Single-source Precursor Approach Research Articles

Formation of uranium disulfide from a uranium thioamidate single-source precursor.

A single-source-precursor approach was developed to synthesize uranium-based materials outside of the typically-studied oxides. This approach allows for shorter reaction times, milder reaction conditions, and control over the chemicals present in synthesis. To this end, the first homoleptic uranium thioamidate complex was synthesized as a precursor for US2 materials. Pyrolysis of the thioamidate results in decomposition via an alkene elimination pathway and formation of γ-US2, which has historically been hard to access without the need for a secondary sulfur source. Despite the oxophilicity of uranium, the method successfully forms US2 without the inclusion of oxygen in the bulk final product. These findings are supported by simultaneous thermal analysis, elemental analysis, powder X-ray diffraction, and uranium L3-edge X-ray absorption fine-structure spectroscopy. This work represents the first example of a single-source precursor approach to target and synthesize actinide materials other than the oxides.

A soft molecular single-source precursor approach to synthesize a nanostructured Co9S8 (pre)catalyst for efficient water oxidation and biomass valorization

A nanocrystalline Co9S8 (pre)catalyst derived from a novel {CoIIS4} complex is reported for the oxygen evolution reaction (OER) and selective biomass valorization. During OER, Co9S8 reconstructs completely into a CoOOH active phase via S-leaching.

A Low-Temperature Synthetic Route Toward a High-Entropy 2D Hexernary Transition Metal Dichalcogenide for Hydrogen Evolution Electrocatalysis.

High-entropy (HE) metal chalcogenides are a class of materials that have great potential in applications such as thermoelectrics and electrocatalysis. Layered 2D transition-metal dichalcogenides (TMDCs) are a sub-class of high entropy metal chalcogenides that have received little attention to date as their preparation currently involves complicated, energy-intensive, or hazardous synthetic steps. To address this, a low-temperature (500 °C) and rapid (1h) single source precursor approach is successfully adopted to synthesize the hexernary high-entropy metal disulfide (MoWReMnCr)S2 . (MoWReMnCr)S2 powders are characterized by powder X-ray diffraction (pXRD) and Raman spectroscopy, which confirmed that the material is comprised predominantly of a hexagonal phase. The surfaceoxidation states and elemental compositions are studied by X-ray photoelectron spectroscopy (XPS) whilst the bulk morphology and elemental stoichiometry with spatial distribution is determined by scanning electron microscopy (SEM) with elemental mapping information acquired fromenergy-dispersive X-ray (EDX) spectroscopy. The bulk, layered material is subsequently exfoliated to ultra-thin, several-layer 2D nanosheets by liquid-phase exfoliation (LPE). The resultingfew-layer HE (MoWReMnCr)S2 nanosheets are found to contain a homogeneous elemental distribution of metals at the nanoscale by high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) with EDX mapping. Finally, (MoWReMnCr)S2 is demonstrated as a hydrogen evolution electrocatalyst and compared to 2H-MoS2 synthesized using the molecular precursor approach. (MoWReMnCr)S2 with 20% w/w of high-conductivity carbon black displays a low overpotential of 229mV in 0.5 M H2 SO4 to reach a current density of 10mA cm-2 , which is much lower than the overpotential of 362mV for MoS2 . From density functional theory calculations, it is hypothesised that the enhanced catalytic activity is due to activation of the basal plane upon incorporation of other elements into the 2H-MoS2 structure, in particular, the first row TMs Cr and Mn.

Transition metal chalcogenide bifunctional catalysts for chemical recycling by plastic hydrocracking: a single-source precursor approach.

Sulfided nickel, an established hydrocracking and hydrotreating catalyst for hydrocarbon refining, was synthesized on porous aluminosilicate supports for the hydrocracking of mixed polyolefin waste. Zeolite beta, zeolite 13X, MCM41 and an amorphous silica-alumina catalyst support were impregnated with the single-source precursor (SSP) nickel (II) ethylxanthate for catalyst support screening. Application of this synthesis method to beta-supported nickel (Ni@Beta), as an alternative to wet impregnation using aqueous nickel (II) nitrate, provided catalytic materials with higher conversion to fluid products at the same mild batch reaction conditions of 330°C with appropriate agitation and 20 bar H2 pressure. Mass balance quantification demonstrated that SSP-derived 5wt%Ni@Beta yielded a greater than 95 wt% conversion of a mixed polyolefin feed to fluid products, compared with 39.8 wt% conversion in the case of 5wt%Ni@Beta prepared by wet impregnation. Liquid and gas products were quantitatively analysed by gas chromatography–flame ionization detection (GC-FID) and gas chromatography–mass spectrometry (GC-MS), revealing a strong selectivity to saturated C4 (37.3 wt%), C5 (21.6 wt%) and C6 (12.8 wt%) hydrocarbons in the case of the SSP-derived catalyst.

