This PDF file contains the front matter associated with SPIE Proceedings Volume 11800, including the Title Page, Copyright information, and Table of Contents.

In-situ fabrication of (magnetic) iron based nanoparticles is demonstrated using ultrashort laser pulses ablation in liquid medium. Using this in-situ method, thermal effects associated with other similar methods are avoided because of pulse duration much beyond 1 ps, avoiding thermal processes as well as chemical contamination and clustering issues during particle fabrication or transfer steps. Size distribution proved to be dependent on laser beam pulse duration and energy, while their surface (zeta-)potential did not. Monitoring of the cellular viability in the presence of the produced nanoparticles showed an excellent bio-compatibility but with rather limited drug loading capabilities in the absence of any surface functionalization.

Understanding how inducing molecular alignment can influence pyrolytic carbon microstructure and functionality is consequential for carbon MEMS microfabrication and applicability. We present a comparative analysis on the effects of compressive stress versus standard tensile treatment of carbon precursors. Different characterization techniques reveal that while subjecting precursor molecules to both types of mechanical stresses will induce graphitization in the pyrolytic carbon, this effect is more pronounced in compressive stress. MEMS functionality of the two carbons was evaluated by characterizing the electrochemical performance of their electrodes. Both carbons exhibited enhanced electrochemical performances. However, the heterogeneous electron transfer rate derived from CV diagrams reveals compression-activated electrode to have remarkably faster kinetics. The results show the versatility of pyrolytic nanocarbons and a synthesis route to tailor functionality for MEMS and Sensors.

Semiconductor Quantum Dots (QDs) have great potential in applications for renewable energy generation due to their size-tunable redox potentials. QDs may also be doped to manipulate their electronic structure. Our group developed a method to dope each quantum dot with an exact number of guest ions by nucleating the QD around an organometallic seed cluster that contains guest ions. As a result, each QD has the same number of dopants, which eliminates problems due to inhomogeneity of the dot stoichiometry. These materials were studied using time-resolved X-ray absorption spectroscopy, which allows us to characterize the electronic and coordination structure in both the ground and excited states. It was found that, when dopants interact with charge carriers, they may alter their bonding to the underlying matrix. This phenomenon of charge carrier modulation of dopant bonding has a strong effect on the conductivity properties of doped semiconductors.

In this work we will review two post-synthetic methods for controlling defects and doping, in 2D-materials, namely hyperthermal ion implantation (HyTII) and helium ion microscopy (HIM) based processing with a focused He-ion beam. HyTII processing ranges in energy between that of plasma processing and traditional ion implantation, however, it benefits from a monoenergetic beam energy, with precise control over energy, direction, and dose. We will discuss the use of HyTII for forming nitrogen doped graphene along with initial doping studies of transition metal dichalcogenides (TMDs). We have utilized HIM processing to create defects and to nano-machine features in a wide range of TMDs. HIM processing will be correlated with changes in the photoluminescence and Raman spectra of WS2 with dose. To conclude, we will review recent results on HIM processing formation of single photon emitters, particularly in MoS2, and summarize future opportunities in ion-beam processing of 2D materials.

I will show our results on improving the growth of topological insulator (TI) van der Waals (vdW) thin films using molecular beam epitaxy. First, lattice-matched trivially-insulating buffer layers can be used to reduce the unintentional doing in the films by improving the crystalline quality of the first few TI layers. Second, substrate pre-treatment can reduce twinning and improve TI morphology by satisfying dangling bonds and changing the surface energy. Finally, when TI films are grown on (001) GaAs, the TI film can be grown in an alternate orientation that has an epitaxial relationship to the substrate and that self-assembled TI nano-columns can be grown without the use of strain or catalysts. Despite the weak film-substrate interaction, the morphology and quality of vdW thin films can be strongly controlled by appropriate choice of growth parameters and substrate.

