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At least 1,369 records · Page 76
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

High-efficiency, air-stable manganese–iron oxide nanoparticle-pigmented solar selective absorber coatings toward concentrating solar power systems operating at 750 °C

Solar selective absorber coating with long-term thermal stability at high temperatures ≥750 °C in air is an important component to reduce the levelized cost of energy (LCOE) of concentrating solar power (CSP) systems toward 50% power efficiency and dispatchable solar electricity. Conventionally, solar spectral selectivity requires multilayer-interference coatings implemented by stringently controlled vacuum deposition, and these coatings degrade significantly at >700 °C in air. Herein, we established a quantitative design approach and demonstrated a proof-of-concept air-stable, manganese–iron oxide nanoparticle (NP)-pigmented solar selective coatings with a high solar absorptance of ~93%, a relatively low thermal emittance of ~52%, and an optical-to-thermal energy conversion efficiency >89% under 1,000× solar concentration at 750 °C toward Generation 3 CSP systems. These coatings demonstrate spectral selectivity using cost-effective spray coating approach, a notable improvement over conventional vacuum-deposited, multilayer solar selective coatings for low-cost, high-efficiency solar thermal receivers. In contrast to the thermal degradation of spectrally non-selective benchmark Pyromark 2500 coatings at 750 °C in air, the solar absorptance of the MnFe 2 O 4 -pigmented coatings on stainless steel 310 (SS 310) substrates is increased to ~92.9% and the optical-to-thermal energy conversion efficiency is improved to 89.7% after serving at 750 °C in air for 700 h. X-ray diffraction results reveal that this improvement is due to the transformation of MnFe 2 O 4 NPs into more thermodynamically stable, non-stoichiometric manganese-rich manganese ferrite and iron-rich manganese–iron oxide phases after 500 h aging at 750 °C. For >1,000 h-endurance testing at 750 °C in air and the subsequent 19 day-night thermal cycling between 750 °C (12 h/cycle) and 25 °C (12 h/cycle) on SS 310 substrates, the thermal degradation is mainly due to the CrO x microflake formation from SS 310 substrates rather than the coatings, which can be suppressed by preoxidizing the surface of SS 310. With lower emittance matrix material and further optimization of pigment NP stoichiometry, concentration, and coating thickness, it is promising to achieve an optimized thermal efficiency ≥92.5% with long-term thermal stability at 750 °C for Generation 3 CSP systems.

14 SOLAR ENERGY↗
DOE OSTI2020

The effects of nanoscale confinement on the behavior of metal laminates

By reducing the layer thickness, h, in metal multilayers from micrometer- to nanometer-levels, scale-dependent transitions in behavior are observed. The role of interface properties in these transitions has been extensively investigated. However, reducing h to the nanoscale regime also imposes a physical geometric confinement on defect-related processes occurring within the layers, an effect that is relatively less explored. Herein, we present illustrative examples of the influence of nanoconfinement on the strength, structure, and radiation response of multilayers. These examples show that nanoconfinement and interface structure and topology act cooperatively in the nanoscale limit, affecting behaviors that cannot be explained based on either factor alone.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Metal DiSulfide thin films synthesized by pulsed ion beam ablation: Self-assembled structures with improved mechanical properties in humid conditions

