Nanoscale surface morphology modulation of graphene - i-SiC heterostructures. - In: Materials today, ISSN 2214-7853, Bd. 53 (2022), 2, S. 289-292
A multitude gratings design consists of gratings with different pitches ranging from the micrometre down to sub 40 nm scale combined with sub 10 nm step heights modulating the surface morphology for length scale measurements is proposed. The surface morphology modulation was performed using electron beam lithography incorporating a standard semiconductor processing technology. The critical dimension, edge roughness, step heights and line morphology in dependence on the grating pitch is studied.
https://doi.org/10.1016/j.matpr.2021.06.427
Localized and programmable material transport and deposition by corona discharge. - Ilmenau : Universitätsbibliothek, 2022. - 1 Online-Ressource (xiii, 133, XXIX Seiten)
Technische Universität Ilmenau, Dissertation 2022
Der Transport von Materialien und Gütern bestimmt unseren Alltag. Materialtransport passiert in globalen Logistikunternehmen, in Fertigungslinien, in alltäglichen Situationen wie dem Einkaufen und selbst in unserem menschlichen Körper. Zu jedem Zeitpunkt werden Materialien von A nach B transportiert, ohne dass uns dies in vielen Situationen bewusst wird. Mit dem Unterschreiten von Größenordnungen im Mikro- und Nanometerbereich wird ein gezielter Materialtransport zu einer Herausforderung, da eine manuelle Manipulation nicht mehr möglich ist. Dennoch wäre es wünschenswert, wenn wir gezielt Material an einen gewünschten Ort transportieren und dort abscheiden könnten, denn diese Lokalisierung wird in vielen Anwendungsfeldern benötigt, um z.B. physikalische Eigenschaften auszunutzen oder chemische oder biologische Reaktionen auszulösen. Daher soll in dieser Arbeit eine Methode zum lokalisierten Materialtransport im Mikro- und Nanometerbereich vorgestellt werden. Diese Methode basiert auf elektrischen Kräften. Einfach ausgedrückt, laden wir ein beliebiges Material elektrisch auf und können es dann in einem elektrischen Feld manipulieren. Negativ geladenes Material wird von negativ geladenen Flächen abgestoßen und von positiv geladenen Flächen angezogen. So lässt sich ein gerichteter Materialtransport erzeugen. Das Material wird mithilfe einer negativen DC Corona-Entladung aufgeladen. Durch die Strukturierung von isolierenden Flächen auf leitfähigen Substraten lassen sich sogenannte elektrodynamische Trichter erzeugen, die das geladene Material zu den gewünschten Orten führen, es dort konzentrieren und abscheiden. Die Nutzung von elektrischen Kräften ermöglicht zudem eine Programmierbarkeit, da durch das Ändern der elektrischen Felder oder durch An- und Ausschalten von Elektroden die Position der Abscheidung, die Abscheideart oder die Abscheiderate geändert werden kann. Die Nutzung der Corona-Entladung ermöglicht eine hohe Freiheit bei der Materialwahl. Die Abscheidung von Metallen, Halbleitern, Isolatoren und biologischen Materialien in einem Größenbereich von Mikropartikeln bis hin zu einzelnen Molekülen wird gezeigt. Die Anwendungsgebiete der vorgestellten Methode sind vielfältig: Sie reichen von Luftüberwachung, Nanodrahtwachstum, Gassensorik bis hin zur Kristallzucht. Die experimentellen Ergebnisse werden mit elektrischen, optischen und materialspezifischen Analysen verifiziert. Darüber hinaus werden Simulationen durchgeführt, um die Art der Lokalisierung zu demonstrieren.
https://doi.org/10.22032/dbt.51508
Ferrimagnetic large single domain iron oxide nanoparticles for hyperthermia applications. - In: Nanomaterials, ISSN 2079-4991, Bd. 12 (2022), 3, 343, S. 1-12
This paper describes the preparation and obtained magnetic properties of large single domain iron oxide nanoparticles. Such ferrimagnetic particles are particularly interesting for diagnostic and therapeutic applications in medicine or (bio)technology. The particles were prepared by a modified oxidation method of non-magnetic precursors following the green rust synthesis and characterized regarding their structural and magnetic properties. For increasing preparation temperatures (5 to 85 ˚C), an increasing particle size in the range of 30 to 60 nm is observed. Magnetic measurements confirm a single domain ferrimagnetic behavior with a mean saturation magnetization of ca. 90 Am2/kg and a size-dependent coercivity in the range of 6 to 15 kA/m. The samples show a specific absorption rate (SAR) of up to 600 W/g, which is promising for magnetic hyperthermia application. For particle preparation temperatures above 45 ˚C, a non-magnetic impurity phase occurs besides the magnetic iron oxides that results in a reduced net saturation magnetization.
https://doi.org/10.3390/nano12030343
Influence of initial temperature and convective heat loss on the self-propagating reaction in Al/Ni multilayer foils. - In: Materials, ISSN 1996-1944, Bd. 14 (2021), 24, 7815, insges. 15 S.
