Search

Study of N-polar GaN surface barrier. Corrigendum: Available online 4 February 2020: Łukasz Janicki, Jan Misiewicz, Marcin Siekacz, Henryk Turski, Joanna Moneta, Sandeep Gorantla, Czesław Skierbiszewski, Robert Kudrawiec; Corrigendum to “Sensitivity of N-polar GaN surface barrier to ambient gases” [Sens. Actuators B: Chem. 281(2019) 561–567]. https://doi.org/10.1016/j.snb.2020.127782, Corrigendum: Available online 4 February 2020: Łukasz Janicki, Jan Misiewicz, Marcin Siekacz, Henryk Turski, Joanna Moneta, Sandeep Gorantla, Czesław Skierbiszewski, Robert Kudrawiec; Corrigendum to "Sensitivity of N-polar GaN surface barrier to ambient gases" [Sens. Actuators B: Chem. 281(2019) 561–567]. https://doi.org/10.1016/j.snb.2020.127782

We demonstrate how to tailor the size-dependent enhanced transmission and absorption of 1D subwavelength semiconductor-based metamaterial high contrast gratings. We focus on 3 spectral regimes generic for most semiconductors, where the refractive index (n) and extinction coefficient (k) of the semiconducting material satisfy the following conditions: n >> k, n ~ k and n < k. We show that the transmission into such structures can be enhanced by reducing the bar width, increasing the grating period, or tapering the grating sidewalls. Moreover, thanks to the slow-light phenomenon, the absorption of the grating can be enhanced as compared to bulk semiconductors.

In this study, the role of a zinc acetate precursor in improving the luminescence stability of purple-emitting CdS nanocrystals is investigated. The oleate-capped core of CdS nanocrystals exhibits intense photodarkening under prolonged UV excitation. From the results of photoluminescence experiments, we can observe that photobleaching is responsible for the degradation of temporal stability, i.e., decline in photoluminescence intensity. Herein, we demonstrate that by adding zinc acetate to the synthesis solution, one can enhance the photoluminescence stability by the complete suppression of the bleaching processes of nanocrystals. We can distinguish between the effects caused by zinc ions and those caused by acetate ligands. Acetate ligands improve the photoluminescence stability of the core of CdS nanocrystals. However, only when zinc acetate is used, the PL stability can be conserved at high excitation power. Simultaneously, we have studied the influence of zinc cations and acetate ligands on the kinetics of nanocrystal growth. The presented results underline the importance of short surface capping ligands and zinc cations in CdS nanocrystal synthesis. This study exhibits a new advantage of exploiting zinc acetate reagents in one-pot nanocrystal synthesis.

In this work modification of TiO2 thin films structure by doping with europium was described. Nanocrystalline films were prepared by high energy magnetron sputtering process. The influence of europium on the microstructure of TiO2 was determined based on the results of X-ray diffraction, transmission electron microscopy, Raman spectroscopy and photoluminescence measurements. It was found that undoped film had rutile structure directly after deposition (without additional annealing), while 0.2 at.% and 0.4 at.% of the dopant was sufficient to receive TiO2 films with anatase form. The type of structure was confirmed with the aid of Raman spectroscopy and by TEM observations. The amount of Eu-dopant had direct impact on PL intensity as well as presence of defect (voids) in the film.

In this paper we show that the surface Fermi level of Ga-polar GaN exhibits a bistable behavior allowing it to be located at two distinct energetic positions at the air/GaN interface which is unusual for other III–V semiconductors such as GaAs or GaSb. To determine the Fermi level position at the air/GaN interface we perform contactless electroreflectance measurements on specially designed UD+ structures [GaN(undoped)/GaN(highly doped)/substrate] doped by Si and Mg. Analyzing the period of Franz-Keldysh oscillation we determine the built-in electric field in the undoped (U) layer. These studies coupled with numerical solutions of the Poisson equation allowed us to determine the position of the Fermi level at the air/GaN interface. We observe a change in the band bending correlated to different Fermi level positions in the doped (D+) layer. We show that depending on the doping type in the D+ layer the Fermi level at the air/GaN interface is located in the upper or lower singularity of surface density of states (SDOS) for Si or Mg doping of D+ layer, respectively. We support our findings with the density functional theory calculations of the SDOS and the dependence of the Fermi level position on the doping concentration in the bulk of a GaN slab.

Understanding the interactions of light with periodic arrays of Si fins is of the utmost importance for nanoelectronics, where laser light is used for the fabrication and metrology of fin field effect transistors (finFETs). However, due to their nanoscale dimensions and periodic arrangement, these structures exhibit complex photonic properties. In this work, we explain theoretically how the reflectance of semi-infinite periodic arrays of Si fins embedded in SiO2 varies with the fin pitch (i.e. spatial periodicity) and fin width. The results are corroborated with band structure calculations, showing that the spectra of both polarizations, parallel (TE) and perpendicular (TM) to the fin sidewalls, can be understood based on the excitation of one waveguide mode. First, we demonstrate that increasing the pitch decreases the reflectance from the arrays, for both polarizations. Moreover, TE spectra resemble that of bulk Si and are much higher as compared to TM, which are similar to the bulk SiO2 spectrum. The difference is attributed to the fact that TE mode is mostly confined inside the fin, whereas TM is spread in the SiO2. Subsequently, we show that the reflectance from the arrays increases as a function of the fin width. TE reflectances are again mostly sensitive towards Si dispersion and higher than TM counterparts. Interestingly, for TM illumination a transition from the SiO2- to Si-like spectra is observed for the fins of increasing width. The transition is caused by the change in the fraction of the electric field propagating inside the fin. The developed insight will facilitate design, fabrication and metrology of optoelectronic, photovoltaic and nanoelectronic devices.

