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Secondary electron emission of graphene-coated copper
DIAMAT-06713; No of Pages 5 Diamond & Related Materials xxx (2016) xxx–xxx
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Secondary electron emission of graphene-coated copper Meng Cao ⁎, Xiu-Sheng Zhang, Wei-Hua Liu, Hong-Guang Wang, Yong-Dong Li Key Laboratory for Physical Electronics and Devices of the Ministry of Education, Department of Electronic Science and Technology, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China
a r t i c l e
i n f o
Article history: Received 28 June 2016 Received in revised form 2 September 2016 Accepted 22 September 2016 Available online xxxx Keywords: Secondary electron emission Graphene Energy distribution Surface barrier Multipactor
a b s t r a c t The secondary electron emission of graphene-coated copper has very interesting properties. In this work, we prepared graphene-coated copper foil and measured both the secondary electron emission yield (SEY) and the secondary electron energy spectra. Compared with the copper base, there is a significantly reduction for the SEY. For the copper base without coating, the max value of SEY is beyond 2.1 for the primary electron energy of 300 eV. However, the corresponding value for the graphene-coated copper is about 1.5, reduced about 25%. For the secondary electron energy distribution, the elastically backscattered peak becomes higher and peak of the true secondary electrons is shifted from about 1.5 eV to 4 eV after coating the copper with graphene. The full width at half maximum of the true secondary electrons peak is also increased. Since the position of the peak is related to the surface potential barrier of the material, we believe that the graphene coating could increase the surface potential barrier of the material. This could also be the reason of SEY reduction because a higher potential barrier can reduce the electron emission from the material. A preliminary theoretical model of the surface potential distribution for copper surface with graphene coating was built and used to analyze the secondary electron emission of metal surface with graphene coating. The graphene coating is found to be an effective method to suppress the secondary electron emission. © 2016 Elsevier B.V. All rights reserved.
1. Introduction As a well-known effect of electron solid interactions, secondary electron (SE) emission plays an important role in an extremely vast field of research spanning from vacuum electronics, microscopic structure analysis, particle accelerators to space research [1–8]. Some devices, such as microchannel plate detector and scanning electron microscope make use of SEs. On the other hand, SE is known to be major cause of degradation of performance for some instruments as such the multipactor effect in high power microwave components, and electron cloud effect in accelerators [6,8]. When secondary electron emission is considered to be harmful, suppression of secondary electron emission is highly desired. Therefore, many investigations have been given into find effective technique of secondary electron emission suppression. Secondary electron yield (SEY, often denoted as σ), which is defined as the ratio of the number of emitted secondary electrons to the number of incident primary electron, is frequently used to characterize the magnitude of secondary emission. Surface treatments are often introduced to reduce the SEY since secondary electron emission strongly depends on the surface condition. For example, carefully desired surface structures, such as micro-porous array structure, rectangle groove, were ⁎ Corresponding author. E-mail address: [email protected] (M. Cao).
found to be effective for decreasing SEY of metal [9–14]. Another approach to secondary electron emission suppression is coating the surface by material with low SEY. The database summarized by Joy [15] can provide SEY data for many materials, which may be helpful for choice of coating material. Graphene was reported to be a material of ultralow secondary electron emission and carbon coating was found to be a way to reduce the SEY [16–18]. It was reported that electron bombardment could also decrease the SEY and the reason was considered to be the surface graphitization due to electron bombardment. It was believed that the sp3 type bond is the nature of SEY decreasing [19–21]. Besides the SEY, the secondary electron energy distribution is also an important knowledge and has been attracting many interests. Fine structures in energy spectra of secondary electrons and their dependence on primary electron energy have been carefully examined for graphene on nickel surface and been used for study the excited states of graphene interfaces [22–25]. Theoretical calculation using a planewave pseudo-potential method based on local density functional theory was also presented to study the graphene-substrate interaction [26]. Knowledge on energy distribution is helpful to understand the mechanism effect of graphene on secondary electron emission. In this work, we studied the effect of graphene coating on the secondary electron emission of copper foil. Both the SEY and secondary electron energy spectra have been considered. Significant reduction for SEY was achieved by graphene coating. Investigation on the
