| 1. |
- Feng, Q., et al.
(författare)
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Novel k2 ti8 o17 anode via na+ /al3+ co-intercalation mechanism for rechargeable aqueous al-ion battery with superior rate capability
- 2021
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Ingår i: Nanomaterials. - : MDPI AG. - 2079-4991. ; 11:9
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Tidskriftsartikel (refereegranskat)abstract
- A promising aqueous aluminum ion battery (AIB) was assembled using a novel layered K2 Ti8 O17 anode against an activated carbon coated on a Ti mesh cathode in an AlCl3-based aqueous electrolyte. The intercalation/deintercalation mechanism endowed the layered K2 Ti8 O17 as a promising anode for rechargeable aqueous AIBs. NaAc was introduced into the AlCl3 aqueous electrolyte to enhance the cycling stability of the assembled aqueous AIB. The as-designed AIB displayed a high discharge voltage near 1.6 V, and a discharge capacity of up to 189.6 mAh g−1 . The assembled AIB lit up a commercial light-emitting diode (LED) lasting more than one hour. Inductively coupled plasma–optical emission spectroscopy (ICP-OES), high-resolution transmission electron microscopy (HRTEM), and X-ray absorption near-edge spectroscopy (XANES) were employed to investigate the intercalation/deintercalation mechanism of Na+ /Al3+ ions in the aqueous AIB. The results indi-cated that the layered structure facilitated the intercalation/deintercalation of Na+ /Al3+ ions, thus providing a high-rate performance of the K2 Ti8 O17 anode. The diffusion-controlled electrochemical characteristics and the reduction of Ti4+ species during the discharge process illustrated the intercala-tion/deintercalation mechanism of the K2 Ti8 O17 anode. This study provides not only insight into the charge–discharge mechanism of the K2 Ti8 O17 anode but also a novel strategy to design rechargeable aqueous AIBs.
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| 2. |
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| 3. |
- Ji, Shaozheng, et al.
(författare)
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Influence of cathode geometry on electron dynamics in an ultrafast electron microscope
- 2017
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Ingår i: Structural Dynamics. - : American Crystallographic Association. - 2329-7778. ; 4:5
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Tidskriftsartikel (refereegranskat)abstract
- Efforts to understand matter at ever-increasing spatial and temporal resolutions have led to the development of instruments such as the ultrafast transmission electron microscope (UEM) that can capture transient processes with combined nanometer and picosecond resolutions. However, analysis by UEM is often associated with extended acquisition times, mainly due to the limitations of the electron gun. Improvements are hampered by tradeoffs in realizing combinations of the conflicting objectives for source size, emittance, and energy and temporal dispersion. Fundamentally, the performance of the gun is a function of the cathode material, the gun and cathode geometry, and the local fields. Especially shank emission from a truncated tip cathode results in severe broadening effects and therefore such electrons must be filtered by applying a Wehnelt bias. Here we study the influence of the cathode geometry and the Wehnelt bias on the performance of a photoelectron gun in a thermionic configuration. We combine experimental analysis with finite element simulations tracing the paths of individual photoelectrons in the relevant 3D geometry. Specifically, we compare the performance of guard ring cathodes with no shank emission to conventional truncated tip geometries. We find that a guard ring cathode allows operation at minimum Wehnelt bias and improve the temporal resolution under realistic operation conditions in an UEM. At low bias, the Wehnelt exhibits stronger focus for guard ring than truncated tip cathodes. The increase in temporal spread with bias is mainly a result from a decrease in the accelerating field near the cathode surface. Furthermore, simulations reveal that the temporal dispersion is also influenced by the intrinsic angular distribution in the photoemission process and the initial energy spread. However, a smaller emission spot on the cathode is not a dominant driver for enhancing time resolution. Space charge induced temporal broadening shows a close to linear relation with the number of electrons up to at least 10 000 electrons per pulse. The Wehnelt bias will affect the energy distribution by changing the Rayleigh length, and thus the interaction time, at the crossover.
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| 4. |
- Ji, Shaozheng, et al.
