TY - JOUR
AU - Tanaka,, N.
AB - Abstract We present further modifications to aberration-corrected environmental transmission electron microscopy (AC-ETEM) for the dynamic HRTEM observation of single atoms. Additional pumping levels that include three additional turbomolecular pumps (TMPs) enable a base pressure of 3.5 × 10−5 Pa in the sample chamber. The effect of these additional TMPs on image resolution was measured in reciprocal space using information limit (Young's fringes) on a standard cross grating sample and also with platinum (Pt) single atoms on an amorphous carbon film (Pt/a-carbon). The Pt/a-carbon was used for measuring the effect of gas pressure on single-atom imaging in addition to the evaluation of vibrations of TMPs, samples, magnetic lenses and a microscope column of the AC-ETEM. TMPs did not affect the ETEM imaging performance when an anti-vibration table was used, and 0.10-nm resolution was achieved. Dynamic ETEM observation of Pt single atoms was achieved in 4.0 × 10−2 Pa of air, using a modified AC-ETEM system and a high-speed CCD camera with a time resolution of 0.05 s. environmental TEM, in situ, gas atmosphere, differential pumping, HRTEM, vibration. Introduction In situ high-resolution transmission electron microscopy (HRTEM) in gas/liquid environments is increasingly proving to be a major field of electron microscopy, principally for studying solid surfaces in gas [1–5], and it is usually known as environmental transmission electron microscopy (ETEM). Many successful results have already been reported, particularly in the field of catalysis [1–13] and cell biology [14–17]. We think these advanced results originated from continuous modifications of previous ETEM systems. Actually, these researchers had modified and used a differential pumping ETEM for their experiments at gas pressure <2000 Pa. Special membrane E-cell holder coupled with differential pumping had been developed by Yaguchi et al. for higher gas pressures up to 1 × 105 Pa [18–21]. Two designs called differential pumping E-cell and membrane E-cell can successfully achieve gas atmospheric conditions in HRTEM. In a basic design of the differential pumping system, several additional small apertures in the microscope column are used to separate regions at different gas pressures, which are pumped separately to maintain high vacuum in the region of the field emission gun (FEG, 8 × 10−7 Pa in our system). Figure 1a shows the basic vacuum setup of the differential pumping and E-cell systems, which are required for ETEM experiments. The design and function of a membrane E-cell holder (Fig. 1b) were given in previous studies [18–23]. Use of such membrane E-cell has recently enabled the demonstration of 1-nm resolution from samples in liquid-phase solvents in conventional transmission electron microscopes [24,25]. Fig. 1. Open in new tabDownload slide Diagrams of the two main types of environmental TEM systems for imaging in gases. (a) Differential pumping E-cell with no membranes, requiring a dedicated ETEM pumping system and (b) enclosed membrane-holder E-cell usable on a standard microscope. Fig. 1. Open in new tabDownload slide Diagrams of the two main types of environmental TEM systems for imaging in gases. (a) Differential pumping E-cell with no membranes, requiring a dedicated ETEM pumping system and (b) enclosed membrane-holder E-cell usable on a standard microscope. ETEM progress had been largely focused on achieving higher resolution imaging at higher gas pressure 10 years ago [2, 3]. Lattice images provided valuable information on crystal structure, shape and orientation and were much improved by image resolution <0.2 nm [4–6,8–13,20–22]. Recently, more researchers started to measure influences of dynamical inelastic scattering in HRTEM images in this ETEM field. It is now widely acknowledged that the achievement of high-resolution TEM imaging becomes more difficult as the gas pressure increases because of the scattering of the electron beam by the gas molecules [1–3,5,13,20,21,26,27]. The pressure dependence of inelastic scattered electrons was measured by electron energy loss spectroscopy and was summarized to be effective for the image quality of HRTEM images as signal-to-noise ratio (SNR) lowering by Jinschek et al. [28]. The relationship between achievable HRTEM image resolution and gas pressure has also been reported in detail with experimental data and calculations by Yaguchi