Strong and Localized Luminescence from Interface Bubbles Between Stacked hBN Multilayers

Van der Waals layered materials and their heterostructures enable extraordinary properties with out-of-plane flexibility, making them attractive for flexible and conformal electronics. During stacking, trapped adsorbates (e.g., hydrocarbons, air, water) form interface bubbles that impose strain and pressure, enabling phenomena such as nano-confined phases and catalyzed reactions, and acting as containers for microscopy. In monolayer transition metal dichalcogenides, bubbles can induce local emitters and optical cavities; however, strain engineering in multilayers has been less explored despite their robust optical performance and distinct bending behavior compared to monolayers or classical plates. Hexagonal boron nitride (hBN), with a ~6 eV band gap and strong near-band-edge ultraviolet emission whose intensity increases with thickness, is a promising platform. Its optical response can be tuned via twist-angle moiré effects, and defect-based single-photon emitters have been observed across a wide spectral range. This study investigates bubbles between stacked hBN multilayers, examining their mechanical behavior, geometry–strain relationship, and resulting localized UV luminescence, and demonstrates tuning of bubble geometry and optical properties via electron-beam-induced modification of trapped material.

• Bubbles in vdW stacks form from trapped adsorbates during transfer, leading to high pressures and unique phenomena (e.g., confined ice, reaction environments) and have been used for liquid-cell EM.
• In monolayer TMDs, bubbles create strain-induced local emitters and optical cavities; monolayer bubbles exhibit a universal, radius-independent aspect ratio governed by adhesion and in-plane stiffness.
• hBN offers strong UV emission with large exciton binding energy; emission intensity increases with thickness. Twist angle in hBN multilayers induces moiré effects that enhance/tune optical response. Single-photon emitters from defects in hBN have been widely reported.
• Multilayers are expected to deviate from membrane behavior due to non-negligible bending rigidity and weak interlayer vdW coupling, positioning their mechanics between classical plate and membrane limits.

• Sample fabrication: Multilayer hBN exfoliated from high-quality single crystals and vertically stacked to form hBN/hBN double-multilayers on Si/SiO2 via modified dry transfer using PDMS/PPC polymer stamps. Post-transfer cleaning in acetone and isopropanol, followed by annealing at 250 °C in Ar for 6 h to coalesce bubbles.
• Bubble characterization: Atomic force microscopy (AFM, tapping mode) to measure bubble height, radius, and multilayer thickness; secondary electron imaging to visualize strain-related contrast.
• Optical characterization: Cathodoluminescence (CL) in SEM (5–10 kV) and STEM (80 keV) for spatially resolved emission; comparison of panchromatic and spectrally resolved maps/line scans across bubbles and flat regions.
• Mechanical modeling: Total energy minimization of a circular bubble at equilibrium with energy terms for in-plane elasticity (E_el), bending (E_bend), adhesion (E_adh), and internal content free energy E(V), with volume–pressure relation. Derived aspect ratio expressions: monolayer (a=(γ/Y)^{1/4}); multilayer (a=(Yh^2/(c1 Y_multi h^2 + c2 κ))^{1/4}), where Y_multi=Y_mono t and c2 κ is a geometry-dependent bending rigidity term. Deflection profile modeled as y(x)/h=(1−(x^2/r^2))^α, with α indicating transition between membrane (α=1) and plate (α=2) behavior.
• Electron-beam modulation: SEM irradiation (1–10 kV) to decompose confined polymeric materials and expand bubbles; pre/post AFM to quantify changes in radius, height, aspect ratio; analysis of deflection exponent α evolution with size.
• Optical cavity modeling: Transfer-matrix method simulations using measured hBN optical constants and literature data for candidate fillers (polycarbonate, air, water) to reproduce wavelength-dependent interference patterns across bubbles and infer filler refractive index.
• Spectroscopy of trapped species: Electron energy loss spectroscopy (EELS) comparing bubble and flat regions to assess carbon bonding states (sp2 signatures).

