Organic Overlayer Research Articles

Ultrathin covalent organic overlayers on metal nanocrystals for highly selective plasmonic photocatalysis

Metal nanoparticle-organic interfaces are common but remain elusive for controlling reactions due to the complex interactions of randomly formed ligand-layers. This paper presents an approach for enhancing the selectivity of catalytic reactions by constructing a skin-like few-nanometre ultrathin crystalline porous covalent organic overlayer on a plasmonic nanoparticle surface. This organic overlayer features a highly ordered layout of pore openings that facilitates molecule entry without any surface poisoning effects and simultaneously endows favourable electronic effects to control molecular adsorption–desorption. Conformal organic overlayers are synthesised through the plasmonic oxidative activation and intermolecular covalent crosslinking of molecular units. We develop a light-operated multicomponent interfaced plasmonic catalytic platform comprising Pd-modified gold nanoparticles inside hollow silica to achieve the highly efficient and selective semihydrogenation of alkynes. This approach demonstrates a way to control molecular adsorption behaviours on metal surfaces, breaking the linear scaling relationship and simultaneously enhancing activity and selectivity.

Long‐Lived Charges in Y6:PM6 Bulk‐Heterojunction Photoanodes with a Polymer Overlayer Improve Photoelectrocatalytic Performance

AbstractPhotogenerating charges with long lifetimes to drive catalysis is challenging in organic semiconductors. Here, the role of a PM6 polymer overlayer on the photoexcited carrier dynamics is investigated in a Y6:PM6 bulk‐heterojunction (BHJ) photoanode undergoing ascorbic acid oxidation. With the additional polymer layer, the hole lifetime is increased in the solid state BHJ film. When the photoanode is electrically coupled to a hydrogen‐evolving platinum cathode, remarkably long‐lived hole polaron states are observed on the timescale of seconds under operational conditions. It is demonstrated that these long‐lived holes enable the organic photoanode with the polymer overlayer to show enhanced ascorbic acid oxidation performance, reaching ≈7 mA cm−2 at 1.23 VRHE without a co‐catalyst. An external quantum efficiency of 18% is observed using 850 nm excitation. It is proposed that the use of an organic overlayer can be an effective design strategy for generating longer charge carrier lifetimes in organic photoanodes for efficient oxidation catalysis.

Molecular Formula Prediction for Chemical Filtering of 3D OrbiSIMS Datasets.

Modern mass spectrometry techniques produce a wealth of spectral data, and although this is an advantage in terms of the richness of the information available, the volume and complexity of data can prevent a thorough interpretation to reach useful conclusions. Application of molecular formula prediction (MFP) to produce annotated lists of ions that have been filtered by their elemental composition and considering structural double bond equivalence are widely used on high resolving power mass spectrometry datasets. However, this has not been applied to secondary ion mass spectrometry data. Here, we apply this data interpretation approach to 3D OrbiSIMS datasets, testing it for a series of increasingly complex samples. In an organic on inorganic sample, we successfully annotated the organic contaminant overlayer separately from the substrate. In a more challenging purely organic human serum sample we filtered out both proteins and lipids based on elemental compositions, 226 different lipids were identified and validated using existing databases, and we assigned amino acid sequences of abundant serum proteins including albumin, fibronectin, and transferrin. Finally, we tested the approach on depth profile data from layered carbonaceous engine deposits and annotated previously unidentified lubricating oil species. Application of an unsupervised machine learning method on filtered ions after performing MFP from this sample uniquely separated depth profiles of species, which were not observed when performing the method on the entire dataset. Overall, the chemical filtering approach using MFP has great potential in enabling full interpretation of complex 3D OrbiSIMS datasets from a plethora of material types.

C60ThSe2/ITO interface formation: photoemission-based charge-transfer recognition for organic electronics applications.

