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Kolhep M, Pantle F, Karlinger M, Wang D, Scherer T, Kübel C, Stutzmann M, Zacharias M. Atomic Layer Deposition and Strain Analysis of Epitaxial GaN-ZnO Core-Shell Nanowires. Nano Lett 2023; 23:6920-6926. [PMID: 37499227 DOI: 10.1021/acs.nanolett.3c01531] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 07/29/2023]
Abstract
We demonstrate the epitaxial coating of GaN NWs with an epitaxial ZnO shell by atomic layer deposition at 300 °C. Scanning transmission electron microscopy proves a sharp and defect-free coherent interface. The strain in the core-shell structure due to the lattice mismatch and different thermal expansion coefficients of GaN and ZnO was analyzed using 4D-STEM strain mapping and Raman spectroscopy and compared to theoretical calculations. The results highlight the outstanding advantages of epitaxial shell growth using atomic layer deposition, e.g., conformal coating and precise thickness control.
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Affiliation(s)
- Maximilian Kolhep
- Laboratory for Nanotechnology, Department of Microsystems Engineering, University of Freiburg, Freiburg, 79110, Germany
| | - Florian Pantle
- Walter Schottky Institut and Physics Department, Technische Universität München, Garching, 85748, Germany
| | - Monika Karlinger
- Walter Schottky Institut and Physics Department, Technische Universität München, Garching, 85748, Germany
| | - Di Wang
- Karlsruhe Nano and Micro Facility (KNMF) and Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, 76344, Germany
| | - Torsten Scherer
- Karlsruhe Nano and Micro Facility (KNMF) and Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, 76344, Germany
| | - Christian Kübel
- Karlsruhe Nano and Micro Facility (KNMF) and Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, 76344, Germany
- Department of Materials and Earth Sciences, Technical University Darmstadt, Darmstadt, 64287, Germany
| | - Martin Stutzmann
- Walter Schottky Institut and Physics Department, Technische Universität München, Garching, 85748, Germany
| | - Margit Zacharias
- Laboratory for Nanotechnology, Department of Microsystems Engineering, University of Freiburg, Freiburg, 79110, Germany
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Rauh F, Pantle F, Stutzmann M. Morphology, Energy Level Alignment, and Charge Transfer at the Protoporphyrin IX-Semiconductor Interface. Langmuir 2023; 39:5095-5106. [PMID: 37010500 DOI: 10.1021/acs.langmuir.3c00085] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/19/2023]
Abstract
The combination of molecular catalysts and semiconductor substrates in hybrid heterogeneous photo- or electrocatalytic devices could yield synergistic effects that result in enhanced activity and long-term stability. The extent of synergy strongly depends on the electronic interactions and energy level alignment between the molecular states and the valence and conduction band of the substrate. These properties of hybrid interfaces are investigated for a model system composed of protoporphyrin IX (PPIX) as a stand-in for molecular catalysts and a variety of semiconductor substrates. Monolayers of PPIX are deposited using Langmuir-Blodgett deposition. Their morphology is studied in dependence of the deposition surface pressure to achieve a high-quality, dense coverage. By making use of ultraviolet-visible spectroscopy and ultraviolet photoelectron spectroscopy, the band alignment is determined by the vacuum level and incorporates an interface dipole of 0.4 eV independent of the substrate. The HOMO, LUMO, and LUMO+1 levels were determined to be at 5.6, 3.7, and 2.7 eV below the vacuum level, respectively. The quenching of PPIX photoluminescence in dependence of the potential gradient between excited state and electron affinity of the semiconductor substrates is overall in good agreement with electron transfer processes occurring at very fast time scales on the order of femtoseconds. Nevertheless, deviations from this model become apparent for narrower band gap semiconductors, which points to an additional relevance of other processes, such as energy transfer. These findings highlight the importance of matching the semiconductor to the molecular catalyst to prevent undesirable deactivation pathways.
