Quinoid structures are considered to be conducive to the charge transport of organic molecules,but this hypothesis is rarely proven at single-molecule level.Herein,as a proof of concept,the single-molecule conductance...Quinoid structures are considered to be conducive to the charge transport of organic molecules,but this hypothesis is rarely proven at single-molecule level.Herein,as a proof of concept,the single-molecule conductance of two furan-based isomers,3,3'-bis(4-(methylthio)phenyl)-2,2'-bifuran(2,2'-SMPBF)and 4,4'-bis(4-(methylthio)phenyl)-3,3'-bifuran(3,3'-SMPBF),is investigated by the scanning tunneling microscopy break junction(STM-BJ)technique and theoretical simulation.2,2'-SMPBF prefers to adopt a nearly planar conformation with intact alternating single and double bonds extended via2,2'-bifuran moiety and therefore exhibits goodπ-conjugation and a prominent quinoid structure.However,theπ-conjugation of 3,3'-SMPBF is interrupted due to ineffective cross-conjugation in the 3,3'-bifuran moiety,leading to the absence of a quinoid structure.2,2'-SMPBF displays switchable multiple conductances induced by the interconversion between folded and unfolded conformations and an abnormal rebound of conductance along with the increases of electrode displacement,which is demonstrated to be caused by the quinoid structure in a nearly planar conformation during the stretching process.However,3,3'-SMPBF without a quinoid structure in unfolded conformation exhibits extremely low conductance that cannot be captured in STM-BJ measurements.These results reveal the significant contribution of quinoid structure to molecular charge transport and provide valuable information on the structure-transport relationship for the design of efficient organic semiconductors.展开更多
The forming processes of 4,40-dipyridyl-based single-molecule junctions and mechanically induced conductance switching as well as the side-group effects are systematically investigated by applying the ab initio-based ...The forming processes of 4,40-dipyridyl-based single-molecule junctions and mechanically induced conductance switching as well as the side-group effects are systematically investigated by applying the ab initio-based adiabatic geometric optimization method and the one-dimensional transmission combined with three-dimensional correction approximation(OTCTCA)method.The numerical results show that for the 4,40-dipyridyl with a p-conjugated phenyl-phosphoryl or diphenylsilyl side group,the pyridyl vertically anchors on the second atomic layer of the pyramid-shaped Au tip electrode at small inter-electrode distances by laterally pushing the apical Au atom aside,which induces stronger pyridyl-electrode coupling and high-conductance state of the formed junctions.As the inter-electrode distance increases,the pyridyl shifts to the apical Au atom of the tip electrode.This apical Au atom introduces additional scatterings to the tunneling electrons and significantly decreases the conductance of the junctions.Furthermore,for the 4,40-dipyridyl with a phenyl-phosphoryl side group,the probability of manifesting the high-conductance state is decreased due to the oxygen atom reducing the probability of the pyridyl adsorbing on the second layer of Au tip electrode.In contrast,for the 4,40-dipyridyl with a nonconjugated cyclohexyl-phosphoryl side group,the steric hindrance from the bulky cyclohexyl group leads the molecule to preferentially form the O-Au contact,which prevents both the high conductance and mechanically induced conductance switching of the junction.Our results provide a theoretical understanding of the side-group effects on electronic transport properties of single-molecule junctions,offering an alternative explanation for the experimental observations.展开更多
Copper–carbon(Cu–C)composites have achieved great success in various fields owing to the greatly improved electrical properties compared to pure Cu,for example,a two-order-of-magnitude increase in current-carrying c...Copper–carbon(Cu–C)composites have achieved great success in various fields owing to the greatly improved electrical properties compared to pure Cu,for example,a two-order-of-magnitude increase in current-carrying capacity(ampacity).However,the frequent fuse failure caused by the poor thermal transport at the Cu–C heterointerface is still the main factor affecting the ampacity.In this study,we unconventionally leverage atomic distortion at Cu grain boundaries to alter the local atomic environments,thereby placing a premium on noticeable enhancement of phonon coupling at the Cu–C heterointerface.Without introducing any additional materials,interfacial thermal transport can be regulated solely through rational microstructural design.This new strategy effectively improves the interfacial thermal conductance by three-fold,reaching the state-of-the-art level in van der Waals(vdW)interface regulation.It can be an innovative strategy for interfacial thermal management by turning the detrimental grain boundaries into a beneficial thermal transport accelerator.展开更多
