18 research outputs found
Site selectivity in carbon monoxide insertion into a PtC σ-bond of the binuclear complex [(CH3)ClPt(μ-Cl) (μ-Ph2PPy) Pt(CH3)(DMSO)]DMSO. Structural characterization of the derivatives [(CH3CO)ClPt(μ-Cl)(μ-Ph2 PPy)Pt(CH3)(DMSO)] and [(CH3)ClPt(μ-Ph2PPy)2Pt(COCH3)]+
No full textTitle full: Site selectivity in carbon monoxide insertion into a PtC σ-bond of the binuclear complex [(CH3)ClPt(μ-Cl) (μ-Ph2PPy) Pt(CH3)(DMSO)]DMSO. Structural characterization of the derivatives [(CH3CO)ClPt(μ-Cl)(μ-Ph2 PPy)Pt(CH3)(DMSO)] and [(CH3)ClPt(μ-Ph2PPy)2Pt(COCH 3)]+. Carbon monoxide reacts with [(CH3)ClPt(μ-Cl)(μ-Ph2PPy)Pt(CH3) (DMSO)]DMSO, 1, to give [(CH3CO)ClPt(μ-Cl)(μ-Ph2 PPy)Pt(CH3)(DMSO)] 2. The insertion of CO into a platinum-carbon σ-bond of 1 shows a site selectivity, occurring at the Pt CH3 bond involving the metal atom connected to the P atom of the 2-(diphenylphosphino)pyridine (Ph2PPy). Under a carbon monoxide atmosphere for about 30 h, 1 forms the ionic compound [(CH3)ClPt(μ-Ph2PPy)2Pt(COCH3 )][Pt(CO)(COCH3)Cl2], 3. Complex 2 changes slowly into 3. It was not possible to obtain crystals of 2 or 3 suitable for X-ray investigations. A crystal species was isolated containing both 2 and 3 in a 1 : 1 ratio. The crystals are triclinic, space group P1 (no. 2), with a = 10.326(4) Å, b = 16.648(4) Å, c = 22.203(4) Å, α = 90.95(2)°, β = 96.50(2)°, γ = 95.44(2)°. The refinements, based on 3255 significant reflections, gave a final R value of 0.0621. The molecular structure of the neutral species 2 is similar to that of the parent compound 1, with a CH3CO group in place of a methyl. The structure of the anion of 3 was incompletely characterized because of disorder. The [(CH3)ClPt(μ-Ph2PPy)2Pt(COCH3 )]+ cation has a direct PtPt bond of 2.728(3) Å bridged by the two Ph2PPy ligands in a head-to-tail fashion. One of the Pt atoms completes its coordination with an acetyl group and is four-coordinate, whereas the second containing both a chlorine atom and a methyl group, is five-coordinate. Semi-empirical MO calculations for the cation have been performed, and show the dative nature of the PtII → PtII bond
Preparation and crystal structure of a selenoantimonate [Fe(en)3][enH]SbSe4
No full textLow temperature synthesis has produced a new selenoantimonate, [Fe(en) 3][enH]SbSe 4 (I). Crystals of I were grown at 160°C in ethylenediamine solution. Compound I crystallizes in triclinic crystal system, space group P-1 (no. 2), with unit cell constants a = 8.916(2) Å, b = 9.679(2) Å, c = 14.345(4) Å, α = 104.71(3)°, β = 92.51(2)°, γ = 110.14(2)°, V = 1112.4(5) Å 3 and Z = 2. Structural refinement using 2850 reflections (F o > 4((F o)] and 204 variables resulted in 0.04 for R1 and 0.06 for wR2. I contains molecular Zintl anions (SbSe 4) 3- and metal coordination complex [Fe(en) 3] 2+ and [enH] + cations. The metal complex is present with 72% as lel 2ob conformer and 28% as lel 3 conformer
Three Cationic: Nonporous CuI-Coordination Polymers: Structural Investigation and Vapor Iodine Capture
No full textThree cationic nonporous copper(I) coordination polymers containing bis-pyrazolyl flexible ligands have been prepared and characterized, namely, [Cu(μ-bdb)1.5](PF6)n (1), [Cu(μ-bpb)2](PF6)n (2), and [Cu(μ-bpmb)2](PF6)n (3) (bdb = 1,4-bis(3,5-dimethylpyrazolyl) methyl)benzene; bpb = 1,4-bis(pyrazolyl)butane; bpmb = 1,4-bis(pyrazolyl)methyl)benzene). All compounds were characterized by infrared, powder X-ray diffraction, elemental and thermal analyses, and single-crystal X-ray diffraction. Compound 1, with methyl-substituted pyrazolyl ligand, forms a chain of alternating rings and ribbons in which the copper(I) centers are three coordinated in distorted trigonal planar geometry. In compounds 2 and 3 copper(I) atoms adopt distorted tetrahedral geometries giving two-dimensional sheet structures with 44-sql topology. Interestingly, iodine sorption experiments show that colorless crystals of 2 and 3 remain unchanged in the presence of iodine vapors, while the three-coordinated compound 1 immediately absorbs iodine and turns dark. Anion exchange behavior of compounds 1 and 2 was also investigated both in solution and in the solid state
