ATOMIC FORCE MICROSCOPY Introduction and theoretical background Jenny Malmstrom

AFM invented by Binning and co-workers in 1986. Belongs to the Scanning Probe Microscopy family Binning et al., Physics Review Letters 1986

AFM invented by Binning and co-workers in 1986. Belongs to the Scanning Probe Microscopy family AFM, atomic force microscopy [1] Contact AFM Non-contact AFM Dynamic contact AFM Tapping AFM BEEM, ballistic electron emission microscopy[2] CFM, chemical force microscopy C-AFM, conductive atomic force microscopy[3] ECSTM electrochemical scanning tunneling microscope[4] EFM, electrostatic force microscopy[5] FluidFM, Fluidic force microscopy[6][7][8][9] FMM, force modulation microscopy[10] FOSPM, feature-oriented scanning probe microscopy[11] KPFM, kelvin probe force microscopy[12] MFM, magnetic force microscopy[13] MRFM, magnetic resonance force microscopy[14] NSOM, near-field scanning optical microscopy (or SNOM, scanning near-field optical microscopy)[15] PFM, Piezoresponse Force Microscopy[16] PSTM, photon scanning tunneling microscopy[17] PTMS, photothermal microspectroscopy/microscopy SCM, scanning capacitance microscopy[18] SECM, scanning electrochemical microscopy SGM, scanning gate microscopy[19] SHPM, scanning Hall probe microscopy[20] SICM, scanning ion-conductance microscopy[21] SPSM spin polarized scanning tunneling microscopy[22] SSM, scanning SQUID microscopy SSRM, scanning spreading resistance microscopy[23] SThM, scanning thermal microscopy[24] STM, scanning tunneling microscopy[25] STP, scanning tunneling potentiometry[26] SVM, scanning voltage microscopy[27] SXSTM, synchrotron x-ray scanning tunneling microscopy[28] SSET Scanning Single-Electron Transistor Microscopy [29] (Wikipedia 2015) Binning et al., Physics Review Letters 1986

PRINCIPLE Physical probe that raster scans a specimen Key elements: 2. Detector & Feedback 3. Piezo actuators http://ssd.phys.strath.ac.uk/index.php/Scanning_tunnelling_luminescence

Scanning Probe Microscopy Scanning Tunneling Microscopy (STM) Atomic Force Microscopy (AFM) Operate by using a small tip (the probe) to scan very closely across a surface, detecting forces present between the surface and the tip. Atomic scale resolution possible Can be operated in air and liquid Slow

STM Allows: To see (1981) To manipulate (1988) Signal origin is quantum tunneling effect 0.1 nm lateral resolution and 0.01 nm depth resolution Can be used to manipulate individual atoms, trigger chemical reactions, or reversibly produce ions by removing or adding individual electrons from atoms or molecules. Very small scan ranges Image of reconstruction on a clean Au(100) surface. STM image of self-assembled supramolecular chains of the organic semiconductor Quinacridone on Graphite. http://en.wikipedia.org/

Scanning tunneling microscopy no limitation to crystalline structures ultimate resolution in ultra-high vacuum also other environments possible such as air and liquids but by far more difficult limited to metal surface AFM problem of sample preparation Surface Science community has started to look at the adsorption of biologically relevant molecules which is interesting in the context of biocompatibility and molecular recognition

Atomic Force Microscopy (AFM) Signal origin from short-range forces between the tip and the sample: van der Waals, capillary, electrostatic 1 nm lateral resolution and 0.1 nm depth resolution Contact mode Tapping mode

AFM COMPONENTS feedback Figures from Wikipedia 10

Cantilevers Length thickness Cantilever dimensions determine how easy to bend it is.

