Low dimensional systems or nanoobjects, having size smaller than certain intrinsic physical length, demonstrate properties that are sometimes completely different from the bulk one. Such nanoobjects can be used as building blocks to form self-organized nanostructures with useful electronic, optical, and magnetic properties. New collective phenomena, that can be explored from such self-organized nanostructures, are mainly related to the interparticle coupling effects in both in-plane and out-of-plane directions, which can be tuned through size, shape, separation and ordering of the nanoobjects.

2D-network of disk-like islands of thiol-coated Au-nanoparticles (DT-AuNPs) on H-Si substrates is evidenced for the first time, directly from a broad peak in grazing incidence small angle X-ray scattering (GISAXS) data and also from atomic force microscopy (AFM) images. Theoretical modeling of the system, carried out based on density-density and height-height correlation functions, supports well the formation of such structures. The structural information of the Langmuir-Schaefer (LS) films, obtained at different surface pressure, helps to infer the growth of Langmuir monolayers of DT-AuNPs, which is very important in understanding the self-assembly process of nanoparticles at the air-water interface and in controlling the growth of 2D-networked nanostructures in large areas. On the surface of water, DT-AuNPs first self-assembled around different points to form disk-like islands of nanometer size and monolayer height, due to the complex balance of long range van der Waals attractions and short-range steric repulsion of the DT-AuNPs, initiated by solvent evaporation and also to optimize the hydrophobic repulsive force of water. On barrier compression, the size and 2D-network of the islands grow due to a combined effect of collision induced coalescence and solidlike behavior resisting deformation of the islands. On the other hand, the separation between the DT-AuNPs either decreases or increases depending upon the competitive effects of packing or buckling [published in RSC Adv. 6, 12326 (2016)].

Structures of LS monolayers of DT-AuNPs deposited on H-terminated and OTS self-assembled Si substrates (of different hydrophobic strength and stability) and their evolution with time under ambient conditions, which plays an important role for their practical use as 2D-nanostructures over large areas, were investigated using the X-ray reflectivity (XR) technique. The strong effect of substrate surface energy (γ) on the initial structures and the competitive role of room temperature thermal energy (kT) and the change in interfacial energy (Δγ) at ambient conditions on the evolution and final structures of the DT-AuNP LS monolayers are evident. The strong-hydrophobic OTS-Si substrate, during transfer, seems to induce strong attraction towards hydrophobic DT-AuNPs on hydrophilic (repulsive) water to form vertically compact partially covered (with voids) monolayer structures (of perfect monolayer thickness) at low pressure and nearly covered buckled monolayer structures (of enhanced monolayer thickness) at high pressure. After transfer, the small kT-energy (in absence of repulsive water) probably fluctuates the DT-AuNPs to form vertically expanded monolayer structures, through systematic exponential growth with time. The effect is prominent for the film deposited at low pressure, where the initial film-coverage and film-thickness are low. On the other hand, the weak-hydrophobic H-Si substrate, during transfer, appears to induce optimum attraction towards DT-AuNPs to better mimic the Langmuir monolayer structures on it. After transfer, the change in the substrate surface nature, from weak-hydrophobic to weak-hydrophilic with time (i.e. Δγ-energy, apart from the kT-energy), enhances the size of the voids and weakens the monolayer/bilayer structure to form a similar expanded monolayer structure, the thickness of which is probably optimized by the available thermal energy [published in Phys. Chem. Chem. Phys. 20, 1051 (2018)].

The structural evolution of DT-AuNP multilayers on a H-passivated Si substrate, formed through a LS deposition process, has been investigated using complementary XR and GISAXS techniques. The fractional coverage multilayers of DT-AuNPs, formed through a multi-transfer process, are found to be quite unstable under ambient conditions. The thickness of these decreases with time and tends to saturate toward a near unique thickness (NUT ≈ 6 nm). Although initial low coverage and their instability create hindrance in the control and formation of desired 3D-nanostructures in the bottom-up approach, the formation of a NUT-layer, through time-evolution, is quite distinctive, thus interesting. It is clear from the evolution that the thermodynamically driven monolayer structures (of DT-AuNPs) at the air-water interface become thermodynamically unstable when transferred sequentially onto the solid substrate. The kT-energy and the partial change in the substrate surface energy (Δγ) create the instability and induce the diffusion in the AuNPs, which in the presence of a net attractive force towards the substrate (arising from anisotropic interaction of the top AuNPs with the other AuNPs and/or hydrophobic substrate) tries to create a thermodynamically favourable and relatively stable NUT-layer through reorganization for a different duration. This happens if the number of AuNPs is less than or equals to the maximum number that can be accommodated within the NUT. The value of the NUT mainly depends on the particle-size and a kT-energy related fluctuation of particles. Furthermore, the formation of the NUT-layer indicates that the hydrophobic-hydrophobic interaction mediated net attraction towards the substrate is long range, while the hydrophilic-hydrophobic interaction mediated repulsion and/or kT-energy induced fluctuation are short range [published in Soft Matter 15, 1869 (2019)].