11 research outputs found
Mechanisms of Contact, Adhesion, and Failure of Metallic Nanoasperities in the Presence of Adsorbates: Toward Conductive Contact Design
The properties of
contacting interfaces are strongly affected not
only by the bulk and surface properties of contacting materials but
also by the ubiquitous presence of adsorbed contaminants. Here, we
focus on the properties of single asperity contacts in the presence
of adsorbates within a molecular dynamics description of metallic
asperity normal contact and a parametric description of adsorbate
properties. A platinum–platinum asperity contact is modeled
with adsorbed oligomers with variable properties. This system is particularly
tailored to the context of nanoelectromechanical system (NEMS) contact
switches, but the results are generally relevant to metal–metal
asperity contacts in nonpristine conditions. Even though mechanical
forces can displace adsorbate out of the contact region, increasing
the adsorbate layer thickness and/or adsorbate/metal adhesion makes
it more difficult for metal asperity/metal surface contact to occur,
thereby lowering the electrical contact conductance. Contact separation
is a competition between plastic necking in the asperity or decohesion
at the asperity/substrate interface. The mechanism which operates
at a lower tensile stress dominates. Necking dominates when the adsorbate/metal
adhesion is strong and/or the adsorbate layer thickness is small.
In broad terms, necking implies larger asperity deformation and mechanical
work, as compared with decohesion. Optimal NEMS switch performance
requires substantial contact conductance and minimal asperity deformation;
these results indicate that these goals can be achieved by balancing
the quantity of adsorbates and their adhesion to the metal surface
Accounting for Nanometer-Thick Adventitious Carbon Contamination in X‑ray Absorption Spectra of Carbon-Based Materials
Near-edge
X-ray absorption fine structure (NEXAFS) spectroscopy
is a powerful technique for characterizing the composition and bonding
state of nanoscale materials and the top few nanometers of bulk and
thin film specimens. When coupled with imaging methods like photoemission
electron microscopy, it enables chemical imaging of materials with
nanometer-scale lateral spatial resolution. However, analysis of NEXAFS
spectra is often performed under the assumption of structural and
compositional homogeneity within the nanometer-scale depth probed
by this technique. This assumption can introduce large errors when
analyzing the vast majority of solid surfaces due to the presence
of complex surface and near-surface structures such as oxides and
contamination layers. An analytical methodology is presented for removing
the contribution of these nanoscale overlayers from NEXAFS spectra
of two-layered systems to provide a corrected photoabsorption spectrum
of the substrate. This method relies on the subtraction of the NEXAFS
spectrum of the overlayer adsorbed on a reference surface from the
spectrum of the two-layer system under investigation, where the thickness
of the overlayer is independently determined by X-ray photoelectron
spectroscopy (XPS). This approach is applied to NEXAFS data acquired
for one of the most challenging cases: air-exposed hard carbon-based
materials with adventitious carbon contamination from ambient exposure.
The contribution of the adventitious carbon was removed from the as-acquired
spectra of ultrananocrystalline diamond (UNCD) and hydrogenated amorphous
carbon (a-C:H) to determine the intrinsic photoabsorption NEXAFS spectra
of these materials. The method alters the calculated fraction of sp<sup>2</sup>-hybridized carbon from 5 to 20% and reveals that the adventitious
contamination can be described as a layer containing carbon and oxygen
([O]/[C] = 0.11 ± 0.02) with a thickness of 0.6 ± 0.2 nm
and a fraction of sp<sup>2</sup>-bonded carbon of 0.19 ± 0.03.
