Intraionic Transitions Research Articles

Microstructural and photoluminescence properties of sol–gel derived Tb3+ doped ZnO nanocrystals

Un-doped and Tb3+ doped ZnO nanocrystals with different concentrations of Tb3+ were synthesized by a sol–gel method and their photoluminescence (PL) properties were investigated. The successful incorporation of Tb3+ ions with different concentration (0.15, 0.5 and 1mol %) in ZnO was realized and confirmed through different characterization. High crystalline ZnO products were produced without the need of a post-heat treatment. Elemental mapping conducted on the as prepared samples using Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDX) revealed homogeneous distribution of Zn, O, and Tb ions. The high resolution transmission electron microscope (HR-TEM) analyses indicated that the un-doped and Tb3+ doped samples were composed of homogeneously dispersed particles of high crystallinity with an average size ranging from 3 to 6nm in diameter, which was in agreement with X-ray diffraction (XRD) analyses. Raman spectroscopy was used to further elucidate the wurtzitic structure of the prepared samples. PL study revealed that among different Tb3+ concentrations, 0.5mol%Tb3+ doped ZnO nanoparticles showed clear emission from the dopant originating from the 4f–4f intra-ionic transitions of Tb3+ while the broad defects emission was dominating in the 0.15 and 1mol%Tb3+ doped ZnO. Optical band gap was extrapolated from the Ultraviolet Visible spectroscopy (UV–Vis) absorption analysis using TAUC’s method and the widening of the optical band gap for the 0.15 and 0.5mol%Tb3+ doped samples and the reduction of the optical band gap for the 1mol%Tb3+ doped sample as compared to the un-doped sample was observed. Energy transfer mechanism between Tb3+ and ZnO is discussed in detail.

Microstructural and optical properties of europium-doped zinc oxide nanowires

Single-crystal Eu3+-doped wurtzite ZnO micro- and nanowires were synthesized by chemical vapor deposition. The nanostructures grew via a self-catalytic mechanism on the walls of an alumina boat. The structure and properties of the doped ZnO were characterized using X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning and transmission electron microscopy, and photoluminescence (PL) methods. A 10-min synthesis yielded vertically grown nanowires of 50–400nm in diameter and several micrometers long. The nanowires grew along the ±[0001] direction. The Eu3+ concentration in the nanowires was 0.8at.%. The crystal structure and microstructure of were compared for Eu3+-doped and undoped ZnO. PL spectra showed a red shift in emission for Eu3+-doped (2.02eV) compared to undoped ZnO nanowires (2.37eV) due to Eu3+ intraionic transitions. Diffuse reflectance spectra revealed widening of the optical bandgap by 0.12eV for Eu3+-doped compared to undoped ZnO to yield a value of 3.31eV. Fourier-transform infrared spectra confirmed the presence of europium in the ZnO nanowires.

Influence of chromium content on the optical and electrical properties of Li1+xCrxTi2−x(PO4)3

Abstract Optical and electrical properties of samples with Li 1 + x Cr x Ti 2 − x (PO 4 ) 3 composition ( x = 0, 0.05 and 0.1) prepared by a low temperature sol-gel method have been investigated. XRD and Raman spectroscopy measurements respectively reveal the rhombohedral-Nasicon structure of the grown samples and a disorder of Li + ions with increasing chromium content. Electrical properties were studied by impedance spectroscopy and both dc and grain boundary ionic conductivity increase with Cr content. A high ionic conductivity value of 1.4·10 − 4 S cm − 1 at room temperature and an activation energy of 0.31 eV have been obtained for x = 0.1, which are comparable to those measured in the best ionic conductors reported to date. Photoluminescence and cathodoluminescence spectra indicate that self-trapped excitons and oxygen defects related to the TiO 6 octahedra structural units are responsible for the observed luminescence in the visible spectral range, explaining the possible electronic origin of a residual conductivity measured in all the samples. Cr incorporation gives rise to infrared emission bands, attributed to different Cr 3 + and Ti 3 + intraionic transitions, that may be related to the better grain connectivity achieved by Cr doping.

