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Laser-induced molybdenum oxide formation by low energy (nJ)–high repetition rate (MHz) femtosecond pulses
Optical Materials 33 (2011) 1648–1653
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Laser-induced molybdenum oxide formation by low energy (nJ)–high repetition rate (MHz) femtosecond pulses M. Cano-Lara a, S. Camacho-López a,⇑, A. Esparza-García b, M.A. Camacho-López c a Departamento de Óptica, Centro de Investigación Científica y de Educación Superior de Ensenada, Carretera Ensenada-Tijuana 3918, Zona Playitas, Ensenada, Baja California 22860, México b Fotofísica y Películas Delgadas, Departamento de Tecnociencias, Centro de Ciencias Aplicadas y Desarrollo Tecnológico, UNAM, Apdo. Postal 70-186, México, DF 04510, México c Facultad de Química, Universidad Autónoma del Estado de México, Tollocan s/n, esq. Paseo Colón, Toluca, Estado de México 50110, México
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Article history: Received 30 December 2010 Received in revised form 20 April 2011 Accepted 23 April 2011 Available online 23 May 2011 Keywords: Femtosecond laser-induced processing Thin films Molybdenum Metal oxides Micro-Raman
a b s t r a c t Experimental results on femtosecond (fs) laser-induced oxidation of molybdenum (Mo) thin films are presented. The Mo thin films were deposited on fused silica substrates by the magnetron DC-sputtering technique. The as-deposited thin films were characterized by X-ray diffraction, which indicates that bbcmolybdenum was grown. The films were irradiated in ambient air, using a femtosecond Ti:Sapphire laser (800 nm, 60 fs pulse duration, 70 MHz and 6.5 nJ per pulse). The molybdenum thin films were laser scanned in the form of several millimeters long straight line traces, by using a per pulse laser fluence well below the (previously reported) ablation threshold. Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) were used to study the laser-induced optical and morphology changes on the exposed zone. Energy Dispersive Spectrometry (EDS) and Micro-Raman Spectroscopy (MRS) were used to determine the degree of oxidation and the phase change across the laser irradiated paths on the Mo thin film. Under the above described experimental conditions our results show that it is possible to laser-induce a specific oxide phase from the molybdenum starting material. Our micro-Raman results clearly demonstrate that the fs-laser irradiation induces the m-MoO2 and o-Mo4O11 crystalline phases at the directly laser irradiated trace and its close proximity. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction There are conventional techniques reported in the literature to transform metals into their metal oxides. For example, to obtain TiO2 thin films a thermal treatment in air or oxygen atmosphere can be applied to titanium films [1,2]. An alternative way to obtain metal oxides from metals is cw-laser or pulsed-laser oxidation. Wautelet [3] published a comparison between isothermal and cw-laser assisted oxidation of metals, where he discusses experimental studies and theoretical models. Pérez del Pino et al. [4,5] have studied the Nd:YAG cw-laser (k = 1064 nm) induced oxidation in titanium plates. The authors report a coloration of the irradiated region that depends on scan velocity of the samples. They have found that a mixture of polycrystalline oxide phases are formed in the laser irradiated region. Dong et al. [6] and Lian et al. [7] have studied the surface oxidation of chromium films. They used Nd:YAG microsecond and millisecond laser pulses at the fundamental wavelength (1064 nm) in air, to obtain Cr2O3 films composed of 30–200 nm grains, whose size depends on laser parameters. Nd:YAG (k = 532 nm) nanosecond pulse laser-induced ⇑ Corresponding author. Tel.: +52 646 1750500; fax: +52 646 1750553. E-mail address: [email protected] (S. Camacho-López). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.04.029
oxidation and microstructuring has been studied in titanium thin films by Camacho-Lopez et al. [8], where the pump light polarization plays an important role in the way the oxide and the microstructure grow; the rutile–TiO2 phase was obtained in the laser irradiated region. Evans et al. [9] showed laser-induced effects on polycrystalline tungsten films, using Nd:YAG (k = 532 nm) nanosecond laser pulses. The authors demonstrated the transformation from the W–W3O crystalline tungsten phase into an amorphous– crystalline form of the WO3; they also report a morphological transformation from an initially homogeneous film into a nanostructured (100–300 nm pore size) one, which results from the laser exposure. A Ti:Sapphire (k = 800 nm) femtosecond laser, delivering amplified pulses, of 120 fs and energy over 1 lJ, at a repetition rate of 250 kHz was used by Yang et al. [10] to laser-induced phase transformation from rutile to anatase, of a titanium oxide single crystal. The authors carried out a Raman spectroscopy study of the laser-induced transformation in the TiO2, where it is shown how the intensity of the Raman active modes changes as a function of the laser exposure time and the incident average power. Tsukamoto et al. [11] used amplified Ti:Sapphire (k = 800 nm) 100 fs laser pulses at a repetition rate of 1 kHz to laser-induce changes in the electrical resistance of TiO2 films. The authors adjusted the delivered fluence to darken the film surface
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and they found that at that stage the electrical resistance of the film is altered without compromising the surface morphology. Femtosecond laser-induced metal surface modifications, beyond laser-induced oxidation, have recently been addressed by several authors. Kuznetsov et al. [12] have studied microstructuring of thin gold films by amplified 30 fs, 0.9 mJ and 1 kHz repetition rate laser pulses (k = 800 nm). Likewise Vorobyev and Guo [13] have reported on various nanostructures produced through direct surface modification on bulk copper, gold and platinum using amplified 65 fs laser pulses (k = 800 nm) of 1 mJ and 1 kHz repetition rate. Wang et al. [14] reported thresholds for surface nano/micro morphology modifications in titanium and copper with amplified 100 fs laser pulses (k = 800 nm) of 1 mJ and 1 kHz repetition rate. The authors carried out a study of how the threshold for surface modification is affected if the experiments are performed in air or water. A large variety of molybdenum oxides, MoOx, have