Adsorbate-induced Surface Stress Research Articles

(Invited) Reaction-Based Microcantilever Sensors

Microcantilevers (MCLs) have emerged as a cost-effective, label-free, and portable analytical technique for detecting chemical and biological species. The method offers notable advantages, particularly its high sensitivity, enabling the precise detection of cantilever motion at sub-nanometer levels. Furthermore, MCLs can be effectively fabricated into a multi-element sensor array, amplifying their capabilities. While many sensors rely on adsorption-induced frequency or surface stress changes of MCLs, there is a notable gap in the literature. Despite numerous review articles on MCLs, none have specifically focused on summarizing these sensors with an emphasis on reactions. Beyond their role in detecting chemical species, MCLs present a distinctive application in characterizing the morphology and mechanical properties of materials at solid-liquid or solid-gas interfaces during reaction processes. I will discuss the reaction based MCL sensors and also their potential applications in monitoring reactions.

(Invited) In-Situ Stress Measurement during Electrochemical Processing

The electrochemical community often uses in situ wafer curvature and cantilever bending techniques to examine stress development during electrochemical processing. These techniques have been used to examine and quantify surface stress induced by surface charge (electrocapillarity), adsorption processes, and growth stress associated with underpotential deposition (UPD) and electrodeposition. The sensitivity of the stress measurement to surface processes makes this technique particularly relevant for electrochemical studies. For example, we have used in situ surface stress measurements to examine the underpotential deposition of a variety of metals (M) onto (111)-textured Au cantilever electrodes. The stress response clearly shows regions of anion desorption, M–Au bond formation, and in some cases, stress relaxation that can be attributed to surface alloying. This talk will focus on the surface and growth stress that develops during a variety of electrochemical processes, including metal deposition, the formation of elastically strained Pt films for electrocatalysis, and adsorbate-induced surface stress, such as carbon monoxide (CO) on Pt.

Compressive Stress and Charge Redistribution during CO Adsorption Onto Pt

Electroorganic reactions, where energy is either stored or released from chemical bonds within carbon-based molecules, are essential to help achieve a circular carbon-based economy. Central to electrochemical conversion of organic molecules is carbon monoxide (CO), which serves as both an intermediate in electrochemical CO2 reduction to hydrocarbons and electrochemical oxidation in fuel cells that operate via oxygenated hydrocarbons (e.g., ethanol, methanol). For the latter, Pt has been the predominate catalyst of choice, and substantial effort has been spent to identify the mechanistic details for complete oxidation of alcohol-based fuels. With CO conversion serving as an important if not rate-limiting step of the oxidative process, it is critical to understand the state of a catalytic surface when CO is present.The change in surface stress associated with the adsorption and oxidative stripping of CO on (111)-textured Pt is examined using the wafer curvature method in 0.1 mol/L KHCO3 electrolyte.1 The curvature of the Pt cantilever electrode was monitored as a function of potential in both CO free and CO-saturated electrolyte. Although CO adsorbs as a neutral molecule, significant compressive stress, up to -1.3 N/m, is induced in the Pt. The magnitude of the stress change correlates directly with the CO coverage, and within the detection limits of the stress measurement, is elastically reversible. Density functional theory calculations of a CO bound Pt surface indicate that charge redistribution from the first atomic layer of Pt to subsurface layers account for the observed compressive stress induced by the charge neutral adsorption of CO. A better understanding of adsorbate-induced surface stress is critical for the development of materials platforms for sensing and catalysis. References D. Raciti, K.A. Schwarz, J. Vinson, and G.R. Stafford, The Journal of Physical Chemistry C , 126, 4446-4457 (2022).

