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

Chirality-Induced Magnet-Free Spin Generation in a Semiconductor

Electrical generation and transduction of polarized electron spins in semiconductors (SCs) are of central interest in spintronics and quantum information science. While spin generation in SCs is frequently realized via electrical injection from a ferromagnet (FM), there are significant advantages in nonmagnetic pathways of creating spin polarization. One such pathway exploits the interplay of electron spin with chirality in electronic structures or real space. Here, utilizing chirality-induced spin selectivity (CISS), the efficient creation of spin accumulation in n-doped GaAs via electric current injection from a normal metal (Au) electrode through a self-assembled monolayer (SAM) of chiral molecules (α-helix l-polyalanine, AHPA-L), is demonstrated. The resulting spin polarization is detected as a Hanle effect in the n-GaAs, which is found to obey a distinct universal scaling with temperature and bias current consistent with chirality-induced spin accumulation. The experiment constitutes a definitive observation of CISS in a fully nonmagnetic device structure and demonstration of its ability to generate spin accumulation in a conventional SC. The results thus place key constraints on the physical mechanism of CISS and present a new scheme for magnet-free SC spintronics.

Inkjet Printing of Nanoscale Functional Patterns on 2D Crystalline Materials and Transfer to Soft Materials

Nanometer-scale control over surface functionality is important in applications ranging from nanoscale electronics to regenerative medicine. However, approaches that provide precise control over surface chemistry at the nanometer scale are often challenging to use with higher throughput and in more heterogeneous environments (e.g., complex solutions, porous interfaces) common for many applications. Here, we demonstrate a scalable inkjet-based method to generate 1 nm-wide functional patterns on 2D materials such as graphite, which can then be transferred to soft materials such as hydrogels. We examine fluid dynamics associated with the inkjet printing process for low-viscosity amphiphile inks designed to maximize ordering with limited residue and show that microscale droplet fluid dynamics influence nanoscale molecular ordering. Additionally, we show that scalable patterns generated in this way can be transferred to hydrogel materials and used to create surface chemical patterns that induce adsorption of charged particles, with effects strong enough to overcome electrostatic repulsion between a charged hydrogel and a like-charged nanoparticle.

Self-Assembled Monolayers of Push-Pull Chromophores as Active Layers and Their Applications

In recent decades, considerable attention has been focused on the design and development of surfaces with defined or tunable properties for a wide range of applications and fields. To this end, self-assembled monolayers (SAMs) of organic compounds offer a unique and straightforward route of modifying and engineering the surface properties of any substrate. Thus, alkane-based self-assembled monolayers constitute one of the most extensively studied organic thin-film nanomaterials, which have found wide applications in antifouling surfaces, the control of wettability or cell adhesion, sensors, optical devices, corrosion protection, and organic electronics, among many other applications, some of which have led to their technological transfer to industry. Nevertheless, recently, aromatic-based SAMs have gained importance as functional components, particularly in molecular electronics, bioelectronics, sensors, etc., due to their intrinsic electrical conductivity and optical properties, opening up new perspectives in these fields. However, some key issues affecting device performance still need to be resolved to ensure their full use and access to novel functionalities such as memory, sensors, or active layers in optoelectronic devices. In this context, we will present herein recent advances in π-conjugated systems-based self-assembled monolayers (e.g., push-pull chromophores) as active layers and their applications.

Spin Polarization in Transport Studies of Chirality-Induced Spin Selectivity

Chirality-induced spin selectivity (CISS) is a recently discovered effect in which structural chirality can result in different conductivities for electrons with opposite spins. In the CISS community, the degree of spin polarization is commonly used to describe the efficiency of the spin filtering/polarizing process, as it represents the fraction of spins aligned along the chiral axis of chiral materials originating from non-spin-polarized currents. However, the methods of defining, calculating, and analyzing spin polarization have been inconsistent across various studies, hindering advances in this field. In this Perspective, we connect the relevant background and the definition of spin polarization, discuss its calculation in different contexts in the CISS, and propose a practical and meaningful figure of merit by quantitative analysis of magnetoresistance in CISS transport studies.

Macropolyhedral syn-B18H22, the "Forgotten" Isomer

The chemistry and physics of macropolyhedral B18H22 clusters have attracted significant attention due to the interesting photophysical properties of anti-B18H22 (blue emission, laser properties) and related potential applications. We have focused our attention on the "forgotten" syn-B18H22 isomer, which has received very little attention since its discovery compared to its anti-B18H22 isomer, presumably because numerous studies have reported this isomer as nonluminescent. In our study, we show that in crystalline form, syn-B18H22 exhibits blue fluorescence and becomes phosphorescent when substituted at various positions on the cluster, associated with peculiar microstructural-dependent effects. This work is a combined theoretical and experimental investigation that includes the synthesis, separation, structural characterization, and first elucidation of the photophysical properties of three different monothiol-substituted cluster isomers, [1-HS-syn-B18H21] 1, [3-HS-syn-B18H21] 3, and [4-HS-syn-B18H21] 4, of which isomers 1 and 4 have been proved to exist in two different polymorphic forms. All of these newly substituted macropolyhedral cluster derivatives (1, 3, and 4) have been fully characterized by NMR spectroscopy, mass spectrometry, single-crystal X-ray diffraction, IR spectroscopy, and luminescence spectroscopy. This study also presents the first report on the mechanochromic shift in the luminescence of a borane cluster and generally enriches the area of rather rare boron-based luminescent materials. In addition, we present the first results proving that they are useful constituents of carbon-free self-assembled monolayers.

