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Directed assembly of nanomaterials using electrospray deposition and substrate-level patterning
Powder Technology 364 (2020) 845–850
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short Communication
Directed assembly of nanomaterials using electrospray deposition and substrate-level patterning Yaqun Zhu, Paul R. Chiarot ⁎ Department of Mechanical Engineering, State University of New York at Binghamton, 4400 Vestal Pkwy E, Binghamton, NY 13902, USA
a r t i c l e
i n f o
Article history: Received 21 May 2019 Received in revised form 22 October 2019 Accepted 22 January 2020 Available online 24 January 2020 Keywords: Electrospray Directed assembly Patterning High aspect ratio Nanoparticles
a b s t r a c t We report on the use of electrospray deposition and electrostatic focusing provided by a patterned photoresist layer to direct the assembly of nanomaterials on to a target substrate. Metallic (silver), semiconducting (titanium dioxide), and dielectric (polystyrene) particles were electrospray printed onto silicon and glass substrates with periodic photoresist patterns. The assembly and structure of the deposit was governed by the spray time, print material, substrate electrical properties, and geometry of the photoresist layer. The deposited particles could maintain an excess electric charge that influenced the assembly of subsequent particles. Tall, tree-like structures with high aspect ratio were formed on the silicon substrate when using silver and titanium dioxide particles. The growth of the deposit was inhibited for the polystyrene particles or when the target was glass. This technique was used to build two-layer deposits of different materials without having to align one layer to the next. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Electrospray deposition has unique capabilities for depositing (i.e. printing) nanoscale materials. In an electrospray, high potential is used to atomize a liquid suspension that may contain a diverse range of nanomaterials. For a sufficiently volatile solvent, the small size of the droplets emitted by electrospray ensures that material is delivered dry (i.e. solvent-free) to the target substrate. The material versatility of electrospray is highlighted by its use in creating films of ceramics, metals, polymers, and biological materials [1–4]. While the creation of films with multiscale structure is important for numerous applications, it is also desirable to create layers consisting of 2-dimensional patterns. Traditionally, this can be achieved using evaporation or sputtering with chemical etching [5], but as a subtractive process this is inherently wasteful and materials restricted [6]. Two unique characteristics of electrospray printing is the high charge of the emitted droplets (particles) and the important role the substrate electrical properties play in governing the deposit morphology. In this work, we leverage these two features to direct the assembly of nanoscale materials to create 2-dimensional patterns on a target substrate. Various methods have been developed to create patterns of charged particles on a substrate. Woven mesh fibers have been used as shadow masks to create periodic deposits [7,8]. Fine lines and periodic nanoscale features can be created using substrate-level charge [9–11] and photoresist [12–15] patterning. ⁎ Corresponding author. E-mail address: [email protected] (P.R. Chiarot).
https://doi.org/10.1016/j.powtec.2020.01.066 0032-5910/© 2020 Elsevier B.V. All rights reserved.
Here we utilize electrospray to deliver metallic (silver, Ag), semiconducting (titanium dioxide, TiO2), and dielectric (polystyrene, PS) particles to a target substrate (silicon, Si or glass) patterned with a layer of photoresist (PR). We focus on the creation of straight lines and evaluate their structure across a range of operating parameters. During electrospray, charge accumulates on the PR surface, creating an electrostatic lens that focuses the nanoscale materials onto the uncoated portion of the target substrate [12–15] (the trench, Fig. 1). Our process is capable of creating thin lines (b10 μm) with high aspect ratios (heights over 50 μm can be achieved) from a variety of different materials. Depending on the material type and processing conditions, the lines can be highly porous. High aspect ratio structures support numerous applications, most notably in the electronics industry [16,17]. Using electrospray with substrate-level patterning, multi-layered and multi-material structures can be built in a layer-by-layer fashion without the challenge of aligning each subsequently deposited layer. Layer-by-layer production of multiple materials has applications in catalysis, optics, energy, membranes, fluid transport, and biomedicine [18,19]. 2. Materials and methods The PR layer was spin coated (thickness of ~6.5 μm) on Si or glass and patterned using photolithography. We created trenches in the PR with a width of ~20 μm. Ag powders (b100 nm), TiO2 particles (average size in precursor ~150 nm), and PS particles (100 nm) were suspended in ethanol with the same volume concentration of 0.05%. Details on our electrospray deposition process is described elsewhere [20,21]. Briefly, the electrospray originated from a glass capillary tube that was pulled
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Fig. 1. (a) Particle deposition using electrospray. Charge accumulation on the PR surface generates convex equipotential lines, which act as electrostatic lenses to direct charged particles onto the exposed substrate surface. (b) Top and (c) cross-sectional SEM images of a thermally sintered Ag line on Si after a PR removal process.
