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Chiral Conductive Polymers as Spin Filters
Prakash Chandra Mondal, Nirit Kantor-Uriel, Shinto P. Mathew, Francesco Tassinari,
Claudio Fontanesi,* and Ron Naaman*
Dr. P. C. Mondal, N. K. Uriel, Dr. S. P. Mathew,
Prof. R. Naaman
Department of Chemical Physics
The Weizmann Institute of Science
Rehovot 76100, Israel
E-mail: ron.naaman@weizmann.ac.il
Dr. F. Tassinari, Prof. C. Fontanesi
Department of Chemical and Geological Science
University of Modena and Reggio Emilia
Via G. Campi 183, Modena 41125, Italy
E-mail: claudio.fontanesi@unimore.it
DOI: 10.1002/adma.201405249
electrode. By controlling the direction of the magnetization of
nickel, it is possible to inject electrons that have mainly one
spin orientation and to verify their transport through the PCT
by monitoring the cyclic voltammetry (CV) curve in the electro-
chemical cell in which the redox couple is not chiral. The CV
curves reflect a steady state in which a double layer is formed
near the working electrode. Another method to monitor the
spin selectivity is by chronoamperometry, in which one moni-
tors time-dependent current at a fixed potential. At the begin-
ning, when the potential is turned on, the current is high,
but it is reduced with time due to the formation of the double
layer.[18,19]
The polymer was deposited on nickel electrodes by electro-
chemical deposition or on Au by dipping (see the Supporting
Information for details). The monolayers of PCT (on Ni, Au)
were characterized using PMIRRAS, AFM, ellipsometry, and
contact angle measurements (see the Supporting Information).
The deposition procedures used were essential for obtaining
spin selectivity, since other deposition methods, like spin
coating or drop casting, induced the formation of relatively
thick layers of polymer that exhibited no spin selectivity effect.
The lack of spin selectivity is due to extensive scattering of elec-
trons in thick film, resulting in spin randomization. The experi-
mental setup is shown schematically in Figure 2A. The PCT-
coated nickel working electrode was magnetized with magneti-
zation pointing up or down, using a permanent magnet of 0.5 T.
The direction of the nickel magnetization determines the spin
direction of the injected electrons. Hence, flipping the magneti-
zation amounts to flipping the direction of the spins injected,
from being antiparallel to the electrons’ velocity to parallel.
The solid-state device is based on a thin gold film (60 nm
thickness) on top of which PCT (in chloroform solution) is
adsorbed by dipping. An Al2O3 film of 1.5–2 nm thickness is
grown on top of the SAM by ALD to act as an efficient tunnel
barrier for the spin injection.[20] A 150 nm Nickel (Ni) layer was
deposited on the Al2O3 layer in order to analyze the spin injec-
tion efficiency. The magnetoresistance (MR) of the device was
measured as a function of an external magnetic field up to 0.5 T,
when the field was applied parallel to the current through
the PCT-L film (perpendicular to the Ni film/sample plane),
using the four-probe method at 1 mA current at various fixed
temperatures. The percentage of magnetoresistance, MR =
100 × [R(H) – R(H=0)]/R(H=0), is calculated from the meas-
ured in-field resistance and the zero-field resistance at fixed
temperatures.
