Metal-organic System Research Articles

Field-induced phase transition in a metalorganic spin-dimer system—a potential model system to study Bose–Einstein condensation of magnons

We report on the results obtained from studying electron spin resonance, magnetic susceptibility, specific heat and thermal expansion experiments on a metalorganic spin-dimer system, C 36 H 48 Cu 2 F 6 N 8 O 12 S 2 (TK91). According to the first principle Density Functional Theory calculations, the compound represents a 3D-coupled dimer system with intradimer coupling J 1 / k B ∼ 10 K and interdimer couplings J 2 / k B ∼ J 3 / k B ∼ 1 K. The measurements have been performed on both pressed powder and single-crystal samples in external magnetic fields up to 12 T and at low temperatures down to ∼ 0.2 K. Susceptibility measurements reveal a spin-gap behavior consistent with the theoretical results. Furthermore, clear indications of a field-induced phase transition have been observed. A similar field-induced phase transition was also detected in an inorganic compound TlCuCl 3 and was interpreted as Bose–Einstein condensation (BEC) of magnons. The possibility of changing both the intradimer and interdimer couplings in TK91 by chemical substitutions makes the system a potentially good system to study BEC of magnons.

Exploring antiferromagnetic S = 1/2 dimer systems in high magnetic fields

We report on measurements of magnetic and thermodynamic properties on two different Cu2+-containing S = 1/2 dimer systems. One is the recently synthesized metal-organic system C36H48Cu2F6N8O12S2 (TK91). At temperatures T < 0.2K and fields around 6T, strong enough to close the spin gap to the excited triplet state, indications for a field-induced phase transition are observed making this system a potential candidate to study the Bose-Einstein condensation of magnons. The second system is the natural mineral Cu3(CO3)2(OH)2(azurite) which was identified as a distorted diamond chain–a realization of a 1D quantum frustrated system. We find a huge acoustic anomaly around 31T accompanying a magnetic soft mode.

Wurtzite ZnSe nanowires: growth, photoluminescence, and single-wire Ramanproperties

Wurtzite ZnSe nanowires were prepared on GaAs substrates in a metal-organic chemicalvapour deposition system. Electron microscopy shows that they are smooth and uniformin size. Both transmission electron microscopy and x-ray diffraction reveal thewurtzite structure of the nanowires, which grows along the direction. Raman scattering studies on individual nanowires were performed in theback-scattering geometry at room temperature. Besides the commonly observedlongitudinal and transverse optical phonon modes, a possible surface mode located at233 cm−1 is also observed in the Raman spectrum. A peak located at 2.841 eV was clearly observed inthe photoluminescence spectra of the nanowires, which can be assigned to near band edgeemissions of wurtzite ZnSe.

Low dark current metal-semiconductor-metal photodiodes based on semi-insulating GaN

Metal-semiconductor-metal photodetectors on semi-insulating GaN templates were demonstrated and compared with photodetectors fabricated on regular GaN templates. Samples were grown on a metal organic chemical vapor deposition system. Devices on semi-insulating template exhibited a dark current density of 1.96×10−10A∕cm2 at 50V bias, which is four orders of magnitude lower compared with devices on regular template. Device responsivities were 101.80 and 88.63A∕W at 50V bias for 360nm ultraviolet illumination for semi-insulating and regular templates, respectively. Incident power as low as 3pW was detectable using the devices that were fabricated on the semi-insulating template.

In situ pendeoepitaxy of GaN using heteroepitaxial AlGaN∕GaN cracks

Pendeoepitaxy on patterned templates has been proven to be efficient for reducing threading dislocation densities in GaN thin films. In this letter, we report on in situ crack-assisted pendeoepitaxy of GaN using spontaneously formed cracks in AlGaN∕GaN heterostructures. Our approach involves the growth of an AlGaN∕GaN template followed by in situ thermal etching and deposition of an amorphous silicon nitride mask in a low pressure metal organic chemical vapor deposition system. Microwirelike GaN seeds are then formed along the crack lines during the initial stage of GaN overgrowth, which act as nucleation stripes for epitaxial lateral overgrowth. Transmission electron microscopy revealed that the lateral overgrowth of the wirelike GaN seeds effectively bends threading dislocations toward ⟨11¯00⟩ directions on the amorphous silicon nitride mask. The threading dislocation density by this method has been reduced from 2×109cm−2 in control samples to 2×108cm−2 in some parts and 5×107cm−2 in other parts of the GaN layer as determined by plan-view transmission electron microscopy which is very encouraging.

