Imaging the Terahertz Nanoscale Conductivity of Polycrystalline CsPbBr₃ Perovskite Thin Films

Abstract

Terahertz (THz) radiation is a valuable tool to investigate the electronic properties of lead halide perovskites (LHPs). However, attaining high-resolution information remains elusive, as the diffraction-limited spatial resolution (∼300 μm) of conventional THz methods prevents a direct analysis of microscopic effects. Here, we employ THz scattering scanning near-field optical microscopy (THz-sSNOM) for nanoscale imaging of cesium lead bromide (CsPbBr3) thin films down to the single grain level at 600 GHz. Adopting a scattering model, we are able to derive the local THz nanoscale conductivity in a contact-free fashion. Increased THz near-field signals at CsPbBr3 grain boundaries complemented by correlative transmission electron microscopy–energy-dispersive X-ray spectroscopy elemental analysis point to the formation of halide vacancies (VBr) and Pb–Pb bonds, which induce charge carrier trapping and can lead to nonradiative recombination. Our study establishes THz-sSNOM as a powerful THz nanoscale analysis platform for thin-film semiconductors such as LHPs.

Ultrabroadband Terahertz Near-Field Nanospectroscopy with a HgCdTe Detector

“…Many fundamental material excitations reside in the energy range of several 10 meV, e.g., phenomena related to phonons, excitons, plasmons, Landau level transitions, charge density waves, and magnons. Consequently, there is a strong desire to extend near-field nanospectroscopy toward the far-infrared and terahertz (THz) spectral range. ,, A successful approach to this goal has been combining scattering-type scanning near-field optical microscopy (s-SNOM) with infrared radiation from accelerator-based light sources. − ,,,− Specifically, synchrotron-based infrared near-field spectroscopy has pushed the spectral limit of nanospectroscopy down to ∼10 THz (320 cm –1 , 31 μm), thereby enabling many new experiments. ,− ,− Additionally, coming from lower frequencies (microwave regime), nanospectroscopy and nanoimaging around and below 1 THz is nowadays more frequently achieved, e.g., via THz time-domain spectroscopy and setups based on Schottky diodes or photoconductive antennas. − Despite such advances, the lack of suitable sources and matching detectors in the 1–10 THz rangethe so-called “THz gap” ,, has so far hampered extending broadband near-field nanospectroscopy into the center of this important spectral region. While examples of near-field nanospectroscopy and polariton interferometry in the 1–10 THz spectral range do exist, ,,,− such studies usually employ high-intensity narrowband laser sources to compensate for less-than-ideal detectors, thereby inducing limitations for spectroscopy.…”

Terahertz s-SNOM Imaging of a Single Cell with Nanoscale Resolution

“…To further achieve spatial resolution toward the nanometer scale, THz scattering-type scanning near-field optical microscopy (THz s-SNOM) has been employed. Offering nanoscale resolution comparable to the tip size, THz s-SNOM has been used to detect the local conductivity of two-dimensional materials, 15 observe in-plane anisotropic plasmon polaritons, 16 and investigate electron motion within nanostructures. 17 In the biomedical field, THz s-SNOM enabled near-field imaging of molecules, including proteins 18 and lactose, 19 as well as bacteria 20 and viruses.…”