1

Designing interfaces for Spin Injection into Organic Molecular Solids: A Surface Science Approach

SESAPS November 11, 2016

Jingying Wang, Drew Deloach, Dan DoughertyDepartment of Physics and Organic and Carbon Electronics LabNorth Carolina State University

Mykhaylo Myahkostupov, Christopher M. Papa, Felix CastellanoDepartment of Chemistry and Organic and Carbon Electronics LabNorth Carolina State University

Wei Jiang and Feng LiuDepartment of Materials Science and Engineering, University of Utah

Technological Motivation: Spintronics

Spintronics =“Spin-electronics” : Applications of spin-dependent charge transport

Already a huge industry:

Chappert et al.Nature Materials 6,813 (2007).

Large change in metallic sensor resistancedue to the small magneticfield of a bit.

Current

RR

2

Fert Grünberg

Nobel Prize in Physics 2007

“For the discovery of Giant Magnetoresistance”

Baibich et al., Phys. Rev Lett. 61, 2472 (1988).

3

Recall Nobel Prizes in 2007

Fert Grünberg

Nobel Prize in Physics 2007

“For the discovery of Giant Magnetoresistance”

Baibich et al., Phys. Rev Lett. 61, 2472 (1988).

Nobel Prize in Chemistry 2007

Ertl

“For his studies of chemical processeson solid Surfaces”

Ertl, Nobel Lecture.

Organic Semiconductors: New Mechanisms in Spintronics ?

4

Organic Semiconductors: highly controllable optoelectronic properties by synthesis

1) Localized hopping transport – new “polaron” mechanisms of spin transport?

2) Synthetic tuning of molecular magnetism is very sophisticated

Pentacene LUMO

e-

Mn12 : S = 10

Motivation: Magnetoresistance in Organic Semiconductors

5

Z. H. Xiong et al. Nature, 427, 821 (2004)

“GMR” in an Organic Spin Valve:

Jiang et al. Phys. Rev. B 77, 035303 (2008).

No MR observed in thick Co/Alq3/Fesandwiches?

6

Interface Effects: Spin Control with Molecular Orbitals?

300% “Tunneling GMR” in Alq3

Nanojunctions

Barraud et al., Nature Physics6, 615 (2011).

* Strong molecularcoupling invertseffective interface polarization

12

Interface Effects: Spin Control with Molecular Orbitals?

Atoderesei et al., Phys. Rev. Lett.106, 066601 (2010).

Spin Polarized STM Imaging

Example: pz-d hybridization can control spin polarization at interfaces

DFT Calculations

13

Metal-Organic Interfaces in Spintronics: A Chemistry View

Roald Hoffman, “A chemical and theoretical way to look at bonding on surfaces”Rev. Mod. Phys. 60, 601 (1988).

Spin Down Spin

Up

I learned about the beauty of these

ideas as a Post-Doc with John Yates

at Pitt

15

Scanning Tunneling Microscopy

Quantum tunneling from a sharp tip

allows atomic-resolution images:

Scanning Tunneling Spectroscopy

(STS)

Tunneling Current:

Density of States:

exp(-f(E,V)z)

16

Spin Polarized STM : High Resolution TMR on Clean Surfaces

R.Wiesendanger, Rev.Mod.Phys. 81,1409 (2011)

costsSASA mmGGG

Spin-polarized conductance:

AGG

GGm

APP

APPs

Spin polarized DOS and spin asymmetry A:

Fe-coated Tungsten tips often giveSpontaneous spin contrast on Cr(001):

Spin Contrast on Cr(001)

We use this to make “model” spin valves!

tip

Cr(001)

19

Clean Cr(001) Topograhic STM image

Surface Electronic Structure of Cr(001) at T= 130 K

50 nm

Tunneling Spectrum of Cr(001)

Surface state peak is due to dz2 orbitals

on Cr(001)

22

Spin Polarized Interface States For Spintronics

A

PP Cr(001)

Alq3

23Wang and Dougherty, Phys. Rev. B 92, 161401R (2015).

