J. Phys. Chem. C 2010, 114, 5637–5644
5637
Single Impacts of C60 on Solids: Emission of Electrons, Ions and Prospects for Surface Mapping† S. V. Verkhoturov,‡ M. J. Eller,‡ R. D. Rickman,§ S. Della-Negra,| and E. A. Schweikert*,‡ Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77842-3012, Halliburton Technology Center, Duncan, Oklahoma 73533, and Institut de Physique Nucle´aire, 91406 Orsay, France ReceiVed: July 31, 2009; ReVised Manuscript ReceiVed: September 28, 2009
We report on the co-emission of secondary ions and electrons resulting from 15 keV C60+ and 30 keV C602+ impacts on targets of Al, Si, Au, CsI, glycine, and guanine. The study has been performed by the combination of an electron emission microscope and a time-of-flight (ToF) mass spectrometer. The electron emission occurs near the kinetic emission threshold, yet yields are notable (>3) for all investigated targets. A key observation for the projectile-target combinations studied is the absence of correlation between the electron emission and the number and type of co-emitted secondary ions for flat and homogeneous samples. This observation validates a novel concept of “positional mass spectrometry”. In this approach a surface is probed in the event-by-event bombardment detection mode. Impacts of an individual C60 projectile are localized via electron emission. The location combined with the corresponding secondary ion information allows to map the distribution of surface molecules. The unique feature of positional mass spectrometry is the ability to identify co-emitted ions from a single projectile impact. To test the concept an electron emission microscope has been combined with a ToF mass spectrometer; the device operates with synchronized detection of electrons and ions. The spatial resolution of the method depends on the kinetic energy and angular distribution of the secondary electrons and the aberrations of the electron optics. Initial tests of positional mass spectrometry showed a spatial resolution of 1.2 µm. Progress is anticipated with improvements in the electron optics used and application of projectiles generating more prolific electron emission. Introduction The distinct features of cluster-SIMS are well documented: enhanced emission of molecular ions, low damage cross section, and reduced molecular fragmentation.1 We refer here in particular to results obtained with C60q+ and Au100qq+ (q ) 1-4). Our aim is to take advantage of these features for probing surfaces at the scale of nanovolumes. This may be achieved by examining the ionized ejecta from single projectile impacts. It has been shown with molecular dynamic simulations and experimentally that the impact of one C60 of 10-20 keV generates secondary ion, SI, emission from a volume of ∼103 nm3.2,3 In practice, to implement nanovolume analysis, the experiments are run in the event-by-event bombardment detection mode: single projectiles are spaced ∼10-3 s apart, the SIs resulting from each impact are recorded individually. With a negative surface bias, electrons are coemitted with the SIs, they can provide the start signal for the identification of the SIs via a time-of-flight (ToF) measurement. The result is spatially resolved negative ion mass spectrometry. A yet-to-be-accomplished step is to localize the individual projectile impacts. Their location combined with the corresponding SI information may then allow one to map the distribution of surface molecules. The map will be over a surface area in the range of 0.01-100 µm2 since massive clusters cannot be focused as well as keV atomic ions. Within the area selected for examination the individual cluster impacts will occur in a stochastic manner. †
Part of the “Barbara J. Garrison Festschrift”. * To whom correspondence should be addressed. E-mail: schweikert@ mail.chem.tamu.edu. ‡ Texas A&M University. § Halliburton Technology Center. | Institut de Physique Nucle´aire.
We present here results from an initial set of experiments designed to assess the concept of “positional mass spectrometry”. The first task was to devise the hardware and software for the localization-identification of SIs from single projectile impacts. The ability to localize single cluster impacts allows to explore fundamental questions about the relationship between SI and electron emissions. Is the coemission of negative SIs and electrons correlated, or are SIs and electrons emitted independently? What is the electron yield when there is coemission of multiple SIs? Little is known about electron emission from massive projectile impacts. An electron emission “threshold” has been mentioned for large water clusters impacting Cu at a velocity of ∼18 km/s, corresponding to an energy per unit mass of ∼1.7 eV/amu.4 Several studies examined electron emission from large molecular ions colliding with solids. Signals were observed from stainless steel and CsI for incident projectiles velocities as low as 3.5 km/s or ∼0.06 eV/amu.5,6 The present study focuses on impacts with C60q+ (q ) 1, 2) at velocities of 63 and 90 km/s. There are very few observations with C60 in this velocity range. They report electron emission from Au and CsI targets “well below the classical kinetic emission threshold”.5,7 In one case the emission was explained as a result of “quasimolecular autoionization”.8 Others have accounted for the deviation from conventional electron emission models by introducing parameters related to the diffusion of electrons and their probable “escape” from the surface.5 A more nuanced understanding may emerge from larger sets of experimental data. The characteristics of electron and corresponding SI emission for positional mass spectrometry are described below. The
10.1021/jp9073703 2010 American Chemical Society Published on Web 11/19/2009
5638
J. Phys. Chem. C, Vol. 114, No. 12, 2010
Figure 1. Schematic of positional mass spectrometry. (a) Schematic 100 ns after the impact of the projectile when the electrons from the impact are extracted toward the position sensitive detector. (b) 10 µs after the impact of the projectile when the secondary ions are detected.
