Showing papers on "Molecular solid published in 2011"

Small Molecule Solution-Processed Bulk Heterojunction Solar Cells†

Abstract: Although most research in the field of organic bulk heterojunction solar cells has focused on combinations of a p-type conducting polymer as a donor and a fullerene-based acceptor, recent work has demonstrated the viability of solution-processed heterojunctions composed entirely of molecular solids. Molecular solids offer potential advantages over conjugated polymer systems in terms of easier purification, amenability to mass-scale production and better batch-to-batch reproducibility. This article reviews the major classes of molecular donors that have been reported in the literature in the past several years and highlights some of key considerations in molecular heterojunction design compared to polymer-based bulk heterojunctions.

ReaxFF-lg: correction of the ReaxFF reactive force field for London dispersion, with applications to the equations of state for energetic materials.

Abstract: The practical levels of density functional theory (DFT) for solids (LDA, PBE, PW91, B3LYP) are well-known not to account adequately for the London dispersion (van der Waals attraction) so important in molecular solids, leading to equilibrium volumes for molecular crystals ∼10-15% too high. The ReaxFF reactive force field is based on fitting such DFT calculations and suffers from the same problem. In the paper we extend ReaxFF by adding a London dispersion term with a form such that it has low gradients (lg) at valence distances leaving the already optimized valence interactions intact but behaves as 1/R^6 for large distances. We derive here these lg corrections to ReaxFF based on the experimental crystal structure data for graphite, polyethylene (PE), carbon dioxide, and nitrogen and for energetic materials: hexahydro-1,3,5-trinitro- 1,3,5-s-triazine (RDX), pentaerythritol tetranitrate (PETN), 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), and nitromethane (NM). After this dispersion correction the average error of predicted equilibrium volumes decreases from 18.5 to 4.2% for the above systems. We find that the calculated crystal structures and equation of state with ReaxFF-lg are in good agreement with experimental results. In particular, we examined the phase transition between α-RDX and γ-RDX, finding that ReaxFF-lg leads to excellent agreement for both the pressure and volume of this transition occurring at ∼4.8 GPa and ∼2.18 g/cm^3 density from ReaxFF-lg vs 3.9 GPa and ∼2.21 g/cm^3 from experiment. We expect ReaxFF-lg to improve the descriptions of the phase diagrams for other energetic materials.

Polymorphic and mechanochromic luminescence modulation in the highly emissive dicyanodistyrylbenzene crystal: secondary bonding interaction in molecular stacking assembly

Abstract: Understanding the role of self-assembly and electronic interactions of constituent molecules in determining optoelectronic properties of a molecular solid is a fundamental and essential issue in material science. Particularly, the correlation between the molecular stacking mode and modulated optical properties has barely been established in spite of its scientific and technological importance. Herein, we present dicyanodistyrylbenzene-based highly luminescent crystals which uniquely exhibit polymorphism and mechanochromism. In these materials, various secondary bonding interactions (local dipole interaction, C–H⋯π interaction, and C–H⋯N hydrogen bond) play key roles in molecular stacking assembly, as well as in polymorphic and mechanochromic behaviors. The two different polymorphic phases of dicyanodistyrylbenzene crystal were correlated to the different modes of local dipole coupling, which caused a substantial alternation of π–π overlap and excited state delocalization to give differently colored fluorescence emission. Most uniquely, the phase transformation between those crystalline phases was effected through thermal and mechanical processes. We have comprehensively carried out in-depth and systematic optical, structural, and photophysical investigations to establish unambiguous structure–property relationships.

Porous organic molecular solids by dynamic covalent scrambling.

Abstract: The main strategy for constructing porous solids from discrete organic molecules is crystal engineering, which involves forming regular crystalline arrays. Here, we present a chemical approach for desymmetrizing organic cages by dynamic covalent scrambling reactions. This leads to molecules with a distribution of shapes which cannot pack effectively and, hence, do not crystallize, creating porosity in the amorphous solid. The porous properties can be fine tuned by varying the ratio of reagents in the scrambling reaction, and this allows the preparation of materials with high gas selectivities. The molecular engineering of porous amorphous solids complements crystal engineering strategies and may have advantages in some applications, for example, in the compatibilization of functionalities that do not readily cocrystallize.

