Reactive deposition of laser ablated FeS_1−x particles on a copper surface

Abstract

Pulsed IR laser ablation of ferrous sulfide (FeS) in a vacuum has been studied using scanning and transmission electron microscopy and X-ray diffraction. This allows the analysis of the surface morphology and composition of the irradiated target and also of the coats deposited on unheated Ta, silica and Cu substrates. It is observed that a noncongruent ablation of FeS results in the deposition of nanosized FeS-containing FeS_1−x amorphous coats on the silica and tantalum, and in the deposition of crystalline copper sulfide-containing amorphous Cu₂S phases on copper. The detected amorphous Cu₂S phase, crystalline and nanocrystalline chalcocite Cu₂S, bornite Cu₅FeS₄, digenite Cu₉S₅ and blaubleibend covellite are formed through the reactive deposition of FeS_1−x particles on the topmost Cu layers. This finding is the first example of the reactive deposition of laser ablated inorganic compounds on unheated surfaces and may spur more interest in using this simple process with various inorganic compounds to achieve reactive modifications of other materials.

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1. Introduction

Laser ablation has been broadly studied in the past 30 years (e.g. ref. 1–5) and has become important in various fields of physics,^1,2 chemistry,³ medicine,⁴ materials science⁵ and mineral analysis.^6,7 The material removal which is induced by high-energy laser pulses, consisting of vaporization or ejection of excited neutral and/or charged fragments which agglomerate and condense at nearby substrate surfaces, has also proven to have great potential for the deposition of smooth and nanostructured films and nanosized particles from elemental, inorganic and organic bulk materials (e.g. ref. 8 and 9). Whether or not these deposits preserve the phase and stoichiometry of the bulk precursors depends on the ability of the ejected particles to survive fast laser excitation and post-pulse cooling during deposition on the substrate surface.

The purposeful laser ablative modification of the morphology and structure of solid materials is achieved via the laser ablation of mixed or dispersed elemental powders^10–14 and polymers (e.g. ref. 15–17), and by laser ablation of bulk elemental solids in the presence of a reactive gas (e.g. oxygen,^18–21 nitrogen,^22–24 carbon monoxide,^25–27 hydrocarbon,^28,29 or hydrogen sulfide³⁰). All these processes are called reactive laser ablation and yield (in the given order) mixed clusters and nanosized products (e.g. intermetallics); structurally modified polymer particles and films; and binary, ternary or multielement (ceramic) compounds like oxides, nitrides, silicon or titanium oxycarbides, carbides and sulfides.

There are plenty of examples of reactive laser ablation for the deposition of various materials with advanced properties. In these processes, materials with a morphology or structure that is different from their bulk progenitors are deposited due to high-temperature reactions taking place on the irradiated target, and/or reactions of ejected fragments and agglomerates in the plume.

The structural or compositional modification of laser ablated particles through reactive collisions with unheated substrate surfaces appears to have attracted little or no attention, although similar phenomenona – reactive deposition epitaxy (e.g. ref. 31 and 32) and deposition of multicomponent nanostructures (e.g. ref. 33–38) involving laser ablative deposition on in-situ or additionally heated substrates – have been given much attention and are now part of high temperature solid–liquid state chemistry.

Thus, there is also missing information on the reactive deposition of laser ablated inorganic compounds on unheated surfaces, in spite of the fact that laser ablation of metal oxides, phosphides, chalcogenides and metal carbonyls has been explored for the formation and interaction of inorganic cluster ions in the plume (e.g. ref. 39–45), and also that the laser ablation of mixed oxides,^34,37 III–V compounds⁴⁶ and CdX (X = S, Se, Te)^35,37 powders was examined for the growth of nanostructures on furnace-heated substrates. The only reported data concern the laser ablative deposition of pyrite (FeS₂) and hematite (Fe₂O₃) on aluminium and silica, and show that the deposited films contain FeS and FeO constituents at both room and higher temperatures.⁴⁷

Herein we report on the pulsed IR laser ablative deposition of ferrous sulfide FeS on unheated silica, tantalum and copper substrates. Ferrous sulfide and its films are of interest due to their environmental reactivity, structural and phase-transition, and electrical and magnetic properties. We present SEM-EDX and XRD investigations of the FeS ablation and reveal that ablated iron sulfide particles form amorphous S-deficient coats on Ta and silica. They also react with unheated copper to produce copper sulfides. These results give support to further investigations into the reactive depositions of laser ablated particles on various unheated substrates.

