A two-dimensional porous network formed from perylene tetracarboxylic diimide (PTCDI) and melamine may be deposited from

solution on the surfaces of highly oriented pyrolytic graphite (HOPG), hexagonal boron nitride (hBN) and molybdenum disulphide (MoS2). Images acquired using high resolution atomic force microscopy (AFM) operating under ambient conditions have revealed that the network forms extended orderedmonolayers (41 lm2) on HOPG and hBN whereas on MoS2 much smaller islands are observed

Bartels

Burke

Clive J. Roberts

Dean

Dmitriev

Geim

Georgiou

Griessl

Gross

Horcas

Karmel

Keeling

Kenji Watanabe

Korolkov

Kudernac

Ludwig

Madueno

Maier

Nath

Neil R. Champness

Perdigão

Peter H. Beton

Rahe

Saul J. B. Tendler

Simon A. Svatek

Slater (née Phillips)

Stepanow

Stephanie Allen

Sweetman

Takashi Taniguchi

Theobald

V Barth

Vladimir V. Korolkov

Yankowitz

Zhang

Korolkov, Vladimir V.

Svatek, Simon A.

Allen, Stephanie

Roberts, Clive J.

Tendler, Saul J.B.

Taniguchi, Takashi

Watanabe, Kenji

Champness, Neil R.

Beton, Peter H.

