Low-dimensional Solids Research Articles

Photoemission as a probe of coexisting and conflicting periodicities in low-dimensional solids

When two different periodic potentials are present at the same time in a solid, the electron wavefunctions must conform to the resulting overall periodicity. It is the case of the broken-symmetry phases which are often observed in low-dimensional systems. The rearrangement of the electronic states has some interesting and perhaps unexpected consequences on the momentum distribution of the spectral weight, which can be measured in an ARPES experiment.

Toward Systematic Understanding of Diversity of Electronic Properties in Low-Dimensional Molecular Solids

Toward Systematic Understanding of Diversity of Electronic Properties in Low-Dimensional Molecular Solids

Interface directed assembly of cyanide-bridged Fe–Co and Fe–Mn square grid networks

Two isostructural square grid networks were formed by reacting a Langmuir monolayer of an amphiphilic pentacyanoferrate (3+) complex with aqueous Co 2+ or Mn 2+ ions dissolved in the subphase. Confinement of the reactants to the air–water interface discriminates against the formation of higher dimensional products and directs the lateral propagation of a polymeric two-dimensional cyanide-bridged network. The network can be transferred to a variety of supports to form monolayer or multilayer lamellar films by the Langmuir–Blodgett (LB) technique. Characterization of both the Fe–Co and Fe–Mn LB film extended networks by FTIR spectroscopy, SQUID magnetometry, and grazing incidence synchrotron X-ray diffraction (GIXD) reveals face-centered square grid structures directed by the defined bond angles of the octahedral metal complexes and the linear cyanide bridges. Magnetic measurements on both samples indicate the presence of anisotropic low temperature magnetic exchange interactions between the paramagnetic centers. The results illustrate the potential utility of an interface as a structure director in the assembly of low dimensional coordinate covalent network solids.

Fermi liquid and non-Fermi liquid spectral lineshapes in low-dimensional solids

We exploited angle-resolved photoemission (ARPES) to investigate the nature of the elementary quasiparticle (QP) excitations in selected low-dimensional compounds. In the model quasi two-dimensional conductor TiTe 2 we observe spectral lineshapes which are consistent with a Fermi liquid (FL) scenario, and the usual QP scattering mechanisms. In contrast, in typical quasi one-dimensional Peierls systems like TTF–TCNQ, (TaSe 4) 2I and K 0.3MoO 3, we find clear non-FL lineshapes. We briefly discuss the possible origins of these unusual properties.

Supramolecular assembly at interfaces: formation of an extended two-dimensional coordinate covalent square grid network at the air-water interface.

Reaction of a Langmuir monolayer of an amphiphilic pentacyanoferrate(3+) complex with Ni(2+) ions from the subphase results in the formation of a two-dimensional iron-nickel cyanide-bridged network at the air-water interface. The network can be transferred to various supports to form monolayer or multilayer lamellar films by the Langmuir-Blodgett (LB) technique. The same network does not form from homogeneous reaction conditions. Therefore, the results demonstrate the potential utility of an interface as a structure director in the assembly of low dimensional coordinate covalent network solids. Characterization of the LB film extended networks by X-ray photoelectron spectroscopy (XPS), FT-IR spectroscopy, SQUID magnetometry, X-ray absorption fine structure (XAFS), and grazing incidence synchrotron X-ray diffraction (GIXD) revealed a face-centered square grid structure with an average domain size of 3600 A(2). Magnetic measurements indicated that the network undergoes a transition to a ferromagnetic state below a T(c) of 8 K.

Electron paramagnetic resonance studies of dimensionality of polyaniline films

The line shape analysis of electron paramagnetic resonance (EPR) spectra for low-dimensional solids are calculated. A comparison is carried out between the experimental EPR results of polyaniline samples and theoretical calculation. The dimension evolution of the samples from nonconducting to conducting states is obtained. It is found that the dimensional properties determined by EPR are in good accordance with those obtained using previous ellipsometric methods.

