Felix J. Brieler

Ordered Arrays of II/VI Diluted Magnetic Semiconductor Quantum Wires: Formation within Mesoporous MCM-41 Silica

We present a novel way of synthesising highly ordered arrays of hollow Cd(1-x)Mn(x)S quantum wires with lateral dimensions of 3-4 nm separated by 1-2 nm SiO2 barriers by forming Cd(1-x)Mn(x)S (0 < or = x < or = 1) semiconductors inside the pore system of mesoporous MCM-41 SiO2 host structures. X-ray diffraction and transmission electron microscopy (TEM) studies reveal the hexagonal symmetry of these arrays (space group p6m) and confirm the high degree of order. Physisorption measurements show the filling of the pores of the MCM-41 SiO2. The X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), electron paramagentic resonance (EPR), and Raman studies confirm the good crystalline quality of the incorporated (Cd,Mn)S guest. The effects of reducing the lateral dimensions on the magnetic and electronic properties of the diluted magnetic semiconductor were studied by photoluminescence (PL) and PL excitation spectroscopy and by SQUID and EPR measurements in the temperature range 2-400 K. Due to the quantum confinement of the excitons in the wires, an increase of about 200 meV in the direct band gap was observed. In addition, the p-d hybridisation-related bowing of the band gap as a function of Mn concentration in the wires is much stronger than in the bulk. This effect is related to the increase in the band gap due to quantum confinement, which shifts the p-like valence band edge closer to the 3d-related states of Mn in the valence band. Thus, the p-d hybridisation and the strength of the band gap bowing are increased. Compared to bulk (II,Mn)VI compounds, antiferromagnetic coupling between the magnetic moments of the Mn2+ ions is weaker. For the samples with high Mn concentrations (x > 0.8) this leads to a suppression of the phase transition of the Mn system from paramagnetic to antiferromagnetic. This effect can be explained by the fact that the lateral dimensions of the wires are smaller than the magnetic length scale of the antiferromagnetic ordering.

Towards ordered arrays of magnetic semiconductor quantum wires

The diluted magnetic semiconductor (Cd, Mn)S has been incorporated into ordered wire-like pores of hexagonal mesoporous silica. X-ray and Raman spectra reveal the wurtzite structure of the incorporated material. Photoluminescence and photoluminescence excitation spectra of the (Cd, Mn)S-wire samples show clearly the optical transitions within the half-filled Mn 3d shell, typical for Mn incorporated in a II–VI host material. The blueshift of the absorption edge of (Cd, Mn)S-wire samples compared to reference crystalline and powder samples of the same Mn content is due to quantum confinement in the nanowires.

Arrays of (Zn,Mn)S quantum wires with well-defined diameters below 10 nm

Zn1−xMnxS with x=1% to 30% were formed inside the ordered pore systems of different mesoporous SiO2 matrices. Due to the highly ordered host structure, regular arrays of Zn1−xMnxS quantum wires with diameters of 3.1 nm and 5.6 nm, respectively, separated by 2 nm SiO2 barriers were obtained. The wires were characterized using photoluminescence (PL), PL excitation (PLE) spectroscopy, and electron paramagnetic resonance (EPR). The 4 T1→6 A1 internal transition of the Mn2+(3d5) ions dominates the PL. The corresponding PLE spectra show higher internal Mn transitions and the band-to-band transition. EPR spectra and the energies of the internal Mn transitions are typical for Mn2+ on a cation site of (II,Mn)VI semiconductors. The crystal field parameters indicate that the wires are tensilely strained. Due to the comparable band gaps of the SiO2 and the Zn1−xMnxS and the small exciton Bohr radius in (Zn,Mn)S, the confinement effects in the wires are less than 150 meV.

