Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “electrostatic doping”

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.

89 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Flat band carrier confinement in magic-angle twisted bilayer graphene

Abstract Magic-angle twisted bilayer graphene has emerged as a powerful platform for studying strongly correlated electron physics, owing to its almost dispersionless low-energy bands and the ability to tune the band filling by electrostatic gating. Techniques to control the twist angle between graphene layers have led to rapid experimental progress but improving sample quality is essential for separating the delicate correlated electron physics from disorder effects. Owing to the 2D nature of the system and the relatively low carrier density, the samples are highly susceptible to small doping inhomogeneity which can drastically modify the local potential landscape. This potential disorder is distinct from the twist angle variation which has been studied elsewhere. Here, by using low temperature scanning tunneling spectroscopy and planar tunneling junction measurements, we demonstrate that flat bands in twisted bilayer graphene can amplify small doping inhomogeneity that surprisingly leads to carrier confinement, which in graphene could previously only be realized in the presence of a strong magnetic field.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Coprecipitation of Fe/Cr Hydroxides at Organic–Water Interfaces: Functional Group Richness and (De)protonation Control Amounts and Compositions of Coprecipitates

Iron/chromium hydroxide coprecipitation controls the fate and transport of toxic chromium (Cr) in many natural and engineered systems. Organic coatings on soil and engineered surfaces are ubiquitous; however, mechanistic controls of these organic coatings over Fe/Cr hydroxide coprecipitation are poorly understood. Here, Fe/Cr hydroxide coprecipitation was conducted on model organic coatings of humic acid (HA), sodium alginate (SA), and bovine serum albumin (BSA). The organics bonded with SiO 2 through ligand exchange with carboxyl (-COOH), and the adsorbed amounts and pK(a) values of -COOH controlled surface charges of coatings. The adsorbed organic films also had different complexation capacities with Fe/Cr ions and Fe/Cr hydroxide particles, resulting in significant differences in both the amount (on HA > SA(-COOH) >> BSA(-NH 2 )) and composition (Cr/Fe molar ratio: on BSA(-NH2) >> HA > SA(-COOH)) of heterogeneous precipitates. Negatively charged -COOH attracted more Fe ions and oligomers of hydrolyzed Fe/Cr species and subsequently promoted heterogeneous precipitation of Fe/Cr hydroxide nanoparticles. Organic coatings containing -NH 2 were positively charged at acidic pH because of the high pK(a) value of the functional group, limiting cation adsorption and formation of coprecipitates. Meanwhile, the higher local pH near the -NH 2 coatings promoted the formation of Cr(OH) 3 . Finally, this study advances fundamental understanding of heterogeneous Fe/Cr hydroxide coprecipitation on organics, which is essential for successful Cr remediation and removal in both natural and engineered settings, as well as the synthesis of Cr-doped iron (oxy)hydroxides for material applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

CO2 Sorption in Aminopolymer-Based Direct Air Capture Composites Through Fluorescent Detection

Aminopolymer-mesoporous oxide composites are becoming widely recognized for their ability to adsorb CO2 directly from the air. However, a better fundamental understanding of the nature of these materials under different environmental operating conditions will be necessary before they become ubiquitous in industrial direct air capture (DAC) systems. For example, gas diffusion in general is coupled with polymer segmental mobility in these composites; diffusion is therefore affected by interactions of the polymer with the pore wall of the mesoporous oxide, by the relative humidity of the air, and by electrostatic crosslinks that develop as a function of CO2 sorption. Disentangling these and additional complicated effects in order to better optimize operating conditions is a major challenge for the field. Here, we present a method for quantifying the adsorption of CO2 and moisture in a polyethylenimine (PEI) - Al2O3 composite using infrared sensing while we simultaneously monitor polymer mobility with a fluorescent probe molecule doped into the composite. Both the fluorescence intensity and shape of the emission spectra are strongly dependent on the mobility of the supporting medium. We monitor the polymer mobility and CO2 adsorption kinetics across a series of different relative humidities (RH) with this technique, and we observed that the relative difference between polymer mobility and sorption kinetics from one RH to the next are quite dramatic. We interpret these results and discuss how they can be used to inform the design of more efficient DAC systems for real-world operating conditions.

