Atmospheric-Microplasma-Assisted Nanofabrication: Metal and Metal–Oxide Nanostructures and Nanoarchitectures

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Atmospheric-Microplasma-Assisted Nanofabrication: Metal and Metal–Oxide Nanostructures and Nanoarchitectures Davide Mariotti, Arumugam Chandra Bose, and Kostya (Ken) Ostrikov

Abstract—In this paper, we report on the fabrication of Mo-oxide nanostructures and nanoarchitectures using an atmospheric-microplasma (AMP) system. This AMP system shows a high degree of flexibility and is capable of producing several different nanostructures and nanoarchitectures by varying the process parameters. The low-cost and simplicity of the process are important characteristics for nanomanufacturing, and AMPs offer such advantages. In addition, AMPs have shown the ability of promoting self-organization of nanostructures. Index Terms—Atmospheric microplasma (AMP), metal–oxide, molybdenum, nanofabrication, nanomanufacturing.

I. I NTRODUCTION

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ANOTECHNOLOGY has recently become one of the fastest growing disciplines that have brought together many researchers from very different backgrounds. Engineers, physicists, and chemists have “nanofabricated” many different varieties of nanostructured materials and proved the applicability of such materials to important fields such as medicine and the energy sector. However, many of the nanofabrication techniques that have been developed have not matured to reach the commercialization stage. Nanofabrication is therefore a valuable research area that still needs to produce avenues for its exploitation in an industrial environment. In other words, nanofabrication techniques still have to become industrially viable “nanomanufacturing” processes. In the last decade, plasma technology enabled the transition from micro to nanoscale synthesis and processing [1], [2]. A low-temperature nonequilibrium plasma is a favorable environment for the formation of nanoscale objects and has Manuscript received November 30, 2008; revised January 13, 2009. This work was supported in part by the National Science Foundation (USA) under Award EEC-0530575, by the Australian Research Council, CSIRO, and by the International Research Network for Deterministic Plasma-Aided Nanofabrication. D. Mariotti was with the National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, Japan. He is now with the Department of Microelectronic Engineering, Rochester Institute of Technology, Rochester, NY 14623 USA (e-mail: [email protected]). A. C. Bose was with the National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, Japan. He is now with the Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, India (e-mail: [email protected]). K. Ostrikov is with the Plasma Nanoscience Centre Australia, CSIRO Materials Science and Engineering, Lindfield, N.S.W. 2070, Australia (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2009.2014067

recently demonstrated the outstanding capability of promoting self-organization [3]–[6]. Self-organization, defined here as simultaneous assembly and functionalization of nanostructures, or directed self-organization (when external energy drivers are applied, e.g., electric fields) can provide a very effective bottom-up nanofabrication mechanism. In addition, in order to meet the nanomanufacturing requirements, plasma research is increasingly giving attention to atmospheric-plasma (AMP) processes, whereby the high density promotes fast reaction rates and, therefore, the increased throughput. The possibility of producing nanostructures at atmospheric pressure has also the advantage of reduced costs of investment and maintenance due to the possibility of working without expensive vacuum equipment and without the need of process confinement. However, the generation of plasmas at atmospheric pressure has challenges and limitations that often compromise part of the advantages of atmospheric-pressure operation for nanofabrication. One of the most promising and practical ways of generating nonequilibrium plasma at atmospheric pressure and minimize such limitations is to generate AMPs. II. AMPs: F EATURES AND A PPLICATION TO N ANOFABRICATION The confinement of plasmas within submillimeter cavities at atmospheric pressure, i.e., AMPs, has recently attracted a significant attention. Numerous studies and reports in the scientific literature have shown that nonequilibrium AMPs have unique characteristics that make them markedly different from the larger scale nonequilibrium plasmas [7]–[11]. One of these characteristics is the possibility of producing a nonequilibrium environment at atmospheric pressure and for a wide range of gas mixtures [12], [13]. Microplasmas also exhibit enhanced fields, species densities, and temperature gradients that contribute further to create highly nonequilibrium environments. In plasmas, the energy coupled to the electrons produce specific electron-energy distributions, which in turn are responsible for molecule fragmentation and, consequently, formation of radicals. Electron-energy distributions in microplasmas are quite different from those found in the larger scale plasmas, signifying that the energy flow can be channeled toward production of different particles and aggregates. One of the challenges in microplasma science is currently the identification of the specific parameter spaces that define the transition into the “microplasma regime.” Although it is generally accepted that the

