Atomic Mechanism in Layer-by-Layer Growth via Surface

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Cite This: Nano Lett. 2019, 19, 4205−4210

Atomic Mechanism in Layer-by-Layer Growth via Surface Reconstruction Jian Yu,†,○ Xiao-Yan Li,○,§,∥ Junjian Miao,§ Wentao Yuan,† Shihui Zhou,‡ Beien Zhu,*,⊥ Yi Gao,*,⊥ Hangsheng Yang,† Ze Zhang,† and Yong Wang*,† †

State Key Laboratory of Silicon Materials and Center of Electron Microscopy, School of Materials Science and Engineering and Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou 310027, China § Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China ⊥ Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China

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‡

S Supporting Information *

ABSTRACT: Layer-by-layer growth played a critical role in the fine design of novel materials and devices. Although it has been widely studied during materials synthesis, the atomic mechanism of the growth remains unclear due to the lack of direct observation at the atomic scale. Here, we report a new mode in layer-by-layer growth via surface reconstruction on MoO2 (011) by environmental transmission electron microscopy and density functional theory calculations. Our in situ environmental transmission electron microscopy results demonstrate that the layer-by-layer growth of MoO2 experiences two steps that occur in an oscillatory manner: (1) the formation of an atomic ledge by transforming a section of the reconstructed layer to the intrinsic surface layer and then (2) the spontaneous reconstruction of the newly formed intrinsic surface section. Thus, the surface reconstruction can be considered as an intermediated phase during the layer-by-layer growth of MoO2. A similar phenomenon was also observed in the MoO2 dissolution procedure. KEYWORDS: Surface reconstruction, layer-by-layer growth, in situ environmental transmission electron microscopy, MoO2 ilm growth is a fundamental topic in the field of physics and chemistry that has been widely studied both theoretically and experimentally.1−5 There are three main growth modes accepted:6 the Volmer−Weber mode (island growth), Frank-van der Merwe mode (layer-by-layer growth), and the Stranski−Krastanov mode (mixed growth). Among these growth modes, the top surface is considered as a simple termination of the bulk material. However, in some cases, the top surface is not stable and forms reconstruction patterns, as is often observed during layer-by-layer film growth by lowenergy electron diffraction (LEED) in reciprocal space.7−9 The surface reconstruction more or less influences the crystalline growth process. For example, Van Vechten suggested that the surface reconstruction on the treads of a growing crystal surfaces could provide the source of growth steps required for the growth to proceed at low supercooling or supersaturating conditions.10 Giling et al. found that the reconstruction structure could change the growth direction of the materials.11 Nevertheless, how does surface reconstruction affect the

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© 2019 American Chemical Society

dynamic crystal growth behavior is still unclear due to the lack of direct dynamic information for reconstruction involved growth process, especially atomic resolution information in real space. Transmission electron microscopy (TEM) is a powerful tool that has been widely employed to investigate growth dynamics and kinetics of nanometer materials, especially with the developments of controlled-atmosphere TEM.12−15 Valuable information at the atomic scale has been achieved and leads to an unprecedented understanding of crystal growth in both solutions and gas environments.13,16−21 In contrast, the dynamic evolution of surface reconstruction during the growth processes and its effects have been rarely reported. In this work, by using environmental transmission electron microscopy (ETEM) as the main tool, we studied the growth and Received: May 11, 2019 Published: May 30, 2019 4205

DOI: 10.1021/acs.nanolett.9b01934 Nano Lett. 2019, 19, 4205−4210

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Figure 1. TEM characterization of the as-synthesized MoO2 samples. Typical HRTEM images of MoO2 viewed along the (a) [100] and (b) [011̅] directions. The insets are the corresponding FFT images, and the green rectangular frame is an enlarged image of the tip, showing the separation of white dots in the image. The electron current density is about 2.7 × 103 Am2−.

