Single molecule performs multiple logic operations simultaneously

 

(Left) The structure of the FG-DTE molecule, which is made of three photochromes that can switch between two different states when irradiated with light of different wavelengths. (Right) A checklist of some of the features of the all-photonic molecular logic device. Image credit: Joakim Andréasson, et al. 2011 American Chemical Society.

While molecules have already been used to perform individual logic operations, scientists have now shown that a single molecule can perform 13 logic operations, some of them in parallel. The molecule, which consists of three chromophores, is operated by different wavelengths of light. The scientists predict that this system, with its unprecedented level of complexity, could serve as a building block of molecular computing, in which molecules rather than electrons are used for processing and manipulating information.

The scientists and engineers, Joakim Andréasson from Chalmers University of Technology in Göteborg, Sweden; Uwe Pischel from the University of Huelva, Spain; and Stephen D. Straight, Thomas A. Moore, Ana L. Moore, and Devens Gust from Arizona State University, have published their study called ?All-Photonic Multifunctional Molecular Logic Devices? in a recent issue of the Journal of the American Chemical Society.

?While previous examples of molecular logic systems have been able to carry out one, or a few differentlogic operations, this molecule can be reconfigured to perform 13 simply by changing the input or output wavelengths,? Gust told PhysOrg.com. ?In addition, it uses light for all inputs and outputs, which avoids some of the problems encountered when using chemicals as inputs.?

In general, chromophores are the parts of a molecule that absorb light of specific wavelengths while transmitting other wavelengths, and are responsible for the molecule?s color. When chromophores can be switched between two different states by being irradiated with light of different wavelengths, they have the ability to perform binary logic operations and effectively serve as transistors. These photoswitchable, bistable chromophores are called photochromes.

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Here, the researchers used three photochromes ? one dithienylethene (DTE) and two fulgmides (FG) ? to build a light-responsive molecule. Each of these photochromes can exist in either an open or closed isomeric form, and can be switched back and forth between forms with light pulses of different wavelengths.

The two forms that each photochrome can take represent the two states that serve as the basis for performing binary logic operations. Various combinations of the three photochromes in different isomeric forms can be used to perform binary arithmetic, such as addition and subtraction. Although previous molecular-based systems have performed binary arithmetic, the FG-DTE molecule is the first that can perform these operations using only two inputs: light with wavelengths of 302 nm and 397 nm. Also, all three photochromes can be reset by green light irradiation (460-590 nm). These features allow the molecule to perform addition and subtraction in parallel, simply by having light convert the photochromes to different isomeric forms.

?All of these 13 logic operations share the same initial state, that is, the molecule is always ?reset? to one and the same state by the use of green light, irrespective of which logic function is to be performed,? said Andréasson. ?This is another unique feature of our molecule.?

The researchers also demonstrated that the FG-DTE molecule can perform non-arithmetic functions. For example, as a digital multiplexer, the molecule can act as a mimic of a mechanical rotary switch to connect any one of several inputs to an output. As a demultiplexer, the molecule can separate two signals that have been multiplexed into one output.

Further, the FG-DTE molecule can perform sequential logic functions, in which inputs must be applied in the correct order, such as for a keypad lock. The molecule can also operate as a transfer gate by transferring the state of an input to that of an output, which is useful for complicated computational operations. The researchers also demonstrated that the molecule can act as an encoder and decoder, by compressing digital information for transmission or storage, and then recovering the information in its original form.

While each of these individual logic operations has previously been performed by molecular systems, the FG-DTE molecule is the first to unite them all in a single molecular platform. Transistors and other more traditional logic devices do not have the same functional flexibility, which the researchers attribute to the chromophores? ability to respond differently to different wavelengths of light and to influence each other?s properties.

As for applications, the researchers note that it?s unlikely that such molecular devices will soon replace electronic computers, but they could have applications in nanotechnology and biomedicine, such as for data storage, labeling and tracking micro-objects, and programmed drug release.

?In the near term, molecular logic devices will complement, rather than compete with, electronic devices,? Gust said. ?In principle, molecular computing could be implemented with extremely small switch sizes, since the operational units are molecules. Photonically operated molecular devices such as the one we describe can also be easily reconfigured to perform a variety of different logic functions, can operate at high speeds, and can be arrayed in three dimensions, rather than the planar arrangements usually found in electronics.

?Molecular logic devices can be employed where electronic ones cannot,? he added. ?For example, they can be used to label and track nanoparticles and nanoscale components of biological organisms. On the other hand, most photochromes currently are not sufficiently stable to stand up to the large number of cycles required for useful full-scale computing. In addition, complex computing will require convenient ways for nanoscale logic devices to communicate with one another.?