This method can be generally applied to the characterization of surfaces
and interfaces in several research fields and technological applications
Stick–Slip Instabilities for Interfacial Chemical Bond-Induced Friction at the Nanoscale
Earthquakes are generally caused
by unstable stick–slip
motion of faults. This stick–slip phenomenon, along with other
frictional properties of materials at the macroscale, is well-described
by empirical rate and state friction (RSF) laws. Here we study stick–slip
behavior for nanoscale single-asperity silica–silica contacts
in atomic force microscopy experiments. The stick–slip is quasiperiodic,
and both the amplitude and spatial period of stick–slip increase
with normal load and decrease with the loading point (i.e., scanning)
velocity. The peak force prior to each slip increases with the temporal
period logarithmically, and decreases with velocity logarithmically,
consistent with stick–slip behavior at the macroscale. However,
unlike macroscale behavior, the minimum force after each slip is independent
of velocity. The temporal period scales with velocity in a nearly
power law fashion with an exponent between −1 and −2,
similar to macroscale behavior. With increasing velocity, stick–slip
behavior transitions into steady sliding. In the transition regime
between stick–slip and smooth sliding, some slip events exhibit
only partial force drops. The results are interpreted in the context
of interfacial chemical bond formation and rate effects previously
identified for nanoscale contacts. These results contribute to a physical
picture of interfacial chemical bond-induced stick–slip, and
further establish RSF laws at the nanoscale
Controlling Nanoscale Friction through the Competition between Capillary Adsorption and Thermally Activated Sliding
We demonstrate measurement and control of nanoscale single-asperity friction by using cantilever probes featuring an <i>in situ</i> solid-state heater in contact with silicon oxide substrates. The heater temperature was varied between 25 and 790 °C. By using a low thermal conductivity sample, silicon oxide, we are able to vary tip temperatures over a broad range from 25 ± 2 to 255 ± 25 °C. In ambient atmosphere with ∼30% relative humidity, the control of friction forces was achieved through the formation of a capillary bridge whose characteristics exhibit a strong dependence on temperature and sliding speed. The capillary condensation is observed to be a thermally activated process, such that heating in ambient air caused friction to increase due to the capillary bridge nucleating and growing. Above tip temperatures of ∼100 ± 10 °C, friction decreased drastically, which we attribute to controllably evaporating water from the contact at the nanoscale. In contrast, in a dry nitrogen atmosphere, friction was not affected appreciably by temperature changes. In the presence of a capillary, friction decreases at higher sliding speeds due to disruption of the capillary; otherwise, friction increases in accordance with the predictions of a thermally assisted sliding model. In ambient atmospheres, the rate of increase of friction with sliding speed at room temperature is sufficiently strong that the friction force changes from being smaller than the response at 76 ± 8 °C to being larger. Thus, an appropriate change in temperature can cause friction to increase at one sliding speed, while it decreases at another speed
Thermally Induced Structural Evolution of Silicon- and Oxygen-Containing Hydrogenated Amorphous Carbon: A Combined Spectroscopic and Molecular Dynamics Simulation Investigation
Silicon-
and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O)
coatings are amorphous thin-film materials composed of hydrogenated
amorphous carbon (a-C:H), doped with silicon and oxygen. Compared
to a-C:H, a-C:H:Si:O exhibits much lower susceptibility to oxidative
degradation and higher thermal stability, making a-C:H:Si:O attractive
for many applications. However, the physical mechanisms for this improved
behavior are not understood. Here, the thermally induced structural
evolution of a-C:H:Si:O was investigated in situ by X-ray photoelectron
and absorption spectroscopy, as well as molecular dynamics (MD) simulations.
The spectroscopy results indicate that upon high vacuum annealing,
two thermally activated processes with a Gaussian distribution of
activation energies with mean value <i>E</i> and standard
deviation σ take place in a-C:H:Si:O: (a) ordering and clustering
of sp<sup>2</sup> carbon (<i>E</i> ± σ = 0.22
± 0.08 eV) and (b) conversion of sp<sup>3</sup>- to sp<sup>2</sup>-bonded carbon (<i>E</i> ± σ = 3.0 ± 1.1
eV). The experimental results are in qualitative agreement with the
outcomes of MD simulations performed using a ReaxFF potential. The
MD simulations also indicate that the higher thermal stability of
a-C:H:Si:O compared to a-C:H (with similar fraction of sp<sup>2</sup>-bonded carbon and hydrogen content) derives from the significantly
lower fraction of strained carbon–carbon sp<sup>3</sup> bonds
in a-C:H:Si:O compared to a-C:H, which are more likely to break at
elevated temperatures
Frictional Behavior of Atomically Thin Sheets: Hexagonal-Shaped Graphene Islands Grown on Copper by Chemical Vapor Deposition
Single asperity friction experiments using atomic force microscopy (AFM) have been conducted on chemical vapor deposited (CVD) graphene grown on polycrystalline copper foils. Graphene substantially lowers the friction force experienced by the sliding asperity of a silicon AFM tip compared to the surrounding oxidized copper surface by a factor ranging from 1.5 to 7 over loads from the adhesive minimum up to 80 nN. No damage to the graphene was observed over this range, showing that friction force microscopy serves as a facile, high contrast probe for identifying the presence of graphene on Cu. Consistent with studies of epitaxially grown, thermally grown, and mechanically exfoliated graphene films, the friction force measured between the tip and these CVD-prepared films depends on the number of layers of graphene present on the surface and reduces friction in comparison to the substrate. Friction results on graphene indicate that the layer-dependent friction properties result from puckering of the graphene sheet around the sliding tip. Substantial hysteresis in the normal force dependence of friction is observed with repeated scanning without breaking contact with a graphene-covered region. Because of the hysteresis, friction measured on graphene changes with time and maximum applied force, unless the tip slides over the edge of the graphene island or contact with the surface is broken. These results also indicate that relatively weak binding forces exist between the copper foil and these CVD-grown graphene sheets
