Dr. Iosif Daniel SIMANDAN

We report large-scale synthesis of monolayer WS2 films obtained by sulfurization of oxidized magnetron sputtered monolayer W precursors. Literature routes typically require similar to 800 degrees C, well above the 400 degrees C limit imposed by back-end-of-line (BEOL) integration. Here, using an enhanced chemical vapor deposition (CVD) approach, the magnetron sputtered ultrathin W precursor (a W monolayer film, 0.27 nm thick, which in ambient air becomes a WOx monolayer) is sulfurized at the lowest possible temperature (450 degrees C) within a microreactor, which consists of a sandwich-like structure formed by the precursor and a clean Si substrate. The obtained WS2 material has a good crystallinity and uniform morphology across the entire growth substrate, as confirmed by detailed characterization. These results highlight the versatility of the method combining magnetron sputtering and microreactor-CVD, facilitating its applications to wafer-scale synthesis of monolayer WS2, heterogeneously integrated into electronic circuits (a major objective for next-generation electronics and optoelectronics). Additionally, we investigate in detail the properties of WS2 films synthesized from a bilayer W precursor (0.43 nm thick), under the same conditions, and we calculated the frequencies of the second-order Raman scattering modes. For electrical measurements, we fabricated WS2/few-layer-graphene heterostructures, whose atomically clean interface yields reliable, low-resistance contacts. These devices exhibit resistive switching behavior, likely governed by vacancy migration, making it a promising candidate for memristive applications. Our results demonstrate that electronics-grade monolayer WS2 can be synthesized at 450 degrees C, approaching the BEOL requirement of 400 degrees C.

Integrating two-dimensional transition-metal dichalcogenides with graphene is attractive for low-power memory and neuromorphic hardware, yet sequential wet transfer leaves polymer residues and high contact resistance. We demonstrate a complementary metal-oxide-semiconductor (CMOS)-compatible, transfer-free route in which an atomically thin amorphous MoS2 precursor is RF-sputtered directly onto chemical vapor-deposited few-layer graphene and crystallized by confined-space sulfurization at 800 degrees C. Grazing-incidence X-ray reflectivity, Raman spectroscopy, and X-ray photoelectron spectroscopy confirm the formation of residue-free, three-to-four-layer 2H-MoS2 (roughness: 0.8-0.9 nm) over 1.5 cm x 2 cm coupons. Lateral MoS2/graphene devices exhibit reproducible non-volatile resistive switching with a set transition (SET) near +6 V and an analogue ON/OFF approximate to 2.1, attributable to vacancy-induced Schottky-barrier modulation. The single-furnace magnetron sputtering + sulfurization sequence avoids toxic H2S, polymer transfer steps, and high-resistance contacts, offering a cost-effective pathway toward wafer-scale 2D memristors compatible with back-end CMOS temperatures.

In the quest for advanced materials suitable for next-generation electronic and optoelectronic applications, tungsten disulfide (WS2) ultrathin films have emerged as promising candidates due to their unique properties. However, obtaining WS2 directly on the desired substrate, eliminating the need for transfer, which produces additional defects, poses many challenges. This paper aims to explore the synthesis of WS2 ultrathin films via physical vapor deposition (PVD) followed by sulfurization in a confined space, addressing the challenge of film formation for practical applications. Precursor layers of tungsten and WS2 were deposited by RF magnetron sputtering. Subsequent sulfurization treatments were conducted in a small, closed, graphite box to produce WS2 films. The physical and chemical properties of these precursor and sulfurized layers were thoroughly characterized using techniques such as X-ray reflectometry (XRR), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The findings reveal notable distinctions in film thickness, structural orientation, and chemical composition, attributable to the different precursor used. Particularly, the sulfurized layers from the tungsten precursor exhibited a preferred orientation of WS2 crystallites with their (00L) planes parallel to the substrate surface, along with a deviation from parallelism in a small angular range. This study highlights the necessity of precise control over deposition and sulfurization parameters to tailor the properties of WS2 films for specific technological applications.

Cu2ZnSnSe4 (CZTSe) is a promising material for thin-film solar cells due to its suitable band gap, high absorption coefficient, and composition of earth-abundant and nontoxic elements. In this study, we prepared CZTSe thin films from Cu/SnSe2 and ZnSe stacks using a two-step annealing process. Initially, Cu-Sn-Se (CTSe) films were synthesized by sequential deposition and annealing of Cu and SnSe2 precursors in either a selenium (Se) or tin-selenium (Sn+Se) atmosphere. After the deposition of a ZnSe layer on top of CTSe films, the stack underwent a second annealing process, again in either a Se or Sn+Se atmosphere, resulting in four distinct annealing combinations: Se -> Se, Sn+Se -> Se, Se -> Sn+Se, and Sn+Se -> Sn+Se. The first annealing step enabled the formation of CTSe, while the second annealing step, performed after ZnSe deposition, led to the formation of the CZTSe phase. Comprehensive characterization including grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electrical measurements was conducted. GIXRD and Raman analysis revealed kesterite CZTSe phase peaks, with some samples showing a split in the main peak at similar to 27 degrees (2 theta), indicating the presence of Cu x Se and ZnSe secondary phases. SEM analysis showed the impact of Sn and Se annealing on grain size, with larger grains observed in films annealed in Sn+Se atmospheres, particularly in the second heat treatment process. EDS results displayed consistent elemental composition across samples, with varying Cu/(Zn+Sn), Zn/Sn and Se/metal ratios influencing the band gap values from 1.09 to 1.63 eV. Hall measurements indicated p-type conductivity with carrier concentrations between 1016 and 1023 cm-3. These results highlight the effectiveness of our two-step annealing process, particularly the Sn+Se atmosphere, in optimizing CZTSe thin films for potential use in high-efficiency thin-film solar cells.

