Browsing by Author "Mehmood, Naveed"
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Item Open Access CVD synthesis and characterization of thin Mo2C crystals(Wiley, 2020) Türker, F.; Caylan, Ö. R.; Mehmood, Naveed; Kasırga, Talip S.; Şevik, C.; Cambaz-Büke, G.In this study, we present an investigation on the growth of thin Mo2C crystals via chemical vapor deposition using CH4. Optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy(AFM), and Raman spectroscopy studies show that the morphology and the thickness of Mo2C crystals are strongly affected by the impurities in the system, the thickness of the copper substrate, and the graphene presence on Cu surface prior to Mo2C formation. Our studies show that during the CVD process, orthorhombic Mo2C crystals grow along the [100] direction on two different regions: directly on Cu surface or on graphene covered regions. Mo2C crystals that form on graphene are found to be thinner and less defective compared to the ones formed on the Cu surface. This is attributed to graphene acting as an additional diffusion barrier for Mo atoms diffusing through the copper. In addition to the graphene beneath the Mo2C crystal, Raman studies indicate that graphene may grow also on top of the Mo2C crystal, forming a graphene/Mo2C/graphene sandwich structure which may offer interesting properties for electronic applications.Item Open Access Electric-field-induced reversible phase transitions in a spontaneously ion-Intercalated 2D metal oxide(American Chemical Society, 2021-05-12) Rasouli, Hamid Reza; Kim, J.; Mehmood, Naveed; Sheraz, Ali; Jo, M. K.; Song, Seungwoo; Kang, K.; Kasırga, Talip SerkanElectric field driven reversible phase transitions in two-dimensional (2D) materials are appealing for their potential in switching applications. Here, we introduce potassium intercalated MnO2 as an exemplary case. We demonstrate the synthesis of large-area single-crystal layered MnO2 via chemical vapor deposition as thin as 5 nm. These crystals are spontaneously intercalated by potassium ions during the synthesis. We showed that the charge transport in 2D K-MnO2 is dominated by motion of hydrated potassium ions in the interlayer space. Under a few volts bias, separation of potassium and the structural water leads to formation of different phases at the opposite terminals, and at larger biases K-MnO2 crystals exhibit reversible layered-to-spinel phase transition. These phase transitions are accompanied by electrical and optical changes in the material. We used the electric field driven ionic motion in K-MnO2 based devices to demonstrate the memristive capabilities of two terminal devices.Item Open Access High elasticity and strength of ultra-thin metallic transition metal dichalcogenides(Royal Society of Chemistry, 2021-05-24) Sheraz, Ali; Mehmood, Naveed; Çiçek, M. M.; Ergün, İ.; Rasouli, H. R.; Durgun, Engin; Kasırga, Talip SerkanMechanical properties of transition metal dichalcogenides (TMDCs) are relevant to their prospective applications in flexible electronics. So far, the focus has been on the semiconducting TMDCs, mostly MoX2 and WX2 (X = S, Se) due to their potential in optoelectronics. A comprehensive understanding of the elastic properties of metallic TMDCs is needed to complement the semiconducting TMDCs in flexible optoelectronics. Thus, mechanical testing of metallic TMDCs is pertinent to the realization of the applications. Here, we report on the atomic force microscopy-based nano-indentation measurements on ultra-thin 2H-TaS2 crystals to elucidate the stretching and breaking of the metallic TMDCs. We explored the elastic properties of 2H-TaS2 at different thicknesses ranging from 3.5 nm to 12.6 nm and find that the Young's modulus is independent of the thickness at a value of 85.9 ± 10.6 GPa, which is lower than the semiconducting TMDCs reported so far. We determined the breaking strength as 5.07 ± 0.10 GPa which is 6% of the Young's modulus. This value is comparable to that of other TMDCs. We used ab initio calculations to provide an insight into the high elasticity measured in 2H-TaS2. We also performed measurements on a small number of 1T-TaTe2, 3R-NbS2 and 1T-NbTe2 samples and extended our ab initio calculations to these materials to gain a deeper understanding on the elastic and breaking properties of metallic TMDCs. This work illustrates that the studied metallic TMDCs are suitable candidates to be used as additives in composites as functional and structural elements and for flexible conductive electronic devices.Item Open Access Optoelectronic and thermal properties of metallic transition