Fabrication routes for one-dimensional nanostructures via block copolymers
© Korea Nano Technology Research Society 2017
Received: 4 March 2017
Accepted: 17 April 2017
Published: 10 May 2017
Nanotechnology is the field which deals with fabrication of materials with dimensions in the nanometer range by manipulating atoms and molecules. Various synthesis routes exist for the one, two and three dimensional nanostructures. Recent advancements in nanotechnology have enabled the usage of block copolymers for the synthesis of such nanostructures. Block copolymers are versatile polymers with unique properties and come in many types and shapes. Their properties are highly dependent on the blocks of the copolymers, thus allowing easy tunability of its properties. This review briefly focusses on the use of block copolymers for synthesizing one-dimensional nanostructures especially nanowires, nanorods, nanoribbons and nanofibers. Template based, lithographic, and solution based approaches are common approaches in the synthesis of nanowires, nanorods, nanoribbons, and nanofibers. Synthesis of metal, metal oxides, metal oxalates, polymer, and graphene one dimensional nanostructures using block copolymers have been discussed as well.
KeywordsBlock copolymer Nanowire Nanorod Nanofiber Nanotechnology
Nanotechnology is a relatively new field which involves control and exploitation of structures with sizes in the nanometer range. Goals in this field include fabrication of materials with properties that are novel and improved and are likely to impact different fields . Nanotechnology can be applied in fields like electronics, environmental science, food safety, biotechnology, medicine, and cosmetic industry [2–7]. Such wide arrayed applications call for the development of nanostructures with well-defined size, form, and crystal structure.
One-dimensional nanostructures like nanowires, nanorods, nanoribbons, and nanofibers are providing a good system to investigate the dependence of thermal and electrical transport or mechanical properties on dimensionality and quantum confinement. They also play an important role as both functional units and interconnects in fabricating electronic, optoelectronic, electromechanical, and electrochemical device in the nanoscale dimension . These nanostructures have been generally synthesized through techniques like chemical vapor deposition, VLS method, lithography, template based and solution based approaches.
Block copolymers are polymer alloys that have rubber-like behavior under normal conditions and can be easily molded at higher temperatures. This is due to the physical crosslinks provided by the glassy domains. Based on the molecular weight, the strength of interaction between the blocks, and segment size, the block copolymers can self-assemble to form nanostructures with specific domain spacing . These properties along with their structure adaptability enable block copolymers to be used in the synthesis of nanostructures, especially one-dimensional nanostructures like nanowires, nanorods nanoribbons, and nanofibers .
2 Block copolymers
Block copolymers have two or more polymer blocks arranged in a specific manner. These blocks are covalently bound together, chemically distinct, and immiscible. Such a structure allows the mechanical, electrical, optical and mass transport behavior of block copolymers to be easily tuned just by tweaking its molecular construction .
It has been found through experimental and theoretical studies that block copolymers can separate into different forms like spheres, cylinders, lamellae and bicontinuous structures. Factors affecting these forms include copolymer composition, w-parameters and the total degree of polymerization .
Block copolymers as compared to other copolymers have superior properties and these properties highly depend on the clustering and structural arrangement of the blocks. They allow the combination of significantly high melting points and elastic properties in the same polymer. Block copolymers also exhibit increased tensile strength, modulus, and elongation by suitable selection of the blocks. Other properties like moisture recapture, ability to absorb dyes also depend on the block copolymer’s structure . The phase behaviour of a block copolymer is dependent on the following factors: overall degree of polymerisation (N), architectural constraints (n) and composition (ƒ) and the Flory–Huggins interaction parameter (χ). The first two factors influence translational and configurational entropy of the copolymer. Increasing χ by lowering the temperature results in less A-B monomer contact. Large N causes some loss of configurational and translational entropy and causes microphase segregation. Decreasing χ and N causes the formation of compositionally disordered phase . In a study, a series of symmetric EPE triblock copolymers were investigated to study the effect of microphases separation on microstructure and mechanical properties of solid state chain. Block copolymers having crystallizable components can undergo microphase separation either due to chemical incompatibility between the blocks causing the formation of a heterogeneous melt or through the expulsion of a block due to crystallisation of another block. The latter is also known as crystallisation induced microphase separation. The tensile strengths of these triblock copolymers depend on the manner of microphase separation. Crystallization-induced segregation causes lower tensile strength than those with templated crystallisation . To realise the actual potential of block copolymer nanostructures, several strategies are required to control and manipulate spatial orientation, connectivity, periodicity, and long range order [11, 24–27]. Topologically  and chemically patterned surfaces , solvent annealing [30, 31], temperature gradients  and electric fields are some of the techniques which utilize physical and chemical constraints to direct self-assembly of block copolymers into highly well-ordered geometries. Electric fields are a versatile and easy way to control long range orientational and translational order of nanostructured block copolymers as well as miscibility and morphology both in thin films, bulk and solutions .
