Luminescent nanocrystal modified photoresin for fabrication of high-aspect ratio three-dimensional microstructures

The organic polymers incorporating inorganic nanoparticles can result in material possessing with unique size-dependent properties, such as mechanical, optical, electrical, thermal and magnetic, that can be istinctively conveyed into the composite by the suitable nanoparticles. When nano-objects such as luminescent nanocrystals (NCs) particles are embedded into photostructurable polymer materials, they provide a new, highly processable material, which enables the engineering of novel, highly integrated micro/nanosystems (MEMS/NEMS) with enhanced intrinsic functional properties. The current effort aims, on one side, to the incorporation of the functional nanoparticles into the polymer matrix and, on the other side, to preserve the characteristic structurability of photosensitive polymers by UV lithography. In this paper we present a novel composite material obtained from the combination of luminescent nanocrystals (NCs) of cadmium selenide coated by a shell of zinc sulfide (CdSe@ZnS) with epoxy-based photosensitive resins. This class of material is well-known for polymer MEMS applications due to its unique structurability by UV lithography and mechanical properties. Luminescent CdSe@ZnS NCs with tuneable size, exhibiting size-dependent optoelectronic properties have been synthesized by means of colloidal routes methods. The presence of an organic ligand shell makes the NCs soluble in different types of solvents and thus enables their processing and incorporation into polymers. For this purpose a range on different solvents were tested in order to optimize the incorporation of the luminescent NCs (4.7 nm) in the epoxy-based resin. Spectroscopical and microscopical investigation were performed on the prepared composite samples and the effects of the NC concentration, size, surface chemistry and solvent composition were investigated in order to evaluate their role on the spectroscopical and structural behaviour of the nanocomposite properties. The NC-doped resin was then spin-coated on a silicon wafer, baked, exposed to UV light through a mask and developed. The results confirm the overall UV-structuring capability of the modified polymer and the NC emission properties were demonstrated to be retained. Furthermore, the luminescent properties of the NCs were retained after patterning by UV photolithography, although some aggregation of NCs was observed. The presence of organic ligand shell at the colloidal NC surface shows to be relevant for the subsequent processing of the doped photo-resin. The presented route looks promising for the microfabrication of polymer MEMS with inherent functionalities. Further optimization needs to be implemented also in view of a possible extension to incorporation of different types of colloidal NCs.

High-aspect ratio three-dimensional photoresin-based microstructures doped with luminescent nanocrystals

Jürgen Brugger, Vahid Fakhfouri

Incorporation of nanoparticles into photostructurable polymer materials enables the engineering of novel, highly integrated micro/nanosystems (MEMS/NEMS) with enhanced intrinsic functional properties. For instance, nanoparticles with (semi)conducting, optical or magnetic properties can be incorporated into polymers in order to convey their inherent functionalities to the final material, which can then be exploited for sensor and actuator applications. The current challenge is two-fold: (a) the incorporation of the functional nanoparticles into the polymer matrix and (b) the maintaining of the structurability of photosensitive polymers by UV lithography. In this paper we present a novel approach to further advance this field by combining luminescent nanocrystals (NCs) of cadmium selenide coated by a shell of zinc sulfide (CdSe@ZnS) with epoxy-based photosensitive resins. This class of material is well-known for polymer MEMS applications due to its superb structurability by UV lithography and mechanical properties. Here, colloidal methods have been used to provide luminescent CdSe@ZnS NCs with tunable size, exhibiting size-dependent optoelectronic properties. The presence of an organic ligand shell makes the NCs soluble in organic solvents and thus enables their processing and incorporation into polymers. The luminescent NCs (4.7 nm) were successfully incorporated in the epoxy-based resin by exploiting their solubility in a suitable common solvent. The NC-doped resin has been spin-coated on a silicon wafer, baked, exposed to UV light through a mask and developed. The same processing was applied to un-doped resin used as reference material. The obtained structures were inspected by means of optical, scanning electron, fluorescent and atomic force microscopy, as well as surface profilemetry. The results confirm the overall UV-structuring capability of the modified polymer. The emission properties of the NCs were demonstrated to be retained upon incorporation in the resin and spin coating. The luminescent properties of the NCs were retained after patterning by UV photolithography, although some aggregation of NCs was observed. The presence of organic ligand shell at the colloidal NC surface shows to be relevant for the subsequent processing of the doped photo-resin. The presented route looks promising for the microfabrication of polymer MEMS with inherent functionalities. Further optimization needs to be implemented also in view of a possible extension to incorporation of different types of colloidal NCs.

