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Graphene (Gr)–transition metal dichalcogenide (TMDC) hybrids are promising platforms for achieving sensitive and ultra-fast photodetection. The process of photo-detection in such van der Waals hybrids is usually dictated by the formation of excitons followed by the transfer of charge and energy from the TMDC layer to graphene, but they have not been explored simultaneously in the same device before. In this work, we have investigated optically excited Gr–WS2(tungsten disulfide) heterostructures using both standard electrical transport and Johnson noise thermometry. At large negative gate voltages, the experimentally observed photoresponse cannot be explained from conventional photogating but was found to host an increase in electron temperature as large as [Formula: see text] K. Time dependence of the transport and the noise reveals that the change in temperature and photoresistance can originate from distinct microscopic processes. The findings can be exploited for the development of Gr–TMDC based ultra-fast bolometers.

(CoFe2O4)x(BaFe12O19)1-x (x= 0.0, 0.4, 0.6, 0.8, 1.0) nano-composites was synthesized via single reaction mixture, utilizing sol-gel auto combustion method, with an intention to have superior structural homogeneity, and eventually to have single magnetic phase with enhanced exchange coupling between CoFe2O4 (soft) − BaFe12O19 (hard) phase. As-synthesized composite was thermally treated at 800 °C/4hrs, needed for the formation of hexagonal phase. X-ray diffraction (XRD) displays presence of both soft and hard phases only. Grain size for BaFe12O19 and CoFe2O4 respectively range between 50.2 − 54.5 nm and 45.5 − 56.5 nm. Obtained value of saturation magnetization (Ms), coercivity (Hc) for the studied samples is respectively: 39.8 Am2/kg − 75.5 Am2/kg and 501.2 Oe − 2587.9 Oe. Observed Ms, Hc values clearly show that single reaction mixture leads to better exchange coupling between soft-hard phases. Ms enhancement was also observed with increasing substitution amount (x). Observed results point out (CoFe2O4)x(BaFe12O19)1-x composite system as a good candidate for their potential applications as hard magnets.

Magnetite (Fe3O4) nano particles have been synthesized via sol-gel auto-combustion method using citric acid as a fuel. Post preparation thermal treatment was done at: 220 °C, 270 °C and, 320 °C for 1 hour, to insure the formation of single phase Fe3O4 nano particles. X-Ray diffraction (XRD) results confirm the formation of Magnetite (Fe3O4), and also the presence of Hematite (α-Fe2O3) as secondary phase. Results reveal that with increasing annealing temperature lattice parameter (a), specific surface area (S), hopping lengths (LA, LB) for tetrahedral (A) and octahedral site (B) increases, whereas x-ray density (ρXRD) and, Scherer’s grain diameter (Ds) varies respectively range between 5286.2−5376.1 kg/m3, 37.2−43.5 nm. Study of Fe3O4 nano particles is of value, due to its prospective applications in medical science - treating metastatic cancer by injecting into the cells.

The Zinc oxide thin films were grown on the amorphous glass substrate using Successive Ionic Layer Adsorption and Reaction (SILAR) technique at room temperature. The various preparative parameters were optimized to obtain good quality films. The as-grown films were annealed at 350° C for 2 h in air. The comparative study of the annealed and the as-grown thin films of ZnO were done for structural, optical and electrical studies. Polycrystalline nature was depicted from the x-ray diffraction (XRD) patterns. The peak intensity of as-grown ZnO thin films enhances after annealing. The average grain size calculated from XRD patterns was 17.66 nm for as-grown, while it increased to 24.66 nm after annealing. The band gap of as-grown ZnO thin films was relatively higher and was found to be 3.80 eV. After air annealing, the band gap decrease to 3.58 eV. The current-voltage characteristics show nearly ohmic behavior with decrease in the film resistance, after annealing in air. N-type conductivity was confirmed from the TEP measurements.The Zinc oxide thin films were grown on the amorphous glass substrate using Successive Ionic Layer Adsorption and Reaction (SILAR) technique at room temperature. The various preparative parameters were optimized to obtain good quality films. The as-grown films were annealed at 350° C for 2 h in air. The comparative study of the annealed and the as-grown thin films of ZnO were done for structural, optical and electrical studies. Polycrystalline nature was depicted from the x-ray diffraction (XRD) patterns. The peak intensity of as-grown ZnO thin films enhances after annealing. The average grain size calculated from XRD patterns was 17.66 nm for as-grown, while it increased to 24.66 nm after annealing. The band gap of as-grown ZnO thin films was relatively higher and was found to be 3.80 eV. After air annealing, the band gap decrease to 3.58 eV. The current-voltage characteristics show nearly ohmic behavior with decrease in the film resistance, after annealing in air. N-type conductivity was confirmed from ...