http://dx.doi.org/10.1016/j.diamond.2016.09.019 0925-9635/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: M. Cao, et al., Secondary electron emission of graphene-coated copper, Diamond Relat. Mater. (2016), http://dx.doi.org/ 10.1016/j.diamond.2016.09.019
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distribution of the secondary electron energy shows that the peak position of energy spectra shifts to higher energy and the full width at half maximum becomes wider. We also analyzed these effects by introducing the effective surface barrier related to the graphene coating. An example of suppression of multipactor in microwave devices is demonstrated as a possible application of graphene coating. 2. Experiment 2.1. Preparation of graphene coating on copper foil Samples of copper foil both with and without graphene coating were prepared. The copper foil substrates with purity 99.8% and thickness 0.025 mm (Thermo Fisher Scientific #13,382) were used. The copper substrates were cleaned with isopropyl alcohol, acetone, ethanol and de-ionized water. Treatment using FeCl3 solution with concentration of 0.01 mol/L was introduced for etching and oxidation of the substrate. About 0.1 mL of FeCl3 solution was dropped onto each 1 cm2 substrate and then dried out in the atmosphere. It was found that the graphene nucleation density can be reduced and the total graphene coverage increase-rate can be increased with the FeCl3 solution treatment [27]. Graphene coating layer was grown using the chemical vapor deposition (CVD) method. The detailed information on growth behavior and characterization was reported in the previous publication [27]. Briefly, in a quartz tube furnace, the substrates were first heated up to 1000 °C and then annealed at this temperature for at least 20 min. In the CVD growth process, a two step introduction of CH4 flows was optimized for high quality graphene growth: 1 sscm for 15 s and 0.1 sccm for 30 min, separately. Throughout the whole process, A 40 sccm H2 flow was maintained. The CVD parameters are sketched in Fig. 1. The quality of samples were checked with scanning electron microscope (SEM) and Raman spectra as shown in Figs. 2 and 3, respectively. In the SEM image, it can be seen a uniform graphene lay was obtained. The Raman spectra indicate a single layer graphene on the substrate. 2.2. Measurement of secondary electron emission The experiments of secondary electron emission measurement were performed in an ultra-high vacuum experimental system outlined in Fig. 4. First, the samples were shaped in a rectangle of about 1 × 1.2 cm2 and loaded into the vacuum chamber with a condition of about 2 × 10−8 Pa. The SEYs of the prepared samples were measured at different primary electron energies with an incident direction perpendicular to the samples. The beam spot size of primary electrons was set to about 200 μm in diameter. Two sample currents Ip and Im were measured at a high positive bias (500 V) and low negative bias (−20 V) of the sample, respectively. When the sample is high positive biased, almost all emitted secondary electrons were dragged back to the sample and thus Ip is equal to the primary electron current. The primary electron current was furthermore calibrated by using a Faraday cup. For the negative bias of the sample, on the other hand, all emitted secondary electron
Fig. 1. Temperature and gas flow in CVD processing of graphene synthesis.
Fig. 2. SEM image of graphene on Cu substrate.
will be push out and we have Im = Ip − Ise, where Ise is the secondary electron current. Therefore, the SEY can be determined as σ= Ise/Ipe = 1 − Im/Ip. Here, three different points on a same sample were measured. The average value was taken as the SEY of the sample at certain condition and the deviations of the maximum SEY was less than 5%. The secondary electron energy spectra was detected at a fixed primary electron energy of 300 eV by the spectrometer (DESA 150 analyzer) vertically above the sample. This spectrometer takes advantage of a cylindrical mirror electrostatic fields. It has a large acceptance solid angle of 6% of 2π, with an energy resolution of less than 0.1 eV. The secondary electron energy spectra collection was lasted for about 0.5 min. Here, a negative bias (−15 V), was applied to the sample to collect secondary electrons more effectively. To avoid the possible influence of primary electrons, the primary electron currents were controlled constantly as about 10 nA and 0.6 nA for measurements of SEY and energy spectra, respectively. 3. Results and discussion 3.1. SEY and energy distribution Fig. 5 gives the measured SEY curves as a function of primary electron energy Epe for copper samples both with and without graphene
Fig. 3. Raman spectra of graphene on Cu substrate.
Please cite this article as: M. Cao, et al., Secondary electron emission of graphene-coated copper, Diamond Relat. Mater. (2016), http://dx.doi.org/ 10.1016/j.diamond.2016.09.019
M. Cao et al. / Diamond & Related Materials xxx (2016) xxx–xxx
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Fig. 4. Schematic of experimental set-up for secondary electron emission measurement in UHV.
