Substrate Engineering Research Articles

Scattering of Quasistatic Plasmons From One-Dimensional Junctions of Graphene: Transfer Matrices, Fresnel Relations, and Nonlocality

It requires substantial computational resources to design plasmonic circuits such as resonators, topological waveguides, modulators, and photonic switches based on graphene and other conducting 2D materials, so a comprehensive study is called for. The authors propose numerical approaches to solve the problem through the transfer-matrix method using reconstructed numerical solutions of Maxwell equations, and find the limits of the method's applicability. The results would facilitate the design of plasmonic circuits by means of electrostatic gating, ultrafast photoexcitation, Moire, and substrate engineering at the visible to near-infrared telecommunication frequencies.

Substrate effect on the photoluminescence of chemical vapor deposition transferred monolayer WSe2

The substrate effect is an important issue in the properties of two-dimensional transition metal dichalcogenides (2D TMDs). Quantitatively determining the dependence of the photoluminescence (PL) emission properties and the excitonic behavior of single-layer 2D materials in a specific dielectric environment would provide helpful guidance for the rational design of substrates for high performance 2D TMD PL emission devices. Here, using a WSe2 monolayer on different substrates as a model system, it is demonstrated that the PL emission intensities can drastically change depending on the substrate effect. From the analysis of the excitonic behavior, the results reveal that the spectral weight between the neutral and charged excitons in the PL spectra is significantly modified by the substrate types, and the weight factor is dependent on the laser excitation density. The charged exciton binding energy is obviously negatively related to the substrate dielectric constant. Furthermore, the change trends of the binding energy of the monolayer WSe2 on different substrates are basically the same for the increase of the excitation density. These results suggest that the choice of the substrate plays a significant role in the modulation of the PL properties and exciton states of atomically thin WSe2; hence, substrate engineering should be carefully considered in the design of future 2D devices.

Extreme learning machine and swarm-based support vector regression methods for predicting crystal lattice parameters of pseudo-cubic/cubic perovskites

Lattice parameters of perovskite compounds play crucial roles in engineering of buffer layers and substrates for heteroepitaxial films. As a result, predictive models that can effectively estimate these lattice parameters are highly desired. Therefore, this work presents elegant methods of predicting lattice parameters of pseudo-cubic/cubic perovskite through the development of the extreme learning machine (ELM) based model and hybridization of the particle swarm optimization (PSO) technique with the support vector regression algorithm (SVRA). The generalization and predictive strengths of the proposed SVRA-PSO and ELM-based models are compared with existing methods such as the Ubic model and the recently developed Sidey model on the basis of root mean square error (RMSE), mean absolute error, mean absolute percentage error (MAPE), and correlation coefficient. The developed SVRA-PSO model performs better than the ELM-based model, the Ubic model, and the Sidey model, with performance improvement of 20.99%, 29.29%, and 33.39%, respectively, on the basis of MAPE. Similarly, the SVRA-PSO, respectively, attains performance improvement of 24.74%, 34%, and 37.89% on the basis of RMSE. Furthermore, the developed ELM-based model outperforms the Ubic and Sidey models with performance improvement of 15.70% and 10.50%, respectively, on the basis of MAPE and percentage enhancement of 17.48% and 12.31% when compared on the basis of RMSE. Although the SVRA-PSO model has the best performance of all the compared models, the developed ELM-based model possesses the advantage of easy implementation in addition to its moderate performance.

Nano-engineering the material structure of preferentially oriented nano-graphitic carbon for making high-performance electrochemical micro-sensors

Direct synthesis of thin-film carbon nanomaterials on oxide-coated silicon substrates provides a viable pathway for building a dense array of miniaturized (micron-scale) electrochemical sensors with high performance. However, material synthesis generally involves many parameters, making material engineering based on trial and error highly inefficient. Here, we report a two-pronged strategy for producing engineered thin-film carbon nanomaterials that have a nano-graphitic structure. First, we introduce a variant of the metal-induced graphitization technique that generates micron-scale islands of nano-graphitic carbon materials directly on oxide-coated silicon substrates. A novel feature of our material synthesis is that, through substrate engineering, the orientation of graphitic planes within the film aligns preferentially with the silicon substrate. This feature allows us to use the Raman spectroscopy for quantifying structural properties of the sensor surface, where the electrochemical processes occur. Second, we find phenomenological models for predicting the amplitudes of the redox current and the sensor capacitance from the material structure, quantified by Raman. Our results indicate that the key to achieving high-performance micro-sensors from nano-graphitic carbon is to increase both the density of point defects and the size of the graphitic crystallites. Our study offers a viable strategy for building planar electrochemical micro-sensors with high-performance.

