Quasi‐Amorphous Inorganic Thin Films: Non‐Crystalline Polar Phases

Quasi‐amorphous thin films of BaTiO₃, SrTiO₃, and BaZrO₃ are the only known examples of inorganic, non‐crystalline, polar materials. The conditions under which they are formed and the origin of their polarity set these materials apart from other classes of inorganic materials. The most important feature of the quasi‐amorphous phase is that the polarity is the result of the orientational ordering of local bonding units but without any detectable spatial periodicity. This mechanism is reminiscent of that observed in ferroelectric polymers and permits compounds that do not have polar crystalline polymorphs, such as SrTiO₃ and BaZrO₃, to form polar non‐crystalline solids. In the present report, we provide an overview of the essential features of these materials including preparation, structure, and chemical composition. The report also reviews our current level of understanding and offers some guidelines for further development and application of non‐crystalline inorganic polar materials.     

钛板表面激光熔覆锆基合金涂层的组织结构

利用激光熔覆技术在Ti板上制备了非晶复合涂层,利用X射线衍射仪、能谱仪、扫描电镜和透射电子显微镜对熔覆层和结合区的组织结构进行了表征。研究发现,熔覆层组织主要由非晶、Zr的金属间化合物、氧化物及局部纳米晶组成。基体与熔覆层结合区由Ti的柱状晶和α(Ti/Zr)固溶体所构成,保证了基体与熔覆层之间有良好的冶金结合。     

Chemical Bonding in Chalcogenides: The Concept of Multicenter Hyperbonding

The precise nature of chemical-bonding interactions in amorphous, and crystalline, chalcogenides is still unclear due to the complexity arising from the delocalization of bonding, and nonbonding, electrons. Although an increasing degree of electron delocalization for elements down a column of the periodic table is widely recognized, its influence on chemical-bonding interactions, and on consequent material properties, of chalcogenides has not previously been comprehensively understood from an atomistic point of view. Here, a chemical-bonding framework is provided for understanding the behavior of chalcogenides (and, in principle, other lone-pair materials) by studying prototypical telluride nonvolatile-memory, “phase-change” materials (PCMs), and related chalcogenide compounds, via density-functional-theory molecular-dynamics (DFT-MD) simulations. Identification of the presence of previously unconsidered multicenter “hyperbonding” (lone-pair–antibonding-orbital) interactions elucidates not only the origin of various material properties, and their contrast in magnitude between amorphous and crystalline phases, but also the very similar chemical-bonding nature between crystalline PCMs and one of the bonding subgroups (with the same bond length) found in amorphous PCMs, in marked contrast to existing viewpoints. The structure–property relationship established from this new bonding-interaction perspective will help in designing improved chalcogenide materials for diverse applications, based on a fundamental chemical-bonding point of view.     

Resonant bonding in crystalline phase-change materials

The identification of materials suitable for non-volatile phase-change memory applications is driven by the need to find materials with tailored properties for different technological applications and the desire to understand the scientific basis for their unique properties. Here, we report the observation of a distinctive and characteristic feature of phase-change materials. Measurements of the dielectric function in the energy range from 0.025 to 3 eV reveal that the optical dielectric constant is 70-200% larger for the crystalline than the amorphous phases. This difference is attributed to a significant change in bonding between the two phases. The optical dielectric constant of the amorphous phases is that expected of a covalent semiconductor, whereas that of the crystalline phases is strongly enhanced by resonant bonding effects. The quantification of these is enabled by measurements of the electronic polarizability. As this bonding in the crystalline state is a unique fingerprint for phase-change materials, a simple scheme to identify and characterize potential phase-change materials emerges.     

