Deprotonation of phenol linked to a silicon dioxide surface using adaptive feedback laser control with a heterodyne detected sum frequency generation signal.

The development of laser-controlled surface reactions has been limited by the lack of decisive methods for detecting evolving changes in surface chemistry. In this work, we demonstrate successful laser control of a surface reaction by combining the adaptive feedback control (AFC) technique with surface sensitive spectroscopy to determine the optimally shaped laser pulse. Specifically, we demonstrate laser induced deprotonation of the hydroxyl group of phenol bound to a silicon dioxide substrate. The experiment utilized AFC with heterodyne detected vibrational sum frequency generation (HD-VSFG) as the surface sensitive feedback signal. The versatile combination of AFC with HD-VSFG provides a route to potentially control a wide range of surface reactions.

Synthesis and Mechanical Properties of sub 5-µm PolyUiO-66 Thin Films on Gold Surfaces.

Metal-organic framework (MOF) thin films currently lack the mechanical stability needed for electronic device applications. Polymer-based metal-organic frameworks (polyMOFs) have been suggested to provide mechanical advantages over MOFs, however, the mechanical properties of polyMOFs have not yet been characterized. In this work, we developed a method to synthesize continuous sub-5 µm polyUiO-66(Zr) films on Au substrates, which allowed us to undertake initial mechanical property investigations. Comparisons between polyUiO-66 and UiO-66 thin films determined polyUiO-66 thin films exhibit a lower modulus but similar hardness to UiO-66 thin films. The initial mechanical characterization indicates that further development is needed to leverage the mechanical property advantages of polyMOFs over MOFs. Additionally, the demonstration in this work of a continuous surface-supported polyUiO-66 thin film enables utilization of this emerging class of polyMOF materials in sensors and devices applications.

Reaction of Dichlorophenylborane with H-Si(100).

Traditional approaches to achieving dopant functionalized Si involve grafting the dopant to the Si substrates through O-Si or C-Si bonds, resulting in indirect attachment of the dopant to the Si. Recently, ultrahigh vacuum work has demonstrated that high densities of direct B-Si bonds enable unprecedented electronic behaviors in Si that make it possible for Si to be used as a next-generation electronic material. As solvothermal approaches are inherently amenable to scale-up, there is currently a push to develop solvothermal approaches for the formation of direct dopant-Si bonds. Thus far, B-Si chemistries for next-generation electronic materials have been demonstrated with boron trichloride and bis(pinacolatodiboron). In this work, we use a combination of experimental work and computational studies to examine the reactivity of a phenyl derivatized boron trichloride, namely dichlorophenylborane, with H-Si(100). We determine that despite the stability and ease for the formation of C-Si bonds, the organic component, the phenyl group remains attached to the B and does not yield competitive formation of products via a Si-C bond. This reaction proved a new solvothermal method for the formation of direct B-Si bonds that, with further work, can be leveraged in developing next-generation electronic materials.

Ultradoping Boron on Si(100) via Solvothermal Chemistry*.

Ultradoping introduces unprecedented dopant levels into Si, which transforms its electronic behavior and enables its use as a next-generation electronic material. Commercialization of ultradoping is currently limited by gas-phase ultra-high vacuum requirements. Solvothermal chemistry is amenable to scale-up. However, an integral part of ultradoping is a direct chemical bond between dopants and Si, and solvothermal dopant-Si surface reactions are not well-developed. This work provides the first quantified demonstration of achieving ultradoping concentrations of boron (∼1e14 cm2 ) by using a solvothermal process. Surface characterizations indicate the catalyst cross-reacted, which led to multiple surface products and caused ambiguity in experimental confirmation of direct surface attachment. Density functional theory computations elucidate that the reaction results in direct B-Si surface bonds. This proof-of-principle work lays groundwork for emerging solvothermal ultradoping processes.

Two-Dimensional versus Three-Dimensional Self-Assembly of a Series of 5-Alkoxyisophthalic Acids.

Physisorbed self-assembled monolayers (SAMs) have been suggested as potential models for three-dimensional (3D) crystallization. This work studies the effect of altering the chain length of 5-alkoxyisophthalic acid (C nISA) on self-assembled morphology in both two-dimensional (2D) and 3D to explore the extent comparisons can be drawn between dimensions. Previous studies of 5-alkoxyisophthalic acid at solid-liquid interfaces (2D) reported different morphologies for C5ISA and C6ISA-alkoxy chains on the one hand and C10ISA and C18ISA on the other. Independently, also in 3D a dependence of morphology on chain length has been reported, including an unexpected inclusion of a solvent in the 3D morphology of C6ISA, while the previous reports of 2D self-assembly were driven only by molecule-molecule and molecule-substrate interactions. However, a complete set of data for comparison has been missing. Here, we report scanning tunneling microscopy (STM) and molecular dynamics simulations performed for C2ISA self-assembled monolayers (SAMs) and STM imaging of C6ISA-C9ISA SAMs, to further examine self-assembly behavior in 2D. In 3D, X-ray diffraction analysis of C2ISA single crystals was carried out to complete the data set. With a complete set of data, it was observed that regardless of the dimension, short-chain-length C nISAs formed H-bonding-dominated structures, mid-chain-length C nISAs exhibited solvent-dependent morphologies, and long-chain-length C nISAs displayed van der Waals-dominated solvent-independent structures. However, the transition point among morphologies occurred at different chain lengths in 2D and 3D regardless of the dominant interaction. The results of this study inform the design of 2D films and guide the application of knowledge from physisorbed SAMs to 3D systems, including mixed-dimensional (2D/3D) van der Waals heterostructures.

Substituent effects on the kinetics of bifunctional styrene SAM formation on H-terminated Si.

Self-assembled monolayers (SAMs) on metal and semiconductor surfaces are of interest in electronic devices, molecular and biosensors, and nanostructured surface preparation. Bifunctionalized molecules, where one functional group attaches to the surface while the other remains free for further modification, allow for the rational design of multilayer chemisorbed thin films. In this study, substituted styrenes acted as a model system for SAM formation through an alkene moiety. Substituents ranging from activating to strongly deactivating for aromatic reactions were used to probe the effect of the electronic properties of functionalizing molecules on the rate of SAM formation. Substituted styrene SAMs were formed on hydrogen-terminated p-type Si(100) and n-type Si(111) via sonochemical functionalization. Monolayers were characterized via ellipsometry, IR spectroscopy, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS). Initial rates of reaction for molecules that selectively attached through the alkene were further studied. A linear relationship was observed between the initial rates of surface functionalization and the substituent electron donating/withdrawing ability for the substituted styrenes, as described by their respective Hammett constants. This study provides precedent for applying well quantified homogeneous chemical reaction relationships to reactions at the solid-liquid interface.