Nanoimaging of Facet-Dependent Adsorption, Diffusion, and Reactivity of Surface Ligands on Au Nanocrystals

Analysis of the influence of dissimilar facets on the adsorption, stability, mobility, and reactivity of surface ligands is essential for designing ligand-coated nanocrystals with optimal functionality. Herein, para-nitrothiophenol and nitronaphthalene were chemisorbed and physisorbed, respectively, on Au nanocrystals, and the influence of different facets within a single Au nanocrystal on ligands properties were identified by IR nanospectroscopy measurements. Preferred adsorption was probed on (001) facets for both ligands, with a lower density on (111) facets. Exposure to reducing conditions led to nitro reduction and diffusion of both ligands toward the top (111) facet. Nitrothiophenol was characterized with a diffusivity higher than that of nitronaphthalene. Moreover, the strong thiol-Au interaction led to the diffusion of Au atoms and the formation of thiol-coated Au nanoparticles on the silicon surface. It is identified that the adsorption and reactivity of surface ligands were mainly influenced by the atomic properties of each facet, while diffusion was controlled by ligand-metal interactions.

A push for mechanochemistry

Chemical reactions can be accelerated by directional local pressure applied to molecules.

Understanding the Two-Dimensional Mixing Behavior of 1-Naphthalenethiol and Octanethiol

A two-dimensional (2D) mixture in the form of a self-assembled monolayer composed of two distinct organothiol compounds was created by sequentially depositing 1-naphthalenethiol (1NT) and octanethiol (OT) on a gold surface. By varying the sequence of deposition, two mixed surface systems were created. The surface structure of the resulting mixed monolayer was characterized with Scanning Tunneling Microscopy (STM) and showed surface disorder across all investigated domains. Elemental analysis was carried out with X-ray Photoelectron Spectroscopy (XPS) and indicated that the 1NT monolayer was prone to significant oxidation. Reductive desorption (RD) was used to characterize the binding strength and electrochemical environments of the molecular components in the mixture, and confirmed disordered molecular layers. Due to the presence of oxidized species in the 1NT monolayer, 1NT was displaced by OT resulting in a novel surface structure composed of either OT or 1NT. Monolayers of OT that were exposed to a solution of 1NT resulted in disordered surface structures with a significant amount of gold vacancy islands. To date, there is no experimental phase diagram explaining the chemical behavior of two-dimensional mixtures. This study addresses the need for an experimental understanding of the phase behavior of mixed organothiol self-assembled monolayers (SAMs).

Exceptional Light Sensitivity by Thiol-Ene Click Lithography

Lithographic patterning, which utilizes the solubility switch of photoresists to convert optical signals into nanostructures on the substrate, is the primary top-down approach for nanoscale fabrication. However, the low light/electron-energy conversion efficiency severely limits the throughput of lithography. Thiol-ene reaction, as a photoinitiated radical addition reaction, is widely known as click reaction in the field of chemistry due to its extremely high efficiency. Here, we introduce a click lithography strategy utilizing the rapid thiol-ene click reaction to realize ultraefficient nanofabrication. This novel approach facilitated by the implementation of ultrahigh-functionality material designs enables high-contrast patterning of metal-containing nanoclusters under an extremely low deep-ultraviolet exposure dose, e.g., 7.5 mJ cm^-2, which is 10-20 times lower than the dose used in the photoacid generator-based photoresist system. Meanwhile, 45 nm dense patterns were also achieved at a low dose using electron beam lithography, revealing the great potential of this approach in high-resolution patterning. Our results demonstrated the high-sensitivity and high-resolution features of click lithography, providing inspiration for future lithography design.

Defect-engineered surfaces to investigate the formation of self-assembled molecular networks

Herein we report the impact of covalent modification (grafting), inducing lateral nanoconfinement conditions, on the self-assembly of a quinonoid zwitterion derivative into self-assembled molecular networks at the liquid/solid interface. At low concentrations where the compound does not show self-assembly behaviour on bare highly oriented pyrolytic graphite (HOPG), close-packed self-assembled structures are visualized by scanning tunneling microscopy on covalently modified HOPG. The size of the self-assembled domains decreases with increasing the density of grafted molecules, i.e. the molecules covalently bound to the surface. The dynamics of domains are captured with molecular resolution, revealing not only time-dependent growth and shrinkage processes but also the orientation conversion of assembled domains. Grafted pins play a key role in initiating the formation of on-surface molecular self-assembly and their stabilization, providing an elegant route to study various aspects of nucleation and growth processes of self-assembled molecular networks.

Wettability-based ultrasensitive detection of amphiphiles through directed concentration at disordered regions in self-assembled monolayers

Various forms of ecological monitoring and disease diagnosis rely upon the detection of amphiphiles, including lipids, lipopolysaccharides, and lipoproteins, at ultralow concentrations in small droplets. Although assays based on droplets' wettability provide promising options in some cases, their reliance on the measurements of surface and bulk properties of whole droplets (e.g., contact angles, surface tensions) makes it difficult to monitor trace amounts of these amphiphiles within small-volume samples. Here, we report a design principle in which self-assembled monolayer-functionalized microstructured surfaces coated with silicone oil create locally disordered regions within a droplet's contact lines to effectively concentrate amphiphiles within the areas that dominate the droplet static friction. Remarkably, such surfaces enable the ultrasensitive, naked-eye detection of amphiphiles through changes in the droplets' sliding angles, even when the concentration is four to five orders of magnitude below their critical micelle concentration. We develop a thermodynamic model to explain the partitioning of amphiphiles at the contact line by their cooperative association within the disordered, loosely packed regions of the self-assembled monolayer. Based on this local analyte concentrating effect, we showcase laboratory-on-a-chip surfaces with positionally dependent pinning forces capable of both detecting industrially and biologically relevant amphiphiles (e.g., bacterial endotoxins), as well as sorting aqueous droplets into discrete groups based on their amphiphile concentrations. Furthermore, we demonstrate that the sliding behavior of amphiphile-laden aqueous droplets provides insight into the amphiphile's effective length, thereby allowing these surfaces to discriminate between analytes with highly disparate molecular sizes.

