Domain Morphology Research Articles

Domain morphology of newly designed lead‐free antiferroelectric NaNbO3‐SrSnO3 ceramics

AbstractReversible antiferroelectric‐ferroelectric phase transitions were recently observed in a series of SrSnO3‐modified NaNbO3 lead‐free antiferroelectric materials, exhibiting well‐defined double polarization hysteresis loops at ambient conditions. Here, transmission electron microscopy was employed to investigate the crystallography and domain configuration of this newly designed system via electron diffraction and centered dark‐field imaging. It was confirmed that antiferroelectricity is maintained in all compositions, manifested by the characteristic ¼ superlattice reflections in the electron‐diffraction patterns. By investigating the antiferroelectric domains and domain boundaries in NaNbO3, we demonstrate that antiphase boundaries are present and their irregular periodicity is responsible for the streaking features along the ¼ superlattice reflections in the electron‐diffraction patterns. The signature domain blocks observed in pure NaNbO3 are maintained in the SrSnO3‐modified ceramics, but disappear when the amount of SrSnO3 reaches 7 mol.%. In particular, a well‐defined and distinct domain configuration is observed in the NaNbO3 sample modified with 5 mol.% SrSnO3, which presents a parallelogram domain morphology.

Enhanced dielectric and piezoelectric properties of manganese-doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals by alternating current poling

Alternating current poling (ACP) was performed on Gen III relaxor-PT Mn-doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (Mn: PIN-PMN-PT) single crystals with the poling direction of [001]. Experimental results proved that ACP could bring property enhancement to both k31 and k33 mode crystals. Compared to those from traditional direct current poling (DCP), ACP with the optimized conditions (20 kV/cm, 0.1 Hz, and 20 cycles) enhanced the dielectric and piezoelectric properties of k31-mode mode crystals by more than 30%, where the enhanced free dielectric constant and piezoelectric coefficient d33 reached 5300 and 1750 pC/N, respectively. Furthermore, replacing DCP with ACP could increase the advantages of Gen III relaxor-PT. The coupling factors k31 and k33 were enhanced to 0.472 and 0.915, the mechanical quality factor Qm was enhanced by 17%, and the depoling temperature was raised by 17 °C to 123 °C. In the following mechanism study, in situ x-ray diffraction (XRD) combined with the temperature-dependent dielectric constant measurement proved the introduction of the monoclinic phases after ACP, while piezoresponse force microscopy (PFM) observation showed “2R”-like “2M” domain morphologies in ACP single crystals. Both these intrinsic and extrinsic factors are believed to be the keys to the mechanisms of property enhancement behind ACP. This study proved that ACP is an effective property enhancement method suitable for Gen III relaxor-PT single crystals and will promote its applications in high-temperature and high-power devices.

Alternating current poling on sliver-mode rhombohedral Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

Sliver-mode 0.73Pb(Mg1/3Nb2/3)O3-0.27PbTiO3 single crystals were poled by the alternating current electric field (ACP) along the [001] direction for low-frequency transducer applications. Under ACP with optimized conditions (2.89 kVrms, 0.1 Hz, 10 cycles), the free dielectric constant (εT33/ε0) was enhanced from 5300 to 7220 and piezoelectric coefficient (d33) was improved from 1410 to 1920 pC/N, both are about 36% higher than those under traditional direct current poling. The piezoresponse force microscopy observation and phase-field simulation show the similar domain morphologies evolution. During the ACP process, the domain morphology first changes from ‘4R’ to ‘2R’ configuration, and then the length of the 109° domain wall keeps increasing, which leads to a longer range of ordering and a stable lamellar domain structure in the end. The correlation between the domain morphology and the coercive field is also found in both the experiment and simulation. These findings on the effect and mechanism of ACP will help develop advanced low-frequency single crystal transducer arrays.

