Magnetic nanoribbons!

Owing to their nanoscale spintronic applications, doping of semiconductor nanocrystals by transition-metal ions has attracted tremendous attention . However, such doping is difficult to achieve in low-dimensional strongly quantum confined nanostructures by conventional growth procedures. In a recent report in Nature materials, researchers from korea in collaboration with american researchers have demonstrate that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process.

The cation-exchange reaction of (CdSe)13 clusters with Mn2+ ions governs the Mn2+ incorporation during the nucleation stage. This highly efficient Mn2+ doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of ~600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. The sign of the s–d exchange is inverted to negative owing to the exceptionally strong quantum confinement in our nanoribbons. figure on the right show the theoretical investigation of Mn2+ doping of CdSe clusters.

This novel nucleation-controlled doping strategy opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications.

Ref: Nature Materials 9, 47 - 53 (2009)

Photoluminescent Silicon Quantum Dots : Chromophore for Biological Imaging

Due to their strong luminescence,Quantum dots of group II/VI such as CdSe quantum dots are well-known, and substantial research has been conducted on these systems .There have, however, been concerns over the toxicity of these quantum dots in the human body. It has been reported in past that cell damage could be caused by an uncoated CdSe core under UV excitation. Because of their low toxicity and also inexpensive nature, silicon nanoparticles emerges as an ideal candidate for biological fluorescence imaging. In a recent article in JACS, researchers from New Zealand and Japan suggested the synthesis of amino-terminated silicon quantum dots made in reverse micelle system and capped with allylamine using a platinum catalyst. Their newly developed technique of surface modification is multistepped based on the chemistry of the terminal double bond on the surface to achieve the target functionalities. Thus formed Silicon quantum dots were characterized by transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS). The capping of the silicon quantum dots has been fully characterized using nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results from cytotoxicity studies indicate that toxicity is dependent on the surface functionality and that the silicon quantum dots as prepared have potential in biological applications such as bioimaging. In summary, silicon quantum dots opens new doors in the field of biological imaging.
Ref: J. Am. Chem. Soc., Article ASAP, DOI: 10.1021/ja906501v

Single Molecule detection by SERS active Gold-Silver-Core-Shell Nanodumbbells

Detection methods using plasmonic nanostructures based on Surface-enhanced Raman scattering (SERS) have been widely investigated for imaging and sensing applications.However, SERS-based single-molecule detection generally faces a problem with structural reproducibility, as particle structure and interparticle distance can markedly affect Raman signals and constructing robust SERS-active nanostructures still remain a challenge.

Recently, in a article in Nature Materials, researchers have reported a high-yield synthetic strategy to obtain gap-tailorable gold–silver core–shell nanodumbbells (GSNDs) and subsequent hot SERS-based single-molecule detection with structurally reproducible dimetric nanostructures.

Gold nanoparticle heterodimers were successfully synthesized in a relatively high yield by means of a single-target-DNA hybridization (displayed in the figure). A single Raman-active Cy3 dye molecule is located between two DNA-tethered particles. In another step, Ag shells were formed on the surface of the dimeric Au nanoparticles, and the Ag shell thickness was controlled on the nanometre scale to generate gap-engineerable, DNA-embedded GSNDs. To detect a Raman signal from each single-DNA-captured GSND, atomic force microscope (AFM)-coupled nano-Raman spectroscopy was used.

To prove that single-DNA detection is possible from a single GSND structure, several characterization experiments were suggested in the article.It has been demonstrated that as formed Raman-active GSNDs have single-molecule sensitivity with high structural reproducibility. This research is important because of following reasons:

- opens new opportunities in the high-yield synthesis of specific nanostructures for materials science and bio-detection applications..

- unlike the conventional strong electrolyte-induced nonspecific nanoparticle aggregation, this synthesis method can be easily scalable to produce targeted SERS-active nanoprobes.

- the nanogap-engineering of GSNDs allows for exploring hot SERS structures in an efficient and straightforward fashion

To summarize, these SERS-active GSNDs could be further modified by other biomolecules (such as proteins) and used as both in vitroand in vivo bio-labelling probes with ultrahigh sensitivity, quantification potential and multiplexing capability.

reference: Nature Materials (13 December 2009) doi:10.1038/nmat2596

Flexible Floating-gate Transistor

Traditionally, high-temperature manufacturing methods are employed for fabricated electronic device using inorganic semiconductors and rigid substrates . Organic semiconductors on the other hand can be processed at low temperatures and on large-area polymeric substrates. This has allowed for the development of a variety of electronic devices on flexible plastic substrates, including solar cells.Most of the organic memory transistors reported to date exploit the electric field–induced remnant polarization in ferroelectric polymer films . A considerable limitation of ferroelectric polymer memory transistors is that the coercive field required to reverse the macroscopic polarization increases with decreasing film thickness, which makes it difficult to obtain a large enough memory window with program and erase voltages below about 20 V. Also, due to the substantial surface roughness of the ferroelectric polymer films, the carrier field-effect mobility in these transistors is usually quite low.

