Correlating Young's Modulus with High Thermal Conductivity in Organic Conjugated Small Molecules.

Attaining elevated thermal conductivity in organic materials stands as a coveted objective, particularly within electronic packaging, thermal interface materials, and organic matrix heat exchangers. These applications have reignited interest in researching thermally conductive organic materials. The understanding of thermal transport mechanisms in these organic materials is currently constrained. This study concentrates on N, N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8), an organic conjugated crystal. A correlation between elevated thermal conductivity and augmented Young's modulus is substantiated through meticulous experimentation. Achievement via employing the physical vapor transport method, capitalizing on the robust C═C covalent linkages running through the organic matrix chain, bolstered by π-π stacking and noncovalent affiliations that intertwine the chains. The coexistence of these dynamic interactions, alongside the perpendicular alignment of PTCDI-C8 molecules, is confirmed through structural analysis. PTCDI-C8 thin film exhibits an out-of-plane thermal conductivity of 3.1 ± 0.1 W m-1 K-1, as determined by time-domain thermoreflectance. This outpaces conventional organic materials by an order of magnitude. Nanoindentation tests and molecular dynamics simulations elucidate how molecular orientation and intermolecular forces within PTCDI-C8 molecules drive the film's high Young's modulus, contributing to its elevated thermal conductivity. This study's progress offers theoretical guidance for designing high thermal conductivity organic materials, expanding their applications and performance potential.

Defect Etching of Phase-Transition-Assisted CVD-Grown 2H-MoTe2.

2D molybdenum ditelluride (MoTe2 ) with polymorphism is a promising candidate to developing phase-change memory, high-performance transistors and spintronic devices. The phase-transition-assisted chemical vapor deposition (CVD) process has been used to prepare large-scale 2H-MoTe2 with large grain size and low density of grain boundary. However, because of the lack of precise control of the growth condition, some defects including the amorphous regions and grain boundaries in 2H-MoTe2 are hardly avoidable. Here, a facile method of selectively etching defects in large-scale CVD-grown 2H-MoTe2 by triiodide ion (I3 - ) solution is reported. The defect etching is attributed to the reduced lattice symmetry, high chemisorption activity and high conductivity of the defects due to the high density of Te vacancies. The treated 2H-MoTe2 shows the suppressed hysteresis in the electrical transfer curve, enhances hole mobility and the higher effective barrier height on the metal contact, suggesting the decreased density of defects. Further chemical analysis indicates that the 2H-MoTe2 is not damaged or doped by I3 - solution during the etching process. This simple and low-cost post-processing method is effective for etching the defects in large-area 2H-MoTe2 for high-performance device applications.

Graphene-assisted electro-optomechanical integration on a silicon-on-insulator platform.

Micro- and nano-optomechanics has attracted broad interest for applications of mechanical sensing and coherent signal processing. For nonpiezoelectric materials such as silicon or silicon nitride, electrocapacitive effects with metals patterned on mechanical structures are usually adopted to actuate the mechanical motion of the micro- or nanomechanical devices. However, the metals have deleterious effects on the mechanical structures because they add an additional weight and also introduce considerable mechanical losses. To solve these problems, we have proposed and experimentally demonstrated a new scheme of electro-optomechanical integration on a silicon-on-insulator platform by using single-layer graphene as a highly conductive coating for electromechanical actuation. Mechanical modes of different groups were electrically actuated and optically detected in a micromechanical resonator, with the mechanical Q > 1000 measured in air. Compatible with CMOS technology, our scheme is suitable for large-scale electro-optomechanical integration and will have wide applications in high-speed sensing, communication, and signal processing.

Observation of Strong J-Aggregate Light Emission in Monolayer Molecular Crystal on Hexagonal Boron Nitride.

