Ordered water structure at hydrophobic graphite interfaces observed by 4D, ultrafast electron crystallography

Interfacial water has unique properties in various functions. Here, using 4-dimensional (4D), ultrafast electron crystallography with atomic-scale spatial and temporal resolution, we report study of structure and dynamics of interfacial water assembly on a hydrophobic surface. Structurally, vertically stacked bilayers on highly oriented pyrolytic graphite surface were determined to be ordered, contrary to the expectation that the strong hydrogen bonding of water on hydrophobic surfaces would dominate with suppressed interfacial order. Because of its terrace morphology, graphite plays the role of a template. The dynamics is also surprising. After the excitation of graphite by an ultrafast infrared pulse, the interfacial ice structure undergoes nonequilibrium “phase transformation” identified in the hydrogen-bond network through the observation of structural isosbestic point. We provide the time scales involved, the nature of ice-graphite structural dynamics, and relevance to properties related to confined water

4D electron diffraction reveals correlated unidirectional Behavior in zinc oxide nanowires

The confined electronic structure of nanoscale materials has increasingly been shown to induce behavior quite distinct from that of bulk analogs. Direct atomic- scale visualization of nanowires of zinc oxide was achieved through their unique pancake- type diffraction by using four- dimensional (4D) ultrafast electron crystallography. After electronic excitation of this wide- gap photonic material, the wires were found to exhibit colossal expansions, two orders of magnitude higher than that expected at thermal equilibrium; the expansion is highly anisotropic, a quasi- one- dimensional behavior, and is facilitated by the induced antibonding character. By reducing the density of nanowires, the expansions reach even larger values and occur at shorter times, suggesting a decrease of the structural constraint in transient atomic motions. This unanticipated ultrafast carrier- driven expansion highlights the optoelectronic consequences of nanoscale morphologies

Scanning ultrafast electron microscopy

Progress has been made in the development of four-dimensional ultrafast electron microscopy, which enables space-time imaging of structural dynamics in the condensed phase. In ultrafast electron microscopy, the electrons are accelerated, typically to 200 keV, and the microscope operates in the transmission mode. Here, we report the development of scanning ultrafast electron microscopy using a field-emission-source configuration. Scanning of pulses is made in the single-electron mode, for which the pulse contains at most one or a few electrons, thus achieving imaging without the space-charge effect between electrons, and still in ten(s) of seconds. For imaging, the secondary electrons from surface structures are detected, as demonstrated here for material surfaces and biological specimens. By recording backscattered electrons, diffraction patterns from single crystals were also obtained. Scanning pulsed-electron microscopy with the acquired spatiotemporal resolutions, and its efficient heat-dissipation feature, is now poised to provide in situ 4D imaging and with environmental capability

Direct role of structural dynamics in electron-lattice coupling of superconducting cuprates

The mechanism of electron pairing in high-temperature superconductors is still the subject of intense debate. Here, we provide direct evidence of the role of structural dynamics, with selective atomic motions (buckling of copper–oxygen planes), in the anisotropic electron-lattice coupling. The transient structures were determined using time-resolved electron diffraction, following carrier excitation with polarized femtosecond heating pulses, and examined for different dopings and temperatures. The deformation amplitude reaches 0.5% of the c axis value of 30 Å when the light polarization is in the direction of the copper–oxygen bond, but its decay slows down at 45°. These findings suggest a selective dynamical lattice involvement with the anisotropic electron–phonon coupling being on a time scale (1–3.5 ps depending on direction) of the same order of magnitude as that of the spin exchange of electron pairing in the high-temperature superconducting phase

Cross-Examination of Photoinitiated Carrier and Structural Dynamics of Black Phosphorus at Elevated Fluences

Revived attention in black phosphorus (bP) has been tremendous in the past decade. While many photoinitiated experiments have been conducted, a cross-examination of bP's photocarrier and structural dynamics is still lacking. In this report, we provide such analysis by examining time-resolved data acquired using optical transient reflectivity and reflection ultrafast electron diffraction, two complementary methods under the same experimental conditions. At elevated excitation fluences, we find that more than 90% of the photoinjected carriers are annihilated within the first picosecond (ps) and transfer their energy to phonons in a nonthermal, anisotropic fashion. Electronically, the remaining carrier density around the band edges induces a significant interaction that leads to an interlayer lattice contraction in a few ps but soon diminishes as a result of the continuing loss of carriers. Structurally, phonon-phonon scattering redistributes the energy in the lattice and results in the generation of out-of-plane coherent acoustic phonons and thermal lattice expansion. Their onset times at ~6 ps are found to be in good agreement. Later, a thermalized quasi-equilibrium state is reached following a period of about 40-50 ps. Hence, we propose a picture with five temporal regimes for bP's photodynamics.Comment: 16 pages, 4 figures for the main tex

4D Scanning Ultrafast Electron Microscopy: Visualization of Materials Surface Dynamics