A QUATERNARY ThCr2Si2 TYPE STRUCTURE: CRYSTAL GROWTH OF KCuZnTe2 FROM MOLTEN SALT
No full textKCuZnTe 2, a quaternary layered metal telluride, was synthesized from a potassium polytelluride flux at 450°C. Single crystal X-ray diffraction analysis showed that the structure of this compound belongs to tetragonal crystal system, space group I4/mmm (no. 139). The unit cell parameters are: a = 4.269(1) Å, c = 14.912(2) Å, V = 271.76(10) Å 3, Z = 2. Direct synthesis at 650°C produced a yield of approximately 25%. KCuZnTe 2 is one of only a few quaternary mixed-metal tellurides having the ThCr 2Si 2-type structure. It is composed of (CuZnTe 2) - layers and K + cations located between the layers. Cu and Zn form a mixed-metal square lattice within each layer by sharing the same crystallographic site. KCuZnTe 2 is electron precise and is therefore most likely a semiconducting material
1,2-<i>eq,eq</i>-[Re<sub>2</sub>(CO)<sub>8</sub>(THF)<sub>2</sub>]: A Reactive Re<sub>2</sub>(CO)<sub>8</sub> Fragment That Easily Activates H−H and C−H Bonds
No full textThe reaction of [Re2(μ-H)2(CO)8] (1) with diazomethane at 193 K in THF-d8 gives the
unstable [Re2(μ-H)(μ-CH3)(CO)8] derivative (2) containing a methyl group arising from the
insertion of CH2 into a Re−H−Re bond. Isotopic perturbation of the equilibria by partial
deuteration demonstrated that the methyl bridges the Re−Re bond in an unsymmetrical
way, with a fast exchange between one agostic and two terminal C−H bonds. At temperatures
higher than 253 K, 2 decomposes, in THF solution, with CH4 elimination to give the novel
red complex 1,2-eq,eq-[Re2(CO)8(THF)2] (3), which was characterized by NMR spectroscopy
and X-ray analysis. In the solid form a staggered conformer of C2 symmetry was found. 13C
NMR analysis revealed the presence, in wet THF, of the aquo complexes [Re2(CO)8(THF)(H2O)] (4) and [Re2(CO)8(H2O)2] (5), whose formation is favored at low temperature (ΔH° for
the formation of 5 from 4: −14.4(2) kJ mol-1). In solution, due to the lability of the THF
ligands, 3 behaves as a “lightly stabilized” Re2(CO)8 fragment, capable of activating different
E−H bonds. Reaction with HCl in THF leads to [Re2(μ-H)(μ-Cl)(CO)8], while with H2 the
unsaturated starting material [Re2(μ-H)2(CO)8] is obtained. In THF solution, at room
temperature, reactions with phenylacetylene, styrene, and acetaldehyde give the derivatives
of C−H activation [Re2(μ-H)(μ-C⋮CPh)(CO)8], [Re2(μ-H)(μ-CHC(H)Ph)(CO)8], and [Re2(μ-H)(μ-η2-C(Me)O)(CO)8]. Moreover, the activation of an sp3 C−H bond in ethyl acetate occurs
slowly when 3 is dissolved in the reactant itself, the resultant product being [Re2(μ-H)(μ-η2-CH2C(O)OEt)(CO)8]
Three Cationic, Nonporous Cu<sup>I</sup>‑Coordination Polymers: Structural Investigation and Vapor Iodine Capture
No full textThree cationic nonporous copper(I)
coordination polymers containing
bis-pyrazolyl flexible ligands have been prepared and characterized,
namely, {[Cu(μ-bdb)1.5](PF6)}n (1), {[Cu(μ-bpb)2](PF6)}n (2), and {[Cu(μ-bpmb)2](PF6)}n (3) (bdb = 1,4-bis(3,5-dimethylpyrazolyl)
methyl)benzene; bpb = 1,4-bis(pyrazolyl)butane; bpmb = 1,4-bis(pyrazolyl)methyl)benzene).
All compounds were characterized by infrared, powder X-ray diffraction,
elemental and thermal analyses, and single-crystal X-ray diffraction.
Compound 1, with methyl-substituted pyrazolyl ligand,
forms a chain of alternating rings and ribbons in which the copper(I)
centers are three coordinated in distorted trigonal planar geometry.
In compounds 2 and 3 copper(I) atoms adopt
distorted tetrahedral geometries giving two-dimensional sheet structures
with 44-sql topology. Interestingly, iodine
sorption experiments show that colorless crystals of 2 and 3 remain unchanged in the presence of iodine vapors,
