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[Hiroshi Amekura](https://orcid.org/0000-0003-2148-8431), Saif Ahmad Khan, Pawan Kumar Kulriya, Debdulal Kabiraj

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[Irradiation Temperature Dependence of Shape Elongation of Metal Nanoparticles in Silica: Counterevidence to Ion Hammering Related Scenario](https://mdr.nims.go.jp/datasets/b4184326-3466-45e7-9b9e-c13bfa21f757)

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Irradiation Temperature Dependence of Shape Elongation of Metal Nanoparticles in Silica: Counterevidence to Ion Hammering Related ScenarioCitation: Amekura, H.; Khan, S.A.;Kulriya, P.K.; Kabiraj, D. IrradiationTemperature Dependence of ShapeElongation of Metal Nanoparticles inSilica: Counterevidence to IonHammering Related Scenario.Quantum Beam Sci. 2023, 7, 12.https://doi.org/10.3390/qubs7020012Academic Editors: Hiroyuki Aoki,Alessandro Genoni and Masaki OuraReceived: 21 February 2023Revised: 20 March 2023Accepted: 24 March 2023Published: 7 April 2023Copyright: © 2023 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).ArticleIrradiation Temperature Dependence of Shape Elongation ofMetal Nanoparticles in Silica: Counterevidence to IonHammering Related ScenarioHiroshi Amekura 1,* , Saif Ahmad Khan 2, Pawan Kumar Kulriya 2,3 and Debdulal Kabiraj 21 National Institute for Materials Science (NIMS), Tsukuba 305-0003, Ibaraki, Japan2 Inter-University Accelerator Centre (IUAC), Aruna Asaf Ali Marg, New Delhi 110067, India;khansaifahmad@gmail.com (S.A.K.); pkkulriya@mail.jnu.ac.in (P.K.K.); d.kabiraj@gmail.com (D.K.)3 School of Physical Sciences, Jawaharlal Nehru University, New Mehrauli Road, New Delhi 110067, India* Correspondence: amekura.hiroshi@nims.go.jp; Tel.: +81-29-863-5479Abstract: Irradiation temperature (IT) dependence of the elongation efficiency of vanadium nanopar-ticles (NPs) in SiO2 was evaluated: The samples were irradiated with 120 MeV Ag9+ ions to a fluenceof 1.0 × 1014 ions/cm2 each at ITs of 300, 433, 515, and 591 K, while the measurements were per-formed at room temperature. The vanadium was selected for the NP species because of the highestbulk m.p. of 1910 ◦C (2183 K) among all the species of the elemental metal NPs in which the shapeelongation was observed. The highest m.p. could contribute negligible size changes of NPs againstinevitable exposure to high temperatures for the IT dependence measurements. The elongation of VNPs was evaluated qualitatively by transmission electron microscopy (TEM) and quantitatively byoptical linear dichroism (OLD) spectroscopy. The electron microscopy studies showed a pronouncedelongation of NPs with ion irradiation at the elevated temperatures. The OLD signal was almostconstant, or even slightly increased with increasing the IT from 300 to 591 K. This IT dependenceprovides a striking contrast to that of the ion hammering (IH) effect, which predicts a steep decreasewith increasing IT. Combined with the other two counterevidence previously reported, the IH-relatedeffect is excluded from the origin of the shape elongation of metal NPs in SiO2.Keywords: shape elongation; metal nanoparticle; swift heavy ion; irradiation temperaturedependence; ion shaping; optical linear dichroism spectroscopy; silica glass1. Introduction1.1. Shape Elongation of Metal Nanoparticles and the Ion Hammering MechanismIn 2003, D’Orleans et al. discovered the shape transformation of metal nanoparticles(NPs) embedded in silica glass (SiO2), from nearly spherical shapes to prolate spheroids ornano-rods, which was induced under swift heavy ion (SHI) irradiation [1]. The researchersprepared nearly spherical Co NPs of a mean diameter of ~10 nm by implanting 160 keV Coions into SiO2 layers on Si at an elevated temperature of 873 K [1]. Then they irradiatedthe spherical Co NPs embedded in SiO2 with swift heavy ions of 200 MeV iodine ions.By increasing the fluence to 1013 ions/cm2, some spherical NPs transformed into lemonshapes. Further increasing to 1014 ions/cm2, they transformed into nano-rods [1]; i.e., NPsexpand parallel to the ion beam and shrink perpendicular to it. An important property ofthis phenomenon was that the major axes of the elongated NPs were parallel with eachother, which were also parallel to the SHI beam.One year later, i.e., in 2004, a similar phenomenon was observed in a different configu-ration: The elongation was induced in the Au-cores of free-standing Au-core/silica-shellcolloidal NPs under SHI irradiation [2], which were synthesized by a wet-chemical method.The spherical gold cores of 14 nm in diameter elongated along