Energy loss and straggling of 1–50 keV H, He, C, N, and O ions passing through few layer graphene

Nuclear Instruments and Methods in Physics Research B 358 (2015) 223–228

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Energy loss and straggling of 1–50 keV H, He, C, N, and O ions passing through few layer graphene Frédéric Allegrini a,b,⇑, Peter Bedworth c, Robert W. Ebert a, Stephen A. Fuselier a,b, Georgios Nicolaou b,a, Steve Sinton c a b c

Southwest Research Institute, Space Science and Engineering Division, San Antonio, TX 78228, USA University of Texas at San Antonio, Physics and Astronomy Department, San Antonio, TX 78249, USA Lockheed Martin Space Technology Advanced Research and Development Laboratories, 3251 Hanover Street, Palo Alto, CA 94304, USA

a r t i c l e

i n f o

Article history: Received 5 March 2015 Received in revised form 5 June 2015 Accepted 24 June 2015 Available online 8 July 2015 Keywords: Graphene Carbon foil Energy loss Energy straggling Space plasma instrument

a b s t r a c t Graphene could be an alternative to amorphous carbon foils, in particular in space plasma instrumentation. The interaction of ions or neutral atoms with these foils results in different effects: electron emission, charge exchange, angular scattering, and energy straggling. We showed in previous studies that (1) the charge exchange properties are similar for graphene and regular carbon foils, and (2) the scattering at low energies (few keVs) is less for graphene than for one of our thinnest practical carbon foils. In this study, we report measurements of the energy loss and straggling of 1–50 keV H, He, C, N, and O ions in graphene. We compare graphene and a carbon foil for hydrogen. We provide simple power law fits to the average energy loss, energy straggling, and skewness of the energy distributions. We find the energy loss for ions transiting through graphene to be reduced compared to thin carbon foils but the energy straggling to be larger, which we attribute to the non-uniformity of the graphene foils used in this study. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Many space plasma instruments use carbon foils to facilitate the detection of ions (e.g., [11,14,21]), neutral atoms (e.g., [13,9,18]), and electrons [10]. The instruments make use of two properties of the interaction of these particles with the carbon foils, namely, electron emission (e.g., [19,1]) and charge exchange (e.g., [12,15]). Other properties, such as energy straggling (e.g., [16]) or angular scattering (e.g., [8]), usually degrade the performance of the instruments and scale with the thickness of the foils. A significant effort has been made to fly foils as thin as possible (e.g., down to 0.5 lg/cm2, or roughly 100 atoms thick). But even so, angular scattering becomes fairly large at keV energies and transmission through the foils decreases rapidly. The emergence of graphene has opened the possibility to reduce even further the thickness of the foils. Graphene is the strongest known material on Earth [17], thus it can be made thinner than the regular carbon foil for equivalent strength. On the one hand, since the angular scattering mainly depends on foil thickness, a thinner graphene foil scatters ions less than a regular carbon foil

[7]. The benefits of reducing angular scattering are, for example, an improvement of the transmission of ions inside the instrument (i.e., increased sensitivity and reduced background noise due to stray trajectories) and of the resolution (e.g., time-of-flight, mass, mass-per-charge). On the other hand, the exit charge state of an ion leaving the foil does not depend on foil thickness and is mainly governed by processes occurring at the exit surface. Therefore, because graphene and regular carbon foils have similar composition, the exit charge state distributions of ions after graphene or after regular carbon foils are similar [3]. This study focuses on the energy loss and straggling of 1– 50 keV ions passing through graphene foils. We used the same few layer graphene (FLG) foils as those for the reports of the angular scattering [7] and exit charge state distribution [3]. We compare the results from the graphene foils with a nominal 0.5 lg/cm2 amorphous carbon foil for hydrogen. While the energy loss of ions through graphene is less than for the 0.5 lg/cm2 carbon foil, the energy straggling is significantly larger for the graphene foils used in this study than for the carbon foil. 2. Experimental setup

