materials & manufacturing research areas
By utilizing the latest electro-optics technology in conjunction with classical optical principles, we achieve solutions to a wide array of problems in the military, academia, and industry.
(1) Surface Enhanced Raman Spectroscopy (SERS)
Raman spectroscopy is a well established technique for research in molecular structure and for unique identification of chemical compounds. Although Raman scattering is inherently very weak, recent advances in enhancement by roughened surfaces have shown the potential for detection of trace amounts of chemicals, even down to the single molecule level. However, construction of surfaces with good enhancement properties has always been a trial and error process. In order to get more reliable and repeatable results, we are developing nano-scale fabrication techniques to consistently produce surfaces with strong enhancement characteristics. The design of surfaces for high sensitivity detection of taggant and tracer chemicals is supported by the Office of Naval Research. Complementary work on detection of chemical warfare agents is supported by the Marine Corps Multi-Sensor Analyzer / Detector Program.
The phenomenon we propose to exploit for taggant detection is surface enhanced Raman spectroscopy (SERS). Kniepp’s recent review provides a thorough account of its discovery, historical advances in experimental findings and theoretical understanding of the phenomenon, and current research thrusts. In 1974, it was discovered that the Raman scattering signal of certain compounds could be enhanced by orders of magnitude proximate to metallic surfaces that have been roughened on the scale of tens to hundreds of nanometers. In the intervening time a great deal of scientific inquiry has gone into the theoretical understanding of the enhancement mechanisms responsible for the observed SERS effect; there has also been a large experimental effort to develop various methods of fabricating so-called SERS-active substrates from silver, gold, and a few other materials, and to find additional compounds whose Raman spectra display the SERS effect. Enhancements in Raman signatures of 103 -106 are common in the literature, and enhancements as great as 1014 have been reported. It is even claimed that single molecule detection may be possible.
Because the Raman signature of every molecular species is unique, it should be possible to track a large number of taggants independently with a single instrument. The technique has the potential to provide unparalleled sensitivity and selectivity in a system configured for field use, as was recently demonstrated by a research group at Oak Ridge National Laboratory that fabricated a field-portable SERS-based CW agent detector and demonstrated its efficacy against an organophosphate pesticide (3,4). Also, because the Raman effect does not constrain the illumination source to certain particular frequencies, the excitation wavelength can be selected for engineering robustness, commercial availability, and affordability.
Based on current theories and the results of certain landmark experiments, the astounding intensity of SERS spectra is believed to arise from a combination of two mechanisms, which are illustrated in the sequence of figures 1-3 below.
Figure 1. - Description of SERS-active Substrate Prepared by Electrochemical Roughening.
Here, One “Hot Spot” Happens To Fall Within The Area Illuminated By The Laser.
Figure 2. - Close up of The “Hot Spot” Illustrating Electromagnetic Enhancement.
Figure 3 - A Closer View Of The Surface Near One Of The Peaks, Where Adsorption
Of The Analyte Facilitates Chemical Enhancement.
Figure 1 illustrates a SERS-active substrate prepared by electrochemical roughening, which yields a random pattern that happens to include some “hot spots” where surface plasmons resonate with the incident radiation–a phenomenon that has been termed Surface Plasmon Resonance (SPR). As is illustrated in figure 2, analyte molecules in this enhanced electromagnetic field are subjected to stronger polarizing effects and thereby support Raman scattering with higher efficiency. Experiments demonstrating SERS enhancements under conditions that were controlled to eliminate the possibility of direct contact between analyte and substrate have lent credence to this concept of electromagnetic enhancement, and based on those experiments and theoretical calculations the SPR is considered to account for the larger component of SERS activity–typically to the order of 103-106. A second effect, called chemical enhancement, is suggested for molecules that become adsorbed onto the surface–thereby coupling the molecule’s valence electron charge density with the substrate. This gives rise to extra bound energy states and presents a greater opportunity for energy coupling with the incident radiation. This mechanism, which is illustrated in figure 3, is not as well established, but is supported by experiments demonstrating enhancement on the order of 101-102 from an analyte adsorbed onto an atomically smooth gold substrate.
The SERS effect selectively enhances the Raman spectra of certain molecular species only, and even certain modes of vibration of a given species, and this fact deserves some elaboration. Operationally, it would be advantageous to have a sensor with preferential sensitivity to the particular compounds or classes of compounds being employed in a taggant application, especially since they must be detected at trace levels against an environmental background that may include higher concentrations of many other compounds. Furthermore, it is the nature of SERS to utilize small areas of substrate at any one time, so it should be feasible to build a field instrument capable of simultaneously scanning some number of substrates having different SERS activity with respect to the pertinent ensemble of taggants and background compounds. This suggests that as experimental findings and theoretical understanding accumulate, it might be possible to engineer SERS-active substrates with preferential sensitivity to certain compounds or classes of compounds. It has already been shown that the enhancement activity of a given SERS substrate toward certain compounds can be modified by coating it with another substance having different adsorption affinities with respect to those analytes.
At present, SERS systems have seen little use in practical, field-portable chemical sensor systems, primarily because of issues associated with the reproducibility of SERS-active substrates. The surface roughness and composition of the sensor surface are of critical importance in obtaining the SERS effect. Multilevel roughness over the scale of ten to a few hundred nanometers is required, and the effect has only been observed on a select group of substrates–chiefly certain metals. Over the past 25 years the general approach to preparing these surfaces has been empirical, with laboratory processes developed to produce surfaces for SERS spectra of analytes of interest. Historically, the first SERS-active substrates were electrochemically-roughened silver electrodes, which strongly enhanced the Raman spectrum of pyridine dissolved in water. Colloidal gold and silver particles suspended in a solution containing certain analytes have proved to be good SERS-active substrates. More recently, groups have succeeded in fabricating SERS-active substrates by depositing silver or gold films over polystyrene nanospheres, lithography, self-assembling nanostructures, and other methods. At this point, there has been no systematic study of the effect of surface characteristics on SERS-activity of substrates.