3 Nanostructures and block copolymers
Block copolymers create well-ordered structures by undergoing microphase separation. Their properties can also be controlled through manipulating the three critical parameters mentioned above. Block copolymers can be used to synthesise different nanostructures due to their fitting size and shape. Their properties are also easily tunable by simply changing the monomer used, their compositions and molecular weight. Using block copolymers for fabricating nanostructures is simple and affordable as well . This section briefly focuses on synthesis of nanowires, nanorods, nanoribbons and nanofibers using block copolymers.
Metal oxalate nanowires like copper oxalate have also been synthesized using block copolymer. Block copolymer material in samples of SBA-15 was used as a reactant. This was oxidized to C2O4 2− in a special aqueous solution containing copper ions. Due to this, copper oxalate nanowires embedded in mesoporous silica channels were produced in situ. Through heating of CuC2O4/SBA-15, CuO and Cu2O nanowires were also produced. These nanowires demonstrated ordered mesostructures and good textural properties through characterization studies using SEM, TEM, and FT-IR. They also possessed excellent electrochemical hydrogen storage capacity . Nanoporous templates have been created using diblock copolymers consisting of different ratios of block copolymers. Gold coated silicon wafers were then spin coated on this template followed by vacuum annealing and intense exposure to UV radiation. In this nanoporous template, NiFe alloys were electrochemically deposited. NiFe has magnetic properties and can be used in magnetic storage devices and as sensors for gas. The wires were further characterized by SEM . Through this technique, molybdenum disulfide nanowires have been fabricated as well. In a study, self-assembled cylindrical diblock copolymer thin films were used to synthesize these wires. It employed selective seeding of ammonium heptamolybdate (AHM) or ammonium tetrathiomolybdate (ATTM) precursors into specific domains of the diblock copolymer [poly(styrene-b-2-vinylpyridine)] thin film. This was later annealed to synthesize molybdenum disulfide nanowires on SiO2/Si substrates. The experimental conditions were modified in such a way that they produced either polycrystalline or amorphous molybdenum disulfide nanowires. These were later characterized through TEM, Raman spectroscopy and X-Ray Photoelectron Spectroscopy (XPS) and XRD. It revealed that at higher annealing temperatures there was a transition from amorphous phase to polycrystalline phase with a decrease in excess elemental sulfur .
Nanorods are nanostructures with characteristic size dependent properties whose effects are significantly observed in the 1–10 nm range. Nanorods also can cause large variations in its composition. They exhibit typical optical, electronic, and catalytic properties which cause them to have many applications. Nanorods like any other nanostructures are also synthesized through ‘top-down’ and ‘bottom-up’ approach. Techniques generally used to synthesize these nanorods include seed-capping, vapor–liquid–solid growth, chemical vapor deposition, solid controlled growth, electrodeposition, sol–gel process, lithography, self-assembly, and template-based synthesis .
Nickel nanorods have been prepared by applying a mask of ordered nanostructured hollow channels in a block copolymer matrix. Through an organized process in block copolymer supramolecular assemblies, the polymeric templates were formed. Onto this template, nickel was deposited via two techniques: electrodeposition and washing-in; and the entire formation process of the nickel nanorods was monitored through AFM and X-Ray Photoelectron Emission Microscopy (XPEM). Through this, it was found that the nickel rods showed metallic behavior despite being synthesized under ambient conditions. Also, no NiO complexes were formed due to the probable protection of Ni nanoparticles against oxidation .