Figure 2. In situ TEM observation of layer-by-layer growth on MoO2 (011) surface at 450 °C with a 5 × 10−2 Pa oxygen pressure. (a) Typical TEM images of the morphology evolution acquired at 0 and 40 s. (b) In situ TEM images showing a typical length difference oscillation for the reconstruction layer and bulk layer during the growth process. (The green arrow and the red arrow point out the growth front of the new bulk layer and new reconstruction layer. The red and green dashed lines display the initial growth front of the new bulk layer and new reconstruction layer.) (c) Quantification of the length of the reconstruction layer (the red curve), bulk layer (the green curve), and their length difference (the blue curve) during the whole growth process. The electron current density is about 2.7 × 103 Am2−.

and then maintaining for 1 h in the ETEM chamber with an oxygen pressure of 5 × 10−2 Pa. To observe the dissolution behavior of MoO2, the temperature was then increased from 450 to 500 °C without changing other experimental conditions. Results and Discussion. Figure 1a shows a typical highresolution TEM (HRTEM) image of the MoO2 sample at 450 °C under an oxygen pressure of 5 × 10−2 Pa, and the corresponding fast Fourier transform (FFT) image viewed along the [100] direction is also shown in the inset. The energy dispersive X-ray (EDX) spectrum characterization of this sample is provided in Figure S1. In this case, the (011) and (011̅) facets are images edge-on. Periodic triangular peaks on

dissolution of MoO2 (011) in an oxygen atmosphere with elevated temperature at the atomic level. Combined with density functional theory (DFT) calculations, it is found that the reconstruction structure acts as an intermediate phase, which not only promotes the layer-by-layer growth mode but also dominates an oscillatory behavior. Experimental Methods. All experiments were carried out in an ETEM (H-9500, Hitachi) with an electron acceleration voltage of 300 kV. The microscope is equipped with a gasheating holder and a differential pumping system, which allows us to heat the sample up to 600 °C with the gas pressure in the sample chamber up to ∼0.1 Pa. MoO2 samples were in situ prepared by heating the Mo grid gradually to 450 °C in 30 min 4206

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Figure 3. In situ TEM observation of layer-by-layer dissolution on MoO2 (011) surface at 500 °C with a 5 × 10−2 Pa oxygen pressure. (a) In situ TEM images showing the dynamic behavior reconstruction layer and bulk layer during the dissolution process. The red and green dashed lines marked out the top surface layer (reconstruction structure) and the subsurface layer (bulk structure), and the green arrow and the red arrow point out the dissolution front of the bulk layer and the reconstruction layer, respectively. (b) Quantification of the length of the reconstruction layer (green line), bulk layer (red line), and their length difference (blue line) during the whole dissolution process. The electron current density is about 2.7 × 103 Am2−.

the (011) surface can be seen as schematically marked out by the red arrows. The height of the peaks is 4.4 Å and the distance between them is 6.8 Å. These peaks have a different contrast compared to the bulk structure and are determined to be a reconstruction structure (seen in Figures S2 and S3), as was often observed from other metal oxides.22−24 Figure 1b is a HRTEM image of another MoO2 sample viewed along [011̅] direction with the (011) and (100) surfaces imaged edge-on. The same reconstruction structure (height of 4.4 Å and distance of 5.2 Å) on the (011) surface is also observed as pointed out by the red arrow. More example images of the reconstruction structure are provided in Figure S4. Such a phenomenon was hardly observed in our previous work, in which the temperature was as high as 600 °C.25 Then, in situ ETEM is used to track the dynamic evolution of the surface reconstruction involved layer-by-layer growth on MoO2 (011) surface. Figure 2a displays the images of the MoO2 sample before and after the complete formation of a new surface layer, showing a typical layer-by-layer growth in which complete layers form prior to growth of subsequent layers. The lattice fringe spacing observed is 3.4 Å, which can be attributed to the MoO2 (011) surface. The height of the surface peaks is also 4.4 Å, which is consistent with that in Figure 1. Thus, the surface discrete dark dots stand for the surface reconstruction layer and the subsurface dark lines stand for the intrinsic crystal layer. Figure 2b shows a typical image sequence of the growth process in which the red and green arrows represent the growth front of the outmost reconstruction layer and intrinsic subsurface layer, respectively. In detail, the growth procedure can be separated into two steps. The first step is the transformation from the reconstructed layer to the intrinsic surface layer, during which an atomic ledge is formed. As seen in Figure 2b, the outmost black line (new formed intrinsic surface structure) propagated to the right a little from 17 to 18 s, and simultaneously, a peak on the right side disappeared. The second step is the formation of a new

