Fabrication of Nanostructured Thin Films Using Microreactors |
CONTENTS
14.1 Microreactor-Assisted Nanomaterial Deposition (MAND)
14.2 MASD for the Deposition of Nanocrystalline thin Films
14.3 MAND for the Deposition of Nanostructured thin Films
14.5 Conclusion and Future Direction
Among the many and diverse opportunities for embedding nanotechnology within industrial products is the opportunity to assemble nanostructured films from nano-material building blocks for many emerging clean energy technologies. Cheap, green, solution-phase oxide, sulfide, and phosphide nanosynthesis chemistries provide opportunities for buffer layers in solar cells [1,2], enhanced catalysis [3] in solar thermal chemical processing, superlubricity [4] in wind turbines, supercapacitance in grid storage batteries [5], and accelerated convection for industrial waste heat recovery [6], among many others. One promising approach is Microreactor-Assisted Nanomaterial Deposition (MAND™) (Figure 14.1), a continuous, liquid-phase alternative to high-temperature, high-vacuum vapor-phase thin-film production such as sputtering and chemical vapor deposition (CVD). MAND has been used by our team to produce high-performance nanoscale coatings on substrates up to 150 mm in dimension with substantially lower processing temperatures.
14.1 MICROREACTOR-ASSISTED NANOMATERIAL DEPOSITION (MAND)
MAND processes combine the merits of microreaction technology with solution-phase nanomaterial synthesis, purification, functionalization, and deposition. MAND architectures are a flexible and versatile nanomanufacturing platform for nanomaterials synthesis and deposition. It can be implemented in various ways for the manufacturing of functional nanostructures. Microreactor-Assisted Solution Deposition (MASD) involves the use of microreactor technology to produce reactive fluxes of short-life, intermediate molecules for heterogeneous growth on a temperature-controlled substrate. Another variant of MAND is Microreactor-Assisted Nanoparticle Deposition (MANpD) involving the use of microreactor technology to implement real-time nucleation, growth, purification, and functionalization of nanoparticles (NPs) for deposition and assembly of NP films and structures. Figure 14.1 illustrates the process concept of MAND and various examples of nanostructured thin films fabricated by the MAND platform.
FIGURE 14.1 (Center) MAND architecture: Microreactor technology generates nanoscale building blocks (e.g., monodispersed, functionalized nanoparticle streams; reacting fluxes; etc.) for precise, economical solution-phase deposition. (Right) MASD and MANpD routes to nanostructured films. (Left) Schematic of a reactor for implementing MANpD.
Current batch processes for NP production are uncoupled from functionalization and deposition processes that can lead to NP agglomeration. Key potential advantages of MANpD over batch synthesis and deposition of NPs include, among others: (1) shorter cycle times, (2) controlled agglomeration, (3) higher conversion of reactants, (4) purer yields, and (5) lower solvent usage. These advantages extend from the accelerated heat and mass transport possible within microchannels, allowing for rapid changes in reaction temperatures and concentrations and more uniform heating and mixing. At Oregon State University and elsewhere, microreactors have been found to yield reductions in the dispersity of nanocrystal size distributions [7] and increases in macromolecular yields [8]. Microreactors also offer to minimize the environmental impact of nanomanufacturing practices through solvent-free mixing, integrated separations, and reagent recycling [9]. Finally, the use of microreactor synthesis at the point of deposition has been demonstrated to eliminate particulate agglomeration/degradation during processing and storage [10].
We have developed several prototypes of continuous flow microreactor systems for the synthesis and deposition of nanomaterials in a number of applications.
14.2 MASD FOR THE DEPOSITION OF NANOCRYSTALLINE THIN FILMS
Chemical bath deposition (CBD) is an aqueous analogue of chemical vapor deposition (CVD). The constituent ions are dissolved in an aqueous solution, and the thin films are produced through a heterogeneous surface reaction. A fundamental understanding of CBD, however, is far less developed than that of CVD. This has limited the development and application of this growth technique. CBD is normally carried out as a batch process in a beaker and involves both heterogeneous and homogeneous reactions. Furthermore, the bath conditions change progressively as a function of time. It is known that CBD is capable of producing an epitaxial layer on a single crystal surface. Many compound semiconductors that are major candidates for solar energy utilization have been deposited by CBD, such as CdSe, Cu2S, SnO, TiO2, ZnO, ZnS, ZnSe, CdZnS, and CuInSe2 [11]. Among these, CBD CdS deposition is the most studied CBD process due to its important role in fabricating CdTe and CuInSe2 thin-film solar cells.
