Tunable D peak in gated graphene

Transcription

1 Nano Research Nano Res 1 DOI /s Tunable D peak in gated graphene Anna Ott, Ivan A. Verzhbitskiy, Joseph Clough, Axel Eckmann, Thanasis Georgiou, and Cinzia Casiraghi ( ) Nano Res., Just Accepted Manuscript DOI: /s on December 12, 2013 Tsinghua University Press 2013 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 Tunable D Peak in Gated Graphene Anna Ott 1, Ivan A. Verzhbitskiy 1, Joseph Clough 2, Axel Eckmann 2, Thanasis Georgiou 2, and Cinzia Casiraghi 1,2,* 1 Freie Universität Berlin, Germany 2 University of Manchester, UK 1 2 The D peak intensity of defective graphene is tunable and reversible with the gate voltage. This is attributed to chemical functionalization of graphene, driven by the water trapped between the substrate and graphene. * Cinzia Casiraghi, cinzia.casiraghi@manchester.ac.uk 1

3 Nano Res DOI (automatically inserted by the publisher) Research Article Tunable D peak in gated graphene Anna Ott 1, Ivan A. Verzhbitskiy 1, Joseph Clough 2, Axel Eckmann 3, Thanasis Georgiou 2, and Cinzia Casiraghi 1,3 ( ) 1 Physics Department, Freie Universität Berlin, Berlin 14195, Germany 2 School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK 3 School of Chemistry, University of Manchester, Manchester M13 9PL, UK Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011 ABSTRACT We report the gate modulated Raman spectrum of defective graphene. We show that the intensity of the D peak can be reversibly tuned by applying a gate voltage. This effect is attributed to chemical functionalization of the graphene crystal lattice, generated by an electro chemical reaction involving the water layer trapped at the interface between silicon and graphene. KEYWORDS graphene, gating, defects, doping, electro chemistry 1 Introduction Graphene shows extraordinary properties that are expected to produce strong technological breakthroughs in various fields such as electronics and photonics, to name a few [1,2]. Since the experimental discovery of graphene, Raman spectroscopy has become one of the most useful non destructive tools for its characterization [3]. This optical technique is able to identify graphene from graphite and few layer graphene [4], to probe doping level [5 8], strain [9, 10], disorder [11 15], chemical derivatives [16 18], the atomic arrangement at the edges [19] and graphene superlattices and hetero structures [20 22]. In particular, gate modulated Raman spectroscopy is a powerful tool to get insights on the Raman scattering process in graphene. For example, gating experiments have provided spectacular demonstration of the existence of a Kohn Anomaly (KA) at the Γ-E 2g phonon (Raman G band) [5,6] and of the non resonant nature of the G peak [23 25], due to the suppression of destructive interference at high doping levels. Recently, peaks with smaller intensities associated to phonons with q 0 have been also investigated by gate modulated Raman spectroscopy [26], showing that phonon renormalization effects are different from what is observed for the zone center phonons, such as the G peak. Despite the wide use of gate modulated Raman spectroscopy, gating has been applied only to defects free graphene. Therefore, the D peak dependence on gating has never been investigated so far. In this Address correspondence to Cinzia Casiraghi, cinzia.casiraghi@manchester.ac.uk 2

