Gate-Tunable Dirac Point of Molecular Doped Graphene Pablo Solís-Fernández,† Susumu Okada,§ Tohru Sato,∥ Masaharu Tsuji,† and Hiroki Ago*,†,‡ †
Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 816-8580, Japan Graduate School of Pure and Applied Sciences, University of Tsukuba, Ibaraki 305-8571, Japan ∥ Department of Molecular Engineering, School of Engineering, Kyoto University, Kyoto 615-8510, Japan ‡ PRESTO, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan §
S Supporting Information *
ABSTRACT: Control of the type and density of charge carriers in graphene is essential for its implementation into various practical applications. Here, we demonstrate the gate-tunable doping effect of adsorbed piperidine on graphene. By gradually increasing the amount of adsorbed piperidine, the graphene doping level can be varied from p- to n-type, with the formation of p−n junctions for intermediate coverages. Moreover, the doping effect of the piperidine can be further tuned by the application of large negative back-gate voltages, which increase the doping level of graphene. In addition, the electronic properties of graphene are well preserved due to the noncovalent nature of the interaction between piperidine and graphene. This gatetunable doping offers an easy, controllable, and nonintrusive method to alter the electronic structure of graphene. KEYWORDS: graphene, transistor, molecular doping, p−n junction
D
Generally, the attained level of doping is directly related with the amount of adsorbed molecules, and hence, it is fixed as long as this amount remains constant.21 However, a further control of the doping level after the functionalization is interesting for practical purposes. Recently, some mechanisms have been developed to alter the doping level of graphene without changing the amount of dopants. These usually rely on external stimulus, such as light irradiation, to modify the electronic properties of the graphene environment.27−29,33−35 Firstprinciples calculations have shown that both the nature of the molecule and its relative orientation with respect to the graphene determine the magnitude of the charge transfer between them.36 Thus, the doping level can be potentially adjusted by controlling the orientation of the molecules, as observed in the case of ammonia adsorbed on graphene.22 Theoretical studies also suggested that external electric fields can influence the magnetic moment of the adsorbed species, causing changes in the amount of transferred charge.37 In the present work, we demonstrate the ability to control the doping level of graphene with adsorbed piperidine by applying a back-gate voltage. Piperidine, a heterocyclic compound containing a secondary amine, has been previously
ue to its excellent physical properties, graphene is expected to become one of the most technologically relevant materials in the forthcoming years. 1,2 However, implementation of graphene or related materials, such as bilayer graphene or graphene nanoribbons, in some current technologies for electronic and optoelectronic applications require a controllable doping.3 Some of the most commonly employed routes to control the doping level of graphene involve the chemical functionalization4−11 and the implantation of heteroatoms in the graphene lattice, such as nitrogen or boron.12−14 However, the disruption of the graphene structure due to the introduction of defects can lead to a severe degradation of graphene’s electronic properties.7,13−15 Thus, a route to dope graphene without degrading its properties is highly desirable. In this sense, molecular doping provides a suitable way to modify the electronic structure of graphene by exploiting its sensitivity to the surroundings.16,17 By controlling the adsorption of chemical species on the surface of graphene, a precise tuning of its doping level can be achieved.18−32 Given the noncovalent nature of the interaction between the graphene and the adsorbed molecules, these kinds of approaches usually result in a better preservation of the electronic properties of graphene than in the case of covalent functionalization. However, the stability of the functionalization becomes an important issue due to the lack of strong bonds between the adsorbate and the graphene.16 © 2016 American Chemical Society
Received: January 4, 2016 Accepted: January 26, 2016 Published: January 26, 2016 2930
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ACS Nano used to induce n-doping in carbon nanotubes.38 As will be shown, when the amount of adsorbed piperidine is gradually increased, graphene can be tuned from an unintentional pdoped state to a highly n-doped state. The inhomogeneity in the adsorption of piperidine leads to the formation of p−n junctions for intermediate coverages. Interestingly, we observed that the charge transfer between piperidine and graphene can be further tuned by the application of a gate-controlled electric field. Thus, the doping level of the graphene can be easily adjusted in a reversible way. Given the noncovalent nature of the interaction, no degradation occurs in the electronic properties of the graphene, either during the functionalization with piperidine or in the subsequent tuning of the doping level. The adsorption of piperidine was highly stable in vacuum conditions, even withstanding a mild annealing. In short, the proposed method offers a new way to reversibly tune the doping level of graphene in a controlled way. This effect is interesting for a wide variety of applications, such as in optoelectronic devices or in volatile memories.
RESULTS AND DISCUSSION n-Doping of Graphene with Piperidine and Formation of p−n Junctions. The impact of the adsorbed piperidine on the charge transport properties of graphene was investigated using graphene field-effect transistors (FETs), according to the schematic depicted in Figure 1a. Uniform single-layer graphene grown by ambient pressure chemical vapor deposition (CVD) was used as channel in the FETs.39 Shown in Figure 1b is the dependence of the graphene resistance (R) on the applied back-gate voltage (VG) for one graphene FET. Before the functionalization, graphene shows a p-type character, with the charge neutrality point (CNP) located at positive gate voltages (∼60 V). This behavior, which can be partially mitigated by annealing in vacuum (see Figure S1 of the Supporting Information), is mainly ascribed to the presence of water molecules or other impurities from the fabrication process, either on the graphene or in the interface between the graphene and the substrate.1,24,40,41 Nonetheless, the main findings of this work do not rely on the initial pdoping level, and similar behavior after exposure to piperidine was obtained for graphene showing no initial doping. The FET was then exposed to different doses of piperidine, as detailed in the Experimental Details section. Small amounts of piperidine were first deposited on the graphene by exposure to piperidine vapor. This decreased the p-character of the graphene as evidenced by the shift of the CNP, and induced the appearance of a shoulder at negative gate voltages of the resistance curve (Figure 1c). The existence of this shoulder is more evident in the first derivative of the resistance (dotted line in the inset of Figure 1c). The density of piperidine molecules adsorbed on the graphene after a second vapor exposure is ∼1.1 × 1013 cm−2, according to XPS measurements (see Figure 2a). Medium concentrations of piperidine were attained by spin coating a water solution of piperidine (10%). At these concentrations, the shoulder at negative voltage becomes the main peak of the resistance plot (Figure 1d). During these stages, both peaks located at positive (CNP+) and negative (CNP−) gate voltages coexist in the resistance curve. Scanning electron microscope (SEM) observations reveal that the presence of these two CNP is probably originated by inhomogeneous coverage of piperidine (right sides of Figure 1b−e). Initially, the graphene surface looks uniform in SEM images, except for the presence of some bright lines that
Figure 1. (a) Schematic of the process of functionalization of a graphene-FET with piperidine. (b−e) Resistance dependence on the back-gate voltage of a same graphene-FET, pristine (b) and with increasing concentrations of piperidine from low (c) to large (e). Blue and red curves are the backward and forward sweeping, respectively. Dotted and full lines in (e) are collected before and after annealing in vacuum (393 K/2 h), respectively. Inset in (c) shows an enlarged image of the CNP located at negative gate voltages, with the dotted line being the first derivative of the backward sweeping. At the right side are shown SEM images of a same graphene area at each of the functionalization stages (scale bars represent 25 μm; the dark circular feature is a hole in the graphene).
correspond to wrinkles formed during the transfer of the graphene to the SiO2 substrate. A small hole can also be seen (dark round feature) in the graphene, which will serve as spatial reference. At low exposures to piperidine, some dark areas appear on the graphene, mainly along graphene wrinkles and other topographic features acting as nucleation sites (Figure 1c). A darker contrast is expected for n-doped graphene, and hence those areas are supposed to correspond to graphene covered by piperidine and are related to the CNP−. On the 2931
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Figure 2. Spectroscopy characterization of the functionalized graphene. (a) XPS N 1s and C 1s peaks of pristine graphene (black), after the 2nd vapor deposition of piperidine (light red), and after the 3rd spin coat of piperidine (red). Spectra were vertically shifted for clarity. The concentration of piperidine was extracted from the evolution of the C/N atomic ratio. (b) G (left) and 2D (right) bands of the Raman spectra of graphene at different stages of functionalization with piperidine. Raman spectra are normalized to the intensity of the G band and vertically shifted for clarity. (c) Evolution of the G fwhm (blue circles) and of the intensity ratio of 2D and G (red squares) with the functionalization. Each dot is the average value of 1600 spectra collected in an area of 20 × 20 μm2.
