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Locally Altering the Electronic Properties of Graphene By Nanoscopically Doping It With Rhodamine 6G Xiaozhu Zhou, Shu He, Keith A Brown, Jose Mendez-Arroyo, Freddy Yin Chiang Boey, and Chad A. Mirkin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl400043q • Publication Date (Web): 13 Mar 2013 Downloaded from http://pubs.acs.org on March 18, 2013

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[Submitted to Nano Letters]

Locally Altering the Electronic Properties of Graphene by Nanoscopically Doping It with Rhodamine 6G Xiaozhu Zhou1,2, ‡, Shu He1, ‡, Keith A. Brown1, Jose Mendez-Arroyo1, Freddy Boey2, Chad A. Mirkin1,* 1

Department of Chemistry and International Institute for Nanotechnology, Northwestern

University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, USA. 2

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang

Avenue, Singapore 639798, Singapore. KEYWORDS. Dip-pen nanolithography, graphene, Rhodamine 6G, Kelvin probe force microscopy, molecular doping

ABSTRACT. We show that Rhodamine 6G (R6G), patterned by dip-pen nanolithography on graphene, can be used to locally n-dope it in a controlled fashion. In addition, we study the transport and assembly properties of R6G on graphene and show that in general the π-π stacking 1

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between the aromatic components of R6G and the underlying graphene drives the assembly of these molecules onto the underlying substrate. However, two distinct transport and assembly behaviors, dependent upon the presence or absence of R6G dimers, have been identified. In particular, at high concentrations of R6G on the tip, dimers are transferred to the substrate and form contiguous and stable lines, while at low concentrations, the R6G is transferred as monomers and forms patchy, unstable, and relatively ill-defined features. Finally, Kelvin probe force microscopy experiments show that the local electrostatic potential of the graphene changes as function of modification with R6G; this behavior is consistent with local molecular doping, highlighting a path for controlling the electronic properties of graphene with nanoscale resolution.

Graphene has generated great interest since its discovery in 2004 due to its unusual and potentially useful electrical, chemical, mechanical, and optoelectronic properties1-5. However, a major challenge in graphene research is to develop reliable and facile methods for tuning its properties for applications in electronics, optics, and sensing4,6,7. In particular, numerous approaches for adjusting the electrical properties of graphene have been investigated8-13, including electrical gating, size constriction, and the generation of defect states. Among those, surface functionalization of graphene with molecules bearing functional groups is especially interesting because graphene is entirely composed of surface atoms. Such functionalization with aromatic molecules has been demonstrated in bulk experiments to be an effective means for changing the chemical potential or even opening a bandgap10-12. One can envision entire arrays of devices created on a single piece of graphene on which different molecules that p- and n-dope the graphene by different amounts are patterned. However, in order to investigate the potential 2


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for realizing this goal by tuning graphene in a local fashion, it is imperative to have the capability to selectively pattern molecules on graphene on the nanoscale. Despite the wide appeal and promise of the local molecular doping paradigm, no direct non-destructive method14,15 for patterning aromatic molecules on graphene has been reported. Dip-pen nanolithography (DPN) and the related techniques, polymer pen lithography (PPL)16 and hard-tip, soft-spring lithography (HSL)17, are direct-write patterning techniques that use inkcoated tips to transfer molecules onto substrates of interest18-21. Since the invention of DPN in 1999, many classes of molecules, including alkanethiols18, DNA22, proteins23, and polymers24 have been successfully patterned on a variety of substrates including metals18, semiconductors25, and insulators26. The versatility of these molecular printing tools27 makes them promising candidates for patterning functional molecules on graphene. It is important to note that the reliable patterning of molecules is highly dependent on the chemistry between the molecule and the surface. For example, the well-known transfer of alkanethiol molecules to a gold substrate is facilitated, in part, by the robust thiol adsorption on gold chemistry28. Indeed, for this reason, such chemistry is used in the vast majority of DPN studies carried out to date. The ability for aromatic molecules to engage in π-π stacking interactions with planar aromatic species make them attractive candidates as novel inks for selectively patterning graphene. In this work, we present a systematic study that evaluates the transport, diffusion, and assembly of such molecules on graphene from a tip in a DPN experiment utilizing Rhodamine 6G (R6G) as a prototypical molecule. In addition, we show that once optimized, this technique can be used to locally modify the electrical properties of graphene. R6G was chosen because it can be easily characterized by Raman spectroscopy when adsorbed on graphene29. Exfoliated graphene30 was chosen as a substrate because it is relatively clean as 3


