Publications by authors named "Martin V Holt"

18 Publications

  • Page 1 of 1

Nanoscale Imaging and Control of Volatile and Non-Volatile Resistive Switching in VO.

Small 2020 12 23;16(50):e2005439. Epub 2020 Nov 23.

Department of Physics, University of California San Diego, La Jolla, CA, 92093, USA.

Control of the metal-insulator phase transition is vital for emerging neuromorphic and memristive technologies. The ability to alter the electrically driven transition between volatile and non-volatile states is particularly important for quantum-materials-based emulation of neurons and synapses. The major challenge of this implementation is to understand and control the nanoscale mechanisms behind these two fundamental switching modalities. Here, in situ X-ray nanoimaging is used to follow the evolution of the nanostructure and disorder in the archetypal Mott insulator VO during an electrically driven transition. Our findings demonstrate selective and reversible stabilization of either the insulating or metallic phases achieved by manipulating the defect concentration. This mechanism enables us to alter the local switching response between volatile and persistent regimes and demonstrates a new possibility for nanoscale control of the resistive switching in Mott materials.
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http://dx.doi.org/10.1002/smll.202005439DOI Listing
December 2020

Perovskite neural trees.

Nat Commun 2020 05 7;11(1):2245. Epub 2020 May 7.

School of Materials Engineering, Purdue University, West Lafayette, IN, 47907, USA.

Trees are used by animals, humans and machines to classify information and make decisions. Natural tree structures displayed by synapses of the brain involves potentiation and depression capable of branching and is essential for survival and learning. Demonstration of such features in synthetic matter is challenging due to the need to host a complex energy landscape capable of learning, memory and electrical interrogation. We report experimental realization of tree-like conductance states at room temperature in strongly correlated perovskite nickelates by modulating proton distribution under high speed electric pulses. This demonstration represents physical realization of ultrametric trees, a concept from number theory applied to the study of spin glasses in physics that inspired early neural network theory dating almost forty years ago. We apply the tree-like memory features in spiking neural networks to demonstrate high fidelity object recognition, and in future can open new directions for neuromorphic computing and artificial intelligence.
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http://dx.doi.org/10.1038/s41467-020-16105-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7206050PMC
May 2020

Ferroelectric Domain Wall Motion in Freestanding Single-Crystal Complex Oxide Thin Film.

Adv Mater 2020 Jan 9;32(4):e1907036. Epub 2019 Dec 9.

Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL, 60439, USA.

Ferroelectric domain walls in single-crystal complex oxide thin films are found to be orders of magnitude slower when the interfacial bonds with the heteroepitaxial substrate are broken to create a freestanding film. This drastic change in domain wall kinetics does not originate from the alteration of epitaxial strain; rather, it is correlated with the structural ripples at mesoscopic length scale and associated flexoelectric effects induced in the freestanding films. In contrast, the effects of the bond-breaking on the local static ferroelectric properties of both top and bottom layers of the freestanding films, such as domain wall width and spontaneous polarization, are modest and governed by the change in epitaxy-induced compressive strain.
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http://dx.doi.org/10.1002/adma.201907036DOI Listing
January 2020

Stabilization of point-defect spin qubits by quantum wells.

Nat Commun 2019 12 6;10(1):5607. Epub 2019 Dec 6.

Wigner Research Centre for Physics, PO Box 49, H-1525, Budapest, Hungary.

Defect-based quantum systems in wide bandgap semiconductors are strong candidates for scalable quantum-information technologies. However, these systems are often complicated by charge-state instabilities and interference by phonons, which can diminish spin-initialization fidelities and limit room-temperature operation. Here, we identify a pathway around these drawbacks by showing that an engineered quantum well can stabilize the charge state of a qubit. Using density-functional theory and experimental synchrotron X-ray diffraction studies, we construct a model for previously unattributed point defect centers in silicon carbide as a near-stacking fault axial divacancy and show how this model explains these defects' robustness against photoionization and room temperature stability. These results provide a materials-based solution to the optical instability of color centers in semiconductors, paving the way for the development of robust single-photon sources and spin qubits.
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http://dx.doi.org/10.1038/s41467-019-13495-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6898666PMC
December 2019

Correlated Nanoscale Analysis of the Emission from Wurtzite versus Zincblende (In,Ga)As/GaAs Nanowire Core-Shell Quantum Wells.

Nano Lett 2019 Jul 5;19(7):4448-4457. Epub 2019 Jun 5.