Within the presented work, we propose a complex photoemission-based approach for the investigation of the C60ThSe2 dyad (C60ThSe2)/indium-tin oxide (ITO) interface formation. For surface topography and basic morphology determination, atomic force microscopy was utilized, and the results showed that C60ThSe2 agglomerated into close-to-spherical crystallites and the island-like growth was the dominant type for fullerene growth on the ITO substrate. Further, detailed X-ray and UV-photoelectron spectroscopies (XPS, UPS) were used for thorough characterization of the chemical and electronic properties of the investigated structures. Experiments were conducted by means of cyclic voltammetry and UV-VIS techniques for both deposition purposes and for determination of the basic electronic structure. As a result, we present the detailed characterization of the chemical and energy structures with a clear designation of the mutual influence of both materials on their counterparts. Among others, the accurate photoemission signal decomposition of the overlapping signals was done with respect to obtaining the energy-related information depth. The obtained data clearly showed that an interface dipole (0.56 eV) was created between the ITO substrate and organic overlayer at the ultrathin coverage stage. Since our results point out the most probable charge-carrier relocation in the vicinity of the interface, this together with the dipole existence should be taken into account while creating energy-level cascades for various (e.g., photovoltaic or organic electronic) applications. The work may also provide insights for engineers working with a vast range of organic-based electronics while designing devices based on fullerene/ITO hybrid structures.

Valorization of Brewery Wastes for the Synthesis of Silver Nanocomposites Containing Orthophosphate.

Brewery wastes from stage 5 (Wort precipitate: BW5) and stage 7 (Brewer’s spent yeast: BW7) were valorized for the synthesis of silver phosphate nanocomposites. Nanoparticles were synthesized by converting silver salt in the presence of brewery wastes at different temperatures (25, 50, and 80 °C) and times (10, 30, and 120 min). Unexpectedly, BW7 yielded Ag3PO4 nanoparticles with minor contents of AgCl and Ag metal (Agmet). Contrastingly, BW5 produced AgCl nanoparticles with minor amounts of Ag3PO4 and Agmet. Nanocomposites with different component ratios were obtained by simply varying the synthesis temperature and time. The morphology of the nanocomposites contained ball-like structures representative of Ag3PO4 and stacked layers and fused particles representing AgCl and Agmet. The capping on the nanoparticles contained organic groups from the brewery by-products, and the surface overlayer had a rich chemical composition. The organic overlayers on BW7 nanocomposites were thinner than those on BW5 nanocomposites. Notably, the nanocomposites exhibited high antibacterial activity against Escherichia coli ATCC 25922. The antibacterial activity was higher for BW7 nanocomposites due to a larger silver phosphate content in the composition and a thin organic overlayer. The growth of Agmet in the structure adversely affected the antimicrobial property of the nanocomposites.

Polarization-Driven Asymmetric Electronic Response of Monolayer Graphene to Polymer Zwitterions Probed from Both Sides.

We investigated the nature of graphene surface doping by zwitterionic polymers and the implications of weak in-plane and strong through-plane screening using a novel sample geometry that allows direct access to either the graphene or the polymer side of a graphene/polymer interface. Using both Kelvin probe and electrostatic force microscopies, we observed a significant upshift in the Fermi level in graphene of ∼260 meV that was dominated by a change in polarizability rather than pure charge transfer with the organic overlayer. This physical picture is supported by density functional theory (DFT) calculations, which describe a redistribution of charge in graphene in response to the dipoles of the adsorbed zwitterionic moieties, analogous to a local DC Stark effect. Strong metallic-like screening of the adsorbed dipoles was observed by employing an inverted geometry, an effect identified by DFT to arise from a strongly asymmetric redistribution of charge confined to the side of graphene proximal to the zwitterion dipoles. Transport measurements confirm n-type doping with no significant impact on carrier mobility, thus demonstrating a route to desirable electronic properties in devices that combine graphene with lithographically patterned polymers.

Cobalt Metal–Organic Framework Ultrathin Cocatalyst Overlayer for Improved Photoelectrochemical Activity of Ti-Doped Hematite

The poor conductivity and sluggish kinetics of hematite (α-Fe2O3) limit its photoelectrochemical (PEC) performance. Herein, a cobalt metal–organic framework (Co-MOF) ultrathin overlayer is in situ-...