Emerging antiferroelectric phases with fascinating dielectric, polarization and strain response in NaNbO3-(Bi0.5Na0.5)TiO3 lead-free binary system

Environmentally-benign antiferroelectrics (AFEs) have attracted a great deal of attention in recent years owing to their special structure and polarization/strain-electric field response, exhibiting giant application advantages in multifunctional devices. In this work, a detailed phase diagram in a binary lead-free system between AFE NaNbO3 (NN) and ferroelectric (FE) (Bi0.5Na0.5)TiO3 (BNT) was established according to the measurement of dielectric and ferroelectric properties in combination with the multiscale structure analysis such as Raman spectra, synchrotron x-ray diffraction and transmission electron microscopy. Multiple AFE phases of different stability, crystal symmetry and domain morphology were stabilized near room temperature, involving AFE orthorhombic P (Pbma) and R (Pnma) phases in NN-rich compositions and AFE tetragonal phase (P4bm) in BNT-rich compositions, as well as field induced metastable AFE monoclinic phase (P21). Owing to the electric field induced AFE-FE phase transformation especially for relaxor AFEs with antipolar nanoregions, tunable and excellent electrical properties of temperature-stable dielectric permittivity, ultrahigh energy-storage density and efficiency and low-hysteresis high strain can be generated in the proximity of these AFEs, showing outstanding potentials in thermally-stable capacitors, dielectric energy storage and large-strain ceramic actuators for NN-BNT solid solutions.

Magnetization processes and magnetic domain structures in Ta/CoFeB/MgO stacks

Magnetization processes and magnetic domain structures in Ta/CoFeB/MgO stacks were studied in a series of samples with various CoFeB thicknesses d ranging from 1.24 to 1.60 nm with a step of 0.04 nm, using polar magneto-optical Kerr effect (PMOKE) magnetometry and microscopy. Thickness dependence of the magnetic anisotropy was evaluated and the first and second order anisotropy constants were quantified for each thickness. Accordingly, this dependence was deduced to result in magnetization reorientation from out-of-plane to in-plane through an easy-cone magnetization region (1.39 nm ≤ d ≤ 1.41 nm) as d was increased. PMOKE imaging of the magnetization reversal processes for stacks with out-of-plane easy axis indicated both a significant increase of the density of nucleation centers and a change in domain morphology with increasing d up to the magnetization reorientation thickness. Magnetization reversal dynamics was described by a thermal activation model consistent with a Barkhausen length of about 120 nm. The thinnest films with d = 1.24 and 1.28 nm exhibited straightened narrow stripe domains resulting from magnetic dipolar repulsion. A thorough study of narrow stripe domains was performed via direct and indirect magnetization reversal processes. The application of such structures as spin wave nano-channels could be promising.

Biobased Thermoplastic Elastomer Based on an SMS Triblock Copolymer Prepared via RAFT Polymerization in Aqueous Medium

In recent years, there has been a great interest in preparing polymers from bioresources, as this practice has several advantages over petrobased polymers. Thermoplastic elastomers (TPEs) are an interesting class of polymers that can be easily processed, reused, and recycled. The terpene, β-Myrcene (MY), is an important vinylic monomer having a conjugated diene in the backbone, which has a resemblance to the isoprene unit, a building block for natural rubber. MY is available from several bioresources like hops, lemongrass, bay plants, and so forth. In this work, reversible addition–fragmentation chain-transfer (RAFT) polymerization was used for the very first time to prepare an ABA-type triblock copolymer, polystyrene-b-poly(β-myrcene)-b-polystyrene (SMS), where poly(β-myrcene) acted as the soft block and polystyrene (PS) as the hard block. For this, styrene was initially polymerized using S,S-dibenzyl trithiocarbonate as a bifunctional RAFT agent, and the resultant PS was then used as a macro-RAFT agent to polymerize MY to attain SMS triblock copolymers. The successful preparation of the block copolymers (BCPs) was confirmed by NMR, FTIR, and GPC analyses. The DSC and AFM analyses showed the phase-separated morphology of hard and soft domains of PS and PMY, respectively. These SMS BCPs showed good tensile properties and a low tension set of 4.12%, indicating their application as a potential TPE. The scrapes of these SMS BCPs were successfully recycled and reused, indicating the sustainability of the process for this potential TPE.

Optimization and Control of Large Block Copolymer Self-Assembly via Precision Solvent Vapor Annealing.

