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Gas Cluster Ion Beams for Secondary Ion Mass Spectrometry AC11CH02_Winograd ARI 3 May 2018 15:10 Annual Review of Analytical Chemistry Gas Cluster Ion Beams for Secondary Ion Mass Spectrometry Nicholas Winograd Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, USA; email: [email protected] Annu. Rev. Anal. Chem. 2018. 11:29–48 Keywords First published as a Review in Advance on bioimaging, cluster ion beams, phospholipids, instrumentation, molecular February 28, 2018 depth profiling, molecular dynamics computer simulations The Annual Review of Analytical Chemistry is online at anchem.annualreviews.org Abstract https://doi.org/10.1146/annurev-anchem- Gas cluster ion beams (GCIBs) provide new opportunities for bioimaging 061516-045249 Annual Rev. Anal. Chem. 2018.11:29-48. Downloaded from www.annualreviews.org and molecular depth profiling with secondary ion mass spectrometry (SIMS). Access provided by Pennsylvania State University on 02/26/19. For personal use only. Copyright c 2018 by Annual Reviews. These beams, consisting of clusters containing thousands of particles, ini- All rights reserved tiate desorption of target molecules with high yield and minimal fragmen- tation. This review emphasizes the unique opportunities for implementing these sources, especially for bioimaging applications. Theoretical aspects of ANNUAL REVIEWS Further the cluster ion/solid interaction are developed to maximize conditions for Click here to view this article's online features: successful mass spectrometry. In addition, the history of how GCIBs have • Download figures as PPT slides become practical laboratory tools is reviewed. Special emphasis is placed on • Navigate linked references • Download citations the versatility of these sources, as size, kinetic energy, and chemical compo- • Explore related articles • Search keywords sition can be varied easily to maximize lateral resolution, hopefully to less than 1 micron, and to maximize ionization efficiency. Recent examples of bioimaging applications are also presented. 29 AC11CH02_Winograd ARI 3 May 2018 15:10 INTRODUCTION Since the discovery by J.J. Thompson over 100 years ago that charged particles could be emitted SIMS: secondary ion from surfaces (1), secondary ion mass spectrometry (SIMS) has become a mainstay for materials mass spectrometry, in characterization. Currently, the technique is unique in its ability to provide surface-specific molec- which secondary ions ular information, to acquire spatially resolved mass spectra with submicron resolution, and to ac- are generated from quire in-depth composition of molecular solids with nanometer resolution. The imaging modality primary ion bombardment is particularly interesting. Originally developed in the SIMS community using focused atomic ion beams to define position (2), the method provides unprecedented chemically specific information at each pixel. Moreover, by stacking images acquired at different sample depths, three-dimensional (3D) information is forthcoming. The SIMS community has retained an unusual buoyancy through the decades, and the recent emergence of cluster ion beams to initiate desorption has had a remarkable effect. With multiple atoms comprising the primary ion, chemical damage to the sam- ple is reduced because each atom carries a smaller share of the incident kinetic energy. Molecular fragmentation is reduced during the desorption process, resulting in nearly fragment-free mass spectra. Molecular depth profiling of complex materials has allowed subsurface information to be acquired for the first time. When combined with imaging, 3D information on the nanoscale is possible to achieve. Cluster projectiles have placed molecular SIMS spectral capabilities, when the m/z < 3,000, nearly on par with other stimulated desorption techniques that do not have these unique aspects (3–5). An excellent comprehensive treatise on this topic has recently been published (6). Although countless cluster projectile varieties have been proposed, evaluated, and espoused, the implementation of gas cluster ion beams (GCIBs) has a special allure. These clusters, created during supersonic expansion, generate clusters that consist of ∼1,000 to >10,000 component atoms or molecules. To allow for beam focusing and to provide sufficient kinetic energy to initiate molecular desorption, these beams are typically accelerated to >10 keV of kinetic energy. Because kinetic energy divides equally among constituents (7), the kinetic energy of each particle is reduced to a value comparable to chemical bond strengths. Hence, molecular fragmentation, subsurface damage, and interlayer mixing are reduced. There is a fundamental difference between GCIBs + + and smaller cluster projectiles such as Bi3 or C60 , where the kinetic energy per atom ranges from a few hundred to a few thousand electron volts. There are practical challenges associated with GCIBs that prevent them from dominating SIMS laboratories. These beams are characterized by a distribution of cluster sizes that renders pulsing for time-of-flight (TOF)-SIMS problematic. Focusing to a submicron spot has not yet been routinely