From Amorphous Macroporous Film to 3D Crystalline Nanorod Architecture: A New Approach to Obtain High‐Performance V2O5 Electrochromism

A 3D crystalline V₂O₅ nanorod architecture on indium tin oxide substrates is prepared by simple annealing treatment of a colloidal crystal‐assisted electrodeposited amorphous three‐dimensionally ordered macroporous film at a low temperature of 350 °C. The crystalline nanorods exhibit a low length/diameter ratio with the typical width range of 80–180 nm, length range of 190–500 nm, and thickness of 30–50 nm. Colloidal sphere‐assisted heterogeneous nucleation during electrodeposition and the anisotropic bonding of the V₂O₅‐layered structure are two important factors for the morphological changes. Because of the large surface area, short lithium ion diffusion distance, and good electrolyte penetration, the 3D crystalline V₂O₅ nanorod architecture exhibits a highly reversible Li‐ion insertion/extraction process (columbic efficiency up to 96.9%) with a five‐color‐change electrochromic performance, good transmittance modulation, and acceptable response times (8.8 s for coloration and 9.3 s for bleaching), making it a promising film electrode for electrochromic devices.     

LASER AND ELECTRON BEAM PROCESSING OF AMORPHOUS SURFACE ALLOYS ON CONVENTIONAL CRYSTALLINE METALS

During last fifteen years various superior surface characteristics including extremely high corrosion resistance and unique electrocatalytic activity have been found for novel melt-spun ribbon-shaped amorphous alloys. Preparation of those amorphous alloys as surface alloys covering bulk conventional crystalline metals has been eagerly awaited for the purpose of utilizing their superior surface characteristics. This is a review of efforts devoted to developing methods for processing amorphous surface alloys by instantaneous melting of a very restricted volume of the surface by irradiation with a CO₂ laser or electron beam and subsequent self quenching by the cold bulk substrates. Processing of a wide area by these high energy density beams requires heating the previously amorphized phase, which is easily crystallized by heating. Consequently, high energy density beam processing is most difficult among various methods for preparation of thermodynamically metastable amorphous alloys. Nevertheless, various amorphous surface alloys have been successfully prepared. The materials consisting of the amorphous surface alloys and bulk crystalline metals are quite suitable for corrosion resistant materials and electrodes for electrolysis of aqueous solutions. A comparison of CO₂ laser and electron beam processing showed the superiority of the latter to the former because of a significantly shorter processing time.     

氧化铪薄膜的制备及其性能研究

采用电子束反应蒸发金属铪、APS离子辅助反应蒸发氧化铪、RF离子辅助反应蒸发氧化铪三种方式制备了单层HfO2薄膜,对样品的光学性能、结构特性以及抗激光损伤特性进行研究,结果表明,离子辅助使氧化铪薄膜更为致密;电子束蒸发氧化铪薄膜为非晶态,离子辅助制备氧化铪薄膜为晶态。选择合适的离子辅助工艺,有利于降低薄膜的缺陷吸收,提高氧化铪薄膜的抗损伤性能。     

Revealing the Origins of Mechanically Induced Fluorescence Changes in Organic Molecular Crystals

Mechanofluorochromic molecular materials display a change in fluorescence color through mechanical stress. Complex structure–property relationships in both the crystalline and amorphous phases of these materials govern both the presence and strength of this behavior, which is usually deemed the result of a mechanically induced phase transition. However, the precise nature of the emitting species in each phase is often a matter of speculation, resulting from experimental data that are difficult to interpret, and a lack of an acceptable theoretical model capable of capturing complex environmental effects. With a combined strategy using sophisticated experimental techniques and a new theoretical approach, here the varied mechanofluorochromic behavior of a series of difluoroboron diketonates is shown to be driven by the formation of low‐energy exciton traps in the amorphous phase, with a limited number of traps giving rise to the full change in fluorescence color. The results highlight intrinsic structural links between crystalline and amorphous phases, and how these may be exploited for further development of powerful mechanofluorochromic assemblies, in line with modern crystal engineering approaches.     

离子束辅助沉积引发互不固溶系非晶相和亚稳晶相形成

利用离子束辅助沉积技术（IBAD）研究了在互不固溶的Cu-Ta和Cu-Nb系统中获得非晶相和亚稳晶体的可能性,结果表明：r(Cu)为30％时Cu含量的Cu-Ta薄膜得到了非晶相;r(Cu)为25％－35％时Cu-Nb薄膜中随辅助等离子束能量的改变出现了fcc相－非晶相转变;r(Cu）和20％时Cu-Nb薄膜中得到了bcc相,说明IBAD技术可以在互不固溶二元合金系统中制备非晶和亚稳晶相,非晶或亚稳晶的形成是由薄膜沉积过程中辅助离子束的作用引起的。     

Microscopy study of laser-induced phase change in Al-Cu film

Microscopy studies of the structural changes of the vapour-quenched Al₇₁Cu₂₉ films induced by laser irradiation are presented along with the discussion of the phase change mechanism. This leads to a high interest in a fast phase-change phenomenon between the amorphous and metastable crystalline states.     