We proposed to exploit phosphomolybdic acid (PMA) as a cost-efficient MoOx source for combined spin-coating/sputtering/spin-coating deposition of a MoOx/Au/MoOx (MAM) composite electrode. The bottom PMA layer provides perfect wetting conditions for ultrathin Au film sputtering and prevents the formation of gold islands on the glass surface, while the top PMA layer helps to reduce light reflection. By optimizing the thickness of ultrathin Au films and PMA layers, we achieved maximum transmittance of 79% at 550 nm and a sheet resistance of only 22 Ω sq−1 which is comparable to the resistance of ITO substrates (20 Ω sq−1). MAM multilayer was explored both as a transparent electrode and as a hole injection layer (HIL) to eliminate ITO and PEDOT:PSS from solution-processed quantum-dot light-emitting diodes (QLEDs). The fabricated MAM-based QLED shows a peak external quantum efficiency (EQE) of 2.7% and maximum brightness of 12 000 cd m−2 at 7 V. By performing bending tests of the polyethylene (PET) substrate coated with MAM electrode, we demonstrate that it is also a promising candidate for flexible transparent optoelectronics.

International audience; In this work, silicon nitride films containing terbium were deposited by reactive magnetron co-sputtering in a nitrogen enriched plasma and subjected to rapid thermal annealing treatments. The influence of annealing temperature on the emission and absorption properties of these films was investigated by photoluminescence, photoluminescence decay and photoluminescence excitation measurements. An increase in the photo-luminescence intensity and photoluminescence decay time was observed upon annealing for the main 5 D 4-7 F 5 transition of Tb 3+ ions. This observation was attributed to decrease of the non-radiative recombination and increase of the number of excited Tb 3+ ions upon annealing. Moreover, high temperature annealing was found to shift the spectral position of absorption bands observed in the photoluminescence excitation spectra. In general, these excitation spectra were shown to have a rather complicated structure and were decomposed into three Gaussian bands. It was suggested that two of these excitation bands might be due to indirect excitation of Tb 3+ ions via defects and the third excitation band could be due to direct 4f-5d transition.

Terbium-doped silicon-rich silicon oxide films were deposited and hydrogenated at elevated temperatures. The influence of hydrogenation on defects-mediated non-radiative recombination of excited Tb 3+ ions was investigated by means of photoluminescence and photoluminescence decay measurements. An increase in photoluminescence intensity and photoluminescence decay time was observed upon hydrogenation for the main 5 D 4 – 7 F 5 transition of Tb 3+ ions. This observation was ascribed to saturation of non-radiative recombination defect centers with hydrogen. It was found that the number of emitted photons increases upon passivation as a result of two effects: (1) a reduction of the non-radiative recombination rate, and (2) optical activation of new Tb 3+ emitters. Based on the obtained results and literature data, Forster energy-transfer was suggested as the interaction responsible for the non-radiative coupling between Tb 3+ ions and defects.

We acknowledge financial support from the Polish Ministry of Science and Higher Education within the “Diamond Grant” project No. DI2012 008642 and by the State of Bavaria. In this report we present experimental studies on the energy transfer between the wetting layer and single large elongated In0.3Ga0.7As/GaAs quantum dots. We obtain insight into the electronic and optical properties of In0.3Ga0.7As/GaAs quantum dots by probing their confined electronic states via photoluminescence excitation spectroscopy on the single dot level. We demonstrate that the energy separation between the states of a quantum dot and the wetting layer states affects the carrier transfer efficiency - reduced transfer efficiency is observed for smaller dots with higher indium content. We also discuss the effects of the excited states and the trapping of carriers on confinement potential fluctuations of the wetting layer. Eventually, the transfer of charge carriers from localized wetting layer states to a single quantum dot is evidenced in temperature-dependent photoluminescence excitation spectroscopy. Postprint

In this work, AlGaAs/GaAs superlattice, with layers’ sequence and compositions imitating the active and injector regions of a quantum cascade laser designed for emission in the terahertz spectral range, was investigated. Three independent absorption-like optical spectroscopy techniques were employed in order to study the band structure of the minibands formed within the conduction band. Photoreflectance measurements provided information about interband transitions in the investigated system. Common transmission spectra revealed, in the target range of intraband transitions, mainly a number of lines associated with the phonon-related processes, including two-phonon absorption. In contrast, differential transmittance realized by means of Fourier-transform spectroscopy was utilized to probe the confined states of the conduction band. The obtained energy separation between the second and third confined electron levels, expected to be predominantly contributing to the lasing, was found to be ~9 meV. The optical spectroscopy measurements were supported by numerical calculations performed in the effective mass approximation and XRD measurements for layers’ width verification. The calculated energy spacings are in a good agreement with the experimental values.

