July 20, 2006

	In This Issue:
	‘Micro-boxes’ of Water Used to Study Single Molecules
	Mercury Atomic Clock Sets Time-Keeping Record
	Novel Nano-Etched Cavity Makes LEDs 7 Times Brighter
	Add Nanotubes and Stir—With the Right Force
	Road to AC Voltage Standard Leads to Important Junction
	NIST Can Help You ‘MBARK’ Onto Better Biometric Systems
	Quick Links
	NIST Director Testifies on Voting Machine Standards

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‘Micro-boxes’ of Water Used to Study Single Molecules

Prodded by optical tweezers, two "hydrosomes" move together and fuse to mix their contents, in an experiment using water droplets as minuscule boxes for manipulating small numbers of biomolecules for nanobiochemistry. Credit: NIST View hi-res version of image. Click here to view avi format video of the experiment.

Researchers at the National Institute of Standards and Technology (NIST) have demonstrated the use of water droplets as minuscule “boxes” for small numbers of biomolecules. The unusually simple containment method may enable easier experiments on single molecule dynamics and perhaps lead to the development of molecule-sorting devices that might be used for medical screening or biotechnology research. The work was reported in the July 3 issue of Applied Physics Letters.

The NIST team creates the boxes by briefly shaking a mixture of water, the biomolecules to be studied, and a fluorocarbon medium. Water droplets form in the oily fluorocarbon and naturally encapsulate one to several biomolecules. The researchers then watch through a microscope while using infrared lasers as “optical tweezers” to manipulate and combine the droplets (dubbed “hydrosomes”) inside a tiny chamber on a slide.

A green laser is then used to excite the molecules in individual droplets, and the light emissions over several seconds are analyzed to count the molecules and observe other phenomena. The researchers use two sets of optical tweezers to move droplets together to fuse them and mix their contents (see accompanying video). The team demonstrated the technique by trapping and manipulating droplets encapsulating various molecules (including a delicate protein that survived the shaking process), detecting the fluorescence signal from dye and protein molecules, and observing the transfer of energy from one end of a specially treated DNA molecule to the other.

Water offers several advantages over other methods for containing single molecules, such as attaching them to surfaces or placing them inside liposomes (artificial cells). The water droplets can be held far from any surface that might interfere, can readily encapsulate biomolecules (which prefer being in water as opposed to the fluorocarbon medium), and can readily fuse together to mix molecules or rapidly change their chemical environment. The water droplets currently average about 300 nanometers in diameter and contain volumes measured in quadrillionths of liters; research is continuing to improve methods for controlling droplet size for different applications.

The work was supported in part by the Office of Naval Research.

* J.E. Reiner, A.M. Crawford, R.B. Kishore, L.S. Goldner, K. Helmerson and M.K. Gilson. 2006. Optically trapped aqueous droplets for single molecule studies. Applied Physics Letters. July 3.

Media Contact:
Laura Ost, laura.ost@nist.gov, (301) 975-4034

 

Mercury Atomic Clock Sets Time-Keeping Record

NIST physicist Jim Bergquist with the world's most accurate clock. The silver cylinder in the foreground is a magnetically shielded cryogenic vacuum system, which holds the heart of the clock— a single mercury ion cooled to near absolute zero. ©Geoffrey Wheeler For a high-resolution version of this photo contact inquiries@nist.gov

An experimental atomic clock based on a single mercury atom is now at least five times more precise than the national standard clock, according to a paper by physicists at the National Institute of Standards and Technology (NIST) in the July 14 issue of Physical Review Letters*. 

The experimental clock, which measures the oscillations of a mercury ion (an electrically charged atom) held in an ultra-cold electromagnetic trap, ticks at “optical” frequencies—much higher than the microwave frequencies measured in cesium atoms in NIST-F1, the national standard and one of the world’s most accurate clocks. Higher frequencies allow time to be divided into smaller units, which increases precision.

 

A prototype mercury optical clock originally was demonstrated at NIST in 2000. Over the last five years its absolute frequency has been measured repeatedly with respect to NIST-F1. The improved version of the mercury clock is the most accurate to date of any atomic clock. The current version of NIST-F1—if operated continuously—would neither gain nor lose a second in about 70 million years. The latest version of the mercury clock would neither gain nor lose a second in about 400 million years.

