Surface Stress Changes Research Articles

Stress and structure at the NiO/Ag(001) interface

We present a combined stress and structural analysis of the growth of epitaxial NiO monolayers on Ag(001). Our experimental results indicate an unexpectedly complex interface formation, where a fraction of the first NiO monolayer (ML) is embedded into the Ag surface. This interface formation induces a tensile surface stress change. Subsequent deposition leads to a layer-by-layer growth of NiO up to 5 ML. Here, the average film stress is compressive $\ensuremath{-}5.8$ GPa, and it corresponds quantitatively to the misfit-induced stress. Our density functional calculations complement the experimental results by identifying the proper O-Ag bonding geometry in the ML regime from two indistinguishable options as provided by the analysis of surface x-ray diffraction data.

Redox-Induced Actuation of Microcantilevers Modified with Ferrocenylalkanethiolate Self-Assembled Monolayers

The electrochemically-activated deflection of free-standing microcantilevers has received considerable interest for chemical sensing and nanoactuation. The development and optimization of this type of microcantilever transduction still requires a molecular-level understanding of the surface stress changes that cause the cantilever to bend. We present here the results of an on-going investigation of the electroactuation dynamics of gold-coated microcantilevers modified with model, redox-active ferrocenylalkanethiolate self-assembled monolayers. The microcantilever transducer enables the observation of the electrochemical transformation of the surface-confined ferrocenes to ferroceniums. We find that the ferrocenylalkanethiolate monolayer induces charge-normalized surface stress changes that are at least 10-fold greater than those generated by multilayers of the conducting polymers commonly used for electroactuation. The effects of the electrolyte anion and ferrocene coverage on the magnitude of the surface stress are presented. A physical model to explain the origin of the observed surface stress changes is described

Aptamer Functionalized Microcantilever Sensors for Cocaine Detection

A cocaine-specific aptamer was used as a receptor molecule in a microcantilever-based surface stress sensor for detection of cocaine molecules. An interferometric technique that relies on measuring differential displacement between two microcantilevers (a sensing/reference pair) was utilized to measure the cocaine/aptamer binding induced surface stress changes. Sensing experiments were performed for different concentrations of cocaine from 25 to 500 μM in order to determine the sensor response as a function of cocaine concentration. In the lower concentration range from 25 to 100 μM, surface stress values increased proportionally to coverage of aptamer/cocaine complexes from 11 to 26 mN/m. However, as the cocaine concentration was increased beyond 100 μM, the surface stress values demonstrated a weaker dependence on the affinity complex surface coverage. On the basis of a sensitivity of 3 mN/m for the surface stress measurement, the lowest detectable threshold for the cocaine concentration is estimated to be 5 μM. Sensing cantilevers could be regenerated and reused because of reversible thermal denaturation of aptamer.

Surface stress changes in the Ir(001)/H system: Density functional theory study

Density functional theory calculations of the surface energy and surface stress changes associated with the formation of the Ir(001)(5 \ifmmode\times\else\texttimes\fi{} 1)-hex clean-surface reconstruction and its transformation to a (5 \ifmmode\times\else\texttimes\fi{} 1)-added row (AR) structure in the presence of adsorbed atomic hydrogen have been performed and compared with experimental results. The calculations clearly show that the clean-surface (1 \ifmmode\times\else\texttimes\fi{} 1)-to-(5 \ifmmode\times\else\texttimes\fi{} 1)-hex reconstruction is not driven by a reduction in tensile surface stress. While the surface energy is reduced by this transformation, the surface stress increases; proper convergence of the calculated surface stress requires the use of slabs containing at least nine atomic layers, due to the substantial subsurface relaxations associated with the (5 \ifmmode\times\else\texttimes\fi{} 1)-hex phase. By contrast, the H-induced reconstruction leads to a calculated reduction in the tensile surface stress in the range 1.76--2.06 Nm${}^{\ensuremath{-}1}$ for an H coverage range of 0.6--0.8 ML, in excellent agreement with the experimentally determined value of 1.7 Nm${}^{\ensuremath{-}1}$.

