Selective Dissolution-Derived Nanoporous Design of Impurity-Free Bi2 Te3 Alloys with High Thermoelectric Performance.

Thermoelectric technology, which has been receiving attention as a sustainable energy source, has limited applications because of its relatively low conversion efficiency. To broaden their application scope, thermoelectric materials require a high dimensionless figure of merit (ZT). Porous structuring of a thermoelectric material is a promising approach to enhance ZT by reducing its thermal conductivity. However, nanopores do not form in thermoelectric materials in a straightforward manner; impurities are also likely to be present in thermoelectric materials. Here, a simple but effective way to synthesize impurity-free nanoporous Bi0.4 Sb1.6 Te3 via the use of nanoporous raw powder, which is scalably formed by the selective dissolution of KCl after collision between Bi0.4 Sb1.6 Te3 and KCl powders, is proposed. This approach creates abundant nanopores, which effectively scatter phonons, thereby reducing the lattice thermal conductivity by 33% from 0.55 to 0.37 W m-1 K-1 . Benefitting from the optimized porous structure, porous Bi0.4 Sb1.6 Te3 achieves a high ZT of 1.41 in the temperature range of 333-373 K, and an excellent average ZT of 1.34 over a wide temperature range of 298-473 K. This study provides a facile and scalable method for developing high thermoelectric performance Bi2 Te3 -based alloys that can be further applied to other thermoelectric materials.

Metal-Free Organic Phosphors toward Fast and Efficient Room-Temperature Phosphorescence.

ConspectusMetal-free purely organic phosphors (POPs) are promising materials for display technologies, solid-state lighting, and sensors platforms because of their advantageous properties such as large design windows, easy processability, and economic material cost. Unlike inorganic semiconductors, creating the conditions for triplet excitons to produce light in organic materials is a demanding task because of the presence of electron spin configurations that undergo spin-forbidden transitions, which is usually facilitated by spin-orbit coupling (SOC). In the absence of heavy metals, however, the SOC efficiency in POPs remains low, and consequently, external nonradiative photophysical processes will also severely affect triplet excitons. Addressing these challenges requires the development of rational molecular design principles to accurately account for how all conceivable structural, electronic, chemical, compositional factors affect materials performance.This Account summarizes important molecular design and matrix engineering strategies to tackle the two key challenges for POPs─boosting SOC efficiencies and suppressing nonradiative decays. We start by reviewing the fundamental understanding of internal and external factors affecting the emission efficiencies of POPs, including the theory behind SOC and the origin of nonradiative decays. Subsequently, we discuss the design of contemporary POP systems on the basis of research insights from our group and others, where SOC is mostly promoted by heavy atom effects and the El-Sayed rule. On one hand, nonmetal heavy atoms including Br, I, or Se provide the heavy atom effects to boost SOC. On the other hand, the El-Sayed rule addresses the necessity of orbital angular momentum change in SOC and the general utilization of carbonyl, heterocyclic rings, and other moieties with rich nonbonding electrons. Because of the slow-decaying nature of triplet excitons, engineering the matrices of POPs is critical to effectively suppress collisional quenching as the major nonradiative decay route, thus achieving POPs with decent room temperature quantum efficiency. For that purpose, crystalline or rigid amorphous matrices have been implemented along with specific intermolecular forces between POPs and their environment.Despite the great efforts made in the past decade, the intrinsic SOC efficiencies of POPs remain low, and their emission lifetimes are pinned in the millisecond to second regime. While this is beneficial for POPs with ultralong emission, designing high-SOC POPs with simultaneous fast decay and high quantum efficiencies is particularly advantageous for display systems. Following the design of contemporary POPs, we will discuss molecular design descriptors that could potentially break the current limit to boost internal SOC in purely organic materials. Our recently developed concept of "heavy atom oriented orbital angular momentum manipulation" will be discussed, accompanied by a rich and expanded library of fast and efficient POP molecules, which serves as a stepping stone into the future of this field. We will conclude this Account by discussing the noteworthy application of POPs in organic light-emitting diodes (OLEDs), solid-state lighting, and sensors, as well as the remaining challenges in the design of fast and efficient POPs.

