Plasma-based processes for planar and 3D surface patterning of functional nanoparticles
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We present a gas-phase process for surface patterning and 3D self-assembly of nanoparticles (NPs) of functional materials such as metals, oxides, and nitrides. The method relies on electrostatic assembly of free-flying NPs with unipolar charge produced in plasma sources. We demonstrate the capability of the process in self-assembly of NPs, with the size in the range 10–60 nm, into arrays of free-standing 3D microstructures with complex morphologies. Considering that the plasma nanoparticle sources are compatible with synthesis of a large library of material NPs, the process introduces a novel approach for 3D printing of various functional NPs, high-precision device integration of NPs on sub-micrometer scales, and large-area parallel surface patterning of NPs.
KeywordsPlasma Sputtering Functional nanoparticles 3D assembly
Directed self-assembly of functional nanoparticles (NPs) has emerged as a powerful bottom-up nano-synthesis approach for creating novel functional materials and surfaces, integrating NPs in miniaturized devices, and also 3D printing of functional materials (Boles et al. 2016; Grzelczak et al. 2010; Yang et al. 2016; Flauraud et al. 2017; Gwo et al. 2016; Huang et al. 2016; Su et al. 2014). A variety of devices have been reported using NP assembly techniques including novel optoelectronic devices (e.g., light-emitting diodes, sensors, solar cells, photodetectors, and transistors), optical and plasmon sensing devices, electrodes in nanoscale electrochemistry, printable chips, nano-pore biosensors, and transparent electrodes (Grzelczak et al. 2010; Yang et al. 2016; Flauraud et al. 2017; Gwo et al. 2016; Huang et al. 2016; Su et al. 2014; Lewis and Ahn 2015; Choi et al. 2016; Xie et al. 2015; Ellis et al. 2014; Momotenko et al. 2016; Blasco et al. 2016). In response to the growing demand in the field, diverse techniques have been developed for 3D self-assembly of ligand-protected colloidal NPs including lithographic patterning, template-based methods, DNA-mediated assembly, dip-pen processes, and capillary assembly (Su et al. 2014; Xie et al. 2015), and also for 2D surface patterning such as etching of close-packed structures and atomic layer deposition (ALD) (Wang et al. 2018; Dendooven et al. 2017; Schmudde et al. 2016; Hou et al. 2018). In 3D printing technology, NPs are now widely used for 3D printing in ink-jet printers that are compatible with printing polymer and metals using colloidal inks (Hirt et al. 2017). The general issues limiting the techniques are the complexity of the NP synthesis (by multi-step chemical methods) and its coupling into the assembly technique, the complexity of the assembly techniques, limited control over geometry especially in 3D architectures, limitations involved with the range of materials, and also limited spatial resolution of the methods.
In a different approach, free-flying NPs produced in a flow-through gas-phase process can be directed onto desired positions with a high spatial resolution (Ma et al. 2007; Fonzo et al. 2000; Popok et al. 2011; Vece 2019; Wegner et al. 2012; Barborini et al. 2008). A common approach is to focus the beam of NPs, produced in the gas phase, through an aerodynamic lens apparatus that allows localized deposition of NPs. Among the gas-phase methods, non-thermal plasma processes have emerged as a viable approach for synthesis of NPs of various functional materials (Kortshagen et al. 2016). NPs extracted from plasmas are negatively charged, which leads to the suppression of NP agglomeration, a serious issue in other gas-phase production methods. Furthermore, it is feasible to use long-range forces such as electric and/or magnetic forces (for magnetic nanoparticles) to manipulate NP trajectory toward the substrate (Ekeroth et al. 2018).
The idea of flow-through high-resolution 3D self-assembly of functional NPs pursued here relies on efficient charging of free-flying NPs and their directed positioning using a designed electric-field configuration placed on a substrate in the vicinity of the assembly zone. In the present work, we employ a hollow cathode pulsed plasma sputtering technique for controlled growth of NPs. In the plasma, the high mobility of the electrons will cause the NPs to become negatively charged. Highly reactive plasma chemistry is generated in the plasma because of the large population of energetic electrons while the gas temperature remains close to ambient temperature. Fast growth of NPs occurs because of the low temperature of the plasma and rapid accumulation of ions onto negatively charged NPs. The simplicity of sputtering from a solid target provides access to a vast library of functional materials including metals, alloys, oxides, and nitrides. Our previous studies show that the method is highly flexible in terms of control over size, chemical composition, and optoelectronic properties of the NPs (Ekeroth et al. 2018; Askari et al. 2018; Pilch et al. 2013; Gunnarsson et al. 2015; Ekeroth et al. 2019).
