Due to the large exciton binding energy,two-dimensional(2D)transition metal dichalcogenides(TMDCs)provide an ideal platform for studying excitonic states and related photonics and optoelectronics.Polarization states l...Due to the large exciton binding energy,two-dimensional(2D)transition metal dichalcogenides(TMDCs)provide an ideal platform for studying excitonic states and related photonics and optoelectronics.Polarization states lead to distinct light-matter interactions which are of great importance for device applications.In this work,we study polarized photoluminescence spectra from intralayer exciton and indirect exciton in WS_(2) and WSe_(2) atomic layers,and interlayer exciton in WS_(2)/WSe_(2) heterostructures by radially and azimuthally polarized cylindrical vector laser beams.We demonstrated the same in-plane and out-of-plane polarization behavior from the intralayer and indirect exciton.Moreover,with these two laser modes,we obtained interlayer exciton in WS_(2)/WSe_(2) heterostructures with stronger out-of-plane polarization,due to the formation of vertical electric dipole moment.展开更多
Bioinspired micro‐/nano‐motors are artificial micro‐/nano‐machines that can convert various forms of energy to propel their movement[1].For example,the motion of some of these micromachines can be precisely contro...Bioinspired micro‐/nano‐motors are artificial micro‐/nano‐machines that can convert various forms of energy to propel their movement[1].For example,the motion of some of these micromachines can be precisely controlled by application of external physical stimuli including magnetic,electric and acoustic fields[2,3].Inspired by the study of microorganisms,researchers have been exploring also the use of available chemical energy from the local environment to trigger and sustain self‐propulsion[4].Within this research direction,Metal‐organic frameworks(MOFs)—a class of extended materials synthesized via a modular approach from inorganic(metal clusters or ions)and organic linkers[5]—offer excellent opportunities for the design and synthesis of self‐propelled micromotors.MOFs typically possess ultra‐high surface areas that allow facile access to densely populated catalytically active sites imbedded within their pore networks.Through careful design these catalytic sites can be exploited to convert chemical energy into kinetic energy resulting in self‐propulsion of the MOF crystal[6].In addition,rigidity,density,crystalline pore organization and pore size of MOFs can be optimized to carry out a swimming‐type motion[7].展开更多
Fluorescent studies of living plant cells such as confocal microscopy and fluorescence lifetime imaging often suffer from a strong autofluorescent background contribution that significantly reduces the dynamic image c...Fluorescent studies of living plant cells such as confocal microscopy and fluorescence lifetime imaging often suffer from a strong autofluorescent background contribution that significantly reduces the dynamic image contrast and the quantitative access to sub-cellular processes at high spatial resolution. Here, we present a novel technique--fluorescence intensity decay shape analysis microscopy (FIDSAM) to enhance the dynamic contrast of a fluorescence image of at least one order of magnitude. The method is based on the analysis of the shape of the fluorescence intensity decay (fluorescence lifetime curve) and benefits from the fact that the decay patterns of typical fluorescence label dyes strongly differ from emission decay curves of autofluorescent sample areas. Using FIDSAM, we investigated Arabidopsis thaliana hypocotyl cells in their tissue environment, which accumulate an eGFP fusion of the plasma membrane marker protein LTI6b (LTI6b-eGFP) to low level. Whereas in conventional confocal fluorescence images, the membranes of neighboring cells can hardly be optically resolved due to the strong autofluorescence of the cell wall, FIDSAM allows for imaging of single, isolated membranes at high spatial resolution. Thus, FIDSAM will enable the sub-cellular analysis of even low-expressed fluorophoretagged proteins in living plant cells. Furthermore, the combination of FIDSAM with fluorescence lifetime imaging provides the basis to study the local physico-chemical environment of fluorophore-tagged biomolecules in living plant cells.展开更多
Water/ethyl acetate/ethanol is widely used as a“green”extractant system.We show that 2 different types of phase separation can be induced upon centrifugation in this ternary system using ethanol as a cosolvent of wa...Water/ethyl acetate/ethanol is widely used as a“green”extractant system.We show that 2 different types of phase separation can be induced upon centrifugation in this ternary system using ethanol as a cosolvent of water and ethyl acetate:centrifuge-induced criticality and centrifuge-induced emulsification.The expected composition profiles of samples after centrifugation can be represented by bent lines in a ternary phase diagram when gravitational energy is added to the free energy of mixing.The experimental equilibrium composition profiles behave qualitatively as expected and can be predicted using a phenomenological theory of mixing.The concentration gradients are small except near the critical point,as expected for small molecules.Nevertheless,they are usable when accompanied by temperature cycles.These findings open new possibilities of centrifugal separation,even if control is delicate during temperature cycles.These schemes are accessible even at relatively low centrifugation speed for molecules that float and sediment with apparent molar masses several hundred times larger than the molecular mass.展开更多