to make a nozzle with outer and inner diameters of 120 and 75 μm, respectively. The nanoparticle suspension was provided to the nozzle via a 3-port manifold at 0.2 μl/min. The nozzle-target separation distance was 15 mm. High voltage (2 kV) was applied through a port in the manifold to generate the electrospray. The emitted droplets evaporated in-flight, forming aggregates [21], which were delivered dry to the target substrate. Depending on the electrical conductivity of the substrate [20–22] and the print material, the deposit can maintain an excess electric charge that influences subsequent material arriving at the surface. The spray time was varied to control the height of the deposits. Once printing of the Ag or TiO2 particles was completed, acetone can be used to remove the PR layer, followed by thermal sintering (Fig. 1). The Ag deposit was sintered at 300 °C for 30 min.
3. Results and discussion The Ag particle suspension was electrosprayed on to PRpatterned Si substrates for spray times from 1 min to 10 min. With a stationary nozzle, material can be delivered over an area of more than 3 cm2. Fig. 2 shows the evolution of the deposited pattern for increasing spray time. The electrostatic focusing effect drove the assembly of the particles on to the exposed substrate to create a projected line width of ~8.5 μm after 10 min. The line length along the trench reached ~20 mm for a spray time of 10 min with a stationary nozzle. The line height was maximum in the center region of the deposit, i.e. under the nozzle. We focus on the center region to probe the evolution of the deposit. Over time, a “tree-like”
Fig. 2. Structure of Ag deposits for spray times of (a) 1 min, (b) 3 min, (c) 5 min, and (d) 10 min. The PR thickness was 6.5 μm. The height of the deposit increased linearly with spray time at a rate of approximately 2.5 μm/min. Top panels are top views and bottom panels are cross-sectional views. Bars are 5 μm.
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Fig. 3. (a) Reducing the PR layer thickness to 1 μm caused the Ag particles to fill the trench from nearly edge-to-edge. (b) When glass is the target substrate, the final deposit is limited to a few particle layers. This is due to the slower dissipation of the excess charge compared to the Si substrate. (c) Periodically activating and deactivating the electrospray provides additional time for the excess charge to dissipate, creating a shorter and wider deposit. Top panels are top views and bottom panels are cross-sectional views. Bars are 5 μm.
structure developed, growing in both height and width. Early in the spray, particles were deposited everywhere, indicating nearly flat equipotential lines above the substrate. As charge accumulated on the PR surface, the equipotential lines became convex above the trench and the electric field lines converged onto the exposed substrate. This decreased the deposit width to form the narrow trunk at the base of the deposit. The trunk attracted additional Ag particles to the growing layer since electrical continuity was provided to the underlying substrate, allowing the excess charge to rapidly dissipate. Particles were attracted at all angles to the evolving layer, forming the branching region of the tree. The formation of the tree-like structure is comparable to the islands formed during film printing reported by our group [21] and others [23–25]. Heterogeneities that form on the film surface during electrospray deposition force additional material to collect, locally increasing the height to form islands [26]. Here, this effect was controlled and amplified by modulating the surface potential using a PRpatterned substrate. The thickness of the PR layer had a significant effect on the deposit structure. Reducing the thickness to 1 μm caused the Ag particles to fill the trench from nearly edge-to-edge (Fig. 3a). The electric field lines converged less for thinner PR layers, weakening the focusing effect. This inhibited the formation of the tree-like structure and the deposit grew more uniformly. The electrical properties of the substrate also govern the deposit structure by regulating the charge dissipation rate [20,21]. When the target substrate was glass
