Nanomaterial Growth Driven by Screw Dislocations
We have discovered a fundamentally different mechanism for the anisotropic growth of nanomaterials that is driven by axial screw dislocations. This is a catalyst-free mechanism, in which the self-perpetuating steps of a screw dislocation spiral provide the fast crystal growth front under low supersaturation (Fig. 1C) to enable the anisotropic crystal growth of nanostructures. A variety of nanostructures, such as 1D nanowires (NWs) and nanotubes (NTs), 2D nanoplates, and 3D hierarchical nanotrees, can be grown via the screw dislocation-driven mechanism (Fig 1).[1] The key is the difference in the step growth rates between different growth fronts, e.g. the velocity of steps at the dislocation core (vc) and those at the outer edges (vo), which leads to different dimensionalities. Moreover, the strain and stress caused by dislocation defects also impact nanomaterial morphologies, which can also be recognized as signatures of dislocation-driven growth.
Figure 1. Formation Pathways for different nanostructures driven by screw dislocations [1]
Briefly, starting from a screw dislocation hillock, when vo is equal to vc , the newly-generated steps near the dislocation core propagate at the same rate with earlier steps at the outer edge of the growth spiral, thus the dislocation hillock spreads in 2D fashion without a step pile up (Fig. 1B), which leads to the formation of highly squashed pyramids that can be approximated as nanoplates.[9]
When vo << vc , the newly-generated spiral steps will catch up, resulting in step bunching and formation of cylinders. When this occurs, highly-anisotropic 1D crystal growth (Figure 1C) is enabled under low supersaturation because axial growth on dislocation spiral steps is preferred while layer-by-layer growth on the sidewalls is energetically unfavored. As the consequence, the crystal can rapidly propagate along the line direction of the dislocation to form a NW.
The fast growing NWs driven by dislocations, when combined with epitaxial overgrowth of NW branches formed via the slower vapor-liquid-solid mechanism, results in unprecedented “Christmas tree” nanostructures of PbS (Fig. 1E, 1F and Fig. 2).[1, 4, 5] These fascinating trees with rotating branches are the clearest demonstration of “Eshelby twist” — the rotation of a crystal lattice around a screw dislocation as the consequence of its associated stress.
Figure 2. SEM micrographs of PbS pine tree nanowires.[4]
We have further elucidated the growth of single-crystal nanotubes (Fig. 1D) due to screw dislocations with large Burgers vectors — axial screw dislocations provide 1D anisotropic crystal growth and their associated strain energy is large enough to favor the creation of a new internal surface, spontaneously forming hollow tubes. Bridging modern nanomaterials research and fundamental crystal growth theory, we compared the prediction from the classical crystal growth theory on dislocation-driven and layer-by-layer growth with the observed growth kinetics to conclusively prove that the anisotropic crystal growth of these 1D nanomaterials is indeed driven by screw dislocations.
These studies will provide the general and unifying concepts for many nanomaterial morphologies that are commonly observed but often unconvincingly explained. Dislocation-driven growth is a general mechanism that is applicable to many nanomaterials grown in solution or vapor phase but has been greatly underappreciated in modern nanomaterials literature. We have since discovered that this mechanism is at play in many more materials, ranging from oxides and hydroxides (Zinc hydroxysulfate,[9] ZnO,[6, 7] FeOOH,[8] Co(OH)2, Ni(OH)2,[9] Cu2O[10]), chalcogenides (CdS,[12] CdSe,[12] CdTe), nitrides (AlN), and metals (Cu[11]), formed via diverse chemical processes, such as CVD, hydrolysis, and reduction oxidation reactions. Even more examples in nitrides (GaN), phosphide (InP), and many other oxides have since been reported and confirmed. Our discoveries and continuing fundamental studies will create a new dimension in the rational design and synthesis of nanomaterials. Furthermore, the general understanding on promoting dislocation-driven nanomaterials growth and the methods we developed in using continuous flow reactors (Fig. 3) and the manipulations of chemical equilibria to control the crystal growth supersaturation enable the rational solution synthesis of a broad class of nanomaterials.
Figure 3. Schematics of supersaturation profiles and a general continuous flow cell reactor (CFR) design for solution synthesis of nanomaterials.[1]
Screw dislocation not only influences the morphologies of nanomaterials, but also impacts their properties, especially in 2D materials where the electronic and optical properties heavily depend on number and stacking of layers. We have shown and explained how the layer stacking of WSe2 and WS2,22,23 representative examples of transition metal dichalcogenides (TMDCs, or MX2), can be modulated by screw dislocations.
In MX2 with screw dislocation, the stacking can be determined by the rotation and number of the elementary triangular dislocation spirals,
which relates to the symmetry of the dislocation spirals and can be categorized in three classes: triangular, hexagonal, and mixed. Triangular spiral plates adopt a noncentrosymmetric phase (AA stacking) with strong SHG signal and enhanced PL, while hexagonal spiral plates adopt the simple centrosymmetric 2H (AB) stacking. Mixed spiral plates fall between these two categories, creating a variety of weakly noncentrosymmetric layer stacking arrangements with new and interesting properties that have not been observed in bulk or previous few-layer materials.
Figure 4. SHG and AFM images of representative WSe2 nanoplates in three classes of spiral plates.[22]
We are now working on using dislocation-driven nanomaterial growth to fabricate more complex mesoscale heterostructures and understanding the relationship between dislocation-driven growth mechanism and other commonly observed crystal defects such as stacking faults. With our advanced understanding on the growth mechanism, it will open up the exploitation of large scale/low cost solution growth for rational catalyst-free synthesis of 1D nanomaterials for a variety of large-scale applications, such as those for renewable energy. [14, 15]
