Molecular Adlayers Research Articles

Toward Gigahertz Photodetection with Transition Metal Dichalcogenides.

ConspectusTransition metal dichalcogenides (TMDCs) exhibit favorable properties for optical communication in the gigahertz (GHz) regime, such as large mobilities, high extinction coefficients, cheap fabrication, and silicon compatibility. While impressive improvements in their sensitivity have been realized over the past decade, the bandwidths of these devices have been mostly limited to a few megahertz. We argue that this shortcoming originates in the relatively large RC constants of TMDC-based photodetectors, which suffer from high surface defect densities, inefficient charge carrier injection at the electrode/TMDC interface, and long charging times. However, we show in a series of papers that rather simple adjustments in the device architecture afford TMDC-based photodetectors with bandwidths of several hundreds of megahertz. We rationalize the success of these adjustments in terms of the specific physical-chemical properties of TMDCs, namely their anisotropic in-plane/out-of-plane carrier behavior, large optical absorption, and chalcogenide-dependent surface chemistry. Just one surprisingly simple yet effective pathway to fast TMDC photodetection is the reduction of the photoresistance by using light-focusing optics, which enables bandwidths of 0.23 GHz with an energy consumption of only 27 fJ/bit.By reflecting on the ultrafast intrinsic photoresponse times of a few picoseconds in TMDC heterostructures, we motivate the application of more demanding chemical strategies to exploit such ultrafast intrinsic properties for true GHz operation in real devices. A key aspect in this regard is the management of surface defects, which we discuss in terms of its dependence on the layer thickness, its tunability by molecular adlayers, and the prospects of replacing thermally evaporated metal contacts by laser-printed electrodes fabricated with inks of metalloid clusters. We highlight the benefits of combining TMDCs with graphene to heterostructures that exhibit the ultrafast photoresponse and large spectral range of Dirac materials with the low dark currents and high responsivities of semiconductors. We introduce the bulk photovoltaic effect in TMDC-based materials with broken inversion symmetry as well as a combination of TMDCs with plasmonic nanostructures as means for increasing the bandwidth and responsivity simultaneously. Finally, we describe the prospects of embedding TMDC photodetectors into optical cavities with the objective of tuning the lifetime of the photoexcited state and increasing the carrier mobility in the photoactive layer.The findings and concepts detailed in this Account demonstrate that GHz photodetection with TMDCs is feasible, and we hope that these bright prospects for their application as next-generation optoelectronic materials motivate more chemists and material scientists to actively pursue the development of the more complicated material combinations outlined here.

Effects of N-functional groups on the electron transfer kinetics of VO2+/VO2+ at carbon: Decoupling morphology from chemical effects using model systems

Carbons and nanocarbons are important electrode materials for vanadium redox flow battery applications, however, the kinetics of vanadium species are often sluggish at these surfaces, thus prompting interest in functionalization strategies to improve performance. Herein, we investigate the effect of N-functionalities on the VO2+/VO2+ redox process at carbon electrodes. We fabricate thin film carbon disk electrodes that are metal-free, possess well-defined geometry and display smooth topography, while featuring different N-site distribution, thus enabling a mechanistic investigation of the intrinsic surface activity towards VO2+/VO2+. Voltammetry and electrochemical impedance spectroscopy show that N-functionalities improve performance, with pyridinic/pyrrolic-N imparting the most significant improvements in charge transfer rates and reversibility, compared to graphitic-N. This was further supported by voltammetry studies on nitrogen-free electrodes modified via aryldiazonium chemistry with molecular pyridyl adlayers. Computational modeling using an electrochemical-chemical mechanism indicates that introduction of surface pyridinic/pyrrolic-N can increase the heterogeneous rate constants by approximately two orders of magnitude relative to those observed at nitrogen-free carbon (k0 = 1.29 × 10−4 vs 9.34 × 10−7 cm/s). Simulations also suggest that these N-functionalities play a role in affecting reaction rates in the chemical step. Our results indicate that nitrogen incorporation via basic functional groups offers an interesting route to the design of advanced carbon electrodes for VRFB devices.