A new synthesis route for colloidal GdF3 nanocrystals was proposed via a thermolysis of trifluoroacetate salts. Its novelty arises from the application of a commercially available rice oil (being a mixture of natural triglycerides) as the synthesis solvent. Rice oil decomposes at ∼254 °C into glycerol and three fatty acids, being a green source of capping ligands. Rapid nucleation with a low concentration of ligands results in monodisperse nanocrystals, while delayed ligand activation results in well-stabilized nanocrystals in the final stage of the synthesis. The nanocrystals show a synthesis time and temperature dependent size (3–11 nm) and shape (spherical and rhombic). We determined a size range (5.6–9.4 nm) in which the nanocrystals show double morphology (spherical and rhombic), while smaller and bigger NCs exist only in a spherical or a rhombic form, respectively. Detailed analysis of the nanocrystal morphology was followed by spectroscopic investigations of their optical properties. We determined a significant influence of a residual oxygen atom and a high surface to volume ratio on the excitation and emission of Eu3+ ions. The contribution of the oxygen diminishes at high temperature and/or with prolonged synthesis.

Multimodal imaging based on nanostructures has become a subject of interest for numerous biomedical laboratories. The main focus was placed on applying nanocrystals for the purpose of two types of clinical imaging (contrast and fluorescent agents) due to their excellent luminescence and/or paramagnetic properties. Such systems should also be characterized by low toxicity and high cellular uptake efficiency. Since bare rare earth fluoride nanocrystals influence the cell membrane integrity, it is expected that their coatings will improve biocompatibility profile, as well as increase hydrophilicity, dispersion and chemical stability. Hence, by synthesis of β-NaGdF4:Yb3+Er3+ nanorods (NRs) coated with noncovalently bounded polyethylene glycol monooleate (PEG-Mo), it should be possible to obtain multimodal imaging biomarkers meeting established criteria. Synthesis of β-NaGdF4:Yb3+Er3+@PEG-Mo NRs was performed by the co-precipitation method. These nanostructures were characterized in terms of their size, morphology, zeta potential, magnetic and optical properties as well as their cytotoxicity profile and cellular internalization was evaluated. It was shown that the shape and size of nanocrystals, namely 20 nm nanorods, present generally accepted parameters for biomedical purpose. Ligand attraction of PEG-Mo 860 resulted in the encapsulation of oleic acid coated NRs and formation of hydrophilic bilayer. Superparamagnetic and luminescence properties were highly efficient. Cytotoxic profiles of normal and cancer cell lines were low and determined by dose and time. Cellular uptake was confirmed by the presence of upconversion luminescence in cell interior. These findings are showing multimodal imaging properties of rod shaped β-NaGdF4:Yb3+Er3+@PEG-Mo NRs which may be useful in some biomedical applications.

In the current paper, the strategy of tuning the optical and structural properties of Eu doped nanocrystals (NCs) is described with the emphasis on obtaining NCs with high brightness and photostability at the desired excitation and emission range. Those NCs are further chemically functionalized and covalently conjugated with lectins, providing specific (but reversible) targeting of pathological glycans on the surface of melanoma cells. Nanoconjugates are then injected intravenously into the blood stream of laboratory mice. Subsequently, histological slices were examined by fluorescence microscopy, whereas whole melanoma tumor were examined by ex vivo fluorescence imaging. Analysis revealed specific targeting, high intensity and photostability of the signal during visualization.

In this paper, we demonstrate the capabilities of Raman spectroscopy for the measurement of the critical dimensions of metallic nanostructures. Using tungsten-based nanogratings, we show that the Raman signal originating from the Si substrate underlying a metallic nanograting is primarily sensitive to the distance between the nanolines, as could be expected for such metallic waveguides. However, surprisingly, when using incident light polarized perpendicular (transverse-magnetic) to the nanolines, the technique becomes strongly sensitive to the width of the lines. Using a simple analytical model, we explain the experimental data quantitatively and show that the very high sensitivity to the width under transverse-magnetic illumination is due to the strong impact of their nanometer variations on the absorption properties of the fundamental transverse-magnetic mode excitable within the nanogratings. This work constitutes an important milestone for the Raman technique to become a routine dimensional metrology tool for nanophotonic and nanoelectronic applications.

In this study, sol-gel derived Zn1-xMgxO (ZMO) is proposed as an electron transport layer (ETL) for solution-processed quantum-dot light-emitting diodes (QLEDs). It is demonstrated that the increase of Mg content in Zn1-xMgxO films from 0% to 20% causes a dramatic suppression of electron current, which is attributed to the lifting of conduction band minimum and reduction of electron mobility. As a result of Mg-doping, the charge carrier balance might be achieved in the QLED with the Zn0.85Mg0.15O layer resulting in maximum external quantum efficiency of 5.74% and current efficiency of 18 cd A−1, which are over 3-fold higher than in the case of the device with ZnO layer. Improved device performance is further explained by reduced exciton quenching at QDs/ZMO interface, which is confirmed by time-resolved PL experiments. Obtained results indicate that sol-gel derived ZMO is a promising candidate for ETL in quantum-dot based optoelectronic devices.