Paul-Drude-Institut für Festkörperelektronik , Leibniz-Institut im Forschungsverbund Berlin e.V. , Hausvogteiplatz 5-7 , 10117 Berlin , Germany.

While the properties of wurtzite GaAs have been extensively studied during the past decade, little is known about the influence of the crystal polytype on ternary (In,Ga)As quantum well structures. We address this question with a unique combination of correlated, spatially resolved measurement techniques on core-shell nanowires that contain extended segments of both the zincblende and wurtzite polytypes. Cathodoluminescence hyperspectral imaging reveals a blue-shift of the quantum well emission energy by 75 ± 15 meV in the wurtzite polytype segment. Nanoprobe X-ray diffraction and atom probe tomography enable · calculations for the specific sample geometry to reveal two comparable contributions to this shift. First, there is a 30% drop in In mole fraction going from the zincblende to the wurtzite segment. Second, the quantum well is under compressive strain, which has a much stronger impact on the hole ground state in the wurtzite than in the zincblende segment. Our results highlight the role of the crystal structure in tuning the emission of (In,Ga)As quantum wells and pave the way to exploit the possibilities of three-dimensional band gap engineering in core-shell nanowire heterostructures. At the same time, we have demonstrated an advanced characterization toolkit for the investigation of semiconductor nanostructures.
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http://dx.doi.org/10.1021/acs.nanolett.9b01241DOI Listing
July 2019

Quantifying Polymer Chain Orientation in Strong and Tough Nanofibers with Low Crystallinity: Toward Next Generation Nanostructured Superfibers.

ACS Nano 2019 05 10;13(5):4893-4927. Epub 2019 May 10.

Department of Mechanical and Materials Engineering , University of Nebraska-Lincoln , Lincoln , Nebraska 68588-0526 , United States.

Advanced fibers revolutionized structural materials in the second half of the 20th century. However, all high-strength fibers developed to date are brittle. Recently, pioneering simultaneous ultrahigh strength and toughness were discovered in fine (<250 nm) individual electrospun polymer nanofibers (NFs). This highly desirable combination of properties was attributed to high macromolecular chain alignment coupled with low crystallinity. Quantitative analysis of the degree of preferred chain orientation will be crucial for control of NF mechanical properties. However, quantification of supramolecular nanoarchitecture in NFs with low crystallinity in the ultrafine diameter range is highly challenging. Here, we discuss the applicability of traditional as well as emerging methods for quantification of polymer chain orientation in nanoscale one-dimensional samples. Advantages and limitations of different techniques are critically evaluated on experimental examples. It is shown that straightforward application of some of the techniques to sub-wavelength-diameter NFs can lead to severe quantitative and even qualitative artifacts. Sources of such size-related artifacts, stemming from instrumental, materials, and geometric phenomena at the nanoscale, are analyzed on the example of polarized Raman method but are relevant to other spectroscopic techniques. A proposed modified, artifact-free method is demonstrated. Outstanding issues and their proposed solutions are discussed. The results provide guidance for accurate nanofiber characterization to improve fundamental understanding and accelerate development of nanofibers and related nanostructured materials produced by electrospinning or other methods. We expect that the discussion in this review will also be useful to studies of many biological systems that exhibit nanofilamentary architectures and combinations of high strength and toughness.
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http://dx.doi.org/10.1021/acsnano.8b08725DOI Listing
May 2019

Homogenized halides and alkali cation segregation in alloyed organic-inorganic perovskites.

Science 2019 02;363(6427):627-631

Department of Nanoengineering, University of California, San Diego, La Jolla, CA 92093, USA.

The role of the alkali metal cations in halide perovskite solar cells is not well understood. Using synchrotron-based nano-x-ray fluorescence and complementary measurements, we found that the halide distribution becomes homogenized upon addition of cesium iodide, either alone or with rubidium iodide, for substoichiometric, stoichiometric, and overstoichiometric preparations, where the lead halide is varied with respect to organic halide precursors. Halide homogenization coincides with long-lived charge carrier decays, spatially homogeneous carrier dynamics (as visualized by ultrafast microscopy), and improved photovoltaic device performance. We found that rubidium and potassium phase-segregate in highly concentrated clusters. Alkali metals are beneficial at low concentrations, where they homogenize the halide distribution, but at higher concentrations, they form recombination-active second-phase clusters.
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http://dx.doi.org/10.1126/science.aah5065DOI Listing
February 2019

Mesoscopic Elastic Distortions in GaAs Quantum Dot Heterostructures.

