Search NASA⌕ Search

SEARCH · Search NASA

Results for “semiconductors electron spectroscopy”

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.

At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Dynamic Nuclear Polarization Enhanced 113 Cd Solid-State Nuclear Magnetic Resonance Spectroscopy Reveals CdSe Nanocrystals with Triangular Two-Dimensional Projections are Terminated by {100} Facets

Surface structure plays an important role in particle growth and determining the chemical and photophysical properties of semiconductor nanocrystals (NCs). Therefore, there is a need for structural tools that can characterize and detect different surface facets. Here, we sought to investigate the structure of {111} CdSe facets by applying dynamic nuclear polarization (DNP) enhanced 113 Cd and 77 Se solid-state nuclear magnetic resonance (SSNMR) spectroscopy to zinc-blende CdSe NCs that exhibit triangular two-dimensional (2D) projections within transmission electron microscope (TEM) images. It was originally hypothesized in the literature that these CdSe NCs were tetrahedral in shape and terminated by facets from the {111} family of lattice planes of the zinc-blende structure. Surprisingly, we observe 113 Cd NMR spectra indicating that the primary facets are from the {100} family of lattice planes. We also obtained DNP-enhanced 113 Cd and 77 Se SSNMR spectra of recently reported right trigonal bipyramidal (rTriBP) CdSe NCs grown by seeded growth. The 113 Cd NMR spectra rTriBP CdSe NCs are consistent with {100} surface facets. TEM images show that rTriBP NCs may also exhibit triangular 2D projections that are similar to the so-called tetrahedral NCs. Based upon these results, we conclude that the so-called tetrahedral NCs are predominantly terminated by {100} surface facets and most likely have the same shape as rTriBP NCs. These results highlight the need for multiple complementary techniques when assigning the shape and surface termination of NCs.

Santhiran, Anuluxan [Ames Laboratory (AMES), Ames,↗
DOE OSTI2021

Direct Visualization and Manipulation of Tunable Quantum Well State in Semiconducting Nb 2 SiTe 4

We report quantum well states (QWSs) can form at the surface or interfaces of materials with confinement potential. They have broad applications in electronic and optical devices such as high mobility electron transistor, photodetector, and quantum well laser. The properties of the QWSs are usually the key factors for the performance of the devices. However, direct visualization and manipulation of such states are, in general, challenging. In this work, by using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we directly probe the QWSs generated on the vacuum interface of a narrow band gap semiconductor Nb 2 SiTe 4 . Interestingly, the position and splitting of QWSs could be easily manipulated via potassium (K) dosage onto the sample surface. Our results suggest Nb 2 SiTe 4 to be an intriguing semiconductor system to study and engineer the QWSs, which has great potential in device applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Near-field imaging of optical resonances in silicon metasurfaces using photoelectron microscopy

Precise control of light–matter interactions at the nanoscale lies at the heart of nanophotonics. However, experimental examination at this length scale is challenging since the corresponding electromagnetic near-field is often confined within volumes below the resolution of conventional optical microscopy. In semiconductor nanophotonics, electromagnetic fields are further restricted within the confines of individual subwavelength resonators, limiting access to critical light–matter interactions in these structures. In this work, we demonstrate that photoelectron emission microscopy (PEEM) can be used for polarization-resolved near-field spectroscopy and imaging of electromagnetic resonances supported by broken-symmetry silicon metasurfaces. We find that the photoemission results, enabled through an in situ potassium surface layer, are consistent with full-wave simulations and far-field reflectance measurements across visible and near-infrared wavelengths. In addition, we uncover a polarization-dependent evolution of collective resonances near the metasurface array edge taking advantage of the far-field excitation and full-field imaging of PEEM. Here, we deduce that coupling between eight resonators or more establishes the collective excitations of this metasurface. All told, we demonstrate that the high-spatial resolution hyperspectral imaging and far-field illumination of PEEM can be leveraged for the metrology of collective, non-local, optical resonances in semiconductor nanophotonic structures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Sub-molecular structural relaxation at a physisorbed interface with monolayer organic single-crystal semiconductors

