Nanostructure | EPFL Graph Search

Une nanostructure est un assemblage d'atomes ou de molécules dont au moins une dimension est comprise entre 0,1 et 100 nanomètres. En pratique, on tolère un certain débordement dans ces limites. Ainsi, un nanofil est une nanostructure dont le diamètre est compris entre 0,1 à 100 nanomètres. Les nanostructures se retrouvent dans la nature et sont étudiées par les nanotechnologies du microscope jusqu'aux visualisations avancées des protéines (une protéine peut faire un nanomètre de diamètre). Elles sont régies par des principes d'économie, de complexité, de propriétés multiples et fonctionnelles : Film sorti en 2020, court-métrage document nature de 55 minutes, de Pascal Moret et Julien Guiol, intitulé Nanosurvies, les pouvoirs invisibles de la nature, présenté dans la chaîne télévisée Ushuaïa le et après. Ce film documentaire explique et montre pourquoi ces nanostructures microscopiques sont étudiées en recherches en physique et chimie, dans les domaines variés de la botanique, d

Scalable fabrication of functional nanostructures on stretchable substrates by capillary-assisted particle assembly and adhesion lithography

Shao-Chi Yu

The fabrication of metallic nanostructures on stretchable substrates enables specific applications that exploit the combination of the nano-scale phenomena and the mechanical tunability of the physical dimensions of the nanostructures. Due to the large difference in the thermal expansion coefficient between metals and polymer-based soft materials, patterning metallic nanostructures directly on a stretchable substrate is a known challenging task. In this thesis, scalable fabrications of metallic nanostructures on stretchable substrates by top-down and bottom-up methods are studied. Metallic nanostructures are first fabricated on Si substrates and subsequently transferred to stretchable substrates.In the first part of this thesis, fabrication of nanogap electrodes (NGEs) on a polydimethylsiloxane (PDMS) substrate using adhesion lithography is reported. With wafer-level processes, the nanogap is created on an Al sacrificial layer by separating two Au electrodes with an Al2O3 nanolayer. By etching the Al sacrificial layer, the NGEs are transferred to the PDMS substrate. Tunneling currents across the nanogap on PDMS are measured under various mechanical deformation statuses of the PDMS substrate. The electrical measurement results show that the nanogap distance is mechanically tunable in the quantum tunneling regime. The NGEs on PDMS might be eventually integrated with micro piezo-electric actuators to become miniaturized tunable NGEs, which could pave the way towards the application of an on-chip single-molecule detector. Apart from developing the fabrication process of the NGEs on PDMS, a yield study of the essential step of the adhesion lithography, e.g., the tape peeling process, is also conducted to understand the design principles of the NGEs. The yield of the wafer-level tape peeling process is larger than 80%.In the second part of this thesis, chip-level (~ 2 x 2 cm2) fabrication of ordered gold nanoparticles (AuNPs) on a PDMS substrate using capillary-assisted particle assembly (CAPA) technique is reported. AuNPs are first assembled on reusable Si templates with pre-defined topographical traps, and then transferred to the PDMS substrate by etching the Al sacrificial layer. The reusable assembly trap has the shape of the funnel which is designed for high assembly yield (> 90%) and precise particle placement (offset ~ 10 nm). The assembly yield, the particle position offset, the yield of the transfer process, and the reusability of the assembly template are systematically studied. Two functional AuNP structures are demonstrated using the reported fabrication process. The first structure is the plasmonic surface lattice resonance (SLR) arrays of 150 and 200 nm AuNP. The optical spectra of the AuNP arrays on PDMS are measured showing pitch-related SLR peaks that agree with the finite element method (FEM) simulation results. The second structure is the dimer of 200 nm AuNPs which has a nanogap between two AuNPs. By assembling two AuNPs in the same funnel-shaped trap, a nanogap is formed between two AuNPs. Combining Au electrodes fabricated using electron beam lithography and the lift-off process, NGEs are fabricated and successfully transferred to a PDMS substrate.

EPFL2022

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

Growth and characterization of specific self-assembled supramolecular architectures at crystal surfaces

Marta Canas Ventura

In the framework of this thesis self-assembled supramolecular architectures of several molecular species on crystal metal surfaces are characterized. These investigations combine results obtained by means of scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), X-Ray Photoelectron Spectroscopy (XPS), and Near-Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS). The interest in controlling the formation of specific arrangements of supramolecular ensembles at surfaces is motivated to a great extent by the prospect to design molecular devices and nanomaterials with customized properties. The goal is a "spontaneous" creation of targeted, well-ordered molecular architectures, which shall be achieved by taking advantage of the molecule's propensity to self-assemble. Control means, in this context, predictability. This term address the accuracy of the prediction of the structures resulting from molecular self-assembly for systems exhibiting different characteristics. Here, system stands for a particular combination of a single-crystal surface and adsorbed molecular species. In a first part, the focus is given to the impact of a Pd(111) surface exhibiting high chemical activity. Due to surface induced deprotonation two resulting chemical states are found for the adsorbed terephthalic acid species. Deprotonation of the carboxyl moieties is observed to affect the topology of the molecular assemblies even though hydrogen bonds (H-bonds) are responsible for the stabilization of assemblies of both, intact and deprotonated molecular species. Therefore, the selective fabrication of a single specific molecular structure becomes a challenge when the surface is prone to induce chemical reactions. Secondly, a molecular species exhibiting conformational flexibility is studied on a largely inert Au(111) surface. It is observed that the characteristic conformational flexibility is also active on the surface, and the molecule is found to adopt different conformations depending on the locally most favorable bonding motif. In particular, phenyl groups may undergo a rotation such that one-dimensional chains can be stabilized by means of intermolecular H-bonds. This phenomenon is very interesting from an adsorbate-substrate interaction point of view, but it limits the predictability of supramolecular structures formed upon self-assembly. In a third step, a comparative study of the self-assembly of three closely related molecular species highlights the impact of concurrent competing intermolecular interactions. Molecules exhibiting an increasing number of possible interaction channels are found to assemble into an increasing number of distinct supramolecular architectures. The most flexible molecular species allowing for different hydrogen-bonding patterns as well as dipolar coupling self-assembles into various structurally complex architectures. Structural predictability, however, is low since the formation of one or the other assembly appears to depend on small variations in sample preparation conditions and on poorly controllable local surface properties. Finally, a system where the resulting structures fully agree with predictions evidences the possibility to control the formation of targeted specific supramolecular architectures. The formation of well ordered bimolecular ribbons and wires over extended length scales is accomplished by the rational choice of both, complementary building blocks for the selective formation of three-fold H-bonds and a nanostructured surface to guide the molecular self-assembly. Furthermore, the impact of H-bond formation on the electronic structure of self-assembled molecular building blocks is probed by STS measurements. The experimental data reveal shifts in the electronic levels depending on the H-bond configuration and on the local characteristics of the underlying surface. In order to improve the precision of such a study, single molecules, homo- and heteromolecular dimers as well as trimers are successfully isolated and assembled by molecular manipulation with the STM tip. Further STS studies on such artificially assembled structures are suggested.

EPFL2008