Search

In this work, co-doping effects of transition (Zn) and rare-earth (La) elements on the crystalline structure, surface morphology, photoluminescence, optical and photosensing properties of NiO thin films are studied. NiO, NiO:Zn(1%), NiO:La(1%), and NiO:Zn(1%):La(1%) thin films are fabricated using the nebulizer spray pyrolysis (NSP) method. X-ray diffraction study revealed the cubic NiO structure of all the films. Photoluminescence (PL) spectra of thin films exhibit various emission peaks centered at the wavelengths of 387, 414, 437, 451, 477, and 521 nm. The optical bandgap energy (Eg) values are found to be 3.46, 3.43, 3.39 and 3.33 eV for NiO, NiO:Zn(1%), NiO:La(1%) and NiO:Zn(1%):La(1%) thin films, respectively. The fabricated (Zn, La) co-doped NiO i.e., NiO:Zn(1%):La(1%) photo-detector exhibits highest responsivity (R), external quantum efficiency (EQE) and detectivity (D*) values of 0.50AW-1, 169% and 14.5 × 109 Jones, respectively as compared to NiO, NiO:Zn(1%) and NiO:La(1%) photo-detectors. The present study revealed that the transition and rare-earth elements co-doping can be an effective approach for tuning the various physical properties of semiconducting oxide films.

We report the effect of Ti-doping on structural, morphological, photoluminescence, optical and photoconductive properties of ZnO thin films. Pure and Ti(1, 3 and 5%)-doped ZnO thin films are deposited by the successive ionic layer adsorption and reaction (SILAR) method. The X-ray diffraction analysis revealed the single-phase hexagonal wurtzite ZnO structure of all the films. Scanning electron microscope images suggest the formation of rod shaped particles in Ti-doped ZnO thin films. Photoluminescence spectra of all the films show emission peaks centered at 398 nm, 413 nm, 438 nm, 477 nm and 522 nm wavelengths. Optical properties support the semiconducting nature of all the films. The optical bandgap values are estimated to be 3.29 eV, 3.26 eV, 3.19 eV and 3.23 eV for ZnO, ZnO:Ti(1%), ZnO:Ti(3%) and ZnO:Ti(5%) thin films, respectively. Photoconductivity study indicates that ZnO:Ti(3%) thin film exhibits high responsivity, external quantum efficiency and detectivity of 0.30 AW-1, 97% and 5.49 × 1010 Jones, respectively, among all the films. The enhanced photoconductivity of Ti-doped ZnO thin films make them useful for optoelectronic applications.

As current electronics makes a transition from bulky and rigid templates to lightweight and flexible ones, the emerging field of soft technology is set to revolutionize our daily life. Currently, polymer based templates dominate this field due to their excellent mechanical characteristics and low cost, but limited thermal budget and stabilities are the major technological bottlenecks resulting in poor performance and short duration of lifetime. Recently, the technology based on van der Waals heteroepitaxy on muscovite is considered as a promising solution to overcome these barriers at once. In this paper, the characteristics of muscovite and the mechanism of van der Waals heteroepitaxy will be introduced, after that, a variety of materials on muscovite via van der Waals heteroepitaxy and the efforts on practical applications are reported. With the outlook of material science and help of advanced measurements, the evidence of heteroepitaxy of muscovite and overlayers has been deeply analyzed. Moreover, various measurements of properties was carried under bending conditions to demonstrate the mechanical stability and reliability. Through this review, we pave the path to develop more material systems on muscovite to make MICAtronics completely.