Thin films composed of transition metal dichalcogenides such as molybdenum disulfide (MoS 2 ) or tungsten disulfide (WS 2 ) are well known for their solid lubricating behavior in dry environments such as space, but deteriorate rapidly in environments where oxygen is present. There have been many attempts over recent decades to improve MoS 2 lubricating performance in humid environments by addition of metals such as titanium to the film. The additions have come in the form of co-sputtering with the MoS 2 , or formation of nano multilayer coatings. We have previously reported on the synthesis of MoS 2 films by high-power ion beam ablation [Prasad et al, Matls. Lett. 65 (2010) 4–6]. The resulting films are observed to contain spheres of pure Mo of 10–100 nm size evenly dispersed throughout the ~1 μm-thick film, a form of self-assembly not before seen in films produced by sputtering or by pulsed laser deposition (PLD). The remaining Mosingle bondS matrix was observed to crystalize in situ under frictional contact into MoS 2 with the basal planes orienting themselves in the direction of sliding underneath the wear surface. The present study concerns metal additions in the form of alternating layers with MoS 2 deposited in between from an ablation target. The addition of Ti resulted in the extension of good tribological performance to humid air conditions, but the path to good performance appears to be relatively narrow. The same distributed Mo spheres were present in the new film, and similar self-assembled morphologies when V was substituted for the Ti, and when multilayers were formed of WS 2 with Zr additions. Furthermore, it appears that addition of Ti resulted in improvements in mechanical film strength that contributed to the wear ability in humid conditions.

Intense ion beams↗
DOE OSTI2021

Synthesis of a planar, multicomponent catalytic surface of Na 2 CO 3 /MnO

Here, O 2 adsorption on MnO(100) precovered with sodium (Na) multilayers was investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Deposition of Na multilayers leads to a first monolayer of oxidic Na followed by metallic Na island growth. XPS results for the oxidation of the metallic Na islands indicate an incomplete oxidation of Na at 125 K. Oxidation at 350 K completely oxidizes the metallic islands producing a mixture of Na 2 O and Na 2 O 2 . Thermal evolution of the oxidation products was examined. After oxidation at 350 K and flashing to 750 K, Na 2 O and Na 2 O 2 are the primary oxides on MnO(100). After flashing to 850 K, a solid state reaction of Na 2 O 2 /Na 2 O and the MnO(100) substrate forms a NaMnO 2 -like surface compound which decomposes above 850 K. Oxygen exchange between CO 2 and Na oxides is observed. The strong interaction between CO 2 and Na oxide islands forms Na 2 CO 3 on MnO(100). Heating the Na 2 CO 3 covered MnO(100) surface to 600 K for 10 min drives oxidic Na in the first monolayer into the MnO subsurface, and produces a surface exposing islands of Na 2 CO 3 on MnO(100).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Comparison of ANN (MLP), ANFIS, SVM, and RF models for the online classification of heating value of burning municipal solid waste in circulating fluidized bed incinerators

Highlights: • A rapid, cost-effective, and comparative methodology was proposed to evaluate the HVs of burning MSW online. • Models ranked in descending order of performance are ANFIS, RF, SVM, MLP. • A well trained model is capable of characterizing the variation trend of HVs accurately. - Abstract: The heating values, particularly lower heating values of burning municipal solid waste are critically important parameters in operating circulating fluidized bed incineration systems. However, the heating values change widely and frequently, while there is no reliable real-time instrument to measure heating values in the process of incinerating municipal solid waste. A rapid, cost-effective, and comparative methodology was proposed to evaluate the heating values of burning MSW online based on prior knowledge, expert experience, and data-mining techniques. First, selecting the input variables of the model by analyzing the operational mechanism of circulating fluidized bed incinerators, and the corresponding heating value was classified into one of nine fuzzy expressions according to expert advice. Development of prediction models by employing four different nonlinear models was undertaken, including a multilayer perceptron neural network, a support vector machine, an adaptive neuro-fuzzy inference system, and a random forest; a series of optimization schemes were implemented simultaneously in order to improve the performance of each model. Finally, a comprehensive comparison study was carried out to evaluate the performance of the models. Results indicate that the adaptive neuro-fuzzy inference system model outperforms the other three models, with the random forest model performing second-best, and the multilayer perceptron model performing at the worst level. A model with sufficient accuracy would contribute adequately to the control of circulating fluidized bed incinerator operation and provide reliable heating value signals for an automatic combustion control system.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2022