2006

Low-Stress UV-Curable Hyperbranched Polymer Nanocomposites for High-Precision Devices

Valérie Geiser

The aim of this thesis was to investigate the process-structure-property relationships of UV-curable hyperbranched polymer (HBP)/silica nanocomposites. Special attention was paid to the interplay between photo-conversion, rheological behavior, shrinkage, and stress dynamics. This knowledge was used to maximize the shape fidelity and dimensional stability of imprinted nano-patterns made with these nanocomposites. Two different processing routes, resulting in different nanocomposite morphologies, were compared. The first and more conventional approach was ultrasonic, solvent-assisted mixing with solid silica nanoparticles followed by UV curing, which led to a discrete dispersion of silica particles in the matrix. The second and more novel approach was a dual-cure process combining UV and sol-gel processing with liquid tetraethyl orthosilicate. The sol-gel process, using a low-viscosity organometallic precursor, overcame the potential processing issue of the highly viscous particulate nanocomposites and led to a hybrid material with a homogeneous silica network at nanometer scale. Rheological analysis of silica nanoparticle suspensions up to the concentrated regime in the HBP showed an exponential increase of the viscosity with the particle fraction for well-dispersed discrete particle systems. A liquid-to-solid transition occurred in the 5 to 10 vol% range, which was correlated with an immobilized layer of polymer hydrogen-bonded to the silanol groups on the surface of the particles, as was confirmed by calorimetric analysis. Polymerization kinetics of HBP nanocomposites and hybrids were analyzed by means of photo differential scanning calorimetry using an autocatalytic model. A time-intensity-superposition principle with power-law dependence was established, which invalidated the classic radiation dose equivalence principle. Gelation and modulus build-up were monitored using photo-rheology. The calorimetric and rheological data were combined in the form of time-intensity-transformation diagrams. It was found that gelation was delayed with respect to conversion up to 36% when lower UV intensities were used, which favored reduce polymerization stresses. The dynamics of polymerization shrinkage and internal stress build-up were investigated using photo-hyphenated interferometry and beam-bending methods. The linear shrinkage of the HBP was as low as 4.5%, which was further reduced to 3.3% with the addition of 20 vol% nanoparticles. The residual stress of HBP/SiO2 nanocomposites was below 5.5 MPa. That is well below the level of standard non-reinforced resins, in spite of an increased stiffness. For the sol-gel hybrids with 20 vol% SiO2, stress reduction by 50% with simultaneous stiffness increase by 20% with respect to corresponding particulate composites was demonstrated. The coefficient of thermal expansion was also lowered by 30%. Finally, nanogratings with a period of 360 nm and a step height of 12 nm were fabricated using UV-nanoimprint lithography with a glass or nickel master in a low-pressure (max. 6 bar) and rapid (≈1 min) process. Stable gratings were imprinted in the composite material containing up to 25 vol% of silica nanoparticles, despite the high viscosity of such compositions. Thanks to the exudation of a HBP-rich surface layer, the shape fidelity in terms of period of the replicated patterns was within 98% of the master, with shape distortion increasing with internal stress. The obtained nanogratings were used as a substrate for grated high refractive index TiO2 waveguides and then applied as wavelength-interrogated optical sensors (WIOS). An immunoassay with the polymer WIOS showed the same ultrahigh detection sensitivity as the standard glass-based devices. Therefore, the novel polymer-based sensor should be useful to probe contaminants in liquids with concentrations as low as a few ppb.

