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Towards Molecular Spintronics: Different conductivity through open- and closed-shell y g p
molecules
NúriaCrivillers; ncrivillers@icmab.esInstitut de Ciència de Materials de Barcelona (CSIC) CIBER-
BBN, Spain
AtMol International Workshop on Architecture & Design of Molecule Logic Gates and Atom Circuitsand Atom Circuits
Barcelona 12th- 13th January
SPINTRONICS is the ability of injecting, manipulating and detectingelectron spins into solid state systems. Devices that specifically exploitsspin properties of electrons instead of its charge.MOLECULAR‐SPINTRONICS, where spin‐polarized currents are carriedthrough molecules, and in turn they can affect the state of the molecule.MOLECULAR‐SPINTRONICS, where spin‐polarized currents are carriedthrough molecules, and in turn they can affect the state of the molecule.
MOLECULAR‐ELECTRONICS Investigates the possibility of makingelectronic devices using organic molecules.
Molecular‐Spintronics: the art of driving spin through molecules by Stefano Sanvito and AlexandreReily RochaMolecular‐Spintronics: the art of driving spin through molecules by Stefano Sanvito and AlexandreReily Rocha
The two possible spin states represent ‘0’ and ‘1’ inlogical operations.g p
To develop this field, one major point is to find novel ways of both generation andconservation of spin polarized current.p p
Why organic molecules?Why organic molecules?
Due to their weak spin‐orbit coupling and hyperfine interactions, organicmolecules are considered to be ideal media for spin transport in which spinmolecules are considered to be ideal media for spin transport, in which spincoherence over time and distance could be preserved much longer than ininorganic materials.
Theoretically: Spin filters, i.e.,as devices favoring transport ofelectrons with either spin up or ORGANIC RADICALS Spin‐polarization and its
conservationspin down.
Limitation: only for situationsin which spin flips can be
l t d
conservation.
Defined as a molecule with oneor more unpaired electrons, andhence it exhibits a magneticneglected.
J. Am. Chem. Soc. 2010, 132, 3682
hence, it exhibits a magneticmoment.
Some very appealing recent findings…
Across NP of the SPM network:
In the lower‐T range: conductance of SPM network <<< SLM network.
Spin‐polarized wire molecules (SPM)
Spinless wire molecule (SLM)
Across NP of the SLM network:Across NP of the SLM network:
Phys. Rev. B. 2008, 77, 235316
This is the first experimental demonstration of the interaction between a singleorganic localized spin with an electron tunneling through the molecule.
Importance of studying the transport through the moleculeImportance of studying the transport through the molecule
Polychlorotriphenylmethyl (PTMs) radicals as bistable and switchablemutifunctionalmolecules
Highly persistentEasily functionalized
ClCl
Cl
C lC l
C l
C lCl
ClCl
C l
C l
C lC l
Cl
Cl
ElectroactiveC lCl
C l
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
ClCl
Cl
Cl Cl
Cl
ClCl
Cl
Cl Cl
Cl
ClCl
Cl
Cl Cl
oxidaciódesprotonaciódeprotonation oxidation
ClCl
Cl Cl Cl
closedclosed--shellshell openopen--shell S=1/2shell S=1/2
SELF-ASSEMBLED MONOLAYERS (SAMs)
Surface properties
Intermolecular lateral
Functional ending group
Intermolecular lateral interactionsSpacer
Well defined thickness
Surface chemisorptionBinding group
Surface Anchoring groups
GoldThiols
disulfidesSulfinic acids
-Well-defined thickness-Acts as a physical barrier-Alters electronic conductivity and local Sulfinic acids
SiO2 Silanes
ITOSilanes
carboxylic or
yoptical properties
yphosphonic acids
Cl ClCl
Cl
Preparation of PTM SAMs: Two different approaches
ClCl
Cl
Cl
ClCl
Cl
Cl
‐Electrochemicalproperties
H H H
ClCl
R
‐Molecular wireAu
RR
H
HHH
R
I) Two-step approachH
HH HH H
II) One-step approachH
H
C-AFM: Three-dimensional Mode:3D for transport measurements
At each tip-sample distance avoltage is applied between twofi d l
Normal Force (Fn) and thecurrent (I) as a function of thebias voltage (V) and the samplefixed values bias voltage (V) and the sampledisplacement distance towardsthe tip (z) are simultaneouslymeasured
In collaboration with Dr. Carmen Munuera, Marcos Paradinas and Prof. Carmen Ocal (ICMAB-CSIC)
measured.
0 0
0.5
1.0
A)V V
At each Fn is possible to obtain the I/V
-2 -1 0 1 2-1.0
-0.5
0.0
I (nA
V
I(V,z)F(V,z)0n
m
0nm 0.0
0.5
1.0
I (nA
)
z
800mV
1.
800mV
1.