metal dichalcogenides(Bilkent University, 2020-11) Mehmood, NaveedAfter the successful isolation of graphene monolayer from its bulky counterpart, there has been tremendous advancement in the field of 2D material. Transition metal dichalcogenides(TMDCs) is family of 2D materials comprising of a transition metal atom sandwiched between two chalcogen atoms. Photoresponse of semiconducting TMDCs has been studied extensively in literature. However, photoresponse from metallic TMDCs is unprecedented and hence has not been studied to explore which mechanism might prevail. Among our findings, we discovered that photocurrent generation through metallic TMDCs is possible and has a photo-thermal origin. Using scanning photo-current microscopy, we were able to obtain spatial photocurrent maps for both, zero biased and biased samples. At zero applied bias, the photocurrent generation is localized to metal-metal junction and governed by Seebeck effect. At finite applied bias, photocurrent from the whole crystal is observed and is due to photobolometric effect. As Photo-bolometric effect relies on photo-thermally induced resistance change of the material, we extended our study to extract thermal conductivity of metallic TMDCs via bolometric effect. As contact of crystal with substrate act as a heat sink, we used suspended crystals over a hole to thermally isolate it from any heat sink. Resistance change via laser induced heating is experimentally measured at the center of the suspended part of crystal. Measured resistance change is matched with expected resistance change which is calculated using thermal conductivity(κ) as a fitting parameter via commercially available finite element method package(COMSOL). This way, thermal conductivity of the metallic TMDCs is calculated with very high accuracy and precision.Item Open Access Photocurrent generation in a metallic transition-metal dichalcogenide(American Physical Society, 2018) Mehmood, Naveed; Rasouli, Hamid Reza; Çakıroǧlu, Onur; Kasırga, T. SerkanPhotocurrent generation is unexpected in metallic 2D layered materials unless a photothermal mechanism is prevalent. Yet, typical high thermal conductivity and low absorption of the visible spectrum prevent photothermal current generation in metals. Here, we report photoresponse from two-terminal devices of mechanically exfoliated metallic 3R-NbS2 thin crystals using scanning photocurrent microscopy (SPCM) both at zero and finite bias. SPCM measurements reveal that the photocurrent predominantly emerges from metal/NbS2 junctions of the two-terminal device at zero bias. At finite biases, along with the photocurrent generated at metal/NbS2 junctions, now a negative photoresponse from all over the NbS2 crystal is evident. Among our results, we realized that the observed photocurrent can be explained by the local heating caused by the laser excitation. These findings show that NbS2 is among a few metallic materials in which photocurrent generation is possible.Item Open Access Real time optical observation and control of atomically thin transition metal dichalcogenide synthesis(Royal Society of Chemistry, 2019) Rasouli, Hamid Reza; Mehmood, Naveed; Çakıroğlu, Onur; Kasırga, T. SerkanUnderstanding the mechanisms involved in chemical vapour deposition (CVD) synthesis of atomically thin transition metal dichalcogenides (TMDCs) requires precise control of numerous growth parameters. All the proposed mechanisms and their relationship with the growth conditions are inferred from characterising intermediate formations obtained by stopping the growth blindly. To fully understand the reaction routes that lead to the monolayer formation, real time observation and control of the growth are needed. Here, we demonstrate how a custom-made CVD chamber that allows real time optical monitoring can be employed to study the reaction routes that are critical to the production of the desired layered thin crystals in salt assisted TMDC synthesis. Our real time observations reveal the reaction between the salt and the metallic precursor to form intermediate compounds which lead to the layered crystal formation. We identified that both the vapour–solid–solid and vapour–liquid–solid growth routes are in an interplay. Furthermore, we demonstrate the role H2 plays in the salt-assisted WSe2 synthesis. Finally, we observed the synthesis of the MoSe2/WSe2 heterostructures optically, and elucidated the conditions required for both lateral and vertical heterostructure syntheses.Item Open Access Study of anisotropic particles in active bath(Bilkent University, 2015) Mehmood, NaveedBrownian (passive) particles undergo random motion due to thermal agitation in the surrounding medium. In recent years, a lot of attention has been devoted to study active Brownian particles, i.e., microscopic particles capable of selfpropelling. Unlike simple passive particles, active particles feature an interplay between random fluctuations and active swimming. Thus, active particles are out of thermal equilibrium and, therefore, they explore their environment completely different from passive particles. Bacteria and other microorganisms are natural examples of active particles that take up energy from their environment and convert into directed (run) motion. Recently, there has been a lot of research progress in the realization of artificial active particles (microswimmers) due to their potential technological applications. We report the anomalous diffusion of anisotropic particles propelled by biological microswimmers (E. Coli Bacteria). The anisotropic particles of various shapes (L-shape, U-shape, cross-shape, Starshape and Z-shape) were fabricated using soft lithography method. We study the motion (translation and rotation) of various anisotropic shaped particles in the thermal and active bath. The results are compared with isotropic (spherical) particles. We observe that shape anisotropy in particle plays a vital role when they are suspended in the bath of E-Coli cells showing different diffusion behavior as the swimming pressure of E-Coli bacteria differs in the shape anisotropy.Item Open Access Synthesis of V2 O3 nanoplates for the exploration of the correlated supercritical state(American Physical Society, 2019) Rasouli, Hamid Reza; Mehmood, Naveed; Çakıroğlu, Onur; Sürmeli, Engin Can; Kasırga, T. SerkanPeculiar features exist in the stress-temperature phase stability diagram of V2O3, such as a first-order phase transition between the paramagnetic insulating and metallic phases that ends with a critical point, quantum phase transition, and a triple point. These features remain largely unexplored, and the exact nature of the phase transitions is not clear due to very limited control over the stress in bulk or film samples. Here, we show the synthesis of single-crystal V2O3 nanoplates using chemical vapor deposition via van der Waals epitaxy. Thickness of the V2O3 nanoplates range from a few to hundreds of nanometers, and they can be mechanically exfoliated from the growth substrate. Using Raman spectroscopy on the nanoplates, we reveal that, upon heating, V2O3 enters a supercritical state for both tensile strained and relaxed crystals with a similar out-of-plane response. Transmission electron microscopy on V2O3 nanoplates hints at the existence of a structural change when the crystals are heated. Our results show that V2O3nanoplates should be useful for studying the physics of the supercritical state and the phase stability of V2O3 to enable new horizons in applications.Item Open Access Thermal conductivity measurements in nanosheets via bolometric effect(IOP Publishing Ltd, 2020) Çakıroğlu, Onur; Mehmood, Naveed; Çiçek, Mert Miraç; Rasouli, Hamid Reza; Durgun, Engin; Kasırga, T. Serkan; Aikebaier, AizimaitiThermal conductivity measurement techniques for materials with nanoscale dimensions require fabrication of very complicated devices or their applicability is limited to a class of materials. Discovery of new methods with high thermal sensitivity are required for the widespread use of thermal conductivity measurements in characterizing materials' properties. We propose and demonstrate a simple non-destructive method with superior thermal sensitivity to measure the in-plane thermal conductivity of nanosheets and nanowires using the bolometric effect. The method utilizes laser beam heating to create a temperature gradient, as small as a fraction of a Kelvin, over the suspended section of the nanomaterial with electrical contacts. Local temperature rise due to the laser irradiation alters the electrical resistance of the device, which can be measured precisely. This resistance change is then used to extract the temperature profile along the nanomaterial using thermal conductivity as a fitting parameter. We measured the thermal conductivity of V2O3 nanosheets to validate the applicability of the method and found an excellent agreement with the literature. Further, we measured the thermal conductivity of metallic 2H-TaS2 for the first time and performed ab initio calculations to support our measurements. Finally, we discussed the applicability of the method on semiconducting nanosheets and performed measurements on WS2 and MoS2 thin flakes.