electron mobility to values similar to those of the samples with low p-doping levels. This is probably due to the removal of the contaminants that caused the initial p-doping. In either case, the fact that the mobility did not decrease is a clear indication that functionalization is not covalent in nature, and it simply proceeds by the adsorption of the piperidine molecules. As will be discussed later, the adsorption of piperidine is highly stable in vacuum, with the n-doping being retained even after a mild annealing (393 K for 2 h) (full lines in Figure 1e). The doping of the functionalized graphene was also verified by Raman spectroscopy (Figure 2b,c). Except for the highest coverage of piperidine after the third spin coat, there is no piperidine-related bands in the Raman spectra, which only show the features commonly associated with the graphene (Figure S2 of the Supporting Information).45 However, increasing the exposure of graphene to piperidine induces a shift of the G band toward higher wavenumbers and a decrease of its full width at half-maximum (fwhm). At the same time, the intensity ratio of the 2D to G bands decreases as the amount of adsorbed piperidine increases (Figure 2c). These trends support the increase of the doping of graphene with the amount of piperidine.46 It is worth noting that the intensity of the D band does not appreciably increase during the exposure of graphene to piperidine (Figure S2 of the Supporting Information), confirming the noncovalent nature of the functionalization. Even though the interaction between the graphene and piperidine is noncovalent, we found that the doping effects are relatively stable. To check the stability of the adsorbed piperidine, after the final spin coating the sample was annealed in vacuum at 393 K during 2 h. This temperature is 14 K above the boiling point of piperidine at atmospheric pressure. The doping level slightly decreased after the annealing (full line curves in Figure 1e), indicating desorption of some of the piperidine. However, the fact that graphene remains in a highly n-doped state indicates a large stability of the piperidine adsorbed on graphene under vacuum conditions. In fact,
other hand, the areas of graphene not yet covered by piperidine (or covered at smaller concentrations) should retain the characteristics of pristine graphene, hence persisting the CNP+ in the resistance plots. We assume that the coexistence of covered and uncovered areas of graphene induce the formation of spatially separated p−n junctions.21,42−44 Thus, functionalization with piperidine provides an easy method for obtaining graphene p−n junctions without the need of local gates. An overall decrease of the resistance with respect to the initial stage was also observed for medium piperidine coverages (Figure 1d). This can be explained by the presence of two different kind of regions for which the graphene Dirac cones are shifted along the energy axis. Thus, there is a general increase of charge carriers in the graphene channel, as the Fermi level will always be positioned far away from at least one of the two Dirac points. Graphene was then exposed to higher piperidine doses, until an estimated molecule density of ∼4.5 × 1013 cm−2 by XPS (Figure 2a, after third spin coat). SEM images showed that graphene is uniformly covered with piperidine at this stage (Figure 1e). In addition, some large piperidine aggregates were also observed at some areas of the surface (dark features). At this stage of high doping, CNP+ disappears from the resistance plot (Figure 1e), and only CNP− remains at approximately −100 V. This corresponds to an electron concentration as high as ∼7.2 × 1012 cm−2 induced by the presence of piperidine. The doping is attributed to a charge transfer occurring between the amine group of the piperidine and the graphene.16,38 Even though the relatively high level of doping attained, the resistance curve shows no sign of degradation of the electronic properties of the graphene. Thus, the mobility of graphene did not decrease after each of the exposures to piperidine. In samples with initial low p-doping levels, mobility maintained values of ∼3000 cm2/(V·s) for both electrons and holes. For samples showing an initial p-doped state, the electron mobility presented low values before exposure to piperidine (∼600 cm2/ (V·s)). In these cases, exposure to piperidine increased the 2932
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Figure 3. Cascade measurements in FETs for pristine (a), mildly (b) and highly doped (c) graphene with piperidine. Left side shows the forward sweepings of the cascades, with the minimum applied gate (V−G) stepping from the initial −40 V (blue curves) to −150 V (red curves). Right graphs show the corresponding position of the CNP for each V−G, with blue and red markers corresponding to the backward and forward sweepings, respectively. Note that position of CNP− for the highly doped graphene (c) lies outside the sweeping range for V−G > −100 V, and hence, no data points are included (a dotted black line of slope equal to unity is a visual aid).
