Self-Assembled hBN Wrinkles as Planar Optofluidic Channels

Optically accessible, scalable planar nanofluidic channels are attractive for studying transport and localization under confinement. Two dimensional (2D) materials provide large area, atomically flat interfaces for generating such platforms, yet achieving long range one-dimensional (1D) confinement with top-down nanofabrication remains challenging because it requires reproducible nanoscale feature control over extended distances, high yield, and low nonspecific adsorption of analytes under aqueous conditions. Here we demonstrate that thermally induced wrinkling of exfoliated hexagonal boron nitride (hBN) produces self-assembled, liquid-accessible, channel-like networks through a lithography-free process. By varying flake thickness and substrate choice, we quantify statistical trends in wrinkle density and morphology, thereby establishing a practical fabrication design space. Atomic force microscopy and electron microscopy reveal wrinkle-derived geometries with vertical confinement ranging from <2 nm to >100 nm depending on flake thickness and substrate. We further employ time-sequence optical imaging upon droplet-contact, which together with Raman mapping of the water OH-stretch band and capacitance-gradient mapping (dC/dz) by scanning dielectric microscopy (KPFM-based) measurements, demonstrates liquid infiltration and long-term liquid retention within the wrinkle network for more than 10 h. We finally show a proof-of-concept biomolecule confinement application in which we integrate a graphene overlayer as a background suppression interface, enabling wide-field fluorescence localization of ATTO647N labeled DNA along hBN wrinkle-induced nanochannels. Overall, this work establishes self-assembled hBN wrinkles as a scalable, and optically addressable planar nanofluidic platform for confinement of fluids and biomolecules.

💡 Research Summary

In this work the authors introduce a lithography‑free method for creating planar nanofluidic channels by exploiting thermally induced wrinkling of exfoliated hexagonal boron nitride (hBN) flakes. The process consists of mechanically exfoliating multilayer hBN onto various substrates (SiO₂/Si, sapphire, quartz), annealing at 1000 °C in high vacuum, and then cooling. Because hBN exhibits a negative in‑plane thermal expansion coefficient, the mismatch with the substrate generates compressive strain that is relieved by out‑of‑plane buckling, producing a network of ridges and junctions. The wrinkles are not random; they tend to be straight over micron‑scale lengths and intersect at angles close to 120°, reflecting crystallographic orientation (often along the arm‑chair direction).

Systematic statistical analysis shows that wrinkle density (total traced length per unit area) depends strongly on both substrate and flake thickness. Softer substrates (quartz) and thicker flakes yield lower densities, while stiffer substrates (sapphire) and thinner flakes produce denser networks. Height measurements reveal a broad range from sub‑2 nm up to >150 nm, while widths lie in the 0.5–2 µm regime. The authors interpret these trends using classical thin‑film buckling theory: the characteristic wavelength scales with film thickness and the stiffness contrast between film and substrate, whereas amplitude grows with the applied compressive strain beyond a critical threshold. Thus, by selecting substrate and controlling hBN thickness, one can statistically tune the vertical gap of the channels to the desired nanometer scale.

To probe the mechanical strain distribution within individual wrinkles, confocal Raman mapping of the hBN E₂g mode (≈1365 cm⁻¹) was performed. The Raman peak shifts to higher wavenumbers at the ridge crests (tensile strain) and to lower wavenumbers at the troughs (compressive strain), confirming a curvature‑driven strain gradient across the cross‑section. This strain landscape is further corroborated by photoluminescence contrast and by prior simulations predicting alternating tensile/compressive zones at wrinkle intersections.

Liquid accessibility was demonstrated by depositing a water droplet onto the wrinkled surface and monitoring infiltration with time‑resolved optical microscopy, Raman imaging of the water OH‑stretch band, and scanning dielectric microscopy (KPFM‑based dC/dz mapping). Water rapidly fills the nanoslits and remains confined for more than 10 hours, indicating that the sub‑nanometer to tens‑nanometer gaps act as capillary traps that prevent evaporation or leakage.

For a proof‑of‑concept biomolecule confinement experiment, a monolayer graphene sheet was transferred onto the hBN surface to serve as an optical background suppressor. Graphene efficiently quenches fluorescence of nearby emitters via non‑radiative energy transfer (RET). ATTO647N‑labeled DNA molecules were introduced; fluorescence was observed only where the DNA lay within ~10 nm of the graphene, effectively highlighting the locations of the DNA along the wrinkle‑defined channels in wide‑field fluorescence images. This demonstrates that the hBN‑graphene heterostructure can be used for high‑contrast, planar nanofluidic studies of single biomolecules.

Overall, the paper delivers four major contributions: (1) a scalable, lithography‑free route to generate long‑range, planar nanofluidic channels via thermally induced hBN wrinkling; (2) a quantitative design space linking substrate choice and flake thickness to wrinkle density, height, and width; (3) multimodal verification of liquid infiltration and long‑term retention using Raman, KPFM, and optical imaging; and (4) a functional demonstration of biomolecule confinement using a graphene overlayer for background suppression. By providing a large‑area, optically accessible, and chemically robust nanofluidic platform, this work opens new avenues for lab‑on‑a‑chip diagnostics, single‑molecule transport studies, and integrated optoelectronic‑nanofluidic devices.