Publications by authors named "Jakob B Wagner"

51 Publications

In situ manipulation of the active Au-TiO interface with atomic precision during CO oxidation.

Science 2021 01;371(6528):517-521

State Key Laboratory of Silicon Materials and Center of Electron Microscopy, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027 China.

The interface between metal catalyst and support plays a critical role in heterogeneous catalysis. An epitaxial interface is generally considered to be rigid, and tuning its intrinsic microstructure with atomic precision during catalytic reactions is challenging. Using aberration-corrected environmental transmission electron microscopy, we studied the interface between gold (Au) and a titanium dioxide (TiO) support. Direct atomic-scale observations showed an unexpected dependence of the atomic structure of the Au-TiO interface with the epitaxial rotation of gold nanoparticles on a TiO surface during carbon monoxide (CO) oxidation. Taking advantage of the reversible and controllable rotation, we achieved in situ manipulation of the active Au-TiO interface by changing gas and temperature. This result suggests that real-time design of the catalytic interface in operating conditions may be possible.
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http://dx.doi.org/10.1126/science.abe3558DOI Listing
January 2021

Visualizing HO molecules reacting at TiO active sites with transmission electron microscopy.

Science 2020 01;367(6476):428-430

State Key Laboratory of Silicon Materials and Center of Electron Microscopy, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027 China.

Imaging a reaction taking place at the molecular level could provide direct information for understanding the catalytic reaction mechanism. We used in situ environmental transmission electron microscopy and a nanocrystalline anatase titanium dioxide (001) surface with (1 × 4) reconstruction as a catalyst, which provided highly ordered four-coordinated titanium "active rows" to realize real-time monitoring of water molecules dissociating and reacting on the catalyst surface. The twin-protrusion configuration of adsorbed water was observed. During the water-gas shift reaction, dynamic changes in these structures were visualized on these active rows at the molecular level.
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http://dx.doi.org/10.1126/science.aay2474DOI Listing
January 2020

Methods for Calibration of Specimen Temperature During Transmission Electron Microscopy Experiments.

Microsc Microanal 2020 Feb;26(1):3-17

Department of Energy Conversion and Storage, Technical University of Denmark, Fysikvej, DK-2800 Kgs. Lyngby, Denmak.

One of the biggest challenges for in situ heating transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) is the ability to measure the local temperature of the specimen accurately. Despite technological improvements in the construction of TEM/STEM heating holders, the problem of being able to measure the real sample temperature is still the subject of considerable discussion. In this study, we review the present literature on methodologies for temperature calibration. We analyze calibration methods that require the use of a thermometric material in addition to the specimen under study, as well as methods that can be performed directly on the specimen of interest without the need for a previous calibration. Finally, an overview of the most important characteristics of all the treated techniques, including temperature ranges and uncertainties, is provided in order to provide an accessory database to consult before an in situ TEM/STEM temperature calibration experiment.
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http://dx.doi.org/10.1017/S1431927619015344DOI Listing
February 2020

Growth kinetics of single-walled carbon nanotubes with a (2, ) chirality selection.

Sci Adv 2019 12 13;5(12):eaav9668. Epub 2019 Dec 13.

Center for Multidimensional Carbon Materials, Institute for Basic Science, UNIST-gil 50, Ulju-gun, Ulsan 44919, Republic of Korea.

The growth kinetics play key roles in determining the chirality distribution of the grown single-walled carbon nanotubes (SWCNTs). However, the lack of comprehensive understandings on the SWCNT's growth mechanism at the atomic scale greatly hinders SWCNT chirality-selective synthesis. Here, we establish a general model, where the dislocation theory is a specific case, to describe the etching agent-dependent growth kinetics of SWCNTs on solid catalyst particles. In particular, the growth kinetics of SWCNTs in the absence of etching agent is validated by both in situ environmental transmission electron microscopy and ex situ chemical vapor deposition growth of SWCNTs. On the basis of the new theory of SWCNT's growth kinetics, we successfully explained the selective growth of (2, ) SWCNTs. This study provides another degree of freedom for SWCNT controlled synthesis and opens a new strategy to achieve chirality-selective synthesis of (2, ) SWCNTs using solid catalysts.
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http://dx.doi.org/10.1126/sciadv.aav9668DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6910834PMC
December 2019

Self-assembled nanofibrils from RGD-functionalized cellulose nanocrystals to improve the performance of PEI/DNA polyplexes.

J Colloid Interface Sci 2019 Oct 3;553:71-82. Epub 2019 Jun 3.