Arranging molecules into highly symmetric, topological crystal structures has been recognized as the best approach to functionalize electronic properties in molecular crystals, where the constituent molecules have been assumed to be rigid in shape. Here, in striking contrast, we demonstrate that the molecules in a monolayer organic crystal can undergo a significant deformation in proximity to the substrate, which is reflected by an asymmetry in the electron density profile. X-ray reflectivity and X-ray absorption spectroscopies in conjunction with density-functional theory calculations reveal that the highly planarized π-core are deformed into a bent shape, while the bulk lattice parameters are maintained. The molecular shape change is found to be perfectly suppressed in a bilayer single crystal, which leads to a 40% increase in mobility in the bilayer crystal. Our finding of a unique, sub-molecular scale shape change in monolayer single crystals can offer possibilities for functionalizing electrical properties via nano-scale physisorption.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Charge Transfer Dynamics in Dye-Sensitized Photocatalysts Using Metal Complex Sensitizers with Long-Wavelength Visible Light Absorption Based on Singlet–Triplet Excitation

An Os(II) polypyridyl complex was applied as a photosensitizer in dye-sensitized photocatalyst systems based on Pt-intercalated HCa 2 Nb 3 O 10 and Pt-loaded TiO 2 . The Os(II) complex exhibits a spin-forbidden but partially allowed triplet metal-to-ligand charge transfer ( 3 MLCT) transition, enabling broad visible light absorption up to 800 nm, which surpasses that of conventional Ru(II)-based dyes. Despite its shorter excited-state lifetime compared to Ru(II) complexes, efficient electron injection from the excited Os(II) dye into the semiconductor was confirmed. Under visible-light irradiation, the Os(II)- sensitized photocatalysts showed higher H 2 evolution activity than the Ru(II)-sensitized photocatalysts when sodium ascorbate was used as an electron donor, demonstrating effective utilization of long-wavelength visible light. In contrast, negligible H 2 evolution was observed when NaI was employed as a redox mediator for Z-scheme water splitting. Transient absorption spectroscopy revealed that the lack of activity stemmed from inefficient electron transfer from I − to oxidized Os(II). These findings highlight the importance of selecting appropriate redox mediators to fully exploit long-wavelength dyes for overall water splitting under visible light.

artificial photosynthesis↗
DOE OSTI2020

Coherent growth and characterization of van der Waals 1T-VSe2 layers on GaAs(111)B using molecular beam epitaxy

We report epitaxial growth of vanadium diselenide (VSe₂) thin films in the octahedrally coordinated (1T) structure on GaAs(111)B substrates by molecular beam epitaxy. Film thickness from a single monolayer (ML) up to 30 ML is demonstrated. Structural and chemical studies using x-ray diffraction, transmission electron microscopy, scanning tunneling microscopy, and x-ray photoelectron spectroscopy indicate high-quality thin films. Further studies show that monolayer VSe₂ films on GaAs are not air stable and are susceptible to oxidation within a matter of hours, which indicates that a protective capping layer should be employed for device applications. This work demonstrates that VSe₂, a candidate van der Waals material for possible spintronic and electronic applications, can be integrated with III-V semiconductors via epitaxial growth for two- and three-dimensional hybrid devices.

2D materials↗
DOE OSTI2025

Linear Dichroism of the Optical Properties of SnS and SnSe Van der Waals Crystals

Abstract Tin monochalcogenides SnS and SnSe, belonging to a family of Van der Waals crystals isoelectronic to black phosphorus, are known as environmentally friendly materials promising for thermoelectric conversion applications. However, they exhibit other desired functionalities, such as intrinsic linear dichroism of the optical and electronic properties originating from strongly anisotropic orthorhombic crystal structures. This property makes them perfect candidates for polarization‐sensitive photodetectors working in near‐infrared spectral range. A comprehensive study of the SnS and SnSe crystals is presented, performed by means of optical spectroscopy and photoemission spectroscopy, supported by ab initio calculations. The studies reveal the high sensitivity of the optical response of both materials to the incident light polarization, which is interpreted in terms of the electronic band dispersion and orbital composition of the electronic bands, dictating the selection rules. From the photoemission investigation the ionization potential, electron affinity and work function are determined, which are parameters crucial for the design of devices based on semiconductor heterostructures.

Chemistry↗
DOE OSTI2022

Influence of Copper Composition on Cu 2 BaSn(S,Se) 4 Solution-Deposited Films and Photovoltaic Devices with Over 5% Efficiency

