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In view of the large-scale production of hydrogen (H2) by means of water electrolysis, it is of utmost importance to develop high-efficiency, stable and low-cost electrocatalysts. We report here on the synthesis and electrocatalytic properties of interconnected porous nanoflakes of the bimetallic catalyst CoMo2S4. These as-grown CoMo2S4 electrodes show higher electrocatalytic ability as compared to the reference CoMoO6 material, requiring a low OER/HER overpotential of 306/162 mV to reach 10 mA cm−2. We ascribe the excellent catalytic efficiency of our novel nanostructured CoMo2S4 catalyst to the combination of an increased number of active sites, enhanced charge transport properties, and a better contact at the CoMo2S4/electrolyte interface. When interconnected CoMo2S4 is used as a bifunctional catalyst in two-electrode electrolyzer cells, it maintains a voltage of 1.65 V to achieve 10 mA cm−2 for at least 10 hours, thus opening a new route for the design of efficient, stable and low-cost catalysts.

Polymorphism is a central phenomenon in materials science that often results in important differences of the electronic properties of organic crystals due to slight variations in intermolecular distances and positions. Although a large number of π-conjugated organic compounds can grow as polymorphs, it is necessary to have at disposal a series of several polymorphs of the same molecule to establish clear and predictive structure–property relationships. We report here on the occurrence of two solvates and three polymorphs in single crystalline form of the organic p-type semiconductor 2,2′,6,6′-tetraphenyldipyranylidene (DIPO). When grown from chlorobenzene or toluene, the DIPO crystals spontaneously capture solvent molecules to form two pseudopolymorphic 1 : 1 binary solvates. Independently, three solvent-free DIPO polymorphs are obtained either from the vapor phase or from acetonitrile and benzene. Surprisingly, single crystal field-effect transistors (SC-FETs) reveal that the DIPO 1 : 1 binary solvate grown from chlorobenzene possesses a higher hole mobility (1.1 cm2 V−1 s−1) than the three solvent-free polymorphs (0.02–0.64 cm2 V−1 s−1). A refined crystallographic analysis combined with a theoretical transport model clearly shows that the higher mobility of the solvate results from an improved π–π overlap. Our observations demonstrate that solvation allows to tune the π–π overlap and transport properties of organic semiconductors by selecting appropriate solvents.

In the quest for mass production of hydrogen from water electrolysis, to develop highly efficient, stable and low-cost catalysts is still the central challenge. When designing a novel catalyst, it is necessary to optimize the exposure and accessibility of its active sites as well as the reaction kinetics. This can be realized by combining an appropriate chemical composition of the material, including doping with metal elements, and a properly nanostructured morphology offering a high surface contact. We report here on the design and performances of cobalt-based oxide and sulfide nanowires as catalysts that can be used for both hydrogen and oxygen evolution reactions (denoted HER and OER respectively) in the same compatible electrolyte. Following a sulfuration process, Co3O4 nanowires are entirely converted into Co3S4 nanowires showing greatly improved OER catalytic performances with an overpotential of 283 mV (instead of 371 mV for Co3O4) to deliver a current density of 10 mA cm-2. Besides, when doping the surface of these Co3S4 nanowires with small amounts of nickel, the resulting Ni:Co3S4 nanowires exhibit an HER overpotential of 199 mV to reach 10 mA cm-2. But most importantly, two-electrode electrolyzer cells combining Co3S4 and Ni:Co3S4 electrodes show operating voltages as low as 1.70 V at 10 mA cm-2 over 40 hours, a value that competes advantageously with the best reported catalysts in 1.0 M KOH. Meanwhile, density functional theory (DFT) calculations show that the free energy of the intermediates has been reduced after the introduction of sulfur and nickel atoms, which have smaller overpotentials and corresponding enhanced electrocatalytic performance. Our results open a new avenue in the quest for overall water splitting using electrochemical systems.

Weak intermolecular interactions in organic semiconducting molecular crystals play an important role in determining molecular packing and electronic properties. Single crystals of metal‐free and metal phthalocyanines were synthesized to investigate how the coordination of the central metal atom affects their molecular packing and resultant electronic properties. Single‐crystal field‐effect transistors were made and showed a hole mobility order of ZnPc>MnPc>FePc>CoPc>CuPc>H2Pc>NiPc. Density functional theory (DFT) and 1D polaron transport theory reach a good agreement in reproducing the experimentally measured trend for hole mobility. Additional detail analysis at the DFT level suggests the metal atom coordination into H2Pc planes can tune the hole mobility via adjusting the intermolecular distances along the shortest axis with closest parallel π stackings. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore)

Carbon-coated hierarchical porous MoO2 nanoflowers are synthetized via a template-free solvothermal technique and investigated as anodes in lithium-ion batteries (LIBs). Such hybrid porous materials possess high porosity and a conductive layer, which have superior lithium ion/electron transport. The voids inside the structure can accommodate huge volume expansion and relieve the stress. The relative MoO2 electrodes present a high reversible capacity of 784.7 mA h g −1 after 60 cycles (707.7 mA h g−1 after 120 cycles for 200 mA g−1) and high coulumbic efficiency (above 98% over 120 cycles). Our study can easily be extended to other materials with a similar hierarchical structure in view of their application in LIBs. MOE (Min. of Education, S’pore)

