Paul Müller

Wet Chemical Synthesis of Graphene

A suitable technology for the preparation of graphene based on versatile wet chemistry is presented for the first time. The protocol allows the wet chemical synthesis of graphene from a new form of graphene oxide that consists of an intact hexagonal σ-framework of C-atoms. Thus, it can be easily reduced to graphene that is no longer dominated by defects.

Spin-State Patterns in Surface-Grafted Beads of Iron(II) Complexes

Novel strategies for the design of functional materials are in increasing demand, as the down-scaling of lithographic processes (the top-down approach) will soon encounter the fundamental physical limits of miniaturization. One of the fascinating perspectives of molecular electronics is information storage at the single-molecule level, on the basis of arrays of molecular switches. Spin-crossover (SCO) compounds hold considerable potential in this context. SCO can occur in octahedral transition-metal complexes in which the metal ion has a d to d electron configuration. The transition may be stimulated externally, by a change in temperature or pressure, or by irradiation. SCO is entropydriven and, in the solid state, is influenced strongly by intermolecular interactions, such as hydrogen bonding or p–p stacking. Such interactions give rise to cooperativity between SCO complexes within the ensemble. High cooperativity can cause the change in spin state to be accompanied by hysteresis, which confers bistability on the system and thus a memory effect. A viable reading/writing procedure, that is, a means of reproducible actuation on the single-molecule level, is a formidable challenge that has yet to be met, but in this way SCO compounds could serve in devices of unsurpassable storage density. In principle, reliable information storage could be achieved even in the absence of hysteresis, provided the energy difference between low-spin state and high-spin state of the complexes within the SCO ensemble is sufficiently large (on the order of several kT). A large number of spin-crossover systems are known, with complexes of iron(II) the most numerous, both in solution and in the solid state. Usually, ferrous iron is in a quasi-octahedral N6 coordination environment, and switching occurs between a low-spin (LS, A1g/t2g , S= 0) and a high-spin state (HS, T2g/ t2g eg , S= 2). SCO systems have been characterized by physical techniques including M ssbauer and UV/Vis spectroscopy, magnetic susceptibility measurements, and diffraction methods, applied to the bulk solids. Many attempts have been made to obtain SCO materials in the form of thin films, multilayers, or nanocrystals. Recent strategies include the sequential assembly of coordination polymers on metal or biopolymer supports (such as gold or chitosan) and the preparation in polymeric matrices, in surface-grown multilayer thin films incorporating iron(II) coordination polymers, and in nanoparticulate iron(II) complexes. Our approach is to use spin-switchable iron(II) complexes of bis(pyrazolyl)pyridine ligands, with a variety of substituents that can serve as surface anchors depending on the kind of substrate. We studied the spin state of adsorbates at the single-molecule level with scanning tunneling microscopy (STM) techniques at room temperature (298 K). For the present study, we chose [Fe(L)2](BF4)2 (1; L= ligand), whose synthesis, solid-state structure, and spin behavior have been reported in detail. The solid-state structure of the dication in 1 is shown in Figure 1a. The magnetic susceptibility of 1

The {FeIII[FeIII(L1)2]3} star-type single-molecule magnet

Star-shaped complex {FeIII[FeIII(L1)2]3} (3) was synthesized starting from N-methyldiethanolamine H2L1 (1) and ferric chloride in the presence of sodium hydride. For 3, two different high-spin iron(III) ion sites were confirmed by Mossbauer spectroscopy at 77 K. Single-crystal X-ray structure determination revealed that 3 crystallizes with four molecules of chloroform, but, with only three molecules of dichloromethane. The unit cell of 3·4CHCl3 contains the enantiomers (Δ)-[(S,S)(R,R)(R,R)] and (Λ)-[(R,R)(S,S)(S,S)], whereas in case of 3·3CH2Cl2 four independent molecules, forming pairs of the enantiomers [Λ-(R,R)(R,R)(R,R)]-3 and [Δ-(S,S)(S,S)(S,S)]-3, were observed in the unit cell. According to SQUID measurements, the antiferromagnetic intramolecular coupling of the iron(III) ions in 3 results in a S = 10/2 ground state multiplet. The anisotropy is of the easy-axis type. EPR measurements enabled an accurate determination of the ligand-field splitting parameters. The ferric star 3 is a single-molecule magnet (SMM) and shows hysteretic magnetization characteristics below a blocking temperature of about 1.2 K. However, weak intermolecular couplings, mediated in a chainlike fashion via solvent molecules, have a strong influence on the magnetic properties. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) were used to determine the structural and electronic properties of star-type tetranuclear iron(III) complex 3. The molecules were deposited onto highly ordered pyrolytic graphite (HOPG). Small, regular molecule clusters, two-dimensional monolayers as well as separated single molecules were observed. In our STS measurements we found a rather large contrast at the expected locations of the metal centers of the molecules. This direct addressing of the metal centers was confirmed by DFT calculations.

