Nanoparticle movement: Plasmonic forces and physical constraints
Highlights
► We investigate the interaction of metal nanoparticles with a high energy electron beam. ► We find forces ranging from 0.1 to 50 pN forces between the metal particles and the beam. ► At moderate distances, dielectric forces are usually small and attractive. ► At sub-Nm distances the forces become repulsive, pushing nanoparticles away from the electron beam. ► While the repulsive behavior is predicted by electromagnetic theory, the detailed origin of the behavior is not yet understood.
Introduction
For many years, electron microscopy in materials science followed two essentially different paths: high magnification imaging of crystalline structures, defined and guided by the conceptual development of Scherzer [1], and microbeam analysis, aimed at obtaining the composition and chemistry of small specimen regions, following the early work of Hillier and Baker [2]. During an exciting and productive time in the 1970s Albert Crewe and his group knit these two fields together by introducing concepts and instrumentation which combined structural imaging and elemental characterization in the same instrument [3], [4]. Further, they demonstrated a capability to image single atoms [5], and introduced methods for understanding that imaging within the context of already existing theory [6], cementing the fundamental unity embodied in the present day Transmission Electron Microscope (TEM) and Scanning Transmission Electron Microscope (STEM). This work contributed greatly to the development of more precise, quantitative methods which today allow us to identify and locate atoms within extended structures through the use of aberration correction to produce sub-Ångstrom probes [7], [8], [9] and through advanced EELS equipment to identify the local bonding environments using Electron Energy Loss Spectroscopy (EELS) in both non-aberration-corrected [10] and aberration-corrected instrumentation [11].
Thus, the modern STEM is increasingly a quantitative tool intended to provide understanding of the relationship among structure, composition, bonding, and function. With the addition of aberration correction, this work is being extended to the investigation of structure–function relationships within small groups of atoms. A good example of this is in the recent identification of catalytic behavior of a single atom [12]. An enabling feature of the aberration corrected system is the very high contrast available in single atom signals, which allows inspection of slowly evolving atomic level processes within the microscope [7], [13]. These investigations are still restricted to time scales of seconds, but it seems not too far in the future that optical excitation of a high brightness source may allow recent very high temporal resolution experiments [14], [15] to be extended to the atomic level.
Recently, we discussed an ubiquitous feature of aberration corrected electron microscopy: atoms and nanoscale objects are often in motion under the electron beam [13]. While this statement may seem a bit surprising to most microscopists, it has been a prominent part of reports on small particle imaging in the past [16], [17]. The movement of atoms under the electron beam is an easy finding to accept, but motion of nanoscale objects was greeted with many questions. What are the primary mechanisms that produce motion? Should electron beam driven motion be considered a damage mechanism, or a variation of naturally occurring motion, a source of additional information about the specimen? With recent additional reports of interesting movement in the microscope [18], [19], these questions have become more interesting and important to address.
We have looked in detail at one type of force in order to understand a primary finding that violent coalescence of well separated nanoscale metal particles can be driven by the dielectric polarization of particle pairs by a passing swift electron. We call these plasmonic forces because much of their spectral strength lies in coupled electromagnetic fields that have plasmonic origin. But they belong to the same family of forces that hold all materials together, including London Dispersion forces, Van der Waals, Debye and other forces that are central to the dynamic behavior of molecules during self-assembly, that transport complex molecules in biology, and that enable DNA and protein measurement using nanopore structures.
Section snippets
Experimental observations
These experiments were performed in a third order aberration corrected VG Microscopes STEM, using 120 keV acceleration energy [7]. The beam current varied from 50–150 pA using a 0.8 Å beam size, which is scanned in a raster fashion to produce 200 ms exposures. Therefore one measure of beam current density is 1×1010 amps/m2 in the small probe, or 1×104 amps/m2 averaged over the image area of 1024×1024 pixels. These are somewhat larger than typical STEM beam current densities used in materials science,
Theory
The behavior described above is apparently driven by an aloof electron beam, and so it is highly likely to be a result of polarization response fields, induced by the field of the passing electron. Other processes might also operate. For instance, electron illumination may raise the local temperature, producing instability, rotation, and structural changes [24]. Significant lateral momentum also may be transferred by electron diffraction, or secondary electron emission. Energy may be injected
Constrained movement
We have seen a few instances where the nanometer Au particle behavior is clearly constrained by local configurations of other polarizable objects. We see that the particles do not accelerate, but move at more or less constant velocity while under an electron beam having fixed flux density. And we see that details of motion, while loosely controllable by the electron beam, also are strongly influenced by the local environment.
In Fig. 9 we show a remarkable sequence wherein particle coalescence
Conclusions
We have discussed in some detail here mechanisms for atomic transport and nanoparticle movement under observation in the electron microscope. These observations have been made feasible by recent advances in aberration correction, but they also follow closely observations made many years ago by Crewe and his co-workers in their pioneering effort to establish Scanning Transmission Electron Microscopy as a quantitative technique that combined the best of electron optical imaging with atom specific
Acknowledgements
We acknowledge financial support from the Basic Energy Sciences Division of the Department of Energy, Award #DE-SC0005132, the Department of Industry of the Basque Government through the ETORTEK project inano, the Spanish Ministerio de Ciencia e Innovaciòn through Project No. FIS2010-19609-C02-01, and the Consejo Nacional de Ciencia y tecnología (Mexico) through Project No. 82073.
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