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Accomplishments in optical measurement methods

New Method for Detecting Motions of Aqueous Biomolecules

Reverse micelle
An illustration of L-proline molecules encapsulated within a reverse aqueous micelle formed by anionic surfactant in terahertz-transparent n-heptane.

The ability of biomolecules to flex and bend is important for their function within living cells. Until recently, researchers interested in understanding how biomolecules such as DNAs and proteins function have had to make inferences from “frozen pictures” of pure crystalline samples. Now, using a new technique based on terahertz spectroscopy, Division scientists have taken the first steps toward revealing the hidden machinations of biomolecules in room temperature water.

Terahertz radiation, which falls between the infrared and microwave spectral regions, probes concerted and large-scale motions of molecules as well as their interactions with surrounding solvent. Unfortunately, room temperature water, the natural solvent for biological molecules, absorbs nearly all terahertz radiation, limiting the utility of terahertz absorption spectroscopy for probing biomolecular function.

To minimize this problem, we used reverse micelles: nanoscale capsules of water suspended in terahertz-transparent organic solvent surrounded by surfactants. The reverse micelles were filled with a solution of water and, for this test, the amino acid L-proline, a protein building block. Spectral measurements validated the hypothesis that the reverse micelles can provide an aqueous environment that allows the amino acid to flex and bend, with minimal water to absorb the terahertz radiation. The terahertz measurements on this simple biomolecule compared well with other studies, further validating the technique.

For more information, contact Edwin Heilweil.

Shape Evolution of Nanoscale Patterns

Line profiles of a pattern made by nanoimprint lithography as measured by scatterometry during the annealing process.

Division researchers successfully demonstrated the application of scatterometry to monitor, in situ, pattern profiles made by thermal embossing nanoimprint lithography. In addition to obtaining real-time measurement of pattern profiles, researchers also gained new insight into the evolution of patterns imprinted in polymers of different molecular weight.

Thermal embossing nanoimprint lithography uses a mold to stamp a nanoscale pattern into a polymer under heat and pressure. This simple process is a low-cost alternative to photolithography, with high throughput and high resolution. This research studies the process to better understand how the shape of the imprinted pattern evolves during thermal annealing, enabling optimization of the imprint process.

Researchers in the Optical Technology, Surface and Microanalysis Science, and Polymer Divisions collaborated to use scatterometry, which combines spectroscopic ellipsometry with rigorous coupled-wave analysis, to extract topographical information from the optical measurement. Scatterometry determines, in situ, the same information about pattern height and shape as the traditional ex situ methods, such as AFM, SEM, and spectral x-ray reflectivity. It also provides a complete record of pattern evolution during annealing of a single sample, whereas ex situ methods require preparation of many samples, annealed for various times, to build up a picture of the pattern decay. Further, scatterometry may provide more sensitive measurements of nanoscale features in the pattern cross-section.

The researchers had expected that patterns imprinted in polymers of high molecular weight to change relatively slowly. However, they observed that the patterns in low molecular weight polymer initially appeared more resistant to change, while the patterns in high molecular weight polymer showed a much faster initial relaxation, consistent with measurements made using ex situ techniques. Furthermore, the scatterometry measurements indicated subtle differences in shape between the annealed high and low molecular weight polymers that were difficult to discern using other methods.

For more information, contact Thomas Germer.

Advances in Hyperspectral Image Projection

Hyperspectral image of coral reef
Image of a coral reef projected by the HIP (above), and the basis functions used for the projected image (below).

Measuring global climate change with remote sensing instruments requires excellent knowledge of their sensors’ performance. For such evaluations, Division scientists have successfully demonstrated the projection capabilities of a Hyperspectral Image Projector (HIP). Using this device, we can generate images in the laboratory with precisely tailored spectra for each pixel, simulating actual scenes in nature without the complications of changing field conditions.

This ability to specify spectra, not just color, is what differentiates the HIP from a common video projector. While the three spectral bands (“red,” “green,” and “blue”) of a video projector are sufficient to create color for the human eye, earth-science cameras need more spectrally detailed test images for their complete calibration.

To demonstrate the HIP’s capability, researchers chose a hyperspectral image of a coral reef located off the coast of Puerto Rico and acquired by an airborne hyperspectral sensor. This image, provided by the University of Puerto Rico at Mayagüez, is an ideal subject for emulating a spectrally and spatially complex scene. We recreated the image using the HIP, and recollected the image using a laboratory imaging spectrometer as a proxy for the type of devices used in the field.