Fig. 6. Measured secondary electron energy distributions of two kinds of samples.
coating. We see that the SEY first increases and then decreases with Epe, for each kind of sample. Evidently, the SEY for sample with graphene coating is smaller than that for copper substrate for Epe up to 1.5 keV. The max value of SEY for copper substrate is about 2.1, while the corresponding value for the substrate with graphene coating is decreased to about 1.5, indicating an obvious suppression effect of graphene coating on secondary electron emission. An important parameter for the SEY curve is the first crossover energies E1 at which the SEY is one. From the inset in Fig. 5, we can see that the value of E1 is increased from 24 eV to 38 eV. Increasing of E1 may increase the power threshold for multipactor in high power microwave devices. Fig. 6 shows the measured energy distributions of the secondary electron for both kinds of samples. There are two peaks for each curve: one is located at 300 eV which is the energy of primary electrons. The other peak's position is less than 50 eV. The first peak at energy of
primary electrons relates to the electrons backscattered elastically by the surface. The second peak with a lower energy relates to the so called true secondary electrons. In Fig. 6, the energy distribution varies due to graphene coating. It is evident that the elastically backscattered peak of the substrate with graphene coating is much higher than that of copper without graphene coating, indicating that more electrons which impinge on the surface of graphene-coated sample were rebounded. To look at the true secondary electrons with lower energy, we compared the energy distribution up to 50 eV in Fig. 7. Two important parameters are often concerned for the energy distribution of true secondary electrons. One is most probable energy (MPE) which is the position of the peak of the true secondary electrons. The other is the full width at half maximum (FWHM) for the peak. We see that graphene coating appears to enlarge the MPE and to broaden the FWHM. In particular, the MPE shifted to higher energy, from about 1.5 eV to about
Fig. 5. Measured curves of SEY as a function of EPE for samples with and without graphene coating, respectively.
Fig. 7. Energy distributions of true secondary electrons with energy less than 50 eV.
Please cite this article as: M. Cao, et al., Secondary electron emission of graphene-coated copper, Diamond Relat. Mater. (2016), http://dx.doi.org/ 10.1016/j.diamond.2016.09.019
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4 eV after graphene. Simultaneously, the FWHM increased markedly from 4.7 eV to 10.9 eV. 3.2. Theoretical analysis We now give a theoretical analysis on the effect of graphene coating on the secondary electron emission. The graphene coating can affect both the primary electrons and the secondary electrons. The variation of the elastic backscattering peak in Fig. 6 reflects the rebounding effect of coated graphene on the primary electrons. The amount of the backscattered electrons is proportional to the area under the elastic backscattering peak. We use the symbols δ and η to denote the SEY related to true secondary electrons and backscattered electrons, respectively. From Fig. 6, we can calculate the ration of η to the total SEY σ. Together with the value of σ given in Fig. 5, we find that the elastic backscattered SEY η is about 0.08 and 0.04 for substrate with and without graphene coating, respectively. The distance between the carbon atoms in graphene layer is 0.142 nm, while the distance between two copper atoms is about 0.261 nm. For the primary electrons, it is more difficult to penetrate a more compact atom layer. Assuming the primary electron current is Ipe, the current that penetrated current is (1− η)Ipe. For the same number of primary electrons, less electrons can really enter the material and thus less internal electrons can be excited. This is one mechanism for suppression of graphene coating on secondary electron emission. Another more important effect from the graphene coating is the influence on emission of inner secondary electrons. After the primary electrons entering the material, electron-atom scatterings occur and some inner secondary electrons are generated after gaining the lose energy of primary electrons in inelastic scattering events. A secondary electron with certain energy will be scattered further until its energy is exhausted. Eventually, some scattered secondary electrons can reach the surface. For the copper substrate without graphene coating, emission of inner secondary electrons need only pass the surface barrier. However, if the surface is coated with graphene, the inner secondary electrons have to further more penetrate the graphene for an effective emission. Effect of graphene on secondary electrons is even more complicated than that on primary electrons. Most of the inner secondary electrons have energies of several eV, which is much lower than the primary electrons. It is more difficult for a low energy electron to penetrate the graphene than a high energy electron. Furthermore, unlike the primary electrons which are perpendicular to the graphene layer, the inner secondary electrons have a certain direction distribution. Only a part of energy Ecos2θ of an inner secondary electron can help for electron emission, where E is the energy of the electron and θ is the angle between the direction of the electron and the normal direction of graphene. We can use an effective barrier to describe the influence of the graphene coating on inner secondary electron emission. Chung and Everhart [28] have shown that the energy distribution of escaped secondary electrons has a form of SðEÞ∝