been reported in literature [15]. For x = 2, the monoclinic phase, mMoO2, is known. m-MoO2 has been synthesized as single crystal [16], powder [17] and in thin film form [18]. For x = 3 three polymorphs have been reported: the thermodynamically stable phase a-MoO3 and the unstable phases b-MoO3 and MoO30 [19]. For x between 2.75 and 3, a set of oxides: Mo4O11 (monoclinic), Mo4O11 (orthorhombic), Mo17O47, Mo5O14, Mo8O23, and Mo9O26 have been reported [20,21]. Due to their optical and electrical properties, molybdenum oxides are attractive in several technological applications [22]. MoO3 in thin film form have been tested as gas sensor material [23–25]. Recently, Aoki et al. have studied optical recording media characteristics of MoO3 films prepared by the Pulsed Laser Deposition (PLD) technique using an ArF excimer laser [26]. mMoO2 and a-MoO3 have been studied as cathode material in lithium micro-batteries [27,28]. In this paper, we present the effect of the exposure of molybdenum thin films, in ambient air, to near infrared (NIR), low energy (nJ), high repetition rate (MHz) femtosecond pulses from a Ti:Sapphire laser. We achieved laser-induced oxidation of the as-deposited molybdenum thin films, using a per pulse fluence of 0.03 J/ cm2. Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) allowed us to study color and morphology changes in the exposed areas; Energy Dispersive Spectrometry (EDS) and Micro-Raman Spectroscopy (MRS) were used to study the stoichiometry and phase transition obtained after laser exposure. Our results incidentally show that the laser ablation threshold of the molybdenum thin films occurs at a lower fluence than previously reported in the literature [29]. We show that it is fairly easy to grow mMoO2 and o-Mo4O11 by exposing molybdenum thin films to the femtosecond laser pulsed irradiation in ambient air as described above. EDS and micro-Raman spectroscopy showed that the femtosecond laser-induced material transformation follows a spatial resolved profile perpendicular to the laser scan direction. 2. Experimental setup 2.1. Deposition of Mo thin film Molybdenum thin films were deposited by using the magnetron DC-sputtering technique. A disk of molybdenum (99.9%, Lesker) was used as target and argon gas to sputter it. Molybdenum was deposited on fused silica substrates at room temperature. The deposition parameters were: Power 150 W, Argon gas: 18 sccm, Pressure 0.48 103 m Bar, and a deposition time of 6 min. The as-deposited molybdenum thin films, were characterized by XRD (Siemens D-5000 diffractometer with the Cu Ka radiation source k = 1.5406 Å) and SEM. The thickness of the films (500 nm) was measured by profilometry and confirmed by Scanning Electron Microscopy.
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2.2. Pulsed laser irradiation of the Mo thin films In our experiments we used a Ti:Sapphire laser oscillator, which output pulses of 60 fs time duration, an energy per pulse of 6.5 nJ and its wavelength centered at 800 nm, to irradiate the Mo thin films at a repetition rate of 70 MHz. We carried out the laser irradiation of the films, in ambient air, at normal incidence and focusing down the slightly elliptical incident laser beam with an aspheric lens (NA = 0.5) of 6 mm focal length; which gave us an elliptically shaped beam waist with FWHM minor and major axes of 3 and 5 lm, respectively. The films were conveniently mounted on a computer controlled XYZ linear stage. Fig. 1 shows a schematic diagram of the experimental set up. We laser exposed the films in the form of a series of straight line traces a few millimeter long, the scan speed was kept fixed at 530 lm/s during exposure. We used an on target (delivered) per pulse energy of 2.4 nJ and therefore per pulse delivered fluence of 0.03 J/cm2. The on target integrated fluence was determined and controlled by the scan speed and the number of scans performed along the same path. We must point out here that we chose the above on target per pulse fluence, since we meant to avoid laser ablation during the exposures, and since we knew from what it was reported by Hermann et al. [29] that the ablation threshold fluence for a 500 nm thick Mo film, deposited on glass, was of 0.11 J/cm2 under 800 nm, 100 fs laser pulse irradiation. However, we obtained from our experimental results (see SEM micrographs below, Fig. 5) that at our chosen on target fluence, which is 1/4 from Hermann’s reported ablation threshold fluence, we still get laser ablation off the Mo film. 2.3. Characterization of the irradiated regions in the Mo thin films The laser irradiated paths on the molybdenum films were analyzed by Optical Microscopy (Olympus BX-41 microscope) to identify texture and color changes; SEM and EDS (Philips XL-30 microscope) were used to study the surface for morphology and stoichiometric changes, respectively; micro-Raman spectroscopy (HR-800-LabRam) was used to identify the laser-induced molybdenum oxide type and its crystalline phase. On the micro-Raman technique the backscattering configuration was used to analyze the laser exposed areas on the Mo films. A linearly polarized mW He–Ne laser (632.8 nm) was used as the excitation source. The He–Ne beam was focused down to a 2 lm diameter spot by using a 100X microscope objective mounted on an Olympus BX-41 microscope. 3. Results and discussion Fig. 2 shows the X-ray diffractogram (a) and a SEM micrograph (b) of an as-deposited molybdenum thin film. A single peak centered at 2h = 40.6° is present in the diffractogram which corresponds to the reflection of the plane (1 1 0). This indicates that the molybdenum grew preferentially in the direction of such a (1 1 0) plane. As one can observe in the SEM micrograph 2b the thin film surface is very smooth and homogeneous. Fig. 3 shows an image acquired with the optical microscope coupled to the micro-Raman system. The optical image corresponds to a fs laser irradiated trace with eight scans along the same trace. There is a clear laser-induced texture and a complex coloration (from center of the trace outwards: light-gray, light-green, dark-green, green, blue, dark-brown, and light-brown) in the vicinity of the directly exposed path (3 lm wide), which actually corresponds to the zone that contains the sharply ablated elliptical spots along the center. The texture and coloration is related to the degree of oxidation and phase of the molybdenum given by
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Fig. 1. Experimental setup.
