O. Yildirim

Monolayer-directed Assembly and Magnetic Properties of FePt Nanoparticles on Patterned Aluminum Oxide

FePt nanoparticles (NPs) were assembled on aluminum oxide substrates, and their ferromagnetic properties were studied before and after thermal annealing. For the first time, phosph(on)ates were used as an adsorbate to form self-assembled monolayers (SAMs) on alumina to direct the assembly of NPs onto the surface. The Al2O3 substrates were functionalized with aminobutylphosphonic acid (ABP) or phosphonoundecanoic acid (PNDA) SAMs or with poly(ethyleneimine) (PEI) as a reference. FePt NPs assembled on all of these monolayers, but much less on unmodified Al2O3, which shows that ligand exchange at the NPs is the most likely mechanism of attachment. Proper modification of the Al2O3 surface and controlling the immersion time of the modified Al2O3 substrates into the FePt NP solution resulted in FePt NPs assembly with controlled NP density. Alumina substrates were patterned by microcontact printing using aminobutylphosphonic acid as the ink, allowing local NP assembly. Thermal annealing under reducing conditions (96%N2/4%H2) led to a phase change of the FePt NPs from the disordered FCC phase to the ordered FCT phase. This resulted in ferromagnetic behavior at room temperature. Such a process can potentially be applied in the fabrication of spintronic devices.

Electrochemical stability of self-assembled alkylphosphate monolayers on conducting metal oxides.

Alkylphosphate self-assembled monolayers (SAMs) were prepared on Nb-doped SrTiO(3) (Nb-STO) conducting metal oxide substrates. Unlike thiols on gold, the alkylphosphate SAMs on Nb-STO exhibited an electrochemical stability over a wide voltage range from -2 to 2 V. Cyclic voltammetry showed that the SAM modification inhibited the electrochemical activity of the underlying conducting substrate with an efficiency dependent on the chain length. Impedance spectroscopy showed that SAM-modified Nb-STO substrates have a significantly higher resistance than bare substrates.

Nano-patterned monolayer and multilayer structures of FePtAu nanoparticles on aluminum oxide prepared by nanoimprint lithography and nanomolding in capillaries

Owing to the superior dielectric property of aluminum oxide, precise patterning of self-assembled monolayers (SAMs) and nanoparticles (NPs) on aluminum oxide substrates is highly interesting for generating SAM- or NP-based electronic devices. This study employed nanoimprint lithography (NIL) and nanomolding in capillaries (NAMIC) for patterning ferromagnetic NPs on aluminum oxide substrates. We demonstrated the fabrication of structured arrays of various SAMs and NPs in micrometre and nanometre ranges. The polymer template generated by NIL behaved as a physical barrier and defined the pattern areas on the substrate. FePtAu NPs were assembled on phosph(on)ate SAM-modified polymer patterned substrates. After polymer removal, nano- and microscale line and dot NP patterns, with controlled layer thickness, were obtained on aluminum oxide substrates. Thick nanolines of NPs were obtained by NAMIC. PO3-terminated FePtAu NPs were assembled on alumina without need of a linker. The magnetic properties of the NPs were addressed by vibrating sample magnetometry and those of the patterned NPs by magnetic force microscopy.

Dielectric behavior of self-assembled monolayers on conducting metal oxides

Pt top contacts have been deposited by pulsed laser deposition (PLD) onto bare and tetradecylphosphate (TDP) self-assembled monolayer (SAM)-modified Nb-doped SrTiO3 (Nb-STO) substrates. For the SAM-modified substrates, electrochemical Cu deposition occurred only at the places where electrical shorts existed between the top contact and the substrate. A nearly perfect yield of top contacts without shorts was obtained, which shows the dense packing and robustness of the SAM. The SAM decreased the leakage current about 500 times compared to the bare substrate. Alkylphosphate SAMs on conducting metal oxide substrates can therefore be used as dielectric thin films for device fabrication.