Date Published: February 26, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Luping Tang, Longbing He, Lei Zhang, Kaihao Yu, Tao Xu, Qiubo Zhang, Hui Dong, Chao Zhu, Litao Sun.
The manipulation and tailoring of the structure and properties of semiconductor nanocrystals (NCs) is significant particularly for the design and fabrication of future nanodevices. Here, a novel domain‐confined growth strategy is reported for controllable fabrication of individual monocrystal hollow NCs (h‐NCs) in situ inside a transmission electron microscope, which enables the atomic scale monitoring of the entire reaction. During the process, the preformed carbon shells serve as nanoreaction cells for the formation of CdSeS h‐NCs. Electron beam (e‐beam) irradiation is demonstrated to be the key activation factor for the solid‐to‐hollow shape transformation. The formation of CdSeS hollow NCs is also found to be sensitive to the volume ratio of the CdSe/CdS NCs to the carbon shell and only those CdSe/CdS NCs with a volume ratio in the range 0.2–0.8 are successfully converted into hollow NCs. The method paves the way to potentially use an e‐beam for the in situ tailoring of individual semiconductor NCs targeted toward future nanodevice applications.
Hollow nanocrystals (h‐NCs) show a unique combination of physical properties including large surface area, low density, and high loading capacity, which enables their application in diverse fields such as micro/nanoreactors,1, 2 catalysis,3 energy storage,4, 5 drug delivery and biomedicine,6 and sensing.7 Consequently, the development of synthesis techniques for the controlled design and fabrication of h‐NCs is of great interest. A variety of synthesis methods have been investigated to obtain hollow NCs. These include techniques that adopt a template‐mediated approach,8, 9 self‐assembly,10 galvanic exchange,11, 12 Kirkendall reaction,13 Ostwald ripening,14 and surface‐protected etching.15 Although these methods have been successful in the synthesis of h‐NCs with different shapes depending upon the reaction and growth in chemical solutions, such liquid phase synthesis strategies currently do not afford fine‐tuning of the resulting structure. Moreover, a detailed understanding of the mechanism of their formation cannot be obtained under these conditions. For example, most of the template‐mediated approaches rely strongly on the quality of the templating material and the method used to etch these templates. Using harsh reaction conditions to remove the core material usually induces unexpected structural collapse of the nanostructure, whereas moderate treatments very often leave residues from the core as impurity.8, 10 In cases where soft‐templates like liquid interfaces or gas bubbles are used, the assembly of hollow shells around these templates cannot be easily controlled since their sizes, shapes, and uniformity are strongly restricted by the preformed emulsion, vesicle/micelle, or gas bubbles as well as their stability.16
This advanced synthesis procedure26 has the advantage that the templating CdSe/CdS NCs have quite a uniform size distribution with an average particle size of around 15.4 nm (Figure S1, Supporting Information). First, several regions of interest are selected and marked by the transmission electron microscope (TEM) stage. Through e‐beam irradiation, the residual surfactant molecules on the CdSe/CdS NC surfaces and the carbohydrate molecules in the TEM chamber are carbonized at the imaging region, and carbon shells are subsequently formed onto the NC surfaces. In this process, the thickness of the carbon shells can be precisely controlled by the e‐beam intensity and the irradiation time (see more details in the Experimental Section). In the experiment here, the thickness of the carbon shells is controlled to be 1–2 nm by monitoring the growth process of the carbon shells. Figure1a,b shows the formation of h‐NCs by e‐beam irradiation of partially sublimated CdSe/CdS NCs at 200 °C (more information on the prior sublimation of CdSe/CdS NCs at 340 °C is given in Figure S2, Supporting Information). It is seen in Figure 1 that a significant number of shape transformations of the residual CdSe/CdS NCs (as marked by the numbered dotted rings) occur as a result of e‐beam irradiation. Interestingly, most of the half‐moon‐like CdSe/CdS NCs gradually turn into bowl‐like or hollow spheres (Nos. 1–12). Others including empty box‐like forms and NCs suffering either very little sublimation or rather severe sublimation show significantly different evolution patterns. For the empty carbon boxes, no significant growth is observed on their inner shells. On the other hand, NCs having very small (Nos. 13 and 14) or very large residual cores (Nos. 15 and 16) eventually turn into discrete islands or spheres, respectively. These evolutions prove that the e‐beam induced regrowth of the partially sublimated CdSe/CdS NCs is mainly restricted inside the carbon shells. The corresponding scanning transmission electron microscope (STEM) image shown in Figure 1c further confirms that no significant nucleation or Ostwald ripening is observed outside the carbon shells. In addition, the energy‐dispersive X‐ray (EDX) spectral data acquired in STEM mode show that the types of chemical elements of the NCs remain unchanged, indicating that Cd, Se, and S still partially remain after the transformation of the solid CdSe/CdS NCs into hollow CdSeS NCs. It is also noted that the NCs maintain their crystalline structure through the entire transformation process. As displayed in Figure 1d, high‐resolution TEM images of the obtained h‐NCs sequentially marked from 1 to 7 in Figure 1b show clear lattice fringes corresponding to CdSeS (002), (101), (102), and (100) crystal planes. The voids formed inside the h‐NCs are rather irregular, which may be due to inhomogeneous nucleation and anisotropic growth.
In summary, a novel domain‐confined growth strategy for controllable fabrication of individual hollow CdSeS NCs has been achieved through an in situ e‐beam activation process in TEM. The recrystallization of the h‐NCs is exclusively confined to inside the carbon shells. The dominant role of electron beam in the formation of CdSeS h‐NCs is demonstrated. Moreover, the volume ratio of the CdSe/CdS NC to the carbon shell is the key factor for the formation of hollow CdSeS NCs. Three types of shape evolution can be observed; only CdSe/CdS NCs with a volume ratio of around 0.2–0.8 can successfully yield CdSeS h‐NCs. Anti‐Ostwald ripening is observed in the formation process of CdSeS h‐NCs and the mechanism of surface energy increase is discussed and found to be related to the e‐beam effect. Our findings here provide valuable knowledge on the interactions between a high‐energy electron beam and semiconductor nanocrystals, as well as a way for better understanding the formation mechanism of hollow nanostructures. With the improvement of preparation technology, such as the material promotion, production cost, and repeatability, the practical application of these individual hollow nanostructures would become more and more extensive.
Preparation of Templating CdSe/CdS NCs: CdSe/CdS core/shell solid NCs were synthesized by epitaxial growth of ≈16 monolayers of CdS shells on the CdSe crystal seeds using the successive ionic layer adsorption and reaction method. The obtained NCs were then washed and purified by precipitating them two to three times with ethanol and redispersing in hexane. More details on this procedure can be found in ref. 25.
The authors declare no conflict of interest.