In non-thermal low-pressure plasmas, electrons have significantly higher thermal velocity than ions because of their higher temperature and lower mass. The electron temperature (Te) in non-thermal plasmas is typically higher than 1 eV (e.g., 1–5 eV), while the ion temperature could be as low as room temperature (typically lower than 0.1 eV). For an object floating inside a plasma, the net (ion + electron) current to the object is 0. Therefore, the electric potential of a floating object relative to the plasma will remain negative so that the excess collection of electrons is retarded. A typical potential for a flat surface (i.e., a wall) is ~4.7 Te eV and for a spherical nanoparticle around 2.4 Te eV (Kortshagen et al. 2016). Operating in a vacuum brings the advantage of efficient charging of NPs in plasma and preserving the charge over long distance from the plasma. The nucleation and growth of InN NPs inside the plasma have been discussed in our previous report (Askari et al. 2018). We expect that a large percentage of the sputtered material are converted into NPs depending on the operating parameters. Owing to the NP charging, positive ions are collected into the growing NPs significantly faster than the neutral atoms. Apart from the NPs, any surface in contact with the plasma will acquire electric charges from it, which can be beneficially used for self-assembly of NPs onto the substrate.
An electrically isolated enclosure around the plasma was used for guiding NPs toward the substrate (Fig. 1a). The enclosure is charged by plasma and therefore helps guide NPs toward the substrate. Several techniques are used for characterizing synthesized NPs of Ag, InN, and ZnO. It includes scanning electron microscopy (SEM) and X-ray diffraction (XRD) for material characterization, photoluminescence (PL) and UV-visible absorption spectroscopy for optical characterization, and also electrical measurements of the NPs (see SI document).
Results and discussion
Figure 1b and c show SEM images of Ag NPs deposited on electrically isolated silicon wafers. For preparing the samples, silicon substrate is placed 5 cm below the anode ring inside the metal enclosure. The deposition time was 30 s. Deposition of NPs formed in the plasma on the substrate is not expected as the substrate is negatively charged. It is likely that ultra-small ~ 1-nm nanoparticles and clusters formed inside the plasma that are not necessarily negatively charged play a major role in formation of such isolated nanoparticles on the substrate.
Surfaces covered with non-close-packed 2D arrangement of NPs are attractive for applications such as optical devices, bio-medical sensors, and catalysis. Several methods have been reported for preparation of such functional surfaces including colloidal lithography and atomic layer deposition (ALD) (Dendooven et al. 2017; Schmudde et al. 2016; Hou et al. 2018). Interestingly, our plasma sputtering method is capable of rapid and large-area deposition of such non-close-pack arrays of NPs.
Charged NPs extracted from the enclosure (Fig. 1a) are positioned on the substrate precisely by applying a pre-designed configuration of the electric field. Two different approaches are examined and will be discussed here: (i) NPs are positioned on an electrically charge-patterned substrate, and (ii) guiding NPs through an external electric field assembly above the substrate surface.
In a similar approach, silver NPs with the size of around 30 nm are positioned on the apex of InGaN pyramids (Fig. 2c, d). The arrays of pyramids that have been fabricated by Houssaine Machhadani et al. were utilized as localized single-photon emitters (Jemsson et al. 2014). The efficiency of the device is expected to enhance with the plasmonic Ag NPs positioned on the tip of the pyramids. The preparation process of the sample is summarized in three steps: (i) the area between the pyramids is spin coated by photoresist with the pyramid tips staying uncovered; (ii) NPs are collected during 45 s on the substrate biased to + 22 V; (iii) photoresist is washed away using solvent.
Further control over the morphology of the 3D features is feasible by using an electrically conductive stencil detached from the surface. The width of the growing pillar is adjustable by changing the voltage applied to the stencil. Figure 3c and d show examples whereas the width of the growing column is controlled by changing the applied voltage to the stencil during the deposition process. The width of the features increases so that the overhanging structures with high aspect ratios are formed. In principle, self-assembly of NPs into any axisymmetric 3D morphology is achievable by varying the electric field. 3D features with ultra-high aspect ratios are achievable by adjusting the gap between the substrate and the stencil and/or the stencil thickness. Features with even more complex morphologies are achievable by moving the substrate during the deposition in all three dimensions.
In summary, NPs of functional materials produced in a plasma source could be assembled into planar patterns or complex 3D structures. Large arrays of 3D features with high aspect ratios are built simply by using a stencil placed on the substrate and exposed to the electron flux from the plasma. Using the combination of long-range electric forces and 3D piezo motion, the process will allow printing features with complex morphologies that are not easily achievable with conventional techniques. Plasma nanoparticle sources equipped with such electrostatic lenses would be capable of rapid and high-resolution surface patterning of NPs with possible application in device fabrication and 3D printing of functional materials in sub-microscales.
We would like to thank Dr. K. Fredrik Karlsson for his help on preparation of substrates with arrays of GaN micropyramids at Linköping University.
Open access funding provided by Linköping University. The authors received financial support from the Knut and Alice Wallenberg Foundation (grant KAW 14.0276) and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (faculty grant SFO-Mat-LiU no. 2009-00971). SA received financial support from ÅForsk Foundation (Swedish grant no. 17-517).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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