Although silicon floating-gate transistors are excellent for high-density data storage, flexible organic floating-gate transistors are potentially useful for large-area sensors and actuators with integrated nonvolatile memory capability.

Using organic transistors with a floating gate embedded in hybrid dielectrics that comprise a 2-nanometer-thick molecular self-assembled monolayer and a 4-nanometer-thick plasma-grown metal oxide, nonvolatile memory arrays on flexible plastic substrates is realized . The small thickness of the dielectrics allows very small program and erase voltages (6 volts) to produce a large, nonvolatile, reversible threshold-voltage shift. The transistors endure more than 1000 program and erase cycles, which is within two orders of magnitude of silicon-based floating-gate transistors widely employed in flash memory. By integrating a flexible array of organic floating-gate transistors with a pressure-sensitive rubber sheet, a sensor matrix that detects the spatial distribution of applied mechanical pressure and stores the analog sensor input as a two-dimensional image over long periods of time has been created.

Ref: Science 11 December 2009: Vol. 326. no. 5959, pp. 1516 - 1519

Bamboo like Carbon Nanorods fabricated by Non-catalytic Approach

Since their discovery in 1990s, Carbon nanotubes (CNTs) are continuously of great interest in both fundamental research and practical applications. As an important member of carbon material family, bamboo-like carbon nanotubes (BCNTs) have been explored extensively due to their unique properties resulting from their hollow compartments inside nanotubes. BCNTs are expected to exhibit excellent electrochemical performances for their high percentage of edge sites along inner wall, compared with the common straight CNTs. For there synthesis, the catalyst is usually used and left inside the final carbon nanotubes, which needs further purification before their applications.

A novel approach for non-catalytic fabrication of BCNT has been suggested by Scientist from China in their recent article in Material Letters. The preparation procedures involve synthesis of monodispersed core–shell structured polymer spheres and the pyrolysis in an argon atmosphere.

The carbon nanotubes with bamboo-like structures are formed via the pyrolysis of PMMA@PDVB core–shell structured spheres. The typical SEM image reveals the one dimensional (1D) twisted nanostructures of BCNTs, which are up to several tens of micrometers in length. The compartments inside the nanotubes are observed from representative TEM images. The wall thickness of the BCNTs is between 20 and 30 nm and the inner diameter is around 200 nm. High-resolution TEM image reveals the lattice fringes with the space of 0.357 nm, which corresponds with the (002) plane lattice parameter of graphitic carbon. X-ray powder diffraction (XRD), Raman spectroscopy and energy dispersive spectroscopy (EDS) measurements were performed to get insight into the structure of BCNTs. Also, the possible mechanism for fabrication has been proposed in the report.

Platinum nanoparticles: Converting homogeneous to heterogeneous catalysis

Uniting the advantages of homogeneous and heterogeneous catalytic processes is a continuing goal in the field of catalysis.

Nanoparticles represent a new frontier in heterogeneous catalysis, where this unification can also be supplemented by the ability to obtain new or divergent reactivity and selectivity. Researchers at University of Berkeley, California reported a novel method for applying heterogeneous catalysts to known homogeneous catalytic reactions through the design and synthesis of electrophilic platinum nanoparticles.

These nanoparticles are selectively oxidized by the hypervalent iodine species PhICl₂, and catalyse a range of π-bond activation reactions previously only catalysed through homogeneous processes. Multiple experimental methods are used to unambiguously verify the heterogeneity of the catalytic process. The discovery of treatments for nanoparticles that induce the desired homogeneous catalytic activity should lead to the further development of reactions previously inaccessible in heterogeneous catalysis. Furthermore, a size and capping agent study revealed that Pt PAMAM dendrimer-capped nanoparticles demonstrate superior activity and recyclability compared with larger, polymer-capped analogues.

reference : Nat. Chem., DOI: 10.1038/nchem.468

Nanodome Solar Cells with High Efficiency!

