We have developed a simple imprinting technique that allows patterning over a nonflat substrate without the need for planarization. In this process, a polymer film is spin coated onto the mold and then transferred to a patterned substrate by imprinting. By selecting polymers with different mechanical properties, either suspended structures over wide gaps or supported patterns on raised features of the substrate can be obtained with high uniformity. It is found that imprinting at a temperature well above the glass transition temperature (Tg) of the polymer causes the thin residue film between features to dewet from the mold, which can greatly simplify the subsequent pattern transfer process. Multilayer three-dimensional polymer structures have also been successfully fabricated using this new imprinting method. The yield and dimensional stability in the multilayer structure can both be improved when polymers with progressively lower Tg are used for different layers. Compared to existing techniques for patterning on nonflat substrates, the current method has a number of advantages, including simplicity, versatility, high resolution, and low pattern distortion.

A reversal imprinting technique was developed in this study. A polymer layer was first spin coated on a patterned hard mold, and then transferred to a substrate under an elevated temperature and pressure. The reversal imprinting method offers an advantage over conventional nanoimprinting by allowing imprinting onto substrates that cannot be easily spin coated, such as flexible polymer substrates. Another unique feature of reversal imprinting is that three different pattern-transfer modes can be achieved by controlling the degree of surface planarization of the mold after spin coating the polymer resist as well as the imprinting temperature. ''Embossing'' occurs at temperatures well above the glass transition temperature (T g) of a polymer; ''inking'' occurs at temperatures around T g with nonplanarized polymer coating surface on the mold; and ''whole-layer transfer'' occurs at temperatures around T g but with a somewhat planarized surface. These three imprinting modes have been quantitatively correlated with the surface planarization of the mold after polymer coating and the imprinting temperature.

High throughput production of nanofibers by means of "Tip-less Electrospinning" (TLES) has been demonstrated using a circular cylinder as the emitting electrode. Electrohydrodynamics instabilities on a thin liquid film under high electrical field can generate artificial liquid jets for the TLES process. Experimental results have shown that the yield of poly(ethylene oxide) nanofibers can be more than 260 times in weight as compared with a single-jet electrospinning process. Parameters affecting the TLES process including applied voltage, polymer solution concentration, electrode-to-substrate distance and the thickness of liquid films have been characterized. As such, TLES has potential for high-throughput, massive production of electrospun nanofibers.

We have developed a technique to create polymer film patterns on substrates with microstructures using an elastomeric polydimethylsiloxane (PDMS) pad, wherein either a continuous or a patterned film on flat PDMS was used for pattern creation. During patterning of the continuous film, a polymer film is first spin coated onto the elastomer pad, and the pad is then brought into contact with the patterned substrate of interest. Since the PDMS can deform elastically around features on the substrate and its surface has low interfacial energy, films on PDMS can be successfully transferred onto the substrate. The resulting profile of the transferred film would depend on the pattern dimensions of the substrate, polymer properties, and conditions for imprinting. Three distinctive film patterns can be achieved, which we designate as continuous film transfer over microstructures, film transfer on both trenches and protrusions, and film transfer on protrusions. Interestingly, a negative replica of patterns on substrates can be simultaneously created on the PDMS pad in the last patterning process and the pattern can be further utilized in subsequent patterning steps. This affords the capability of transferring a patterned film from the PDMS to the sidewalls of the topographic features. Because of the great versatility of this patterning technique, it can be used to rapidly form channels, conformal coating on a patterned substrate, and micro- or nanometer sized patterns inside trenches of a patterned substrate.

Metallic microstructures are the essential building blocks in microelectronics as electric interconnection between different functioning parts. [1] In the blooming field of optoelectronics, metallic microstructures also play a key role due to surfaceplasmon-plariton-related effects, [2,3] applications in miniaturization of photonic circuits, [4,5] near-field optics, [6] and singlemolecule optical sensing. [7,8] The electromagnetic resonance in metallic microstructures may offer unique properties that do not exist in natural materials, such as negative refractive index. [9,10] In previous studies, metallic microstructures were usually fabricated by photolithography, which was time-consuming and costly. [11] Template-assisted electrodeposition is an easy way to fabricate microstructures. For example, anodic aluminum oxide (AAO) [12][13][14] and polymeric membranes [15] have been used as molds, and the size of the channels in these systems can reach a few nanometers.[16] However, the destructivity in removal template renders it difficult to preserve the spatial order among the nanowires, [17,18] hence limits their applications in optoelectronics. We once introduced a selective electrodeposition method to fabricate two-dimensional metallic structures, where the substrate surface was modified by stripes of lipid monolayers, on which nucleation of metal crystallites was easier.[19] However, in that case the width of the metallic wires was limited by the geometrical shape of templates, that is, the width of metallic wires could not be tuned unless a new template was used. Furthermore, previously fabricated wires [19] were essentially flat belts lying on the substrate, which would not be effective if applied to sensors where a large specific surface area is usually expected. [20] Therefore, one of the important challenges in fabricating metallic microstructures is to find an easy, repeatable, and controllable method to meet the increasing demands in optoelectronics and plasmonics.In this communication, we report a new template-assisted electrochemical approach to fabricate arrays of metallic nanowires. Unlike conventional template-assisted growth, where the generated wires are confined by the size of template, in our case the width of the metallic wires can be tuned by changing the control parameters of electrodeposition. By imprinting polymer stripes on a silicon surface, the concave corner of polymer stripes and silicon substrate provides a preferential nucleation site for the formation of metal nanowires. The width of wires can be tuned from 25 nm to a few hundred nanometers. Further, we demonstrate that this method can be applied for fabricating more complicated structures rather than straight lines only.In our experiments, the metallic nanowires are electrodeposited with the help of polymer stripes embossed on silicon surfaces. To form the patterned substrate, a thin film of poly(methylmethacrylate) (PMMA) (mr-I 7030E M w ¼ 75 kDa) is initially spin-coated on the silicon wafer. The film thickness is about 300 nm. Th...

Articles you may be interested inSidewall-angle dependent mold filling of three-dimensional microcavities in thermal nanoimprint lithography J. Vac. Sci. Technol. B 30, 06FB09 (2012); 10.1116/1.4764096 Duo-mold imprinting of three-dimensional polymeric structures