MoS2 has proven its efficacy in flexible electronics, transistor devices, and various biological and chemical applications. However, it is still challenging to achieve large-area MoS2 monolayers with desired material quality and electrical properties to fulfill the requirement for practical applications. Moreover, the main strategy for the preparation of a 2D heterostructure it is based on the sequential stacking of the layered materials using wet or dry transfer methods which introduces many defects. This paper presents an economically viable and straightforward two-step methodology to obtain MoS2 thin films, encompassing magnetron sputtering deposition of Mo and subsequent annealing in a sulfur-rich environment. This approach successfully yielded MoS2 thin films on Si\SiO2 substrates. Additionally, heterostructures consisting of few layer graphene (FLG) and MoS2 were directly obtained using the same method. The utilization of grazing incidence X-ray diffraction verified the formation of the hexagonal MoS2 phase, a finding further confirmed by Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) investigations revealed the successful sulfurization process, with surface-bound oxides forming only subsequent to air exposure. Comprehensive assessment involving X-ray reflectivity, atomic force microscopy and XPS collectively inferred the fabrication of thin films comprised of a small number of MoS2 layers covering the entire substrate. Electrical assessments exhibited an electrical hysteresis, demonstrating its potential for memristor applications. Overall, this study outlines a cost-effective fabrication method for producing nanoscale MoS2 thin films with excellent properties, avoiding the use of toxic gases such as H2S. These findings contribute to the potential development of cutting-edge applications.

The development of kesterite (Cu2ZnSn(S,Se)4, CZTSSe) thin films for photovoltaic applications is highly necessary, given their composition of Earth-abundant, environmentally friendly elements and their compatibility with established photovoltaic technologies. This study presents a novel synthesis approach for CZTSSe films with varied S/(S+Se) ratios, ranging from 0.83 to 0.44, by a two-step magnetron sputtering deposition/annealing process. The first step consists in an initial deposition of stacked Mo/SnS2/Cu layers, which, upon thermal treatment in a sulfur atmosphere, were transformed into Cu2SnS3 (CTS) films. In the second step, further deposition of ZnSe and subsequent annealing in a tin and selenium atmosphere resulted in the formation of a CZTSSe phase. These processes were optimized to fabricate high-quality and single-phase CZTSSe films, thereby mitigating the formation of secondary phases. Characterization techniques, including scanning electron microscopy, demonstrated a clear correlation between decreased S/(S+Se) ratios and enhanced film densification and grain size. Moreover, grazing incidence X-ray diffraction and Raman spectroscopy confirmed a compositional and structural transition from close to CZTS to nearly a CZTSe phase as the S/(S+Se) ratios decreased. This study advances kesterite-based solar cell technology by enhancing the structural properties and crystallinity of the absorber layer, necessary for improving photovoltaic performance.

Considering the increasing need for sustainable and economical energy storage solutions, the integration of layered materials such as MoS2 into these systems represents an important step toward enhancing energy sustainability and efficiency. Exploring environmentally responsible fabrication techniques, this study assesses wrinkled MoS2 thin films synthesized from distinct Mo and MoS2 targets, followed by sulfurization conducted in a graphite box. We utilized magnetron sputtering to deposit precursor Mo and MoS2 films on Si substrates, achieving thicknesses below 20 nm. This novel approach decreases sulfur by up to tenfold during sulfurization due to the confined space technique, contributing also to avoiding the formation of toxic gases such as SO2 or the necessity of using H2S, aligning with sustainable materials development. Thinner MoS2 layers were obtained post-sulfurization from the MoS2 precursors, as shown by X-ray reflectometry. Raman spectroscopy and grazing X-ray diffraction analyses confirmed the amorphous nature of the as-deposited films. Post-sulfurization, both types of films exhibited crystalline hexagonal MoS2 phases, with the sulfurized Mo showing a polycrystalline nature with a (100) orientation and sulfurized MoS2 displaying a (00L) preferred orientation. The X-ray photoelectron spectroscopy results supported a Mo:S ratio of 1:2 on the surface of the films obtained using the MoS2 precursor films, confirming the stoichiometry obtained by means of energy dispersive X-ray spectroscopy. Scanning electron microscopy and atomic force microscopy images revealed micrometer-sized clusters potentially formed during rapid cooling post-sulfurization, with an increased average roughness. These results open the way for the further exploration of wrinkled MoS2 thin films in advanced energy storage technologies.

