Theo Woike

Technische Universität Dresden, Dresden, Saxony, Germany

Are you Theo Woike?

Claim your profile

Publications (104)239.46 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Ferroelectric (FE) domain walls (DWs) have gained substantial interest in the last few years given by novel discoveries such as domain wall conductivity (DWC) at room temperature, and novel DW topologies. DWC is tightly connected to charged DWs, which might fully penetrate across the whole FE bulk insulator, hence offering a bright perspective for applications in nano- optoelectronics. Both thin-film [1] and single-crystalline (sc) ferroics [2] were discovered to possess such ac or dc DWC [3], however, depending on oxygen deficiency, doping, DW inclination angle, external stimulus and others more. We investigated here the influence of the geometric properties of DWs on the conductivity, and found, to our surprise, that a broad range of different complex DW topologies can be generated in our lithium niobate (LNO) single crystals. We thus report on how to generate these different DW topologies in LNO, and furthermore, how it is possible to quantify these topologies using a combination of surface- and bulk-sensitive techniques. Of importance is, for instance, the head-to-head or tail-to-tail character, as necessary for any electronic transport in DWs, and probed here by scanning force methods. Conversely, we applied 3- dimensional Cherenkov second harmonic imaging (CSHG) to track the DW topologies into the sc-LNO bulk [4]. Recent developments pushing CSHG to an interferometric detection adds more benefits for topology tracking, allowing quasi real-time imaging at an increased optical resolution. [1] J. Seidel et al., Adv. Mat., 26 (25), 4376 (2014). [2] M. Schröder et al., Adv. Funct. Mat., 22 (18), 3936 (2012). [3] M. Schröder et al., Mat. Res. Expr. 1 (3), 035012 (2014). [4] T. Kämpfe et al., Phys. Rev. B 89 (3), 035314 (2014).
    E-MRS Spring Meeting, Lille; 05/2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Four cyano-bridged bimetallic complexes, {[Mn(salpn)]2[Fe(CN)5NO]}n (1), {[Mn(salpn)(CH3OH)]4[Mn(CN)5NO]}[C(CN)3]•3H2O (2), {[Mn(dapsc)]Fe(CN)5NO]•0.5CH3OH•0.25H2O}n (3) and {[Mn(salpn)(CH3OH)]4[Fe(CN)5NO]}(ClO4)2•4H2O (4), where salpn2−= N,N'-1,3-propylenebis(salicylideneiminato) dianion, dapsc = 2,6-diacetylpyridine bis(semicarbazone), have been synthesized and structurally characterized by single crystal X-ray diffraction. In 1, the nitroprusside anion [Fe(CN)5NO]2− coordinates with four [Mn(salpn)]+ via four co-planar CN− groups, whereas each [Mn(salpn)]+ links two [Fe(CN)5NO]2− ions, which results in a two-dimensional network. The structure of 3 contains two independent neutral infinite chains {[Mn(dapsc)][Fe(CN)5(NO)]}∞ consisting of alternating cationic [MnII(dapsc)]2+ and anionic [FeII(CN)5(NO)]2− units connected through cyanide bridges. The cation complexes 2 and 4 have the pentanuclear molecular structure in which four [Mn(salpn)(MeOH)]+ fragments are linked by the [Mn(CN)5NO]3− or [Fe(CN)5(NO)]2− moieties, respectively. The magnetic and photochromic properties of 1, 3 have been studied. The thermal magnetic behaviour of the complexes indicates the presence of weak antiferromagnetic interactions between Mn3+ or Mn2+ mediated by diamagnetic [Fe(CN)5NO]2− bridges. Irradiation of 1, 3 with light gives birth to the long-lived metastable states of nitroprusside.
    CrystEngComm 04/2015; 17(20). DOI:10.1039/C5CE00354G · 3.86 Impact Factor
  • Source
  • Source
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Multiple light-induced reversible metastable NO linkage isomers (PLIs) have been detected in the dinitrosyl compound [RuCl(NO)2(PPh3)2]BF4 by a combination of photocrystallographic and IR analysis. The IR signature of three PLI states has been clearly identified, with estimated populations of 59% (PLI-1), 8% (PLI-2) and 5% (PLI-3) for a total population of the metastable state of 72%. The structural configuration of the major component (PLI-1) has been derived by X-ray photocrystallography. In the ground state, the structure is characterized by a bent and a linear nitrosyl, the bent one being oriented towards the linear equatorial nitrosyl with an Ru-N-O angle of 133.88 (9)°. X-ray Fourier difference maps indicate a selectivity of the photo-isomerization process in PLI-1: only the bent NO ligand changes its position, while the linear NO is unaffected. After irradiation at 405 nm, the orientation is changed by rotation towards the Cl ligand opposite the linear NO, with an Ru-N-O angle in this new position of 109 (1)°. The photocrystallographic analysis provides evidence that, in the photo-induced metastable state, the bent NO group is attached to the Ru atom through the N atom (Ru-N-O), rather than in an isonitrosyl Ru-O-N binding mode. In the IR spectra, the asymmetric NO vibrational band shifts by -33 cm(-1) to a lower value, whereas the symmetric band splits and shifts by 5 cm(-1) to a higher value and by -8 cm(-1) to a lower value. The down shift is a clear indication of the structural change, and the small upward shift in response to the new electronic configuration of the metastable structure. Variable-temperature IR kinetic measurements in the range 80-114 K show that the decay of the PLI-1 state follows an Arrhenius behaviour with an activation energy of 0.22 eV.
