Passivation Effect of Gold Nanoparticles on Uniform Beam-Induced

Jul 6, 2017 - Passivation effect of heterogeneous Au nanoparticles (AuNPs) on the structural changes at the nanoscale (or so-called nanoinstability) o...
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Passivation Effect of Gold Nanoparticles on Uniform Beam-Induced Structural Changes of Amorphous SiOx Nanowire Jiangbin Su†,‡ and Xianfang Zhu*,† †

China-Australia Joint Laboratory for Functional Nanomaterials & Physics Department, Xiamen University, Xiamen 361005, People’s Republic of China ‡ Experiment Center of Electronic Science and Technology, School of Mathematics and Physics, Changzhou University, Changzhou 213164, People’s Republic of China ABSTRACT: Passivation effect of heterogeneous Au nanoparticles (AuNPs) on the structural changes at the nanoscale (or so-called nanoinstability) of amorphous SiOx nanowire (a-SiOx NW) as athermally induced by uniform electron beam (e-beam) irradiation is investigated in an in situ transmission electron microscope. It is found that at room temperature the straight and uniform a-SiOx NW demonstrates a considerable uniform plastic elongation and an accelerated uniform radial shrinkage at the nanoscale. However, once being modified with AuNPs, the nanocurved sidewall surface of a-SiOx NW becomes intriguingly passivated. As a consequence, both the elongation and the radial shrinkage of the AuNPs-modified a-SiOx NW are greatly retarded. A new mechanism of athermal diffusion and plastic flow combined with athermal evaporation as driven by the nanocurvature of a-SiOx NW and the beam-induced soft mode and instability of atomic vibration, which are different from the existing knock-on mechanism, is proposed to elucidate the observed new phenomena.

1. INTRODUCTION Nanowires (NWs) are regarded as one group of the most promising materials due to their unique quasi-one-dimensional cylinder-like structure and the related unique physical and chemical properties. In recent years, a great deal of effort has been put into cutting, welding, or penetrating of thin crystalline metal and semiconductor NWs1−4 by energetic electron beam (e-beam) irradiation and studying of the resultant structural changes for their potential applications in future nanodevices or nanotechnology. In these studies, the structural changes of the NWs which were largely unique to the nanoscale were described in terms of the existing knock-on mechanism and the subsequent atom reconstructions mostly along with some related simulations.1−5 However, it was recently observed that the radius (half of the diameter) of amorphous SiOx (a-SiOx) NW shrank in an increasingly faster manner when it reduced down to the nanoscale during e-beam irradiation at room temperature.6 Such an accelerated shrinkage confirmed that the previously predicted effect of NW curvature at the nanoscale7,8 could be indeed much exceeded that predicted from the reported theories and simulations (we called this unique curvature at the nanoscale as nanocurvature for short6,8). Moreover, it was also observed that with such a nanocurvature effect, the a-SiOx NW could exhibit an extraordinary plastic flow and elongation6 at room temperature under e-beam irradiation. This confirmed the previously predicted, energetic beam-induced, athermal (nonthermal) activation effect (or called as energetic beam-induced atomic vibration soft mode and instability)8,9 on NW. Both the nanocurvature effect and the beam-induced atomic vibration soft mode and instability © 2017 American Chemical Society

effect seem to be intrinsically of nonequilibrium, disorder (amorphous-like structure), and nonlinearity nature, which in principle, cannot be adequately accounted for by existing theories such as the reported knock-on mechanism or simulations. This is because the reported theories and simulations were normally based on the nature of equilibrium, symmetry, periodicity, and linearity of bulk crystalline structure or its approximations where the nanocurvature effect or the beam-induced atomic vibration soft mode and instability effect has attracted much less attention.6 It has been reported that both the nanocurvature effect and the beam-induced atomic vibration soft mode and instability effect are much more obvious in an amorphous low dimensional nanostructure (LDN) than in a crystalline one.6 Thus, it can be further predicted that heterogeneous crystalline nanoparticles (NPs) such as metal NPs decorated on the surface of an amorphous NW may passivate the beam-induced structural changes such as the aforementioned elongation and radial shrinkage. Up until now, there are some reports about the e-beam-assisted tensile pulling-induced elongation10−12 and the e-beam purely induced elongation6 of NWs. Even so, there is still no experimental evidence to demonstrate the passivation effect of metal NPs on the elongation and the relevant radial shrinkage of amorphous NW and thus the underlying physical mechanism has not been explored yet. In this regard, study on the passivation effect of metal NPs on the amorphous NW is Received: April 4, 2017 Revised: June 18, 2017 Published: July 6, 2017 15977

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Figure 1. Sequential TEM images showing the structural changes with the irradiation time as induced by irradiation of a uniform beam spread over the whole area with beam current density of 5.6 A/cm2 on a pristine a-SiOx NW.

