Prafull Purohit

High Dynamic Range Pixel Array Detector for Scanning Transmission Electron Microscopy

We describe a hybrid pixel array detector (electron microscope pixel array detector, or EMPAD) adapted for use in electron microscope applications, especially as a universal detector for scanning transmission electron microscopy. The 128×128 pixel detector consists of a 500 µm thick silicon diode array bump-bonded pixel-by-pixel to an application-specific integrated circuit. The in-pixel circuitry provides a 1,000,000:1 dynamic range within a single frame, allowing the direct electron beam to be imaged while still maintaining single electron sensitivity. A 1.1 kHz framing rate enables rapid data collection and minimizes sample drift distortions while scanning. By capturing the entire unsaturated diffraction pattern in scanning mode, one can simultaneously capture bright field, dark field, and phase contrast information, as well as being able to analyze the full scattering distribution, allowing true center of mass imaging. The scattering is recorded on an absolute scale, so that information such as local sample thickness can be directly determined. This paper describes the detector architecture, data acquisition system, and preliminary results from experiments with 80-200 keV electron beams.

A Medium-Format, Mixed-Mode Pixel Array Detector for Kilohertz X-ray Imaging

An x-ray pixel array detector (PAD) capable of framing up to 1 kHz is described. This hybrid detector is constructed from a 3-side buttable, 128×128 pixel module based upon the mixed-mode pixel array detector (MMPAD) chip developed jointly by Cornell and Area Detector Systems Corporation (Poway, CA). The chip uses a charge integrating front end for a high instantaneous count rate yet with single photon sensitivity. In-pixel circuitry utilizing a digital overflow counter extends the per frame dynamic range to >4×107 x-rays/pixel. Results are shown from a base configuration of a 2×3 module array (256×384 pixels).

Time-resolved x-ray diffraction techniques for bulk polycrystalline materials under dynamic loading

We have developed two techniques for time-resolved x-ray diffraction from bulk polycrystalline materials during dynamic loading. In the first technique, we synchronize a fast detector with loading of samples at strain rates of ~10(3)-10(4) s(-1) in a compression Kolsky bar (split Hopkinson pressure bar) apparatus to obtain in situ diffraction patterns with exposures as short as 70 ns. This approach employs moderate x-ray energies (10-20 keV) and is well suited to weakly absorbing materials such as magnesium alloys. The second technique is useful for more strongly absorbing materials, and uses high-energy x-rays (86 keV) and a fast shutter synchronized with the Kolsky bar to produce short (~40 μs) pulses timed with the arrival of the strain pulse at the specimen, recording the diffraction pattern on a large-format amorphous silicon detector. For both techniques we present sample data demonstrating the ability of these techniques to characterize elastic strains and polycrystalline texture as a function of time during high-rate deformation.

High-speed X-ray imaging pixel array detector for synchrotron bunch isolation

A high-speed pixel array detector for time-resolved X-ray imaging at synchrotrons has been developed. The ability to isolate single synchrotron bunches makes it ideal for time-resolved dynamical studies.

Electron ptychography of 2D materials to deep sub-ångström resolution

Aberration-corrected optics have made electron microscopy at atomic resolution a widespread and often essential tool for characterizing nanoscale structures. Image resolution has traditionally been improved by increasing the numerical aperture of the lens (α) and the beam energy, with the state-of-the-art at 300 kiloelectronvolts just entering the deep sub-ångström (that is, less than 0.5 ångström) regime. Two-dimensional (2D) materials are imaged at lower beam energies to avoid displacement damage from large momenta transfers, limiting spatial resolution to about 1 ångström. Here, by combining an electron microscope pixel-array detector with the dynamic range necessary to record the complete distribution of transmitted electrons and full-field ptychography to recover phase information from the full phase space, we increase the spatial resolution well beyond the traditional numerical-aperture-limited resolution. At a beam energy of 80 kiloelectronvolts, our ptychographic reconstruction improves the image contrast of single-atom defects in MoS2 substantially, reaching an information limit close to 5α, which corresponds to an Abbe diffraction-limited resolution of 0.39 ångström, at the electron dose and imaging conditions for which conventional imaging methods reach only 0.98 ångström.Combining an electron microscope pixel-array detector that collects the entire distribution of scattered electrons with full-field ptychography greatly improves image resolution and contrast compared to traditional techniques, even at low beam energies.