The fundamental aspects of CBD are similar to those of the CVD process. It involves mass transport of reactants, adsorption, surface diffusion, reaction, desorption, nucleation, and growth. The earlier studies suggested a colloidal-by-colloidal growth model. However, investigations by Ortega-Borges and Lincot [12], based on initial rate studies using a quartz crystal microbalance, suggested different growth kinetics. They identified three growth regimes: an induction period with no growth observed, a linear growth period, and finally, a colloidal growth period, followed by the depletion of reactants. They proposed a molecular level heterogeneous reaction mechanism given in Equations 14.1 to 14.3.
(14.1) |
(14.2) |
(14.3) |
This model has provided a good foundation for an understanding of the CBD process at the molecular level. It is well known that the particle formation plays an important role in the CBD process. Thus there is a need to find a method to decouple the homogeneous particle formation and deposition from the molecular level heterogeneous surface reaction. For this reason, we have developed the MASD process and implemented it first for CBD CdS, and investigated the fundamental reaction kinetics and growth mechanism using MASD as a tool.
A schematic diagram of a laboratory-scale continuous flow microreactor for MASD is shown in Figure 14.2. The reactant streams A and B were dispensed through two syringe pumps and allowed to mix in an interdigital micromixer [13]. Stream A consists of CdCl2, NH4OH, and NH4Cl, and stream B consists of thio-urea. A detailed schematic diagram of an interdigital micromixer is shown in the inset. Fluids A and B to be mixed are introduced into the mixing element as two counterflows and enter interdigital channels (~20–50 μm), then split into many interpenetrated substreams. The substreams leave the interdigital channel perpendicular to the direction of the feed flows, initially with a multilayered structure. Fast mixing through diffusion will soon follow due to small thickness of the individual layers. The resulting mixture from the micromixer then is passed through a temperature-controlled microchannel before it is delivered to a temperature-controlled substrate. The homogeneous chemistry of the reacting flux is controlled precisely by the inlet concentrations, temperature, and residence time, as illustrated in Figure 14.2.
FIGURE 14.2 A schematic diagram of a continuous flow microreactor for CBD. (From Y.J. Chang et al., Electrochem. Solid-State Lett., 12(7): H244−H247 (2009). With permission.)
Previous results indicated that for CBD CdS deposition, small particles were forming and growing even at the beginning of the deposition process, as supported by real-time dynamic light scattering measurements and transmission electron microscopy (TEM) characterization [12]. We have observed a similar result using the continuous flow microreactor. Experiments were carried out by preheating the precursor solutions (streams A and B) at 80°C. At this temperature, thiourea releases more sulfide ions through hydrolysis. Free sulfide ions react with free cadmium ions to form CdS particles at these operation conditions. The source chemicals were maintained at room temperature before they entered the micromixer in order to obtain a reacting flux without the homogeneous particle formation. The mixed reactants were maintained at 80°C using heat exchanging fluid from a constant temperature circulator. TEM samples were obtained by collecting drops of hot solution from the PEEK tube on the lacey carbon-coated TEM copper grid. TEM images (Figure 14.3) indicate that at very short residence times (e.g., 1 s), there was no evidence of particle formation on the surface of the grid under these processing conditions. This suggests the reaction was carried out within the induction timescale for homogeneous particle formation. Using this particle-free flux, study of the CdS deposition kinetics was simplified by focusing on the heterogeneous surface reaction through a molecule-by-molecule growth mechanism.
FIGURE 14.3 TEM evidence showing no nanoparticle formation in the chemical solution.
A series of CdS thin-film deposition experiments at different residence times (1, 3.5, 7, 35, and 70 s) were performed. The film thickness was determined by a surface profiler. Figure 14.4(a) shows the deposited CdS thin-film thickness versus the deposition time at different residence times. These growth rate results in Figure 14.4(b) clearly indicate that a lower CdS thin-film growth rate was obtained (~77 Ǻ/min) when a 1 s residence time was used. The growth rate increases significantly (about four times higher) when a 3.5 s residence time was used compared to a 1 s time. The thin-film growth rate increases gradually from 3.5 s to 35 s residence time. However, when a 70 s residence time was applied, a decrease of the growth rate was observed (due to particle formation). This deposition result suggests that ions formed through the thiourea hydrolysis reaction are the dominant sulfide ion source responsible for the CdS deposition, rather than thiourea itself, which had been widely discussed in almost all of the previous literature. This finding could not be observed previously by a conventional CBD batch setup because all the reactant solutions were sequentially pulled into the reaction beaker and mixed all at once. These results also demonstrate the capability of MASD in controlling the reactant flux through varying the initial concentration and residence time for continuous growth of thin films. It is not possible to continuously grow high-quality films using the batch CBD process since the reactant flux is changing as a function of time and depletes in limited time.