while the three-coordinated compound 1 immediately absorbs
iodine and turns dark. Anion exchange behavior of compounds 1 and 2 was also investigated both in solution
and in the solid state
Addressing and tailoring the electronic properties of semiconductor nanostructures: nanowires and transition metal dichalcogenides
No full textSemiconductor materials played and still play a pivotal role in the technological development of modern life. From personal computer to data storage (e.g. solid-state disk-drives), from solar cells and cell phones to LEDs and biological sensors, there has always been a new system to study, a novel application to develop, and a solution to an otherwise unsolvable problem in which semiconductors play an essential role. All those technological goals have been achieved thanks to the synergic works carried out by basic researches in materials science, in particular in the semiconductor field. In the last decades, tremendous efforts have been made to miniaturize semiconductor devices at a nanometer scale, aiming at obtaining more compact devices with optimized speed and reduced power consumption. Unfortunately, or fortunately, the physical properties of any material dramatically change when the material dimensions are reduced to nanometer-lengths. Therefore, many efforts are required to understand the properties of any desired nanostructure, if they have to play a central role in technological applications.
In recent years, a great interest has grown in the investigation and applications of nanowires (NWs). NWs are several micron-long filamentary-crystals whose diameters range from few to hundreds of nanometers. Their dimensions make NWs suitable to bridge the gap between the microscopic and the nanoscopic world in both research and technology fields. Although several types of materials can be grown in a NW form, e.g. metals, insulators, and semiconductors, the latter are the most interesting and promising materials. As a matter of fact, owing to their peculiar shape and dimensions, semiconductor NWs are valuable candidates for novel nanoscale devices, in which they act as both functionalized components and interconnects. Moreover, semiconductor NWs represent nanostructured systems for which some key parameters in device engineering, e.g. chemical composition, size, and crystal phase, are well controlled nowadays. This is mainly due to the technique used to grow NWs. NWs are usually fabricated via the vapor-liquid-solid (VLS) technique, in which metal nanoparticles are used as catalyst seeds to induce a one-dimensional crystal growth. This well-controlled process allows for the synthesis of a wide range of semiconductor systems in the NW form, ranging from IV-IV to II-VI, with a high degree of manageability of both the chemical composition and morphology. In addition, under suitable VLS conditions, non-nitride III-V NWs can crystallize in the hexagonal wurtzite (WZ) structure in materials that, instead, are notoriously stable in the cubic zinc-blende (ZB) structure. The opportunity to controllably grow NWs in different crystal phases, namely, the polytypism, adds a new degree of freedom in device engineering. The presence of a WZ crystal phase in many III-V NWs offers also the opportunity to address the electronic band structure of this poorly known structure,
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whose presence itself is a subject of fundamental interest in materials science and chemistry. As an example, there is no experimental information concerning the variation of the spin and transport properties, i.e. gyromagnetic factors and carrier effective-masses, respectively, when the phase transition from ZB to WZ occurs. Even the fundamental band-gap value of some WZ semiconductor materials have not been determined, yet. Therefore, a comprehensive study aimed at the investigation of the correlation between the NW electronic properties and NW crystal structure is mandatory nowadays.