concept is illustrated with maps of specified SI emission including of coemitted SIs. Experimental Section The schematic of positional mass spectrometry is shown in Figure 1. A single projectile impact causes the emission of electrons and negative ions. The emitted electrons are separated by a magnetic field and reach the position sensitive detector within ∼100 ns of emission (Figure 1a). The coordinates of the detected electrons (x,y) correspond to the projectile impact coordinates (X,Y) (Figure 1b). The acquisition time of the position sensitive detector must be fast enough to avoid delaying of ToF measurement, which has a typical repetition rate of 2 pixels and be round in shape. This procedure avoids counting of randomly flashed single pixels as original electron spots. In practice, given the large effective number of pixels, and the procedure used for
Figure 5. Total electron distribution (9), electron distribution when no (b), one (2), two (1), three (pink triangle), four (green triangle), and five ([) ions were detected for (a) electron probability distribution P(n) from a glycine target bombarded by C60+ at 15 keV total impact energy and (b) C602+ at 30 keV total impact energy.
TABLE 1: Yields of Electrons from Individual C60+ and C602+ Impacts at 15 keV and 30 keV, Respectively analyte
C60+
C602+
glycine guanine Si wafer Al wafer Au wafer CsI on 400 mesh grid
3.2 3.1 3.1 3.6 3.9 5.6
3.9 3.6 4.1 5.7 5.9 7.8
“image recognition”, the measured distribution P(n) is not distorted by the camera sensor. Thus, the measured yield of electrons Yexp is related to the yield of emitted electrons Y by
Yexp )
∑ nP(n) ) τY
(4)
n
The measured yields are shown in the Table 1. The yields are large (>3) for all investigated targets. The CMOS camerabased mode of scintillator mediated electron detection used here is remarkable in its dynamic range. Most importantly, it provides an accurate count of impact events. The highest yields are observed for CsI target, with values of 5.6 and 7.8 for 15 keV C60+ (V1 ) 63.4 km/s) and 30 keV C602+ (V2 ) 89.6 km/s),
Single Impacts of C60 on Solids
J. Phys. Chem. C, Vol. 114, No. 12, 2010 5641
P(m|n) ) P(n)
(5)
where m and n are, respectively, the number of ions and electrons detected per projectile impact. The probability distributions for codetected electrons and ions P(m,n) can thus be expressed by a function of P(m) and P(n) as follows
P(m, n) ) P(m)P(m|n) ) P(m)P(n)
(6)
We now consider the probability distributions of the electrons as a function of the type of coemitted SI. Figures 7 and 8 present the data for C60+ and C602+ impacts on glycine and on gold targets, respectively. The data show that the number of electrons emitted is independent of the type of coemitted SIs. Again, similar behavior has also been observed in the nominal guanine, gramicidin S, aluminum, silicon, and CsI targets (data not shown). As an initial test of positional mass spectrometry, we present the case of a flat (electroformed) nickel grid (Industrial Netting) covered with vapor-deposited CsI. The grid was mounted on a hollowed out sample holder, such that there was no supporting material beneath the grid openings. The CsI was deposited on the mounted grid. The tests were run with 120 000 impacts of 30 keV C602+ on a grid area of ∼140 µm in diameter. A map
Figure 6. Total electron distribution (9), electron distribution when no (b), one (2), two (1), three (pink triangle), four (green triangle), and five ([) ions were detected for (a) electron probability distribution P(n) from a gold target bombarded by C60+ at 15 keV total impact energy and (b) C602+ at 30 keV total impact energy.
