Martin Steinhart | Universität Osnabrück (original) (raw)

Papers by Martin Steinhart

Chemistry of Materials, 2007

Ordered mesoporous materials 1,2 combine a large specific surface area with well-defined pore geo... more Ordered mesoporous materials 1,2 combine a large specific surface area with well-defined pore geometry and have been used in energy storage, energy conversion, separation, and catalysis. 3 Their synthesis is based on the self-assembly of block copolymers and surfactants. Whereas the mesopore arrays thus formed may show a well-developed local order, the control over shape and arrangement of the mesoporous entities on a macroscopic scale remains a challenge. A strategy that addresses these problems involves the synthesis of mesoporous silica nanowires inside the pores of porous alumina, 4,5 thus adapting an established method for the preparation of one-dimensional nanostructures based on the use of porous templates. 6 It is highly desirable to apply this concept to amorphous mesoporous carbon, 3,7-9 which exhibits superior resistance to acids and bases, excellent heat resistance, and high intrinsic electrical conductivity compared to mesoporous silica. Potential applications for hybrid membranes consisting of mesoporous carbon within porous alumina include size-selective electrosorption, electrosynthesis of nanostructures, catalysis, separation, and storage. Moreover, released mesoporous carbon microwires and nanowires may be used as lightweight functional filler material that allows tuning of the mechanical and electrical properties of nanocomposites.

Advanced Materials, 2006

Ordered mesoporous materials fabricated by exploiting self-assembled surfactants as molecular tem... more Ordered mesoporous materials fabricated by exploiting self-assembled surfactants as molecular templates have been intensively investigated. However, their integration into device architectures still remains a challenge. Recent efforts have focused on the synthesis of MCM (Mobil Crystalline Material) and SBA (Santa Barbara amorphous) type materials inside porous supports characterized by straight, aligned pores oriented normal to the surface. For this purpose, porous alumina matrices, either ordered or disordered, have predominantly been employed. The hybrid systems thus obtained are promising components for device architectures in the fields of catalysis and separation, where macroscopic membranes that consist of aligned pores with diameters of a few nanometers and high aspect ratios are required. The mesoporous rods can be released by a wet-chemical-etching step that destroys the alumina matrix. When the solvent of the suspension thus obtained is evaporated, capillary forces that act between the mesoporous rods result in the occurrence of large aggregates in otherwise disordered powders.

Angewandte Chemie-international Edition, 2006

Chemistry of Materials, 2008

An emerging approach toward the integration of mesoporous materials into functional device archit... more An emerging approach toward the integration of mesoporous materials into functional device architectures is based on the self-assembly of amphiphilic block copolymers (BCPs) inside hard templates containing arrays of aligned cylindrical nanochannels. [1][2][3][4][5][6] The morphology defined by the soft template is fixated by cross-linking precursors for silica or amorphous carbon after their selective segregation to the hydrophilic domains. After gelation, organic components are removed by calcination, and mesoporous fibers consisting of silica 1-5 or amorphous carbon 6 are obtained. However, solvent evaporation and high-temperature calcination lead to substantial volume shrinkage. The resulting inhomogeneous filling of the hard templates with the mesoporous scaffold is a serious drawback for any application requiring mechanically stable membranes with uniform mesopore structure.

