Infrared optoelectronics in twisted black phosphorus

Twisted van der Waals heterostructures, with atomically sharp interfaces and tunable twist angles, enable unconventional physical phenomena and novel device functionalities, including superconductivity, moiré excitons, bulk photovoltaic effects (BPVE), and pseudo-magnetic fields. Black phosphorus (BP), with a puckered lattice and lower crystalline symmetry than graphene, exhibits highly anisotropic properties; its light emission is linearly polarized along the armchair direction and strictly forbidden along the zigzag direction due to optical selection rules. Thin-film BP has a mid-infrared bandgap (0.3–0.4 eV), a spectral range with scarce materials. The study aims to manipulate recombination (radiative transitions) and separation (photocarrier collection) of electron–hole pairs in BP via twisted interfaces, hypothesizing that twisting will break symmetry to brighten zigzag-polarized emission and generate interfacial spontaneous polarization enabling zero-bias photocarrier separation. The work explores these effects across a broad range of twist angles and thicknesses, beyond strictly atomically thin layers.

Prior research established that twisted vdW structures can lead to emergent phenomena and device concepts (e.g., moiré excitons, BPVE, superconductivity). BP’s intrinsic anisotropy yields linear dichroism with emission allowed only along the armchair direction and forbidden along zigzag; thin-film BP provides tunable mid-IR optoelectronics but mid-IR materials remain scarce. Earlier works demonstrated BP-based emitters, detectors, and bandgap tuning, and twisted BP devices enabling other functionalities (e.g., resonant tunneling diodes, polarimeters). These set the context that manipulating interfaces and symmetry in BP could unlock new optical transitions and photovoltaic effects.

Sample preparation: BP and hBN flakes were mechanically exfoliated (all-dry viscoelastic stamping) onto PDMS/glass, then the bottom BP was transferred onto Si/SiO2 (300 nm SiO2). The top BP was aligned and transferred onto the bottom BP to form twisted BP, capped with hBN. Samples were annealed at 200 °C for 20 min to enhance vdW interactions. All BP handling occurred in a nitrogen-filled glovebox to prevent oxidation and photodegradation. Optical characterizations: Photoluminescence (PL) was measured with a Bruker FTIR spectrometer (Invenio-R) and Hyperion-1000 IR microscope, using a chopped 532 nm laser (~25 µm spot radius, 572 Hz) as excitation. PL was collected with a 15× objective and analyzed by FTIR; an infrared polarizer in the detection path and a half-wave plate for incident polarization enabled polarization-resolved PL. Temperature-dependent PL was performed in vacuum with incident laser power ~80 W/cm^2. Photocurrent mapping and spectroscopy: Photocurrent mapping was conducted in vacuum (MStarter-200 system) using a 520 nm laser (1 µm spot radius, 300 µW), 1 µm scan step, and lock-in detection at 200 Hz. Mid-IR photocurrent spectra were measured with the Bruker FTIR/Hyperion microscope using an internal blackbody source, chopped at 604 Hz, focused to ~100 µm diameter; a lock-in amplifier captured amplitude and phase and interfaced with FTIR for spectral transformation. Photocurrents were mapped for different electrode pairs to probe collection directions (near Cx vs Cy) at zero bias. Theoretical calculations: Density functional theory (DFT) using VASP with PBE-GGA, energy cutoff 500 eV, and total energy convergence <1e-5 eV. k-point meshes: 12×12×1 (3-layer BP), 5×5×1 (90° twisted heterostructure), and 2×2×1 (36.78° twisted heterostructure with 480 P atoms/unit cell). Calculations included band structures, state distributions, and differential charge densities to evaluate spontaneous polarization and symmetry of transition matrix elements.