(Fig. 3b), material still assembled in the trench, but there was no tree-like structure and the final deposit was limited to a few particle layers. The reduction in the deposit height was balanced by a larger deposit size compared to the Si substrate. In fact, many particles were found to collect at the edge of the glass substrate on the underlying metal counter-electrode. The higher electrical conductivity of Si permits more rapid charge dissipation and deposit growth. For glass, the excess charge on the deposited particles is unable to dissipate at a sufficient rate relative to the arrival rate of the particles. Due to Coulombic repulsion, the thickness of the deposit is limited and the deposit grows laterally across the glass substrate surface. However, even for glass, the number of particles deposited on to the PR layer is negligible. To further probe the influence of the excess charge on the deposit morphology, material was delivered to a Si target by periodically activating and deactivating the electrospray. This process provided additional time for the accumulated charge on the surface to dissipate. Each period consisted of a 15 s spray followed by a 2minute rest. A total of 40 cycles was used to match the 10-min spray time for the continuous electrospray process in Fig. 2d. A 14.5-μm high tree with a wider trunk and larger crown was formed (Fig. 3c) compared to the 21.4-μm high tree in Fig. 2d. This indicates that the extra time for charge dissipation flattens the convex equipotential lines above the trench. Therefore, each period starts with a wider deposition area across the trench, resulting in a shorter, wider tree for the same amount of mass loading. These results
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Fig. 4. Top and cross-sectional SEM images of (a) PS and (b, c) TiO2 deposits for (b) 5 min and (a, c) 10 min sprays on Si substrates. Bars are 5 μm.
highlight the importance of accumulated charge and substrate electrical properties in determining the deposit structure. The electrical property of the sprayed material also governs the deposit structure. To compare with the Ag deposits, we electrosprayed PS and TiO2 particles at the same operating parameters onto PRpatterned Si. The PS particles filled the trench from edge-to-edge and the deposit height was limited to only a few particle diameters (Fig. 4a). The low electrical conductivity of PS prevented the excess charge from dissipating once the particles were deposited into the trench. This inhibited the deposition of additional particles, similar to the deposition of Ag particles onto glass (Fig. 3b). The TiO2 deposit had a height of 15.8 μm and 40 μm and projected width of 6.7 μm and 16 μm after 5 min and 10 min of spraying, respectively (Fig. 4b and c). The morphology of the Ag and TiO2 deposits is comparable; for example, both possess a tree-like structure with a narrow trunk and wide crown. The trunk initially formed due to the focusing effect from the PR layer. Then, once it was established, the trunk attracted additional material since the field lines concentrated at its top surface. However, there are notable differences in deposit structure for the two materials due to their difference in electrical conductivity (the sheet resistance of electrosprayed Ag and TiO2 thin films before sintering is 4.5 × 103 Ω/□ and 1.4 × 1010 Ω/□, respectively). The Ag particles tended to fill a large portion of the trench including along its sidewalls, while the TiO2 particles created only a thin layer along the trench. The Ag particles assembled on the growing tree, but also had the opportunity to deposit at the base due to the rapid dissipation of charge for the deposited particles. In contrast, the TiO2 particles landed preferentially at the apex of
the growing tree and not at the base due to the slower dissipation of charge. This variation in particle accumulation accounts for the difference in height between the Ag and TiO2 deposits. We also contend that it accounts for the difference in microscale morphology along the surface of the tree. The Ag particles deposited everywhere along the surface of the tree, while the TiO2 particles deposited on the top, creating a smoother surface along the deposit height. A unique feature of this directed assembly approach is the ability to create multi-layered deposits with different materials without aligning one layer to the next. By leaving the PR layer in place, materials can be stacked using electrostatic focusing. We created 2-layer patterns by depositing TiO2 on top of Ag (Fig. 5a-d) and Ag on top of TiO2 (Fig. 5eh) using 5 min sprays for each material. Backscattered electron (BSE) imaging and X-ray spectroscopy (SEM-EDX) mapping show that the second layer forms a cap on the first. The interval between delivery of these two materials was over 5 min, which was sufficient for the excess charge on the first layer to dissipate. 2-layer printing highlights the differences in morphology for Ag and TiO2 deposits first seen in Figs. 2 and 4. The morphology of the first layer in Fig. 5 (Ag or TiO2) is comparable to the single layer deposit. When the second layer is TiO2, material almost exclusively assembled on top of the Ag trunk and produced a deposit with a smooth sidewall. When the second layer is Ag, a large amount of material preferentially deposited near the top of the first layer, but a significant amount also approached the trunk from all angles and accumulated over the entire TiO2 trunk. Ultimately, the structure of the 2-layer deposits share similar features to single layer deposits for both material systems.
