D. S. Negi

High thermoelectric performance in tellurium free p-type AgSbSe2

Enhanced electrical transport and ultra low thermal conductivity resulted in a high thermoelectric figure of merit, ZT, of ∼1 and ∼1.15 at ∼680 K in 4 mol% Pb and 2 mol% Bi doped AgSbSe2, which are 150 and 190% higher compared to that of the pristine sample, respectively. With this excellent thermoelectric performance, p-type AgSbSe2, constituting earth abundant Se, offers promise to replace traditional metal tellurides containing expensive and scarce Te for mid temperature (350–700 K) thermoelectric applications.

Nanostructuring, carrier engineering and bond anharmonicity synergistically boost the thermoelectric performance of p-type AgSbSe2–ZnSe

Thermoelectric “waste heat-to-electrical energy” generation is an efficient and attractive option for robust and environmentally friendly renewable energy production. Simultaneous tailoring of interdependent thermoelectric parameters, i.e. electrical conductivity, thermopower and thermal conductivity, to improve the thermoelectric figure of merit is the utmost challenge in this field. Another important aspect is to develop high performance materials based on cheap and earth abundant materials. We have chosen AgSbSe2, a homologue of AgSbTe2 containing earth abundant selenium, as a model system for thermoelectric investigation due to its low thermal conductivity and favourable valence band structure. Herein, we show that by integrating different but synergistic concepts: (a) carrier engineering, (b) second phase endotaxial nanostructuring and (c) bond anharmonicity, we can achieve a maximum ZT of ∼1.1 at 635 K in AgSbSe2–ZnSe (2 mol%), which is significantly higher than that of pristine AgSbSe2. The above system therefore offers promise to replace traditional metal tellurides for mid-temperature power generation. We demonstrate a design strategy which provides simultaneous enhancement of electrical transport through optimized doping, superior thermopower by the convergence of degenerate valence bands, and glass-like thermal conductivity due to the effective scattering of phonons by nanostructuring, bond anharmonicity and a disordered cation sublattice.

Robust room temperature ferromagnetism in epitaxial CoO thin film

Co vacancy (VCo) induced robust room temperature ferromagnetism (Ms ∼ 60 emu/cm3 and coercivity ∼ 603 Oe) is experimentally realized in rock-salt CoO epitaxial thin film (∼110 nm) grown by pulsed laser deposition. Co charge state is found to be higher ∼+3.2 (from Co L3/L2 white line ratio) and this is due to the VCo induced charge transfer from the neighboring Co-3d to O-2p states in order to compensate for the hole formation. O-K and cathodoluminescence spectra corroborate the existence of VCo and higher charge state. Temperature dependent magnetization and exchange bias experiments confirm the coexistence of ferromagnetic and antiferromagnetic phases.

Direct evidence of strong local ferroelectric ordering in a thermoelectric semiconductor

It is thought that the proposed new family of multi-functional materials, namely, the ferroelectric thermoelectrics may exhibit enhanced functionalities due to the coupling of the thermoelectric parameters with ferroelectric polarization in solids. Therefore, the ferroelectric thermoelectrics are expected to be of immense technological and fundamental significance. As a first step towards this direction, it is most important to identify the existing high performance thermoelectric materials exhibiting ferroelectricity. Herein, through the direct measurement of local polarization switching, we show that the recently discovered thermoelectric semiconductor AgSbSe2 has local ferroelectric ordering. Using piezo-response force microscopy, we demonstrate the existence of nanometer scale ferroelectric domains that can be switched by external electric field. These observations are intriguing as AgSbSe2 crystalizes in cubic rock-salt structure with centro-symmetric space group (Fm–3m), and therefore, no ferroelect...

Native defect induced charge and ferromagnetic spin ordering and coexisting electronic phases in CoO epitaxial thin film

We report on the observation of Co vacancy (VCo) induced charge ordering and ferromagnetism in CoO epitaxial thin film. The ordering is associated with the coexistence of commensurate, incommensurate, and discommensurate electronic phases. Density functional theory calculation indicates the origin of ordering in Co atoms undergoing high spin to low spin transition immediately surrounding the VCo(16.6 at. %). Electron magnetic chiral dichroism experiment confirms the ferromagnetic signal at uncompensated Co spins. Such a native defects induced coexistence of different electronic phases at room temperature in a simple compound CoO is unique and adds to the richness of the field with the possibility of practical device application.

Spatially resolved quantitative magnetic order measurement in spinel CuCr2S4 nanocrystals

We have utilized spatially resolved high resolution electron energy loss spectroscopy to quantify the relative percentage of ferromagnetic order in the core and the surface regions of CuCr2S4 nanoparticles with nanocube and nanocluster morphology. The organic capping layer is found to play a significant role in restoring magnetic order at the surface. The technique is based on recording the fine features of the Cr L3 absorption edge and matching them with the theoretical spectra. The nanoscale probing technique we have developed is quite versatile and can be extended to understand magnetic ordering in a number of nanodimensional magnetic materials.

Bulk Single Crystal‐Like Structural and Magnetic Characteristics of Epitaxial Spinel Ferrite Thin Films with Elimination of Antiphase Boundaries

Spinel ferrite NiFe2 O4 thin films have been grown on three isostructural substrates, MgAl2 O4 , MgGa2 O4 , and CoGa2 O4 using pulsed laser deposition. These substrates have lattice mismatches of 3.1%, 0.8%, and 0.2%, respectively, with NiFe2 O4 . As expected, the films grown on MgAl2 O4 substrate show the presence of the antiphase boundary defects. However, no antiphase boundaries (APBs) are observed for films grown on near-lattice-matched substrates MgGa2 O4 and CoGa2 O4 . This demonstrates that by using isostructural and lattice-matched substrates, the formation of APBs can be avoided in NiFe2 O4 thin films. Consequently, static and dynamic magnetic properties comparable with the bulk can be realized. Initial results indicate similar improvements in film quality and magnetic properties due to the elimination of APBs in other members of the spinel ferrite family, such as Fe3 O4 and CoFe2 O4 , which have similar crystallographic structure and lattice constants as NiFe2 O4 .

Surface spin canting in Fe3O4 and CoFe2O4 nanoparticles probed by high-resolution electron energy loss spectroscopy

High resolution electron energy loss spectroscopy (HR-EELS) is utilized to probe the surface spin canting in nanoparticles of two technologically important magnetic materials, i.e. Fe3O4 and CoFe2O4 (CFO). A soft experimental technique is developed that is capable of extracting EELS spectra with one atomic plane resolution recorded in a single frame. This yields information at different depth of the nanoparticle from the surface to the core regions with high signal to noise ratio and without beam damage. This enables comparing the fine structures between the surface and core regions of the nanoparticles. The results confirm earlier observations of uniformly oriented spin canting structure for CFO with additional information on atom site-selective spin canting information. In case of Fe3O4 preferred canting orientation forming core and shell structure is deduced. Unlike earlier reports based on polarized spin-flip neutron scattering measurement, it is possible to narrow down the possible canting angles for Fe3O4 (Td, Oh tilts 40{\deg}, 40{\deg}) and CFO (Td, Oh tilts 17{\deg}, 17{\deg}) from the experimental spectra combined with the first principle based calculation considering non-collinear magnetism. In addition, the role of Dzyaloshinskii-Moriya interaction in stabilizing the spin canting at the nanoparticle surface is discussed. The results demonstrate that HREELS can be a powerful technique to probe the magnetic structure in nano-dimensional systems and has advantages over neutron based techniques in terms of superior spatial resolution, site specific information and easy of sample preparation.