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Direct Laser Writing from Aqueous Precursors for Nano to Microscale Topographical Control, Integration, and Synthesis of Nanocrystalline Mixed Metal Oxides Cody Kindle, Alexander Castonguay, Shannon McGee, John Tomko, Patrick E. Hopkins, and Lauren D. Zarzar ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00360 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Nano Materials

Direct Laser Writing from Aqueous Precursors for Nano to Microscale Topographical Control, Integration, and Synthesis of Nanocrystalline Mixed Metal Oxides

Cody Kindle1, Alexander Castonguay1, Shannon McGee1, John A. Tomko2, Patrick E. Hopkins3, and Lauren D. Zarzar*1,4 1.

Department of Chemistry, The Pennsylvania State University, University Park, PA 16802

2.

Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904

3.

Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904

4.

Materials Research Institute, The Pennsylvania State University, University Park, PA 16802

*corresponding author, [email protected]

Abstract As evidenced by the growth of additive manufacturing, there is extensive value in customization, both in terms of chemistry and structure. Direct laser writing is a promising approach to the maskless patterning of a broad range of nano to microscale materials. We employ a methodology using laser-induced solvothermal voxels, which combines the control of solvothermal chemistry for nanoparticle synthesis with the patterning abilities of direct laser writing, enabling the integration of nanocrystalline oxides, metals, and mixed metal oxides and alloys. We present an expanded range of materials that can be easily fabricated through this method and explore how this process can be used for 2.5D topographical patterning and layering with potential applications for catalysis and sensing.

Keywords Direct laser writing, additive manufacturing, nanocrystalline, mixed metal oxides, solvothermal chemistry

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The development of synthesis and processing approaches that facilitate the spatially controlled integration of a variety of materials into complex nano- to micro-scale architectures is increasingly important as higher resolutions and enhanced customizability of production have become necessary1. Direct laser writing (DLW), an approach in which the focus of a laser is used to deposit or etch in various materials, is growing in popularity and practicality due its ability to customize patterning without the need for molds or masks. The DLW of inorganic materials is of particular interest for integration into devices and applications including flexible electronics, photonic devices, sensors, and catalysts2. Often, approaches to DLW of metals and oxides involve laser-assisted photothermal sintering using precursors such as nanoparticle dispersions3 or the laser-induced thermal pattern-transfer of solid thin films (e.g. via laser-induced forward transfer4 or focused laser spike excitation5). For these techniques, the laser is used for patterning rather than material synthesis, and films or nanoparticles must be produced in an additional processing step. Laser writing in which solution-phase chemical precursors can be employed for simultaneous synthesis and deposition of inorganic nanomaterials in-situ are therefor of interest and may provide enhanced control over aspects such as chemical composition and crystallinity. General approaches to using laser writing for the simultaneous synthesis and patterning of inorganic nanomaterials include both photochemical and photothermal routes. Examples of DLW of inorganic materials involving synthesis from fluid precursors include the photoreduction of metal salts (e.g. silver, platinum)6-7 and laser-induced photothermal chemical liquid growth of zinc oxide, titanium dioxide, and iron oxides8-9 using an optically absorbing thin-film metallic substrate. Photoreduction pathways are more chemistry-specific, while thermal routes tend to be applicable to a broader range of materials. However, research on laser-assisted photothermal growth of inorganic materials in liquids has been largely focused on the patterned, aligned 3D growth of nanowires of select oxides; there remains extensive opportunities for exploring how other hydrothermal chemistries can be employed to access a far broader range of nanomaterials. Recently, an approach of using laserinduced thermal voxels (LITV) was introduced as a method by which to synthesize and pattern more diverse inorganic materials10; however, few such materials have been previously characterized and the accessible chemistries, patterning, and resolution capabilities have not been studied. Expanding the range of nanomaterials, including alloys and mixed-metal oxides, and developing integration strategies compatible with DLW approaches will be important for the future integration of diverse inorganic materials into nanoscale devices for applications such as sensing or catalysis.