    01/2015; 2(Pt 1):35-44. DOI:10.1107/S2052252514023598
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We observed a multiphoton luminescence contrast between virgin and single-switched domains in Mg-doped LiNbO3 (LNO) and LiTaO3 (LTO) single crystals with different doping levels of 0–7 mol. % and 0–8 mol. %, respectively. A luminescence contrast in the range of 3% was measured between as-grown and electrically inverted domain areas in Mg:LNO samples, while the contrast reaches values of up to 30% for the Mg:LTO case. Under annealing, an exponential decay of the do- main contrast was observed. The activation energy of about 1 eV being determined for the decay allowed a comparison with reported activation energies of associated defects, clearly illustrating a strong connection between thermal contrast decay and the Hþ and Liþ-ion mobility. Finally, per- forming similar experiments on oxidized samples undoubtedly demonstrated that the origin of the reported luminescence contrast is strongly connected with lithium ions.
    Applied Physics Letters 09/2014; 12(105):122906. DOI:10.1063/1.4896579 · 3.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Two different structure types were found for a series of mononuclear dinitrosyl complexes of the general formula [RuL2(NO)2X]BF4 (L = monodentate phosphane, X = Cl, Br, I). The {Ru(NO)2}(8)-type target compounds were prepared by the reduction of the respective {RuNO}(6) precursors and subsequent oxidative addition of (NO)BF4. About one half of the new compounds share their molecular structure with the hitherto only representative of this class of dinitrosyls, Pierpont and Eisenberg's [RuCl(NO)2(PPh3)2]PF6·C6H6 (Inorg. Chem., 1972, 11, 1088-1094). The Cs-symmetric cations exhibit both a linear and a bent Ru-N-O fragment, in line with a formal 6 + 2 split of the {Ru(NO)2}(8) electron sum in the sense of a [Ru(II)(NO(+))((1)NO(-))](2+) bonding. The coordination entity's configuration in this subgroup is described by IUPAC's polyhedral symbol SPY-5. Continuous shape measures (CShM) as defined by Alvarez et al. (Coord. Chem. Rev., 2005, 249, 1693-1708) reveal a uniform deviation from the L-M-L angles expected for SPY-5, in a narrower sense, towards a vacant octahedron (vOC-5). DFT calculations confirmed that Enemark and Feltham's analysis (Coord. Chem. Rev., 1974, 13, 339-406) of the electronic situation of the {Ru(NO)2}(8) group remains adequate. The same holds for the second subclass of new compounds the existence of which had been predicted in the same paper by Enemark and Feltham, namely C2v-symmetric, TBPY-5-type cations with two almost equally bonded nitrosyl ligands. In agreement with an 8 + 0 distribution of the relevant electrons, the formal [Ru(0)(NO(+))2](2+) entities are found for L/X couples that donate more electron density on the central metal. Two solid compounds (/, /) were found in both structures including the special case of the P(i)Pr3/Br couple /, which led to crystals that contained both structure types in the same solid. Conversely, four compounds showed a single form in the solid but both forms in dichloromethane solution in terms of the solutions' IR spectra. The irradiation of crystalline with blue laser light resulted in the photoisomerisation of, mainly, the bent (1)NO(-) ligand in terms of low-temperature IR spectroscopy.
    Dalton Transactions 07/2014; 43(35). DOI:10.1039/c4dt01506a · 4.20 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on differentiating antiparallel ferroelectric domains in congruent Mg-doped LiNbO3 (Mg:LNO) single crystals through a multiphoton photoluminescence technique. Sample illumination with femtosecond laser pulses at λ = 790 nm results in a broad multiphoton emission spectrum revealing a domain contrast of >3% between virgin and inverted domains. The contrast decreases via annealing and shows an exponential decay in the temperature range from 80 to 150 °C. Our findings give clear ground of a thermally induced structural change by surpassing a specific activation energy. Hence, the reported contrast dynamics must be closely connected to the thermal activation of charged defects, which dramatically alters the internal bias field of these defects. This explanation is also supported when using single crystal LNO of different Mg doping levels showing much lower multiphoton effects for a < 5% Mg concentration. Based on this effect of multiphoton luminescence, it becomes easy to microscopically monitor and quantify virgin and switched domains in LNO and other samples.