Multiphase equation of state of hydrogen from ab initio calculations in the range 0.2 to 5 g/cc up to 10 eV

Abstract: We construct a multiphase equation of state (EoS) of hydrogen in the range 0.2 to 5 g/cc and up to 10 eV based on ab initio electronic structure calculations. In the molecular solid, cold curve and phonon spectra calculations are performed for various structures, proposed in the literature, to cover the stability field up to 500 GPa. A weak structural dependence is observed, and the solid EoS is averaged over these data. In the dissociating molecular fluid and in the dense plasma, calculations are made to complete the abundant data set in the literature. Two physical models are used to fit these calculations: a double-Debye model for the solid phase and a one-component plasma model with a mass action law for dissociation to implicitly access the molecular phase in the fluid state. The output of the calculations; energy, pressure, temperature, and density are perfectly reproduced with thermodynamical consistency. This model also allows us to access to the total free energy. The ionic quantum zero-point contribution is taken into account. The present hydrogen EoS is shown to reproduce most of the existing experimental data very well: the solid compression curve, the Hugoniot curve, the sound velocity in the molecular fluid, and the melting curve. The usefulness of this EoS is illustrated by the computation of an interesting isotopic shift on the melting curve and of an isentropic compression path reaching temperatures lower than 1000 K in the terapascal range.

The electronic properties of superatom states of hollow molecules.

Abstract: Electronic and optical properties of molecules and molecular solids are traditionally considered from the perspective of the frontier orbitals and their intermolecular interactions. How molecules condense into crystalline solids, however, is mainly attributed to the long-range polarization interaction. In this Account, we show that long-range polarization also introduces a distinctive set of diffuse molecular electronic states, which in quantum structures or solids can combine into nearly-free-electron (NFE) bands. These NFE properties, which are usually associated with good metals, are vividly evident in sp2 hybridized carbon materials, specifically graphene and its derivatives.The polarization interaction is primarily manifested in the screening of an external charge at a solid/vacuum interface. It is responsible for the universal image potential and the associated unoccupied image potential (IP) states, which are observed even at the He liquid/vacuum interface. The molecular electronic properties that ...

“Stubborn” triaminotrinitrobenzene: Unusually high chemical stability of a molecular solid to 150 GPa

Abstract: We report an unexpectedly high chemical stability of molecular solid 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) under static high pressures. In contrast to the high-pressure behavior of the majority of molecular solids, TATB remains both chemically stable and an insulator to 150 GPa--well above the predicted metallization pressure of 120 GPa. Single crystal studies have shown that TATB exhibits pressure-induced Raman changes associated with two subtle structural phase transitions at 28 and 56 GPa. These phase transitions are accompanied by remarkable color changes, from yellow to orange and to dark red with increasing pressure. We suggest that the high-stability of TATB arises as a result of its hydrogen-bonded aromatic two-dimensional (2D) layered structure and highly repulsive interlayer interaction, hindering the formation of 3D networks or metallic states.

Plasmon dispersion in molecular solids: Picene and potassium-doped picene

Abstract: We investigate the dynamic response of pristine and potassium-doped picene, the first example of a new family of organic molecular superconductors, by combining first-principles calculations and state-of-the-art experimental tools. We find that charge-carrier plasmons in K${}_{3}$ picene have a negative or almost negligible dispersion, which deviates from the traditional picture of metals based on the homogeneous electron gas. We show how this finding is the result of the competition between metallicity and electronic localization on the molecular units. Conduction electrons alone give rise to the negative dispersion, which is reduced by molecular polarization and crystal local-field effects. This analysis allows us to obtain a general picture of the plasmon dispersion in metallic molecular crystals.

Exciton character in picene molecular solids

Abstract: We have studied the low-energy electronic excitations of solid picene at 20 K using momentum dependent electron energy-loss spectroscopy. Our results demonstrate the presence of five excitonic features below the transport energy gap of picene, which all are characterized by a negligible dispersion. One of these excitons has not been observed in the optical absorption spectrum of picene molecules in solution and thus is assigned to a (solid-state induced) charge transfer exciton. This conclusion is supported by the momentum dependent intensity variation of this exciton which clearly signals a significant dipole forbidden contribution, in contrast to the other low energy excitations.

