Melanie Homberger

On the application potential of gold nanoparticles in nanoelectronics and biomedicine

Ligand-stabilized gold nanoparticles (AuNPs) are of high interest to research dedicated to future technologies such as nanoelectronics or biomedical applications. This research interest arises from the unique size-dependent properties such as surface plasmon resonance or Coulomb charging effects. It is shown here how the unique properties of individual AuNPs and AuNP assemblies can be used to create new functional materials for applications in a technical or biological environment. While the term technical environment focuses on the potential use of AuNPs as subunits in nanoelectronic devices, the term biological environment addresses issues of toxicity and novel concepts of controlling biomolecular reactions on the surface of AuNPs.

Field-Emission Resonances at Tip/α,ω-Mercaptoalkyl Ferrocene/Au Interfaces Studied by STM

The electrical properties of alpha,omega-mercaptoalkyl ferrocenes with different alkyl chain lengths embedded in a self-assembled host matrix of alkanethiols on Au(111) are studied by scanning tunneling microscopy and spectroscopy. Based on current-distance spectroscopy, as well as on the evaluation of Fowler-Nordheim tunneling current oscillations, the apparent barrier height of ferrocene is determined independently by two methods. The electronic coupling of the ferrocene moiety to the Au(111) substrate is shown to depend on the length of the alkane-spacer chain. In a double tunnel junction model our experimental findings are explained, addressing the role of the different molecular moieties of the mercaptoalkyl ferrocenes.

Differential Adsorption of Gold Nanoparticles to Gold/Palladium and Platinum Surfaces

Integration of molecule-capped gold nanoparticles (AuNP) into nanoelectronic devices requires detailed knowledge about the AuNP-electrode interface. Here, we report the pH-dependent adsorption of amine or carboxylic acid-terminated gold nanoparticles on platinum or gold/palladium (30% Pd) alloy, respectively. We synthesized amine-terminated AuNP, applying a new solid phase supported approach, as well as AuNP exhibiting carboxylic acid as terminal groups. The pH-induced agglomeration of the synthesized AuNP was investigated by UV-vis, DLS, and ζ-potential measurements. Depending on the pH and the ionic strength of the AuNP solution a preferential adsorption on the different metals occurred. Thereby, we demonstrate that by choosing the appropriate functional group and adjusting the pH as well as the ionic strength a directed binding can be achieved, which is an essential prerequisite for applications of these particles in nanoelectronics. These findings will pave the way for a controlled designing of the interface between molecule-capped AuNP and metallic electrodes for applications in nanoelectronics.

Self Assembly of Mixed Monolayers of Mercaptoundecylferrocene and Undecanethiol studied by STM

Mixed monolayers of mercaptoundecylferrocene and undecanethiol were deposited from solution by coadsorption and by a two-step insertion method, using the alkanethiol monolayer as insulating matrix. The resulting layers were characterized by UHV-STM, showing molecular resolution. For insertion-processed samples, a mesh-like surface structure of ferrocenes was observed, due to the preferential incorporation of molecules along domain boundaries and defect sites of the alkanethiol monolayer. For monolayers in the intermediate coverage regime, a crystalline phase was observed.

Striped phase of mercaptoalkylferrocenes on Au(111) with a potential for nanoscale surface patterning.

The molecular structure of submonolayer-coverage phases of 3-(thioacetyl)-propanoylferrocene and 5-ferrocenylpentanethiol in mixed layers with alkanethiols on Au(111) was resolved by scanning tunneling microscopy. The ferrocenes formed a striped surface phase, similar to the lying-down structures of alkanethiols, resulting in equally spaced rows of the ferrocene moieties. The obtained nanoscale lattice of functional groups on the surface offers an interesting potential for the patterning of small, periodic structures with precise distance control via a hydrocarbon spacer.

Directed Self-Assembly and Infrared Reflection Absorption Spectroscopy Analysis of Amphiphilic and Zwitterionic Janus Gold Nanoparticles.

