Photoinduced Modulation of Polymeric Interfacial Behavior Controlling Thin-Film Block Copolymer Wetting

The tunable surface-wetting properties of photosensitive random copolymer mats were used to spatially control the orientations of thin-film block copolymer (BCP) structures. A photosensitive mat was produced via thermal treatment on spin-coated random copolymers of poly­(styrene-ran-2-nitrobenzyl methacrylate-ran-glycidyl methacrylate), synthesized via reversible-deactivation radical polymerization. The degree of UV-induced deprotection of the nitrobenzyl esters in the mat was precisely controlled through the amount of UV-irradiation energy imparted to the mat. The resulting polarity switching of the constituents collectively altered the interfacial wetting properties of the mat, and the tunability allowed lamellar or cylinder-forming poly­(styrene-b-methyl methacrylate) BCP thin films, applied over the mat, to change the domain orientation from perpendicular to parallel at proper UV exposures. UV irradiation passing through a photomask was capable of generating defined regions of BCP domains with targeted orientations

Resist Free Patterning of Nonpreferential Buffer Layers for Block Copolymer Lithography

We report the design of a direct electron beam patternable buffer layer to spatially control the orientation of the microdomains in an overlaying polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) block copolymer (BCP) film. The buffer layer consists of a surface anchored low molecular weight PS-b-PMMA, with the PMMA segment anchored to the surface and a short PS block at the buffer layer/BCP interface. The block architecture of the buffer layer combines the essential features of “bottom up” and “top down” approaches as it functions as a nonpreferential layer to dictate perpendicular orientation of BCP domains from the substrate interface and as an e-beam resist to allow top-down lithographic process to spatially define the buffer layer on the substrate. The composition of the buffer layer can be tuned by changing the relative block lengths to create a nonpreferential surface which effectively induces perpendicular orientation of domains in an overlying BCP film. The grafted block copolymer can be locally shaved by e-beam lithography resulting in spatial control of domain orientation in the BCP film. The direct patterning approach reduces the number of steps involved in forming chemical patterns by conventional lithography

A Single-Component Inimer Containing Cross-Linkable Ultrathin Polymer Coating for Dense Polymer Brush Growth

We have developed a highly versatile universal approach to grow polymer brushes from a variety of substrates with high grafting density by using a single-component system. We describe a random copolymer which consists of an inimer, <i>p</i>-(2-bromoisobutyloylmethyl)­styrene (BiBMS), copolymerized with glycidyl methacrylate (GMA) synthesized by reversible addition–fragmentation chain-transfer (RAFT) polymerization. Thermal cross-linking created a mat that was stable during long exposure in organic solvent even with sonication or during Soxhlet extraction. The absolute bromine density was determined via X-ray photoelectron spectroscopy (XPS) to be 1.86 ± 0.12 Br atoms/nm³. The ratio of experimental density to calculated absolute initiator density suggests that ∼25% of the bromine is lost during cross-linking. Surface-initiated ATRP (SI-ATRP) was used to grow PMMA brushes on the substrate with sacrificial initiator in solution. The brushes were characterized by ellipsometry, XPS, and atomic force microscopy (AFM) to determine thickness, composition, and homogeneity. By correlating the molecular weight of polymer grown in solution with the brush layer thickness, a high grafting density of 0.80 ± 0.06 chains/nm² was calculated. By synthesizing the copolymer before cross-linking on the substrate, this single-component approach avoids any issues with blend miscibility as might be present for a multicomponent curable mixture, while resulting in high chain density on a range of substrates

Segmental Dynamics of an Isolated Component Polymer Chain in Polymer Blends Near the Glass Transition