Decomposing the Spin Polarized Interface States for Alq2/Cr(001)

F-LUMO

Cr dz2

Surface

state

Enhances Fermi level spin polarization at the interface!

Similar to the interaction mechanism in our previous study of PTCDA/Cr(001):

24

Hybridization Mechanism: Alq3 on Cr(001)

Alq3

Cr(001)

charge transfer

Similar to mechanism in

Wang and Dougherty,

Phys. Rev. B 92, 161401R (2015).

25

Alq3 vs. Crq3: Changing d-orbital content

25

HOMO

LUMO

Alq3: p frontier orbitals Crq3: p+d frontier orbitals

DFT Calculations from W. Jiang and F. Liu, University of Utah MSE

S = 0 S = 3/2

26

Cr(001)

Crq3

Cr

SS is gone!

27

dz2 surface state

LUMO

d orbital of Cr

dds*

dds

Crq3

Cr(001)

Hybridization Mechanism for Crq3 on Cr(001): Orbital Mixing

Something like a conventional

covalent bond forms here

28

Crq3

Alq3

Comparative Density Functional Theory Studies of Alq3,Crq3 on Cr(001)

zb = 0.262 nm

zb = 0.282 nm

29

Connecting DFT Trends with SPSTM Spectroscopy

Crq3

Alq3

Substrate PDOS Molecule PDOS

30

31

Summary: Tuning from Metallic to Resistive Spin Filters

Spin polarized interface states form at Alq3 and Crq2 interfaces with Cr(001)

The nature of the interfaces is surprisingly sensitive to details of orbitals

We can tune from a metallic to a resistive interface within a single

Charge transfer

Interaction (p-sp)

Metallic interface

Local covalent bonding

Interaction (d-d)

Resistive interface

See e.g. Raman

Applied Physics Reviews1, 031101 (2014).

32

SPSTM Funded by DOE – BES Electron and ScannedProbe Microscopy Program(DE-SC0010324)

Dr. Jingying WangDrew Deloach

Synthesis: Mykhaylo Myahkostupov, Christopher Papa Phil Castellano

DFT modeling: Wei Jiang Feng Liu

Acknowledgments

Dept. of Chemistry

NC State

Dept. of Physics

NC State

Dept. of Materials Science and Engineering

University of Utah

The Organic and Carbon Electronics Lab (ORaCEL)

33

A new interdisciplinary center supporting collaborative efforts in organic semiconductors,

conjugated polymers, graphene, carbon nanotubes and hybrid carbon materials

Shared instrumentation, seminar series, proposal and course development

Funded by the NC Carbon Materials Initiative

Director: Prof. Harald Ade

https://oracel.wordpress.ncsu.edu/

Science Collaborations?

Broader Impact support

network?

34

35

-1 0 1-2

-1

0

1

2

PD

OS

(Sta

tes/e

V)

Energy(eV)

dxy

dyz

dz2

dxz

dx2-y2

total

-1 0 1-2

-1

0

1

2

PD

OS

(Sta

tes/e

V)

Energy(eV)

dxy

dyz

dz2

dxz

dx2-y2

total

Surface of Cr(001) Sub-Surface of Cr(001)

dz2 surface state near Fermi level

These calculations show evidence of a dz2 –derived surface state near the Fermi Level (

While not in perfect agreement with experiment, it is at least qualitatively reproducing the symmetry, spin polarization,

and existence of the surface state

No dz2

36

-2 -1 0 1-3.0

-1.5

0.0

1.5

3.0

P

DO

S(S

tate

s/e

V)

Energy(eV)

Cr-(Sub)-d

Cr-(Crq3)-d

O-p

N-p

C-p

37

Traditional Semiconductors: highly controllable electronic properties by doping

1) Band transport with challenging spin injection:

2) Magnetic doping is a difficult materials science problem

Appelbaum et al., Nature 447, (2007).