theoretical practice on grazing-exit energy dispersive X-ray spectroscopy as a surface analysis

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1 A theoretical practice on grazing-exit energy dispersive X-ray spectroscopy as a surface analysis strategy to investigate BiVO4 nano-films This is the peer reviewed author accepted manuscript (post print) version of a published work that appeared in final form in: Golmojdeh, H, Zanjanchi, MA, Sohrabnejad, S, Mazloom, J & Hojati, Talemi Pejman 2014 'A theoretical practice on grazing-exit energy dispersive X-ray spectroscopy as a surface analysis strategy to investigate BiVO4 nano-films' X-ray spectrometry, vol. 43, no. 3, pp This un-copyedited output may not exactly replicate the final published authoritative version for which the publisher owns copyright. It is not the copy of record. This output may be used for noncommercial purposes. The final definitive published version (version of record) is available at: Persistent link to the Research Outputs Repository record: General Rights: Copyright and moral rights for the publications made accessible in the Research Outputs Repository are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognize and abide by the legal requirements associated with these rights. Users may download and print one copy for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the persistent link identifying the publication in the Research Outputs Repository If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

2 This is the accepted version of the following article: Golmojdeh, Hosein et al (2014), A theoretical practice on grazing-exit energy dispersive X-ray spectroscopy as a surface analysis strategy to investigate BiVO4 nanofilms, X-ray spectrometry, vol. 43, no. 3, pp , which has been published in final form at

3 A theoretical practice on Grazing Exit-Energy Dispersive X-ray Spectroscopy (GE-EDS) as a surface analysis strategy to investigate BiVO 4 nano-films Short running title: A Theoretical practice on GE-EDS as a surface analysis strategy Hosein Golmojdeh a, *, Mohammad Ali Zanjanchi a, Shabnam Sohrabnejad a, Jamal Mazloom b, Pejman Hojati-Talemi c a. Department of Chemistry, Faculty of Science, University of Guilan, P.O. Box 1914, Rasht,Iran b. Department of Physics, Faculty of Science, University of Guilan, P.O. Box 1914, Rasht,Iran c. Mawson Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Corresponding author: H. Golmojdeh: golmojdeh@guilan.ac.ir Keywords: BiVO 4, nano-film, MC X-Ray, CASINO, simulation, Electron beam Acknowledgment: The authors wish to acknowledge the developers of MC X-Ray and CASINO for preparation of the main codes and offering them as sharewares. 1

4 Abstract In the current work a thin film of bismuth vanadate was defined over a silicon substrate and a calculative Monte Carlo approach was followed to achieve the best grazing-exit angle to acquire compositional data from top few nanometers of surface. This strategy is very beneficial in order to increase X-ray signals originated from surface and diminish the background X-ray signals started off from the substrate. In this regard, GE-EDS can be considered as an accessible and economical analytical tool to investigate thin films and nano-layers. The major advantage of this method is that just by applying a re-arrangement in a scanning electron microscope, it can be used to study compositional properties of thin layers. In this contribution a theoretical approach using Monte Carlo models was used to simulate the behavior of electron beams impinging onto BiVO 4 nano-layers with thickness of 50 nm and electron trajectories inside the film. Characteristic X-rays and spatial energy distribution of the backscattered electrons were also calculated. Under grazing-exit angle of around 0.5º the best surface signal/background noise ratio was achieved. Introduction: Scanning electron microscopy has become a widely useful tool to study surface morphology, uniformity and also chemical composition of various materials and thin films. [1]. In a typical electron microscope, an electron beam with a spot size of around one to tens of nanometers is impinging onto the surface of a sample. Various interactions between the incident electrons and the sample generate various signals that produce the images or spectra. The size and shape of a given interaction volume depends on compositional parameters of sample (such as density, chemical composition and atomic weights) and technical operating parameters of microscope and electron beam (such as operating voltage, electron beam diameter and tilt angle of specimen, etc.). [2] Incident electron beam with enough energy can excite atoms and result in emitting characteristic X-rays. Energy dispersive X-ray spectroscopy is usually considered as semiquantitative analysis method for bulk samples. However, in order to obtain a reliable analysis of the top layers of the sample strategies needs to be considered. Re-arranging the position of X-ray detectors toward surface is one of the strategies for achieving smaller take-off angles (or grazing- 2

5 exit angles). This method is dependent on the fact that characteristic X-rays of substrate under grazing exit arrangement of detector are more attenuated than those originated from the surface layer. On the other hand the refractive index of X-rays is slightly less than 1, that means X-rays originated from deeper substrate material are refracted at a larger exit angle than the X-rays from the surface layers. [3] Some pioneering experiments have been reported by Tsuji et al. [4-7] The main idea behind this technique is that detecting and measuring the emitted X-rays under an exit angle of less than 1 degree, allows for analysis of chemical composition of the materials just beneath the surface. Under these conditions X-rays emitted from the substrate beneath surface thin layer decrease and almost vanish. Geometry and arrangement of conventional and grazing exit electron probe microanalysis is shown in Fig. 1. Fig. 1. Arrangement of X-ray detector in (a) conventional and (b) grazing-exit electron probe micro analysis. Take off angle of detector (θ) under grazing exit is usually under 1º. We have studied experimentally some of the properties of BiVO 4 material so far. [8-9] Here BiVO 4 was chosen to be investigated via a theoretical approach. Bismuth vanadate compounds recently are being used as visible-light activated photocatalysts. [10-13] These compounds have been prepared in three crystalline forms including tetragonal zircon, monoclinic sheelite and tetragonal sheelite. It is claimed that the monoclinic form has the best photocatalytic performance. [14] In line with our previous work on studying of BiVO 4, investigating the interaction of electron beams over thin films of this material seems as a complementary step that allows for accurate analysis and characterization of this catalyst. Bismuth vanadate is a promising photoactive material and it might have potential application in solar devices. 3

6 Developing innovative analysis routs might facilitate its studying, though GE-EDS is a general methodology and might apply to all thin films. Monte Carlo approach uses random numbers and weighting factors to predict the statistical distributions of physical phenomena. [1] Mathematical basis of Monte Carlo approach for this purposes is described previously. [1, 15, 16] Some of the first works on the calculations regarding the electron beam-solid state matter interactions are investigated by Bethe [17] and Mott and Massey [18] that are being used as the basis of some modern softwares. As a general perspective, when an energetic electron beam impacts onto a surface, it can penetrate and interact with individual atoms in elastic and inelastic ways. Some of the elastically scattered electrons can finally escape from the surface and generate backscattered electrons (BSE). Electrons undergoing inelastic scattering processes can produce secondary electron and some analytical signals especially characteristic X-rays. The spatial, angular and energy distributions of backscattered electrons can be calculated and recorded. Validity of such strategy is discussed in some contributions. [19] Everhart et al. [20] used a chemical etching technique to visualize pearshaped interaction volumes inside a polymethylmethacrilate sample. For denser materials this process is not useful and the Monte Carlo calculations can be used to envisage and track electron trajectories and for this purpose the simulation softwares seem to be very useful in interpretation of the structural properties of materials. Methods and procedures: Simulation We used CASINO (Version 2.42) and MC X-ray (Version ) to define a nano-layer of BiVO 4 by 50 nm over a silicon substrate and to reproduce characteristic X-rays and electron trajactories. CASINO developed by Drouin group at Université de Sherbrooke. [21] Gauvin group at McGill University has also developed a software called win X-ray that enables prediction of energy dispersive x-ray spectra (EDS) for a virtually defined sample. [22] MC X-ray (a new extension of win X-ray) was used to simulate the EDS spectra under grazing exit conditions and CASINO was employed to compare relative intensities of characteristic X-rays for three elements of Bi, V and Si. Accelerating voltage of 20 KeV was chosen as operational voltage based on common experimental observations. After defining the virtual sample for softwares, the microscope settings were adjusted as follow: 4

7 Accelerating voltage (20 KeV), angle of the incident beam (0, normal to the sample surface), probe diameter of 10 nm and take-off angle of the X-ray detector (40, 10, 5, 1, 0.5 and 0.1 ) are defined. At the final steps of the process of defining the data acquiring conditions, the physical model, number of simulated electrons and the minimum energy to which the trajectory is followed by software, are defined. A huge number of electron trajectories should be predicted to have a statistically acceptable replication of the physical routs happening in real event. We calculated 10 4 electron trajectories for our simulation. The estimated of relative error for this number of electrons is about 1 %. The trajectory of each individual electron is followed till its energy drops below the minimum value of 0.5 KeV. Results and Discussion: Fig. 2 shows the simulated spectra calculated for BiVO 4 thin film by defining 40, 10, 5, 1, 0.5 and 0.1 as take-off angles of detector. It can be seen that by reaching the grazing angle (less that 1º) the signals associated with the vanadium and bismuth are increasing while the Si characteristic X-ray intensity is declining. 5

8 Fig. 2. (A) A schematic depiction of thin film of BiVO 4 over a substrate of silicon defined virtually in current study, (B) Simulated energy dispersive X-ray spectra (EDS) at 20 KeV accelerating voltage for different take-off angles of detectors: (a) 45º, (b) 10º, (c) 5º, (d) 1º, (e) 0.5º and (f) 0.1º. 6

9 Shape and depth of the interaction volume A beam of electron with energy of 20 KeV was impinged onto a sample consisted of a 50 nm BiVO4 thin film over a silicon substrate. Fig. 3 shows that the most of the back-scattered electrons come about within the top 500 nm of specimen. Apparently the total interaction volume of electron is about 4 µm3. It is can be observed that the interaction volume grows very fast right after the electrons passed the thin film. This can be partially because of the lower density of silicon compared to that of BiVO4.[1] Interaction volumes of 20 KeV electron beam under the same conditions in pure Si and BiVO4 are shown in Fig. 4. BiVO4 with a density of around 6.25 g/cm3 shows a semi-spherical interaction volume of about 1 µm3 while silicon with a density around 2.3 g/cm3 demonstrates an elongated pear-shaped interaction volume of about 5 µm3. Fig. 3. Simulation of of 20 KeV electrons impacting into the 50 nm thin film of monoclinic bismuth vanadate coated over silicon substrate. Only 300 simulated electron trajectories are depicted here. Backscattered electron trajectories are depicted as red lines, (A) close-up view and (B) the whole interaction volume. 7

10 Fig. 4. Simulation of of 20 KeV electrons impinging into (A) pure silicon and (B) BiVO 4. All other conditions are the same as mentioned for Fig. 3. Sampling volume of backscattered electrons Fig. 5 shows the distribution of the penetration depth of the electrons and the backscattered electrons for the specimen at 20 KeV. Over 90 percent of electrons are originated from a volume spread over 3 µm alongside Z axis (electron beam direction). The maximum sampling depth of backscattered electrons is around 1.2 µm at 20 KeV with a considerable maximum at the interface of nano-layer and substrate (about 47 nm of depth). Fig. 5 (A) and Fig. 5 (B) show the distribution of total electrons and backscattered electrons alongside Z axis, respectively. Fig. 5 (C) shows the angular distribution of backscattered electrons. It can be seen that most of these electrons are escaping the surface at the angles of 45º. Fig. 5 (D) shows the energy distribution of the backscattered electrons at accelerating voltage 20 KeV. Two distributions of electrons can be recognized here. One at 19 KeV and the other at 16 KeV, originated from the thin film and the 8

11 substrate, respectively. In another approach we increased the thickness of BiVO 4 layer from 50 nm to 100, 200 and 500 nm. Associated energy distributions and interaction volumes are depicted in Fig. 6. By increasing the thickness of the thin film energy distribution maximum around 16 KeV disappears and more backscattered electrons show energy values close to the initial electron beam energy which is in agreement with some previous reports. [2] Fig. 5. Distribution of penetration depth of (A) all electrons and (B) backscattered electrons at accelerating voltage of 20 KeV. (C) Angular and (D) energy distributions of backscattered electrons. 9