Importance of accurate spectral simulations for the analysis of terahertz spectra: citric acid anhydrate and monohydrate.

Abstract: The terahertz (THz) spectra of crystalline solids are typically uniquely sensitive to the molecular packing configurations, allowing for the detection of polymorphs and hydrates by THz spectroscopic techniques. It is possible, however, that coincident absorptions may be observed between related crystal forms, in which case careful assessment of the lattice vibrations of each system must be performed. Presented here is a THz spectroscopic investigation of citric acid in its anhydrous and monohydrate phases. Remarkably similar features were observed in the THz spectra of both systems, requiring the accurate calculation of the low-frequency vibrational modes by solid-state density functional theory to determine the origins of these spectral features. The results of the simulations demonstrate the necessity of reliable and rigorous methods for THz vibrational modes to ensure the proper evaluation of the THz spectra of molecular solids.

Roles of 2D Liquid in Reduction of the Glass-Transition Temperature of Thin Molecular Solid Films

Abstract: To clarify the properties of a liquidlike phase formed on the surface, self-diffusion of molecules in monolayer and multilayer films has been investigated by measuring their uptake behaviors in porous Si substrates using time-of-flight secondary ion mass spectrometry and temperature programmed desorption. It is found that self-diffusion of water, ethanol, and 3-methylpentane in the first monolayer commences at temperatures of ca. 110, 85, and 50 K, respectively, which are considerably lower than the corresponding glass-transition temperatures (Tg) in the bulk. The surface mobility of water appears to be not influenced by hydrogen bonds with substrates: The molecules form droplets on the H-terminated Si substrate at 110 K, whereas they start to diffuse into pores of the OH-terminated Si substrate at the same temperature. The ethanol and 3-methylpentane molecules tend to diffuse into pores of both hydrophilic and hydrophobic Si substrates, although a small number of much slower diffusers remain on the surfa...

Investigation of the Low-Frequency Vibrations of Crystalline Tartaric Acid Using Terahertz Spectroscopy and Solid-State Density Functional Theory

Abstract: The room temperature and cryogenic terahertz (THz) spectra (10–95 cm–1) of l-tartaric acid and dl-tartaric acid were investigated. At 293 K, the l-tartaric acid spectrum showed four absorption features at 36.4, 61.6, 78.7, and 87.3 cm–1 in the experimental spectrum. Once cooled to 78 K, these features narrowed and shifted to 35.9, 63.4, 81.1, and 90.1 cm–1. The THz spectrum of dl-tartaric acid is significantly different, containing only a single absorption at 79.9 cm–1 at room temperature, which shifts to 82.9 cm–1 at 78 K. Solid-state density functional theory calculations [B3LYP/6-311G(2d,2p)] were performed to simulate the crystalline structure of both molecular solids and to assign the observed spectral features to specific atomic motions. The THz spectrum of l-tartaric acid is particularly interesting in that it contains a theoretically unaccounted for spectral feature that may arise from second-order phonon processes and also exhibits an anomalous red-shifting absorption feature with cooling that is...

Fluid Flow and Effusive Desorption: Dominant Mechanisms of Energy Dissipation after Energetic Cluster Bombardment of Molecular Solids.

Abstract: The angular distribution of intact organic molecules desorbed by energetic C60 primary ions was probed both experimentally and with molecular dynamics computer simulations For benzo[a]pyrene, the angular distribution of intact molecules is observed to peak at off-normal angles Molecular dynamics computer simulations on a similar system show the mechanism of desorption involves fast deposition of energy followed by fluid-flow and effusive-type emission of intact molecules The off-normal peak in the angular distribution is shown to arise from emission of intact molecules from the rim of a crater formed during the cluster impact This signature is unique for molecules because fragmentation processes remove molecules that would otherwise eject at directions near-normal to the surface