Figure 3A presents the CV curve obtained with a nickel
working electrode coated with PCT-L when achiral ferrocene
(Fc) serves as the redox couple in methanol solution. The results
are striking, since when the magnet points up, one clearly sees
that the current flips its sign upon changing from oxidation to
reduction, whereas when the magnet points down, the current
The introduction of conducting polymers to organic electronics
has revolutionized the technology and conducting polymers are
now part of the OLED based appliances.[1–4]
Recently spin-LED/
OLED,[5–7]
has been suggested for improving the efficiency of
LEDs and OLEDs and a spin-OLED system was indeed suc-
cessfully demonstrated.[8]
According to this new concept, the
electrons injected into and from the light-emitting species
have their spin predetermined; therefore, the formation of
“dark,” nonemitting triplet states is avoided. This is expected to
enhance significantly the efficiency of the LEDs, a critical issue
in view of the desire to reduce the energy consumption of elec-
tronic devices. The control of the spin in a spin-LED or spin-
OLED device typically requires a magnetic element that defines
the spin orientation. Combining magnetic components with
LED or OLED technology is challenging both because of the
material constraints and the interfaces between the different
components that may affect charge and mainly the spin injec-
tion. Here, we show that by using chiral conductive polymers
(CCPs)[9–12]
it is possible to inject spins with very high spin
selectivity. This finding introduces the possibility of producing
spin-OLED without any magnetic component and without
introducing additional production restrictions. The spin selec-
tivity observed results from the chiral-induced spin selectivity
(CISS) effect reported recently.[13–15]
In the CISS effect the elec-
tron's spin and linear momentum are coupled by the chiral
potential so that one spin is preferred for electrons moving
in one direction in the potential, whereas the opposite spin is
preferred for electrons moving in the opposite direction. The
precise spin orientation is determined by the molecular hand-
edness. The spin selectivity is demonstrated here by using a
spin-specific electrochemistry method and a solid-state device.
Here, we investigated spin-selective electron transport
through poly{[methyl N-(tert-butoxycarbonyl)-S-3-thienyl-L-
cysteinate]-cothiophene}[17]
(PCT-L, Figure 1A–C). Measure-
ments were performed in two configurations: in an electro-
chemical cell at room temperature (Figure 2A) or in a solid-
state device (Figure 2B) at various temperatures. In the case of
the electrochemical cell, the polymer was deposited on a nickel
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does not change direction, indicating a much higher barrier for
electron conduction from the nickel electrode towards the solu-
tion. In addition, in the reduction direction the ratio between
the current of the two spins exceeds a factor
of two at certain potentials. Clearly this is not
the case when electrons are not spin polar-
ized, as shown in Figure 3B. Hence, the PCT
is a very efficient spin filter, allowing mainly
electrons with a certain spin to be conducted
in a certain direction, as expected in the CISS
effect.[13]
Direct proof that the polymer deposited
on the surface is indeed chiral is given in
Figure 4. The CV curves were recorded for
nonmagnetic electrodes (Au) coated either
with PCT-D or PCT-L when the solution con-
tained either the R or S chiral N,N-dimethyl-
1-ferrocenyl-ethylamine. Unequivocally, there
was an enantio-selective interaction between
the adsorbed polymer and the chiral redox
probes in the solution. The adsorbed PCT-D
interacts strongly with S-ferrocene (red
curve), whereas PCT-L interacts stronger with
R ferrocene (blue curve).
To demonstrate unambiguously the spin
selectivity in electron transport through the
polymer, the chronoamperometric meas-
urements were performed at two different
potentials when the nickel magnetization
points up or down. The curves obtained are
shown in Figure 5. The spin polarization,
SP, is defined as SP
I I
I I
=
−
+
↑ ↓
↑ ↓
, where I↑ and I↓
represent the current when the nickel is mag-
netized up or down, respectively. The curves
were taken at potentials that reflect mainly
the oxidation (0.32 V) and the reduction
(0.12 V). Indeed, in the first case the current
has a positive sign, whereas in the second the
sign is negative.
At short times after the voltage pulse
(2–3 s) the spin polarization is 34% and
–50% for potentials of 0.32 and 0.12 V,
respectively. At longer times (about 100 s)
the spin polarization disappeared completely
for the 0.32 V signal and became –16% at
0.12 V. As shown in Figure 3A, the spin fil-
tering strongly depends on the voltage and
varies dramatically with it. Figure 5 also
indicates that the formation of the double
layer on the nickel working electrode affects
the spin filtering properties of the polymer.
This may result from the electrical poten-
tial built up as a function of time,[21]
which
changes the conformation of the polymer
and as a result, reduces its spin-filter prop-
erties. This phenomenon of field-induced
reduction in chirality was indeed recently
reported when the same polymers were
deposited in the presence of an electric field.[22]
The spin filtering ability of PCT was also demonstrated by
fabricating a solid-state device (Figure 2B).[23]
Figure 6 shows
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Figure 1. A) Structure of poly{[methyl N-(tert-butoxycarbonyl)-S-3-thienyl-L-cysteinate]-cothio-
phene} (PCT-L). B) Hydrogen bond-based interactions between the substituents. C) Helical
structure formed by PCT-L in the solid state, owing to the hydrogen bonding between the polar
substituents and the π staking of the thiophene rings.