⇑ Corresponding author at: Southwest Research Institute, P.O. Drawer 28510, San Antonio, TX 78228, USA. Tel.: +1 210 522 6029; fax: +1 210 520 9935. E-mail address: [email protected] (F. Allegrini). http://dx.doi.org/10.1016/j.nimb.2015.06.028 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

The graphene foils were mounted on nickel grids (333 lines per inch, 13.1 lines per mm). Due to fragility of the foils, we floated two

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FLG foils on top of each other (the second FLG foil after the first one had dried). The FLG foil is formed on a copper substrate and consists of a field of graphene nano plates resulting in a thickness of 3–7 atomic layers, with each monolayer corresponding to 0.345 nm. The thickness was determined by Raman spectroscopy using the shape of the 2D peak and by optical transmission prior to transfer from the copper substrate to the nickel grid. Later scanning transmission electron microscopy (STEM) analysis, performed on material that has been collected in the same manner as the foils in the experiment, revealed thicker areas of up to 20 atomic layers (for up to about 25% of the surface) with typical spacing from 250 to 1000 nm. The analysis shows that the transition between the thinner and thicker areas is smooth. The regular carbon foil (nominal 0.5 lg/cm2, 100 atoms thick), to which we compare the graphene results, was purchased from the Arizona Carbon Foil company, and was mounted on similar nickel grids. The experiments were performed in a vacuum chamber having a pressure of 107 torr. The ion beam (H, He, C, N, or O) is created by a duo-plasmatron source and can be accelerated to 1– 50 keV/q (see Appendix A of [18]). We measure the ion time-of-flight (TOF) after the foil with the setup described in Allegrini et al. [5]. A collimator downstream of the foil selects only trajectories within ±2.7° of the foil normal. To calibrate our TOF measurement, we also use a high resolution energy analyzer (DE/E  1.6% at FWHM). 3. Results 3.1. Comparison between graphene and regular carbon foil Fig. 1 shows the energy distributions obtained with our high-resolution energy analyzer for a 50-keV proton beam (black) and a 50-keV proton beam after passing through a graphene (red) and carbon foils (blue). The peak energy after the graphene foil is higher than after the carbon foil, which means that the energy loss for graphene is lower than for this carbon foil. From a simple foil thickness estimate, this result is expected. A 2FLG foil represents a thickness of about 2.1 nm (=2  3  0.345 nm) to 4.8 nm (=2  7  0.345 nm), neglecting the thicker areas mentioned above. For the carbon foil, the actual thickness is larger than the nominal thickness [8,2], typically by a factor of 2 or greater. Assuming a density of 1.8 g/cm3 [19] and a thickness of >1.1 lg/cm2 for the carbon foil, the thickness estimate is >6.1 nm. The energy straggling is defined as the full-width half max (FWHM) of the energy distribution after the foil. Here, the energy straggling after graphene is larger than after the carbon foil. The thickness variations of the graphene could be the reason why the straggling is so much larger than for the regular carbon foil. Note that the FWHMs indicated in the figure are the convolution of the straggling due to the foils and the energy resolution of the analyzer. The straggling due to the foils only is given in Table 1. We measured the ion TOF after graphene and the carbon foil and calculated the average energy loss (see also [5]). Fig. 2 shows the result for hydrogen between 1 and 50 keV. The blue data points correspond to the carbon foil (C8) and the red (A9) and yellow– green (A15) data points to two different graphene foils that were obtained using the same process. The crosses at 50 keV represent the high resolution energy analyzer measurements as shown in Fig. 1 (for foils C8 and A9). The curves are fits using the following equation:

DE ¼ a0 E0ð0:5a1 E0 Þ ;

ð1Þ

where DE is the average energy loss, E0 is the incident energy, and a0 and a1 are the free parameters (given in Table 1). The choice for

Fig. 1. 50 keV hydrogen beam energy measurement (black) and after 2FLG (top red) and 0.5 lg/cm2 carbon foil (bottom blue) performed with a high resolution energy analyzer. The curves are Gaussian fits to the data points. The peak energy after the 2FLG is higher than that of the carbon foil (i.e., less energy loss), whereas the energy straggling is more for the 2FLG foil than for the carbon foil.