Fibre and Particle Engineering Research Unit, University of Oulu, P.O. Box 4300, FI-90014 Oulu, Finland. Electronic address:

Cellulose nanocrystals (CNCs) are promising bio-derived nanomaterials for the bottom-up fabrication of biomedical constructs. In this report, dicarboxylic acid-functionalized CNC (DCC) was functionalized with arginylglycylaspartic acid (RGD) tripeptide as a motif for improved cell adhesion and targeting. The product (DCC-RGD) self-assembled into a more elongated nanofibrillar structure through lateral and end-to-end association. When added into poly(ethylene imine) (PEI)/pDNA polyplex solution, nanocelluloses interacted electrostatically with positively charged polyplexes without affecting their integrity. The constructs were tested for their potentials as non-viral transfection reagents. Cell viability and transfection efficiency of fibroblast NIH3T3 cells were monitored as a function of CNC concentration where, in general, viability increased as the CNC concentration increased, and transfection efficiency could be optimized. Using wild-type MDCK and αV-knockout MDCK cells, the construct was able to provide targeted uptake of polyplexes. The findings have potential applications, for example, cell-selective in vitro or ex vivo transfection of autologous mesenchymal stem cells for cell therapy, or bottom-up design of future innovative biomaterials.
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http://dx.doi.org/10.1016/j.jcis.2019.06.001DOI Listing
October 2019

Rationally Designed PdAuCu Ternary Alloy Nanoparticles for Intrinsically Deactivation-Resistant Ultrafast Plasmonic Hydrogen Sensing.

ACS Sens 2019 05 10;4(5):1424-1432. Epub 2019 May 10.

Department of Physics , Chalmers University of Technology , 412 96 Göteborg , Sweden.

Hydrogen sensors are a prerequisite for the implementation of a hydrogen economy due to the high flammability of hydrogen-air mixtures. They are to comply with the increasingly stringent requirements set by stakeholders, such as the automotive industry and manufacturers of hydrogen safety systems, where sensor deactivation is a severe but widely unaddressed problem. In response, we report intrinsically deactivation-resistant nanoplasmonic hydrogen sensors enabled by a rationally designed ternary PdAuCu alloy nanomaterial, which combines the identified best intrinsic attributes of the constituent binary Pd alloys. This way, we achieve extraordinary hydrogen sensing metrics in synthetic air and poisoning gas background, simulating real application conditions. Specifically, we find a detection limit in the low ppm range, hysteresis-free response over 5 orders of magnitude hydrogen pressure, subsecond response time at room temperature, long-term stability, and, as the key, excellent resistance to deactivating species like carbon monoxide, notably without application of any protective coatings. This constitutes an important step forward for optical hydrogen sensor technology, as it enables application under demanding conditions and provides a blueprint for further material and performance optimization by combining and concerting intrinsic material assets in multicomponent nanoparticles. In a wider context, our findings highlight the potential of rational materials design through alloying of multiple elements for gas sensor development, as well as the potential of engineered metal alloy nanoparticles in nanoplasmonics and catalysis.
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http://dx.doi.org/10.1021/acssensors.9b00610DOI Listing
May 2019

Metal-polymer hybrid nanomaterials for plasmonic ultrafast hydrogen detection.

Nat Mater 2019 05 1;18(5):489-495. Epub 2019 Apr 1.

Department of Physics, Chalmers University of Technology, Göteborg, Sweden.

Hydrogen-air mixtures are highly flammable. Hydrogen sensors are therefore of paramount importance for timely leak detection during handling. However, existing solutions do not meet the stringent performance targets set by stakeholders, while deactivation due to poisoning, for example by carbon monoxide, is a widely unsolved problem. Here we present a plasmonic metal-polymer hybrid nanomaterial concept, where the polymer coating reduces the apparent activation energy for hydrogen transport into and out of the plasmonic nanoparticles, while deactivation resistance is provided via a tailored tandem polymer membrane. In concert with an optimized volume-to-surface ratio of the signal transducer uniquely offered by nanoparticles, this enables subsecond sensor response times. Simultaneously, hydrogen sorption hysteresis is suppressed, sensor limit of detection is enhanced, and sensor operation in demanding chemical environments is enabled, without signs of long-term deactivation. In a wider perspective, our work suggests strategies for next-generation optical gas sensors with functionalities optimized by hybrid material engineering.
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http://dx.doi.org/10.1038/s41563-019-0325-4DOI Listing
May 2019

Direct In Situ TEM Visualization and Insight into the Facet-Dependent Sintering Behaviors of Gold on TiO.

Angew Chem Int Ed Engl 2018 Dec 21;57(51):16827-16831. Epub 2018 Nov 21.

