TY - JOUR
AU - Kondo,, Y.
AB - Abstract An aberration-corrected electron microscope developed in CREST project has been applied for imaging atoms and clusters buried inside crystals. The resolution of the microscope in scanning transmission electron microscopy (STEM) has experimentally proved to be better than 47 pm by use of a cold-field emission gun at 300 kV. The high resolution has given an advantage for imaging light elements such as lithium atoms discriminating one by one. Moreover, a three-dimensional structure imaging has been demonstrated for dopant clusters by a sub-50 pm STEM, using its high depth resolution. Introduction Aberration-corrected electron microscopes have been used to study modern material sciences and nanotechnology with very good results which have never been obtained without a correction technique. Aberration correction in transmission electron microscopy (TEM) [1] and scanning transmission electron microscopy (STEM) [2] is a ‘conventional’ technique in modern electron microscopy, because it increases the image resolution; aberration-corrected STEM (hereafter abbreviated as AbC-STEM) can realize an electron probe as sharp as the diffraction limit, 0.61λ/θ, where λ is the wavelength of the electrons and θ the convergent semi-angle of the probe. A sharp probe benefits high-contrast imaging of individual atoms [3], which enables us to discriminate elements according to their Z-number [4]. The depth resolution increases with increasing resolution. Thus, resolution benefits three-dimensional imaging of specimens. Therefore, AbC-STEM allows us one-by-one imaging of atoms, and three-dimensional imaging of clusters which localized randomly in matrices. Imaging local nanostructures buried in matrices at three dimensions with their dynamic changes is enabled by the AbC-STEM. Here we show the development of a new aberration-corrected microscope which has achieved a 47 pm resolution by AbC-STEM [5]. The microscope, R005, has enabled Z-contrast imaging of lithium, carbon, oxygen, silicon, copper, gallium, germanium, arsenic, antimony, tungsten and gold atoms. Here, AbC-STEM imaging of lithium atoms and arsenic/antimony dopants are shown. In addition to high-angle annular dark-field (HAADF) imaging [4], we used annular bright-field (ABF) imaging [6,7]. A new R005 microscope The microscope newly developed uses a new cold-field emission gun (CFEG) and new aberration correctors for the probe forming and image forming, as shown in Fig. 1 [5]. Figure 2 shows an HAADF image of the ‘47 pm’ spaced ‘dumbbell’ image of the [114] atomic columns in a germanium crystal. The simulated intensity profile of the electron probe (Fig. 2b) supports HAADF imaging at the sub-50 pm resolution. Fig. 1. Open in new tabDownload slide An R005 microscope for STEM–TEM imaging at the sub-50 pm resolution. Fig. 1. Open in new tabDownload slide An R005 microscope for STEM–TEM imaging at the sub-50 pm resolution. Fig. 2. Open in new tabDownload slide A 47 pm spaced dumbbell image of a germanium crystal. Each dumbbell is the pair of the [114] atomic columns. (a) HAADF image. (b) Simulated profile of the ABC-STEM probe, where energy spread ΔE is 0.4 eV, Cs = 0, Cc = 1.65 mm, with a wavelength of 1.97 pm and a Gaussian probe with an FWHM of 16 pm. Fig. 2. Open in new tabDownload slide A 47 pm spaced dumbbell image of a germanium crystal. Each dumbbell is the pair of the [114] atomic columns. (a) HAADF image. (b) Simulated profile of the ABC-STEM probe, where energy spread ΔE is 0.4 eV, Cs = 0, Cc = 1.65 mm, with a wavelength of 1.97 pm and a Gaussian probe with an FWHM of 16 pm. The development of a new aberration corrector with an automatic correction system called SRAM [8] has enabled the sub-50 pm resolution. This R005 microscope