Over the past nearly 90 years,scientists have gradually revealed the mysteries of intramolecular motion and chemical bonds through continuous research and innovation.In 1928,Irish scientist E.Synge proposed a method t...Over the past nearly 90 years,scientists have gradually revealed the mysteries of intramolecular motion and chemical bonds through continuous research and innovation.In 1928,Irish scientist E.Synge proposed a method to overcome the limit of classical optical resolution,which laid the foundation for the subsequent development of scanning near-field optical microscopes.Near-field optical microscopy continues to push optical resolution to new records,well beyond the diffraction limit of traditional optical microscopy.展开更多
High-performance multiphoton-pumped lasers based on cesium lead halide perovskite nanostructures are promising for nonlinear optics and practical frequency upconversion devices in integrated photonics. However, the pe...High-performance multiphoton-pumped lasers based on cesium lead halide perovskite nanostructures are promising for nonlinear optics and practical frequency upconversion devices in integrated photonics. However, the performance of such lasers is highly dependent on the quality of the material and cavity, which makes their fabrication challenging. Herein, we demonstrate that cesium lead halide perovskite triangular nanorods fabricated via vapor methods can serve as gain media and effective cavities for multiphoton-pumped lasers. We observed blue-shifts of the lasing modes in the excitation fluence-dependent lasing spectra at increased excitation powers, which fits well with the dynamics of Burstein-Moss shifts caused by the band filling effect. Moreover, efficient multiphoton lasing in CsPbBr3 nanorods can be realized in a wide excitation wavelength range (700-1,400 nm). The dynamics of multiphoton lasing were investigated by time-resolved photoluminescence spectroscopy, which indicated that an electron-hole plasma is responsible for the multiphoton-pumped lasing. This work could lead to new opportunities and applications for cesium lead halide perovskite nanostructures in frequency upconversion lasing devices and optical interconnect systems.展开更多
We investigate nanorod assemblies of two 64-substituted pentacenes, namely (2,3-X2-9,10-Y2)-substituted pentacenes with X -- Y = OCH3 (MOP) and with X = F, Y-- OCH3 (MOPF), grown on Au(111) single crystals. By...We investigate nanorod assemblies of two 64-substituted pentacenes, namely (2,3-X2-9,10-Y2)-substituted pentacenes with X -- Y = OCH3 (MOP) and with X = F, Y-- OCH3 (MOPF), grown on Au(111) single crystals. By using a multi-technique approach based on ultraviolet photoelectron spectroscopy X-ray photoelectron spectroscopy; and X-ray absorption, we find evidence for charge transfer screening at the interface with gold. Furthermore, the MOP and MOPF nanorods show a rough surface morphology, which was investigated with atomic force microscopy. We use molecular simulation techniques to investigate the energetic barriers to diffusion and to traverse step-edges to estimate their influence on the nanorod roughness. We find that barriers to surface diffusion on a terrace are anisotropic and that their direction favors the formation of nanorods in these materials.展开更多
Metal-free organic radicals are fascinating materials owing to their unique properties. Having a stable magnetic moment coupled to light elements makes these materials central to develop a large variety of application...Metal-free organic radicals are fascinating materials owing to their unique properties. Having a stable magnetic moment coupled to light elements makes these materials central to develop a large variety of applications. We investigated the magnetic spinterface coupling between the surface of a single rutile TiO2(110) crystal and a pyrene-based nitronyl nitroxide radical, using a combination of thickness-dependent X-ray photoelectron spectroscopy and ab initio calculations. The radicals were physisorbed, and their magnetic character was preserved on the (almost) ideal surface. The situation changed completely when the molecules interacted with a surface defect site upon adsorption. In this case, the reactivity of the defect site led to the quenching of the molecular magnetic moment. Our work elucidates the crucial role played by the surface defects and demonstrates that photoemission spectroscopy combined with density functional theory calculations can be used to shed light on the mechanisms governing complex interfaces, such as those between magnetic molecules and metal oxides.展开更多