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compared with CVD-grown graphene31. In a typical DPN experiment (Figure 1A), an AFM tip was dip-coated in an inkwell containing 1 mM R6G in deionized water. The uniform inking of the tips was confirmed by fluorescence microscopy. To obtain consistent ink transfer from tip to substrate, multiple spots were written prior to the beginning of each reported experiment to remove the excess ink from the tip (Figure S1, Supporting Information). Subsequently, arrays of lines were written on graphene and characterized by a combination of AFM topographical and phase imaging and Raman mapping (Figure 1B and C). The lines were determined to be 45 ± 5 nm in width, and their chemical identity was verified using Raman mapping of R6G’s characteristic band29 at 1650 cm-1 (additional spectra for graphene and R6G can be found in Figure S2, Supporting Information). Interestingly, R6G does not transfer onto the underlying SiO2 substrate, as evidenced by the abrupt end of the lines at the top edge of the graphene sheet (Figure 1B and 1C). This observation led us to conclude that the π-π stacking between R6G and graphene is responsible for driving the ink diffusion onto the graphene surface. Having established that patterning of R6G on graphene was possible, we explored the transport and assembly dynamics of R6G in an effort to make features of controlled size. After removing excess ink from the tip (as described above), lines were patterned with writing speeds between 0.02 and 0.2 µm/s (Figure 2A). In stark contrast with conventional DPN processes32, the widths of all lines were determined to be 233 ± 18 nm, showing no dependence on writing speed. Examination of the dots written as the tips were moved into contact with the surface revealed that the diameter of the dots was 220 ± 9 nm, nearly the same value as the line width. This observation can be attributed to two possible reasons: the aggregated R6G on the tip increased the effective tip size so that the tip was acting as a stamp during patterning, and as a result the line width is determined by the effective tip size; or, the water meniscus formed between the tip 4


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and the substrate constrained the ink spreading so that the size of the water meniscus dictates the line width. Nevertheless, we are able to pattern robust R6G lines on graphene. Interestingly, when we further depleted the amount of ink on the tip, we began to observe a dependence of line width on writing speed (Figure 2B). The lines were 90, 130, and 180 nm wide when the writing speeds were 0.1, 0.05, and 0.02 µm/s, respectively (Figure S3). The emergence of a dependence of line width on writing speed implies that we were now operating in the conventional regime of DPN, where the tip is acting as a point source. Noticeably, thin traces of R6G (arrowed and dashcircled in Figure 2B) are left parallel to the main patterned line. We attribute these features to R6G that is transported along the air-water interface as the tip is scanned across the surface, meaning that these features potentially correspond to the size of the water meniscus, a hypothesis that is in accordance with previous observations of more widely studied alkanethiolate inks33. An important feature of the patterns of R6G written on graphene is that the height of patterned lines is 0.9 ± 0.1 nm, in agreement with an estimate of the thickness of the R6G dimer on graphene (0.7 nm)34 and the dimer-graphene substrate distance (0.3 nm). In support of this observation, dimers of R6G have been reported in aqueous environments at concentrations higher than 100 µM35, and the dimer configuration is maintained even in thin films upon drying36. It is therefore reasonable that R6G dimers remain stable when coated on AFM tips and also when transported to the graphene substrate. Since at concentrations greater than 100 µM, dimers are the stable configuration of R6G in water, we hypothesized that by inking the tip with a more dilute solution (~10 µM), it might be possible to pattern graphene substrates with monomers. Indeed, when tips were inked with solutions at this concentration, we observed monomer transfer (Figure 3), but surprisingly, the diffusion characteristics were strikingly different than in the dimer case. From AFM imaging 5


