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Assembly of Suspended Graphene on Carbon Nanotube Scaffolds with Improved Functionalities

Sharon Xiaodai Lim1,2, Gavin Kok Wai Koon2,3, Da Zhan5, Zexiang Shen5, Barbaros Özyilmaz1,2,3,4,

Chornghaur Sow1,2 () 1 NUS Graduate School for Integrative Sciences and Engineering (NGS), Centre for Life Sciences (CeLS), #05-01, 28 Medical Drive,

Singapore 117456 2 Department of Physics, Blk S12, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542 3 NanoCore, 4 Engineering Drive 3, National University of Singapore, Singapore 117576 4 Graphene Research Centre, National University of Singapore, Singapore 117542 5 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore

637371 Received: 15 July 2012 / Revised: 28 August 2012 / Accepted: 24 September 2012 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2012

ABSTRACT With self-assembly being an efficient and often preferred process to build micro- and nano-materials into ordered macroscopic structures, we report a simple method to assemble monolayer graphene onto densified vertically aligned carbon nanotube (CNT) micropillars en route to unique functional three-dimensional microarchitecture. This hybrid structure provides new means of studying strain induced in suspended graphene. The strain induced could be controlled by the size and number of supporting microstructures, as well as laser-initiated localised relaxation of the graphene sheet. The assembled structure is also able to withstand high-energy electron irradiation with negligible effect on the electrical properties of the hybrid system. The hybrid system was further functionalised with quantum dots on the CNTs with the assembled top graphene layer as a transparent electrode. Significant improvements in photocurrent were achieved in this system.

KEYWORDS Carbon nanotubes, graphene, electron beam irradiation, strain, photocurrent

1. Introduction

As the representatives of one- and two-dimensional graphitic nanomaterials, carbon nanotubes [1] (CNTs), and graphene [2] are mutually complementary in many attributes. In addition, they also share some common properties ranging from high mechanical strength to high electrical conductivity [3]. To fully extract the merits of these materials, many researchers have

attempted to combine these two materials together [3–6], resulting in the formation of all carbon-based hybrid materials. Dong et al. [7] and Lee et al. [8] reported the fabrication of such hybrids through chemical vapor deposition (CVD) or plasma-enhanced CVD growth on silicon nanoparticle-decorated copper foil or graphene oxide. Attributes of these hybrids include the fabrication of aligned CNTs synthesised by Dong et al. [7] and control over inter-nanotube

Nano Res. 2010, 3(9): 676–684 ISSN 1998-0124DOI 10.1007/s12274-012-0262-x CN 11-5974/O4Research Article

Address correspondence to physowch@nus.edu.sg

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spacing between the CNTs produced by Lee et al. [8] Given the demand for compact sized microelectronics with better performance, one way to meet such demand would be the creation of controlled three-dimensional (3D) interconnects on the micro/nanoscale. CNTs have long been targeted as a potential candidate for such applications. Moreover, many researchers have shown that superior conductance can be achieved through exploiting liquid-induced densification of vertically aligned CNTs [9–12]. With the addition of a monolayer of graphene [13]—an excellent transparent electrical conductor—that bridges across these vertical CNTs, one could create hybrid system with potentially interesting applications.

Production of suspended monolayer graphene over metallic scaffolds [14] or SiO2/Si trenches [15] pro- vided a good reference system to probe the intrinsic or induced properties of the graphene sheet. Interest in studying such locally induced strain effect on graphene lies in the potential of using such a strain engineering approach for the development of graphene- based electronic devices [16]. Through mechanical or thermal deformation of the substrates or through geometrical patterning of the substrates, the strained material can become a topological insulator. This approach resulted in the opening of significant energy gaps in graphene’s electronic structure [17, 18]. Induced magnetic fields and modified local potential of gra- phene as a result of controlled manipulation of intrinsic ripples in graphene has also been reported [19].

Herein, we present a simple approach to create suspended graphene [13] across highly packed vertically aligned and patterned array of CNT microstructures by exploiting the simplicity and efficiency of liquid induced self-assembly process. Placing graphene on top of CNT microstructures allows us to probe unique strained pattern, which is dictated by the pre-designed supports. With the flexibility in patterning or removal of the supporting CNT array with a focused laser beam, this method offers a new way to study the effect of strain induced on the suspended graphene sheet by the supporting microstructures.

Beyond the studies of strain in graphene, the introduction of quantum dots (QDs) brings with it another perspective of using this hybrid material for applications in photocurrent studies. QDs were chosen

for their ability to generate multiple charge carriers with a single photon [20, 21]. With the CNT array fashioned into periodic pillared microstructures, their light-trapping ability [22] was found to be effective in extending the dwell time of the incident photons in the material. This enhances the photocurrent generated due to the multiple adsorption opportunities. Through capillary induced assembly [10], QDs can be positioned along the walls of the vertically aligned CNT micropillars. Combining the merits of these materials, this hybrid structure is expected to produce significant improvements in the generation of photocurrents. After adding graphene, with its high degree of transparency [13, 23, 24] as the top electrode, this form of hybrid material holds promise as an opto- electronic device.

