XCVII Congresso Nazionale The Fe(001)-p(1x1)O surface: scanning tunneling microscopy (STM) results and related issues Lamberto Duò CNISM - Dipartimento di Fisica Politecnico di Milano
Oxides and thin (few nanometers) oxide films 2 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
Motivations 3 (2011) Understand & control the physics of TMO thin films advances in e.g. nanoelectronics, heterogeneous catalysis and high density magnetic data storage Microscopic morphology by STM
How to make TMO thin films 4 Exposing a Transition Metal to O 2 atmosphere (undefined stoichiometry & thickness, unhomogeneities, only TMO/TM) Mid 90s: Reactive deposition single crystal TMO thin films TM evaporation on a monocrystalline substrate in gas (O 2 ) atmosphere Proper choice of substrate & its surface, TM, gas, growth conditions (temperature, pressure, evaporation rate, time), after growth treatments Noble metals Ag(001) MnO(001), CoO(001), NiO(001) Au(111) NiO(111) Near noble Pd(001) CoO(001), NiO(001) Pt(111) MnO(111), FeO(111) metals Transition Fe(001) CoO(001), NiO(001) Rh(001) MnO(001) metals Ir(001) CoO(111) Insulators MgO(001) CoO(001), NiO(001) ( for spectroscopy and STM) Metal substrate more reactive than the TM used for TMO film??
Fe(001)-p(1x1)O surface Outline 5 Single adlayer (O 2 @700 K + anneal @900 K) O atoms in the Fe hollow sites: p(1x1) O-induced surface stress O atoms higher than topmost Fe atoms High chemical stability TMO growth Bertacco et al., APL 72, 2050 (1998) Enhancement of the surface spindependent effects Bertacco & Ciccacci, PRB 59, 4207 (1999) Efficient new electron polarimeters Bertacco et al., RSI 73, 3867 (2002) Winkelmann et al., RSI 79, 083303 (2008) Expt: Legg et al., PRB 16, 5271 (1977); Parihar et al., PRB 81, 075428 (2010) Theory: Chubb & Pickett, PRL 58, 1248 (1987) Blonski et al., Surf. Sci. 590, 88 (2005) (Å) Side view O Fe Fe O Fe Investigation with atomic-scale resolution scanning tunneling microscopy (STM) & spectroscopy (STS) Fe Fe Fe Fe(001)-p(1x1)O Fe/Fe(001)-p(1x1)O CoO/Fe(001)-p(1x1)O
Fe(001)-p(1x1)O vs Fe(001): STM at mesoscopic scale 6 Oxygen-induced coalescence step bunching (SB) Reversible mechanism 100 nm Monoatomic steps Ideal surface (no stress) repulsive short range force between steps 7 ML Oxygen-induced stress extra attractive long range force, causing SB at high T Picone et al., PRB 83, 235402 (2011)
Height [pm] 1.45 nm Fe(001)-p(1x1)O vs Fe(001): STM at atomic scale 7 [010] 25 20 Fe(001)-p(1x1)O: experimental calculated [100] 0.287 nm Fe(001): surface corrugation < 2 pm @RT Hofer et al., Surf. Sci. 466, L795 (2000) No atomic resolution with our STM Constant integrated DOS surface at average distance z = 2 Å above the sample naïf topographic interpretation But also chemical effects: 15 10 5 0 0.0 0.5 1.0 1.5 Lateral displacement [nm] (V b = 0.16 V; I = 3.2 na) Donati et al., PRB 79, 195430 (2009)
Fe(001)-p(1x1)O: search for corrugation reversal 8 O Fe z 5 Å topographic effects O bright Side view Need a reference! Defect Oxygen vacancy increasing tip/sample distance z 7 Å corrugation reversal charge distribution (O more electronegative) faster decay in vacuum Fe bright reversal of defect Picone et al., PRB 81, 115450 (2010)
Intensity (arb.un.) Fe(001)-p(1x1)O vs Fe(001): Scanning Tunneling Spectroscopy Fixed position, I(V) di/dv (differential conductivity) Local DOS Fe(001) surface V s = 1 V; I = 2 na Raw-data - 0.5 0.9-0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 E - E F (ev) See: Stroscio et al., PRL 75, 2960 (1995) Minority-spin surface state 0.2 ev above E F Donati et al., PRB 79, 195430 (2009) Huge surface sensitivity Theoretical efforts Fe 3d minority-spin surface states Non-uniform weight of states with different k
Fe(001)-p(1x1)O vs Fe(001): Fe homoepitaxy 10 Fe Island 0.25 ML 3.5 ML Same morphology No O-driven decrease of intralayer 1 nm mass transport Wedding-cake vs single-layer steps Fe(001)-p(1x1)O STS layer-by-layer growth O surfactant action O floats on top of islands Picone et al., PRB 83, 235402 (2011) O-driven increase of interlayer mass transport!
