Department of Physics

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  • ItemOpen Access
    Understanding the suitable alloying conditions for highly efficient Cu- and Mn-doped Zn1-xCdxS/ZnS core-shell quantum dots
    (Elsevier B.V., 2023-10-20) Kaur, Manpreet; Sharma, Ashma; Erdem, Onur; Kumar, A.; Demir, Hilmi Volkan; Sharma, M.
    Doping of alloyed colloidal quantum dots (QDs) has garnered significant attention for providing tunable and Stokes-shifted emission. By alloying the host semiconductor nanocrystals (NCs), their band gap can be tuned. With the specific addition of dopant ions, these NCs can emit tunable emissions within the visible spectrum. However, while doped and alloyed quantum dots (QDs) have shown promise for tunable emissions, their emission qualities have not been consistent across the spectrum. Here, we report the synthesis of high-quality Cu- and Mn-doped ZnxCd1-xS (x = 0–1) alloyed QDs by a colloidal non-injection method. In this study, we examined the effect of different dopant ions on the optical properties of similar alloyed nanocrystals. The deposition of a ZnS shell on these doped QDs significantly improves their quantum yield (QY), increasing it from 7.0 % to 50.0 % for Cu-doped QDs and from 30.0 % to 80.0 % for Mn-doped QDs. The Cu-doped QDs exhibit tunable emission from green to red across the visible spectrum by varying the Zn/Cd ratio, whereas the Mn-doped QDs show a fixed orange emission. Interestingly, the Cu-doped alloyed QDs show a contrasting trend in quantum yield (QY) compared to those of Mn-doped QDs when the amount of Cd in ZnCdS alloyed QDs is systematically changed. As the amount of Cd increases in the ZnCdS alloyed QDs, the Cu-doped QDs show both an increase in average lifetime and an increase in QY. In contrast, for the Mn-doped QDs, the decay lifetime values remain fairly constant for different amounts of Cd in the ZnCdS alloyed QDs, but the QY decreases as the amount of Cd increases. The results of this study may facilitate the design of optimal alloying combinations for Cu/Mn-doped QDs in optoelectronic applications. © 2023 The Authors
  • ItemOpen Access
    Deep-learning-enabled electromagnetic near-field prediction and inverse design of metasurfaces
    (Optica Publishing Group (formerly OSA), 2023-10-16) Kanmaz, Tevfik Bülent; Öztürk, E.; Demir, Hilmi Volkan; Gündüz-Demir, Ç.
    Metasurfaces generate desired electromagnetic wavefronts using sub-wavelength structures that are much thinner than conventional optical tools.However, their typical design method is based on trial and error, which is adversely inefficient in terms of the consumed time and computational power. This paper proposes and demonstrates deep-learning-enabled rapid prediction of the full electromagnetic near-field response and inverse prediction of the metasurfaces from desired wavefronts to obtain direct and rapid designs. The proposed encoder-decoder neural network was tested for different metasurface design configurations. This approach overcomes the common issue of predicting only the transmission spectra, a critical limitation of the previous reports of deep-learning-based solutions. Our deep-learning-empowered near-field model can conveniently be used as a rapid simulation tool for metasurface analyses as well as for their direct rapid design. © 2023 Optica Publishing Group.
  • ItemOpen Access
    Single-material MoS2 thermoelectric junction enabled by substrate engineering
    (Nature Research, 2023-05-26) Razeghi, Mohammadali; Spiece, J.; Oğuz, Oğuzhan; Pehlivanoğlu, Doruk; Huang, Y.; Sheraz, Ali; Başçı, U.; Dobson, P. S.; Weaver, J. M. R.; Gehring, P.; Kasırga, Talip Serkan
    To realize a thermoelectric power generator, typically, a junction between two materials with different Seebeck coefficients needs to be fabricated. Such differences in Seebeck coefficients can be induced by doping, which renders it difficult when working with two-dimensional (2d) materials. However, doping is not the only way to modulate the Seebeck coefficient of a 2d material. Substrate-altered electron–phonon scattering mechanisms can also be used to this end. Here, we employ the substrate effects to form a thermoelectric junction in ultrathin, few-layer MoS2 films. We investigated the junctions with a combination of scanning photocurrent microscopy and scanning thermal microscopy. This allows us to reveal that thermoelectric junctions form across the substrate-engineered parts. We attribute this to a gating effect induced by interfacial charges in combination with alterations in the electron–phonon scattering mechanisms. This work demonstrates that substrate engineering is a promising strategy for developing future compact thin-film thermoelectric power generators. © 2023, The Author(s).