Optical torque induces magnetism at the molecular level.

We report experimental observations of a mechanism that potentially supports and intensifies induced magnetization at optical frequencies without the intervention of spin-orbit or spin-spin interactions. Energy-resolved spectra of scattered light, recorded at moderate intensities (108 W/cm2) and short timescales (<150 fs) in a series of non-magnetic molecular liquids, reveal the signature of torque dynamics driven jointly by the electric and magnetic field components of light at the molecular level. While past experiments have recorded radiant magnetization from magneto-electric interactions of this type, no evidence has been provided to date of the inelastic librational features expected in cross-polarized light scattering spectra due to the Lorentz force acting in combination with optical magnetic torque. Here, torque is shown to account for unpolarized rotational components in the magnetic scattering spectrum under conditions that produce only polarized vibrational features in electric dipole scattering, in excellent agreement with quantum theoretical predictions.

Molecular Design Approach Managing Molecular Orbital Superposition for High Efficiency without Color Shift in Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes.

Molecular design principles of thermally activated delayed fluorescent (TADF) emitters having a high quantum efficiency and a color tuning capability was investigated by synthesizing three TADF emitters with donors at different positions of a benzonitrile acceptor. The position rendering a large overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) enhances the quantum efficiency of the TADF emitter. Regarding the orbital overlap, donor attachments at 2- and 6-positions of the benzonitrile were more beneficial than 3- and 5-substitutions. Moreover, an additional attachment of a weak donor at the 4-position further increased the quantum efficiency without decreasing the emission energy. Therefore, the molecular design strategy of substituting strong donors at the positions allowing a large molecular orbital overlap and an extra weak donor is a good approach to achieve both high quantum efficiency and a slightly increased emission energy.

Design principles for the energy level tuning in donor/acceptor conjugated polymers.

To identify reliable molecular design principles for energy level tuning in donor/acceptor conjugated polymers (CPs), we studied the governing factors by means of ab initio calculations based on density-functional theory (DFT). We investigated a series of CPs in which we independently and systematically varied the electron withdrawing power of the acceptor unit and the electron donating power of the donor unit, while maintaining the same conjugated chain conformation. We observed that the introduction of a stronger acceptor unit, while keeping the same donor unit in the CP, lowers the LUMO level, but leaves the HOMO level almost unchanged. Conversely, enhancing the strength of the donor unit for the same acceptor unit raises the HOMO level, while maintaining the LUMO level. We identified strong correlations between the frontier orbital energy levels and the degree of orbital localization, which depends on the electron donating or withdrawing power of the molecular groups carrying the orbitals. Moreover, the HOMO/LUMO gap of the CP is directly proportional to the charge transfer between donating and accepting units, which provides a robust design criterion for CPs.

Design of a simple paper-based colorimetric biosensor using polydiacetylene liposomes for neomycin detection.

We developed a paper-based analytical device (µPAD) combined with self-signaling polydiacetylene (PDA) liposomes for convenient visual neomycin detection. The simple dot array type of µPAD was fabricated by the wax printing technique, and the PDA liposomes in the aqueous solution were facilely immobilized onto the hydrophilic dot region of the paper substrate. We found that, when the PDA liposomes were inserted to the paper matrix, the stability of the PDA liposomes can be significantly enhanced by adding a hydrophilic reagent such as polyvinyl alcohol and glycerol to the liposome solution. In particular, polyvinyl alcohol (PVA) provides the best stabilization among the various hydrophilic reagents tested in this contribution, and the enhanced stability sharply increased the sensitivity of the PDA liposomes in the paper matrix. Based on the above results, we successfully detected neomycin through both naked-eye observation and fluorescence measurement of PDA signals. The detection limit was 1 ppm and was selective to non-aminoglycoside antibiotics.