carbon capture↗
DOE OSTI2024

Comment on “Tunneling-tip-induced collapse of the charge gap in the excitonic insulator Ta 2 NiSe 5 ”

In this study, we investigate the discrepancy between the estimate of Q. He [], who observed a remarkable collapse of the exciton gap in Ta 2 NiSe 5 due to the electrostatic field between the scanning tunneling microscope (STM) tip and the sample, and that of a recent angle-resolved photoemission spectroscopy investigation [C. Chen , ]. It is proposed that a critical factor contributing to this discrepancy is due to He 's assumption of a constant work function of the STM tip. This assumption led to an underestimation of the tip-induced electric field. Using a literature value for the sample work function, a more substantial electric field strength is obtained, which resolves the apparent conflict between the doping estimates of these two techniques. Furthermore, our findings highlight the importance of the STM tip condition, which can significantly impact the tip work function and, consequently, influence the doping estimation in experiments involving tip-induced electric fields. Published by the American Physical Society 2024

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects

Individual single-walled carbon nanotubes with covalent sidewall defects have emerged as a class of photon sources whose photoluminescence spectra can be tailored by the carbon nanotube chirality and the attached functional group/molecule. Here we present electroluminescence spectroscopy data from single-tube devices based on (7, 5) carbon nanotubes, functionalized with dichlorobenzene molecules, and wired to graphene electrodes. We observe electrically generated, defect-induced emissions that are controllable by electrostatic gating and strongly red-shifted compared to emissions from pristine nanotubes. The defect-induced emissions are assigned to excitonic and trionic recombination processes by correlating electroluminescence excitation maps with electrical transport and photoluminescence data. At cryogenic conditions, additional gate-dependent emission lines appear, which are assigned to phonon-assisted hot-exciton electroluminescence from quasi-levels. Similar results were obtained with functionalized (6, 5) nanotubes. We also compare functionalized (7, 5) electroluminescence data with photoluminescence of pristine and functionalized (7, 5) nanotubes redox-doped using gold(III) chloride solution. This work shows that electroluminescence excitation is selective toward neutral defect-state configurations with the lowest transition energy, which in combination with gate-control over neutral versus charged defect-state emission leads to high spectral purity.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Water Formation Reaction under Interfacial Confinement: Al0.25Si0.75O2 on O-Ru(0001)

Confined nanosized spaces at the interface between a metal and a seemingly inert material, such as a silicate, have recently been shown to influence the chemistry at the metal surface. In prior work, we observed that a bilayer (BL) silica on Ru(0001) can change the reaction pathway of the water formation reaction (WFR) near room temperature when compared to the bare metal. In this work, we looked at the effect of doping the silicate with Al, resulting in a stoichiometry of Al0.25Si0.75O2. We investigated the kinetics of WFR at elevated H2 pressures and various temperatures under interfacial confinement using ambient pressure X-ray photoelectron spectroscopy. The apparent activation energy was lower than that on bare Ru(0001) but higher than that on the BL-silica/Ru(0001). The apparent reaction order with respect to H2 was also determined. The increased residence time of water at the surface, resulting from the presence of the BL-aluminosilicate (and its subsequent electrostatic stabilization), favors the so-called disproportionation reaction pathway (*H2O + *O ↔ 2 *OH), but with a higher energy barrier than for pure BL-silica.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Electronic Structure Modification of Rectangular Phosphorene Quantum Dots Via Edge Passivation