respectively. For reference, the electron yield recorded with a (nonimaging) setup designed for such measurements is 6.5 for a CsI target bombarded with 15 keV C60+.5 This value is consistent with the yield of 5.6 in this study. The ratio of yields measured at the two velocities is 0.72 (Table 1). This ratio correlates with the ratio of velocities V1/V2 ) 0.71. A more detailed study of the yield dependence versus velocity requires measurement over a range of the velocities starting from the threshold velocity (∼20 km/s) for which data are available.7 Implementation of positional mass spectrometry rests on the relationship between the electron and SI emissions. Pertinent data are shown in Figure 5 for C60+ and C602+ impacts in glycine. The total electron probability distributions and conditional electron probability distributions when the number of codetected SIs varies from 0 to 5 are shown in panels a and b of Figure 5, respectively. For either of the projectiles the number of electrons emitted is independent of the number of coemitted SIs. Similar observations have been made for C60 impacts on gold (Figure 6) and for targets nominally of guanine, aluminum, silicon, and CsI (data not shown). The equality, regardless of the number of ions detected, between the conditional electron probability distributions and the total electron probability distributions can be expressed as follows
Figure 7. (a) Electron probability distribution P(n) from a glycine target bombarded by C60+ at 15 keV total impact energy: total electron distribution (9), electron distribution when H- (b), CN- (2), and (GlyH)- (1) was detected. (b) C602+ at 30 keV total impact energy: total electron distribution (9), electron distribution when H- (b), CN- (2), and (Gly-H-) (1) was detected.
5642
J. Phys. Chem. C, Vol. 114, No. 12, 2010
Figure 8. (a) Electron probability distribution P(n) from a gold target bombarded by C60+ at 15 keV total impact energy: total electron distribution (9), electron distribution when H- (b), Au- (2), and (C2H)- (1) was detected. (b) C602+ at 30 keV total impact energy: total electron distribution (9), electron distribution when H- (b), Au- (2), and Au2- (1) was detected.
of the electrons detected form a 400 mesh grid at 100× magnification is shown in Figure 9a. A line scan of the electron data across ∼140 µm and encompassing two grid wires is shown in Figure 9b. The width of the grid wires at their respective half heights is ∼14 µm, in agreement with their nominal width. The scan shows the electron emission along a ∼140 µm line, which is the approximate diameter of the area of C602+ bombardment. In our mode of operation the impacts occur in a stochastic fashion and is evident in the scanned line. There is no focusing within the 140 µm diameter, hence the difference in the number of electrons emitted from the two wires is due to different numbers of C602+ impacts and reveals inhomogeneity in the spatial distribution of C602+. Clearly, at the level of single impacts it is important to ascertain any bias in the sampling of the area exposed for examination. The electron emissions localized in the grid openings are attributed to “chromatic” and spherical aberrations in electrostatic lenses. The ratio of signal from the grids vs the opening in-between is >6. Each impact location has SI information as an attachment. To obtain the location of emission of a specified SI, we select from the files of the individual spectra, the impacts where the
Verkhoturov et al.
Figure 9. (a) Map and (b) line scan of electrons detected from 400 mesh grid at 100× magnification.
Figure 10. Mass spectrum of a Ni 400 mesh grid coated by CsI bombarded by C602+ with corresponding time window for I- shown.
species of interest was detected (Figure 10). For instance, the selection of impacts where I- was detected yields a map shown in Figure 11a. Similarly, one can obtain a map of locations from which (CsI)I- was emitted (Figure 11b). A truly unique
Single Impacts of C60 on Solids
J. Phys. Chem. C, Vol. 114, No. 12, 2010 5643
Figure 11. (a) Selected electron impacts when I- was detected; (b) selected electron impacts when (CsI)I- was detected; and (c) selected electron impacts when I- and (CsI)I- were detected in coincidence.
capability of the method described here, is the ability to localize sites of coemission of two selected SIs. A map of sites of Iand (CsI) I- coemission is shown in Figure 11c. The ultimate limit in spatial resolution would be the diameter of the area of SI emission from a single projectile impact, i.e., ∼10 nm.2,3 In practice, the spatial resolution is limited by aberrations due to the emission characteristics of the electrons and the electron optics. The spatial resolution, G, can be defined as follows
G ) R/MG
TABLE 2: Average Size of an “Electron Cluster” and Corresponding Spatial Resolution for Different Magnifications on CsI average size of cluster (pixels) projectile 15 keV C60+ 30 keV 602+
50× 26.7 28.9
100× 36.1 42.5
400× 41.8 43.1
spatial resolution (µm) 50× 6.4 6.4
100× 4.3 4.3
400× 1.3 1.3
the yield of electrons by increasing the kinetic energy of the projectile will allow restriction of the aperture.