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Fig. 5. 2-layer deposit composed of Ag and TiO2 particles on Si substrates. (a)-(d) Ag particles were deposited for 5 min followed by TiO2 for 5 min. The TiO2 particles deposit almost exclusively on top of the Ag trunk to form a cap. (e)-(h) TiO2 particles were deposited for 5 min followed by Ag for 5 min. Ag preferentially deposits near the top of the TiO2 layer, but a significant amount also approaches the trunk from all angles. (a, e) Top view and (b, f) cross-sectional SEM images with (c, g) BSE and (d, h) SEM-EDX mapping of the 2-layer deposits. Bars are 5 μm.
4. Conclusions
Acknowledgements
We have shown that electrospray deposition can be used to assemble nanoscale materials onto a substrate with electrostatic focusing provided by PR patterns. The structure of the deposit was governed by the spray time, print material, substrate electrical properties, and geometry of the PR layer. The width of the deposited features was smaller than the original PR pattern and the technique was not limited by the target substrate material. To the best of our knowledge, this work represents the first study on the formation of high aspect ratio features using electrospray deposition with electrostatic focusing provided by PR patterns. Compared to traditional sputtering or evaporation followed by the lift-off process, electrospray deposition is more materials conservative. SEM imaging showed that the number of particles on the PR layer after 10 min of electrospraying was very low, in particular in regions away from the trench. For the Ag and TiO2 case, we estimate that the mass of the sprayed material was of the same order of magnitude as the mass delivered to the trench. This was determined by considering the dimensions and porosity (estimated from SEM images) of the deposited features. An important issue associated with the high aspect ratio structures is the mechanical strength, which can be improved through post processing (e.g. sintering for the Ag and TiO2 deposits). This method can also be used for the layer-by-layer deposition of multiple materials, without the need of registering one layer to the next. Applications of high aspect ratio and/or highly porous multi-material structures are found in numerous industries.
Support for this research was provided by the National Science Foundation (CAREER Award #1554038).
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Short Communication
Directed assembly of nanomaterials using electrospray deposition and substrate-level patterning Yaqun Zhu, Paul R. Chiarot ⁎ Department of Mechanical Engineering, State University of New York at Binghamton, 4400 Vestal Pkwy E, Binghamton, NY 13902, USA
a r t i c l e
i n f o
Article history: Received 21 May 2019 Received in revised form 22 October 2019 Accepted 22 January 2020 Available online 24 January 2020 Keywords: Electrospray Directed assembly Patterning High aspect ratio Nanoparticles
a b s t r a c t We report on the use of electrospray deposition and electrostatic focusing provided by a patterned photoresist layer to direct the assembly of nanomaterials on to a target substrate. Metallic (silver), semiconducting (titanium dioxide), and dielectric (polystyrene) particles were electrospray printed onto silicon and glass substrates with periodic photoresist patterns. The assembly and structure of the deposit was governed by the spray time, print material, substrate electrical properties, and geometry of the photoresist layer. The deposited particles could maintain an excess electric charge that influenced the assembly of subsequent particles. Tall, tree-like structures with high aspect ratio were formed on the silicon substrate when using silver and titanium dioxide particles. The growth of the deposit was inhibited for the polystyrene particles or when the target was glass. This technique was used to build two-layer deposits of different materials without having to align one layer to the next. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Electrospray deposition has unique capabilities for depositing (i.e. printing) nanoscale materials. In an electrospray, high potential is used to atomize a liquid suspension that may contain a diverse range of nanomaterials. For a sufficiently volatile solvent, the small size of the droplets emitted by electrospray ensures that material is delivered dry (i.e. solvent-free) to the target substrate. The material versatility of electrospray is highlighted by its use in creating films of ceramics, metals, polymers, and biological materials [1–4]. While the creation of films with multiscale structure is important for numerous applications, it is also desirable to create layers consisting of 2-dimensional patterns. Traditionally, this can be achieved using evaporation or sputtering with chemical etching [5], but as a subtractive process this is inherently wasteful and materials restricted [6]. Two unique characteristics of electrospray printing is the high charge of the emitted droplets (particles) and the important role the substrate electrical properties play in governing the deposit morphology. In this work, we leverage these two features to direct the assembly of nanoscale materials to create 2-dimensional patterns on a target substrate. Various methods have been developed to create patterns of charged particles on a substrate. Woven mesh fibers have been used as shadow masks to create periodic deposits [7,8]. Fine lines and periodic nanoscale features can be created using substrate-level charge [9–11] and photoresist [12–15] patterning. ⁎ Corresponding author. E-mail address: [email protected] (P.R. Chiarot).