Advanced Materials, 2009

Block copolymers (BCPs) self-assemble into ordered arrays of nanoscopic domains, the nature of wh... more Block copolymers (BCPs) self-assemble into ordered arrays of nanoscopic domains, the nature of which depends on the constituents of the BCPs and their molecular architecture. [1] BCPs have been exploited as precursors for nanoporous materials and as templates for the rational design of nanoscopic architectures with periods from below 10 nm up to the 100-nm-range in thin-film configurations. [3] Whereas molds containing arrays of aligned cylindrical nanochannels with hard confining walls, such as self-ordered anodic aluminum oxide (AAO), [4] have been used to fabricate one-dimensional nanostructures from a plethora of materials, the self-assembly of BCPs in cylindrical confinement has only recently emerged as an access to nanorods exhibiting ordered nanoscopic fine structures. Sol-gels containing precursors for inorganic scaffold materials and BCPs as structuredirecting soft templates were infiltrated into AAO. Subsequent gelation and calcination yielded nanorods consisting of various inorganic oxides and amorphous carbon exhibiting ordered mesoporous fine structures. [7] However, direct infiltration of microphase-separated BCP melts into AAO [8] has received much less attention, mainly because of a lack of obvious applications of the solid BCP nanorods thus obtained. Only recently, BCP nanorods have been converted into mesoporous polymeric nanorods by degrading sacrificial blocks and by selective swelling. BCPs confined to AAO with pore diameters about one order of magnitude larger than their characteristic bulk periods have been reported to retain bulk-like morphologies, such as cylinders oriented along the pore axes and concentric-cylindrical lamellae.

Journal of Materials Research, 2006

Lead titanate (PbTiO 3 ) nano-and microtubes were fabricated by wetting ordered porous alumina an... more Lead titanate (PbTiO 3 ) nano-and microtubes were fabricated by wetting ordered porous alumina and macroporous silicon with precursor oligomers coupled with templated thermolysis. The diameters of the PbTiO 3 tubes range from a few tens of nanometers up to one micron. The proper selection of the template allowed for a precise adjustment of their size over two orders of magnitude. Electron microscopy and x-ray diffraction revealed that the tube walls were polycrystalline. The generic approach presented here can be adapted for the fabrication of tubes and rods from a multitude of functional inorganic oxides.

Chemistry of Materials, 2006

Nanostructured semiconductor materials are of great interest because of their novel physical, che... more Nanostructured semiconductor materials are of great interest because of their novel physical, chemical, optical, and optoelectronic properties, 1 as well as their applications in fabricating nanoscale electronic, photonic, electromechanical, and biomedical devices. 2-5 Cadmium selenide (CdSe)

Polymer Engineering and Science, 2001

The aim was to prepare fibers with diameters below the micrometer range characterized by specific... more The aim was to prepare fibers with diameters below the micrometer range characterized by specific bulk morphologies and surface topologies. Such materials are of interest for various applications including reinforcement, sensors or filtration as well as the formation of functional tubes by the use of fiber templates. We were able to manufacture highly structured submicrometer fibers by electrospinning from ternary solutions using polylactide (PLA) and polyvinylpyrrolidone (PVP) as polymer model components. Co-continuous phase morphologies resulted from phase separation processes taking place during fiber formation. In a subsequent step, specific surface topologies or fine pores were generated by selective removal of one of the components.