2. Results and discussion

2.1 Suspended graphene across densified carbon nanotube scaffolds

To assemble graphene onto the CNT scaffolds, the steps illustrated in Fig. 1(a) were carried out. A focused laser beam [25] was used to create micropillars, microplatforms as well as microwalls made out of an aligned array of CNTs on a Si substrate. Subsequently, a patterned CNT sample was attached onto a small piece of glass slide and used to scoop up the graphene that was floating on the water surface (Fig. 1(a)). Both graphene and CNTs display hydrophobic surface properties. To prevent the layer of graphene from sliding off the surface of the CNT scaffolds, it is crucial to allow a small corner of the graphene to be attached onto the glass slide during this transfer process.

The sample with the attached graphene was re- moved from the glass slide and left to dry in ambient conditions before being placed into an empty beaker. A droplet of acetone (~3.0 L) was then introduced onto the edge of the sample. The purpose of the acetone was to remove the poly (methyl methacrylate) (PMMA). As acetone spread across the CNTs, the resultant capillary force allowed the graphene to be attached firmly onto the CNT surface. To ensure the complete removal of the PMMA coating on the graphene, more acetone was added until the sample

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was submerged in the solvent. After 30 min, a syringe was used to extract the remaining acetone from the beaker and the graphene–CNT (GCNT) sample was left to dry in ambient conditions.

During the final drying phase, evaporation of acetone from the CNT sample resulted in the densification of the CNT micropillars while allowing the graphene to be suspended neatly over the CNT micropillars at the same time. It has been reported that densified CNTs exhibit much better electrical conductivity than un-densified CNTs [9–12]. Hence, using this simple yet effective way for the creation of suspended graphene across densified CNT scaffolds, one can fabricate

unique 3D hybrid architecture comprising graphene and CNTs. As there are no additional steps required for the densification of CNTs, the resultant 3D architecture is expected to exhibit enhanced electrical properties.

In an effort to study the effect of densification on the CNT scaffolds, the assembly process was carried out in the absence of the graphene layer. Un-densified micropillars with dimensions of 25 18 m2 (L W) and inter-pillar spacing of 30 m were laser-pruned from an array of CNTs (with an average height of 40 m). Figure 1(b) shows an SEM image of one such micropillar. From micropillars with straight edges,

Figure 1 (a) Schematic illustration of the assembly process to suspend graphene across the CNT micropillars. Scanning electron microscope (SEM) image of (b) un-densified and (c) densified micropillars without the presence of graphene. Inserts of (b) and (c) show the inter-nanotube distance for the respective type of micropillars. (d) Graphene suspended across micropillars with initial dimensions of 23 23 m2. (e) A higher magnification view of the intersection between the graphene sheet and a micropillar. (f) Graphene across microplatforms with initial dimension of 46 46 m2. (g) Graphene sheet molded over one such microplatform (the insert is a microplatformwithout any graphene sheet). Images (b)–(g) were taken at 40o tilt. (h) Graphene across CNT microwalls. (i) Side profile of the assembly in (h)

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the densified micropillars were transformed into one shaped like an hour-glass. The resultant shape is believed to play a critical role in the successful assembly of the suspended graphene across these micropillars, since it provides a wide base for the suspended graphene, allowing it to have a firm hold onto the supporting micropillars.

To illustrate the versatility of such an assembly method, monolayer graphene sheet was suspended across different types of CNT scaffolds (each displaying a cross section in the general shape of an hour-glass). In Fig. 1(d), monolayer graphene is observed to bridge across densified CNT micropillars with initial dimensions of 23 23 µm2. A close up of the inter- section between the monolayer and the CNT micropillar can be observed in Fig. 1(e). The appearance of “tight-fitting” feature in the figure suggests strong and effective contact between the graphene and the top of the CNT scaffold. Increasing the dimensions of the CNT micropillars to 46 46 m2 gave rise to the microplatform. The graphene successfully bridged across the platforms and molded itself over the CNT microplatforms as shown in Figs. 1(f) and 1(g). Insert of Fig. 1(g) shows an image of the densified micro- platform without any graphene. In addition to having pillared microstructures, monolayer graphene sheet can also be bridged across CNT microwalls (Fig. 1(h)) with a gap of 30 m. A side view of the assembly shows a more prominent view of the suspension (Fig. 1(i)).

2.2 Raman analyses of graphene–CNT hybrid systems

The densification process gave rise to distinct eventual shapes from the micropillar, microplatform or microwalls. For example, the surfaces of the micropillars appear to be convex while those of the microplatforms appear to be concave. These structural differences induce different degrees of strain at different regions on the suspended graphene. Moreover, we can use a focused laser beam to selectively trim the suspended graphene sheet to further study the strain effect induced by different numbers of supporting pillars/platforms/walls on the suspended graphene sheet.