14
-2 -1 0 1 2-1.0
-0.5
V
15
68
101214
nN)
-10
-5
0
5
10
15
I (nA
)
0246
F(n
-2 -1 0 1 2
-15
V
100
0 1 2 3 4 5
z(nm)Voltage range: +2VPiezo movement: 5nm
-50
0
50
I (nA
)
Piezo movement: 5nm-2 -1 0 1 2
-100
V
From C. Munuera
PTM Radical : nonMson Gold SurfacesI) Two‐step approach: non‐conjugated PTM SAM on gold
Adsorption Organization
Cl
Cl
Cl
Cl
Cl
Cl
Cl
ClCl
Cl
Cl Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
ClCl
Cl
Cl Cl
NH2 NH2 NH2
SHH2N
Cl
Cl
Cl
Cl
NH
O
NH2 NH2
Cl
Cl
Cl
Cl
NH
O
Au
Au
S S S
Contact angle = 64°
Contact angle = 84°
AuS S S S
Contact angle = 84
Adv. Mater. 2009, 21, 1177
Transport properties through radical and non‐radical SAMs
SAMLL difference in theirmolecular structure but LARGEmolecular structure but LARGEdifference in the electronicstructure
R(H) =280 MΩR(r) =35 MΩ
R(H) =200 MΩR(r) =12 M Ω
Energylevels of non‐radical and radical SAMsDFT calculationson PTM‐(CO)‐NH‐Ph
Spin filter???Non resonant Spin filter???Non‐resonant tunneling
Radical SAM: LUMO assistedtransport contribution of resonantRadical SAM: LUMO assistedtransport, contribution of resonanttunneling mechanism
II) One‐step approach i.e., direct anchoringDesign and Synthesis:
MeS AcSCHO CHO1.MeSNa,HMPA
2.AcCl
Design and Synthesis:
Cl ClCl
ClCl
Cl
Cl
Cl
SAcH
Cl ClCl
ClCl
Cl
Cl
Cl CH2PPh3 BrH 1.tBuOK = SAcHHH H
ClCl
Cl
ClCl
ClCl
ClClCl Cl
ClClClCl Cl Cl
Cl2) AgNO3
1) (nBu)4NOH
2. 1, THF
ClCl
Cl ClCl
Cl
Cl
Cl
Cl Cl
S
Cl Cl
ClClCl
Cl
Cl
Cl
ClCl
S
= S-S
ClCl Cl Cl
Transport properties through conjugated PTM SAMs with C‐SFM
10000
H
Junction resistance (R) as a function of applied force
6000
8000 Radical
)
2000
4000
R (M
4eV
2eV
0 2 4 6 8 100
Fn (nN)
R(H)/R(rad) is one order ofmagnitud higher than the non-conjugated SAM
Non‐radical SAM: non‐resonanttunnelingRadical SAM:resonant tunneling
conjugated SAM
(B3LYP13 hybrid functional and a 6‐31G(d,p) basis set)
Radical SAM:resonant tunneling
ChemCommun. 2011, 47, 4664
100Negative differential resistance (NDR):
I/V for radical PTM SAM:
40
60
80
100decreasing current through a junctionat increasing voltage.
*Attributed to resonant tunneling
40
-20
0
20
F= 0.5nN
I(nA
)
Attributed to resonant tunnelingbetween molecular orbitals and the metaldelocalized states.
*Junctions exhibiting nonlinear current‐
-100
-80
-60
-40 F 0.5nNF= 0.75nNF= 1nNF= 1.25nN
Junctions exhibiting nonlinear currentvoltage properties such as NDR couldserve as nanoscale analogues ofmultistate electronic switches (J. Am.Chem. Soc. 2004,126,295)
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Voltage(V)
, , )
Diff t i i f NDRDifferent origins for NDR:
-Conformational changes.
-Charging of the molecule followed by thelocalization and delocalization of orbitals.
Polaron formation in redox active molecules-Polaron formation in redox active molecules.
I/V for radical PTM SAM:100
I/V for non-radical PTM SAM:
100
0
50
A)
0
50
)
-50
0
1.25 nN 1 nN 0.75nN
I(nA
-50
0
9 nN 8 nN3.5 nN
I(nA
)
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-100
Voltage (V)-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-100
Voltage (V)
In these systems, the redox character does not determine the NDR
Representative I/V for both SAMs at differentapplied loads, at about 2nN higher for thenon‐radical.
R(αH) ~ 4000 MΩR(rad) ~ 100 MΩ
PTM 1H PTM 1 d at V 0Vat V ‐1.5VNegative differential resistances (NDR) in both I-V curves
PTM 1H PTM 1rad
ChemCommun. 2011, 47, 4664
Proposed transport mechanism
At low bias Non resonant Resonant tunnelingAt low bias Non‐resonant tunneling
Resonant tunnelingmediated by LUMO‐β
At higher bias Some resonant tunneling with the At higher biasunoccupied orbitals
PTM 1H PTM 1rad
Conclusions
Open‐shell form is significantly more conducting than the closed‐shellderivativederivative.
Larger conductivity is observed for the conjugated radical in agreementith l h b idi ti ith th t l fwith a larger hybridization with the metal surface.
The redox character does not determine the NDR phenomena.
LUMO‐β plays an important role in the transport which could be exploitedfor spintronics.
These type of comparatives measurements can help the fundamentalunderstanding of the transport mechanism.
Acknowledgments
Co‐workers:Dr Cla dia Simao
Financial support:•Dr. Claudia Simao
•Dr. Marta Mas‐Torrent
•Prof. Concepció Rovira p
•Prof. Jaume Veciana
Collaborations:
Marcos Paradinas (ICMAB Barcelona)Marcos Paradinas (ICMAB, Barcelona)
Dr. Carmen Munuera (ICMAB, Barcelona)
Prof. Carmen Ocal (ICMAB, Barcelona)
Prof. Stefan Bromley (ICREA, U. Barcelona)
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