FIGURE 14.4 CdS thin-film thickness versus deposition time using flux at different residence times.
Using the MASD reactor, we obtained a particle-free flux that is capable of depositing high-quality CdS thin films. We were able to deposit smooth, dense, and highly oriented nanocrystalline CdS thin films in shorter time than typical CBD approaches [14]. Figure 14.5 shows the AFM images of the CBD-deposited CdS surfaces for a (a) continuous flow microreactor and (b) batch process. These images demonstrate the capability of using the continuous flow microreactor to obtain a dense and uniform deposition [15]. The x-ray diffraction (XRD) pattern given in Figure 14.5(c) clearly shows a highly (111)-oriented cubic CdS thin film produced by the continuous flow microreactor at low temperature.
Enhancement-mode CdS thin film transistors (TFTs) were fabricated using this continuous flow microreactor at low temperature (80–90°C) without any post-deposition annealing [16]. The schematic diagram of the transistor and the transfer characteristics are shown in Figure 14.6. An effective mobility, μeff ≅ 1.5 cm2/V-s, and an on-off ratio of 105 were obtained from these devices. These values verify that the quality of films deposited by MASD at low temperature is suitable for electronic applications. This new approach could be adopted for low-temperature deposition of other compound semiconductor thin films using chemical solution deposition with better control over processing chemistry and reduced waste production.
FIGURE 14.5 AFM images of the CBD-deposited CdS surfaces for (a) MASD, (b) batch process, and (c) XRD pattern for CdS film deposited by MASD. (From P.-H. Mugdur et al., J. Electrochem. Soc., 154(9): D482–D488 (2007). With permission.)
Using the continuous flow microreactor, we were able to deposit uniform, dense, and highly oriented CdS thin films in shorter time with excellent conformal coverage, even on 6-inch highly textured substrates using a particle-free solution (see Figure 14.7a). Figure 14.7b shows a cross-sectional transmission electron microscopy image of nanocrystalline CdS thin film deposited on a highly textured fluorine-doped tin oxide (FTO) substrate using our scale-up continuous flow microreactor [17].
McPeak and Baxter [18] demonstrated the deposition of dense arrays of well-aligned, single-crystal ZnO nanowires using a continuous flow microreactor (Figure 14.8). They utilized the spatial resolution of the microreactor to enable rapid and direct correlation of material properties to growth conditions. They observed that nanowire lengths decreased, morphology changed from pyramidal tops to flat tops, and the growth mechanism transitioned from two-dimensional nuclei to spiral growth.
FIGURE 14.6 (a) CdS thin-film transistor fabricated by MASD at low temperature. (a) Schematic diagram of the CdS TFT. (b) Transfer characteristics (Ids-Vds at different Vg values) curve of the CdS TFT. (From Y.-J. Chang et al., Electrochem. Solid-State Lett., 9(5): G174–G177 (2006). With permission.)
FIGURE 14.7 CdS thin film deposited by MASD on 6-inch FTO-coated glass fabricated by MASD at low temperature. (a) Optical image. (b) Cross-sectional high resolution transmission electron microscopy (HRTEM) image and electron diffraction pattern.
14.3 MAND FOR THE DEPOSITION OF NANOSTRUCTURED THIN FILMS
Another variant of MAND is Microreactor-Assisted Nanoparticle Deposition (MANpD) involving the use of microreactor technology to implement real-time nucleation, growth, purification, and functionalization of NPs for the deposition and assembly of NP films and structures. We have also used this approach to deposit various nanostructured thin films. This technique follows a thin-film growth mechanism based on nanoparticle formation and deposition. We have fabricated a nanoporous ZnO film [19] that is highly transparent using MANpD. Figure 14.9a shows a top-view SEM image of a ZnO film deposited by MANpD. The film consists of uniformly distributed nanoparticles and nanopores. The cross-sectional image shows a uniform nanoporous film with a thickness around 24 nm (see Figure 14.9b). The nanoporous ZnO thin film is highly transparent with an optical bandgap of 4.35 and 3.27 eV before and after thermal annealing, respectively (see Figure 14.9c).