Structural and Optical Properties of Chemically Deposited Metal Chalcogenide Thin Film CrS and its Photovoltaic Application

In this work, chromium sulfide (CrS) thin films were grown on the acetic acid substrates by chemical bath deposition to prepare non-toxic photovoltaic devices. The combined single-source precursor approach has been developed for the deposition method using tris(diethyldithiocarbamato)chromium(III) for the deposition of CrS thin films grown at bath temperatures of 30, 60 and 90 ºC and at a constant deposition time of 30-120 min. The sufrace mophology of the prepared films have been analyzed by SEM and HR-TEM techniques. The study of the films indicate the distributed roughness and nano bundled hexagonal structures. The energy dispersive X-ray (EDX) spectroscopy analysis conformed the presence of Cr and S. The polycrystalline behaviour of the films was studied by an XRD study which revealed the mixed phases with a predicted crystallite size of 20 nm. The optical measurements showed films had a maximum transmittance of 90% in the visible region and the evaluated energy band varied in the range of 2.2-2.378 eV with the change of bath temperatures. This suggests that CrS thin film prepared at 90 ºC has enhanced crystalline superiority. According to photoluminescence (PL) analysis, the green emission can be attributed to the presence of several deep trap states or defects in the CrS structure. Moreover, natural dye sensitized solar cells (DSSCs) in CrS thin film prepared at 90 ºC, Jsc (28.0 mA/cm2) produced a larger voltage in the short circuit as compared to synthetic dye sensitized solar cells (DSSCs) using CrS thin film Jsc (22.5 mA/cm2).

Single-Source Alkoxide Precursor Approach to Titanium Molybdate, TiMoO5, and Its Structure, Electrochemical Properties, and Potential as an Anode Material for Alkali Metal Ion Batteries.

Transition-metal oxide nanostructured materials are potentially attractive alternatives as anodes for Li ion batteries and as photocatalysts. Combining the structural and thermal stability of titanium oxides with the relatively high oxidation potential and charge capacity of molybdenum(VI) oxides was the motivation for a search for approaches to mixed oxides of these two metals. Challenges in traditional synthetic methods for such materials made development of a soft chemistry single-source precursor pathway our priority. A series of bimetallic Ti-Mo alkoxides were produced by reactions of homometallic species in a 1:1 ratio. Thermal solution reduction with subsequent reoxidation by dry air offered in minor yields Ti2Mo2O4(OMe)6(OiPr)6 (1) by the interaction of Ti(OiPr)4 with MoO(OMe)4 and Ti6Mo6O22(OiPr)16(iPrOH)2 (2) by the reaction of Ti(OiPr)4 with MoO(OiPr)4. An attempt to improve the yield of 2 by microhydrolysis, using the addition of stoichiometric amounts of water, resulted in the formation with high yield of a different complex, Mo7Ti7+xO31+x(OiPr)8+2x (3). Controlled thermal decomposition of 1–3 in air resulted in their transformation into the phase TiMoO5 (4) with an orthorhombic structure in space group Pnma, as determined by a Rietveld refinement. The electrochemical characteristics of 4 and its chemical transformation on Li insertion were investigated, showing its potential as a promising anode material for Li ion batteries for the first time. A lower charge capacity and lower stability were observed for its application as an anode for a Na ion battery.

Protective Spinel Coating for Li1.17Ni0.17Mn0.50Co0.17O2 Cathode for Li-Ion Batteries through Single-Source Precursor Approach.