Figure 2. Sequential TEM images showing the structural changes with the irradiation time under the same irradiation as that in Figure 1 on a AuNPs-modified a-SiOx NW.

imperative and crucial not only to the precise and flexible controlling of the e-beam-induced NW structural changes but also to the fundamental understanding of some new phenomena and concepts in LDNs.

sputter coater (BAL-TEC SCD 005). Both the pristine and the AuNPs-modified a-SiOx NWs were first ultrasonically detangled in ethanol and then dripped onto the holey carbon film on Cu grids for the TEM studies. The e-beam irradiation and the in situ observation were carried out at room temperature in a fieldemission Tecnai F-30 TEM operated at 300 kV. The irradiation was always targeted on segments of single wires protruding into the open space of the holes in the carbon film of microscopy grid so that local structure transformations or deformation of the wires were not affected by any support within the holes. During the experiment, if there was any deviation of the wires from this position (actually rarely occurred in the experiment in this paper), we always tilted the sample to make sure that axis of the wires is normal to the electron beam by checking whether focus of the wire sidewall is uniform along the axis direction. In doing so, we ensured that there is no reduction in

2. EXPERIMENTAL SECTION The a-SiOx NWs with the desired diameter were fabricated by our improved chemical vapor deposition technique where x is determined to be 2.3.13 As-fabricated pristine NWs can be straight and uniform in diameter all the way along the axis direction with a smooth surface to avoid as much as possible the effect of nonuniformity of nanocurvature on the sidewall of wire. In order to study the passivation effect of metal NPs, some uniformly distributed AuNPs were further deposited onto the a-SiOx NWs surface for a total duration of 1 s by a cool 15978

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The Journal of Physical Chemistry C the projected or measured length of the wires which may be caused by any upward or downward tilting of the wires. During each irradiation, the e-beam current density was kept at about 5.6 A/cm2 (flux: 3.5 × 105 nm−2 s−1), which was uniform over an area (∼600 nm) larger than the zone or NW observed. The beam was spread to an around 100 times weaker intensity for the observation or taking a picture. Furthermore, we took each picture within a time interval less than 10 s before returning to the normal irradiation. In this way, the irradiation effect (dose or time) during the observation or taking a picture can be minimized to a negligible degree and at the same time the image contrast can also be improved. It should be noted that the electron irradiation would not heat a specimen of LDN by more than a few degrees5,6,12,14 due to its extremely large ratio of surface to volume. Thus, the NW essentially remained at room temperature throughout the irradiation and we could consider that the dominant irradiation effect should be nonthermal or athermal. Because of the minor fluctuation in local beam current density or the little nonuniformity in local wire structure or both during the irradiation, some local neckings or small deformations can be observed in an elongated and shrunk aSiOx NW after a long period of irradiation (see Figures 1 and 2). To account for this nonuniformity in the wire, the radius of the a-SiOx NW was taken as the average value measured at several representative sites of the wire. Meanwhile, the length and the volume of the wire segment were taken as the average values measured and calculated between two red mark dots in Figures 1 and 2 along the wire axis. The two red dots were carefully marked at the locations of two feature points and by further checking the relative positions of their surrounding feature points on the wire surface (see the arrows in Figures 1 and 2).

Figure 3. Typical HRTEM micrographs and FFT images (insets) showing the structure evolution of two AuNPs under e-beam irradiation.

and the radial shrinkage. During the irradiation, when two (or more) AuNPs come together with the NW diameter shrinkage, the two types of nanocurvature-driven nanoripening of the AuNPs would take place. That is, the AuNPs would rotate and coalesce into a bigger and bigger AuNP (see Figure 3a−d) and/ or the larger AuNPs may grow by taking up of the adjacent smaller ones before the coalescing. Nevertheless, during the nanoripening as shown in Figure 2a−h and 3a−d, the AuNPs seemed to keep their crystalline structure and rarely evaporate even after the surrounding a-SiOx material was gradually polished or dug out (the details of the nanoripening is to be published in a separated paper). A similar irradiation on other pristine and AuNPs-modified NW segments was repeated several times. It was observed that the features of the structural changes were essentially the same as shown in Figures 1 and 2. The detailed evolutions of length of the pristine and AuNPsmodified NW segments against the irradiation time were plotted in Figure 4. For the pristine NW, the segment elongated quickly from 85.8 to 109.5 nm within 1170 s

