Crystalline Transition Research Articles

Phase transitions of liquid crystal confined in electrospun polymer nanofibres

Phase behaviour as well as phase transitions of $$4'$$ -pentyl-4-biphenylcarbonitrile (5CB) liquid crystal (LC) confined in an amorphous polymer matrix of electrospun nanofibres were investigated. The nanofibres were fabricated from simple monoaxial electrospinning using 5CB/polymer mixtures as well as coaxial electrospinning, where the polymer solution and neat 5CB constituted the shell- and core-forming fluids, respectively. The 5CB was found to be miscible with polystyrene (PS) and poly(4-vinyl pyridine) (P4VP). This was evident from the sharp drop in the glass transition temperature ( $$T_{\mathrm{g}}$$ ) of PS and P4VP in their mixtures with 5CB. Hence, the phase transitions of 5CB were completely suppressed in its mixture with PS and P4VP in electrospun nanofibres as ascertained from DSC and polarized optical microscopy measurements. However, the electrospun nanofibres composed of poly(vinyl pyrrolidone) (PVP) and 5CB showed phase-separated morphology. The phase-separated morphology was unambiguously characterized using SEM and TEM measurements. Furthermore, the phase separation resulted in 5CB exhibiting its liquid crystalline characteristics. However, the radial constraint of the nanofibres led to the formation of small-sized 5CB domains with limited spatial connectivity, which resulted in deviation from the known phase-transition characteristics of 5CB. It was also observed that the inherent orientation of the nanofibres favours the nematic to crystalline transition in the blend nanofibres. The present study gives new insight and understanding about phase behaviour of LC in electrospun polymer fibre and has technological relevance for the design of LC-based flexible fibrous components with tunable optical, thermal and dielectric properties.

Citrate-mediated synthesis of highly crystalline transition metal hexacyanoferrates and their Na ion storage properties

In this work, highly crystalline transition-metal hexacyanoferrates (MHCFs) have been synthesized in the presence of citrate, which is found not only to mediate the precipitation reaction rate but also prevent the MHCF crystals from aggregation. This method is applicable to a wide range of MHCFs with different transition metals (e.g., Co, Ni, Mn and Zn). The highly crystalline MHCFs with little Fe(CN)6 vacancies and rich alkaline ions exhibited significantly improved electrochemical performance for Na ion storage in both aqueous and organic electrolytes. The effect of transition metals in MHCFs on the Na ion storage performance has been investigated in an aqueous electrolyte. Results showed that the CoHCF nanocube sample displayed the highest electrochemical performance than other MHCFs samples synthesized in this work. This CoHCF sample exhibits a specific capacity of 85 mAh g−1 at 1 A g−1. The CoHCF sample also displays a good cycling stability with capacity retentions of 92% after 12,000 cycles in aqueous electrolyte (0.5 M Na2SO4) and 78% after 10,000 cycles in organic electrolyte (NaClO4 in PC + EC with 5% FEC).

Reconfigurable Reflective Colors in Holographically Patterned Liquid Crystal Gels

Thin film holographic gratings have attracted significant attention due the tailorability of their photonic properties. The ability of holography to create materials with unique photonic properties is particularly promising when combined with stimuli-responsive liquid crystals. Holographic polymer-dispersed liquid crystals (H-PDLCs) are an example of such a system where nonuniform irradiation is used to template alternating polymer-rich and polymer-poor regions with a periodicity related to the illumination wavelength. These systems typically use nonmesogenic monomers at a concentration range where a significant volume fraction of the final system is polymer. Here, we present a reconfigurable material that was generated by holographically polymer stabilizing liquid crystals (H-PSLCs) using liquid crystalline monomers, thereby enabling both alternating regions to be liquid crystalline. The “stabilizing effect” of the liquid crystalline polymer leads to a material with regions of periodic transition temperatures. As written, the polymer-rich regions have a higher isotropic transition temperature than the polymer-poor regions. Upon heating these samples beyond the isotropic temperature of the polymer-poor regions, a vivid wavelength specific reflection appears due to the refractive index mismatch between the ordered liquid crystalline polymer-rich regions and the disordered isotropic polymer-poor regions. When these materials are further heated to the isotropic temperature of the polymer-rich regions, the samples again return to a transparent state because the refractive index modulation between the alternating isotropic regions is negligible. By tailoring the liquid crystalline isotropic transition temperature to be near-room temperature, modest AC-fields can be used as an Ohmic heat source to drive the thermal color change.

