Organic Molecular Materials Research Articles

Transition metal oxide thin films for nonvolatile resistive random access memory applications

Resistive random access memory (RRAM) based on reversible bistable resistance switching effect is attracting much attention for its superior properties such as nonvolatility, long retention time, simple device structure, small size, and low operating voltage. The unique resistance switching properties have been observed in some binary transition metal oxides, perovskite oxides, and organic molecular materials. This paper briefly reviews the status and new progress on binary transition metal oxide thin film materials such as NiO, TiO 2 , Zr0 2 , ZnO, and their multilayered thin films and metal nanocomposite thin films, for resistive random access memory applications, and also presents some of our own research work in this field. There is no doubt that study on transition metal oxide thin films for RRAM application will have been a topic of great interest in the forthcoming years, and it is expected that better understanding of physical mechanisms for the bistable resistance switching of the transition metal oxide thin films sandwiched between two metal electrodes can be achieved.

Hydrothermal Synthesis, Structure and Fluorescence of a New Triazole-bridging Zinc(II) 2D Layered Framework

Metal-organic frameworks (MOFs) are of considerable interest due to their potential applications in the areas of ionexchange, sorption, catalysis and nonlinear optics. Their diversified structural features greatly rely on the selection of the metal centers, organic building blocks, and the reaction pathways. The selection of organic building blocks exhibiting various connection fashions is the most effective strategy to assemble MOFs. Complexes containing heterocyclic πconjugated 1,2,4-triazole moiety are one of the most extensively explored systems, whose researches lead to some molecular functional materials, such as magnetic materials, photochemically driven molecular devices, information storage, molecular recognition media, and supramolecular devices. 1,2,4-Triazole ring being a 6-electron π-excessive system, its emission region locates near blue light, implying that 1,2,4-triazole derivatives can act as a carrier to investigate the possibility in the design strategies for the heterocycle-based organic blue emission molecular materials. 3-Amino-1H-1,2,4-triazole (Httr) is chosen as the organic building blocks in the presented work, which is a twoconnecting neutral ligand or three-connecting (deprotonated) anionic ligand in the construction of MOFs. Its coordination chemistry is helpful in the studies of the “simple highsymmetry” structures and new topological networks. Though Httr can be purchased from commercial sources or synthesized in laboratories, the examples of the complexes containing Httr is not rich as expected. Therefore, our interest is in the investigation of fabricating metal-organic frameworks with desired optical properties. Herein, a 2D layered compound, [Zn2(ttr)2(OAc)2]n (1), was yielded from the hydrothermal reaction of Httr with Zn(OAc)2·2H2O, whose structure and fluorescence were characterized.

Langmuir–Blodgett films of molecular organic materials

Langmuir–Blodgett methods are perhaps the original approach for achieving controlleddeposition of organic thin films. Molecules are first organized into a monolayer array on thesurface of water before transfer as a monolayer onto solid supports. Molecular monolayers,multilayers, and multilayered heterostructures can be achieved. The capability of exercisingsuch control over thin film assemblies has attracted materials chemists and physicists todevelop Langmuir–Blodgett films for studies on organic conductors, magnets,non-linear optics, rectifiers, and intermolecular electron transfer. This article reviewsobjectives in each of these areas and selects some specific examples from the literatureto highlight the state of the art, mostly from the point of view of the chemicalsystems that are studied. Mixed organic/inorganic hybrid films represent a newdirection for Langmuir–Blodgett films in materials science, combining conventionalinorganic solid-state phenomena with the properties of the organic networks,and recent examples, taken principally from the authors’ work, are highlighted.

Modelling organic molecular crystals by hybrid quantum mechanical/molecular mechanical embedding

We present a hybrid quantum mechanical/molecular mechanical (QM/MM) embedding scheme to model organic molecular materials. The method employs specialist QM and MM codes as modules in an integrated way that enables the dynamic simulation of large numbers of molecules at a MM level with a smaller fraction treated at a higher QM level of theory. In this preliminary study our QM/MM methodology is applied to the high performance organic (opto)electronic material dithiophene–tetrathiafulvalene (DT–TTF). The QM/MM methodology is tested and confirmed with respect to experiment and previous cluster calculations. The effects of the more realistic treatment of the molecular environment are highlighted.