has new designs of microscope column, electronic circuits and mechanical anti-vibration system. The new spherical aberration corrector has an asymmetric geometry in order to achieve small chromatic aberration for STEM probe (Cc = 1.65 mm) and for TEM imaging (Cc = 1.54 mm). In addition, the asymmetric Cs corrector compresses the parasitic aberrations much more than the symmetric ones [1]. Development of a new CFEG is another important factor for this achievement. The new CFEG has achieved 0.32 eV energy spread at 300 kV acceleration voltage and a constant emission current for 8 h of continuous observation, which enabled quantitative intensity analysis of the present cluster imaging by HAADF and lithium column imaging by ABF. In the present HAADF and ABF imaging, we used the convergent semi-angle of the incident probe of 30 mrad and the probe current is ranging from 8 to 40 pA. The smallest probe diameter [full-width at half-maximum (FWHM)] was estimated to be about 40 pm at 8 pA. Lithium atom imaging Lithium atoms and ions are of increasing interest because of their behavior in rechargeable batteries [9]. Yet they have not been detected individually by electron microscopy, although elaborate works have been done in TEM imaging [10,11]. During imaging, lithium columns containing a few atoms were detected by the R005 microscope [12]. Figure 3 shows the structure and an ABF image of the LiV2O4 crystal with the spinel structure (space group, Fd−3m) viewed from the [110]-direction. In this view, atomic columns of vanadium (V) atoms, oxygen (O) atoms or lithium (Li) atoms are imaged with dark-contrast imaging. The positions of dark dots agree well with the projected structure of LiV2O4. The simulation by multi-slice calculation reproduces the observed intensity profile as given in Reference [12], although oxygen columns gave less intensity than that expected from the simulation. In the simulation, an ADF image observed simultaneously was used to determine the thickness of the specimen. The detector angle used was 20–30 mrad for ABF imaging, and it was 75–200 mrad for ADF imaging. Fig. 3. Open in new tabDownload slide (a) Bird's view of LiO2V4 crystal and (b) ABF image of LiO2V4 crystal, viewed from the [110]-direction. Dark dots are the images of vanadium columns (α and β sites), oxygen columns and lithium columns. Lithium atomic columns are located on the long-diagonal line of the rhombic unit, separated by 0.1 nm from the oxygen column. Specimen thickness is around 4 nm. Fig. 3. Open in new tabDownload slide (a) Bird's view of LiO2V4 crystal and (b) ABF image of LiO2V4 crystal, viewed from the [110]-direction. Dark dots are the images of vanadium columns (α and β sites), oxygen columns and lithium columns. Lithium atomic columns are located on the long-diagonal line of the rhombic unit, separated by 0.1 nm from the oxygen column. Specimen thickness is around 4 nm. The sub-50 pm resolution is critically required for the present lithium imaging, since the lithium columns are separated only by 0.1 nm from the oxygen column. The contrast and width of the column images were determined, after simulation, by the effective probe diameter. Here, the CFEG with small emitter size, which affects the probe diameter, is critical for imaging lithium columns. The R005 microscope working with CFEG has shown imaging capability of a few lithium ions by ABF, which promises lithium microscopy and microanalysis. Arsenic- and antimony-dopant clusters in a silicon crystal Clustering of dopant atoms (antimony and arsenic) in the silicon crystal was studied by HAADF. Cluster imaging is an interesting issue of STEM, although imaging of individual heavy dopants had already been done [13,14]. The dopant clustering in silicon-based transistors, having 