Statistical Raman Microscopy and Atomic Force Microscopy on Heterogeneous Graphene Obtained after Reduction of Graphene Oxide

Graphene oxide can be used as a precursor to graphene, but the quality of graphene flakes is highly heterogeneous. Scanning Raman microscopy (SRM) is used to characterize films of graphene derived from flakes of graphene oxide with an almost intact carbon framework (ai-GO). The defect density of these flakes is visualized in detail by analyzing the intensity and full width at half-maximum of the most pronounced Raman peaks. In addition, we superimpose the SRM results with AFM images and correlate the spectroscopic results with the morphology. Furthermore, we use the SRM technique to display the amount of defects in a film of graphene. Thus, an area of 250 × 250 μm2 of graphene is probed with a step-size increment of 1 μm. We are able to visualize the position of graphene flakes, edges and the substrate. Finally, we alter parameters of measurement to analyze the quality of graphene in a fast and reliable way. The described method can be used to probe and visualize the quality of graphene films.

A new family of 1D exchange biased heterometal single-molecule magnets: observation of pronounced quantum tunneling steps in the hysteresis loops of quasi-linear {Mn2Ni3} clusters.

First members of a new family of heterometallic Mn/Ni complexes [Mn(2)Ni(3)X(2)L(4)(LH)(2)(H(2)O)(2)] (X = Cl: 1; X = Br: 2) with the new ligand 2-{3-(2-hydroxyphenyl)-1H-pyrazol-1-yl}ethanol (H(2)L) have been synthesized, and single crystals obtained from CH(2)Cl(2) solutions have been characterized crystallographically. The molecular structures feature a quasi-linear Mn(III)-Ni(II)-Ni(II)-Ni(II)-Mn(III) core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. 1 and 2 exhibit quite strong ferromagnetic exchange interactions throughout (J(Mn-Ni) ≈ 40 K (1) or 42 K (2); J(Ni-Ni) ≈ 22 K (1) or 18 K (2)) that lead to an S(tot) = 7 ground state, and a sizable uniaxial magnetoanisotropy with D(mol) values -0.55 K (1) and -0.45 K (2). These values are directly derived also from frequency- and temperature-dependent high-field EPR spectra. Slow relaxation of the magnetization at low temperatures and single-molecule magnet (SMM) behavior are evident from frequency-dependent peaks in the out-of-phase ac susceptibilities and magnetization versus dc field measurements, with significant energy barriers to spin reversal U(eff) = 27 K (1) and 22 K (2). Pronounced quantum tunnelling steps are observed in the hysteresis loops of the temperature- and scan rate-dependent magnetization data, but with the first relaxation step shifted above (1) or below (2) the zero crossing of the magnetic field, despite the very similar molecular structures. The different behavior of 1 and 2 is interpreted in terms of antiferromagnetic (1) or ferromagnetic (2) intermolecular interactions, which are discussed in view of the subtle differences of intermolecular contacts within the crystal lattice.

