SEARCH · Engineering Papers
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.
Knudsen effusion studies of the sublimation of polycrystalline GeSe have been performed employing mass spectrometry. The results demonstrate that GeSe vaporizes congruently under present experimental conditions according to the reaction: GeSe(s) yields GeSe(g). The mean values for the third-law heat and second-law entropy of reaction based on direct mass-loss data are 42.0 + or - 1.5 kcal/mole and 42.3 + or - 1.6 eu respectively. From these data the standard heat of formation was calculated to be -10.1 + or - 2.0 kcal/mole, and the standard absolute entropy was determined to be 16.9 + or - 2.0 eu.
The thermal expansion of GeSe has been studied above room temperature up to the melting point of 670 plus or minus 5 C by X-ray diffraction techniques using a 190 mm Unicam high temperature camera. The thermal expansion of the crystallographic axes is linear with a distinct change in the expansion coefficients for all axes above 400 C. The relative changes in the axes indicate a rearrangement of the structure towards cubic symmetry with increasing temperature. The transformation of GeSe from the orthorhombic to a normal NaCl-type structure is observed at 651 plus or minus 5 C. The lattice parameter of the cubic form of GeSe is a 0 ? 5.730 plus or minus 0.003 A at 656 C. The GeSe lattice remains cubic up to the melting point.
The GeSe:GeI4 vapor crystal growth system was characterized mass spectroscopically. A steady-state Knudsen effusion technique was developed to simulate the equilibrium conditions at one end of a vapor transport ampoule. It was found that the previously neglected equilibrium GeSe2(s) = GeSe(v) + 1/2Se2(v) reduces the Se2(v) concentration to an extent that sublimation/condensation of GeSe becomes the dominant transport mechanism. At total pressures near 1 atm the concentration of an additional Ge-Se-I vapor species becomes comparable to that of GeSe(v).
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.
The analysis of the STS-7 flight experiments of the GeSe-xenon system yielded positive results which surpass the original objectives of these experiments. The mass transport rates of GeSe observed in microgravity environment are in close agreement with theoretically predicted values for diffusion limited mass transport. This supports the earlier proposed hypothesis for the interpretation of flux anomalies observed in previous space experiments. In addition, the STS-7 flight experiments led to the observation of unexpected crystal growth phenomena. The largest GeSe single crystals obtained in microgravity grew in the ampul without direct wall contact which could suggest homogeneous nucleation. The space grown crystals are much larger and have considerably improved surface and bulk morphologies relative to corresponding ground control specimens. The combined results are of basic scientific importance and of technological significance.
GeSe-xenon experiments performed aboard the D1 mission at xenon pressures of 2 and 6 atm confirm the crystal growth pattern, sizes, and surface morphology of crystals previously grown aboard STS-7 for different pressures. Besides the deposition and growth of GeSe crystals on the ampoule wall, several large single-crystalline GeSe platelets with lateral dimensions much greater than those of crystals on the wall and obtained on the ground are found. The present results reemphasize the question concerning the nucleation phenomena in microgravity.
Previous work on the mass transport rate of the GeSe-GeI4 system has been reevaluated using the mass spectrometric results of Buchan and Rosenberger (1987), and considering the presence of GeSe(s) and GeSe2(s) phases in the source material. The present transport rate study confirms the previously derived dominance of the sublimation of GeSe at lower pressures of GeI4, and of chemical vapor transport at higher pressures of GeI4. It is noted that experimentally observed flux anomalies are model-independent.
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.
One of the key goals of NASA’s astrophysics program is to answer the question: How did galaxies evolve into the spiral, elliptical, and irregular galaxies that we see today? We describe a space mission concept called Galaxy Evolution Spectroscopic Explorer (GESE) to help address this question by making a large ultraviolet spectroscopic survey of galaxies at a redshift, z approximately 1 (look-back time of approximately 8 billion years). GESE is a 1.5-m space telescope with an near-ultraviolet (NUV) multi-object slit spectrograph covering the spectral range, 0.2-0.4 micrometers (0.1-0.2 micrometers as emitted by galaxies at a redshift, z approximately 1) at a spectral resolution of delta lambda=6 A.
Physical vapor transport experiments on GeSe in the presence of 2 atm xenon and for a nominal temperature difference of 600-500 C were performed under 1 g, 5 g, and 10 g acceleration conditions. Under high acceleration and destabilizing conditions, the GeSe crystals are generally larger than those under 1 g, stabilizing, and up to three orders of magnitude larger in surface area than those under 1 g, destabilizing conditions. The mass transport rates of the 5 g and 10 g destabilizing experiments are considerably greater than those of the 10 g, stabilizing, and 1 g experiments. The observed increase in mass flux (under destabilizing conditions) with acceleration is significantly greater than the anticipated dependence (mass flux proportional to g exp 1/4) for laminar, boundary-layer driven free convection. In view of the considerable convection under high acceleration, destabilizing conditions, the surface morphology and bulk crystallinity of the large crystal platelets are unexpectedly good.
One of the key goals of NASA's astrophysics program is to answer the question: How did galaxies evolve into the spirals and elliptical galaxies that we see today? We describe a space mission concept called Galaxy Evolution Spectroscopic Explorer (GESE) to address this question by making a large spectroscopic survey of galaxies at a redshift, z is approximately 1 (look-back time of approximately 8 billion years). GESE is a 1.5-meter space telescope with an ultraviolet (UV) multi-object slit spectrograph that can obtain spectra of hundreds of galaxies per exposure. The spectrograph covers the spectral range, 0.2-0.4 micrometers at a spectral resolving power, R approximately 500. This observed spectral range corresponds to 0.1-0.2 micrometers as emitted by a galaxy at a redshift, z=1. The mission concept takes advantage of two new technological advances: (1) light-weighted, wide-field telescope mirrors, and (2) the Next- Generation MicroShutter Array (NG-MSA) to be used as a slit generator in the multi-object slit spectrograph.
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.