Physics of ultra-thin phthalocyanine films on semiconductors

This paper focuses on the structural arrangements, as seen in scanning tunneling microscopy, chemical and electric properties, as studied especially by high resolution synchrotron radiation photoelectron spectroscopy and energy loss spectroscopy of sub-monolayer up to few monolayer thin films of phthalocyanines adsorbed on narrow band gap III–V semiconductors where they form ordered superstructures, as well as on silicon surfaces. Special emphasis will be put on non-planar metal phthalocyanines where the central atom is either lead or tin. These molecules have been studied in detail from the experimental point of view as well as theoretically by ab initio quantum calculations. Comparison with metal free and metal substituted planar phthalocyanines (H₂Pc, CuPc, CoPc, …) adsorbed on semiconductors or noble metal surfaces is included. Perspectives and novel potential applications are finally outlined.     

Single-Crystal Superstructures via Hierarchical Assemblies of Giant Rubik's Cubes as Tertiary Building Units

Intricate superstructures possess unusual structural features and promising applications. The preparation of superstructures with single-crystalline nature are conducive to understanding the structure–property relationship, however, remains an intriguing challenge. Herein we put forward a new hierarchical assembly strategy towards rational and precise construction of intricate single-crystal superstructures. Firstly, two unprecedented superclusters in Rubik's cube's form with a size of ≈2×2×2 nm³ are constructed by aggregation of eight {Pr₄Sb₁₂} oxohalide clusters as secondary building units (SBUs). Then, the Rubik's cubes further act as isolable tertiary building units (TBUs) to assemble diversified single-crystal superstructures. Importantly, intermediate assembly states are captured, which helps illustrate the evolution of TBU-based superstructures and thus provides a profound understanding of the assembly process of superstructures at the atomic level.     

Syntheses and Crystal Structures of CaCuGe,CaAuIn, and CaAuSn – Three Different Superstructures of the KHg2 Type

The intermetallic compounds CaCuGe, CaAuIn, and CaAuSn can be prepared from the elements in sealed tantalum tubes or in glassy carbon crucibles in a high-frequency furnace. Their crystal structures were determined from single crystal X-ray data. The three compounds crystallize with the same subcell structure (KHg₂), however, they form three clearly perceptible superstructures with different unit cells, but all in space groups Pnma: a = 2124.9(6) pm, b = 436.0(2) pm, c = 749.4(5) pm, Z = 12, wR2 = 0.0789, 1303 F² values, 56 variables for CaCuGe (own structure type), a = 738.2(1) pm, b = 459.4(1) pm, c = 839.4(2) pm, Z = 4, wR2 = 0.0651, 656 F² values, 20 variables for CaAuIn (TiNiSi type), a = 3690.3(3) pm, b = 470.5(1) pm, c = 813.6(2) pm, Z = 20, wR2 = 0.1294, 1730 F² values, 92 variables for CaAuSn (new structure type). The three structures may be considered as superstructures of the KHg₂ type with an ordered arrangement of the transition metal and germanium (indium, tin) atoms on the mercury position. Each calcium atom in the structures of CaCuGe, CaAuIn, and CaAuSn has an distinctly ordered near-neighbor environment of six transition metal (T) and six p element (X) atoms in the form of two counter-tilted T₃X₃ hexagons. All known superstructures of the KHg₂ type are described in terms of a group-subgroup scheme.     

Supramolecular ordering of amide dendrons in lyotropic and thermotropic conditions

Self-assembled superstructures of amide dendrons, from first to third generation including monodendrons and covalently linked dimers, were extensively examined, and the supramolecular ordering processes in thermotropic and lyotropic conditions were compared. The superstructures as determined by X-ray diffraction and DSC revealed that the first and second generation dendrons showed nearly identical superstructures regardless of the assembly conditions. But, the third generation dendrons showed a more sensitive self-organizing behavior. The structure obtained from the gel state was lamellar with a more extended conformation, while the structure from the melt state revealed the columnar superstructures of contracted branches. The superstructure formed from the gel state also showed a structural change upon raising the temperature and assumed a structure similar to the thermotropically driven one, implying that the structure formed from the gel is thermodynamically unstable. The formation of lamellar- or cylinder-type superstructures from amide dendrons was primarily dependent on the shape of dendrons, which is associated with the branch size (generation) and the surrounding conditions.     