4 work, we analyze the Raman spectrum of gated defective graphene. We will show that under particular conditions, the intensity of the D peak can be reversibly tuned by applying a gate voltage. This effect is attributed to chemical functionalization of the graphene crystal lattice, generated by an electro chemical reaction involving the water layer trapped at the interface between silicon and graphene. The electric field breaks the water molecules into H + or OH, which gets chemisorbed to graphene, depending on the intensity and polarity of the applied field. Furthermore, the silicon oxide layer may acts as catalyst by further enhancing chemisorption. 2 Results and discussion Figure 1(a) shows the optical picture of the graphene samples used in this study. A few bubbles and ripples are visible. These are typically observed on large graphene flakes when placed on a silicon substrate [27]. Far from the bubbles, the surface appears clean without any optically visible structural damage (inset in Fig. 1(b)). However, the Raman spectrum clearly shows a D peak, indicating that this is a defective area, Figure 1(b). I(D)/I(G) is in average , but can reach up to 1.4 in isolated spots. We selected this defective area to perform our gating experiments. The evolution of the first order Raman spectrum at different gate voltage is illustrated in Fig. 2(a), where the intensities have been normalized to the G peak. Figure 2(b) shows that I(D)/I(G) increases by a factor 5 when tuning the gate voltage from 0 to 20V. A Dʹ peak can be clearly seen at relatively high voltage, Fig. 2(c). If the increase in I(D)/I(G) would be an intrinsic effect caused by the simple introduction of extra charge in the crystal, then we would expect to see a D peak also when gating a defect free pristine graphene at relatively high voltage, but none of the previous works reported the appearance of a D peak in the Raman spectrum. Therefore, this effect can be seen as characteristic of defective graphene, although it may depend on the exact nature of the defects in the crystal. We now look at the overall evolution of the gate modulated Raman spectra of defective graphene. Figure 3 shows: (a) the position, (b) width and (c) intensity of the G, 2D and D peaks (top, middle and bottom panels, respectively). The G and 2D peaks shows qualitatively the same behavior observed in gated defects free graphene: the G peak position increases for increasing charge concentration, due to the (non adiabatic) removal of the Kohn anomaly at Γ [5, 6], while the 2D peak position remains constant for relatively low doping [8, 28]. No strong variations have been observed for the G peak FWHM, indicating that the graphene flake is already doped. This is in agreement with previous studies on large graphene flakes, where strong doping has been observed and attributed to water trapped at the interface between graphene and the silicon substrate [10, 27]. The G peak FWHM decreases for increasing doping and saturates when the electron hole gap becomes higher than the phonon energy [5, 6]. Figure 3 shows that the D peak is also sensitive to gating: its position slightly decreases and its FWHM increases by 4 cm 1, as observed for non center phonons [27]. However, these are very small changes almost comparable with the resolution of the spectrometer. In contrast,, the D peak intensity strongly increases with increasing voltage, showing a very different behavior as compared to the G and 2D peaks intensities [29, 30]. This shows that the D peak intensity is responsible for the increase in I(D)/I(G) with gating. In order to get more insights on the D peak intensity tunability with the gate voltage, we performed cycling gating, i.e. we tuned back and forward the gate voltage twice. We did not use higher number of cycles to avoid dielectric breakdown and current leakage. Figure 4 shows I(D)/I(G) obtained in the two cycles. One can observe that the curves are reversible: both curves are almost symmetric with respect to 0V, and saturation is reached in both cases when the voltage goes above 10V. Some hysteresis appears in cycle 2, in particular at low voltage. Note that the data depicted in Fig. 2 were recorded at different positions on the flake, compared to Fig. 4. This 3

5 shows that the initial amount of defects in the flake strongly affects the D peak intensity enhancement: an initial I(D)/I(G) of 0.2 gives a factor 5 in enhancement, while an initial I(D)/I(G) of 0.1, gives a factor 2 in the same range of voltage, Fig. 2(b) and Fig. 4. In the second case we also observe saturation at high voltage. In our experiments we observed the D peak intensity to be tunable and reversible through the application of a gate voltage. We attribute this effect to chemisorption induced by an electrochemical reaction controlled by the gate voltage. For instance, hydrogen chemisorption is known to modify the hybridization from sp 2 to sp 3 [16]. The sp 3 site breaks the translation symmetry of the crystal by activating the D peak [16]. Recent studies have reported the strong influence of disorder on chemical reactivity. Defects on the graphene lattice decrease the chemisorption energy in the surrounding area and thus become centers of chemical activity [31]. This could explain why a tunable D peak is observed only in defective graphene. Note that the variation in the D peak intensity could be attributed to the electron phonon coupling, i.e. to the effect of doping on the Kohn anomaly at the K point [8, 32]. However, when defects are introduced in the crystal lattice, they change the electronic structure (for example a fully hydrogenated graphene is a wide band gap semiconductor [16]) and so the phonon affected by the Kohn anomaly. Therefore, it is questionable to apply the theory developed for defect free gated graphene, where the Kohn anomaly plays an important role, to defected graphene. The Kohn anomaly is expected to depend on the amount and type of defects, not only to the amount of charges. Therefore, samples with well defined amount and type of defects, such as defective graphene produced by ion bombardment [11, 12], should be used to investigate the role of the electron phonon coupling on the D peak intensity. We propose the following chemisorption mechanism. It is well known that a water layer is trapped between the silicon oxide wafer and graphene, in particular for large graphene flakes [27, 33, 34]. In the presence of an electrostatic field, H2O molecules are split into H + and OH reactive ions that diffuse towards the carbon monolayer sheet, according to the applied polarity. This assumption is in agreement with the bipolar character of the tunable D peak, Fig. 2(b). Furthermore, the silicon dioxide may act as a catalyst for water splitting [35, 36]. A recent computational study shows that patterning of the graphene surface by oxygen adatoms can be simply modulated by the applied gate voltage [37]. Hydroxyl groups can attach to graphene in a way similar to the single side hydrogenation process, leading to change of hybridization. In other words, this mechanism allows tunable functionalization of graphene. Note that a similar electro chemical effect was observed in transport measurements of double gated graphene field effect devices [38] and in graphene based ph sensors, where any sign of ph response has been attributed to imperfections in the graphene crystal [39, 40]. No specific binding of ions is expected in the ideal case of a perfect graphene [39]. Following our argument, the gate voltage activates the D peak through formation of sp 3 sites. In a recent work [14, 15], we have shown that in the limit of low defect concentration, Raman spectroscopy is able to probe the nature of defects in graphene. In particular, the intensity ratio between the D and Dʹ peak, I(D)/I(Dʹ), is expected to be ~13 for sp 3 sites. The Dʹ peak is less intense than the D peak, but well visible in our spectra when measured at relatively high gate voltage (e.g. Fig. 2(c)). We found that I(D)/I(Dʹ) ~13 and this ratio does not change with the gate voltage. By taking long measurements, we observed that also the isolated defective region at 0V is characterized by similar I(D)/I(Dʹ). This shows that the initial defects in our samples are sp 3 sites and their concentration increases with the voltage, in agreement with our model. By converting I(D)/I(G) of Fig. 2(b) into defects concentration [15] and by fitting the data for small gate voltage (V<< 10V ) we obtain that about 1.4x10 9 defects/ cm 2 are created per unit of voltage. Note however that there is a strong difference between the initial defects in graphene and the ones introduced by gating: the first ones cannot be removed (I(D)/I(G) never goes to zero), while the second type of defects is tunable and reversible with the gate voltage. 4

