Molecule-substrate Interface Research Articles

Triboelectric Charging Properties of the Functional Groups of Common Pharmaceutical Materials Using Density Functional Theory Calculations.

Triboelectrification is a ubiquitous and poorly understood phenomenon in powder processing, particularly for pharmaceutical powders. Charged particles can adhere to vessel walls, causing sheeting; they can also cause agglomeration, threatening the stability of powder formulations, and in extreme cases electrostatic discharges, which present a serious fire and explosion hazard. Triboelectrification is highly sensitive to environmental and material conditions, which makes it very difficult to compare experimental results from different publications. In this work, density functional theory (DFT) is used to investigate the charge transfer characteristics of several functional groups of paracetamol in order to better understand the mechanisms of charging at the nanoscale and the influence of the environmental and material properties on charge transfer. This is achieved by studying the structure and electronic properties at the molecule-substrate interface. Using this molecule-substrate approach, the charging contributions of individual functional groups are explored by examining the Hirschfeld charges, the charge density difference between the molecule and substrate, the density of states, and the location of the frontier orbitals (HOMO and LUMO) of a paracetamol molecule. Charge density difference calculations indicate a significant transfer of charge from the molecule to the surface. Observable regions of electron density enrichment and depletion are evident around the electron-donating and -withdrawing groups, respectively. The density of states for the paracetamol molecule evolves as it approaches the surface, and the band gap disappears upon contact with the substrate. Hirshfeld charge analysis reveals asymmetry in the charge redistribution around the molecule, highlighting the varying charging tendencies of different atoms.

The Influence of the Substrate on the Functionality of Spin Crossover Molecular Materials.

Spin crossover complexes are a route toward designing molecular devices with a facile readout due to the change in conductance that accompanies the change in spin state. Because substrate effects are important for any molecular device, there are increased efforts to characterize the influence of the substrate on the spin state transition. Several classes of spin crossover molecules deposited on different types of surface, including metallic and non-metallic substrates, are comprehensively reviewed here. While some non-metallic substrates like graphite seem to be promising from experimental measurements, theoretical and experimental studies indicate that 2D semiconductor surfaces will have minimum interaction with spin crossover molecules. Most metallic substrates, such as Au and Cu, tend to suppress changes in spin state and affect the spin state switching process due to the interaction at the molecule-substrate interface that lock spin crossover molecules in a particular spin state or mixed spin state. Of course, the influence of the substrate on a spin crossover thin film depends on the molecular film thickness and perhaps the method used to deposit the molecular film.

Defective Fe Metal-Organic Frameworks Enhance Metabolic Profiling for High-Accuracy Diagnosis of Human Cancers.

Cancers heavily threaten human life; therefore, a high-accuracy diagnosis is vital to protect human beings from the suffering of cancers. While biopsies and imaging methods are widely used as current technologies for cancer diagnosis, a new detection platform by metabolic analysis is expected due to the significant advantages of fast, simple, and cost-effectiveness with high body tolerance. However, the signal of molecule biomarkers is too weak to acquire high-accuracy diagnosis. Herein, precisely engineered metal-organic frameworks for laser desorption/ionization mass spectrometry, allowing favorable charge transfer within the molecule-substrate interface and mitigated thermal dissipation by adjusting the phonon scattering with metal nodes, are developed. Consequently, a surprising signal enhancement of ≈10 000-fold is achieved, resulting in diagnosis of three major cancers (liver/lung/kidney cancer) with area-under-the-curve of 0.908-0.964 and accuracy of 83.2%-90.6%, which promises a universal detection tool for large-scale clinical diagnosis of human cancers.

Interfacial electronic properties of FeTPP-Cl on HOPG

Conjugated tetrapyrrole complexes have potential for novel spintronic and optoelectronic devices. Detailed understanding of electronic properties at molecule–substrate interface is essential for their potential applications. In this report, electronic properties of iron (III) chloride tetraphenylporphyrin (FeTPP-Cl, C44H28ClFeN4) thin films on highly oriented pyrolytic graphite (HOPG) have been investigated using photoemission and X-ray absorption spectroscopy. Photoemission analysis shows that no significant charge transfer takes between FeTPP-Cl and HOPG. Fe 2P3/2 core level analysis indicates towards dechlorination of FeTPP-Cl on HOPG in the monolayer regime. Fe L2,3 edge X-ray absorption spectroscopy reveal that iron oxidation state changes from +2 to +3 due to adsorption on to HOPG, suggesting a substrate driven dechlorination of FeTPP-Cl. Curve fitting analysis of XPS Fe 2p3/2 spectrum for the deposition of FeTPP-Cl on HOPG in the monolayer regime confirms +2 oxidation state of central metal atom. An interface dipole of 0.2 eV has been found at FeTPP-Cl/HOPG interface suggesting weaker molecule–substrate interactions.