Eq. (1) is motivated by the fact that it fits the data reasonably well. Eq. (1) is empirical and the parameter a1 does not have a physical meaning. Fig. 1 already showed that the energy loss through graphene (foil ‘‘A9’’ red curve) was less than through the carbon foil (blue curve) at 50 keV. Fig. 2 shows that it is consistent all the way down to 1 keV. Fig. 3 shows the energy straggling, X, i.e., the FWHM of the energy distribution after the foil, for 1–50 keV protons transiting these foils. The straggling for graphene is about a factor of 2.3 larger than for the carbon foil. The curves are fit to the data points using:

X ¼ b0 E0:5 0 ;

ð2Þ

where b0 is the free parameter (also given in Table 1). 3.2. Energy loss, straggling, and skewness for H, He, C, N, and O To determine the energy distribution of the ions after the graphene, we use a four-step process that has been developed for this

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F. Allegrini et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 223–228 Table 1 Foil thickness, scattering constant, energy loss and straggling parameters derived for 1–50 keV hydrogen ions interacting with graphene and carbon foils. Foil ID

Nominal thickness 0.5 lg/cm 2 FLG 2 FLG

C8 A9 A15

2

kF [keV °] 11.1 ± 1.5 – –

DE at 50 keV [keV]

X at 50 keV [keV]

1.266 ± 0.001 0.995 ± 0.001 1.075 ± 0.001

0.805 ± 0.016 1.798 ± 0.075 1.914 ± 0.096

a0 [keV] 0.209 ± 0.006 0.150 ± 0.005 0.184 ± 0.007

a1 [keV1]

b0 [keV] 4

(7.9 ± 5.2) * 10 (3.2 ± 4.8) * 104 (9.7 ± 6.2) * 104

0.113 ± 0.008 0.254 ± 0.017 0.271 ± 0.018

. The skewness is a measure of the asymmetry of the distribution. For example, a Gaussian-like distribution has zero skewness. The larger R (in absolute), the more skewed the distribution. We use this measure to quantify the skewness as observed in Fig. 4, for example. In this energy range, all three quantities follow a power law that we fit with:

y ¼ c0 Ec01

ð5Þ

 X2, or R3, and c0 and c1 are the free parameters where y is either DE, (given in Table 2). The power index varies as a function of species for the average energy loss and the straggling, but is relatively constant around 1 for R3, except for He at 0.77.

4. Discussion We measured the average energy loss, energy straggling, and skewness of the energy distribution of 1–50 keV ions after passing through 2FLG, and compared the average energy loss and straggling with a regular amorphous carbon foil of nominal thickness 0.5 lg/cm2 for hydrogen. We find that the energy loss is less for graphene than for the regular carbon foils. If the energy loss difference was solely due Fig. 2. Average energy loss as a function of incident energy for two different 2FLG foils (red and yellow–green) and a nominal 0.5 lg/cm2 carbon foil (blue). The curves are fits using Eq. (1). The crosses are the measurement from the high resolution analyzer shown in Fig. 1.

experimental setup (see [5] for details). In short, the first step consists of correcting the TOF spectra for all offsets. In the second step, we fit asymmetric Gaussians [20] to the TOF peaks. In the third step, we remove the contribution of the start electrons TOF dispersion from the TOF peaks. This step only matters for narrow TOF peaks (typically for high energy H or He), and has a negligible effect on heavier species for which the TOF peak width is much larger than the electron TOF dispersion. Finally, in the fourth step, the fits to the TOF peaks are converted to energy distributions. An example of the last three steps is illustrated in Fig. 4 (H at 7 keV incident energy), where the black histogram in the top panel is the corrected TOF spectrum, the red curve is the fit to the histogram with the asymmetric Gaussian, the green curve is the fit with the electron TOF dispersion subtracted, and the bottom panel shows the TOF fit converted to an energy distribution. Note that the blue curve is truncated at the incident beam energy. Figs. 5–7 show the average energy loss, straggling, and a measure of the skewness of the energy distribution, respectively. The skewness is defined as (e.g., [16]):

