New research advances the integration of hybrid organic-inorganic materials into molecular devices
(Nanowerk News) A group of researchers from Bell Labs, Rutgers Dept. of Chemistry, Stanford's Dept. of Chemical Engineering and the University of Konstanz's Dept. of Physics conducted the first experimental research
that systematically correlates the electrical properties of SAM (self-assembled molecular layers) based
molecular devices with the microscopic details of the
metal–molecule interface. They demonstrated that the
generation of defects can be dramatically reduced for certain
combinations of metals and molecules by changing the surface
topography and growth conditions at the interface. The findings were published in the Feb. 7, 2006 online issue of Nanotechnology with the title "Control of topography, stress and diffusion at molecule–metal interfaces".
"Our research addresses some of the major issues that are slowing the integration of hybrid organic-inorganic materials into practical device structures" Nikolai Zhitenev of Bell Labs told Nanowerk. "The methods described in our paper substantially improve the yields in the fabrication of devices with molecular monolayers active channels and significantly reduce the density of defects caused by metal penetration."
Fabrication of templates and molecular junctions. a) The stencil mask is
defined within an insulating stack of SiO2/Si3N4 layers grown on Si substrate. A layer of
SiO2 (light blue, 10 nm in 5 µm by 5 µm central windows, 200 nm elsewhere on the
wafer) is grown on Si (light purple) to isolate the substrate from the devices followed by
Si3N4 (∼400 nm, cyan) and SiO2 (∼150 nm, blue) mask forming layers. The desired
pattern is defined by photolithography and etching of the top SiO2 layer. Si3N4 is
selectively etched undercutting the top SiO2. The mask is used to define the pattern of
evaporated metal electrodes (yellow). The metal accumulated on the top surface of the
mask is electrically isolated from the evaporated electrodes. b) Fabrication of molecular
junction using the template. Bottom electrodes are defined on the substrate by
evaporation. Molecular layer is deposited from solution. Evaporation of second electrode
from an angle completes the junction. c) SEM images of the template (one out of four
shown samples is shaded) and examples of small and large junctions. (Source: Bell Labs)
Reliable and scalable integration of organic molecules within
nanoscale electronic devices has the potential to dramatically
expand available device functionality. The electronic properties of molecular
devices that are just a few atomic layers thick are determined not solely by the properties
of the molecules but are equally dependent on dopants, defects and electronic states at the interfaces.
The incorporation of molecules in
small devices calls for the simultaneous solutions of many
interrelated material, electronic and chemical issues.
The researchers studied the phenomena affecting the conductance of molecular devices by systematically varying the growth
conditions at the metal-molecule interface. Transport properties of metal-molecule-metal junctions containing monolayer of
conjugated and saturated molecules with characteristic dimensions in the range of 30-300 nm are correlated with microscopic
topography, stress and chemical bonding at metal-molecule interfaces. Small shadow masks are used to obtain features below
the usual lithographic limits.
It was demonstrated that the defects/interfaces can be
rationally controlled and that their properties are often more important than the electronic properties of molecules in
determining the device conductance. The density of defects caused by metal penetration into monolayers is significantly reduced increasing the yield to >90%. Their statistically significant data set allowed the researchers to conclude that the conductivity of organic molecules ∼1.5 nm long is at least 4 orders of magnitude lower than is commonly believed.
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