Initial Stage of para-Hexaphenyl Thin-Film Growth Controlled by the Step Structure of the Ion-Beam-Modified TiO2(110) Surface

Organic electronics require a precise control over properties of a molecule–substrate interface as well as film growth morphology, from both fundamental points of view, when a clean vacuum environm...

Millimeter-Sized Two-Dimensional Molecular Crystalline Semiconductors with Precisely Defined Molecular Layers via Interfacial-Interaction-Modulated Self-Assembly.

The newly emerging field in organic electronics is to control the molecule-substrate interface properties at a two-dimensional (2D) limit via interfacial interactions, which paves the way for driving the molecular assembly for highly ordered 2D molecular crystalline films with precise molecular layers and large-area uniformity. Here, by exploiting molecule-substrate van der Waals (vdW) interactions, we demonstrate thermally induced self-assembly of 2D organic crystalline films exhibiting well-defined molecular layer number over a millimeter-sized area. The organic field-effect transistors (OFETs) with bilayer films show excellent electrical performance with a maximum mobility of 12.8 cm2 V-1 s-1. Moreover, we find that the monolayer films can act as interfacial molecular templates to construct heterojunctions with well-balanced ambipolar transport behaviors. The capability of thermally induced self-assembly of 2D molecular crystalline films with controllable molecular layers and scale-up coverage opens up a way for realizing complicated electronic applications, such as lateral heterojunctions and superlattices.

Quasi-ordered C60 molecular films grown on the pseudo-ten-fold (1 0 0) surface of the Al13Co4 quasicrystalline approximant

The growth of C60 films on the pseudo-ten-fold (1 0 0) surface of the orthorhombic Al13Co4 quasicrystalline approximant was studied experimentally by scanning tunneling microscopy, low-energy electron diffraction and photoemission spectroscopy. The (1 0 0) surface terminates at bulk-planes presenting local atomic configurations with five-fold symmetry—similar to quasicrystalline surfaces. While the films deposited at room temperature were found disordered, high-temperature growth (up to 693 K) led to quasi-ordered molecular films templated on the substrate rectangular unit mesh. The most probable adsorption sites and geometries were investigated by density functional theory (DFT) calculations. A large range of adsorption energies was determined, influenced by both symmetry and size matching at the molecule-substrate interface. The quasi-ordered structure of the film can be explained by C60 adsorption at the strongest adsorption sites which are too far apart compared to the distance minimizing the intermolecular interactions, resulting in some disorder in the film structure at a local scale. Valence band photoemission indicates a broadening of the molecular orbitals resulting from hybridization between the substrate and overlayer electronic states. Dosing the film at temperature above 693 K led to molecular damage and formation of carbide thin films possessing no azimuthal order with respect to the substrate.

C60 as an Atom Trap to Capture Co Adatoms

C60 molecules were used to trap Co adatoms and clusters on a Au(111) surface using atomic/molecular manipulation with a scanning tunneling microscope. Two manipulation pathways (successive integration of single Co atoms in one molecule or direct integration of a Co cluster) were found to efficiently allow the formation of complexes mixing a C60 molecule with Co atoms. Scanning tunneling spectroscopy reveals the robustness of the π states of C60 that are preserved after Co trapping. Scanning tunneling microscopy images and density functional theory calculations reveal that dissociated Co clusters of up to nine atoms can be formed at the molecule–substrate interface. These results open new perspectives in the interactions between metal adatoms and molecules, for applications in metal–organic devices.

Self-Organized Molecular Films with Long-Range Quasiperiodic Order

Self-organized molecular films with long-range quasiperiodic order have been grown by using the complex potential energy landscape of quasicrystalline surfaces as templates. The long-range order arises from a specific subset of quasilattice sites acting as preferred adsorption sites for the molecules, thus enforcing a quasiperiodic structure in the film. These adsorption sites exhibit a local 5-fold symmetry resulting from the cut by the surface plane through the cluster units identified in the bulk solid. Symmetry matching between the C60 fullerene and the substrate leads to a preferred adsorption configuration of the molecules with a pentagonal face down, a feature unique to quasicrystalline surfaces, enabling efficient chemical bonding at the molecule-substrate interface. This finding offers opportunities to investigate the physical properties of model 2D quasiperiodic systems, as the molecules can be functionalized to yield architectures with tailor-made properties.