E ðE þ E F þ ϕÞ2
;
accurately. However, we can determine this value by experimental result of secondary electron energy spectrum. It is easy to determine the peak position of the energy distribution is ϕ/3 from Eq. (1). Therefore, the value of Δϕ should be Δϕ ¼ 3 Ep1 −Ep2 ;
ð3Þ
where Ep1 and Ep2 are the peak positions for substrate with and without graphene coating. From Fig. 7, we can get Δϕ =7.5 eV. Using this value, we calculated the energy distribution for copper substrate with graphene coating, the results are compared with the measurement data in Fig. 8. The solid lines in the figure are the calculation and the marks are measurement results. Good agreements have been achieved. It should be noted that the value of Δϕ obtained from the peak shift of secondary electron energy spectrum is just a phenomenological result which reflects the comprehensive effect of graphene layer. It may not directly correspond to the change of work function of the material since its magnitude is much bigger than other measurement result of graphene/metal interface [29]. A comprehensive analysis on the effect should consider information such as the band structure and the electron scattering at the interface. We will try to have a detailed study in the future. 3.3. A possible application The suppression effect of graphene coating on secondary electron emission can be used to increase the multipactor threshold of microwave comments. To demonstrate this application, the multipactor of parallel plate transmission line was examined. The properties of the secondary electron emission of the wall of the parallel plate were set to the former measurement data of copper with and without graphene coating, respectively. The susceptibility zones in fd-V plane for both cases were calculated using the method given in references [30,31] and results were shown as in Fig. 9. Here f is the frequency of the microwave, d the distance between the two plates, and V the maximum voltage between the plates. The shaded areas denote the susceptibility zones of multipactor. It can be seen that the susceptibility zone for copper substrate with graphene coating is much smaller than that of the copper.
ð1Þ
where E is the electron energy, EF the fermi level of the material and ϕ the work function. To reflect the effect of the graphene coating, an effective potential barrier Δϕ should be added to the work function and thus the energy distribution of secondary electron for copper substrate with graphene coating is SðEÞ∝
E ðE þ E F þ ϕ þ ΔϕÞ2
:
ð2Þ
The value of Δϕ depends on many factors that relate to the interface between the copper and graphene and is hard to be calculated
Fig. 8. Comparison of theoretical and measurement results of true secondary electron energy distributions.
Please cite this article as: M. Cao, et al., Secondary electron emission of graphene-coated copper, Diamond Relat. Mater. (2016), http://dx.doi.org/ 10.1016/j.diamond.2016.09.019
M. Cao et al. / Diamond & Related Materials xxx (2016) xxx–xxx
Fig. 9. The susceptibility zones of multipactor.
4. Conclusions Graphene coating has been used to suppress the secondary electron emission. The graphene is synthesized by using CVD method. Measurement results for SEY and secondary electrons energy distribution show that the secondary electron emission is decreased significantly. The maximum value of SEY is reduced from about 2.1 to about 1.5. For the energy spectra, the elastically backscattered peak becomes much higher. The true secondary electrons peak position is shifted from about 1.5 eV to 4 eV and the FWHM is also increased. A model of effective surface barrier is used to explain the effect of graphene on secondary electron emission and good agreement is achieved between the theoretical and measurement results. A demonstration of simulation shows a possible application of graphene coating for suppression of multipactor in high power microwave comment. Acknowledgments Project supported by the National Natural Science Foundation of China (Grant No. U1537210, 11375139). The authors would like to thank Dr. Lin from Xi'an Jiaotong University for his help on calculation of the susceptibility zones of multipactor. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.diamond.2016.09.019. References [1] R. Cimino, L.A. Gonzalez, R. Larciprete, A. Di Gaspare, G. Iadarola, G. Rumolo, Detailed investigation of the low energy secondary electron yield of technical Cu and its relevance for the LHC, Phys. Rev. Spec. Top. Accel Beams 18 (5) (2015), 051002. http:// dx.doi.org/10.1103/PhysRevSTAB.18.051002 pp. [2] J.E. Yater, J.L. Shaw, K.L. Jensen, T. Feygelson, R.E. Myers, B.B. Pate, J.E. Butler, Secondary electron amplification using single-crystal CVD diamond film, Diam. Relat. Mater. 20 (5–6) (2011) 798, http://dx.doi.org/10.1016/j.diamond.2011.03.040. [3] S. Gupta, E. Heintzman, J. Jasinski, Secondary electron intensity contrast imaging and friction properties of micromechanically cleaved graphene layers on insulating substrates, J. Electron. Mater. 43 (9) (2014) 3458, http://dx.doi.org/10.1007/s11664014-3277-0. [4] D. Maric, M. Savic, J. Sivos, N. Skoro, M. Radmilovic-Radjenovic, G. Malovic, Z.L. Petrovic, Gas breakdown and secondary electron yields, Eur. Phys. J. D 68 (6) (2014), 155. http://dx.doi.org/10.1140/epjd/e2014-50090-x pp.
