Formation and characterization of three-dimensional tetrahedral MoS2 thin films by chemical vapor deposition. - In: Crystal growth & design, ISSN 1528-7505, Bd. 22 (2022), 9, S. 5229-5238
A method to synthesize the three-dimensional arrangement of bulk tetrahedral MoS2 thin films by solid source chemical vapor deposition of MoO3 and S is presented. The developed synthesizing recipe uses a temperature ramping with a constant N2 gas flow in the deposition process to grow tetrahedral MoS2 thin film layers. The study analyses the time-dependent growth morphologies, and the results are combined and presented in a growth model. A combination of optical, electron, atomic force microscopy, Raman spectroscopy, and X-ray diffraction are used to study the morphological and structural features of the tetrahedral MoS2 thin layers. The grown MoS2 is c-axis oriented 2H-MoS2. Additionally, the synthesized material is further used to fabricate back-gated field-effect transistors (FETs). The fabricated FET devices on the tetrahedral MoS2 show on/off current ratios of 10^6 and mobility up to ∼56 cm^2 V^-1 s^-1 with an estimated carrier concentration of 4 × 10^16 cm-3 for VGS = 0 V.
https://doi.org/10.1021/acs.cgd.2c00333
Temperature dependence of the local electromagnetic field at the Fe site in multiferroic bismuth ferrite. - In: Physical review, ISSN 2469-9969, Bd. 106 (2022), 5, 054416, S. 054416-1-054416-15
In this paper, we present a study of the temperature-dependent characteristics of electromagnetic fields at the atomic scale in multiferroic bismuth ferrite (BiFeO3 or BFO). The study was performed using time differential perturbed angular correlation (TDPAC) spectroscopy on implanted 111In (111Cd) probes over a wide temperature range. The TDPAC spectra show that substitutional 111In on the Fe3+ site experiences local electric polarization, which is otherwise expected to essentially stem from the Bi3+ lone pair electrons. Moreover, the TDPAC spectra show combined electric and magnetic interactions below the Néel temperature TN. This is consistent with simulated spectra. X-ray diffraction (XRD) was employed to investigate how high-temperature TDPAC measurements influence the macroscopic structure and secondary phases. With the support of ab initio DFT simulations, we can discuss the probe nucleus site assignment and can conclude that the 111In (111Cd) probe substitutes the Fe atom at the B site of the perovskite structure.
https://doi.org/10.1103/PhysRevB.106.054416
Confirming the unusual temperature dependence of the electric-field gradient in Zn. - In: Crystals, ISSN 2073-4352, Bd. 12 (2022), 8, 1064, S. 1-8
The electric-field gradient (EFG) at nuclei in solids is a sensitive probe of the charge distribution. Experimental data, which previously only existed in insulators, have been available for metals with the development of nuclear measuring techniques since about 1970. An early, systematic investigation of the temperature dependence of the EFG in metals, originally based on results for Cd, but then also extended to various other systems, has suggested a proportionality to T3/2. However, later measurements in the structurally and electronically similar material Zn, which demonstrated much more complex behavior, were largely ignored at the time. The present experimental effort has confirmed the reliability of this unexpected behavior, which was previously unexplained.
https://doi.org/10.3390/cryst12081064
A review on photothermal conversion of solar energy with nanomaterials and nanostructures: from fundamentals to applications. - In: Advanced sustainable systems, ISSN 2366-7486, Bd. 6 (2022), 9, 2200115, S. 1-19
Solar energy is a green, sustainable, and de facto inexhaustible energy source for mankind. The conversion of solar energy into other forms of energy has attracted extensive research interest due to climate change and the energy crisis. Among all the solar energy conversion technologies, photothermal conversion of solar energy exhibits unique advantages when applied for water purification, desalination, high-temperature heterogeneous catalysis, anti-bacterial treatments, and deicing. In this review, the various photothermal conversion mechanisms based on different forms of heat release are summarized and some of the latest examples are presented. In addition, the necessary prerequisites for solar-driven photothermal materials toward their practical applications are also discussed. Further, the latest advances in photothermal conversion of solar energy are discussed, focusing on different types of photothermal applications. Finally, a summary is given and the challenges and opportunities in the photothermal conversion of solar energy are presented. This review aims to give a comprehensive understanding of emerging solar energy conversion technologies based on the photothermal effect, especially by using nanomaterials and nanostructures.
