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Nearly twenty years ago, Dunitz and Bernstein described a selection of intriguing cases of polymorphs that disappear. The inability to obtain a crystal form that has previously been prepared is indeed a frustrating and potentially serious problem for solid-state scientists. This Review discusses recent occurrences and examples of disappearing polymorphs (as well as the emergence of elusive crystal forms) to demonstrate the enduring relevance of this troublesome, but always captivating, phenomenon in solid-state research. A number of these instances have been central issues in patent litigations. This Review, therefore, also highlights the complex relationship between crystal chemistry and the law.

Now you see it, now you don't: Some of the most captivating accounts of organic solid-state chemistry in recent years concern disappearing polymorphs. This Review features notorious examples and underlines the misconceptions in understanding this phenomenon—both among scientists and in the court of law.

Gallium-nitride-based light-emitting diodes have enabled the commercialization of efficient solid-state lighting devices. Nonplanar nanomaterial architectures, such as nanowires and nanowire-based heterostructures, have the potential to significantly improve the performance of light-emitting devices through defect reduction, strain relaxation, and increased junction area. In addition, relaxation of internal strain caused by indium incorporation will facilitate pushing the emission wavelength into the red. This could eliminate inefficient phosphor conversion and enable color-tunable emission or white-light emission by combining blue, green, and red sources. Utilizing the waveguiding modes of the individual nanowires will further enhance light emission, and the properties of photonic structures formed by nanowire arrays can be implemented to improve light extraction. Recent advances in synthetic methods leading to better control over GaN and InGaN nanowire synthesis are described along with new concept devices leading to efficient white-light emission.

The drive for ever more efficient light sources is at the base of fast growing industry, producing gallium nitride light-emitting diodes at cost and efficiencies that outperform most commercial solutions. GaN nanowires and heterostructures provide a new device architecture, promising to revolutionize white and color-tunable light emitters to be used in future lightening and display technologies.

2D vertical stacking and lateral stitching growth of monolayer (ML) hexagonal transition-metal dichalcogenides are reported. The 2D heteroepitaxial manipulation of MoS₂ and WS₂ MLs is achieved by control of the 2D nucleation kinetics during the sequential vapor-phase growth. It enables the creation of hexagon-on-hexagon unit-cell stacking and hexagon-by-hexagon stitching without interlayer rotation misfits.

Atomically thin 2D layered transition metal dichalcogenides (TMDs) have been extensively studied in recent years because of their appealing electrical and optical properties. Here, the fabrication of ReS₂ field-effect transistors is reported via the encapsulation of ReS₂ nanosheets in a high-κ Al₂O₃ dielectric environment. Low-temperature transport measurements allow to observe a direct metal-to-insulator transition originating from strong electron–electron interactions. Remarkably, the photodetectors based on ReS₂ exhibit gate-tunable photoresponsivity up to 16.14 A W⁻¹ and external quantum efficiency reaching 3168%, showing a competitive device performance to those reported in graphene, MoSe₂, GaS, and GaSe-based photodetectors. This study unambiguously distinguishes ReS₂ as a new candidate for future applications in electronics and optoelectronics.

Few-layer ReS₂ is successfully synthesized via chemical vapor deposition. Top-gated FET devices, back-gated four-terminal devices, and photodetectors are built based on the as-grown high-quality materials. All of them show great device performance, which distinguishes ReS₂ a great platform for future applications in electronic and optoelectronic devices.

The synthesis and unique tunable optical properties of core/crown nanoplatelets having an inverted Type-I heterostructure are presented. Here, colloidal 2D CdS/CdSe heteronanoplatelets are grown with thickness of four monolayers using seed-mediated method. In this work, it is shown that the emission peak of the resulting CdS/CdSe heteronanoplatelets can be continuously spectrally tuned between the peak emission wavelengths of the core only CdS nanoplatelets (421 nm) and CdSe nanoplatelets (515 nm) having the same vertical thickness. In these inverted Type-I nanoplatelets, the unique continuous tunable emission is enabled by adjusting the lateral width of the CdSe crown, having a narrower bandgap, around the core CdS nanoplatelet, having a wider bandgap, as a result of the controlled lateral quantum confinement in the crown region additional to the pure vertical confinement. As a proof-of-concept demonstration, a white light generation is shown by using color conversion with these CdS/CdSe heteronanoplatelets having finely tuned thin crowns, resulting in a color rendering index of 80. The robust control of the electronic structure in such inverted Type-I heteronanoplatelets achieved by tailoring the lateral extent of the crown coating around the core template presents a new enabling pathway for bandgap engineering in solution-processed quantum wells.