Evaluation of capacitive surface stress biosensors

The evaluation of a capacitive type biosensor array, consisting of a total of 256 biosensing elements, in the detection of single oligonucleotide mutations is presented. The biosensor takes advantage of surface stress changes during biological interactions and is able to translate them into a capacitive signal. The array is organized in a 16i?16 matrix of distinct biosensing elements thus allowing for the concurrent sensing of multiple biological targets. In this work the sensing elements of the array are spotted with three different oligonucleotides (CD8, CD17 and CD19) and their hybridization is detected using 36nM PCR. Moreover tests with CD19 revealed the ability of the biosensor to detect the hybridization of the oligo with sample concentrations of 36, 18 and 9nM.

Probing Insertion and Solubilization Effects of Lysolipids on Supported Lipid Bilayers Using Microcantilevers

The interaction of surfactants with lipid membranes can result in composition change, area expansion, solubilization, or the formation of protrusion features of the membranes. Amphipathic surfactant molecules are simplified analogues to membrane-active drugs and peptides which are known for inserting into lipid bilayers; however, the effect of these amphipathic molecules on supported membranes is not well characterized. In this paper we explore the use of microcantilever sensors to quantify surfactants' effects on lipid membranes. We use microcantilevers which are coated with lipid membranes to probe the interactions between lysolipids and supported lipid bilayers (SLBs). In particular, we investigate the effects of four zwitterionic surfactants similar to phospholipids: lysolipids of different aliphatic chain lengths (lysophosphocholines, lysoPCs, 12:0, 14:0, 16:0, and 18:0) on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine-supported lipid bilayers. By monitoring the deflection of the microcantilevers, real-time free energy changes in the SLBs upon the addition of lysolipids can be detected. Additionally, the bending direction reveals whether the lysoPCs incorporate into or solubilize the SLB. When the bulk lysoPC concentration is less than its critical micelle concentration (CMC), we observe a compressive bending of the microcantilever, indicating adsorption to the SLB. Additionally, the change in surface stress is found to be proportional to the amount of membrane-bound lysoPCs. For bulk concentrations greater than the CMC, lysoPCs 12:0 and 14:0, there is tensile bending, indicating that the lysoPCs begin to solubilize and destroy the SLBs. Interestingly, this is not observed for lysoPCs with longer chain lengths. This new method of using microcantilevers for detecting and quantifying the surfactant insertion and solubilization of SLBs offers additional insights into the interactions between small amphipathic molecules and lipid membranes.

Growth of bent carbon nanotubes by in-situ control of cantilever bending

A new and simple method for in-situ control of the growth direction of carbon nanotubes (CNTs) on cantilevers has been developed using plasma enhanced chemical vapor deposition (PECVD). Plasma-induced surface stresses in PECVD processes tend to cause bending of the cantilevers, which significantly changes the electric field distribution near the free-end of the cantilever. By adjusting the flow ratio of the feed gases during CNT growth, the degree of cantilever bending can be controlled due to the change in the plasma-induced surface stress, and in doing so manipulating the field line direction, as well as the growth direction of CNTs. Combining this in-situ tunable CNT growth technique with electron beam induced deposition of catalyst patterns, we have fabricated a bent CNT on a cantilever in one single, continuous deposition run.

Simulating fault slip areas of mining induced seismic tremors using static boundary element numerical modelling

