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Tip characterization
SIN in Japan 2011 1
Teemu Hynninen M. Bieletzki, T. Soini, C. Barth, C. Henry, F. Esch, A. S. Foster, U. Heiz
Tip characterization in nc-AFM and KPFM imaging of
thinfilms
Tip characterization
SIN in Japan 2011 2
AFM reversed: can the surface “image” the tip? Of course, but can we say something quantitative?
Tip characterization
SIN in Japan 2011 3
Methodology
MgO(001) grown on Ag(001)• Ag sputtered and annealed• Mg evaporation (0.1 ML/min)
in O2 atmosphere at 300 oC
frequency modulated nc-AFM and KPFM
• constant detuning mode• room temperature• 10-10 mbar UHV
EXPERIMENTS CALCULATIONS density functional theory
• VASP code• plane wave basis• PAW potentials• PBE functional
Tip characterization
SIN in Japan 2011 4
Topography: experimental
Ag surface
MgO islands
Relative height of islands (2 Å):
supported
embedded
MgO
Ag
MgO
Mg OAg
Mg OAg
images in constant df mode 0.3 ML MgO coverage
Bieletzki et al., Phys. Chem. Chem. Phys. 12 (2010) 3203
Tip characterization
SIN in Japan 2011 5
+-
+
–
+
–
+
–
+
–
+
–
+
–
Topography: simple model Polar/charged tip: attractive interaction with metal
Bieletzki et al., Phys. Chem. Chem. Phys. 12 (2010) 3203
Dipole layer on the MgO island breaks charge symmetry
+- positive
+- negative
+-
+- positive
+- negative
+-
+
–
+
–
+
–
+
–
+
–
+
–
+-
relative height:Ag > MgO
relative height:Ag < MgO
Tip characterization
SIN in Japan 2011 6
Topography: calculations
non-stoichiometric tips (polar)
stoichiometric tips
Bieletzki et al., Phys. Chem. Chem. Phys. 12 (2010) 3203
+-positive
+-
negative
neutral
“Long-range” electrostatic interaction for polar tip and MgO film Relative height differences of several Ångströms
Tip characterization
SIN in Japan 2011 7
Kelvin: experimentalTopography Kelvin voltage
neutral
+- negative
+- positive
Contrast Tip
Tip characterization
SIN in Japan 2011 8
Kelvin: simple model
+-
+-
-
--
-
+
+++
Tip characterization
SIN in Japan 2011 9
Kelvin: simple model
= + +
energy polarization charge-capacitor capacitor
dipole charge
L. Kantorovich et al. Surf Sci 445, 283–299 (2000).
Tip characterization
SIN in Japan 2011 10
Kelvin: simple model
-80 -60 -40 -20 0 20 40 60 800
200
400
600
800Equal charges 5 nm apart
Lateral distance (Å)
Ke
lvin
vo
lta
ge
(m
V)
-80 -60 -40 -20 0 20 40 60 80-800
-400
0
400
800Opposite charges 5 nm apart
R = 5 nm, z = 1 nm
R = 20 nm, z = 1 nm
R = 5 nm, z = 3 nm
R = 20 nm, z = 3 nm
Lateral distance (Å)
+ -+ +
T. Hynninen et al. E-J. Surf. Sci. Nanotech. 9 6 (2011)
Tip characterization
SIN in Japan 2011 11
Kelvin: beyond the simple model
bias potentialin a realisticgeometry (FEM)
10 cm 1 nm
field fed toatomisticmodels (DFT?)
Tip characterization
SIN in Japan 2011 12
Tip characterization: SummaryTopography contrast
(neutral vs. positive surface)Tip charge/polarization Average Kelvin voltage(w.r.t contact potential)
+- negative
MgO > Ag negative shift
+- positive
MgO < Ag positive shift
neutralMgO = Ag no shift
Thank you!