In order to solve the problem of maintaining acidic environment and continuous loss of iron ions in traditional homogeneous Fenton reaction, an advanced photocatalytic-Fenton coupling system was developed on the basis of 2D/2D S-scheme Fe2O3/Bi2WO6 and Fe2O3/Bi2MoO6 catalysts fabricated by a facile hydrothermal method. X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy analyses of the synthesized catalyst displayed that the hexagonal nanosheets of Fe2O3 were successfully deposited on Bi2WO6 and Bi2MoO6 nanosheets. Under visible light irradiation, the photo-Fenton catalytic activities of Fe2O3/Bi2WO6 and Fe2O3/Bi2MoO6 were significantly higher than those of individual components. By virtue of large surface area, adequate active sites and efficient charge transfer mechanism, the construction of 2D/2D Fe2O3/Bi2WO6 and Fe2O3/Bi2MoO6 heterojunction observably enhanced the separation efficiency of photogenerated carriers. Simultaneously, highly efficient charge mobility can lead to continuous Fe3+/Fe2+ conversion, promoting a cooperative effect between the photocatalysis and Fenton reaction. What’s more, a novel S-scheme model was proposed to expound the charge transfer process in the photo-Fenton catalytic reaction. We believe that Fe2O3/Bi2WO6 and Fe2O3/Bi2MoO6 composites can be a valuable guide for designing and constructing 2D/2D S-scheme heterojunction photo-Fenton catalysts.

Various metallic and dielectric thin films are used in silver-based mirrors for astronomical telescopes. The topmost surface of such silver-based mirrors needs to be shielded by a protection coating. Conventionally, the protection coating is deposited at room temperature to minimize thermal stress to which the entire mirror is subjected. Nevertheless, various thin film deposition techniques offer protection coatings with improved characteristics when carried out at elevated temperatures. This paper describes a study of high-performance protected silver-based mirrors annealed at various temperatures in assessing the rationality of introducing post-fabrication annealing with the aim of improving overall optical performance and durability of protected silver-based mirrors.

Depositing thin films is often limited to a specific deposition process by which precursors are transported in a deposition environment. In other words, a deposition environment in which two deposition processes complementary to each other are unified may offer new insights in designing thin film structures. This view motivated us to combine atomic layer deposition (ALD) and magnetron sputtering (SPU) in a single chamber – sputtering atomic layer augmented deposition (SALAD). The SALAD system offers benefits of consistently delivering precursors in ALD and freely choosing chemical elements in SPU. In this paper, the SALAD system is employed to deposit nanocomposites consisting of multiple layers of aluminum oxide deposited by ALD and copper layers deposited by SPU. Distinctive dispersion features seen in optical properties of the nanocomposites are analyzed to reveal the interrelationship between structural properties and electronic properties of the nanocomposites.

Lasers are often used to process materials. For example, crystallization of amorphous semiconductor can be induced by having laser light interact with the semiconductor and having amorphous semiconductor undergo a liquid-solid phase transition – laser-induced crystallization. While laser-induced crystallization is predominantly utilized in preparing thin films made of such single chemical elements as silicon and germanium, extending its use for semiconductors that contain multiple chemical elements (e.g., metal oxides) unfolds applications that have yet to be envisioned. In this paper, a continuous-wave laser diode with a micrometer-scale chevron-shaped beam profile – micro-chevron laser beam (μ-CLB) – was exploited to convert amorphous CuO thin films prepared on fused silica substrates into single-crystal Cu2O stripes under various crystallization conditions. The dependence of the crystallization on laser power density and laser scan rate was investigated by Raman spectroscopy and ana

The use of van der Waals substrates, in which the epitaxial growth is achieved through weak dipolar interactions, can result in a significant relaxation of the epilayer strain, facilitating at the same time layer detachment. Here, we study the case of GaN layers grown on graphene and muscovite mica. Morphology, surface potential and strain relaxation of GaN are addressed. In the case of graphene, we show it experiences interesting transformations during the growth of GaN, resulting in the intercalation of metal atoms below the graphene layer. In the case of mica, we find that part of the strain accumulated in the GaN layer relaxes by the formation of three-dimensional structures in the shape of telephone cord buckles, straight blisters or by more complex arrangements. Their characteristics are studied in relation to the initial compressive strain and the elastic parameters of the materials.

Topological insulators (TIs) are a class of materials which exhibit unique electronic states delocalized on the outer surface of the material. Due to the strong spin-orbit coupling in TIs, these states are spin-polarized and robust against scattering. The robust nature of these states could present a potential solution to the problem of decoherence in qubits. This has motivated research into creating quantum information storage/transport devices using TIs. In order to create such a device, we first must understand how to isolate these states energetically, and this can be done using quantum confinement. Our first step involves the creation of quantum confined TIs. Here, we report on the growth of TI quantum dots grown by droplet epitaxy. We chose the TI Bi2Se3, a commonly studied 3D topological insulator, and growths were performed on GaAs in a molecular beam epitaxy chamber. We varied the growth conditions of bismuth deposition time and substrate temperature to determine the degree to which TI nanoparticle dimensions could be controlled. Within the growth window, we observed particles in the range of 5-15 nm tall and 500-3000 nm2 in area (equivalent circular radius ~12-30 nm). We observed various trends in particle height, area, density and polydispersity as a result of varying growth conditions and adjusting the growth procedure (ex. adding a long anneal step between bismuth and selenium deposition steps). Overall, these results will be helpful in aiding further development of topological insulator devices.