45 nm gate length in commercial and 20 nm in research, is a key issue in nano-device technology, which affects carrier density and localization. An additional scattering signal by the dopant is detected in HAADF images, provided a silicon atom is substituted by a dopant. The scattering signal is intense when the 50 pm electron probe is focused at the dopant. The columns showing high intensity in an HAADF image thus give a location map of the dopant atoms [15]. When dopants substitute, say, two neighboring silicon sites to form a cluster, the substituted columns should give different column intensity if the two locate at different depths. Here, aberration correction and CFEG are basically important and critical, because a sharp probe gives a higher column intensity and a higher depth resolution [15,16]. Figure 4a and b shows the HAADF images of antimony (Sb)-doped silicon [16] and arsenic (As)-doped silicon [15] at ∼5 × 1020 cm−3 concentration (1%), respectively. The images are the [001] view of the doped specimens, and the columns are separated by ∼0.2 nm from one another. The intensity of non-doped Si columns had a Gaussian distribution due to their thickness variation and statistical noise [17]. The Si columns containing Sb dopants had a higher intensity than the non-doped columns. Their column intensity varies depending on the depth of the Sb dopants [16] or on the distance (z) from the in-focus plane. The calculated column intensity variation on the distance, Ic(z), was found to be approximately as Ic(z) = ISb exp(−z2/2d2) + ISi. The depth resolution (d) was 1.36 nm. The Sb dopant in the Si columns showed the maximum intensity (ISb + ISi), which was two times higher than that of the non-doped columns (ISi): ISb/ISi = 2. For a smaller convergent angle of the incident electron wave, we obtained smaller ISb/ISi and larger d. In a cyclic focus series of HAADF images (change in ±z), we confirmed that cyclic change of an Sb-doped column intensity is given by Ic(±z), which enables us to obtain a three-dimensional map of the dopant distribution. Fig. 4. Open in new tabDownload slide (a) HAADF image of a silicon crystal with antimony-dopant clusters and (b) with arsenic dopants, viewed from the [001]-direction. The [001] atomic column images are displayed by bright contrast. Dopant atoms at the depth of in-focus condition give high contrast. The spacing of the columns is 0.2 nm. (c) The FDP cluster and (d) the projected structure of the FDP on the focal plane of the HAADF image. Fig. 4. Open in new tabDownload slide (a) HAADF image of a silicon crystal with antimony-dopant clusters and (b) with arsenic dopants, viewed from the [001]-direction. The [001] atomic column images are displayed by bright contrast. Dopant atoms at the depth of in-focus condition give high contrast. The spacing of the columns is 0.2 nm. (c) The FDP cluster and (d) the projected structure of the FDP on the focal plane of the HAADF image. As shown from the intense columns of Fig. 4a, some neighboring pairs have particular arrangements which relate to the geometry of the Sb-dopant clusters. Since the intensity difference between two pairs tells us the depth difference, we could determine ‘cluster probability’ of the two dopants to be in the same depth (the pair of two dopants locating within the layer of the unit cell height) [16]. A new cluster structure of facing donor pair (FDP), whose band structure was theoretically calculated to be electrically inactive [18], is illustrated in Fig. 4c and d. The FDP was confirmed to be formed based on the quantitative analysis of the intensity in Fig. 4a. The FDP cluster looks the ‘Keima (knight)’ pattern in Shogi (chess) game in view of the (100)-direction: two steps horizontally and one step