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(Figure 3B and C), thinner branched structures (0.4 nm high) instead of contiguous, well-formed lines were observed at all writing speeds. Similar diffusion behavior has been observed when molecules exhibit weak interactions with an underlying substrate37. It is important to mention that the reproducibility of these experiments was poor, since coating the tip with this low concentration ink often resulted in no significant molecular transport and therefore patterned features. The results presented so far show a rich variety of patterning behavior dependent upon the concentration and dimerization of R6G. To reconcile this diverse behavior, we present a model based upon the intermolecular interactions of R6G and the interaction between R6G and graphene. According to the molecular structure of R6G, each molecule bears a positive charge that is delocalized through the conjugated structure and favorably resides on the imine groups and is associated with a negatively charged chloride ion. Once two molecules form a dimer through π-π stacking between the xanthene backbones, the positive charges of the dimer are distributed evenly to opposite ends of the dimer (Figure 4A). The distribution of charge on dimers and their interaction with chloride anions make it electrostatically favorable for them to form 1D chains. When immobilized on graphene, these chain structures may pack parallel to each other as a consequence of van der Waals interactions to form a densely packed 2D network. The network is held together strongly to the graphene surface by the π-π stacking arising from the extended π-electron system of the R6G dimers. Taken together, this physical picture explains why these dimers, once patterned and packed, do not spread on the graphene substrate (confirmed by AFM imaging three days after patterning). However, the R6G monomer, on average only bears half of a positive charge on each end (Figure 4B), such that even if two monomers were electrostatically bonded to form a pair, the electrostatic interaction between the 6


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monomers and chloride ions will be much weaker; therefore they are less likely to propagate to form strongly bound 1D chains, as in the dimer case. This lack of strong intermolecular attraction may lead to monomers being more mobile on the graphene substrate, as evidenced by their branched structure and subsequent spreading after patterning (Figure S4, Supporting Information). Having shown that R6G can be patterned on a graphene substrate, we now aimed to explore the interactions between these patterned molecules and the graphene substrate. Of particular interest were the electrostatic interactions as other molecules have been shown to electrostatically dope graphene, for example by changing the chemical potential relative to the Dirac point10. Since R6G has both electron-donating and withdrawing groups, it is not obvious how it will electronically affect graphene. In order to probe the doping capability of R6G, we performed back-gate field-effect measurements on two terminal graphene flakes before and after bulk functionalization with R6G (Figure 5A). The shift of the Dirac point to more negative gate voltages indicates that R6G has an n-doping effect on graphene. The shift in Dirac point is consistent in magnitude with the shift observed with other molecular monolayers10,38. In addition, analysis of the peak positions and relative intensities in Raman spectra of pristine and R6Gcoated graphene corroborates the role of R6G as an n-dopant for graphene (Figure S2D)10,39. To explore whether R6G can locally n- dope graphene, we performed Kelvin probe force microscopy (KPFM), an AFM-based technique that is used to measure the contact potential difference (CPD) between a surface and a conducting AFM probe (detailed description of KPFM can be found in Supporting Information), on the R6G patterned graphene substrates. KPFM is widely used in many areas of surface science because of its ability to non-destructively

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characterize the electrostatic properties of materials under ambient conditions40 and has been used to study the uniform chemical doping of graphene41. To probe the electrical effects of the patterned R6G on graphene, we performed KPFM of a graphene surface with lines of R6G dimers written by DPN. In this experiment, all lines were written at a speed of 0.1 µm/s. Line cuts from the topographical image show an average height of 1 nm and width of 225 nm (Figure 5B). In order to characterize the electrical properties of the R6G locally-modified graphene, we performed KPFM on the same region (Figure 5C). The measured CPD on R6G lines decreases 22 mV compared to that of the unmodified graphene area. The negative shift in CPD is characteristic of n-doping, similar to what has been reported when graphene is functionalized with electron-donating molecules42,43. In contrast, random debris (circled in Figure 5B) that was left on graphene did not affect the local CPD (Figure 5C), indicating that the π-π interaction between R6G and graphene is important for the shift in CPD. The CPD decrease associated with patterned R6G on graphene is further shown to be dependent on the interaction between R6G and graphene by the observation that R6G increases the local CPD when patterned SiO2 due to its charge (Figure S7). Additionally, a quantitatively consistent negative 20-25 mV CPD shift (additional KPFM data shown in Figure S8, Supporting Information) was observed in all high quality dimer patterns. These data show that the R6G is affecting the local electronic environment of graphene, suggesting a pathway for tailoring the electrical properties of graphene on the nanoscale. In summary, we have developed an understanding of the transport, diffusion, and assembly behavior of R6G on graphene, and shown that the patterned R6G can locally alter the properties of graphene. This important proof-of-concept experiment suggests that it will be possible to find a series of molecules that, when combined with DPN, can be used to n- and p- dope graphene 8


