Photocatalytic Nanostructuring of Graphene Guided by Block

Letter www.acsami.org

Photocatalytic Nanostructuring of Graphene Guided by Block Copolymer Self-Assembly Zhongli Wang,†,‡ Tao Li,*,† Lars Schulte,†,‡ Kristoffer Almdal,†,‡ and Sokol Ndoni*,†,‡ †

Department of Micro- and Nanotechnology and ‡Center for Nanostructured Graphene, Technical University of Denmark, 2800 Konegs Lyngby, Denmark S Supporting Information *

ABSTRACT: Nanostructured graphene exhibits many intriguing properties. For example, precisely controlled graphene nanomeshes can be applied in electronic, photonic, or sensing devices. However, fabrication of nanopatterned graphene with periodic supperlattice remains a challenge. In this work, periodic graphene nanomesh was fabricated by photocatalysis of single-layer graphene suspended on top of TiO2-covered nanopillars, which were produced by combining block copolymer nanolithography with atomic layer deposition. Graphene nanoribbons were also prepared by the same method applied to a line-forming block copolymer template. This mask-free and nonchemical/ nonplasma route offers an exciting platform for nanopatterning of graphene and other UV-transparent materials for device engineering. KEYWORDS: suspended graphene, block copolymer nanolithography, TiO2-covered nanopillars, selective-area photocatalysis, graphene nanomesh

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photocatalysis without using any plasma or chemical agents. In our approach, we use a fully scalable block copolymer lithography process for the preparation of the nanopillar array,19,20 which allows for the fabrication of graphene nanomesh with significantly smaller pitch than state-of-theart.15 Graphene patterning can be controlled with great accuracy by using uniform and periodic nanotemplates fabricated by block copolymer lithography. In addition, the photocatalytic process can be easily adapted to large scale production, which is a very appealing feature of the proposed method. Figure 1 is a schematic of the fabrication process of 2D graphene nanomesh based on patterned graphene photocatalysis. First the periodically patterned substrate (silicon oxycarbide nanopillars) was fabricated employing a recently developed block copolymer nanolithography process:19,20 polystyrene-b-polydimethylsiloxanes (SD) was directly spincast on SiO2/Si substrate without any preliminary surface modification (like, for example, the ubiquitous surface grafting of a brush layer). Selective solvent vapor annealing (SVA) was then applied to generate well-ordered hexagonal cylinder pattern with perpendicular orientation and without film defects over large area. The following O2 dry etching enables simultaneous formation of the hard silicon oxycarbide nanopillars by oxidation of the PDMS block and removal of the PS block. A thin layer of TiO2 (∼4 nm) was conformably

ince 2004 graphene has attracted a wide range of interests because of its fascinating electronic, optical, mechanical, and thermal properties.1 Graphene provides a great opportunity especially for the development of high-performance electronic devices because of its extremely high carrier mobility, flexibility, transparency, and chemical stability.2 However, it is a semimetal with intrinsic zero bandgap, which prohibits to achieve high on/off current ratios required for many electronic components, e.g., field-effect transistors.3−5 Many efforts so far have focused on the creation of an energy bandgap by cutting graphene into nanomesh6 and nanoribbons.7 Nanopatterned graphene shows also a high potential for applications such as enhanced fluid sensor technology,8 one-atom-thick separation membranes,9 controlled absorption of infrared light,10 or as substrate for plasmonics.11 To date, several approaches have been reported for fabricating graphene superlattices.6,12 Most of these methods are based on the use of lithography masks in complicated and time-consuming procedures. It was recently shown that carbon bonds of graphene can be cut by oxidation.13 Oxidation and decomposition of carbonaceous materials, using metal oxide semiconductor photocatalysts, such as TiO2 and ZnO, are well-known.14−18 Patterned photodegradation of graphene by nanostructured photocatalysts is an emerging procedure to produce graphene nonomesh. TiO2based photocatalysis works like chemical scissors that can be employed to cut the stable sp2 carbon bonds of graphene.15 This method provides a clean and dry photocatalytic process enabling a versatile tailoring of graphene. Here we report a simple approach to pattern graphene suspended on top of nanopillar arrays via selective-area © XXXX American Chemical Society