J-aggregates are widely used in studies of light-matter interaction and organic optoelectronic devices. Although J-aggregate films can be fabricated on salt by epitaxial growth method, the size is limited to hundreds of nanometer. In this work, with hexagonal boron nitride (h-BN) as a substrate, highly crystalline J-aggregate ultrathin films of N,N'-ditridecylperylene 3,4,9,10-tetracarboxylic diimide (PTCDI-C13) are achieved by physical vapor transport (PVT) method. Significant bathochromically shifted absorption band and narrowed 0-0 transition are observed in the monolayer PTCDI-C13 crystal on h-BN. The exciton coherence number Ncoh of monolayer J-aggregate film extracted from the photoluminescence (PL) spectrum is up to 15 at T = 140 K, which is higher than that of the epitaxially grown layer on salt. Beyond the first molecular layer, the multilayer crystal on h-BN is dominated by H-aggregates. Further study shows that that the first molecular layer on h-BN adopts the highly ordered face-on configuration, while the overlayers adopt the edge-on motif. As a comparison, only H-aggregate PTCDI-C13 ultrathin films are found on SiO2 substrates, but no J-aggregates. The results suggest that high-quality J-aggregates can be prepared by utilizing appropriate substrates via physical vapor transport.

1T' Transition Metal Telluride Atomic Layers for Plasmon-Free SERS at Femtomolar Levels.

Plasmon-free surface enhanced Raman scattering (SERS) based on the chemical mechanism (CM) is drawing great attention due to its capability for controllable molecular detection. However, in comparison to the conventional noble-metal-based SERS technique driven by plasmonic electromagnetic mechanism (EM), the low sensitivity in the CM-based SERS is the dominant barrier toward its practical applications. Herein, we demonstrate the 1T' transition metal telluride atomic layers (WTe2 and MoTe2) as ultrasensitive platforms for CM-based SERS. The SERS sensitivities of analyte dyes on 1T'-W(Mo)Te2 reach EM-comparable ones and become even greater when it is integrated with a Bragg reflector. In addition, the dye fluorescence signals are efficiently quenched, making the SERS spectra more distinguishable. As a proof of concept, the SERS signals of analyte Rhodamine 6G (R6G) are detectable even with an ultralow concentration of 40 (400) fM on pristine 1T'-W(Mo)Te2, and the corresponding Raman enhancement factor (EF) reaches 1.8 × 109 (1.6 × 108). The limit concentration of detection and the EF of R6G can be further enhanced into 4 (40) fM and 4.4 × 1010 (6.2 × 109), respectively, when 1T'-W(Mo)Te2 is integrated on the Bragg reflector. The strong interaction between the analyte and 1T'-W(Mo)Te2 and the abundant density of states near the Fermi level of the semimetal 1T'-W(Mo)Te2 in combination gives rise to the promising SERS effects by promoting the charge transfer resonance in the analyte-telluride complex.

Construction of 3D Skeleton for Polymer Composites Achieving a High Thermal Conductivity.

Owing to the growing heat removal issue in modern electronic devices, electrically insulating polymer composites with high thermal conductivity have drawn much attention during the past decade. However, the conventional method to improve through-plane thermal conductivity of these polymer composites usually yields an undesired value (below 3.0 Wm-1 K-1 ). Here, construction of a 3D phonon skeleton is reported composed of stacked boron nitride (BN) platelets reinforced with reduced graphene oxide (rGO) for epoxy composites by the combination of ice-templated and infiltrating methods. At a low filler loading of 13.16 vol%, the resulting 3D BN-rGO/epoxy composites exhibit an ultrahigh through-plane thermal conductivity of 5.05 Wm-1 K-1 as the best thermal-conduction performance reported so far for BN sheet-based composites. Theoretical models qualitatively demonstrate that this enhancement results from the formation of phonon-matching 3D BN-rGO networks, leading to high rates of phonon transport. The strong potential application for thermal management has been demonstrated by the surface temperature variations of the composites with time during heating and cooling.

The physics and chemistry of graphene-on-surfaces.

Graphene has demonstrated great potential in next-generation electronics due to its unique two-dimensional structure and properties including a zero-gap band structure, high electron mobility, and high electrical and thermal conductivity. The integration of atom-thick graphene into a device always involves its interaction with a supporting substrate by van der Waals forces and other intermolecular forces or even covalent bonding, and this is critical to its real applications. Graphene films on different surfaces are expected to exhibit significant differences in their properties, which lead to changes in their morphology, electronic structure, surface chemistry/physics, and surface/interface states. Therefore, a thorough understanding of the surface/interface properties is of great importance. In this review, we describe the major "graphene-on-surface" structures and examine the roles of their properties and related phenomena in governing the overall performance for specific applications including optoelectronics, surface catalysis, anti-friction and superlubricity, and coatings and composites. Finally, perspectives on the opportunities and challenges of graphene-on-surface systems are discussed.

Correction: The physics and chemistry of graphene-on-surfaces.