Determination of fractional dimension of low-dimensional solids by electron paramagnetic resonance

We propose a new concept of the determination of the fractional dimension of low-dimensional solids by electron paramagnetic resonance (EPR). First, we present the model and the theory for determining the fractional dimension by EPR. The line shapes of EPR for different fractional–dimensional solids were calculated. Second, we describe the result of the EPR measurement of polyaniline samples with different conductivity, and the determination of the values of the dimension. This result is compared with that determined by the spectroscopic ellipsometry.

Molecular and crystal engineering of a new class of inorganic cadmium-thiocyanate polymers with host–guest complexes as organic spacers, controllers, and templates

This review provides an account of the structures of polymeric anionic cadmium-thiocyanate coordination solids with cationic crown-ether–metal complexes as spacers, controllers, and templates. Specifically, depending upon the size, shape, symmetry, and charge of the cationic host–guest complexes, this highly interesting series of coordination solids gives rise to a wide variety of structures, ranging from one-dimensional (1-D) single-chain structures (as exemplified by [(18C6)K][Cd(SCN) 3] and [(18C6) 2Na 2(H 2O) 2] 1/2[Cd(SCN) 3]), to 1-D double-chain structures (as observed in [(15C5) 2K][Cd 2(SCN) 5] and [(15C5) 2Na 2(H 2O) 2] 1/2[Cd 2(SCN) 5]), to two-dimensional (2-D) layered structures (as observed in [(12C4) 2Cd][Cd 2(SCN) 6]and [(12C4) 2Cd][Cd 3(SCN) 8]). These low-dimensional coordination solids exhibit highly anisotropic physical properties such as nonlinear optical (NLO) behavior. The arrangement and/or alignment of these polymeric cadmium-thiocyanates can be controlled and/or induced by the host–guest complexes. In this regard, for 1-D single- or double-chain cadmium-thiocyanate coordination solids, the cations serve as the spacer/controller, dictating the crystal structure and symmetry, thereby giving rise to desirable properties of these crystals such as nonlinear optical (NLO) behavior (as observed in [(18C6)K][Cd(SCN) 3]). In the case of 2-D cadmium-thiocyanate coordination solids, the cationic complex such as [(12C4) 2Cd] 2+ serves not only as the ‘spacer’ and ‘controller’ of the crystal packing, but also as an anisotropic ‘template’ for the formation of various layered structures with highly interesting motifs. It is hoped that the inorganic polymers with organic spacers (IPOS) concept will not only lead to new and interesting materials but also to the development of new strategies in materials fabrication at the molecular engineering level and the discovery of new patterns of crystallization at the crystal engineering level. Furthermore, the general strategies and structural principles developed here can also be extended to other IPOS systems.

Behaviour of the one-dimensional, inorganic polymer 1∞[MPS4]− anions (M=Ni, Pd) in organic solutions

The behaviour of the KMPS 4 (M=Ni, Pd) salts and some solid-solution phases, KNi x Pd 1–x PS 4 , when dissolved in dimethylformamide (DMF) have been studied. For KNiPS 4 -DMF the novel re-arranged molecular bowl-like [Ni 3 P 3 S 12 ] 3– trianion can be formed from solutions that have been heated at 323 K for 2 days and K + exchanged by larger organic cations such as PPh 4 + (tetraphenylphosphonium), TMA + (tetramethylammonium), TEA + (tetraethylammonium), and MEM + (methylethylmorpholinium). This remarkable dispersion/auto-fragmentation/re-arrangement sequence extends the concept of mere excision in solution of monomeric molecular motifs from low-dimensional inorganic solids. Investigations by electrospray mass spectrometry (ESMS), 31 P solution-state NMR and optical microscopy under polarized light of KNiPS 4 in DMF have shown that chains of[MPS 4 ] – exist intact in freshly prepared solutions. These solutions show transient birefringence under stress. ESMS and NMR studies of KNiPS 4 solutions show that the [NiPS 4 ] – chains are quickly broken down at ambient temperature and that cyclic [Ni 3 P 3 S 12 ] 3– trianions are formed. By contrast, the [PdPS 4 ] – chain remains intact at temperatures up to 323 K. This behavioural difference can be understood in terms of the Ni-S and Pd-S bond strengths. ESMS spectra of aged KNi x Pd 1–x PS 4 /DMF solutions indicate that mixed metal trianions are formed, such as [Ni 2 PdS 12 ] 3– , [NiPd 2 P 3 S 12 ] 3– and [Ni 3 P 3 S 12 ] 3– , but no [Pd 3 P 3 S 12 ] 3– species have yet been observed.