Nottingham ePrints

Korolkov, Vladimir V. and Svatek, Simon A. and Allen, Stephanie and Roberts, Clive J. and Tendler, Saul J.B. and Taniguchi, Takashi and Watanabe, Kenji and Champness, Neil R. and Beton, Peter H. (2014) Bimolecular porous supramolecular networks deposited from solution on layered materials: graphite, boron nitride and molybdenum disulphide. Chemical Communications, 50 (64). pp. 8882-8885. ISSN 1364-548X Access from the University of Nottingham repository: http://eprints.nottingham.ac.uk/31728/1/korolkov_bimolecular.pdfCopyright and reuse: The Nottingham ePrints service makes this work by researchers of the University of Nottingham available open access under the following conditions.· Copyright and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners.· To the extent reasonable and practicable the material made available in Nottingham ePrints has been checked for eligibility before being made available.· Copies of full items can be used for personal research or study, educational, or not-for-profit purposes without prior permission or charge provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way.· Quotations or similar reproductions must be sufficiently acknowledged.Please see our full end user licence at: http://eprints.nottingham.ac.uk/end_user_agreement.pdf A note on versions: The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the repository url above for details on accessing the published version and note that access may require a subscription.For more information, please contact eprints@nottingham.ac.ukThis journal is©The Royal Society of Chemistry 2014 Chem. Commun.Cite this:DOI: 10.1039/c4cc03720kBimolecular porous supramolecular networksdeposited from solution on layered materials:graphite, boron nitride and molybdenumdisulphide†Vladimir V. Korolkov,a Simon A. Svatek,a Stephanie Allen,b Clive J. Roberts,bSaul J. B. Tendler,b Takashi Taniguchi,c Kenji Watanabe,c Neil R. Champnessd andPeter H. Beton*aA two-dimensional porous network formed from perylene tetra-carboxylic diimide (PTCDI) and melamine may be deposited fromsolution on the surfaces of highly oriented pyrolytic graphite(HOPG), hexagonal boron nitride (hBN) and molybdenum disulphide(MoS2). Images acquired using high resolution atomic force micro-scopy (AFM) operating under ambient conditions have revealed thatthe network forms extended orderedmonolayers (41 lm2) on HOPGand hBN whereas on MoS2 much smaller islands are observed.Coinage-metal surfaces and highly oriented pyrolytic graphite(HOPG) have been used in many of the studies of two-dimensional supramolecular organisation which have beenreported over the past decade.1–3 These conducting substrates arecompatible with scanning tunnelling microscopy (STM), and havestructural properties which are understood in great detail, allowingexperimental studies to focus on stabilisation of networks throughhydrogen bonding4–8 and metal co-ordination.9,10 An extension ofthese studies to other substrates, particularly non-conductingsurfaces, is highly desirable and would support investigations ofa much wider range of properties going beyond the determinationof structural and molecular organisation. The investigation ofsupramolecular networks on non-metallic substrates requires theuse of atomic force microscopy (AFM) rather than STM. Thepotential of AFM for the acquisition of high resolution images ofmolecular structure has been convincingly demonstrated underultra-high vacuum (UHV) conditions,11–14 including hydrogen-bonded systems,15,16 but to date AFM images of two-dimensionalsupramolecular organisation with molecular resolution have onlybeen acquired under ambient conditions for trimesic acid17 and,using a torsional tapping mode, the bimolecular network formedby PTCDI (perylene tetracarboxylic di-imide) and melamine;18 inthese studies HOPG and gold, respectively, were used as substrates.In this paper we show that, under ambient conditions, AFMimages of supramolecular networks may be acquired with aresolution which is similar to that achieved using STM underthe same conditions. A nanoporous honeycomb network formedfrom PTCDI and melamine network, previously studied on theAg/Si(111)-surface under UHV conditions,19 and on Au(111) firstunder UHV20 and then from solution18,21 was chosen for thisstudy. Recently the same network was formed on graphenemultilayers grown on SiC under UHV conditions.22 Here wecompare the deposition of PTCDI and melamine from solutionon three layered materials, HOPG, molybdenum disulphide(MoS2) and exfoliated flakes of hexagonal boron nitride (hBN).The molecular structures of PTCDI and melamine, togetherwith the hydrogen-bonded PTCDI–melamine junction and thehexagonal unit cell of the resulting supramolecular structureare shown in Fig. 1. Each melamine interacts with three PTCDImolecules via a total of nine hydrogen bonds. We deposit thenetworks by immersion of the substrate into solutions of themolecular components which, as previous studies have shownfor deposition on Au(111),18,21 leads to much larger domainsthan those obtained by UHV deposition.Images of the networks are acquired using AFM, which forthe layers deposited on graphite may be complemented by STM.Fig. 1 Chemical structure of (a) PTCDI, (b) melamine; (c) schematic ofPTCDI–melamine hydrogen bonded vertex, (d) unit cells of the derivedsupramolecular network.a School of Physics & Astronomy, The University of Nottingham,Nottingham NG7 2RD, UK. E-mail: peter.beton@nottingham.ac.ukb School of Pharmacy, The University of Nottingham, Nottingham, NG7 2RD, UKc The National Institute for Materials Science, Advanced Materials Laboratory,1-1 Namiki, Tsukuba, Ibaraki-305-0044, Japand School of Chemistry, The University of Nottingham, Nottingham NG7 2RD, UK† Electronic supplementary information (ESI) available: Additional images ofnetwork. See DOI: 10.1039/c4cc03720kReceived 15th May 2014,Accepted 22nd June 2014DOI: 10.1039/c4cc03720kwww.rsc.org/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView JournalChem. Commun. This journal is©The Royal Society of Chemistry 2014In this work we use, predominantly, PeakForce tapping modeAFM, which was recently introduced by Bruker. In this mode thetip oscillates at a frequency far below resonance (2 kHz) and aforce–distance curve is acquired at each pixel. The maximumforce detected during the approach cycle is used as a set pointwhich, in conjunction with a feedback loop, regulates the probeheight. The values for the set point are typically in the (repulsive)range of 1–10 pN, allowing for a very low force while imagingdelicate systems. As the force curve is recorded at every singlepixel it is also possible to determine the spatial variation ofmechanical properties, such as adhesion, of the array. AFMimages were acquired in ambient conditions using standardsilicon tips (Multi75Al, Multi75Al-G supplied by Budget sensors;typical spring constant, 2.8 N m!1). We have also used a CypherAFM (Asylum Research) to acquire images (in AC mode) of somesamples. STM imaging was performed utilizing tips cut fromPt/Ir (20% Ir) wire. WsXM V5.0 software was used to process andanalyse all AFM images.23The PTCDI–melamine network was prepared via adsorptionin a dimethylformamide (DMF) solution at 373 K.18,21 Themelamine and PTCDI concentrations were 0.66 mM andB0.5 mM respectively. Each sample was rinsed with 5 ml offresh DMF and blown dry in N2-stream. The same protocol wasused for all substrates.Fig. 2 shows AFM images following molecular deposition onHOPG (purchased from NT-MDT (20 minute immersion time)).The AFM images show clearly that a regular hexagonal frameworkis formed on the HOPG surface with the structure and dimensionsexpected for the PTCDI–melamine network shown in Fig. 1.Fig. 2a shows a hexagonal array of nanopores in a single domainwhich extends over 250 nm (in larger area images we observeislands with dimensions up to 0.5 mm; see ESI†). The size of thesedomains, and the fraction of the surface covered (490%) by thenetwork is much greater than that achieved in previous UHVstudies,19,20,22 but is comparable to those previously reported forsolution deposition on Au(111).18,21 Themeasured lattice constant is3.5 " 0.1 nm, very close to the values of 3.46 nm and 3.5 nmreported for adsorption on Ag/Si(111)19 and Au(111)20,21 surfaces. Aheight profile extracted from a higher magnification image (Fig. 2b)is shown in the inset; the topographic height of the molecules isB0.1 nm. Fig. 2c shows, for comparison, an STM image of a surfaceprepared using the same (highly reproducible) protocol. Interest-ingly, there is very little difference between the resolution and imagequality using these two modalities (see ESI† for additional images).Fig. 2d shows an adhesion map which is extracted from theforce–distance curves measured during the acquisition of Fig. 2b;the adhesion is defined as themaximum attractive force during theretraction of the cantilever. This image shows stronger adhesion atthe centres of the hexagonal cavities formed in the array, where theHOPG surface is exposed, with the network structure appearing asa dark honeycomb mesh (darker features correspond to loweradhesion). Similar behaviour was observed for trimesic acid mono-layers