reconstruction structure on the intrinsic surface. As shown in Figure 2b, from 18 to 19 s, the appearance of a new dark dot, which represents the new formed reconstruction structure, was observed. It is worthwhile to note that the surface reconstruction structure was directly formed on the intrinsic surface structure during growth rather than transformed from intrinsic surface structure, as previous studies reported.22,25−27 Next, the length of the new formed intrinsic surface layer (the green arrow and the green curve) and that of the new surface reconstruction layer (the red arrow and the red curve) were investigated, and the results are displayed in Figure 2b,c. It is of interest to find that the length difference between the two layers (the distance between the red and green arrows) oscillated with time. For example, at 17 s, the length difference was 5.4 Å, and 1 s later, the length difference increased to 10.5 Å, and then it returned back to 6.1 Å at 19 s. Such oscillation behavior was observed throughout the whole growth process. This phenomenon is different from our previous report, in which there was no reconstruction structure observed on the MoO2 (011) surface and the vertical growth mode was also observed together with the layer-by-layer growth mode.28 As the opposite procedure of its growth, the dissolution of MoO2 was also in situ studied, which can provide additional perspective to further understand the effect of the surface reconstruction structure. When the temperature was increased to 500 °C, the oxide surface became unstable and started to decompose. Figure 3a is a series of HRTEM snapshots that show the surface morphological changes during the MoO2 layer-by-layer dissolution at 500 °C with a 5 × 10−2 Pa oxygen pressure. At 0 s, the surface layer fully reconstructed, which showed periodical triangular peaks, and the subsurface layer was the bulk layer. At 3 s, the MoO2 dissolution started with the triangle at the top right corner been destroyed as the red arrow indicated. Evidently the dissolution continued from right to left in this case, and the second triangle disappeared at 4 s. At 5 s, the dissolution of the reconstruction layer stopped, and 4207

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Figure 4. (a) Modeled reconstructed molybdenum oxide surface with a ledge. (b) The intrinsic MoO2 surface is formed alongside the ledge. (c) The intrinsic MoO2 surface is formed on the top of the ledge. (d) A triangular peak is formed on the top of the newly formed intrinsic surface. (e) The intrinsic surface is extended by repeating the structural evolution from panel a to panel b.

ing, hydrocarbon contamination, and so on.29 Especially when gas is introduced to TEM, the beam effect becomes more complicated because the interaction between electrons and gas molecules. One of the most important results is the ionization of gas molecules, which leads to increased reactivity.30−33To evaluate the beam effect, we employed two methods, which were widely applied in the in situ TEM experiments:34−36 (1) block the beam except the time capturing pictures and (2) observe the growth behavior with different electron current densities. First, we studied the growth behaviors for the MoO2 samples growing for 30 and 60 min at 450 °C in 5 × 10−2 Pa O2 without beam irradiation. It was found that surface reconstruction structures can always be observed on the snapshots we took (beam on just for a fast picture capturing), indicating that the reconstruction structure is always on the surface during the growth (refer to Figure S4). To have a general idea of how the samples grow without beam irradiation, we made a statistic of the lengths vertical to the MoO2 (011) surface after growing for 30 and 60 min. As the results shown in Figure 5 support, the length vertical to the

a new triangle formed at the original subsurface layer, as marked by a green arrow. Thus, the dissolution of MoO2 can also be separated into two processes: (1) the destroying of the reconstruction layer and (2) the removal of surface atoms together with the formation of new reconstruction structure at the original subsurface layer (the new top surface layer). Thus, the reconstruction structure also acts as an intermediate phase during the dissolution procedure. Meanwhile, it is worth noting that the length difference between the top surface reconstruction layer and the subsurface bulk layer also fluctuates with time, as the blue line shows in Figure 3b. To explain and help in understanding the experimental observations, DFT calculations were performed based on the atomic model obtained from the high-resolution ETEM image. As shown in Figure S3, the simulated image of the modeled surface agrees very well with the experimental image. A stepped reconstructed surface with an atomic ledge was further set as the initial surface to mimic the growth process observed in the experiments. A pair of possible intermediate surfaces were considered for the following growth. One forms the intrinsic surface alongside the ledge (Figure 4b), and the other forms it on top of the ledge (Figure 4c). The details of the models and calculations settings can be found in the Supporting Information. As is clearly shown in Figure 4, the existing ledge helps stabilize the intrinsic surface forming alongside it. As a comparison, the on-top one is isolated and undergoes deformation due to the instability. The formation energy difference between these two surfaces can be calculated as: ΔEc−b = Ec + EO2 − E bx