Enhanced Electrochromic Properties by Improvement of Crystallinity for Sputtered WO3 Film

Tungsten oxide (WO3) is widely used as a functional material for “smart windows” due to its excellent electrochromic properties, however it is difficult to overcome the conflict between its optical modulation and cyclic stability. In this work, WO3 thin films with different crystal structures were prepared by DC reactive magnetron sputtering method. The effects of substrate temperature on the structure, composition, and electrochromic properties of WO3 films were investigated. The results show that the crystallinity of the WO3 film increases with increasing deposition temperature, indicating that temperature plays an important role in controlling the structure of the WO3 film. For WO3 thin films formed at a substrate temperature of 573 K, the film is in an amorphous state to a crystalline transition state. From X-ray diffraction (XRD) analysis, the thin film showed a weak WO3 crystallization peak, which was in the composite structure of amorphous and nanocrystalline. Which has the best electrochromic properties, with modulation amplitude of 73.1% and bleached state with a coloration efficiency of 42.9 cm2/C at a wavelength of 550 nm. Even after 1500 cycles, the optical modulation still contains 65.4%, delivering the best cyclic stability.

ZnO–PVA Polymer Matrix with Transition Metals Oxide Nano-fillers for High Dielectric Mediums

Polymer-based nanocomposites are considered promising candidates for flexible modern electronic devices. ZnO–PVA polymer nanocomposites have gained recent research interest due to excellent optical and dielectric properties. Transition metals (Cr, Mn, Fe, Ni, Cu) oxide nanoparticles incorporated ZnO–PVA composite matrix are investigated in this work for a comprehensive study of its structure, morphology, optical and dielectric parameters. Crystallographic information revealed the presence of crystalline zinc oxide and transition metals oxides reflections in specific orientations in the PVA matrix. The inter-particle distance and interfacial interaction of transition metal oxide nanoparticles in host polymer are observed as important factors for improved dipole polarization. The nanoparticles of metal oxides in matrix induce the enhanced negative charge carriers and exhibit robust interactions with positive carriers of the polymer surface. A sharp and substantial increment in real dielectric permittivity from 74 for ZnO–PVA to maximum 350 for ZnO–PVA–Fe2O3 composition was observed by the addition of transition metal oxides in the PVA matrix with same loss factor value of 0.1.

Structure and strain tunings of topological anomalous Hall effect in cubic noncollinear antiferromagnet Mn3Pt epitaxial films

Antiferromagnets (AFMs) with chiral noncollinear spin structure have attracted great attention in recent years. However, the existing research has mainly focused on hexagonal chiral AFMs, such as Mn3Sn, Mn3Ga, Mn3Ge with low crystalline symmetry. Here, we present our systematical study for the face-centered cubic noncollinear antiferromagnetic Mn3Pt. By varying the alloy composition ( x ), we have successfully fabricated antiferromagnetic Mn1− x Pt x epitaxial films on MgO substrates and have observed a crystalline structure transition from L 10 MnPt to L 12 Mn3Pt. The Mn3Pt exhibits a large anomalous Hall effect, which is in the same order of magnitude as those of ferromagnetic materials. Moreover, a large thickness-evolved strain effect is revealed in Mn3Pt films by X-ray diffraction (XRD) analysis based on the Scherrer method. Our work explores Mn3Pt as a promising candidate for topological antiferromagnetic spintronics.

