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18 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Layered transition metal thiophosphates /MPX3/ as photoelectrodes in photoelectrochemical cells

Layered crystals of the transition metal thiophosphates were synthesized and characterized for use as photoelectrodes in photoelectrochemical cells. Crystals incorporating tin and manganese show n-type response while those with iron and nickel show p-type response. These materials have a measured indirect bandgap of about 2.1 eV. They show ability to photoelectrolyze water in acid solutions with onset potentials which change in a Nernstian way as the PH of the solution changes. The onset potential is near zero volts versus a saturated calomel electrode at pH 2. At n-type crystals, oxygen could be evolved upon irradiation at underpotentials of 850 mV and at p-type crystals, hydrogen could be evolved at underpotentials of 400 mV, indicating a net gain in energy conversion. All crystals were unstable in basic solution. Liquid junction photovoltaic cells in iodide-triiodide acid solution using these layered materials were also constructed and found to have low efficiences.

Byvik, C. E.↗
DOE OSTI2024

Understanding and tuning magnetism in van der Waals-type metal thiophosphates

Over the past two decades, significant progress in two-dimensional (2D) materials has invigorated research in condensed matter and material physics in low dimensions. While traditionally studied in three-dimensional systems, magnetism has now been extended to the 2D realm. Recent breakthroughs in 2D magnetism have attracted substantial interest from the scientific community, owing to the stable magnetic order achievable in atomically thin layers of the van der Waals (vdW)-type layered magnetic materials. These advances offer an exciting platform for investigating related phenomena in low dimensions and hold promise for spintronic applications. Consequently, vdW magnetic materials with tunable magnetism have attracted significant attention. Specifically, antiferromagnetic metal thiophosphates MPX 3 (M = transition metal, P = phosphorus, X = chalcogen) have been investigated extensively. These materials exhibit long-range magnetic order spanning from bulk to the 2D limit. The magnetism in MPX 3 arises from localized moments associated with transition metal ions, making it tunable via substitutions and intercalations. In this review, we focus on such tuning by providing a comprehensive summary of various metal- and chalcogen-substitution and intercalation studies, along with the mechanism of magnetism modulation, and a perspective on the development of this emergent material family.

42 ENGINEERING↗
DOE OSTI2025

Tuning magnetism in Ising-type van der Waals magnet FePS 3 by lithium intercalation

Recently, layered transition metal thiophosphate MPX 3 (M = transition metals, X = S or Se) have gained significant attention because of their rich magnetic, optical, and electronic properties. Specifically, the diverse magnetic structures and the robustness of magnetism in the two-dimensional (2D) limit have made them prominent candidates to study 2D magnetism. Numerous efforts such as substitutions and interlayer intercalations have been adopted to tune the magnetic properties of these materials, which has greatly deepened the understanding of the underlying mechanisms that govern the properties. In this work, we focus on modifying the magnetism of Ising-type antiferromagnet FePS 3 using electrochemical lithium intercalation. Furthermore, our work demonstrate the effectiveness of electrochemical intercalation as a controllable tool to modulating magnetism, including tuning magnetic ordering temperature and inducing low temperature spin-glass state, offering an approach for implementing this material into applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Controlling magnetic exchange and anisotropy by nonmagnetic ligand substitution in layered 𝑀⁢P⁢𝑋 3 (𝑀 = Ni, Mn; 𝑋 = S, Se)

Recent discoveries in two-dimensional magnetism have intensified the investigation of van der Waals magnetic materials and further improved our ability to tune their magnetic properties. Tunable magnetism has been widely studied in antiferromagnetic metal thiophosphates 𝑀⁢P⁢𝑋 3 . Substitution of metal ions 𝑀 has been adopted as an important technique to engineer the magnetism in 𝑀⁢P⁢𝑋 3 . In this work, we have studied the previously unexplored chalcogen 𝑋 substitutions in 𝑀⁢P⁢𝑋 3 (𝑀 = Mn/Ni; 𝑋 = S/Se). We synthesized the single crystals of MnP⁢S 3−𝑥 ⁢Se 𝑥 (0 ≤ 𝑥 ≤ 3) and NiP⁢S 3−𝑥 ⁢Se 𝑥 (0 ≤ 𝑥 ≤ 1.3) and investigated the systematic evolution of the magnetism with varying 𝑥. Our study reveals the effective tuning of magnetic interactions and anisotropies in both MnPS 3 and NiPS 3 upon Se substitution. Such efficient engineering of the magnetism provides a suitable platform to understand the low-dimensional magnetism and develop future magnetic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Tuning the Structural and Magnetic Properties in Mixed Cation Mn x Co 2– x P 2 S 6

