Charge Transport through Molecular Rods with Reduced π‐Conjugation

A series of oligophenylene rods of increasing lengths is synthesized to investigate the charge‐transport mechanisms. Methyl groups are attached to the phenyl rings to weaken the electronic overlap of the π‐subsystems along the molecular backbones. Out‐of‐plane rotation of the phenyl rings is confirmed in the solid state by means of X‐ray analysis and in solution by using UV/Vis spectroscopy. The influence of the reduced π‐conjugation on the resonant charge transport is studied at the single‐molecule level by using the mechanically controllable break‐junction technique. Experiments are performed under ultra‐high‐vacuum conditions at low temperature (50 K). A linear increase of the conductance gap with increasing number of phenyl rings (from 260 meV for one ring to 580 meV for four rings) is revealed. In addition, the absolute conductance of the first resonant peaks does not depend on the length of the molecular wire. Resonant transport through the first molecular orbital is found to be dominated by charge‐carrier injection into the molecule, rather than by the intrinsic resistance of the molecular wire length.     

Preparation and Acid‐Responsive Photophysical Properties of T‐Shaped π‐Conjugated Molecules Containing a Benzimidazole Junction

T‐shaped π‐conjugated molecules with an N‐methyl‐benzimidazole junction have been synthesized and their acid‐responsive photophysical properties owing to the change in the π‐conjugation system are discussed. T‐shaped π‐conjugated molecules consist of two orthogonal π‐conjugated systems including a phenyl thiophene extended from the 2‐position and alkyl phenylenes connected through various π‐spacers from the 4,7‐positions of the N‐methyl‐benzimidazole junction. The π‐spacers, such as thiophene, ethyne, and ethane, have an effect on the acid response of photophysical properties in terms of changes in conformation, excited‐state energy and charge‐transfer (CT) characteristics. In particular, the π‐conjugated molecule with ethynyl spacers exhibited a marked redshift in the fluorescence spectrum with a large Stokes shift upon the addition of acid, whereas the other molecules showed substantial quenching. The redshift in emission was studied in detail by temperature‐dependent fluorescence measurements, which indicated the transition to a CT state over the finite activation energy at the excited state. The change in the frontier molecular orbitals upon acid addition was further discussed by means of DFT calculations.     

Solution route to inorganic nanobelt‐conducting organic polymer core‐shell nanocomposites

A facile and versatile solution‐based approach was developed to prepare semiconductor metal oxide nanobelt‐conducting organic polymer core‐shell nanocomposites. Well‐defined nanobelts of several types of oxide nanobelts were combined with conducting polymer [polypyrrole (PPy) and polyaniline (PANi)] via in situ polymerization in aqueous solution to obtain a new type of inorganic–organic composite nanostructure. Samples were characterized by using X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, electron energy loss spectra, high‐resolution transmission electron microscopy, and ultraviolet–visible techniques. Electron energy loss spectra revealed the existence of C?C and C? N bonds in coating layers to prove the encapsulation of PPy or PANi. The red‐shift of absorption band at high‐energy was observed for PPy‐encapsulated composites via ultraviolet–visible spectroscopy, and significant absorption band shifts were also encountered to PANi‐encapsulated composites, which suggest possibilities of band‐gap tuning of such metal oxide‐conducting polymer composites to be applied especially in solar cell devices. However, the sacrifice of nanobelts‐core led to hollow structures of PPy and PANi, which expands the synthetic strategies to prepare conducting polymer nanotubes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2892–2900, 2005     

Multisite bond and overlap treatment of polymer‐chain band structure

The usual tight‐binding (TB) approximation, employed in electronic structure calculations, is extended to include the more‐distant neighbor sites than the first. In doing so, a simple power law is adopted to describe the bond and overlap contributions, which enables the energy dispersion relation to be obtained in a closed analytic form. The resulting energy band structure has markedly different features from its TB counterpart to which it reduces in the appropriate limits. In particular, local extrema always occur at the TB stationary values, although these may not both be band edges. Another intermediate critical point may arise, which is one of the band edges, the other being located at one of the TB extrema. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 341–346, 2003     