R3 ¼ ð1=N0 Þ

Z

   3 dE; ðdN=dEÞ E  E

ð3Þ

where

N0 ¼

Z

ðdN=dEÞdE

ð4Þ

Fig. 3. Energy straggling as a function of incident energy for two different 2FLG foils (red and yellow–green) and a nominal 0.5 lg/cm2 carbon foil (blue). The lines are fits using Eq. (2). The crosses are the measurement from the high resolution analyzer (Fig. 1).

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our measurements and solving for the roughness, we find that rt is between 3.1 and 3.4 nm. This range is compatible with the thickness description of the 2FLG given in Section 2. The fact that the energy loss distribution does not show a double peak (due to the thin and thick areas) confirms the results from the SEM analysis that show smooth transition between the thin and thick areas. If the transition were abrupt, then we would probably see more than a single peak in the distribution. The thickness variations of the 2FLG foils should also affect the angular scattering to some extent. Thicker regions should scatter more than thinner regions. In Ebert et al. [7], we found that the angular scattering at low energies (1–5 keV) was better for 2FLG foils than for nominal 0.5 lg/cm2 carbon foils. However, at higher energies (10–40 keV) both foils had similar angular scattering properties. We argue that the thickness non-uniformity of the 2FLG foils can cause such differences in our experimental setup. For that, we start by assuming that the foil has two different thicknesses of equal surface area, and then we simulate the effect that that would have on the scattering distribution. The result is that the scattering half width is about half way between that of the two different thicknesses at all energies, and it follows the same trend as that for the individual thicknesses. Thus, it does not explain the different trend that is observed in Ebert et al. [7]. However, that simulation does not account for the fact that in the angular scattering data analysis from our measurements, the part of the distribution near zero scattering is disregarded due to contamination from pinholes (that obviously do not scatter ions). When going from low to high energies, the scattering of the thin part of the foils becomes smaller, and comparable to the region where there is no scattering from the pinhole. Since the scattering part near zero is not used to constrain the fit to the scattering distributions, the contribution from the thin part of the foil progressively decreases. This leads to an overestimate of the average scattering at high energies, because it is mostly determined by the thicker part of the foil. When we assume that there are pinholes in our simulated foils and we disregard the near-zero

Fig. 4. Example of conversion from TOF spectrum to energy distribution for H at 7 keV. See text for details.

to the thickness of the foil, a simple thickness estimate for both the graphene and the regular carbon foil supports this. We also find that the energy straggling through the graphene foils used in this study was larger than for the regular carbon foil. One of the factors that determine the energy straggling is thickness uniformity. The 2FLG foils used here probably have larger relative thickness variations (3–7 atomic layers for most of the graphene, but presence of much thicker areas of up to 20 atomic layers) than the regular carbon foil to which we compare our results. A more uniform thickness for graphene would likely reduce straggling and probably make it comparable to, or even less than, that of the carbon foil. To verify this assumption, we evaluate the additional roughness required to explain the differences. If we consider that the straggling is the quadratic sum of a term due to statistical fluctuations, Xcol, and a term due to surface roughness, Xroughness, then we can write (e.g., [6]) 2

X2 ¼ X2col þ X2roughness ¼ X2col þ ðdE=dxÞ ð2:355rt Þ2 ;

ð6Þ

where dE/dx is the stopping power and rt is the standard deviation of the foil thickness. We assume that the carbon foil is smooth and that the difference in straggling between 2FLG and the carbon foil is solely due to the contribution from the roughness. We obtain a proxy for the stopping power from TRIM [22] using carbon. Using

Fig. 5. Average energy loss (first moment) of H (red crosses), He (orange stars), C (green diamonds), N (violet triangles), O (blue squares) ions passing through 2FLG as a function of incident energy. The lines are power law fits (linear fits in log–log space).