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locally and quantitatively. Additionally, by using parallel patterning tools such as PPL or HSL16,17,27, it should be possible to rapidly chemically modify graphene substrates to create devices on a large scale. An important lesson from this work is highlighted in the large disparity between the transport and assembly behavior of R6G monomers and dimers. This observation underscores how important molecular and aggregate structure can be in any type of molecular printing experiment, a variable that must be considered in all patterning and local doping experiments.

Figure 1. (A) Schematic illustration depicting the patterning of Rhodamine 6G (R6G) on graphene by dip-pen nanolithography (DPN). The inset shows the chemical structure of R6G. (B) Atomic force microscopy (AFM) image of 45 ± 5 nm R6G lines patterned on graphene. (C) Raman mapping at 1650 cm-1 confirms the chemical identity of the lines. This mapping corresponds to the same region as (B).

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Figure 2. (A) AFM topographic image of R6G lines patterned on graphene, using writing speeds of 0.02, 0.05, 0.1, and 0.2 µm/s, from left to right. The line widths for four lines are 233 ± 18 nm. In this case, the line width is determined by the actual size of the tip due to R6G coating or the size of the water meniscus. (B) AFM topographic image of R6G lines patterned on graphene using writing speeds of 0.1, 0.05, and 0.02 µm/s and corresponding line widths of 90 ± 10, 130 ± 18 and 180 ± 19 nm, from left to right. In this case, the tip had been partially de-inked and serves as a point source resulting in features that incompletely fill the water meniscus and depend on the writing speed. The height of the lines in (A) and (B) was 0.9 ± 0.1 nm, corresponding to the height of R6G dimers adsorbed on graphene.

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Figure 3. (A) AFM topographic image of R6G lines patterned on graphene. This image contains four repetitions of a set of three lines (the dashed box encompassing one such set) written using writing speeds of 0.1, 0.05, and 0.02 µm/s, from left to right. The height is ~0.4 nm, corresponding to the height of R6G monomers. AFM topographic image (B) and phase image (C) of one line array, zoomed in from (A).

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Figure 4. (A) Schematic of strongly connected dimers held together by strong ionic interactions along the 1D chain and van der Waals forces between neighboring chains to form 2D networks. (B) Schematic of monomers that are weakly bound because of the weak ionic interactions. Therefore, the monomers are not likely to propagate to form 1D chains, as in the dimer case.

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Figure 5. (A) Ids-Vg curves of the same graphene device (shown in the inset; the gold contact was prepared by evaporation of gold through a shadow mask) before (Red) and after (Blue) bulk functionalization with R6G. The bias voltage applied is 1 mV for both measurements. The Dirac point moved from ~ 32 to ~ -6 V. AFM topographic image (B) and surface potential image (C) of lines of R6G dimers written on graphene. Average line cuts below (B) and (C) show the average height profile and the average surface potential profile for lines of R6G dimers written on graphene.

ASSOCIATED CONTENT Supporting Information. Experimental details for graphene preparation, DPN patterning, electrical measurements, and Kelvin probe force microscopy. Additional Raman mapping and

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Raman spectra, writing speed-line width dependence curve, AFM images, KPFM, and optical micrographs of graphene.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Author Contributions ‡

These authors contributed equally.

ACKNOWLEDGMENT. This material is based upon work supported by DoD/NSSEFF/NPS Awards N00244-09-1-0012 and N00244-09-1-0071, AOARD Award FA2386-10-1-4065, Department of the Navy ONR Award N00014-11-1-0729, AFOSR Award FA9550-09-1-0294, and the Non-equilibrium Energy Research Center (NERC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Award DE-SC0000989.

KAB gratefully acknowledges support from the Northwestern

University’s International Institute for Nanotechnology. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the sponsors.

REFERENCES AND NOTES 14


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ToC

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