FIGURE 14.8 Cross-sectional SEM images of ZnO nanowires grown at flow rates of 0.72 ml/h (top row) and 2.88 ml/h (bottom row) with an equimolar inlet concentration of 0.025 M zinc nitrate and hexamethylenenetetramine (HMT). Images were taken at positions (a, e) 0, (b, f) 6, (c, g) 12, and (d, h) 18 mm downstream from the inlet. Scale bar applies to all panels. (From K.M. McPeak and J.B. Baxter, Crystal Growth Des., 9: 4538–4545 (2009). With permission.)
A variety of nanostructures could be fabricated using MAND by changing the process parameters. For example, we were able to produce a variety of ZnO structures using our continuous flow microreactor by varying the concentration of NaOH [20]. In this experiment, 0.05 M zinc acetate dehydrate and 0.25 M ammonium acetate were mixed with the four different concentrations of NaOH varying from 0.005 M to 0.15 M. The SEM images of the synthesized ZnO at the four different NaOH concentrations are shown in Figure 14.10. It can be clearly observed that the concentration of NaOH exerted a great influence on the morphology of the synthesized ZnO. From Figure 14.10, several types of structures can be clearly seen from the SEM images. The flower-like ZnO structure was fabricated on the substrate at a higher NaOH concentration, while the chrysanthemum-like ZnO structure was grown at a lower one. The petals of the flower-like ZnO structures became narrower in width, smaller in size, sharper in tip, and simpler in shape as the concentration of NaOH was decreased from 0.15 M to 0.05 M. Meanwhile, the synthesized ZnO has chrysanthemum-like ZnO structures if the synthesis of the ZnO microstructure was carried out at a concentration of NaOH less than 0.05 M. The synthesized chrysanthemum-like ZnO structure at 0.005 M NaOH has a lot of petals, and its petals are narrow and nanosized. These assembled ZnO structures have been deposited onto a substrate.
FIGURE 14.9 Nanoporous ZnO thin films deposited by MANpD. (a) Top-view SEM image. (b) Cross-sectional SEM image. (c) UV-Vis absorption and optical image (inset). (From S.-Y. Han et al., Electrochem. Solid-State Lett., 10(1): K1–K5 (2007). With permission.)
Figure 14.11(a) shows the SEM image of a flower-like ZnO structure, which was synthesized by delivering the mixed solution on the substrates for 1 min. Figure 14.10(b) shows the SEM image of a ZnO structure after 10 min of depositions. It can be seen clearly that the size of the flower-like ZnO structures increases as a function of deposition time. The conventional batch CBD ZnO process would produce films from large aggregates that exhibit coarse morphologies that dominate homogeneous reactions. The conventional batch CBD ZnO process could also form a mixture of heterogeneously grown ZnO columns or rods, along with homogeneously grown large aggregated particles. In addition, the growth rate of the solid phase on the substrates decreases rapidly as the time duration for the reaction is prolonged. The deposited ZnO structures continue to grow in the MAND method because a fresh and constant solution is continuously fed to the active sites of ZnO.
FIGURE 14.10 SEM images of flower-like ZnO structures synthesized with four different concentrations of NaOH at 90°C for 5 min: (a) 0.005 M, (b) 0.01 M, (c) 0.1 M, and (d) 0.15 M. (From J.Y. Jung, N.-K. Park, S.-Y. Han, G.B. Han, T.J. Lee, S.O. Ryu, C.-H. Chang, The growth of the flower-like ZnO structure using a continuous flow microreactor, Curr. Appl. Phys., 8(6): 720−724. 2008. With permission.)
These flower-like nanostructures show enhanced pool boiling critical heat fluxes (CHFs) at reduced wall superheat [6]. In particular, we observed a pool boiling CHF of 82.5 W/cm2 with water as the fluid for ZnO on Al versus a CHF of 23.2 W/cm2 on a bare Al surface with a wall superheat reduction of 25–38°C. These CHF values on ZnO surfaces correspond to a heat transfer coefficient of ~23,000 W/m2K.