The segmental dynamics of a component chain isolated in its blending partner chains is examined using the reorientation of polymer-tethered fluorescent probes near the glass transition. It is found that the temperature dependence of the dynamics of an isolated component follows that of the other component, with a horizontal shift corresponding to the glass transition temperature modification, which may result from a local composition of ≈10% isolated component. On the contrary, the dynamic heterogeneity, another key dynamic feature near the glass transition, shows that the local dynamic environment of an isolated component becomes either as heterogeneous as a more inherently heterogeneous component or more heterogeneous than either. These observations emphasize that not only the chain connectivity but also the dynamic modulation of a component by the other component needs to be addressed in order to understand the segmental dynamics of an isolated component in polymer blends

Electronic Transport and Raman Scattering in Size-Controlled Nanoperforated Graphene

We demonstrate the fabrication and study of the structure–property relationships of large-area (>1 cm²) semiconducting nanoperforated (NP) graphene with tunable constriction width (<i>w</i> = 7.5–14 nm), derived from CVD graphene using block copolymer lithography. Size-tunable constrictions were created while minimizing unintentional doping by using a dual buffer layer pattern-transfer method. An easily removable polymeric layer was sandwiched between an overlying silicon oxide layer and the underlying graphene. Perforation-size was controlled by overetching holes in the oxide prior to pattern transfer into graphene while the polymer protected the graphene from harsh conditions during oxide etching and lift off. The processing materials were removed using relatively mild solvents yielding the clean isolation of NP graphene and thereby facilitating Raman and electrical characterization. We correlate the D to G ratio as a function of <i>w</i> and show three regimes depending on <i>w</i> relative to the characteristic Raman relaxation length. Edge phonon peaks were also observed at 1450 and 1530 cm^–1 in the spectra, without the use of enhancement methods, due to high density of nanoconstricted graphene in the probe area. The resulting NP graphene exhibited semiconducting behavior with increasing ON/OFF conductance modulation with decreasing <i>w</i> at room temperature. The charge transport mobility decreases with increasing top-down reactive ion etching. From these comprehensive studies, we show that both electronic transport and Raman characteristics change in a concerted manner as <i>w</i> shrinks

Utilization of Polymer-Tethered Probes for the Assessment of Segmental Polymer Dynamics near the Glass Transition

The approach of utilizing polymer-tethered fluorescent molecules in probing segmental dynamics of polymers near the glass transition was validated by the examination of the rotational dynamics of the probes that were randomly dispersed in the same polymer hosts as the tethered polymers. Poly(alkyl methacrylate)- and polystyrene-tethered fluorescent probes, located either at the end or in the middle of a polymer chain, were tethered either with flexible dodecyl or hexyl alkyl chains by atom transfer radical polymerization and post-polymerization modification, respectively. Different polymeric systems with different glass transition temperature and fragility differing by ≈100 K and ≈80, respectively, were studied. Although the polymer-tethered probes report increased average rotational relaxation times compared to segmental dynamics of polymers, the temperature dependence of the polymer dynamics reported by the probe was not altered as confirmed by the goodness-of-fit test of the Vogel–Fulcher–Tammann (VFT) equation. Through the comparison of β reported by a bigger untethered probe in the same system, the origin of the vertical shift of VFT was interpreted as the result of an increased restriction of probes upon tethering, which was not associated with an increase in the probing length scale. To summarize, the rotational dynamics of the tethered probe accurately captures the degree of non-Arrhenius temperature dependence and nonexponential relaxation of the host polymers regardless of Tg and fragility of the system or the tethering condition

Photoimageable Organic Coating Bearing Cyclic Dithiocarbonate for a Multifunctional Surface