We study the size-dependent optical properties of periodic arrays of semiconducting nanolines in the near-infrared to near-ultraviolet spectral range, where the absorption of the semiconductor increases. Using band structure calculations, we demonstrate that specific dimensions allow the slow down of the light, resulting in an enhanced absorption as compared to bulk material once the extinction coefficient of the semiconductor becomes comparable to its refractive index. Further, the refractive properties of the arrays can be tailored beyond the values of the constituting materials when the extinction coefficient of the semiconductor exceeds its refractive index. To confirm our theoretical findings, we propose a simple semi-analytical model for the light interactions with such structures and validate it with experimental reflectance spectra collected on arrays for the next-generation transistors. ispartof: OPTICS EXPRESS vol:28 issue:5 pages:6781-6793 ispartof: location:United States status: published

In this work, we discuss a method to control the polarization anisotropy of spontaneous emission from neutral excitons confined in quantum-dot-like nanostructures, namely single epitaxial InAs quantum dashes emitting at telecom wavelengths. The nanostructures are embedded inside lithographically defined, in-plane asymmetric photonic mesa structures, which generate polarization-dependent photonic confinement. First, we study the influence of the photonic confinement on the polarization anisotropy of the emission by photoluminescence spectroscopy, and we find evidence of different contributions to a degree of linear polarization (DOLP), i.e., from the quantum dash and the photonic mesa, in total giving rise to $\mathrm{DOLP}=0.85$. Then, we perform finite-difference time-domain simulations of photonic confinement, and we calculate the DOLP in a dipole approximation showing well-matched results for the established model. Furthermore, by using numerical calculations, we demonstrate several types of photonic confinements where highly linearly polarized emission with DOLP of about 0.9 is possible by controlling the position of a quantum emitter inside the photonic structure. Then, we elaborate on anisotropic quantum emitters allowing for exceeding $\mathrm{DOLP}=0.95$ in an optimized case, and we discuss the ways towards efficient linearly polarized single photon source at telecom bands.

We report on the experimental demonstration of triggered single-photon emission at the telecom O-band from In(Ga)As/GaAs quantum dots (QDs) grown by metal-organic vapor-phase epitaxy. Micro-photoluminescence excitation experiments allowed us to identify the p-shell excitonic states in agreement with high excitation photoluminescence on the ensemble of QDs. Hereby we drive an O-band-emitting GaAs-based QD into the p-shell states to get a triggered single photon source of high purity. Applying pulsed p-shell resonant excitation results in strong suppression of multiphoton events evidenced by the as measured value of the second-order correlation function at zero delay of 0.03 (and ~0.005 after background correction).

TiO2 and TiO2:Nd thin films were deposited using reactive magnetron sputtering process from mosaic Ti–Nd targets with various Nd concentration. The thin films were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and spectroscopic techniques. Photoluminescence (PL) in the near infrared obtained upon 514.5 nm excitation was also examined. The relationship between the Nd concentration, structural, optical and photoluminescence properties of prepared thin films was investigated and discussed. XRD and TEM measurements showed that an increase in the Nd concentration in the thin films hinders the crystal growth in the deposited coatings. Depending on the Nd amount in the thin films, TiO2 with the rutile, mixed rutile–amorphous or amorphous phase was obtained. Transmittance measurements revealed that addition of Nd dopant to titania matrix did not deteriorate optical transparency of the coatings, however it influenced on the position of the fundamental absorption edge and therefore on the width of optical band gap energy. All TiO2:Nd thin films exhibited PL emission that occurred at ca. 0.91, 1.09 and 1.38 μm. Finally, results obtained for deposited coatings showed that titania with the rutile structure and 1.0 at.% of Nd was the most efficient in VIS to NIR photon conversion.

Luminescent ZnSe Nanocrystals Mateusz Banski,*,† Mohammad Afzaal,‡ Mohammad A. Malik, Artur Podhorodecki,† Jan Misiewicz,† and Paul O’Brien* †Department of Experimental Physics, Wroclaw University of Technology, Wroclaw 50-370, Poland ‡Center of Research Excellence in Renewable Energy, King Fahd University of Petroleum and Minerals, PO Box 1292, Dhahran 31261, Saudi Arabia The School of Materials and The School of Chemistry, The University of Manchester, Manchester M13 9PL, United Kingdom

Titanium dioxide thin films, each doped with the same amount of neodymium (1 at.%) were deposited by Low Pressure Hot Target Reactive Sputtering and High Energy Reactive Magnetron Sputtering processes in order to obtain anatase and rutile thin film structures respectively. The microstructure and phase composition were analyzed using the transmission electron microscopy method including high resolution electron microscopy imaging. The measurements of the optical properties showed, that both prepared thin films were transparent in the visible light range and had a low extinction coefficient of ca. 3 ⋅ 10−3. The thin film with the anatase structure had a lower cut-off wavelength and refractive index and a higher value of optical energy band gap as-compared to the TiO2:Nd coating with the rutile structure. Simultaneously, more efficient photoluminescence emission was observed for the rutile thin films.

