Mapping of the Electrostatic Potentials in a Fully Processed Led Device with nm-Scale Resolution by In Situ off-Axis Electron Holography.

The optoelectronic properties of a fully processed red emitting AlGaInP micro-diode device is measured using standard I-V and luminescence measurements. A thin specimen is then prepared for in situ transmission electron microscopy analysis by focused ion beam milling, then the changes of electrostatic potential as a function of applied forward bias voltage are mapped by off-axis electron holography. We demonstrate that the quantum wells in the diode sit on a potential gradient until the threshold forward bias voltage for light emission is reached; at which point the quantum wells are aligned at the same potential. From simulations, a similar effect for the band structure can be demonstrated, where the quantum wells are aligned at the same energy level, and contain electrons and holes that are available for radiative recombination at this threshold voltage. We demonstrate that off-axis electron holography can be used to directly measure the potential distribution in optoelectronic devices, and is a powerful tool to help better understand their performance and to improve simulations.

Automated Fabrication of Streptavidin-Based Self-assembled Materials for High-Content Analysis of Cellular Response to Growth Factors.

The automation of liquid-handling routines offers great potential for fast, reproducible, and labor-reduced biomaterial fabrication but also requires the development of special protocols. Competitive systems demand for a high degree in miniaturization and parallelization while maintaining flexibility regarding the experimental design. Today, there are only a few possibilities for automated fabrication of biomaterials inside multiwell plates. We have previously demonstrated that streptavidin-based biomimetic platforms can be employed to study cellular behaviors on biomimetic surfaces. So far, these self-assembled materials were made by stepwise assembly of the components using manual pipetting. In this work, we introduce for the first time a fully automated and adaptable workflow to functionalize glass-bottom multiwell plates with customized biomimetic platforms deposited in single wells using a liquid-handling robot. We then characterize the cell response using automated image acquisition and subsequent analysis. Furthermore, the molecular surface density of the biomimetic platforms was characterized in situ using fluorescence-based image correlation spectroscopy. These measurements were in agreement with standard ex situ spectroscopic ellipsometry measurements. Due to automation, we could do a proof of concept to study the effect of heparan sulfate on the bioactivity of bone morphogenetic proteins on myoblast cells, using four different bone morphogenetic proteins (BMPs) (2, 4, 6, and 7) in parallel, at five increasing concentrations. Using such an automated self-assembly of biomimetic materials, it may be envisioned to further investigate the role of a large variety of extracellular matrix (ECM) components and growth factors on cell signaling.

Filling of Nanometric Pores with Polymer by Initiated Chemical Vapor Deposition.

The integration of porous thin films using microelectronic compatible processes sometimes requires the protection of the interior of the pores during the critical integration steps. In this paper, the polymerization of neo-pentyl methacrylate (npMA) is performed via initiated chemical vapor deposition (iCVD) on a porous organosilicate (SiOCH) and on a dense SiOCH. The characterizations by Fourier-transform infrared spectroscopy, spectroscopic ellipsometry, and time-of-flight secondary ion mass spectrometry of the different stacks show that iCVD is a powerful technique to polymerize npMA in the nanometric pores and thus totally fill them with a polymer. The study of the pore filling for very short iCVD durations shows that the polymerization in the pores is complete in less than ten seconds and is uniform in depth. Then, the poly(npMA) film growth continues on top of the filled SiOCH layer. These characteristics make iCVD a straightforward and very promising alternative to other infiltration techniques in order to fill the porosity of microporous thin films.

New approach for the molecular beam epitaxy growth of scalable WSe2 monolayers.

The search for high-quality transition metal dichalcogenides mono- and multi-layers grown on large areas is still a very active field of investigation. Here, we use molecular beam epitaxy to grow WSe2 on 15 × 15 mm large mica in the van der Waals regime. By screening one-step growth conditions, we find that very high temperature (>900 °C) and very low deposition rate (<0.15 Å min-1) are necessary to obtain high quality WSe2 films. The domain size can be as large as 1 µm and the in-plane rotational misorientation of 1.25°. The WSe2 monolayer is also robust against air exposure, can be easily transferred over 1 cm2 on SiN/SiO2 and exhibits strong photoluminescence signal. Moreover, by combining grazing incidence x-ray diffraction and transmission electron microscopy, we could detect the presence of few misoriented grains. A two-dimensional model based on atomic coincidences between the WSe2 and mica crystals allows us to explain the formation of these misoriented grains and gives insight to achieve highly crystalline WSe2.

