Somnath Koley - Academia.edu (original) (raw)

Papers by Somnath Koley

Proceedings of the nanoGe Spring Meeting 2022, 2022

Electronic coupling and hence hybridization of atoms serve as the basis for the rich properties o... more Electronic coupling and hence hybridization of atoms serve as the basis for the rich properties of the endless library of naturally occurring molecules. Colloidal quantum dots (CQDs) manifesting quantum strong confinement, possess atomic like characteristics with s and p electronic levels, which popularized the notion of CQDs as artificial atoms. Continuing this analogy, when two atoms are close enough to form a molecule so that their orbitals start overlapping, the orbitals' energies start to split into bonding and anti-bonding states made out of hybridized orbitals. The same concept is also applicable for two fused core-shell nanocrystals in close proximity. Their band-edge states, which dictate the emitted photon energy, start to hybridize changing their electronic and optical properties. Thus, an exciting direction of artificial molecules emerges leading to a multitude of possibilities for creating a library of new hybrid nanostructures with novel optoelectronic properties w...

Nano Letters, 2021

Indium phosphide (InP) nanocrystals are emerging as an alternative to heavy metal containing nano... more Indium phosphide (InP) nanocrystals are emerging as an alternative to heavy metal containing nanocrystals for optoelectronic applications but lag behind in terms of synthetic control. Herein, luminescent wurtzite InP nanocrystals with narrow size distribution were synthesized via a cation exchange reaction from hexagonal Cu3P nanocrystals. A comprehensive surface treatment with NOBF4 was performed, which removes excess copper while generating stoichiometric In/P nanocrystals with fluoride surface passivation. The attained InP nanocrystals manifest a highly resolved absorption spectrum with a narrow emission line of 80 meV, and photoluminescence quantum yield of up to 40%. Optical anisotropy measurements on ensemble and single particle bases show the occurrence of polarized transitions directly mirroring the anisotropic wurtzite lattice, as also manifested from modeling of the quantum confined electronic levels. This shows a green synthesis path for achieving wurtzite InP nanocrystals with desired optoelectronic properties including color purity and light polarization with potential for diverse optoelectronic applications.

Coupled colloidal quantum dot (CQD) dimers represent a new class of artificial molecules composed... more Coupled colloidal quantum dot (CQD) dimers represent a new class of artificial molecules composed of fused core/shell semiconductor nanocrystals. The electronic coupling and wavefunction hybridization is enabled by the formation of an epitaxial connection with a coherent lattice between the shells of the two neighboring quantum dots where the shell material and its dimensions dictate the quantum barrier characteristics for the charge carriers. Herein we introduce a colloidal approach to control the neck formation at the interface between the two CQDs in such artificial molecular constructs. This allows the tailoring of the neck barrier in pre-linked homodimers formed via fusion of multifaceted wurtzite CdSe/CdS CQDs. The effects of reaction time, temperature and excess ligands is studied. The neck filling process follows an intraparticle ripening mechanism at relatively mild reaction conditions while avoiding inter-particle ripening. The degree of surface ligand passivation plays a key role in activating the surface atom diffusion to the neck region. The degree of neck filling strongly depends also on the initial relative orientation of the two CQDs, where homonymous plane attachment allows for facile neck growth, unlike the case of heteronymous plane attachment. Upon neck-filling, the observed redshift of the absorption and fluorescence measured both for ensemble and single dimers, is assigned to enhanced hybridization of the confined wavefunction in CQD dimer molecules, as supported by quantum calculations. The fine tuning of the particle interface introduced herein provides therefore a powerful tool to further control the extent of hybridization and coupling in CQD molecules.

