RESUMO

Methylammonium lead halide perovskite (CH3NH3PbI3) films, with high PbI2 concentration, were grown by the two-step spin coating method. The influence of the precursor concentration and annealing time on the optical and structural properties of the perovskite films was analyzed by optical absorption, photoluminescence, X-ray diffraction and scanning electron microscopy. The results showed that, in addition to the CH3NH3PbI3 and PbI2 phases, intermediate phases, such as (MA)2(DMF)2Pb3I8, were formed in the films, depending on the time and temperature of annealing, which can tune the optical absorption in the visible spectra. This intermediate phase induced the formation of perovskite nanowires, identified by SEM images, and their growth may be associated with the presence of the DMF solvent remaining in the PbI2 film.

RESUMO

We study the effect of Co co-doping on the optical properties of Mn-doped ZnTe nanocrystals (NCs) embedded in a glass matrix. Optical absorption (OA) and crystal field theory strongly indicated the substitutional incorporation of Co2+ ions into these semiconducting NCs as well as the characteristic transitions of these ions in the visible and near infrared spectral region. Transmission electron microscopy (TEM) images revealed an invariant NC lattice parameter with the incorporation of Mn2+ and Co2+ ions. The same was confirmed by X-ray diffraction (XRD). The photoluminescence (PL) spectra showed that the characteristic emission bands of Co2+ ions (E1Co2+ and E2Co2+) are intense and evident at low temperatures. Indeed, Raman spectra showed that the dependence of luminescence intensity on temperature is due to the electron-phonon interaction that arises from multiphonon relaxation processes. The redshifts in the PL spectra from green to orange with the incorporation of Mn2+ ions into ZnTe NCs, and in the near infrared with increasing Co-concentration, result from sp-d exchange interactions associated with the increase in Mn2+ and Co2+ ions in tetrahedral sites of ZnTe NCs, which may be very interesting for applications in luminescent devices. These observations provide strong evidence that higher Co-concentrations inhibit the incorporation of Mn2+ into ZnTe NCs, suggesting that there may be competition between Co2+ and Mn2+ ions substituting Zn2+ ions and, furthermore, that these ions replace zinc vacancies (VZn) in these NCs.

RESUMO

Semimagnetic Pb1-xCoxSe nanocrystals were synthesized by a fusion protocol in a glass matrix and characterized by optical absorption (OA), transmission electron microscopy (TEM), and photoluminescence (PL) techniques. OA spectra and TEM images strongly indicated the formation of Pb1-xCoxSe magnetic phases in the glass system and the quantum dot size was manipulated by tuning the annealing time. The OA spectra together with crystal field theory indicate that Co(2+) is located in the tetrahedral site (Td) and the PL of the Pb1-xCoxSe nanocrystals presents characteristic recombination in the visible (∼700 nm) and near-IR (1300-1600 nm) electromagnetic spectral range. With temperature decreasing, the PL spectra, in the visible spectral range, indicate an excited-state crossover yielding PL changes from (4)T1(P) → (4)A2(F) broadband emission to (2)E(G) → (4)A2(F) narrow-line emission. This phenomenon was explained on the basis of a configurational energy model. The OA and PL spectra of PbSe:Co(2+) indicate that the localized energy transition of Co(2+) ((4)A2(F) ↔ (4)T1((4)F)) can be tuned from the band-gap energy to the conduction-band energy of PbSe NCs by changing the NC size by increasing the thermal annealing time. In the near-IR spectral range, the temperature-dependent PL spectra show that the process of thermal activation of localized electrons in Co(2+) states can be transferred to the conduction band of the NCs. This process depends on the energy distance between extended and localized states, which can be controlled by the sample annealing time.

RESUMO

We studied the effect of Mn concentration on the optical, morphological and magnetic properties of Zn1-xMnxTe NCs grown in a glass matrix produced by the fusion method. The physical properties of these materials were determined by optical absorption (OA), transmission electron microscopy (TEM), atomic/magnetic force microscopy (AFM/MFM) and photoluminescence (PL). An analysis of the OA spectra, based on the crystal field theory (CFT), showed strong evidence that Mn(2+) ions were substitutionally incorporated into the Zn1-xMnxTe NCs until reaching the solubility limit (concentration, x = 0.100). Above this nominal concentration, TEM showed the onset of Mn-related phases, such as MnO and α-MnO2, in the PZABP glass system. AFM images showed that NC density on the surface of the glass matrix decreased as x-content increased. It is probable that MnO and MnO2 NCs would outnumber Zn1-xMnxTe NCs at higher concentrations - a conclusion that was corroborated by the OA spectra and TEM images. MFM images revealed that samples with Mn(2+) ions responded to magnetization from an MFM probe. This implied that Mn(2+) ions were incorporated within the Zn1-xMnxTe NCs and gave rise to the diluted magnetic semiconductor (DMS) structure. The PL spectra not only confirmed the evidence obtained by OA, CFT, TEM and AFM/MFM, but also showed that Mn(2+) concentration could be used to tune (4)T1((4)G) → (6)A1((6)S) emission energy.

