Flower-like Na2O nanotip synthesis via femtosecond laser ablation of glass
© Samarasekera et al.; licensee Springer. 2012
Received: 8 May 2012
Accepted: 3 July 2012
Published: 18 July 2012
The current state-of-the-art in nanotip synthesis relies on techniques that utilize elaborate precursor chemicals, catalysts, or vacuum conditions, and any combination thereof. To realize their ultimate potential, synthesized nanotips require simpler fabrication techniques that allow for control over their final nano-morphology. We present a unique, dry, catalyst-free, and ambient condition method for creating densely clustered, flower-like, sodium oxide (Na2O) nanotips with controllable tip widths. Femtosecond laser ablation of a soda-lime glass substrate at a megahertz repetition rate, with nitrogen flow, was employed to generate nanotips with base and head widths as small as 100 and 20 nm respectively, and lengths as long as 10 μm. Control of the nanotip widths was demonstrated via laser dwell time with longer dwell times producing denser clusters of thinner nanotips. Energy dispersive X-ray analysis reveals that nanotip composition is Na2O. A new formation mechanism is proposed, involving an electrostatic effect between ionized nitrogen and polar Na2O. The synthesized nanotips may potentially be used in antibacterial and hydrogen storage applications.
KeywordsFemtosecond laser ablation Nanostructure Formation mechanism Nonmetallic glasses (silicates) Na2O 81 Materials science; 81.07.-b nanoscale materials and structures: fabrication and characterization; 81.16.-c methods of micro- and nanofabrication and processing
Nanotips are a subset of nanostructured materials harboring size-dependent properties that make them uniquely suited to a wide variety of applications in biosensing [1, 2], high efficiency solar cells [3, 4], and light emission/detection [5, 6]. Similarly, soda-lime silicate glasses are among the most ubiquitous of commercial glasses found in windows, containers, bioactive implants , and as low cost substrates for thin-film photovoltaics . Previous studies have shown that increasing the Na2O content (a major component of soda-lime silicates) of glass can induce a cytotoxic response in cells , and randomly oriented nanoscale rod-like structures have demonstrated cell death through reduced cell adhesion . Thus, nanostructured soda-lime silicates may open up new avenues of relevance in antibacterial applications. Additionally, it has been shown that up to 3.0 wt.% hydrogen can be reversibly absorbed by Na2O . Improving the performance of such hydrogen storage materials may lay in the enhanced reactivity through increased surface area that the nanostructuring can provide.
Sodium (Na) is frequently used in the process of synthesizing nanostructures and usually constitutes part of a compound nanomaterial in the form of nanowires [12, 13] and nanocubes . Nanostructured Na2O can be purchased in the form of spherical nanopowders; however, there is no report on the synthesis of Na2O nanotips or nanowires. This may be indicative of a reluctance to work with sodium oxide due to its reactivity with aqueous media. However, this need not be a limiting factor. In antibacterial applications where the shape of the nanostructure is believed to cause reduced adhesion of cells, the nano-morphology must be preserved. In the case of Na2O, the preservation of its nanostructure and resistivity to aqueous media can be accomplished by depositing a thin film of material (i.e., sputter coating) over the nanostructured surface. The hydrogen storage work of Xu et. al. required no interaction with aqueous media, and the experimenters worked around Na2O reactivity by storing samples in a dry box under argon.
Flower-like arrangements of nanoscale structures have previously been synthesized from other metals or metal oxides, such as ZnO [15–18], CuO , Cd(OH)2, Ni(OH)2, Fe3O4, TiO2[23–25], titanate , Pt [27, 28], Ni , and SiOx. The bulk of the aforementioned nanostructures were formed using wet techniques such as sol-gel [23, 24], hydrothermal [15, 16, 22, 25, 26], solvothermal , chemical bath [19, 21], galvanic displacement , electrodeposition , or liquid phase pulsed laser ablation (LP-PLA)  synthesis. Besides the high cost of raw materials, most wet techniques also require long processing times and/or the use of hazardous organic solutions, which is likely to result in chemically contaminated products. Even the relatively simple LP-PLA method requires careful choices in liquid media and surfactant. A few researchers have used dry-deposition systems such as radio-frequency plasma-assisted magnetrons , pulsed laser deposition , or chemical vapor deposition (CVD) . These techniques operate under vacuum conditions that can cause added complications in production scale up, and CVD requires the use of catalysts.