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Affiliation(s)
- Felix Rauh
- Walter Schottky Institute and Physics Department, Technical University of Munich, Am Coulombwall 4, 85748 Garching, Germany
| | - Florian Pantle
- Walter Schottky Institute and Physics Department, Technical University of Munich, Am Coulombwall 4, 85748 Garching, Germany
| | - Martin Stutzmann
- Walter Schottky Institute and Physics Department, Technical University of Munich, Am Coulombwall 4, 85748 Garching, Germany
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Pantle F, Becker F, Kraut M, Wörle S, Hoffmann T, Artmeier S, Stutzmann M. Correction: Selective area growth of GaN nanowires and nanofins by molecular beam epitaxy on heteroepitaxial diamond (001) substrates. Nanoscale Adv 2023; 5:2119. [PMID: 36998650 PMCID: PMC10044745 DOI: 10.1039/d3na90031b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/10/2023] [Accepted: 03/10/2023] [Indexed: 06/19/2023]
Abstract
[This corrects the article DOI: 10.1039/D1NA00221J.].
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Affiliation(s)
- Florian Pantle
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Fabian Becker
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Max Kraut
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Simon Wörle
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Theresa Hoffmann
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Sabrina Artmeier
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Martin Stutzmann
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
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Pantle F, Wörle S, Karlinger M, Rauh F, Kraut M, Stutzmann M. Environmental sensitivity of GaN nanofins grown by selective area molecular beam epitaxy. Nanotechnology 2023; 34:175501. [PMID: 36669201 DOI: 10.1088/1361-6528/acb4f6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/31/2022] [Accepted: 01/20/2023] [Indexed: 06/17/2023]
Abstract
Nanostructures exhibit a large surface-to-volume ratio, which makes them sensitive to their ambient conditions. In particular, GaN nanowires and nanofins react to their environment as adsorbates influence their (opto-) electronic properties. Charge transfer between the semiconductor surface and adsorbed species changes the surface band bending of the nanostructures, and the adsorbates can alter the rate of non-radiative recombination in GaN. Despite the importance of these interactions with the ambient environment, the detailed adsorption mechanisms are still not fully understood. In this article, we present a systematic study concerning the environmental sensitivity of the electrical conductivity of GaN nanofins. We identify oxygen- and water-based adsorbates to be responsible for a quenching of the electrical current through GaN nanofins due to an increased surface band bending. Complementary contact potential difference measurements in controlled atmospheres on bulkm- andc-plane GaN reveal additional complexity with regard to water adsorption, for which surface dipoles might play an important role besides an increased surface depletion width. The sensitive reaction of the electrical parameters to the environment and surface condition underlines the necessity of a reproducible pre-treatment and/or surface passivation. The presented results help to further understand the complex adsorption mechanisms at GaN surfaces. Due to the sensitivity of the nanofin conductivity on the environment, such structures could perform well as sensing devices.