WO3 quantum-dots electrochromism

In the past decades, as an alternative to traditional sputtering deposition process, atmospheric pressure solution-based deposition (APSD), considered as a cost-effective method for the construction of nanostructured electrochromic (EC) films with improved EC performance, is widely studied. However, the inadequate EC performance of the films, especially the poor cycle stability, impedes the development of solution-processed EC films. This paper reports excellent EC performance results for WO3 quantum-dots films prepared by a common APSD process with either Li+ or Al3+ electrolyte: a large optical contrast (97.8% and 94.1% at 633 nm), a fast switching speed (4.5 s and 13.5 s for coloring, 4 s and 10 s for bleaching) and an ultralong cycle life (10000 cycles with 10% optical contrast loss and 20000 cycles without degeneration at 633 nm). The excellent EC performance can be attributed to the ultrasmall size in all three-dimensions and no organic overlayer of WO3 quantum dots, which would greatly shorten the diffusion paths of intercalation ions, decrease interface barrier, provide fast charge-transport and electron-transfer kinetics and high reaction rates. Trivalent Al3+, as an alternative to common monovalent insertion ions (H+, Li+, Na+), was proven to be as an effective insertion ion for WO3 quantum dots. Compared with Li+ electrolyte, the films possess longer cycle life in Al3+ electrolyte, which can be attributed to the smaller ionic radius and the ability to support multi-electron redox reactions of Al3+. This research is an important first step for the fabrication of inexpensive EC smart windows, and should shape the future research on solution-based processes.

Far- and deep-ultraviolet surface plasmon resonance using Al film for efficient sensing of organic thin overlayer

Recently, a focus has been placed on plasmonics utilizing ultraviolet light because of its higher energy and shorter wavelength, compared to visible light. Al is a suitable metal for plasmonics in the ultraviolet region. However, Al is easily oxidized, and accurate control for Al film deposition is difficult. In this study, organic thin films were formed on the Al films, and their surface plasmon resonance (SPR) wavelengths were measured by our original attenuated total reflectance spectroscopic system. With an increase in the organic film thickness, the SPR wavelength shifted to longer wavelengths. The minimum detection thickness of the organic overlayer reached 2 nm for three Al films under different conditions (i.e., different Al thickness and oxidation effects), and the spectra were reproduced by simulations based on the Fresnel equations. These results showed the practical advantage of the Al-based SPR sensor without accurate control of the chemical state of the Al film.

Exciton Energy Transfer in Hybrid Organics—Semiconductor Nanostructure

We consider a hybrid heterostructure containing an inorganic quantum well in close proximity with organic material as overlayer. The resonant optical pumping of Frenkel exciton can lead to an efficient indirect pumping of Wannier excitons. As organic material in such a hybrid structure, we consider crystalline tetracene. In tetracene, the singlet exciton energy is close to twice the one of triplet exciton state and singlet exciton fission into two triplets can be efficient. This process in tetracene is thermally activated and we investigate here how the temperature-dependent exciton energy transfer affects the functional properties of hybrid organic-inorganic nanostructures. We have obtained the exact analytical solution of diffusion equation for organics at different temperatures defining different diffusion lengths of excitons. The effectiveness of energy transfer in hybrid with tetracene was calculated by definite method for two selected temperatures that open possibility to operate in full region of temperatures. Temperature dependence of energy transfer opens a new possibility to turn on and off the indirect pumping due to energy transfer from the organic subsystem to the inorganic subsystem.

Metal–Insulator Transition and Heterostructure Formation by Glycines Self-Assembled on Defect-Patterned Graphene

This study unveils critical physical and chemical processes taking place at the interface between an amino acid, glycine, and defected graphene. Although glycine interacts weakly through van der Waals attraction with pristine graphene, it can set rather strong bonding at the close proximity of low-coordinated C atoms of single and triple vacancies and also at the edges of nanoribbons. The adsorption of a glycine molecule first leads to a reconstruction of the defect through a concerted process, which in turn induces magnetic metal–nonmagnetic insulator transition. This way, glycine can be pinned at the defect site with well-defined atomic configuration and electronic structure. In particular, libration frequency of the adsorbed glycine is attributed to significant restoring forces, which is essential for the self-assembly of glycines. Organic overlayer produced by self-assembled glycines on defect-patterned graphene constitutes a junction (heterostructure) with bilateral electronic and optical properties ...

A simple approach to measuring thick organic films using the XPS inelastic background

In this work, we explore the possibility of finding a simple mathematical function that describes XPS background intensities as a function of energy and depth of emission. We describe the shape of the global structure of the background in XPS survey spectra for organic overlayer films with thicknesses greater than ~10 nm. In this situation, the shape of XPS backgrounds becomes statistical, and the detailed structure of, for example, energy‐loss functions and elastic‐scattering cross sections can be disregarded. Using Irganox 1010 as a model organic overlayer on silicon dioxide, gold, and silver substrates, a consistent description of the XPS background can be found, and overlayer thicknesses greater than 50 nm can be measured in the case of gold substrates. Furthermore, by using a suitable subset of these spectra, it is possible to separate the influence of overlayer thickness, electron kinetic energy, and intensity on the global shape of the spectrum. We show that it is possible to fully describe the shape of calibrated XPS survey spectra with reasonable accuracy for samples that have organic overlayers using only binding energy and sensitivity factor data. The same functions are shown to provide a reasonable description of the XPS spectra of silicon with an oxide overlayer.

