Atomically precise metal nanoclusters self assembly dynamics colloidal superstructures and advanced device fabrication

Abstract

Nanomaterials are fascinating because of their small size and high surfacearea-to-volume ratio, which cause them to act completely differently fromtheir bulk counterparts and display unique optical and photophysicalproperties. To fully comprehend their unique characteristics, in-depthcompositional and structural investigations are necessary. However, in thecase of metallic nanoparticles (NPs), one problem faced by researchers is thelack of monodispersity. Later, size-focused synthesis of NPs led to thediscovery of atomically precise metal nanoclusters (NCs). NCs consist of ametal core protected by organic ligands, with a core size of less than 2 nm.Since the number of metal atoms in the core and ligands is inite, they can bedenoted as [MxLy]z, where x and y represent the number of metal atoms andligands (e.g., L= thiolate (SR)), and z denotes the overall charge of the NC. NCsexhibit strong quantum con inement effects owing to their ultrasmall size,resulting in molecule-like properties such as distinct optical absorptionfeatures, photoluminescence, and enhanced catalytic activity. This makesthem valuable for diverse applications in catalysis, sensing, energy, andbiology. Recently, the self-assembly of NCs has become a “hot topic” in bothfundamental and applied research. Many nanoscale molecular interactionsat the NC surface play crucial roles in self-assembly, which can occasionallyresult in the crystallization of NCs and provide information about theirstructural and property relationships. The self-assembly of NCs intohierarchical superstructures led to enhanced photophysical properties dueto the coupling effect of neighboring NCs. One could foresee that suchenhanced photophysical properties are important for the development ofadvanced devices in the near future.This thesis is organized into seven chapters. Chapter 1 offers an overview ofNCs, covering their synthesis, isolation, properties, and applications. The second half of this chapter will discuss in detail the hierarchical self-assembly of NCs and their properties and applications. Chapter 2 discussesthe materials and experimental methods used for our investigations.Chapter 3 discusses the synthesis and photon-assisted reversible self-assembly of thiolated azobenzene-stapled Au25 NCs. Photoactivation offunctionalized NCs ([Au25C3AMT)18]−) in dichloromethane by irradiatingultraviolet light at 345 nm resulted in a visual change and formation of disc-like colloidal superstructures (d ~100-1000 nm). The superstructures arereadily disassembled into individual NCs upon irradiating with the visiblelight at 435 nm. Systematic changes in both the electronic absorption bandsand nuclear magnetic resonance spectra of chromophores in solutionsuggest that the photoisomerization of surface ligands drives the self-assembly. High-resolution transmission electron microscopy, electrontomographic reconstruction, dynamic light scattering, and small-angle X-raypowder diffraction show that the disc-like superstructures contain denselypacked NCs. Long-range self-assembly and disassembly under ultravioletand visible light, respectively, demonstrate reversible photo-switching inNCs.Chapter 4 discusses the role of chromophore-spacer length in the light-induced self-assembly. Herein, the self-assembly is studied in detail usingtwo photoswitchable NCs (C3-NC and C9-NC) fabricated using the thiolated-azobenzene molecule with different spacer lengths (C3-AMT and C9-AMT).The core and molecular information of both NCs were meticulously exploredusing various spectroscopic and microscopic techniques. Photoactivation ofboth NC solutions at ambient temperature using 345 nm light resulted in theformation of self-assembled superstructures. The C9-NCs show a fastassembly due to their improved photoswitching ef iciency resulting from thenegligible steric hindrance experienced at the NC-chromophore interface. Chapter 5 discusses the room temperature fabrication of a stable, lexible,nontoxic, and low-cost precision NC-based luminescent ink for the stencilprinting of an optically unclonable security label. NC-based printing inkshowed brilliant photoluminescence owing to its extended C− H···π/π···πinteractions. Spectroscopic and microscopic investigations show thatintercalated NC in printed security labels are highly stable as their opticalfeatures and molecular compositions are unaffected. The exceptionalmechanical, thermal, photo, and aqueous stabilities of the printed securitylabels endorsed to demonstrate the printing and smartphone-basedelectronic reading of the quick response code on a currency. Finally,con idential information protection and decryption under a precise windowof light have been achieved by adopting the optical contrast illusion. Thepolyurethane elastomeric binder helped to hold the printed QR code oncurrency even after multiple folding and wetting. The overall cost of thesecurity label is found to be approximately 0.013 USD per stamp.Chapter 6 discusses a highly stable, solid-state emitting bimetallic copper-gold nanocluster [Au2Cu6(Sadm)6(DPPEO)2], using adamantane thiol and1,2-bis(diphenylphosphino)ethane as primary and secondary ligands.During the synthesis, only one phosphine is bound to gold and the secondone has oxidized to P = O bond. The strong hydrogen bonding between P = Oand the adamantane hydrogen atom facilitated rapid crystallization. The NCilm and crystals show better photostability compared to its solution. Due totheexceptionalphotostabilityandsolid-statephotoluminescence,[Au2Cu6(Sadm)6(DPPEO)2] NC has been successfully employed to fabricatean electroluminescent light-emitting diode without additional hostmaterials. The fabricated electroluminescent light-emitting diode shows amaximum brightness of 1246 cd/m2 and an external quantum ef iciency of12.6 %.