Gaurab Rimal

Spin–orbit torque-assisted switching in magnetic insulator thin films with perpendicular magnetic anisotropy

As an in-plane charge current flows in a heavy metal film with spin–orbit coupling, it produces a torque on and thereby switches the magnetization in a neighbouring ferromagnetic metal film. Such spin–orbit torque (SOT)-induced switching has been studied extensively in recent years and has shown higher efficiency than switching using conventional spin-transfer torque. Here we report the SOT-assisted switching in heavy metal/magnetic insulator systems. The experiments used a Pt/BaFe12O19 bilayer where the BaFe12O19 layer exhibits perpendicular magnetic anisotropy. As a charge current is passed through the Pt film, it produces a SOT that can control the up and down states of the remnant magnetization in the BaFe12O19 film when the film is magnetized by an in-plane magnetic field. It can reduce or increase the switching field of the BaFe12O19 film by as much as about 500 Oe when the film is switched with an out-of-plane field.

Giant photocurrent enhancement by transition metal doping in quantum dot sensitized solar cells

A huge enhancement in the incident photon-to-current efficiency of PbS quantum dot (QD) sensitized solar cells by manganese doping is observed. In the presence of Mn dopants with relatively small concentration (4 at. %), the photoelectric current increases by an average of 300% (up to 700%). This effect cannot be explained by the light absorption mechanism because both the experimental and theoretical absorption spectra demonstrate several times decreases in the absorption coefficient. To explain such dramatic increase in the photocurrent we propose the electron tunneling mechanism from the LUMO of the QD excited state to the Zn2SnO4 (ZTO) semiconductor photoanode. This change is due to the presence of the Mn instead of Pb atom at the QD/ZTO interface. The ab initio calculations confirm this mechanism. This work proposes an alternative route for a significant improvement of the efficiency for quantum dot sensitized solar cells.

Steady state and time resolved optical characterization studies of Zn2SnO4 nanowires for solar cell applications

Nanowires are a promising option for sensitized solar cells, sensors, and display technology. Most of the work thus far has focused on binary oxides for these nanowires, but ternary oxides have advantages in additional control of optical and electronic properties. Here, we report on the diffuse reflectance, Low Temperature and Room Temperature Photoluminescence (PL), PL excitation spectrum, and Time Resolved PL (TRPL) of Zinc Tin Oxide (ZTO) nanowires grown by Chemical Vapor Deposition. The PL from the ZTO nanowires does not exhibit any band gap or near gap emission, and the diffuse reflectance measurement confirms that these ZTO nanowires have a direct forbidden transition. The broad PL spectrum reveals two Gaussian peaks centered at 1.86 eV (red) and 2.81 eV (blue), representing two distinct defect states or complexes. The PL spectra were further studied by the Time Resolved Emission Spectrum and intensity dependent PL and TRPL. The time resolved measurements show complex non-exponential decays at all w...

Effects of Mn dopant locations on the electronic bandgap of PbS quantum dots

Dilute magnetic semiconductors (DMSs) are typically made by doping semiconductors with magnetic transition metal elements. Compared to the well-understood bulk and thin film DMS, the understanding of the magnetic element doping effects in semiconducting quantum dots (QDs) is relatively poor. In particular, the influence of the dopant locations is rarely explored. Here, we present a comprehensive study of the effects of Mn doping on the electronic density of states of PbS QDs. Based on the results observed by scanning tunneling microscopy, X-ray diffraction, electron paramagnetic resonance, and density functional theory calculations, it is found that the Mn doping causes a broadening of the electronic bandgap in the PbS QDs. The sp-d hybridization between the PbS host material and Mn dopants is argued to be responsible for the bandgap broadening. Moreover, the locations of the Mn dopants, i.e., on the surface or inside the QDs, have been found to play an important role in the strength of the sp-d hybridization, which manifests as different degrees of the bandgap change.Dilute magnetic semiconductors (DMSs) are typically made by doping semiconductors with magnetic transition metal elements. Compared to the well-understood bulk and thin film DMS, the understanding of the magnetic element doping effects in semiconducting quantum dots (QDs) is relatively poor. In particular, the influence of the dopant locations is rarely explored. Here, we present a comprehensive study of the effects of Mn doping on the electronic density of states of PbS QDs. Based on the results observed by scanning tunneling microscopy, X-ray diffraction, electron paramagnetic resonance, and density functional theory calculations, it is found that the Mn doping causes a broadening of the electronic bandgap in the PbS QDs. The sp-d hybridization between the PbS host material and Mn dopants is argued to be responsible for the bandgap broadening. Moreover, the locations of the Mn dopants, i.e., on the surface or inside the QDs, have been found to play an important role in the strength of the sp-d hybridiza...