Highly Stable Bonding of Thiol Monolayers to Hydrogen-Terminated Si via Supercritical Carbon Dioxide: Toward a Super Hydrophobic and Bioresistant Surface

Grafting of Organophosphonic Acid Monolayers on Hydrogen-Terminated Silicon Surface and Secondary Functionalization in Supercritical Carbon Dioxide Media

The monolayer grafting on the oxide-free Si surface is challenging due to vulnerability of the surface against oxide formation in an ambient atmosphere. Most of the conventional studies focused on organic solvent-based chemistry and solvent and substrate interfaces, and residual solvents after the monolayer grafting play a key role in producing the highly stable monolayers. CO2 in its supercritical state (SCCO2) provides an elegant engineering solution for the problem faced as it can be used as inert processing environment and as carrier fluid for monolayer grafting taking up the role of organic solvents. In this work, monolayers of alkyl organophosphonic acids (OPAs) and functional OPAs were grafted on hydrogen-terminated oxide-free Si surfaces using the SCCO2 process. Grafted monolayers were physically and chemically characterized to verify the successful monolayer formation and determine the nature of the covalent binding configuration on the surface. To broaden the prospects of practical utility of the process and the OPA monolayer, the (3-bromopropyl)phosphonic acid (BPPA) monolayer was demonstrated to undergo secondary functionalization by terminal group substitution to convert the Br terminal group to the OH terminal group and secondary monolayer grafting to assemble 4-fluorothiophenol on top of the BPPA monolayer. The ability of monolayers to sustain secondary functionalization processing qualitatively hints toward ordered and stable monolayers of OPAs. The developed SCCO2 process in this work presents a single-step, green, and scalable method to graft the OPA monolayer on oxide-free Si which can employed in the future for monolayer doping, highly selective biochemical sensors, and targeted biological interactions.

Universal Single-Step Approach to the Immobilization of Cyclodextrins in a Supercritical Medium for Capturing Drug, Dye, and Metal Nanoclusters

By utilizing nanoreactor-like structures, the immobilization of macromolecules such as calixarenes and cyclodextrins (CD) with bucket-like structures provides new possibilities for engineered surface-molecule systems. The practical use of any molecular system depends on the availability of a universal procedure for immobilizing molecules with torus-like structures on various surfaces while maintaining identical operating parameters. There are currently several steps, including toxic solvent-based approaches using modified β-CD to covalently attach to surfaces with multistep reactions. However, the existing multistep process results in molecular orientation, restricts the accessibility of the hydrophobic barrel of β-CD's for practical use, and is effectively unable to use the surfaces immobilized with β-CD for a variety of applications. In this study, it was demonstrated that β-CD attached to the oxide-based semiconductor and metal surfaces through a condensation reaction between the hydroxyl-terminated oxide-based semiconductor/metal oxide and β-CD in supercritical carbon dioxide (SCCO₂) as a medium. The primary benefit of SCCO₂-assisted grafting of unmodified β-CD on various oxide-based metal and semiconductor surfaces is that it is a simple, efficient, one-step process and that it is ligand-free, scalable, substrate-independent, and uses minimal energy. Various physical microscopy and chemical spectroscopic methods were used to analyze the grafted β-CD oligomers. The application of the grafted β-CD films was demonstrated by the immobilization of rhodamine B (RhB), a dye, and dopamine, a drug. The in situ nucleation and growth of silver nanoclusters (AgNCs) in the molecular systems were studied for antibacterial and tribological properties by utilizing the guest-host interaction ability of β-CD.

Silicon - single molecule - silicon circuits

In 2020, silicon - molecule - silicon junctions were fabricated and shown to be on average one third as conductive as traditional junctions made using gold electrodes, but in some instances to be even more conductive, and significantly 3 times more extendable and 5 times more mechanically stable. Herein, calculations are performed of single-molecule junction structure and conductivity pertaining to blinking and scanning-tunnelling-microscopy (STM) break junction (STMBJ) experiments performed using chemisorbed 1,6-hexanedithiol linkers. Some strikingly different characteristics are found compared to analogous junctions formed using the metals which, to date, have dominated the field of molecular electronics. In the STMBJ experiment, following retraction of the STM tip after collision with the substrate, unterminated silicon surface dangling bonds are predicted to remain after reaction of the fresh tips with the dithiol solute. These dangling bonds occupy the silicon band gap and are predicted to facilitate extraordinary single-molecule conductivity. Enhanced junction extendibility is attributed to junction flexibility and the translation of adsorbed molecules between silicon dangling bonds. The calculations investigate a range of junction atomic-structural models using density-functional-theory (DFT) calculations of structure, often explored at 300 K using molecular dynamics (MD) simulations. These are aided by DFT calculations of barriers for passivation reactions of the dangling bonds. Thermally averaged conductivities are then evaluated using non-equilibrium Green's function (NEGF) methods. Countless applications through electronics, nanotechnology, photonics, and sensing are envisaged for this technology.