the cumulative temperature gradient established by the laser heat deposition and its diffusion process during the exposure. We must point out here that the thin film was ablated away preserving the exact footprint (shape and size) of the laser beam waist, which is characteristic for ablation with fs laser pulses. We have here a clear evidence of two distinct physical mechanisms of light–matter interaction; on the one hand the ultrashort pulse laser ablation nature, and on the other hand the thermal component provided by the long envelope of the MHz pulse train used in the exposure. Fig. 4 shows the Raman spectra that correspond to the asdeposited molybdenum thin film and the fs laser irradiated traces. In order to obtain the structure of the material in each region with different color, the spectra were obtained by using a low power He–Ne laser (1.2 mW, 10 kW/cm2), this is so for serving the purpose to avoid inducing any additional structural changes in the probed material while running the Raman characterization. It is worth noting that no additional transformation was observed, in the fs-laser-irradiated material, during Raman characterization runs even for higher probing He–Ne laser power. The spectrum 4a corresponds to the as-deposited molybdenum thin film (zone III, Fig. 3); as it is expected for all metals no Raman peaks are present. Fig. 4b shows a representative Raman spectrum taken at the center of a fs-laser irradiated trace (zone I, Fig. 3). One can observe that the spectrum 4b is constituted by several peaks located at 204, 209, 231, 347, 351, 366, 425, 461, 471, 498, 571, 588 and 744 cm1, in good agreement with the Raman spectrum of the m-MoO2 phase reported for single crystal by Srivastava and Chase [16], powder by Camacho-López et al. [30], and thin films by Spevack and McIntyre [18]. This result indicates that the Molybdenum thin film transforms into m-MoO2 after low energy–high repetition rate fs-laser irradiation. In this manner, the irradiated molybdenum suffers an oxidation process acquiring the monoclinic structure. EDS measurements (at that same zone I) confirmed the stoichiometric relation MoO2. Raman spectrum 4c corresponds to the dark-green region (zone II, Fig. 3). Raman bands are located at 211, 275, 310, 339, 382, 416, 431, 455, 500, 745, 795, 836, 849, 862, 909, 941,
986 cm1. According to the work reported by Dieterle et al. [31] and Dieterle and Mestl [32] and Blume [21], these Raman bands indicate that the material in the dark-green (zone II, Fig. 3) is constituted by the orthorhombic (o-Mo4O11) crystalline phase. Additional peaks marked with ⁄, located at 1006 and 1014 cm1 are present in the spectrum 4c. These two peaks (at 1003 and 1012 cm1) have been previously reported by Mestl et al. for MoO3 [33]. They attribute those peaks to Mo@O stretching vibrations. Fig. 5 shows SEM micrographs of laser irradiated traces on the molybdenum thin films. As we addressed above, although we used an on target (per pulse) delivered fluence of 1/4 the previously reported ablation threshold fluence, for Mo thin films, we still get well defined ablation at this significant lower fluence (see Fig. 5a). Therefore, we can conclusively state that the ablation threshold fluence for molybdenum thin films (deposited on fused silica substrates) under 800 nm, 60 fs laser pulses, is 0.03 J/cm2. The most likely cause of the lower ablation threshold we observe here is the fact that, since we are using a multiple pulse exposure at a very high repetition rate, there are incubation effects involved; it is well known for different materials, including metals, that this effect will lower down the ablation threshold [34,35]. In our case, the incubation effects are of thermal nature provided by the pulse train envelope used during the irradiation. A dominant effect in the interaction studied in this paper is laser heating given by the high repetition rate (MHz) delivery of pulses, which will produce heat accumulation and therefore temperature rise. Nonetheless, given the ultrashort nature of every single pulse within the pulse train; it is worth pointing out the reported work on lower ablation thresholds for metals which results of multi-photon absorption processes [36,37]. This scenario is possible to occur at the high peak laser irradiances involved during the Mo thin film irradiation discussed in this paper. The effect of performing an increasing number of scans along the same path, is the growth of a couple of sideways tracks (see Fig. 5b and c) composed of scatter grains of 1 lm and smaller
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Fig. 4. Micro-Raman spectra corresponding to the colored regions showed in Fig. 3. (a) As-deposited Mo thin film; (b) light-gray region at the center of the trace (zone I, see Fig. 3); (c) dark-green stripe (zone II, see Fig. 3) besides the light-gray region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) X-ray diffractogram and (b) SEM micrograph corresponding to the asdeposited on fused silica molybdenum thin films.
Fig. 3. Optical image of a femtosecond laser irradiated (eight scan) trace on a molybdenum thin film. The dashed circles show the regions probed by the laser in the micro-Raman experiment.
sizes. The (a–c) SEM micrographs show laser exposed paths to a single scan (a) and to five scans (b–c); we must note that Fig. 5c shows one end of the trace exposed to five scans, and there is
not ablation signs. The reason why laser ablation did not occur at one end of the path is that, for a millimeter long laser scan, and a very small Rayleigh range, the laser beam-waist eventually takes off or dives in the film surface. The above scenario makes the on target delivered fluence to fall (eventually below the ablation threshold), since the laser beam cross section intersecting the film surface is larger than the cross section of the beam-waist in either case. The laser beam-waist takes off or dives in the sample over a long scan because the sample does not run perfectly perpendicular to the laser incidence. SEM of the laser exposed traces reveals well defined ablation spots which are an exact footprint of the elliptically shaped laser beam waist (of 3 lm and 5 lm short and long axes) incident on the film. Notice that the elliptically shaped ablation spots (Figs. 3 and 5a) are periodically distributed, this obeys to an unexpected software-electronic failure in our translation stage system that occurred during the laser irradiation, which caused the scan to pause periodically. So, the ablated spots correspond to the positions where the scan paused and therefore those spots on the thin film were exposed to a larger number of pulses than everywhere else along the scan. This actually reinforces the explanation of the lower ablation threshold as a result of incubation effects, since the effect depends on the number of pulses, i.e. the exposure time. We can also see how the effect of the laser irradiation extends as far as 25 lm sideways; these irradiated traces show grain regions, which correspond in the optical images to pattern colored fringes of different widths between 2 and 10 lm wide, caused by the temperature gradient established by the laser heat deposition and the heat diffusion perpendicular to the laser scan direction. It is important to point out the advantages of the ultrafast (ultrashort pulse) laser-processing technique over the conventional thermal treatment for obtaining metallic thin film oxides. In general, metal oxides can be produced by using conventional furnaces and ambient air, where usually several hours of thermal heating at different temperatures are necessary to achieve a given type of oxide and crystalline structure [1,2,38,39]. When using this thermal heating technique, for example with metallic thin films, the whole sample is homogeneously oxidized. Lasers on the contrary allow rapid and very well spatially confined rising of temperature; if using pulsed lasers the heat deposition and consequently the temperature rise occurs within the pulse duration, making it possible to rapidly oxidize a metal when exposing it to a long enough single pulse or to a series of short laser pulses [40,41].
