Comments by Tan Dun on His Music

Guest of honour at the Musimarch 2002 festival at McGill University, Montreal, the Chinese composer Tan Dun gave, amongst others, a conference on his music that developed into a passionate dialogue with the audience. We have transcribed his ideas on his music, his influences, his personal development and his impressions on Western music.Invité d’honneur du festival Musimars 2002 à l’Université McGill, Montréal, le compositeur chinois Tan Dun y donnait, entre autres, une conférence sur son oeuvre qui s’est transformée en un passionnant dialogue avec l’auditoire. Nous avons retranscrit ses propos sur sa musique, ses influences, son cheminement et ses impressions sur la musique occidentale

Supramolecular Approaches to Nanoscale Morphological Control in Organic Solar Cells

Having recently surpassed 10% efficiency, solar cells based on organic molecules are poised to become a viable low-cost clean energy source with the added advantages of mechanical flexibility and light weight. The best-performing organic solar cells rely on a nanostructured active layer morphology consisting of a complex organization of electron donating and electron accepting molecules. Although much progress has been made in designing new donor and acceptor molecules, rational control over active layer morphology remains a central challenge. Long-term device stability is another important consideration that needs to be addressed. This review highlights supramolecular strategies for generating highly stable nanostructured organic photovoltaic active materials by design

Understanding and controlling the growth of metals and molecules on an insulating surface

Noncontact atomic force microscopy (NC-AFM) was applied to investigating the creation of monatomic depth rectangular pits, the growth of metals, and the templated growth of molecules on the KBr (001) surface under ultrahigh vacuum conditions. The pits were produced by a new method where the sample is exposed to a controlled dose of charge from an electron beam evaporator. The structure and size distribution of the pits was characterized by NC-AFM. For the metal growth studies, gold, tantalum, and palladium were deposited onto KBr by electron beam deposition. The gold produced tall multiply twinned and epitaxial nanoparticles, while the tantalum formed flatter fractal islands. The palladium growth resulted in the creation of rectangular KBr islands in addition to palladium nanoparticles. Despite the use of a charge deviating grid, charge played an important role during the metal growth. In particular, the number density of gold nanoparticles followed nearly the same temperature dependence as the pits, suggesting that the metal nanoparticles nucleate predominantly at defect sites created by incident charge. The effect of charge was also seen in the tantalum system where pits surrounded the nanoparticles prepared at elevated temperatures. By creating pits before depositing gold, it was shown that the pits edges can be used to template the growth of metals. It was also shown that the pits can be used to trap PTCDA molecules and to align C60 molecules with the <100> direction of the substrate. Molecular resolution NC-AFM measurements were used to determine the structures and lattice constants of the molecular nanostructures. Experiments involving the sequential growth of metals and molecules showed that the order of deposition and the strength of the molecule-metal interaction are key factors in determining the nature of the growth. Furthermore, it was shown that metal structures can be used to nucleate the growth of sufficiently strongly interacting molecules

Temperature dependence of the energy dissipation in dynamic force microscopy

The dissipation of energy in dynamic force microscopy is usually described in terms of an adhesion hysteresis mechanism. This mechanism should become less efficient with increasing temperature. To verify this prediction we have measured topography and dissipation data with dynamic force microscopy in the temperature range from 100 K up to 300 K. We used 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) grown on KBr(001), both materials exhibiting a strong dissipation signal at large frequency shifts. At room temperature, the energy dissipated into the sample (or tip) is 1.9 eV/cycle for PTCDA and 2.7 eV/cycle for KBr, respectively, and is in good agreement with an adhesion hysteresis mechanism. The energy dissipation over the PTCDA surface decreases with increasing temperature yielding a negative temperature coefficient. For the KBr substrate, we find the opposite behaviour: an increase of dissipated energy with increasing temperature. While the negative temperature coefficient in case of PTCDA agrees rather well with the adhesion hysteresis model, the positive slope found for KBr points to a hitherto unknown dissipation mechanism

Role of van der Waals forces in the adsorption and diffusion of organic molecules on an insulating surface

The adsorption and diffusion of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules on a nanostructured KBr (001) surface were investigated by combining noncontact atomic force microscopy (NC-AFM) and first-principles calculations. Atomically resolved measurements demonstrate trapping of PTCDA molecules in intentionally created rectangular monolayer-deep substrate pits and a preferential adsorption at kink sites. In order to understand the experimental results, we found that it was essential to include a first-principles treatment of the van der Waals interactions. We show that at some sites on the surface, 85% of the molecular binding is provided by van der Waals interactions, and in general it is always the dominant contribution to the adsorption energy. It also qualitatively changes molecular diffusion on the surface. Based on the specificity of the molecular interaction at kink sites, the species of the imaged ionic sublattice in the NC-AFM measurements could be identified.Peer reviewe