Thermal, morphological and optical investigations of Cu(DAB)2 thin films produced by matrix-assisted pulsed laser evaporation and laser-induced forward transfer for sensor development

Many hybrid metal-organic complex materials which exhibit crystalline nature, nonlinear optical properties and chemoselective behavior generate interest as choice materials in various applications. In this paper we report results on Cu(II) 2,2′-dihydroxyazobenzene thin films deposited on silicon and quartz substrates by matrix assisted pulsed laser evaporation using a Nd:YAG laser, at 266 and 355 nm laser wavelengths. Thermal analysis, atomic force microscopy, scanning electron microscopy and spectroscopic ellipsometry were performed in order to investigate thin film properties. Micrometric pixels of the compound have been transferred on glass plates by laser-induced forward transfer for chemoselective sensor development purposes.     

Substrate and annealing temperature effects on the crystallographic and optical properties of MoO3 thin films prepared by laser assisted evaporation

MoO₃ thin Films were prepared using the assisted laser evaporation technique. Samples were grown on glass and silicon substrates at different substrates temperatures. The effect on structural and optical properties of the substrate and on annealing temperatures was evaluated. A phase transition was found around 200 °C in all samples from the amorphous to the β phase with a small percentage of α phase, and another one was found around 500 °C from the α + β to the α phase. The percentage errors between the lattice parameter a₀ of the crystallographic index card for the MoO₃ alpha phase and the indexed lattice parameters were 1.4% and 0.3% for the samples deposited on glass and silicon respectively, indicating the crystalline structure of the silicon substrate favors the formation of the MoO₃ alpha orthorhombic phase. The spectral variation of the refractive index and the absorption coefficient were theoretically determined. The amorphous samples presented a constant gap of 3.2 eV while the optical properties critically depended on the substrate and annealing temperatures.     

Thin films of semiconductors and dielectrics produced by laser evaporation

Thin films of several III–V and II–VI compounds as well as of some dielectrics have been vacuum-deposited using a focused beam of a CO₂ or ruby laser to evaporate these materials. The crystallinity, morphology and the chemical composition of the produced thin films have been examined by various analytical methods. Films produced by the ruby laser were in most cases polycrystalline and stoichiometric, while films produced by the CO₂ laser were amorphous and non-stoichiometric. Different mechanisms of evaporation leading to the observed differences in characteristics of thin films are discussed.     

Comparative study of crystallization processes in Sb2Te3 films using laser and thermal annealing techniques

Adherent and pin-hole free amorphous Sb₂Te₃ thin films have been obtained by vacuum evaporation at substrate temperatures ≤25 °C. The films have been crystallized by thermal and laser annealing, and the crystallization processes monitored as a function of annealing temperature and laser scan speed. A comparative study of topography reveals disk-shaped crystallized areas in thermal crystallization and dendrite growth in the laser induced process. The crystallized films in both cases contain a single Sb₂Te₃ phase. Activation energy of 2 eV for crystallization, determined using differential scanning calorimetery indicates good room temperature stability of the amorphous states.     

Amorphous/Crystalline Hetero‐Phase Pd Nanosheets: One‐Pot Synthesis and Highly Selective Hydrogenation Reaction

Similar to heterostructures composed of different materials, possessing unique properties due to the synergistic effect between different components, the crystal‐phase heterostructures, one variety of hetero‐phase structures, composed of different crystal phases in monometallic nanomaterials are herein developed, in order to explore crystal‐phase‐based applications. As novel hetero‐phase structures, amorphous/crystalline heterostructures are highly desired, since they often exhibit unique properties, and hold promise in various applications, but these structures have rarely been studied in noble metals. Herein, via a one‐pot wet‐chemical method, a series of amorphous/crystalline hetero‐phase Pd nanosheets is synthesized with different crystallinities for the catalytic 4‐nitrostyrene hydrogenation. The chemoselectivity and activity can be fine‐tuned by controlling the crystallinity of the as‐synthesized Pd nanosheets. This work might pave the way to preparing various hetero‐phase nanostructures for promising applications.     

