Mahasin Alam Sk

Molybdenum phosphide as an efficient electrocatalyst for the hydrogen evolution reaction

Electrochemical production of hydrogen from water has been directed to the search for non-noble metal based and earth-abundant catalysts. In this work, we propose a novel cost-effective catalyst, molybdenum phosphide that exhibits high activity towards the hydrogen evolution reaction (HER) in both acid and alkaline media even in bulk form. Comparative analysis of Mo, Mo3P and MoP as catalysts for HER clearly indicates that phosphorization can potentially modify the properties of the metal and different degrees of phosphorization lead to distinct activities and stabilities. Theoretical calculations by density functional theory also show that a simple phosphorization of molybdenum to form MoP introduces a good ‘H delivery’ system which attains nearly zero binding to H at a certain H coverage. With the combination of experimental results and theoretical calculations, this work has enlightened a new way of exploring cost-effective catalysts for HER.

Revealing the tunable photoluminescence properties of graphene quantum dots

Graphene quantum dots (GQDs) are a new class of fluorescent reporters promising various novel applications such as bio-imaging, optical sensing and photovoltaics. They have recently attracted enormous interest because of their extraordinary and tunable optical, electrical, chemical and structural properties. However, the widespread use of GQDs is hindered by the poor understanding of their photoluminescence (PL) mechanisms. Using density-functional theory (DFT) and time-dependent DFT calculations, we reveal that the PL of a GQD can be sensitively tuned by its size, edge configuration, shape, attached chemical functionalities, heteroatom doping and defects. In addition, it is discovered that the PL of a large GQD consisting of heterogeneously hybridized carbon network is essentially determined by the embedded small sp2 clusters isolated by sp3 carbons. This study not only provides an explanation to the previous experimental observations but also provides insightful guidance to develop methods for the controllable synthesis and engineering of GQDs.

Scalable and Effective Enrichment of Semiconducting Single-Walled Carbon Nanotubes by a Dual Selective Naphthalene-Based Azo Dispersant

Semiconducting single-walled carbon nanotubes (s-SWNTs) have emerged as a promising class of electronic materials, but the metallic (m)-SWNTs present in all as-synthesized nanotube samples must be removed for many applications. A high selectivity and high yield separation method has remained elusive. A separation process based on selective chemistry appears to be an attractive route since it is usually relatively simple, but more effective chemicals are needed. Here we demonstrate the first example of a new class of dual selective compounds based on polycyclic aromatic azo compounds, specifically Direct Blue 71 (I), for high-purity separation of s-SWNTs at high yield. Highly enriched (~93% purity) s-SWNTs are produced through the simple process of standing arc-discharge SWNTs with I followed by centrifugation. The s-SWNTs total yield is up to 41%, the highest yet reported for a solution-based separation technique that demonstrates applicability in actual transistors. 91% of transistor devices fabricated with these s-SWNTs exhibited on/off ratios of 10(3) to 10(5) with the best devices showing mobility as high as 21.8 cm(2)/V s with on/off ratio of 10(4). Raman and X-ray photoelectron spectroscopic shifts and ultraviolet-visible-near-infrared (UV-vis-NIR) show that I preferentially complexes with s-SWNTs and preferentially suspends them. Preferential reaction of naphthyl radicals (generated from I with ultrasonication) with m-SWNTs is confirmed by changes in the D-band in the Raman spectroscopy, matrix-assisted desorption-ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and molecular simulation results. The high selectivity of I stems from its unique dual action as both a selective dispersion agent and the generator of radicals which preferentially attack unwanted metallic species.

Oxygen vacancies in self-assemblies of ceria nanoparticles

Cerium dioxide (CeO2, ceria) nanoparticles possess size-dependent chemical properties, which may be very different from those of the bulk material. Agglomeration of such particles in nanoarchitectures may further significantly affect their properties. We computationally model the self-assembly of CenO2n particles (n = 38, 40, 80) – zero-dimensional (0D) structures – in one- and two-dimensional (1D and 2D) nanoarchitectures by employing density-functional methods. The electronic properties of 1D Ce80O160 and 2D Ce40O80 resemble those of larger 0D crystallites, Ce140O280, rather than those of their building blocks. These 0D, 1D and 2D nanostructures are employed to study the size dependence of the formation energy of an oxygen vacancy, Ef(Ovac), a central property in ceria chemistry. We rationalize within a common electronic structure framework the variations of the Ef(Ovac) values, which are computed for the CenO2n nanostructures with different sizes and dimensionalities. We identify: (i) the bandwidth of the unoccupied density of states projected onto the Ce 4f levels as an important factor, which controls Ef(Ovac); and (ii) the corner Ce atoms as the structural motif essential for a noticeable reduction of Ef(Ovac). These results help to understand the size dependent behaviour of Ef(Ovac) in nanostructured ceria.