The possibility of ttming between red, green and broadband white color of up-conversion was demonstrated in thermally 450 ℃ treated Yb/Er co-doped nano NaYF4 phosphor. The color variability was studied by means of po...The possibility of ttming between red, green and broadband white color of up-conversion was demonstrated in thermally 450 ℃ treated Yb/Er co-doped nano NaYF4 phosphor. The color variability was studied by means of power dependence of luminescence, which exhibited unusual behavior. Large hysteresis, as well as discrepancy from a power law indicated the important role the increased heating played during the up-conversion in nano-sized materials.展开更多
The study focuses on the in flue nee of Ni and Bi on alkali me etha nol oxidati on reacti on (EOR) activities, stabilities and structure characteristics of carb on supported Pd-based nano catalysts (Pd/C, Pd6oNi4o/C, ...The study focuses on the in flue nee of Ni and Bi on alkali me etha nol oxidati on reacti on (EOR) activities, stabilities and structure characteristics of carb on supported Pd-based nano catalysts (Pd/C, Pd6oNi4o/C, Pd6oBi4o/C, Pd6oNi2oBi2o/C) by cyclic voltammetry/chr ono amperometry using rotating disk electrode and various physico-chemical methods such as X-ray powder diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy coupled with energy dispersive X-ray spectroscopy and inductively coupled plasma optical emission spectrometry. Nickel generates more adsorbed OH on the Pd catalyst surface than Bi and promotes the oxidation of adsorbed ethanol species. This results in a low onset potential toward ethanol oxidation with high current density. The presenee of Bi facilitates high toleranee toward various reaction in termediates resulting from the incomplete etha nol oxidation, but might also initiate the agglomerati on of Pd nano particles. The no vel Pd60Ni20Bi20/C nanocatalyst displays exceptional byproduct toleranee, but only satisfying catalytic activity toward ethanol oxidation in an alkaline medium. Therefore, the EOR performanee of the novel carbon supported ternary PdxNiyBiz anode catalyst with various atomic variations (Pd70Ni25Bi5/C, Pd70Ni20Bi10/C, Pd80Ni10Bi10/C and Pd40Ni20Bi40/C) using the common instant reduction synthesis method was further optimized for the alkaline direct ethanol fuel cell. The carbon supported Pd:Ni:Bi nano catalyst with atomic ratio of 70:20:10 displays outsta nding catalytic activity for the alkaline EOR compared to the other PdxNiyBiy/C nanocatalysts as well as to the benchmarks Pd/C, Pd60Ni40/C and Pd60Bi40/C. The synergy and the optimal content in consideration of the oxide species of Pd, Ni and Bi are crucial for the EOR kinetic enhancement in alkaline medium.展开更多
We show the power of spirally polarized doughnut beams as a tool for tuning the field distribution in the focus of a high numerical aperture (NA) lens. Different and relevant states of polarization as well as field ...We show the power of spirally polarized doughnut beams as a tool for tuning the field distribution in the focus of a high numerical aperture (NA) lens. Different and relevant states of polarization as well as field distributions can be created by the simple turning of a λ/2 retardation wave plate placed in the excitation path of a micro- scope. The realization of such a versatile excitation source can provide an essential tool for nanotechnology investigations and biomedical experiments.展开更多
Understanding the fundamental charge carrier dynamics is of great significance for photodetectors with both high speed and high responsivity.Devices based on two-dimensional(2D)transition metal dichalcogenides can exh...Understanding the fundamental charge carrier dynamics is of great significance for photodetectors with both high speed and high responsivity.Devices based on two-dimensional(2D)transition metal dichalcogenides can exhibit picosecond photoresponse speed.However,2D materials naturally have low absorption,and when increasing thickness to gain higher responsivity,the response time usually slows to nanoseconds,limiting their photodetection performance.Here,by taking time-resolved photocurrent measurements,we demonstrated that graphene/MoTe_(2) van der Waals heterojunctions realize a fast 10 ps photoresponse time owing to the reduced average photocurrent drift time in the heterojunction,which is fundamentally distinct from traditional Dirac semimetal photodetectors such as graphene or Cd_(3)As_(2) and implies a photodetection bandwidth as wide as 100 GHz.Furthermore,we found that an additional charge carrier transport channel provided by graphene can ef-fectively decrease the photocurrent recombination loss to the entire device,preserving a high responsivity in the near-infrared region.Our study provides a deeper understanding of the ultrafast electrical response in van der Waals heterojunctions and offers a promising approach for the realization of photodetectors with both high responsivity and ultrafast electrical response.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant Nos.91850116,51772084,52022029,and U19A2090)Hunan Provincial Natural Science Foundation of China(Grant Nos.2018RS3051 and 2018WK4004)the Key Program of the Hunan Provincial Science and Technology Department,China(Grant No.2019XK2001).
