Heat Turns Tiny Perovskite Crystals into Unexpected Shapes in a Microfluidic Reactor
A new study has shown that tiny crystals designed to emit bright green light can be coaxed into radically different shapes simply by changing the temperature at two moments during their formation. The finding gives researchers an unusually detailed look at how formamidinium lead bromide, or FAPbBr₃, nanocrystals grow inside a microfluidic system—a network of miniature channels that can control chemical reactions with precision. Depending on the temperature used during an initial growth stage and a later second-stage treatment, the crystals became elongated and anisotropic, cube-like, or large and irregular. At the most extreme second-stage temperature tested, the average particle size reached about 70 nanometres. The work, published in the Journal of Nanoparticle Research, could help turn a notoriously sensitive nanomaterial synthesis into a more predictable manufacturing process for light-emitting technologies.
FAPbBr₃ belongs to the metal-halide perovskite family, a class of semiconductors known for absorbing and emitting light efficiently. Its crystal structure consists of a three-dimensional framework of lead and bromide ions, with positively charged formamidinium ions occupying the spaces within that framework. At nanoscale dimensions, the material’s optical behaviour is influenced not only by its chemical composition but also by particle size, shape, surface chemistry and crystal quality. These properties make FAPbBr₃ nanocrystals attractive candidates for green emitters in displays, light-emitting devices and other optoelectronic systems. When a nanocrystal absorbs energy, electrons and positively charged holes are created; when they recombine, the material can release that energy as light. The efficiency and colour of this photoluminescence depend on how effectively those charge carriers remain confined and avoid defects that would dissipate energy without producing light.
The researchers from Hangzhou Dianzi University focused on a problem that has limited the practical development of these materials: the growth process is fast, complex and difficult to observe or control using conventional batch chemistry. In an ordinary flask, the reaction mixture can experience local variations in concentration, mixing and temperature. Those differences can cause crystals to form at slightly different times and grow under different conditions, producing a broad distribution of sizes and shapes. A microfluidic reactor offers a way around some of these difficulties. Reactants flow through channels whose small dimensions promote rapid mixing and efficient heat transfer. The reaction environment can therefore be adjusted by controlling flow, residence time and temperature, allowing scientists to examine how a crystal’s history affects its final morphology. In this study, the growth was deliberately separated into two thermal stages rather than treated as one continuous event.
That distinction proved decisive. During the first stage, the team tested initial growth temperatures of 80 and 160 degrees Celsius. The second stage was then conducted at higher temperatures, including 130 °C, 210 °C and 250 °C. This design allowed the researchers to ask whether a later thermal treatment could erase the structural decisions made when the nanocrystals first emerged. The answer was largely no. When the initial growth temperature was only 80 °C, the particles preferentially developed anisotropic forms—shapes that grow more strongly in one direction than another. Raising the temperature later to either 130 °C or 210 °C did not readily eliminate that early tendency. The result suggests that the first stage establishes a growth pattern that becomes difficult to reverse, even when the particles subsequently encounter substantially hotter conditions.
The behaviour reflects the competition between nucleation and growth. Nucleation is the moment when atoms, ions or molecular building blocks assemble into stable, embryonic crystals. Growth follows as additional material attaches to their surfaces. In perovskite nanocrystal synthesis, different crystal faces can have different surface energies and can interact unequally with ligands—molecules that bind to the surface and regulate how quickly new material is added. Temperature changes the rates of precursor conversion, diffusion, ligand binding and ion movement. At a lower initial temperature, the balance among these processes may favour rapid growth on selected faces, producing rods, plates or other elongated structures instead of compact particles. Once those faces and interfaces have formed, later heating may accelerate growth without reconstructing the entire crystal. The particles can become larger while retaining the anisotropic character encoded during their earliest development.
The experiments at 160 °C revealed a more nuanced form of temperature control. A similar anisotropic-growth phenomenon appeared when the initial stage was carried out at that temperature, unless the researchers prolonged the duration of the first stage. With a longer initial growth period, cube-like nanocrystals formed. Cubes are closer to an isotropic morphology, meaning their dimensions are more nearly balanced in different directions. This observation indicates that temperature alone does not determine the outcome. Time spent at a given temperature is also important because it allows competing growth pathways to develop, surface-bound ligands to redistribute and unstable shapes to evolve toward more energetically favourable structures. In effect, the reactor does not simply provide a hot or cold environment; it supplies a carefully timed sequence in which the material’s growth history can be programmed.
