Dipankar Sarkar

Threefold Electron Scattering on Graphite Observed with C60-Adsorbed STM Tips

The scanning tunneling microscope (STM) has been used to observe threefold symmetric electron scattering from point defects on a graphite surface. These theoretically predicted electronic perturbations could not be observed with a bare metal tip but could only be imaged when a fullerene (C60) molecule was adsorbed onto the tunneling region (apex) of an STM tip. Functionalizing an STM tip with an appropriate molecular adsorbate alters the density of states near the Fermi level of the tip and changes its imaging characteristics.

Diffusion of silver in C60 thin films

The diffusion of silver atoms into C60 solid thin films is observed. The diffusion process is significant at temperatures just above room temperature, with an activation energy of 2.5±0.5 eV (60 kcal/mol). Temperature dependent conductivity measurements indicate that silver atoms form an impurity band in solid C60 with an activation energy of 0.26 eV. The ready interdiffusion of silver and C60 may have important implications in the formation of C60/metal contacts or C60/metal layered structures or materials.

Dember effect in C60 thin films

Abstract Photoinduced diffusion of charge carriers is observed in the form of a potential difference across thin films of photoexcited C 60 . The threshold photon energy for macroscopic separation of mobile charge carriers in C 60 is 2.3 ± 0.1 eV. This is also a conclusive demonstration of the fact that the optical absorption at 2.3 eV is responsible for electronic excitations across the C 60 “bandgap”. Experimental evidence suggests a bimolecular process for charge transport and carrier dynamics in C 60 solids. The polarity of the observed photovoltage confirms earlier published results that the negative charge carriers (electrons) are significantly more mobile than the positive charge carriers (holes) in C 60 solids. The estimated mobility of the electrons in C 60 is ≈ 1 cm 2 V −1 . The diffusion length for electrons in C 60 is estimated to be ≈ 10 microm.

Direct observation of fullerene‐adsorbed tips by scanning tunneling microscopy

We have succeeded in imaging individual C60 molecules that have been adsorbed onto the tunneling region of a scanning tunneling microscopy tip. The individual tip‐adsorbed molecules are imaged by scanning the fullerene‐adsorbed tip over a defect covered graphite surface. The defects are generated by low energy argon ion bombardment and protrude from the graphite surface. These nanometer‐size defects serve as a surface tip array which inverse images the molecules adsorbed to the tip when the surface is scanned.

Scanning tunneling microscopy and spectroscopy with fullerene coated tips

We report a marked improvement in STM images of graphite surfaces taken in air when scanning is performed with tunneling tips which have been previously exposed to thin films of fullerenes. Spectroscopy, TEM measurements, and previous studies on the bonding of C60 to various substrates indicate that such treatment results in the adsorption of fullerene molecules on the tip. The adsorbed fullerene molecules stay attached to the tip, and once prepared are stable with respect to sample transfer. The reduced noise and enhanced corrugation of the images of graphite surfaces obtained with these fullerene-coated tips suggest that a modified Pethica mechanism may play an important role in obtaining these images.

Fullerene functionalized scanning tunneling microscope tips- preparation, characterization and applications

Abstract We have succeeded in adsorbing individual C60 molecules onto the tunneling region of an STM tip. The individual tip-adsorbed molecules are imaged by scanning the fullerene-adsorbed tip over a defect covered graphite surface. These tips were subsequently used to observe three fold symmetric electron scattering from point defects on a graphite surface, an effect that could not be observed using bare metal tips. Functionalizing an STM tip with an appropriate molecular adsorbate alters the density of states near the Fermi level of the tip and modifies its imaging characteristics.