Atomic‐Level Elucidation of the Initial Stages of Self‐Assembled Monolayer Metallization and Nanoparticle Formation

Density Functional Theory Study of Pd Aggregation on a Pyridine-Terminated Self-Assembled Monolayer

By using density functional theory calculations, the initial steps towards Pd metal cluster formation on a pyridine-terminated self-assembled monolayer (SAM) consisting of 3-(4-(pyridine-4-yl)phenyl)propane-1-thiol on an Au(1 1 1) surface are investigated. Theoretical modelling allows the investigation of structural details of the SAM surface and the metal/SAM interface at the atomic level, which is essential for elucidating the nature of Pd-SAM and Pd-Pd interactions at the liquid/solid interface and gaining insight into the mechanism of metal nucleation in the initial stage of electrodeposition. The structural flexibility of SAM molecules was studied first and the most stable conformation was identified, planar molecules in a herringbone packing, as the model for Pd adsorption. Two binding sites are found for Pd atoms on the pyridine end group of the SAM. The strong interaction between Pd atoms and pyridines illustrates the importance of SAM functionalisation in the metal nucleation process. Consistent with an energetic driving force of approximately -0.3 eV per Pd atom towards Pd aggregation suggested by static calculations, a spontaneous Pd dimerisation is observed in ab initio molecular dynamic studies of the system. Nudged elastic band calculations suggest a potential route with a low energy barrier of 0.10 eV for the Pd atom diffusion and then dimerisation on top of the SAM layer.

Coordination controlled electrodeposition and patterning of layers of palladium/copper nanoparticles on top of a self-assembled monolayer

A scheme for the generation of bimetallic nanoparticles is presented which combines electrodeposition of one type of metal, coordinated to a self-assembled monolayer (SAM), with another metal deposited from the bulk electrolyte. In this way PdCu nanoparticles are generated by initial complexation of Pd²⁺ to a SAM of 3-(4-(pyridine-4-yl)phenyl)propane-1-thiol (PyP3) on Au/mica and subsequent reduction in an acidic aqueous CuSO₄ electrolyte. Cyclic voltammetry reveals that the onset of Cu deposition is triggered by Pd reduction. Scanning tunneling microscopy (STM) shows that layers of connected particles are formed with an average thickness of less than 3 nm and lateral dimensions of particles in the range of 2 to 5 nm. In X-ray photoelectron spectra a range of binding energies for the Pd 3d signal is observed whereas the Cu 2p signal appears at a single binding energy, even though chemically different Cu species are present: normal and more noble Cu. Up to three components are seen in the N 1s signal, one originating from protonated pyridine moieties, the others reflecting the SAM-metal interaction. It is suggested that the coordination controlled electrodeposition yields layers of particles composed of a Pd core and a Cu shell with a transition region of a PdCu alloy. Deposited on top of the PyP3 SAM, the PdCu particles exhibit weak adhesion which is exploited for patterning by selective removal of particles employing scanning probe techniques. The potential for patterning down to the sub-10 nm scale is demonstrated. Harnessing the deposition contrast between native and PdCu loaded PyP3 SAMs, structures thus created can be developed into patterned continuous layers.

Surface Modification of a n-Si(111) Electrode through Aldehyde Grafting and Subsequent Metallization: Theory and Experiment

Motivated by our previous studies on metallic substrates, in the present work we addressed the functionalization and the subsequent metallization of a hydrogen-terminated n-Si(111) electrode. DFT provides atomistic insights on the grafting mechanism of 4-pyridinecarboxaldehyde (C6H5NO) what encouraged electrochemical investigations, i. e. cyclic voltammetry and in-situ STM, combined with XPS measurements which together provide evidence for a successful transfer of the so far obtained knowledge from metal single crystal to semiconductor surfaces.

Influence of water on the properties of an Au/Mpy/Pd metal/molecule/metal junction

The geometric and electronic structure of the metal-molecule interface in metal/molecule/metal junctions is of great interest since it affects the functionality of such units in possible nanoelectronic devices. We have investigated the interaction between water and a palladium monolayer of a Au(111)/4-mercaptopyridine/Pd junction by means of DFT calculations. A relatively strong bond between water and the palladium monolayer of the Au/Mpy/Pd complex is observed via a one-fold bond between the oxygen atom of the water molecule and a Pd atom. An isolated H(2)O molecule adsorbs preferentially in a flat-lying geometry on top of a palladium atom that is at the same time also bound to the nitrogen atom of a Mpy molecule of the underlying self-assembled monolayer. The electronic structure of these Pd atoms is considerably modified which is reflected in a reduced local density of states at the Fermi energy. At higher coverages, water can be arranged in a hexagonal ice-like bilayer structure in analogy to water on bulk metal surfaces, but with a much stronger binding which is dominated by O-Pd bonds.