Chalcogenide Layers Research Articles

Real-Time Unsupervised Learning and Image Recognition via Memristive Neural Integrated Chip Based on Negative Differential Resistance of Electrochemical Metallization Cell Neuron Device.

Spiking neurons are essential for building energy-efficient biomimetic spatiotemporal systems because they communicate with other neurons using sparse and binary signals. However, the achievable high density of artificial neurons having a capacitor for emulating the integrate function of biological neurons has a limit. Furthermore, a low-voltage operation (<1.0V) is essential for connecting with modern complementary metal-oxide-semiconductor-field-effect-transistor-based (C-MOSFET-based) integrated circuits. Here, a capacitorless memristive-neural integrated chip (MnIC) based on the negative differential resistance of the electrochemical metallization cell designed using a 28-nm C-MOSFET process in a foundry is reported. The fabricated MnIC exhibits extremely low-voltage operation (<0.7V) via the rupture dynamics of Ag filaments formed in the GeS2 chalcogenide layer, with a nonlinear increase in the action potential in a manner similar to a human sensory system. Moreover, to construct a fully-structured spiking neural network (SNN), an oxygenated amorphous carbon-based (α-COx-based) synaptic device having 32 multi-level conductance states is designed. The designed MnIC and α-COx-based synaptic device demonstrate real-time unsupervised learning via a spike-timing-dependent plasticity learning rule with an SNN. Using the trained SNN, the real-time hand-written digit image of a cell phone obtained from a live webcam is successfully classified, which suggests practical applications for brain-like neuromorphic chips.

Ferroelectricity with concomitant Coulomb screening in van der Waals heterostructures.

Interfacial ferroelectricity emerges in non-centrosymmetric heterostructures consisting of non-polar van der Waals (vdW) layers. Ferroelectricity with concomitant Coulomb screening can switch topological currents or superconductivity and simulate synaptic response. So far, it has only been realized in bilayer graphene moiré superlattices, posing stringent requirements to constituent materials and twist angles. Here we report ferroelectricity with concomitant Coulomb screening in different vdW heterostructures free of moiré interfaces containing monolayer graphene, boron nitride (BN) and transition metal chalcogenide layers. We observe a ferroelectric hysteretic response in a BN/monolayer graphene/BN, as well as in BN/WSe2/monolayer graphene/WSe2/BN heterostructure, but also when replacing the stacking fault-containing BN with multilayer twisted MoS2, a prototypical sliding ferroelectric. Our control experiments exclude alternative mechanisms, such that we conclude that ferroelectricity originates from stacking faults in the BN flakes. Hysteretic switching occurs when a conductive ferroelectric screens the gating field electrically and controls the monolayer graphene through its polarization field. Our results relax some of the material and design constraints for device applications based on sliding ferroelectricity and should enable memory device or the combination with diverse vdW materials with superconducting, topological or magnetic properties.

Transport Properties of Doped Wide Band Gap Layered Oxychalcogenide Semiconductors Sr2GaO3CuCh, Sr2ScO3CuCh, and Sr2InO3CuCh (Ch = S or Se).

The structural, electrical, and optical properties of a series of six layered oxychalcogenides with the general formula Sr2 MO3CuCh, where M = Ga, Sc, or In and Ch = S or Se, have been investigated. From this set, we report the structure and properties of Sr2GaO3CuSe for the first time, as well as the full structural details of Sr2ScO3CuSe, which have not previously been available. A systematic study of the suitability of all of the Sr2 MO3CuCh phases as p-type conductors has been carried out, after doping with both sodium and potassium to a nominal composition of A 0.05Sr1.95 MO3CuCh, (A = Na or K), to increase the hole carrier concentration. Density functional theory calculations were used to determine the electronic band structure and predict the transport properties, while optical properties were determined using UV-vis spectroscopy, and structures were confirmed using Rietveld refinement against powder X-ray diffraction data. Room-temperature conductivity measurements were carried out on both pristine samples and doped samples, 18 compositions in total, using four-point probe measurements. We found that the most conductive sample was K0.05Sr1.95GaO3CuSe, with a measured conductivity of 0.46 S cm-1, collected from a sintered pellet. We have also been able to identify a relationship between the conductivity and the geometry of the copper chalcogenide layer within the Sr2 MO3CuCh series of compounds. As this geometry can be controlled through the material composition, the identification of this structure-property relationship highlights a route to the selection and identification of materials with even higher conductivities.

Research on the Performance of Fmoc-amino Acid Passivated Peroverskite Solar Cells

This study investigates the effect of surface passivation of chalcogenide solar cells by introducing Fmoc-Gly-OH amino acids. The results show that the moderate amount of Fmoc-Gly-OH modification can significantly improve the photovoltaic performance and stability of chalcogenide solar cells. With the increase of Fmoc-Gly-OH concentration, the photovoltaic conversion efficiency firstly increases and then decreases, and the best effect is reached when the concentration is 1.0 mg/ml.XRD and SEM analyses show that the crystal structure of the Fmoc-Gly-OH modified chalcocite thin films is more regular, and the defect density is obviously reduced.XPS analysis shows that the Fmoc-Gly-OH modification is able to reduce the defect state density of the chalcogenide layer. The reduction of defects was further verified by steady-state fluorescence spectroscopy analysis. In addition, the electrochemical impedance spectra and dark-state I-V curve results of the devices showed that the Fmoc-Gly-OH modification was able to reduce the non-radiative complexation and charge transport resistance, and increase the fill factor and open-circuit voltage. Finally, the effectiveness of Fmoc-Gly-OH surface passivation in enhancing the performance of chalcogenide solar cells was confirmed by the analysis of J-V curves and box plots. This study provides new ideas and methods for surface engineering of chalcogenide solar cells, which has some practical application prospects.

Multishell Silver Indium Selenide-Based Quantum Dots and Their Poly(methyl methacrylate) Composites for Application in Red-Light-Emitting Diodes.

In this work, the production of novel multishell silver indium selenide quantum dots (QDs) shelled with zinc selenide and zinc sulfide through a multistep synthesis precisely designed to develop high-quality red-emitting QDs is explored. The formation of the multishell nanoheterostructure significantly improves the photoluminescence quantum yield of the nanocrystals from 3% observed for the silver indium selenide core to 27 and 46% after the deposition of the zinc selenide and zinc sulfide layers, respectively. Moreover, the incorporation of the multishelled QDs in a poly(methyl methacrylate) (PMMA) matrix via in situ radical polymerization is investigated, and the role of thiol ligand passivation is proven to be fundamental for the stabilization of the QDs during the polymerization step, preventing their decomposition and the relative luminescence quenching. In particular, the role of interface chemistry is investigated by considering both surface passivation by inorganic zinc chalcogenide layers, which allows us to improve the optical properties, and organic thiol ligand passivation, which is fundamental to ensuring the chemical stability of the nanocrystals during in situ radical polymerization. In this way, it is possible to produce silver-indium selenide QD-PMMA composites that exhibit bright red luminescence and high transparency, making them promising for potential applications in photonics. Finally, it is demonstrated that the new silver indium selenide QD-PMMA composites can serve as an efficient color conversion layer for the production of red light-emitting diodes.

Modulation of insulator metal transition of VO₂ films grown on Al2O3 (001) and TiO2 (001) substrates by the crystallization of capping Ge2Sb2Te5 layer

We demonstrate modulation of insulator metal transition (IMT) of VO2 films grown on single crystalline substrates through the effect of in-plane compression with crystallization of capping chalcogenide layer on the targeted VO2 films. Chalcogenide germanium–antimony–telluride (Ge2Sb2Te5: GST), which shows large volume reduction of 6.8% with its phase change from amorphous to crystal, was deposited on VO2 films grown on Al2O3 (001) and TiO2 (001) substrates, where V–V atoms along the cR-axis in the tetragonal VO2 phase align parallel and perpendicular to the substrate surfaces, respectively. As a result, counter shifts in temperature-dependence of resistance characteristics, to lower and higher directions, were observed for VO2 films on Al2O3 (001) and TiO2 (001), consistent with the lattice modulation of VO2 films by the in-plane compression introduced by GST crystallization. The obtained results open a way to realize large resistance change of IMT under constant temperature by controlling GST phases.