Characterization of CdTe Sensors with Schottky Contacts Coupled to Charge-Integrating Pixel Array Detectors for X-Ray Science

Pixel Array Detectors (PADs) consist of an x-ray sensor layer bonded pixel-by-pixel to an underlying readout chip. This approach allows both the sensor and the custom pixel electronics to be tailored independently to best match the x-ray imaging requirements. Here we present characterizations of CdTe sensors hybridized with two different charge-integrating readout chips, the Keck PAD and the Mixed-Mode PAD (MM-PAD), both developed previously in our laboratory. The charge-integrating architecture of each of these PADs extends the instantaneous counting rate by many orders of magnitude beyond that obtainable with photon counting architectures. The Keck PAD chip consists of rapid, 8-frame, in-pixel storage elements with framing periods < 150 ns. The second detector, the MM-PAD, has an extended dynamic range by utilizing an in-pixel overflow counter coupled with charge removal circuitry activated at each overflow. This allows the recording of signals from the single-photon level to tens of millions of x-rays/pixel/frame while framing at 1 kHz. Both detector chips consist of a 128 × 128 pixel array with (150 μm)2 pixels.

Strain Mapping of Two-Dimensional Heterostructures with Subpicometer Precision

Next-generation, atomically thin devices require in-plane, one-dimensional heterojunctions to electrically connect different two-dimensional (2D) materials. However, the lattice mismatch between most 2D materials leads to unavoidable strain, dislocations, or ripples, which can strongly affect their mechanical, optical, and electronic properties. We have developed an approach to map 2D heterojunction lattice and strain profiles with subpicometer precision and the ability to identify dislocations and out-of-plane ripples. We collected diffraction patterns from a focused electron beam for each real-space scan position with a high-speed, high dynamic range, momentum-resolved detector-the electron microscope pixel array detector (EMPAD). The resulting four-dimensional (4D) phase space data sets contain the full spatially resolved lattice information on the sample. By using this technique on tungsten disulfide (WS2) and tungsten diselenide (WSe2) lateral heterostructures, we have mapped lattice distortions with 0.3 pm precision across multimicron fields of view and simultaneously observed the dislocations and ripples responsible for strain relaxation in 2D laterally epitaxial structures.

Reconstruction of Polarization Vortices by Diffraction Mapping of Ferroelectric PbTiO3 / SrTiO3 Superlattice Using a High Dynamic Range Pixelated Detector

Kayla X. Nguyen, Prafull Purohit, Ajay Yadav, Mark W. Tate, Celesta S. Chang, Ramamoorthy Ramesh, Sol M. Gruner, David A. Muller School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Department of Physics, Cornell University, Ithaca, NY, USA Department of Material Science and Engineering, University of California, Berkeley Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY USA

Electron Diffraction from a Single Atom and Optimal Signal Detection

Imaging single atoms against a supporting background, made of many more atoms, is a common challenge for many areas of materials characterization from detecting dopants in semiconductors [1,2] to understanding the behavior of catalysts [3]. Electron energy loss spectroscopy and x-ray analysis can help to suppress the background support. While single atom detection is possible [2], their low cross sections imply they are not dose efficient or well-suited to radiation-sensitive materials. The elastic signal is the largest atomic-resolution signal available, but efficient discrimination of the target atom against the support atoms can be system specific. In the best case of single atoms on a graphene or uniform 2D support, the problem is well posed, and the optimal single-channel STEM detector is a low-angle annular dark field (ADF) detector [4]. The inclusion of a thick support considerably complicates matters, but detection of heavy atoms against a light matrix is possible using high-angle ADF imaging [1-3]. The question is can we do better by recording the full scattering distribution?

An Electron Microscope Pixel Array Detector as a Universal STEM Detector

Complete information about the scattering potential of a sample is in principle encoded in the distribution of scattered electrons from a localized beam propagating through it. A new generation of high speed imaging detectors brings us closer to this goal and will allow us to explore practical limits and identify the most promising methods of analysis. We have recently developed an electron microscope pixel array detector (EMPAD) that functions as a compact and high-speed, high dynamic range electron diffraction camera (Figure 1a). It has single electron sensitivity with a signal/noise ratio of 140 for a single electron at 200 keV [1,2]. It has a dynamic range of 10 6 for primary electrons– i.e a pixel can detect from 1 to 1,000,000 electrons, and reads out an image frame in 0.86 ms. These properties allow us to record essentially an image of all the transmitted electrons, from the unscattered beam to out beyond the HOLZ lines, and do so for every probe position in a real-space, atomic resolution image. Not only does this allow quantitative and simultaneous annular dark and bright field signals on an absolute scale, but from the analysis of the spatially-resolved diffraction patterns we can extract thickness, strain and tilt, octahedral rotations, polarity and even electric and magnetic fields.