Conformationally engineering flexible peptides on silver nanoparticles

Molecular conformational engineering is to engineer flexible non-functional molecules into unique conformations to create novel functions just like natural proteins fold. Obviously, it is a grand challenge with tremendous opportunities. Based on the facts that natural proteins are only marginally stable with a net stabilizing energy roughly equivalent to the energy of two hydrogen bonds, and the energy barriers for the adatom diffusion of some metals are within a similar range, we propose that metal nanoparticles can serve as a general replacement of protein scaffolds to conformationally engineer protein fragments on the surface of nanoparticles. To prove this hypothesis, herein, we successfully restore the antigen-recognizing function of the flexible peptide fragment of a natural anti-lysozyme antibody on the surface of silver nanoparticles, creating a silver nanoparticle-base artificial antibody (Silverbody). A plausible mechanism is proposed, and some general principles for conformational engineering are summarized to guide future studies in this area.

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A silver NP-based artificial antibody is created by conformational engineering

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Function emerges on NPs from non-functional peptide by mimicking the protein folding

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A general mechanism is proposed for the conformational engineering on metal NPs

Extraction of Hidden Science from Nanoscale Images

Scanning probe microscopies and spectroscopies enable investigation of surfaces and even buried interfaces down to the scale of chemical-bonding interactions, and this capability has been enhanced with the support of computational algorithms for data acquisition and image processing to explore physical, chemical, and biological phenomena. Here, we describe how scanning probe techniques have been enhanced by some of these recent algorithmic improvements. One improvement to the data acquisition algorithm is to advance beyond a simple rastering framework by using spirals at constant angular velocity then switching to constant linear velocity, which limits the piezo creep and hysteresis issues seen in traditional acquisition methods. One can also use image-processing techniques to model the distortions that appear from tip motion effects and to make corrections to these images. Another image-processing algorithm we discuss enables researchers to segment images by domains and subdomains, thereby highlighting reactive and interesting disordered sites at domain boundaries. Lastly, we discuss algorithms used to examine the dipole direction of individual molecules and surface domains, hydrogen bonding interactions, and molecular tilt. The computational algorithms used for scanning probe techniques are still improving rapidly and are incorporating machine learning at the next level of iteration. That said, the algorithms are not yet able to perform live adjustments during data recording that could enhance the microscopy and spectroscopic imaging methods significantly.

Nanometer-scale patterning of hard and soft interfaces: from photolithography to molecular-scale design

Many areas of modern materials chemistry, from nanoscale electronics to regenerative medicine, require design of precisely-controlled chemical environments at near-molecular scales on both hard and soft surfaces.

Lipids: An Atomic Toolkit for the Endless Frontier

The evolution of lipids in nanoscience exemplifies the powerful coupling of advances in science and technology. Here, we describe two waves of discovery and innovation in lipid materials: one historical and one still building. The first wave leveraged the relatively simple capability for lipids to orient at interfaces, building layers of functional groups. This simple form of building with atoms yielded a stunning range of technologies: lubricant additives that dramatically extended machine lifetimes, molecules that enabled selective ore extraction in mining, and soaps that improved human health. It also set the stage for many areas of modern nanoscience. The second wave of lipid materials, still growing, uses the more complex toolkits lipids offer for building with atoms, including controlling atomic environment to control function (e.g., pK_a tuning) and the generation of more arbitrary two-dimensional and three-dimensional structures, including lipid nanoparticles for COVID-19 mRNA vaccines.

Doping of the hydrogen-passivated Si(100) electronic structure through carborane adsorption studied using density functional theory

Adsorption of molecular materials with tailored chemical properties represents a new and promising avenue to non-destructively dope silicon. Dithiocarboranes possess large permanent dipoles and readily form stable monolayers on a variety of substrates. Here we use density functional theory to investigate the doping of hydrogen-passivated Si(100) substrates through the adsorption of dithiocarborane molecules. We find that dithiocarboranes can both physisorb and chemisorb on the substrate. Chemisorbed structures arise when a S atom in the molecular linker group replaces a surface H atom. We establish the formation of these Si-molecule bonds and characterize their mechanical and thermal stability. Analysis of the calculated electronic structure of adsorbed interfaces shows that carborane adsorption does not result in interface gap states. The band gap in adsorbed junctions is defined by Si states and its magnitude is almost unchanged with respect to the clean Si slab. The large carborane electrostatic dipole results in the downwards shift of Si spectral features by 0.3 eV, reducing the hole injection barrier by this amount with respect to the pristine Si substrate. Molecular dynamics simulations reveal these structural and electronic features to be stable at room temperature. Our work shows that molecular adsorbates having large electrostatic dipoles are a promising strategy to non-destructively dope semiconductor substrates.

Molecular patterns of oligopeptide hydrocarbons on graphite

Graphitic materials including graphene, carbon nanotubes and fullerenes, are promising for use in nanotechnology and biomedicine. Non-covalent functionalization by peptides and other organic molecules allows changing the properties of graphitic surfaces in a controlled manner and represents a big potential for fundamental research and applications. Recently described oligopeptide-hydrocarbon derivative N,N'-(decane-1,10-diyl)bis(tetraglycineamide) (GM) is highly prospective for the development of graphitic interfaces in biosensor application as well as in structural biology for improving the quality of high-resolution atomic force microscopy (AFM) visualization of individual biomacromolecules. However, molecular organization of GM on graphitic surfaces is still unknown. In this work, the molecular model of GM at the water/highly oriented pyrolytic graphite (HOPG) interface has been developed basing on the high-resolution AFM and full-atom molecular modeling data. This model explains two periodicities observed in AFM images by GM self-assembly on a HOPG surface with formation of the stacks with the lateral shifts. The obtained results reveal the particular patterns and dynamics of GM molecules adsorbed on graphite and unravel the puzzle of peptide self-assembly on graphitic surfaces.