and Yoshida [20,27]. The contrast transfer function of objective lens on the aberration-corrected environmental transmission electron microscopy (AC-ETEM) has been compared at various pressures of argon gas by Hansen et al. [29]. Then, we reported about additional effects of ionized gas molecules on the image quality of ETEM, and recently achieved 0.22-nm resolution at 1600 Pa for the pressure of nitrogen gas [30]. Although interesting, such studies of electron–gas interactions are not enough for true progress in AC-ETEM research because image quality and resolution in ETEM experiments are often limited by experimental factors. These include sample drift (tends to be much worse during heating experiments or after changing the gas flow rate), CCD noise (if fast acquisition time or low-dose imaging is required) and basic system performance (the stability of magnetic lenses and mechanical vibrations). Such factors, related to system engineering and design, seem more open to progress than the dynamic processes occurring as electrons pass through a gas phase. These factors should also be minimized for future products such as atomic-scale ETEM, high-speed/sensitive CCD camera and stable goniometer. Here we achieve engineering modifications to the pumping line and anti-vibration system of a recent differential pumping AC-ETEM. This paper presents a new design of the differential pumping AC-ETEM with lower base pressure and mechanical vibration for the dynamical study of single atoms diffusing in the gas phase. Experimental procedure The differential pumping AC-ETEM (Titan ETEM 80–300, FEI Company) was operated with a modified S-Twin objective lens (pole piece gap and gas path length of 5.4 mm), CETCOR image aberration corrector, thermal field emission gun and 300-kV electrons. The conventional Titan ETEM that used one turbomolecular pump (TMP) (260 l/s) was used temporarily for sample exchange. Six TMPs (two 260 l/s TMPs and two sets of 260 and 33 l/s) were continuously used for the differential pumping system to allow up to 2000 Pa of gas in the sample area while maintaining ∼8 × 10−7 Pa of vacuum in the FEG area during ETEM observation. During the ETEM observation using conventional differential pumping lines (pumping levels 1 and 2 in Fig. 1a), base pressure in the sample area was ∼1 × 10−1 Pa at equilibrium. Although such base pressure was sufficiently lower than partial pressures in conventional ETEM research (1–20 Pa) or E-cell research (1 × 105 Pa), a lower atmosphere in the sample of <1 × 10−3 Pa was of much interest to achieve the dynamic visualization of single atoms and clusters. Because weaker phase contrasts of single atoms and clusters are easily diffused by gas molecules in a ray path. The volume ratio between platinum single atom and nitrogen gas molecules recommended a partial pressure of <1 × 10−1 Pa in the case of the present AC-ETEM system having gas a pass of 5.4 mm. In addition, the developing target of base pressure for achieving 99% nitrogen concentration of introduced gas was estimated to be 1 × 10−3 Pa. Therefore, we added zero-level pumping line that can evacuate the sample area directly using a set of 260 l/s TMP and 33 l/s TMP as shown in Fig. 1a. This pumping system goes back to the design of the first differential pumping ETEM based on Phillips CM30T, which was reported by Boyes et al. in 1997 [1]. Recent new technologies on vacuum sealing, TMP and valves connection enabled us to use a constant base pressure of <1 × 10−4 Pa. The present system can achieve base pressures of 1 × 10−4 and 3.5 × 10−5 Pa with waiting times of 1 and 3 h, respectively, after gas exchange with the sample. Achievable partial pressures of a conventional differential pumping E-cell, a window type E-cell and the modified E-cell are provided in Table 1. The present system achieved low pressures in the sample area due to progress in base vacuum pressure. Table 1. Effective gas pressure ranges allowed by differential pumping, window and modified pumping systems E-cell type . Maximum gas pressure (Pa) . Base vacuum pressure (Pa) . Achievable partial pressure for ETEM (Pa) . Differential pumping 2.0 × 103 10−1 101–103 Window 2.0 × 103 n/a >105 Modified pumping 102 10−5 10−3–102 E-cell type . Maximum gas pressure (Pa) . Base vacuum pressure (Pa) . Achievable partial pressure for ETEM (Pa) . Differential pumping 2.0 × 103 10−1 101–103 Window 2.0 × 103 n/a >105 Modified pumping 