    Journal of Applied Physics 06/2014; 115(21):213509-213509-5. DOI:10.1063/1.4881496 · 2.19 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Cherenkov second-harmonic generation (CSHG) is a powerful tool for three-dimensional domain wall profiling in ferroic bulk crystals. Here, we apply this noninvasive technique for tracking head-to-head charged domain walls (CDWs) across millimeter-thick ferroelectric single-crystalline lithium niobate. CSHG sensitively reveals the inclination α >0 of any such CDW with a superb optical resolution. Moreover, we deduce fully charged head-to-head CDWs (α = 90∘) to be much rougher and to show protrusions, domain inclusions, and novel topologies. Our findings provide insight into the mechanisms of electron transport and charge trapping in CDWs, as is mandatory for their use in prospective nanoelectronic devices.
    Physical Review B 01/2014; 89(3):035314. DOI:10.1103/PhysRevB.89.035314 · 3.74 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The structure of the title compound is redetermined by single crystal XRD at 100 K (monoclinic, P2/n, Z = 2).
    ChemInform 11/2013; 44(47):no-no. DOI:10.1002/chin.201347006
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present an experimental approach to study the structure and dynamics of molecular functional complexes in porous host matrices. Combining the results of Solid-State NMR and pair distribution function analysis based on total X-ray scattering data the structural arrangement and dynamical behaviour of Na2[Fe(CN)5NO]$2H2O (SNP) embedded in amorphous SiO2 matrix is investigated. We show that the SNP complexes are embedded as isolated complexes within the SiO2 pores and propose a structural model for the cation and anion arrangement. Additionally the NMR results demonstrate the rotational dynamics of the SNP complexes.
    RSC Advances 10/2013; 3(48):26132-26141. DOI:10.1039/c3ra45347b · 3.84 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The structure of the photoluminescent compound hexaaquadichloridoneodymium(III) chloride has been redetermined from single-crystal X-ray diffraction data at 100 K, with the aim of providing an accurate structural model for the bulk crystalline material. The crystal structure may be described as a network of [NdCl2(H2O)6](+) cations with distorted square-antiprism geometry around the Nd(III) centre. The Nd(III) cation and the nonbonded Cl(-) anion are both located on twofold symmetry axes. The crystal packing consists of three different neodymium pairs linked by a three-dimensional network of O-H...Cl intermolecular interactions. The pair distribution function (PDF) calculated from the experimentally determined structure is used for the discussion of the local structure.
    Acta Crystallographica Section C Crystal Structure Communications 09/2013; 69(Pt 9):1002-5. DOI:10.1107/S0108270113021367 · 0.54 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: UV–Vis- and infrared femtosecond spectroscopy makes it possible to reveal all different steps of photochemical reactions after the electronic excitation. The electronic relaxations are observed in the UV–Vis spectral range whereas the nuclear motions are monitored in the infrared spectral range. We used femtosecond time-resolved infrared spectroscopy to demonstrate the photoisomerization of the NO ligand photoinduced by a visible femtosecond pulse in a Na2[Fe(CN)5NO]·2H2O single crystal occurs in about 350 fs. The analysis of data makes it possible to unravel the mechanism leading to the photoisomerization of the NO ligand.
    Chemical Physics Letters 11/2012; 552:64–68. DOI:10.1016/j.cplett.2012.09.071 · 1.99 Impact Factor
  • Source
  • Source
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ferroic materials play an increasingly important role in novel (nano)electronic devices. Recently, research on domain walls (DWs) receives a big boost by the discovery of DW conductivity (DWC) in BiFeO3 and Pb(ZrxTi1-x)O3 ferroic thin films. Here, it is demonstrated that DWC is not restricted to thin films, but equally applies to millimeter-thick wide-bandgap, ferroic single crystals, such as LiNbO3. In this material transport along DWs can be switched by super-bandgap illumination and tuned by engineering the tilting angle of DWs with respect to the polar axis. The results are consistently obtained using conductive atomic force microscopy to locally map the DWC and macroscopic contacts, thereby in addition investigating the temperature dependence, DW transport activation energies, and relaxation behavior.