The Li1.17Ni0.17Mn0.50Co0.17O2 Li-rich NMC positive electrode (cathode) for lithium-ion batteries has been coated with nanocrystals of the LiMn1.5Co0.5O4 high-voltage spinel cathode material. The coating was applied through a single-source precursor approach by a deposition of the molecular precursor LiMn1.5Co0.5(thd)5 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) dissolved in diethyl ether, followed by thermal decomposition at 400 °C inair resulting in a chemically homogeneous cubic spinel. The structure and chemical composition of the coatings, deposited on the model SiO2 spheres and Li-rich NMC crystallites, were analyzed using powder X-ray diffraction, electron diffraction, high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive X-ray (EDX) mapping. The coated material containing 12 wt.% of spinel demonstrates a significantly improved first cycle Coulombic efficiency of 92% with a high first cycle discharge capacity of 290 mAhg−1. The coating also improves the capacity and voltage retention monitored over 25 galvanostatic charge–discharge cycles, although a complete suppression of the capacity and voltage fade is not achieved.

The Attractiveness of the Ternary Rh-Pd-Pt Alloys for CO Oxidation Process

Ternary alloys of platinum group metals attract a growing interest due to their unique catalytic properties. The present research is aimed to synthesize a series of Rh-Pd-Pt alloys with varied ratios of metals using a single-source precursor approach. Rhodium and palladium are partly miscible metals, while each of these metals is unlimitedly miscible with platinum. Thermolysis of complex salts used as a precursor results in the formation of metastable systems. The 3D nanostructure alloys are being formed after the complete decomposition of the single-source precursor. High-resolution transmission electron microscopic studies have shown that the nanoalloys are composed of interconnected polycrystalline ligaments with a mean diameter of 50 nm. The single-phase composition is confirmed by an X-ray diffraction analysis. The ratio of metals plays an important role in determining the catalytic activity of alumina-supported alloys and their thermal stability. According to UV-vis spectroscopy data, the higher palladium portion corresponds to worse dispersion of initially prepared, fresh catalysts. Treatment of the catalysts under prompt thermal aging conditions (up to 800 °C) causes redispersion of palladium-rich alloyed nanoparticles, thus leading to improved catalytic activity and thermal stability.

Electrochemical Performance of Photovoltaic Cells using HDA Capped-SnS Nanocrystal from bis (N-1,4-Phenyl-N-Morpho-Dithiocarbamato) Sn(II) Complexes.

Great consideration is placed on the choice of capping agents’ base on the proposed application, in order to cater to the particular surface, size, geometry, and functional group. Change in any of the above can influence the characteristics properties of the nanomaterials. The adoption of hexadecylamine (HDA) as a capping agent in single source precursor approach offers better quantum dots (QDs) sensitizer materials with good quantum efficiency photoluminescence and desirable particles size. Structural, morphological, and electrochemical instruments were used to evaluate the characterization and efficiency of the sensitizers. The cyclic voltammetry (CV) results display both reduction and oxidation peaks for both materials. XRD for SnS/HDA and SnS photosensitizers displays eleven peaks within the values of 27.02° to 66.05° for SnS/HDA and 26.03° to 66.04° for SnS in correlation to the orthorhombic structure. Current density–voltage (I–V) results for SnS/HDA exhibited a better performance compared to SnS sensitizers. Bode plot results indicate electrons lifetime (τ) for SnS/HDA photosensitizer have superiority to the SnS photosensitizer. The results connote that SnS/HDA exhibited a better performance compared to SnS sensitizers due to the presence of HDA capping agent.

Heterometallic trinuclear oxo-centered clusters as single-source precursors for synthesis of stoichiometric monodisperse transition metal ferrite nanocrystals.