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F. Allegrini et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 223–228 Table 2 Fit parameters of Eq. (5) for Figs. 5–7.

X2

 DE

H He C N O

R3

c0 ± r

c1 ± r

c0 ± r

c1 ± r

c0 ± r

c1 ± r

0.148 ± 0.001 0.242 ± 0.002 0.520 ± 0.007 0.415 ± 0.005 0.566 ± 0.006

0.504 ± 0.009 0.372 ± 0.014 0.276 ± 0.020 0.375 ± 0.020 0.249 ± 0.016

0.0428 ± 0.0006 0.0998 ± 0.0011 0.508 ± 0.013 1.31 ± 0.12 0.930 ± 0.038

1.13 ± 0.03 0.960 ± 0.018 0.718 ± 0.040 0.52 ± 0.14 0.557 ± 0.066

(0.17 ± 0.01) * 102 (1.5 ± 0.1) * 102 (4.1 ± 0.1) * 102 (6.1 ± 0.7) * 102 (5.0 ± 0.4) * 102

1.12 ± 0.05 0.77 ± 0.06 1.01 ± 0.05 1.02 ± 0.16 0.96 ± 0.13

Fig. 6. Straggling, X2, as a function of incident energy. The lines are power law fits (linear fits in log–log space). Fig. 7. Skewness, |R3|, as a function of incident energy. The lines are power law fits (linear fits in log–log space).

scattering region, we are able to reproduce a similar trend as observed in Ebert et al. [7]. From this study and the two previous ones, we comment on the likelihood of graphene being a substitute for regular carbon foils in space plasma instrumentation. 1. Graphene performs better than regular ultra-thin (e.g., nominal 0.5 lg/cm2) carbon foils in terms of scattering at low energies. For that reason, it is an interesting option when it comes to measuring low energy (sub keV) energetic neutral atoms (e.g., [9]) and ions. 2. Charge exchange is similar for both graphene and regular carbon foils. Thus, graphene could also be used for charge conversion in a neutral atom instrument. 3. The average energy loss is less for graphene than for regular carbon foils. Thus, it is likely that transmission of ions at low energies is higher for graphene than for regular carbon foils. This would increase the sensitivity at low energies but also allow a lower detection threshold of energetic neutral atoms. 4. The straggling is larger for graphene than for regular carbon due to thickness non-uniformities. A uniform graphene may straggle less than carbon foils because the atomic layers would drive the thickness and its uniformity, whereas for carbon foils, the process of arc deposition cannot guaranty that carbon is built up in layers. Less energy straggling will lead to a better resolution (e.g., energy or time-of-flight). Thus, it is highly desirable that the graphene thickness is more uniform than the samples we used in this and the previous studies.

Therefore, the challenges ahead are to produce few layer (<15) graphene foils that have less pinholes and that are uniform in thickness. We chose 2FLG to ensure better grid coverage by free-float transfer than we might have achieved using single-layer (SLG) or bi-layer graphene (BLG) that can be prepared by known synthesis recipes. Sacrificial supporting layer transfer methods are an option to effectively transfer SLG or BLG to substrates but add challenges associated with post-transfer removal of supporting layer material. For all types of CVD graphene, pinholes are a potential issue that can be mitigated by multiple transfers. Finally, more characterization, such as electron yields (e.g., [19]) or resistance to pressure [4], are also needed before graphene becomes a strong candidate in space plasma instruments.

Acknowledgments We would like to thank everyone who has been involved with or has contributed to this project in one way or another (in alphabetical order): Chip Beebe, Dave Cronk, Ed Patrick, Amanda Richter-Walther, Ben Rodriguez, Ken Smith. This work was supported by the Southwest Research Institute Internal Research & Development program and NASA Grant NNX13AG23G.

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