Emergence of On-Surface Magnetochemistry

The control of exchange coupling across the molecule–substrate interface is a key feature in molecular spintronics. This Perspective reviews the emerging field of on-surface magnetochemistry, where coordination chemistry is applied to surface-supported metal porphyrins and metal phthalocyanines to control their magnetic properties. The particularities of the surface as a multiatomic ligand or “surface ligand” are introduced. The asymmetry involved in the action of a chemical ligand and a surface ligand on the same planar complexes modifies the well-established “trans effect” to the notion of the “surface-trans effect”. As ad-complexes on ferromagnetic substrates are usually exchange-coupled, the magnetochemical implications of the surface-trans effect are of particular interest. The combined action of the different ligands allows for the reproducible control of spin states in on-surface supramolecular architectures and opens up new ways toward building and operating spin systems at interfaces. Notably, sp...

Site-Dependent Donation/Backdonation Charge Transfer at the CoPc/Ag(111) Interface

The organic/metal interface formed upon adsorption of cobalt(II) phthalocyanine (CoPc) molecules on a flat Ag(111) single crystal was investigated using a combination of scanning tunneling microscopy (STM) and photoemission spectroscopy (PES). A flat-lying molecular adsorption with the π conjugated phthalocyanine ligand parallel to the substrate was found to lead to an effective molecule-substrate coupling which governs a template-guided molecular growth. A voltage polarity dependence at the cobalt ion site was emphasized and correlated with the Co 2p core level spectra evolution which sustains an interface-confined reduction effect of the cobalt oxidation state. The formation of interface dipoles was observed via monitoring the changes in the work function (WF) upon deposition. The observations are discussed on the basis of a site-dependent donation/backdonation charge transfer at the molecule-substrate interface.

Control of the axial coordination of a surface-confined manganese(III) porphyrin complex

The organization and thermal lability of chloro(5,10,15,20-tetraphenylporphyrinato)manganese(III) (Cl-MnTPP) molecules on the Ag(111) surface have been investigated under ultra-high vacuum conditions, using scanning tunnelling microscopy, low energy electron diffraction and x-ray photoelectron spectroscopy. The findings reveal the epitaxial nature of the molecule–substrate interface, and moreover, offer a valuable insight into the latent coordination properties of surface-confined metalloporphyrins. The Cl-MnTPP molecules are found to self-assemble on the Ag(111) surface at room temperature, forming an ordered molecular overlayer described by a square unit cell. In accordance with the threefold symmetry of the Ag(111) surface, three rotationally equivalent domains of the molecular overlayer are observed. The primitive lattice vectors of the Cl-MnTPP overlayer show an azimuthal rotation of ±15° relative to those of the Ag(111) surface, while the principal molecular axes of the individual molecules are found to be aligned with the substrate and crystallographic directions. The axial chloride (Cl) ligand is found to be orientated away from the Ag(111) surface, whereby the average plane of the porphyrin macrocycle lies parallel to that of the substrate. When adsorbed on the Ag(111) surface, the Cl-MnTPP molecules display a latent thermal lability resulting in the dissociation of the axial Cl ligand at ∼423 K. The thermally induced dissociation of the Cl ligand leaves the porphyrin complex otherwise intact, giving rise to the coordinatively unsaturated Mn(III) derivative. Consistent with the surface conformation of the Cl-MnTPP precursor, the resulting (5,10,15,20-tetraphenylporphyrinato)manganese(III) (MnTPP) molecules display the same lattice structure and registry with the Ag(111) surface.

Ion‐Beam‐Induced Desorption as a Method for Probing the Stability of the Molecule‐Substrate Interface in Self‐Assembled Monolayers

Ion-induced desorption and neutral fragments mass spectrometry are used to analyze the chemical stability of the molecule–substrate interface in a homologue series of selenium-based aromatic SAMs on a Au(111) substrate (see picture). The AuSe bond is more stable than the AuS bond and the stability of the molecule–substrate interface shows an odd–even effect.

Inducing Nonlocal Reactions with a Local Probe

The scanning tunneling microscope (STM) has evolved continually since its invention, as scientists have expanded its use to encompass atomic-scale manipulation, momentum-resolved electronic characterization, localized chemical reactions (bond breaking and bond making) in adsorbed molecules, and even chain reactions at surfaces. This burgeoning field has recently expanded to include the use of the STM to inject hot electrons into substrate surface states; the injected electrons can travel laterally and induce changes in chemical structure in molecules located up to 100 nm from the STM tip. We describe several key demonstrations of this phenomenon, including one appearing in this issue of ACS Nano by Chen et al. Possible applications for this technique are also discussed, including characterizing the dispersion of molecule-substrate interface states and the controlled patterning of molecular overlayers.