Modelling anisotropic adsorption-induced coal swelling and stress-dependent anisotropic permeability

To investigate the anisotropy of coal swelling, this study proposes an effective stress model for saturated, adsorptive fractured porous media by considering gas adsorption induced surface stress change on solid-fluid interface. The effective stress model can be used to capture the anisotropic swelling of coal combining anisotropic mechanical properties and to link with the anisotropic permeability. Direction dependent fracture compressibility is used to describe the evolution of anisotropic stress-dependent permeability behaviour. Particularly, the impact of gas adsorption on fracture compressibility is considered in the model. The proposed models were tested against experimental results and compared to relevant existing models available in literatures. The model predicts that the coal swelling in the direction perpendicular to the bedding plane, is greater than that in the parallel plane. Coal permeability in each direction can be affected by the stress changes in any directions. The permeability parallel to the bedding plane is more sensitive to change in stresses than in perpendicular to the bedding due to higher fracture compressibility. The cleat compressibility could increase with gas adsorption, especially for carbon dioxide. Permeability loss in the direction parallel to the bedding plane is more significant than that in the direction perpendicular to the bedding plane. The presented models provide a tool for quantifying gas adsorption-induced anisotropic coal swelling and permeability behaviours.

Compressive Stress and Charge Redistribution during CO Adsorption onto Pt.

The change in surface stress associated with the adsorption and oxidative stripping of carbon monoxide (CO) on (111)-textured Pt is examined using the wafer curvature method in 0.1 mol/L KHCO3 electrolyte. The curvature of the Pt cantilever electrode was monitored as a function of potential in both CO-free and CO-saturated electrolytes. Although CO adsorbs as a neutral molecule, significant compressive stress, up to -1.3 N/m, is induced in the Pt. The magnitude of the stress change correlates directly with the CO coverage and, within the detection limits of the stress measurement, is elastically reversible. Density functional theory calculations of a CO-bound Pt surface indicate that charge redistribution from the first atomic layer of Pt to subsurface layers accounts for the observed compressive stress induced by the charge neutral adsorption of CO. A better understanding of adsorbate-induced surface stress is critical for the development of material platforms for sensing and catalysis.

Modulation rate on adsorption and catalysis of 2D Pt: the effects of adsorbate-induced surface stress

Competition between intrinsic surface stress and adsorbate-induced surface stress.

Elastic properties of two-dimensional Pt with adsorbed oxygen

Previous studies on elastic properties of two-dimensional (2D) metals have focused on thin slabs with a clean surface. However, metal surfaces are full of binding sites for adsorption, which makes them interesting materials in catalysis and sensing. In this contribution, we study Young's moduli of ultrathin oxygen-adsorbed 2D Pt slabs with a (111) surface by density functional theory methods. We show that upon oxygen adsorption, 2D Pt slabs experience significant lattice expansion due to adsorbate-induced additional surface stress. Young's moduli of the studied 2D slabs typically decrease when surface oxygen coverage increases. Moreover, thinner slabs in general exhibit higher stiffness. We show that the nonlinear elastic properties of both the core and surface regions of the considered 2D structures underlie the overall effective elastic properties.

Using Dynamic Stress Analysis to Quantify Adsorbate-Induced Surface Stress

Wafer curvature and cantilever bending techniques have been used by the electrochemical community to examine stress development during electrochemical processing. Surface stress changes as low as 10−3 N/m can typically be resolved from cantilever electrodes immersed in solution and under potential control. Such resolution makes this measurement useful for examining virtually all aspects of electrochemistry; i.e., electrocapillarity, adsorption processes, underpotential deposition, electrodeposition, etc. Often these processes occur either simultaneously or in rapid succession and we are often limited to measuring the influence of the dominant process in the time-scale of the experiment. In the case of electrocapillarity (charge-induced stress), the figure of merit is the stress-charge coefficient (ς) which captures the fundamental surface mechanics associated with charging the electrode surface. It has been well documented in the literature that ς is influenced by the anion in solution and its concentration.1,2 However, a steady state measurement cannot separate the contributions of anion adsorption from that of simple capacitive charging. Similar to electrochemical impedance spectroscopy (EIS) where electrochemical processes with different characteristic time constants can be separated, dynamic stress analysis (DSA) allows us to study the dynamics of any particular stress-generating process and link the stress to specific electrochemical and surface phenomena. We have demonstrated the technique by examining the electrocapillarity of both Pt and Au in HClO4 electrolyte.3,4 ς can be obtained from the following equation, ς = jωYsZe where Ze is the electrochemical impedance, Ys is the stress admittance (with units of N/(V-m), ω is the angular frequency, and j =√-1. In order to include adsorbate-induced contributions to the surface stress, one considers an equivalent circuit that adequately describes both the double layer and adsorption contributions to the electrochemical impedance. One can then obtain unique stress-charge coefficients that capture both electrocapillarity and anion adsorption as a function of potential. This will be demonstrated using (111)-textured Au cantilever electrodes in both sulfate and perchlorate electrolyte. References W. Haiss, R.J. Nichols, J.K. Sass, and K.P. Charle, J. Electroanal. Chem., 452, 199, (1998).R.N. Viswanath, D. Kramer, and J. Weissmüller, Langmuir, 21, 4604 (2005).M. C. Lafouresse, U. Bertocci, C. R. Beauchamp, and G. R. Stafford, J. Electrochem. Soc. 159, H816 (2012).M. C. Lafouresse, U. Bertocci, and G. R. Stafford, J. Electrochem. Soc. 160, H636 (2013).