Interface Doping for Ohmic Organic Semiconductor Contacts Using Self‐Aligned Polyelectrolyte Counterion Monolayer

Contact resistance limits the performance of organic field‐effect transistors, especially those based on high‐mobility semiconductors. Despite intensive research, the nature of this phenomenon is not well understood and mitigation strategies are largely limited to complex schemes often involving co‐evaporated doped interlayers. Here, this study shows that solution self‐assembly of a polyelectrolyte monolayer on a metal electrode can induce carrier doping at the contact of an organic semiconductor overlayer, which can be augmented by dopant ion‐exchange in the monolayer, to provide ohmic contacts for both p‐ and n‐type organic field‐effect transistors. The resultant 2D‐doped profile at the semiconductor interface is furthermore self‐aligned to the contact and stabilized against counterion migration. This study shows that Coulomb potential disordering by the polyelectrolyte shifts the semiconductor density‐of‐states into the gap to promote extrinsic doping and cascade carrier injection. Contact resistivities of the order of 0.1–1 Ω cm2 or less have been attained. This will likely also provide a platform for ohmic injection into other advanced semiconductors, including 2D and other nanomaterials.

Charge transfer quantification in a SnOx/CuPc semiconductor heterostructure: investigation of buried interface energy structure by photoelectron spectroscopies.

A tin oxide/copper phthalocyanine (CuPc) layer stack was investigated with two complementary photoemission methods. Non-destructive analysis of the electronic properties at the SnOx/CuPc interface was performed applying angle-dependent measurements with X-ray photoelectron spectroscopy (ADXPS) and energy-resolved photoemission yield spectroscopy (PYS). The different components (related to oxide layer and organic overlayer as well as to contamination features) observed in the spectra were assigned to a particular layer by relative depth plot analysis. ADXPS allowed determination of the chemical and electronic structure of the investigated samples. The addition of the organic ultra-thin film to the oxide layer caused a significant increase of the structure's photoemission yield. The combination of ADXPS and PYS allowed determination of the work function of constituent layers, and charge transfer phenomena at the SnOx/CuPc buried interface. An interface dipole of 0.23 eV was detected, assigned to charge transfer across the interface from the oxide layer towards the organic film. The energy level alignment at the SnOx/CuPc interface was determined, and presented in a band-like diagram, together with depth-dependent changes of the core energy levels of the structure's constituents. Finally the role of the oxide's defect-related energy levels in the charge transfer was discussed. The results obtained exhibit significance ranging from investigation, basic understanding and application of such hybrid films. Application of these results in hybrid electronic devices can help understanding and furthering this technology.

Electronic properties of the boroxine-gold interface: evidence of ultra-fast charge delocalization.

We performed a combined experimental and theoretical study of the assembly of phenylboronic acid on the Au(111) surface, which is found to lead to the formation of triphenylboroxines by spontaneous condensation of trimers of molecules. The interface between the boroxine group and the gold surface has been characterized in terms of its electronic properties, revealing the existence of an ultra-fast charge delocalization channel in the proximity of the oxygen atoms of the heterocyclic group. More specifically, the DFT calculations show the presence of an unoccupied electronic state localized on both the oxygen atoms of the adsorbed triphenylboroxine and the gold atoms of the topmost layer. By means of resonant Auger electron spectroscopy, we demonstrate that this interface state represents an ultra-fast charge delocalization channel. Boroxine groups are among the most widely adopted building blocks in the synthesis of covalent organic frameworks on surfaces. Our findings indicate that such systems, typically employed as templates for the growth of organic films, can also act as active interlayers that provide an efficient electronic transport channel bridging the inorganic substrate and organic overlayer.