https://doi.org/10.1016/j.powtec.2020.01.066 0032-5910/© 2020 Elsevier B.V. All rights reserved.
Here we utilize electrospray to deliver metallic (silver, Ag), semiconducting (titanium dioxide, TiO2), and dielectric (polystyrene, PS) particles to a target substrate (silicon, Si or glass) patterned with a layer of photoresist (PR). We focus on the creation of straight lines and evaluate their structure across a range of operating parameters. During electrospray, charge accumulates on the PR surface, creating an electrostatic lens that focuses the nanoscale materials onto the uncoated portion of the target substrate [12–15] (the trench, Fig. 1). Our process is capable of creating thin lines (b10 μm) with high aspect ratios (heights over 50 μm can be achieved) from a variety of different materials. Depending on the material type and processing conditions, the lines can be highly porous. High aspect ratio structures support numerous applications, most notably in the electronics industry [16,17]. Using electrospray with substrate-level patterning, multi-layered and multi-material structures can be built in a layer-by-layer fashion without the challenge of aligning each subsequently deposited layer. Layer-by-layer production of multiple materials has applications in catalysis, optics, energy, membranes, fluid transport, and biomedicine [18,19]. 2. Materials and methods The PR layer was spin coated (thickness of ~6.5 μm) on Si or glass and patterned using photolithography. We created trenches in the PR with a width of ~20 μm. Ag powders (b100 nm), TiO2 particles (average size in precursor ~150 nm), and PS particles (100 nm) were suspended in ethanol with the same volume concentration of 0.05%. Details on our electrospray deposition process is described elsewhere [20,21]. Briefly, the electrospray originated from a glass capillary tube that was pulled
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Fig. 1. (a) Particle deposition using electrospray. Charge accumulation on the PR surface generates convex equipotential lines, which act as electrostatic lenses to direct charged particles onto the exposed substrate surface. (b) Top and (c) cross-sectional SEM images of a thermally sintered Ag line on Si after a PR removal process.
to make a nozzle with outer and inner diameters of 120 and 75 μm, respectively. The nanoparticle suspension was provided to the nozzle via a 3-port manifold at 0.2 μl/min. The nozzle-target separation distance was 15 mm. High voltage (2 kV) was applied through a port in the manifold to generate the electrospray. The emitted droplets evaporated in-flight, forming aggregates [21], which were delivered dry to the target substrate. Depending on the electrical conductivity of the substrate [20–22] and the print material, the deposit can maintain an excess electric charge that influences subsequent material arriving at the surface. The spray time was varied to control the height of the deposits. Once printing of the Ag or TiO2 particles was completed, acetone can be used to remove the PR layer, followed by thermal sintering (Fig. 1). The Ag deposit was sintered at 300 °C for 30 min.
3. Results and discussion The Ag particle suspension was electrosprayed on to PRpatterned Si substrates for spray times from 1 min to 10 min. With a stationary nozzle, material can be delivered over an area of more than 3 cm2. Fig. 2 shows the evolution of the deposited pattern for increasing spray time. The electrostatic focusing effect drove the assembly of the particles on to the exposed substrate to create a projected line width of ~8.5 μm after 10 min. The line length along the trench reached ~20 mm for a spray time of 10 min with a stationary nozzle. The line height was maximum in the center region of the deposit, i.e. under the nozzle. We focus on the center region to probe the evolution of the deposit. Over time, a “tree-like”
Fig. 2. Structure of Ag deposits for spray times of (a) 1 min, (b) 3 min, (c) 5 min, and (d) 10 min. The PR thickness was 6.5 μm. The height of the deposit increased linearly with spray time at a rate of approximately 2.5 μm/min. Top panels are top views and bottom panels are cross-sectional views. Bars are 5 μm.
