Chuan-Fu Sun

BaNbO(IO3)5: A New Polar Material with a Very Large SHG Response

By combination of Nb(5+) (having a d(0) electronic configuration) and the lone-pair-containing iodate anion, a new SHG material, BaNbO(IO(3))(5), has been prepared. It exhibits a very large SHG response (approximately 14 times that of KH(2)PO(4) and approximately 660 times that of alpha-SiO(2)) and is phase-matchable. The material has high thermal stability and a wide transparent region.

Explorations of new second-order nonlinear optical materials in the potassium vanadyl iodate system.

Four new potassium vanadyl iodates based on lone-pair-containing IO(3) and second-order Jahn-Teller distorted VO(5) or VO(6) asymmetric units, namely, α-KVO(2)(IO(3))(2)(H(2)O) (Pbca), β-KVO(2)(IO(3))(2)(H(2)O) (P2(1)2(1)2(1)), K(4)[(VO)(IO(3))(5)](2)(HIO(3))(H(2)O)(2)·H(2)O (P1), and K(VO)(2)O(2)(IO(3))(3) (Ima2) have been successfully synthesized by hydrothermal reactions. α-KVO(2)(IO(3))(2)(H(2)O) and β-KVO(2)(IO(3))(2)(H(2)O) exhibit two different types of 1D [VO(2)(IO(3))(2)](-) anionic chains. Neighboring VO(6) octahedra in the α-phase are corner-sharing into a 1D chain with the IO(3) groups attached on both sides of the chain in a uni- or bidentate bridging fashion, whereas those of VO(5) polyhedra in the β-phase are bridged by IO(3) groups into a right-handed helical chain with remaining IO(3) groups being grafted unidentately on both sides of the helical chain. The structure of K(4)[(VO)(IO(3))(5)](2)(HIO(3))(H(2)O)(2)·H(2)O contains novel isolated [(VO)(IO(3))(5)](2-) units composed of one VO(6) octahedron linked to five IO(3) groups and one terminal O(2-) anion. The structure of K(VO)(2)O(2)(IO(3))(3) exhibits a 1D [(VO)(2)O(2)(IO(3))(3)](-) chain in which neighboring VO(6) octahedra are interconnected by both oxo and bridging iodate anions. Most interestingly, three of four compounds are noncentrosymmetric (NCS), and K(VO)(2)O(2)(IO(3))(3) displays a very strong second-harmonic generation response of about 3.6 × KTP, which is phase matchable. It also has high thermal stability, a wide transparent region and moderate hardness as well as an excellent growth habit. Thermal analyses and optical and ferroelectric properties as well as theoretical calculations have also been performed.

Covalently functionalized double-walled carbon nanotubes combine high sensitivity and selectivity in the electrical detection of small molecules.

Atom-thick materials such as single-walled carbon nanotubes (SWCNTs) and graphene exhibit ultrahigh sensitivity to chemical perturbation partly because all of the constituent atoms are surface atoms. However, low selectivity due to nonspecific binding on the graphitic surface is a challenging issue to many applications including chemical sensing. Here, we demonstrated simultaneous attainment of high sensitivity and selectivity in thin-film field effect transistors (TFTs) based on outer-wall selectively functionalized double-walled carbon nanotubes (DWCNTs). With carboxylic acid functionalized DWCNT TFTs, we obtained excellent gate modulation (on/off ratio as high as 4000) with relatively high ON currents at a CNT areal density as low as 35 ng/cm(2). The devices displayed an NH(3) sensitivity of 60 nM (or ~1 ppb), which is comparable to small molecule aqueous solution detection using state-of-the-art SWCNT TFT sensors while concomitantly achieving 6000 times higher chemical selectivity toward a variety of amine-containing analyte molecules over that of other small molecules. These results highlight the potential of using covalently functionalized double-walled carbon nanotubes for simultaneous ultrahigh selective and sensitive detection of chemicals and illustrate some of the structural advantages of this double-wall materials strategy to nanoelectronics.

Hoop-Strong Nanotubes for Battery Electrodes

The engineering of hollow nanostructures is a promising approach to addressing instabilities in silicon-based electrodes for lithium-ion batteries. Previous studies showed that a SiOx coating on silicon nanotubes (SiNTs) could function as a constraining layer and enhance capacity retention in electrodes with low mass loading, but we show here that similarly produced electrodes having negligible SiOx coating and significantly higher mass loading show relatively low capacity retention, fading quickly within the early cycles. We find that the SiNT performance can still be enhanced, even in electrodes with high mass loading, by the use of Ni functional coatings on the outer surface, leading to greatly enhanced capacity retention in a manner that could scale better to industrially relevant battery capacities. In situ transmission electron microscopy studies reveal that the Ni coatings suppress the Si wall from expanding outward, instead carrying the large hoop stress and forcing the Si to expand inward toward the hollow inner core. Evidence shows that these controlled volume changes in Ni-coated SiNTs, accompanied by the electrochemically inert nature of Ni coatings, unlike SiOx, may enhance the stability of the electrolyte at the outer surface against forming a thick solid electrolyte interphase (SEI) layer. These results provide useful guidelines for designing nanostructured silicon electrodes for viable lithium-ion battery applications.

