Intraband Transitions of Nanocrystals Transforming from Lead Selenide to Self-doped Silver Selenide Quantum Dots by Cation Exchange.

In search of heavy metal-free mid-IR active colloidal materials, self-doped silver selenide colloidal quantum dots (CQDs) can be an alternative offering tunable mid-IR wavelength with a narrow bandwidth. One of the challenges in the study of the intraband transition is developing a method to widen the intraband transition energy range as well as reducing the toxicity of the materials. Here, we present AgxSe (x > 2) CQDs exhibiting an intraband transition up to 0.39 eV, produced by the cation exchange (CE) method from PbSe CQDs. The major electronic transition efficiently changes from the SWIR band gap of PbSe CQDs to the mid-IR intraband transition of the AgxSe CQDs by the CE. The intraband exciton is verified by examining the absorption and emission of the CE AgxSe CQDs as well as their applications on electrochemical mid-IR luminescence and mid-IR intraband photodetectors.

Midwavelength Infrared Colloidal Nanowire Laser.

Realizing bright colloidal infrared emitters in the midwavelength infrared (or mid-IR), which can be used for low-power IR light-emitting diodes (LEDs), sensors, and deep-tissue imaging, has been a challenge for the last few decades. Here, we present colloidal tellurium nanowires with strong emission intensity at room temperature and even lasing at 3.6 µm (ω) under cryotemperature. Furthermore, the second-harmonic field at 1.8 µm (2ω) and the third-harmonic field at 1.2 µm (3ω) are successfully generated thanks to the intrinsic property of the tellurium nanowire. These unique optical features have never been reported for colloidal tellurium nanocrystals. With the colloidal midwavelength infrared (MWIR) Te nanowire laser, we demonstrate its potential in biomedical applications. MWIR lasing has been clearly observed from nanowires embedded in a human neuroblastoma cell, which could further realize deep-tissue imaging and thermotherapy in the near future.

Beyond the Bandgap Photoluminescence of Colloidal Semiconductor Nanocrystals.

Intraband transitions of colloidal semiconductor nanocrystals, or the electronic transitions occurring in either the conduction band or valence band, have recently received considerable attention because utilizing the intraband transitions provides new approaches for applications such as photodetectors, imaging, solar cells, lasers, and so on. In the past few years, it has been revealed that observing the intraband transition is not limited for temporal measurement such as ultrafast spectroscopy but available for steady-state measurement even under ambient conditions with the help of self-doped semiconductor nanocrystals. Considering the large absorption coefficient of the steady-state intraband transition comparable to that of the bandgap transition, the use of the intraband transition will be promising for both fundamental and application studies. Here, we summarize the recent progress in studies on intraband photoluminescence of self-doped semiconductor nanocrystals and discuss key questions to be addressed in future research.

Investigation of Morphology-Controlled Ultrafast Relaxation Processes of Aggregated Porphyrin.

Here, we have synthesized rod and flake shaped morphology of porphyrin aggregates from 5, 10, 15, 20-tetra (4-n-octyloxyphenyl) porphyrin (4-opTPP) molecule which are evident from scanning electron microscopy (SEM). The formation of J-type aggregation is evident from steady state and time-resolved fluorescence spectroscopic studies. Ultrafast transient absorption spectroscopic studies reveal that the excited state lifetime is controlled by the morphology and the time constant for S1→S0 relaxation changes from 3.05 ps to 744 ps with changing the shape from rod to flake, respectively. In spite of similar exciton coupling energy in both the aggregates, the flake shaped aggregates undergo a faster exciton relaxation process and the non-radiative relaxation channels are found to depend on the shape of aggregates. The fundamental understanding of morphology controlled ultrafast relaxation processes of aggregated porphyrin is important for designing efficient light harvesting devices.

Ultrafast carrier dynamics in 2D-2D hybrid structures of functionalized GO and CdSe nanoplatelets.