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structure of distinct stoichiometry and crystalline phases across the laser affected zone and beyond (see Fig. 3). At present, we do not have a direct experimental method for estimating the temperatures achieved in the Mo thin film at different laser fluences, however, it is well know from the literature the range of temperatures at which the different Mo oxides reported in this paper synthesize. MoO2, an intermediate oxide, and MoO3 synthesize within the temperature range 800–1200 K [43,44]; therefore we can estimate that in our case the laser-induced heating profile (transversal to the laser scanning direction), achieves the above average temperatures across the regions I and II (shown in Fig. 3) where we observe the formation of m-MoO2 and oMo4O11. According to Floquet et al. [43] the MoO2 forms at the highest temperature, therefore our results are consistent to such fact since the highest laser-induced temperature must be achieved at the center of the laser exposed region, i.e. along region I in Fig. 3; while a lower temperature should be achieved (by heat diffusion) in the close proximity of the directly laser-irradiated region, which gives place to the formation of the intermediate oxide o-Mo4O11. If the laser beam is tightly focused on the sample very fine micro or even nano patterning is possible. For the particular case of ultrashort pulses (femtosecond) and very high repetition rates (MHz), quite low energies and very tight focusing allow both high enough fluences and heat cumulative effects as to reach the needed temperatures, over a few seconds exposure, to obtain very fine patterning of the m-MoO2, the intermediate o-Mo4O11 and even the aMoO3 crystalline phases. All the above demonstrate dramatic advantages of the ultrashort pulse and high repetition rate laserinduced oxidation of metallic thin films as compared to the conventional thermal oxidation technique. Thermally obtained metallic oxides such as TiOx, ZnOx, BiOx, WOx, and MoOx have been studied extensively showing that those kind of oxides are usually photochromic [45], and/or electrochromic [46,47] and/or gasochromic [48]. Based on some of these properties of metallic oxides a variety of technological applications have been either suggested or demonstrated, such as optically based gas sensors, transducer based gas sensors, and optically recording devices for storage [49,50,26]. It also has been demonstrated that the electrical features, say the resistivity, of a metallic oxide can be modified by exposing it to femtosecond laser pulses [11]. On the applied side of the work presented here, we must mention that the study and characterization of both the electrical and the optical properties of the fs laser-induced MoOx is currently underway, and will be reported on a future paper.
4. Conclusions
Fig. 5. SEM micrographs of a laser exposed molybdenum thin film. (a) Single scan laser exposure with an on target per pulse fluence of 0.03 J/cm2, ablation occurs at this fluence; (b) five scans laser exposure, notice the grain structure formed to the sides of the ablated track; (c) the same five scans laser exposure at the edge of the scanned path, notice the absence of any ablation signs and the enhanced grain structure formation.
Additionally, lasers posses a wide selection of parameters to finely tune and control a chemical reaction such as a metal oxidation. For instance, we can chose the right single pulse fluence to achieve a desired peak temperature while, on multiple pulse exposure, the pulse repetition rate drives heat accumulation effects [41,42] and therefore the average temperature which would lead to a specific stoichiometry and crystalline phase in the case of laser-induced metallic oxides. Also, due to heat diffusion effects it is possible to obtain a lateral heat distribution which gives place to a complex
We demonstrated, for the first time to the best of our knowledge, the transformation of metallic molybdenum into a complex pattern of molybdenum oxides by using very low energy femtosecond laser pulses delivered at a very high repetition rate. Both laserinduced oxidation and crystalline–crystalline phase transformation was achieved, on as-deposited (1 1 0) cubic-molybdenum thin films, by using low energy (nJ)-high repetition rate (MHz) femtosecond pulses. Our results show solid evidence of the transformation from c-Mo into m-MoO2 and o-Mo4O11. The m-MoO2 forms along the directly laser-exposed trace and its close proximity, which extends 5 lm sideways and looks light-gray under optical microscope imaging; the o-Mo4O11 forms a dark-green stripe 2 lm wide, right besides the m-MoO2 trace. There is a complex color pattern formed, as a result of the fs-laser exposure, which includes, from center of the trace outwards: light-gray, light-green, dark-green, green, blue, dark-brown, and light-brown. Neither the stoichiometry nor the phase has been identified yet for these colored regions, but those of the light-gray (m-MoO2) and
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dark-green (o-Mo4O11) regions. A detailed Raman study to identify the types of Mo oxide and stoichiometry of the remaining of the colored pattern is underway within our research group. The pulsed-laser exposure technique described throughout this work probes to be an efficient and rapid way of obtaining molybdenum oxides, in the form of microstructured patterns. We used tightly focusing of the laser beam such that it is feasible to obtain an on demand micro-pattern made of laser-induced MoOx i.e. to selectively obtain a micro-pattern made of stripes of molybdenum dioxide and intermediate oxides up to molybdenum trioxide. The results presented in this work could find applications in technology areas as gas sensing, recording and displays were metal oxides have proved themselves useful given their photochromic, gasochromic and electrochromic features. Acknowledgments The authors acknowledge partial support to this work from CONACyT Grant 57309 and AFOSR-CONACyT Grant FA9550-10-10212. M. Cano-Lara thanks CONACyT for PhD scholarship granted. We are also very grateful to Dr. Manuel Herrera Zaldivar for his support for the SEM facilities, his great scientific advice and useful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Laser-induced molybdenum oxide formation by low energy (nJ)–high repetition rate (MHz) femtosecond pulses M. Cano-Lara a, S. Camacho-López a,⇑, A. Esparza-García b, M.A. Camacho-López c a Departamento de Óptica, Centro de Investigación Científica y de Educación Superior de Ensenada, Carretera Ensenada-Tijuana 3918, Zona Playitas, Ensenada, Baja California 22860, México b Fotofísica y Películas Delgadas, Departamento de Tecnociencias, Centro de Ciencias Aplicadas y Desarrollo Tecnológico, UNAM, Apdo. Postal 70-186, México, DF 04510, México c Facultad de Química, Universidad Autónoma del Estado de México, Tollocan s/n, esq. Paseo Colón, Toluca, Estado de México 50110, México