Influence of the hydrophilic–hydrophobic contrast of porous surfaces on the enzymatic performance

Herein we report the dependency of the performance (uptake, activity, stability) of hydrophilic glucose-6-phosphate dehydrogenase (G6PDH) onto mesoporous cellular foams (MCF) grafted with aminosilanes of different chain length. The resulting hydrophobic-hydrophilic contrast was carefully evaluated by combined argon and water sorption and quantified with a newly developed hydrophilicity index suitable for the characterization of large-pore materials. The enzymatic behaviour was influenced by electrostatic and hydrophobic interactions between both reaction partners due to the creation of specific microenvironments inside the pore system. The microenvironment created by the materials with high hydrophobicity did not contribute to the beneficial electrostatic interactions of the charged amine groups by increased hydrophobic forces instead a competition of both forces was present. Thus an improved biocatalytic performance was not observed for materials with high hydrophobicity but short-chain functionalized MCFs led to highly stable and active biocatalysts.

Magneto-spectroscopy of ordered arrays of magnetic semiconductor quantum wires

Abstract Hexagonally ordered arrays of quantum wires of the diluted magnetic semiconductor Cd 1−x Mn x S (0 have been obtained by incorporating the semiconductor alloy into the ordered wire-like pores of mesoporous silica hosts (MCM-41). Electron paramagnetic resonance measurements indicate that the crystalline structure of the Cd 1− x Mn x S quantum wires changes from zincblende to wurtzite with increasing x . Photoluminescence and photoluminescence excitation spectroscopy have been performed at liquid helium temperatures in external magnetic fields up to 7.5 T . The photoluminescence studies of the transitions within the half-filled Mn 3d-shell reveal the existence of at least two kinds of Mn-centres with different crystal fields. A blue shift of more than 200 meV of the absorption edge of the Cd 1− x Mn x S wires compared to that of bulk material is found due to the lateral quantum confinement in the nanowires. In addition, the p–d exchange-induced bowing of the band gap with x is enhanced in the wire structures due to the confinement-induced shift of the lowest valence band state towards the Mn 3d-states.

Modification of the magnetic and electronic properties of ordered arrays of (II, Mn)VI quantum wires due to reduced lateral dimensions

We present a novel way of synthesising highly ordered arrays of Cd 1-x Mn x S and Cd 1-x Mn x Se quantum wires with lateral dimensions of 3 nm separated by 2 nm SiO 2 barriers by incorporating the (II,Mn)VI semiconductor with 0 ≤ x ≤ 1 into the pore system of mesoporous MCM-41 SiO 2 matrices. The electronic and excitonic properties were studied using photoluminescence and photoluminescence excitation spectroscopy at low temperatures and in magnetic fields up to 7.5 T. Due to the quantum confinement of the excitons in the wires an increase of the direct band gap by about 200 meV for (Cd, Mn)S and by about 350 meV for (Cd, Mn)Se is observed. In addition, we observe a much stronger, p-d hybridisation related band gap bowing as a function of Mn-concentration in the wires compared to bulk. This effect is related to the increase of the band gap due to the quantum confinement which shifts the p-like valence band edge closer to the Mn-3d-related states in the valence band. Surprisingly, the s,p-d exchange induced giant Zeeman splitting of the excitons in the (Cd,Mn)Se wires compared to those in bulk material appears to be very small. The magnetic properties of the samples were studied by SQUID and electron paramagnetic resonance measurements in the temperature range from 2 to 300 K. Compared to the bulk (II, Mn)VI compounds, a reduced antiferromagnetic coupling between the magnetic moments of the Mn 2+ ions is found. For x > 0.8, a suppression of the paramagnetic to antiferromagnetic phase transition of the Mn-system is observed because the lateral dimensions of the wires are smaller than the magnetic length scale of the antiferromagnetic ordering.