RESUMO

Thermal lens spectroscopy (TLS), differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) techniques were applied to the thermo-optical property analysis of a new phosphate glass matrix PANK with nominal composition 40P2O5·20Al2O3·35Na2O·5K2O (mol%), doped with different Nd(3+) compositions. This glass system, synthesized by the fusion protocol, presents high transparency from UV to the near infrared, excellent thermo-optical properties at room temperature and high fluorescence quantum efficiency. Thermal lens phase shift parameters, thermal diffusivity and the DSC signal present pronounced changes at about 61 °C for the PANK glass system. This anomalous behavior was associated with a phase transition in the nanostructured glass materials. The FTIR signal confirms the presence of isolated PO4 tetrahedron groups connected to different cations in PANK glass. As a main result, our experimental data suggest that these tetrahedron groups present a structural phase transition, paraelectric-ferroelectric phase transition, similar to that in potassium dihydrogen phosphate, KH2PO4, nanocrystals and which TLS technique can be used as a sensitive method to investigate changes in the structural level of nanostructured materials.

RESUMO

Semimagnetic Pb(1-x)Mn(x)Se nanocrystals were synthesized by a fusion method in a glass matrix and characterized by optical absorption (OA), atomic/magnetic force microscopy (AFM/MFM), and photoluminescence techniques. MFM images strongly indicated the formation of Pb(1-x)Mn(x)Se magnetic phases in the glass system. Quantum dot size was manipulated by tuning annealing time. It was shown that Mn(2+) impurity affects nucleation, where Mn(2+)-doped samples present a redshift of the OA peak after a short annealing time and a blueshift after long annealing time compared to undoped PbSe NCs. This behavior was linked to the dependence of band-gap energy and the absorption selection rule on Mn(2+) concentration. Photoluminescence in the Pb(1-x)Mn(x)Se nanocrystals increases as the temperature rises up to a point and then decreases at higher temperatures. Anomalous increases in emission efficiency were analyzed by considering temperature induced carrier-transfer in semimagnetic Pb(1-x)Mn(x)Se quantum dots nanocrystals of different sizes.

RESUMO

Zn(1-x)Mn(x)Te nanocrystals (NCs), at various concentrations x, were successfully grown in a host glass matrix by the fusion method after appropriate annealing. Growth of these NCs was evidenced by optical absorption (OA), X-Ray Diffraction (XRD), magnetic force microscopy (MFM) and photoluminescence (PL) measurements. From the room temperature OA spectra, it was possible to observe the formation of two well defined, different sized groups of NCs, one attributed to quantum dots (QDs) and the other to bulk-like nanocrystals (NCs). XRD results have confirmed that the cubic zincblend structure of nanoparticles is not altered by the substitutional incorporation of Mn(2+) ions into the ZnTe NCs. MFM images supported the OA spectra results and thus provided additional confirmation of the formation of Zn(1-x)Mn(x)Te magnetic nanoparticles in the host glass matrix. The two groups of NCs were also observed in the PL spectra as well as deep defects attributed to the presence of oxygen centers in the electronic structure of the Zn(1-x)Mn(x)Te NCs. Strong agreement between the fitting model, based on rate equation, and experimental PL intensity data at different temperatures demonstrates that this model adequately describes the energy transfer processes between the NCs and the defects of the Zn(1-x)Mn(x)Te system at different temperatures.

Assuntos

RESUMO

Cd(1-x)Mn(x)S nanoparticles (NPs) were successfully grown in a glass matrix and investigated by optical absorption (OA), magnetic circularly polarized photoluminescence (MCPL) measurements, and magnetic force microscopy (MFM). The room temperature OA spectra have revealed the formation of two groups of Cd(1-x)Mn(x)S NPs with different sizes: bulk-like nanocrystals (NCs) and quantum dots (QDs). The MCPL spectra were recorded at 2.0 K with several magnetic fields up to 15 T, allowing a detailed comparison between the degrees of circular polarization of the two groups of NPs. The different behaviours of magneto-optical properties of bulk-like NCs and QDs were explained by taking into account a considerable alteration of exchange interaction between the carrier spins and the substitutional doping magnetic ions incorporated into the NPs. As a main result, we have demonstrated that self-purification is the dominant mechanism that controls the doping in semiconductor QDs grown by the melting-nucleation synthesis approach due to the relatively high temperature that was used in thermal annealing of samples.

RESUMO

Optical absorption (OA), magnetic force microscopy (MFM), and photoluminescence (PL) measurements were employed to study Cd(1-x)Mn(x)S nanoparticles (NPs), grown in a glass matrix, at different x-concentrations. The formation of two well defined groups of NPs with different sizes was detected by OA spectra at room temperature and confirmed by MFM images, from which they were identified as quantum dots (QDs) and bulk-like nanocrystals (NCs). Emissions from luminescent states were observed in the temperature dependent PL spectra of both groups of NPs, including those from deep defects which were attributed to the presence of divacancies (V(Cd)-V(S)) in the hexagonal wurtzite structure. Furthermore, we have come up with a model based on rate equations that describes energy transfers involving the excitonic states of QDs, the conduction band of bulk-like NCs, and the shallow virtual levels of NPs. This model was used to fit the integrated PL intensity of the corresponding NP groups, and a good agreement between them confirms that the model suitably describes the temperature dependent carrier dynamics of Cd(1-x)Mn(x)S NPs.