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Highlights Confirmation of 2-electron model for anodic dissolution of Ge in H2O2 Etching mechanism for Ge is significantly different compared to silicon Etch processing techniques cannot be transferred between silicon and germanium Addition of alcohol to H2O2 and agitation improves specularity and etch quality

Mg shows great potential as a metal hydride for switchable optical response and hydrogen detection due to its ability to stably incorporate significant amounts of hydrogen into its lattice. However, this thermodynamic stability makes hydrogen removal difficult. By alloying Mg with secondary elements, the hydrogenation kinetics can be increased. Here, we report the dynamic optical, loading, and stress properties of three Mg alloy systems (Mg-Al, Mg-Ti, and Mg-Ni) and present several novel phenomena and three distinct device designs that can be achieved with them. We find that these materials all have large deviations in refractive index when exposed to H2 gas, with a wide range of potential properties in the hydride state. The magnitude and sign of the optical property change for each of the alloys are similar, but the differences have dramatic effects on device design. We show that Mg-Ti alloys perform well as both switchable windows and broadband switchable light absorbers, where Mg0.87Ti0.13 and Mg0.85Ti0.15 can achieve a 40% transmission change as a switchable window and a 55% absorption change as a switchable solar absorber. We also show how different alloys can be used for dynamically tunable color filters, where both the reflected and transmitted colors depend on the hydrogenation state. We demonstrate how small changes in the alloy composition (e.g., with Mg-Ni) can lead to dramatically different color responses upon hydrogenation (red-shifting vs blue-shifting of the resonance). Our results establish the potential for these Mg alloys in a variety of applications relating to hydrogen storage, detection, and optical devices, which are necessary for a future hydrogen economy.

While there are several bottlenecks in hybrid organic–inorganic perovskite (HOIP) solar cell production steps, including composition screening, fabrication, material stability, and device performance, machine learning approaches have begun to tackle each of these issues in recent years. Different algorithms have successfully been adopted to solve the unique problems at each step of HOIP development. Specifically, high-throughput experimentation produces vast amount of training data required to effectively implement machine learning methods. Here, we present an overview of machine learning models, including linear regression, neural networks, deep learning, and statistical forecasting. Experimental examples from the literature, where machine learning is applied to HOIP composition screening, thin film fabrication, thin film characterization, and full device testing, are discussed. These paradigms give insights into the future of HOIP solar cell research. As databases expand and computational power improves, increasingly accurate predictions of the HOIP behavior are becoming possible. [ABSTRACT FROM AUTHOR]

Structural color filters have recently blossomed as a superior alternative to organic dyes or chemical pigments owing to their remarkable durability and compactness. With appropriate design, nanostructure‐induced photonic or plasmonic resonance modes can give rise to either enhancement in transmission or suppression in the reflection within specific wavelength ranges in the optical regime, generating distinctive colors. However, the static optical properties due to fixed structural geometry and size after fabrication hinder their deployment in many cutting‐edge technologies requiring adaptive complexion changes. Here, a multilayer thin film‐based color filter incorporating Mg and MgO, earth‐abundant and biodegradable materials, is devised. The devices display vivid hues spanning a broad gamut via the control of the film thickness. They also exhibit minimal color changes with varying angle views up to 40°–50°. Moreover, the tones fade away instantly upon immersion in water and then progressively transition to a different hue with the complete removal of the Mg‐containing layers, realizing transient color responses. This approach holds great promise for alternative pixels with irreversible color‐change capability as well as zero‐power consumption and low cost, while making use of biodegradable materials. [ABSTRACT FROM AUTHOR]

Metal halide perovskites (MHP) can be made more stable through the addition of small amounts of cesium. Despite the improvement, these multication absorbers still display strong environmental sensitivity to any combination of factors, including water, oxygen, bias, temperature, and light. Here, the relationship is elucidated between light absorption, charge carrier radiative recombination, and relative humidity (rH) for the Cs0.05FA0.79MA0.16Pb(I0.83Br0.17)3 composition, revealing partially reversible reductions in the extinction coefficient and fully reversible 25× enhancements in absolute light emission registered across the same humidity cycles up to 70% rH. With in situ excitation wavelength‐dependent measurements, irreversible changes are identified in the perovskite after a single cycle of humidity‐dependent photoluminescence (PL) performed with 450 nm excitation. The in situ measurement platform can be extended to test the effect of other stressors on thin films' optical behavior. [ABSTRACT FROM AUTHOR]