Nano Lett 2018 05 23;18(5):2780-2786. Epub 2018 Apr 23.

Department of Materials Science & Engineering , University of Wisconsin-Madison , Madison , Wisconsin 53706 , United States.

Quantum devices formed in high-electron-mobility semiconductor heterostructures provide a route through which quantum mechanical effects can be exploited on length scales accessible to lithography and integrated electronics. The electrostatic definition of quantum dots in semiconductor heterostructure devices intrinsically involves the lithographic fabrication of intricate patterns of metallic electrodes. The formation of metal/semiconductor interfaces, growth processes associated with polycrystalline metallic layers, and differential thermal expansion produce elastic distortion in the active areas of quantum devices. Understanding and controlling these distortions present a significant challenge in quantum device development. We report synchrotron X-ray nanodiffraction measurements combined with dynamical X-ray diffraction modeling that reveal lattice tilts with a depth-averaged value up to 0.04° and strain on the order of 10 in the two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. Elastic distortions in GaAs/AlGaAs heterostructures modify the potential energy landscape in the 2DEG due to the generation of a deformation potential and an electric field through the piezoelectric effect. The stress induced by metal electrodes directly impacts the ability to control the positions of the potential minima where quantum dots form and the coupling between neighboring quantum dots.
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http://dx.doi.org/10.1021/acs.nanolett.7b04603DOI Listing
May 2018

Domain alignment within ferroelectric/dielectric PbTiO/SrTiO superlattice nanostructures.

Nanoscale 2018 Feb;10(7):3262-3271

Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA.

The ferroelectric domain pattern within lithographically defined PbTiO/SrTiO ferroelectric/dielectric heteroepitaxial superlattice nanostructures is strongly influenced by the edges of the structures. Synchrotron X-ray nanobeam diffraction reveals that the spontaneously formed 180° ferroelectric stripe domains exhibited by such superlattices adopt a configuration in rectangular nanostructures in which domain walls are aligned with long patterned edges. The angular distribution of X-ray diffuse scattering intensity from nanodomains indicates that domains are aligned within an angular range of approximately 20° with respect to the edges. Computational studies based on a time-dependent Landau-Ginzburg-Devonshire model show that the preferred direction of the alignment results from lowering of the bulk and electrostrictive contributions to the free energy of the system due to the release of the lateral mechanical constraint. This unexpected alignment appears to be intrinsic and not a result of distortions or defects caused by the patterning process. Our work demonstrates how nanostructuring and patterning of heteroepitaxial superlattices allow for pathways to create and control ferroelectric structures that may appear counterintuitive.
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http://dx.doi.org/10.1039/c7nr07203aDOI Listing
February 2018

Phase Coexistence and Kinetic Arrest in the Magnetostructural Transition of the Ordered Alloy FeRh.

Sci Rep 2018 01 29;8(1):1778. Epub 2018 Jan 29.

X-ray Science Division, Argonne National Laboratory, Argonne, IL, 60439, USA.

In materials where two or more ordering degrees of freedom are closely matched in their free energies, coupling between them, or multiferroic behavior can occur. These phenomena can produce a very rich phase behavior, as well as emergent phases that offer useful properties and opportunities to reveal novel phenomena in phase transitions. The ordered alloy FeRh undergoes an antiferromagnetic to ferromagnetic phase transition at ~375 K, which illustrates the interplay between structural and magnetic order mediated by a delicate energy balance between two configurations. We have examined this transition using a combination of high-resolution x-ray structural and magnetic imaging and comprehensive x-ray magnetic circular dichroism spectroscopy. We find that the transition proceeds via a defect-driven domain nucleation and growth mechanism, with significant return point memory in both the structural and magnetic domain configurations. The domains show evidence of inhibited growth after nucleation, resulting in a quasi-2 order temperature behavior.
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http://dx.doi.org/10.1038/s41598-018-20101-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5789070PMC
January 2018

Measuring Three-Dimensional Strain and Structural Defects in a Single InGaAs Nanowire Using Coherent X-ray Multiangle Bragg Projection Ptychography.

Nano Lett 2018 02 24;18(2):811-819. Epub 2018 Jan 24.

Materials Science Division, Argonne National Laboratory , Argonne, Illinois 60439, United States.

