Probing Exciton Complexes and Charge Distribution in Inkslab-Like WSe₂ Homojunction

By virtue of the layer-dependent band structure and valley-selected optical/electronic properties, atomically layered transition-metal dichalcogenides (TMDs) exhibit great potentials such as in valleytronics and quantum devices, and have captured significant attentions. Precise control of the optical and electrical properties of TMDs is always the pursuing goal for real applications, and constructing advanced structures that allow playing with more degrees of freedom may hold the key. Here, we introduce a triangular inkslab-like WSe₂ homojunction with a monolayer in the inner surrounded by a multilayer frame. Benefit from this interesting structure, the photoluminescence (PL) peaks redshift up to 50 meV and the charge density increases about 6 times from the center to the edge region of the inner monolayer. We demonstrated that the Se-deficient multilayer frame offers the excessive free electrons for the generation of the electron density gradient inside the monolayer, which also results in the spatial variation and distribution gradient of a series of exciton complexes. Furthermore, we observed the strong rectifying characteristic and clear photovoltaic response across the homojunction through measuring and mapping the photocurrent of the devices. Our result provides another route for efficient modulation of the exciton-complex emissions of TMDs, which is exceptionally desirable for the “layer- and charge-engineered” photonic and optoelectronic devices

Oxygen Vacancy: An Electron–Phonon Interaction Decoupler to Modulate the Near-Band-Edge Emission of ZnO Nanorods

Through in situ control of the growth condition in the vapor phase transport and condensation, we intentionally prepared ZnO nanorods with different concentrations of oxygen vacancy (<i>V</i>_O), as confirmed by the X-ray photoelectron spectra. A spectral shift in the ultraviolet (UV) emission between these nanorods, as large as ∼80 meV, has been observed in the room temperature photoluminescence (PL) spectra, showing strong correlation to the <i>V</i>_O concentration. With the help of the variable-temperature PL, this spectral shift is clearly attributed to the different spectral contributions of the free exciton emission and its phonon replicas. Furthermore, a remarkable variation in the electron–phonon interaction strength among these samples is unambiguously revealed by the Raman spectra, which is in good consistence with the Huang–Rhys parameters obtained from the PL. This study implies that <i>V</i>_O in ZnO nanostructures not only modulates the visible emission as intensively previously investigated but also significantly suppresses the electron–phonon interaction strength and therefore tailor the UV (near-band-edge) emission property. This finding is useful to design and fabricate ZnO-based high-performance short-wavelength photonic and optoelectronic devices on the nanoscale

High-Throughput Fabrication of Ultradense Annular Nanogap Arrays for Plasmon-Enhanced Spectroscopy

The confinement of light into nanometer-sized metallic nanogaps can lead to an extremely high field enhancement, resulting in dramatically enhanced absorption, emission, and surface-enhanced Raman scattering (SERS) of molecules embedded in nanogaps. However, low-cost, high-throughput, and reliable fabrication of ultra-high-dense nanogap arrays with precise control of the gap size still remains a challenge. Here, by combining colloidal lithography and atomic layer deposition technique, a reproducible method for fabricating ultra-high-dense arrays of hexagonal close-packed annular nanogaps over large areas is demonstrated. The annular nanogap arrays with a minimum diameter smaller than 100 nm and sub-1 nm gap width have been produced, showing excellent SERS performance with a typical enhancement factor up to 3.1 × 10⁶ and a detection limit of 10^–11 M. Moreover, it can also work as a high-quality field enhancement substrate for studying two-dimensional materials, such as MoSe₂. Our method provides an attractive approach to produce controllable nanogaps for enhanced light–matter interaction at the nanoscale

Great Disparity in Photoluminesence Quantum Yields of Colloidal CsPbBr₃ Nanocrystals with Varied Shape: The Effect of Crystal Lattice Strain

Understanding the big discrepancy in the photoluminesence quantum yields (PLQYs) of nanoscale colloidal materials with varied morphologies is of great significance to its property optimization and functional application. Using different shaped CsPbBr₃ nanocrystals with the same fabrication processes as model, quantitative synchrotron radiation X-ray diffraction analysis reveals the increasing trend in lattice strain values of the nanocrystals: nanocube, nanoplate, nanowire. Furthermore, transient spectroscopic measurements reveal the same trend in the defect quantities of these nanocrystals. These experimental results unambiguously point out that large lattice strain existing in CsPbBr₃ nanoparticles induces more crystal defects and thus decreases the PLQY, implying that lattice strain is a key factor other than the surface defect to dominate the PLQY of colloidal photoluminesence materials