SCANNING-, FIELD ION- AND ELECTRON MICROSCOPY FOR ANALYSIS OF SURFACES CREATED BY KLAUS DOBREZBERGER TARGET AUDIENCE UNIVERSITY

CONTENT GENERAL INFORMATIONS HISTORICAL BACKROUND ELECTRON MICROSCOPY SCANNING ELECTRON MICROSCOPY SCANNING TUNNELING MICROSCOPY Further methods ATOMIC FORCE MICROSCOPY 2

HISTORICAL DEVELOPMENT Maingroups of microscopy LIGHT MICROSCOPY ELECTRON MICROSCOPY FIELD-ION MICROSCOPY: Atomic resolution for the first time! SCANNING TUNNELING MICROSCOPY ATOMIC FORCE MICROSCOPY 3

HISTORICAL EXCURSUS: FIELD-ION MICROSCOPE Developed by Werner Müller in 1955 Atomic resolution for the first time! Use of a thin metallic tip as sample Cooling with inert gas in vacuum Tunneling through a high electric field is possible Inert gas (for example He) has been ionised and disgusted Visualisation on a phosphor screen and microchannels 4

GENERAL INFORMATIONS OF SEM NEW METHODS of SURFACE ANALYSIS Open the barriers of light microscopy Using of electrons instead of photons  ELECTRON MICROSCOPY Developed in 1930 by Ruska, Knoll and Rüdenberg (Pics from above to below) Microscopical possibilities using intermolecular forces: Atomic force microscopy (AFM) 5

ELECTRON MICROSCOPY Shootings on the surface with electrons Electron gun produces electrons from a source  accelerated to the anode Some kV to MV voltage Electron lense is needed Vacuum is needed and the sample must be conductive Detectors for collection of electrons and secondary electrons are used Backscattered electrons produce a picture  Scanning electron microscope (SEM) Transmitted electrons produce a picture  Transmission electron microscope (TEM) 6

INTERDEPENDENCY: ELECTRONS AND SAMPLE Producing of heat Electrons through sample as sample current Exit of secondary electrons Exit of backscattered electrons Characteristic X-Ray: For X-ray fluorescence analysis Auger electrons 7

DETECTORS IN USE Secondary electron detector for contrast of topography Backscattered electron detector for elemental contrast EDX-detector for fast elemental analysis WDX-detector for exactly elemental analysis Auger electron detector for elemental analysis of thin layers 8

COMPARISON REM VS. TEM SCANNING ELECTRON MICROSCOPYTRANSMISSION ELECTRON MICROSCOPY 9

COMPARISON REM VS. TEM: FUNCTION SCANNING ELECTRON MICROSCOPY Electron beam is „rasterised“ over surface Escaped (secondary) and backscattered electrons are detected Increasing atomic number  reflexion of electrons is more intensive  brighter picture Backscattered electrons in keV-area TRANSMISSION ELECTRON MICROSCOPY Transmitting the sample Thin sample is needed Few nm to some mm thickness Principle is similar to light microscope The higher the atomic number and the lower the acceleration of voltage is, the thinner the sample must be. 10

COMPARISION REM VS. TEM: APPLICATIONS SCANNING ELECTRON MICROSCOPY Conductive samples Good resolution of surface Vacuum: No living objects Alternative to light microscope TRANSMISSION ELECTRON MICROSCOPY Very thin samples (100 nm) Very good resolution Determination of dimensions Especially suitable for nanomaterials 11

DISADVANTAGES OF ELECTRON MICROSCOPY Investigation in vacuum: Just living material can be measured Complex preparation of sample (Sputtering with Gold): Artifacts possible Damage of sample due to electron beam Electron microscopy is expensive in buying and service 12

