Substrate Contact Research Articles

Functions of Self-Assembled Ultrafine TiO2 Nanocrystals for High Efficient Dye-Sensitized Solar Cells

In this paper, we demonstrate a simple approach of self-assembled process to form a very smooth and compacted TiO2 underlayer film from ultrafine titanium oxide (TiO2) nanocrystals with dimension of 4 nm for improving the electrical properties and device performances of dye-sensitized solar cells (DSSCs). Because the TiO2 film self-assembles by simply casting the TiO2 on fluorine-doped tin oxide (FTO) substrate, it can save a lot of materials in the process. As compared with control DSSC without the self-assembled TiO2 (SA-TiO2) layer, short-circuit current density (Jsc) improves from 14.9 mA/cm(2) for control DSSC to 17.3 mA/cm(2) for masked DSSC with the SA-TiO2 layer. With the very smooth SA-TiO2 layer, the power conversion efficiency is enhanced from 8.22% (control) to 9.35% for the DSSCs with mask and from 9.79% (control) to 11.87% for the DSSCs without mask. To explain the improvement, we have studied the optical properties, morphology, and workfunction of the SA-TiO2 layer on FTO substrate as well as the impedance spectrum of DSSCs. Importantly, we find that the SA-TiO2 layers have better morphology, uniformity, and contact with FTO electrode, increased workfunction and optical transmission, as well as reduced charge recombination at the contact of FTO substrate contributing to the improved device performances. Consequently, our results show that the simple self-assembly of TiO2 ultrafine nanocrystals forms a very good electron extraction layer with both improved optical and electrical properties for enhancing performances of DSSCs.

Structural Mapping of a Chaperone–Substrate Interaction Surface

AbstractNMR spectroscopy is used to detect site‐specific intermolecular short‐range contacts in a membrane–protein–chaperone complex. This is achieved by an “orthogonal” isotope‐labeling scheme that permits the unambiguous detection of intermolecular NOEs between the well‐folded chaperone and the unfolded substrate ensemble. The residues involved in these contacts are part of the chaperone–substrate contact interface. The approach is demonstrated for the 70 kDa bacterial Skp‐tOmpA complex.

Real-time monitoring of cell mechanical changes induced by endothelial cell activation and their subsequent binding with leukemic cell lines

Endothelial cell (EC) activation and their subsequent binding with different cells have various mechanical consequences that, if monitored real time, can serve as a functional biomarker of many pathophysiological response mechanisms. This work presents an innovative and facile strategy to conduct such monitoring using quartz crystal microbalance (QCM), thereby relating the shifts in its frequency and motional resistance to morphological changes upon cell–cell and cell–substrate interactions. By activating ECs with TNF-α and then characterizing their binding with HL-60 and KG-1 leukemia cells, we are able to induce the mechanical changes in ECs especially in the region of cell–substrate contact which resulted in dynamically coupled mass and viscoelastic changes representing the extent of both activation and binding. The activated ECs suffered a decrease of cellular contact area, leading to positive frequency shift and decreased motional resistance. The binding of leukemia cells onto pre-activated ECs exerted a mechanical force to regain the cell surface contact which resulted in the obvious QCM responses opposite to that of activation, and proportional to the number of cells added, in spite of the fact that these added cells are extremely outside the extinction boundary of the shear wave generated by QCM. Different cell lines demonstrate different attachment behavior, which was detected by the QCM. Despite these variations are quite subtle, yet the sensitivity of the technique for dynamic changes at the interface makes them detectable. Moreover, the reproducibility of the generated data determined at each step by deviation measurements (<10%) in response plot was very high despite the high possible heterogeneity in cell populations. The results are explained on the basis of simple theoretical and physical models, although, the development of a more quantitative and precise model is underway in our laboratory.

