Piezoelectric and ferroelectric properties in the two-dimensional (2D) limit are highly desired for nanoelectronic, electromechanical, and optoelectronic applications. Here we report the first experimental evidence of out-of-plane piezoelectricity and ferroelectricity in van der Waals layered α-InSe nanoflakes. The noncentrosymmetric R3m symmetry of the α-InSe samples is confirmed by scanning transmission electron microscopy, second-harmonic generation, and Raman spectroscopy measurements. Domains with opposite polarizations are visualized by piezo-response force microscopy. Single-point poling experiments suggest that the polarization is potentially switchable for α-InSe nanoflakes with thicknesses down to ∼10 nm. The piezotronic effect is demonstrated in two-terminal devices, where the Schottky barrier can be modulated by the strain-induced piezopotential. Our work on polar α-InSe, one of the model 2D piezoelectrics and ferroelectrics with simple crystal structures, shows its great potential in electronic and photonic applications.

The wetting behavior of thin films of symmetric poly(styrene-block-methyl methacrylate) was investigated on self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) that were chemically modified by exposure to X-rays in the presence of air. The concentration of aldehyde and hydroxyl groups on the surface of exposed OTS increases with increasing dose. The polarity and surface tension of the SAMs were tuned to control the wetting behavior of the polymer films. Symmetric, neutral, and asymmetric wetting of the films were observed for doses of 400-1000, 1200, and 1400-2000 mJ/cm 2 , respectively. The wetting behavior was qualitatively described by estimating the interfacial energies between each block of the copolymer and exposed SAMs and the surface tensions of both blocks using the Fowkes-van Oss-Chaudhury-Good model of surface tension. These results lay the foundation for controlling the morphology of thin films of block copolymers over macroscopic dimensions by nanopatterning substrates with regions of different wetting behavior using advanced lithography.

We report a method to fabricate high-quality patterned magnetic dot arrays using block copolymer lithography, metal deposition, and a dry lift-off technique. Long-range order of cylindrical domains oriented perpendicular to the substrate and in hexagonal arrays was induced in the block copolymer films by prepatterning the substrate with topographic features and chemically modifying the surface to exhibit neutral wetting behaviour towards the blocks of the copolymer. The uniformity of the domain size and row spacing of block copolymer templates created in this way was improved compared to those reported in previous studies that used graphoepitaxy of sphere-forming block copolymers. The pattern of block copolymer domains was transferred to a pattern of magnetic metal dots, demonstrating the potential of this technology for the fabrication of patterned magnetic recording media.

Self-assembled films of octadecyltrichlorosilane were patterned with regions of different chemical functionality using extreme ultraviolet interferometric lithography. Unexposed regions of the imaging layers remain terminated in methyl groups, and exposed regions are modified so as to be terminated with polar, oxygen-containing terminal groups. Thin films of symmetric poly(styrene-b-methyl methacrylate) were deposited on the substrates and annealed. Unexposed and exposed regions are preferentially wet by the polystyrene block and poly(methyl methacrylate) block of the copolymer, respectively. The dimensions of the grating patterns on the substrate had periods (L s) from 1400 to ∼60 nm. If L s ≫ L o (L o = bulk lamellar period of the block copolymer), then the surface pattern was replicated in the topography of the polymer film with a maximum difference in film thickness of 1/2 L o on adjacent regions. The topographic pattern of the polymer film was a result of lamellae oriented parallel to the substrate with symmetric wetting on unexposed regions (thickness = nL o) and with asymmetric wetting on exposed regions (thickness = (n + 1/2)L o). As the dimension of L s approached L o, the replication of the surface pattern in the topography of the film continued to be observed, but with decreasing difference in thickness over adjacent exposed and unexposed regions. For a surface pattern with L s ≈ L o, the lamellae oriented perpendicular to the substrate and were macroscopically aligned with the surface pattern.