102 10−5 10−3–102 Open in new tab Table 1. Effective gas pressure ranges allowed by differential pumping, window and modified pumping systems E-cell type . Maximum gas pressure (Pa) . Base vacuum pressure (Pa) . Achievable partial pressure for ETEM (Pa) . Differential pumping 2.0 × 103 10−1 101–103 Window 2.0 × 103 n/a >105 Modified pumping 102 10−5 10−3–102 E-cell type . Maximum gas pressure (Pa) . Base vacuum pressure (Pa) . Achievable partial pressure for ETEM (Pa) . Differential pumping 2.0 × 103 10−1 101–103 Window 2.0 × 103 n/a >105 Modified pumping 102 10−5 10−3–102 Open in new tab However, those eight TMPs that are close to the microscope column to ensure high-pumping efficiency on connection to the differential pumping system were expected to increase mechanical vibrations with rotation frequencies of 1, 60 and 90 kHz. First, the effect of running these TMPs on the modified system and on the information limit of HRTEM images was studied for clarifying the character of the AC-ETEM with the zero-level pumping line. The AC-ETEM was installed on an active vibration isolation system (α4G-201L-2020-FE1, Tokkyokiki). Vibration measurements were performed using a servo acceleration meter/micro vibration gauge (MG-102S/OSP-06, Tokkyokiki) fixed at two different locations on the AC-ETEM system including the anti-vibration table, TEM column and TMPs. Information limit (equal to the point resolution because of the image corrector) of the modified AC-ETEM system was measured at 860kx magnification using a slow-scan CCD camera (US1000, Gatan, Inc.) on an energy filter (GIF Tridiem 863, Gatan, Inc.). All AC-ETEM images shown in this study are unfiltered images. The alignment sample is a widely available cross grating sample (Cross Grating Replica S106, Agar Scientific) consisting of a high density of AuPd metal crystallites on an amorphous carbon film. We used an exposure time of 1 s, shifting the image by 1 nm halfway through. Fast Fourier transforms (diffractograms) of the resulting images were taken using the Gatan Digital Micrograph software and the extent of Young's Fringes was measured. To record movie files of the Young's Fringes, video capture software (VisualDub, licensed under the GNU General Public License) was set up with an image size of 620 pixel × 480 pixel and a time resolution of 2 s. The performance of the present AC-ETEM on dynamic single-atom visualization was evaluated using typical Pt single atoms on Pt/a-carbon, which are prepared by conventional argon sputtering (Ar Ion Sputter E-1030, Hitachi) and chemical vapor deposition (CVD coater NC5 Turbo, Enomoto AV) [31]. Dynamic HRTEM movie files were taken with a high-speed CCD camera (Orius CCD, GatanInc) and using VisualDub (same as above but with a time resolution of 0.05 s). Pixel size is 0.035 nm and frame rate is 20 fps. The local current density is estimated to be 6.5 × 103 A/cm2 (300 kV). Results and discussions Influence of TMP vibrations on HRTEM information transfer Figure 2 shows transmission electron micrographs of the cross grating sample used for the measurement of information transfer. First, the magnification of the CCD camera was calibrated using the grid pattern of the grating sample as shown in Fig. 2a. Then, current flowing through the fluorescent screen was used to measure the electron beam current (this reading is calibrated in the factory), and the beam current was adjusted by changing the spot size (C1 lens). As much as possible, the beam convergence (i.e. current density) was kept constant. HRTEM images for evaluating information transfer were always obtained at similar sample regions, which consist of 75 ± 5% of AuPd lattices and 15 m 5% amorphous carbon patterns as shown in Fig. 2b. Fig. 2. Open in new tabDownload slide Representative HRTEM image of an AuPd/carbon cross grating sample taken in a high vacuum of 3.5 × 10−5 Pa with (a) 10 k and (b) 620 k times. Fig. 2. Open in new tabDownload slide Representative HRTEM image of an AuPd/carbon cross grating sample taken in a high vacuum of 3.5 × 10−5 Pa with (a) 10 k and (b) 620 k times. Figure 3a and b shows measured diffractograms with TMPs on and off. Both diffractograms show information transfer out to 0.1 nm. This result means that TMPs of our differentially pumped ETEM do not have any major effect on the achievable resolution of HRTEM images. Therefore, such additional direct pumping of the sample chamber could prove valuable for