the SHI beam direction andtransformed to rods of 6 nm in diameter and 54 nm in length under 30 MeV selenium (Se)Quantum Beam Sci. 2023, 7, 12. https://doi.org/10.3390/qubs7020012 https://www.mdpi.com/journal/qubshttps://doi.org/10.3390/qubs7020012https://doi.org/10.3390/qubs7020012https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/qubshttps://www.mdpi.comhttps://orcid.org/0000-0003-2148-8431https://orcid.org/0000-0001-5563-7584https://doi.org/10.3390/qubs7020012https://www.mdpi.com/journal/qubshttps://www.mdpi.com/article/10.3390/qubs7020012?type=check_update&version=1Quantum Beam Sci. 2023, 7, 12 2 of 11ion irradiation to a fluence of 2 × 1014 ions/cm2. At the same time, the silica shells exhibiteda completely different shape-transformation to oblate spheroids, i.e., shrinkage parallelto and expansion perpendicular to the SHI beam [2]. Since pure silica NPs were knownto exhibit the same shape transformation to the oblate spheroids by the ion hammering(IH) effect [3], the shape transformation of the silica shells to the oblate spheroids wasalso ascribed to the IH effect [2]. To clarify the relationship between silica shells and theelongation of the Au-cores, core/shell NPs with different shell thicknesses ranging from15 to 72 nm, but fixing the core diameter to 14 nm were synthesized and irradiated with30 MeV Se ions. The elongation in the Au cores was observed with the particles havingsilica shells thicker than 26 nm only. The Au cores with thinner silica shells did not show anelongation. From these observations, it was considered that the silica shells were essentialto induce the elongation of Au cores. The elongation of the Au-cores could be induced bythe deformation of the silica shells, which could be induced by the IH effect [2]. The samemechanism was presumed for the elongation of the metal NPs embedded in films/bulkof silica.However, it was pointed out that the IH effect was able to build up stress to the orderof 100 MPa only, which is too low to induce the clearly observed deformation of solid AuNPs. Therefore, any radiation-induced softening of Au NPs was assumed. However, theobserved large NP elongation cannot be ascribed to any known mechanisms of radiation-induced softening [4]. Then the synergy effect [5] was proposed between the in-plane stressaccumulation by the IH effect and the transient melting, i.e., a kind of softening, of NPs bythe inelastic thermal spike (i-TS) effect [6].In this model, an isolated impact of SHI on the sample cannot induce the elongation ofNP. Multiple impacts are necessary to induce the elongation: The first impact, and possiblysome successive impacts, generate the in-plane stress in a certain region of the samplevia the IH effect. When an ion finally hits an NP located within the previously generatedin-plane stress, the NP becomes a molten phase by the i-TS effect and deforms following thein-plane stress. Because the elongation requires the multiple impacts of SHIs, the fluencedependence of the elongation of NPs should be non-linear with the fluence and could havea threshold along the fluence.1.2. Counterevidence to the Ion Hammering MechanismContrarily, Leino et al. have developed the two-temperature molecular dynamics (TT-MD) model, which is a combination of the classical molecular dynamics and the inelasticthermal spike model, and have reproduced the shape elongation of Au NP in SiO2 [7].The simulations show that an NP is elongated by only one impact of an SHI. The authorsascribed the elongation mechanism to “thermal pressure and flow”. The elongation of NPsis induced by the mass transfer of molten metal through the low-density cores in silicasimultaneously generated by the i-TS effect [7].To judge which mechanism is more probable, we have measured the fluence depen-dence of the NP elongation down to low fluences of ~1 × 1011 ions/cm2, where trackoverlaps are negligible. Optical linear dichroism (OLD) spectroscopy [8] was applied sincethis is a sensitive method to detect the elongation of metal NPs even at low fluences. In thismethod, the optical absorption is detected under the illumination of linearly polarized lighteither parallel or perpendicular to the major axes of the NPs. Then the difference betweenthe two polarizations (0 and 90◦) is evaluated. The difference should be null for sphericalNPs because there is no special direction in a sphere. However, once the elongation isinduced, the difference signal is detected. In this method, a region of a few mm in diameterwas illuminated. Since the signal is averaged over a macroscopic number of NPs includedin the millimeter dimension, the