Spin Filtering Along Chiral Polymers

Article abstract of DOI:10.1002/anie.202006570

Spin-dependent conduction and polarization in chiral polymers were studied for polymers organized as self-assembled monolayers with conduction along the polymer backbone, namely, along its longer axis. Large spin polarization and magnetoresistance effects

Full text of DOI:10.1002/anie.202006570

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
www.angewandte.org
Accepted Article
Title: Spin Filtering Along Chiral Polymers
Authors: Ron Naaman, Suryakant Mishra, Amit Kumar Mondal, Eilam
Z. B. Smolinsky, Katsuhiro Maeda, Tatsuya Nishimura,
Tsuyoshi Taniguchi, Takumu Yoshida, Kokoro Takayama,
and Eiji Yashima
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Angewandte Chemie International Edition
10.1002/anie.202006570
Spin Filtering Along Chiral Polymers
1
1
1
1*
Suryakant Mishra, Amit Kumar Mondal, Eilam Z. B. Smolinsky, Ron Naaman,
Katsuhiro Maeda,2,3* Tatsuya Nishimura, Tsuyoshi Taniguchi, Takumu Yoshida, Kokoro
3*
3
3
Takayama, Eiji Yashima4
3
1
.
Department of Chemical and Biological Physics, Weizmann Institute, Rehovot 76100,
WPI Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, Kakuma-
Israel.
2
.
machi, Kanazawa 920-1192, Japan.
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-
machi, Kanazawa 920-1192, Japan.
Department of Molecular and Macromolecular Chemistry, Graduate School of
3
.
4
.
Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan.
Abstract
Spin dependent conduction and polarization in chiral polymers were studied, when the
polymers are organized as self-assembled monolayers and the conduction is along the polymer
backbone, namely along its longer axis. Large spin polarization and magnetoresistance effects
were observed, which showed a clear dependence on the secondary structure of the polymer.
The results indicate that the spin polarization process does not include spin flipping and hence
it results from backscattering probabilities for the two spin states.
*
Corresponding authors:
Ron Naaman email: ron.naaman@weizmann.ac.il
Katsuhiro Maeda email: maeda@se.kanazawa-u.ac.jp
Tatsuya Nishimura email: nishimura@se.kanazawa-u.ac.jp
TOC Graphics:
1
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Introduction
In recent years it was established that when electron is transmitted through chiral molecules
one spin is preferred over the other. This effect was termed the Chiral Induced Spin Selectivity
(CISS) effect and it results from the coupling between the linear momentum of the moving
electron and its spin degree of freedom by the special symmetry properties of the chiral
1
2,3
4
potential. The CISS effect was observed in bio-molecules like DNA, peptides, and proteins
5
,6
7
as well as in helicene, chiral perovskites, chiral polymers, and other molecules. In the case
of polymers and supramolecular structures, several CISS related studies were performed. In
the first, a thin film of chiral polymer was deposited on the surface and the magneto resistance
8
effect was measured. In another working electrode coated with chiral molecules was used for
9
enhancing hydrogen production in water splitting process. In a recent study, the effect of
secondary chiral structures of supramolecular systems on the spin selective transmission was
investigated and correlated with the circular dichroism (CD) intensity for nanofiber-based
materials.10
While for most of the chiral molecules the spin selectivity was measured when the
molecules are adsorbed on a solid substrate and the measurements were performed along the
main molecular axis (the longest dimension), perpendicular to the substrate plane, for polymers
this configuration is problematic. Therefore, typically in the case of chiral polymers and wires,
the measurements were conducted when the long axis of the molecules are in plane and the
spin polarization was measured perpendicular to the molecular main axis (Figure 1A). These
measurements necessarily were lacking precision regarding the paths of the electrons in the
film studied and they did not probe the effect of the polymer backbone on the spin polarization.
The present study aims to overcome the deficiencies in former studies of the CISS effect
in polymers. Here the polymers of well controlled length were adsorbed on a substrate when
the long axis of the polymers is about perpendicular to the substrate plane (Figure 1B). The
spin polarization was measured for electron transferred along the main axis of the polymer,
hence probing the polymer backbone itself as a spin filter.