Cu2SnSe3 (CTSe) is a polyvalent material that can be used as an absorber layer for thin film solar cells or as a starting layer for the synthesis of CZTSe or CZTSSe compounds. Obtaining CTSe single phase films with opti-mized properties for thin film solar cells is a difficult task. A systematic study using both metallic and binary chalcogenides precursors for the formation of the CTSe phase was not performed. The films consisting of four different stacks (Sn\Cu, SnSe2\Cu, Sn\Cu2Se, and SnSe2\Cu2Se) were prepared by magnetron sputtering on soda lime glass (SLG) and molybdenum (Mo) coated SLG substrates, followed by annealing at 550 degrees C under Sn + Se atmosphere. X-ray diffraction and Raman spectroscopy results indicated the formation of a single CTSe phase in most of the stacks deposited on both substrates. Scanning electron microscopy images showed compact surfaces with large grains in the films deposited on Mo substrate, while the films on SLG have more voids on their sur-faces. The elemental analysis measured by energy dispersive spectroscopy revealed stoichiometric films on Mo, and copper and tin rich compositions on SLG substrates. The band gap values inferred by conventional spec-troscopy are between 0.81 and 1.95 eV. It was found that the SnSe2\Cu and Sn\Cu2Se stacks are preferred for the formation of a single CTSe phase, with dense surface morphology, a stoichiometric composition, and an optimal absorber layer band gap. This study opens the way to comprehend the formation reactions during the seleni-zation of metallic and binary chalcogenides precursors towards the optimization of kesterite absorber for photovoltaic device fabrication.

One of the new materials for next-generation thin film solar cells is Cu2ZnSnSe4 (CZTSe). However, achieving a single-phase CZTSe compound remains a challenge. This study describes the development of Cu2ZnSnSe4 thin films through the sequential deposition of stacked films of non-stoichiometric Cu2SnSe3 (CTSe) and ZnSe by magnetron sputtering. The structural, morphological, and electrical properties as well as the surface chemistry of the films were investigated and compared depending on the growth sequence of the thin films. By using Raman spectroscopy and grazing incidence X-ray diffraction, the tetragonal CZTSe structure was confirmed. Scanning electron microscopy and energy-dispersive spectroscopy measurements of the morphological and compositional properties indicated large grains and dense surfaces with an elemental composition close to the desired stoichiometry in SLG\SnSe2\Cu\ZnSe and SLG\SnSe2\Cu2Se\ZnSe stacks. To ascertain the surface chemistry and unique characteristics of the produced films, additional X-ray photoemission spectroscopy experiments were carried out. The optimal band gap values for the absorber layers were found using conventional spectroscopy, and they ranged from 0.88 to 1.47 eV. According to the electrical measurements, all the films were p-type and have high carrier concentrations between 1016 and 1020 cm-3. Our findings demonstrate that employing a sequential deposition approach and annealing in different atmospheres can yield CZTSe absorber layers with desirable properties, overcoming the challenge of non-stoichiometric CTSe precursors.

Cu2ZnSnSe4 thin films have been synthesized by employing two magnetron-sputtering depositions, interlaced with two sequential post-deposition heat treatments in low vacuum, Sn+Se and Se-rich atmospheres at 550 degrees C. By employing successive structural analysis methods, namely Grazing Incidence X-Ray Diffraction (GIXRD) and Raman Spectroscopy, secondary phases such as ZnSe coexisting with the main kesterite phase have been identified. SEM peered into the surface morphology of the samples, detecting structural defects and grain profiles, while EDS experiments showed off-stoichiometric elemental composition. The optical bandgaps in our samples were calculated by a widely used extrapolation method from recorded transmission spectra, holding values from 1.42 to 2.01 eV. Understanding the processes behind the appearance of secondary phases and occurring structural defects accompanied by finding ways to mitigate their impact on the solar cells' properties is the prime goal of the research beforehand.

Cu2ZnSnS4 (CZTS) is regarded as one of the emerging materials for next-generation thin film solar cells. However, its synthesis is complex, and obtaining a single-phase CZTS thin film is difficult. This work reports the elaboration of Cu2ZnSnS4 thin films by a sequential magnetron sputtering deposition of Cu2SnS3 (CTS) and ZnS as stacked films. Initially, the CTS films were prepared on a soda lime glass substrate by annealing Cu and SnS2 stacked layers. Second, ZnS was deposited by magnetron sputtering on the CTS films. The CTS\ZnS stacks were then annealed in Sn + S or S atmospheres. The tetragonal CZTS structure was obtained and confirmed by grazing incidence X-ray diffraction and Raman spectroscopy. The morphological and compositional characteristics, measured by scanning electron microscopy and energy-dispersive spectroscopy, revealed large grains and dense surfaces with the elemental composition close to the intended stoichiometry. Additional X-ray photoemission spectroscopy measurements were performed to determine the surface chemistry and particularities of the obtained films. The optical properties, determined using conventional spectroscopy, showed optimal absorber layer band gap values ranging between 1.38 and 1.50 eV. The electrical measurements showed that all the films are p-type with high carrier concentrations in the range of 10(15) to 10(20) cm(-3). This new synthesis route for CZTS opens the way to obtain high-quality films by an industry-compatible method.