Broadening the scope of biocatalytic C-C bond formation.

The impeccable control over chemo-, site-, and stereoselectivity possible in enzymatic reactions has led to a surge in the development of new biocatalytic methods. Despite carbon-carbon (C-C) bonds providing the central framework for organic molecules, development of biocatalytic methods for their formation has been largely confined to the use of a select few lyases over the last several decades, limiting the types of C-C bond-forming transformations possible through biocatalytic methods. This Review provides an update on the suite of enzymes available for highly selective biocatalytic C-C bond formation. Examples will be discussed in reference to the (1) native activity of enzymes, (2) alteration of activity through protein or substrate engineering for broader applicability, and (3) utility of the biocatalyst for abiotic synthesis.

Self-Assembled Monolayers with Embedded Dipole Moments for Work Function Engineering of Oxide Substrates

Self-assembled monolayers (SAMs) are frequently used for work function (WF) engineering of different materials. For this, typically dipolar groups are attached to the molecule terminus at the SAM-a...

Influence of the Substrate on the Optical and Photo-electrochemical Properties of Monolayer MoS2.

Substrates influence the electrical and optical properties of monolayer (ML) MoS2 in field-effect transistors and photodetectors. Photoluminescence (PL) and Raman spectroscopy measurements have shown that conducting substrates can vary the doping concentration and influence exciton decay channels in ML-MoS2. Doping and exciton decay dynamics are expected to play a major role in the efficiency of light-driven chemical reactions, but it is unclear to what extent these factors contribute to the photo(electro)catalytic properties of ML-MoS2. Here, we report spatially resolved PL, Raman, and photo-electrochemical current measurements of 5-10 μm-wide ML-MoS2 triangles deposited on pairs of indium-doped tin oxide (ITO) electrodes that are separated by a narrow insulating quartz channel [i.e., an ITO interdigitated array (IDA) electrode]. Optical microscopy images and atomic force microscopy measurements revealed that the ML-MoS2 triangles lie conformally on the quartz and ITO substrates. The PL spectrum of MoS2 shifts and decreases in intensity in the ITO region, which can be attributed to differences in nonradiative and radiative exciton decay channels. Raman spectra showed no significant peak shifts on the two substrates that would have indicated a substrate-induced doping effect. We spatially resolved the photo-electrochemical current because of hole-induced iodide oxidation and observed that ML-MoS2 produces lower photocurrents in the quartz region than in the ITO region. The correlated PL, Raman, and photocurrent mapping data show that the MoS2/quartz interface diminishes fast nonradiative exciton decay pathways but the photocurrent response in the quartz region is likely limited by inefficient in-plane carrier transport to the ITO electrode because of carrier recombination with S vacancies in MoS2 or charged impurities in the quartz substrate. Nonetheless, the experimental methodology presented herein provides a framework to evaluate substrate engineering strategies to tune the (photo)electrocatalytic properties of two-dimensional materials.

Power loss of hot Dirac fermions in silicene and its near equivalence with graphene

The power loss P of hot Dirac fermions through the coupling to the intrinsic intravalley and intervalley acoustic and optical phonons is analytically investigated in silicene as a function of electron temperature T e and density n s . At very low T e , the power dissipation is found to follow the Bloch–Grüneisen power-law and , as in graphene, and for 20–30 K, the power loss is predominantly due to the intravalley acoustic phonon scattering. On the other hand, dispersionless low energy intervalley acoustic phonons begin to dominate the power transfer at temperatures as low as ∼30 K, and optical phonons dominate at K, unlike the graphene. The total power loss increases with T e with a value of ∼1010 eV s−1 at 300 K, which is the same order of magnitude as in graphene. The power loss due to intravalley acoustic phonons increases with n s at higher T e , whereas due to the intervalley acoustic and optical phonons is found to be independent of n s . Interestingly, the energy relaxation time in silicene is about four times higher than that in graphene. For this reason, silicene may be superior over graphene for its applications in bolometers and calorimeters. Power transfer to the surface optical phonons is also studied as a function of T e and n s for silicene on Al2O3 substrate and it is found to be greater than the intrinsic phonon contribution at higher T e . Substrate engineering is discussed to reduce .