sensitivity is quite high.In fact, we have observed an elongation signal from irradiated Zn NPs in silica down to1 × 1011 ions/cm2, where the track overlaps are negligible [8]. This observation contradictsthe synergy effect of the IH and the NP melting and rather supports that the elongation isinduced by a single impact of SHI. Furthermore, silica shows radiation-induced compactionQuantum Beam Sci. 2023, 7, 12 3 of 11(densification) at very low fluences where overlaps of tracks are negligible. When tracks areoverlapped, the compaction transforms into the IH effect [9]. Consequently, the observedlinear fluence dependence of the NP elongation without the threshold is totally inconsistentwith the IH effect [10].Another counterevidence to the IH effect is the relationship between the ion incidentangles and the elongation angles of NPs [11]. Since the angular dependence of the defor-mation tensor A of the IH effect has off-diagonal components, the ion incident angle is notalways expected to be the same as the elongation angle of the NPs due to the IH effect.While the IH effect suggested a complicated relationship between the ion incident angleand the elongation angle of NPs, the same angles were observed within experimental errorsbetween the ion incident angles and the elongation angles of NPs [12].While the above two observations have been proposed as counterevidence to theIH effect involved in the mechanism of the elongation of NPs, we propose in this paperthe third counterevidence to the IH effect, i.e., the different irradiation temperature (IT)dependences between the elongation of NPs and the IH effect. While the deformationefficiency of the IH effect monotonically decreases with the IT from 300 to 650 K [9], theelongation efficiency of NPs was almost constant or rather slightly increases between 300 Kand 591 K as described below.2. Materials and MethodsTo study the IT dependence of the elongation efficiency of metal NPs, the NPs ofvanadium were selected, because of its highest bulk melting point (m.p.) of 1910 ◦C(2183 K) [13] among all the species of elemental metal NPs which have shown shapeelongation. It is known that some changes induced in metals by increasing temperatureT can be normalized by a ratio of T/Tm between different metals, where Tm denotes themelting temperature. Therefore, the highest melting temperature of V is advantageous tominimize the thermally induced size changes in NPs. In fact, we have compared the thermalstability of elongated Zn and V NPs in SiO2 under isochronal annealing, both of whichwere irradiated with 200 MeV Xe ions to the same fluence [14] prior to the annealing. Theelongated shapes of Zn NPs were maintained up to 400 ◦C (673 K) but were recovered tothe spherical shapes exceeding 400 ◦C (673 K). Contrarily, V NPs maintained the elongatedshapes up to a much higher temperature of 800 ◦C (1073 K) [14], which indicates muchhigher thermal stability of V NPs over Zn NPs.Vanadium NPs in nearly spherical shapes were fabricated in silica glass of KU-1type (OH concentration of ~820 ppm) by implanting 60 keV V+ ions to a fluence of1.0 × 1017 ions/cm2. V NPs were formed without post-implantation annealing. Accordingto our past literature [15], the NPs with a mean diameter of 9.0 nm and a standard deviationof 2.9 nm were formed within the surface layer of ~70 nm in thickness.The V NPs embedded in SiO2 were irradiated with 120 MeV Ag9+ ions to a fluenceof 1 × 1014 ions/cm2 at constant temperatures of 300, 433, 515, and 591 K using a high-temperature irradiation chamber [16] in the Inter-University Accelerator Centre (IUAC),New Delhi, India. The samples were irradiated at an incident angle of 45◦ from the surfacenormal to evaluate the degree of the elongation by the optical linear dichroism (OLD)spectroscopy [8,14], in addition to transmission electron microscopy (TEM).Since the high fluence of 1 × 1014 ions/cm2 required a relatively long irradiationtime of ~5 h, the NPs were maintained at a higher temperature than room temperature(RT) in the irradiation chamber in a vacuum for ~5 h during the irradiation. These high-temperature processes might change the NP sizes. This point is important because theinitial sizes of the NPs affect the elongation efficiency [17]. Therefore, three or four setsof samples were prepared for each IT. See Table 1: One set of the samples was annealedin a vacuum at 591 K, i.e., the highest IT, for 5 h before the irradiations at each IT, tomaximize the thermally induced coarsening of NPs, if induced. The 2nd set of samples wasirradiated at each IT without pre-irradiation annealing. The 3rd