    Advanced Functional Materials 09/2012; 22(18). DOI:10.1002/adfm.201290110 · 11.81 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ferroic materials play an increasingly important role in novel (nano)electronic devices. Recently, research on domain walls (DWs) receives a big boost by the discovery of DW conductivity (DWC) in BiFeO3 and Pb(ZrxTi1-x)O3 ferroic thin films. Here, it is demonstrated that DWC is not restricted to thin films, but equally applies to millimeter-thick wide-bandgap, ferroic single crystals, such as LiNbO3. In this material transport along DWs can be switched by super-bandgap illumination and tuned by engineering the tilting angle of DWs with respect to the polar axis. The results are consistently obtained using conductive atomic force microscopy to locally map the DWC and macroscopic contacts, thereby in addition investigating the temperature dependence, DW transport activation energies, and relaxation behavior.
    Advanced Functional Materials 09/2012; 22(18):3936-3944. DOI:10.1002/adfm.201201174 · 11.81 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In mononitrosyl complexes of transition metals two long-lived metastable states corresponding to linkage isomers of the nitrosyl ligand can be induced by irradiation with appropriate wavelengths. Upon irradiation, the N-bound nitrosyl ligand (ground state, GS) turns into two different conformations: isonitrosyl O bound for the metastable state 1 (MS1) and a side-on nitrosyl conformation for the metastable state 2 (MS2). Structural and spectroscopic investigations on [RuCl(NO)py(4)](PF(6))(2)·1/2H(2)O (py = pyridine) reveal a nearly 100% conversion from GS to MS1. In order to identify the factors which lead to this outstanding photochromic response we study in this work the influence of counteranions, trans ligands to the NO and equatorial ligands on the conversion efficiency: [RuX(NO)py(4)]Y(2)·nH(2)O (X = Cl and Y = PF(6)(-) (1), BF(4)(-) (2), Br(-)(3), Cl(-) (4); X = Br and Y = PF(6)(-) (5), BF(4)(-) (6), Br(-)(7)) and [RuCl(NO)bpy(2)](PF(6))(2) (8), [RuCl(2)(NO)tpy](PF(6)) (9), and [Ru(H(2)O)(NO)bpy(2)](PF(6))(3) (10) (bpy = 2,2'-bipyridine; tpy = 2,2':6',2"-terpyridine). Structural and infrared spectroscopic investigations show that the shorter the distance between the counterion and the NO ligand the higher the population of the photoinduced metastable linkage isomers. DFT calculations have been performed to confirm the influence of the counterions. Additionally, we found that the lower the donating character of the ligand trans to NO the higher the photoconversion yield.
    Inorganic Chemistry 07/2012; 51(14):7492-501. DOI:10.1021/ic202702r · 4.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Mono- and dianionic oxocarboxylate Ru-Mn12 clusters, [RuNO(en)2Cl]-[Mn12O12(O2CR)16(H2O)4]2 (R = Ph (1)), [RuNO(en)2Cl][Mn12O12(O2CR)16(H2O)4] (R = C6F5 (2), CH2Cl (3), R = CHCl2 (4)), and [RuNO(NH3)4OH][Mn12O12(O2CR)16(H2O)4] (R = CH2Cl (5), CHCl2 (6)) containing photochromic mononitrosyl ruthenium complexes as cations were synthesized. The magnetic properties of the complexes in static and alternating magnetic fields were studied, and compounds 1–6 were found to be molecular nanomagnets. By IR spectroscopy, they were shown to exhibit also photochromic properties.
    Russian Chemical Bulletin 06/2012; 60(6). DOI:10.1007/s11172-011-0170-3 · 0.51 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We study the properties of photoinduced metal-nitrosyl linkage isomers in sodium nitroprusside (SNP) as a function of particle size. By embedding the molecular complex at various concentrations into mesopores of silica xerogels the size of the particles can be adjusted. The ground state is characterized by X-ray diffraction, absorption and infrared spectroscopy. The physical properties of the photoswitched molecules were analysed by steady-state low-temperature absorption, infrared spectroscopy and by nanosecond transient absorption spectroscopy. The electronic structure as well as the activation energies of the metastable linkage isomers are independent of the particle size down to single isolated molecules, indicating that the SNP complexes are quasi-free inside the pores of the gel.
    Physical Chemistry Chemical Physics 02/2012; 14(11):3775-81. DOI:10.1039/c2cp23607a · 4.20 Impact Factor

Publication Stats

1k Citations
239.46 Total Impact Points

Institutions

  • 2012–2014
    • Technische Universität Dresden
      Dresden, Saxony, Germany
  • 2013
    • University of Lorraine
      Nancy, Lorraine, France
  • 1992–2010
    • University of Cologne
      • • I. Institute of Physics
      • • Institute of Physics
      • • Institute of Geology and Mineralogy
      • • Institute of Crystallography
      Köln, North Rhine-Westphalia, Germany
  • 2006–2009
    • University of Bonn
      • Physics Institute
      Bonn, North Rhine-Westphalia, Germany