Vapor-Phase Passivation of Chlorine-Terminated Ge(100) Using Self-Assembled Monolayers of Hexanethiol

Continued scaling of electronic devices shows the need to incorporate high mobility alternatives to silicon, the cornerstone of the semiconductor industry, into modern field effect transistor (FET) devices. Germanium is well-poised to serve as the channel material in FET devices as it boasts an electron and hole mobility more than twice and four times that of Si, respectively. However, its unstable native oxide makes its passivation a crucial step toward its potential integration into future FETs. The International Roadmap for Devices and Systems (IRDS) predicts continued aggressive scaling not only of the device size but also of the pitch in nanowire arrays. The development of a vapor-phase chemical passivation technique will be required to prevent the collapse of these structures that can occur because of the surface tension and capillary forces that are experienced when tight-pitched nanowire arrays are processed via liquid-phase chemistry. Reported here is a vapor-phase process using hexanethiol for the passivation of planar Ge(100) substrates. Results benchmarking it against its well-established liquid-phase equivalent are also presented. X-ray photoelectron spectroscopy was used to monitor the effectiveness of the developed vapor-phase protocol, where the presence of oxide was monitored at 0, 24, and 168 h. Water contact angle measurements compliment these results by demonstrating an increase in hydrophobicity of the passivated substrates. Atomic force microscopy monitored the surface topology before and after processing to ensure the process does not cause roughening of the surface, which is critical to demonstrate suitability for nanostructures. It is shown that the 200 min vapor-phase passivation procedure generates stable, passivated surfaces with less roughness than the liquid-phase counterpart.

Supercritical Fluid Applications in the Design of Novel Antimicrobial Materials

Bacterial resistance to antibiotics is one of the biggest problems in the modern world. The prevention of bacterial spreading from hospitals to the community and vice versa is an issue we have to deal with. This review presents a vast potential of contemporary high-pressure techniques in the design of materials with antimicrobial activity. Scientists from all over the world came up with ideas on how to exploit extraordinary properties of supercritical fluids in the production of advantageous materials in an environmentally friendly way. The review summarizes reported methods and results.

Spontaneous S-Si bonding of alkanethiols to Si(111)-H: towards Si-molecule-Si circuits

We report the synthesis of covalently linked self-assembled monolayers (SAMs) on silicon surfaces, using mild conditions, in a way that is compatible with silicon-electronics fabrication technologies. In molecular electronics, SAMs of functional molecules tethered to gold via sulfur linkages dominate, but these devices are not robust in design and not amenable to scalable manufacture. Whereas covalent bonding to silicon has long been recognized as an attractive alternative, only formation processes involving high temperature and/or pressure, strong chemicals, or irradiation are known. To make molecular devices on silicon under mild conditions with properties reminiscent of Au–S ones, we exploit the susceptibility of thiols to oxidation by dissolved O₂, initiating free-radical polymerization mechanisms without causing oxidative damage to the surface. Without thiols present, dissolved O₂ would normally oxidize the silicon and hence reaction conditions such as these have been strenuously avoided in the past. The surface coverage on Si(111)–H is measured to be very high, 75% of a full monolayer, with density-functional theory calculations used to profile spontaneous reaction mechanisms. The impact of the Si–S chemistry in single-molecule electronics is demonstrated using STM-junction approaches by forming Si–hexanedithiol–Si junctions. Si–S contacts result in single-molecule wires that are mechanically stable, with an average lifetime at room temperature of 2.7 s, which is five folds higher than that reported for conventional molecular junctions formed between gold electrodes. The enhanced “ON” lifetime of this single-molecule circuit enables previously inaccessible electrical measurements on single molecules.

Spontaneously formed Si–S bonds enable monolayer and single-molecule Si–molecule–Si circuits.