Nonclassical crystallization: mesocrystals and morphology change of CaCO3 crystals in the presence of a polyelectrolyte additive

Crystallization of calcite from differently concentrated calcium chloride solutions by the CO2 gas diffusion technique in the presence of polystyrene sulfonate yields crystal superstructures with unusual morphology. From the typical calcite rhombohedra as a starting situation, the morphology can be systematically varied via rounded edges and truncated triangles to finally concavely bended lens-like superstructures. Although these "crystals" are apparently well-faceted in light microscopy, electron microscopy analysis and BET reveal that the structures are highly porous and are composed of almost perfectly 3D-aligned calcite nanocrystals scaffolded to the final, partly nicely curved superstructures. At high supersaturations, superstructures with changed symmetry indicative of dipolar interaction potentials between the building blocks are found. The present model case also gives evidence for the importance of nonclassical, mesoscopic processes in crystallization in general.     

Coverage-Controlled Superstructures of C3-Symmetric Molecules: Honeycomb versus Hexagonal Tiling

The competition between honeycomb and hexagonal tiling of molecular units can lead to large honeycomb superstructures on surfaces. Such superstructures exhibit pores that may be used as 2D templates for functional guest molecules. Honeycomb superstructures of molecules that comprise a C₃ symmetric platform on Au(111) and Ag(111) surfaces are presented. The superstructures cover nearly mesoscopic areas with unit cells containing up to 3000 molecules, more than an order of magnitude larger than previously reported. The unit cell size may be controlled by the coverage. A fairly general model was developed to describe the energetics of honeycomb superstructures built from C₃ symmetric units. Based on three parameters that characterize two competing bonding arrangements, the model is consistent with the present experimental data and also reproduces various published results. The model identifies the relevant driving force, mostly related to geometric aspects, of the pattern formation.     

Coverage‐Controlled Superstructures of C3‐Symmetric Molecules: Honeycomb versus Hexagonal Tiling

The competition between honeycomb and hexagonal tiling of molecular units can lead to large honeycomb superstructures on surfaces. Such superstructures exhibit pores that may be used as 2D templates for functional guest molecules. Honeycomb superstructures of molecules that comprise a C₃ symmetric platform on Au(111) and Ag(111) surfaces are presented. The superstructures cover nearly mesoscopic areas with unit cells containing up to 3000 molecules, more than an order of magnitude larger than previously reported. The unit cell size may be controlled by the coverage. A fairly general model was developed to describe the energetics of honeycomb superstructures built from C₃ symmetric units. Based on three parameters that characterize two competing bonding arrangements, the model is consistent with the present experimental data and also reproduces various published results. The model identifies the relevant driving force, mostly related to geometric aspects, of the pattern formation.     

Chemical Intergrowth of Dimeric and Trimeric Clusters in Reduced Tin Oxomolybdates: A study by high-resolution electron microscopy

By high-resolution electron microscopy, chemical intergrowth was studied of dimeric Mo₁₀O₁₆ and trimeric Mo₁₄O₂₂ clusters in reduced tin oxomolybdates within the Sn_xMo₁₀O₁₆? Sn_yMo₁₄O₂₂ system. The variety of observed intergrowth patterns includes polysynthetically twinned structures as well as a number of different superstructures. For an orthorhombic superstructure formed by alternating layers of dimeric and trimeric clusters, the lattice parameters and the Sn content were determined. The chemical reason for the amazing abundance of structures in the considered system is discussed.     