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Fig. 3. (a) Reducing the PR layer thickness to 1 μm caused the Ag particles to fill the trench from nearly edge-to-edge. (b) When glass is the target substrate, the final deposit is limited to a few particle layers. This is due to the slower dissipation of the excess charge compared to the Si substrate. (c) Periodically activating and deactivating the electrospray provides additional time for the excess charge to dissipate, creating a shorter and wider deposit. Top panels are top views and bottom panels are cross-sectional views. Bars are 5 μm.
structure developed, growing in both height and width. Early in the spray, particles were deposited everywhere, indicating nearly flat equipotential lines above the substrate. As charge accumulated on the PR surface, the equipotential lines became convex above the trench and the electric field lines converged onto the exposed substrate. This decreased the deposit width to form the narrow trunk at the base of the deposit. The trunk attracted additional Ag particles to the growing layer since electrical continuity was provided to the underlying substrate, allowing the excess charge to rapidly dissipate. Particles were attracted at all angles to the evolving layer, forming the branching region of the tree. The formation of the tree-like structure is comparable to the islands formed during film printing reported by our group [21] and others [23–25]. Heterogeneities that form on the film surface during electrospray deposition force additional material to collect, locally increasing the height to form islands [26]. Here, this effect was controlled and amplified by modulating the surface potential using a PRpatterned substrate. The thickness of the PR layer had a significant effect on the deposit structure. Reducing the thickness to 1 μm caused the Ag particles to fill the trench from nearly edge-to-edge (Fig. 3a). The electric field lines converged less for thinner PR layers, weakening the focusing effect. This inhibited the formation of the tree-like structure and the deposit grew more uniformly. The electrical properties of the substrate also govern the deposit structure by regulating the charge dissipation rate [20,21]. When the target substrate was glass
(Fig. 3b), material still assembled in the trench, but there was no tree-like structure and the final deposit was limited to a few particle layers. The reduction in the deposit height was balanced by a larger deposit size compared to the Si substrate. In fact, many particles were found to collect at the edge of the glass substrate on the underlying metal counter-electrode. The higher electrical conductivity of Si permits more rapid charge dissipation and deposit growth. For glass, the excess charge on the deposited particles is unable to dissipate at a sufficient rate relative to the arrival rate of the particles. Due to Coulombic repulsion, the thickness of the deposit is limited and the deposit grows laterally across the glass substrate surface. However, even for glass, the number of particles deposited on to the PR layer is negligible. To further probe the influence of the excess charge on the deposit morphology, material was delivered to a Si target by periodically activating and deactivating the electrospray. This process provided additional time for the accumulated charge on the surface to dissipate. Each period consisted of a 15 s spray followed by a 2minute rest. A total of 40 cycles was used to match the 10-min spray time for the continuous electrospray process in Fig. 2d. A 14.5-μm high tree with a wider trunk and larger crown was formed (Fig. 3c) compared to the 21.4-μm high tree in Fig. 2d. This indicates that the extra time for charge dissipation flattens the convex equipotential lines above the trench. Therefore, each period starts with a wider deposition area across the trench, resulting in a shorter, wider tree for the same amount of mass loading. These results
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Fig. 4. Top and cross-sectional SEM images of (a) PS and (b, c) TiO2 deposits for (b) 5 min and (a, c) 10 min sprays on Si substrates. Bars are 5 μm.
