Saturday, May 12, 2012

"Tunneling Currents That Increase with Molecular Elongation".

"Tunneling Currents That Increase with Molecular Elongation".

In this article, they present a model molecular system with an unintuitive transport extension behavior in which the tunneling current increases with forced molecular elongation. The molecule consists of two complementary aromatic units (1,4-anthracenedione and 1,4-anthracenediol) hinged via two ether chains and attached to gold electrodes through thiolterminated alkenes. The transport properties of the molecule as it is mechanically elongated in a single-molecule pulling setting are computationally investigated using a combination of equilibrium molecular
dynamics simulations of the pulling with gDFTB computations of the transport properties in the Landauer limit. 

Contrary to the usual exponential decay of tunneling currents with increasing molecular length, the simulations indicate that upon elongation electronic transport along the molecule increases 10-fold. The structural origin of this inverted trend in the transport is elucidated via a local
current analysis that reveals the dual role played by H-bonds in both stabilizing π-stacking for selected extensions and introducing additional electronic couplings between the complementary aromatic rings that also enhance tunneling currents across the molecule.

The simulations illustrate an inverted electromechanical single-molecule switch that is based on a novel class of transport extension behavior that can be achieved via mechanical manipulation and highlight the remarkable sensitivity of conductance measurements to the molecular conformation.

Tunneling Currents That Increase with Molecular Elongation.
Ignacio Franco,Gemma C. Solomon, George C. Schatz and Mark A. Ratner.
J. Am. Chem. Soc. 2011, 133, 15714–15720 | 

Friday, May 11, 2012

"Assessment of a nanoparticle bridge platform for molecular electronics measurements".

"Assessment of a nanoparticle bridge platform for molecular electronics measurements".

A combination of electron beam lithography, photolithography and focused ion beam milling was used to create a nanogap platform, which was bridged by gold nanoparticles in order to make electrical measurements and assess the platform under ambient conditions. Non-functionalized electrodes were tested to determine the intrinsic response of the platform and it was found that creating devices in ambient conditions requires careful cleaning and awareness of the contributions contaminants may make to measurements. The platform was then used to make measurements on octanethiol (OT) and biphenyldithiol (BPDT) molecules by functionalizing the nanoelectrodes with the molecules prior to bridging the nanogap with nanoparticles. Measurements on OT show that it is possible to make measurements on relatively small numbers of molecules, but that a large variation in response can be expected when one of the metal–molecule junctions is physisorbed, which was partially explained by attachment of OT molecules to different sites on the surface of the Au electrode using a density functional theory calculation. On the other hand, when dealing with BPDT, high yields for device creation are very difficult to achieve under ambient conditions. Significant hysteresis in the I–V curves of BPDT was also observed, which was attributed primarily to voltage induced changes at the interface between the molecule and the metal.

"Assessment of a nanoparticle bridge platform for molecular electronics measurements".
S H M Jafri, T Blom, K Leifer, M Strømme, H Löfås, A Grigoriev, R Ahuja and K Welch.

Nanotechnology 21 (2010) 435204 (10pp). 

Fine tuning of the electronic structure of π-conjugated molecules for molecular electronics.

Fine tuning of the electronic structure of π-conjugated molecules for molecular electronics.

Molecular components with their inherent scalability are expected to be promising supplements for nanoscale electronic devices. Here we report on how to specifically tune the electronic structure of chemisorbed molecules and thus to gain control of molecular transport properties. The electronic structure of our prototype π-conjugated carboxylic acid anchored on the Cu(110) surface is modified systematically by inserting nitrogen atoms in a six-membered aromatic ring, a carboxylic functional group at the aromatic ring or both. Depending on the specific nature of the substituent, the relative position of the occupied or unoccupied electronic states with respect to the Fermi level can be specifically controlled and thus the transport properties of the studied molecular systems are modified intentionally, as proven by our scanning tunneling spectroscopy measurements. On the basis of the insight gained by our systematic experiment and first-principles calculations we are also able to predict the specific molecular character (σ or π) of the orbitals involved in the transport process of a carboxylate–Cu(110) system, depending on the functionalization pattern employed.

Fine tuning of the electronic structure of π-conjugated molecules for molecular electronics.
V Caciuc, M C Lennartz, N Atodiresei, S Karthauser and S Bl ¨ ugel ¨
Nanotechnology 22 (2011) 145701 (9pp) 

"Nanomanipulation set-up assembles single-nanoparticle electronics".

"Nanomanipulation set-up assembles single-nanoparticle electronics".

