SEARCH · Search NASA
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
We report the synthesis of layered anisotropic semiconductor GeSe and GeSe 2 nanomaterials through low temperature and atmospheric pressure chemical vapor deposition using halide based precursors. The crystal phase is controlled by simply changing selenium vapor pressure.
Here, we have used highly accurate quantum Monte Carlo methods to determine the chemical structure and electronic band gaps of monolayer GeSe. Two-dimensional (2D) monolayer GeSe has received a great deal of attention due to its unique thermoelectric, electronic, and optoelectronic properties with a wide range of potential applications. Density functional theory (DFT) methods have usually been applied to obtain optical and structural properties of bulk and 2D GeSe. For the monolayer, DFT typically yields a larger band-gap energy than for bulk GeSe but cannot conclusively determine if the monolayer has a direct or indirect gap. Moreover, the DFT-optimized lattice parameters and atomic coordinates for monolayer GeSe depend strongly on the choice of approximation for the exchange-correlation functional, which makes the ideal structure-and its electronic properties-unclear. In order to obtain accurate lattice parameters and atomic coordinates for the monolayer, we use a surrogate Hessian-based parallel line search within diffusion Monte Carlo to fully optimize the GeSe monolayer structure. The DMC-optimized structure is different from those obtained using DFT, as are calculated band gaps. The potential energy surface has a shallow minimum at the optimal structure. This, combined with the sensitivity of the electronic structure to strain, suggests that the optical properties of monolayer GeSe are highly tunable by strain.
Abstract Performance of the group IV monochalcogenide GeSe in solar cells, electronic, and optoelectronic devices is expected to improve when high‐quality single crystalline material is used rather than polycrystalline films. Crystalline flakes represent an attractive alternative to bulk single crystals as their synthesis may be developed to be scalable, faster, and with higher overall yield. However, large – and especially large and thin – single crystal flakes are notoriously hard to synthesize. Here it is demonstrated that vapor‐liquid‐solid growth combined with direct lateral vapor‐solid incorporation produces high‐quality single crystalline GeSe ribbons with tens of micrometers size and controllable thickness. Electron microscopy shows that the ribbons exhibit perfect equilibrium (AB) van der Waals stacking order without extended defects across the entire thickness, in contrast to the conventional case of substrate‐supported flakes where material is added via layer‐by‐layer nucleation and growth on the basal plane. Electrical measurements show anisotropic transport and a high Hall mobility of 85 cm 2 V −1 s −1 , on par with the best single crystals to date. Growth from mixed GeSe and SnSe vapors, finally, yields ribbons with unchanged structure and composition but with jagged edges, promising for applications that rely on ample chemically active edge sites, such as catalysis or photocatalysis.
The emergence of atomically thin crystals has allowed extending materials integration to lateral heterostructures where different 2D materials are covalently connected in the plane. The concept of lateral heterostructures can be generalized to thicker layered crystals, provided that a suitably faceted seed crystal presents edges to which a compatible second van der Waals material can be attached layer by layer. Here, we examine the possibility of integrating multilayer crystals of the group IV monochalcogenides SnS and GeSe, which have the same crystal structure, small lattice mismatch, and similar bandgaps. In a two-step growth process, lateral epitaxy of GeSe on the sidewalls of multilayer SnS flakes (obtained by vapor transport of a SnS 2 precursor on graphite) yields heterostructures of laterally stitched crystalline GeSe and SnS without any detectable vertical overgrowth of the SnS seeds and with sharp lateral interfaces. Combined cathodoluminescence spectroscopy and ab initio calculations show the effects of small band offsets on carrier transport and radiative recombination near the interface. Further, the results demonstrate the possibility of forming atomically connected lateral interfaces across many van der Waals layers, which is promising for manipulating optoelectronics, photonics, and for managing charge- and thermal transport.
