The Fe(001)-p(1x1)O surface: scanning tunneling

microscopy (STM) results and related issues

XCVII Congresso Nazionale

Lamberto Duò

CNISM - Dipartimento di Fisica – Politecnico di Milano

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

2Oxides…and “thin” (few nanometers) oxide films

• Important class of materials among insulators

• Finite-size effects systems exhibit different properties w.r.t. bulk

“Intrinsic” dimension smaller than a relevant length scale

(e.g. “thin”/transparent vs “thick”/opaque)

“Surface related” surface atoms have reduced coordination

(e.g. different binding energy surface core-level shift)

“Chemical” & “Structural” differences between bulk and surface

(e.g. surface segregation, relaxation, reconstruction)

• Most studied 3d Transition Metal Oxide (TMO) thin films

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

3Motivations

• Understand & control the physics of TMO thin films

advances in e.g. nanoelectronics, heterogeneous catalysis

and high density magnetic data storage

(2011)

Microscopic morphology by STM

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

4

( for spectroscopy and STM)

single crystal TMO thin films

TM evaporation on a monocrystalline substrate in gas (O2) atmosphere

(undefined stoichiometry & thickness, unhomogeneities, only TMO/TM)

How to make TMO thin films

• Exposing a Transition Metal to O2 atmosphere

• Mid ’90s: “Reactive deposition”

• Proper choice of substrate & its surface, TM, gas, growth conditions

(temperature, pressure, evaporation rate, time), after growth treatments

Ag(001) MnO(001), CoO(001), NiO(001) Au(111) NiO(111)

Pd(001) CoO(001), NiO(001)

Rh(001) MnO(001)

Ir(001) CoO(111)

Pt(111) MnO(111), FeO(111)

Noble

Near noble

Fe(001) CoO(001), NiO(001)Transition

MgO(001) CoO(001), NiO(001)

metals

metals

metals

Insulators

• Metal substrate more reactive than the TM used for TMO film??

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

5Fe(001)-p(1x1)O surface

Single adlayer (O2 @700 K + anneal @900 K)

O atoms in the Fe hollow sites: p(1x1)

Expt: Legg et al., PRB 16, 5271 (1977);

Parihar et al., PRB 81, 075428 (2010)

• High chemical stability TMO growth

• Enhancement of the surface spin-

dependent effects

Bertacco et al., APL 72, 2050 (1998)

Bertacco & Ciccacci, PRB 59, 4207 (1999)

• Efficient new electron polarimetersBertacco et al., RSI 73, 3867 (2002)

Winkelmann et al., RSI 79, 083303 (2008)

Fe(001)-p(1x1)O

Investigation with atomic-scale resolution

scanning tunneling microscopy (STM)

& spectroscopy (STS)

Outline

Fe

Fe Fe

Side view

Fe Fe

Fe

O O

Theory: Chubb & Pickett, PRL 58, 1248 (1987)

• O-induced surface stress

• O atoms “higher” than topmost Fe atoms

(Å)

Blonski et al., Surf. Sci. 590, 88 (2005)

CoO/Fe(001)-p(1x1)OFe/Fe(001)-p(1x1)O

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

Fe(001)-p(1x1)O vs Fe(001): STM at mesoscopic scale 6

100 nm

Picone et al., PRB 83, 235402 (2011)

Oxygen-induced

coalescence

step bunching (SB)

Reversible mechanism

Ideal surface (no

stress) repulsive

short range force

between stepsMonoatomic steps

7 ML

Oxygen-induced stress

extra attractive long

range force, causing

SB at “high” T

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

7Fe(001)-p(1x1)O vs Fe(001): STM at atomic scale

Fe(001): surface corrugation < 2 pm @RT

0.0 0.5 1.0 1.5

0

5

10

15

20

25

Heig

ht [p

m]

Lateral displacement [nm]

0.287 nm

1.4

5 n

m

[100]

[010]

Fe(001)-p(1x1)O: experimental

(Vb = 0.16 V; I = 3.2 nA)

calculated

Donati et al., PRB 79, 195430 (2009)

Constant integrated DOS surface

at average distance z = 2 Å above

the sample

naïf “topographic” interpretation

But also “chemical” effects:

No atomic resolution with our STM

Hofer et al., Surf. Sci. 466, L795 (2000)

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

8Fe(001)-p(1x1)O: search for corrugation reversal

Side view

O bright

incre

asin

g tip

/sa

mp

le d

ista

nce

Oxygen vacancy

corrugation reversal

topographic effects

Fe bright

charge distribution (O

more electronegative)

faster decay in vacuum

reversal of defect

8

O

Fe

Need a reference!