A Weyl semimetal carries topological charges at the Weyl nodes; a light beam can also carry a topological charge, when it has an orbital angular momentum (OAM). Recently there has been a lot of interest in understanding how the spin angular momentum (SAM) of light interacts with materials to induce photocurrents (circular photogalvanic effect, CPGE), but not many studies have focused on photocurrents generated by the OAM of light. Here we report a unique orbital photogalvanic effect (OPGE) in a type-II Weyl semimetal WTe2, featured by a photocurrent winding around the axis of OAM-carrying beams, whose intensity is directly proportional to the topological winding number of the light field, and can be attributed to a discretized dynamical Hall effect. In addition to obtaining evidence of OAM induced electron excitations, our measurements show promise for on-chip detection of the phase of light.

The gain in Avalanche Photodiodes (APDs) and Single Photon Avalanche Diodes (SPADs) is dependent on the probability of photo-generated carriers to trigger an avalanche process, which is correlated to the depth where a photon is absorbed by the photodiode. For silicon photodiodes, most of the photons with wavelengths in the visible spectrum are absorbed near the surface in the highly doped contact regions where the recombination rate is high. Thus, they do not contribute significantly to the avalanche multiplication process. By integrating photon-trapping nanostructures, we facilitate deeper penetration of photons into the devices, enhancing light absorption to generate more carriers that can trigger the avalanche process. This improves the gain-bandwidth of silicon APDs and SPADs significantly. Photon-trapping nanoholes can reduce the thickness of silicon without compromising its quantum efficiency, while a perforated surface reduces the device capacitance improving the bandwidth. Therefore, the manipulation of light penetration depth using photon-trapping nanoholes leads to ultrafast high-gain photodetectors capable of detecting faint light signals particularly useful for low light applications such as fluorescent lifetime imaging microscopy and time-of-flight positron emission tomography.

Understanding and mitigating the adverse effects of radiation on semiconductor devices remains an active area of research motivated by increased use of electronics in high radiation environments. When a device is exposed to energetic particles, a variety of structural defects are created and then redistributed through diffusion, cluster formation, and recombination. How the defects are distributed in the device can profoundly affect its electrical characteristics, yet methods which can reliably image this distribution are lacking. In the first part of my presentation, I will describe how EBIC in the scanning electron microscope can spatially identify defects produced by a 300 keV He+ beam in model n-MOSFET devices. By analyzing the EBIC signal through the bulk pn-junction and through the gate, radiation damage in the bulk Si or the SiO2 gate can be identified, respectively. Correlation of the defect distribution maps with device electrical characteristics analyzed using Silavaco softwa

We investigated the difference between a macro scale PL and μPL (excitation and detection area ≤ 5μm²). Low-temperature micro-photoluminescence (μPL) is used to evaluate structural perfection of high current density InGaAs/AlAs/InP resonant tunnelling diodes (RTD) structure on different length scales. The thin and highly strained quantum wells (QWs) is subject to monolayer fluctuations in well and barrier thickness that can lead to random fluctuations in their band profile. μPL is performed reducing the laser spot size using a common photolithography mask to reach typical RTD mesa size (a few square microns). We observed that for spot size around 1μm² the PL line shape present strong differences on multiple points on the wafer. These variations in the PL is investigated by line-shape fitting and discussed in terms of variations in long-range disorder brought about by strain relaxation processes. We also highlight this μPL as a powerful and cost-effective non-destructive characterization method for RTD structures.