Low-temperature properties of glassy and crystalline states of n-butanol

Abstract: We present low-temperature measurements of the specific heat and the thermal conductivity for the three solid phases of n-butanol, namely glass, crystal and “glacial” phases. Whereas crystal and glass ones are found to exhibit the expected thermal behavior for crystalline and non-crystalline solids, respectively (i.e. Debye theory for crystals, and universal low-temperature properties with a boson peak and a concomitant plateau in the thermal conductivity for glasses), the so-called “glacial phase” behaves as a very defective crystal, confirming earlier work that identifies it as a mixed phase of nanocrystallites and a disordered matrix. We have also measured longitudinal and transverse sound velocities at low temperatures for the glass phase. The elastic Debye coefficient and Debye temperature of the glass determined from these measurements are found to agree very well with the calorimetric ones obtained from a SPM analysis of the specific heat.

Phase diagram of carbon dioxide: update and challenges

Abstract: We present the phase diagram of carbon dioxide with the most recent finding of coesite-like carbon dioxide, a missing analog to SiO2, address several controversies on phase VII and phase IV in terms of the phase metastabilities and thermal path-dependent phase transitions, and discuss the implications to the generalized phase diagram of simple molecular solids.

Electron Density Topology of Crystalline Solids at High Pressure

Abstract: This chapter focuses on the electron density in solids under external stress. The modifications of electronic configurations and chemical bonding are studied through analysis of direct space indicators. Examples are given of elemental solids, ionic compounds and molecular crystals, including many experimental evidences.

The effect of the H:C ratio on the sputtering of molecular solids by fullerenes

Abstract: The understanding of the process by which molecular solids are sputtered by keV clusters such as C 60 is of great importance if cluster SIMS is to be routinely used to depth profile a wide range of molecular materials. Computer simulations of the impact of a C 60 cluster on a molecular solid show that an impact crater is produced. At the edges of the crater, a reaction zone is created which contains fragmented and cross-linked molecules. It has been shown previously that this'reaction zone' can be extensive for a molecular material like fullerite, and yet, for benzene it appears to be smaller. It is well known that fullerite cross-links through cycloaddition under compression in which the normal sp 2 bonding in the fullerene molecules takes on a tetragonal sp 3 pattern when two molecules are forced together creating a strong cross-link between them. It is this process which leads to such an extensive reaction zone after impact. The presence of hydrogen in other molecular systems could resist this process. The purpose of the investigation reported here is to observe the nature of the reaction zone for different molecular systems with varying hydrogen content and different initial coordination. Molecular dynamics simulations of molecular solids of octane, octatetraene, benzene and fullerite struck by 15 keV C 60 were performed. Octane is initially four-fold coordinated, whilst the other molecular solids are all three-fold coordinated. The reaction zone of the crater formed by the impact was measured and it was concluded that the H: C ratio influences the nature of the reaction zone, whereas the initial coordination has only a secondary effect on the reaction zone.

Solid phases and pairing in a mixture of polar molecules and atoms

Abstract: We consider a mixture of hard-core bosonic polar molecules, interacting via repulsive dipole-dipole interaction, and one atomic bosonic species. The mixture is confined on a two-dimensional square lattice and, at low enough temperatures, can be described by the two-component Bose-Hubbard model. The latter displays an extremely rich phase diagram including solid, superfluid, and supersolid phases. Here, we mainly focus on the checkerboard molecular solid, stabilized by the long-range dipolar interaction, and study how the presence of atoms affects its robustness both at zero and finite temperatures. We find that, due to atom-molecule interactions, solid phases can be stabilized at both (much) lower strengths of dipolar interaction and higher temperatures, than when no atoms are present. As a byproduct, atoms also order in a solid phase with the same melting temperatures as for molecules. Finally, we find that for large enough interaction between atoms and molecules, a paired supersolid phase can be stabilized.