Expanding the Cage of 2D Bromide Perovskites by Large A-Site Cations

Two-dimensional (2D) halide perovskites have outstanding optoelectronic properties, and they feature a variety of organic cation spacers and cage A-site cations that can be incorporated into their structures. It has recently been reported that the Goldschmidt tolerance factor can be relaxed and expanded in iodide 2D perovskites. Bromide 2D perovskites, whose multilayer structures and optical properties are much less studied, provide a great platform for studying structure-property relationships for 2D perovskites with large Asite cations. Herein, we report the synthesis and structure of three new 2D bromide perovskites-(BA)(2)(MHy)(2)Pb3Br10 (BA, butylammonium; MHy, methylhydrazinium), (BA)(2)(EA)(2)Pb3Br10 (EA is ethylammonium), and (BA)(2)(DMA)Pb2Br7 (DMA is dimethylammonium). We compared them with other 2D perovskites with different A-site cations but with the same spacer and layer thickness. Single-crystal structures show that the Pb-Br bonds are elongated to accommodate the large A-site cations. Additionally, the octahedra in (BA)(2)(MHy)(2)Pb3Br10 and (BA)(2)(EA)(2)Pb3Br10 are highly distorted, and their different stacking patterns of the inner and outer layers lead to the formation of the n = 3 phases. Density functional theory calculations show that 2D perovskites with larger A-site cations (e.g., DMA, MHy, and EA) have smaller band dispersions and larger effective masses than those with Cs+ and MA. (BA)(2)(MHy)(2)Pb3Br10 also exhibits one of the largest Rashba splittings in the literature. Structures with large cage cations also exhibit high band gaps within the same n number and short photoluminescence (PL) lifetimes. Temperature- and power-dependent PL measurements reveal that the broad shoulder in the PL peak originates from the trap states.

Emission↗
DOE OSTI2021

Hydrophobe Containing Polypeptoids Complex with Lipids and Induce Fusogenesis of Lipid Vesicles

The hydrophobic effect of alkyl group insertion into phospholipid bilayers is exploited in modifying and modulating vesicle structure. We show that amphiphilic polypeptoids (peptide mimics) with n-decyl side chains, which we term as hydrophobe-containing polypeptoids (HCPs), can insert the alkyl hydrophobes into the membrane bilayer of phospholipid-based vesicles. Such insertion leads to disruption of the liposomes and the formation of HCP–lipid complexes that are colloidally stable in aqueous solution. Interestingly, when these complexes are added to fresh liposomes, remnant uncomplexed hydrophobes (the n-decyl groups) bridge liposomes and fuse them. The fusion leads to the engulfing of liposomes and the formation of multilayered vesicles. The morphology of the liposome system can be changed from stopping fusion and forming clustered vesicles to the continued formation of multilayered liposomes simply by controlling the amount of the HCP–lipid complex added. The entire procedure occurs in aqueous systems without the addition of any other solvents. There are several implications to these observations including the biological relevance of mimicking fusogenic proteins such as the SNARE proteins and the development of new drug delivery technologies to impact delivery to cell organelles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Highly Asymmetric Graphene Layer Doping and Band Structure Manipulation in Rare Earth–Graphene Heterostructure by Targeted Bonding of the Intercalated Gadolinium