Cu2SnS3 (CTS) is emerging as a promising absorber for the next generation thin film solar cells (TFSC) due to its excellent optical and electronic properties, earth-abundance and eco-friendly elemental composition. In addition, CTS can be used as precursor films for the Cu2ZnSnS4 (CZTS) synthesis. The optical properties of CTS are influenced by stoichiometry, crystalline structure, secondary phases and crystallite size. Routes for obtaining CTS films with optimized properties for TFSC are still being sought. Here, the CTS thin films synthesized by magnetron sputtering on soda lime glass (SLG) using Cu and SnS2 targets in two different stacks, were studied. The SLG\Cu\SnS2 and SLG\SnS2\Cu stacks were annealed in S and Sn + S atmospheres, at various temperatures. Both stacks show a polymorphic structure, and higher annealing temperatures favor the monoclinic CTS phase formation. Morphology is influenced by the stacking order since a SnS2 top layer generates several voids on the surface due to the evaporation of SnS, while a Cu top layer provides uniform and void-free surfaces. The films in the copper-capped stack annealed under Sn + S atmosphere have the best structural, morphological, compositional and optical properties, with tunable band gaps between 1.18 and 1.37 eV. Remarkably, secondary phases are present only in a very low percent (< 3.5%) in samples annealed at higher temperatures. This new synthesis strategy opens the way for obtaining CTS thin films for solar cell applications, that can be used also as intermediary stage for CZTS synthesis.

Ge2Sb2Te5 (GST-225) is a chalcogenide material with applications in nonvolatile memories. However, chalcogenide material properties are dependent on the deposition technique. GST-225 thin films were prepared using three deposition methods: magnetron sputtering (MS), pulsed laser deposition (PLD) and a deposition technique that combines MS and PLD, namely MSPLD. In the MSPLD technique, the same bulk target is used for sputtering but also for PLD at the same time. The structural and optical properties of the as-deposited and annealed thin films were characterized by Rutherford backscattering spectrometry, X-ray reflectometry, X-ray diffraction, Raman spectroscopy and spectroscopic ellipsometry. MS has the advantage of easily leading to fully amorphous films and to a single crystalline phase after annealing. MS also produces the highest optical contrast between the as-deposited and annealed films. PLD leads to the best stoichiometric transfer, whereas the annealed MSPLD films have the highest mass density. All the as-deposited films obtained with the three methods have a similar optical bandgap of approximately 0.7 eV, which decreases after annealing, mostly in the case of the MS sample. This study reveals that the properties of GST-225 are significantly influenced by the deposition technique, and the proper method should be selected when targeting a specific application. In particular, for electrical and optical phase change memories, MS is the best suited deposition method.

ZnS is a wide band gap material which was proposed as a possible candidate to replace CdS as a buffer layer in solar cells. However, the structural and optical properties are influenced by the deposition method. ZnS thin films were prepared using magnetron sputtering (MS), pulsed laser deposition (PLD), and a combined deposition technique that uses the same bulk target for sputtering and PLD at the same time, named MSPLD. The compositional, structural, and optical properties of the as-deposited and annealed films were inferred from Rutherford backscattering spectrometry, X-ray diffraction, X-ray reflectometry, Raman spectroscopy, and spectroscopic ellipsometry. PLD leads to the best stoichiometric transfer from target to substrate, MS makes fully amorphous films, whereas MSPLD facilitates obtaining the densest films. The study reveals that the band gap is only slightly influenced by the deposition method, or by annealing, which is encouraging for photovoltaic applications. However, sulphur vacancies contribute to lowering the bandgap and therefore should be controlled. Moreover, the results add valuable information towards the understanding of ZnS polymorphism. The combined MSPLD method offers several advantages such as an increased deposition rate and the possibility to tune the optical properties of the obtained thin films.

The lack of order in amorphous chalcogenides offers them novel properties but also adds increased challenges in the discovery and design of advanced functional materials. The amorphous compositions in the Si-Ge-Te system are of interest for many applications such as optical data storage, optical sensors and Ovonic threshold switches. But an extended exploration of this system is still missing. In this study, magnetron co-sputtering is used for the combinatorial synthesis of thin film libraries, outside the glass formation domain. Compositional, structural and optical properties are investigated and discussed in the framework of topological constraint theory. The materials in the library are classified as stressed-rigid amorphous networks. The bandgap is heavily influenced by the Te content while the near-IR refractive index dependence on Ge concentration shows a minimum, which could be exploited in applications. A transition from a disordered to a more ordered amorphous network at 60 at% Te, is observed. The thermal stability study shows that the formed crystalline phases are dictated by the concentration of Ge and Te. New amorphous compositions in the Si-Ge-Te system were found and their properties explored, thus enabling an informed and rapid material selection and design for applications.