Targeted Reengineering of Protein Geranylgeranyltransferase Type I Selectivity Functionally Implicates Active-Site Residues in Protein-Substrate Recognition

Posttranslational modifications are vital for the function of many proteins. Prenylation is one such modification, wherein protein geranylgeranyltransferase type I (GGTase-I) or protein farnesyltransferase (FTase) modify proteins by attaching a 20- or 15-carbon isoprenoid group, respectively, to a cysteine residue near the C-terminus of a target protein. These enzymes require a C-terminal Ca1a2X sequence on their substrates, with the a1, a2, and X residues serving as substrate-recognition elements for FTase and/or GGTase-I. While crystallographic structures of rat GGTase-I show a tightly packed and hydrophobic a2 residue binding pocket, consistent with a preference for moderately sized a2 residues in GGTase-I substrates, the functional impact of enzyme-substrate contacts within this active site remains to be determined. Using site-directed mutagenesis and peptide substrate structure-activity studies, we have identified specific active-site residues within rat GGTase-I involved in substrate recognition and developed novel GGTase-I variants with expanded/altered substrate selectivity. The ability to drastically alter GGTase-I selectivity mirrors similar behavior observed in FTase but employs mutation of a distinct set of structurally homologous active-site residues. Our work demonstrates that tunable selectivity may be a general phenomenon among multispecific enzymes involved in posttranslational modification and raises the possibility of variable substrate selectivity among GGTase-I orthologues from different organisms. Furthermore, the GGTase-I variants developed herein can serve as tools for studying GGTase-I substrate selectivity and the effects of prenylation pathway modifications on specific proteins.

Functionalization of Natural Cork Composite with Microcapsules after Plasma Treatment

This research aims to study the chemical and physical modifications of natural cork agglomerate after plasma treatment using dielectric barrier discharge (DBD). Different experimental techniques were used to evaluate the surface alterations of the pretreated samples with DBD plasma, as well as the adsorption and adhesion of microcapsules in the substrate, namely, static and dynamic contact angle, surface energy, energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). Plasma discharge greatly increases the wettability and surface energy of the samples. Chemical and physical analyses of the cork agglomerate confirmed considerable surface modification. All these surface changes of the cork after plasma treatment led to a remarkable increase in microcapsule adsorption and adhesion when compared with the untreated cork sample.

Anisotropy in wetting of oriented sapphire surfaces by liquid Al–Cu alloys

The coexistence of a liquid with a solid and a gas phase causes a contact angle at the triple line and results in a certain work of adhesion. These properties were studied for liquid Al, Cu, and their alloys on single-crystalline sapphire surfaces with C(0001)-, A(11-20)-, and R(1-102)- orientation. Measurements were performed at 1100 °C and under \(3 \cdot 10^4\, \hbox{Pa}\) Ar atmosphere in a sessile drop apparatus. There, the sample was heated and melted separately from the substrate within a drop dispenser. Only after the desired measurement conditions were reached, the liquid metal was released. Depending on the alloy composition, the wetting angle approached a constant value within few minutes after the contact of droplet and substrate was established: For pure Cu the contact angle increased to an equilibrium value of 116° ± 5°, which is identical for all the studied sapphire surfaces. For pure Al an anisotropy of the contact angle with regard to these surfaces is found: time evolution of the Al contact angle is only observed for wetting of C-surfaces. Wetting of A- and R-surfaces shows no pronounced time dependence. In these cases, a smaller contact angle of about 90° is observed. Wetting of the different sapphire surfaces by Al–Cu alloys corresponds qualitatively to their wetting by pure Al: again, only for C-surfaces a time-dependent increase of the contact angle is observed. On A- and R-surfaces wetting is not time-dependent and the contact angle increases with Cu content of the alloy.

Analysis of Plastic Strain between Substrate and Micro-Cantilever under Different Deformation Characteristics

The contact between substrate and micro-cantilever simplified as an ideal flat substrate contact with a micro-cantilever rough surface. A three-dimensional adhesive contact model was established on isotropic rough surfaces exhibiting fractal behavior, and the equivalent plastic strain was discussed using the finite element analysis. The maximum equivalent plastic strain and its depth were presented with the different paths of rough solid when loading. The result show that the equivalent plastic strain versus different depth which at different locations showed different laws, in the top area of the asperities versus different depth, the maximum equivalent plastic strain occurs in the subsurface range about 0.5μm from the surface or on the surface. In addition, with different deformation characteristics, the degree of the equivalent plastic strain was different.. The contact model between micro-cantilever rough surface and flat substrate will lay a foundation to further research on the substance of the process of friction and wear.

