Nanotechnology Research Center (NANOTAM)
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Item Open Access Foerster-Type nonradiative energy transfer in media with complex permittivity(META Conference, 2023) Hernandez-Martinez, Pedro Ludwig; Yucel, Abdulkadir C.; Demir, Hilmi VolkanWe present the effects of the complex permittivity of a background medium on Foerster-type nonradiative energy transfer (FRET) and the changes in FRET as a function of the relative permittivity of the medium. We discuss examples of enhanced FRET via tuning the complex permittivity of the medium and illustrate that FRET can significantly increase when the denominator of the FRET screening factor approaches zero. © 2023, META Conference. All rights reserved.Item Open Access Oriented colloidal quantum wells: Pushing the limits, breaking records(META Conference, 2023) Demir, Hilmi Volkan; Lalanne, P.; Zouhdi, S.We introduce a powerful, large-area self-assembly technique for orienting colloidal quantum wells in all face-down configuration. We demonstrate three-dimensional constructs of such oriented self-assemblies with monolayer precision. We present the most recent examples of LEDs and lasers using these oriented assemblies for lighting and displays. Here we also show record high efficiency from their LEDs and record thin gain medium from their laser structures. These solution-processed quantum wells hold great promise to challenge their epitaxial thin-film counterparts in semiconductor optoelectronics. © 2023, META Conference. All rights reserved.Item Open Access Improvement of fatigue properties of EN AW 6082 aluminum alloy using different deep rolling directions(Universiti Malaysia Pahang, 2023-06-30) Görtan, Mehmet Okan; Yüksel, BerkayDeep rolling (DR) is an effective mechanical surface treatment method to improve the fatigue properties of engineering components. In this method, the surface of the component was rolled using a roller with a predetermined force to obtain reduced roughness, hardness increases and compressive residual stresses in the surface region. These alterations allow for increasing the fatigue lives of the components in industrial applications. In the current study, DR was applied in tangential and longitudinal directions on specimens that were manufactured using EN-AW 6082-T6 aluminum. The resulting roughness, hardness and residual stresses were determined experimentally. Fatigue tests were carried out to determine the improvements in fatigue properties after DR. It was found that DR-induced compressive residual stresses depend on DR direction considerably. Due to this reason, fatigue strength improvements were found to be different for different DR direction applications. Longitudinal rolling resulted in a 23% fatigue strength increase compared to a 7% increase for tangential rolling. For both DR direction applications, fatigue cracks were shown to initiate at the sub-surface region, whereas the as-turned specimens exhibited surface crack initiation.Item Open Access Nanometer-thick ınsertion layer for the effective passivation of surface traps and ımproved edge acuity for AlGaN/GaN HEMTs(Institute of Electrical and Electronics Engineers, 2023-09-30) Odabaşı, Oğuz; Ghobadi, Amir; Ghobadi, Türkan Gamze Ulusoy; Güneysu, Efkan; Urfalı, Emirhan; Yağlıoğlu, Gül; Bütün, Bayram; Özbay, EkmelIn AlGaN/GaN high electron mobility transistors (HEMTs), the existence of long lifetime surface traps can cause several adverse effects, including threshold voltage ( Vth ) instability and current collapse. Therefore, understanding the nature and lifetime of these traps is crucial to provide effective passivation. In this work, the nature of these traps is scrutinized by combining femtosecond transient optical and multiple structural analyses. Later, using a nanometer-thick Al2O3 insertion layer, these traps are effectively passivated. In order to observe the effect of the proposed passivation on device performance, HEMT devices were fabricated. As a result of this passivation, better edge acuity in ohmic contacts and protection of the surface of the epitaxy were achieved. The lag performance of the HEMT devices was significantly improved. It was found that the drain lag was reduced from 37.1% (for the standard SiNx passivated design) to 10.4% for the modified HEMT design. In operating this transistor as a power amplifier, nearly no change in the quiescent bias point was observed after consecutive load–pull measurements, which shows the stability of the fabricated device.Item Open Access 