adsorbed on HOPG.17 At this stage a quantitative interpreta-tion, which requires detailed knowledge of the probe radius, is notpossible. However, we include these observations to highlight thepotential for AFM to provide additional modalities in future studiesof host–guest interactions in supramolecular networks.We have also successfully deposited the PTCDI–melaminenetwork on hBN. These surfaces are prepared by exfoliatinghBN flakes from small mm-sized crystals grown at the NationalInstitute for Materials Science, and transferring them to a siliconwafer on which a 300 nm thick oxide layer has been grown. Theflakes are then heated in an Ar/H2 (95 : 5) atmosphere at 400 1C for4–6 hours to remove organic residues. This process is very similarto that used by other researchers to prepare hBN substrates foruse in graphene research.24,25 hBN is a layered compound with astructure similar to graphite, except that the two atom carbonbasis set is replaced by two heteroatoms, boron and nitrogen. As alarge band-gap semiconductor hBN is not suitable for study withSTM and all images were acquired using AFM.The PTCDI–melamine network could be readily formed onthis substrate by immersion in the solution described above for1 minute; this was found to be sufficient to grow an extendedmonolayer of the network with typical islands with widthsB0.3 mm(see ESI†). Fig. 3a shows a large area image in which the honeycomblattice is clearly resolved and we determine the lattice constant as3.5" 0.1 nm, the same value, within experimental error as observedfor the network onHOPG. Higher magnification images acquired inPeakForce tapping mode which show the honeycomb network arepresented in Fig. 3b and c. In both Fig. 3a and b there are darkerfeatures with facetted edges which appear within the molecularnetwork. These are defects in the molecular layer and tend to growduring the acquisition of AFM images in AC mode, so that afterprolonged imaging the network becomes discontinuous.MoS2 is a layered semiconductor currently attracting interestdue to its potential for integration with graphene.27 We haveFig. 2 AFM and STM scans of a PTCDI–melamine network on HOPG inair. Images (a) and (b) are topographic AFM scans. Image (c) is a constantcurrent STM image acquired at I = 5 pA and sample voltage 0.5 V. Image (d)shows adhesion distribution of the upper half of the area in (b). Inset onimage (a) shows cross-section taken along the line shown in image (b).Communication ChemCommOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThis journal is©The Royal Society of Chemistry 2014 Chem. Commun.also demonstrated that the PTCDI–melamine network may beformed on this surface as shown in Fig. 4 (PTCDI on MoS2 haspreviously been studied by Ludwig et.al.26 using STM). A similarprotocol is used as for the formation on HOPG and hBN, but onthe MoS2 surface much smaller islands are formed. After adeposition time of 1 minute we see only islands of typicallateral dimensions of o100 nm. Increase of the immersiontime to 10 minutes leads to the formation of an extendedsupramolecular layer with a hexagonal symmetry and a latticeconstant of 3.5 " 0.1 nm consistent with the formation of thehydrogen-bonded structure in Fig. 1. We have also tried toimage this surface using STM but have not succeeded inacquiring images showing the molecular network.The most significant difference which we identify foradsorption on MoS2 lies in the dimensions of the network.This may be due to differences in chemistry; in particular thepresence of a layer of sulphur atoms at the surface may lead tostronger interactions with both environmental gaseous species,and also with the adsorbed molecules themselves. Nevertheless,our results confirm that the hydrogen-bonding which leads tothe stabilisation of the honeycomb network is effective on theMoS2 surface.The network co-exists with three dimensional features forwhich we cannot obtain molecular resolution; these may be dueto adsorption of molecules in a non-planar configuration, or,possibly, the formation of multilayers, and are present is slightlygreater concentration on the MoS2 surface. Possibly due to thesmaller islands and greater disorder observed on this surface, it ismore difficult to acquire images, and we have not been able toacquire adhesion maps with nm-scale MoS2 substrates (adhesionmaps for hBN are included in the ESI;† these show a similarcontrast to Fig. 2d). For all substrates the network structureremains sufficiently stable (when kept in a sealed vial filled withnitrogen) for further images to be obtained for over a week.In our study we have successfully demonstrated that hydrogen-bonded supramolecular networks can be readily deposited on thesurfaces of HOPG, hBN andMoS2. Importantly the structures maybe resolved using AFM using conventional micromachined canti-levers. The formation of the honeycomb PTCDI–melamine net-work and its stability under ambient conditions is particularlyimportant since this offers the prospect of integrating molecularrecognition and host–guest interactions with a family of layeredmaterials, including semimetals, semiconductors and insulators,which are currently receiving great attention as candidates for anew generation of optoelectronic devices.27,28Financial support was provided by the UK Engineering andPhysical Sciences Research Council (EPSRC; grants EP/K01773X/1and EP/K005138/1).Notes and references1 L. Bartels, Nat. Chem., 2010, 2, 87–95.2 T. Kudernac, S. Lei, J. A. A. W. Elemans and S. De Feyter, Chem. Soc.Rev., 2009, 38, 402–421.3 A. G. Slater (ne´e Phillips), P. H. Beton and N. R. Champness,Chem. Sci., 2011, 2, 1440.4 J. V Barth, J. Weckesser, C. Cai, P. Gu¨nter, L. Bu¨rgi, O. Jeandupeuxand K. Kern, Angew. Chem., Int. Ed., 2000, 1230–1234.5 S. B. Lei, C. Wang, S. X. Yin, H. N. Wang, F. Xi, H. W. Liu, B. Xu,L. J. Wan and C. L. Bai, J. Phys. Chem. B, 2001, 105, 10838–10841.6 S. Griessl, M. Lackinger, M. Edelwirth, M. Hietschold andW. M. Heckl, Single Mol., 2002, 3, 25–31.7 K. G. Nath, O. Ivasenko, J. A. Miwa, H. Dang, J. D. Wuest, A. Nanci, D. F.Perepichka and F. Rosei, J. Am. Chem. Soc., 2006, 128, 4212–4213.8 D. L. Keeling, N. S. Oxtoby, C. Wilson, M. J. Humphry, N. R. Champnessand P. H. Beton, Nano Lett., 2003, 3, 9–12.9 S. Stepanow, M. Lingenfelder, A. Dmitriev, H. Spillmann, E. Delvigne,N. Lin, X. Deng, C. Cai, J. V Barth and K. Kern, Nat. Mater., 2004, 3,229–233.10 A. Dmitriev, H. Spillmann, N. Lin, J. V Barth and K. Kern, Angew.Chem., Int. Ed., 2003, 42, 2670–2673.11 L. Gross, F. Mohn, N. Moll, P. Liljeroth and G. Meyer, Science, 2009,325, 1110–1114.12 P. Rahe, R. Lindner, M. Kittelmann, M. Nimmrich and A. Ku¨hnle,Phys. Chem. Chem. Phys., 2012, 14, 6544–6548.13 S. Burke, J. Mativetsky, S. Fostner and P. Gru¨tter, Phys. Rev. B, 2007,76, 035419.14 S. Maier, L.-A. Fendt, L. Zimmerli, T. Glatzel, O. Pfeiffer, F. Diederichand E. Meyer, Small, 2008, 4, 1115–1118.15 J. Zhang, P. Chen, B. Yuan, W. Ji, Z. Cheng and X. Qiu, Science, 2013,342, 611–614.Fig. 3 AFM scans of the PTCDI–melamine network on the hBN surface inair. Image (a) was acquired in AC mode on a Cypher AFM (AsylumResearch). Images (b) and (c) were taken in PeakForce Tapping Mode.Fig. 4 AFM images of PTCDI–melamine network on MoS2 surface after(a) 1 minute and (b) 10 minute adsorption times.ChemComm CommunicationOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineChem. Commun. This journal is©The Royal Society of Chemistry 201416 A. M. Sweetman, S. P. Jarvis, H. Sang, I. Lekkas, P. Rahe, Y. Wang,J. Wang, N. Champness, L. Kantorovich and P. Moriarty, Nat. Commun.,2014, 5, 3931.17 V. V. Korolkov, S. Allen, C. J. Roberts and S. J. B. Tendler, J. Phys.Chem. C, 2012, 116, 11519–11525.18 V. V. Korolkov, N. Mullin, S. Allen, C. J. Roberts, J. K. Hobbs andS. J. B. Tendler, Phys. Chem. Chem. Phys., 2012, 14, 15909.19 J. A. Theobald, N. S. Oxtoby, M. A. Phillips, N. R. Champness andP. H. Beton, Nature, 2003, 424, 1029–1031.20 L. M. A. Perdiga˜o, E. W. Perkins, J. Ma, P. A. Staniec, B. L. Rogers,N. R. Champness and P. H. Beton, J. Phys. Chem. B, 2006, 110,12539–12542.21 R. Madueno, M. T. Ra¨isa¨nen, C. Silien and M. Buck, Nature, 2008,454, 618–621.22 H. J. Karmel, T. Chien, V. Demers-Carpentier, J. J. Garramone andM. C. Hersam, J. Phys. Chem. Lett., 2014, 5, 270–274.23 I. Horcas, R. Ferna´ndez, J. M. Go´mez-Rodrı´guez, J. Colchero,J. Go´mez-Herrero and A. M. Baro, Rev. Sci. Instrum., 2007,78, 013705.24 C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei,K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard and J. Hone,Nat. Nanotechnol., 2010, 5, 722–726.25 M. Yankowitz, J. Xue, D. Cormode, J. D. Sanchez-Yamagishi,K. Watanabe, T. Taniguchi, P. Jarillo-Herrero, P. Jacquod andB. J. LeRoy, Nat. Phys., 2012, 8, 382–386.26 C. Ludwig, B. Gompf, J. Petersen, R. Strohmaier andW. Eisenmenger,Z. Phys. B, 1994, 93, 365–373.27 A. K. Geim and I. V. Grigorieva, Nature, 2013, 499, 419–425.28 T. Georgiou, R. Jalil, B. D. Belle, L. Britnell, R. V. Gorbachev,S. V. Morozov, Y.-J. Kim, A. Gholinia, S. J. Haigh, O. Makarovsky,L. Eaves, L. a. Ponomarenko, A. K. Geim, K. S. Novoselov andA. Mishchenko, Nat. Nanotechnol., 2013, 8, 100–103.Communication ChemCommOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online