Compact model to efficiently characterize TSV-to-transistor noise coupling in 3D ICs

A methodology is proposed to characterize through silicon via (TSV) induced noise coupling in three-dimensional (3D) integrated circuits. Different substrate biasing schemes (such as a single substrate contact versus regularly placed substrate contacts) and TSV fabrication methods (such as via-first and via-last) are considered. A compact π model is proposed to efficiently estimate the coupling noise at a victim transistor. Each admittance within the compact model is approximated with a closed-form expression consisting of logarithmic functions. The methodology is validated using the 3D transmission line matrix (TLM) method, demonstrating, on average, 4.8% error. The compact model and the closed-form expressions are utilized to better understand TSV induced noise as a function of multiple parameters such as TSV type, placement of substrate contacts, signal slew rate and voltage swing. The effect of differential TSV signaling is also investigated. Design guidelines are developed based on these results.

Distinction in binding of peptides (P2E) and its mutations (P2G, P2Q) to a graphene sheet via a hierarchical coarse-grained Monte Carlo simulation

A hierarchical coarse-grained approach is used to study the binding of peptides (P2E: (1)E(2)P(3)L(4)Q(5)L(6)K(7)M) and variants (P2G: (1)G(2)P(3)L(4)Q(5)L(6)K(7)M and P2Q: (1)Q(2)L(3)P(4)M(5)E(6)K(7)L) with a graphene sheet. Simulation-based residue-substrate and hydropathy index-based residue-residue interaction is used as input to a phenomenological interaction potential for peptide chains to execute the stochastic motion with a graphene sheet at the center of a box. Large-scale Monte Carlo simulations are performed at a range (low to high) of temperatures to identify peptides binding with the graphene sheet with a constant peptide concentration (Cp = 0.01). A number of local (energy, mobility, and substrate contact profiles) and global (density profiles, mean square displacement of the center of mass of a peptide and its radius of gyration) physical quantities are examined to monitor the patterns. We find that each peptide can bind to a graphene sheet at low temperatures but the residues that can anchor their binding vary among these three peptides. For example, P2E is anchored by (1)E, (4)Q, and (6)K, P2Q by (1)Q, (5)E, and (6)K, and P2G by nearly all its residues with about the same strength except (1)G and (2)P. The site-specific binding is reflected in the thermal response of the radius of gyration of the peptides. Despite the lack of a large difference in binding patterns, a systematic variation in radius of gyration and surface binding profile with the temperature reveals the distinction in their binding: the probability of P2E binding is the highest and that of P2G is the lowest.

Nanoscale Interfacial Interactions of Graphene with Polar and Nonpolar Liquids

While mechanical and frictional properties of graphene in air have been extensively studied, graphene's nanomechanical behavior in liquids, vital for its operation in rechargeable batteries, supercapacitors, and sensors, is still largely unexplored. In this paper, we investigate the nanomechanics of normal (adhesive and elastic) and tangential (friction) forces between a stationary, moving, and ultrasonically excited nanoscale atomic force microscope (AFM) tip and exfoliated few layer graphene (FLG) on SiO2 substrate as a function of surrounding media-air, polar (water), and nonpolar (dodecane) liquids. We find that, while the friction coefficient is significantly reduced in liquids, and is always lower for FLG than SiO2, it is higher for graphene in nonpolar dodecane than highly polar water. We also confirm that in ambient environment the water meniscus dominates high adhesion for both hydrophobic FLG and the more hydrophilic SiO2 surface, with the lowest adhesion observed in liquids, in particular for FLG in dodecane, reflecting the low interface energy of this system. By using nanomechanical probing via ultrasonic force microscopy (UFM), we observed a profound reduction of graphene rippling and increase of graphene-substrate contact area in liquid environment. Friction force dependence on ultrasonic modulation amplitude suggests that dodecane at the graphene interface produces a solid-like "cushion" of approximately 2 nm thickness, whereas, in water immersion, the same dependence shows a remarkable similarity with the ambient environment, confirming the presence of a water meniscus in air, and suggesting negligible thickness of a similar water "cushion" on graphene. Dependence of friction on local environment opens new pathways for friction management in microfluidic and micro- and nanoelectromechanical systems.