Figure 2. A) The electrochemistry setup in which Ni is the working electrode (WE) and platinum
and saturated calomel electrode (SCE) are the counter (CE) and the reference (RE) electrodes,
respectively. The polymer is adsorbed on the Ni, which is magnetized by an external magnetic
field (H) with the magnetic dipole pointing up or down. B) The scheme of the solid-state device
used. The chiral PCT polymer polarizes the spin distribution of the transmitted electrons and
the spin is probed by the magnetic field dependence of the resistivity through the Ni layer. The
chiral layer that defines the spin being transferred is not affected by the external magnetic field.
Figure 3. A) The CV plots for a PCT-L-coated Ni working electrode when the redox couple is
achiral ferrocene. The black and red curves correspond to a plot taken when the Ni is mag-
netized down or up, respectively. For a comparison, the CV plot is shown in B) for a PCT-L-
coated gold working electrode, when electrons are not spin polarized. The voltammograms
were recorded for 0.5 × 10–3
M ferrocene (Fc) in 0.07 M tetrabutylammonium tetrafluoroborate
(TBA-TFB) in methanol.

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that the MR measured at various temperatures is not symmetric
with respect to the zero magnetic field, as in common magneto-
resistance devices,[24]
but rather, it is antisymmetric, i.e., the
device’s resistance depends on the sign of the magnetic field,
unlike in common magnetoresistance devices where the resist-
ance depends only on the absolute magnitude of the field and
not on its sign. This indicates that PCT transmits only one spin
orientation, independent of the external magnetic field. When
no external magnetic field is applied, the magnetic domains of
the Ni are oriented mostly in plane and the net magnetization
is zero.[25]
When the magnetization of the Ni is parallel to the
direction of the spin favored for transmission through PCT, the
device has high resistance, since there are fewer unoccupied
majority spin states near the Fermi level in the Ni.[26]
However,
when the magnetization of the Ni points in the opposite direc-
tion, the device has low resistance since there are many unoc-
cupied minority spin states near the Fermi level in the Ni.[26]
When the external magnetic field is tuned
up from zero, more domains in the Ni are
magnetized in the field direction and hence
the magnetoresistance increases or decreases
depending on the direction of the external
field. The asymmetry is opposite when the
current flows in the opposite direction. The
magnetoresistance (MR) of the device with
spin-coated polymer is symmetric with
respect to zero field and has a value of 0.1%
at 2 T which is typical to Ni (see Supporting
Information).[27]
This observation shows
that the spin coated polymer film cannot act
as spin filter. The basics of the operation of
CISS-based magnetoresistance devices are
explained in ref. [15].
The MR observed is about 8% and it varies
slightly with temperature mainly due to the
change in the resistance of the device. The variation in temper-
ature observed in the present device is very small as compared
with ordinary magnetoresistance devices.[28]
We found that
although devices that are composed of PCT-L exhibit reproduc-
ible results, those composed of PCL-D (under identical condi-
tions) had highly varying properties and many of them were not
stable after being measured for a short time. We attribute this
observation to the fact that the PCT-D polymer has different
chain length than the PCT-L and hence has higher tendency to
form aggregates of several chains, both in solution and as solid.
As a result, its chiral secondary structure is less stable than that
of PCT-L (see Supporting Information for details).
The spin-dependent electron transmission results obtained
with the solid-state device confirm those obtained electro-
chemically. The magnitude of the MR, which is below the spin
polarization obtained in the electrochemical cell, may result
from pinholes in the polymer layer that allow for conduction
of unpolarized spins. It is important to appreciate that whereas
in the electrochemical set-up the polymer is adsorbed directly
on the nickel, in the solid-state device it is adsorbed on the gold
Adv. Mater. 2015, 27, 1924–1927
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Figure 4. A) CV curves obtained with PCT-D when the redox couple is either S-ferrocene (red)
or R-ferrocene (black). B) CV curves obtained when PCT-L is used with S-ferrocene (green) and
R-ferrocene (blue). The working electrode in this case is gold.
