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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.

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.

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.

Thermally activated delayed fluorescence (TADF) materials have attracted wide attention due to the full utilization of triplet and singlet excitons. Beyond the typical donor–acceptor (D–A) TADF compounds, D–π–A–π–D TADF emitters were reported recently. In this paper, we study the dynamic nature of excited states of four D–π–A–π–D TADF molecules with full color emission at the first principles level. It shows that MAC-PN has good TADF efficiency benefiting from the large reverse intersystem crossing (RISC, krisc) and radiative decay (kr) rates. PX-PN and MCZ-DPPN molecules have a reduced TADF efficiency, owing to the largely increased non-radiative decay rate (knr) by two orders of magnitude and decreased krisc, respectively, while PX-CNP shows the worst TADF performance, because knr is five orders of magnitude larger than kr. Moreover, reorganization energy analysis reveals that the RISC process is largely manipulated by the torsional vibration of the dihedral angle, while the non-radiative process mainly happens through the channels of bond stretching vibration. These calculations agree well with the experimental efficiency trend among the four TADF molecules and uncover the intrinsic TADF mechanisms, providing the possibility of further engineering the molecules with improved TADF efficiency.

Exploiting high-performance non-doped organic light-emitting diodes (OLEDs) is a step towards future commercial application requirements, but great challenges remain due to quenching related to intermolecular triplet interaction. In this work, a novel strategy of exploiting high-performance non-doped electroluminescence via tuning intermolecular hydrogen bonding is demonstrated. Suitable intermolecular hydrogen bonding enables formation of a 3D supramolecular framework, which not only evidently restricts the nonradiative process and suppresses the triplet exciton quenching caused by π–π stacking of triplets, but also favors the horizontal molecular orientations especially in their non-doped states. The non-doped OLED based on the thermally activated delayed fluorescence emitter mTPy-PXZ with such suitable intermolecular hydrogen bonds exhibits the state-of-the-art performance with maximum external quantum efficiency of up to 23.6% with only 7.2% roll-off at 1000 cd m−2. Moreover, it is the first report that the performance of an OLED with a non-doped emitting layer can surpass its corresponding optimized doped device. It is believed that this hydrogen bond-modulated mechanism can not only provide a new pathway for designing emitters for high-performance non-doped organic electroluminescence, but also has great potential in other solid-state luminescence applications.

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 ...

International audience; The understanding of hydrodynamic heat transport in finite-sized graphitic materials remains elusive due to the lack of an efficient methodology. In this paper, we develop a computational framework enabling an accurate description of heat transport in anisotropic graphite ribbons by a kinetic theory approach with full quantum mechanical first-principles input. A unified analysis of the size scaling of the thermal conductivity in the longitudinal and transverse directions of the system is made within the computational framework complemented with a macroscopic hydrodynamic approach. As a result, we demonstrate a strong end effect on the phonon Knudsen minimum, as a hallmark of the transition from ballistic to hydrodynamic heat transports, along a rectangular graphite ribbon with finite length and width. The phonon Knudsen minimum is found to take place only when the ribbon length is ∼5-10 times the upper limit of the width range in the hydrodynamic regime. This paper contributes to a unique methodology with high efficiency and a deeper understanding of the size effect on phonon hydrodynamics, which would open opportunities for its theoretical and experimental investigation in graphitic micro-and nanostructures.

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., Surface of colloidal quantum dot is sensitive to water, and the interaction could potentially alter its chemical environments. Here, Shi et al. investigate how the interaction effects the nanostructures and carrier dynamic in CQDs, and subsequently introduce meniscus-guided coating technique to mitigate CQD fusion triggered by water adsorption.

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.

International audience; The understanding and modeling of inelastic scattering of thermal phonons at a solid/solid interface remain an open question. We present a fully quantum theoretical scheme to quantify the effect of anharmonic phonon-phonon scattering at an interface via nonequilibrium Green's function (NEGF) formalism. Based on the real-space scattering rate matrix, a decomposition of the interfacial spectral energy exchange is made into contributions from local and nonlocal anharmonic interactions, of which the former is shown to be predominant for high-frequency phonons whereas both are important for low-frequency phonons. The anharmonic decay of interfacial phonon modes is revealed to play a crucial role in bridging the bulk modes across the interface. The overall quantitative contribution of anharmonicity to thermal boundary conductance is found to be moderate. The present work promotes a deeper understanding of heat transport at the interface and an intuitive interpretation of anharmonic phonon NEGF formalism.