(författare)
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Influence of strain on an ultrafast phase transition
- 2022
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 15:1, s. 304-312
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Tidskriftsartikel (refereegranskat)abstract
- The flexibility of 2D materials combined with properties highly sensitive to strain makes strain engineering a promising avenue for manipulation of both structure and function. Here we investigate the influence of strain, associated with microstructural defects, on a photo-induced structural phase transition in Td-WTe2. Above threshold photoexcitation of uniform, non-strained, samples result in an orthorhombic Td to a metastable orthorhombic 1T* phase transition facilitated by shear displacements of the WTe2 layers along the b axis of the material. In samples prepared with wrinkle defects WTe2 continue its trajectory through a secondary transition that shears the unit cell along the c axis towards a metastable monoclinic 1T ' phase. The time scales and microstructural evolution associated with the transition and its subsequent recovery to the 1T* phase is followed in detail by a combination of ultrafast electron diffraction and microscopy. Our findings show how local strain fields can be employed for tailoring phase change dynamics in ultrafast optically driven processes with potential applications in phase change devices.
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| 5. |
- Ji, Shaozheng, et al.
(författare)
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Manipulation of Stacking Order in Td-WTe2 by Ultrafast Optical Excitation
- 2021
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 15:5, s. 8826-8835
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Tidskriftsartikel (refereegranskat)abstract
- Subtle changes in stacking order of layered transition metal dichalcogenides may have profound influence on the electronic and optical properties. The intriguing electronic properties of Td-WTe2 can be traced to the break of inversion symmetry resulting from the ground-state stacking sequence. Strategies for perturbation of the stacking order are actively pursued for intentional tuning of material properties, where optical excitation is of specific interest since it holds the potential for integration of ultrafast switches in future device designs. Here we investigate the structural response in Td-WTe2 following ultrafast photoexcitation by time-resolved electron diffraction. A 0.23 THz shear phonon, involving layer displacement along the b axis, was excited by a 515 nm laser pulse. Pump fluences in excess of a threshold of similar to 1 mJ/cm(2) result in formation, with an similar to 5 ps time constant, of a new stacking order by layer displacement along the b axis in the direction toward the centrosymmetric 1T* phase. The shear displacement of the layers increases with pump fluence until saturation at similar to 8 pm. We demonstrate that the excitation of the shear phonon and the stabilization of the metastable phase are decoupled when using an optical pump as evidenced by observation of a transition also in samples with a pinned shear phonon. The results are compared to dynamic first-principles simulations and the transition is interpreted in terms of a mechanism where transient local disorder is prominent before settling at the atomic positions of the metastable phase. This interpretation is corroborated by results from diffuse scattering. The correlation between excitation of intralayer vibrations and interlayer interaction demonstrates the importance of including both short- and long-range interactions in an accurate description of how optical fields can be employed to manipulate the stacking order in 2-dimensional transition metal dichalcogenides.
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| 6. |
- Ji, Shaozheng
(författare)
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Photo-induced Structural Dynamics in Transition Metal Dichalcogenides
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ultrafast electron microscope (UEM), a combination of transmission electron microscopy and laser-based pump-probe techniques, facilitates ultrafast imaging, diffraction, and electron-spectroscopy with high spatial resolution. The unique advantages of UEM enable local ultrafast dynamic studies in materials, nano-system, and biology. The performance of UEM, such as its temporal and energy resolutions and coherence, is largely determined by the quality of electron beam. In this thesis, the beam dynamics in our UEM with a thermionic gun was studied. The influence of cathode geometry and Wehnelt bias voltage on the electron pulse dynamics is determined through experiments and finite element simulations. A guard ring cathode can effectively address the problem of shank-emitted electrons in traditional truncated tip geometries, allowing UEM operation at minimum Wehnelt bias and improving the temporal resolution under realistic conditions. A sub-ps temporal resolution can be reached with few electrons in one pulse. Compared to the 300 fs laser pulse width, the temporal duration of the electron pulse is nevertheless elongated during the propagation in the UEM column. The simulations show that the initial energy spread and the angular distribution from the photoemission process are the dominant factors in this temporal dispersion.Utilizing our UEM, the structural dynamics including photo-induced phase transitions and coherent phonon excitation were studied in two transition metal dichalcogenides (TMDs), 1T-TaSe2 and Td-WTe2. 