Metallic nanostructures and thin films are fundamental building blocks for next‐generation nanophotonics. Yet, the fixed permittivity of pure metals often imposes limitations on the materials employed and/or on device performance. Alternatively, metallic mixtures, or alloys, represent a promising pathway to tailor the optical and electrical properties of devices, enabling further control of the electromagnetic spectrum. In this Review, a survey of recent advances in photonics and plasmonics achieved using metal alloys is presented. An overview of the primary fabrication methods to obtain subwavelength alloyed nanostructures is provided, followed by an in‐depth analysis of experimental and theoretical studies of their optical properties, including their correlation with band structure. The broad landscape of optical devices that can benefit from metallic materials with engineered permittivity is also discussed, spanning from superabsorbers and hydrogen sensors to photovoltaics and hot electron devices. This Review concludes with an outlook of potential research directions that would benefit from the on demand optical properties of metallic mixtures, leading to new optoelectronic materials and device opportunities. [ABSTRACT FROM AUTHOR]

Perovskite semiconductors are an exciting class of materials due to their promising performance outputs in photovoltaic devices. To boost their efficiency further, researchers introduce additives during sample synthesis, such as KI. However, it is not well understood how KI changes the material and, often, leaves precipitants. To fully resolve the role of KI, multiple microscopy techniques are applied and the electrical and chemical behavior of a Reference (untreated) and a KI‐treated perovskite are compared. Upon correlation between electrical and chemical nanoimaging techniques, it is discovered that these local properties are linked to the macroscopic voltage enhancement of the KI‐treated perovskite. The heterogeneity revealed in both the local electrical and chemical responses indicates that the additive partially migrates to the surface, yet surprisingly does not deteriorate the performance locally, rather, the voltage response homogeneously increases. The research presented within provides a diagnostic methodology, which connects the nanoscale electrical and chemical properties of materials, relevant to other perovskites, including multication and Pb‐free alternatives. [ABSTRACT FROM AUTHOR]

Metal halide perovskites exhibit optimal properties for optoelectronic devices, ranging from photovoltaics to light‐emitting diodes, utilizing simple fabrication routes that produce impressive electrical and optical tunability. As perovskite technologies continue to mature, an understanding of their fundamental properties at length scales relevant to their morphology is critical. In this review, an overview is presented of the key insights into perovskite material properties provided by measurement methods based on the atomic force microscopy (AFM). Specifically, the manner in which AFM‐based techniques supply valuable information regarding electrical and chemical heterogeneity, ferroelectricity and ferroelasticity, surface passivation and chemical modification, ionic migration, and material/device stability is discussed. Continued advances in perovskite materials will require multimodal approaches and machine learning, where the output of these scanning probe measurements is combined with high spatial resolution structural and chemical information to provide a complete nanoscale description of materials behavior and device performance. [ABSTRACT FROM AUTHOR]

Abstract: Surface plasmon polaritons (SPPs) enable the deep subwavelength confinement of an electromagnetic field, which can be used in optical devices ranging from sensors to nanoscale lasers. However, the limited number of metals that satisfy the required boundary conditions for SPP propagation in a metal/dielectric interface severely limits its occurrence in the visible range of the electromagnetic spectrum. We introduce the strategy of engineering the band structure of metallic materials by alloying. We experimentally and theoretically establish the control of the dispersion relation in Ag–Au alloys by varying the film chemical composition. Through X‐ray photoelectron spectroscopy (XPS) measurements and partial density‐of‐states calculations we deconvolute the d band contribution of the density‐of‐states from the valence band spectrum, showing that the shift in energy of the d band follows the surface plasmon resonance change of the alloy. Our density functional theory calculations of the alloys band structure predict the same variation of the threshold of the interband transition, which is in very good agreement with our optical and XPS experiments. By elucidating the correlation between the optical behavior and band structure of alloys, we anticipate the fine control of the optical properties of metallic materials beyond pure metals. [ABSTRACT FROM AUTHOR]

