Method for fabricating nanoporous polymer thin film and corresponding method for fabricating nanoporous thin film

A method for fabricating nanoporous polymer thin film includes steps as follows. A polymer thin film is provided, wherein a polymer solution including a polymer is coated on a substrate to form the polymer thin film. A swelling and annealing process is provided, wherein the polymer thin film is disposed inside a chamber with a vapor of a first solvent, the polymer thin film is swollen and annealed to form a swollen polymer thin film, and the swollen polymer thin film includes the polymer and the first solvent. A freezing process is provided, wherein the swollen polymer thin film is cooled to a temperature less than or equal to a crystallization temperature of the first solvent to crystallize the first solvent. A first solvent removing process is provided, wherein the first solvent is removed with a second solvent, such that a nanoporous polymer thin film is obtained.

RELATED APPLICATIONS

This application claims priority to Taiwan Application Serial Number 107105826, filed Feb. 21, 2018, which is herein incorporated by reference.

BACKGROUND

Technical Field

The present disclosure relates to a thin film fabricating method. More particularly, the present disclosure relates to a method for fabricating nanoporous polymer thin film and a corresponding method for fabricating nanoporous thin film.

Description of Related Art

Nanoporous materials are widely used in many fields, such as the optical filed, the biology field and medical field, due to the superior performance thereof. A conventional nanoporous material can be fabricated by an inverted method and a phase separation method, etc. However, such methods have disadvantages of complication and long process time.

Therefore, how to develop a method for fabricating a nanoporous thin film having cost-effective and short process time becomes a pursuit target for practitioners.

SUMMARY

The present disclosure provides a method for fabricating nanoporous polymer thin film including steps as follows. A polymer thin film is provided, wherein a polymer solution including a polymer coated on a substrate to form the polymer thin film. A swelling and annealing process is provided, wherein the polymer thin film is kept inside a chamber with a vapor of a first solvent, and the polymer thin film is swollen and annealed to form a swollen polymer thin film including the polymer and the first solvent. A freezing process is provided, wherein the swollen polymer thin film is cooled to a temperature less than or equal to a crystallization temperature of the first solvent to crystallize the first solvent. A first solvent removing process is provided, wherein the first solvent is removed with a second solvent such that a nanoporous polymer thin film is obtained.

The present disclosure provides corresponding method for fabricating nanoporous thin film including steps of providing a template, providing a filling process and providing a template removing process. The template includes a nanoporous polymer thin film fabricated by the abovementioned method. In the filling process, a mixture film is fabricated. The mixture film includes a first material and the nanoporous polymer thin film, and the first material is filled in a plurality of pores of the nanoporous polymer thin film. In the template removing process, the nanoporous polymer thin film is removed to form a nanoporous thin film composed of the first material.

DETAILED DESCRIPTION

Method for Fabricating Nanoporous Polymer Thin Film

FIG. 1shows a flow chart of a method100for fabricating nanoporous polymer thin film according to one embodiment of the present disclosure.FIG. 2shows a schematic illustration of the method100ofFIG. 1.FIG. 3shows a relationship between thickness and time of Step120ofFIG. 1. Refer toFIG. 1,FIG. 2andFIG. 3, the method100for fabricating nanoporous polymer thin film includes Step110, Step120, Step130and Step140.

In Step110, a polymer thin film220is provided. A polymer solution210including a polymer211is coated on a substrate300to form the polymer thin film220.

In Step120, a swelling and annealing process is provided. The polymer thin film220is kept inside a chamber400with a vapor510of a first solvent530, and the polymer thin film220is swollen and annealed to form a swollen polymer thin film230including the polymer211and the first solvent530.

In Step130, a freezing process is provided. The swollen polymer thin film230is cooled to a temperature less than or equal to a crystallization temperature of the first solvent530to crystallize the first solvent530.

In Step140, a first solvent removing process is provided. The first solvent530is removed with a second solvent such that a nanoporous polymer thin film240is obtained.

Therefore, the polymer thin film220is converted to the swollen polymer thin film230owing to the phase separation between the first solvent530and the polymer211occurred in the swelling and annealing process. Moreover, the nanoporous polymer thin film240can be fabricated through the freezing process and the first solvent removing process. The detail of the method100will be described below.

The polymer211included in the polymer thin film220can be polystyrene, and the substrate300can be a silicon wafer in the embodiment. The polymer solution210is fabricated by mixing polystyrene with neutral solvents such as chlorobenzene. The polymer solution210is spin-coated on the substrate300and then baked in a vacuum oven, such that the polymer thin film220including the polymer211is retained. In other embodiment not shown, the polymer211can be, but not limited to, poly(methyl-methacrylate), polysulfone or polycarbonate. Preferably, the polymer211can be amorphous polymers.