Spin Crossover in a Vacuum-Deposited Submonolayer of a Molecular Iron(II) Complex

Spin-state switching of transition-metal complexes (spin crossover) is sensitive to a variety of tiny perturbations. It is often found to be suppressed for molecules directly adsorbed on solid surfaces. We present X-ray absorption spectroscopy measurements of a submonolayer of [Fe(II)(NCS)2L] (L: 1-{6-[1,1-di(pyridin-2-yl)ethyl]-pyridin-2-yl}-N,N-dimethylmethanamine) deposited on a highly oriented pyrolytic graphite substrate in ultrahigh vacuum. These molecules undergo a thermally induced, fully reversible, gradual spin crossover with a transition temperature of T1/2 = 235(6) K and a transition width of ΔT80 = 115(8) K. Our results show that by using a carbon-based substrate the spin-crossover behavior can be preserved even for molecules that are in direct contact with a solid surface.

Enhanced Macroscopic Quantum Tunneling in Bi~2Sr~2CaCu~2O~8~+~d~e~l~t~a Intrinsic Josephson-Junction Stacks

We have investigated macroscopic quantum tunneling in Bi(2)Sr(2)CaCu(2)O(8 + delta) intrinsic Josephson junctions at millikelvin temperatures using microwave irradiation. Measurements show that the escape rate for uniformly switching stacks of Nu junctions is about Nu(2) times higher than that of a single junction having the same plasma frequency. We argue that this gigantic enhancement of the macroscopic quantum tunneling rate in stacks is boosted by current fluctuations which occur in the series array of junctions loaded by the impedance of the environment.

Interface screening and imprint in poly(vinylidene fluoride/trifluoroethylene) ferroelectric field effect transistors

We investigated the imprint effect in ferroelectric capacitors and field effect transistors (FETs) with a poly(vinylidene fluoride/trifluoroethylene) [P(VDF-TrFE)] ferroelectric insulator. The shift in switching voltages and the change in the ferroelectric FET (FeFET) channel conductance were measured as a function of time and the thickness of the ferroelectric layer. Analyzing our experimental data, we show that the imprint originates from interface-induced processes, which effectively screen polarization charges in P(VDF-TrFE). This phenomenon significantly influences the retention of FeFET channel conductance and the memory functionality of FeFET with P(VDF-TrFE).

Self-assembly and layer-by-layer deposition of metallosupramolecular perylene bisimide polymers

2,2′:6′,2″-Terpyridine (tpy)-functionalized red and green perylene bisimide (PBI) building blocks were synthesized, and their metal-ion-directed self-assembly was studied in detail by 1H NMR, DOSY NMR, and UV-vis spectroscopy. These studies revealed that the newly synthesized ditopic bis(tpy)-PBI ligands, in which the tpy units are directly connected to PBI moieties at the imide positions, form supramolecular coordination polymers upon addition of one equivalent of Zn(II) ions. Atomic force microscopy (AFM) investigations confirmed the formation of extended, rigid polymers with average chain lengths of up to 30–35 repeat units. These coordination polymers create closely packed films with linear arrangement on mica and HOPG surfaces. The Zn(II) centers were mapped by applying current imaging tunnelling spectroscopy (CITS) to the red-coloured polymer deposited on graphite surfaces. An enhanced local density of unoccupied states close to the Fermi energy was observed in positions with metal ions. Alternate multilayer films of two different coordination polymers were prepared by the layer-by-layer deposition technique and surface densities Γ of perylene bisimide chromophores in these films were determined.

Magnetic anisotropy of a supramolecular Cu(II) [3×3] grid

Abstract The magnetic anisotropy of the nona-Cu(II) [3×3] grid structure [Cu 9 (2POAP–H) 6 ](NO 3 ) 12 ·9H 2 O was investigated by means of torque magnetometry. The analysis of the high-field part of torque vs. field curves measured at 1.75 K allowed a precise determination of all anisotropy parameters of the ground state. These could be related to the magnetic parameters of the microscopic Hamiltonian by a suitable perturbation theoretical treatment. We found strong evidence for a sizeable anisotropic exchange interaction. The observed anisotropy of the g -factor could not be explained by simple ligand field arguments which is attributed to the unusual quasi-octahedral coordination environment of the Cu ions.