Amorphous metal chalcogenides have good switching properties for resistive memories, but have low thermal stability. In this work, the response to rapid thermal stress, as high as 550 degrees C, of amorphous Cu-GeSe, Ag-GeSe, Cu-GeTe, Ag-GeTe thin films, is investigated. Metal-GeTe films, which are amorphous up to 280 degrees C, are the most stable. Metal-GeSe films start to crystallize at 190 degrees C and a Cu1.59Se phase, with 20.5% Cu vacancies and a structure similar to the c-Cu2-xSe superionic conductor, is formed. This might boost the performance of memory devices. Silver atoms migration is facilitated in Ag-GeSe by poor crystallization (below 5%, at all temperatures). Difussion of Ag is enhanced in Ag-GeTe, due to the crystallization of the cubic (Ag2Te)(4)-GeTe2 (Ag8GeTe6) phase, which has Ag+ vacancies. In Cu-GeTe, the formation of stoichiometric polycrystalline Cu0.67Ge0.33 Te might hinder diffusion. An unusual anisotropic behaviour (increase in thickness, simultaneously with contraction of surface) is observed at 100 degrees C in Cu-GeSe and Cu-GeTe thin films, which suggests the orientation of the amorphous clusters package along a preferential direction.

This paper presents the study of the development of complex organic materials deposited by the Langmuir Blodgett technique. We have synthetized Langmuir Blodgett multilayers for the recognition of toxic chemicals in the air and / or ultraviolet radiation. The sensitive materials are based on multilayers of stearic acid metal salts combined with nanocarbon and metalloporphyrin structures. We prepared and obtained by the Langmuir Blodgett method films with nano-metric thicknesses combined in different concentrations of metal salts of fatty acids, Nano carbon structures and metalloporphyrins. Further we have characterized and tested the materials obtained for the sensitivity and selectivity of multilayers under the influence of various toxic gases and / or ultraviolet radiation obtaining high results in the field of sensors.

Surface plasmon resonance containing amorphous As2S3 film is proposed as a chemical sensor to highlight several liquid hydrocarbons. Method is label-free and is based on detection of small changes in the refractive index. As2S3 operates as plasmonic waveguide which confines the probing beam to the interface with liquid hydrocarbons. The method can easily distinguish hydrocarbons with very close refractive indices. The film thicknesses were optimized to obtain the best sensitivity and resolving power Minimum reflectance of SPR less than 1 % was found for optimal calculated film thicknesses, the sensitivity to the refractive index changes being 2.10(-5).

Cu2ZnSnS4 (CZTS) is a complex quaternary material, and obtaining a single-phase CZTS with no secondary phases is known to be challenging and dependent on the production technique. This work involves the synthesis and characterization of CZTS absorber layers for solar cells. Thin films were deposited on Si and glass substrates by a combined magnetron sputtering (MS) and pulsed laser deposition (PLD) hybrid system, followed by annealing without and with sulfur powder at 500 degrees C under argon (Ar) flow. Three different Cu2S, SnS2, and ZnS targets were used each time, employing a different target for PLD and the two others for MS. The effect of the different target arrangements and the role of annealing and/or sulfurization treatment were investigated. The characterization of the absorber films was performed by grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR), Raman spectroscopy, scanning electron microscopy, and regular transmission spectroscopy. The film with ZnS deposited by PLD and SnS2 and Cu2S by MS was found to be the best for obtaining a single CZTS phase, with uniform surface morphology, a nearly stoichiometric composition, and an optimal band gap of 1.40 eV. These results show that a new method that combines the advantages of both MS and PLD techniques was successfully used to obtain single-phase Cu2ZnSnS4 films for solar cell applications.

Cu2ZnSnS4 (CZTS) is an economically and environmentally friendly alternative to other toxic and expensive materials used for photovoltaics, however, the variation in the composition during synthesis is often followed by the occurrence of the secondary binary and ternary crystalline phases. These phases produce changes in the optical absorption edge important in cell efficiency. We explore here the secondary phases that emerge in a combinatorial Cu2S-ZnS-SnS2 thin films library. Thin films with a composition gradient were prepared by simultaneous magnetron sputtering from three binary chalcogenide targets (Cu2S, SnS2 and ZnS). Then, the samples were crystallized by sulfurization annealing at 450 degrees C under argon flow. Their composition was measured by energy dispersive X-ray spectroscopy (EDX), whereas the structural and optical properties were investigated by grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy and optical transmission measurements. As already known, we found that annealing in a sulfur environment is beneficial, increasing the crystallinity of the samples. Raman spectroscopy revealed the presence of CZTS in all the samples from the library. Secondary crystalline phases such as SnS2, ZnS and Cu-S are also formed in the samples depending on their proximity to the binary chalcogenide targets. The formation of ZnS or Cu-S strongly correlates with the Zn/Sn and Cu/Zn ratio of the total sample composition. The presence of these phases produces a variation in the bandgap between 1.41 eV and 1.68 eV. This study reveals that as we go further away from CZTS in the composition space, in the quasi-ternary Cu2S-ZnS-SnS2 diagram, secondary crystalline phases arise and increase in number, whereas the bandgap takes values outside the optimum range for photovoltaic applications.

