V. Sundararajan

Bio: V. Sundararajan is an academic researcher from Centre for Development of Advanced Computing. The author has contributed to research in topics: Pseudopotential & Local-density approximation. The author has an hindex of 4, co-authored 6 publications receiving 518 citations. Previous affiliations of V. Sundararajan include Tohoku University.

Ultra-stable nanoparticles of CdSe revealed from mass spectrometry

Abstract: Nanoparticles under a few nanometres in size have structures and material functions that differ from the bulk because of their distinct geometrical shapes and strong quantum confinement. These qualities could lead to unique device applications. Our mass spectral analysis of CdSe nanoparticles reveals that (CdSe)(33) and (CdSe)(34) are extremely stable: with a simple solution method, they grow in preference to any other chemical compositions to produce macroscopic quantities. First-principles calculations predict that these are puckered (CdSe)(28)-cages, with four- and six-membered rings based on the highly symmetric octahedral analogues of fullerenes, accommodating either (CdSe)(5) or (CdSe)(6) inside to form a three-dimensional network with essentially heteropolar sp(3)-bonding. This is in accordance with our X-ray and optical analyses. We have found similar mass spectra and atomic structures in CdS, CdTe, ZnS and ZnSe, demonstrating that mass-specified and macroscopically produced nanoparticles, which have been practically limited so far to elemental carbon, can now be extended to a vast variety of compound systems.

Ab initio molecular-dynamics studies of doped magic clusters and their interaction with atoms

Abstract: We present results of the atomic and electronic structure of icosahedral ${\mathrm{Al}}_{12}X$ $(X=\mathrm{S}\mathrm{i}$, Ge, and Sn) clusters using the ab initio molecular-dynamics method within the local density functional theory. Substitutional doping of a ${\mathrm{Al}}_{13}$ cluster by a tetravalent atom leads to a substantial gain in energy in all the cases studied. Tin is found to have a lower energy at a vertex site in contrast to the central site for Si and Ge, leading to surface segregation of Sn in these clusters. Also in the case of a ${\mathrm{Al}}_{13}\mathrm{Si}$ cluster, Si occupies the central site of a capped icosahedral structure. These results when interpreted in terms of the interaction of closed shell clusters with atoms leads to a relatively strong interaction of ${\mathrm{Al}}_{12}\mathrm{Si}$ with $\mathrm{Al}$ as compared to the weak interaction of rare gas atoms with other elements.

Optical properties of zinc selenide clusters from first-principles calculations

Abstract: The optical properties of bare and passivated ${\text{Zn}}_{n}{\text{Se}}_{n}$ $(n=1--13)$ clusters have been studied within the framework of time-dependent local density approximation. The atomic structure of the clusters has been obtained using projector augmented wave pseudopotential method, with generalized gradient approximation for the exchange-correlation energy. The small clusters with $n$ up to 5 have two-dimensional (2D) structure and for larger sizes, cagelike 3D structures become favorable. At $n=13$, the clusters start getting an atom inside the cage to attain bulklike local structure. For the bare clusters, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gap increases from a small value for ZnSe dimer and beyond $n=3$, the variation is small. On the other hand, the HOMO-LUMO gap of the clusters passivated with partially charged hydrogen atom decreases nearly monotonically with increasing size, though the value remains higher compared with that of the bare clusters even for the case of $n=13$. Further, the optical absorption spectra and the corresponding optical gap have been calculated and a decreasing trend as a function of the increasing cluster size has been obtained. This compares well with the experimental results available on larger clusters in the literature though the calculated values underestimate the optical absorption gap as expected within the local density approximation framework.

Ab initio molecular dynamics study of antimony clusters

Abstract: We present an ab initio molecular dynamics study of the atomic and electronic structure of SbN (N=2–8 and 12) clusters within the local density approximation and pseudopotential representation of the electron–ion interaction. Simulated annealing calculations have been done for 6‐, 7‐, 8‐, and 12‐atom clusters. While for Sb4 a bent rhombus is about 2 eV higher in energy than a regular tetrahedron, we find that it plays an important role in the structure of larger clusters. For Sb8 we obtain two weakly interacting tetrahedra to be of lowest energy. However, this is nearly degenerate with a bent rhombus interacting with a distorted tetrahedron. Further, our calculations suggest a bent rhombus based structure for Sb12 cluster indicating the observation of Sb4n clusters in Sb vapor condensation cell to be due to abundance of Sb4 clusters. A large gap is found to exist between the highest and the next occupied Kohn–Sham eigenvalues of the lowest energy isomers of 3‐, 5‐, and 7‐atom clusters. This is in agreemen...