A series of novel tetrasubstituted dipyranylidenes (DIPO) consisting of large π-extended moieties located at the four edge of the quinoidal core were synthesized and characterized with UV–visible, photoluminescence spectroscopies and cyclic voltammetry. Tetraphenyldipyranylidene compound has a high molar absorption coefficient with e = 8.73 × 104 l/mol.cm at 456 nm. Larger π-extended DIPO derivatives show a bathochromic absorption peak with lower molar absorption coefficient. In the presence of large π-extended moieties, these molecules present a higher first oxidation potential, leading to a deeper HOMO level. Remarkably, naphthyl, benzothienyl and benzofuryl-substituted DIPO show a third oxidation potential at 0.91 V, 0.99 V and 1.08 V respectively. These novel compounds which combined both aromatic and quinoidal characters are attractive for their applications as multiredox active materials.

When grown according to the conventional one-step annealing process at 550 °C, WO3 exhibits low photocurrent density and inferior photocurrent retention. In contrast, after a two-step annealing process at first 550 °C and second 700 °C, WO3 contains a higher concentration of oxygen deficiencies acting as shallow donors, thus leading to improved charge separation. Importantly, this results in a substantial photocurrent increase and higher material stability, two criteria that are of utmost importance for efficient photoelectrochemical water splitting.

In the quest for mass production of hydrogen from water electrolysis, to develop highly efficient, stable and low-cost catalysts is still the central challenge. When designing a novel catalyst, it is necessary to optimize the exposure and accessibility of its active sites as well as the reaction kinetics. This can be realized by combining an appropriate chemical composition of the material, including doping with metal elements, and a properly nanostructured morphology offering a high surface contact. We report here on the design and performances of cobalt-based oxide and sulfide nanowires as catalysts that can be used for both hydrogen and oxygen evolution reactions (denoted HER and OER respectively) in the same compatible electrolyte. Following a sulfuration process, Co3O4 nanowires are entirely converted into Co3S4 nanowires showing greatly improved OER catalytic performances with an overpotential of 283 mV (instead of 371 mV for Co3O4) to deliver a current density of 10 mA cm-2. Besides, when doping the surface of these Co3S4 nanowires with small amounts of nickel, the resulting Ni:Co3S4 nanowires exhibit an HER overpotential of 199 mV to reach 10 mA cm-2. But most importantly, two-electrode electrolyzer cells combining Co3S4 and Ni:Co3S4 electrodes show operating voltages as low as 1.70 V at 10 mA cm-2 over 40 hours, a value that competes advantageously with the best reported catalysts in 1.0 M KOH. Meanwhile, density functional theory (DFT) calculations show that the free energy of the intermediates has been reduced after the introduction of sulfur and nickel atoms, which have smaller overpotentials and corresponding enhanced electrocatalytic performance. Our results open a new avenue in the quest for overall water splitting using electrochemical systems.

Centimetre-scale CH3NH3PbI3 perovskite single crystals of high structural quality are grown by a new versatile method based on the use of a binary solvent mixture at a temperature as low as 70 °C. The growth rate is fast enough for a millimeter scale seed crystal to grow to the centimeter scale in a few hours.

Weak intermolecular interaction in organic semiconducting molecular crystals plays an important role in molecular packing and electronic properties. Here, four five‐ring‐fused isomers were rationally designed and synthesized to investigate the isomeric influence of linear and angular shapes in affecting their molecular packing and resultant electronic properties. Single‐crystal field‐effect transistors showed mobility order of 5,7‐ICZ (3.61 cm2 V−1 s−1) >5,11‐ICZ (0.55 cm2 V−1 s−1) >11,12‐ICZ (ca. 10−5 cm2 V−1 s−1) and 5,12‐ICZ (ca. 10−6 cm2 V−1 s−1). Theoretical calculations based on density functional theory (DFT) and polaron transport model revealed that 5,7‐ICZ can reach higher mobilities than the others thanks to relatively higher hole transfer integral that links to stronger intermolecular interaction due to the presence of multiple NH⋅⋅⋅π and CH⋅⋅⋅π(py) interactions with energy close to common NH⋅⋅⋅N hydrogen bonds, as well as overall lower hole‐vibrational coupling owing to the absence of coupling of holes to low frequency modes due to better π conjugation. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore)

A hierarchical multi-shell structure of MoO2 is synthesized through a template-free one-pot solvothermal method. The relative MoO2 electrode exhibits high reversible specific capacity of 780 mA h g−1 after 40 cycles, good stability, high capacity retention and improved rate performance in the lithium ion battery application. This material has a unique hierarchical architecture which is a combination of pure hollow structure and core-shell structure. Such a hierarchical multi-shell structure can not only amplify active mass-electrolyte contact area and minimizes lithium ion diffusion path, but also provides a unique interstitial void space to relieve the huge volume change of material. Materials with a hierarchical multi-shell structure are expected to be promising candidates in lithium ion batteries application. MOE (Min. of Education, S’pore)