We report a study related to the influence of heat treatment (up to 300 degrees C) on the structure of GaSb \GeTe, SnSe\GeTe and GaSb\SnSe stacked phase change memory films and of their counterparts with Hf thin film barrier between the layers. Samples were prepared by pulsed laser deposition and investigated by X-ray reflectometry and X-ray diffraction in order to evaluate the inter-films diffusion and the temperature threshold where this process is initiated. The thickness and mass density variations of films after each heat treatment, as well as the efficiency of hafnium barrier film, to eliminate potential atomic diffusion issues, were investigated.

Chalcogenide amorphous materials, such as GeTe, are known to exhibit deposition dependent optical and structural properties. The formation of a single and homogeneous amorphous GeTe (a-GeTe) phase is questionable since the deposited films can be mixtures of monoelemental nanoclusters. In this work, we employed two deposition techniques, pulsed laser deposition from a polycrystalline GeTe target and co-sputtering from two distinct Ge and Te targets, respectively, to obtain a-GeTe films. To improve the homogeneity of the amorphous phase obtained by magnetron sputtering, the substrate temperature was varied from room temperature up to 180 degrees C. The samples were investigated by X-ray diffraction, X-ray reflectometry, X-ray photoelectron spectroscopy and spectroscopic ellipsometry. It was found that the film mass density, optical bandgap, refractive index and absolute reflectivity become progressively larger with increasing substrate temperature, due to the minimization of voids fraction and the number of dangling bonds in the amorphous structure. Moreover, X-ray photoelectron spectroscopy results prove the formation of Ge-Te bonds and therefore of the GeTe alloy at the optimal substrate temperature of 180 degrees C. This study reveals the importance of optimizing the deposition conditions for obtaining a specific amorphous phase, which enables the atomic rearrangements responsible for fast phase-change needed in memory applications.

Thin amorphous As2S3 films show a giant photoexpansion of similar to 5% upon femtosecond laser illumination and this expansion remains after switching off the laser beam. To understand the structural modifications which occur during the photoexpansion process we assumed a molecular cluster structure of the amorphous As2S3. The focused femtosecond laser beam induces electrical charge redistribution on the sulfur atoms of each cluster, which increases the electrical repulsion between sulfur atoms, and thus induces an expansion of the clusters, by network reconfiguration, without breaking the bonds. An increase of the van der Waals distance between molecular clusters also takes place. (C) 2016 Elsevier B.V. All rights reserved.

Thin amorphous films based on gallium-chalcogen (Ga-Ch), namely Ga2S3, Ga2Se3, Ga2Te3, and GaTe have been prepared by pulsed laser deposition (PLD). The films were characterized by X-ray diffraction, extended X-ray absorption fine structure (EXAFS), energy-dispersive X-ray spectroscopy (EDX), optical transmission spectroscopy, ellipsometry, and electrical measurements. Structural measurements showed that Ga is threefold coordinated, except the Te-based alloys were, it seems, only twofold coordinated, while the chalcogen is usually twofold coordinated. In all the compositions, layered and chain-like structures are assumed. The bandgaps range between 1.09 eV for Ga2Te3 and 2.21 eV for Ga2Se3. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The aim of this research is to prepare amorphous thin films of undoped gallium sulphide and doped with rare-earth sulphides, of rare-earth sulphides and to investigate their physical properties. We have prepared thin amorphous films of Ga2S3, EuS, Er2S3, Gd2S3, and Ga2S3 doped with rare-earth sulphides (Ga2S3:EuS, Ga2S3:Er2S3, Ga2S3:Gd2S3) by Pulsed laser Deposition (PLD). The corresponding targets for preparation of amorphous thin films were obtained by Spark Plasma Sintering (SPS) from commercially available powders of binary sulphides. The structural results for the undoped and doped Ga2S3 thin films indicate a packing of disordered layers similar to that of amorphous As2S3. Femtosecond laser irradiation of the Ga2S3 thin films shows a photoexpansion effect at low laser power (85-100 mW) and an ablation effect at higher laser power (above 105 mW). The threshold between low power and high power pulses is situated at higher value for Ga2S3 (100 mW) in comparison with the case of As2S3 thin films (20 mW). (C) 2014 Elsevier B.V. All rights reserved.

Bulk ceramics of Ga2S3 and rare-earth sulfides (EuS, Gd2S3, Er2S3) as well as combinations thereof have been prepared by spark plasma sintering (SPS). The disk-shaped ceramics were used as targets for pulsed laser deposition (PLD) experiments to obtain amorphous thin films. The properties of these new bulks and amorphous thin films have been investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), optical transmission spectroscopy, and atomic force microscopy (AFM). In order to test the photoexpansion effect in Gd2S3 and the possibility to create planar arrays of microlenses, the film was irradiated with femtosecond laser pulses at different powers. For low laser power pulses (up to 100mW power per pulse) a photoexpansion effect was observed, which leads to formation of hillocks with a height of 40-50 nm. EuS doped Gd2S3 thin film shows luminescence properties, which recommend them for optoelectronic applications.