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China.

Preventing sintering of supported nanocatalysts is an important issue in nanocatalysis. A feasible way is to choose a suitable support. However, whether the metal-support interactions promote or prevent the sintering has not been fully identified. Now, completely different sintering behaviors of Au nanoparticles on distinct anatase TiO surfaces have been determined by in situ TEM. The full in situ sintering processes of Au nanoparticles were visualized on TiO (101) surface, which coupled the Ostwald ripening and particle migration coalescence. In contrast, no sintering of Au on TiO anatase (001) surface was observed under the same conditions. This facet-dependent sintering mechanism is fully explained by the density function theory calculations. This work not only offers direct evidence of the important role of supports in the sintering process, but also provides insightful information for the design of sintering-resistant nanocatalysts.
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http://dx.doi.org/10.1002/anie.201811933DOI Listing
December 2018

2D Transition Metal Carbides (MXenes) for Carbon Capture.

Adv Mater 2019 Jan 4;31(2):e1805472. Epub 2018 Nov 4.

Thin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83, Linköping, Sweden.

Global warming caused by burning of fossil fuels is indisputably one of mankind's greatest challenges in the 21st century. To reduce the ever-increasing CO emissions released into the atmosphere, dry solid adsorbents with large surface-to-volume ratio such as carbonaceous materials, zeolites, and metal-organic frameworks have emerged as promising material candidates for capturing CO . However, challenges remain because of limited CO /N selectivity and long-term stability. The effective adsorption of CO gas (≈12 mol kg ) on individual sheets of 2D transition metal carbides (referred to as MXenes) is reported here. It is shown that exposure to N gas results in no adsorption, consistent with first-principles calculations. The adsorption efficiency combined with the CO /N selectivity, together with a chemical and thermal stability, identifies the archetype Ti C MXene as a new material for carbon capture (CC) applications.
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http://dx.doi.org/10.1002/adma.201805472DOI Listing
January 2019

Accuracy of surface strain measurements from transmission electron microscopy images of nanoparticles.

Adv Struct Chem Imaging 2017 25;3(1):14. Epub 2017 Oct 25.

Department of Physics, Technical University of Denmark, Fysikvej, Building 311, 2800 Kongens Lyngby, Denmark.

Strain analysis from high-resolution transmission electron microscopy (HRTEM) images offers a convenient tool for measuring strain in materials at the atomic scale. In this paper we present a theoretical study of the precision and accuracy of surface strain measurements directly from aberration-corrected HRTEM images. We examine the influence of defocus, crystal tilt and noise, and find that absolute errors of at least 1-2% strain should be expected. The model structures include surface relaxations determined using molecular dynamics, and we show that this is important for correctly evaluating the errors introduced by image aberrations.
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http://dx.doi.org/10.1186/s40679-017-0047-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5656738PMC
October 2017

Grain boundary mediated hydriding phase transformations in individual polycrystalline metal nanoparticles.

Nat Commun 2017 10 20;8(1):1084. Epub 2017 Oct 20.

Department of Physics, Chalmers University of Technology, Göteborg, 412 96, Sweden.

Grain boundaries separate crystallites in solids and influence material properties, as widely documented for bulk materials. In nanomaterials, however, investigations of grain boundaries are very challenging and just beginning. Here, we report the systematic mapping of the role of grain boundaries in the hydrogenation phase transformation in individual Pd nanoparticles. Employing multichannel single-particle plasmonic nanospectroscopy, we observe large variation in particle-specific hydride-formation pressure, which is absent in hydride decomposition. Transmission Kikuchi diffraction suggests direct correlation between length and type of grain boundaries and hydride-formation pressure. This correlation is consistent with tensile lattice strain induced by hydrogen localized near grain boundaries as the dominant factor controlling the phase transition during hydrogen absorption. In contrast, such correlation is absent for hydride decomposition, suggesting a different phase-transition pathway. In a wider context, our experimental setup represents a powerful platform to unravel microstructure-function correlations at the individual-nanoparticle level.
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http://dx.doi.org/10.1038/s41467-017-00879-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5651804PMC
October 2017

Direct observation of nanowire growth and decomposition.

Sci Rep 2017 09 26;7(1):12310. Epub 2017 Sep 26.

Department of Applied Physics, Aalto University School of Science, Puumiehenkuja 2, 00076, Aalto, Finland.