The use of heterobimetallic metal complexes as molecular single-source precursors is a promising strategy for the targeted synthesis of phase-pure stoichiometric ternary metal oxide nanocrystals. However, the design and synthesis of these precursors is not trivial and can require considerable effort. Using spinel metal ferrite nanocrystals of formula MFe2O4 (M = Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) as a model system, this paper evaluates the efficacy of the single-source precursor approach by comparing directly nanocrystals synthesized from the solvothermal reaction of heterobimetallic trinuclear oxo-bridged clusters of formula MIIFeIII2(μ3-O)(μ2-O2CR)6(H2O)3, R = CF3 to nanocrystals synthesized from the solvothermal reaction of stoichiometric mixtures of multi-source precursors, such as metal acetate or nitrate salts. For each M explored here, the clusters form phase-pure MFe2O4 nanocrystals with significantly narrower size distributions than nanocrystals synthesized from multi-source-precursors. In the case of M = Cu, the multi-source metal salt precursors produce a mixture of CuO and CuFe2O4. Additionally, changing the electronic nature of the R-group on the bridging carboxylate ligand from electron withdrawing (CF3) to electron donating (CH3 or C(CH3)3) decreases the average diameter of the resulting nanocrystals by a factor of two. The cluster molecules therefore offer superior control over both morphology and composition for transition metal ferrite nanocrystals. We hypothesize that this remarkable performance arises from the presence of pre-formed M2+-O-Fe3+ moieties in the cluster molecules that enable direct nucleation of MFe2O4 and preclude nucleation of binary oxide impurities.

Solution-processed amorphous yttrium aluminium oxide YAlxOy and aluminum oxide AlxOy, and their functional dielectric properties and performance in thin-film transistors

Yttrium aluminium oxide (YAlxOy) dielectric is accessible using a molecular single-source precursor approach. Processing using deep UV leads to a functional amorphous dielectric with functionality in a thin-film transistor device.

Improved chemical water oxidation with Zn in the tetrahedral site of spinel-type ZnCo2O4 nanostructure

Spinel oxides with the composition of AIIBIII2O4 (A and B are metal ions) represent an important class of anode material for water splitting to replace the currently used noble-metal catalysts. Although spinel electrocatalysts have widely been investigated for electrochemical water oxidation, the role of octahedral and tetrahedral sites influencing catalytic performance has been a topic of discussion for a long time and still under debate. Lately, this issue has been addressed by substituting redox-inert cation to the tetrahedral sites of cobalt spinels and comparing the electrochemical activity between them. However, rapid surface structural transformation of the catalysts under operating electrochemical conditions makes it difficult to infer the exact contribution of tetrahedral and octahedral sites for water oxidation. Herein, for the first time, we utilize the oxidant-driven water oxidation approach to reveal the responsible active sites using two spinel-type nanostructures, ZnIICo2IIIO4 and CoIICo2IIIO4 (Co3O4), synthesized by using a single-source precursor approach. Strikingly, a superior O2 production rate (0.98 mmolO2 molCo−1 s−1) following first-order reaction kinetics was achieved for ZnCo2O4 in the presence of CeIV as sacrificial electron acceptor compared to Co3O4 spinel (0.29 mmolO2 molCo−1 s−1). The structural and morphological stability of the ZnCo2O4 and Co3O4 post water oxidation catalysis confirms that the catalytic activity is strictly controlled by the geometry and electronic structure of the active site of the spinel structure. The higher performance of ZnCo2O4 over Co3O4 further indicates that the presence of CoII is not essential for catalytic water oxidation. The presence of redox inert ZnII at the tetrahedral site of ZnCo2O4 can facilitate the stabilization of a high-valent CoIV intermediate via oxidation of CoIII (situated at the octahedral site), and this intermediate can be regarded as the active species for water oxidation catalyst along with structural defects caused by surface Zn leaching.

Near-Infrared Ag2S quantum dots loaded in phospholipid nanostructures: Physical properties, stability and cytotoxicity

In this paper, we report the encapsulation of near-infrared emitting Ag2S quantum dots (QDs) in the phospholipid nanostructures with different functional head-groups of phospholipids. Ag2S QDs were prepared using single-source precursor approach by thermal decomposition of silver diethyldithiocarbamate (Ag(DDTC)). Water dispersion of the QDs were accomplished by their encapsulation within cationic, anionic, neutral or PEGylated phospholipid nanostructures. The influence of different head-groups of phospholipids on physicochemical properties, encapsulation efficiency, luminescence efficiency, type of aggregates and stability of phospholipid nanostructures containing Ag2S QDs was determined and discussed. The basic biocompability of Ag2S QDs preparates was tested in vitro using NIH3T3 and HCT116 cell lines. In this regard the viability was assessed using MTS assay along with induction of apoptosis in cultures treated with selected phospholipid formulations of Ag2S. The anionic and PEGylated phospholipids showed greater encapsulation efficacy, stability and lower cytotoxicity than neutral and cationic phospholipids used for QDs encapsulation. Such low-toxic phospholipid structures can be used as carriers for different types of QDs. Our study revealed that the DSPE-PEG2000-based formulations of Ag2S QDs are well suited for applications in living biological systems Combination of outstanding optical properties of QDs and phospholipids biocompatibility makes such particles potentially useful in fluorescent microscopy as fluorescent contrast agents.