In the swelling and annealing process of Step120, the vapor510of the first solvent530is taken in the chamber400first, and then the polymer thin film220is put into the chamber400. In other word, the vapor510formed by the first solvent530under saturation pressure is included in the chamber400, and the first solvent530can be crystallizable solvents such as N,N-dimethylformamide. As shown inFIG. 3, the polymer thin film220is swollen when contacting with the vapor510, such that a thickness of the polymer thin film220is increased and remained at a certain value. After continuous vapor anneal, the polymer thin film220is converted to the swollen polymer thin film230. The vapor anneal facilitates inducing phase separation at nanoscale between the polymer211and the first solvent530of the swollen polymer thin film230. In other embodiment not shown, the first solvent530can be, but not limited to, dimethyl sulfoxide. Preferably, the first solvent530can be crystallizable solvent which is a liquid at room temperature and can be crystalized at low temperature.

In the freezing process of Step130, a liquid nitrogen540can be introduced into the chamber400to quickly freeze the swollen polymer thin film230. In other embodiment not shown, the swollen polymer thin film230can be frozen by any method which can freeze the swollen polymer thin film230, and the present disclosure will not be limited thereto. When a temperature of the swollen polymer thin film230is lower than the crystallization temperature of the first solvent530, the first solvent530will be crystallized and no reaction will occur. Hence, the phase separation between the polymer211and the first solvent530is remained.

In Step140, methanol can be severed as the second solvent to remove the first solvent530. The temperature of the swollen polymer thin film230will raise when the first solvent530is removed, which facilitates the removal of the first solvent530. In other embodiment not shown, the second solvent can be ethanol or isopropyl alcohol. Preferably, the second solvent can be lower aliphatic alcohols.

Therefore, the swollen polymer thin film230can be converted to the nanoporous polymer thin film240after freezing and removal of the first solvent530. The nanoporous polymer thin film240includes polymer211and a plurality of pores212which are net spaces left by removal of the first solvent530.

In the method100for fabricating nanoporous polymer thin film, when the polymer thin film220is exposed in the vapor510of the first solvent530, the first solvent530diffuses into the polymer thin film220such that the polymer thin film220is swollen, and the swollen polymer thin film230including the polymer211and the first solvent530is formed. A homogeneous spatial distribution of the first solvent530and the polymer211occurs initially; subsequently, the phase separation between the first solvent530and the polymer211begins. Finally, the developed morphologies are kinetically trapped by freezing in the liquid nitrogen540. The first solvent530is crystalized in low temperature, and the nanoporous polymer thin film240including the plurality of pores212can be formed by removing the first solvent530with the second solvent.

FIG. 4shows a relationship between different uptake ratios of the first solvent530and the thickness of the polymer thin film220. Refer toFIG. 4, a density of the pores212of the nanoporous polymer thin film240can be adjusted by the swelling and annealing process in Step120. Precisely, in the swelling and annealing process, a gas520is used for adjusting a mole fraction of the vapor510of the first solvent530in the chamber400to change a weight percentage of the first solvent530in the swollen polymer thin film230. Therefore, the uptake ratio of the first solvent530absorbed by the polymer thin film220, which is also the weight percentage of the first solvent530in the swollen polymer thin film230, is changed according to the mole fraction of the vapor510in the chamber400adjusted by the gas520. As shown inFIG. 4, the thickness of the polymer thin film220after swelling is changed, and the density (porosity) of the pores212of the nanoporous thin film240is changed accordingly.

In one example, when the mole fraction of the vapor510in the chamber400is changed to give the weight percentage of the first solvent530in the swollen polymer thin film230being 38% (under a condition that a processing time of the swelling and annealing process being 5 minutes), an average porosity of the nanoporous polymer thin film240is 40%. In another example, when the mole fraction of the vapor510in the chamber400is changed to give the weight percentage of the first solvent530in the swollen polymer thin film230being 24% (under a condition that a processing time of the swelling and annealing process being 5 minutes), the average porosity of the nanoporous polymer thin film240is 30%. It is clear that the porosity of the nanoporous polymer thin film240is increased as the uptake ratio of the first solvent530absorbed by the polymer thin film220is increase. Preferably, the weight percentage of the first solvent530in the swollen polymer thin film230is equal to or greater than 6%. The gas can be nitrogen or other gas which does not react with the polymer211.

Additionally, the size of the pore212(pore size) can be controlled by controlling the processing time of the swelling and annealing process. The size of the pore212is smaller as the processing time of the polymer thin film220in the vapor510is shorter. On the other hand, the pore size is larger as the processing time of the polymer thin film220in the vapor510is longer. Preferably, the processing time of the swelling and annealing process is in a range of 5 minutes to 240 minutes; particularly, the processing time of the swelling and annealing process is in a range of 5 minutes to 60 minutes.

