Smart surfaces by initiated chemical vapor deposition

The ability to modify the surface of materials with functional and responsive coatings is a powerful tool for the fabrication of smart devices for biotechnology, microfluidics, membrane technology, sensors and drug delivery systems. A recently developed method for the deposition of polymeric thin films, called initiated chemical vapor deposition (iCVD) is reviewed here. The authors will describe the high versatility of iCVD in driving application-specific properties into the material, creating a platform for the implementation of polymeric coatings into device fabrication. The significant impact of this polymerization technique lies in the possibility of obtaining polymers with chemical structure similar to the one of the polymers synthesized by conventional techniques with the advantages of a solvent-free deposition, which is totally substrate independent. Deposition has been demonstrated on paper, metal, plastics, porous substrates and very recently liquids. Tuning the process parameters allows to obtain controlled and uniform thickness over 3D substrates. Future outlook and iCVD scale-up approaches are also discussed. </jats:p

Super-Hydrophobic and Oloephobic Crystalline Coatings by Initiated Chemical Vapor Deposition

Preferred crystallographic orientation (texture) in thin films frequently has a strong effect on the properties of the materials and it is important for stable surface properties. Organized molecular films of poly-perfluorodecylacrylate p(PFDA) were deposited by initiated Chemical Vapor Deposition (iCVD). The high tendency of p(PFDA) to crystallize has been fully retained in the polymers prepared by iCVD. The degree of crystallinity and the preferred orientation of the perfluoro side chains, either parallel or perpendicular to the surface, were controlled by tuning the CVD process parameters (i.e. initiator to monomer flow rate ratio, filament temperature, and substrate temperature). Super- hydrophobicity (advancing water contact angle, WCA, of 160°, low hysteresis of 5°), and oleophobicity (advancing CA with mineral oil of 120°) were achieved. Low hysteresis was associated with high crystallinity, particularly when the orientation of the crystallites resulted in the perfluoro side groups being oriented parallel to the surface. The latter texture resulted in smoother film (RMS roughness < 30 nm) than the texture with the chains oriented perpendicularly to the surface. This can be very advantageous for applications that require smooth but still crystalline films.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract DAAD-19-02D-0002

Smart Core-Shell Nanostructures for Force, Humidity, and Temperature Multi-Stimuli Responsiveness

A force, humidity, and temperature-responsive electronic skin is presented by combining piezoelectric zinc oxide (ZnO) and poly-N-vinylcaprolactam-co-di(ethylene glycol) divinyl ether hydrogel into core-shell nanostructures using state-of-the-art dry vapor-based techniques. The proposed concept is realized with biocompatible materials in a simplified design that delivers multi-stimuli sensitivity with high spatial resolution, all of which are prerequisites for an efficient electronic skin. While the piezoelectricity of ZnO provides sensitivity to external force, the thermoresponsiveness of the hydrogel core provides sensitivity to surrounding temperature and humidity changes. The hydrogel core exerts mechanical stress onto the ZnO shell, which is translated to a measurable piezoelectric signal. A localized force sensitivity of 364 ± 66 pC N−1 is achieved with very low cross talk between 0.25 mm2 pixels. Additionally, the sensor's sensitivity to humidity is demonstrated at 25 and 40&nbsp;°C, i.e., above and below the hydrogel's lower critical solution temperature (LCST) of 34&nbsp;°C. The largest response to temperature is obtained at high humidity and below the hydrogel's LCST. The sensor response to force, humidity, and temperature is significantly faster than the system's intrinsic or excitation-induced time scale. Finally, the sensor response to touch and breath demonstrates its applicability as e-skin in real-life environment

Thickness-Dependent Swelling Behavior of Vapor-Deposited Smart Polymer Thin Films

In this contribution, the temperature-dependent swelling behavior of vapor-deposited smart polymer thin films is shown to depend on cross-linking and deposited film thickness. Smart polymers find application in sensor and actuator setups and are mostly fabricated on delicate substrates with complex nanostructures that need to be conformally coated. As initiated chemical vapor deposition (iCVD) meets these specific requirements, the present work concentrates on temperature-dependent swelling behavior of iCVD poly­(<i>N</i>-isopropylacrylamide) thin films. The transition between swollen and shrunken state and the corresponding lower critical solution temperature (LCST) was investigated by spectroscopic ellipsometry in water. The films’ density in the dry state evaluated from X-ray reflectivity could be successfully correlated to the position of the LCST in water and was found to vary between 1.1 and 1.3 g/cm³ in the thickness range 30–330 nm. This work emphasizes the importance of insights in both the deposition process and mechanisms during swelling of smart polymeric structures