Originally the CISS effect was observed when electron transmission through chiral
molecules was found to be spin dependent. However, later it was realized that the CISS effect
results also in transient spin polarization that is accompanied by charge polarization in chiral
molecules. Namely, when the electric field is applied on a chiral molecule, or when a chiral
molecule interacts with another molecule or with a surface, charge reorganization occurs in the
molecule. This spin dependent charge reorganization (SDCR) affects the reactivity of chiral
molecules and contributes to the enantio-selectivity in the interaction between chiral
2
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Angewandte Chemie International Edition
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1
1
molecules. It also affects the interaction of chiral molecules with ferromagnetic surfaces and
it can be applied for separating enantiomers.12
Figure 1: (A) The typical measuring configuration for spin dependent electron transport
through chiral polymers. In this case, the conduction perpendicular to the long axis of the
polymer is measured. (B) The measuring configuration applied in the current study, where the
chiral polymers are organized as a self-assembled monolayer and the spin dependent
conduction is measured along the long axis of the polymer.
Here we are presenting the study of the spin filtering properties of a set of helical
poly(phenylacetylene)s with a thiol group (-SH) at a terminal end synthesized using a novel
living polymerization system.13 The polymers are poly(4-ethynylbenzoyl-L-alanine decyl
ester) (poly-1L),14 poly(4-ethynylbenzoyl-D-alanine decyl ester) (poly-1D),14 and poly(4-
ethynylbenzoyl-2-methylalanine decyl ester) (poly-2) (Figure 2A, more in Supporting
1
5
Information). These polymers are adsorbed as a self-assembled monolayer (SAM) through
thiol to gold coated substrate and the spin filtering properties are measured when the electrons
are conducted along the backbone of the polymer. Both uniformity of surface and layer
thickness are required to achieve stable spin polarization and its measurements. The SAMs of
the polymers used here, which have precisely controlled length, are especially suitable for
measuring the spin filtering properties.
The poly-1L and poly-1D have enantiomeric one-handed helical structures that are
induced by the enantiomeric chiral side groups, while poly-2 bearing achiral side groups
consists of an equal mixture of right- and left-handed helical structures. The sense of
handedness of poly-1, right- or left-handed helix, depends on the handedness of the side groups
and the solvent used.16 We investigated how the spin filtering properties depend on the
macromolecular helicity and on its inversion when the monolayer is exposed to toluene vapors.
The spintronics properties were measured by magnetic conductive probe atomic force
microscopy (mcp-AFM), by Hall, and by magnetoresistance devices.
3
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10.1002/anie.202006570
Result & Discussion
Figure 2: (A) The structures of the polymers studied and a scheme showing the binding of the
polymers to the gold surface. (B) An AFM image of a monolayer of poly-1L providing the total
thickness of the SAM which is about 6 nm. (C) IR spectra recorded for the SAM on gold in
reflection mode (top) and drop-cast film in transmittance mode (bottom). (D) The CD and
absorption spectra of the polymers in THF.
The SAMs of the given polymers were characterized by various methods. Figure 2A
presents a scheme of the polymer used in this study and Figure 2B shows the atomic force
microscope (AFM) image of the SAM of poly-1L, which indicates a densely packed monolayer
with a thickness of about 6 nm. The monolayer was also characterized by IR spectroscopy in
reflection mode (Figure 2C), where one can clearly see the peaks related to the amide and ester
-1
at about 1650 and 1736 cm , respectively. The spectrum of the monolayer can be compared
with transmittance mode spectrum of the drop-cast film (bottom panel). Figure 2D shows the
CD and absorption spectra of the polymers in THF at concentration of ~0.3 mM (0.1 mg/mL).
As expected, poly-1L and poly-1D showed characteristic CD peaks in the polymer backbone
region (250–450 nm) that are just the mirror image of each other, which can be assigned to the
1
6
left- and right- handed helical structures, respectively, whereas no CD peak was observed for
the optically inactive poly-2.
4
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Figure 3: (i) Scheme of magnetic conducting probe AFM (mcp-AFM) setup. Panels A and B
present the current versus voltage (I-V) curves averaged over about 100 scans (see the
Supporting Information) and recorded for poly-1L and poly-1D, respectively when the magnet
north pole is pointing up (blue) or down (red). Panels C and D show the calculated spin
polarization obtained for poly-1L and poly-1D, respectively.
Figure 3 presents the results obtained by magnetic–conductive probe atomic force
microscopy (mcp-AFM) where the SAMs of the polymers were absorbed on a ferromagnetic
substrate (Ni/Au) (see details in the Supporting Information). An electric potential is varied
between the substrate and the grounded Pt tip of the AFM and the current through the SAM is
measured as a function of the potential for the magnetic substrate magnetized with its north
pole up or down. About 100 current-voltage (I-V) curves were recorded on each sample and
their average is plotted in Figures 3A and 3B (see Figure S5 in the Supporting Information).