Two mining induced tremors that occurred in a South African deep‐level gold mine are considered, with the aim to simulate the shear slip area and source mechanism using static boundary element numerical modelling. Large seismic tremors, induced by the extraction of the tabular ore body typically 1 m high over horizontal spans of kilometres, are a major seismic hazard in South African gold mines. For the cases considered here, virgin stress levels are ∼54 MPa, and seismic related damage is mainly due to geological faults failing in shear under gravitational loading. The tremors occurred ∼2 km below surface, 700 m apart in space, and in consecutive months. Seismic recordings and underground observations suggested that the tremors, of local magnitudes 3·0 and 4·0 respectively, were associated with normal slip on similar geological faults. A boundary element numerical model was implemented to some degree of accuracy, since the fault geometries could be inferred from extensive mining spans and fault intersections, and an in situ stress state could be estimated from other available information. Furthermore, the rock mass stress changes and associated surface stress changes on faults could be modelled from monthly measured advance of the mining excavations. Simulation of the seismic sources entailed finding appropriate Mohr–Coulomb strength parameters to allow yielding of the faults at the actual mining stage, and quantitatively in agreement with the seismic moment as inferred from seismic waveform recordings. A critical input into the model was the use of non‐zero cohesion in addition to friction angle as specified in the Mohr–Coulomb shear failure criterion, allowing the simulation of coseismic slip. Hence, the breaking of fault cohesion and subsequent stress transfer to the surrounding fault area is simulated, albeit statically, conforming to the perceived triggering of mining related seismic tremors by induced stress changes. The back analysis furthermore suggested that coseismic stope closure can comprise a significant part of the total recorded seismic moment, depending on the proximity of the hypocentre to mining excavations. From the back analysis a modelling methodology and input parameters are proposed, aiming at identifying vulnerable mining excavations subject to seismic hazard posed by geological structures, hence providing relevant knowledge towards safer mining.

Microcantilever Biosensors: Probing Biomolecular Interactions at the Nanoscale

Microcantilever based biosensors have attracted much attention as a label-free, real-time and highly sensitive approach to the detection of biomolecules. When specific biomolecular interactions occur between a receptor immobilized on one side of a cantilever and a target analyte in solution, a mechanical bending of the cantilever results due to a change in surface stress, thereby translating biochemical interactions into a concentration-dependent nanomechanical response of the microcantilevers. In this review, we present a general overview of the operation principles, fabrication and preparation of microcantilever biosensors and discuss their uses for a wide range of biosensing applications. The focus of the review is given to probing the biomolecular interactions at the solid-liquid interface by measuring the cantilever bending. In addition to the label-free detection of DNA, protein and cells, the emphasis is also made to discuss about the new development of microcantilever biosensors for label-free probing nanoscale conformational changes of DNA, protein and polymers, and real-time detection of nanomechanical forces from living cells. Finally, the outlook for future development work and challenges is also discussed. Keywords: Microcantilever biosensors, molecular interactions, conformational changes, microarrays, ELISAs, atomic force microscopy (AFM), label-free biosensor, microfabrication, Device Fabrication, AFM, Surface Functionalization, BIOSENSING APPLICATIONS, DNA hybridization reaction, single nucleo-tide polymorphisms (SNPs),, prostate-specific antigen (PSA), human serum albumin, human plasminogen, CDK2, drug vancomycin, mucopetide analogues, piezoelectric cantilever

Microcantilevers Modified with Ferrocene-Terminated Self-Assembled Monolayers: Effect of Molecular Structure and Electrolyte Anion on the Redox-Induced Surface Stress

The redox-activated deflection of microcantilevers has attracted interest for chemical sensing and nanoactuation. However, the development and optimization of this type of microcantilever transduction requires a better understanding of the effect of the particular system parameters on the surface stress changes that cause the measured bending response. We investigate here the effects of the adsorbate structure and electrolyte anion on the surface stress generated by the electrochemical oxidation of ferrocene-terminated self-assembled monolayers (SAMs) chemisorbed to the surface of gold-coated microcantilevers. Ferrocenylalkanethiolate-modified cantilevers are an interesting system for study as the electroactive monolayer can induce charge-normalized surface stress changes that are at least 10-fold greater than those generated by multilayers of the conducting polymers commonly used for electroactuation. The resonance angle shifts measured by surface plasmon resonance spectroscopy in the presence of ClO4− suggest that the extent of the oxidation-induced SAM reorganization is the same for short (n = 6) and long (n = 12) chain ferrocenylalkanethiolate (FcCnSAu) SAMs and for a long chain carbonyl derivative (Fc(CO)C11SAu). The magnitude of the measured cantilever deflection is however not the same for the different SAMs, reflecting differences in the tensile contributions to the overall surface stress of the SAM elasticity and wettability versus the compressive lateral pressure generated by the collective molecular reorientations induced by the pairing of anions to the surface-confined, oxidized ferrocenium cations. The hydrophobic anions PF6− and ClO4−, which form 1:1 contact ion pairs with the SAM-bound ferrocenium cations, give reversible cantilever deflections and the largest compressive surface stress changes. By contrast, oxidation of the FcC11SAu SAM in NO3− and F−, which exhibit the poorest ion-pairing abilities of the anions investigated, results in an irreversible deformation of the cantilever bending and smaller stress changes. The surface phenomena that give rise to the observed differences in the surface stress as a function of the ion pairing ability are discussed.