Figure 5. Chronoamperometric curves of Ni electrodes coated with PCT-L
in an aqueous solution of 3.8 × 10–3 M K4[Fe(CN)6]/K3[Fe(CN)6] redox
couple in 0.4 M KCl. The black and red curves correspond to nickel mag-
netization pointing up or down, respectively. The upper curves (0.32 V)
were taken for the oxidation process, whereas the lower curves (0.12 V)
were for the reduction process.
Figure 6. Magnetoresistance curves obtained with a solid-state device
composed of PCT-L using an external magnetic field up to 0.5 T at dif-
ferent temperatures.

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electrode and the electrons tunnel through the Al2O3 layer. This
may cause some spin scattering.
The present work evidently shows that a 3-nm-thick film of
chiral conductive polymer can serve as a very efficient spin
filter at room temperature. This phenomenon opens up the
possibility of constructing an organic spin-OLED that does not
require any ferromagnetic elements.
Experimental Section
Electrochemical Measurements: Cyclic voltammetry (CV) and
chronoamperometric measurements were performed using a Bio-Logic
potentiostat SAS (Model SP-200) with a software EC Lab (V 10.36), by
employing a typical three-electrode electrochemical cell arrangement. The
composition of the solutions used in the electrochemical measurements:
with (a) a chiral redox probe of 0.8 × 10–3
M (R or S) ferrocene in a
10 × 10–3
M potassium phosphate buffer solution at pH 7, (b) an achiral
redox probe of 0.5 × 10–3
M ferrocene in 0.07 M tetrabutylammonium
tetrafluoroborate (TBA-TFB) in methanol. Chronoamperometric
measurements were performed in an aqueous solution of 3.8 × 10–3
M
K4[Fe(CN)6]/K3[Fe(CN)6] redox couple in 0.4 M KCl. Polymer-coated Ni or
Au, a Pt wire, and a saturated calomel electrode (SCE) were used as the
working, counter, and reference electrodes, respectively. Spin-dependent
electrochemical measurements were carried out in the presence of
a permanent magnet having a magnetic field strength of up to 0.5 T,
which was placed underneath the Ni working electrode, followed by
changing the magnet's direction (UP/DOWN). All the electrochemical
measurements were carried out at room temperature.
Solid-State Device Fabrication and Magnetoresistance Measurements: The
device is based on a vertical structure in which the bottom electrode is
composed of gold coated with the PCT polymer, on top of which an Al2O3
layer is deposited, followed by a layer of nickel. The devices were prepared
by photolithography, followed by e-beam evaporation, on a silicon substrate
with thermally grown 300 nm SiO2 (〈100〉, >400 Ω cm–2
). The 1-µm-wide,
2-mm-long, and 60-nm-thick Au line was evaporated on an 8-nm-thick
Cr adhesion layer. An Al2O3 layer with a thickness of about 2 nm was
deposited on top of the SAM at 100 °C by atomic layer deposition. The top
150-nm-thick, 50-µm-wide Ni line was evaporated without any adhesion
layer. 150-nm-thick gold contact pads for wire-bonding were evaporated.
The solid-state device was attached to a sample holder and electrically
connected to the measuring units so that electrons will be injected from
the Au electrode through the chiral PCT layer into the Ni electrode. It
was placed in-between the magnetic poles and on a cold finger that
could be cooled down to 14 K. A magnetic field up to 0.5 T could be
applied perpendicular to the sample plane by an electromagnet. The
temperature of the sample holder was controlled by a PID temperature
controller with a temperature stability of 0.1% at 14 K and 0.3% at 300 K.
The resistance of the device was measured at a relative accuracy of more
than 10 ppm using a standard four-probe method with cross bridge
geometry. Typically, a dc current of 1 mA from a Keithley 6221 current
source was passed through the device. The voltage drop across the
junction was measured using a Keithley Nanovoltmeter 2182A device.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
P.C.M. and N.K.-U. contributed equally to this work. The research was
supported by an ERC-Adv grant, by the Israel Ministry of Science, and by
the VW Foundation.
Received: November 17, 2014
Revised: December 17, 2014
Published online: January 23, 2015
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