 

Improved time and frequency standards have many applications, including improved synchronization in navigation and positioning systems, telecommunications networks, and wireless and deep-space communications. Better frequency standards also can be used to improve probes of magnetic and gravitational fields for security and medical applications, and to measure whether “fundamental constants” used in scientific research might be varying over time—a question that has enormous implications for understanding the origins and ultimate fate of the universe.

 

Funding for the research was provided by NIST and the Office of Naval Research. For more detail, see: http://www.nist.gov/public_affairs/releases/mercury_atomic_clock.htm.

 

* W.H. Oskay, S.A. Diddams, E.A. Donley, T.M. Fortier, T.P. Heavner, L. Hollberg, W.M. Itano, S.R. Jefferts, M.J. Jensen, K. Kim, F. Levi, T.E. Parker and J.C. Bergquist. 2006. A single-atom optical clock with high accuracy. Physical Review Letters. July 14.

Media Contact:
Laura Ost, laura.ost@nist.gov, (301) 975-4034

 

 

Novel Nano-Etched Cavity Makes LEDs 7 Times Brighter

Etched nanostructured rings around an LED can make it more than seven times brighter. The novel technique developed at NIST may have applications in areas such as in biomedical imaging where LED brightness is crucial. View hi-res version of image. Credit: NIST

Researchers at the National Institute of Standards and Technology (NIST) have made semiconductor light-emitting diodes (LEDs) more than seven times brighter by etching nanoscale grooves in a surrounding cavity to guide scattered light in one direction. The novel nanostructure, which may have applications in areas such as in biomedical imaging where LED brightness is crucial, is described in the July 17 issue of Applied Physics Letters.*

Semiconductor LEDs are used increasingly in displays and many other applications, in part because they can efficiently produce light across a broad spectrum, from near-infrared to the ultraviolet. However, they typically emit only about two percent of the light in the desired direction: perpendicular to the diode surface. Far more light skims uselessly below the surface of the LED, because of the extreme mismatch in refraction between air and the semiconductor. The NIST nanostructured cavity boosts useful LED emission to about 41 percent and may be cheaper and more effective for some applications than conventional post-processing LED shaping and packaging methods that attempt to redirect light.

 

The NIST team fabricated their own infrared LEDs consisting of gallium arsenide packed with "quantum dots" of assorted sizes made of indium gallium arsenide. Quantum dots are nanoscale semiconductor particles that efficiently emit light at a color determined by the exact size of the particle. The LEDs were backed with an alumina mirror to reflect the light emitted backwards. The periphery of each LED was turned into a cavity etched with circular grooves, in which the light reflects and interferes with itself in an optimal geometry.

 

The researchers experimented with different numbers and dimensions of grooves. The brightest output was attained with 10 grooves, each about 480 nanometers (nm) wide and 150 nm deep, and spaced 40 nm apart. (Note correction.**) The team spent several years developing the design principles and perfecting the manufacturing technique. The principles of the method are transferable to other LED materials and emission wavelengths, as well as other processing techniques, such as commercial photolithography, according to lead author Mark Su.

 

* M.Y. Su and R.P. Mirin. Enhanced light extraction from circular Bragg grating coupled microcavities. Applied Physics Letters. July 17.
** As a result of a calculation error, this item when originally published on July 20, 2006, stated that "The brightest output was attained with 10 grooves, each about 240 nanometers (nm) wide ..." The correct value is 480 nanometers. We apologize for the error. August 8, 2006

Media Contact:
Laura Ost, laura.ost@nist.gov, (301) 975-4034

 

Add Nanotubes and Stir—With the Right Force

NIST researchers have mapped the relationship between stirring force and nanotube arrangement, an advance key to the processing of new nanocomposite materials. Images show the evolution from solid-like nanotube networks (a) to macroscopically shear banded fluids (b) to small isolated nanotube aggregates (c) to individually dispersed and aligned carbon nanotubes (d). View hi-res version of image. Credit: NIST

Polymer scientists at the National Institute of Standards and Technology have some stirring results to share with researchers and companies developing new, advanced composite materials with carbon nanotubes—mix carefully.

