Surface electronic characterization with spm
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Surface electronic characterization with spm

  • This presentation will probably involve audience discussion, which will create action items. Use PowerPoint to keep track of these action items during your presentation

  • In Slide Show, click on the right mouse button

  • Select “Meeting Minder”

  • Select the “Action Items” tab

  • Type in action items as they come up

  • Click OK to dismiss this box

  • This will automatically create an Action Item slide at the end of your presentation with your points entered.

Surface Electronic Characterization with SPM

Sidney Cohen

Franco-Israel Conference on Nanocharacterization


Preview
Preview which will create action items. Use PowerPoint to keep track of these action items during your presentation

  • Introduction to modes of SPM electronic characterization -

  • current/voltage spectroscopy (I/V), scanning spreading resistance microscopy (SSRM), scanning capacitance microscopy (SCM), scanning Kelvin microscopy (SKPM).

  • Examples:

    • Study of electronic states in Quantum dots

    • Study of electron transport in thin organic films

    • Investigation of transport at grain boundaries

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Why use these techniques
Why use these techniques? which will create action items. Use PowerPoint to keep track of these action items during your presentation

  • Combination of high resolution imaging with electronic characterization

  • Possible to identify, characterize, modify, and characterize again with same probe.

  • BUT !!…Need to consider interaction of probe with sample.

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Current voltage spectroscopy
Current-Voltage Spectroscopy which will create action items. Use PowerPoint to keep track of these action items during your presentation

I

  • dI/dV gives directly local density of electronic states.

  • Possible influence of measurement (band bending, charging)

  • Difference between I/V in STM/SFM

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I v spectroscopy
I/V spectroscopy which will create action items. Use PowerPoint to keep track of these action items during your presentation

Fermi Level

DOS

eV =Bias voltage

= energy

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Contact Resistance which will create action items. Use PowerPoint to keep track of these action items during your presentation

(a=contact radius,

=electron mean free path

Contact resistance)

1. Spreading resistance,

2. Sharvin (Ballistic) transport,

Note:

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For typical experimental values, metals: which will create action items. Use PowerPoint to keep track of these action items during your presentation

But measure

(Contaminants, oxidation, etc)

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Modes based on capacitative force
Modes based on capacitative force which will create action items. Use PowerPoint to keep track of these action items during your presentation

  • Scanning Capacitance Microscopy

  • Scanning Kelvin Probe Microscopy

  • Forces are long-range. Finite size of tip causes broadening of features.

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Scanned probe measurements of cdse quantum dot structures
Scanned Probe Measurements of CdSe Quantum dot Structures* which will create action items. Use PowerPoint to keep track of these action items during your presentation

  • Want to correlate size of dot with electronic properties

  • Due to confinement, gap varies inversely with size:

  • bulk Eg

  • *Alperson, Cohen,Rubinstein, Hodes, Phys. Rev. B 52

Localization

energy

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I v spectroscopy on cdse q dot
I/V spectroscopy on CdSe Q. Dot which will create action items. Use PowerPoint to keep track of these action items during your presentation

Gap 1 0.15 eV

Gap 2, 0.2 eV

Eg=2.1 V

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Double capacitor configuration
Double capacitor configuration which will create action items. Use PowerPoint to keep track of these action items during your presentation

4 nm gaps in parallel

gives C = 6e-19.

This translates to charging

energy of 0.15 eV

Supports premise that each peak corresponds to addition of electron to quantum dot “Coulomb Charging”

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Size distribution vs msd energy gap
Size Distribution which will create action items. Use PowerPoint to keep track of these action items during your presentationvs. Msd. Energy Gap

TEM

This Exp.,with

Calculated Gap

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Work function variations on thin film surfaces
Work Function Variations on thin film surfaces* which will create action items. Use PowerPoint to keep track of these action items during your presentation

  • May be expected due to microscopic domain structure

  • SKPM can be used to detect domains with different work function down to 50 nm size.

  • Evidence supports domain existence:

    • Macroscopic Kelvin Msmts. Cannot give the spatial resolution

    • * Cohen, Efimov, Dimitrov, Trakhtenberg, Naaman,

    • submitted

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Microscopic domain structure in mixed film
Microscopic Domain Structure in Mixed Film which will create action items. Use PowerPoint to keep track of these action items during your presentation

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Results show no variation of signal across surface
Results show NO variation of signal across surface which will create action items. Use PowerPoint to keep track of these action items during your presentation

Topography

Raw SKPM

Contrast

< 5 mV

Corrected

SKPM

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Contact potential differences on different surfaces
Contact Potential Differences on Different Surfaces which will create action items. Use PowerPoint to keep track of these action items during your presentation

Monolayer types

CN MIX3 MIX2 MIX 1 RL858 OM

Mstm. 1 -600 -260 210 20 410 450 (mV)

Msmt. 2 -640 -300 190 -40 360 500 (mV)

Monolayer = Lewis Acid Lewis Base

CPD is of tip relative to surface. More negative CPD therefore corresponds to higher work function because monolayers have extracted electrons from the gold substrate.

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Electron Transport at grain boundaries which will create action items. Use PowerPoint to keep track of these action items during your presentation

in semiconductors*

For polycrystalline semiconductors, the

electron transport properties across grain

boundaries play a significant role in solar cell

function, and particularly in their degradation.

Crystallites can be a fraction of a micron

in size, making it difficult to determine these

transport properties by conventional means.

Scanning Spreading Resistance, I/V

spectroscopy, and SKPM can give

this information

*I. Visoly-Fisher, D. Cahen, S. Cohen (samples

from C. Farakadis

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Electronic properties of Grain which will create action items. Use PowerPoint to keep track of these action items during your presentation

Boundaries can be measured by:

1. Comparing I/V curves across the grain boundary

2. Monitoring change in surface potential

across boundary with SKPM

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Spatially-resolved I/V spectroscopy on CdTe film which will create action items. Use PowerPoint to keep track of these action items during your presentation

Using conducting SFM

2

1

3

Forward-biased currents are highest near grain boundary.

May be due to lower gap energy or higher carrier concentration

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SKPM - Contrast in CPD image which will create action items. Use PowerPoint to keep track of these action items during your presentation

CdTe with Molecular Layer

contrast = 15 meV

Uncoated CdTe

contrast = 30 meV

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Conclusions
Conclusions which will create action items. Use PowerPoint to keep track of these action items during your presentation

  • SPM can give useful information on the nanoscale surface electronic properties

  • Correlation can be made between topography and electronic characteristic

  • Knowledge of the effect of measurement on the system is required to interpret results

  • Many possibilities untouched here (photo-effects, direct capacitance msmt., STM UHV work)

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Acknowledgements which will create action items. Use PowerPoint to keep track of these action items during your presentation

Quantum Dot Work - I. Rubinstein, G. Hodes, B. Alperson

Organic Films - R. Naaman, D. Dimitrov

Photovoltaics - D. Cahen, I. Visoli-Fisher

All work performed at:

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