"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

dx.doi.org/10.1021/ja205908q | 

"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). 
doi:10.1088/0957-4484/21/43/435204.

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) 

doi:10.1088/0957-4484/22/14/145701

"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).

doi:10.1088/0957-4484/23/16/165201.

"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).

doi:10.1088/0957-4484/18/14/145502.

"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.

Using Oppositely Charged Ions To Operate a Three-Station [2]Rotaxane in Two Different Switching Modes.

Highly Selective and High-Yielding Rotaxane Synthesis via Aminolysis of Prerotaxanes Consisting of a Ring Component and a Stopper Unit.

Los rotaxanos pueden ser sintetizados por métodos orgánicos y en general son caracterizados por resonancia magnética nuclear. A continuación se incluirá un ejemplo de síntesis que llevará a la manufactura de un rotaxano. En éste caso particular el método de síntesis será la aminólisis de prerotaxanos los cuales estarán compuestos por una unidad fenólica de éter pseudo corona  y un grupo voluminoso benzoilo como una unidad de frenado.

Si la aminólisis procede vía ataque nucleofílico de la amina (4)  desde la parte de atrás del éter corona del prerotaxano  3a-e, los rotaxanos correspondientes 5a-f pueden ser obtenidos. El método de síntesis consiste de dos pasos:

En primer lugar, la esterificación del éter corona fenólico con ácido clorhídrico  y en segundo lugar la aminólisis con un compuesto amino que contiene un grupo voluminoso. Varios rotaxanos fueron sintetizados con buen porcentaje de rendimiento por éste método.

            Los prerotaxanos 3a-f fueron preparados por la acilación de un éter corona 1a-e con cloruro de benzoilo 2a ó 2b en presencia de KO´Bu en THF. El procedimiento de la reacción desde los prerotaxanos 3a-f a los correspondientes rotaxanos 5a-f fue muy simple. Una mezcla de un prerotaxano y la amina 4 en un solvente apropiado fue agitada a temperatura ambiente. En las aminólisis 3a y 3c  teniendo los compuestos pseudo-21-corona-7 y pseudo-27-corona-9 ningún rotaxano fue aislado porque el tamaño de los anillos no fue óptimo (o muy corto o muy largo) para la formación del rotaxano. Solo la estructura “dumbell” correspondiente al compuesto 6a y el ester corona 1a ó 1c fueron formados.

Highly Selective and High-Yielding Rotaxane Synthesis via Aminolysis of Prerotaxanes Consisting of a Ring Component and a Stopper Unit.

Keiji Hirose, Keiji Nishihara, Naoki Harada, Yamato Nakamura, Daisuke Masuda,

Masami Araki, and Yoshito Tobe.

Org. Lett., Vol. 9, No. 16, 2007

"Highly Selective and High-Yielding Rotaxane Synthesis via Aminolysis of Prerotaxanes Consisting of a Ring Component and a Stopper Unit".

Using Oppositely Charged Ions To
Operate a Three-Station [2]Rotaxane in
Two Different Switching Modes.

El desarrollo de nuevos métodos para la operación de los rotaxanos como switches moleculares  y actuadores ha atraído mucha atención porque dichos sistemas tienen aplicaciones potenciales en el sensado , entrega de drogas, memoria molecular y transporte de fluídos. Para los switches moleculares que son operados químicamente, el estímulo externo es frecuentemente de especies catiónicas (iones metálicos, protones) porque el ion dipolo o las interacciones de enlace introducidos por éstos aditivos pueden ser suficientemente fuertes para invertir la unidad de preferencia  del macrociclo entrelazado para los sitios de enlace. En contraste, el uso de aniones como controladores es muy raro.

            A pesar que el reconocimiento de cationes y aniones  es posible, aún se intenta operar switches moleculares entrelazados  en diferentes modos de switching  a través de la aplicación de iones de cargas opuestas. En teoría éste tipo de switches moleculares puede ser construída por la adición de dos o más estaciones de reconocimiento  permitiendo al macrociclo migrar específicamente hacia la presencia de una carga particular.

En la figura de la izquierda, se puede apreciar un rotaxano en el cuál el 2,2´-bipiridilo y el carbamato sirven como estadiones de unidades adicionales de reconocimiento que permiten  al anillo migrar fuera de la estación de guanidinio ocupada con la presencia o remoción de iones Zn²⁺ y PO₄^3- respectivamente.

