Amit Dalui

Mechanism of Versatile Catalytic Activities of Quaternary CuZnFeS Nanocrystals Designed by a Rapid Synthesis Route

Quaternary alloyed nanocrystals (NCs) composed of earth abundant, environment friendly elements are of interest for energy-harvesting applications. These complex NCs are useful as catalysts for the degradation of multiple refractory organic pollutants as well as nitro-organic reduction at a rapid rate. Here, a remarkably fast (∼30 s) and facile synthesis of crystalline quaternary chalcopyrite copper-zinc-iron-sulfide (CZIS) NCs is reported. These NCs show excellent catalytic properties by degrading a number of refractory organic dyes and converting nitro-compounds at a rapid rate. The valence and conduction band information of the newly designed NCs are extracted using scanning tunneling spectroscopy and ultraviolet photoelectron spectroscopy, which reveal energy levels suitable for performing redox chemistry by generating reactive radicals establishing NCs as efficient catalyst with multiple uses. Rapid synthesis of high quality phase-controlled CZIS NCs with robust catalytic activities could be useful for organic waste treatment.

Efficient Solid-State Light-Emitting CuCdS Nanocrystals Synthesized in Air

Semiconductor nanocrystals (NCs) possess high photoluminescence (PL) typically in the solution phase. In contrary, PL rapidly quenches in the solid state. Efficient solid state luminescence can be achieved by inducing a large Stokes shift. Here we report on a novel synthesis of compositionally controlled CuCdS NCs in air avoiding the usual complexity of using inert atmosphere. These NCs show long-range color tunability over the entire visible range with a remarkable Stokes shift up to about 1.25 eV. Overcoating the NCs leads to a high solid-state PL quantum yield (QY) of ca. 55% measured by using an integrating sphere. Unique charge carrier recombination mechanisms have been recognized from the NCs, which are correlated to the internal NC structure probed by using extended X-ray absorption fine structure (EXAFS) spectroscopy. EXAFS measurements show a Cu-rich surface and Cd-rich interior with 46% Cu(I) being randomly distributed within 84% of the NC volume creating additional transition states for PL. Color-tunable solid-state luminescence remains stable in air enabling fabrication of light-emitting diodes (LEDs).

Shape-controlled cobalt phosphide nanoparticles as volatile organic solvent sensor

Here we demonstrate shape-controlled cobalt phosphide nanostructures through a one-pot synthetic approach in a single step with precise control over the size, shape and composition. We found that the ratio of the cobalt precursor, phosphorus sources, capping ligands, and annealing time play a key role in determining the morphology of the cobalt phosphide nanostructures. For instance, a zero-to-one-dimensional nanostructure transformation was observed with an increase in annealing time. The aspect ratio of one-dimensional nanorods could flexibly be tuned by a subtle balance of cobalt precursor, phosphorous source and capping ligands. Furthermore, a rod-to-hollow sphere transformation could be observed upon a decrease in the phosphorous source. Of these nanostructures, branched nanowires showed an excellent volatile organic solvent vapor sensing performance selective to benzene compared to the linear chain hydrocarbon hexane. A novel yet simple synthesis strategy with wide varieties of a controlled morphology of cobalt phosphide is expected to open up a new avenue to design other metal phosphide nanostructures, which would offer superior applications in the field of energy harvesting and sensing.

Current rectification by a single ZnS nanorod probed using a scanning tunneling microscopic technique

We report on the rectification properties from a single ZnS nanorod measured using the UHV-SPM technique. The rectification behavior is evidenced from the current–voltage characteristics measured on a single ZnS nanorod. We propose a tunneling mechanism where the direct tunneling mechanism is dominant at lower applied bias voltages followed by resonant tunneling through discrete energy levels of the nanorod. A further increase in the bias voltage changes the tunneling mechanism to the Fowler–Nordheim tunneling regime enabling rectification behavior. Realizing rectification from a single ZnS nanorod may provide a means of realizing a single nanorod based miniaturized device.

Facile synthesis of composition and morphology modulated quaternary CuZnFeS colloidal nanocrystals for photovoltaic application

Quaternary semiconductor CuZnFeS (CZIS) nanocrystals (NCs) with controlled size, shape and composition composed of earth abundant elements have been successfully synthesized using the colloidal synthesis method. The size, shape and composition of the NCs have been controlled by tuning the reaction parameters to obtain NCs in the form of dots, triangles, hexagons, sheets, rods and wires. These quaternary CZIS NCs show high light absorbing properties towards visible to near infrared light with a high absorption coefficient suitable for photovoltaic applications. Utilizing layer-by-layer deposition of CIZS NCs films, heterojunction devices consisting of ITO/PEDOT : PSS/CZIS NCs/Al are fabricated for photovoltaic performance. The devices exhibit excellent rectification behavior (rectification ratio of ∼150) and good photoresponsitivity (on/off ratio of ∼55). The broad range of absorptions with strong extinction coefficient properties of CZIS NCs has been utilized to fabricate quantum dot sensitized solar cells (QDSSCs). Our synthesis protocol marks an advance for chalcopyrite NCs based solar cells and offers a possible template for the synthesis of other ternary and quaternary NCs with robust photoelectric properties.

Shape Dependent Synthesis and Field Emission Induced Rectification in Single ZnS Nanocrystals

We report on the synthesis of shape controlled ZnS nanocrystals designed into nanodots, nanorods, and nanowires retaining the same diameter and crystallographic phase. We used UHV scanning tunneling microscopy and spectroscopy to study rectification behavior from single nanocrystals. The nanorod and nanowire show large tunneling current at the negative bias in comparison to the positive bias demonstrating current rectification, while the nanodot shows symmetric current-voltage behavior. We proposed a tunneling mechanism where direct tunneling is followed by resonant tunneling mechanism through ZnS nanocrystal at lower applied bias voltages. Stimulation of field emission in Fowler-Nordheim tunneling regime at higher negative bias voltages enables the rectification behavior from the ZnS nanorod or nanowire. Absence of rectification from the ZnS nanodot is associated with spherical shape where the field emission becomes less significant. Realizing functional electronic component from such shape dependent single ZnS nanocrystal may provide a means in realizing nanocrystal based miniaturized devices.