The self-assembly of ultra-high molecular weight (UHMW) block copolymers (BCPs) remains a complex and time-consuming endeavor owing to the high kinetic penalties associated with long polymer chain entanglement. In this work, we report a unique strategy of overcoming these kinetic barriers through precision solvent annealing of an UHMW polystyrene-block-poly(2-vinylpyridine) BCP system (Mw: ∼800 kg/mol) by fast swelling to very high levels of solvent concentration (ϕs). Phase separation on timescales of ∼10 min is demonstrated once a thickness-dependent threshold ϕs value of ∼0.80–0.86 is achieved, resulting in lamellar feature spacings of over 190 nm. The threshold ϕs value was found to be greater for films with higher dry thickness (D0) values. Tunability of the domain morphology is achieved through controlled variation of both D0 and ϕs, with the kinetically unstable hexagonal perforated lamellar (HPL) phase observed at ϕs values of ∼0.67 and D0 values of 59–110 nm. This HPL phase can be controllably induced into an order–order transition to a lamellar morphology upon further increase of ϕs to 0.80 or above. As confirmed by grazing-incidence small-angle X-ray scattering, the lateral ordering of the lamellar domains is shown to improve with increasing ϕs up to a maximum value at which the films transition to a disordered state. Thicker films are shown to possess a higher maximum ϕs value before transitioning to a disordered state. The swelling rate is shown to moderately influence the lateral ordering of the phase-separated structures, while the amount of hold time at a particular value of ϕs does not notably enhance the phase separation process. These large period self-assembled lamellar domains are then employed to facilitate pattern transfer using a liquid-phase infiltration method, followed by plasma etching, generating ordered, high aspect ratio Si nanowall structures with spacings of ∼190 nm and heights of up to ∼500 nm. This work underpins the feasibility of a room-temperature, solvent-based annealing approach for the reliable and scalable fabrication of sub-wavelength nanostructures via BCP lithography.

In-situ variable reflectance spectra model of two-dimensional material prepared by CVD

Here we introduce the spectral measurement model of online reflectance spectra aimed at the variable temperature environment. The model combines the traditional differential reflectance spectroscopy (DRS) model with the temperature error compensation method. The blank experiments and molybdenum disulfide (MoS2) growth experiment prepared by chemical vapor deposition (CVD) were monitored in situ by using the new DRS model proposed in this work. The characteristic peaks of MoS2 at 1.82 eV and 1.99 eV were successfully discovered in the spectra processed by the temperature error compensation method. This observation reveals that MoS2 thin films have grown. The morphology and Raman properties of MoS2 domains were off-line characterized using optical microscopy and Raman spectroscopy. The experimental results indicate that the spectral model and the temperature error compensation method can contribute to the real-time growth characterization of two-dimensional material (2D). It is of great significance for the preparation and application of 2D materials.

3D Quantification of Microstructural Properties of LiNi0.5Mn0.3Co0.2O2 High‐Energy Density Electrodes by X‐Ray Holographic Nano‐Tomography

AbstractDespite significant progress in the field of tomography, capturing the carbon binder domain (CBD) morphology presented in the Li‐ion electrode remains challenging, due to its low attenuation coefficient. In this work, quantitative phase contrast X‐ray nano‐holotomography is used as a straightforward approach that provides a large reconstructed volume, where the CBD can be resolved along with the active materials and the pore space. As a result, a complete quantitative analysis of the microstructures of three LiNi0.5Mn0.3Co0.2O2 high energy density electrodes, including the characterization of each phase separately along with the statistical quantification of their inter‐connectivity at particle scale, is performed. The microstructural heterogeneities are quantified and comparison between different electrodes is done. The results from this work suggest reasons for the negative impacts of the CBD excess on the electrode performance at high C‐rates. Those results are true in the case of high energy density electrodes, and are due to the reduction of the electrochemical active surface area. This sheds light on the optimization of the electrode design to improve the power rate of high energy density electrodes.

Depolarization field tuning of nanoscale ferroelectric domains in (001)PbZr0.4Ti0.6O3/SrTiO3/PbZr0.4Ti0.6O3 epitaxial heterostructures

The effect of tuning the depolarization field in (001)-oriented ultrathin epitaxial PbZr0.4Ti0.6O3/SrTiO3/PbZr0.4Ti0.6O3 ferroelectric heterostructures is investigated. The thickness of the dielectric spacer (SrTiO3) is maintained constant at 2 unit cells. The ferroelectric layer thickness in the heterostructure (each PbZr0.4Ti0.6O3) layer varied from 8 to 15 nm is exploited as the parameter to tune the depolarization field. Piezoresponse force microscopy reveals a domain evolution from continuous labyrinthine domains to individual nanoscale bubble domains under the influence of an increasing depolarization field. A statistical analysis of the domain features (i.e., domain wall length and domain fraction) reveals that this change in domain morphology also affects the wall roughness and its associated disorder. The local coercive voltage obtained using switching spectroscopy piezoresponse force microscopy finds the 15 nm film to have the lowest coercive voltage. This is attributed to both a strain-induced increase in tetragonality and the depolarization field-induced changes in the domain morphology.

Effects of anisotropic misfit strains on equilibrium phases and domain structures in (111)-oriented ferroelectric PbTiO3 films

Advances in high-index-oriented ferroelectric films offer the exciting possibility of utilizing strain engineering to tune the desired properties related to the nanostructures. Previous studies often considered ferroelectric films grown on cubic substrates (isotropic strain), while the effects of anisotropic strains introduced by orthorhombic substrates are still not well understood. In our recent work, we grew (111)-oriented PbTiO3 (PTO) films on orthorhombic GdScO3 substrates and studied the domain structure evolution with film thickness. However, the theoretical understanding is lacking. In this work, the physical mechanism of the formation and evolution of domain structures was clearly revealed from the viewpoint of energy competition. Moreover, through the stereographic projection (SP) analysis of polarization vectors, a wide variety of ferroelectric phases for (111)-oriented PTO films under anisotropic strains were identified, and the misfit strain-misfit strain domain stability diagram was then constructed at room temperature. It was indicated that anisotropic misfit strains not only lead to the separation of ferroelectric domain variants that could co-exist under isotropic strain states for tetragonal-like d and monoclinic ma phases, but also stabilize the monoclinic mb phase. Their representative domain morphologies with separated domain variants under different anisotropic strains were analyzed in detail. These results provide guidance for interpreting and understanding more experimental data as well as the designing of (111)-oriented epitaxial ferroelectrics under anisotropic strains.

Photo-induced bond breaking during phase separation kinetics of block copolymer melts: a dissipative particle dynamics study.

Using a dissipative particle dynamics (DPD) simulation method, we study the phase separation dynamics in block copolymer (BCP) melts in d = 3, subjected to external stimuli such as light. An initial homogeneous BCP melt is rapidly quenched to a temperature T < Tc, where Tc is the critical temperature. We then allow the system to undergo alternate light "on" and "off" cycles. An on-cycle breaks the stimuli-sensitive bonds connecting both the blocks A and B in the BCP melt, and during the off-cycle, the broken bonds recombine. By simulating the effect of light, we isolate scenarios where phase separation begins with the light off (set 1); the cooperative interactions within the system allow it to undergo microphase separation. When the phase separation starts with the light on (set 2), the system undergoes macrophase separation due to bond breaking. Here, we report the role of alternate cycles on domain morphology by varying the bond-breaking probability for both set 1 and set 2, respectively. We observe that the scaling functions depend upon the conditions mentioned above that change the time scale of the evolving morphologies in various cycles. However, in all the cases, the average domain size respects the power-law growth: R(t) ∼tφ at late times, where φ is the dynamic growth exponent. After a short-lived diffusive growth (φ∼ 1/3) at early times, φ illustrates a crossover from the viscous hydrodynamic (φ∼ 1) to the inertial hydrodynamic (φ∼ 2/3) regimes at late times.

Effect of Crystalline Microstructure Evolution on Thermoelectric Performance of PEDOT : PSS Films

Although organic polymer thermoelectric (TE) materials have witnessed explosive advances in the recent decade, the molecular mechanism of crystallization engineering of TE performance, even for the most successful polymer of poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS), is still far from clear. Here, we deepen the understanding of the role of annealing-induced crystalline microstructure evolution on TE performance of the PEDOT : PSS film with thickness of 10 μ m, which is usually more effective than thin ones in applications. Annealed at optimized temperature of 220°C, the film displays a power factor of 162.5 times of that of the pristine film before annealing. The enhanced TE performance is associated with the changes of crystallographic and morphologic microstructures, including increased crystallinity and crystal grain size, a domain morphology transformation from granular to crystalline nanofibril, and reduced insulating PSS in the skin layer. These variances facilitate the carrier transport by a transition from 3D to 1D hopping, reduce the activation energy, and improve the carrier mobility. The mechanism of crystallization engineering reported here can be conceptually extended to other TE polymers and guides the future rational design of preparation principles for organic and composite TE materials.

Toughening Polylactic Acid by a Biobased Poly(Butylene 2,5-Furandicarboxylate)-b-Poly(Ethylene Glycol) Copolymer: Balanced Mechanical Properties and Potential Biodegradability.