Phosphorene is an elemental two-dimensional material that shows great promise as a metal-free photocatalyst owing to its high carrier mobility and direct band gap. Doping and edge functionalization of phosphorene can be exploited to tune its band gap and alter other properties. Herein, the electronic properties of a series of edge-passivated monolayer phosphorene quantum dots are investigated using an array of functional groups (OH, SH, NH 2 , SCH 3 , OCN, CN, and Cl) that are known to have electron-donating or electron-withdrawing capabilities. By employing density functional theory calculations, we demonstrate that edge functionalization can perturbatively shift the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels while simultaneously preserving the geometric structure and other favorable properties of pristine phosphorene. We use the average overall electrostatic potential of these systems to successfully predict the relative positioning of the HOMO and LUMO energy levels while presenting qualitative explanations on potential jumps introduced by the dipole layers formed due to edge functionalization. Furthermore, our results suggest that selective functionalization permits considerable control over the band edge positions, which could be utilized for applications in energy and optoelectronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Sensor Distortion Effects in Photon Monte Carlo Simulations

We present a detailed method to simulating sensor distortions using a photon and electron Monte Carlo method. We use three-dimensional electrostatic simulations to parameterize the perturbed electric field profile for nonideal sensor details. We follow the conversion of simulated photons, and the subsequent response of the converted electrons to the electric field pattern. These nonideal sensor details can be implemented efficiently in a Monte Carlo approach. We demonstrate that the nonideal sensor distortions have a variety of observable consequence including the modification of the astrometric pattern; the distortion of the electron diffusion size and shape; and the distortion of flats. We show analytic validation of the diffusion physics, reproduce two kinds of edge distortion, and show qualitative validation of field-free regions, lithography errors, and fringing. We also demonstrate that there are two related effects of doping variation having different observable consequences. We show that field distortions from accumulated electrons lead to intensity-dependent point-spread functions and the sublinear variance in flats. The method is implemented in the Photon Simulator and the code is publicly available.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2021

Interplay between charge ordering and geometric ferroelectricity in LuFe 2 O 4 /LuFeO 3 superlattices

Oxide superlattices have drawn great attentions owing to the intriguing coupling among elastic, electrical, and magnetic orderings at the interfaces and the emerging of improper ferroelectricity. Here, superlattices composed of hexagonal LuFeO 3 (h-LuFeO 3 ) and LuFe 2 O 4 are investigated via density functional theory calculations. h-LuFeO 3 is a well-known multiferroic material that is stable only in thin-film or doped bulk state, while LuFe 2 O 4 is a charge ordered material where the existence of ferroelectricity is still in controversy. We have found that the charge ordering (CO) induced polarizations in LuFe 2 O 4 layers coexist with the geometric polarizations in h-LuFeO 3 layers in the (LuFe 2 O 4 ) m /(LuFeO 3 ) n superlattices with different periodicities, and the ferroelectric states are generally preferred over the antiferroelectric states for LuFe 2 O 4 in superlattices. The out-of-plane polarizations in h-LuFeO 3 and LuFe 2 O 4 layers tend to be aligned in parallel, and the overall polarization increases with the ratio of h-LuFeO 3 . The influence of layered polarizations on the local electrostatic potential is not significant except the detected small trend caused by the CO-induced polarization within a FeO bilayer. Besides, the local electronic structures show that the Fermi level position in a certain layer can be tuned by the valences of Fe in this layer and the polarization distributions in neighboring layers. LuFe 2 O 4 layers sandwiched between thick h-LuFeO 3 layers are more susceptible. The calculated configurations of the superlattices are supported by atomic-resolution transmission electron microscopy experiments. Our results pave a way for tunable ferroelectricity in superlattice systems and create a new playground for manipulating the coupling between various degrees of freedom.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Spin orbit torque in ferromagnet/topological-quantum-material heterostructures