Recrystallization‐Induced Self‐Assembly for the Growth of Cu2O Superstructures

The assembly of inorganic nanoparticles (NPs) into 3D superstructures with defined morphologies is of particular interest. A novel strategy that is based on recrystallization‐induced self‐assembly (RISA) for the construction of 3D Cu₂O superstructures and employs Cu₂O mesoporous spheres with diameters of approximately 300 nm as the building blocks has now been developed. Balancing the hydrolysis and recrystallization rates of the CuCl precursors through precisely adjusting the experimental parameters was key to success. Furthermore, the geometry of the superstructures can be tuned to obtain either cubes or tetrahedra and was shown to be dependent on the growth behavior of bulk CuCl. The overall strategy extends the applicability of recrystallization‐based processes for the guided construction of assemblies and offers unique insights for assembling larger particles into complicated 3D superstructures.     

Post-polymerization of preorganized assemblies for creating shape-controlled functional materials

Combination of supramolecular chemistry with molecular recognition has been successfully applied to creating large superstructures with a wide variety of morphologies. Control of shapes and patterns of ordered molecular assemblies in nano and micro scales has attracted considerable interest as promising bottom-up technology. It is known, however, that these molecular assembling superstructures are fragile, reflecting the characteristic of the non-covalent interaction, a driving force operating in these molecular systems. In fact, they easily collapse or change by small perturbation in the environmental conditions. Thus, over the last decade, researchers have been seeking possible methods for the immobilization these superstructures. This critical review focuses on recent advances in in situ post-modification under the influence of the molecular assemblies as templates and polymerization of ordered molecular assemblies such as organogel fibers and crystals to preserve their original superstructures and intensify their mechanical strength.     

Hydrothermal synthesis and photoluminescent properties of stacked indium sulfide superstructures

Unusual hierarchical stacked superstructures of cubic beta-In2S3 were fabricated via a facile hydrothermal process in the presence of a surfactant cetyltrimethylammonium bromide CTAB; the 3D superstructures were developed by helical propagation of surface steps from microflakes of 10-20 nm thickness.     

Porous CuO superstructure: precursor-mediated fabrication, gas sensing and photocatalytic properties

A facile route was developed for the large-scale preparation of porous CuO superstructures based on a hydrothermal route with subsequent calcination. The CuO superstructures show "box-like" shape and are composed of microplatelets with high porosity resulting from the thermal decomposition of the precursor. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform spectroscopy (FT-IR) and Brunauer-Emmett-Teller N(2) adsorption-desorption analyses were employed to characterize the microstructure, size and crystalline phase of the porous cupric oxide product. The porous CuO superstructures with pore size of about 20 nm have a surface area of 18.2 m(2)/g. The gas-sensing measurements of the porous CuO superstructures demonstrate that the obtained CuO product exhibits higher sensing response to ethanol, propanol and acetone than commercial CuO powder. In addition, the enhanced photocatalytic activity of the porous CuO superstructures was also demonstrated with the photocatalytic degradation of methylene blue as a probe reaction. It is believed that the enhanced gas-sensing and catalytic properties are originated from their unique openly porous microstructure, which is highly beneficial to the reagent diffusion and mass transportation.     

Unique Ordered TiO(2) Superstructures with Tunable Morphology and Crystalline Phase for Improved Lithium Storage Properties

Unique ordered TiO(2) superstructures with tunable morphology and crystalline phase were successfully prepared by the use of different counterions. Dumbbell-shaped rutile TiO(2) and nanorod-like rutile mesocrystals constructed from ultrathin nanowires, and quasi-octahedral anatase TiO(2) mesocrystals built from tiny nanoparticle subunits were achieved. Interestingly, the obtained anatase mesocrystals have a fine microporous structure and a large surface area. The influence of the counterions in the reaction system is discussed and possible mechanisms responsible for the formation of the unique ordered TiO(2) superstructures with different morphologies and crystalline phases are also proposed based on a series of experimental results. The obtained TiO(2) superstructures were used as anode materials in lithium ion batteries, and exhibited higher capacity and improved rate performance; this is attributed to the intrinsic characteristics of the mesoscopic TiO(2) superstructures, which have a single-crystal-like and porous nature.     