Here, we report an approach to use infrared reflection absorption spectroscopy (IRRAS) for the unambiguous proof of the presence as well as the spatial distribution of organic ligands on the Janus gold nanoparticle (AuNP) surface. For this purpose we synthesized amphiphilic and zwitterionic Janus AuNPs and immobilized these on pretreated gold surfaces by directed self-assembly, exploiting hydrophilic/hydrophobic or electrostatic interactions, respectively. Thus, we obtained macroscopic two-dimensional arrays of Janus AuNPs exhibiting a specific orientation. These arrays were investigated by IRRAS, and the obtained spectra revealed only peaks of the ligands facing the IR beam, while the ligands facing the gold substrate were not detected due to reflection of the IR beam on the AuNP cores. Thus, we describe a straightforward spectroscopic procedure to prove the Janus character of zwitterionic and amphiphilic AuNPs in the size range of 10-15 nm.

Polydiacetylene stabilized gold nanoparticles – extraordinary high stability and integration into a nanoelectrode device

A new diacetylene containing photopolymerizable ligand molecule was developed, and tailored for applications in nanoelectronic devices based on gold nanoparticles. This ligand molecule consists of a thiol group, a diacetylene unit and a terminal carboxylic group. The thiol group guarantees preferred binding to the gold nanoparticles surface whereas at the same time the carboxylic group enables electrostatic stabilization. Applying this ligand molecule, gold nanoparticles in the size range of 12–13 nm were prepared. The diacetylene unit was polymerized upon UV irradiation leading to a polymeric ligand shell. Investigations including colloidal stability towards NaCl, DTT displacement reactions, and temperature were performed and indicate an extraordinary high degree of steric and electrostatic stabilization. Individual or at least a few of these particles were immobilized in between nanoelectrodes, thus forming nanoelectronic devices, which were characterized by transport measurements.

DNA-Mediated Assembly of Metal Nanoparticles: Fabrication, Structural Features, and Electrical Properties

Many different synthetic routes have been developed in order to obtain metal nanoparticles of different sizes and shapes. The evolution of high-resolution physical measurements together with the elaboration of theoretical methods applicable to mesoscopic systems inspired many scientists to create fascinating ideas about how these nanoparticles can provide new technological breakthroughs; for example, in nanoelectronic, diagnostic, or sensing devices (de Jongh 1994; Schon and Simon 1995; Simon 1998; Feldheim and Foss 2002; Schmid 2004; Willner and Katz 2004; Rosi and Mirkin 2005). Nanoparticles with a diameter between one and several tens of nanometres possess an electronic structure that is an intermediate of the discrete electronic levels of an atom or molecule and the band structure of a bulk material. The resulting size-dependent change of physical properties is called the quantum size effect (QSE) or size quantization effect (Halperin 1986). 1

Electrochemical and Electronic Charge Transport Properties of Ni-Doped LiMn2O4 Spinel Obtained from Polyol-Mediated Synthesis

LiNi0.5Mn1.5O4 (LNMO) spinel has been extensively investigated as one of the most promising high-voltage cathode candidates for lithium-ion batteries. The electrochemical performance of LNMO, especially its rate performance, seems to be governed by its crystallographic structure, which is strongly influenced by the preparation methods. Conventionally, LNMO materials are prepared via solid-state reactions, which typically lead to microscaled particles with only limited control over the particle size and morphology. In this work, we prepared Ni-doped LiMn2O4 (LMO) spinel via the polyol method. The cycling stability and rate capability of the synthesized material are found to be comparable to the ones reported in literature. Furthermore, its electronic charge transport properties were investigated by local electrical transport measurements on individual particles by means of a nanorobotics setup in a scanning electron microscope, as well as by performing DFT calculations. We found that the scarcity of Mn3+ in the LNMO leads to a significant decrease in electronic conductivity as compared to undoped LMO, which had no obvious effect on the rate capability of the two materials. Our results suggest that the rate capability of LNMO and LMO materials is not limited by the electronic conductivity of the fully lithiated materials.

Polyol mediated synthesis and electrochemical performance of nanostructured LiMn2O4 cathodes

Nanoparticulate single phase LiMn2O4 spinel was prepared via polyol method and applied as a cathode in a lithium ion battery. The effects of calcination temperature (250 °C 800 °C) as well as of postsynthetic treatment by ball milling on the physiochemical and electrochemical properties of LiMn2O4 were studied by means of powder XRD, SEM, cyclic voltammetry and charge/discharge cycling. With increasing calcination temperature, the electrochemical activity and discharge capacity increased. The measurements revealed that the electrochemical performance of LiMn2O4 can be further improved by ball milling before calcination. Furthermore, the ball milling process allowed reducing the calcination temperature needed to obtain electrochemically active material.