https://doi.org/10.1002/adsu.202200115
Evaluating the optical response of heavily decorated black silicon based on a realistic 3D modeling methodology. - In: ACS applied materials & interfaces, ISSN 1944-8252, Bd. 14 (2022), 31, S. 36189-36199
Combining black silicon (BS), a nanostructured silicon containing highly roughened surface morphology with plasmonic materials, is becoming an attractive approach for greatly enhancing light-matter interactions with promising applications of sensing and light harvesting. However, precisely describing the optical response of a heavily decorated BS structure is still challenging due to the increasing complexity in surface morphology and plasmon hybridization. Here, we propose and fully characterize BS-based multistacked nanostructures with randomly distributed nanoparticles on the highly roughened nonflat surface. We demonstrate a realistic 3D modeling methodology based on parametrized scanning electron microscopy images that provides high-precision morphology details, successfully linking the theoretical analysis with experimental optical response of the complex nanostructures. Far-field calculations very nicely reproduce experimental reflectance spectra, revealing the dependency of light trapping on the thickness of the conformal reflector and the atop nanoparticle size. Near-field analysis clearly identifies three types of stochastic “hotspots”. Their contribution to the overall field enhancement is shown to be very much sensitive to the nanoscale surface morphology. The simulated near-field property is then used to examine the measured surface-enhanced Raman scattering (SERS) response on the multistacked structures. The present modeling approach combined with spectroscopic characterizations is expected to offer a powerful tool for the precise description of the optical response of other large-scale highly disordered realistic 3D systems.
https://doi.org/10.1021/acsami.2c05652
Plasma-assisted fabrication of molecularly imprinted NiAl-LDH layer on Ni nanorod arrays for glyphosate detection. - In: ACS applied materials & interfaces, ISSN 1944-8252, Bd. 14 (2022), 31, S. 35704-35715
An inorganic-framework molecularly imprinted NiAl layered double hydroxide (MI-NiAl-LDH) with specific template molecule (glyphosate pesticide, Glyp) recognition ability was prepared on Ni nanorod arrays (Ni NRAs) through electrodeposition followed by a low-temperature O2 plasma treatment. The freestanding Ni/MI-NiAl-LDH NRA electrode had highly enhanced sensitivity and selectivity. The electrocatalytic oxidation of Glyp was proposed to occur at Ni3+ centers in MI-NiAl-LDH, and the current response depended linearly on the Glyp concentration from 10.0 nmol/L to 1.0 μmol/L (R2 = 0.9906), with the limit of detection (LOD) being 3.1 nmol/L (S/N = 3). An exceptional discriminating capability with tolerance for other similar organophosphorus compounds was achieved. Molecular imprinting (N and P residues) affected the electronic structure of NiAl-LDH, triggering the formation of highly active NiOOH sites at relatively lower anodic potentials and substantially enhancing the electrocatalytic oxidation ability of the NiAl-LDH interface toward the C-N bonds in Glyp. In combination with the surface enrichment effect of MI-NiAl-LDH toward template molecules, the electrochemical oxidation signal intensity of Glyp increased significantly, with a greater peak separation from interfering molecules. These results challenge the common belief that the excellent performance of inorganic-framework molecularly imprinted interfaces arises from their specific adsorption of template molecules, providing new insight into the development of high-performance organic-pollutant-sensing electrodes.
https://doi.org/10.1021/acsami.2c08500
Improving silicon photocathode performance for water reduction through dual interface engineering and integrating ReS2 photocatalyst. - In: ACS applied energy materials, ISSN 2574-0962, Bd. 5 (2022), 7, S. 8222-8231
Photoelectrochemical (PEC) water splitting for H2 production is a possible alternative for fossil energy in the future. However, there exists three problems in PEC water splitting with the silicon (Si) photocathode: poor light absorption of the untreated Si substrate, bad stability in strong acid solution, and poor photocatalytic activity of Si. Here, a strategy of dual interface engineering and photocatalyst deposition is proposed to improve the PEC performance, which consists of fabricating black Si (b-Si) by reactive ion etching, depositing of TiO2 on the b-Si by atomic layer deposition, and growing ReS2 on top of the TiO2 by chemical vapor deposition. Owing to the suitable band alignment of b-Si, TiO2, and ReS2, the ReS2/TiO2/b-Si shows obviously enhanced PEC performance compared to b-Si, TiO2/b-Si, and ReS2/b-Si photocathodes. Results of electrochemical impedance spectroscopy and Mott-Schottky plot analysis demonstrate that the TiO2 layer plays an important role and the charge-transfer kinetics of the system is clearly improved. Transient photocurrent measurements indicate that the ReS2/TiO2/b-Si photocathode has the most remarkable photocurrent response. In addition, the ReS2/TiO2/b-Si photocathode also shows excellent stability after being operated for 25 h.
