INVESTIGATION OF THIN FILMS AND

CLUSTERS BY SCANNING PROBE TECHNIQUES

Natascha Niermann, concerning the phd progamm FB Physik,

Surface Physics Universität Osnabrück, 49069 Osnabrück

contentcontent• problem: informations of materials• description of used analysis methods • some results and the problems I had so

far within the tests and tries• ideas how to improve the techniques?• future plans regarding different

materials to investigate and different possibilities of type of exploring

• summary and outlook

types of scanning probe types of scanning probe microsopymicrosopy

• origin of all scanning probe techniques is the > scanning tunneling microscopy, invented by Binning and Rohrer in the 80‘s

• from this time on there have been developed a number of variations or further developments

• in my work used techniques now are AFM(atomic force microscopy) and MFM(magnetic force microscopy) and spin polarized STM

scanning tunneling microscopy

• STM uses quantum mechanical tunneling effect of electron waves between two electrodes

• a tunneling current that can be measured bringing a tip close to the surface, no need for contact, gap ~ 10 Å

etched tip e.g. of tungsten is used to scan the surface, piezo allows small movements

scanning force microscopyAFM in air or in a liquid cell:- contact mode, topography, normal force and lateral force

- dynamic mode (intermittent contact mode or Tapping mode™), topography, phase shift, amplitude

AFM in UHV:- contact mode, topography, normal force and lateral force

- dynamic mode (non contact mode), topography, damping, frequency shift

Nanoscope Multimode 3

STM/AFM/MFM

Atomic force microscopy > light detector system

Fotodiode

Reflektierter Laserstrahl

Einfallender Laserstrahl

Probe auf dem ScannerSpitze

Cantilever

function of the four quadrant photodiode:

•the normal force FN respectively the frequency is detected vertically

•the lateral force FL is detected horizontally

Static mode

the distance is regulated by a fixed force (setpoint FN) >you acquire an equiforce plane

Dynamic mode

an oscillating quartz drives the cantilever the cantilever oscillates with the resonance frequencywith decreasing distance the resonance frequency decreasesthe distance is regulated according to a fixed shift of the frequency a plane of constant frequency shift (force gradient) is measured

Schwingquartz

AFM/MFM – amplitude regulation

• principle of the dynamic mode in UHV (FM-MODE)

• Cantilever working Eigenfrequecy

• Excitation frequency nearby resonance frequency

• Oscillation frequency depends on force gradient

• amplification by loop gain to keep constant amplitude

distance regulation AFM/MFM

• Excitation frequency by mixing frequency measured at photodetector with reference frequency

• resonance frequency of the cantilever between 10 and 500 kHz

• frequency shift by force gradient of the sample is used for controlling the distance

• the shift is held constant through the correction of distance between sample and cantilever

first resultsfirst results

• STM topography of tempo molecules on a clean gold surface in air, tempo solved at 2x10-4 in ethanol

overview of used samples• we have used two

different types of amorphous gold surfaces as substrate exhibiting flat (111) terraces < 1nm

• molecules with different properties and sizes, like TEMPO

• single crystal surfaces with magnetic metallic sub-mono-layers

NCH3

CH3CH3

CH3

O

stolen from Prof. Walder

Structure Of TEMPO

untreated amorphous gold surface

• quasi 3D view1 nm2 of amorphous gold, surface oxidated

amorphous gold: flash annealed

• corrugation of the gold surface after „flashing“:

• height differences about 50 nm

• cell-type structure of the flat terraces

• deep valleys in between of up to 10 nm

5 µm

5 µm

preparation of SAMs on surfaces

SAM( > means self assembling monolayer) :-preparation of the substrate itself ( e.g. cleaning or hydroxylisation )solution for coating:-making a solution with OTS, thiols or tempo with a appropriate solvent -coating eventually in a clean, e.g. nitrogen, environmentTypical parameters of preparation: concentration of water in the

solvent, age of the solution, temperature, type of annealing processsubstrate, chain length and concentrationMethods of investigation: STM,AFM,MFM, Ellipsometry, test of chemical properties (e.g. pH)

thiols on gold

• image taken with a Nanoscope

• gold surface with monolayer of thiols

• in holes in the layer the thickness of the layer could be measured (ca. 2nm)

OTS islands on gold

• OTS (C18H37Cl3Si) sub monolayer, height of the layer with 1,8nm

• on SiOx comparable height but different types of islands

UHV deposited gold

• view of a gold terrace made by MBE at about 10-7 mbar pressure,

• 2nm height differences

TEMPO on gold

• overview of different structures directly

• three types of structuring

first ideas about structure

• aggregate directly upon deposition of molecules

After a while: more order

• period of the pattern is 1 nm

• matches with theoretical sizes

after filtering

• correlation filtering• code by Bas Hulsken from Nijmegen

submomolayers of magnetic materials

• comparison between cobalt clusters on the left and iron clusters on silver(100) on the right (STM-measurement)

sub monolayer cobalt on silver(100)

5-15 Angström: diameter of the clusters, 1-2 atomic layers

5-15 Angström: diameter of the clusters, 1-2 atomic layers

topographic and magnetic information

this example shows a comparison betweeen corrugation and magnetic structure information detected with an MFM cantilever, distance for the measurement varies, identical position

this example shows a comparison betweeen corrugation and magnetic structure information detected with an MFM cantilever, distance for the measurement varies, identical position

corrugation 4nm Scan of 500 x 500 micrometer

troubleshootingtroubleshooting

• UHV at about 1x10-10 mbar

• leaks (most annoying) • repair or upgrade

means breaking the vacuum ... Afterwards bake out necessary

Plans for the futurePlans for the future

• uncovering the structure of theTEMPO layer, • characterization of the binding• magnetic properties (investigation with MFM and

spin polarized STM)

in cooperation with Prof Jaitner• structure and the magnetic properties of

vanadium on silver as a comparison to cobalt and iron

• VT AFM/STM