3. RESULTS AND DISCUSSION The sequential TEM micrographs in Figures 1 and 2, respectively, show the typical structural changes of a pristine (without AuNPs) and a AuNPs-modified (with AuNPs) a-SiOx NW segment during the uniform e-beam irradiation at room temperature. Prior to the irradiation, as shown in (a) of Figures 1 and 2, both the pristine and the AuNPs-modified a-SiOx NW segments presented a well-defined straight and uniform cylinder shape with the same initial diameter of ∼18 nm but different surface morphologies. For the pristine NW, the surface is smooth and clean (see Figure 1a), while for the AuNPs-modified NW, the surface is uniformly decorated by many tiny, dispersive NPs with an average size of ∼2 nm (see Figure 2a). The energy dispersive X-ray spectra (not shown), high resolution TEM (HRTEM) micrographs and fast Fourier transform (FFT) images (see Figure 3a) respectively confirmed the composition and texture of crystalline AuNPs. With the irradiation turned on, as experimentally demonstrated in parts a−h of Figures 1 and 2, sequential consistent extension in the segment length and shrinkage in the segment diameter after each dose of the beam were occurring in both the pristine and the AuNPs-modified NWs. Moreover, both NWs almost kept their straight and uniform cylinder shape during the whole duration of 1170 s, which indicated that the elongation and radial shrinkage proceeded uniformly. However, relative to the pristine NW segment, it was clearly observed that the AuNPsmodified one demonstrated a less and slower elongation and radial shrinkage especially after 900 s irradiation, which indicated an intriguing passivation effect on both the elongation

Figure 4. Elongation of the pristine and the AuNPs-modified a-SiOx NWs with the irradiation time as measured from Figures 1 and 2. The length is measured as the length of the irradiated wire segments between the two red feature dots as shown in Figures 1 and 2 to trace the changes of wire length. 15979

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surface would greatly passivate the accelerated radial shrinkage as well as the elongation of a-SiOx NW at the nanoscale. The above observations indeed demonstrated a typical uniform elongation and accelerated radial shrinkage in the SiOx NW no matter whether it was modified with AuNPs or not. More intriguingly, the passivation effect of the AuNPs on both the elongation and the accelerated radial shrinkage of the a-SiOx NW was also experimentally demonstrated. Obviously, such beam-induced nanophenomena can not be simply explained by the existing knock-on mechanism or simulations1−5 as originally based on classical bulk crystalline theories or their approximations where the intrinsic nanocurvature effect7,8 of the a-SiOx NW and the beam-induced atomic vibration soft mode and instability effect8,9 are normally neglected. Actually, from the following analysis, we can conceive that the nanocurvature effect of the a-SiOx NW and the e-beam-induced atomic vibration soft mode and instability effect were the key factors to induce the observed structural changes of the a-SiOx NWs. For the nanocurvature effect of a NW, as discussed in literature,6−8 when the radius of a NW approaches its atomic bond length, a positive nanocurvature on the highly curved wire surface will become appreciable. Such a positive nanocurvature would cause an additional tensile stress on the electron cloud structure of surface atoms which would lead to a dramatic increase in surface energy of the NW. This dramatically increased surface energy would give rise to a strong tendency of self-compression on the nanocurved wire surface and thus provide a thermodynamic driving force for the wire to contract in the radius direction and massive atom plastic flow as self-extruded toward the two ends of the wire. Nevertheless, only the nanocurvature effect is not enough to cause the structural changes for the a-SiOx NW at room temperature. For the transition from one metastable structure configuration to another, the a-SiOx NW also needs to be softened or overcome some energy barrier. With the assistance of energetic e-beaminduced athermal activation, it can be reasonably assumed that under the highly energetic e-beam irradiation the energy deposition rate could be faster with respect to frequencies of thermal vibration of atoms in condensed matter. Thus, there would be no enough time for the atoms of a-SiOx NW to transfer the beam-deposited energy into atom vibration energy and the mode of atom thermal vibration would become softened or the vibration of atoms would lose stability.6,8,9 The as-induced atomic vibration soft mode and instability would suppress the energy barrier greatly or even make it totally disappear. In doing so, the irradiation would induce athermal diffusion (even athermal plastic flow) or/and athermal evaporation (even athermal ablation) of the wire atoms. With the nanocurvature effect, the former would lead to the overall elongation of the NW while the both would cause the radial shrinkage. Moreover, since the nanocurvature distributed over the wire surface is uniform along the wire axis and a uniform e-beam is applied for the irradiation, the as-induced athermal diffusion and evaporation of surface atoms and athermal plastic flow of massive atoms would proceed uniformly along the wire axis. In this way, as demonstrated in Figure 1 and further illustrated in Figure 6a, a uniform axial elongation and a uniform radial shrinkage are occurring in the a-SiOx NW during the irradiation of the uniform e-beam. In particular, with the increase of irradiation time, the NW became thinner and demonstrated an accelerated radial shrinkage at the nanoscale as shown in Figure 5. This could be attributed to the dramatically increased

