Suresh Sarkar

Ultrasmall Color-Tunable Copper-Doped Ternary Semiconductor Nanocrystal Emitters†

Synthesis of light-emitting dispersed semiconductor nanocrystals with tunable emission has been widely studied in the last two decades because of their potential applications in photovoltaics, optoelectronics, and biology. Soon after the development of high-quality CdSe nanocrystals with spectacular size-dependent tunable excitonic emission in the entire visible window, simplification of the synthetic method, stabilization of the emission, surface functionalization of the nanocrystals, design of non-cadmium nanocrystal emitters, fabrication of different kinds of composition-tunable multifunctional alloy nanocrystals, and related photophysical properties have been widely investigated for both fundamental understanding and their implementation in day-to-day developing technology. Analysis of up to date literature reports reveals that biological applications require strongly emitting, small and nontoxic nanocrystals preferably with excitation in the visible window, light-emitting diodes require nanocrystals having large Stokes shift and high quantum efficiency, and for solar cells nanocrystals having visible/near-IR (NIR) absorption and/or ternary/ quaternary nanocrystals with excess of either of the charge carriers (electron or hole) 11] are preferred. So far, no nanocrystal emitters having all such required properties have been reported, and thus further investigations are required to obtain new materials with new properties that would be suitable for versatile applications. We have now designed a new series of ultrasmall (< 2.5 nm), nearly fixed size, alloyed nanocrystals composed of Cu–Zn–In–Se ions which show composition-dependent tunable emissionover most of the visible window. In addition, these nanocrystals are cadmium-free and have aqueous dispersibility, photostability, large Stokes shifts, and high emission intensity (quantum yield (QY) = 25–30%), which makes them a versatile light-emitting nanoscale materials providing one-step solutions for various applications. The fundamental designing principle of these nanocrystals involves a mechanism whereby composition-variable alloy formation tunes the optical bands from lower to higher energy and vice versa. Here we report details of the synthesis, chemistry of formation, and composition-variable optical tuning of these fixed-size alloy nanocrystals. In addition, aqueous dispersibility and photovoltaic properties of these nanocrystals were investigated. The alloy nanocrystals were synthesized by simultaneous precipitation and surface cation-exchange protocols. Injection of a selenium precursor into a mixture of Zn, In, and Cu salts at 220 8C (see Experimental Section) results in copperdoped zinc indium selenide alloy nanocrystals whose absorption and emission wavelengths are determined by the In:Zn ratio of the reaction mixture. Further addition of Zn with continuous annealing slowly shifts both absorption and emission bands to the blue in a surface ion-exchange process. Successive photoluminescence (PL) spectra, absorption (UV/ Vis) spectra, and a schematic model of surface cation exchange for a typical alloying process are shown in Figure 1a–c. With an initial Zn:In ratio of 1:2, the emission appears at about 660 nm soon after injection of the Se precursor and is tuned up to 575 nm (Figure 1a) on introduction of additional Zn precursor, while for an initial Zn:In ratio of 1:1, the emission appears at about 620 nm and is tuned further to the blue, to 540 nm (Figure 1 b), that is, a total

Influence of doping on semiconductor nanocrystals mediated charge transfer and photocatalytic organic reaction

Doped and undoped ZnS semiconductor nanocrystals having different recombination pathways are explored to study the charge transfer reaction between the nanocrystals and the 4-nitrophenol/sodium borohydride redox couple.

Subnanometer Thin β-Indium Sulfide Nanosheets

Nanosheets are a peculiar kind of nanomaterials that are grown two-dimensionally over a micrometer in length and a few nanometers in thickness. Wide varieties of inorganic semiconductor nanosheets are already reported, but controlling the crystal growth and tuning their thickness within few atomic layers have not been yet explored. We investigate here the parameters that determine the thickness and the formation mechanism of subnanometer thin (two atomic layers) cubic indium sulfide (In2S3) nanosheets. Using appropriate reaction condition, the growth kinetics is monitored by controlling the decomposition rate of the single source precursor of In2S3 as a function of nucleation temperature. The variation in the thickness of the nanosheets along the polar [111] direction has been correlated with the rate of evolved H2S gas, which in turn depends on the rate of the precursor decomposition. In addition, it has been observed that the thickness of the In2S3 nanosheets is related to the nucleation temperature.