Structure and compositional trends in alkali-metal containing titanium and vanadium oxide chalcogenides and the new van der Waals phase Ti2Te2O

We report the solid-state synthesis of all eighteen layered oxide chalcogenides in the structural family AM2Q2O (A = K/Rb/Cs; M = Ti/V; Q = S/Se/Te), allowing the determination of trends in composition and reactivity within the series. All materials are isostructural, crystallising in the primitive tetragonal space group P4/mmm as reported previously for five compounds in the series. The titanium or vanadium ions have intermediate valency on a single crystallographic site, leading to temperature-independent paramagnetism and correlated electronic behaviour which is influenced by the compositional variation. Furthermore, the alkali metal ions in ▪ and ▪ can be removed by oxidative deintercalation using ▪ at room temperature to produce a new metastable van der Waals layered phase, ▪. During the deintercalation reaction the oxide chalcogenide layers undergo a relative shift by (0.5,0.5) in the ab plane such that ▪ is body-centered with space group I4/mmm.

Topochemical Anionic Subunit Insertion Reaction for Constructing Nanoparticles of Layered Oxychalcogenide Intergrowth Structures.

Intergrowth compounds contain alternating layers of chemically distinct subunits that yield composition-tunable synergistic properties. Synthesizing nanoparticles of intergrowth structures requires atomic-level intermixing of the subunits rather than segregation into stable constituent phases. Here we introduce an anionic subunit insertion reaction for nanoparticles that installs metal chalcogenide layers between metal oxide sheets. Anionic [CuS]- subunits from solution replace interlayer chloride anions from LaOCl to form LaOCuS topochemically with retention of crystal structure and morphology. Sodium acetylacetonate helps extract Cl- concomitant with the insertion of S2- and Cu+ and is generalized to other oxychalcogenides. This topochemical reaction produces nanoparticles of ordered mixed-anion intergrowth compounds and expands nanoparticle ion exchange chemistry to anionic subunits.

Plasmon-enhanced photostimulated diffusion in a thin-layer Ag–GeSe2 structure

This work studies photostimulated diffusion of silver, enhanced by a plasmon field, into GeSe2 thin films. To ensure the excitation of surface plasmon polaritons (SPP) at the interface between Ag and GeSe2, silver diffraction gratings substrates were used, on which chalcogenide layers were deposited by thermal evaporation in vacuum. The samples were exposed to the p-polarized radiation of a He-Ne laser. The kinetics of photostimulated processes was studied by recording the changes in SPP characteristics with exposure. It has been established that at the initial stage of the photodiffusion process, SPP excitation at the Ag/GeSe2 interface during exposure results in a fourfold increase in the photostimulated flux of silver ions. The concentrations of photodissolved silver in the chalcogenide layer are estimated. The mechanism of photostimulated diffusion of Ag in GeSe2 and its features in such thin-layer structures, as well as a possible mechanism of plasmon-stimulated acceleration of this process, is discussed.

Superstructures and Chemical Bonding in Rare Earth Metal Polytellurides RETe2‐δ (RE=La−Nd; Sm−Tm; 0≤δ≤0.2)

AbstractPolychalcogenides REX2‐δ (X=S, Se, Te; 0≤δ≤0.2) of trivalent rare earth metals RE have been investigated in recent years to shed light on the structural diversity as a function of compositional, metric, thermodynamic, and electronic situation. Whereas the former aspects have comparable influence on the structures of all polychalcogenides REX2‐δ, the bonding situation was assumed different for tellurides due to tellurium's higher tendency to delocalize electrons. The crystal structures generally contain puckered [REX] double slabs and planar [X] layers, the latter hosting different distortions from a square‐like arrangement. The distortion patterns of sulfides and selenides can be understood by a Zintl‐type approach; they are dominated by localization of valence electrons in mono‐ (X2−) or dinuclear (X22−) anions only. This review discusses crystal structures of some rare‐earth metal polytellurides RETe2‐δ (0≤δ≤0.2) and bonding features in the chalcogenide layers and relates them to their sulfide and selenide counterparts.

Anion redox as a means to derive layered manganese oxychalcogenides with exotic intergrowth structures

Topochemistry enables step-by-step conversions of solid-state materials often leading to metastable structures that retain initial structural motifs. Recent advances in this field revealed many examples where relatively bulky anionic constituents were actively involved in redox reactions during (de)intercalation processes. Such reactions are often accompanied by anion-anion bond formation, which heralds possibilities to design novel structure types disparate from known precursors, in a controlled manner. Here we present the multistep conversion of layered oxychalcogenides Sr2MnO2Cu1.5Ch2 (Ch = S, Se) into Cu-deintercalated phases where antifluorite type [Cu1.5Ch2]2.5- slabs collapsed into two-dimensional arrays of chalcogen dimers. The collapse of the chalcogenide layers on deintercalation led to various stacking types of Sr2MnO2Ch2 slabs, which formed polychalcogenide structures unattainable by conventional high-temperature syntheses. Anion-redox topochemistry is demonstrated to be of interest not only for electrochemical applications but also as a means to design complex layered architectures.

