Trends in high spatial high spectral resolution material characterization A. Passian, L. Tetard, R. Farahi, B. Davison, A. Lereu, T. Thundat, S. Gleason, K. Tobin Measurement Science and Systems Engineering Division (MSSE) Oak Ridge National Laboratory, Oak Ridge, TN USA
[email protected] Morphological and spectral properties of surfaces and interfaces are often closely related at small spatial scales. In the case of nanomaterials, such as those currently used in medicine, electronics, aeronautics, automotives, and energy an understanding of the physical properties is required often at the scale of these novel materials. These circumstances have driven the development of new analytical tools. Given the very small dimensions, concentrations, quantities, densities etc., a key challenge is the quantitative characterization of these engineered or natural, chemical or biological materials within the composite material or biological system. Two fundamental properties of interest – location and identification – demand high spatial resolution (both surface and subsurface) and chemical speciation. While high resolution may benefit from scanning probe microscopy technologies, chemical speciation may be realized through spectroscopy. In this paper we discuss some potential approaches to achieving high spatial and high spectral material characterization. State-of-the-art near-field and remote-standoff instrumentation for diverse applications in biofuel production, nanotoxicology, plasmon nano-devices, food quality control, and chemical threat agent detection are discussed. Keywords-microscopy, spectroscopy, subsurface, standoff, nanomaterial, nanotoxicology, biofuel, atomic force microscopy, near-field scanning optical microscopy
I.
INTRODUCTION
Undoubtedly, modern measurement technologies and tools are heavily impacted by a need to provide an understanding of material properties at scales relevant to the development of applications that can uniquely use specific properties to achieve a function or perform a task. Visual inspection has always been an important first approach to understanding materials. However, historically understanding the atomic fabric of materials was not instrumental for the evolution of our species rendering our eyesight limited to about 0.35 mm. As a result, optical microscopy broke this first visual barrier, only to be declared limited by the diffraction to half of the wavelength of the light as stated by Abbe [1]. Many attempts have been since made to break Abbe’s barrier resulting in Scanning Tunneling Microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, Photon Scanning Tunneling Microscopy, Near-Field Scanning Optical Microscopy (NSOM), Atomic Force Microscopy (AFM), going beyond the diffraction limit. Probe based microscopy
further offered the potential advantage of nondestructive approaches where the sample is not disturbed. However, three immediate challenges continue to resurface in many applications: 1) acquiring sample chemical information, 2) lack of physical access to the sample, and 3) acquiring sample subsurface information. This paper addresses how some potential approaches can overcome these challenges. Section II describes hyperspectral mode synthesizing atomic force microscopy (hMSAFM) for nanoscale point contact characterization. In doing so, we first describe our imaging methodologies and their potential applications in biofuel energy research and nanotoxicology. Imaging providing physical and mechanical properties such as morphology and elastic response of the materials is augmented with novel spectroscopic approaches to provide chemical information. Many future devices will require a suite of integrated tools, such as lab-on-a chip. In Section III, we therefore describe how plasmonics may aid the creation of such integrated tools. In cases where physical contact with the sample is not an option, reliable tools are needed to obtain information remotely. Such processes may provide information ranging from simple false/positive detection of a given known species to a more complex chemical and spectral recognition and identification of remote object surface and subsurface properties. In Section IV, we discuss remote hyperspectral imaging (RHI) for standoff spectral investigations. Here we discuss potential applications in quality control and detection of chemical and biological threat agents as examples. Finally, concluding remarks are provided in Section V. II.