Correction for 'The physics and chemistry of graphene-on-surfaces' by Guoke Zhao, Xinming Li, Meirong Huang et al., Chem. Soc. Rev., 2017, 46, 4417-4449.

Precise, Self-Limited Epitaxy of Ultrathin Organic Semiconductors and Heterojunctions Tailored by van der Waals Interactions.

Precise assembly of semiconductor heterojunctions is the key to realize many optoelectronic devices. By exploiting the strong and tunable van der Waals (vdW) forces between graphene and organic small molecules, we demonstrate layer-by-layer epitaxy of ultrathin organic semiconductors and heterostructures with unprecedented precision with well-defined number of layers and self-limited characteristics. We further demonstrate organic p-n heterojunctions with molecularly flat interface, which exhibit excellent rectifying behavior and photovoltaic responses. The self-limited organic molecular beam epitaxy (SLOMBE) is generically applicable for many layered small-molecule semiconductors and may lead to advanced organic optoelectronic devices beyond bulk heterojunctions.

Quantitative Analysis of Scattering Mechanisms in Highly Crystalline CVD MoS2 through a Self-Limited Growth Strategy by Interface Engineering.

The electrical performance of highly crystalline monolayer MoS2 is remarkably enhanced by a self-limited growth strategy on octadecyltrimethoxysilane self-assembled monolayer modified SiO2 /Si substrates. The scattering mechanisms in low-κ dielectric, including the dominant charged impurities, acoustic deformation potentials, optical deformation potentials), Fröhlich interaction, and the remote interface phonon interaction in dielectrics, are quantitatively analyzed.

Probing Carrier Transport and Structure-Property Relationship of Highly Ordered Organic Semiconductors at the Two-Dimensional Limit.

One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two dimensionally in the first few molecular layers near the dielectric interface. Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered single-crystalline mono- to tetralayer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagonal BN. We find that the charge transport is dominated by hopping in the first conductive layer, but transforms to bandlike in subsequent layers. Such an abrupt phase transition is attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations. The structural modulation becomes negligible beyond the second conductive layer, leading to a mobility saturation thickness of only ∼3 nm. Highly ordered organic ultrathin films provide a platform for new physics and device structures (such as heterostructures and quantum wells) that are not possible in conventional bulk crystals.

Variable electronic properties of lateral phosphorene-graphene heterostructures.

Phosphorene and graphene have a tiny lattice mismatch along the armchair direction, which can result in an atomically sharp in-plane interface. The electronic properties of the lateral heterostructures of phosphorene/graphene are investigated by the first-principles method. Here, we demonstrate that the electronic properties of this type of heterostructure can be highly tunable by the quantum size effects and the externally applied electric field (Eext). At strong Eext, Dirac Fermions can be developed with Fermi velocities around one order smaller than that of graphene. Undoped and hydrogen doped configurations demonstrate three drastically different electronic phases, which reveal the strongly tunable potential of this type of heterostructure. Graphene is a naturally better electrode for phosphorene. The transport properties of two-probe devices of graphene/phosphorene/graphene exhibit tunnelling transport characteristics. Given these results, it is expected that in-plane heterostructures of phosphorene/graphene will present abundant opportunities for applications in optoelectronic and electronic devices.

Effects of 3d transition-metal doping on electronic and magnetic properties of MoS2 nanoribbons.

The electronic and magnetic properties of MoS2 nanoribbons doped with 3d transitional metals (TMs) were investigated using first-principles calculations. Clean armchair MoS2 nanoribbons (AMoS2NRs) are nonmagnetic semiconductors whereas clean zigzag MoS2 nanoribbons (ZMoS2NRs) are metallic magnets. The 3d TM impurities tend to substitute the outermost cations of AMoS2NRs and ZMoS2NRs, which are in agreement with the experimental results reported. The magnetization of the 3d-TM-impurity-doped AMoS2NRs and ZMoS2NRs is configuration dependent. The band gap and carrier concentration of AMoS2NRs can be tuned by 3d-TM doping. Fe-doped AMoS2NRs exhibit ferromagnetic characteristics and the Curie temperature (T(C)) can be tuned using different impurity concentrations. Co-doped ZMoS2NRs are strongly ferromagnetic with a T(C) above room temperature.

Configuration-dependent electronic and magnetic properties of graphene monolayers and nanoribbons functionalized with aryl groups.

