Abstract Electron beam (e-beam) irradiation is an inev-itable, but crucial issue for electron microscopy. Ourinvestigation results show the e-beam-induced in situstructural transformations in silicon (Si) nanowires andzinc oxide (ZnO) nanowires (NWs), respectively. Crystalto amorphous structure transition was revealed in Si NWsutilizing high resolution electron microscopy and electronenergy loss spectroscopy. Reconstruction at the (10 ? 10)surface of ZnO NWs was also observed in the transmissionelectron microscope (TEM) using aberration-correctedelectron microscopy. These e-beam-induced in situ struc-tural transformations prove that the electron beam irradi-ation effect is able to be used for the local modification ofone-dimensional nanomaterials.

Keywords Electron beam effect ? In situ electronmicroscopy ? Structure transformation ? Si nanowires ?ZnO nanowires

1 Introduction

One-dimensional nanomaterials, especially nanowires(NWs), are ideal candidates for fundamental research andimportant building blocks in high-performance nanodevices[1–8].Electronmicroscopyisaneffectivemethodtoprovidemorphological and structural characterization of nanoma-terials. Additionally, the chemical, electronic, and magnetic properties of nanomaterials can also be obtained with therecent developments in electron microscopy [9–11]. Inparticular, the high energy electron beam (e-beam) withinthe electron microscope is able to be used for the modifica-tion and fabrication of nanostructures. For instance, twocarbon nanotubes have been welded together to form ajunction with the aid of electron beam (e-beam) [12].Nanopores with single nanometer precision can also befabricated on the silicon oxide under the e-beam irradiation[13]. Here, two examples of our research are provided toshow the e-beam-induced structural transformation in sili-con (Si) NWs and zinc oxide (ZnO) NWs.

2 E-beam-induced structure transformation in Si NWs

Previous reports have demonstrated that Si NWs exhibitlinear elastic behavior and prone to fracture without anyappreciable plastic deformation [14, 15]. This may increasethe difficulties for processing Si–NW-based nanodevices.Takeda et al. [16] have found that the crystalline toamorphous transition of bulk Si is possible to take placewith the irradiation of a 2-MeV e-beam. Furthermore, itwas revealed by our group [17] that the crystal to amor-phous structure transition can also be induced in Si NWs,while exposed to normal intensity e-beam irradiation in thetransmission electron microscope (TEM). A currentintensity (J) of 3.2 A/cm 2 is sufficient to cause this struc-tural transition, while J of a typical high resolution trans-mission electron microscopy (HRTEM) observation isusually on the order of 10 A/cm 2 . The results in Fig. 1clearly present the in situ structure transition in an indi-vidual Si NW. Before the e-beam irradiation, both theelectron diffraction pattern (Fig. 1a) and the HRTEMimage (Fig. 1b) demonstrate the single crystallinity of this Si NW. However, after 900 s of irradiation, the Si NW hasbeen transformed into an amorphous structure (Fig. 1c) atthe center where the e-beam was focused (the circular areain Fig. 1a). Meanwhile, some mass of the NW wasremoved due to the transmission sputtering phenomenon[18]. The electron energy loss spectrum (EELS) in Fig. 1dis collected from the amorphous part in Fig. 1c. The Si L 2,3edge of the amorphous part is consistent with the standardspectrum of amorphous silicon [19]. This result indicatesthat the observed amorphization process is a transformationfrom single crystalline silicon (c-Si) to amorphous silicon(a-Si).

Interestingly, the mechanical behavior of the Si NWcan be modified by its structural transformation. Aspresented in Fig. 1e, g, electrostatic force was used tobend the as-irradiated Si NW. As we know, Si NWs aretypical semiconductor materials and the charge accu-mulation occurs under continuous e-beam illumination.Therefore, the electric field from the e-beam is able toexert an electrostatic force on the NW [17, 20]. It wasfound that Si NW was bent gradually under e-beamirradiation. Meanwhile, an abrupt crack and shaperecovery did not happen as the e-beam was spread totake images under low magnification. According to theHRTEM images in Fig. 1f, h, it is clear that the entire plastic deformation was created at the amorphous regionof the Si NW.