There exist a myriad of experimental studies on resistive switching devices that consist of a dielectric film inserted between a pair of electrodes. These resistive switching devices display reversible multi-state switching behaviors pertinent to a range of applications including neuromorphic computing. However, coherent understanding of physical and chemical origins of their distinctive electrical properties has yet to be completed and needs to be further investigated to improve overall performance and endurance of resistive switching devices. In this paper, phase-field methodology was used to study the formation and annihilation of electrically conducting channels in a dielectric film of a resistive switching device. The study focuses on the progressive evolution of domains made of electrical charges – charge clusters – under the influence of spatially varying electric-field and temperature. The study also sheds light on the retention loss – a degradation by which resistive switching

We present our recent research on color centers in Aluminum Gallium Nitride which emit single photons up to room temperature. The mature processing technology which is available for group-III-nitrides and the host material’s optical transparency in the visible and infra-red opens up the possibility of novel applications in nanophotonics and quantum devices. We are working to create suspended photonic devices, including waveguides and photonic crystal cavities, which we will show can guide and enhance the color center emission.

Realizing optically and/or electrically tunable plasmonic resonances in the visible to ultraviolet (UV) spectral region is particularly important for reconfigurable photonic device applications. Ultrathin layered group-III chalcogenides, such as GaS, GaSe, GaTe, Sb2S3, are particularly intriguing 2D materials that are revealing exotic phase-change properties with great promise for application in next generation reconfigurable electronics and optoelectronic devices. In this contribution, we present experimental and calculated results obtained on low-loss layered phase-change semiconducting materials of GaS, GaSe, GaTe, Sb2S3, which shows in addition to the conventional amorphous to crystalline phase transition (like the GST family), order-order (polytypes), metal-to-insulator transitions that can be triggered electrically, optically and via plasmonic coupling with alternative phase-change plasmonic metals.

We are developing a wearable sensing platform that provides rapid and quantitative measurements of a panel of inflammatory biomarkers. The sensor detects interleukin-6 (IL-6) and C-reactive protein (CRP) levels, which are found to be associated with adverse clinical outcomes and death in critically ill SARS-CoV-2 patients. Although wearable technology has entered the fight against COVID-19, the devices are limited to monitoring physical attributes and rely on syndromic case finding. Hence, asymptomatic but contagious individuals with no early symptoms remain undetected and transmit the virus/bacteria. In this regard, our sensor would result in a powerful public health weapon that will monitor biochemical attributes in real-time and diagnose an infection before symptoms appear.

We report on the achievement of a new type of ultraviolet light-emitting diodes (LEDs) using AlInN nanowire heterostructures. The molecular beam epitaxial grown AlInN nanowires have relatively high internal quantum efficiency of > 52% at 295nm. The peak emission wavelength is in the range of 280 - 355nm. Moreover, we show that the light extraction efficiency of AlInN nanowire LEDs could reach ~ 63% for hexagonal photonic crystal nanowire structures which is significantly higher compared to the random nanowire arrays. This study provides significant insights into the design and fabrication of new type of high performance AlInN nanowire ultraviolet light-emitters.

Semiconductor nanowires are routinely grown on high-priced crystalline substrates as it is extremely challenging to grow directly on plastics and flexible substrates due to high temperature requirements and substrate preparation. At the same time, plastic substrates can offer many advantages such as extremely low price, light weight, mechanical flexibility, shock and thermal resistance, and biocompatibility. We explore the direct growth of InSb nanowires on flexible plastic substrates by metal-organic vapor phase epitaxy (MOVPE). We synthesize InSb nanowires on polyimide and show that the fabricated NWs are optically active with strong light emission even at RT. Overall, we demonstrate that InSb nanowires can be synthesized directly on flexible plastic substrates inside a MOVPE reactor, and we believe that our results will further advance the development of the nanowire-based flexible electronic devices.

We report on a combined spectroscopic/structural study of MOVPE-grown GaAs-AlGaAs core-multishell nanowires, containing thin GaAs quantum well tubes (QWTs) wrapped around the central GaAs core. Low temperature (7K) cathodoluminescence (CL) spectroscopic imaging combined with Z-contrast scanning transmission electron microscopy (STEM) tomography performed on single core-multishell nanowires allowed robust correlation between QWT emission and the nanowire inner structure down to the nano-scale. Besides the core luminescence and minor defects-related contributions, each nanowire showed one or more QWT peaks in the 1.53-1.65 eV spectral region, which correlated with sections of the nanowire trunk having different diameters. Average values of QWT thickness (in the 3-7 nm range) were thus extracted from measured nanowire diameter through the application of a multishell growth model, the latter validated against experimental data (core/nanowire diameter, shell thicknesses) obtained from 3-dimensional (3D) reconstructed STEM tomograms of single QWT nanowires. Our data evidenced that the QWT emissions appear redshifted (by about 40-120 meV) with respect to values expected for uniform QWTs of the same thickness. CL mapping evidenced nanoscale localization of QWT exciton emissions along the nanowire, demonstrating that their emission is affected by carrier localization at confinement-potential inhomogeneity. The latter have been ascribed to azimuthal asymmetries as well as (azimuthal and axial) random fluctuations of the GaAs QWT thickness within each nanowire, as evidenced by detailed statistical analysis of the 3D tomograms.