essentially unchanged (Figure 3a). A small dependence can be appreciated for low piperidine coverages in the shoulder at negative voltages (Figure S3 of the Supporting Information), while for higher piperidine coverages the gate-tuning is evident (Figure 3b,c). In the case of mild coverages of piperidine, CNP− shifted by approximately −90 V as V−G was varied from −40 to −150 V (Figure 3b). In contrast, CNP+ shifted less than −4 V, and thus, the separation between the two maxima grows up to ∼126 V for V−G = −150 V. This corresponds to a difference in energy of ∼454 meV between the Dirac points of the areas covered by piperidine and those not covered (details of this calculation can be found in the Experimental Details section).46 The gate-tuning of CNP− was maintained for high piperidine coverages (Figure 3c), and even after the sample is annealed in vacuum. As previously indicated, for this high piperidine concentrations CNP+ is absent from the resistance plot. It is worth noting that in contrast to the tuning obtained when changing V−G, varying the positive gate limit (V+G) did not significantly affect in the position of the CNP (Figure S4 of the Supporting Information). We will show below that the position of the CNP depends not only on the applied gate voltage but also on the time period that this voltage is applied, and hence indirectly in the sweeping speed. However, it is worth noting that the present method allows for a fine and precise tuning of the doping level of graphene. Thus, once that the experimental conditions have been fixed (e.g. the sweeping speed), calibration curves such as
adsorption of piperidine was proved to be completely stable when the sample was stored in vacuum, as no appreciable decrease in the doping level was observed after 2 weeks of the functionalization (see Figure S8a of the Supporting Information). A more comprehensive study indicated that the effects of piperidine diminish in ambient conditions and by contact with water (see Supporting Information). However, we would like to remark that even though functionalization of graphene with piperidine is not completely stable in ambient or humid conditions, passivation with protective layers can aid in maintaining the integrity of the doping.38 Gate-Induced Tunability of the Dirac Point. Apart from the n-type doping, functionalization with piperidine shows an interesting effect which allowed us to further control the doping level by applying a gate voltage. To demonstrate this gatecontrolled tuning of the doping, graphene FETs were subjected to cascade measurements, which are described in detail in the Supporting Information. Briefly, a cascade consist of repeated measurements of the FET transfer characteristics for which the sweeping range is increased on each iteration. Specifically, we found that varying the minimum gate voltage (V−G) to more negative values strongly influenced the FET characteristics. Cascades of the pristine graphene and of graphene submitted to mild and high piperidine exposures are shown in Figure 3, for which V−G ranges from −40 to −150 V. As expected, in the case of pristine graphene, the resistance did not show any dependence on V−G and the position of the CNP remained 2933
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piperidine, differences between sweeping directions become more evident, increasing with the exposure to piperidine (Figure 1c−e), and with the decrease of V−G (right panels of Figure 3b,c and Figure S5 of the Supporting Information). Interestingly, the hysteresis is always more pronounced around CNP−, suggesting that this peak is associated with the presence of piperidine. While the difference in the position of CNP+ between opposite sweeping directions never exceeds 5 V, for CNP− it can be as large as 40 V (Figure S5). This corresponds to differences in the charge density between opposite sweeping directions of ∼2.9 × 1012 cm−2. Such large hysteresis occurs even with the relatively fast sweep rates used (20 V/s), which demand a fast mechanism to origin it.21 The exact nature of hysteresis in graphene FETs is a topic that is still discussed in the literature, with several mechanisms proposed.21,47−49 In some cases, hysteresis can be well explained by the presence of polar species, such as water, trapped below the graphene or adsorbed on it.21,47 In the case of piperidine, we consider that the most probable reason for the hysteresis is a rearrangement of the piperidine molecules due to the interactions of their dipole moments with the electric field from the gate. To support this hypothesis, first principles calculations were carried out for the piperidine/graphene system (see Figure S6 of the Supporting Information). Our calculations show that the presence of an electric field vertical to the graphene modifies the adsorption energies of the piperidine on the graphene. In the absence of an electric field, the most stable of the studied configurations of the piperidine/graphene system is that with the piperidine lying flat over the graphene (flat configuration of Figure S6). The energy of this flat configuration is ∼73 meV/ molecule lower than that of the piperidine oriented vertically and with the amine group pointing toward the graphene (vertical configuration). Interestingly, the energy difference between those two configurations gradually decreases with the intensity of the electric field, until the vertical configuration becomes more stable. These calculations reinforce our speculation that the reorientation of the piperidine molecules contribute to the observed hysteresis effects, and ultimately to the tunability of the doping level. Previous reports have shown that the relative position of molecules adsorbed on graphene can be controlled by the application of a gate voltage.22,50 Particularly, a similar mechanism has been claimed to occur for graphene doped with ammonia