irradiation, which demonstrated a total extension in the segment length of 23.7 nm and an average extension rate of ∼2.0 × 10−2 nm/s. However, for the AuNPs-modified NW, the segment elongated much more slowly from 86 to 90.8 nm within 1170 s irradiation, which showed a total extension in the segment length of only 4.8 nm and an average extension rate of ∼4.1 × 10−3 nm/s. Obviously, the elongation of AuNPsmodified NW is almost 5 times slower than that of the pristine one. Since the irradiation conditions and the original radius of the NWs were totally the same, such a big decline in the elongation rate of SiOx NW should be attributed to the existence of AuNPs on the wire surface. This phenomena was referred here to a passivation effect of heterogeneous metal NPs on the beam-induced elongation of a-SiOx NW. As shown in Figure 5, the comparative study of radius evolutions between the pristine and the AuNPs-modified SiOx

Figure 5. Radius shrinkage of the pristine and the AuNPs-modified aSiOx NWs with the irradiation time as measured from Figures 1 and 2. The dashed lines are the fitting results with a value of the constant b at about 3.3 for SiOx NW and 2.4 for AuNPs-modified a-SiOx NW.

NWs further demonstrated another passivation effect on the radial shrinkage of a-SiOx NW. It was observed that the pristine NW shrank in radius from 9.1 to 3.8 nm at an average rate of ∼4.5 × 10−3 nm/s whereas the AuNPs-modified NW shrank in radius more slowly from 9 to 6.3 nm at an average rate of ∼2.3 × 10−3 nm/s. The radial shrinkage of the AuNPs-modified NW is almost 2 times slower than that of the pristine one. Even more intriguingly, as also demonstrated in Figure 5, the shrinkage of the pristine NW started to become faster and faster when the radius reduced down to about 7 nm after 900 s irradiation whereas the accelerated radial shrinkage tendency of the AuNPs-modified NW seemed much less pronounced. According to a new modeling on kinetic relationship between the shrinking wire radius and the beam irradiation time where a key parameter b (the definition is similar to that in the case of single-walled carbon nanotube (SWCNT) 15) has been particularly established for the nanocurvature effect of NW on the NW surface energy,19 we obtained a well-fitted value of the constant b at ∼2.4 for the AuNPs-modified NW and ∼3.3 for the pristine NW as shown in Figure 5. The fitting result that the constant b for the AuNPs-modified NW is much smaller than that for the pristine NW indicated that the nanocurvedsurface energy for the AuNPs-modified NW became much smaller than that for the pristine NW once being decorated by AuNPs on the wire surface. In this way, the AuNPs on the wire 15980