ZnO nanorods have also been synthesized in a solution based approach. Effect of the diblock copolymer poly(ethylene oxide)-b-poly(propylene oxide) (PEO-b-PPO) has been investigated on the morphological control of radial spherical ZnO nanorods. Solutions of different molar ratios of the block copolymer was added to the aqueous solution of Zn(C2H3O2)2·2H2O followed by addition of NaOH solution to the PEO-b-PPO modified zinc solution. SEM was used to further characterize the precipitate obtained to reveal ZnO nanorods. The length and the diameter of the hexagonal facet of each rod decreased with increase in copolymer concentrations. The effectiveness of photocatalytic degradation of the nanorods also increased with increase in their surface areas. However, there was no improvement in the antibacterial activity of the nanorods due to their sizes . In another solution based approach, size-tunable CdS nanorods were synthesized at low temperature via the reaction of air-insensitive inorganic precursors sodium sulfide and cadmium acetate in an aqueous phase in the presence and absence of a surfactant. Here, nonionic amphiphilic triblock copolymers were used as structure-directing agents. In the absence of surfactant, CdS nanorods were synthesized by refluxing at high temperatures using ethylene glycol, ethylene glycol dimethyl ether or a mixture of the two chemicals as the solvent. The inorganic precursors were added followed by refluxing which led to the formation of a yellow precipitate. In the presence of a surfactant, the amphiphilic triblock copolymer was dissolved in deionized water at different ratios. The precursors were added in the solvent and the entire reaction was carried out in a N2 atmosphere. The temperature was gradually increased and the resulting solution was centrifuged to obtain yellow to reddish orange precipitates. These were then washed with ether/ethanol mixture. SEM analysis revealed a uniform flat-ended rod whose average length was greater than 4 µm and diameter of 0.2–0.6 µm. It was also found that the diameter of the CdS nanorods were easily controlled by changing the surfactant species. However, in the absence of a surfactant, the morphology of the product changed to microrods with flat ends, cotton-ball microparticles, and dumbbell-shaped microrods. The typical diameter of the nanorods obtained were 5–7 nm and was 30–90 nm in length .
Nanoribbons are 1-D nanostructures that have gained attention due to their unique flat geometries which enable them to prompt positive changes in terms of pore size distribution and shape. It also enables them to improve their anisotropic mass and heat transport and strengthens their mechanical properties . This section briefly focuses on the synthesis of graphene nanoribbons through block copolymers.
Graphene nanoribbons are strips of graphene with dimensions less than 10 nm . Lithographic, sonochemical, chemical and unzipping methods like high current pulse burning and catalytic cutting methods have been developed to fabricate graphene nanoribbons .
In another study, FET characteristics and photoelectric properties of graphene nanoribbons having 9–12 nm ribbon widths that were fabricated through cylindrical PS-b-PDMS block copolymer as a lithographic mask to pattern graphene single layers of graphene grown through CVD on Si substrates which were heavily p-doped with an SiO2 layer was demonstrated. Here, a PEO brush replaced PDMS brush to prevent the formation of a PDMS layer at the graphene surface. The graphene nanoribbons fabricated into FETs through this technique displayed a higher on/off ratio and a stronger temperature dependence of the current as compared to the FETs having no patterned graphene nanoribbons . Graphene is atomically thin which causes the development of a unique surface energy characteristic called ‘wetting-transparency (WT)’. This property can be exploited for graphene nanoribbon array. A research group has used this property to demonstrate a robust procedure to fabricate graphene nanoribbons by placing the cross-linked SNT (Surface Neutralized Treatment) layer under the graphene, where the SNT can control the orientation of block copolymer due to WT. The procedure involved the growth of graphene monolayer on a copper foil followed by transferring of this monolayer onto an SNT-coated Si wafer through PMMA-assisted transfer method. The diblock copolymer PS-b-PMMA [poly(styrene)-b-poly(methyl methacrylate)] was spin-coated on the mild plasma treated graphene substrate followed by annealing to achieve perpendicular domain orientation. The PMMA domain was then selectively removed through a CO2 based etching process and the pattern formed was subsequently transferred into the underlying graphene. The residual PS was removed by soaking it in THF, followed by drying of the sample at room temperature under vacuum. Characterization studies by AFM and SEM revealed successful removal of the PMMA block and SNT layers. Since graphene is placed between the block copolymer and SNT, this observation showed the successful etching of graphene thus transforming it into graphene nanoribbons. Also, the structure of graphene nanoribbons was identical to the PS fingerprinting pattern, thus confirming the pattern transfer from the block copolymer mask to fabricate the nanoribbons .
