How far the chemistry of self-assembled monolayers on gold surfaces affects their work function?

Self-assembled monolayers composed of various long-chain aliphatic molecules and different tail functional groups have been synthesized on the Au(111) surface and characterized by Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Carboxy, amino, thio and methyl terminal groups have been considered in the design of self-assembled monolayers with different aliphatic chain lengths (from C6 to C16). Work function measurements by Kelvin probe force microscopy have been carried out under a controlled and room atmosphere. Remarkably, a reduction of the relative humidity from 40% to 3% has induced a work function shift of up to 0.3 eV. As expected, the changes of the chain length of the aliphatic moiety and of the tail group have a significant impact on the tuning of the measured work function (3.90 eV for dodecanethiol versus 4.57 eV for mercaptohexadecylamine). Surprisingly, the change of the net dipole moment of the tail group (sign and amplitude) does not dominate the work function variations. In contrast, the change of the chain length and the possibility of the tail group to form a complex hydrogen bond network between molecules lead to significant modulations of the work function. In order to interpret these original findings, density functional theory models of equivalent self-assembled monolayers adsorbed on the Au(111) surface have been developed at an unprecedented level of description with large supercells including simultaneously 27 co-adsorbed molecules and weak van der Waals interactions between them. Such large systems have allowed the theoretical modeling of complex hydrogen bond networks between molecules when possible (carboxy tail group). The comparison between computed and measured work functions shows a striking agreement, thus allowing the disentanglement of the previously mentioned competing effects. This consistency between experiment and theory will help in designing the electronic properties of self-assembled monolayers in the context of molecular electronics and organic transistors.

Modulating the Electronic Properties of Au-MoS2 Interfaces Using Functionalized Self-Assembled Monolayers

Molybdenum disulfide (MoS₂) is a transition-metal dichalcogenide with many applications including in electronic devices and sensors. A critical issue in the development of these devices is the high resistance between the metal contact and the molybdenum disulfide layer. In this study, we employ Raman spectroscopy and X-ray photoelectron spectroscopy to investigate the modification of Au-MoS₂ contact properties using functionalized alkanethiolate self-assembled monolayers (SAMs). We demonstrate that both 2H and 1T MoS₂ strongly interact with the underlying Au substrate. The electronic properties of the interface are mediated by the dipole moment of the alkanethiolate SAM, which have a -CH₃, -CO₂C₆F₅, -OH, or -COOH terminal group. Finally, we demonstrate the site-selective deposition of 2H and 1T MoS₂ on micropatterned SAMs to form conducting-semiconducting patterned MoS₂ films.

Photovoltaic Effect of a Ferroelectric-Luminescent Heterostructure under Infrared Light Illumination

In this report, a ferroelectric-luminescent heterostructure is designed to convert infrared light into electric power. We use BiFeO₃ (BFO) as the ferroelectric layer and Y₂O₃:Yb,Tm (YOT) as the upconversion layer. Different from conventional ferroelectric materials, this heterostructure exhibits switchable and stable photovoltaic effects under 980 nm illumination, whose energy is much lower than the band gap of BFO. The energy transfer mechanism in this heterostructure is therefore studied carefully. It is found that a highly efficient nonradiative energy transfer process from YOT to BFO plays a critical role in achieving the below-band-gap photon-excited photovoltaic effects in this heterostructure. Our results also indicate that by introducing asymmetric electrodes, both the photovoltage and photocurrent are further enhanced when the built-in field and the depolarization field are aligned. The construction of ferroelectric-luminescent heterostructure is consequently proposed as a promising route to enhance the photovoltaic effects of ferroelectric materials by extending the absorption of the solar spectrum.

Organic Spin-Valves and Beyond: Spin Injection and Transport in Organic Semiconductors and the Effect of Interfacial Engineering

Since the first observation of the spin-valve effect through organic semiconductors, efforts to realize novel spintronic technologies based on organic semiconductors have been rapidly growing. However, a complete understanding of spin-polarized carrier injection and transport in organic semiconductors is still lacking and under debate. For example, there is still no clear understanding of major spin-flip mechanisms in organic semiconductors and the role of hybrid metal-organic interfaces in spin injection. Recent findings suggest that organic single crystals can provide spin-transport media with much less structural disorder relative to organic thin films, thus reducing momentum scattering. Additionally, modification of the band energetics, morphology, and even spin magnetic moment at the metal-organic interface by interface engineering can greatly impact the efficiency of spin-polarized carrier injection. Here, progress on efficient spin-polarized carrier injection into organic semiconductors from ferromagnetic metals by using various interface engineering techniques is presented, such as inserting a metallic interlayer, a molecular self-assembled monolayer (SAM), and a ballistic carrier emitter. In addition, efforts to realize long spin transport in single-crystalline organic semiconductors are discussed. The focus here is on understanding and maximizing spin-polarized carrier injection and transport in organic semiconductors and insight is provided for the realization of emerging organic spintronics technologies.