molecules and submitted to voltage pulses during a few seconds.22 This ultimately can affect the amount of transferred charge, and consequently the doping level of graphene.36 Another possibility that could account for the hysteresis is that the piperidine is undergoing some redox reaction at large negative gate voltages, which would also justify the slow recovery of the CNP shift shown below.51,52 Figure 5a shows the time dependence of the transfer curve after the CNP− has been shifted by the application of a large negative gate voltage (− 150 V). Once that a shift has been induced in the CNP−, the transfer curve becomes relatively stable, with the CNP− slowly drifting back only a few volts per hour (red curve in Figure 5a). Thus, piperidine doping of graphene could be applied for the fabrication of memory devices.53 We note that it is possible to accelerate the recovery time by the application of a fixed gate voltage. Plotted in Figure 5b are the shift of the CNP and the related variation in the density of charge carriers (Δn) in the graphene when different fixed VG values are kept for 1 h. These are calculated from the measured variation of the FET resistance during the keeping periods. The application of a large positive gate voltage (+60 V)
those at the right side of Figure 3 provide the exact gate voltage that needs to be applied to each particular FET in order to reach the desired doping level. The observed shift of CNP− in the cascades (Figure 3b,c) implies that graphene is being more electron doped as the applied gate voltage becomes more negative. The fact that the position of CNP+ is barely affected by the gate voltage indicates that the tuning is related to the effect of the gate voltage in the interaction between graphene and the piperidine molecules. The levels of charge carrier density (electron) that can be induced by high concentrations of piperidine are ∼7.20 × 1012 cm−2 before applying large negative gate voltages. With the gate-tuning effect, the carrier density increases to ∼1.04 × 1013 cm−2 after applying V−G = −150 V. This value can even be increased to ∼1.30 × 1013 cm−2 by increasing the gate limit to −180 V, although at such gate values, some of the devices start to fail due to dielectric breakdown of the silicon dioxide. These values are only slightly smaller than those recently attained for graphene doped with ethylene amine molecules (∼1.4 × 1013 cm−2),32 which rank among the highest values reported for ndoping of graphene by chemical functionalization. Thus, piperidine can be considered an efficient n-type dopant for graphene, with the advantage of allowing to tune the doping level by applying a gate voltage. A quantitative comparison of the CNP tuning for different piperidine coverages can be done from the slopes of the V−G− CNP graphs (right panels of Figure 3), which indicate the ratios of the CNP shifts and the changes in V−G causing them (Figure 4). Given the relation between the position of the CNP and the
Figure 4. Ratios of the CNP shifts and the variations of V−G causing them for pristine graphene, and at different piperidine coverages. Ratios for CNP+ and CNP− are included, both for backward (blue) and forward (red) sweep directions.
charge carrier density n, it is also possible to determine the change in the carrier density induced by V−G from those slopes (right axis of Figure 4). Although the doping level increases with the concentration of piperidine, the tuning effect (i.e., the shift of CNP by V−G) is similar for mild to high piperidine concentrations (Figure 4). It is also confirmed from Figure 4 that the gate voltage has a larger effect on CNP− than on CNP+. Every resistance curve consists of a backward sweeping, for which the gate voltage is swept from V+G to V−G, immediately followed by a forward sweeping, for which the direction is reverted. Figure 4 shows that in the forward sweeps (red bars) the CNP shifts are remarkably larger than those in the backward (blue bars), indicating an increasing hysteresis in the transfer characteristics as V−G is decreased. Hysteresis is absent in the resistance curves of pristine graphene, as seen in Figure 1b and in the right side of Figure 3a. After exposure to 2934
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procedures, showing the expected shifts in the CNP position in the backward sweepings (Figure 5c). The forward sweepings (collected just after the corresponding backward ones) look similar in each case, with the CNP already shifted due to the back voltage applied during the backward sweep (inset of Figure 5c). This proves that the voltage keeping procedures have not damaged the device and that their effects are completely reversible. The fact that the position of the CNP− depends not only on the applied gate voltage, but also on the period that this voltage is applied, may bring concerns about whether the doping level can be precisely tuned. It should be noted here that the sweeping rate does not have a large impact on the tuning of the doping level (Figure S7 of the Supporting Information). Moreover, as already pointed, once that the experimental conditions have been fixed, data as that of Figure 3 can be used as precise calibration curves. Another proof of the high control of the doping level can be seen in Figure 6a,b. These figures show a cascade measurement done on a graphene FET exposed to piperidine (Figure 6a). To follow the short-term temporal evolution of the resistance right after each of the sweepings, the gate voltage was kept at 0 V for 2 min (Figure 6b) after each of the measurements of Figure 6a. The CNP shifts toward more negative values after each of the sweeps in the cascade, while the resistance at VG = 0 V decreases ∼25% from the first to the last of the sweepings (Figure 6b). This corresponds to an increase in the density of charge carriers of ∼3 × 1012 cm−2. However, the