Colloid mobilization by fluid displacement fronts in channels

Understanding colloid mobilization during transient flow in soil is important for addressing colloid and contaminant transport issues. While theoretical descriptions of colloid detachment exist for saturated systems, corresponding mechanisms of colloid mobilization during drainage and imbibition have not been considered in detail. In this work, theoretical force and torque analyses were performed to examine the interactive effects of adhesion, drag, friction, and surface tension forces on colloid mobilization and to outline conditions corresponding to the mobilization mechanisms such as lifting, sliding, and rolling. Colloid and substrate contact angles were used as variables to determine theoretical criteria for colloid mobilization mechanisms during drainage and imbibition. Experimental mobilization of hydrophilic and hydrophobic microspheres with drainage and imbibition fronts was investigated in hydrophilic and hydrophobic channels using a confocal microscope. Colloid mobilization differed between drainage and imbibition due to different dynamic contact angles and interfacial geometries on the contact line. Experimental results did not fully follow the theoretical criteria in all cases, which was explained with additional factors not included in the theory such as presence of aggregates and trailing films. Theoretical force and torque analyses resulted in similar mobilization predictions and suggested that all mobilization mechanisms contributed to the observed colloid mobilization.

Zirconium adsorption and incorporation on a reconstructed Al-T4 AlN(0001) surface

We discuss the energetic stability and electronic structure of zirconium adsorption and incorporation on a 2×2 reconstructed AlN(0001) surface. We employ density-functional theory within the generalized-gradient approximation to study such effects. First-principles total energy calculations reveal that the most stable positions of a Zr adatom on the reconstructed 2×2-T4 AlN(0001) surface are at the S1 and Sp substitutional sites. In addition, calculating the relative surface energy of several configurations, we constructed a phase diagram showing the energetically most stable surfaces as a function of the aluminium chemical potential. Based on these results, we find that incorporation at Al-substitutional sites is energetically favorable compared to adsorption on the top layers. In particular, we find that Zr-S1 and Zr-Sp structures are stable over the entire chemical potential range. This effect can lead to the formation of a non-reactive interfacial ZrN(111) layer on the AlN(0001) surface, which can offer a good interfacial combination between AlN substrate and other metal contacts, i.e. zirconium.

Resonant reflection of a surface acoustic wave from strip waveguides

A numerical study is performed of the oblique reflection of a surface acoustic wave from a strip of finite width deposited on the surface of a half-infinite substrate. The finite element method is used. If the strip–substrate contact supports waveguide modes with the velocity exceeding the surface wave velocity on the free surface of the substrate, then an interval of angles of incidence exists where the surface wave efficiently excites a waveguide mode. The excitation of the waveguide mode is accompanied by a singular behavior of the reflection and the transmission coefficients. The dependence of the magnitude and the phase of the coefficients on the angle of incidence, the frequency, the width and the thickness of the strip is examined. In particular, it is found that the magnitude of the reflection coefficient abruptly almost vanishes and abruptly increases almost to unity within the resonance interval of angles of incidence.

Interface structural stability of Zr on the AIN(0001) surface: An ab‐initio study

AbstractAb‐initio density functional theory calculations are carried out to investigate the role of zirconium (Zr) impurity atoms during AlN(0001) surface growth. Adsorption and diffusion of Zr atoms on AlN(0001)‐2 × 2 surface is examined and it is shown that Zr atoms preferentially adsorb at the T4 sites at low and high coverage (from 1/4 up to 1 monolayer). We found that the Zr adatom diffusion energy barrier between the T4 and H3 sites is around ∼0.4 eV, which is an indication of a significant Zr adatom diffusion on this surface. In addition, calculating the relative surface energy of several configurations and various Zr concentrations, we constructed a phase diagram showing the energetically most stable surfaces as a function of the Zr and Al chemical potentials. Based on these results, we find that incorporation of Zr adatoms in the Al‐substitutional site is energetically more favorable compared with the adsorption on the top layers. This effect leads to the formation of a non‐reactive interfacial ZrN(111) layer on the AlN(0001) surface, which can offer a good interfacial combination between AlN substrate and other metal contacts, i.e. zirconium.