Raman spectroscopy is a key diagnostic tool for the characterisation of graphene [26] and the investigation

of the effects of strain [16, 27–29]. The Raman spectrum of graphenes comprises three main active bands. The G mode (at ~1580 cm–1), the D mode (~1350 cm–1) and the 2D (or G’) overtone mode (~2700 cm–1). Figure 2(a) shows a comparison of the Raman spectra (collected in ambient conditions) of the densified CNT pillar, GSiO2 (graphene on SiO2/Si substrate), and GCNT sample. The cross in Fig. 2(b) indicates the location from which the Raman spectrum was obtained in the case of GCNT sample. Given that the laser spot size is ~1 m, the Raman spectrum obtained from that location will not be affected by any signals from the nearby densified CNT pillars.

The Raman spectrum of GSiO2 shows a 2D peak at 2684 cm–1 with a full width at half maximum (FWHM) of ~30 cm–1. This suggests graphene exists as a mono- layer. At 1586 cm–1, a sharp peak with a FWHM of ~20 cm–1 was visible. A small D peak can be seen at 1343 cm–1 with FWHM of 18 cm–1. In the case of the Raman spectrum of CNT, broad 2D and G peaks were identified at 2701 cm–1 with FWHM of ~181 cm–1

and at 1595 cm–1 with FWHM of ~82 cm–1. The presence of the D peak for CNT is detected at 1350 cm–1 with FWHM of ~65 cm–1. While the general features are the same, a few differences can be distinguished between GSiO2 and GCNT. Firstly, a ~2.6 times enhancement in the relative intensity of the 2D peak is observed. Secondly, a redshift (by 3 cm–1) of the G peak of the GCNT sample was detected. These observations are consistent with the decrease of self-doping upon removal of the substrate [26]. In addition, the redshift could be induced by presence of anisotropic strain and residual uniaxial tensile stress that exist between the micropillars (Fig. 2(b)) [15]. At the same time, the G peak of the GCNT sample experienced a peak broadening effect. A multipeak Lorentzian fit shown in the insert indicated that this G peak could be decomposed into two peaks, at 1584 cm–1 and 1619 cm–1 (insert of Fig. 2(a)). The peak at 1619 cm–1 is associated with the D’ peak, which together with the D peak, is a defect-induced Raman feature [30, 31]. Lastly, a larger D peak was detected from GCNT sample as compared to the GSiO2 sample. While the presence of disorder within the graphene sample is a contributing factor to this observation, more detailed investigations are required to pinpoint the precise nature of the

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disorder [26]. In an attempt to understand strain distribution across

both suspended and supported regions of graphene, spatial maps of G (Fig. 2(e)) and 2D (Fig. 2(f)) peaks were obtained from the sample shown in Fig. 2(b). Due to overlapping of G peaks as mentioned above, two Lorentzian curves were used to fit the spectra and the frequency of the G peak was found to be 1584 cm–1. Although the frequency shift of the Raman band is related to the uniaxial strain and the shear strain, the much smaller contribution from the shear strain allows it to be neglected for simplicity [32]. In Figs. 2(e) and 2(f), the pronounced redshift of both G and 2D peaks coming from the four corners of the graphene supported by the CNT pillars, indicates that strain is induced on the graphene which is in direct contact with the CNT. In addition to the reasons mentioned above, micro/nano features on the supporting CNT

pillars could also contribute to the observed strain. Given the flexibility of monolayer graphene, it could easily mold itself to the features present on the CNT pillar. This bending of graphene plays a significant role in redshifting both the G and 2D peaks. Figure 2(g) shows a high magnification scanning electron microscope (SEM) image of graphene molded to the features present on one of the CNT pillars (indicated by the white arrow in Fig. 2(f).

While an insignificant shift in G peak is observed from the suspended regions of the graphene sheet, clear signs of redshift of the 2D peak from the same region can be seen from Fig. 2(f). This difference can be attributed to the large Raman shift in the 2D peak, which allows even smaller stress/strain variations to be easily detected [16]. Interestingly, along the path traced out by the black lines in Fig. 2(e), a blueshift of the G peak, indicating the presence of compressive

Figure 2 (a) Raman spectra of GSiO2, densified CNT micropillars, and GCNT. Insert is a multipeak Lorentzian fit of the G peak from the GCNT sample. (b) SEM image of a typical GCNT sample used in other characterisations. The black cross indicates the location from which the GCNT spectrum in (a) was obtained. (c) and (d) Higher magnification SEM images from the area enclosed in box A and B of (b), taken at 60 tilt. Spatial Raman map of the (e) G and (f) 2D mode from (b). (g) SEM image from one GCNT pillar as indicated by the arrow in (f), taken at 60 tilt

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strain, is observed. A SEM image of the region enclosed in box A of Figs. 2(b), 2(e), and 2(f) is shown in Fig. 2(c). Taken at a tilted angle of 60, a line of compression cutting across the ripples on the graphene can clearly be seen. Ripples formed on suspended graphene are suggested to be a way to relieve compressive strain [29]. Such rippling effect can be seen from the SEM image (Fig. 2(d)) taken from the region enclosed in box B (Fig. 2(b)). The corresponding spatial map of the G and 2D peaks from the same region shows small redshift of both peaks. This is in good agreement with the above discussion.