We report on a new phenoxazine derivative, 10-butyl-phenoxazine-3-carboxylic acid (BPCA), that we designed to operate as a molecular relay in semiconductor-sensitized solar cells (SSCs). After BPCA surface modification and in the presence of a cobalt-bipyridyl complex acting as a redox mediator, both TiO

The accumulation of positive charges at the anodic interface considerably limits the efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs). Interfacial layers (IFLs) such as PEDOT:PSS improve charge injection but have no effect on the unbalanced electron/hole transport across the BHJ. We report here the use of 2,2′,6,6′-tetraphenyl-dipyranylidene (DIPO-Ph4), a planar quinoïd compound, as an efficient anodic IFL in organic solar cells based on BHJs made of poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric-acid-methyl-ester (P3HT:PCBM). When deposited under vacuum onto a glass substrate, DIPO-Ph4 thin films are constituted of densely packed and vertically aligned crystalline needles. Current-sensing atomic force microscopy (CS-AFM) reveals a considerable increase of the hole-carrying pathways in DIPO-Ph4 thin films as compare to PEDOT:PSS, thus revealing their hole transporting/electron blocking properties. Inserting a 10 nm thick IFL of DIPO-Ph4 in combination with a 5 nm thick PEDOT:PSS between the ITO electrode and the P3HT:PCBM film leads to photocurrent densities up to 11.5 mA/cm2 under AM 1.5G and conversion efficiencies up to 4.6%, that is substantially higher than PEDOT:PSS-only devices. MOE (Min. of Education, S’pore)

In this article the Wells-Dawson polyoxometalate K6[P2W18O62] is grown as an interfacial layer between indium tin oxide and bulk heterojunction of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The structure of the POM layers depends on the thickness and shows a highly anisotropic surface organization. The films have been characterized by atomic force microscopy and X-ray photoelectron spectroscopy (XPS) to gain insight into their macroscopic organization and better understand their electronic properties. Then, they were put at the anodic interface of a P3HT:PCBM organic solar cell and characterized on an optical bench. The photovoltaic efficiency is discussed in terms of the benefit of the polyoxometalate at the anodic interface of an organic photovoltaic cell., Comment: 7 pages, 6 figures

The conductance properties of 1,3-(trimethylsilyl)-1-tridecene-6,12-diyne, a non-conjugated trimethylsil-acetylene molecule have been investigated both experimentally and theoretically. Based on scanning tunnelling spectroscopy experiments, a discussion on the mechanisms controlling the charge transfer through this linear molecule is carried out. A specific property of the studied molecule is that it contains localized molecular orbitals. The shifts of the MOs energy levels caused by the applied voltage as well as a distant superexchange coupling between the respective localized MOs are shown to become determining in the formation of a nonlinear hole current through the molecule.

We report on a new phenoxazine derivative, 10-butyl-phenoxazine-3-carboxylic acid (BPCA), that we designed to operate as a molecular relay in semiconductor-sensitized solar cells (SSCs). After BPCA surface modification and in the presence of a cobalt-bipyridyl complex acting as a redox mediator, both TiO2/CdS/BPCA and TiO2/CdSe/BPCA SSCs exhibit enhanced photovoltaic performance and stability. In particular, the power conversion efficiencies of CdS and CdSe-based solar cells are improved by 90 % and 57 %, respectively. Furthermore, after 300 s the JSC of TiO2/CdS/BPCA SSCs is stabilized at 30 % of its initial value, while in the same time CdS-based devices retain only 1 % of their initial JSC. The origin of the improvement arises from the excellent electron-donating property of BPCA and its role as a powerful molecular relay in non-polysulfide based SSCs. MOE (Min. of Education, S’pore) NRF (Natl Research Foundation, S’pore)

International audience; We report on the use of 2,2′,6,6′-tetraphenyldipyranylidene (DIPO-Ph4), a large quinoidal planar π-conjugated heterocycle, as a simple, easy-to-synthetize, and efficient dopant-free hole-transport material in perovskite solar cells (PSCs). PSCs using pristine DIPO-Ph4 show photon conversion efficiencies up to 10.1 %, which is higher than a PSC utilizing dopant-free spiro-OMeTAD (5.1 %). DIPO-Ph4-based PSCs exhibit a short-circuit current density of 19.52 mA cm−2 and an open-circuit voltage of 0.933 V, values that are superior to dopant-free spiro-OMeTAD and comparable to doped spiro-OMeTAD. These better performances find their origin in a higher HOMO level (−4.74 eV) and a much higher hole mobility (2×10−2 cm2 V−1 s−1). Finally, PSCs based on DIPO-Ph4 possess a superior stability compared to doped-spiro-OMeTAD-based devices when tested over 600 h.