In a paper for Physical Review Letters,* they explain how the amount of force applied while mixing carbon nanotube suspensions influences the way the tiny cylinders ultimately disperse and orient themselves. In turn, the final arrangement of the nanotubes largely dictates the properties of the resultant materials.

 

Measuring only a few nanometers in diameter (the width of a handful of atoms), carbon nanotubes possess many superior properties that make them highly desirable additives in composites, a class of engineered materials made by blending polymers and fibers or by combining other types of unlike materials. Mixed in polymeric materials, carbon nanotubes can provide incredible strength, toughness and electrical conductivity. The trouble is, nanotubes stick to each other and form networks that tend to stay fixed in place. Apply enough force, the networks will flow but usually end up in tangled clumps. The resultant nanocomposites are difficult to mold or shape, and their properties fall short of expectations.

 

In an elegantly simple result, NIST researchers Erik Hobbie and Dan Fry found that networks of carbon nanotubes respond predictably to externally applied force. The networks also showed behavior reminiscent of more conventional materials that align spontaneously under the forces of Brownian motion—the random motion of particles in a fluid famously described mathematically by Einstein.

 

The response was so predictable that the scientists mapped out the relationship in the form of a phase diagram, the materials science equivalent of a recipe. Using their “phase diagram of sticky nanotube suspensions,” other researchers can estimate the order that will result when applying a certain amount of force when mixing a polymer fluid with a particular concentration of nanotubes. The recipe can be used to prevent entanglement and to help achieve the nanotube arrangement and orientation associated with a desired set of properties.

 

* E.K. Hobbie and D.J. Fry, 2006. Nonequilibrium phase diagram of sticky nanotube suspensions. Physical Review Letters, July 21. Posted on-line at http://prl.aps.org/ on July 18.

Media Contact:
Mark Bello, mark.bello@nist.gov, (301) 975-3776

 

Road to AC Voltage Standard Leads to Important Junction

 

After 10 years of research, the National Institute of Standards and Technology (NIST) has unveiled the world’s first precision instrument for directly measuring alternating current (AC) voltages. The instrument is being tested for use in NIST’s low-voltage calibration service, where it is expected to increase significantly the measurement precision of industrial voltmeters, spectrum analyzers, amplifiers and filters.

Described July 14 at the Conference on Precision Electromagnetic Measurements in Turin, Italy,* the patented instrument** is based on the same “Josephson junction” technology used in NIST’s widely used direct current (DC) voltage standards, offering high precision based on quantum physics. A Josephson junction consists of two superconducting pieces of metal separated by a thin insulator or normal metal. When a fixed DC voltage is applied across it, a junction responds by generating an AC current that oscillates like a wave at a frequency exactly proportional to the applied voltage.

The new instrument uses arrays of junctions to generate AC pulses in precisely measured voltage units over a range of audio frequencies. Arbitrary waveforms can be generated at different voltage levels for different applications. The new standard would establish an entirely new method for AC voltage metrology. Until now, AC voltage calibrations have been performed indirectly, by measuring the heat delivered by an instrument to a resistor, and comparing that measurement to the heat delivered by a known DC voltage. At low voltages (such as 2 millivolts), the new AC Josephson junction voltage standard should improve measurement accuracy as much as 1,000-fold.

The concept for the new device was co-invented by researchers at NIST and Northrop-Grumman in the mid-1990s.*** A number of innovations since then have led to the first practical system. For instance, to increase the output voltage, NIST developed “nano-stacked” arrays of Josephson junctions, in which the spacing between junctions is reduced to less than 100 nanometers by stacking the junctions on top of each other. Using this technique NIST can make programmable voltage standard integrated circuits with over 130,000 junctions on a single chip. The new AC instrument currently has a maximum output of 100 millivolts; NIST researchers hope eventually to increase that level to 1 volt.

