1. Franco-Israel Conference on Nanocharacterization Surface Electronic Characterization with SPM Sidney Cohen 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.

2. Franco-Israel Conference on Nanocharacterization Preview 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

3. Franco-Israel Conference on Nanocharacterization Why use these techniques? 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.

4. Franco-Israel Conference on Nanocharacterization Current-Voltage Spectroscopy dI/dV gives directly local density of electronic states. Possible influence of measurement (band bending, charging) Difference between I/V in STM/SFM I

5. Franco-Israel Conference on Nanocharacterization I/V spectroscopy DOS Fermi Level eV = Bias voltage = energy

6. Franco-Israel Conference on Nanocharacterization Contact Resistance 1. Spreading resistance, 2. Sharvin (Ballistic) transport, Note: (a=contact radius,=electron mean free path Contact resistance)

7. Franco-Israel Conference on Nanocharacterization For typical experimental values, metals: But measure(Contaminants, oxidation, etc)

8. Franco-Israel Conference on Nanocharacterization Scanning Capacitance Microscopy Scanning Kelvin Probe Microscopy Forces are long-range. Finite size of tip causes broadening of features. Modes based on capacitative force

9. Franco-Israel Conference on Nanocharacterization 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 Scanned Probe Measurements of CdSe Quantum dot Structures* Localization energy

10. Franco-Israel Conference on Nanocharacterization I/V spectroscopy on CdSe Q. Dot Gap 1 0.15 eV Gap 2, 0.2 eV Eg=2.1 V

11. Franco-Israel Conference on Nanocharacterization Double capacitor configuration Supports premise that each peak corresponds to addition of electron to quantum dot “Coulomb Charging” 4 nm gaps in parallel gives C = 6e-19. This translates to charging energy of 0.15 eV

12. Franco-Israel Conference on Nanocharacterization Size Distribution vs. Msd. Energy Gap TEM This Exp.,with Calculated Gap

13. Franco-Israel Conference on Nanocharacterization Work Function Variations on thin film surfaces* 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

14. Franco-Israel Conference on Nanocharacterization Microscopic Domain Structure in Mixed Film

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16. Results show NO variation of signal across surface Topography Raw SKPM Corrected SKPM Contrast < 5 mV

17. Franco-Israel Conference on Nanocharacterization Contact Potential Differences on Different Surfaces CN MIX3MIX2 MIX 1 RL858 OM Mstm. 1 -600 -260 210 20 410 450 (mV) Msmt. 2 -640 -300 190 -40 360 500 (mV) Monolayer types 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. Monolayer = Lewis Acid Lewis Base

18. Franco-Israel Conference on Nanocharacterization Electron Transport at grain boundaries 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

19. Franco-Israel Conference on Nanocharacterization Electronic properties of Grain 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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21. 1 2 3 Spatially-resolved I/V spectroscopy on CdTe film Using conducting SFM Forward-biased currents are highest near grain boundary. May be due to lower gap energy or higher carrier concentration

22. Franco-Israel Conference on Nanocharacterization Uncoated CdTe contrast = 30 meV CdTe with Molecular Layer contrast = 15 meV SKPM - Contrast in CPD image

23. Franco-Israel Conference on Nanocharacterization Conclusions 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)

24. Franco-Israel Conference on Nanocharacterization Acknowledgements 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: