Electron Beam Etching of CaO Crystals Observed Atom by Atom

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Electron Beam Etching of CaO Crystals Observed Atom by Atom Yuting Shen, Tao Xu, Xiaodong Tan, Jun Sun, Longbing HE, Kuibo Yin, Yilong Zhou, Florian BANHART, and Litao Sun Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02498 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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Electron Beam Etching of CaO Crystals Observed Atom by Atom Yuting Shen1, Tao Xu1,2, Xiaodong Tan1, Jun Sun1, Longbing He1,2, Kuibo Yin1,2, Yilong Zhou1, Florian Banhart3 & Litao Sun1, 2* 1

SEU–FEI Nano–Pico Center, Key Lab of MEMS of Ministry of Education, Collaborative

Innovation Center for Micro/Nano Fabrication, Device and System, Southeast University, Nanjing 210096, P. R. China. 2

Center for Advanced Materials and Manufacture, Joint Research Institute of Southeast University and Monash University, Suzhou 215123, P. R. China

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Institut de Physique et Chimie des Matériaux, Université de Strasbourg, CNRS, UMR 7504, 67034 Strasbourg, France

*Address all correspondence to this author. e–mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

With the rapid development of nanoscale structuring technology, the precision in the etching reaches the sub-10 nm scale today. However, with the ongoing development of nanofabrication, the etching mechanisms with atomic precision still have to be understood in detail and improved. Here we observe, atom by atom, how preferential facets form in CaO crystals that are etched by an electron beam in an in situ high resolution transmission electron microscope (HRTEM). An etching mechanism under electron beam irradiation is observed that is surprisingly similar to chemical etching and results in the formation of nano-facets. The observations also explain the dynamics of surface roughening. Our findings show how electron beam etching technology can be developed to ultimately realize tailoring of the facets of various crystalline materials with atomic precision.

KEYWORDS. Electron beam etching, in situ, TEM, atomic precision, surface roughness.

Etching is an important production step in semiconductor technology and the emerging nano-electronics industry.1 The widely-used traditional etching methods are wet and dry etching with a precision that doesn’t go far beyond the micrometer scale.2, 3 Alternatively, focused ion beam (FIB) etching and gas-assisted electron beam induced etching (EBIE) are structuring techniques at the sub-10 nm scale.1, 4-6 FIB often results in unavoidable ion implantation, staining and surface damage.1 Although EBIE can avoid such degradation of the material, it requires gas injection and exhaust systems,1,

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and might lead to contamination with different gaseous

precursors.5-8 In addition, surface roughening caused by FIB etching9-11 and EBIE7,

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dominates the morphology of the surfaces and limits the structural precision. The precision of 10 nm does not satisfy future needs of nanofabrication, where the sub-nanometer or even atomic scale has to be reached. Thus, it is essential to understand the mechanisms of etching and surface roughening, so that nanostructures with a high level of structural perfection can be achieved. Electron beam irradiation with high electron energies in the transmission electron microscope (TEM) has been applied to tailor materials with nanometric16-19 or even sub-nanometric20, 21 precision. Unlike in EBIE, atom displacements caused by knock-on collisions with the energetic electrons cannot be neglected so that the electron beam can also serve as an etching tool.22-24 This method doesn't lead to implantation or requires any gaseous precursors. Furthermore, in situ observation in the TEM can provide insight into the etching process, which allows us to investigate the roughening mechanism in detail. Electron beam etching has already been used to sculpture nanostructures in different materials, including various metals (Au, Mg, Ni/Cr alloy)17-19, silica16, graphene25,

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, molybdenum

disulfide21 and other transition-metal dichalcogenides20. However, due to either the experimental limits or the material itself, it is difficult to record the specific etching processes at the atomic scale. CaO can act as recyclable sorbent to capture carbon dioxide (CO2) during hydrogen production27, 28and coal gasification processes29, and from exhaust gas30; it can act as solid base catalyst for biodiesel production31-33, which has great significance in the development of renewable energy and the reduction of CO2 emissions. During the CO2 capture and release process, the sintering leads to the reduction in surface area and pore volume, which causes the decay in sorbent reactivity and CO2 capture efficiency.28-30 In addition, the surface area is important for the catalyst activity during biodiesel production.31-33 Through electron beam irradiation, CaCO3 can be easily transformed to CaO, even at low electron beam energy,34 which