In-situ study of the interface formation in organic multilayers

Organic molecular beam epitaxy (OMBE) is the growth technique assuring high quality films and properly designed multilayers of organic molecular materials in view of both their fundamental studies and applications. To monitor in-situ the OMBE growth, we have applied reflectance anisotropy spectroscopy (RAS), which is particularly effective and sensitive, being non destructive for organic materials. In this work, RAS is used to monitor, in-situ, the growth of organic heterostructures. The experiment is carried out using organic single crystals as substrates and oligothiophenes as molecular materials to build a stack of layers. A layer-by-layer growth of the films is demonstrated, together with the formation of interfaces with peculiar optical behavior and structure.

Accurate molecular structures of chlorothiazide and hydrochlorothiazide by joint refinement against powder neutron and X-ray diffraction data

The compounds chlorothiazide and hydrochlorothiazide (crystalline form II) have been studied in their fully hydrogenous forms by powder neutron diffraction on the GEM diffractometer. The results of joint Rietveld refinement of the structures against multi-bank neutron and single-bank X-ray powder data are reported and show that accurate and precise structural information can be obtained from polycrystalline molecular organic materials by this route.

Dependence of charge transfer reorganization energy on carrier localisation in organic molecular crystals

Taking the organic molecular material dithiophene-tetrathiafulvalene (DT-TTF) as an example of a high mobility organic molecular material, we use density functional calculations to calculate the dependency of the reorganization energy associated with charge carrier transport on: (i) the geometric and electronic responsiveness of the local molecular crystal environment, and, (ii) the local spatial extent of the charge carrier. We find that in our most realistic extended models the charge transfer reorganization energy is strongly dependent on carrier localization. In particular, whereas highly localized carriers are found to be highly susceptible to their charge transfer efficiency being affected by changes in the local crystal environment, more delocalized carriers are better able to maintain their low reorganization energies. Considering that maintaining a relatively small charge transfer reorganization energy magnitude is an important factor in achieving high carrier mobilities, we suggest that those materials better able to sustain carriers with short-range thermally resistant intermolecular delocalisation should be sought for device applications.

Characterization of C-14 in PWR Radioactive Wastes

Characterization of C-14 in PWR Radioactive Wastes has been researched and formation mechanisms of C-14 have been discussed. It was found from the research results that the chemical form of C-14 existed in primary coolant was organic and was low molecule compounds which are soluble in water. On the other hand, most of C-14 components existed in condensed liquid waste and existed on solid waste were insoluble in water and chemically stable. The insoluble C-14 component was considered to be produced by activation reaction between neutron and substances with nitrogen. Those were thought to be decomposition substances escaped from high molecular organic materials, such as ion exchange resin, diaphragm seal, etc.

Growth-related properties and postgrowth phenomena in organic molecular thin films

The problem of monitoring the structural and morphological evolutions of thin films of organic molecular materials during their growth by organic molecular beam epitaxy and in the postgrowth stage is addressed here by a combination of in situ optical reflectance anisotropy measurements, ex situ optical and morphological investigations, and theoretical simulation of the material optical response. For alpha-quaterthiophene, a representative material in the class of organic molecular semiconductors, the results show that molecules crystallize in the first stage of growth in metastable structures, even when deposition is carried out at room temperature. In the postdeposition stage, the film structure evolves within a few days to the known equilibrium structure of the low temperature polymorph. When deposition is carried out at low substrate temperatures, an evolution of the film morphology is also demonstrated.

