Balavinayagam Ramalingam

Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on TiO2 Single Crystals

A facile development of highly efficient Pt-TiO(2) nanostructured films via versatile gas-phase deposition methods is described. The films have a unique one-dimensional (1D) structure of TiO(2) single crystals coated with ultrafine Pt nanoparticles (NPs, 0.5-2 nm) and exhibit extremely high CO(2) photoreduction efficiency with selective formation of methane (the maximum CH(4) yield of 1361 μmol/g-cat/h). The fast electron-transfer rate in TiO(2) single crystals and the efficient electron-hole separation by the Pt NPs were the main reasons attributable for the enhancement, where the size of the Pt NPs and the unique 1D structure of TiO(2) single crystals played an important role.

Ultrafine sputter-deposited Pt nanoparticles for triiodide reduction in dye-sensitized solar cells: impact of nanoparticle size, crystallinity and surface coverage on catalytic activity

This paper presents a detailed electrochemical impedance spectroscopy and cyclic voltammetry (CV) investigation into the electrocatalytic activity of ultrafine (i.e., smaller than 2 nm) platinum (Pt) nanoparticles generated on a fluorine-doped tin oxide (FTO) surface via room temperature tilted target sputter deposition. In particular, the Pt-decorated FTO electrode surfaces were tested as counter electrode candidates for triiodide (I3(-)) reduction in dye-sensitized solar cells (DSSCs). We observed a direct correlation between size-dependent Pt nanoparticle crystallinity and the I3(-) reduction activity underlying DSSC performance. CV analysis confirmed the higher electrocatalytic activities of sputter-deposited crystalline Pt nanoparticles (1-2 nm) compared with either sub-nanometre Pt clusters or a continuous Pt thin film. While the low catalytic activity and DSSC performance of Pt clusters smaller in size than 1 nm is believed to arise from their non-crystalline nature and charge-trapping attributes, we attribute the high catalytic performance of larger Pt nanoparticles in the 1-2 nm regime to their well-defined crystallinity and fast electron transfer kinetics. For DSSC applications, the optimized Pt loading was calculated to be ~2.54 × 10(-7) g cm(-2), which corresponds to surface coverage by ~1.6 nm sized Pt nanoparticles.

Charge storage characteristics of ultra-small Pt nanoparticle embedded GaAs based non-volatile memory

Charge storage characteristics of ultra-small Pt nanoparticle embedded devices were characterized by capacitance-voltage measurements. A unique tilt target sputtering configuration was employed to produce highly homogenous nanoparticle arrays. Pt nanoparticle devices with sizes ranging from ∼0.7 to 1.34 nm and particle densities of ∼3.3–5.9 × 1012 cm−2 were embedded between atomic layer deposited and e-beam evaporated tunneling and blocking Al2O3 layers. These GaAs-based non-volatile memory devices demonstrate maximum memory windows equivalent to 6.5 V. Retention characteristics show that over 80% charged electrons were retained after 105 s, which is promising for device applications.

Enhanced water photolysis with Pt metal nanoparticles on single crystal TiO2 surfaces.

Two novel deposition methods were used to synthesize Pt-TiO(2) composite photoelectrodes: a tilt-target room temperature sputtering method and aerosol-chemical vapor deposition (ACVD). Pt nanoparticles (NPs) were sequentially deposited by the tilt-target room temperature sputtering method onto the as-synthesized nanostructured columnar TiO(2) films by ACVD. By varying the sputtering time of Pt deposition, the size of deposited Pt NPs on the TiO(2) film could be precisely controlled. The as-synthesized composite photoelectrodes with different sizes of Pt NPs were characterized by various methods, such as SEM, EDS, TEM, XRD, and UV-vis. The photocurrent measurements revealed that the modification of the TiO(2) surface with Pt NPs improved the photoelectrochemical properties of electrodes. Performance of the Pt-TiO(2) composite photoelectrodes with sparsely deposited 1.15 nm Pt NPs was compared to the pristine TiO(2) photoelectrode with higher saturated photocurrents (7.92 mA/cm(2) to 9.49 mA/cm(2)), enhanced photoconversion efficiency (16.2% to 21.2%), and increased fill factor (0.66 to 0.70). For larger size Pt NPs of 3.45 nm, the composite photoelectrode produced a lower photocurrent and reduced conversion efficiency compared to the pristine TiO(2) electrode. However, the surface modification by Pt NPs helped the composite electrode maintain higher fill factor values.

