We report the colloidal synthesis of ~3 tungsten-oxygen (W-O) layer thick (~1 nm), two-dimensional (2D) WO 3-x nanoplatelets (NPLs) (x ~ 0.55 Ð 1.03), which display tunable near-infrared localized surface plasmon resonances (LSPR) spectra and high free electron density (N e ) that arises predominantly from their large shape factor. Importantly, the W to O composition ratios inferred from their LSPR measurements show much higher percentage of oxygen vacancies than those determined by X-ray diffraction analysis, suggesting that the aspect ratio of ultrathin WO 3-x NPLs is the key to producing an unprecedentedly large N e , although synthesis temperature is also an independent factor. We find that NPL formation is kinetically controlled, whereas thermodynamic parameter manipulation leads to N e as high as 4.13 X 10 22 cm -3 , which is close to that of plasmonic noble metals, and thus our oxide-based nanostructures can be considered as quasi-metallic. The unique structural properties of 2D nanomaterials along with the high N e of WO 3-x NPLs provide an attractive alternative to plasmonic noble metal nanostructures for energy conversions photochromic nanodevices.

Surface passivating ligands, although ubiquitous to colloidal nanocrystal (NCs) synthesis, play a role in assembling NCs into higher-order structures and hierarchical superstructures, which has not been demonstrated yet for colloidal CsPbX3, (X= Cl, Br, and I) NCs. In this work, we report that functional polyethylene glycols (PEG6-Y, Y =-COOH and-NH2) represent unique surface passivating ligands enabling the synthesis of near uniform CsPbBr3 NCs with diameters of 3.0 nm. The synthesized NCs are assembled into individual pearl necklaces, bundled pearl necklaces, lamellar, and nanorice superstructures, in-situ. It is believed a variety of forces, including van der Waals attractions between hydrophilic PEG tails in a nonpolar solvent and dipole-dipole attraction between NCs, drive mesoscale assembly to form superstructures. Furthermore, post-synthetic ligand treatment strengthens the argument for polymer-assisted mesoscale assembly as pearl necklace assemblies can be successfully converted into either lamellar or nanorice structures. We observe an ~240 meV bathochromic shift in the lowest energy absorption peak of CsPbBr3 NCs when they are present in the lamellar and nanorice assemblies, representing strong inter-NC electronic coupling. Moreover, pearl necklace structures are spontaneously assembled into micrometer length scale twisted ribbon hierarchical superstructures during storage of colloidal CsPbBr3 NCs. The results show that the self-assembled superstructures of CsPbBr3 NCs are now feasible to prepare via template free synthesis, as self-assembled structures emerge in the bulk solvent, a process that mimics biological systems except for the use of non-biological surface ligands (PEG6-Y). Taken together, emergent optoelectonic properties and higher-order superstructures of CsPbBr3 NCs should aid their potential use in solid-state devices and simplify scalable manufacturing.

As interest continues to grow in Ti3C2T x and other related MXenes, advancement in methods of manipulation of their surface functional groups beyond synthesis-based surface terminations (T x : −F, −OH, and O) can provide mechanisms to enhance solution processability as well as produce improved solid-state device architectures and coatings. Here, we report a chemically important surface modification approach in which “solvent-like” polymers, polyethylene glycol carboxylic acid (PEG6-COOH), are covalently attached onto MXenes via esterification chemistry. Surface modification of Ti3C2T x with PEG6-COOH with large ligand loading (up to 14% by mass) greatly enhances dispersibility in a wide range of nonpolar organic solvents (e.g., 2.88 mg/mL in chloroform) without oxidation of Ti3C2T x two-dimensional flakes or changes in the structure ordering. Furthermore, cooperative interactions between polymer chains improve the nanoscale assembly of uniform microstructures of stacked MXene-PEG6 flakes into ordered thin films with excellent electrical conductivity (∼16,200 S·cm–1). Most importantly, our covalent surface modification approach with ω-functionalized PEG6 ligands (ω-PEG6-COOH, where ω: −NH2, −N3, −CHCH2) allows for control over the degree of functionalization (incorporation of valency) of MXene. We believe that installing valency onto MXenes through short, ion conducting PEG ligands without compromising MXenes’ features such as solution processability, structural stability, and electrical conductivity further enhance MXenes surface chemistry tunability and performance and widens their applications.

The localized surface plasmon resonance (LSPR) properties of nanocrystals (NCs) allow manipulation of optical responses by controlling their morphology, free carrier density, and local dielectric environment. In this context, semiconductor NCs, in which plasmonic properties arise due to various types of doping, provide unique opportunities in tailoring LSPR properties for a wide range of applications as viable alternatives to expensive noble metal NCs. Although extensive works have been done to control the LSPR properties of semiconductor NCs via doping, the role of surface ligand chemistry in the enhancement of LSPR properties remains poorly understood. Incomplete passivation of surface atoms creates dangling bonds and surface trap states that together could compromise the free carrier density and thus optoelectronic properties. Here, we report the impact of metal–ligand bonding interactions on the free electron density (N e) and the LSPR response of monoclinic, sub-stoichiometric, and two-dimensional tungsten oxide (WO3–x ) nanoplatelets (NPLs). The LSPR properties of WO3–x NPLs arise from the presence of free electrons in the conduction band as a result of oxygen vacancies in the monoclinic crystal. In situ surface passivation of unpurified colloidal WO3–x NPLs with X-type alkylphosphonate (R-PO3 2–) produces an LSPR peak in the near-infrared region of the electromagnetic spectrum. X-ray photoelectron, electron paramagnetic, and Raman spectroscopic data support the presence of a tridentate PO3–W3 bonding motif that allows increased passivation of shallow surface trap states, leading to an experimentally determined N e value of 8.4 × 1022 cm–3. Furthermore, experimentally determined bonding characteristics are correlated with density functional theory calculations. The effect of the high N e values of NPLs on their refractive index sensitivity is also evaluated. Together, the knowledge gained regarding surface-ligand-chemistry-controlled manipulation of the plasmonic properties in semiconducting metal oxide NPLs and the high N e values of WO3–x NPLs achieved may result in sizable advancement in various LSPR-driven applications such as sensing and energy storage and conversion schemes.

Single- and few-layered two-dimensional (2D) nanomaterials have attracted intense research interest in the last two decades due to their unique electronic and optoelectronic properties leading to various potential applications. Herein, we report the colloidal synthesis of Bi-based 2D perovskite nanosheets (PEG6-NH3 +) n Cs3–n Bi2X9, where X = Cl, Br, and I, through careful design of reaction conditions and selection of poly­(ethylene glycol) (PEG6) surface passivating ligands. The 2D nanosheets are ∼5 nm in thickness with micron-sized lateral dimensions and display composition-dependent band gap and work function modulation. Small-angle X-ray scattering analysis substantiates that the individual inorganic crystal layer, Cs3–n Bi2X9, is separated by the spacer, PEG6 ligand. Additionally, we determined that PEG6-NH2 is an essential passivating ligand and spacer for the formation of Bi-based 2D nanosheets. Most importantly, controlled crystallization of the colloidal dispersion of nanosheets results in the formation of superlattice microstructures of the quasi-Ruddlesden–Popper phase. These microstructures can be exfoliated to ultrathin nanosheets by overcoming the van der Waals interaction between the organic passivating layers. The controlled synthesis of lead-free 2D perovskite nanosheets presented here can expand their utility to photocatalytic and optoelectronic applications with reduced toxicity.