a r t i c l e
i n f o
Article history: Received 30 December 2010 Received in revised form 20 April 2011 Accepted 23 April 2011 Available online 23 May 2011 Keywords: Femtosecond laser-induced processing Thin films Molybdenum Metal oxides Micro-Raman
a b s t r a c t Experimental results on femtosecond (fs) laser-induced oxidation of molybdenum (Mo) thin films are presented. The Mo thin films were deposited on fused silica substrates by the magnetron DC-sputtering technique. The as-deposited thin films were characterized by X-ray diffraction, which indicates that bbcmolybdenum was grown. The films were irradiated in ambient air, using a femtosecond Ti:Sapphire laser (800 nm, 60 fs pulse duration, 70 MHz and 6.5 nJ per pulse). The molybdenum thin films were laser scanned in the form of several millimeters long straight line traces, by using a per pulse laser fluence well below the (previously reported) ablation threshold. Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) were used to study the laser-induced optical and morphology changes on the exposed zone. Energy Dispersive Spectrometry (EDS) and Micro-Raman Spectroscopy (MRS) were used to determine the degree of oxidation and the phase change across the laser irradiated paths on the Mo thin film. Under the above described experimental conditions our results show that it is possible to laser-induce a specific oxide phase from the molybdenum starting material. Our micro-Raman results clearly demonstrate that the fs-laser irradiation induces the m-MoO2 and o-Mo4O11 crystalline phases at the directly laser irradiated trace and its close proximity. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction There are conventional techniques reported in the literature to transform metals into their metal oxides. For example, to obtain TiO2 thin films a thermal treatment in air or oxygen atmosphere can be applied to titanium films [1,2]. An alternative way to obtain metal oxides from metals is cw-laser or pulsed-laser oxidation. Wautelet [3] published a comparison between isothermal and cw-laser assisted oxidation of metals, where he discusses experimental studies and theoretical models. Pérez del Pino et al. [4,5] have studied the Nd:YAG cw-laser (k = 1064 nm) induced oxidation in titanium plates. The authors report a coloration of the irradiated region that depends on scan velocity of the samples. They have found that a mixture of polycrystalline oxide phases are formed in the laser irradiated region. Dong et al. [6] and Lian et al. [7] have studied the surface oxidation of chromium films. They used Nd:YAG microsecond and millisecond laser pulses at the fundamental wavelength (1064 nm) in air, to obtain Cr2O3 films composed of 30–200 nm grains, whose size depends on laser parameters. Nd:YAG (k = 532 nm) nanosecond pulse laser-induced ⇑ Corresponding author. Tel.: +52 646 1750500; fax: +52 646 1750553. E-mail address: [email protected] (S. Camacho-López). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.04.029
oxidation and microstructuring has been studied in titanium thin films by Camacho-Lopez et al. [8], where the pump light polarization plays an important role in the way the oxide and the microstructure grow; the rutile–TiO2 phase was obtained in the laser irradiated region. Evans et al. [9] showed laser-induced effects on polycrystalline tungsten films, using Nd:YAG (k = 532 nm) nanosecond laser pulses. The authors demonstrated the transformation from the W–W3O crystalline tungsten phase into an amorphous– crystalline form of the WO3; they also report a morphological transformation from an initially homogeneous film into a nanostructured (100–300 nm pore size) one, which results from the laser exposure. A Ti:Sapphire (k = 800 nm) femtosecond laser, delivering amplified pulses, of 120 fs and energy over 1 lJ, at a repetition rate of 250 kHz was used by Yang et al. [10] to laser-induced phase transformation from rutile to anatase, of a titanium oxide single crystal. The authors carried out a Raman spectroscopy study of the laser-induced transformation in the TiO2, where it is shown how the intensity of the Raman active modes changes as a function of the laser exposure time and the incident average power. Tsukamoto et al. [11] used amplified Ti:Sapphire (k = 800 nm) 100 fs laser pulses at a repetition rate of 1 kHz to laser-induce changes in the electrical resistance of TiO2 films. The authors adjusted the delivered fluence to darken the film surface
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and they found that at that stage the electrical resistance of the film is altered without compromising the surface morphology. Femtosecond laser-induced metal surface modifications, beyond laser-induced oxidation, have recently been addressed by several authors. Kuznetsov et al. [12] have studied microstructuring of thin gold films by amplified 30 fs, 0.9 mJ and 1 kHz repetition rate laser pulses (k = 800 nm). Likewise Vorobyev and Guo [13] have reported on various nanostructures produced through direct surface modification on bulk copper, gold and platinum using amplified 65 fs laser pulses (k = 800 nm) of 1 mJ and 1 kHz repetition rate. Wang et al. [14] reported thresholds for surface nano/micro morphology modifications in titanium and copper with amplified 100 fs laser pulses (k = 800 nm) of 1 mJ and 1 kHz repetition rate. The authors carried out a study of how the threshold for surface modification is affected if the experiments are performed in air or water. A large variety of molybdenum oxides, MoOx, have been reported in literature [15]. For x = 2, the monoclinic phase, mMoO2, is known. m-MoO2 has been synthesized as single crystal [16], powder [17] and