  • ItemOpen Access
    Enhanced generation of higher harmonic from Halide Perovskite Metasurfaces
    (META Conference, 2023) Tonkaev, P.; Koshelev, K.; Masharin, Mikhail A.; Makarov S.; Kruk S.; Kruk S.
    Many outstanding properties of halide perovskites provided their applications in optoelectronics. Perovskite films demonstrate outstanding nonlinear properties with large optical nonlinearities comparable to the nonlinear constants of conventional semiconductor materials. Meanwhile, nonlinear properties can be enhanced by the metaphotonic approach. Here we demonstrate a two-order enhancement of fifth-harmonic generation in halide perovskite nonlocal metasurfaces due to high-quality resonance at the generated harmonic wavelength in the visible frequency range. © 2023, META Conference. All rights reserved.
  • ItemOpen Access
    Sub-wavelength silicon nano-structuring with direct laser writing
    (META Conference, 2023) Sabet, Rana Asgari; Tokel, Onur
    We present a novel laser nano-lithography method inside the bulk of silicon. We exploit nanosecond laser pulses of 1.55-μm wavelength which are modulated with a spatial light modulator. The created Bessel beams enable direct and highly-controlled subsurface fabrication capability in Si. Using this technique, we demonstrate for the first time, the fabrication of sub-wavelength nano-modifications deep inside Si. We further illustrate nanopatterns of 200-nm width and of sub-micron separation. Such 3D control on sub-wavelength structures inside Si offer exciting possibilities for Si-photonics devices, meta-material and meta-surface in-chip technologies. © 2023, META Conference. All rights reserved.
  • ItemOpen Access
    Foerster-Type nonradiative energy transfer in media with complex permittivity
    (META Conference, 2023) Hernandez-Martinez, Pedro Ludwig; Yucel, Abdulkadir C.; Demir, Hilmi Volkan
    We 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.
  • ItemOpen Access
    Observation of enhanced generation of a fifth harmonic from halide perovskite nonlocal metasurfaces
    (American Chemical Society, 2023-03-11) Tonkaev, P.; Koshelev, K.; Masharin, Mikhail A.; Makarov, S. V.; Kruk, S. S.; Kivshar, Y.
    Lead halide perovskites are widely employed in photonic and light-emitting devices because of their rich optoelectronic properties and simplicity of fabrication based on low-cost flexible technologies. Perovskite bulk crystals and films demonstrate outstanding nonlinear characteristics with large optical nonlinearities exceeding the nonlinear susceptibilities of conventional semiconductor materials by several orders of magnitude. One of the promising approaches for further enhancement of the nonlinear response of perovskites as dielectric photonic materials is to employ optical resonances of structured surfaces, or metasurfaces. Nonlocal metasurfaces supporting lattice modes over many unit cells provide a new approach to both spatial and spectral control of light fields, and they may deliver improved characteristics of nonlinear effects for a wide range of applications associated with broadband excitation of multiple high-quality resonances. Here we report on the first observation of enhanced fifth harmonic generation in MAPbBr3 halide perovskite nonlocal metasurfaces driven by high-quality resonances at the generated harmonic wavelength in the visible frequency range. The demonstrated enhancement is about 2 orders of magnitude compared to an unpatterned MAPbBr3 film of the same thickness, and is broadband by virtue of the excitation of multiple resonant modes in the highly nonlocal regime. Our work suggests a novel approach for achieving parametric processes in resonant dielectric structures with high efficiency.
  • ItemOpen Access
    Ultrafast control of the optical transition in type-II colloidal quantum wells
    (American Chemical Society, 2023-04-21) Yu, J.; Durmusoglu, E. G.; Wang, Y.; Sharma, M.; Demir, Hilmi Volkan; Dang, C.