Similar spatial Raman mapping was also conducted on different GCNT samples, in an effort to study strain effects induced by the size and different number of supporting CNT microstructures on the suspended graphene sheet. The scale of all the Raman maps in Figs. 3(e)–3(h) ranges from 2670 to 2710 cm–1. Phonon softening induced by tensile strain will result in a redshift of the 2D peak while compression-induced phonon hardening will result in a blueshift of the 2D peak [16]. A 2D Raman map of the suspended graphene across different types and numbers of supporting CNT microstructures indicates the locations and type of strain induced on the suspended graphene sheet.

Figures 3(a), 3(b), and 3(e)–3(g) show SEM images of graphene on SiO2 substrates, as well as across different CNT scaffolds. The corresponding Raman maps for the 2D peak are presented in Figs. 3(c), 3(d), and 3(h)–3(j). The Raman map of the 2D peak of graphene on SiO2 (Figs. 3(a) and 3(c)) shows substrate- induced uniform redshifts in the 2D peaks detected across the graphene sample. On the other hand, from the Raman maps obtained, variations in the size of the supporting microstructures induced different degrees of strain on the suspended graphene. The larger microplatforms (Figs. 3(b) and 3(d)) caused relatively weaker strain on the graphene compared to those supported by the smaller micropillars (Figs. 3(e) and 3(h)). From the same sample, another set of graphene on four micropillars was created (Fig. 3(f)). For this set, a 10 mW 532 nm focused laser beam was used to create a hole at the center of the graphene sheet (insert of Fig. 3(f)). This was carried out with the intention of relaxing the suspended graphene at the chosen site. Assuming that the same type of supporting structures

should induce similar types of strain on the suspended graphene, the resultant Raman map shows com- pression (blueshift of the 2D peak, Fig. 3(i)) of the suspended graphene sheet around the laser spot. After removing one supporting micropillar by a focused laser beam (Fig. 3(g)), the location of the strain was redistributed across the suspended graphene (Fig. 3(j)).

While many studies of the strain of suspended graphene over etched Si/SiO2 trenches have been reported [33–35], we believe that this hybrid material offers unique handles for the study of induced strain on suspended graphene. In a single system, we can study induced strain on the graphene as a result of

Figure 3 (a) SEM image of graphene on SiO2 substrate. SEM image (60 tilt) of graphene supported by (b) four microplatforms; (e) four micropillars; (f) another set of four micropillars with a hole (marked by the black cross in (i)) created by focused laser beam at the center of the suspended graphene (the insert is the SEM image of the hole); and (g) the same set of sample shown in (f) but with one densified CNT micropillar removed via laser pruning. (c), (d), (h)–(j) Raman maps of the 2D peak for the respective samples shown in (a), (b), (e)–(g)

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both micro- and nano-features on the supporting CNT microstructures. Moreover, we can easily vary the sizes/number and locations of the supporting structures to study its effect on the strain on the suspended graphene.

2.3 Electron-beam irradiation of suspended graphene

In this work, we have also studied the effect of high energy or high dosage electron beam (e-beam) irradiation on the suspended graphene. High energy or high dosage of e-beam irradiation of carbon nano- structures, particularly graphene, has been known to generate various structural changes. The magnitude of these changes was found to be larger in the presence of a supporting substrate. Childres et al. [36] reported that at room temperature and in the absence of a supporting substrate, the shift in the charge-neutral

point (CNP) was substantially reduced. A study of the effect of electron bombardment on

both suspended and supported graphene was carried out. In this study, GCNT similar to that presented in Fig. 2(b) was used. For reference, a graphene sheet was transferred onto SiO2/Si wafers (GSiO2). Standard electron beam lithography on positive resist PMMA was used to pattern the graphene into 50 50 m2 squares. Unwanted areas of graphene were etched using a reactive ion etching (RIE) system. A second electron beam lithography step was used to write the areas for gold (Au) deposition, which served as electrodes for electrical measurements.

Raman spectra of GSiO2 with different dosages show an increase in the intensity of the D peak relative to the G peak as the dosage increases (Fig. 4(a)). This indicates that e-beam irradiation is damaging the graphene

Figure 4 Raman spectra of (a) GSiO2 and (b) suspended graphene, with different amount of e-beam dosage. (c) Ratios of the intensitiesof Id/Ig peaks with different dosage for GSiO2 and suspended graphene. (d) I–V measurements of GSiO2 and GCNT (insert) with differente-beam dosage

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lattice structure. On the other hand, increasing e-beam dosage did not result in significant changes in the relative intensities of the D and G peaks for the suspended samples (Fig. 4(b)). Taking ratio of the intensity of the D to G peak, Id/Ig (Fig. 4(c)), the values changed from ~0.1 to ~1.9 to ~5.3 as the dosage was increased from 0 to 1000 to 5000 C·cm–2 for GSiO2. For similar dosages, the ratio of the peak intensities remained at ~1.2 for the suspended graphene. This result is consistent with that reported by Childres et al. [36], who showed that the presence of substrates intensifies e-beam induced damage to graphene lattice structures.