Image quality depends on tip size and shape trace contact point Slide courtesy of Duncan Sutherland

Influence of tip sharpness trace Tip trace Slide courtesy of Duncan Sutherland

Contact Mode AFM Laser Detector Z Cantilever Tip X,Y Simple In contact mode, the tip is mounted onto the end of a flexible cantilever and raster scans the surface of the sample. The tip-surface interaction deflects the cantilever, which gives information about the surface topography.  Samples can be analyzed in air, liquids or vacuum. Resolution in liquid and vacuum is increased because of the absence of strong capillary forces due to a thin liquid film on all samples in air.  Unfortunately, biological samples are challenging to study in contact mode because they are generally soft, weakly bound to the surface, and damaged easily. Not too affected by humidity Operation in liquid Feedback: Deflection of cantilever Damage to soft samples Slide courtesy of Duncan Sutherland

Tapping Mode AFM Feedback: Oscillation amplitude Detector Piezoelectric material drives oscillations Cantilever oscillates can be driven at a resonant frequency ~10-500 KHz 10-100nm The surface acts to damp the resonance Feedback: Oscillation amplitude Slide courtesy of Duncan Sutherland

Repulsive regime Attractive regime

Phase imaging in tapping mode Delay related to tip surface interaction Phase shift Two regions on the surface with different Tip- surface interactions Slide courtesy of Duncan Sutherland

Phase only really means something in the repulsive mode (more contributions in the attractive mode) Many of the more advanced imaging modes has to be run in the repulsive mode Not always possible

MFP-3D Origin AFM Imaging in contact and tapping mode. Force curves in single or multiple points. Image large area with the Origin AFM (80um scan size), 15-20 um z-piezo Can do future liquid imaging (if we buy a ‘skirt’)

Cypher ES Image relatively fast to a very high resolution of flat samples (e.g. nanofiber on mica) Image samples with features up to 5 um in height Flow inert gas into the imaging chamber Control the temperature of the sample stage (0-150 deg) Image by tapping mode in liquid (using blue drive) Applications available that needs a bit of learning and that will not work for all samples: AMFM EAFM SKPM Things we can modify the system to do in the future: Using perfusion cell to exchange solutions Electrochemistry (ports for electrodes available)

  AM-FM Viscoelastic Mapping Mode Quantitatively maps both the storage modulus (elastic response) and loss modulus or loss tangent (viscous response) with nanoscale resolution based on tapping mode, so it’s gentle and high resolution. Works in both gas and liquid environments Using both ground tone resonance and first overtone. Several feedbacks (A, A1, f). Need to calibrate tip on known sample.

EFM: Electrical AFM High resolution technique. Has to be done in air. Need conductive probe. First tap along one raster line. Then do a non-contact trace (fly) with applied electric field and monitor forces on cantilever. “Nap” mode Can “see” hidden conductive elements under the surface of the material (carbon nanotubes in polymer) SKPM: Scanning Kelvin Probe Microscopy Monitors surface potential. Less resolution than EFM. Uses a conductive probe and applied a bias to the tip. Detects potential difference between tip and sample. “Nap-mode” SKPM is QUANTITATIVE PFM: Piezo responsive force microscopy Tune the tip in contact with the surface – which gives the resonance of the ‘combined system’. This can be used to investigate internal dipoles in the system and can also be used as lithography technique if the dipoles are rewritable.

Improved optics – smaller laser spot New developments: Improved optics – smaller laser spot Smaller cantilevers – faster scanning Fast scanning cantilevers are 10–20x shorter than conventional cantilevers The Cypher laser spot is perfectly sized for even the smallest conventional cantilevers.

blueDrive Photothermal excitation

SUMMARY AFM is a member of the scanning probe family. It uses a sharp tip on the end of a flexible cantilever to ‘feel’ the sample surface. Things AFM cannot do: Image very rough samples Image something inside another material Image something that cannot be deposited on a solid material Things AFM is particularly suitable for: Imaging hard nanostructures Imaging soft nanostructures (proteinfibers, polymers) Imaging single molecules on a flat substrate Imaging adherent cells