Corresponding semi log plots are presented in the insets which show the non-linear dependence
of the current on the applied potential and the difference in the threshold for the currents of the
electrons with the two spin polarization. Figures 3C and 3D present the spin polarization, SP
(
Iup − Idown
)
)
(
%) = (Iup
x 100, as a function of the applied voltage, when ꢀꢁꢂ and ꢀꢃꢄꢅꢆ are the
+ Idown
currents measured when the magnetic north pole of the substrate is pointing towards or away
from the monolayer, respectively. The absolute spin polarization of about 50% indicates a ratio
of about 1:3 in the spin transmission. For the optically inactive poly-2, no difference was
observed in the averaged I-V curves when the magnet north pole is pointing up or down (see
Figure S6 in the Supporting Information).
5
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10.1002/anie.202006570
The non-linear dependent of the current on the applied voltage was observed before in
CISS based mcp-AFM measurements.1,17,18 It is also clear that a different threshold for charge
injection is found for the two spins. The difference is on the order of 100±10 meV and which
spin is injected at lower potential depends on the handedness of the polymers. For the poly-1L
(
poly-1D), the lower spin corresponds to magnet pointing up (down), namely the spins injected
are polarized antiparallel (parallel) to the electron's velocity. The difference in threshold for
injection of the two spin is an indication that during the electron transmission through the
polymers, no spin flipping occurs.19
Figure 4: Chemical activated helix switching using toluene vapor. (A) CD and absorption
spectra of poly-1L recorded in THF (solid line) and toluene (dotted line). (B) Averaged I-V
curve of poly-1L after toluene treatment when the magnet north pole is pointing up (blue) or
down (red).
There are already indications for the correlation between the optical activity of chiral
molecules and the extent of the CISS effect in them.3,20 This correlation is manifested also in
the current study. While the results shown in Figure 3 are obtained when the poly-1 is initially
dissolved in THF, the CD spectrum of poly-1 changes the sign upon dissolving the polymer in
toluene (Figure 4A). The toluene apparently induces a switch in the handedness of the
1
6
secondary helical structure of poly-1 as was verified in the past studies. It is important to
appreciate that when a monolayer of the poly-1L, made from the THF solution, is exposed to
toluene vapors which switch the helicity of the main chain from the left- to right-handed helix,
similarly to the process occurring in solution. The spin dependent current was probed for poly-
1
L sample before and after toluene treatment. Before the toluene treatment the current was
higher when the magnet is down (Figure 3A) while after being exposed to toluene, the current
6
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Angewandte Chemie International Edition
10.1002/anie.202006570
is higher for magnet pointing up (Figure 4B). The results show again the correlation between
changes in the CD spectra and the CISS effect. Despite the fact that the monomer has the same
chirality, the secondary structure of the polymer depends on the solvent and affects the spin
selectivity.
While the mcp-AFM studies described above, were performed in ambient, the next set
of experiments were done in a solution using the Hall based methods.6,21,22 These Hall devices
are made of GaN/AlGaN structure with two dimensional electron gas (2DEG) interface layer.
A common Hall device is applied for monitoring magnetic field. It is based on a constant
current applied between source and drain electrodes. When a magnetic field is acting
perpendicular to the surface of the device, as a result of the Lorentz force, electric potential can
be measured perpendicular to the source-drain current and to the magnetic field. This potential
is the “Hall potential”. In the device used here, no external magnetic field is applied, but instead
chiral polymers are adsorbed on the surface of the device. Details on the device fabrication and
experimental setup are given in the Supporting Information. Two types of measurements were
conducted. In the first type, the polymers are adsorbed on the device which is placed within the
electrolytic solution using PDMS cell. Externally top gold electrode placed on the top of the
solution facing the device and is used to apply the electric field. On the application of the
electric field, the polymer is charge polarized and if this charge polarization is accompanied by
spin polarization, the spins that are injected from the polymer into the device create a magnetic
field which can be monitored by the Hall probes (Figure 5i). The mechanism by which this
device is operated is described in details in reference 22. In the second type of experiments,
spin dependent electrochemistry measurements were conducted (Figure 5ii). Here, the Hall
device served as the working electrode in a common electrochemical cell that includes also a
counter and reference electrodes. When the electrochemical cell operates, electrons have to
pass through the polymer’s SAM on their way from or to the working electrodes. If the current
is spin polarized, it will result in Hall potential. Hence, this scheme provides in addition to the
regular C-V plots, also the Hall potential dependent on the voltage. Namely indication for the
spin polarization of the current in the electrochemical cell. It is important to appreciate the
difference between the two modes of experiments used the Hall device. In the first type of
measurements, namely the monitoring of the spin polarization that accompanied the charge
polarization, no current is flowing and the spin injected into the Hall device is due to the gate
voltage that inject charge, electrons or hole, into the device. In the second type of studies, the
spin dependent electrochemistry, constant current is flowing and the spin associated with this
current is monitored.