Considerable attention has been paid to designing graphene based 2D hybrid nanostructures for their potential applications in various areas from healthcare to energy harvesting. Herein, we have prepared 2D-2D hybrid structures of 2D CdSe nanoplatelets (NPLs) with thiol (-SH) functionalized reduced graphene oxide (G-Ph-SH). Microscopic and spectroscopic studies reveal that the G-Ph-SH surface is successfully decorated by CdSe NPLs through a thiophenol (-SH) linker. The significant photoluminescence quenching (65%) and the shortening of decay time from 1 ns to 0.4 ns of CdSe NPLs are observed after adding 100 µg of G-Ph-SH. Furthermore, the femto-second transient absorption spectroscopic (fs-TAS) study reveals that the growth time of CdSe NPLs in the composite is reduced to 0.4 ps from 0.8 ps due to faster hot electron cooling. A faster component of 1.4 ps in the kinetic parameters of the composite system further suggests that the ultrafast electron transfer occurs from the conduction band of CdSe NPLs to surface functionalized reduced graphene oxide. This type of 2D-2D hybrid structure may open up new possibilities in light harvesting applications.

Dual release kinetics in a single dosage from core-shell hydrogel scaffolds.

The development of drug delivery systems with microencapsulated therapeutic agents is a promising approach to the sustained and controlled delivery of various drug molecules. The incorporation of dual release kinetics to such delivery devices further adds to their applicability. Herein, novel core-shell scaffolds composed of sodium deoxycholate and trishydroxymethylaminomethane (NaDC-Tris) have been developed with the aim of delivering two different drugs with variable release rates using the same delivery vehicle. Data obtained from XRD studies, sol-gel transition temperature measurement, rheology and fluorescence studies of the core-shell systems indicate a significant alteration in the core and the shell microstructural properties in a given system as compared to the pure hydrogels of identical compositions. The release of the model drugs Fluorescein (FL) and Rhodamine B (RhB) from the shell and the core, respectively, of the two core-shell designs studied exhibited distinctly different release kinetics. In the 25@250 core-shell system, 100% release of FL from the shell and 19% release of RhB from the core was observed within the first 5 hours, while 24.5 hours was required for the complete release of RhB from the core. For the 100@250 system, similar behaviour was observed with varied release rates and a sigmoidal increase in the core release rate upon disappearance from the shell. Cell viability studies suggested the minimal toxicity of the developed delivery vehicles towards NMuMG and WI-38 cells in the concentration range investigated. The reported core-shell systems composed of a single low molecular weight gelator with dual release kinetics may be designed as per the desired application for the consecutive release of therapeutic agents as required, as well as combination therapy commonly used to treat diseases such as diabetes and cancer.

Correction: An efficient charge separation and photocurrent generation in the carbon dot-zinc oxide nanoparticle composite.

Correction for 'An efficient charge separation and photocurrent generation in the carbon dot-zinc oxide nanoparticle composite' by Monoj Kumar Barman et al., Nanoscale, 2017, 9, 6791-6799.

An efficient charge separation and photocurrent generation in the carbon dot-zinc oxide nanoparticle composite.

The development of light harvesting systems based on heterostructures for efficient conversion of solar energy to renewable energy is an emerging area of research. Here, we have designed heterostructures by using carbon dots (C-dots) and zinc oxide nanoparticles (ZnO NP) to develop an efficient light harvesting system. Interestingly, the conduction band and the valence band positions of ZnO NP are lower than the LUMO and HOMO positions of C-dots in this type II heterostructure of C dot-ZnO NP, which causes efficient charge separation and photocurrent generation. Steady state and time resolved spectroscopic studies reveal that an efficient photoinduced electron transfer occurs from C dots to ZnO NP and a simultaneous hole transfer occurs from the valence band of ZnO NP to the HOMO of C dots. The calculated rate of electron transfer is found to be 3.7 × 109 s-1 and the rate of hole transfer is found to be 3.6 × 107 s-1. The enhancement of photocurrent (11 fold) under solar light irradiation of the C dot-ZnO NP heterostructure opens up new possibilities to design efficient light harvesting systems.

Light Harvesting and Photocurrent Generation in a Conjugated Polymer Nanoparticle-Reduced Graphene Oxide Composite.

