Correcting force error-induced underestimation of lattice thermal conductivity in machine learning molecular dynamics.

Machine learned potentials (MLPs) have been widely employed in molecular dynamics simulations to study thermal transport. However, the literature results indicate that MLPs generally underestimate the lattice thermal conductivity (LTC) of typical solids. Here, we quantitatively analyze this underestimation in the context of the neuroevolution potential (NEP), which is a representative MLP that balances efficiency and accuracy. Taking crystalline silicon, gallium arsenide, graphene, and lead telluride as examples, we reveal that the fitting errors in the machine-learned forces against the reference ones are responsible for the underestimated LTC as they constitute external perturbations to the interatomic forces. Since the force errors of a NEP model and the random forces in the Langevin thermostat both follow a Gaussian distribution, we propose an approach to correcting the LTC by intentionally introducing different levels of force noises via the Langevin thermostat and then extrapolating to the limit of zero force error. Excellent agreement with experiments is obtained by using this correction for all the prototypical materials over a wide range of temperatures. Based on spectral analyses, we find that the LTC underestimation mainly arises from increased phonon scatterings in the low-frequency region caused by the random force errors.

Low Content Ga2O3 Enables the Direct Methane Conversion.

The direct conversion of methane (CH4), a main greenhouse gas, to value-added chemicals has attracted increasing attention in order to alleviate the current energy crisis and environmental concern. Nevertheless, the oriented conversion of CH4 to target product is formidably challenging due to the inertness of CH4. In this work, we demonstrate that zeolite modified by a low amount of Ga2O3 (GS-1) can serve as a highly active and stable catalyst for direct conversion to hydrogen (H2) and solid carbon. The optimal GS-1 with 0.62 wt % of Ga displays a CH4 conversion rate of 70.6 mol/gGa/h with a H2 productivity of 134 mol/gGa/h at 800 °C. Analysis on NH3 temperature-programmed desorption (TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) suggests that the introduction of Ga2O3 can poison the acidic site of zeolite and promote the dehydrogenation of CH4. This work reports a highly active and stable catalyst for direct methane conversion, which may provide a feasible strategy for the sustainable utilization of CH4.

Efficient Polytelluride Anchoring for Ultralong-Life Potassium Storage: Combined Physical Barrier and Chemisorption in Nanogrid-in-Nanofiber.

Metal tellurides (MTes) are highly attractive as promising anodes for high-performance potassium-ion batteries. The capacity attenuation of most reported MTe anodes is attributed to their poor electrical conductivity and large volume variation. The evolution mechanisms, dissolution properties, and corresponding manipulation strategies of intermediates (K-polytellurides, K-pTex) are rarely mentioned. Herein, we propose a novel structural engineering strategy to confine ultrafine CoTe2 nanodots in hierarchical nanogrid-in-nanofiber carbon substrates (CoTe2@NC@NSPCNFs) for smooth immobilization of K-pTex and highly reversible conversion of CoTe2 by manipulating the intense electrochemical reaction process. Various in situ/ex situ techniques and density functional theory calculations have been performed to clarify the formation, transformation, and dissolution of K-pTex (K5Te3 and K2Te), as well as verifying the robust physical barrier and the strong chemisorption of K5Te3 and K2Te on S, N co-doped dual-type carbon substrates. Additionally, the hierarchical nanogrid-in-nanofiber nanostructure increases the chemical anchoring sites for K-pTex, provides sufficient volume buffer space, and constructs highly interconnected conductive microcircuits, further propelling the battery reaction to new heights (3500 cycles at 2.0 A g-1). Furthermore, the full cells further demonstrate the potential for practical applications. This work provides new insights into manipulating K-pTex in the design of ultralong-cycling MTe anodes for advanced PIBs.

Enhancing the Coherent Phonon Transport in SiGe Nanowires with Dense Si/Ge Interfaces.

The manipulation of phonon transport with coherent waves in solids is of fundamental interest and useful for thermal conductivity design. Based on equilibrium molecular dynamics simulations and lattice dynamics calculations, the thermal transport in SiGe superlattice nanowires with a tuned Si/Ge interface density was investigated by using the core-shell and phononic structures as the primary stacking layers. It was found that the thermal conductivity decreased with the increase of superlattice period lengths (Lp) when Lp was larger than 4 nm. This is because introducing additional Si/Ge interfaces can enhance phonon scattering. However, when Lp<4 nm, the increased interface density could promote heat transfer. Phonon density-of-state analysis demonstrates that new modes between 10 and 14 THz are formed in structures with dense Si/Ge interfaces, which is a signature of coherent phonon transport as those modes do not belong to bulk Si or Ge. The density of the newly generated modes increases with the increase of interface density, leading to an enhanced coherent transport. Besides, with the increase of interface density, the energy distribution of the newly generated modes becomes more balanced on Si and Ge atoms, which also facilitates heat transfer. Our current work is not only helpful for understanding coherent phonon transport but also beneficial for the design of new materials with tunable thermal conductivity.

