Pressure Treatment Reshapes Perovskite Film Surfaces Without Touching the Crystal Core

Metal-halide perovskite thin films can look deceptively simple: a well-defined cubic crystal lattice, a bright green glow under laser excitation, and a composition that barely changes from sample to sample. Yet a new study of methylammonium lead bromide (MAPbBr3) films grown by close-spaced sublimation shows that pressing the films after deposition, at 100 °C for three hours under pressures ranging from 1,000 to 5,000 pounds per square inch, leaves the bulk crystal framework untouched while reshaping the film’s near-surface region in ways that are anything but predictable. The work, published in Frontiers in Physics, reports that surface chemistry, surface energetics, and photoluminescence each vary strongly and non-monotonically across the pressure series, even as X-ray diffraction confirms that every film retains the characteristic cubic perovskite structure.

The research team, led by authors from the Universidad Autónoma de Sinaloa in Mexico and The University of Texas at Dallas, prepared MAPbBr3 films by close-spaced sublimation, a solvent-free vapor-phase deposition method in which lead bromide is first sublimed onto glass and methylammonium bromide is then deposited on top to form the perovskite. The two-step process used independently controlled heaters for the crucible and substrate, a source-to-substrate separation of just five millimeters, and a nitrogen working pressure kept at or below 90 millitorr. After deposition, the films were subjected to a hot-press treatment at 100 °C for three hours under applied pressures of 1 to 5 kPsi, with two reference conditions included: an untreated film (NA) and a film heated at 100 °C without applied pressure (HP). All treatments were carried out in a nitrogen-filled glovebox, and more than ten film specimens were prepared for each processing condition.

The first key result concerns what did not change. X-ray diffraction patterns of all films, from the untreated NA condition through the HP treatment and the full 1 to 5 kPsi pressure series, show the same main reflections at approximately 15, 21, 30, 34, 37, 43, and 46 degrees, indexed to the (100), (110), (200), (210), (211), (220), and (300) planes of perovskite-type MAPbBr3 and consistent with the cubic reference structure. No additional reflections from a new crystalline phase appear after any treatment. Analysis of the dominant (100) reflection yields apparent coherent domain sizes between roughly 18.8 and 22.0 nanometers across the series, with a moderate non-monotonic variation. Cross-sectional scanning electron microscopy shows continuous micrometric layers with local thicknesses spanning approximately 2.6 to 3.8 micrometers, again without a simple monotonic dependence on the applied pressure.

Where the differences emerge is at the surface. High-resolution X-ray photoelectron spectroscopy, performed after argon-ion sputter cleaning with binding energies referenced to adventitious carbon at 284.8 electronvolts, shows that all films preserve the same core-level features: the lead, bromine, nitrogen, carbon, and oxygen contributions expected for MAPbBr3 remain detectable throughout the series. The elemental identity of the near-surface region is therefore maintained. What changes are the standard XPS observables themselves, including relative intensity, line shape, spectral width, and apparent binding-energy position, which vary in a condition-dependent manner across the Pb 4f, Br 3d, O 1s, and C 1s regions. The authors note that the O 1s surface fraction, consistent with oxygen-containing species in the near-surface region, varies non-monotonically among the processing conditions.

One of the most striking findings concerns the lead 4f spectra. In the untreated NA film, only the dominant Pb2+ doublet is resolved, at 137.9 and 142.8 electronvolts. Across the processed series, this dominant doublet remains confined to a narrow 137.9 to 138.2 and 142.8 to 143.1 electronvolt window, but an additional low-binding-energy doublet develops at 135.9 to 136.2 and 140.8 to 141.0 electronvolts, roughly 1.8 to 2.1 electronvolts below the principal state. This separation is characteristic of two electronically inequivalent lead populations, and the low-binding-energy interval coincides closely with metallic Pb0 components previously reported for MAPbBr3. Quantitatively, the Pb0 contribution accounts for 4.0 percent of the total Pb 4f area in the HP condition, 1.4 to 1.7 percent for the 1 to 3 kPsi conditions, 3.9 percent at 4 kPsi, and 3.1 percent at 5 kPsi, while no Pb0 component was resolved in the untreated film.

The authors are careful about what this reduced lead contribution means. Metallic Pb0 in lead-halide perovskites can form under X-ray irradiation, and argon-ion sputter cleaning has also been shown to generate a metallic-lead contribution during surface preparation. Because all samples underwent the same XPS acquisition and surface-cleaning history, the team treats the variations in low-binding-energy spectral weight as changes in the relative near-surface lead-state population under a common protocol, rather than assigning the reduction pathway exclusively to the applied pressure. They also caution that lead-core-level interpretation in lead-halide perovskites must be handled carefully, since measurement-induced surface modifications and electronic-structure effects such as band bending can complicate the assignment of binding-energy shifts purely to local chemical changes.