Fluorescent quantum dots derived from PEDOT and their applications in optical imaging and sensing

We herein demonstrate a facile and general approach to synthesize novel polymer quantum dots (QDs) derived from non-fluorescent poly(3,4-ethylenedioxythiophene) (PEDOT). Such a PEDOT-QD is of molecular size and exhibits excellent photostability, two-photon excitation properties, and biocompatibility. Furthermore, we demonstrate the use of this new type of fluorescent reporter for cellular imaging and sensitive and specific optical detection of mercury ions. The synthetic route demonstrated here can be easily modified (e.g., using different polymer precursors, ionic liquids for polymerization, and solvents for exfoliation) to produce different types of polymer QDs with various interesting properties.

Modulating the electronic properties of germanium nanowires via applied strain and surface passivation

We report a systematic study on the surface passivation and strain effects on the electronic properties of hydrogenated germanium nanowires (H-GeNWs) with different growth orientations and diameters using density functional theory calculations. We show that increasing the coverage percentage of halogen passivations--fluorine (F) and chlorine (Cl) in particular--reduces the band gap of the GeNWs drastically but not linearly, depending on the chemical environment of the passivation sites. Moreover, we find that in general, applying strain--either compression or tensile--can only induce a decreased band gap in GeNWs but exception is found in <110> GeNWs: an increased band gap can be induced which is determined to be related to their surface structures. The current work reveals that electronic response upon structural changes caused by external factors is more sensitive in <110> GeNWs than in <100> GeNWs, suggesting that GeNWs with selected growth orientation can be applied in specialized applications that require different degrees of sensitivity or robustness.

Thiophene-derived polymer dots for imaging endocytic compartments in live cells and broad-spectrum bacterial killing

In this work, we present a new type of polymer dot synthesized from bithiophene (pTh-Pdot) via a simple and high yield procedure. Owing to their high brightness, excellent photo-stability, good biocompatibility, and molecular size, pTh-Pdots are promising for bioimaging applications. As a proof-of-concept demonstration, pTh-Pdots are herein employed to track the endocytic pathway in live cells. Furthermore, we show that these polymeric nanoparticles can serve as potent antibacterial agents by making use of their ability to disrupt bacterial membranes and high peroxidase mimicking activity.

Controlling armchair and zigzag edges in oxidative cutting of graphene

Density-functional theory (DFT) calculations reveal that the formation of an armchair epoxy chain on a graphene sheet is energetically favorable when oxidation occurred on both sides of the graphene sheet. However, the formation of a zigzag epoxy chain is favorable when oxidation occurred on the same side of the graphene sheet. In addition, the zigzag epoxy chain formation on the graphene sheet becomes energetically more favorable when external strain is applied on graphene. Our theoretical calculations show that the edge (armchair or zigzag) of graphene nanoribbons (GNRs) and graphene quantum dots (GQDs) can be synthetically controlled by (a) experimental conditions which allow the oxidation of graphene to occur on either one or both sides of the graphene sheet and (b) engineering the strain on the graphene sheet.

Band-gap manipulations of monolayer graphene by phenyl radical adsorptions: a density functional theory study.

Phenyl radical (Ph˙) adsorption on monolayer graphene sheets is used to investigate the band-gap manipulation of graphene through density functional theory. Adsorption of a single Ph˙ on graphene breaks the aromatic π-bond and generates an unpaired electron, which is delocalized to the ortho or para position. Adsorption of a second radical at the ortho or para position saturates the radical by electron pairing and results in semiconducting graphene. Adsorption of a second radical at the ortho position (ortho-ortho pairing) is found to be more favorable than adsorption at the para position (ortho-para pairing), and the ortho-ortho pairing has stronger effects on band-gap opening compared with ortho-para pairing. Adsorption of even numbers of Ph˙ on graphene by ortho-ortho and ortho-para pairings, in general, increases the band gap. Our study shows promise of band-gap manipulation in monolayer graphene by Ph˙ adsorption, leading to potential wider applications of graphene.