Band Engineering of Thermoelectric Materials

Lead chalcogenides have long been used for space‐based and thermoelectric remote power generation applications, but recent discoveries have revealed a much greater potential for these materials. This renaissance of interest combined with the need for increased energy efficiency has led to active consideration of thermoelectrics for practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity. The simple high symmetry NaCl‐type cubic structure, leads to several properties desirable for thermoelectricity, such as high valley degeneracy for high electrical conductivity and phonon anharmonicity for low thermal conductivity. The rich capabilities for both band structure and microstructure engineering enable a variety of approaches for achieving high thermoelectric performance in lead chalcogenides. This Review focuses on manipulation of the electronic and atomic structural features which makes up the thermoelectric quality factor. While these strategies are well demonstrated in lead chalcogenides, the principles used are equally applicable to most good thermoelectric materials that could enable improvement of thermoelectric devices from niche applications into the mainstream of energy technologies.     

Elastic and mechanical properties of intrinsic and doped PbSe and PbTe studied by first-principles

Lead chalcogenides, most notably lead selenide (PbSe) and lead telluride (PbTe), have become an active area of research due to their thermoelectric (TE) properties. The high figure of merit of these materials has brought much attention to them, due to their ability to convert waste heat into electricity. Recent efforts, such as applying pressure or doping, have shown an increase in TE efficiency. Variation in application and synthesis conditions gives rise to a need for analysis of mechanical properties of these materials. In addition to the rocksalt (NaCl) structure at ambient conditions, lead chalcogenides have an orthorhombic (Pnma) intermediate pressure phase and a higher pressure CsCl phase. By using first-principles calculations, performed within density functional theory, we study the structural, elastic and mechanical properties of PbTe and PbSe in their three phases. For each phase, elastic constants, bulk modulus, shear modulus, and Young's modulus are calculated, and the NaCl phase is studied with typical dopants, both n-type (Bi and I) and p-type (Na, In, and Tl). Pugh's ratio is employed to give insight on the brittleness of the materials and phase studied. The results presented here will be useful to guide future experiments toward the search for structurally stable TE materials.     

Statistical analysis of atmospheric trace metals and particulate fractions in Islamabad, Pakistan

Airborne suspended particulate matter was collected on glass fibre filters in urban atmosphere of Islamabad, Pakistan, using high volume sampler. The particulate samples were analysed for 10 selected metals (Fe, Na, Zn, K, Pb, Mn, Cr, Ni, Co and Cd) by FAAS method. Maximum mean contribution was noted for Fe (1.761microg/m(3)), followed by Na (1.661microg/m(3)), Zn (1.021microg/m(3)), K (0.488microg/m(3)) and Pb (0.128microg/m(3)). The particle size determination on vol.% basis for nine fractions (PM(<1.0), PM(1.0-2.5), PM(2.5-5), PM(5-10), PM(10-15), PM(15-25), PM(25-50), PM(50-100) and PM(>100)) was carried out using Mastersizer. PM(5.0-10) were found to be most abundant in the local atmosphere followed by PM(2.5-5.0) and PM(15-25) while coarse/giant particles (PM(50-100) and PM(>100)) showed lower contribution. The trace metals were found to be mainly associated with smaller particulate fractions up to PM(10-15). Among the climatic parameters temperature has significant relationship with fine particles and airborne metal levels while relative humidity showed negative correlation. The source identification was carried out by principal component analysis and cluster analysis. Five metal sources were identified: industrial, vehicular emissions, metallurgical operations, garbage incineration and soil derived dust. The metal levels were also compared with those reported for other rural and urban parts around the world.     

Transition metal chalcogenides exhibiting quasi-one-dimensional behaviour

This paper presents a survey of transition metal chalcogenides (mainly sulphides and selenides) that exhibit unidimensional structural features and electronic properties arising therefrom. The survey indicates that linear, single-atom, chains of transition metals are formed in chalcogenides by sharing faces of MX₆ (X=chalcogen) trigonal prisms or octahedra as well as corners or edges of MX₄ tetrahedra. Besides these single-atom chain compounds, chalcogenides possessing multiple-atom chains are known among the early members of the transition series when the transition metal is in a low formal oxidation state. Typical examples of this class are Ti₅Te₄ and TlMo₃Se₃. Contribution No. 223 from the Solid State and Structural Chemistry Unit.     