60W stacked-HEMT based asymmetric X-band GaN SPDT switch for single chip T/R modules(IEEE - Institute of Electrical and Electronics Engineers, 2023-10-25) Ertürk, Volkan; Gürdal, Armağan; Çankaya Akoğlu, Büşra; Özbay, EkmelThis paper presents a high-power, asymmetric single-pole double-throw (SPDT) monolithic microwave integrated circuit (MMIC) switch using high electron mobility transistors (HEMT) with AlGaN/GaN technology for single chip X-band T/R modules. The SPDT switch is designed in series-shunt topology for high-power handling and low-loss performance. For high-power handling, shunt-stacked HEMTs on the transmit (Tx) path and series-stacked HEMTs on the receive (Rx) path are used. In its Tx mode, the switch has achieved an insertion loss better than 0.75 dB throughout the 6-13 GHz bandwidth with a return loss of 14 dB and an isolation of 28 dB. It can handle more than 60 W RF input power at 0.1 dB compression. In its Rx mode, the switch can receive signals with an insertion loss lower than 1.15 dB with 14 dB return loss and 19 dB isolation. With its low insertion and high-power handling capacity from C-band to Ku-band, this switch shows state-of-the-art performance for communication systems.Item Open Access Photocatalytic CO2 conversion: Beyond the earth(Elsevier, 2023-07-25) Low, J.; Zhang, C.; Karadaş, Ferdi.; Xiong, Y.The issue of climate change attributed to CO2 emissions has led to increased attention towards the study and development of artificial photosynthesis through photocatalytic CO2 conversion to recon‐struct the broken carbon cycle in nature. Photocatalytic CO2 conversion can simultaneously reduce the CO2 concentration in the atmosphere and produce valuable hydrocarbon fuels. With the recent discovery of abundant reserves of CO2 and water at extraterrestrial sites, it has been proposed that photocatalytic CO2 conversion can also be implemented at extraterrestrial sites to build up an artificial carbon cycle for providing propellants and life support for space missions. This comment presents our perspectives on the development of photocatalytic CO2 conversion beyond Earth, with a focus on its general principles and potential challenges that may arise at extraterrestrial sites. Finally, a brief overview of the future research directions in this field is presented.Item Open Access High-figure-of-merit biosensing and enhanced excitonic absorption in an mos2-integrated dielectric metasurface(MDPI, 2023-02-01) Hajian, H.; Rukhlenko, I. D.; Bradley, A. L.; Özbay, EkmelAmong the transitional metal dichalcogenides (TMDCs), molybdenum disulfide (MoS2) is considered an outstanding candidate for biosensing applications due to its high absorptivity and amenability to ionic current measurements. Dielectric metasurfaces have also emerged as a powerful platform for novel optical biosensing due to their low optical losses and strong near-field enhancements. Once functionalized with TMDCs, dielectric metasurfaces can also provide strong photon–exciton interactions. Here, we theoretically integrated a single layer of MoS2 into a CMOS-compatible asymmetric dielectric metasurface composed of TiO2 meta-atoms with a broken in-plane inversion symmetry on an SiO2 substrate. We numerically show that the designed MoS2-integrated metasurface can function as a high-figure-of-merit ((Formula presented.)) van der Waals-based biosensor due to the support of quasi-bound states in the continuum. Moreover, owing to the critical coupling of the magnetic dipole resonances of the metasurface and the A exciton of the single layer of MoS2, one can achieve a (Formula presented.) enhanced excitonic absorption by this two-port system. Therefore, the proposed design can function as an effective biosensor and is also practical for enhanced excitonic absorption and emission applications. © 2023 by the authors.Item Embargo Microheater-integrated spectrally selective multiband mid-infrared nanoemitter for on-chip optical multigas sensing(American Chemical Society, 2023-11-10) Rahimian Omam, Zahra; Ghobadi, Amir; Khalichi, Bahram; Güneş, Burak; Özbay, EkmelTraditional optical gas sensors often require multiple components such as broadband infrared sources, detectors, and band-pass filters to detect various target gases, resulting in bulky and expensive sensor designs. A streamlined optical gas-sensing platform utilizing a narrowband thermal emitter with a spectrally selective response, capable of accommodating various target gases, has the potential to supplant current bulky designs. Through the