A great interest has grown also in the field of layered materials. Since the discovery of graphene in 2004, it has been understood the great potential of layered systems for advanced-technological applications. As a matter of fact, layered materials thinned to their physical limits -and usually referred to as two-dimensional (2D) materials- exhibit properties quite different from those of their bulk counterparts. A very wide spectrum of 2D materials has been then investigated. The most studied material is graphene because of its exceptional electronic and mechanical properties. Group VI transition metal dichalcogenides (TMDs) have also attracted the attention of researchers involved in the semiconductor field. TMDs have a crystal structure similar to that of graphite. Their layered structure, X-M-X, where M is the transition metal and X is the chalcogen atom, is characterized by weak interlayer van der Waals bonds and strong intralayer covalent bonds. That structure allows for an easy mechanical exfoliation, as in the graphene case, which is a major advantage of 2D materials, together with their synthesis techniques, cheap and easy as compared to the molecular-beam-epitaxy or metal-organic chemical-vapor-deposition techniques used for the fabrication of other nanostructured systems. The most surprising feature observed in 2D TMDs is the transition from an indirect band-gap in the infrared region to a direct band-gap in the visible region when they are thinned to the mono- layer limit. That feature, coupled with the TMD extremely high flexibility, elasticity, and resistance, makes TMDs suitable in the field of low-dimensional optoelectronic devices. In addition, the TMD high surface-to-volume ratio is valuable in biological fields, as they can be used as highly reactive sensors. Besides, the TMD unique properties in the single-layer limit of valley-valley coupling and valley-spin coupling render TMDs the suitable candidates for novel technologies based on valleytronic and spintronic. However, almost all these aforementioned properties are at the early stage of investigation and systematic studies are necessary before TMDs could be exploited in future applications.
In this thesis, the electronic properties of InP NWs and MX2 TMDs, with M=Mo or W and X=S or Se, are thoroughly investigated mainly by means of optical spectroscopy, in particular photoluminescence (PL) in combination with external perturbations, e.g. high magnetic fields. The response of semiconductor TMDs to hydrogen irradiation is studied, too. The thesis is therefore structured in two parts, the first one, from chap. 1 to chap. 3, is devoted to InP NWs, the second one, from chap. 4 to chap. 6, is devoted to 2D TMDs.
• In the first chapter, the high degree of freedom achieved in NW fabrication is presented and accounted for by the VLS technique, which is also discussed in details together with its recent development: the selective-area-epitaxy technique. Then, the differences between the structural, electronic, and optical
ix
properties of WZ and ZB crystal phases are discussed. The striking variation induced in the band structure by the crystal phase-transition is highlighted, too. Moreover, the different optical anisotropies of the two crystal phases are summarized. The chapter is concluded by a review of the technological applications of semiconductor NWs in the fields of optoelectronic, energy conversion, biosensoring, and as probes of elusive quantum effects.
