BCP Thin Films Research Articles

Feature size control using surface reconstruction temperature in block copolymer lithography for InAs nanowire growth

Here we present a method to control the size of the openings in hexagonally organized BCP thin films of poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) by using surface reconstruction. The surface reconstruction is based on selective swelling of the P4VP block in ethanol, and its extraction to the surface of the film, resulting in pores upon drying. We found that the BCP pore diameter increases with ethanol immersion temperature. In our case, the temperature range 18 to 60 °C allowed fine-tuning of the pore size between 14 and 22 nm. A conclusion is that even though the molecular weight of the respective polymer blocks is fixed, the PS-b-P4VP pore diameter can be tuned by controlling temperature during surface reconstruction. These results can be used for BCP-based nanofabrication in general, and for vertical nanowire growth in particular, where high pattern density and diameter control are of importance. Finally, we demonstrate successful growth of indium arsenide InAs vertical nanowires by selective-area metal-organic vapor phase epitaxy (MOVPE), using a silicon nitride mask patterned by the proposed PS-b-P4VP surface reconstruction lithography method.

Defect Removal by Solvent Vapor Annealing in Thin Films of Lamellar Diblock Copolymers

Solvent vapor annealing (SVA) is known to be a simple, low-cost and highly efficient technique to produce defect-free diblock copolymer (BCP) thin films. Not only can the solvent weaken the BCP segmental interactions, but it can vary the characteristic spacing of the BCP microstructures. We carry out systematic theoretical studies on the effect of adding solvent into lamellar BCP thin films on the defect removal close to the BCP order-disorder transition. We find that the increase of the lamellar spacing, as is induced by addition of solvent, facilitates more efficient removal of defects. The stability of a particular defect in a lamellar BCP thin film is given in terms of two key controllable parameters: the amount of BCP swelling and solvent evaporation rate. Our results highlight the SVA mechanism for obtaining defect-free BCP thin films, as is highly desired in nanolithography and other industrial applications.

Physical characterization of 5′,5″-dibromo-o-cresolsulfophthalein (BCP) spin-coated thin films and BCP/p-Si based diode

The organic 5′,5″-dibromo-o-cresolsulfophthalein (BCP) compound thin films were deposited simply through spin coating technique from a pre-prepared solution of different molarities. Differential thermal analysis (DSC) and thermogravimetric analysis (TGA) of the starting powder compound were investigated to obtain the thermal stability and phase transformation of the compound. The structure, morphology, and optical properties were analyzed for all the prepared films of different molarities. Structural analyses revealed the nanocrystalline composition of all the prepared thin films. The resulted thin films nanostructure feature is verified by utilizing both the field emission scanning-electron-microscope (FESEM) and the high-resolution transmission electron microscope (HRTEM). Optical absorption exploration of BCP thin films was carried out in the limit of 200–2500 nm wavelength. The results revealed no changes in optical properties with molarity change. Al/p-Si/BCP/Au junction was prepared and investigated electrically in dark conditions and the diode parameters were extracted. The obtained diode can be employed in many applications such as rectifiers, clipper circuits, clamping circuits, reverse current protection circuits, logic gates, voltage multipliers, flexible electronics, and many other optoelectronic applications.

Block copolymer ordering on elastomeric substrates of tunable surface energy

We report a range of surface characteristics from strong and partial dewetting to complete wetting for thin films of cylinder and lamellae forming block copolymers (c-BCP and l-BCP, respectively) of polystyrene-b-polymethylmethacrylate (PS-b-PMMA) on flexible substrates. BCP of various molecular weights (Mw) was applied on elastomeric polydimethylsiloxane (PDMS) substrates of distinct cross-link densities and various surface energy (SE) obtained by exposure to UV-ozone. We demonstrate that lower Mw c-BCP films dewet faster than l-BCP films of higher Mw applied on PDMS of lowest cross-link density and SE < 25 mJ/m2. Partial dewetting was observed for c-BCP films applied on the lowest cross-link density substrates with 25 mJ/m2 < SE < 32 mJ/m2, with a sharper transition from dewetting to wetting for l-BCP films. Finally, complete wetting (stable films) occurred for both c-BCP and l-BCP films on PDMS substrates with lowest cross-link density and SE > 32 mJ/m2, as well as on PDMS of the highest cross-link density with SE > 25 mJ/m2. In this extended SE wetting regime, c-BCP show vertical orientation when applied on both types of cross-linked PDMS in a narrow neutral SE range (39–40) mJ/m2, while being oriented parallel to the substrate when SE > 40 mJ/m2. The ratio of vertically oriented l-BCP however decreases gradually when applied on substrates with a higher SE range (40–65 mJ/m2) in comparison to the sharply transitioning c-BCP orientation, reflecting the intrinsic stability of vertical lamellae over a wider range of substrate SE, consistent with theoretical estimates. These results have important ramifications for design of next-generation flexible electronics utilizing BCP thin films.

