Iman Santoso

Effect of Oxygen Plasma on the Optical Properties of Monolayer Graphene

The significant alteration of absorption (A) of monolayer graphene under mild oxygen plasma exposure has been observed. The first important effect is the reduction of the excitonic resonance peak at 4.64 eV. Secondly, in the near infrared range, A is gradually suppressed below an exposure-dependent threshold in sense that A << A0. Quantity A0 (given by πα and α is the fine structure constant) denotes constant absorption and relates to universal optical conductivity σ0. The suppression of A0 can be thought as the weakening of electron-hole interaction as displayed by the reduction of the excitonic resonance peak at 4.64 eV. The weakening of this interaction is due to the disorder introduced by the oxygen plasma exposure.

Polarity tuning of spin-orbit-induced spin splitting in two-dimensional transition metal dichalcogenides

The established spin splitting in monolayer (ML) of transition metal dichalcogenides (TMDs) that is caused by inversion symmetry breaking is dictated by mirror symmetry operations to exhibit fully out-of-plane direction of spin polarization. Through first-principles density functional theory calculations, we show that polarity-induced mirror symmetry breaking leads to new sizable spin splitting having in-plane spin polarization. These splittings are effectively controlled by tuning the polarity using biaxial strain. Furthermore, the admixtures of the out-of-plane and in-plane spin-polarized states in the strained polar systems are identified, which is expected to influence the spin relaxation through the Dyakonov-Perel mechanism. Our study clarified that the polarity-induced mirror symmetry breaking plays an important role in controlling the spin splitting and spin relaxation in the TMDs ML, which is useful for designing future spintronic devices.

The Role of the Oxygen Impurity on the Electronic Properties of Monolayer Graphene: A Density-Functional Study

Oxidation during the fabrication of a two-dimensional carbon-based system (graphene) can not be avoided so that will affect the electronic properties of the system. In this study, we investigate the effect of the oxygen impurity on the electronic properties of monolayer graphene using density functional theory calculations. The calculations were performed using the (3×3) supercell model and were validated by the (1×1) supercell calculations. We find that by increasing the oxygen concentration from 6%, 11%, 17%, 33% and 50%, the band gap enhanced from 0.1 to 2.48 eV. An analysis using the partial density-of-states projected to the atoms confirmed that strong hybridizations between O − Py and C − Pz orbitals are responsible for inducing the enhancement of the bandgap. Therefore, we conclude that oxygen atoms are highly sensitive to changes in the electronic properties of monolayer graphene.

Effect of Chemical Reduction Temperature on Optical Properties of Reduced Graphene Oxide (rGO) and its Potentials Supercapacitor Device

Reduced graphene oxide (rGO) has been successfully synthesized from graphite powder using Hummer’s Method. The epoxy group in GO structure was reduced by hydrazine 80 wt% at a temperature of 70o, 80o, and 90°C. The optical properties of produced rGO were characterized by using Uv-Vis and FTIR spectrometer. From Uv-Vis spectroscopy we observe that the absorbance of rGO decreased as increasing the reduction temperature. This is because the higher reduction temperature yields a high degree of rGO defect. The rGO produced at a reduction temperature of 70oC has very close similiraties absorbance spectrum with rGO produced by Sigma Aldrich Company. The Uv-Vis absorbance of rGO was used to calculated optical constant, and by using Kramers-Kronig relation we got refractive index values of rGO. The decreasing of absorbance may also stem out from the reducing of C=C bonding with sp2 hybridization due to the presence of energetic Hydrazine as indicated by the decrease of FTIR spectrum at ~1600 cm-1. Our produced rGO then used to fabricated the supercapacitor device with a mass of 0,03 gram using Polivinyl Alcohol (PVA) as a binder. From cyclic voltammetry measurement, we obtain the specific capacitance of our rGO-based supercapacitor is 2.45 F which is still in the category of supercapacitive although the optimization of rGO and PVA composition is still required. Our result shows the exciting potential of rGO based supercapacitor as electrical energy storage.