2. Experimental

The IR laser irradiation experiments were conducted in a vacuum (better than 10⁻³ Torr) in a previously described Pyrex reactor (70 mL in volume)⁴⁸ which consisted of a tube fitted at each end with NaCl windows and a valve connecting to the vacuum manifold and pressure transducer. We used a pulsed TEA CO₂ laser (model 1300 M, Plovdiv University) operating with a frequency of 1 Hz on the P(20) line of the 0001–1000 transition (944.19 cm⁻¹), a full width at half maximum (FWHM) of 150 ns and a pulse energy of 1.4 J. The radiation was focused with a NaCl lens (f.l. 15 cm) on a FeS pellet positioned in the centre of the reactor, above which were accommodated copper, tantalum and silica substrates at a distance of 1 cm from the target. A simple irradiation scheme is given in Fig. 1. The effect of the distance of the Cu substrate on the morphology of the deposited coats was examined for substrate–target distances of 1 mm, 3 cm, 7 cm and 9 cm. The focus point was adjusted at the target to get an incident energy fluence of 140 J cm⁻². After irradiation with 700 pulses, the reactor was opened to the atmosphere and the substrates covered with the deposited coats were transferred to allow measurement of the coat properties by electron microscopy and X-ray diffraction analysis. Electron microscopy studies were performed on a SEM Tescan Indusem (Bruker Quantax) microscope equipped with an EDAX detector, and on a JEOL JEM 3010 TEM microscope.

Fig. 1 Scheme of laser ablative deposition.

Transmission electron microscopy (TEM) analysis (particle size and phase analysis) was carried out with a JEOL JEM 3010 microscope operating at 300 kV and equipped with an EDS detector (INCA/Oxford) and CCD Gatan (Digital Micrograph software), on scraped samples that were subsequently dispersed in ethanol followed by the application of a drop of diluted suspension on a polymer/carbon coated Cu grid. The diffraction patterns were evaluated using the JCPDS-2 database⁴⁹ and Process Diffraction software package.⁵⁰

Diffraction patterns were collected with a PANalytical X'Pert PRO diffractometer equipped with a conventional X-ray tube (Co K_a radiation, 40 kV, 30 mA, point focus), an X-ray monocapillary with diameter of 0.1 mm, and a multichannel detector X'Celerator with an anti-scatter shield. A sample holder for single crystal XRD measurements was adopted by adding z-(vertical) axis adjustment (Huber 1005 goniometer head). We assumed that the layer produced was very thin, so we decided to fix the angle of the incident beam (omega) to 1.5 degrees, to suppress the penetration depth and to enhance the signal from the layer. The diffraction patterns were taken between 10 and 90° 2θ with 0.0334° step and 2000 s counting time per step, which produced a total counting time of about 12 hours. XRD patterns were not pre-treated before interpretation, as no background correction was needed. Qualitative analysis was performed with the HighScore software package (PANalytical, the Netherlands, version 1.0d), the Diffrac-Plus software package (Bruker AXS, Germany, version 8.0) and the JCPDS PDF-2 database.⁴⁹

Raman and FTIR spectra were also measured, but they did not show any characteristic features. The FeS pellet was made at 100 atm. on a hydraulic press from a commercially available iron sulfide powder (FeS, 99% Fe, Aldrich).