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Here, we explore the LITV method as a means to expand the scope of nanomaterials accessible for DLW directly from liquid chemical precursors by describing pathways for synthesis and laser deposition of metals and oxides, exploring how processing conditions affect topographical control and resolution, and introducing the capabilities of this technique for the direct writing of mixed metal oxides from multi-component precursors. For the first time, we characterize in detail the inorganic materials produced and compare them to the products of thermal decomposition for insight as to the deposition mechanism and reaction conditions. To expand morphological and structural control, we demonstrate opportunities for 2.5D patterning and material layering by tuning laser power during scanning. Because the precursors are all aqueous, creating precursor mixtures is facile, but the resultant mixed metal materials deposited exhibit non-trivial composition; we use cross-sectional analysis to examine elemental composition as a function of material thickness and also identify opportunities for the synthesis of mixed metal oxides such as ferrites with this technique. We demonstrate that this non-equilibrium synthesis approach can also be used for the deposition of nanocrystalline or amorphous oxides and metastable phases, which are promising for catalysis and sensing. We expect that this laser writing method will find use in applications where customizable combination of chemically diverse inorganic nanomaterials are desirable. In this LITV direct writing approach, a continuous wave Ti:Sapphire laser operating at 760 nm is focused through a microscope objective onto an optically absorbing medium to create a femtoliter scale reaction volume (Figure 1, Scheme S1). Evidence of glass melting within laser scanned regions experimentally demonstrates that we reach temperatures exceeding 600 °C while previous efforts in modeling the reaction volume suggest temperature rises of over 2000 K are possible at the laser focus10. Thus, this LITV acts as a miniature solvothermal reactor where reagents in the surrounding fluid are locally decomposed and nanoparticles are simultaneously synthesized and sintered together on the glass substrate surface, locally generating bubbles in the process of deposition. As the laser is translated, the previously-deposited material film serves as the optical absorber, thereby propagating the deposition as the material is “written” on the substrate (Figure 1). By simply rinsing out the precursor fluid and replacing it with another, we can sequentially deposit a variety of materials with high precision and resolution10. A unique aspect of this LITV method is that a wide variety of precursors can be utilized to influence the chemistry of the material deposited. In order to identify the specific oxides or metals accessible through this approach as a function of precursor, we characterized a diversity of materials deposited from aqueous solutions of metal salts. The precursor chemicals, concentrations, laser deposition conditions, and resultant compounds examined



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are listed in Table 1 while X-ray diffraction results (XRD) and Raman spectra used for chemical identification are included in Figures S1 and S2. As evident in Table 1, we have chosen to use nitrate or ammonium salts as the precursors where possible as they have high water solubility and the nitrate hydrates tend to have low thermal decomposition temperatures11 as compared with other salts. However, other precursors could be used, and the list of precursors is not exclusionary. All aqueous metal salt precursors except those of platinum, palladium, iridium, rhenium, rhodium, and silver deposit as oxides. Gold from (NH4)AuCl4 aqueous solution only deposited consistently in the presence of a reducing agent, sodium citrate (Table 1).

Figure 1. Schematic of direct laser writing with laser-induced thermal voxels. A Ti:Sapphire continuous wave laser (760 nm) is focused through a microscope objective onto a substrate (usually a glass coverslip) within a precursor solution containing the salts listed in Table 1. An inverted configuration is used where the laser is introduced from the bottom through the glass substrate. Either the solution or the surface contains an optical absorber, thereby generating a local hot spot at the laser focus at which deposition can be initiated. As the laser is translated, the process is self-propagating because the material deposited now serves as the new optical absorber. By rinsing and replacing the precursor solution, we can sequentially pattern any number of materials together on a substrate. Precursor Al(NO3)3 (with 0.5 M NaOH) TiOSO4 · xH2SO4 · yH2O VOSO4 Cr(NO3)3 Mn(NO3)2 Fe(NO3)3 FeCl2 Co(NO3)2 Ni(NO3)2

Concentration (M) 0.5 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0


Laser Power (mW) 75 45 30 30 30 30 50 30 30

Material Deposited Al2O3 TiO2 VO2 Cr2O3 Mn2O3 Fe2O3 Fe3O4 Co3O4 NiO*

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Cu(NO3)2 Zn(NO3)2 ZrONO3

1.0 1.0 1.0

30 50 65

(NH4)6Mo7O24

0.25

50

(NH4)2RuCl6 (NH4)3RhCl6 (NH4)2PdCl6 AgNO3 (NH4)10W12O41 (NH4)3ReO4 Na3IrCl6 (NH4)2PtCl6 (NH4)AuCl4 (with 0.5 M sodium citrate)