文摘Due to the large exciton binding energy,two-dimensional(2D)transition metal dichalcogenides(TMDCs)provide an ideal platform for studying excitonic states and related photonics and optoelectronics.Polarization states lead to distinct light-matter interactions which are of great importance for device applications.In this work,we study polarized photoluminescence spectra from intralayer exciton and indirect exciton in WS_(2) and WSe_(2) atomic layers,and interlayer exciton in WS_(2)/WSe_(2) heterostructures by radially and azimuthally polarized cylindrical vector laser beams.We demonstrated the same in-plane and out-of-plane polarization behavior from the intralayer and indirect exciton.Moreover,with these two laser modes,we obtained interlayer exciton in WS_(2)/WSe_(2) heterostructures with stronger out-of-plane polarization,due to the formation of vertical electric dipole moment.
基金Lei GanPaolo Falcaro acknowledge European Union’s Horizon 2020 Program(FP/2104‐2020)/ERC Grant Agreement(771834 POPCRYSTAL).
文摘Bioinspired micro‐/nano‐motors are artificial micro‐/nano‐machines that can convert various forms of energy to propel their movement[1].For example,the motion of some of these micromachines can be precisely controlled by application of external physical stimuli including magnetic,electric and acoustic fields[2,3].Inspired by the study of microorganisms,researchers have been exploring also the use of available chemical energy from the local environment to trigger and sustain self‐propulsion[4].Within this research direction,Metal‐organic frameworks(MOFs)—a class of extended materials synthesized via a modular approach from inorganic(metal clusters or ions)and organic linkers[5]—offer excellent opportunities for the design and synthesis of self‐propelled micromotors.MOFs typically possess ultra‐high surface areas that allow facile access to densely populated catalytically active sites imbedded within their pore networks.Through careful design these catalytic sites can be exploited to convert chemical energy into kinetic energy resulting in self‐propulsion of the MOF crystal[6].In addition,rigidity,density,crystalline pore organization and pore size of MOFs can be optimized to carry out a swimming‐type motion[7].
文摘Fluorescent studies of living plant cells such as confocal microscopy and fluorescence lifetime imaging often suffer from a strong autofluorescent background contribution that significantly reduces the dynamic image contrast and the quantitative access to sub-cellular processes at high spatial resolution. Here, we present a novel technique--fluorescence intensity decay shape analysis microscopy (FIDSAM) to enhance the dynamic contrast of a fluorescence image of at least one order of magnitude. The method is based on the analysis of the shape of the fluorescence intensity decay (fluorescence lifetime curve) and benefits from the fact that the decay patterns of typical fluorescence label dyes strongly differ from emission decay curves of autofluorescent sample areas. Using FIDSAM, we investigated Arabidopsis thaliana hypocotyl cells in their tissue environment, which accumulate an eGFP fusion of the plasma membrane marker protein LTI6b (LTI6b-eGFP) to low level. Whereas in conventional confocal fluorescence images, the membranes of neighboring cells can hardly be optically resolved due to the strong autofluorescence of the cell wall, FIDSAM allows for imaging of single, isolated membranes at high spatial resolution. Thus, FIDSAM will enable the sub-cellular analysis of even low-expressed fluorophoretagged proteins in living plant cells. Furthermore, the combination of FIDSAM with fluorescence lifetime imaging provides the basis to study the local physico-chemical environment of fluorophore-tagged biomolecules in living plant cells.
