Virendra Kumar Parashar

Biography

Virendra Kumar Parashar received doctorate degree from Indian Institute of Technology, Delhi, India. From 1989 to 2000, he worked at National Physical Laboratory, New Delhi, India. He was an associate researcher and visiting faculty to Republic of Korea from 2000 to 2001. In 2001, he joined the Institute of Microelectronics & Microsystemes (EPFL) where he is responsible for the technology development of fabrication of micro- and nano- structures in glasses & ceramics.

Official source

https://people.epfl.ch/virendrakumar.parashar

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Ripening of two-dimensional colloidal CdSe nanocrystals into zero-dimensional nanodots

Martinus Gijs, Xiaopeng Huang, Virendra Kumar Parashar

Understanding the ripening of two-dimensional (2D) colloidal nanocrystals (NCs) is important for the controllable synthesis of NCs with desired morphology and properties. In this study, we systematically investigate the ripening behavior of the 2D CdSe NCs in the presence of a short-chain acetate ligand and a long-chain oleate ligand. We find that a low acetate/oleate ratio, a low Cd/Se ratio, and a low monomer concentration help in the ripening of the 2D NCs to form 0D NCs. Moreover, a porous nanosheet intermediate is observed when there is a high Cd/Se ratio, whereas in the case of a low Cd/Se ratio, the ripening starts from the edge of the nanosheets, resulting in a saw-like nanosheet intermediate. These findings provide necessary insights into the growth and ripening of 2D CdSe NCs that allow for the controlled synthesis of 0D and 2D CdSe NCs.

CELL PRESS2021

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An in-depth understanding of the growth mechanism of nanocrystals (NCs) is of great significance for shape control of colloidal semiconductor nanostructures. In this study, we elucidate the formation mechanism of anisotropic CdSe NCs by systematically investigating their growth in the presence of dioctylamine and carboxylate ligands with different chain lengths. A morphological transition from nanotubes and nanosheets to irregular nanorods and finally to nanodots was observed when the carbon number of the carboxylate ligands increased from 2 to 18. The traditional understanding of the growth of anisotropic CdSe nanostructures is mainly based on the monomer addition, which is difficult to explain the phenomena observed. We found that both short- and long-chain carboxylate ligands can lead to the anisotropic growth of CdSe NCs by attachment of early-formed building blocks, which is a nonclassical particle-mediated growth approach. The use of different carboxylate ligands plays a key role in the formation of different building blocks by affecting the local monomer supersaturation. We provide both experimental observations and first-principles simulations to support this hypothesis.

AMER CHEMICAL SOC2020

Spontaneous Formation of CdSe Photoluminescent Nanotubes with Visible-Light Photocatalytic Performance

Martinus Gijs, Xiaopeng Huang, Virendra Kumar Parashar

Two-dimensional (2D) colloidal CdSe nanocrystals (NCs) with precise atomic-scale thickness have attracted intensive attention in recent years due to their optical properties and quantum confinement effects originating from their particular band structure. Here, we report a solution-based and template-free protocol to synthesize CdSe nanotubes (NTs) having 3-6 walls, each of which has 3.5 molecular monolayers. Their crystal structure is zincblende, with Cd-terminated {100} planes at the top and bottom surfaces of each wall, which are passivated by short-chain Angular Attachment acetate ligands. After verifying the prominent role of the acetate ligand for NT synthesis, we elucidated the formation mechanism of these NTs. It starts by heterogeneous nucleation of 2D plateletlike nanoseeds from the amorphous Cd precursor matrix, followed by the growth via lateral and angular attachment of nanoplatelet building blocks into curved nanosheets, eventually resulting in NTs with sharp absorption and photoluminescence peak at around 460 nm. Moreover, the NTs show remarkable visible-light photocatalytic activity, as demonstrated by the reduction of the reddish Rhodamine B into its leuco form with a conversion rate of 92% in 1 min.

2019