In this article we report the synthesis of Na2O nanotip clusters using femtosecond laser irradiation of soda-lime glass. Laser irradiation focused inside soda-lime silicates has been suggested to cause the precipitation of Na nanoparticles . However, to the best of our knowledge, this work is the first instance describing the synthesis of flower-like Na2O nanotip clusters. The ultrafast laser technique used in this work results in immediate nanotip processing and requires no surfactant, catalyst, nor pressure chamber. The definitive goal for fabricators is the ability to control growth, geometry, and size of the nanostructures. To achieve such bridling, an understanding of a nanostructure's formation mechanism is required. This paper reports on the morphologies of nanotip structures created on soda-lime glass via femtosecond laser ablation, control of nanotip thickness by changing the amount of time over which the laser delivered pulses to the sample, the importance of Na2O in nanotip synthesis, and also proposes formation mechanisms based on the observations of nanotip composition and structure using scanning/transmission electron microscopy (S/TEM) and energy dispersive X-ray spectroscopy (EDX).
Glass samples were cut from standard 75 × 25 mm Corning soda-lime glass microscope slides (Ted Pella Inc, Redding, CA) (72% SiO2, 15% Na2O, 5% CaO, 4% MgO, 2% Al2O3, 1% K2O, and 1% all other constituents). The laser processed samples were first sputter-coated with gold and then examined under SEM. The copper substrate grids were dragged across the surface of ablated samples to collect nanostructures and subsequently observed under TEM. For chemical characterization of the nanostructures, EDX was employed.
Morphology of nanotips
Composition of nanotips
The presence of several types of structures (single/clustered nanotips, microspheres, and nanoscale aggregates) would appear to indicate multiple formation mechanisms. Much is understood about the material breakdown processes during laser ablation . The optical breakdown of materials is achieved through avalanche ionization of electrons that transfer energy to the lattice of a target material. The energy from a femtosecond laser pulse is deposited into a target faster than the electron-phonon scattering time. Under laser irradiation a solid target will undergo several phenomena simultaneously: direct vaporization of the solid forming a rapidly expanding plasma , direct fragmentation of the bulk solid into nanoparticles , and material melt forming a thin (few microns thick) molten pool that will explosively boil at high laser intensity and eject microspheres . We shall consider every type of observed structure and see how their formation mechanisms complement each other and fit within the context of laser ablation. Figure 9 illustrates the proposed formation mechanisms together with the experimental evidence. All SEM images were of samples collected at 8 MHz repetition rate, 15 W average power, 10 SCFH nitrogen flow rate, and varying dwell times of 2 and 10 ms.
where d0 is the laser spot diameter at the focal plane and λ is the wavelength. At a distance of 10 μm above the focal plane, the beam intensity drops by approximately 10% resulting in a tenfold decrease in the rate of nitrogen ionization. At a distance of 130 μm above the sample surface, the beam diameter expands to 20 μm, and the beam intensity is too low to ionize nitrogen. As shall be explained later, this layer of ionized nitrogen may play a critical role in the formation of nanotips.
Proposed formation mechanism
The Si formations on nanotip fragments observed under TEM are evidently the result of aggregating nanoparticles and are usually attributed to nucleation and condensation of the vaporized material . In this process, beams of sufficiently high laser fluence heat the region of a target material and vaporize atoms and molecules creating plasma. This plasma will expand as a plume due to continued heating by the laser. The vapor plume begins to cool as it propagates outward interacting with the ambient environment and as a result, the vaporized atoms condense and particles begin to aerosolize (see Figure 9, ii). The particles may then continue to collide with each other forming larger nanoscale aggregates .