Implementation of non-planar aromatic motifs in the molecular design of π-conjugated materials represents a paradigm shift that is interesting from a fundamental research viewpoint but remains largely underdeveloped. To further facilitate this cause, we report on the synthesis and properties of two large polycyclic aromatic compounds (38 carbon atoms). In one compound the planar π-system pyrene is linked to bowl-shaped hydrocarbon corannulene through a C=C bond, which is referred to as “corannulene–vinylpyrene”. Photochemical treatment of this compound results in annulation of both aromatic nuclei and formation of the other large fused structure with ten six-membered aromatic rings, referred to as “corannulene–benzopyrene”. Photophysical and electrochemical studies reveal that the properties of these two structurally related hybrid materials arise from an extended π-conjugation between the planar and non-planar motifs and varies depending upon their molecular structure.

A new acceptor moiety, γ-lactone-Pechmann dye is synthesized and used in the construction of two new conjugated D-A polymers. The optical and electrochemical characterization shows a broad absorption band extended to 1000 nm and narrow band gaps with deep LUMO energy levels. The chemical structures of the side chains appended from the donor units affect the intermolecular aggregation and the charge transport in thin-film field-effect transistors. The high degree of planarity, low bandgap, and strong electron accepting properties could make γ-lactone-Pechmann dye as a good candidate for the construction of conjugated D-A polymers.

Layered double hydroxides (LDHs) are bimetallic hydroxides that currently attract considerable attention as co-catalysts in photoelectrochemical (PEC) systems in view of water splitting under solar light. A wide spectrum of LDHs can be easily prepared on demand by tuning their chemical composition and structural morphology. We describe here the electrochemical growth of NiFe-LDH overlayers on Cu2O electrodes and study their PEC behavior. By using the modified Cu2O/NiFe-LDH electrodes we observe a remarkable seven-fold increase of the photocurrent intensity under an applied voltage as low as −0.2 V vs Ag/AgCl. The origin of such a pronounced effect is the improved electron transfer towards the electrolyte brought by the NiFe-LDH overlayer due to an appropriate energy level alignment. Long-term photostability tests reveal that Cu2O/NiFe-LDH photocathodes show no photocurrent loss after 40 hours of operation under light at −0.2 V vs Ag/AgCl low bias condition. These improved performances make Cu2O/NiFe-LDH a suitable photocathode material for low voltage H2 production. Indeed, after 8 hours of H2 production under −0.2 V vs Ag/AgCl the PEC cell delivers a 78% faradaic efficiency. This unprecedented use of Cu2O/NiFe-LDH as an efficient photocathode opens new perspectives in view of low biasd or self-biased PEC water splitting under sunlight illumination.

In this work, we describe the synthesis of a molecule in which fullerene C60 is covalently attached to a bowl-shaped polycyclic aromatic hydrocarbon, corannulene. The developed synthetic route is comprised of four linear steps with isolated yields ranging from 41–91%. The first three steps relate to the preparation of a hydrazone derivative of corannulene carrying a butyric acid methyl ester group. This key compound generates a reactive diazo moiety under base-induced decomposition process. The generated diazoalkane then adds to the C60 molecule to furnish the targeted corannulene-C61-butyric acid methyl ester (CCBM) molecule. This compound is structurally characterized by NMR spectroscopy and mass spectrometry, whereas material properties are determined with the help of optical absorption spectroscopy and cyclic voltammetry. Finally, some preliminary aspects on its application are examined in a poly(3-hexyl thiophene)-based photovoltaic device.

Quantum-dot-sensitized solar cells (QDSCs) are promising solar-energy-conversion devices, as low-cost alternatives to the prevailing photovoltaic technologies. Compared with molecular dyes, nanocrystalline quantum dot (QD) light absorbers exhibit higher molar extinction coefficients and a tunable photoresponse. However, the power-conversion efficiencies (PCEs) of QDSCs are generally below 9.5 %, far behind their molecular sensitizer counterparts (up to 13 %). These low PCEs have been attributed to a large free-energy loss during sensitizer regeneration, energy loss during the charge-carrier transport and transfer processes, and inefficient charge separation at the QD/electrolyte interfaces, and various interfacial engineering strategies for enhancing the PCE and cell stability have been reported. Herein, we review recent progress in the interfacial engineering of QDSCs and discuss future prospects for the development of highly efficient and stable QDSCs.