Microprobe analysis, iono- and photo-luminescence of Mn2+ activated ZnGa2O4 fibres

Cubic ZnGa2O4 fibres have been grown by the laser floating zone technique with different pulling rates. In fibres activated with manganese ions, the room temperature photo- and iono-luminescence is dominated by an intense green emission which is observed by the naked eye. The green band is due to an overlap of the 4T1→6A1 intraionic transitions of the Mn2+ ions in different sites in the gallate host. The fibres’ photoluminescence spectra have been found to be dependent on the excitation energy. Additionally, the intensity of the green photo- and iono-luminescence is strongly sensitive to the measurement temperature and proton irradiation time. Micro PIXE analysis was used in order to verify the homogeneous distribution of the Mn luminescence activators and determine its concentration as well as for verification of impurity contents that may have been incorporated during the fibres growth. The potential of ionoluminescence measurements for characterization of optical materials is discussed.

MBE Growth and Magnetooptical Properties of (Zn,Co)Te Layers

The (Zn,Co)Te is a one of diluted magnetic semiconductor (DMS) with particularly strong interaction of magnetic ions and free holes. The interaction can be described by p d exchange integral N0β = −3.03 ± 0.15 eV [1], so the absolute value is two or three times larger than for the other Co or Mn based DMSs (e.g. for (Cd,Mn)Te N0β = −0.88 eV [2]). High optical quality bulk (Zn,Co)Te samples studied in Ref. [1] were grown using the Bridgman method, with cobalt concentration between 0.05 and 1.06%. Other technique to introduce Co into ZnTe was thermal di usion [3], which allowed infrared spectroscopy of intraionic Co transitions [3, 4]. Main di culty in doping of the ZnTe crystals during growth is high evaporation temperatures of Co as compared with the evaporation temperatures of Zn and Te [3]. For molecular beam epitaxy (MBE) this di culty should be not as important as for other crystal growth techniques, but for our best knowledge, there was no reports on MBE (Zn,Co)Te. It would be very interesting to enhance magnetooptical e ects using higher Co concentrations and epitaxial nanostructures such as quantum wells, quantum dots, or distributed Bragg reectors. This is the reason why we equipped our new MBE machine (SVT model 26-V-3) in high temperature e usion cell.

Optical Properties of Au-Centers in CsBr and CsI Single Crystals

The temperature dependence of the absorption and luminescence spectra has been investigated for Au - centers in CsBr and CsI single crystals with the CsCl structure. In addition to the A, B, C, and D absorption bands, the A' and B' luminescence bands caused by the intraionic transitions in the Au - ion are observed at 12 K. The luminescence bands are characterized by a small Stokes shift and a narrow halfwidth, reflecting a weak electron–phonon interaction. Furthermore, the photoexcitation at the energies corresponding to the A, B, and C absorption bands induces luminescence bands in the visible energy region. The A', B', and visible luminescence bands change with temperature while complementing each other. The features of the Au - centers in cesium halides and the nonradiative transitions between the relaxed excited states are discussed.

Specific Features and Nature of the 890 nm Photoluminescence Band Detected in SiO x Films after Low-Temperature Annealing

A band with a peak at 890 nm is detected in the photoluminescence spectra of SiO x (x ≈ 1.3) films deposited by thermal evaporation of SiO and annealed in air at 650–1150°C. The 890-nm band appears after low-temperature (∼650°C) annealing and exhibits a number of features: (i) as the annealing temperature is elevated to 1150°C, the position of the band peak remains unchanged, whereas the intensity increases by two orders of magnitude; (ii) the effects of the annealing atmosphere (air, vacuum) and the excitation wavelength and power density on the intensity of the 890-nm band differ from the corresponding effects on the well-known bands observable in the ranges 600–650 and 700–800 nm; and (iii) the photoluminescence decay is first fast and then much slower, with corresponding lifetimes of ∼9 and ∼70 μs. The observed features are inconsistent with the interpretation of photoluminescence observed in SiO x so far. Specifically, the earlier observed photoluminescence was attributed to transitions between the band and defect states in the matrix and between the states of band tails, transitions inside Si nanoclusters, and intraion transitions in rare-earth impurity ions. Therefore, we consider here the possibility of attributing the 890-nm band to transitions in local centers formed by silicon ions twofold- and/or threefold-coordinated with oxygen; i.e., we attempt to interpret the 890-nm band in the same manner as was done for luminescence in SiO2 glasses and films slightly deficient in oxygen.