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plasma confinement within submillimeter volumes is required to produce glowlike AMPs, the mechanisms that determine the specific plasma state are still far from being understood. Despite a significant lack of such understanding, AMPs have already demonstrated their usefulness in many different application fields. Microplasmas have been used for many years in display technology and only in the last couple of decades have been considered for a broader and continuously increasing number of applications such as materials processing and nanofabrication [7], [14]. AMPs have favorable characteristics for nanofabrication such as low cost of implementation, atmospheric-pressure operation, nonequilibrium, and unique plasma chemistry that generate a suitable processing environment [7], [8], [12], [15], [16]. The possibility for the microplasma to operate at atmospheric pressure drastically reduces the cost of implementation of microplasma systems. Moreover, atmospheric pressure leads to a high density of the plasma species (including reactive radicals) that result into fast reaction rates and, eventually, the increased process throughput. All these factors play an important role to improve the industrial viability of AMP-based processes. Microplasma processing can be seen by industries as a low-risk investment that would facilitate the introduction of new bottom-up nanoscale synthesis techniques in the industrial fabrication processes. Moreover, AMPs as much as the larger scale plasmas are capable of producing self-organized nanoarchitectures [3], [5], [17]. This is why AMPs represent a unique process environment that cannot be achieved in any other processing technique. Furthermore, microplasmas can facilitate the formation of exotic functional nanoobjects, contributing to the exploration of new nanomaterial systems. The nonequilibrium environment produced by AMPs can also promote the formation of metastable phases that would be difficult to produce otherwise [18]–[20]. In this paper, we report on the application of AMPs for the fabrication of a wide range of Mo-oxide nanostructures and nanoarchitectures. The process used is directly transferrable to the formation of many other metal and metal–oxide nanostructures. Some of these applications have been already demonstrated for nanostructures made of Au, Cu, and W [20]– [22]. The AMP-based process is also analyzed in terms of the scalability to process large-area surfaces cost-effectively and reliably. It is shown that this AMP system can be considered as a versatile tool for nanomanufacturing of metal and metal–oxide nanomaterials. Nanostructured metal and metal–oxides produced with this simple nanomanufacturing system can meet the needs of many different applications that include energy storage, environmental remediation, hydrogen production, smart coatings for buildings, water purification and desalination, dye-sensitized solar cells, and several other photovoltaic applications. These applications also require the development of new nanoarchitectures that are capable of improving the efficiencies and effectiveness of the technologies used. Some of these improvements are likely to be offered by nanostructured metastable phases which require either a highly controlled fabrication or a nonequilibrium environment where the process follows kinetic pathways rather than a thermodynamic course. It will be shown