Heterostructures consisting of vertically stacked two-dimensional (2D) materials have recently gained large attention due to their highly controllable electronic properties and resulting quantum phases. In contrast to the mechanically stacked multilayered systems, which offer exceptional control over a stacking sequence or interlayer twist angles, the epitaxially grown 2D materials express unprecedented quality and stability over wafer-scale lengths. However, controlling the growth conditions remains a major obstacle toward the formation of complex, epitaxial heterostructures with well-defined electronic properties. Here, we synthesized a trilayer graphene heterostructure on the SiC(0001) substrate with two specific interlayer locations occupied by gadolinium. We applied multitechnique methodology based on low-temperature scanning tunneling microscopy/spectroscopy (STM/S) and angle-resolved photoelectron spectroscopy (ARPES) to determine the intercalant’s locations in the complex, epitaxial graphene heterostructure. Our approach relies on very high quality and large, micrometer-scale homogeneity of the synthesized system. The experimentally determined electronic structure is dominated by the two topmost graphene layers. Overall, our spectroscopic results show quantitative agreement between global ARPES, local STM/S, and density functional theory predictions. The characterized electronic properties primarily reflect highly anisotropic doping levels between the two corresponding graphene layers, which significantly affect the band structure topology. Two pairs of hybridized massive Dirac bands from our initial synthesis–the bilayer graphene on the SiC(0001) substrate–are transformed upon Gd intercalation into two pairs of massless Dirac bands with a new hybridization region in between. Our results open perspectives in the realization of exotic 2D quantum materials via atomically precise synthesis of epitaxial, multilayered graphene–rare earth heterostructures.

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

First-Principles-Based Study of the Decomposition of Phenol and Hydroquinone on Pt(111) Combined with Quantitative Information from XPS Spectra to Address the Impact of Coverage and Number of Hydroxyl Functional Groups

A combined first-principles-based and experimental X-ray photoelectron spectroscopy approach was used to investigate the thermal decomposition of two model biofuel compounds, phenol and hydroquinone, on Pt(111) at both low and high coverages. The DFT-based approach yields adsorption geometries and energies, activation barriers and core-level binding energy shifts for C 1s and O 1s. Increasing the coverage in the theoretical model leads to slight shifts in core-level binding energies─toward higher values for C 1s and lower values for O 1s. It also alters the energy profiles of the decomposition reaction pathway, resulting in weaker adsorption energies and changes in both reaction and activation barriers. At low temperatures, we observe a multilayer for phenol and hydroquinone upon adsorption, with desorption occurring at 200 and 270 K, respectively. Following desorption of the multilayer, decomposition proceeds via initial O–H bond scission, followed by two parallel pathways involving either C–H or C–C bond scission, whereby in the case of phenol C–H bond scission occurs first. Here, we further provide characteristic core level binding energies by theoretical calculations that are subsequently used in experimental analyses, establishing a reference database for key spectra of phenolic functionalities applicable to a range of catalytic reactions.

09 BIOMASS FUELS↗
DOE OSTI2018

Machine-Learning Approach for the Development of Structure-Energy Relationships of ZnO Nanoparticles

The structure–energy relationships for the zinc oxide morphologies were investigated using a newly developed fragment-based energy decomposition approach. In this approach, the local chemical compositions of a material are abstracted as fragment types that serve as the material’s genes with respect to its thermodynamic properties. A machine learning-based fragment recognition scheme was developed to learn about the fragment-related knowledge from a relatively small training set consisting of computationally viable ultrasmall nanoparticles. The knowledge gained including the fragment geometries and fragment energy parameters can be used for the classification and energy expression of the test sets consisting of different polymorphs and morphologies of that material at various scales. The stabilities of ZnO nanoparticles with different morphologies were expressed explicitly as functions of the particle size. The size-related phase transitions among various morphologies including wurtzite prisms, wurtzite octahedrons, body-centered tetragonal particles, sodalite-like particles, single-layered cages, multilayered cages, and nonpolar hexagonal prisms were predicted. The multilayered cages with nonpolar surfaces exhibit superior stability among the low-energy morphologies, but wurtzite nanoparticles are more favorable under practical synthesis and growth conditions under the control of the kinetics. The growth mechanism for ZnO clusters, ultrasmall nanoparticles, nanocrystals, and bulk-sized particles is proposed based on the synergy between the size-related phase transitions and external factors that affect the surface energies of the particles. Our interpretation of the Wulff theorem at a fragment-sized resolution provides new chemical insight for understanding the structural phase transition and particle growth for ZnO at various scales.