Chemistry of Materials, 2005

Lithium niobate (LiNbO 3 ) has been used for all-optical wavelength conversion 1 and ultrafast op... more Lithium niobate (LiNbO 3 ) has been used for all-optical wavelength conversion 1 and ultrafast optical signal processing 2-5 because of its outstandingly rapid nonlinear optical response behavior, low switching power, and broad conversion bandwidth. Periodically poled photonic crystals 6 and inverse artificial opals made of LiNbO 3 7 have been reported. Ordered macroporous silicon (Si) 8-10 is an extensively investigated two-dimensional photonic band gap material. Here we report on the preparation of hybrid systems consisting of LiNbO 3 and macroporous Si. Such composites are of considerable interest because of the integration of LiNbO 3 into silicon photonics. For instance, the band gap of macroporous Si could be tuned by poling LiNbO 3 and thus changing its dielectric constant. Moreover, we investigated whether released LiNbO 3 tubes are accessible using macroporous Si as a sacrificial template. We have adapted a methodology involving the use of porous matrixes as molds, as investigated by Martin 11-13 and others. 14-20 We dropped a 5% solution of lithium [tetraethoxy(1-phenyl-1,3butanedione)niobate] (Scheme 1) in ethanol at ambient conditions on macroporous Si with a pore diameter, D p , of 1 µm. The synthesis and the properties of this single-source precursor for LiNbO 3 are described elsewhere. 21 The Si substrate into which the macropores were etched was a single-crystalline (100)-oriented wafer. We removed the residual liquid from the template surface. As the solvent evaporates, a layer of adsorbed molecules remains at the pore walls, which can be converted to the target compound by thermolysis. This procedure may be repeated once or several times. The conversion to crystalline LiNbO 3 was performed by annealing the wetted templates at 550, 650, and 750°C, respectively, for 5 h. Then, the samples were cooled to room temperature at a rate of 2 K/min. Note that all these steps were performed under an argon atmosphere. The templates may be etched with 30 wt % aqueous KOH either partially at room temperature or completely at 70°C. represents LiNbO 3 tubes annealed at 550°C, prepared by two successive wetting-crystallization-cooling cycles. shows a scanning electron microscopy (SEM) image of the openings of LiNbO 3 tubes partially embedded in the template. Their wall thickness is approximately 30 nm. A completely released LiNbO 3 tube is depicted in . Its aspect ratio (length/diameter) equals 100. The inset (top right of ) shows its open end at higher magnification. is a bright field transmission electron microscopy (TEM) image, and is the corresponding dark field micrograph of a detail of a tube wall.

Advanced Functional Materials, 2007

Segmented nanotubes and nanorods exhibiting a variation in their composition along their long axe... more Segmented nanotubes and nanorods exhibiting a variation in their composition along their long axes represent a new and exciting class of nanomaterials. It is shown that bidirectional template wetting enables the integration of functional and complex polymeric materials into segmented nanofibers. First, a template is wetted under conditions in which a solid polymeric thread with adjustable length fills a pore segment starting from one template surface. Subsequently, a second wetting step starting from the opposite template surface yields segmented nanofibers. The exploitation of different wetting mechanisms results in the formation of tube/rod hybrid nanofibers.

Sensors and Actuators B-chemical, 2005

Lithium niobate (LiNbO 3 ) exhibits outstanding photorefractive, piezo-electric, electrooptic and... more Lithium niobate (LiNbO 3 ) exhibits outstanding photorefractive, piezo-electric, electrooptic and non-linear properties, as well as pronounced mechanical and chemical stability and a wide transparency range. We prepared LiNbO 3 nanotubes with diameters ranging from a few tens to a few hundreds of nanometers and a wall thickness on the order of 20 nm. Ordered porous alumina templates were wetted with solutions of a single source precursor, which was converted to LiNbO 3 by thermolysis. The LiNbO 3 nanotubes can be aligned within highly ordered arrays. They were characterized by X-ray diffraction and electron microscopy. LiNbO 3 nanotubes may be employed as building blocks for microand nanoelectronic devices such as ferroelectric nonvolatile memories, sensor systems, and micro-electromechanical systems (MEMS).

Advanced Functional Materials, 2007

Amorphous silica formed by thermally oxidizing silicon is commonly eligible for lithographic patt... more Amorphous silica formed by thermally oxidizing silicon is commonly eligible for lithographic patterning or can be grown on patterned substrates. However, it is still a challenge to combine controlled microstructuring with controlled crystallization of SiO2. Here, it is shown that traces of volatile lithium species transported through the gas phase catalyze the crystallization of silica integrated into common silicon microstructures. The selected crystallization temperature determines which polymorph forms. As an example, the formation of quartz, tridymite, and cristobalite microtubes by thermally oxidizing macroporous silicon is investigated. Lithium-induced crystallization may extend state-of-the-art silicon technology and yield nano- and microstructures consisting of different silica polymorphs, which are, in contrast to many functional oxides, nontoxic.