1T-TaSe2 is a room temperature commensurate charge density wave (C-CDW) material. The C-CDW phase undergoes a phase transition to an incommensurate charge density wave (IC-CDW) at 473 K featured by a rotation of the superstructure. Under photoexcitation, the C-CDW in 1T-TaSe2 can be suppressed within sub-ps time scale. A recovery time-constant of ~0.7 ps is observed for the commensurate periodic lattice distortion (PLD) at a pump fluence insufficient to drive a phase transition into the IC-CDW phase. At higher pump fluence, sufficient to drive nucleation of the IC-CDW phase, there is a ~1 ps delay between the extinction of the C-CDW phase and the onset for formation of the IC-CDW phase. Within the ~1 ps, a transient unreconstructed state may exist. The ~1 ps delay time for the nucleation of the IC-CDW phase implies that a phononic thermalization is involved in the decay of this highly perturbed photoinduced transient state. During the nucleation of the IC-CDW phase, a face-centered cubic (FCC) like stacking order is observed already at ~4 ps after photoexcitation. Such rapid stacking order formation indicates that the nucleation of the IC-CDW phase in the adjacent layers is not independent but coupled together. We can infer that the nucleation of the IC-CDW is inherently 3-dimensional (3D). The highly 3D feature of CDW in 1T-TaSe2 indicates a strong interlayer interaction that establish long range out-of-plane stacking order.Both in 1T-TaSe2 and Td-WTe2, a coherent shear phonon is observed by photoexcitation. In 1T-TaSe2, the coherent shear mode is along the stacking direction of the C-CDW phase. We analyze the launching mechanism in terms of hot/cold spots on the Se-sublattice that result from the rapid melting of the PLD. During the melting, a difference in Se-phonon amplitudes results in shear forces between the layers. For a perfect trigonal stacking, the force will be compensated. However, there always remain uncompensated restoring forces in stacking-order direction because of the domain structure in out-of-plane direction. The excitation of a coherent shear phonon is even stronger in Td-WTe2. The shear direction is along the b axis where there is a stacking displacement for the adjacent layers.In Td-WTe2, a photo-induced phase transition from orthorhombic Td to orthorhombic T* phase is observed which involves a stacking order change in the out-of-plane direction by a layer shear displacement along the b axis direction. Upon photoexcitation with pump fluence higher than a critical value, the change in interlayer potential results in the formation of a new metastable phase with a ~4 ps time constant. The shear displacement of the adjacent layers increases linearly with the increase of pump fluence and stabilize at ~ 8 pm when the pump fluence is higher than ~2 mJ/cm2. The photo-induced phase transition in Td-WTe2 can be influenced by local defect structures. In a ripple defect rich sample, a new phase transition from orthorhombic T* to monoclinic T’ phase will occur following the Td to T* phase transition. It can be inferred that strain fields in the sample can modulate the photo-induced phase stability. This effect has potential application in strain engineering of 2 dimensional TMDs.The observed photo-induced phase transition and coherent shear phonon in 1T-TaSe2 and Td-WTe2, demonstrate the importance of inter-layer interaction in TMDs.
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| 7. |
- Ji, Shaozheng, et al.
(författare)
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Structural Dynamics in Td-WTe2 Induced by Ultrafast Impulsive Excitation
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The stacking order in layered transition metal dichalcogenides exhibit stark influence on the electronic and optical properties. Perturbation of the stacking order is actively pursued in strategies for intentional tuning of material properties, where optical excitation is of specific interest since it holds the potential for integration of ultrafast switching in future device designs. A consequence of the ground state Td-WTe2 stacking sequence is a break of inversion symmetry, with intriguing electronic properties. Here we investigate the structural dynamics in Td-WTe2 following ultrafast photoexcitation by time resolved electron diffraction. A shear phonon, involving layer displacement along the b axis, was excited by a 515 nm laser pulse. Pump fluences in excess of a threshold of ~ 1 mJ/cm2 results in formation, ~4 ps time constant, of a novel metastable phase where the layers are displaced along the b axis in the direction towards the centro-symmetric 1T* phase. The shear displacement of the layers increases with pump fluence until saturation at ~8 pm. The results are compared to dynamic first-principles simulations and the transition is interpreted in terms of a mechanism where the material transiently explore a large phase space before settling at the atomic positions of the metastable phase. This interpretation is corroborated by results from diffuse scattering. The correlation between excitation of intralayer vibrations and interlayer interaction demonstrates the importance of including both short- and long-range interactions in an accurate description of ultrafast phase transitions in 2-dimensional transition metal dichalcogenides.