The complex chalcogenides with excellent memory switching properties are mainly situated close to the border of glass formation domain. The simulation of the structural changes occurring during the memory switching process of a ternary chalcogenide composition has been carried out. The transition of a high resistivity GeAs4Te7 amorphous cluster with 120 atoms to a low resistivity crystalline cluster was analyzed. The coordination of atoms changes from that corresponding to 8-N coordination rule (two for tellurium, three for arsenic, and four for germanium) in the amorphous phase to six (the same for all atoms) in metastable crystalline phase. Because of spatial constraints exercised by the amorphous matrix, the amorphous cluster cannot expand. In these circumstances Te atoms seem to be over-coordinated (up to sixfold-coordinated). During the switching process, the atoms are moving on distances up to 4.0 angstrom. The average displacement is of 2.36 angstrom.

A novel structure of metalloporphyrin, namely: 5,10,15,20-tetrakis(3,4-dimethoxyphenyl)-porphyrin Fe(III) chloride, was successfully synthesized. UV sensitive structure based on barium stearate functionalized with the novel synthesized porphyrin and carbon nanotubes was obtained. A five monolayers structure was deposited from solution by Langmuir - Blodgett technique onto a ceramic substrate with interdigital platinum electrodes. The complex film shows sensitivity to UV radiation due to the fact that an electron is excited from a Fe d orbital into a porphyrin antibonding pi orbital and is moved onto the ligand that is attached, facilitating the charge separation, while the presence of SWCNT activates the transport of the charge carriers.

Photovoltaic structures based on P3HT:PCBM(1:0.8) mixed with barium stearate (BS), carbon nanotubes (CNT) and different phtalocyanines (Coperphthalocyanine (CuPc), Zincphthalocyanine (ZnPc) and Magnesiumphthalocyanine (MgPc) were successfully prepared by spin-coating technique onto optical glass substrates covered with ITO and PEDOT:PSS. The goal of this study was to investigate the influence of barium stearate, carbon nanotubes and different small molecules of dyes mixed with the P3HT:PCBM "conventional" blend on the photovoltaic response of the "custom" obtained structures. The current-voltage characteristics were drawn in the dark and illumination and the external quantum efficiency was determined for each from the prepared samples.

A complex structure of four double layers of Ag / As2S3 has been deposited by Pulsed Laser Deposition method on a glass substrate. The effects of thermal annealing on the structural and optical properties were investigated. An effect of layer mixing has been evidenced.

A sandwich structure of four double layers of Ag (5 nm)/As2S3 (82.7 nm) has been prepared by Pulsed Laser Deposition (PLD) method. The effect of broadband light on the multilayer structure has been investigated. The X-ray reflectivity (XRR) patterns after different irradiation times have been measured. Although the fully intermixing of Ag and As2S3 layers should be expected during irradiation with visible light, however even after 5 h of irradiation the intermixing is not completed. The characteristic features of XRR diagrams for long irradiation times have been interpreted by scattering of X-ray radiation on clusters of Ag or Ag-As2S3. (c) 2013 Elsevier B.V. All rights reserved.

We have prepared complex multilayer films based on Barium stearate. The multilayer samples have been deposited by the Langmuir-Blodgett technique. The complex multilayers were also deposited on body sensor. The sample was then subjected to the illumination with UV light. We observed changes in the resistivity of the multilayer during the illumination

Thin films of arsenic sulphide have been obtained by Pulsed Laser Deposition from bulk As(2)S(3). A very thin layer of Eu(2)O(3) was deposited by PLD from a different target in the next deposition process. After a heat treatment at 180 degrees C for 45 minutes in inert atmosphere a structural transformation of the amorphous As(2)S(3) to realgar (As(2)S(2)) occurred and the film develops a strong luminescence effect.

New sensors based on single wall carbon nanotubes (SWCNTs) and porphyrins have been tested for sensitivity to NO(2) gas. SWCNTs embedded in barium stearate multilayers were deposited with the help of Langmuir-Blodgett technique. It was demonstrated a sensitive effect of the sensor for NO(2) gas. The coverage of the sensor with metalloporphyrin (Mn) leads to strong enhancing of the sensitive effect to NO(2), besides, the selectivity was preserved both around room temperature and at larger operating temperature (up to 200 degrees C). No sensitivity to CO and CH(4) was evidenced. The NO(2) sensitivity strongly increased at 100 degrees C but at high operation temperatures (150-200 degrees C) a different behaviour is observed, probably due to the structural change of porphyrin or porphyrin-SWCNT interaction. The reversibility of the sensor resistivity is reasonably good at the optimum operating temperature (100 degrees C).