Multichannel Conductance of Folded Single‐Molecule Wires Aided by Through‐Space Conjugation

Deciphering charge transport through multichannel pathways in single‐molecule junctions is of high importance to construct nanoscale electronic devices and deepen insight into biological redox processes. Herein, we report two tailor‐made folded single‐molecule wires featuring intramolecular π–π stacking interactions. The scanning tunneling microscope (STM) based break‐junction technique and theoretical calculations show that through‐bond and through‐space conjugations are integrated into one single‐molecule wire, allowing for two simultaneous conducting channels in a single‐molecule junction. These folded molecules with stable π–π stacking interaction offer conceptual advances in single‐molecule multichannel conductance, and are perfect models for conductance studies in biological systems, organic thin films, and π‐stacked columnar aggregates.     

High‐Performance,Stretchable, Wire‐Shaped Supercapacitors

A general approach toward extremely stretchable and highly conductive electrodes was developed. The method involves wrapping a continuous carbon nanotube (CNT) thin film around pre‐stretched elastic wires, from which high‐performance, stretchable wire‐shaped supercapacitors were fabricated. The supercapacitors were made by twisting two such CNT‐wrapped elastic wires, pre‐coated with poly(vinyl alcohol)/H₃PO₄ hydrogel, as the electrolyte and separator. The resultant wire‐shaped supercapacitors exhibited an extremely high elasticity of up to 350 % strain with a high device capacitance up to 30.7 F g⁻¹, which is two times that of the state‐of‐the‐art stretchable supercapacitor under only 100 % strain. The wire‐shaped structure facilitated the integration of multiple supercapacitors into a single wire device to meet specific energy and power needs for various potential applications. These supercapacitors can be repeatedly stretched from 0 to 200 % strain for hundreds of cycles with no change in performance, thus outperforming all the reported state‐of‐the‐art stretchable electronics.     

Radical‐Enhanced Charge Transport in Single‐Molecule Phenothiazine Electrical Junctions

We studied the single‐molecule conductance through an acid oxidant triggered phenothiazine (PTZ‐) based radical junction using the mechanically controllable break junction technique. The electrical conductance of the radical state was enhanced by up to 200 times compared to the neutral state, with high stability lasting for at least two months and high junction formation probability at room‐temperature. Theoretical studies revealed that the conductance increase is due to a significant decrease of the HOMO–LUMO gap and also the enhanced transmission close to the HOMO orbital when the radical forms. The large conductance enhancement induced by the formation of the stable PTZ radical molecule will lead to promising applications in single‐molecule electronics and spintronics.     

Low‐Temperature Solution Synthesis of Few‐Layer 1T ′‐MoTe2 Nanostructures Exhibiting Lattice Compression

Molybdenum ditelluride, MoTe₂, is emerging as an important transition‐metal dichalcogenide (TMD) material because of its favorable properties relative to other TMDs. The 1T ′ polymorph of MoTe₂ is particularly interesting because it is semimetallic with bands that overlap near the Fermi level, but semiconducting 2H‐MoTe₂ is more stable and therefore more accessible synthetically. Metastable 1T ′‐MoTe₂ forms directly in solution at 300 °C as uniform colloidal nanostructures that consist of few‐layer nanosheets, which appear to exhibit an approx. 1 % lateral lattice compression relative to the bulk analogue. Density functional theory calculations suggest that small grain sizes and polycrystallinity stabilize the 1T ′ phase in the MoTe₂ nanostructures and suppress its transformation back to the more stable 2H polymorph through grain boundary pinning. Raman spectra of the 1T ′‐MoTe₂ nanostructures exhibit a laser energy dependence, which could be caused by electronic transitions.     