Dielectric constant extraction of graphene nanostructured on SiC substrates from spectroscopy ellipsometry measurement using Gauss–Newton inversion method

The extraction of the dielectric constant of nanostructured graphene on SiC substrates from spectroscopy ellipsometry measurement using the Gauss-Newton inversion (GNI) method has been done. This study aims to calculate the dielectric constant and refractive index of graphene by extracting the value of ψ and Δ from the spectroscopy ellipsometry measurement using GNI method and comparing them with previous result which was extracted using Drude-Lorentz (DL) model. The results show that GNI method can be used to calculate the dielectric constant and refractive index of nanostructured graphene on SiC substratesmore faster as compared to DL model. Moreover, the imaginary part of the dielectric constant values and coefficient of extinction drastically increases at 4.5 eV similar to that of extracted using known DL fitting. The increase is known due to the process of interband transition and the interaction between the electrons and electron-hole at M-points in the Brillouin zone of graphene.

Preparation of graphene oxide/poly (3,4-ethylenedioxytriophene): Poly (styrene sulfonate) (PEDOT:PSS) electrospun nanofibers

Graphene oxide (GO)/Poly (3,4-Ethylenedioxytriophene):Poly (styrene Sulfonate) (PEDOT:PSS)nanofibers have been successfully fabricated by a simple electrospinning technique to develop conductive nanofibers with polyvinyl alcohol (PVA) act as a carrier solution. Graphene oxide hasbeen synthesized by Hummer’s method and has been confirmed by Raman Spectroscopy, FTIR, and UV-Vis Spectroscopy. The structural and morphological properties of GO/PEDOT:PSS composite nanofiberswere characterized by Scanning Electron Microscopy (SEM). The result of SEM showed that GO/PEDOT:PSS nanofibers have a relatively uniform morphology nanofiber with adiameterof 180u2005nm - 340u2005nm with smooth nanofiber surface. The produced nanofibers from this study can be utilized for various applicationssuch as aflexible, conductive and transparent electrode.

Drude Lorentz model for dielectric constant of multilayer epitaxial graphene on C-face SiC measured by synchrotron radiation

We report on the procedure for determining the dielectric constant of multilayer epitaxial graphene (MEG) film grown epitaxially on the carbon face (C-face) of SiC substrate. The dielectric constant is determined by fitting the reflectance spectra of C-face MEG film measured from synchrotron radiation over a broad wavenumber from 5000 – 165000u2005cm−1 by using Drude-Lorentz (DL) model. To calculate the dielectric constants of C-face MEG, the system is modeled with Fresnel coefficient for an optically stratified multilayer film system. The model is composed of the air, a layer that represents C-face MEG film, and a layer that represents SiC substrate. We have used forty DL oscillators and have found all parameters that give the best match to the reflectance data. Of those forty oscillators, we find five oscillators (i.e., at 5000u2005cm−1, 40000u2005cm−1, 45000u2005cm−1, 50000u2005cm−1, and 90000u2005cm−1) that contribute dominantly in the dielectric constant of MEG Film. Those dominant oscillators that appear at low wavenumbers...

Calculation of dielectric constant of buffer layer graphene on SiC measured by spectroscopy ellipsometry using Gauss-Newton numerical inversion method

Calculation of dielectric constant of buffer layer graphene on SiC substrate measured by Spectroscopic Ellipsometry using Gauss-Newton inversion method has been done. The data obtained from spectroscopic ellipsometry measurement are amplitude ratio (Ψ) and phase difference (Δ). The results show that Gauss-Newton numerical inversion method can be used to extract the dielectric constant of nanostructured graphene on SiC substrates (in this case buffer layer graphene). Through the Gauss-Newton numerical inversion method, we obtain the thickness of buffer layer is around 0.5 to 1 ML and has different dielectric constant value as compared to that of graphene films.

Calculation of refractive index of multilayer epitaxial graphene on C-face SiC measured by synchrotron using Kramers-Kronig and Newton-Raphson method

Calculation of the refractive index of Multilayer Epitaxial Graphene (MEG) on C-face SiC using Kramers-Kronig and Newton-Raphson method has been done. Calculation of complex refractive index performed through the reflectance R data from Synchrotron radiation measurement with the incident angle of 17.5 °. The result of Kramers-Kronig transformation of R is phase shift (δ), and the equations that are used in the extracting refractive index of MEG on C-face SiC involve Fresnel and Snell’s equations. The problem in determining the root of Fresnel equation is resolved by using Newton-Raphson method. The results show that Kramers-Kronig and Newton-Raphson method can be used to extract the complex refractive index of nanostructure material, i.e., MEG on C-face SiC substrate. At lower wavenumber (up to 40000u2005cm−1), the complex refractive index resembles that of graphene on SiO2 and epitaxial graphene on Si-Face obtained using low energy spectroscopic ellipsometry and absorption spectroscopy, in which the refracti...