Characterizing the Conformational Distribution in an Amorphous Film of an Organic Emitter and Its Application in a "Self-Doping" Organic Light-Emitting Diode.

The conformational distribution and mutual interconversion of thermally activated delayed fluorescence (TADF) emitters significantly affect the exciton utilization. However, their influence on the photophysics in amorphous film states is still not known due to the lack of a suitable quantitative analysis method. Herein, we used temperature-dependent time-resolved photoluminescence spectroscopy to quantitatively measure the relative populations of the conformations of a TADF emitter for the first time. We further propose a new concept of "self-doping" for realizing high-efficiency nondoped OLEDs. Interestingly, this "compositionally" pure film actually behaves as a film with a dopant (quasi-equatorial form) in a matrix (quasi-axial form). The concentration-induced quenching that may occur at high concentrations is thus expected to be effectively relieved. The "self-doping" OLED prepared with the newly developed TADF emitter TP2P-PXZ as a neat emitting layer realizes a high maximum external quantum efficiency of 25.4 % and neglectable efficiency roll-off.

The effect of water on colloidal quantum dot solar cells.

Almost all surfaces sensitive to the ambient environment are covered by water, whereas the impacts of water on surface-dominated colloidal quantum dot (CQD) semiconductor electronics have rarely been explored. Here, strongly hydrogen-bonded water on hydroxylated lead sulfide (PbS) CQD is identified. The water could pilot the thermally induced evolution of surface chemical environment, which significantly influences the nanostructures, carrier dynamics, and trap behaviors in CQD solar cells. The aggravation of surface hydroxylation and water adsorption triggers epitaxial CQD fusion during device fabrication under humid ambient, giving rise to the inter-band traps and deficiency in solar cells. To address this problem, meniscus-guided-coating technique is introduced to achieve dense-packed CQD solids and extrude ambient water, improving device performance and thermal stability. Our works not only elucidate the water involved PbS CQD surface chemistry, but may also achieve a comprehensive understanding of the impact of ambient water on CQD based electronics.

Anomalous thermal conductivity enhancement in low dimensional resonant nanostructures due to imperfections.

Nanophononic metamaterials have broad applications in fields such as heat management, thermoelectric energy conversion, and nanoelectronics. Phonon resonance in pillared low-dimensional structures has been suggested to be a feasible approach to reduce thermal conductivity (TC). In this work, we study the effects of imperfections in pillared nanostructures based on graphene nanoribbons (GNR), using classical molecular dynamics simulations and harmonic lattice dynamics. The TC of perfect pillared GNR is only about 13% of that of pristine GNR due to the strong phonon resonant hybridization in pillared GNR. However, introducing imperfections such as vacancy defects and mass mismatch between the pillars and the base material, and alloy disorder in the pillars, can weaken the resonant hybridization and abnormally increase the TC. We show that both vacancy defects and mass mismatch can reduce the penetration of the resonant modes from the pillars into the base material, while the alloy disorder in the pillars can scatter the phonons inside them, which turns regular resonance into a random one with weaker hybridization. Our work provides useful insight into the phonon resonance mechanisms in experimentally relevant low dimensional nanostructures containing various imperfections.

Nonconjugated Triptycene-Spaced Donor-Acceptor-Type Emitters Showing Thermally Activated Delayed Fluorescence via Both Intra- and Intermolecular Charge-Transfer Transitions.

Thermally activated delayed fluorescence (TADF) emitters have aroused considerable attention, particularly for their great potential in organic light-emitting diodes (OLEDs). In typical TADF molecules, intramolecular charge transfer (CT) between electron-donor (D) and electron-acceptor (A) moieties is the dominant transition. Actually, CT transitions can possibly occur between different molecules as well. Herein, we used a nonconjugated triptycene (TPE) moiety to space D and A moieties and developed two novel emitters tBuDMAC-TPE-TRZ and tBuDMAC-TPE-TTR to explore the roles of intra- and intermolecular CT transitions. Along with weak intramolecular CT transitions, intermolecular CT transitions are dominant for tBuDMAC-TPE-TRZ and tBuDMAC-TPE-TTR neat films. Particularly, tBuDMAC-TPE-TRZ showed a high maximum external quantum efficiency of 10.0% in a nondoped solution-processed OLED, which was evidently higher than that of a corresponding 10 wt % tBuDMAC-TPE-TRZ-doped OLED with 4,4',4″-tris(carbazol-9-yl)triphenylamine (TCTA) as the host matrix. The results prove that intermolecular CT transitions indeed participate in the CT transition process in these systems and they are helpful to enhance the electroluminescence performance of emitting systems with weak intramolecular CT transitions.