Heterobimetallic Single-Source Precursors: A Springboard to the Synthesis of Binary Intermetallics

Intermetallics are atomically ordered crystalline compounds containing two or more main group and transition metals. In addition to their rich crystal chemistry, intermetallics display unique properties of interest for a variety of applications, including superconductivity, hydrogen storage, and catalysis. Because of the presence of metals with a wide range of reduction potentials, the controlled synthesis of intermetallics can be difficult. Recently, soft chemical syntheses such as the modified polyol and ship-in-a-bottle methods have helped advance the preparation of these materials. However, phase-segregated products and complex multistep syntheses remain common. Here, we demonstrate the use of heterobimetallic single-source precursors for the synthesis of 10–15 and 11–15 binary intermetallics. The coordination environment of the precursor, as well as the exact temperature used play a critical role in determining the crystalline intermetallic phase that is produced, highlighting the potential versatility of this approach in the synthesis of a variety of compounds. Furthermore, we show that a recently developed novel plasma-processing technique is successful in removing the surface graphitic carbon observed in some of the prepared compounds. This new single-source precursor approach is a powerful addition to the synthesis of atomically ordered intermetallic compounds and will help facilitate their further study and development for future applications.

From a Molecular Single‐Source Precursor to a Selective High‐Performance RhMnOx Catalyst for the Conversion of Syngas to Ethanol

AbstractThe first molecular carbonyl RhMn cluster Na2[Rh3Mn3(CO)18] 2 with highly labile CO ligands and predefined Rh‐Mn bonds could be realized and successfully used for the preparation of the silica (davisil)‐supported RhMnOx catalysts for the conversion of syngas (CO, H2) to ethanol (StE); it has been synthesized through the salt metathesis reaction of RhCl3 with Na[Mn(CO)5] 1 and isolated in 49 % yields. The dianionic Rh3Mn3 cluster core of 2 acts as a molecular single‐source precursor (SSP) for the low‐temperature preparation of selective high‐performance RhMnOx catalysts. Impregnation of 2 on silica (davisil) led to three different silica‐supported RhMnOx catalysts with dispersed Rh nanoparticles tightly surrounded by a MnOx matrix. By using this molecular SSP approach, Rh and MnOx are located in close proximity on the oxide support. Therefore, the number of tilted CO adsorption sites at the RhMnOx interface increased leading to a significant enhancement in selectivity and performance. Investigations on the spent catalysts after several hours time‐on‐stream revealed the influence of rhodium carbide RhCx formation on the long‐term stability.

Metal-catalyst-free access to multiwalled carbon nanotubes/silica nanocomposites (MWCNT/SiO2) from a single-source precursor.

The present study introduces a facile single-source precursor preparative access to bamboo-like multiwalled carbon nanotubes (MWCNTs) highly dispersed within a mesoporous silica-rich matrix. The metal-free single-source precursor was synthesized via a one-pot sol-gel process using tetramethyl orthosilicate (TMOS) and 4,4'-dihydroxybiphenyl (DHBP) and converted subsequently via pyrolysis under an argon atmosphere into MWCNT/silica nanocomposites. The in situ segregation of the highly defective bamboo-like MWCNTs was carefully investigated and has been shown to occur within the mesopores of the silica-rich matrix at relatively low temperatures and without the use of a metal catalyst. The experimental results have been supported by extensive computational simulations, which correlate the molecular architecture of the single-source precursor with the structural features of the carbon phase segregating from the silica matrix. Furthermore, the role of hydrogen in the stability of the prepared nanocomposites as well as in the high-temperature evolution and morphology of the segregated MWCNTs has been discussed based on vibrational spectroscopy, calorimetric studies and empirical potential calculations. The results obtained within the present study may allow for designing highly-defined nanocarbon-containing composites with tailored structural features and property profiles.

