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Department of Materials Science and Engineering

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This figure is reproduced with permission from Ref. a) Schematic illustration of the ligand exchange process with NOBF4. This figure is reproduced with permission from Ref. b) Small-angle X-ray diffraction measured from superlattices of 7 nm PbS nanocrystals. Comparison of the FT-IR spectra of (a) organic-capped and Et3OBF4-treated UCNPs, and (b) organic-capped and MoS42-capped FePt NPs.

Illustrated models of matchstick nanosurfactants capped with oleylamine (top), tetradecylamine (middle) and dodecylamine (bottom).

Overview on Synthesis and Self-Organization of Charged Nanocrystals

Synthesis of charged nanocrystals through surface engineering

  • Solution-phase synthesis of colloidal nanocrystals
  • Synthesis of charged nanocrystals by the surface modification
  • Introduction of inorganic ligands
  • Ligand stripping process

The removal of the previous organic ligands allows the dispersion in a polar solvent, as the positive metal cations are exposed on the surfaces. The surfaces of the ligand-stripped NCs can be further modified by the introduction of secondary ligands due to the weak binding affinity of the BF4 anions. - polar solvent. The surface charge of the Fe3O4 NCs was changed from positive to negative according to the introduction of PtS anions (Figure 1.17).17. a) Schematic representation of the ligand exchange process with NOBF4.

Schematic illustration of the secondary ligand exchange process showing the surface functionalization of BF4--modified Fe3O4 NCs with different capping molecules.

Figure 1.1. (a) The schematic image depicting the stages of nucleation and growth for the preparation  of monodisperse NCs in the framework of the La Mer model
Figure 1.1. (a) The schematic image depicting the stages of nucleation and growth for the preparation of monodisperse NCs in the framework of the La Mer model

Self-organization of the charged nanocrystals

  • Self-assembly of nanocrystals
  • Electrostatic interaction between charged nanocrystals
  • Amphiphilic nanocrystals

Note that the fraction of the free energy of the system varies with the surface area or the number of broken bonds remaining after assembly. Evolution of the effective pair interaction potential U at the interparticle distance r for the nanocrystals from the start (darkest trace, scattered state) to the end point (lightest trace, close-packed state) during the self-assembly experiment. Schematic diagram of the self-assembly process during the early stages of drying (not to scale) showing how the nanocrystals are trapped by the rapidly receding liquid-air interface.

The structural hierarchy and compositional adaptability of the various NC superstructures obtained provide new functionality that enables access to wide application fields. Charged NCs of the same sign are colloidally dispersed in the high dielectric medium so that the repulsive double-layer force acts on the Debye length. The magnitude and length scale of interparticle interactions are determined by various factors such as the dielectric constant of the solvent, the valences of the surface ligands, and the types and concentration of counterions.

The dielectric screening of the solvent is weakened by the addition of salts or solvent mixing of lower dielectric constants resulting in the collapse of the counterion layer and the aggregation of electrostatically stabilized NCs. Despite the lack of steric repulsive force, self-assembly of all-inorganic NCs into long-range ordered phases is achievable with precise control of interparticle interactions.59 The interparticle interaction of all-inorganic NCs was well regulated by different factors. such as particle size, salt concentration and valence, and anion association on the surface of NCs (Figure 1.25). Precise control of the interparticle interaction allows narrow and intermediate depth of the potential well, which enables reversible attachment of NCs and rearrangement with crystal structures.

For oppositely charged NCs, the building blocks are attracted to each other through Coulombic forces, resulting in rapid agglomeration and the formation of non-equilibrium structures such as clusters and gels.61 The strong attractive force between oppositely charged NCs can be weakened by steric repulsion of forces with polymer ligands and reduction of electrostatic interaction with by the addition of salt (Figure 1.26).21 The addition of salt enables fine-tuning of the interaction between particles, which results in an ordered assembly. a) Estimated vapor potential for 4.5-nm Au NCs in the presence of different 1:3 electrolyte concentrations. The amphiphilic nature leads to self-assembly, and the self-assembly morphology varies depending on the surface coating agent and lobe size (Figure 1.29).66 Furthermore, as nanosurfactants prepared with functional building blocks, droplets prepared with Au-Fe3O4 show multi-responsibility for extrinsic stimuli (Figure 1.30).67.

Figure 1.18. Superlattice morphology and free energy landscape of nanocrystal self-assembly at the  disorder−order transition
Figure 1.18. Superlattice morphology and free energy landscape of nanocrystal self-assembly at the disorder−order transition

Colloidal suprastructures self-organized from oppositely charged all-inorganic

Results and discussion

Initially, the effect of the content ratio of oppositely charged NPs was determined by varying the number of negatively charged FePt NPs and using a fixed number of positively charged UCNPs. At low content of negatively charged FePt NPs, the zeta potential gradually decreased with increasing content of FePt NPs. The TEM image showed that some FePt NPs were sparsely adsorbed on the surfaces of UCNPs (Figure 2.1c), resulting in the formation of non-uniform particles.

The TEM image of the precipitates further confirmed that the oppositely charged NPs bridged the negatively charged FePt NPs, resulting in the formation of large agglomerates (Figure 2.1d). The zeta potential of the colloid showed a high negative value that saturated at −31.4 mV, with an increase in negatively charged particles. As shown in the TEM image, the negatively charged NPs uniformly covered the surfaces of the high-density UCNPs and eventually restored the colloidal stability (Figure 2.1e).

The IEPs were shifted toward the higher number ratio of the oppositely charged NPs with an increase in the size ratio of FePt/UCNPs. This phenomenon can be attributed to the exposure of the negatively charged NPs to the outer surfaces of the agglomerates at low concentrations. Moreover, this result reveals that the suprastructures at the lower concentrations can be more colloidally stable over a wider range of the number ratio of the oppositely charged NPs.