ChemistrySelect, 2018

Certain commercial entities, equipment, products, or materials are identified in tliis document i... more Certain commercial entities, equipment, products, or materials are identified in tliis document in order to describe a procedure or concept adequately or to trace the history of the procedures and practices used. Such identification is not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or equipment are necessarily the best available for the purpose. Nor does such identification imply a finding of fault or negligence by the National Institute of Standards and Technology. Disclaimer No. 2 The policy of NIST is to use the International System of Units (metric units) in all publications. In this document, however, units are presented in metric units or the inch-pound system, whichever is prevalent in the discipline. Disclaimer No. 3 Pursuant to section 7 of the National Construction Safety Team Act, the NIST Director has determined that certain evidence received by NIST in the course of this Investigation is "voluntarily provided safety-related information" that is "not directly related to the building failure being investigated" and that "disclosure of that information would inhibit the voluntary provision of that type of information" (15 USC 7306c). In addition, a substantial portion of the evidence collected by NIST in the course of the Investigation has been provided to NIST under nondisclosure agreements. Disclaimer No. 4 NIST takes no position as to whether the design or construction of a WTC building was compliant with any code since, due to the destruction of the WTC buildings, NIST could not verify the actual (or as-built) construction, the properties and condition of the materials used, or changes to the original construction made over the life of the buildings. In addition, NIST could not verify the interpretations of codes used by applicable authorities in determining compliance when implementing building codes. Where an Investigation report states whether a system was designed or installed as required by a code provision, NIST has documentary or anecdotal evidence indicating whether the requirement was met, or NIST has independently conducted tests or analyses indicating whether the requirement was met. Use in Legal Proceedinqs No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter mentioned in such report (15 USC 281a; as amended by P.L. 107-231).

Angewandte Chemie International Edition, 2021

Supporting information for this article is given via a link at the end of the document.

Accounts of Chemical Research, 2021

Metrics & More Article Recommendations CONSPECTUS: Electronic coupling and hence hybridization of... more Metrics & More Article Recommendations CONSPECTUS: Electronic coupling and hence hybridization of atoms serves as the basis for the rich properties for the endless library of naturally occurring molecules. Colloidal quantum dots (CQDs) manifesting quantum strong confinement possess atomiclike characteristics with s and p electronic levels, which popularized the notion of CQDs as artificial atoms. Continuing this analogy, when two atoms are close enough to form a molecule so that their orbitals start overlapping, the orbitals energies start to split into bonding and antibonding states made out of hybridized orbitals. The same concept is also applicable for two fused core−shell nanocrystals in close proximity. Their band edge states, which dictate the emitted photon energy, start to hybridize, changing their electronic and optical properties. Thus, an exciting direction of "artificial molecules" emerges, leading to a multitude of possibilities for creating a library of new hybrid nanostructures with novel optoelectronic properties with relevance toward diverse applications including quantum technologies. The controlled separation and the barrier height between two adjacent quantum dots are key variables for dictating the magnitude of the coupling energy of the confined wave functions. In the past, coupled double quantum dot architectures prepared by molecular beam epitaxy revealed a coupling energy of few millielectron volts, which limits the applications to mostly cryogenic operation. The realization of artificial quantum molecules with sufficient coupling energy detectable at room temperature calls for the use of colloidal semiconductor nanocrystal building blocks. Moreover, the tunable surface chemistry widely opens the predesigned attachment strategies as well as the solution processing ability of the prepared artificial molecules, making the colloidal nanocrystals as an ideal candidate for this purpose. Despite several approaches that demonstrated enabling of the coupled structures, a general and reproducible method applicable to a broad range of colloidal quantum materials is needed for systematic tailoring of the coupling strength based on a dictated barrier This Account addresses the development of nanocrystal chemistry to create coupled colloidal quantum dot molecules and to study the controlled electronic coupling and their emergent properties. The simplest nanocrystal molecule, a homodimer formed from two core/shell nanocrystal monomers, in analogy to homonuclear diatomic molecules, serves as a model system. The shell material of the two CQDs is structurally fused, resulting in a continuous crystal. This lowers the potential energy barrier, enabling the hybridization of the electronic wave functions. The direct manifestation of the hybridization reflects on the band edge transition shifting toward lower energy and is clearly resolved at room temperature. The hybridization energy within the single homodimer molecule is strongly correlated with the extent of structural continuity, the delocalization of the exciton wave function, and the barrier thickness as calculated numerically. The hybridization impacts the emitted photon statistics manifesting faster radiative decay rate, photon bunching effect, and modified Auger recombination pathway compared to the monomer artificial atoms. Future perspectives for the nanocrystals chemistry paradigm are also highlighted.

Epitaxial growth of a protective semiconductor shell on a colloidal quantum dot (QD) core is the ... more Epitaxial growth of a protective semiconductor shell on a colloidal quantum dot (QD) core is the key strategy for achieving high fluorescence quantum efficiency and essential stability for optoelectronic applications and biotagging with emissive QDs. Herein we investigate the effect of shell growth rate on the structure and optical properties in blue-emitting ZnSe/ ZnS QDs with narrow emission line width. Tuning the precursor reactivity modifies the growth mode of ZnS shells on ZnSe cores transforming from kinetic (fast) to thermodynamic (slow) growth regimes. In the thermodynamic growth regime, enhanced fluorescence quantum yields and reduced on−off blinking are achieved. This high performance is ascribed to the effective avoidance of traps at the interface between the core and the shell, which are detrimental to the emission properties. Our study points to a general strategy to obtain high-quality core/shell QDs with enhanced optical properties through controlled reactivity yielding shell growth in the thermodynamic limit.