3. Results and discussion

The IR laser irradiation of the FeS pellet in the vacuum results in pellet ablation and a visible luminescence (plasma) observed as a bluish plume of ca. 1 cm wide and 7 cm long. Similar plumes of excited fragments during IR laser ablations were previously observed for the IR laser ablation of metals⁵¹ due to the transient formation of metal atoms and ions.⁵² The ablation leads to the development of a crater and the deposition of ejected particles on the adjacent Ta, silica and Cu substrates (Fig. 2), where they form layers of different appearances. The coats on the Ta and silica are smooth, but those on the copper are not homogeneous and show areas which are detached from the surface.

Fig. 2 The coated Ta (a), silica (b) and Cu (c) substrates.

The SEM images of the irradiated spot (Fig. 3) reveal melting, formation of column-like features pushed from a ca. 1 mm² sized crater, and ejection of several μm-sized objects and droplets that are deposited on the crater periphery. The images of the coats deposited on silica and Ta show μm-sized round-shape objects in continuous surroundings (Fig. 4a and b), whereas those of the coat deposited on Cu show large flat peeled-off bodies occurring near shapeless agglomerates (Fig. 4c). These features are in accordance with vaporized and plasma-produced clusters, and liquid droplets expelled from the target surface and quenched upon deposition. The round-shaped objects on the silica and Ta indicate the rapid cooling of the solidifying droplets, which may occur in a metastable amorphous state. Furthermore, the high energy of these particles prior to collisions with the substrate surface is proved by splattering and trails. The flat entities extending from the Cu surface, occurring together with irregular agglomerates and droplets, indicate the formation of a new phase.

Fig. 3 SEM images of the FeS powder (a) the irradiated spot (b) and the crater periphery (c).

Fig. 4 SEM images of coats deposited on Ta (a), silica (b) and Cu (c) substrates.

EDX-SEM analyses allow assessment of the composition of the coats. They have been performed on the crater and its periphery, the μm-sized particles near the crater and the deposited coats (droplets, agglomerates and smooth areas, Table 1). The S/Fe atomic percent ratios of the crater and its periphery (0.85 ± 0.02) and of the droplets near the crater (0.74 ± 0.06) are somewhat lower than those of the initial FeS sample (0.94 ± 0.02). These values indicate some decomposition of FeS. Such incongruent deposition of S-deficient coats is also observed on the Ta and silica, where the μm-sized droplets have S/Fe ratios even lower than those near the crater. Here the smooth areas have more S than Fe, which is attributed to a fast diffusion of Fe to the lower Ta and SiO₂ layers.

Table 1 EDX-SEM analysis^a of the coats deposited on Ta, silica and Cu

Object/substrate	Ta	SiO2	Cu
	S/Fe	S/Fe	S/Fe	Cu/Fe
a S/Fe and Cu/Fe atomic percent ratios.
μm-sized droplets	0.75 ± 0.07	0.50	0.64 ± 0.03	0–0.2
Smooth area	2.0 ± 0.50	1.35 ± 0.05	—
Flat bodies	—	—	23–32	54–58
Shapeless area	—	—	8.0–14.4	14.7–27.0

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Regarding the coats on Cu, the S/Fe and Cu/Fe atomic percent ratios for the flat bodies and the proximal shapeless areas reflect similar Cu/S ratios and highly diminished Fe contents in both features. There are very few several-μm sized isolated droplets on the Cu surface which have an S/Fe ratio of 0.64 and which correspond to the particles seen on the Ta and silica. We assume that the flat and shapeless areas were produced by collisions of both FeS_1–x clusters and μm-sized FeS_1–x particles with the Cu surface, and that their different appearance arises from the charge- or energy-inhomogeneous flow of colliding FeS_1−x species.

It is known that plasma vapor and liquid droplets move at high speeds and that their flight distances and cooling rates after an impact with a surface diminish with their mass. The SEM images (Fig. 5) and the EDX analysis of the deposited areas at different distances between the Cu substrate and the target reveal a substantial deposition for areas 1 mm and 3 cm from the target, a low deposition for areas at a distance of 7 cm and no deposition for those at a distance of 9 cm (not shown). The large several μm-sized flattened bodies and the droplets with an S/Fe atomic percent ratio 0.67 occur near the target together with coated areas that have an S/Fe value of 3.5 and a Cu/Fe value of 98. These features respectively correspond to incongruent FeS particles and the reactive deposition of smaller particles on the copper. The flat and amorphous features without large droplets are seen at a distance of 3 cm from the target, and they consist of Cu and S (Cu/S = 5.1–5.4) and a very low amount of Fe (ca. 0.1 percent of S). These large plates and the agglomerates have the same composition and are also in line with reactive deposition. Farther areas at a 7 cm distance show no Fe and their Cu/S ratio of 24–25 indicates a thin layer with a prevalence of Cu substrate.