TABLE I AMP USED FOR NANOFABRICATION

that the parameters of the microplasma discharges can be used to control the composition, crystal structure, phase, and the specific nanoarchitecture produced. Mo-oxide materials exhibit interesting properties that have been studied and applied by several authors [23]–[27]. This is mainly due to a large number of interesting and unique catalytic, electronic, chromic, and field-emission properties. The wide range of applications include chemical synthesis, petroleum refining, sensing devices (e.g., for CO, NH3 , NO, NO2 , H2 , CH4 ), electrochromic systems, optical and optoelectronic devices, catalysis involving hydrogen and oxygen, smart windows, and display applications. Initial studies of AMPs as a potential tool for material processing started in early 2000s with the application to silicon etching [28] and the deposition of carbon films [29], [30]. These research works employed microplasmas sustained in a hollow-cathode configuration or between coplanar electrodes. The interest increased quite rapidly, and presently, there are a large number of publications that report on the use of AMPs for different material-processing purposes [31]–[34]. One of the first AMP-assisted nanofabrication processes was reported in [35], whereby different sized carbon nanoparticles were produced using an AMP jet configuration. Several authors have then reported on the fabrication of nanostructures and nanoarchitectures using various AMP systems. Most of these systems featured a jet configuration which is quite similar to the AMP system of our interest here. Table I summaries the main reports found in the literature. The ability to process metal, metal–oxides, Si, and C nanomaterials has been widely reported. In particular, very high processing rates have been achieved for Si nanocrystals [41] and Mo-oxides [19]. III. M ICROPLASMA S YSTEM C ONFIGURATIONS AND D IAGNOSTICS Suitable AMP configurations for the fabrication of metal and metal–oxide nanostructures are shown in Fig. 1(a)–(d). These configurations exhibit a high degree of flexibility and allow different ways of power coupling to generate and sustain the

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Fig. 2. Ignition power and sustain power for Ar microplasma at 20-sccm flow rate. The distance refers to the distance between the substrate (on top of the power electrode) and the Ni tubing as shown in Fig. 1(d).

Fig. 1. (a)–(d) Typical basic configuration of a microplasma system for the fabrication of metal and metal–oxide nanostructures using different powercoupling schemes. Two different approaches to nanofabrication are also depicted, (e) remotely or (f) with the microplasma in direct contact with the processing surface.

plasma. Purely inductive [Fig. 1(a)], purely capacitive [Fig. 1(b) and (d)], or hybrid [Fig. 1(c)] power coupling can be used to generate and sustain the plasma. Each power-coupling mode, together with the parameter settings, offers a wide range of processing conditions. This in turns allows for a wide range of nanostructures and nanoarchitectures to be produced. AMPs can be easily sustained in a low (e.g., up to 20 kHz)- or a high (e.g., 450 MHz)-frequency range using the configurations shown in Fig. 1(a)–(d). The use of many different processing gases is also possible, including Ar, N2 , CH4 , He, H2 , O2 , and several mixtures thereof. Gas mixtures are flown through the capillary vessel which is used to confine the plasma into a submillimeter-sized cavity. A typical internal diameter of the capillary is 0.3–0.7 mm. The capillary is generally chosen to be made of quartz for diagnostic purposes; however, alumina and other metals have also been adopted. A metal wire can be inserted into the capillary to produce solid precursors for the fabrication of metal/metal–oxide nanostructures [Fig. 1(e) and (f)]. The metallic wire or part of it resides within the plasma volume which then allows for the reactions to take place on its surface. Power consumption is generally very low, and using ultrahigh-frequency (UHF, 450 MHz) power supply, microplasmas can be sustained with UHF powers as low as 2 W. However, microplasma ignition generally requires higher power, and to facilitate the generation of the microplasma, a highvoltage (5 kV) dc pulse is used. Therefore, in order to start the microplasma, the UHF power is first set to a specified value (ignition power), and then, the dc pulse is applied. Upon extinction of the dc high-voltage pulse, the microplasma is sustained by the UHF power which can then be reduced. Fig. 2 shows the ignition power, and the sustain power for the microplasma system is shown in Fig. 1(d). In this case, the tubing was made