Precursor and processing effects on BaPbO3 formation kinetics

The synthesis of BaPbO3 from a wide range of mixtures containing metalorganic precursors, nitrate precursors, lead oxides, barium oxide and peroxide was investigated, and the kinetics was analyzed using the Johnson–Mehl–Avrami (JMA) equation. It was found that Ba and Pb stearate soaps and Pb oxalate that were used as metalorganic precursors formed BaCO3 and PbO or Pb3O4 after firing at 440 °C. The formation rate of BaPbO3 from a metalorganic precursor system is not higher than that from the conventional BaCO3–PbO system and does not depend on mixing methods or the kinds of metalorganic precursors but instead on the synthesis atmosphere. In the case of the BaCO3–PbO system, the Avrami exponent (n) is ∼1, indicating that the reaction is controlled by the phase-boundary-contraction interface reaction. For the BaO2–PbO2 system, n has two values ∼1 and ∼0.3, depending on the reaction temperature and time, indicating that the reaction is either controlled by the phase-boundary-contraction interface reaction or diffusion-controlled reaction. In the Ba nitrate–Pb nitrate system, phase-pure BaPbO3 is obtained at 550 °C, which is 250 °C lower than in the case of the BaCO3–PbO system. The value of n for the nitrate system is ∼1.5, indicating that the reaction is controlled by a three-dimensional (3D) diffusion-controlled nucleation mechanism. In the BaO–PbO system, the formation of BaPbO3 started at 350 °C by an exothermic reaction and the content of BaPbO3 in the product was ∼40 wt%, which is independent of reaction temperature as well as time in the temperature range of 350–500 °C.

Ge/GaAs heterostructure matrix detector

In this paper we present a novel germanium/gallium arsenide heterostructure X-ray detector with the active volume of germanium . The heterostructure is fabricated by depositing a gallium arsenide layer on a high-purity germanium wafer in a vertical metalorganic vapor-phase epitaxy system. This approach provides a new alternative to traditional lithium diffused n+ contact which is not easily applicable for finely pixelated detectors. The detector chip fabrication utilizing this kind of heterostructure is straightforward and only standard lithographic processes need to be applied. Electrical properties of the small format detector matrices are studied. Very low reverse biased current at 77 K is observed. It is concluded that the diffusion of arsenic in germanium results in an n-type germanium layer under the epitaxial gallium arsenide.

Defect influence on luminescence efficiency of GaN-based LEDs

Wafers with normal light-emitting diode structure were grown by metal organic chemical vapor deposition system. The pressure and temperature were varied during growth of buffer layer in order to grow different types of epilayers. The cathodoluminescence results show that the interface distortion of quantum well plays an important role in radiant efficiency. The electroluminescence detections indicate that the dislocations also influence the external quantum efficiency by lowering the electron injection efficiency.

Structural diversity and chemical trends in hybrid inorganic–organic framework materials

Hybrid framework compounds, including both metal-organic coordination polymers and systems that contain extended inorganic connectivity (extended inorganic hybrids), have recently developed into an important new class of solid-state materials. We examine the diversity of this complex class of materials, propose a simple but systematic classification, and explore the chemical and geometrical factors that influence their formation. We also discuss the growing evidence that many hybrid frameworks tend to form under thermodynamic rather than kinetic control when the synthesis is carried out under hydrothermal conditions. Finally, we explore the potential applications of hybrid frameworks in areas such as gas separations and storage, heterogeneous catalysis, and photoluminescence.