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can effectively avoid the negative impact of sintering. Moreover, the surface roughening caused by electron beam etching will effectively improve the surface area of CaO. In this article, we utilized electron beam irradiation to in situ etch CaO in a TEM, which permits simultaneous fabrication and imaging with atomic resolution. Our finding gives insight into the etching mechanism: it is similar to a close-to-equilibrium process, as in chemical etching, leaving well-defined facets; and the etching preferentially takes place at {200} facets and leads to etch pits with favourable {111} facets. The morphology changes on the surfaces explain the origin of surface roughening, which is inevitable unless the diameter of the electron beam reaches the atomic level.

Results and Discussion In our in situ investigation, an aberration-corrected TEM (FEI Titan 80-300) operating with an acceleration voltage of 300 kV was employed. The etching processes were carried out by a parallel electron beam (current density ~2.3 × 105 A ·m-2), i.e. not focused onto a spot, to limit the etching rate. Figure 1A shows a high resolution TEM (HRTEM) image of the edge of a CaO crystal. Figure 1B is the Fast Fourier transformation (FFT) of the structure in the red square in Figure 1A, where the red and yellow reflexes correspond to the {111} and {200} planes of CaO, respectively. The measured interplanar spacings match values reported for {111} and {200} lattice spacings (2.78 Å and 2.40 Å); and the measured angles fit well the angles between (11ത1) and (111ത) planes (70.52°) and between (111ത) and (200) planes (54.74°) of cubic CaO along . Figure 1C is the magnified HRTEM image of the area in the red square, and the structure matches the stick-and-ball model in Figure 1E, where the light yellow balls indicate the Ca atoms and the


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green balls indicate the O atoms. Figure 1D is the corresponding image simulation (see Methods) of CaO with NaCl structure along , which corresponds to the HRTEM image in Figure 1C. The bending of lattices in Figure 1C is caused by the internal stacking fault, which is not taken into account in the image simulation. Since the contrast of the O atoms is much weaker than that of Ca atoms, we mainly see the Ca atoms in the HRTEM image. Nevertheless, the etching process that is visible here corresponds to the removal of both Ca and O atoms. The energy dispersive X-ray (EDS) mapping images of CaCO3 and CaO in Figures S1 and S2 shows that they both include Ca, O and C. From the energy-loss spectroscopy (EELS) in Figure S3, CaCO3 transforms to CaO under the 300 keV electron beam, which agrees with the results reported in the literature.34 As shown in Figure 2, there are specific etching processes of two different edge structures. Figure 2A-E shows the etching of a surface; the atoms indicated in red are those that are sputtered in the next frame. At first, one of the atom columns in the outermost layer is sputtered and generates a vacancy-like defect in Figure 2B, the atoms next to the defect (in red) become less stable and are sputtered in the next exposure (5 s later). The atoms adjacent to the notch (Figure 2C) are in less stable positions and ejected by the electron beam as well. Diffusing surface atoms may adsorb at the edge to heal some defects randomly, which is a close-to-equilibrium process (dark blue balls in Figure 2D). The adatoms are ejected as seen in the next frame in Figure 2E. The further etching process is shown in Figure 2F-J. When the surface layer is sputtered off, atoms in the second layer become surface atoms and are successively sputtered (Figure 2G). With ongoing etching, {111} facets are gradually exposed, and the etching goes beyond the {200} facets. Finally a notch is formed on the CaO surface (Figure 2J). The detailed etching process is presented in Figure S5 and Video 1. Notably, the