Luminescence properties of KCl:Ag − crystals excited near the fundamental absorption edge

Luminescence properties of KCl single crystals doped with Ag − centers have been investigated under various excitation energies around the fundamental absorption edge at low temperatures. Under the excitation at 6.89 eV, which is lower than the intrinsic exciton energy by 0.87 eV, the A ′ luminescence band due to the intraionic transition in the Ag − ion is dominantly observed at 2.91 eV. On the other hand, the excitation at 6.66 eV induces a broad luminescence band at 2.60 eV in addition to the A ′ luminescence band. From the comparison with the localized excitons in KCl:I crystals, the 2.60 eV luminescence band is attributed to the two-center type localized exciton related with the Ag − ion. The adiabatic potential energy surfaces of the excited states in the Ag − center and the localized exciton in KCl:Ag − are discussed.

Cathodoluminescence of rare earth implantedGa2O3 andGeO2 nanostructures

Rare earth (RE) doped gallium oxide and germanium oxide micro- and nanostructures,mostly nanowires, have been obtained and their morphological and opticalproperties have been characterized. Undoped oxide micro- and nanostructureswere grown by a thermal evaporation method and were subsequently doped withgadolinium or europium ions by ion implantation. No significant changes in themorphologies of the nanostructures were observed after ion implantation andthermal annealing. The luminescence emission properties have been studied withcathodoluminescence (CL) in a scanning electron microscope (SEM). Bothβ-Ga2O3 andGeO2 structuresimplanted with Eu show the characteristic red luminescence peak centered at around 610 nm, due tothe 5D0–7F2 Eu3 + intraionictransition. Sharpening of the luminescence peaks after thermal annealing is observed in Eu implantedβ-Ga2O3, which is assigned tothe lattice recovery. Gd3 + as-implanted samples do not show rare earth related luminescence. After annealing, optical activationof Gd3 + is obtained in both matrices and a sharp ultraviolet peak centered at around 315 nm, associated withthe Gd3 + 6P7/2–8S7/2 intraionic transition, is observed. The influence of the Gd ion implantation and theannealing temperature on the gallium oxide broad intrinsic defect band has beenanalyzed.

Photoluminescence of zircon (ZrSiO4) doped with REE3+ (REE = Pr, Sm, Eu, Gd, Dy, Ho, Er)

The photoluminescence properties of synthetic zircon, ZrSiO4, doped with REE3+ (REE = Pr, Sm, Eu, Gd, Dy, Ho, Er) were investigated using combined excitation and emission spectroscopy. All samples showed luminescence characteristics of intra-ion energy transitions, similar to other lanthanide-doped materials. However, the relative intensities were dependent on the energy of excitation and the presence of charge-transfer bands were inferred from excitation spectra. From the data, we conclude that the lanthanides in zircon occur in more than one type of coordination. Energy transfer between different lanthanides was observed in some co-doped samples and emissions that were unassigned in previous studies have been assigned to specific lanthanides based on excitation spectroscopy.