(1)

where Ec and Eb are the total energies of the two slabs and EO2 is the energy of a gas phase O2. The calculated ΔEc−b value is 8.42 eV, which means that structure b is much more energetically favorable than structure c. On the basis of structure b, we further considered two following growth modes. One forms a reconstructed triangular peak on top of the newly formed intrinsic surface section (Figure 4d), and the other extends the growth of the intrinsic surface (Figure 4e). Following eq 1, ΔEe−d is calculated to be 3.63 eV, which means the newly formed intrinsic surface will be reconstructed prior to its extension. Thus, the experimentally observed oscillatory growth is explained. It is revealed that the ledge works like a zipper head, leading to the layer-by-layer growth. Finally, it is necessary to discuss the effect of beam irradiation on the growth behavior as beam irradiation has various effects, such as heating, electrostatic charging, ionization damage (radiolysis), displacement damage, sputter-

Figure 5. Statistic diagrams of the sample length vertical to the MoO2 (011) surface after 30 min (black squares) and 60 min (red squares) growth at 450 °C in 5 × 10−2 Pa O2.

(011) surface increased with time evidently, which indicates the growth of MoO2 (011) surface with surface reconstruction proceeds without beam irradiation. Second, in situ TEM observations with different electron current densities were carried out. As shown in Figures S5−S7, our in situ TEM observations of the MoO2 growth process with different electron current densities display no significant difference of 4208

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by Shanghai Supercomputer Center, National Supercomputer Centers in Tianjin, Guangdong.

growth rate, implying the electron beam played an insignificant role in the growth. Third, we have also observed the MoO2 samples under lower pressure (1 × 10−4 Pa) for comparison. We decreased the pressure to 1 × 10−4 Pa and the surface structure of MoO2 was badly damaged, which showed significant beam irradiation effect. As shown in Figures S8− S10, the surface of MoO2 was reduced to Mo clusters under beam irradiation. This is completely different from the phenomenon we observed in 5 × 10−2 Pa O2. Interestingly, the damaged surface will be recovered by reintroducing oxygen to 5 × 10−2 Pa (refer to Figures S8 and S9). Based on these results, it can be concluded that this surface reconstruction involved layer-by-layer growth mode under 5 × 10−2 Pa O2 is intrinsic rather than being controlled by electron-beam irradiation. In summary, we demonstrate for the first time in real space by ETEM a step-wise layer-by-layer growth of MoO2 with surface reconstruction involved. During MoO2 growth and dissolution, the formation of an atomic ledge by transforming a section of reconstructed layer to a new intrinsic surface layer and the formation of new reconstruction quickly on the intrinsic surface layer occur in an oscillatory manner. It is further unveiled that this atomic ledge acts like a zip head leading to the layer-by-layer growth. Our results provide new insight for crystal growth and dissolution research.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b01934. Figures showing EDX spectra, HRTEM simulations, the atomic structure of MoO2 (011) surface reconstruction, TEM results, MoO2 (011) top layer length, and HRTEM results (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Beien Zhu: 0000-0002-0126-0854 Yi Gao: 0000-0001-6015-5694 Hangsheng Yang: 0000-0002-5968-9223 Yong Wang: 0000-0002-9893-8296 Author Contributions ○

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of the National Natural Science Foundation of China (grant nos. 91645103, 51390474, 11327901, 11604357, 21773287, 51872260, and 11574340), the Zhejiang Provincial Natural Science Foundation of China (grant no. LD19B030001), the Natural Science Foundation of Shanghai (grant no. 16ZR1443200), and the Youth Innovation Promotion Association CAS and the Ministry of Science and Technology of China (grant no. 2016YFE0105700). The computational resources utilized in this research were provided 4209

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