• Strong and localized UV luminescence arises from interface bubbles in hBN multilayers, exhibiting a sharp asymmetric band at 300–400 nm with a zero-phonon line at 303 nm (4.09 eV) and phonon replicas spaced by ~180 meV.
• Near-band-edge excitonic emission at 215 nm (5.77 eV) is observed from all regions; flat double-multilayer regions show an additional symmetric peak at 320 nm (3.9 eV) associated with a 29° twist angle.
• CL is strongly enhanced at bubbles under low accelerating voltage (5 kV SEM) but not at high voltage (80 keV STEM), indicating the emission originates from the strained top multilayer.
• Multilayer bubble aspect ratios (e.g., t=70 nm) are ~0.01–0.02, an order of magnitude smaller than monolayer hBN bubbles (~0.11), and increase with radius, unlike the radius-independent monolayer case.
• Deflection profiles of multilayer bubbles fit y/h=(1−(x^2/r^2))^α with α≈1.5 (between membrane and plate limits), differing from monolayers (α≈1).
• Theoretical model predicts and experiments confirm that aspect ratio increases with radius and decreases with multilayer thickness; geometry-dependent bending rigidity (c2 κ) controls the slope.
• Extracted thickness dependence of bending rigidity lies between κ∝t (no interlayer interaction) and κ∝t^3 (classical plate), consistent with partial interlayer sliding in vdW multilayers.
• Electron-beam irradiation expands bubbles (example: ~5× volume, 1.6× aspect ratio increase) while preserving intrinsic mechanical parameters (c2 κ≈2–4×10^9 eV); deflection exponent α decreases toward 1 as radius increases, indicating membrane-like behavior at larger expansion.
• Interference fringes in CL around bubbles are reproduced by transfer-matrix simulations when the bubble filler has refractive index n≥1.65 at ~300 nm, consistent with polycarbonate-like polymer residues from transfer.
• EELS from bubbles shows sp2 carbon π* peak at ~285 eV and a relatively narrow σ* feature (~295 eV), contrasting with broader amorphous carbon-like features in flat regions, suggesting beam-induced or strain-assisted carbon incorporation/modification near bubbles.

Findings demonstrate that vdW multilayer bubbles act as strain-engineered, radially symmetric optical cavities with geometry and emission properties determined by multilayer bending mechanics. Unlike monolayers with constant aspect ratio, multilayer bubbles exhibit radius- and thickness-dependent aspect ratios and deflection profiles governed by a finite bending rigidity arising from weak interlayer interactions, enabling tunable strain distributions and edge stress concentrations. The localized 300–400 nm emission with phonon replicas likely originates from carbon-related defect states in the strained top hBN multilayer, consistent with (i) depth-dependent CL showing top-layer origin, (ii) polymeric filler inferred from optical interference (n≥1.65) and its electron-beam-decomposition-driven bubble expansion, and (iii) EELS evidence for sp2 carbon features within bubbles. Electron-beam control of bubble volume/shape enables tuning of optical standing waves and intensity via cavity thickness modulation. The mechanical model accurately predicts bubble aspect ratio, strain at the center, and shape evolution across sizes and thicknesses, offering a route to engineer optical responses in vdW multilayers by selecting thickness, bubble radius, and controlled irradiation. These insights are relevant for fundamental studies of vdW bending mechanics and for designing robust UV photonic elements.

The study reveals strong, localized UV luminescence from interface bubbles in stacked hBN multilayers and establishes a mechanics-based framework linking bubble geometry, strain, and thickness-dependent bending rigidity in vdW multilayers. Multilayer bubbles show distinct, tunable aspect ratios and deflection profiles between membrane and plate limits, enabling systematic strain engineering via radius and thickness. Electron-beam irradiation of trapped polymeric fillers expands bubbles without altering intrinsic mechanical parameters, enhancing luminescence and forming optical standing waves. Transfer-matrix modeling and EELS indicate organic filler with high refractive index and sp2 carbon signatures, consistent with carbon-related defect emission in strained hBN. These results open pathways to create and modulate ultraviolet emitters and microcavities using vdW multilayer bubbles. Future directions include: designing ordered bubble arrays as photonic crystals; integrating different fillers or dopants (e.g., Ni, Cu) to shift emission wavelengths; extending the mechanics–optics framework to other vdW materials (e.g., WS2); and exploring controlled interlayer coupling and sliding to tailor bending rigidity and strain distributions.

• The precise chemical identity and distribution of the trapped material are inferred from optical modeling and processing history; direct chemical quantification is limited.
• The proposed luminescence mechanism (carbon-related defects in strained hBN) is supported by indirect evidence (EELS, depth-dependent CL) but not conclusively isolated to specific defect configurations.
• The bending rigidity’s thickness dependence is extracted as an effective, geometry-dependent term (c2 κ) without a unique closed-form scaling law; interlayer friction/sliding parameters are not independently measured.
• Electron-beam modulation is demonstrated, but dose–response and potential beam-induced defect formation in hBN are not fully mapped, which may influence optical properties.