Frontiers of Materials Science (v.9, #2)

Bismuth telluride nanostructures: preparation, thermoelectric properties and topological insulating effect by Eric Ashalley; Haiyuan Chen; Xin Tong; Handong Li; Zhiming M. Wang (103-125).
Bismuth telluride is known to wield unique properties for a wide range of device applications. However, as devices migrate to the nanometer scale, significant amount of studies are being conducted to keep up with the rapidly growing nanotechnological field. Bi2Te3 possesses distinctive properties at the nanometer level from its bulk material. Therefore, varying synthesis and characterization techniques are being employed for the realization of various Bi2Te3 nanostructures in the past years. A considerable number of these works have aimed at improving the thermoelectric (TE) figure-of-merit (ZT) of the Bi2Te3 nanostructures and drawing from their topological insulating properties. This paper reviews the various Bi2Te3 and Bi2Te3-based nanostructures realized via theoretical and experimental procedures. The study probes the preparation techniques, TE properties and the topological insulating effects of 0D, 1D, 2D and Bi2Te3 nanocomposites. With several applications as a topological insulator (TI), the topological insulating effect of the Bi2Te3 is reviewed in detail with the time reversal symmetry (TRS) and surface state spins which characterize TIs. Schematics and preparation methods for the various nanostructural dimensions are accordingly categorized.
Keywords: Bi2Te3 nanostructure; thermoelectric property; topological insulator (TI)

Previously we reported the synthesis of novel organic-inorganic composite indium tin oxide (ITO) foam precursor leading to the formation of “sponge-like” ITO by burning away the organics. This newly made sponge-like ITO possesses relatively high electrical conductivity due to phonon confinement with reasonable pore structure and may have potential application as functional materials in semiconducting dye absorbing layer in dye-sensitized solar cell (DSSC) and also as the receptor of electrons injected from the quantum dots (QDs) of organic-inorganic hybrid QD based solar cell. This report is a short review of “sponge-like” ITO described as a lecture note on its future use as an alternative new prospective material for photoanode of solar cell in the domain of sustainable energy.
Keywords: sponge-like indium tin oxide (ITO); sol-gel ITO; nanoporous ITO; smoke like ITO film; indium nitrate-PVA foam

Hexanoyl chitosan and polystyrene blends are immiscible by the elucidation of the glass transition temperature (T g) as well as the viscometric and morphological analyses. Selective localization of the lithium salt in hexanoyl chitosan phase as the percolation pathway enhanced the conductivity in the blends as compared to the neat hexanoyl chitosan. The ionic conductivity of a polymer electrolyte is described by σ = enµ. Thus, estimation of charge carrier density (n) and mobility (µ) is important in order to assess the performance. In this work, these parameters are calculated using impedance spectroscopy and FTIR.
Keywords: hexanoyl chitosan; polystyrene; polymer electrolyte; conductivity; percolation pathway

Impact of surface phase coexistence on the development of step-free areas on Si(111) by Andreas Fissel; Ayan Roy Chaudhuri; Jan Krügener; Philipp Gribisch; H. Jörg Osten (141-146).
The step-flow growth condition of Si on Si(111) near the (7×7)-“1×1” surface phase transition temperature T C are analyzed within the framework of Burton-Cabrera-Frank theory. In particular, coexistence of both surface phases well below T C and their specific influence on the step-flow growth behavior are considered. We presume that under dynamical condition of growth, the surface initially covered by only the (7×7) phase separates into domains surrounded by “1×1” areas. On such a surface, the overall supersaturation should be reduced drastically compared to a surface with only (7×7), resulting in much larger critical terrace width for nucleation.
Keywords: molecular beam epitaxy; step-flow growth mode; surface superstructure; silicon

Synthesis, crystal structure and thermal analysis of a new stilbazolium salt crystal by Bing Teng; Weijin Kong; Ke Feng; Fei You; Lifeng Cao; Degao Zhong; Lun Hao; Qing Sun; Sander van Smaalen; Wenhui Gong (147-150).
A new organic crystal of 4-N, N-dimethylamino-4′-N′-methyl-stilbazolium benzene sulfonate (DASBS) was synthesized and characterized for the first time. It is a derivative of 4-N, N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) with the benzene sulfonate replacing p-toluenesulfonate. Single crystal XRD demonstrated that the crystal structure of DASBS·H2O was triclinic. The thermal analysis of this new crystal was also conducted, and the melting point was obtained to be 232°C.
Keywords: organic crystal; crystal growth; single crystal X-ray diffraction; thermal analysis