Structural Order of the Molecular Adlayer Impacts the Stability of Nanoparticle-on-Mirror Plasmonic Cavities

Immense field enhancement and nanoscale confinement of light are possible within nanoparticle-on-mirror (NPoM) plasmonic resonators, which enable novel optically activated physical and chemical phenomena and render these nanocavities greatly sensitive to minute structural changes, down to the atomic scale. Although a few of these structural parameters, primarily linked to the nanoparticle and the mirror morphology, have been identified, the impact of molecular assembly and organization of the spacer layer between them has often been left uncharacterized. Here, we experimentally investigate how the complex and reconfigurable nature of a thiol-based self-assembled monolayer (SAM) adsorbed on the mirror surface impacts the optical properties of the NPoMs. We fabricate NPoMs with distinct molecular organizations by controlling the incubation time of the mirror in the thiol solution. Afterward, we investigate the structural changes that occur under laser irradiation by tracking the bonding dipole plasmon mode, while also monitoring Stokes and anti-Stokes Raman scattering from the molecules as a probe of their integrity. First, we find an effective decrease in the SAM height as the laser power increases, compatible with an irreversible change of molecule orientation caused by heating. Second, we observe that the nanocavities prepared with a densely packed and more ordered monolayer of molecules are more prone to changes in their resonance compared to samples with sparser and more disordered SAMs. Our measurements indicate that molecular orientation and packing on the mirror surface play a key role in determining the stability of NPoM structures and hence highlight the under-recognized significance of SAM characterization in the development of NPoM-based applications.

Chemical Lift-Off Lithography of Metal and Semiconductor Surfaces

Chemical lift-off lithography (CLL) is a subtractive soft-lithographic technique that uses polydimethylsiloxane (PDMS) stamps to pattern self-assembled monolayers of functional molecules for applications ranging from biomolecule patterning to transistor fabrication. A hallmark of CLL is preferential cleavage of Au-Au bonds, as opposed to bonds connecting the molecular layer to the substrate, i.e., Au-S bonds. Herein, we show that CLL can be used more broadly as a technique to pattern a variety of substrates composed of coinage metals (Pt, Pd, Ag, Cu), transition and reactive metals (Ni, Ti, Al), and a semiconductor (Ge) using straightforward alkanethiolate self-assembly chemistry. We demonstrate high-fidelity patterning in terms of precise features over large areas on all surfaces investigated. We use patterned monolayers as chemical resists for wet etching to generate metal microstructures. Substrate atoms, along with alkanethiolates, were removed as a result of lift-off, as previously observed for Au. We demonstrate the formation of PDMS-stamp-supported bimetallic monolayers by performing CLL on two different metal surfaces using the same PDMS stamp. By expanding the scope of the surfaces compatible with CLL, we advance and generalize CLL as a method to pattern a wide range of substrates, as well as to produce supported metal monolayers, both with broad applications in surface and materials science.

Manipulating chemistry through nanoparticle morphology

We demonstrate that the protonation chemistry of molecules adsorbed at nanometer distances from the surface of anisotropic gold nanoparticles can be manipulated through the effect of surface morphology on the local proton density of an organic coating. Direct evidence of this remarkable effect was obtained by monitoring surface-enhanced Raman scattering (SERS) from mercaptobenzoic acid and 4-aminobenzenethiol molecules adsorbed on gold nanostars. By smoothing the initially sharp nanostar tips through a mild thermal treatment, changes were induced on protonation of the molecules, which can be observed through changes in the measured SERS spectra. These results shed light on the local chemical environment near anisotropic colloidal nanoparticles and open an alternative avenue to actively control chemistry through surface morphology.

Conformal Ultrathin Film Metal-Organic Framework Analogues: Characterization of Growth, Porosity, and Electronic Transport

Thin-film formation and transport properties of two copper-paddlewheel metal-organic framework (MOF) -based systems (MOF-14 and MOF-399) are investigated for their potential integration into electrochemical device architectures. Thin-film analogs of these two systems are fabricated by the sequential, alternating, solution-phase deposition of the inorganic and organic ligand precursors that result in conformal films via van der Merwe-like growth. Atomic force microscopy reveals smooth film morphologies with surface roughnesses determined by the underlying substrates and linear film growth of 1.4 and 2.2 nm per layer for the MOF-14 and MOF-399 systems, respectively. Electrochemical impedance spectroscopy is used to evaluate the electronic transport properties of the thin films, finding that the MOF-14 analog films demonstrate low electronic conductivity, while MOF-399 analog films are electronically insulating. The intrinsic porosities of these ultrathin MOF analog films are confirmed by cyclic voltammetry redox probe characterization using ferrocene. Larger peak currents are observed for MOF-399 analog films compared to MOF-14 analog films, which is consistent with the larger pores of MOF-399. The layer-by-layer deposition of these systems provides a promising route to incorporate MOFs as thin films with nanoscale thickness control and low surface roughness for electrochemical devices.

Do-It-Yourself Pyramidal Mold for Nanotechnology

The handcrafted fabrication of a pyramidal mold on a silicon wafer for nanopatterning was investigated. This process started with the manual delivery of an aqueous glycerol solution onto the SiO₂/Si wafer using a micropipette and subsequent drying to form a hemisphere whose diameter is in the range of hundreds of micrometers. A coating of polystyrene (PS) onto this wafer generates a circular hole caused by dewetting. Subsequently, anisotropic wet-etching with the PS film as a mask produces a pyramidal trench, whose apex approaches hundreds of nanometers. Various elastomeric materials were casted into this pyramidal mold. A pyramidal tip mounted on a simple micropositioner was used for electrochemistry and patterning of a protein. First, an agarose hydrogel was cast with a hydrogel pen for the electrochemical reaction (HYPER). The redox reaction at the HYPER-electrode interface demonstrated the characteristics of an ultramicroelectrode or bulk electrode based on the contact area. Second, the pyramidal polydimethylsiloxane served as a polymer pen for the contact printing of silane on a glass substrate. After the successive immobilization of biotin and avidin with fluorescence labeling, the resulting fluorescence image demonstrated the successful patterning of the protein. This new process for the creation of a pyramidal mold, referred to as a "do-it-yourself" process, offers advantages to nonspecialists in nanotechnology compared to conventional lithography, specifically simplicity, rapidity, and low cost.

