Ionic Diffusion-Driven Ionovoltaic Transducer for Probing Ion-Molecular Interactions at Solid-Liquid Interface

Ion-solid surface interactions are one of the fundamental principles in liquid-interfacing devices ranging from various electrochemical systems to electrolyte-driven energy conversion devices. The interplays between these two phases, especially containing charge carriers in the solid layer, work as a pivotal role in the operation of these devices, but corresponding details of those effects remain as unrevealed issues in academic fields. Herein, an ion-charge carrier interaction at an electrolyte-semiconductor interface is interrogated with an ion-dynamics-induced (ionovoltaic) energy transducer, controlled by interfacial self-assembled molecules. An electricity generating mechanism from interfacial ionic diffusion is elucidated in terms of the ion-charge carrier interaction, originated from a dipole potential effect of the self-assembled molecular layer (SAM). In addition, this effect is found to be modulated via chemical functionalization of the interfacial molecular layer and transition metal ion complexation therein. With the aiding of surface analytic techniques and a liquid-interfacing Hall measurement, electrical behaviors of the device depending on the magnitude of the ion-ligand complexation are interrogated, thereby demonstrating the ion-charge carrier interplays spanning at electrolyte-SAM-semiconductor interface. Hence, this system can be applied to study molecular interactions, including chemical and physical influences, occurring at the solid-liquid interfacial region.

Probing an Interfacial Ionic Pairing-Induced Molecular Dipole Effect in Ionovoltaic System

A surficial molecular dipole effect depending on ion-molecular interactions has been crucial issues regarding to an interfacial potential, which can modulate solid electronic and electrochemical systems. Their properties near the interfacial region can be dictated by specific interactions between surface and adsorbates, but understandings of the corresponding details remain at interesting issues. Here, intuitive observations of an ionic pair formation-induced interfacial potential shifts are presented with an ionovoltaic system, and corresponding output signal variations are analyzed in terms of the surficial dipole changes on self-assembled monolayer. With aiding of photoelectron spectroscopies and density function theory simulation, the ionic pair formation-induced potential shifts are revealed to strongly rely on a paired molecular structure and a binding affinity of the paired ionic moieties. Chemical contributions to the binding event are interrogated in terms of polarizability in each ionic group and consistent with chaotropic/kosmotropic character of the ionic groups. Based on these findings, the ionovoltaic output changes are theoretically correlated with an adsorption isotherm reflecting the molecular dipole effect, thereby demonstrating as an efficient interfacial molecular probing method under electrolyte interfacing conditions.

Surface chemistry effects on work function, ionization potential and electronic affinity of Si(100), Ge(100) surfaces and SiGe heterostructures

We combine density functional theory and many body perturbation theory to investigate the electronic properties of Si(100) and Ge(100) surfaces terminated with halogen atoms (-I, -Br, -Cl, -F) and other chemical functionalizations (-H, -OH, -CH3) addressing the absolute values of their work function, electronic affinity and ionization potential. Our results point out that electronic properties of functionalized surfaces strongly depend on the chemisorbed species and much less on the surface crystal orientation. The presence of halogens at the surface always leads to an increment of the work function, ionization potential and electronic affinity with respect to fully hydrogenated surfaces. On the contrary, the presence of polar -OH and -CH3 groups at the surface leads to a reduction of the aforementioned quantities with respect to the H-terminated system. Starting from the work functions calculated for the Si and Ge passivated surfaces, we apply a simple model to estimate the properties of functionalized SiGe surfaces. The possibility of modulating the work function by changing the chemisorbed species and composition is predicted. The effects induced by different terminations on the band energy line-up profile of SiGe surfaces are then analyzed. Interestingly, our calculations predict a type-II band offset for the H-terminated systems and a type-I band offset for the other cases.

Binary functionalization of H:Si(111) surfaces by alkyl monolayers with different linker atoms enhances monolayer stability and packing

Alkyl monolayer modified Si forms a class of inorganic-organic hybrid materials with applications across many technologies such as thin-films, fuel/solar-cells and biosensors. Previous studies have shown that the linker atom, through which the monolayer binds to the Si substrate, and any tail group in the alkyl chain, can tune the monolayer stability and electronic properties. In this paper we study the H:Si(111) surface functionalized with binary SAMs: these are composed of alkyl chains that are linked to the surface by two different linker groups. Aiming to enhance SAM stability and increase coverage over singly functionalized Si, we examine with density functional theory simulations that incorporate vdW interactions, a range of linker groups which we denote as -X-(alkyl) with X = CH2, O(H), S(H) or NH(2) (alkyl = C6 and C12 chains). We show how the stability of the SAM can be enhanced by adsorbing alkyl chains with two different linkers, e.g. Si-[C, NH]-alkyl, through which the adsorption energy is increased compared to functionalization with the individual -X-alkyl chains. Our results show that it is possible to improve stability and optimum coverage of alkyl functionalized SAMs linked through a direct Si-C bond by incorporating alkyl chains linked to Si through a different linker group, while preserving the interface electronic structure that determines key electronic properties. This is important since any enhancement in stability and coverage to give more densely packed monolayers will result in fewer defects. We also show that the work function can be tuned within the interval of 3.65-4.94 eV (4.55 eV for bare H:Si(111)).