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.25

50 50 50 50 30 50 50 50 50

CuO ZnO ZrO2 MoO2,Mo4O11, (NH4)2Mo4O13 Ru0 Rh0 Pd0 Ag0 WO3 Re0** Ir0 Pt0 Au0

Table 1. Typical deposition conditions and identification of material deposited from given precursors. All samples were deposited with a laser scanning speed of 300 μm/s (rastering with the galvo scanner) and stage translation speed of 20 μm/s. If the scanning rate was too fast, or the laser power too low, the deposition was not consistent and any gaps in the deposited lines halted further deposition as there would be no absorber present. Therefore, the concentrations, laser power, and scan rates represent exemplary conditions that we find to yield consistent depositions rather than describing the minimum values required for deposition. Laser power is measured at the back of the objective (60x 1.4 NA) which focuses the laser onto the sample. Material identification was made based on XRD and Raman data which are included in the Supporting Information, Figures S1 and S2. *Nickel oxide was reported and analyzed in previous work10. **Rhenium was challenging to deposit in large enough areas to collect XRD data due to poor glass adhesion; however, Raman analysis did not reveal the presence of any oxide and EDS identified Re, hence the designation as Re0.

Given that this is a thermal processing method, we aimed to investigate whether the products of this synthesis approach would coincide with those products formed via thermal decomposition or hydrothermal reactions of the respective hydrated precursors. At the same time, this laser synthesis process involves much higher peak temperatures and much more rapid heating and cooling than a traditional hydrothermal synthesis, which might lead to the formation of different products or metastable phases. Based on our analysis, we find that the deposition products listed in Table 1 are largely consistent with those observed from thermal decomposition of the respective hydrated precursors11-14 with some exceptions. Interestingly, VO2 deposited from vanadyl sulfate as identified by both XRD and Raman spectroscopy (Supporting Information Figures S1 and S2), which, given the multi-valence



nature of vanadium oxides and the fact that VO2 is one of the less stable forms, was unexpected15. However, it has also been found that for spray pyrolysis of hydrated VOSO4, the composition of the gaseous environment significantly influences the resultant oxide produced, and VO2 is formed preferably in environments of high water vapor, similar to the reaction environment in this LITV method16. Deposition from ammonium heptamolybdate, (NH4)6Mo7O24, yielded a mixture of (NH4)2Mo4O13, Mo4O11, and MoO2. Literature thermal decomposition studies of ammonium heptamolybdate similarly show the presence of ammonium tetramolybdate—(NH4)2Mo4O13—as an intermediate, as well as small amounts of Mo4O11 present in the final product; however, the main decomposition product is MoO3, not reduced MoO2 as we find17. The exact cause of this deviation from previous thermal decomposition studies is unknown, but Wienold et al. showed that ammonium heptamolybdate decomposes to MoO2 in a reducing atmosphere, suggesting that such a condition may exist within the LITV for this precursor17. Generally, we also found that materials which are not strongly optically absorbing in the near-infrared (e.g. TiO2, Al2O3, ZnO, ZrO2)18 could not be deposited in a self-propagating fashion when using a near-infrared laser (wavelength 760-780 nm), which is expected given the proposed deposition mechanism. Thus, in order to deposit and probe the composition of near-IR reflective materials, we layered them on top of a laser-deposited platinum film6 where platinum served as the absorber. One approach to address this limitation for IR reflective materials that will be explored in the future is to use a laser with a more strongly absorbed wavelength, such as in the UV range. Once the chemical identity of the deposited materials was determined, we aimed to gain insight of the factors contributing to the resolution of deposition for varying materials. Minimum line widths achievable with a 60x 1.4 N.A. objective for representative oxides and metals are listed in Table S1 and range from 2.5 µm to 14.5 µm. We also examined how the line width varied as a function of material for consistent laser deposition conditions and did observe material-dependent effects (Table S2). To test if variations in thermal conductivity could account for the material-dependent resolution19, we measured the thermal conductivities for several oxides (Table S3) but found no correlation. Instead, we experimentally observe that bubbles, generated due to heating at the reaction site, seem to significantly influence achievable line widths; materials for which bubbles do not adhere tend to have higher resolutions (e.g. Mn2O3, Video S1) while for those materials where bubbles persist pinned to the deposition site contribute to lower resolutions (e.g. Cr2O3, Video S2), likely a result of lateral heat deflection and spreading at the bubble-water interface20.