It was assumed that the negatively charged NPs were completely adsorbed on the surfaces of the positively charged NPs in the coexistence lines and distributed evenly (calculation details in the supplementary information). Comparison of FT-IR spectra of (a) organic-capped and Et3OBF4-treated CdS nanoplates, and (b) organic-capped and MoS42-capped PbSe NPs.

Figure 2.1. (a) Schematic illustration of two-phase surface treatment and constructed suprastructures  in the identified three phases
Figure 2.1. (a) Schematic illustration of two-phase surface treatment and constructed suprastructures in the identified three phases

Conclusion

Experimental section

The NaGdF4 NPs were washed several times with ethanol and then re-dispersed in cyclohexane. 22 nm- and 28 nm-sized NaYF4 NPs were synthesized according to the previously reported procedure with minor modification31. The NaYF4 NPs were washed several times with ethanol and then re-dispersed in cyclohexane.

40 nm NaGdF4 NPs were synthesized according to the previously reported procedure with minor modifications. Synthesized Bi NPs were washed three times with excess acetone and finally dispersed in hexane. The sulfur-octylamine complex was prepared by dissolving 4.5 mmol of elemental sulfur in 5 mL of octylamine at room temperature, and the color of the resulting solution was reddish brown.

Fe3O4 NPs with a size of 5 nm were synthesized according to the literature reported by Park et al. PbSe NPs with a size of 4 nm were synthesized following the literature reported by Rosen et al.27 TOP-Se was prepared by dissolving 5.23 mmol of elemental Se in 13.2 ml of TOP with vigorous stirring overnight. Ligand removal of Fe3O4 NPs was performed with the same procedure for other NOBF4 NPs.

The suprastructure of oppositely charged all-inorganic NPs was prepared with the integration of negatively charged FePt NP solution into the positively charged UCNPs solution. Zeta potential and hydrodynamic size of the suprastructures were characterized with the addition of negatively charged FePt NPs to the fixed number of positively charged UCNPs to characterize the effect of the content ratio of oppositely charged NPs.

Table 2.1. Synthesis conditions of upconversion nanoparticles (UCNPs - NaGdF 4 , NaYF 4 )
Table 2.1. Synthesis conditions of upconversion nanoparticles (UCNPs - NaGdF 4 , NaYF 4 )

Self-assembly of matchstick-shaped inorganic nano-surfactants with controlled

Results and discussion

As shown in TEM images (Figure 3.14), the self-assemblies of the nanosurfactants aged for 10 min and 24 h were identical in microstructures. Hydrodynamic sizes of the lamellar-structured self-assemblies of the match-shaped nanosurfactants aged at 10 min and 24 h in NMF. The orientation of the nano-matches was also investigated using energy dispersive X-ray spectroscopy (EDS).

In addition, TEM images of the lamellar structures at different tilt angles reveal that the edges of the lamellar structures are surrounded by laterally aligned matchstick nanosurfactants without rolling or bending (Figure 3.19). The colloidal stability of the matches was preserved for all studied nano-surfactants of alkylamine ligands (C12~C18) (Figure 3.24a). Instead, the overall morphology and curvatures were very similar to those observed for nanosurfactant self-assemblies with an aspect ratio of 2.

The controlled length and surface hydrophobicity of the nanosurfactants define d and γ in the formed self-assemblies. The systematic reduction of the length of the hydrophobic segment reduces d, leading to a smaller radius of curvature R for the self-assemblies. This decrease in γ of the nanosurfactants reduces R, which explains the phase transition from the lamellar structure to cylinders and micelles.

As seen in this study, the aspect ratio and surface states of nanosurfactants play critical roles in determining the phases and morphologies of the self-assemblies. Second, the slow self-assembly of nano-surfactants can improve the uniformity of the self-assembled structures.

Figure 3.2. (a) Schematic illustration of synthesis of nano-matchstick and matchstick-shaped nano- nano-surfactants
Figure 3.2. (a) Schematic illustration of synthesis of nano-matchstick and matchstick-shaped nano- nano-surfactants

Conclusion

Experimental section

먼저 교수님에게 감사의 말씀을 전하고 싶습니다. 내가 아무것도 모르는데도 당신은 나에게 매우 다양한 분야에서 많은 것을 가르쳐 주었습니다. 연구실을 떠난 뒤에도 잊지 않고 마음 속에 간직하겠습니다.

덕분에 연구실에 잘 적응할 수 있었습니다. 형우야, 같은 분야를 연구하면서 많은 이야기를 나눴는데, 형우의 존재가 나에게 힘이 됐다. 상민님, 편하게 대해주셔서 정말 감사드립니다.

연구자로서 열심히 일하는 모습을 보며 많은 동기부여를 받았습니다. 성헌님, 연구실의 중심이 되어 사람들을 대하는 모습이 보기 좋았습니다. 우용씨, 같은 팀에 저희밖에 없어서 얘기를 많이 했어요.

덕분에 가끔 웃으며 연구실에서 시간을 보낼 수 있었습니다. 성은이가 최근 연구실에 적응해서 밝아지는 모습이 보기 좋네요.

수치

Figure  1.2.  Hot-injection  method  for  the  synthesis  of  monodisperse  nanoparticles
Figure 1.3. Nanocrystal Ligand Binding Motifs According to the Covalent Bond Classification Method
Figure 1.15. (a) FT-IR spectra of Fe 3 O 4  NCs before and after NOBF 4  treatment, respectively
Figure 1.23. From discrete to extended electronic state of QD solids according to the interdot distance
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