Magnetic properties of magnetically frustrated polycrystalline Mn1.5Fe1.5Al Heusler type alloy with a β-Mn structure were systematically studied using dc magnetization, ac susceptibility, magnetic relaxation and magnetic memory effect measurements to understand its magnetic ground state. The Mn1.5Fe1.5Al alloy system exhibits a sharp peak at low temperature around 34.5 K with strong antiferromagnetic correlations ( θ CW ∼ −639 K) and a high degree of frustration ( f p ∼ 18.7) which suggest the presence of competing magnetic interactions. Despite of strong antiferromagnetic correlations, this peak has spin-glass character as confirmed by the observations of bifurcation of the zero-field-cooled and field-cooled magnetization curves below the irreversibility temperature, field dependent shift of the irreversibility temperature, unidirectional exchange bias effect on the field-cooled M-H hysteresis loop, exponential decay of the temperature variation of remanence and coercivity, frequency dependent shift of the spin glass freezing temperature (Tf) as per the dynamic scaling laws given by the critical slowing down model and Vogel-Fulcher law, magnetic relaxation and magnetic memory effect below Tf. The observation of a clear asymmetric response with respect to the temperature cycling in the magnetic relaxation measurements validates the hierarchical model of the spin-glass magnetic relaxation in Mn1.5Fe1.5Al alloy. Since the behaviour exhibited by the Mn1.5Fe1.5Al alloy has perfect similarity with the well-known atomic spin glasses like CuMn, AuFe, therefore, it may be considered as canonical spin-glass system.

Stoichiometric gallium ferrite (GFO) is a promising single-phase multiferroic material at room temperature. Nevertheless, simultaneous magnetism and ferroelectricity in a single GFO at room temperature has not been demonstrated yet. In this work, single-phase GFO thin films with different amount of excessive Fe have been successfully grown using a dual target pulsed laser deposition (PLD) process, and the magnetic transition temperature is found to be above room temperature with excessive Fe distributed among the available cation sites of GFO unit cell. Ferroelectricity of GFO films have been confirmed by second harmonic generation (SHG), polarization hysteresis, pyroelectric reversal, and piezoresponse force microscopy (PFM), and magnetoelectric (ME) coupling has been demonstrated by increase in piezoresponse induced via external magnetic field applied in-plane, all measured at room temperature. From the data, it is theorized that the magnetic properties of GFO originate from the superexchange interaction mediated via the Fe1-O-Fe2 bond, and an additional Fe1-O-Fe* bond can form from excess Fe atoms. Under a magnetic field applied along c axis, the magnetic moment of Fe2 and Fe* increase while that of Fe1 decreases, and these changes in magnetic moment result in larger distortion of Fe2 and Fe* octahedrons along the b-axis, and thus enhanced polarization and piezoresponse. This series of studies thus confirm single-phase GFO with excessive Fe as a multiferroic material at room temperature with ME coupling, paving way for its potential functional applications in microelectronic and spintronic devices.

In this work, we report the structural, morphological, optical and photo-sensing properties of pure as well as Al, Sn and Sb-doped ZnS thin films, with the prospect of photo-sensing and photo-detection applications. ZnS, ZnS:Al(1%), ZnS:Sn(1%) and ZnS:Sb(1%) thin films are fabricated by the spray pyrolysis method. These thin films exhibit polycrystalline nature and crystallize in hexagonal wurtzite ZnS structure. Optical properties reveal the semiconductor behavior of all the films. The optical bandgap (Eg) values, which are around 3.67 eV, 3.43 eV, 3.32 eV and 3.56 eV for ZnS, ZnS:Al(1%), ZnS:Sn(1%) and ZnS:Sb(1%) thin films, respectively, decrease with Al, Sn and Sb-doping. On the other hand, doping of Al, Sn and Sb into the ZnS lattice enhances the photo-sensing parameters such as responsivity (R) and external quantum efficiency (EQE). Al-doping into the ZnS does not affect detectivity (D*) but Sb-doping results in a decrement of D*-value. ZnS:Sn(1%) photo-detector exhibits the highest R, EQE and D*-values of 1.05 × 10−1 AW−1, 33.9% and 4.29 × 1010 Jones, respectively, among all the fabricated photo-devices. Thus doping of post-transition (Al, Sn) and metalloid (Sb) elements into the ZnS host lattice significantly alter the optical and photo-sensing properties of the present thin films.