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As this LITV method is a non-equilibrium synthesis method characterized by both rapid heating and cooling, we hypothesized that materials deposited with this approach may be nanocrystalline or amorphous. Generally, we find that many oxides deposited using this DLW approach (e.g. Mn2O3, VO2, Fe2O3, Co3O4, Cr2O3) have weak and broad XRD signals or Raman spectra (Supporting Information Figures S1 and S2) suggestive of nanocrystalline materials. Peak fitting of the noble metals (Ag, Au, Ir, Pt, Pd, Rh) showed that the deposited materials were nanocrystalline with crystallite sizes estimated to be on the order of 15 to 200 nm. To observe effects of thermal annealing on crystallinity, we focused on the iron oxides as a case study given their more complex chemistry and polymorphs21-22. The material deposited from iron nitrate in particular showed a featureless diffraction pattern (Figure 2a(i)) but the broad peaks in the Raman spectrum were suggestive of maghemite (γFe2O3) with three active modes observed at 370 cm-1 (T2g), 520 cm-1 (Eg) and 700 cm-1 (A1g) (Figure 2b(i))23-24. Increasing the laser intensity during micro-Raman analysis was sufficient to locally trigger phase transitions and crystallization. As we gradually increased the power of the 532 nm laser from 0.45 mW (Figure 2b(i)) to 11.25 mW (Figure 2b(v)) we observed a disappearance of the maghemite phase and an eventual transition to hematite as indicated by peaks at 224 cm-1 (A1g), 242 cm-1 (Eg), 290 cm-1 (Eg), 410 cm-1 (Eg), 500 cm-1 (A1g), 612 cm-1 (Eg), and 660 cm-1 (LO Eu). The 660 cm-1 peak is assigned to a Raman forbidden and IR active longitudinal optical (LO) Eu mode which is proposed to be activated by disorder in the hematite crystal23; we see this peak appear with high relative intensity after low laser power exposure (1.44 mW, Figure 2b(ii)) but almost disappear entirely after 11.25 mW laser exposure, indicating increased ordering and crystallinity in the lattice. Annealing of the iron oxide sample in an oven at 450 °C for 12 hours (Figure 2a(ii)) similarly showed a conversion to hematite and a much sharper peaks in the XRD spectrum indicating crystallization. These results suggest that the LITV approach is capable of producing metastable phases with amorphous to nanocrystalline structure, likely due to the non-equilibrium processing approach, which could prove advantageous for sensing and catalyst applications.



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Figure 2. XRD, Raman, and SEM analysis of material deposited from iron nitrate aqueous precursor. a, XRD spectra of the as-deposited sample (i) and after annealing at 450 °C (ii). Crystallization and emergence of hematite peaks is observed after annealing. b, Micro-Raman analysis and in-situ laser annealing. Each plot corresponds to exposure of the same local region of surface exposed to 532 nm laser powers of (i) 0.45 mW, (ii) 1.44 mW, (iii) 2.25 mW, (iv) 4.5 mW, (v) 11.25 mW as focused through an objective (100x, N.A. 0.9). In (ii-v) the surface was exposed to the increased laser power for 10 seconds and then the spectrum was collected at 0.45 mW. Increased exposure time did not affect the results. The longitudinal optical (LO) mode, which is Raman forbidden and activated by disorder in the hematite lattice, appears with high relative intensity at lower laser power exposure and subsequently disappears after higher laser power exposure. The spectra are consistent with a transformation from maghemite to hematite. c, Scanning electron micrograph (SEM) of the sample used for (a, b) (scale, 500 μm) with a high magnification inset (scale, 1 μm). No apparent visual changes to the surface morphology were evident by SEM after thermal annealing at 450 °C