GeSe crystallizes in the orthorhombic Pnma space group. The structure is two-dimensional and consists of two GeSe sheets oriented in the (0, 0, 1) direction. Ge2+ is bonded in a 3-coordinate geometry to three equivalent Se2- atoms. There are one shorter (2.59 Å) and two longer (2.61 Å) Ge–Se bond lengths. Se2- is bonded in a distorted trigonal non-coplanar geometry to three equivalent Ge2+ atoms.
GeSe is Halite, Rock Salt structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Ge2+ is bonded to six equivalent Se2- atoms to form a mixture of corner and edge-sharing GeSe6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Ge–Se bond lengths are 2.83 Å. Se2- is bonded to six equivalent Ge2+ atoms to form a mixture of corner and edge-sharing SeGe6 octahedra. The corner-sharing octahedral tilt angles are 0°.
Co2(GeSe)3 is Hausmannite-derived structured and crystallizes in the trigonal R3 space group. The structure is three-dimensional. there are four inequivalent Co sites. In the first Co site, Co is bonded to three equivalent Ge and three equivalent Se atoms to form CoGe3Se3 octahedra that share corners with six equivalent CoGe3Se3 octahedra, corners with six GeCo2Se2 tetrahedra, and corners with six SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 52–58°. All Co–Ge bond lengths are 2.35 Å. All Co–Se bond lengths are 2.37 Å. In the second Co site, Co is bonded to three equivalent Ge and three equivalent Se atoms to form CoGe3Se3 octahedra that share corners with six equivalent CoGe3Se3 octahedra, corners with six GeCo2Se2 tetrahedra, and corners with six SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 51–58°. All Co–Ge bond lengths are 2.35 Å. All Co–Se bond lengths are 2.36 Å. In the third Co site, Co is bonded to three Ge and three Se atoms to form CoGe3Se3 octahedra that share corners with six CoGe3Se3 octahedra, corners with six GeCo2Se2 tetrahedra, and corners with six SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 51–58°. There are two shorter (2.33 Å) and one longer (2.35 Å) Co–Ge bond lengths. There are a spread of Co–Se bond distances ranging from 2.37–2.41 Å. In the fourth Co site, Co is bonded to three Ge and three Se atoms to form CoGe3Se3 octahedra that share corners with six CoGe3Se3 octahedra, corners with six GeCo2Se2 tetrahedra, and corners with six SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 51–58°. There are one shorter (2.32 Å) and two longer (2.34 Å) Co–Ge bond lengths. There are a spread of Co–Se bond distances ranging from 2.37–2.40 Å. There are four inequivalent Ge sites. In the first Ge site, Ge is bonded to two Co and two Se atoms to form distorted GeCo2Se2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four GeCo2Se2 tetrahedra, corners with six SeCo2Ge2 tetrahedra, and an edgeedge with one GeCo2Se2 tetrahedra. The corner-sharing octahedra tilt angles range from 70–71°. There are one shorter (2.57 Å) and one longer (2.75 Å) Ge–Se bond lengths. In the second Ge site, Ge is bonded to two Co and two Se atoms to form distorted GeCo2Se2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four GeCo2Se2 tetrahedra, corners with six SeCo2Ge2 tetrahedra, and an edgeedge with one GeCo2Se2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–72°. There are one shorter (2.58 Å) and one longer (2.74 Å) Ge–Se bond lengths. In the third Ge site, Ge is bonded to two Co and two Se atoms to form distorted GeCo2Se2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four GeCo2Se2 tetrahedra, corners with six SeCo2Ge2 tetrahedra, and an edgeedge with one GeCo2Se2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–72°. There are one shorter (2.58 Å) and one longer (2.73 Å) Ge–Se bond lengths. In the fourth Ge site, Ge is bonded to two Co and two Se atoms to form distorted GeCo2Se2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four GeCo2Se2 tetrahedra, corners