Impact of Polyamide Surface Preparation on the Formation of Mixed CdS-CdTe Layers

CdTe-CdS layers were formed on polyamide (PA) 6 films with different surface modifications using the sorption-diffusion method. Part of the samples of the PA films was boiled in distilled water for 2 h and the other ones were stored in concentrated acetic acid at 20 °C for 0.5 h. After this stage, all the PA 6 films were chalcogenized at 20 °C for 1 or 5 h using an acidified 0.1 mol/L solution of K2TeS4O6. Then, the chalcogenized samples were treated with a 0.1 mol/L solution of cadmium acetate at 70, 80 or 90 °C for 10 min. The elemental and phase composition and the morphological and optical properties of the obtained films were determined. XRD analysis showed that cadmium chalcogenide layers consist of four phases: hexagonal CdTe, orthorhombic CdS, rhombohedral Te and orthorhombic S18. The average crystallite size among the obtained layers was very similar and was in the range of 36–42 nm. The concentrations of cadmium, sulfur and tellurium in the layers on PA 6 and the optical properties of the CdTe-CdS layers were dependent on the method of preparation of the polyamide film, the duration of chalcogenization and the temperature of the Cd(CH3COO)2 solution.

Read Disturbance in Cross-Point Phase-Change Memory Arrays—Part II: Array Simulations Considering External Currents

In Part I of this study, we demonstrated that when a high-read current from phase-change memory (PCM) programmed at a high-SET current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {SET}}{)}$ </tex-math></inline-formula> is achieved, melting dynamics in the chalcogenide layer are promoted, thus leading to read disturbances. Therefore, we analyzed additional external current sources that worsened the read disturbances by considering the PCM cell and additional selector and core circuits used in practical applications. By means of HSPICE simulations, the impact of two noticeable external overshoot and inrush currents generated during respective SET and read operations in one-selector and one-PCM (1S-1R) array was investigated. Our findings show that resistance-related components mainly affect the magnitude of the inflowing external current to the 1S-1R cell. The PCM can thus be easily heated up by the current, making the memory state vulnerable to the melting process. For these reasons, we showed that the disturbance primarily observed in the high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {SET}}$ </tex-math></inline-formula> operated PCM can also occur even at low- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {SET}}$ </tex-math></inline-formula> settings, given the actual cross-point array environment. Finally, we explored the maximum achievable array size that ensures disturbance-free read endurance by quantifying the read current to examine the location wherein the melting occurs.

Direct Laser Writing of Computer-Generated Holograms by Photodissolution of Silver in Arsenic Trisulfide

Photodissolution is a process that is well known for its ability to cause inclusion of silver into the matrix of a chalcogenide layer, changing its optical properties. In this paper, using e-beam deposition, we developed Ag (74 nm)/As2S3 (355 nm) bilayers and characterized the photodissolution kinetics when exposed to actinic radiation. We showed that local complete silver photodissolution at the micron scale can be achieved. Based on this result, we then developed amplitude-based computer-generated holograms using direct laser writing. CW lasers with beam shaping and short pulse lasers with beam scanning were both implemented. Elements with 8.5 µm and &lt;1 µm spatial resolution and close to theoretical intensity distribution, respectively, were successfully demonstrated.

Reliable Low Voltage Selector Device Technology Based on Robust SiNGeCTe Arsenic-Free Chalcogenide

Modern data-centric computing applications are increasingly demanding in terms of memory density and performance. The cross-point memory architecture based on the two-terminal one-selector/one-resistor memory cell is an attractive solution for high-density and 3D-compatible embedded memory. In this letter, we introduce a new arsenic-free chalcogenide material, namely SiNGeCTe, for low voltage selector applications, and we report on its remarkable reliability characteristics. The inclusion of the Si dopant allows for an enhanced material robustness in terms of stable high current operation and extended thermal stability (≥450°C, 30 min), and reduces the First Fire impact on the subthreshold conduction. We investigate the threshold voltage drift and reveal a marked trend of the drift slope with the thickness of the chalcogenide layer. Finally, based on the reported drift trend, the optimized device with 5 nm thickness is highlighted for low threshold voltage and low drift applications, demonstrating low threshold voltage drift with slope <30 mV/dec. The optimized device proves an excellent cycling endurance ≥1e10 cycles.