Improving the control of heat flow at the nanoscale is essential for promoting its applications in many fields, such as energy conversion, thermal informatics, and communication technologies. Here we perform a systematic study on the synergistic effect of screw dislocations and surface resonators on thermal transport of Si nanowires and the corresponding mechanisms based on molecular dynamics simulations. We uncover that screw dislocations reduce the thermal conductivity by enhancing the anharmonicity of nanowires due to the nonhomogeneous stress field. For resonant structures, we demonstrate that the suppression of relaxation time is the main mechanism for thermal conductivity reduction. The suppression of relaxation time by more than two orders of magnitude below 4 THz dramatically reduces the resonant structure thermal conductivity, while the previously proposed group velocity reduction mechanism can only impede phonon transport beyond 4 THz slightly. By comparing the mechanisms produced by dislocations and resonators, we find that the resonators have a stronger effect over screw dislocations in impeding the phonon transport at low frequencies while it becomes opposite at high frequencies. As a result, they can be combined together to manipulate phonon transport synergistically at all frequencies. Our findings not only provide insights into the mechanisms of thermal conductivity engineering by screw dislocations and surface resonators, but they also illustrate a paradigm for ultralow thermal conductivity design through the tailoring of the entire frequency range of phonon transport.

The understanding and modeling of inelastic scattering of thermal phonons at a solid/solid interface remain an open question. We present a fully quantum theoretical scheme to quantify the effect of anharmonic phonon-phonon scattering at an interface via non-equilibrium Green's function (NEGF) formalism. Based on the real-space scattering rate matrix, a decomposition of the interfacial spectral energy exchange is made into contributions from local and non-local anharmonic interactions, of which the former is shown to be predominant for high-frequency phonons whereas both are important for low-frequency phonons. The anharmonic decay of interfacial phonon modes is revealed to play a crucial role in bridging the bulk modes across the interface. The overall quantitative contribution of anharmonicity to thermal boundary conductance is found to be moderate. The present work promotes a deeper understanding of heat transport at the interface and an intuitive interpretation of anharmonic phonon NEGF formalism.

We propose a way to properly interpret the apparent thermal conductivity obtained for finite systems using equilibrium molecular dynamics simulations (EMD) with fixed or open boundary conditions in the transport direction. In such systems the heat current autocorrelation function develops negative values after a correlation time which is proportional to the length of the simulation cell in the transport direction. Accordingly, the running thermal conductivity develops a maximum value at the same correlation time and eventually decays to zero. By comparing EMD with nonequilibrium molecular dynamics (NEMD) simulations, we conclude that the maximum thermal conductivity from EMD in a system with domain length 2L is equal to the thermal conductivity from NEMD in a system with domain length L. This facilitates the use of nonperiodic-boundary EMD for thermal transport in finite samples in close correspondence to NEMD., 7pages, 8 figures

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.

Monolayer borophene, a novel kind of two-dimensional (2D) crystal, has been receiving intensive attention owing to its atomic thickness and metallic characteristics. Rational tuning the anisotropic electronic transport properties is essential to the application of monolayer borophene in electronic and optoelectronic devices. Herein, we developed an oxidation strategy to tune the anisotropic transport properties of borophene by changing O-defect coverage, using density functional theory combined with the nonequilibrium Green’s function formalism. It was found that for monolayer borophene, the preferable current flowing direction between armchair and zigzag could be reversed by modulating the surface O-defect coverage between 0 and 100%. The tunable anisotropic transport properties of oxidized borophene could be attributed to the interplay among several factors, including the surface charge transfer between O-defects and borophene layer, the scattering effects related to the coverage and orientation of O-B-O interfaces, and the additional transport channels through O-defects. Our work unveils the great potential of oxidization strategy in tuning the anisotropic electronic transport properties of monolayer borophene and is of significance to its application in high-performance electronic and optoelectronic nanodevices.

Redox-active polymers are promising materials for rechargeable batteries because of their structural diversity and resource sustainability. We present in this work the feasibility of manipulation of the rigidity of polymer chains to alter the ion diffusion behaviour in polymeric materials. The results indicate that the ionic diffusion coefficient is enhanced by orders of magnitude if a proper twisted group is introduced to interrupt the rigid backbone of the polymers. Both greater galvanometric and volumetric power/energy densities have been achieved for the polymer with more flexible chains, regardless of its lower surface area and electrical conductivity compared to the one with a more rigid chain structure. Such a strategy might be useful for the design of polymeric battery materials with high power density without sacrificing the volumetric energy density.