Graphene monolayers functionalized with aryl groups exhibit configuration-dependent electronic and magnetic properties. The aryl groups were adsorbed in pairs of neighboring atoms in the same sublattice A (different sublattices) of graphene monolayers, denoted as the M2 (AA) (M2 (AB)) configuration. The M2 (AA) configuration behaved as a ferromagnetic semiconductor. The band gaps for the majority and minority bands were 1.1 eV and 1.2 eV, respectively. The M2 (AB) configuration behaved as a nonmagnetic semiconductor with a band gap of 0.8 eV. Each aryl group could induce 1 Bohr magneton (µB) into the molecule-graphene system. Armchair graphene nanoribbons (GNRs) exhibited the same configuration-dependent magnetic properties as the graphene monolayers. The net spin of the functionalized zigzag GNRs was mainly localized on the edges demonstrating an adsorption site-dependent magnetism. For the zigzag GNRs, both the M2 (AA) and M2 (AB) configurations possibly had a magnetic moment. Each aryl group could induce 1.5-3.5 µB into the molecule-graphene system. There was a metal-to-insulator transition after adsorption of the aryl groups for the zigzag GNRs.

Poly(ionic liquid)s: A Promising Matrix for Thermal Interface Materials.

The swift progression of high-density chiplet packaging, propelled by the artificial intelligence revolution, has precipitated a critical need for high-performance chip-scale thermal interface materials (TIMs). The elevated thermal resistance, limited interfacial adhesion, and mechanical flexibility intrinsic to silicone systems present a substantial challenge in meeting reliability standards amidst chip warpage. This particular matter underscores a significant performance bottleneck within existing high-end TIMs. In this study, we present poly(ionic liquid)s (PILs) as an innovative matrix for TIMs. Our findings highlight the unique properties of PILs, showcasing a low elastic modulus (60 kPa), exceptional flexibility and stretchability (>3800%), high adhesion to diverse substrates (up to 4.10 MPa), favorable filler compatibility, remarkable thermal stability, and prompt self-healing capabilities coupled with recyclability. The collective findings suggest that the PIL serves as an ideal matrix for heat transfer. As a proof of concept, PIL blended with liquid metal was straightforwardly combined to produce a TIM, exhibiting exceptional performance within practical encapsulated structures. The PIL-based TIM demonstrates substantial elongation at break (>350%), coupled with sustained high adhesion strength (up to 1.70 MPa), and exhibits favorable thermal conductivity in package testing. This study presents an innovative TIM matrix with the potential to enhance existing TIM systems, delivering significant performance benefits compared to silicones. Besides elucidating their multifaceted characteristics, this study forecasts an expanded range of applications for PILs, along with laying the groundwork for advancing next-generation TIMs.

Monolayer Organic Crystals for Ultrahigh Performance Molecular Diodes.

Molecular diodes are of considerable interest for the increasing technical demands of device miniaturization. However, the molecular diode performance remains contact-limited, which represents a major challenge for the advancement of rectification ratio and conductance. Here, it is demonstrated that high-quality ultrathin organic semiconductors can be grown on several classes of metal substrates via solution-shearing epitaxy, with a well-controlled number of layers and monolayer single crystal over 1 mm. The crystals are atomically smooth and pinhole-free, providing a native interface for high-performance monolayer molecular diodes. As a result, the monolayer molecular diodes show record-high rectification ratio up to 5 × 108 , ideality factor close to unity, aggressive unit conductance over 103 S cm-2 , ultrahigh breakdown electric field, excellent electrical stability, and well-defined contact interface. Large-area monolayer molecular diode arrays with 100% yield and excellent uniformity in the diode metrics are further fabricated. These results suggest that monolayer molecular crystals have great potential to build reliable, high-performance molecular diodes and deeply understand their intrinsic electronic behavior.

[Effect of Lycium barbarum polysaccharides on the retinal ultrastructure of streptozocin-induced diabetic rats].