We report the fabrication of photocross-linkable and surface-functionalizable polymeric thin films using reactive cyclic dithiocarbonate (DTC)-containing copolymers. The chemical functionalities of these material surfaces were precisely defined with light illumination. The DTC copolymers, namely, poly­(dithiocarbonate methylene methacrylate–random-alkyl methacrylate)­s, were synthesized via reversible addition–fragmentation chain transfer polymerization, and the reaction kinetics was thoroughly analyzed. The copolymers were cross-linked into a coating using a bifunctional urethane cross-linker that contains a photolabile o-nitrobenzyl group and releases aniline upon exposure to light. The nucleophilic attack of the aromatic amine opens the DTC group, forming a carbamothioate bond and generating a reactive thiol group in the process. The surface concentrations of the unreacted DTC and thiol were effectively controlled by varying the amounts of the copolymer and the cross-linker. The use of methacrylate comonomers led to additional reactive surface functionality such as carboxylic acid via acid hydrolysis. The successful transformations of the resulting DTC, thiol, and carboxylic acid groups to different functionalities via sequential nucleophilic ring opening, thiol–ene, and carbodiimide coupling reactions under ambient conditions were confirmed quantitatively using X-ray photoelectron spectroscopy. The presented chemistries were readily adapted to the immobilization of complex molecules such as a fluorophore and a protein in lithographically defined regions, highlighting their potential in creating organic coatings that can have multiple functional groups under ambient conditions

Perovskite Pattern Formation by Chemical Vapor Deposition Using Photolithographically Defined Templates

Thin film fabrication is necessary to realize the device integration of organic–inorganic hybrid perovskites (OIHPs), and solution-based crystallization methods have been employed widely to this end. Despite the versatility of the solution approach, device fabrication using typical “top-down” lithography is generally incompatible with as-prepared OIHPs films because of the low stability of perovskites to polar solvents involved in the lithographic process. Moreover, solution-prepared perovskites usually exhibit irregular surface roughness, implying the existence of randomly oriented crystal domains with a large density of grain boundaries, which are ultimately detrimental to the material properties. Here, we report a patterning of CH3NH3PbI3 (MAPbI3) thin films using a photolithographically fabricated cross-linked copolymer template on Si or SiO2 substrates via a chemical vapor deposition (CVD) method. Perovskite patterning is accomplished by growing PbI2 precursor layers selectively on template patterns and subsequently converting to MAPbI3 using CH3NH3I (MAI) in the vapor phase. We confirm that [0001]-oriented PbI2 nanoplatelets nucleate primarily on a Si or SiO2 surface and grow by surface diffusion from a polymer surface. The MAPbI3 conversion process preserves the original pattern morphology through the vapor–solid intercalation of MAI. Prototype photodetector arrays based on MAPbI3 patterns are also demonstrated. Our results highlight the advantages of the CVD patterning of perovskite materials in large-scale production for a range of optoelectronic applications

Light-Driven Reversible Modulation of Doping in Graphene

We report a route to noncovalently latch dipolar molecules on graphene to create stable chromophore/graphene hybrids where molecular transformation can be used as an additional handle to reversibly modulate doping while retaining high mobilities. A light switchable azobenzene chromophore was tethered to the surface of graphene via π–π interactions, leading to p-doping of graphene with an hole concentration of ∼5 × 10¹² cm^–2. As the molecules switch reversibly from trans to cis form the dipole moment changes, and hence the extent of doping, resulting in the modulation of hole concentration up to ∼18% by alternative illumination of UV and white light. Light-driven conductance modulation and control experiments under vacuum clearly attribute the doping modulation to molecular transformations in the organic molecules. With improved sensitivities these “light-gated” transistors open up new ways to enable optical interconnects

How Tethered Probes Report the Dynamics of a Polymer near the Glass Transition

How tethered probes report dynamics of host polymers near the glass transition was investigated by changing the length of the flexible linkers and the number of tethering points via imaging rotational fluorescence correlation microscopy and compared with free probes of different sizes. The results show that tethering did not alter the temperature-dependence of polymer dynamics and the shape of the correlation decay reported by the probe; however, the rotation slowed down up to ≈1 decade when both ends of the probe were restricted with short alkyl chain linkers. Upon comparison with the bigger free probe, the mechanism of the slowdown was attributed to the restricted motion upon tethering for tethered probes compared to averaging over different regions of the dynamic heterogeneity for the bigger probe. If the size of the probe was comparable to that of the dynamic heterogeneity of the system, tethered probes accurately report dynamics relevant to glass transition, regardless of tethering conditions