Reversible modulation of spontaneous emission by strain in silicon nanowires

Coexistence of the Radial-Guided Mode and WGM in Azimuthal-Grating-Integrated Microring Lasers

Whispering-gallery mode (WGM) resonators, renowned for their high Q-factors and narrow line widths, are widely utilized in integrated photonics. Integrating diffraction gratings onto WGM cavities has gained significant attention because these gratings function as azimuthal refractive index modulators, enabling single-mode WGM emissions and supporting beams with orbital angular momentum (OAM). The introduction of curved grating structures facilitates guided mode resonances by coupling high-order diffracted waves with leaking modes from the waveguide. These gratings act as wavelength-selective mirrors and support concentric circular radial-guided modes. This study investigates the coexistence and interaction between OAM-carrying WGMs and radial-guided modes with Bessel beam characteristics in an active cladding grating-integrated microring laser. These phenomena are examined through both three-dimensional simulations and experiments. The active layer enhances the radial-guided modes at the microring's center, where cylindrical waves from the active cladding produce strong guided mode resonance at specific wavelengths corresponding to radial modes. Additionally, general WGMs are formed and confined within the microring. The effects of grating depth and microring size on radial-guided mode resonance are evaluated through two-dimensional simulations and experiments. These insights pave the way for integrating functional lasers into photonic circuits and advancing technologies for topological optical vortex emission and manipulation.

Photoelectrochemical Lithium Extraction from Waste Batteries

The amount of global hybrid-electric and all electric vehicle has increased dramatically in just five years and reached an all-time high of over 10 million units in 2022. A good deal of waste lithium (Li)-containing batteries from dead vehicles are invaluable unconventional resources with high usage of Li. However, the recycle of Li by green approaches is extremely inefficient and rare from waste batteries, giving rise to severe environmental pollutions and huge squandering of resources. Thus, in this mini review, we briefly summarized a green and promising route-photoelectrochemical (PEC) technology for extracting the Li from the waste lithium-containing batteries. This review first focuses on the critical factors of PEC performance, including light harvesting, charge-carrier dynamics, and surface chemical reactions. Subsequently, the conventional and PEC technologies applying in the area of Li recovery processes are analyzed and discussed in depth, and the potential challenges and future perspective for rational and healthy development of PEC Li extraction are provided positively.

Lattice Strain and Defects Analysis in Nanostructured Semiconductor Materials and Devices by High-Resolution X-Ray Diffraction: Theoretical and Practical Aspects

The reliability of semiconductor materials with electrical and optical properties are connected to their structures. The elastic strain field and tilt analysis of the crystal lattice, detectable by the variation in position and shape of the diffraction peaks, is used to quantify defects and investigate their mobility. The exploitation of high-resolution X-ray diffraction-based methods for the evaluation of structural defects in semiconductor materials and devices is reviewed. An efficient and non-destructive characterization is possible for structural parameters such as, lattice strain and tilt, layer composition and thickness, lattice mismatch, and dislocation density. The description of specific experimental diffraction geometries and scanning methods is provided. Today's X-ray diffraction based methods are evaluated and compared, also with respect to their applicability limits. The goal is to understand the close relationship between lattice strain and structural defects. For different material systems, the appropriate analytical methods are highlighted.

Nanometer-Scale Ge-Based Adaptable Transistors Providing Programmable Negative Differential Resistance Enabling Multivalued Logic

The functional diversification and adaptability of the elementary switching units of computational circuits are disruptive approaches for advancing electronics beyond the static capabilities of conventional complementary metal-oxide-semiconductor-based architectures. Thereto, in this work the one-dimensional nature of monocrystalline and monolithic Al-Ge-based nanowire heterostructures is exploited to deliver charge carrier polarity control and furthermore to enable distinct programmable negative differential resistance at runtime. The fusion of electron and hole conduction together with negative differential resistance in a universal adaptive transistor may enable energy-efficient reconfigurable circuits with multivalued operability that are inherent components of emerging artificial intelligence electronics.

Gunn-Hilsum Effect in Mechanically Strained Silicon Nanowires: Tunable Negative Differential Resistance

Gunn (or Gunn-Hilsum) Effect and its associated negative differential resistivity (NDR) emanates from transfer of electrons between two different energy subbands. This effect was observed in semiconductors like GaAs which has a direct bandgap of very low effective mass and an indirect subband of high effective mass which lies ~300 meV above the former. In contrast to GaAs, bulk silicon has a very high energy spacing (~1 eV) which renders the initiation of transfer-induced NDR unobservable. Using Density Functional Theory (DFT), semi-empirical 10 orbital (sp³d⁵s^*) Tight Binding and Ensemble Monte Carlo (EMC) methods we show for the first time that (a) Gunn Effect can be induced in silicon nanowires (SiNW) with diameters of 3.1 nm under +3% strain and an electric field of 5000 V/cm, (b) the onset of NDR in the I-V characteristics is reversibly adjustable by strain and (c) strain modulates the resistivity by a factor 2.3 for SiNWs of normal I-V characteristics i.e. those without NDR. These observations are promising for applications of SiNWs in electromechanical sensors and adjustable microwave oscillators. It is noteworthy that the observed NDC is different in principle from Esaki-Diode and Resonant Tunneling Diodes (RTD) in which NDR originates from tunneling effect.