Polylactic acid (PLA) is a biodegradable thermoplastic polyester produced from natural resources. Because of its brittleness, many tougheners have been developed. However, traditional toughening methods cause either the loss of modulus and strength or the lack of degradability. In this work, we synthesized a biobased and potentially biodegradable poly(butylene 2,5-furandicarboxylate)-b-poly(ethylene glycol) (PBFEG50) copolymer to toughen PLA, with the purpose of both keeping mechanical strength and enhancing the toughness. The blend containing 5 wt % PBFEG50 exhibited about 28.5 times increase in elongation at break (5.5% vs 156.5%). At the same time, the tensile modulus even strikingly increased by 21.6% while the tensile strength was seldom deteriorated. Such a phenomenon could be explained by the stretch-induced crystallization of the BF segment and the interconnected morphology of PBFEG50 domains in PLA5. The Raman spectrum was used to identify the phase dispersion of PLA and PBFEG50 phases. As the PBFEG50 content increased, the interconnected PBFEG50 domains start to separate, but their size increases. Interestingly, tensile-induced cavitation could be clearly identified in scanning electron microscopy images, which meant that the miscibility between PLA and PBFEG50 was limited. The crystallization of PLA/PBFEG50 blends was examined by differential scanning calorimetry, and the plasticizer effect of the EG segment on the PLA matrix could be confirmed. The rheological experiment revealed decreased viscosity of PLA/PBFEG50 blends, implying the possible greener processing. Finally, potential biodegradability of these blends was proved.

Microstructural phase coexistence kinetics near the polymorphic phase boundary

By implementing a novel multiphase field model for ferroelectric systems, the phase coexistence of the tetragonal (T) and rhombohedral (R) phases in Pb-free BZT-40BCT was analyzed. Metastable coexistence of the T and R phases is predicted between a thermodynamic upper limit at TC,R=49.90∘C and a kinetic lower limit determined by the time-temperature-transformation behaviour. Predicted domain microstructures exhibit faceted domain walls and curved T-R phase interfaces that are consistent with recent TEM studies in the vicinity of the polymorphic phase boundary (PPB). Further, miniaturization of the domain structure near the PPB is a result of the relatively low interfacial energies and a pinning effect caused by the large metastable phase fraction that originates from the vanishing macroscopic driving force for phase transformation. Particularly, the vanishing of rhombohedral domain wall energies as T→TC,R enables a phase transformation-induced polarization rotation mechanism and predicts a hierarchical domain morphology for the R phase. These results are in agreement with the higher piezoelectric response reported near the maximum temperature for R+T coexistence and the observations of a miniaturized nanodomains structure within micron-sized, wedge-shaped domains in the R phase for the BZT-xBCT system.

Kinetics of domain growth and aging in a two-dimensional off-lattice system.

We have used molecular dynamics simulations for a comprehensive study of phase separation in a two-dimensional single-component off-lattice model where particles interact through the Lennard-Jones potential. Via state-of-the-art methods we have analyzed simulation data on structure, growth, and aging for nonequilibrium evolutions in the model. These data were obtained following quenches of well-equilibrated homogeneous configurations, with density close to the critical value, to various temperatures inside the miscibility gap, having vapor-"liquid" as well as vapor-"solid" coexistence. For the vapor-liquid phase separation we observe that ℓ, the average domain length, grows with time (t) as t^{1/2}, a behavior that has connection with hydrodynamics. At low-enough temperature, a sharp crossover of this time dependence to a much slower, temperature-dependent, growth is identified within the timescale of our simulations, implying "solid"-like final state of the high-density phase. This crossover is, interestingly, accompanied by strong differences in domain morphology and other structural aspects between the two situations. For aging, we have presented results for the order-parameter autocorrelation function. This quantity exhibits data collapse with respect to ℓ/ℓ_{w},ℓ, and ℓ_{w} being the average domain lengths at times t and t_{w} (≤t), respectively, the latter being the age of a system. Corresponding scaling function follows a power-law decay: ∼(ℓ/ℓ_{w})^{-λ} for t≫t_{w}. The decay exponent λ, for the vapor-liquid case, is accurately estimated via the application of an advanced finite-size scaling method. The obtained value is observed to satisfy a bound.

Effect of Molecular Architecture on the Self-Assembly of Bottlebrush Copolymers.

