Tribochemical Wear of Diamond-Like Carbon-Coated Atomic Force Microscope Tips

Atomistic Wear Mechanisms in Diamond: Effects of Surface Orientation, Stress, and Interaction with Adsorbed Molecules

Despite its unrivaled hardness, diamond can be severely worn during the interaction with others, even softer materials. In this work, we calculate from first-principles the energy and forces necessary to induce the atomistic wear of diamond and compare them for different surface orientations and passivation by oxygen, hydrogen, and water fragments. The primary mechanism of wear is identified as the detachment of the carbon chains. This is particularly true for oxidized diamond and diamonds interacting with silica. A very interesting result concerns the role of stress, which reveals that compressive stresses can highly favor wear, making it even energetically favorable.

Tribochemical nanolithography: selective mechanochemical removal of photocleavable nitrophenyl protecting groups with 23 nm resolution at speeds of up to 1 mm s-1

We describe the mechanochemical regulation of a reaction that would otherwise be considered to be photochemical, via a simple process that yields nm spatial resolution. An atomic force microscope (AFM) probe is used to remove photocleavable nitrophenyl protecting groups from alkylsilane films at loads too small for mechanical wear, thus enabling nanoscale differentiation of chemical reactivity. Feature sizes of 20-50 nm are achieved repeatably and controllably at writing rates up to 1 mm s^-1. Line widths vary monotonically with the load up to 2000 nN. To demonstrate the capacity for sophisticated surface functionalisation provided by this strategy, we show that functionalization of nanolines with nitrilo triacetic acid enables site-specific immobilization of histidine-tagged green fluorescent protein. Density functional theory (DFT) calculations reveal that the key energetic barrier in the photo-deprotection reaction of the nitrophenyl protecting group is excitation of a π-π* transition (3.1 eV) via an intramolecular charge-transfer mechanism. Under modest loading, compression of the adsorbate layer causes a decrease in the N-N separation, with the effect that this energy barrier can be reduced to as little as 1.2 eV. Thus, deprotection becomes possible via either absorption of visible photons or phononic excitation transfer, facilitating fast nanolithography with a very small feature size.

Insight into Drug Loading Regulated Micellar Rigidity by Nuclear Magnetic Resonance

The rigidity of polymeric micelles plays an important role in their biological behaviors. However, how drug loading affects the rigidity of polymeric micelles remains elusive. Herein, the indomethacin (IMC)-loaded Pluronic F127 micelle is used as a model system to illustrate the impact of drug loading on the rigidity and biological behaviors of polymeric micelles. Against expectations, micelles with moderate drug loading show higher cellular uptake and more severe cytotoxicity as compared to both high and low drug loading counterparts. Extensive one- and two-dimensional nuclear magnetic resonance (NMR) measurements are employed to reveal that the higher drug loading induces stronger interaction between IMC and hydrophilic block to boost the micellar rigidity; consequently, the moderate drug loading imparts micelles with appropriate rigidity for satisfactory cellular uptake and cytotoxicity. In summary, NMR spectroscopy is an important tool to gain insight into drug loading regulated micellar rigidity, which is helpful to understand their biological behaviors.

Insights into Superlow Friction and Instability of Hydrogenated Amorphous Carbon/Fluid Nanocomposite Interface

Hydrogenated amorphous carbon (a-C:H) film exhibits the superlubricity phenomena as rubbed against dry sliding contacts. However, its antifriction stability strongly depends on the working environment. By composting with the fluid lubricant, the friction response and fundamental mechanisms governing the low-friction performance and instability of a-C:H remain unclear, while they are not accessible by experiment due to the complicated interfacial structure and the lack of advanced characterization technique in situ. Here, we addressed this puzzle with respect to the physicochemical interactions of a-C:H/oil/graphene nanocomposite interface at atomic scale. Results reveal that although the friction capacity and stability of system are highly sensitive to the hydrogenated degrees of mated a-C:H surfaces, the optimized H contents of mated a-C:H surfaces are suggested in order to reach the superlow friction or even superlubricity. Interfacial structure analysis indicates that the fundamental friction mechanism attributes to the hydrogenation-induced passivation of friction interface and squeezing effect to fluid lubricant. Most importantly, the opposite diffusion of fluid oil molecules to the sliding direction is observed, resulting in the transformation of the real friction interface from a-C:H/oil interface to oil/oil interface. These outcomes enable an effective manipulation of the superlow friction of carbon-based films and the development of customized solid-fluid lubrication systems for applications.

Thermal Activation of Nanoscale Wear

Nanoscale wear tracks on ionic crystals are created by reciprocating single asperity scratch tests using atomic force microscopy. The wear characteristics are analyzed by the scratch depth as a function of surface temperature from 25 to 300 K. The average wear depth shows a nonmonotonic behavior as a function of temperature, with a transition between two different regimes characterized by the occurrence of quasiperiodic ripple formation. A thermally activated bond breaking model quantitatively explains the wear data in the low temperature, nonripple regime, but fails above the temperature threshold. This discrepancy is resolved with a geometric separation of the ripple mounds from the troughs, leading to full agreement with Arrhenius kinetics over the full temperature range.

