The Study Of Nanofabrication Methods Based On The Phase Separation Of Polymer Blends And Their Applications

Nano materials have attracted much attention for the unique properties,such as the small-size effect,the surface effect,the quantum size effect the macroscopic quantum tunneling effect,etc.,with broad potential applications.In particular,the adjustment of nanostructures is an important development orientation of nano materials.Recently,nanofabrication technology has blossomed a lot,contributing to the further research of the adjustment of nanostructures and the corresponding properties.However,to save energy and meet the needs of industrialization,it is still urgent and a challenge to develop nanofabrication methods with simple processes,low cost,high productivity and large scale.Hence,this article demonstrated a simple and effective approach based on the phase separation of polymer blends.Immisible polymers were dissolved in the common solvent of tolunene and subsequently spin-coated on the substrate forming a polymer blend film.The phase separation occurred with the rapid volatilization of the solvent during the spin-coating process and the phase-separated morphology was gradually fixed with the depletion of the solvent.By adjusting the factors that affect the phase-separated nanostructures,including polymer polarization,molecular weight,weight ratio,solvent polarization,solution concentration,spin speed,temperature,solubilizer,substrate tension and air humidity,various controllable nanostructures could be obtained,which were nano-disordered and macro-uniform.Generally,it is a challenge to obtain sub-100 nm nanostructures even by adjusting the abovementioned factors.Furthermore,the low aspect ratio of the phase-separated nanostructures and similarity of the polymers limited the pattern transfer to the target substrate with nanofabrication methods,such as developing,reaction ion etching,metal deposition,lift-off,etc.Three different silicon-containing polymers,polydimethylsiloxane(PDMS),polyphenylsilsequioxane(PPSQ)and acrylated polysiloxane,were used to produce polymer blends with PS in toluene.Spin-coated thin films of the polymer blends were treated by O2 reactive-ion etching(RIE).The PS constituent was selectively removed by O2 RIE,whereas the silicon-containing phase remained because of the high etching resistance of silicon.This selective removal of PS substantially enhanced the contrast of the phase separation morphologies for better pattern transfer in the following processing steps.We investigated the effects of the silicon-containing constituents,polymer blend composition,concentration of the polymer blend solution,surface tension of the substrate,and the spin-coating speed on the ultimate morphologies of phase separation.The average domain size,ranging from 20 nm to 10 ?m,was tuned through an interplay of these factors.In addition,by forming the polymer blend film on a pure organic layer or combining the metal-assisted chemical etching(MaCE)technology,more varieties of nanostructures such as nanowires and overhang nanopillars were obtained and the aspect ratio of the phase separation morphologies was further amplified by a selective etching process,with broad potential applications.Then,to verify the application possibilities,we demonstrated the applications in surface modification of photoanodes,anti-reflective surfaces,super-hydrophobic surfaces and surface enhanced Raman scattering substrates.The main research contents can be stated as follows:The nanofabrication method based on the phase separation of silicon-containing polyer/PS polymer blends.Three silicon-containing polyers with different characteristics,PDMS,PS and liquid UV-curable acrylated polysiloxane,were used to produce polymer blends with polystyrene(PS).Firstly,PDMS(water contact angle of 109.2°)is much more hydrophobic than PS(water contact angle of 89.2°),both of which were solved into toluene and subsequently spin-coated onto the hydrophilic Si substrate(36.1°).After the O2 RIE process,micro-and nano-poles were achieved.In addition,the effect of the weight ratio of PS and PDMS on morphologies were further investigated.Then,the morphology of PS/PPSQ polymer blend layer was investigated in this article.After the O2 RIE process,PPSQ nanopillars and bicontinuous PPSQ+PS phase residual layer were achieved.By investigating the effects of solution concentrations,weight ratio of polymer constituents,spin speeds and surface energy of substrates on the morphologies,we mastered the exact parameters to control the feature size and distribution of nanopillars,and thus the nanopillar diameters ranged from 16 nm to 260 nm was obtained.Additionally,to obtain dense and high-aspect-ration sub-50nm nanopillars,we employed the liquid prepolymer of cross-linkable acrylated polysiloxane with a low viscosity of about 100 cP as one constituent to provide a relatively good miscibility with PS.Meanwhile,the surface of PS(83°)and polysiloxane(89°)exhibited similar water contact angles,which indicated the similar polarity of the two materials.After the O2 RIE process,dense and high-aspect-ration sub-50nm nanopillars were achieved.Then,the effects of solution concentrations,weight ratio of polymer constituents and spin speeds on the morphologies were investigated.By adjusting the phase separation parameters,high-aspect-ratio(3:1)and dense nanopillars(?1011 cm-2)with average diameter ranging from 17 nm to 80 nm were achieved.The investigation of charge behavior mechanism and surface modification of n-Si photoanodes with the bilayer adjustable etching template route based on phase separation of PS/PPSQ polymer blend.By forming a dual-phase polymer with hierarchical structure of PPSQ nanopillars and bicontinuous PPSQ+PS phase residual layer on the Si/PMMA/SiO2 