Grids for Applications in High-Temperature High-Resolution Transmission Electron Microscopy

New TEM grids coated with ultrathin amorphous Al 2 O 3 ﬁlms have been developed using atomic layer deposition technique. The amorphous Al 2 O 3 ﬁlms can withstand temperatures over 600 ◦ C in air and 900 ◦ C in vacuum when the thickness of the Al 2 O 3 ﬁlm is 2 nm, and up to 1000 ◦ C in air when the thickness is 25nm, which makes heating TEM grids with nanoparticles up to 1000 ◦ C in air and immediate TEM observation without interrupting the nanoparticles possible. Such coated TEM grids are very much desired for applications in high-temperature high-resolution transmission electron microscopy.

Unfortunately such ideal ultra-thin and mechanically stable supporting films are not available. The conventional ultra-thin carbon supporting films with thickness of several nanometers are suitable for HRTEM imaging and electron diffraction patterning at room temperature and in vacuum [12]. However, the carbon supporting films are not mechanically stable at high temperatures, always break in vacuum or burn away in air upon heating. In order to improve mechanical stability of the carbon supporting films, a thermally stable silicon oxide was deposited on the conventional carbon films using sputtering method [13]. The deposited oxide films were too thick and blurred HRTEM images while the quality of the deposited oxide films was hard to be controlled because of the sputtering deposition technique. Gold-platinum alloys [14] and germanium films [15] were also sputtered on the carbon supporting films, but the coated films were only suitable for HRTEM up to 500 • C in vacuum and could not resist oxidization in air at high temperatures. In order to withstand temperatures higher than 500 • C, silicon-based membrane window grids, like silicon oxide membranes [16] and silicon nitride membranes [17], have recently been developed to meet the new requirement of high-temperature TEM experiments. These commercial membrane window grids can withstand temperatures up to 1000 • C in air and the thickness of these membranes is usually several ten nanometers. The quality of obtained HRTEM images from these membranes is better than those from the sputtering films while is worse than those from the ultra-thin carbon films.
Atomic layer deposition (ALD) can deposit very uniform thin films on various substrates based on a sequential use of a gas-phase chemical process [18]. Large-area, ultra-thin, and uniform films can be produced with precise control of the thickness and composition at atomic level. The thickness of the ALD films can be precisely controlled from several Angströns to several hundred nanometers.
Here, we successfully utilized the ALD technique to fabricate Al 2 O 3 TEM-supporting films with thickness varying from 2 nm to 25 nm. Such new TEM grids coated with amorphous Al 2 O 3 films are mechanically stable over 600 • C in air when the thickness of ALD films is 2 nm and stable up to 1000 • C in air when the thickness is 25 nm, and even higher in vacuum, suitable for high-temperature HRTEM investigations.