Continuously tunable photoluminescence in a model platform of inverted core/crown nanoplatelets is demonstrated with a tunability range of 90 nm. The unique continuous tunable emission is achieved by controlling the lateral width of the CdSe crown around the seed CdS nanoplatelet template because of the finely tuned lateral quantum confinement in the crown region additional to the pure vertical confinement.

Protons, as one of the world's smallest ions, are able to trigger the charge effect without obvious lattice expansion inside inorganic materials, offering a unique and important test-bed for controlling their diverse functionalities. Arising from the high chemical reactivity of hydrogen (easily losing an electron) with various main group anions (easily accepting a proton), the hydric effect provides a convenient and environmentally benign route to bring about fascinating new physicochemical properties, as well as to create new inorganic structures based on the “old lattice” without dramatically destroying the pristine structure, covering most inorganic materials. Moreover, hydrogen atoms tend to bond with anions or to produce intrinsic defects, both of which are expected to inject extra electrons into lattice framework, promising advances in control of bandgap, spin behavior, and carrier concentration, which determine functionality for wide applications. In this review article, recently developed effective hydric strategies are highlighted, which include the conventional hydric reaction under high temperature or room temperature, proton irradiation or hydrogen plasma treatment, and gate-electrolyte-driven adsorption or doping. The diverse physicochemical properties brought by the hydric effect via modulation of the intrinsic electronic structure are also summarized, finding wide applications in nanoelectronics, energy applications, and catalysis.

The hydric effect demonstrates great potential to bring about fine modulation of intrinsic physical behavior, as well as to create novel hydric inorganic nanomaterials. Recently developed methods are summarized regarding the synthesis of hydric nanomaterials, their electronic-structure modulation, and their application for nanoelectronics and energy applications.

Wearable double-twisted fibrous perovskite solar cells are developed based on flexible carbon nanotube fiber electrodes, which exhibit a maximum power conversion efficiency of 3.03% and bending stability larger than 1000 cycles, and maintain 89% efficiency after 96 h in ambient conditions if sealed by a transparent polymer layer. The obtained superior performance can shed light on future self-powering e-textiles.

The effect of the presence of unreacted PbI₂ on the perovskite solar cells efficiency is reported. N,N-Dimethylformamide vapor treatment is introduced to study the influence of complete conversion to a power conversion efficiency of the device. It is discovered that the optimized morphology of the PbI₂ under layer is essential to form a dense perovskite layer preventing recombination by direct contact between TiO₂ and a hole transporting layer, and to increase the charge collection efficiency. The present findings provide an insight into the morphology and growth mechanism of perovskite layer, the correlation between the device performance, and the film deposition process.

The influence of the unreacted PbI₂ to the performance of perovskite solar cells is investigated. The optimized morphology of the PbI₂ under layer is found to be essential to form a dense perovskite layer preventing recombination by direct contact between TiO₂ and a hole transporting layer, and increase the charge collection efficiency.

A novel strategy for the controlled synthesis of 2D MoS₂/C hybrid nanosheets consisting of the alternative layer-by-layer interoverlapped single-layer MoS₂ and mesoporous carbon (m-C) is demonstrated. Such special hybrid nanosheets with a maximized MoS₂/m-C interface contact show very good performance for lithium-ion batteries in terms of high reversible capacity, excellent rate capability, and outstanding cycling stability.

Two pseudohalide thiocyanate ions (SCN⁻) have been used to replace two iodides in CH₃NH₃PbI₃, and the resulting perovskite material was used as the active material in solar cells. In accelerated stability tests, the CH₃NH₃Pb(SCN)₂I perovskite films were shown to be superior to the conventional CH₃NH₃PbI₃ films as no significant degradation was observed after the film had been exposed to air with a relative humidity of 95 % for over four hours, whereas CH₃NH₃PbI₃ films degraded in less than 1.5 hours. Solar cells based on CH₃NH₃Pb(SCN)₂I thin films exhibited an efficiency of 8.3 %, which is comparable to that of CH₃NH₃PbI₃ based cells fabricated in the same way.

The replacement of two iodides by two pseudohalide thiocyanate ions in CH₃NH₃PbI₃ results in a perovskite material with superior tolerance to moisture when used as the active material of a solar cell. For the CH₃NH₃Pb(SCN)₂I perovskite films, no significant degradation was observed after the film had been exposed to air with a relative humidity of 95 % for over four hours, whereas the CH₃NH₃PbI₃ films degraded in less than 1.5 hours.

Amorphous black phosphorus (a-BP) ultrathin films are deposited by pulsed laser deposition. a-BP field-effect trans­istors, exhibiting high carrier mobility and moderate on/off current ratio, are demonstrated. Thickness dependence of the bandgap, mobility, and on/off ratio are observed. These results offer not only a new nanoscale member in the BP family, but also a new opportunity to develop nano­electronic devices.