F-Center-Mediated Growth of Patterned Organic Semiconductor Films on Alkali Halides

Organic semiconductors combine flexible tailoring of their optoelectronic properties by synthetic means with strong light-matter coupling, which is advantageous for organic electronic device applications. Although spatially selective deposition has been demonstrated, lateral patterning of organic films with simultaneous control of molecular and crystalline orientation is lacking as traditional lithography is not applicable. Here, a new patterning approach based on surface-localized F-centers (halide vacancies) generated by electron irradiation of alkali halides is presented, which allows structural control of molecular adlayers. Combining optical and atomic force microscopy, X-ray diffraction, and density functional theory (DFT) calculations, it is shown that dinaphthothienothiophene (DNTT) molecules adopt an upright orientation on pristine KCl surfaces, while the F-centers stabilize a recumbent orientation, and that these orientations are maintained in thicker films. This specific nucleation results also in different crystallographic morphologies, namely, densely packed islands and jagged fibers, each epitaxially aligned on the KCl surface. Spatially selective surface irradiation can also be used to create patterns of F-centers and thus laterally patterned DNTT films, which can be further transferred to any (including elastomer) substrate due to the water solubility of the alkali halide growth templates.

Nanopatterning Graphite Surface by Diazoniums Using Electrochemical Method

Molecular functionalization of graphitic surfaces with nanopatterned structures is regarded as one of the effective bottom-up techniques to tune their electronic properties towards electronics applications. Diazonium molecules have been often employed to covalently functionalize graphene and highly oriented pyrolytic graphite (HOPG) substrates. However, controlling the structure of the molecular adlayers is still challenging. In this contribution, we demonstrated an inconventional approach for covalent functionalization the HOPG surface by using mixture of 4-nitrobenzenediazonium (4-NBD) and 3,5-bis-tert-butylbenzenediazonium (3,5-TBD) molecules in which the former tends to polimezise and physisorb while the later chemically anchors on surface. The physisorbed features can be removed by washing with hot toluene and water. As a result, the HOPG surface is patterned in a quasi-periodic fashion. The efficiency of this development was verified by a combination of cyclic voltametry (CV) and atomic force microscopy (AFM) methods. This finding represents a convenient strategy for creating nanoconfined templates that might serve as nano-playgrounds for further supramolecular self-assembly and other on-surface reactions.

On the Role of Collective Electrostatic Effects in Electronic Level Pinning and Work Function Changes by Molecular Adlayers: The Case of Partially Fluorinated DNTTs Adsorbed Flat‐Lying on Various Metals and Hetero‐Structures (Adv. Mater. Interfaces 18/2022)

Buried Interface Dipoles The formation of a buried interface dipole at a copper/2D cesium/organic hetero-interface, which can be manipulated by the outer organic monolayer is reported in article number 2200361 by Maximilian Dreher, Gregor Witte, and co-workers. By combined experimental and theoretical methods the contribution of the inner and outer interface dipoles could be deciphered and their influence on the overall work function shift identified.

On the Role of Collective Electrostatic Effects in Electronic Level Pinning and Work Function Changes by Molecular Adlayers: The Case of Partially Fluorinated DNTTs Adsorbed Flat‐Lying on Various Metals and Hetero‐Structures

AbstractModifying the work function of metal electrodes by monolayers of molecules with specifically tailored electronic properties is a versatile tool, but such chemical modifications often also affect the adsorption geometry and packing density, making microscopic modeling difficult. Using scanning tunneling microscopy, it is shown that the recently synthesized partially fluorinated dinaphthothienothiophenes (DNTTs) adopt the same interface structure on different metal substrates independent of the degree of fluorination. Combining Kelvin probe measurements and density functional theory (DFT) calculations, a highest occupied molecular orbital (HOMO) pinning effect for such FxDNTTs on Au(111) and Ag(111) induced by collective electrostatic interactions in the monolayer is observed. Since the adsorption of weakly interacting molecules such as the FxDNTTs is not restricted to specific surfaces as is the case with SAMs, this concept is extended to metal substrates with quite different work function values. For a low work function surface such as Cs(110), a lowest unoccupied molecular orbital (LUMO) pinning effect is predicted at the theoretical level. Since such alkali metal surfaces are not experimentally accessible, a well‐defined Cs monolayer on Cu(100) as a low work function substrate is used instead. For this substrate, however, a variation is observed in the LUMO energies and the work function as a function of the degree of fluorination. This is attributed to the formation of a second interface dipole at the buried Cs/Cu interface, which is modulated with the degree of fluorination and competes with the dipole at the outer molecule/Cs interface. Such a second internal interface dipole, which can be modified by the top layer, has to be considered when going to more complex heterointerfaces.