Fast scintillation processes in CsCl crystals comprising semiconductor nanocrystals

We analyzed the scintillation mechanism in CsCl single crystals comprising CsPbCl 3-like semiconductor nanocrystals. A decay component with a subnanosecond lifetime was observed and ascribed to the exciton luminescence in the nanocrystals. This is the first observation of scintillation from semiconductor nanocrystals. In addition, slower components were observed and ascribed to the intraionic transition at isolated Pb 2+ ions and the Auger-free luminescence in the CsCl matrix. Furthermore, it was shown that the nanocrystals absorbed and re-emitted the scintillation photons from the isolated Pb 2+ ions. The results in this report clearly indicates that the semiconductor nanocrystals function as ultrafast “luminescent centers,” and complicated scintillation dynamics are observed due to the luminescence from different components in the crystal and the radiative energy transfer between them.

Influence of the Next-Nearest-Neighbor Cations on Tl+ Ions Doped in Ammonium Chloride Crystals

The optical spectra of Tl + centers doped in NH 4 Cl and ND 4 Cl single crystals have been investigated up to the vacuum ultraviolet energy range. The A, B, and C absorption bands due to the intraionic transitions in Tl + ions are observed at the same energy positions in NH 4 Cl:Tl + and ND 4 Cl:Tl + . In both the crystals, the successive absorption bands appear in the higher-energy region above the A band. The energy intervals between these successive absorption bands are about 190 meV for NH 4 Cl:Tl + and about 150 meV for ND 4 Cl:Tl + . The interval energies are almost equal to the vibrational energies of the respective bending modes in the NH 4 + and ND 4 + ions. This implies that the electronic states in the Tl + ion undergo a harmonic interaction with the internal vibrations of the ammonium ions corresponding to the next-nearest-neighbor cations.

Synthesis and optical properties of a lithium niobiosilicate glass doped with europium

The sol–gel method was used to prepare a lithium niobiosilicate glass doped with europium ions, 91SiO 2–4Li 2O–4Nb 2O 5–1Eu 2O 3 (% mole). The dried gel was heat-treated between 500 °C and 750 °C and glass ceramics were obtained. X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman studies show that crystallization starts at temperatures above 600 °C. The LiNbO 3 and EuNbO 4 nanocrystals were found in the heat-treated samples as corroborated by the structural, morphological and optical properties. Room temperature photoluminescence (PL) studies allow us the identification of multiple Eu-related optical centres and the strongest red 5D 0 → 7F 2 intraionic transitions occur in the samples heat-treated at 600 °C where the LiNbO 3 crystal phase was detected.

Chitosan Clad Manganese Doped Zing Sulphide Nanocrystallites for Biolabeling

The synthesis of fluorescent nanocrystals is receiving a lot of attention for potential application in biological labeling as well as phosphors for field emission devices. Zinc sulphide doped with manganese (ZnS:Mn2+) is one of the most efficient electroluminescent phosphor displaying a wide emission band centred around 590 nm resulting from the intra-ionic transition in Mn2+ ions. We report a unique synthesis of zinc sulphide nanoparticles doped with manganese using a biocompatible passivating agent ‘chitosan’, with bright luminescence peaking at 590 nm. This high luminescence efficiency of the synthesized nanocrystals are ideal for quantum dot based bio-labeling applications. Synthesis of the nanoparticles was carried out by precipitation reaction in aqueous media of zinc acetate and sodium sulphide where manganese acetate was added as the dopant. The obtained nanoparticles were around 4 to 6 nm in size and were found to be stable for months of shelf life. The photoluminescence intensity did not degrade when the colloid was heated up to 65 oC for prolonged periods.

Intrinsic magnetism in Zn1−xCoxO (0.03≤x≤0.10) thin films prepared by ultrasonic spray pyrolysis

