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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.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Surface anchoring requirements for vanadia clusters on titanium oxide surfaces and their impact on activity for oxidative dehydrogenation of ethanol

Titanium oxide site requirements for the anchoring of catalytically active forms of dispersed vanadia species are investigated. Using a series of oxygen-modified titanium nitride materials, we systematically modify the available vanadia-anchoring moieties on the titanium-bearing support surface. This strategy in turn results in a very narrow size distribution of well-dispersed vanadia clusters anchored on the support. This synthesis technique ensures that only highly dispersed vanadia is present on the catalyst surface even at nominal monolayer coverages. A deeper insight into the catalytic behavior for ethanol partial oxidation of the active vanadia species is obtained by comparing intrinsic kinetic and thermodynamic parameters of kinetically relevant reaction steps (heat of ethanol adsorption, hydrogen abstraction activation energy, and the energetics of catalyst reoxidation). In conclusion, it is found that these parameters are dependent on the relative amount of oxygen in the titanium-bearing support and the resulting distribution of vanadia species, which validates the premise of a potential participation of lattice oxygen atoms of the titania support into the catalytic activity of active vanadia species.

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

Structure-Dependent Lithium Metal Reactivity of Lithium Lanthanum Titanium Oxide Solid Electrolytes

Ongoing efforts to design stable, ionically conductive solid-state electrolytes (SSEs) for next-generation solid-state batteries make it clear that both long and short-range structural order strongly influence materials performance. However, clear structure-property relationships are generally lacking, making it difficult to develop design rules for improving the (electro) chemical stability of SSEs. Here, in this work, we synthesize epitaxial, single-crystal lithium lanthanum titanium oxide (LLTO) films and demonstrate that the kinetics of Ti 4+ reduction and lithium intercalation depend sensitively on the crystal orientation, with electrochemical stability increasing as LLTO (001) < (110) similar to (112) < (100). However, thermodynamic stability is ultimately unaffected-all orientations fully reduce after extended contact with Li metal. In contrast, amorphous LLTO films exhibit minimal, self-limiting reactivity that results in an interface that is stable to extended contact with Li metal. The results demonstrate the potential to engineer crystal lattice strain and long-range order to differentially tune the stability of the solid electrolyte toward reactive lithium metal and cathode materials, suggesting strategies for enabling the wider deployment of LLTO and other ionically conductive ceramic films in advanced energy storage technologies

Ketter, Benjamin [Argonne National Laboratory (ANL↗
DOE OSTI2024

Vapor Phase Infiltration of Titanium Oxide into P3HT to Create Organic–Inorganic Hybrid Photocatalysts

Herein, we report for the first time the use of vapor phase infiltration (VPI) to infuse conducting polymers with inorganic metal oxide clusters that together form a photocatalytic material. While vapor infiltration has previously been used to electrically dope conjugated polymers, this is the first time, to our knowledge, that the resultant hybrid material has been demonstrated to have photocatalytic properties. The system studied is poly(3-hexylthiophene-2,5-diyl) (P3HT) vapor infiltrated with TiCl 4 and H 2 O to create P3HT-TiO x organic–inorganic hybrid photocatalytic materials. X-ray photoelectron spectroscopy analysis shows that P3HT-TiO x VPI films consist of a partially oxidized P3HT matrix, and the infiltrated titanium inorganic is in a 4+ oxidation state with mostly oxide coordination. Upon visible light illumination, these P3HT-TiO x hybrids degrade methylene blue dye molecules. The P3HT-TiO x hybrids are 4.6× more photocatalytically active than either the P3HT or TiO 2 individually or when sequentially deposited (e.g., P3HT on TiO 2 ). On a per surface area basis, these hybrid photocatalysts are comparable or better than other best in class polymer semiconductor photocatalysts. VPI of TiCl 4 + H 2 O into P3HT makes a unique hybrid structure and idealized photocatalyst architecture by creating nanoscale TiO x clusters concentrated toward the surface achieving extremely high catalytic rates. The mechanism for this enhanced photocatalytic rate is understood using photoluminescence spectroscopy, which shows significant quenching of excitons in P3HT-TiO x as compared to neat P3HT, indicating that P3HT acts as a photosensitizer for the TiO x catalyst sites in the hybrid material. This work introduces a new approach to designing and synthesizing organic–inorganic hybrid photocatalytic materials, with expansive opportunities for further exploration and optimization.

