Jonathan Zachary Tischler

Nanosecond resolution time-resolved x-ray study of silicon during pulsed-laser irradiation

We have used the pulsed time structure of the Cornell High-Energy Synchrotron Source (CHESS) to carry out a nanosecond resolution time-resolved x-ray study of silicon during pulsed-laser irradiation. Time-resolved temperature distributions and interfacial overheating and undercooling were measured on and silicon during 25 ns UV laser pulses through the analysis of thermal expansion induced strain. The temperature gradients were found to be >10/sup 7/ K/cm at the liquid--solid interface and the temperature distributions have been shown to be in agreement with numerical heat flow calculations for these laser conditions. The combined overheating and undercooling (during approx.10 m/s melting and approx.6 m/s regrowth) was measured to be 110 +- 30 K on oriented silicon and 50 +- 25 K on silicon. These values have been interpreted in terms of velocity coefficients of overheating and undercooling.

Elliptical x-ray microprobe mirrors by differential deposition

A differential coating method is described for fabricating high-performance x-ray microfocusing mirrors. With this method, the figure of ultrasmooth spherical mirrors can be modified to produce elliptical surfaces with low roughness and low figure errors. Submicron focusing is demonstrated with prototype mirrors. The differential deposition method creates stiff monolithic mirrors which are compact, robust, and easy to cool and align. Prototype mirrors have demonstrated gains of more than 104 in beam intensity while maintaining submilliradian divergence on the sample. This method of producing elliptical mirrors is well matched to the requirements of an x-ray microdiffraction Kirkpatrick–Baez focusing system.

Short focal length Kirkpatrick-Baez mirrors for a hard x-ray nanoprobe

We describe progress in the fabrication of short-focal-length total-external-reflection Kirkpatrick-Baez x-ray mirrors with ultralow figure errors. The short focal length optics produce nanoscale beams (<100nm) on conventional (∼64m long) beamlines at third generation synchrotron sources. The total-external reflection optics are inherently achromatic and efficiently focus a white (polychromatic) or a tunable monochromatic spectrum of x rays. The ability to focus independent of wavelength allows novel new experimental capabilities. Mirrors have been fabricated both by computer assisted profiling (differential polishing) and by profile coating (coating through a mask onto ultra-smooth surfaces). A doubly focused 85×95nm2 hard x-ray nanobeam has been obtained on the UNICAT beamline 34-ID at the Advanced Photon Source. The performance of the mirrors, techniques for characterizing the spot size, and factors limiting focusing performance are discussed.

Metallization of vanadium dioxide driven by large phonon entropy

Phase competition underlies many remarkable and technologically important phenomena in transition metal oxides. Vanadium dioxide (VO2) exhibits a first-order metal–insulator transition (MIT) near room temperature, where conductivity is suppressed and the lattice changes from tetragonal to monoclinic on cooling. Ongoing attempts to explain this coupled structural and electronic transition begin with two alternative starting points: a Peierls MIT driven by instabilities in electron–lattice dynamics and a Mott MIT where strong electron–electron correlations drive charge localization. A key missing piece of the VO2 puzzle is the role of lattice vibrations. Moreover, a comprehensive thermodynamic treatment must integrate both entropic and energetic aspects of the transition. Here we report that the entropy driving the MIT in VO2 is dominated by strongly anharmonic phonons rather than electronic contributions, and provide a direct determination of phonon dispersions. Our ab initio calculations identify softer bonding in the tetragonal phase, relative to the monoclinic phase, as the origin of the large vibrational entropy stabilizing the metallic rutile phase. They further reveal how a balance between higher entropy in the metal and orbital-driven lower energy in the insulator fully describes the thermodynamic forces controlling the MIT. Our study illustrates the critical role of anharmonic lattice dynamics in metal oxide phase competition, and provides guidance for the predictive design of new materials.

Interplay between Ferroelastic and Metal−Insulator Phase Transitions in Strained Quasi-Two-Dimensional VO2 Nanoplatelets

Formation of ferroelastic twin domains in vanadium dioxide (VO(2)) nanosystems can strongly affect local strain distributions, and hence couple to the strain-controlled metal-insulator transition. Here we report polarized-light optical and scanning microwave microscopy studies of interrelated ferroelastic and metal-insulator transitions in single-crystalline VO(2) quasi-two-dimensional (quasi-2D) nanoplatelets (NPls). In contrast to quasi-1D single-crystalline nanobeams, the 2D geometric frustration results in emergence of several possible families of ferroelastic domains in NPls, thus allowing systematic studies of strain-controlled transitions in the presence of geometrical frustration. We demonstrate the possibility of controlling the ferroelastic domain population by the strength of the NPl-substrate interaction, mechanical stress, and by the NPl lateral size. Ferroelastic domain species and domain walls are identified based on standard group-theoretical considerations. Using variable temperature microscopy, we imaged the development of domains of metallic and semiconducting phases during the metal-insulator phase transition and nontrivial strain-driven reentrant domain formation. A long-range reconstruction of ferroelastic structures accommodating metal-insulator domain formation has been observed. These studies illustrate that a complete picture of the phase transitions in single-crystalline and disordered VO(2) structures can be drawn only if both ferroelastic and metal-insulator strain effects are taken into consideration and understood.