improving the vacuum level in normal TEM imaging. It would be of interest for microscopists in a HR(S)TEM field who need a clean sample chamber with less contamination. Here we report another modification to reply to an engineering question, which is where TMPs can be placed in relation to the electron microscope for maximum pumping rate and minimum mechanical vibration. TMPs are needed on the ETEM to allow gas to be introduced to the sample area, so it is important that they do not cause vibrations that lead to a major drop in image resolution [32,33]. The active vibration isolation system is expected to improve the damping of low-frequency vibrations (below ∼20 Hz) without the amplification of vibrations <100 Hz, which is demanding on the product design. In general, taller electron microscope columns are more likely to sway, which can increase sensitivity to low-frequency floor vibrations caused by road and rail traffic, heavy industry, ocean waves or microscope ancillary equipment. In addition, many buildings have their natural frequencies at ∼5–10 Hz. Microscopes are sensitive to low-frequency vibration, in the range of a few Hertz, and these vibrations are the most difficult to eliminate. The active vibration isolation system has provided fairly good performance for this environment without any amplification at resonance. Fig. 3. Open in new tabDownload slide Measured point resolution (equal to the information limit) with (a) TMPs off and (b) TMPs on. Fig. 3. Open in new tabDownload slide Measured point resolution (equal to the information limit) with (a) TMPs off and (b) TMPs on. TMPs were initially placed on this anti-vibration table and were later relocated to a separate frame attached to the room floor (the floor rack), where they are isolated from the microscope by the anti-vibration table. Vibration measurements were performed using a servo acceleration meter/micro vibration gauge (MG-102S/OSP-06) fixed to two different locations on the system, shown in Fig. 4. Point (i) is on the frame to which turbomolecular pumps are attached and point (ii) is on the top surface of the anti-vibration table. Vibration levels at each frequency were measured with the TMP rack first fixed on the anti-vibration table (Fig. 4a) and then fixed on the room floor outside the anti-vibration table (Fig. 4b). Fig. 4. Open in new tabDownload slide Titan ETEM system diagrams showing the two locations of the TMP rack (a) and (b) used for testing and the vibration measurement points marked (i) and (ii). Additional turbomolecular pumps used for differential pumping are shown in orange. Fig. 4. Open in new tabDownload slide Titan ETEM system diagrams showing the two locations of the TMP rack (a) and (b) used for testing and the vibration measurement points marked (i) and (ii). Additional turbomolecular pumps used for differential pumping are shown in orange. Measured vibration spectra presumably caused by a combination of transmitted ground vibrations, acoustic vibrations and moving parts on the microscope itself are shown in Fig. 5. The measurement system is set up such that both the input and output vibration levels at 0 dB are 1 × 10−5 m/s2. An increase of 20 dB means a 10-fold increase in measured (output) acceleration, so 20 dB = 1 × 10−4 m/s2 and 60 dB = 1 × 10−2 m/s2. All vibrations in Fig. 5 were measured with the anti-vibration table switched off. The upper and lower graphs show different frequency ranges. Measurements were taken at two positions on the system, shown in Fig. 4 as (i) and (ii). Figure 5a shows vibrations present on the TMP rack (including those induced by running TMPs), and Fig. 5b indicates the extent to which these vibrations are transmitted to the base of the microscope. In Fig. 5a, we can see the main pump rotation frequency of 1 kHz, which stays constant when TMPs are moved. However, many of the lower frequency vibrations on the TMP rack are reduced by fixing TMPs to the floor, probably because of a more solid foundation for the rack on the room floor, reducing resonance and other parasitic effects. Figure 5b shows similar measurements at the base of the microscope, which is more significant in predicting the influence on imaging performance. The isolation of TMPs by moving them to the floor rack results in the highest vibration reduction at 1 kHz (from 50 to 30 dB) and also improvement at 600–800 Hz. Average vibration level at 0–100 Hz decreased ∼25%, from 20.35 