7
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10.1002/anie.202006570
(A) 16
(B)16
1
0V
1
2
8
4
0
4
8
12
8
8V
6
V
4V
4
2V
0
-
-
2V
-4
-8
-12
-16
(
i)
4
V
6V
8
V
-
12
16
1
0V
poly-1L
poly-1D
100
-
0
100
200
0
200
Time (sec)
Time (sec)
(C)
4
0
poly-1D
3
2
1
0
-
1
Counter
Electrode
-2
Reference
Electrode
-
3
4
(ii)
-
poly-1L
0
2
4
6
8
10
Gate Voltage (V)
(D)
(E) 7
0
1
0
8
6
4
2
0
0
6
0
0
-
-
-
-
-
-
2
5
4
poly-1L
poly-1D
40
30
poly-1L
poly-1D
6
20
8
10
0
10
12
-10
0
.0 0.1 0.2 0.3 0.4
0
.0
0.2
0.4
EG(V vs Ag/AgCl)
E (V vs Ag/AgCl)
G
Figure 5: Schematic presentation of the experimental set of Hall measurement in the
polarization (i) and electrochemical mode (ii). Hall potential recorded in polarization mode as
a function of time for (A) poly-1L and (B) poly-1D for various gate pules. (C) The Hall voltage
as a function of the gate voltage for monolayers of poly-1L (blue) or poly-1D (red). (D) Hall
potential recorded in the electrochemical mode as a function of the voltage when the working
electrode (the Hall device) is coated with monolayers of poly-1L (blue) or poly-1D (red). (E)
The CV curves obtained in the electrochemical process when the working electrode is coated
with monolayers of poly-1L (blue) or poly-1D (red). Note: All the electrochemical
measurements were performed using ferrocene as redox probe in water. Pt wire was used as
the counter electrode, the drain electrode of the Hall device was used as the working electrode
and silver wire used as a reference electrode.
Figures 5A and 5B show the Hall signal as a function of time, obtained for poly-1L and
Poly-1D, respectively, when pulses of varying voltage are applied between the gate electrode
and the Hall device. Clearly the magnitude of the Hall potential measured, correlates with the
voltage applied (see Figure 5C) and its sign with the handedness of the polymers. Indicating
that upon the charge polarization, the polymer is also spin polarized and which spin
corresponds to which electric pole depends on the handedness of the polymers.
8
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10.1002/anie.202006570
In the electrochemistry related experiments, we measured the spin selective electron
transfer through the polymers. All the electrochemical measurements were performed using
ferrocene as redox probe in PBS buffer. Pt wire was used as the counter electrode, the drain
electrode of the Hall device was used as the working electrode and silver wire used as a
reference electrode. Figure 5D shows the Hall potential obtained when the potential in the
electrochemical cell was varied between 0 to 0.4 V. The corresponding cyclic voltammetry
(CV) plots are shown in Figure 5E. While the CV plots are basically identical for the two
enantiomers, the Hall potentials obtained having opposite signs. The small hysteresis observed
in the Hall potential plots is an indication for the asymmetry between the spin polarization in
the oxidation and reduction currents that may result from differences in the interfaces on the
two sides of the polymers, one being the solid substrate and the other the electrolyte.
(i)
SQUID-MR
(A)
(B)
2
MR-3.3%
MR-2.1%
3
1
0
2
1
0
poly-1L
poly-1D
-
1
2
-
-0.6 -0.3 0.0 0.3 0.6
-0.6 -0.3 0.0 0.3 0.6
Field (T)
Field (T)
Figure 6: (i) Scheme of the 4-probe magnetoresistance (MR) measurement setup with bottom
gold and top Ni electrode. The inset shows the combination of layers in the MR device. The
MR signal of (A) poly-1L and (B) poly-1D devices.