CoO/Fe(001)-p(1x1)O: morphology 11 Substrate step terraces preserved 3 layer-high islands up to t CoO = 5 ML For t CoO > 5 ML, layer-plus-islands How does CoO grow? LEED 250 250 nm 2 a) Fe(001)-p(1x1)O b) + 2 ML CoO deposition @RT Brambilla et al., Surf. Sci. 605, 95 (2011) Fe @ 120 ev CoO @ 120 ev Only for t CoO > 5 ML Brambilla et al., Thin Sol. Films. 516, 7519 (2008)
Intensity (arb. units) CoO/Fe(001)-p(1x1)O: stoichiometry & interface chemistry 12 Co2p XPS Fe2p O1s Co 2p Fe 2p O 1s c) Fe(001)-p(1x1)O CoO(001) Fe(001)?? CoO 810 800 790 780 770 730725720715710705700 545 540 535 530 525 520 Binding Energy (ev) Binding energy (ev) RT growth 20 ML CoO b) substrate a) Fe 2p Interface Fe oxides thickness comparable to CoO thickness Fe(001)-p(1x1)O CoO(001) Fe oxides Fe(001) Brambilla et al., Thin Sol. Films. 516, 7519 (2008) 2 ML CoO
Normalized di/dv (arb. un.) CoO/Fe(001)-p(1x1)O: localization of Fe oxides 13 0.5 ML CoO on Fe(001)-p(1x1)O STS Position A (Fe substrate) Position B Position C Fe(001)-p(1x1)O B 1 ML-thick C 3 ML-thick 5 nm A substrate Co oxides -1.5-1.0-0.5 0.0 0.5 1.0 1.5 Sample Bias (V) Stability of Fe(001)-p(1x1)O confirmed Fe oxides develop below CoO islands Fe & Co may promote O 2 dissociation
Conclusions 14 Significance of transition metal thin films oxides What can we learn with STM / STS Properties of Fe(001)-p(1x1)O surface vs Fe(001) Fe homoepitaxy on Fe(001)-p(1x1)O CoO/Fe(001)-p(1x1)O Interesting perspectives for applications BUT.
Acknowledgements 15 Dipartimento di Fisica Laboratory for Epitaxial Nanostructures on Silicon and for Spintronics Franco Ciccacci Marco Finazzi Alberto Brambilla Paolo Sessi Andrea Picone Alberto Calloni Alberto Ferrari Gianlorenzo Bussetti Dipartimento di Energia Andrea Li Bassi Fabio Donati Matteo Passoni Carlo Casari Carlo Bottani Simona Achilli Mario Italo Trioni Guido Fratesi L. Maini
and Fe/CoO/Fe(001)-p(1x1)O 16 5ML CoO/Fe(001) 30 30 nm 2 150 150 nm 2 10 ML Fe/10 ML CoO/Fe(001) 30 30 nm 2
17 K. O. Legg et al., PRB 16, 5271 (1977) Jona & Marcus, Sol. St. Comm. 64, 667 (1987)