Nucleation of Elusive Crystal Polymorphs at the Solution–Substrate Contact Line

While metastable crystal forms are kinetically favored at higher supersaturation conditions, more stable polymorphs often crystallize under the control of competing kinetic processes such as solvent-mediated polymorphic transformation and/or preferential molecular self-assembling in solution. In such cases, alternative methodologies are required to facilitate nucleation and growth of a metastable polymorph. In this study, the metastable polymorphs of d-mannitol (δ form) and glycine (β form) — which are usually difficult to isolate by classical solution crystallization methods — were obtained by slow evaporation of aqueous solution microdroplets. The metastable forms preferentially nucleated at the “contact line” where the solution–air interface meets the glass substrate surface. In contrast, the thermodynamically stable polymorph of mannitol (β form) and kinetically stable polymorph of glycine (α form) crystallized within the solution drop. Upon nucleation at the contact line, δ mannitol grew out from the solution drop by entrapping the solution in the region between the dendrite-shaped crystals. Evaporation of the thin film of solution effectively removed the water around the crystals and consequently prevented any solution-mediated transformation to the stable form. On the other hand, β glycine crystals nucleated at the contact line partially transformed to the α form through solvent-mediated transformation. By analyzing the nucleation behavior of mannitol and glycine polymorphs under fast solvent evaporation conditions, we surmise that preferential nucleation of the metastable polymorphs from aqueous solution microdroplets is not controlled solely by the rate of supersaturation generation. Alternatively, development of higher supersaturation at the solution–substrate contact line via a Marangoni-driven convective solute transport in the solution drop could influence crystallization of the metastable polymorph.

Examining the usefulness of a Y-maze choice method to measure the preferences of laying hens

Measurement of animal preferences can be used as an indirect, but persuasive, method for assessing animal welfare on the basis that preferences may identify resources and behaviours that might be important to animals. The present experiment examined the usefulness of a Y-maze methodology, incorporating alteration of motivational state through prior restriction of resources of potentially differing value in assessing and understanding animal preferences. The choice behaviour of laying hens for feed, a dustbath substrate (sawdust; ‘dust’) and social contact was measured under three pairwise comparisons in a Y-maze apparatus. In each of the three experiments, 48 birds (HyLine Brown Strain; n = 24 per experiment) were offered a choice of two resources in a Y-maze test, with one of the three possible resource pairings. In each experiment, after Y-maze training, an equal number of birds was deprived in a factorial design of Resource 1, Resource 2, both, or neither resource. Analysis of choices over 24 trials per bird in each experiment revealed that birds preferred feed over social contact or dust, irrespective of restriction of any of these resources, and further, were quicker (P &lt; 0.01) to make feed choices than dust or social-contact choices. In the social contact and dust comparison, restriction of dust significantly (P &lt; 0.05) increased choice for dust in the 24 trials (38 vs 53% dust choice), suggesting that dust restriction increased the birds’ motivation to access dust. This result potentially highlights the impact of the resource of comparison in pairwise tests on overall choice response to restriction. The inclusion of measurements of speed of movement through the Y maze proved a useful aspect of the methodology, providing results consistent with choice behaviour in all three experiments. Although overall choices for dust and social contact were not significantly (P = 0.328) different from random, birds were quicker (P &lt; 0.05) to make dust choices than social-contact choices, suggesting that speed of choice may be a particularly sensitive correlate of current motivation levels. The consistency of results across these experiments, together with results reported in the literature on laying-hen preferences, suggests that the methodology is a promising option for assessing the relative preference for resources for laying hens. Additional evidence, particularly on the occurrence of abnormal behaviour, stress physiology and health, when restricted of the resource of interest is necessary to provide a more comprehensive assessment of the impact of the restriction on animal welfare.

GaSb Quantum Dots growth by Liquid Phase Epitaxy

采用改进的快速推舟液相外延技术在GaAs衬底上成功地生长了GaSb量子点材料.通过原子力显微镜观测了不同生长参数下GaSb量子点材料的形貌(形状、尺寸、密度、尺寸分布均匀性等).分析了不同衬底、不同生长源配比、生长源与衬底的不同接触时间等生长条件参数对GaSb量子点生长的影响.研究表明在 GaAs衬底上、富镓生长源配比以及较短的生长源和衬底接触时间下更易获得高质量的GaSb量子点.上述生长条件的摸索和研究对于GaSb量子点器件应用具有重要意义.;The results on the growth of GaSb quantum dots (QDs) by liquid phase epitaxy (LPE) were reported. The dot Morphology in term of size, shape, density and uniform was studied by atomic force microscopy (AFM). The effects of growth conditions such as substrate, melt composition, and melt-substrate contact time on the morphology of GaSb QDS were investigated. It was found that it’s easier to get high quality GaSb QDs in condition of GaAs substrate, Ga-rich melt and shorter of contact time.