Fundamental aspects in surface self-assembly: theoretical implications of molecular polarity and shape

We investigate fundamental aspects of structure formation in molecular self-assembly, by examining the emergence of order upon adsorption of a series of model molecules. It is known that strongly polar diatomic molecules form three-dimensional crystals in the absence of a substrate. This tendency can be disrupted upon assembly on a solid surface, and various other types of order may arise. Depending on the relative strength of the interactions, disordered phases, two-dimensional crystals commensurate to the surface, and unmodified crystals were observed upon adsorption of simple dipoles in the present work. Introduction of steric features, in the form of a longer backbone or substituents external to the polar pair, led to even richer phase diagrams. The formation of two-dimensional phases with nematic (parallel) or antiparallel alignment was accomplished by altering the polarity of the end groups on needle-like molecules, whereas embedded charged groups made two-dimensional structure unstable for even very long molecules. These molecules preferred to align in long, often desorbed, molecular wires. The wealth of phases observed here parallel the results of experimental systematic or incidental studies of the relationships between molecular interactions and self-assembled patterns, and provide some insight into the molecular handles that self-assembly researchers can wield to guide the process towards a desired structural outcome.

Protein attachment to silane-functionalized porous silicon: A comparison of electrostatic and covalent attachment

Porous silicon (pSi) is a prosperous biomaterial, biocompatible, and biodegradable. Obtaining regularly functionalized pSi surfaces is required in many biotechnology applications. Silane-PEG-NHS (triethoxysilane-polyethylene-glycol-N-hydroxysuccinimide) is useful for single-molecule studies due to its ability to attach to only one biomolecule. We investigate the functionalization of pSi with silane-PEG-NHS and compare it with two common grafting agents: APTMS (3-aminopropylotrimethoxysilane) as electrostatic linker, and APTMS modified with glutaraldehyde as covalent spacer. We show the arrangement of two proteins (collagen and bovine serum albumin) as a function of the functionalization and of the pore size. FTIR is used to demonstrate correct functionalization while fluorescence confocal microscopy reveals that silane-PEG-NHS results in a more uniform protein distribution. Reflection interference spectroscopy (RIfS) is used to estimate the attachment of linker and proteins. The results open a way to obtain homogenous chemical modified silicon supports with a great value in biosensing, drug delivery and cell biology.

Effect of surface potential on extracellular matrix protein adsorption

Extracellular matrix (ECM) proteins, such as fibronectin, laminin, and collagen IV, play important roles in many cellular behaviors, including cell adhesion and spreading. Understanding their adsorption behavior on surfaces with different natures is helpful for studying the cellular responses to environments. By tailoring the chemical composition in binary acidic (anionic) and basic (cationic) functionalized self-assembled monolayer (SAM)-modified gold substrates, variable surface potentials can be generated. To examine how surface potential affects the interaction between ECM proteins and substrates, a quartz crystal microbalance with dissipation detection (QCM-D) was used. To study the interaction under physiological conditions, the ionic strength and pH were controlled using phosphate-buffered saline at 37 °C, and the ζ potentials of the SAM-modified Au and protein were determined using an electrokinetic analyzer and phase analysis light scattering, respectively. During adsorption processes, the shifts in resonant frequency (f) and energy dissipation (D) were acquired simultaneously, and the weight change was calculated using the Kelvin-Voigt model. The results reveal that slightly charged protein can be adsorbed on a highly charged SAM, even where both surfaces are negatively charged. This behavior is attributed to the highly charged SAM, which polarizes the protein microscopically, and the Debye interaction, as well as other short-range interactions such as steric force, hydrogen bonding, direct bonding, charged domains within the protein structure, etc., that allow adsorption, although the macroscopic electrostatic interaction discourages adsorption. For surfaces with a moderate potential, proteins are not significantly polarized by the surface, and the interaction can be predicted through simple electrostatic attraction. Furthermore, surface-induced self-assembly of protein molecules also affects the adsorbed structures and kinetics. The adsorbed layer properties, such as rigidity and packing behaviors, were further investigated using the D-f plot and phase detection microscopy (PDM) imaging.

Investigation into the effect of varied functional biointerfaces on silicon nanowire MOSFETs

A biocompatible and functional interface can improve the sensitivity of bioelectronics. Here, 3-aminopropyl trimethoxysilane (APTMS) and 3-mercaptopropyl trimethoxysilane (MPTMS) self-assembled monolayers (SAMs) were independently modified on the surface of silicon nanowire metal-oxide-semiconductor field effect transistors (NW-MOSFETs). Those SAMs-modified silicon NW-MOSFETs were used to discriminate various pH solutions and further verify which modified regime was capable of providing better electrical signals. The APTMS-SAM modified NW-MOSFETs showed better electrical responses in pH sensing. Biomolecules on APTMS-SAM modified NW-MOSFETs also gave better signals for the corresponding proteind in physiological buffer solutions. Atomic force microscopy (AFM) clarified those electrical phenomena and found biomolecules on APTMS-SAM were relatively uniformly modified on NW-MOSFETs. Our results showed that more uniform modification contributed to better signal response to protein interactions in physiological buffer solutions. It suggests that suitable surface modifications could profoundly affect the sensing response and sensitivity.