A two-dimensional numerical model for self-propagating reactions in Al/Ni multilayer foils was developed. It was used to study thermal properties, convective heat loss, and the effect of initial temperature on the self-propagating reaction in Al/Ni multilayer foils. For model adjustments by experimental results, these Al/Ni multilayer foils were fabricated by the magnetron sputtering technique with a 1:1 atomic ratio. Heat of reaction of the fabricated foils was determined employing Differential Scanning Calorimetry (DSC). Self-propagating reaction was initiated by an electrical spark on the surface of the foils. The movement of the reaction front was recorded with a high-speed camera. Activation energy is fitted with these velocity data from the high-speed camera to adjust the numerical model. Calculated reaction front temperature of the self-propagating reaction was compared with the temperature obtained by time-resolved pyrometer measurements. X-ray diffraction results confirmed that all reactants reacted and formed a B2 NiAl phase. Finally, it is predicted that (1) increasing thermal conductivity of the final product increases the reaction front velocity; (2) effect of heat convection losses on reaction characteristics is insignificant, e.g., the foils can maintain their characteristics in water; and (3) with increasing initial temperature of the foils, the reaction front velocity and the reaction temperature increased.
https://doi.org/10.3390/ma14247815
Chemoheteroepitaxy of 3C-SiC(111) on Si(111): influence of predeposited Ge on structure and composition. - In: Physica status solidi, ISSN 1862-6319, Bd. 218 (2021), 24, 2100399, S. 1-10
Secondary ion mass spectroscopy, Fourier transformed infrared spectroscopy, ellipsometry, reflection high energy diffraction and transmission electron microscopy are used to gain inside into the effect of Ge on the formation of ultrathin 3C-SiC layers on Si(111) substrates. Accompanying the experimental investigations with simulations it is found that the ultrathin single crystalline 3C-SiC layer is formed on top of a gradient Si1-x-yGexCy buffer layer due to a complex alloying and alloy decomposition processes promoted by carbon and germanium interdiffusion and SiC nucleation. This approach allows tuning residual stress at very early growth stages as well as the interface properties of the 3C-SiC/Si heterostructure. Useful yields of secondary ions of Ge in Si matrix and Si dimer are estimated.
https://doi.org/10.1002/pssa.202100399
Photoluminescence kinetics of dark and bright excitons in atomically thin MoS2. - In: Physica status solidi, ISSN 1862-6270, Bd. 15 (2021), 10, 2100263, insges. 14 S.
The fine structure of the exciton spectrum, containing optically allowed (bright) and forbidden (dark) exciton states, determines the radiation efficiency in nanostructures. Time-resolved microphotoluminescence in MoS2 monolayers (MLs) and bilayers (BLs), both unstrained and compressively strained, in a wide temperature range (10-300 K), is studied to distinguish between exciton states optically allowed and forbidden, both in spin and in momentum, as well as to estimate their characteristic decay times and contributions to the total radiation intensity. The decay times are found to either increase or decrease with increasing temperature, indicating the lowest bright or lowest dark state, respectively. The results unambiguously show that, in an unstrained ML, the spin-allowed state is the lowest for a series of A excitons (1.9 eV), with the dark state being <2 meV higher, and that the splitting energy can increase several times at compression. In contrast, in the indirect exciton series in BLs (1.5 eV), the spin-forbidden state is the lowest, being about 3 meV below the bright one. The strong effect of strain on the exciton spectrum can explain the large scatter among the published data and must be taken into account to realize the desired optical properties of 2D MoS2.
https://doi.org/10.1002/pssr.202100263
Three-dimensional platinum nanoparticle-based bridges for ammonia gas sensing. - In: Scientific reports, ISSN 2045-2322, Bd. 11 (2021), 12551, S. 1-9
This study demonstrates the fabrication of self-aligning three-dimensional (3D) platinum bridges for ammonia gas sensing using gas-phase electrodeposition. This deposition scheme can guide charged nanoparticles to predetermined locations on a surface with sub-micrometer resolution. A shutter-free deposition is possible, preventing the use of additional steps for lift-off and improving material yield. This method uses a spark discharge-based platinum nanoparticle source in combination with sequentially biased surface electrodes and charged photoresist patterns on a glass substrate. In this way, the parallel growth of multiple sensing nodes, in this case 3D self-aligning nanoparticle-based bridges, is accomplished. An array containing 360 locally grown bridges made out of 5 nm platinum nanoparticles is fabricated. The high surface-to-volume ratio of the 3D bridge morphology enables fast response and room temperature operated sensing capabilities. The bridges are preconditioned for ˜ 24 h in nitrogen gas before being used for performance testing, ensuring drift-free sensor performance. In this study, platinum bridges are demonstrated to detect ammonia (NH3) with concentrations between 1400 and 100 ppm. The sensing mechanism, response times, cross-sensitivity, selectivity, and sensor stability are discussed. The device showed a sensor response of ˜ 4% at 100 ppm NH3 with a 70% response time of 8 min at room temperature.