to 15.25 dB. For comparison, Fig. 6 shows information transfer stability with the anti-vibration table off, and it is clear how important and effective the anti-vibration table is for removing ground vibrations. Although little benefit was seen from the vibration isolation of TMPs, they may produce higher vibrations when pumping higher gas pressures and may also mechanically degrade over time as the bearings wear down. Therefore, isolating TMPs from the microscope column is recommended. Fig. 5. Open in new tabDownload slide Vibration spectra from two locations on the ETEM system with all TMPs running: (a) measured on the TMP rack at point (i) and (b) measured on the anti-vibration table at point (ii) in Fig. 3. In each graph the measurements with the TMP rack fixed in the two different locations are compared. Fig. 5. Open in new tabDownload slide Vibration spectra from two locations on the ETEM system with all TMPs running: (a) measured on the TMP rack at point (i) and (b) measured on the anti-vibration table at point (ii) in Fig. 3. In each graph the measurements with the TMP rack fixed in the two different locations are compared. Fig. 6. Open in new tabDownload slide Information limit measured every 2 s for almost 1 min with different TMP conditions. Fig. 6. Open in new tabDownload slide Information limit measured every 2 s for almost 1 min with different TMP conditions. The long-term stability of the information transfer is most important for in situ studies because changes of the microstructure over time are recorded during experiments, for example changes due to reactions with gas. The information transfer stability was measured on captured video using an exposure time of 1 s and performing a 1-nm image shift every 0.5 s. Measured information transfer with TMPs on and off is shown in Fig. 6, when the TMPs were fixed to the anti-vibration table and floor. This shows no measurable change in information transfer and only a small drop in information stability with an exposure time of 0.5 s that is a reasonable value for conventional HRTEM observation. One can say that the measured small difference is probably within the noise for this measurement and the influence of TMPs when running normally is very small, even with TMPs fixed on top of the anti-vibration table, and therefore, it is directly linked to the microscope column. The most important result, clarified firstly by the present vibration analysis, is that both systems, which are shown in Fig. 6, enable us to achieve continuous atomic resolution <0.1 nm. The separation of TMPs can improve the long-term stability of the information transfer for in situ atomic-scale observation. Dynamic ETEM observation of platinum single atom The present AC-ETEM achieved a base pressures of 3.5 × 10–5 Pa and continuous information transfer <0.1 nm. This suggested that we could increase the frame rate for the dynamic observation of single atoms. Here we report a trial using a high-speed CCD camera for visualizing Pt single atoms on amorphous carbon in environmental gas atmosphere. We know that the image on a recording device is determined by point spread function (PSF). In the case of HRTEM, image intensity I(r) can be described as follows: (1) (2) (3) The total envelop function to explain the image blur can be defined in reciprocal space as shown in previous studies reported by Van Dyck et al. [34,35]. Individual envelop functions shown in equation (3) are: for chromatic aberration. for the source dependence due to the small spread of angles from the probe. for specimen drift. for specimen vibration. for the detector. Identification of such blur functions could permit fast high-resolution restoration. We sorted these terms out for discussions about effects of sample motion and frame rate. ⁠: for the anti-vibration table ⁠: for the detector including MTF and NPS, etc. Δt: exposure time The term was generally understood as temporally regulated functions such as modulation transfer function (MTF) and noise power spectrum (NPS) in reciprocal space. Stabilities of magnetic lenses and gun (particularly for objective and projection lenses) are not significant in this, because recent spot drift measurements were <0.1 nm/min, much lower than the conventional specimen movement (0.1 nm/s) due to specimen drift, holder vibration and specimen vibration. For dynamical imaging of single atoms, we considered time dependence of envelop functions