Nanomechanics of pH-Responsive, Drug-Loaded, Bilayered Polymer Grafts
Stimuli-responsive
polymer films play an important role in the development of smart antibacterial
coatings. In this study, we consider complementary architectures of
polyelectrolyte films, including a thin chitosan layer (CH), poly(acrylic
acid) (PAA) brushes, and a bilayer structure of CH grafted to PAA
brushes (CH/PAA) as possible candidates for targeted drug delivery
platforms. Atomic force microscopy (AFM) was employed to study the
structure–mechanical property relationship for these mono-
and bi-layered polymer grafts at pH 7.4 and 4.0, corresponding to
physiological and biofilm formation conditions, respectively. Herein,
the surface interactions between polymer grafts and the negatively
charged silica colloid attached to an AFM lever are considered as
representative interactions between the antibacterial coating and
a bacteria/biofilm. The bilayered structure of CH/PAA showed significantly
reduced adhesive interactions in comparison to pure CH but slightly
higher interactions in comparison to PAA films. Among PAA and CH/PAA
films, upon grafting CH over the PAA brushes, the normal stiffness
increased by 10-fold at pH 7.4 and 20-fold at pH 4.0. Notably, the
study also showed that the addition of an antibiotic drug such as
multicationic Tobramycin (TOB) impacts the mechanical properties of
the antibacterial coatings. Competition between TOB and water molecules
for the PAA chains is shown to determine the structural properties
of PAA and CH/PAA films loaded with TOB. At high pH (7.4), the TOB
molecules, which remain multicationic, strongly interact with polyanionic
PAA, thereby reducing the film’s compressibility. On the contrary,
at low pH (4.0), the water molecules preferentially interact with
TOB in comparison to uncharged PAA chains and, upon TOB release, results
in a stronger film collapse together with an increase in adhesive
interactions between the probe, the surface, and the elastic modulus
of the film. The bacterial proliferation on these platforms when compared
to the measured mechanical properties shows a direct correlation;
hence, understanding nanomechanical properties can provide insights
into designing new antibacterial polymer coatings
Heterogeneity in the Small-Scale Deformation Behavior of Disordered Nanoparticle Packings
Atomic force microscopy-based nanoindentation
is used to image and probe the local mechanical properties of thin
disordered nanoparticle packings. The probed region is limited to
the size of a few particles, and an individual particle can be loaded
and displaced to a fraction of a single particle radius. The results
demonstrate heterogeneous mechanical response that is location-dependent.
The weak locations may be analogous to the “soft spots”
previously predicted in glasses and other disordered packings
Fluorination of Graphene Enhances Friction Due to Increased Corrugation
The addition of a single sheet of
carbon atoms in the form of graphene
can drastically alter friction between a nanoscale probe tip and a
surface. Here, for the first time we show that friction can be altered
over a wide range by fluorination. Specifically, the friction force
between silicon atomic force microscopy tips and monolayer fluorinated
graphene can range from 5−9 times higher than for graphene.
While consistent with previous reports, the combined interpretation
from our experiments and molecular dynamics simulations allows us
to propose a novel mechanism: that the dramatic friction enhancement
results from increased corrugation of the interfacial potential due
to the strong local charge concentrated at fluorine sites, consistent
with the Prandtl-Tomlinson model. The monotonic increase of friction
with fluorination in experiments also demonstrates that friction force
measurements provide a sensitive local probe of the degree of fluorination.
Additionally, we found a transition from ordered to disordered atomic
stick–slip upon fluorination, suggesting that fluorination
proceeds in a spatially random manner
Tribochemical Wear of Diamond-Like Carbon-Coated Atomic Force Microscope Tips
Nanoscale wear is a critical issue
that limits the performance of tip-based nanomanufacturing and nanometrology
processes based on atomic force microscopy (AFM). Yet, a full scientific
understanding of nanoscale wear processes remains in its infancy.
It is therefore important to quantitatively understand the wear behavior
of AFM tips. Tip wear is complex to understand due to adhesive forces
and contact stresses that change substantially as the contact geometry
evolves due to wear. Here, we present systematic characterization
of the wear of commercial Si AFM tips coated with thin diamond-like
carbon (DLC) coatings. Wear of DLC was measured as a function of external
loading and sliding distance. Transmission electron microscopy imaging,
AFM-based adhesion measurements, and tip geometry estimation via inverse
imaging were used to assess nanoscale wear and the contact conditions
over the course of the wear tests. Gradual wear of DLC with sliding
was observed in the experiments, and the tips evolved from initial
paraboloidal shapes to flattened geometries. The wear rate is observed
to increase with the average contact stress, but does not follow the
classical wear law of Archard. A wear model based on the transition
state theory, which gives an Arrhenius relationship between wear rate
and normal stress, fits the experimental data well for low mean contact
stresses (<0.3 GPa), yet it fails to describe the wear at higher
stresses. The wear behavior over the full range of stresses is well
described by a recently proposed multibond wear model that exhibits
a change from Archard-like behavior at high stresses to a transition
state theory description at lower stresses