Photoluminescence of bridged polysilsesquioxanes containing urea groups: Doping with Safranine‐O to generate red‐light emission

Bridged polysilsesquioxanes containing urea groups (ureasils) exhibit a broad photoluminescent emission in the visible spectrum. For a particular bridged polysilsesquioxane synthesized by the hydrolytic condensation of a precursor prepared by reaction of 4,4′‐[1,3 phenylenebis‐(1‐methylethylidene)]bis(aniline) (BSA) with 3‐(isocyanatepropyl)triethoxysilane (IPTES), we show that the broad photoluminescent band could be transformed into a relatively narrow emission band located in the red region of the spectra, by doping with Safranine‐O. The spectral overlap between the host emission and the dye absorption generated an efficient energy transfer process (antenna effect). Absorption, emission, and triplet–triplet absorption spectra revealed an efficient dispersion of the dye in the ureasil as well as the presence of strong host‐guest specific interactions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 289–296, 2008     

Chemical surface passivation of 3C‐SiC nanocrystals: A first‐principle study

The effect of the chemical surface passivation, with hydrogen atoms, on the energy band gap of porous cubic silicon carbide (PSiC) was investigated. The pores are modeled by means of the supercell technique, in which columns of Si and/or C atoms are removed along the [001] direction. Within this supercell model, morphology effects can be analyzed in detail. The electronic band structure is performed using the density functional theory based on the generalized gradient approximation. Two types of pores are studied: C‐rich and Si‐rich pores surface. The enlargement of energy band gap is greater in the C‐rich than Si‐rich pores surface. This supercell model emphasizes the interconnection between 3C‐SiC nanocrystals, delocalizing the electronic states. However, the results show a clear quantum confinement signature, which is contrasted with that of nanowire systems. The calculation shows a significant response to changes in surface passivation with hydrogen. The chemical tuning of the band gap opens the possibility plenty applications in nanotechnology. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2455–2461, 2010     

Laser effects in excitons in a quantum wire

We investigate the effects of laser field intensity over the ground state binding energy of light and heavy hole excitons confined in GaAs/Ga_1?xAl_x As cylindrical quantum wire. We have applied the variational method using 1s‐hydrogenic wave functions, in the framework of the single band effective mass approximation with the spatial dielectric function. The polaronic effects are included in the calculation to compute the exciton binding energy as a function of the wire radius for different field of laser intensity. The valence‐band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The dressed laser donor binding energies are calculated and compared with the results of binding energy of excitons. The results show that (i) the binding energy is found to increase with decrease with the wire radius, and decrease with increase with the value of laser field amplitude, (ii) the heavy‐hole exciton in a cylindrical quantum wire is more strongly bound than the light‐hole exciton, (iii) the values of ground state binding energy for the laser field amplitude α₀ = 10 Å resemble with the values of heavy hole exciton binding energy, and (iv) the binding energy of the impurity for the narrow well wire is more sensitive to the laser field amplitude. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Energy band alignment of SnO2/SrTiO3 epitaxial heterojunction studied by X‐ray photoelectron spectroscopy

(200) SnO₂//(111) SrTiO₃ (STO) epitaxial heterojunctions were fabricated using the laser molecular epitaxy technique. Sharp junction interface was confirmed by the high‐resolution transmission electron microscopy. The valence band offset and conduction band offset of the SnO₂/STO heterojunction were determined to be 0.85 ± 0.20 and 0.45 ± 0.20 eV using XPS, respectively. It is found that a type‐II band alignment forms at the SnO₂/STO interface. Copyright © 2015 John Wiley & Sons, Ltd.     

High‐Performance,Stretchable, Wire‐Shaped Supercapacitors

A general approach toward extremely stretchable and highly conductive electrodes was developed. The method involves wrapping a continuous carbon nanotube (CNT) thin film around pre‐stretched elastic wires, from which high‐performance, stretchable wire‐shaped supercapacitors were fabricated. The supercapacitors were made by twisting two such CNT‐wrapped elastic wires, pre‐coated with poly(vinyl alcohol)/H₃PO₄ hydrogel, as the electrolyte and separator. The resultant wire‐shaped supercapacitors exhibited an extremely high elasticity of up to 350 % strain with a high device capacitance up to 30.7 F g^?1, which is two times that of the state‐of‐the‐art stretchable supercapacitor under only 100 % strain. The wire‐shaped structure facilitated the integration of multiple supercapacitors into a single wire device to meet specific energy and power needs for various potential applications. These supercapacitors can be repeatedly stretched from 0 to 200 % strain for hundreds of cycles with no change in performance, thus outperforming all the reported state‐of‐the‐art stretchable electronics.     