Synthesis, dielectric properties and application in a thin film transistor device of amorphous aluminum oxide AlxOy using a molecular based precursor route

Generation of dielectric amorphous aluminum oxide using a novel chimie douce molecular precursor route is reported.

Synthesis, oxide formation, properties and thin film transistor properties of yttrium and aluminium oxide thin films employing a molecular-based precursor route.

Combustion synthesis of dielectric yttrium oxide and aluminium oxide thin films is possible by introducing a molecular single-source precursor approach employing a newly designed nitro functionalized malonato complex of yttrium (Y-DEM-NO21) as well as defined urea nitrate coordination compounds of yttrium (Y-UN 2) and aluminium (Al-UN 3). All new precursor compounds were extensively characterized by spectroscopic techniques (NMR/IR) as well as by single-crystal structure analysis for both urea nitrate coordination compounds. The thermal decomposition of the precursors 1–3 was studied by means of differential scanning calorimetry (DSC) and thermogravimetry coupled with mass spectrometry and infrared spectroscopy (TG-MS/IR). As a result, a controlled thermal conversion of the precursors into dielectric thin films could be achieved. These oxidic thin films integrated within capacitor devices are exhibiting excellent dielectric behaviour in the temperature range between 250 and 350 °C, with areal capacity values up to 250 nF cm−2, leakage current densities below 1.0 × 10−9 A cm−2 (at 1 MV cm−1) and breakdown voltages above 2 MV cm−1. Thereby the increase in performance at higher temperatures can be attributed to the gradual conversion of the intermediate hydroxy species into the respective metal oxide which is confirmed by X-ray photoelectron spectroscopy (XPS). Finally, a solution-processed YxOy based TFT was fabricated employing the precursor Y-DEM-NO21. The device exhibits decent TFT characteristics with a saturation mobility (μsat) of 2.1 cm2 V−1 s−1, a threshold voltage (Vth) of 6.9 V and an on/off current ratio (Ion/off) of 7.6 × 105.

Synthesis of sulfur-rich nitrogen dots from a single source precursor and its application in dual-mode sensing

In this work, a strategy of sulfur-rich doped nitrogen dots (S-Ndots) based on a single-source precursor (SSP) was developed and applied to sensing of Hg2+. Fluorescent S-Ndots were synthesized by microwave-assisted method using 2-azidothiazole as a single-source precursor, which served as carbon, nitrogen and sulfur source simultaneously. This SSP approach guarantee efficient core doping and improve the effect of doping in a molecular level, which is more efficient and simpler than the doping strategy with multiple precursors. The results showed that the as-synthesized S-Ndots have high sulfur component up to 24.6%, and their specific structure effectively promoted artificial peroxidase activity and coordination interaction. Interestingly, the fluorescence intensity could be quenched obviously and the peroxidase activity of the S-Ndots were selectively inhibited by Hg2+, then the S-Ndots were applied to the dual-mode sensing of Hg2+ with both fluorometric and colorimetric readout. The S-Ndots-based sensing assay with simplicity and rapidity showed high selectivity to Hg2+ with the detection limit of 0.1 μmol/L by fluorescence spectroscopy and a distinguishable change in the color was observed by the naked eye. In addition, the accuracy and precision were evaluated based on the quantitative detection of Hg2+ in industrial waste water samples with satisfactory results. Moreover, the S-Ndots with good biocompatibility were further explored for imaging of Hg2+ in A549 cells. As a novel member of nitrogen quantum dots family, the S-Ndots hold great promise to broaden its applications in environmental and biological sensing.

A single-source precursor approach to solution processed indium arsenide thin films.

This paper reports the synthesis of the novel single-source precursor, [{(MeInAs t Bu)3}2(Me2InAs( t Bu)H)2] and the subsequent first report of aerosol-assisted chemical vapour deposition of InAs thin films. Owing to the use of the single-source precursor, highly crystalline and stoichiometric films were grown at a relatively low deposition temperature of 450 °C. Core level XPS depth profiling studies showed some partial oxidation of the film surface, however this was self-limiting and disappeared on etch profiles. Valence band XPS analysis matched well with the simulated density of state spectrum. Hall effect measurements performed on the films showed that the films were n-type with promising resistivity (3.6 × 10-3 Ω cm) and carrier mobility (410 cm2 V-1 s-1) values despite growth on amorphous glass substrates.