Picosecond Control of Quantum Dot Laser Emission by Coherent Phonons

A picosecond acoustic pulse can be used to control the lasing emission from semiconductor nanostructures by shifting their electronic transitions. When the active medium, here an ensemble of (In,Ga)As quantum dots, is shifted into or out of resonance with the cavity mode, a large enhancement or suppression of the lasing emission can dynamically be achieved. Most interesting, even in the case when gain medium and cavity mode are in resonance, we observe an enhancement of the lasing due to shaking by coherent phonons. In order to understand the interactions of the nonlinearly coupled photon-exciton-phonon subsystems, we develop a semiclassical model and find an excellent agreement between theory and experiment.

Nanofabrication of arrays of silicon field emitters with vertical silicon nanowire current limiters and self-aligned gates

We developed a fabrication process for embedding a dense array (10(8) cm(-2)) of high-aspect-ratio silicon nanowires (200 nm diameter and 10 μm tall) in a dielectric matrix and then structured/exposed the tips of the nanowires to form self-aligned gate field emitter arrays using chemical mechanical polishing (CMP). Using this structure, we demonstrated a high current density (100 A cm(-2)), uniform, and long lifetime (>100 h) silicon field emitter array architecture in which the current emitted by each tip is regulated by the silicon nanowire current limiter connected in series with the tip. Using the current voltage characteristics and with the aid of numerical device models, we estimated the tip radius of our field emission arrays to be ≈4.8 nm, as consistent with the tip radius measured using a scanning electron microscope (SEM).

Quantum mechanical modeling the emission pattern and polarization of nanoscale light emitting diodes

Understanding of the electroluminescence (EL) mechanism in optoelectronic devices is imperative for further optimization of their efficiency and effectiveness. Here, a quantum mechanical approach is formulated for modeling the EL processes in nanoscale light emitting diodes (LED). Based on non-equilibrium Green's function quantum transport equations, interactions with the electromagnetic vacuum environment are included to describe electrically driven light emission in the devices. The presented framework is illustrated by numerical simulations of a silicon nanowire LED device. EL spectra of the nanowire device under different bias voltages are obtained and, more importantly, the radiation pattern and polarization of optical emission can be determined using the current approach. This work is an important step forward towards atomistic quantum mechanical modeling of the electrically induced optical response in nanoscale systems.

Photodegradation of organic dyes based on anatase and rutile TiO2 nanoparticles

Dye degradation study using anatase and rutile titania suggests maximum efficiency of about 88% using anatase phase for MB under short UV light however, the maximum degradation under long UV light was not more than 65% (anatase/ rutile/EBT).

Reversible strain-induced electron-hole recombination in silicon nanowires observed with femtosecond pump-probe microscopy

Strain-induced changes to the electronic structure of nanoscale materials provide a promising avenue for expanding the optoelectronic functionality of semiconductor nanostructures in device applications. Here we use pump-probe microscopy with femtosecond temporal resolution and submicron spatial resolution to characterize charge-carrier recombination and transport dynamics in silicon nanowires (NWs) locally strained by bending deformation. The electron-hole recombination rate increases with strain for values above a threshold of ∼1% and, in highly strained (∼5%) regions of the NW, increases 6-fold. The changes in recombination rate are independent of NW diameter and reversible upon reduction of the applied strain, indicating the effect originates from alterations to the NW bulk electronic structure rather than introduction of defects. The results highlight the strong relationship between strain, electronic structure, and charge-carrier dynamics in low-dimensional semiconductor systems, and we anticipate the results will assist the development of strain-enabled optoelectronic devices with indirect-bandgap materials such as silicon.

A complex study of the fast blue luminescence of oxidized silicon nanocrystals: the role of the core

Silicon nanocrystals (SiNCs) smaller than 5 nm are a material with strong visible photoluminescence (PL). However, the physical origin of the PL, which, in the case of oxide-passivated SiNCs, is typically composed of a slow-decaying red-orange band (S-band) and of a fast-decaying blue-green band (F-band), is still not fully understood. Here we present a physical interpretation of the F-band origin based on the results of an experimental study, in which we combine temperature (4-296 K), temporally (picosecond resolution) and spectrally resolved luminescence spectroscopy of free-standing oxide-passivated SiNCs. Our complex study shows that the F-band red-shifts only by 35 meV with increasing temperature, which is almost 6 times less than the red-shift of the S-band in a similar temperature range. In addition, the F-band characteristic decay time obtained from a stretched-exponential fit decreases only slightly with increasing temperature. These data strongly suggest that the F-band arises from the core-related quasi-direct radiative recombination governed by slowly thermalizing photoholes.

Strain-induced large exciton energy shifts in buckled CdS nanowires

Strain engineering can be utilized to tune the fundamental properties of semiconductor materials for applications in advanced electronic and photonic devices. Recently, the effects of large strain on the properties of nanostructures are being intensely investigated to further expand our insights into the physics and applications of such materials. In this Letter, we present results on controllable buckled cadmium-sulfide (CdS) optical nanowires (NWs), which show extremely large energy bandgap tuning by >250 meV with applied strains within the elastic deformation limit. Polarization and spatially resolved optical measurements reveal characteristics related to both compressive and tensile regimes, while microreflectance spectroscopy clearly demonstrates the effect of strain on the different types of excitons in CdS. Our results may enable strained NWs-based optoelectronic devices with tunable optical responses.