A Ru-Pt alloy electrode to suppress leakage currents of dynamic random-access memory capacitors.

Rutile TiO2, a high temperature phase, has attracted interest as a capacitor dielectric in dynamic random-access memories (DRAMs). Despite its high dielectric constant of >80, large leakage currents caused by a low Schottky barrier height at the TiO2/electrode interface have hindered the use of rutile TiO2 as a commercial DRAM capacitor. Here, we propose a new Ru-Pt alloy electrode to increase the height of the Schottky barrier. The Ru-Pt mixed layer was grown by atomic layer deposition. The atomic ratio of Ru/Pt varied in the entire range from 100 at.% Ru to 100 at.% Pt. Rutile TiO2 films were inductively formed only on the Ru-Pt layer containing ≤43 at.% Pt, while anatase TiO2 films with a relatively low dielectric constant (∼40) were formed at Pt compositions > 63 at.%. The Ru-Pt (40-50 at.%) layer also attained an increase in work function of ∼0.3-0.4 eV, leading to an improvement in the leakage currents of the TiO2/Ru-Pt capacitor. These findings suggested that a Ru-Pt layer could serve as a promising electrode for next-generation DRAM capacitors.

Waveguiding characteristics of surface enhanced Raman scattering signals along crystalline organic semiconducting microrod.

The waveguiding of surface enhanced Raman scattering (SERS) signals was demonstrated by using organic semiconducting microrods (MRs) hybridized with functionalized gold nanoparticles (Au-NPs). Organic semiconducting 1,4-bis(3,5-bis(trifluoromethyl) styryl)-2,5-dibromobenzene (TSDB) crystalline MRs were fabricated as active optical waveguiding system using a self-assembly method. The static SERS effect and the enhancement of photoluminescence were simultaneously observed for the TSDB MRs hybridized with Au-NPs. The waveguiding characteristics of the SERS signals through the hybrid MR of TSDB/Au-NPs were investigated using a high-resolution laser confocal microscope (LCM) system. The enhanced output Raman characteristic modes of TSDB molecules were clearly observed along the hybrid MR of TSDB/Au-NPs, which is attributed to stronger scattering of the light and the increased coupling efficiency of waveguiding due to the presence of Au-NPs. The waveguiding of the SERS signals exhibited different decay constants for the corresponding characteristic Raman modes, such as -C = C- aromatic, -CF3, and C-Br stretching modes. The observed waveguiding characteristics of various SERS modes enable multi-modal waveguiding with relatively narrow spectral resolution for nanophotonic information.

Room-Temperature-Phosphorescence-Based Dissolved Oxygen Detection by Core-Shell Polymer Nanoparticles Containing Metal-Free Organic Phosphors.

The highly sensitive optical detection of oxygen including dissolved oxygen (DO) is of great interest in various applications. We devised a novel room-temperature-phosphorescence (RTP)-based oxygen detection platform by constructing core-shell nanoparticles with water-soluble polymethyloxazoline shells and oxygen-permeable polystyrene cores crosslinked with metal-free purely organic phosphors. The resulting nanoparticles show a very high sensitivity for DO with a limit of detection (LOD) of 60 nm and can be readily used for oxygen quantification in aqueous environments as well as the gaseous phase.

High thermal conductivity in amorphous polymer blends by engineered interchain interactions.

Thermal conductivity is an important property for polymers, as it often affects product reliability (for example, electronics packaging), functionality (for example, thermal interface materials) and/or manufacturing cost. However, polymer thermal conductivities primarily fall within a relatively narrow range (0.1-0.5 W m(-1) K(-1)) and are largely unexplored. Here, we show that a blend of two polymers with high miscibility and appropriately chosen linker structure can yield a dense and homogeneously distributed thermal network. A sharp increase in cross-plane thermal conductivity is observed under these conditions, reaching over 1.5 W m(-1) K(-1) in typical spin-cast polymer blend films of nanoscale thickness, which is approximately an order of magnitude larger than that of other amorphous polymers.