A top-down, two probe technique was implemented for current–voltage (I–V) measurements of the GCNT hybrid (insert of Fig. 5(a)) while a side two probe measurement technique was used on the GSiO2 samples (insert of Fig. 5(b)). I–V measurements of the samples showed significant deteriorations in electrical conductivity with increase in e-beam dosage for GSiO2 (Fig. 4(d)). At an applied voltage of 1 V, an appro- ximately two-fold reduction in current was detected with an accumulated e-beam dosage of 1000 C·cm–2. Increasing the dosage to 5000 C·cm–2 yielded an approximately four-fold reduction in current. E-beam dosages appear to have little or no effect on the current measured for graphene suspended on CNT micropillars. As seen in the insert of Fig. 4(d), the current detected at 1 V applied voltage remained approximately constant at 0.08 mA. The I–V measure- ments were consistent with the results of the Raman analyses. Here, an increase in the Id/Ig ratio is inversely proportional to the reduction in current measured at 1 V applied voltage. This relation thus provides evidence for the scattering of carriers by the e-beam induced defects in the graphene lattice structures. This also further emphasises the potential of suspended graphene for the development of radiation-hard graphene-based electronics.

2.4 Photocurrent effect of graphene–CNT hybrid systems

This hybrid system can be further functionalised to increase its potential applications. There are four main motivations behind the creation of such a

system. Namely, (1) densified CNTs have already shown potential for application in interconnects [9–12]; (2) pillared CNTs have been shown to exhibit superior light trapping ability in comparison to the unpatterned counterpart [22]; (3) by decorating the CNT pillars with controlled assembly of QDs, the quantum efficiency of such a system is expected to be enhanced [20, 21]; (4) with graphene, given its high degree of transparency, serving as an electrode [13, 23, 24], we seek to combine the unique characteristics of the different components and create a hybrid material that shows potential in opto-electronic applications. For the analysis of this functionalised material, similar top-down, two probe techniques to those mentioned above were implemented for all I–V measurements. All electrical measurements involving CNT or GCNT or GCNT–QD hybrid micropillars were obtained from the same sample. This eliminates any discrepancies in the I–V measurements as a result of differences in CNT properties. All I–V data presented are an average obtained from 3–5 different measurements.

To determine the efficiency of using graphene as a transparent electrode to bridge across different CNT micropillars, a simple I–V measurement was conducted. At an applied voltage of 1 V, a current of 1 A was detected from a single as-grown CNT pillar. Densifying the CNT micropillar resulted in enhancement of current by an order of magnitude. Linking four densified CNT micropillars with the graphene sheet, resulted in an additional approximately eight-fold improvement in the current detected. Considering that the hybrid system comprises parallel circuits and assuming that the graphene bridging the four micropillars is a good conductor, with an applied voltage of 1 V, a current of 0.05 mA is expected. However, from Fig. 5(a), the measured value was 0.09 mA. One possible explanation would be the smaller contact resistance existing between tungsten and graphene as compared to that between tungsten and CNTs. Comparing the current measured across the supported and suspended graphene of similar dimensions (Fig. 5(b)), an appro- ximately two-fold improvement in the current was detected for the suspended graphene. As such, it is evident that a better electrical conductor has been created from the GCNT hybrid system.

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Furthermore, by first densifying the CNT micro- pillars using a solution containing suspended QDs, we created a simple structure comprising of densified CNT micropillars with QDs that are arranged along the nanotube length (as a result of capillary induced self-assembly). With the graphene subsequently assembled on top, each of these pillars is connected via a transparent conductor. Figure 5(c) shows a typical SEM image obtained at the junction of the graphene sheet and a densified CNT micropillar. From the region indicated by the arrow in the image, the graphene sheet appears to be supported by an array of randomly oriented, bent CNT strands. Given that this assembly process is carried out in ambient conditions at room

temperature, it is believed that only physical contacts exist between the graphene and the CNT strands. However, there are many CNTs on top of the CNT array and many of them are in physical contact with the graphene sheet on top (the region enclosed within the dotted box in (Fig. 5(d)). The multiple contacts facilitate the transport of charge carriers and hence the assembled hybrid GCNTs was found to exhibit improved electrical properties. The transparency of the graphene sheet on top of the CNT micropillars is further emphasised in the SEM image shown in Fig. 5(d). In that SEM image, the CNT strands can be clearly seen through the graphene sheet.

Figure 6(a) shows an SEM image of such a structure.

Figure 5 (a) Top-down, two probe I–V measurements [1] of N doped silicon on which the CNT scaffolds were grown; [2] of 1 as-grownCNT micro-pillar; [3] of 1 densified CNT micro-pillar and [4] of graphene bridging across 4 densified CNT micro-pillars. The insert shows a schematic view of the top-down measurement setup. (b) I–V measurements of supported GSiO2 and suspended graphene across four densified CNT micropillars. The insert shows a schematic view of the setup. High magnification SEM images (40º tilt) taken (c) at the edge of the CNT pillar with a portion of the graphene suspended beyond the pillar; and (d) at the center of the pillar. The arrow in (c)shows graphene resting on the bent portion of the CNT bundle. This effect is also shown in the region enclosed within the dotted box in (d)

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The presence of the QDs on this exact same structure is shown in the fluorescence microscope image (FM) (Fig. 6(b)), with the corresponding photoluminescence spectrum placed as an insert. The region outlined by the dotted line shows the location of the suspended graphene. From the FM, luminescence from the regions of the CNT–QD micropillars covered by the graphene sheet appears to be quenched. This hints at the poten- tial for using this hybrid material for photocurrent applications.