MODE SYNTHESIZING ATOMIC FORCE MICROSCOPY
The ability to noninvasively explore subsurface and surface composition and inhomogeneities in a given sample constitutes a major challenge in material characterization. Understanding the composition and behavior of complex systems requires access to physical, structural and chemical information with high spatial resolution. The AFM provides a powerful approach to noninvasive nanoscale characterization. AFM imaging is based on the scanning of a sharp tip (tens of nanometers in diameter) located at the end of a microcantilever probe, over the surface of the sample. The tipsample interaction and the consequent changes in the probe’s elastic state are monitored by a sensitive read-out technique. The resulting signal is then converted into a contrast map or an
image, that can be interpreted as a spatial representation of the forces measured at each point of the scanned area. The AFM can reach atomic resolution, offering important insight on the morphology and physical properties of the surface of the sample. However, the success of AFM has predominantly been limited to surfaces and physically accessible interfaces. Recently, we have demonstrated that the AFM can be customized to reach subsurface information. By combining the high spatial resolution of the AFM and the nondestructive nature of propagation and scattering of acoustic waves, we successfully demonstrated its potential for subsurface imaging and coined the term Mode Synthesizing Atomic Force Microscopy (MSAFM), shown in Fig. 1. MSAFM operates on the basis of increasing the dynamic attributes of the probe and the sample of an AFM to boost the frequency content of the system. Actuators, located at the base of the cantilever and the sample, exert mechanical forcings on the probe and the substrate, respectively. The non-linear nature of the tip-sample interaction causes sum and difference mixing of the frequencies in the system, creating a wealth of new operational modes for imaging [2].
Figure 1. Probe-sample excitation in mode synthesizing atomic force microscopy. In MSAFM, both probe and the sample to be examined are mechanically excited into oscillations at multiple frequencies. The nonlinear tip-surface interaction then generates a host of mixed frequencies that can be used for surface and subsurface imaging. The figure shows possible deformation eigenmodes of the triangular probe and the sample.
Such capabilities have diverse applications, for example in renewable energy research and nanotoxicology, as described below. A. Biofuel applications Recent trends in energy production continue to dictate needs for sophisticated scientific instrumentation that encompasses a number of research areas. Biofuel energy production from renewable sources such as biomass is one such trend that has been particularly emphasized by the US DOE [3]. One outstanding challenge has been the efficient extraction of cellulose contained in the plant cell walls [4]. To overcome this challenge, instruments that would aid plant biologists to better examine complex samples at high spatial and spectral resolutions in a nondestructive manner would be of high priority. The factors limiting the penetration of enzymes and chemicals used to release cellulose in biomass BESC. (sponsors)
involve the nanoscale structure of the plant cell wall: ligninhemicellulose coating of the cellulosic microfibrils, cellulose crystallinity, etc. However, a detailed model of the processes at the molecular level is still lacking. Based on this need, we performed MSAFM imaging, a technique that allows more intensive mechanical characterization of materials at the nanoscale, on cross sections of Populus stem samples. This preliminary work revealed the complexity of the system underpinning the need for specific complementary characterization of the samples for mechanical, structural and chemical information within the cell wall. The cell wall of fresh Populus stem sample is presented in Fig. 2. Interestingly, the MSAFM higher resolution images (bottom) exhibit different features within the cell wall. Although chemical characterization is beyond the scope of this article, the results will be presented elsewhere.
Figure 2. Analysis of cross sections of fresh poplar wood with various modes of MSAFM.
B. Nanotoxicology applications Engineered nanomaterial is at the heart of one the most exciting research opportunities in the field of Nanotoxicology [5][6][7]. We have shown that MSAFM (Fig. 3) can play an important role in this rapidly developing field, as for example highlighted by the NIEHS of NIH [5][6][7]. Our preliminary results (Fig. 4) have shown that to understand the deposition of nanoparticles within cell structure of animal cells, it is necessary to have the capability to probe sub-cellular features, to access subsurface information, and to have capabilities to study the chemical composition of the features detected. Additional measurements using micro-Raman spectroscopy were performed on the samples. The results have been reported in [5] and corroborate the presence of carbon nanohorns inside the cells. Furthermore, the MSAFM was shown to be capable of producing size-distribution for the intracellular populations of nanoparticles.
III.
Figure 3. Seeing below the cell surface. The dynamics of the cantilever is monitored using an optical readout system (laser). The signal is analyzed in the lockin amplifier using the difference frequency as reference. The images emerging from MSAFM reveal the presence of nanoparticles below the cell surface.
However, higher spatial resolution for the spectroscopic measurements will be necessary for advanced studies on such systems. The potential adverse effects of engineered nanomaterials raise the same concern in plant biology. The repercussion of this work reaches important areas of interest to major research programs (see for example US-DOE roadmap).
