Transient charging phenomena in graphite

1. Transient charging phenomena in graphite Tatiana Makarova1, Tanzina Chowdhury1, Christina Hacke1, Anna Zyryanova2 1Umeå University, Sweden 2St. Petersburg Academy of Industrial Art and Design, Russia

2. Motivation • EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions To separate water and graphite contributions S. Sadewasser and Th. Glatzel, ”Comment on EFM...” PRL 98 269701 (2007) R. Proksh,”Multifrequency, repulsive-mode amplitude-modulated AFM”, APL 89 113121 (2006) After 24 hours the high contrast regions became visible, consistent with growth of a water layer on different regions of the graphite surface The relative humidity in the laboratory varied between 40% and 60% during this time. The high grade HOPG did not show these regions, only the lower grade with a higher mosaic angle.

3. Motivation• EFM• Graphite edges • Graphite defects • Graphite planes • Conclusions • The microscope was placed under a dry nitrogen atmosphere in order to avoid: • artefacts • charge leakage due to adsorbed surface water • anodic oxidation during the charge injection experiments

4. Motivation• EFM• Graphite edges • Graphite defects • Graphite planes • Conclusions 3 types of signals • The tip-substrate capacitive signal f t-s is proportional to (VEFM-VSP)2 • The increase f of the capacitive force gradient when the tip is moved over the nanoparticle in the linear mode. Also scales as (VEFM - VSP)2 • A shift fQassociated with the nanoparticle charge Q.This shift corresponds to the interaction between capacitive charges on the tip apex and the Q stored charges in the nanoparticle. This quantity is thus proportional to both (VEFM- VSP) and Q, and can be made either positive (repulsive force gradient) for (VEFM- VS) Q > 0, or negative (attractive force gradient) provided (VEFM- VS)Q<0 T. Melin, H. Diesinger, D. Deresmes and D. Stievenard, Phys. Rev. B 69, 035321 (2004)

5. Motivation• EFM• Graphite edges • Graphite defects • Graphite planes • Conclusions Q Charged particle (V=0) dipole-dipole interaction ~Q² Charged particle (V≠0V) dipole-charge interaction ~ QV Uncharged particle (V≠0) capacitive interaction ~V² No charges Charges

6. Capacitor model of the graphite plane edge

7. Capacitor model of the graphite plane edge F’ = d2C/dz2(Vsp+Vbias)2 + 2dC/dz (Vsp+Vbias) Model calculations: V bias = x Vsp= 1 eV F’ = a (x-1)2 + b(x-1) We take model coefficients “a” and “b” instead of the first and second derivative

8. Motivation• EFM• Graphite edges • Graphite defects • Graphite planes • Conclusions Phase shift versus VEFM Contact potential difference = 0.25 eV Contact potential difference = 0.25 eV

9. Motivation• EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions EFM of graphite steps

10. Motivation• EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions EFM height (left) and phase (right) images of graphite, V = +3V, V = −3V. ZYX graphite SPI-1 graphite,

11. Motivation• EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions Conclusion #1 • We did not find a difference between the EFM images of zigzag and armchair edges.

12. Motivation• EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions Hidden charged defects • Hidden charged defects are not frequent on the graphite surface. They usually appear near big surface destructions. We were able to find an extremely flat piece of the sample surface filled with charged defects. All features observed on the AFM topographic images are very low: about 1 Å. +4 V -4 V

13. Motivation• EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions Dendritic images near the defects + 4V - 4V

14. Motivation• EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions Conclusion #2 • The grain boundaries in HOPG reveal itself as hidden charged defects. The EFM images of charged grain boundaries are stable in time. • Peeling off the top layer near the grain boundaries leads to appearance of quickly changing dendritic-like images. • These images visualize the redustrbution of charges

15. Motivation • EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions EFM of defect-free surfaces of the highest quality HOPG AFM +2 V -2 V 5 • 5 µm scan

16. Motivation • EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions Charge dynamics on the HOPG surface

17. Motivation • EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions

18. Anodic oxidaton of the surface

19. Nanolithography using Anodic Oxidation When the AFM tip is brought close to the surface, water from the ambient humidity forms a droplet between the tip and the substrate. To drive the anodic oxidation process, a voltage (5 to 15 volts) is applied between the tip and the substrate. The high electric field ionizes the water droplet and the OH- ions produced provide the oxidant for the chemical reaction.

20. Motivation • EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions Possible explanation Graphene as an electronic membrane Eun-Ah Kim and A. H. Castro Neto, ArXiv:cond-mat/0702562v2 The curvature generates spatially varying electrochemical potential. The charge inhomogeneity in turn stabilizes ripple formation. The curvature causes a misalignment between π orbitals and π-σ rehybridzation between nearest neighbors. Variations in the next to- nearest neighbor hopping integral lead to local changes in the electrochemical potential

21. Motivation • EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions Graphene as an electronic membrane Eun-Ah Kim and A. H. Castro Neto, ArXiv:cond-mat/0702562v2 Local electrochemical potential variation V (x, y) on HOPG surface Local electrochemical potential variation V (x, y).

22. The understanding of the electronic behavior of charges in graphite is a crucial issue in electronic applications Electrostatic force microscopy is a dedicated tool to map and study the spatial distributions of electric field and charges at the nanometer scale, but up to now only few studies have applied this technique to graphite Our results reveal an unknown behavior of graphite and warn on interpretations of transport data based on assumptions of translation invariance and homogeneous current . Motivation • EFM • Graphite edges • Graphite defects • Graphite planes • Conclusions Conclusions