A New,Thorough Look on Unusual and Neglected Group III-VI Compounds Toward Novel Perusals

The attention on group III-VI compounds in the last decades has been centered on the optoelectronic properties of indium and gallium chalcogenides. These outstanding properties are leading to novel advancements in terms of fundamental and applied science. One of the advantages of these compounds is to present laminated structures, which can be exfoliated down to monolayers. Despite the large knowledge gathered toward indium and gallium chalcogenides, the family of the group III-VI compounds embraces several other noncommon compounds formed by the other group III elements. These compounds present various crystal lattices, among which a great deal is offered from layered structures. Studies on aluminium chalcogenides show interesting potential as anodes in batteries and as semiconductors. Thallium (Tl), which is commonly present in the +1 oxidation state, is one of the key components in ternary chalcogenides. However, binary Tl–Q (Q = S, Se, Te) systems and derived films are still studied for their semiconducting and thermoelectric properties. This review aims to summarize the biggest features of these unusual materials and to shed some new light on them with the perspective that in the future, novel studies can revive these compounds in order to give rise to a new generation of technology.     

In Situ Repair of 2D Chalcogenides under Electron Beam Irradiation

Molybdenum disulfide (MoS₂) and bismuth telluride (Bi₂Te₃) are the two most common types of structures adopted by 2D chalcogenides. In view of their unique physical properties and structure, 2D chalcogenides have potential applications in various fields. However, the excellent properties of these 2D crystals depend critically on their crystal structures, where defects, cracks, holes, or even greater damage can be inevitably introduced during the preparation and transferring processes. Such defects adversely impact the performance of devices made from 2D chalcogenides and, hence, it is important to develop ways to intuitively and precisely repair these 2D crystals on the atomic scale, so as to realize high‐reliability devices from these structures. Here, an in situ study of the repair of the nanopores in MoS₂ and Bi₂Te₃ is carried out under electron beam irradiation by transmission electron microscopy. The experimental conditions allow visualization of the structural dynamics of MoS₂ and Bi₂Te₃ crystals with unprecedented resolution. Real‐time observation of the healing of defects at atomic resolution can potentially help to reproducibly fabricate and simultaneously image single‐crystalline free‐standing 2D chalcogenides. Thus, these findings demonstrate the viability of using an electron beam as an effective tool to precisely engineer materials to suit desired applications in the future.     

Ground state properties and structural phase transition of beryllium chalcogenides

The ground state properties and the structural phase transformation of beryllium chalcogenides (BeS, BeSe, and BeTe) have been investigated using first principle full potential-linearized augmented plane wave method (FP-LAPW) within density functional theory. We used local density approximation with and without generalized gradient correction as well as the Engel Vosko’s GGA formalism to find band gap. From the obtained band structures, the electron (hole) valence and conduction effective masses are deduced. We have determined the full set of first-order elastic constant, which have not been established experimentally. We have also calculated the energy–volume relations for these compounds in the zinc-blende (B3) and the NiAs (B8) phases. Hence we have obtained the lattice parameters, bulk modulus, pressure derivative of bulk modulus and cohesive energy as well as structural transition pressure. The calculated ionicity parameter which expected from the charge density behavior compared well with the Phillips’ ionicity scale.     

Heat Capacity and Mössbauer Study of Self-Flux Grown FeTe Single Crystal

We report mainly the heat capacity and Mössbauer study of self-flux grown FeTe single crystal, which is a ground state compound of the Fe chalcogenides superconducting series i.e. FeTe_1?x(Se/S) _x The as grown FeTe single crystal is large enough to the tune of a few centimetres and the same crystallizes in tetragonal structure having space group of P4/nmm. FeTe shows the structural/magnetic phase transition at 70 K in both magnetic and resistivity measurements. Heat capacity measurement also confirms the coupled structural/magnetic transition at the same temperature. The Debye model fitting of low temperature (below 70 K) heat capacity exhibited Debye temperature (?? _D ) to be 324 K. Mössbauer spectra are performed at 300 and 5 K. The 300-K spectra showed two paramagnetic doublets and the 5-K spectra exhibited hyperfine magnetic sextet with an average hyperfine field of 10.6 Tesla matching with the results of Yoshikazu Mizuguchi et al.     