Thermodynamic Analysis and Experimental Study on the Oxidation of PbX (X = S, Se) Nanostructured Layers.

Heat treatment in an oxygen-containing medium is a necessary procedure in the technology of forming photodetectors and emitters based on lead chalcogenides. Lead chalcogenide layers (PbS, PbSe) were prepared via a chemical bath deposition method. Surface oxidation of lead chalcogenide layers was analyzed using X-ray diffraction and Raman spectroscopy methods, and thermodynamic analysis of the oxidation of PbSe and PbS layers was also performed. The calculated phase diagrams from 20 °C to 500 °C showed good agreement with the experimental results. According to the thermodynamic analysis, the oxidation products depend on the initial composition of the layers and temperature of the annealing. In some cases, the formation of a separate metallic phase Pb is possible along with the formation of lead oxide PbO and other oxides. The performed thermodynamic analysis makes it possible to substantiate the two-stage annealing temperature regimes which ensure an increase in the speed of photodetectors.

2D Non‐Van Der Waals Transition‐Metal Chalcogenide Layers Derived from Vanadium‐Based MAX Phase for Ultrafast Zinc Storage

AbstractAlthough 2D non‐van der Waals (vdW) layers show many intriguing physical and chemical properties as well as wide applications in the fields of electronics, catalysis, and energy storage, they still lack efficient synthetic approaches owing to their three‐dimensionally bonded structures. Here, a facile approach to produce 2D non‐vdW transition‐metal chalcogenide (TMC) layers based on the conversion of vanadium‐based MAX phase (V2GeC) at high temperatures in hydrogen sulfide gas is developed. Associated with the etching of the germanium layers from the MAX phase, the vanadium layers are transformed into 2D non‐vdW V3S4 layers. This originates from the self‐intercalation of ordered V atoms within the vdW space of intermediated vdW vanadium disulfide layers during the conversion reaction. Owing to the ultrathin character, highly exposed active surface, and unique vacancy‐enriched structure, the resultant 2D non‐vdW V3S4 layers deliver a high reversible capacity of 341 mAh g−1, good rate capabilities, and long‐term cycling performance for zinc storage.

Layered Organic Metal Chalcogenides (OMCs): From Bulk to Two‐Dimensional Materials

AbstractThe modification of inorganic two‐dimensional (2D) materials with organic functional motifs is in high demand for the optimization of their properties, but it is still a daunting challenge. Organic metal chalcogenides (OMCs) are a type of newly emerging 2D materials, with metal chalcogenide layers covalently anchored by long‐range ordered organic functional motifs, these materials are extremely desirable but impossible to realize by traditional methods. Both the inorganic layer and organic functional motifs of OMCs are highly designable and thus provide this type of 2D materials with enormous variety in terms of their structure and properties. This Minireview aims to review the latest developments in OMCs and their bulk precursors. Firstly, the structure types of the bulk precursors for OMCs are introduced. Second, the synthesis and applications of OMC 2D materials in photoelectricity, catalysis, sensors, and energy transfer are explored. Finally, the challenges and perspectives for future research on OMCs are discussed.

Layered Organic Metal Chalcogenides (OMCs): From Bulk to Two-Dimensional Materials.

The modification of inorganic two-dimensional (2D) materials with organic functional motifs is in high demand for the optimization of their properties, but it is still a daunting challenge. Organic metal chalcogenides (OMCs) are a type of newly emerging 2D materials, with metal chalcogenide layers covalently anchored by long-range ordered organic functional motifs, these materials are extremely desirable but impossible to realize by traditional methods. Both the inorganic layer and organic functional motifs of OMCs are highly designable and thus provide this type of 2D materials with enormous variety in terms of their structure and properties. This Minireview aims to review the latest developments in OMCs and their bulk precursors. Firstly, the structure types of the bulk precursors for OMCs are introduced. Second, the synthesis and applications of OMC 2D materials in photoelectricity, catalysis, sensors, and energy transfer are explored. Finally, the challenges and perspectives for future research on OMCs are discussed.

Promising developments of chalcogenide nanoresists for optical, x-ray and electron beam lithography

The results of application of combined systems chalcogenide layer - silver layer as nanoresists are presented. With the help of electron-beam lithography, elements of drawings with a size of 4-7 nm are obtained. These experimental results put chalcogenide combined systems among the most promising lithographic nanoresists.