Systematic Preparation of Solids Using Vapor Deposition Techniques
Vapor deposition techniques allow the high-purity fabrication of a huge variety of materials with the ratio of the components easily controlled by adjusting the flux of the evaporation sources. In the Mannhart department, oxides with special transport or magnetic properties or oxide heterostructures are synthesized with particular emphasis on the pulsed laser deposition (PLD) technique. Here, for the systematic preparation of solids, we use two modified approaches: direct synthesis from the elements and evaporation of compounds or mixtures. In particular the solid state reactions are investigated in-situ by depositing the components onto a substrate at very low temperature. In addition to the evolution of crystal-structure formation, the physical properties (electrical/optical) of the products are studied in detail. By varying the experimental parameters, we explore systematic routes to prepare desired compounds.
For the preparation, several UHV chambers are available with a special shuttle system, which allows sample transfer while maintaining vacuum and low temperature. The synthesis chamber contains effusion cells, an electron-beam evaporator, and an ECR plasma-source for the activation of gases. One of the other deposition chambers is combined with a new laser deposition equipment, a scanning multicomponent PLD facility based on a femtosecond laser (femto-PLD).
The samples are investigated in-situ on an X-ray diffractometer as a function of temperature. The diffractometer is equipped with a detector for the X-ray diffraction and fluorescence analysis in combination with a Raman spectrometer. In-situ TEM investigations are performed using a special cryo-vacuum-holder that is directly adapted to our deposition chamber.
Elements: Gallium Allotropes
Systematics of the allotrope formation in elemental gallium films
Elemental gallium forms unusual allotropes, whose structures all differ distinctly from the close-packing of spheres typical for nearly all elemental metals. At standard pressure, four different crystalline allotropes are known of which the α-Ga allotrope is the thermodynamically stable one. Since the other allotropes can only be obtained under special conditions such as spatial confinement or undercooling, the structural relation among the gallium allotropes is an open question.
We investigated systematically the dependence of the crystal structure formation in elemental gallium films on the deposition temperature and the subsequent annealing procedures using in situ X-ray powder diffraction and Raman measurements, complemented by ab initio calculations. The films were prepared with the femtosecond pulsed-laser-deposition technique on a substrate kept at constant temperature in the range of –190 °C to 25 °C, followed by cooling+heating cycles in the same temperature range.
Unsuspectedly, at room temperature amorphous gallium and below –60 °C the α-Ga allotrope, respectively, is formed in the deposited films (Figure 1). The most surprising discovery, however is the new β'-Ga allotrope (see Figure 1, violet color), which we could identify and characterize from the X-ray powder patterns. It forms a distorted derivative of the β-Ga crystal structure starting from amorphous deposits between 25 and –60 °C. The existence of this new allotrope was also supported by ab initio calculations of the equations of state of the various Ga-allotropes, which revealed nearly identical energies for α-Ga, β-Ga and β'-Ga.
D. Fischer, B. Andriyevsky, J.C. Schön
Mater. Res. Express 6 (2019) 116401
Biological Composites: Ivory
Bio-inspired synthetic ivory as a sustainable material for piano keys
Ivory has traditionally been the material of choice for piano key surfaces because it provides a desirable grip for the fingers and has a characteristic warm color and luster. However, as it has become largely unavailable due to the vulnerable status and corresponding protection of elephants, there is request for an alternative, sustainable material with comparable functional properties. The global demand for ivory has caused the population of African forest elephants to shrink in the past decade by 60%, mainly due to poaching.
To obtain such a material, we synthesized a composite that is chemically identical to natural ivory. The synthesis consists of a solution-based process that starts from powders of gelatin and hydroxylapatite (see Figure 2). The Biocomposite is fabricated from abundant materials in an environmentally friendly process and is furthermore biodegradable. The material is sturdy, machinable, and allows to match the grip of keys to the preferences of individual pianists as well as the ivory color (cf. Figure 2). Furthermore, during the fabrication process pigments can be incorporated into the matrix to achieve virtually any color, including color patterns. In the same manner, additives such as antibacterial compounds or markers yielding fluorescent or DNA-based fingerprints for identification of the material may be added.
Possible applications extend beyond our use case of piano keys and could conceivably include jewelry, and alternatives to hydrocarbon-based plastics for applications that require superior tactile properties or biodegradability.
D. Fischer, S.C. Parks, J. Mannhart
Sustainability 11 (2019) 6538
Metal-Organic Frameworks (MOF): ZIF-8
Deposition of porous MOF thin films (ZIF-8) by femtosecond pulsed-laser deposition (femto-PLD)
Metal-organic frameworks are exciting because of their use as tunable photocatalytic platforms and in optoelectronic devices. A state-of-the-art MOF is zeolitic imidazolate framework ZIF-8, one of the four metal−organic frameworks being manufactured commercially. For the first time, we have succeeded in growing crystalline films of the zeolitic imidazolate framework ZIF-8. The films were deposited using polyethylene glycol 400 (PEG) as an impregnated “vehicle” during femto-PLD (Figure 3). The remaining PEG additive in the films can be easily removed by washing with ethanol, leading to pure ZIF-8 films on substrates. Thus, for the first time films of a porous MOF were prepared by a physical vapor deposition technique, which opens new strategies for the fabrication of MOF films.
D. Fischer, A. von Mankowski, A. Ranft, S.K. Vasa, R. Linser, J. Mannhart, B. V. Lotsch
Chem. Mater. 29 (2017) 5148−5155.
Oxide Films: Tuning TiO2 films / D. Fischer / Thin Solid Films 598 (2016) 204-213.