Optically stimulated luminescence from Al2O3:C irradiated with relativistic heavy ions.

Optically stimulated luminescence (OSL) from Al2O3:C (ALOC) irradiated with selected heavy ions (4He, 12C, 40Ar, and 56Fe) was examined for discussion on the effectiveness of ALOC for space radiation protection dosimetry. The OSL efficiency on the absorbed dose basis was almost unity for He (LETinfinity x H2O: 2.2 keV x microm(-1)) and decreased with increasing LET for C (14 keV x microm(-1)), Ar (91 keV x microm(-1)), and Fe (198 keV x microm(-1)); a notable reduction greater than 60%, was observed for Fe ions. The linearity in dose response and the angular independence for the heavy ions were fairly good (+/- <15%) Although further experimental studies are clearly necessary, these results suggest that small ALOC chips can be a part of an integrating dosimetry system in future space missions.     

Three-in-One Strategy Constructing the First High-Performance Nonlinear Optical Sulfide Crystallizing with the P43 Space Group

The balance between large nonlinear optical (NLO) effect and wide bandgap is the key scientific issue for the exploration of infrared NLO materials. Targeting this issue, two new pentanary chalcogenides KGaGe_1.37Sn_0.63S₆ ( 1 ) and KGaGe_1.37Sn_0.63Se₆ ( 2 ) are obtained by the three-in-one strategy, viz. three types of fourfold-coordinated metal elements co-occupying the same site. They crystallize in the tetragonal P4₃ ( 1 ) and monoclinic Cc ( 2 ) space group. Their structures can be evolved from benchmark AgGaS₂ (AGS) by suitable substitution. Remarkably, 1 is the first NLO sulfide crystallizing with the P4₃ space group, representing a new structure-type NLO material. The structural relationship between 1 and 2 and the evolution from 1 , 2 to AGS are also analyzed. Both 1 and 2 show balanced NLO properties. Specifically, 1 exhibits phase-matchable SHG response of 0.6 × AGS, a wide bandgap of 3.50 eV, and a high laser damage threshold of 6.24 × AGS. Theoretical calculation results suggest that the Ga/Ge/Sn element ratios of the co-occupied sites of 1 and 2 are the most appropriate for stabilizing the structures. The strategy adopted here will provide some inspiration for exploring new high-performance NLO materials.     

Electroanalytical performance of nitrogen-containing tetrahedral amorphous carbon thin-film electrodes

Tetrahedral amorphous carbon (ta-C) consists of a mixture of sp(3)- and sp(2)-bonded carbon ranging from 60 to 40% (sp(3)/sp(3)+sp(2)) depending on the deposition conditions. The physical, chemical, and electrochemical properties depend on the sp(2)/sp(3) bonding ratio as well as the presence of incorporated impurities, such as hydrogen or nitrogen. The ability to grow ta-C at lower temperatures (25-100 °C) on a wider variety of substrates as compared to CVD diamond is an advantage of this material. Herein, we report on the structural and electrochemical properties of nitrogen-incorporated ta-C thin films (ta-C:N). The incorporation of nitrogen into the films decreases the electrical resistivity from 613 ± 60 (0 sccm N(2)) to 1.10 ± 0.07 Ω-cm (50 sccm N(2)), presumably by increasing the sp(2)-bonded carbon content and the connectedness of these domains. Similar to boron-doped diamond, these materials are characterized by a low background voltammetric current, a wide working potential window (~ 3 V), and relatively rapid electron-transfer kinetics for aqueous redox systems, including Fe(CN)(6)(-3/-4) and Ru(NH(3))(6)(+3/+2), without conventional pretreatment. Additionally, there is weak molecular adsorption of polar molecules (methylene blue) on the ta-C surface. Overall, the properties of the ta-C and ta-C:N electrodes are such that they could be excellent new choices for electroanalytical measurements.     

Understanding the Structure and Properties of Sesqui‐Chalcogenides (i.e., V2VI3 or Pn2Ch3 (Pn = Pnictogen,Ch = Chalcogen) Compounds) from a Bonding Perspective

A number of sesqui‐chalcogenides show remarkable properties, which make them attractive for applications as thermoelectrics, topological insulators, and phase‐change materials. To see if these properties can be related to a special bonding mechanism, seven sesqui‐chalcogenides (Bi₂Te₃, Bi₂Se₃, Bi₂S₃, Sb₂Te₃, Sb₂Se₃, Sb₂S₃, and β‐As₂Te₃) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui‐chalcogenides (Bi₂Te₃, Bi₂Se₃, Sb₂Te₃, and β‐As₂Te₃) show an unconventional bond‐breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal‐like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui‐chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum‐chemistry‐based bonding analysis employing charge partitioning, which reveals that the four sesqui‐chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.     