Defect

Picone et al., PRB 81, 115450 (2010)

≈ 7 Å

≈ 5 Å

z

z

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

dI/dV

(differential conductivity) ÷ Local DOS

Fe(001)-p(1x1)O vs Fe(001): Scanning Tunneling Spectroscopy

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Fe(001) surface

Vs = 1 V; I = 2 nA

Inte

ns

ity

(a

rb.u

n.)

E - EF (eV)

Donati et al., PRB 79, 195430 (2009)

• Fixed position, I(V)

See: Stroscio et al., PRL 75, 2960 (1995)

• Minority-spin surface

state ≈ 0.2 eV above EF

• Huge surface sensitivity

- 0.5

0.9

• Theoretical efforts

Fe 3d minority-spin

surface states

• Non-uniform weight of

states with different k||

Raw-data

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

10Fe(001)-p(1x1)O vs Fe(001): Fe homoepitaxy 10

0.25 ML

Fe

3.5 ML

layer-by-layer growth O surfactant action O floats on top of islands

Picone et al., PRB 83, 235402 (2011)

Island

1 nm

No

O-driven decrease of

intralayer mass transport

“Wedding-cake” vs

single-layer steps

Same morphology

O-driven increase of

interlayer mass transport!

STS

Fe(001)-p(1x1)O

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

1111CoO/Fe(001)-p(1x1)O: morphology

250 × 250 nm2

a) Fe(001)-p(1x1)O

b) + 2 ML CoO

deposition @RT

• For tCoO > 5 ML, layer-plus-islands

Brambilla et al., Surf. Sci. 605, 95 (2011)

Fe @ 120 eV CoO @ 120 eV

Brambilla et al., Thin Sol. Films. 516, 7519 (2008)

LEED

Only for tCoO > 5 ML

• 3 layer-high islands up to tCoO = 5 ML

How does CoO grow ?

• Substrate step terraces preserved

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

CoO/Fe(001)-p(1x1)O: stoichiometry & interface chemistry

XPS

730725720715710705700 545 540 535 530 525 520810 800 790 780 770

Inte

nsity (

arb

. u

nits)

Co2p O1sFe2p

Binding Energy (eV)

a)

b)

c)

Co 2p Fe 2p O 1s

Binding energy (eV)

substrate

RT growth

Fe(001)

CoO(001)

CoO

Fe(001)-p(1x1)O

Fe 2p

??

Fe(001)

Fe oxides

CoO(001)

Fe(001)-p(1x1)O

Interface Fe

oxides thickness

comparable to

CoO thickness

Brambilla et al., Thin Sol. Films. 516, 7519 (2008)

12

2 ML CoO

20 ML CoO

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

CoO/Fe(001)-p(1x1)O: localization of Fe oxides

5 nm

A

B

C

0.5 ML CoO on Fe(001)-p(1x1)O

substrate

1 ML-thick

3 ML-thick

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

No

rma

lize

d d

I/d

V (

arb

. u

n.)

Sample Bias (V)

Position A (Fe substrate)

Position B

Position C

STS

Fe(001)-p(1x1)O

Co oxides

Fe oxides develop below CoO islands

Fe & Co may promote O2 dissociation

Stability of Fe(001)-p(1x1)O confirmed

13

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

Conclusions 14

• Significance of transition metal thin films oxides

• Properties of Fe(001)-p(1x1)O surface vs Fe(001)

• Fe homoepitaxy on Fe(001)-p(1x1)O

• What can we learn with STM / STS

• CoO/Fe(001)-p(1x1)O

• Interesting perspectives for applications

BUT….

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

1515Acknowledgements

Dipartimento di Fisica

• Franco Ciccacci

• Marco Finazzi

• Alberto Brambilla

• Paolo Sessi

• Andrea Picone

• Alberto Calloni

• Alberto Ferrari

• Gianlorenzo Bussetti

Laboratory for Epitaxial

Nanostructures on Silicon

and for Spintronics

• Andrea Li Bassi

• Fabio Donati

• Matteo Passoni

• Carlo Casari

• Carlo Bottani

• Simona Achilli

• Mario Italo Trioni

• Guido Fratesi

• L. Maini

Dipartimento di Energia

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

1616

5ML CoO/Fe(001)

30×30 nm2

150×150 nm2

10 ML Fe/10 ML CoO/Fe(001)

30×30 nm2

… and Fe/CoO/Fe(001)-p(1x1)O

Workshop ISM-CNR – 21.11.2008 Franco CiccacciCongresso SIF 2011 – L’Aquila Lamberto Duò

17

K. O. Legg et al., PRB 16, 5271 (1977) Jona & Marcus, Sol. St. Comm. 64, 667 (1987)