Dendrimers are highly branched, nanometer-sized molecules with fascinatingly symmetrical fractal morphologies. The word dendrimer (coined by Tomalia et al. [21]) is derived from the Greek words dendri (branch, tree-like) and meros (part of). Dendrimers consist of a core unit, branching units, and end groups located on their peripheries. Their dendritic architecture presents great potential for a wide variety of applications. For example, they hold great promise as building blocks for complex supramolecular structures with specifically designed functions. They can be considered versatile nanoscale components for building nanoscale structures. Dendrimers can act as nanoscale carrier molecules in drug delivery, where nanoparticles and nanocapsules are gaining popularity. Structural variety, yielding molecules having differing optical, electrical, and chemical properties, makes dendrimers potentially even more versatile than the alternatives. The molecules can be assembled with startling precision, a necessity when the goal is construction of nanoscale structures or devices with sophisticated and complex functionality. Along with targeting tumor cells and drug delivery systems, dendrimers have shown promising results as tools for MRI [22,23,24] and gene transfer techniques. Also, dendrimer-based nanocomposites are being studied as possible antimicrobial agents to fight Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli [25]. Dendrimers have been shown to act as scavengers of metal ions, offering the potential to meet environmental cleanup needs. Their size allows them to be filtered out post-extraction using common ultrafiltration techniques.
FIGURE 14.11 SEM images of flower-like ZnO structures synthesized at 90°C (0.1 M NaOH) (a) for 1 min and (b) for 10 min. (From J.Y. Jung, N.-K. Park, S.-Y. Han, G.B. Han, T.J. Lee, S.O. Ryu, C.-H. Chang, The growth of the flower-like ZnO structure using a continuous flow microreactor, Curr. Appl. Phys., 8(6): 720−724. 2008. With permission.)
The synthesis of dendrimer is precisely controlled, so a certain generation of dendrimer is characterized with a single size and molecular weight. This property distinguishes dendrimers from typical polymers.
The general synthetic strategy of dendrimers includes divergent and convergent approaches. The divergent approach, arising from the independent discovery in parallel works from Buhleier et al. [26] and Tomalia et al. [27], initiates growth at the core of the dendrimer and continues outward by several repetitions of activation and coupling steps. Convergent synthesis, first reported by Fréchet et al. in 1989 [28], initiates growth from the exterior of the molecule, and progresses inward by coupling focal point to each branch of the monomer. Usually, the divergent approach generates more side products with uncompleted addition of branches due to the steric hindrance of multifunctional reaction sites on the surface. This will result in difficulty for the separation of products. On the other hand, the convergent approach will provide a much purer product, because only one reaction site exists for each reactant molecule.
For dendrimers to realize their full potential, methods must be developed in the production of these macromolecules. We have demonstrated a convergent approach to the synthesis of polyamide dendrons and dendrimers, and the deposition of the G1 dendron on the aminosilanized glass surfaces, by using a continuous flow microreactor. The microreactor has proven to be an effective tool to synthesize the polyamide dendrons and dendrimers. The microreactor demonstrated several advantages over a conventional batch reactor. One of the most attractive advantages is that the reaction time was reduced tremendously from a few hours to seconds or minutes [29]. Therefore the continuous flow microreactor could potentially reduce the production cost, which will enhance the opportunities for the large-scale application of dendrimers in various fields. In addition, the required reaction conditions are easier to implement.
We have demonstrated an approach that used the microreactor to activate and deposit G1 dendrons directly on the silanized glass slides [30]. The process scheme is illustrated in Figure 14.12. A solution containing dendron G1 and another solution containing thionyl chloride were mixed through the micromixer. The activated G1 dendron from the microreactor was impinged onto a spinning silanized glass slide that was mounted on a heated rotating disk substrate holder. The entire deposition process using the microreactor took about 2 min, and the residence time in the outlet tubing was 17 s. Figure 14.13 shows the time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectrum and corresponding ion images of the deposited dendrons.
14.5 CONCLUSION AND FUTURE DIRECTION
Microreactor-Assisted Nanomaterial Depostion (MAND) processes combine the merits of microreaction technology with solution-phase nanomaterial synthesis, purification, functionalization, and deposition. MAND architectures are a flexible and versatile nanomanufacturing platform for nanomaterial synthesis and deposition. Microreactors offer exciting opportunities for high levels of process control over nanomaterial synthesis via precise and rapid changes in reaction conditions.