• The second chapter comprehends a systematic investigation of InP NWs in both the ZB and WZ crystal-phases. The morphological characteristics of the investigated samples as accessed through scanning-electron-microscopy, transmission-electron-microscopy, and selective-area-diffraction patterning are also presented. The basic optical properties of InP in both crystal phases are assessed by either PL or μ-PL experiments as a function of lattice temperature and power excitation. Polarization-resolved measurements are shown, too. The three lowest-energy critical points of the WZ band-structure are investigated by PL excitation (PLE) as a function of lattice temperature. A quantitative reproduction of those spectra allows for establishing the temperature depen- dence of the A, B, and C inter-band transitions. A comparison with ZB results is made, too. Finally, the hot-carrier effect in NWs is found and its dependence on NW morphology is investigated.
• In the third chapter, the transport and spin properties of WZ InP are assessed by PL spectroscopy under high magnetic fields (up to 28 T ). A brief review of the effects that a magnetic field has on the energy and symmetry of exciton recombinations and of free-electron-to-acceptor and donor-to-acceptor transi- tions in WZ crystal is presented. Both diamagnetic shift and Zeeman splitting depend on the magnetic-field direction with respect to the NW symmetry-axis, namely the WZ cˆ-axis. That dependence has been investigated by applying the magnetic field either parallel or orthogonal to the NW axis. The obtained results are compared with the literature of both theoretical models of WZ InP and experimental results in other WZ compounds, such as GaN, InN, and ZnO. Finally, the non-linearity observed in the Zeeman splitting for magnetic fields above 10T and parallel to the NW axis is compared to a theoretical prediction.
• In the fourth chapter, the lattice, electronic, and vibrational properties of 2D TMDs are described. In particular, the lattice structures of several polytypes are shown, with special emphasis on the 2H polytype, whose electronic and vibrational properties are investigated and its different properties in the bulk and single-layer regimes highlighted. Then, several methods aimed at reaching the mono-layer limit are presented and top-down exfoliations from bulk mate- rials are singled out from bottom-up syntheses. The chapter ends with a brief review of the technological applications of semiconductor 2D TMDs in the fields of optoelectronic, energy conversion and storage, and molecular sensing.
• The fifth chapter comprehends a systematic investigation of the effects of hydrogen irradiation on the emission properties of single- and bi-layer TMDs, such as MoSe2 and WSe2. Firstly, a wide variety of experimental results con-
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cerning MX2 optical band-gaps and vibrational mode-energies are summarized. A brief description of the investigated samples is presented, too. The optical properties of pristine samples are assessed by means of either μ-Raman or μ-PL experiments whose room- and low-temperature results agree well with the existing literature. Then, the pristine flakes are irradiated with progressively increasing doses of hydrogen and the results thus obtained are reported. In the single-layer regime, a worsening of the material optical quality is observed together with the appearances of very sharp peaks below the band-gap energy. Conversely, a small improvement in the PL efficiency is obtained in the bi-layer regime. Finally, a solution to the worsening of the optical quality observed in hydrogenated single-layer flakes is provided.
• In the sixth chapter, the effects of hydrogen irradiation on the morphological and optical properties of multi-layer TMDs are discussed. Surprisingly, hy- drogenation favors unique conditions for the production and accumulation of molecular hydrogen just one or few layers beneath the crystal surface of all the multi-layer MX2 compounds investigated. That turns into the creation of atomically-thin domes filled with hydrogen molecules. The results of an atomic-force-microscopy and optical investigation of these new fascinating nanostructures are discussed. Finally, the possibility to tailor the dome posi- tion, size, and density is demonstrated, which provides a tool to manage the mechanical and electronic structure of 2D materials.
• The main results obtained in this work are summarized in the conclusive remarks.
• In the appendix, the theoretical basis of the optical-spectroscopy techniques here used, such as PL, PLE, magneto-PL, and Raman spectroscopy, are provided. PL and PLE are complementary techniques that enable a complete characterization of the electronic states of any optically-efficient material. Indeed, PL is an extremely sensitive probe of low-density electronic states, such as impurities or defects, while PLE can address the full density of states, i.e, it mimics absorption measurements, at least under certain approximations. On the other hand, PL spectroscopy under magnetic field allows for the determination of carrier effective-masses and g-factors, while Raman allows for getting information about the lattice properties of solids. A description of all the used experimental setups is also given. Finally, a description of the experimental apparatus used for hydrogen irradiation and atomic-force- microscopy measurements is provided.