Interfacial microfluidic transport on micropatterned superhydrophobic textile

Textile-enabled interfacial microfluidics, utilizing fibrous hydrophilic yarns (e.g., cotton) to guide biological reagent flows, has been extended to various biochemical analyses recently. The restricted capillary-driving mechanism, however, persists as a major challenge for continuous and facilitated biofluidic transport. In this paper, we have first introduced a novel interfacial microfluidic transport principle to drive three-dimensional liquid flows on a micropatterned superhydrophobic textile (MST) platform in a more autonomous and controllable manner. Specifically, the MST system utilizes the surface tension-induced Laplace pressure to facilitate the liquid motion along the hydrophilic yarn, in addition to the capillarity present in the fibrous structure. The fabrication of MST is simply accomplished by stitching hydrophilic cotton yarn into a superhydrophobic fabric substrate (contact angle 140 ± 3°), from which well-controlled wetting patterns are established for interfacial microfluidic operations. The geometric configurations of the stitched micropatterns, e.g., the lengths and diameters of the yarn and bundled arrangement, can all influence the transport process, which is investigated both experimentally and theoretically. Two operation modes, discrete and continuous transport, are also presented in detail. In addition, the gravitational effect as well as the droplet removal process have been also considered and quantitatively analysed during the transport process. As a demonstration, an MST design has been implemented on an artificial skin surface to collect and remove sweat in a highly efficient and facilitated means. The results have illustrated that the novel interfacial transport on the textile platform can be potentially extended to a variety of biofluidic collection and removal applications.

Identification of a His-Asp-Cys Catalytic Triad Essential for Function of the Rho Inactivation Domain (RID) of Vibrio cholerae MARTX Toxin

Vibrio cholerae is the causative agent of the severe diarrheal disease cholera. For V. cholerae to colonize the intestinal epithelium, accessory toxins such as the multifunctional autoprocessing repeats-in-toxin (MARTX(Vc)) toxin are required. MARTX toxins are composite toxins comprised of arrayed effector domains that carry out distinct functions inside the host cell. Among the three effector domains of MARTX(Vc) is the Rho inactivation domain (RID(Vc)) known to cause cell rounding through inactivation of small RhoGTPases. Using alanine scanning mutagenesis in the activity subdomain of RID(Vc), four residues, His-2782, Leu-2851, Asp-2854, and Cys-3022, were identified as impacting RID(Vc) function in depolymerization of the actin cytoskeleton and inactivation of RhoA. Tyr-2807 and Tyr-3015 were identified as important potentially for forming the active structure for substrate contact but are not involved in catalysis or post translational modifications. Finally, V. cholerae strains modified to carry a catalytically inactive RID(Vc) show that the rate and efficiency of MARTX(Vc) actin cross-linking activity does not depend on a functional RID(Vc), demonstrating that these domains function independently in actin depolymerization. Overall, our results indicate a His-Asp-Cys catalytic triad is essential for function of the RID effector domain family shared by MARTX toxins produced by many Gram-negative bacteria.

Optimization of the Sensitivity of FET-Based Biosensors via Biasing and Surface Charge Engineering

It has been shown that the sensitivity of FET-based biosensors is highest when the biosensor is biased in the subthreshold regime. Given a substrate contact and a solution electrode, there is a wide range of voltages that bias the sensor this way, and not all of them are optimal; we use simulations to clarify the proper bias point. We also show that screening in the buffer solution is bias dependent, and we describe the origin of this dependence. Furthermore, we show that phenomena in the FET part and screening at the outer surface of the biosensor may have differing optimization criteria in terms of biasing. To make the most of the biosensor, both have to be optimized simultaneously. We show that this may require engineering of the charges at the surface of the biosensor, possibly affecting the choice of receptor molecules, linker molecules, and insulator material.