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

Composition-Dependent Reconstructions of Titanium Oxide Clusters and Cu(111) Support via Cluster-Adatom Interactions

Mass-selected cluster deposition and scanning tunneling microscopy (STM) were used to investigate the surface structure and thermal stability of Ti 3 O y (y = 5, 6) clusters on a Cu(111) surface. STM imaging shows that the substoichiometric Ti 3 O 5 clusters aggregate into large dendritic assemblies at step edges, while the stoichiometric Ti 3 O 6 clusters are highly dispersed as single or small groupings of clusters. Cluster deposition also results in extensive reconstruction of the Cu(111) surface. The Ti 3 O 5 /Cu(111) surface exhibits one Cu-layer deep pits that are ringed by cluster assemblies that nucleate and grow at the perimeter step edges. By comparison, the Ti 3 O 6 /Cu(111) surface consists of irregular step edges and small one Cu-layer islands that are decorated with clusters. The restructuring of the Cu(111) surface is attributed to strong Cu-cluster interactions that release Cu adatoms from the step edges. This mechanism is supported by density functional theory (DFT) calculations, which predict that the Cu adatom attachment to the clusters is energetically favorable. Annealing the surfaces at elevated temperatures (400-700 K) results in higher densities of irregular steps, pits and isolated Cu islands as well as increased cluster aggregation. In conclusion, this work highlights adatom-cluster interactions as another manifestation of strong cluster-support interactions that can significantly alter the nature of the oxide-metal interface.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Role of nitrogen in the high-temperature oxidation of titanium alloys

Here, we characterized the oxidation of pure Ti, Ti-0.8Si, and Ti-2Nb at 800 °C in N 2 -20% O 2 and in sequential isotopically labeled N 2 -20% O 2 atmospheres to clarify how nitrogen influences the high-temperature oxidation mechanisms of titanium alloys. The alloys always exhibited a nitride layer beneath a compact oxide scale with localized nitrogen concentrations in the underlying metal. Nitrogen is found to limit the oxygen concentrations in the metal. The synergy between alloying and nitrogen helps maintain compact oxide scales, preventing the formation of the lamellar scales observed during oxidation in nitrogen-free atmosphere and the oxidation of pure Ti regardless of the atmosphere.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Optical and Plasmonic Properties of High-Electron-Density Epitaxial and Oxidative Controlled Titanium Nitride Thin Films

The present paper reports on the fabrication, detailed structural characterizations, and theoretical modeling of titanium nitride (TiN) and its isostructural oxide derivative, titanium oxynitride (TiNO) thin films that have excellent plasmonic properties and that also have the potential to overcome the limitation of noble metal and refractory metals. The TiNO films deposited at 700 °C in high vacuum conditions have the highest reflectance (R = ~ 95%), largest negative dielectric constant (ε 1 = –161), and maximal plasmonic figure of merit (FoM = –ε 1 /ε 2 ) of 1.2, followed by the 600 °C samples deposited in a vacuum (R = ~ 85%, ε 1 = –145, FoM = 0.8) and 700 °C–5 mTorr sample (R = ~ 82%, ε 1 = –8, FoM = 0.3). To corroborate our experimental observations, we calculated the phonon dispersions and Raman active modes of TiNO by using the virtual crystal approximation. From the experimental and theoretical studies, a multilayer optical model has been proposed for the TiN/TiNO epitaxial thin films for obtaining individual complex dielectric functions from which many other optical parameters can be calculated. The advantages of oxide derivatives of TiN are the continuation of similar free electron density as in TiN and the acquisition of additional features such as oxygen-dependent semiconductivity with a tunable bandgap.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Defining the Shape Density of Cerium Oxide and Titanium Dioxide Aerosol

The “shape density” of CeO 2 (cerium oxide) powder (American Elements, Los Angeles CA) is estimated to be 0.82 ± 0.02 g cm -3 and the shape density for TiO 2 (titanium dioxide) powder (American Elements, Los Angeles CA) is estimated as 0.54 ± 0.01 g cm -3 . The “shape density” is defined as the measured loose density (ASTM 2018 and ASTM 2021) of a powder divided by the ICRP (International Commission on Radiological Protection) default aerosol shape factor (Valetin 2002). This information is used for experimental measurements of the RRF (respirable release factor) and other respirable source term parameter (US DOE 1994) experiments at the Los Alamos National Laboratory (LANL). The shape density is the input parameter to the TSI Inc (Shoreview MN) AIM (Aerosol Instrument Manager) software for the TSI model 3321 APS (Aerodynamic Particle Sizer) (Figure 1).