highlight the importance of accumulated charge and substrate electrical properties in determining the deposit structure. The electrical property of the sprayed material also governs the deposit structure. To compare with the Ag deposits, we electrosprayed PS and TiO2 particles at the same operating parameters onto PRpatterned Si. The PS particles filled the trench from edge-to-edge and the deposit height was limited to only a few particle diameters (Fig. 4a). The low electrical conductivity of PS prevented the excess charge from dissipating once the particles were deposited into the trench. This inhibited the deposition of additional particles, similar to the deposition of Ag particles onto glass (Fig. 3b). The TiO2 deposit had a height of 15.8 μm and 40 μm and projected width of 6.7 μm and 16 μm after 5 min and 10 min of spraying, respectively (Fig. 4b and c). The morphology of the Ag and TiO2 deposits is comparable; for example, both possess a tree-like structure with a narrow trunk and wide crown. The trunk initially formed due to the focusing effect from the PR layer. Then, once it was established, the trunk attracted additional material since the field lines concentrated at its top surface. However, there are notable differences in deposit structure for the two materials due to their difference in electrical conductivity (the sheet resistance of electrosprayed Ag and TiO2 thin films before sintering is 4.5 × 103 Ω/□ and 1.4 × 1010 Ω/□, respectively). The Ag particles tended to fill a large portion of the trench including along its sidewalls, while the TiO2 particles created only a thin layer along the trench. The Ag particles assembled on the growing tree, but also had the opportunity to deposit at the base due to the rapid dissipation of charge for the deposited particles. In contrast, the TiO2 particles landed preferentially at the apex of
the growing tree and not at the base due to the slower dissipation of charge. This variation in particle accumulation accounts for the difference in height between the Ag and TiO2 deposits. We also contend that it accounts for the difference in microscale morphology along the surface of the tree. The Ag particles deposited everywhere along the surface of the tree, while the TiO2 particles deposited on the top, creating a smoother surface along the deposit height. A unique feature of this directed assembly approach is the ability to create multi-layered deposits with different materials without aligning one layer to the next. By leaving the PR layer in place, materials can be stacked using electrostatic focusing. We created 2-layer patterns by depositing TiO2 on top of Ag (Fig. 5a-d) and Ag on top of TiO2 (Fig. 5eh) using 5 min sprays for each material. Backscattered electron (BSE) imaging and X-ray spectroscopy (SEM-EDX) mapping show that the second layer forms a cap on the first. The interval between delivery of these two materials was over 5 min, which was sufficient for the excess charge on the first layer to dissipate. 2-layer printing highlights the differences in morphology for Ag and TiO2 deposits first seen in Figs. 2 and 4. The morphology of the first layer in Fig. 5 (Ag or TiO2) is comparable to the single layer deposit. When the second layer is TiO2, material almost exclusively assembled on top of the Ag trunk and produced a deposit with a smooth sidewall. When the second layer is Ag, a large amount of material preferentially deposited near the top of the first layer, but a significant amount also approached the trunk from all angles and accumulated over the entire TiO2 trunk. Ultimately, the structure of the 2-layer deposits share similar features to single layer deposits for both material systems.
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Fig. 5. 2-layer deposit composed of Ag and TiO2 particles on Si substrates. (a)-(d) Ag particles were deposited for 5 min followed by TiO2 for 5 min. The TiO2 particles deposit almost exclusively on top of the Ag trunk to form a cap. (e)-(h) TiO2 particles were deposited for 5 min followed by Ag for 5 min. Ag preferentially deposits near the top of the TiO2 layer, but a significant amount also approaches the trunk from all angles. (a, e) Top view and (b, f) cross-sectional SEM images with (c, g) BSE and (d, h) SEM-EDX mapping of the 2-layer deposits. Bars are 5 μm.
4. Conclusions
Acknowledgements
We have shown that electrospray deposition can be used to assemble nanoscale materials onto a substrate with electrostatic focusing provided by PR patterns. The structure of the deposit was governed by the spray time, print material, substrate electrical properties, and geometry of the PR layer. The width of the deposited features was smaller than the original PR pattern and the technique was not limited by the target substrate material. To the best of our knowledge, this work represents the first study on the formation of high aspect ratio features using electrospray deposition with electrostatic focusing provided by PR patterns. Compared to traditional sputtering or evaporation followed by the lift-off process, electrospray deposition is more materials conservative. SEM imaging showed that the number of particles on the PR layer after 10 min of electrospraying was very low, in particular in regions away from the trench. For the Ag and TiO2 case, we estimate that the mass of the sprayed material was of the same order of magnitude as the mass delivered to the trench. This was determined by considering the dimensions and porosity (estimated from SEM images) of the deposited features. An important issue associated with the high aspect ratio structures is the mechanical strength, which can be improved through post processing (e.g. sintering for the Ag and TiO2 deposits). This method can also be used for the layer-by-layer deposition of multiple materials, without the need of registering one layer to the next. Applications of high aspect ratio and/or highly porous multi-material structures are found in numerous industries.
Support for this research was provided by the National Science Foundation (CAREER Award #1554038).
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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