    Manipulating the optical transition in semiconductors at ultrashort timescales is of both fundamental interest and central importance for emerging photonic applications. Traditionally, this manipulation is realized by electrostatic gating via Stark effects or band-gap renormalizations. Here, we report an ultrafast and all-optical route to engineer an indirect transition in core–crown colloidal quantum wells (CQWs), namely, CdSe/CdTe, with a type-II band alignment. Following the intense laser pulse excitation, the indirect band transition energy exhibits a pronounced blueshift–redshift crossover on the picosecond timescale, stemming from the formation and dissipation of the transient electric field (E-field) that forms upon photoexcitation to compensate for the driving force provided by the band offsets. Both the energy shift and dynamics of the transient E-field can be modulated optically by tuning the laser pulse excitation fluence. Our finding demonstrates a strong analogy between the type-II heterojunction and a p–n junction with respect to carrier equilibrium processes, which holds promise to facilitate the integration of CQWs within optical switching networks.
  • ItemOpen 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.
  • ItemOpen Access
    High-frequency EPR and ENDOR spectroscopy of Mn2+ ions in CdSe/CdMnS nanoplatelets
    (American Chemical Society, 2023-02-20) Babunts, Roman A.; Uspenskaya, Yulia A.; Romanov, Nikolai G.; Orlinskii, Sergei B.; Mamin, Georgy V.; Shornikova, Elena V.; Yakovlev, Dmitri R.; Bayer, M.; Işık, Furkan; Shendre, S.; Delikanlı, Savaş; Demir, Hilmi Volkan
    Semiconductor colloidal nanoplatelets based of CdSe have excellent optical properties. Their magneto-optical and spin-dependent properties can be greatly modified by implementing magnetic Mn2+ ions, using concepts well established for diluted magnetic semiconductors. A variety of magnetic resonance techniques based on high-frequency (94 GHz) electron paramagnetic resonance in continuous wave and pulsed mode were used to get detailed information on the spin structure and spin dynamics of Mn2+ ions in core/shell CdSe/(Cd,Mn)S nanoplatelets. We observed two sets of resonances assigned to the Mn2+ ions inside the shell and at the nanoplatelet surface. The surface Mn demonstrates a considerably longer spin dynamics than the inner Mn due to lower amount of surrounding Mn2+ ions. The interaction between surface Mn2+ ions and 1H nuclei belonging to oleic acid ligands is measured by means of electron nuclear double resonance. This allowed us to estimate the distances between the Mn2+ ions and 1H nuclei, which equal to 0.31 ± 0.04, 0.44 ± 0.09, and more than 0.53 nm. This study shows that the Mn2+ ions can serve as atomic-size probes for studying the ligand attachment to the nanoplatelet surface.
  • ItemEmbargo
    Optically detected magnetic resonance spectroscopic analyses on the role of magnetic ions in colloidal nanocrystals
    (AIP Publishing LLC, 2023-08-15) Dehnel, Joanna; Harchol, Adi; Barak, Yahel; Meir, Itay; Horani, Faris; Shapiro, Arthur; Strassberg, Rotem; de Mello Donegá, Celso; Demir, Hilmi Volkan; Gamelin, Daniel R.; Sharma, Kusha; Lifshitz, Efrat
    Incorporating magnetic ions into semiconductor nanocrystals has emerged as a prominent research field for manipulating spin-related properties. The magnetic ions within the host semiconductor experience spin-exchange interactions with photogenerated carriers and are often involved in the recombination routes, stimulating special magneto-optical effects. The current account presents a comparative study, emphasizing the impact of engineering nanostructures and selecting magnetic ions in shaping carrier-magnetic ion interactions. Various host materials, including the II-VI group, halide perovskites, and I-III-VI2 in diverse structural configurations such as core/shell quantum dots, seeded nanorods, and nanoplatelets, incorporated with magnetic ions such as Mn2+, Ni2+, and Cu1+/2+ are highlighted. These materials have recently been investigated by us using state-of-the-art steady-state and transient optically detected magnetic resonance (ODMR) spectroscopy to explore individual spin-dynamics between the photogenerated carriers and magnetic ions and their dependence on morphology, location, crystal composition, and type of the magnetic ion. The information extracted from the analyses of the ODMR spectra in those studies exposes fundamental physical parameters, such as g-factors, exchange coupling constants, and hyperfine interactions, together providing insights into the nature of the carrier (electron, hole, dopant), its local surroundings (isotropic/anisotropic), and spin dynamics. The findings illuminate the importance of ODMR spectroscopy in advancing our understanding of the role of magnetic ions in semiconductor nanocrystals and offer valuable knowledge for designing magnetic materials intended for various spin-related technologies.