Soda-lime glass composition by weight and corresponding molecular densities at 25°C
2.2 – 2.6
Depending on their size, these spherical particles can be attributed to spallation and backcoil pressure (1 to 10 μm) [34, 35]. In the first scenario, after a melt layer is created (see Figure 9 iii), the tensile stresses induced by the laser create defects (cavities) along the solid-melt interface; these defects combine causing the ejection of droplets. Backcoil pressure is the result of a quickly expanding vapor plume, pushing liquid melt out from the irradiated spot. If the momentum of the melt is higher than the surface tension, droplets will be ejected around a formed rim as shown in Figure 9, ix.
Sodium oxide is the least dense compound in soda-lime glass. Therefore, regardless of the melt ejection process, we can expect sodium oxide to be at the melt surface (see Figure 9, iv). Furthermore, Na2O is a polar molecule. The previously described layer of ionized nitrogen may cause an electrostatic effect that attracts the polar liquid phase Na2O (see Figure 9, v). As this liquid is drawn out towards the nitrogen ions, it cools and solidifies into nanotips. The drawing process is evidenced by the tapered structure of the nanotips (see Figure 9, vi). In this experiment the nitrogen gas was provided from one nozzle and in only one direction. As laser dwell time increases, the population of nitrogen ions would grow (see Figure 9, vii). This increase in ions would offer more sources of electrostatic attraction for the polar Na2O. Thus, a greater number of tips would be drawn from a droplet of the same volume; the formed cluster would have a higher density of tips, but they would be thinner in width. However, this increased ion density does not affect the length of the nanotips (see Figure 9, viii).
The formation of nanotips into clusters larger than the beam diameter (see Figure 3) suggests that the ionization of nitrogen has an expanded area of effect, which can be accounted for by the constant flow of nitrogen disturbing the population of ions and moving them away from the ablation zone. The fewer observed flower-like nanotips at 12 MHz repetition rates are congruent with a lower quantity of ionized nitrogen due to the lower laser pulse intensity. The observed scarcity of nanotips at a 25 MHz repetition rate can be explained from little to no nitrogen ionizing due to the laser pulse intensity dropping below the ionization threshold of nitrogen.
We have previously shown  that without nitrogen flow, similar ablation conditions will not result in nanotip but rather nanofiber formation. Given that the threshold intensity for ionization of air is lower than that of nitrogen [36, 39], this may seem surprising. However, the high pressure plume formed during vaporization creates a shockwave that pushes the ambient gas away, creating a vacuum [40, 41]. Thus, the ablation zone would be devoid of ionized air. The introduction of nitrogen flow during the ablation process forces a mixing of nitrogen gas into the plume. The nitrogen flow also results in rapid cooling, which would aid in halting the vaporization process and initiating melt ejection processes.
Although silicon dioxide (SiO2) is similar in density to Na2O, the SiO2 molecules are nonpolar and, therefore, unaffected by the electrostatic effect, hence the lack of silicon in the nanotip structure. However, this explanation would not hold true for highly ionic calcium oxide molecules (CaO). The higher density of CaO must impede its ability to be drawn into nanotip structures. The melt that has not solidified into nanotips is likely low in Na2O; a delayed melt expulsion process (such as backcoil pressure) would then explain the formation of spherical particles rich in SiO2 and higher density molecules (see Figure 9, ix). The crater and rim structures are typically evidence of a backcoil pressure event with microspheres and nanotip clusters around the rim (see Figure 9, x).
We demonstrate a novel method for synthesis of flower-like nanotips on soda-lime glass via femtosecond laser ablation. Nanotips with head widths as small as 20 nm were obtained. The composition of these nanotips has been investigated and shown to be primarily sodium oxide. From the morphology and composition of the nanotips and their surrounding nano and micro structures, a formation mechanism has been proposed. The process begins with a melting of the glass substrate and separation by density of molecules. This is quickly followed by melt ejection of polar Na2O particles that interact electrostatically with the ionized nitrogen, creating the characteristic nanotip shape. Further work is being conducted to determine the cytotoxic potential of these Na2O nanotip structures for applications in the controlled localized growth of cells (i.e., patterned growth) and their use as antibacterial surfaces. The determination of the hydrogen absorptivity of these Na2O nanotips is also planned for possible energy storage opportunities.