set was mounted in thehigh-temperature irradiation chamber and maintained at the IT for 5 h but not irradiated.Quantum Beam Sci. 2023, 7, 12 4 of 11From the comparisons of the two groups (with and without pre-irradiation annealing), theeffect of the thermally induced size changes on NPs was evaluated and concluded as small.Table 1. Thermal history of each sample. O: applied, X: not applied.Pre-Irradiation Annealingat 591 K for 5 hIrradiationTemperature (K)Ion Irradiation at SpecifiedTemperature (120 MeV Ag9+Ions to 1.0 × 10 14 ions/cm2)X300OX XO OO XX433OX XO OX515OX XO OX591OX XO OA standard ultraviolet- and visible-range (UV-vis) dual-beam spectrophotometer wasused for the OLD spectroscopy measurements in the wavelength region of 215–800 nm witha resolution of 1 nm; a pair of optical polarizers (extinction ratio < 5 × 10−5 each) were used.All the OLD spectroscopy measurements were carried out at RT. A sample was set betweentwo polarizers, P and A, and illuminated by linearly polarized monochromatic light fromthe spectrophotometer through the first polarizer P. Light transmitted through the samplewas detected through the second polarizer A, whose polarization angle was set to the sameas the polarizer P. The second polarizer A removes the birefringence signal [18] from theOLD signal, while the former is much weaker than the latter. The detected transmittancewas shown in the form of optical density (OD = −log10T) without reflection correction,where T denotes the transmittance. To represent a value of the elongation degree from apair of polarized spectra at each IT, the elongation η was defined as,η(Tirrad) =∫ 800 nm340 nmOD(λ, 0◦, Tirrad)− OD(λ, 90◦, Tirrad)dλ. (1)The elongation η was determined with the spectra measured at the polarizationangle of 0 and 90◦. When the polarization plane includes the major axes of NPs, thepolarization angle is defined as 0◦. Since the point-defect-related absorption was detectedat the wavelength region shorter than 340 nm, the shorter bound of the integral intervalwas set to 340 nm.Some samples were thinned down to a thickness of less than 100 nm by using30 keV Ga+ focused ion beam (FIB) milling. TEM observation was conducted usingJEOL JEM-2100 transmission electron microscopes (JEOL, Tokyo, Japan) with an oper-ating voltage of 200 kV.3. Results3.1. XTEM ObservationFigure 1a exhibits cross-sectional TEM (XTEM) images of a silica sample implantedwith 60 keV V+ ions to a fluence of 1.0 × 1017 ions/cm2, i.e., as-implanted sample. Nearlyspherical NPs were observed with a certain size distribution. Since the NPs overlap witheach other in the image, the determination of the size distribution was not easy.Quantum Beam Sci. 2023, 7, 12 5 of 11Quantum Beam Sci. 2023, 7, x FOR PEER REVIEW 5 of 11   3. Results 3.1. XTEM Observation Figure 1a exhibits cross-sectional TEM (XTEM) images of a silica sample implanted with 60 keV V+ ions to a fluence of 1.0  1017 ions/cm2, i.e., as-implanted sample. Nearly spherical NPs were observed with a certain size distribution. Since the NPs overlap with each other in the image, the determination of the size distribution was not easy.  Figure 1. Cross-sectional transmission electron microscopy (XTEM) images of vanadium nanopar-ticles (NPs) embedded in SiO2: (a) as-implanted state without annealing, (b) annealed in a vacuum at 591 K for 5 h. (c,d) irradiated at 300 K with 120 MeV Ag9+ ion to a fluence of 1.0  1014 ions/cm2, (c) without and (d) with the pre-irradiation annealing in a vacuum at 591 K for 5 h. (e) irradiated at 591 K with 120 MeV Ag9+ ions without the pre-irradiation annealing. Since the IT dependence is the target of this paper, the thermally induced size change of the NPs is a critical issue, particularly when the elongation is induced at high ITs. To estimate the degree of the size changes, another set of as-implanted samples was annealed in a vacuum using the high-temperature irradiation chamber at the highest IT of 591 K for the typical irradiation duration of 5 h. The corresponding XTEM image is shown in Figure 1b. Since vanadium has high thermal stability and the annealing temperature of 591 K is Figure 1. Cross-sectional transmission electron microscopy (XTEM) images of vanadium nanoparti-cles (NPs) embedded in SiO2: (a) as-implanted state without annealing, (b) annealed in a vacuum at591 K for 5 h. (c,d) irradiated at 300 K with 120 MeV Ag9+ ion to a fluence of 1.0 × 1014 ions/cm2,(c) without and (d) with the pre-irradiation annealing in a vacuum at 591 K for 5 h. (e) irradiated at591 K with 120 MeV Ag9+ ions without the pre-irradiation annealing.Since the IT