Polar or Non-Polar? Syntheses, Crystal Structures, and Optical Properties of Three New Palladium(II) Iodates

Three new novel palladium(II) iodates with square-planar PdO(4) units, namely, Pd(IO(3))(2), AgPd(IO(3))(3), and BaPd(IO(3))(4), have been successfully hydrothermally synthesized. They represent the first ternary and quaternary palladium(II) iodates and display three different structural types. Pd(IO(3))(2) exhibits a novel two-dimensional (2D) layered structure in which each PdO(4) square further connects with four neighboring ones by four bridging IO(3) groups. AgPd(IO(3))(3) exhibits a unique three-dimensional (3D) network based on unique one-dimensional (1D) [Pd(IO(3))(3)](-) anionic chains along the c-axis which are further interconnected by Ag(+) cations. BaPd(IO(3))(4) is isostructural with KAu(IO(3))(4), and its structure features zero-dimensional (0D) [Pd(IO(3))(4)](2-) anionic units that are interconnected by Ba(2+) cations. These materials can be polar or non-polar depending on the different alignments of the lone pairs of the I(V) atoms. Pd(IO(3))(2) and AgPd(IO(3))(3) are non-polar and centrosymmetric, hence not second-harmonic generation (SHG) active. BaPd(IO(3))(4) is polar and displays a moderate SHG response of about 0.4× KTP. Thermal analyses, optical, luminescent, and ferroelectric properties as well as electronic structure calculations have also been performed.

Syntheses and crystal structures of a series of alkaline earth vanadium selenites and tellurites.

Six new novel alkaline-earth metal vanadium(V) or vanadium(IV) selenites and tellurites, namely, Sr(2)(VO)(3)(SeO(3))(5), Sr(V(2)O(5))(TeO(3)), Sr(2)(V(2)O(5))(2)(TeO(3))(2)(H(2)O), Ba(3)(VO(2))(2)(SeO(3))(4), Ba(2)(VO(3))Te(4)O(9)(OH), and Ba(2)V(2)O(5)(Te(2)O(6)), have been prepared and structurally characterized by single crystal X-ray diffraction analyses. These compounds exhibit six different anionic structures ranging from zero-dimensional (0D) cluster to three-dimensional (3D) network. Sr(2)(VO)(3)(SeO(3))(5) features a 3D anionic framework composed of VO(6) octahedra that are bridged by SeO(3) polyhedra. The oxidation state of the vanadium cation is +4 because of the partial reduction of V(2)O(5) by SeO(2) at high temperature. Ba(3)(VO(2))(2)(SeO(3))(4) features a 0D [(VO(2))(SeO(3))(2)](3-) anion. Sr(V(2)O(5))(TeO(3)) displays a unique 1D vanadium(V) tellurite chain composed of V(2)O(8) and V(2)O(7) units connected by tellurite groups, forming 4- and 10-MRs, whereas Sr(2)(V(2)O(5))(2)(TeO(3))(2)(H(2)O) exhibits a 2D layer consisting of [V(4)O(14)] tetramers interconnected by bridging TeO(3)(2-) anions with the Sr(2+) and water molecules located at the interlayer space. Ba(2)(VO(3))Te(4)O(9)(OH) exhibits a one-dimensional (1D) vanadium tellurite chain composed of a novel 1D [Te(4)O(9)(OH)](3-) chain further decorated by VO(4) tetrahedra. Ba(2)V(2)O(5)(Te(2)O(6)) also features a 1D vanadium(V) tellurites chain in which neighboring VO(4) tetrahedra are bridged by [Te(2)O(6)](4-) dimers. The existence of V(4+) ions in Sr(2)(VO)(3)(SeO(3))(5) is also confirmed by magnetic measurements. The results of optical diffuse-reflectance spectrum measurements and electronic structure calculations based on density functional theory (DFT) methods indicate that all six compounds are wide-band gap semiconductors.

Explorations of New Second-Order Nonlinear Optical Materials in the K-I-M-II -I-V-O Systems

Explorations of new second-order nonlinear optical (NLO) materials in the K(I)-M(II) -I(V)-O systems led to four novel mixed metal iodates, namely, K(2)M(IO(3))(4)(H(2)O)(2) (M = Mn, Co, Zn, Mg). The four compounds are isostructural and crystallize in space group I2 which is in the chiral and polar crystal class 2. Their structure features zero-dimensional {M(IO(3))(4)(H(2)O)(2)}(2-) anions that are separated by K(+) cations. The M(II) centers are ligated by two aqua ligands in trans fashion and four monodentate iodate anions. The K(+) cation is eight-coordinated by two iodate anions in bidentate chelating fashion and four other iodates in a unidentate fashion. Second harmonic generation (SHG) measurements indicate that K(2)Zn(IO(3))(4)(H(2)O)(2) and K(2)Mg(IO(3))(4)(H(2)O)(2) display moderate SHG responses that are approximately 2.3 and 1.4 times of KH(2)PO(4) (KDP), respectively, and they are also phase-matchable. The SHG response of K(2)Co(IO(3))(4)(H(2)O)(2) is much weaker (about 0.3 x KDP), and no obvious SHG signal was detected for K(2)Mn(IO(3))(4)(H(2)O)(2). Results of optical property calculations for the Zn and Mg phases revealed SHG responses of approximately 5.3 and 4.7 times of KDP, respectively, the order of Zn > Mg is in good agreement with the experiment data.