Molecular-Scale Understanding of Charge Storage Mechanisms in Positive Electrode Materials for Rechargeable Aluminum Metal Batteries

Rechargeable aluminum metal batteries are an emerging energy storage technology with great promise: aluminum has among the highest capacities of common metal electrodes and is low cost, earth abundant, environmentally friendly, and inherently safe. Despite these opportunities, their technological development has been hindered due to fundamental challenges associated with aluminum electrochemistry. Few electrolytes enable the reversible electrodeposition of aluminum metal at room temperature, while few positive electrode materials have been demonstrated that exhibit high energy density and cycle life in those electrolytes. Here, recent progress will be discussed in the development and characterization of positive electrode materials for rechargeable aluminum metal batteries, including graphites, organic materials, sulfur, and crystalline transition metal compounds. Molecular-scale understanding of their charge storage mechanisms will be elucidated, revealed by a combination of electrochemical, spectroscopic, diffraction, imaging, and theoretical methods, such as multi-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, in operando X-ray diffraction (XRD), electron microscopy, and density functional theory (DFT). The diversity of electrochemical charge storage mechanisms possible for aluminum battery chemistries will be highlighted. Overall, the results are aimed at developing next-generation rechargeable aluminum metal batteries for diverse energy storage applications.

Fabrication of Nanoporous Metals through Thermal Decomposition of Transition Metal Dichalcogenides

Nanoporous metals (np-M) owing to their high surface area-to-volume ratio and electrical/thermal conductivity, have emerged as promising materials for catalysis. There exists a group of processing methodologies by which np-M are formed through a top-down nanostructure evolution driven by the selective removal of a sacrificial component, all of which are a variation of dealloying. The driving force for the traditional electrochemical/chemical dealloying is difference of redox potentials and dissolution tendencies amongst the constituent metals. This limits the process to few noble metals e.g. Pt, Pd, Au etc. More recently liquid metal dealloying and vapor phase dealloying have shown success in mimicking the process of traditional dealloying based on difference of affinities of thermal dissolution in a liquid metal, and vapor pressures of the constituent metals, respectively. Nanoporosity evolution through current dealloying methodologies, however, is governed by strict requirements including sufficient separation in “reactivity” of the participating components and a homogeneous solid solution precursor alloy. This limits the viable alloy systems that may be used and the range of np-M's that may be formed with reasonable porosity length scales. Here, we report thermal decomposition of crystalline transition metal dichalcogenides (TMDs) as a new processing methodology for np-M formation, adding to the spectrum of dealloying protocols. We demonstrate application of this process to the formation of a broader class of np-M including W, Re, Mo, and Ta with feature sizes below 100 nm. The presented facile thermal treatment of TMDs offers a new methodology for the evolution of nanoporosity in a broad range of metals.

Elucidating Aluminum-Ion Intercalation and Its Effects on the Local Crystalline Framework Using Solid-State NMR Spectroscopy