Se puede observar también que en la primera etapa, la sustracción de un ion Zn²⁺ promueve un cambio de conformación de forma cis a trans ayudando a que la corona se cambie de posición a la estación de guanidinio. Posteriormente, el ión fosfato es introducido para el desplazamiento de la corona y la unión del anión al guanidinio que vendrá seguida de la unión de la corona a la estación de carbamato.

Para más información:

Using Oppositely Charged Ions To Operate a Three-Station [2]Rotaxane in Two Different Switching Modes.

Org. Lett., Vol. 9, No. 16, 2007.

DOI. 10.1021/ol070999w.

"DNA Machines: Bipedal Walker and Stepper"

"DNA Machines: Bipedal Walker and Stepper"

La figura de abajo muestra la operación de una máquina de ADN que funge como un “caminante”.  La operación principal de éstas máquinas envuelve la construcción de 4 plantillas de ácidos nucléicos en una placa de ADN y una señal de activación para la molvilidad del caminante atado a dos de las plantillas. El funcionamiento general básicamente es de 4 puntos principales:

            1.- Formación de un complejo Timina(T)-Hg²⁺-Timina (T).

            2.- Separación por cisteína (proceso inverso).

            3.- Disociación/formación de una estructura “i-motif” por estimulación de H⁺ y  OH^-.

            4.- El funcionamiento es regido por las estabilidades relativas de los duplexes nucléicos y las plantillas.

            La estructura del sistema está conformada por cuatro 4 plantillas (1-4) en las cuales están adheridas fluoróforos F₁, F₂, F₃ y F₄ en la parte terminal de cada plantilla. 4 ácidos nucléicos complementarios (1´-4´). 2 hombros (6y7). Una placa de sostén (8) que sostendrá a los hombros. La placa de sostén llevará adherido 2 quenchers Q₁ y Q₂ que estarán pegados por las partes de los carbonos 3´y 5´.

            En primer lugar, el estado inicial del caminador es estabilizado por las plantillas I y II por hibirdación. Un tratamiento de iones Hg²⁺ ocasiona la formación de un complejo de coordinación T-Hg^2+-T. La fuerza impulsora de la formación de éste complejo está dada por la estabilidad de los enlaces T-Hg²⁺-T que son más fuertes que los puentes de hidrógeno formados Timina-Citosina ó Adenina-Guanina. EL proceso inverso involucra la adición de cisteína al sistema lo que ocasionará que el caminante dé de nuevo un paso hacia atrás. La cisteína es últil debido a que contiene un átomo de azufre que  es muy afín al mercurio. Cuando el caminante no pisa ciertas plantillas, se emitirá fluorescencia como puede observarse en la figura superior. Dichas emisiones producidas, son observadas mediante espectroscopia de UV.  Cada emisión está acompañada de un color que puede ser rojo, naranja, verde ó azul. El siguiente paso es la adición de protones (pH=5.2) el cuál ocasionará el re arreglo de la hebra 2´a una estructura “i-motif cuádruplex”. La adición de protones provocará la ruptura de puentes de hidrógeno los cuáles darán el estímulo necesario para que el caminante dé un paso. Finalmente el tratamiento con iones OH- provocará una estabilización energética de la hebra lo que hará que el caminante dé un paso hacia atrás.

            En base a estos principios otras estructuras pueden ser diseñadas de tal forma que funjan como engrantes que puedan realizar movimientos horarios y anti horarios como se ve en la figura inferior.  Para éste caso, se aprecia una estructura en forma de engrane la cuál contiene los siguientes elementos:

            1.- ADN circular (9).

            2.- 4 plantillas (V-VIII).

            3.- 2 plantillas base (V y VI).

            4.-  Hebras de sostén 10, 11, 12 y 13 las cuáles evitarán el colapso de la estructura.

            5.- Una plantilla de sostén (8) a la cual serán adheridas los quenchers.

            6.- Plantillas de sostén 14,15 y 16 las cuáles proveerán de un soporte general al sistema.

No hay que olvidar que los quenchers absorben las radiaciones emitidas por los fluoróforos, es por ello que cuando son puestos en contacto (una pisada) el espectro UV muestra dicha parte “apagada”. Para ésta estructura se siguen los mismos tratamientos que el caminante lo que hará que el círculo comience a moverse.

Para más información consulte:

"DNA Machines: Bipedal Walker and Stepper"

Nano Lett. 2011, 11, 304—309.

Zhen-Gang Wang, Johann Elbaz and Itamar Willner.

DOI: 10.1021/nl104088s