Browsing by Author "Vempati, S."

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Open Access
Electron-phonon interaction in bulk layered graphene and its oxide in the presence of alcohols in a device: Equilibrium molecular doping
(The Royal Society of Chemistry, 2014-08-22) Vempati, S.; Celebioglu, A.; Uyar, T.
We report on electron phonon interactions in bulk layered graphene (GRA) and its oxide (GO) under bias when exposed to 1° or 2° alcohol vapors, where we have focused on the change of Raman intensity of G and D bands as a function of the bias across the device. In addition to the softening of phonons we have observed a systematic variation in the intensity for D and G bands which is directly related to guest molecules and intrinsic surface nature of GRA and GO. Although the guest molecules withdraw electrons from GRA or GO, the intrinsic nature of the host material has caused mutually contrasting behaviour in IV-characteristics, where the conductance of the former decreases while it increases for the latter. The results from IV-spectra and the intensity maps of D and G bands are juxtaposed and the changes are analyzed with respect to surface and functional group interactions. In the context of doping, it is interesting to see that under equilibrium molecular charge transfer (top-gate like), the intensity ratios of 2D and G bands are not constant in contrast to a previous study [Phys. Rev. B., 2009, 80, 165413] in which such a ratio is invariant in the field effect configuration. © the Partner Organisations 2014.
Open Access
Electrospinning combined with atomic layer deposition to generate applied nanomaterials: A review
(American Chemical Society, 2020) Vempati, S.; Ranjith, K. S.; Topuz, Fuat; Bıyıklı, Necmi; Uyar, Tamer
Combining different material processing techniques is one of the keys to obtain materials that depict synergistic properties. In this review, we have reviewed a combination of two highly potential techniques, namely, electrospinning and atomic layer deposition (ALD), in the view of various applications. Over the past 10 years, our research groups are involved in the exploration of employing this combination for a range of applications. We also include some basic information on both the processes and diversity of nanostructures as a result of their combination. Nonwoven nanofiber membranes are excellent candidates for a wide range of applications. Also, they can act as templates to produce various other kinds of nanostructures when combined with ALD in small/large scale production. These nanostructures could be used as such or further subjected to other processing techniques yielding hierarchical structures. In this review, we exclusively survey and highlight the unique capabilities of combined electrospinning and ALD for applications in catalysis, photocatalysis, solar cells, batteries and gas sensors.
Open Access
Fluorescence from graphene oxide and the influence of ionic, π π interactions and heterointerfaces: electron or energy transfer dynamics
(Royal Society of Chemistry, 2014-08-29) Vempati, S.; Uyar, T.
2D crystals such as graphene and its oxide counterpart have sought good research attention for their application as well as fundamental interest. Especially graphene oxide (GO) is quite interesting because of its versatility and diverse application potential. However the mechanism of fluorescence from GO is under severe discussion. To explain the emission in general two interpretations were suggested, viz localization of sp2 clusters and involvement of oxygeneous functional groups. Despite this disagreement, it should be acknowledged that the heterogeneous atomic structure, synthesis dependent and uncontrollable implantation of oxygen functional groups on the basal plane make such explanations more difficult. Nevertheless, a suitable explanation enhances the applicability of the material which also enables the design of novel materials. At this juncture we believe that given the complexity in understanding the emission mechanism it would be very useful to review the literature. In this perspective we juxtapose various results related to fluorescence and influencing factors so that a conclusive interpretation may be unveiled. Apparently, the existing interpretations have largely ignored the factors such as self-rolling, byproduct formation etc. Vis-a-vis previous reviews did not discuss the interfacial charge transfer across heterostructures and the implication on the optical properties of GO or reduced graphene oxide (rGO). Such analysis would be very insightful to determine the energetic location of sub band gap states. Moreover, ionic and π-π type interactions are also considered for their influence on emission properties. Apart from these, quantum dots, covalent modifications and nonlinear optical properties of GO and rGO were discussed for completeness. Finally we made concluding remarks with outlook. © the Partner Organisations 2014.
Open Access
Graphene oxide-doped PEDOT:PSS as hole transport layer in inverted bulk heterojunction solar cell
(Springer, 2020) Özcan, Şefika; Erer, M. C.; Vempati, S.; Uyar, Tamer; Toppare, L.; Çırpan, A.
Transparent poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) is widely used hole conducting material in optoelectronic devices. Secondary doping of PEDOT:PSS enables the tunability of its electronic properties. In this work, graphene oxide (GO) was used as a secondary dopant for PEDOT:PSS and the doped materials (composites) were tested for their efficiency as hole transport material in inverted bulk heterojunction (BHJ) solar cell. The composites were studied to unveil the effects of Coulombic interaction between GO and PEDOT:PSS where we note some segregation of PEDOT phase. We found that the GO majorly interacts with PSS through oxygeneous functional groups which promote the detachment of PEDOT from PSS and segregation of PEDOT. Electrochemical properties with and without illumination revealed some photo-induced changes to surface of the samples. Device performances showed about 2.2% efficiency enhancement when GO doping level was 0.25 (v:v) when compared to that of pristine PEDOT:PSS.