Quantum-dot-sensitized solar cells (QDSCs) are promising solar-energy-conversion devices, as low-cost alternatives to the prevailing photovoltaic technologies. Compared with molecular dyes, nanocrystalline quantum dot (QD) light absorbers exhibit higher molar extinction coefficients and a tunable photoresponse. However, the power-conversion efficiencies (PCEs) of QDSCs are generally below 9.5 %, far behind their molecular sensitizer counterparts (up to 13 %). These low PCEs have been attributed to a large free-energy loss during sensitizer regeneration, energy loss during the charge-carrier transport and transfer processes, and inefficient charge separation at the QD/electrolyte interfaces, and various interfacial engineering strategies for enhancing the PCE and cell stability have been reported. Herein, we review recent progress in the interfacial engineering of QDSCs and discuss future prospects for the development of highly efficient and stable QDSCs. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore)

One inherent limitation to the efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs) is the accumulation of positive charges at the anodic interface. The unsymmetrical charge collection of holes and electrons dramatically decreases the short-circuit current. Interfacial layers (IFLs) such as poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) have no effect on the unbalanced electron/hole transport across the BHJ. We report here on the use of dithiapyrannylidenes (DITPY), a new class of planar quinoid compounds, as efficient hole-transporting/electron-blocking layers in organic solar cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) BHJs. Inserting a 15-nm-thick IFL of 4,4'-bis(diphenyl-2,6-thiapyrannylidene) (DITPY-Ph(4)) between the indium-tin oxide electrode and the P3HT:PCBM BHJ prevents detrimental space-charge effects and favors recombination-limited currents. Current-sensing atomic force microscopy reveals a drastic increase of the hole-carrying pathways in DITPY-Ph(4) compared to PEDOT:PSS. In ambient conditions, photovoltaic cells using DITPY-Ph(4) exhibit an 8% increase in the current density, although the conversion efficiency remains slightly lower compared to PEDOT:PSS-based devices. Finally, we present a detailed analysis of the photocurrent generation, showing that DITPY-Ph(4) IFLs induce a transition from unproductive space-charge-limited currents to recombination-limited currents.

The two-dimensional (2D) crystal engineering of molecular architectures on surfaces requires controlling various parameters related respectively to the substrate, the chemical structure of the molecules, and the environmental conditions. We investigate here the influence of temperature on the self-assembly of hexakis(n-dodecyl)-peri-hexabenzocoronene (HBC-C(12)) adsorbed on gold using scanning tunneling microscopy (STM) at the liquid/solid interface. We show that the packing density of 2D self-assembled HBC-C(12) can be precisely tuned by adjusting the substrate temperature. Increasing the temperature progressively over the 20-50 degrees C range induces three irreversible phase transitions and a 3-fold increase of the packing density from 0.111 to 0.356 molecule/nm(2). High-resolution STM images reveal that this 2D packing density increase arises from the stepwise desorption of the n-dodecyl chains from the gold surface. Such temperature-controlled irreversible phase transitions are thus a versatile tool that can then be used to adjust the packing density of highly ordered functional materials in view of applications in organic electronic devices.

Self-assembled monolayers (SAMs) of para- and ortho-fluorothiophenol (p- and o-FTP) spontaneously formed on Au(111) substrate have been contrasted through investigation by a scanning tunneling microscope (STM) at room temperature. High-resolution STM imaging reveals that p-FTP adopts a 6 x radical3R30 degrees molecule arrangement containing six molecules. Two different kinds of p-FTP molecule dimer line structures have been formed on Au(111) by intermolecular pi-pi stacking along 112 substrate directions, besides a single p-FTP molecule line. In contrast, o-FTP molecules self-assemble into a much looser wave-like SAM, which can be described as a 5 x 3 radical3R30 degrees structure containing two molecules. Periodic density functional theory (DFT) calculations for the two systems suggest that these kinds of FTP molecules preferentially take the asymmetrical positions between 3-fold face-centered cubic (fcc) hollow and bridge sites on Au(111), tilting from the substrate surface. Theoretical simulation gives apparent average tilted angles of 58 degrees and 68 degrees for p-FTP and o-FTP with respect to the surface normal, respectively. This simulation shows that o-FTP is more inclined to lie down toward the Au(111) surface compared to p-FTP. The difference between p-FTP and o-FTP SAM structures can be qualitatively understood in terms of the variation of intermolecular dipole-dipole orientation. This suggests that, besides well-known Au-S and pi-pi interactions, electrostatic interactions including dipole-dipole, quadrupole-quadrupole, and dipole-quadrupole interactions might also play an important role in influencing the SAM structures formed by aromatic thiols with a permanent dipole moment.

2D supramolecular structures on HOPG by self-assembly of physisorbed amphiphile nucleotides have been successfully imaged by high resolution STM. The organization of the systems depends on the nature of nucleic bases. In the case of the thymidine derivative a head-to-tail self-assembly is observed, whereas the amphiphile adenosine affords head-to-head nanostructures. The co-adsorption of complementary A+T amphiphile molecules induces the formation of a third head-to-head 2D supramolecular structure stabilized via base pairing. Molecular modelling calculations including the graphite surface provide models for all the 2D supramolecular systems observed.