Bimolecular porous supramolecular networks deposited from solution on layered materials: graphite, boron nitride and molybdenum disulphide

A two-dimensional porous network formed from perylene tetracarboxylic diimide (PTCDI) and melamine may be deposited from\ud
solution on the surfaces of highly oriented pyrolytic graphite (HOPG), hexagonal boron nitride (hBN) and molybdenum disulphide (MoS2). Images acquired using high resolution atomic force microscopy (AFM) operating under ambient conditions have revealed that the network forms extended orderedmonolayers (41 lm2) on HOPG and hBN whereas on MoS2 much smaller islands are observed

Name not available

Nottingham eTheses

Korolkov, Vladimir V. and Svatek, Simon A. and Allen, Stephanie and Roberts, Clive J. and Tendler, Saul J.B. and Taniguchi, Takashi and Watanabe, Kenji and Champness, Neil R. and Beton, Peter H. (2014) Bimolecular porous supramolecular networks deposited from solution on layered materials: graphite, boron nitride and molybdenum disulphide. Chemical Communications, 50 (64). pp. 8882-8885. ISSN 1364-548X Access from the University of Nottingham repository: http://eprints.nottingham.ac.uk/31728/1/korolkov_bimolecular.pdfCopyright and reuse: The Nottingham ePrints service makes this work by researchers of the University of Nottingham available open access under the following conditions.This article is made available under the Creative Commons Attribution licence and may be reused according to the conditions of the licence.  For more details see: http://creativecommons.org/licenses/by/2.5/A note on versions: The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the repository url above for details on accessing the published version and note that access may require a subscription.For more information, please contact eprints@nottingham.ac.ukThis journal is©The Royal Society of Chemistry 2014 Chem. Commun.Cite this:DOI: 10.1039/c4cc03720kBimolecular porous supramolecular networksdeposited from solution on layered materials:graphite, boron nitride and molybdenumdisulphide†Vladimir V. Korolkov,a Simon A. Svatek,a Stephanie Allen,b Clive J. Roberts,bSaul J. B. Tendler,b Takashi Taniguchi,c Kenji Watanabe,c Neil R. Champnessd andPeter H. Beton*aA two-dimensional porous network formed from perylene tetra-carboxylic diimide (PTCDI) and melamine may be deposited fromsolution on the surfaces of highly oriented pyrolytic graphite(HOPG), hexagonal boron nitride (hBN) and molybdenum disulphide(MoS2). Images acquired using high resolution atomic force micro-scopy (AFM) operating under ambient conditions have revealed thatthe network forms extended orderedmonolayers (41 lm2) on HOPGand hBN whereas on MoS2 much smaller islands are observed.Coinage-metal surfaces and highly oriented pyrolytic graphite(HOPG) have been used in many of the studies of two-dimensional supramolecular organisation which have beenreported over the past decade.1–3 These conducting substrates arecompatible with scanning tunnelling microscopy (STM), and havestructural properties which are understood in great detail, allowingexperimental studies to focus on stabilisation of networks throughhydrogen bonding4–8 and metal co-ordination.9,10 An extension ofthese studies to other substrates, particularly non-conductingsurfaces, is highly desirable and would support investigations ofa much wider range of properties going beyond the determinationof structural and molecular organisation. The investigation ofsupramolecular networks on non-metallic substrates requires theuse of atomic force microscopy (AFM) rather than STM. Thepotential of AFM for the acquisition of high resolution images ofmolecular structure has been convincingly demonstrated underultra-high vacuum (UHV) conditions,11–14 including hydrogen-bonded systems,15,16 but to date AFM images of two-dimensionalsupramolecular organisation with molecular resolution have onlybeen acquired under ambient conditions for trimesic acid17 and,using a torsional tapping mode, the bimolecular network formedby PTCDI (perylene tetracarboxylic di-imide) and melamine;18 inthese studies HOPG and gold, respectively, were used as substrates.In this paper we show that, under ambient conditions, AFMimages of supramolecular networks may be acquired with aresolution which is similar to that achieved using STM underthe same conditions. A nanoporous honeycomb network formedfrom PTCDI and melamine network, previously studied on theAg/Si(111)-surface under UHV conditions,19 and on Au(111) firstunder UHV20 and then from solution18,21 was chosen for thisstudy. Recently the same network was formed on graphenemultilayers grown on SiC under UHV conditions.22 Here wecompare the deposition of PTCDI and melamine from solutionon three layered materials, HOPG, molybdenum disulphide(MoS2) and exfoliated flakes of hexagonal boron nitride (hBN).The molecular structures of PTCDI and melamine, togetherwith the hydrogen-bonded PTCDI–melamine junction and thehexagonal unit cell of the resulting supramolecular structureare shown in Fig. 1. Each melamine interacts with three PTCDImolecules via a total of nine hydrogen bonds. We deposit thenetworks by immersion of the substrate into solutions of themolecular components which, as previous studies have shownfor deposition on Au(111),18,21 leads to much larger domainsthan those obtained by UHV deposition.Images of the networks are acquired using AFM, which forthe layers deposited on graphite may be complemented by STM.Fig. 1 Chemical structure of (a) PTCDI, (b) melamine; (c) schematic ofPTCDI–melamine hydrogen bonded vertex, (d) unit cells of the derivedsupramolecular network.a School of Physics & Astronomy, The University of Nottingham,Nottingham NG7 2RD, UK. E-mail: peter.beton@nottingham.ac.ukb School of Pharmacy, The University of Nottingham, Nottingham, NG7 2RD, UKc The National Institute for Materials Science, Advanced Materials Laboratory,1-1 Namiki, Tsukuba, Ibaraki-305-0044, Japand School of Chemistry, The University of Nottingham, Nottingham NG7 2RD, UK† Electronic supplementary information (ESI) available: Additional images ofnetwork. See DOI: 10.1039/c4cc03720kReceived 15th May 2014,Accepted 22nd June 2014DOI: 10.1039/c4cc03720kwww.rsc.org/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView JournalChem. Commun. This journal is©The Royal Society of Chemistry 2014In this work we use, predominantly, PeakForce tapping modeAFM, which was recently introduced by Bruker. In this mode thetip oscillates at a frequency far below resonance (2 kHz) and aforce–distance curve is acquired at each pixel. The maximumforce detected during the approach cycle is used as a set pointwhich, in conjunction with a feedback loop, regulates the probeheight. The values for the set point are typically in the (repulsive)range of 1–10 pN, allowing for a very low force while imagingdelicate systems. As the force curve is recorded at every singlepixel it is also possible to determine the spatial variation ofmechanical properties, such as adhesion, of the array. AFMimages were acquired in ambient conditions using standardsilicon tips (Multi75Al, Multi75Al-G supplied by Budget sensors;typical spring constant, 2.8 N m!1). We have also used a CypherAFM (Asylum Research) to acquire images (in AC mode) of somesamples. STM imaging was performed utilizing tips cut fromPt/Ir (20% Ir) wire. WsXM V5.0 software was used to process andanalyse all AFM images.23The PTCDI–melamine network was prepared via adsorptionin a dimethylformamide (DMF) solution at 373 K.18,21 Themelamine and PTCDI concentrations were 0.66 mM andB0.5 mM respectively. Each sample was rinsed with 5 ml offresh DMF and blown dry in N2-stream. The same protocol wasused for all substrates.Fig. 2 shows AFM images following molecular deposition onHOPG (purchased from NT-MDT (20 minute immersion time)).The AFM images show clearly that a regular hexagonal frameworkis formed on the HOPG surface with the structure and dimensionsexpected for the PTCDI–melamine network shown in Fig. 1.Fig. 2a shows a hexagonal array of nanopores in a single domainwhich extends over 250 nm (in larger area images we observeislands with dimensions up to 0.5 mm; see ESI†). The size of thesedomains, and the fraction of the surface covered (490%) by thenetwork is much greater than that achieved in previous UHVstudies,19,20,22 but is comparable to those previously reported forsolution deposition on Au(111).18,21 Themeasured lattice constant is3.5 " 0.1 nm, very close to the values of 3.46 nm and 3.5 nmreported for adsorption on Ag/Si(111)19 and Au(111)20,21 surfaces. Aheight profile extracted from a higher magnification image (Fig. 2b)is shown in the inset; the topographic height of the molecules isB0.1 nm. Fig. 2c shows, for comparison, an STM image of a surfaceprepared using the same (highly reproducible) protocol. Interest-ingly, there is very little difference between the resolution and imagequality using these two modalities (see ESI† for additional images).Fig. 2d shows an adhesion map which is extracted from theforce–distance curves measured during the acquisition of Fig. 2b;the adhesion is defined as themaximum attractive force during theretraction of the cantilever. This image shows stronger adhesion atthe centres of the hexagonal cavities formed in the array, where theHOPG surface is exposed, with the network structure