Fig. 5 SEM images of coats on the Cu substrate, at 1 mm (a), 3 cm (b) and 7 cm (c) from the target.

The X-ray diffraction analysis of the commercial FeS sample allows the estimation of the relative amounts of crystalline troilite (∼63%), pyrrhotite (∼24%) and alpha-Fe (13%) it contains. The XRD analysis of the coat deposited on Ta shows only FeS (troilite) and no diffraction lines on the silica, the latter being consistent with an amorphous phase. The analysis of the coat deposited on Cu reveals the presence of two Cu-containing sulfides, specifically digenite and bornite (Fig. 6).

Fig. 6 XRD patterns of the FeS target (a) and the coats deposited on the Ta (b), silica (c) and Cu (d) substrates.

The TEM and ED examinations of the coats on the Ta, silica and Cu reveal a number of areas with both discrete diffraction lines and halo patterns, which are in line with the occurrence of nanocrystalline and nanoamorphous regions. These analyses (Fig. 7) indicate ca. 2 nm-sized hexagonal FeS nanograins on the Ta and silica, and show that the nanograins on the silica escaped identification by XRD analysis. There are more distinct FeS diffraction rings on the silica than on the Ta, which is in line with less amorphization of nanograins on the silica due to a slower cooling of solidifying fragments on the low thermal conductivity substrate.

Fig. 7 TEM and electron diffraction images of the coats on Ta (a) and silica (b).

The examination of the coats on Cu shows the occurrence of amorphous and crystalline nanophases. The observed single crystal diffraction patterns as well as the polycrystalline ring diffraction patterns are respectively consistent with ca. μm-sized objects and polycrystals composed of nm-sized grains. Both were found in peeled-off plates and shapeless bodies (Fig. 8). The ED analysis is consistent with the occurrence of chalcocite, digenite, bornite and blaubleibend covellite.

Fig. 8 TEM and electron diffraction images of the coats on Cu: (a) and (b) respectively relate to peeled-off plates and irregular surface-attached agglomerates; (c) shows ED images found on both regions.

We note that the absence of any characteristic features in the IR and Raman spectra of the coats on Cu can be therefore ascribed to the fact that the copper sulfides are poorer IR absorbers and also have weaker Raman scattering, due to the fact that the symmetrical Cu₂S is not amenable to the internal dipole changes required for Raman analysis.

These complementary analyses are therefore in line with the deposition of a FeS nanograin-containing amorphous FeS_1−x phase on Ta and silica; and additionally with the reactive deposition of μm- and several nm-sized crystalline sulfides and amorphous copper sulfides on Cu. The morphological inhomogeneity of the Cu coats, manifesting itself as shapeless and flat-body-containing areas, is apparently due to the reactive collisions of FeS_1−x particles and clusters, which differ in mass, energy and charge.^13,14,41 We suggest that the high-energy interactions of colliding FeS_1−x particles and clusters are a driving force for the FeS_1−x–Cu interdiffusion and crystalline sulfidic phase formation, while the interactions of less energetic particles can be sufficient only for the formation of the amorphous phase.