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etching happens at {200} facets, where the etching speed is higher than at {111} facets, as shown in Figure S11A and S11B. There is another morphology of the CaO surface, which is a pyramidal island, as shown in Figure 2K-O. The etching process of the pyramid is different from the flat surfaces. Some representative frames from Figure S7 are shown here (the detailed etching process from Video 3). Edge atoms (red labels) with fewer neighbours, are sputtered preferentially, as shown in Figure 2K-M. In Figure 2M and 2N, the adatoms (blue) occupy unstable sites and are ejected later. As illustrated in Figure S4A-E, the atoms are sputtered layer by layer from the pyramid. As shown in Figures S5 to S8 and Videos 1 to 4, the different edge structures inevitably formed {111} facets, leading eventually to surface roughening. The number of atom columns in each layer was counted to statistically analyze the etching processes. Figure 3A is a flat surface from Video 2, where some atoms of the outermost layer had been sputtered; while Figure 3B is a pyramid from Video 3. The layers from the outside to inside are numbered 1-7 with different colors; each color corresponds to one layer. Figure 3C and 3D display the complete etching processes with the disappearance of atom columns from Figure 3A and 3B. The detailed etching processes are shown in Figures S6 and S7. It can be concluded that the undamaged facets have a certain stability; the atoms are not sputtered until a defect appears. As we know, TEM images are the projections of the sample, which only provide evidence for the etching of atom columns. To achieve more insight into the atomic processes, the calculations were carried out for the configurations with different atomic vacancies in Figure S14. We focus on one column of Ca atoms and the adjacent column of O atoms, which highlighted in Figure 4A-F. Figure 4A shows the pristine CaO surface. According to the energy analysis in Figure S14,


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the O atom labeled by the orange dash circle is ejected first (Figure 4B), followed by the neighboring Ca atom (Figure 4C), and so on. It can be concluded that the etching process is carried out in the order of O-Ca-O-Ca-O, as illustrated from Figure 4B to 4F. We can reveal the etching mechanism of CaO from the different etching processes described above. As in chemical etching, the stable {111} facets form at the expense of less stable {002} facets. The {002} surface of CaO is therefore roughened instead of a layer-by-layer removal. Figure 4(G-L) summarizes the etching processes of the flat surface of CaO. The corresponding etching mechanism is illustrated in Figure 4(M-R). First, the electron beam sputters some atoms from the outermost layer and generates vacancies (Figure 4M); then the atoms around the defects are sputtered (Figure 4N). After a certain time, new vacancies in the second layer appear (Figure 4O). The regions between the notches form pyramids when the notches become wider and deeper under electron beam irradiation (Figure 4P). Etching from the side walls of the pyramids happens by ejecting the atoms gradually from the facets. Finally this leads to surface roughening. Moreover, surface roughening is inevitable unless the size of the electron beam is as small as the interlayer distance. It is well known that elastic electron scattering in solids causes knock-on damage, where the energetic electrons transfer energy and momentum to displace the atoms. Inelastic electron-electron scattering results in damage due to electronic excitation and ionization.35, 36 For non-conductive oxides and under moderate irradiation (105 A ·m-2), the damage mechanism is often a combination of knock-on displacements and ionization or dissociation (radiolysis).36, 37 In our experiments, for the high incident beam energy (300 keV), the knock-on damage is expected to be the dominant mechanism.36-40 The threshold displacement energy (Ed) is the minimum energy for permanent displacements to form stable Frenkel pairs,41 which includes


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both the atomic bonding energy and the energy to move an atom from its lattice position to an interstitial site where recombination doesn't occur.36 Due to the Coulomb interaction between a charged ion and a vacancy, oxides usually have larger Ed, so that it is less likely that atoms in the bulk are displaced. The displacement threshold for surface atoms is generally lower than in the bulk since fewer bonds with neighboring atoms exist and the surface as well as the open space act as sinks for displaced atoms. The minimum electron energy (Emin) required to induce sputtering is calculated by: T=