Fabrication of 2D Hetero-Complexes With Nucleic-Acid-Base Adenine and Fatty-Acid Stearic Acid at Liquid/Solid Interface

Designing and fabricating hetero-complexes composed of organic and biological compounds had become an exciting area referring to biological recognition, molecular devices etc. Here, hydrogen-bonded complex of nucleic-acid-base (adenine, A) and fatty-acid (stearic acid, SA) was designed, fabricated and investigated at liquid/solid interface. The interesting striped-shaped structure composed of SA-A-SA trimers was formed after introducing adenine molecules. Meanwhile, the primary lamella-shape characteristic of the assembly of SA molecules was kept because of the collaboration of non-covalent interactions of molecule-molecule and molecule-substrate. With a series of experimental characterization and theoretical simulation, the origination of the as-prepared 2D hetero-complexes was gradually exhibited from the assembled structures of two building blocks of stearic acid and adenine. Our study provides a blueprint for designing additional multi-component complexes based on the existing molecular assembled architectures.

Spin Selectivity in Photoinduced Charge-Transfer Mediated by Chiral Molecules

Optical control and readout of electron spin and spin currents in thin films and nanostructures have remained attractive yet challenging goals for emerging technologies designed for applications in information processing and storage. Recent advances in room-temperature spin polarization using nanometric chiral molecular assemblies suggest that chemically modified surfaces or interfaces can be used for optical spin conversion by exploiting photoinduced charge separation and injection from well-coupled organic chromophores or quantum dots. Using light to drive photoexcited charge-transfer processes mediated by molecules with central or helical chirality enables indirect measurements of spin polarization attributed to the chiral-induced spin selectivity effect and of the efficiency of spin-dependent electron transfer relative to competitive relaxation pathways. Herein, we highlight recent approaches used to detect and to analyze spin selectivity in photoinduced charge transfer including spin-transfer torque for local magnetization, nanoscale charge separation and polarization, and soft ferromagnetic substrate magnetization- and chirality-dependent photoluminescence. Building on these methods through systematic investigation of molecular and environmental parameters that influence spin filtering should elucidate means to manipulate electron spins and photoexcited states for room-temperature optoelectronic and photospintronic applications.

Photoinduced Charge Transfer in Single-Molecule p-n Junctions

We measured photoinduced charge separation in isolated individual C₆₀-tethered 2,5-dithienylpyrrole triad (C₆₀ triad) molecules with submolecular resolution using a custom-built laser-assisted scanning tunneling microscope. Laser illumination was introduced evanescently into the tunneling junction through total internal reflection, and the changes in tunneling current and electronic spectra caused by photoexcitation were measured and spatially resolved. Photoinduced charge separation was not detected for all C₆₀ triad molecules, indicating that the conformations of the molecules may affect the excitation probability, lifetime, and/or charge distribution. A photoinduced signal was not observed for dodecanethiol molecules in the surrounding matrix or for control molecules without C₆₀ moieties, as neither absorbs incident photons at this energy. This spectroscopic imaging technique has the potential to elucidate detailed photoinduced carrier dynamics, which are inaccessible via ensemble-scale (i.e., averaging) measurements, which can be used to direct the rational design and optimization of molecular p-n junctions and assemblies for energy harvesting.

Tuning of the gold work function by carborane films studied using density functional theory

Using density functional theory including van der Waals interactions, we calculate the adsorption and electronic properties of dithiol-dicarba-closo-dodecaboranes chemisorbed on Au(111) surfaces. Carborane molecules consist of a cage-like structure made of boron and carbon atoms and possess a large intrinsic dipole. We consider two functionalized carborane positional isomers, with thiol linker groups attached to either carbon or boron backbone atoms, such that when adsorbed on the Au substrate, the molecular dipole points towards the metal surface or away from it. We investigate a large number of junction geometries and find that carborane adsorption can induce significant changes in the work function of the Au substrate, in the range of 1 eV. These changes depend strongly on the interface geometry at the atomistic level. From the analysis of these junction structures, we provide a picture of the driving mechanisms that determine adsorption geometries, and relate them to interface electronic structure and resulting work function modification. In particular, our results highlight the important role played in these interface quantities by distortions in the Au surface layer induced by carborane adsorption.

Adsorption of Stearic Acid at the Iron Oxide/Oil Interface: Theory, Experiments, and Modeling

Improved friction performance is an important objective of equipment manufacturers for meeting improved energy efficiency demands. The addition of friction-reducing additives, or friction modifiers (FMs), to lubricants is a key part of the strategy. The performance of these additives is related to their surface activity and their ability to form adsorbed layers on the metal surface. However, the extent of surface coverage (mass per unit area) required for effective friction reduction is currently unknown. In this article, we show that full coverage is not necessary for significant friction reduction. We first highlight various features of surface adsorption that can influence the surface coverage, packing, and free energy of adsorption of organic FMs on iron oxide surfaces. Using stearic acid in heptane and hexadecane as model lubricant formulations, we employ a combination of experiments and molecular dynamics (MD) simulations to show how the dimerization of acid molecules in the bulk solvent and the crystallographic orientation of the surface modifies surface adsorption. In addition, we show that the solvent can strongly influence the adsorption kinetics, and MD simulations reveal that hexadecane tends to align on the surface, increasing the energy barrier for the adsorption of stearic acid to the surface. Furthermore, we present a combined approach using MD and molecular thermodynamic theory to calculate adsorption isotherms for stearic acid on iron oxide surfaces, which agrees well with experimental data obtained with a quartz crystal microbalance (QCM). Our results suggest that while the friction of systems lubricated with organic FMs decreases with increasing coverage, complete coverage of the surface is neither practically achievable nor necessary for effective friction reduction for the systems and conditions studied here.

Hierarchically patterned striped phases of polymerized lipids: toward controlled carbohydrate presentation at interfaces

Microcontact printing can be used to generate well-defined microscopic areas of striped phases of both single-chain and dual-chain amphiphiles.