Scanning-probe-based science and technology.

Scanning-probe-based science and technology.

An embedded ring approach to the vibrational dynamics of low-dimensional amorphous solids with applications to graphitic carbon materials

A theoretical approach has been developed to model the vibrational modes of amorphous, two-dimensional materials. The method considers that the vibrational density of states is composed primarily of states originating from embedded ring structures of medium-range order. The materials are modeled as continuous random networks comprised of a statistical distribution of symmetric, planar rings with four to eight members. The rings are treated as local structural units embedded in the material, similar to molecules within a solid. The ring potentials are approximated with a valence force model (bond-stretching and bond-angle-bending force constants) modified by a third harmonic, effective force constant coupling the rings to the surrounding network. The molecular dynamics of the rings are analyzed with the use of group theory, and the frequencies are calculated using a normal coordinate treatment. The utility of the embedded ring approach lies in its use of the material's ring statistics and ring mode frequencies, allowing determination of the structure of an amorphous material from its vibrational spectrum or prediction of vibrational spectra and density of states from structural models. The method is applied here to various graphitic carbon materials as a test case and predicts a set of force constants and Raman spectra consistent with previous data and structural models.

Low-frequency transport and relaxation of charge density waves in (TaSe4)2I crystals

A charge density wave (CDW) occurs in low-dimensional solids as a consequence of the Peierls transition. Various field- and frequency-dependent effects, as well as those of excess noise, are related to CDW transport. We investigate low-frequency (10-10 6 Hz) dielectric relaxation as well as voltage fluctuations as a result of internal dynamic or sliding CDW in quasi one-dimensional chain compound (TaSe4)21 single crystals. The activation energy for normal conduction obtained from low field resistivity was ΔE = 0.122 ± 0.004eV. The most useful expression of dielectric relaxation for a strong damped system with one degree of freedom is e(ω) α(1 + iωτ 0 ) -1 , where relaxation times are distributed at the average τ 0 . τ 0 versus 1/T shows thermal activation ΔE = 0.124 ± 0.004eV. Voltage fluctuations produced by a sliding CDW are found to originate from random thermally activated transitions between metastable states. Noise power spectra between 500Hz and 600kHz at temperatures between 165 and 215K have S(f) α f -α dependence. The distribution of relaxation times and the temperature-dependent shortest relaxation time reflect CDW internal dynamics and the importance of free carriers on it.

Probing complex low-dimensional solids with scanning probe microscopes: From charge density waves to high-temperature superconductivity

Scanning probe microscopy (SPM) studies of low-dimensional solids have yielded significant advances in the understanding of collective phenomena in these complex materials. In this article, SPM studies from the authors’ laboratory of charge density waves (CDWs) in metal dichalcogenide materials and of high-temperature copper–oxide superconductors (HTSCs) are reviewed. Temperature-dependent SPM studies of the structural properties of CDW phases in metal-doped tantalum disulfide (TaS2) and tantalum diselenide (TaSe2) have directly illuminated the problem of weak and strong CDW pinning. These studies have also led to the unexpected discovery of a hexatic CDW phase in niobium-doped TaS2. SPMs have also been used to probe the structural and electronic properties of HTSCs. SPM studies of metal-doped Bi2Sr2CaCu2O8 (BSCCO) superconductors have been used to elucidate the complex crystal chemistry of this system. Comparisons of these data with electron diffraction measurements also highlight the importance of local crystallography in these complex materials. In addition, SPM studies have been used to elucidate the interaction of magnetic flux lines with surface defects, thus enabling a quantitative assessment of pinning by surface roughness. Finally, etching has been studied by in situ SPM and shown to be a promising means for controlling the properties of HTSC surfaces.