Syntheses, Crystal Structures, and Properties of Five New Transition Metal Molybdenum(VI) Selenites and Tellurites

Five new transition metal molybdenum(VI) selenites or tellurites, namely, TM(MoO(3))(SeO(3))(H(2)O) (TM = Mn, Co), Fe(2)(Mo(2)O(7))(SeO(3))(2)(H(2)O), Cu(2)(MoO(4))(SeO(3)), and Ni(3)(MoO(4))(TeO(3))(2), have been prepared and structurally characterized. They belong to five different types of structures. Mn(MoO(3))(SeO(3))(H(2)O) and Ni(3)(MoO(4))(TeO(3))(2) are non-centrosymmetric and crystallize in the orthorhombic space groups Pmc2(1) and P2(1)2(1)2(1), respectively, whereas Co(MoO(3))(SeO(3))(H(2)O), Fe(2)(Mo(2)O(7))(SeO(3))(2)(H(2)O), and Cu(2)(MoO(4))(SeO(3)) are centrosymmetric and crystallize in P1, C2/c, P2(1)/c, respectively. The Mo(6+) cations in Mn(MoO(3))(SeO(3))(H(2)O), Co(MoO(3))(SeO(3))(H(2)O), and Fe(2)(Mo(2)O(7))(SeO(3))(2)(H(2)O) are in severely distorted octahedral geometry whereas those in Cu(2)(MoO(4))(SeO(3)) and Ni(3)(MoO(4))(TeO(3))(2) are in a slightly distorted tetrahedral geometry. Second-Harmonic Generation (SHG) measurements revealed that (MoO(3))(SeO(3))(H(2)O) displays a moderate SHG signal of about 3 x KH(2)PO(4) (KDP) whereas the SHG response of Ni(3)(MoO(4))(TeO(3))(2) is much weaker than that of KDP.

Interfacial Oxygen Stabilizes Composite Silicon Anodes

Silicon can store Li(+) at a capacity 10 times that of graphite anodes. However, to harness this remarkable potential for electrical energy storage, one has to address the multifaceted challenge of volume change inherent to high capacity electrode materials. Here, we show that, solely by chemical tailoring of Si-carbon interface with atomic oxygen, the cycle life of Si/carbon matrix-composite electrodes can be substantially improved, by 300%, even at high mass loadings. The interface tailored electrodes simultaneously attain high areal capacity (3.86 mAh/cm(2)), high specific capacity (922 mAh/g based on the mass of the entire electrode), and excellent cyclability (80% retention of capacity after 160 cycles), which are among the highest reported. Even at a high rate of 1C, the areal capacity approaches 1.61 mAh/cm(2) at the 500th cycle. This remarkable electrochemical performance is directly correlated with significantly improved structural and electrical interconnections throughout the entire electrode due to chemical tailoring of the Si-carbon interface with atomic oxygen. Our results demonstrate that interfacial bonding, a new dimension that has yet to be explored, can play an unexpectedly important role in addressing the multifaceted challenge of Si anodes.

Second-Order Nonlinear Optical Materials Based on Metal Iodates, Selenites, and Tellurites

In this chapter, the syntheses, structures, and Second Harmonic Generation (SHG) properties of metal iodates, selenites, and tellurites all of which contain a lone pair cation in an asymmetric coordination geometry were reviewed. A second asymmetric building unit such as distorted octahedra of the d0 transition-metal (TM) cations such as V5+, Mo6+, other cations with a stereochemically active lone pair such as Pb2+ and Bi3+, and tetrahedral groups such as BO 4 5− and PO 4 3− , can be introduced into metal iodates, selenites, and tellurites. The combination of d0 transition-metal cations with the iodate groups afforded a large number of new metal iodates, a number of which display excellent SHG properties due to the additive effects of polarizations from both types of the asymmetric units. Introducing other lone-pair cations such as Pb2+ and Bi3+ into the metal iodates is also an effective strategy to design new SHG materials. With respect to the metal selenite or tellurite systems, many compounds in the alkali or alkaline earth-d0 TM–Se(IV)/Te(IV)–O systems can also exhibit excellent SHG properties due to the additive effects of polarizations from both types of asymmetric units. Lanthanide or posttransition metal main group element-d0 TM–Se(IV)/Te(IV)–O compounds are usually structurally centrosymmetric and not SHG active, but they can also display abundant structural diversities and interesting magnetic or luminescent properties. Metal tellurites and selenites containing tetrahedral groups of the main group elements such as BO4 and PO4 may also form NCS structures with excellent SHG properties.