La0.5Sr0.5CoO3−δ system displaying coexistence of metallicity and ferromagnetism, and the interplay between distinct spin states of Co ions has gained significant attention from the fundamental as well as the applications point of view. In this regard, the effect of V5+ doping on the dynamics of spin and oxidation states of Co ions in the La0.5Sr0.5CoO3−δ system has been studied using structural, X-ray photoemission spectroscopy, magnetic and transport measurements. The La0.5Sr0.5Co1−xVxO3−δ (x = 0, 0.01, 0.02 and 0.1) series was prepared by conventional solid-state reaction method and its chemical homogeneity was established by inductively coupled plasma mass spectroscopy. The increase of lattice parameters/volume with increasing V5+ content in rhombohedrally crystallized (R 3 - c) samples is attributed to the presence of Co2+ ions . The presence of Co2+ ions and the reduction of Co3+ ion concentration with V substitution is verified from the results of core-level photoemission spectra of Co. Detailed magnetic measurements show an increase in Curie temperature, effective magnetic moment, soft magnetic nature and suggesting strengthening of underlying magnetic interactions. As a result of multiple oxidation states of Co ions and the delocalization of electrons, rise in the conductivity is noticed. Such enhancement in the conductivity and ferromagnetic ordering temperature, simultaneously, with V doping implies a strong coupling between spin and charge degrees of freedom. To understand the underlying mechanism more clearly, different possible scenarios have been discussed. The most appropriate scenario i.e., the conversion of Co3+ ions into Co2+ ions and admixture of Co4+ ion spin states with V5+ doping was found to be consistent with all the experimental observations.

In the present era of “Internet-of-Things”, the demand for flexible, light-weight, low-cost, low-power consumption, multifunctional, and environmentally friendly electronics has moved to the forefront of materials science research. Numerous compounds with unique material properties in epitaxial thin film form hold key to future technologies. van der Waals epitaxy (vdWE) involving two-dimensional layered materials can play a crucial role in the expansion of thin film epitaxy by overcoming the bottleneck of material combinations due to lattice/thermal matching conditions inherent to conventional epitaxy. Among the layered materials, mica is a well-known phyllosilicate mineral that can have a remarkable impact on flexible electronics. We confine ourselves to the validity of vdWE of functional oxides on muscovite mica throughout this treatise. These heterostructures with excellent properties are flexible and exhibit high-temperature stability. With such demonstrations, it is anticipated that MICAtronics, vdWE on mica, can reveal unusual properties and emergent phenomena in the realm of high-performance flexible device applications.

In this communication, we report on how the co-doping of two transition metal ions (Fe and Mn) effectively improves the structural, morphological, optical, electrical and photodetection properties of CdO thin films than individual doping. For this, pure-CdO, Fe-doped, Mn-doped and (Fe-Mn) co-doped CdO thin films are deposited by spray pyrolysis technique. X-ray diffraction (XRD) patterns reveal that these films possess cubic structure and belong to the Fm-3 m space group. The Field Emission Scanning Electron micrographs hold a qualitative agreement with the crystalline sizes obtained from the XRD analysis. Energy Dispersive X-Ray Analysis approved the existence of constituent elements in the prepared films. The photoluminescence spectra identified the quality of crystal structure and the presence of defects in the prepared films. Analyzing the absorption spectra unveiled the direct bandgap values in the range 2.32 eV (for pure-CdO) - 1.96 eV (for CdO:Fe(1%):Mn(1%)). The Hall Effect measurements established n-type behavior with the values of resistivity (ρ), carrier concentration (n) and carrier mobility values in the range 5.8–7.1 mΩcm, 6.7−2.4 × 1019 cm−3 and 16−37 cm2/Vs, respectively. Three different transport mechanisms such as ohmic, recombination-tunneling and space-charge limited current are found to play roles in the three distinct ranges of the I–V data measured for the films under study. The ideality factor values are obtained from the region where the recombination-tunneling mechanism is prevalent. The photoelectric parameters such as photo-responsivity, and external quantum efficiency are obtained. The values of these quantities pertaining to pure-CdO, CdO:Mn, CdO:Fe and CdO:Fe:Mn films vary in the range 128.4–561.3 mA/W, and 30–131.1 %, respectively. From the current study, it was found that photoresponsivity and external quantum efficiency are enhanced by a factor of four in codoped CdO films than the undoped CdO film and can find potential photodetector applications.