Polychromatic X-ray Microdiffraction Studies of Mesoscale Structure and Dynamics

Polychromatic X-ray microdiffraction is an emerging tool for studying mesoscale structure and dynamics. Crystalline phase, orientation (texture), elastic and plastic strain can be nondestructively mapped in three dimensions with good spatial and angular resolution. Local crystallographic orientation can be determined to approximately 0.01 degree and elastic strain tensor elements can be measured with a resolution of approximately 10(-4) or better. Complete strain tensor information can be obtained by augmenting polychromatic microdiffraction with a monochromatic measurement of one Laue-reflection energy. With differential-aperture depth profiling, volumes tens to hundreds of micrometers below the surface are accessible so that three-dimensional distributions of crystalline morphology including grain boundaries, triple points, second phases and inclusions can all be mapped. Volume elements below 0.25 microm3 are routinely resolved so that the grain boundary structure of most materials can be characterized. Here the theory, instrumentation and application of polychromatic microdiffraction are described.

Doping-Based Stabilization of the M2 Phase in Free-Standing VO2 Nanostructures at Room Temperature

A new high-yield method of doping VO(2) nanostructures with aluminum is proposed, which renders possible stabilization of the monoclinic M2 phase in free-standing nanoplatelets in ambient conditions and opens an opportunity for realization of a purely electronic Mott transition field-effect transistor without an accompanying structural transition. The synthesized free-standing M2-phase nanostructures are shown to have very high crystallinity and an extremely sharp temperature-driven metal-insulator transition. A combination of X-ray microdiffraction, micro-Raman spectroscopy, energy-dispersive X-ray spectroscopy, and four-probe electrical measurements allowed thorough characterization of the doped nanostructures. Light is shed onto some aspects of the nanostructure growth, and the temperature-doping level phase diagram is established.

Electromechanical actuation and current-induced metastable states in suspended single-crystalline VO2 nanoplatelets

Current-induced electromechanical actuation enabled by the metal-insulator transition in VO(2) nanoplatelets is demonstrated. The Joule heating by a sufficient current flowing through suspended nanoplatelets results in formation of heterophase domain patterns and is accompanied by nanoplatelet deformation. The actuation action can be achieved in a wide temperature range below the bulk phase transition temperature (68 °C). The observed current-sustained heterophase domain structures should be interpreted as distinct metastable states in free-standing and end-clamped VO(2) samples. We analyze the main prerequisites for the realization of a current-controlled actuator based on the proposed concept.

Experimental characterization of the mesoscale dislocation density tensor

The dislocation density tensor has been an important variable in the theoretical characterization of dislocations in deformed crystals since its introduction over 5 decades ago. However, the non-destructive, three-dimensional (3D) measurements of lattice rotations and elastic strain needed to determine dislocation density tensors with micron spatial resolution over mesoscopic length scales have until now not been available. We have used 3D X-ray microscopy with sub-micron point-to-point spatial resolution to demonstrate 3D, spatially resolved measurements of the dislocation density tensor in elastically and plastically deformed silicon single crystal plates. Measurements were made of the dislocation density tensor along a line in a ∼35 µm thick silicon plate that was bent (elastically) to a 5.42 mm radius of curvature at room temperature, and in a similar sample deformed plastically by annealing to 700°C under bending stress. We discuss the theoretical background for the dislocation density tensor with respect to lattice rotation and elastic strain, we describe the X-ray microscopy technique used to make non-destructive measurement of local rotations and elastic strains with sub-micron resolution in 3D, and we discuss the analysis procedures for extracting dislocation tensors on mesoscopic length scales.

Spatially resolved Poisson strain and anticlastic curvature measurements in Si under large deflection bending

A scanning-monochromatic form of differential-aperture x-ray microscopy (DAXM) has been developed that provides micron-resolution depth-resolved dilatational strain measurements. This scanning-monochromatic DAXM technique is applied to measurements of Poisson dilatational strain in 25-μm-thick Si bent into an arch with an apex radius of R=3 mm. Poisson strain measurements agree with anisotropic linear elasticity calculations for a Searle parameter as large as β=1009. Local anticlastic bend radii were shown to oscillate across the arch and reach the R/ν limit for distances less than the plate thickness from the edges, where ν is the anisotropic Poisson’s ratio.