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| 8. |
- Ji, Shaozheng, et al.
(författare)
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Transient three-dimensional structural dynamics in 1T -TaSe2
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:9
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Tidskriftsartikel (refereegranskat)abstract
- We report on thermal and optically driven transitions between the commensurate (C) and incommensurate (IC) charge-density wave (CDW) phases of 1T−TaSe2. Optical excitation results in suppression of the C-CDW on a subpicosecond timescale. The optically driven C to IC transition involves a short-lived (∼1 ps) unreconstructed phase. Nucleation of an IC phase stacking order is observed already at ∼4 ps following photoexcitation. The short timescales involved in establishing the stacking order implies that the nucleation of the IC phase is influenced by the local geometry of the adjacent layers such that the stacking direction of the C phase determines the stacking direction of the IC phase. From this follows that the nucleation of the IC-CDW is inherently three dimensional (3D). We observe the activation of a coherent shear mode in the optically driven transitions to the transiently stabilized unreconstructed phase. The activation mechanism starts with a rapid lifting of the periodic lattice distortions (PLD) of the Ta sublattice which results in formation of local transient velocity disparities in the Se sublattice. The local differences in Se-phonon amplitudes result in noncompensated shear forces between the layers. This is an example of a multistep coherent launching mechanism. The energy of the optically excited electronic state dissipates energy into modes of the PLD through strong electron-phonon coupling. The rapid suppression of the PLD launches the third step, a coherent vibrational shear mode with low dissipation. The results highlight the importance in considering the 3D nature of the CDWs in the analysis of both structure and dynamics in transition-metal dichalcogenides.
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| 9. |
- Kim, Ye-Jin, et al.
(författare)
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Femtosecond-resolved imaging of a single-particle phase transition in energy-filtered ultrafast electron microscopy
- 2023
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 9:4
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Tidskriftsartikel (refereegranskat)abstract
- Using an energy filter in transmission electron microscopy has enabled elemental mapping at the atomic scale and improved the precision of structural determination by gating inelastic and elastic imaging electrons, respec-tively. Here, we use an energy filter in ultrafast electron microscopy to enhance the temporal resolution toward the domain of atomic motion. Visualizing transient structures with femtosecond temporal precision was achieved by selecting imaging electrons in a narrow energy distribution from dense chirped photoelectron packets with broad longitudinal momentum distributions and thus typically exhibiting picosecond durations. In this study, the heterogeneous ultrafast phase transitions of vanadium dioxide (VO2) nanoparticles, a repre-sentative strongly correlated system, were filmed and attributed to the emergence of a transient, low-symmetry metallic phase caused by different local strains. Our approach enables electron microscopy to access the time scale of elementary nuclear motion to visualize the onset of the structural dynamics of matter at the nanoscale.
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| 10. |
- Prasad, Amit Kumar, et al.
(författare)
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Nonequilibrium Phonon Dynamics and Its Impact on the Thermal Conductivity of the Benchmark Thermoelectric Material SnSe
- 2023
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 17:21, s. 21006-21017
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Tidskriftsartikel (refereegranskat)abstract
- Thermoelectric materials play a vital role in the pursuit of a sustainable energy system by allowing the conversion of waste heat to electric energy. Low thermal conductivity is essential to achieving high-efficiency conversion. The conductivity depends on an interplay between the phononic and electronic properties of the nonequilibrium state. Therefore, obtaining a comprehensive understanding of nonequilibrium dynamics of the electronic and phononic subsystems as well as their interactions is key for unlocking the microscopic mechanisms that ultimately govern thermal conductivity. A benchmark material that exhibits ultralow thermal conductivity is SnSe. We study the nonequilibrium phonon dynamics induced by an excited electron population using a framework combining ultrafast electron diffuse scattering and nonequilibrium kinetic theory. This in-depth approach provides a fundamental understanding of energy transfer in the spatiotemporal domain. Our analysis explains the dynamics leading to the observed low thermal conductivity, which we attribute to a mode-dependent tendency to nonconservative phonon scattering. The results offer a penetrating perspective on energy transport in condensed matter with far-reaching implications for rational design of advanced materials with tailored thermal properties.
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