Immobilization of alcohol dehydrogenase from E. coli onto mesoporous silica for application as a cofactor recycling system

Alcohol dehydrogenase (ADH) from Escherichia coli (E. coli) was electrostatically immobilized onto the surface of aminopropyl modified mesoporous cellular siliceous foams (MCF‐NH2). To ascertain the optimal pH for the immobilization, several pH values (pH 6.5, 7.0, 7.5) were examined. The MCF‐NH2 amount applied for immobilization was also varied to identify the most suitable enzyme to host ratio. ADH uptakes, long‐term stabilities, and leaching behavior were monitored, and the forward and back reactions were studied. ADH activity after 24 h of immobilization decreased with increasing pH. Independent of the pH, the best long‐term stabilities were obtained with an enzyme to silica ratio of 1:10. The back reaction is always preferred and is thus much faster. Michaelis–Menten kinetics of the free and the immobilized ADH of the forward as well as the back reaction were elaborated for each immobilization pH and by employing an ADH to MCF‐NH2 ratio of 1:10. Turnover numbers and the enzyme efficiencies (EE) were determined. From the kinetic aspects the immobilization was most promising when using a pH 6.5 buffer (forward reaction) or pH 7.0 buffer (back reaction) for immobilization. Thus, depending on the coenzyme that needs to be recycled (NADP+ or NADPH) the pH of the immobilization has to be selected.

Regular Arrays of (Zn,Mn)S Quantum Wires with Well-Defined Diameters in the Nanometer Range

Zn1−xMnxS, with x varying between 0.01 and 0.30, were formed inside the ordered pore systems of different mesoporous SiO2 matrices. Because of the highly ordered structure of the hosts, regular arrays of Zn1−xMnxS quantum wires with lateral dimensions of 3 and 5.5 nm, respectively, separated by 2-nm SiO2 barriers were obtained. The wires were characterized using photoluminescence (PL) and PL excitation (PLE) spectroscopy at liquid Helium temperatures. The PL of the wires is dominated by the 4T1 → 6A1 internal transition of the Mn2+(3d5) ions. The corresponding PLE spectra show higher internal Mn transitions as well as the band to band transition. The energies of the internal Mn transitions are typical for Mn2+ on a cation site of (II,Mn)VI semiconductors. Because of the comparable bandgaps of the SiO2 and the Zn1−xMnxS as well as the small exciton Bohr radius in (Zn,Mn)S quantum confinement effects in the wires are less than about 150 meV.

Properties of water confined in periodic mesoporous organosilicas - nanoimprinting the local structure

The properties of materials confined in porous media are important in scientific and technological aspects. Topology, size, and surface polarity of the pores play a critical role in the confinement effects, however, knowledge regarding the guest-pore interface structure is still lacking. Herein, we show that the molecular mobility of water confined in periodic mesoporous organosilicas (PMOs) is influenced by the polarity of the organic moiety. Multidimensional solid-state NMR spectroscopy directly probes the spatial arrangement of water inside the pores, showing that water interacts either with only the silicate layer or with both silicate and organic layers depending on the alternating surface polarity. A modulated and a uniform pore filling mode are proposed for different types of PMOs.

Modified Magnetic Properties of Paramagnetic (Zn,Mn)S at Reduced Dimensions

We present a novel way of synthesizing highly ordered arrays of magnetic nanostructures with lateral dimensions of 2 to 10\,nm by incorporation into mesoporous SiO\(_2\) matrices. Such nanostructure arrays are suitable for studying magnetic phenomena at reduced dimensions, which is also important in the context of device miniaturization. Exemplarily we study (Zn,Mn)S nanowires. Changes of the macroscopic observables (e.g. Curie-Weiss parameter \(\Theta\) and line width \(\Delta H\) of the electron paramagnetic resonance) of the paramagnetic phase due to reduced dimensions are observed. The microscopic coupling between the Mn-ions (e.g. the exchange constants \(J_\mathrm{nn}\) and \(J_\mathrm{nnn}\)) is not altered to a first approximation. The macroscopic modifications arise mainly due to geometrical restrictions, i.e. the number of neighbors in the cation shells around a Mn-ion in the surface region are considerably reduced compared to a Mn-ion in the bulk. Similar effects are also anticipated for antiferromagnetic and ferromagnetic wires.