Research

1. Dynamic Study of Materials

臨場觀測技術可獲得材料動態變化的訊息，結合電子顯微鏡的高解析能力可得原子級材料結構的改變，是材料研發獨特且關鍵技術。我們已成功利用加熱和通電方式解開材料擴散、相變化和原子遷移等材料現象，對於材料應用開發有極大的幫助。

我們曾探討銦在氧化鋅奈米線中的擴散行為[Movie 1-1]，除觀察到材料完全取代的現象，也得到不同晶面對擴散速度的影響；透過動力學探討得知此材料系統擴散是界面控制反應[Fig. 1-1]。另外，金屬通電時原子電遷移造成導線失效是迫切的問題，我們透過臨場觀測了解銀奈米線電遷移現象，也知材料內的疊差能有效抑制電遷移 [Fig. 1-2]。

In-situ TEM observation provides not only high image resolution for probing the dynamic changes of materials at atomic scale, which is unique and crucial for research and development. We conducted the experiments to study the material phenomena such as diffusion, phase change and the migration by heating and electrical biasing. We believe in-situ TEM observations have great contributions to the developments of material applications.

Previously, we studied the diffusion behavior of indium in ZnO nanowires [Movie 1-1]. In addition to observing the complete replacement of materials, the influences of crystal planes on diffusion were discussed. Also, interface-controlled reaction was determined through kinetic studies. [Figure 1-1] Furthermore, the failure of metal interconnects caused by the atomic electromigration (EM) under biasing is the serve issue for semiconductor device operation. We realized the EM phenomenon of silver nanowires through in-situ TEM observation, and also learned that the stacking faults in materials can effectively inhibit the EM. [Figure 1-2]

Movie 1-1. The ledge migration at the indium–ZnO interface.

Fig. 1-1. (a) A magnified ADF HR-STEM image and atomic model of hexagonal indium–Wurzite ZnO crystalline interface. (b) Diffusion rate of indium in ZnO: distance of diffused indium in [001]- and [100]-ZnO NW as a function of time.

Fig. 1-2. Necking of Ag nanowire with/without stacking faults.

S. C. Wang,∥ M. Y. Lu,∥* et al. Nano Lett. 14, 3241-3246 (2014)Y. H. Hsueh and M. Y. Lu et al Adv. Electron. Mater. 9, 2201054 (2023)

2. Atomic Imaging and Analysis

電子顯微術是材料研究的重要利器，可提供材料內部結構與成分的訊息，幫助材料開發，且因材料微小化的研發趨勢，電子顯微術重要性更勝過去。近年電子顯微分析能力大幅度提升，高解析能力可得到原子級結構訊息，藉此了解原子的排列和缺陷型態。

WS2和MoS2為重要的二維半導體材料，可用於太陽能電池與電晶體等應用。Fig. 2-1為WS2-MoS2 Heterojunction高解析掃描穿透電子顯微（HR STEM）影像，界面原子排列完整且清楚，從影像對比高低也可分辨WS2和MoS2。此外，PdSe2是另一種的二維半導體材料，可用於光電元件應用，而能隙會因堆疊層數不同，可從STEM影像中原子對比判斷層數 [Fig. 2-2]。

另外，銀為導電性最佳的金屬，可作為金屬導線，透過TEM分析可發現疊差(Stacking Faults)在銀奈米線中，且由原子排列可確定其為本質型(intrinsic)疊差。[Fig. 2-3]

Transmission electron microscopy (TEM) is a powerful technique for material science research, it provides the information about the structures and compositions of material. Due to the downsizing of the materials, TEM plays a much more important role than ever. Recently, the resolving power and the capability of TEM is the dramatically improved, we can obtain the structural information of the materials, such as the atomic arrangement and defects, at the atomic scale from high-resolution TEM images.

WS2 and MoS2 are important 2-D semiconductors, which have the potential applications on solar cell and transistors, and so on. The high resolution scanning transmission electron microscopy (STEM) image in figure 2-1reveals the atomic structure of the interface between WS2 and MoS2. The details of atomic arrangements at the boundary are obviously seen. We can distinguish WS2 and MoS2 from the contrasts of the image. In addition, is another 2-D semiconductor, the physical properties depend on the number of stacking layers, and it can be distinguished from the contrast of STEM image [Figure 2-2]. On the other hand, Ag, the best electrically conductive material, can be used us the interconnects in semiconductor devices. the stacking faults in Ag nanowires are found in STEM image and the type of the stacking faults is determined via TEM analysis [Figure 2-3].

Fig. 2-1. WS2-MoS2 heterojunction.

Fig. 2-2. Stacking faults in Ag nanowire.

Fig. 2-3. PdSe nanoflakes.

G. J. Lai, et al. Nano Energy 81, 105608 (2021)S. H. Tseng, et al. Chem Comm 56, 5593 (2020)L. S. Lu et al. ACS Nano 14, 4963-4972 (2020)

3. Applications of Low-Dimensional Semiconductors

低維度材料因尺寸效應、高表面積和量子效應等特殊性，使之可應用於電子、光電和能源元件。太陽光是乾淨的能源之一，如何能源轉換並儲存是研究的重點之一。我們曾結合ZnO奈米線和Au奈米顆粒，因優異光電特性和表面電漿共振，得到高轉換效率的染料敏化太陽能元件 [Fig. 3-1]；另外，我們也合成WS2-MoS2異質結構，因type II能隙對應關係適合用於能源轉換應用，結果顯示此異質結構能有效的以太陽光將水分解產生氫氣 [Fig. 3-2]。奈米異質結構有多元的應用，有著無能耗且高敏感度。我們也將CdS奈米線和MoTe2二維薄片結合成異質結構，並應用於自驅動光偵測元件，可達到無能耗且高敏感度的優異表現 [Fig. 3-3]。

Low-dimensional materials can be applied to electronic, optoelectronic and energy devices due to their unique features, such as size effect, larger surface area, quantum effect and etc. Sunlight is one of the clean energies, and how to convert and store the energy becomes the major issue for the applications. Previously, the heterostructures composed of ZnO nanowires and Au nanoparticles are used as dye-sensitized solar cells, due to its excellent optoelectronic property and surface plasmon resonance, the optimized energy conversion efficiency is obtained [Fig. 3-1]. In addition, we also probed the study on hydrogen production of WS2-MoS2 heterostructures. Because of the type II energy band alignment of WS2 and MoS2, the results showed that the heterostructure can efficiently split water into hydrogen by sunlight [Fig. 3-2]. Be the way, self-driven sensors have the potential to be the majority type of the sensors, since the sensors are operated without applied energy, we combined CdS nanowires and MoTe2 2-D flake into the heterostructures for self-driven photodetectors, which possesses the excellent performance and high sensitivity under light illumination [Fig. 3-3].

Fig. 3-1. Dye-sensitized solar cells.

Fig. 3-2. Hydrogen evolution with the heterostructures.

Fig. 3-3. Self-driven mixed-dimensional photodetectors.

M. Y. Lu, et al. Nano Energy 20, 264-271 (2016) G. J. Lai, M. Y. Lu et al. Nano Energy 81, 105608 (2021) M. Y. Lu, et al. Small 14, 1802302 (2018)

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