High-resolution scanning tunneling microscopy...

Materials Modelling, DL, March 20, 2007

High-resolution scanning tunneling microscopy and spectroscopy

Werner HoferSurface Science Research Centre, LINSET

The University of LiverpoolLiverpool L69 3BX

Website: http://wernerhofer.eu


Scanning tunnelling microscope

Gerd Binnig

Heinrich Rohrer


STM imaging and manipulation

Cu(111) Surface statescattering

Kanji (Atom)

IBM Almaden


STM developments 1982 - 2002

Terraces 1982 Single atoms 1986 Atomic species 1993

Electron waves 1993Bond vibration 2001

Reactions 1999


(Some) theoretical models

J. Cerda et al, PRB 56, 15885 (1997)M. Brandbyge et al, PRB 65, 165401 (2002)

Extended Huckel theory:coupled system,

local orbitals, arbitary offset

Keldysh formalism:coupled system,

local orbitals,no arbitrary offset

General problem in STM theory: the arbitrary offset in experimental scans and

the high sensitivity of the instrument to surface electronic properties


! Theory! Tip Design! Local Spin Detection! Single Atom Contacts! Molecular Corrals! Outlook


Multiple scattering technique in STM

Keldyshnon-equilibrium

Landauerthermal equilibrium

Contactsfrom Greens functions

Dysonhigher order

J. Phys: Cond. Mat. 17, 2705 (2005)


Greensfunctions of the interface

Zero OrderGreensfunction

Surface

STM tip

Zero ordercurrent


Perturbation approach (Bardeen method)

Bardeenmatrix element

Eigenvalues(V = 0)

Bardeenapproach

Bardeen approach: zero order approximation to scattering


First order transport through vacuum barrier

First orderGreensfunction

Interactionenergy

First ordercurrent

Current status: developed to first order


First-principles theory of SP-STM

Spin-polarizedcurrent

Angle betweensurface and tipmagnetization

Tunnellingmatrix element

Ferromagnetic/Anti-ferromagn.current

General approach for arbitrary magnetic structuresincluding the properties of the STM tip

Progr. Surf. Sci. 71, 147 (2003)


Theoretical proceduresCurrent contours

χSTM tipmodel

1pA

10pA

100pA

DFT(VASP)

bSKANBias voltage

Surface

ψDFT(VASP)

Rev. Mod. Phys. 75, 1287 (2003)


The importance of efficient programming:

! Typical STM simulation:"No of k-points: 36"No of G-vectors: 100"No of gridpoints: > 1000"No of integrations: 362 x 1002 x 1000 = 1010

Required: highly efficient parallel code with minimum operations in the integration kernel. Development time: about three years

Required: large computer arrays


Typical requirement for simulations

! Group of eight PostDocs/PhD students" Typical job profile: 8CPU/24-48hours" Typical number of jobs: 2/person" Typical CPU-hours/year: > 500000" Typical data storage: > 50Gb/person

! Data sharing:" Visual program output: OpenDX" Numbers: text files" Inputs: text files


Relation to the NW-Grid: the future

! Required:"Data transfer to experimental collaborators"Data storage of calculated structures"Data sharing with theoretical collaborators

A grid environment is potentially the most suitableWay to organise cross-institutional collaborations.

Caveats: - not yet developed in our research area

- Needs to operate across national boundaries


! Theory! Tip Design! Local Spin Detection! Single Adatom Contacts! Molecular Corrals! Outlook


Perylene molecules on Ag(110)

Adsorption site STM experiments and simulations

With a clean STM tip the scans show the molecular states


Functional STM tip

Perylene molecule adsorbedon a tungsten tip

A functional tip shows the statesof the clean silver surface

PRL 96, 156102 (2006)


Tip design: revealing subsurface atoms

Si(111) 7x7 Silicon tip Charge density New tip X

‘Infinitely sharp’ Even sharper:rest atoms resolved

Two dangling bondsResolved by STM

CPL 420, 177 (2006) PRL 94, 106106 (2005)


Tip design: spectroscopy tips

Problem: tungsten tips distort the electronic surface structure

PRB 71, 085401 (2005)

The prediction:

•The same tip is suitable for topography – high contrast-and spectroscopy – minimum distortion of the surface structure. 10

20

-1000 500 0 500 1000

dI/d

V [n

A/V

]

Bias voltage [mV]

Cu: -380mVAg: -60mV

Au: -520mV

High k-space resolution: 547 k-points in IBZ


Co-islands on Pt(111): magnetic contrast

Tunnel Magneto Resistance: 50% Tunnel Magneto Resistance: 850%APL 87, 162514 (2005).


The reason? Adsorbates

Cr tip with Cr apexdz = 0.45 Angstrom

Cr tip with H at surfacedz = 0.00 Angstrom

Cr tip with H at apexdz = 1.00 Angstrom

-12

-8

-4

0

20 40 60 80

Tota

l Ene

rgy

[eV

]

Field [meV/A]

Hydrogen on top

Hydrogen at surface

Crossover:50meV/A

Cr tip with Co apexdz = 0.18 Angstrom

Standard tip Non-magnetic tip

Cr tip with O at surfacedz = 0.00 Angstrom

High-contrast tip

Cr tip with O at apexdz = 0.88 Angstrom

PRL, submitted


Stability of giant magnetic contrast

Giant magnetic contrast stable over wide bias range


! Theory! Tip Design! Local Spin Detection! Single Adatom Contacts! Molecular Corrals! Outlook


Magnetic semiconductors: Mn-doped GaAs

-2V As +2V Ga

Mn atoms in low concentration form regular pattern on GaAs


Localization of Mn statesGaAs GaMnAs Simulated/measured spectra

Mn states in the bandgap Mn states do not couple to lattice


Magnetic contrast of Mn-states measured with spin-polarized tip

Nanotechnology 18, 044006 (2007)

SP-STS potentially suitable to detect the spin-state of Mn atoms(caveat: magnetic anisotropy energy very low)


Nanopatterning with polar molecules

The principle: take a halogenated molecule, adsorb it ona surface, and by a voltage pulse of 2-4V detach the halogen

Surf. Sci. 547, 324 (2003)


Patterning of silicon surfaces

Proof of principle:

The molecular arrangementcan be used to pattern thesurface.

Chem. Rev. 106, 4321 (2006)


But somehow … we were sidetracked

Where is this atom?

Why is thisatom fuzzy?

ChlorododecaneClC12H25 @ Si(111)V = -1.5V, I = 0.2nA


Type I corral

Type ISurf. Sci. Lett. 600, L47 (2006)


Simulation of type I corral

Adsorption energy: ~ -1.5 eV/molecule

Surf. Sci. Lett. 600, L47 (2006)


Surface Dipoles

lar interactions at the surface lead to charge redistributionand a shift of the DOS in the range of one eV.

Atomicallyresolved DOS


STM simulations

@ -1.5V

The corralled atom appears lower by 0.4 Angstrom, in line with experiments


Switching currents: type II corral

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12 14 16

Tunn

ellin

g cu

rren

t (nA

)

B

Time (ms)

Bright=ON

Dark=OFF

Chlorododecanes adsorb in pairs on Si(111) and createa corral-like molecular structure.

JACS 128, 16791 (2006)Nanotechnology 18, 044012 (2007)


Switch driven by conformation changes

DOS shifted by 1eV:Induced surface dipole


Outlook

! Multiple scattering through an interface

! STM induced photon emission

! Design of tips for optimum performance


Thank you for your attention

High-resolution scanning tunneling microscopy...

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