Structural Origin of the Band Gap Anomaly of Quaternary Alloy Cd_<i>x</i>Zn_1–<i>x</i>S_<i>y</i>Se_1–<i>y</i> Nanowires, Nanobelts, and Nanosheets in the Visible Spectrum

Single-crystalline alloy II–VI semiconductor nanostructures have been used as functional materials to propel photonic and optoelectronic device performance in a broad range of the visible spectrum. Their functionality depends on the stable modulation of the direct band gap (<i>E</i>_g), which can be finely tuned by controlling the properties of alloy composition, crystallinity, and morphology. We report on the structural correlation of the optical band gap anomaly of quaternary alloy Cd_<i>x</i>Zn_1–<i>x</i>S_<i>y</i>Se_1–<i>y</i> single-crystalline nanostructures that exhibit different morphologies, such as nanowires (NWs), nanobelts (NBs), and nanosheets (NSs), and cover a wide range of the visible spectrum (<i>E</i>_g = 1.96–2.88 eV). Using pulsed laser deposition, the nanostructures evolve from NWs <i>via</i> NBs to NSs with decreasing growth temperature. The effects of the growth temperature are also reflected in the systematic variation of the composition. The alloy nanostructures firmly maintain single crystallinity of the hexagonal wurtzite and the nanoscale morphology, with no distortion of lattice parameters, satisfying the virtual crystal model. For the optical properties, however, we observed distinct structure-dependent band gap anomalies: the disappearance of bowing for NWs and maximum and slightly reduced bowing for NBs and NSs, respectively. We tried to uncover the underlying mechanism that bridges the structural properties and the optical anomaly using an empirical pseudopotential model calculation of electronic band structures. From the calculations, we found that the optical bowings in NBs and NSs were due to residual strain, by which they are also distinguishable from each other: large for NBs and small for NSs. To explain the origin of the residual strain, we suggest a semiempirical model that considers intrinsic atomic disorder, resulting from the bond length mismatch, combined with the strain relaxation factor as a function of the width-to-thickness ratio of the NBs or NSs. The model agreed well with the observed optical bowing of the alloy nanostructures in which a mechanism for the maximum bowing for NBs is explained. The present systematic study on the structural–optical properties correlation opens a new perspective to understand the morphology- and composition-dependent unique optical properties of II–VI alloy nanostructures as well as a comprehensive strategy to design a facile band gap modulation method of preparing photoconverting and photodetecting materials

Simultaneous Enhancement of Upconversion and Downshifting Luminescence via Plasmonic Structure

We describe a metal nanodisk–insulator–metal (MIM) structure that enhances lanthanide-based upconversion (UC) and downshifting (DS) simultaneously. The structure was fabricated using a nanotransfer printing method that facilitates large-area applications of nanostructures for optoelectronic devices. The proposed MIM structure is a promising way to harness the entire solar spectrum by converting both ultraviolet and near-infrared to visible light concurrently through resonant-mode excitation. The overall photoluminescence enhancements of the UC and DS were 174- and 29-fold, respectively

On-Demand Drug Release from Gold Nanoturf for a Thermo- and Chemotherapeutic Esophageal Stent

Stimuli-responsive delivery systems for cancer therapy have been increasingly used to promote the on-demand therapeutic efficacy of anticancer drugs and, in some cases, simultaneously generate heat in response to a stimulus, resulting in hyperthermia. However, their application is still limited due to the systemic drawbacks of intravenous delivery, such as rapid clearance from the bloodstream and the repeat injections required for sustained safe dosage, which can cause overdosing. Here, we propose a gold (Au)-coated nanoturf structure as an implantable therapeutic interface for near-infrared (NIR)-mediated on-demand hyperthermia chemotherapy. The Au nanoturf possessed long-lasting doxorubicin (DOX) duration, which helps facilitate drug release in a sustained and prolonged manner. Moreover, the Au-coated nanoturf provides reproducible hyperthermia induced by localized surface plasmon resonances under NIR irradiation. Simultaneously, the NIR-mediated temperature increase can promote on-demand drug release at desired time points. For <i>in vivo</i> analysis, the Au nanoturf structure was applied on an esophageal stent, which needs sustained anticancer treatment to prevent tumor recurrence on the implanted surface. This thermo- and chemo-esophageal stent induced significant cancer cell death with released drug and hyperthermia. These phenomena were also confirmed by theoretical analysis. The proposed strategy provides a solution to achieve enhanced thermo-/chemotherapy and has broad applications in sustained cancer treatments