on-chip integration of a narrowband metamaterial perfect absorber with a microelectromechanical system (MEMS) heater, a selective infrared source emitter could be designed. In this paper, a multiband metamaterial absorber with resonance modes located at different gas absorption signatures is developed for optical multi-gas-sensing applications. The proposed nanoemitter supports penta-band light absorption through the simultaneous excitation of phononic modes (within the hexagonal boron nitride (hBN) topmost layer) and plasmonic modes (with the spectrally selective underlying metal-insulator-metal (MIM) absorber stack). It achieves five near-perfect sharp absorption resonance peaks compatible with the H2S, CH4, CO2, NO, and SO2 gas absorption signatures in the mid-infrared (MIR) spectral range. This spectrally engineered multiwavelength absorption behavior is achieved by simultaneously coupling the optical phonons (OPhs) and the plasmonic modes in the vicinity of the OPh region of hBN and by exciting plasmonic modes with the help of the spacer (ZnTe: zinc telluride) and the metallic nanogratings. Finally, this low-cost and efficient penta-band absorber is combined with a MEMS-based microheater. The microheater uses a Peano-shaped configuration to provide a highly uniform surface temperature, which is crucial for accurate and reliable gas sensing. The proposed platform demonstrates excellent potential in terms of cost-effectiveness, source-free operation, and suitability for multi-gas-sensing platforms.Item Open Access Effect of Si-rich SiXNY multilayer passivation material on the DC electrical characteristics of AlGaN/GaN HEMTs(Springer, 2023-08-16) Dinçer, Ahmet Serhat; Haliloğlu, Mehmet Taha; Toprak, Ahmet; Altındal, Ş.; Özbay, EkmelIn this study, the effect of SiXNY bilayer passivation materials on the electrical properties of an AlGaN high electron mobility transistor (HEMT) was investigated. AlGaN/GaN HEMTs were grown on 3-inch silicon carbide by the metal organic chemical vapor deposition method, which is one of the chemical vapor deposition methods. SiXNY passivation materials with two different Si concentrations, which were 50/1 and 70/3 (Silane—SiH4/Ammonia—NH3), were used. The passivation material coating process was carried out with the plasma enhanced chemical vapor deposition (PECVD) system. The first sample was a coated single layer with 70/3 (SiH4/NH3) passivation material at 75 nm and the second sample was coated with bilayer (two layers) passivation materials wherein the first layer was coated with 15 nm 50/1 (SiH4/NH3) and the second layer was coated with 60 nm 70/3 (SiH4/NH3). The obtained results were compared. Experimental results show that the drain leakage current (Id) and gate leakage current (Ig) decreases; current density (Idss) and transconductance (gm) increases with bilayer passivation.Item Open Access First principles calculations of electronic and optical properties of InSe nanosheets doped with noble metal atoms(Elsevier, 2023-04-05) Narin, P.; All, J. M. Abbas; Kutlu, E. Narin; Lisesivdin, S.B.; Özbay, EkmelMonolayer Indium Selenide (ML-InSe) is studied for 4x4 supercell structure through ab initio calculations. The electronic and optical properties of ML-InSe for both pristine and substitutional doped ML-InSe with Palladium (Pd), Platinum (Pt), Silver (Ag), and Gold (Au) atoms have been calculated. With substitutional doping, ML-InSe has been observed to have a spin-dependent electronic structure. The flat energy bands near the Fermi level are observed in ML-InSe with doping elements placed in In site. The flat bands of d orbitals of some noble metal atoms are formed by the projected density of states (PDOS). The PDOS calculations show that the s-orbital of In and p-orbital of Se form the conduction band edge. The energetically favorable position of doping atoms is found to be the PtIn substitution atom according to formation energy calculations. For each studied structure, the bond length of the first neighbor of doping atoms in doped ML-InSe, static dielectric constant (ε0), refractive index, and energy band gap have been calculated. In the structure ML-InSe with AuSe, ε0 reaches ∼ 8.15. Another important result is that substitutional doping induces some peaks in the lower energy region of the imaginary part of the dielectric function. These peaks mainly refer to the absorption in related regions and may be important for the optoelectronic properties of ML-InSe.Item Open Access An ab initio study of vertical heterostructures formed by CdO