On the Phosphorescence of 1H‐Phenalen‐1‐one

Combining the effects of heavy atom and low temperature, the phosphorescence spectrum from 1H‐phenalen‐1‐one has been unveiled. The 0‐0 band is located at 649 nm in methylcyclohexane and shifted to 646 nm in ethanol, which sets the triplet‐energy level to 185 and 186 kJ⋅mol⁻¹, respectively. The emission is unambiguously identified as phosphorescence originating from 1H‐phenalen‐1‐one through complementary transient absorption and emission studies. The quantum yield for triplet formation is confirmed to be unity.     

Energy transfer from fluorene‐based conjugated polyelectrolytes to on‐chain and self‐assembled porphyrin units

A new water soluble fluorene‐based polyelectrolyte containing on‐chain porphyrin units has been synthesized via Suzuki coupling, for use in optoelectronic devices. The material consist of a random copolymer of poly{1,4‐phenylene‐[9,9‐bis(4‐phenoxy butylsulfonate)]fluorene‐2,7‐diyl} (PBS‐PFP) and a 5,15‐diphenylporphyrin (DPP). The energy transfer process between the PBS‐PFP units and the porphyrin has been investigated through steady state and time‐resolved measurements. The copolymer PBS‐PFP‐DPP displays two different emissions one located in the blue region of the spectra, corresponding to the fluorene part and another in the red due to fluorescent DPP units either formed directly or by exciton transfer. However, relatively inefficient energy transfer from the PFP to the on‐chain porphyrin units was observed. We compare this with a system involving an anionic blue light‐emitting donor PBS‐PFP and a anionic red light‐emitting energy acceptor meso‐tetrakisphenylporphyrinsulfonate (TPPS), self‐assembled by electrostatic attraction induced by Ca²⁺. Based on previous studies related to chain aggregation of the anionic copolymer PBS‐PFP, two different solvent media were chosen to further explore the possibilities of the self‐assembled system: dioxane–water and aqueous nonionic surfactant n‐dodecylpentaoxyethylene glycol ether (C₁₂E₅). In contrast, with the on‐chain PBS‐PFP‐DPP system the strong overlap of the 0‐0 emission peak of the PBS‐PFP and the Soret absorption band of the TPPS results in an efficient Förster transfer. This is strongly dependent on the solvent medium used. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

T‐Jump/time‐of‐flight mass spectrometry for time‐resolved analysis of energetic materials

We describe a new T‐Jump/time‐of‐flight (TOF) mass spectrometer for the time‐resolved analysis of rapid pyrolysis chemistry of solids and liquids, with a focus on energetic materials. The instrument employs a thin wire substrate which can be coated with the material of interest, and can be rapidly heated (10⁵ K/s). The T‐Jump probe is inserted within the extraction region of a linear TOF mass spectrometer, which enables multiple spectra to be obtained during a single reaction event. By monitoring the electrical characteristics of the heated wire, the temperature could also be obtained and correlated to the mass spectra. As examples, we present time‐resolved spectra for the ignition of nitrocellulose and RDX. The fidelity of the instrument is demonstrated in the spectra presented which show the temporal formation and decay of several species in both systems. The simultaneous measurement of temperature enables us to extract the ignition temperature and the characteristic reaction time. The time‐resolved mass spectra obtained show that these solid energetic material reactions, under a rapid heating rate, can occur on a time scale of milliseconds or less. While the data sampling rate of 10 000 Hz was used in the present experiments, the instrument is capable of a maximum scanning rate of up to ～30 kHz. The capability of high‐speed time‐resolved measurements offers an additional analytical tool for the characterization of the decomposition, ignition, and combustion of energetic materials. Copyright © 2008 John Wiley & Sons, Ltd.     