Role of Adatoms for the Adsorption of F4TCNQ on Au(111).

Organic adlayers on inorganic substrates often contain adatoms, which can be incorporated within the adsorbed molecular species, forming two-dimensional metal–organic frameworks at the substrate surface. The interplay between native adatoms and adsorbed molecules significantly changes various adlayer properties such as the adsorption geometry, the bond strength between the substrate and the adsorbed species, or the work function at the interface. Here, we use dispersion-corrected density functional theory to gain insight into the energetics that drive the incorporation of native adatoms within molecular adlayers based on the prototypical, experimentally well-characterized system of F4TCNQ on Au(111). We explain the adatom-induced modifications in the adsorption geometry and the adsorption energy based on the electronic structure and charge transfer at the interface.

Molecular Imaging of Viologen Adlayers and In Situ Monitoring Structural Transformations at Electrode-Electrolyte Interfaces.

The effects of temperature and molecular concentration on the ordering of two-dimensional (2D) nanostructures have been investigated at the well-defined Au(111)-electrolyte interface. In comparison to the assembly of thiolated alkanes or hydrogen-bonded nonthiolated molecules, fabricating large aromatic thiolated molecules into a highly ordered adlayer on a surface remained a challenge. In this study, we demonstrated the importance of controlling the assembly conditions and procedures for the formation of ordered adlayers of redox-active viologen derivatives. The assembly conditions that were explored include the variation of molar concentration of assembly solutions, assembly time, and thermal annealing. We report that the optimal assembly conditions for creating highly ordered thiolated viologen derivatives on a Au(111)-(1 × 1) electrode surface are to limit the time in which the electrode is immersed in a deoxygenated 0.05 mM ethanolic viologen solution (preheated to 70 °C) to 45 s, followed by thermal annealing in absolute ethanol for 12 h. Highly ordered molecular adlayers were imaged by electrochemical scanning tunneling microscopy (STM), revealing the molecular packing of low-coverage adlayers. Furthermore, in situ STM combined with cyclic voltammetry (CV) allowed for the exploration of the structural transformation and potential limit of reductive and "oxidative" desorption of adlayers within the electrochemical potential range of the sample potential (ES) from -0.95 V to -0.10 V vs SCE.

Tailoring Superconductivity in Large-Area Single-Layer NbSe2 via Self-Assembled Molecular Adlayers.

Two-dimensional transition metal dichalcogenides (TMDs) represent an ideal testbench for the search of materials by design, because their optoelectronic properties can be manipulated through surface engineering and molecular functionalization. However, the impact of molecules on intrinsic physical properties of TMDs, such as superconductivity, remains largely unexplored. In this work, the critical temperature (TC) of large-area NbSe2 monolayers is manipulated, employing ultrathin molecular adlayers. Spectroscopic evidence indicates that aligned molecular dipoles within the self-assembled layers act as a fixed gate terminal, collectively generating a macroscopic electrostatic field on NbSe2. This results in an ∼55% increase and a 70% decrease in TC depending on the electric field polarity, which is controlled via molecular selection. The reported functionalization, which improves the air stability of NbSe2, is efficient, practical, up-scalable, and suited to functionalize large-area TMDs. Our results indicate the potential of hybrid 2D materials as a novel platform for tunable superconductivity.

Controlled Covalent Functionalization of 2 H-MoS2 with Molecular or Polymeric Adlayers.