of Ni, and the measurements were taken for the microplasma with Ar flow rate of 20 sccm. The distance shown in Fig. 2 refers to the substrate-tubing distance. The ignition power is the lowest power required to ignite the plasma with the dc pulse. The sustain power represents the lowest power that could sustain the microplasma discharge. In these AMP systems, the plasma is generated and used in a jet fashion, and few processing parameters (e.g., flow rate, substrate distance) can be varied to control the degree of interaction between the plasma and the processing surface. In particular, we have studied two different power-coupling schemes which allow the microplasma to be either remote from the deposition surface [Fig. 1(e)] or interact with the processing surface [Fig. 1(f)]. Both microplasma-system configurations are sustained by a UHF (450 MHz) power supply through a matching network [Fig. 1(e) and (f)]. Optical diagnostic studies have determined the processing temperatures and the effective electron temperatures, thus confirming the nonequilibrium state of the AMPs of our interest [10], [12]. The gas temperature can be easily controlled by the processing parameters and can be varied from room temperature up to 2000 K, while the effective electron temperature can reach 14 eV [10], [12]. Optical emission has also been used to analyze the plasma species and identify the elementary processes responsible for the formation of specific nanostructures [18]. The following sections will provide specific details of the nanofabrication capabilities of the systems shown in Fig. 1(e) and (f). The focus will be on Mo-oxide nanostructures and nanoarchitectures. This is why a Mo wire (0.3-mm diameter) was used to obtain all the results reported as follows. We stress that this approach is generic, and by replacing the Mo wire with another metallic wire, quite similar nanostructures of metals and metal–oxides can be produced. A large number of such nanostructures and nanoarchitectures have already been demonstrated [17]–[22]. IV. Mo-O XIDE N ANOSTRUCTURES AND N ANOARCHITECTURES A. Nanoparticles and Nanorods Using the AMP system shown in Fig. 1(e), Mo-oxide nanoparticles have been synthesized. Specifically, Ar gas mixed

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Fig. 3. SEM images of Mo-oxide nanoparticles produced using the microplasma system by varying the gas flow and with 1% oxygen in Ar: MoO2 at 10 sccm, MoO3 at 20 sccm, and MoO3 at 30 sccm. Fig. 5.

High-resolution TEM image of a nanorod.

Fig. 4. TEM images and diffraction patterns of Mo-oxide nanostructures produced with the microplasma system by varying the gas flow rates and with 1% oxygen in Ar: MoO2 at 10 sccm, MoO3 at 20 sccm, and MoO3 at 30 sccm.

with 2% O and 20-W UHF power were used to produce nanoparticles with different sizes, phases, and crystal structures [18]. An interesting observation is that an increasing flow rate resulted in a significant decrease of the average particle size from approximately 100-nm diameter (at 5 sccm) to less than 20-nm diameter (at 30 sccm). In addition, the phase composition has changed from pure MoO2 (5 sccm) to a mixed phase (10 sccm) and to pure MoO3 (20 sccm and 30 sccm). Interestingly, quite similar results were produced when a W wire was used, and the same relationship between the particle size and the flow rate was revealed [21]. For this particular study, the quartz capillary was pinched near the exit to form a 70-μm diameter nozzle so that the nanostructures deposition could be localized to small line features. This initial study showed that by only varying a single process parameter, i.e., the gas flow rate, it was possible to control the size and the phase composition of the produced nanostructures. Further study was then carried out by decreasing the oxygen concentration to 1%, and the effect of the gas flow was studied while maintaining the same input power (10 W). The produced nanostructures changed quite drastically. Representative scanning electron microscope (SEM) images of Mo-oxide nanoparticles at different gas flow rates are shown in Fig. 3. In these experiments, Mo-oxide nanoparticles could be produced at 10 sccm. However, at 20 sccm, nanorod-shaped structures were produced instead, and at higher flow rates, mixed nanostructures were observed. The phase transition occurred also in this case, whereby the structures produced at 10 sccm were MoO2 , whereas the MoO3 phase was produced at gas flow rates higher than 20 sccm. These results were all confirmed by transmission electron microscopy (TEM); Fig. 4 shows the corresponding TEM images and diffraction patterns. Of particular interest are the nanorods, as such elongated nanostructures often require the presence of catalyst particles to enable the vapor–liquid–solid (VLS) growth mechanism. The

Fig. 6. Mo-oxide nanostructures produced by AMP at different processing conditions.