We modeled the equilibrium structure, at 0 K temperature, of some spherical As(2)S(3) molecules with only 12 rings of ten member (alternating As and S atoms) and 20 triangular faces: As(60)S(90) (like C(60)), As(140)S(210) (like C(140)), As(320)S(480) (like C(320)), grown on carbon fullerene and As(20)S(30) (like C(20)). We used a Monte-Carlo relaxation procedure in the frame of valence force fields theory. The onion-like configurations of the arsenic sulphide was demonstrated as possible. The deposited films of As(2)S(3) seen in the cross-section prove the presence of onion-like configurations even in pure As(2)S(3) material.

Photo-diffusion of silver across an As2S3 thin film layer is studied, as continuing some earlier work about surface morphology modifications in double-layer structures of Ag and As2S3, and some qualitative preliminary results on lateral Ag diffusion in As2S3 thin films. An estimation of the Ag film's dissolution rate and the diffusion front's advancement rate in As,S3 is given based on crosssection measurements. In order to understand better the diffusion mechanism of Ag in As2S3, a structural model of the As2S3 matrix is used to estimate the amount of silver which might be hosted by an As2S3 layer in terms of molar ratio, and an interaction mechanism of the Ag with the chalcogenide matrix is proposed. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

There was discovered a material with strong photoconduction effect. Langmuir-Blodgett film with barium stearate, and selenium carbazole has been prepared and tested for the change of conductivity (decrease) under the influence of UV-radiation. Change of resistance in a time scale of 0-400 seconds was evidenced. The UV radiation determines an increase of the electrical resistence and after switching off the illumination a slow decrease of the resistance occurs.

Non-linear phenomena have been observed in the Ag(paint)-As2S3 system. This behaviour is useful in rectifier devices and in dynamical switching devices based on chalcogenides.

Networks of dots made of As2S3 chalcogenide has been obtained by Vacuum Evaporation and Pulsed Laser Deposition from bulk glassy As2S3 by the adapted method of SCEV (screen evaporation method). A very thin layer of Eu2O3 was deposited from a different target in the final deposition process. After a heat treatment at 180 degrees C for 90 minutes in nitrogen atmosphere a structural transformation of the amorphous As2S3 to realgar (AsS) occurred in the PLD deposited dots and the network of dots develops a luminescence effect. (Received November 25, 2011; accepted December 2, 2011)

Chalcogenide phase change materials are one of the major contenders for the new non-volatile memory applications. Here is reported that the switching speed of Ge2Sb2Te5 is strongly dependent on the percent of defects in the material. Using cellular automata simulations it was shown that the size of the percolation cluster is minimum, thus the switching speed is maximum, for a percent of around 25% defects in the material. This is a property of the Ge2Sb2Te5 that can be useful for new phase change materials and devices design with better switching properties.

The synthesis of the chalcogenide glass (Ge5As2S13) doped by Nd3+ has been carried out. The bulk glass develops a small amount of Nd2O3 and As2O3 crystallites besides some amount of (NdCl3)*6H(2)O crystalline phase initially introduced during synthesis. The optical absorption spectra demonstrate the presence of the Nd3+ both in the amorphous phase and in crystalline combinations. The formation of Nd2O3 crystallites was explained by the reaction of water with neodymium chloride. The presence of As2O3 is due to the oxidation of As from the glass during preparation.

The electrical switching in solid compositions based on chalcogenide materials is discussed. Different mechanisms proposed for switching effect are reviewed. A new description of the switching phenomenon is done. The switching is regarded as due to formation and breaking of the links between the dendrites of crystalline nuclei in bulk materials, as a consequence of the energy pumped by an electrical field. This mechanism explains the very short switching time (<20ns), the possibility to get smart memories based on multisteps of resistivity and the high number of cycles supported by the cell (10(16)).

As2S3 on Ag heterostructures deposited onto microscope glass substrates have been prepared. The quality of the hetero-structures has been investigated by careful analysis of the scanning electron microscope pictures taken on the native cross-sections, produced after the vacuum deposition sequence and various green laser irradiation times. The As2S3 layer is not uniformly grown, but reveals a columnar-like structure and multi-scale aggregation.

Barium stearate multilayer samples have been deposited by the Langmuir-Blodgett technique. Carbon nanotubes have been added in the deposition solution. The structural properties have been studied. The effect of the doped multilayers has been tested against the solution of ammonium nitrate in water. The effect of (5,10,15,20-tetraphenyl)-porphinato manganese (III) chloride, (MnTPP)Cl, on the multilayer sample has been tested. The change of resistivity of the multilayer samples modified by manganese porphyrin, as a function of the ultraviolet radiation has been discovered and investigated. The increase of the resistance during irradiation is a reversible process, although a slow one. This effect could give a basis for applications in UV sensors and switches. Finally, a photo-resistive effect (induced by UV light) has been discovered in Barium stearate layers doped by silver nitrate.

Langmuir films and Langmuir-Blodgett multilayers based on fatty acid (stearic acid, barium stearate) mixed with carbon nanotubes have been prepared and investigated by isothermal compression (for Langmuir films) and X-ray diffraction (for Langmuir-Blodgett multilayers). An irreversible modification has been observed in the case of Langmuir films doped by carbon nanotubes, due to reordering of the nanotubes in the film matrix.