dependence is the target of this paper, the thermally induced size changeof the NPs is a critical issue, particularly when the elongation is induced at high ITs. Toestimate the degree of the size changes, another set of as-implanted samples was annealedin a vacuum using the high-temperature irradiation chamber at the highest IT of 591 Kfor the typical irradiation duration of 5 h. The corresponding XTEM image is shown inFigure 1b. Since vanadium has high thermal stability and the annealing temperature of591 K is not so high, the size distribution of the annealed sample looks like that of thesample without annealing. Therefore, the thermal-induced size changes of NPs in thepresent experiments are small.To further confirm the thermally induced effects on the elongation of V NPs, a pairof V-implanted samples with/without the annealing at 591 K for 5 h were irradiated with120 MeV Ag9+ ions at RT under the same conditions. XTEM images of the samples areQuantum Beam Sci. 2023, 7, 12 6 of 11shown in Figure 1c,d without and with the pre-annealing, respectively. Comparing theunirradiated samples (Figure 1a,b) with the irradiated samples (Figure 1c,d), the over-whelming elongation starting from the unirradiated spherical NPs is clear. As later shownin Section 3.2, the degree of the elongation determined by OLD spectroscopy was compara-ble between the samples with/without the pre-irradiation annealing. This conclusion issupported by the TEM images of Figure 1c,d.As shown in Figure 1e, the sample irradiated at 591 K exhibits nearly comparablebut slightly clearer elongation than the samples irradiated at 300 K. This observation iscounterevidence to the IH effect, since the deformation yield A of the IH effect in silica at590 K decreases to 37% of that at 300 K [9]. However, the elongation at 591 K is comparableto or slightly higher than that at 300 K as shown in Figure 1c,e. A slightly higher degree ofelongation in the sample irradiated at 591 K is confirmed by OLD spectroscopy, as shownin Section 3.2.3.2. Optical Linear Dichroism (OLD) SpectroscopyFigure 2 exhibits optical density spectra of V NPs embedded in silica, all of whichwere measured at RT, while the irradiations with 120 MeV Ag9+ ions had been carried outat a specified temperature between 300 and 591 K. A pair of samples was prepared for eachIT: Both the samples were mounted in the high-temperature irradiation chamber and weremaintained at the specified temperature. Only one of the two samples was irradiated with120 MeV Ag9+ ions to the fixed fluence of 1.0 × 1014 ions/cm2. The other sample was notirradiated but maintained at the same temperature during the ion irradiation periods of~5 h. After cooling, with the sample temperature down to nearly RT, the pair of sampleswas extracted from the chamber. Then the optical density spectra were collected later usinglinearly polarized light with the polarization angle of 0 and 90◦.The spectra collected at the polarization angle of 0 and 90◦ are indicated by solidcurves and broken curves in Figure 2, respectively. With maintaining the samples atany temperatures between 300 and 591 K without irradiations, almost no deviation wasobserved between the 0◦- and the 90◦-polarizations. While these observations are quitereasonable, they are necessary to avoid criticism.After the ion irradiations, the 0◦-spectra and 90◦-spectra increase and decrease, respec-tively, and the basements of the curves almost maintain the shapes of the curves. These aretypical changes in the OLD spectra against the elongation of V NPs [15].It should be noted that a strong peak appeared around 5 eV after 120 MeV Ag9+ ionirradiation as shown in Figure 2. However, this peak is ascribed to point defects of silica,which are called oxygen-deficient centers of type II (ODC-II) [19]. This peak is observedin SiO2 irradiated with SHIs [20] and even irradiated with low energy ions of 60 keV Bdimmer ions [21]. It is reported that ODC-II absorption steeply decreases with raisingthe temperature to 200 ◦C (473 K) [22]. According to our unpublished data of isochronalvacuum annealing of silica irradiated with 200 MeV Xe ions, the absorption intensity of theODC-II does not decrease even after annealing at 373 K for 10 min. However, the isochronalannealing for 10 min each at 473, 573, and 673 K, the absorption intensity decreased to 60,25, and less than 5% of the value without annealing, respectively. Since the ODC-II defectsmay be formed during the irradiation period, a direct comparison is difficult between theisochronal annealing and the irradiation at high temperature. A decrease in the ODC-IIabsorption