The continuous electron beam of conventional scanning electron microscopes (SEM) limits the temporal resolution required for the study of ultrafast dynamics of materials surfaces. Here, we report the development of scanning ultrafast electron microscopy (S-UEM) as a time-resolved method with resolutions in both space and time. The approach is demonstrated in the investigation of the dynamics of semiconducting and metallic materials visualized using secondary-electron images and backscattering electron diffraction patterns. For probing, the electron packet was photogenerated from the sharp field-emitter tip of the microscope with a very low number of electrons in order to suppress space–charge repulsion between electrons and reach the ultrashort temporal resolution, an improvement of orders of magnitude when compared to the traditional beam-blanking method. Moreover, the spatial resolution of SEM is maintained, thus enabling spatiotemporal visualization of surface dynamics following the initiation of change by femtosecond heating or excitation. We discuss capabilities and potential applications of S-UEM in materials and biological science

Strong Coupling of Self-Trapped Excitons to Acoustic Phonons in Bismuth Perovskite Cs3Bi2I9\textrm{Cs}_{3}\textrm{Bi}_{2}\textrm{I}_{9}

To assess the potential optoelectronic applications of metal-halide perovskites, it is critical to have a detailed understanding of the nature, strength, and dynamics of the interactions between carriers and the polar lattices. Here, we report the electronic and structural dynamics of bismuth-based perovskite $\textrm{Cs}_{3}\textrm{Bi}_{2}\textrm{I}_{9}$ revealed by transient reflectivity and ultrafast electron diffraction. A cross-examination of these experimental results combined with theoretical analyses allows the identification of the major carrier-phonon coupling mechanism and the associated time scales. It is found that carriers photoinjected into $\textrm{Cs}_{3}\textrm{Bi}_{2}\textrm{I}_{9}$ form self-trapped excitons on an ultrafast time scale. However, they retain most of their energy and their coupling to Fr\"ohlich-type optical phonons is limited at early times. Instead, the long-lived excitons exert an electronic stress via deformation potential and develop a prominent, sustaining strain field as coherent acoustic phonons in 10 ps. From sub-ps to ns and beyond, a similar extent of the atomic displacements is found throughout the different stages of structural distortions, from limited local modulations to a coherent strain field to the Debye-Waller random atomic motions on longer times. The current results suggest the potential use of bismuth-based perovskites for applications other than photovoltaics to take advantage of carriers' stronger self-trapping and long lifetime.Comment: 21 pages, 4 figures for the main tex

Ultrafast Electron Crystallography: Principles and Applications

During the last 20 to 30 years, the development and application of time-resolved experimental techniques with a femtosecond temporal resolution have brought to us much knowledge about the fundamental processes in physics, chemistry and biology. Nevertheless, standard spectroscopic methods have their limitation in the determination of the transient structures during ultrafast dynamics at the atomic level, because the spatial resolution is restricted by the wavelength of the probe pulse used. In contract, with the scheme of femtosecond optical initiation and electron probing and through the diffraction phenomenon, ultrafast electron crystallography (UEC) was recently developed as a time-resolved structure-probing technique for condensed-matter studies. The short wavelength and small pulse duration of the highly accelerated electrons used provide the atomic-scale spatiotemporal resolution. In addition, the large electron–matter interaction enables the detection of small transient changes as well as the investigation of surface and interfacial phenomena. This thesis describes the principles of UEC and its applications to a variety of systems, ranging from nanometer-scale structures to highly correlated materials and to interfacial assemblies. By using a prototype semiconducting material, we elucidated the fundamental processes at work in different parts of the femtosecond-to-nanosecond time range; this investigation led to a conceptual change from the consideration of laser-induced heating to the examination of nonequilibrium structural modifications as a result of the transient dynamical changes in, e.g., carriers, the crystal potential, and phonons. On the basis of such an understanding, we observed and understood the colossal unidirectional expansion induced by the photoexcitation of nanostructures to be a potential-driven result rather than a thermal one. For highly correlated materials, we showed the effectiveness of UEC in resolving the transient intermediate structures during phase transformations as well as identifying new phases in the nonequilibrium state. An important breakthrough made by UEC was the confirmation of the anisotropic involvement of lattice in the electron pairing mechanism for high-temperature superconductors. In interfacial assemblies, we also found a nonequilibrium phase transformation in water and the phenomenon of ultrafast annealing for a better order in a self-assembled monolayer. With these successful experiences, we expect more condensed-matter studies by UEC to come.</p

Ultrafast electron crystallography of the cooperative reaction path in vanadium dioxide

Time-resolved electron diffraction with atomic-scale spatial and temporal resolution was used to unravel the transformation pathway in the photoinduced structural phase transition of vanadium dioxide. Results from bulk crystals and single-crystalline thin-films reveal a common, stepwise mechanism: First, there is a femtosecond V-V bond dilation within 300 fs, second, an intracell adjustment in picoseconds and, third, a nanoscale shear motion within tens of picoseconds. Experiments at different ambient temperatures and pump laser fluences reveal a temperature-dependent excitation threshold required to trigger the transitional reaction path of the atomic motions. (C) 2016 Author(s)