This research is funded by Natural Sciences and Engineering Research Council of Canada. CS appreciates Mugunthan Sivayoganathan at Ryerson University for his helpful discussions on particle generation mechanisms.
- Zheng XT, Li CM: Single living cell detection of telomerase over-expression for cancer detection by an optical fiber nanobiosensor. Biosens Bioelectron 2010, 25(6):1548–1552. 10.1016/j.bios.2009.11.008View ArticleGoogle Scholar
- Lo H-C, Hsiung H-I, Chattopadhyay S, Han H-C, Chen C-F, Leu JP, Chen K-H, Chen L-C: Label free sub-picomole level DNA detection with Ag nanoparticle decorated Au nanotip arrays as surface enhanced Raman spectroscopy platform. Biosens Bioelectron 2011, 26(5):2413–2418. 10.1016/j.bios.2010.10.022View ArticleGoogle Scholar
- Yang Z, Xu T, Ito Y, Welp U, Kwok WK: Enhanced electron transport in dye-sensitized solar cells using short ZnO nanotips on a rough metal anode. J Phys Chem C 2009, 113(47):20521–20526. 10.1021/jp908678xView ArticleGoogle Scholar
- Liu C-H, Chen C-H, Chen S-Y, Yen Y-T, Kuo W-C, Liao Y-K, Juang J-Y, Kuo H-C, Lai C-H, Chen L-J, Chueh Y-L: Large scale single-crystal Cu(In, Ga)Se2 nanotip arrays for high efficiency solar cell. Nano Lett 2011, 11(10):4443–4448. 10.1021/nl202673kView ArticleGoogle Scholar
- Hsieh Y-P, Chen H-Y, Lin M-Z, Shiu S-C, Hofmann M, Chern M-Y, Jia X, Yang Y-J, Chang H-J, Huang H-M, Tseng S-C, Chen L-C, Chen K-H, Lin C-F, Liang C-T, Chen Y-F: Electroluminescence from ZnO/Si-nanotips light-emitting diodes. Nano Lett 2009, 9(5):1839–1843. 10.1021/nl803804aView ArticleGoogle Scholar
- Chang SJ, Hsiao CH, Wang SB, Cheng YC, Li TC, Chang SP, Huang BR, Huang SC: A quaternary ZnCdSeTe nanotip photodetector. Nanoscale Res Lett 2009, 4(12):1540–1546. 10.1007/s11671-009-9432-5View ArticleGoogle Scholar
- Day D: Using glass in the body. Am Ceram Soc Bull 1995, 74(12):64–68.Google Scholar
- Bosio A, Menossi D, Mazzamuto S, Romeo N: Manufacturing of CdTe thin film photovoltaic modules. Thin Solid Films 2011, 519(21):7522–7525. 10.1016/j.tsf.2010.12.137View ArticleGoogle Scholar
- Wallace KE, Hill RG, Pembroke JT, Brown CJ, Hatton PV: Influence of sodium oxide content on bioactive glass properties. J Mater Sci: Mater Med 1999, 10(12):697–701. 10.1023/A:1008910718446Google Scholar
- Lee J, Chu BH, Chen K-H, Ren F, Lele TP: Randomly oriented upright SiO2 coated nanorods for reduced adhesion of mammalian cells. Biomaterials 2009, 30(27):4488–4493. 10.1016/j.biomaterials.2009.05.028View ArticleGoogle Scholar
- Xu Q, Wang R, Kiyobayashi T, Kuriyama N, Kobayashi T: Reaction of hydrogen with sodium oxide: a reversible hydrogenation/dehydrogenation system. J Power Sources 2006, 155(2):167–171. 10.1016/j.jpowsour.2005.05.019View ArticleGoogle Scholar
- Wang H, Wang W, Ren Y, Huang K, Liu S: A new cathode material Na2V6O16.xH2O nanowire for lithium ion battery. J Power Sources 2012, 199: 263–269.View ArticleGoogle Scholar