12 Fig. 6. (A-1), (B-1) and (C-1): Energy distribution of the backscattered electrons for different thickness of BiVO4 over silicon substrate. (A-2), (B-2) and (C-2): associated interaction volumes. Inset of (C-2) is depicting a magnification of the interaction volume. Sampling volume of characteristic X-rays When electron energy is enough to satisfy the excitation energies of certain elements, characteristic X-rays can be produced. It can be seen from Fig. 7 that the majority of X-ray characteristics are generated near electron impact point. The sampling volume can be considered 10

13 about 4 µm 3 at 20 KeV accelerating voltage for analytical purposes under current theoretical conditions. This sampling volume is dependent on the initial accelerating voltage and also on the density of the target. Fig. 8. shows the calculated ratios of intensities of each X-ray characteristic line of each element to the ones calculated for pure elements under the same conditions. We can track these relative intensities to find the best theoretical take-off angle of detector. A reducing trend for Si intensity and increasing trend for V and Bi intensities were calculated by reducing the take-off angle of detector and approaching toward grazing-exit angle. For take-off angle of 0.5º the least intensity of Si and was achieved. Although the intensities of the Bi and V characteristic X-rays are diminishing too, but the decreasing rate of Si signal is more considerable and results in higher signal/noise ratio for surface layer. We suggest this angle as the best grazing-exit angle of X-ray detector to collect the most data from surface because of the least interference from substrate characteristic X-rays. Fig. 7. Depth of distribution for X-rays at accelerating voltage of 20 KeV at different take off angles of detector: (A) and (B) silicon K line, (D) bismuth M line and (B) vanadium K a line. 11

14 Figure 8. Calculated relative intensities of Si, V and Bi in thin film (50 nm) to the intensities of each characteristic X-ray in pure Si or BiVO 4. Conclusion: In the current contribution, we employed two sofwares which are based on Monte Carlo approach for simulating the electron-solid interactions in a specimen consisted of 50 nm bismuth vanadate thin film over silicon substrate applying the accelerating voltages of 20 KeV. The shape and size of the interaction volume, the spatial and energy distribution of backscattered electrons and characteristic X-rays were elucidated. It was found that the size of the interaction volume and sampling volume of the backscattered electrons is highly dependent on thickness and density of surface layer and substrate. The interaction volume for pure bismuth vanadate was found to be more spherical-ellipsoidal rather than a pear-shaped which was found for pure silicon. The 12

15 distribution of backscattered electrons and characteristic X-rays is more probable near the electron impact point at the surface. The maximum depth of the backscattered electrons is around 25% of the interaction volume depth and the sampling volume can be considered about 4 µm 3 at 20 KeV accelerating voltage. By decreasing the take-off angle of X-ray detector the optimum grazing angle of 0.5º was found at which V and Bi signals are relatively dominant while Si signal is almost vanished. References [1] J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L. Sawyer, J. Michael, Scanning Electron Microscopy and X-Ray Microanalysis, Kluwer Academic/Plenum Publishers, New York, [2] H. S. Wong, N. R. Buenfeld, Cem. Concr. Res. 2006; 36, [3] Z. Spolnik, K. Tsuji, K. Saito, K. Asami, K. Wagatsuma, X-Ray Spectrom. 2002; 31, [4] K. Tsuji, K. Wagatsuma, R. Nullens, R. E. Van Grieken, Anal. Chem. 1999; 71, [5] K. Tsuji, Z. Spolnik, K. Wagatsuma, J. Zhang, R. E. Van Grieken, Spectrochim. Acta Part B, 1999; 54, [6] Tsuji K, Murakami Y, Wagatsuma K, Love G.X-Ray Spectrom. 2001; 30, 123. [7] K. Tsuji, in X-Ray Spectrometry: Recent Technological Advances, (Eds: K. Tsuji, J. Injuk, R. Van Grieken), John Wiley & Sons Ltd, Chichester, 2004, pp. 293 [8] H. Golmojdeh, M. A. Zanjanchi, Cryst. Res. Technol. 2012; 47, [9] H. Golmojdeh, M.A. Zanjanchi, M. Arvand, Photochem. Photobiol. 2013; 89: [10] L. Dong, S. Guo, S. Zhu, D. Xu, L. Zhang, M. Huo, X. Yang, Catal. Commun. 2011; 16, [11] G. S. Li, D. Q. Zhang, J. C. Yu, Chem. Mater. 2008; 20, [12] D. Jing, M. Liu, J. Shi, W. Tang, L. Guo, Catal. Commun. 2010; 12, [13] Y. Zhou, K. Ville, A. Heel, B. Probest, R. Kontic, G. R. Patzke, Appl. Catal. A, 2010; 375, [14] M. Shang, W. Wang, L. Zhou, S. Sun, W. Yin, J. Hazardous Mater. 2009; 172,