Nonlocal Chemical Reactivity at Organic−Metal Interfaces

Understanding electron transport at the organic-inorganic interface is crucial for many research fields including surface physics and chemistry. In this article, we report the nonlocal chemical reactivity of one monolayer copper hexadecafluorophthalocyanine (F(16)CuPc) adsorbed on two different substrates, Ag(111) and Au(111), by injecting hot electrons from a scanning tunneling microscope tip. On the basis of experimental and theoretical results, the nonlocal reactions are proposed to strongly depend on hot-electron transport through molecule-substrate interface states. This observation of nonlocal reactivity increases our understanding of electron transport at organic-metal interfaces.

Adsorption-Induced Switching of Magnetic Anisotropy in a Single Iron(II) Phthalocyanine Molecule on an Oxidized Cu(110) Surface

We examined the zero-field splitting of an iron(II) phthalocyanine (FePc) attached to clean and oxidized Cu(110) surfaces and the dependence on an applied magnetic field by inelastic electron tunneling spectroscopy with STM. The symmetry of the ligand field surrounding the Fe atom is lowered on the oxidized surface, switching the magnetic anisotropy from the easy plane of the bulk to the easy axis. The zero-field splitting was not observed for FePc on a clean Cu(110) surface, and the spin state converts from triplet to singlet due to the strong coupling of Fe d states with the Cu substrate, as is also confirmed by photoelectron spectroscopy. These findings demonstrate the importance of coupling at the molecule-substrate interface for manipulating the magnetic properties of adsorbates.

Spin Excitation of a Single Iron (II) Phthalocyanine Molecule on the Cu(110) Surface

We investigated the zero field splitting (ZFS) of a single iron (II) phthalocyanine (FePc) molecule on clean Cu(110) and Cu(110) (2×1)-O surfaces mainly by using inelastic electron tunneling spectroscopy (IETS) with STM. On the Cu(110) (2×1)-O surface, the excitations between the spin states were observed. Adsorption of FePc on the (2×1)-O surface reduces the symmetry of the ligand field for the Fe atom, resulting in the switching of the easy-magnetization direction from the easy-plane to easy-axis. On the clean Cu(110) surface, there is no signal relevant to the excitation. The spin state converts from triplet to singlet due to the stronger coupling with the surface as is confirmed by photoelectron spectroscopy. These findings demonstrate the importance of coupling at molecule-substrate interface for manipulating the magnetic properties of adsorbates.

Submolecular Potential Profiling Across Organic Monolayers

Potential profiles across molecular layers are constructed by means of noncontact electrically stimulated photoelectron spectroscopy, probing for the first time the molecule-substrate interface potential and resolving local screening effects across inner phenyl groups.

Studies on Surface and Interface with X-ray Absorption Fine Structure (XAFS). XAFS Study of Self-Assembled Monolayers.

The molecular orientation and the molecule-substrate interface structure of self-assembled monolayers of alkanethiol and its related molecules have been investigated by use of X-ray absorption fine structure (XAFS). The molecular orientation is mainly dependent on the molecular density, while for the ordering of molecular arrangement, which is necessary for self-assembly, the key factor is the balance between structures demanded by intermolecular and molecule/substrate interactions. The internal degree of freedom of the molecule plays an important role in the balancing. In the case of alkanethiols this condition is fulfilled at the saturated coverage, while in the case of a thiophene-substituted alkanethiol, the interaction between thiophene moieties prevent its monolayer from self-assembling at the saturated coverage but leads to form a self-assembled monolayer at a lower coverage. The molecular orientation in a monolayer exerts a strong effect on the orientation in a multilayer that is formed on the monolayer. This suggests that the structure of three-dimensionally self-assembled molecular aggregation can be controlled by using self-assembled monolayers as the template.

Local structures and electrical properties of organic molecular films investigated by non-contact atomic force microscopy

Local structures and electrical properties of ferroelectric molecular films on a KCl(0 0 1) surface as well as π-conjugated oligothiophene thin films on a metal substrate were investigated by non-contact atomic force microscopy (NC-AFM). The ‘ferroelectric’ molecules, vinylidene fluoride (VDF) oligomers [CF 3–(CH 2CF 2) 17–I], with large dipole moment due to the difference in the electron affinity between the fluorine and the hydrogen atoms have recently attracted much interest in terms of the dipole control of the single molecule. NC-AFM investigations of the VDF oligomer thin films revealed the initial stage of the epitaxial growth process on the KCl substrate. Furthermore, Kelvin probe force microscopy (KFM) was used to map the local surface potential of single and double molecular films of oligothiophene molecules deposited on a platinum substrate. The results indicated that the charge transfer at the molecule–substrate interface caused the surface potential of 190 mV on the molecular film against the Pt substrate in a dark environment. Photoinduced potential of the film under ultraviolet light irradiation was also investigated.