Rechargeable multivalent-ion batteries are safe and low-cost alternatives to traditional lithium-ion batteries. However, higher Coulombic charge density of the multivalent ions results into sluggish diffusion of their respective cations within crystalline transition metal compounds. The Chevrel phases Mo6X8, where X is a chalcogen atom (e.g., S or Se) are among the few materials known to reversibly and electrochemically intercalate both divalent (e.g. Mg2+, Zn 2+, etc.) and trivalent (Al3+) ions. Thus, understanding the ion intercalation and charge transfer mechanisms within these unique compounds may provide insights into the molecular-level design of new intercalation electrodes for rechargeable multivalent ion batteries. Here, we use multi-nuclear solid-state magic-angle-spinning (MAS) NMR spectroscopy to reveal the ion intercalation and charge transfer mechanisms of zinc and aluminum intercalation in Chevrel electrodes Mo6S8 and Mo6Se8. Ex situ single pulse solid-state 27Al MAS NMR measurements were performed on the thio-chevrel electrodes at different states-of-charge to observe changes in the environments and to quantify the local populations of aluminum ions within the host crystal structure, revealing both intercalated ions and additional aluminum species associated with surface layers and/or decomposition products. Al-Mo6Se8 batteries were made for the first time and their electrochemical properties were compared to the thio-chevrel Mo6S8. Ex situ 77Se and 95Mo measurements on the seleno-chevrel Mo6Se8 at charged and discharged states revealed that the electrons are transferred to the anionic chalcogen framework the electron charge transfer mechanism and its effects on the local crystalline framework environments upon aluminum ion intercalation. These results were then generalized for zinc intercalation into chevrel phase electrodes.Overall, the solid-state NMR measurements combined with the electrochemical measurements yield molecular-level insights into the coupled aluminum ion intercalation and electron charge transfer processes that occur in a model crystalline transition metal electrode. Unique reversible anionic redox reactions were observed for multivalent ion intercalation into chevrel, which is very different that typically observed for lithium-ion intercalation into transition metal oxides, where the electrons are stored into the d-orbitals of the transition metal. The results suggest materials design principles aimed at designing multivalent-ion intercalation electrodes with improved electrochemical properties. Figure 1

Polymorphic Transition of Pre-oriented Polybutene-1 under Tensile Deformation: In Situ FTIR Study

Deformation-induced phase transition of Form II to Form I in polybutene-1 (PB-1) has been investigated by time-resolved Fourier transform infrared (FTIR) spectroscopy over a wide temperature range from 25 °C to 105 °C. The initial film sample containing orientated lamellae is prepared by pre-stretching of PB-1 melt followed by solidification. This is to realize a homogeneity of subsequent deformation at the mesoscale of lamellar stacks by avoiding large-scale spherulites. The deformation induced phase transition is recognized to occur with two stages: first, Form II undergoes the lamellar fragmentation, slipping or local melting after yielding to activate its transition to Form I, which may be realized by releasing the restrictions on chains translational movements in crystalline phase; second, the phase transition proceeds with a continuous dissipation of external work and determines the tensile mechanical response of film. To quantify the relationship between crystalline transition of Form II to Form I and external tensile field, a simple kinetic equation is well established based on FTIR measurement. The equation can describe not only the dependence of crystal transitional degree on applied specific work, but also the retardation effect of elevating temperature on phase transition.

From complex to simple : hierarchical free-energy landscape renormalized in deep neural networks

We develop a statistical mechanical approach based on the replica method to study the design space of deep and wide neural networks constrained to meet a large number of training data. Specifically, we analyze the configuration space of the synaptic weights and neurons in the hidden layers in a simple feed-forward perceptron network for two scenarios: a setting with random inputs/outputs and a teacher-student setting. By increasing the strength of constraints,~i.e. increasing the number of training data, successive 2nd order glass transition (random inputs/outputs) or 2nd order crystalline transition (teacher-student setting) take place layer-by-layer starting next to the inputs/outputs boundaries going deeper into the bulk with the thickness of the solid phase growing logarithmically with the data size. This implies the typical storage capacity of the network grows exponentially fast with the depth. In a deep enough network, the central part remains in the liquid phase. We argue that in systems of finite width N, the weak bias field can remain in the center and plays the role of a symmetry-breaking field that connects the opposite sides of the system. The successive glass transitions bring about a hierarchical free-energy landscape with ultrametricity, which evolves in space: it is most complex close to the boundaries but becomes renormalized into progressively simpler ones in deeper layers. These observations provide clues to understand why deep neural networks operate efficiently. Finally, we present some numerical simulations of learning which reveal spatially heterogeneous glassy dynamics truncated by a finite width N effect.

Effects of Nanofillers on the Hydrolytic Degradation of Polyesters.

