The conductive properties of nanodots of model porphyrins were investigated using conductive-probe atomic force microscopy (CP-AFM). Porphyrins provide excellent models for preparing surface structures that can potentially be used as building blocks for devices. The conjugated, planar structure of porphyrins offers opportunities for tailoring the electronic properties. Two model porphyrins were selected for studies, 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) (TPC) and its metal-free analog 5,10,15,20-tetraphenyl-21H,23H-porphine (TPP). Nanodots of TPP and TPC were prepared within a dodecanethiol resist on gold using particle lithography. The nanopatterned surfaces exhibit millions of reproducible test structures of porphyrin nanodots. The porphyrin nanodots have slight differences in dimensions at the nanoscale, to enable size-dependent measurements of conductive properties. The size of the nanodots corresponds to ∼5-7 layers of porphyrin. The conductivity along the vertical direction of the nanodots was measured by applying a bias voltage between the gold surface and a metal-coated AFM cantilever. The TPP nanodots exhibited semi-conductive profiles while the TPC nanodots exhibited profiles that are typical of a conductive film or molecular wire. The engineered nanostructures of porphyrins provide an effective platform for investigation and measurement of conductive properties.
Surface structures
of magnetic nanorings were made using electroless
deposition of Ni onto patterned templates of an amine-functionalized
organosilane. Samples were prepared by chemical approaches based on
colloidal lithography employing a surface mask of size-sorted, monodisperse
silica spheres. Surface changes were evaluated after key points of
the reactions using imaging modes of atomic force microscopy (AFM).
Nanopatterns of 3-aminopropyltriethoxysilane (APTES) were prepared
on Si(111) by applying a heated vapor to a surface mask of silica
spheres. After rinsing, the particle mask was removed to reveal ring-shaped
nanopatterns presenting amine groups at the interface. Organosilane
nanopatterns were then immersed in a solution of Pd catalyst followed
by treatment in a Ni plating bath. Changes in surface morphology after
each reaction step were characterized ex situ using
tapping-mode AFM to follow the time course of nanofabrication. Images
of the Ni nanorings acquired with AFM were compared with SEM micrographs
to further elucidate the morphology of the metal coatings. The magnetic
character of the nanostructures was investigated with magnetic sample
modulation (MSM-AFM), which is a hybrid of contact mode AFM combined
with magnetic actuation of samples. Surface maps of the vibration
of diamagnetic Pd and magnetic Ni nanorings were obtained with MSM-AFM,
providing insight on processes of electroless plating. Fine details
of the surface corrugation and grain structure of the Ni coated areas
of the sample detected with SEM were sensitively resolved with MSM-AFM
that were not apparent in AFM topography frames. Chemistry-based steps
with electroless deposition (ELD) of metal and colloidal lithography
provide a practical route for reproducible nanofabrication of highly
regular geometries with high-throughput.
The surface assembly of 2,3,7,8,12,13,17,18-octaethylporphyrin
(OEP) using silicon tetrachloride as a coupling agent was investigated
using atomic force microscopy (AFM). Nanopatterned films of Si-OEP
were prepared by protocols of colloidal lithography to evaluate the
morphology, thickness, and molecular orientation for samples prepared
on Si(111). The natural self-stacking of porphyrins can pose a challenge
for molecular patterning. When making films on surfaces, porphyrins
will self-associate to form co-planar configurations of random stacks
of molecules. There is a tendency for the flat molecules to orient
spontaneously in a side-on arrangement that is mediated by physisorption
to the substrate as well as by π–π interactions
between macrocycles to form a layered arrangement of packed molecules,
analogous to a stack of coins. When silicon tetrachloride is introduced
to the reaction vessel, the coupling between the surface and porphyrins
is mediated through covalent Si–O bonding. For these studies,
surface structures of Si-OEP were formed that are connected with a
Si–O–Si motif to a silicon atom coordinated to the center
of the porphyrin macrocycles. Protocols of colloidal lithography were
used as a tool to prepare surface structures and films of Si-OEP to
facilitate surface characterizations. Conceptually, by arranging the
macrocycles of porphyrins with defined orientation, local AFM surface
measurements can be enabled to help address mechanistic questions
about how molecules self-assemble and bind to substrates.
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