The Redox Chemistry at the Interface for Retrieving and Brightening the Emission of Doped Semiconductor Nanocrystals.

Photo-oxidation of semiconductor quantum dots is the prime concern during their processability, as it often induces nonradiative states and quenches the band edge excitonic emission. Nevertheless, similar effects have been observed for light emitting doped semiconductor nanocrystals, and the dopant emissions are also quenched due to the surface oxidation. This is more pronounced for selenide-based host semiconductors. To overcome this, we study the interface chemistry of Cu-doped and Mn-doped ZnSe nanocrystals and report here the retrieving and brightening of the emission from completely quenched months old doped nanocrystals. This has been obtained by treating the doped nanocrystals with appropriate organic thiol ligands which remove the surface oxidative states as well as resist further oxidation of the nanocrystals. Here, we investigate details of the redox chemistry at the interface and study related photophysics in retrieving the dopant emission.

Efficient Superionic Conductor Catalyst for Solid in Solution-Solid-Solid Growth of Heteronanowires.

How efficient could a superionic conductor catalyst be? Beyond the traditionally used molecular precursors when the solution dispersed solid nanomaterials of variable size, shape and phase are introduced under certain reaction condition; the catalyst is found to digest all these structures in minutes irrespective of their phase and morphology, resulting unique heteronanowires. This has been inspected here by employing different ZnSe nanostructures as precursor for Ag2Se nanocrystal catalyst in its superionic conductor phase to obtain the Ag2Se-ZnSe heteronanowires. This dissolution and formation process of these nanostructures is correlated with the change in the reaction temperature profile, the phase of the catalyst, the shape/phase and surface ligands of the source nanostructures, and the possible mechanism of the unique heteronanowires growth has been investigated.

Tuning the Growth Pattern in 2D Confinement Regime of Sm2O3 and the Emerging Room Temperature Unusual Superparamagnetism

Programming the reaction chemistry for superseding the formation of Sm2O3 in a competitive process of formation and dissolution, the crystal growth patterns are varied and two different nanostructures of Sm2O3 in 2D confinement regime are designed. Among these, the regular and self-assembled square platelets nanostructures exhibit paramagnetic behavior analogous to the bulk Sm2O3. But, the other one, 2D flower like shaped nanostructure, formed by irregular crystal growth, shows superparamagnetism at room temperature which is unusual for bulk paramagnet. It has been noted that the variation in the crystal growth pattern is due to the difference in the binding ability of two organic ligands, oleylamine and oleic acid, used for the synthesis and the magnetic behavior of the nanostructures is related to the defects incorporated during the crystal growth. Herein, we inspect the formation chemistry and plausible origin of contrasting magnetism of these nanostructures of Sm2O3.

Chemical Sealing of Nanotubes: A Case Study on Sb2S3

Implementing the solution chemistry, herein, we report the sealing of both ends of Sb2 S3 semiconductor nanotubes following the diffusion-controlled deposition of the sealing material, AgSbS2 . As a consequence, unique dumbbell-shaped hollow nanocapsules having a binary-ternary epitaxial heterojunction were formed in solution. Whereas these capsule-shaped nanostructures were obtained by the introduction of Ag(0) nanocrystals just after the formation of Sb2 S3 nanotubes, the addition of Ag(0) at the beginning of the process, prior to the formation of nanotubes, changed the growth pattern, and solid nanorods of Sb2 S3 were formed. The details of the chemistry involved in the formation of these nanostructures were investigated and are discussed herein.

Vortex‐Pattern Self‐Assembly in Mn‐Doped ZnSe Nanorods

Spontaneous patterning of anisotropic nanostructures into ordered assemblies remains a challenging quest, which requires controlled innovative approaches. One way to achieve such ordering of 1D nanorods is by manipulating the varieties of interactions (attractive and repulsive forces) present in colloidal solutions of anisotropic nanocrystals. The other ingenuous pathway is solvent-evaporation-mediated self-organization of the 1D nanorods. By following the second protocol, we have achieved exclusive pillar self-assembled patterns of visible-light-emitting Mn-doped ZnSe nanorods. The nanorods also exhibit intriguing vortex patterning observed by directional solvent evaporation from the nanorod solution. The effect of solvent evaporation to generate such unique morphologies on the TEM grid is discussed and the reported procedure to obtain the assembled patterns of visible-light-emitting, doped nanorods might be useful for future technological applications.