substrate via a spin-coating phase separation process,the bilayer adjustable etching template was generated,which could be tailored by selectively etching the patterns of PPSQ and PS and adjusting the phase separation parameters.This proposed method with low cost,simple processes and controllable nanostructures allowed us to deposit multifunctional layers with tunable nanostructures by electron beam deposition method onto surface of Si electrodes and deeply understand the mechanism of charge separation and transfer at semiconductor/electrolyte interface after modification by functional layers with different microstructures.Controllably etching the polymer template allowed us to physically deposit functional Ni layers with tunable nanostructures of nanonets,broken nanonets and dispersed nanodots with various coverage and to investigate the nanostructural effects on the performance.We found that the dispersed Ni nanoparticles on Si surface provided better performance,and the Si/SiOx/10%Ni nanodots photoanode provided the optimal performance of high saturated photocurrent density of 40 mA cm-2 and low onset potential of 1.0 VRHE.By the deep investigation of the open circuit photovoltage,charge injection and separation efficiency,surface potential distribution of the Si/SiOx/32%Ni broken nanonets and the Si/SiOx/38%Ni nanonets with similar light transimissivity,we concluded the dispersed Ni nanoparticles may induce the inhomogeneous barrier height driven efficient charge extraction,greatly contributing to the high performance.In addition,introducing NiOx layer with high work function to form Ni/NiOx nanostructures should lead to higher energy barrier and greater band bending,resulting in enhanced charge extraction efficiency and decreased charge recombination behavior under the influence of inhomogeneous barrier height effect.Moreover,introducing NiOx layer provided more high-efficiency catalysis which promoted the oxygen evolution reaction.In the combination of the abovementioned two positive effects,the 32%Ni/NiOx broken nanonets modified Si photoanode exhibited a low onset potential of 0.93 V vs.reversible hydrogen electrode,a high saturated photocurrent density of 40 mA cm-2 and a stability of 50 h in 1.0 M K-borate under AM 1.5 G illumination.Our proposed dual-phase lithographic method may open a new avenue to easily assemble functional layers on photoelectrodes for efficient solar energy conversion.The fabrication of various high-aspect-ratio nanostructures.To broaden the potential applications of the proposed lithographic method based on phase separation of polymer blends,we further enhanced the aspect ratio of the nanostructures and obtained various nanostructures with different morphologies.On the one hand,the SU-8 photoresist layer was introduced as the transfer layer,combined with the phase separation of PS/PPSQ polymer blend,to obtain the high-aspect-ratio nanopillars(7.5:1)and more varieties of nanostructures such as nanotapers and overhang nanopillars.On the other hand,the phase separation of PS/acrylated polysiloxane polymer blend assisted with the metal assisted chemical etching(MaCE)technology fabricated dense high-aspect-ratio Si nanowires.The application of anti-reflective surface.The fabricated 4 in.Si wafer with high-aspect-ratio(1000:1)nanowires by the phase separation of PS/acrylated polysiloxane polymer blend and MaCE technology could provide effective light trapping,and thus showed extremely low reflectance(0.1%)over the entire spectral ranging from 200 to 800 nm.This manifested the practical application of the lithographic method in anti-reflective surface.The application of super-hydrophobic surface.The dense and high-aspect-ration sub-50 nm nanopillars fabricated by the phase separation of PS/acrylated polysiloxane polymer blend offered high surface roughness.After modified with the trichloro(1H,1H,2H,2H-perfluorooctyl)silane layer with low surface energy,the surface provided super-hydrophobic property with static water contact angle of 150.5°.On the other hand,the overhang nanopillars fabricated by the phase separation of PS/PPSQ polymer blend on the SU-8 photoresist transfer layer were used to overcome the energy barrier to maintain the Cassie state of liquid droplets.Hence,after the surface modification of trichloro(1H,1 H,2H,2H-perfluorooctyl)silane,the substrate with overhang nanopillars provided super-hydrophobic and nearly super-hydrophilic properties.The application of surface enhanced Raman scattering substrates.The dense and high-aspect-ration sub-50 nm nanopillars fabricated by the phase separation of PS/acrylated polysiloxane polymer blend provided a large amount of nanogaps(?10 nm).After coating with a 30 nm-thick Au layer,the nanogaps between polymer nanopillars were changed into nanogaps between Au layers and many "hot spots" were obtained.Hence,the abovementioned substrate should be suitable as a surface enhanced Raman scattering(SERS)substrate in theory.To verify the real effect,we demonstrated the Raman spectra of trans-1,2-bis(4pyridyl)ethylene(BPE)with the abovementioned SERS substrate,which showed high homogeneity and high enhancement factors of 108-109.Nanofabrication method based on the phase separation is a combination of "top-down" and "bottom-up" methods,which is espectially suitable to fabricate the nanostructural materials and devices with high tolerability of order degree,regularity and dimensional distribution.The size effect of the nanostructures can enhance the macro-performance of these materials and devices,such as anti-reflective coatings,transmissivity enhancement,sensing,catalysis,etc.In addition,the disordered pattern and dimentional distribution can further enlarge the spectral response and eliminate the rainbow effect resulted from the ordered nanostructures.