Synthesis of achiral PEG‐PANI rod‐coil block copolymers and their helical superstructures

Poly(ethylene glycol) (PEG) was modified with aniline groups at both the end, and then PEG‐PANI rod‐coil block polymers have been synthesized by polymerization of the aniline with the aniline‐modified PEG. FTIR, NMR, and elemental analysis provided the chemical strucutre of the as‐prepared polymers. The achiral rod‐coil copolymer could form different superstructures by means of self‐assembly when adding diethyl ether into its THF solution and the length of PANI segments is a key factor to the superstructures. AFM measurements revealed that they form spring‐like helical superstructures from the short PANI‐containing copolymers while these form fibrous helical superstructures from the longer PANI‐containing copolymer. A possible mechanism of the helical superstructures is suggested in this article and the driving force is believed the π–π stacking of the rigid segment of the copolymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 12–20, 2008     

Surfactant-assisted alignment of ZnO nanocrystals to superstructures

Self-organization of ZnO nanoparticles into various superstructures (sheet, platelet, ring) has been achieved with the assistance of micelles formed by surfactant cetyltrimethylammonium bromide (CTAB) under one-pot condition. The CTAB-modified zinc hydroxy double salt (Zn-HDS) mesocrystals act as intermediates to form ZnO hexagonal superstructures at temperatures as low as 50 degrees C. The decomposition temperature of Zn-HDS mesocrystals is much lower than that of the corresponding bulk crystals because the organic additive CTAB effectively decreases the degree of crystallinity. Taking advantage of temperature-induced phase transformation of micelles, two-stage self-organization can form ZnO platelets and ring mesocrystals, that is, ZnO ellipsoidal superstructures formed through vertical attachment on (0001) facets of basic units can further assemble to form ZnO platelets and rings through vertical attachment on (0001) facets of ZnO ellipsoidal superstructures. The structural transformation of micelles as shape templates can offer a new route for self-assembly of nonspherical colloids into three-dimensional photonic crystals. ZnO sheet, ring, and platelet mesocrystals with a high population of polar Zn-(0001) plane are expected to have high photocatalytic activity.     

Topotactic transformations of superstructures: from thin films to two-dimensional networks to nested two-dimensional networks

Design and synthesis of super-nanostructures is one of the key and prominent topics in nanotechnology. Here we propose a novel methodology for synthesizing complex hierarchical superstructures using sacrificial templates composed of ordered two-dimensional (2D) nanostructures through lattice-directed topotactic transformations. The fabricated superstructures are nested 2D orthogonal Bi(2)S(3) networks composed of nanorods. Further investigation indicates that the lattice matching between the product and sacrificial template is the dominant mechanism for the formation of the superstructures, which agrees well with the simulation results based on an anisotropic nucleation and growth analysis. Our approach may provide a promising way toward a lattice-directed nonlithographic nanofabrication technique for making functional porous nanoarchitectures and electronic devices.     

Light‐Directed Dynamic Chirality Inversion in Functional Self‐Organized Helical Superstructures

Helical superstructures are widely observed in nature, in synthetic polymers, and in supramolecular assemblies. Controlling the chirality (the handedness) of dynamic helical superstructures of molecular and macromolecular systems by external stimuli is a challenging task, but is of great fundamental significance with appealing morphology‐dependent applications. Light‐driven chirality inversion in self‐organized helical superstructures (i.e. cholesteric, chiral nematic liquid crystals) is currently in the limelight because inversion of the handedness alters the chirality of the circularly polarized light that they selectively reflect, which has wide potential for application. Here we discuss the recent developments toward inversion of the handedness of cholesteric liquid crystals enabled by photoisomerizable chiral molecular switches or motors. Different classes of chiral photoresponsive dopants (guests) capable of conferring light‐driven reversible chirality inversion of helical superstructures fabricated from different nematic hosts are discussed. Rational molecular designs of chiral molecular switches toward endowing handedness inversion to the induced helical superstructures of cholesteric liquid crystals are highlighted. This Review is concluded by throwing light on the challenges and opportunities in this emerging frontier, and it is expected to provide useful guidelines toward the development of self‐organized soft materials with stimuli‐directed chirality inversion capability and multifunctional host–guest systems.     