Self-Alignment of Cylinder-Forming Silicon-Containing Block Copolymer Films

Well-ordered perpendicular cylinders of star-block copolymers (BCPs) composed of polystyrene and poly(dimethylsiloxane) blocks with high aspect ratio can be achieved by using a combination of architecture effect (entropy effect) and surface air plasma treatment (enthalpy effect). An interesting morphological evolution from disordered texture to hexagonally packed cylinders with perpendicular orientation has been observed, in which perpendicular cylinders are initiated at the top surface and bottom substrate of the thin film, followed by a self-alignment process, resulting in span-thru cylinders through the thin film. The self-alignment process is systematically examined by a combination of real space imaging using transmission electron microscopy, reciprocal space imaging using grazing incidence small-angle X-ray scattering, and three-dimensional tomography. The experimental results reveal a unique mechanism for the formation of highly ordered BCP thin films via surface-induced nucleation of perpendicular...

Extraordinary boundary morphologies of large-scale ordered domains of spheres in thin films of a narrowly dispersed diblock copolymer via thermodynamic control.

Long-range ordering of body centered cubic (BCC) spheres and various extraordinary morphologies at the boundaries of the adjacent orderly oriented domains are observed in thermally annealed thin films of a series of specific narrowly dispersed diblock copolymers, poly(dimethylsiloxane)-b-poly{2,5-bis[(4-butoxyphenyl)oxycarbonylstyrene} (PDMS-b-PBPCS, DB). The series of asymmetrical DB block copolymers (BCPs) with volume fractions of PDMS (f(PDMS)'s) from 10% to 23% self-assemble into thermodynamically stable body centered cubic (BCC) nanostructures in bulk at ambient temperature after thermal annealing. The thin films of these BCPs with a relatively large film thickness on a carbon-film coated substrate are annealed in a vacuum at 180 °C for 3 days and are characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). For all thin films of these BCPs, micrometer-scale domains with a rectangular unit cell similar to the projection of the BCC lattice along the [110] direction to the substrate are observed. And the XPS results indicate that the surface layers of the thin films are composed of both PDMS and PBPCS blocks. For the thin films of the BCPs with f(PDMS) values of 10% and 13%, the neighboring [110]-oriented BCC domains match well with each other, and the boundaries are defect-free. For the thin film of the BCP with a f(PDMS) value of 23%, the PDMS spheres in the [110]-oriented BCC domains in the TEM micrograph are overlapped with each other, and interesting morphologies including defect-free interfaces, interfaces with line defects, and domains with defects and local ordering are observed at the boundaries of the neighboring [110]-oriented domains.

On the Self-Assembly of Brush Block Copolymers in Thin Films

We describe a simple route to fabricate two dimensionally well-ordered, periodic nanopatterns using the self-assembly of brush block copolymers (brush BCPs). Well-developed lamellar microdomains oriented perpendicular to the substrate are achieved, without modification of the underlying substrates, and structures with feature sizes greater than 200 nm are generated due to the reduced degree of chain entanglements of brush BCPs. A near-perfect linear scaling law was found for the period, L, as a function of backbone degree of polymerization (DP) for two series of brush BCPs. The exponent increases slightly from 0.99 to 1.03 as the side chain molecular weight increases from ∼2.4 to ∼4.5 kg/mol(-1) and saturated with further increase in the side chain molecular weight due to the entropic penalty associated with the packing of the side chains. Porous templates and scaffolds from brush BCP thin films are also obtained by selective etching of one component.