Finely Tunable Surface Wettability by Two-Dimensional Molecular Manipulation

Local molecular environment governs material interface properties, especially the substrate's exposing behavior and overall functionality expression. Although current techniques can provide efficient surface property modification, challenges in molecule spatial distribution and composition controls limited the generation of homogeneous and finely tunable molecular environment. In this study, Au-thiolate rupturing operation in chemical lift-off lithography (CLL) is used to manipulate the substrate interface molecular environment. The creation of randomly distributed artificial self-assembled monolayer defects generates vacancies for substrate property modification through back-insertion of molecules with opposite functionalities. Surface wettability adjustment is utilized as an example, where well-controllable molecule distribution provides finely tunable substrate affinity toward liquids with different physical properties. The distinct property difference between two surface regions assists microdroplet formation when liquids flow through, not only water solution but also low-surface-tension organic liquids. These microdroplet arrays become a template to guide material assembly in its formation process and act as pH-sensitive platforms for high-throughput detection. Furthermore, the tunability of the molecular pattern in this approach helps minimize the coffee-ring effect and the sweet-spot issue in matrix-assisted laser desorption/ionization mass spectrometry. Two-dimensional molecular manipulation in the CLL operation, therefore, holds the capability toward controlling homogeneous material surface property and toward exhibiting behavior adjustments.

Supramolecular Assemblies on Surfaces: Nanopatterning, Functionality, and Reactivity

Understanding how molecules interact to form large-scale hierarchical structures on surfaces holds promise for building designer nanoscale constructs with defined chemical and physical properties. Here, we describe early advances in this field and highlight upcoming opportunities and challenges. Both direct intermolecular interactions and those that are mediated by coordinated metal centers or substrates are discussed. These interactions can be additive, but they can also interfere with each other, leading to new assemblies in which electrical potentials vary at distances much larger than those of typical chemical interactions. Earlier spectroscopic and surface measurements have provided partial information on such interfacial effects. In the interim, scanning probe microscopies have assumed defining roles in the field of molecular organization on surfaces, delivering deeper understanding of interactions, structures, and local potentials. Self-assembly is a key strategy to form extended structures on surfaces, advancing nanolithography into the chemical dimension and providing simultaneous control at multiple scales. In parallel, the emergence of graphene and the resulting impetus to explore 2D materials have broadened the field, as surface-confined reactions of molecular building blocks provide access to such materials as 2D polymers and graphene nanoribbons. In this Review, we describe recent advances and point out promising directions that will lead to even greater and more robust capabilities to exploit designer surfaces.

Small-Molecule Patterning via Prefunctionalized Alkanethiols

Interactions between small molecules and biomolecules are important physiologically and for biosensing, diagnostic, and therapeutic applications. To investigate these interactions, small molecules can be tethered to substrates through standard coupling chemistries. While convenient, these approaches co-opt one or more of the few small-molecule functional groups needed for biorecognition. Moreover, for multiplexing, individual probes require different surface functionalization chemistries, conditions, and/or protection/deprotection strategies. Thus, when placing multiple small-molecules on surfaces, orthogonal chemistries are needed that preserve all functional groups and are sequentially compatible. Here, we approach high-fidelity small-molecule patterning by coupling small-molecule neurotransmitter precursors, as examples, to monodisperse asymmetric oligo(ethylene glycol)alkanethiols during synthesis and prior to self-assembly on Au substrates. We use chemical lift-off lithography to singly and doubly pattern substrates. Selective antibody recognition of pre-functionalized thiols was comparable to or better than recognition of small molecules functionalized to alkanethiols after surface assembly. These findings demonstrate that synthesis and patterning approaches that circumvent sequential surface conjugation chemistries enable biomolecule recognition and afford gateways to multiplexed small-molecule functionalized substrates.

Acid-Base Control of Valency within Carboranedithiol Self-Assembled Monolayers: Molecules Do the Can-Can

We use simple acid-base chemistry to control the valency in self-assembled monolayers of two different carboranedithiol isomers on Au{111}. Monolayer formation proceeds via Au-S bonding, where manipulation of pH prior to or during deposition enables the assembly of dithiolate species, monothiol/monothiolate species, or combination. Scanning tunneling microscopy (STM) images identify two distinct binding modes in each unmodified monolayer, where simultaneous spectroscopic imaging confirms different dipole offsets for each binding mode. Density functional theory calculations and STM image simulations yield detailed understanding of molecular chemisorption modes and their relation with the STM images, including inverted contrast with respect to the geometric differences found for one isomer. Deposition conditions are modified with controlled equivalents of either acid or base, where the coordination of the molecules in the monolayers is controlled by protonating or deprotonating the second thiol/thiolate on each molecule. This control can be exercised during deposition to change the valency of the molecules in the monolayers, a process that we affectionately refer to as the "can-can." This control enables us to vary the density of molecule-substrate bonds by a factor of 2 without changing the molecular density of the monolayer.

Se-C Cleavage of Hexane Selenol at Steps on Au(111)

Selenols are considered as an alternative to thiols in self-assembled monolayers, but the Se-C bond is one limiting factor for their usefulness. In this study, we address the stability of the Se-C bond by a combined experimental and theoretical investigation of gas-phase-deposited hexane selenol (CH₃(CH₂)₅SeH) on Au(111) using photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory (DFT). Experimentally, we find that initial adsorption leaves atomic Se on the surface without any carbon left on the surface, whereas further adsorption generates a saturated selenolate layer. The Se 3d component from atomic Se appears at 0.85 eV lower binding energy than the selenolate-related component. DFT calculations show that the most stable structure of selenols on Au(111) is in the form of RSe-Au-SeR complexes adsorbed on the unreconstructed Au(111) surface. This is similar to thiols on Au(111). Calculated Se 3d core-level shifts between elemental Se and selenolate in this structure nicely reproduce the experimentally recorded shifts. Dissociation of RSeH and subsequent formation of RH are found to proceed with high barriers on defect-free Au(111) terraces, with the highest barrier for scissoring R-Se. However, at steps, these barriers are considerably lower, allowing for Se-C bond breaking and hexane desorption, leaving elemental Se at the surface. Hexane is formed by replacing the Se-C bond with a H-C bond by using the hydrogen liberated from the selenol to selenolate transformation.