?In addition, the application of molecular logic in biological systems, such as the human body, is still relatively unexplored, although molecular systems are better suited for this purpose compared to electronic devices,? said Andréasson.

In the future, the researchers plan to address some of the biggest challenges facing molecular logic, such as the efficient wiring (concatenation) of logic switches.

?One of the major challenges of molecular logic is concatenation of logic operations,? Gust said. ?In electronics, this can be done simply by wiring the output of one element to the input of the next. We need to find ways of achieving similar results in molecules.?

 

Secrets of mysterious metal hotspots uncovered by new single molecule imaging technique

Secrets of mysterious metal hotspots uncovered by new single molecule imaging technique

Enlarge

Electron micrograph showing multiple nano-sized electromagnetic hotspots on an aluminum film. Credit: courtesy of Xiang Zhang research group

The secrets behind the mysterious nano-sized electromagnetic "hotspots" that appear on metal surfaces under a light are finally being revealed with the help of a BEAST. Researchers at the DOE's Lawrence Berkeley National Laboratory have developed a single molecule imaging technology, dubbed the Brownian Emitter Adsorption Super-resolution Technique (BEAST), that has made it possible for the first time to directly measure the electromagnetic field inside a hotspot. The results hold promise for a number of technologies including solar energy and chemical sensing.

"With our BEAST method, we were able to map the electromagnetic field profile within a single hotspot as
small as 15 nanometers with an accuracy down to 1.2 nanometers, in just a few minutes," says Xiang Zhang,
a principal investigator with Berkeley Lab's Materials Sciences Division and the Ernest S. Kuh Endowed Chaired Professor at the University of California (UC), Berkeley. "We discovered that the field is highly localized and, unlike a typical electromagnetic field, does not propagate through space. The field also has an exponential shape that rises steeply to a peak and then decays very fast."

Zhang, who directs the Center for Scalable and Integrated NanoManufacturing (SINAM), a National Science Foundation Nano-scale Science and Engineering Center at UC Berkeley, is the corresponding author of a paper on this research that appears in the journal Nature under the title "Mapping the Distribution of Electromagnetic Field Inside a 15nm Sized Hotspot by Single Molecule Imaging." Co-authoring the paper with Zhang were Hu Cang, Anna Labno, Changgui Lu, Xiaobo Yin, Ming Liu and Christopher Gladden.

Under optical illumination, rough metallic surfaces will become dotted with microscopic hotspots, where the light is strongly confined in areas measuring tens of nanometers in diameter, and the Raman (inelastic) scattering of the light is enhanced by up to 14 orders of magnitude. First observed more than 30 years ago, such hotspots have been linked to the impact of surface roughness on plasmons (electronic surface waves) and other localized electromagnetic modes.
However, during the past three decades, little has been learned about the origins of these hotspots.

"Amazingly, despite thousands of papers on this problem and various theories, we are the first to experimentally determine the nature of the electromagnetic field inside of such a nano-sized hotspots," says Hu Cang, lead author on the Nature paper and a member of Zhang's research group. "The 15 nanometer hotspot we measured is about the size of a protein molecule. We believe there are hotspots that may even be smaller than a molecule."

Because the size of these metallic hotspots is far smaller than the wavelength of incident light, a new technique was needed to map the electromagnetic field within a hotspot. The Berkeley researchers developed the BEAST method to capitalize on the fact that individual fluorescent dye molecules can be localized with single nanometer accuracy. The fluorescence intensity of individual molecules adsorbed on the surface provides a direct measure of the electromagnetic field inside a single hotspot. BEAST utilizes the Brownian motion of single dye molecules in a solution to make the dyes scan the inside of single hotspot stochastically, one molecule at a time.

"The exponential shape we found for the electromagnetic field within a hotspot is direct evidence for the existence of a localized electromagnetic field, as opposed to the more common form of Gaussian distribution," Cang says. "There are several competing mechanisms proposed for hotspots and we are now working to further examine these fundamental mechanisms."

BEAST starts with the submerging of a sample in a
solution of freely diffusing fluorescent dye. Since the diffusion of the dye is much faster than the image acquisition time (0.1 milliseconds vs. 50-to-100 milliseconds), the fluorescence produces a homogeneous background. When a dye molecule is adsorbed onto the surface of a hotspot, it appears as a bright spot in images, with the intensity of the spot reporting the local field strength.

"By using a maximum likelihood single molecule localization method, the molecule can be localized with single nanometer accuracy," Zhang says. "After the dye molecule is bleached (typically within hundreds of milliseconds), the fluorescence disappears and the hotspot is ready for the next adsorption event."