Control of the initial growth in atomic layer deposition of Pt films by surface pretreatment.

The controllability of the nucleation behavior of Pt in atomic layer deposition (ALD) by surface pretreatments with H2O, H2S, and NH3 was investigated. The H2O pretreatment on SiO2 and TiO2 surfaces had little effect on the nucleation of Pt. The H2S pretreatment on the SiO2 and TiO2 surfaces significantly delayed the nucleation of Pt on them, while the NH3 pretreatment on the TiO2 surface led to fluent nucleation of Pt. In particular, a continuous Pt film was successfully formed even at an ultrathin thickness of approximately 2.2 nm by NH3 pretreatment. This work suggests that the pretreatment with H2S and NH3 is an efficient way to control the nucleation of Pt in ALD without the support of any reactive species, such as plasma or O3. Such a strategy enables the easy control of the size and distribution density of Pt nanoparticles for a wide range of applications.

The effects of extended conjugation length of purely organic phosphors on their phosphorescence emission properties.

We synthesized a series of purely organic phosphors, bromobenzaldehyde derivatives, with varying conjugation length to investigate the effects of conjugation length on their phosphorescence emission properties. As the conjugation length increases phosphorescence efficiency decreases with a redshift in the emission color at 77 K. Our computational results imply that this correlation is related to the intersystem crossing rate and that the rate is determined by spin-orbit coupling strength rather than by simply the energy difference between the lowest lying singlet and triplet states. TD-DFT calculations show that the S1 → T1 transition occurs more dominantly than the S1 → T2 transition for all cases. Moreover, singlet excited states are localized on the aldehyde functional group, regardless of the conjugation length, while triplet excited states are evenly distributed over the conjugated backbone. Consequently, as the conjugation length increases, the larger spatial separation between singlet and triplet states diminishes the spin-orbit coupling efficiency, resulting in reduced phosphorescence.

Synthesis, copolymerization and peptide-modification of carboxylic acid-functionalized 3,4-ethylenedioxythiophene (EDOTacid) for neural electrode interfaces.

BACKGROUND: Conjugated polymers have been developed as effective materials for interfacing prosthetic device electrodes with neural tissue. Recent focus has been on the development of conjugated polymers that contain biological components in order to improve the tissue response upon implantation of these electrodes. METHODS: Carboxylic acid-functionalized 3,4-ethylenedioxythiophene (EDOTacid) monomer was synthesized in order to covalently bind peptides to the surface of conjugated polymer films. EDOTacid was copolymerized with EDOT monomer to form stable, electrically conductive copolymer films referred to as PEDOT-PEDOTacid. The peptide GGGGRGDS was bound to PEDOT-PEDOTacid to create peptide functionalized PEDOT films. RESULTS: The PEDOT-PEDOTacid-peptide films increased the adhesion of primary rat motor neurons between 3 and 9 times higher than controls, thus demonstrating that the peptide maintained its biological activity. CONCLUSIONS: The EDOT-acid monomer can be used to create functionalized PEDOT-PEDOTacid copolymer films that can have controlled bioactivity. GENERAL SIGNIFICANCE: PEDOT-PEDOTacid-peptide films have the potential to control the behavior of neurons and vastly improve the performance of implanted electrodes. This article is part of a Special Issue entitled Organic Bioelectronics-Novel Applications in Biomedicine.

A molecular design principle of lyotropic liquid-crystalline conjugated polymers with directed alignment capability for plastic electronics.

