| 1. |
- Adolph, David, 1971, et al.
(författare)
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Hybrid ZnO/GaN distributed Bragg reflectors grown by plasma-assisted molecular beam epitaxy
- 2016
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 4:8, s. 086106-
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate crack-free ZnO/GaN distributed Bragg reflectors (DBRs) grown by hybrid plasma-assisted molecular beam epitaxy using the same growth chamber for continuous growth of both ZnO and GaN without exposure to air. This is the first time these ZnO/GaN DBRs have been demonstrated. The Bragg reflectors consisted up to 20 periods as shown with cross-sectional transmission electron microscopy. The maximum achieved reflectance was 77% with a 32 nm wide stopband centered at 500 nm. Growth along both (0001) and (000-1) directions was investigated. Low temperaturegrowth as well as two-step low/high-temperature deposition was carried out where the latter method improved the DBR reflectance. Samples grown along the (0001) direction yielded a better surface morphology as revealed by scanningelectron microscopy and atomic force microscopy. Reciprocal space maps showed that ZnO(000-1)/GaN reflectors are relaxed whereas the ZnO(0001)/GaN DBRs are strained. The ability to n-type dope ZnO and GaN makes the ZnO(0001)/GaNDBRs interesting for various optoelectronic cavity structures.
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| 2. |
- Ahlberg, Patrik, et al.
(författare)
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Defect formation in graphene during low-energy ion bombardment
- 2016
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 4:4
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Tidskriftsartikel (refereegranskat)abstract
- This letter reports on a systematic investigation of sputter induced damage in graphene caused by low energy Ar+ ion bombardment. The integral numbers of ions per area (dose) as well as their energies are varied in the range of a few eV's up to 200 eV. The defects in the graphene are correlated to the dose/energy and different mechanisms for the defect formation are presented. The energetic bombardment associated with the conventional sputter deposition process is typically in the investigated energy range. However, during sputter deposition on graphene, the energetic particle bombardment potentially disrupts the crystallinity and consequently deteriorates its properties. One purpose with the present study is therefore to demonstrate the limits and possibilities with sputter deposition of thin films on graphene and to identify energy levels necessary to obtain defect free graphene during the sputter deposition process. Another purpose is to disclose the fundamental mechanisms responsible for defect formation in graphene for the studied energy range.
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| 3. |
- Armakavicius, Nerijus, 1989-, et al.
(författare)
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Electron effective mass in GaN revisited: New insights from terahertz and mid-infrared optical Hall effect
- 2024
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 12:2
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Tidskriftsartikel (refereegranskat)abstract
- Electron effective mass is a fundamental material parameter defining the free charge carrier transport properties, but it is very challenging to be experimentally determined at high temperatures relevant to device operation. In this work, we obtain the electron effective mass parameters in a Si-doped GaN bulk substrate and epitaxial layers from terahertz (THz) and mid-infrared (MIR) optical Hall effect (OHE) measurements in the temperature range of 38-340 K. The OHE data are analyzed using the well-accepted Drude model to account for the free charge carrier contributions. A strong temperature dependence of the electron effective mass parameter in both bulk and epitaxial GaN with values ranging from (0.18 +/- 0.02) m(0) to (0.34 +/- 0.01) m(0) at a low temperature (38 K) and room temperature, respectively, is obtained from the THz OHE analysis. The observed effective mass enhancement with temperature is evaluated and discussed in view of conduction band nonparabolicity, polaron effect, strain, and deviations from the classical Drude behavior. On the other hand, the electron effective mass parameter determined by MIR OHE is found to be temperature independent with a value of (0.200 +/- 0.002) m(0). A possible explanation for the different findings from THz OHE and MIR OHE is proposed. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)
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| 4. |
- Asahina, Shunsuke, et al.
(författare)
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Direct observation and analysis of york-shell materials using low-voltage high-resolution scanning electron microscopy : Nanometal-particles encapsulated in metal-oxide, carbon, and polymer
- 2014
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 2:11, s. 113317-
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Tidskriftsartikel (refereegranskat)abstract
- Nanometal particles show characteristic features in chemical and physical properties depending on their sizes and shapes. For keeping and further enhancing their features, the particles should be protected from coalescence or degradation. One approach is to encapsulate the nanometal particles inside pores with chemically inert or functional materials, such as carbon, polymer, and metal oxides, which contain mesopores to allow permeation of only chemicals not the nanometal particles. Recently developed low-voltage high-resolution scanning electron microscopy was applied to the study of structural, chemical, and electron state of both nanometal particles and encapsulating materials in york-shell materials of Au@C, Ru/Pt@C, Au@TiO2, and Pt@Polymer. Progresses in the following categories were shown for the york-shell materials: (i) resolution of topographic image contrast by secondary electrons, of atomic-number contrast by back-scattered electrons, and of elemental mapping by X-ray energy dispersive spectroscopy; (ii) sample preparation for observing internal structures; and (iii) X-ray spectroscopy such as soft X-ray emission spectroscopy. Transmission electron microscopy was also used for characterization of Au@C. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
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| 5. |
- Cohen-Azarzar, Dana, et al.