Cover Picture: Excitonic and Vibrational Properties of Single‐Walled Semiconducting Carbon Nanotubes (Adv. Funct. Mater. 17/2007)

AbstractThe cover picture illustrates excited state dynamics of semiconducting single‐walled carbon nanotubes studied theoretically. Sergai Tretiak and Svetlana Kilina report on p. 3405 that absorption of the light quantum leads to spatially delocalized photoexcitation, which can be described as tightly bound excitons and characterized by 2D plots of transition density. The photoexcitation is coupled to the vibrational degrees of freedom, leading to complex exciton‐phonon dynamics, which can be monitored experimentally using ultrafast spectroscopic probes.We review quantum‐chemical studies of the excited‐state electronic structure of finite‐size semiconducting single‐walled carbon nanotubes (SWCNTs) using methodologies previously successfully applied to describe conjugated polymers and other organic molecular materials. The results of our simulations are in quantitative agreement with available spectroscopic data and show intricate details of excited‐state properties and photoinduced vibrational dynamics in carbon nanotubes. We analyze in detail the nature of strongly bound first and second excitons in SWCNTs for a number of different tubes, emphasizing emerging size‐scaling laws. Characteristic delocalization properties of excited states are identified by the underlying photoinduced changes in charge densities and bond orders. Due to the rigid structure, exciton–phonon coupling is much weaker in SWCNTs compared to typical molecular materials. Yet we find that, in the ground state, a SWCNT's surface experiences the corrugation associated with electron–phonon interactions. Vibrational relaxation following photoexcitation reduces this corrugation, leading to a local distortion of the tube surface, which is similar to the formation of self‐trapped excitons in conjugated polymers. The calculated associated Stokes shift increases with enlargement of the tube diameters. Such exciton vibrational phenomena are possible to detect experimentally, allowing for better understanding of photoinduced electronic dynamics in nanotube materials.

An improved SCPF scheme for polarization energy calculations

Convergence of the Self-Consistent Polarization Field (SCPF) method of polarization energy calculations for organic molecular materials is analysed. Use of the Conjugate Gradients method for solving the SCPF equations is proposed. Efficiency of both the original and the newly proposed approach is compared for selected model systems. Brief discussion of the factors influencing the performance of Krylov-space-based methods for polarization energy calculations is presented.

Size and Temperature Dependence of Phase Stability in Nanocrystalline Pentacene Thin Films

Pentacene thin films are frequently used to fabricate organic molecular electronic materials. It is believed that the orthorhombic phase is transformed into the triclinic phase in a process that is dominated by thin film thickness and substrate temperature. In this work, a thermodynamic model was established to describe the polymorphic behavior of pentacene thin films with respect to contributions in surface (interface) energy and surface stress to the total Gibbs free energy. The accuracy of the developed model was verified with the reported experimental data. It reveals that a lower surface energy and surface stress coupled with a higher volume Gibbs free energy induces crystallization of the orthorhombic phase on the nanometer scale.

Excitonic and Vibrational Properties of Single‐Walled Semiconducting Carbon Nanotubes

AbstractWe review quantum‐chemical studies of the excited‐state electronic structure of finite‐size semiconducting single‐walled carbon nanotubes (SWCNTs) using methodologies previously successfully applied to describe conjugated polymers and other organic molecular materials. The results of our simulations are in quantitative agreement with available spectroscopic data and show intricate details of excited‐state properties and photoinduced vibrational dynamics in carbon nanotubes. We analyze in detail the nature of strongly bound first and second excitons in SWCNTs for a number of different tubes, emphasizing emerging size‐scaling laws. Characteristic delocalization properties of excited states are identified by the underlying photoinduced changes in charge densities and bond orders. Due to the rigid structure, exciton–phonon coupling is much weaker in SWCNTs compared to typical molecular materials. Yet we find that, in the ground state, a SWCNT's surface experiences the corrugation associated with electron–phonon interactions. Vibrational relaxation following photoexcitation reduces this corrugation, leading to a local distortion of the tube surface, which is similar to the formation of self‐trapped excitons in conjugated polymers. The calculated associated Stokes shift increases with enlargement of the tube diameters. Such exciton vibrational phenomena are possible to detect experimentally, allowing for better understanding of photoinduced electronic dynamics in nanotube materials.