III-As nanowires are candidates for near-infrared light emitters and detectors that can be directly integrated onto silicon. However, nanoscale to microscale variations in structure, composition, and strain within a given nanowire, as well as variations between nanowires, pose challenges to correlating microstructure with device performance. In this work, we utilize coherent nanofocused X-rays to characterize stacking defects and strain in a single InGaAs nanowire supported on Si. By reconstructing diffraction patterns from the 21̅1̅0 Bragg peak, we show that the lattice orientation varies along the length of the wire, while the strain field along the cross-section is largely unaffected, leaving the band structure unperturbed. Diffraction patterns from the 011̅0 Bragg peak are reproducibly reconstructed to create three-dimensional images of stacking defects and associated lattice strains, revealing sharp planar boundaries between different crystal phases of wurtzite (WZ) structure that contribute to charge carrier scattering. Phase retrieval is made possible by developing multiangle Bragg projection ptychography (maBPP) to accommodate coherent nanodiffraction patterns measured at arbitrary overlapping positions at multiple angles about a Bragg peak, eliminating the need for scan registration at different angles. The penetrating nature of X-ray radiation, together with the relaxed constraints of maBPP, will enable the in operando imaging of nanowire devices.
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http://dx.doi.org/10.1021/acs.nanolett.7b04024DOI Listing
February 2018

Nanoscale Detection of Intermediate Solid Solutions in Equilibrated LiFePO Microcrystals.

Nano Lett 2017 12 30;17(12):7364-7371. Epub 2017 Nov 30.

Department of Chemistry, University of Illinois at Chicago , Chicago, Illinois 60607, United States.

Redox-driven phase transformations in solids determine the performance of lithium-ion batteries, crucial in the technological transition from fossil fuels. Couplings between chemistry and strain define reversibility and fatigue of an electrode. The accurate definition of all phases in the transformation, their energetics, and nanoscale location within a particle produces fundamental understanding of these couplings needed to design materials with ultimate performance. Here we demonstrate that scanning X-ray diffraction microscopy (SXDM) extends our ability to image battery processes in single particles. In LiFePO crystals equilibrated after delithiation, SXDM revealed the existence of domains of miscibility between LiFePO and LiFePO. These solid solutions are conventionally thought to be metastable, and were previously undetected by spectromicroscopy. The observation provides experimental verification of predictions that the LiFePO-FePO phase diagram can be altered by coherency strain under certain interfacial orientations. It enriches our understanding of the interaction between diffusion, chemistry, and mechanics in solid state transformations.
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http://dx.doi.org/10.1021/acs.nanolett.7b03086DOI Listing
December 2017

Strain imaging of nanoscale semiconductor heterostructures with x-ray Bragg projection ptychography.

Phys Rev Lett 2014 Apr 23;112(16):165502. Epub 2014 Apr 23.

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.

We report the imaging of nanoscale distributions of lattice strain and rotation in complementary components of lithographically engineered epitaxial thin film semiconductor heterostructures using synchrotron x-ray Bragg projection ptychography (BPP). We introduce a new analysis method that enables lattice rotation and out-of-plane strain to be determined independently from a single BPP phase reconstruction, and we apply it to two laterally adjacent, multiaxially stressed materials in a prototype channel device. These results quantitatively agree with mechanical modeling and demonstrate the ability of BPP to map out-of-plane lattice dilatation, a parameter critical to the performance of electronic materials.
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http://dx.doi.org/10.1103/PhysRevLett.112.165502DOI Listing
April 2014

The range and intensity of backscattered electrons for use in the creation of high fidelity electron beam lithography patterns.

Nanotechnology 2013 Aug 2;24(30):305302. Epub 2013 Jul 2.

Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Avenue Building 440, Argonne, IL 60439, USA.

We present a set of universal curves that predict the range and intensity of backscattered electrons which can be used in conjunction with electron beam lithography to create high fidelity nanoscale patterns. The experimental method combines direct write dose, backscattered dose, and a self-reinforcing pattern geometry to measure the dose provided by backscattered electrons to a nanoscale volume on the substrate surface at various distances from the electron source. Electron beam lithography is used to precisely control the number and position of incident electrons on the surface of the material. Atomic force microscopy is used to measure the height of the negative electron beam lithography resist. Our data shows that the range and the intensity of backscattered electrons can be predicted using the density and the atomic number of any solid material, respectively. The data agrees with two independent Monte Carlo simulations without any fitting parameters. These measurements are the most accurate electron range measurements to date.
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http://dx.doi.org/10.1088/0957-4484/24/30/305302DOI Listing
August 2013

Thermal diffuse scattering as a probe of large-wave-vector phonons in silicon nanostructures.