during exposure time Δt. Envelop function was described as a multiplication of three envelop terms such as influence of holder/specimen vibrations influence of specimen drift and A new term was added for determination of single-atom movement (or sample motion due to structural change). We know that larger drift rate and vibration amplitude generally enhance dumping of these envelop functions. Influence of the anti-vibration table on image blur is separately described from shown in equation (3). In our system, mechanical vibration of the anti-vibration table decreased 25% by the separation of TMPs and is the same level as that of general electron microscopes working with ion getter pumps. Envelop functions and simply suggested that increase in the frame rate (shorter exposure time) provides less image blur (better resolution). Actually, the SNR should also be considered for recording high-speed videos. SNR includes CCD background noise and Pt/a-carbon sample's contrast, which can be described using the number of Pt and C atoms and the atomic scattering factor of each atom. First, we minimized CCD background noise for the present electron density and current frame rates to achieve adequate SNR written by Rose in 1948 [36]. In addition, we controlled the thickness of the Pt/a-carbon <5 nm, although it is almost the measuring limit of electron energy loss spectroscopy. Effects of object motion and camera shutter on PSF have been reported by researchers in computer graphics and image processing [37,38]. Following experiment was achieved to confirm the time dependence of and SNR. Figure 7a shows selected area captured (SAC) images of a movie obtained from the Pt/a-carbon samples in high vacuum (3.5 × 10−5 Pa). The original movie was recorded with 20 fps (time resolution of 0.05 s). The original movie file can be downloaded from http://sirius.cirse.nagoya-u.ac.jp/~tanakalab/~kenta/jem2012/singleatoms/index.html. Fig. 7. Open in new tabDownload slide (a) AC-ETEM image of the Pt/a-carbon. (b)–(g) and (h)–(m) are SAC images taken at locations for (i) single atoms and (ii) surface atoms with frame rates of 20, 10, 5, 3, 2 and 1. Fig. 7. Open in new tabDownload slide (a) AC-ETEM image of the Pt/a-carbon. (b)–(g) and (h)–(m) are SAC images taken at locations for (i) single atoms and (ii) surface atoms with frame rates of 20, 10, 5, 3, 2 and 1. Figure 7b–g and h–m shows SAC images obtained from square areas in Fig. 7a with frame rates of 20, 10, 5, 3, 2 and 1 fps. These frame rates correspond to time resolution or exposure times of 0.05, 0.1, 0.2, 0.3, 0.5 and 1 s. By decreasing the frame rate, CCD camera noise and the Poisson noise as seen in the granular pattern of amorphous carbon were reduced. This can be seen by comparing Fig. 7b and c. Some trapped Pt single atoms on defect sites of the Pt/a-carbon also showed much better contrast, as in Fig. 7c. However, too low a frame rate decreased the image resolution of SAC images. The symmetry and contrast of the granular pattern that were clearly observed in SAC images with 20, 10 and 5 fps became less clear in Fig. 7f and g obtained with 3, 2 and 1 fps, respectively. We think that the decrease of image resolution following the decrease of frame rate (increase of exposure time) is due to the term described previously, although there is no experimental evidence for the effect of low-frequency noise on the magnetic field of the present system for ultra-high resolution environmental transmission electron microscopy. The present specimen drift rate was measured to be 0.06 nm/s (1.8 nm/35 s) using x–y shifts of gravity points of three particles shown in Fig. 7(a). The value was reasonable in comparison with other references measured with the same experimental conditions such as vacuum level (3.0-4.0 × 10−5 Pa), temperature and electron beam irradiation (6.0–6.9 × 103 A/cm2). Effective values of specimen drift rate on 5–20 fps (0.003-0.012 nm) were smaller than the CCD camera pixel size (0.019 nm/pixel). It was reasonable that the anti-vibration table still had vibrations with frequencies of 1–20 Hz and vibration amplitudes bigger than pixel size because TMP vibration was not zero even when the anti-vibration table was active and because lens currents of each lens did not show any fluctuation >1 Hz. In addition, results indicated that the frame rate should be increased for dynamic ETEM observation of atoms. With a frame rate of 1 fps, some