In the calculations of lattice thermal conductivity ($\kappa_{\text{L}}$), vital contributions stemming from four-phonon scattering are often neglected. The significance of four-phonon scattering in the thermal transport properties of monolayer (ML) MoS$_{2}$ has been unraveled using first-principles calculations combined with the Boltzmann transport equation. If only three-phonon scattering processes are considered then the $\kappa_{\text{L}}$ is found to be significantly overestimated ($\sim$ 115.8 Wm$^{-1}$K$^{-1}$ at 300 K). With the incorporation of the four-phonon scattering processes, the $\kappa_{\text{L}}$ reduces to 24.6 Wm$^{-1}$K$^{-1}$, which is found to be closer to the experimentally measured $\kappa_{\text{L}}$ of 34.5 Wm$^{-1}$K$^{-1}$. Four-phonon scattering significantly impacts the carrier lifetime ($\tau$) of the low-energy out-of-plane acoustic mode (ZA) phonons and thereby, suppresses its contribution in $\kappa_{\text{L}}$ from 64% (for three-phonon scattering) to 16% (for both three- and four-phonon scatterings). The unusually high four-phonon scattering rate ($\tau_{4}^{-1}$) of the ZA phonons is found to result from the simultaneous effect of the acoustic-optical frequency gap, strong anharmonicity, and the reflection symmetry imposed selection rule. The strong coupling between the quadratic dispersion of the ZA mode and the $\tau_{4}^{-1}$ is discovered by the application of mechanical strain. The strain induced increase in the linearity of the ZA mode dispersion dramatically reduces the significance of the four-phonon scattering in the strained ML-MoS$_{2}$, both qualitatively and quantitatively. These conclusions will provide significant insights into the thermal transport phenomena in ML-MoS$_{2}$, as well as any other 2D material.

First principles density functional theory based calculations have been performed to investigate the strain and temperature induced tunability of the thermoelectric properties of monolayer (ML) MoS2. Modifications in the electronic and phononic transport properties, under two anisotropic uniaxial strains along the armchair (AC) and zigzag (ZZ) directions, have been explored in detail. Considering the intrinsic carrier-phonon scattering, we found that the charge carrier mobility (μ) and relaxation time (τ) increase remarkably for strains along the ZZ direction. Concomitantly, strain along the ZZ direction significantly reduces the lattice thermal conductivity (κL) of ML-MoS2. The combined effect of shortened phonon relaxation time and group velocity, and the reduced Debye temperature is found to be the driving force behind the lowering of κL. The large reduction in κL and increase in τ, associated with the strains along the ZZ direction, act in unison to result in enhanced efficiency and hence, improved thermoelectric performance. Nearly 150% enhancement in the thermoelectric efficiency can be achieved with the optimal doping concentration. We, therefore, highlight the significance of in-plane tensile strains, in general, and strains along the ZZ direction, in particular, in improving the thermoelectric performance of ML-MoS2.

First-principles calculations have been performed on the 2H and the 1T' phase of single-layer MoS2. We have addressed the strain-induced tunability of the electronic and phononic properties of both phases, and compared their stability. For the 2H phase, a direct to indirect band gap transition, followed by the band gap lowering leading to a semiconductor to metal transition, is observed. The applied strain destroys the semimetallic nature of the 1T' phase. Significant strain-induced changes in the frequency and the dispersion of the phonon branches are observed. With increasing strain, the out-of-plane acoustic mode (ZA) turns imaginary, indicating a possibility of phase transition or instability of the crystal structure. The 2H phase appears to withstand a larger amount of strain indicating better stability compared to the 1T' phase. We highlight the significance of strain-engineering in tuning the electronic and phononic properties and the stability limit in different polymorphs of monolayer MoS2.

Density functional theory based calculations have been performed for solving the phonon Boltzmann transport equation to investigate the thermal transport properties of monolayer (ML) ZnO under in-plane isotropic biaxial tensile strain. The in-plane lattice thermal conductivity ($\kappa_{\text{L}}$) of ML-ZnO increases dramatically in response to the biaxial tensile strain ranging from 0% to 10%, conflicting with the general belief. The strain-induced stiffening of the ZA phonon mode and the resulting concomitant increase in group velocity and decrease in phonon population is found to play a significant role behind the unusual enhancement of $\kappa_{\text{L}}$. The mode resolved analysis shows the tensile strain driven competitive behavior between different phonon properties, mainly the group velocity and phonon lifetimes, being responsible for the observed unusual enhancement in $\kappa_{\text{L}}$. Additionally, the phonon scattering calculations show the importance of inclusion of 4-phonon scattering in the thermal transport calculations suggesting the significance of higher-order anharmonicity in ML-ZnO. A strikingly high 4-phonon scattering strength in ML-ZnO primarily results from the strong anharmonicity, quadratic ZA mode dispersion, large frequency gap in phonon dispersion, and reflection symmetry induced selection rule. The incorporation of 4-phonon scattering significantly alters the transport characteristics of all the phonon modes, in general and ZA phonons, in particular. At large strains, a linear dispersion of the ZA mode and closure of the frequency gap is observed, which results in a significant reduction of 4-phonon scattering strength in ML-ZnO.