Please be noted that the observed morphological development (phase separation) is a typical behavior of the spinodal decomposition kinetics. Spinodal decomposition in polymer blends or solutions is a spontaneous phase separation process that occurs when an infinitesimally small fluctuation in the system from homogeneity provokes an exponential growth of the starting fluctuations because of a lowering in the free energy of the system resulting from the phase separation process.

The polymer thin film is metastable under saturated swelling condition because of the first solvent initially, and then undergoes a rapid phase separation when there is an infinitesimal compositional fluctuation. In addition, the annealing results in a rapid evolution of features.

Example

Please refer toFIGS. 5A, 5B, 5C, 5D, 5E and 5F.FIG. 5Ashows a top view of a nanoporous polymer thin film of a 1st example of the present disclosure.FIG. 5Bshows a cross-sectional view of the nanoporous polymer thin film of the 1st example ofFIG. 5A.FIG. 5Cshows a top view of a nanoporous polymer thin film of a 2nd example of the present disclosure.FIG. 5Dshows a cross-sectional view of the nanoporous polymer thin film of the 2nd example ofFIG. 5C.FIG. 5Eshows a top view of a nanoporous polymer thin film of a 4th example of the present disclosure.FIG. 5Fshows a cross-sectional view of the nanoporous polymer thin film of the 4th example ofFIG. 5E.FIGS. 5A to 5Fare SEM images.

In the 1st example to the 4th example, the polymer is polystyrene with a molecular weight of 280,000 g/mol from Scientific polymer products, Inc. The polystyrene is mixed with chlorobenzene (99% GC) from Methanol Alfa Aesar to form the polymer solution, and the weight percentage of the polystyrene is 7%. In fabrication, the polymer is spin-coated onto the substrate at 2000 rpm to form the polymer thin film, and then the polymer thin film is disposed in the vacuum oven for 1 minute.

Subsequently, the polymer thin film is kept into the chamber for the swelling and annealing process, and the chamber includes the vapor of the first solvent which is N,N-dimethylformamide (98% GC) from JT Baker. The vapor is under saturation pressure of the first solvent. The processing time of the swelling and annealing process is 5 minutes in the 1st example. The processing time of the swelling and annealing process is 30 minutes in the 2nd example. The processing time of the swelling and annealing process is 45 minutes in the 3rd example. The processing time of the swelling and annealing process is 60 minutes in the 4th example.

As shown inFIGS. 5A to 5F, the nanoporous polymer thin film with a plurality of pores can be fabricated by the method100of the present disclosure. Moreover, compare to the 1st example, the pore size of the 2nd example is larger owing to the longer processing time of the swelling and annealing process, and the nanoporous thin film of the 4th example whose processing time of the swelling and annealing time is the longest has the largest pore size.

FIG. 6shows small-angle X-ray scattering (SAXS) measuring results of the 1st example, the 2nd example, the 3rd example and the 4th example. InFIG. 6, the small-angle X-ray scattering with an X-ray wavelength of 0.1555 nm is conducted. Broad scattering peak is observed in the SAXS profile of the nanoporous polymer thin films of each of the 1st example, the 2nd example, the 3rd example and the 4th example, which is characteristic of a disordered, phase-separated state with structural heterogeneities on mesoscale. The structures obtained by the spinodal decomposition mechanism at different compositional fluctuations, i.e., at different processing time of the swelling and annealing (especially the time kept in the chamber after swelling), would diffract the light with wavenumber q=2π/d where d represents the interdomain spacing of the nanoporous polymer thin film.

FIGS. 7A to 7Dshow Brunauer-Emmett-Telle (BET) measuring results of the 1st example, the 2nd example, the 3rd example and the 4th example. InFIGS. 7A to 7D, N2sorption isotherm measurements at 77 K were conducted. Because the N2adsorption of the nanoporous polymer thin films will reach a maximum (p/p0=0.99), it is proved that the pore development can reach the inner region of the nanoporous polymer thin film.

FIG. 8shows pore size measuring results of the 1st example, the 2nd example, the 3rd example and the 4th example by Barrett-Joyner-Halenda (BJH) method. As shown inFIG. 8, the nanoporous polymer thin films of the 1st example, the 2nd example, the 3rd example and the 4th example have different pore size. Therefore, through the control of the swelling and annealing time, not only does the nanoporous polymer thin film have high specific surface area, but also the pore size can be effectively controlled.

Table 1 shows structural parameters of the nanoporous polymer thin films of the 1st example to the 4th example. The structural parameters such as a framework size, an interdomain spacing, a pore diameter, a porosity and a BET specific surface area are shown.