Molecular chains and carpets of sexithiophenes onAu(111)

The two organic molecular materials $\ensuremath{\alpha}$-sexithiophene (6T) and $\ensuremath{\alpha},\ensuremath{\omega}$-dihexylsexithiophene (DH6T) adsorbed on $\mathrm{Au}(111)$ in the (sub)monolayer range were investigated by scanning tunneling microscopy (STM) in order to explore the effect of alkyl substitution on the self-assembly at surfaces. Metal substrate step edges are identified as preferred nucleation sites for 6T, while stable nucleus formation for DH6T occurs at kinks of the $\mathrm{Au}(111)$ herringbone reconstruction. At low coverage, 6T forms continuous chains of single-molecular width along Au step edges, involving molecular conformation changes by rotations around C-C bonds of neighboring thiophene units. In contrast, DH6T exhibits no ordered structures in the submonolayer range. At monolayer coverage, substantially different structures were observed for the two molecules. 6T forms rows of molecules with parallel long molecular axes, whereas DH6T forms lines along these axes, where the conjugated cores are embedded in a matrix of hexyl chains. Because of different preferred nucleation sites, 6T forms a continuous molecular carpet on extended $\mathrm{Au}(111)$ terraces, whereas DH6T resembles a patchworklike carpet as domain boundaries are induced by the $\mathrm{Au}(111)$ herringbone surface structure, leading to reduced domain sizes. Alkylation of 6T thus drastically changes the adsorption behavior and the resulting layer structure on the Au surface. These results should be valuable for developing new directed self-assembly schemes on prepatterned surfaces.

Charge-Transfer Excitations in One-Dimensional Dimerized Mott Insulators

We investigate the optical properties of one-dimensional (1D) dimerized Mott insulators using the 1D dimerized extended Hubbard model. Numerical calculations and a perturbative analysis from the decoupled-dimer limit clarify that there are three relevant classes of charge-transfer (CT) states generated by photoexcitation: interdimer CT unbound states, interdimer CT exciton states, and intradimer CT exciton states. This classification is applied to understanding the optical properties of an organic molecular material, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which is known for its photoinduced transition from the dimerized spin-singlet phase to the regular paramagnetic phase. We conclude that the lowest photoexcited state of TTTA is the interdimer CT exciton state and the second lowest state is the intradimer CT exciton state.

Epitaxial growth of organic heterostructures: Morphology, structure, and growth mode

For organic molecular materials the definition of the condition of epitaxial growth has been subjected to several misconceptions and a great debate, leading in any case to less restrictive requirements than for inorganic materials. Here, the deposition of oligothiophene films by molecular beam epitaxy on properly grown organic single crystals is discussed and all-organic rigorous epitaxy demonstrated for both sub-monolayer and several-monolayer thick films. The possibility of growing all-organic nanostructures directly follows from these results.

Electronic states of organic molecular materials II

Electronic states of organic molecular materials II

Electronic structure of organic molecular materials I

Electronic structure of organic molecular materials I

Synthesis and properties of π conjugated organic molecular one-dimensional nanomaterials

Much attention is turning to nanoscale organic materials, due to many unique properties such as flexibility, high photoconductivity, and nonlinear optical effects that may offer novel applications in nano-optoelectronic devices. We have synthesised new π conjugated organic molecular aggregates using various methods, including template, solid phase organic reaction, thermal evaporation method and organic vapour-solid phase reaction. The dimension-dependent emission properties were observed in the nanowires and nanorods. The approaches are expected to form new general routes for the controlled morphosynthesis of organic molecular materials in restricted dimensions, with controlled size and shape, the solid-state physical properties of which are of great interest.

A multi-technique approach for probing the evolution of structural properties during crystallization of organic materials from solution

We are engaged in a multidisciplinary study of fundamental aspects of the crystallization of organic molecular materials from solution, focusing on polymorphic systems under the recognition that such systems represent an ideal opportunity for obtaining a systematic understanding of competing pathways in crystallization processes. The range of techniques employed in this work are sensitive to structural properties on different length scales and are thus appropriate for mapping the changes that occur at different stages of the crystallization process, starting from the early aggregation events in solution (probed by solution-state NMR and molecular dynamics simulations, including studies of diffusion properties), leading to the growth of molecular aggregates (probed by small-angle neutron scattering), then the emergence of solid microcrystals dispersed in the crystallization solution (probed by small-angle neutron scattering and solid-state NMR) and finally the formation of the bulk solid crystalline phase (probed by powder X-ray diffraction). This paper reports preliminary results on the application of this multi-technique approach to study the crystallization of glycine (which has three known polymorphic forms under ambient conditions) from aqueous solution.