single atoms on carbon film indicated by arrows in Fig. 7g lost their contrast and surface atoms showed the weaker streaked contrast indicated in Fig. 7m. These images define that the single atoms (black arrowed in Fig. 7(b)) are trapped on defect sites on amorphous carbon much strongly than ones that disappeared at least for 1 s and are evidence of diffusion of Pt atoms. First, we mention that movements of atoms are enhanced drastically in environmental conditions. Figure 8a and b shows SAC images obtained from the Pt/a-carbon sample in room air of 4.0 × 10−2 and 100 Pa of partial pressures, respectively. The original movies are available at http://sirius.cirse.nagoya-u.ac.jp/~tanakalab/~kenta/jem2012/roomair/index.html. Fig. 8. Open in new tabDownload slide AC-ETEM images of the Pt/a-carbon in room air of (a) 4.0 × 10−2 and (b) 100 Pa. Fig. 8. Open in new tabDownload slide AC-ETEM images of the Pt/a-carbon in room air of (a) 4.0 × 10−2 and (b) 100 Pa. Platinum (Pt) single atoms on a 5-nm thick layer of amorphous carbon are still visible in room air of 4.0 × 10−2 Pa. However, the number of visible single atoms that are stable for at least 0.05 s during exposure decreases to 37% from the density of 2.81 × 1013 atoms/cm2 measured in high vacuum (3.5 × 10−5 Pa). These single atoms were invisible in room air of 100 Pa. One can say that this kind of dynamic observation visualizes only discrete states of the surface diffusion and is unable to say anything about the growth process. But we believe that the dynamic analysis using a movie file demonstrated in Fig. 7 enables us to achieve relative evaluations of samples from the point of view of trap site density, trapping time and moving unit (single atom, bi-atom and cluster) in various gases at various temperatures. This information is particularly important in catalysis because trapping sites of Pt atoms could also function as reactive sites. In the present differential pumping AC-ETEM, Pt atoms became invisible in room air of 100 Pa. Joerg et al. have clarified the defocus of HRTEM image due to inelastic scattered electron and achieved 0.2 nm resolution in 1900 Pa of nitrogen gas [28]. Then, Bright et al. reported about additional effects of ionized gas molecules on the image quality of ETEM, and recently achieved 0.22 nm resolution in 1600 Pa of nitrogen gas [33]. Our results indicate that movement (diffusion) dominates image quality on the dynamic ETEM observation of single atoms. In other words, the diffusion speed of single atoms is the limiting factor for dynamic AC-ETEM observation. The evidence of the sample movement can be seen in the supporting movies. Pt nanoparticles also rotate and migrate in room air of 100 Pa. This phenomenon of increased Pt atom diffusion in a gas atmosphere needs further discussion in order to clarify the development of future products. In the present study, estimated diffusion rate of a single atom was 4 nm/s on amorphous carbon in room air of 4.0 × 10−2 Pa, which is the highest pressure at which we could distinguish single atoms, and it was 400 times higher than the thermal drift of the Pt/a-carbon itself. Low-dose and summation technique can be useful when the irradiation damage is mainly caused by the thermal effect by inelastic scattered electrons. In addition, we suspect that the summation of low-dose images makes images similar to those in Fig. 7 depending on the number of EM images because the diffusion of single or surface atoms are phenomena belonging to the elastic scattering process. Pulse electron sources [39,40] also would not give us improvements for the dynamic single-atom observation in gas atmosphere. The summation of low-dose images has a disadvantage when the ionization of gas molecules makes further background noise and reduces SNR [41]. The most reasonable modification is the improvement of the frame rate on the CCD camera. Finally, we discuss the maximum frame rates that are achievable with each future development. The high-speed CCD camera (Orius SC 200, GatanInc) was used in Fast mode, reading out the two halves of the Orius CCD camera separately (using 2 ports). Binning 4 was used, so each half was 512 × 256 pixels. Using the Orius CCD camera, 20 fps has been available on the recorded movie file even when each frame includes ∼3.7 × 106 bits (512 × 512 × 14). However, the processing speed of the main PC is now limiting the frame rate because of some other tasks on the CPU