Phys Rev Lett 2013 May 14;110(20):205503. Epub 2013 May 14.

Materials Science and Engineering and Materials Science Program, University of Wisconsin, Madison, Wisconsin 53706, USA.

Large-wave-vector phonons have an important role in determining the thermal and electronic properties of nanoscale materials. The small volumes of such structures, however, have posed significant challenges to experimental studies of the phonon dispersion. We show that synchrotron x-ray thermal diffuse scattering can be adapted to probe phonons with wave vectors spanning the entire Brillouin zone of nanoscale silicon membranes. The thermal diffuse scattering signal from flat Si nanomembranes with thicknesses from 315 to 6 nm, and a sample volume as small as 5 μm(3), has the expected linear dependence on the membrane thickness and also exhibits excess intensity at large wave vectors, consistent with the scattering signature expected from low-lying large-wave-vector modes of the membranes.
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http://dx.doi.org/10.1103/PhysRevLett.110.205503DOI Listing
May 2013

A hard X-ray nanoprobe beamline for nanoscale microscopy.

J Synchrotron Radiat 2012 Nov 5;19(Pt 6):1056-60. Epub 2012 Sep 5.

Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60441, USA.

The Hard X-ray Nanoprobe Beamline (or Nanoprobe Beamline) is an X-ray microscopy facility incorporating diffraction, fluorescence and full-field imaging capabilities designed and operated by the Center for Nanoscale Materials and the Advanced Photon Source at Sector 26 of the Advanced Photon Source at Argonne National Laboratory. This facility was constructed to probe the nanoscale structure of biological, environmental and material sciences samples. The beamline provides intense focused X-rays to the Hard X-ray Nanoprobe (or Nanoprobe) which incorporates Fresnel zone plate optics and a precision laser sensing and control system. The beamline operates over X-ray energies from 3 to 30 keV, enabling studies of most elements in the periodic table, with a particular emphasis on imaging transition metals.
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http://dx.doi.org/10.1107/S0909049512036783DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3579591PMC
November 2012

Structural consequences of ferroelectric nanolithography.

Nano Lett 2011 Aug 5;11(8):3080-4. Epub 2011 Jul 5.

Department of Materials Science and Engineering and Materials Science Program, University of Wisconsin, Madison, Wisconsin 53706, United States.

Domains of remnant polarization can be written into ferroelectrics with nanoscale precision using scanning probe nanolithography techniques such as piezoresponse force microscopy (PFM). Understanding the structural effects accompanying this process has been challenging due to the lack of appropriate structural characterization tools. Synchrotron X-ray nanodiffraction provides images of the domain structure written by PFM into an epitaxial Pb(Zr,Ti)O(3) thin film and simultaneously reveals structural effects arising from the writing process. A coherent scattering simulation including the superposition of the beams simultaneously diffracted by multiple mosaic blocks provides an excellent fit to the observed diffraction patterns. Domains in which the polarization is reversed from the as-grown state have a strain of up to 0.1% representing the piezoelectric response to unscreened surface charges. An additional X-ray microdiffraction study of the photon-energy dependence of the difference in diffracted intensity between opposite polarization states shows that this contrast has a crystallographic origin. The sign and magnitude of the intensity contrast between domains of opposite polarization are consistent with the polarization expected from PFM images and with the writing of domains through the entire thickness of the ferroelectric layer. The strain induced by writing provides a significant additional contribution to the increased free energy of the written domain state with respect to a uniformly polarized state.
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http://dx.doi.org/10.1021/nl2009873DOI Listing
August 2011

Framework for three-dimensional coherent diffraction imaging by focused beam x-ray Bragg ptychography.

Opt Lett 2011 Jun;36(12):2227-9

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.

We present the framework for convergent beam Bragg ptychography, and, using simulations, we demonstrate that nanocrystals can be ptychographically reconstructed from highly convergent x-ray Bragg diffraction. The ptychographic iterative engine is extended to three dimensions and shown to successfully reconstruct a simulated nanocrystal using overlapping raster scans with a defocused curved beam, the diameter of which matches the crystal size. This object reconstruction strategy can serve as the basis for coherent diffraction imaging experiments at coherent scanning nanoprobe x-ray sources.
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http://dx.doi.org/10.1364/OL.36.002227DOI Listing
June 2011
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