X-ray nanoscopy study on metal nano-structure formation at a metal-organic interface

Scanning transmission X-ray microscopy (STXM) with ~30-nm spatial resolution revealed at annealing effect on the metal-organic interface of an Al layer with a poly (3-hexylthiophene) (P3HT) organic semiconducting layer with and without phenyl-C61-butyric acid methyl ester (PCBM). In the case of a P3HT/Al layer with a high-tensile-strain organic over-layer, the overall interface changed to a zagged morphology with Al nano pillars after thermal annealing. On the contrary, a P3HT:PCBM/Al layer with a less-tensile-strain organic over-layer showed a smooth interface except for a few nano pillars. These interfacial morphology changes were related to the initial strain status and to the relaxation process and the phase separation of P3HT crystals relating to the PCBM. Grazing-incident wide-angle X-ray scattering (GI-WAXS) measurements were also conducted to examine the residual strain and the crystalline properties of P3HT in the presence of PCBM.

Ordered Au Nanoparticle Array on Au(111) through Coverage Control of Precursor Metal–Organic Chains

Metal–organic overlayer structures formed by 1,4-phenylene-diisocyanide (PDI) and Au adatoms on Au(111) in UHV, their stability in air, and the tip-induced Au nanoparticle formation on PDI–Au(111) surfaces in air were investigated using scanning tunneling microscopy (STM) and vibrational spectroscopy. This study reveals that the distribution of Au nanoparticles created during tip-induced release of Au atoms from molecule-Au adatom complexes shows strong dependence on the PDI coverage. Ordered Au nanoparticle arrays form in the medium-coverage regime, while more disordered distributions are observed at low and saturation coverages. The different distributions of Au nanoparticles are a direct consequence of the coverage-dependent assembly of (PDI–Au)n chains, their different stability in air, and a templating effect of the Au(111) surface, which is most pronounced for medium coverage, where phases of densely packed (PDI–Au)n chains and disordered PDI–Au assemblies are confined, respectively, to the fcc and hcp regions of the (22 × √3) surface reconstruction of Au(111). The Au nanoparticles nucleate preferentially in the disordered or defective regions of the PDI–Au precursor overlayer, and their formation requires ambient air and high negative tip-bias, suggesting an electrochemical initiation of Au release from the molecule–Au adatom complexes.

Tuning and stabilizing topological insulator Bi2Se3 in the intrinsic regime by charge extraction with organic overlayers

In this work, we use charge extraction via organic overlayer deposition to lower the chemical potential of topological insulator (TI) Bi2Se3 thin films into the intrinsic (bulk-insulating) regime. We demonstrate the tuning and stabilization of intrinsic topological insulators at high mobility with low-cost organic films. With the protection of the organic charge extraction layers tetrafluorotetracyanoquinodimethane or tris(acetylacetonato)cobalt(III) (Co(acac)3), the sample is stable in the atmosphere with chemical potential ∼135 meV above the Dirac point (85 meV below the conduction band minimum, well within the topological insulator regime) after four months, which is an extraordinary level of environmental stability. The Co complex demonstrates the use of an organometallic for modulating TI charge density. The mobility of surface state electrons is enhanced as high as ∼2000 cm2/V s. Even at room temperature, a true topologically insulating state is realized and stabilized for months' exposure to the atmosphere.

Self-assembled monolayer engineered interfaces: Energy level alignment tuning through chain length and end-group polarity

We explore the different mechanisms through which self-assembled monolayers can tailor energy level alignment at metal–organic semiconductor interfaces. We show that the large work function variation that can be induced by the self-assembled monolayer on gold has limited ability to tailor the interface energy level alignment of a subsequent organic semiconductor overlayer.

Organic–inorganic heterostructures for nonlinear optics

We consider a hybrid heterostructure made of an inorganic quantum well in close proximity with an organic material overlayer, whereby the latter is used to funnel excitation energy to the former in order to exploit the optical nonlinearities of the two-dimensional Wannier excitons. The resonant optical pumping of the Frenkel excitons and their diffusion to the organic–inorganic interface can lead to an efficient indirect pumping of the inorganic quantum well turning on the corresponding nonlinearities. As organic material we consider a layer of anthracene or of tetracene. In the latter case, the singlet exciton has an energy which is close to twice the one of a triplet exciton and singlet exciton fission into two triplets can be efficient. In tetracene based hybrid heterostructures, the temperature dependence of fission opens the possibility to turn on and off the indirect pumping due to energy transfer from the organic into the inorganic subsystem. Finally, we show how a generic mechanism of dipole–dipole hybridization may lead to the formation of virtual heterodimers of organic molecules with an enhanced nonlinear optical response.