Single layer growth of sub-micron metal–organic framework crystals observed by in situ atomic force microscopyw Neena S. John,z a Camilla Scherb,z b Maryiam Sho¨ aˆ ee`,z a Michael W. Anderson, a Martin P. Attﬁeld* a and Thomas Bein* b Received (in Cambridge, UK) 27th April 2009, Accepted 12th August 2009 First published as an Advance Article on the web 7th September 2009 DOI: 10.1039/b908299a In situ atomic force microscopy was used to directly investigate the growth processes of the oriented metal–organic framework HKUST-1 grown on self-assembled monolayers on gold. Crystalline nanoporous metal–organic frameworks (MOFs), constructed from metal ions and organic linkers, oﬀer vast potential for the design of porous materials with molecularly selective interfaces, novel physical properties, enormous surface areas and a diverse array of functionality. 1 The metamorphosis of MOFs to functional materials necessitates a detailed understanding of their crystal growth to produce desirable crystal forms, for instance defect-free crystallites or single crystal ﬁlms, that will open new avenues for application. Atomic force microscopy (AFM) has emerged as a powerful tool to investigate such crystal growth and has been applied to a wide variety of crystalline materials including nanoporous inorganic 2 and more recently hybrid materials. 3 In particular, in situ AFM is an essential technique to provide deﬁnitive real-time evidence about crystal growth including the mechanism, rates, fundamental growth units, and the eﬀect of the surface form, supersaturation and impurities on growth. 3d,4 Crucial fundamental knowledge can often only be gained from in situ AFM measurements if individual processes can be monitored without inﬂuence from other growing entities. This is attainable if additional surface nucleation is restricted through use of small relatively defect-free crystals. Growing such crystals and anchoring them during in situ AFM measurements is non-facile. Previously, we have used in situ AFM to provide important information concerning the crystal growth of a large single crystal of the copper trimesate Cu 3 (C 9 H 3 O 6 ) 2 (H 2 O) 3 HKUST-1 5 with a high surface defect concentration and multiple surface nucleation sites. 3d However, extraction of information con- cerning the individual growth processes was limited due to fast growth rates and the inﬂuence of multiple growing features on the crystal surface. Here we report, for the ﬁrst time, the direct observation of single layer growth on a monolayer-supported low-defect HKUST-1 crystal by in situ AFM. HKUST-1 is a signiﬁcant crystalline nanoporous MOF 6 built from Cu 2 (H 2 O) 2 units and benzene-1,3,5-tricarboxylate (BTC) groups to form a cubic framework with a three- dimensional nanoporous channel system (see Fig. S1, ESIw). HKUST-1 crystals were grown under ambient conditions in an oriented manner on gold substrates functionalized with self-assembled monolayers (SAMs). 7 They provide a unique platform for in situ AFM studies since the crystals are ﬁrmly anchored by direct attachment to a gold-coated glass substrate, but more importantly, the orientation of the crystals can be tuned by using diﬀerent functional groups for surface functionalization, such that the growth of the {111} face can be monitored directly (Scheme 1). High-resolution ex situ AFM images of a similar system only have been reported. 3c The high-resolution AFM deﬂection images of the growing {111} facet of a ca. 600 nm sized crystal of HKUST-1 as a function of time are presented in Fig. 1 (see Fig. S3 (ESIw) for additional micrographs). The crystal growth could be clearly monitored 56 min after injection of the growth solution. The image at 56 min (Fig. 1a) reveals an extremely ﬂat and relatively defect-free crystal surface, compared to the crystal surfaces observed in our previous studies, 3b,d exemplifying the utility of this synthetic protocol to produce high quality crystal surfaces. In the subsequent images, growth of the surface is seen to proceed by a two-dimensional (2D) crystal growth mechanism 8 in which each new crystal layer nucleates at the same point on the crystal surface, indicated by an asterisk in Fig. 1b. A defect on the crystal edge that is beyond the resolution limits of the microscope is likely to be present at this point. The growth steps emerge from this single nucleation point and spread across the entire exposed crystal face. Scheme 1 Attachment of copper and BTC species during terrace growth on a {111} face (d 111 steps are shown) of octahedral HKUST-1 crystals attached to 16-mercapto-1-hexadecanol-SAM on a gold substrate in [111] orientation (see Fig. S2 (ESIw) for X-ray data of oriented HKUST-1 crystals). a School of Chemistry, The University of Manchester, Brunswick Street, Manchester, UK M13 9PL. E-mail: m.attﬁeld@manchester.ac.uk; Fax: +44 (0)161-275-4598 b Department for Chemistry and Biochemistry, University of Munich, Butenandtstr. 11, 81377 Munich, Germany. E-mail: bein@lmu.de; Fax: +49 89 2180 77622; Tel: +49 89 2180 77621 w Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/b908299a z These authors contributed equally to the work. 6294 | Chem. Commun., 2009, 6294–6296 This journal is  c The Royal Society of Chemistry 2009 COMMUNICATION www.rsc.org/chemcomm | ChemComm Published on 07 September 2009. 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