• Finally, a list of the publications to which the author of this thesis has contributed is provided, along with a list of poster/oral contributions to international conferences given by the author of this thesis during his PhD studies
New Type of Polymeric Indium Tellurides: Low-Temperature Synthesis and Structure Characterization of [M(en)<sub>3</sub>]In<sub>2</sub>Te<sub>6</sub> (M = Fe, Zn) and α- and β-[Mo<sub>3</sub>(en)<sub>3</sub>(μ<sub>2</sub>-Te<sub>2</sub>)<sub>3</sub>(μ<sub>3</sub>-Te)(μ<sub>3</sub>-O)]In<sub>2</sub>Te<sub>6</sub>
No full textCrystal growth of metal tellurides and tellurometalates employing
solvothermal reactions at temperatures below
200 °C have resulted in four new indium tellurium phases,
[Fe(en)3](In2Te6)
(I),
[Zn(en)3](In2Te6)
(II), and α-
and
β-[Mo3(en)3(μ2-Te2)3(μ3-Te)(μ3-O)]In2Te6
(III-α and III-β).
Single crystal X-ray diffraction analyses show
that I and II are isostructural and belong to the
orthorhombic crystal system, space group
P212121
(No. 19).
Compound I: a = 11.654 (1) Å,
b = 12.968(2) Å, c = 16.273(2) Å,
Z = 4. Compound II: a =
11.662(2) Å,
b = 12.948(2) Å, c = 16.285(1) Å,
Z = 4. The two polymorphs
III-α and III-β
crystallize in the monoclinic
system. Compound III-α: a =
11.815(2) Å, b = 21.769(3)Å, c =
14.498(4) Å, β = 95.43(2)°, Z = 4,
space
group P21/c (No. 14).
Compound III-β: a =
22.154(3) Å, b = 11.550(2)Å, c =
14.230(2) Å, β = 99.05(1)°, Z
= 4, space group
P21/a (No.
14). All four are Zintl compounds containing novel
one-dimensional polymeric
chains of
[(In2Te6)2-]
that can be described as alternating fused five-membered rings
[(In3+)2(Te22-)(Te2-)],
joined at the In atoms
Measurement of the cross-section ratio σ(χc2)/σ(χc1) for prompt χc production at √s=7 TeV
No full textThe prompt production of the charmonium χc1 and χc2 mesons has been studied in proton–proton collisions at the Large Hadron Collider at a centre-of-mass energy of . The χc mesons are identified through their decays χc→J/ψγ with J/ψ→μ+μ− using 36 pb−1 of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for the two χc spin states, σ(χc2)/σ(χc1), has been determined as a function of the J/ψ transverse momentum, , in the range from 2 to 15 GeV/c. The results are in agreement with the next-to-leading order non-relativistic QCD model at high and lie consistently above the pure leading-order colour-singlet prediction
Measurement of the ratio of prompt χ c to J / ψ production in pp collisions at √s = 7 TeV
No full textThe prompt production of charmonium χ c and J / ψ states is studied in proton-proton collisions at a centre-of-mass energy of √s = 7 TeV at the Large Hadron Collider. The χ c and J / ψ mesons are identified through their decays χ c → J / ψ γ and J / ψ → μ + μ - using 36 pb - 1 of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for χ c and J / ψ, σ (χ c → J / ψ γ) / σ (J / ψ), is determined as a function of the J / ψ transverse momentum in the range 2 < p T J / ψ < 15 GeV / c. The results are in excellent agreement with next-to-leading order non-relativistic expectations and show a significant discrepancy compared with the colour singlet model prediction at leading order, especially in the low p T J / ψ region