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Affiliation(s)
- Florian Pantle
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Simon Wörle
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Monika Karlinger
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Felix Rauh
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Max Kraut
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Martin Stutzmann
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
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Kraut M, Pantle F, Wörle S, Sirotti E, Zeidler A, Eckmann F, Stutzmann M. Influence of environmental conditions and surface treatments on the photoluminescence properties of GaN nanowires and nanofins. Nanotechnology 2021; 32:495703. [PMID: 34399419 DOI: 10.1088/1361-6528/ac1dd1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/20/2021] [Accepted: 08/16/2021] [Indexed: 06/13/2023]
Abstract
Due to their intrinsically large surface-to-volume ratio, nanowires and nanofins interact strongly with their environment. We investigate the role of the main air constituents nitrogen, oxygen and water on the efficiency of radiative recombination in GaN nanostructures as a function of different surface treatments and at temperatures up to 200 °C. Oxygen and water exposures exhibit a complex behavior as they can both act quenching and enhancing on the photoluminescence intensity dependent on the temperature. For oxygen, these characteristics are already observed for low concentrations of below 0.5% in nitrogen. While the photoluminescence intensity changes induced by oxygen occur independently of illumination, the influence of water is light-induced: it evolves within tens of seconds under ultraviolet light exposure and is heavily influenced by the nanostructure pre-treatment. In contrast to observations in dry atmospheres, water prevents a recovery of the photoluminescence intensity in the dark. Combined measurements of the electrical current through GaN nanofins and their photoluminescence intensity reveal the environmental influence on the interaction of non-radiative recombination processes and changes in the surface band bending of the nanostructures. Several investigated solvents show an enhancing effect on the PL intensity increase, peaking in c-hexane with a 26-fold increase after 6 min of light exposure. Stabilization of the PL intensity was achieved by a passivation of the GaN surface with GaxOy, and ZnO shells. Surprisingly, Al2O3coatings resulted in a highly instable PL intensity during the first minutes of illumination. Our findings reveal the high importance of controlled environmental conditions for the investigation of nanostructures, especially when aimed at their applications in the fields of environmental sensing, photo-catalysis and light-emitting diodes.
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Affiliation(s)
- Max Kraut
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Florian Pantle
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Simon Wörle
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Elise Sirotti
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Andreas Zeidler
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Felix Eckmann
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
| | - Martin Stutzmann
- Walter Schottky Institut and Physics Department, Technische Universität München, Am Coulombwall 4, D-85748 Garching, Germany
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Pantle F, Becker F, Kraut M, Wörle S, Hoffmann T, Artmeier S, Stutzmann M. Selective area growth of GaN nanowires and nanofins by molecular beam epitaxy on heteroepitaxial diamond (001) substrates. Nanoscale Adv 2021; 3:3835-3845. [PMID: 36133019 PMCID: PMC9417268 DOI: 10.1039/d1na00221j] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/23/2021] [Accepted: 05/04/2021] [Indexed: 05/12/2023]
Abstract
GaN-on-diamond is a promising route towards reliable high-power transistor devices with outstanding performances due to better heat management, replacing common GaN-on-SiC technologies. Nevertheless, the implementation of GaN-on-diamond remains challenging. In this work, the selective area growth of GaN nanostructures on cost-efficient, large-scale available heteroepitaxial diamond (001) substrates by means of plasma-assisted molecular beam epitaxy is investigated. Additionally, we discuss the influence of an AlN buffer on the morphology of the GaN nanostructures. The nanowires and nanofins are characterized by a very high selectivity and controllable dimensions. Low temperature photoluminescence measurements are used to evaluate their structural quality. The growth of two GaN crystal domains, which are in-plane rotated against each other by 30°, is observed. The favoring of a certain domain is determined by the off-cut direction of the diamond substrates. By X-ray diffraction we show that the GaN nanostructures grow perpendicular to the diamond surface on off-cut diamond (001) substrates, which is in contrast to the growth on diamond (111), where the nanostructures are aligned with the substrate lattice. Polarity-selective wet chemical etching and Kelvin probe force microscopy reveal that the GaN nanostructures grow solely in the Ga-polar direction. This is a major advantage compared to the growth on diamond (111) and enables the application of GaN nanostructures on cost-efficient diamond for high-power/high-frequency applications.
