Rafael Valiente

Luminescence upconversion mechanisms in Yb3+-Tb3+systems

Abstract Tb 3+ ( 5 D 4 → 7 F J ) luminescence has been observed in single crystals of SrCl2:Tb3+(1%):Yb3+(1%) and Cs3Tb2Br9:Yb3+(1%) under excitation energies in the region of Yb3+ absorption. Previous reports of similar systems have postulated two possible upconversion mechanisms: (1) cooperative sensitization and (2) a sequence of two cooperative absorption steps, or GSA/ESA. We present results of emission, excitation and kinetic measurements at low and high temperatures. These measurements allow assignment of the mechanism responsible for upconversion in these systems. For temperatures greater than T=100 K process (1) is the dominant mechanism, in agreement with previous assignments. However, for T K in Cs3Tb2Br9:Yb3+(1%), the upconversion occurs through a GSA/ESA sequence, which is shown here in a clear manner for the first time. The efficiency of the Yb3+–Tb3+ upconversion process for SrCl2:Tb3+(1%):Yb3+(1%), at room temperature, under 2.4(10)4W/cm2, is on the order of 10−4 and decreases with decreasing temperature by four orders of magnitude.

Multiwalled Carbon Nanotubes Display Microtubule Biomimetic Properties in Vivo, Enhancing Microtubule Assembly and Stabilization

Microtubules are hollow protein cylinders of 25 nm diameter which are implicated in cytokinetics and proliferation in all eukaryotic cells. Here we demonstrate in vivo how multiwalled carbon nanotubes (MWCNTs) interact with microtubules in human cancer cells (HeLa) blocking mitosis and leading to cell death by apoptosis. Our data suggest that, inside the cells, MWCNTs display microtubule biomimetic properties, assisting and enhancing noncentrosomal microtubule polymerization and stabilization. These features might be useful for developing a revolutionary generation of chemotherapeutic agents based on nanomaterials.

Room-temperature green upconversion luminescence in LaMgAl11O19:Mn2+, Yb3+ upon infrared excitation

We present a spectroscopic study of the green upconversion luminescence of Mn2+ upon near infrared Yb3+ excitation in LaMgAl11O19 codoped with manganese and ytterbium. Excitation at 975 nm with a diode laser resonant with the F27/2→F25/2 Yb3+ transition induces a broad emission band centered at 514 nm and assigned to the T41→A61 transition of tetrahedral Mn2+. The upconversion luminescence can be seen by the naked eye up to 500 K. This finding is a promising step in order to use the Mn2+–Yb3+ system as a potential candidate for display phosphors by upconversion.

Phonon-assisted cooperative sensitization of Tb3+ in SrCl2:Yb, Tb

Excitation into the Yb 3+ 2 F5/2 excited states leads to visible-by-eye green luminescence spanning the spectral region from 490 to 790 nm, with a quadratic power dependence. Optical absorption, luminescence, and excitation spectroscopy as well as pulsed measurements on single-crystal SrCl2:Yb(1%), Tb(1%) are used to determine the upconversion (UC) properties of this system. The upconverted luminescence is easily detectable by eye from RT to 100 K, at which point the intensity drops significantly and a change in colour from green to blue is observed. Pulsed measurements coupled with excitation spectroscopy lead to the unambiguous assignment of a phononassisted cooperative sensitization mechanism as the dominant UC process for T> 50 K with VIS NIR photon ratios on the order of 10 −2 % for a laser power of 56 W mm −2 . Below 50 K, the dominant UC emission becomes the wellknown Yb–Yb cooperative luminescence around 500 nm, with a consequential reduction of Tb 3+ emission by more than three orders of magnitude from RT to 10 K.

Upconversion luminescence in Yb3+ doped CsMnCl3: Spectroscopy, dynamics, and mechanisms

Single crystals of CsMnCl3 doped with 0.9% Yb3+ were grown from the melt by the Bridgman technique and studied by means of variable temperature optical spectroscopy. At cryogenic temperatures, near-infrared Yb3+-excitation around 1 μm leads to intense Mn2+ upconversion luminescence in the red spectral region. This very efficient upconversion process is possible because of magnetic Yb3+–Mn2+ exchange interactions, and a new type of upconversion mechanism is found to be active in this system. The upconversion properties of Yb3+:CsMnCl3 are compared to those of Yb3+:RbMnCl3 and Yb3+:CsMnBr3. The upconversion efficiencies at cryogenic temperatures differ by many orders of magnitude. The bridging geometry between Yb3+ and Mn2+ is found to be a key factor for the efficiency of the process. The highest efficiency is observed for the title compound, and this is correlated with the most likely linear Yb3+–Cl−–Mn2+ arrangement in this crystal. At 15 K the dominant upconversion mechanism in the title compound involv...

Exchange Interactions at the Origin of Slow Relaxation of the Magnetization in \{TbCu3\} and \{DyCu3\} Single-Molecule Magnets

New {TbCu3} and {DyCu3} single-molecule magnets (SMMs) containing a low-symmetry Ln(III) center (shape measurements relative to a trigonal dodecahedron and biaugmented trigonal prism are 2.2-2.3) surrounded by three Cu(II) metalloligands are reported. SMM behavior is confirmed by frequency-dependent out-of-phase ac susceptibility signals and single-crystal temperature and sweep rate dependent hysteresis loops. The ferromagnetic exchange interactions between the central Ln(III) ion and the three Cu(II) ions could be accurately measured by inelastic neutron scattering (INS) spectroscopy and modeled effectively. The excitations observed by INS correspond to flipping of Cu(II) spins and appear at energies similar to the thermodynamic barrier for relaxation of the magnetization, ~15-20 K, and are thus at the origin of the SMM behavior. The magnetic quantum number M(tot) of the cluster ground state of {DyCu3} is an integer, whereas it is a half-integer for {TbCu3}, which explains their vastly different quantum tunneling of the magnetization behavior despite similar energy barriers.

