• Today– Homework #4 Due– Scanning Probe Microscopy, Optical

Spectroscopy– 11 am NanoLab Tour

• Tomorrow– Fill out project outline– Quiz #3 in regular classroom

• Next week– Energy and Nanotechnology

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Characterization of Nanomaterials

NANO 101Introduction to Nanotechnology

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Characterization Techniques

• Structural Characterization• Scanning electron microscopy• Transmission electron microscopy• Scanning probe microscopy

• Chemical Characterization• Optical spectroscopy• Electron spectroscopy

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Scanning Probe Microscopy (SPM)

• AFM & STM• Measure feedback from atomically defined

tip• Many types of feedback (dependent on tip)

– Magnetic Force Microscopy• Magnetic material (iron) coated tip

• magnetized along tip axis

– Scanning Thermal Microscopy– Scanning Capacitance Microscopy

• Capacity changes between tip and sample

– Scanning Acoustic Microscopy

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Scanning Tunneling Microscopy (STM)

• Developed by Binnig and Rohrer in 1982• Tunneling• Very dependent on distance between the

two metals or semiconductorsBy making the distance 1 nm smaller, tunneling can increase 10X

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Scanning Tunneling Microscopy (STM)

Instrument: Scanning Tip– Extremely sharp– Metal or metal alloys (Tungsten); Conductive– Mounted on stage that controls position of tip in

x, y, z– Typically kept 0.2 - 0.6 nm from surface

Tunneling Current: ~ 0.1 - 10 nAResolution:

0.01 nm (in X and Y directions)0.002 nm in Z direction Source: Univ. of Michigan

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Scanning Tunneling Microscopy (STM)

Constant Current Mode:– As tip moves across the surface, it constantly

adjusts height to keep the tunneling current constant

– Uses a feedback mechanism– Height is measured at each point

Constant Height Mode:– As tip moves across surface, it keeps height

constant– Tunneling current is measured at each point– No feedback loop

STM

• STM is measuring electron density and not nuclear position

8http://www.aist-nt.com/content/stm

STM video

• Notice: size, complexity of equipment, sample prep

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Atomic Force Microscopy (AFM)

• Can be used for most samples• Measures:

– Small distances:• Van der Waals interactions

– Larger distances:• Electrostatic interactions (attraction, repulsion)

• Magnetic interactions

• Capillary forces (condensation of water between sample and tip)

Source: NanosurfSource: photonics.com

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Atomic Force Microscopy (AFM)

• Scan tip across surface with constant force of contact• Measure deflections of cantilever

http://content.answers.com/main/content/wp/en/1/1a/Atomic_force_microscope_block_diagram.png

AFM • Atmospheric technique• Easy sample prep

12AFM at NIST in MDhttp://www.nist.gov/cnst/nanofab/nanofab_afm3000.cfm

Protein surface/ contact AFM

Low Temp needed for atomic resolutionhttp://cen.acs.org/articles/91/i51/Atomic-Force-Microscopy-Provides-Astonishing.html

Common Feedback Modes

• Contact– Tip is dragged across sample, adjusted for

constant force against tip

• Tapping– Tip oscillates at a certain frequency which is

sensitive to distance from sample– Used for more delicate samples

13http://virtual.itg.uiuc.edu/training/AFM_tutorial/

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Scanning Probe Techniques

Other tip-surface force microscopes:• Magnetic force microscope• Scanning capacitance microscope• Scanning acoustic microscope

Uses:• Imaging of surfaces• Measuring chemical/physical properties of surfaces• Fabrication/Processing of nanostructures• Nanodevices

Some instruments combine STM and AFM

MFM

• Image magnetic domains, Rare earth – Transition metal thin film

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http://www.science.uva.nl/research/cmp/qem/research_projects/patterned_magnetic_films.html

Scanning Acoustic Microscope

• Good for finding cracks and voids in material• Failure Analysis

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http://www.soest.hawaii.edu/HIGP/Faculty/zinin/Zi-SAM.html

Scanning Capacitance Microscope

• Capacitance is used for feedback loop• Ability to store electrical charge 17

http://www.pa.msu.edu/~ghosh/printresearchSiC.htmlhttp://www.ma-tek.com/service_detail.php?path=65

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Characterization Techniques

• Structural Characterization– Scanning electron microscopy– Transmission electron microscopy– Scanning probe microscopy

• Chemical Characterization– Optical spectroscopy– Electron spectroscopy

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Chemical Characterization

• Optical Spectroscopy– Absorption– Photoluminescence (PL)– Infrared Spectroscopy (IR or FTIR)– Raman Spectroscopy

• Electron Spectroscopy– Energy-Dispersive X-ray Spectroscopy (EDS)– Auger Electron Spectroscopy (AES)

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Optical Spectroscopy:Absorbance/Transmittance

• Absorbance: electron excited from ground to excited state

• Emission: electron relaxed from excited state to ground state

• Transmittance: “opposite” of absorbance: A = -log(T)

N&N Fig. 8.10

-Information about electronic structure-Nano -> size dependent electronic structure

Abs/Emission

• Abs/PL are complimentary

• Both are size dependent

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Diameter vs absorption and photoluminescence of various sizes of CdSe0.34Te0.66 QDshttp://www.azom.com/article.aspx?ArticleID=10454

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Summary: Techniques used to study

nanostructures• Bulk/ensemble characterization techniques

– Information is average for all particles

• Surface/individual characterization techniques– Information about individual nanostructures