such as the control of lenses and monitoring the vacuum in the current system. We have not achieved the catalog spec of >30 fps for the Orius SC 200. The separation of the high-speed CCD camera from the microscope and setting it on the individual PC would enable 50 fps (exposure time of 0.002 s), which was confirmed on the monitor screen. Total data volume for 1 s is 1.9 × 108 bits and this is small in comparison with the maximum throughputs of USB 3.0 (5.0 × 109 bit/s) and IEEE 1394 (3.2 × 109 bit/s). There are still 3,200 electrons on each CCD pixel for 0.02 s with the present experimental conditions (pixel size of 0.019 nm/pixel, current density of 6.5 × 103 A/cm2 and frame rate of 50 fps). Dynamic observation with 50 fps is expected to contribute greatly to the studies of the diffusion of atoms and clusters. There would be a new world with a higher frame rate than that in conventional in situ observation using a TV rate CCD (33 fps). To achieve frame rates >100 fps that are needed for the visualization of molecules' rotation and vibration, two advanced technologies would be applied to CCD cameras for electron microscopes in a manner similar to that of optical high-speed CCD cameras: dividing the output signal for parallel readout and recording using an external memory. Optical CCD cameras are achieving continuous recording with 1000 fps using an external memory. Frame rates of 100 fps are already on the road map for developments of high-speed CCD cameras in electron microscopy. New CCD cameras that can visualize (a TEM image of) a hundred electrons will be essential for ultra-high-speed CCD cameras that can achieve 1000 fps. Each CCD pixel sensor should precisely detect several tens of electrons and record a data volume of 3.7 Mb (512 pixel × 512 pixel, 14 bit and 1000 fps) into memory for 1 s. Background noise must be reduced to almost zero. This will require further advances of technology in the future; however, it is a key technology not only for the atomic scale visualization of adsorption and reaction of molecules and single atoms on catalysts' surfaces but also for single-atom visualization in various reactive gases with partial pressures of over 100 Pa. Conclusions Aberration-corrected environmental transmission electron microscopy (AC-ETEM, Titan ETEM, FEI Company) for dynamic HRTEM observation at atomic scale was newly developed. An additional level of pumping that includes three TMPs enabled a base pressure lower than 3.5 × 10−5 Pa in the sample chamber. The vibrations from the additional TMPs were minimized by the separation of the pumps from the microscope column, which allowed a continuous image resolution of 0.10 nm under normal operating conditions. A high-speed CCD camera (Orius CM200, Gatan, Inc.) achieved dynamic AC-ETEM observation, which we recorded as movie files with a frame rate of 20 fps. The AC-WTEM images obtained with an exposure time of 0.05 s had drastically improved the contrast of single atoms and surface atoms in comparison with conventional AC-ETEM images with an exposure time of 0.5 or 1 s when atoms are diffusing rapidly on surfaces of Pt/a-carbon and Pt nanoparticles. We estimated an effective partial pressure for Dynamic AC-ETEM observation of the Pt single atom. The maximum pressure for room air was 4.0 × 10−2 Pa with a frame rate of 20 fps. Future development for higher frame rate CCD was also discussed from the point of view of single-atom diffusion in high-pressure gas. Funding This work was supported by Grant-in-Aid for Young Scientific Researcher (B) 24710110 and 23760030 from the Japan Society for the Promotion of Science (JSPS) and Grant-in-Aid for Scientific Research (A) 23241036 from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The ETEM facility is partly supported by a grant from the Chubu Economic Federation and Aichi Prefecture. Acknowledgements We thank Dr. A. N. Bright, FEI Company Japan Ltd, for his great contribution on information transfer measurement. Author (K.Y.) thanks Young Leaders Cultivation Program of Nagoya University. References 1 Boyes E D , Gai P L . 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TI - Key factors for the dynamic ETEM observation of single atoms
JF - Microscopy
DO - 10.1093/jmicro/dft033
DA - 2013-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/key-factors-for-the-dynamic-etem-observation-of-single-atoms-ZVUzaFUxn3
SP - 571
EP - 582
VL - 62
IS - 6
DP - DeepDyve
ER -