A fractal array of room-temperature (RT) luminescent Si nanowires (NWs) is realized by thin-film metal-assisted chemical etching, a cost-effective, fast, and maskless Si technology compatible approach. This process permits obtaining Si NWs with interesting structural and optical features for a wide range of applications, from photonics to sensing. For what concern photonics, the possibility to fabricate artificial fractals based on Si NWs that integrate other interesting elements is reported. In particular, an artificial fractal based on the Er:Y₂O₃ decoration of Si NWs where the Er emission can be tuned as a function of the decoration angle is shown. In the sensor field, the use of Si NW luminescence can represent an interesting and innovative sensing mechanism for the realization of a novel class of sensing platform. In this work, a light-emitting Si NWs-based label-free sensor for both selective isolation and ultrasensitive quantification of small extracellular vesicles (sEVs) is reported opening the route toward liquid biopsy applications.

Controllable doping in semiconductor nanowires is essential for development of optoelectronic devices. Despite great progress, a fundamental challenge remains in controlling the uniformity of doping, particularly in the presence of relatively high levels of geometrical inhomogeneity in bottom-up growth. A relatively high doping level of 1E18 cm-3 corresponds to just ~1000 activated dopants in a 2µm long, 50nm diameter nanowire. High-throughput photoluminescence spectroscopy enables the collection of doping distributions across many (>10k) nanowires, but geometric variation adds additional uncertainty to the modelling. We present an approach that uses large datasets of doping and emission intensity to infer both doping and diameter across a growth, and apply Bayesian methods to study the underlying distributions in Zn-doped aerotaxy-grown GaAs nanowires. This new big-data enabled approach provides a route to exploit inherent inhomogeneity to reveal fundamental recombination mechanisms.

In this keynote talk, I will describe the properties and device applications of quasi-2D and quasi- 1D quantum van der Waals (vdW) materials. The 2D vdW materials include TMDs, which exfoliate into quasi-2D atomic layers. The focus will be on 1T-TaS2, a unique material with charge-density-wave (CDW) phases observed above room temperature. The 1D vdW materials include members of the TMT family, which exfoliate into quasi-1D atomic threads. I will discuss switching among various CDW phases and possibilities of their device applications; the use of the current fluctuations for observing phase transitions; and current carrying capacity of TaSe3 atomic bundles.

Non-hydrogenic Rydberg series associated with excitons have been identified in ultraclean monolayer TMDCs. Here, we investigated the radiative properties of the excitonic Rydberg series in monolayer MoTe2 based devices and the influence of Fermi level position on the same. Using low temperature (4K) photoluminescence measurements, we observed bright emission from the first three states of the excitonic Rydberg series, namely A1s, 2s and 3s. Upon doping on either electron or hole side, oscillator strengths are rapidly transferred to the corresponding trion (charged exciton/attractive polaron) states associated with the aforementioned neutral excitonic resonances. Energy shifts between different states are observed as a function of gate voltage, indicating strong band-structure renormalization. Our work identifies MoTe2 as a novel platform to realize highly tunable bright light sources or electro-optic modulators in the NIR.

In this work we present PL and time resolved PL (TRPL) measurements of three of these materials: BA2PbI4, BA2MA1Pb2I7, and BA2CuCI4 where BA2 represents (CH3(CH2)3NH3)2, and MA: CH3NH3. Both BA2PbI4 and BA2CuCI4 have a single layer of perovskite material separated by an organic cation layer while BA2MA1Pb2I7 has two atomic layers of perovskite. Our observations indicate the existence of both free and trapped excitons in these systems. Additionally, BA2PbI4 displays two sets of peaks for both trapped and free excitons that evolve with temperature, indicating that as the temperature is reduced the system begins, but does not complete, a phase change from a tetragonal to an orthorhombic crystal lattice. Our result provides new insights on the low temperature behavior of this phase transition, as well as exploring the exciton spectra as a function of both temperature and magnetic field. This material is based upon work supported by the Air Force Office of Scientific Research under awar