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.

International audience; We propose a novel phonon resonator for hindering the thermal transport of nanowires (NWs), based on a screw threadlike helical nanowall. Results from molecular dynamic simulations reveal that the thermal conductivity and phonon transmission of the screw threadlike Si NWs continuously decrease with an increase in the period density of the helical nanowall. The reduction can reach as high as 36% for the NW with six circles of the helical nanowall, which is much larger than in the case of NWs with straight nanowalls (12%) and nanopillars (15%). This phenomenon is due to that the helical nanowall has a larger contact area with the base structure, which leads to a larger volume of the resonating substructure with a constant height and width. Phonon dispersion relations show the formation of flat bands, which confirms the occurrence of phonon resonances due to the surface screw threadlike structures. The phonon spatial distribution reveals mode localization in the helical resonator at the resonant frequency. With regard to suppressing the phonon propagation, the helical nanowall as a phonon resonator exhibits superiority over the straight nanowalls or conventional nanopillars, because (1) it may be easier to fabricate a larger contact area with the base structure and (2) it avoids the problem of nanopillars or nanowalls touching each other, which maintains the ability of generating localized modes. The obtained results provide a novel design of efficient surface phonon resonators to realize nanowires with ultralow thermal conductivity.

The potential application of monolayer MS2 (M = Mo, W) as thermoelectric material has been widely studied since the first report of successful fabrication. However, their performances are hindered by the considerable band gap and the large lattice thermal conductivity in the pristine 2H phase. Recent discoveries of polymorphism in MS2s provide new opportunities for materials engineering. In this work, phonon and electron transport properties of both 2H and 1T′ phases were investigated by first-principle calculations. It is found that upon the phase transition from 2H to 1T′ in MS2, the electron transport is greatly enhanced, while the lattice thermal conductivity is reduced by several times. These features lead to a significant enhancement of power factor by one order of magnitude in MoS2 and by three times in WS2. Meanwhile, the figure of merit can reach up to 0.33 for 1T′MoS2 and 0.68 for 1T′WS2 at low temperature. These findings indicate that monolayer MS2 in the 1T′ phase can be promising materials for thermoelectric devices application. Meanwhile, this work demonstrates that phase engineering techniques can bring in one important control parameter in materials design. Keywords: Phase engineering, Thermoelectric, Transition metal dichalcogenides

p-n heterostructure (HTS) is a fundamental component for high-performance electronic and optoelectronic device. Vertical stacking through van der Waals (vdW) force is emerging as a feasible technique to construct p-n HTS. Herein, we designed a novel kind of direct-bandgap C3N monolayer, via adjusting the arrangement of C and N atoms in C3N hexagonal cell. On the basis of the density functional theory combined with the non-equilibrium Green’s function method, we built two-dimensional vdW-contact phosphorene (BP)/C3N p-n HTS, and analyzed its electronic and optical properties in comparison with the inplane-jointed ones. The strong charge transfer between BP and C3N segments results in a wide bandgap of 0.48 eV for joint-contact type BP/C3N HTS, whereas the effective interlayer coupling in vdW-contact type leads to an improved light adsorption as compared to the isolated C3N monolayer. By fabricating dual-gated BP/C3N HTS field-effect transistors (FETs), the dynamic transport behaviors demonstrated that the band bending under a lower threshold voltage makes band-to-band tunneling possible for vdW-contact type. Our work suggests that vdW-contact type is superior to joint-contact type in constructing p-n HTS for high-performance electronic and optoelectronic devices.

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 b...

Engineering low-frequency phonon transport in nanostructures with the phonon resonant mechanism has become an important research direction. On the basis of non-equilibrium molecular dynamics simulations, the thermal transport in pristine and resonant Si-membranes bounded with {100}, {110} and {111} facets is investigated. It is found that the creation of surfaces can introduce anisotropic thermal transport due to the lattice symmetry breaking. Besides, ballistic phonon transport is found in pristine membranes with lengths up to 500 nm at low-frequencies with a critical frequency mainly dependent on the crystallographic orientation. Moreover, although surface resonances can dramatically reduce the thermal conductivity of all membranes, the resonant effect strongly relies on membrane orientation. Among the three studied membrane orientations, the resonant effect is maximized in the {111}-membrane, where the thermal conductivity is tuned from the largest one to the smallest one among the three membrane types by resonant pillars. The large thermal conductivity reduction in the {111}-membranes by resonances originated from the reduced spectral heat flux between 3 and 12 THz. Furthermore, the resonant coupling strength can be tuned by the interface vacancy between resonant pillars and the base material, which can enhance phonon transport at an intermediate frequency range. Our work provides further insights on thermal transport engineering by phonon resonances and could be useful for thermal conductivity engineering with surface orientations and resonances.