By applying a 1 V bias voltage and introducing a 405 nm broad beam laser with a laser power of 18 mW at 5 s intervals, I–t graphs were collected as shown in Fig. 6(c). The insert of Fig. 6(c) shows a schematic view of the experimental setup. Measured currents obtained from the samples were normalised. In the absence of QDs, a single densified CNT micropillar showed negligible response to the introduction of the excitation source (405 nm laser). With the introduction of the QDs, one CNT-QD micropillar showed a one order improvement in the current measured upon excitation. Using graphene to bridge across the CNT– QD micropillars, an additional three-fold improvement

in the measured current was detected upon excitation.

3. Conclusion

A simple liquid driven self-assembly process has been developed to create suspended graphene across highly packed vertically aligned CNT micropillars/ microplatforms and microwalls. Raman mapping of the 2D peak from the suspended sample showed distinct differences in strain distributions across the suspended graphene. Using a localised focused laser beam, site-specific relaxation could be induced on the suspended graphene. Control of induced strain could also be achieved via selective removal of CNT supports, thereby allowing the induced strain to be redistributed across the suspended graphene. High energy e-beam irradiation of the suspended graphene was found to have a negligible effect on the electrical properties of the graphene sheet. This further emphasised the potential of implementing suspended graphene for the development of radiation-hard graphene-based electronics. Lastly, the hybrid system was functionalised with quantum dots and the resultant system exhibited

Figure 6 (a) SEM image of graphene bridging across six CNT– QD pillars. (b) FM image showing orange color emission from the GCNT–QD hybrid system. The insert is the PL spectrum obtained from one of the micropillars. (c) Photocurrent studies: I–t graphs obtained from one densified CNT-pillar, from one CNT–QD pillar and from the GCNT–QD hybrid system. The insert is a schematic view of the experimental setup. A 405 nm broad beam laser introduced at 5 s intervals served as an excitation source for the system

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three orders of magnitude improvement in the current measured upon irradiation by an external laser source. This result thus illustrates the potential of this hybrid material for future applications in photocurrent devices.

4. Experimental

4.1 MWNTs and laser pruning

Aligned multi-walled CNTs (MWNTs) with typical height of 30–40 m were grown on clean n-type silicon (2.5 × 2.5 mm2, (100) Si) substrates containing a native oxide layer. Before growth, a layer of iron film (~17 nm) was coated on the Si substrates as catalyst using a magnetron sputtering system (Model: RF Magnetron Denton Discovery 18). These MWNTs were synthesised using a plasma enhanced chemical vapor deposition (PECVD) system and details of the growth process have been reported elsewhere [37, 38]. A laser pruning technique [25] involving a focused laser beam of wavelength 660 nm with 3 mm initial beam diameter and a computer controlled sample stage was used to create platforms and patterns made up of arrays of MWNTs. By focusing the laser beam using an optical lens, the beam size can be reduced. Using 50 mW focused laser beam, MWNT micropillars and microwalls were controllably pruned from the MWNT array.

4.2 Preparation of the graphene samples for the transfer process

Monolayer CVD graphene synthesised on both sides of copper foils was purchased from the Graphene Supermart. First of all, a protection layer of PMMA resist (~350 nm) was spin-coated onto one side of the copper foils. Next, a RIE system (20 standard cubic centimeters per minute (sccm) of O2 and 20 sccm of Ar at 50 W for 2 min followed by 20 sccm of O2 at 50 W for another 2 min) was used to etch away the unprotected graphene, located on the opposite side of the copper foils. This exposes the copper foils for the next etching process. The exposed copper layer was etched away using a solution of ammonium persulphate (7 g/L) and rinsed in deionised water; finally the resulting structure (PMMA and graphene)

was transferred onto the desired substrate (SiO2/Si wafers or CNT) and acetone was used to remove the PMMA layer leaving behind a layer of graphene.

4.3 Quantum dots

Cadmium selenide (CdSe) core–shell quantum dots coated with zinc sulphide (ZnS) (CdSe/ZnS QDs) of various sizes and hence emission colors were pur- chased from Evident Technologies. The size of QDs used was 4.0 nm with a concentration of 20.7 nmol/mL with an emission peak at 600.0 nm (i.e., an orange color). Their outer shells were covered with a C16 hydrocarbon ligand. The size of the QDs corresponded to the diameter of the CdSe core and the ZnS shell was a few monolayers in thickness. These commercially purchased QDs were suspended in toluene, which was found to spread rapidly across the MWNTs.

4.4 Electron beam irradiation

Dosage experiments were conducted in an SEM machine under high vacuum (10–6 Torr). The selected area of the graphene was exposed to the e-beam with energy of 30 keV. The beam current (Ie) used ranged from 6–2500 pA and the samples were exposed for different durations, depending on the dosage required. Accumulated dosages on the samples were calculated from the product of Ie with accumulated exposure time, Te. For both sets of samples, dosages of 0, 1000, and 5000 C·cm–2 were analysed.