Axial coordination and electronic structure of diatomic NO, CO, and O2 molecules adsorbed onto Co-tetraphenylporphyrin on Au(111), Ag(111), and Cu(111): a density-functional theory study

Based on density functional theory calculations, we investigated the axial bindings of diatomic molecules (NO, CO, and O₂) to metalloporphyrins and their spin switching behaviors. These binding reactions provide the ways to control molecular states and spins in metalloporphyrin systems that can be used in bio-sensing and spintronic applications. To microscopically understand the non-trivial axial binding structures and spin-switching behaviors of diatomic molecules (NO, CO, and O₂) adsorbed onto Co-tetraphenylporphyrin (CoTPP), we performed spin-polarized density functional theory (DFT) calculations for various CoTPP systems on Au(111), Ag(111), and Cu(111) substrates. We also systematically evaluated the effects of van der Waals and Hubbard U corrections by directly comparing the electronic structure with the results of scanning tunnelling microscopy. From the calculation results, we have found that a NO molecule, almost 60° tilted away from the axial direction, is coordinated with CoTPP with a binding energy of 1.2-1.7 eV depending on substrates, and the spin state of CoTPP is completely switched off due to the charge transfer from NO to CoTPP. On the other hand, CO and O₂ molecules rather weakly interact with a binding energy of 0.4-0.8 eV, and a spin polarization of ∼1μ_B is still present at CoTPP. A CO molecule is expected to be almost straightly coordinated along the axial direction of CoTPP, but O₂ is tilted similarly to the NO case. Regarding the substrate effects, we have found that there is noticeable charge transfer from Ag(111) and Cu(111) to CoTPP, but no significant charge transfer from Au(111) to CoTPP. These findings of the axial coordination and spin states for NO, CO, and O₂ adsorbed CoTPP systems will be useful for understanding bio-sensing mechanisms and designing molecular spintronic systems.

Band alignment at the interface between Ni-doped Cr2O3 and Al-doped ZnO: implications for transparent p-n junctions

The realization of transparent electronic and optoelectronic devices requires the use of transparent p-n junctions. In this context, understanding the band alignment at the interface between the p- and n-components represents a fundamental step towards the realization of high performance devices. In this work, the band alignment at the interface between Al-doped ZnO (AZO) and Ni-doped Cr2O3 has been analysed. The formation and evolution of the core levels as the interface progressively forms have been followed by means of x-ray Photoelectron Spectroscopy, x-ray diffraction and x-ray reflectivity. A type two (staggered) band alignment was identified, with the valence band offset and conduction band offset found to be 2.6 eV and 2.5 eV, respectively. The electrical behaviour will be discussed in terms of the position of the bands, the presence of band bending and the expected built-in potential and how these can be engineered in order to achieve the maximum performance for this hetero-structure.

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)).

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.

Flexible power fabrics made of carbon nanotubes for harvesting thermoelectricity

Thermoelectric energy conversion is very effective in capturing low-grade waste heat to supply electricity particularly to small devices such as sensors, wireless communication units, and wearable electronics. Conventional thermoelectric materials, however, are often inadequately brittle, expensive, toxic, and heavy. We developed both p- and n-type fabric-like flexible lightweight materials by functionalizing the large surfaces and junctions in carbon nanotube (CNT) mats. The poor thermopower and only p-type characteristics of typical CNTs have been converted into both p- and n-type with high thermopower. The changes in the electronic band diagrams of the CNTs were experimentally investigated, elucidating the carrier type and relatively large thermopower values. With our optimized device design to maximally utilize temperature gradients, an electrochromic glucose sensor was successfully operated without batteries or external power supplies, demonstrating self-powering capability. While our fundamental study provides a method of tailoring electronic transport properties, our device-level integration shows the feasibility of harvesting electrical energy by attaching the device to even curved surfaces like human bodies.

From the bottom up: dimensional control and characterization in molecular monolayers

Self-assembled monolayers are a unique class of nanostructured materials, with properties determined by their molecular lattice structures, as well as the interfaces with their substrates and environments. As with other nanostructured materials, defects and dimensionality play important roles in the physical, chemical, and biological properties of the monolayers. In this review, we discuss monolayer structures ranging from surfaces (two-dimensional) down to single molecules (zero-dimensional), with a focus on applications of each type of structure, and on techniques that enable characterization of monolayer physical properties down to the single-molecule scale.