Biopolymer matrices reinforced with nanoparticles or nanofillers have received a great deal of attention over the past decades due to their various roles such as the augmentation of thermal, electrical, mechanical, and surface properties in tissue engineering, drug delivery, and implantation. Understanding the degradation kinetics of these polyesters is very important to successful applications of them. Hydrolysis is a widely agreed mechanism for the polyester degradation. According to this mechanism, hydrolytic degradation of these polyesters can be affected by the autocatalytic action of carboxyl groups as well as other factors such as hydrophilicity, crystallinity, and glass transition temperature. In this article, the effects of nanofillers on the autocatalytic action of carboxyl groups and the foregoing factors are examined. A particular attention is paid to carbon nanotubes (CNTs), which are a favorite candidate in a wide range of applications due to their unique thermal and electrical properties. Contradictory degradation results with CNTs are reported and analyzed. Finally, a future research perspective on these polyesters is discussed. Impact statement Nanoparticles have attracted a great deal of attention over the past decades due to their various roles, such as the augmentation of thermal, electrical, mechanical, and surface properties in tissue engineering, drug delivery, and implantation. In this article, the effects of nanoparticles or nanofillers* on the degradation behavior of the polyester matrices (tissue scaffolds, drug delivery devices) are examined with a focus on the autocatalytic action of carboxyl groups, and the factors such as hydrophilicity, crystallinity, and glass transition temperature. Among many nanofillers, carbon nanotubes (CNTs), which are a favorite candidate in a wide range of applications, are found to produce some contradictory results on the degradation of the polyester matrix with CNTs incorporated, and the underlying reason for this finding is discussed for the first time in this article.

In-situ uniaxial drawing of poly-L-lactic acid (PLLA): Following the crystalline morphology development using time-resolved SAXS/WAXS

Simultaneous synchrotron small- and wide-angle X-ray scattering (SAXS/WAXS) was used to follow the crystalline morphology evolution of poly-L- lactic acid (PLLA) during uniaxial deformation at various draw temperatures (Td). The mechanical behaviour of PLLA, was found to be strongly dependent on Td. 2D SAXS/WAXS data taken during the draw showed that at low Tds cavitation and voiding occurred and the initial crystallites underwent ‘overdrawing’ where they slip and are partially destroyed. SEM confirmed that surface voiding and cavitation had occurred at Td = 60 and 65 °C but was absent at higher Tds. During the draw, no long-range macromolecular lamellar structure was seen in the SAXS, but small crystallites of the disordered α′ crystal form of PLLA were observed in the WAXS at all Tds. The PLLA samples were then step annealed in a second processing stage (post-draw) to develop the oriented crystalline lamellar structure and increase the amount of the stable α crystalline form. SAXS/WAXS data showed that a highly oriented lamellar stack macrostructure developed on annealing, with increased crystallite size and crystallinity at all Tds. Furthermore, step annealing drove the crystalline transition in all samples from the disordered α′ crystal form to the stable α crystal form. Therefore, varying pre- and post-processing parameters can significantly influence the mechanical properties, orientation, crystalline morphology and crystal phase transition of the final PLLA material.

Template-free Synthesis of Mesoporous and Crystalline Transition Metal Oxide Nanoplates with Abundant Surface Defects

Summary Mesoporous transition metal oxides with crystalline walls show promising applications in catalysis and energy storage. Syntheses of those crystalline mesoporous oxides, however, currently remain as an unmet challenge because of the large volume shrinkage during crystallization in a sol-gel process. Here, we demonstrate a facile and general template-free method to prepare six mesoporous transition metal oxides, e.g., CuO, CoO, and spinel MCo2O4 (M = Co, Cu, Mn, Zn), with highly crystalline walls and continuous mesopores in the forms of two-dimensional nanoplates or nanosheets. The method is based on the thermal crystalline transformation from basic carbonates to mesoporous metal oxides. The crystal interconversion not only endows the crystallinity and mesoporosity of oxides but also generates abundant surface-step defects that show remarkable enhancement of the catalytic activity of porous oxides. Our method likely provides an alternative paradigm for cost-effective and universal synthesis of crystalline mesoporous oxides beyond the traditional templating methods.