The hydrostatic pressure induced changes in the transport properties of monolayer (ML) MoS$_2$ have been investigated using first-principles density functional theory based calculations. The application of pressure induces shift in the conduction band minimum (CBM) from K to $\Lambda$, while retaining the band extrema at K in around the same energy at a pressure of 10 GPa. This increase in valley degeneracy is found to have a significant impact on the electronic transport properties of ML-MoS$_2$ via enhancement of the thermopower (S) by up to 140\% and power factor (S$^{2}$$\sigma$/$\tau$) by up to 310\% at 300 K. Besides, the very low deformation potential (E$_\text{DP}$) associated with the CB-$\Lambda$ valley results in a remarkably high electronic mobility ($\mu$) and relaxation time ($\tau$). Additionally, the application of pressure reduces the room temperature lattice thermal conductivity ($\kappa_\text{L}$) by 20\% of its unstrained value, owing to the increased anharmonicity and resulting increase in the intrinsic phonon scattering rates. The hydrostatic pressure induced increase in power factor (S$^{2}$$\sigma$) and the decrease in $\kappa_\text{L}$ act in unison to result in a substantial improvement in the overall thermoelectric performance (zT) of ML-MoS$_2$. At 900 K with an external pressure of 25 GPa, zT values of 1.63 and 1.21 are obtained for electron and hole doping, respectively, which are significantly higher compared to the zT values at zero pressure. For the implementation in a thermoelectric module where both n-type and p-type legs should be preferably made of the same material, the concomitant increase in zT of ML-MoS$_2$ for both types of doping with hydrostatic pressure can be highly beneficial.

•Self-organized epitaxial γ-FeSi2 nanowires on 3° miscut vicinal on Si(111)-7 × 7.•γ-FeSi2 nanowires grow in three 〈110〉 equivalent in-plane directions on Si(111)-7 × 7.•Epitaxial CoSi2 growth on γ-FeSi2 nanowire decorated vicinal Si(111)-7 × 7 surfaces.•Self-organized cobalt disilcide preferably grows as triangular islands.•Self-organized distinct FeSi2 and CoSi2 fabrication on same vicinal on Si(111)-7 × 7.

First-principles calculations based on density functional theory, coupled with the semi-classical Boltzmann transport theory, have been performed to investigate the strain- and temperature-induced tunability of the thermoelectric properties of monolayer (ML) MoS 2 . The electronic band gap reduces with increasing tensile strain, and a semiconductor to metal transition occurs for 10% biaxial strain. Tensile strains, in most of the cases, are seen to have a diminishing effect on the thermopower and power factor of monolayer MoS 2 , with larger impact on the p-type carriers. Performing an-harmonic phonon calculations, it is seen that tensile strain, in general and uniaxial tensile strain along the zig-zag direction, in particular, significantly reduces the thermal conductivity of ML-MoS 2 . The combined effect of reduced phonon relaxation time, ZA-optical phonon frequency gap and the Debye temperature is found to be the driving force behind the lowering of the thermal conductivity. The large reduction in thermal conductivity and increase in power factor under the action of tensile strains along the zigzag direction act in concert to result in an enhanced efficiency and hence, improved thermoelectric performance. The thermoelectric efficiency of ML-MoS 2 is seen to increase with increasing temperature, suggesting its use as a high-temperature thermoelectric material. Nearly 75% enhancement in the thermoelectric efficiency can be achieved with optimal doping concentration. We, therefore, highlight the significance of in-plane tensile strains in improving the thermoelectric performance of ML-MoS 2 , which could open avenues for its application in emerging areas in 2D-thermoelectrics.

Epitaxial gamma-FeSi2 and CoSi2 nanostructures, grown on a vicinal Si(1 1 1)-7 x 7 surface by sequentially depositing 1-monolayer (ML) of Fe and 1ML of Co on a hot Si substrate under ultrahigh vacuum condition, have been investigated by in-situ scanning tunneling microscopy, ex-situ field emission scanning electron microscopy and x-ray photoemission spectroscopy (XPS). While gamma-FeSi2 nanostructures have grown as nanowires along the three equivalent (1 1 0) directions on the Si(1 1 1) surface, CoSi2 nanoislands have grown as equilateral triangular and trapezoidal islands. Such self-organized nanostructures may find applications in nanoscale devices including S-F hybrids for quantum technology, as CoSi2 is a superconductor (5) and gamma-FeSi2 is a ferromagnet (F).