Structural Polymorphism as the Result of Kinetically Controlled Self-Assembly

Traditionally, the goal of self-assembly and supramolecular chemistry is to engineer an equilibrium structure with a desired geometry and functionality; this is achieved through careful choice of molecular monomers, growth conditions, and substrate. Supramolecular assemblies produced under nonequilibrium conditions, in contrast, can form metastable structures with conformations quite different from those accessible in equilibrium self-assembly. The study of nonequilibrium growth of clusters potentially impacts the study of nucleation in atmospheric aerosols, nucleation in organic crystallization, and mesoscale organization for systems ranging from biological molecules to molecular electronics. In our experiments, we prepare surface monolayers of small organic and organometallic molecules through direct injection of a solution onto a substrate in high vacuum. During this process, the rapid evaporation of small solution droplets in high vacuum can lead to nonequilibrium growth conditions. The resulting structures are then characterized by scanning tunneling microscopy. Among the features observed in these experiments are cyclic, hydrogen-bonded pentamers. For carboxylic acids, the two-molecule ring dimer is the common binding motif. Large, cyclic hydrogen-bonded systems are uncommon, especially so for rings with five members. Despite this, pentagonal clusters appear to be a general phenomenon for systems containing adjacent strong and weak hydrogen-bonding elements on five-member aromatic rings. Regular pentamers have been observed as metastable structures for ferrocenecarboxylic acid, indole-2-carboxylic acid, and isatin (1-H-indole-2,3-dione). Electronic structure calculations confirm the relative stability of these structures with respect to the dimer or catemer conformations which are observed in the solid-state crystal structures. For ferrocenecarboxylic acid, cyclic pentamers undergo further self-assembly, resulting in long-range order in conjunction with local 5-fold rotational symmetry. This system is the first reported self-assembled molecular quasicrystal, and it remains the only example of a hydrogen-bonded quasicrystal. This supramolecular structure forms as a result of the cocrystallization of hydrogen-bonded cyclic pentamers with intercalated molecular dimers. The shared bonding to a single dimer is responsible for locking the adjacent pentamers in specific distances and orientations, which produces the quasicrystal. Careful analysis of experimental data provides evidence that, in some cases, metastable clusters are formed in solution and then subsequently adsorb on the surface. This is a unusual mechanism for supramolecular assembly, and it has important implications for understanding questions in crystal growth, namely: what the initial stages of crystal growth are as molecules are first precipitating from solution; what role the solvent plays in determining crystal structure; and whether solvent-mediated clustering is important in the broader phenomenon of solid-state polymorphism.

Self-standing aptamers by an artificial defect-rich matrix

The classical alkanethiol post-passivation can prevent nonspecific binding of nucleotide bases onto supporting substrates and help aptamers transition from a "lying down" to a "standing up" orientation. However, the surface probes display lower binding affinity towards targets than those in bulk solutions due to unsatisfied hybridization spaces on the alkanethiol passivated substrate. To overcome this challenge, an artificial defect-rich matrix possessing an aptamer "self-standing" property created by chemical lift-off lithography (CLL) is demonstrated. This approach provided artificial defects on a hydroxyl-terminated alkanethiol self-assembled monolayer (SAM), which allowed the insertion of thiolated aptamers. The diluted surface molecular environment assisted aptamers not only to "self-stand" on the surface, but also to separate from each other, providing a suitable surface aptamer density and sufficient space for capturing targets. With this approach, the binding affinity of the aptamer towards a target was comparable to solution-type probes, showing higher recognition efficiency than that in conventional methods.

Hierarchically Patterned Noncovalent Functionalization of 2D Materials by Controlled Langmuir-Schaefer Conversion

Noncovalent monolayer chemistries are often used to functionalize 2D materials. Nanoscopic ligand ordering has been widely demonstrated (e.g., lying-down lamellar phases of functional alkanes); however, combining this control with micro- and macroscopic patterning for practical applications remains a significant challenge. A few reports have demonstrated that standing phase Langmuir films on water can be converted into nanoscopic lying-down molecular domains on 2D substrates (e.g., graphite), using horizontal dipping (Langmuir-Schaefer, LS, transfer). Molecular patterns are known to form at scales up to millimeters in Langmuir films, suggesting the possibility of transforming such structures into functional patterns on 2D materials. However, to our knowledge, this approach has not been investigated, and the rules governing LS conversion are not well understood. In part, this is because the conversion process is mechanistically very different from classic LS transfer of standing phases; challenges also arise due to the need to characterize structure in noncovalently adsorbed ligand layers <0.5 nm thick, at scales ranging from millimeters to nanometers. Here, we show that scanning electron microscopy enables diynoic acid lying-down phases to be imaged across this range of scales; using this structural information, we establish conditions for LS conversion to create hierarchical microscopic and nanoscopic functional patterns. Such control opens the door to tailoring noncovalent surface chemistry of 2D materials to pattern local interactions with the environment.

Large Area Nanoparticle Alignment by Chemical Lift-Off Lithography

Nanoparticle alignment on the substrate attracts considerable attention due to its wide application in different fields, such as mechanical control, small size electronics, bio/chemical sensing, molecular manipulation, and energy harvesting. However, precise nanoparticle positioning and deposition control with high fidelity are still challenging. Herein, a straightforward strategy for high quality nanoparticle-alignment by chemical lift-off lithography (CLL) is demonstrated. This technique creates high resolution self-assembled monolayer (SAM) chemical patterns on gold substrates, enabling nanoparticle-selective deposition and precise alignment. The fabricated nanoparticle arrangement geometries and dimensions are well-controllable in a large area. With proper nanoparticle surface functionality control and adequate substrate molecular manipulation, well-defined nanoparticle arrays with single-particle-wide alignment resolution are achieved.