appearing asa dark honeycomb mesh (darker features correspond to loweradhesion). Similar behaviour was observed for trimesic acid mono-layers adsorbed on HOPG.17 At this stage a quantitative interpreta-tion, which requires detailed knowledge of the probe radius, is notpossible. However, we include these observations to highlight thepotential for AFM to provide additional modalities in future studiesof host–guest interactions in supramolecular networks.We have also successfully deposited the PTCDI–melaminenetwork on hBN. These surfaces are prepared by exfoliatinghBN flakes from small mm-sized crystals grown at the NationalInstitute for Materials Science, and transferring them to a siliconwafer on which a 300 nm thick oxide layer has been grown. Theflakes are then heated in an Ar/H2 (95 : 5) atmosphere at 400 1C for4–6 hours to remove organic residues. This process is very similarto that used by other researchers to prepare hBN substrates foruse in graphene research.24,25 hBN is a layered compound with astructure similar to graphite, except that the two atom carbonbasis set is replaced by two heteroatoms, boron and nitrogen. As alarge band-gap semiconductor hBN is not suitable for study withSTM and all images were acquired using AFM.The PTCDI–melamine network could be readily formed onthis substrate by immersion in the solution described above for1 minute; this was found to be sufficient to grow an extendedmonolayer of the network with typical islands with widthsB0.3 mm(see ESI†). Fig. 3a shows a large area image in which the honeycomblattice is clearly resolved and we determine the lattice constant as3.5" 0.1 nm, the same value, within experimental error as observedfor the network onHOPG. Higher magnification images acquired inPeakForce tapping mode which show the honeycomb network arepresented in Fig. 3b and c. In both Fig. 3a and b there are darkerfeatures with facetted edges which appear within the molecularnetwork. These are defects in the molecular layer and tend to growduring the acquisition of AFM images in AC mode, so that afterprolonged imaging the network becomes discontinuous.MoS2 is a layered semiconductor currently attracting interestdue to its potential for integration with graphene.27 We haveFig. 2 AFM and STM scans of a PTCDI–melamine network on HOPG inair. Images (a) and (b) are topographic AFM scans. Image (c) is a constantcurrent STM image acquired at I = 5 pA and sample voltage 0.5 V. Image (d)shows adhesion distribution of the upper half of the area in (b). Inset onimage (a) shows cross-section taken along the line shown in image (b).Communication ChemCommOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThis journal is©The Royal Society of Chemistry 2014 Chem. Commun.also demonstrated that the PTCDI–melamine network may beformed on this surface as shown in Fig. 4 (PTCDI on MoS2 haspreviously been studied by Ludwig et.al.26 using STM). A similarprotocol is used as for the formation on HOPG and hBN, but onthe MoS2 surface much smaller islands are formed. After adeposition time of 1 minute we see only islands of typicallateral dimensions of o100 nm. Increase of the immersiontime to 10 minutes leads to the formation of an extendedsupramolecular layer with a hexagonal symmetry and a latticeconstant of 3.5 " 0.1 nm consistent with the formation of thehydrogen-bonded structure in Fig. 1. We have also tried toimage this surface using STM but have not succeeded inacquiring images showing the molecular network.The most significant difference which we identify foradsorption on MoS2 lies in the dimensions of the network.This may be due to differences in chemistry; in particular thepresence of a layer of sulphur atoms at the surface may lead tostronger interactions with both environmental gaseous species,and also with the adsorbed molecules themselves. Nevertheless,our results confirm that the hydrogen-bonding which leads tothe stabilisation of the honeycomb network is effective on theMoS2 surface.The network co-exists with three dimensional features forwhich we cannot obtain molecular resolution; these may be dueto adsorption of molecules in a non-planar configuration, or,possibly, the formation of multilayers, and are present is slightlygreater concentration on the MoS2 surface. Possibly due to thesmaller islands and greater disorder observed on this surface, it ismore difficult to acquire images, and we have not been able toacquire adhesion maps with nm-scale MoS2 substrates (adhesionmaps for hBN are included in the ESI;† these show a similarcontrast to Fig. 2d). For all substrates the network structureremains sufficiently stable (when kept in a sealed vial filled withnitrogen) for further images to be obtained for over a week.In our study we have successfully demonstrated that hydrogen-bonded supramolecular networks can be readily deposited on thesurfaces of HOPG, hBN andMoS2. Importantly the structures maybe resolved using AFM using conventional micromachined canti-levers. The formation of the honeycomb PTCDI–melamine net-work and its stability under ambient conditions is particularlyimportant since this offers the prospect of integrating molecularrecognition and host–guest interactions with a family of layeredmaterials, including semimetals, semiconductors and insulators,which are currently receiving great attention as candidates for anew generation of optoelectronic devices.27,28Financial support was provided by the UK Engineering andPhysical Sciences Research Council (EPSRC; grants EP/K01773X/1and EP/K005138/1).Notes and references1 L. Bartels, Nat. Chem., 2010, 2, 87–95.2 T. Kudernac, S. Lei, J. A. A. W. Elemans and S. De Feyter, Chem. Soc.Rev., 2009, 38, 402–421.3 A. G. Slater (ne´e Phillips), P. H. Beton and N. R. Champness,Chem. Sci., 2011, 2, 1440.4 J. V Barth, J. Weckesser, C. Cai, P. Gu¨nter, L. Bu¨rgi, O. Jeandupeuxand K. Kern, Angew. Chem., Int. Ed., 2000, 1230–1234.5 S. B. Lei, C. Wang, S. X. Yin, H. N. Wang, F. Xi, H. W. Liu, B. Xu,L. J. Wan and C. L. Bai, J. Phys. Chem. B, 2001, 105, 10838–10841.6 S. Griessl, M. Lackinger, M. Edelwirth, M. Hietschold andW. M. Heckl, Single Mol., 2002, 3, 25–31.7 K. G. Nath, O. Ivasenko, J. A. Miwa, H. Dang, J. D. Wuest, A. Nanci, D. F.Perepichka and F. Rosei, J. Am. Chem. Soc., 2006, 128, 4212–4213.8 D. L. Keeling, N. S. Oxtoby, C. Wilson, M. J. Humphry, N. R. Champnessand P. H. Beton, Nano Lett., 2003, 3, 9–12.9 S. Stepanow, M. Lingenfelder, A. Dmitriev, H. Spillmann, E. Delvigne,N. Lin, X. Deng, C. Cai, J. V Barth and K. Kern, Nat. Mater., 2004, 3,229–233.10 A. Dmitriev, H. Spillmann, N. Lin, J. V Barth and K. Kern, Angew.Chem., Int. Ed., 2003, 42, 2670–2673.11 L. Gross, F. Mohn, N. Moll, P. Liljeroth and G. Meyer, Science, 2009,325, 1110–1114.12 P. Rahe, R. Lindner, M. Kittelmann, M. Nimmrich and A. Ku¨hnle,Phys. Chem. Chem. Phys., 2012, 14, 6544–6548.13 S. Burke, J. Mativetsky, S. Fostner and P. Gru¨tter, Phys. Rev. B, 2007,76, 035419.14 S. Maier, L.-A. Fendt, L. Zimmerli, T. Glatzel, O. Pfeiffer, F. Diederichand E. Meyer, Small, 2008, 4, 1115–1118.15 J. Zhang, P. Chen, B. Yuan, W. Ji, Z. Cheng and X. Qiu, Science, 2013,342, 611–614.Fig. 3 AFM scans of the PTCDI–melamine network on the hBN surface inair. Image (a) was acquired in AC mode on a Cypher AFM (AsylumResearch). Images (b) and (c) were taken in PeakForce Tapping Mode.Fig. 4 AFM images of PTCDI–melamine network on MoS2 surface after(a) 1 minute and (b) 10 minute adsorption times.ChemComm CommunicationOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineChem. Commun. This journal is©The Royal Society of Chemistry 201416 A. M. Sweetman, S. P. Jarvis, H. Sang, I. Lekkas, P. Rahe, Y. Wang,J. Wang, N. Champness, L. Kantorovich and P. Moriarty, Nat. Commun.,2014, 5, 3931.17 V. V. Korolkov, S. Allen, C. J. Roberts and S. J. B. Tendler, J. Phys.Chem. C, 2012, 116, 11519–11525.18 V. V. Korolkov, N. Mullin, S. Allen, C. J. Roberts, J. K. Hobbs andS. J. B. Tendler, Phys. Chem. Chem. Phys., 2012, 14, 15909.19 J. A. Theobald, N. S. Oxtoby, M. A. Phillips, N. R. Champness andP. H. Beton, Nature, 2003, 424, 1029–1031.20 L. M. A. Perdiga˜o, E. W. Perkins, J. Ma, P. A. Staniec, B. L. Rogers,N. R. Champness and P. H. Beton, J. Phys. Chem. B, 2006, 110,12539–12542.21 R. Madueno, M. T. Ra¨isa¨nen, C. Silien and M. Buck, Nature, 2008,454, 618–621.22 H. J. Karmel, T. Chien, V. Demers-Carpentier, J. J. Garramone andM. C. Hersam, J. Phys. Chem. Lett., 2014, 5, 270–274.23 I. Horcas, R. Ferna´ndez, J. M. Go´mez-Rodrı´guez, J. Colchero,J. Go´mez-Herrero and A. M. Baro, Rev. Sci. Instrum., 2007,78, 013705.24 C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei,K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard and J. Hone,Nat. Nanotechnol., 2010, 5, 722–726.25 M. Yankowitz, J. Xue, D. Cormode, J. D. Sanchez-Yamagishi,K. Watanabe, T. Taniguchi, P. Jarillo-Herrero, P. Jacquod andB. J. LeRoy, Nat. Phys., 2012, 8, 382–386.26 C. Ludwig, B. Gompf, J. Petersen, R. Strohmaier andW. Eisenmenger,Z. Phys. B, 1994, 93, 365–373.27 A. K. Geim and I. V. Grigorieva, Nature, 2013, 499, 419–425.28 T. Georgiou, R. Jalil, B. D. Belle, L. Britnell, R. V. Gorbachev,S. V. Morozov, Y.-J. Kim, A. Gholinia, S. J. Haigh, O. Makarovsky,L. Eaves, L. a. Ponomarenko, A. K. Geim, K. S. Novoselov andA. Mishchenko, Nat. Nanotechnol., 2013, 8, 100–103.Communication ChemCommOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online

