Thin SiO 2 films were grown on a Ru(0001) single crystal and studied by photoelectron spectroscopy, infrared spectroscopy and scanning probe microscopy. The experimental results in combination with density functional theory calculations provide compelling evidence for the formation of crystalline, double-layer sheet silica weakly bound to a metal substrate. DOI: 10.1103/PhysRevLett.105.146104 PACS numbers: 68.35.Àp, 68.47.Gh, 68.55.Àa Silicon dioxide (SiO 2 ) plays a key role in many modern technologies and applications that range from insulating layers in integrated circuits to supports for metal and oxide clusters in catalysts. For better understanding of structureproperty relationships on silica-based materials, particularly of reduced dimensions, thin silica films grown on metal single crystal substrates are suggested as suitable model systems that allow the facile application of many ''surface science'' techniques. It has recently been shown that crystalline silica films and nanowires can be grown on Mo(112) [1][2][3][4][5]. The ultrathin film consists of a monolayer honeycomblike network of corner-sharing [SiO 4 ] tetrahedra, thus resulting in a SiO 2:5 stoichiometry of the film. The Si atoms in these films can be partly substituted by Al in the course of preparing metal supported aluminosilicate films [6], which is the first step towards experimental modeling of catalytic centers in zeolitelike materials. However, attempts to grow thicker silica films on the Mo substrates resulted in amorphous structures [7][8][9], most likely due to the formation of strong Si-O-Mo bonds at the interface that govern the growth mode [9]. Recently, the preparation of crystalline silica films on other supports such as Pd(100) [10] and Ni(111) [11] has been reported. However, the atomic structure of the films, film surface termination, and the nature of the silica-metal interface were not determined.In this Letter, we report on the preparation and the atomic structure of well-defined silica films on Ru(0001). The experimental results, obtained by photoelectron and vibrational spectroscopies and high-resolution scanning probe microscopy, are complemented by density functional theory calculations which together provide compelling evidence for the formation of a double-layer sheet silicate, with a SiO 2 stoichiometric composition, weakly bound to a metal support. The results open new perspectives for employing a ''surface science'' approach to understand the reactivity of silicate surfaces consisting of hydrophobic Si-O-Si bonds, such as those of microporous all-silica zeolites [12]. Also, these films can be used as model supports for catalytically active metal and oxide clusters [4,13].The experiments were performed in an ultrahigh vacuum chamber equipped with low energy electron diffraction (LEED) and Auger electron spectroscopy, x-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), and scanning tunneling microscopy (STM). Atomically resolved atomic force microscopy (AFM) and STM image...

Loading with guest molecules is a crucial step for most applications of porous materials. For metal-organic frameworks, which are one of the most intensely investigated classes of porous materials, the experimentally determined rate of mass transfer into the material may vary by several orders of magnitude for different samples of the same material. This phenomenon is commonly attributed to the presence of so-called surface barriers, which appear to be omnipresent but poorly understood. Here we quantitatively study this phenomenon with a quartz crystal microbalance, using well-defined, highly crystalline, epitaxially grown thin films of metal-organic frameworks as a model system. Our results clearly demonstrate that surface barriers are not an intrinsic feature of metal-organic frameworks, as pristine films do not exhibit these limitations. However, by destroying the structure at the outer surface, for instance by exposure to air or water vapour, surface barriers are created and the molecular uptake rate is reduced.

Metal-organic frameworks offer tremendous potential for efficient separation of molecular mixtures. Different pore sizes and suitable functionalizations of the framework allow for an adjustment of the static selectivity. Here we report membranes which offer dynamic control of the selectivity by remote signals, thus enabling a continuous adjustment of the permeate flux. This is realized by assembling linkers containing photoresponsive azobenzene-side-groups into monolithic, crystalline membranes of metal-organic frameworks. The azobenzene moieties can be switched from the trans to the cis configuration and vice versa by irradiation with ultraviolet or visible light, resulting in a substantial modification of the membrane permeability and separation factor. The precise control of the cis:trans azobenzene ratio, for example, by controlled irradiation times or by simultaneous irradiation with ultraviolet and visible light, enables the continuous tuning of the separation. For hydrogen:carbon-dioxide, the separation factor of this smart membrane can be steplessly adjusted between 3 and 8.

In recent years, a particularly attractive and novel strategy has become available for the fabrication of photoresponsive, porous, and crystalline molecular solids from appropriately functionalized chromophoric linkers. The crystallinity of these materials allows a rather straightforward description and analysis using theoretical methods, thus tremendously accelerating the design of novel materials. This new class of crystalline molecular solids is referred to as metal-organic frameworks, MOFs (or porous coordination polymers, PCPs). [1] MOFs are constructed from metal-/metal-oxo nodes and organic linkers (Figure 1). The incorporation of photoactive species into MOFs may be realized by using them as linkers (method L) or attaching them to a linker (method A), as shown in Figure 1a. In addition, the porosity of MOFs allows the loading of chromophoric compounds as guests (method G) into the pores of this interesting framework material.In this review article, we mainly focus on organic photoactive species, which are either simply loaded as guests into porous MOFs or used after appropriate functionalization as building blocks for the construction of the framework (methods L, A, and G). [2] It is important to note that in the context of photoresponsive behavior, MOFs carry a potential which by far exceeds that of nonporous coordination polymers. This is because simple loading of guest/solvent molecules of different size/polarity/functionality in the MOF pores can impart a large optical response, which is useful, e.g., for sensing applications. [3] As an example, the solvent-dependent optical response or solvatochromism [4] is illustrated in Figure 1b. Such effects cannot be realized for nonporous coordination polymers.The different types of photoresponsive molecules addressed in this review can be grouped into two classes. The first contains molecules where the structure remains essentially unchanged upon light-induced electronic excitation, and the second contains molecular species that change their structure or conformation upon absorption of photons (molecular switches).Light with a wavelength in the range 200-800 nm (6.2-1.5 eV) can excite a molecule to a transient excited electronic state. The transient state then decays to a low-energy state, either the parent ground state or another longer-lived excited state. Decay time scales are in the range 10 −12 -10 1 s, and the released energy can be radiative or nonradiative in nature. For many applications, nonradiative energy loss is unwanted, When fabricating macroscopic devices exploiting the properties of organic chromophores, the corresponding molecules need to be condensed into a solid material. Since optical absorption properties are often strongly affected by interchromophore interactions, solids with a well-defined structure carry substantial advantages over amorphous materials. Here, the metal-organic framework (MOF)-based approach is presented. By appropriate functionalization, most organic chromophores can be converted to function as linkers, which can coo...

Nanoporous solids are attractive materials for energetically efficient and environmentally friendly catalytic and adsorption separation processes. Although the performance of such materials is largely dependent on their molecular transport properties, our fundamental understanding of these phenomena is far from complete. This is particularly true for the mechanisms that control the penetration rate through the outer surface of these materials (commonly referred to as surface barriers). Recent detailed sorption rate measurements with Zn(tbip) crystals have greatly enhanced our basic understanding of such processes. Surface resistance in this material has been shown to arise from the complete blockage of most of the pore entrances on the outer surface, while the transport resistance of the remaining open pores is negligibly small. More generally, the revealed correlation between intracrystalline diffusion and surface permeation provides a new view of the nature of transport resistances in nanoporous materials acting in addition to the diffusion resistance of the regular pore network, leading to a rational explanation of the discrepancy which is often observed between microscopic and macroscopic diffusion measurements.