Nanoparticles possess several advantages over 1D and 2D structures. For instance, in nanoparticles the carrier travelling distance is short and the carrier lifetime is long. However, connecting electrodes to a single nanoparticle is not a trivial task, which presents an obstacle to progress in areas such as electronics and optoelectronics. Devices containing randomly spread particles atop closely spaced leads made by break junction or tilt-angle evaporation techniques have been demonstrated, but developers would prefer a more routine approach.

Recently, researchers from the Institute of Physics, Academia Sinica, in Taiwan, have proposed and demonstrated a reliable approach for producing nanoparticle devices. The circuits fabricated by the team contain a single ZnO particle embedded in a nanopore structure and exhibit photovoltaic functionality with a fill factor of 48%.
Suits most materials

What’s more, the method provides a route for making electronic devices containing a single nanoparticle of virtually any material. Based on the device fabrication process, the team is now developing a technique that allows the chemical potential of an embedded nanoparticle to be tuned by a surrounding gate electrode. In this way, field-effect transistors containing a single semiconductor nanoparticle could be reproducibly constructed.

In a related project, the scientists have also used the manipulation probes to pick up selected objects such as nanowires, nanotubes and graphene sheets and place them on top of pre-prepared electrodes crossing the holes on a chip. This would allow for correlated structural TEM inspection and rigorous electrical characterization on the same specimen.

"Nanomanipulation set-up assembles single-nanoparticle electronics".
Linh-Nam Nguyen, Ming-Chou Lin, Horng-Shyang Chen, Yann-Wen Lan, Cen-Shawn Wu, Kuei-Shu Chang-Liao and Chii-Dong Chen.
Nanotechnology 23 (2012) 165201 (6pp).

"Electronic transport through apo- and holoferritin".

"Electronic transport through apo- and holoferritin".

The incredible molecular architectures seen in many protein molecules, responsible for numerous biological functions, can provide inspiration for synthetic design. Perhaps an even more exciting prospect – potentially offering immediate access to biological attributes – is the direct exploitation of biological species via successful interfacing with an electronic device.

The Jason Davis group in Oxford is exploring bio-recognition and sensing along with novel materials for molecular electronics including biomolecules. Ferritin is an interesting iron-storage protein, central to the control of iron chemistry within the cell. It is a relatively large and robust protein that could serve as a paradigm for a biomolecule-based device. The present work demonstrates how the electronic properties of the ferritin protein change dramatically depending on the presence or absence of the central mineral core. We have also shown how the electronic behaviour can be linked to the contrasting mechanical properties of the core and the protein.

We hope that our understanding of the mechanism of charge transfer in large biomolecules, fundamental to essential biological processes, will advance – thus providing the knowledge necessary for successful bio-electronic interfacing and improved synthetic models exploiting some of nature’s advanced chemistry.
Conductive probe atomic force microscopy (CP-AFM) has been used to investigate electronic transport through the protein ferritin in both its holo and apo forms. The presence of the iron oxide core has a notable effect on both conductance and the molecular response to probe-induced compression. This response can also be contrasted with that of the much smaller metalloprotein cytochrome c, across which electron transport can be simulated by a single non-resonant tunnel barrier model. Tapping mode AFM imaging, in different compressional regimes, reveals both the mineral core of holoferritin and significant collapse of the hollow protein cavity of apoferritin. These topographic findings correlate well with CP-AFM conductance data and facilitate a clearer description of electron transport across these molecules.

Electron flux through apo-and holoferritin.
Danny N Axford and Jason J Davis.
Nanotechnology 18 (2007) 145502 (7pp).

"Laser writer makes graphene supercapacitors."

"Laser writer makes graphene supercapacitors."

Researchers in the US have employed a routine laser-writing technique to create sheets of graphene on the surface of a DVD. The graphene sheets can then be joined together to make electrochemical capacitors (or supercapacitors) that are able to store as much energy as a conventional battery but that can be charged 100–1000 times faster. The capacitors are completely flexible and robust, which makes them ideal energy-storage systems for next-generation flexible and portable electronics.
Capacitors are devices that store electric charge. Electrochemical capacitors – also known as supercapacitors or electric double-layer capacitors – can store much more charge thanks to the double layer formed at an electrolyte-electrode interface when voltage is applied. Although promising energy-storage materials, they still lag behind traditional batteries (which store energy through electrochemical reactions) in terms of energy densities: just 4 to 5 Wh/kg as opposed to 10 to 150 Wh/kg. They do, however, have a much longer shelf- and cycle life than batteries and can deliver large amounts of power much more quickly.
Now, a research team led by Richard Kaner and Maher El-Kady at the University of California, Los Angeles, say they have developed a graphene-based device that combines both the power performance of capacitors with the high energy density of batteries. The researchers have come up with a new process that involves coating an ordinary DVD disc with a film of graphite oxide supported on a sheet of plastic.

