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Concept

Moiré (physique)

Résumé

thumb|Móvil (Mobile, Eusebio Sempere, 1972) ; sculpture de plein air exploitant l'effet de moiré (Madrid). Le moiré est un effet de contraste changeant avec la déformation d'un objet, indépendamment des effets d'ombre. On parle souvent du moiré d'une étoffe (par exemple de la soie). On peut obtenir un effet similaire en superposant deux voiles à maille régulière, ou bien lorsque l'on observe deux grillages l'un derrière l'autre, ou encore deux rambardes de pont, à une certaine distance. D'une manière générale, le moiré est une figure composée de lignes sombres et claires résultant de la superposition de deux réseaux (ensemble de lignes globalement parallèles). Il s'agit en fait d'un phénomène d'interférences spatiales entre les deux réseaux. Ce phénomène peut être utilisé pour analyser la déformation d'un objet ; il permet aussi d'expliquer le phénomène de tramage que l'on a lorsque l'on numérise (« scanne ») une image composée de points (comme une photo de quotidien), ou bien l'effe

Source officielle

https://fr.wikipedia.org/wiki/Moir%C3%A9_(physique)

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Chargement

Moiré effects produced by superpositions of micro-lens arrays

Thomas Walger

Moiré effects result from the superimposition of repetitive structures. The beauty of these moirés comes from the fact that macroscopic shapes seem to appear from the superposition of layers containing microscopic patterns. In addition, the moirés are dynamic and change when the layers are moved, or when the fabricated moiré samples are tilted. Aside from their visual appeal, moirés can also be used in security application, where their requirement of high resolution and fabrication accuracy are interesting to fight counterfeiting. In the context of this thesis, we focus on three moiré types. The first is called the level-line moiré. It displays a static shape whose intensities or colours vary depending on the superposition conditions of its two constituting layers. One layer is a grating of straight lines, while the other is a grating of the same period, to which distorsions have been purposefully applied. The distortions are dictated by an elevation profile, whose level lines are visible when the two layers are superposed. The second moiré type we cosider is the 1D moiré. The layers to be superposed are the base and the revealer. The base is a grating of rectangular tiles that contain the same symbol, shape or text. The revealer is a grating of lines, whose period is slightly different from that of the base. The base is only seen through the transparent parts of the revealer and is therefore sampled only at specific locations. The 1D moiré shape is thus a magnified version of the base tiles and is repeated in a given direction. When the revealer is translated relative to the base, the moiré shapes translate in a certain direction. Finally, 2D moirés are obtained in a similar fashion to the 1D moirés. The base tile is compressed and replicated in both directions to create the base array. Similarly, the revealer is an array of dots or sampling holes instead of lines. The moiré shape is a magnified version of the base tile and it can move in any direction, depending on the displacement of the revealer. Moreover, each of these moirés can be modified by applying transformations to the moirés or to their constituting layers. This is for example how spiraling moirés are designed. We have expanded the range of moiré features by creating a combination of these moiré types. The new moiré effects we created combine level-line moiré and 1D moiré or level-line moiré and 2D moiré. The moiré shapes can now both move and display varying intensities when the revealer is translated relative to the base. To obtain moiré samples, several methods can be employed. First, the two layers can be printed on transparencies. Although a fast method, the resulting moirés lack intensity and cannot be obtained at very high resolutions. An alternative is to combine a printed (or micro-fabricated) base with a lens-based revealer. The micro-lens arrays increase the dynamic range of the moirés, which is particularly important for 1D and 2D moirés. Finally, the new method we have developed relies on micro-fabrication technologies to fabricate moiré samples that are only made of transparent materials. In these new samples, both the base and the revealer are made of micro-lenses, which are fabricated on both sides of a transparent substrate. The three moiré types can be fabricated with this method and the samples are now brighter, more intriguing and the moiré intensities depend strongly on the viewing and illumination conditions.