A solution processible polymer- poly(3,3'''-didodecylquaterthiophene) (PQT-12) is investigated at the liquid/solid interface using the scanning tunneling microscopy (STM). Two-dimensional ordered films made up of self-assembled domains, with dimensions of 100 nm x 50 nmadsorbed on highly oriented pyrolytic graphite (HOPG) were formed. These domains consist of parallel lamellar polymer chains, with the alkyl chains forming interdigitated structures, along with U-shaped and closed ring segments of the polymer chains. A polymer chain packing model is proposed herein, which attempts to propose a correlation between the packing of long chains and charge mobilities. These STM results could help in understanding the relationship between the extended conjugation and molecular organization of the PQT-12 chains.

The variation in the refractive index of nanoporous anodic aluminum oxide (AAO) films regarding the sulfur anion incorporation is studied in this work. For this purpose, different samples are grown under potentiostatic conditions at different voltages and concentrations of sulfuric acid in the electrolyte. The samples are analyzed by Rutherford backscattering spectroscopy and infrared spectrometry, confirming the presence of sulfate anions and water embedded into the nanoporous AAO films. The incorporation of sulfate ions into the alumina matrix varies from 6.3 up to 11.7% regarding aluminum content. We have studied Fabry-Pérot optical interferences by shining incident monochromatic light in specular reflectance conditions. The reflected monochromatic light waves interact in internal reflectivity generating constructive and destructive interferences known as Fabry-Pérot optical interferences. An iterative method based on the equation for constructive interferences conditions in thin films and Snell equations is developed in order to calculate the refractive index of the nanoporous AAO films as a function of the wavelength. The calculated refractive indices increase when the sulfur content of the nanoporous AAO films decreases. The variation of the calculated refractive index is 0.08 and remains constant in the wavelength range 400-1200 nm., Work supported by the Spanish Ministry of Science and Innovation under project MAT2010-20798-05-01 and 05-05. The authors wish to thank the support from the Ministry of Education - Singapore under the AcRF Tier 2 (MOE2014-T2-1-132). Ignacio Mínguez Bacho thanks the Spanish National Research Council (CSIC) for a JAE-preDoc fellowship co-financed by the European Social Fund. Manuel Hernández-Vélez thanks financial support under project CCG08-UAM/MAT-4024.

International audience; The charge transport properties of 2,2 0 ,6,6 0-tetraphenyldipyranylidene (DIPO-Ph4 ), a large planar quinö ıd p-conjugated heterocycle, are investigated in field-effect transistor (FET) configuration and by conductive atomic force microscopy (c-AFM). The FET properties show a clear p-type behavior with a hole mobility up to 2 X 10 2 cm 2 V 11 and on/off ratio of 10 4. The transfer characteristics I d /V g present a clear hysteresis typical of a resistive memory effect. This memory effect is again observed by means of c-AFM in lateral mode using a nearby gold top-contact as the counter-electrode. The c-AFM current response recorded for variable distances d ¼ 0.5–9.0 mm between the AFM tip and the top electrode shows a resistive switching behavior in the low-voltage 0.0–3.0 V region. Repeated " write-read-erase-read " cycles performed at low frequency reveal a non-volatile memory effect in the form of high-resistance and low-resistance states with a stable on/off ratio of 102 during cycling operation.

Solution-processed n-type organic field-effect transistors (OFETs) based on the fullerene derivative {6}-1-(3-(2- thienylethoxycarbonyl)-propyl)-{5}-l-phenyl-[5,6]-C61 (TEPP) and phenyl-C61-butyric acid methyl ester (PCBM) in a multiring source/drain structure are reported. Devices with TEPP show high electron mobility up to 7.8 x 10-2 cm2/Vs in the saturation regime for bottom-contact OFETs with Au S/D electrodes with a solution-processed fullerene derivative. The ON/OFF ratios reported in this letter, which are in the range of 105 -106, are among the highest values reported for such devices. This mobility is always higher compared to PCBM devices prepared in identical conditions. The mobility of TEPP and PCBM increased with increasing temperatures in the range of 100-300 K with activation energy of 78 and 113 meV, respectively, which suggests that the thermally activated hopping of electrons is dominant in TEPP.

As an answer to the increasing energy demand of mobile battery-powered electronic devices, we propose a new approach offering an autonomous power source. Comprising a thin film organic or hybrid solar cell connected to a Lithium-polymer (Li-polymer) battery, the so-called EURO-PSB device possesses attractive characteristics like low weight (