Given that this LITV process is a thermal deposition mechanism, we aimed to investigate if incident laser power would significantly influence deposition thickness and thus allow us to control surface topography (Figure 3). The steady state temperature rise (ΔΤ) at the edge of the absorber in the laser focus can be estimated10,

25

as a

function of P0 (the average laser power), α (the absorptivity of the material deposited at 760 nm), w0 (the 1/e2 radius of the focused laser spot), and κ (the thermal conductivity of the glass substrate), where 𝛥𝑇 =

𝛼𝑃0 2𝑟0𝜅 𝜋

As incident laser power is increased, higher steady state temperatures can be reached thus expanding the volume of the thermal voxel and hence, we expected, the deposition thickness. We investigated two materials, Co3O4 and


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Mn2O3, in detail for their laser power and thickness correlation by translating the sample while continuously altering laser power during laser scanning. Laser powers reported were measured at the back of the 60x (1.4 NA) objective and material deposition thicknesses were analyzed by optical profilometry. Co3O4 deposited with a minimum thickness of 2 µm at 30 mW and reached a thickness plateau of approximately 10 µm at 165 mW (Figure 3a) with a linear dependence on incident laser power within this range. Similar results were observed with Mn2O3 (Figure 3a). Higher incident laser power did not contribute significantly to increases in thickness, which is expected because the laser is passing through the deposited film and thicker films block the irradiation. By dynamically tuning laser power while scanning, we could create varying surface topographies as seen in Figure 3b. A triangle pattern of Co3O4 was created by gradually varying the power of the scanning laser between 30 mW and 165 mW while translating the stage (Figure 3b(i)) and, similarly, a sawtooth pattern was created by slowly ramping up and then quickly decreasing the laser power during stage translation (Figure 3b(ii)). For both materials, changes in laser power contributed to changes in thickness at the rate of approximately 0.05 µm/mW. While we had previously demonstrated high-resolution patterning of diverse materials in 2D with the LITV technique10, we were interested in exploring whether we could integrate materials in layers to yield 2.5D structures, allowing more control over material interfaces which would be necessary for integrated devices. Because higher laser powers allow for thicker depositions, we hypothesized that layers of multiple materials may be possible by switching precursors and re-scanning over existing films with higher laser power. As shown in Figure 3c, we first deposited triangle or sawtooth patterns in Co3O4 from aqueous Co(NO3)2 and then exchanged the precursor with aqueous Mn(NO3)2, scanning the laser over the existing structure to “backfill” them with Mn2O3. Three, or more, layers could be created with this approach; we used scanning electron microscopy (SEM) and energy dispersive Xray spectroscopy (EDS) to examine a Co3O4 and Mn2O3 tri-layer cross-section prepared by focused ion beam milling (Figure 3d). This cross-section shows clear separation of the Mn and Co containing layers, indicating that this technique could be promising for the design of more complex integrated device architectures.



Figure 3. Topographical control and layering of oxides. a, Optical profilometry maps of Co3O4 and Mn2O3 as deposited from Co(NO3)2 and Mn(NO3)2, respectively, by translating the sample stage at 5 µm/s while simultaneously increasing the laser power from 30 mW to 165 mW at 0.2 mW/s as measured at the back of the 60x, 1.4 N.A. objective. The linear increase in power was correlated with a linear increase in film thickness within this power range yielding a thickness increase of approximately 0.05 µm/mW. The color scale shown is applicable to all figure sections. Scale, 100 μm. b, Co3O4 was patterned in (i) triangle and (ii) sawtooth topographic patterns. Side view and top view profilometry images are shown for each pattern. Scale, 100 μm. c, By rinsing out the Co(NO3)2 precursor and replacing it with Mn(NO3)2 and scanning over half the (i) triangle or (ii) sawtooth patterns from (b), we could “backfill” the Co3O4 trenches with Mn2O3. 45° and top view profilometry images are shown. A top-view SEM and corresponding energy dispersive X-ray spectroscopy (EDS) maps of Co and Mn elements. Scales, 100 μm. d, A tri-layer sectioned with a focused ion beam imaged with SEM and the corresponding EDS maps. Scale, 5 μm.