In the present work, various concentrations of Ti (1, 2, 3, 4 and 5 at %) doped SnO2 thin films were grown onto glass substrate using cost effective sol-gel spin coating method and subsequently investigated the effect of Ti doping concentration on structural, optical and electrical properties. X-ray diffraction studies revealed that all deposited films exhibit polycrystalline tetragonal rutile structure with fundamental orientation peak along (110) direction. Moreover, Ti doped SnO2 films were started growing along (211) direction and it was more pronounced with increasing of Ti concentration. The average grain size was decreased with the increase of Ti concentration, confirmed by XRD and AFM studies. The UV–visible spectrometer measurements shown that the average transmittance of un-doped SnO2 film was above 85%, whereas Ti doped SnO2 films were found to be a decrease of transmittance up to 77% in 5 at% Ti:SnO2 film. The optical band gap energy values were considerably decreased from 3.91 to 3.73 eV with increase of Ti content. Further, the sheet resistance (Rsh) and resistivity (ρ) values were found to be decreased with the increase of Ti doping up to 3 at % then it was slightly increased in 4 and 5 at % of Ti:SnO2 films. The efficiency parameter figure of merit (φ) was also estimated for all deposited films with the function of Ti doping.

Electrical-transport and magneto-transport in bulk nanocrystalline (BNC) La 1 - x Ca x MnO 3 + δ (x = 1/8 with either δ = 0.00 or δ = 0.06, x = 3/8 with δ = 0.00) (LCMO) manganite system have been studied treating the bulk crystalline (BC) counterparts as the reference systems. Irrespective of the value of δ in under hole-doped (x = 1/8) LCMO, the metal-insulator (M-I) transition (not observed in the BC counterpart) appears at a temperature T MI ≪ T C , where T C is the temperature at which the ferromagnetic (FM)–paramagnetic (PM) second-order phase transition occurs. Oxygen off-stoichiometry makes the M-I transition more pronounced and reduces the overall magnitude of resistivity. Reducing the average crystallite size d below 150 nm, lowers both T MI and T C in the BNC samples with x = 1/8 and δ = 0.00 or 0.06 but decreases T MI while increasing T C in the optimally hole-doped x = 3/8 LCMO. The single-orbital double-exchange model, which predicts the coincidence, or otherwise, of the M-I and FM-PM transitions with some success in the BC LCMO, fails to correctly describe the above-mentioned results in BNC LCMO. The resistivity minima at low temperatures ( T min ), completely absent in BC, occur in BNC LCMO, regardless of the value of x , δ , d and the strength of the magnetic field (H). The mechanism for resistivity, ρ (T), changes from the adiabatic small polaron nearest-neighbor hopping (for x = 3/8) or from the Shklovskii-Efros variable range hopping, SE-VRH, (for x = 1/8), prevalent in BC, to the Mott-VRH in the PM insulating state in the BNC counterparts when d falls below 150 nm. At T T min , the contributions due to the Mott-VRH (present only in the samples with x = 1/8 and δ = 0.00 or δ = 0.06) and the enhanced electron-electron Coulomb interaction (in all the BNC LCMO samples) are responsible for the negative temperature coefficient of resistivity (TCR). By contrast, the coherent electron-magnon scattering accounts for the positive TCR observed at T > T min . H suppresses the mean hopping energy difference between the hopping sites but increases the electron localization length. The field-induced enhancement in the localization length far outweighs the field-induced drop in the mean hopping energy difference to produce an increase in the mean hopping distance with H.