Recently,the strategy of tuning the dielectric parameters of absorbers for their excellent electromag-netic wave absorption(EMA)performance has attracted much attention.Among those candidates used for EMA application,...Recently,the strategy of tuning the dielectric parameters of absorbers for their excellent electromag-netic wave absorption(EMA)performance has attracted much attention.Among those candidates used for EMA application,high-entropy oxides(HEOs)can be implemented with this strategy due to their rich composition modulability.In this work,a series of implementation approaches varying from elemental design to structural modulation are employed to modulate the dielectric parameters of HEOs,resulting in their excellent EMA performance.The addition of Ti element optimizes the dipole distribution at the microscopic scales,improving the dielectric polarization of the materials.Moreover,a composite mate-rial is constructed by physically blending HEO with acetylene black(ACET),which significantly improves the macroscopic conduction loss of the material.The optimization of the dielectric genes of HEO/ACET is achieved with the blending effect and excellent EMA performance could be obtained.Among them,HEO with 17.5%ACET addition exhibits dual-band absorption,while Ti-HEO containing Ti element exhibits not only low-frequency absorption with reflection loss(RL)up to-29.81 dB at C-band but broadband absorption over 6 GHz as well as an optimal RL value up to-52.31 dB.In addition to the development of innovative EMA materials,this study offers a new perspective on how the EMA characteristics can be effectively regulated.展开更多
Transpiration element is included in the integrated stomatal conductance photosynthesis model by considering gaseous transfer processes, so the present model is capable to simulate the influence of boundary layer con...Transpiration element is included in the integrated stomatal conductance photosynthesis model by considering gaseous transfer processes, so the present model is capable to simulate the influence of boundary layer conductance. Leuning in his revised Ball's model replaced relative humidity with VPD s (the vapor pressure deficit from stomatal pore to leaf surface) and thereby made the relation with transpiration more straightforward, and made it possible for the regulation of transpiration and the influence of boundary layer conductance to be integrated into the combined model. If the differences in water vapor and CO 2 concentration between leaf and ambient air are considered, VPD s , the evaporative demand, is influenced by stomatal and boundary layer conductance. The physiological responses of photosynthesis, transpiration, and stomatal function, and the changes of intercellular CO 2 and water use efficiency to environmental factors, such as wind speed, photon flux density, leaf temperature and ambient CO 2, are analyzed. It is shown that if the boundary layer conductance drops to a level comparable with stomatal conductance, the results of simulation by the model presented here differ significantly from those by the previous model, and, in some cases, are more realistic than the latter.展开更多
基金financially supported by the National Natural Science Foundation of China(Nos.U23A20594,22375066 and 21788102)Guang Dong Basic and Applied Basic Research Foundation(No.2023B1515040003)。
文摘Quinoid structures are considered to be conducive to the charge transport of organic molecules,but this hypothesis is rarely proven at single-molecule level.Herein,as a proof of concept,the single-molecule conductance of two furan-based isomers,3,3'-bis(4-(methylthio)phenyl)-2,2'-bifuran(2,2'-SMPBF)and 4,4'-bis(4-(methylthio)phenyl)-3,3'-bifuran(3,3'-SMPBF),is investigated by the scanning tunneling microscopy break junction(STM-BJ)technique and theoretical simulation.2,2'-SMPBF prefers to adopt a nearly planar conformation with intact alternating single and double bonds extended via2,2'-bifuran moiety and therefore exhibits goodπ-conjugation and a prominent quinoid structure.However,theπ-conjugation of 3,3'-SMPBF is interrupted due to ineffective cross-conjugation in the 3,3'-bifuran moiety,leading to the absence of a quinoid structure.2,2'-SMPBF displays switchable multiple conductances induced by the interconversion between folded and unfolded conformations and an abnormal rebound of conductance along with the increases of electrode displacement,which is demonstrated to be caused by the quinoid structure in a nearly planar conformation during the stretching process.However,3,3'-SMPBF without a quinoid structure in unfolded conformation exhibits extremely low conductance that cannot be captured in STM-BJ measurements.These results reveal the significant contribution of quinoid structure to molecular charge transport and provide valuable information on the structure-transport relationship for the design of efficient organic semiconductors.
基金supported by the National Natural Science Foundation of China(Grant Nos.12474286,22173052,and 12204281).