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2024

Electrochemical and Chemical Reactivities of Titanium Oxide-Based Materials with a Chloroaluminate Ionic Liquid Electrolyte for Aluminum Batteries

Here, by selecting three different types of electrode materials, we intended to better understand the Al 3+ intercalation chemistry of titanium oxide-based frameworks with an acidic chloroaluminate electrolyte. In agreement with previous reports, we confirmed that the native interstitial sites of anatase TiO 2 are less prone to accommodate Al 3 + than Li + or Na + ions, while introducing cationic vacancies largely increases the electrochemical storage capacity. Upon the first cycle, the highest reversible capacity, up to 277 mAh/g, was obtained for a hydrated layered structure featuring cationic vacancies. Total scattering data showed that the insertion of Al 3+ ions induced a strong distortion of the framework. In addition, combined 27 Al MAS NMR and DFT calculations revealed that in oxy-hydroxylated vacant sites, the coordination mode of Al 3+ ions depends on the arrangement of anions around vacancies inducing the occurrence of 4-, 5-, and 6-fold coordination modes. Further cycling experiments revealed a progressive capacity fading for all electrode materials. Using cyclic voltammetry on the used electrolyte, we evidenced that a partial dissolution has occurred, which is more pronounced for the layered hydrate compound, and that solubilized species are electrochemically active, giving rise to specific signatures in both CVs and galvanostatic experiments. Raman spectroscopy enabled us to characterize these species, which are derived from the Ti-Cl system. The solubilized species, however, eventually precipitated, as shown by a purple deposit observed on the separator and tentatively assigned to TiCl 3 , known to be insoluble in this medium. By providing further information on the Al 3+ intercalation chemistry and a better understanding of the electrochemical and chemical reactivities of electrode materials, this work will enable progress to be made in the development of aluminum-ion batteries.

Al3+-ion insertion chemistry↗
DOE OSTI2025

Nucleation-promoting and growth-limiting synthesis of disordered rock-salt Li-ion cathode materials

Disordered rock-salt oxides and oxyfluorides are promising positive electrode materials for high-performance lithium-ion batteries free of nickel and cobalt. However, conventional synthesis methods rely on post-synthesis pulverization to achieve cycling-appropriate particle sizes, offering limited control over particle microstructure and crystallinity. This accelerates degradation and complicates secondary particle processing. Here we present a synthesis strategy that enhances nucleation while suppressing particle growth and agglomeration across various disordered rock-salt compositions, including lithium–manganese–titanium oxide, lithium–manganese–niobium oxide, and lithium–nickel–titanium oxide systems. Applied to Li 1.2 Mn 0.4 Ti 0.4 O 2 , this method yields highly crystalline, well-dispersed sub-200 nm particles that form homogeneous electrode films with stable cycling behavior. Tested in cells with lithium metal as the counter electrode, these electrodes deliver ~200 mAh/g with 85% capacity retention relative to the first cycle after 100 cycles (20 mA/g, 1.5–4.8 V), and an average discharge voltage loss of 4.8 mV per cycle, compared to 38.6% retention and 7.5 mV loss per cycle for electrodes derived from pulverized solid-state particles. This approach suggests a route to enhance the performance and durability of disordered rock-salt electrodes for sustainable lithium-ion batteries.

Batteries↗
DOE OSTI2024

Oxide-Encapsulated Ruthenium Oxide Catalysts for Selective Oxygen Evolution in Unbuffered pH-Neutral Seawater

