Scanning Probe Microscopy. Amanda MacMillan, Emmy Gebremichael, & John Shamblin Chem 243: Instrumental Analysis Dr. Robert Corn March 10, 2010

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1 Scanning Probe Microscopy Amanda MacMillan, Emmy Gebremichael, & John Shamblin Chem 243: Instrumental Analysis Dr. Robert Corn March 10, 2010

2 Scanning Probe Microscopy High-Resolution Surface Analysis Technique Through use of a probe, SPM allows researchers to image, characterize, and manipulate surfaces at the nanometer scale The interaction of the probe with the surface defines the type of SPM used Several Different SPM Techniques Exist Scanning Tunneling Microscopy (STM) Atomic Force Microscopy (AFM) Near-field Scanning Optical Microscopy (NSOM or SNOM) Magnetic Force Microscopy (MFM) Chemical Force Microscopy (CFM) Kelvin Probe Force Microscopy (KPFM) Founded with the invention of STM in 1981 SPM has allowed for several achievements in nanotechnology

3 Scanning Tunneling Microscopy Allows for atomic level imaging of surfaces Invented by Gerd Binnig and Heinrich Rohre in 1981 Nobel Prize in Physics in 1986 Based on several principles: Quantum Tunneling Piezoelectric Effect Feedback Loops Used in UHV, liquids, or gases Wide temperature range Two modes Constant-Current Constant-Height

4 STM: Principles Quantum Electrons have wavelike properties Tunneling is the movement of particles through a barrier In STM (depending on experimental set-up): Electrons start at either tip or sample Barrier is the gap between (vacuum, air, or liquid) Electrons end up in the second region, either the sample or tip

5 STM: Principles Piezoelectric Driver Probe tip attached to driver Consists of three (x,y and z) mutually perpendicular piezoelectric tranducers Scans in xy plane and z plane Allows for controlled motion over small distances Feedback Loop Monitors the tunneling current Makes adjustments to maintain a constant tunneling current Coordinates the current and how the tip is the positioned

6 STM: Procedure Extremely fine metal wire tip brought within ~ nm of the sample surface Bias voltage applied Electrons begin to tunnel between tip and sample Tunneling current exponential function of distance High STM sensitivity Tip slowly scans over surface Changes in surface results in changes in current Piezoelectric driver adjusts to maintain constant current Tip height plotted against position yields an image of the

7 Atomic Force Microscopy A physical probe is used to scan a surface and determine topography A laser is pointed at the tip and reflected to a sensor The interaction as a function of position is measured Since deflection of the lever, depends on the stiffness of the cantilever, Hook s law ( F = -kz)

8 Technical Aspects Resolution of fractions of a nanometer Field of view from nm to ~100 µm and vertical view of 8-10 µm Information about height High contrast image Surface may be conductive or nonconductive Standard tip 3 µm tall pyramid with ~30nm radius made of Si or Si 3 N 4 Tips may be modified by deposition of material by electron beam or microlithography techniques to improve aspect ratio and radius of the apex

9 Uses Characterize structural, dynamic and thermodynamic properties of solid surfaces The cantilever may be used to form defined nano objects and alter surfaces Before After In-situ measurements at solid-liquid interfaces

10 Data Acquisition Stage/ scanner, driven by piezoelectric material which expands and contracts proportionally to an applied voltage, to scan across a line and back Reflected beam is directed to a position sensitive detector which is composed of two photodectors and the difference in two photodector signals is collected by differential amplifier Feedback loop which measures and regulates force to maintain a constant force

11 Modes Contact mode- Tip scanned maintaining contact Tapping mode- Measures repulsive forces between tip and sample by tapping sample Lateral force mode- Measures twist as well as deflection of cantilever Non contact mode- Measures attractive forces by oscillating cantilever above sample detecting changes to the resonant frequency or amplitude of the cantilever Phase imaging- Oscillating cantilever is used to differentiate properties such as friction and adhesion to determine phase Force modulation- Sample elasticity determined by high frequency oscillations

12 Near-field Scanning Optical Microscopy (NSOM) NSOM is a light source and/or collector attached to a SPM. It delivers position measurements and optical measurements. The best resolution of optical light is about 200nm NSOM has a nm optical resolution determined by the size of the light aperture When people speak of the resolution limit imposed by wavelength, they technically mean far field resolution limit NSOM uses evanescent light (near field standing waves) The near field is about λ/3 away from the light source Source: Observation of nanostructure by scanning near-field optical microscope with small sphere probe Oshikane

13 Near-field Scanning Optical Microscopy (NSOM) According to the Maxwell equations, the properties of EM waves (E,H,D,B) can not be discontinuous. This rule holds even at a boundary. So if a wave passes through or reflects off of a boundary they form evanescent waves to make their properties continuous at the boundary What are evanescent waves: They are not travelling waves Their amplitude decays exponentially with distance (i.e. they are not sinusoidal waves) They have complex wave vectors

14 Near-field Scanning Optical Microscopy (NSOM) There are many methods to produce and collect the light used A few examples: Transmission: Light source travels through the probe aperture, and transmits through sample. Requires a transparent sample. Reflection: Light source travels through the probe aperture, and reflects from the surface. Lower light intensity, and tip-dependent, but allows for opaque samples. Collection: Sample is illuminated from large outside light source, and the probe collects the reflected light. Illumination/Collection: The probe both illuminates the sample and collects the reflected light Nanoscience Instruments, Inc. Spectrometers, APD and Photomultiplier tubes are used to collect the light emitted by the sample Source:

15 Near-field Scanning Optical Microscopy (NSOM) Practical uses Multiple forms of microscopy can performed at once NSOMs can perform Raman spectroscopy and fluorescence spectroscopy It can be used to perform fluorescence experiments on single molecules Source: Nanonics Imaging Source: Near-field polarimetric characterization of polymer crystallites Goldner, et al. Source: Imaging phase-separated domains in conducting polymer blend films with near-field scanning optical microscopy Jeeseong Hwang, et al.