https://doi.org/10.1038/s41598-021-91975-w
Efficient fabrication of MoS2 nanocomposites by water-assisted exfoliation for nonvolatile memories. - In: Green chemistry, ISSN 1463-9270, Bd. 23 (2021), 10, S. 3642-3648
Efficient and green exfoliation of bulk MoS2 into few-layered nanosheets in the semiconducting hexagonal phase (2H-phase) remains a great challenge. Here, we developed a new method, water-assisted exfoliation (WAE), for the scalable synthesis of carboxylated chitosan (CC)/2H-MoS2 nanocomposites. With facile hand grinding of the CC powder, bulk MoS2 and water followed by conventional liquid-phase exfoliation in water, this method can not only efficiently exfoliate the 2H-MoS2 nanosheets, but also produce two-dimensional (2D) CC/2H-MoS2 nanocomposites. Interestingly, the intercalated CC in MoS2 nanosheets increases the interlayer spacing of 2H-MoS2 to serve as good candidates for the semiconductor devices. 2D CC/2H-MoS2 nanocomposites show superior electronic rectification effects in nonvolatile write-once-read-many-times memory (WORM) behavior with an ON/OFF ratio over 103, which can be rationally controlled by the weight ratios of CC and MoS2. These findings by the WAE method would open tremendous potential opportunities to prepare commercially available semiconducting 2D nanocomposites for promising high-performance device applications.
https://doi.org/10.1039/D1GC00162K
Ultrafast formation of single phase B2 AlCoCrFeNi high entropy alloy films by reactive Ni/Al multilayers as heat source. - In: Materials and design, ISSN 1873-4197, Bd. 206 (2021), 109790, insges. 12 S.
High entropy alloy films of AlCoCrFeNi B2-ordered structure are formed during an ultrafast heating process by reactive Ni/Al multilayers. The self-propagating high-temperature reaction occurring in reactive Ni/Al multilayers after ignition represents an ultrafast heat source which is used for the transformation of a thin films Al/CoFe/CrNi multilayer structure into a single-phase high entropy alloy film. The materials design of the combined multilayers thus determines the phase formation. Conventional rapid thermal annealing transforms the multilayer into a film with multiple equilibrium phases. Ultrafast combustion synthesis produces films with ultrafine-grained single-phase B2-ordered compound alloy. The heating rates during the combustion synthesis are in the order of one million K/s, much higher than those of the rapid thermal annealing, which is about 7 K/s. The results are compared with differential scanning calorimetry experiments with heating rates ranging from about 100 K/s up to 25000 K/s. It is shown that the heating rate clearly determines the phase formation in the multilayers. The rapid kinetics of the combustion prevents long-range diffusion and promotes the run-away transformation. Thus, multilayer combustion synthesis using reactive Ni/Al multilayers as heat source represents a new pathway for the fabrication of single phase high-entropy alloy films.
https://doi.org/10.1016/j.matdes.2021.109790
Nanoparticle gas phase electrodeposition: fundamentals, fluid dynamics, and deposition kinetics. - In: Journal of aerosol science, ISSN 1879-1964, Bd. 151 (2021), 105652, S. 1-15
This communication uncovers missing fundamental elements and an expanded model of gas phase electrodeposition; a relatively new and in large parts unexplored process, which combines particle generation, transport zone and deposition zone in an interacting setup. The process enables selected area deposition of charged nanoparticles that are dispersed and transported by a carrier gas at atmospheric pressure conditions. Two key parameters have been identified: carrier gas flow rate and spark discharge power. Both parameters affect electrical current carried by charged species, nanoparticle mass, particle size and film morphology. In combination, these values enable to provide an estimate of the gas flow dependent Debye length. Together with Langmuir probe measurements of electric potential and field distribution, the transport can be described and understood. First, the transport of the charged species is dominated by the carrier gas flow. In close proximity, the transport is electric field driven. The transition region is not fixed and correlates with the electric potential profile, which is strongly dependent on the deposition rate. Considering the film morphology, the power of the discharge turns out to be the most relevant parameter. Low spark power combined with low gas flow leads to dendritic film growth. In contrast, higher spark power combined with higher gas flow produces compact layers.
https://doi.org/10.1016/j.jaerosci.2020.105652