Direct seawater electrolysis is a promising approach to producing green hydrogen in water-scarce environments using renewable energy. However, the undesirable chlorine evolution reaction and hypochlorite evolution reaction compete with the desired oxygen evolution reaction (OER) at the anode electrocatalyst. This issue is most pronounced in unbuffered pH-neutral solutions due to local acidification resulting from the OER. To overcome this challenge, this study provides a comprehensive evaluation of the use of silicon oxide (SiO x ) and titanium oxide (TiO x ) nanoscale overlayers coated on metallic ruthenium (Ru) and ruthenium oxide (RuO x ) thin film electrodes and their ability to block chloride ions from reaching active sites during operation in an unbuffered 0.6 M NaCl electrolyte. Using a combination of (electro)analytical techniques, encapsulated RuO x anodes are shown to effectively suppress Cl - transport to buried catalyst active sites while allowing for the desired OER to occur, leading to increases in OER faradaic efficiency at moderate overpotentials. Evidence for the ability of SiO x overlayers to block Cl - ions from reaching the active buried interface was obtained by monitoring the ν(O-H) stretching mode of OH adsorbates using in situ Raman spectroscopy. Here, this study also reports trade-offs between the activity, selectivity, and stability of bare and encapsulated Ru and RuO x electrocatalysts, finding that the magnitude of these trade-offs strongly depends on the complex interplay between electrode architecture, material properties, and catalytic performance, especially in unbuffered pH-neutral seawater.

58 GEOSCIENCES↗
DOE OSTI2024

Multiscale Modeling of Nanoparticle Precipitation in Oxide Dispersion-Strengthened Steels Produced by Laser Powder Bed Fusion

Laser Powder Bed Fusion (LPBF) enables the efficient production of near-net-shape oxide dispersion-strengthened (ODS) alloys, which possess superior mechanical properties due to oxide nanoparticles (e.g., yttrium oxide, Y-O, and yttrium-titanium oxide, Y-Ti-O) embedded in the alloy matrix. To better understand the precipitation mechanisms of the oxide nanoparticles and predict their size distribution under LPBF conditions, we developed an innovative physics-based multiscale modeling strategy that incorporates multiple computational approaches. These include a finite volume method model (Flow3D) to analyze the temperature field and cooling rate of the melt pool during the LPBF process, a density functional theory model to calculate the binding energy of Y-O particles and the temperature-dependent diffusivities of Y and O in molten 316L stainless steel (SS), and a cluster dynamics model to evaluate the kinetic evolution and size distribution of Y-O nanoparticles in as-fabricated 316L SS ODS alloys. The model-predicted particle sizes exhibit good agreement with experimental measurements across various LPBF process parameters, i.e., laser power (110–220 W) and scanning speed (150–900 mm/s), demonstrating the reliability and predictive power of the modeling approach. The multiscale approach can be used to guide the future design of experimental process parameters to control oxide nanoparticle characteristics in LPBF-manufactured ODS alloys. Additionally, our approach introduces a novel strategy for understanding and modeling the thermodynamics and kinetics of precipitation in high-temperature systems, particularly molten alloys.

Wang, Zhengming (ORCID:0000000241627112)↗
DOE OSTI2024

Enhanced performance of Bi 2 S 3 /TiO 2 heterostructure composite films for solar cell applications

Day to day energy production is shifting towards renewable energy sources as these sources become more economically viable while being less polluting to operate; solar energy has become one of the major sources of renewable energy. However, it currently relies on ultra-pure silicon ingots to produce commercial silicon photovoltaics, which prevents the cost of electricity being produced to compete with non-renewable energy production. A viable low-cost alternative for silicon based cells would be dye-sensitized solar cells (DSSCs), which are easier and cheaper to manufacture as they do not require expensive and delicate raw materials to make. Moreover, they could be made semi-flexible which allows for a greater variety of applications. A DSSC consists of three components, a photo-electrode, an electrolyte and a counter-electrode. When exposed to incident light, the complex photosensitizers in the photoelectrode release electrons which are transported to the external load, leaving the photoelectrode in an oxidized state. The electrons are collected by the counter electrode and used to reduce the electrolyte. This charged electrolyte then reduces the positively charged photoelectrode, allowing the process to begin again. To improve the efficiency of this process, we explore the use of bismuth sulfide (Bi 2 S 3 ) and titanium oxide (TiO 2 ) composite as photoelectrode material and investigate their impact on the efficiency of DSSC.