The main objective of this proposal was to investigate the spin-orbital driven phenomena in topological insulator (TI)/ferromagnet (FM) or transition metal (TM) heterostructures. In particularly, we tried to utilize the spin-momentum locking properties on the surface of TIs to achieve ideal spin-orbit torques (SOTs). In practice, we found that the surface properties have been greatly modified, resulting in much more complicated magneto-transport behaviors than what is expected in SOT-driven magnetotransport. Through systematic and extended investigations, we start to unravel that the nonlinear magnetotransport properties, which was, in the past, mainly attributed to SOT-driven phenomena, can be used to infer the modification of the electronic band structures of TIs and spin textures in FMs which are closely related with charge-spin interconversion phenomena. Regarding the band structures In TIs, we find under electrostatic perturbations imposed by deposition of nonmagnetic TMs, the quantum well states formed on the surface of TIs develop substantial Rashba spin-splitting, which is detected as a bilinear magnetoresistance (BMR). Under magnetic perturbations, the nonlinear planar Hall effect (NPHE) in the topological surface states is greatly enhanced, possibly reflecting the modified surface band structure due to time-reversal symmetry breaking. In a double HM/FM heterostructure where the magnetic anisotropy can be robustly tuned, we observed second-harmonic Hall voltage responses corresponding to domain wall structures with fixed chirality, stabilized by interfacial Dzyaloshinskii–Moriya interaction (DMI). In addition, in the linear transport regime, we utilized inverse-SHE-induced anomalous Hall effect in Pt to probe the exchange bias on the surface of a doped magnetic insulator Tm 3 Fe 5 O 12 .

36 MATERIALS SCIENCE↗
DOE OSTI2022

Optical Response of Periodic Arrays of Graphene Nanodisks

Doped graphene nanostructures are a promising platform for photonics due to their exceptionally strong and tunable plasmonic resonances. When placed in a periodic array configuration, the plasmons supported by the individual nanostructures interact with each other and, under the appropriate conditions, can give rise to a collective mode known as a lattice resonance. Here, for this study, we perform a comprehensive analysis of the response of periodic arrays of graphene nanodisks and identify the conditions under which the system is able to support lattice resonances. We find that the ratio between the period of the array and the wavelength of the plasmon completely determines the behavior of the system. As a consequence, strong lattice resonances are achieved for micron-size nanodisks in the terahertz regime. We develop a theoretical model valid beyond the electrostatic approximation and use it to derive closed analytical expressions for the strength, the wavelength, and the width of the optical resonance of the arrays. The theoretical framework developed in this work paves the way for facile design and discovery of emerging properties of periodic arrays of graphene nanostructures that could enable applications in photonics and plasmonics.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Recent trends in all-organic polymer dielectrics for high-temperature electrostatic energy storage capacitors

Electrostatic energy storage (EES) capacitors are critical for renewable energy and high-power systems, driving the search for dielectric materials that combine superior electrical insulation, mechanical flexibility, low density, cost-effectiveness, and processability. Polymer-based dielectrics have emerged as leading candidates, particularly for high electric field applications. However, conventional polymers often fail to meet the demands of high-temperature environments due to increased electrical conductivity and reduced discharged energy density at elevated temperatures, resulting in energy loss and reduced performance. High glass transition temperature (T g) polymers show promise but require further optimization to enhance their energy storage capabilities under thermal and electrical stress. This review provides a comprehensive update on recent advancements in high-T g polymer-based dielectrics for EES capacitors, focusing on both intrinsic polymers and all-organic composites. It outlines key design principles, critical performance parameters, and innovative strategies—such as nanofiller doping, layered architectures, physical blending, and chemical crosslinking—to improve electrical, thermal, and mechanical properties. The review also highlights emerging trends, including the integration of machine learning algorithms to explore novel polymer structures and expand the chemical design space. By bridging the gap between academic research and industrial application, this review aims to accelerate the development of next-generation dielectric materials capable of balancing multiple performance metrics for high-temperature EES capacitors.

