Andrew J. Yost

Built-in Electric Field Induced Mechanical Property Change at the Lanthanum Nickelate/Nb-doped Strontium Titanate Interfaces

The interactions between electric field and the mechanical properties of materials are important for the applications of microelectromechanical and nanoelectromechanical systems, but relatively unexplored for nanoscale materials. Here, we observe an apparent correlation between the change of the fractured topography of Nb-doped SrTiO3 (Nb:STO) within the presence of a built-in electric field resulting from the Schottky contact at the interface of a metallic LaNiO3 thin film utilizing cross-sectional scanning tunneling microscopy and spectroscopy. The change of the inter-atomic bond length mechanism is argued to be the most plausible origin. This picture is supported by the strong-electric-field-dependent permittivity in STO and the existence of the dielectric dead layer at the interfaces of STO with metallic films. These results provided direct evidence and a possible mechanism for the interplay between the electric field and the mechanical properties on the nanoscale for perovskite materials.

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.

Coexistence of Two Electronic Nano-Phases on a CH3NH3PbI3–xClx Surface Observed in STM Measurements

Scanning tunneling microscopy is utilized to investigate the local density of states of a CH3NH3PbI3-xClx perovskite in cross-sectional geometry. Two electronic phases, 10-20 nm in size, with different electronic properties inside the CH3NH3PbI3-xClx perovskite layer are observed by the dI/dV mapping and point spectra. A power law dependence of the dI/dV point spectra is revealed. In addition, the distinct electronic phases are found to have preferential orientations close to the normal direction of the film surface. Density functional theory calculations indicate that the observed electronic phases are associated with local deviation of I/Cl ratio, rather than different orientations of the electric dipole moments in the ferroelectric phases. By comparing the calculated results with experimental data we conclude that phase A (lower contrast in dI/dV mapping at -2.0 V bias) contains a lower I/Cl ratio than that in phase B (higher contrast in dI/dV).

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...