An IR Spectroscopy Study of the Degradation of Surface Bound Azido-Groups in High Vacuum.

Controlled surface functionalization with azides to perform on surface "click chemistry" is desired for a large range of fields such as material engineering and biosensors. In this work, the stability of an azido-containing self-assembled monolayer in high vacuum is investigated using in situ Fourier transform infrared spectroscopy. The intensity of the antisymmetric azide stretching vibration is found to decrease over time, suggesting the degradation of the azido-group in high vacuum. The degradation is further investigated at three different temperatures and at seven different nitrogen pressures ranging from 1 × 10-6 mbar to 5 × 10-3 mbar. The degradation is found to increase at higher temperatures and at lower nitrogen pressures. The latter supporting the theory that the degradation reaction involves the decomposition into molecular nitrogen. For the condition with the highest degradation detected, only 63% of azides is found to remain at the surface after 8 h in vacuum. The findings show a significant loss in control of the surface functionalization. The instability of azides in high vacuum should therefore always be considered when depositing or postprocessing azido-containing layers.

Depositing ALD-oxides on MLD-metalcones: enhancing initial growth through O2 plasma densification.

Flexible devices are experiencing a steady increase in popularity, which brings the need of suitable protective/functional coatings for these applications. On the one hand, Atomic Layer Deposition (ALD) produces thin films with great purity, few pinholes and good conformality, but flexibility is rather limited. On the other hand, Molecular Layer Deposition (MLD) can produce partially/fully organic coatings with good flexibility, but stability concerns limit their applications. Therefore, combining ALD and MLD to obtain materials with good flexibility and improved characteristics holds great potential. In this article, we utilised O2 plasma treatments on various metalcone films to improve the compatibility of sequential ALD/MLD depositions. During plasma modification, in situ spectroscopic ellipsometry measurements (in situ SE) suggested that mainly the near-surface region of the metalcone layer was affected by the plasma treatment, locally converting the metalcone into a metal-oxide structure. This structure shielded the underlying metalcone layer from the plasma, thus resulting in a saturative-type behaviour even during extended plasma exposures. X-Ray reflectivity measurements (XRR) could only be fitted with bilayer models, while Fourier-Transform InfraRed spectroscopy (FTIR) showed an absorption decrease in the C-O band and an increase in the CO region. Additionally, film air stability seemed improved by this treatment. ALD-oxides were grown on these plasma-treated metalcones (PT-metalcones), and results were compared to pristine ones. While ALD growth on pristine metalcones always suffered from a delay, after which linear growth was achieved, oxides on PT-metalcones exhibited linear growth immediately, from cycle one. We therefore conclude that, upon O2 plasma exposure, metalcones are densified into a metalcone/oxide bilayer, where the oxide shields the underlying film from further oxidation. And, if an ALD oxide coating is to be deposited on top of these structures, this plasma treatment will make the structure more suitable for post-processing. In applications that require the combination of ALD/MLD multistacks, the use of an intermittent plasma treatment can prove useful.

Integrated PbS Colloidal Quantum Dot Photodiodes on Silicon Nitride Waveguides.

Colloidal quantum dots (QDs) have become a versatile optoelectronic material for emitting and detecting light that can overcome the limitations of a range of electronic and photonic technology platforms. Photonic integrated circuits (PICs), for example, face the persistent challenge of combining active materials with passive circuitry ideally suited for guiding light. Here, we demonstrate the integration of photodiodes (PDs) based on PbS QDs on silicon nitride waveguides (WG). Analyzing planar QDPDs first, we argue that the main limitation WG-coupled QDPDs face is detector saturation induced by the high optical power density of the guided light. Using the cladding thickness and waveguide width as design parameters, we mitigate this issue, and we demonstrate WG-QDPDs with an external quantum efficiency of 67.5% at 1275 nm that exhibit a linear photoresponse for input powers up to 400 nW. In the next step, we demonstrate a compact infrared spectrometer by integrating these WG-QDPDs on the output channels of an arrayed waveguide grating demultiplexer. This work provides a path toward a low-cost PD solution for PICs, which are attractive for large-scale production.

Riemannian classification of single-trial surface EEG and sources during checkerboard and navigational images in humans.

OBJECTIVE: Different visual stimuli are classically used for triggering visual evoked potentials comprising well-defined components linked to the content of the displayed image. These evoked components result from the average of ongoing EEG signals in which additive and oscillatory mechanisms contribute to the component morphology. The evoked related potentials often resulted from a mixed situation (power variation and phase-locking) making basic and clinical interpretations difficult. Besides, the grand average methodology produced artificial constructs that do not reflect individual peculiarities. This motivated new approaches based on single-trial analysis as recently used in the brain-computer interface field. APPROACH: We hypothesize that EEG signals may include specific information about the visual features of the displayed image and that such distinctive traits can be identified by state-of-the-art classification algorithms based on Riemannian geometry. The same classification algorithms are also applied to the dipole sources estimated by sLORETA. MAIN RESULTS AND SIGNIFICANCE: We show that our classification pipeline can effectively discriminate between the display of different visual items (Checkerboard versus 3D navigational image) in single EEG trials throughout multiple subjects. The present methodology reaches a single-trial classification accuracy of about 84% and 93% for inter-subject and intra-subject classification respectively using surface EEG. Interestingly, we note that the classification algorithms trained on sLORETA sources estimation fail to generalize among multiple subjects (63%), which may be due to either the average head model used by sLORETA or the subsequent spatial filtering failing to extract discriminative information, but reach an intra-subject classification accuracy of 82%.

Atomic Layer Deposition on Polymer Thin Films: On the Role of Precursor Infiltration and Reactivity.

Inorganic barriers grown by atomic layer deposition (ALD) can overcome the stability issues originating from the permeation of foreign species (water and oxygen) into polymer thin films. Alternatively, infiltration of ALD species into the bulk of the polymer can be used to modify its characteristic properties. In this study, the feasibility of growing an inorganic barrier with ALD on polystyrene, poly(methyl methacrylate), and poly(ethylene terephthalate glycol) thin films is evaluated. The nucleation and growth of the ALD layer, including the infiltration into the polymer thin film, are monitored in situ using spectroscopic ellipsometry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy for Al2O3-ALD with trimethylaluminum as the Al precursor and H2O as the reactant. The results show that the deposition temperature and the presence and location of functional groups in the polymer chain exert the strongest influence on the infiltration behavior and as such allow us to manipulate (i.e. to prevent or expedite) the infiltration into the polymer thin film.

Adding memory to pressure-sensitive phosphors.

Mechanoluminescence (ML) is the phenomenon describing the emission of light during mechanical action on a solid, leading to applications such as pressure sensing, damage detection and visualization of stress distributions. In most cases, this mechanical action releases energy that was previously stored in the crystal lattice of the phosphor by means of trapped charge carriers. A drawback is the need to record the ML emission during a pressure event. In this work, we provide a method for adding a memory function to these pressure-sensitive phosphors, allowing an optical readout of the location and intensity of a pressure event in excess of 72 h after the event. This is achieved in the BaSi2O2N2:Eu2+ phosphor, where a broad trap depth distribution essential for the process is present. By merging optically stimulated luminescence (OSL), thermoluminescence (TL) and ML measurements, the influence of light, heat and pressure on the trap depth distribution is carefully analysed. This analysis demonstrates that mechanical action can not only lead to direct light emission but also to a reshuffling of trap occupations. This memory effect not only is expected to lead to new pressure sensing applications but also offers an approach to study charge carrier transitions in energy storage phosphors.