Self-assembled block copolymer nanofibers are excellent materials having applications as viscosifying agents in complex fluids or in biomedical field. The preparation of polymeric nanofibers is based on an aqueous emulsion polymerization using the RAFT method. RAFT agents were synthesized followed by the synthesis of the block copolymers using these RAFT agents under different pH and salt concentrations. The effect of these parameters on the formation of nanofibers was studied by TEM. With RAFT agents, at acidic pH or high salt concentrations, non-spherical morphologies were favored. The formation of the nanofibers also depended on the monomer concentration. With the conversion, the morphology of the nanofibers changed from spherical to worm-like due to increasing viscosity .
Formation of nanofibers using block copolymers under cylindrical confinement has been reported as well. The two-fluid coaxial spinning technique was used to encapsulate the desired block copolymer as the core with another protective polymer as the shell. Advantages of the two-fluid electrospinning include (a) The shell fluid itself being electro-spinnable serves as a process aid for the block copolymers. Removal of these shells, results in ultrafine block copolymer fibers, (b) smaller diameter fibers are produced due to the independent control of the two fluids that permit wider changes in the diameter of the core fiber, (c) the integrity of the fiber does not get compromised due to the cylindrical confinement of the shell polymer which has a higher glass or melt transition temperature, thus creating a temperature window. TEM images confirm the continuous core–shell nature of the nanofibers. It also shows well-defined structures consisting of concentric layers because of curving of the lamellar phase due to fiber confinement .
Polymer/Fe2O3 hybrid nanofibers have been prepared by processing films of the triblock copolymer polystyrene-block-poly(2-cinnamoyloxyethyl methacrylate)-block-poly(tert-butyl acrylate) (PS-b-PCEMA-b-PtBA). This preparation involved blocks in thin films getting self-assembled into concentric PCEMA and PtBA cylinders dispersed in a matrix of PS. These cylindrical structures were locked through by photocrosslinking the PCEMA shells. When these shells were dissolved in THF, nanofibers were produced with PtBA cores, PCEMA middle layers, and PS coronas. TEM images revealed PtBA cores that are 20 nm in diameter. Fe2O3 loading in the caused the nanofiber dispersion to become red. These nanofibers had Fe2O3 content to be about 0.28 g. TEM images of Fe2O3 nanofibers revealed fibers around 20 nm, which as the same as the polymer nanofiber .
Nanotechnology has seen many advances in recent years and several synthesis techniques for nanostructures have been developed. These techniques can be mainly classified into top down and bottom-up approaches. Block copolymers have two or more blocks arranged in a defined manner. This arrangement is dependent on factors like Flory–Huggins interaction parameter, degree of polymerization, architectural constraints, and composition. By varying these parameters, it is possible to control the phase behavior and morphology of block copolymers. Such a flexibility enables block copolymers to be used for fabrication of nanostructures as a template or in a solution form. They can also be used to lithographically fabricate nanostructures. One dimensional nanostructures like nanowires, nanorods, nanoribbons, and nanofibers have unique electronic, chemical, optical, magnetic, and mechanical properties due to their nanostructure which enable them to have several applications in different fields of Science. Synthesis of such one-dimensional nanostructures using block copolymers has been discussed in this review. The techniques discussed show the versatility and effortlessness of block copolymers to synthesis these nanostructures.
DR is responsible for the conception, critical revision of the article and approval of the final version of the article. MT has contributed in the design of the work, data collection and drafting of the article. GP has contributed in design and proofreading of the article. All authors read and approved the final manuscript.
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