4.5 Further characterisation

Further characterisation of the samples was conducted using a JEOL JSM-6701F field emission SEM and a “Cascade Microtech” optical microscope.

References

[1] Iijima, S. Helical microtubules of graphitic carbon. Nature 1991, 354, 56–58.

[2] 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.

[3] Li, C. Y.; Li, Z.; Zhu, H. W.; Wang, K. L.; Wei, J. Q.; Li, X.

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12

A.; Sun, P. Z.; Zhang, H.; Wu, D. H. Graphene

nano-"patches" on a carbon nanotube network for highly

transparent/conductive thin film applications. J. Phys. Chem. C. 2010, 114, 14008–14012.

[4] Lee, S. H.; Lee, D. H.; Lee, W. J.; Kim, S. O. Tailored

assembly of carbon nanotubes and graphene. Adv. Funct. Mater. 2011, 21, 1338–1354.

[5] Patra, N.; Song, Y. B.; Kral, P. Self-assembly of graphene

nanostructures on nanotubes. ACS Nano 2011, 5, 1798–1804.

[6] Hong, T. K.; Lee, D. W.; Choi, H. J.; Shin, H. S.; Kim, B. S.

Transparent, flexible conducting hybrid multi layer thin films

of multiwalled carbon nanotubes with graphene nanosheets.

ACS Nano 2010, 4, 3861–3868.

[7] Dong, X. C.; Li, B.; Wei, A.; Cao, X. H.; Chan-Park, M. B.;

Zhang, H.; Li, L. J.; Huang, W.; Chen, P. One-step growth

of graphene-carbon nanotube hybrid materials by chemical

vapor deposition. Carbon 2011, 49, 2944–2949.

[8] Lee, D. H.; Kim, J. E.; Han, T. H.; Hwang, J. W.; Jeon, S.;

Choi, S. Y.; Hong, S. H.; Lee, W. J.; Ruoff, R. S.; Kim, S. O.

Versatile carbon hybrid films composed of vertical carbon

nanotubes grown on mechanically compliant graphene films.

Adv. Mater. 2010, 22, 1247–1252.

[9] Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu,

Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.;

Iijima, S. Shape-engineerable and highly densely packed

single-walled carbon nanotubes and their application as

super-capacitor electrodes. Nature Mater. 2006, 5, 987–994.

[10] Lim, X. D.; Foo, H. W. G.; Chia, G. H.; Sow, C. H.

Capillarity-assisted assembly of carbon nanotube

microstructures with organized initiations. ACS Nano 2010, 4, 1067–1075.

[11] Hayamizu, Y.; Yamada, T.; Mizuno, K.; Davis, R. C.; Futaba,

D. N.; Yumura, M.; Hata, K. Integrated three-dimensional

microelectromechanical devices from processable carbon

nanotube Wafers. Nature Nanotech. 2008, 3, 289–294.

[12] Kaur, S.; Sahoo, S.; Ajayan, P.; Kane, R. S. Capillarity-

driven assembly of carbon nanotubes on substrates into dense

vertically aligned arrays. Adv. Mater. 2007, 19, 2984–2987.

[13] Bae, S.; Kim, H.; Lee, Y.; Xu, X. F.; Park, J. S.; Zheng, Y.;

Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I. et. al.

Roll-to-roll production of 30-inch graphene films for

transparent electrodes. Nature Nanotech. 2010, 5, 574–578.

[14] Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K.

S.; Booth, T. J.; Roth, S. The structure of suspended graphene

sheets. Nature 2007, 446, 60–63.

[15] Berciaud, S.; Ryu, S.; Brus, L. E.; Heinz, T. F. Probing

the intrinsic properties of exfoliated graphene: Raman

spectroscopy of free-standing monolayers. Nano Lett. 2009,

9, 346–352.

[16] Frank, O.; Mohr, M.; Maultzsch, J.; Thomsen, C.; Riaz, I.;

Jalil, R.; Novoselov, K. S.; Tsoukleri, G.; Parthenios, J.;

Papagelis, K. et. al. Raman 2D-band splitting in graphene:

Theory and experiment. ACS Nano 2011, 5, 2231–2239.

[17] Pereira, V. M.; Neto, A. H. C. Strain engineering of graphene's

electronic structure. Phys. Rev. Lett. 2009, 103, 046801.

[18] Guinea, F.; Katsnelson, M. I.; Geim, A. K. Energy gaps and

a zero-field quantum hall effect in graphene by strain

engineering. Nature Phys. 2010, 6, 30–33.

[19] Bao, W. Z.; Miao, F.; Chen, Z.; Zhang, H.; Jang, W. Y.;

Dames, C.; Lau, C. N. Controlled ripple texturing of suspended

graphene and ultrathin graphite membranes. Nature Nanotech. 2009, 4, 562–566.