Scanning microwave microscopy applied to semiconducting GaAs structures.

A calibration algorithm based on one-port vector network analyzer (VNA) calibration for scanning microwave microscopes (SMMs) is presented and used to extract quantitative carrier densities from a semiconducting n-doped GaAs multilayer sample. This robust and versatile algorithm is instrument and frequency independent, as we demonstrate by analyzing experimental data from two different, cantilever- and tuning fork-based, microscope setups operating in a wide frequency range up to 27.5 GHz. To benchmark the SMM results, comparison with secondary ion mass spectrometry is undertaken. Furthermore, we show SMM data on a GaAs p-n junction distinguishing p- and n-doped layers.

Doping porous silicon with erbium: pores filling as a method to limit the Er-clustering effects and increasing its light emission.

Er clustering plays a major role in hindering sufficient optical gain in Er-doped Si materials. For porous Si, the long-standing failure to govern the clustering has been attributed to insufficient knowledge of the several, concomitant and complex processes occurring during the electrochemical Er-doping. We propose here an alternative road to solve the issue: instead of looking for an equilibrium between Er content and light emission using 1-2% Er, we propose to significantly increase the electrochemical doping level to reach the filling the porous silicon pores with luminescent Er-rich material. To better understand the intricate and superposing phenomena of this process, we exploit an original approach based on needle electron tomography, EXAFS and photoluminescence. Needle electron tomography surprisingly shows a heterogeneous distribution of Er content in the silicon thin pores that until now couldn't be revealed by the sole use of scanning electron microscopy compositional mapping. Besides, while showing that pore filling leads to enhanced photoluminescence emission, we demonstrate that the latter is originated from both erbium oxide and silicate. These results give a much deeper understanding of the photoluminescence origin down to nanoscale and could lead to novel approaches focused on noteworthy enhancement of Er-related photoluminescence in porous silicon.

Exchange of methyl- and azobenzene-terminated alkanethiols on polycrystalline gold studied by tip-enhanced Raman mapping.

Mixed thiol self-assembled monolayers (SAMs) presenting methyl and azobenzene head groups were prepared by chemical substitution from the original single-component n-decanethiol or [4-(phenylazo)phenoxy]hexane-1-thiol SAMs on polycrystalline gold substrates. Static contact-angle measurements were carried out to confirm a change in the hydrophobicity of the functionalized surfaces following the exchange reaction. The mixed SAMs presented contact-angle values between those of the more hydrophobic n-decanethiol and the more hydrophilic [4-(phenylazo)phenoxy]hexane-1-thiol single-component SAMs. By means of tip-enhanced Raman spectroscopy (TERS) mapping experiments, it was possible to highlight that molecular replacement takes place easily and first at grain boundaries: for two different mixed SAM compositions, TERS point-by-point maps with <50 nm step sizes showed different spectral signatures in correspondence to the grain boundaries. An example of the substitution extending beyond grain boundaries and affecting flat areas of the gold surface is also shown.

Evidence of light depolarization in grazing incidence germanium attenuated total reflection prisms.

Attenuated total reflection (ATR) infrared absorption spectroscopy is a well-known vibrational spectroscopy technique for many different applications. In recent years this technique has been used to detect thin layer(s) lying on a solid substrate. Such a sample needs high pressure to ensure good optical contact between sample and prism and a p-polarization to enhance the signal to be detected. Such conditions have not been detailed in the literature regarding the effect of high pressure on the ATR measurement. This study shows the detrimental effect of high pressure on the ATR spectra. This effect is related to light depolarization induced by the germanium prism under high pressure. Moreover, the importance of polarizer position in the optical bench is highlighted. Indeed, due to the pressure-induced depolarization of the prism, the polarizer has to be placed before the prism to limit undesirable effects on the ATR spectrum baseline.