Thin films of metal-organic compounds and metal nanoparticle-embedded polymers for nonlinear optical applications

Thin films based on two very different metal-organic systems are developed and some nonlinear optical applications are explored. A family of zinc complexes which form perfectly polar assemblies in their crystalline state are found to organize as uniaxially oriented crystallites in vapor deposited thin films on glass substrate. Optical second harmonic generation from these films is investigated. A simple protocol is developed for thein-situ fabrication of highly monodisperse silver nanoparticles in a polymer film matrix. The methodology can be used to produce free-standing films. Optical limiting capability of the nanoparticle-embedded polymer film is demonstrated

Effects of growth pressure on AlGaN and Mg-doped GaN grown using multiwafer metal organic vapor phase epitaxy system

We investigated the effects of growth pressure on AlGaN growth and material properties of GaN:Mg using a multiwafer metal organic vapor phase epitaxy (MOVPE) reactor. We developed a three-layer laminar flow gas injection reactor to control the pre-reaction between adducts in GaN MOVPE. Using this reactor, we could grow Al 0.09Ga 0.91N at a growth rate of 0.8 μm/h, and GaN:Mg with a carrier concentration of 1.4×10 18 cm −3 at a growth rate of 3.5 μm/h at atmospheric pressure. While we grew Al 0.24Ga 0.76N at a growth rate of 0.8 μm/h at 300 Torr, we found that the hole carrier concentration of GaN:Mg grown at 300 Torr is more than one order of magnitude lower than that of GaN:Mg grown at atmospheric pressure.

Design of Extended Surface-Supported Chiral Metal-Organic Arrays Comprising Mononuclear Iron Centers

A design strategy for fabricating a surface-supported chiral metal-organic system comprising a regular arrangement of mononuclear iron centers and nanocavities is presented. By sequential deposition of 1,2,4-benzenetricarboxylic acid (tmla) molecules and iron atoms on a Cu(100) surface under ultrahigh vacuum conditions, chiral square-planar Fe(tmla)4 metal-organic complexes are generated, which order in extended homochiral arrays. Structure formation and envisioned functionality of such metal-organic architectures are discussed.

Towards surface-supported supramolecular architectures: tailored coordination assembly of 1,4-benzenedicarboxylate and Fe on Cu(100).

We present a comprehensive investigation of the modular assembly of surface-supported metal-organic coordination systems with specific topologies and high structural stability formed by vapor deposition of 1,4-benzenedicarboxylic acid molecules and iron atoms on a Cu(100) surface under ultra-high vacuum conditions. By making use of the two carboxylate moieties available for lateral linkage to Fe atoms, we succeeded in the fabrication of distinct Fe-carboxylate coordination architectures at the surface by carefully adjusting the ligand and metal concentration ratio and the temperature of the post-deposition annealing treatment. The mononuclear, 1D-polymeric and fully 2D-reticulated metallosupramolecular arrangements obtained were characterized in situ at the single-molecule level by scanning tunneling microscopy.

First-principle molecular dynamics with ultrasoft pseudopotentials: parallel implementation and application to extended bioinorganic systems.

We present a plane-wave ultrasoft pseudopotential implementation of first-principle molecular dynamics, which is well suited to model large molecular systems containing transition metal centers. We describe an efficient strategy for parallelization that includes special features to deal with the augmented charge in the contest of Vanderbilt's ultrasoft pseudopotentials. We also discuss a simple approach to model molecular systems with a net charge and/or large dipole/quadrupole moments. We present test applications to manganese and iron porphyrins representative of a large class of biologically relevant metalorganic systems. Our results show that accurate density-functional theory calculations on systems with several hundred atoms are feasible with access to moderate computational resources.

Crystal engineering. From molecules to materials

Crystal engineering is target oriented and property directed synthesis of molecular crystals. The subject may be approached from several directions. To an organic chemist, it is supramolecular retrosynthesis with the synthon approach. To a crystallographer, it is crystal design based on hydrogen bonds and intermolecular interactions. To the materials scientist, it is a new approach wherein ease and modularity of construction is accompanied by diversity and range of properties. In this article, synthetic strategy and property optimisation from a materials viewpoint in some organic and metal-organic systems are discussed.