International audience; We use multiscale modelings to investigate the heat dissipation in 2D transistors based on phosphorene and silicene. First, molecular dynamics (MD) simulations were used to calculate the thermal interface resistance R int between the 2D materials (phosphorene and silicene) and dielectrics substrates (SiO 2 and TiO 2). The calculated R int of these systems are close to that between graphene and SiO 2 and are insensitive to the temperature. The MD values then served as inputs for finite-element simulations at the device scale. It is found that the heat-dissipation ability of the 2D transistors can be improved by increasing the thermal conductivities of the 2D materials as well as of the substrate. However, in contrast to the common belief, it is difficult to largely reduce the hot-spot temperature by tuning the interface thermal resistance. Finally, we show that the cooling performance of silicene/SiO 2 system can be significantly improved with a few-layer graphene heat spreader. These results provide important information for the novel design of 2D transistors in terms of thermal management.

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.

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 C

We report that the interface composition diffusion, which often occurs in the synthesis of two-dimensional lateral heterostructures, can significantly suppress the thermal transport property. Our molecular dynamics simulations show that the thermal conductivity of graphene/h-BN lateral heterostructures can be largely tuned by varying the interface composition diffusion length. The underling mechanism is explained by Anderson localization of phonons, which is corroborated by the exponential decay of the phonon transmission with composition gradient length. Phase breaking interactions are shown to delocalize the modes at elevated temperatures. The findings in this work suggest that composition graded interfaces can be used to tune the thermal transport of heterostructures via phonon localization, which is important for their applications in electronics, thermoelectrics and thermal insulators.

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.

Lithium (Li) metal is the most promising anode material for the next-generation high-energy density battery due to its high specific capacity and low redox potential. However, its practical applications is challenged by uncontrolled Li dendrite growth and volume expansion. In this work, we propose a cobalt-enhanced nitrogen doped porous carbon nanofibers (PCNF–Co/N) with an enhanced lithiophilic property. The Co/N co-doping is achieved through pyrolysis of self-assembled Co/Zn metal-organic framework (MOF) nanosheets on carbon nanofibers (CNF). The synergistic effect between the simultaneously introduced Co and N defects not only boosts the Li affinity of pyridinic and pyrrolic N but also transfers the graphitic N into lithiophilic, providing crucial roles for uniform plating of Li. Meanwhile, the porous structure formed by Zn sublimation provides sufficient buffer space for internal stress caused by Li plating/stripping. As a result, the PCNF–Co/N@Li anode delivers a long lifespan over 1400 h with a low voltage hysteresis. When paired with a LiFePO4 cathode, the full cell demonstrates decent cyclic stability and rate capability, demonstrating the capability of PCNF–Co/N in developing composite Li-metal anode with enhanced lithiophilicity and low internal stress.

International audience; This perspective describes the key physical concepts at play capturing transport regimes in gaps with sizes ranging from the micron down to theÅngström. The roles of photons, phonons and electrons are highlighted and illustrated across several examples from the literature. Particularly, two situations for which atomistic calculations provide insights in the microscopic mechanisms at play in thermal transport across nanoscale gaps will be expounded, namely thermal transport across silica clusters and in 2D materials. While experimental investigations are highly challenging and a vast field of opportunities still remains open in this matter, several related domains of application such as Thermophotovoltaics, Thermoionics, Thermal management, Imaging and 2D materials are involved. *