Reactions of Plasma-Polymerised Pentafluorophenyl Methacrylate with Simple Amines

Ultra thin films of pp-PFM offer highly reactive ester groups throughout the structure of the film that allow for subsequent reactions with amino-group containing reagents. The reactivity of this surface towards simple primary amines was studied in DI-H2O and in PBS using IRRAS and XPS. Plasma-polymerised PFM films are typically characterised by a high roughness. Immersion in aqueous media leads to a smoothening of the surface as was observed using SEM and scanning force microscopy. Chemical reactivity towards primary (1°), secondary (2°) and tertiary (3°) amines was investigated using IRRAS in combination with MC sensors. While IRRAS provided information on the chemistry occurring at the surface, the MC sensors technique gave insights into the surface stress induced by the changes in the chemistry of the polymer film upon reaction. These changes in surface stress were related to reactions occurring either at the liquid–solid interface or within the matrix of the polymer. The reactivity of the pp-PFM towards amines seems to be influenced not only by the chemical structure of the amines but also by their size, which influences the diffusion into the polymer film.

Research of Micromechanism on Residual Stress in Aluminum Alloy Thick Plate

The research of micromechanism of stress evolvement is presented by analyzing fluctuation and change of surface stress in aluminum alloy thick plate with X-ray diffraction(XRD). From the micro view, a model of “rotation looping and shearing grain” was proposed to analyze and discuss the inhomogeneous distribution of micro-stress in location area, due to inhomogeneity of organization in material. The results present that the inhomogeneity of plastic deformation is direct cause lead to residual stress in plate, and complication of micro-stress distribution results from non-uniformity of organization.

Sequential population of adsorption sites driven by surface stress

We performed a combined experimental and theoretical study of surface stress and structure of the adsorption of oxygen on Ir(001). We find that up to an oxygen coverage of half a monolayer oxygen adsorbs in bridge sites, whereas at larger coverage between 0.5 and 0.75 adsorption occurs in both bridge and hollow sites. A compressive surface stress change of $\ensuremath{-}2.2\text{ }\text{N}/\text{m}$ is measured, and substantial lattice relaxations are identified by surface x-ray diffraction. Density-functional theory calculations reveal that anisotropic O-induced surface stress is a key factor for the sequential occupation of the adsorption sites.

H- and O-induced compressive surface stress on Pt(111): Experiments and density functional theory calculations

Surface-stress changes induced by the adsorption of oxygen and hydrogen on clean Pt(111) are measured at 320 K under UHV conditions with an optical crystal curvature method and are calculated using density-functional theory. A compressive surface-stress change is measured upon exposure of Pt(111) to either ${\text{H}}_{2}$ or ${\text{O}}_{2}$. The magnitude of the adsorbate-induced surface-stress change is in quantitative agreement with our calculations. Our results indicate that both electropositive (H) and electronegative (O) adsorbates may induce a compressive surface-stress change. The significance of these results for the understanding of adsorbate-induced surface stress is discussed.

Temperature dependence of surface stress across an order-disorder transition:p(1×2)O/W(110)

Strain relaxations of a $p(1\ifmmode\times\else\texttimes\fi{}2)$ ordered oxygen layer on W(110) are measured as a function of temperature across the disordering transition using low-energy electron diffraction. The measured strains approach values of 0.027 in the $[1\overline{1}0]$ and $\ensuremath{-}0.053$ in the [001] direction. On the basis of the measured strain relaxations, we give quantitative information on temperature-dependent surface stress using the results of ab initio calculations. From the surface formation energy for different strains, determined by first-principles calculations, we estimate that surface stress changes from $\ensuremath{-}1.1$ for the ordered phase to $\ensuremath{-}0.2\text{ }\text{N}/\text{m}$ for the disordered one along $[1\overline{1}0]$, and from 5.1 to 3.4 N/m along [001]. Moreover, our observation that the strains scale inversely with domain size confirms that the strain relaxation takes place at the domain boundaries.