Formation of H2+ by Ultra-Low-Energy Collisions of Protons with Water Ice Surfaces

Abstract: The molecular ion of dihydrogen (H2 + ) is produced by 1 eV collisions of protons (H + ) onamorphous water ice surfaces.The reaction is also observed on crystalline ice surfaces, but with lower efficiency. Collisions of D + on amorphous H2O and D2O ices yield D2 + on the former, subsequent to isotope exchange on the H2O surface. Ultra-low-energy collision-induced dihydrogen ion production is also observed from alkanol surfaces, with decreasing efficiency asthe alkyl chainlength increases.Thereis nocorrespond- ing reaction on solid hexane. This endothermic reaction, with implications for interstellar chemistry and plasma etching processes, is proposed to occur as a resultofstabilizationoftheotherreactionproduct,ahydroxylradical(OH  ),on water surfaces through hydrogen-bonding interactions with the surface. These results point to an interesting chemistry involving ultra-low-energy ions on molecular solids.

Molecular ions in cluster bombardment: what clues do the molecular dynamics simulations provide?

Abstract: Molecular dynamics simulations of C 60 bombardment of molecular solids suggest that there is a space-time divide between the fragmentation/ion creation process and the ejection of intact molecules via fluid flow physics. This space-time divide leads to the conclusion that molecular ions are preformed prior to ejection. Consequently, to increase the ion yield significantly, the sample must be altered to create more preformed ions.

Applying the Z method to estimate temperatures of melting in structure II clathrate hydrates.

Abstract: Equilibrium melting temperatures for structure II THF hydrate and argon/xenon (Ar/Xe) binary hydrate have been calculated using molecular dynamics using two melting techniques, namely the Z method [Belonoshko et al., Phys. Rev. B, 2006, 73, 012201] (applied for the first time to complex molecular solids) and direct phase coexistence simulations. The two methods give results in moderate agreement: calculations with the Z method give Tfus to be 250.7 K (0.77 katm) for THF and 244.3 K (1.86 katm) for Ar/Xe hydrate respectively; the corresponding direct phase coexistence calculations give Tfus in the range 235–240 K (0.77 katm) for THF and 240–252.5 K (1.86 katm) for Ar/Xe hydrate. The Z method was found to define the key thermodynamic states with high precision, although required long simulation times with these multicomponent molecular systems to ensure the complete melting required by the method. In contrast, the direct phase coexistence method did bracket the equilibrium temperature with little difficulty, but small thermodynamic driving forces close to phase equilibrium generated long-lived fluctuations, that obscured the precise value of phase coexistence conditions within the bracketed range.

Low-Temperature Magnetization Relaxation in Magnetic Molecular Solids

Abstract: The low temperature relaxation of the magnetization in magnetic molecular solids such as Fe$_8$ is studied using Monte Carlo simulations. A set of rate equations is developed to understand the simulations, and the results are compared. The simulations show that the magnetization of an initially saturated samples deviates as a square-root in time at short times, as observed experimentally, and this law is derived from the rate equations analytically.

Dirac Electrons in Molecular Solids

Abstract: Electrons in solids are characterized by the energy bands, which indicate that electrons are considered to be "elementary particles" with specific effective masses and g-factors reflecting features of each solid. There are cases where these particles obey dispersion relationship similar to those of Dirac electrons. Examples include graphite and bismuth both of which are known for many years, together with graphene, a single layer of graphite, recently addressed intensively after its realization. Another recent example is a molecular solid, alpha-ET2I3, which is described by an equation similar to Weyl equation with massless Dirac cones but the coordinate axis is tilted because of the location of cones at off-symmetry points. Orbital susceptibility of such Dirac electrons in graphite and bismuth has been known to have striking features not present in ordinary band electrons but resulting from the inter-band matrix elements of magnetic field. Results of theoretical studies on not only orbital susceptibility but also Hall effect of such Dirac electrons in molecular solids with tilting are introduced in this paper.

Dirac Electrons in Molecular Solids

Abstract: Electrons in solids are characterized by the energy bands, which indicate that electrons are considered to be "elementary particles" with specific effective masses and g-factors reflecting features of each solid. There are cases where these particles obey dispersion relationship similar to those of Dirac electrons. Examples include graphite and bismuth both of which are known for many years, together with graphene, a single layer of graphite, recently addressed intensively after its realization. Another recent example is a molecular solid, α-ET2I3, which is described by an equation similar to Weyl equation with massless Dirac cones but the coordinate axis is tilted because of the location of cones at off-symmetry points. Orbital susceptibility of such Dirac electrons in graphite and bismuth has been known to have striking features not present in ordinary band electrons but resulting from the inter-band matrix elements of magnetic field. Results of theoretical studies on not only orbital susceptibility but also Hall effect of such Dirac electrons in molecular solids with tilting are introduced in this paper.