Chen, Mingyang↗
DOE OSTI2020

Structure and Desorption Kinetics of Acetonitrile Thin Films on Pt(111) and on Graphene on Pt(111)

Acetonitrile thin films were prepared on Pt(111) and on graphene on Pt(111) using molecular beam techniques. Temperature programed desorption (TPD) experiments of acetonitrile on Pt(111) displayed first-order kinetics for monolayer desorption and zero-order kinetics for multilayer desorption. We used reflection adsorption infrared spectroscopy (RAIRS) to characterize the orientation of the monolayer on Pt(111). Acetonitrile molecules have been shown to adopt a bridging structure where the C=N bond breaks to form two single bonds to underlying platinum atoms. When Tdeposition = 120 K our RAIRS spectra are consistent with this bridging configuration. However, when Tdeposition was lowered to 60 K molecules retained the C=N bond and adopted a vertical orientation with the nitrogen closest to the platinum. On graphene, the TPD experiments show that both the monolayer and multilayer desorb with zero-order kinetics. Our RAIRS spectra suggest that the monolayer consists of acetonitrile molecules oriented with their molecular axes parallel to the graphene substrate. This orientation did not change when Tdeposition was varied from 60 to 120 K. Our results provide a more complete picture of how acetonitrile molecules interact with Pt(111) and with graphene surfaces.

desorption tin films, molecules, thin films, desor↗
DOE OSTI2021

Investigations into Spin- and Unpolarized Secondary Electron-Induced Reactions in Self-Assembled Monolayers of Cysteine

Cysteine is the simplest thiolated, chiral amino acid and is often used as the anchor for studies of self-assembled monolayers (SAMs) of complex biomolecules such as peptides. Understanding the interaction of SAMs of cysteine with low-energy secondary electrons (SEs) produced by X-rays can further our understanding of radiation damage in biomolecules. In particular, if the electrons are polarized, chiral-selective chemistry could have bearing on the origin of homochirality in nature. In the present paper, we use synchrotron radiation-based X-ray photoelectron spectroscopy to determine the changes that occur in the bonding of self-assembled layers of cysteine on gold as a result of soft X-ray irradiation. To investigate the possibility of chiral selectivity resulting from the interaction of low-energy, spin-polarized SEs (SPSEs), measurements were conducted on cysteine adsorbed on a 3 nm-thick gold layer deposited on a CoPt thin-film multilayer with perpendicular magnetic anisotropy. Time-dependent measurements of the C 1s, N 1s, O 1s, S 2p, and Au 4f core levels are used to follow the changes in surface chemistry and determine reaction cross-sections as a function of SE exposure. Analysis of the data results in cross-sections in the range of 5-7 Mb and suggests possible reaction pathways. Changing the magnetization direction of the CoPt multilayer produces SPSEs with opposite polarity. Some evidence of spin-dependent reactions is indicated but is inconclusive. Possible reasons for the discrepancy are posited.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2022

Ultrasensitive Sub-monolayer Palladium Induced Chirality Switching and Topological Evolution of Skyrmions

Chiral spin textures are fundamentally interesting, with promise for device applications. Stabilizing chirality is conventionally achieved by introducing Dzyaloshinskii-Moriya interaction (DMI) in asymmetric multilayers, where the thickness of each layer is at least a few monolayers. Here we report an ultrasensitive chirality switching in (Ni/Co) n multilayer induced by capping with only 0.22 monolayer of Pd. Using spin-polarized low-energy electron microscopy, we monitor the gradual evolution of domain walls from left-handed to right-handed Néel walls and quantify the DMI induced by the Pd capping layer. We also observe the chiral evolution of a skyrmion during the DMI switching, where no significant topological protection is found as the skyrmion winding number varies. This corresponds to a minimum energy cost of <1 attojoule during the skyrmion chirality switching. Here, our results demonstrate the detailed chirality evolution within skyrmions during the DMI sign switching, which is relevant to practical applications of skyrmionic devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Monolayer Ti 3 C 2 T x as an Effective Co-catalyst for Enhanced Photocatalytic Hydrogen Production over TiO 2