The metal thiophosphates (MTP), M 2 P 2 S 6, are a versatile class of van der Waals materials, which are notable for the possibility of tuning their magnetic properties with the incorporation of different transition-metal cations. Further, they also offer opportunities to probe the independent and synergistic role of the magnetically active cation sublattice when coupled to P 2 Q 6 polyhedra. Herein, we report the structural, magnetic, and electronic properties of the series of MTPs, Mn x Co 2– x P 2 S 6 ( x = 0.25, 0.5, 1, 1.5, 1.75) synthesized by the P 2 S 5 flux method. In this work, structural and elemental analysis indicates a homogeneous stoichiometry in the Mn x Co 2– x P 2 S 6 compounds. We observe that a correlation is apparent between the intensities of specific Raman modes and Raman shifts with respect to the alloying ratio between Mn and Co. Magnetic susceptibility measurements indicate that the alloyed systems adopt an ordered antiferromagnetic (AFM) configuration with a dependence of the Néel temperature on the alloying ratio. A possible magnetic frustration behavior was observed for the composition MnCoP 2 S 6 due to magnetic moment compensation as the alloying ratio between Mn and Co approaches parity. Interestingly, mixed oxidation states of the metal cation species are also observed in Mn x Co 2– x P 2 S 6 along with a linear dependence of the work function on the alloying ratio of Mn and Co.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Highly sensitive spin-flop transition in antiferromagnetic van der Waals material M PS 3 ( M = Ni and Mn)

Recent developments in two-dimensional (2D) magnetism have motivated the search for van der Waals (vdW) magnetic materials to explore magnetic phenomena in the 2D limit. Metal thiophosphates, MPX 3 , are a class of magnetic vdW materials with antiferromagnetic ordering persisting down to the atomically thin limit. The magnetism in this material family has been found to be highly dependent on the choice of transition metal M. In this paper, we have synthesized the intermediate compounds Ni 1–x Mn x PS 3 (0 ≤ x ≤ 1) and investigated their magnetic properties. Our study reveals that the variation of Ni and Mn content in Ni 1–x Mn x PS 3 can efficiently tune the spin-flop transition, likely due to the modulation of the magnetic anisotropy. Such effective tuning offers a promising candidate to engineer 2D magnetism for future device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Dynamic Stabilization of Metastable States in Triple-Well Ferroelectric Sn 2 P 2 S 6

Polarization dynamics in ferroelectric materials is governed by the effective potential energy landscape of the order parameter. The unique aspect of ferroelectrics compared to many other transitions is the possibility of more than two potential wells, leading to complicated energy landscapes with new fundamental and functional properties. In this work, direct dynamic evidence is revealed of a triple-well potential in the metal thiophosphate Sn 2 P 2 S 6 compound using multivariate scanning probe microscopy combined with theoretical simulations. The key finding is that the metastable zero polarization state can be accessed through a gradual switching process and is stabilized over a broad range of electric fields. Simulations confirm that the observed zero polarization state originates from a kinetic stabilization of the nonpolar state of the triple-well, as opposed to domain walls. Dynamically, the triple-well of Sn 2 P 2 S 6 becomes equivalent to antiferroelectric hysteresis loops. Therefore, this material combines the robust and well-defined domain structure of a proper ferroelectric with dynamic hysteresis loops present in antiferroelectrics. Moreover, the triple-well enhances mem-capacitive effects in Sn 2 P 2 S 6 , which are forbidden for ideal double-well ferroelectrics. These findings provide a path to tunable electronic elements for beyond binary high-density computing devices and neuromorphic circuits based on dynamic properties of the triple-well.

36 MATERIALS SCIENCE↗
DOE OSTI2026

First-Principles Discovery of Novel LiInP 2 S 6 Polymorphs with Promising Optoelectronic Responses

Recent advances in two-dimensional (2D) van der Waals (vdW) metal thiophosphates have attracted considerable attention due to their promising ionic conductivity, optical characteristics, and tunable physical properties. Within this material family, LiInP 2 S 6 has emerged as an intriguing candidate, not only because of its sensitivity to air and moisture but also due to its suitable band gap within the UV−vis range, enabling potential optoelectronic and photocatalytic applications. In this study, through comprehensive first-principles investigations, we unveil two previously unreported polymorphs of LiInP 2 S 6 in the monoclinic C2/c and trigonal P3̅1c (in-gap) space groups, in addition to examining the experimentally synthesized P3̅1c (in-layer) phase. Our studies identify the C2/c structure as the ground state, lying 9 meV per unit cell lower in energy than the experimentally realized trigonal P3̅1c (in-layer) phase. Further, we systematically examine the elastic, mechanical, thermodynamical, dynamical, electronic, and optical properties of all three polymorphs, confirming their mechanical, thermal, and dynamical stability. Notably, the P3̅1c (in-gap) phase exhibits enhanced stiffness, while the calculated indirect band gaps and strong photon absorption in the UV−vis range (∼3 eV) highlight the potential of the studied LiInP 2 S 6 phases for iontronic devices and optoelectronic applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Observation of Giant Optical Linear Dichroism in a Zigzag Antiferromagnet FePS 3