Wafer-scale bioactive substrate patterning by chemical lift-off lithography

The creation of bioactive substrates requires an appropriate interface molecular environment control and adequate biological species recognition with minimum nonspecific attachment. Herein, a straightforward approach utilizing chemical lift-off lithography to create a diluted self-assembled monolayer matrix for anchoring diverse biological probes is introduced. The strategy encompasses convenient operation, well-tunable pattern feature and size, large-area fabrication, high resolution and fidelity control, and the ability to functionalize versatile bioarrays. With the interface-contact-induced reaction, a preformed alkanethiol self-assembled monolayer on a Au surface is ruptured and a unique defect-rich diluted matrix is created. This post lift-off region is found to be suitable for insertion of a variety of biological probes, which allows for the creation of different types of bioactive substrates. Depending on the modifications to the experimental conditions, the processes of direct probe insertion, molecular structure change-required recognition, and bulky biological species binding are all accomplished with minimum nonspecific adhesion. Furthermore, multiplexed arrays via the integration of microfluidics are also achieved, which enables diverse applications of as-prepared substrates. By embracing the properties of well-tunable pattern feature dimension and geometry, great local molecular environment control, and wafer-scale fabrication characteristics, this chemical lift-off process has advanced conventional bioactive substrate fabrication into a more convenient route.

Nanopore Electrochemistry: A Nexus for Molecular Control of Electron Transfer Reactions

Pore-based structures occur widely in living organisms. Ion channels embedded in cell membranes, for example, provide pathways, where electron and proton transfer are coupled to the exchange of vital molecules. Learning from mother nature, a recent surge in activity has focused on artificial nanopore architectures to effect electrochemical transformations not accessible in larger structures. Here, we highlight these exciting advances. Starting with a brief overview of nanopore electrodes, including the early history and development of nanopore sensing based on nanopore-confined electrochemistry, we address the core concepts and special characteristics of nanopores in electron transfer. We describe nanopore-based electrochemical sensing and processing, discuss performance limits and challenges, and conclude with an outlook for next-generation nanopore electrode sensing platforms and the opportunities they present.

Surface functional DNA density control by programmable molecular defects

Spatially programmable molecular-level defects via straightforward chemical lift-off manipulation leads to the direct regulation of complex surface DNA densities.

Understanding coordination equilibria in solution and gel-phase [2]rotaxanes

An active-metal template approach has been use to synthesise solution and surface bound addressable [2]rotaxanes giving unique insights into thermodynamic equilibria in interlocked structures.

Boronic acids as molecular inks for surface functionalization of polyvinyl alcohol substrates

Boronic acids are proposed to be used as molecular inks for surface functionalization of polyvinyl alcohol substrates using marker pen applicators.

Artificial antibody created by conformational reconstruction of the complementary-determining region on gold nanoparticles

Mimicking protein-like specific interactions and functions has been a long-pursued goal in nanotechnology. The key challenge is to precisely organize nonfunctional surface groups on nanoparticles into specific 3D conformations to function in a concerted and orchestrated manner. Here, we develop a method to graft the complementary-determining regions of natural antibodies onto nanoparticles and reconstruct their “active” conformation to create nanoparticle-based artificial antibodies that recognize the corresponding antigens. Our work demonstrates that it is possible to create functions on nanoparticles by conformational engineering, namely tuning flexible surface groups into specific conformations. Our straightforward strategy could be used further to create other artificial antibodies for various applications and provides a new tool to understand the structure and folding of natural proteins.

To impart biomedical functions to nanoparticles (NPs), the common approach is to conjugate functional groups onto NPs by dint of the functions of those groups per se. It is still beyond current reach to create protein-like specific interactions and functions on NPs by conformational engineering of nonfunctional groups on NPs. Here, we develop a conformational engineering method to create an NP-based artificial antibody, denoted “Goldbody,” through conformational reconstruction of the complementary-determining regions (CDRs) of natural antibodies on gold NPs (AuNPs). The seemingly insurmountable task of controlling the conformation of the CDR loops, which are flexible and nonfunctional in the free form, was accomplished unexpectedly in a simple way. Upon anchoring both terminals of the free CDR loops on AuNPs, we managed to reconstruct the “active” conformation of the CDR loops by tuning the span between the two terminals and, as a result, the original specificity was successfully reconstructed on the AuNPs. Two Goldbodies have been created by this strategy to specifically bind with hen egg white lysozyme and epidermal growth factor receptor, with apparent affinities several orders of magnitude stronger than that of the original natural antibodies. Our work demonstrates that it is possible to create protein-like functions on NPs in a protein-like way, namely by tuning flexible surface groups to the correct conformation. Given the apparent merits, including good stability, of Goldbodies, we anticipate that a category of Goldbodies could be created to target different antigens and thus used as substitutes for natural antibodies in various applications.

Patterning of supported gold monolayers via chemical lift-off lithography

The supported monolayer of Au that accompanies alkanethiolate molecules removed by polymer stamps during chemical lift-off lithography is a scarcely studied hybrid material. We show that these Au-alkanethiolate layers on poly(dimethylsiloxane) (PDMS) are transparent, functional, hybrid interfaces that can be patterned over nanometer, micrometer, and millimeter length scales. Unlike other ultrathin Au films and nanoparticles, lifted-off Au-alkanethiolate thin films lack a measurable optical signature. We therefore devised fabrication, characterization, and simulation strategies by which to interrogate the nanoscale structure, chemical functionality, stoichiometry, and spectral signature of the supported Au-thiolate layers. The patterning of these layers laterally encodes their functionality, as demonstrated by a fluorescence-based approach that relies on dye-labeled complementary DNA hybridization. Supported thin Au films can be patterned via features on PDMS stamps (controlled contact), using patterned Au substrates prior to lift-off (e.g., selective wet etching), or by patterning alkanethiols on Au substrates to be reactive in selected regions but not others (controlled reactivity). In all cases, the regions containing Au-alkanethiolate layers have a sub-nanometer apparent height, which was found to be consistent with molecular dynamics simulations that predicted the removal of no more than 1.5 Au atoms per thiol, thus presenting a monolayer-like structure.