Solar cells of nanostructures such as nanocrystals and nanowires have attracted much attention due to their potential for improving charge collection efficiency, fabricating small-scale power sources, enabling novel conversion mechanisms, and using low-cost processes.
Recently in a Nano letters report, Researchers at Stanford University have demonstrate novel nanodome solar cells, which have periodic nanoscale modulation for all layers from the bottom substrate, through the active absorber to the top transparent contact. These devices combine many nanophotonic effects to both efficiently reduce reflection and enhance absorption over a broad spectral range.
Nanodome solar cells with only a 280 nm thick hydrogenated amorphous silicon (a-Si:H) layer can absorb 94% of the light with wavelengths of 400−800 nm, significantly higher than the 65% absorption of flat film devices. Because of the nearly complete absorption, a very large short-circuit current of 17.5 mA/cm² is achieved in our nanodome devices. Excitingly, the light management effects remain efficient over a wide range of incident angles, favorable for real environments with significant diffuse sunlight. Nanodome devices demonstrated with a power efficiency of 5.9%, which is 25% higher than the flat film control. The nanodome structure is not in principle limited to any specific material system and its fabrication is compatible with most solar manufacturing; hence it opens up exciting opportunities for a variety of photovoltaic devices to further improve performance, reduce materials usage, and relieve elemental abundance limitations.
Also, these nanodome devices when modified with hydrophobic molecules present a nearly superhydrophobic surface and thus enable self-cleaning solar cells.

Mistery of Crystallization

For many years scientists have scratched surfaces to promote crystallization. Crystals are observed to nucleate along the scratches. Despite the popularity of this technique, our understanding of why crystals nucleate in scratches is very limited. There have been no quantitative measurements of the nucleation rate in a groove, and there is no data on the microscopic details of how the nucleation occurs here.

In a recent report in JACS, Researcher from U.K. studied the heterogeneous nucleation of a crystal in a wedge-shaped groove via computer simulation.They found that nucleation in these grooves is indeed many orders of magnitude faster than on a flat surface. It is also been found that there is a competition between the angle of the wedge, and the angles that are intrinsic to the lattice of the crystal. This competition results in a wedge angle at which the nucleation rate goes through a maximum, a phenomenon that is not seen in the nucleation of liquids.

Therefore, generically we expect crystallization to start in grooves or pits in surfaces, not on the flat parts of surfaces. This expectation is consistent with the common observation that crystallization readily occurs on a scratched surface. It been found that nucleation is highly sensitive to the wedge angle β. This is because there are angles that are intrinsic to crystals, such as the 70.5° angle between the close-packed planes in an fcc lattice. As this effect is a direct consequence of the crystal lattice it has been expected to be a general feature of crystallization, even with more complex molecules, for example proteins. Nucleation is faster when the wedge angle is such that a defect-free unstrained piece of the crystal fits perfectly into the wedge. As different crystal polymorphs have different intrinsic angles, this may provide a way to control the polymorph that nucleates. A wedge into which the desired polymorph fits perfectly, but which has the wrong angle for other polymorphs, will favor nucleation only of the desired polymorph.

Reference : J. Am. Chem. Soc., 2009, 131 (48), pp 17550–17551

Nanoplasmonic Probes of Catalytic Reactions

Scientists from Sweden have recently published a report in Science, which discuss about their newly developed plasmonic sensing method for heterogeneous catalytic reactions based on arrays of nanofabricated gold disks, covered by a thin (~10 nanometer) coating (catalyst support) on which the catalyst nanoparticles are deposited.

In heterogeneous catalysis, reactants in gas or liquid phase are converted to desired product molecules on the surface of a solid catalyst, which is usually composed of catalytically active nanoparticles (1 to 10 nm) dispersed on a porous, high-surface-area support material. In order to understand and improve these systems, it is important to be able to monitor the catalyst’s state and to follow the reaction in real time. An important quantity is the surface coverage of reactants. However, experimental difficulties arise from the complexity of the catalyst and the atmospheric or higher pressures in which the reactions occur. Model systems (commonly single-crystal surfaces) and model reactions are frequently investigated at idealized ultrahigh vacuum (UHV) conditions, allowing use of powerful experimental probes (such as electrons, photons, or ions). A major challenge is to correlate results obtained by using the idealized and thoroughly scrutinized model catalysts in UHV with those of the less characterized real nanostructured catalysts at real reaction conditions.

This Report describe a method that, with a simple optical transmission (or reflection) measurement, can follow catalytic reactions in real time for both model and real supported catalysts. The principle is "nanoplasmonic" [localized surface plasmon resonance (LSPR)] sensing, currently intensely explored for biosensing, down toward single-molecule sensitivity. It is shown that LSPR can monitor changes in adsorbate coverages on "realistic" supported catalysts with a sensitivity corresponding to much less than 0.1 monolayer (ML).

The LSPR sensing structures are nanoscale disks of gold or platinum. The light transmitted through the sample has an intensity minimum (maximum extinction) at the wavelength at which the LSPR excitation in the sensing particles is strongest. The excitation creates a strongly enhanced electromagnetic near field, which acts as a probe of the nanoparticle’s surrounding.

For detailed experimental study please goto Science, Vol. 326. no. 5956, pp. 1091 - 1094