Microreactor-Assisted Solution Deposition (MASD) involves the use of micro-reactor technology to produce reactive chemical fluxes of short-life, intermediate molecules for heterogeneous growth on a temperature-controlled substrate. MASD is a new approach that could be adopted for many chemical solution deposition processes. It could enable low-temperature deposition of many compound semiconductor thin films. We have successfully used MASD approaches for the deposition of CuS, CuSe [31], CuInS2 [32], and CuInSe2 [33]. Another variant of MAND is Microreactor-Assisted Nanoparticle Deposition (MANpD) involving the use of microreactor technology to implement real-time nucleation, growth, purification, and functionalization of NPs for deposition and assembly of NP films and structures. A variety of nanostructured thin films could be fabricated using MANpD. For example, nanoporous ZnO, flower-like ZnO, and ZnO nanorod arrays were fabricated by the MANpD technique and used as a channel layer for thin-film transistors [10], as a nanotextured surface for enhanced boiling [6], and as an antireflection coating layer for solar cells [34], respectively. In addition to inorganic materials, MAND is also suitable for the synthesis and deposition of organic nanomaterials. We have demonstrated the feasibility of microreactor-based continuous synthesis and deposition of dendrimers.
FIGURE 14.12 (a) A schematic diagram of the experimental setup. (b) A schematic diagram of the interdigital micromixer. (From S.-H. Liu and C.-H. Chang, Chem. Eng. Technol., 30(3): 334–340 (2007). With permission.)
FIGURE 14.13 TOF-SIMS schematic diagrams of the dendron deposition process via a microreactor.
Rapid, continuous flow, high yield and selectivity, and most importantly, a facility for numbering up the process for industrial production scale, MAND opens the possibility to scale the production of nanomaterials. One approach to the high-production scaling of nanomaterial deposition is to use an equal-down/equal-up approach [35]. This approach starts from the process or product to be realized at the commercial scale, where the main requirements and parameters are identified for successful application in the marketplace. These key requirements (e.g., film properties, cycle times, etc.) are specified by industrial partners and used to specify design goals for laboratory-scale devices in an equal-down step. The design goals provide the demonstration requirements necessary to enable scale-up. The reaction conditions of the laboratory design are designed to be equal to the commercial-scale design with respect to the governing heat transfer, mass transfer, and reaction kinetics, leading to similarity with respect to key reactor geometries and materials, fluid dynamics, mixing methods, reaction engineering approaches, and thermal management strategies. A consequence of developing the laboratory-scale reactor and process chemistry, which controls the microscale heat transfer, mass transfer, and reaction kinetics, is the definition of the shape and structure of the active unit reactor cell that can be replicated to produce higher chemical production volumes. Unit cell results obtained from the laboratory-scale demonstration are then used for the detailed design of the commercial-scale reactor in an equal-up step. The key parameters of the unit cell, such as channel width, channel length, and modified residence time, are made to be the same in the commercial-scale reactor, with the only difference being the number of channels, the size of headers, and perhaps, the techniques used to fabricate the reactor.
The authors acknowledge financial support from the W.M. Keck Foundation, National Science Foundation CAREER CTS-0348723, CBET-0654434, Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE), Industrial Technology Program (ITP), Nanomanufacturing Activity through award NT08847 DOE ITP, instrumentation equipment grant from the Murdock Charitable Trust (2010004), and Oregon Nanoscience and Microtechnology Institute (ONAMI).
1. A. Ennaoui, M. Weber, R. Scheer, and H.J. Lewerenz. Chemical-bath ZnO buffer layer for CuInS sub(2) thin-film solar cells. Sol. Energ. Mater. Sol. Cell, 54(1/4): 277–286 (1998).
2. C.H. Huang, S.S. Li, W.N. Shafarman, C.-.H Chang, E.S. Lambers, and L. Rieth. Study of Cd-free buffer layers using Inx(OH,S)y on CIGS solar cells. Sol. Energ. Mater. Sol. Cells., 69(2): 131–137 (2001).
3. C.H.T. Tseng, B.K. Paul, C.-H. Chang, and M.H. Engelhard. Continuous precipitation of ceria nanoparticles from a continuous flow micromixer. Int. J. Adv. Manuf. Technol., accepted (2012).
4. J.J. Hu and J.S. Zabinski. Nanotribology and lubrication mechanisms of inorganic fullerene-like MoS2 nanoparticles investigated using lateral force microscopy (LFM). Tribology Letters, 18(2): 49–57 (2005).
5. S. Stewart, P. Albertus, V. Srinivasan, I. Plitz, N. Pereira, G. Amatucci, and J. Newman. Optimizing the performance of lithium titanate spinel paired with activated carbon or iron phosphate. J. Electrochem. Soc., 155(3): A253–A261 (2008).
