D. S. Sutar

Electronic structure of graphene oxide and reduced graphene oxide monolayers

Graphene oxide (GO) monolayers obtained by Langmuir Blodgett route and suitably treated to obtain reduced graphene oxide (RGO) monolayers were studied by photoelectron spectroscopy. Upon reduction of GO to form RGO C1s x-ray photoelectron spectra showed increase in graphitic carbon content, while ultraviolet photoelectron spectra showed increase in intensity corresponding to C2p-π electrons (∼3.5 eV). X-ray excited Auger transitions C(KVV) and plasmon energy loss of C1s photoelectrons have been analyzed to elucidate the valence band structure. The effective number of (π+σ) electrons as obtained from energy loss spectra was found to increase by ∼28% on reduction of GO.

Study of simultaneous reduction and nitrogen doping of graphene oxide Langmuir?Blodgett monolayer sheets by ammonia plasma treatment

Graphene oxide (GO) monolayer sheets, transferred onto Si by the Langmuir-Blodgett technique, were subjected to ammonia plasma treatment at room temperature with the objective of simultaneous reduction and doping. Scanning electron microscopy and atomic force microscopy studies show that plasma treatment at a relatively low power (∼10 W) for up to 15 min does not affect the morphological stability and monolayer character of GO sheets. X-ray photoelectron spectroscopy has been used to study de-oxygenation of GO monolayers and the incorporation of nitrogen in graphitic-N, pyrrolic-N and pyridinic-N forms due to the plasma treatment. The corresponding changes in the valence band electronic structure, density of states at the Fermi level and work function have been investigated by ultraviolet photoelectron spectroscopy. These studies, supported by Raman spectroscopy and electrical conductivity measurements, have shown that a short duration plasma treatment of up to 5 min results in an increase of sp²-C content along with a substantial incorporation of the graphitic-N form, leading to the formation of n-type reduced GO. Prolonged plasma treatment for longer durations results in a decrease of electrical conductivity, which is accompanied by a substantial decrease of sp²-C and an increase in defects and disorder, primarily attributed to the increase in pyridinic-N content.

Effect of ammonia plasma treatment on graphene oxide LB monolayers

Graphene oxide monolayer sheets were transferred on Si and SiO2/Si substrates by Langmuir-Blodgett technique and were exposed to ammonia plasma at room temperature. The monolayer character of both graphene oxide and plasma treated graphene oxide sheets were ascertained by atomic force microscopy. X-ray photoelectron spectroscopy and Raman spectroscopy revealed that ammonia plasma treatment results in enhancement of graphitic carbon content along with the incorporation of nitrogen. The conductivity of graphene oxide monolayers, which was in the range of 10−6-10−7 S/cm, increased to 10−2-10−3 S/cm after the ammonia plasma treatment. These results indicate that the graphene oxide was simultaneously reduced and N-doped during ammonia plasma treatment, without affecting the morphological stability of sheets.

GO and RGO based FETs fabricated with Langmuir-Blodgett grown monolayers

Graphene oxide (GO) monolayers were transferred onto SiO2/Si substrates by Langmuir-Blodgett (LB) technique and were converted to reduced graphene oxide (RGO) by exposure to hydrazine vapors followed by various durations of heat treatment at 400 °C in Ar atmosphere. Bottom gated FETs were fabricated with LB grown monolayers before and after reduction and were electrically characterized. The conductivity of RGO monolayers has been found to be in the range of 3 to 5 Scm−1. The RGO devices showed p-type behavior with a hole mobility of 0.07cm2/Vs and Ion/Ioff ratio of 2.

Effect of subphase pH on Langmuir-Blodgett deposition of graphene oxide monolayers on Si and SiO2/Si substrates

Graphene oxide monolayers were transferred onto Si and SiO2/Si substrates by Langmuir-Blodgett technique. Effect of subphase pH changes on the GO Langmuir isotherm and the morphology of monolayers deposited on RCA-1 treated and untreated Si and SiO2/Si substrates is reported. No GO transfer was observed on untreated Si substrate but well defined GO sheets could be transferred at subphase pH ∼ 5.5 on RCA-1 treated Si. GO sheets transferred on untreated SiO2/Si surface exhibit curled up edges, implying poor adhesion. Adherent GO sheets with planar morphology were transferred on RCA-1 treated SiO2/Si substrates at all subphase pH values in the range 3.5-6.5.

Reduction of graphene oxide monolayers transferred on Si and Ti substrates by LB technique

Graphene oxide sheets were transferred on Si and Ti substrates by Langmuir-Blodgett technique and were reduced by hydrazine vapor treatment followed by annealing in vacuum/inert atmosphere. A comparison of Raman spectra of reduced graphene oxide on Si and Ti shows that the reduction process is more effective in the later case and results in the formation of RGO with higher graphitic carbon content, exhibiting spectral features closer to those of graphene monolayers.