Amongst various metal oxides, Zinc Oxide (ZnO) is widely used semiconducting material amongst various II-VI group materials due to its various properties like wide energy band gap (3.3 eV), strong binding energy of 60 meV, higher transparency, low electron affinity sensitivity and selectivity for device grade applications. For this study purpose, for this study purpose (ZnO) thin films have been deposited by Successive Ionic Layer Adsorption and Reaction (SILAR) technique on amorphous glass substrate at room temperature. These films were then annealed for about 2 hours in air atmosphere. The structural studies reveal the increase in grain size upto 400°C. The change in crystallite size were observed in the Atomic Force Microscopy (AFM) images. The optical studies were also in well agreement with the structural analysis.

Amongst various semiconductors, Bi2S3 is one of the most widely used compounds, used for various types of applications like thermoelectric and optoelectronic applications as well as biological and chemical sensors due to its various types of properties. Bi2S3 is a challenging material because of its midway band gap (Eg=1.74eV) and absorption coefficient of the order of 104 cm-1. There are a lot of techniques have developed for synthesizing the Bi2S3 thin films. Amongst them, Zhehao Zhu et al have revealed that as annealing temperature increases in sulfurvapour atmosphere, the light absorption and charge separation efficiencies enhances by achieving carrier diffusion length that is comparable to the light absorption depth, leading to high solar energy conversion efficiencies in Bi2S3. R.S. Mane et al also have reported that, the Photoelectrochemical (PEC) performance of chemically grown Bi2S3 is improved with increase in film thickness and grain size.

Optical materials exhibiting a large third-order optical nonlinearity are in great demands because of their functional applications in optical limiting, optical switching, optical data storage etc. A well-known single Z-scan technique is employed to determine third-order nonlinear optical properties of nonlinear optical materials. Z-scan is a simple experimental technique to measure intensity dependent nonlinear susceptibilities of third-order nonlinear optical materials. It was originally introduced by Sheik Bahae et.al. In this technique, the sample is translated in the z-direction along the axis of a focused Gaussian beam, and the far field intensity is measured as function of sample position. Consequently, increases and decreases in the maximum intensity incident on the sample produce wavefront distortions created by nonlinear optical effects. This is a simple and sensitive single beam technique to measure the sign and magnitude of both real and imaginary part of the third order nonlinear susceptibility χ(3) of nonlinear optical materials.

Soft−hard (CoFe2O4)x(SrFe12O19)1-x (0.0≤x ≤1.0) nano-composite(s) were prepared by novel one pot reaction sol-gel auto-combustion route and annealed at 800 °C for 4 hours. X-ray diffraction (XRD), and vibration sample magnetometer (VSM) were utilized to monitor the effect of substitution on structural, and magnetic properties. XRD confirms the existence of soft (CoFe2O4), and hard (SrFe12O19) phase with average grain size (D) ranging between 57.7 nm to 59.4 nm for hexagonal and 34.6 to 64.5 nm for cubic phase. The magnetization of nano-composites demonstrates hysteresis loop that is characteristic of the exchange coupled system showing structurally two phases, but magnetically single phase. As a result of exchange coupling, between Co, and Sr ferrite enhanced saturation magnetization (70.65 Am2/kg), and remanence (26.29 Am2/kg) were observed as compared to pure SrFe12O19. Observed enhancement in energy product (BH)max (0.83 − 28.96 kJ/m3) of nano composite also favors good exchange coupling between hard and soft phase.