Polymer-graphene nanocomposites are promising candidates for light harvesting applications such as photocatalysis and photovoltaics, where significant charge separation occurs due to photoinduced electron transfer. Much attention has been paid to using reduced graphene oxide (r-GO) as template for anchoring various nanomaterials due to its efficient electron accepting and transport properties. Here, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) nanoparticles are prepared from MEH-PPV polymer and the change in photophysical properties upon formation of polymer nanoparticles (PNPs) from the molecular state are investigated by using steady-state and time-resolved spectroscopy. Nanocomposites are constructed by adding hexadecylamine-functionalized positively charged MEH-PPV PNPs to a solution of negatively charged r-GO. Steady-state and time-resolved spectroscopy are also used to study the electronic interactions between PNPs and r-GO. Ultrafast femtosecond up-conversion and transient absorption spectroscopy unequivocally confirms the electron transfer process from the excited state of MEH-PPV PNPs to r-GO at the interface of the nanocomposite. Analysis reveals that the charge separation time is found to be pulse-width-limited (<100 fs). Due to charge separation in these nanocomposites, an increase (2.6 fold) of photocurrent under visible light illumination is obtained. The fundamental understanding of the charge transfer dynamics affords new opportunities to design efficient light-harvesting systems based on inorganic-organic hybrids.

Photon Harvesting in Sunscreen-Based Functional Nanoparticles.

The ultraviolet light component in the solar spectrum is known to cause several harmful effects, such as allergy, skin ageing, and skin cancer. Thus, current research attention has been paid to the design and fundamental understanding of sunscreen-based materials. One of the most abundantly used sunscreen molecules is Avobenzone (AB), which exhibits two tautomers. Here, we highlight the preparation of spherically shaped nanoparticles from the sunscreen molecule AB as well as from sunscreen-molecule-encapsulated polymer nanoparticles in aqueous media and study their fundamental photophysical properties by steady-state and time-resolved spectroscopy. Steady-state studies confirm that the AB molecule is in the keto and enol forms in tetrahydrofuran, whereas the enol form is stable in the case of both AB nanoparticles and AB-encapsulated poly(methyl methacrylate) (PMMA) nanoparticles. Thus, the keto-enol transformation of AB molecules is restricted to a nanoenvironment. An enhancement of photostability in both the nanoparticle and PMMA-encapsulated forms under UV light irradiation is observed. The efficient excited energy transfer (60 %) from AB to porphyrin molecules opens up further prospects in potential applications as light-harvesting systems.

2D Hybrid Nanostructure of Reduced Graphene Oxide-CdS Nanosheet for Enhanced Photocatalysis.

Graphene-based hybrid nanostructures have recently emerged as a new class of functional materials for light-energy conversion and storage. Here, we have synthesized reduced graphene oxide (RGO)-semiconductor composites to improve the efficiency of photocatalysis. Zero-dimensional CdS nanoparticles (0D), one-dimensional CdS nanorods (1D), and two-dimensional CdS nanosheets (2D) are grafted on the RGO sheet (2D) by a surface modification method using 4-aminothiophenol (4-ATP). Structural analysis confirms the attachment of CdS nanocrystals with RGO, and the strong electronic interaction is found in the case of a CdS nanosheet and RGO, which has an influence on photocatalytic properties. The degradation of dye under visible light varies with changing the dimension of nanocrystals, and the catalytic activity of the CdS NS/RGO composite is ∼4 times higher than that of CdS nanoparticle/RGO and 3.4 times higher than that of CdS nanorod/RGO composite samples. The catalytic activity of the CdS nanosheet/RGO composite is also found to be ∼2.5 times than that of pure CdS nanosheet samples. The unique 2D-2D nanoarchitecture would be effective to harvest photons from solar light and transport electrons to reaction sites with respect to other 0D-2D and 1D-2D hybrid systems. This observation can be extended to other graphene-based inorganic semiconductor composites, which can provide a valuable opportunity to explore novel hybrid materials with superior visible-light-induced catalytic activity.

Immobilization of palladium in mesoporous silica matrix: preparation, characterization, and its catalytic efficacy in carbon-carbon coupling reactions.