ALTERNATIVE POSSIBILITIES OF ELECTRON MICROSCOPY Scanning tunneling microscopy (STEM) Developed by Heinrich Rohrer and Gerd Binnig in1981 Bases at the physical principles of tunneling effects Low energy electron microscopy (LEEM) Low skin depth of electrons to the sample Photoemission electron microscopy (PEEM) Bases at the physical principles of photoelectric effect 13

SCANNING TUNNELING MICROSCOPY (STEM) Metallic tip is leaded over surface Voltage between tip and sample  Flow of current The value of the tunneling current depends on Distance between sample and tip Electron density between sample and tip Bias voltage Chemical and geometrical properties are mixed Example: Arsen- and Germanium-surface atoms looks lower und higher although identical geometrical arrangement  As has 5 external electrons and Ge only 3 14

THEORETICAL BACKROUND The tunneling effect of electrons is the theorie behind electron microscopy. Electrons are tunneling through the ferminiveau (E F ) to free levels cause of invested voltage on the sample  tunneling current (I T ) 15

SCANNING TUNNELING MICROSCOPY MODE OF OPERATION Constant Hight Changing the tunneling current depending on surface topographie Constant tunneling current The position of height of the tip is changed depending the surface topopgrahie 16

SCANNING TUNNELING MICROSCOPY: EXAMPLES Tantaldisulfid-surface with hexagonal lattice IBM-Logo created by nanomanipulation 17

LOW ENERGY ELECTRON MICROSCOPY (LEEM) Developed bei Ernst Bauer in 1962 Microscopical measurements of atomic flat surfaces, atomic-surface- interactions of thin cristalline film Larg-area lighting of the sample with electrons of some keV of energy  Very small skin depth  very high surface selectivity (few atomic layers) Picture is made visible by the electron detector and recorded by a camera 18

PHOTOEMISSION ELECTRON MICROSOPY (PEEM) On basic of photoelectric effect: Discovered by Albert Einstein (nobel price!) Beaming of a metal surface with photons of certain energy take electrons of the metalconstruct Kinetic energy of electrons depends on the energy of caved ray and the bounding energy of the electrons The electrons are collected through a system of lenses With more electrons, the area get brigther 19

ATOMIC FORCE MICROSCOPY (AFM) Tip (Picture above) rasterise over surface Intermolecular forces are effective: Rise and batter Recording of the surface topography is possible Prevention of underground noises Springing of equipment Good conductivity of the sample isn´t necessary The ideal case produces atomic resolution 20

EVALUATION OF MEASUREMENT Atomical fine tip rasterise surface. It is fixed at a cantliver. The diversion is measured by a laser beam 21

ATOMIC FORCE MICROSCOPY: OPERATIONAL MODE Tapping Mode (TM-AFM): Sample surface is touched at each vibration Lower damage of sample comparing normal contact mode Non-contact Mode (NC-AFM) – No touch AFM Van der Waals forces causes a change of the vibration frequency Measurement without damage with atomic resolution 22

ATOMIC FORCE MICROSCOPY: EXAMPLE Tip is scanning through a molecule (Picture above) Atomic resolution is possible! (Picture below) Figure of a thin filament of Gold Gold atoms ordered in a chain are visible 23

ALTERNATIVE METHODS OF ATOMIC FORCE MICROSCOPY Magnet force microscopy (MFM): Measurement of hard disks Friction force measurement (LFM): For example measurement of oil at OMV Chemical force microscopy (CFM): Chemical modified groups on the tip (Picture right) 24

SOURCES Pictures on master foil MAUSCHITZ GERD: Vorlesungsunterlagen „Technologie der Nanopartikeln“ ; ; Picture on foil 2, 7, 9, 13, 20, 21, 22: MAUSCHITZ GERD: Vorlesungsunterlagen „Technologie der Nanopartikeln“ Pictures on foil 5: ; ; ; Picture on foil 15: ;

SOURCES Picture on foil 4 and 16: ; Picture on foil 17: ; Picture on foil 19: ; Picture on foil 23: ; http:// Picture on foil 24: ;