Single-step exfoliation and chemical functionalisation of graphene and hBN nanosheets with nickel phthalocyanine† James Thompson, a Alison Crossley, a Peter D. Nellist a and Valeria Nicolosi * b Received 23rd July 2012, Accepted 10th September 2012 DOI: 10.1039/c2jm34854c Versatile routes to functionalise few-layer graphene and hBN with nickel phthalocyanine (Ni-Pc) were achieved using liquid phase exfoliation and sonication methods. EDX performed on the graphene//Ni- Pc specimen showed nickel on the flake surface whilst Raman spectroscopy revealed a prominent D peak indicating the presence of basal plane defects. New Raman active modes were also found in the hBN//Ni-Pc complex. X-ray photoelectron spectroscopy showed a charge transfer for both graphene and hBN confirming that the flake edges and basal-planes were covalently functionalised by the phthalocyanine molecules. Transmission electron microscopy confirmed the only presence of single and few-layer flakes of hBN and graphene in solution, demonstrating that the exfoliation yield was not affected by the functionalisation step. We therefore proved that tuning of the electronic and optical properties of graphene and hBN nanosheets is indeed conceivable. We used the Z-scan technique to prove the nonlinear optical (NLO) behaviour of the functionalised graphene sheets. Based on such optical response, we demonstrate an optical limiting effect for nanosecond laser pulses at 532 nm, proving these materials to be a suitable candidate for photonic and optoelectronic applications. Introduction Since its experimental discovery in 2004 (ref. 1) graphene has established itself as a viable candidate for replacing silicon technology and being used in the next generation of electronic devices. The sustained interest in graphene is due to its excellent electronic properties. It is a zero-gap semiconductor with its valence and conduction bands touching in k-space. 1 Graphene films show a strong ambipolar electric field effect where both electrons and holes can be the conducting entities. 1 Mobilities as high as 15 000 cm 2 V 1 s 1 can be reached even at extremely high carrier concentrations. 1–4 The single and bi-layer forms are similar in that they are both zero-gap semiconductors with only a hole and an electron as charge carriers. For few-layer graphene a few more charge carriers appear as the valence and conduction bands begin to overlap, however, it can still be considered unique from the bulk material. 1,5 The 3D limit is at about 10 atomic layers. 6 Being a zero-gap semiconductor, a flat mono- or few- layer graphene is almost transparent and exhibits very low elec- trical resistivity at room temperature. 7 Electronics is where the majority of research is currently focussed, with graphene having promising applications in conducting films, FETs and logic gates in nanoelectronics as well as the potential in battery efficiency. 8 As an addition to graphene, a wide range of 2-dimensional (2-D) atomic crystals exist in nature, including transition metal dichalcogenides and hexagonal boron nitride (hBN) nano- sheets. 9 The unique 2D structure of these materials gives rise to a variety of useful mechanical, optical and particularly electrical properties. 1,2,10 The electronic structures of hBN and graphene are very different. The B–N bond shows some ionic character both in and out of the plane. 11 This polarity accounts for the different stacking sequence in BNNSs as well as slightly greater inter-layer forces. More importantly it means that, despite their similar crystal structure, hBN is an electrical insulator with a direct gap of about 5.8 eV. 12,13 Mechanically hBN sheets have been shown to be fairly strong; 1 nm thick BN sheets were found to have a breaking stress of 8.8 Nm 1 and a Young’s modulus of 233 Nm 1 within the plane of the sheet. 13 Potential applications are vast; the most immediate are likely to be in composites. 14 A likely application of pure hBN nano- sheets is as a complementary dielectric to graphene in nano- electronic devices, due to its large band gap. 15 Moreover, bulk hBN materials are currently used in a variety of diodes and other optoelectronic and electronic devices and the respective two- dimensional counterpart is expected to play a competing role in these types of applications. 16 The scale investigation of these 2D nanomaterials has only recently become possible due to their increased availability through liquid phase exfoliation. 17,18 This process allows the quick and cheap high yield production of single layers of a Department of Materials, University of Oxford, Parks Road, OX1 3PH, Oxford, UK b School of Chemistry, School of Physics & CRANN, Trinity College Dublin, Dublin 2, Ireland. E-mail: nicolov@tcd.ie; Tel: +353 (0)1 896 4408 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c2jm34854c 23246 | J. Mater. Chem., 2012, 22, 23246–23253 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2012, 22, 23246 www.rsc.org/materials PAPER Published on 11 September 2012. Downloaded by Trinity College Dublin on 28/03/2014 12:32:58. View Article Online / Journal Homepage / Table of Contents for this issue