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

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

  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   SThM, scanning thermal microscope   STM, scanning tunneling microscope   SVM, scanning voltage microscope   SHPM, scanning Hall probe microscope SPM Types

Scanning tunneling microscope - STM powerful technique for viewing surfaces at the atomic level powerful technique for viewing surfaces at the atomic level invented in 1981 by Gerd Binnig and Heinrich Rohrer (at IBM Zürich) invented in 1981 by Gerd Binnig and Heinrich Rohrer (at IBM Zürich) Nobel Prize in Physics in 1986 Nobel Prize in Physics in 1986 probes the density of states of a material using tunneling current probes the density of states of a material using tunneling current good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution 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 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 based on the concept of quantum tunneling 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

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

movie:by courtesy of Dr. Dirk Sander, Max Planck Institute Halle, Germany

2 inch The Real Thing

Surface of Au: atoms are visible!

N. Knorr, A. Schneider Dept. Prof. Kern, MPI Stuttgart, Germany Co atoms on a Copper surface Atoms and Molecules at Surfaces 4.2 Kelvin

Moving Things Around

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

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

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

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

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

Basic principle cantilever with a sharp tip (probe) is used to scan the specimen surface cantilever with a sharp tip (probe) is used to scan the specimen surface 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 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 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… 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… deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes 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 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 sample is mounted on a piezoelectric tube which moves it z direction for maintaining a constant force, and x and y for scanning sample is mounted on a piezoelectric tube which moves it z direction for maintaining a constant force, and x and y for scanning AFM can be operated in a number of modes, depending on the application AFM can be operated in a number of modes, depending on the application possible imaging modes are divided into static (also called Contact) modes and dynamic possible imaging modes are divided into static (also called Contact) modes and dynamic (or non-contact) modes where the cantilever vibrates (or non-contact) modes where the cantilever vibrates AFM can be used to image and manipulate atoms and structures on surfaces AFM can be used to image and manipulate atoms and structures on surfaces

Atomic Force Microscope (AFM)

Laser Beam Deflection

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

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

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

DualScope™ Microscope Sample size: Ø 50 mm Sample size: Ø 50 mm Sample height: 5 mm Sample height: 5 mm Scan size: 40 x 40 µm Scan size: 40 x 40 µm Z range: 2.7 µm Z range: 2.7 µm

Bearing

Wooden Fibres

Landing Zone of Hard Disk

Motheye

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

Operating Principle optical microscopy 1870 Ernst Abbe : d > λ / (2sinθ) 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 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 hole 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 light passes through a sub- wavelength diameter aperture and illuminates a sample that is placed within its near field (distance much less than the wavelength of the light) light passes through a sub- wavelength diameter aperture and illuminates a sample that is placed within its near field (distance much less than the wavelength of the light) achieved resolution is far better than conventional optical microscopy achieved resolution is far better than conventional optical microscopy

Modes of Operation Modes of Operation 1)Transmission mode imaging - sample is illuminated through the probe - light passing through the sample is collected and detected 2)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 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 instead of using a small aperture, a metal tip is used to provide local excitation instead of using a small aperture, a metal tip is used to provide local excitation 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 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 enhancement is localized to the tip, which has a typical diameter of 10 nm enhancement is localized to the tip, which has a typical diameter of 10 nm as this tip is scanned over the surface, an image can be formed with a resolution as fine as the tip as this tip is scanned over the surface, an image can be formed with a resolution as fine as the tip

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

SPM Advantages resolution not limited by diffraction, but only by the size of the probe-sample interaction volume ( few picometers) resolution not limited by diffraction, but only by the size of the probe-sample interaction volume ( few picometers) ability to measure small local differences in object height (like that of 135 picometre steps on silicon) ability to measure small local differences in object height (like that of 135 picometre steps on silicon) probe-sample interaction extends only across the tip atom or atoms involved in the interaction probe-sample interaction extends only across the tip atom or atoms involved in the interaction interaction can be used to modify the sample to create small structures (nanolithography) interaction can be used to modify the sample to create small structures (nanolithography) 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. 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 SPM Disadvantages 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) 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) generally slower in acquiring images due to the scanning process generally slower in acquiring images due to the scanning process 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 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 The maximum image size is generally smaller The maximum image size is generally smaller not useful for examining buried solid-solid or liquid-liquid interfaces not useful for examining buried solid-solid or liquid-liquid interfaces

The End