This demonstration also revealed the HIP’s potential for other applications. A HIP could be used to determine the threshold of detection of changes in scenes. This is a key area of concern for measurement of regional and global climate change. The HIP may also serve as a tool in the development of medical imaging by providing a standard tissue phantom for medical imagers.

For more information, contact Joseph Rice.

Near-Infrared Reflectance Standards for Satellite Remote Sensing

Field Measurements of reflectance standard
A researcher measures the solar radiance from a calibration plaque reflectance standard prior to measuring the reflectance of the terrain.

Accurate reflectance standards in the solar radiation band from the ultraviolet (250 nm) through the shortwave infrared (2500 nm) are important for calibrating satellite measurements of surface albedo, atmospheric aerosols, ocean color, and other environmental variables for weather, climate, and geospatial imagery applications. A new method has been developed to determine the reflectance factor for white polytetrafluoroethylene (PTFE) diffuse reflectance standards from 1100 nm to 2500 nm at the 0°/45° geometry (0° incident and 45° reflected angles).

Diffuse reflectance standards are used, for example, to calibrate the spectral reflectance of solar-illuminated PTFE diffusers deployed on the Moderate-resolution Imaging Spectroradiometer (MODIS) and the Seaviewing Wide Field-of-view Sensor (SeaWiFS) satellite systems, and planned for the future Visible  and Infrared Imaging and Radiometry Suite (VIIRS), Advanced Baseline Imager (ABI), and Operational Land Imager (OLI) satellite missions. They are also important for calibrating ground-based measurements of surface reflectance used to validated and calibrate satellite measurements. An example of such a groundbased measurement is presented in Fig. 7.

This work extended the long-wavelength limit of the NIST reflectance factor scale from 1100 nm to 2500 nm, allowing NIST to provide white reflectance standards with low measurement uncertainties over the entire solar band. The uncertainties of the absolute diffuse (non-specular) reflectances are < 1 % from 1100 nm to 2450 nm and 2.5 % at 2500 nm.

The major challenge in measuring diffuse, non-specular reflectance in the short-wave infrared is the low reflected signal. The present effort exploits advances in extended-range indium gallium arsenide detectors and NIST-designed low-noise preamplifiers that together allow noise equivalent powers of approximately 15 fW with a 1 s time constant.

For more information, contact David Allen.

Tuning the Properties of Engineered Cobalt Nanoparticles

Cobalt Nanoparticles
Transmission Electron Microscope (TEM) images of cobalt nanocubes (upper) and spherical Co nanoparticles (lower) at different stages: growing (left), freshly prepared (middle), and aged (right).

Cobalt nanoparticles possess large magnetic moments and unique catalytic properties. Potential applications for these nanomaterials exist in information storage, energy, and medicine.

We have applied a colloidal synthesis route to obtain monodisperse and highly crystalline cobalt nanoparticles. Our results demonstrate that the properties of these engineered cobalt nanoparticles can be tuned by changing the shape of cobalt nanoparticles from spherical to cubic, and by simply aging cobalt colloids in air.

The properties of cobalt nanocubes were compared with their spherical counterparts (see figure). One striking difference was their relaxation behavior upon the removal of external magnetic fields. As the strength of the external field is gradually reduced to zero, the magnetization of the spherical nanoparticles also decreases gradually. The magnetization of nanocubes, on the other hand, decreases in a much slower fashion until the formation of vortex states, resulting in a sudden drop in their magnetization.

The other difference is their stability upon the exposure to the illuminating electron beam. While the spherical nanoparticles (15 nm diameter) remain intact, the nanocubes (50 nm across) melt together and form nanowires. The smaller particles are expected to have a lower melting point. However, the sharp edges and corners in the nanocubes could be the locations that initiate melting.

Transmission electron microscopy and magnetic measurements have been combined to probe the growth and aging of spherical colloidal cobalt nanoparticles (figure, lower panels). While their growth process involves the conversion of diamagnetic and paramagnetic intermediates, and small, weakly interacting nanoparticles into larger, strongly interacting nanoparticles, their aging process involves surface oxidation, resulting in a hysteresis loop shift (exchange bias) in their magnetic measurements. The exchange bias created by this simple aging process can be used to tune the nanoparticles’ magnetic properties and to improve the thermal stability of their magnetic moments.

For more information, contact Angela Hight Walker.


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