Contactless electroreflectance spectroscopy has been applied to study the surface potential barrier in AlxGa1−xN/n-AlxGa1−xN structures with 0 ≤ x ≤ 0.25 grown by metaloorganic vapor phase epitaxy. A strong band-to-band transition followed by Franz–Keldysh oscillation (FKO) was clearly observed for all samples. The value of built-in electric field was determined from the period of FKO, and the surface potential barrier was calculated with knowledge of the thickness of undoped AlxGa1−xN layer. For a set of GaN/n-GaN structures with different thickness of undoped GaN layer (30–120 nm) an analysis of built-in electric field gives the surface potential barrier of 0.59 ± 0.03 eV. For AlxGa1−xN(60 nm)/n-AlxGa1−xN structures with 0.09 ≤ x ≤ 0.25 the surface potential barrier does not vary with Al concentration within the experimental accuracy. In this case the position of the Fermi level at the surface has been estimated to be ∼1.4 eV. The increase of surface potential barrier from 0.59 eV for GaN to 1.4 eV for AlGaN with 0.09 ≤ x ≤ 0.25 is attributed to low concentration of n-type non-intentional residual dopants in the undoped AlxGa1−xN layer as well as the presence of Al atoms at the surface and their higher sensitivity to oxidation and/or creation of defects states which are not present at GaN surfaces.

Contactless electroreflectance (CER) spectroscopy was applied to study optical transitions in ZnO/ZnMgO quantum wells (QWs) of various widths and barrier contents (Mg = 10, 20, and 30%) grown on sapphire substrates with a Zn0.9Mg0.1O or ZnO buffer layer. Spectral features related to absorption in ZnO QW region and ZnMgO barrier were clearly observed in CER spectra. The spectral position of these features correlates very well with photoluminescence peaks related to emission from ZnO QW and ZnMgO barrier. Besides CER resonances, which are associated with optical transitions, some spectral features related to the Fabry–Perot (F–P) interference effect were clearly observed for these samples. They appear for these structures due to a strong contrast of refractive index between the sapphire and Zn(Mg)O layers and they are observed in the region of transparency for Zn(Mg)O epilayers. For some samples the F–P related signal is quite strong and interferes with CER resonances related to QW transitions. However the two contributions (i.e., CER signal related to optical transitions and the Fabry–Perot effect) can be recognized when the proper analysis of CER spectra is performed. This analysis is presented and discussed in this paper.

The impact of Ga and N fluxes on optical quality of InGaN multiple quantum wells grown by plasma assisted molecular beam epitaxy is studies by means of photoluminescence (PL) and time resolved photoluminescence (TRPL) technique. This investigation reveals that both PL spectrum intensity and broadening are affected by Ga and N fluxes but the optimal growth conditions for the smallest broadening and highest intensity can be different. These effects are explained by the changes in the concentration of nonradiative recombination centres. The experimental results are supported by simulation via hopping excitons model. The theoretical studies of impact of quantum wells inhomogeneity and concentration of nonradiative centres on photoluminescence intensity, decay times and quenching are presented and discussed. Both experimental and theoretical results show that nonradiative processes are present also at low temperature and the recombination efficiency can be lower than 100% even at low temperatures.

In this work we report co-thermolysis as a suitable method for nanomaterial synthesis which allows the creation of hexagonal upconverting nanocrystals, NaGdF4:Yb(3+),Er(3+), in a wide range of sizes (20-120 nm). Only a very high Yb(3+) concentration (above 70%) results in pure cubic-phase nanocrystals with irregular shape. Additionally, we showed that the impact of Yb(3+), Er(3+) and Gd(3+) ions on the size and optical properties of nanocrystals is significant. We found that the main changes in optical properties do not depend on the nanocrystal size mostly, but are determined by the ion-ion interactions which include both Er(3+)-Er(3+) and Er(3+)-Yb(3+) cross relaxation.

Silicon-rich silicon oxide films, with various Si concentrations, were deposited by plasma enhanced chemical vapor deposition and annealed at 1100 °C in order to form silicon nanocrystals. For these films, it has been found that the absorption edge shifts as a function of the nanocrystal size due to the quantum confinement of exciton. This result showed that the size-related effects are present in the investigated films. Next, we examined the influence of the nanocrystal size on the vibrational modes. In this case, the Raman line related to silicon nanocrystals significantly down-shifts as a function of the nanocrystal size, which is against the predictions of the phonon confinement model. It has been shown that this effect is due to stress exerted on the nanocrystals. It has been also found that this stress changes from compressive to tensile, and the stress character depends on the film stoichiometry.