in thin film form [18]. For x = 3 three polymorphs have been reported: the thermodynamically stable phase a-MoO3 and the unstable phases b-MoO3 and MoO30 [19]. For x between 2.75 and 3, a set of oxides: Mo4O11 (monoclinic), Mo4O11 (orthorhombic), Mo17O47, Mo5O14, Mo8O23, and Mo9O26 have been reported [20,21]. Due to their optical and electrical properties, molybdenum oxides are attractive in several technological applications [22]. MoO3 in thin film form have been tested as gas sensor material [23–25]. Recently, Aoki et al. have studied optical recording media characteristics of MoO3 films prepared by the Pulsed Laser Deposition (PLD) technique using an ArF excimer laser [26]. mMoO2 and a-MoO3 have been studied as cathode material in lithium micro-batteries [27,28]. In this paper, we present the effect of the exposure of molybdenum thin films, in ambient air, to near infrared (NIR), low energy (nJ), high repetition rate (MHz) femtosecond pulses from a Ti:Sapphire laser. We achieved laser-induced oxidation of the as-deposited molybdenum thin films, using a per pulse fluence of 0.03 J/ cm2. Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) allowed us to study color and morphology changes in the exposed areas; Energy Dispersive Spectrometry (EDS) and Micro-Raman Spectroscopy (MRS) were used to study the stoichiometry and phase transition obtained after laser exposure. Our results incidentally show that the laser ablation threshold of the molybdenum thin films occurs at a lower fluence than previously reported in the literature [29]. We show that it is fairly easy to grow mMoO2 and o-Mo4O11 by exposing molybdenum thin films to the femtosecond laser pulsed irradiation in ambient air as described above. EDS and micro-Raman spectroscopy showed that the femtosecond laser-induced material transformation follows a spatial resolved profile perpendicular to the laser scan direction. 2. Experimental setup 2.1. Deposition of Mo thin film Molybdenum thin films were deposited by using the magnetron DC-sputtering technique. A disk of molybdenum (99.9%, Lesker) was used as target and argon gas to sputter it. Molybdenum was deposited on fused silica substrates at room temperature. The deposition parameters were: Power 150 W, Argon gas: 18 sccm, Pressure 0.48 103 m Bar, and a deposition time of 6 min. The as-deposited molybdenum thin films, were characterized by XRD (Siemens D-5000 diffractometer with the Cu Ka radiation source k = 1.5406 Å) and SEM. The thickness of the films (500 nm) was measured by profilometry and confirmed by Scanning Electron Microscopy.
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2.2. Pulsed laser irradiation of the Mo thin films In our experiments we used a Ti:Sapphire laser oscillator, which output pulses of 60 fs time duration, an energy per pulse of 6.5 nJ and its wavelength centered at 800 nm, to irradiate the Mo thin films at a repetition rate of 70 MHz. We carried out the laser irradiation of the films, in ambient air, at normal incidence and focusing down the slightly elliptical incident laser beam with an aspheric lens (NA = 0.5) of 6 mm focal length; which gave us an elliptically shaped beam waist with FWHM minor and major axes of 3 and 5 lm, respectively. The films were conveniently mounted on a computer controlled XYZ linear stage. Fig. 1 shows a schematic diagram of the experimental set up. We laser exposed the films in the form of a series of straight line traces a few millimeter long, the scan speed was kept fixed at 530 lm/s during exposure. We used an on target (delivered) per pulse energy of 2.4 nJ and therefore per pulse delivered fluence of 0.03 J/cm2. The on target integrated fluence was determined and controlled by the scan speed and the number of scans performed along the same path. We must point out here that we chose the above on target per pulse fluence, since we meant to avoid laser ablation during the exposures, and since we knew from what it was reported by Hermann et al. [29] that the ablation threshold fluence for a 500 nm thick Mo film, deposited on glass, was of 0.11 J/cm2 under 800 nm, 100 fs laser pulse irradiation. However, we obtained from our experimental results (see SEM micrographs below, Fig. 5) that at our chosen on target fluence, which is 1/4 from Hermann’s reported ablation threshold fluence, we still get laser ablation off the Mo film. 2.3. Characterization of the irradiated regions in the Mo thin films The laser irradiated paths on the molybdenum films were analyzed by Optical Microscopy (Olympus BX-41 microscope) to identify texture and color changes; SEM and EDS (Philips XL-30 microscope) were used to study the surface for morphology and stoichiometric changes, respectively; micro-Raman spectroscopy (HR-800-LabRam) was used to identify the laser-induced molybdenum oxide type and its crystalline phase. On the micro-Raman technique the backscattering configuration was used to analyze the laser exposed areas on the Mo films. A linearly polarized mW He–Ne laser (632.8 nm) was used as the excitation source. The He–Ne beam was focused down to a 2 lm diameter spot by using a 100X microscope objective mounted on an Olympus BX-41 microscope. 3. Results and discussion Fig. 2 shows the X-ray diffractogram (a) and a SEM micrograph (b) of an as-deposited molybdenum thin film. A single peak centered at 2h = 40.6° is present in the diffractogram which corresponds to the reflection of the plane (1 1 0). This indicates that the molybdenum grew preferentially in the direction of such a (1 1 0) plane. As one can observe in the SEM micrograph 2b the thin film surface is very smooth and homogeneous. Fig. 3 shows an image acquired with the optical microscope coupled to the micro-Raman system. The optical image corresponds to a fs laser irradiated trace with eight scans along the same trace. There is a clear laser-induced texture and a complex coloration (from center of the trace outwards: light-gray, light-green, dark-green, green, blue, dark-brown, and light-brown) in the vicinity of the directly exposed path (3 lm wide), which actually corresponds to the zone that contains the sharply ablated elliptical spots along the center. The texture and coloration is related to the degree of oxidation and phase of the molybdenum given by
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Fig. 1. Experimental setup.