Optimizing organic molecular deposition conditions to improve charge transport is of great importance for applications in OLED. Based on molecular dynamics simulations, 4,4-bis(Ncarbazolyl)-1,1-biphenyl (CBP), a linear bipolar host OLED molecule, are deposited with different substrate temperature Tsub and inserting molecule velocity vd. It is found that in the investigated parameter range, both lower substrate temperature and higher initial velocity of inserting molecules are beneficial for obtaining more ordered morphology. Further studies based on kinetic Monte Carlo simulations reveal that the deposited structures with higher molecule order possess higher charge mobilities, which arises from the reduced energetic disorder and improved spatial correlation of site energies. It is also found that the effect of molecular ordering on electron transport is more obvious than that on hole transport. With electrical field of 1.6 × 10 5 V / c m , the electron and hole mobilities can be enhanced by 100% and 42% when Tsub is increased from 200 K to 300 K (vd = 0.002 A/fs), respectively. While their mobilities can also be enhanced by 30% and 17% with the increase of vd from 0.002 to 0.008 A/fs (Tsub = 300 K). The larger enhancement of electron mobility can result in more balanced electron and hole mobilities in CBP, thus improve its bipolar transport characteristic. Our work not only connects the molecular morphology with charge transport properties, but also provides a strategy for improving charge mobility through the optimization of molecular deposition conditions.

Thermally activated delayed fluorescence (TADF) chiral emitters attract widespread attention due to their high exciton utilization as well as potential applications. In this work, we developed a pair of chiral donors, (R) and (S)-9-methyl-2,9-diphenyl-9,10-dihydroacridine (PMAc), for general TADF molecular design. The enantiomers (R) and (S)-TTR-PMAc are accordingly constructed. Interestingly, they are not only TADF emitters, but also possess dual stable conformations, i.e. nearly planar and nearly orthogonal conformations. (R) and (S)-TTR-PMAc show similar physical properties under conventional non-polarized environment. While under chiroptical environments, they exhibit obvious mirror-like circular dichroism (CD) and circularly polarized luminescence (CPL) properties. Moreover, by further comparing the CPL properties originated from each conformer, we confirm that molecular conformations can significantly influence the chiroptical performance.

International audience; Screw dislocations are known to impede the thermal transport of homogeneous nanowires by reducing the phonon relaxation time without affecting the phonon group velocity. By using molecular dynamics simulations in this study, we show that the impact of screw dislocation on the thermal conductivity of the SiGe superlattice nanowires depends on the period length. The analysis of phonon transmission spectra and phonon mean free paths indicate that strong phonon-screw dislocation scatterings occur for phonons in the frequency range of 3-8 THz. The screw dislocations change the phonon scattering mechanisms, which is the main cause of the thermal conductivity reduction. Contrary to the case of homogeneous nanowires, a sizable decrease in the phonon group velocity is found in superlattices with screw dislocations. This phenomenon is attributed to the larger number of Si-Ge bonds in the vicinity of the interface due to the slipping of the atomic planes. In contrast to the decreased thermal conductivity, the phonon propagation in the interface region of the nanowires is enhanced by screw dislocations. Our findings provide critical insights into the understanding of dislocation-heat transfer relationship in materials, especially in heterostructures where interfaces are vital for thermal transport.

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.

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 TiO

Monolayer transition metal dichalcogenides/metal (MX2/metal) based transistors have been widely studied. However, further development is hindered by the large contact resistance between MX2 and metal contact. In this paper, we demonstrated that interfacial charge transfer between MX2 and metal is the key for tuning contact resistance. With the lattice misfit criterion applied to screen combination of MX2s and metals, it has been found out that both phase stability of MX2 and contact nature between MX2 and metal will be sensitively affected by interfacial charge transfer. Additionally, we have identified seven MX2/metal systems that can potentially form zero Schottky barrier contacts utilizing phase engineering. On base of interfacial charge calculations and contact resistance analysis, we have presented three types of MX2/metal contacts that can be formed with distinguished contact resistance. Our theoretical results not only demonstrate various choice of MX2/metal designs in order to achieve different amounts of interfacial charge transfer as well as manipulate contact resistance, but also shed light on designing ohmic contacts in MX2/metal systems. Interfacial charge calculations enable the prediction of the contact resistance behaviour of MX2/metal structures. A team led by Bin Ouyang at the University of California Berkeley performed a systematic theoretical investigation of the interplay between interface interactions and phase stability in atomically thin MX2/metal systems, where M is a transition metal and X is a chalcogenide. A combination of interfacial charge calculations and contact resistance analysis allowed the identification of twenty-eight MX2/metal structures that can be further categorised in three groups according to their contact nature. Notably, the first type of contact possesses zero tunnel barrier between MX2 and the metal, whereas the second type enables substantial charge transfer accompanied to a 2H-to-1T’ structural phase transition in MX2. These results highlight viable design routes for contact resistance manipulation in MX2 transistors.