Sub-2 nm size and density tunable platinum nanoparticles using room temperature tilted-target sputtering

This paper describes a tilted-target RF magnetron sputter deposition system to grow nanoparticles in a controlled way. With detailed characterization of ultra-high density (up to 1.1 × 10¹³ cm⁻²) and ultra-small size Pt nanoparticles (0.5-2 nm), it explains their growth and crystalline properties on amorphous Al₂O₃ thin films. It is shown that Pt nanoparticle size and number density can be precisely engineered by varying selected experimental parameters such as target angle, sputtering power and time of deposition to control the energy of the metal atoms in the deposition flux. Based on rate equation modelling of nanoparticle growth, three distinct growth regimes, namely nucleation dependent, coalescence dependent and agglomeration dependent regimes, were observed. The correlation between different nanoparticle growth regimes and the consequent crystal structure transformation, non-crystalline clusters → single crystalline nanoparticles → polycrystalline islands, is also discussed.

Room temperature observation of size dependent single electron tunneling in a sub-2?nm size tunable Pt nanoparticle embedded metal?oxide?semiconductor structure

In this paper we report size dependent single electron tunneling behavior at room temperature in a metal-oxide-semiconductor structure with uniformly sized Pt nanoparticles embedded in an Al(2)O(3) dielectric. The sub-2 nm size Pt nanoparticles sandwiched between the Al(2)O(3) layers are deposited by a unique tilted target sputter deposition technique which produces metal nanoparticles as small as 0.5 nm with narrow size distributions at room temperature. The charging behavior of these nanoparticles shows clear single electron tunneling peaks due to the Coulomb blockade effect. Moreover, the average single electron addition energy and height of the single electron tunneling current strongly depend on the size of the Pt nanoparticle. These controllable single electron tunneling behaviors suggest a new route for fabrication of single electron devices.

Hydrogen spillover at sub-2 nm Pt nanoparticles by electrochemical hydrogen loading

Hydrogen generation and storage is an essential component in the increasingly important field of energy storage. Electrochemical generation of hydrogen atoms at the surface of Pt like metals at select potentials is a widely accepted phenomenon. However, moving these adsorbed hydrogen atoms to high surface area support systems for storage is an issue. We show spillover of these adsorbed hydrogen atoms to the supporting structure for sub-2 nm Pt nanoparticles sputtered on Fluorine Doped Tin Oxide (FTO) and on few layer graphene (FLG) supports. Evidence of size-dependent hydrogen spillover was observed for Pt nanoparticles deposited using tilted target sputtering and a correlation between nanoparticle size, crystallinity, support characteristics, and hydrogen spillover is also reported. Evidence of C–H bonds formed on the FLG surface due to H spillover from 0.9 nm Pt nanoparticles was also confirmed through XPS analysis.

Large sensitivity enhancement in semiconducting organic field effect transistor sensors through incorporation of ultra-fine platinum nanoparticles

We report remarkable improvement in sensitivity of pentacene-based field effect transistor devices towards trace nitro-aromatic explosive vapors through the incorporation of high density, sub-2 nm platinum nanoparticles (NPs) within these structures. Exploiting the unique electronic properties of these NPs, we have demonstrated a detection limit of 56.6 parts per billion of 2,4-dinitrotoluene (DNT) vapor while control samples without any embedded NPs showed no observable sensitivity to DNT vapor. We attribute this remarkable enhancement in sensitivity to the ability of these NPs to function as discrete nodes, participating in the charge transfer with adsorbed nitro-aromatic molecules.

Layer-by-layer charging in non-volatile memory devices using embedded sub-2 nm platinum nanoparticles

In this work, we demonstrate multi-level operation of a non-volatile memory metal oxide semiconductor capacitor by controlled layer-by-layer charging of platinum nanoparticle (PtNP) floating gate devices with defined gate voltage bias ranges. The device consists of two layers of ultra-fine, sub-2 nm PtNPs integrated between Al2O3 tunneling and separation layers. PtNP size and interparticle distance were varied to control the particle self-capacitance and associated Coulomb charging energy. Likewise, the tunneling layer thicknesses were also varied to control electron tunneling to the first and second PtNP layers. The final device configuration with optimal charging behavior and multi-level programming was attained with a 3 nm Al2O3 initial tunneling layer, initial PtNP layer with particle size 0.54 ± 0.12 nm and interparticle distance 4.65 ± 2.09 nm, 3 nm Al2O3 layer to separate the PtNP layers, and second particle layer with 1.11 ± 0.28 nm PtNP size and interparticle distance 2.75 ± 1.05 nm. In this devi...