Xie, Zongliang↗
DOE OSTI2023

Vacuum-Healing of Grain Boundaries in Sodium-Doped CuInSe 2 Solar Cell Absorbers

Alkali metal doping and grain boundaries (GB) have been at the center of attention within the Cu(In,Ga)(S,Se) 2 photovoltaics community for years. This study provides the first experimental evidence that the GB of sodium-doped CuInSe 2 thin films may undertake reversible oxidation even at room temperature, whereas undoped films may not. The findings are corroborated by cathodoluminescence imaging, secondary ion mass spectrometry, and Kelvin probe force microscopy on air-exposed films subsequently subject to vacuum. A thermochemical assessment identifies the likely solid-gas equilibria involved. These reactions open new research questions with respect to the beneficial role played by alkali metal dopants in chalcopyrite solar cells and may steer the community toward new breakthroughs.

14 SOLAR ENERGY↗
DOE OSTI2024

Mitigating Band Tailing in Kesterite Solar Absorbers: Ab Initio Quantum Dynamics

Open-circuit voltage deficits are limiting factors in kesterite solar cells. Addressing this issue by suppressing band tailing and nonradiative charge recombination is essential for enhancing the performance. We employ ab initio nonadiabatic molecular dynamics to elucidate the origin of band tailing and charge losses and propose a mitigation strategy. The simulations show that Cu–Zn disorder, associated with antisite defect clusters [Cu Zn +Zn Cu ], is a significant source of band tailing in kesterites, as evidenced by the much larger Urbach energy in disordered than ordered kesterites. Cu–Zn disorder gives rise to new sulfur-centered coordination polyhedra, increases structural inhomogeneity, changes electrostatic potential at sulfur centers, and shifts the S(3p) orbital energy. Differences in the S(3p)/Cu(3d) and S(3p)/Sn(5s) hybridization strengths and the S(3p) orbital energy shift reduce the band gap by 0.37 eV. Furthermore, Cu–Zn disorder enhances vibrational motion of sulfur anions and surrounding cations, increasing band gap fluctuations by 15 meV. The stronger electron–phonon interactions reduce charge carrier lifetimes and limit the kesterite solar cell efficiency. Partial substitution of Zn with Cd facilitates structural ordering and significantly suppresses band tailing, particularly in disordered systems. The improvement can be attributed to the larger atomic radius and mass of Cd, which weakens bonding around the anion, suppresses S-related vibrations within the covalent tetrahedra, and reduces nonadiabatic coupling, thereby increasing charge carrier lifetimes. The reported results establish the key influence of cation disorder on band tailing and reduced charge carrier lifetimes in kesterites and highlight cation disorder engineering as a strategy to achieve high-efficiency kesterite solar cells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Edge‐Driven Fringe‐Field Effects, Reduced Screening, and Bandgap Widening in Graphene Nanoribbons Enable Single‑Molecule Sensitivity

Graphene nanoribbons (GNRs) offer promising platforms for single‐molecule sensing due to their quasi‐1D channels and discrete electronic states, providing superior sensitivity toward molecular perturbations. While prior studies emphasize smoother edges as essential for optimal performance, the potential benefits of controlled edge roughness remain largely unexplored. Additionally, most investigations focus on GNR arrays, leaving critical edge‐ and width‐dependent factors, including fringe fields, bandgap widening, interactions between adsorbing molecules and GNR atoms, density of states (DOS) suppression, and electrostatic screening lengths, and their collective impact on sensitivity, poorly understood. Here, in this work, we fabricated field‐effect transistors using individual GNRs (widths: 200–20 nm) and characterized their response to molecular adsorption with perfluorooctanoic acid as the model analyte. Narrower ribbons displayed significantly enhanced sensitivity, yielding a coverage‐normalized response of 116 ± 10 mV per molecule in 20 nm‐wide GNRs (from calibrated ensemble Dirac‐point shifts). Experimental and theoretical analyses reveal that this heightened sensitivity arises from stronger fringe fields, width‐dependent quantum confinement effects, reduced DOS, and increased edge roughness that facilitates molecular anchoring, enhanced orbital overlap, and improved charge transfer efficiency. Our findings challenge the conventional assumption that smoother edges inherently enhance sensor performance, demonstrating that controlled edge disorder substantially boosts molecular sensitivity in GNR sensors.

36 MATERIALS SCIENCE↗