14 SOLAR ENERGY↗
DOE OSTI2025

Commodity Thermoplastic Elastomer-Enabled Templated Synthesis of Large-Pore Ordered Mesoporous Materials

Fabrication of ordered mesoporous materials (OMMs) has predominantly relied on templating-based methods. However, these methods are constrained by several limitations, especially the limited pore sizes attainable with commercially available surfactants used as structure-directing agents. To unlock the full potential of the OMMs, it is essential to develop synthetic strategies that facilitate the production of large-pore OMMs using scalable processes and cost-effective precursors. This work demonstrates the use of thermoplastic elastomer (TPE)-derived carbon replicas for synthesizing ordered mesoporous silica (OMS) and metal oxides (OMMOs) via precursor infiltration and template removal. The nanostructural evolution of the resulting inorganic materials was systematically investigated. Specifically, using tetraethyl orthosilicate (TEOS) as a silica precursor, this method can produce an OMS with relatively large pores. To establish the generalizability of this process, the fabrication approach was extended to other commercially available TPEs with varied chemical compositions and molecular weights while consistently resulting in ordered structures. Additionally, this synthetic strategy can be successfully applied to the production of OMMOs, including tin and titanium oxide matrix chemistries, yielding pore sizes of 16.0 and 19.2 nm, respectively. By developing a general method and using low-cost precursors, this work presents a scalable approach for fabricating large-pore OMMs with tunable pore textures and matrix chemistries.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Role of Metal Cocatalysts in the Photocatalytic Production of Hydrogen from Water Revisited

The use of photocatalysts to promote the production of molecular hydrogen from water, following the so-called water splitting reaction, continues to be a promising route for the green production of fuels. The molecular basis of this photocatalysis is the photoexcitation of electrons from the valence band of semiconductors to their conduction band, from which they can be transferred to chemical reactants, protons in the case of water, to promote a reduction reaction. The mechanism by which such a process takes place has been studied extensively using titanium oxide, a simple material that fulfills most requirements for water splitting. However, photocatalysis with TiO 2 tends to be highly inefficient; a cocatalyst, commonly a late transition metal (Au, Pt) in nanoparticle form, needs to be added to facilitate the production of H 2 . The metal is widely believed to help with the scavenging of the excited electrons from the conduction band of the semiconductor in order to prevent their recombination with the accompanying hole formed in the valence band, a step that cancels the initial photon absorption and competes with the photolytic chemical reduction. Here we review and analyze the molecular basis for that mechanism and argue for an alternative explanation, that the role of the metal is to help with the recombination of the atomic hydrogen atoms produced by proton reduction on the semiconductor surface instead. First, we summarize what is known about the electronic structure of these photocatalysts and how the electronic levels need to line up for the reduction of protons in water to be feasible. Next, we review the current understanding of the dynamics of the steps associated with the absorption of photons, the de-excitation via electron–hole pair recombination and fluorescence decay, and the electronic transitions that lead to proton reduction, and contrast those with the rates of the chemical steps required to produce molecular hydrogen. The following section addresses the changes introduced by the addition of the metal cocatalyst, comparatively evaluating its role as either an electron scavenger or a promoter of the recombination of hydrogen atoms. A discussion of the viable chemical mechanisms for the latter pathway is included. Finally, we briefly mention other associated aspects of this photocatalysis, including the possible promotion of H 2 production with visible light via resonant excitation of the surface plasmon of Au nanoparticles, the use of single-metal (Au, Pt) atom catalysts and of yolk–shell nanostructures, and the reduction of organic molecules. We end with a brief personal perspective on the possible generality of the concepts introduced in this Critical Review.

08 HYDROGEN↗
DOE OSTI2025

Resistive Switching of Spinel Li 4 Ti 5 O 12 Lithium-Ion Battery Material for Neuromorphic Computing

The rapid rise of AI has exposed significant limitations in conventional Von Neumann computing architecture, particularly in regard to speed and energy efficiency. To address these challenges, researchers are exploring a brain-inspired neuromorphic architecture that mimics biological neural networks, enabling massive parallel processing with reduced power consumption for complex AI computational demands. Recent interest has focused on utilizing battery electrodes and solid electrolyte materials for their resistive switching properties in developing a neuromorphic architecture. These properties are precisely tuned through local- and bulk-level chemical composition modifications via voltage bias stimuli. In this study, we demonstrate fabricating a three-terminal lithium-ion electrochemical transistor based on lithium titanium oxide (Li 4 Ti 5 O 12 ), a popular lithium-ion battery anode material. We deposited and characterized LTO thin films using RF sputtering, demonstrating a 6 orders of magnitude increase in electronic conductivity upon lithiation, with conductivity plateauing after 20% lithiation. Density functional theory calculations revealed transformation from the insulating to conducting state, supported by experimental characterization through X-Ray Photoelectron Spectroscopy (XPS) and Direct Current (DC) polarization analyses. The fabricated transistor consisted of LTO as the channel layer, gold as source/drain terminals, lithium phosphorus oxynitride (LiPON) as the lithium-ion conductor, and copper as the gate terminal. The device exhibited clear hysteresis in transfer characteristics due to lithium insertion/extraction processes. Long-term potentiation (LTP) and long-term depression (LTD) measurements showed an asymmetric ratio of 1.425 and maximum/minimum conductance ratio of 7.83. When implemented in a deep neural network (DNN) for MNIST handwritten digit recognition, the device achieved 92.03% accuracy over 20 training epochs. Detailed transport mechanism analysis revealed the crucial role of oxygen vacancies and interface effects in device operation. Our preliminary findings establish LTO-based lithium-ion electrochemical transistors as promising candidates for energy-efficient neuromorphic computing applications, offering potential solutions to traditional Von Neumann architecture limitations.