Material science aspects of phase change optical recording

Chalcogenide thin films are used as the recording medium for phase change-type optical memory discs. The films are switched between amorphous and crystalline states using the heat of a focussed laser beam. Large reflectivity differences between amorphous and crystalline states are then used to store and retrieve the information. An active chalcogenide layer for this purpose should have a high optical absorption coefficient (α), and good structural and thermal stability. It should be possible to switch the chalcogenide layer between amorphous and crystalline states repeatedly within a short duration, the optical contrast should be high, and the material must have large cycling capability. Keeping the above requirements in mind, we have carried out systematic investigation of structural, optical and crystallization behaviour of thin films of various compositions of GaGeTe, Sb₂Te₃ and BiSe. These studies have shown that these materials can be good candidates for use as recording media in erasable phase-change optical recording.     

激光拉曼光谱在CVD金刚石薄膜质量表征中的应用

激光拉曼光谱是一种优异的，灵敏的，应用广泛的无损表征技术，对于评估碳物质质量及其键合状态，也是有效的手段。     

Impact of Stoichiometry on the Structure of van der Waals Layered GeTe/Sb2Te3 Superlattices Used in Interfacial Phase‐Change Memory (iPCM) Devices

Van der Waals layered GeTe/Sb₂Te₃ superlattices (SLs) have demonstrated outstanding performances for use in resistive memories in so‐called interfacial phase‐change memory (iPCM) devices. GeTe/Sb₂Te₃ SLs are made by periodically stacking ultrathin GeTe and Sb₂Te₃ crystalline layers. The mechanism of the resistance change in iPCM devices is still highly debated. Recent experimental studies on SLs grown by molecular beam epitaxy or pulsed laser deposition indicate that the local structure does not correspond to any of the previously proposed structural models. Here, a new insight is given into the complex structure of prototypical GeTe/Sb₂Te₃ SLs deposited by magnetron sputtering, which is the used industrial technique for SL growth in iPCM devices. X‐ray diffraction analysis shows that the structural quality of the SL depends critically on its stoichiometry. Moreover, high‐angle annular dark‐field‐scanning transmission electron microscopy analysis of the local atomic order in a perfectly stoichiometric SL reveals the absence of GeTe layers, and that Ge atoms intermix with Sb atoms in, for instance, Ge₂Sb₂Te₅ blocks. This result shows that an alternative structural model is required to explain the origin of the electrical contrast and the nature of the resistive switching mechanism observed in iPCM devices.     

Revealing the Simultaneous Effects of Conductivity and Amorphous Nature of Atomic‐Layer‐Deposited Double‐Anion‐Based Zinc Oxysulfide as Superior Anodes in Na‐Ion Batteries

Although sodium‐ion batteries (SIBs) are considered promising alternatives to their Li counterparts, they still suffer from challenges like slow kinetics of the sodiation process, large volume change, and inferior cycling stability. On the other hand, the presence of additional reversible conversion reactions makes the metal compounds the preferred anode materials over carbon. However, conductivity and crystallinity of such materials often play the pivotal role in this regard. To address these issues, atomic layer deposited double‐anion‐based ternary zinc oxysulfide (ZnOS) thin films as an anode material in SIBs are reported. Electrochemical studies are carried out with different O/(O+S) ratios, including O‐rich and S‐rich crystalline ZnOS along with the amorphous phase. Amorphous ZnOS with the O/(O+S) ratio of ≈0.4 delivers the most stable and considerably high specific (and volumetric) capacities of 271.9 (≈1315.6 mAh cm^?3) and 173.1 mAh g^?1 (≈837.7 mAh cm^?3) at the current densities of 500 and 1000 mA g^?1, respectively. A dominant capacitive‐controlled contribution of the amorphous ZnOS anode indicates faster electrochemical reaction kinetics. An electrochemical reaction mechanism is also proposed via X‐ray photoelectron spectroscopy analyses. A comparison of the cycling stability further establishes the advantage of this double‐anion‐based material over pristine ZnO and ZnS anodes.