Prediction of Self-Assembled Dewetted Nanostructures for Photonics Applications via a Continuum-Mechanics Framework

When a liquid film lies on a nonwettable substrate, the configuration is unstable, and the film spontaneously ruptures to form droplets. This phenomenon, known as dewetting, commonly leads to undesirable morphological changes. Nevertheless, recent works, combining spontaneous dewetting triggered by thermal annealing and topographic pattern-directed dewetting, demonstrate the possibility of harnessing dewetting with a degree of precision on par with that of advanced lithographic processes for high-performance nanophotonic applications. Since resonant behavior is highly sensitive to geometrical changes, predicting quantitatively dewetting dynamics is of high interest. Here, we develop a continuum model that predicts the evolution of a thin film on a patterned substrate, from the initial reflow to the nucleation and growth of holes. We provide an operative framework based on macroscopic measurements to model the intermolecular interactions at the origin of the dewetting process, involving length scales that span from sub-nanometer to micrometer. A comparison of experimental and simulated results shows that the model can accurately predict the final distributions, thereby offering predictive tools to tailor the optical response of dewetted nanostructures.

Ordered nanostructures on silicon substrates

Jelena Vukajlovic Plestina

Silicon is today the main material used in electronics. It is a very advanced and mature technology. It is therefore clear that new technological concepts and materials should be introduced through the integration on the silicon platform. III-V semiconductors, such as GaAs and InAs, are high performance semiconductors, with direct bandgap and high carrier mobility, what makes them very prominent candidates for future electronics and optoelectronics. Lattice, thermal and polarity mismatch have been for decades limiting their integration on Si in the form of thin film technology. The nanowire geometry brings new prospects in this direction by reducing the contact area between mismatched materials,. Now, while growth of defect-free III-V structures on silicon is important, use in applications requires the nanostructures to be placed deterministically following a certain design/order. In this thesis we have studied different mechanisms to obtain ordered nanostructures on a silicon substrate, with the goal of increasing its functionality. The core part of this work was dedicated to the integration of III-V nanowires on Si in the formof ordered arrays. We have considered both GaAs and InAs nanowires. This process has several requirements: substrate fabrication and the growth process should be CMOS compatible, nanowires within the arrays should be grown vertically with a yield close to 100% and nanowires should be clones-looking and performing the same. An additional point to consider is that the fabrication process should be compatible with large scale techniques. The substrate requirements for obtaining ordered arrays of Ga-assisted GaAs nanowires on silicon were identified. In particular, we focused on the initial stages of growth, that turned to be key for achieving a high yield in the arrays. We found that the positioning of the Ga droplet within the predefined holes on the substrate determined whether the nanowire would grow perpendicularly to the substrate. Our HRTEMstudies on the titled nanowires show that the initial nanowire seeds seem to nucleate at the corner of the nanoscale holes. Instead, the crystal seeds of vertical nanowires occupy homogeneously the nanoscale holes. Achieving high yield in the growth of GaAs arrays on silicon also allowed us to study their evolution in time. We demonstrated that there is an incubation time for nanowires to start growing. This incubation time is different from NW to NWand leads to a relatively broad length distribution. We proposed a solution and showed how an increase in the supersaturation in the Ga droplet leads to the formation of homogeneous arrays. Growth of InAs nanowires in ordered arrays on silicon was based on the concept of guided growth. We used a SiO2 nanotube templates to promote vertical growth. Due to the directionality of MBE, achieving growth inside a nanotube was especially challenging. Our results provided new insights on the role of different pathways of the In and As4 adatoms during growth. In this part, large scale patterning by phase shift photolithography was demonstrated as an alternative to the conventionally used electron beam lithography. Stain etching method for producing porous silicon was applied on top-bottom fabricated Si micropillars. This led to geometrically driven electrochemical dissolution of silicon trough the center of the microstructure forming microtubes. The optical properties were also studied and used to produce functional 3D LED.

EPFL2017

An Experimental Setup for Heterogeneous Catalysis on Atomically Defined Metal Nanostructures

Jean-Guillaume De Groot

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