Characterization of mesoporous rice husk ash (RHA) and adsorption kinetics of metal ions from aqueous solution onto RHA

The present study deals with the characterization of low-cost rice husk ash (RHA) for its various physico-chemical properties and adsorption characteristics of metal ions. The average particle size of RHA was 150.47mum. Proximate analysis showed the presence of high amount of ash in RHA. Bulk density and the heating value of RHA were 104.9kg/m(3) and 9.68MJ/kg, respectively. The pore size distribution results showed that the RHA was predominantly mesoporous. The BET surface area was 36.44m(2)/g. The average pore diameter by BET was 42.603A. The BJH pore area showed 80% of the pore area due to the mesopores. The polar groups present on the RHA surface imparted considerable cation exchange capacity to it. RHA was found to be an effective adsorbent for the removal of cadmium (Cd(II)), nickel (Ni(II)) and zinc (Zn(II)) metal ions from aqueous solutions. The pH(0) approximately 6.0 is found to be the optimum for the removal of individual cations from the aqueous solutions by RHA at an optimum dose of 10kg/m(3). The kinetics of adsorption showed that the metal ions adsorption on RHA is a gradual process with quasi-equilibrium being attained in 5h. The pseudo-second-order kinetics represents the equilibrium data well. The effective diffusion coefficient of the cations onto the RHA is of the order of 10(-13)m(2)/s.     

Isotope and affinity tags in photoreactive substance P analogues to identify the covalent linkage within the NK-1 receptor by MALDI-TOF analysis

Photoreactive analogues of substance P (biotin sulfone-spacer (amino pentanoic or Gly(3))-Arg-Pro-Lys-Pro-(pBzl)Phe-Gln-Phe-Phe-Gly-Leu-Met(O(2))NH(2)) with or without isotope (deuterium) labeling have been synthesized. Deuteriums were present on (d)-biotin or epibiotin sulfone (D(3)), on the Gly(3) spacer linker (D(6)), or on the Gly in position 9 of SP (D(2)). Therefore, peptide analogues could be either unlabeled or tri-, penta-, or hexadeuterated. Results obtained with the use of these peptide analogues show that (d)-biotin sulfone and epibiotin sulfone are not recognized with the same affinity by streptavidin, with (d)-biotin sulfone displaying better affinity for the protein. Photolabeling of the human NK-1 receptor with a 1:1 molar ratio of nondeuterated and deuterated photoreactive substance P (SP) analogues in position 5, followed by combined digestions, purification, and MALDI-TOF mass spectrometry analysis, made the identification of the domain of the receptor covalently linked by the photoreactive SP analogue easier. Indeed, doublets in mass spectra were specific for the covalent complex whereas single peaks could be attributed to contaminating species. This method is particularly suitable when minute amounts of complex have to be analyzed, as in the case of highly hydrophobic G-protein coupled receptors.     

Effect of TiO2 nanotube parameters on field emission properties

The dependence of field emission properties of titanium dioxide (TiO(2)) nanotubes (NTs) has been studied as a function of NT diameter (D) and height (h), which varied in the ranges 18-500 nm and 500-12,000 nm, respectively. The studies showed a strong dependence of the field emission on these parameters. With an increase of NT diameter, the field enhancement factor increased monotonically from 120 to 3800; the current density also increased until D = 320 (current density ～ 3.8 mA cm( - 2)), with subsequent decrease for larger diameters. The field emission properties initially improved with NT height until h = 5 μm, and later remained unchanged with further increases in h.     