ଶா೘೔೙ (ா೘೔೙ ାଶ௠೐ ௖ మ ) ெ௖ మ

(1)

where T is the energy required to sputter a Ca or O atom.22 For simplicity, here we use the binding energy to measure the threshold energy T, which is defined as a difference between the energy of the CaO sheet and the energy of its reconstructed configuration with the vacancy plus energy of the removed Ca or O atom. The maximum binding energy of Ca and O in Figure S14 is 10.9 eV and 10.3 eV, respectively. Thus, the electron energy Emin for displacing a Ca atom is 171 keV, and that of an O atom is 71 keV, which are both clearly below the incident electron beam energy of 300 keV. We can therefore assume that Ca and O atoms can both be sputtered under electron irradiation while O atoms are sputtered preferentially. During the etching process, some defects appeared at the surface which might be caused by radiolysis. Electron beam excited O atoms may generate a peroxyl linkage, resulting in bond breakage followed by the displacement of an atom that causes a point defect.36 When the sample contains many point defects (e.g. vacancies), amorphization may appear.42 In our work, no amorphization of the crystal appeared during the etching processes, which indirectly proves that the ionization damage is not dominant in the system. However, it does not mean that the


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ionization damage has no effect on the sample. Considerable radiolitic movement of the atoms may still occur.37 And the displacement of atoms is the combined effect of knock-on and ionization damage, while knock-on damage plays a more important role under 300 kV electron irradiation.37 Different electron beam current densities (1.2 × 106 A ·m-2 and 2 × 106 A ·m-2) were applied to etch the CaO surface, as shown in Figure S9. The etching process is the same as in the above-mentioned case, while the etching rate increases with beam current density (Figure S10). As reported, in knock-on displacement processes, the displacement rate is proportional to the beam current density,22 which is consistent with our results. It is known that the chemical etching process itself is isotropic, but the etching of crystalline materials with different packing densities on different facets lead to preferential etching of certain orientations and different etching rates at different surfaces.2,

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Basically, the same

mechanism happens in electron beam etching, which attacks non-favourable {002} facets preferentially and forms islands with more favourable {111} facets. The difference between chemical and electron beam etching is that chemical etching has high selectivity, which determines whether a dissolution reaction takes place by specific interaction between the solution and the material.2 Electron beam etching does not have the advantage of various etchants, but it can be used for more precise etching at the atomic scale, which can be used for, e.g. reducing the surface roughness. In the future, electron beam etching will allow to realize higher spatial control and higher selectivity when the diameter of the electron beam is on the sub-nanometer scale. As a further aspect, appropriate masks44 are required for chemical etching to produce 3D structures, while for electron beam etching, just a control of the electron beam is needed.


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Conclusion To conclude, we have investigated in real time the specific etching processes of CaO crystals at the atomic scale. Preferential sputtering happens at surface defects, while undamaged facets are more stable; and adatoms with a certain mobility heal surface defects at the same time. Although the irradiation leads to a situation that is locally far from equilibrium, the overall dynamics of surface migration is thermal and can, therefore, be described as a close-to-equilibrium process. During the whole etching process, the sputtering is preferable at {200} planes and leads to pits, where the side walls are the favourable {111} planes. Moreover, the evolving {111} faceting leads to surface roughening. The result of electron beam etching is similar to that of chemical etching although the atomic mechanisms are different. Our study on in situ electron beam etching has visualized the etching mechanism with atomic precision, which is of significant importance in the understanding of traditional etching and surface roughening. Due to the limits of CCD recording, the interval between the images is about 2-3 seconds, which limits the information about the temporal evolution. More insight could be gained by using high-speed direct electron recording. In the future, higher control of the etching can be achieved by reducing the diameter of the electron beam down to the sub-nanometer scale, which could avoid the roughening of less favourable surfaces such as {001} by atomic-scale selectivity. Moreover, nanostructuring with single atom precision can be exploited through electron beam processing techniques.