First-principles density functional theory-based calculations have been performed to investigate the strain-induced modifications in the electronic and vibrational properties of monolayer (ML)-ZnO. A wide range of in-plane tensile and compressive strain along different directions are applied to analyse the modifications in detail. The electronic band gap decreases under both tensile and compressive strain, and a direct-to-indirect band gap transition occurs for high values of biaxial tensile strain. The relatively low rate of decrease of band gap and large required strain for direct-to-indirect band gap transition compared to other 2D materials are analysed. Systematic decrease in the frequency of the in-plane and increase in the out-of-plane optical phonon modes with increasing tensile strain are observed. The in-plane acoustic modes show linear dispersion for unstrained as well as strained cases. However, the out-of-plane acoustic mode (ZA), which shows quadratic dispersion in the unstrained condition, turns linear with strain. The dispersion of the ZA mode is analysed using the shell elasticity theory and the possibility of ripple formation with strain is analysed. The strain-induced linearity of the ZA mode indicates the absence of rippling under strain. Finally, the stability limit of ML-ZnO is investigated and found that for 18% biaxial tensile strain the structure shows instability with the emergence of imaginary phonon modes. Furthermore, the potential of ML-ZnO to be a good thermoelectric material is analysed in an intuitive way based on the calculated electronic and phononic properties. Our results, thus, not only highlight the significance of strain-engineering in tailoring the electronic and vibrational properties but also provide a thorough understanding of the lattice dynamics and mechanical strength of ML-ZnO.

Using in-situ scanning tunneling microscopy (STM) we have investigated the evolution of Si(111)-7×7 surfaces, prepared under ultrahigh vacuum condition, upon Ar+ ion sputtering and subsequent annealing. We have monitored how the surface atomic arrangement changes upon annealing of the sputtered Si(111)-7×7 surface. Sputtering renders the Si(111)-7×7 surface amorphous and rough. Annealing at 500˚C causes no recrystallization, although the surface roughness is reduced. When the sample is annealed at 600˚C, recrystallization starts producing short-range orders. Flat-top nanoislands with a height distribution appear. The top surface of these nanoislands is ordered; most islands have Si(111)-7×7 surface reconstruction, while there are also islands with other surface reconstructions, such as 5×5, 2×2 etc. Smaller silicon nanodots grow at the edges of these flat-top islands.

Quantum phenomena in nanostructures, especially epitaxial structures, are utilized for the fabrication of quantum devices. Here we discuss self organized growth of epitaxial quantum structures on silicon and their electronic structures, such as electronic density of states, and electronic transport in such systems. We discuss nanodots and nanowires, or an equivalent of zero-dimensional (0D) and one-dimensional (1D) structures in more details, and in general structures in all dimensional cross-over regimes. Further arrangements of quantum dots (QDs) in particular structures offers to form quantum bits and quantum dot molecules (QDMs). This could lead to exploration of new physics and new quantum devices.

We report the observation of inhomogeneous superconductivity (ISC) in the recently discovered high-density nonmagnetic (NM) phase of Co in thin films below an onset temperature ( ) of 5.4 K in the absence of external magnetic field, via four-probe measurements of resistivity. Further, the point-contact spectroscopy studies also confirm superconductivity in this system. We attribute the observed ISC to the presence of nanoscale grains of high-density non-magnetic Co (FCC structure) in a thin film of conductive, normal Co (HCP structure) which is magnetic. Incomplete superconducting transition found in the bulk measurements suggests that the observed phenomenon is due to ISC of nanoscale grains of NM phase of Co. In addition, using first-principles density functional and BCS theoretical analysis of Co under hydrostatic and volume-preserving-biaxial strains, we demonstrate that superconducting of its NM phase increases anomalously with strain near its transformation to ferromagnetic phase, as a result of softening of N phonon due to strong electron-phonon coupling that is further enhanced with the biaxial strain.

The reactive deposition epitaxy growth of self-organized cobalt silicide nanowires (NWs) on clean Si(110) surfaces has been investigated by in situ scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) as well as by scanning electron microscopy (SEM). Half a monolayer of cobalt was deposited on the Si(110) surfaces at ∼600 °C substrate temperature. Following cobalt deposition, the substrates have been annealed for different durations. Cobalt forms aligned cobalt disilicide nanowires upon reaction with the silicon substrate, following the twofold substrate symmetry. With increasing duration of annealing, the NWs have been found to grow with larger aspect ratio (length/width), eventually producing narrower NWs. These self-organized unidirectional NWs of sub-hundred nanometer width and ∼4–7 nm height produce a Schottky barrier with the silicon substrate and are expected to find applications in nanoelectronic devices.