PLASMONIC NANO-DEVICES
Noble metal nanoparticles are well-known for their strong tunable optical coupling to surface plasmons governed by the material and geometry of the nanoparticles. This phenomenon opens up opportunities for nanoparticles to be advantageously used for the optical control of very small scale physical processes, including plasmonic-induced thermal processes, in future nano-devices with applications such as biosensing and data communications. For example, as the incremental benefits of downscaling electronic integrated circuits continue to become more limited, plasmonic nano-circuits composed of a few noble metal nanoparticles could become the technology of choice for the next generation of integrated circuits. To this end, we developed the concept of thermoplasmonics to deal with the optical and electronic excitation of nanoparticles and their thermal consequences [4][9][10][11][12][13][14][15] relevant to plasmonic nano-devices. The development of these nano-devices also requires specialized tools capable of stimulating, probing, and manipulating the nano-device in order to study and understand their characteristics. Fortunately near-field microscopy has the necessary versatility for such nanoscale characterization, with advanced photon scanning tunneling microscopy (PSTM) and near-field scanning optical microscopy (NSOM) under development shown in Fig. 5. The delivery and collection of highly localized light from a probe, positioned in the near-field regime of a sample surface, enable optical microscopy and spectroscopy with resolution beyond the diffraction limit of the illuminating light. Various types of probes can be precisely controlled to stimulate and interrogate the nano-device with highly confined electromagnetic energy.
Figure 5. Principle of advanced near-field microscopy. Various types of probes are currently under development for electromagnetic confinement and enhancement that would aid single molecule spectroscopy and imaging. Use of nanostructured surfaces and nanoparticles in conjunction with geometrically designed resonant nanoantenna probes allow a host of detection and characterization schemes. Figure 4. MSAFM images revealing nanoparticles within the cells. Topography (A) and MSAFM (B) images of macrophages exposed to silica nanoparticles. Topography (C) and MSAFM (D) images of macrophages exposed to carbon nanohorns.
Nano-devices such as nanoantennas have received recent attention for their potential applications in sensing attributable to their confined and enhanced local electric field distribution due to excitation of surface plasmons. In Fig. 6, a near-field study of gold nano-rods confirms the polarization dependence of local field enhancement, where NSOM provides a confined
polarized light source and simultaneously measures the optical response. Optical reflection, optical transmission, and atomic force topography images of nano-rods aligned with their major axis of symmetry parallel to the polarization (Fig. 6 top) and perpendicular to the polarization (Fig. 6 bottom) are shown. The results show greater field enhancement in the parallel case as expected, with the transmission images in particular demonstrating this effect.
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Figure 7. Schematics of the principle of remote hyperspectral imaging. The distance, d, between the excitation source/detection plane and the target plane can range from millimeters to meters.
Figure 6. Gold nanoantenna arrays stimulated by a near-field optical probe. The local field distributions due to the creation of surface plasmons optically excited by a localized quasi-linear light source (shown as yellow arrows) reveal polarization dependency.
IV.
REMOTE HYPERSPECTRAL IMAGING
Detection tools that can be used outside a laboratory setting in the field, and be able to accurately and rapidly detect trace amounts of substances without requiring precise handling of the sample to be examined is a major challenge. In order to be feasible, the working distance between the detector and the target cannot be in the near-field regime, but in the range of millimeters to meters for “standoff” operation. A versatile instrument capable of standoff detection of chemical and biological agents with a working distance (d), called remote hyperspectral imaging (RHI), is under development at ORNL. Shown in Fig. 7, a remote radiation source emitting a range of wavelengths, S, is aimed at a target plane with suspected surface contaminant. The scattered radiation is captured by a remote detector, D. The analysis of the obtained reflectance spectra enables the identification of the contaminant at the illumination spot. The spot is raster scanned over an area of the target to produce an image representing the spectra at each point, called hyperspectral imaging. The RHI technology has potential security applications in food and water safety, as well as threat agent containment.