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Please cite this article as: M. Cao, et al., Secondary electron emission of graphene-coated copper, Diamond Relat. Mater. (2016), http://dx.doi.org/ 10.1016/j.diamond.2016.09.019
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Secondary electron emission of graphene-coated copper Meng Cao ⁎, Xiu-Sheng Zhang, Wei-Hua Liu, Hong-Guang Wang, Yong-Dong Li Key Laboratory for Physical Electronics and Devices of the Ministry of Education, Department of Electronic Science and Technology, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China
a r t i c l e
i n f o
Article history: Received 28 June 2016 Received in revised form 2 September 2016 Accepted 22 September 2016 Available online xxxx Keywords: Secondary electron emission Graphene Energy distribution Surface barrier Multipactor
a b s t r a c t The secondary electron emission of graphene-coated copper has very interesting properties. In this work, we prepared graphene-coated copper foil and measured both the secondary electron emission yield (SEY) and the secondary electron energy spectra. Compared with the copper base, there is a significantly reduction for the SEY. For the copper base without coating, the max value of SEY is beyond 2.1 for the primary electron energy of 300 eV. However, the corresponding value for the graphene-coated copper is about 1.5, reduced about 25%. For the secondary electron energy distribution, the elastically backscattered peak becomes higher and peak of the true secondary electrons is shifted from about 1.5 eV to 4 eV after coating the copper with graphene. The full width at half maximum of the true secondary electrons peak is also increased. Since the position of the peak is related to the surface potential barrier of the material, we believe that the graphene coating could increase the surface potential barrier of the material. This could also be the reason of SEY reduction because a higher potential barrier can reduce the electron emission from the material. A preliminary theoretical model of the surface potential distribution for copper surface with graphene coating was built and used to analyze the secondary electron emission of metal surface with graphene coating. The graphene coating is found to be an effective method to suppress the secondary electron emission. © 2016 Elsevier B.V. All rights reserved.
1. Introduction As a well-known effect of electron solid interactions, secondary electron (SE) emission plays an important role in an extremely vast field of research spanning from vacuum electronics, microscopic structure analysis, particle accelerators to space research [1–8]. Some devices, such as microchannel plate detector and scanning electron microscope make use of SEs. On the other hand, SE is known to be major cause of degradation of performance for some instruments as such the multipactor effect in high power microwave components, and electron cloud effect in accelerators [6,8]. When secondary electron emission is considered to be harmful, suppression of secondary electron emission is highly desired. Therefore, many investigations have been given into find effective technique of secondary electron emission suppression. Secondary electron yield (SEY, often denoted as σ), which is defined as the ratio of the number of emitted secondary electrons to the number of incident primary electron, is frequently used to characterize the magnitude of secondary emission. Surface treatments are often introduced to reduce the SEY since secondary electron emission strongly depends on the surface condition. For example, carefully desired surface structures, such as micro-porous array structure, rectangle groove, were ⁎ Corresponding author. E-mail address: [email protected] (M. Cao).
found to be effective for decreasing SEY of metal [9–14]. Another approach to secondary electron emission suppression is coating the surface by material with low SEY. The database summarized by Joy [15] can provide SEY data for many materials, which may be helpful for choice of coating material. Graphene was reported to be a material of ultralow secondary electron emission and carbon coating was found to be a way to reduce the SEY [16–18]. It was reported that electron bombardment could also decrease the SEY and the reason was considered to be the surface graphitization due to electron bombardment. It was believed that the sp3 type bond is the nature of SEY decreasing [19–21]. Besides the SEY, the secondary electron energy distribution is also an important knowledge and has been attracting many interests. Fine structures in energy spectra of secondary electrons and their dependence on primary electron energy have been carefully examined for graphene on nickel surface and been used for study the excited states of graphene interfaces [22–25]. Theoretical calculation using a planewave pseudo-potential method based on local density functional theory was also presented to study the graphene-substrate interaction [26]. Knowledge on energy distribution is helpful to understand the mechanism effect of graphene on secondary electron emission. In this work, we studied the effect of graphene coating on the secondary electron emission of copper foil. Both the SEY and secondary electron energy spectra have been considered. Significant reduction for SEY was achieved by graphene coating. Investigation on the