[20] Schaller, R. D.; Agranovich, V. M.; Klimov, V. I. High-

efficiency carrier multiplication through direct photogeneration

of multi-excitons via virtual single-exciton states. Nature Phys. 2005, 1, 189–194.

[21] Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P. R.;

Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Highly

efficient multiple exciton generation in colloidal PbSe and

PbS quantum dots. Nano Lett. 2005, 5, 865–871.

[22] Camacho, R. E.; Morgan, A. R.; Flores, M. C.; McLeod, T. A.;

Kumsomboone, V. S.; Mordecai, B. J.; Bhattacharjea, R.;

Tong, W.; Wagner, B. K.; Flicker, J. D. et. al. Carbon nanotube

arrays for photovoltaic applications. JOM 2007, 59, 39–42.

[23] Wang, X.; Zhi, L. J.; Mullen, K. Transparent, conductive

graphene electrodes for dye-sensitized solar cells. Nano Lett. 2008, 8, 323–327.

[24] Tung, V. C.; Chen, L. M.; Allen, M. J.; Wassei, J. K.; Nelson,

K.; Kaner, R. B.; Yang, Y. Low-temperature solution

processing of graphene-carbon nanotube hybrid materials

for high-performance transparent conductors. Nano Lett. 2009, 9, 1949–1955.

[25] Lim, K. Y.; Sow, C. H.; Lin, J. Y.; Cheong, F. C.; Shen, Z. X.;

Thong, J. T. L.; Chin, K. C.; Wee, A. T. S. Laser pruning of

carbon nanotubes as a route to static and movable structures.

Adv. Mater. 2003, 15, 300–303.

[26] Ferrari, A. C. Raman spectroscopy of graphene and graphite:

Disorder, electron-phonon coupling, doping and nonadiabatic

effects. Solid State Commun. 2007, 143, 47–57.

[27] Frank, O.; Tsoukleri, G.; Parthenios, J.; Papagelis, K.; Riaz, I.;

Jalil, R.; Novoselov, K. S.; Galiotis, C. Compression behavior

of single-layer graphenes. ACS Nano 2010, 4, 3131–3138.

[28] Tsoukleri, G.; Parthenios, J.; Papagelis, K.; Jalil, R.; Ferrari,

A. C.; Geim, A. K.; Novoselov, K. S.; Galiotis, C. Subjecting

a graphene monolayer to tension and compression. Small 2009, 5, 2397–2402.

[29] Chen, C. C.; Bao, W. Z.; Theiss, J.; Dames, C.; Lau, C. N.;

Cronin, S. B. Raman spectroscopy of ripple formation in

Nano Res

13

suspended graphene. Nano Lett. 2009, 9, 4172–4176.

[30] Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.;

Cancado, L. G.; Jorio, A.; Saito, R. Studying disorder in

graphite-based systems by Raman spectroscopy. Phys. Chem. Chem. Phys. 2007, 9, 1276–1291.

[31] Mohiuddin, T. M. G.; Lombardo, A.; Nair, R. R.; Bonetti,

A.; Savini, G.; Jalil, R.; Bonini, N.; Basko, D. M.; Galiotis,

C.; Marzari, N. et.al. Uniaxial strain in graphene by Raman

spectroscopy: G peak splitting, gruneisen parameters, and

sample orientation. Phys. Rev. B 2009, 79, 205433.

[32] Reich, S.; Jantoljak, H.; Thomsen, C. Shear strain in carbon

nanotubes under hydrostatic pressure. Phys. Rev. B 2000,

61, 13389–13392.

[33] Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank,

I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.;

McEuen, P. L. Electromechanical resonators from graphene

sheets. Science 2007, 315, 490–493.

[34] Huang, M. Y.; Pascal, T. A.; Kim, H.; Goddard, W. A.;

Greer, J. R. Electronic-mechanical coupling in graphene

from in situ nanoindentation experiments and multiscale

atomistic simulations. Nano Lett. 2011, 11, 1241–1246.

[35] Eichler, A.; Moser, J.; Chaste, J.; Zdrojek, M.; Wilson-Rae, I.;

Bachtold, A. Nonlinear damping in mechanical resonators

made from carbon nanotubes and graphene. Nature Nanotech. 2011, 6, 339–342.

[36] Childres, I.; Jauregui, L. A.; Foxe, M.; Tian, J. F.; Jalilian,

R.; Jovanovic, I.; Chen, Y. P. Effect of electron-beam

irradiation on graphene field effect devices. Appl. Phys. Lett. 2010, 97, 173109.

[37] Zhu, Y. W.; Cheong, F. C.; Yu, T.; Xu, X. J.; Lim, C. T.;

Thong, J. T. L.; Shen, Z. X.; Ong, C. K.; Liu, Y. J.; Wee, A.

T. S. et.al. Effects of CF4 plasma on the field emission

properties of aligned multi-wall carbon nanotube films.

Carbon 2005, 43, 395–400.

[38] Wang, Y. H.; Lin, J.; Huan, C. H. A.; Chen, G. S. Synthesis

of large area aligned carbon nanotube arrays from C2H2-H2

mixture by rf plasma-enhanced chemical vapor deposition.

Appl. Phys. Lett. 2001, 79, 680–682.