Nils W. Rosemann

A highly efficient directional molecular white-light emitter driven by a continuous-wave laser diode

Converting infrared into visible There is a constant societal drive to develop new light sources that are both efficient and environmentally benign. Rosemann et al. developed an amorphous material that emits a broadband (warm white) spectrum of light upon excitation with an infrared laser via highly nonlinear processes. Inorganic nanocrystals form the core of their material and are coated with organic ligands on the surface. When excited with infrared light, nonlinear optical processes cause the material to emit broadband white light. Science, this issue p. 1301 A tailored compound can transform near-infrared light into broadband white light. Tailored light sources have greatly advanced technological and scientific progress by optimizing the emission spectrum or color and the emission characteristics. We demonstrate an efficient spectrally broadband and highly directional warm-white-light emitter based on a nonlinear process driven by a cheap, low-power continuous-wave infrared laser diode. The nonlinear medium is a specially designed amorphous material composed of symmetry-free, diamondoid-like cluster molecules that are readily obtained from ubiquitous resources. The visible part of the spectrum resembles the color of a tungsten-halogen lamp at 2900 kelvin while retaining the superior beam divergence of the driving laser. This approach of functionalizing energy-efficient state-of-the-art semiconductor lasers enables a technology complementary to light-emitting diodes for replacing incandescent white-light emitters in high-brilliance applications.

Synthesis, Crystal Structure, and Photoluminescence Studies of a Ruthenocenyl-Decorated Sn/S Cluster

Recently, we reported on ferrocenyl-decorated Sn/S clusters; herein, we present the extension of our investigations by attachment of ruthenocenyl units to an according cluster skeleton. The latter was realized upon improvement of the synthesis of acetylruthenocene, its conversion to a hydrazone derivative, and the subsequent reaction with a keto-functionalized Sn/S precursor complex. The report comprises the crystal structures of acetylruthenocene and the ruthenocenyl-terminated Sn/S cluster [(R(Rc)Sn)4Sn2S10] (R(Rc) = CMe2CH2C(Me)═N-N═C(Me)Rc), as well as the discussion of the electrochemical properties of the latter and its behavior during time-resolved photoluminescence investigations.

Organotetrel Chalcogenide Clusters: Between Strong Second-Harmonic and White-Light Continuum Generation

Highly directional white-light generation was recently reported for the organotin sulfide cluster [(StySn)4S6] (Sty = p-styryl). This effect was tentatively attributed to the amorphous nature of the material in combination with the specific combination of an inversion-symmetry-free T/E cluster core (T = tetrel, E = chalcogen) with the attachment of ligands that allow π delocalization of the electron density. Systematic variation of T and the organic ligand (R) that runs from T = Si through Ge to Sn and from R = methyl through phenyl and p-styryl to 1-naphthyl provides a more comprehensive view. According to powder X-ray data, only [(PhSi)4S6] is single-crystalline among the named combinations. Here we demonstrate the fine-tuneability of the nonlinear response, i.e., changing from white-light generation to second-harmonic generation as well as controlling the white-light properties. These are investigated as a function of T, π delocalization of the electron density within R, and the order within the molecular solids.

Excitonic transitions in highly efficient (GaIn)As/Ga(AsSb) type-II quantum-well structures

The excitonic transitions of the type-II (GaIn)As/Ga(AsSb) gain medium of a “W”-laser structure are characterized experimentally by modulation spectroscopy and analyzed using microscopic quantum theory. On the basis of the very good agreement between the measured and calculated photoreflectivity, the type-I or type-II character of the observable excitonic transitions is identified. Whereas the energetically lowest three transitions exhibit type-II character, the subsequent energetically higher transitions possess type-I character with much stronger dipole moments. Despite the type-II character, the quantum-well structure exhibits a bright luminescence.

Revisiting [(RSnIV)6SnIII2S12]: Directed Synthesis, Crystal Transformation, and Luminescence Properties

We report the synthesis of the mixed-valence cluster [(R(5)Sn(IV))6Sn(III)2S12] [1; R(5) = CMe2CH2C(O)Me] under optimization of the reaction conditions. A new crystalline form of 1 in the orthorhombic space group Pbca was found at 250 K, which undergoes crystal transformation into the known monoclinic one at lower temperature. Further, we have studied the luminescence properties of 1. Time-resolved photoluminescence measurements confirm the lability of the tin-chalcogenide bonds to UV irradiation, while the organic ligands are much less affected by it.