Fundamental concepts of the crystal formation suggest that the growth and decomposition are determined by simultaneous embedding and removal of the atoms. Apparently, by changing the crystal formation conditions one can switch the regimes from the growth to decomposition. To the best of our knowledge, so far this has been only postulated, but never observed at the atomic level. By means of in situ environmental transmission electron microscopy we monitored and examined the atomic layer transformation at the conditions of the crystal growth and its decomposition using CuO nanowires selected as a model object. The atomic layer growth/decomposition was studied by varying an O partial pressure. Three distinct regimes of the atomic layer evolution were experimentally observed: growth, transition and decomposition. The transition regime, at which atomic layer growth/decomposition switch takes place, is characterised by random nucleation of the atomic layers on the growing {111} surface. The decomposition starts on the side of the nanowire by removing the atomic layers without altering the overall crystal structure, which besides the fundamental importance offers new possibilities for the nanowire manipulation. Understanding of the crystal growth kinetics and nucleation at the atomic level is essential for the precise control of 1D crystal formation.
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http://dx.doi.org/10.1038/s41598-017-12381-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5615043PMC
September 2017

Growth Termination and Multiple Nucleation of Single-Wall Carbon Nanotubes Evidenced by in Situ Transmission Electron Microscopy.

ACS Nano 2017 05 17;11(5):4483-4493. Epub 2017 Apr 17.

Center for Electron Nanoscopy, Technical University of Denmark , Fysikvej 307, 2800, Kgs. Lyngby, Denmark.

In order to controllably grow single-wall carbon nanotubes (SWCNTs), a better understanding of the growth processes and how they are influenced by external parameters such as catalyst and gaseous environment is required. Here, we present direct evidence of growth termination of individual SWCNTs and successive growth of additional SWCNTs on Co catalyst particles supported on MgO by means of environmental transmission electron microscopy. Such in situ observations reveal the plethora of solid carbon formations at the local scale while it is happening and thereby elucidate the multitude of configurations resulting from identical external synthesis conditions, which should be considered in the quest for controlled SWCNT growth. Using CO and a mixture of CO and H as carbon sources, we show that the growth of SWCNTs terminates with a reduced tube-catalyst adhesion strength. Two main reasons for the cessation are proposed: insufficient active carbon species and a certain amount of stress exerted at the tube-catalyst interface. Interestingly, it was observed that catalyst particles stayed active in terms of nucleating additional solid carbon structures after growth termination of the first SWCNT. These observations elucidate the importance of an in-depth understanding of the role of catalysts and carbon sources in the continued growth of SWCNTs. Furthermore, it serves as a guide for further control of carbon nanostructure synthesis via catalyst engineering and synthesis optimization.
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http://dx.doi.org/10.1021/acsnano.6b05941DOI Listing
May 2017

Stability of a Bifunctional Cu-Based [email protected] Shell Catalyst for Dimethyl Ether Synthesis Under Redox Conditions Studied by Environmental Transmission Electron Microscopy and In Situ X-Ray Ptychography.

Microsc Microanal 2017 06 5;23(3):501-512. Epub 2017 Apr 5.

1Institute for Chemical Technology and Polymer Chemistry,Karlsruhe Institute of Technology,76131 Karlsruhe,Germany.

When using bifunctional [email protected] catalysts, the stability of both the shell and core-shell interface is crucial for catalytic applications. In the present study, we elucidate the stability of a CuO/ZnO/[email protected] [email protected] material, used for one-stage synthesis of dimethyl ether from synthesis gas. The catalyst stability was studied in a hierarchical manner by complementary environmental transmission electron microscopy (ETEM), scanning electron microscopy (SEM) and in situ hard X-ray ptychography with a specially designed in situ cell. Both reductive activation and reoxidation were applied. The core-shell interface was found to be stable during reducing and oxidizing treatment at 250°C as observed by ETEM and in situ X-ray ptychography, although strong changes occurred in the core on a 10 nm scale due to the reduction of copper oxide to metallic copper particles. At 350°C, in situ X-ray ptychography indicated the occurrence of structural changes also on the µm scale, i.e. the core material and parts of the shell undergo restructuring. Nevertheless, the crucial core-shell interface required for full bifunctionality appeared to remain stable. This study demonstrates the potential of these correlative in situ microscopy techniques for hierarchically designed catalysts.
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http://dx.doi.org/10.1017/S1431927617000332DOI Listing
June 2017

Correction to: Accuracy of surface strain measurements from transmission electron microscopy images of nanoparticles.

Adv Struct Chem Imaging 2017 7;3(1):16. Epub 2017 Nov 7.

1Department of Physics, Technical University of Denmark, Fysikvej, Building 311, 2800 Kongens Lyngby, Denmark.