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distribution of the secondary electron energy shows that the peak position of energy spectra shifts to higher energy and the full width at half maximum becomes wider. We also analyzed these effects by introducing the effective surface barrier related to the graphene coating. An example of suppression of multipactor in microwave devices is demonstrated as a possible application of graphene coating. 2. Experiment 2.1. Preparation of graphene coating on copper foil Samples of copper foil both with and without graphene coating were prepared. The copper foil substrates with purity 99.8% and thickness 0.025 mm (Thermo Fisher Scientific #13,382) were used. The copper substrates were cleaned with isopropyl alcohol, acetone, ethanol and de-ionized water. Treatment using FeCl3 solution with concentration of 0.01 mol/L was introduced for etching and oxidation of the substrate. About 0.1 mL of FeCl3 solution was dropped onto each 1 cm2 substrate and then dried out in the atmosphere. It was found that the graphene nucleation density can be reduced and the total graphene coverage increase-rate can be increased with the FeCl3 solution treatment [27]. Graphene coating layer was grown using the chemical vapor deposition (CVD) method. The detailed information on growth behavior and characterization was reported in the previous publication [27]. Briefly, in a quartz tube furnace, the substrates were first heated up to 1000 °C and then annealed at this temperature for at least 20 min. In the CVD growth process, a two step introduction of CH4 flows was optimized for high quality graphene growth: 1 sscm for 15 s and 0.1 sccm for 30 min, separately. Throughout the whole process, A 40 sccm H2 flow was maintained. The CVD parameters are sketched in Fig. 1. The quality of samples were checked with scanning electron microscope (SEM) and Raman spectra as shown in Figs. 2 and 3, respectively. In the SEM image, it can be seen a uniform graphene lay was obtained. The Raman spectra indicate a single layer graphene on the substrate. 2.2. Measurement of secondary electron emission The experiments of secondary electron emission measurement were performed in an ultra-high vacuum experimental system outlined in Fig. 4. First, the samples were shaped in a rectangle of about 1 × 1.2 cm2 and loaded into the vacuum chamber with a condition of about 2 × 10−8 Pa. The SEYs of the prepared samples were measured at different primary electron energies with an incident direction perpendicular to the samples. The beam spot size of primary electrons was set to about 200 μm in diameter. Two sample currents Ip and Im were measured at a high positive bias (500 V) and low negative bias (−20 V) of the sample, respectively. When the sample is high positive biased, almost all emitted secondary electrons were dragged back to the sample and thus Ip is equal to the primary electron current. The primary electron current was furthermore calibrated by using a Faraday cup. For the negative bias of the sample, on the other hand, all emitted secondary electron
Fig. 1. Temperature and gas flow in CVD processing of graphene synthesis.
Fig. 2. SEM image of graphene on Cu substrate.
will be push out and we have Im = Ip − Ise, where Ise is the secondary electron current. Therefore, the SEY can be determined as σ= Ise/Ipe = 1 − Im/Ip. Here, three different points on a same sample were measured. The average value was taken as the SEY of the sample at certain condition and the deviations of the maximum SEY was less than 5%. The secondary electron energy spectra was detected at a fixed primary electron energy of 300 eV by the spectrometer (DESA 150 analyzer) vertically above the sample. This spectrometer takes advantage of a cylindrical mirror electrostatic fields. It has a large acceptance solid angle of 6% of 2π, with an energy resolution of less than 0.1 eV. The secondary electron energy spectra collection was lasted for about 0.5 min. Here, a negative bias (−15 V), was applied to the sample to collect secondary electrons more effectively. To avoid the possible influence of primary electrons, the primary electron currents were controlled constantly as about 10 nA and 0.6 nA for measurements of SEY and energy spectra, respectively. 3. Results and discussion 3.1. SEY and energy distribution Fig. 5 gives the measured SEY curves as a function of primary electron energy Epe for copper samples both with and without graphene
Fig. 3. Raman spectra of graphene on Cu substrate.
Please cite this article as: M. Cao, et al., Secondary electron emission of graphene-coated copper, Diamond Relat. Mater. (2016), http://dx.doi.org/ 10.1016/j.diamond.2016.09.019
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Fig. 4. Schematic of experimental set-up for secondary electron emission measurement in UHV.