  • ItemOpen 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, Ekmel
    In 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.
  • ItemOpen Access
    Highly directional highly efficient solution processed light emitting diodes of all face down
    (Wiley-VCH Verlag GmbH & Co. KGaA, 2023-04-06) Baruj, Hamed Dehghanpour; Bozkaya, İklim; Canımkurbey, Betül; Işık, Ahmet Tarık; Shabani, Farzan; Delikanlı, Savaş; Shendre, S.; Erdem, Onur; Işık, Furkan; Demir, Hilmi Volkan
    Semiconductor colloidal quantum wells (CQWs) provide anisotropic emission behavior originating from their anisotropic optical transition dipole moments (TDMs). Here, solution-processed colloidal quantum well light-emitting diodes (CQW-LEDs) of a single all-face-down oriented self-assembled monolayer (SAM) film of CQWs that collectively enable a supreme level of IP TDMs at 92% in the ensemble emission are shown. This significantly enhances the outcoupling efficiency from 22% (of standard randomly-oriented emitters) to 34% (of face-down oriented emitters) in the LED. As a result, the external quantum efficiency reaches a record high level of 18.1% for the solution-processed type of CQW-LEDs, putting their efficiency performance on par with the hybrid organic-inorganic evaporation-based CQW-LEDs and all other best solution-processed LEDs. This SAM-CQW-LED architecture allows for a high maximum brightness of 19,800 cd m$^{−2}$ with a long operational lifetime of 247 h at 100 cd m$^{−2}$ as well as a stable saturated deep-red emission (651 nm) with a low turn-on voltage of 1.7 eV at a current density of 1 mA cm$^{−2}$ and a high J$_{90}$ of 99.58 mA cm$^{−2}$. These findings indicate the effectiveness of oriented self-assembly of CQWs as an electrically-driven emissive layer in improving outcoupling and external quantum efficiencies in the CQW-LEDs.
  • ItemOpen Access
    Gradient Type-II CdSe/CdSeTe/CdTe Core/Crown/Crown heteronanoplatelets with asymmetric shape and disproportional excitonic properties
    (Wiley-VCH Verlag GmbH & Co. KGaA, 2023-01-17) Shabani, Farzan; Hernandez Martinez, P. L.; Shermet, Nina; Korkut, Hilal; Sarpkaya, İbrahim; Baruj, Hamed Dehghanpour; Delikanlı, Savaş; Işık, Furkan; Durmuşoğlu, E. G.; Demir, Hilmi Volkan
    Characterized by their strong 1D confinement and long-lifetime red-shifted emission spectra, colloidal nanoplatelets (NPLs) with type-II electronic structure provide an exciting ground to design complex heterostructures with remarkable properties. This work demonstrates the synthesis and optical characterization of CdSe/CdSeTe/CdTe core/crown/crown NPLs having a step-wise gradient electronic structure and disproportional wavefunction distribution, in which the excitonic properties of the electron and hole can be finely tuned through adjusting the geometry of the intermediate crown. The first crown with staggered configuration gives rise to a series of direct and indirect transition channels that activation/deactivation of each channel is possible through wavefunction engineering. Moreover, these NPLs allow for switching between active channels with temperature, where lattice contraction directly affects the electron–hole (e–h) overlap. Dominated by the indirect transition channels over direct transitions, the lifetime of the NPLs starts to increase at 9 K, indicative of low dark-bright exciton splitting energy. The charge transfer states from the two type-II interfaces promote a large number of indirect transitions, which effectively increase the absorption of low-energy photons critical for nonlinear properties. As a result, these NPLs demonstrate exceptionally high two-photon absorption cross-sections with the highest value of 12.9 × 10$^{6}$ GM and superlinear behavior.