Substrate Engineering of Inductors on SOI for Improvement of Q-Factor and Application in LNA

High Q-factor inductors are critical in designing high performance RF/microwave circuits on SOI technology. Substrate losses is a key limiting factor when designing inductors with high Q-factors. In this context, we report a substrate engineering method that enables improvement of quality factors of already fabricated inductors on SOI. A novel femtosecond laser milling process is utilized for the fabrication of locally suspended membranes of inductors with handler silicon completely etched. Such flexible membranes suspended freely on the BOX show up to 92 % improvement in Q-factor for single turn inductor. The improvement in Q-factor is reported on large sized inductors due to reduced parallel capacitance which allows enhanced operation of inductors at high frequencies. A compact model extraction methodology has been developed to model inductor membranes. These membranes have been utilized for the improvement of noise performance of LNA working in the 4.9 - 5.9 GHz range. A 0.1 dB improvement in noise figure has been reported by taking an existing design and suspending the input side inductors of the LNA circuit. The substrate engineering method reported in this work is not only applicable to inductors but also to active circuits, making it a powerful tool for enhancement of RF devices.

Tailoring the transfer characteristics and hysteresis in MoS2 transistors using substrate engineering.

We demonstrate a novel form of transfer characteristics in substrate engineered MoS2 field effect transistors. Robust hysteresis with stable threshold voltages and a large gate voltage window is observed, which is suppressed at low temperatures. We analyse the dependence of the device characteristics on gate voltage range, gate stressing and sweep rates. We infer that the hysteresis originates from artificially created charged traps near the MoS2-SiO2 interface. These charge traps act as long range Coulomb scatterers and are screened at high carrier densities. The hysteresis is strongly suppressed in measurements on wafers devoid of the substrate treatment, providing a new extrinsic route to carefully tune the transfer characteristics.

Modified Zinc Oxide Nanoparticles for Corrosion Resistance Applications

Atmospheric corrosion is a big threat to the steel structures. This is because it compromises its structural integrity, aesthetic aspects and overall efficiency. An attempt has been made to counteract this through surface engineering of substrates including glass and steel by using modified zinc oxide nanoparticles to increase hydrophobicity. The synthesis of zinc oxide nanoparticles is carried out by using sol-gel method, thereafter these particles were modified by using stearic acid; a fatty acid. The zinc oxide nanoparticles were characterized by using X-ray diffraction analysis (XRD) which confirms the presence of hexagonal wurtzite structure. Moreover, the Scanning Electron microscopy (SEM) reveals the hexagonal wurtzite morphology of as prepared nanoparticles. Fourier transform infrared spectroscopy (FTIR) confirms the grafting of stearic acid on the surface of ZnO in bidentate form. The water Contact Angle obtained by using sessile drop method gives a statistical value of 140o which is of great interest due to higher water repellency and lower surface contact area. Finally, corrosion test was carried out on the coated steel substrate by means of conventional corrosion testing technique and it is observed that the coated sample decays three times slower than that of its bare steel counterpart.

Modified Zinc Oxide Nanoparticles for Corrosion Resistance Applications

Atmospheric corrosion is a big threat to the steel structures. This is because it compromises its structural integrity, aesthetic aspects and overall efficiency. An attempt has been made to counteract this through surface engineering of substrates including glass and steel by using modified zinc oxide nanoparticles to increase hydrophobicity. The synthesis of zinc oxide nanoparticles is carried out by using sol-gel method, thereafter these particles were modified by using stearic acid; a fatty acid. The zinc oxide nanoparticles were characterized by using X-ray diffraction analysis (XRD) which confirms the presence of hexagonal wurtzite structure. Moreover, the Scanning Electron microscopy (SEM) reveals the hexagonal wurtzite morphology of as prepared nanoparticles. Fourier transform infrared spectroscopy (FTIR) confirms the grafting of stearic acid on the surface of ZnO in bidentate form. The water Contact Angle obtained by using sessile drop method gives a statistical value of 140o which is of great interest due to higher water repellency and lower surface contact area. Finally, corrosion test was carried out on the coated steel substrate by means of conventional corrosion testing technique and it is observed that the coated sample decays three times slower than that of its bare steel counterpart.

Role of interface properties in organic solar cells: from substrate engineering to bulk-heterojunction interfacial morphology

Surface properties control provides tools for tailoring the active layer self-assembly and phase separation kinetics, which plays an important role in the morphology by directing phase separation during film deposition, drying and annealing.

Engineering cytochrome P450 enzyme systems for biomedical and biotechnological applications.