https://doi.org/10.1021/acsaem.2c00761
Grundlagenuntersuchung zur ultraschallunterstützten Aufbautechnik für Elektronik. - Ilmenau, 2022. - 121 Blätter
Technische Universität Ilmenau, Dissertation 2022
Die Einpresstechnik ist ein gut entwickelter Prozess, um eine elektrische Kontaktierung zwischen Pin und Leiterplatte herzustellen. Ein Vorteil der Einpresstechnik ist das Herstellen der Verbindung ohne weitere Hilfsmittel wie Lotpaste oder Wärme. Nachteile des Verfahrens sind zum Teil hohe Einpresskräfte, die zu Beschädigungen an der Leiterplatte führen können oder die Verwendung von zusätzlichen Beschichtungen, um ausreichend hohe Festigkeiten zu erzielen. Im Rahmen der vorliegenden Arbeit wird ein neuer Prozess, ultraschallunterstützte Einpresstechnik, untersucht. Dabei wird die Leiterplatte während oder nach dem Einpressen mit Ultraschall angeregt. Das verwendete Materialsystem Kupfer- bzw. Aluminium-basierter Pin und Kupfer-Hülse weist keine zusätzliche metallische Beschichtung auf. Es werden verschiedene Einflussfaktoren des Prozesses auf Einpresskraft, Festigkeit und Qualität der Verbindungen untersucht. Der Prozess wird weiterentwickelt, um eine Reproduzierbarkeit der Ergebnisse zu gewährleisten und die Prozessparameter werden optimiert. Weiterhin wird eine neue geeignete Pingeometrie entwickelt und ihr Verhalten mit einer flexiblen Pingeometrie verglichen. Der Wirkmechanismus des Ultraschalls wird anhand der Verformung der Fügezone und Veränderungen im Werkstoff erläutert. Zu weiteren Erklärung werden statistische und thermische Modelle aufgestellt. Damit ist eine Voraussage der Auspresskraft und der Temperatur in der Fügezone möglich.
Anisotropy of the electric field gradient in two-dimensional α-MoO3 investigated by 57Mn(57Fe) emission Mössbauer spectroscopy. - In: Crystals, ISSN 2073-4352, Bd. 12 (2022), 7, 942, S. 1-13
Van der Waals α-MoO3 samples offer a wide range of attractive catalytic, electronic, and optical properties. We present herein an emission Mössbauer spectroscopy (eMS) study of the electric-field gradient (EFG) anisotropy in crystalline free-standing α-MoO3 samples. Although α-MoO3 is a two-dimensional (2D) material, scanning electron microscopy shows that the crystals are 0.5-5-µm thick. The combination of X-ray diffraction and micro-Raman spectroscopy, performed after sample preparation, provided evidence of the phase purity and crystal quality of the samples. The eMS measurements were conducted following the implantation of 57Mn (t1/2 = 1.5 min), which decays to the 57Fe, 14.4 keV Mössbauer state. The eMS spectra of the samples are dominated by a paramagnetic doublet (D1) with an angular dependence, pointing to the Fe2+ probe ions being in a crystalline environment. It is attributed to an asymmetric EFG at the eMS probe site originating from strong in-plane covalent bonds and weak out-of-plane van der Waals interactions in the 2D material. Moreover, a second broad component, D2, can be assigned to Fe3+ defects that are dynamically generated during the online measurements. The results are compared to ab initio simulations and are discussed in terms of the in-plane and out-of-plane interactions in the system.
https://doi.org/10.3390/cryst12070942
Compositional dependence of epitaxial L10-Mnx Ga magnetic properties as probed by 57Mn/Fe and 119In/Sn emission Mössbauer spectroscopy. - In: Physica status solidi, ISSN 1521-3951, Bd. 259 (2022), 7, 2200121, S. 1-11
The magnetic properties of Mn x Ga alloys critically depend on composition x, and the atomic-scale origin of those dependences is still not fully disclosed. Molecular beam epitaxy has been used to produce a set of Mn x Ga samples (x = 0.7 ÷ 1.9) with strong perpendicular magnetic anisotropy, and controllable saturation magnetization and coercive field depending on x. By conducting 57Mn/Fe and 119In/Sn emission Mössbauer spectroscopy at ISOLDE/CERN, the Mn and Ga site-specific chemical, structural, and magnetic properties of Mn x Ga are investigated as a function of x, and correlated with the magnetic properties as measured by superconducting quantum interference device magnetometry. Hyperfine magnetic fields of Mn/Fe (either at Mn or Ga sites) are found to be greatly influenced by the local strain induced by the implantation. However, In/Sn probes show clear angular dependence, demonstrating a huge transferred dipolar hyperfine field to the Ga sites. A clear increase of the occupancy of Ga lattice sites by Mn for x > 1 is observed, and identified as the origin for the increased antiferromagnetic coupling between Mn and Mn at Ga sites that lowers the samples' magnetization. The results shed further light on the atomic-scale mechanisms driving the compositional dependence of magnetism in Mn x Ga.
https://doi.org/10.1002/pssb.202200121