Choosing the right concentration of the dye molecules enables the adsorption rate on the surface of a hotspot
to be controlled so that only one adsorbed molecule emits photons at a time. Since BEAST uses a camera to record the single molecule adsorption events, multiple hotspots within a field of view of up to one square millimeter can be imaged in parallel.

In their paper, Zhang and his colleagues see hotspots being put to use in a broad range of applications, starting with the making of highly efficient solar cells and devices that can detect weak chemical signals.

"A hotspot is like a lens that can focus light to a small spot with a focusing power well beyond any conventional optics," Cang says. "While a conventional lens can only focus light to a spot about half the wavelength of visible light (about 200-300 nanometers), we now confirm that a hotspot can focus light to a nanometer-sized spot."

Through this exceptional focusing power, hotspots could be used to concentrate sun light on the photocatalytic sites of solar devices, thereby helping to maximize light- harvesting and water-splitting efficiencies. For the detection of weak chemical signals, e.g., from a single
molecule, a hotspot could be used to focus incident light so that it only illuminates the molecule of interest, thereby enhancing the signal and minimizing the background.

BEAST also makes it possible to study the behavior of light as it passes through a nanomaterial, a critical factor for the future development of nano-optics and metamaterial devices. Current experimental techniques suffer from limited resolution and are difficult to implement on the truly nanoscale.

"BEAST offers an unprecedented opportunity to measure how a nanomaterial alters the distribution of light, which will guide the development of advanced nano-optics devices," says Cang. "We will also use BEAST to answer some challenging problems in surface science, such as where and what are the active sites in a catalyst, how the energy or charges transfer between molecules and a nanomaterial, and what determine surface hydrophobicity. These problems require a technique with electron-microscopy level resolution and optical spectroscopy information. BEAST is a perfect tool for these problems."

Scientists identify spontaneously chain-reacting molecule

Scientists identify spontaneously chain-reacting molecule

In the burgeoning field of nano-science there are now many ways of 'writing' molecular-scale messages on a surface, one molecule at a time. The trouble is that writing a molecule at a time takes a very long time.

"It is much better if the molecules can be persuaded to gather together and imprint an entire pattern simultaneously, by themselves. One such pattern is an indefinitely long line, which can then provide the basis for the ultimately thin molecular 'wire' required for nano-circuitry," says John Polanyi of the University of Toronto's Department of Chemistry, co- author of the paper to be published on Nature Chemistry this week.

The paper describes, for the first time, a simple molecule that each time it chemically reacts with a surface prepares a hospitable neighbouring site at which the next incoming molecule reacts. Accordingly, these molecules, when simply dosed (blindly) on the surface, spontaneously grow durable 'molecular-chains'. These molecular chains are the desired prototypes of nano-wires.

The experiments were conducted by graduate student Tingbin Lim in the John Polanyi Scanning Tunneling Microscopy laboratory at U of T, in conjunction with theory performed by postdoctoral fellow Dr. Wei Ji in the Hong Guo laboratory in the Department of Physics, McGill University. The experiments in Toronto yielded visual evidence of the chains, and the theory at McGill explained why the chains spontaneously grew.

"Early-on, far-sighted Xerox Research Centre Canada (XRCC) recognized this opportunity for imprinting patterns at the molecular scale, thereby persuading Ontario Centres of Excellence (OCE) and the federal Natural Sciences and Engineering Research Council (NSERC), through its Strategic Grant program, to fund the bulk of the research costs in our lab," says Polanyi.

"The experiments constituted the doctoral work of a recent PhD student in the Toronto laboratory, Dr. Tingbin Lim an outstanding student who came from Singapore to join our group and now makes his home as a scientist in Canada."

Secrets of mysterious metal hotspots uncovered by new single molecule imaging technique

Secrets of mysterious metal hotspots uncovered by new single molecule imaging technique

Enlarge

Electron micrograph showing multiple nano-sized electromagnetic hotspots on an aluminum film. Credit: courtesy of Xiang Zhang research group

The secrets behind the mysterious nano-sized electromagnetic "hotspots" that appear on metal surfaces under a light are finally being revealed with the help of a BEAST. Researchers at the DOE's Lawrence Berkeley National Laboratory have developed a single molecule imaging technology, dubbed the Brownian Emitter Adsorption Super-resolution Technique (BEAST), that has made it possible for the first time to directly measure the electromagnetic field inside a hotspot. The results hold promise for a number of technologies including solar energy and chemical sensing.

"With our BEAST method, we were able to map the electromagnetic field profile within a single hotspot as
small as 15 nanometers with an accuracy down to 1.2 nanometers, in just a few minutes," says Xiang Zhang,
a principal investigator with Berkeley Lab's Materials Sciences Division and the Ernest S. Kuh Endowed Chaired Professor at the University of California (UC), Berkeley. "We discovered that the field is highly localized and, unlike a typical electromagnetic field, does not propagate through space. The field also has an exponential shape that rises steeply to a peak and then decays very fast."