[This corrects the article DOI: 10.1186/s40679-017-0047-0.].
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http://dx.doi.org/10.1186/s40679-017-0049-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5676803PMC
November 2017

Current status and future directions for in situ transmission electron microscopy.

Ultramicroscopy 2016 11 6;170:86-95. Epub 2016 Aug 6.

Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899-6203, USA. Electronic address:

This review article discusses the current and future possibilities for the application of in situ transmission electron microscopy to reveal synthesis pathways and functional mechanisms in complex and nanoscale materials. The findings of a group of scientists, representing academia, government labs and private sector entities (predominantly commercial vendors) during a workshop, held at the Center for Nanoscale Science and Technology- National Institute of Science and Technology (CNST-NIST), are discussed. We provide a comprehensive review of the scientific needs and future instrument and technique developments required to meet them.
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http://dx.doi.org/10.1016/j.ultramic.2016.08.007DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5100813PMC
November 2016

In Situ Ptychography of Heterogeneous Catalysts using Hard X-Rays: High Resolution Imaging at Ambient Pressure and Elevated Temperature.

Microsc Microanal 2016 Feb;22(1):178-88

1Institute for Chemical Technology and Polymer Chemistry,Karlsruhe Institute of Technology,76131 Karlsruhe,Germany.

A new closed cell is presented for in situ X-ray ptychography which allows studies under gas flow and at elevated temperature. In order to gain complementary information by transmission and scanning electron microscopy, the cell makes use of a Protochips E-chipTM which contains a small, thin electron transparent window and allows heating. Two gold-based systems, 50 nm gold particles and nanoporous gold as a relevant catalyst sample, were used for studying the feasibility of the cell. Measurements showing a resolution around 40 nm have been achieved under a flow of synthetic air and during heating up to temperatures of 933 K. An elevated temperature exhibited little influence on image quality and resolution. With this study, the potential of in situ hard X-ray ptychography for investigating annealing processes of real catalyst samples is demonstrated. Furthermore, the possibility to use the same sample holder for ex situ electron microscopy before and after the in situ study underlines the unique possibilities available with this combination of electron microscopy and X-ray microscopy on the same sample.
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http://dx.doi.org/10.1017/S1431927615015573DOI Listing
February 2016

Stable amorphous georgeite as a precursor to a high-activity catalyst.

Nature 2016 Mar 15;531(7592):83-7. Epub 2016 Feb 15.

Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK.

Copper and zinc form an important group of hydroxycarbonate minerals that include zincian malachite, aurichalcite, rosasite and the exceptionally rare and unstable--and hence little known and largely ignored--georgeite. The first three of these minerals are widely used as catalyst precursors for the industrially important methanol-synthesis and low-temperature water-gas shift (LTS) reactions, with the choice of precursor phase strongly influencing the activity of the final catalyst. The preferred phase is usually zincian malachite. This is prepared by a co-precipitation method that involves the transient formation of georgeite; with few exceptions it uses sodium carbonate as the carbonate source, but this also introduces sodium ions--a potential catalyst poison. Here we show that supercritical antisolvent (SAS) precipitation using carbon dioxide (refs 13, 14), a process that exploits the high diffusion rates and solvation power of supercritical carbon dioxide to rapidly expand and supersaturate solutions, can be used to prepare copper/zinc hydroxycarbonate precursors with low sodium content. These include stable georgeite, which we find to be a precursor to highly active methanol-synthesis and superior LTS catalysts. Our findings highlight the value of advanced synthesis methods in accessing unusual mineral phases, and show that there is room for exploring improvements to established industrial catalysts.
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http://dx.doi.org/10.1038/nature16935DOI Listing
March 2016

Bottom-Up Nanofabrication of Supported Noble Metal Alloy Nanoparticle Arrays for Plasmonics.

ACS Nano 2016 Feb 5;10(2):2871-9. Epub 2016 Feb 5.

Department of Physics, Chalmers University of Technology , 412 96 Göteborg, Sweden.

Mixing different elements at the nanoscale to obtain alloy nanostructures with fine-tuned physical and chemical properties offers appealing opportunities for nanotechnology and nanoscience. However, despite widespread successful application of alloy nanoparticles made by colloidal synthesis in heterogeneous catalysis, nanoalloy systems have been used very rarely in solid-state devices and nanoplasmonics-related applications. One reason is that such applications require integration in arrays on a surface with compelling demands on nanoparticle arrangement, uniformity in surface coverage, and optimization of the surface density. These cannot be fulfilled even using state-of-the-art self-assembly strategies of colloids. As a solution, we present here a generic bottom-up nanolithography-compatible fabrication approach for large-area arrays of alloy nanoparticles on surfaces. To illustrate the concept, we focus on Au-based binary and ternary alloy systems with Ag, Cu, and Pd, due to their high relevance for nanoplasmonics and complete miscibility, and characterize their optical properties. Moreover, as an example for the relevance of the obtained materials for integration in devices, we demonstrate the superior and hysteresis-free plasmonic hydrogen-sensing performance of the AuPd alloy nanoparticle system.
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http://dx.doi.org/10.1021/acsnano.5b08057DOI Listing
February 2016

Radial Nanowire Light-Emitting Diodes in the (AlxGa1-x)yIn1-yP Material System.