Most air-stable 2D materials are relatively inert, which makes their chemical modification difficult. In particular, in the case of MoS2 , the semiconducting 2 H-MoS2 is much less reactive than its metallic counterpart, 1T-MoS2 . As a consequence, there are hardly any reliable methods for the covalent modification of 2 H-MoS2 . An ideal method for the chemical functionalization of such materials should be both mild, not requiring the introduction of a large number of defects, and versatile, allowing for the decoration with as many different functional groups as possible. Herein, a comprehensive study on the covalent functionalization of 2 H-MoS2 with maleimides is presented. The use of a base (Et3 N) leads to the in situ formation of a succinimide polymer layer, covalently connected to MoS2 . In contrast, in the absence of base, functionalization stops at the molecular level. Moreover, the functionalization protocol is mild (occurs at room temperature), fast (nearly complete in 1 h), and very flexible (11 different solvents and 10 different maleimides tested). In practical terms, the procedures described here allow for the chemist to manipulate 2 H-MoS2 in a very flexible way, decorating it with polymers or molecules, and with a wide range of functional groups for subsequent modification. Conceptually, the spurious formation of an organic polymer might be general to other methods of functionalization of 2D materials, where a large excess of molecular reagents is typically used.

Doping-Induced Electron Transfer at Organic/Oxide Interfaces: Direct Evidence from Infrared Spectroscopy.

Charge transfer at organic/inorganic interfaces critically influences the properties of molecular adlayers. Although for metals such charge transfers are well documented by experimental and theoretical results, in the case of semiconductors, clear and direct evidence for a transfer of electrons or holes from oxides with their typically high ionization energy is missing. Here, we present data from infrared reflection–absorption spectroscopy demonstrating that despite a high ionization energy, electrons are transferred from ZnO into a prototype strong molecular electron acceptor, hexafluoro-tetracyano-naphthoquinodimethane (F6-TCNNQ). Because there are no previous studies of this type, the interpretation of the pronounced vibrational red shifts observed in the experiment was aided by a thorough theoretical analysis using density functional theory. The calculations reveal that two mechanisms govern the pronounced vibrational band shifts of the adsorbed molecules: electron transfer into unoccupied molecular levels of the organic acceptor and also the bonding between the surface Zn atoms and the peripheral cyano groups. These combined experimental data and the theoretical analysis provide the so-far missing evidence of interfacial electron transfer from high ionization energy inorganic semiconductors to molecular acceptors and indicates that n-doping of ZnO plays a crucial role.

Probing Molecular Nanostructures of Aromatic Terephthalic Acids Triggered by Intermolecular Hydrogen Bonds and Electrochemical Potential.

Self-assembly provides unique routes to create supramolecular nanostructures at well-defined surfaces. In the present work, we employed scanning tunneling microscopy (STM) in combination with electrochemical techniques to explore the adsorption and phase formation of a series of aromatic carboxylic acids (ACAs) at Au(111)/0.1 M HClO4. Specific goals are to elucidate the roles of electrochemical potential and directional hydrogen-bonding on the structures and orientation of individual ACAs that form nanoarchitectures. ACAs are prototype materials for supramolecular self-assemblies via stereospecific hydrogen bonds between neighboring molecules. In this study, we mainly focus on a special ACA, terephthalic acid (TPA), which is almost insoluble in water, making the assembly of this molecule from aqueous solution challenging. Depending on the applied electric field, TPA molecules form distinctly different, highly ordered adlayers on Au(111) triggered by directional intermolecular hydrogen bonds. At low electrochemical potentials, TPA molecules are planar oriented, forming a potentially infinite hydrogen-bonded adlayer without any observed domain boundaries. The increase of the electrode potential triggers the deprotonation of one carboxylic acid functional group of TPA; additionally, this is accompanied by an orientation change of molecules from planar to perpendicular. In contrast, structural "defects" and multiple domain boundaries were found at this positively charged surface. The assembled nanostructures of TPA are compared with other ACAs (trimesic acid, benzoic acid, and isophthalic acid), and corresponding adsorption models were built for all molecular adlayers, showing that intermolecular hydrogen-bonding plays a determining role in the formation of two-dimensional ACA nanostructures.

Electrochemical metallization of molecular adlayers

Electrochemical metallization of molecular adlayers allows the sandwiching of a group of molecules between a metallic substrate and a metal nanoisland or nanocluster through a wet process not requiring expensive vacuum equipment and with precise control through the electrode potential. These metal–molecule–metal sandwiches could provide a simple means to integrate molecules in electronic circuits or to explore effects arising because of quantum confinement of electrons due to the reduced dimensions of the islands or clusters deposited on the adlayer. Although initial attempts resulted in penetration through the adlayer and direct deposition on the metal substrate, the development 14 years ago of a two-step technique involving preadsorption of the target cation on the self-assembled monolayer before its reduction in an electrolyte not containing that cation opened a whole new range of possibilities. In this brief review, we discuss the development and recent advances in the implementation of this technique and briefly discuss the outlook for future work.