Methods TEM and EDX


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Transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) observations, and Electron energy-loss spectroscopy (EELS) were carried out in an aberration-corrected TEM (FEI Titan 80-300) operated at 300 kV acceleration voltage. The images were recorded by a charge coupled device camera (2k×2k, Gatan UltraScan 1000) with an exposure time of 0.5-1 s. Energy dispersive spectroscopy (EDX) was acquired by a Titan Cubed G2 with Super-X. The experiment under the 80 keV electron beam irradiation was carried out in an aberration-corrected TEM (FEI Titan G2 60-300) operated at 80 kV. Multislice Image simulations were performed based on the codes provided in the book “Advanced Computing in Electron Microscopy”45, using the same parameters as the experiment. Computational methods Our calculations were performed using the Vienna ab initio simulation package (VASP) code based on the density functional theory (DFT). The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) form was employed to describe the exchange-correlation term. The core electrons were treated with projector augmented wave (PAW) potentials. The plane wave cutoff energy was set to 400 eV, and further increasing this value had little effect on the results. The structures were relaxed until the energy and force on each atom were less than 10-6 eV and 0.01 eV/Å, respectively. The Brillouin zone integration was performed on a 9×3×3 gamma-centered k-point mesh.


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Figure 1. Atomic structure of a CaO crystal: (A) HRTEM image of the CaO edge structure. (B) FFT of the image in the red square in (A). The red and yellow reflexes correspond to the {111} and {200} plane, respectively. (C) Magnified HRTEM image of the region labeled by red square in (A). (D) The corresponding TEM imaging simulation. (E) Stick-and-ball models of CaO, where the light yellow balls indicate the Ca atoms and the light green balls indicate the O atoms.


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Figure 2. Etching processes of different edge structures. (A-E) Image series illustrating the in situ electron beam etching process of the surface layer of CaO. The further etching process is shown in (F-J). (K-O) The etching process of a pyramidal island on the surface. In order to distinguish the two etching processes, the Ca atoms were labeled by yellow and purple balls in the two edges. The left side is the stick-and-ball model, and the right side is the HRTEM image. The red balls indicate those atoms that are sputtered in the next frame, whereas the dark blue balls indicate the adatoms. Scale bar is 1 nm.


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Figure 3. Time-dependent number of atom columns in each layer, shown for a flat surface (A, Video 2) and a pyramid (B, Video 3). The layers from outside to inside are numbered 1-7. (C, D)


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Image series display the complete processes of time-resolved atom columns disappearance in A and B. The yellow overlays label the atoms that have been ejected.

Figure 4. Etching mechanism. (A-F) Detailed etching process atom by atom concluded from calculation: pristine surface (A), surface with one O atom vacancy(B), one O and one Ca atom vacancies (C), two O and one Ca atom vacancies (D), two O and two Ca atom vacancies (E), three O and two Ca atom vacancies (F). The orange dash circles indicate the Ca and O vacancies. (G-L) Representative time-lapse images of the whole etching process of the flat surface; the yellow overlays label the atoms that have been ejected. (M-R) Schematics of the formation mechanism for surface roughening. The light yellow balls indicate the Ca atoms and the light green balls indicate the O atoms. The Ca and O atoms that would be ejected are highlighted with red and green balls. ASSOCIATED CONTENT


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Supporting Information. Supplementary figures include EDS mapping and EELS spectra of samples, HRTEM image series of the detailed etching processes at the four different CaO surfaces, etching processes with different beam current densities, etching rate of {200} and {111} facets, etching process under the 80 keV electron beam, calculations on the binding energy. Supplementary videos include raw TEM videos showing the etching dynamics. The following files are available free of charge. Support information (PDF) Videos 1-6 (AVI) AUTHOR INFORMATION Corresponding Author *Litao Sun. E–mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Y. Shen carried out the TEM characterization and analysis; X. Tan performed the theoretical calculations; T. Xu and J. Sun took part in the discussion; L. Sun supervised the project, and revised the manuscript with L. He, K. Yin, Y. Zhou, and F. Banhart. All the authors contributed to the discussion and preparation of the manuscript. Funding Sources The research was supported by the National Natural Science Foundation of China (Grant No: 51420105003, 11525415, 11327901 and 61274114).