with six SeCo2Ge2 tetrahedra, and an edgeedge with one GeCo2Se2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–71°. There are one shorter (2.58 Å) and one longer (2.74 Å) Ge–Se bond lengths. There are four inequivalent Se sites. In the first Se site, Se is bonded to two Co and two Ge atoms to form distorted SeCo2Ge2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four SeCo2Ge2 tetrahedra, corners with six GeCo2Se2 tetrahedra, and an edgeedge with one SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–74°. In the second Se site, Se is bonded to two Co and two Ge atoms to form distorted SeCo2Ge2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four SeCo2Ge2 tetrahedra, corners with six GeCo2Se2 tetrahedra, and an edgeedge with one SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 70–74°. In the third Se site, Se is bonded to two Co and two Ge atoms to form distorted SeCo2Ge2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four SeCo2Ge2 tetrahedra, corners with six GeCo2Se2 tetrahedra, and an edgeedge with one SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–73°. In the fourth Se site, Se is bonded to two Co and two Ge atoms to form distorted SeCo2Ge2 tetrahedra that share corners with four CoGe3Se3 octahedra, corners with four SeCo2Ge2 tetrahedra, corners with six GeCo2Se2 tetrahedra, and an edgeedge with one SeCo2Ge2 tetrahedra. The corner-sharing octahedra tilt angles range from 69–73°.
The layered semiconductor tin selenide (SnSe) has received extensive interest due to its promising thermoelectric and ferroelectric properties. Integrating SnSe with other layered crystals in heterostructures can enable the modification of charge- and thermal transport, electrical polarization, and other properties such as chemical stability, optoelectronics, and photonics. Here, we demonstrate the vapor transport synthesis of single-crystalline SnSe monochalcogenide flakes that are spontaneously encapsulated in a thin layered SnSe 2-x S x dichalcogenide shell. In a second growth step, such SnSe-SnSe 2-x S x heterostructures are integrated with the monochalcogenide GeSe, which is laterally stitched to the SnSe side facets while preserving the dichalcogenide shell across the basal facets. This architecture is confirmed by optical microscopy, electron microscopy and diffraction, energy dispersive X-ray and Raman spectroscopies, as well as cathodoluminescence spectroscopy. Furthermore, the results extend our capabilities for materials integration by forming complex heterostructures with both vertical van der Waals interfaces and covalent lateral interfaces between layered semiconductors.
The 3D integration technology in semiconductor fabrication requires a key component, the ovonic threshold switching (OTS) selector, to suppress the current leakage. The As doped amorphous (a-) GeSe glass is a commercialized OTS material in 3D phase-change memory, but the understanding of such a doping mechanism is still inadequate. Here we systematically explore the effect of As doping on the structural, bonding, and dynamics properties of a-GeAsSe using ab initio molecular dynamics simulations. The results reveal that As atoms form strong bonds with both Ge and Se atoms. The distorted octahedral structures and the 5-fold rings linked by atoms are increased. All of these structural features lead to a more disordered configuration. Moreover, as atoms have notably slowed down the atomic mobility, rendering a-GeAsSe a high stability. Overall, our studies offer insightful understanding of As-doping in OTS materials, paving the way for the design and application of advanced selector devices.
Analyses and modeling of the T -dependent thermal properties of Zn 2 GeSe 4 reveal an ultra-low thermal conductivity over a large temperature range due to low-frequency soft modes as well as strong lattice anharmonicity.