文摘Water/ethyl acetate/ethanol is widely used as a“green”extractant system.We show that 2 different types of phase separation can be induced upon centrifugation in this ternary system using ethanol as a cosolvent of water and ethyl acetate:centrifuge-induced criticality and centrifuge-induced emulsification.The expected composition profiles of samples after centrifugation can be represented by bent lines in a ternary phase diagram when gravitational energy is added to the free energy of mixing.The experimental equilibrium composition profiles behave qualitatively as expected and can be predicted using a phenomenological theory of mixing.The concentration gradients are small except near the critical point,as expected for small molecules.Nevertheless,they are usable when accompanied by temperature cycles.These findings open new possibilities of centrifugal separation,even if control is delicate during temperature cycles.These schemes are accessible even at relatively low centrifugation speed for molecules that float and sediment with apparent molar masses several hundred times larger than the molecular mass.
文摘Over the past nearly 90 years,scientists have gradually revealed the mysteries of intramolecular motion and chemical bonds through continuous research and innovation.In 1928,Irish scientist E.Synge proposed a method to overcome the limit of classical optical resolution,which laid the foundation for the subsequent development of scanning near-field optical microscopes.Near-field optical microscopy continues to push optical resolution to new records,well beyond the diffraction limit of traditional optical microscopy.
基金Acknowledgements All authors are grateful to the National Natural Science Foundation of China (Nos. 51525202, 61574054, 61505051 and 61474040), the Hunan province science and technology plan (Nos. 2014FJ2001 and 2014TT1004), the Aid program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, and the Fundamental Research Funds for the Central Universities.
文摘High-performance multiphoton-pumped lasers based on cesium lead halide perovskite nanostructures are promising for nonlinear optics and practical frequency upconversion devices in integrated photonics. However, the performance of such lasers is highly dependent on the quality of the material and cavity, which makes their fabrication challenging. Herein, we demonstrate that cesium lead halide perovskite triangular nanorods fabricated via vapor methods can serve as gain media and effective cavities for multiphoton-pumped lasers. We observed blue-shifts of the lasing modes in the excitation fluence-dependent lasing spectra at increased excitation powers, which fits well with the dynamics of Burstein-Moss shifts caused by the band filling effect. Moreover, efficient multiphoton lasing in CsPbBr3 nanorods can be realized in a wide excitation wavelength range (700-1,400 nm). The dynamics of multiphoton lasing were investigated by time-resolved photoluminescence spectroscopy, which indicated that an electron-hole plasma is responsible for the multiphoton-pumped lasing. This work could lead to new opportunities and applications for cesium lead halide perovskite nanostructures in frequency upconversion lasing devices and optical interconnect systems.
文摘We investigate nanorod assemblies of two 64-substituted pentacenes, namely (2,3-X2-9,10-Y2)-substituted pentacenes with X -- Y = OCH3 (MOP) and with X = F, Y-- OCH3 (MOPF), grown on Au(111) single crystals. By using a multi-technique approach based on ultraviolet photoelectron spectroscopy X-ray photoelectron spectroscopy; and X-ray absorption, we find evidence for charge transfer screening at the interface with gold. Furthermore, the MOP and MOPF nanorods show a rough surface morphology, which was investigated with atomic force microscopy. We use molecular simulation techniques to investigate the energetic barriers to diffusion and to traverse step-edges to estimate their influence on the nanorod roughness. We find that barriers to surface diffusion on a terrace are anisotropic and that their direction favors the formation of nanorods in these materials.
文摘Metal-free organic radicals are fascinating materials owing to their unique properties. Having a stable magnetic moment coupled to light elements makes these materials central to develop a large variety of applications. We investigated the magnetic spinterface coupling between the surface of a single rutile TiO2(110) crystal and a pyrene-based nitronyl nitroxide radical, using a combination of thickness-dependent X-ray photoelectron spectroscopy and ab initio calculations. The radicals were physisorbed, and their magnetic character was preserved on the (almost) ideal surface. The situation changed completely when the molecules interacted with a surface defect site upon adsorption. In this case, the reactivity of the defect site led to the quenching of the molecular magnetic moment. Our work elucidates the crucial role played by the surface defects and demonstrates that photoemission spectroscopy combined with density functional theory calculations can be used to shed light on the mechanisms governing complex interfaces, such as those between magnetic molecules and metal oxides.