Received: January 25, 2016 Accepted: March 21, 2016

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DOI: 10.1021/acsami.6b01021 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the process for selective-area photocatalytic graphene patterning. (a) Spin-coating of block copolymer thin film directly on SiO2/Si substrate. (b) Structure alignment via solvent vapor annealing (SVA) of block copolymer. (c) Fabrication of hard silicon oxycarbide nanopillars through oxidation of PDMS and simultaneous removal of PS under oxygen plasma. (d) Atomic layer deposition (ALD) of TiO2 thin film on top of the silicon oxycarbide nanopillars. (e) Transfer of PMMA-supported single-layer graphene onto the TiO2-covered nanopillars. (f) Photocatalytic creation of graphene nanomesh via UV irradiation at ambient conditions. (g) Transfer of graphene nanomesh onto SiO2/Si substrate after floating the graphene nanomesh by HF and final removal of PMMA by acetone. HF was used as wet etchant of the nanopillars after completion of the photoreaction.

nanopillars (Figures 2b and Figure S1b). Compared to the silicon oxycarbide nanopillars array with a period of 34 nm and average pillar diameter of 22 nm (Figure S1a), the period of TiO2-covered nanopillars remains unchanged and the diameter increases to about 29 nm (Figure S1b). The increased diameter is compatible with a 3.5 nm thick TiO2 layer. Figure 2c shows the AFM image of the monolayer CVD graphene on top of the TiO2-covered nanopillars. The apparent nanopillar diameter as imaged through graphene (Figure 2c) is smaller than the diameter of the TiO2-covered nanopillars (Figure 2b) because of the decreased lateral resolution of AFM in the presence of the graphene film. The photocatalytic reaction on graphene was then performed under Xe-lamp irradiation. Because of the direct contact between nanopillars and graphene, active •OH species generated on TiO2 locally react with carbon atoms in contact with TiO2, breaking the sp2 carbon bond network. Graphene cutting most likely begins from the center of each nanopillar and then extends to the edge as the irradiation time increases. Figure 2d shows the AFM image of the sample after 12 h of UV-irradiation. Most areas of Figure 2d show hexagonally packed round shaped features, ascribed to the nanopillar top with diameter a bit smaller than the full pillar diameter of Figure 2b. The smaller diameter hints at a pillar profile getting narrower at the top, somewhat like in the schematic drawing of Figure 1. In addition, the contrast of the image in panel d is significantly improved as compared to the image in panel c, which we interpret as due to conformal photocatalytic degradation of graphene by the supporting TiO2

deposited onto these silicon oxycarbide nanopillars by ALD. The obtained samples were then thermally annealed in air at 500 °C for 2 h in order to induce crystallization of TiO2 in the photocatalitacally active anatase phase.21 CVD graphene supported by poly(methyl methacrylate) (PMMA) thin film was transferred onto such a patterned substrate. Then, the graphene sample was irradiated with ultraviolet light of wavelength 315−400 nm and power density of 5 mW/cm2 in the presence of oxygen at room temperature for nanopatterning. Descriptions of the photocatalytic mechanism can be found elsewhere.18,22,23 Briefly, electron−hole pairs are created by the UV photons on TiO2, which then generate highly reactive •OH free radicals in reaction with the air oxygen and water molecules. The •OH free radicals oxidize and ultimately break the graphene skeleton covalent bonds. Finally, the graphene nanomesh was transferred onto a flat target substrate after dissolving the TiO2-covered nanopillars in HF solution. The PMMA thin film was used as transfer medium, and subsequently removed by acetone. As a result, graphene nanopatterning is defined by the block copolymer prepatterned substrates. We have successfully fabricated graphene nanomesh and graphene nanoribbons with different periodicities by varying the morphology and length scale of the photocatalytic substrate. The surface topography of the nanopillar array and the graphene sheet were characterized by AFM and SEM, as shown in Figure 2 and Figure S1, respectively. It is seen that TiO2 thin film was deposited uniformly on the silicon oxycarbide B


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Figure 2. Graphene nanomesh formation on TiO2-covered nanopillars (steps c−f in Figure 1). AFM images of (a) silicon oxycarbide nanopillars. (b) TiO2-covered nanopillars. (c) Graphene sheet transferred on TiO2-covered nanopillars and (d) graphene sheet after 12 h of UV irradiation. The supporting PMMA film was removed by acetone before AFM imaging in b and c. Scale bars: 200 nm.