  • ItemOpen Access
    Stable harmonic modelocked laser operation by damping the temporal displacement of optical pulses in analogy to Brownian particles
    (IEEE - Institute of Electrical and Electronics Engineers, 2023-09-04) Laçin, Mesut; Repgen, Paul; Şura, Aladin; Şenel, Ç.; İlday, Fatih Ömer
    Since the first report of modelocking in 1964, every parameter of a typical oscillator has been improved by several orders of magnitude. The exception is the repetiton rate, which is still roughly in the order of the initial 56 MHz. Some applications, such as ablation-cooled material processing, require high pulse repetition rates. Harmonic modelocking presents itself as a solution. However, it has had a limited impact outside the laser research laboratory to date, but its utility can be improved significantly if its reliable, repeatable, and stable operation can be ensured.
  • ItemEmbargo
    Rational design of an acceptor-chromophore-relay-catalyst tetrad assembly for water oxidation
    (Royal Society of Chemistry, 2023-12-25) Chalil Oglou, Ramadan; Ulusoy Ghobadi, T. Gamze; Saylam, A.; Bese, D.; Bese, C.; Yağlıoğlu, Halime Gül; Özçubukçu, S.; Özbay, Ekmel; Karadaş, Ferdi
  • ItemOpen Access
    Thermodynamic silver doping of core/shell colloidal quantum wells imparted with paramagnetic properties emitting at near-infrared
    (American Chemical Society, 2023-05-29) Shabani, Farzan; Ahmad, Muhammad; Kumar, Satish; Delikanlı, Savaş; Işık, Furkan; Bhattacharya, A.; Petrou, A.; Demir, Hilmi Volkan
    Two-dimensional (2D) core/shell nanoplatelets (NPLs) synthesized via the hot-injection method provide excellent thermal and chemical stability for high-temperature doping, where an expanded and flexible lattice is required. Here, a thermodynamic approach toward silver doping of these NPLs is proposed and demonstrated, which previously proved to be challenging due to the fast self-purification of the dopants with the introduction of the shell. Maintaining the doping procedure in the reversible regime ensured the integrity of the NPLs and allowed a high level of doping; however, the equilibrium condition is further complicated by environmental factors that affect the chemical activity of the cations and the surface composition of the NPLs. Two main deterioration mechanisms in the irreversible regime were observed: ZnS-shelled NPLs suffered preferential etching, while CdS-shelled NPLs underwent cleavage and fragmentation. Alloying of the shell minimized both mechanisms for CdZnS-shelled NPLs and preserved the metastable state of the NPLs, including their 2D shape and crystalline structure. Distribution of silver ions in the lattice of the NPLs directly affected the recombination dynamics and enabled fine-tuning of the near-infrared emission beside the exciton confinement. These silver-doped CdZnS-shelled NPLs are shown further to exhibit enhanced paramagnetic properties with Zeeman splitting and Brillouin-like bound-exciton polarization as a function of the magnetic field, critical for spintronic applications
  • ItemEmbargo
    Aqueous colloidal nanoplatelets for imaging and improved ALA-based photodynamic therapy of prostate cancer cells
    (Royal Society of Chemistry, 2023-07-10) Onbaşlı, K.; Demirci, G.; Işık, Furkan; Durmuşoğlu, Emek G.; Demir, Hilmi Volkan; Acar, Havva Y.
  • ItemOpen Access
    Defining a critical temperature of a crossoverfrom BEC to the normal phase
    (World Scientific Publishing Company, 2023-01-25) Rakhimov, A.; Khudoyberdiev, A.; Narzikulov, Z.; Tanatar, Bilal
    We address the problem of identifying the critical temperature in a crossover from the Bose-Einstein condensed (BEC) phase to the normal phase. For this purpose we study the temperature dependence of magnetization of spin-gapped quantum magnets described by BEC of triplons. We have calculated the heat capacity CH at constant field and fluctuations in magnetization in a spin-gapped quantum magnet using the Hartree-Fock-Bogouliubov approximation and found optimized parameters of the Hamiltonian of triplon gas. In the region of phase transition, the heat capacity CH is smeared out due to the Dzyaloshinsky-Moriya (DM) interaction. The sharp maximum of the fluctuations in the magnetization is identified as the critical temperature of the crossover. © 2023 World Scientific Publishing Company.
  • ItemOpen 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, Ekmel
    Among 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.