Approaches to increasing surface stress for improving signal-to-noise ratio of microcantilever sensors.

Microcantilever sensor technology has been steadily growing for the last 15 years. While we have gained a great amount of knowledge in microcantilever bending due to surface stress changes, which is a unique property of microcantilever sensors, we are still in the early stages of understanding the fundamental surface chemistries of surface-stress-based microcantilever sensors. In general, increasing surface stress, which is caused by interactions on the microcantilever surfaces, would improve the S/N ratio and subsequently the sensitivity and reliability of microcantilever sensors. In this review, we will summarize (A) the conditions under which a large surface stress can readily be attained and (B) the strategies to increase surface stress in case a large surface stress cannot readily be reached. We will also discuss our perspectives on microcantilever sensors based on surface stress changes.

Influence of random roughness on cantilever curvature sensitivity

In this work we explore the influence of random surface roughness on the cantilever sensitivity to respond to curvature changes induced by changes in surface stress. The roughness is characterized by the out-of-plane roughness amplitude w, the lateral correlation length ξ, and the roughness or Hurst exponent H(0<H<1). The cantilever sensitivity is found to decrease with increasing roughness (decreasing H and/or increasing ratio w/ξ) or equivalently increasing local surface slope. Finally, analytic expressions of the cantilever sensitivity as a function of the parameters w, ξ, and H are derived in order to allow direct implementation in sensing systems.

Cantilever-based sensing: the origin of surface stress and optimization strategies

Many interactions drive the adsorption of molecules on surfaces, all of which can result in ameasurable change in surface stress. This article compares the contributions of variouspossible interactions to the overall induced surface stress for cantilever-based sensingapplications. The surface stress resulting from adsorption-induced changes in the electronicdensity of the underlying surface is up to 2–4 orders of magnitude larger than thatresulting from intermolecular electrostatic or Lennard-Jones interactions. We reveal thatthe surface stress associated with the formation of high quality alkanethiol self-assembledmonolayers on gold surfaces is independent of the molecular chain length, supporting ourtheoretical findings. This provides a foundation for the development of new strategiesfor increasing the sensitivity of cantilever-based sensors for various applications.

A piezoresistive bridge-microcantilever biosensor by CMOS process for surface stress measurement

Microsensors based on piezoresistive microcantilevers have been proposed to measure the surface stress change by molecule adsorption from biochemical reaction on an immobilized surface. However, the sensor performance is adversely influenced by the biaxial effect of the surface stress and the thermal effect of the piezoresistor during operation. A bridge-microcantilever design based on standard CMOS fabrication process is developed to improve the measurement sensitivity. The design is an immobilized cantilever connected to the sensing cantilever by a transmitter such that biaxial surface stress can be converted into uniaxial strain similar to AFM operation and the thermal effect can be isolated. The design based on standard CMOS process also facilitates on-chip signal processing electronics and stable batch fabrication. Analyses show that the sensitivity can be increased by the design parameters of the length ratio of the two cantilevers and the transmitter rigidity.

Flexible and tactile sensor based on a photosensitive polymer

This paper presents a novel design of tactile sensing arrays with integrated strain gauges for the measurement of contact force. Surface stress or strain changes on the sensor area due to applied force are measured by the encapsulated Au gauges. The fabricated tactile sensors are highly flexible and durable so that they can conform to more complex surfaces without damaging the skin structure and the metal interconnects on the sensing array. The experimental results show the output characteristics are linear with contact force from 0 to 700 gf and a sensitivity of 3%/100 gf within the full scale range of 700 gf. The effect of electrode structure and position on the enhancement of sensitivity are also numerically simulated by a finite element method and verified experimentally. The measured tactile sensors are robust enough for direct contact with human and contaminants without undue care.