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)