Experimental
Commercial holey carbon TEM grids coated with a continuous ultra-thin carbon film were loaded into an ALD chamber (Savanna 100, Cambridge NanoTech Inc.), pumped down to 1 × 10 −3 Pa and heated to 120 • C. Then, Al 2 O 3 amorphous films were deposited on the carbon-coated TEM grids, using water and trimethylaluminum (TMA) as the sources for oxygen and aluminum, respectively. The ALD process was performed by alternatively supplying pulses of nitrogen gas containing water and TMA with each cycle consisting of a 15milliseconds water pulse, a 5-second nitrogen purge period, a 15-millisecond TMA pulse, and a 5-second nitrogen purge period. The deposition rate of Al 2 O 3 was 0.10 nm per cycle. The thickness of the amorphous Al 2 O 3 films was controlled by the cycle number. After deposition, the TEM grids were cooled down to 100 • C, taken out from the ALD chamber, plasma cleaned, and heated in air at 300 • C to burn the continuous ultra-thin carbon film over the holes of the holey carbon film that were exposed to air. In order to test the mechanical stability of the amorphous Al 2 O 3 films, the ALD-coated TEM grids were heated in a tube furnace in air at different temperatures up to 1000 • C, or heated in vacuum (10 −5 Pa) in TEM column using a Gatan double tilt heating holder (Model 652, Gatan Inc.) at different temperatures up to 1200 • C. Microstructures of the amorphous ALD Al 2 O 3 films were characterized on a JEOL-2010F TEM equipped with an Oxford energy dispersive X-ray spectrometer, operated at an accelerating voltage of 200 kV.  (Figures 1(a)-1(c)). Energy-dispersive X-ray spectroscopy (EDS) indicates that the ALD film over a hole of the holey carbon film consists of oxygen and aluminum without carbon within the experimental errors (bottom curve in inset of Figure 1(b)). EDS also indicates that the holey carbon film is still there (top curve in inset of Figure 1(b)). The holey carbon film supports the ALD-Al 2 O 3 film. The ratio of aluminum and oxygen is calculated from EDS, Al : O = 2.0 : 3.2, close to the element ratio of Al 2 O 3 . The slightly high oxygen content may come from the oxygen absorbed on the ALD film surfaces. The ratio of aluminum and oxygen is spatially uniform on the whole ALD film. HRTEM and selected area electron diffraction (SAED) (Figure 1(d)) indicate that the ALD Al 2 O 3 film is amorphous. HRTEM of the ALD film does not show any thickness contrast, indicating the thickness of the Al 2 O 3 film is uniform at nanoscale. Atomic force microscopy indicated that such grown ALD Al 2 O 3 thin films are smooth at atomic level with a root-mean-squared roughness of several Angströns [18,19].

Results and Discussion
The ultra-thin ALD Al 2 O 3 films are mechanically stable at high temperature in air and in vacuum. Figure 2 is a plan view of the amorphous ALD Al 2 O 3 films after being heated at various temperatures. The amorphous Al 2 O 3 film is still continuous after being heated at 600 • C for 5 minutes in air (Figure 2(a)). SAED indicates that the ALD film is still amorphous after being heated. After being heated at 700 • C for 5 minutes in air, big voids are formed in the ALD film, and the ALD Al 2 O 3 film is broken (Figure 2(b)).
The ALD Al 2 O 3 film is more mechanically stable in vacuum (< 1 × 10 −5 Pa) than in air. After being heated at 900 • C for over 10 minutes in vacuum, the morphology of the ALD Al 2 O 3 film is same as before being heated (Figure 2(c)). The heated ALD Al 2 O 3 film is still amorphous. After being heated at 985 • C for 10 minutes in vacuum, the ALD Al 2 O 3 film breaks and rolls up (Figure 2(d)).
Crystallization of amorphous ALD Al 2 O 3 films on silicon wafers was investigated in air by X-ray diffraction [20][21][22]. It was found that amorphous ALD Al 2 O 3 films crystallize into γ-Al 2 O 3 at 900 • C [20,22] and into α-Al 2 O 3 at 1100 • C [21,22]. Because of the density difference between the crystalline phase and amorphous phase, the crystallization causes a shrink of film thickness [20], affecting the mechanical stability of the Al 2 O 3 films. The crystallization would affect the mechanical stability of the ALD films on TEM grids above 900 • C, like shown in Figure 2(d). Atomic force microscopy shows that the roughness of the Al 2 O 3 films increases after high-temperature annealing over 1000 • C [21,22]. When the roughness of the locally crystallized films is higher than the thickness of the unannealed amorphous films, voids appear in the Al 2 O 3 films because of the shrink of thickness. The mechanical stability of the Al 2 O 3 films depends on the properties of the voids (size and number per volume). Therefore, the thicker films are more mechanically stable. Amorphous films with 10 nm thickness are mechanically stable above 1200 • C in vacuum, while stable below 985 • C when thickness is 2 nm. However, the mechanical un-stability at high temperature in air (like shown in Figure 2(b) where temperature is 700 • C, below the crystallization temperature of 900 • C) should come from other reasons, not from the crystallization.
The thermal stability is dependent on the thickness of the ALD Al 2 O 3 films. Here, we define a mechanically stable temperature T s at which the ALD Al 2 O 3 films break. Figure 3 shows the thickness dependence of T s . TEM observation indicates that the thicker the ALD Al 2 O 3 films, the higher the T s . For example, the ALD Al 2 O 3 films with 10 nm thickness are mechanically stable up to 700 • C in air while the films with 25 nm thickness are stable up to 1000 • C in air.
In order to examine the ALD Al 2 O 3 amorphous films further, commercial silver nanoparticles with diameter less than 20 nm were deposited on a ALD Al 2 O 3 amorphous film and heated in situ up to 900 • C in vacuum. Figure 4 shows an in situ HRTEM image and SAED of a silver nanoparticle taken at 500 • C in vacuum, showing the polycrystalline nature of the silver nanoparticle. Quality of the obtained HRTEM image is almost the same as those obtained using the conventional ultra-thin carbon films with thickness of several nanometers. The diffraction halos of the ALD Al 2 O 3 film are very weak and hardly seen under our experimental conditions (inset of Figure 4).