Method for Fabricating Nanoporous Thin Film

FIG. 9shows a flow chart of a method600for fabricating nanoporous thin film according to another embodiment of the present disclosure.FIG. 10shows a schematic illustration of the method600ofFIG. 9. Refer toFIGS. 9 and 10, the method600includes Step610, Step620and Step630.

In Step610, a template is provided, wherein the template includes a nanoporous polymer thin film240fabricated by the abovementioned method100.

In Step620, a filling process is provided. A mixture film is fabricated. The mixture film includes a first material700and the nanoporous polymer thin film240, and the first material700is filled in a plurality of pores212of the nanoporous polymer thin film240.

In Step630, a template removing process is provided. The nanoporous polymer thin film240is removed to form a nanoporous thin film composed of the first material700.

Therefore, the nanoporous thin film can be fabricated by the low-cost nanoporous polymer thin film240, and the nanoporous polymer thin film240has large specific surface area. In one embodiment, the first material can be oxide silicon (SiO2).

By executing the sol-gel reaction of SiO2precursors (e.g., tetraethyl orthosilicate) within the 3D co-continuous nano-channels, i.e., the pores212, of the nanoporous polymer thin film240, the mixture film including the polymer and SiO2can be fabricated. In Step630, after calcination of the mixture film at high temperature, the nanoporous polymer thin film240can be removed to obtain the nanoporous SiO2thin film with high porosity and high specific surface area. In other embodiment, the first material can be oxide titanium.

In another embodiment, the first material is nickel. In Step620, the formation of Ni from electroless plating is an autocatalytic reduction process in an aqueous Ni ion solution. Subsequently, a mixture film including the polymer and nickel with co-continuous metallic networks can be fabricated. In Step630, the nanoporous Ni thin film can be fabricated after the removal of the template (nanoporous polymer thin film) by solvent treatment.

In other embodiment, the first material can be, but not be limited to, other metals, or a ceramic.

Refer toFIGS. 11A, 11B and 11C.FIG. 11Ashows one nanoporous thin film fabricated by the method600ofFIG. 9.FIG. 11Bshows another nanoporous thin film fabricated by the method600ofFIG. 9.FIG. 11Cshows yet another nanoporous thin film fabricated by the method600ofFIG. 9.FIGS. 11A to 11Care SEM images.

SiO2is severed as the first material of the method600inFIG. 11Asuch that a nanoporous SiO2thin film can be fabricated. Because the nanoporous polymer thin film of the 1st example (with porosity 29%) is served as the template, a predict porosity of the nanoporous SiO2thin film is 71%. The measured porosity of the nanoporous SiO2thin film is 63% and is closed to the predict porosity, which can prove that the nanoporous SiO2thin film fabricated by the method600has high porosity. The nanoporous SiO2thin film exhibits excellent anti-reflection and high transmission property; hence, its low refractive index might improve the efficiency of the opto-electronic devices by increasing the light transmission.

TiO2is severed as the first material of the method600inFIG. 11Bsuch that a nanoporous TiO2thin film can be fabricated. The nanoporous TiO2thin film can be applied to the perovskite solar cell to increase effective dispersion of active perovskite materials in the solar cell. Additionally, the perovskite solar cell integrated with the nanoporous TiO2thin film is expected to increase the efficient many folds with enhanced quantum efficiency.

InFIG. 11C, nickel is severed as the other metallic materials that a nanoporous nickel thin film can be fabricated. The nanoporous nickel thin film exhibits excellent catalytic properties over the hydrogenation of aromatic and unsaturated compounds with very high selectivity and turnover frequencies.

Based on the above embodiment and examples, it can be known that the pore size and the porosity of the nanoporous polymer thin film can be adjusted precisely to obtain the nanoporous polymer thin film having a well-interconnected co-continuous network structure with pore size ranging from 10 nm to 100 nm. Particularly, the phase-separation between the polymer and the first solvent occurs in the swelling and annealing process, and the nanoporous polymer thin film having the well-interconnected co-continuous network structure can be fabricated by freezing the phase-separated swollen polymer thin film and removal of the frozen first solvent; therefore, the method is facile and cost-effective.

Furthermore, the pore size of the nanoporous thin film can be well controlled by solvent annealing kinetics. In the swelling and annealing process, the nanoporous polymer thin film is found to have wide range of pore size control resulting from the co-continuous phase formation by spinodal decomposition in nanoscale. The increase in the processing time of the swelling and annealing can increase the pore size of the nanoporous polymer thin film, and the porosity can be controlled efficiently by changing the uptake ratio of the first solvent absorbed in the swelling and annealing process.