Titanium dioxide (TiO 2 ) represents a promising candidate for hydrogen production via photocatalysis. Yet, its large bandgap and fast charge recombination limits its efficiency. To overcome this limitation, we explored in this work two-dimensional titanium carbide MXene, Ti 3 C 2 T x ( T x = O, OH, F), as feasible co-catalysts for hydrogen production with TiO 2 as the photocatalyst. We synthesized a series of Ti 3 C 2 T x /TiO 2 composite photocatalysts with monolayer Ti 3 C 2 T x as the co-catalyst to improve the separation of photoinduced electrons and holes. The physicochemical properties of the Ti 3 C 2 T x /TiO 2 composites were investigated by a variety of characterization techniques, and the effect of the monolayer Ti 3 C 2 T x on the photocatalytic performance of the Ti 3 C 2 T x /TiO 2 composites is elucidated by comparison to the multilayer counterpart. The photocatalytic hydrogen evolution rate of the optimized monolayer Ti 3 C 2 T x /TiO 2 composite is over 9 times higher than that of the pure TiO 2 and 2.5 times higher than the multilayer counterpart. The significantly enhanced activity is attributed to the superior electrical conductivity of monolayer Ti 3 C 2 T x and charge-carrier separation at the MXene/TiO 2 interface. A mechanism of photocatalytic hydrogen evolution over the Ti 3 C 2 T x /TiO 2 system is proposed. Our work reports on the potential of monolayer MXenes as effective co-catalysts for photocatalysis and further broadens the applications of the MXene family of two-dimensional materials.

08 HYDROGEN↗
DOE OSTI2023

Uptake of Pb and the Formation of Mixed (Ba,Pb)SO 4 Monolayers on Barite During Cyclic Exposure to Lead-Containing Sulfuric Acid

Barite (BaSO 4 ) is a common additive in lead-acid batteries, where it acts as a nucleating agent to promote the reversible formation and dissolution of PbSO 4 during battery cycling. However, little is known about the molecular-scale mechanisms the nucleation and cyclic evolution of PbSO 4 over a battery’s lifetime. In this study, we explore the responses of a barite (001) surface to cycles of high and low lead concentrations in the presence of 100 mM sulfuric acid using in-situ atomic force microscopy and high-resolution X-ray reflectivity. Here we find that PbSO 4 epitaxial films readily nucleate on the barite surface, even from solutions that are undersaturated relative to bulk PbSO 4 . Despite this, barite (001) proves to be an ineffective nucleator of bulk PbSO 4 , as multilayer growth is suppressed even in highly supersaturated solutions. Instead, we find evidence that Pb 2+ ions can directly exchange with Ba 2+ to create mixed (Ba,Pb)SO 4 surfaces. These chemically mixed surfaces do not host PbSO4 monolayers as readily as pristine barite, and the original reactivity is not regained until a fresh surface is reestablished by aggressive etching. Our results can be partly explained by traditional models of Stranski-Krastanov (S-K) growth, in which monolayer films are stabilized by a reduction in surface energy, but multilayer growth is inhibited by epitaxial strain. Complementary density functional theory calculations confirmed the basic energetic-terms of traditional S-K models, but also showed evidence form more complex, thickness-dependent, energetics than would be predicted from the S-K models. The experimental results can be better understood by extending the S-K model to consider the formation of mixed surfaces and films, which can have reduced strain and interfacial energies relative to pure films, while also being stabilized by entropy of mixing. These insights into non-stoichiometric heteroepitaxy will enable better predictions of how barite affects PbSO 4 nucleation in battery environments.