Figure 3. Graphene nanomesh transferred onto SiO2/Si substrate after 12 h of UV irradiation (step g in Figure 1). (a) SEM image and (b) AFM image. Scale bars: (a) 200 and (b) 100 nm.

nanopillars. The missing single-pillar resolution observable in some areas of panel d is most probably related to graphene folds that impede proper contact with the nanopillars. In the case of SEM there is no clear distinction between the images of graphene before (Figure S1c) and after (Figure S1d) UV irradiation. The lower contrast of these images compared to the image in Figure S1b may be ascribed to absorption of SEM secondary electrons by graphene. The necessary presence of TiO2 for a successful nanopatterning of graphene is substantiated by a control experiment in which CVD single-layer graphene was transferred directly on silicon oxycarbide nanopillars, without TiO2 coverage, as shown in Figure S2a. After 12 h of irradiation by UV, we found that the Raman spectrum of graphene remained unchanged; there was no defect D peak generated (see the discussion of Raman

spectroscopy after Figure 3). This result confirms the crucial role of TiO2 for the photoreaction. Figure 3 exhibits typical (a) SEM and (b) AFM images of the patterned graphene after transfer onto a flat SiO2/Si substrate. These images clearly demonstrate the successful fabrication of graphene nanomesh after UV-irradiation, resulting in the formation of porous sheets after the photocatalytic process. The period of the nanomesh in panels a and b was measured to 34 nm, which is the same as the period of the nanotemplate (Figure 2). This is strong evidence that the selective-area graphene nanopatterning is defined by the underlying nanotemplate. Graphene superlattice with a neck width of approximately 19 nm (Figure 3a) was formed after 12 h of photoreaction. SEM and AFM images of the graphene nanomesh formed after 14 h of UV are shown in Figures S3c, d, respectively. The average neck width for the 14 h UVC


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Figure 4. Characterization by Raman spectroscopy of the effect of photooxidation on graphene after 0, 6, 9, 12, 14, and 24 h of UV irradiation. (a) Raman spectra. (b) Evolution of the peak intensity ratios ID/IG (black) and I2D/IG (red). (c) Evolution of fwhm (G) (black) signal together with fwhm (2D) (red).

maximum of the 2D peak (fwhm(2D)) changes slightly after 6 h of UV-irradiation and then clearly increases, revealing high defect density at irradiation times exceeding 14 h (Figure 4c).26 A higher defect density is also visible in the SEM image of the 14 h irradiated graphene in Figure S3c as compared to the sample irradiated for 12 h (Figure S3a). This feature has been captured by the abrupt increase in the intensity ratio ID/IG plotted in Figure 4b at irradiation time of 14 h. At extended irradiation times all the Raman peaks characteristic of graphene diminish and broaden, as shown from the Raman spectrum of the 24 h irradiated sample (Figure 3a). These features indicate that most of the graphene bond conjugation network has been destroyed at prolonged photocatalytic oxidation followed by severe damage of the 2D material. The measurement of peak parameters was very uncertain for this sample. To quantify the amount of defects introduced in the photocatalytic process, we analyze the obtained Raman spectra accounting for the defect that may be present in the system. The patterning itself could introduce a characteristic crystallite length scale related to the boundaries of the GNM and this is a measure of the average inverse nanocrystallite size 1/La.27 For our analysis, we use the Tuinstra−Koenig relation to obtain La

treated sample in Figure S3c was measured to 14 nm. Moreover, XPS data shown in Figure S4 point to no titanium left on the final graphene nanomesh transferred onto SiO2/Si substrate. As a powerful tool for characterizing ordered/disordered crystal structures of carbonaceous materials, Raman spectroscopy was utilized to examine the defects generated during photocatalysis. At least four measurements at different locations were made for each sample. Figure 4a shows the Raman spectra of graphene after 0 h (black curve), 6 h (red), 9 h (blue), 12 h (magenta), 14 h (olive), and 24 h (navy) of UV-irradiation, revealing the evolution of defect formation. Only a very small Raman D peak is observed for the pristine CVD graphene used in this study, indicative of its good quality. After the photoreaction in the presence of TiO2, a prominent disorderinduced D peak appears at 1358 cm−1. In addition, the doubleresonance 2D peak is weakened. These observations suggest the presence of a larger number of defects, which originate from active species •OH oxidizing the carbon atoms under UV illumination.22,23 It is argued that highly reactive •OH radicals work as photoscissors for the graphenic carbon network.15,24 Also the intensity ratio between Raman D and G peaks ID/IG (commonly used to characterize disorder in graphene), is observed to increase with the time of UV-irradiation. At increasing irradiation time, we observed a significant increase of ID/IG ratio, from the initial average of 0.03 for 0 h to 0.40 for 6 h, 0.49 for 9 h, 0.58 for 12 h, 1.20 for 14 h, and 1.35 for 24 h as shown in Figure 4b. Moreover disorder leads to a decrease of the I2D/IG as shown in Figure 4b.25 The full width half-

La(nm) = (2.4 × 10−10)λlaser 4(ID/IG)−1

(1)