(7)
where, R is the radius of the area containing the spots in the video frame depicting the electron emission; M is the magnification of the electron optics. We present in Table 2 the average sizes of the “area of spots” and the spatial resolution obtained in the initial tests. The spatial resolution reported here is 1.2 µm with a magnification of 400×. Further developments of the method will focus on improving the spatial resolution and increasing the yields of electrons and ions. The secondary electron optics could be modified to decrease aberrations. One way to reduce the chromatic aberrations is to modify the design of the lenses and to significantly increase the target bias from the current 5 to 20-30 kV. Another possible step is to operate the electron optics with magnetic lenses. They can be designed with substantially lower chromatic and spherical aberrations and will thus improve spatial resolution. Last but not least, increasing
Conclusion The objective of this study was to evaluate the emission of electrons and ions from single impacts of C60 for positional mass spectrometry. We measured emission of multiple electrons at projectile energies as low as ∼300 eV/atom. A collective effect operates, the mechanism involved remains to be elucidated. The notable observation for homogeneous flat targets is that the electrons are emitted independently from the type and the number of co-emitted ions. Electron-ion emisstion characteristics from surfaces of more complex chemical composition and/or physical features remain a topic for future investigation. The unique feature of positional mass spectrometry is the ability to identify negative ions coemitted from a single projectile impact. This information is spatially resolved in the size of the area of co-emission. Its localization via the concurrent emission of electrons depends on their kinetic and angular distributions and
5644
J. Phys. Chem. C, Vol. 114, No. 12, 2010
on the performance of the electron optics and signal detection and processing scheme. We anticipate significant progress in the localization of single impacts with projectiles generating more prolific electron emission, i.e., higher kinetic energy and/ or more massive projectiles such as Au4004+. Abundant electron emission events should have randomized radial and translational velocities which should result in a more precise computation of the impact coordinates. Acknowledgment. This work was supported by the National Science Foundation (Grant No. CHE-0750377) and the Robert A. Welch Foundation (Grant No. A-1482). References and Notes (1) Winograd, N.; Postawa, Z.; Cheng, J.; Szakal, C.; Kozole, J.; Garrison, B. J. Appl. Surf. Sci. 2006, 252 (19), 6836–6843.
Verkhoturov et al. (2) Czerwinski, B.; Rzeznik, L.; Paruch, R.; Garrison, B. J.; Postawa, Z. Vacuum 2009, 83 (Suppl. 1), S155–S158. (3) Li, Z.; Verkhoturov, S. V.; Locklear, J. E.; Schweikert, E. A. Int. J. Mass Spectrom. 2008, 269 (1-2), 112–117. (4) Beuhler, R. J. J. Appl. Phys. 1983, 54 (7), 4118–4126. (5) Brunelle, A.; Chaurand, P.; Della-Negra, S.; Le Beyec, Y.; Parilis, E. Rapid Commun. Mass Spectrom. 1997, 11 (4), 353–362. (6) Westmacott, G.; Ens, W.; Standing, K. G. Nucl. Instrum. Methods Phys. Res., Sect. B 1996, 108 (3), 282–9. (7) Toeglhofer, K.; Aumayrae, F.; Kurz, H.; Winter, H.; Scheier, P.; Mark, T. D. Europhys. Lett. 1993, 22 (8), 597–602. (8) Winter, H. P.; Vana, M.; Betz, G.; Aumayr, F.; Drexel, H.; Scheier, P.; Mark, T. D. Phys. ReV. A: At., Mol., and Opt. Phys. 1997, 56 (4), 3007– 3010. (9) Morrison, G. H.; Slodzian, G. Anal. Chem. 1975, 47(11), 932A936A, 938A, 940A-943A. (10) Liebl, H. Scanning Electr. Microsc. 1984, (2), 519–28. (11) Rickman, R. D.; Verkhoturov, S. V.; Schweikert, Emile A. Appl. Surf. Sci. 2004, 231-232, 54–58.
JP9073703