Kaner and colleagues begin by reducing the graphite oxide to graphene using a standard “LightScribe” DVD drive head – usually used to optically etch labels and images on DVD media discs. The process can easily be monitored as the golden-brown-coloured graphite oxide turns into black-coloured graphene. The plastic (now coated with graphene) is subsequently peeled off and cut with scissors to make different devices.
Electrochemical capacitors are made by simply “gluing” together two identical pieces of graphene sheet (which can be used as electrodes without the need for any additional binders or additives) with a little polymer gel electrolyte that is placed between them. “We also tested a variety of other electrolytes confirming that the material can be used in a number of device systems for different applications,” said Kaner.

More information:
Laser writer makes graphene supercapacitors.
Maher F. El-Kady Veronica Strong,  Sergey Dubin,  Richard B. Kaner.
Science 16 March 2012:  Vol. 335 no. 6074 pp. 1326-1330
DOI: 10.1126/science.1216744

"Smart sandwich enables fundamental studies in molecular electronics".

"Smart sandwich enables fundamental studies in molecular electronics".

Research in molecular electronics is working towards the bottom-up fabrication of single-molecule devices. However, the electronic properties of such devices may depend as much on the chemical structure of the molecule as on the electrode-molecule interface. To attain a better understanding of molecular electronics it is necessary to tune both the electrical potential and the geometry of metal-molecule-metal junctions. Researchers in the Netherlands based at Delft University of Technology and Leiden University have now developed a new type of gated mechanical break junction (MCBJ) that enables such studies.

Their devices build on the mechanical break junction principle, in which a flexible substrate is bent to stretch and break a metal wire on its top. As the suspended wire breaks at a prefabricated constriction, two fresh fracture surfaces are formed. If the wire is made of gold – the standard electrode material in molecular electronics – the breaking leads to two atomically sharp tips that are small enough to contact a single molecule. The bending of the substrate can then be used to control the distance between the electrodes with subangstrom precision.

The Dutch team used advanced nanolithography to fabricate such a gold wire directly on top of a gate insulator and a gate electrode. Thanks to this sandwich-type architecture, the devices are exceptionally stable and versatile. The gold electrodes can be broken and tuned independently of the gate, which makes it possible to actively contact single molecules in a three-terminal configuration.

Initial low-temperature measurements on a nanoscale cluster indicate that charge transport can be tuned independently in the new devices, both by bending the substrate and by applying a voltage to the gate electrode. In the future, the gated mechanical break junctions will be used to unravel structure-property relations in large conjugated molecules.

More information:
"Smart sandwich enables fundamental studies in molecular electronics".
Christian A Martin, Jan M van Ruitenbeek and Herre S J van der Zant.
Nanotechnology 21 (2010) 265201 (8pp). doi:10.1088/0957-4484.

Una mirada al futuro desde el pasado.

Por lo visto, de las tantas fuentes renovables y eficientes de energía. El hidrógeno esta ganando campo para las producción energética casera. En Japón,Matsushita Electronic, el fabricante de Panasonic ha podido producir una celda de combustible de hidrogeno muy eficiente.

"El sistema de Matsushita está basado en una tecnología de electrolitos polímeros que, según el Nikkei, es más eficiente que los modelos existentes. De esta manera, por medio de una reacción química, el nuevo sistema es capaz de convertir más del 39% para generar un máximo de 750 vatios de energía hasta un 37% menos de emisiones contaminantes que los generadores tradicionales."

Por lo visto, iban a hacer pruebas de eficiencia(2009) y si todo quedaba dentro de los estándares de producción y eficiencia las iban a sacar al mercado.Ya han pasado tres años, y tal parece que el poder de tomar la abundancia del elemento mas simple a nuestro favor y en nuestras casas aun esta lejos. Desde el lado de vista económico-gubernamental hay una disruptiva entre eficiencia e interés. Si todos los hogares tuvieran celdas de hidrogeno u otra fuente de energia renovable entonces serian eficientes y no se desperdiciarian recursos. Pero por otro lado el gobienro no tendria als recaudaciones de impuestos de compañias paraestatales o privadas energéticas. No se puede tener a todos contentos.