5
The copper grids used to support the TEM-supporting films usually evaporate at high temperature and contaminate the TEM-supporting films [13]. Different from conventional coated copper grids, no copper contamination is observed on the Al 2 O 3 films after the ALD films are heated at 1000 • C in vacuum. One possible explanation is that the condense Al 2 O 3 film deposited on the copper grids limits the high vaporization of copper. In order to avoid the potential grid contamination, especially when heated in air, other kinds of TEM grids with low vapor pressure, like molybdenum and nitride TEM grids can be employed to support the Al 2 O 3 films.
When an intense electron beam with 500 pA/cm 2 irradiates on the Al 2 O 3 films, a local temperature gradient is produced. Because of low thermal conductivity of Al 2 O 3 , the Al 2 O 3 films, especially the thin Al 2 O 3 films, wrap under the irradiation. The behavior is different from the conventional carbon supporting films in which the intense electron beam usually burns a hole on the carbon films. We examined the wrapped Al 2 O 3 films using SAED and HRTEM and no crystallization is observed on the irradiated Al 2 O 3 films. Under our TEM experimental conditions where the beam is less than 350 pA/cm 2 , the Al 2 O 3 amorphous films are mechanically stable, even at high temperatures up to 1000 • C in vacuum, under an irradiation of the electron beam.
Compared with other TEM-supporting films, the ALD Al 2 O 3 films are more suitable for HRTEM imaging of nanoparticles at high temperatures. The ALD films not only mechanically support the observed nanoparticles at high temperatures but also are thin enough for HRTEM imaging. Graphene sheets have been commercially used for TEM-supporting films and are super for atomic-resolution transmission electron microscopy because of its thickness [23]. However, the graphene sheets are not thermally stable at high-temperature, especially oxidized in air at high temperatures, limiting high temperature application. Silicon-based membranes with thickness down to 10 nm are commercially available and can stand over 1000 • C in air. However, the thickness of the membranes is still too thick to obtain clear HRTEM images of nanoparticles with a diameter of several nanometers. More unfortunately, it is hard to further reduce the thickness of the silicon-based membranes because of complex etching technology. So the ALD Al 2 O 3 films are super than the graphene sheets and the siliconbased membrane windows after considering the mechanical stability at high temperatures and the atomic-resolution imaging of nanoparticles. Additionally, the fabrications of the graphene sheets and the silicon-based membranes are complex and these TEM-supporting films are expensive. On the contrary, the ALD Al 2 O 3 films can be fabricated much easier and can be massively produced in a short time. The produced Al 2 O 3 supporting films should be much cheaper than the graphene sheets and the membrane windows.

Conclusions
In summary, ultra-thin, oxidization resistant, amorphous Al 2 O 3 supporting films are deposited on conventional TEM grids by atomic layer deposition. The amorphous films with 2 nm thickness are mechanically stable up to 600 • C in air and over 900 • C in vacuum, suitable for high-temperature HRTEM and high-temperature SAED. The coated TEM grids can withstand higher temperature when the thickness of Al 2 O 3 supporting films increases.