Stephan Heer, Karsten Kömpe, Hans-Ulrich Güdel and Markus Haase
Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide-Doped NaYF₄ Nanocrystals
Adv. Mater. 16, 2102-2105 (2004)                       

Abstract: (Article without Abstract)
Graphical Abstract: Highly efficient photon upconversion in solution is demonstrated for the first time. Transparent colloidal solutions of Yb/Er and Yb/Tm doped NaYF₄ nanocrystals show bright upconversion emission in the red, green, and blue spectral region by excitation around 1 mm with a laser of moderate power. These new systems show a substantial improvement on the order of 10⁸ compared to previously reported systems.

Inside Front Cover Picture: A synthesis route to lanthanide-doped NaYF₄ nanocrystals, an important new class of materials showing efficient multicolor upconversion emission in transparent colloidal solution, is reported. Solutions of Yb/Er- and Yb/Tm-doped NaYF₄ nanocrystals show bright upconversion emission in the red, green, and blue spectral region after excitation with a laser of moderate power. These new systems show a substantial improvement on the order of 10⁸ compared to previously reported systems.

G. Chaboussant, A. Sieber, S. Ochsenbein, H.-U. Güdel, M. Murrie, A. Honecker, N. Fukushima and B. Normand
Exchange interactions and high-energy spin states in Mn₁₂-acetate
Phys. Rev. B 70, 104422/1-16 (2004)                 

Abstract: We perform inelastic neutron scattering measurements on the molecular nanomagnet Mn₁₂-acetate to measure the excitation spectrum up to 45  meV (500  K). We isolate magnetic excitations in two groups at 5–6.5  meV (60–75  K) and 8–10.5  meV (95–120  K), with higher levels appearing only at 27  meV  (310  K) and 31  meV  (360  K). From a detailed characterization of the transition peaks we show that all of the low-energy modes appear to be separate S = 9 excitations above the S = 10 ground state, with the peak at 27  meV  (310  K) corresponding to the first S = 11 excitation. We consider a general model for the four exchange interaction parameters of the molecule. The static susceptibility is computed by high-temperature series expansion and the energy spectrum, matrix elements, and ground-state spin configuration by exact diagonalization. The theoretical results are matched with experimental observation by inclusion of cluster anisotropy parameters, revealing strong constraints on possible parameter sets. We conclude that only a model with dominant exchange couplings J₁ J₂ 5.5  meV  (65  K) and small couplings J₃ J₄ 0.6  meV  (7  K) is consistent with the experimental data.

Colette Boskovic, Andreas Sieber, Grégory Chaboussant, Hans U. Güdel, Jürgen Ensling, Wolfgang Wernsdorfer, Antonia Neels, Gael Labat, Helen Stoeckli-Evans and Stefan Janssen
Synthesis and Characterization of a New Family of Bi-, Tri-, Tetra-, and Pentanuclear Ferric Complexes
Inorg. Chem. 43, 5053-5068 (2004)                             

Abstract: Nine members of a new family of polynuclear ferric complexes have been synthesized and characterized. The reaction of Fe(O₂CMe)₂ with polydentate Schiff base proligands (H₂L) derived from salicylidene-2-ethanolamine, followed in some cases by reaction with carboxylic acids, has afforded new complexes of general formulas [Fe₂(pic)₂(L)₂] (where pic^– is the anion of 2-picolinic acid), [Fe₃(O₂CMe)₃(L)₃], [Fe₄(OR)₂(O₂CMe)₂(L)₄], and [Fe₅O(OH)(O₂CR)₄(L)₄]. The tri-, tetra-, and pentanuclear complexes all possess unusual structures and novel core topologies. Mössbauer spectroscopy confirms the presence of high-spin ferric centers in the tri- and pentanuclear complexes. Variable-temperature magnetic measurements suggest spin ground states of S = 0, 1/2, 0, and 5/2 for the bi-, tri-, tetra-, and pentanuclear complexes, respectively. Fits of the magnetic susceptibility data have provided the magnitude of the exclusively antiferromagnetic exchange interactions. In addition, an easy-axis-type magnetic anisotropy has been observed for the pentanuclear complexes, with D values of approximately -0.4 cm^–1 determined from modeling the low-temperature magnetization data. A low-temperature micro-SQUID study of one of the pentanuclear complexes reveals magnetization hysteresis at nonzero field. This is attributed to an anisotropy-induced energy barrier to magnetization reversal that is of molecular origin. Finally, an inelastic neutron scattering study of one of the trinuclear complexes has revealed that the magnetic behavior arises from two distinct species.

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