EPFL2010

Nanocomposites based on luminescent colloidal nanocrystals in polymers: new opportunities for micro and nano fabrication

Jürgen Brugger, Vahid Fakhfouri

In recent years a wide range of materials including metals, semiconductors and metal oxides have been synthesized with nanometer dimensions. In particular II–VI semiconductor nanocrystals (NCs) are receiving considerable attention for fundamental studies as zerodimensional quantum confined material and for their potential application in optoelectronics and photonics. The colour tunability of the emission of semiconductor NCs as a function of size is one of their most attractive properties. A current bottle-neck of these materials is their poor compatibility with micropatterning methods in general and MEMS processes in particular. Inorganic colloidal nanoparticles encapsulated in polymer matrix may lead to versatile hybrid micro- and nanoscale devices made by simple techniques. Colloidal methods provide effective routes to prepare semiconductor NCs that are soluble in organic solvents with a narrow size distribution. Moreover the ability to manipulate their surface chemistry is a powerful tool to place the NCs in almost any chemical environment, due to the possibility of properly engineering their surface by ligand exchange techniques. Recent efforts have been directed towards the fabrication of polymer nanocomposites containing nanometer sized inorganic semiconductor. Such interest is due to the combination the processability and the structural properties of the polymeric matrix, i.e. thermoplastic or photosensitive materials, and the unique optoelectronic characteristics of the inorganic moiety, that can be thus used as emitters for many purposes such as optical switches, sensors, electro-luminescent devices, lasers and biomedical tags. In particular, microelectromechanical systems (MEMS) that are currently built mainly from Silicon, dielectrics, polymers and some selected metals will benefit from new functional polymeric nanocomposit materials, which combine simple UV processing and superior material properties such as biocompatibility, chemical stability, luminescence, photochromism and more. This paper describes the combination of synthesis and functionalization of luminescent colloidal NCs for polymer modification, with micro- and nanofabrication methods to form basic elements of advanced polymer MEMS/NEMS with increased inherent functionality. We prepared nanocomposites based on highly luminescent II-VI semiconductors colloidal CdS, CdSe, and CdSe@ZnS, core shell type. The NCs were incorporated in poly-methyl methacrylate (PMMA) a thermoplastic material, and SU-8, a photosensitive polymer with superior lithographic properties, respectively. Spectroscopical (UV-vis absorption and emission), structural (TEM) and morphological analysis were performed on both types of nanocomposite solutions as well as the thin films spin coated onto silicon and quartz substrates. The effect of NC composition, concentration, size, and surface chemistry was evaluated in order to understand the role played by these factors on the optical properties of the investigated nanocomposites. The presence of organic ligand shell at colloidal NC surface shows to be critical for their incorporation either into SU8 and PMMA. Incorporation of the different types of luminescent NCs in PMMA and SU-8 did not show any relevant change of the optical properties of the NCs. The excitonic peaks as well as the emission properties were retained. Preliminary spinning, UV exposure and development experiments were performed on the SU-8 based nanocomposites. The obtained structures were inspected by optical microscope and confirmed that the overall UVstructuring capability of the modified polymer was not macroscopically affected by the NC incorporation in the polymer.

2005

Low-Stress UV-Curable Hyperbranched Polymer Nanocomposites for High-Precision Devices

Valérie Geiser

EPFL2010

Nanocomposites based on luminescent colloidal nanocrystals in polymers: new opportunities for micro and nano fabrication

Jürgen Brugger, Vahid Fakhfouri

2005

High-aspect ratio three-dimensional photoresin-based microstructures doped with luminescent nanocrystals

Jürgen Brugger, Vahid Fakhfouri

2006