The characteristics of a new architecture of bottlebrush copolymers (BBCPs) self-assembly were studied using self-consistent field theory. In this molecule, a series of AB linear diblock side chains were connected at the diblock junction using a C backbone. The control over the linker length and its chemical nature created an additional constraint on the intrinsic AB diblock microphase separation. Increasing side-chain crowding by increasing the grafting density and total degree of polymerization induced improved phase separation. This was reflected in the overall reduction in the effective interaction parameter between the diblocks as well as the abrupt increase in phase density when crossing the order-disorder transition. Side-chain crowding resulted in an increase in the equilibrium domain spacing compared to a linear diblock. On the other hand, the localization of block C at the AB interface generated a diffuse domain boundary and reduction in side-chain stretching. The unique behavior of BBCPs was observed in 1D confined systems where the molecule showed the natural tendency to orient domains parallel to neutral confinement in contrast to the behavior of confined diblocks. Such behavior largely depended on the degree of incompatibility between the AB blocks and BBCP structure. A ternary phase diagram was constructed for different proportions of each block. Rich morphologies of core-shell domains and tiling patterns were observed including octagonal and pentagonal polygons. The unique architecture of this bottlebrush molecule and its improved nanoscale properties make it an attractive candidate for various applications of nanotechnology.

Cross-examining Polyurethane Nanodomain Formation and Internal Structure.

Structural and morphological interplay between hard and soft phases determine the bulk properties of thermoplastic polyurethanes. Commonly employed techniques rely on different physical or chemical phenomena for characterizing the organization of domains, but detailed structural information can be difficult to derive. Here, total scattering pair distribution function (PDF) analysis is used to determine atomic-scale insights into the connectivity and molecular ordering and compared to the domain size and morphological characteristics measured by AFM, TEM, SAXS, WAXS, and solid-state NMR 1H–1H spin-diffusion. In particular, density distribution functions are highlighted as a means to bridging the gap from the domain morphology to intradomain structural ordering. High real-space resolution PDFs are shown to provide a sensitive fingerprint for indexing aromatic, aliphatic, and polymerization-induced bonding characteristics, as well as the hard phase structure, and indicate that hard phases coexist in both ordered and disordered states.

The role of cellulose nanowhiskers in controlling phase segregation, crystallization and thermal stimuli responsiveness in PCL-PEGx-PCL block copolymer-based PU for human tissue engineering applications

In this research, two groups of polyurethane (PU) nanocomposites were developed based on PCL500-PEGx-PCL500 tri-block polyols and cellulose nanowhisker (CNW), as a cross-linker and controller of microphase separation of blocks. The effect of PEGx block length on phase segregation and crystallization of blocks was evaluated. The impact of tuned crystallization and hard domain morphology on shape memory parameters was studied in detail. PU2000-0.25 % was selected as the optimum specimen with shape fixity and recovery of 100 %. CNW nanorods were found to act as a controlling agent of the microphase segregation, possibly through changing the spatial organization of hard segments from a 3D self-assembled to a shell-like structure. Three different cell lines (HepG2, HFF, and mesenchymal stem cells) were used to culture on the prepared 2D specimens, resulting in different behaviors. The newly synthesized biomaterials with different cellular responses presented a great potential for a variety of tissue engineering applications.

The heterogeneous nucleation of threading dislocations on partial dislocations in III-nitride epilayers

III-nitride compound semiconductors are breakthrough materials regarding device applications. However, their heterostructures suffer from very high threading dislocation (TD) densities that impair several aspects of their performance. The physical mechanisms leading to TD nucleation in these materials are still not fully elucidated. An overlooked but apparently important mechanism is their heterogeneous nucleation on domains of basal stacking faults (BSFs). Based on experimental observations by transmission electron microscopy, we present a concise model of this phenomenon occurring in III-nitride alloy heterostructures. Such domains comprise overlapping intrinsic I1 BSFs with parallel translation vectors. Overlapping of two BSFs annihilates most of the local elastic strain of their delimiting partial dislocations. What remains combines to yield partial dislocations that are always of screw character. As a result, TD nucleation becomes geometrically necessary, as well as energetically favorable, due to the coexistence of crystallographically equivalent prismatic facets surrounding the BSF domain. The presented model explains all observed BSF domain morphologies, and constitutes a physical mechanism that provides insight regarding dislocation nucleation in wurtzite-structured alloy epilayers.