(författare)
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Scalable and highly tunable conductive oxide interfaces
- 2023
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Ingår i: APL Materials. - : American Institute of Physics (AIP). - 2166-532X. ; 11:11
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Tidskriftsartikel (refereegranskat)abstract
- Conducting oxide interfaces have attracted considerable attention, motivated by both fundamental science and potential for oxide electronic devices. An important gap for maturing such device technology is scalability and routes to control the electronic properties, which can narrow the device engineering space. Here, we demonstrate and explain the mechanisms of highly tunable conductive oxide interfaces. We synthesized amorphous-crystalline Al2O3/SrTiO3 interfaces using the scalable and industry-compatible atomic layer deposition (ALD) technique. An NH3 plasma pretreatment is employed in the ALD chamber, and its duration is used as a tuning parameter for the electrical properties, where a span of three orders of magnitude in the sheet resistance is observed at room temperature. For the most conductive sample, our results are comparable to the highest carrier density values reported for all-crystalline oxide interfaces prepared with state-of-the-art epitaxial growth techniques, such as pulsed laser deposition. We pinpoint the origin of conductivity to oxygen vacancies caused by the SrTiO3 reduction by the NH3 plasma pretreatment. These results present a simple, scalable, and industry-compatible route for realizing conductive oxide interfaces, with a broad parameter space, offering a versatile and flexible toolkit for oxide device engineering.
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| 6. |
- Dombrovskis, Johanna, 1986, et al.
(författare)
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Optimization of fuel cell membrane electrode assemblies for transition metal ion-chelating ordered mesoporous carbon cathode catalysts
- 2014
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 2:12, s. Art. no. 121102-
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal ion-chelating ordered mesoporous carbon (TM-OMC) materials were recently shown to be efficient polymer electrolyte membrane fuel cell (PEMFC) catalysts. The structure and properties of these catalysts are largely different from conventional catalyst materials, thus rendering membrane electrode assembly (MEA) preparation parameters developed for conventional catalysts not useful for applications of TM-OMC catalysts. This necessitates development of a methodology to incorporate TM-OMC catalysts in the MEA. Here, an efficient method for MEA preparation using TM-OMC catalyst materials for PEMFC is developed including effects of catalyst/ionomer loading and catalyst/ionomer-mixing and application procedures. An optimized protocol for MEA preparation using TM-OMC catalysts is described.
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| 7. |
- Dong, Y. B., et al.
(författare)
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Transfer-free, lithography-free, and micrometer-precision patterning of CVD graphene on SiO 2 toward all-carbon electronics
- 2018
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 6:2
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Tidskriftsartikel (refereegranskat)abstract
- A method of producing large area continuous graphene directly on SiO 2 by chemical vapor deposition is systematically developed. Cu thin film catalysts are sputtered onto the SiO 2 and pre-patterned. During graphene deposition, high temperature induces evaporation and balling of the Cu, and the graphene "lands onto" SiO 2 . Due to the high heating and growth rate, continuous graphene is largely completed before the Cu evaporation and balling. 60 nm is identified as the optimal thickness of the Cu for a successful graphene growth and μm-large feature size in the graphene. An all-carbon device is demonstrated based on this technique.
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| 8. |
- Esbjörnsson Elgh, Björn Neo, 1980, et al.
(författare)
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Controlling morphology, mesoporosity, crystallinity, and photocatalytic activity of ordered mesoporous TiO2 films prepared at low temperature
- 2014
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 2:11, s. Art. no. 113313-
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Tidskriftsartikel (refereegranskat)abstract
- Partly ordered mesoporous titania films with anatase crystallites incorporated into the pore walls were prepared at low temperature by spin-coating a microemulsion-based reaction solution. The effect of relative humidity employed during aging of the prepared films was studied using SEM, TEM, and grazing incidence small angle X-ray scattering to evaluate the mesoscopic order, porosity, and crystallinity of the films. The study shows unambiguously that crystal growth occurs mainly during storage of the films and proceeds at room temperature largely depending on relative humidity. Porosity, pore size, mesoscopic order, crystallinity, and photocatalytic activity of the films increased with relative humidity up to an optimum around 75%.
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| 9. |
- Fenwick, O., et al.
(författare)
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Efficient red electroluminescence from diketopyrrolopyrrole copolymerised with a polyfluorene
- 2013
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 1:3, s. Art. no. 032108-
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Tidskriftsartikel (refereegranskat)abstract
- We report the synthesis, characterization, and device incorporation of copolymers based on a common green-emitting polyfluorene but containing a small proportion of a low energy gap donor-acceptor-donor unit for red emission in photo- and electroluminescence. At just 1%-3% random incorporation, the low-gap unit is not present on all chains, yet we demonstrate that efficient charge and energy transfer can yield electroluminescent devices with 1% quantum efficiency and a color that can be tuned by adjusting the density of low-gap units to achieve primary red (National Television System Committee). The high current density tail off in the efficiency is reduced by replacing the hole-injection layer with a photochemically cross-linked electron-blocking layer.
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| 10. |
- Fernandez, Yuri A. Diaz, 1978, et al.
(författare)
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Research Update: Progress in synthesis of nanoparticle dimers by self-assembly
- 2014
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 2:1
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Tidskriftsartikel (refereegranskat)abstract
- This article highlights recent advances in the controlled self-assembly of nanoparticles to produce dimeric nanoparticle structures. The relevance of this emergent field is discussed in terms of recent applications in plasmonics and chemical catalysis. The concept of bond-valence applied to nanoparticles will be discussed, emphasizing some general approaches that have been successfully used to build these structures. Further, the asymmetric functionalization of nanoparticles surfaces as a path to drive selective aggregation, the use of biomolecules to self-assemble nanoparticles into dimers in solution, and the confinement of aggregates in small cavities are discussed.
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