and SnC monolayers(Elsevier, 2024-01-30) Seyedmohammadzadeh, Mahsa; Mobaraki, Arash; Tanatar, B.; Gülseren, OğuzAssembling two dimensional (2D) materials in vertical heterostructures is one of the main techniques for tuning electronic and optical properties. In most cases, known as van der Waals heterostructures (vdWHs), the interlayer distances are larger than typical covalent bond lengths resulting in weak interlayer interactions. It has been shown that reducing the distance between the layers can significantly alter the properties of separated layers, which is not so noticeable in vdWHs and thus creates a new platform for controlling the physical properties of 2D materials. Motivated by enhanced properties of 2D vertical heterostructures, employing ab-initio calculations based on density functional theory we examined CdO/SnC systems in four different configurations. Our results reveal that in spite of thermodynamic and mechanical stabilities of all considered structures, according to the calculated phonon frequencies, only the structure formed by placing the Sn atom on the O atom and the C atom on the Cd atom is dynamically stable at zero temperature. This structure has an interlayer distance of 2.52 Å which is smaller than the interlayer distance in typical vdWHs. We investigated the electronic and optical properties of this dynamically stable structure utilizing GW approximation and solving Bethe–Salpeter equation. Unlike the monolayer CdO which possesses a single optical absorption peak close to the red light energy, the considered CdO/SnC structure has an optical band gap of 1.14 eV, and it can absorb 13% of incident light in the blue light region.Item Open Access Light-sensitive monolayer-thick nanocrystal skins of face-down self-oriented colloidal quantum wells(Royal Society of Chemistry, 2023-10-20) Bozkaya, Taylan; Işık, Furkan; Bozkaya, İklim; Delikanlı, Savaş; Ünal, Emre; Demir, Hilmi VolkanColloidal quantum wells (CQWs), a quasi-two-dimensional, atomically-flat sub-family of semiconductor nanocrystals, are well suited to produce excellent devices for photosensing applications thanks to their extraordinarily large absorption cross-sections. In this work, we propose and demonstrate a new class of light-sensitive nanocrystal skins (LS-NS) that employ a monolayer of face-down orientation-controlled self-assembled CQWs as the active absorbing layer in the UV-visible range. This CQW LS-NS platform enables non-conventional photosensing operation that relies on the strong optical absorption of the monolayered assembly of CQWs and the subsequent photogenerated potential build-up across the device, allowing for self-powered operation. Here such self-oriented CQWs reduce the surface roughness in their monolayer-thick film, essential to high device performance. Owing to their ease of fabrication and low cost, these devices hold great promise for large-scale use in semi-transparent photosensing surfaces.Item Open Access Phase-change Fano resonator for active modulation of thermal emission(Royal Society of Chemistry, 2023-06-10) Khalichi, Bahram; Ghobadi, Amir; Kalantari Osgouei, Ataollah; Rahimian Omam, Zahra; Kocer, Hasan; Ozbay, EkmelOptical modulation of heat emission using spectrally selective infrared (IR) metasurface nanoantenna designs has found potential applications in various fields, including radiative cooling and thermal camouflage. While radiative cooling requires emitters to emit within atmospheric transmissive windows (mainly located at 8–14 μm), thermal camouflage structures have to operate within the non-transmissive window (5–8 μm) to hide an object from thermal imaging systems and cameras. Therefore, a passive nanoantenna structure cannot satisfy both conditions simultaneously. In this paper, we propose an adaptive nanoantenna emitter made of samarium nickelate (SmNiO3) phase change material to cover both functionalities with a single Fano resonator-based design. As the temperature rises, the thermal signature of the nanoantenna at the transmissive window is suppressed; therefore, a better camouflage performance is achieved. The dynamic tunability of switching from radiative cooling to thermal camouflage of the proposed Fano resonator-based design is quantitatively demonstrated using emissive power calculations under different conditions.Item Open Access Manipulating intermetallic charge transfer for switchable external stimulus-enhanced water oxidation electrocatalysis(John Wiley and Sons Inc, 2023-10-26) Chalil Oglou, Ramadan; Ulusoy Ghobadi, Türkan