Growth of large crystalline grains of vanadyl-phthalocyanine without epitaxy on graphene

The performance of organic semiconductor thin films in electronic devices is related to their crystal structure and morphology, with charge transport mobility dependent on the degree of crystallinity and on the crystallographic orientation. Here organic molecular beam deposition of vanadyl phthalocyanine is studied on graphene and it is shown that crystalline grains up to several micrometers across can be formed at substrate temperatures of 155 °C, compared to room temperature grain sizes of ≈30 nm. Transmission electron microscopy confirms the presence of long range order at elevated substrate temperatures and reveals that the molecules are stacked in an edge-on orientation, but are not epitaxially aligned to the graphene. The crystalline grain sizes are significantly larger on graphene than on disordered substrates such as graphene oxide and silicon oxide. The effect on charge transport is probed by conducting atomic force microscopy, with the high temperature films on graphene showing increased mobility and uniformity and decreased trap density. These results illustrate an important advantage for the integration of graphene electrodes with organic semiconductor devices: the homogeneous surface of graphene results in high diffusion and low nucleation rates for thin film growth, encouraging the formation of highly crystalline films even with nonepitaxial growth

High on-off conductance switching ratio in optically-driven self-assembled conjugated molecular systems

A new azobenzene-thiophene molecular switch is designed, synthesized and used to form self-assembled monolayers (SAM) on gold. An "on/off" conductance ratio up to 7x1E3 (with an average value of 1.5x1E3) is reported. The "on" conductance state is clearly identified to the cis isomer of the azobenzene moiety. The high "on/off" ratio is explained in terms of photo-induced, configuration-related, changes in the electrode-molecule interface energetics (changes in the energy position of the molecular orbitals with respect to the Fermi energy of electrodes) in addition to changes in the tunnel barrier length (length of the molecules). First principles DFT calculations demonstrate a better delocalization of the frontier orbitals, as well as a stronger electronic coupling between the azobenzene moiety and the electrode for the cis configuration over the trans one. Measured photoionization cross-sections for the molecules in the SAM are close to the known values for azobenzene derivatives in solution.Comment: 1 file with main text, figure and suppementary informatio

Substrate Templating upon Self-Assembly of Hydrogen-Bonded Molecular Networks on an Insulating Surface

Rahe P, Nimmrich M, Kühnle A. Substrate Templating upon Self-Assembly of Hydrogen-Bonded Molecular Networks on an Insulating Surface. Small. 2012;8(19):2969-2977.Molecular self-assembly on insulating surfaces, despite being highly relvant to many applications, generally suffers from the weak moleculesurface interactions present on dielectric surfaces, especially when benchmarked against metallic substrates. Therefore, to fully exploit the potential of molecular self-assembly, increasing the influence of the substrate constitutes an essential prerequisite. Upon deposition of terephthalic acid and trimesic acid onto the natural cleavage plane of calcite, extended hydrogen-bonded networks are formed, which wet the substrate. The observed structural complexity matches the variety realized on metal surfaces. A detailed analysis of the molecular structures observed on calcite reveals a significant influence of the underlying substrate, clearly indicating a substantial templating effect of the surface on the resulting molecular networks. This work demonstrates that choosing suitable molecule/substrate systems allows for tuning the balance between intermolecular and moleculesurface interactions even in the case of typically weakly interacting insulating surfaces. This study, thus, provides a strategy for deliberately exploiting substrate templating to increase the structural variety in molecular self-assembly on a bulk insulator at room temperature

P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance

Each year we are bombarded with B.Sc. and Ph.D. applications from students that want to improve the world. They have learned that their future depends on changing the type of fuel we use and that solar energy is our future. The hope and energy of these young people will transform future energy technologies, but it will not happen quickly. Organic photovoltaic devices are easy to sketch, but the materials, processing steps, and ways of measuring the properties of the materials are very complicated. It is not trivial to make a systematic measurement that will change the way other research groups think or practice. In approaching this chapter, we thought about what a new researcher would need to know about organic photovoltaic devices and materials in order to have a good start in the subject. Then, we simplified that to focus on what a new researcher would need to know about poly-3-hexylthiophene:phenyl-C61-butyric acid methyl ester blends (P3HT: PCBM) to make research progress with these materials. This chapter is by no means authoritative or a compendium of all things on P3HT:PCBM. We have selected to explain how the sample fabrication techniques lead to control of morphology and structural features and how these morphological features have specific optical and electronic consequences for organic photovoltaic device applications