Surface Structure Dependence of Mechanochemical Etching: Scanning Probe-Based Nanolithography Study on Si(100), Si(110), and Si(111)

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.

Photothermal Atomic Force Microscopy Coupled with Infrared Spectroscopy (AFM-IR) Analysis of High Extinction Coefficient Materials: A Case Study with Silica and Silicate Glasses

Photothermal atomic force microscopy coupled with infrared spectroscopy (AFM-IR) brings significant value as a spatially resolved surface analysis technique for disordered oxide materials such as glasses, but additional development and fundamental understanding of governing principles is needed to interpret AFM-IR spectra, since the existing theory described for organic materials does not work for materials with high extinction coefficients for infrared (IR) absorption. This paper describes theoretical calculation of a transient temperature profile inside the IR-absorbing material considering IR refraction at the interface as well as IR adsorption and heat transfer inside the sample. This calculation explains the differences in peak positions and amplitudes of AFM-IR spectra from those of specular reflectance and extinction coefficient spectra. It also addresses the information depth of the AFM-IR characterization of bulk materials. AFM-IR applied to silica and silicate glass surfaces has demonstrated novel capability of characterizing subsurface structural changes and surface heterogeneity due to mechanical stresses from physical contacts, as well as chemical alterations manifested in surface layers through aqueous corrosion.

Effect of Native Oxide Layer on Mechanochemical Reaction at the GaN-Al2O3 Interface

Mechanochemical reactions at the gallium nitride-alumina (GaN-Al₂O₃) interface at nanoscale offer a significant beneficial reference for the high-efficiency and low-destruction ultra-precision machining on GaN surface. Here, the mechanochemical reactions on oxide-free and oxidized GaN surfaces rubbed by the Al₂O₃ nanoasperity as a function of the ambient humidity were studied. Experimental results reveal that oxidized GaN exhibits a higher mechanochemical removal rate than that of oxide-free GaN over the relative humidity range of 3-80%. The mechanical activation in the mechanochemical reactions at the GaN-Al₂O₃ interface is well-described by the mechanically-assisted Arrhenius-type kinetics model. The analysis indicates that less external mechanical activation energy is required to initiate the mechanochemical atomic attrition on the oxidized GaN surface compared with the oxide-free GaN surface. These results may not only gain a deep understanding of the mechanochemical removal mechanism of GaN but also provide the basic knowledge for the optimization of the oxidation-assisted ultra-precision machining.

Physical Origin of the Mechanochemical Coupling at Interfaces

We used density functional theory calculations to investigate the physical origin of the mechanochemical response of material interfaces. Our results show that the mechanochemical response can be decomposed into the contribution from the interface itself (deformation of interfacial bonds) and a contribution from the underlying solid. The relative contributions depend on the stiffness of these regions and the contact geometry, which affects the stress distribution within the bulk region. We demonstrate that, contrary to what is commonly assumed, the contribution to the activation volume from the elastic deformation of the surrounding bulk is significant and, in some case, may be dominant. We also show that the activation volume and the mechanochemical response of interfaces should be finite due to the effects on the stiffness and stress distribution within the near-surface bulk region. Our results indicate that the large range of activation volumes measured in the previous experiments even for the same material system might originate from the different degrees of contributions probed from the bulk vs interface.

Atomic-Level Material Removal Mechanisms of Si(110) Chemical Mechanical Polishing: Insights from ReaxFF Reactive Molecular Dynamics Simulations

Reactive molecular dynamics (ReaxFF) simulations are performed to explore the atomistic mechanism of chemical mechanical polishing (CMP) processes on a Si(110) surface polished with an a-SiO₂ particle. The Si surface is oxidized by reacting with water before the CMP process, and the O atoms of the oxidized Si surface mainly exist in the form of Si-O- dangling bonds and Si-O-Si bonds. In the CMP process, the insertion of O atoms into the surface, the formation of interfacial Si-O-Si and Si-Si bridge bonds, and the adsorption of H atoms on the surface-saturated Si atoms can all cause the surface bond breakage and even the Si atomic removal. The contributions of the four different kinds of tribochemical wear mechanisms to the surface wear decrease in turn and are much larger than that of mechanical wear. The results indicate that the material removal in the actual Si CMP process is the combined results of multiple atomic-level wear mechanisms. Furthermore, we find that the oxide layer of the Si surface plays an important role in the surface wear mainly by providing O atoms to insert into the surface, rather than by providing additional reaction pathways to form interfacial Si-O-Si bridge bonds. This work provides new and further insights into the process and mechanism of silicon removal during CMP.

In situ generating Cu2O/Cu heterointerfaces on the Cu2O cube surface to enhance interface charge transfer for the Rochow reaction

Cubic Cu/Cu₂O with heterointerfaces showed enhanced catalytic performance for the Rochow reaction. The resulting Schottky junction enhanced charge transfer efficiency and contributed to easier cleavage of Si–Si bond along {110} crystal plane.

Thickness and Structure of Adsorbed Water Layer and Effects on Adhesion and Friction at Nanoasperity Contact

Most inorganic material surfaces exposed to ambient air can adsorb water, and hydrogen bonding interactions among adsorbed water molecules vary depending on, not only intrinsic properties of material surfaces, but also extrinsic working conditions. When dimensions of solid objects shrink to micro- and nano-scales, the ratio of surface area to volume increases greatly and the contribution of water condensation on interfacial forces, such as adhesion (Fa) and friction (Ft), becomes significant. This paper reviews the structural evolution of the adsorbed water layer on solid surfaces and its effect on Fa and Ft at nanoasperity contact for sphere-on-flat geometry. The details of the underlying mechanisms governing water adsorption behaviors vary depending on the atomic structure of the substrate, surface hydrophilicity and atmospheric conditions. The solid surfaces reviewed in this paper include metal/metallic oxides, silicon/silicon oxides, fluorides, and two-dimensional materials. The mechanism by which water condensation influences Fa is discussed based on the competition among capillary force, van der Waals force and the rupture force of solid-like water bridge. The condensed meniscus and the molecular configuration of the water bridge are influenced by surface roughness, surface hydrophilicity, temperature, sliding velocity, which in turn affect the kinetics of water condensation and interfacial Ft. Taking the effects of the thickness and structure of adsorbed water into account is important to obtain a full understanding of the interfacial forces at nanoasperity contact under ambient conditions.