The most dramatic transformation occurred when the second-stage temperature rose to 250 °C. Under those conditions, the study reports the formation of large nanocrystals with irregular morphologies and an average size of approximately 70 nm. Such particles are considerably larger than the compact quantum-confined nanocrystals typically associated with strong size-dependent optical effects. At elevated temperature, precursor molecules and ions move more rapidly, and crystal ripening can become more pronounced. Larger particles may grow at the expense of smaller ones because reducing total surface area lowers the system’s energy, a process commonly described as Ostwald ripening. High-temperature treatment may also destabilize the ligand layer that normally limits growth and helps preserve a regular shape. The resulting particles can therefore expand quickly while losing the geometric uniformity seen under milder conditions.
The significance of the work extends beyond producing a catalogue of shapes. In semiconductor nanocrystals, morphology is closely connected to function. A cube, plate or elongated particle presents different proportions of crystal facets and surface area, which can alter ligand coverage, charge transport and the likelihood of defect formation. Shape also affects how particles pack into films and how they interact with electromagnetic fields. For light-emitting applications, researchers often seek nanocrystals that are uniform in size and composition because variations can broaden emission spectra and reduce device consistency. The study’s two-stage approach offers a framework for separating the moment when shape is established from the later period when particles increase in size. That separation could make it easier to tune morphology without changing the material’s chemical identity.
Microfluidics may be especially valuable for this kind of optimisation because it converts synthesis into a sequence of controllable reaction zones. Rather than heating an entire vessel and hoping every particle experiences the same history, a continuous-flow device can expose the moving reaction mixture to distinct temperatures for defined residence times. Small changes in channel length, flow rate or heating profile can then be used to map a large experimental parameter space. The approach also has potential advantages for scale-up: many identical microchannels can theoretically operate in parallel, maintaining the uniform heat and mass transfer that are difficult to achieve in large batch reactors. The present findings do not establish that the process is ready for commercial production, but they show why continuous-flow systems can reveal mechanistic information as well as generate materials.
The researchers’ conclusions also highlight a practical caution for nanomaterial design: a final high-temperature treatment cannot necessarily rescue an unfavourable beginning. Low-temperature initial growth left a lasting imprint on FAPbBr₃ morphology, while extended treatment at 160 °C was required to obtain cube-like particles. Excessive heating, meanwhile, produced particles that were both larger and less regular. Future work will need to connect these shape changes with detailed optical measurements, long-term stability and device performance, as well as determine how precursor concentration, solvent, flow conditions and surface ligands interact with temperature. The study provides a map of that terrain rather than a finished recipe. But by showing that nanocrystals remember their earliest moments—and that a few degrees and minutes can redirect their development—it brings controlled production of bright perovskite emitters closer to an engineering discipline than a chemical guessing game.
Subject of Research: Microfluidic synthesis and temperature-controlled morphological evolution of formamidinium lead bromide nanocrystals
Article Title: Microfluidic synthesis and temperature-controlled morphological evolution of formamidinium lead bromide nanocrystals
Article References: Jiang, H., Liu, B., Huang, B. et al. “Microfluidic synthesis and temperature-controlled morphological evolution of formamidinium lead bromide nanocrystals.” Journal of Nanoparticle Research 28, 227 (2026). Original research article
Image Credits: AI Generated
DOI: 10.1007/s11051-026-06750-7
Keywords: FAPbBr₃ nanocrystals, microfluidics, perovskites, two-stage growth, temperature control, crystal morphology, anisotropic growth, green photoluminescence
Tags: anisotropic perovskite nanostructureslight-emitting perovskite nanomaterialsMicrofluidic nanocrystal synthesisnanocrystal morphology control in microfluidicsscalable manufacturing of perovskite nanocrystalsshape transformation of FAPbBr₃ nanocrystalsshape-dependent optical properties of perovskitessize and shape tuning of lead bromide nantemperature effects on nanocrystal formationtemperature-controlled perovskite crystal growth