Direct optical probing of the antiferromagnetic order parameter in atomically thin samples is challenging, for example, via magneto-optical spectroscopy, due to the lack of net magnetization. Here, we report zigzag-antiferromagnetism (AFM) induced optical linear dichroism (LD) in layered transition-metal thiophosphate FePS3 down to the monolayer limit. The observed LD is giant despite having the optical wave vector parallel to the Neel vector. The LD is at least one order of magnitude larger than those reported in other antiferromagnetic systems, where the optical wave vector is orthogonal to the Neel vector. The large LD enables the probe of 60 degrees orientated zigzag-AFM domains. The optical anisotropy in FePS 3 originates from an electronic anisotropy associated with the zigzag direction of the AFM order and is independent of the spin-pointing direction. Furthermore, our findings point to a new optical approach for the investigation and control of zigzag or stripe magnetic order in strongly correlated systems.

2D magnet↗
DOE OSTI2022

Nanoscale Control of Polar Surface Phases in Layered van der Waals CuInP 2 S 6

Antiferroelectric (AFE) materials, in which alternating dipole moments cancel out to a zero net macroscopic polarization, can be used for high-density energy storage and memory applications. The AFE phase can exist in bulk CuInP 2 Se 6 , CuBiP 2 S 6 , and a few other transition-metal thiophosphates below 200 K. The required low temperature poses challenges for practical applications. In this work, we report the coexistence of ferrielectric (FE) states and a stable surface phase that does not show piezoelectric response (“zero-response phase”) in bulk CuInP 2 S 6 at room temperature. Using piezoresponse force microscopy (PFM) tomographic imaging together with density functional theory, we find that direct and alternating voltages can locally and stably convert FE to zero-response phases and vice versa. While PFM loops show pinched hystereses reminiscent of antiferroelectricity, PFM tomography reveals that the zero-response areas form only on top of the FE phase in which the polarization vector is pointing up. Theoretical calculations suggest that the zero-response phase may correspond to AFE ordering where stacked CuInP 2 S 6 layers have alternating polarization orientations thereby leading to a net-zero polarization. Consistent with experimental findings, theory predicts that the FE polarization pointing down is robust up to the top surface, whereas FE polarization pointing up energetically favors the formation of an AFE surface layer, whose thickness is likely to be sensitive to local strains. AFE order is likely to be more robust against detrimental size effects than polar order, therefore providing additional opportunities to create multifunctional heterostructures with 2D electronic materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Emergence of ferrimagnetism in Li-intercalated NiPS 3

AbstractIntercalation has become a powerful approach to tune the intrinsic properties and introduce novel phenomena in layered materials. Intercalating van der Waals (vdW) magnetic materials is a promising route to engineer the low-dimensional magnetism. Recently, metal thiophosphates, MPX 3 , has been widely studied because their magnetic orders are highly tunable and persist down to the two-dimensional limit. In this work, we used electrochemical technique to intercalate Li into NiPS 3 single crystals and found the emergence of ferrimagnetism at low temperature in Li-intercalated NiPS 3 . Such tuning of magnetic properties highlights the effectiveness of intercalation, providing a novel strategy to manipulate the magnetism in vdW magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1983

Semiconductor photoelectrochemistry

Semiconductor photoelectrochemical reactions are investigated. A model of the charge transport processes in the semiconductor, based on semiconductor device theory, is presented. It incorporates the nonlinear processes characterizing the diffusion and reaction of charge carriers in the semiconductor. The model is used to study conditions limiting useful energy conversion, specifically the saturation of current flow due to high light intensity. Numerical results describing charge distributions in the semiconductor and its effects on the electrolyte are obtained. Experimental results include: an estimate rate at which a semiconductor photoelectrode is capable of converting electromagnetic energy into chemical energy; the effect of cell temperature on the efficiency; a method for determining the point of zero zeta potential for macroscopic semiconductor samples; a technique using platinized titanium dioxide powders and ultraviolet radiation to produce chlorine, bromine, and iodine from solutions containing their respective ions; the photoelectrochemical properties of a class of layered compounds called transition metal thiophosphates; and a technique used to produce high conversion efficiency from laser radiation to chemical energy.