Advanced Functional Materials, 2005

Polymer/liquid-crystal (LC) tubes consisting of an approximately 30 nm thick poly(methyl methacry... more Polymer/liquid-crystal (LC) tubes consisting of an approximately 30 nm thick poly(methyl methacrylate) (PMMA) layer on the outside and a 5 to 10 nm thick discotic liquid-crystalline layer on the inside of the tube walls have been prepared by wetting ordered porous alumina templates with a pore diameter of 400 nm. Decreasing the pore diameter to 60 nm results in a confinement-induced transition from a wetting state to a non-wetting state, and solid rods with a sequential morphology are obtained. The texture of the mesophase depends on the morphology type and the thermal history. Under certain conditions the LC mesophase exhibits a dominant, well-ordered planar texture where the discotic columns are aligned with the long axes of the tubes. The controlled generation of one-dimensional nano-objects possessing mesoscopic fine structures and intrinsic anisotropy should be the first step towards a rational design of miniaturized building blocks.

Nachrichten Aus Der Chemie, 2004

Es muss nicht immer Kohlenstoff sein: Auch Polymere bilden präzise eindimensionale Nanostrukturen... more Es muss nicht immer Kohlenstoff sein: Auch Polymere bilden präzise eindimensionale Nanostrukturen, die das Potential zu vielseitigen Bausteinen der Nanotechnologie haben und in Feldern wie Katalyse, Sensorik, Fluidik und Optoelektronik, aber auch in der Umweltforschung und der Medizin eingesetzt werden können. Komposite aus Polymer und Additiv erweitern das Spektrum möglicher Anwendungen zusätzlich.

Nature Nanotechnology, 2008

Nanoporous anodic aluminium oxide has traditionally been made in one of two ways: mild anodizatio... more Nanoporous anodic aluminium oxide has traditionally been made in one of two ways: mild anodization or hard anodization. The first method produces self-ordered pore structures, but it is slow and only works for a narrow range of processing conditions; the second method, which is widely used in the aluminium industry, is faster, but it produces films with disordered pore structures. Here we report a novel approach termed "pulse anodization" that combines the advantages of the mild and hard anodization processes. By designing the pulse sequences it is possible to control both the composition and pore structure of the anodic aluminium oxide films while maintaining high throughput. We use pulse anodization to delaminate a single as-prepared anodic film into a stack of well-defined nanoporous alumina membrane sheets, and also to fabricate novel three-dimensional nanostructures.

Advanced Materials, 2001

COMMUNICATIONS Fig. 4. SEM micrographs of fibers with regular surface and pore structures obtaine... more COMMUNICATIONS Fig. 4. SEM micrographs of fibers with regular surface and pore structures obtained by electrospinning of solutions in dichloromethane. a) PC fibers. b) Polyvinylcarbazole fibers.

Angewandte Chemie-international Edition, 2007

Mesoporous materials are currently being investigated as versatile support and template structure... more Mesoporous materials are currently being investigated as versatile support and template structures for a broad range of applications in the fields of catalysis, electronics, storage, filtration, nanobiotechnology, and drug delivery. Their preparation by the self-assembly of structure-directing soft templates, such as surfactants and block copolymers, has been intensively investigated, and di-and triblock copolymers containing poly(ethylene oxide) blocks have been used to produce mesoporous structures with various morphologies and adjustable pore sizes ranging from a few nanometers to a few tens of nanometers. Nanoporous hard templates, such as self-ordered porous alumina, contain arrays of aligned nanochannels with uniform diameters in the range from about 25 to 400 nm and lengths of up to several hundreds of micrometers. Such templates have been used to form nanowires and nanotubes from a variety of materials. The exploitation of templated self-assembly processes, such as mesophase formation, crystallization, liquid/liquid decomposition, and the microphase separation of block copolymers inside nanoporous hard templates, allows the rational generation of onedimensional nanostructures that exhibit specific mesoscopic fine structures, which, in turn, determine their properties. In the case of block copolymers, bulk-like morphologies form in pores with diameters larger than 100-200 nm. Mesoporous silica nanowires are thus accessible by a hierarchical templating process involving the self-assembly of a blockcopolymer soft template inside a nanoporous hard tem-[*] Dr.