Self-ordering degree of anodic alumina nanopores is related to the volume expansion of the aluminum oxide. However, little is known about how the ionic species derived from electrolyte affect parameters inducing self-ordering of the nanopores. The influence of sulfur incorporation into nanoporous anodic aluminum oxide (AAO) films on volume expansion and self-ordering degree has been investigated under potentiostatic conditions (14-25 V) in different sulfuric acid electrolytes (3-20 wt %), the average current densities of each anodization being in the range of 0.1-10 mA cm. Rutherford backscattering spectroscopy (RBS) reveals that the incorporation of sulfur species into AAO, as well as the volume expansion factor (VEF), follows a logarithmic dependence on the average current density regardless of the applied voltage and sulfuric acid concentration. The relationship between volume expansion and the S/Al ratio is linear for each concentration of acid in the electrolyte. Furthermore, self-ordering regimes are also revealed for each acid concentration at VEF in the range of 1.50-1.66. We suggest that plasticity, enhanced by sulfur incorporation, counterbalances the high mechanical stress generated by volume expansion, thus inducing new self-ordering regimes. These new regimes are dependent not only on VEF but also on a subtle equilibrium between stress and plasticity of the nanoporous AAO films., Work supported by the Spanish Ministry of Science and Innovation under project MAT2010-20798-05-01 and 05-05. The authors wish to thank the support from the Ministry of Education in Singapore under the AcRF Tier 2 (MOE2014-T2-1-132). I.M.B. thanks the Spanish National Research Council (CSIC) for a JAE-preDoc fellowship cofinanced by the European Social Fund. M.H.-V. thanks financial support under project CCG08-UAM/MAT-4024.

Cuprous oxide Cu2O is a promising p-type semiconductor for photoelectrochemical (PEC) solar hydrogen generation because it has a suitable bandgap (Eg = 2.0-2.2 eV) and a band alignment adapted to water reduction. In addition, metallic Cu is earth-abundant thus making Cu2O a low-cost material. However, the reduction potential of Cu2O into metallic Cu (0.47 V versus RHE) is lower than that of water which induces a severe instability under irradiation in a PEC cell. Therefore, our recent efforts focused on the growth of a protective overlayer on top of Cu2O in order to stabilize Cu2O when used as a photocathode in an aqueous electrolyte. Among potential protective materials cuprous sulphide Cu2S is another p-type semiconductor with a 1.2 eV bandgap and an appropriate energy level alignment with Cu2O that would allow electrons flowing to the interface. We present here an original and simple method aimed at protecting a compact layer (CL) or nanowires (NWs) of Cu2O with a Cu2S coating. Our method is based on the ions exchange reaction (IER) of O(2-) into S(2-) at the surface of Cu2O itself in a solution-containing Na2S as the sulphur source. The local surface IER implies the formation of a conformal and uniform coating independently on the starting Cu2O morphology, CLs or NWs. As expected, coating Cu2O photocathodes by a conformal Cu2S layer improves their stability and PEC performances.

Electroabsorption (EA) studies at room temperature on organic thin films based on a dicyanovinyl-quaterthiophene 4T-V(CN) 2 are reported. An electric field modulation is applied to the samples for two different electrode geometries, i.e. sandwich and coplanar versus the organic layer. Changes in optical absorption coefficient of 4T-V(CN) 2 based thin films are measured and analyzed to determine the character of the optical transition in the visible range (400–800 nm). Depending on the experimental electrode configuration, magnitude of electroabsorption responses are different, possibly due to different distribution of the externally applied electric field. The results indicate a higher resolution of EA response for the sandwich electrode configuration and confirm the charge transfer exciton character of 4T-V(CN) 2 in contrast to the unsubstituted quaterthiophene 4T. Finally, a third-order nonlinear susceptibility χ (3) (− ω ; ω , 0, 0) of 16 × 10 −12 e.s.u. is obtained.

Molecular organization and morphology of vacuum deposited (VD) thin films of the europium complex of europium(dibenzoylmethanato) 3 (bathophenanthroline) (Eu(DBM) 3 bath) from metal-organic planar light-emitting nanocomposites consisting of gold island films and VD Eu(DBM) 3 bath film were investigated by FTIR spectroscopy and scanning tunneling microscopy (STM). It has been found that thermal evaporation in vacuum does not cause chemical decomposition of Eu(DBM) 3 bath, and that the obtained films are essentially amorphous. Reflection–absorption IR spectra show no predominant orientation of Eu(DBM) 3 bath molecules in the films with respect to the substrate. With the STM method, the morphology and structure of the near-surface layers of Eu(DBM) 3 bath films were characterized, and it was found that they consist of self-assembled molecular dimers closely packed along the Au [110] steps on the Au(111) terrace. With increase of the Eu(DBM) 3 bath film thickness, the ordered structure is lost.

Single-crystal organic semiconductors represent perfect systems not only for studying basic science associated with transport of polaronic charge carriers but also for investigating the upper limits of mobilities in thin-film organic devices for flexible displays and other emerging electronic applications. The field-effect transistor provides an important tool to explore the transport of field-induced charge carriers at the surface of organic semiconductors. [1] Special care, however, must be taken during the device fabrication to avoid degradation of the critical interface between the semiconductor and the dielectric. [2,3] High material purity, excellent crystalline quality, and nanoscale morphological smoothness are among the characteristics of this interface that are crucial to obtaining high-performance devices [4,5] and test structures for studies of intrinsic phenomena. Local surface measurements of organic semiconductors at the active interfaces of conventional transistors are difficult to perform, if not impossible, due to the inaccessibility of the semiconductor/gate dielectric interface by many classes of useful probes. [6–9] A new method for building transistors on the pristine surfaces of bulk single crystals uses free-space dielectrics (i.e., vacuum or air) to eliminate completely any contact or processing of the active surface of the semiconductor. [10] This technique, which provides a reversible ability to assemble and dissemble the devices repetitively, recently enabled observation of intrinsic transport and transport anisotropy in rubrene. [10,11] Other groups, using different techniques