resistance barely changes within each of the 2 min between the sweepings, and so the doping level is being maintained (Figure 6b). Thus, a differentiated value of the doping level has been set by each of the sweepings. Figure 6c shows another cascade collected in a different device. This cascade was collected in three steps, until V−G = −100 V (blue curves), until V−G = −150 V (green curves) and until V−G = −180 V (red curves). Immediately after each of the steps, the same sweeping range was repeated 5 times in each occasion, i.e., V−G was kept constant at −100, −150, and −180 V, respectively (Figure 6d). The CNP shifted when V−G became more negative (Figure 6c), but the sweeping curves overlapped to each other when V−G was kept constant (Figure 6d). Hence, the doping level is maintained when the sweeping limit is kept constant. Together, Figure 6 shows the stability and controllability of the gate controlled tuning of the graphene doping. Therefore, once a set of experimental conditions is fixed, the doping level of graphene exposed to piperidine can be precisely adjusted by a gate voltage. This makes the piperidine a suitable candidate for tuning of the doping level in practical applications. To better illustrate the mechanism of the doping level tuning, Figure 7 shows the transfer characteristics of a piperidine-doped FET for which V−G = −150 V, with the backward (blue) and forward (red) sweeps placed in the sequential order at which they were acquired. The inset graph shows a conventional view of both the transfer characteristics (full lines) and the resistance plots (dotted lines). At the beginning (1) the graphene is ndoped due to the effect of the piperidine. This is depicted in the correspondent band schematic by the Dirac point lying below the Fermi level (indicated by a dotted line). The current decreases as the sweeping proceeds and the Dirac point approaches the Fermi level, with the minimum current occurring when they both coincide at approximately −100 V (2). From there, an increase of the current should occur as the gate voltage continues decreasing.1 However, the current barely increases until the end of the backward sweep (3), and the
Figure 5. (a) Resistance curve of a FET just before (dotted black) and after being shifted by applying a large negative gate voltage (black). Red curve is the resistance measured again after 1 h. The inset show the evolution of the resistance at VG = 0 V during the time between both measurements. (b) Calculated variation of the CNP position and of the charge density when keeping VG fixed at 0 V (black), 60 V (blue), and −90 V (red). (c) Backward sweeps before (black), after keeping a VG of 60 V during 1 h (blue), and after keeping −90 V for an additional hour (red). Inset shows the subsequent forward sweepings.
results in a faster recovering of the CNP than in the case of a natural recovering (i.e., no gate voltage applied). On the other hand, the application of a negative gate (−90 V) increases the electron carrier density, hence increasing the n-doping level of the graphene. In the case of keeping +60 and −90 V (blue and red curves respectively), the time variations of Δn can be well fitted by three exponential decays in the form y = y0 + A1 e−t/τ1 + A2 e−t/τ2 + A3 e−t/τ3, where τ1 ∼ 5 s, τ2 ∼ 100 s, and τ3 ∼ 1200−2000 s. For the case of fixing a gate of 0 V (black curve), the fitting can be done with only one of the exponentials, with τ ∼ 1700 s. Considering the different phenomenon involved in the measurements, the shortest decay constant (5 s) can be ascribed to the abrupt change in the gate voltage from 0 V to the chosen kept value (and hence it is absent in the case of keeping 0 V). A charge transfer between the piperidine and the graphene is possibly the mechanism behind the largest decay constant (>1000 s). Regarding the intermediate decay constant (∼100 s), its order of magnitude is similar to that of the times for collecting a backward/forward sweeping. Thus, we assume that its origin is the same as that of the hysteresis present in the sweeping curves. Calculations from Figure 5b were confirmed by the collection of the resistance plots just after each of the keep 2935
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Figure 6. (a) Cascade measurement in a piperidine-doped graphene FET. (b) Time variation in the resistance at VG = 0 V before (black line) and after each of the sweepings in (a) (blue to red lines). (c) Cascade in a piperidine-doped graphene. The cascade was interrupted at V−G = −100 V (blue), −150 V (green), and −180 V (red), and the same sweeping range was repeated for 5 times (d).
level of graphene by charge accumulations in the substrate.35,54 We speculate that the gate-tuning in the piperidine-functionalized graphene involves charge trapping not by the substrate, but by the adsorbed piperidine. This trapped charge counteracts the effect of the back-gate, producing a local gating similar to that observed in dual-gated FET configurations.44,55 This accounts for the fact that only the peak corresponding to graphene covered by piperidine (CNP−) shifts, while that of uncovered graphene (CNP+) is almost unaffected by the gate. Although it is not clear which is the precise mechanism causing the absence of hole carriers from the conduction channel, this only happens when the Dirac point of graphene is located at certain level over the Fermi level (region between (2) and (3) in Figure 7). Thus, it seems that charge transfer is only allowed for certain energy alignments between graphene and piperidine band levels. Thus, we consider that the reorientation of the piperidine under the influence of the electric field may also play a role in the tuning of the Dirac point. In this sense, Figure S6 shows that the stability of the different orientations of adsorbed piperidine change when the Fermi level lies below the Dirac point of graphene.