Terbium doped YAlO3 composites were fabricated by the co-precipitation method in a porous anodic alumina (PAA) films grown on silicon at three different Tb concentrations: 0.23, 0.87 and 2.11 at.%. To investigate the emission thermal quenching for all samples, we measured the emission spectra as a function of temperature in the range from 10 up to 500 K at 266 and 488 nm excitation wavelengths. Based on obtained results we proposed the physical model explaining the mechanism of Tb3+ emission quenching in YAlO3 composites deposited into PAA film.

This project has received funding from the European Commission's Horizon 2020 Research and Innovation Programme iCspec under grant agreement No. 636930 and has also been supported by the National Science Centre of Poland within Grant No. 2014/15/B/ST7/04663. Two designs of active region for an interband cascade laser, based on double or triple GaInSb/InAs type II quantum wells (QWs), were compared with respect to passive mode-locked operation in the mid-infrared range around 4 µm. The layer structure and electron and hole wavefunctions under external electric field were engineered to allow controlling the optical transition oscillator strength and the resulting lifetimes. As a result, the investigated structures can mimic absorber-like and gain-like sections of a mode-locked device when properly polarized with opposite bias. A significantly larger oscillator strength tuning range for triple QWs was experimentally verified by Fourier-transform photoreflectance. Publisher PDF

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 636930 (iCspec). The work proposes possible designs of active regions for a mode-locked interband cascade laser emitting in the mid infrared. For that purpose we investigated the electronic structure properties of respectively modified GaSb-based type II W-shaped quantum wells, including the effect of external bias in order to simultaneously fulfil the requirements for both the absorber as well as the gain sections of a device. The results show that introducing multiple InAs layers in type II InAs/GaInSb quantum wells or introducing a tensely-strained GaAsSb layer into “W-shaped” type II QWs offers significant difference in optical transitions’ oscillator strengths (characteristic lifetimes) of the two oppositely polarized parts of such a laser, being promising for utilization in mode-locked devices. Publisher PDF

The work has been supported by the grant No. 2013/10/M/ST3/00636 of the National Science Centre in Poland and the QuCoS Project of Deutsche Forschungsgemeinschaft No. RE1110/16‐1. We have investigated optical properties of hybrid two‐dimensional‐zero‐dimensional (2D‐0D) tunnel structures containing strongly elongated InAs/InP(001) quantum dots (called quantum dashes), emitting at 1.55 μm. These quantum dashes (QDashes) are separated by a 2.3 nm‐width barrier from an InGaAs quantum well (QW), lattice matched to InP. We have tailored quantum‐mechanical coupling between the states confined in QDashes and a QW by changing the QW thickness. By combining modulation spectroscopy and photoluminescence excitation, we have determined the energies of all relevant optical transitions in the system and proven the carrier transfer from the QW to the QDashes, which is the fundamental requirement for the tunnel injection scheme. A transformation between 0D and mixed‐type 2D‐0D character of an electron and a hole confinement in the ground state of the hybrid system have been probed by time‐resolved photoluminescence that revealed considerable changes in PL decay time with the QW width changes. The experimental discoveries have been explained by band structure calculations in the framework of the eight‐band k⋅p model showing that they are driven by delocalization of the lowest energy hole state. The hole delocalization process from the 0D QDash confinement is unfavorable for optical devices based on such tunnel injection structures. Publisher PDF

The lateral interdot coupling is investigated in high density (~10 cm−2 ) self-assembled InAs quantum dots (QDs) grown on an InP substrate. Two types of structures are selected for this study, in which QDs are embedded into an InAlAs matrix, forming nearly twice stronger confinement for an electron and a hole than expected for an InAlGaAs counterpart. Resonantly injected low carrier population in these families of QDs gives very different spectral and temporal response in the temperature range of 5-30 K. While InAs/InAlGaAs QDs show monotonic temperature quench of photoluminescence (PL), the process seems to be ineffective in the family of InAs/InAlAs dots. Moreover, the PL decay traces for InAs/InAlGaAs QDs reveal a two-exponential decay as compared to a mono-exponential one observed for InAs/InAlAs dots. While a short decay component of ≤1.9 ns has been attributed to recombination of an electron-hole pair confined in the dot, the long one of >2.4 ns, observed exclusively for InAs/InGaAlAs QDs, is attributed to recombination of spatially separated electron-hole pairs formed due to carrier exchange between adjacent dots., Journal of Physics: Conference Series, 906 (1), ISSN:1742-6588, ISSN:1742-6596

The influence of precursor composition on the crystallization of LixNa1−xGdF4 is investigated and discussed. Nanocrystals are prepared from the thermal decomposition of trifluoroacetates in the presence of trioctylphosphine oxide to provide control over particle size. A crystal phase transition from hexagonal to cubic and to tetragonal is observed by increasing lithium trifluoroacetate (Li-TFA) in the solution. Controlling the composition of LixNa1−xGdF4 nanocrystals results in modified crystal field symmetry and emission properties from doped europium (Eu3+) ions. We report that for lithium (Li+) substitution