Chemistry of Materials, 2007

Ordered mesoporous materials 1,2 combine a large specific surface area with well-defined pore geo... more Ordered mesoporous materials 1,2 combine a large specific surface area with well-defined pore geometry and have been used in energy storage, energy conversion, separation, and catalysis. 3 Their synthesis is based on the self-assembly of block copolymers and surfactants. Whereas the mesopore arrays thus formed may show a well-developed local order, the control over shape and arrangement of the mesoporous entities on a macroscopic scale remains a challenge. A strategy that addresses these problems involves the synthesis of mesoporous silica nanowires inside the pores of porous alumina, 4,5 thus adapting an established method for the preparation of one-dimensional nanostructures based on the use of porous templates. 6 It is highly desirable to apply this concept to amorphous mesoporous carbon, 3,7-9 which exhibits superior resistance to acids and bases, excellent heat resistance, and high intrinsic electrical conductivity compared to mesoporous silica. Potential applications for hybrid membranes consisting of mesoporous carbon within porous alumina include size-selective electrosorption, electrosynthesis of nanostructures, catalysis, separation, and storage. Moreover, released mesoporous carbon microwires and nanowires may be used as lightweight functional filler material that allows tuning of the mechanical and electrical properties of nanocomposites.

Advanced Materials, 2006

Ordered mesoporous materials fabricated by exploiting self-assembled surfactants as molecular tem... more Ordered mesoporous materials fabricated by exploiting self-assembled surfactants as molecular templates have been intensively investigated. However, their integration into device architectures still remains a challenge. Recent efforts have focused on the synthesis of MCM (Mobil Crystalline Material) and SBA (Santa Barbara amorphous) type materials inside porous supports characterized by straight, aligned pores oriented normal to the surface. For this purpose, porous alumina matrices, either ordered or disordered, have predominantly been employed. The hybrid systems thus obtained are promising components for device architectures in the fields of catalysis and separation, where macroscopic membranes that consist of aligned pores with diameters of a few nanometers and high aspect ratios are required. The mesoporous rods can be released by a wet-chemical-etching step that destroys the alumina matrix. When the solvent of the suspension thus obtained is evaporated, capillary forces that act between the mesoporous rods result in the occurrence of large aggregates in otherwise disordered powders.

Angewandte Chemie-international Edition, 2006

Chemistry of Materials, 2008

An emerging approach toward the integration of mesoporous materials into functional device archit... more An emerging approach toward the integration of mesoporous materials into functional device architectures is based on the self-assembly of amphiphilic block copolymers (BCPs) inside hard templates containing arrays of aligned cylindrical nanochannels. [1][2][3][4][5][6] The morphology defined by the soft template is fixated by cross-linking precursors for silica or amorphous carbon after their selective segregation to the hydrophilic domains. After gelation, organic components are removed by calcination, and mesoporous fibers consisting of silica 1-5 or amorphous carbon 6 are obtained. However, solvent evaporation and high-temperature calcination lead to substantial volume shrinkage. The resulting inhomogeneous filling of the hard templates with the mesoporous scaffold is a serious drawback for any application requiring mechanically stable membranes with uniform mesopore structure.