In situ surface stress measurement and computational analysis examining the oxygen reduction reaction on Pt and Pd

Dynamic electrochemical surface stress response during the oxygen reduction reaction (ORR) on Pt and Pd cantilever electrodes in HClO4 and KOH was examined to elucidate surface binding configurations during O2 reduction electrocatalysis. Upon reduction of O2, the surface of Pt exhibits a compressive surface stress response, ΔStress, in both acid and base electrolytes due to adsorption of the ORR reactant and intermediates (O2, O, and OH). The magnitude of compressive ΔStress on Pt is greater in acid relative to base. On the other hand, the surface of Pd exhibits a negligible ΔStress in acid and a slight compressive ΔStress in base. Thus, magnitudes of the compressive ΔStress (surface expansion) during the ORR follow the order of Pt (acid) > Pt (base) > Pd (base) > Pd (acid) ∼ 0. Density functional theory (DFT) calculations of adsorbate-induced excess surface stress on Pt(111) and Pd(111) surfaces imply a greater compressive surface stress induced on Pt(111) for nearly all adsorbate geometries examined. This trend, which agrees with the experimental observations, can be correlated to a greater tensile intrinsic surface stress of Pt(111) relative to Pd(111) resulting from difference in bond strength and bulk modulus of two metals. On stepped Pt(221) and Pd(221) surfaces, both the intrinsic tensile stress of the clean surface and the adsorbate-induced excess compressive stress are significantly reduced due to the presence of less coordinated, flexible step sites. Moreover, this difference between surface stress at terrace and step sites is more pronounced on Pt, which exhibits a greater intrinsic surface stress.

Adsorption-induced surface stress on a c(4 × 2) adatom surface by an alkanethiol

We investigate the surface stresses induced by an alkanethiol adsorbing onto a Au(1 1 1) surface in a supercell. The surface stresses of the adatom and clean surfaces are determined using density functional theory calculations with the opt-B86b vdW and PBE functionals. We find that the Au(1 1 1) surface with adatoms leads to a reduction in the intrinsic tensile surface stresses, which is in agreement with analytical results from elastic field theory. When methanethiols adsorb onto the clean Au(1 1 1) surface, charge directly redistributes from the surface gold atoms to the methanethiols. On the other hand, when methanethiols adsorb onto the adatom Au(1 1 1) surface, the adatoms shield the effect of charge redistribution from the surface gold atoms to the methanethiols. This shielding mechanism weakens Au–S bonding, and results in lower adsorption-induced compressive surface stresses than for the clean surface.