文摘The forming processes of 4,40-dipyridyl-based single-molecule junctions and mechanically induced conductance switching as well as the side-group effects are systematically investigated by applying the ab initio-based adiabatic geometric optimization method and the one-dimensional transmission combined with three-dimensional correction approximation(OTCTCA)method.The numerical results show that for the 4,40-dipyridyl with a p-conjugated phenyl-phosphoryl or diphenylsilyl side group,the pyridyl vertically anchors on the second atomic layer of the pyramid-shaped Au tip electrode at small inter-electrode distances by laterally pushing the apical Au atom aside,which induces stronger pyridyl-electrode coupling and high-conductance state of the formed junctions.As the inter-electrode distance increases,the pyridyl shifts to the apical Au atom of the tip electrode.This apical Au atom introduces additional scatterings to the tunneling electrons and significantly decreases the conductance of the junctions.Furthermore,for the 4,40-dipyridyl with a phenyl-phosphoryl side group,the probability of manifesting the high-conductance state is decreased due to the oxygen atom reducing the probability of the pyridyl adsorbing on the second layer of Au tip electrode.In contrast,for the 4,40-dipyridyl with a nonconjugated cyclohexyl-phosphoryl side group,the steric hindrance from the bulky cyclohexyl group leads the molecule to preferentially form the O-Au contact,which prevents both the high conductance and mechanically induced conductance switching of the junction.Our results provide a theoretical understanding of the side-group effects on electronic transport properties of single-molecule junctions,offering an alternative explanation for the experimental observations.
基金financial support from the National Natural Science Foundation of China(Nos.52222602 and 52476052)Fundamental Research Funds for the Central Universities(FRF-TP-22-001C1 and FRF-EYIT-23-05).
文摘Copper–carbon(Cu–C)composites have achieved great success in various fields owing to the greatly improved electrical properties compared to pure Cu,for example,a two-order-of-magnitude increase in current-carrying capacity(ampacity).However,the frequent fuse failure caused by the poor thermal transport at the Cu–C heterointerface is still the main factor affecting the ampacity.In this study,we unconventionally leverage atomic distortion at Cu grain boundaries to alter the local atomic environments,thereby placing a premium on noticeable enhancement of phonon coupling at the Cu–C heterointerface.Without introducing any additional materials,interfacial thermal transport can be regulated solely through rational microstructural design.This new strategy effectively improves the interfacial thermal conductance by three-fold,reaching the state-of-the-art level in van der Waals(vdW)interface regulation.It can be an innovative strategy for interfacial thermal management by turning the detrimental grain boundaries into a beneficial thermal transport accelerator.
基金supported by the National Natural Science Foundation of China(Nos.52372289 and 52102368)the Guangdong Special Fund for key Areas(No.20237DZX3042)+1 种基金the State Key Laboratory of New Ceramic Materials Tsinghua University(No.KF202415)the Shenzhen Stable Support Project.
文摘Recently,the strategy of tuning the dielectric parameters of absorbers for their excellent electromag-netic wave absorption(EMA)performance has attracted much attention.Among those candidates used for EMA application,high-entropy oxides(HEOs)can be implemented with this strategy due to their rich composition modulability.In this work,a series of implementation approaches varying from elemental design to structural modulation are employed to modulate the dielectric parameters of HEOs,resulting in their excellent EMA performance.The addition of Ti element optimizes the dipole distribution at the microscopic scales,improving the dielectric polarization of the materials.Moreover,a composite mate-rial is constructed by physically blending HEO with acetylene black(ACET),which significantly improves the macroscopic conduction loss of the material.The optimization of the dielectric genes of HEO/ACET is achieved with the blending effect and excellent EMA performance could be obtained.Among them,HEO with 17.5%ACET addition exhibits dual-band absorption,while Ti-HEO containing Ti element exhibits not only low-frequency absorption with reflection loss(RL)up to-29.81 dB at C-band but broadband absorption over 6 GHz as well as an optimal RL value up to-52.31 dB.In addition to the development of innovative EMA materials,this study offers a new perspective on how the EMA characteristics can be effectively regulated.
文摘Transpiration element is included in the integrated stomatal conductance photosynthesis model by considering gaseous transfer processes, so the present model is capable to simulate the influence of boundary layer conductance. Leuning in his revised Ball's model replaced relative humidity with VPD s (the vapor pressure deficit from stomatal pore to leaf surface) and thereby made the relation with transpiration more straightforward, and made it possible for the regulation of transpiration and the influence of boundary layer conductance to be integrated into the combined model. If the differences in water vapor and CO 2 concentration between leaf and ambient air are considered, VPD s , the evaporative demand, is influenced by stomatal and boundary layer conductance. The physiological responses of photosynthesis, transpiration, and stomatal function, and the changes of intercellular CO 2 and water use efficiency to environmental factors, such as wind speed, photon flux density, leaf temperature and ambient CO 2, are analyzed. It is shown that if the boundary layer conductance drops to a level comparable with stomatal conductance, the results of simulation by the model presented here differ significantly from those by the previous model, and, in some cases, are more realistic than the latter.