Discovery of a nonstoichiometric Zn 11 MnSe 13 magnetic magic quantum dot from ab initio calculations

Abstract: Ab initio calculations on ZnSe quantum dots (QDs) doped with one Mn atom predict a new nonstoichiometric magnetic magic Zn${}_{11}$MnSe${}_{13}$ structure in contrast to QDs of undoped ZnSe that are stoichiometric and exhibit magic behavior for Zn${}_{n}$Se${}_{n}$ with $n$ $=$ 13 and 34. Our results suggest that such doping would lead to a high abundance of only one specie (the magic QD) that would be produced preferentially. The stoichiometric Zn${}_{n\ensuremath{-}1}$MnSe${}_{n}$ QDs have a large magnetic moment of 5 ${\ensuremath{\mu}}_{B}$ that is predominantly localized on the Mn site. However, nonstoichiometic QD has a reduced magnetic moment of 3 ${\ensuremath{\mu}}_{B}$ due to strong covalent bonding of the Mn atom with the excess Se atom and a small gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Charging this magic QD with two electrons leads to a large HOMO-LUMO gap of 1.8 eV and 5 ${\ensuremath{\mu}}_{B}$ magnetic moment. These results together with calculations on Mn-doped $n$ $=$ 34 QD provide a possible growth mechanism of larger doped QDs and a new ground for understanding such QDs of compound semiconductors.

Doping semiconductor nanocrystals

Abstract: Doping--the intentional introduction of impurities into a material--is fundamental to controlling the properties of bulk semiconductors. This has stimulated similar efforts to dope semiconductor nanocrystals. Despite some successes, many of these efforts have failed, for reasons that remain unclear. For example, Mn can be incorporated into nanocrystals of CdS and ZnSe (refs 7-9), but not into CdSe (ref. 12)--despite comparable bulk solubilities of near 50 per cent. These difficulties, which have hindered development of new nanocrystalline materials, are often attributed to 'self-purification', an allegedly intrinsic mechanism whereby impurities are expelled. Here we show instead that the underlying mechanism that controls doping is the initial adsorption of impurities on the nanocrystal surface during growth. We find that adsorption--and therefore doping efficiency--is determined by three main factors: surface morphology, nanocrystal shape, and surfactants in the growth solution. Calculated Mn adsorption energies and equilibrium shapes for several nanocrystals lead to specific doping predictions. These are confirmed by measuring how the Mn concentration in ZnSe varies with nanocrystal size and shape. Finally, we use our predictions to incorporate Mn into previously undopable CdSe nanocrystals. This success establishes that earlier difficulties with doping are not intrinsic, and suggests that a variety of doped nanocrystals--for applications from solar cells to spintronics--can be anticipated.

White-light emission from magic-sized cadmium selenide nanocrystals.

Abstract: Magic-sized cadmium selenide (CdSe) nanocrystals have been pyrolytically synthesized. These ultra-small nanocrystals exhibit broadband emission (420−710 nm) that covers most of the visible spectrum while not suffering from self absorption. This behavior is a direct result of the extremely narrow size distribution and unusually large Stokes shift (40−50 nm). The intrinsic properties of these ultra-small nanocrystals make them an ideal material for applications in solid state lighting and also the perfect platform to study the molecule-to-nanocrystal transition.

Crystal structure prediction from first principles

Abstract: The prediction of structure at the atomic level is one of the most fundamental challenges in condensed matter science. Here we survey the current status of the field and consider recent developments in methodology, paying particular attention to approaches for surveying energy landscapes. We illustrate the current state of the art in this field with topical applications to inorganic, especially microporous solids, and to molecular crystals; we also look at applications to nanoparticulate structures. Finally, we consider future directions and challenges in the field.

Kinetics and Mechanisms of Aggregative Nanocrystal Growth

Abstract: The aggregative growth and oriented attachment of nanocrystals and nanoparticles are reviewed, and they are contrasted to classical LaMer nucleation and growth, and to Ostwald ripening. Kinetic and mechanistic models are presented, and experiments directly observing aggregative growth and oriented attachment are summarized. Aggregative growth is described as a nonclassical nucleation and growth process. The concept of a nucleation function is introduced, and approximated with a Gaussian form. The height (Γmax) and width (Δtn) of the nucleation function are systematically varied by conditions that influence the colloidal stability of the small, primary nanocrystals participating in aggregative growth. The nucleation parameters Γmax and Δtn correlate with the final nanocrystal mean size and size distribution, affording a potential means of achieving nucleation control in nanocrystal synthesis.