Modification of a H-terminated silicon surface by organic sulfide molecules: the mechanism and origin of reactivity

For the reactions of disulfide molecules (RSSR), the steric effect rather than the electronic effect of the R group is the main origin of the different reactivity. In the reactions of sulfide molecules (RSXR′, X = S, P, Si, O, N, C), charges on the S atom and dissociation energies of the S–X bonds have a great impact on the reactivity of these reactions.

n-Alkanethiols Directly Grown on a Bare Si(111) Surface: From Disordered to Ordered Transition

We observed the growth phase transition of n-alkanethiols (AT), CH₃(CH₂)_n-1SH, n = 4-16, directly implanted on a bare Si(111) surface, forming an AT monolayer. These monolayers were characterized with static water-contact angle, high-resolution X-ray photoelectron spectroscopy, near-edge X-ray fine-structure spectroscopy, and grazing-angle reflection absorption Fourier-transform infrared spectroscopy. The integrated spectral results indicated that the implanted n-AT molecules formed a self-oriented and densely packed monolayer through formation of an S-Si bond. With the number of carbons in the alkyl chain at six or more, namely beginning at hexanethiol, the molecular monolayer began to develop an orientation-ordered structure, which is clearly shorter than that for AT monolayers on Au and Ag. This result implies that, with a stronger molecule-substrate interaction, an ordered molecular monolayer can form with a short chain.

Light-Converting Polymer/Si Nanocrystal Composites with Stable 60-70% Quantum Efficiency and Their Glass Laminates

Thiol-ene polymer/Si nanocrystal bulk hybrids were synthesized from alkyl-passivated Si nanocrystal (Si NC) toluene solutions. Radicals in the polymer provided a copassivation of "dark" Si NCs, making them optically active and leading to a substantial ensemble quantum yield increase. Optical stability over several months was confirmed. The presented materials exhibit the highest photoluminescence quantum yield (∼65%) of any solid-state Si NC hybrid reported to date. The broad tunability of thiol-ene polymer reactivity provides facile glass integration, as demonstrated by a laminated structure. This, together with extremely fast polymerization, makes the demonstrated hybrid material a promising candidate for light converting applications.

One-Step Synthesis of Hydrophobic Multicompartment Organosilica Microspheres with Highly Interconnected Macro-mesopores for the Stabilization of Liquid Marbles with Excellent Catalysis

The combination of an emulsion template with polymerization is a very convenient approach to the one-step realization of both simple control porous structures via a change in emulsion formulation and easy functionalization via the concomitant choice of an on-demand monomer. A major challenge of this approach is the inherent instability of the oil/water interface in emulsions, especially the occurrence of chemical reactions in oil or aqueous phases. This study reports the pioneering preparation of highly interconnected macro-mesopores and multicompartment (HIMC) vinyl organosilica microspheres with hydrophobicity by the one-step formation of W/O/W emulsions acting as a template. The emulsion system consists of acidified deionized water, a stabilizer, and vinyltriethoxysilane (VTEO) in which VTEO can be used to produce an organosilica skeleton of the resultant microsphere by a sol-gel process. The study demonstrated that the marvelous stability of W/O/W emulsions aids the formation of multicompartment organosilica microspheres with highly interconnected macro-mesopores by emulsion droplets rather than single-compartment (SC) microspheres. Meanwhile, the internal porous structure and surface morphology of as-prepared organosilica microspheres could be largely tuned by a simple variation of the pH value, the volume fraction of the water phase, and the stabilizer concentration in the initiating multiemulsions. Benefiting from such a well-orchestrated structure and the existence of numerous vinyl groups on the surface, HIMC organosilica microspheres exhibit very high hydrophobicity (with a water contact angle larger than 160°), which allows them to stabilize liquid marbles with excellent stability and high mechanical robustness. Because of its strong catalyst, Ag nanoparticles within HIMC organosilica microspheres enable Ag/HIMC-vinyl organosilica microsphere-based liquid marbles to be an efficient catalytic microreactor, realizing the complete degradation of MB to leuco methylene blue by NaBH₄ in 10 min. The result of this work could provide some guidance for the easy, low-cost, benign preparation of HIMC microspheres having the potential to be excellent supporter of metal nanoparticles or other functionalized compounds for applications in sensing, optoelectronics, and catalysis.