Highly disordered cobalt oxide nanostructure induced by sulfur incorporation for efficient overall water splitting

Exploitation of cost-efficient active electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) plays a significant role for scalable electricity-to-hydrogen energy conversion. Crystalline transition metal oxides as the promising non-noble catalysts, however, are often suffering from the large excess overpotential and unsatisfactory performance. To boost their intrinsic catalytic property, we report here an incorporation of electronegative sulfur into crystalline cobalt oxide (S-CoO x ) to create structural disorder via a facile room-temperature ion exchange strategy. Compared with its crystalline form, the disorder in S-CoO x catalyst enables the increased low oxygen coordination and rich defect sites, which endows S-CoO x a superior catalytic activity for both OER and HER in alkali. Intriguingly, a water electrolyser adopting S-CoO x as both OER and HER electrode catalysts requires mere 1.63 V to reach a current density of 10 mA cm −2 in 1 M KOH. This work highlights the effectiveness of designing high-performing electrocatalysts for water electrolysers based on disordered structural materials. • The structurally disordered S-CoO x catalyst was synthesized via a facile room-temperature sulfur ion exchange strategy. • The structurally disordered catalyst shows increased low oxygen coordination and more defect sites. • Experimental and DFT calculation results reveal that a preferable electronic state was generated for the disorderedS-CoO x catalyst. • This novel structurally disordered strategy enables the S-CoO x catalyst a superior catalytic activity for both OER and HER in alkaline electrolyte.

Outlooks for development of silicon nanoparticle memory cells

Phase change memory is based on changes in the optical, electrical or other properties of materials during phase transitions, e.g. an amorphous to crystalline transition. Currently existing and potential applications of this memory are primarily based on multicomponent alloys of metals and semiconductors. However single-component nanoparticles including Si ones are also of interest as promising nanosized memory cells. The potential for developing this type of memory cells is confirmed by the fact that the optical absorption index of bulk amorphous silicon is of the same order of magnitude as that of crystalline silicon. Certainly this phenomenon can hardly be implemented with a single nanoparticle the size of which is within light wavelength. Using molecular dynamics and the Stillinger-Weber potential we have studied the regularities of melting and the conditions of crystallization of silicon nanoparticles containing within 105 atoms. We have shown that cooling of nanosized silicon drops at a 0.2 TK/s rate or higher rates causes their amorphous transition whereas single-component nanosized metallic drops crystallize in molecular dynamics experiments even at a 1 TK/s rate. Further heating of amorphous silicon nanoparticles containing above 5 ∙ 104 atoms causes their crystallization in a specific temperature range from 1300 to 1400 K. We have concluded that there is a possibility of developing phase change memory cells on the basis of the above phase transitions. An amorphous transition of a nanoparticle can be achieved by its melting and further cooling to room temperature at a 0.2 TK/s rate whereas a crystalline transition, by its heating to 1300–1400 K at a 0.2 TK/s rate followed by cooling. Results of molecular dynamics experiments suggest there is a minimum silicon nanoparticle size for which the development of phase change memory cells becomes theoretically impossible at a given temperature change rate. For a 0.2 TK/s temperature change rate this minimum size is 12.4 nm (number of atoms approx. 5 ∙ 104).

Anisotropic Epitaxial Behavior in the Amorphous Phase-Mediated Hydroxyapatite Crystallization Process: A New Understanding of Orientation Control.