Influence of hydrostatic pressure on the Jahn–Teller effect in the 4T2g excited state of CrCl63− doped Cs2NaScCl6

The 4T2g→4A2g luminescence of a 4.1% Cr3+ doped Cs2NaScCl6 crystal is studied as a function of hydrostatic pressure at room temperature and 15 K. The vibrational fine structure observed in the low-temperature variable pressure emission spectra is analyzed with a two configurational coordinate approach, involving the totally symmetric a1g and the eg Jahn–Teller normal coordinate. Increasing hydrostatic pressure is found to reduce the tetragonal distortion of the CrCl63− unit in the electronic 4T2g state resulting from the Jahn–Teller effect. Additionally, pressure impedes expansion along the a1g coordinate of the CrCl63− complex upon 4A2g→4T2g photo-excitation, and thus has a greater influence on the 4T2g excited state than on the 4A2g ground state. The absolute Cr3+–Cl− average distance reduction upon increasing pressure is estimated using a simple point charge model.

Synthesis, structure and luminescence of Er3+-doped Y3Ga5O12 nano-garnets

A novel Y3(1−x)Er3xGa5O12 nanocrystalline garnet has been synthesized by a sol–gel technique and a complete structural, morphological, vibrational, and optical characterization has been carried out in order to correlate the local structure of the Er3+ ions with their optical properties. The synthesized nanocrystals are found in a single-phase garnet structure with an average grain size of around 60 nm. The good crystalline quality of the garnet structure is confirmed by FTIR and Raman measurements, since the phonon modes of the nano-garnet are similar to those found in the single crystal garnet. Under blue laser excitation, intense green and red visible and 1.5 μm infrared luminescences are observed, whose relative intensities are very sensitive to the Er3+ concentration. The dynamics of these emissions under pulsed laser excitations are analyzed in the framework of different energy transfer interactions. Intense visible upconverted luminescence can be clearly observed by the naked eye for all synthesized Er3+-doped Y3Ga5O12 nano-garnets under a cw 790 nm laser excitation. The power dependency and the dynamics of the upconverted luminescence confirm the existence of different two-photon upconversion processes for the green and red emissions that strongly depend on the Er3+ concentration, showing the potential of these nano-garnets as excellent candidates for developing new optical devices.

Optical spectroscopy of Al2O3:Ti3+ single crystal under hydrostatic pressure. The influence on the Jahn-Teller coupling

This work investigates the effect of hydrostatic pressure on the excitation and emission spectra, as well as on the lifetime, of Al2O3:Ti3+ at room temperature. The aim is to establish correlations between the pressure-induced band shifts and the corresponding local structural changes undergone by the TiO6 complex. A blue-shift of 8.52 and 6.86 cm−1 kbar−1 was found for the lower (E1) and upper (E2) energy components of the excitation band at 17 760 and 20 500 cm−1, respectively, and blue-shifts of 5.93 and 5.40 cm−1 kbar−1 for the two overlapping bands of the emission spectrum located at 12 680 and 14 210 cm−1. We explain these results on the basis of a reduction of the TiO6 Jahn–Teller distortion upon increasing the pressure. In contrast, the increase of the overall Stokes shift, which is mainly associated with electron–vibration coupling to the totally symmetric a1g vibration, is explained by the increase of the excited-state stabilization energy, Sa1g hωa1g , with increasing pressure. The luminescence lifetime is also found to be pressure dependent, varying from 2.6 μs at ambient conditions to 3.2 μs at 80 kbar. This increase is caused by a diminution of the transition oscillator strength that is related to the odd vibration assistance mechanism. The softening of the transition mechanism is interpreted in terms of the blue-shift experienced by the O2− -> Ti3+ charge transfer transition energy upon increasing the pressure.

Polarized electronic spectra of the (CH3NH3)2Cd1-xMnxCl4 (x = 0-1) perovskite layer doped with Cu2+ : Study of the Cl- → Cu2+ charge transfer intensity enhancement along the series

Abstract The polarized optical absorption spectra of the 2-D A 2 Cd 1 − x Mn x Cl 4 ( x = 0–1; A = CH 3 NH 3 ) crystals doped with Cu 2+ are investigated. The analysis of both the charge transfer and crystal field spectra indicates that the copper impurities form CuCl 6 4− complexes with an elongated D 2h (nearly D 4h ) symmetry. A salient feature is the enhancement of charge transfer band intensity as well as the presence of new intense bands at 21000 and 25000cm −1 observed on passing from x = 0 to x = 1 along the series. These bands are associated with Mn-Cu aggregates whose superexchange pathways involve the short (equatorial) CuCl bond (25000 cm −1 ) and the long (axial) CuCl bond (21000 cm −1 ) of the CuCl 6 4− complex. A noteworthy fact of these exchange coupled MnCu systems is that the transition energy of the first Mn 2+ excitations are resonant with the Cl − → Cu 2+ charge transfer transitions. This effect seems to play a fundamental role in the enhancement of intensity. The temperature dependence of the oscillator strength allowed us to estimate a ground state exchange constant J ≈ 70cm −1 for the MnCu pair. The results are compared with those obtained in MnCu aggregates in fluorides.