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conceive exactly how the metal-bonded Au atoms interact with the bridge-bonded Si−O−Si of the nanocurved wire sidewall. Nevertheless, it has demonstrated an extraordinary structural stability of the crystalline AuNPs and their enhancement in the stability of the highly curved a-SiOx NW. On the basis of these findings, similar to the case of SWCNT,17 we can further predict that a metal NP catalyst attached to the free end of aSiOx NW would also passivate the wire axial shrinkage in ref 18. Thus, it may be a good clue to the explanation of its passivation effect as well as for its catalyzing effect on the a-SiOx NW nucleation and growth.13 From ref 17, we can also further predict that metal NPs could be able to passivate radial shrinkage and axial elongation of single-walled or multi-walled carbon nanotube in a way similar to that in the present paper. We need a further experiment to test this predication. In this sense, the study on the passivation effect of the metal NPs on the structural changes of a-SiOx NW in this paper may also open an alternative route to study the metal NP catalyzing effect on the a-SiOx NW nucleation and growth. In addition, one may also envision that e-beam irradiation would lead to the formation of oxygen vacancies. Although there is no report found so far as to whether e-beam irradiation would lead to the formation of oxygen vacancies, it is possible that the as-produced oxygen vacancies would enhance the beam-induced atomic vibration soft mode and instability or atom plastic flow of the a-SiOx NWs in this paper. This is because in contrast to a crystalline structure such as the AuNPs the concentration of vacancies in an amorphous structure is very variable due to its nonequilibrium nature.8,9 To further reveal the passivation effect of AuNPs respectively on the atom diffusion or flow and the atom evaporation, we compared the diffused and evaporated volumes between the pristine and the AuNPs-modified a-SiOx NWs. For simplicity, we made the following two assumptions: (i) the net loss of materials is attributed to the atom evaporation; (ii) the elongation of wire is caused by the atom diffusion or flow. Accordingly, as illustrated in Figure 7a, we could obtain the evaporated volume V evap and the diffused volume V diff respectively

Figure 6. Schematic illustrations showing the passivation effect of AuNPs on the beam-induced uniform elongation and radial shrinkage of a-SiOx NW segment of any fixed length of L: (a) SiOx NW without AuNPs; (b) SiOx NW with AuNPs.

nanocurvature effect of the NW when it became thinner with a radius down to the nanoscale. Similarly, as demonstrated in Figure 2, the above uniform elongation and radial shrinkage could be also observed in the AuNPs uniformly modified a-SiOx NW. However, as further compared in Figures 4 and 5 and illustrated in Figure 6a,b, the AuNPs greatly reduced surface energy of the a-SiOx NW and thus passivated the nanocurved wire sidewall surface of the NW. That is, the AuNPs could weaken the diffusion, plastic flow and evaporation of the wire atoms. Thus, the elongation and the radial shrinkage of the aSiOx NW were retarded and a less number of atoms diffused and flowed to the two ends of the wire and evaporated from the wire surface, which would lead to slowing down of the elongation and the radial shrinkage of the a-SiOx NW. In contrast to the amorphous SiOx NW, the crystalline AuNPs were much more resistant to the irradiation with their metal crystal structure. This would be due to the fact that a crystal metal of metal bond would hardly become amorphized merely by the e-beam irradiation except by an ultrafast quenching from its melt state or an ion beam irradiation with cascade effect at low temperature.8,16 In this way, the surface structure of the crystalline AuNPs of metal bond tends to be atom-faceted; this atom-faceted surface structure probably could avoid the deformation of the electron cloud of surface atoms8,16 and thus the surface strain energy as induced by the nanocurvature on the a-SiOx NW surface. However, the sidewall surface of the a-SiOx NW of Si−O−Si bridge bond has no such a atomfaceted surface structure. Thus, the electron cloud on the sidewall surface of the a-SiOx NW should be highly deformed and thus induces surface strain energy by nanocurvature when the surface is curved to nanoscale. From the above analysis, we can conceive that the surface energy of the AuNPs should be lower than that of a-SiOx NW if they are all curved at the same nanoscale or even the AuNPs are much more curved. The above structure differences probably lead to the fact that the nanocurvature effect and the beam-induced atomic vibration soft mode and instability effect of the AuNPs are much weaker than these of a-SiOx NW. In this way, the AuNPs on the a-SiOx NW surface become the most stable and tend to keep their crystalline structures and thus their passivation effect throughout the irradiation. For the a-SiOx NW without AuNPs as shown in Figure 1, it can be regarded as an amorphous cylinder-like structure of Si− O−Si bridge bond. After being modified with AuNPs as shown in Figure 2, part of the smooth, amorphous sidewall surface of the a-SiOx NW is uniformly replaced by the atom-faceted, crystalline AuNPs surface. At this stage, it is still hard to

Vevap = Vr0 , L0 − Vr1, L1 = π (r0 2L0 − r12L1)

(1)

Vdiff = Vr1, L1− L0 = πr12(L1 − L0)