A. Food quality control application The development of new diagnostic tools to detect chemical and biological agents in the food and water supply has been recognized as a critical security objective [18]. Chemical agents that can be introduced into food and water include toxins and pesticides. Food-borne and water-borne viral and bacterial pathogens include influenza, Salmonella, Escherichia coli (E. coli) and Listeria monocytogenes. Effective technologies for early detection of these chemical and biological agents would need to be able to rapidly identify the target analyte at the site of the suspected contamination.
Figure 8. Background-corrected transmission FTIR spectroscopy of bacteria incubated for 24 hrs and dried on a ZnSe substrate. The legend is described as follows: top10 – E. coli TOP10, 10625 – Citrobacter braakii Brenner et al. deposited as Salmonella sp. type Ballerup ATCC® No: 10625, 53647 – Salmonella enterica subsp. enterica ATCC® No: 53647, 53648 – Salmonella enterica subsp. enterica ATCC® No: 53648, 700720 – Salmonella enterica subsp. enterica ATCC® No: 700720, baa836 – Salmonella subterranean ATCC® Number: BAA-836, cn32 – Shewanella putrefaciens CN32, dh5a – E. coli DH5alpha, mr1 – Shewanella oneidensis MR-1
Spectroscopy can be a viable speciation method for identification, as shown in Fig. 8, where FTIR spectroscopy of various strains of Salmonella and other bacteria may be used
to identify the microorganisms. Leveraging from the evolving body of knowledge of spectra of microorganisms and chemical compounds, RHI can be a potentially valuable instrument for laboratory, industrial and consumer monitoring of food quality. B. Standoff chemical and biological threat agent detection application The control of substances, both chemical and biological, at the numerous ports of entry into the United States is a major security initiative. Chemical and biological threat agents have the potential to be dissiminated in large areas, affecting populations with highly toxic effects. Chemical threat agents target the vesicant, blood, nerve and the respiratory system. They include, but are not restricted to, phosgenes, Sarin, cyanogen compounds, etc. Biological threat agents consist of bacteria, viruses, spores and toxins. Bacillus anthracis, also known as anthrax, Smallpox, Ricin have been under close scrutiny in the past few years. As part of a comprehensive effort in managing port security, new technologies for rapidly and accurately detecting trace amounts of controlled substances are important. This is a complex problem due to the multiple requirements a standoff sensor should meet in order to be viable for general use, in particular sensitivity and selectivity, low false positive, possibility of long-range systems, study of mixtures, real-time monitoring, safety of use (eye, skin), possibility for use as a surveillance device. RHI technology is a promising candidate to detect an agent, characterize it (chemical fingerprint and quantity), and create a spatial mapping of the chemical content. In addition the technology can potentially be used in various environment, in particular gaseous or liquid environments. V.
CONCLUSION
Advancements in microscopy and spectroscopy tools continue to be made in conjunction with the new challenges in nanoscience and nanotechnology, specifically in fields such as pharmaceuticals, electronics, forensics, security, and fabrication. Some of these challenges are the subsurface detection and imaging of nanomaterials in a biological system, the development of plasmonic nano-devices, the rapid detection of pathogens in food, and the remote detection of explosives. Undoubtedly, the future instruments to address the existing problems such as those discussed here will have to operate on multiple spatial and temporal scales, be of high spectral resolution and sensitivity, withstand noisy and/or harsh environments. In a typical future measurement scenario, based on the approaches described here, one may envision the detection and spectral surveillance of a single 5nm nanoparticle taken up by a live cell in a fluidic environment on time scales close to those dictated by the stochastic nature of the system and/or by the chemical reaction rates of the cellular response. The technologies addressed here play an important
role in the future of instrumentation for material characterization in general and biological studies in particular. ACKNOWLEDGMENT This research was sponsored in part by the Oak Ridge National Laboratory (ORNL) BioEnergy Science Center (BESC) and in part by the Laboratory Directed Research and Development Program of ORNL. The BioEnergy Science Center is a US Department of Energy (DOE) Bioenergy Research Center supported by the Office of Biological and Environmental Research in the DOE Office of Science. ORNL is managed by UT-Battelle, LLC, for the US DOE under contract DE-AC05-00OR22725. REFERENCES [1] [2]
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