25 ENERGY STORAGE↗
DOE OSTI2026

Syngas Production at Si Hybrid Photoelectrodes Modified with Re(I) and Mn(I) Tricarbonyl Phenanthroline Complexes Containing Reactive Aryl Azide Groups

We installed molecular CO 2 reduction (CO 2 R) catalysts directly onto Si (photo)electrodes. The highly reactive M(5-azido-1,10-phenanthroline)(CO) 3 X (where M = Mn or Re, X = Br or Cl) complexes readily bubbled when dissolved in polar organic solvents, in both the presence and absence of an ultraviolet light source. When placed on hydrogen-terminated Si (H-Si) and native silicon oxide (SiOx), similar amounts of the complex were attached to the surface under illumination (367 nm, 50–200 mW/cm 2 ) or in the dark. Surprisingly, these films revealed submonolayer coverages instead of the multilayered structures we expected. DFT analyses support monolayer formation, showing that the triplet-state nitrene of the complex is more energetically favorable than the singlet state. Using controlled-potential electrolysis experiments, we showed that Re- and Mn-containing films on pSi photoelectrodes generated small amounts of CO when exposed to 1 atm of CO 2 and 1 sun illumination. These amounts of CO were an order of magnitude greater than control surfaces, producing 5.59 × 10 –7 mol CO/h for Re(az-phen) and 7.83 × 10 –7 mol CO/h for Mn(az-phen) films. Much of the charge passed at the pSi electrodes was consumed by the competing hydrogen evolution reaction, which we attribute to the low molecular coverage and the presence of native oxide on the electrode surface after attachment. Finally, this work demonstrates the feasibility of reacting azide-containing ligands with Si surfaces. Still, it highlights the need for alternative ligand structures and reaction conditions to form multilayer films.

azides↗
DOE OSTI2019

High-Temperature Selective Emitter Design and Materials: Titanium Aluminum Nitride Alloys for Thermophotovoltaics

The efficiency of a thermophotovoltaic (TPV) system depends critically upon the spectral selectivity and stability of an emitter, which may operate most effectively at temperatures in excess of 1000 °C. Here, we computationally design and experimentally demonstrate a novel selective emitter design based on multilayer nanostructures, robust to off-normal emission angles. A computational search of the material and temperature compatibility space of simple emitter designs motivates new material classes and identifies several promising multilayer nanostructure designs for both TPV absorber and emitter applications. One such structure, comprising a thin (<100 nm) tunable Ti x Al 1-x N (TiAlN) absorber and refractory oxide Bragg reflector is grown on W metal foil. In agreement with simulations, the emitter achieves record spectral efficiency (43.4%) and power density (3.6 W/cm 2 ) for an emitter with at least 1 h of high temperature (>800 °C) operation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Growth Mechanisms of Anisotropic Layered Group IV Chalcogenides on van der Waals Substrates for Energy Conversion Applications

Two-dimensional group IV monochalcogenide semiconductors (SnX, GeX; X = S, Se) are of fundamental interest due to their anisotropic crystal structure and predicted unique characteristics such as very large exciton binding energies and multiferroic order possibly up to above room temperature. Whereas growth on reactive supports produces mostly standing flakes, deposition on van der Waals (vdW) substrates can yield basal-plane oriented layered crystals. But so far, this approach invariably resulted in flakes that are several atomic layers thick, and the synthesis of monolayers has remained elusive. Here, we use in situ microscopy during molecular beam epitaxy of SnS on graphite and graphene to establish the origin of this predominant multilayer growth. The enhanced reactivity of group IV chalcogenide layers causes adsorption of precursor molecules primarily on the initial SnS nuclei instead of the vdW support. On graphite, this unusual imbalance in the material supply is the primary cause for fast vertical growth. Experiments on graphene/Ru(0001) suggest increased adsorption on the vdW substrate, which enables enhanced lateral SnS growth. As a result, the fundamental insight obtained here provides a basis for identifying conditions for the scalable synthesis of single-layer group IV monochalcogenides and guides the growth of high-quality multilayer films of interest for applications in energy conversion, optoelectronics, and thermoelectrics.

2D crystal↗