foundation (ESF) under the EUROCORES Program EuroGRAPHENE (GOSPEL), MoQuaS (Moquas Molecular Quantum Spintronics FP7-ICT-2013-10), the EU Career Integration Grant 334324 LIGHTER, and by the Max Planck Society. We gratefully acknowledge the support of the Graduate School of Excellence Materials Science in Mainz (MAINZ) GSC 266 and the Center for INnovative and Emerging MAterials (CINEMA). We acknowledge the valuable help for TOC picture design from Marc-Jan van Zadel, and discussions with I. Ivanov and Dr E. Canovas.

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Figure 1. (A) Scheme of the optical pump THz probe (OPTP) spectroscopy on graphene supported by fused silica (no electrode, no backgate, device type I); (B) The OPTP carrier dynamics evolution in graphene (sample A) as a function of pulsed laser irradiation treatment time. The sample is measured under N2 purging by a 400 nm excitation with a fixed fluence of 2 µJ/cm2; (C) The peak magnitude of the photo-induced THz change (-∆E/E ~0.5 ps after the excitation pulse) over time under laser irradiation treatment. Graphene clearly evolves from having p-doping (red shaded area) to n-doping (green shaded area); (D) The peak magnitude of the photo-induced THz change (-∆E/E) over air exposure time after blocking the laser pump reveals that the doping change is completely reversible.


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Figure 2. (A) The device (type II) geometry for monitoring the resistance change under laser irradiation treatment with the graphene layer indicated in black; (B) The photochemical doping effect in graphene (sample B) on the resistance by 400 nm pulsed laser irradiation treatment. In the period of time labeled in gray area, the laser is intentionally blocked to monitor the corresponding resistance response; (C) Resistance (and/or doping) relaxation after stopping the laser irradiation treatment and exposing the graphene device to air; (D-E) Back-gate (BG) measurements during photochemical doping and air exposure processes.



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Figure 3. (A) The OPTP carrier dynamics in graphene (sample C) in 3 typical atmospheres (O2 partial pressure); (B) The peak value of OPTP dynamics at different O2 partial pressure; (C) A schematic demonstration of oxygen desorption and (re-)adsorption equilibrium upon laser pulse excitation; (D) Carrier dynamics in graphene (sample D) after different irradiation treatments under 4.5% O2 partial pressure. All the dynamics are recorded with a pump fluence of 2 µJ/cm2 immediately after optical


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treatments. The inset shows the peak values of the dynamics; (E) The doping relaxation processes from the illumination condition fluence 40µJ/cm2 to 2 µJ/cm2 monitored by OPTP. The sample has been treated with 40 µJ/cm2 first to reach the doping saturation. Then the pump fluence is changed to 2 µJ/cm2 to monitor the OPTP dynamics evolution over time. The measurement is conducted under 4.5% O2 partial pressure.

Figure 4. (A) Spatial doping profile control by photochemical treatment via focusing laser light on only one electrode (in contrast to illuminate both source and drain electrode as shown in figure 2 (A)); (B) Back-gate measurements (sample E) after 1 hour pulsed 400nm laser treatment with a fluence of 2 and 20 µJ/cm2, respectively.



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TOC Graphic


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Reversible Photochemical Control of Doping ... - ACS Publications

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