Nature Communications, 2019

Coupling of atoms is the basis of chemistry, yielding the beauty and richness of molecules. We ut... more Coupling of atoms is the basis of chemistry, yielding the beauty and richness of molecules. We utilize semiconductor nanocrystals as artificial atoms to form nanocrystal molecules that are structurally and electronically coupled. CdSe/CdS core/shell nanocrystals are linked to form dimers which are then fused via constrained oriented attachment. The possible nanocrystal facets in which such fusion takes place are analyzed with atomic resolution revealing the distribution of possible crystal fusion scenarios. Coherent coupling and wave-function hybridization are manifested by a redshift of the band gap, in agreement with quantum mechanical simulations. Single nanoparticle spectroscopy unravels the attributes of coupled nanocrystal dimers related to the unique combination of quantum mechanical tunneling and energy transfer mechanisms. This sets the stage for nanocrystal chemistry to yield a diverse selection of coupled nanocrystal molecules constructed from controlled core/shell nanocr...

The Journal of Chemical Physics, 2019

Paper published as part of the special topic on Colloidal Quantum Dots Note: This paper is part o... more Paper published as part of the special topic on Colloidal Quantum Dots Note: This paper is part of the JCP Special Topic on Colloidal Quantum Dots.

The Journal of Physical Chemistry Letters, 2019

Integrity of the fluorescent carbon dot (FCD) emission deserves its highest appreciation when sam... more Integrity of the fluorescent carbon dot (FCD) emission deserves its highest appreciation when sample purification is performed with extreme care. Several controversial phenomena of FCD fluorescence including excitation dependent emission, spectral migration with time and thereby violation of Kasha-Vavilov rule, those sparked intense debate during recent reports, disappeared when we rigorously purified the as-synthesized FCD sample. Purification was performed by first visual silica column chromatography (observing the emissions under UV-illumination) and subsequently a prolonged membrane dialysis. Most of the surprising phenomena of FCD fluorescence reported earlier apparently arose from a ground state spectral heterogeneity of FCD sample containing a large amount of fluorescent impurities (mostly polymeric or oligomeric in nature). Observation of our ensemble spectroscopic measurements albeit nicely matched with recent reports based on single-particle experiments, but differed largely from other ensemble measurements. Our results reconciled a number of long-standing controversies on FCD emission mostly by emphasizing the urgency of sample purification with more scientific rigor.

The Journal of Physical Chemistry C, 2018

Time resolved area normalized emission spectra (TRANES) of a photo-acid dye HPTS, confined within... more Time resolved area normalized emission spectra (TRANES) of a photo-acid dye HPTS, confined within the water-pools of reverse micelles (RMs), are analyzed for a direct and model free observation of excited state proton transfer (ESPT) kinetics. When area normalized emission spectra of HPTS at different times are overlapped in a single window, we find population of RO-* (PTS-*) form of HPTS increases at the cost of a concomitant decreasing of the population of ROH* (HPTS*) form as time progresses. Migration of excited state population from ROH* form to RO-* form causes the emergence of an isoemissive point in TRANES, which retains throughout the entire course of ESPT process. An estimation of ESPT timescale is obtained either from the population depleting rate of ROH* form or from the population increasing rate of RO-* form; both are practically same here. Emergence of an isoemissive point in TRANES is implying that there are only two kinetically and reversibly coupled emitting species (ROH* and RO-*) of HPTS are present within the water pools of RMs. Continuous spectral relaxations of HPTS due to excited state solvation dynamics apparently have no effect on the ESPT kinetics of HPTS within the RMs; otherwise spectral shifting, caused by the excited state solvation, would have destroyed the isoemissive point of TRANES.