- Yu A, Qian J, Liu L, Pan H, Zhou X: Surface sprouting growth of Na2Nb2O6. H2O nanowires and fabrication of NaNbO3 nanostructures with controlled morphologies. Appl Surf Sci 2012, 258(8):3490–3496. 10.1016/j.apsusc.2011.08.133View ArticleGoogle Scholar
- Zhang C, Chen J: Facile EG/ionic liquid interfacial synthesis of uniform RE3+ doped NaYF4 nanocubes. Chem Commun 2010, 46(4):592–594. 10.1039/b919044aView ArticleGoogle Scholar
- Yang B, Kumar A, Zheng H, Feng P, Katiyar RS, Wang Z: Growth of ZnO nanostructures on metallic and semiconducting substrates by pulsed laser deposition technique. J Phys D 2009, 42(4):045415. 10.1088/0022-3727/42/4/045415View ArticleGoogle Scholar
- Ostrikov K, Kumar S, Cheng QJ, Rider AE, Yajadda MMA, Han ZJ, Seo DH, van der Laan TA, Yick S, Tam E, Levchenko I: Different nanostructures from different plasmas: nanoflowers and nanotrees on silicon. IEEE Trans Plasma Sci 2011, 39(11):2796–2797.View ArticleGoogle Scholar
- Zhao H, Su X, Xiao F, Wang J, Jian J: Synthesis and gas sensor properties of flower-like 3D ZnO microstructures. Mater Sci Eng B 2011, 176(7):611–615. 10.1016/j.mseb.2011.01.019View ArticleGoogle Scholar
- Cao Y, Hu X, Wang D, Sun Y, Sun P, Zheng J, Ma J, Lu G: Flower-like hierarchical zinc oxide architectures: synthesis and gas sensing properties. Mater Lett 2012, 69: 45–47.View ArticleGoogle Scholar
- Zaman S, Asif MH, Zainelabdin A, Amin G, Nur O, Willander M: CuO nanoflowers as an electrochemical pH sensor and the effect of pH on the growth. J Electroanal Chem 2011, 662(2):421–425. 10.1016/j.jelechem.2011.09.015View ArticleGoogle Scholar
- Singh SC, Gopal R: Nano architectural evolution from laser-produced colloidal solution: growth of various complex cadmium hydroxide architectures from simple particles. J Phys Chem C 2010, 114(20):9277–9289. 10.1021/jp1018907View ArticleGoogle Scholar
- Cheng Z, Xu J, Zhong H, Li D, Zhu P: A facile and novel synthetic route to Ni(OH)2 nanoflowers. Superlattices Microstruct 2010, 48(2):154–161. 10.1016/j.spmi.2010.05.013View ArticleGoogle Scholar
- Ramesh R, Rajalakshmi M, Muthamizhchelvan C, Ponnusamy S: Synthesis of Fe3O4 nanoflowers by one pot surfactant assisted hydrothermal method and its properties. Mater Lett 2012, 70(1):73–75.View ArticleGoogle Scholar
- Zhao B, Chen F, Huang Q, Zhang J: Brookite TiO2 nanoflowers. Chem Commun 2009, 34: 5115–5117.View ArticleGoogle Scholar
- Cong W, Xu L: Research on performance of TiO2 nano flower structure DSSC. In ICECE 2011. 2nd International Conference on Electrical and Control Engineering: 16–18 September 2011. Yichang. IEEE Xplore Digital Library, Piscataway; 2011:318–321.View ArticleGoogle Scholar
- Haouemi K, Touati F, Gharbi N: Characterization of a new TiO2 nanoflower prepared by the sol-gel process in a reverse microemulsion. J Inorg Organomet Polym Mater 2011, 21(4):929–936. 10.1007/s10904-011-9587-2View ArticleGoogle Scholar
- Huang J, Cao Y, Liu Z, Deng Z, Tang F, Wang W: Efficient removal of heavy metal ions from water system by titanate nanoflowers. Chem Eng J 2012, 180: 75–80.View ArticleGoogle Scholar