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ACKNOWLEDGMENT The research was supported by the National Natural Science Foundation of China (Grant No: 51420105003, 11525415, 11327901 and 61274114). In situ transmission electron microscopy studies (300 kV) and EELS were carried out at SEU–FEI Nano–Pico Center at Southeast University. The authors are indebted to Jie Li from Zhejiang University and Peng Wang from Nanjing University for their helps on 80 kV electron beam irradiation and EDS mapping. ABBREVIATIONS CaO, Calcium Oxide; HRTEM, high resolution transmission electron microscope; FIB, focused ion beam; EBIE, electron beam induced etching; EDX, energy dispersive X-ray; EELS, energy-loss spectroscopy; STEM, scanning transmission electron microscope; FFT, Fast Fourier transformation; Ed; threshold displacement energy; Emin, minimum electron energy. REFERENCES 1. Utke, I.; Hoffmann, P.; Melngailis, J. J. Vac. Sci. Technol., B 2008, 26, 1197-1276. 2. Köhler, M., Wet-Chemical Etching Methods. In Etching in Microsystem Technology, Wiley-VCH Verlag GmbH: Weinheim, New York, Chichester, Brisbane, Singapore, Toronto, 2007; pp 29-110. 3. Köhler, M., Dry-Etching Methods. In Etching in Microsystem Technology, Wiley-VCH Verlag GmbH: Weinheim, New York, Chichester, Brisbane, Singapore, Toronto, 2007; pp 111-171. 4. Tseng, A. A. Small 2005, 1, 924-939.


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Figure 1. Atomic structure of a CaO crystal: (A) HRTEM image of the CaO edge structure. (B) FFT of the image in the red square in (A). The red and yellow reflexes correspond to the {111} and {200} plane, respectively. (C) Magnified HRTEM image of the region labeled by red square in (A). (D) The corresponding TEM imaging simulation. (E) Stick-and-ball models of CaO, where the light yellow balls indicate the Ca atoms and the light green balls indicate the O atoms. 177x166mm (300 x 300 DPI)


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Figure 2. Etching processes of different edge structures. (A-E) Image series illustrating the in situ electron beam etching process of the surface layer of CaO. The further etching process is shown in (F-J). (K-O) The etching process of a pyramidal island on the surface. In order to distinguish the two etching processes, the Ca atoms were labeled by yellow and purple balls in the two edges. The left side is the stick-and-ball model, and the right side is the HRTEM image. The red balls indicate those atoms that are sputtered in the next frame, whereas the dark blue balls indicate the adatoms. Scale bar is 1 nm. 177x105mm (300 x 300 DPI)


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Figure 3. Time-dependent number of atom columns in each layer, shown for a flat surface (A, Video 2) and a pyramid (B, Video 3). The layers from outside to inside are numbered 1-7. (C, D) Image series display the complete processes of time-resolved atom columns disappearance in A and B. The yellow overlays label the atoms that have been ejected. 177x203mm (300 x 300 DPI)


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Figure 4. Etching mechanism. (A-F) Detailed etching process atom by atom concluded from calculation: pristine surface (A), surface with one O atom vacancy(B), one O and one Ca atom vacancies (C), two O and one Ca atom vacancies (D), two O and two Ca atom vacancies (E), three O and two Ca atom vacancies (F). The orange dash circles indicate the Ca and O vacancies. (G-L) Representative time-lapse images of the whole etching process of the flat surface; the yellow overlays label the atoms that have been ejected. (M-R) Schematics of the formation mechanism for surface roughening. The light yellow balls indicate the Ca atoms and the light green balls indicate the O atoms. The Ca and O atoms that would be ejected are highlighted with red and green balls. 177x104mm (300 x 300 DPI)


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TOC 34x26mm (300 x 300 DPI)


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Electron Beam Etching of CaO Crystals Observed Atom by Atom

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