Comparison of luminescent efficiency of InGaAs quantum well structures grown on Si, GaAs, Ge, and SiGe virtual substrate

In order to study the luminescent efficiency of InGaAs quantum wells on Si via SiGe interlayers, identical In0.2Ga0.8As quantum well structures with GaAs and Al0.25Ga0.75As cladding layers were grown on several substrates by an atmospheric metalorganic vapor deposition system. The substrates used include GaAs, Si, Ge, and SiGe virtual substrates. The SiGe virtual substrates were graded from Si substrates to 100% Ge content. Because of the small lattice mismatch between GaAs and Ge (0.07%), high-quality GaAs-based thin films with threading dislocation densities &amp;lt;3×106 cm−2 were realized on these SiGe substrates. Quantitative cathodoluminescence was used to compare the luminescent efficiency of the quantum well structure on the different substrates and cross-sectional transmission electron microscopy was used to characterize dislocation densities. Our results show that the InGaAs quantum wells grown on the GaAs substrates have the highest luminescent efficiencies due to the lowest dislocation densities. Interestingly, InGaAs quantum wells grown on the SiGe virtual substrates outperform those on Ge substrates, both in terms of luminescent efficiency and dislocation density. This difference is attributed to the variation in thermal expansion coefficient (α) and its impact on defect structure during the process cycle. The SiGe virtual substrate has a smaller α compared to a Ge substrate because of the smaller α of the Si substrate, which helps minimize compressive strain in the quantum well layer during the temperature decrease from the growth temperature. Consequently, fewer misfit dislocations are created between the quantum well and cladding interfaces. These misfits can greatly affect the luminescent efficiency since they can act as recombination sites. In general, the efficiencies of the quantum wells on the SiGe and Ge substrates were affected only by higher misfit dislocation densities, whereas the quantum wells on the Si substrate had low efficiency due to high threading dislocation density.

MOMBE growth studies of GaN using metalorganic sources and nitrogen

Abstract Using modulated beam mass spectrometry we have investigated the species in the gas phase within the reaction chamber of an ultra-high vacuum metal organic vapour phase epitaxy system, when growing gallium nitride (GaN). We use high purity 14 N and 15 N to identify the species containing nitrogen within the post-reaction products and have identified a number of parasitic reactions which consume the active nitrogen and thus limit the growth rate at a level below the expected value.

Growth and optical properties of high-quality ZnTe homoepitaxial layers by metalorganic vapor phase epitaxy

Homoepitaxial growth of ZnTe layers have been investigated by changing the total gas flow rate in a metalorganic vapor phase epitaxial system using a horizontal reactor. Correlations between the growth process and the photoluminescence properties of the layers have been clarified. A high-quality ZnTe homoepitaxial layer with strong free excitonic emission was obtained in this experiment. The binding energy of the free exciton and the temperature dependence of the band gap has been investigated using this sample. The resistivity of a sample was estimated to be as high as 5×10 8 Ω cm supporting a high-quality epitaxial layer.

New control method of low deposition temperature limit of metal-organic chemical vapor deposition (MOCVD) by the introduction of organic vapor—Application to SrRuO 3 film preparation

A new control method for lowering the deposition temperature limit of the film prepared by metalorganic chemical vapor deposition (MOCVD) using organic vapor was proposed. This method was applied to Sr(C 11H 19O 2) 2(C 8H 23N 5) x –Ru(C 11H 19O 2) 3–O 2 system. As a result, the lowest limit of the deposition temperature of Sr element in the film decreased from 350 to 250 °C by the introduction of the vapors of 1-Hexen-3-ol and 2,4-Heaxadien-1-ol. However, obvious change was not detected by the introduction of n-Propanol and 3-Hexanol. This shows that CC bonds in the organic material play an important role. On the other hand, any remarkable change was not detected for the deposition rate of Ru element by the introduction of 1-Hexen-3-ol and 2,4-Hexadien-l-ol. This shows the selectivity of the constituent element. This method can be applied to improve the unbalance of the deposition rate of each constituent element at low deposition temperature from the multi metal-organic source gas system.

EXAFS investigation of metal organic synthesis tools

An EXAFS spectroscopical study of three metal organic systems is presented. First, a series of differently concentrated Grignard compounds in solution were measured in order to obtain the degree of association and to find an eventual concentration dependence of the structure. No significant changes in the structure could be detected in the measured concentration range. The second investigated system Mg(CHBrCH 2Ph) 2, dissolved in THF, belongs to the relatively new class of carbenoids. A structural model of this compound, especially of the local environment of the α-carbon atom is deduced. Thirdly, Reformatsky compounds in solution with different ester groups, solvents and concentrations were studied. It is shown that the structure of these compounds depends on the polarity of the solvents.