Cu 2 BaSn(S,Se) 4 is currently in the spotlight for prospective environmentally friendly, stable, thin-film solar cell application, with demonstrated device power conversion efficiency (PCE) exceeding 5% for vacuum-deposited absorbers. As suggested by first-principles calculations, experimental studies involving related Cu 2 ZnSn(S,Se) 4 and Cu(In,Ga)(S,Se) 2 absorbers prove that the detailed chemical composition typically plays a sensitive role in altering defects and electronic properties of these complicated compound semiconductors. Herein, the copper composition of Cu 2 BaSn(S,Se) 4 has been systematically modified, employing a solution-based deposition approach, to provide a more complete picture of the phase stability and optoelectronic property sensitivity for this material. X-ray diffraction and scanning electron microscopy show that phase purity is preserved over a film Cu content range of nominally 0.94 ≤ [Cu]/[Ba + Sn] ≤ 1.01. Terahertz spectroscopy and Hall effect measurements reveal that the majority carrier hole density of ~10 13 cm –3 and mobility (~5 cm 2 /V s), as well as the minority carrier lifetime (a bulk lifetime of 180 ps and a surface recombination velocity >106 cm/s), are nominally independent of Cu content. The champion PCEs exceed 4.7% for all copper compositions in the phase-pure region, with a record value of 5.1%, similar to the reported values for record vacuum-deposited devices. Furthermore, these results suggest that Cu 2 BaSn(S,Se) 4 films and solar cells (at the current performance level) may be less sensitive to Cu stoichiometry compared to kesterite materials and therefore may provide a more stable material platform to prepare thin-film solar cells.

(CBTSSe)↗
DOE OSTI2022

Electronic Structure of InAs and InSb Surfaces: Density Functional Theory and Angle-Resolved Photoemission Spectroscopy

The electronic structure of surfaces plays a key role in the properties of quantum devices. However, surfaces are also the most challenging to simulate and engineer. Here, in this work, the electronic structure of InAs(001), InAs(111), and InSb(110) surfaces is studied using a combination of density functional theory (DFT) and angle-resolved photoemission spectroscopy (ARPES). Large-scale first principles simulations are enabled by using DFT calculations with a machine-learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. To facilitate direct comparison with ARPES results, a “bulk unfolding” scheme is implemented by projecting the calculated band structure of a supercell surface slab model onto the bulk primitive cell. For all three surfaces, a good agreement is found between DFT calculations and ARPES. For InAs(001), the simulations clarify the effect of the surface reconstruction. Different reconstructions are found to produce distinctive surface states, which may be detected by ARPES with low photon energies. For InAs(111) and InSb(110), the simulations help elucidate the effect of oxidation. Owing to larger charge transfer from As to O than from Sb to O, oxidation of InAs(111) leads to significant band bending and produces an electron pocket, whereas oxidation of InSb(110) does not. The combined theoretical and experimental results may inform the design of quantum devices based on InAs and InSb semiconductors, for example, topological qubits utilizing the Majorana zero modes.

42 ENGINEERING↗
DOE OSTI2024

Operando XPS for Plasma Process Monitoring: A Case Study on the Hydrogenation of Copper Oxide Confined under h-BN

Here, we demonstrate that ambient pressure x-ray photoelectron spectroscopy (APXPS) can be used for in situ studies of dynamic changes in surface chemistry in a plasma environment. This opens a new and vast application space for XPS and greatly complements modern spectroscopy techniques to probe plasma-solid/liquid interactions relevant to process monitoring in the semiconductor industry, bio-medical plasma applications and plasma remediation technologies. Hexagonal boron nitride (h-BN) grown on Cu was used in this study as a well-defined model system for plasma process monitoring and because of its unique chemical, optical and electrical properties that make it a prospective material for advanced electronics. To better understand the stability and surface chemistry of h-BN during plasma assisted processing, we track in real time the plasma-induced chemical state changes of B, N and the underlying Cu substrate using APXPS equipped with an AC discharge plasma source operating at 13 Pa. Residual gas analysis (RGA) mass-spectra were concurrently collected during plasma-XPS to track reaction products formed during plasma exposure. A clear reduction of Cu x O is seen, while an h-BN layer remains intact, suggesting hydrogen radical (H • ) species can attack the exposed and h-BN covered Cu oxide patches and partially reduce the underlying substrate without significantly damaging the overlaying h-BN, which is of practical importance for development of h-BN encapsulated devices and interfaces. In addition to demonstration of plasma-XPS capabilities we discuss the observed challenges (e.g., parasitic plasma-chamber walls reactions and charging effects) and propose potential solutions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Millimeter-Scale Exfoliation of hBN with Tunable Flake Thickness for Scalable Encapsulation