6 Furthermore one can observe that the data in Fig. 4, where saturation is reached, follows relatively well the Langmuir absorption isotherm given by Θ=kC/(1+kC), where Θ is the fractional coverage, k is the rate constant of adsorption/desorption and C is the concentration of the molecules that can be chemisorbed on the surface. By taking into account that C and Θ are proportional to the gate voltage and I(D)/I(G), respectively, the data in Fig. 4 can be well fitted with the Langmuir absorption isotherm, further confirming that the tunability of the D peak is correlated to chemisorption of species on graphene. Note that the Langmuir absorption isotherm is based on several assumptions [41]. For example they assume the surface to be flat and no interactions between the particles. These hypotheses are probably a good approximation at low coverage but they break closer to saturation and after the first cycle. In order to further validate our model, we performed two types of control experiments: i) gating of a partially hydrogenated graphene. This sample was specifically selected to investigate if defective graphene containing sp 3 sites is showing a tunable D peak. In this specific case, we could not observe any controlled change in the Raman spectra and in particular of the D peak intensity (Supporting Information). The spectra are very unstable with time indicating that probably the sample is interacting with adsorbed species or mobile ions. This shows that a tunable D peak can be achieved only under particular conditions, probably strictly related to the nature of the defects and to the presence of a water layer between graphene and the substrate. ii) In the second control experiment we induced splitting of water by applying an electric field directly to the drop, placed on a defective graphene in device configuration. We observed the appearance of a D peak, confirming that water splitting generated by an electric field (either through the substrate or directly to water) can be used to functionalize graphene (Supporting Information). In conclusion, we have demonstrated that the D peak can be tuned by an applied gate voltage. We have shown that the D peak is generated by chemical functionalization with non permanent defects, i.e. H + and OH ions, produced by splitting the water trapped between the silicon substrate and graphene. This effect allows to easily functionalize graphene and to use Raman spectroscopy as optical readout in solution gated and ultra fast biosensors and chemical sensors. Methods Graphene samples of a few hundreds of microns in size were placed on an oxidized silicon wafer by micro mechanical exfoliation. Typically, this production method gives high quality graphene, i.e. I(D)/I(G) is below 1% [19]. However, sometimes it is possible to observe area with relatively intense D peak. We selected this type of defective graphene in our study. The partially hydrogenated graphene sample was obtained as described in Ref. 14. Graphene was identified by Raman spectroscopy. We used a micro Raman WITec spectrometer with spectral resolution of 2 3 cm 1. This is equipped with a 532 nm (2.33 ev) laser and 100x long distance objective. The laser power was kept well below 1 mw to avoid damaging or heating the sample. The laser spot size is nm. The Raman measurements were taken just after changing the gate voltage and they were performed in air and at room temperature. A Lorentzian shape function was used to fit the recorded spectra. The intensity is the integrated area of the peak. Back gating configuration was used for all samples, where the contacts were made with silver paste by hand to avoid contamination during lithography. Acknowledgements The authors acknowledge useful discussions with R. Gorbachev, K. S. Novoselov, A. K. Geim and R. A. Dryfe. This work is funded by the Alexander von Humboldt Foundation in the framework of the Sofja Kovalevskaja Award, endowed by the Federal Ministry for Education and Research of Germany. 5

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9 Figure 1 (color on-line) (a) Optical micrograph of the graphene sample used in this study; (b) typical Raman spectrum taken on the area in the dotted circle in (a) (Inset: high magnification optical micrograph of this area showing no macroscopic damage). 8

10 Figure 2 (color on-line) (a) Evolution of the Raman spectrum measured at different gate voltage; (b) I(D)/I(G) as a function of the gate voltage; (c) Comparison between the first order Raman spectrum measured at 0 and +20V. Enhancement of the D peak intensity and appearance of the D' peak are visible with increasing gate voltage. 9

11 Figure 3 (a) Position, (b) FWHM and (c) intensity of G, 2D and D peak (top, middle, bottom panel, respectively) as a function of the gate voltage. 10

12 Figure 4 (color on-line) I(D)/I(G) as a function of the gate voltage obtained by cycling gating (2 cycles). The D peak intensity is tunable and reversible. 11