Mixed resolution model for C60 cluster bombardment of solid benzene

Abstract: Molecular dynamics simulations of C60 cluster bombardment have been instrumental in elucidating physical phenomena related to the sputtering process; however, chemical phenomena can also play an important role in C60 cluster bombardment of molecular solids. Therefore, a mixed resolution model of C60 cluster bombardment is being developed, where the reactive zone is represented by an all atom region, and the remaining part of the target is described by a coarse-grained representation. A reactive many body potential describes the interactions among atoms; whereas, pair potentials describe the interactions between coarse-grained beads and between coarse-grained beads and atoms. Solid benzene is used to develop the methodology of blending the potentials. The blending of potentials is evaluated by the differences in the velocities of the pressure waves (generated by the C60 impact) between the all atom benzene, coarse-grained benzene and the mixed resolution benzene systems. Initial testing with 1 keV C60 cluster bombardment simulations show a smooth transition between regions.

Solid‐State Nuclear Magnetic Resonance

Abstract: The first experimental verification of nuclear magnetic resonance (NMR) in the solid state was carried out by Edward M. Purcell, Henry C. Torrey, and Robert V. Pound in 1945. Since then, new developments have transformed this form of spectroscopy from an interesting physical curiosity at its inception to one of the most widely used analytical tools for physical, chemical, biological, and materials research and applications. The capabilities of NMR include the following: identification of molecules in all phases of matter by their atomic connectivities; probing the temperature-dependent conformation of molecules from cyclohexane to complex 50 kDa proteins; molecular dynamics investigations from the picosecond timescale for rotation of small molecules to the second timescale for atoms diffusing throughout the three-dimensional nanoscopic channels of zeolites; elucidation of chemical reaction mechanisms; the determination of organization, reorientation, and diffusion of molecular solids; the identification of phase changes and morphology of solids; the measurement of bond lengths and interatomic distances between nonbonded atoms; the assessment of pore size in nanoporous solids as well as the identification of adsorbates and their diffusivities; the estimation of physicochemical parameters of molecular systems such as equilibrium and kinetic rate constants, enthalpies and entropies of a reaction, and activation energies; the capability to follow a metabolic pathway taking place inside living cells; the quantification of the alignment of molecules in partially ordered systems; the determination of nanoscale domain sizes in heterogeneous materials; and the noninvasive imaging of all phases and forms of matter under various temperature, pressure, and mechanical conditions. The scope of this article is very selective, covering some of the most widely used techniques and more unusual areas of application of solid-state NMR. These topics are covered at different levels of detail, and some are just mentioned by reference to published work. This article is not intended to present a comprehensive review of all state-of-the-art NMR techniques; for these, the reader should consult references included in the text. Instead, this article provides an historical background and the fundamental concepts of magnetic resonance that preceded solid-state NMR, a general overview of the basic principles with emphasis on the visualization of anisotropic spin interactions, a review of the most widely used methods for investigating solids by NMR, and practical information to provide a modicum of guidance for application of the methods. There are several redundancies in terminology so that different sections can be read independently. The references are an important component of the article and should be consulted for more complete and often original accounts of the theory and techniques for the application of NMR to solids. A modest attempt has been made to include original and important references, but more importantly, many have been included to provide examples of chemical and physical insights obtained in whole or in part by application of NMR techniques to the investigation of the problem. For figures and tables, if no reference or acknowledgment has been provided, the spectrum or photograph in the figure or the table is previously unpublished. The article is directed at a general audience of chemical and physical analytical scientists with an interest in NMR. In addition, NMR spectroscopists may find some interesting details of studies on metals and magnetic materials. Keywords: solid; tensor; induced magnetic field; resonance surface; magic angle spinning; anisotropy; spin relaxation; shielding; Knight shift; quadrupole moment; Rabi field; molecular beam; T-field