A series of thiophene-based oligomers are investigated by electroabsorption spectroscopy in terms of influence of chain length and peripheral substitution for the first time. Polycrystalline thin films of nonsubstituted oligomers, 8T, 6T and 4T, and of oligothiophenes with electron-withdrawing cyano substituents, 4T-V(CN) 2 and CN-TET-CN, are measured and analyzed to determine the character of optical transitions. The results of the nonsubstituted oligomers suggest that the transitions at 500–600 nm present a neutral (Frenkel exciton) character, while those of oligothiophenes with electron-withdrawing substituents suggest that the transitions present a polar (charge-transfer exciton) one, indicating significant contribution of noncentrosymmetric substitution. Third-order nonlinear susceptibilitiy χ (3) (− ω ; ω , 0, 0) is also evaluated, and it is shown that the longest unsubstituted oligomer 8T presents a high value of χ (3) ∼ 45 × 10 −12 esu, and substituted one CN-TET-CN presents further higher value of χ (3) ∼ 250 × 10 −12 esu. These investigations indicate multiple significance of electron-withdrawal substitution in molecular design of an oligothiophene-based nonlinear optical material.

The synthesis of a new linear benzo[3]phenylene–[60]fullerene dyad 1 is achieved over 10 steps in 15% overall yield by using an efficient sequence combining a double cobalt(I)-mediated cyclotrimerization with a Bingel reaction.

The growth and structure of self-assembled adlayers of hexakis(n-dodecyl)-peri-hexabenzocoronene (HBC-C12) adsorbed on highly ordered pyrolitic graphite (HOPG) decorated by an n-pentacontane (n-C50H102) monolayer have been investigated by scanning tunneling microscopy (STM). Whereas on HOPG the HBC-C12 molecules readily self-assemble into a unique stable 2D structure, on the [n-C50H102 monolayer/graphite] system we observe morphological phase transitions with formation of time dependent alpha, beta, and gamma phases (alpha--beta--gamma). The initial alpha-phase is similar to that obtained on bare graphite, while intermediate beta- and final gamma-structures present molecular dimers and rows, respectively. The observed two-dimensional polymorphism is due to weak interaction between HBC-C12 molecules and n-C50H102-modified graphite substrate. Our results constitute an important step toward the control of the growth and structure of highly ordered monolayers of functional conjugated molecules by modifying the graphite surface with an n-alkane monolayer of appropriate chain length.

Anchoring of discotic liquid crystals on atomically flat surfaces is important in the field of film growth and for a better understanding of charge transport across interfaces. The presented comparative scanning tunneling microscopy study shows that self-organized monolayers of 2,3,6,7,10,11-undecalkoxy-triphenylene (noted T 11 ) reveal a drastically different packing on Au(1 1 1) than on highly oriented pyrolitic graphite (HOPG). A hexagonal structure is found on HOPG while on Au(1 1 1) T 11 molecules form periodic parallel rows of dimers. These rows consist of molecules lying flat on the surface and aligned along specific directions of the underlying substrate. The presented results suggest that the interactions between the alkoxy chains of T 11 and the Au(1 1 1) substrate drive the formation of the observed supramolecular rows.

We present a detailed study on the structure and morphology of highly oriented thin films of octithiophene (8T), the longest non-substituted oligothiophene so far. 8T thin films are vacuum-deposited on glass substrates and oriented either vertically by adjusting deposition rate and substrate temperature or horizontally by using a mechanical rubbing technique. UV-Vis spectroscopy performed under polarized light leads to a quantitative analysis of the orientation in 8T films through the calculated dichroic ratio value (R). Both types of films are investigated by optical and electron microscopies, electron diffraction and atomic force microscopy (AFM). Vertically oriented 8T thin films show highly crystalline ordered grains with the unit cells vertically oriented relative to the substrate plane. Rubbed 8T thin films are highly birefringent and electron diffraction indicates the presence of a preferential arrangement with the 8T molecular axis parallel to each other and parallel to the rubbing direction with an 8T molecules chevron-type structure. (C) 2003 Elsevier Science B.V. All rights reserved.

The adsorption of C60 molecules on Au(1 1 1) surfaces has been investigated by using scanning tunneling microscopy (STM) and spectroscopy (STS) at a liquid–solid interface and at room temperature (RT). At low coverage, C60 decorates the step edges, forming two-dimensional islands on the terraces which indicate that the molecules have a high mobility even at RT. When the coverage is increased, the C60 molecules form a close-packed 2√3×2√3R30° structure on reconstructed Au(1 1 1) surface. It is demonstrated that STM in a liquid environment facilitates the performance of spectroscopy measurements and allows the manipulation of single molecules.