REU Wednesdays at One Scanning Probe Microscopy Susan

REU Wednesdays at One: Scanning Probe Microscopy Susan Enders Department of Engineering Mechanics June 30 th, 2010

Microscopy Optical Electron Scanning Probe § uses visible light and § uses a particle beam § forms images of surfaces system of lenses to magnify § oldest and simplest design § new digital microscopes use CCD camera § magnification up to 2000 times of electrons to illuminate a specimen § create a highly- magnified image § uses electrostatic and electromagnetic lenses § magnification up to 2 million times using a physical probe that scans the specimen § surface image produced by mechanically moving probe in a raster scan of the specimen and recording probe-surface interaction as a function of position § atomic resolution § was founded in 1981

SPM Types § § § § § § AFM, atomic force microscope BEEM, ballistic electron emission microscope EFM, electrostatic foce microscope ESTM, electrochemical scanning tunneling microscope FMM, force modulation microscope KPFM, kelvin probe force microscope MFM, magnetic force microscope MRFM, magnetic resonance force microscope NSOM, Near-Field scanning optical microscope (or SNOM, scanning near-field optical microscopy) PFM, Piezo Force Microscopy PSTM, photon sanning tunneling microscope PTMS, photothermal microspectroscopy/microscope SAP, scanning atom probe SECM, scanning electrochemical microscope SCM, scanning capacitance microscope SGM, scanning gate microscope SICM, scanning ion-conductance microscope SPSM, spin polarized tunneling microscope STh. M, scanning thermal microscope STM, scanning tunneling microscope SVM, scanning voltage microscope SHPM, scanning Hall probe microscope

Scanning tunneling microscope - STM n powerful technique for viewing surfaces at the atomic level n invented in 1981 by Gerd Binnig and Heinrich Rohrer (at IBM Zürich) n Nobel Prize in Physics in 1986 n probes the density of states of a material using tunneling current n good resolution is considered to be 0. 1 nm lateral resolution and 0. 01 nm depth resolution n can be used not only in ultra high vacuum but also in air and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to a few hundred degrees Celsius n based on the concept of quantum tunneling

Classic “Tunnel Effect”

Quantum Mechanics: The Tunnel Effect

Using the Tunnel Effect for Imaging Binnig & Rohrer, 1982, Nobel Prize in Physics 1986

Using the Tunnel Effect for Imaging Binnig & Rohrer, 1982, Nobel Prize in Physics 1986

Using the Tunnel Effect for Imaging piezo-céramique pm = 0. 00001 mm pointe échantillon

movie: by courtesy of Dr. Dirk Sander, Max Planck Institute Halle, Germany sander@mpi-halle. de, www. mpi-halle. de

The Real Thing 2 inch

Surface of Au: atoms are visible!

Atoms and Molecules at Surfaces 4. 2 Kelvin Co atoms on a Copper surface N. Knorr, A. Schneider Dept. Prof. Kern, MPI Stuttgart, Germany

Moving Things Around

Fe atoms on Cu (111) IBM Almaden , D. Eigler

Fe atoms on Cu (111) IBM Almaden , D. Eigler

Xe / Ni(110) D. Eigler & E. Schweizer, Nature 344, 524 (1990)

Atomic Force Microscope AFM n very high-resolution type of SPM n resolution of fractions of a nanometer - 1000 times better than the optical diffraction limit n STM precursor to the AFM n Binnig, Ouate and Gerber invented the first AFM in 1986 n one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale n information is gathered by "feeling" the surface with a mechanical probe n Piezoelectric elements facilitate tiny but accurate and precise movements enable the very precise scanning

Cantilevers and their properties n Typically made of Six. Ny n Spring constants in the range of 1 40 N/m (Forces from 0, 1 n. N – 20 µN) n Tips range from a pyramid to very sharp, high aspect ratio tips, to flat punches. J. E. Sader, Review of Scientific Instruments -- April 2003 -- Volume 74, Issue 4, pp. 2438 -2443

Basic principle n cantilever with a sharp tip (probe) is used to scan the specimen surface n tip is brought into proximity of a sample surface -> forces between tip and sample lead to a deflection of the cantilever according to Hooke’s law n forces measured in AFM include mechanical contact force, Van der Waals forces, capillar forces, chemical bonding, electrostatic forces, magnetic forces, Casimir forces, solvation forces etc… n deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes n if tip was scanned at a constant height -> risk that the tip collides with the surface -> damage -> feedback mechanism adjusts the tip-to-sample distance to maintain a constant force between tip and sample n sample is mounted on a piezoelectric tube which moves it z direction for maintaining a constant force, and x and y for scanning n AFM can be operated in a number of modes, depending on the application n possible imaging modes are divided into static (also called Contact) modes and dynamic (or non-contact) modes where the cantilever vibrates n AFM can be used to image and manipulate atoms and structures on surfaces

Atomic Force Microscope (AFM)

Laser Beam Deflection

Laser Beam Deflection

Laser Beam Deflection

Imaging modes Static mode n the static tip deflection is used as a feedback signal n measurement of a static signal is prone to noise and drift -> low stiffness cantilevers used to boost the deflection signal n close to the surface of the sample, attractive forces can be quite strong -> tip 'snaps-in' to the surface n static mode AFM is almost always done in contact where overall force is repulsive n technique called 'contact mode‘ n force between the tip and the surface is kept constant during scanning by maintaining a constant deflection