We analyse the functional capabilities of new crystals, BaGa{sub 2}GeS{sub 6} (BGGS) and BaGa{sub 2}GeSe{sub 6} (BGGSe), which are used for nonlinear optical frequency conversion in their transparency range. The wavelengths at which maximum conversion efficiencies can be obtained and the tuning range for difference-frequency generation are found. It is shown that there are wavelength combinations at which the effective nonlinearity coefficient varies only slightly in a wide frequency band. (nonlinear optical phenomena)
Not Available
While traditional ferroelectrics are based on polar crystals in bulk or thin film form, two-dimensional and layered materials can support mechanisms for symmetry breaking between centrosymmetric building blocks, e.g., by creating low-symmetry interfaces in van der Waals stacks. Here, we introduce an approach toward symmetry breaking in van der Waals crystals that relies on the spontaneous incorporation of stacking faults in a nonpolar bulk layer sequence. The concept is realized in nanowires consisting of Se-rich group IV monochalcogenide (GeSe 1–x S x ) alloys, obtained by vapor–liquid–solid growth. The single crystalline wires adopt a layered structure in which the nonpolar A-B bulk stacking along the nanowire axis is interrupted by single-layer stacking faults with local A-A' stacking. Density functional theory explains this behavior by a reduced stacking fault formation energy in GeSe (or Se-rich GeSe 1–x S x alloys). Computations demonstrate that, similar to monochalcogenide monolayers, the inserted A-layers should show a spontaneous electric polarization with a switching barrier consistent with a Curie temperature above room temperature. Second-harmonic generation signals are consistent with a variable density of stacking faults along the wires. Here, our results point to possible routes for designing ferroelectrics via the layer stacking in van der Waals crystals.
We compute the spectrum of pure spin current injection in ferroelectric single-layer SnS, SnSe, GeS, and GeSe. The formalism takes into account the coherent spin dynamics of optically excited conduction states split in energy by spin–orbit coupling. The velocity of the electron's spins is calculated as a function of incoming photon energy and angle of linearly polarized light within a full electronic band structure scheme using density functional theory. We find peak speeds of 520, 360, 270 and 370 Km s -1 for SnS, SnSe, GeS and GeSe, respectively which are an order of magnitude larger than those found in bulk semiconductors, e.g., GaAs and CdSe. Interestingly, the spin velocity is almost independent of the direction of polarization of light in a range of photon energies. Our results demonstrate that single-layer SnS, SnSe, GeS and GeSe are candidates to produce on demand spin-current in spintronics applications.
Group-IV monochalcogenides are a class of layered ferroelectric semiconductors that have demonstrated spontaneous intrinsic polarization above room temperature. Here, in this study, we use the multi-atomic cluster expansion (MACE) machine learning architecture to train and test a force field capable of modeling the structural properties and second-order ferroelectric-to-paraelectric phase transition in a Group-IV monochalcogenide, GeSe. The model captures the double-well potential energy surface associated with the onset of macroscopic polarization in bulk GeSe within 12.5 meV/atom, as well as near-equilibrium properties like the phonon dispersion. The development of this quantitatively accurate force field enables long-time molecular dynamics simulations, which predict the critical temperature of the ferroelectric-to-paraelectric phase transition in bulk GeSe to be T c = 600 K. This study demonstrates the capabilities of equivariant force-fields to accurately describe phenomena associated with structural symmetry breaking.
Research on phase change materials is predominantly focused on their application as memory devices or for temperature control which requires low phase change temperature. The Ge–Se binary chalcogenide glass system with its wide glass-forming region is a potential candidate for high-temperature and high-radiation phase change applications. In this study, the concept of employing Ge x Se 100– x glasses to monitor high temperature (450–528 °C) using the phase change effect, is reported. Materials selection, device structure, and performance of prototype sensors are analyzed. In addition, the effect of heavy ion irradiation by Xe ions with energies of 200, 600, and 1000 keV (fluence ≈10 14 cm –2 ) on the Ge x Se 100– x ( x = 30, 33, 40) thin films and phase change devices is studied. The irradiation effect on the amorphous and crystalline structure of the thin films is evaluated by Raman spectroscopy and X-ray diffraction (XRD). Although the changes in the structural units of amorphous films are negligible, in crystalline films orthorhombic-GeSe 2 crystals are found to be most affected by irradiation and a new phase, orthorhombic GeSe is found in the thin films after irradiation. The performance of a sensor with an active film of Ge 40 Se 60 is also shown as an example.