9
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The ability to apply the polymers for spintronics application was probed by producing
a spin valve which operates because of its magnetoresistance (MR) effect. The device is
presented schematically in Figure 6i. A crossbar geometry was employed, when a 2 mm-wide
1
0/40 nm thick Ti/Au bottom electrode was deposited on silicon oxide wafer using optical
lithography. On top of this electrode a monolayer of the helical polymer was deposited
followed by an insulating buffer layer of 1.5 nm-thick magnesium oxide (MgO) and then a
ferromagnetic nickel/gold (30/8 nm) 15 mm wide electrode. MR measurements were carried
out using a SQUID-MPMS3 (Quantum Design).
The crossbar geometry enables us to measure accurately the resistance of the device by
the standard 4-probe configuration. The magnetoresistance is defined as
(
RH − RH=ꢇ)
MR (%) =
x 100, where ꢈ is the measured infield resistance and ꢈꢉꢊꢋ is the zero-
ꢉ
(
RH=ꢇ)
field resistance. Figure 6 presents the MR results measured at the ambient condition with a
maximum field of 0.6 T and a constant current of 1 mA, for poly-1L and poly-1D. The MR
plot is asymmetric relative to the field sign, not like in the common MR devices. The reason
for the asymmetry is that only one ferromagnetic electrode is used, while the chiral polymers
transmit mainly one spin. The MR results are consistent with the other spin dependent
measurements, however the relative low values result probably from pin holes in the
monolayers and from electron scattering. As observed before, there is no simple correlation
between the magnitude of the MR signal and the magnitude of the spin polarization as
measured by the mcp-AFM. This is because the MR signal depends on the precise geometry,
the width of the electrodes used and the quality of the monolayers, while the mcp-AFM signal
typically only one or very few molecules are measured and the measurement is much less
sensitive to pin-holes in the monolayer. We have observed a clear correlation between the MR
studies and the conduction through the polymers measured with the mcp-AFM. For example
in the case poly-1L, using the mcp-AFM, the current is lower and the resistance in MR is higher
when the magnet is in UP configuration (Figure 6A).
Conclusion
8
Past studies on chiral polymers were conducted either on chiral polymers, polythiophene, or
1
0
on helical supramolecular π-conjugated nanofibers. In both cases the polymers were lying on
the surface. In the case of the polythiophene, the spin polarization was measured to be about
3
4% and the magnetoresistance measured was less than 1%. In the case of the supramolecular
nanofibers, very high spin polarization of about 85% was measured. The difference is attributed
to the fact that while in the present case the current is conducted along the polymer backbone,
1
0
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in the case of the supramolecular wires the conduction is across the diameter, and the process
involves sequential hopping between the molecules. However the magnetoresistance in this
case was around one order of magnitude lower than in the current study. The low values of the
magnetoresistance observed in the past are due to the ill-defined organization of the polymers
on the surfaces that allows for pin-holes. While in the present study the organization is closely
packed and well defined.
The present study confirms that when current is transmitted through the backbone of chiral
polymer, the transmission is spin dependent, similarly to what was observed when the current
is transmitted perpendicular to the backbone. The results observed here are very stable and the
variation among various measurements is small. This stability is a result of the uniformity in
the length of the polymer and its structure. The correlation between the change in the secondary
structure and the spin polarization point to the important role the secondary structure plays in
defining the magnitude of the CISS effect. The results also confirm that the spin polarization
does not involve spin flipping and they are consistent with recent findings that the spin
polarization is a result of suppression of back scattering for the favoured spin, while the other
spin is scattered back.10
Finally, we see that the common CISS effect is going hand to hand with the spin dependent
charge reorganization (SDCR) and with asymmetric MR effects. Hence well controlled chiral
polymers are good candidates for organic-spintronics CISS-based applications.
Acknowledgement
EZBS and RN acknowledge the help of Dr. G. Leitus in conducting the MR measurements.
SM, AKM, EZBS, and RN acknowledge the partial support from the Israel Ministry of Science,
from Israel Science Foundation, and from the John Templeton Foundation. This work was also
supported in part by JSPS KAKENHI (Grant-in-Aid for Specially Promoted Research, no.
1
8H05209 (K.M. and E.Y.) and Grants-in-Aid for Scientific Research (C), no. 18K04723
(
T.N.)) and World Premier International Research Center Initiative (WPI) of the Ministry of
Education, Culture, Sports, Science and Technology (MEXT), Japan.
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10.1002/anie.202006570
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