6. T.J. Hendricks, S. Krishnan, C. Choi, C.-H. Chang, and B.K. Paul. Enhancement of pool boiling heat transfer using nanostructured surfaces on aluminum and copper. Int. J. Heat Mass Transfer, 53(15–16): 3357–3365 (2010).
7. S. Krishnadasan, J. Tovilla, R. Vilar, A.J. deMello, and J.C. deMello. On-line analysis of CdSe nanoparticle formation in a continuous flow chip-based microreactor. J. Mater. Chem., 14: 2655 (2004); E.M. Chan, R.A. Mathies, and A.P. Alivisato. Size-controlled growth of CdSe nanocrystals in microfluidic reactors. Nano Letters, 3: 199–201 (2003); H. Nakamura, Y. Yamaguchi, M. Miyazaki, H. Maeda, M. Uehara, and P. Mulvaney. Preparation of CdSe nanocrystals in a micro-flow-reactor. Chem. Commun., 2844–2845 (2002); B.K.H. Yen, N.E. Stott, K.F. Jensen, and M.G. Bawendi, A continuous-flow microcapillary reactor for the preparation of a size series of CdSe nanocrystals. Adv. Mater., 15: 1858 (2003).
8. S. Liu, C. Chang, B.K. Paul, and V.T. Remcho. Convergent synthesis of polyamido dendrimer using a continuous flow microreactor. Chem. Eng. J., 135(1): S333–S337 (2008); S.-H. Liu, C.-H. Chang, B.K. Paul, V.T. Remcho, and B. Abhinkar. Convergent synthesis of dendrimers using continuous flow microreactors. Presented at AIChE Spring National Meeting, Orlando FL, April 24 (2006); C.-H. Chang, V.T. Remcho, B.K. Paul, et al. Progress towards chip-based high-throughput dendrimer synthesis. Presented at IMRET 8, Atlanta, GA (2005).
9. C.-H. Chang, B.K. Paul, V.T. Remcho, S. Atre, and J.E. Hutchinson. Synthesis and post-processing of nanomaterials using microreaction technology. J. Nanoparticle Res., 10(6): 965–980 (2008).
10. S.-Y. Han, Y.-J. Chang, D.-H. Lee, S.-O. Ryu, T.J. Lee, and C-H. Chang. Chemical nanoparticle deposition of transparent ZnO thin films. Electrochem. Solid-State Lett., 10(1): K1–K5 (2007).
11. G. Hodes. Chemical Solution Deposition of Semiconductor Films. Marcel Dekker, New York, 2003.
12. R. Ortega-Borges and D. Lincot. Mechanism of chemical bath deposition of cadmium-sulfide thin-films in the ammonia-thiourea system—In-situ kinetic study and modelization. J. Electrochem. Soc., 140(12): 3464–3473 (1993).
13. H. Löwe, W. Ehrfeld, V. Hessel, T. Richter, and J. Schiewe. Micromixing technology. In Proceedings of the 4th International Conference on Microreaction Technology (IMRET 4), 2000, pp. 31–47.
14. Y.-J. Chang, Y.-W. Su, D.-H. Lee, S.-O. Ryu, and C.-H. Chang. Investigate the reacting flux of chemical bath deposition by a continuous flow microreactor. Electrochem. Solid-State Lett., 12(7): H244–H247 (2009).
15. P.-H. Mugdur, Y.-J. Chang, S.-Y. Han, A.A. Morrone, S.-O. Ryu, T.J. Lee, and C.-H. Chang. A comparison of chemical bath deposition of CdS from a batch reactor and a continuous flow microreactor. J. Electrochem. Soc., 154(9): D482–D488 (2007).
16. Y.-J. Chang, P.-H. Mugdur, S.-Y. Han, A.A. Morrone, S.-O. Ryu, T.J. Lee, and C.-H. Chang. Nanocrystalline CdS MISFETs fabricated by a novel continuous flow microreactor. Electrochem. Solid-State Lett., 9(5): G174–G177 (2006).
17. B. K. Paul, C.L. Hires, Y.-W. Su, C.-H. Chang, S. Ramprasad and D. Palo, A uniform residence time flow cell for the microreactor-assisted solution deposition of CdS on a FTO-glass substrate, Crystal Growth Des. (2012), in press.