the cumulative temperature gradient established by the laser heat deposition and its diffusion process during the exposure. We must point out here that the thin film was ablated away preserving the exact footprint (shape and size) of the laser beam waist, which is characteristic for ablation with fs laser pulses. We have here a clear evidence of two distinct physical mechanisms of light–matter interaction; on the one hand the ultrashort pulse laser ablation nature, and on the other hand the thermal component provided by the long envelope of the MHz pulse train used in the exposure. Fig. 4 shows the Raman spectra that correspond to the asdeposited molybdenum thin film and the fs laser irradiated traces. In order to obtain the structure of the material in each region with different color, the spectra were obtained by using a low power He–Ne laser (1.2 mW, 10 kW/cm2), this is so for serving the purpose to avoid inducing any additional structural changes in the probed material while running the Raman characterization. It is worth noting that no additional transformation was observed, in the fs-laser-irradiated material, during Raman characterization runs even for higher probing He–Ne laser power. The spectrum 4a corresponds to the as-deposited molybdenum thin film (zone III, Fig. 3); as it is expected for all metals no Raman peaks are present. Fig. 4b shows a representative Raman spectrum taken at the center of a fs-laser irradiated trace (zone I, Fig. 3). One can observe that the spectrum 4b is constituted by several peaks located at 204, 209, 231, 347, 351, 366, 425, 461, 471, 498, 571, 588 and 744 cm1, in good agreement with the Raman spectrum of the m-MoO2 phase reported for single crystal by Srivastava and Chase [16], powder by Camacho-López et al. [30], and thin films by Spevack and McIntyre [18]. This result indicates that the Molybdenum thin film transforms into m-MoO2 after low energy–high repetition rate fs-laser irradiation. In this manner, the irradiated molybdenum suffers an oxidation process acquiring the monoclinic structure. EDS measurements (at that same zone I) confirmed the stoichiometric relation MoO2. Raman spectrum 4c corresponds to the dark-green region (zone II, Fig. 3). Raman bands are located at 211, 275, 310, 339, 382, 416, 431, 455, 500, 745, 795, 836, 849, 862, 909, 941,
986 cm1. According to the work reported by Dieterle et al. [31] and Dieterle and Mestl [32] and Blume [21], these Raman bands indicate that the material in the dark-green (zone II, Fig. 3) is constituted by the orthorhombic (o-Mo4O11) crystalline phase. Additional peaks marked with ⁄, located at 1006 and 1014 cm1 are present in the spectrum 4c. These two peaks (at 1003 and 1012 cm1) have been previously reported by Mestl et al. for MoO3 [33]. They attribute those peaks to Mo@O stretching vibrations. Fig. 5 shows SEM micrographs of laser irradiated traces on the molybdenum thin films. As we addressed above, although we used an on target (per pulse) delivered fluence of 1/4 the previously reported ablation threshold fluence, for Mo thin films, we still get well defined ablation at this significant lower fluence (see Fig. 5a). Therefore, we can conclusively state that the ablation threshold fluence for molybdenum thin films (deposited on fused silica substrates) under 800 nm, 60 fs laser pulses, is 0.03 J/cm2. The most likely cause of the lower ablation threshold we observe here is the fact that, since we are using a multiple pulse exposure at a very high repetition rate, there are incubation effects involved; it is well known for different materials, including metals, that this effect will lower down the ablation threshold [34,35]. In our case, the incubation effects are of thermal nature provided by the pulse train envelope used during the irradiation. A dominant effect in the interaction studied in this paper is laser heating given by the high repetition rate (MHz) delivery of pulses, which will produce heat accumulation and therefore temperature rise. Nonetheless, given the ultrashort nature of every single pulse within the pulse train; it is worth pointing out the reported work on lower ablation thresholds for metals which results of multi-photon absorption processes [36,37]. This scenario is possible to occur at the high peak laser irradiances involved during the Mo thin film irradiation discussed in this paper. The effect of performing an increasing number of scans along the same path, is the growth of a couple of sideways tracks (see Fig. 5b and c) composed of scatter grains of 1 lm and smaller
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Fig. 4. Micro-Raman spectra corresponding to the colored regions showed in Fig. 3. (a) As-deposited Mo thin film; (b) light-gray region at the center of the trace (zone I, see Fig. 3); (c) dark-green stripe (zone II, see Fig. 3) besides the light-gray region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) X-ray diffractogram and (b) SEM micrograph corresponding to the asdeposited on fused silica molybdenum thin films.
Fig. 3. Optical image of a femtosecond laser irradiated (eight scan) trace on a molybdenum thin film. The dashed circles show the regions probed by the laser in the micro-Raman experiment.
sizes. The (a–c) SEM micrographs show laser exposed paths to a single scan (a) and to five scans (b–c); we must note that Fig. 5c shows one end of the trace exposed to five scans, and there is
not ablation signs. The reason why laser ablation did not occur at one end of the path is that, for a millimeter long laser scan, and a very small Rayleigh range, the laser beam-waist eventually takes off or dives in the film surface. The above scenario makes the on target delivered fluence to fall (eventually below the ablation threshold), since the laser beam cross section intersecting the film surface is larger than the cross section of the beam-waist in either case. The laser beam-waist takes off or dives in the sample over a long scan because the sample does not run perfectly perpendicular to the laser incidence. SEM of the laser exposed traces reveals well defined ablation spots which are an exact footprint of the elliptically shaped laser beam waist (of 3 lm and 5 lm short and long axes) incident on the film. Notice that the elliptically shaped ablation spots (Figs. 3 and 5a) are periodically distributed, this obeys to an unexpected software-electronic failure in our translation stage system that occurred during the laser irradiation, which caused the scan to pause periodically. So, the ablated spots correspond to the positions where the scan paused and therefore those spots on the thin film were exposed to a larger number of pulses than everywhere else along the scan. This actually reinforces the explanation of the lower ablation threshold as a result of incubation effects, since the effect depends on the number of pulses, i.e. the exposure time. We can also see how the effect of the laser irradiation extends as far as 25 lm sideways; these irradiated traces show grain regions, which correspond in the optical images to pattern colored fringes of different widths between 2 and 10 lm wide, caused by the temperature gradient established by the laser heat deposition and the heat diffusion perpendicular to the laser scan direction. It is important to point out the advantages of the ultrafast (ultrashort pulse) laser-processing technique over the conventional thermal treatment for obtaining metallic thin film oxides. In general, metal oxides can be produced by using conventional furnaces and ambient air, where usually several hours of thermal heating at different temperatures are necessary to achieve a given type of oxide and crystalline structure [1,2,38,39]. When using this thermal heating technique, for example with metallic thin films, the whole sample is homogeneously oxidized. Lasers on the contrary allow rapid and very well spatially confined rising of temperature; if using pulsed lasers the heat deposition and consequently the temperature rise occurs within the pulse duration, making it possible to rapidly oxidize a metal when exposing it to a long enough single pulse or to a series of short laser pulses [40,41].