Gamze; Hegner, F. S. .; Galán-Mascarós, J. R.; López, N; Özbay, Ekmel; Karadaş, FerdiElectrocatalytic processes involving the oxygen evolution reaction (OER) present a kinetic bottleneck due to the existence of linear-scaling relationships, which bind the energies of the different intermediates in the mechanism limiting optimization. Here, we offer a way to break these scaling relationships and enhance the electrocatalytic activity of a Co−Fe Prussian blue modified electrode in OER by applying external stimuli. Improvements of ≈11 % and ≈57 % were achieved under magnetic field (0.2 T) and light irradiation (100 mW cm−2), respectively, when working at fixed overpotential, η=0.6 V at pH 7. The observed enhancements strongly tie in with the intermetallic charge transfer (IMCT) intensity between Fe and Co sites. Density Functional Theory simulations suggest that tuning the IMCT can lead to a change of the OER mechanism to an external stimuli-sensitive spin crossover-based pathway, which opens the way for switchable electrocatalytic devices.Item Open Access Adaptive thermally tunable radiative cooling with angle insensitivity using phase-change-material-based metasurface(Institute of Physics Publishing Ltd., 2023-11-17) Boşdurmaz, Ekin Bircan; Ghobadi, Amir; Özbay, EkmelRadiative cooling is the passive cooling of a material with the help of a specific spectral response to emit thermal energy into space through atmospheric transparency windows. However, most of the proposed designs have no dynamically tunable emission response. In this paper, we present a feasible inverse pyramid structure made of a phase change material (PCM) on top of a metallic mirror to realize an adaptive radiative cooler with almost angle-independent emission response. The design uses the thermally controlled PCM called Samarium nickelate (SmNiO3) to actively tune the spectral response of the design, which, in turn, allows the design to radiatively cool itself. The emission response of the design is compatible with atmospheric transmissive windows. As the design heated up to higher temperatures, the peak of the emission spectrum red-shifts and moves toward the atmospheric transparency window.Item Embargo Dual-Resonance nanostructures for color downconversion of colloidal quantum emitters(American Chemical Society, 2023-12-12) Ha, Son Tung; Lassalle, Emmanuel; Liang, Xiao; Do, Thi Thu Ha; Foo, Ian; Shendre, Sushant; Durmusoglu, Emek G.; Valuckas, Vytautas; Adhikary, Sourav; Paniagua-Dominguez, Ramon; Demir, Hilmi Volkan; Kuznetsov, Arseniy I.We present a dual-resonance nanostructure made of a titanium dioxide (TiO2) subwavelength grating to enhance the color downconversion efficiency of CdxZn1-xSeyS1-y colloidal quantum dots (QDs) emitting at ∼530 nm when excited with a blue light at ∼460 nm. A large mode volume can be created within the QD layer by the hybridization of the grating resonances and waveguide modes, resulting in large absorption and emission enhancements. Particularly, we achieved polarized light emission with a maximum photoluminescence enhancement of ∼140 times at a specific angular direction and a total enhancement of ∼34 times within a 0.55 numerical aperture (NA) of the collecting objective. The enhancement encompasses absorption, Purcell and outcoupling enhancements. We achieved a total absorption of 35% for green QDs with a remarkably thin color conversion layer of ∼400 nm. This work provides a guideline for designing large-volume cavities for absorption/fluorescence enhancement in microLED display, detector, or photovoltaic applications. © 2023 American Chemical Society.Item Open Access Microneedle technology as a new standpoint in agriculture: treatment and sensing(Elsevier, 2023-09) Ece, Emre; Eş, İsmail; İnci, FatihPreventing plant loss and improving their health status are essential for agricultural industry. Correspondingly, the deprivation of plants severely impacts our ecological system. As such, global efforts have been intensely made to promote the development of advanced sensing and treatment platforms to forestall plant loss. Existing technologies mainly encounter a number of challenges in providing results in a non-invasive, rapid turnaround, and affordable fashions. Accordingly, notable progressions in innovative approaches—particularly biosensing and delivery platforms, are vastly required for agriculture realm. In this regard, microneedles have emerged as a pivotal technological tool that plays multifaceted roles in biosensing and delivery systems, with attention of