Tuning the Anisotropic Thermal Transport in {110}-Silicon Membranes with Surface Resonances.

Understanding the thermal transport in nanostructures has important applications in fields such as thermoelectric energy conversion, novel computing and heat dissipation. Using non-homogeneous equilibrium molecular dynamic simulations, we studied the thermal transport in pristine and resonant Si membranes bounded with {110} facets. The break of symmetry by surfaces led to the anisotropic thermal transport with the thermal conductivity along the [110]-direction to be 1.78 times larger than that along the [100]-direction in the pristine structure. In the pristine membranes, the mean free path of phonons along both the [100]- and [110]-directions could reach up to ∼100 µm. Such modes with ultra-long MFP could be effectively hindered by surface resonant pillars. As a result, the thermal conductivity was significantly reduced in resonant structures, with 87.0% and 80.8% reductions along the [110]- and [100]-directions, respectively. The thermal transport anisotropy was also reduced, with the ratio κ110/κ100 decreasing to 1.23. For both the pristine and resonant membranes, the thermal transport was mainly conducted by the in-plane modes. The current work could provide further insights in understanding the thermal transport in thin membranes and resonant structures.

Thermal transport in amorphous small organic materials: a mechanistic study.

Understanding the thermal transport mechanisms in amorphous organic materials is of great importance to solve hot-spot issues in organic-electronics nanodevices. Here we studied thermal transport in two popular molecular electronic materials, N,N-dicarbazolyl-3,5-benzene (mCP) and N,N'-diphenyl-N,N'-di(3-methylphenyl)-(1,1'-biphenyl)-4,4'diamine (TPD), in the amorphous state by molecular dynamics simulations. We found that due to the softness of organic materials, the low thermal conductivity of both systems can be greatly enhanced under pressure. Notably, in such systems, the convective term of heat flux provides an important contribution to thermal transport as it cross-correlates with the Virial term in the Green-Kubo formula. Mode diffusivity calculations reveal that low-frequency modes can contribute significantly to thermal transport in both mCP and TPD. By increasing the pressure, the sound velocity and relaxation time of such low-frequency modes can be enhanced, and a part of these modes converts from diffusons to propagons. The cooperation of these three effects is responsible for the strong pressure dependence of thermal transport in amorphous organic systems. Molecular pair heat flux calculations demonstrate that heat transfer mainly happens between pairs of molecules with distances below 1.4 nm. This work paves the way for the optimization of thermal transport in amorphous organic materials widely used in opto-electronics, e.g. as OLED and OPV.

Theoretical Studies of Bipolar Transport in CnBTBT-FmTCNQ Donor-Acceptor Cocrystals.

The development of crystals with bipolar transport characteristics is essential for high-performance organic field effect transistor (OFET) devices. In this work, we theoretically investigated the bipolar transport behaviors in CnBTBT-FmTCNQ cocrystals. It is found that bipolar transport can be realized in C8BTBT-TCNQ and C12BTBT-TCNQ cocrystals with room-temperature electron/hole mobility up to 1.8/0.75 and 2.5/1.8 cm2 V-1 s-1, respectively. The comparable electron- and hole-transfer integrals between the nearest-neighbor molecule pairs as well as the small hole reorganization energy of the TCNQ molecule are responsible for the balanced electron and hole mobilities. Moreover, because of the π-π stacking between neighboring molecules, all cocrystals show strong anisotropic transport characteristic for both electron and hole transport with the mobility along the π-π stacking direction much larger than those along the other two directions. This work provides the possibility of high-performance OFET engineering and also enriches the OFET families with bipolar transport characteristics.

Influence of thermostatting on nonequilibrium molecular dynamics simulations of heat conduction in solids.