Unraveling the Friction Evolution Mechanism of Diamond-Like Carbon Film during Nanoscale Running-In Process toward Superlubricity

Diamond-like carbon (DLC) films are capable of achieving superlubricity at sliding interfaces by a rapid running-in process. However, fundamental mechanisms governing the friction evolution during this running-in processes remain elusive especially at the nanoscale, which hinders strategic tailoring of tribosystems for minimizing friction and wear. Here, it is revealed that the running-in governing superlubricity of DLC demonstrates two sub-stages in single-asperity nanocontacts. The first stage, mechanical removal of a thin oxide layer, is described quantitatively by a stress-activated Arrhenius model. In the second stage, a large friction decrease occurs due to a structural ordering transformation, with the kinetics well described by the Johnson-Mehl-Avrami-Kolmogorov model with a modified load dependence of the activation energy. The direct observation of a graphitic-layered transfer film formation together with the measured Avrami exponent reveal the primary mechanism of the ordering transformation. The findings provide fundamental insights into friction evolution mechanisms, and design criteria for superlubricity.

Non-Empirical Law for Nanoscale Atom-by-Atom Wear

Wear of contact materials results in energy loss and device failure. Conventionally, wear is described by empirical laws such as the Archard's law; however, the fundamental physical and chemical origins of the empirical law have long been elusive, and moreover empirical wear laws do not always hold for nanoscale contact, collaboratively hindering the development of high-durable tribosystems. Here, a non-empirical and robustly applicable wear law for nanoscale contact situations is proposed. The proposed wear law successfully unveils why the nanoscale wear behaviors do not obey the description by Archard's law in all cases although still obey it in certain experiments. The robustness and applicability of the proposed wear law is validated by atomistic simulations. This work affords a way to calculate wear at nanoscale contact robustly and theoretically, and will contribute to developing design principles for wear reduction.

Role of interfacial adhesion on minimum wear particle size and roughness evolution

Adhesion between two bodies is a key parameter in wear processes. At the macroscale, strong adhesive bonds are known to lead to high wear rates, as observed in clean metal-on-metal contact. Reducing the strength of the interfacial adhesion is then desirable, and techniques such as lubrication and surface passivation are employed to this end. Still, little is known about the influence of adhesion on the microscopic processes of wear. In particular, the effects of interfacial adhesion on the wear particle size and on the surface roughness evolution are not clear and are therefore addressed here by means of molecular dynamics simulations. We show that, at short timescales, the surface morphology and not the interfacial adhesion strength dictates the minimum size of wear particles. However, at longer timescales, adhesion alters the particle motion and thus the wear rate and the surface morphology.

Wear comparison of critical dimension-atomic force microscopy tips

Nanoscale wear affects the performance of atomic force microscopy (AFM)-based measurements for all applications including process control measurements and nanoelectronics characterization. As such, methods to prevent or reduce AFM tip wear is an area of active research. However, most prior work has been on conventional AFMs rather than critical dimension AFM (CD-AFM). Hence, less is known about CD-AFM tip-wear. Given that tip-wear directly affects the accuracy of dimensional measurements, a basic understanding of CD-AFM tip wear is needed. Toward this goal, we evaluated the wear performance of electron beam deposited CD-AFM tips. Using a continuous scanning strategy, we evaluated the overall wear rate and tip lifetime and compared these with those of silicon-based CD-AFM tips. Our data show improved tip lifetime of as much as a factor of five and reduced wear rates of more than 17 times. Such improvements in wear rate means less measurement variability and lower cost.

Sliding History-Dependent Adhesion of Nanoscale Silicon Contacts Revealed by in Situ Transmission Electron Microscopy

Nanoscale asperity-on-asperity sliding experiments were conducted using a nanoindentation apparatus inside a transmission electron microscope, allowing for atomic-scale resolution of contact formation, sliding, and adhesive separation of two silicon nanoasperities in real time. The formation and separation of the contacts without sliding revealed adhesion forces often below detectable limits (ca. 5 nN) or at most equal to values expected from van der Waals forces. Lateral sliding during contact by distances ranging from 3.7 μm down to as little as 20 nm resulted in an average 19× increase in the adhesive pull-off force, with increases as large as 32× seen. Adhesion after sliding increased with both the sliding speed and the applied normal contact stress. Unlike cold welding, where irreversible material changes like flow occur, these effects were repeatable and reversible multiple times, for multiple pairs of asperities. We hypothesize that sliding removes passivating surface terminal species, most likely hydrogen or hydroxyl groups, making sites available to form strong covalent bonds across the interface that increase adhesion. Upon separation, repassivation occurs within the experimentally limited lower bound time frame of 5 s, with full recovery of low adhesion. The results demonstrate the strong sliding history-dependence of adhesion, which hinges on the interplay between tribologically induced removal of adsorbed species and repassivation of unsaturated bonds on freshly separated surfaces by dissociative chemisorption.