GaN metal-oxide-semiconductor field-effect transistors on AlGaN/GaN heterostructure with recessed gate by Qingpeng Wang; Jin-Ping Ao; Pangpang Wang; Ying Jiang; Liuan Li; Kazuya Kawaharada; Yang Liu (151-155).
GaN metal-oxide-semiconductor field-effect transistors (MOSFETs) on AlGaN/GaN heterostructure with a recess gate were fabricated and characterized. The device showed good pinch-off characteristics and a maximum field-effect mobility of 145.2 cm2·V−1·s−1. The effects of etching gas of Cl2 and SiCl4 were investigated in the gate recess process. SiCl4-etched devices showed higher channel mobility and lower threshold voltage. Atomic force microscope measurement was done to investigate the etching profile with different etching protection mask. Compared with photoresist, SiO2-masked sample showed lower surface roughness and better profile with stepper sidewall and weaker trenching effect resulting in higher channel mobility in the MOSFET.
Keywords: gallium nitride; MOSFET; recess gate; dry etching

2.45 GHz 0.8 mW voltage-controlled ring oscillator (VCRO) in 28 nm fully depleted silicon-on-insulator (FDSOI) technology by Gilles Jacquemod; Alexandre Fonseca; Emeric de Foucauld; Yves Leduc; Philippe Lorenzini (156-162).
MOS bulk transistor is reaching its limits: sub-threshold slope (SS), drain induced barrier lowering (DIBL), threshold voltage (VT) and VDD scaling slowing down, more power dissipation, less speed gain, less accuracy, variability and reliability issues. Fully depleted devices are mandatory to continue the technology roadmap. FDSOI technology relies on a thin layer of silicon that is over a buried oxide (BOx). Called ultra thin body and buried oxide (UTBB) transistor, FDSOI transistors correspond to a simple evolution from conventional MOS bulk transistor. The capability to bias the back-gate allows us to implement calibration techniques without adding transistors in critical blocks. We have illustrated this technique on a very low power voltage-controlled oscillator (VCO) based on a ring oscillator (RO) designed in 28 nm FDSOI technology. Despite the fact that such VCO topology exhibits a larger phase noise, this design will address aggressively the size and power consumption reduction. Indeed we are using the efficient back-gate biasing offered by the FDSOI MOS transistor to compensate the mismatches between the different inverters of the ring oscillator to decrease jitter and phase noise. We will present the reasons which led us to use the FDSOI technology to reach the specifications of this PLL. The VCRO exhibits a 0.8 mW power consumption, with a phase noise about −94 dBc/Hz@1 MHz.
Keywords: nanoelectronics; FDSOI; UTBB; VCO; PLL

The nanocrystalline Fe64Ni36 thin films were prepared by molecular-beam-vapor deposition under different magnetic flux densities. The microstructure and magnetic properties of thin films were examined by AFM, TEM, HRTEM and VSM. The results show that with the increase of magnetic flux densities, the changing trend of the average particle size is the same as the coercive force except 6 T. Under 6 T condition, the thin film became the mixture of bcc and fcc phases, which leads to slight increase of the coercive force. In addition, the HRTEM result shows the short-range ordered clusters (embryos) or nucleation rate of thin films increase with increasing magnetic flux densities.
Keywords: magnetic field intensity; microstructure; soft magnetic property; coercive force; crystallinity

The beginnings of plasmomechanics: towards plasmonic strain sensors by Thomas Maurer; Joseph Marae-Djouda; Ugo Cataldi; Arthur Gontier; Guillaume Montay; Yazid Madi; Benoît Panicaud; Demetrio Macias; Pierre-Michel Adam; Gaëtan Lévêque; Thomas Bürgi; Roberto Caputo (170-177).
This article exposes the beginnings of a new field which could be named as “plasmomechanics”. Plasmomechanics comes from the convergence between mechanics and plasmonics. Here we discuss a relatively recent topic whose technological aim is the development of plasmonic strain sensors. The idea is based on the ability to deduce Au nanoparticles (NPs) distance distributions from polarized optical extinction spectroscopy which could thus give access to material strains. Variations of interparticle distances distributions can indeed lead to variations of plasmonic coupling and thus to material color change as shown here experimentally and numerically for random Au NP assemblies deposited onto elastomer films.
Keywords: localized surface plasmon resonance (LSPR); metallic nanoparticle; strain; composite material; elastomeric film

Bio-inspired artificial cilia with magnetic dynamic properties by Leilei Sun; Yongmei Zheng (178-184).
Inspired by the structure and properties of natural cilia, we focused on a facile template-free approach to prepare magnetic artificial cilia grown on the substrate (glass, PDMS, or others). In an applied magnetic field, the cilia formed spontaneously and immediately from magnetic nanoparticles and elastomeric polymer in a liquid solvent by bottom-up self-assembly. The length of prepared cilia could be in the scale of millimeter and reach a high aspect ratio of even over 100. We studied the effect of the magnetic strength applied and the size of nanoparticles to get tunable scale of cilia. The cilia show reversibly bending in an external magnetic field and this bending actuation gave some important functions: to transport macroscopic nonmagnetic materials on the cilia and to mix liquids.
Keywords: artificial cilia; magnetic nanoparticle; actuation; liquids mixing