Recent theoretical and experimental work on monolayer transition-metal dichalcogenides show that optical excitation and strain leads to a transition from an excitonic to electron-hole liquid (EHL) phase. This phase transition is accompanied by a huge (23-fold) increase in photoluminescence (PL) but so far a mechanism has not been confirmed. Here, authors investigate how dark excitons beyond the light cone may influence the PL response of 1L-MoS2 in the excitonic vs EHL regime. They predict that in the excitonic to plasma transition, intraband collisions redefine the effective light cone of optically accessible carriers. Also, sample strain is shown to impact the spectral positions of bright and dark exciton transitions by way of altering the momentum space band positions of 1L-MoS2, increasing the ratio of bright carriers within the light cone.

Two-Dimensional (2D) monolayer transition metal dichalcogenides (TMDs) enable distinct quantum optical properties compared to bulk analogs. The pervasive appeal of 2D TMDs is underpinned by the nascent ability to scalably isolate mono to few layer TMDs from bulk constituents via exfoliation strategies. To-date, the optical characterization of films from exfoliated TMDs has been scarce, especially in relation to the quality of the optical response (i.e., refractive index, n, and extinction coefficient, k) and associated physical material tolerances. In this work, we report the optical properties of representative liquid phase exfoliated MoS2 films and identify important considerations toward maximizing associated low-dimensional optical performance. Understanding processing impact on material quality post-exfoliation and on the resulting optical performance of such films is expected to further enable application-ready quantum nanophotonic technologies.

One of the driving forces of the ongoing nanotechnology revolution is the ever-improving ability to understand and control the properties of quantum matter down to the atomic scale. Key drivers in this revolution are quantum materials, such as the layered materials of the transition metal dichalcogenide (TMD) family. The realization of novel TMD-based devices relies heavily on understanding the relationship between structural and electrical properties at the nanoscale. The ultimate goal is that of crafting TMD nanostructures in a way that makes possible the tailored control of their properties. In this talk, I will present recent studies illustrating novel fabrication approaches of TMD nanostructures based on combining top-down and bottom-up methods. These allow to control of the resulting geometries and material combinations, making possible the realization of novel functionalities such as metallic edge states arising in MoS2 nanowalls and nanowires, enhanced nonlinear response in vertically-oriented MoS2 nanostructures, and surface plasmons in WS2 nanoflowers. I will emphasize the crucial role that cutting-edge electron microscopy techniques play in these studies.

Indium Selenide (InSe) is a remarkable two-dimensional quantum material whose characteristic properties include a bandgap in the near infrared region that increases with fewer layers. InSe is known to crystallize in either the β-, γ- or the ε-phase. Of these three crystalline phases, only the β and γ exhibit a direct bandgap, which makes them suitable for optoelectronic applications. The β-phase is easily distinguished from the others by means of Transmission Electron Microscopy (TEM), whereas the γ- and ε-phases appear very similar. We determine the crystalline phase present in these InSe specimens by systematic investigation with High Resolution TEM. We further assess the local electronic properties using Electron Energy-Loss Spectroscopy (EELS) by mapping relevant features in the spectra. Finally, we deploy Machine Learning techniques for a model-independent subtraction of the Zero Loss Peak, making it possible to identify features in the ultra-low-loss region of the EELS spectra.

Metal halide perovskites in the form of nanocrystals are highly efficient light emitters at visible-NIR wavelengths. In this work, the optical properties of single nanocrystals and ensembles will be discussed, as also several applications in nanophotonics. At low temperatures, single nanocrystals can be also single photon emitters if blinking and spectral diffusion is conveniently reduced. In the case of nanocrystal assemblies, stimulated emission can be observed with thresholds lower than 10 μJ/cm² under nanosecond laser excitation at low temperatures, whose physical origin is attributed to single exciton recombination. Finally, the coupling of perovskite nanocrystals to the optical modes of hyperbolic metaldielectric metamaterials has been studied and demonstrated an important Purcell enhancement of the exciton radiative emission by more than a factor three for CsPbI3 and around factor two for FAPbI₃ when the distance between the emitters and HMM is 10 nm.

