Transparence (physique) | EPFL Graph Search

La transparence désigne la capacité d'un matériau à ne pas interagir avec une onde. Dans le cas de l'optique, un matériau transparent a pour propriété de ne pas absorber la lumière. Cette propriété du matériau dépend cependant de la longueur d'onde. Différence entre matériau transparent et translucide Translucidité L'appellation translucide est spécifiquement employée pour les ondes lumineuses du domaine visible. Les matériaux translucides ont la propriété de diffuser la lumière lors de sa transmission, ce qui rend l'observation au travers difficile ou impossible. À une échelle macroscopique, où les dimensions étudiées sont bien plus grandes que la longueur d'onde, les matériaux transparents obéissent aux lois de Snell-Descartes, tandis que les matériaux translucides ne les respectent pas puisqu'ils diffusent partiellement ou totalement la lumière. Des matériaux transparents peuvent se comporter comme des matériaux translucides sous certaines conditions de changement d'ét

Electronic transport properties of vertical and lateral graphene-based heterostructures

Roberto Urcuyo Solorzano

Graphene has some drawbacks, like the absence of an electronic band gap which leads to only small gate switching ratios, and its fast carrier recombination that limits its use in optoelectronics. In order to overcome these issues, the aim of this thesis was to tune the properties of graphene by two different modification approaches. The first approach involved chemical functionalization to create lateral graphene heterostructures, whereas the second approach was directed toward vertical heterostructures that combine graphene with another 2D material. For the former case, two novel chemical functionalization methods were investigated. The first method relied upon hyperthermal molecular ion collisions with 4,4¿-azobis(pyridine). This enabled the covalent functionalization of graphene with a functionalization degree of up to 3%, as demonstrated by Raman in combination with XPS. AFM studies revealed that the functionalized sheets retain their topographic integrity. Thus obtained stripe-like patterns of covalently graphene within extended graphene sheets enabled enhanced on/off ratios upon gate switching. In the second functionalization, OsO4 was used to selectively introduce hydroxyl groups to graphene. Best results were obtained using UV light activation, which yielded graphene of high covalent functionalization degree, as concluded from its optical transparency and gate-induced on/off switching ratios of up to 500. Temperature dependent electrical measurements revealed 2D hopping as the dominant transport mechanism. The first type of vertical heterostructure, gr-TiOx-Ti diodes were fabricated and their electrical properties in the dark and under visible light illumination studied. In contrast to conventional MIM diodes, the performance of the graphene-based diodes was found to increase with decreasing thickness of the oxide insulator. Bias-induced modulation of the work function of graphene was identified as the key to the operation mechanism of the graphene diodes, enabling them to reach a very high asymmetry and nonlinearity (9000 and 8). The diodes compete well with state-of-the-art MIM diodes. Furthermore, the graphene-TiOx-Ti diodes could be operated as photovoltaic cell, with a maximum open-circuit voltage of 0.3 V and a short-circuit photocurrent of 14 nA under global illuminationt. This finding constitutes the first proof-of-principle of hot carrier extraction from graphene, based on the photovoltaic effect. The second type of vertical heterostructures was based on black phosphorus (BP). As a first step, thin BP sheets were combined with n-GaAs into novel pn devices. They displayed pronounced rectification behavior that, in the low bias regime, approaches that of ideal diodes. Moreover, when operated as photodiodes they reached external quantum efficiencies (EQE) above 30% under higher reverse bias. On this basis, the bP was then combined with gr into a p+/p heterojunction. Thus obtained devices achieved a maximum internal quantum efficiency (IQE) and responsivity of 13% and 10.7 mA/W, respectively. This IQE is the highest thus far reported for diodes comprising BP and another 2D material. Moreover, with the aid of a top gate it was possible to modulate both, the photo-conversion efficiency and photocurrent generation mechanism of the black phosphorous-graphene diodes. Together with the possibility to tune the photocurrent response by the thickness of the black phosphorous, these diodes emerge as promising photodetectors.

EPFL2016

Plasmonic Nanohole Arrays on a Robust Hybrid Substrate for Highly Sensitive Label-Free Biosensing

Hatice Altug, Mickaël Pierre Busson, Arif Engin Cetin, Yasa Eksioglu Özok, Dordaneh Etezadi

Plasmonic nanohole arrays have received significant attention, as they have highly advantageous optical properties for ultrasensitive and label-free biosensing applications. Currently, most of these subwavelength periodic apertures are mainly implemented on transparent materials, which results in multiple spectrally close transmission resonances. However, this spectral characteristic is not ideal for biosensing applications, as it complicates monitoring spectral variations. In this article, utilizing a hybrid substrate composed of a high refractive index dielectric interlayer over a transparent material, we show that gold nanohole arrays support spectrally isolated and well-defined plasmonic resonances that are easy to track. Compared to conventional configurations on transparent material, nanoholes on a hybrid substrate also exhibit plasmonic modes with well-preserved amplitudes, which is useful for reliable spectral monitoring. We show that nanohole arrays on a hybrid substrate are more sensitive to changes in surface conditions. Using a spectral integration method, which evaluates wavelength shifts in a large spectral window instead of monitoring only the plasmonic resonance wavelength, we obtain a detection limit as low as 2 x 10^(-5) RIU. Furthermore, we successfully demonstrate real-time monitoring of biomolecular binding interactions even at sub-1 ng/mL levels.

Amer Chemical Soc2015

Gallium Nitride as Transparent Electron-Selective Contact in Silicon Heterojunction Solar Cells

Christophe Ballif, Mathieu Gérard Boccard, Tzu-Chin Chang Chien, Angela Nicole Fioretti, Yashi Xiao

Carrier-selective, passivating contacts have allowed silicon heterojunction (SHJ) cells to reach recordbreaking efficiencies particularly in all-back-contacted designs. However, two-side-contacted SHJ cell efficiency has been limited due in part to parasitic absorption losses up to 3 mA/cm2 in the aSi:H layers. More transparent materials could reduce this current loss while minimizing process complexity. Gallium nitride (GaN), with a bandgap of 3.4 eV and an advantageous band alignment with silicon, could be applied as a transparent electron-selective layer. Here, we report on SHJ solar cells utilizing PECVD GaN layers grown at 200°C as electron-selective contact. First devices exhibited open-circuit voltages of ~575 mV due to poor passivation, and low conductivity of the as-yet undoped GaN layers induced high series resistance (Rs). However, this Voc suggests the potential for electron selectivity if appropriate passivation and doping strategies are implemented.

2019

Electronic transport properties of vertical and lateral graphene-based heterostructures

Roberto Urcuyo Solorzano

EPFL2016