We have studied the absorption behaviour at the L-edge of the recently discovered high density nonmagnetic (HDNM) cobalt in a polycrystalline cobalt thin film. Soft X-ray absorption spectroscopy (SXAS) experiment has shown a higher edge jump energy for HDNM cobalt compared to normal ferromagnetic cobalt. Electron energy loss spectroscopy (EELS) measurement has shown a lower absorption cross-section for HDNM cobalt compared to normal cobalt. As the thin film contained layers of HDNM Co and normal Co at different depths (Sci. Rep. 7, 41856 (2017)) a depth selective SXAS experiment was performed. In SXAS experiment the thin film sample was sputtered in order to obtain absorption spectra from different depths. We clearly observe a shift of the L-III edge energy to lower energy as soon as the high density layer is removed and normal Co layer is exposed to the X-ray beam. EELS experiments were carried out on high density grains, identified by transmission electron microscopy and Co fluorescence imaging. The results were compared with those from lower density or normal density grains. The energy loss peaks are less intense for high density cobalt.

We have obtained the essential features of x-ray waveguide formation in an ion-irradiated cobalt thin film on Si substrate. Ion irradiation was carried out with 1 MeV Si+ ions at a fluence of 5 x 10(16) ions cm(-2). X-ray reflectivity (XRR) experiment shows characteristic dips in the total external reflection region indicating formation of resonant electromagnetic modes-an essential requirement for x-ray waveguides. XRR analysis further reveals formation of a low electron-density layer sandwiched between higher electron-density layers. These are two key requirements for an x-ray waveguide. Computation ofx-ray intensity within the irradiated sample shows about 15 times flux enhancement of the electric field within the low electron-density guiding layer.

We have detected size-dependent modifications in magnetic ordering in charge ordered nanoparticles through the study of magnetocaloric effect. Due to prominent surface induced magnetism in nanoparticles, it is very difficult to probe the magnetic ordering in charge ordered antiferromagnetic nanoparticles from direct magnetization study, especially at lower magnetic fields, though at higher magnetic fields the antiferromagnetic/charge ordered transition can be visualized properly. We show that, at comparatively lower magnetic fields, the magnetocaloric effect provides a sensitive way for the detection of such ordering transitions.

We have investigated the evolution of magnetic behavior with ion fluence in ion irradiation of a Si/Ni/Si sandwich system. An oscillatory behavior of the saturation magnetic moment and the coercive field has been observed with increasing ion fluence. As reported earlier, with increasing ion fluence, this system also shows an oscillatory amorphization and recrystallization of the Si substrate on which the Ni layer was grown. Ion beam induced Ni redistribution, associated with the oscillatory behavior of amorphization and recrystallization of Si, appears to be responsible for the oscillatory magnetic behavior in the sandwich structure.

We have observed a significantly large magnetocaloric effect near room temperature for polycrystalline ferromagnetic La0.83Sr0.17MnO3 compound. The maximum value of the magnetic entropy change at the vicinity of the paramagnetic to ferromagnetic ordering temperature is about 8.0 J/kg-K in the presence of a 70 kOe external magnetic field. The present study indicates that the magnetic entropy change near room temperature is comparable to (or larger than) that for the suggested refrigerant materials reported earlier.

Various samples of Teflon chemically doped with lithium and sodium have been irradiated with hydrogen ions in order to study the depth profile using Rutherford backscattering (RBS) spectrometry which can detect even microgram of sample. Studies have also been done to see the effects of irradiation on the samples after and before doping them with sodium or lithium. Co-60 gamma-rays (1.17 and 1.33 MeV) and 1 MeV electrons have been used for irradiation.

Recently high density (HD) nonmagnetic cobalt has been discovered in a nanoscale cobalt thin film, grown on Si(111) single crystal. This form of cobalt is not only nonmagnetic but also superconducting. These promising results have encouraged further investigations of the growth of the nonmagnetic (NM) phase of cobalt. In the original investigation, the cobalt film had a natural cobalt oxide at the top. We have investigated whether the growth of HD NM cobalt layers in the thin film depends on (i) a capping layer on the cobalt film, (ii) the thickness of the cobalt film and (iii) the nature of the substrate on which the cobalt film is grown. The results of such investigations indicate that for cobalt films capped with a thin gold layer, and for various film thicknesses, HD NM cobalt layers are formed. However, instead of a Si substrate, when the cobalt films are grown on oxide substrates, such as silicon oxide or cobalt oxide, HD NM cobalt layers are not formed. The difference is attributed to the nature-crystalline or amorphous-of the substrate.