The transfer of charge carriers across the optically excited hetero-interface of graphene and semiconducting transition metal dichalcogenides (TMDCs) is the key to convert light to electricity, although the intermediate steps from the creation of excitons in TMDC to the collection of free carriers in the graphene layer are not fully understood. Here, we investigate photo-induced charge transport across graphene–MoS2 and graphene–WSe2 hetero-interfaces using time-dependent photoresistance relaxation with varying temperature, wavelength, and gate voltage. In both types of heterostructures, we observe an unprecedented resonance in the inter-layer charge transfer rate as the Fermi energy ( E F) of the graphene layer is tuned externally with a global back gate. We attribute this to a resonant quantum tunneling from the excitonic state of the TMDC to E F of the graphene layer and outline a new method to estimate the excitonic binding energies ( E b) in the TMDCs, which are found to be 400 meV and 460 meV in MoS2 and WSe2 layers, respectively. The gate tunability of the inter-layer charge transfer timescales may allow precise engineering and readout of the optically excited electronic states at graphene–TMDC interfaces.

Atomically-abrupt interfaces in transition metal oxide (TMO) heterostructures could host a variety of exotic condensed matter phases that may not be found in the bulk materials at equilibrium. A critical step in the development of such atomically-sharp interfaces is the deposition of atomically-smooth TMO thin films. Optimized deposition conditions exist for the growth of perovskite oxides. However, the deposition of rutile oxides, such as VO 2, with atomic-layer precision has been challenging. In this work, we used pulsed laser deposition to grow atomically-smooth VO 2 thin films on rutile TiO 2 (101) substrates. We show that an optimal substrate preparation procedure followed by the deposition of VO 2 films at a temperature conducive for step-flow growth mode is essential for achieving atomically-smooth VO 2 films. The films deposited at optimal substrate temperatures show a step and terrace structure of the underlying TiO 2 substrate. At lower deposition temperatures, there is a transition to a mixed growth mode comprised of island growth and layer-by-layer growth modes. VO 2 films deposited at optimal substrate temperatures undergo a sharp metal to insulator transition, similar to that observed in bulk VO 2, but at a transition temperature of ∼ 325 K with ∼ 10 3 times increase in resistance.

We report a detailed study on low-frequency 1/f-noise in large-area molecular-beam epitaxy grown thin (10 nm) films of topological insulators as a function of temperature, gate voltage, and magnetic field. When the Fermi energy is within the bulk valence band, the temperature dependence reveals a clear signature of generation-recombination noise in the defect states in the bulk band gap. However, when the Fermi energy is tuned to the bulk band gap, the gate voltage dependence of noise shows that the resistance fluctuations in surface transport are caused by correlated mobility-number density fluctuations due to the activated defect states present in the bulk of the topological insulator crystal with a density of D$_{it}=3.2\times10^{17}$ cm$^2$eV$^{-1}$. In the presence of a magnetic field, noise in these materials follows a parabolic dependence, which is qualitatively similar to mobility and charge-density fluctuation noise in non-degenerately doped trivial semiconductors. Our studies reveal that even in thin films of (Bi,Sb)$_2$Te$_3$ with thickness as low as 10 nm, the intrinsic bulk defects are the dominant source of noise., Comment: 15 pages, 4 figures

We report detailed temperature and gate-voltage dependence of 1/f resistance noise in magnetically doped topological insulators (TI) Crx(Bi,Sb)2−xTe3. The noise is remarkably sensitive to the gate voltage, increasing rapidly as the chemical potential is moved towards the charge neutrality point. Unlike in identically prepared (Bi,Sb)2Te3 films, where mobility-fluctuations in the surface states is the dominant mechanism, the noise in the magnetic Crx(Bi,Sb)2−xTe3 originates from transport in the localized band tail of the bulk valence band. A strong increase in noise with decreasing temperature supports this scenario. At higher temperature (≥10 K), we observed large noise peaks at gate voltage-dependent characteristic temperature scales. In line with similar observations in other non-magnetic TI systems, we attribute these peaks to generation-recombination in the Cr-impurity band.

Measurements of the dc transport properties and the low-frequency conductivity noise in films of charge ordered Nd_0.5Ca_0.5MnO_3 grown on Si subtrate reveal the existence of a threshold field in the charge ordered regime beyond which strong non linear conduction sets in along with a large broad band conductivity noise. Threshold-dependent conduction disappears as T -> T_{CO}, the charge ordering temperature. This observation suggests that the charge ordered state gets depinned at the onset of the non-linear conduction., 3 pages of two-column text and 4 eps figures

We report measurement of low frequency 1/f noise in molybdenum di-sulphide (MoS2) field-effect transistors in multiple device configurations including MoS2 on silicon dioxide as well as MoS2-hexagonal boron nitride (hBN) heterostructures. All as-fabricated devices show similar magnitude of noise with number fluctuation as the dominant mechanism at high temperatures and density, although the calculated density of traps is two orders of magnitude higher than that at the SiO2 interface. Measurements on the heterostructure devices with vacuum annealing and dual gated configuration reveals that along with the channel, metal-MoS2 contacts also play a significant role in determining noise magnitude in these devices.