Palladium(0) has been immobilized into the silica-based mesoporous material to develop catalyst Pd(0)-MCM-41, which is found to be highly active in carbon-carbon coupling reactions. [Pd(NH3)4]2+ ions have been incorporated into the mesoporous material during synthesis of MCM-41 and subsequently upon treatments with hydrazine hydrate Pd2+ ions present in mesoporous silica matrix were reduced to Pd(0) almost instantaneously. The catalyst has been characterized by small-angle X-ray diffraction, N2 sorption, and transmission electron microscopy (TEM). TEM and surface area measurements clearly demonstrate that the immobilization of Pd(0) into the mesoporous silica has a significant effect on pore structure of the catalyst. Nevertheless, after immobilization of palladium the meso-porosity of the material is retained, as evidenced in the nitrogen sorption measurement. The TEM micrograph shows that both MCM-41 and Pd(0)-MCM-41 have similar types of external surface morphology; however, Pd(0)-MCM-41 was less ordered. Pd(0)-MCM-41 showed high catalytic activity toward carbon-carbon bond formation reactions like Heck and Sonogashira coupling, as evidenced in high turn-over numbers. In contrast to many other Pd-based catalysts reported so far, Pd(0)-MCM-41 acts as a truly heterogeneous catalyst in C-C coupling reactions. Notably, the new heterogeneous catalyst is found to be efficient in the activation of arylchloride to give impressive conversion in cross coupling (15-45% for Heck and 30% for Sonogashira) reactions under mild conditions.

Oxo-vanadium(IV) dihydrogen phosphate: preparation, magnetic study, and heterogeneous catalytic epoxidation.

A layered oxo-vanadium(IV) dihydrogen phosphate, {VO(H2PO 4)2} n has been synthesized hydrothermally and characterized by several physicochemical methods. Single-crystal X-ray analysis (crystal system, tetragonal; space group, P4/ ncc; unit cell dimensions, a = b = 8.9632(4), c = 7.9768(32) A) of {VO(H2PO4) 2} n reveals that the compound has an extended two-dimensional structure. The VO2+ moieties are connected through bridging H 2PO4 (-) ions, and this type of connection propagates parallel to the crystallographic ab plane which gives rise to a layered structure. The layers are staked parallel to the crystallographic c axis with a separation between the layers of ca. 4.0 A. Magnetic susceptibility of {VO(H2PO4)2} n has been measured in the temperature range 2-300 K on a SQUID magnetometer. The magnetic property of {VO(H2PO4)2} n is explicable in the light of a two-dimensional quantum Heisenberg antiferromagnet model. Magnetic pathways are available through the dihydrogen-phosphato bridges within the layer and provide for weak antiferromagnetic interactions. Notably {VO(H2PO4)2} n catalyzes the epoxidation reaction of alkenes with tert-BuOOH in acetonitrile medium under heterogeneous condition.

Anchoring of copper complex in MCM-41 matrix: a highly efficient catalyst for epoxidation of olefins by tert-BuOOH.

A complex moiety containing copper (II) has been anchored covalently into the organic-modified Si-MCM-41 to prepare a new catalyst. The amine group containing organic moiety 3-aminopropyl-triethoxysilane has been first anchored on the surface of Si-MCM-41 via silicon alkoxide route. The amine group upon condensation with salicyldehyde affords a bidentate ligand in the mesoporous matrix for anchoring copper(II) ions. The prepared catalyst has been characterized by UV-vis, electron paramagnetic resonance (EPR), and infrared (IR) spectroscopic analysis, small-angle X-ray diffraction, and N2 sorption study. A remarkable difference in the pore structure has been observed after the immobilization of copper(II) complex in Si-MCM-41. The catalyst showed excellent catalytic efficiency in epoxidation reactions with various olefinic compounds including styrene and allyl alcohol, using tert-BuOOH as oxidant. Notably, styrene shows unprecedented high conversion (97%) as well as epoxide selectivity (89%) with tert-BuOOH over the Cu-MCM-41 catalyst.