(2)

where Vr0,L0 is the volume of wire segment before irradiation with a diameter of r0 and a length of L0, Vr1,L1 is the volume of wire segment after irradiation with a diameter of r1 and a length of L1, and Vr1,L1 −L0 is the volume of elongated wire part with a diameter of r1 and a length of L1 − L0. Here, it should be noted that the real diffused volumes would be larger or even much larger than the as-calculated results by eq 2. This is because part of the diffused atoms which aggregate on the wire sidewall would further be evaporated and lost in the subsequent irradiation. In spite of this, it still can provide some references and suggestions which will be discussed in the following. As shown in Figure 7b, the evaporated and diffused volumes of the SiOx NWs with and without AuNPs were respectively plotted with the irradiation time for a detailed comparison. It was found that the following is true: (i) The real-time evaporated volume of the NW with AuNPs is nearly the same as that without AuNPs during the initial 900 s irradiation, whereas it becomes increasingly smaller in the subsequent irradiation. It indicates that 15981

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AuNPs-modified NW should be similar to that on the pristine NW within the initial 900 s irradiation and much smaller during the subsequent irradiation. Thus, the total diffused volume of the AuNPs-modified NW including the calculated part as shown in Figure 7b and the reevaporation part is obviously smaller than that of pristine NW during the whole irradiation duration. It thus indicates an overall passivation effect of AuNPs on the atom diffusion of a-SiOx NW during the e-beam irradiation.

4. SUMMARY In this work, the passivation effect of heterogeneous AuNPs on the structural changes of a-SiOx NW as athermally induced by uniform e-beam irradiation was investigated in an in situ transmission electron microscope. It was found that at room temperature the straight and uniform a-SiOx NW demonstrated a considerable uniform plastic elongation and an accelerated uniform radial shrinkage at the nanoscale. However, once being modified with AuNPs, the nanocurved sidewall surface of aSiOx NW became intriguingly passivated and both the elongation and the radial shrinkage were greatly retarded. The above processes involve accelerated, directional, and ultrafast atom transportations and cannot be adequately explained by the existing knock-on mechanism. But it can be well interpreted by a novel mechanism of athermal diffusion and plastic flow along with athermal evaporation of wire atoms as driven by the nanocurvature of a-SiOx NW and the e-beaminduced soft mode and instability of atom vibration. It is worth noting that the nanocurvature effect and the beam-induced atomic vibration soft mode and instability effect for the LDNs including a-SiOx NW instability would be intrinsically of nonequilibrium, disorder (amorphous-like structure), and nonlinearity nature. Such a study on these effects can help to better understanding of the fundamentals of some new phenomena and concepts in LDNs.

Figure 7. (a) Schematic illustration showing the uniform length and diameter changes of a NW segment after irradiation; (b) comparative study showing the different evaporated and diffused volumes between the SiOx NWs with and without AuNPs.

within the initial 900 s duration there is little or even no passivation effect of the AuNPs on the atom evaporation of a-SiOx NW. After this, the accelerated increasing of the evaporated volume as observed in the pristine NW is greatly retarded when being modified with AuNPs which demonstrates a pronounced passivation effect of the AuNPs on the atom evaporation. The start time for such a change in the passivation effect is about 900 s, at which the radius of the NW without AuNPs reduces down to 7 nm and an accelerated radial shrinkage at the nanoscale is demonstrated. We repeated the experiments several times and obtained the same conclusions. Thus, it can be concluded that the passivation effect of AuNPs on the atom evaporation of a-SiOx NW starts to work when the radius of a-SiOx NW reduces down to the nanoscale although the mechanism is still unknown. (ii) The diffused volume of the AuNPs-modified NW is always smaller than that of the pristine NW throughout the whole irradiation. We further note that the diffused volumes of the pristine and the AuNPs-modified NWs both exhibit two different evolution periods: first increase and then decrease with the irradiation time. Such an abnormal decrease of the diffused volume is probably due to the further evaporation of the diffused atoms during the subsequent irradiation especially when the radius reduces down to the nanoscale with a high evaporation rate (see Figure 7b). In this regard, we should consider the re-evaporation effect of the diffused atoms when we attempt to compare the real diffused volumes. As demonstrated in Figure 7b, within the initial 900 s irradiation, the real-time atom evaporation rates of the pristine and the AuNPs-modified NW keep consistently the same, while from 900 to 1170 s, the atom evaporation rate of the AuNPs-modified NW becomes much smaller than that of the pristine NW. Accordingly, the re-evaporation amount of the diffused atoms on the

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AUTHOR INFORMATION

Corresponding Author

*(X.F.Z.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC Project under Grant No. 11574255, the Science and Technology Plan (Cooperation) Key Project from Fujian Province Science and Technology Department under Grant No. 2014I0016, and the National Key Basic Science Research Program (973 Project) under Grant No. 2007CB936603.

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