In vitro immunotoxicity of hydrophobic sodium fluoride-based nanocrystals (NCs) doped with lanthanide ions was examined in this study. Although there is already a significant amount of optical and structural data on NaYF4 NCs, data on safety assessment are missing. Therefore, peripheral whole blood from human volunteers was used to evaluate the effect of 25 and 30 nm hydrophobic NaYF4 NCs dissolved in cyclohexane (CH) on lymphocytes, and of 10 nm NaYF4 NCs on phagocytes. In the concentration range 0.12–75 µg cm−2 (0.17–106 µg ml−1), both 25 and 30nm NaYF4 NCs did not induce cytotoxicity when measured as incorporation of [3H]-thymidine into DNA. Assessment of lymphocyte function showed significant suppression of the proliferative activity of T-lymphocytes and T-dependent B-cell response in peripheral blood cultures (n = 7) stimulated in vitro with mitogens phytohemagglutinin (PHA) and pokeweed (PWM) (PHA > PWM). No clear dose–response effect was observed. Phagocytic activity and respiratory burst of leukocytes (n = 5–8) were generally less affected. A dose-dependent suppression of phagocytic activity of granulocytes in cultures treated with 25 nm NCs was observed (vs. medium control). A decrease in phagocytic activity of monocytes was found in cells exposed to higher doses of 10 and 30 nm NCs. The respiratory burst of phagocytes was significantly decreased by exposure to the middle dose of 30 nm NCs only. In conclusion, our results demonstrate immunotoxic effects of hydrophobic NaYF4 NCs doped with lanthanide ions to lymphocytes and to lesser extent to phagocytes. Further research needs to be done, particularly faze transfer of hydrophobic NCs to hydrophilic ones, to eliminate the solvent effect. Copyright © 2014 John Wiley & Sons, Ltd.

Surface photovoltage (SPV) spectra were measured for GaN 0.014 As 0.986 layers at room temperature and compared with room temperature photoreflectance (PR) and contactless electroreflectance (CER) measurements. Spectral features related to E − and E + transitions were clearly observed in SPV spectra at energies corresponding to PR and CER resonances. In this way it has been shown that SPV spectroscopy is an alternative absorption-like technique to study both the E − and E + transitions in dilute nitrides. The observation of E + transition in SPV spectra means that it is a direct optical transition at the Γ point of GaNAs band structure which can be explained by the band anticrossing interaction between the localized states of N and the extended conduction band states of the GaAs host.

There are presented optical properties of strongly in-plane elongated nanostructures the so called quantum dashes made in InAs/InP material system by molecular beam epitaxy. They have been investigated systematically by a spectroscopic manner on both the entire ensemble and on the single dash level. Their properties are discussed with respect to the fundamental electronic and optical properties as the polarization of emission and the corresponding driving factors, exciton fine structure splitting, biexciton binding energy, the characteristic exciton to biexciton lifetimes ratio and exciton decoherence via interaction with acoustic phonons. The experimental results are analyzed supported by previous energy level calculations within the eight-band kp theory and the rate equation modeling of the exciton kinetics.

Thin (50-90 m) non-doped and doped (by Al atoms) Mg 2 Sn 0.6 Si 0.4 and Mg 2 Sn 0.4 Si 0.6 films with roughness of 1.9-3.7 nm have been grown by multiple deposition and single annealing at 150 °C of multilayers formed by repetition deposition of three-layers (Si-Sn-Mg) on Si(111) p-type wafers with 45 Ω-cm resistivity. Transmission electron microscopy has shown that the first forming layer is an epitaxial layer of hex-Mg 2 Sn(300) on Si(111) substrate with thickness not more than 5-7 nm. Epitaxial relationships:hex-Mg 2 Sn(300)|| Si(111), hex-Mg 2 Sn[001]|| Si[-112] and hex-Mg2Sn[030]||Si[110] have been found for the epitaxial layer. But inclusions of cub-Mg 2 Si were also observed inside hex-Mg 2 Sn layer. It was found that the remaining part of the film thickness is in amorphous state and has a layered distribution of major elements: Mg, Sn and Mg without exact chemical composition. It was established by optical spectroscopy data that both type films are semiconductor with undispersed region lower 0.18 eV with n o = 3.59 ± 0.01, but only two direct interband transitions with energies 0.75-0.76 eV and 1.2 eV have been determined. The last interband transition has been confirmed by photoreflectance data at room temperature. Fourier transmittance spectroscopy and Raman spectroscopy data have established the formation of stannide, silicide and ternary compositions.

Si/Ca2Si/Si(111) and Si/Ca3Si4/Si(111) heterostructures have been formed by procedure of layer-by-layer deposition of Ca at 130 °C or 500 °C on previously formed layer of amorphous Si or on a polycrystalline Si layers. Features of electronic structure, optical and electrical properties were established by in situ and ex situ methods. Strong direct interband transitions at 0.89 and 0.912 eV were firstly found for Ca3Si4 film grown at 500 °C. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work we investigate the electronic structure of coupled quantum dashes. The respective con ned state energy levels are calculated for various cross-sectional shapes and sizes of the dashes and in function of the lateral distance between them. The results are confronted with the existing experimental data on the optical transitions in such structures. It has been concluded that for realistic system parameters (geometry and spatial in-plane separation) the obtained direct coupling is weak and in most of the applications the dashes can be considered individually, as long as the ensemble is strongly inhomegeneous.