Because of the lack of a complete, fundamental understanding of the SERS effect, and because of the difficulty in control of surface chemistry and multi-scale morphology, the experience of the community with regard to the reproducibility of SERS-active substrates has been marginal. Despite recent advances in reproducibility, fabrication of high-performance SERS surfaces remains an art form, and results are very inconsistent, even spatially within a single sample of substrate.
(2) Aerosol & Biological Agent Detection
A light-weight, portable device is under development for monitoring the levels of breathable aerosols and for fluorescence detection of biological activity. The focus of this work, which is supported by the Marine Corps Multi-Sensor Analyzer / Detector Program, is to build a low cost, rugged, rapid response instrument as a first alert detector for field operation.
(3) Shearography / Non-Destructive Evaluation
Speckle shear interferometry, or “shearography,” is a highly sensitive method for full field measurement of differential displacement over a surface under perturbation. It is a robust technique that is well suited for use outside of the laboratory environment, and it has been applied to non-destructive evaluation (NDE) for identification of bonding failure in composite structures such as honeycomb and laminated panels. Developments at Penn State ARL on optical phase shifting have led to a greatly improved noise immunity and visualization capability of this technique. In current work, funded under a Cooperative Agreement with NASA Goddard Space Flight Center, new techniques have been developed to extend the applicability of shearography NDE to objects with specular (mirror-like) surfaces and to new types of structural evaluation.
(4) Precision Optical Metrology
Phase Shifted Projected Fringe Profilometry (PSPFP) is a non-contact optical method for high precision measurement of surfaces. As a full-field technique, it offers a high measurement speed over a dense array of points. Resolution is related to the size of the inspection area, scaling from about 1 micron for a 1 cm square up to about 100 microns for a one meter square. A data fusion technique is under development to merge individual measurement patches, enabling the complete surface of an object to be assembled from a set of coordinated viewpoints.
(5) Optical Fiber
Through the Office of Naval Research Mantech Program, we are developing new manufacturing technology to substantially reduce the cost of attaching sensors and associated electrical connections to optical fiber array cables. In addition, a training program on optical fiber technology is being developed for Navy personnel.
(6) Lidar / Remote Sensing
ARL has had a strong involvement in the advancement of lidar technology. Developments include the LAPS system, a ruggedized UV/visible Raman lidar, designed and built for Navy shipboard operation and demonstrated in at-sea trials. This versatile instrument has been used in a number of atmospheric research campaigns for profiling various properties such as water vapor, temperature, ozone, aerosols, and optical extinction, and is capable of ranges of up to 5 kilometers or more. Other investigations include underwater lidar to characterize light scattering and propagation, and surf zone aerosol studies.
(7) Laser Induced Breakdown Spectroscopy
Metals and metal oxides have long been added to paints for many purposes including pigmentation, film strength, spreading quality, and weathering resistance. However, studies in the past thirty years have shed light on the dangers of many common metal-containing paints. The most notorious metal is of course lead, but others, such as chromium and cadmium, pose health dangers. Danger is presented to workers when paint is disturbed and made airborne, usually during removal, and it enters the body either through direct ingestion or breathing. On an industrial site, work is often delayed because the paint composition on a structure is unknown. To minimize dangerous exposure yet keep downtime to a minimum, there is a need to accurately determine the paint’s heavy metal content, on-site, in a timely manner.
The only in situ methods available for testing paint are X-Ray Fluorescence (XRF) and chemical spot tests. XRF is susceptible to errors from metal substrates, and layers of lead-free paint that contain other metals can obscure lead-bearing paint underneath. In addition, all radioactive sources must be federally registered with specially certified operators, and must be properly disposed of and replaced annually. Chemical spot tests, containing a rhodizonate dye reagent that changes color in the presence of lead, are relatively inexpensive and readily available. The degree of color change is difficult to quantify, however, and lead-bearing paint will not be detected if other paint layers are above it. Compositional analysis techniques, such as atomic absorption or emission spectrophotometry, are conducted in off-site laboratories. These techniques require careful extraction techniques that do not cause sample contamination, and require timely delivery of the samples to the laboratory. Costs incurred due to the delays involved with sample collection, transport, and preparation can be prohibitive.
The prototype LIBS system.
The portable LIBS system accurately measured the amount of lead, cadmium, and chromium from samples prepared by NIST and PSU/ARL. Detection limits were determined to be 0.5 wt % for cadmium and lead, and 1 wt % for chromium. These limits are acceptable since the trigger for abatement procedures is typically 1 wt %. The portable system was demonstrated at a military facility on a painted vehicle. Results showed the presence of chromium in two particular paints, and no lead or cadmium in any. Two other potential applications–identification of scrap metals for sorting, and measurement of heavy metals in grit blast for proper material handling and disposal–were suggested. The ability to quickly determine accurate compositional data for a variety of harmful metals in a portable system makes LIBS an important tool for in situ paint analysis.
For this work, a portable LIBS system was developed and the study was narrowed to three particular metals: lead, cadmium, and chromium. Intended for use in shipyards, LIBS would be a screening tool where a worker would take measurements to determine the presence of chosen metals. If the paint contains these metals, a decision would be made to enforce abatement and/or protection procedures.