Figure 7. Transfer characteristics of a graphene FET taken after doping with large doses of piperidine. Backward (blue) and forward (red) sweeps are located in the sequential order of acquisition. (1−4) The band levels of the graphene are schematically represented at different stages of the measurement, showing the relative shift with respect to the Fermi level (black dotted line). Upper inset shows a conventional view of the transfer characteristics (full lines) and the corresponding resistance (dotted lines).
curve exhibits a large asymmetry between the electron and hole conduction. This indicates a decrease in the amount of hole carriers from the graphene channel, presumably due to charge trapping by the piperidine molecules or by their surroundings. As the forward sweep proceeds, the current experiences a sharp decrease until reaching its minimum value at −140 V. The CNP results significantly shifted with respect to the backward sweep, although the minimum current is still the same. Moreover, the forward sweep shows a higher symmetry for the electron and hole sides. At the end of the sweep, the current significantly increased with respect to the initial state, indicating a higher n-doping level (4). Figure 7 clearly shows that the tuning of the doping level occurs in the region between (2) and (3), when VG < VCNP. We discard that the effect is related to the metal electrodes or from impurities induced during their fabrication, as the tuning of the CNP is closely related to the presence of piperidine and it was never observed in its absence. It is worth noting that some recent works already show that it is possible to alter the doping
CONCLUSIONS Controlled n-doping of single-layer graphene has been realized by exposure to piperidine. FET, Raman and XPS measurements indicate that piperidine adsorbs on the graphene, inducing an electron doping that is attributed to charge transfer between piperidine and the graphene. Given the noncovalent nature of the interaction between piperidine and graphene, no further damage in the electronic properties of the latter is observed. The amount of adsorbed piperidine determines the level of doping, with the graphene experiencing a gradual change from an initially hole-doped state to a large electron-doped state as the piperidine coverage increases. For intermediate coverages, p−n junction behavior can be observed in the transfer characteristics of the FETs. Piperidine is proved to be an efficient dopant, attaining electron-doping levels comparable to 2936
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induced in the graphene was calculated from the position of the CNP, according to the expression n = CoxVCNP/e for a parallel plate capacitor model (where e is the elementary charge).1 The energy shift of the Dirac points of the areas covered (CNP−) and not covered (CNP+) by piperidine was obtained from the value of the charge carrier density in each case by the expression ΔEF = ℏvF(π|n|)1/2, where vF = 1.1 × 106 ms−1 is the Fermi velocity of graphene.46 Further characterization was done by confocal Raman microscopy with laser excitation 532 nm, in a Nanofinder 30 spectrometer (Tokyo Instruments, Inc.). Raman mappings were conducted on areas of 20 × 20 μm2, from which representative spectra were collected. SEM images were collected in a S-4800 apparatus from Hitachi High-Technologies Co. XPS spectra were measured in a Kratos Axis 165 spectrometer equipped with a monochromatic Al X-ray source at pressures of ∼6 × 10−7 Pa. Self-consistent geometric and electronic structure calculations were performed using the framework of density functional theory (DFT)58,59 with the Simulation Tool for Atom Technology (STATE) package.60 To express the exchange correlation potential among the interacting electrons, the local density approximation (LDA) was applied with the Perdew−Zunger functional generated by a homogeneous electron gas,61,62 since the LDA is known to reproduce intermolecular spacing between hydrocarbon molecules and graphene. We uses an ultrasoft pseudopotential to describe the interactions between the valence electrons and ions generated by the Vanderbilt scheme.63 The valence wave functions and charge density were expanded with a plane-wave basis set with cutoff energies of 25 and 225 Ry, respectively. To integrate the Brillouin zone, we took equidistant 2 × 2 k-point mesh for piperidine adsorbed on a graphene sheet with 4 × 4 lateral periodicity. The effective screening medium method was applied to investigate the electronic properties of graphene with piperidine under the hole injection within the framework of DFT using the periodic boundary condition.64 Geometric structures of graphene with piperidine are fully optimized until the remaining force acting on each atom is less than 5 meV/A.
those recently reported for other molecules. Additionally, the doping level can be precisely tuned by the application of a gate voltage, which represents a clear advantage of the piperidine compared to other molecules that provide only fixed doping. We speculate that this tuning is produced by an enhancement in the charge transfer between piperidine and graphene for certain alignments between their band levels. The tuning was found to be completely reversible, showing a short-term stability of a few hours. Thus, the total degree of doping attained can be tuned both by the amount of piperidine and by the applied gate voltage. This provides a new route to further tune the electronic structure of graphene in a controllable way, which can be exploited for applications such as the fabrication optoelectronic devices and volatile memories.