Conjugated polymers with a one-dimensional p-orbital overlap exhibit optoelectronic anisotropy. Their unique anisotropic properties can be fully realized in device applications only when the conjugated chains are aligned. Here, we report a molecular design principle of conjugated polymers to achieve concentration-regulated chain planarization, self-assembly, liquid-crystal-like good mobility and non-interdigitated side chains. As a consequence of these intra- and intermolecular attributes, chain alignment along an applied flow field occurs. This liquid-crystalline conjugated polymer was realized by incorporating intramolecular sulphur-fluorine interactions and bulky side chains linked to a tetrahedral carbon having a large form factor. By optimizing the polymer concentration and the flow field, we could achieve a high dichroic ratio of 16.67 in emission from conducting conjugated polymer films. Two-dimensional grazing-incidence X-ray diffraction was performed to analyse a well-defined conjugated polymer alignment. Thin-film transistors built on highly aligned conjugated polymer films showed more than three orders of magnitude faster carrier mobility along the conjugated polymer alignment direction than the perpendicular direction.

Strain-assisted, low-temperature synthesis of high-performance thermoelectric materials.

Utilizing internal energy artificially implemented by cold-pressing in the specimens, we demonstrate a way to synthesize high-quality bulk thermoelectric materials at otherwise too low a temperature to approach to an equilibrium state. This low-temperature synthesis technique will provide a new opportunity to integrate high-performance thermoelectric materials into various electronic devices for a built-in energy source, as well as to develop low-cost fabrication methods.

Optical properties of 4-bromobenzaldehyde derivatives in chloroform solution.

In this work we give a deeper insight into the electronic structure of a series of purely organic molecules that were recently employed as building blocks in crystals with very efficient phosphorescent emission. With this purpose, the low-lying excited states of a series of 4-bromobenzaldehyde derivatives in chloroform solution are explored by means of time-dependent density functional theory (TDDFT) calculations, together with the absorption, fluorescence, and phosphorescence experimental spectra. The optical properties of the studied molecular models are extensively discussed, in terms of the frontier molecular orbitals involved in the relevant electronic transitions, the recorded and simulated absorption profiles, and the molecular geometries and transition energies of the emitting states. The calculations eventually help in the assignment of the character of the lowest lying singlet and triplet emitting states for these compounds.

Tailoring intermolecular interactions for efficient room-temperature phosphorescence from purely organic materials in amorphous polymer matrices.

Herein we report a rational design strategy for tailoring intermolecular interactions to enhance room-temperature phosphorescence from purely organic materials in amorphous matrices at ambient conditions. The built-in strong halogen and hydrogen bonding between the newly developed phosphor G1 and the poly(vinyl alcohol) (PVA) matrix efficiently suppresses vibrational dissipation and thus enables bright room-temperature phosphorescence (RTP) with quantum yields reaching 24%. Furthermore, we found that modulation of the strength of halogen and hydrogen bonding in the G1-PVA system by water molecules produced unique reversible phosphorescence-to-fluorescence switching behavior. This unique system can be utilized as a ratiometric water sensor.

Polydopamine Adhesion: Catechol, Amine, Dihydroxyindole, and Aggregation Dynamics.

While polydopamine (PDA) possesses the surface-independent adhesion property of mussel-binding proteins, significant differences exist between them. Particularly, PDA's short and rigid backbone differs from the long and flexible protein sequence of mussel-binding proteins. Given that adhesion relies on achieving a conformal contact with large surface coverage, PDA has drawbacks as an adhesive. In our study, we investigated the roles of each building block of PDA to build a better understanding of their binding mechanisms. Initially, we anticipated that catecholamine oligomers form specific binding with substrates. However, our study showed that the universal adhesion of PDA is initiated by the solubility limit of growing oligomers by forming agglomerates, complemented by multiple binding modes of catechol. Notably, in the absence of amines, poly(catechol) either remained in solution or formed minor suspensions without any surface coating, underscoring the essential role of amines in the adhesion process by facilitating insoluble aggregate formation. To substantiate our findings, we induced poly(catechol) aggregation using quaternized poly(4-vinylpyridine) (qPVP), leading to subsequent surface adhesion upon agglomerate formation.