AMP process is not likely to offer the required conditions for the VLS growth mechanism. However, its nonequilibrium state has allowed the formation of structures that are generally nonfavorable in thermodynamic processes and without the presence of a catalyst seed particle. The TEM analysis (Fig. 5) has revealed fine details of the crystal structure and suggested that the MoO3 nanorods preferentially grow along the (011) planes. B. Deposition of Nanostructured Films: Nanosheets and Other Nanoarchitectures Using the AMP system shown in Fig. 1(e) can be a very efficient way of producing many different kinds of metal/metal– oxide nanostructures that can be eventually dispersed in colloids with the associated advantages of colloidal dispersions. However, the AMP system shown in Fig. 1(e) is lacking the ability of organizing nanostructures in functional nanoarchitectures. For this reason, the configuration in Fig. 1(f) has been developed and tested for nanofabrication. Using this AMP system, different nanostructures and nanoarchitectures have been produced. Representative examples are shown in Fig. 6. The deposition of the nanostructured films was possible by scanning a single AMP element over the substrate. Surface areas of up to 3 mm long and 0.3–0.5 mm wide have been processed using this technique. The accurate control of the processing conditions allowed us to alter the desired shape and the Mo-oxide phase composition. In addition, we were able to produce films with very high surface area as shown in Fig. 7(a).

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V. C ONCLUSION

Fig. 7. (a) Monoclinic MoO3 nanosheetlike structures produced by AMP and (b) similar nanosheetlike nanostructures subsequently decorated with spherical particles in a single-step process.

Fig. 8.

Self-alignment of Mo-oxide nanoparticles under AMP exposure.

A detailed characterization has determined that the nanosheetlike material, similar to that in Fig. 7(a), is made of monoclinic MoO3 , and its application in Li-ion batteries has been also demonstrated following the optimization of the AMP process [19]. The nanosheet-based films have exhibited very high Li-ion storage capacity, mainly due to two main factors: 1) a large surface area of the nanostructured film that allows Li-ion storage within reachable depth and 2) the unusual β-monoclinic metastable phase of MoO3 with a crystal structure that features large channels for the intercalation and deintercalation of Li ions. Both properties are strongly correlated to the nonequilibrium AMP process and demonstrate the importance of this technique. The combination of different processes in sequence has produced composite nanoarchitectures [17]. A representative SEM image of such composite nanoarchitectures is shown in Fig. 7(b), whereby the nanosheets could be decorated with spherical particles. C. Self-Organization of Nanoparticles One of the most interesting aspects of plasma-aided nanofabrication is the possibility of directed self-organization on plasma-exposed surfaces [44], [45]. Several reports can be found in the literature on self-organization of nanostructures under plasma exposure due to the charging effect and to the formation of strong electric fields [5], [6]. It has been demonstrated that the AMP can also produce similar effects and, as such, can be used to synthesize self-organized nanoarchitectures [3], [5]. In addition, for the case of Mo-oxide, we have observed alignment of nanoparticles in preferential directions such as those shown in Fig. 8. These mechanisms of self-organization are not fully understood and cannot be controlled yet. Nonetheless, they suggest interesting possibilities to guide large-scale self-organization of nanostructures. Computer simulations of self-alignment of nanoparticles on plasma-exposed surfaces are in progress, and the results will be reported elsewhere.

The AMP system described in this paper has shown the capabilities of producing a large variety of nanostructures and nanostructured films. The power consumption of this system has varied from extremely low power (2 W) up to 30 W. The deposition yield is extremely high, whereby a 3-mm-long and 0.3–0.5-mm-wide area could be processed in less than 45 s. The system setup is inexpensive if compared to low-pressure deposition system. In the deposition of Mo-oxide nanostructures reported in this paper, argon and oxygen have been utilized. However, a wide range of gases can be used and have already been employed, including hydrogen and gaseous carbon precursors [3], [10], [12], [20], [35], [39]. Ambient temperature and humidity may affect the processing conditions and will need detailed investigations. Nevertheless, the nanostructured films that have been produced have shown very high reproducibility [19], considering the experimental setup where gas feed lines had no temperature control. Before AMPs can succeed as a nanomanufacturing tool, there exist challenges that need to be addressed such as film uniformity and adhesion to the underlying substrate. In addition, the applicability of AMPs to process large-area surfaces depends on the possibility of organizing single AMP elements in arrays. However, AMPs prove to be a versatile and cost-effective technique to produce nanostructured materials. In this contribution, we have shown that AMPs have demonstrated unique capabilities to produce metal/metal–oxide nanostructured materials, offering a viable alternative for low-cost nanomanufacturing.

