Self-assembly of nanoparticles

Nanoparticles are classified as having at least one of three dimensions be in the range of 1-100 nm. The small size of nanoparticles allows them to have unique characteristics which may not be possible on the macro-scale. Self-assembly is the spontaneous organization of smaller subunits to form larger, well-organized patterns. For nanoparticles, this spontaneous assembly is a consequence of interactions between the particles aimed at achieving a thermodynamic equilibrium and reducing the system’s free energy. The thermodynamics definition of self-assembly was introduced by Nicholas A. Kotov. He describes self-assembly as a process where components of the system acquire non-random spatial distribution with respect to each other and the boundaries of the system. This definition allows one to account for mass and energy fluxes taking place in the self-assembly processes. This process occurs at all size scales, in the form of either static or dynamic self-assembly. Static self-ass

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

Bottom-up approaches for organizing nanoparticles with polymers

Caterina Minelli

This thesis describes some of three years'work carried at the Centre Suisse d'Electronique et de Microtechnique (Neuchâtel, Switzerland) under the supervision of Dr. Martha Liley and in collaboration with Prof. Horst Vogel of the Ecole Polytechnique Fédérale of Lausanne (Switzerland). The goal of this work is to contribute to the development of innovative technologies in the treatment of surfaces, for the lateral organization of various materials on the micrometer and nanometer scale. The aim is therefore to offer a valid alternative to common lithographic techniques that often require expensive capital equipment and infrastructures. In spite of the high-quality and precision of their products, these techniques often have a limit of ~70 nm on the resolution of the minimum feature size. In this work, techniques based on the ability of some systems to self-organize were used. In particular, the tendency of immiscible polymer mixtures and of block copolymers to form separate phases was exploited. Films made of different polymer phases formed templates for the organization of various materials, with length scale that varied from some 10 µm to some 10 nm. The materials that were most investigated were particles of the diameter of some nanometer, made of semiconductors (CdSe and ZnS) and of metals (Au, CoPt3 and Co). These nanoparticles have different properties from the corresponding bulk material. This fact is due to two main phenomena: in a nanoparticle the electronic energy levels of electrons are discrete because of their spatial confinement. Also, surface effects arise due to the high surface to volume atom ratio compared to the bulk material. As an example, semiconductor particles of the diameter of some nanometers are fluorescent. Semiconductor nanoparticle fluorescence is usually characterized by long fluorescence lifetimes. This fact promoted the investigation, in the framework of this thesis, of the fluorescence properties of Mn-doped ZnS nanoparticles. These particles turned out to have exceptionally long fluorescence lifetimes compared to the fluorophores commonly used as fluorescent markers in biological techniques. This singular property allowed the construction of a simple and cost effective instrument for time-resolved detection of the nanoparticle fluorescence. This technique allows a remarkable increase of the signal-to-noise ratio compared to conventional detection methods. Two approaches were explored to laterally organize nanoparticles on polymer surfaces. In the first method, the nanoparticles were added into a mixture of immiscible polymers and a film was formed from the solution by spin-coating, typically on silicon or glass slides. Numerical simulations by other authors indicate that if the particles have a higher affinity for one of the two polymers, they will be distributed in the corresponding phase. This behaviour was confirmed by our experiments. The lateral dimensions of the patterns in which the nanoparticles organized could be changed with continuity from some 10 µm to se sub-micrometer range. Their shape was modelled from a stochastic to an ordered type. The second approach explored for the organization of the nanoparticles consisted of the pre-formation of the polymer films and their subsequent decoration with the nanoparticles. Due of the different interactions of the two polymers with the particle-solvent system, different absorption behaviours of the nanoparticles were found on the two polymer phases. This technique allowed nanoparticle organization on homopolymer demixed films in patterns having typical sizes in the micron and in the submicron range. Alternatively, the nanoparticles were organized on block copolymer films in regular patterns having typical periodicities of the order of 100 nm. Nanoparticles organized on thin block copolymer films could be transferred on the hard substrate via removal of the polymer molecules by oxygen plasma etching. This process did not affect the nanoparticle organization. Under particular conditions, an aggregation of 10 nm gold nanoparticles was induced using oxygen plasma. This technique allowed the formation of gold nanowires and nanostructures both on polymer layers and on the hard substrate. Their width varied from about 25 nm to the micrometer, while their length extended for various micrometers. They presented a fingerprint like structure or, alternatively, quasi-parallel nanowires extended for several µm2, the typical periodicities being about 100 nm. The conductivity of these nanowires and nanostructures was demonstrated using SEM.

EPFL2004

Molecular Engineering Strategies for Morphology Control in Organic Semiconductors for Optoelectronics

Muhammad Aiman Bin Rahmanudin Hamdan

It is understood that the optoelectronic performance of organic electronic devices is determinant on the macroscopic solid state self-assembly of organic semiconductors (OSC), which is driven by the supramolecular Ï-Ï stacking interactions between the conjugated segments in its molecular structure. Molecular engineering approaches directly impacts the intrinsic self-assembling properties of the OSCs, but the flexibility of organic chemistry on modulating the backbone architecture of the Ï-conjugated system has led a vast library of OSCs with various functionalities, which fundamentally changes the electronic properties of the Ï-conjugated system. It is highlighted that introducing conjugation break spacers (flexible linkers) between the Ï-conjugated segments in the OSC, showed great promise in controlling supramolecular self-assembly without altering the semiconducting core of the OSC. Previous examples on controlling the morphology of the molecular OSC, DPP(TBFu)2 using horizontal and vertical dimers, and a horizontal flexibly linked polymer analogues, have shown great promise in effecting the self-assembly behaviour and stabilizing the morphology of the parent (non-flexibly linked) DPP(TBFu)2 molecule, when used as an additive. In this thesis, an extension of this work is presented whereby the flexible linking approach is used to design and synthesize two molecular compatibilizers that consists of the donor component, DPP(TBFu)2 that is linked with an aliphatic spacer to an acceptor component, based on a fullerene and perylenediimde small molecule OSC. In chapter 2, a comparison between the compatiblizer (CP) and an in-situ linker approach (ISL) was explored to elucidate the impact of stabilizing a multi-component bulk heterojunction (BHJ) morphology and its device performance for organic photovoltaics (OPVs). It was concluded that the CP approach shows the most promise in stabilizing the BHJ morphology for OPVs, which was then applied onto a highly crystalline BHJ system with a perylenediimde acceptor to demonstrate its versatility as described in chapter 3. Taking leverage from this demonstration, chapter 4 discusses how the CP approach is used to tune the phase-domain size of the BHJ that is processed from a homogeneous single-phase melt, to obtain a photoactive BHJ in an OPV device. This unique demonstration, ultimately opens up vast new possibilities for solvent free âgreenâ processing of OPVs. Lastly in Chapter 5, the approached used to address BHJ morphological stabilization is slightly different from that of previous chapters, where the (kinetic) stability of a binary donor-acceptor BHJ is addressed. In this chapter, a fully-conjugated block copolymer (BCP) consisting of donorâacceptor blocks is used to demonstrate its applicability for a single-component BHJ for OPVs. However, the main challenge for this approach is in the synthetic methodology, and to overcome this, a modular synthetic strategy using Heck coupling between two functionalized donor and accepting marcromonomers showed promise in obtaining a fully conjugated BCP for OPVs

EPFL2018