*S.P. Benz, C.J. Burroughs, P.D. Dresselhaus, T.E. Lipe and J.R. Kinard. 2006. 100 mv AC-DC transfer standard measurements with a pulse-driven AC Josephson voltage standard. Presented at Conference on Precision Electromagnetic Measurements, July 9-14, Turin, Italy.

**S.P. Benz, C.J. Burroughs, C.A. Hamilton, T.E. Harvey. U.S. Patent 6,236,344 (issued 5/22/01) “AC And DC Bipolar Voltage Source Using Quantized Pulses.”

*** J.X. Przybysz, S.P. Benz, C.A. Hamilton, A. Worsham. U.S. Patent 5,812,078 (issued 9/22/98) “Josephson Junction Digital to Analog Converter for Accurate AC Waveform Synthesis.”

Media Contact:
Laura Ost, laura.ost@nist.gov, (301) 975-4034

 

NIST Can Help You ‘MBARK’ onto Better Biometric Systems

Once a tool primarily used by law enforcement to help identify criminals, biometric technologies increasingly are being used by government and the private sector to authenticate a person’s identity, provide security at the nation’s borders and restrict access to secure sites—both buildings and computer networks. New software and other tools that can be used to help build improved biometric applications are now available from the National Institute of Standards and Technology (NIST).

Most biometric systems are “unimodal,” meaning they rely on a single distinguishing physical characteristic—such as a fingerprint—for authenticating identity. But using a single feature can present problems. Poor illumination could make a face image unrecognizable; dirty or damaged sensor plates could affect fingerprint equipment. A multimodal system that has several sources of information, including fingerprint, face and iris data, can be more flexible and reliable. But most biometric equipment, including the sensors that capture data and the database that stores the information, are not interoperable. Organizations must either purchase a complete system or develop “middleware”—custom integration software—to link together applications.   

NIST’s new Multimodal Biometric Application Resource Kit (MBARK) provides a solution. Originally envisioned as a tool to develop a large database of face, fingerprint and iris images for performance testing of biometric systems, MBARK has evolved into a standardized, flexible middleware package that will enable organizations to plug in sensors from different manufacturers, saving dollars and time. The package, which includes example applications and public-domain source code, can help reduce the complexity and costs of building multimodal biometric applications. MBARK also can used by government and industry to develop standards and tests for biometric system interoperability and usability.

MBARK was developed as part of NIST’s homeland security responsibilities and was funded by the Science and Technology Directorate of the U.S. Department of Homeland Security.

The USA Patriot Act and the Enhanced Border Security and Visa Entry Reform Act calls for NIST to develop and certify standards for verifying the identity of individuals and determining the accuracy of biometric technologies, including fingerprints, facial recognition and iris recognition.

For more information and to download MBARK, go to http://www.itl.nist.gov/iad/894.03/nigos/nigos.html.

 

Media Contact:
Jan Kosko, kosko@nist.gov, (301) 975-2767

 

 

Quick Links

NIST Director Testifies on Voting Machine Standards

On July 19, National Institute of Standards and Technology (NIST) Director Bill Jeffrey testified before the House Committee on Science and the Committee on House Administration on “Voting Machines: Will New Standards and Guidelines Help Prevent Future Problems?” As part of his testimony, Jeffrey discussed the Voluntary Voting System Guidelines (VVSG) of 2005, delivered to the Election Assistance Commission in May 2005. Jeffrey said, "The VVSG 2005 built upon the strengths of the previous Voting Systems Standards and enhanced areas needing improvement and added new material. The new material adds more formalism and precision to the requirements using constructs and language commonly used in rigorous, well-specified standards. The new material focuses primarily on usability, accessibility, and security." Jeffrey also addressed the VVSG 2007, currently planned for delivery to the EAC in July 2007.  He said, "The VVSG 2007 builds upon the VVSG 2005, but takes a fresh look at many of the requirements. It will be a larger, more comprehensive standard, with more thorough treatments of security areas and requirements for equipment integrity and reliability." For Jeffrey's complete testimony, see http://www.nist.gov/testimony/06index.html.

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Editor: Michael Baum

Date created: 7/19/06
Date updated: 8/8/06
Contact: inquiries@nist.gov