Mariyappan Shanmugam

Scalable synthesis of two-dimensional antimony telluride nanoplates down to a single quintuple layer

Scalable syntheses of two-dimensional topological insulators are critical to material exploration. We demonstrate a controlled assembly of a two-dimensional V–VI group compound, Sb2Te3 nanoplates (NPs), through a vapor–solid growth process. The physical thickness of Sb2Te3 NPs can be rationally controlled in a wide range, from hundreds of nm down to sub-10 nm. Single-quintuple-layer Sb2Te3 NPs were obtained, with a high domain density of ∼2.465 × 108 cm−2 over a large surface area (1 cm × 1 cm) of a SiO2/Si substrate, verifying a scalable synthesis method. Extensive material analyses were conducted to explore the basic properties of Sb2Te3 NPs using SEM and AFM, etc. HRTEM analysis confirms that the NP samples exhibit a highly crystalline structure and XPS analysis confirms the chemical composition and material stoichiometry. The growth of 2D topological insulator nanostructures may open up new opportunities in surface-state studies and potential applications in low-dissipative electronic systems.

On the Physics of Dispersive Electron Transport Characteristics in SnO2 Nanoparticle Based Dye Sensitized Solar Cells

The present study elucidates dispersive electron transport mediated by surface states in tin oxide (SnO2) nanoparticle-based dye sensitized solar cells (DSSCs). Transmission electron microscopic studies on SnO2 show a distribution of ∼10 nm particles exhibiting (111) crystal planes with inter-planar spacing of 0.28 nm. The dispersive transport, experienced by photo-generated charge carriers in the bulk of SnO2, is observed to be imposed by trapping and de-trapping processes via SnO2 surface states present close to the band edge. The DSSC exhibits 50% difference in performance observed between the forward (4%) and reverse (6%) scans due to the dispersive transport characteristics of the charge carriers in the bulk of the SnO2. The photo-generated charge carriers are captured and released by the SnO2 surface states that are close to the conduction band-edge resulting in a very significant variation; this is confirmed by the hysteresis observed in the forward and reverse scan current-voltage measurements under AM1.5 illumination. The hysteresis behavior assures that the charge carriers are accumulated in the bulk of electron acceptor due to the trapping, and released by de-trapping mediated by surface states observed during the forward and reverse scan measurements.

Molybdenum trioxide thin film recombination barrier layers for dye sensitized solar cells

A physical vapor deposition based molybdenum trioxide (MoO3) thin film is demonstrated as an efficient reverse-electron recombination barrier layer (RBL) at the fluorine doped tin oxide (FTO)/titanium dioxide (TiO2) interface in dye sensitized solar cells (DSSCs). Thin films of MoO3 show an average optical transmittance of ∼77% in a spectral range of 350–800 nm with bandgap value of ∼3.1 eV. For an optimum thickness of MoO3, deposited for 5 minutes, the resulting DSSCs showed 15% enhancement in efficiency (η) compared to the reference DSSC which did not use MoO3 RBL; this suggests that MoO3 is effectively suppressing interfacial recombination at the FTO/TiO2 interface. Further, increasing the thickness of MoO3 RBL at the FTO/TiO2 interface (20 minutes deposition) is observed to impede charge transport, as noticed with 55% reduction in η compared to the reference DSSC. Thin film MoO3 RBL with an optimum thickness value at the FTO/TiO2 interface efficiently blocks the leaky transport pathways in the mesoporous TiO2 nanoparticle layer and facilitates efficient charge transport as confirmed by electrochemical impedance spectroscopy.