Dynamic Stress Analysis at Solid Electrodes

Wafer curvature and cantilever bending techniques have been used by the electrochemical community to examine stress development during electrochemical processing. Surface stress changes as low as 10−3 N/m can typically be resolved from cantilever electrodes immersed in solution and under potential control. Such resolution makes this measurement useful for examining virtually all aspects of electrochemistry; i.e., electrocapillarity, adsorption processes, underpotential deposition, electrodeposition, battery reactions, and metal hydride formation. Often these processes occur either simultaneously or in rapid succession and we are often limited to measuring the influence of the dominant process in the time-scale of the experiment. Electrochemical impedance spectroscopy (EIS) is a technique in which one measures the impedance of an electrochemical system over a range of frequencies, revealing information about the reaction mechanisms: different reaction steps will dominate at frequencies dictated by their relaxation times. Whereas traditional EIS determines the transfer function of the current in response to potential modulation, Dynamic Stress Analysis (DSA) examines the transfer function of the stress (as measured from cantilever curvature) to the same potential modulation. This will provide us with the time constants for the stress response that might enable us to directly link the stress to specific electrochemical phenomena. We first use DSA to examine the electrocapillarity (surface stress vs. charge density) behavior of (111)-textured Au and Pt cantilever electrodes in 0.1 M HClO4 electrolyte. Our measurements confirm that a direct relationship between the surface stress and the charge density exists over a wide range of frequencies and potentials. We define a general stress-charge coefficient ς = jωYsZ e that can be obtained from experimental values of the electrochemical impedance ( Ze ) and the stress admittance ( Ys ), where j and ω have their usual meaning. For both Au and Pt in the double layer region, the surface stress was found to be 180° out of phase with respect to the charge density, yielding ς  of -2.0 V and -0.7 V, respectively. These values are in agreement with first-principles calculations that appear in the literature.1, 2 In the hydrogen adsorption region, stress and charge are in phase ( ς is positive; i.e., negative charge induces compressive stress). This compressive stress response for H adsorption is contrary to charge distribution models for adsorbate-induced surface stress but supports first-principles calculations for hydrogen adsorption onto Pt (111).3, 4 We also apply DSA to electrodeposition, making use of separate stress-charge coefficients for electrocapillarity and metal deposition. Since these stress-generating mechanisms have dramatically different frequency dependency, Cu deposition is a nice demonstration that highlights the attributes of DSA; i.e., using frequency to separate the various stress contributions. 1. J.-M. Albina, C. Elsasser, J. Weissmüller, P. Gumbsch and Y. Umeno, Phys. Rev. B 85, 125118 (2012). 2. Y. Umeno, C. Elsässer, B. Meyer, P. Gumbsch, M. Nothacker, J. Weissmüller and F. Evers, EPL 78, 13001 (2007). 3. P. J. Feibelman, Phys. Rev. B 56, 2175 (1997). 4. H. Ibach, J. Vac. Sci. Technol. A12, 2240 (1994).

Adsorption-Induced Surface Stresses of the Water/Quartz Interface: Ab Initio Molecular Dynamics Study.

Adsorption-induced deformation is expansion or contraction of a solid due to adsorption on its surface. This phenomenon is important for a wide range of applications, from chemomechanical sensors to methane recovery from geological formations. The strain of the solid is driven by the change of the surface stress due to adsorption. Using ab initio molecular dynamics, we calculate the surface stresses for the dry α-quartz surfaces, and investigate how these stresses change when the surfaces are exposed to water. We find that the nonhydroxylated surface shows small and approximately isotropic changes in stress, while the hydroxylated surface, which interacts more strongly with the polar water molecules, shows larger and qualitatively anisotropic (opposite sign in xx and yy) surface stress changes. All of these changes are several times larger than the surface tension of water itself. The anisotropy and possibility of positive surface stress change can explain experimentally observed surface area contraction due to adsorption.

Electrochemical Surface Stress Development during CO and NO Oxidation on Pt

The adsorbate-induced surface stress during the electrochemical oxidation of CO and NO on Pt is studied with in situ surface stress measurements and density functional theory (DFT) calculations. The changes in the surface stress response, Δstress, demonstrate the interplay between the adsorbed species during the oxidation process, which is determined by the coverage and the nature of the adsorbates. The oxidation of adsorbed CO, COads, shows a nonlinear surface stress response in both acidic and alkaline electrolytes, with the greatest tensile Δstress observed in the beginning of the oxidation where the CO coverage is the highest. Once a significant amount of CO is removed, OH starts to populate the surface and the Δstress becomes compressive. This surface stress development profile—the nonlinear stress development at high COads coverages and the inflection point due to coadsorption of CO and OH—is further interrogated by DFT calculations. While a tensile to compressive switch in Δstress is observed during CO oxidation, the oxidation of another strongly bound diatomic adsorbate, NOads, shows a continuous compressive Δstress. DFT calculations show that this behavior is attributed to the adsorption of the oxidation product, NO3–, which induces a similar magnitude of compressive Δstress compared to that of NOads. Hence, the compressive Δstress from the oxide and hydroxide on the surface governs the surface stress response.