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Affiliation(s)
- Florian Pantle
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Fabian Becker
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Max Kraut
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Simon Wörle
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Theresa Hoffmann
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Sabrina Artmeier
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
| | - Martin Stutzmann
- Walter Schottky Institut and Physics Department, Technische Universität München Am Coulombwall 4 85748 Garching Germany
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Kraut M, Pantle F, Winnerl J, Hetzl M, Eckmann F, Sharp ID, Stutzmann M. Photo-induced selective etching of GaN nanowires in water. Nanoscale 2019; 11:7967-7975. [PMID: 30968077 DOI: 10.1039/c8nr10021g] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
Nanowire (NW) based devices for solar driven artificial photosynthesis have gained increasing interest in recent years due to the intrinsically high surface to volume ratio and the excellent achievable crystal qualities. However, catalytically active surfaces often suffer from insufficient stability under operational conditions. To gain a fundamental understanding of the underlying processes, the photochemical etching behavior of hexagonal and round GaN NWs in deionized water under illumination are investigated. We find that the crystallographic c-plane remains stable, whereas the m-planes are photochemically etched with rates up to 11 nm min-1, depending on the applied UV light intensity. By investigating nanowalls, we achieve control of the exposed crystallographic facets and find an enhanced stability of the a-plane compared to the m-plane. Photo-excited holes, which drift to the side facets due to the upward surface band bending in nominally n-type (not intentionally doped) GaN, are identified as the driving force of the process, which allows the development of concepts for the stabilization of the nanostructures. A geometrically enhanced absorption of periodic NW arrays is correlated with a dependence of the etch rate on the NW pitch and diameter. Further, we find selective photochemical etching of the NW base in the presence of sub-band gap illumination, which is attributed to defect-related absorption in this region. These results provide improved understanding of the roles of inhomogeneous defect distribution, light excitation profiles, and different surface facets on the photochemical stability of nanostructures and provide viable strategies for improving stabilities under light-driven reaction conditions.
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Affiliation(s)
- Max Kraut
- Walter Schottky Institut and Physik-Department, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany.
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Schlipf J, Askar AM, Pantle F, Wiltshire BD, Sura A, Schneider P, Huber L, Shankar K, Müller-Buschbaum P. Top-Down Approaches Towards Single Crystal Perovskite Solar Cells. Sci Rep 2018; 8:4906. [PMID: 29559737 PMCID: PMC5861077 DOI: 10.1038/s41598-018-23211-x] [Citation(s) in RCA: 26] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2017] [Accepted: 03/02/2018] [Indexed: 12/02/2022] Open
Abstract
Solar cells employing hybrid perovskites have proven to be a serious contender versus established thin-film photovoltaic technologies. Typically, current photovoltaic devices are built up layer by layer from a transparent substrate (bottom-up approach), while the deposition of the perovskite layer itself comes with many challenges including the control of crystal size, nucleation density and growth rate. On the other hand, single crystals have been used with great success for studying the fundamental properties of this new class of optoelectronic materials. However, optoelectronic devices fabricated from single crystals often employ different materials than in their thin film counterparts. Here, we demonstrate various top-down approaches for low-temperature processed organic-inorganic metal halide perovskite single crystal devices. Our approach uses common and well-established material combinations that are often used in polycrystalline thin film devices. The use of a polymer bezel allows easier processing of small crystals and the fabrication of solution-processed, free-standing perovskite single crystal devices. All in all these approaches can supplement other measurements of more fundamental material properties often requiring perovskite single crystals by rendering a photovoltaic characterization possible on the very same crystal with comparable material combinations as in thin film devices.
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Affiliation(s)
- Johannes Schlipf
- Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, 85748, Garching, Germany
| | - Abdelrahman M Askar
- Department of Electrical and Computer Engineering, University of Alberta, 9211-116 St, Edmonton, AB T6G 1H9, Canada
| | - Florian Pantle
- Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, 85748, Garching, Germany
| | - Benjamin D Wiltshire
- Department of Electrical and Computer Engineering, University of Alberta, 9211-116 St, Edmonton, AB T6G 1H9, Canada
| | - Anton Sura
- Department of Electrical and Computer Engineering, University of Alberta, 9211-116 St, Edmonton, AB T6G 1H9, Canada
| | - Peter Schneider
- Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, 85748, Garching, Germany
| | - Linus Huber
- Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, 85748, Garching, Germany
| | - Karthik Shankar
- Department of Electrical and Computer Engineering, University of Alberta, 9211-116 St, Edmonton, AB T6G 1H9, Canada
| | - Peter Müller-Buschbaum
- Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, 85748, Garching, Germany.
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