Symmetry control of nanorod superlattice driven by a governing force

Nanoparticle self-assembly promises scalable fabrication of composite materials with unique properties, but symmetry control of assembled structures remains a challenge. By introducing a governing force in the assembly process, we develop a strategy to control assembly symmetry. As a demonstration, we realize the tetragonal superlattice of octagonal gold nanorods, breaking through the only hexagonal symmetry of the superlattice so far. Surprisingly, such sparse tetragonal superstructure exhibits much higher thermostability than its close-packed hexagonal counterpart. Multiscale modeling reveals that the governing force arises from hierarchical molecular and colloidal interactions. This force dominates the interactions involved in the assembly process and determines the superlattice symmetry, leading to the tetragonal superlattice that becomes energetically favorable over its hexagonal counterpart. This strategy might be instructive for designing assembly of various nanoparticles and may open up a new avenue for realizing diverse assembly structures with pre-engineered properties.

-Page 4, line 74-77: The following statement should be rephrased: "Such self-assembly strategy has been successfully extended to a…… universal and broadly applicable for…superstructures". -To be universal the process should be applicable to all systems including the surfactant. However, based on the results the current process did not work when R6B was replaced by RB.
Reviewer #2 (Remarks to the Author): The manuscript 'Symmetry control of nanoparticle superlattice driven by a governing force' presents experimental data on the self-assembly of gold nano-rods (GNR) into tetragonal structures, which display superior thermal stability to similar assemblies with hexagonal arrangements. The findings are supported by simulation studies.
In detail: The authors show that DNRs that are stabilised via a double-layer of the common surfactant CTAB will spontaneously self-assemble into a layered structure with each layer having a hexagonal symmetry, as would be expected for hard-rod behaviour forming a smectic liquid crystal at high enough concentrations. Their surprising finding is, that when the sterically stabilising layer of CTAB is partially replaced by the dye molecule Rhodamine such that it intercalates into the CTAB double-layer the effective inter-particle interactions change. As a result the particles will now self-assemble into a tetragonal symmetry, meaning the smectic layers have a square symmetry now. This is reminiscent of the self assembly of micelles made of Pluronics (PEO-PPO-PEO), which show the less dense BCC packing for shorter chains with low aggregation number, while longer chains with higher aggregation number will display FCC packing, because they behave more like hard spheres. The author's simulation studies on the effective interaction potential support the idea that a less dense packing will be assumed for particles with a 'softer' potential. In analogy, Landau and Lifshitz have argued that atoms with a more soft potential will form BCC (for spherical symmetry).
In addition the authors also show that the tetragonally assembled GNR-crystals display superior stability against melting. This may again be due to the fact that the Rhodamine dye remains stabilising the rods as opposed to the purely CTAB-stabilized particles, which start to melt and fuse at lower temperatures.
Both results are new and very exciting, and therefore deserve publication.
However, I do ask the authors to have the English improved considerably.

Point to point response letter
Reviewer #1: Q1: After viewing from the top, the terminal end shape of the gold nanorods in Fig. 1a looks quite different from the Fig 1b. In Fig 1a, most of the nanorods terminal end look like penta shaped whereas in Fig1b is spherical. Had the experiments been done from different batches of prepared gold-nanorods? If this is the case then the results presented for symmetry transformation from hexagonal to tetragonal superlattice will not be comparable.

A1:
Thanks for the careful observation on the graphs. GNRs as shown in Supplementary Figure 3 (HRTEM images of PC GNRs) are single crystal, determining their symmetry is impossible to be five-fold symmetry. The different terminal end shapes of the gold nanorods in Fig.1a and Fig. 1b are caused by the differences in image conditions, such as the imaging angle and the image magnification. In addition, we have checked our experimental records again and further confirmed that the same batch of rods was used in Fig.1a and Fig. 1b. We thank the referee for this comment. Q2: The process of making R6G adsorbed gold nanorods follows centrifugation of CTAB capped gold nanorods (12,000 rpm for 5 min), removal of CTAB and adsorption of R6G. In this process, it is quite possible that the CTAB will be removed from the gold nanorod surface from random sites and R6G will take over that place. If the positioning of R6G is random on the surface of nanorods; then maintaining uniformity of microenvironment around each nanorod will be very difficult. Without such uniformity, precise control over the self-assembly into particular geometry will be hard to achieve. A2: We appreciate the remarks. As shown in Preparation of the GNRs of Methods, the "CTAB partially covered" GNRs were obtained via centrifugation process. In such process, with the same centrifugation condition (such as centrifugation force, centrifugation time, centrifugation tube), we think CTAB desorption from the rods is uniform rather than random. We have made the revision in the main text and Methods to clarify this further.

Q3:
Page 5, line 104-106: Authors need to explain in more details about the statement "increasing concentration of R6G, no influence on Zeta potential is observed (Fig. 1f) because the adsorbed R6G may be screened by CTAB bilayer" A3: We are grateful for this helpful remark. For the "CTAB partially covered" GNRs, with increasing concentration of R6G, we have not observed the influence of adsorbed R6G on the Zeta potential of the rods (Fig. 1f). This is because the Zeta potential of the rods is mainly determined by the CTAB bilayer due to the following two reasons. First, the thickness of CTAB bilayer is much larger than the length of the single R6G molecule. Second, the coverage of CTAB on the rods is still dominated. Thus, the influence of R6G on Zeta potential can be ignored. We have explained it in more details in the revised manuscript.
its intensity damped when R6G concentration increases, implying the change in the surrounding dielectric environment (Fig.1d)". -This statement needs to be rewritten. Change in the surrounding dielectric environment could also lead to increase in intensity and blue shift. A4: Following this suggestion, we have rewritten the sentence to "With increasing R6G concentration, the LSPR band is gradually red-shifted with a slight decrease in intensity (Fig.1d) because the adsorption of R6G increases the local dielectric constant on rod surface.". Page 7, Fig 2a: On the top left and bottom left some tetragonal arrangements of the gold nanorods can be seen which has been synthesized using CTAB gold nanorods only. Why?