Orientation Control of Block Copolymer Thin Films Placed on Ordered Nanoparticle Monolayers

We investigate orientation and lateral ordering of poly(styrene-block-methyl methacrylate) (PS-b-PMMA) diblock copolymer (diBCP) thin films placed on ordered nanoparticle (NP) monolayers. The densely packed NP monolayers were prepared on silicon substrates with the Langmuir–Blodgett (LB) deposition technique. The perpendicular domain orientation of BCP thin films is obtained on the ordered NP monolayers due to the nanoscale regular roughness which exerts the elastic deformation on the BCP nanodomains and suppresses the substrate-induced parallel orientation. The effect of BCP film thickness as well as the NP size on the orientation of BCP nanodomains was systematically investigated. We also demonstrate the defect-tolerant ordering of the perpendicular orientation of BCP thin films on the NP-vacant sites.

Directed Assembly of Nanoparticles in Block Copolymer Thin Films: Role of Defects

The structure of A-b-B diblock copolymer (BCP) thin films is often exploited for “tailoring” the self-assembly of nanoparticles into various geometries, characterized by long-range order. Mechanistically, the nanoparticles are sequestered within the A or B domains of the copolymer, and the domains act as scaffolds to direct the assembly of the nanoparticles. We show that in BCP thin films, which order at suboptimal thicknesses, dislocations play a dominant role in determining the spatial organization of sufficiently large nanoparticles. The nanoparticles preferentially segregate to the core of edge dislocations, which are ubiquitous in these materials. To this end, the two-dimensional planar shape of the islands and holes determine the self-assembled geometry of the nanoparticles. We also show that the so-called “healing length”, λ, increases with film thickness, h, in a manner consistent with λ ∝ h1/2, in accordance with theory. In films of thickness between L < h < 3L, where L is the domain spacing, λ l...

Enhanced performance of a CuPc: PCBM based solar cell using bathocuproine BCP or nanostructured TiO2 as hole‐blocking layer

AbstractThe roles played by a hole‐blocking layer, TiO2 or BCP are investigated for heterojunction photovoltaic devices based on CuPc and the fullerene derivative PCBM. The insertion of TiO2 or BCP thin films in the CuPc:PCBM based solar cells resulted in a higher open circuit voltage (VOC) and a higher fill factor which is due to the increase in the shunt resistance as well as to the improved collection of the electrons at the aluminum (Al) cathode. The energy conversion efficiency of the device having TiO2 hole‐blocking layer is two times greater than that having BCP.

Phase-field modeling of the formation of lamellar nanostructures in diblock copolymer thin films under inplanar electric fields

Recent experiments show that external inplanar electric field can be employed to guide the molecular self-assembly in diblock copolymer (BCP) thin films to form lamellar nanostructures with potential applications in nanotechnology. We study this self-assembly process through a detailed coarse-grained phase-separation modeling. During the process, the free energy of the BCP films is modeled as the Ginzburg-Landau free energy with nonlocal interaction and electrostatic coupling. The resulting Cahn-Hilliard (CH) equation is solved using an efficient semi-implicit Fourier-spectral algorithm. Numerical results show that the morphology of order parameter formed in either symmetric or asymmetric BCP thin films is strongly influenced by the electric field. For symmetrical BCPs, highly ordered lamellar nanostructures evolved along the direction of the electric field. Phase nucleation and dislocation climbing in the BCP films predicted by the numerical simulation are in a good agreement with those observed in recent BCP electronanolithography. For asymmetrical BCPs, numerical simulation shows that nanodots are guided to align to the electric field. Furthermore, in the case of high electric field, nanodots formed in asymmetrical BCPs may further convert into highly ordered lamellar nanostructures (sphere-to-cylinder transition) parallel to the electric field. Effects of the magnitude of electric field, BCP asymmetry, and molecular interaction of BCPs on the self-assembly process are examined in detail using the numerical scheme developed in this study. The present study can be used for the prediction of the formation of nanostructures in BCP thin films and the quality control of BCP-based nanomanufacturing through optimizing the external electric fields.

The characterization of the optical functions of BCP and CBP thin films by spectroscopic ellipsometry

The optical properties of 2,9-dimethyl-4,7-diphenyl-l,10-phenanthroline (bathocuproine or BCP) and 4,4′-bis(carbazol-9-yl)biphenyl (CBP) thin films were characterized by spectroscopic ellipsometry (SE). Thin films of BCP and CBP were evaporated on silicon substrates. Spectroscopic ellipsometry measurements were performed in the wavelength range of 300–1000 nm. The refractive indices and extinction coefficients of BCP and CBP were determined by fitting the obtained data using Lorentz model and Relaxed Lorentz (RL) model with variable thicknesses. The extinction coefficients obtained from the fitting of ellipsometry data were in excellent agreement with absorbance measurement results.