16 [15] R. Shimizu, D. Ze-Jun, Rep. Prog. Phys. 1992; 55, [16] D. C. Joy, Monte Carlo Modeling for Microscopy and Microanalysis, Oxford University Press, New York, [17] H. A. Bethe, Ann. Phys. (Leipzig) 1930; 5, [18] N. F. Mott, H. S. W. Massey, The Theory of Atomic Collisions, Oxford University Press, Oxford, [19] D. F. Kyser, H. Niedrig, D. E. Newbury, R. Shimizu, Electron Beam Interactions with Solids for Microscopy, Microanalysis, and Microlithography, Scanning Electron Microscopy Inc., Chicago, [20] T. E. Everhart, R. F. Herzog, M. S. Chang, W. J. DeVore, Proceedings of the 6th International Conference on X-ray Optics and Microanalysis. University of Tokyo Press, Tokyo, [21] D. Drouin, A. R eal Couture, D. Joly, X. Tastet, V. Aimez, R. Gauvin, Scanning, 2007; 29, 92. [22] R. Gauvin, E. Lifshin, E. Demers, P. Horny, H. Campbell, Microsc. Microanal. 2006; 12, [23] G. H. Bernstein, A. D. Carter, D. C. Joy, Scanning, 2013; 35, 1. 14

17 Figure Captions: Figure. 1. Arrangement of X-ray detector in (a) conventional and (b) grazing-exit electron probe micro analysis. Take off angle of detector (θ) under grazing exit is usually under 1º. Figure. 2. (A) A schematic depiction of thin film of BiVO 4 over a substrate of silicon defined vitually in current study, (B) Simulated energy dispersive X-ray spectra (EDS) at 20 KeV accelerating voltage for different take off angles of detectors: (a) 45º, (b) 10º, (c) 5º, (d) 1º, (e) 0.5º and (f) 0.1º. Figure. 3. Simulation of of 20 KeV electrons impinging into 50 nm thin film of monoclinic bismuth vanadate coated over silicon substrate. Only 300 simulated electron trajectories are depicted here. Backscattered electron trajectories are depicted as red lines, (A) close-up view and (B) the whole interaction volume. Figure. 4. Simulation of of 20 KeV electrons impinging into pure (A) Silicon and (B) BiVO 4. All other conditions are the same as mentioned for Figure. 3. Figure. 5. Distribution of penetration depth of (A) all electrons and (B) backscattered electrons at accelerating voltage of 20 KeV. (C) Angular and (D) energy distributions of backscattered electrons. Figure. 6. (A-1), (B-1) and (C-1): Distribution of penetration depth backscattered electrons for different thickness of BiVO 4 over silicon substrate. (A-2), (B-2) and (C-2): associated interaction volumes. Inset of (C-2) is depicting a magnification of the interaction volume. Figure. 7. Depth of distribution for X-rays at accelerating voltage of 20 KeV at different take off angles of detector: (A) and (B) silicon K line, (D) bismuth M line and (B) vanadium K a line. Figureure 8. Calculated relative intensities of Si, V and Bi in thin film (50 nm) to the intensities of each characteristic X-ray in pure Si or BiVO 4. 15