Gold-Induced Desulfurization in a Bis(ferrocenyl) Alkane Dithiol

Thiol-modified ferrocenes on gold have been archetypical model systems for many fundamental charge transfer and other studies, since both thiol-gold and ferrocene redox chemistry are considered to be well-understood. Thus unexpectedly, we found that for a representative of a new class of flexibly linked bis-ferrocenyl compounds, namely, 1-10-bis(1-ferrocenyl)decane dithiol, surface immobilization on gold failed. Instead, in the presence of gold, molecular decomposition took place, resulting in sulfur-based adlayers and well-defined molecular elimination products, for which we provide spectroscopic evidence. Careful control experiments and comparison with related ferrocene compounds provide insight into the mechanism of the observed elimination reactions, as a combined effect of the molecular structure and the nature of the gold/sulfur bond. These findings, thus, have a broader impact on the design of molecular adlayers, for example, in the context of surface functionalization in sensing or the synthesis o...

Noncontact AFM and differential reflectance spectroscopy joint analyses of bis-pyrenyl thin films on bulk insulators: Relationship between structural and optical properties

The combined characterization of structural and optical properties of organic interfaces adsorbed on inorganic solid substrates down to the molecular scale is crucial from a fundamental point of view, but also if one tackles efficient applied devices. In this work, a set of joint structural and optical analyses of self-assemblies of $\ensuremath{\pi}$-conjugated bis-pyrene derivatives upon adsorption on two alkali halides bulk insulators is reported. The structural analysis is performed by means of noncontact atomic force microscopy in ultrahigh vacuum either at room or at the liquid-nitrogen temperature with molecular resolution. The surface coverage ranges from the submonolayer (ML) regime up to 5 ML. In situ optical spectroscopy is performed by means of differential reflectance (DR) spectroscopy. A thorough fitting methodology of the DR spectra allows us to derive the complete dielectric function of the molecular adlayers treated in an anisotropic formalism, albeit restricted to an uniaxial approximation. Conclusions regarding the process of condensation of the molecules into H aggregates from its early stages up to the solid molecular phase are drawn. This work highlights three main reasons to bridge high-resolution structural and optical characterization of the molecular layers, which all point towards the necessity to constrain the fitting process, namely, (i) characterizing the growth mode of the molecules, (ii) identifying the structural order of the resulting assemblies, and (iii) discriminating their constitutive phases by means of molecular resolution imaging.

Insights into the self-assembly of aromatic dinitroso derivatives on gold surface

Adsorption of compounds with exposed aromatic C-nitroso groups on metal surfaces provides a way for fabrication of self-assembled bilayers (SABs) owing to ability of these compounds to dimerize to azodioxides. Here, we have investigated the possibility of self-assembly and dimerization of sulfur-containing aromatic dinitroso derivatives on an Au(111) surface by using polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS), atomic force microscopy (AFM), scanning tunneling microscopy (STM), ellipsometry and water contact angle goniometry. While aromatic dinitroso derivatives are present as E-isomeric oligomers in solid state, PM-IRRAS spectra of their adlayers on gold surface revealed the presence of signals characteristic for nitroso monomers and E-azodioxy dimers. AFM and STM images of molecular adlayers showed the appearance of separate domains on an Au(111) surface suggesting formation of monolayer and bilayer-type structures. These results were further corroborated by ellipsometry measurements, which pointed to the presence of a non-uniform molecular adlayer covering gold surface. Comparison with previously studied systems of mononitroso derivatives indicates that adsorption of aromatic dinitroso derivatives on gold surface results in the formation of less ordered adlayers probably due to the steric hindrance of meta-situated nitroso groups.

