Application of Scanning Tunneling Microscopy and Spectroscopy in the Studies of Colloidal Quantum Qots

Colloidal quantum dots display remarkable optical and electrical characteristics with the potential for extensive applications in contemporary nanotechnology. As an ideal instrument for examining surface topography and local density of states (LDOS) at an atomic scale, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) has become indispensable approaches to gain better understanding of their physical properties. This article presents a comprehensive review of the research advancements in measuring the electronic orbits and corresponding energy levels of colloidal quantum dots in various systems using STM and STS. The first three sections introduce the basic principles of colloidal quantum dots synthesis and the fundamental methodology of STM research on quantum dots. The fourth section explores the latest progress in the application of STM for colloidal quantum dot studies. Finally, a summary and prospective is presented.

Plasmonic metal oxides and their biological applications

Metal oxides modified with dopants and defects are an emerging class of novel materials supporting the localized surface plasmon resonance across a wide range of optical wavelengths, which have attracted tremendous research interest particularly in biological applications in the past decade. Compared to conventional noble metal-based plasmonic materials, plasmonic metal oxides are particularly favored for their cost efficiency, flexible plasmonic properties, and improved biocompatibility, which can be important to accelerate their practical implementation. In this review, we first explicate the origin of plasmonics in dopant/defect-enabled metal oxides and their associated tunable localized surface plasmon resonance through the conventional Mie-Gans model. The research progress of dopant incorporation and defect generation in metal oxide hosts, including both in situ and ex situ approaches, is critically discussed. The implementation of plasmonic metal oxides in biological applications in terms of therapy, imaging, and sensing is summarized, in which the uniqueness of dopant/defect-driven plasmonics for inducing novel functionalities is particularly emphasized. This review may provide insightful guidance for developing next-generation plasmonic devices for human health monitoring, diagnosis and therapy.

Tumor-targeting multi-shelled hollow nanospheres as drug loading platforms for imaging-guided combinational cancer therapy

In this work, we developed multi-shelled hollow nanospheres [RGD@am-ZnO@CuO@Au@DOX HNSs] as multifunctional therapeutic agents to achieve effective and targeted Zn2+/Cu2+ therapy, induced drug delivery under low pH/red-light conditions, and enhanced phototherapy under single red-light. The photothermal and photodynamic performance of am-ZnO@CuO@Au HNSs was enhanced relative to that of am-ZnO nanoparticles (NPs) or am-ZnO@CuO HNSs by utilizing the resonance energy transfer process and broad red-light absorption. The pH-sensitive am-ZnO@CuO@Au HNSs were dissolved to Zn2+/Cu2+ in the acidic endosomes/lysosomes of cancer cells, resulting in a cancer cell killing effect. The release performance of doxorubicin (DOX) from RGD@am-ZnO@CuO@Au@DOX HNSs was evaluated under low pH and red-light-irradiated conditions, and targeting of HNSs was confirmed by dual-modal imaging (magnetic resonance/fluorescence) of the tumor area. Moreover, in vivo synergistic therapy using RGD@am-ZnO@CuO@Au@DOX HNSs was further evaluated in mice bearing human pulmonary adenocarcinoma (A549) cells, achieving a remarkable synergistic antitumor effect superior to that obtained by monotherapy. This study validated that RGD@am-ZnO@CuO@Au@DOX HNSs can be a promising candidate for efficient postoperative cancer therapy.

One-Dimensional Cuprous Selenide Nanostructures with Switchable Plasmonic and Super-ionic Phase Attributes

Cuprous selenide nanocrystals have hallmark attributes, especially tunable localized surface plasmon resonances (LSPRs) and super-ionic behavior. These attributes of cuprous selenide are now integrated with a one-dimensional morphology. Essentially, Cu₂ Se nanowires (NWs) of micrometer-scale lengths and about 10 nm diameter are prepared. The NWs exhibit a super-ionic phase that is stable at temperatures lower than in the bulk, owing to compressive lattice strain along the radial dimension of the NWs. The NWs can be switched between oxidized and reduced forms, which have contrasting phase transition and LSPR characteristics. This work thus makes available switchable, one-dimensional waveguides and ion-conducting channels.

Size Dependence of Doping by a Vacancy Formation Reaction in Copper Sulfide Nanocrystals

Doping of nanocrystals (NCs) is a key, yet underexplored, approach for tuning of the electronic properties of semiconductors. An important route for doping of NCs is by vacancy formation. The size and concentration dependence of doping was studied in copper(I) sulfide (Cu₂ S) NCs through a redox reaction with iodine molecules (I₂ ), which formed vacancies accompanied by a localized surface plasmon response. X-ray spectroscopy and diffraction reveal transformation from Cu₂ S to Cu-depleted phases, along with CuI formation. Greater reaction efficiency was observed for larger NCs. This behavior is attributed to interplay of the vacancy formation energy, which decreases for smaller sized NCs, and the growth of CuI on the NC surface, which is favored on well-defined facets of larger NCs. This doping process allows tuning of the plasmonic properties of a semiconductor across a wide range of plasmonic frequencies by varying the size of NCs and the concentration of iodine. Controlled vacancy doping of NCs may be used to tune and tailor semiconductors for use in optoelectronic applications.

Copper Sulfide Nanocrystal Level Structure and Electrochemical Functionality towards Sensing Applications

The level structure of copper sulfide nanocrystals of different sizes was investigated by correlating scanning tunneling spectroscopy and cyclic voltammetry data in relation to sensing applications. Upon oxidation of Cu2 S nanocrystals in the low-chalcocite phase, correlated changes are detected by both methods. The cyclic voltammetry oxidation peak of Cu(1+) down shifts, while in-gap states, adjacent to the valence-band edge, appeared in the tunneling spectra. These changes are attributed to Cu vacancy formation leading to a Cu depleted phase of the nanocrystals. The relevance of the oxidation to the use of copper sulfide nanocrystals in hydrogen peroxide sensing was also addressed, showing that upon oxidation the sensitivity vanishes. These findings bare significance to the use of copper sulfide nanocrystals in glucose sensing applications.