A two-dimensional porous network formed from perylene tetracarboxylic diimide (PTCDI) and melamine may be deposited from solution on the surfaces of highly oriented pyrolytic graphite (HOPG), hexagonal boron nitride (hBN) and molybdenum disulphide (MoS2). Images acquired using high resolution atomic force microscopy (AFM) operating under ambient conditions have revealed that the network forms extended ordered monolayers (&gt;1 μm2) on HOPG and hBN whereas on MoS2 much smaller islands are observed

Roberts, Clive J

Tendler, Saul J. B.

University of Birmingham Research Portal

English

Bimolecular porous supramolecular networks deposited from solution on layered materials:graphite, boron nitride and molybdenum disulphide

Solution deposition and molecular resolution AFM of bimolecular supramolecular networks on layered materials including the insulating boron nitride surface.</p

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Chemical Communications

A two-dimensional porous network formed from perylene tetracarboxylic diimide (PTCDI) and melamine may be deposited fromsolution on the surfaces of highly oriented pyrolytic graphite (HOPG), hexagonal boron nitride (hBN) and molybdenum disulphide (MoS2). Images acquired using high resolution atomic force microscopy (AFM) operating under ambient conditions have revealed that the network forms extended orderedmonolayers (41 lm2) on HOPG and hBN whereas on MoS2 much smaller islands are observed