growing towards agriculture. Simply put, microneedles offer several advantages over conventional methods for being less invasive, rapid, and highly precise. In this review, recent advancements in microneedle technologies including their implementations in agriculture are highlighted coherently. In particular, extracting DNA from plant leaves and expressing transient genes using microneedles are elaborated in details. Microneedle-based sensing platforms for detecting essential compounds and secondary metabolites are discussed as well. Recent advances focusing the delivery of agrochemicals and nanotherapeutics via microneedles are elaborated. By this means, this review aims to bridge the existing gaps between microneedles and agriculture precisely.Item Open Access Development of AZO TCOs with ALD for HEMT and HJSC solar cell applications(Gazi univ, 2021-02-11) Tugrul, D.; Cakmak, H.; Özbay, Ekmel; Imer, B.Transparent Conductive Oxide (TCO) films are widely used in optoelectronic devices, such as solar cells, LEDs, and Lasers. Utilization of these contacts directly affects the device efficiencies. Purpose of this study is to produce and optimize properties of Aluminum doped Zinc Oxide (AZO) using a vapor phase technique, Atomic Layer Deposition (ALD) for (n+) a-Si:H surface of silicon Heterojunction Solar Cells (HJSCs) and High Electron Mobility Transistor (HEMT) applications. This study is focused on the effect of the deposition temperature and aluminum atomic concentration on structural, electrical and optical properties of ALD grown AZO ohmic contact films. The results show that as-deposited films have 80-90% transmittance in the visible spectra, low resistance (2.04x10(-3) ohm.cm) and mobility value of 5.25 cm(2)/V.s.Item Open Access Low-temperature synthesis of silicon oxynitride-doped si for tunable bragg gratings homogeneously deposited on Si, SiO2, and borosilicate substrates and the tip of SM and PM optical fibers(John Wiley and Sons Inc, 2023-06-19) Karatutlu, Ali; Tabaru, Timuçin Emre; Ortaç, BülendOptical tunability and repeatability are essential in fabricating optoelectronic devices from waveguides to Bragg gratings (BGs) for high-energy, high-power, mode-locking, and sensing applications. For this purpose, a controlled adjustment in the optical properties, including the refractive index of the deposited nanolayers, becomes critical. This study reveals that silicon oxynitride (SiON) doping into silicon (Si) offers a new way for the preparation of novel Si-based devices with an emphasis on the BGs for filtering a particular portion of an electromagnetic spectrum, including the wavelengths of 800, 976, 1550, and 1840 nm. Control on the incident angle dependence of the BGs is demonstrated at Watt-level for the wavelength of 976 nm. Amorphous SiON-doped Si layers on alternating SiO2 can be synthesized on bulk substrates and different optical fibers at relatively low temperatures with wide and narrow bandwidths. The high reflectivity of the novel Si-based BGs reveals over −22 dB reflection using typical optical fibers, including standardsingle-mode fibers and high-birefringent polarization-maintaining (PM) fibers. The polarized transmission measurement over the BG on the PMfiber shows the BGs do not deteriorate the PM properties, strongly yielding a beat length of 1.68 mm and birefringence of 9.2 × 10−4 at the telecom C band.Item Open Access A high-power and broadband gan spdt mmic switch using gate-optimized hemts(Institute of Electrical and Electronics Engineers, 2022-05-22) Erturk, V.; Gurdal, A.; Özbay, EkmelA high-power, broadband monolithic microwave integrated circuit (MMIC) single-pole double-throw (SPDT) switch is designed with gate-optimized high-electron-mobility transistors (HEMTs) using AlGaN/Gallium nitride (GaN) technology. The foot length of the gate is varied from 200 to 250 nm, and the head length is varied from 500 to 750 nm in the T-gate structure to optimize the radio frequency (RF) performance. The SPDT switch is designed in a series-shunt-shunt topology using gate topology as a design parameter. The switch has achieved an insertion loss better than 0.75 dB throughout the 3.5-13.5-GHz bandwidth. It can transmit 30-W output power at 0.1-dB compression point and handle 47.5-dBm input power at P-1dB. The isolation is above 25 dB, and the return loss is better than 11 dB. With its low insertion and high power-handling capacity in broadband, the SPDT switch shows state-of-the-art performance for high-power communication systems and radar applications.