EPFL2020

Chargement

Transfer of Graphene under Ultra-High Vacuum

Darius Constantin Merk

This thesis reports on a novel method for graphene transfer that is fully UHV compatible, which has remained a challenge for a long time due to the necessity of supporting graphene for transfer and most often requiring supporting layer removal post-transfer. Our graphene transfer method is based on the wafer-bonding approach, where graphene is stamped onto a target surface, from a support to which it is weakly bound. We use a bilayer of graphene supported by a teflon tape for the low adhesion between graphene layers and the flexibility of teflon tape to allow adaptability to the target surface.We demonstrate successful transfer onto Cu(100) and Ir(111) single crystal surfaces as well as on gr/Ir(111), thus forming a (partial) graphene bilayer on Ir(111). For in-situ characterization, we use Auger electron spectroscopy and STM. Auger measurements confirm that we are able to transfer an average 0.8-0.9 ML of carbon, by comparison to CVD grown samples. After transfer onto Ir(111), we show that by annealing to 1000 °C, we are able to observe by STM the characteristic moiré pattern formed by CVD grown graphene on Ir(111). Auger spectra show that the graphene-Iridium interaction undergoes a change from physisorption to chemisorption upon annealing. XAS and Raman spectroscopy demonstrate that the annealing step taken post transfer to observe the moiré by STM is only necessary to induce a change in interaction strength but not to heal graphene defects, to increase its domain size, or to make it flat. After transfer onto Cu(100), Raman spectra show that the transferred graphene is of the same quality as the CVD grown graphene before transfer, without further annealing. Comparison of synchrotron based high resolution XAS spectra of transferred gr/Ir(111) to CVD grown samples confirms that the transferred graphene is flat on top of the Ir(111) crystal surface.This enables sample and device fabrication, usually hampered by impurities, to be carried out in UHV leading to fully reproducible results. Clean twisted bilayer graphene samples can be made and studied in UHV. Further, surface science can be extended to the third dimension, by capping layers and continuing growth, etc., all under clean conditions.

EPFL2022

Chargement

The main objective of this PhD thesis is to link the atomic scale structure to the catalytic properties of self-assembled nanostructures. The growth and characterization of these nanostructures was carried out in an ultra-high vacuum (UHV) chamber initially designed for the study of the kinetics of epitaxial growth of thin films and nanostructures by variable temperature scanning tunneling microscopy. During this PhD thesis, we built a very sensitive and selective gas detector based on a quadrupole mass spectrometer and adapted to the geometry of this UHV system. The proper functioning of the gas detector is confirmed by temperature programmed desorptions of Xe on Ag(100) which demonstrate the very high sensitivity of the device of about

10^{-6}

~monolayer (ML). The zero-order desorption parameters obtained for the first monolayer of Xe adsorbed on Ag(100) are

E_d=0.22-0.23

~eV with a prefactor

\nu_0

between

4\times10^{12}

^{-1}

and

1 \times10^{13}

^{-1}

. An exchange process between Fe adatoms and Ag atoms from the Ag(100) surface is identified by means of temperature programmed desorption of N

_2

. Desorption spectra suggest that the exchange is complete from an annealing temperature of 140~K. Our study of the catalytic activity of metal nanostructures focuses on the ability of passivated Fe clusters to adsorb N

_2

molecules without dissociating them, which is a fundamental step in the bio-inspired heterogeneous catalysis of ammonia. The first system investigated for the growth of Fe clusters is MgO/Ag(100). Diffusion of Fe adatoms deposited on a MgO monolayer is observed from an annealing temperature of 280~K onwards. Fe clusters with an average size of

12.1\pm 0.5

~atoms are obtained on a MgO monolayer by thermal ripening of 0.072~ML of Fe deposited at 40~K. No sign of N

_2

adsorption was observed on these clusters by thermal programmed desorption investigation. The second system chosen for the growth of Fe clusters is graphene/Ir(111) obtained by chemical vapor deposition. The deposition of 0.3 and 0.4 ML of Fe on a graphene/Ir(111) sample at 50~K, followed by annealing at 300~K, generates a superlattice of Fe clusters which matches the periodicity of the moiré formed by graphene/Ir(111). The theoretically expected mean cluster size is 26 atoms for 0.3~ML of Fe, and 35 atoms for 0.4~ML of Fe. The temperature stability of the clusters obtained with 0.4~ML Fe is investigated from 300~K to 600~K. It is established that Smoluchowski ripening takes place as the annealing temperature increases and the superlattice structure has completely disappeared after annealing at 600~K. Temperature programmed desorption of molecularly adsorbed N

_2

at 50~K on a sample with 0.4~ML of Fe deposited on graphene/Ir(111) reveals 3 desorption peaks at 120~K, 92~K, and 60~K. In addition, slow dissociation of nitrogen molecules at 50~K adsorbed on the higher energy adsorption sites is demonstrated.

EPFL2021

Chargement

10^{-6}

~monolayer (ML). The zero-order desorption parameters obtained for the first monolayer of Xe adsorbed on Ag(100) are

E_d=0.22-0.23

~eV with a prefactor

\nu_0

between

4\times10^{12}

^{-1}

and

1 \times10^{13}

^{-1}

. An exchange process between Fe adatoms and Ag atoms from the Ag(100) surface is identified by means of temperature programmed desorption of N

_2

_2

12.1\pm 0.5

~atoms are obtained on a MgO monolayer by thermal ripening of 0.072~ML of Fe deposited at 40~K. No sign of N

_2

_2

EPFL2021

Transfer of Graphene under Ultra-High Vacuum

Darius Constantin Merk

EPFL2022

Moiré effects produced by superpositions of micro-lens arrays

Thomas Walger

EPFL2020