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This journal is©The Royal Society of Chemistry 2014 Chem. Commun.Cite this:DOI: 10.1039/c4cc03720kBimolecular porous supramolecular networksdeposited from solution on layered materials:graphite, boron nitride and molybdenumdisulphide†Vladimir V. Korolkov,a Simon A. Svatek,a Stephanie Allen,b Clive J. Roberts,bSaul J. B. Tendler,b Takashi Taniguchi,c Kenji Watanabe,c Neil R. Champnessd andPeter H. Beton*aA two-dimensional porous network formed from perylene tetra-carboxylic diimide (PTCDI) and melamine may be deposited fromsolution on the surfaces of highly oriented pyrolytic graphite(HOPG), hexagonal boron nitride (hBN) and molybdenum disulphide(MoS2). Images acquired using high resolution atomic force micro-scopy (AFM) operating under ambient conditions have revealed thatthe network forms extended orderedmonolayers (41 lm2) on HOPGand hBN whereas on MoS2 much smaller islands are observed.Coinage-metal surfaces and highly oriented pyrolytic graphite(HOPG) have been used in many of the studies of two-dimensional supramolecular organisation which have beenreported over the past decade.1–3 These conducting substrates arecompatible with scanning tunnelling microscopy (STM), and havestructural properties which are understood in great detail, allowingexperimental studies to focus on stabilisation of networks throughhydrogen bonding4–8 and metal co-ordination.9,10 An extension ofthese studies to other substrates, particularly non-conductingsurfaces, is highly desirable and would support investigations ofa much wider range of properties going beyond the determinationof structural and molecular organisation. The investigation ofsupramolecular networks on non-metallic substrates requires theuse of atomic force microscopy (AFM) rather than STM. Thepotential of AFM for the acquisition of high resolution images ofmolecular structure has been convincingly demonstrated underultra-high vacuum (UHV) conditions,11–14 including hydrogen-bonded systems,15,16 but to date AFM images of two-dimensionalsupramolecular organisation with molecular resolution have onlybeen acquired under ambient conditions for trimesic acid17 and,using a torsional tapping mode, the bimolecular network formedby PTCDI (perylene tetracarboxylic di-imide) and melamine;18 inthese studies HOPG and gold, respectively, were used as substrates.In this paper we show that, under ambient conditions, AFMimages of supramolecular networks may be acquired with aresolution which is similar to that achieved using STM underthe same conditions. A nanoporous honeycomb network formedfrom PTCDI and melamine network, previously studied on theAg/Si(111)-surface under UHV conditions,19 and on Au(111) firstunder UHV20 and then from solution18,21 was chosen for thisstudy. Recently the same network was formed on graphenemultilayers grown on SiC under UHV conditions.22 Here wecompare the deposition of PTCDI and melamine from solutionon three layered materials, HOPG, molybdenum disulphide(MoS2) and exfoliated flakes of hexagonal boron nitride (hBN).The molecular structures of PTCDI and melamine, togetherwith the hydrogen-bonded PTCDI–melamine junction and thehexagonal unit cell of the resulting supramolecular structureare shown in Fig. 1. Each melamine interacts with three PTCDImolecules via a total of nine hydrogen bonds. We deposit thenetworks by immersion of the substrate into solutions of themolecular components which, as previous studies have shownfor deposition on Au(111),18,21 leads to much larger domainsthan those obtained by UHV deposition.Images of the networks are acquired using AFM, which forthe layers deposited on graphite may be complemented by STM.Fig. 1 Chemical structure of (a) PTCDI, (b) melamine; (c) schematic ofPTCDI–melamine hydrogen bonded vertex, (d) unit cells of the derivedsupramolecular network.a School of Physics & Astronomy, The University of Nottingham,Nottingham NG7 2RD, UK. E-mail: peter.beton@nottingham.ac.ukb School of Pharmacy, The University of Nottingham, Nottingham, NG7 2RD, UKc The National Institute for Materials Science, Advanced Materials Laboratory,1-1 Namiki, Tsukuba, Ibaraki-305-0044, Japand School of Chemistry, The University of Nottingham, Nottingham NG7 2RD, UK† Electronic supplementary information (ESI) available: Additional images ofnetwork. See DOI: 10.1039/c4cc03720kReceived 15th May 2014,Accepted 22nd June 2014DOI: 10.1039/c4cc03720kwww.rsc.org/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView JournalChem. Commun. This journal is©The Royal Society of Chemistry 2014In this work we use, predominantly, PeakForce tapping modeAFM, which was recently introduced by Bruker. In this mode thetip oscillates at a frequency far below resonance (2 kHz) and aforce–distance curve is acquired at each pixel. The maximumforce detected during the approach cycle is used as a set pointwhich, in conjunction with a feedback loop, regulates the probeheight. The values for the set point are typically in the (repulsive)range of 1–10 pN, allowing for a very low force while imagingdelicate systems. As the force curve is recorded at every singlepixel it is also possible to determine the spatial variation ofmechanical properties, such as adhesion, of the array. AFMimages were acquired in ambient conditions using standardsilicon tips (Multi75Al, Multi75Al-G supplied by Budget sensors;typical spring constant, 2.8 N m!1). We have also used a CypherAFM (Asylum Research) to acquire images (in AC mode) of somesamples. STM imaging was performed utilizing tips cut fromPt/Ir (20% Ir) wire. WsXM V5.0 software was used to process andanalyse all AFM images.23The PTCDI–melamine network was prepared via adsorptionin a dimethylformamide (DMF) solution at 373 K.18,21 Themelamine and PTCDI concentrations were 0.66 mM andB0.5 mM respectively. Each sample was rinsed with 5 ml offresh DMF and blown dry in N2-stream. The same protocol wasused for all substrates.Fig. 2 shows AFM images following molecular deposition onHOPG (purchased from NT-MDT (20 minute immersion time)).The AFM images show clearly that a regular hexagonal frameworkis formed on the HOPG surface with the structure and dimensionsexpected for the PTCDI–melamine network shown in Fig. 1.Fig. 2a shows a hexagonal array of nanopores in a single domainwhich extends over 250 nm (in larger area images we observeislands with dimensions up to 0.5 mm; see ESI†). The size of thesedomains, and the fraction of the surface covered (490%) by thenetwork is much greater than that achieved in previous UHVstudies,19,20,22 but is comparable to those previously reported forsolution deposition on Au(111).18,21 Themeasured lattice constant is3.5 " 0.1 nm, very close to the values of 3.46 nm and 3.5 nmreported for adsorption on Ag/Si(111)19 and Au(111)20,21 surfaces. Aheight profile extracted from a higher magnification image (Fig. 2b)is shown in the inset; the topographic height of the molecules isB0.1 nm. Fig. 2c shows, for comparison, an STM image of a surfaceprepared using the same (highly reproducible) protocol. Interest-ingly, there is very little difference between the resolution and imagequality using these two modalities (see ESI† for additional images).Fig. 2d shows an adhesion map which is extracted from theforce–distance curves measured during the acquisition of Fig. 2b;the adhesion is defined as themaximum attractive force during theretraction of the cantilever. This image shows stronger adhesion atthe centres of the hexagonal cavities formed in the array, where theHOPG surface is exposed, with the network structure appearing asa dark honeycomb mesh (darker features correspond to loweradhesion). Similar behaviour was observed for trimesic acid mono-layers adsorbed on HOPG.17 At this stage a quantitative interpreta-tion, which requires detailed knowledge of the probe radius, is