Optical Properties of Graphene – Effect of Compressive Strain

Mondal, Manodeep and J, Suryanarayana (2015) Optical Properties of Graphene – Effect of Compressive Strain. Masters thesis, Indian Institute of Technology Hyderabad.

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Abstract

Graphene is single layer of graphite with 2D honeycomb lattice (sp2 hybridization form) shows remarkable mechanical, electrical, thermal and optical properties. It is a potential candidate for nano-electronics owing to its exotic electronic properties. However, lacking a bandgap in this semimetal is greatly narrowing its application in logic devices. Hence, it is of our interest see if we can open up band gap in graphene by applying compressive strain. For this purpose we have used a method ball milling technique to apply compressive strain. We have characterized the ball milled samples for their structural and optical properties using powder x – ray diffraction, Raman spectroscopy and UV – Vis NIR spectrometer. Using the samples which were milled at different times was used to prepare thin films. Solar cell I – V characteristics were also measured in order to find the efficiency. From the powder x – ray diffraction technique, we could confirm that the graphene which is used for the present measurements consists of hexagonal structure and with space group P63mc. In addition to the intense reflections (002) from graphene, we also could see a peak which is related to graphene oxide (001). Hence, we believe that the properties which we would discuss will be of graphene + graphene oxide. From the Raman spectroscopy, the G peak which corresponds to breathing of sp2 bond shifts to lower wave number which is due to compressive strain, as a result of increase in graphene layer number. Such thing can happen due to the agglomeration of graphene flakes for long milling times. D peak which corresponds to defects in graphene remains constant with respect to wave number. However, the intensity increases as a result of increase in defects. ID/IG ratio increases, which essentially means that the defect density increases with respect to ball milling time. 2D peak which corresponds to double phonon scattering remains constant and there is no shift for the above. Calculated band gap values from absorption spectra are in the range of 0.4 – 2.25 eV. If we think logically, as graphene is zero band gap semiconductor or semi metal, it should not give any band gap value. However, from the measurements that we performed we could see a gap opening of 0.4 eV in the parent form. What we believe is that the gap opening of 0.4 eV may have contribution from graphene oxide, which we could realize from x – ray diffraction measurements. Using these ball milled graphene we could prepare thin films on FTO substrate using drop casting method. Such films were used to measure the efficiency of solar cells. We also could see the surface morphology of films using 3D ZETA profiler and looks like the surface is smooth everywhere with the thickness of 18μm. As the ball milling time increases, the efficiency of solar cell increases from 8 x 10-5 – 1.8 x 10-3, which is the indication that band gap of graphene is varying with respect to milling time. We also have performed theoretical calculations using Gaussian 09 software, which is to calculate the available Raman modes with respect to change in the bond angles or bond lengths. As bond length between two sp2 hybridized carbon changes, the G peak of graphene shifts to lower wave number, which is in accordance with our experimental results. However, the values of wave number don’t match with respect to experimental one which could be due to the kind of structure that we have used in our simulations.

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