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Davide Mariotti received the B.Eng. degree in electronic systems from the University of Ulster, Antrim, U.K., the Laurea degree in electronic engineering from the Università Politecnica delle Marche, Ancona, Italy, and the Ph.D. degree from the University of Ulster with a thesis entitled “Characterization of a micro-plasma devices sensor using electrical measurement and emission spectroscopy.” He was a Research Fellow with the National Institute of Advanced Science and Technology, Tsukuba, Japan, a Research Associate with Loughborough University, Leicestershire, U.K., a Research Assistant with the University of Ulster, and the Project Leader with Sensor Technology and Devices, U.K., a spin-out company in U.K. He is currently an Assistant Professor with the Department of Microelectronic Engineering, Rochester Institute of Technology, Rochester, NY, where he is currently working on the development of atmospheric-microplasma systems for nanomanufacturing and for hydrogen production. His interests are particularly oriented toward the study of nanofabrication self-organization mechanisms and with attention to metal, metal–oxides, carbon nanotubes, and silicon nanocrystals for applications in nanoelectronics and solar cells.

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This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. MARIOTTI et al.: ATMOSPHERIC-MICROPLASMA-ASSISTED NANOFABRICATION

Arumugam Chandra Bose was born in India in 1971. He received the Ph.D. degree in physics (physics-materials science interdisciplinary) from the University of Madras, Chennai, India, in 2002, with a thesis on nanomaterial preparations and characterizations of oxide nanomaterials for gas sensing applications. He was with National Institute of Material Science, Tsukuba, Japan, where he did postdoctoral work on surface science. He was with the National Institute of Advanced Industrial Science and Technology, Tsukuba, where he did postdoctoral work microplasma. He is currently with the Department of Physics, National Institute of Technology, Tiruchirappalli, India, where he became an Assistant Professor in 2007. He is the author of 30 scientific articles and 35 contributions to seminars and workshops. His current research interests are in the field of oxide nanomaterials materials for potential applications and microplasma for MEMS applications.

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Kostya (Ken) Ostrikov received the Doctor of Science (Habilitation) degree in 1996. He is currently the Leader of the Plasma Nanoscience Centre Australia, CSIRO Materials Science and Engineering, Australia, of the Plasma Nanoscience Team, University of Sydney, Australia and also of the International Network for Deterministic Plasma-Aided Nanofabrication. His research interests are in the areas of deterministic plasmabased control of self-organized nanomatter; plasma generation, creation, and manipulation of atomic and nanoscale building blocks; description of plasma–solid interactions and complex self-organized plasma–solid systems used for plasma processing; and synthesis of nano and biomaterials, as well as complex (dusty) plasmas, surface science, materials science, nanoplasmonics, and nanoparticle-related phenomena in space physics and astrophysics. His research is related to the physics of low-temperature plasmas and nanoscale synthesis and a broad range of applications including new-generation self-assembled nanomaterials, nanoelectronic and photonic structures, and devices for future computer chips, solar cells, communication systems, and biosensors. Dr. Ostrikov was the recipient of six prestigious fellowships, after receiving professorial appointment in 1996 to work with leading research universities in the U.K., Germany, Japan, Singapore, and Australia. He was also the recipient of the Best Young Scientist of Ukraine Award of the Academy of Sciences of Ukraine and the Pawsey Medal of the Australian Academy of Sciences.

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