The surface energetic landscape tells an equally non-monotonic story. Kelvin probe measurements, referenced to gold with a work function of 5.15 electronvolts and averaged over five independent films per condition, show that the work function rises from 4.80 plus or minus 0.014 electronvolts in the untreated film to 4.88 plus or minus 0.012 electronvolts after the heat-only treatment, then falls progressively to 4.82, 4.71, and 4.63 electronvolts at 1, 2, and 3 kPsi. At 4 kPsi the trend reverses sharply, with the work function jumping to 4.97 plus or minus 0.013 electronvolts and reaching 5.00 plus or minus 0.014 electronvolts at 5 kPsi. The overall 0.37-electronvolt variation in the mean work function represents a substantial change in the energetic separation between the Fermi and vacuum levels, and the authors emphasize that the response clearly does not scale linearly with applied pressure. They also note that Kelvin probe measurements alone cannot determine whether the variation arises from a Fermi-level shift, a change in the surface contribution to the vacuum level, or both.

Combining the Kelvin probe data with photoelectron yield spectroscopy in air, which determines the ionization energy over a 4 to 6 electronvolt range, the team derived the separation between the Fermi level and the valence-band maximum. This derived quantity falls from 0.52 electronvolts in the untreated film to 0.46 electronvolts after the heat-only treatment, climbs to 0.49, 0.60, and 0.68 electronvolts between 1 and 3 kPsi, and then drops to 0.32 and 0.28 electronvolts at 4 and 5 kPsi. Because the ionization energy stays within a narrow 5.28 to 5.34 electronvolt interval while the work function spans 4.63 to 5.00 electronvolts, the work-function variation dominates the derived separation range. The authors caution that quantitative uncertainty estimates are not available for the PESA measurements, a relevant limitation given that the low density of states near the valence-band maximum of MAPbBr3 can affect frontier-level determination.

Photoluminescence completes the picture of a surface that responds in its own way to each processing condition. All films emit a broad, asymmetric band around 547 nanometers, consistent with the band-edge photoluminescence commonly reported for MAPbBr3, when excited at 405 nanometers under identical settings. But the integrated intensity, calculated without normalization over the 500 to 600 nanometer interval from four independent specimens per condition, varies dramatically and non-monotonically: from 215,709 plus or minus 8,724 counts-nanometers in the untreated film, it falls to 92,671 after the heat-only treatment, drops further to 47,853 and 12,724 at 1 and 2 kPsi, rebounds to 80,836 plus or minus 6,801 at 3 kPsi, and collapses again to 11,487 and 16,606 at 4 and 5 kPsi. Among the pressure-treated conditions, the 3 kPsi treatment stands out, combining the largest apparent coherent domain size, the lowest resolved Pb0 contribution, and the highest integrated photoluminescence intensity within the 1 to 5 kPsi series.

The broader significance lies in what the study demonstrates about where processing effects live in these films. Post-deposition pressure does not induce an evident transformation of the main crystalline phase, yet the near-surface state exhibits condition-dependent, non-monotonic variations in local chemistry, surface energetics, and emissive response, each measured independently. Because the work function and ionization energy help set the energetic boundary conditions at perovskite contacts, and because surface and interface properties strongly influence charge recombination, band alignment, and operational stability in metal-halide perovskites, these findings are relevant to the integration of MAPbBr3 films in light-emitting, photodetection, and radiation-detection architectures. The study is an association between a processing variable and measured near-surface properties rather than a demonstration of device performance, and the authors attribute thickness differences only partly to pressure, noting that film-to-film variability from deposition and local morphology may also contribute. Still, the message is clear: in MAPbBr3, the surface is an active part of the material, and pressing it changes that active layer in ways the crystal core never reveals.

Subject of Research: Post-deposition pressure effects on the near-surface properties of CSS-grown MAPbBr3 perovskite thin films

Article Title: Non-monotonic near-surface chemistry, surface energetics, and photoluminescence in CSS-grown MAPbBr3 films after post-deposition pressure treatment

Article References: Martinez-Falomir, G. G., Quevedo-Lopez, M. A., Flores-Valenzuela, J., Mendivil-Escalante, J. M., López-Lazcano, C. A., Vargas-Arana, C., Rodriguez-Guerrero, M. J., Aguirre-Tostado, F. S., & Leal-Perez, J. E. (2026). Non-monotonic near-surface chemistry, surface energetics, and photoluminescence in CSS-grown MAPbBr3 films after post-deposition pressure treatment. Frontiers in Physics, 14, 1931500. https://doi.org/10.3389/fphy.2026.1931500

Image Credits: AI Generated

DOI: 10.3389/fphy.2026.1931500

Keywords: MAPbBr3, perovskite thin films, close-spaced sublimation, post-deposition pressure, X-ray photoelectron spectroscopy, work function, Kelvin probe, photoluminescence, surface chemistry, surface energetics, X-ray diffraction, metal halide perovskites