Nano Lett 2016 Jan 31;16(1):656-62. Epub 2015 Dec 31.

Solid State Physics and NanoLund, Lund University , Box 118, SE-221 00, Lund, Sweden.

Nanowires have the potential to play an important role for next-generation light-emitting diodes. In this work, we present a growth scheme for radial nanowire quantum-well structures in the AlGaInP material system using a GaInP nanowire core as a template for radial growth with GaInP as the active layer for emission and AlGaInP as charge carrier barriers. The different layers were analyzed by X-ray diffraction to ensure lattice-matched radial structures. Furthermore, we evaluated the material composition and heterojunction interface sharpness by scanning transmission electron microscopy energy dispersive X-ray spectroscopy. The electro-optical properties were investigated by injection luminescence measurements. The presented results can be a valuable track toward radial nanowire light-emitting diodes in the AlGaInP material system in the red/orange/yellow color spectrum.
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http://dx.doi.org/10.1021/acs.nanolett.5b04401DOI Listing
January 2016

Hysteresis-free nanoplasmonic Pd-Au alloy hydrogen sensors.

Nano Lett 2015 May 30;15(5):3563-70. Epub 2015 Apr 30.

†Department of Applied Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden.

The recent market introduction of hydrogen fuel cell cars and the prospect of a hydrogen economy have drastically accelerated the need for safe and accurate detection of hydrogen. In this Letter, we investigate the use of arrays of nanofabricated Pd-Au alloy nanoparticles as plasmonic optical hydrogen sensors. By increasing the amount of Au in the alloy nanoparticles up to 25 atom %, we are able to suppress the hysteresis between hydrogen absorption and desorption, thereby increasing the sensor accuracy to below 5% throughout the investigated 1 mbar to 1 bar hydrogen pressure range. Furthermore, we observe an 8-fold absolute sensitivity enhancement at low hydrogen pressures compared to sensors made of pure Pd, and an improved sensor response time to below one second within the 0-40 mbar pressure range, that is, below the flammability limit, by engineering the nanoparticle size.
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http://dx.doi.org/10.1021/acs.nanolett.5b01053DOI Listing
May 2015

In Situ Observations during Chemical Vapor Deposition of Hexagonal Boron Nitride on Polycrystalline Copper.

Chem Mater 2014 Nov 20;26(22):6380-6392. Epub 2014 Oct 20.

Department of Engineering, University of Cambridge , Cambridge CB3 0FA, U.K.

Using a combination of complementary in situ X-ray photoelectron spectroscopy and X-ray diffraction, we study the fundamental mechanisms underlying the chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) on polycrystalline Cu. The nucleation and growth of h-BN layers is found to occur isothermally, i.e., at constant elevated temperature, on the Cu surface during exposure to borazine. A Cu lattice expansion during borazine exposure and B precipitation from Cu upon cooling highlight that B is incorporated into the Cu bulk, i.e., that growth is not just surface-mediated. On this basis we suggest that B is taken up in the Cu catalyst while N is not (by relative amounts), indicating element-specific feeding mechanisms including the bulk of the catalyst. We further show that oxygen intercalation readily occurs under as-grown h-BN during ambient air exposure, as is common in further processing, and that this negatively affects the stability of h-BN on the catalyst. For extended air exposure Cu oxidation is observed, and upon re-heating in vacuum an oxygen-mediated disintegration of the h-BN film via volatile boron oxides occurs. Importantly, this disintegration is catalyst mediated, i.e., occurs at the catalyst/h-BN interface and depends on the level of oxygen fed to this interface. In turn, however, deliberate feeding of oxygen during h-BN deposition can positively affect control over film morphology. We discuss the implications of these observations in the context of corrosion protection and relate them to challenges in process integration and heterostructure CVD.
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http://dx.doi.org/10.1021/cm502603nDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4311958PMC
November 2014

Doping GaP Core-Shell Nanowire pn-Junctions: A Study by Off-Axis Electron Holography.

Small 2015 Jun 5;11(22):2687-95. Epub 2015 Feb 5.

Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kgs., Lyngby, Denmark.

The doping process in GaP core-shell nanowire pn-junctions using different precursors is evaluated by mapping the nanowires' electrostatic potential distribution by means of off-axis electron holography. Three precursors, triethyltin (TESn), ditertiarybutylselenide, and silane are investigated for n-type doping of nanowire shells; among them, TESn is shown to be the most efficient precursor. Off-axis electron holography reveals higher electrostatic potentials in the regions of nanowire cores grown by the vapor-liquid-solid (VLS) mechanism (axial growth) than the regions grown parasitically by the vapor-solid (VS) mechanism (radial growth), attributed to different incorporation efficiency between VLS and VS of unintentional p-type carbon doping originating from the trimethylgallium precursor. This study shows that off-axis electron holography of doped nanowires is unique in terms of the ability to map the electrostatic potential and thereby the active dopant distribution with high spatial resolution.
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http://dx.doi.org/10.1002/smll.201403361DOI Listing
June 2015

Structure identification in high-resolution transmission electron microscopic images: an example on graphene.

Microsc Microanal 2014 Dec 12;20(6):1772-81. Epub 2014 Nov 12.

1Department of Applied Mathematics and Computer Science,Technical University of Denmark,Building 324/130,Richard Petersens Plads,2800 Kgs Lyngby,Denmark.

A connection between microscopic structure and macroscopic properties is expected for almost all material systems. High-resolution transmission electron microscopy is a technique offering insight into the atomic structure, but the analysis of large image series can be time consuming. The present work describes a method to automatically estimate the atomic structure in two-dimensional materials. As an example graphene is chosen, in which the positions of the carbon atoms are reconstructed. Lattice parameters are extracted in the frequency domain and an initial atom positioning is estimated. Next, a plausible neighborhood structure is estimated. Finally, atom positions are adjusted by simulation of a Markov random field model, integrating image evidence and the strong geometric prior. A pristine sample with high regularity and a sample with an induced hole are analyzed. False discovery rate-controlled large-scale simultaneous hypothesis testing is used as a statistical framework for interpretation of results. The first sample yields, as expected, a homogeneous distribution of carbon-carbon (C-C) bond lengths. The second sample exhibits regions of shorter C-C bond lengths with a preferred orientation, suggesting either strain in the structure or a buckling of the graphene sheet. The precision of the method is demonstrated on simulated model structures and by its application to multiple exposures of the two graphene samples.
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http://dx.doi.org/10.1017/S1431927614013464DOI Listing
December 2014

In situ study of noncatalytic metal oxide nanowire growth.

Nano Lett 2014 Oct 24;14(10):5810-3. Epub 2014 Sep 24.

Department of Applied Physics, Aalto University School of Science , Puumiehenkuja 2, 00076 Espoo, Finland.

The majority of the nanowire synthesis methods utilize catalyst particles to guide the nanowire geometry. In contrast, catalyst-free methods are attractive for facile fabrication of pure nanowires without the need for catalyst preparation. Nonetheless, how nanowire growth is guided without a catalyst is still widely disputed and unclear. Here, we show that the nanowire growth during metal oxidation is limited by a nucleation of a new layer. On the basis of in situ transmission electron microscope investigations we found that the growth occurs layer by layer at the lowest specific surface energy planes. Atomic layers nucleate at the edge of twin boundary ridges and form a long-range ordering along the twin boundary. We anticipate our study to be a starting point to employ defects for nanowire growth control and consequently shaping the geometry of nanowires in a similar manner as in the catalyst-assisted growth method.
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http://dx.doi.org/10.1021/nl502687sDOI Listing
October 2014

Extremely confined gap surface-plasmon modes excited by electrons.

Nat Commun 2014 Jun 18;5:4125. Epub 2014 Jun 18.

1] Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark [2] Center for Nanostructured Graphene (CNG), Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.

High-spatial and energy resolution electron energy-loss spectroscopy (EELS) can be used for detailed characterization of localized and propagating surface-plasmon excitations in metal nanostructures, giving insight into fundamental physical phenomena and various plasmonic effects. Here, applying EELS to ultra-sharp convex grooves in gold, we directly probe extremely confined gap surface-plasmon (GSP) modes excited by swift electrons in nanometre-wide gaps. We reveal the resonance behaviour associated with the excitation of the antisymmetric GSP mode for extremely small gap widths, down to ~5 nm. We argue that excitation of this mode, featuring very strong absorption, has a crucial role in experimental realizations of non-resonant light absorption by ultra-sharp convex grooves with fabrication-induced asymmetry. The occurrence of the antisymmetric GSP mode along with the fundamental GSP mode exploited in plasmonic waveguides with extreme light confinement is a very important factor that should be taken into account in the design of nanoplasmonic circuits and devices.
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http://dx.doi.org/10.1038/ncomms5125DOI Listing
June 2014

Controlled environment specimen transfer.