25 ENERGY STORAGE↗
DOE OSTI2025

Etching-Chemistry-Driven Ruthenium Doping on Ti 3 C 2 T x MXene for Optimizing Electrochemical Performance

We demonstrate that the etching chemistry used during MXene synthesis from Ti 3 AlC 2 MAX phase significantly influences surface functionalization and structural vacancies, which in turn affect ruthenium (Ru) ion interactions. Using hydrofluoric acid (HF) and ammonium bifluoride (NH 4 HF 2 ) as etchants, we obtained MXene surfaces with distinct functional groups and Ti vacancies that impact Ru ion interactions and electrochemical performance. Both MXene variants (labeled MX(H) and MX(N), respectively) exhibited negative zeta potentials in their pristine state, but upon the addition of Ru the zeta potential for MX(H) reached 12.9 mV while that for MX(N) remained negative at −6.4 mV. This adsorption resulted in a 14.4-fold increase in the specific capacitance of MX(H)/Ru compared to pristine MX(H), whereas MX(N)/Ru exhibited only a 4.4-fold increase over its pristine counterpart. X-ray diffraction analysis identified the formation of ammonium titanium oxide fluoride, (NH 4 ) 3 TiOF 5 , on MX(N), which likely contributed to its reduced Ru adsorption. X-ray photoelectron spectroscopy suggested the presence of Ti vacancies in both MXene variants; however, their behavior toward Ru accommodation differed markedly, with MX(H) showing the most obvious shift in the Ti 2p peak in the XPS survey spectrum, while MX(N) showed the most obvious shift in the C 1s peak. Electron paramagnetic resonance spectroscopy further demonstrated a distinct alteration in the spectral signatures of MX(H) upon Ru addition, in contrast to the negligible changes in MX(N), indicating effective passivation of the Ti defect sites in MX(H) via vacancy-assisted Ru doping. Cyclic voltammetry showed that Ru-incorporated MX(H) nanocomposites exhibit more efficient redox-active sites, as reflected in their higher capacitance values. These findings highlight the pivotal role of MXene surface chemistry in controlling cation adsorption, providing valuable insights for the rational design of high-performance electrodes.

2D surface engineering↗
DOE OSTI2026

Engineering diamond interfaces free of dark spins

Nitrogen-vacancy (NV) centers in diamond are utilized extensively as quantum sensors for imaging fields at the nanoscale. The ultrahigh sensitivity of NV magnetometers has enabled the detection and spectroscopy of individual electron spins, with potentially far-reaching applications in condensed matter physics, spintronics, and molecular biology. However, the surfaces of these diamond sensors naturally contain electron spins, which create a background signal that can be hard to differentiate from the signal of the target spins. In this study, we develop a surface modification approach that eliminates the unwanted signal of these so-called dark electron spins. Our surface passivation technique, based on coating diamond surfaces with a thin titanium oxide (Ti⁢O 2 ) layer, reduces the dark spin density. The observed reduction in dark spin density aligns with our findings on the electronic structure of the diamond-Ti⁢O 2 interface. The reduction, from a typical value of 2000 μm −2 to a value below that set by the detection limit of our NV sensors (200 μm −2 ), results in a twofold increase in Hahn-echo coherence time of near surface NV centers. Furthermore, we derive a comprehensive spin model that connects dark spin relaxation with NV coherence, providing additional insights into the mechanisms behind the observed spin dynamics. Our findings are directly transferable to other quantum platforms, including nanoscale solid-state qubits and superconducting qubits.

74 ATOMIC AND MOLECULAR PHYSICS↗