Conjugation through Si-O-Si bonds, silsesquioxane (SQ) half cage copolymers, extended examples via SiO0.5/SiO1.5 units: multiple emissive states in violation of Kasha's rule.

We previously reported that phenyl- and vinyl-silsesquioxanes (SQs), [RSiO1.5]8,10,12 (R = Ph or vinyl) functionalized with three or more conjugated moieties show red-shifted absorption- and emission features suggesting 3-D conjugation via a cage centered LUMOs. Corner missing [PhSiO1.5]7(OSiMe3)3 and edge opened, end capped [PhSiO1.5]8(OSiMe2)2 (double decker, DD) analogs also offer red shifted spectra again indicating 3-D conjugation and a cage centered LUMO. Copolymerization of DD [PhSiO1.5]8(OSiMevinyl)2 with multiple R-Ar-Br gives copolymers with emission red-shifts that change with degree of polymerization (DP), exhibit charge transfer to F4TNCQ and terpolymer averaged red-shifts suggesting through chain conjugation even with two (O-Si-O) end caps possibly via a cage centered LUMO. Surprisingly, ladder (LL) SQ, (vinylMeSiO2)[PhSiO1.5]4(O2SiMevinyl) copolymers offer emission red-shifts even greater for analogous copolymers requiring a different explanation. Here we assess the photophysical behavior of copolymers of a more extreme SQ form: the half cage [PhSiO1.5]4(OSiMe2Vinyl)4, Vy4HC SQs. We again see small red-shifted absorptions coupled with significant red-shifted emissions, even with just a half cage, thus further supporting the existence of pπ-dπ and/or σ*-π* conjugation through Si-O-Si bonds and contrary to most traditional views of Si-O-Si linked polymers. These same copolymers donate an electron to F4TCNQ generating the radical anion, F4TCNQ-. as further proof of conjugation. Column chromatographic separation of short from longer chain oligomers reveals a direct correlation between DP and emission λmax red-shifts as another indication of conjugation. Further, one- and two-photon absorption and emission spectroscopy reveals multiple excited fluorescence-emitting states in a violation of Kasha's rule wherein emission occurs only from the lowest excited state. Traditional modeling studies again find HOMO LUMO energy levels residing only on the aromatic co-monomers rather than through Si-O-Si bonds as recently found in related polymers.

Unlocking the Potential of Porous Bi2Te3-Based Thermoelectrics Using Precise Interface Engineering through Atomic Layer Deposition.

Porous thermoelectric materials offer exciting prospects for improving the thermoelectric performance by significantly reducing the thermal conductivity. Nevertheless, porous structures are affected by issues, including restricted enhancements in performance attributed to decreased electronic conductivity and degraded mechanical strength. This study introduces an innovative strategy for overcoming these challenges using porous Bi0.4Sb1.6Te3 (BST) by combining porous structuring and interface engineering via atomic layer deposition (ALD). Porous BST powder was produced by selectively dissolving KCl in a milled mixture of BST and KCl; the interfaces were engineered by coating ZnO films through ALD. This novel architecture remarkably reduced the thermal conductivity owing to the presence of several nanopores and ZnO/BST heterointerfaces, promoting efficient phonon scattering. Additionally, the ZnO coating mitigated the high resistivity associated with the porous structure, resulting in an improved power factor. Consequently, the ZnO-coated porous BST demonstrated a remarkable enhancement in thermoelectric efficiency, with a maximum zT of approximately 1.53 in the temperature range of 333-353 K, and a zT of 1.44 at 298 K. Furthermore, this approach plays a significant role in enhancing the mechanical strength, effectively mitigating a critical limitation of porous structures. These findings open new avenues for the development of advanced porous thermoelectric materials and highlight their potential for precise interface engineering through the ALD.