Nonequilibrium molecular dynamics (NEMD) has been extensively used to study thermal transport at various length scales in many materials. In this method, two local thermostats at different temperatures are used to generate a nonequilibrium steady state with a constant heat flux. Conventionally, the thermal conductivity of a finite system is calculated as the ratio between the heat flux and the temperature gradient extracted from the linear part of the temperature profile away from the local thermostats. Here, we show that, with a proper choice of the thermostat, the nonlinear part of the temperature profile should actually not be excluded in thermal transport calculations. We compare NEMD results against those from the atomistic Green's function method in the ballistic regime and those from the homogeneous nonequilibrium molecular dynamics method in the ballistic-to-diffusive regime. These comparisons suggest that in all the transport regimes, one should directly calculate the thermal conductance from the temperature difference between the heat source and sink and, if needed, convert it into the thermal conductivity by multiplying it with the system length. Furthermore, we find that the Langevin thermostat outperforms the Nosé-Hoover (chain) thermostat in NEMD simulations because of its stochastic and local nature. We show that this is particularly important for studying asymmetric carbon-based nanostructures, for which the Nosé-Hoover thermostat can produce artifacts leading to unphysical thermal rectification.

Thermal Transport Engineering in Graphdiyne and Graphdiyne Nanoribbons.

Understanding the details of thermal transport in graphdiyne and its nanostructures would help to broaden their applications. On the basis of the molecular dynamics simulations and spectrally decomposed heat current analysis, we show that the high-frequency phonons in graphdiyne can be strongly hindered in nanoribbons because of the boundary scattering. The isotropic transport in graphdiyne can be switched to anisotropic along the armchair and zigzag directions. Adding side chains onto the nanoribbon edges further reduces the thermal conductivity (TC) along both armchair and zigzag directions thanks to the reduction of heat current carried by low-frequency modes, a mechanism that arises from the phonon resonances. The uniaxial tensile strain plays a different role in the TC of graphdiyne, armchair nanoribbons, and zigzag nanoribbons. Tensile strain causes the thermal conductivities of graphdiyne, and armchair nanoribbons increase first and then get reduced, whereas for zigzag nanoribbons, the TC decreases with strain first and reaches to a plateau. The different low-frequency phonon response on strain is the main reason for the different TC behavior. For graphdiyne and armchair nanoribbons, the low-frequency heat current is enhanced gradually first and then get reduced with the increase of strain, while that of zigzag nanoribbons decreases with strain and then increases slightly. The current studies could help us understand the phonon transport in graphdiyne and its nanoribbons, which is useful for their TC engineering.

Memory phototransistors based on exponential-association photoelectric conversion law.

Ultraweak light detectors have wide-ranging important applications such as astronomical observation, remote sensing, laser ranging, and night vision. Current commercial ultraweak light detectors are commonly based on a photomultiplier tube or an avalanche photodiode, and they are incompatible with microelectronic devices for digital imaging applications, because of their high operating voltage and bulky size. Herein, we develop a memory phototransistor for ultraweak light detection, by exploiting the charge-storage accumulative effect in CdS nanoribbon. The memory phototransistors break the power law of traditional photodetectors and follow a time-dependent exponential-association photoelectric conversion law. Significantly, the memory phototransistors exhibit ultrahigh responsivity of 3.8 × 109 A W-1 and detectivity of 7.7 × 1022 Jones. As a result, the memory phototransistors are able to detect ultraweak light of 6 nW cm-2 with an extremely high sensitivity of 4 × 107. The proposed memory phototransistors offer a design concept for ultraweak light sensing devices.

Orbital-dependent redox potential regulation of quinone derivatives for electrical energy storage.

Electrical energy storage in redox flow batteries has received increasing attention. Redox flow batteries using organic compounds, especially quinone-based molecules, as active materials are of particular interest owing to the material sustainability, tailorable redox properties, and environmental friendliness of quinones and their derivatives. In this report, various quinone derivatives were investigated to determine their suitability for applications in organic RFBs. Moreover, the redox potential could be internally regulated through the tuning of σ and π bonding contribution at the redox-active sites. Furthermore, the binding geometry of some selected quinone derivatives with metal cations was studied. These studies provide an alternative strategy to identify and design new quinone molecules with suitable redox potentials for electrical energy storage in organic RFBs.

Author Correction: Thermal transport crossover from crystalline to partial-crystalline partial-liquid state.

The original version of this Article incorrectly omitted an affiliation of Sebastian Volz: 'LIMMS/CNRS-IIS(UMI2820) Institute of Industrial Science, University of Tokyo 4-6-1 Komaba, Meguro-ku Tokyo 153-8505 JAPAN'. This has now been corrected in both the PDF and HTML versions of the Article.

Thermal transport crossover from crystalline to partial-crystalline partial-liquid state.