Synthesis of novel material often times requires novel analysis and characterization techniques. The possibility of combining sputtering (SPU) and Atomic Layer Deposition (ALD) in the same chamber, Sputtering Atomic Layer Augmented Deposition (SALAD), has produced interesting meta-dielectric nanocomposite systems that have unique optical and electronic properties, which may find novel applications [1]. Scanning Microwave Impedance Microscopy (sMIM) is a relatively novel characterization tool which permits assessment of local impedance. More recently, the utilization of microwaves in the near field regime has been an exciting topic in the field of high-resolution microscopy. We were able to demonstrate 1 nm resolution using scanning Microwave Impedance Microscopy (sMIM) where a spontaneously forming water meniscus concentrated the microwave fields in small regions [2]. Here we analyzed numerically sMIM with Finite Element Method (FEM) to investigate complex metal-dielectric structures created in a SALAD system. sMIM measurements provide information on both real and imaginary parts of the reflected microwave signal, which can be associated with the local conductivity and permittivity. Yet, these quantities can be influenced by the local topography, so extraction of the electronic contribution is a challenge. In this work, we perform tip-surface distance scans in order to gain a better understanding of the substrate response and compare with the FEM results.

The optical properties of periodic graded GaN/InGaN are studied. We have designed graded InGaN quantum well (QW) structures with the indium composition increasing then decreasing in a zigzag pattern. Through polarization doping, this naturally creates alternating p-type and n-type regions. Separate structures are designed by varying the number of repeating periods (1 to 3), while maintaining constant overall structure thicknesses. Calculation of the transition probabilities and the electron and hole wave-functions between the conduction band and the valence band reveals a complex energy structure which predicts the photoluminescence peaks for band to band transitions.

Owing to the Purcell effect, optical micro-structures can control the radiative decay of the quantum emitters in transition metal dichalcogenide (TMDC) media. However, conventional optical microstructures change the local density of optical states (LDOS) not only at the photoluminescence (PL) wavelength of the TMDC quantum emitters and but also at the pump wavelength simultaneously and thus cause an inevitable influence on the excitation conditions. We propose and experimentally demonstrate a reflective metallic metasurface for independently engineering the excitation and radiation of quantum emitters in the TMDC monolayer

The usage of the capping layer on InAs/GaAs quantum dots (QDs) to improve the optical and structural characteristics has been a common practice for decades. Especially in vertically coupled QD structures, the strain plays a significant role in determining the size and shape of the QDs. In this study, we have used GaAsN capping layer with 0% (Sample N1), 0.1% (Sample N2), 1.2% (Sample N3) and 1.8% (Sample N4) Nitride composition to analyze its effects on the emission wavelength, band structure, and the strain build-up in a ten-layer InAs/GaAs Stranksi-Krastanov (SK) QDs that are electronically coupled to six stack submonolayer (SML) QDs. Research revealed that GaAsN capping enables the growth of coupled QDs with high quality and uniformity. GaAsN capping allowed a redshift in the PL emission wavelength with 1072 nm, 1090 nm, 1129 nm, and 1184 nm for samples N1, N2, N3, and N4, respectively. In a self-assembled QD system, the relief in strain is often attributed to the redshift in the emission wavelength. However, the obtained hydrostatic compressive strain indicates that the strain for all these samples N1 to N4 inside the SK QD remains the same. In lieu, we conclude that the redshift in the PL emission wavelength is due to the reduction in the conduction band energy level in the GaAsN capping layer with respect to the GaAs layer that reduces the electron confinement in QDs. The GaAsN capping offering a strong red shift of 1184 nm with minimum strain for coupled QD system can further be used in optoelectronic device studies.

A detailed theoretical study of optical and structural properties of heterogeneously coupled Stranski-Krastanov (SK) on submonolayer (SML) QDs heterostructure with In0.18Ga0.82AsYSb1-Y capping on InAs SK QD has been done using Nextnano++ software. Variation of Sb composition was taken as 10, 15, 20 and 25%. By solving 3-D Schrodinger’s equation the biaxial and hydrostatic strain distribution, energy band diagram and PL peak observation is done. It has been noted that increasing Sb composition contributes in transition from type I to type II. This transition can be discovered in positions of probability density function for electron and holes and from energy band illustrations. The biaxial strain is responsible for energy splitting in light hole (LH) and heavy hole (HH) and its distribution in SK QD increases with increasing Sb composition. The hydrostatic strain is compressive strain in nature, which is responsible for carrier confinement in conduction band. With increasing Sb composition the magnitude of hydrostatic strain is observed as diminishing. From the ground state energy levels of electron-hole Eigen state (E1-H1) PL emission wavelength have been observed for all the four structures. It has been noted that higher the composition of Sb, higher the wavelength emission. In overall analysis it has been observed that type-II has lower compressive strain inside the QDs and higher wavelength emission. The composition selection demonstrates both type-I and type-II energy band profile which can be advantageous in many optoelectronic device properties. This theoretical study can be useful in optimization of strain coupled heterostructures for better crystalline quality.