18. K.M. McPeak and J.B. Baxter, ZnO Nanowires grown by chemical bath deposition in a continuous flow microreactor, Crystal Growth Des, 9: 4538–4545, (2009).
19. S.-Y. Han, Y.-J. Chang, D.-H. Lee, S.-O. Ryu, T.J. Lee, and C-H. Chang. Chemical nanoparticle deposition of transparent ZnO thin films. Electrochem. Solid-State Lett., 10(1): K1–K5 (2007).
20. J.Y. Jung, N.-K. Park, S.-Y. Han, G.B. Han, T.J. Lee, S.O. Ryu, C.-H. Chang, The growth of the flower-like ZnO structure using a continuous flow microreactor, Curr. Appl. Phys., 8(6): 720–724 (2008).
21. D.A. Tomalia, J.R. Dewald, M.R. Hall, S.J. Martin and P.B. Smith, Preprints of the 1st SPSJ International Polymer Conference, Society of Polymer Science Japan, Kyoto, 1984; p 65.
22. C. Wu, M.W. Breshbiel, R.W. Kozack, and O.A. Gansow. Metal-chelate-dendrimer-antibody constructs for use in radioimmunotherapy and imaging. Bioorg. Med. Chem. Lett., 4(3): 449–454 (1994).
23. L.D. Margerum, B.K. Campion, M. Koo, N. Shargill, J. Lai, A. Marumoto, and P.C. Sontum. Gadolinium(III) DO3A macrocycles and polyethylene glycol coupled to dendrimers—Effect of molecular weight on physical and biological properties of macromolecular magnetic resonance imaging contrast agents. J. Alloys Comp., 249(1–2): 185 (1997).
24. Y. Kim and S.C. Zimmerman. Applications of dendrimers in bio-organic chemistry. Curr. Opin. Chem. Biol., 2(6): 733–742 (1998).
25. A.R. Menjoge, R.M. Kannan, D.A Tomalia, Dendrimer-based drug and imaging conjugates: design considerations for nanomedical applications, Drug Discovery Today, 15(5–6): 171–185 (2010).
26. E. Buhleier, W. Wehner, and F. Vögle. Cascade-chain-like and nonskid-chain-like syntheses of molecular cavity topologies. Synthesis, 2: 155 (1978).
27. D.A. Tomalia, H. Baker, J. Dewald, M. Hall, G. Kallos, S. Martin, J. Roeck, J. Ryder, and P. Smith. A new class of polymers—Starburst-dendritic macromolecules. Polym. J., 17(1): 117 (1985).
28. J.M.J. Fréchet, Y. Jiang, C.J. Hawker, and A.E. Philippides. In Proceedings of IUPAC International Symposium on Macromolecules, Seoul, 1989, pp. 19–20.
29. S.-H. Liu, C.-H. Chang, B.K. Paul, and V.T. Remcho. Convergent synthesis of polyamido dendrimer using a continuous flow microreactor. Chem. Eng. J., 135S: S333–S337 (2008).
30. S.-H. Liu and C.-H. Chang. High rate convergent synthesis and deposition of polyamide dendrimers. Chem. Eng. Technol., 30(3): 334–340 (2007).
31. C.R. Kim, S.Y. Han, C.H. Chang, T.J. Lee, and S.O. Ryu. A study on copper selenide thin films for photovoltaics by a continuous flow microreactor. Mol. Crystals Liquid Crystals, 532: 455–463 (2010).
32. M.S. Park, S.-Y. Han, E.J. Bae, T.J. Lee, C.H. Chang, and S.O. Ryu. Synthesis and characterization of polycrystalline CuInS2 thin films for solar cell devices at low temperature processing conditions. Curr. Appl. Phys., 10(3): S379–S382 (2010).
33. C.R. Kim, S.Y. Han, C.H. Chang, T.J. Lee, and S.O. Ryu. Synthesis and characterization of CuInSe2 thin films for photovoltaic cells by a solution-based deposition method. Curr. Appl. Phys., 10(3): S383–S386 (2010).
34. K. Mullins. Moth eyes inspire better optics. Northwest Science and Technology, Spring 2010.
35. F. Becker, J. Albrecht, G. Markowz, R. Schütte, S. Schirrmeister, K.J. Caspary, H. Döring, T. Schwartz, and E. Schüth. DEMiS: Results from the development and operation of a pilot-scale microreactor on the basis of laboratory measurements. IMRET 8, TK 131 f.2005.