In this work, terbium and europium doped YAlO3 phosphors were synthesized by sol–gel method. It has been shown that after annealing at 1000 °C these phosphors crystallize in the cubic garnet phase, similar to Y3Al5O12. As evidenced by the time-resolved photoluminescence spectroscopy and Maximum Entropy Method, the photoluminescence decay of both europium 5D0 and terbium 5D4 states has two lifetime components: short and long, both of the order of milliseconds. Moreover, for both phosphors the short photoluminescence lifetime was ascribed to the ions occupying low-symmetry sites in the vicinity of the nanocrystal surface, while the long decay component was related to the ions present in the bulk of YAlO3 nanocrystal. For both dopants, the long lifetime component of Tb3+ and Eu3+ emission is longer than the equivalent lifetimes in bulk Y3Al5O12 crystal.

The purely hexagonal phase of ultrasmall (~10 nm) NaYF(4) nanocrystals (NCs), containing different Eu concentrations, has been obtained by a modified co-thermolysis method. Detailed investigations of the excitation and relaxation mechanisms of the Eu ions in such NCs are reported. Based on the photoluminescence excitation, absorbance, photoluminescence and emission decay times measured as a function of the excitation wavelengths, it has been shown that two Eu sites with different excitation and relaxation characteristics are present in the case of ultrasmall NaYF(4) NCs. It has been shown that, when the Eu concentration increases, strong ion-ion interactions influence the relaxation phenomena in Eu ions, changing their optical properties. Moreover, these ion-ion interactions enable connections between the surface ions and the internal ones via energy transfer from the surface to the NCs core. Furthermore, it has been proposed that the different kinetic properties of the surface Eu ions are mainly caused by the formation of a charge transfer state between the ions and ligand groups attached to the NCs surface.

The influence of the nanocrystal matrix on the optical properties of lanthanide dopants is investigated with europium ions used as local crystal field probes. The analysis is performed on small NaYF4 and NaGdF4 nanocrystals obtained by the thermolysis of the corresponding metal trifluoroacetates. An important role in the synthesis is played by trioctylphosphine oxide which induces the crystallization of nanocrystals with small diameters (∼5 to 6 nm). In such small particles, the energy transfer from gadolinium to europium ions is studied with photoluminescence, photoluminescence excitation and time-resolved experiments. We demonstrate that excited gadolinium ions efficiently transfer their energy to europium, and their photoluminescence spectra depend on the nanocrystal size. This is contrary to the direct excitation of Eu3+ ions, which produces size-dependent emission corresponding to the surface to volume ratio of europium sites. Finally, we propose that Gd3+ ions transfer their energy mainly to the Eu3+ in the core of the nanocrystals. These observations provide a base for the optically controlled emission from only the core of the nanocrystals.

P.M., J.M. and G. S. acknowledge support from the grant of National Science Centre of Poland No. 2011/02/A/ST3/00152 (Maestro), whereas M.Z. acknowledges support from the Polish National Science Centre under grant No. 2015/18/E/ST3/005 (Sonata Bis). The experiments have partially been performed within the Wroclaw University of Science and Technology laboratory infrastructure financed by the Polish Ministry of Science and Higher Education Grant No. 6167/IA/119/2012. The fundamental electronic and optical properties of elongated InAs nanostructures embedded in quaternary InGaAlAs barrier are investigated by means of high-resolution optical spectroscopy and many-body atomistic tight-binding theory. These wire-like shaped self-assembled nanostructures are known as quantum dashes and are typically formed during the molecular beam epitaxial growth on InP substrates. In this work we study properties of excitonic complexes confined in quantum dashes emitting in a broad spectral range from below 1.2 to 1.55 μm. We find peculiar trends for the biexciton and negative trion binding energies, with pronounced trion binding in smaller size quantum dashes. These experimental findings are then compared and qualitatively explained by atomistic theory. The theoretical analysis shows a fundamental role of correlation effects for the absolute values of excitonic binding energies. Eventually, we determine the bright exciton fine structure splitting (FSS), where both the experiment and theory predict a broad distribution of the splitting varying from below 50 to almost 180 μeV. We identify several key factors determining the FSS values in such nanostructures including quantum dash size variation and composition fluctuations. Postprint

We would like to acknowledge the National Science Centre of Poland for support within Grant No. 2014/15/B/ST7/04663. Optical properties of AlSb/InAs/GaInSb/InAs/AlSb quantum wells (QWs) grown on an InAs substrate were investigated from the point of view of room temperature emission in the mid- and long-wavelength infrared ranges. By means of two independent techniques of optical spectroscopy, photoreflectance and temperature-dependent photoluminescence, it was proven that the main process limiting the performance of such InAs substrate-based type II structures is related to the escape of carriers from the hole ground state of the QW. Two nonradiative recombination channels were identified. The main process was attributed to holes tunneling to the valence band of the GaAsSb spacing layer and the second one with trapping of holes by native defects located in the same layer. Publisher PDF