We present temperature dependent I-V measurements of short channel MoS2 field effect devices at high source-drain bias. We find that, although the I-V characteristics are ohmic at low bias, the conduction becomes space charge limited at high V-DS, and existence of an exponential distribution of trap states was observed. The temperature independent critical drain-source voltage (V-c) was also determined. The density of trap states was quantitatively calculated from V-c. The possible origin of exponential trap distribution in these devices is also discussed. (C) 2013 AIP Publishing LLC.

Metal-based piezoresistive sensing devices could find a much wider applicability if their sensitivity to mechanical strain could be substantially improved. Here, we report a simple method to enhance the strain sensitivity of metal films by over two orders of magnitude and demonstrate it on specially designed microcantilevers. By locally inhomogenizing thin gold films using controlled electromigration, we have achieved a logarithmic divergence in the strain sensitivity with progressive microstructural modification. The enhancement in strain sensitivity could be explained using non-universal tunneling-percolation transport. We find that the Johnson noise limited signal-to-noise ratio is an order of magnitude better than silicon piezoresistors. This method creates a robust platform for engineering low resistance, high gauge factor metallic piezoresistors that may have profound impact on micro and nanoscale self-sensing technology. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4761817]

We report on the fabrication and characterization of a device which allows the formation of an antidot lattice (ADL) using only electrostatic gating. The antidot potential and Fermi energy of the system can be tuned independently. Well defined commensurability features in magnetoresistance as well as magnetothermopower are obsereved. We show that the thermopower can be used to efficiently map out the potential landscape of the ADL., 4 pages, 3 figures; to appear in Appl. Phys. Lett

Due to the limited sensitivity of many nuclear magnetic resonance (NMR) applications, careful consideration must be given to the effect of NMR data processing on spectral noise. This work presents analytical relationships as well as simulated and experimental results characterizing the propagation of noise by unsymmetric covariance NMR processing, which concatenates two NMR spectra along a common dimension, resulting in a new spectrum showing spin correlations as cross peaks that are not directly measured in either of the two input spectra. It is shown how the unsymmetric covariance spectrum possesses an inhomogeneous noise distribution across the spectrum with the least amount of noise in regions whose rows and columns do not contain any cross or diagonal peaks and with the largest amount of noise on top of signal peaks. Therefore, methods of noise estimation commonly used in Fourier transform spectroscopy underestimate the amount of uncertainty in unsymmetric covariance spectra. Different data processing procedures, including the Z-matrix formalism, thresholding, and maxima ratio scaling, are described to assess noise contributions and to reduce noise inhomogeneity. In particular, determination of a Z score, which measures the difference in standard deviations of a statistic from its mean, for each spectral point yields a Z matrix, which indicates whether a given peak intensity above a threshold arises from the covariance of signals in the input spectra or whether it is likely to be caused by noise. Application to an unsymmetric covariance spectrum, obtained by concatenating two 2D (13)C-(1)H heteronuclear, single quantum coherence (HSQC) and (13)C-(1)H heteronuclear, multiple bond correlation (HMBC) spectra of a metabolite mixture along their common proton dimension, reveals that for sufficiently sensitive input spectra the reduction in sensitivity due to covariance processing is modest.

The correlations between properties of hydrogenated diamond like carbon films and their Raman spectra have been investigated. The films are prepared by plasma deposition technique, keeping different hydrogen to methane ratio during the growth process. The hydrogen concentration, sp$^3$ content, hardness and optical Tauc gap of the materials have been estimated from a detail analysis of their Raman spectra. We have also measured the same parameters of the films by using other commonly used techniques, like sp$^3$ content in films by x-ray photoelectron spectroscopy, their Tauc gap by ellipsometric measurements and hardness by micro-hardness testing. The reasons for the mismatch between the characteristics of the films, as obtained by Raman measurements and by the above mentioned techniques, have been discussed. We emphasize on the importance of the visible Raman spectroscopy in reliably predicting the above key properties of DLC films.