Temperature-dependent quantum efficiency of Ga(N,As,P) quantum wells

The photoluminescence quantum efficiencies of a series of Ga(N,As,P)/GaP multiple quantum wells are analyzed. The external quantum efficiencies are derived from the absorbed and the emitted light intensities measured using an integrating sphere mounted inside a closed-cycle helium cryostat. By taking into account the device layer sequences as well as internal reflections and reabsorption, the internal quantum efficiencies yield values above 90% for all samples at cryogenic temperatures. The temperature-dependence of the quantum efficiencies as a function of active quantum well layer design reveal the internal interfaces as remaining growth challenge in these heterostructures.

Syntheses and Properties of Gold–Organotin Sulfide Clusters

We report that reactions of the binary organotin sulfide cluster [(R1Sn)3S4]Cl [A; R1 = CMe2CH2C(O)Me] with gold(I) phosphane complexes yield discrete ternary complexes [(R1Sn)2(AuPMe3)2S4] (1) and [(R2Sn)2(AuPMe3)2S4] [2; R2 = CMe2CH2C(NNH2)Me], which are related to recently published complexes [(R1,2Sn)2(AuPPh3)2S4] (B and C). Further, we present a binary tin sulfide cluster that cocrystallizes with a structure-directing salt of a gold phosphane complex in [Au(dppe)2][(R3Sn)4S6Cl] [3; R3 = CMe2CH2C(NNHPh)Me]. The nature of the product depends on the choice of the phosphane ligand as well as the addition of hydrazine hydrate or phenylhydrazine. Additionally, we report on the photophysical properties of 1, 2, B, and C, which indicate that the different phosphane ligands only have a slight influence on the optical responses. The structure, however, has a significant impact on the luminescence efficiency.

Low-threshold steady-state generation of directional white light

Tailored light sources are used in a wide variety of applications, such as sensors or illumination devices. In this field, the discovery of the phenomenon upon which LEDs are based,1 as well as the development of LEDs into a commercial product, are considered as ground-breaking steps. Furthermore, the emergence of gallium-nitride-based LEDs (which emit in the UV region) led to a huge leap forward,2 as these are the basis of ubiquitous white-light LEDs (in which different types of phosphor are used to convert UV radiation into a broad visible spectrum). Whitelight LEDs are efficient and reasonably priced. In addition, their emission profile is nearly Lambertian (i.e., uniform in radiation intensity, regardless of the position of the observer), which is a feature that is preferred for illumination applications.3, 4 However, there are many applications (such as microscopy and optical coherence tomography) where it is preferable to have light sources that emit virtually collimated beams and therefore provide illumination with a limited angular extent. Although lasers can provide illumination with limited angular range, their light is emitted only over a narrow spectral range (and they can even be virtually monochromatic). Nonlinear effects such as self-phase modulation can be used to generate supercontinua (i.e., sources emitting over a broad spectral range that exhibit properties of a laser beam, such as collimation and temporal coherence). To achieve these effects, however, a high field strength—and therefore pulsed or high-power lasers—are required.5 Supercontinua are thus expensive to generate and are almost exclusively used in medical or scientific applications. Figure 1. (a) Structure of the molecular cluster, where tin (Sn) and sulfur (S) atoms are represented by blue and yellow spheres, respectively, and carbon and hydrogen atoms in the ligands are denoted by gray and white lines, respectively. (b) Photograph of the compound (in powder form) that comprises the molecular clusters. (c) Photograph of the powder encapsulated in a polymer film, sandwiched between two glass slips, and excited in the bright center spot using an 800nm laser.

Structural investigations of the α12Si–Ge superstructure

X-ray diffraction-based structural analysis results of possible direct bandgap Si/Ge superlattices composed of monolayer thin deposits are presented, together with theoretical predictions and first optical measurements.

Structural investigations of the α12 Si-Ge superstructure.

X-ray diffraction-based structural analysis results of possible direct bandgap Si/Ge superlattices composed of monolayer thin deposits are presented, together with theoretical predictions and first optical measurements.