SOD is a type of conventional doping technique where diffusion of dopant atom takes place from the liquid source to film by thermal annealing of sample. The study shows the SOD process is a cost effective, less destructive and an efficient way to dope ZnO film. We have doped ZnO films with phosphorus atom by simply annealing it in atmospheric furnace up to 600°C for 4 hrs. After in-situ annealing SOD process, sample has also been ex-situ annealed at 900°C in oxygen ambient for 10 secs. The elemental analysis of phosphorus 2p peak at 132.62 eV ensures the existence of P-O bond for doped sample which shows phosphorus replacing Zn and bonding with oxygen in to the lattice in order to make Pzn-2Vzn an acceptor complex. The doped samples showed the photoluminescence peak at 3.32eV and 3.35eV, which attributed to free electron to acceptor (FA) and acceptor-bound exciton (A₀X) energy as an evidence of acceptor doping in ZnO film. The ex-situ annealing of doped sample further improves in passivation of deep level defects of film. All sample has (002) orientation, and a compressive stress to be found in the doped sample due to phosphorus replacing Zn, are confirmed by analysis of XRD results.

Zinc magnesium oxide is a ternary compound wide bandgap semiconductor. Incorporation of Mg into ZnO helps in increasing the of p-type conductivity by affecting the background n-type nature of ZnO. This is possible because Mg incorporation in ZnO elevates the conduction band edge which in turn increases the distance between the shallow donor level and conduction band minima, resulting increase of activation energy for background donor. In this work, we report Spin-on Dopant technique to dope phosphorus in Zn_0.85Mg_0.15O lattice. The undoped ZnMgO thin film (sample A) was deposited using RF sputtering. The SOD sample (sample B) was prepared using P509 spin on dopant and kept approximate 1cm above ZnMgO film at 600°C for four hours. The doped sample was annealed at temperature 700°C (sample C) in oxygen ambient to see the high temperature annealing effect on doping. In studies of high-resolution x-ray diffraction, a dominant (002) peak was observed in sample A, B, and C at 34.173°, 34.624°, and 34.638° respectively. The shifting of (002) peak at higher angle for doped samples indicates the phosphorus doping in film. The XPS spectra of phosphorus 2p peak are appears at ~134 eV indicates the presence of P atoms as P-O bonds in ZnMgO lattice. The Donor-Acceptor pair (DAP) transition peak around 3.473eV and free Acceptor (AX°) peak around 3.588eV were found in photoluminescence spectra of sample B revels the phosphorus doping in ZnMgO.

The capping layer (CL) overgrowth process is a very mandate step to preserve the quantum dot (QD) basic parameters necessary for enhanced device performance. It is a well-known aspect that the strain fields inside QD(CL) gets altered both along [100] (perpendicular) and [001] (parallel) directions, when employed with an appropriate CL thickness and composition (%). In this study, we report on the InAs Sub-monolayer (SML) QDs capped by a tensile-strained CL: GaAs_1−xN_x (2 ML thick), to lessen the net compressive strain in the system. A 8-band k.p simulation was performed in this regard to understand the change in optical and strain properties, for varying dilute Nx contents (x): 1.5, 1.8, 2.2 and 2.5% respectively. Firstly, the hydrostatic (biaxial) strain along [001] inside QD region increases (decreases) with increasing x(%) and this supposedly should blueshift photoluminescence (PL) spectra. Secondly, the changes in band structure across the conduction and valence bands (CB, VB) gave some clear insights on the PL redshift. The CB minima in CL gets lowered, accounting for reduced carrier confinement and not due to the strain counterparts. The tensile strain nature of CL has a larger band gap bowing parameter that helps in this stronger redshift in PL energy. By this lowering of CB energy in CL, the electron eigen levels inside QD shift downwards, reducing the bandgap of the same. Finally, the simulated PL energy values at 19 K for varying N% was found to be 1.06, 1.02, 0.96, and 0.92 eV finding it suitable for laser applications.