- Kawasaki H, Yao T, Suganuma T, Okumura K, Iwaki Y, Yonezawa T, Kikuchi T, Arakawa R: Platinum nanoflowers on scratched silicon by galvanic displacement for an effective SALDI substrate. Chem Eur J 2010, 16(35):10832–10843. 10.1002/chem.201001038View ArticleGoogle Scholar
- Jia W, Su L, Lei Y: Pt nanoflower/polyaniline composite nanofibers based urea biosensor. Biosens Bioelectron 2011, 30(1):158–164. 10.1016/j.bios.2011.09.006View ArticleGoogle Scholar
- Zhang G, Zhao X, Zhao L: Preparation of single-crystalline nickel nanoflowers and their potential application in sewage treatment. Mater Lett 2012, 66(1):267–269. 10.1016/j.matlet.2011.08.052View ArticleGoogle Scholar
- Zhou CW, Cai KF, Yin JL: Synthesis and characterization of silicon oxide nanoflowers. Curr Nanosci 2011, 7(4):598–602. 10.2174/157341311796196763View ArticleGoogle Scholar
- Jiang N, Su D, Qiu J, Spence JCH: On the formation of Na nanoparticles in femtosecond-laser irradiated glasses. J Appl Phys 2010, 107(6):064301. 10.1063/1.3346858View ArticleGoogle Scholar
- Gattass RR, Mazur E: Femtosecond laser micromachining in transparent materials. Nat Photonics 2008, 2: 219–225. 10.1038/nphoton.2008.47View ArticleGoogle Scholar
- Lushnikov AA: Laser induced aerosols. J Aerosol Sci 1996, 27(Suppl. 1):S377-S378.View ArticleGoogle Scholar
- Webb RL, Dickinson JT, Exarhos GJ: Characterization of particulates accompanying laser ablation of NaNO3. Appl Spectrosc 1997, 51(5):707–717. 10.1366/0003702971940855View ArticleGoogle Scholar
- Yoo JH, Jeong SH, Greif R, Russo RE: Explosive change in crater properties during high power nanosecond laser ablation of silicon. J Appl Phys 2000, 88(3):1638–1649. 10.1063/1.373865View ArticleGoogle Scholar
- Couairon A, Mysyrowicz A: Femtosecond filamentation in air. In Progress in Ultrafast Intense Laser Science Volume I. Edited by: Yamanouchi K, Chin SL, Agostini P, Ferrante G. Springer Berlin, Heidelberg; 2006:235–258.View ArticleGoogle Scholar
- Pedrotti FL: Pedrotti LS: Introduction to Optics. Prentice Hall, New Jersey; 1993.Google Scholar
- Sivakumar M, Venkatakrishnan K, Tan B: Synthesis of glass nanofibers using femtosecond laser radiation under ambient condition. Nanoscale Res Lett 2009, 4(11):1263–1266. 10.1007/s11671-009-9390-yView ArticleGoogle Scholar
- Sun J, Longtin JP: Inert gas beam delivery for ultrafast laser micromachining at ambient pressure. J Appl Phys 2001, 89(12):8219–8224. 10.1063/1.1372157View ArticleGoogle Scholar
- Harilal SS, Bindhu CV, Tillack MS, Najmabadi F, Gaeris AC: Internal structure and expansion dynamics of laser ablation plumes into ambient gases. J Appl Phys 2003, 93(5):2380–2388. 10.1063/1.1544070View ArticleGoogle Scholar
- Sasaki K, Watarai H: Reaction between laser ablation plume and ambient gas studied by laser-induced fluorescence imaging spectroscopy. J Phys: Conf Ser 2007, 59(1):60.Google Scholar
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