Download Organic Nanofiber Impedance Spectroscopy

Organic Nanofiber Impedance Spectroscopy
Organic semiconductors constitute an important class of materials for electronic and
opto-electronic devices such as organic field-effect transistors (OFETs) and lightemitting diodes (OLEDs). For example, organic semiconductors are used for flexible
electronics and in organic flat panel displays. Typically, the organic semiconductor is
processed into a thin film and electrical contact is realized through a pair of
lithographically defined metal electrodes. However, this often results in ‘large’ devices
with dimensions of several hundred micrometers or more. For future devices with a
higher level of integration – such as microsystems for biochemical sensing - smaller
devices with nanoscale dimensions are required.
In the NanoSYD research group at the Mads Clausen Institute, we are working with a
particular class of organic semiconducting nanofibers. The basic building block is a small
molecule (such as hexaphenylene as seen in fig. 1a), which can be assembled into a
nanofiber structure (fig. 1b) through a special self-assembly process. These nanofibers
with cross sectional dimensions of the order of 100 nanometers have a range of
interesting properties such a being semiconducting and able to emit and guide light.
Among other things, our research is focused on fabricating nanoscale OFETs and OLEDs
from nanofibers. This requires a detailed investigation of the electrical carrier injection
and transport. One experimental technique for making such investigations is impedance
spectroscopy, where the frequency dependence of the device impedance is examined.
Figure 1 (a) Space-fill model of a para-hexaphenylene (p6P) molecule. (b) Top view of a p6P
nanofiber structure model. The individual molecules are stacked in a nanofiber structure with a
typical width between 100 and 400 nm, a typical height between 20 and 50 nm, and an as-grown
length of typically some hundred micrometers. (c) Scanning electron microscope (SEM)
image of a single p6P nanofiber contacted with two metal electrodes for electrical
characterization.
In this project, you will work on constructing a set-up for impedance spectroscopy
measurements and use this to investigate the electrical properties of such nanofibers. The
system consisting of one (or several parallel) nanofiber(s) between two metal electrodes
can be considered as a one-port, which should be characterized within a bandwidth
between a few Hz and a few tens of kHz. By fitting the measured impedance to an
equivalent circuit model, it is possible to extract device parameters that can be used to
investigate the carrier transport mechanism in detail.
The project will involve the following tasks:

Literature study of:
o different measurement techniques for impedance spectroscopy
o device modeling of organic semiconductor devices

Realization and test of measurement set-up
o Design, construct, and characterize a suitable measurement set-up for
nanofiber impedance spectroscopy
o Set-up and program a commercial lock-in amplifier (Labview
programming) to compare with the constructed set-up
o Determine appropriate comparison parameters (e.g. bandwidth, signal
levels etc.)
o Compare the performance of the two systems using different “known”
components

Investigate the nanofiber impedance frequency response and extract device
model parameters
Supervisors
Jakob Kjelstrup-Hansen [email protected]
Horst-Günter Rubahn [email protected]