Phase-change materials (crystalline at low temperatures and partial-crystalline partial-liquid state at high temperatures) are widely used as thermoelectric converters and battery electrodes. Here, we report the underlying mechanisms driving the thermal transport of the liquid component, and the thermal conductivity contributions from phonons, vibrations with extremely short mean free path, liquid and lattice-liquid interactions in phase-changed Li2S. In the crystalline state (T ≤ 1000 K), the temperature dependent thermal conductivity manifests two different behaviors, i.e., a typical trend of 1/T below 800 K and an even faster decrease between 800 and 1000 K. For the partial-crystalline partial-liquid Li2S when T ≥ 1100 K, the contributions of liquid and lattice-liquid interactions increase significantly due to the fluidization of Li ions, and the vibrations with extremely short mean free path, presumably assimilated to diffusons, can contribute up to 46% of the total thermal conductivity at T = 1300 K.

TiO2-Photoanode-Assisted Direct-Solar-Energy Harvesting and Storage in a Solar-Powered Redox Cell Using Halides as Active Materials.

The rapid deployment of renewable energy is resulting in significant energy security, climate change mitigation, and economic benefits. We demonstrate here the direct solar-energy harvesting and storage in a rechargeable solar-powered redox cell, which can be charged solely by solar irradiation. The cell follows a conventional redox-flow cell design with one integrated TiO2 photoanode in the cathode side. Direct charging of the cell by solar irradiation results in the conversion of solar energy in to chemical energy. Whereas discharging the cell leads to the release of chemical energy in the form of electricity. The cell integrates energy conversion and storage processes in a single device, making the solar energy directly and efficiently dispatchable. When using redox couples of Br2/Br- and I3-/I- in the cathode side and anode side, respectively, the cell can be directly charged upon solar irradiation, yielding a discharge potential of 0.5 V with good round-trip efficiencies. This design is expected to be a potential alternative toward the development of affordable, inexhaustible, and clean solar-energy technologies.

The impact of light irradiation timing on the efficacy of nanoformula-based photo/chemo combination therapy.

Photo/chemo combination therapy has been demonstrated to be a generally more powerful strategy for treating cancers than a single treatment modality. However, it is unknown whether the timing of light irradiation has any impact on therapeutic efficacy. We designed a carrier-free and self-monitoring nanodrug to monitor the entire dual-drug release profile and determined the impact of photodynamic therapy (PDT) at different time points. The designed nanodrug consists of the chemotherapeutic doxorubicin (DOX) and the photosensitizer pheophorbide A (PhA). The drugs form a fluorescence resonance energy transfer (FRET) pair (DOX transferring energy to PhA) when present at a precise ratio in the combination nanodrug. Due to the FRET effect, the DOX-PhA nanoparticles (NPs) show PhA fluorescence in a normal pH environment (such as cytoplasm). However, the FRET effect is lost when the NPs are disassembled in an acidic environment (such as lysosomes), and the DOX fluorescence is recovered. By real-time fluorescence variation monitoring, we determined the key time points when the drugs reached various subcellular locations, which helped us to determine the PDT-triggering time points and investigate the impact on the therapeutic effect in the combination therapy. Furthermore, the PDT was triggered at these established time points both in vitro and in vivo, which revealed that the best PDT-triggering time point in the combination therapy was achieved after nuclear entry of DOX. The study suggests that the optimization of combination therapy, not only photo/chemo but also chemo/chemo combination therapy, may require not only a controlled drug ratio but also a controlled drug release profile and target arrival time.

MoS2 heterostructure with tunable phase stability: strain induced interlayer covalent bond formation.

The structural phase transition in MoS2 promises applications in novel nanoelectronic devices. Elastic strain engineering can not only serve as a potential route for phase transition engineering, but also reveal potential ferroelastic behavior of MoS2 nanostructures. However, the elastic strain required for phase transition in monolayer MoS2 is far beyond its elastic limit, thus inhibiting the experimental realization. In this study, employing density functional theory calculations, we uncover that by forming heterostructure with buckled 2D materials, such as silicene, germanene and stanene, the critical phase transition strain required in monolayer MoS2 can be drastically reduced. Particularly when MoS2 forms sandwiched structures with silicene or stanene, the uniaxial and biaxial critical strain can be reduced to ∼0.06 and ∼0.03, respectively, which is well below the experimental elastic limit. This theoretical study not only proposes an experimental achievable strategy for flexible phase transition design in MoS2 nanostructure, but also identifies those MoS2 heterostructures as 2D candidates for potential shape memory devices and pseudoelasticity applications.