As a two-dimensional (2D) dielectric material, hexagonal boron nitride (hBN) is in high demand for applications in photonics, nonlinear optics, and nanoelectronics. Unfortunately, the high-throughput preparation of macroscopic-scale, high-quality hBN flakes with controlled thickness is an ongoing challenge, limiting device fabrication and technological integration. In this report we present a metal thin-film exfoliation method to prepare hBN flakes with millimeter-scale dimension, near-unity yields, and tunable flake thickness distribution from 1-7 layers, a substantial improvement over scotch tape exfoliation. The single crystallinity and high quality of the exfoliated hBN are demonstrated with optical microscopy, atomic force microscopy, Raman spectroscopy, and second harmonic generation. We further explore a possible mechanism for the effectiveness and selectivity based on thin-film residual stress measurements, density functional theory calculations, and transmission electron microscopy imaging of the deposited metal films. We find that the magnitude of the residual tensile stress induced by thin film deposition plays a key role in determining exfoliated flake thickness in a manner which closely resembles 3D semiconductor spalling. Lastly, we demonstrate that our exfoliated, large-area hBN flakes can be readily incorporated as encapsulating layers for other 2D monolayers. Altogether, this method brings us one step closer to the high throughput, mass production of hBN-based 2D photonic, optoelectronic, and quantum devices.

2D materials↗
DOE OSTI2023

Insulators at fractional fillings in twisted bilayer graphene partially aligned to hexagonal boron nitride

At partial fillings of its flat electronic bands, magic-angle twisted bilayer graphene (MATBG) hosts a rich variety of competing correlated phases that show sample-to-sample variations. Divergent phase diagrams in MATBG are often attributed to the sublattice polarization energy scale, tuned by the degree of alignment of the hexagonal boron nitride (hBN) substrates typically used in van der Waals devices. Unaligned MATBG exhibits unconventional superconductor and correlated insulator phases, while nearly perfectly aligned MATBG/hBN exhibits zero-field Chern insulating phases and lacks superconductivity. Here we use scanning tunneling microscopy and spectroscopy (STM/STS) to observe gapped phases at partial fillings of the flat bands of MATBG in a new intermediate regime of sublattice polarization, observed when MATBG is only partially aligned (θ Gr-hBN ≈ 1.65°) to the underlying hBN substrate. Under this condition, MATBG hosts not only phenomena that naturally interpolate between the two sublattice potential limits, but also unexpected gapped phases absent in either of these limits. At charge neutrality, we observe an insulating phase with a small energy gap (Δ < 5 meV) likely related to weak sublattice symmetry breaking from the hBN substrate. In addition, we observe new gapped phases near fractional fillings ν = ±1/3 and ν = ±1/6, which have not been previously observed in MATBG. Importantly, energy-resolved STS unambiguously identifies these fractional filling states to be of single-particle origin, possibly a result of the super-superlattice formed by two moiré superlattices. Finally, our observations emphasize the power of STS in distinguishing single-particle gapped phases from many-body gapped phases in situations that could be easily confused in electrical transport measurements, and demonstrate the use of substrate engineering for modifying the electronic structure of a moiré flat-band material.

Physics↗
DOE OSTI2023

Measurement of minority carrier diffusion length in p -GaN using electron emission spectroscopy (EES)

Electron emission spectroscopy was performed on metalorganic chemical vapor deposition grown p-n – -n + junctions with p-thicknesses ranging from 50 to 300 nm, doped with [Mg] = 3.5 × 10 19 cm –3 . By measuring the decreasing emitted electron intensity from a cesiated p-GaN surface with increasing p-thickness, we were able to extract the minority carrier diffusion length of electron in p-type GaN, L e = 26 ± 3 nm. The measured value is in good agreement with literature reported values. In conclusion, the extrapolated electron current at the n – region–p-GaN interface is in reasonable agreement with the simulated electron current at the interface.

36 MATERIALS SCIENCE↗
NASA NTRS1993

Ballistic Electron Emission Microscopy of Metal/Group IV Interfaces

Ballistic electron emission microscopy and spectroscopy, together with related techniques, have been applied with great success to the study of buried interfaces. These probes, known collectively as BEEM, have yielded important information on interface transport, interface band structure, and carrier scattering, with lateral spatial resolution on the nanometer scale. Recent applications of the technique to polycrystalline metal/semiconductor interfaces have demonstrated an ability to spatially map both conduction band and valence band semiconductor structure. BEEM studies of epitaxial silicide/silicon interfaces have been particularly fruitful, as the rich silicide band structure results in complex and often surprising transport behavior...

