Browsing by Subject "Impurities"
Now showing 1 - 7 of 7
Results Per Page
Sort Options
Item Open Access Ab initio study of hydrogenic effective mass impurities in Si nanowires(Institute of Physics Publishing, 2017-01) Peelaers, H.; Durgun, Engin; Partoens, B.; Bilc, D. I.; Ghosez, P.; Van De Walle C. G.; Peeters, F. M.The effect of B and P dopants on the band structure of Si nanowires is studied using electronic structure calculations based on density functional theory. At low concentrations a dispersionless band is formed, clearly distinguishable from the valence and conduction bands. Although this band is evidently induced by the dopant impurity, it turns out to have purely Si character. These results can be rigorously analyzed in the framework of effective mass theory. In the process we resolve some common misconceptions about the physics of hydrogenic shallow impurities, which can be more clearly elucidated in the case of nanowires than would be possible for bulk Si. We also show the importance of correctly describing the effect of dielectric confinement, which is not included in traditional electronic structure calculations, by comparing the obtained results with those of G0W0 calculations.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 Effect of in-material losses on terahertz absorption, transmission, and reflection in photonic crystals made of polar dielectrics(A I P Publishing LLC, 2015) Serebryannikov, A. E.; Nojima, S.; Alici, K. B.; Özbay, EkmelThe effect of the material absorption factor on terahertz absorption (A), transmittance (T), and reflectance (R) for slabs of PhC that comprise rods made of GaAs, a polar dielectric, is studied. The main goal was to illustrate how critical a choice of the absorption factor for simulations is and to indicate the importance of the possible modification of the absorption ability by using either active or lossy impurities. The spectra of A, T, and R are strongly sensitive to the location of the polaritonic gap with respect to the photonic pass and stop bands connected with periodicity that enables the efficient combination of the effects of material and structural parameters. It will be shown that the spectra can strongly depend on the utilized value of the material absorption factor. In particular, both narrow and wide absorption bands may appear owing to a variation of the material parameters with a frequency in the vicinity of the polaritonic gap. The latter are often achieved at wideband suppression of transmission, so that an ultra-wide stop band can appear as a result of adjustment of the stop bands having different origin. The results obtained at simultaneous variation of the absorption factor and frequency, and angle of incidence and frequency, indicate the possibility of the existence of wide ranges of tolerance, in which the basic features do remain. This allows for mitigating the accuracy requirements for the absorption factor in simulations and promises the efficient absorption of nonmonochromatic waves and beams with a wide angular spectrum. Suppression of narrowband effects in transmission is demonstrated at rather large values of the absorption factor, when they appear due to either the defect modes related to structural defects or dispersion inspired variations of the material parameters in the vicinity of the polaritonic gap. Comparison with auxiliary structures helps one to detect the common features and differences of homogeneous slabs and slabs of a PhC, which are made of GaAs. © 2015 AIP Publishing LLC.Item Open Access Hall conductance in graphene with point defects(2013) İslamoǧlu, S.; Oktel, M. Ö.; Gülseren, O.We investigate the Hall conductance of graphene with point defects within the Kubo formalism, which allows us to calculate the Hall conductance without constraining the Fermi energy to lie in a gap. For pure graphene, which we model using a tight-binding Hamiltonian, we recover both the usual and the anomalous integer quantum Hall effects depending on the proximity to the Dirac points. We investigate the effect of point defects on Hall conduction by considering a dilute but regular array of point defects incorporated into the graphene lattice. We extend our calculations to include next nearest neighbor hopping, which breaks the bipartite symmetry of the lattice. We find that impurity atoms which are weakly coupled to the rest of the lattice result in gradual disappearance of the high conductance value plateaus. For such impurities, especially for vacancies which are decoupled from the lattice, strong modification of the Hall conductance occurs near the E = 0 eV line, as impurity states are highly localized. In contrast, if the impurities are strongly coupled, they create additional Hall conductance plateaus at the extremum values of the spectrum, signifying separate impurity bands. Hall conductance values within the original spectrum are not strongly modified.Item Open Access Light-induced paramagnetism in colloidal Ag+-doped CdSe nanoplatelets(American Chemical Society, 2021-03-25) Najafi, A.; Sharma, Manoj; Delikanlı, Savaş; Bhattacharya, A.; Murphy, J. R.; Pientka, J.; Sharma, A.; Quinn, A. P.; Erdem, Onur; Kattel, S.; Kelestemur, Y.; Kovalenko, M. V.; Rice, W. D.; Demir, Hilmi Volkan; Petrou, A.We describe a study of the magneto-optical properties of Ag+-doped CdSe colloidal nanoplatelets (NPLs) that were grown using a novel doping technique. In this work, we used magnetic circularly polarized luminescence and magnetic circular dichroism spectroscopy to study light-induced magnetism for the first time in 2D solution-processed structures doped with nominally nonmagnetic Ag+ impurities. The excitonic circular polarization (PX) and the exciton Zeeman splitting (ΔEZ) were recorded as a function of the magnetic field (B) and temperature (T). Both ΔEZ and PX have a Brillouin-function-like dependence on B and T, verifying the presence of paramagnetism in Ag+-doped CdSe NPLs. The observed light-induced magnetism is attributed to the transformation of nonmagnetic Ag+ ions into Ag2+, which have a nonzero magnetic moment. This work points to the possibility of incorporating these nanoplatelets into spintronic devices, in which light can be used to control the spin injection.Item Open Access Mutual energy transfer in a binary colloidal quantum well complex(American Chemical Society, 2019) Yu, J.; Sharma, Manoj; Delikanlı, Savaş; Birowosuto, M. D.; Demir, Hilmi Volkan; Dang, C.Förster resonance energy transfer (FRET) is a fundamental process that is key to optical biosensing, photosynthetic light harvesting, and down-converted light emission. However, in total, conventional FRET in a donor–acceptor pair is essentially unidirectional, which impedes practical application of FRET-based technologies. Here, we propose a mutual FRET scheme that is uniquely bidirectional in a binary colloidal quantum well (CQW) complex enabled by utilizing the d orbital electrons in a dopant–host CQW system. Steady-state emission intensity, time-resolved, and photoluminescence excitation spectroscopies have demonstrated that two distinct CQWs play the role of donor and acceptor simultaneously in this complex consisting of 3 monolayer (ML) copper-doped CQWs and 4 ML undoped CQWs. Band-edge excitons in 3 ML CQWs effectively transfer the excitation to excitons in 4 ML CQWs, whose energy is also harvested backward by the dopants in 3 ML CQWs. This binary CQW complex, which offers a unique mutual energy-transfer mechanism, may unlock revolutionary FRET-based technologies.Item Open 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 VolkanTwo-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