Zhang, who directs the Center for Scalable and Integrated NanoManufacturing (SINAM), a National Science Foundation Nano-scale Science and Engineering Center at UC Berkeley, is the corresponding author of a paper on this research that appears in the journal Nature under the title "Mapping the Distribution of Electromagnetic Field Inside a 15nm Sized Hotspot by Single Molecule Imaging." Co-authoring the paper with Zhang were Hu Cang, Anna Labno, Changgui Lu, Xiaobo Yin, Ming Liu and Christopher Gladden.

Under optical illumination, rough metallic surfaces will become dotted with microscopic hotspots, where the light is strongly confined in areas measuring tens of nanometers in diameter, and the Raman (inelastic) scattering of the light is enhanced by up to 14 orders of magnitude. First observed more than 30 years ago, such hotspots have been linked to the impact of surface roughness on plasmons (electronic surface waves) and other localized electromagnetic modes.
However, during the past three decades, little has been learned about the origins of these hotspots.

"Amazingly, despite thousands of papers on this problem and various theories, we are the first to experimentally determine the nature of the electromagnetic field inside of such a nano-sized hotspots," says Hu Cang, lead author on the Nature paper and a member of Zhang's research group. "The 15 nanometer hotspot we measured is about the size of a protein molecule. We believe there are hotspots that may even be smaller than a molecule."

Because the size of these metallic hotspots is far smaller than the wavelength of incident light, a new technique was needed to map the electromagnetic field within a hotspot. The Berkeley researchers developed the BEAST method to capitalize on the fact that individual fluorescent dye molecules can be localized with single nanometer accuracy. The fluorescence intensity of individual molecules adsorbed on the surface provides a direct measure of the electromagnetic field inside a single hotspot. BEAST utilizes the Brownian motion of single dye molecules in a solution to make the dyes scan the inside of single hotspot stochastically, one molecule at a time.

"The exponential shape we found for the electromagnetic field within a hotspot is direct evidence for the existence of a localized electromagnetic field, as opposed to the more common form of Gaussian distribution," Cang says. "There are several competing mechanisms proposed for hotspots and we are now working to further examine these fundamental mechanisms."

BEAST starts with the submerging of a sample in a
solution of freely diffusing fluorescent dye. Since the diffusion of the dye is much faster than the image acquisition time (0.1 milliseconds vs. 50-to-100 milliseconds), the fluorescence produces a homogeneous background. When a dye molecule is adsorbed onto the surface of a hotspot, it appears as a bright spot in images, with the intensity of the spot reporting the local field strength.

"By using a maximum likelihood single molecule localization method, the molecule can be localized with single nanometer accuracy," Zhang says. "After the dye molecule is bleached (typically within hundreds of milliseconds), the fluorescence disappears and the hotspot is ready for the next adsorption event."

Choosing the right concentration of the dye molecules enables the adsorption rate on the surface of a hotspot
to be controlled so that only one adsorbed molecule emits photons at a time. Since BEAST uses a camera to record the single molecule adsorption events, multiple hotspots within a field of view of up to one square millimeter can be imaged in parallel.

In their paper, Zhang and his colleagues see hotspots being put to use in a broad range of applications, starting with the making of highly efficient solar cells and devices that can detect weak chemical signals.

"A hotspot is like a lens that can focus light to a small spot with a focusing power well beyond any conventional optics," Cang says. "While a conventional lens can only focus light to a spot about half the wavelength of visible light (about 200-300 nanometers), we now confirm that a hotspot can focus light to a nanometer-sized spot."

Through this exceptional focusing power, hotspots could be used to concentrate sun light on the photocatalytic sites of solar devices, thereby helping to maximize light- harvesting and water-splitting efficiencies. For the detection of weak chemical signals, e.g., from a single
molecule, a hotspot could be used to focus incident light so that it only illuminates the molecule of interest, thereby enhancing the signal and minimizing the background.

BEAST also makes it possible to study the behavior of light as it passes through a nanomaterial, a critical factor for the future development of nano-optics and metamaterial devices. Current experimental techniques suffer from limited resolution and are difficult to implement on the truly nanoscale.

"BEAST offers an unprecedented opportunity to measure how a nanomaterial alters the distribution of light, which will guide the development of advanced nano-optics devices," says Cang. "We will also use BEAST to answer some challenging problems in surface science, such as where and what are the active sites in a catalyst, how the energy or charges transfer between molecules and a nanomaterial, and what determine surface hydrophobicity. These problems require a technique with electron-microscopy level resolution and optical spectroscopy information. BEAST is a perfect tool for these problems."