Tunable micro- and nanomechanical resonators.

Advances in micro- and nanofabrication technologies have enabled the development of novel micro- and nanomechanical resonators which have attracted significant attention due to their fascinating physical properties and growing potential applications. In this review, we have presented a brief overview of the resonance behavior and frequency tuning principles by varying either the mass or the stiffness of resonators. The progress in micro- and nanomechanical resonators using the tuning electrode, tuning fork, and suspended channel structures and made of graphene have been reviewed. We have also highlighted some major influencing factors such as large-amplitude effect, surface effect and fluid effect on the performances of resonators. More specifically, we have addressed the effects of axial stress/strain, residual surface stress and adsorption-induced surface stress on the sensing and detection applications and discussed the current challenges. We have significantly focused on the active and passive frequency tuning methods and techniques for micro- and nanomechanical resonator applications. On one hand, we have comprehensively evaluated the advantages and disadvantages of each strategy, including active methods such as electrothermal, electrostatic, piezoelectrical, dielectric, magnetomotive, photothermal, mode-coupling as well as tension-based tuning mechanisms, and passive techniques such as post-fabrication and post-packaging tuning processes. On the other hand, the tuning capability and challenges to integrate reliable and customizable frequency tuning methods have been addressed. We have additionally concluded with a discussion of important future directions for further tunable micro- and nanomechanical resonators.

Adsorbate-induced surface stress, surface strain and surface reconstruction: CH3S on Cu(100) and Cu(111)

Adsorbate-induced surface stress, surface strain and surface reconstruction: CH3S on Cu(100) and Cu(111)

In Situ Stress Measurements Using Cantilever Curvature

The electrodeposition community has been very active in the development of in situ stress measurements during electrochemical processing. The simplest and most widely used methods involve the measurement of the deflection of a flexible cathode, typically in a direction that is perpendicular to the in-plane stress generated in the film. These in situ stress measurements have been used to examine adsorbate-induced surface stress, underpotential deposition, electrochemical intercalation reactions, and bulk film deposition.Intrinsic stress is caused by the manner in which the film grows and can arise from lattice-mismatched epitaxial growth, nuclei coalescence and grain growth during deposition, and other processes that occur during thickening of the continuous film. For example, Cu grows on Au by a Stranski–Krastanov mechanism where island coalescence is the primary source of stress generation. Figure 1 shows the average in-plane stress associated with the bulk deposition of copper from an additive-free sulfate electrolyte. The large tensile stress generated in the early stages of deposition is consistent with nuclei coalescence and subsequent grain boundary formation. As the deposit thickens, the average film stress decreases rather quickly and even becomes compressive in deposits formed at small deposition overpotentials. If deposition is interrupted in this compressive stage (Fig. 1 inset), a rapid relaxation of the stress occurs. When deposition is resumed the stress level is re-established as if the interruption had never occurred. This behavior is consistent with compressive stress generation due to the reversible diffusion of ad-atoms on the surface into grain boundaries.Electrochemical impedance spectroscopy (EIS) is often used to separate electrochemical processes with different characteristic time constants. A sinusoidal voltage (typically of the order of a few mV) is applied to an electrochemical system and the current response is measured. The frequency of the applied voltage is varied so that different processes can be singled out at specific frequencies. Dynamic stress analysis (DSA) is analogous to EIS, however in addition to the current, the cantilever’s curvature is also measured as a function of frequency. The aim of DSA is to study the dynamics of any particular stress-generating process and link the stress to specific electrochemical and surface phenomena. We have demonstrated the technique by examining the electrocapillarity (charge-induced stress) of both Pt and Au in HClO4 electrolyte.1 In this case the figure of merit is the stress charge coefficient (ξ) which captures the fundamental surface mechanics associated with charging the electrode surface. As expected for capacitive charging, there is no frequency dependence (below the resonant frequency of the cantilever). In this paper we apply DSA to the electrodeposition of Cu, our first attempt to add a stress-generating faradaic reaction to the electrocapillarity response. DSA should be sensitive to not only the stress generating mechanisms that contribute to the steady-state stress but to relaxation processes that may occur as well.1. (a) Lafouresse, M. C.; Bertocci, U.; Beauchamp, C. R.; Stafford, G. R., Simultaneous Electrochemical and Mechanical Impedance Spectroscopy Using Cantilever Curvature. J. Electrochem. Soc. 2012, 159, H816; (b) Lafouresse, M. C.; Bertocci, U.; Stafford, G. R., Dynamic Stress Analysis Applied to (111)-Textured Pt in HClO4 Electrolyte. J. Electrochem. Soc. 2013, 160, H636.