Furthermore, the in situ TEM-STM (scanning tunnelingmicroscopy) holder (Nanofactory Instrument AB Com-pany, Sweden) was utilized to apply mechanical force onan individual Si NW. As shown in Fig. 2a, a 30-nm-thickSi NW was attached on the gold substrate and can bemanipulated by a W tip, which can achieve three-dimen-sional movements on the other side. The initial NW wasfirst bent from a straight line into a hook shape. Withoutany special irradiation process, the NW showed greatflexibility and would recover to its original straight shape ifthe W tip moved away. This is in accordance with thereported elastic mechanical behavior of Si NWs [14, 15].Then, if we reloaded the W tip to keep the Si NW at a bentstate and applied 900 s of irradiation at the corner of thehook (Fig. 2b), the bent deformation was retained in the SiNW after the tip was removed (Fig. 2c). It is reasonablethat the crystalline to amorphous transformation has takenplace in this Si NW due to the e-beam irradiation. Thedangling bonds in an a-Si region were able to be restruc-tured under sufficient energy from the e-beam. Accordingto the results of Egerton et al. [21], the energy transformedfrom a 200-keV electron to a silicon atom is about 19.0 eV.This energy is adequate to drive a silicon atom displaced from its original site in a-Si in that the correspondingdisplacement energy is only about 15.8–21.0 eV [22].Therefore, additional energy from the e-beam was able tomotivate the bond reformation process at the irradiatedareas and the elastic energy can be mildly released throughthis dynamic motion. Abrupt cracks can be avoided, andthe elastic–plastic transition has been achieved in Si NWdue to this structural evolution.

3 E-beam-induced structural transformationat the surface of ZnO NWs

Another example showing the in situ structural transfor-mation is the reconstruction at the surface of ZnO NWs[23]. Changes at this surface are able to modify the vari-ation of physical and chemical properties in ZnO NWs,applicable for functional materials [24]. However, stillmuch less is understood about surface reconstruction at theZnO (10 ? 10) surface, one of the most basic surfaces in ioniccrystals. The first experimental evidence for the reversiblewurtzite-body-centered tetragonal (WZ-BCT) reconstruc-tion at the ZnO (10 ? 10) surface by using an aberration-corrected Titan 80-300 TEM was reported by He et al. [23]in our research group. Single crystalline nanoislands can beformed at the ZnO NW surface during intense e-beamirradiation [25]. HRTEM observations were performed toacquire the atomic profile of the clean (10 ? 10) surface uti-lizing the negative spherical aberration (C s ) imagingtechnique [26]. Calculations of equilibrium structures andenergetics of the ZnO (10 ? 10) surface were also carried outusing the projector augmented wave method with densityfunction theory (DFT) calculation, implemented in theVienna ab-initio simulation package (VASP) code [27–29].

Figure 3a shows the (10 ? 10) surface with the originalWZ structure based on six-atom rings in ZnO NW, whilein Fig. 3b, a different lattice was manifested at the out-ermost surface layers with alternating four-atom andeight-atom rings, which is the characteristic configurationof the BCT structure. The corresponding HRTEM simu-lations were performed using the multislice methodaccording to the reconstructed surface from DFT calcu-lations, and the results are presented at the right side ofFig. 3c. The left side of Fig. 3c is the enlarged HRTEMimage of Fig. 3b. It is notable that one-to-one corre-spondence of intensity peaks is realized between theexperimental image and the simulated positions of atomiccolumns. Moreover, the measured projective distance ofZn 1 –O 2 (0.98 A˚ ) and Zn 2 –O 1 (0.98 A ˚ ) is quantitatively inagreement with the relaxed BCT structure from DFTcalculations showing Zn 1 –O 2 (0.985 A˚ )and Zn 2 –O 1(0.978 A˚ ), as illustrated in Fig. 3c. Also the transforma-tion process was recorded in Fig. 3d, showing theneighboring WZ and BCT structure at the surface and thecoexistence of Zn 1 –O 2 atomic pairs at the boundarybetween a WZ domain and a BCT domain. It was alsorevealed that the Zn 1 –O 2 atomic pair could move towardeither the WZ or the BCT domain under e-beamirradiation.

Wang et al. [30] have showed a stress-induced phasetransformation from a WZ structure to a BCT structure inZnO NWs by molecular dynamic (MD) simulations.According to their work, the WZ-BCT structural transfor-mation is closely related to the mechanical properties ofZnO NWs. Therefore, the WZ-BCT structural transfor-mation induced by the e-beam in our results may have itssignificance in the modification of the mechanical behaviorof ZnO NWs.

4 Conclusions

To summarize, two examples of e-beam-induced in situstructural transformation were observed in Si NWs and ZnONWs. The additional energy from the e-beam is able toreconfigure the structure and the bonding condition in one-dimensional nanomaterials. The e-beam effect has beenproved as a valid method to achieve local modifications ofnanomaterials by our work and some other reports [31–33].We have also demonstrated that in situ TEM and aberration-correctedHRTEMareverypowerfultechniquesforstudyingthese local microscopic structural transformations.

Acknowledgements This work was supported by the NationalBasic Research Program of China (2009CB623701) and the NationalNatural Science Foundation of China (11374174, 51390471). Thiswork made use of the resources of the Beijing National Center forElectron Microscopy.Conflict of interest The authors declare that they have no conflictof interest.

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