基金Project supported by Polish National Science Centre (N N507 584938,N N507599038)the Foundation for Polish Science
文摘The possibility of ttming between red, green and broadband white color of up-conversion was demonstrated in thermally 450 ℃ treated Yb/Er co-doped nano NaYF4 phosphor. The color variability was studied by means of power dependence of luminescence, which exhibited unusual behavior. Large hysteresis, as well as discrepancy from a power law indicated the important role the increased heating played during the up-conversion in nano-sized materials.
文摘The study focuses on the in flue nee of Ni and Bi on alkali me etha nol oxidati on reacti on (EOR) activities, stabilities and structure characteristics of carb on supported Pd-based nano catalysts (Pd/C, Pd6oNi4o/C, Pd6oBi4o/C, Pd6oNi2oBi2o/C) by cyclic voltammetry/chr ono amperometry using rotating disk electrode and various physico-chemical methods such as X-ray powder diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy coupled with energy dispersive X-ray spectroscopy and inductively coupled plasma optical emission spectrometry. Nickel generates more adsorbed OH on the Pd catalyst surface than Bi and promotes the oxidation of adsorbed ethanol species. This results in a low onset potential toward ethanol oxidation with high current density. The presenee of Bi facilitates high toleranee toward various reaction in termediates resulting from the incomplete etha nol oxidation, but might also initiate the agglomerati on of Pd nano particles. The no vel Pd60Ni20Bi20/C nanocatalyst displays exceptional byproduct toleranee, but only satisfying catalytic activity toward ethanol oxidation in an alkaline medium. Therefore, the EOR performanee of the novel carbon supported ternary PdxNiyBiz anode catalyst with various atomic variations (Pd70Ni25Bi5/C, Pd70Ni20Bi10/C, Pd80Ni10Bi10/C and Pd40Ni20Bi40/C) using the common instant reduction synthesis method was further optimized for the alkaline direct ethanol fuel cell. The carbon supported Pd:Ni:Bi nano catalyst with atomic ratio of 70:20:10 displays outsta nding catalytic activity for the alkaline EOR compared to the other PdxNiyBiy/C nanocatalysts as well as to the benchmarks Pd/C, Pd60Ni40/C and Pd60Bi40/C. The synergy and the optimal content in consideration of the oxide species of Pd, Ni and Bi are crucial for the EOR kinetic enhancement in alkaline medium.
文摘We show the power of spirally polarized doughnut beams as a tool for tuning the field distribution in the focus of a high numerical aperture (NA) lens. Different and relevant states of polarization as well as field distributions can be created by the simple turning of a λ/2 retardation wave plate placed in the excitation path of a micro- scope. The realization of such a versatile excitation source can provide an essential tool for nanotechnology investigations and biomedical experiments.
基金This work was supported by the National Natural Science Foundation of China(Grants No.52022029,91850116,51772084,and U19A2090)the Sino-German Center for Research Promotion(Grant No.GZ1390)the Hunan Provincial Natural Science Foundation of China(Grants No.2018RS3051 and 2019XK2001)。
文摘Understanding the fundamental charge carrier dynamics is of great significance for photodetectors with both high speed and high responsivity.Devices based on two-dimensional(2D)transition metal dichalcogenides can exhibit picosecond photoresponse speed.However,2D materials naturally have low absorption,and when increasing thickness to gain higher responsivity,the response time usually slows to nanoseconds,limiting their photodetection performance.Here,by taking time-resolved photocurrent measurements,we demonstrated that graphene/MoTe_(2) van der Waals heterojunctions realize a fast 10 ps photoresponse time owing to the reduced average photocurrent drift time in the heterojunction,which is fundamentally distinct from traditional Dirac semimetal photodetectors such as graphene or Cd_(3)As_(2) and implies a photodetection bandwidth as wide as 100 GHz.Furthermore,we found that an additional charge carrier transport channel provided by graphene can ef-fectively decrease the photocurrent recombination loss to the entire device,preserving a high responsivity in the near-infrared region.Our study provides a deeper understanding of the ultrafast electrical response in van der Waals heterojunctions and offers a promising approach for the realization of photodetectors with both high responsivity and ultrafast electrical response.