First-principles surface stress calculations and multiscale deformation analysis of a self-assembled monolayer adsorbed on a micro-cantilever.

Micro-cantilever sensors are widely used to detect biomolecules, chemical gases, and ionic species. However, the theoretical descriptions and predictive modeling of these devices are not well developed, and lag behind advances in fabrication and applications. In this paper, we present a novel multiscale simulation framework for nanomechanical sensors. This framework, combining density functional theory (DFT) calculations and finite element method (FEM) analysis, is capable of analyzing molecular adsorption-induced deformation and stress fields in the sensors from the molecular scale to the device scale. Adsorption of alkanethiolate self-assembled monolayer (SAM) on the Au(111) surface of the micro-cantilever sensor is studied in detail to demonstrate the applicability of this framework. DFT calculations are employed to investigate the molecular adsorption-induced surface stress upon the gold surface. The 3D shell elements with initial stresses obtained from the DFT calculations serve as SAM domains in the adsorption layer, while FEM is employed to analyze the deformation and stress of the sensor devices. We find that the micro-cantilever tip deflection has a linear relationship with the coverage of the SAM domains. With full coverage, the tip deflection decreases as the molecular chain length increases. The multiscale simulation framework provides a quantitative analysis of the displacement and stress fields, and can be used to predict the response of nanomechanical sensors subjected to complex molecular adsorption.

Intrinsic stress evolution during amorphous oxide film growth on Al surfaces

The intrinsic stress evolution during formation of ultrathin amorphous oxide films on Al(111) and Al(100) surfaces by thermal oxidation at room temperature was investigated in real-time by in-situ substrate curvature measurements and detailed atomic-scale microstructural analyses. During thickening of the oxide a considerable amount of growth stresses is generated in, remarkably even amorphous, ultrathin Al2O3 films. The surface orientation-dependent stress evolutions during O adsorption on the bare Al surfaces and during subsequent oxide-film growth can be interpreted as a result of (i) adsorption-induced surface stress changes and (ii) competing processes of free volume generation and structural relaxation, respectively.

Adsorbate-Induced Surface Stress of Methanol Monolayer on Au(III) Surface

Adsorbate-Induced Surface Stress of Methanol Monolayer on Au(III) Surface

Chain Length Effect on Surface Stress of Alkanethiolates Adsorbed onto AU(111) Surface: a van der Waals Density Functional Study

ABSTRACTFirst-principles calculations were employed to investigate the adsorption-induced surface stress of self-assembled alkanethiolate monolayers on a Au(111) surface as a function of the alkyl chain length. A recently developed fully nonlocal van der Waals density functional was used to accurately account for the chain-chain interactions. We found that surface charge redistribution produces compressive surface stress, while chain-chain interactions produce tensile surface stress. The stress induced by surface charge redistribution is about one order of magnitude greater than that of chain-chain interactions. We observed that the chain-chain interactions play an important role in determining the molecular configuration during adsorptions, and also contribute significantly to the induced anisotropic tensile (positive) surface stress. As the chain length increases the tensile stress increases at a rate of ∼ 0.32 (∼ 0.18) N/m for the direction perpendicular (parallel) to the chain tilt direction.