vertically (or two steps vertically and one step horizontally) [16]. The Keima is found to be the unique pattern that can appear for the FDP cluster. Although ‘cluster probability’ had not been analyzed for arsenic dopants, a pair of intense columns (in Fig. 4b) of ‘Keima’ pattern seems to be the FDP cluster. High visibility of arsenic dopants in Fig. 4b has been enabled by the present AbC-STEM with CFEG [15]. Localization and inhomogeneity of atoms and clusters in materials have been paid attention for decades [19], since they control property of materials more dominantly as the length scale becomes smaller. The AbC-STEM can be a new means, useful as secondary ion mass spectroscopy and atom probe. Dynamics of chemical change and local structure change An important area of electron microscopic study is dynamic changes of structures and chemical compositions. Figure 5 demonstrates a dynamic change in structure and chemical modification of a model catalyst, gold (Au) nanoparticles deposited on a TiO2 support, where Au/TiO2 was exposed to oxygen gas under high pressure [20]. The model catalyst was found as a CO oxidation catalyst [21], and an active site has long been discussed to be a periphery of Au/TiO2 [22] and/or gold surface [23]. Fig. 5. Open in new tabDownload slide (a) Schematic view of the reaction process of a model catalyst, a gold particle supported on the TiO2 substrate, during exposure to oxygen gas at 100 Pa. (b) TEM image at the beginning (upper panel) and after (lower panel) oxygen exposure of Au/TiO2. Note formation of a pillar between Au and TiO2. (c) EEL spectrum observed from the pillar region. Fig. 5. Open in new tabDownload slide (a) Schematic view of the reaction process of a model catalyst, a gold particle supported on the TiO2 substrate, during exposure to oxygen gas at 100 Pa. (b) TEM image at the beginning (upper panel) and after (lower panel) oxygen exposure of Au/TiO2. Note formation of a pillar between Au and TiO2. (c) EEL spectrum observed from the pillar region. A pillar has nucleated and grown between a gold particle and a TiO2 substrate. The pillar areas did not have regular lattices, and the structure image was neither stationary nor regular. Comparative observations with and without electron beam shower showed more evidently that the pillar growth has never been excited without oxygen gas at high pressures. The electron energy loss (EEL) spectra, Ti-L23 edge and O-K edge after subtraction of the background showed a higher O/Ti concentration ratio at the pillar region (Fig. 5c(a)) than that at thick TiO2 areas (Fig. 5c(b, c and d); integrated area of core-loss peak of Ti and that of O were compared. In addition, EEL spectra indicated a chemical shift of the Ti-L23 peak to a lower energy by 1.5 eV from those at thick areas which is the same as the TiO2 crystal. Thus, the pillar area is supposed to have rather a Ti3+ state than the Ti4+ ionic state of the TiO2 crystal. In coincident with the chemical shift of Ti ions at the pillar region, oxygen ions seem to change their ionic state. Thus, STEM-EELS investigation has shown a chemical shift of Ti and O ions at local areas with a disordered structure, which correlates with the in situ TEM observation revealing pillar growth which could be a key procedure for CO oxidation under the mixture of O2 and CO gases at Au/TiO2 model catalyst. Concluding remarks Aberration-corrected scanning electron microscopy has enabled us to image atoms and cluster structures in matrices at high resolutions both in lateral and in depth. The R005 microscope showed atom-imaging capability in both STEM and TEM (not shown in this review) mode of observation. Long-life CFEG with the asymmetric aberration corrector benefits present quantitative observations; the