Franck–Condon Blockade and Aggregation‐Modulated Conductance in Molecular Devices Using Aggregation‐Induced Emission‐Active Molecules

We report an effective modulation of the quantum transport in molecular junctions consisting of aggregation‐induced‐emission(AIE)‐active molecules. Theoretical simulations based on combined density functional theory and rate‐equation method calculations show that the low‐bias conductance of the junction with a single tetraphenylethylene (TPE) molecule can be completely suppressed by strong electron–vibration couplings, that is, the Franck‐Condon blockade effect. It is mainly associated with the low‐energy vibration modes, which is also the origin of the fluorescence quenching of the AIE molecule in solution. We further found that the conductance of the junction can be lifted by restraining the internal motion of the TPE molecule by either methyl substitution on the phenyl group or by aggregation, a mechanism similar to the AIE process. The present work demonstrates the correlation between optical processes of molecules and quantum transport in their junction, and thus opens up a new avenue for the application of AIE‐type molecules in molecular electronics and functional devices.     

Exploring concerted effects of base pairing and stacking on the excited‐state nature of DNA oligonucleotides by DFT and TD‐DFT studies

We have taken (dA)₅, (dT)₅, and (dA)₅·(dT)₅ as model systems to study concerted effects of base pairing and stacking on excited‐state nature of DNA oligonucleotides using density functional theory (DFT) and time dependent DFT methods. The spectroscopic states are determined to be of a partial A → A charge‐transfer nature in the A·T oligonucleotides. The T → T charge‐transfer transitions produce dark states, which are hidden in the energy region of the steady‐state absorption spectra. This is different from the previous assignment that the T → T charge‐transfer transition is responsible for a shoulder at the red side of the first strong absorption band. The A → T charge‐transfer states were predicted to have relatively high energies in the A·T oligonucleotides. The present calculations predict that the T → A charge‐transfer states are not involved in the spectra and excited‐state dynamics of the A·T oligonucleotides. In addition, the influence of base pairing and stacking on the nature of the ¹nπ* and ¹ππ* states are discussed in detail. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A Bipodal Dicyano Anchor Unit for Single‐Molecule Spintronic Devices

The conductance through single 7,7,8,8‐tetracyanoquinodimethane (TCNQ) connected to gold electrodes is studied with the nonequilibrium Green’s function method combined with density functional theory. The aim of the study is to derive the effect of a dicyano anchor group, ?C(CN)₂, on energy level alignment between the electrode Fermi level and a molecular energy level. The strong electron‐withdrawing nature of the dicyano anchor group lowers the LUMO level of TCNQ, resulting in an extremely small energy barrier for electron injection. At zero bias, electron transfer from electrodes easily occurs and, as a consequence, the anion radical state of TCNQ with a magnetic moment is formed. The unpaired electron in the TCNQ anion radical causes an exchange splitting between the spin‐α and spin‐β transmission spectra, allowing the single TCNQ junction to act as a spin‐filtering device.     

Pressure‐Induced Emission Enhancement,Band‐Gap Narrowing,and Metallization of Halide Perovskite Cs3Bi2I9

Low‐toxicity, air‐stable bismuth‐based perovskite materials are attractive substitutes for lead halide perovskites in photovoltaic and optoelectronic devices. The structural, optical, and electrical property changes of zero‐dimensional perovskite Cs₃Bi₂I₉ resulting from lattice compression is presented. An emission enhancement under mild pressure is attributed to the increase in exciton binding energy. Unprecedented band gap narrowing originated from Bi?I bond contraction, and the decrease in bridging Bi‐I‐Bi angle enhances metal halide orbital overlap, thereby breaking through the Shockley–Queisser limit under relatively low pressure. Pressure‐induced structural evolutions correlate well with changes in optical properties, and the changes are reversible upon decompression. Considerable resistance reduction implies a semiconductor‐to‐conductor transition at ca. 28 GPa, and the final confirmed metallic character by electrical experiments indicates a wholly new electronic property.