Temperature-Dependent Mechanochemical Wear of Silicon in Water: The Role of Si-OH Surfacial Groups

Mechanochemical wear has attracted much attention due to its critical role in micro/nanodevice applications, reliable microscopy, and ultraprecision manufacturing. As a process of stress-associated chemical reactions, mechanochemical wear strongly depends on temperature; however, the impact mechanism is not fully understood at any length scale. Here, we reported different water-temperature dependence of mechanochemical wear on two typical single crystal silicon (Si) surfaces, involving oxide-covered Si partially terminated with Si-OH groups and oxide-free Si fully terminated with Si-H groups. As the water temperature increased from 10 to 80 °C, the mechanochemical wear of the oxide-covered Si underwent a process from no obvious surface damage to significant material removal but that occurring at all temperatures decreased gradually on the oxide-free Si surface. The opposite temperature-dependence was found to have a strong relation to the growth or degeneration of the Si-OH surfacial groups. The mechanochemical wear on the both Si surfaces decreased with the Si-OH coverage rising, which facilitated the growth of strongly hydrogen-bonded ordered water and then suppressed the chemical reaction between the sliding interfaces. These results can provide new insight into the mechanism of the surrounding temperature affecting the reliable micro/nanodevices, manufacturing, and microscopy.

Nanotribological Printing: A Nanoscale Additive Manufacturing Method

Additive manufacturing methods are transforming the way components and devices are fabricated, which in turn is opening up completely new vistas for conceiving and designing products and engineered systems. Small-scale (submicrometer) additive manufacturing methods are largely in their infancy. While a number of methods exist, a particular challenge lies in finding methods that can produce a range of materials while obtaining sufficiently robust mechanical properties. In this paper, we describe a novel nanoscale additive manufacturing technique deemed "Nanotribological Printing" (NTP), which creates structures through tribomechanical and tribochemical surface interactions at the contact between a substrate and an atomic force microscope probe, where material pattern formation is driven by normal and shear contact stresses. The "ink" consists of nanoparticles or molecules dispersed in a carrier fluid surrounding the atomic force microscope (AFM) probe, which are entrained into the contact during sliding. Being stress-driven, patterning only occurs locally within regions which experience contact and sufficiently high stresses. Thus, imaging and measurement to characterize the morphology and properties of the deposited structures can be conducted in situ during the manufacturing process. Moreover, using local mechanical energy as the kinetic driver activating the solidification process, the method is compact and does not require application of a bias voltage or laser exposure and can be performed at ambient temperatures. We demonstrate (1) control of pattern dimensions with sub-100 nm lateral and sub-5 nm thickness control through variations in contact size and applied stress, (2) creation of amorphous, polycrystalline, and nanocomposite structures including sequential multimaterial deposition, and (3) formation of manufactured structures which exhibit mechanical properties approaching those of bulk counterparts. The ability to create nanoscale patterns using standard AFM cantilever probes and operation modes (contact mode scanning in fluid) with commercial AFM instruments, independent of substrate, establishes NTP as a versatile and easily accessible method for nanoscale additive manufacturing.

Metrology for the next generation of semiconductor devices

The semiconductor industry continues to produce ever smaller devices that are ever more complex in shape and contain ever more types of materials. The ultimate sizes and functionality of these new devices will be affected by fundamental and engineering limits such as heat dissipation, carrier mobility and fault tolerance thresholds. At present, it is unclear which are the best measurement methods needed to evaluate the nanometre-scale features of such devices and how the fundamental limits will affect the required metrology. Here, we review state-of-the-art dimensional metrology methods for integrated circuits, considering the advantages, limitations and potential improvements of the various approaches. We describe how integrated circuit device design and industry requirements will affect lithography options and consequently metrology requirements. We also discuss potentially powerful emerging technologies and highlight measurement problems that at present have no obvious solution.

Exploring wear at the nanoscale with circular mode atomic force microscopy

The development of atomic force microscopy (AFM) has allowed wear mechanisms to be investigated at the nanometer scale by means of a single asperity contact generated by an AFM tip and an interacting surface. However, the low wear rate at the nanoscale and the thermal drift require fastidious quantitative measurements of the wear volume for determining wear laws. In this paper, we describe a new, effective, experimental methodology based on circular mode AFM, which generates high frequency, circular displacements of the contact. Under such conditions, the wear rate is significant and the drift of the piezoelectric actuator is limited. As a result, well-defined wear tracks are generated and an accurate computation of the wear volume is possible. Finally, we describe the advantages of this method and we report a relevant application example addressing a Cu/Al₂O₃ nanocomposite material used in industrial applications.