The Zn1−xCoxO thin films were prepared on a commercial glass substrate at400 °C by ultrasonic spray pyrolysis. The effect of doping concentration on thestructural, optical and magnetic properties of these films has been studied.The structural and optical properties of these films reflect the fact that theCo2+ ions havesubstituted the Zn2+ ions without changing the wurtzite structure of ZnO. We have not observed any secondaryphases even after doping the 10 at.% concentration of Co. All the films were highly orientedwith preferred (002) direction. By increasing the Co content, a systematic change in both thec-axis lattice constant and the fundamental band gap energy was observed. Threeabsorption bands were found in the transmission spectra of the Co doped ZnO films at657 nm (1.89 eV), 610 nm (2.03 eV) and 567 nm (2.20 eV). The absorption bands wereattributed to d–d intraionic transitions of tetrahedrally coordinated high-spinCo2+ ions. Magnetic studies reveal the absence of room temperatureferromagnetic behaviour in these films. The values of coercivity(HC)∼145 and 123 Oe andremanent magnetization (Mr)∼4.32 × 10−5 and 7.28 emu cm−2 wereobserved at 5 K for Zn0.95Co0.05O and Zn0.93Co0.07O thin films.

ParamagneticGaN:Feand ferromagnetic(Ga,Fe)N: The relationship between structural, electronic, and magnetic properties

We report on the metalorganic chemical vapor deposition of $\mathrm{GaN}:\mathrm{Fe}$ and $(\mathrm{Ga},\mathrm{Fe})\mathrm{N}$ layers on $c$-sapphire substrates and their thorough characterization via high-resolution x-ray diffraction, transmission electron microscopy (TEM), spatially resolved energy dispersive x-ray spectroscopy (EDS), secondary-ion mass spectroscopy (SIMS), photoluminescence (PL), Hall-effect, electron-paramagnetic resonance (EPR), and magnetometry employing a superconducting quantum interference device (SQUID). A combination of TEM and EDS reveals the presence of coherent nanocrystals presumably ${\mathrm{Fe}}_{x}\mathrm{N}$ with the composition and lattice parameter imposed by the host. From both TEM and SIMS studies, it is stated that the density of nanocrystals and, thus the Fe concentration increases towards the surface. According to Hall effect measurements, electrons from residual donors are trapped by midgap Fe acceptor states in the limit of low iron content $x\ensuremath{\lesssim}0.4%$, indicating that the concentration of ${\mathrm{Fe}}^{2+}$ ions increases at the expense of Fe ions in the $3+$ charge state. This effect is witnessed by PL measurements as changes in the intensity of the ${\mathrm{Fe}}^{3+}$-related intraionic transition, which can be controlled by codoping with Si donors and Mg acceptors. In this regime, EPR of ${\mathrm{Fe}}^{3+}$ ions and Curie-like magnetic susceptibility are observed. As a result of the spin-orbit interaction, ${\mathrm{Fe}}^{2+}$ does not produce any EPR response. However, the presence of Fe ions in the $2+$ charge state may account for a temperature-independent Van Vleck--type paramagnetic signal that we observe by SQUID magnetometry. Surprisingly, at higher Fe concentrations, the electron density is found to increase substantially with the Fe content. The coexistence of electrons in the conduction band and Fe in the $3+$ charge state is linked to the gradient in the Fe concentration. In layers with iron content $x\ensuremath{\gtrsim}0.4%$ the presence of ferromagnetic signatures, such as magnetization hysteresis and spontaneous magnetization, have been detected. A set of precautions has been undertaken in order to rule out possible sources of spurious ferromagnetic contributions. Under these conditions, a ferromagneticlike response is shown to arise from the $(\mathrm{Ga},\mathrm{Fe})\mathrm{N}$ epilayers, it increases with the iron concentration, it persists up to room temperature, and it is anisotropic---i.e., the saturation value of the magnetization is higher for in-plane magnetic field. We link the presence of ferromagnetic signatures to the formation of Fe-rich nanocrystals, as evidenced by TEM and EDS studies. This interpretation is supported by magnetization measurements after cooling in and without an external magnetic field, pointing to superparamagnetic properties of the system. It is argued that the high temperature ferromagnetic response due to spinodal decomposition into regions with small and large concentration of the magnetic component is a generic property of diluted magnetic semiconductors and diluted magnetic oxides showing high apparent Curie temperature.