Advanced Materials, 2009

Block copolymers (BCPs) self-assemble into ordered arrays of nanoscopic domains, the nature of wh... more Block copolymers (BCPs) self-assemble into ordered arrays of nanoscopic domains, the nature of which depends on the constituents of the BCPs and their molecular architecture. [1] BCPs have been exploited as precursors for nanoporous materials and as templates for the rational design of nanoscopic architectures with periods from below 10 nm up to the 100-nm-range in thin-film configurations. [3] Whereas molds containing arrays of aligned cylindrical nanochannels with hard confining walls, such as self-ordered anodic aluminum oxide (AAO), [4] have been used to fabricate one-dimensional nanostructures from a plethora of materials, the self-assembly of BCPs in cylindrical confinement has only recently emerged as an access to nanorods exhibiting ordered nanoscopic fine structures. Sol-gels containing precursors for inorganic scaffold materials and BCPs as structuredirecting soft templates were infiltrated into AAO. Subsequent gelation and calcination yielded nanorods consisting of various inorganic oxides and amorphous carbon exhibiting ordered mesoporous fine structures. [7] However, direct infiltration of microphase-separated BCP melts into AAO [8] has received much less attention, mainly because of a lack of obvious applications of the solid BCP nanorods thus obtained. Only recently, BCP nanorods have been converted into mesoporous polymeric nanorods by degrading sacrificial blocks and by selective swelling. BCPs confined to AAO with pore diameters about one order of magnitude larger than their characteristic bulk periods have been reported to retain bulk-like morphologies, such as cylinders oriented along the pore axes and concentric-cylindrical lamellae.

Journal of Materials Research, 2006

Lead titanate (PbTiO 3 ) nano-and microtubes were fabricated by wetting ordered porous alumina an... more Lead titanate (PbTiO 3 ) nano-and microtubes were fabricated by wetting ordered porous alumina and macroporous silicon with precursor oligomers coupled with templated thermolysis. The diameters of the PbTiO 3 tubes range from a few tens of nanometers up to one micron. The proper selection of the template allowed for a precise adjustment of their size over two orders of magnitude. Electron microscopy and x-ray diffraction revealed that the tube walls were polycrystalline. The generic approach presented here can be adapted for the fabrication of tubes and rods from a multitude of functional inorganic oxides.

Chemistry of Materials, 2006

Nanostructured semiconductor materials are of great interest because of their novel physical, che... more Nanostructured semiconductor materials are of great interest because of their novel physical, chemical, optical, and optoelectronic properties, 1 as well as their applications in fabricating nanoscale electronic, photonic, electromechanical, and biomedical devices. 2-5 Cadmium selenide (CdSe)

Polymer Engineering and Science, 2001

The aim was to prepare fibers with diameters below the micrometer range characterized by specific... more The aim was to prepare fibers with diameters below the micrometer range characterized by specific bulk morphologies and surface topologies. Such materials are of interest for various applications including reinforcement, sensors or filtration as well as the formation of functional tubes by the use of fiber templates. We were able to manufacture highly structured submicrometer fibers by electrospinning from ternary solutions using polylactide (PLA) and polyvinylpyrrolidone (PVP) as polymer model components. Co-continuous phase morphologies resulted from phase separation processes taking place during fiber formation. In a subsequent step, specific surface topologies or fine pores were generated by selective removal of one of the components.

Chemistry of Materials, 2005

Lithium niobate (LiNbO 3 ) has been used for all-optical wavelength conversion 1 and ultrafast op... more Lithium niobate (LiNbO 3 ) has been used for all-optical wavelength conversion 1 and ultrafast optical signal processing 2-5 because of its outstandingly rapid nonlinear optical response behavior, low switching power, and broad conversion bandwidth. Periodically poled photonic crystals 6 and inverse artificial opals made of LiNbO 3 7 have been reported. Ordered macroporous silicon (Si) 8-10 is an extensively investigated two-dimensional photonic band gap material. Here we report on the preparation of hybrid systems consisting of LiNbO 3 and macroporous Si. Such composites are of considerable interest because of the integration of LiNbO 3 into silicon photonics. For instance, the band gap of macroporous Si could be tuned by poling LiNbO 3 and thus changing its dielectric constant. Moreover, we investigated whether released LiNbO 3 tubes are accessible using macroporous Si as a sacrificial template. We have adapted a methodology involving the use of porous matrixes as molds, as investigated by Martin 11-13 and others. 14-20 We dropped a 5% solution of lithium [tetraethoxy(1-phenyl-1,3butanedione)niobate] (Scheme 1) in ethanol at ambient conditions on macroporous Si with a pore diameter, D p , of 1 µm. The synthesis and the properties of this single-source precursor for LiNbO 3 are described elsewhere. 21 The Si substrate into which the macropores were etched was a single-crystalline (100)-oriented wafer. We removed the residual liquid from the template surface. As the solvent evaporates, a layer of adsorbed molecules remains at the pore walls, which can be converted to the target compound by thermolysis. This procedure may be repeated once or several times. The conversion to crystalline LiNbO 3 was performed by annealing the wetted templates at 550, 650, and 750°C, respectively, for 5 h. Then, the samples were cooled to room temperature at a rate of 2 K/min. Note that all these steps were performed under an argon atmosphere. The templates may be etched with 30 wt % aqueous KOH either partially at room temperature or completely at 70°C. represents LiNbO 3 tubes annealed at 550°C, prepared by two successive wetting-crystallization-cooling cycles. shows a scanning electron microscopy (SEM) image of the openings of LiNbO 3 tubes partially embedded in the template. Their wall thickness is approximately 30 nm. A completely released LiNbO 3 tube is depicted in . Its aspect ratio (length/diameter) equals 100. The inset (top right of ) shows its open end at higher magnification. is a bright field transmission electron microscopy (TEM) image, and is the corresponding dark field micrograph of a detail of a tube wall.

