Graphene based optoelectronics in the visible spectrum

Ankara : The Department of Physics and the Graduate School of Engineering and Science of Bilkent University, 2015.Thesis (Ph. D.) -- Bilkent University, 2015.Includes bibliographical references leaves 86-95.Graphene, a two dimensional crystal of carbon atoms, emerges as a viable material for optoelectronics because of its electrically-tunable broadband optical properties. Optical response of graphene at visible and near infrared frequencies is defined by inter-band electronic transitions. By electrical tuning of the Fermi energy, the inter-band transitions can be blocked due to Pauli blocking. However, controlling inter-band transitions of graphene in the visible and near infrared wavelengths, has been an outstanding challenge. We developed a new device to control optical properties of graphene in the visible spectra. Our device relies on a graphene supercapacitor which includes two parallel graphene electrodes and electrolyte between them. Mutual gating between graphene electrodes enables us to fabricate optical modulators which can operate in the visible and near-infrared. Single layer graphene, however, has performance limits due to its small optical absorption defined by fundamental constants. We extend our method and we developed a new class of electrochromic devices using multilayer graphene. Fabricated devices undergo a reversible color change with the electrically controlled intercalation process. The electrical and optical characterizations of the electrochromic devices reveal the broadband optical modulation up to 55 per cent in the visible and near-infrared. Integration of semiconducting materials on unconventional substrates enables optoelectronic devices with new mechanical functionalities that cannot be achieved with wafer-based technologies. As a novel application, we demonstrate ultra thin electronic paper displays using the multilayer graphene as a reconfigurable optical medium. We anticipate that the developed devices would find wide range of applications in optoelectronics.Polat, Emre OzanPh.D

Broadband optical modulators based on graphene supercapacitors

Optical modulators are commonly used in communication and information technology to control intensity, phase or polarization of light. Electro-optic, electroabsorption and acousto-optic modulators based on semiconductors and compound semiconductors have been used to control the intensity of light. Due to gate tunable optical properties, graphene introduces new potentials for optical modulators. The operation wavelength of graphene based modulators, however, is limited to infrared wavelengths due to inefficient gating schemes. Here, we report a broadband optical modulator based on graphene supercapacitors formed by graphene electrodes and electrolyte medium. The transparent supercapacitor structure allows us to modulate optical transmission over a broad range of wavelengths from 450 nm to 2 µm under ambient conditions. We also provide various device geometries including multilayer graphene electrodes and reflection type device geometries that provides modulation of 35%. The graphene supercapacitor structure together with the high modulation efficiency can enable various active devices ranging from plasmonics to optoelectronics

Standardized uptake values highly correlate with tumor size and Fuhrman grade in patients with clear cell renal cell carcinoma

Background: We investigated the correlation between standardized uptake value (SUVmax), tumor size and Fuhrman grade in patients with renal cell carcinoma (RC). Materials and Methods: We retrospectively analyzed the data of 54 patients with clear cell renal cell carcinoma histopathologically diagnosed who underwent fluorine-18 fluoro-2 deoxyglucose positron emission tomography/computed tomography (F-18 FDG PET/CT) between January 2005 and March 2014. Results: Avarage tumor sizes were 5.64±1.85, 6.85±2.24 and 7.98±2.45 in low, medium and high SUVmax groups, respectively. The Spearman's correlation coefficient between the tumor size and SUVmax was 0.385 (p=0.004) and between the Fuhrman grade and SUVmax was 0.578 (p<0.001). Conclusions: SUVmax appears highly correlated with tumor size and Fuhrman grade in patients with histopathologically confirmed clear cell RC. Multicenter studies are needed to provide larger series for more accurate results

Low oxalobacter formigenes colonization is associated with reduced bone mineral density in urinary stone forming patients

Introduction: Lower bone mineral density (BMD) and reduced Oxalobacter formigenes colonization are common findings in urolithiasis patients. But none of the studies conducted investigated the relationship between decreased bone mineral density and reduced Oxalobacter colonization. Here we evaluated the relation between BMD and O. formigenes colonization in urolithiasis patients. Materials and Methods: 50 stone formers (48.9 ± 11.9 years) and 50 control (47.2 ± 13.4 years) adult male subjects were included in the study. Alterations in O. formigenes colonization were determined as absolute O. formigenes count from fecal samples by real time polymerase chain reaction using species specific primers. BMD was evaluated from t- and z- scores calculated by using dual energy absorptiometry in the total femoral neck and lumbar spine (L2-L4). Results: Low BMD was observed in 18 (36%) urinary stone forming patients and in 7 (14%) control subjects in the lumbar area (p < 0.05). The mean O. formigenes count in stone formers and control subjects were 19,257 (5,791 ± 1,117.93) and 143,850 (2,815,725 ± 3,946,044.7) (p < 0.05) respectively. We observed a correlation between decreased lumbar BMD and O. formigenes colonization and testosterone levels in stone formers. Our results indicated that diminished O. formigenes colonization in the gut of urinary stone forming subjects was associated with reduced BMD

Transducer Technologies for Biosensors and Their Wearable Applications

The development of new biosensor technologies and their active use as wearable devices have offered mobility and flexibility to conventional western medicine and personal fitness tracking. In the development of biosensors, transducers stand out as the main elements converting the signals sourced from a biological event into a detectable output. Combined with the suitable bio-receptors and the miniaturization of readout electronics, the functionality and design of the transducers play a key role in the construction of wearable devices for personal health control. Ever-growing research and industrial interest in new transducer technologies for point-of-care (POC) and wearable bio-detection have gained tremendous acceleration by the pandemic-induced digital health transformation. In this article, we provide a comprehensive review of transducers for biosensors and their wearable applications that empower users for the active tracking of biomarkers and personal health parameters

Challenging transitions.

We asked young scientists these questions: Have you ever encountered a particularly stark difference between an old and new position in your education or career? What was the difference between the positions, and what advice would you give to someone making a similar transition? Here, respondents share the challenges they faced when they took on new responsibilities and roles, changed fields, or moved to new places. To others in similar situations, they advise: Be confident, prepared, and patient; communicate; and always ask for help when needed. —Jennifer Sill