confirms that the IT certainly increases.To express the degree of the shape elongation of NPs by a single numerical value,the quantity “elongation η” defined by Equation (1) was plotted in Figure 3 for each IT.As described in Section 2, two sets of samples were prepared: While one set of sampleswas irradiated with the SHIs at specified Its after pre-irradiation annealing at 591 K for5 h, the other set of samples was irradiated with the same conditions without the pre-irradiation annealing.Quantum Beam Sci. 2023, 7, 12 7 of 11Quantum Beam Sci. 2023, 7, x FOR PEER REVIEW 7 of 11    Figure 2. Optical density spectra of V NPs in silica, unirradiated and irradiated with 120 MeV Ag9+ ions to the fixed fluence of 1.0  1014 ions/cm2 at the temperatures indicated in the figure. Unirradi-ated samples were not irradiated but maintained at the indicated temperatures for a duration of 5 h. All the spectra were measured at room temperature after cooling down from the irradiated tem-peratures. The solid (broken) curves indicate the spectra measured with linearly polarized light at a polarization angle of 0° (90°). The 0° polarization plan includes the major axes of the NPs, if elon-gated, while the 90° polarization plane is perpendicular to it. The spectra with different irradiation temperatures or fluences are vertically shifted with each other for clarity. Horizontal lines indicate the offsets of each spectrum. A peak at ~5 eV is ascribed to the ODC(II) point defects of silica [19]. The spectra collected at the polarization angle of 0 and 90° are indicated by solid curves and broken curves in Figure 2, respectively. With maintaining the samples at any temperatures between 300 and 591 K without irradiations, almost no deviation was ob-served between the 0°- and the 90°-polarizations. While these observations are quite rea-sonable, they are necessary to avoid criticism. After the ion irradiations, the 0°-spectra and 90°-spectra increase and decrease, re-spectively, and the basements of the curves almost maintain the shapes of the curves. These are typical changes in the OLD spectra against the elongation of V NPs [15]. It should be noted that a strong peak appeared around 5 eV after 120 MeV Ag9+ ion irradiation as shown in Figure 2. However, this peak is ascribed to point defects of silica, Figure 2. Optical density spectra of V NPs in silica, unirradiated and irradiated with 120 MeVAg9+ ions to the fixed fluence of 1.0 × 1014 ions/cm2 at the temperatures indicated in the figure.Unirradiated samples were not irradiated but maintained at the indicated temperatures for a durationof 5 h. All the spectra were measured at room temperature after cooling down from the irradiatedtemperatures. The solid (broken) curves indicate the spectra measured with linearly polarized lightat a polarization angle of 0◦ (90◦). The 0◦ polarization plan includes the major axes of the NPs,if elongated, while the 90◦ polarization plane is perpendicular to it. The spectra with differentirradiation temperatures or fluences are vertically shifted with each other for clarity. Horizontal linesindicate the offsets of each spectrum. A peak at ~5 eV is ascribed to the ODC(II) point defects ofsilica [19].Quantum Beam Sci. 2023, 7, 12 8 of 11Quantum Beam Sci. 2023, 7, x FOR PEER REVIEW 8 of 11   which are called oxygen-deficient centers of type II (ODC-II) [19]. This peak is observed in SiO2 irradiated with SHIs [20] and even irradiated with low energy ions of 60 keV B dimmer ions [21]. It is reported that ODC-II absorption steeply decreases with raising the temperature to 200 °C (473 K) [22]. According to our unpublished data of isochronal vac-uum annealing of silica irradiated with 200 MeV Xe ions, the absorption intensity of the ODC-II does not decrease even after annealing at 373 K for 10 min. However, the isochro-nal annealing for 10 min each at 473, 573, and 673 K, the absorption intensity decreased to 60, 25, and less than 5% of the value without annealing, respectively. Since the ODC-II defects may be formed during the irradiation period, a direct comparison is difficult be-tween the isochronal annealing and the irradiation at high temperature. A decrease in the ODC-II absorption confirms that the IT certainly increases. To express the degree of the shape elongation of NPs by a single numerical value, the quantity “elongation η” defined by Equation (1) was plo�ed in Figure 3 for each IT. As described in Section 2, two sets of samples were prepared: While one set of samples was irradiated with the SHIs at specified Its after pre-irradiation annealing at 591 K for 5 h, the other set of samples was irradiated with the same conditions without the pre-irradiation annealing.  