Abstract: Optoelectronic materials are the backbone of today's high‐tech industry. To customize their response, one can directly modify the atomic arrangement, chemical composition, lattice strain, or doping of the semiconductor. However, these processes frequently cause undesirable effects resulting from induced defects. Here, a novel concept is demonstrated to actively tune the optoelectronic response of a material through tailored photon recycling. Without altering the material's intrinsic structure, doping, or temperature, the reabsorption of emitted photons within GaAs is modulated to control its carrier density. This approach is used to create a diode that can change its emission wavelength, a solar cell with improved open‐circuit voltage, and an actively controlled, gate‐free current modulator. These results represent a new platform to enable materials with tailored optoelectronic response based on photonic manipulation rather than semiconductor engineering. [ABSTRACT FROM AUTHOR]

This study demonstrates a technique for facile encapsulation and adhesion of micro- and nanoobjects on arbitrary substrates, stencils, and microstructured surfaces by ultrathin graphene oxide membranes via a simple drop casting of graphene oxide solution. A self-assembled encapsulating membrane forms during the drying process at the liquid-air and liquid-solid interfaces and consists of a water-permeable quasi-2D network of overlapping graphene oxide flakes. Upon drying and interlocking between the flakes, the encapsulating coating around the object becomes mechanically robust, chemically protective, and yet highly transparent to electrons and photons in a wide energy range, enabling microscopic and spectroscopic access to encapsulated objects. The characteristic encapsulation scenarios have been demonstrated on a set of representative inorganic and organic micro- and nanoobjects and microstructured surfaces. Different coating regimes can be achieved by controlling the pH of the supporting solution, and the hydrophobicity and morphology of interfaces. Several specific phenomena, such as compression of encased objects by contracting membranes as well as hierarchical encapsulations, have been observed. Finally, electron/optical microscopy, analysis of encapsulated objects, and the effect of membrane on the image contrast formation and signal attenuation are discussed. [ABSTRACT FROM AUTHOR]

A novel imaging platform to determine the open‐circuit voltage of solar cells with nanoscale spatial resolution is presented. Here, a variant of illuminated Kelvin probe force microscopy can be implemented to quantify local variations in the voltage of different solar cells. The new metrology can be applied to any optoelectronic device, and works in ambient environments. [ABSTRACT FROM AUTHOR]

The atomistic mechanism for lithiation/delithiation in all-solid-state batteries is still an open question, and the ‘holy grail’ to engineer devices with extended lifetime. Here, by combining real-time scanning electron microscopy in ultra-high vacuum with electrochemical cycling, we quantify the dynamic degradation of Al anodes in Li-ion all-solid-state batteries, a promising alternative for ultra lightweight devices. We find that AlLi alloy mounds are formed on the top surface of the Al anode and that degradation of battery capacity occurs because of Li trapped in them. Our approach establishes a new platform for probing the real-time degradation of electrodes, and can be expanded to other complex systems, allowing for high throughput characterization of batteries with nanoscale resolution. [ABSTRACT FROM AUTHOR]

We have fabricated an In0.52Al0.48As solar cell lattice-matched to InP with efficiency higher than 14% and maximum external quantum efficiency equal to 81%. High quality, dislocation-free InxAl1-xAs alloyed layers were used to fabricate the single junction solar cell. Photoluminescence of InxAl1-xAs showed good material quality and lifetime of over 200 ps. A high band gap In0.35Al0.65As window was used to increase light absorption within the p-n absorber layer and improve cell efficiency, despite strain. The InAlAs top cell reported here is a key building block for an InP-based three junction high efficiency solar cell consisting of InAlAs/InGaAsP/InGaAs lattice-matched to the substrate. [ABSTRACT FROM AUTHOR]