Imaging modes -Dynamic mode n tip of the cantilever does not contact the sample surface n cantilever is oscillated at a frequency slightly above its resonance frequency (amplitude ~ few nanometers (<10 nm)) n van der Waals forces (strongest from 1 nm to 10 nm above surface) n or other long range force which extends above the surface acts to n decrease the resonance frequency of the cantilever n decrease in resonance frequency combined with feedback loop system maintains a constant oscillation amplitude or frequency by adjusting the average tip-to-sample distance n Measuring tip-to-sample distance at each (x, y) data point allows software to construct topographic image of sample surface n AFM does not suffer from tip or sample degradation effects n non-contact AFM preferable to contact AFM for measuring soft samples n in case of rigid samples, contact and non-contact images may look the same

Advantages and disadvantages n AFM provides a true 3 D surface profile n samples viewed by AFM do not require special treatments n Most AFM modes work perfectly well in ambient air or even a liquid n Study of biological macromolecules and even living organisms n gives true atomic resolution in ultra-high vacuum (UHV) and liquid environments n high resolution AFM is comparable in resolution to STM and TEM n disadvantage of AFM is image size (maximum height in mm and maximum scanning area around 150 by 150 mm) n incorrect choice of tip for required resolution can lead to image artifacts n relatively slow rate of scanning during AFM imaging often leads to thermal drift in the image n AFM images can be affected by hysteresis of the piezoelectric material and cross-talk between the (x, y, z) axes ->may require software enhancement and filtering n filtering could "flatten" out real topographical features n AFM probes cannot measure steep walls or overhangs

Dual. Scope™ Microscope n n Sample size: Ø 50 mm Sample height: 5 mm Scan size: Z range: 40 x 40 µm 2. 7 µm www. dme-spm. dk

Bearing www. dme-spm. dk

Wooden Fibres www. dme-spm. dk

Landing Zone of Hard Disk www. dme-spm. dk

Motheye www. dme-spm. dk

Near-field scanning optical microscope NSOM/SNOM n nanostructure investigation that breaks the far field resolution limit by exploiting the properties of evanescent waves n done by placing the detector very close (<< λ) to the specimen surface n allows for the surface inspection with high spatial, spectral and temporal resolving power n resolution of the image is limited by the size of the detector aperture and not by the wavelength of the illuminating light n lateral resolution of 20 nm and vertical resolution of 2 -5 nm has been demonstrated n contrast mechanism can be adapted to study different properties (refractive index, chemical structure, local stress) n Dynamic properties can also be studied at a sub-wavelength scale

Operating Principle optical microscopy 1870 Ernst Abbe : d > λ / (2 sinθ) d = distance between the two objects λ = wavelength of the incident light 2θ = angle through which the light is collected. best resolution with optical light is about 200 nm

Operating Principle SNOM n if a subwavelength hole in a metal sheet is scanned close to an object, a super-resolved image can be built up from the detected light that passes through the hole n light passes through a subwavelength diameter aperture and illuminates a sample that is placed within its near field (distance much less than the wavelength of the light) n achieved resolution is far better than conventional optical microscopy

Modes of Operation 1) 2) 3) 4) 5) 6) 7) 8) 9) Transmission mode imaging - sample is illuminated through the probe - light passing through the sample is collected and detected Reflection mode imaging - sample is illuminated through the probe - light reflected from the sample surface is collected and detected 3) Collection mode imaging - sample is illuminated with a macroscopic light source from the top or bottom - probe is used to collect the light from the sample surface 4) Illumination/collection mode imaging - probe is used for both the illumination of sample and for collection of reflected signal

SNOM - Limit Amount of light that can be transmitted by a small aperture poses a limit on how small it can be made before nothing gets though To a degree this can be lived with, as more optical power can be generated, but the cutoff is so severe that it cannot be made smaller.

Next Step Field Enhancement Microscopy n instead of using a small aperture, a metal tip is used to provide local excitation n if a sharp metal tip is placed in the focus of a laser beam, an effect called local field enhancement will cause the electric field to become roughly 1000 times stronger n enhancement is localized to the tip, which has a typical diameter of 10 nm n as this tip is scanned over the surface, an image can be formed with a resolution as fine as the tip

Optical scans made with Bio. Lyser SNOM Shear Force Imaging Bovine Kidney Cells, non-contact mode: 10 µm scan 3 D view of Bovine Kidney cell sample Aluminum projection pattern on glas Julien Toquant, University of Basel

SPM Advantages n resolution not limited by diffraction, but only by the size of the probe-sample interaction volume ( few picometers) n ability to measure small local differences in object height (like that of 135 picometre steps on <100> silicon) n probe-sample interaction extends only across the tip atom or atoms involved in the interaction n interaction can be used to modify the sample to create small structures (nanolithography) n do not require a partial vacuum but can be observed in air at standard temperature and pressure or while submerged in a liquid reaction vessel.

SPM Disadvantages n detailed shape of the scanning tip difficult to determine (effect particularly noticeable if the specimen varies greatly in height over lateral distances of 10 nm or less) n generally slower in acquiring images due to the scanning process n embedding of spatial information into a time sequence leads to uncertainties in metrology (lateral spacings and angles) which arise due to time-domain effects like specimen drift, feedback loop oscillation, and mechanical vibration n The maximum image size is generally smaller n not useful for examining buried solid-solid or liquid-liquid interfaces

The End