A novel waist-regulable dumbbell-like nanosuperstructure of (3-carboxy-1-acyl-propyl)-ferrocene

Dumbbell-like nanosuperstructures of (3-carboxy-1-acyl-propyl)-ferrocene were successfully prepared by ultrasonic–pHcontrolling–reprecipitation method. The scanning electron microscope shows that as-obtained products were composed of nanorods. The length of the products is about 6–12 μm, the diameter of the two polar coronas and the middle part (waist) are respectively about 2–8 μm and 0.5–2 μm. The diameter of the nanorods that are the basic unit of the superstructures, is about 0.3 μm. X-ray diffraction analysis of the products indicated that the superstructures keep original crystal structure. The optical properties of the products were characterized by FT-IR spectrometer and UV spectrometer, and they display quantum size effect. This paper provides a novel method that has potential application in preparing other organometallic compounds superstructures, and more importantly, the as-obtained superstructures have widely applications. The possible mechanism was also proposed.     

Peptide Conjugates for Directing the Morphology and Assembly of 1D Nanoparticle Superstructures

Designed peptide conjugates molecules are used to direct the synthesis and assembly of gold nanoparticles into complex 1D nanoparticle superstructures with various morphologies. Four peptide conjugates, each based on the gold‐binding peptide (AYSSGAPPMPPF; PEP_Au), are prepared: C₁₂H₂₃O‐AYSSGAPPMPP ( 1 ), C₁₂H₂₃O‐AYSSGAPPMPPF ( 2 ), C₁₂H₂₃O‐AYSSGAPPMPPFF ( 3 ), and C₁₂H₂₃O‐AYSSGAPPMPPFFF ( 4 ). The affect that C‐terminal hydrophobic F residues have on both the soft‐assembly of the peptide conjugates and the resulting assembly of gold nanoparticle superstructures is examined. It is shown that the addition of two C‐terminal F residues ( 3 ) leads to thick, branched 1D gold nanoparticle superstructures, whereas the addition of three C‐terminal F residues ( 4 ) leads to bundling of thin 1D nanoparticle superstructures.     

Synthesis and Enhanced Photocatalytic Activity of a Hierarchical Porous Flowerlike p–n Junction NiO/TiO2 Photocatalyst

Hierarchical flowerlike β‐Ni(OH)₂ superstructures composed of intermeshed nanoflakes are synthesized by hydrothermal treatment with a mixed solution of C₂H₄(NH₂)₂, NaOH, and Ni(NO₃)₂. The as‐prepared β‐Ni(OH)₂ superstructures could be easily changed into NiO superstructures without great morphology change by calcination at 400 °C for 5 h. Furthermore, the TiO₂ nanoparticles can be homogeneously deposited on the surface of NiO superstructures by dispersing β‐Ni(OH)₂ powders in Ti(OC₄H₉)₄–C₂H₅OH mixed solution and then vaporizing to remove the ethanol at 100 °C, and finally calcination at 400 °C for 5 h. The prepared NiO/TiO₂ p–n junction superstructures show much higher photocatalytic activity for photocatalytic degradation of p‐chlorophenol aqueous solution than conventional TiO₂ powders and NiO superstructures prepared under the same experimental conditions. An obvious enhancement in the photocatalytic activity can be related to several factors, including formation of hierarchical porous structures, dispersion of TiO₂ particles on the surface of NiO superstructures, and production of a p–n junction. Further results show that NiO/TiO₂ composite superstructures can be more readily separated from the slurry system by filtration or sedimentation after photocatalytic reaction and re‐used, compared with conventional powder photocatalysts. After many recycling experiments for the photodegradation of p‐chlorophenol, the NiO/TiO₂ composite sample does not exhibit any great activity loss, confirming that NiO/TiO₂ sample is stable and not photocorroded.