Structural and optical properties ofEr3+ ion insol–gel grown LiNbO3

Er-doped lithium niobate (LiNbO3) ceramics embedded in silica were grown by the sol–gel method. The dried gel91SiO2–4Li2O–4Nb2O5–Er2O3 (mol%) was subsequently heat treated between 500 and750 °C. X-ray diffraction and Raman spectroscopy have detected the crystallization ofLiNbO3 andErNbO4 in samples treated attemperatures above 600 °C. At least two Er-related optical centres have been identified by low-temperature opticalabsorption and photoluminescence studies. The dominant intra-ionic transition arises from theEr3+ ion, inC2v site symmetry,in the ErNbO4 lattice.

Photoionization energies of Cr 3+-doped LiNbO 3

We report on a study of intra-ion and electron transfer transitions in Cr 3+-doped LiNbO 3. The energies of the meta-stable emitting levels of Cr 3+ on both Li and the Nb sites are determined by standard optical spectroscopy techniques. Photoconductive measurements are used to determine the ionization energies of both species of Cr 3+ ions. From these results, energy level diagrams are created which give the energies of the optically active levels with respect to the host valence and conduction bands. The emitting 4T 2 level of Cr 3+ on a Li site and on a Nb site is found to lie 1.5 and 1.2 eV below the conduction band bottom, respectively. These results lead to the suggestion that the lack of laser action found in LiNbO 3:Cr 3+ may be due to strong excited state absorption transitions from the meta-stable 4T 2 level to the host conduction band.

Photoluminescence of Au− formed in 12CaO · 7Al2O3 single crystal by Au+-implantation

Au+ ion implantation with fluences from 1×1014 to 3×1016cm−2 into 12CaO·7Al2O3 (C12A7) single crystals was carried out at a sample temperature of 600°C. The implanted sample with the fluence of 1×1015cm−2 exhibited photoluminescence (PL) bands peaking at ∼3.1 and ∼2.3eV at ⩽150K when excited by He–Cd laser (325nm). This was the first observation of PL from C12A7. These two PL bands are possibly due to intra-ionic transitions of an Au− ion having the electronic configuration of 6s2, judged from their similarities to those reported on Au− ions in alkali halides. However, when the concentration of the implanted Au ions exceeded the theoretical maximum value of anions encaged in C12A7 (∼2.3×1021cm−3), surface plasmon absorption appeared in the optical absorption spectrum, suggesting Au colloids were formed at such high fluences. These observations indicate that negative gold ions are formed in the cages of C12A7 by the Au+ implantation if an appropriate fluence is chosen.

Optical activation of Eu ions in nanoporous GaN films

A systematic optical activation study of Eu-implanted nanoporous GaN films has been carried out as a function of ion dose and annealing temperature. The nanoporous GaN films are prepared by photoelectrochemical etching of n-type GaN films in HF-based electrolyte. Eu ions are implanted in both n-type GaN and n-type porous GaN films at 200keV with doses ranging from 5×1014to5×1015cm−2. For the implantation damage recovery and optical activation of Eu3+ ions, rapid thermal annealing is performed in the temperature range of 900–1200°C under nitrogen ambient. The surface morphology of implanted porous GaN after different processing steps is characterized by scanning electron microscopy and the results show that porous morphology remains uniform even after ion implantation and high temperature processing. Microphotoluminescence and micro-Raman techniques have been used to investigate the optical properties of these Eu-implanted nanoporous films. Postimplantation annealing of both as-grown GaN and porous GaN films leads to the observation of strong photoluminescence (PL) peak around 622nm, which is associated with the D05–F27 intraionic transition of Eu3+ ions. We have observed that PL intensity of Eu-related luminescence peaks increases with annealing temperature up to 1100°C. In addition, due to efficient light extraction by surface nanostructuring, Eu-implanted porous GaN films show much stronger luminescence when compared to Eu-implanted as-grown GaN. Raman spectral analyses also indicate the optimum annealing condition for the implantation damage recovery and the compressive stress state in the Eu-implanted films.