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structure of distinct stoichiometry and crystalline phases across the laser affected zone and beyond (see Fig. 3). At present, we do not have a direct experimental method for estimating the temperatures achieved in the Mo thin film at different laser fluences, however, it is well know from the literature the range of temperatures at which the different Mo oxides reported in this paper synthesize. MoO2, an intermediate oxide, and MoO3 synthesize within the temperature range 800–1200 K [43,44]; therefore we can estimate that in our case the laser-induced heating profile (transversal to the laser scanning direction), achieves the above average temperatures across the regions I and II (shown in Fig. 3) where we observe the formation of m-MoO2 and oMo4O11. According to Floquet et al. [43] the MoO2 forms at the highest temperature, therefore our results are consistent to such fact since the highest laser-induced temperature must be achieved at the center of the laser exposed region, i.e. along region I in Fig. 3; while a lower temperature should be achieved (by heat diffusion) in the close proximity of the directly laser-irradiated region, which gives place to the formation of the intermediate oxide o-Mo4O11. If the laser beam is tightly focused on the sample very fine micro or even nano patterning is possible. For the particular case of ultrashort pulses (femtosecond) and very high repetition rates (MHz), quite low energies and very tight focusing allow both high enough fluences and heat cumulative effects as to reach the needed temperatures, over a few seconds exposure, to obtain very fine patterning of the m-MoO2, the intermediate o-Mo4O11 and even the aMoO3 crystalline phases. All the above demonstrate dramatic advantages of the ultrashort pulse and high repetition rate laserinduced oxidation of metallic thin films as compared to the conventional thermal oxidation technique. Thermally obtained metallic oxides such as TiOx, ZnOx, BiOx, WOx, and MoOx have been studied extensively showing that those kind of oxides are usually photochromic [45], and/or electrochromic [46,47] and/or gasochromic [48]. Based on some of these properties of metallic oxides a variety of technological applications have been either suggested or demonstrated, such as optically based gas sensors, transducer based gas sensors, and optically recording devices for storage [49,50,26]. It also has been demonstrated that the electrical features, say the resistivity, of a metallic oxide can be modified by exposing it to femtosecond laser pulses [11]. On the applied side of the work presented here, we must mention that the study and characterization of both the electrical and the optical properties of the fs laser-induced MoOx is currently underway, and will be reported on a future paper.
4. Conclusions
Fig. 5. SEM micrographs of a laser exposed molybdenum thin film. (a) Single scan laser exposure with an on target per pulse fluence of 0.03 J/cm2, ablation occurs at this fluence; (b) five scans laser exposure, notice the grain structure formed to the sides of the ablated track; (c) the same five scans laser exposure at the edge of the scanned path, notice the absence of any ablation signs and the enhanced grain structure formation.
Additionally, lasers posses a wide selection of parameters to finely tune and control a chemical reaction such as a metal oxidation. For instance, we can chose the right single pulse fluence to achieve a desired peak temperature while, on multiple pulse exposure, the pulse repetition rate drives heat accumulation effects [41,42] and therefore the average temperature which would lead to a specific stoichiometry and crystalline phase in the case of laser-induced metallic oxides. Also, due to heat diffusion effects it is possible to obtain a lateral heat distribution which gives place to a complex
We demonstrated, for the first time to the best of our knowledge, the transformation of metallic molybdenum into a complex pattern of molybdenum oxides by using very low energy femtosecond laser pulses delivered at a very high repetition rate. Both laserinduced oxidation and crystalline–crystalline phase transformation was achieved, on as-deposited (1 1 0) cubic-molybdenum thin films, by using low energy (nJ)-high repetition rate (MHz) femtosecond pulses. Our results show solid evidence of the transformation from c-Mo into m-MoO2 and o-Mo4O11. The m-MoO2 forms along the directly laser-exposed trace and its close proximity, which extends 5 lm sideways and looks light-gray under optical microscope imaging; the o-Mo4O11 forms a dark-green stripe 2 lm wide, right besides the m-MoO2 trace. There is a complex color pattern formed, as a result of the fs-laser exposure, which includes, from center of the trace outwards: light-gray, light-green, dark-green, green, blue, dark-brown, and light-brown. Neither the stoichiometry nor the phase has been identified yet for these colored regions, but those of the light-gray (m-MoO2) and
M. Cano-Lara et al. / Optical Materials 33 (2011) 1648–1653
dark-green (o-Mo4O11) regions. A detailed Raman study to identify the types of Mo oxide and stoichiometry of the remaining of the colored pattern is underway within our research group. The pulsed-laser exposure technique described throughout this work probes to be an efficient and rapid way of obtaining molybdenum oxides, in the form of microstructured patterns. We used tightly focusing of the laser beam such that it is feasible to obtain an on demand micro-pattern made of laser-induced MoOx i.e. to selectively obtain a micro-pattern made of stripes of molybdenum dioxide and intermediate oxides up to molybdenum trioxide. The results presented in this work could find applications in technology areas as gas sensing, recording and displays were metal oxides have proved themselves useful given their photochromic, gasochromic and electrochromic features. Acknowledgments The authors acknowledge partial support to this work from CONACyT Grant 57309 and AFOSR-CONACyT Grant FA9550-10-10212. M. Cano-Lara thanks CONACyT for PhD scholarship granted. We are also very grateful to Dr. Manuel Herrera Zaldivar for his support for the SEM facilities, his great scientific advice and useful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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