OBJECTIVE: To study the retinal ultrastructure of streptozocin (STZ)-induced diabetic rats and the intervention effect of Lycium Barbarum Polysaccharides (LBP). METHODS: The STZ-induced diabetic SD rat model was established. LBP was given to those in the treatment group by gastrogavage. Changes of body weight, blood glucose, and retinal ultrastructure at 24-week were observed. RESULTS: Early retinal changes covered mitochondrion changes, cell degeneration and apoptosis of retinal neurons and neuroglia cells in the diabetic rats. No change of body weight or blood glucose was observed between the LBP group and the diabetic model group (P > 0.05). The ultrastructural changes were obviously relieved by LBP, and limited to the inner nuclear layer. CONCLUSIONS: LBP could obviously relieve pathological changes of mitochondrion, hinder neural cell apoptosis. Its effect might not be achieved by lowering blood glucose. It was expected to be used in preventing and treating early diabetic retinal neuropathy.

Contact-engineered reconfigurable two-dimensional Schottky junction field-effect transistor with low leakage currents.

Two-dimensional (2D) materials have been considered promising candidates for future low power-dissipation and reconfigurable integrated circuit applications. However, 2D transistors with intrinsic ambipolar transport polarity are usually affected by large off-state leakage currents and small on/off ratios. Here, we report the realization of a reconfigurable Schottky junction field-effect transistor (SJFET) in an asymmetric van der Waals contact geometry, showing a balanced and switchable n- and p-unipolarity with the Ids on/off ratio kept >106. Meanwhile, the static leakage power consumption was suppressed to 10-5 nW. The SJFET worked as a reversible Schottky rectifier with an ideality factor of ~1.0 and a tuned rectifying ratio from 3 × 106 to 2.5 × 10-6. This empowered the SJFET with a reconfigurable photovoltaic performance in which the sign of the open-circuit voltage and photo-responsivity were substantially switched. This polarity-reversible SJFET paves an alternative way to develop reconfigurable 2D devices for low-power-consumption photovoltaic logic circuits.

Creating Biomimetic Central-Radial Skeletons with Efficient Mass Adsorption and Transport.

Porous skeletons play a crucial role in various applications. Their fundamental significance stems from their remarkable surface area and capacity to enhance mass adsorption and transport. Freeze-casting is a commonly utilized methodology for the production of porous skeletons featuring vertically aligned channels. Nevertheless, the resultant single-oriented skeleton displays anisotropic mass transfer characteristics and suboptimal mechanical properties. Our investigation was motivated by the intricate microstructures observed in botanical organisms, leading us to devise an advanced freeze-casting methodology. A novel central-radial skeleton with significantly enhanced capabilities has been successfully engineered. The central-radial architecture demonstrates superior refinement and uniformity in its pore structure, featuring an axial mass transfer axis and meticulously arranged radial channels. This microstructure endows the porous skeleton with a higher compression resilience, superior adsorption rate, and structural maintenance capacity. Through a rigorous examination of the thermal conductivity of skeleton-filled composites coupled with comprehensive COMSOL simulations, the exceptional characteristics of this unique structural arrangement have been definitively ascertained. Furthermore, the efficacy of implementing this skeleton in chip cooling and photothermal conversion has been convincingly substantiated. Our pioneering method of microstructure preparation, employing freeze-casting, holds immense potential in expanding its applicability and inspiring innovative concepts for the advancement of novel structures.

Organic Conjugated Small Molecules with High Thermal Conductivity as an Effective Coupling Layer for Heat Transfer.

As the features of electronics are miniaturized, the need for interfacial thermal coupling layers to enhance their thermal transfer efficiency and improve device performance becomes critical. Organic conjugated small molecules possess a unique combination of periodic crystal structures and conjugated units with π electrons, resulting in notable thermal conductivities and molecular structure orientation that facilitates directed heat transfer. Nevertheless, there is a noticeable gap in literatures regarding the thermal properties of organic conjugated small molecules and their potential applications in nanoscale thermal management. Herein, we report the fabrication of high-quality thin films of organic conjugated small molecules. The result reveals that the 2D organic conjugated small molecule thin films exhibit a high cross-plane thermal conductivity of 3.2 W/m K. The increased thermal conductivity is attributed to the well-organized lattice structure and existence of π-electrons induced by conjugated systems. The studied conjugated small molecules engage in π-π stacking interactions with carbon materials and efficiently exchange energy with electrons in metals, promoting rapid interfacial heat transfer. These molecules act as coupling layers, significantly enhancing thermal transfer efficiency between graphite-based thermal pads and copper heat sinks. This pioneering research represents the inaugural investigation of the thermal performance of conjugated organic small molecules. These findings highlight the potential of conjugated small molecules as thermal coupling layers, offering tunable combinations of desirable properties.