The mechanism of the prolific formation of the copper sulfidic phase can be discussed in relation to some previous findings on metal exchange and the formation of these entities in other systems. It is known that heavy metal (M) ions react with insoluble FeS in aqueous solutions via an FeS + M²⁺ → MS + Fe²⁺ reaction,⁵³ and that discrete particles of copper sulphide Cu_1.8S (digenite) are precipitated from the high temperature oxidizing environment during the welding of sulfur-bearing steels in the presence of soluble copper. This is thought to be due to a reaction between copper and the less-stable ferrous sulphide which are present in these steels (2FeS + 4Cu + O₂ → 2Cu₂S + 2FeO).⁵⁴ Our finding of copper sulfide formation in the absence of air is consistent with metal–metal sulfide exchange through the reduction of Fe^II and the oxidation of Cu⁰. This is likely enhanced by both high energy and charge-bearing FeS_1−x clusters diffusing into the copper surface.

The presented results have three implications. First, the stoichiometric FeS, occurring as troilite and the less-stable amorphous phase, experiences phase transitions upon annealing prior to its decomposition at 1260 K,⁵⁵ even though a partial decomposition to FeS_0.91 begins above 500 °C.⁵⁶ The observed IR laser ablation which leads to the FeS_1−x coats may be optimized for the fabrication of more homogeneous films of various compositions, and also for films which are structurally similar to mackinawite (whose properties, contrary to those of pyrrhotite (Fe_1−xS)-type films and nanostructures,⁵⁷ have not been yet studied). In this regard, the use of other irradiation wavelengths and pulse durations emitted by other lasers (e.g. excimer, near IR, or pico- and femtosecond) to create the conditions for more photochemical than thermochemical action, may be highly hopeful.

Second, copper sulfides are usually obtained by solid-state reactions of elements, metathesis or self-propagating high-temperature synthesis. Digenite (Cu₉S₅), which occurs as high-, metastable- and low-polymorphs, is usually prepared by a melting method, in solution, or by mechanical alloying. The low-digenite is only stable at room temperature, if it contains a small amount of Fe, while the high-digenite, which is isostructural with bornite (Cu₅FeS₄), is only stable^58–60 above 73 °C. These materials may decompose to binary alloys which can become promising thermoelectric materials, e.g. Cu₉S₅ decomposing⁶¹ to Cu_1.96S. Further optimization of the formation of copper sulfides and other metal sulfides, by using reactive laser deposition at various laser wavelengths and fluences, is therefore worth examining.

Third, bornite and digenite formation upon fast quenching during the reactive deposition of the FeS_1−x particles, illustrates that the process of reactive laser deposition may help to understand the formation of intermediary compositions in the course of important thermodynamically balanced geological processes, like exsolution of metal sulfides⁶² and other natural ore assemblages. The technique of laser ablative deposition of mineral ores or synthesized model samples may serve for this purpose.

Our results encourage more investigation into the reactive deposition of laser ablated particles that are rapidly quenched on unheated substrates. This is especially true for materials which are unstable at higher temperatures and can offer specific reactivity towards energized ablated particles at room temperature. More work on a possible use of the technique for examination of such solid-state chemistry is in progress.

4. Conclusion

Pulsed IR laser ablation of ferrous sulfide (FeS) results in the deposition of coats of amorphous FeS_1−x, which contains FeS nanocrystals and is poor in sulfur, on silica and tantalum. It also results in the deposition of amorphous Cu₂S phase which contains nanocrystalline copper sulfides (chalcocite Cu₂S, bornite Cu₅FeS₄, digenite Cu₉S₅ and blaubleibend covelite), on copper.

These coats are formed by the somewhat noncongruent ablation of FeS and amorphization of high-energy particles, which rapidly cool upon collisions with the ambient-temperature substrates.

The coats on Cu result from reaction(s) between high-energy FeS_1−x particles and the copper surface, and their morphological inhomogeneity obviously reflects the collisions of particles which differ in energy and size.

The results encourage further exploration of the reactive laser deposition of ablated inorganic compounds on unheated surfaces, due to the potential of this technique for the synthesis of inorganic films and recognition of rapidly quenched intermediate stages during the exsolution of minerals.

Acknowledgements

This research was carried out within the CENTEM project, reg. no. CZ.1.05/2.1.00/03.0088, co-funded by the ERDF as part of the Ministry of Education, Youth and Sports' OP RDI programme.

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