Cytochrome P450 enzymes (P450s) are broadly distributed among living organisms and play crucial roles in natural product biosynthesis, degradation of xenobiotics, steroid biosynthesis, and drug metabolism. P450s are considered as the most versatile biocatalysts in nature because of the vast variety of substrate structures and the types of reactions they catalyze. In particular, P450s can catalyze regio- and stereoselective oxidations of nonactivated C-H bonds in complex organic molecules under mild conditions, making P450s useful biocatalysts in the production of commodity pharmaceuticals, fine or bulk chemicals, bioremediation agents, flavors, and fragrances. Major efforts have been made in engineering improved P450 systems that overcome the inherent limitations of the native enzymes. In this review, we focus on recent progress of different strategies, including protein engineering, redox-partner engineering, substrate engineering, electron source engineering, and P450-mediated metabolic engineering, in efforts to more efficiently produce pharmaceuticals and other chemicals. We also discuss future opportunities for engineering and applications of the P450 systems.

Graphene Growth and Characterization: Advances, Present Challenges and Prospects

It is about a decade since graphene became a material for serious research by researchers in condensed matter of various nationalities making significant progress. This paper on graphene growth and characterization: advances, present challenges and prospects is therefore timely. Basic topics such as graphene and graphene technology, history and trend of graphene as well as graphene growth and synthesis have been discussed. Also presented are fundamental and mechanical properties, structural and morphological property characterization using different techniques. Graphene in biomedical and radio frequency applications, transparent electronics, integrated circuits, quantum dots, frequency multiplier, optical modulator and piezoelectricity and as a battery super capacitor are some applications and uses of graphene that have been considered. The lowering of the growth temperature of graphene has been found to be beneficial for the compartibility with other materials and processes and could also decrease the impact of cooling-induced wrinkling on the morphology of graphene; the growth on dielectric substrates; being able to resolve many problems associated with metallic growth substrates; better control of both the formation and the extension of additional layers on the graphene through substrate engineering that will result in approaches of graphene that is envisaged are some of the advances and future prospects. Also, the proposed tunable bandgap for graphene which is essential for microelectronics which contributes one of the present challenges is likely to be achieved in the very near future. Although theoretical and computational analyses have proved to have solved the zero bandgap problem of graphene, more convincing approaches that will solve the problem and give way for the fabrication of high performance graphene device are being awaited.

Strain Engineering of a Defect-Free, Single-Layer MoS2 Substrate for Highly Efficient Single-Atom Catalysis of CO Oxidation.

Single-atom catalysts (SACs) are of great scientific and technical importance due to their low cost, high site density, and high specificity to enhance chemical reactions. Nevertheless, a major issue that severely limits the practical exploration of SACs is their instability, i.e., the preference of sintering and clustering over a defect-free substrate during operation. Here, we employ first-principles calculations to investigate how substrate engineering can stabilize SACs by strain-tuning the electronic interactions between the metal and the substrate using two Pd adatoms on a defect-free, single-layer MoS2 as a typical example. It is identified that the Pd2 dimer is prone to dissociate and form highly efficient SACs for CO oxidation due to the enhanced charge transfer and orbital hybridization with the MoS2 substrate under a suitable tensile strain. The straining induces a semiconductive-to-metallic phase transition of the substrate. Moreover, low-cost elements, such as Ag, Ni, Cu, and Cr, can also be stabilized into high-performance SACs for CO oxidation with tunable reaction barriers by straining. The present findings offer a new avenue to inhibit the transition metal atoms from clustering into nanoclusters/particles and provide a clear guidance for the development of highly cost-efficient and stable SACs on defect-free substrates.

Suppression of twinning and enhanced electronic anisotropy of SrIrO3 films

The spin–orbit coupling and electron correlation in perovskite SrIrO3 (SIO) strongly favor new quantum states and make SIO very attractive for next generation quantum information technology. In addition, the small electronic bandwidth offers the possibility to manipulate anisotropic electronic transport by strain. However, twinned film growth of SIO often masks electronic anisotropy, which could be very useful for device applications. We demonstrate that the twinning of SIO films on (001) oriented SrTiO3 substrates can be strongly reduced for thin films with thickness t less than 30 nm by using substrates displaying a TiO2-terminated surface with step-edge alignment parallel to the a- or b-axis direction of the substrate. This allows us to study electronic anisotropy of strained SIO films, which hitherto has been reported only for bulklike SIO. For films with t < 30 nm, electronic anisotropy increases with increasing t and becomes even twice as large compared to nearly strain-free films grown on (110) DyScO3. The experiments demonstrate the high sensitivity of electronic transport toward structural distortion and the possibility to manipulate transport by substrate engineering.