Since using homogenous precursor mixtures is trivial with this method, we investigated the possibility that the LITV process could lead to facile deposition of homogenous systems containing arbitrary ratios of metals or metal oxides. To test this, depositions from binary mixtures containing varying combinations of precursor salts—


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Cr(NO3)3, Ni(NO3)2, Cu(NO3)2, Co(NO3)3, Mn(NO3)2, Zn(NO3)2, Al(NO3)3, and Fe(NO3)3—were investigated using EDS and XRD. We observed that most depositions exhibited a homogenous distribution of metal atoms through the depth of the deposited layer at an atomic ratio consistent with that of the precursor (example given in Figure 4a), except for mixtures containing Fe(NO3)3 which instead displayed non-homogenous atomic distributions with a depletion of the non-iron element at the surface (Figure 4b). Furthermore, XRD analysis showed that iron-containing systems yielded a distinct mixed-metal oxide ferrite structure, while non-iron systems appear to create mixtures of the constituent metal oxides, although we note that the presence of a NiO phase in Figure 4c(i) does not rule out the possibility that some Ni substitution in Co3O4 occurred; the resulting NiCo2O4 would be difficult to distinguish from Co3O4 in XRD. Future studies aim to investigate the role of precursor atomic ratio and deposition power on the thickness of the depleted layer in iron mixtures to further understand what causes the observed non-homogeneity.

Figure 4. EDS of mixed metal oxide cross sections and XRD patterns. Cross-sectional SEM micrographs and EDS spectra of films on glass substrates deposited from a, a 2:1 molar ratio Ni(NO3)2 : Co(NO3)3 precursor mixture, and b, a 2:1 molar ratio Fe(NO3)3 : Cu(NO3)2 precursor mixture. Although the atomic ratios from EDS in (a) were found to be 2:1 Co:Ni, matching the molar ratio of the precursors, the atomic ratios for (b) were 50:1 Fe:Cu in the copper depleted surface layer and 2:1 Fe:Cu in the bulk. Copper EDS signal present in the glass substrate as seen in (b) is due to overlap of copper Lα emission with sodium Kα emission. Particular interest is taken in the depletion of



copper at the surface of mixtures containing iron, such as in (b), while no such depletion is seen for mixtures without iron, such as in (a). c, XRD patterns of materials from (a and b), respectively. c(i) shows two distinguishable oxides, while c(ii) shows the formation of a new mixed oxide ferrite structure. Scale bars are 10 µm.

In summary, we have described a method for the direct-writing of various metal and metal oxide materials directly from aqueous precursor salt solutions using a continuous wave Ti:Sapphire laser to create a high temperature laser-induced thermal voxel (LITV). We conducted chemical and structural characterization of a wide range of amorphous to nanocrystalline metals and oxides via Raman, XRD, and EDS analysis, and found that most products were consistent with what would have been predicted from thermal decomposition, with notable exceptions for the vanadium, iron, and molybdenum salts. We explored in-depth the thermal transitions within iron oxides to demonstrate the fabrication of metastable phases, and using cross-sectional imaging we probed the chemical composition through the depth of the deposited layers. Given the diversity of inorganic materials accessible with this approach, we suspect that it will be of use for applications benefiting from high degrees of customization or structural and chemical complexity, such as devices with multiple mixed oxides for combinatorial sensing or catalysts with tailored chemical and morphological structure.

Associated Content The Supporting Information is available through the ACS Publications website. Supporting Information includes detailed experimental methods, XRD, Raman spectra, thermal conductivity data, and line width data.

Acknowledgments C.K., A.C., S.M., and L.D.Z. gratefully acknowledge support from the Department of Chemistry and the Materials Research Institute at Penn State, and the 3M Non-Tenured Faculty Award. J.A.T. and P.E.H. appreciate support from the Office of Naval Research, Grant # N00014-15-1-2769.

References

1. Ivanova, O.; Williams, C.; Campbell, T., Additive manufacturing (AM) and nanotechnology: promises and challenges. Rapid Prototyping Journal 2013, 19 (5), 353-364. 12 ACS Paragon Plus Environment

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