Thursday, May 10, 2012

Synthesis and formation mechanism of hydrogenated boron clusters B12Hn with controlled hydrogen content

Maximiliano De La Higuera Macías

     We present the formation of hydrogen-content-controlled B12 Hn+  clusters through the decomposition and ion-molecule reactions of the decaborane _ B10 H14_  and diborane _ B2 H6_ molecules in an external quadrupole static attraction ion trap. The hydrogen- and boron-contents of the B10−y Hx+  cluster are controlled by charge transfer from ambient gas ions. In the process of ionization, a certain number of hydrogen and boron atoms are detached from decaborane ions by the energy caused by charge transfer. The energy caused by the ion-molecule reactions also induces H atom detachment. Ambient gas of Ar leads to the selective generation of B10 H6+ . The B10 H6+ clusters react with B2 H6  molecules, resulting in the selective formation of B12 H8+  clusters. Ambientgas of Ne _ He_  leads to the generation of B10−y Hx+  clusters with x =4–10 and y =0–1 _ with x =2–10 and y =0–2_ , resulting in the formation of B12 Hn+  clusters with n =4–8 _n =2,4–8_ . The introduction of ambient gas also increases the production of clusters. PBE0/ 6-311+G_ d_/ / B3LYP/ 6-31G_ d_ -level density functional theory calculations are conducted to investigate the structure and the mechanism of formation of B10−y Hx+  and B12 Hn+  clusters.
     The analysis of the mass spectrum of B10−y Hx+  ions, which are generated by charge transfer from noble gas ions _ Ar, Ne, and He_  to B10 H14  molecules in the EQSIT, revealed that B10 H6 +  is generated with Ar, B10 H4–12 +  and B9 Hx +  are generated with Ne, and B10 H2–12+ , B9 Hx+ , and B8 Hx+  are generated with He. The B10 Hx+  ions react with B2 H6  molecules, and the analysis of the mass spectrum shows that B12 Hn+  with n =8, n =4–8, and n =2–8 are produced withthe ambient gas of Ar, Ne, and He, respectively. PBE0/ 6-311+G_ d_/ / B3LYP/ 6-31G_ d_ -level DFT calculations were conducted to investigate the ionization process of B10 H14 . On the basis of the experimentally observed derivative ions of B10 H14 and their calculated energies, the charge transfer energy _ECT_  was estimated. ECT  is generated when charge is transferred from noble gas ions to B10 H14  molecules. When ECT  is in the range of 0.96–12.00 eV for He, 0.96–9.18 eV for Ne, and 3.94–5.36 for Ar, the computationally expected products are in agreement with the experimental result. The formation process of B12 Hn+  was also calculated.
     The reaction energies, _E_x_ , of B10 Hx+  and B2 H6 were calculated. _E _ 6, 4, and 2_  were estimated to be 3.85, 6.38, and 7.50 eV. The calculations of the pathway of hydrogen detachment from icosahedral B12 Hx+6+ _x =6,4,2_ indicated that B12 Hn +  with n =8, 6, and 4 are produced by the detachment of two hydrogen molecules with ambient gas of Ar, Ne, and He, respectively. The remaining ECT  is considered to be the reason of the formation of B12 Hn +  clusters with fewer H atoms in the experiment than in the prediction. The introduction of ambient gas was shown to be effective for producing B- and H-atom-controlled ions and clusters. The DFT calculation of the reaction process of B12 Hn+  indicates that further reducing of the hydrogen atoms in decaborane ion leads to the formation of planar B12 Hn+  with n =0–3. This means that control of the number of the H atoms in decaborane ion leads to the control of the structures of boron clusters. In addition, the production of clusters increases dramatically upon introducing the ambient gas. These results open up the possibility of fabricating nanostructured materials by the deposition of clusters.

 Toshihiko Kanayama, et al. "Synthesis And Formation Mechanism Of Hydrogenated Boron Clusters B12hn With Controlled Hydrogen Content." Journal Of Chemical Physics 133.7 (2010): 074305. Academic Search Complete. Web. 10 May 2012.