Buoncristiani, A. M.↗
DOE OSTI2022

Ionic Control over Ferroelectricity in 2D Layered van der Waals Capacitors

The van der Waals layered material CuInP 2 S 6 features interesting functional behavior, including the existence of four uniaxial polarization states, polarization reversal against the electric field through Cu ion migration, a negative-capacitance regime, and reversible extraction of Cu ions. At the heart of these characteristics lies the high mobility of Cu ions, which also determines the spontaneous polarization. Therefore, Cu migration across the lattice results in unusual ferroelectric behavior. Here, we demonstrate how the interplay of polar and ionic properties provides a path to ionically controlled ferroelectric behavior, achieved by applying selected DC voltage pulses and subsequently probing ferroelectric switching during fast triangular voltage sweeps. Using current measurements and theoretical calculations, we observe that increasing DC pulse duration results in higher ionic currents, the buildup of an internal electric field that shifts polarization loops, and an increase in total switchable polarization by ~50% due to the existence of a high polarization phase which is stabilized by the internal electric field. Apart from tuning ferroelectric behavior by selected square pulses, hysteretic polarization switching can even be entirely deactivated and reactivated, resulting in three-state systems where polarization switching is either inhibited or can be performed in two different directions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Strongly adhesive dry transfer technique for van der Waals heterostructure

That one can stack van der Waals materials with atomically sharp interfaces has provided a new material platform of constructing heterostructures. The technical challenge of mechanical stacking is picking up the exfoliated atomically thin materials after mechanical exfoliation without chemical and mechanical degradation. Chemically inert hexagonal boron nitride (hBN) has been widely used for encapsulating and picking up vdW materials. However, due to the relatively weak adhesion of hBN, assembling vdW heterostructures based on hBN has been limited. Here, we report a new dry transfer technique. We used two vdW semiconductors (ZnPS 3 and CrPS 4 ) to pick up and encapsulate layers for vdW heterostructures, which otherwise are known to be hard to fabricate. By combining with optimized polycaprolactone (PCL) providing strong adhesion, we demonstrated various vertical heterostructure devices, including quasi-2D superconducting NbSe 2 Josephson junctions with atomically clean interface. The versatility of the PCL-based vdW stacking method provides a new route for assembling complex 2D vdW materials without interfacial degradation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Revisiting two thiophosphate compounds constituting d 0 transition metal HfP 2 S 6 and d 10 transition metal α-Ag 4 P 2 S 6 as multifunctional materials for combining second harmonic generation response and photocurrent response

Two acentric thiophosphate compounds, HfP 2 S 6 and α-Ag 4 P 2 S 6 , are revisited and studied as infrared nonlinear optical materials. HfP 2 S 6 and α-Ag 4 P 2 S 6 were structurally characterized without any property measurements. Here, in this work, HfP 2 S 6 and α-Ag 4 P 2 S 6 were synthesized via high temperature salt flux reactions. Low-temperature polymorph acentric α-Ag 4 P 2 S 6 was purified and grown as mm-sized crystals with the aid of AgBr flux. The AgBr flux was revealed to play an important role in stabilizing the acentric α-Ag 4 P 2 S 6 . The acentric α-Ag 4 P 2 S 6 transforms to centrosymmetric β-Ag 4 P 2 S 6 at 850(5) K, which is revealed by differential scanning calorimetry (DSC) analysis and powder X-ray diffraction experiments. HfP 2 S 6 and α-Ag 4 P 2 S 6 are discovered by UV-vis spectrum measurements as indirect bandgap semiconductors with bandgaps of 2.2(1) eV and 2.5(1) eV, respectively, which is supported by DFT calculations and TB-LMTO-ASA calculations. The bonding pictures of α-Ag 4 P 2 S 6 were studied by crystal orbital Hamilton population calculations (COHP) coupled with electron localization function (ELF) analysis. DFT calculations predict that HfP 2 S 6 and α-Ag 4 P 2 S 6 would exhibit different optical performances regardless of being constructed from identical [P 2 S 6 ] motifs. HfP 2 S 6 exhibits a low second harmonic generation (SHG) response, ~0.21 × AGS (for the sample of particle size of 25 μm). α-Ag 4 P 2 S 6 possesses moderate SHG response, ~0.61 × AGS (for the sample of particle size of 225 μm) coupled with a high laser damage threshold (LDT) of ~3.2 × AGS. Characteristics of high ambient stability, moderate bandgap and SHG response, type-I phase-matching capability, and high LDT together with the easy growth of large crystals make α-Ag 4 P 2 S 6 attractive for future infrared nonlinear optical applications. Photocurrent measurements found that α-Ag 4 P 2 S 6 and β-Ag 4 P 2 S 6 have high photocurrent response, 165 nA cm -2 and 135 nA cm -2 , respectively. α-Ag 4 P 2 S 6 is a new multifunctional material of the ternary Ag–P–S system, which combines nonlinear optical (NLO) properties and photocurrent response.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Investigation of glass-ceramic lithium thiophosphate solid electrolytes using NMR and neutron scattering