present work by the R005 microscope is the first step for hydrogen atom imaging to be realized. Rich information of materials is given by choosing configured detectors for STEM [6], such as ABF imaging. ABF imaging for thin specimens is useful for imaging light elements, as demonstrated by lithium ion counting one by one [24]. Imaging molecules by ABF may open the door between electron microscopy and nano/molecular/biotechnology. Computer-aided three-dimensional structure imaging is promising for ADF imaging, because it can clarify chemical/atomic structure of interfaces, buried at a few nanometers below the top surface. Electron microscopy with a configured detector really becomes powerful when electronic/optical/chemical nature of nanostructures could be obtained at high speed in combination with in situ observation of their dynamic behavior. Funding The work was supported by CREST program from the Japan Science and Technology (JST). Acknowledgements The authors acknowledge T. Ishizawa, M. Kawazoe, T. Sannomiya and T. Nakamichi, for development of the R005 microscope. References 1 Haider M , Rose H , Kabius B , Urban K . Electron microscopy image enhances , Nature , 1998 , vol. 392 (pg. 768 - 769 ) Google Scholar Crossref Search ADS WorldCat Crossref 2 Krivanek O L , Chisholm M F , Nicolosi V , Pennycook T J , Corbin G J , Dellby N , Murfit M F , Own C S , Szilagyi Z S , Oxley M P , Pentelides S T , Pennycook S J . Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy , Nature , 2010 , vol. 464 (pg. 571 - 574 ) Google Scholar Crossref Search ADS PubMed WorldCat Crossref 3 Crew A V . A system for the correction of axial aperture aberrations in electron lenses , Optik , 1982 , vol. 60 (pg. 271 - 281 ) WorldCat 4 Pennycook S J , Jesson D E . High-resolution Z-contrast imaging of crystals , Ultramicroscopy , 1991 , vol. 37 (pg. 14 - 38 ) Google Scholar Crossref Search ADS WorldCat Crossref 5 Sawada H , Tanishiro Y , Ohashi N , Tomita T , Hosokawa F , Kaneyama T , Kondo Y , Takayanagi K . STEM imaging of 47pm separated atomic columns by a spherical aberration corrected electron microscope with a 300keV cold field emission gun , J. Electron Microsc. , 2009 , vol. 108 (pg. 357 - 361 ) Google Scholar Crossref Search ADS WorldCat Crossref 6 Cowley J M . Configurated detectors for STEM imaging of thin specimens , Ultramicroscopy , 1993 , vol. 49 (pg. 4 - 13 ) Google Scholar Crossref Search ADS WorldCat Crossref 7 Okunishi E , Ishikawa I , Sawada H , Hosokawa F , Hori M , Kondo Y . Visualization of light elements at ultrahigh resolution by STEM annular bright field microscopy , Microsc. Microanal. , 2009 , vol. 15 (pg. 164 - 165 ) Google Scholar Crossref Search ADS WorldCat Crossref 8 Sawada H , Sannomiya T , Hosokawa F , Nakamichi T , Kaneyama T , Tomita T , Kondo Y , Tanaka T , Oshima Y , Tanishiro Y , Takayanagi K . Measurement method of aberration from Ronchigram by autocorrelation function , Ultramicroscopy , 2008 , vol. 108 (pg. 1467 - 1475 ) Google Scholar Crossref Search ADS PubMed WorldCat Crossref 9 Thackeray M M , de Kock A , Rossouw M H , Liles D , Birrihm R , Hoge D . Spinel electrodes from the Li–Mn–O system for rechargeable lithium battery application , J. Electrochem. Soc. , 1992 , vol. 139 (pg. 363 - 366 ) Google Scholar Crossref Search ADS WorldCat Crossref 10 Rossell M D , Erni R , Asta M , Radmilovic V , Dahmen U . Atomic-resolution imaging of lithium in Al3Li precipitates , Phys. Rev. B , 2000 , vol. 80 (pg. 024110-1 - 024110-6 ) WorldCat 11 Shao-Horn Y , Croguennec L , Delmas C , Nelson E C , O'Keef M A . Atomic resolution of lithium ions in LiCoO2 , Nat. Mater. , 2003 , vol. 2 (pg. 464 - 467 ) Google Scholar Crossref Search ADS PubMed WorldCat Crossref 12 Oshima Y , Sawada H , Hosokawa F , Okunishi E , Kaneyama T , Kondo Y , Niitaka S , Takagi H , Tanishiro Y , Takayanagi K . Direct imaging of lithium atoms in LiV2O4 by spherical aberration-corrected electron microscopy , J. Electron Microsc. , 2010 , vol. 59 (pg. 457 - 461 ) Google Scholar Crossref Search ADS WorldCat Crossref 13 Voyles P M , Muller D A , Grazul J L , Citrin P H , Gossmann H-J L . Atomic-scale imaging of individual dopant atoms and clusters in highly n-type bulk Si , Nature (Lond) , 2002 , vol. 416 (pg. 826 - 829 ) Google Scholar Crossref Search ADS WorldCat Crossref 14 Voyles P M , Grazul J L , Muller D A . Imaging individual atoms inside crystals with ADF-STEM , Ultramicroscopy , 2003 , vol. 96 (pg. 251 - 273 ) Google Scholar Crossref Search ADS PubMed WorldCat Crossref 15 Oshima Y , Hashimoto Y , Tanishiro Y , Takayanagi K . Detection of arsenic dopant atoms in a silicon crystal using a spherical aberration corrected scanning transmission electron microscope , Phys. Rev. B , 2010 , vol. 81 (pg. 035317-1 - 035317-5 ) Google Scholar Crossref Search ADS WorldCat 16 Kim S , Oshima Y , Sawada H , Hashikawa N , Asayama K , Kaneyama T , Kondo Y , Tanishiro Y , Takayanagi K . A dopant cluster in a highly antimony doped silicon crystal , Appl. Phys. Express , 2010 , vol. 3 (pg. 081301-1 - 081301-3 ) WorldCat 17 Kim S , Oshima Y , Sawada H , Kaneyama T , Kondo Y , Takeguchi M , Nakayama Y , Tanishiro Y , Takayanagi K . Quantitative annular dark-field STEM images of a silicon crystal using a large-angle convergent electron probe with a 300-kV cold field-emission gun , J. Electron Microsc. , 2011 , vol. 60 (pg. 109 - 116 ) Google Scholar Crossref Search ADS WorldCat Crossref 18 Mueller D C , Fichtner W . Highly n-doped silicon: deactivating defects of donors , Phys. Rev. B , 2004 , vol. 70 (pg. 245207-1 - 245207-8 ) Google Scholar Crossref Search ADS WorldCat 19 Asenov A . Random dopant induced threshold voltage lowering and fluctuations in sub-50 nm MOSFETs: a statistical 3D ‘atomistic’ simulation study , Nanotechnology , 1999 , vol. 10 (pg. 153 - 158 ) Google Scholar Crossref Search ADS WorldCat Crossref 20 Tanaka T , Sano K , Ando M , Sumiya A , Sawada H , Hosokawa F , Okunishi E , Kondo Y , Takayanagi K . Oxygen-rich Ti1−xO2 pillar growth at a gold nanoparticle-TiO2 contact by O2 exposure , Surf. Sci. , 2010 , vol. 604 (pg. L75 - L78 ) Google Scholar Crossref Search ADS WorldCat Crossref 21 Harura M , Tsubota S , Kobayashi T , Kageyama H , Genet M J , Delmon B . Low-temperature oxidation of CO over gold supported on TiO2, α-Fe2O3, and Co3O4 , J. Catal. , 1993 , vol. 144 (pg. 175 - 192 ) Google Scholar Crossref Search ADS WorldCat Crossref 22 Haruta M . When gold is not noble: catalysis by nanoparticles , Chem. Rec. , 2003 , vol. 3 (pg. 75 - 87 ) Google Scholar Crossref Search ADS PubMed WorldCat Crossref 23 Chen M S , Goodman D W . Catalytically active gold on ordered titania supports , Chem. Soc. Rev. , 2008 , vol. 37 (pg. 1860 - 1870 ) Google Scholar Crossref Search ADS PubMed WorldCat Crossref 24 Lee S , Oshima Y , Sawada H , Hosokawa F , Okunishi E , Kaneyama T , Kondo Y , Niitaka S , Takagi H , Tanishiro Y , Takayanagi K . Counting lithium ions in the diffusion channel of an LiV2O4 crystal , J. Appl. Phys. , 2011 , vol. 109 pg. 113530 Google Scholar Crossref Search ADS WorldCat Crossref © The Author 2011. Published by Oxford University Press [on behalf of Japanese Society of Microscopy]. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com
TI - Electron microscopy at a sub-50 pm resolution
JF - Journal of Electron Microscopy
DO - 10.1093/jmicro/dfr048
DA - 2011-08-01
UR - https://www.deepdyve.com/lp/oxford-university-press/electron-microscopy-at-a-sub-50-pm-resolution-2Vu4uBCDJ5
SP - S239
EP - S244
VL - 60
IS - suppl_1
DP - DeepDyve
ER -