Substrate Specificity and Engineering of Mevalonate 5-Phosphate Decarboxylase.

A bifurcation of the mevalonate (MVA) pathway was recently discovered in bacteria of the Chloroflexi phylum. In this alternative route for the biosynthesis of isopentenylpyrophosphate (IPP), the penultimate step is the decarboxylation of (R)-mevalonate 5-phosphate ((R)-MVAP) to isopentenyl phosphate (IP), which is followed by the ATP-dependent phosphorylation of IP to IPP catalyzed by isopentenyl phosphate kinase (IPK). Notably, the decarboxylation reaction is catalyzed by mevalonate 5-phosphate decarboxylase (MPD), which shares considerable sequence similarity with mevalonate diphosphate decarboxylase (MDD) of the classical MVA pathway. We show that an enzyme originally annotated as an MDD from the Chloroflexi bacterium Anaerolinea thermophila possesses equal catalytic efficiency for (R)-MVAP and (R)-mevalonate 5-diphosphate ((R)-MVAPP). Further, the molecular basis for this dual specificity is revealed by near atomic-resolution X-ray crystal structures of A. thermophila MPD/MDD bound to (R)-MVAP or (R)-MVAPP. These findings, when combined with sequence and structural comparisons of this bacterial enzyme, functional MDDs, and several putative MPDs, delineate key active-site residues that confer substrate specificity and functionally distinguish MPD and MDD enzyme classes. Extensive sequence analyses identified functional MPDs in the halobacteria class of archaea that had been annotated as MDDs. Finally, no eukaryotic MPD candidates were identified, suggesting the absence of the alternative MVA (altMVA) pathway in all eukaryotes, including, paradoxically, plants, which universally encode a structural and functional homologue of IPK. Additionally, we have developed a viable engineered strain of Saccharomyces cerevisiae as an in vivo metabolic model and a synthetic biology platform for enzyme engineering and terpene biosynthesis in which the classical MVA pathway has been replaced with the altMVA pathway.

Positioning-Group-Enabled Biocatalytic Oxidative Dearomatization.

Biocatalysts have the potential to perform reactions with exceptional selectivity and high catalytic efficiency while utilizing safe and sustainable reagents. Despite these positive attributes, the utility of a biocatalyst can be limited by the breadth of substrates that can be accommodated in the active site in a reactive pose. Proven strategies exist for optimizing the performance of a biocatalyst toward unnatural substrates, including protein engineering; however, these methods can be time intensive and require specialized equipment that renders these approaches inaccessible to synthetic chemists. Strategies accessible to chemists for the expansion of a natural enzyme’s substrate scope, while maintaining high levels of site- and stereoselectivity, remain elusive. Here, we employ a computationally guided substrate engineering strategy to expand the synthetic utility of a flavin-dependent monooxygenase. Specifically, experimental observations and computational modeling led to the identification of a critical interaction between the substrate and protein which is responsible for orienting the substrate in a pose productive for catalysis. The fundamental hypothesis for this positioning group strategy is supported by binding and kinetic assays as well as computational studies with a panel of compounds. Further, incorporation of this positioning group into substrates through a cleavable ester linkage transformed compounds not oxidized by the biocatalyst SorbC into substrates efficiently oxidatively dearomatized by the wild-type enzyme with the highest levels of site- and stereoselectivity known for this transformation.

S,N dual-doped carbon nanotubes as substrate to enhance the methanol oxidation performance of NiO nanoparticles

Surface atomic engineering of substrates provides an efficient way to further improve the catalytic activity and stability of coupled catalysts for methanol oxidation. In this paper, S,N dual-doped carbon nanotubes (S,N-CNTs) with high doping levels of S (4.78 at%) and N (3.58 at%) were generated by laser irradiation, and used as substrate to in-situ load NiO nanoparticles (NPs). S and N atoms in CNTs provided abundant anchor sites for NiO NPs to restrain their growth, strengthen the interaction between NiO NPs and CNTs, and promote the interfacial electron transportation. Therefore, electrochemical tests demonstrated that NiO/S,N-CNTs shown a high primary methanol oxidation activity of 2200 mA/mg, a long-term durability with 65.8% of mass activity maintained after 40000s, and an excellent methanol saturation concentration of 13 M. This work provides a facile strategy to fabricate S,N-CNTs and helps to understand the effect of heteroatoms in carbonaceous substrates on boosting the electroactivity of coupled catalysts.