Figure 3. Irradiation temperature dependence of the degree of the shape elongation of V NPs in SiO2, defined by Equation (1), induced by irradiation with 120 MeV Ag9+ ions to a fluence of 1.0  1014 ions/cm2, without pre-annealing (open circles) and with pre-annealing of 591 K for 5 h (closed circles). The dependence of the deformation yield A of SiO2 ascribed to the ion hammering effect was collected from Ref. [9]. Solid lines and horizontal broken lines are guides for eyes. The data points of the elongation η with/without the pre-annealing were plo�ed with open and closed circles, respectively. The errors of the elongation were set as ±10% of the elongation value. The main source of the errors comes from the fluctuation of the ion flu-ence via that of the beam current since the irradiation duration is very long at nearly 5 h. The data with and without the pre-irradiation annealing at the same Its were the same within the experimental errors. The averaged values were almost constant or slightly in-creased with rising the IT from 300 to 591 K. This behavior is consistent with the results of TEM observation shown in Figure 1, as the elongation of NPs looks clearer in the sample irradiated at 591 K than those irradiated at 300 K. Figure 3. Irradiation temperature dependence of the degree of the shape elongation of V NPs in SiO2,defined by Equation (1), induced by irradiation with 120 MeV Ag9+ ions to a fluence of 1.0 × 1014ions/cm2, without pre-annealing (open circles) and with pre-annealing of 591 K for 5 h (closedcircles). The dependence of the deformation yield A of SiO2 ascribed to the ion hammering effect wascollected from Ref. [9]. Solid lines and horizontal broken lines are guides for eyes.The data points of the elongation η with/without the pre-annealing were plotted withopen and closed circles, respectively. The errors of the elongation were set as ±10% ofthe elongation value. The main source of the errors comes from the fluctuation of the ionfluence via that of the beam current since the irradiation duration is very long at nearly5 h. The data with and without the pre-irradiation annealing at the same Its were thesame within the experimental errors. The averaged values were almost constant or slightlyincreased with rising the IT from 300 to 591 K. This behavior is consistent with the resultsof TEM observation shown in Figure 1, as the elongation of NPs looks clearer in the sampleirradiated at 591 K than those irradiated at 300 K.It is known that the deformation of silica by the IH effect becomes less prominentat higher IT [9]. The IT dependence of the deformation yield A of the IH effect in silicairradiated with 340 MeV Xe ions was collected from Ref. [9] and plotted in Figure 3by triangles. The comparisons of the IT dependences of the elongation of NPs and thedeformation of silica by the IH effect indicate that the former slightly increases but thelatter steeply decreases with increasing the IT, indicating that the IH effect is not the mainmechanism of the shape elongation of NPs.4. DiscussionWe have pointed out three observations which are inconsistent with the IH effect asthe origin of the elongation of NPs: (i) and (ii) were reported in past literature [8,10,11] and(iii) is in this work.(i) It is confirmed that the shape elongation of NPs is induced even at the first collisionof the ion to each NP [8], but the single ion impact is not expected to induce NP elongationaccording to the synergy model with the IH effect and NP melting by i-TS effect. Sincethis observation required the detection of small elongation at low fluences, the detectionwas attained using OLD spectroscopy [8,23]. Furthermore, silica shows compaction withthe first ion impact and the IH effect is only induced after almost all the surface is coveredby ion impacts. Therefore, it has again been supported that the elongation induced withQuantum Beam Sci. 2023, 7, 12 9 of 11the first ion impacts cannot be explained by the IH effect [10]. Since the NPs in silica areformed by low energy (60 keV) ion implantation, we might be criticized that the IH effectcould be induced from the beginning of the SHI irradiation, because the full of compactionwould be built up by the 60 keV metal ion implantation. To answer this criticism, some ofthe samples prepared by Ion implantation were annealed at 600 ◦C (873 K) to recover themfrom the compacted state and then irradiated them with SHIs. Similar fluence dependencewas observed, indicating that NP elongation does not depend on the existence of thecompaction or the IH effect.