Microsc Microanal 2014 Aug 14;20(4):1038-45. Epub 2014 May 14.

1Center for Electron Nanoscopy,Technical University of Denmark,Kgs. Lyngby DK-2800,Denmark.

Specimen transfer under controlled environment conditions, such as temperature, pressure, and gas composition, is necessary to conduct successive complementary in situ characterization of materials sensitive to ambient conditions. The in situ transfer concept is introduced by linking an environmental transmission electron microscope to an in situ X-ray diffractometer through a dedicated transmission electron microscope specimen transfer holder, capable of sealing the specimen in a gaseous environment at elevated temperatures. Two catalyst material systems have been investigated; Cu/ZnO/Al2O3 catalyst for methanol synthesis and a Co/Al2O3 catalyst for Fischer-Tropsch synthesis. Both systems are sensitive to ambient atmosphere as they will oxidize after relatively short air exposure. The Cu/ZnO/Al2O3 catalyst, was reduced in the in situ X-ray diffractometer set-up, and subsequently, successfully transferred in a reactive environment to the environmental transmission electron microscope where further analysis on the local scale were conducted. The Co/Al2O3 catalyst was reduced in the environmental microscope and successfully kept reduced outside the microscope in a reactive environment. The in situ transfer holder facilitates complimentary in situ experiments of the same specimen without changing the specimen state during transfer.
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http://dx.doi.org/10.1017/S1431927614000853DOI Listing
August 2014

Surface-enhanced Raman scattering on aluminum using near infrared and visible excitation.

Chem Commun (Camb) 2014 Apr;50(28):3744-6

Danmarks Tekniske Universitet DTU, Department of Micro- and Nanotechnology, 2800 Kgs. Lyngby, Denmark.

We observed strong surface-enhanced Raman scattering on discontinuous nanostructured aluminum films using 785 nm excitation even though dielectric constants of this metal suggest plasmon supported spectroscopy in the ultraviolet range. The excitation of SERS correlates with plasmon resonances in the 1.3-2.5 eV range identified in electron energy loss spectra.
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http://dx.doi.org/10.1039/c4cc00010bDOI Listing
April 2014

Strong Schottky barrier reduction at Au-catalyst/GaAs-nanowire interfaces by electric dipole formation and Fermi-level unpinning.

Nat Commun 2014 ;5:3221

1] Division of Solid State Physics and The Nanometer Structure Consortium ([email protected]), Lund University, Box 118, SE-221 00 Lund, Sweden [2] Department of Mathematics, Physics and Electrical Engineering, Halmstad University, Box 823, SE-301 18 Halmstad, Sweden.

Nanoscale contacts between metals and semiconductors are critical for further downscaling of electronic and optoelectronic devices. However, realizing nanocontacts poses significant challenges since conventional approaches to achieve ohmic contacts through Schottky barrier suppression are often inadequate. Here we report the realization and characterization of low n-type Schottky barriers (~0.35 eV) formed at epitaxial contacts between Au-In alloy catalytic particles and GaAs-nanowires. In comparison to previous studies, our detailed characterization, employing selective electrical contacts defined by high-precision electron beam lithography, reveals the barrier to occur directly and solely at the abrupt interface between the catalyst and nanowire. We attribute this lowest-to-date-reported Schottky barrier to a reduced density of pinning states (~10(17) m(-2)) and the formation of an electric dipole layer at the epitaxial contacts. The insight into the physical mechanisms behind the observed low-energy Schottky barrier may guide future efforts to engineer abrupt nanoscale electrical contacts with tailored electrical properties.
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http://dx.doi.org/10.1038/ncomms4221DOI Listing
April 2015

Measurements of local chemistry and structure in Ni(O)-YSZ composites during reduction using energy-filtered environmental TEM.

Chem Commun (Camb) 2014 Feb 8;50(15):1808-10. Epub 2014 Jan 8.

Interdisciplinary Centre for Electron Microscopy, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.

Energy-filtered transmission electron microscopy images are acquired during the reduction of a NiO-YSZ composite in H2 up to 600 °C. Temperature-resolved quantitative information about both chemistry and structure is extracted with nm spatial resolution from the data, paving the way for the development of detailed reduction models.
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http://dx.doi.org/10.1039/c3cc46682eDOI Listing
February 2014
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