Chemistry - An Asian Journal, 2018

Analyses of Förster Resonance Energy Transfer (FRET) in nanoparticles are often complicated by a ... more Analyses of Förster Resonance Energy Transfer (FRET) in nanoparticles are often complicated by a number of factors; like, close emission peak positions of donor and acceptor, presence of simultaneous photoinduced electron transfer (PET), fluorescence blinking, natural excited state decay processes and so on. Addressing these concerns mostly from material chemists and biologists, herein we propose a state of the art FRET analysis method utilizing the bright green emission of a CdSe/ZnS core/shell quantum dot (QD). Uniqueness of our model free FRET analysis demonstrates the potential of this QD to be a part of an efficient FRET assay in molecular ruler applications. Molecular distance calculations relying on proposed FRET analysis complemented nicely with the expected donor-to-acceptor separation distances; where QD as a photoluminescent marker is electrostatically attached to a compatible fluorescent dye rhodamine-6g (R6g). The beneficial aspect of our model free FRET analysis enables many possibilities including the use of low cytotoxic QD based FRET assays as a next generation molecular ruler for a precise estimation of distances inside a biological system.

Langmuir : the ACS journal of surfaces and colloids, Jan 25, 2017

Molecular recognitions, causing supramolecular complex formation between a hyperbranched polymer ... more Molecular recognitions, causing supramolecular complex formation between a hyperbranched polymer molecule (polyamidoamine (PAMAM) dendrimer generation 3) with oppositely charged surfactant sodium dodecyl sulfate (SDS) in aqueous solution, were studied by using various spectroscopic techniques and calorimetric titration of heat change measurements. Spectroscopic measurements were performed using dynamic Stokes shift (DSS), rotational anisotropy decay, and translational diffusion of a fluorescent probe molecule coumarin 153 (C153) noncovalently attached to the dendrimer-surfactant complex. All these studies unanimously confirm that the critical aggregation concentration (CAC) of SDS falls to ∼0.8 mM (from its critical micelle concentration (CMC) ∼ 8 mM) in the presence of ∼0.2 mM dendrimer. Further studies of isothermal titration calorimetry (ITC) measurement show that the CAC of SDS in the presence of dendrimer remains invariant to the dendrimer concentration. Complexation reaction b...

The journal of physical chemistry. B, Jan 2, 2017

Tightly confined water within a small droplet behaves differently from bulk water. This notion is... more Tightly confined water within a small droplet behaves differently from bulk water. This notion is obtained on the basis of several reports showing unusual behaviors of water droplet residing at the core of a reverse micelle. In this study, we have shown a well-known hydrophobic dye, coumarin 153 (C153), which prefers to reside at the water-rich region inside the dendrimer molecule. Optical density (OD) measurement at the absorption peak of C153 shows that it is almost insoluble in bulk water but highly soluble in aqueous dendrimer solution. The OD of C153 increases several times in the latter case as compared to that in the former. We found the most interesting observation when we compared the data from fluorescence correlation spectroscopy (FCS) with the fluorescence anisotropy decay of C153 in aqueous dendrimer solution. The FCS measurement reveals a much slower translational diffusion time (τD) of C153 attached to a dendrimer molecule as compared to that of free C153 in bulk wate...

Physical chemistry chemical physics : PCCP, Jan 14, 2016

The PAMAM dendrimer with a cage like structure acts as an excellent electron donor in the presenc... more The PAMAM dendrimer with a cage like structure acts as an excellent electron donor in the presence of an electron deficient molecule. Electron transfer (ET) causes significant quenching of dendrimer fluorescence. Trapping of quencher molecules within the dendrimer cage helps ET to take place through an expeditious route. Utilization of intrinsic fluorescence and sensing applications of dendrimers have been established here.

Physical chemistry chemical physics : PCCP, Jan 30, 2016

An insight study reveals the strong synergistic solvation behaviours from reporter dye molecules ... more An insight study reveals the strong synergistic solvation behaviours from reporter dye molecules within the acetonitrile (ACN)-water (WT) binary mixture. Synergism of a binary mixture refers to some unique changes of the physical and thermodynamic properties of the solvent mixture, originating from the interactions among its cosolvents, which are absent within the pure cosolvents. Synergistic solvation of a binary mixture is likely to be fundamental for greater stabilization of an excited state solute dipole; at least to some extent greater as compared to one stabilized by any of its cosolvents alone. A dynamic Stokes shift due to the solvation of an excited dipole in the ACN-WT binary mixture is found to be highly relevant to the ground state physical properties of the solute molecule (polarity, hydrophilicity, acidity, etc.). Largely different solvation times in the ACN-WT mixture are observed from different dye molecules with widely varying polarities. However, earlier study show...