Fig. 6. Measured secondary electron energy distributions of two kinds of samples.
coating. We see that the SEY first increases and then decreases with Epe, for each kind of sample. Evidently, the SEY for sample with graphene coating is smaller than that for copper substrate for Epe up to 1.5 keV. The max value of SEY for copper substrate is about 2.1, while the corresponding value for the substrate with graphene coating is decreased to about 1.5, indicating an obvious suppression effect of graphene coating on secondary electron emission. An important parameter for the SEY curve is the first crossover energies E1 at which the SEY is one. From the inset in Fig. 5, we can see that the value of E1 is increased from 24 eV to 38 eV. Increasing of E1 may increase the power threshold for multipactor in high power microwave devices. Fig. 6 shows the measured energy distributions of the secondary electron for both kinds of samples. There are two peaks for each curve: one is located at 300 eV which is the energy of primary electrons. The other peak's position is less than 50 eV. The first peak at energy of
primary electrons relates to the electrons backscattered elastically by the surface. The second peak with a lower energy relates to the so called true secondary electrons. In Fig. 6, the energy distribution varies due to graphene coating. It is evident that the elastically backscattered peak of the substrate with graphene coating is much higher than that of copper without graphene coating, indicating that more electrons which impinge on the surface of graphene-coated sample were rebounded. To look at the true secondary electrons with lower energy, we compared the energy distribution up to 50 eV in Fig. 7. Two important parameters are often concerned for the energy distribution of true secondary electrons. One is most probable energy (MPE) which is the position of the peak of the true secondary electrons. The other is the full width at half maximum (FWHM) for the peak. We see that graphene coating appears to enlarge the MPE and to broaden the FWHM. In particular, the MPE shifted to higher energy, from about 1.5 eV to about
Fig. 5. Measured curves of SEY as a function of EPE for samples with and without graphene coating, respectively.
Fig. 7. Energy distributions of true secondary electrons with energy less than 50 eV.
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4 eV after graphene. Simultaneously, the FWHM increased markedly from 4.7 eV to 10.9 eV. 3.2. Theoretical analysis We now give a theoretical analysis on the effect of graphene coating on the secondary electron emission. The graphene coating can affect both the primary electrons and the secondary electrons. The variation of the elastic backscattering peak in Fig. 6 reflects the rebounding effect of coated graphene on the primary electrons. The amount of the backscattered electrons is proportional to the area under the elastic backscattering peak. We use the symbols δ and η to denote the SEY related to true secondary electrons and backscattered electrons, respectively. From Fig. 6, we can calculate the ration of η to the total SEY σ. Together with the value of σ given in Fig. 5, we find that the elastic backscattered SEY η is about 0.08 and 0.04 for substrate with and without graphene coating, respectively. The distance between the carbon atoms in graphene layer is 0.142 nm, while the distance between two copper atoms is about 0.261 nm. For the primary electrons, it is more difficult to penetrate a more compact atom layer. Assuming the primary electron current is Ipe, the current that penetrated current is (1− η)Ipe. For the same number of primary electrons, less electrons can really enter the material and thus less internal electrons can be excited. This is one mechanism for suppression of graphene coating on secondary electron emission. Another more important effect from the graphene coating is the influence on emission of inner secondary electrons. After the primary electrons entering the material, electron-atom scatterings occur and some inner secondary electrons are generated after gaining the lose energy of primary electrons in inelastic scattering events. A secondary electron with certain energy will be scattered further until its energy is exhausted. Eventually, some scattered secondary electrons can reach the surface. For the copper substrate without graphene coating, emission of inner secondary electrons need only pass the surface barrier. However, if the surface is coated with graphene, the inner secondary electrons have to further more penetrate the graphene for an effective emission. Effect of graphene on secondary electrons is even more complicated than that on primary electrons. Most of the inner secondary electrons have energies of several eV, which is much lower than the primary electrons. It is more difficult for a low energy electron to penetrate the graphene than a high energy electron. Furthermore, unlike the primary electrons which are perpendicular to the graphene layer, the inner secondary electrons have a certain direction distribution. Only a part of energy Ecos2θ of an inner secondary electron can help for electron emission, where E is the energy of the electron and θ is the angle between the direction of the electron and the normal direction of graphene. We can use an effective barrier to describe the influence of the graphene coating on inner secondary electron emission. Chung and Everhart [28] have shown that the energy distribution of escaped secondary electrons has a form of SðEÞ∝
E ðE þ E F þ ϕÞ2
;
accurately. However, we can determine this value by experimental result of secondary electron energy spectrum. It is easy to determine the peak position of the energy distribution is ϕ/3 from Eq. (1). Therefore, the value of Δϕ should be Δϕ ¼ 3 Ep1 −Ep2 ;
ð3Þ
where Ep1 and Ep2 are the peak positions for substrate with and without graphene coating. From Fig. 7, we can get Δϕ =7.5 eV. Using this value, we calculated the energy distribution for copper substrate with graphene coating, the results are compared with the measurement data in Fig. 8. The solid lines in the figure are the calculation and the marks are measurement results. Good agreements have been achieved. It should be noted that the value of Δϕ obtained from the peak shift of secondary electron energy spectrum is just a phenomenological result which reflects the comprehensive effect of graphene layer. It may not directly correspond to the change of work function of the material since its magnitude is much bigger than other measurement result of graphene/metal interface [29]. A comprehensive analysis on the effect should consider information such as the band structure and the electron scattering at the interface. We will try to have a detailed study in the future. 3.3. A possible application The suppression effect of graphene coating on secondary electron emission can be used to increase the multipactor threshold of microwave comments. To demonstrate this application, the multipactor of parallel plate transmission line was examined. The properties of the secondary electron emission of the wall of the parallel plate were set to the former measurement data of copper with and without graphene coating, respectively. The susceptibility zones in fd-V plane for both cases were calculated using the method given in references [30,31] and results were shown as in Fig. 9. Here f is the frequency of the microwave, d the distance between the two plates, and V the maximum voltage between the plates. The shaded areas denote the susceptibility zones of multipactor. It can be seen that the susceptibility zone for copper substrate with graphene coating is much smaller than that of the copper.