EXPERIMENTAL DETAILS Single-layer graphene was synthesized on a Cu(111) film by atmospheric pressure CVD.39 A PMMA film was then spin coated on the graphene, and the Cu film was etched in a FeCl3 solution. Once that the Cu was completely etched, the PMMA/graphene was repeatedly rinsed in water and then transferred to a Si substrate with a thermally grown 300 nm SiO2 dielectric layer. Finally, the PMMA was removed by immersing the sample in hot acetone. Piperidine was purchased from Sigma-Aldrich (ReagentPlus grade). For low degrees of functionalization, vapor deposition of piperidine was realized by keeping the graphene in an enclosed atmosphere at 70 °C during 30 min, and in the presence of 50 μL of piperidine.11,32,56 To attain higher degrees of functionalization, piperidine aqueous solutions at different concentrations (10% or 50%) were directly spin coated on the graphene. Each of the graphene samples was subjected to functionalization in sequential steps (see Figure 1a), consisting of two consecutive exposures to piperidine vapor, two consecutive spin coatings of piperidine solution (10%), and a last spin coat at a higher concentration (50%). After the last spin coat, the samples used for FET measurements were annealed in vacuum (393 K during 2 h). To test the stability of the piperidine, some samples were also washed twice by sequential immersion in water and isopropanol, first during 5 min and then during 5 h. Given the similarities of the FET measurements, for simplicity the samples are grouped into three different levels of exposure to piperidine, labeled as low (vapor exposure), mild (10% spin coat), and high (50% spin coat). When the precise step of the functionalization procedure can be relevant, such as in XPS or Raman measurements, it will be indicated in the text and in the corresponding figure. The electronic properties of the functionalized graphene were characterized using FETs. For this, graphene was first patterned into 10 μm wide parallel lines by photolithography followed by O2 plasma. After removal of the photoresist in acetone, the source and drain electrodes were patterned by photolithography and a ∼50 nm thick layer of Au was deposited by thermal evaporation in vacuum, while the Si substrate was used as the gate electrode. Transport properties of the back-gated graphene FETs were measured in a B1500A semiconductor analyzer (Keysight Technologies) at room temperature and under vacuum conditions (∼1.5 × 10−4 Pa). To obtain the transfer characteristics of the FETs, back gate voltage was always swept starting from the maximum applied voltage (backward sweep), and then in the reverse direction (forward sweep). This was done to prevent any disturbance in the measurements, as the piperidine-treated samples proved to be sensitive to the application of large negative voltages. In all the measurements, the drain to source voltage (VD) was fixed at 0.1 V. Relatively fast sweeping speeds (20 V/s) were used to obtain the curves. According to the Drude model of electrical conduction in metals, the field effect mobility was calculated as μ = σ/ e|n| = (L/WCoxVD)(∂ID/∂VG) where L and W are, respectively, the length and width of the graphene channel (50 and 10 μm, respectively), Cox = εε0/t is the dielectric capacitance of the SiO2 layer (thickness t = 300 nm), and ∂ID/∂VG is the slope in the linear part of the transfer characteristics.57 The charge carrier density
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00064. Complementary FET, Raman, SEM measurements and other related data (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work is supported by PRESTO-JST, KAKENHI (15H03530, 15K13304) and JSPS Funding Program for Next Generation World-Leading Researchers (NEXT Program, GR075). We thank Dr. Miura, of the Center of Advanced Instrumental Analysis of Kyushu University, for the XPS measurements. REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (2) Ferrari, A. C.; Bonaccorso, F.; Fal’ko, V.; Novoselov, K. S.; Roche, S.; Bøggild, P.; Borini, S.; Koppens, F. H. L.; Palermo, V.; Pugno, N.; Garrido, J. A.; Sordan, R.; Bianco, A.; Ballerini, L.; Prato, M.; Lidorikis, E.; Kivioja, J.; Marinelli, C.; Ryhänen, T.; Morpurgo, A.; et al. Science and Technology Roadmap for Graphene, Related Two2937
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