Plasma-assisted molecular beam epitaxy of ZnO on in-situ grown GaN/4H-SiC buffer layers by David Adolph; Tobias Tingberg; Thorvald Andersson; Tommy Ive (185-191).
Plasma-assisted molecular beam epitaxy (MBE) was used to grow ZnO (0001) layers on GaN(0001)/4H-SiC buffer layers deposited in the same growth chamber equipped with both N- and O-plasma sources. The GaN buffer layers were grown immediately before initiating the growth of ZnO. Using a substrate temperature of 440°C–445°C and an O2 flow rate of 2.0–2.5 sccm, we obtained ZnO layers with smooth surfaces having a root-mean-square roughness of 0.3 nm and a peak-to-valley distance of 3 nm shown by AFM. The FWHM for X-ray rocking curves recorded across the ZnO(0002) and $$ZnO(10ar 15)$$ reflections were 200 and 950 arcsec, respectively. These values showed that the mosaicity (tilt and twist) of the ZnO film was comparable to corresponding values of the underlying GaN buffer. It was found that a substrate temperature > 450°C and a high Zn-flux always resulted in a rough ZnO surface morphology. Reciprocal space maps showed that the in-plane relaxation of the GaN and ZnO layers was 82.3% and 73.0%, respectively and the relaxation occurred abruptly during the growth. Room-temperature Hall-effect measurements showed that the layers were intrinsically n-type with an electron concentration of 1019 cm−3 and a Hall mobility of 50 cm2·V−1·s−1.
Keywords: ZnO; molecular beam epitaxy (MBE); epitaxy

The nanosized hydroxyapatite substituted by fluoride and carbonate ions (CFHA) had been synthesized by aqueous precipitation method. CFHA had been considered as potential bone graft material for orthopedic and dental applications. The objective of this study was to determine the effects of simultaneously incorporated CO 3 2− and F on the substitution type and content. The morphologies of CFHAs were observed by TEM. The carbonate substitution type and content were characterized by FTIR. The fluoride contents were determined by F-selective electrode. The phase compositions and crystallinity of the samples were investigated by XRD. The fluoride and carbonate contents of CFHA increase with the dopant concentrations nonlinearly. The carbonate substitution has much more obvious effect on morphology compared with the fluoride substitution. The co-existence of CO 3 2− and F ions can influence the corresponding substitution fraction. The isomorphic substitution of sodium for calcium in the substitution process of CO 3 2− can improve crystal degree and favor the B-type substitutions. Due to the closeness of the ion radii and equivalent substitution of F and OH, F will occupy the OH sites of HA crystals more easily, compelling most of the CO 3 2− to be located in the B sites.
Keywords: hydroxyapatite; co-substitution; carbonate; fluoride; substitution type; substitution content

Electrochemical performance of overlithiated Li1+xNi0.8Co0.2O2: structural and oxidation state studies by Roshidah Rusdi; Norlida Kamarulzaman; Kelimah Elong; Hashlina Rusdi; Azilah Abd-Rahman (199-205).
Pure, layered compounds of overlithiated Li1+xNi0.8Co0.2O2 (x = 0.05 and 0.1) were successfully prepared by a modified combustion method. XRD studies showed that cell parameters of the material decreased with increasing the lithium content. SEM revealed that the morphology of particles changed from rounded polyhedral-like crystallites to sharp-edged polyhedral crystals with more doped lithium. EDX showed that the stoichiometries of Ni and Co agrees with calculated synthesized values. Electrochemical studies revealed the overlithiated samples have improved capacities as well as cycling behavior. The sample with x = 0.05 shows the best performance with a specific capacity of 113.29 mA·h·g−1 and the best capacity retention of 92.2% over 10 cycles. XPS results showed that the binding energy of Li 1s is decreased for the Li doped samples with the smallest value for the x = 0.05 sample, implying that Li+ ions can be extracted more easily from Li1.05Ni0.8Co0.2O2 than the other stoichiometries accounting for the improved performance of the material. Considerations of core level XPS peaks for transition metals reveal the existence in several oxidation states. However, the percentage of the +3 oxidation state of transition metals for the when x = 0.1 is the highest and the availability for charge transition from the +3 to +4 state of the transition metal during deintercalation is more readily available.
Keywords: overlithiation; LiNi0.8Co0.2O2 ; interstitial doped; Li1.05Ni0.8Co0.2O2 ; Li1.1Ni0.8Co0.2O2

The relation between the microstructure, observed using an electron probe microanalyzer, and the domain structure, observed using a Kerr microscope, was established to evaluate the effects of hot rolling and the addition of Ti-C on the c-axis orientation and the magnetization process of hot-rolled Nd-Fe-B-Ti-C melt-spun ribbons. The addition of Ti-C promotes the c-axis orientation and high coercivity in the ribbons. Elemental mapping suggests a uniform elemental distribution; however, an uneven distribution of Ti was observed in an enlarged grain with Ti-enriched points inside the grain. The reversal domains that nucleated at the Ti-enriched point inside the grain cause low coercivity.
Keywords: magnetic domain; microstructure; Kerr effect microscope; magneto-optical effect; Nd-Fe-B permanent magnet