Strong Hydrogen Bonds at the Interface between Proton-Donating and -Accepting Self-Assembled Monolayers on Au(111)

Hydrogen-bonding heterogeneous bilayers on substrates have been studied as a base for new functions of molecular adlayers by means of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), and density functional theory (DFT) calculations. Here, we report the formation of the catechol-fused bis(methylthio)tetrathiafulvalene (H2Cat-BMT-TTF) adlayer hydrogen bonding with an imidazole-terminated alkanethiolate self-assembled monolayer (Im-SAM) on Au(111). The heterogeneous bilayer is realized by sequential two-step immersions in solutions for the individual Im-SAM and H2Cat-BMT-TTF adlayer formations. In the measurements by AFM, a grained H2Cat-BMT-TTF adlayer on Im-SAM is revealed. The coverage and the chemical states of H2Cat-BMT-TTF on Im-SAM are specified by XPS. On the vibrational spectrum measured by IRAS, the strong hydrogen bonds between H2Cat-BMT-TTF and Im-SAM are characterized by the remarkably red-shifted OH stretching mode at 3140 cm-1, which is much lower than that for hydrogen-bonding water (typically ∼3300 cm-1). The OH stretching mode frequency and the adsorption strength for the H2Cat-BMT-TTF molecule hydrogen bonding with imidazole groups are quantitatively examined on the basis of DFT calculations.

An experimental and theoretical study of adenine adsorption on Au(111).

A model study of adenine adsorption on the Au(111) surface is reported for molecular adlayers prepared by evaporation in vacuum and deposition from saturated aqueous solution. The electronic structure and adsorption geometry of the molecular films were studied experimentally by X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy. Adsorption models are proposed for the adlayers arising from the different preparation methods. Density functional theory calculations were used to examine both parallel and upright adenine adsorption geometries, supply additional information on the bond strength, and identify which atom is involved in bonding to Au(111). In the case of deposition in vacuum, the adenine molecule is bound via van der Waals forces to Au(111) with the molecular plane parallel to the surface, consistent with the published scanning tunneling microscopy data on this system. The most stable parallel adenine configuration was found to have an adsorption energy of ca. -1.1 eV using the optB86b-vdW functional. For adenine deposition from aqueous solution, the adlayer is disordered, with molecules in an upright geometry, and with an adsorption energy of ca. -1.0 eV, coordinated via the imino N3 nitrogen atom. The present study contributes to the substantial literature of model studies of adenine on Au(111), complementing the existing knowledge with information on electronic structure, bonding geometry and adsorption energy of this system.

Adatom Coadsorption with Three-Dimensional Cyclophanes on Ag(111)

The structure of molecular adlayers is of great interest for surface functionalization. As molecular complexity increases, the subtle interplay of the relevant interactions becomes more difficult to unravel. Here, we present a scanning tunneling microscope (STM) and atomic force microscope study along with free-energy calculations using density functional theory on two closely related NDI-cyclophane molecules. These three-dimensional double-decker molecules are designed to attach to the substrate with one subunit while the other functional moiety is exposed to the environment. The molecular arrangements obtained on Ag(111) are rationalized by the inclusion of adatoms from the substrate into the structure. The presence of adatoms is identified by a drastic change in corrugation of the STM images that takes place at moderate bias voltages. Our calculations using density functional theory of the system’s free-energy yield that two adatoms favorably coadsorb with the molecules.

Formation of heterocaryotic and homonuclear bridged–dimeric complexes on surface

Abstract The formation of coordinated dimeric complexes bridged by axial ligands on surface is observed with the help of a 1,3,5-tris(10-carboxydecyloxy)benzene (TCDB) template through scanning tunneling microscopy (STM). STM images of molecular adlayers of zinc tetraphenylporphyrin (ZnTPP), zinc phthalocyanine (ZnPc), and their mixture are reported. ZnTPP and ZnPc can spontaneously form highly an ordered structure with a 1:1 molar ratio, which is different from that of individual ZnPc. The coordinated bimolecular complexes bridged with axial ligands, simply as ZnPc–DPP–ZnTPP and ZnPc–DPE–ZnPc, are presented and the corresponding surface structures are compared. ZnPc and ZnTPP can be connected by an axial ligand DPP and formed assembled structures out of surface. Two types of arrays with entirely new structure are obtained for the ZnPc–DPE–ZnPc complex. These bridged hybrid complexes provide an example of design of self-organized crystals on the basis of coordination through non-covalent interactions.