ARTICLES PUBLISHED ONLINE: 1 NOVEMBER 2009 | DOI: 10.1038/NMAT2567 Nanostructured arrays of semiconducting octahedral molecular sieves by pulsed-laser deposition Anais E. Espinal 1 , Lichun Zhang 2 , Chun-Hu Chen 3 , Aimee Morey 3 , Yuefeng Nie 4 , Laura Espinal 1 , Barrett O. Wells 4 , Raymond Joesten 3 , Mark Aindow 1,2 and Steven L. Suib 1,2,3 * Cryptomelane-type manganese oxide (OMS-2) has been widely used to explore the semiconducting and catalytic properties of molecular sieves with mixed-valent frameworks. Selective synthesis of patterned thin films of OMS-2 with hierarchical nanostructures and oriented crystals is challenging owing to difficulties in preserving the mixed valence, porosity and crystalline phase. Here, we report that pulsed-laser ablation of OMS-2 in an oxygen-rich medium produces a three-dimensional nanostructured array of parallel and inclined OMS-2 fibres on bare substrates of (001) single-crystal strontium titanate. Both parallel and inclined OMS-2 fibres elongate along the [001] OMS-2 direction. The parallel fibres interact strongly with the substrate and grow epitaxially along 〈110〉 STO with lattice misfits of less than 4%, whereas the inclined fibres are oriented with (301) parallel to the substrate surface. The spontaneous orientation of the crystalline OMS-2 domains over the STO surface opens up a new avenue in lattice-engineered synthesis of multilayer materials. P orous materials, such as molecular sieves and zeolites, show exceptionally useful properties in catalysis, petroleum re- fining, gas cleansing, separation, membranes and chemical sensors 1–3 , owing to their stable frameworks composed of micro, meso and macroporous tunnels that selectively allow access to specific ions, molecules and clusters. Controlled fabrication of highly oriented three-dimensional (3D) films in which functional crystal surfaces are more accessible further enhances and op- timizes their already novel functionalities 2 as in membranes 3,4 , photocatalysts 5 and sensors 5 . Conventional methods for preparing oriented films involve lengthy liquid-phase chemistry approaches or hydrothermal treat- ments that require the use of templates or seeds to direct crystallization 3–5 . Cancrinite 6 and chabazite 7 have been reported to grow epitaxially on sodalite using hydrothermal treatment for the formation of the zeolite film. Thin films of other zeolites 8–10 and molecular sieves 11,12 have been prepared by combining pulsed-laser deposition (PLD) for the seeding step with hydrothermal treatment for the formation of the film. Amorphous titanium oxide has been grown in a nanocolumnar array by PLD, but such a structure required a polystyrene colloidal monolayer as a template 5 . Other procedures require indirect methods of deposition that involve in situ decomposition of target compounds through laser ablation that induce deposition of the actual desired material. Thin films of La 0.5 Sr 0.5 FeO 3 fibres perpendicular to strontium titanate (SrTiO 3 , STO) substrates have been prepared by PLD through decompo- sition of a La 0.5 Sr 0.5 FeO 3 target 13 , although both the target and substrate materials have the same dense perovskite structure. The development of procedures that produce 3D structures of porous materials by means of self-assembly (seedless) and direct deposition from a target of the corresponding material would facilitate scale-up and improve cost-effectiveness. Cryptomelane-type manganese oxide is the most extensively studied of the manganese oxide octahedral molecular sieves (OMS) owing to its excellent catalytic activity and semiconductor 1 Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136, USA, 2 Chemical, Materials & Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269-3222, USA, 3 Chemistry Department, University of Connecticut, Storrs, Connecticut 06269-3060, USA, 4 Department of Physics, University of Connecticut, Storrs, Connecticut 06269-3046, USA. *e-mail:Steven.Suib@uconn.edu. properties 14 . The oxide is designated OMS-2 and has a composition of KMn 4+ 7 Mn 3+ O 16 , where the charge imbalance on the octahedral framework owing to reduction of Mn 4+ to Mn 3+ is balanced by K + in one dimensional tunnels along the c axis. This mixed valence in the structure is what gives OMS-2 materials their good semiconducting properties 15 . A myriad of synthetic methods and morphologies have been reported for OMS-2 (ref. 16). Although oriented films of OMS-2 would be highly desirable for numerous applications, preparation of such films has proven to be very challenging owing to the difficulty in maintaining the mixed valence, porosity and crystal phase of the cryptomelane structure. To address the need for the development of highly oriented thin films of OMS-2 with a 3D nanostructured architecture, we have used a method that involves deposition of OMS-2 onto single-crystal (001)STO by means of self-assembly using direct laser ablation of a target made of OMS-2 powder material. STO has the cubic perovskite structure (a = 0.391 nm) and is an excellent substrate for the epitaxial growth of high-temperature superconductors and many other oxide-based thin films 17 . PLD was used in this work owing to its ability to produce thin films of a wide variety of complex oxides in which the stoichiometry of the target is replicated in the film. This technique uses a high-energy ultraviolet laser, with laser pulses as short as 10–15 fs, capable of ablating a thin surface layer from a target while leaving the remainder of the target virtually unheated 18 . The present work shows the direct formation of a nanostructured array of OMS-2 with an open architecture of parallel and inclined fibres, which provide more surface accessibility both for sensing applications and for the deposition of other functional materials. In a typical experiment, a single-crystal (001)STO substrate is placed in the PLD chamber and heated at a rate of 20 ◦ C min −1 up to 600 ◦ C under a vacuum of ∼1 × 10 −6 torr. Then a KrF excimer laser (wavelength = 248 nm, pulselength = 20 ns) is used to ablate the OMS-2 target material using a partial pressure of oxygen of ∼200 mtorr for the desired deposition time. At the end of the 54 NATURE MATERIALS | VOL 9 | JANUARY 2010 | www.nature.com/naturematerials © 2010 Macmillan Publishers Limited. All rights reserved.