ð1Þ
where E is the electron energy, EF the fermi level of the material and ϕ the work function. To reflect the effect of the graphene coating, an effective potential barrier Δϕ should be added to the work function and thus the energy distribution of secondary electron for copper substrate with graphene coating is SðEÞ∝
E ðE þ E F þ ϕ þ ΔϕÞ2
:
ð2Þ
The value of Δϕ depends on many factors that relate to the interface between the copper and graphene and is hard to be calculated
Fig. 8. Comparison of theoretical and measurement results of true secondary electron energy distributions.
Please cite this article as: M. Cao, et al., Secondary electron emission of graphene-coated copper, Diamond Relat. Mater. (2016), http://dx.doi.org/ 10.1016/j.diamond.2016.09.019
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Fig. 9. The susceptibility zones of multipactor.
4. Conclusions Graphene coating has been used to suppress the secondary electron emission. The graphene is synthesized by using CVD method. Measurement results for SEY and secondary electrons energy distribution show that the secondary electron emission is decreased significantly. The maximum value of SEY is reduced from about 2.1 to about 1.5. For the energy spectra, the elastically backscattered peak becomes much higher. The true secondary electrons peak position is shifted from about 1.5 eV to 4 eV and the FWHM is also increased. A model of effective surface barrier is used to explain the effect of graphene on secondary electron emission and good agreement is achieved between the theoretical and measurement results. A demonstration of simulation shows a possible application of graphene coating for suppression of multipactor in high power microwave comment. Acknowledgments Project supported by the National Natural Science Foundation of China (Grant No. U1537210, 11375139). The authors would like to thank Dr. Lin from Xi'an Jiaotong University for his help on calculation of the susceptibility zones of multipactor. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.diamond.2016.09.019. References [1] R. Cimino, L.A. Gonzalez, R. Larciprete, A. Di Gaspare, G. Iadarola, G. Rumolo, Detailed investigation of the low energy secondary electron yield of technical Cu and its relevance for the LHC, Phys. Rev. Spec. Top. Accel Beams 18 (5) (2015), 051002. http:// dx.doi.org/10.1103/PhysRevSTAB.18.051002 pp. [2] J.E. Yater, J.L. Shaw, K.L. Jensen, T. Feygelson, R.E. Myers, B.B. Pate, J.E. Butler, Secondary electron amplification using single-crystal CVD diamond film, Diam. Relat. Mater. 20 (5–6) (2011) 798, http://dx.doi.org/10.1016/j.diamond.2011.03.040. [3] S. Gupta, E. Heintzman, J. Jasinski, Secondary electron intensity contrast imaging and friction properties of micromechanically cleaved graphene layers on insulating substrates, J. Electron. Mater. 43 (9) (2014) 3458, http://dx.doi.org/10.1007/s11664014-3277-0. [4] D. Maric, M. Savic, J. Sivos, N. Skoro, M. Radmilovic-Radjenovic, G. Malovic, Z.L. Petrovic, Gas breakdown and secondary electron yields, Eur. Phys. J. D 68 (6) (2014), 155. http://dx.doi.org/10.1140/epjd/e2014-50090-x pp.
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Please cite this article as: M. Cao, et al., Secondary electron emission of graphene-coated copper, Diamond Relat. Mater. (2016), http://dx.doi.org/ 10.1016/j.diamond.2016.09.019