notpossible. However, we include these observations to highlight thepotential for AFM to provide additional modalities in future studiesof host–guest interactions in supramolecular networks.We have also successfully deposited the PTCDI–melaminenetwork on hBN. These surfaces are prepared by exfoliatinghBN flakes from small mm-sized crystals grown at the NationalInstitute for Materials Science, and transferring them to a siliconwafer on which a 300 nm thick oxide layer has been grown. Theflakes are then heated in an Ar/H2 (95 : 5) atmosphere at 400 1C for4–6 hours to remove organic residues. This process is very similarto that used by other researchers to prepare hBN substrates foruse in graphene research.24,25 hBN is a layered compound with astructure similar to graphite, except that the two atom carbonbasis set is replaced by two heteroatoms, boron and nitrogen. As alarge band-gap semiconductor hBN is not suitable for study withSTM and all images were acquired using AFM.The PTCDI–melamine network could be readily formed onthis substrate by immersion in the solution described above for1 minute; this was found to be sufficient to grow an extendedmonolayer of the network with typical islands with widthsB0.3 mm(see ESI†). Fig. 3a shows a large area image in which the honeycomblattice is clearly resolved and we determine the lattice constant as3.5" 0.1 nm, the same value, within experimental error as observedfor the network onHOPG. Higher magnification images acquired inPeakForce tapping mode which show the honeycomb network arepresented in Fig. 3b and c. In both Fig. 3a and b there are darkerfeatures with facetted edges which appear within the molecularnetwork. These are defects in the molecular layer and tend to growduring the acquisition of AFM images in AC mode, so that afterprolonged imaging the network becomes discontinuous.MoS2 is a layered semiconductor currently attracting interestdue to its potential for integration with graphene.27 We haveFig. 2 AFM and STM scans of a PTCDI–melamine network on HOPG inair. Images (a) and (b) are topographic AFM scans. Image (c) is a constantcurrent STM image acquired at I = 5 pA and sample voltage 0.5 V. Image (d)shows adhesion distribution of the upper half of the area in (b). Inset onimage (a) shows cross-section taken along the line shown in image (b).Communication ChemCommOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThis journal is©The Royal Society of Chemistry 2014 Chem. Commun.also demonstrated that the PTCDI–melamine network may beformed on this surface as shown in Fig. 4 (PTCDI on MoS2 haspreviously been studied by Ludwig et.al.26 using STM). A similarprotocol is used as for the formation on HOPG and hBN, but onthe MoS2 surface much smaller islands are formed. After adeposition time of 1 minute we see only islands of typicallateral dimensions of o100 nm. Increase of the immersiontime to 10 minutes leads to the formation of an extendedsupramolecular layer with a hexagonal symmetry and a latticeconstant of 3.5 " 0.1 nm consistent with the formation of thehydrogen-bonded structure in Fig. 1. We have also tried toimage this surface using STM but have not succeeded inacquiring images showing the molecular network.The most significant diﬀerence which we identify foradsorption on MoS2 lies in the dimensions of the network.This may be due to diﬀerences in chemistry; in particular thepresence of a layer of sulphur atoms at the surface may lead tostronger interactions with both environmental gaseous species,and also with the adsorbed molecules themselves. Nevertheless,our results confirm that the hydrogen-bonding which leads tothe stabilisation of the honeycomb network is eﬀective on theMoS2 surface.The network co-exists with three dimensional features forwhich we cannot obtain molecular resolution; these may be dueto adsorption of molecules in a non-planar configuration, or,possibly, the formation of multilayers, and are present is slightlygreater concentration on the MoS2 surface. Possibly due to thesmaller islands and greater disorder observed on this surface, it ismore diﬃcult to acquire images, and we have not been able toacquire adhesion maps with nm-scale MoS2 substrates (adhesionmaps for hBN are included in the ESI;† these show a similarcontrast to Fig. 2d). For all substrates the network structureremains suﬃciently stable (when kept in a sealed vial filled withnitrogen) for further images to be obtained for over a week.In our study we have successfully demonstrated that hydrogen-bonded supramolecular networks can be readily deposited on thesurfaces of HOPG, hBN andMoS2. Importantly the structures maybe resolved using AFM using conventional micromachined canti-levers. The formation of the honeycomb PTCDI–melamine net-work and its stability under ambient conditions is particularlyimportant since this offers the prospect of integrating molecularrecognition and host–guest interactions with a family of layeredmaterials, including semimetals, semiconductors and insulators,which are currently receiving great attention as candidates for anew generation of optoelectronic devices.27,28Financial support was provided by the UK Engineering andPhysical Sciences Research Council (EPSRC; grants EP/K01773X/1and EP/K005138/1).Notes and references1 L. Bartels, Nat. Chem., 2010, 2, 87–95.2 T. Kudernac, S. Lei, J. A. A. W. Elemans and S. De Feyter, Chem. Soc.Rev., 2009, 38, 402–421.3 A. G. Slater (ne´e Phillips), P. H. Beton and N. R. Champness,Chem. Sci., 2011, 2, 1440.4 J. V Barth, J. Weckesser, C. Cai, P. Gu¨nter, L. Bu¨rgi, O. Jeandupeuxand K. Kern, Angew. Chem., Int. Ed., 2000, 1230–1234.5 S. B. Lei, C. Wang, S. X. Yin, H. N. Wang, F. Xi, H. W. Liu, B. Xu,L. J. Wan and C. L. Bai, J. Phys. Chem. B, 2001, 105, 10838–10841.6 S. Griessl, M. Lackinger, M. Edelwirth, M. Hietschold andW. M. Heckl, Single Mol., 2002, 3, 25–31.7 K. G. Nath, O. Ivasenko, J. A. Miwa, H. Dang, J. D. Wuest, A. Nanci, D. F.Perepichka and F. Rosei, J. Am. Chem. Soc., 2006, 128, 4212–4213.8 D. L. Keeling, N. S. Oxtoby, C. Wilson, M. J. Humphry, N. R. Champnessand P. H. Beton, Nano Lett., 2003, 3, 9–12.9 S. Stepanow, M. Lingenfelder, A. Dmitriev, H. Spillmann, E. Delvigne,N. Lin, X. Deng, C. Cai, J. V Barth and K. Kern, Nat. Mater., 2004, 3,229–233.10 A. Dmitriev, H. Spillmann, N. Lin, J. V Barth and K. Kern, Angew.Chem., Int. Ed., 2003, 42, 2670–2673.11 L. Gross, F. Mohn, N. Moll, P. Liljeroth and G. Meyer, Science, 2009,325, 1110–1114.12 P. Rahe, R. Lindner, M. Kittelmann, M. Nimmrich and A. Ku¨hnle,Phys. Chem. Chem. Phys., 2012, 14, 6544–6548.13 S. Burke, J. Mativetsky, S. Fostner and P. Gru¨tter, Phys. Rev. B, 2007,76, 035419.14 S. Maier, L.-A. Fendt, L. Zimmerli, T. Glatzel, O. Pfeiﬀer, F. Diederichand E. Meyer, Small, 2008, 4, 1115–1118.15 J. Zhang, P. Chen, B. Yuan, W. Ji, Z. Cheng and X. Qiu, Science, 2013,342, 611–614.Fig. 3 AFM scans of the PTCDI–melamine network on the hBN surface inair. Image (a) was acquired in AC mode on a Cypher AFM (AsylumResearch). Images (b) and (c) were taken in PeakForce Tapping Mode.Fig. 4 AFM images of PTCDI–melamine network on MoS2 surface after(a) 1 minute and (b) 10 minute adsorption times.ChemComm CommunicationOpen Access Article. Published on 23 June 2014. Downloaded on 07/07/2014 13:05:09.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineChem. Commun. This journal is©The Royal Society of Chemistry 201416 A. M. Sweetman, S. P. Jarvis, H. Sang, I. Lekkas, P. Rahe, Y. Wang,J. Wang, N. Champness, L. Kantorovich and P. Moriarty, Nat. Commun.,2014, 5, 3931.17 V. V. Korolkov, S. Allen, C. J. Roberts and S. J. B. Tendler, J. Phys.Chem. 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Bimolecular porous supramolecular networks deposited from solution on layered materials: graphite, boron nitride and molybdenum disulphide

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