Advanced Functional Materials, 2007

Segmented nanotubes and nanorods exhibiting a variation in their composition along their long axe... more Segmented nanotubes and nanorods exhibiting a variation in their composition along their long axes represent a new and exciting class of nanomaterials. It is shown that bidirectional template wetting enables the integration of functional and complex polymeric materials into segmented nanofibers. First, a template is wetted under conditions in which a solid polymeric thread with adjustable length fills a pore segment starting from one template surface. Subsequently, a second wetting step starting from the opposite template surface yields segmented nanofibers. The exploitation of different wetting mechanisms results in the formation of tube/rod hybrid nanofibers.

Sensors and Actuators B-chemical, 2005

Lithium niobate (LiNbO 3 ) exhibits outstanding photorefractive, piezo-electric, electrooptic and... more Lithium niobate (LiNbO 3 ) exhibits outstanding photorefractive, piezo-electric, electrooptic and non-linear properties, as well as pronounced mechanical and chemical stability and a wide transparency range. We prepared LiNbO 3 nanotubes with diameters ranging from a few tens to a few hundreds of nanometers and a wall thickness on the order of 20 nm. Ordered porous alumina templates were wetted with solutions of a single source precursor, which was converted to LiNbO 3 by thermolysis. The LiNbO 3 nanotubes can be aligned within highly ordered arrays. They were characterized by X-ray diffraction and electron microscopy. LiNbO 3 nanotubes may be employed as building blocks for microand nanoelectronic devices such as ferroelectric nonvolatile memories, sensor systems, and micro-electromechanical systems (MEMS).

Advanced Functional Materials, 2007

Amorphous silica formed by thermally oxidizing silicon is commonly eligible for lithographic patt... more Amorphous silica formed by thermally oxidizing silicon is commonly eligible for lithographic patterning or can be grown on patterned substrates. However, it is still a challenge to combine controlled microstructuring with controlled crystallization of SiO2. Here, it is shown that traces of volatile lithium species transported through the gas phase catalyze the crystallization of silica integrated into common silicon microstructures. The selected crystallization temperature determines which polymorph forms. As an example, the formation of quartz, tridymite, and cristobalite microtubes by thermally oxidizing macroporous silicon is investigated. Lithium-induced crystallization may extend state-of-the-art silicon technology and yield nano- and microstructures consisting of different silica polymorphs, which are, in contrast to many functional oxides, nontoxic.