Different Response Kinetics to Temperature and Water Vapor of Acrylamide Polymers Obtained by Initiated Chemical Vapor Deposition

Thermoresponsive polymers undergo a reversible phase transition at their lower critical solution temperature (LCST) from a hydrated hydrophilic state at temperatures below the LCST to a collapsed hydrophobic state at higher temperatures. This results in a strong response to temperature when in aqueous environment. This study shows that hydrogel thin films synthesized by initiated chemical vapor deposition show fast and strong response to temperature also in water vapor environment. Thin films of cross-linked poly­(<i>N</i>-isopropylacrylamide), p­(NIPAAm), were found to have a sharp change in thickness by 200% in water vapor at temperatures above and below the LCST. Additionally, the stimuli-responsive poly­(<i>N</i>,<i>N</i>-diethylacrylamide) was investigated and compared to results found for p­(NIPAAm). Analysis of the swelling kinetics performed with in situ spectroscopic ellipsometry with variable stage temperature shows differences for swelling and deswelling processes, and a hysteresis in the thickness profile was found as a function of temperature and of temperature change rate

Universal software for the real-time control of sequential processing techniques

Sequential process control is essential for many thin film processing techniques such as atomic1 and molecular layer deposition2 (ALD and MLD), molecular beam epitaxy,3 and atomic layer etching.4 In these processes, valves or shutters are used to dose precursors in a vacuum reactor. The dosing has to happen in a fully automated and reproducible way and often has to be quite short (e.g., around ∼10 ms for metalor- ganic precursors in ALD). Furthermore, novel hybrid materials require exact control of the composition and grading of their various components to obtain the desired properties.

Opto-chemical control through thermal treatment of plasma enhanced atomic layer deposited ZnO

Properties and performance of materials are closely connected. In order to obtain piezoelectric and lasing optical quality, ZnO has to be free of defects and highly crystalline. Instead, conductivity depends upon such defects, making it not trivial to aim at a specific set of properties in a single step. In this regard, we studied in situ the effect of temperature as an additional knob to finely control such properties. In this contribution, plasma enhanced atomic layer deposited (PE-ALD) zinc oxide (ZnO) layers, deposited between 25 °C and 250 °C, were studied in situ during annealing in air, and the opto-chemical and structural characteristics of the oxides were followed as a function of temperature. In situ spectroscopic ellipsometry (SE) and X-ray diffraction (XRD) were adopted to identify temperature windows where major structural and optical changes in the material occurred. Two temperature regions were identified for the effusion of adsorbed gases and minor structural rearrangements (180–280 °C) and for the growth/coalescence of ZnO crystals and its densification (360–500 °C). The results were corroborated by ex situ SE, XRD, UV–Vis and X-ray photoelectron spectroscopy. The in situ study revealed differences among the ZnO layers deposited at different temperatures, giving additional insights on the material properties deposited by PE-ALD

On the transformation of "zincone"-like into porous ZnO thin films from sub-saturated plasma enhanced atomic layer deposition

The synthesis of nanoporous ZnO thin films is achieved through annealing of zinc-alkoxide (“zincone”-like) layers obtained by plasma-enhanced atomic layer deposition (PE-ALD). The zincone-like layers are deposited through sub-saturated PE-ALD adopting diethylzinc and O2 plasma with doses below self-limiting values. Nanoporous ZnO thin films were subsequently obtained by calcination of the zincone-like layers between 100–600 °C. Spectroscopic ellipsometry (SE) and X-ray diffraction (XRD) were adopted in situ during calcination to investigate the removal of carbon impurities, development of controlled porosity, and formation and growth of ZnO crystallites. The layers developed controlled nanoporosity in the range of 1–5%, with pore sizes between 0.27 and 2.00 nm as measured with ellipsometric porosimetry (EP), as a function of the plasma dose and post-annealing temperature. Moreover, the crystallinity and crystallite orientation could be tuned, ranging from a powder-like to a (100) preferential growth in the out-of-plane direction, as measured by synchrotron-radiation grazing incidence XRD. Calcination temperature ranges were identified in which pore formation and subsequent crystal growth occurred, giving insights in the manufacturing of nanoporous ZnO from Zn-based hybrid materials