Overview: Systematic Preparation of Solides 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 cart 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 our new combining laser deposition (CLD) equipment, a scanning multicomponent PLD facility based on a femtosecond laser.
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.
Metal-Organic Frameworks (MOF):
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 (Fig. 1). 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.
Tuning TiO2 films
TiO2 is a key functional oxide. Its polymorphs are used as photocatalysts, for example in self-cleaning surfaces, for water splitting, and as defining ingredient in many sunscreens. Thin films of various TiO2 polymorphs with different colors can be formed by deposition of titanium with activated oxygen. The substrate temperature chosen during growth determines the polymorph. We have found that transparent and black amorphous films with a predefined rutile arrangement are formed at -190 and 30°C which then crystallize above 350°C forming pure anatase polymorph or a mixture of anatase with brookite, respectively (see Figure 2). At a substrate temperature of 200°C opaque polycrystalline films are obtained. Surprisingly, the preparation of the black TiO2 film does not require a reduction step, e.g. with hydrogen. These films seem to be a product of two competing reaction parameters: oxygen adsorption and surface species mobility. Furthermore, we found that silver-doping stabilizes the anatase polymorph and generates reduced titanium species in the films.
Thin Solid Films 598 (2016) 204-213.
Laser ablation of molecular carbon nitride compounds
The remarkable mechanical, optoelectronic and structural properties of polymeric carbon nitrides have been attractive targets for both fundamental and applied research. We demonstrated the deposition of molecular precursors of polymeric carbon nitrides, namely dicyandiamide (C2N4H4), melamine (C3N6H6), and melem (C6N10H6), on sapphire substrates using the femto-pulsed laser deposition technique at different temperatures. The resulting polycrystalline, opaque films show that in all aspects the films resemble the bulk material; e.g., the IR spectra of all deposited samples largely coincide with the ones of the corresponding powders (see Figure 3). These results open the possibility to form thin films of temperature-sensitive or reactive materials that are difficult to deposit, which applies in particular to compounds based on carbon nitride networks.
D. Fischer, K. Schwinghammer, C. Sondermann, V.W. Lau, J. Mannhart,
Appl. Surf. Sci. 349 (2015) 353-60.