Advanced Functional Materials, 2005

Polymer/liquid-crystal (LC) tubes consisting of an approximately 30 nm thick poly(methyl methacry... more Polymer/liquid-crystal (LC) tubes consisting of an approximately 30 nm thick poly(methyl methacrylate) (PMMA) layer on the outside and a 5 to 10 nm thick discotic liquid-crystalline layer on the inside of the tube walls have been prepared by wetting ordered porous alumina templates with a pore diameter of 400 nm. Decreasing the pore diameter to 60 nm results in a confinement-induced transition from a wetting state to a non-wetting state, and solid rods with a sequential morphology are obtained. The texture of the mesophase depends on the morphology type and the thermal history. Under certain conditions the LC mesophase exhibits a dominant, well-ordered planar texture where the discotic columns are aligned with the long axes of the tubes. The controlled generation of one-dimensional nano-objects possessing mesoscopic fine structures and intrinsic anisotropy should be the first step towards a rational design of miniaturized building blocks.

Nachrichten Aus Der Chemie, 2004

Es muss nicht immer Kohlenstoff sein: Auch Polymere bilden präzise eindimensionale Nanostrukturen... more Es muss nicht immer Kohlenstoff sein: Auch Polymere bilden präzise eindimensionale Nanostrukturen, die das Potential zu vielseitigen Bausteinen der Nanotechnologie haben und in Feldern wie Katalyse, Sensorik, Fluidik und Optoelektronik, aber auch in der Umweltforschung und der Medizin eingesetzt werden können. Komposite aus Polymer und Additiv erweitern das Spektrum möglicher Anwendungen zusätzlich.

Nature Nanotechnology, 2008

Nanoporous anodic aluminium oxide has traditionally been made in one of two ways: mild anodizatio... more Nanoporous anodic aluminium oxide has traditionally been made in one of two ways: mild anodization or hard anodization. The first method produces self-ordered pore structures, but it is slow and only works for a narrow range of processing conditions; the second method, which is widely used in the aluminium industry, is faster, but it produces films with disordered pore structures. Here we report a novel approach termed "pulse anodization" that combines the advantages of the mild and hard anodization processes. By designing the pulse sequences it is possible to control both the composition and pore structure of the anodic aluminium oxide films while maintaining high throughput. We use pulse anodization to delaminate a single as-prepared anodic film into a stack of well-defined nanoporous alumina membrane sheets, and also to fabricate novel three-dimensional nanostructures.

Advanced Materials, 2001

COMMUNICATIONS Fig. 4. SEM micrographs of fibers with regular surface and pore structures obtaine... more COMMUNICATIONS Fig. 4. SEM micrographs of fibers with regular surface and pore structures obtained by electrospinning of solutions in dichloromethane. a) PC fibers. b) Polyvinylcarbazole fibers.

Angewandte Chemie-international Edition, 2007

Mesoporous materials are currently being investigated as versatile support and template structure... more Mesoporous materials are currently being investigated as versatile support and template structures for a broad range of applications in the fields of catalysis, electronics, storage, filtration, nanobiotechnology, and drug delivery. Their preparation by the self-assembly of structure-directing soft templates, such as surfactants and block copolymers, has been intensively investigated, and di-and triblock copolymers containing poly(ethylene oxide) blocks have been used to produce mesoporous structures with various morphologies and adjustable pore sizes ranging from a few nanometers to a few tens of nanometers. Nanoporous hard templates, such as self-ordered porous alumina, contain arrays of aligned nanochannels with uniform diameters in the range from about 25 to 400 nm and lengths of up to several hundreds of micrometers. Such templates have been used to form nanowires and nanotubes from a variety of materials. The exploitation of templated self-assembly processes, such as mesophase formation, crystallization, liquid/liquid decomposition, and the microphase separation of block copolymers inside nanoporous hard templates, allows the rational generation of onedimensional nanostructures that exhibit specific mesoscopic fine structures, which, in turn, determine their properties. In the case of block copolymers, bulk-like morphologies form in pores with diameters larger than 100-200 nm. Mesoporous silica nanowires are thus accessible by a hierarchical templating process involving the self-assembly of a blockcopolymer soft template inside a nanoporous hard tem-[*] Dr.