Electronic and atomic structure of the AlnHn+2 clusters

Maximiliano De La Higuera Macías

     The electronic and atomic structure of the family of hydrogenated Al clusters Aln Hn+2  with n =4–11 has been studied using the density functional theory with the generalized gradient approximation _ GGA_  for exchange and correlation. All these clusters have substantial gaps between the highest occupied and the lowest unoccupied molecular orbitals _ HOMO-LUMO_ and, consequently, they are chemically very stable. The largest gap of 2.81 eV occurs for Al6 H8 . Five clusters of the family, Al4 H6 , Al5 H7 , Al6 H8 , Al7 H9 , and Al10 H12 , fulfill the Wade–Mingos rule. That is, in Aln Hn+2 , the Al matrix forms a polyhedron of n  vertices and n  H atoms form strong H–Al terminal bonds; one pair of electrons is involved in each of those bonds. The remaining n +1 electron pairs form a delocalized cloud over the surface of the Al cage. The clusters fulfilling the Wade–Mingos rule have wider HOMO-LUMO gaps and are chemically more stable. The trends in the gap have some reflections in the form of the photoabsorption spectra, calculated in the framework of time-dependent density functional theory using the GGA single-particle energies and orbitals and a local density approximation exchange-correlation kernel.
     Motivated by the interest on the Al4 H6  cluster,9  we have studied the family of hydrogenated Al clusters Aln Hn+2  with n =4–11 using the DFT with the GGA approximation for XC. All these clusters have 2n +1 valence electron pairs. The main characteristic is that the clusters have sizable H-L gaps and, consequently, they are chemically very stable. The largest gap of 2.81 eV occurs for Al6 H8 . Five clusters of the family, Al4 H6 , Al5 H7 , Al6 H8 , Al7 H9 , and Al10 H12 , fulfill the WM rule, as earlier discovered for Al4 H6  by Li et al.9  That is, in the Aln Hn+2 , the Al matrix forms a polyhedron of n  vertices and n  hydrogen atoms of the cluster form strong H–Al terminal bonds. One pair of electrons is involved in the formation of each of those bonds. The remaining n +1 electron pairs form a cloud delocalized over the surface of the Al cage.
     The other three clusters, Al8 H10 , Al9  H11 , and Al11 H13 do not conform to this rule. The clusters fulfilling the WM rule have wider H-L gaps and are then chemically more stable _ although the gap of Al10 H12  is smaller than the gap of Al8 H10 , it is a local maximum with respect to neighbor clusters. The trends in the H-L gap have some reflection in the behavior of the photoabsorption spectrum. This has been calculated by the Casida method in the frame of TDDFT, using PBE single-particle energies and orbitals and an LDA XC kernel.

 Martínez, J. I., and J. A. Alonso. "Electronic And Atomic Structure Of The Alnhn+2 Clusters." Journal Of Chemical Physics 129.7 (2008): 074306. Academic Search Complete. Web. 10 May 2012.

Study of the relative performance of silicon and germanium nanoparticles embedded gate oxide in metal–oxide–semiconductor memory devices

Maximiliano De La Higuera Macías

     In the present work, we have investigated a comparative performance of the silicon (Si) and germanium (Ge) nanoparticles embedded SiO2  floating gate MOS memory devices. In such devices for low applied fields, the tunneling current is dominated by the direct tunneling mechanism, whereas for higher electric fields, the Fowler–Nordheim tunneling mechanism dominates. As the device dimensions get smaller, problem arises in the conventional MOS memory devices due to the leakage through the thin tunnel oxide. This leakage can be reduced via charge trapping by embedding nanoparticles in the gate dielectric of such devices. Here one objective is to prevent the leakage due to the direct tunneling mechanism and the other objective is to reduce the write voltage, by lowering the onset voltage of the Fowler–Nordheim tunneling mechanism. Our simulations for the current voltage characteristics covered both the low and the high applied field regions.
     Simulations showed that both the Si and the Ge nanoparticles embedded gate dielectrics offer reduction of the leakage current and a significant lowering of the writing or programming onset voltage, compared to the pure SiO2 gate dielectric. In terms of the comparative performance, the Germanium nanoparticles embedded gate dielectric showed better results compared to the silicon nanoparticles embedded one. The results of the simulations are discussed in the light of recent experimental results.
     Here we have presented a simulation study based on a compact, analytical model of a deep submicron MOS structure with a gate dielectric embedded with nanocrystals of Si/Ge. The gate current due to the combination of the direct and the F–N tunneling in such a MOS device has been simulated using the WKB approximation and the Maxwell–Garnett effective medium theory. The study indicated that due to the addition of nanocrystalline Ge/Si the onset voltage of F–N tunneling decreases and the direct tunneling current is slightly lowered compared to a device with pure SiO2 gate dielectric. Consequently the performance of the MOS nonvolatile memory (NVM) device is improved in terms of the lowering of the write voltage and the slight suppression of the leakage. The results of our simulation studies have also been discussed in light of recent experimental reports.

 C. K. Sarkar, et al. "Study Of The Relative Performance Of Silicon And Germanium Nanoparticles Embedded Gate Oxide In Metal-Oxide-Semiconductor Memory Devices." Journal Of Applied Physics 109.6 (2011): 064504. Academic Search Complete. Web. 10 May 2012.