• Symmetry-forbidden emission brightened by twisting: In individual BP flakes, PL is strictly forbidden along zigzag and maximized along armchair. For a 90° twisted sample (Sample LA; top: 19 layers, bottom: 41 layers), PL spectra of the twisted region detected along the x-direction (zigzag of bottom BP) show a broad, multi-peak emission spanning 2.7–4.2 µm, exceeding the sum of individual layers’ PL (I_twisted >> I_top+bot) along zigzag, indicating new allowed transitions. Individual flake PL peaks: top at 3.38 µm and bottom at 3.60 µm.
• Decomposition suggests bottom-layer contribution: Estimated contribution from the bottom BP in the twisted region (I_twist_bot = I_twist − A·I_top with A=1.7) shows a redshift compared to the individual bottom BP, consistent across A values.
• DFT supports symmetry breaking and indirect transitions: For a 3L/4L BP heterostructure at 90°, direct transitions at Γ persist (CB0→VB), but the conduction minimum shifts to CB1^D at Γ–Y, enabling lower-energy indirect transitions (CB1^D→VB) that are localized in the thicker (bottom) BP. Odd/even symmetries of states near the interface are broken along both x and y, yielding finite dipole moments along zigzag and permitting both direct and indirect zigzag-polarized emission.
• Temperature dependence evidences indirect processes: As temperature increases, individual BP PL intensities decrease monotonically (enhanced e–ph scattering) and blue-shift due to thermal-stress effects. In twisted BP, PL intensity vs temperature is non-monotonic: initial decrease (direct transition dominant) followed by increase (phonon-assisted indirect transition becomes stronger with more phonons), consistent with DFT-derived band structure.
• Interfacial spontaneous polarization (BPVE): DFT at twist angle 36.78° shows nonzero differential charge densities and spontaneous polarization P maximized along Cx and vanishing along Cy, indicating an interfacial BPVE axis. At 90°, charge distributions are mirror symmetric along Cx and Cy with vanishing polarization.
• Zero-bias photocurrent maps consistent with BPVE: In a device with α≈47° (Sample PA), zero-bias photocurrent collected along Cx shows unchanged polarity across the channel, distinct from Schottky or photo-thermoelectric profiles which exhibit opposite signs near contacts and vanish mid-channel. Along Cy, maps show conventional profiles dominated by Schottky or photo-thermoelectric effects.
• Broadband photoresponse: Zero-bias responsivity extends up to ~4 µm with photocurrent peaks near 520 nm and 2.6 µm matching absorption peaks.
• Twist-angle dependence: Across >24 samples, zigzag-polarized PL brightening is pronounced at large twist angles (e.g., 75°, 30°) but absent at small angles (e.g., 6°). Conversely, BPVE is stronger at smaller twist angles (e.g., ~10°) and vanishes at 90°, aligning with theory that interfacial polarization increases as twist angle decreases.
• Beyond atomically thin limit: The effects are observed in thin-film BP tens of nanometers thick and do not require precise angle alignment.

The results validate that twisting BP layers breaks optical selection-rule symmetry and generates interfacial spontaneous polarization, addressing the goal of simultaneously controlling recombination and separation of carriers. The emergence of zigzag-polarized mid-IR emission in twisted regions, absent in pristine BP, directly supports symmetry-breaking of transition matrix elements. The red-shifted, non-monotonic temperature behavior and DFT-predicted CB minimum relocation corroborate the indirect transition component localized in the thicker bottom layer. Interfacial BPVE is evidenced by theory (nonzero polarization along Cx at non-90° twists) and zero-bias photocurrent maps showing uniform polarity independent of contact position, which contrasts with Schottky and photo-thermoelectric behaviors. Control analyses discount alternative explanations: interfacial bubbles induce tensile strain that would blue-shift PL and preserve zigzag suppression; measured PL across bubble intensities remains broad and similar. Thermal-expansion-induced uniform strain maintains centrosymmetry and cannot produce BPVE; single-flake BP control shows conventional zero-bias photocurrent profiles. The twist-angle dependence—PL brightening at large angles, stronger BPVE at small angles—fits the mechanism that stronger spontaneous polarization at small angles enhances carrier separation (photocurrent) while suppressing recombination (PL).

Twisted BP interfaces enable two key advances in mid-IR optoelectronics: (1) brightening of symmetry-forbidden zigzag-polarized emission, including lower-energy indirect transitions localized in the thicker layer; and (2) interfacial spontaneous polarization that drives a bulk photovoltaic effect for zero-bias photocarrier separation. These phenomena persist over a range of twist angles and layer thicknesses beyond atomically thin limits, simplifying device fabrication. The approach offers a general strategy to engineer optoelectronic properties via vdW twisted interfaces and can be extended to other vdW materials systems. Future work could optimize interfacial polarization strength, device geometries for enhanced responsivity, precise twist-angle control to tailor emission vs BPVE dominance, and explore other material combinations for tunable mid-IR functionalities.

• The BPVE responsivity is modest, likely because the effect predominantly influences carriers generated near the twisted interface and because the interfacial polarization is not very strong.
• Mid-IR photocurrent measurements used a relatively large light spot (~100 µm), making it difficult to completely eliminate contributions from Schottky junctions and photo-thermoelectric effects, though experimental designs minimized them.
• Interfacial bubbles and strain may be present; while analyses argue against them as primary causes, they could still subtly affect local band structure and PL.
• Angle-dependent optimization and precise control of twist and interface quality may further improve performance but were not the focus here.