Here La is the average separation between defects, λ is the Raman excitation wavelength (455 nm in our case) and ID/IG is the peak intensity ratio. Supposing that the disorder originates from graphene boundaries, Equation 1 provides an estimate for the nanomesh neck width of 18 nm after 12 h and 9 nm after D


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In summary, uniform graphene nanomesh and nanoribbons were fabricated using local photocatalysis of graphene in contact with vertically aligned TiO2-covered nanopillar and nanowire arrays. AFM and SEM images demonstrated the successful formation of the graphene nanomesh and nanoribbons. Disorder-quantification by analyses of Raman spectroscopy data provided additional information about the type and distribution of defects generated around the contact interface between graphene and TiO 2. The presented substrate engineering technique allows for a well-controlled periodic modification of graphene and possibly of other UV transparent 2D materials and thin films. Such nanostructured materials hold promise for applications in electronics, photonics, sensing, and molecular separation technologies.

14 h of UV-irradiation. A neck width of 18 nm is close to the one estimated from the SEM image in Figure 3a, i.e. 19 nm. However, the average neck width after 14 h of UV treatment as measured by SEM (14 nm, Figure S3c) is significantly bigger than the result estimated by eq 1 for the same sample, i.e., 9 nm. Now, as already mentioned above the TiO2 covered nanopillars have a diameter of 29 nm (Figure S1b), which allows to estimate the closest distance between neighbor pillars as equal to 5 nm. Under the assumptions that graphene photooxidation happens in close contact with the catalyst and that the TiO2-covered nanopillars are cylinders (same diameter at different heights), we would therefore have expected a minimum neck width of 5 nm. The SEM images of Figure 3a and Figure S3c show neck widths of 19 and 14 nm, which are significantly bigger than 5 nm, thus supporting a scenario where graphene degradation starts from the center of the nanopillar tops and advances radially. Therefore, as already noted in the discussion of Figure 2, the local contact of graphene with each nanopillar is smaller than a circular area of diameter of 29 nm. On the basis of these considerations the discrepancy between the SEM measured neck width and that estimated from eq 1 for the 14 h irradiated sample could be interpreted as being due to random oxidation of graphene around the holes, with conversion of sp2 bonds to sp3. To check this explanation further, we analyzed the Raman spectra using a model that describes average distances between point defects Lp.18 Lp can be estimated by the following expression18,19 Lp2(nm) = 3600/E lasesr 4(ID/IG)−1

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01021. Details on the experimental procedures for the fabrication and characterization of the photocatalytic substrates templated by block copolymers; details on the fabrication of graphene nanomesh and the graphene nanoribbons, as well as additional characterization data of the nanostructured graphene by SEM, AFM, Raman spectroscopy, and XPS (PDF)

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(2)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Elaser = hv = hc/λ = 2.72 eV (λ = 455 nm is the wavelength, h is the Planck constant, c the speed of light in vacuum, and v the frequency) is the energy of the laser photons. From eq 2 the average distance between point defects Lp after 12 and 14 h of UV irradiation is 11 and 7 nm, respectively. These values are smaller than those estimated by SEM or by eq 1, which supports the existence of a ring of randomly oxidized graphene around the holes observed by SEM. Other nanostructured substrates such as low-density nanopillars, nanospheres, and nanoribbons have already been fabricated in our lab by application of block copolymer lithography.19,20,28 Starting with a photocatalytic substrate prepared by ALD of TiO2 on silicon oxycarbide nanowires19 (Figure S5a) and following a similar procedure as for the graphene nanomesh, 20 nm wide graphene nanoribbons with a periodicity of 33 nm have also been successfully fabricated, as shown in Figure S5. From the above experimental observations, we conclude that a selective-area photoreaction has taken place on graphene, originating from the TiO2-assisted photocatalytic reaction. The nanostructure of graphene can be locally modulated by the structural design of the underlying photocatalytic substrate. This offers a straightforward way to fabricate graphene superlattices by using densely packed diblock copolymer selfassembled structures. The advantages of the presented method are (i) the nanoscale photomask is easy and efficient to fabricate; (ii) the feature density and geometry can be controlled by appropriate choice of the block copolymer composition and molecular weight; (iii) uncontrolled diffusion of defects can be limited by strict control of the irradiation time; (iv) the generated ordered nanostructures are free from plasma and chemical contamination.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Danish National Research Foundation Center for Nanostructured Graphene, CNG (DNRF103), and by the Department. of Micro and Nanotechnology at the Technical University of Denmark.

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REFERENCES

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Photocatalytic Nanostructuring of Graphene Guided by Block

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