(ii) Coincidence of the ion incident angle and the NP elongation angle. See [11].(iii) The different irradiation temperature dependencies between the IH effect and theNP elongation. It should be noted that the degree of elongation was determined by OLDspectroscopy.When we reported observation (i), one of the criticisms was whether the OLD signalreally came from the elongation of NPs or not. This criticism could be applied to the presentcase of IT dependence since it was detected by OLD spectroscopy. The single ion impactcould form certain kinds of anisotropic point defects in SiO2, which possess optical lineardichroism. The observed OLD signal could not be from the elongated NPs but from theanisotropic point defects. However, this possibility was easily excluded: The OLD spectrahave material-dependent spectral shapes, i.e., the OLD spectra of Zn NPs have completelydifferent shapes from those of V NPs. Furthermore, the OLD spectra are approximatelycalculated from the Rayleigh theory of optical extinction spectra of ellipsoids [24]. Thecalculated OLD spectra for elongated metal NPs matched well with the experimentallyobserved ones. These facts strongly support that the observed OLD signal is ascribed toelongated metal NPs, not from anisotropic point defects in silica. In addition, the TEMresults also show the elongation of NPs in the present cases.5. ConclusionsIrradiation temperature dependence of the shape elongation efficiency of V NPs inSiO2 was evaluated between 300 and 591 K. The V NPs were formed in SiO2 by V ionimplantation of 60 keV to the fluence of 1.0 × 1017 ions/cm2. Vanadium was selectedbecause of its highest m.p. of 1910 ◦C (2183 K) among all the metal NPs which exhibitshape elongation. Because of the high m.p., thermal changes in sizes in metal NPs arenegligible compared with other low m.p. metal NPs. The evaluation of the NP elongationwas carried out by TEM observation and OLD spectroscopy. Since the V NPs were formedby low energy ion implantation of 60 keV, the interparticle distances were so short thatthe disassembly of the overlapped images to each NP was quite difficult. However, TEMimages showed that the elongation in the sample irradiated at 591 K was slightly morepronounced than that irradiated at 300 K. This observation was qualitatively inconsistentwith the previous literature on the IH effect of silica, which reported the reduction of thedeformation yield A of ~37% with increasing the temperature from 300 to 590 K.The quantitative evaluation was carried out using OLD spectroscopy. According to theOLD signal, the elongation degree of V NPs was almost constant or slightly increases withincreasing the irradiation temperature from 300 to 591 K. The deformation yield A of theIH effect in SiO2 steeply decreases with increasing the IT. Consequently, the IT dependenceof the NP elongation and that of the IH effect are qualitatively different. Therefore, the IHeffect cannot be considered as the origin of the NP elongation.Author Contributions: Basic conceptualization, H.A.; realistic implementation plan, H.A., S.A.K.,P.K.K. and D.K.; sample preparation, H.A.; temperature calibration, S.A.K., P.K.K. and D.K.; ionirradiation, S.A.K., P.K.K. and D.K.; Characterization, H.A. All authors have read and agreed to thepublished version of the manuscript.Funding: This research was funded by JSPS-KAKENHI, Grant number 22K04990.Institutional Review Board Statement: Not applicable.Quantum Beam Sci. 2023, 7, 12 10 of 11Informed Consent Statement: Not applicable.Data Availability Statement: The datasets and materials generated during the current study areavailable from the corresponding author on reasonable request.Acknowledgments: SHI irradiations were carried out in IUAC, New Delhi via BTR No 57401 and60401. The authors are grateful to the staff of the accelerator facilities at IUAC. 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Absorption and Scattering of Light by Small Particles; John Wiley & Sons, Inc.: New York, NY, USA, 1983.Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individualauthor(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.http://doi.org/10.1063/1.5050080http://doi.org/10.1063/1.2978215http://doi.org/10.2109/jcersj.99.923http://doi.org/10.1088/0957-4484/25/43/435301http://www.ncbi.nlm.nih.gov/pubmed/25288109 Introduction  Shape Elongation of Metal Nanoparticles and the Ion Hammering Mechanism  Counterevidence to the Ion Hammering Mechanism  Materials and Methods  Results  XTEM Observation  Optical Linear Dichroism (OLD) Spectroscopy  Discussion  Conclusions  References