Nanoparticle movement: Plasmonic forces and physical constraints

Abstract

Nanoparticle structures observed in aberration-corrected electron microscopes exhibit many types of behavior, some of which are dominated by intrinsic conditions, unrelated to the microscope environment. Some behaviors are clearly driven by the electron beam, however, and the question arises as to whether these are similar to intrinsic mechanisms, useful for understanding nanoscale behavior, or whether they should be regarded as unwanted modification of as-built specimens. We have studied a particular kind of beam–specimen interaction – plasmon dielectric forces caused by the electric fields imposed by a passing swift electron – identifying four types of forced motion, including both attractive and repulsive forces on single nanoparticles, and coalescent and non-coalescent forces in groups of two or more nanoparticles. We suggest that these forces might be useful for deliberate electron beam guided movement of nanoparticles.

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.