The surface energy and growth kinetics during Ge deposition on Si(001) were modified by growing the films in a phosphine environment. Islands were formed under a H2 flux as well as in a PH3/H2 atmosphere, but the morphologies were different. The presence of PH3 not only affects the island shape and size but also the composition profile. The dramatical inhibition of Ge/Si intermixing during growth leads to islands richer in Ge compared to undoped islands. [ABSTRACT FROM AUTHOR]

Hafnium silicide islanding occurs spontaneously when metallic Hf is deposited on a Si(001) surface and subsequently annealed at 750 °C. Different coverages were investigated by scanning tunneling microscopy (STM) in order to verify distinct stages of island formation. Small islands occurred for 0.26 ML of deposited Hf, coalescing into flat top islands for longer deposition times. Atomic resolution STM and nanobeam electron diffraction allowed the determination of the mature islands top face structure, identified as the (061) surface of the HfSi2 in the C49 phase. Finally, scanning tunneling spectroscopy was carried out over the smallest islands, demonstrating their metallic behavior. [ABSTRACT FROM AUTHOR]

Metal Halide Perovskites: Imaging Metal Halide Perovskites Material and Properties at the Nanoscale (Adv. Keywords: artificial intelligence; atomic force microscopy; functional imaging; halide perovskites; nanoscale properties EN artificial intelligence atomic force microscopy functional imaging halide perovskites nanoscale properties 1 1 1 07/16/20 20200714 NES 200714 As halide perovskite technologies continue to mature, an understanding of their fundamental properties at length scales relevant to their morphology is critical. Artificial intelligence, atomic force microscopy, functional imaging, halide perovskites, nanoscale properties. [Extracted from the article]

Patterned planar silicon (Si) semiconductor structures able to conform around 3D surfaces are promising candidates for wearable devices from solar cells to displays. Despite the known anisotropic material properties of crystalline semiconductors, prior evaluations have assumed isotropic stretchable mechanical behavior. The effect of structural anisotropy on the mechanical stretching behavior of {100} single crystalline Si planes is demonstrated through models and experiments. 3D finite element analysis results show serpentines fabricated and strained loaded on the dense {111} family of planes will have maximum von Mises peak stresses that are 20% higher than those fabricated in the <100> direction on the {100} family of planes. A fabrication process is presented to release in‐plane Si serpentine test structures from (100) silicon‐on‐insulator wafers aligned parallel to the <110> and <100> crystallographic orientations. The released test structures can accommodate strains to 84%. Raman spectroscopy is used to characterize anisotropic effects on the internal stresses of Si serpentines. Raman measurements confirm two different maximum stress locations on <110> and <100> Si serpentines, resulting in two different break locations. All results show anisotropy plays a critical role in the mechanical behavior of stretchable Si serpentines and must be considered during design of stretchable semiconductor structures. [ABSTRACT FROM AUTHOR]

Andrei Kolmakov and co‐workers demonstrate a facile dropcasting technique for encapsulation and adhesion of micro‐ and nanoobjects on arbitrary substrates, stencils, and microstructured surfaces by ultrathin graphene oxide membranes in article 1600734. Such membranes are mechanically robust, chemically protective, and yet highly transparent to electrons and photons. Thus, the membranes enable electron/optical microscopy and spectroscopy to be performed on encapsulated objects. [ABSTRACT FROM AUTHOR]

An approach for an all lattice-matched multijunction solar cell optimized design is presented with 5.807 Å lattice constant, together with a detailed analysis of its performance by means of full device modeling. The simulations show that a (1.93 eV)In0.37Al0.63As/(1.39 eV)In0.38Ga0.62As0.57P0.43/(0.94 eV)In0.38Ga0.62As 3-junction solar cell can achieve efficiencies >51% under 100-suns illumination (with Voc = 3.34 V). As a key proof of concept, an equivalent 3-junction solar cell lattice-matched to InP was fabricated and tested. The independently connected single junction solar cells were also tested in a spectrum splitting configuration, showing similar performance to a monolithic tandem device, with Voc = 1.8 V. [ABSTRACT FROM AUTHOR]