Creating gold nanoprisms directly on quartz crystal microbalance electrodes for mercury vapor sensing

A novel electrochemical route is used to form highly {111}-oriented and size-controlled Au nanoprisms directly onto the electrodes of quartz crystal microbalances (QCMs) which are subsequently used as mercury vapor sensors. The Au nanoprism loaded QCM sensors exhibited excellent response-concentration linearity with a response enhancement of up to ～ 800% over a non-modified sensor at an operating temperature of 28?°C. The increased surface area and atomic-scale features (step/defect sites) introduced during the growth of nanoprisms are thought to play a significant role in enhancing the sensing properties of the Au nanoprisms toward Hg vapor. The sensors are shown to have excellent Hg sensing capabilities in the concentration range of 0.123-1.27 ppm(v) (1.02-10.55 mg m(-3)), with a detection limit of 2.4 ppb(v) (0.02 mg m(-3)) toward Hg vapor when operating at 28?°C, and 17 ppb(v) (0.15 mg m(-3)) at 89?°C, making them potentially useful for air monitoring applications or for monitoring the efficiency of Hg emission control systems in industries such as mining and waste incineration. The developed sensors exhibited excellent reversible behavior (sensor recovery) within 1 h periods, and crucially were also observed to have high selectivity toward Hg vapor in the presence of ethanol, ammonia and humidity, and excellent long-term stability over a 33 day operating period.     

Physicochemical characterization and Bioremediation perspective of textile effluent, dyes and metals by indigenous Bacteria

Physicochemical and bacteriological status of a local textile mill effluent showed considerably high values of temperature (40 degrees C), pH (9.50), EC (3.57mus/m), BOD (548mgl(-1)), COD (1632mgl(-1)), TSS (5496mgl(-1)), TDS (2512mgl(-1)), heavy metals ions (0.28-6.36mgl(-1)) and color above the prescribed fresh water limits. However, a considerable decline in almost all pollution indicators from source to sink indicated signs of natural remediation. Ten bacteria strains isolated from effluent showed comparatively higher resistance (MRL) (mgl(-1)) (average) for 10 heavy metals than against four structurally different dyes tested on solid media of mineral salt. Overall bacterial resistance was quite high against Fe(3+) (2820), Cr(3+) (1203), Zn(2+) (1122), Mn(2+) (804) and Pb(2+) (435), whereas, it varied amid 300-500 in four dyes. Bacterial decolorization/degradation of dyes indicated on solid media was confirmed through experiments carried out in liquid broth.     

Enhanced photocatalytic of N/F-doped-NaTaO3 photocatalyst synthesized by hydrothermal method

N/F-doped NaTaO₃ powders were synthesized via a hydrothermal method at 160 °C. The influences of N/F co-doping on the crystal structure, morphology and photocatalytic properties of the NaTaO₃ powders were investigated systematically. Rietveld refinement of X-ray diffraction data confirm that the nanoparticles of pure NaTaO₃ have orthorhombic structures with Pc21n:bca and Ta₂O₅ can be obtained by doping N/F molar ratio (NaOH/NH₄F ≤ 1) with the orthorhombic P2 mm:cab space group. The NaTaO₃ powders are transformed from orthorhombic Pc21n:bca into monoclinic P2/m:b space group with the NaOH/NH₄F molar ratio (NaOH/NH₄F > 1). The well-defined cubic block NaTaO₃ single-crystalline particles with the size of 0.5 μm are determined using TEM and grow preferentially along (101) and (010) crystal planes. The photocatalytic activities were evaluated by the degradation of Rhodamine B under UV-light irradiation. The possible mechanism is proposed to discuss the enhanced photocatalytic activity based on the increasing separation efficiency of photoinduced charge. The results illustrate that the obtained monoclinic N/F-NaTaO₃ (NaOH/NH₄F = 1.5/1.0) powder exhibits the best photocatalytic activity with the UV-light irradiation for 210 min.     

First-principles investigations of structural,optoelectronic and thermoelectric properties of Cu-based chalcogenides compounds

Journal of Materials Science - The structural, electronic, optical and thermoelectric properties of copper-based ternary chalcogenides ACuSe2 (A?=?Sc, Y and La) were investigated within...     

Pressure effects on the phonon modes in beryllium chalcogenides

We report first-principles calculations of the structural, lattice-dynamical and dielectric properties for zinc-blend beryllium chalcogenides: BeS, BeSe, and BeTe. The ground state properties, such as the lattice structure and bulk modulus, are calculated using a plane wave pseudopotential method within the density functional theory. A linear response approach is employed in order to derive the high-frequency dielectric constants, Born effective charges and phonon frequencies. Furthermore, the pressure dependence of phonon modes is also detailed.