demonstrated, compromising the ability to acquire high spatial resolution chemical images. Moreover, ionization efficiency is often poor even though molecular desorption is highly Annual Rev. Anal. Chem. 2018.11:29-48. Downloaded from www.annualreviews.org Access provided by Pennsylvania State University on 02/26/19. For personal use only. efficient. Because of these issues, most SIMS researchers utilize the GCIB as an effective erosion source for molecular depth profiling but employ the smaller cluster sizes for spectral characteri- zation and imaging. As it turns out, however, none of the aforementioned challenges is without solution. Instrumental developments now allow high-quality mass spectra to be acquired easily with GCIBs, and sources focused to at least one micron are becoming available. The great flexi- bility of tuning the chemistry of the GCIB also has implications for enhancing ionization. In this review, prospects for the exclusive use of GCIBs as the ion source of choice for molecular SIMS experiments are considered by examining each of these factors in detail. THEORETICAL CONSIDERATIONS The emergence of GCIB technology has enormously expanded the space available for projectile choice. Variables include size, composition, energy, and incidence angle. How can we optimize 30 Winograd AC11CH02_Winograd ARI 3 May 2018 15:10 the experiment? Several important properties need to be considered. First, the sample needs to receive sufficient kinetic energy to initiate molecular desorption. Second, the kinetic energy must be low enough so as not to initiate collision-induced fragmentation. Third, there must be sufficient energy provided to the system to allow ionization during desorption when precharged species are not present. If there are no ions, there is no signal. Finally, because 3D imaging is an important goal, the depth resolution during erosion needs to be optimized by tuning either the kinetic energy or the angle of incidence. The basic challenge is, then, how to find the sweet spot that maximizes all of these demands. Is there enough parameter space to find some set of optimized conditions? Can there be a theory that leads us in the right direction? To begin, a basic picture of the cluster-solid interaction is needed. This type of picture is most vividly provided by molecular dynamics computer simulations, which for several decades have provided molecular-level information about the energy dissipation process (8–12). An example that illustrates the unique aspect of the GCIB when compared to other atomic or cluster ion projectiles is shown in Figure 1. In this case, the substrate is an Ag(111) crystal, and the projectile 4 is accelerated to a kinetic energy of 15 keV (13) (Ar872 at 15 keV moves at 10 m/s). Hence, the energy per atom varies from 15,000 eV for Ga to 17.2 eV for Arn, where n is the number of atoms in the cluster; in this case, it is 872. Note that both Ga and Au3 penetrate deeply into the Ag crystal, leaving considerable damage and atomic mixing. The displacements are characterized by binary collisions producing a collision cascade of atoms. For C60, the impact results in the formation of a well-defined crater, much like that of a meteor striking the earth. This mesoscale phenomenon is not well described by a collision cascade because the diameter of the C60 molecule (0.7 nm) is larger than the interatomic spacing of the Ag crystal (0.23 nm). This observation has led to the development of analytical theories of particle emission based upon mesoscale fluid flow that are successful for predicting a range of parameters (14, 15). For the GCIB bombardment, the crater is of similar diameter to C60, although the depth is much lower. In addition, Ar872 is large enough to block emission of substrate atoms. Hence, most of the particles are emitted from the crater edge at off-normal angles. The yield of substrate atoms is approximately one-third that of C60, presumably due to this blocking effect and to the lower kinetic energy per particle in the cluster, E/n. It is this parameter that may also affect ionization probability, which is generally lower for GCIBs than for C60. Many other graphical pictures and videos, acquired using molecular dynamics, have been published over the last several years (9–12, 16–20). With all of the variables associated with GCIBs, and with the possibility of generating clusters with different chemistry (e.g., C60), there has been an effort to find universal relationships that allow for a predictive model using a single equation. A successful approach involves plotting experimentally measured Y/n versus E/n for Ar–GCIBs. Here, Y is the yield of neutral material 3 Annual Rev. Anal. Chem. 2018.11:29-48. Downloaded from www.annualreviews.org from a molecular solid in volume units (nm ), and E is the kinetic energy of the cluster (21–25). Access provided by Pennsylvania State University on 02/26/19. For personal use only. Such a plot is shown for a variety of materials in Figure 2. Note that the value of Y/n can vary over four orders of magnitude for organic solids and polymers, and even more for atomic solids such as Si, SiO2, and Au.
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