Hecht, M. H.↗
DOE OSTI2025

Evolution of the Electronic Gap of Directly Synthesized Versus Mechanically Transferred WS 2 Monolayer to Multilayer Films

The electronic properties of 2D materials play a critical role in determining their potential for device applications. Despite rapid developments in 2D semiconductors, studies of fundamental electronic parameters, including the electronic gap and ionization energy, are limited, with significant discrepancies in reported values. The study focuses on tungsten disulfide (WS₂) and investigates the electronic structure of films comprising an increasing number of layers deposited with two different methods: direct synthesis via metal–organic chemical vapor deposition (MOCVD) and additive mechanical transfer of exfoliated single layers. The films are characterized via Raman, UV–vis, and photoluminescence spectroscopies, as well as ultraviolet photoelectron and inverse photoemission spectroscopies (UPS/IPES). The electronic gap of WS₂ is found to decrease from 2.43 eV for the monolayer to 1.97 eV for the trilayer, indicating a bulk transition at the trilayer thickness. This reduction in the electronic gap is primarily due to the downward shift of the conduction band minimum relative to the valence band maximum. A comparative analysis with MOCVD-grown WS₂ reveals a slightly larger electronic gap for MOCVD-grown samples, attributed to differences in defect densities. The electronic levels evaluated through UPS/IPES highlight the significant influence of preparation methods on the electronic properties of WS₂.

2D materials↗
DOE OSTI2025

Possible evidence of excitonic condensation in a topological insulator

The transient excitonic condensate is a nonequilibrium electron-hole Bardeen-Cooper-Schrieffer state in a photoexcited semiconductor and semimetal, where electron-hole pairs undergo a phase transition and condense into a single coherent quantum state. Despite numerous experimental works to realize the predicted excitonic condensation phase, experimental evidence still remains elusive. This is largely due to the absence of direct measurements of a material's transient momentum-dependent electronic structure and the excitonic state in the condensation regime. Here, using time and angle-resolved photoemission spectroscopy, we find direct evidence of a transient excitonic condensate in the spin-polarized spatially indirect excitonic topological states in Bi2Te3. Accompanying the formation of the excitonic topological states by photoexcitation, we reveal a splitting of the hole's and electron's quasi-equilibrium chemical potential followed by the band flattening and backbending of the transient topological surface state. Moreover, within the same momentum range, we report a reshaping of the bulk valence band in the form of a Mexican-hat-like Bogoliubov dispersion-hallmarks of the excitonic condensation, followed by the opening of an energy gap at the Fermi level. The fluence and temperature dependence of these renormalization effects are reminiscent of excitonic condensation within Bardeen-Cooper-Schrieffer (BCS)-like behavior. These results, together with theoretical simulation, point to the possible formation of a transient excitonic condensate and provide opportunities to manipulate topologically protected Bose condensates with light.

ARPES↗
DOE OSTI2024

Atomic-Scale Characterization of Dilute Dopants in Topological Insulators via STEM–EDS Using Registration and Cell Averaging Techniques

Magnetic dopants in three-dimensional topological insulators (TIs) offer a promising avenue for realizing the quantum anomalous Hall effect (QAHE) without the necessity for an external magnetic field. Understanding the relationship between site occupancy of magnetic dopant elements and their effect on macroscopic property is crucial for controlling the QAHE. By combining atomic-scale energy-dispersive X-ray spectroscopy (EDS) maps obtained by aberration-corrected scanning transmission electron microscopy (AC-STEM) and novel data processing methodologies, including semi-automatic lattice averaging and frame registration, we have determined the substitutional sites of Mn atoms within the 1.2% Mn-doped Sb 2 Te 3 crystal. More importantly, the methodology developed in this study extends beyond Mn-doped Sb 2 Te 3 to other quantum materials, traditional semiconductors, and even electron irradiation sensitive materials.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Atomic Manipulation of Single Erbium on GaAs(110): From Adatom Motion to Creation and Control of Bistable Substitutions

We demonstrate atomic-scale control over the position and configuration of single erbium (Er) atoms at the GaAs(110) surface using scanning tunneling microscopy (STM). By applying voltage pulses from the STM tip, we drive Er adatoms to move sequentially among three distinct adsorption sites and controllably substitute into Ga lattice sites. The resulting substitutional Er atoms exhibit two isoelectronic configurations that differ by less than one angstrom in lateral position and can be reversibly switched by additional voltage pulses. Differential conductance spectroscopy shows differences in the unoccupied local electronic states for the two substitutional configurations, and an absence of mid-gap states. Preliminary density functional theory (DFT) calculations support these findings, identifying the most stable adsorption and substitutional sites, their relative binding energies, charge states, and local density of states (LDOS) characteristics. Together, the experimental and theoretical results establish a method for precise, reversible manipulation of rare-earth dopants at semiconductor surfaces, opening a pathway to engineer atomic-scale quantum and optoelectronic functionalities.

Poche, Steven [Ohio State University]↗