Solid-state Li batteries require solid electrolytes which have high Li+ conductivity and good chemical/mechanical compatibility with Li metal anodes and high energy cathodes. Structure/function correlations which relate local bonding to macroscopic properties are needed to guide development of new solid electrolyte materials. This study combines diffraction measurements with solid-state nuclear magnetic resonance spectroscopy (ssNMR) and neutron pair distribution function (nPDF) analysis to probe the short-range vs. long-range structure of glass-ceramic Li 3 PS 4 -based solid electrolytes. Here, we demonstrate how different synthesis conditions (e.g., solvent selection and thermal processing) affect the resulting polyanionic network. More specifically, structures with high P coordination numbers (e.g., PS 4 3– and P 2 S 7 4– ) correlate with higher Li + mobility compared to other polyanions (e.g., (PS 3 ) n n– chains and P 2 S 6 4– ). Overall, this work demonstrates how ssNMR and nPDF can be used to draw key structure/function correlations for solid-state superionic conductors.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Atomic layer deposited aluminum nitride for interface protection of lithium germanium thiophosphate solid electrolytes

Lithium germanium thiophosphate (LGPS) is an attractive solid-state electrolyte material due to its exceptionally high ionic conductivity (~1.2 × 10 –2 S cm –1 ), comparable to many organic liquid electrolytes commonly used in batteries. Despite the high conductivity of LGPS, the susceptibility of LGPS to deleterious degradation reactions has impeded its commercial adoption into solid-state batteries. In particular, the poor voltage stability of LGPS with high-voltage cathode or lithium metal potentials often results in dramatically increasing cell impedance during galvanic cycling. Here, we use aluminum nitride (AlN) as a protection layer for Li metal anode, applied directly to the LGPS at 250 °C using plasma-enhanced atomic layer deposition, to enhance cell performance by preventing LGPS-Li metal reactions. We compare the surface chemistry and electrochemical cycling performance of atomic layer-deposited AlN grown using both plasma N 2 and NH 3 precursors. Galvanostatic cycling and electrochemical impedance spectroscopy show that AlN-coated LGPS cells perform better than bare LGPS cells in contact with Li metal anodes, with the AlN able to improve cycling longevity by over a factor of 3 in certain cases. Finally, we utilize x-ray photoelectron spectroscopy (XPS) line scans to highlight the slow room-temperature reactivity between AlN and evaporated lithium metal, and a computational model is built to aid further XPS analysis.

25 ENERGY STORAGE↗
DOE OSTI2025

Exploring the Electrochemical Stability Window of an All-Solid-State Composite Cathode via a Novel Operando Tender XPS Setup

All-solid-state batteries (ASSBs) have the potential to provide greater energy density than conventional batteries based on liquid electrolytes. Here, an operando ASSB cell setup for tender X-ray photoelectron spectroscopy (XPS) was developed, and the interface of a Ni-rich layered transition metal oxide cathode active material (CAM) and an Li 6 PS 5 Cl (LPSCl) solid electrolyte (SE) was evaluated during initial charge/discharge cycles. After validating the cell performance against a conventional pouch cell operated at high compression, intermittent galvanostatic cycling was performed, and XPS data were recorded as a function of state of charge (SOC). Upon the initial charge of the cell to ≈3.3 V Li , the LPSCl appears to decompose into LiCl, Li 3 PS 4 , and polysulfides, whose amount gradually increases with potential. Upon further charge, at a potential higher than ≈3.8 V Li , initially, present sulfate and sulfite impurities decompose, and at ≈74% SOC (corresponding to a cathode potential of ≈4.10 V Li ), surface reconstruction of the CAM particles due to lattice oxygen release is detected. In addition, at potentials beyond ≈4.6 V Li , a decrease of the S 1s counts of the sum of the LPSCl, the thiophosphate, and polysulfide species suggests the formation of elemental sulfur that is lost via sublimation into the vacuum chamber.

25 ENERGY STORAGE↗