The precise control of crystallization is a key in the construction and engineering of crystalline materials, especially in biomineralization. Although it is generally accepted that biomineral crystals have evolved from their amorphous precursors, there are intense debates about crystallographic orientation control. By using in situ high-resolution transmission electron microscopy, we herein reveal that hydroxyapatite (HAP) is produced through its epitaxial growth from amorphous calcium phosphate with a preferential c-axis orientation. Abnormally but interestingly, this anisotropic epitaxial crystallization priority along the c-axis is not affected by the existing HAP crystalline substrate, which is exactly the same on either {002} or {100} facets. Molecular dynamics simulations suggest this preference is correlated with the interfacial energetic controls at the amorphous-crystalline transition frontier. The orientation control of biominerals here shows the key role of the interface energy, rather that the organic molecules or matrices, which provides a complementary understanding of the general c-axis orientation control of HAP in various biomineralization cases and aids in the development of an alternative strategy for crystallization control of functional materials.

Effects of neodymium doping on the structural and magnetic properties of the double perovskite Sr2−yNdyFe1+xMo1−xO6 compound

Curie temperature (TC) of the ferromagnetic compound Sr2FeMoO6 can be modified by increasing the itinerant electrons at the Fermi level. Nd doping increases the electron density but the charge neutrality should be taken into account. In this work, a series of double perovskites Sr[Formula: see text]Nd[Formula: see text]Fe[Formula: see text]Mo[Formula: see text]O6 ([Formula: see text]y[Formula: see text]=[Formula: see text]0.0, 0.25, 0.5, 0.75 and 1.0) for both cases, the off-stoichiometric ([Formula: see text]x[Formula: see text]=[Formula: see text]0) and the stoichiometric ([Formula: see text]x[Formula: see text][Formula: see text][Formula: see text]0 with x[Formula: see text]=[Formula: see text]y/2) one, were synthesized via solid state route and discussed in detail. A crystalline structure transition was confirmed by X-ray diffraction analysis, showing a change of the lattice parameters and the unit cell volume as a function of Nd doping. It was also found that the nominal valence in Fe is mainly Fe[Formula: see text] and in Mo is Mo[Formula: see text]. The TC increases continuously with Nd doping in the stoichiometric case. The increase of the TC follows a competition with the substitution of Sr[Formula: see text] ions by Nd[Formula: see text] ions and the charge neutrality considered in our system.

Nanoporous metals from thermal decomposition of transition metal dichalcogenides

Nanoporous metals (np-M) have emerged as promising materials owing to their high surface area-to-volume ratio and electrical/thermal conductivity. There exists a group of processing methodologies by which np-M are formed through a top-down nanostructure evolution driven by the selective removal of a sacrificial component, all of which are a variation of dealloying. Nanoporosity evolution through current dealloying methodologies, however, is governed by strict requirements including sufficient separation in “reactivity” of the participating components and a homogeneous solid solution precursor alloy. This limits the viable alloy systems that may be used and the range of np-M's that may be formed. Here, we report thermal decomposition of crystalline transition metal dichalcogenides (TMDs) as a new processing methodology for np-M formation, adding to the spectrum of dealloying protocols. We demonstrate application of this process to the formation of a broader class of np-M including W, Re, Mo, and Ta with feature sizes below 100 nm. The presented facile thermal treatment of TMDs offers a new methodology for the evolution of nanoporosity in a broad range of metals.

Nitrogen plasma treatment of a TiO2 layer for MIS ohmic contact on n-type Ge substrate

This work investigates the characteristics of ohmic contacts on an n-type Ge by combining TiO2 with different metals. Metal-insulator-semiconductor (MIS) structures easily crystallize after post-metal annealing (PMA), with the subsequent characteristics dependent on the insulator thickness and process temperature. If 2 nm thick TiO2 is doped with nitrogen, there is no obvious difference in the current-voltage characteristics before/after PMA. The transmission electron microscope (TEM) analysis also verified that a lightly nitrogen-doped process is useful in suppressing the crystalline transition. In this study, the tungsten (W) contact showed the best thermal stability. Therefore, this technique could be effective in suppressing TiO2 crystallization with improved thermal stability.