New Approach to Cofacially Assembled Partially Oxidized Metal Phthalocyanine Low Dimensional Solids: Synthesis, Structure, and Electrical Conductivity Properties of the Fe(IV) Containing Species [(PcFe)2C](I3)0.66 Obtained by I2 Doping of .mu.-Carbido-bis[phthalocyaninatoiron(IV)

New Approach to Cofacially Assembled Partially Oxidized Metal Phthalocyanine Low Dimensional Solids: Synthesis, Structure, and Electrical Conductivity Properties of the Fe(IV) Containing Species [(PcFe)2C](I3)0.66 Obtained by I2 Doping of .mu.-Carbido-bis[phthalocyaninatoiron(IV)

Infrared Detectors - New Trends

Recent trends in infrared detectors are towards large, electronically ad- dressed two-dimensional arrays and higher operating temperature. This will lead both to higher performance and to smaller, lighter and more afford- able IR systems. Cooling requirements are the main obstacle to the more widespread use of infrared systems based on semiconductor photodetectors, particularly in the civil field. Fundamental limitations to performance of IR photodetectors are due to the statistical nature of generation—recombination processes and resulting noise. Ways to overcome the limitations are discussed including use of the optical immersion and optical resonant cavity. Finally, the progress in a mode of operation is presented, in which the thermal gen- eration of carriers is suppressed by the use of stationary depletion of narrow gap semiconductors. The practical near room temperature narrow gap semi- conductor photodetectors are reported. Competitive technologies based on Schottky barrier devices and low dimensional solids are considered briefly.

LOW-DIMENSIONAL ELECTRONIC STATES AT METAL SURFACES: QUANTUM WELLS AND QUANTUM WIRES

Several possibilities of “engineering” low-dimensional solids on the atomic scale are discussed. The electronic and magnetic structure of such materials is explored for two classes, i.e., multilayers and “wires” attached to step edges. Magnetic multilayers represent a particularly promising case, since quantum effects have macroscopic consequences. Quantization perpendicular to the layers is connected with oscillatory magnetic coupling, which in turn is important for obtaining “giant” magnetoresistance. This effect is being applied towards the fabrication of magnetoresistive reading heads for magnetically stored data. Extensions towards lateral superlattices and quantum wires are explored, where a stepped surface acts as a template. It is found that electrons can be trapped at step edges, and level shifts of the order 0.5 eV are observed for atoms adsorbed at step edges.

Magnetic resonance of low dimensional magnetic solids

On montre l'utilite des la RPE et de la RMN pour etudier les composes solides magnetiques de basse dimensionalite. On rappelle brievement les bases de la Resonance Magnetique dans ces systemes, et on montre l'importance de la diffusion de spin et de l'anisotropie magnetique. On presente quelques resultats obtenus sur des chaines metal-radical et sur des clusters

Electron-Phonon Vs. Electron-Electron Interactions in Low Dimensional Solids

Abstract We present a general method to investigate the effect of electron-phonon coupling on the vibratlonal spectra of low-dimensional solids. The method, originally developed to deal with charge transfer crystals, is extended to conjugated polymers and to metal-halogen chains. We show that quadratic electron-phonon coupling is essential to properly define the reference state and to determine sets of transferable e-ph coupling constants.

Electronic factors in the design and reactivity of low-dimensional solids

Electronic factors in the design and reactivity of low-dimensional solids

Low-dimensional solids: an interface between molecular and solid-state chemistry? The example of chainlike niobium and tantalum chalcogenides

The synthesis of new low-dimensioned solids relies on difficult chemistry that simultaneously takes into account many different factors, some of them largely conflicting. The design of chainlike niobium and tantalum chalcogenides reported here illustrates the experimental approaches that can be used to stabilize chainlike arrangements or to strengthen low-dimensional character when starting from a given compound.